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NACE 

INTERNATIONAL 



TRAINING & 
CERTIFICATION 




CP 3-Cathodic Protection 
Technologist 

COURSE IMANUAL 



J uly 2008 
© NACE International, 2005 



Ac kno wiedgements 



The time and expertise of a many members of NACE International have gone 
into the development of this course. Their dedication and efforts are greatly 
appreciated by the authors of this course and by those who have assisted in 
making this work possible. 

The scope, desired learning outcomes and performance criteria were prepared 
by the NACE Cathodic Protection Subcommittee under the auspices of the 
NACE Certification and Education Committees. Special thanks go to this 
subcommittee. 

Cathodic Protection Subcommittee 

Paul Nichols Shell Global Solutions, Houston, Texas 

Brian Holtsbaum CC Technologies, Calgary, Alberta 

Don Mayfield Dominion Transmission, Delmont, Pennsylvania 

Steve Nelson Columbia Gas Transmission, Charleston, WestVirginia 

Kevin Parker CC Technologies, Mt. Pleasant, Michigan 

David A. Schramm ENEngineering, Woodridge, Illinois 

Steve Zurbuchen OneOK Inc., Topeka, Kansas 

This group of NACE members worked closely with the contracted course 
developers, Rob Wakelin, CorrEng Consulting Service, Inc., (Downsview, 
Ontario), Bob Gummow, CorrEng Consulting Service, Inc., (Downsview, Ontario) 
and Tom Lewis, Loresco International (Hattiesburg, Mississippi). 



IMPORTANT NOTICE: 

Neither the NACE International, its officers, directors, nor members thereof 
accept any responsibility for the use of the methods and materials discussed 
herein. No authorization is implied concerning the use of patented or copyrighted 
material. The information is advisory only and the use of the materials and 
methods is solely at the risk of the user. 

Printed in the United States. All rights reserved. Reproduction of contents in 
whole or part or transfer into electronic or photographic storage without 
permission of copyright owner is expressly forbidden. 



NACE COATINGS NETWORK 

(NCN) 



NACE has established the NACE Coatings Network, an electronic list serve that 
is free to the public. It facilitates communications among professionals who work 
in all facets of corrosion prevention and control. 

If you subscribe to the NACE Coatings Network, you will be part of an 
E-mail-driven open discussion forum on topics A-Z in the coatings industry. Got 
a question? Just ask. Got the answer? Share it\ The discussions sometimes 
will be one-time questions, and sometimes there will be debates. 

What do you need to join? An E-mail address. That's all! Then: 



1 . To Subscribe, send a blank e-mail to: 

Join-coatings@nacecorrosionnetwork.com 

To Unsubscribe, send a blank e-mail to: 
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You're done! You'll get an e-mail back telling you how to participate, but 
it's so easy that you'll figure it out without any help. 



Table of Contents 



CP 3-Cathodic Protection Technologist 
Course ivjanual 

Table of Contents 



General Course Information 



Introduction 



Cathodic Protection Technologist Certification 
Application 



Chapter 1 
Mechanisms of Corrosion 



1.1 Thermodynamic Considerations 1:1 

1.2 The Pourbaix Diagram 1:5 

1.3 The Electrode Potential 1:8 

1.3.1 The Electromotive Force Series 1:9 

1.3.2 T lie Nernst Equation 1:10 

1.3.3 Common Reference Electrodes 1:10 

1.3.4 Effect of Temperature on Reference Electrode Potentials 1:11 

1.3.5 Converting Measured Potentials Between Reference Electrodes 1:12 

1.4 The Corrosion Cell 1:13 

1.4.1 Corrosion Cell Components 1:15 

1.4.2 Corrosion Cell Kinetics (Polarization) 1:16 

1.5 Faraday's Law 1:22 

1.6 Corrosion Potential 1:26 

1.7 Factors Affecting the Operation of a Corrosion Cell 1:30 

1.7.1 Depolarization of a Corrosion Cell 1:31 

(a) Cathode Depolarization 1:32 

(b) Anode Depolarization 1:33 

1.7.2 Increased Polarization in a Corrosion Cell 1:34 

(a) Increased Polarization atthe Cathode of a Corrosion Cell.... 1:35 

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J uly 2008 



Table of Contents 



(b) Increased Polarization at the Anode of a Corrosion Cell 1:35 

1.7.3 Circuit Resistance Changes 1:36 

(a) Increase in Resistance in a Corrosion Cell 1:37 

(b) Decrease in Resistance in a Corrosion Cell 1:38 

1.7.4 Effect of Driving Voltage on a Corrosion Cell 1:39 

1.7.5 EffectofTime on a Corrosion Cell 1:40 

1.7.6 Randies Circuit Model for an Electrode Interface in a 

Corrosion Cell 1:41 

1.7.7 Types of Corrosion 1:43 

(a) Galvanic Corrosion 1:45 

Experiment 1.1 - To Demonstrate Polarization in a Corrosion Cell 1:47 



Chapter 2 
Cathodic Protection Theory 



2,1 Definition 



2:1 



2.2 Criteria 2:4 

2.2.1 Potential Criterion (-850 mVcsE) 2:5 

2.2.2 Polarization Shift Critshon (100 mV) 2:9 

2.2.3 Factors Affecting Validity of Criteria 2:11 

2.2.3.1 Tenrperature 2:11 

2.2.3.2 Sulfate Reducing Bacteria 2:11 

2.2.3.3 Alternating Current (AC) Density. 2:12 

2.2.3.4 Type of Metal 2:14 

2.2.3.5 Mixed Metals 2:17 

2.2.3. 6 S tress Corrosion Cracking (SCC) 2:17 

2.2.3.7 Disbonded Coatings 2:18 

2.3 Typical Cathodic Polarization Characteristics 2:21 

2.3.1 Cathodic Polarization Cutve 2:21 

2.3.2 Activation and Concentration Polarization 2:27 

2.3.3 F actors Affecting Polarization 2:31 

2.3.3.1 Aeration (Oxygen) 2:34 

2.3.3.2 Agitation (Velocity) 2:35 

2.3.3.3 Temperature 2:36 

2.3.3.4 pH 2:37 

2.3.3.5 Surface Area 2:40 

2.3.3.6 Effect of Tinne 2:43 

2.4 Types of Cathodic Protection Systems 2:46 

2.4.1 Galvanic (Sacrificial) Anodes 2:47 

2.4.1.1 Aluminum Anodes 2:47 

2.4.1.2 Magnesium Anodes 2:50 



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C P 3- Cathodic P ro tec tion Technologist 

J uly 2008 



Table of Contents 



III 



2.4.1.3 Zinc Anodes 2:52 

2.4.1.4 Polarization Diagram 2:55 

2.4.1.5 Badcfiii 2:56 

2.4.1.6 Typical Uses 2:58 

2.4.2 Impressed-Current Anodes 2:58 

2.4.2.1 Massive (Large) Anodes 2:59 

2.4.2.2 Dimensionally Stable Anodes 2:63 

a. Platinum Anodes 2:63 

b. Mixed-Metal Oxide Anodes 2:65 

c. Polymer Anodes 2:66 

2.4.3 Polarization Diagram 2:66 

2.4.4 Carbon Backfill 2:67 

2.4.5 Typical Uses 2:70 

2.4.6 I mpressed-C urrent P ower Supplies 2:71 

2.4.6.1 a. Standard Transformer/Rectifiers 2:71 

b. Silicon Controlled Rectifiers (SCR) 2:75 

c. Switching-Mode Rectifiers 2:78 

2.4.6.2 Solar Power Supplies 2:79 

2.4.6.3 Wind-Driven Generators 2:81 

2.4.6.4 Batteries 2:82 

2.4.6.5 Thermoelectric Generators (TEG) 2:82 

2.4.6.6 Fuel Cells 2:83 

2.4.6.7 Modes of Operation 2:85 

Experiment 2-1: Demonstrate the Use of a Sacrificial Anode to 

M itigate Corrosion in a L ocal-A ction Cell 2:85 



Chapters 
Intetference 



3.1 Introduction 3:1 

3.2 Detecting Stray Current 3:11 

3.2.1 Effects of Stray Current on Metallic Structures 3:13 

a. At Area of Current Pick-Up 3:13 

b. Along the Structure 3:15 

c. Effects at the Stray Current Discharge Location 3:17 

3.2.2 Mitigation of Interference Effects from Impressed Current 
Cathodic Protection Systems 3:19 

a. Source Removal or OutputReduction 3:19 

b. Installation of Isolating Fittings 3:20 

c. Burying a Metallic Shield Next to the Interfered -with 

Structure 3:21 

d. Installation of Galvanic Anodes on Interfered -with 



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J uly 2008 



Table of Contents iv 



Structure atPointof Stray CurrentDischarge 3:22 

e. Installation of an Impressed Current Distribution 

System on the Interfered -with Structure at Point 

of Stray CurrentDischarge 3:25 

f. Installing a Bond Between the Interfered -with and 

Interfering Structures 3:25 

g. Use of Coatings In the Mitigation of Interference Effects 3:27 
3.2.3 Other Sources of DC Stray Current 3:28 

a. DC Transit Systems 3:28 

I. Mitigation ofTransltSystem Stray Currents 3:31 

b. High Voltage Direct Current (HVDC) Electrical 

Transmission Systems 3:35 

c. DC Welding Operations 3:37 

3.3 AC Interference 3:38 

3.3.1 Introduction 3:38 

3.3.2 Conductive Coupling Due to Faults 3:40 

a. Description 3:40 

b. Deleterious Effects 3:41 

c. Prediction and Mitigation 3:42 

3.3.3 Electrostatic (Capacltlve) Coupling 3:47 

a. Description 3:47 

b. Deleterious Effects and Mitigation 3:49 

3.3.4 Electromagnetic (Inductive Coupling) 3:50 

a. Description 3:50 

b. AC Corrosion 3:52 

c. Electrical Shock Hazards 3:54 

d. Prediction 3:55 

e. Mitigation 3:65 

3.4 Telluric Current Interference 3:66 

3.4.1 Interference Effects 3:67 

3.4.2 Mitigation ofTellurIc Current Effects 3:74 

Experiment 3-1: To Demonstrate DC Interference 

and Its Mitigation 3:77 



Chapter 4 
CP Design Fundamentals 



4.1 Design Objectives 4:1 

4.2 Determining Current Requirements 4:4 

4.2.1 Current Density 4:4 

4.3 Current Requirement Estimating Methods 4:8 

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Table of Contents 



4.3.1 Literature Sources 4:9 

4.3.2 Experience on Similar Structures in Similar Conditions 4:11 

4.3.3 Determining Current R equirements on a Coated Structure 

by Estimating the Percentage Bare 4:11 

4.3.4 Minimum Voltage Drop Method 4:13 

a. Field Test to Determine Current Required on a 

Pipeline Based on Minimum Voltage Drop 4:14 

b. Using P ipe-to-E arth R esistance to Determine Current 

Required by the Voltage Drop Method 4:15 

c. Calculation of Current Required to Achieve a Minimum 

Voltage Drop on a Coated Structure Based on 

Coating Resistance 4:15 

4.3.5 Polarization TestMethod 4:17 

4.3.6 Polarization ShiftMethod 4:20 

4.4 Calculation of Cathodic ProtECtion Circuit Resistances 4:21 

4.4.1 Resistance of a Single Rod Shaped Anode Positioned 

Vertically in the Earth 4:23 

4.4.2 Resistance of Multiple Vertical Anodes Connected to a 

Common Header Cable orStructure 4:25 

4.4.3 Resistance of a Single Rod Shaped Anode Positioned 

Horizontally in the Earth 4:27 

4.4.4 Resistance of Multiple Horizontal Anodes Connected on 

a Common Header Cable 4:29 

4.4.5 Calculating Pipe Resistance to Remote Earth 4:31 

4.4.6 Calculation of Cable and Pipe Lineal Resistances 4:32 

4.5 Calculating System Capacity & Life 4:34 

4.6 Calculation of System Life 4:36 

4.7 Calculating Number of Anodes 4:38 

4.8 Calculating System Driving Voltage 4:39 

4.8.1 Galvanic (Sacrificial) System 4:39 

4.8.2 Impressed CurrentSystem 4:40 

4.9 Sample Cathodic Protection Designs 4:41 

4.9.1 Galvanic Anodes 4:41 

a. Example No. 1 4:41 

b. Example No. 2 4:44 

4.9.2 Impressed CurrentSystem 4:48 

a. Example No. 1 4:48 

b. Example No. 2 4:51 

4.10 Design of Performance Monitoring Facilities 4:56 

4.11 Current Distribution 4:57 



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J uly 2008 



Table of Contents 



V! 



4.11.1 Introduction 4:57 

4.11.2 Attenuation 4:60 

4.11.3 Effect of Coating on Current Distribution 4:64 

4.11.4 Effect of Anode-to-Structure Spacing on Current 

Distribution 4:68 

4.11.5 Effectof Structure Arrangement on Current Distribution . 4:70 

4.11.6 Effectof Electrolyte Resistivity Variation on Current 

Distribution 4:71 

4.11.7 E ffect of C urrent Distribution on Holidays on a 

Coated Structure 4:73 

4.11.8 Effectof Polarization (Time) on Current Distribution 4:76 

4.11.9 Summary of Current Distribution Factors 4:76 

Team Project 4:78 



Chapters 
Evaluation of CP System Performance 



5.0 Introduction 



5:1 



5.1 The PotentialMeasurement 5:1 

5.1.1 Copper-Copper Sulfate Reference Electrode 5:2 

5.1.2 Buried Reference Electrodes 5:4 

5.1.3 Polarity Considerations 5:5 

5.1.4 The Potential Measurement Circuit and Measurement Error 5:6 

5.2 Voltage Drop Errors External to the Metering Circuit 5:10 

5.2.1 Voltage Drop Errors in the Potential Measurement due to 

Current in the Earth 5:10 

5.2.2 Voltage Drop Errors in the Potential Measurement due to 

Current in the Pipeline 5:13 

5.3 Methods of Minimizing Voltage Drop Errors in the Potential 

Measurement 5:14 

5.3.1 Current Interruption Method 5:14 

5.3.2 Step-Wise Current Reduction Method of Determining the 
Amount of Soil IR Drop in the On-Potential 5:18 

5.3.3 Reference Electrode Placement Close to the Structure 5:21 

5.3.4 Using Coupons to Minimize Voltage Drop Errors in 

the Potential Measurement 5:23 

5.4 Measurement of Polarization Potential Shift 5:29 

5.5 Current Measurement 5:31 

5.5.1 Using an Ammeterto Measure Current 5:31 



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Table of Contents 



Vll 



5.5.2 Using a Shuntto Determine Magnitude 5:33 

5.5.3 Zero Resistance Ammeter 5:34 

5.5.4 Clamp-on Ammeter 5:35 

5.5.5 Pipeline Current Measurements 5:36 

5.6 Close Interval Potential Survey 5:38 

5.7 Coating Condition Surveys 5:43 

5.7.1 Voltage Gradient Method of Detecting Holidays in a 

Pipe Coating 5:43 

5.7.2 Coating Conductance Method of Evaluating Coating Quality 5:44 

5.8 Troubleshooting Cathodic Protection Systems 5:48 

5.8.1 Polarization Changes 5:48 

a. Structure Depolarization 5:48 

5.8.2 Anode Polarization 5:50 

5.8.3 Increased Resistance 5:51 

5.8.4 Power Supply Changes 5:54 

a. Zero Current and Voltage Outputs 5:57 

b. Zero Current Output with Unchanged Voltage Output.. 5:58 

c. Significant Current Change with Unchanged Voltage... 5:58 

d. Significant Changes in Both Voltage and Current 

Outputs 5:59 

e. Transformer-Rectifier Efficiency 5:59 

5.8.5 Cathodic Protection Troubleshooting Flow Chart 5:60 

Class Exercise 5-1: Evaluating Cathodic Protection Performance 5:62 

Experiment 5-1: To Demonstrate Four Methods of Minimizing 

IR Drop Error in a Potential Measurement 5:65 



Appendices 



Appendix A Anode Specifications 

Appendix B Pipe Data Table 

Appendix C Metric Conversion Table 

Appendix D ShuntTable 

Appendix E Glossary 

Appendix F NACE Standards 



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J uly 2008 



Table of Contents 



viu 



List of Figures 

Chapter 1 
Mechanisms of Corrosion 



Fig. 1-1 Iron Oxide to Steel to Iron Oxide Cycle 1:2 

Fig, 1-2 Unit Cell Atomic Arrangement in ivjetal Crystal Structures ,,,, 1:2 

Fig, 1-3 Metal/Aqueous Solution Interface 1:4 

Fig, 1-4 Corrosion Forming Ferrous Hydroxide 1:4 

Fig, 1-5 Potential Difference Across Metal/Water Interface due 

to Corrosion 1:5 

Fig, 1-6 Theoretical Conditions of Corrosion, Immunity and 

Passivation of Iron - Simplified pH Pourbaix Diagram for 

Iron in Waterat25eC 1:5 

Fig, 1-7 Pourbaix Diagram for Iron in Water at 25 ^C Showing Fe"^"^ 

Solubility Lines 1:7 

Fig. 1-8 Pourbaix Diagram for Iron in Water at 252C Showing 

Corrosion Rates 1:7 

Fig. 1-9 Measurement of Metal Potential with respect to a 

Reference Electrode 1:8 

Fig. 1-10 Standard Hydrogen Electrode 1:8 

Fig. 1-11 Reference Electrode Conversion Scale 1:12 

Fig. 1-12 Iron Corrosion Cell 1:13 

Fig. 1-13 Direction of Conventional Current (positive charge flow) 1:14 

Fig. 1-14 Charge Movement in a Corrosion Cell 1:16 

Fig. 1-15 Direction of Conventional Current (-fve Charges) in a 

Corrosion Cell 1:16 

Fig. 1-16 Open Circuit Corrosion Cell 1:17 

Fig. 1-17 Closed Circuit Corrosion Cell 1:18 

Fig. 1-18 Evans' Diagram fora Corrosion Cell 1:19 

Fig. 1-19 Evans' Diagram for a Corrosion Cell Under Anodic Control 

and Mixed Control 1:20 

Fig. 1-20 Simple DC Circuit Representing a Corrosion Cell 1:20 

Fig. 1-21 Polarization Diagram fora Corrosion Cell 1:21 

Fig. 1-22 Corrosion Potential Measurement to a Remote Reference 

Electrode 1:26 

Fig. 1-23 Corrosion Potential Indicated on the Polarization Diagram... 1:27 

Fig. 1-24 Measurement of Corrosion Potential (Mixed Potential) 1:28 

Fig. 1-25 Measurement of Corrosion Potential on a Pipeline with 

Two Holidays 1:28 

Fig. 1-26 Polarization Diagram Showing Charge Transfer Reactions .. 1:31 

Fig. 1-27 Effectof Depolarization at Botli Anode and Catliode 1:31 

Fig. 1-28 Cathode Depolarization in a Corrosion Cell 1:32 

Fig. 1-29 Anode Depolarization in a Corrosion Cell 1:33 

Fig. 1-30 Increased Polarization at Both the Anode and Catliode of 

A Corrosion Cell 1:34 

Fig. 1-31 Increased Polarization atthe Cathode in a Corrosion Cell.... 1:35 



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J uly 2008 



Table of Contents 



IX 



Fiq. 


1-32 


Fiq. 


1-33 


Fiq. 


1-34 


Fiq. 


1-35 


Fiq. 


1-36 


Fiq. 


1-37 


Fiq. 


1-38 


Fiq. 


1-39 



Fiq. 1-40 
Fiq. 1-41 



Fiq. 1-42 



Increased Polarization atthe Anode in a Corrosion Cell 1:36 

Increase in Resistance in a Corrosion Cell 1:37 

Soil Resistivity vs. Moisture Concentration 1:38 

Resistivity vs. Temperature for Four Soil Types 1:38 

Decrease in Resistance in a Corrosion Cell 1:39 

Effect of Drivinq Voltaqe (EM F) on a Corrosion Cell 1:40 

Effect of Time on a Corrosion Cell 1:41 

Randle's Electrical Circuit Model of a Metal/Electrolyte 

Interface 1:41 

Randle's Electrical Circuit Model for a Typical Corrosion 

Cell on an Unprotected Steel Surface 1:42 

a. Uniform Corrosion 1:43 

b. Pittinq Corrosion 1:43 

c. Crevice Corrosion 1:43 

d. Galvanic Corrosion 1:43 

e. Environmentally Induced Crackinq 1:44 

f. De-alloyinq and Dezincification 1:44 

q. Erosion-corrosion and Frettinq 1:44 

Galvanic Series forSeawater 1:46 



Chapter 2 
Cathodic Protection Theory 



Fig, 


2-1 


Fig. 


2-2 


Fig. 


2-3 


Fig, 


2-4 


Fig, 


2-5 


Fig, 


2-6 


Fig. 


2-7 


Fig. 


2-8 


Fig. 


2-9 


Fig. 


2-10 


Fig. 


2-11 


Fig. 


2-12 


Fig. 


2-13 


Fig. 


2-14 


Fig. 


2-15 


Fig. 


2-16 


Fig. 


2-17 


Fig. 


2-18 


Fig. 


2-19 


Fig. 


2-20 


Fig. 


2-21 


Fig. 


2-22 


NACE 





Simple Corrosion Cell 2:2 

Application of CP Current 2:2 

Corrosion Stopped 2:3 

Range of Polarized Potentials for Protection at Each Site 2:7 

Range of Polarization for Protection at Each Site 2:10 

AC Voltage vs. Holiday Size for iac =100 A/m^ 2:14 

Polarization Curves for Iron Corrosion in Acid 2:23 

Evans' Diagram for Corrosion Cell 2:24 

Evans' Diagram for Corrosion Cell with CP 2:25 

Polarization Curves in Aerated and Deaerated Solutions 

0fpH7 2:30 

Effect of Increasing Oxygen Concentration 2:34 

Effectof Increasing Agitation 2:36 

Laminar Flow Versus Turbulent Flow 2:36 

Effectof Increasing Temperature 2:37 

Effectof Decreasing pH 2:40 

Effectof Increasing Surface Area 2:42 

Effectof Increasing Surface Area (Current Density) 2:42 

Variation of pH with Distance 2:44 

Current Capacity Vs. Current Density for Aluminum 2:49 

Current Capacity of AZ63 Magnesium Alloy vs. Current 

Density 2:52 

Polarization Diagram for Galvanic Anode System 2:55 

Polarization of Structure and CP Anode 2:56 



© NACE International, 2005 



C P 3- Cathodic P ro tec tion Technologist 

J uly 2008 



Table of Contents 



Fiq. 


2-23 


Fig. 


2-24 


Fiq. 


2-25 


Fiq. 


2-26 


Fiq. 


2-27 


Fiq. 


2-28 


Fiq. 


2-29 


Fiq. 


2-30 


Fiq. 


2-31 


Fiq. 


2-32 


Fiq. 


2-33 



Fig. 2-34 



Polarization Diagram for Impressed Current CP System 2:67 

Current Discharge Paths from Anode Surface 2:68 

Single-Phase Bridge Rectifier Circuit 2:72 

Three-Phase Bridge Rectifier Circuit 2:72 

Single-Phase Diode Bridge 2:74 

Input and OutputSignals from Single-Phase Diode Bridge... 2:74 

Silicon-Controlled Rectifier (SCR) 2:75 

Single-Phase SCR Controlled Bridge 2:76 

Full-Wave SCR Bridge Output Waveforms 2:77 

Block Diagram for Switching-Mode and Standard Rectifiers. 2:78 

Solar Power Supply System 2:81 

Experiment to Demonstrate Corrosion Mitigation of Local- 
Action Cells by Galvanic Anode Cathodic Protection 2:85 



Chapters 
Interference 



Fig 
Fig 

Fig 
Fig 

Fig 



3-1 Parallel Current Patlis in tine Eartli 3:1 

3-2 Parallel Current Patlis in a Pipeline Cathodic Protection 

Section 3:2 

3-3 Parallel Current Paths in Vertically Stratified Soil Conditions 3:3 

3-4 Parallel Current Paths in Horizontally Stratified Soil 

Conditions 3:3 

3-5 Stray Current in a Metallic Structure Parallel to a 

Cathodically Protected Structure 3:5 

Voltage vs, Distance from a Vertically Oriented Anode 3:6 

Voltage Gradient in the Earth Around a Cathodically 

Protected Bare Pipeline 3:7 

Cathodic Protection Circuit Model 3:8 

Cathodic Protection Circuit Model with Foreign Structure 

Intercepting the Anode Gradient 3:8 

Stray Current in a Foreign Metallic Structure that Intercepts 

both the Anodic and Cathodic Voltage Gradient 3:9 

Cathodic Protection Circuit Model with Foreign Structure 

Intercepting both Anodic and Cathodic Voltage Gradient,,, 3:10 
Stray Current in a Foreign Metallic Structure that Intercepts 

the Cathodic Protection Gradient 3:10 

Cathodic Protection Circuit Model for Foreign Structure 

Intercepting the Cathodic Voltage Gradient 3:11 

Typical Potential Profile on an Interfered-with Structure 
that Intersects both Anodic and Cathodic Voltage 

Gradientwith the CurrentSource Interrupted 3:12 

Fig, 3-15 Current Changes In and Near an Interfered-with Structure ,,, 3:12 



Fig 


3-6 


Fig 


3-7 


Fig 


3-8 


Fig 


3-9 


Fig 


3-10 


Fig 


3-11 


Fig 


3-12 


Fig 


3-13 


Fig 


3:14 



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C P 3- Cathodic P ro tec tion Technologist 

J uly 2008 



Table of Contents 



xz 



Fig. 3-16 Comparison of Zn and Al Coatings for Corrosion Resistance 

as Functions of pH 3:13 

Fig. 3-17 Typical Section Through a J oint in Two Types of PCCP Pipe 

a. Lined CylinderPipe 3:14 

b. Embedded CylinderPipe 3:14 

Fig. 3-18 Cathodic Blistering/Disbondment of Protective Coating 3:15 

Fig. 3-19a Stray Current Discharge and Pick-up Around an 

Electrically Discontinuous J ointThrough the Earth 3:16 

Fig. 3-19b Stray Current Discharge and Pick-up Through the Internal 

Aqueous Medium Around an Electrically Discontinuous 

Bell and Spigot J oint on Cast Iron Piping 3:16 

Fig. 3-20 Stray Current Circuit in an AC Electrical Distribution System 3:16 
Fig. 3-21 Current Discharge from a Metal Structure to Earth via an 

Oxidation Reaction 3:17 

Fig. 3-22 Current Discharge from a Cathodically Protected Metal 

Structure to Earth via an Oxidation Reaction 3:18 

Fig. 3-23 Stray Current Arising from Installation of Isolating Fittings.... 3:21 
Fig. 3-24 Using a Buried Metallic Cable or Pipe as a Shield to 

Reduce Stray Current Interference 3:21 

F ig. 3-25 C athodic P rotection C urrent M odel for a B uried M etallic 

S hield C onnected to the N egative Terminal of the 

Transformer-Rectifier 3:22 

Fig. 3-26 Interference Mitigation using Galvanic Anodes atStray 

Current Discharge Location 3:23 

Fig. 3-27 Electrical Circuit Model for Mitigating Stray Current 

Interference at a Stray Current Discharge Site Using 

Galvanic Anodes 3:24 

Fig. 3-28 Potential Profile Changes on a Pipeline where Stray 

Current is Discharging in an End-Wise Pattern 3:25 

Fig. 3-29 Interference Mitigation Using a Resistance Bond 3:26 

Fig. 3-30 Use of a Dielectric Coating to Mitigate Interference 3:27 

Fig. 3-31 Typical Stray Current Paths Around a DC Transit System.... 3:29 
Fig. 3-32 Typical Structure-to-Soil Potential Recording with Time 

Caused by Interference from a DC Transit System 3:30 

Fig. 3-33a Typical Embedded Track Installation 3:31 

Fig. 3-33b Typical Direct-Fixation Isolating Fastener 3:31 

Fig. 3-34 Typical Utilities Drainage System at a Transit Substation 3:32 

F ig. 3-35 S chematic S howing C irculating C urrent between Transit 

Substations Through Direct Bonds to Utilities 3:33 

Fig. 3-36 Forced Drainage Bonds Using a Potential Controlled 

Rectifier 3:34 

Fig. 3-37 Electrical Schematic for a HVDC System 3:35 

Fig. 3-38 Potential-Time Plot for a Metallic Structure being 

Interfered-with by a HVDC System 3:37 

Fig. 3-39 Stray Current Caused by DC Welding Operations 3:38 

Fig. 3-40 Typical Three-Phase AC Powerline (Horizontal Configuration 

with Two Shield Wires) 3:39 

Fig. 3-41 AC Voltage Waveforms in a Three-Phase Circuit 3:40 



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Table of Contents 



xzz 



Fig. 
Fig. 

Fig. 
Fig. 
Fig. 

Fig. 
Fig. 



3-42 
3-43 

3-44 
3-45 
3-46 

3-47 
3-48 



Fig. 3-49 
Fig. 3-50 
Fig. 3-51 
Fig. 3-52 

Fig. 3-53 

Fig. 3-54 

Fig. 3-55 



g. 3-56 
g. 3-57 
g. 3-58 
g. 3-59 
g. 3-60 
g. 3-61 
g. 3-62 
g. 3-63 



Fig. 3-64 
Fig. 3-65 
Fig. 3-66 
Fig. 3-67 

Fig. 3-68 

Fig. 3-69 

Fig. 3-70 

Fig. 3-71 

Fig. 3-72 

Fig. 3-73 

Fig. 3-74 



Conductive Coupling During Line-to-G round Fault Conditions 3:41 
Example of Touch and Step Voltages at an Energized 

Grounded Structure 3:42 

Lockable Test Station 3:44 

'Dead-Front' Test Station 3:44 

Mitigation of Hazardous Touch Potentials at Aboveground 

Appurtenances 3:47 

Elements of a Capacitor 3:48 

Pipeline During Construction Represented as a Capacitive 

Voltage Divider 3:48 

Resistive Voltage Divider 3:49 

Capacitive Voltage Divider 3:49 

Electromagnetic Field Created by Current Flow in a Wire 3:51 

Electromagnetic Induction in a Multi-turn, Iron-Core 

Transformer 3:51 

Electromagnetic Induction in a Single-turn, Air-Core 

Transformer 3:52 

Electromagnetic Induction in a Pipeline due to an AC 

Powerline 3:52 

AC Voltage Required to Produce 100 A/m^ Current Density 

fora Variety of Holiday Sizes and Soil Resistivities 3:53 

Pipeline Anomaly Due to AC Corrosion (Before Cleaning).... 3:54 

PipelineAnomaly Dueto AC Corrosion (After Cleaning) 3:54 

AC Model of PipeSection 3:56 

Simplified AC Model of Pipe Section 3:56 

Network Analysis of Two-Section Pipe Model 3:57 

Network Analysis of Simplified Two-Section Pipe Model 3:57 

AC Voltage Profile Along Pipeline with 'Polarity' Indicated.... 3:57 
AC Voltage Profile Along Pipeline as Measured Using 

AC Voltmeter 3:57 

Effect of E lectrical Length of P ipeline on AC Voltage P ipeline 3:58 

Simple Pipeline-Powerline Corridor (Plan View) 3:59 

Field Estimation of LEF 3:59 

AC Voltage Profile Along an Electrically Short Pipeline 

(Uniform Conditions - No Grounding) 3:61 

AC Voltage Profile Along an Electrically Short Pipeline 

(Non-Zero Resistance Ground at Distance =0) 3:61 

AC Voltage Profile Along an Electrically Short Pipeline 

(Zero Resistance Ground at Distance =0) 3:61 

AC Voltage Profile Along an Electrically Short Pipeline 

(Grounds Evenly Distributed oratBotli Ends) 3:61 

AC Voltage Profile Along an Electrically Long or Lossy 

Pipeline (Uniform Conditions - No Grounding) 3:63 

AC Voltage Profile Along an Electrically Long or Lossy 

Pipeline (Zero Resistance Ground at Distance =0) 3:63 

AC Voltage P rofile Along E lectrically S hort P ipeline with 

Insulator at Midpoint 3:64 

AC Voltage Profile Along Electrically Long Pipeline with 



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Insulator at Midpoint 3:64 

Fig. 3-75 S olid-State DC Decoupling Device 3:65 

Fig. 3-76 Interaction of Solar Particles on the Earth's Magnetic Field .. 3:66 
Fig. 3-77 Schematic of Geomagnetic Induction Directly into a Pipeline 

and the R esulting C hange in P ipeline P otential that is 

Produced 3:67 

Fig. 3-78 Example Day of Geomagnetically Induced Potential 

Fluctuations on a Pipeline 3:68 

Fig. 3-79 Typical Geomagnetically Induced Potential Profile on a 

Well Coated and Poorly Coated Pipeline 3:68 

F ig. 3-80 Typical P ipe-to-S oil P otential M easurement S ituation Where 

Telluric Current Activity is Present 3:69 

Fig. 3-81 Current Flow and Calculated Off Potentials During a GIC 

Incident 3:70 

Fig. 3-82 Example Test Station in Which Coupon Does Not Require 

Disconnection to Minimize IR Drop Error in the Potential 

Measurement 3:71 

Fig. 3-83 Example of Difference in Potential-time Recording Between 

Reference Electrode at Grade and Inside a Soil Tube with 

a Coupon (e.g. Figure 3-82) 3:72 

F ig. 3-84 P ipe-to-S oil P otential M easurement M ethod to C ompensate 

for Telluric Current Effects During a Close Interval CP 

Survey 3:73 

Fig. 3-85 Mitigation of Telluric Current Discharge Effects Using 

Galvanic Anodes 3:74 

F ig. 3-86 S chematic of P otentially C ontrolled C athodic P rotection 

System Used to Mitigate Telluric Current Effects 3:75 

Fig. 3-87 Pipe Potential and Rectifier Current Output vs. Time for an 

Impressed Current System Operating in Potential Control. 3:76 

Experiment Schematic No. 1 3:77 

Experiment Schematic No. 2 3:78 

Experiment Schematic No. 3 3:79 



Chapter 4 
CP Design Fundamentals 



Fig. 4-1 An Example of a Cathodic Protection Design Procedure. 

Fig, 4-2 Coupon Current Density as a Function of Off-Potential,,,, 
Fig, 4-3 Instant-off Potential vs. Current Density for Carbon Steel 
Electrode in Sand and Clay Soil from Long Term 

Polarization Test Results 

Fig, 4-4 Polarization Curves in Aerated and Deaerated Solutions 
ofpH7 



4:2 
4:5 



4:5 
4:6 



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XIV 



Fig. 4-5 Surface Areas and Current Densities for Different Zones 

on an UncoatedJ acl<et Off-sliore Drilling Platform 4:8 

Fig. 4-6 FBE Coating Conductance vs. Bare Area Normalized for a 

Soil Resistivity of 1000 ohm-cm as Related to General 

Coating Quality in Table 4-4 4:13 

Fig. 4-7 Voltage Drop Method of Determining Current Requirements 4:14 
F ig. 4-8 C athodic P olarization P lot and Determination of C athodic 

Protection Current (Up) Required 4:18 

Fig. 4-9 Effect of Time on the Shape of a Dynamic Cathodic 

Polarization Curve 4:19 

Fig. 4-10 Current Density in Clay Soil for lOOmV Polarization 

Shifl:vs.% Bare on Coated Steel 4:21 

Fig. 4-11 Relative Economic Range for Galvanic and Impressed 

Current Systems as a Function of Current Required and 

Soil Resistivity 4:22 

Fig. 4-12 Electrical Schematic for an Operating Galvanic Cathodic 

Protection System 4:22 

Fig. 4-13 Electrical Schematic for an Operating Impressed Current 

Cathodic Protection System 4:23 

Fig. 4-14 Anode Placed Vertically in Earth at Grade 4:24 

Fig. 4-15 Multiple Vertical Anodes Connected to a Common 

Header Cable 4:25 

Fig. 4-16 Anode Placed Horizontally Below Grade 4:27 

Fig. 4-17a Multiple Horizontal Anodes in a Coke Trench Connected 

to a Common HeaderCable 4:28 

Fig. 4-17b Multiple Horizontal Anodes Connected to a Common 

HeaderCable 4:29 

Fig. 4-18 Resistance of a Horizontal Pipe Section 4:31 

Fig. 4-19 Polarization Diagram for a Galvanic Cathodic Protection 

System 4:39 

Fig. 4-20 Polarization Diagram for an Impressed Current Cathodic 

Protection System (cable resistances (Re) are ignored) 4:40 

Fig. 4-21 Single Groundbed Design 4:54 

Fig. 4-22 Typical Current Distribution with a Vertical Cylindrical Anode 4:58 

Fig. 4-23 Current Patli Resistances for Ideal Current Distribution 4:59 

Fig. 4-24 Current in Structure Under Ideal Conditions 4:60 

Fig. 4-25 Current Patli Resistance Including Resistance of Pipeline.... 4:61 
Fig. 4-26 Current and Voltage Attenuation Away from the Drain Point. 4:61 
Fig. 4-27 Effect of Attenuation Constanta on Attenuation 

Characteristics 4:62 

Fig. 4-28 Leakage Resistance to Remote Earth on a Coated Structure 4:64 
Fig. 4-29 Current Distribution with a Close Anode-to-Structure 

Spacing 4:68 

F ig. 4-30 Anode-to-Anode S pacing to Achieve R elatively U niform 

Current Distribution with Close Anode-to-Structure 

Spacing 4:69 

Fig. 4-31 Distributed Impressed Current Anodes Around 

Underground Storage Tanks 4:70 



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Table of Contents 


Fig. 


4-32 


Fig. 


4-33 


Fig. 


4-34 


Fig. 


4-35 


Fig. 


4-36 


Fig. 


4-37 



XV 



Currentiviax/iviin Ratio forTwo Parallel Structures 4:71 

Localized Low-Resistivity Path in a High Resistivity Soil 4:72 

C urrent Distribution on a Well C asing with Variable 

Resistivities 4:73 

Cathodic Protection Current Distribution to a Well Coated 

Pipe with a Holiday 4:74 

Effect of Polarization with Time on Attenuation Profile 4:76 

Typical Anode Arrangements 4:77 



Chapters 
Evaluation ofCP System Performance 

Fig, 5-1 Illustration of a Typical Pipe-to-Soil Potential Measurement . 5:1 

Fig, 5-2 Copper-Copper Sulfate Reference Electrode 5:2 

Fig,5-3a Effect of CI" Concentration on CSE Potential 5:3 

Fig. 5-3b EffectofCopper-Sulfate Concentration on CSE Potential 5:3 

Fig. 5-4 Structure-to-Soil Potential Measurements 5:5 

Fig. 5-5 Electrical Schematic of the Pipe-to-Soil Measurement Circuit 5:6 
Fig. 5-6 Methods of Minimizing Reference Electrode Contact 

Resistance 5:8 

Fig. 5-7 Voltage and Current Lines Around a Pipeline Receiving 

Cathodic Protection Current 5:10 

Fig. 5-8 Electrical Schematic Illustrating Soil Voltage Drop in the 

Potential Measurement 5:11 

Fig. 5-9 Current and Voltage Lines Around a Holiday on a Coated 

Pipeline 5:12 

Fig. 5-10 Current and Voltage Lines in Immediate Vicinity of a 

Holiday 5:12 

Fig. 5-11 Voltage Drop in a Pipeline Carrying Current 5:13 

Fig. 5-12 Electrical Schematic to Illustrate Potential Measurement 

Error due to CP Currentin a Pipeline 5:13 

Fig. 5-13 Graphical Illustration of tine Current Interruption Metliod of 

Minimizing Voltage Drop Error in the Potential Measurement 5:15 
Fig. 5-14 Example of Close Interval Potential Survey Data Plotted 

Vs. Distance for Botli 'ON' and 'Instant-off' Potentials 5:16 

Fig. 5-15 Illustration of Current Interruption Technique for Minimizing 

Voltage Drop Errors Using Randle's Model of Electrode/ 

Electrolyte Interface 5:16 

Fig. 5-16 Illustration of Positive Spike in Potential when CP Current 

is Interrupted 5:17 

Fig. 5-17 Illustration of Re-circulating Current Activity afterthe 

Interruption of CP Current 5:18 

Fig. 5-18 Field Test Arrangement for tine Step-Wise Current Reduction 

Method of Determining the Amount of IR Drop in the 

On-Potential 5:19 

Fig. 5-19 Data Plotfor Step-Wise Current Reduction Technique 5:20 

Fig. 5-20 Reference Electrode Placed Close to Pipe Surface to 



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Minimize IR Drop Error in Potential jvieasurement 5:21 

Fig. 5-21 Reference Electrode Placed Close to a Bare Riser Pipe 5:22 

Fig. 5-22 Reference Electrode Placed Close to a Coated Pipe Surface 5:22 
Fig. 5-23 Using a Soil Tube to Minimize IRe Drop in a Potential 

Measurement on Bare Pipe 5:23 

Fig. 5-24 Using a Steel Coupon to Simulate a Holiday on a Pipeline... 5:24 

Fig. 5-25 Schematic of an Integrated Coupon Test Station 5:25 

Fig. 5-26 Difference Between Coupon Disconnected Potential and 

Coupon-Pipe Potential with ICCP Interrupted 5:25 

Fig. 5-27 Difference Between Coupon Disconnected Potential 

Measured with Reference on Grade Versus in the 

Soil Tube 5:26 

Fig. 5-28 Pipe vs. Coupon Off-Potential 5:26 

Fig. 5-29a Photo of Vertical IR Drop Coupon 5:28 

Fig. 5-29b Schematic of Vertical IR Drop Coupon 5:28 

Fig. 5-30 Potential vs. Time Plot for Determining Polarization 

Potential Shift 5:29 

Fig. 5-31 Measurement of CP Current Using an Ammeter 5:31 

Fig. 5-32 Current Measurement in Parallel Drain Conductors 5:32 

Fig. 5-33 Use of Shunts for Current Measurements in Parallel 

Conductors 5:33 

Fig. 5-34 Current Measurement Using a Zero Resistance Ammeter 

(ZRA) 5:34 

Fig. 5-35 Using a Clamp-on Ammeter to Measure Current 5:35 

Fig. 5-36 Schematic of tine Hall Effect for Conventional Current 

Direction 5:35 

Fig. 5-37 Swain Meter 5:36 

Fig. 5-38 Calibrating a Pipeline CurrentSpan 5:36 

Fig. 5-39 Length of Pipe Sampled in a Pipe-to-Soil Potential 

Measurement 5:38 

F ig. 5-40 Length of Bare P ipe Over Which P otentials are Averaged 

as a Function of Burial Depth 5:38 

Fig. 5-41 Relative Circumferential Sampling Distance as a Function 

of Pipe Diameter-to-Pipe Depth Ratio 5:39 

Fig. 5-42 Error in Potential Measurement Introduced by Reference 

Electrode Lead Conductor Contacting the Eartli 5:40 

Fig. 5-43 Error in Potential Measurement Introduced by Current 

in the Pipeline 5:41 

Fig. 5-44 Error in Potential Measurement due to Interaction with a 

Parallel Interconnected Pipeline 5:42 

Fig. 5-45 Coating Holiday Detection Using Voltage Gradient Method .. 5:43 
Fig. 5-46 Arrangement for a Pipeline Coating Resistance 

(Conductance) Test 5:44 

Fig. 5-47 Structure Depolarization in a Galvanic CP System 5:59 

Fig. 5-48 Increased Anode Polarization in an Impressed Current 

System 5:50 

Fig. 5-49 Increased Anode Polarization in a Galvanic System 5:50 

Fig. 5-50 Increased Resistance in a Galvanic CP System 5:52 



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Fig. 5-51 Increased Resistance in an Impressed Current System 5:52 

Fig. 5-52 Seasonal Effects on a Galvanic Cathodic Protection 

System due to Drying of the Soil Resulting in 

Increased Resistance and Depolarization 5:54 

Fig. 5-53 Typical Singe Phase Tap Set Transformer-Rectifier 5:56 

Fig. 5-54 Locating a Short Circuit in an ICCP System 5:58 

Fig. 5-55 Cathodic Protection Troubleshooting Flow Chart 5:61 

Exercise Schematic 5-1: Plan & Potential Profile for an Underground 

Cathodically Protected Pipeline 5:64 

Experiment Schematic 5-1 5:65 

Experiment Schematic 5-2 5:66 

Experiment Schematic 5-3 5:67 



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J uly 2008 



Table of Contents 



xwzi 



List of Tables 

Chapter 1 
Mechanisms of Corrosion 



Table 1-1 Standard Free Energy (-aG^) of Formation of Some Oxides 

Kcal/mole of Oxide at27sc 1:1 

Table 1-2 The Electromotive Series at252C (772F) (also called EMF 

Series) 1:9 

Table 1-3 Common Reference Electrodes and Tlieir Potentials and 

Temperature Coefficients 1:11 

Table 1-4 Theoretical Consumption Rates of Various Metals and 

Substances on An Ampere-Yr, Basis 1:23 

Table 1-5 Electrochemical and Current Density Equivalence with 

Corrosion Rate 1:25 



Table 2-1 


Table 2-2 


Table 2-3 


Table 2-4 


Table 2-5 


Table 2-6 


Table 2-7 


Table 2-8 


Table 2-9 


Table 2-10 


Table 2-11 


Table 2-12 


Table 2-13 


Table 2-14 



Chapter 2 
Cathodic Protection Theory 

Potential Criteria from British Standard BS 7361 2:15 

Potential Criteria from German Standard DIN 30676 2:16 

Factors Controlling Polarization Response 2:31 

Factors Controlling Equilibrium Potential, Em/m+ 2:32 

Factors Controlling Exchange Current Density, io 2:32 

Factors Controlling Linniting Current Density, k 2:33 

Factors Controlling Polarization Slope, p 2:33 

Relationship Between [H'], [OH"], and pH 2:39 

Aluminum Anode Alloys 2:48 

Magnesium Anode Alloys 2:51 

Zinc Anode Alloys 2:53 

Galvanic Anode Backfills 2:57 

Massive-Type Impressed Current Anodes 2:62 

Mixed-Metal Oxide Anodes 2:65 



Chapters 
Interference 



Table 3-1 Types of R everse C urrent S witches 



3:33 



Chapter 4 
CP Design Fundamentals 

Table 4-1 Approximate C urrent R equirements for C athodic P rotection 

of Steel 4:9 



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J uly 2008 



Table of Contents 



xzx 



Table 4-2 Typical C urrent Density R equirements for C athodic 

Protection 4:10 

Table 4-3 Typical C urrent Density R equirements to C athodically 

Polarize Various Metals to -850 mVcse 4:11 

Table 4-4 Typical Specific Pipe to Earth Leakage Conductance for 

Dielectric Protective Coatings in 1000 Ohm-cm Soil 4:16 

Table 4-5 Concentric Stranded Copper Single Conductors Direct 

Burial Sen/ice Suitably Insulated 4:33 

Table 4-6 Typical Consumption Rate and Capacities of Different 

Anode Materials in Soils or Fresh Waters 4:36 

Table 4-7 Some Factors Which Affect Relative Current Distribution 4:60 

Table 4-8 Specific Leakage Resistances and Conductances 4:67 

Table 4-9 Summary of the Effect on Current Distribution of 

Various Factors 4:77 



Chapters 
Evaluation of System Performance 



Table 5 
Table 5 



List of Copper-Copper Sulfate Maintenance Items 5:2 

Summary of Relative Interaction of Two Paralleling 

Pipelines on Potential Measurements 5:42 

Table 5-3 Typical Specific Leakage Conductance for Dielectric 

Protective Coatings in 1000 ohm-cm Soil 5:47 



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Instructions for Completing the ParSCORE™ Student Enrollment Sheet/Score Sheet 

1. Use a Number 2 pencil, 

2. F ill in aM of the following information and the corresponding bubbles for each category: 

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V PHONE: Your phone number. The last four digits of this number will be your 

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V LAST NAME: Your last name (surname) 

V FIRST NAME: Your first name (or name by which you are called) 

V M.I.: Middle initial (if applicable) 

V TEST FORM: This is the version of the exam you are taking 

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V NAM E : (fill in your entire name) 

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V DATE : (date you are taking exam) 

3. The next section of the form (1 to 200) is for the answers to your exam questions, 

• All answers MUST be bubbled in on the ParSCORE™ Score Sheet . Answers 
recorded on the actual exam will NOT be counted. 

• If changing an answer on the ParSCORE^" sheet, be sure to erase completely . 

• Bubble only one answer per question and do not fill in more answers than the exam 
contains. 



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WEB INSTRUCTIONS 
FOR ACCESSING STUDENT GRADES 

It is NACE policy to not disclose student grades via the telephone, e-mail, or fax. Students 
will receive a grade letter, by regular mail or through a company representative, in 
approximately 6 to 8 weeks after the completion of the course. However, in most cases, 
within 7 to 10 business days following receipt of exams at NACE Headquarters, students may 
access their grades via the NACE Web site. The following are instructions for this process: 

To access grades on the NACE Web site go to: www.nace.org 

Choose: Education 

Students Only 

Grades 

Access Scores Online 

F ind your Course ID Number 

(Example 07C44222 or 42407002) in the drop down menu. 
Type in your Student ID or Temporary Student ID 

(Example 123456 or 4240700217)*. 
Type in your4-digit Password 

(Normally the last four digits of the telephone number entered on your exam form) 
Click on Search 

Prior to leaving the class , write down your course and student information in the 
spaces provided below: 



STUDENT ID COURSE CODE 



PASSWORD (Only Four Digits) 



*Note that the Student ID number forNACE members will be the same as their NACE 
membership number unless a Temporary Student ID number is issued at the course. For 
those who register through NACE Headquarters, the Student ID will appear on their course 
confirmation form, student roster provided to the instructor, and/or students' name badges. 

For In-House, Licensee, and Section-Registered courses, a Temporary ID number will be 
assigned atthe course forthe purposes of accessing scores online only. 

For In-House courses, this information may not be posted until payment has been received 
from the hosting company. 

Any questions, please contact Carol Steele at carol.steele@ nace.orq or at 281-228-6244. 



Introduction 



Audience (Who Should Attend) 

This course is designed for an individual who has extensive CP field experience and 
a strong technical background in cathodic protection and who intends to become a 
certified Cathodic Protection Technologist. 

Prerequisites 

To attend this training course, students must meet the following prerequisites: 

PATHl 

8 years Cathodic PratectiGn work experience with progressively increasing 
technical responsibilities 

PLUS 

High school diploma or GED 

PLUS 

Algebra and logarithm training 

CP 2-Cathodic Protection Technician certification or equivalent training 

PATH 2 

6 years Cathodic PratectiGn work experience with progressively increasing 
technical responsibilities 

PLUS 

2 years post high school training from an approved math or science technical/trade 
school including algebra and logarithm training 

PLUS 

CP 2- Cathodic Protection Technician certification or equivalent training 



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© NACE Intsmational, 2005 CP 3-Cathodic Protection Technologist 

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Introduction 



PATH 3 

3 years Cathodic PratectiGn work experience with progressively increasing 
technical responsibilities 

PLUS 

4-year physical science or engineering degree 

PLUS 

CP 2- Cathodic Protection Technician or equivalent training 

Length 

The course begins on Sunday at 10:00 am and concludes on Friday at 5 pm. 

Examination 

This course will conclude with a written final examination. This examination 
consists of 2 parts- Part A and Part B . Part A is multiple choice format and Part B is 
essay format. A combined score (Part A and Part B combined) of 70% or greater is 
required for successful completion of this course. 

The final examination is open book and students may bring reference materials and 
notes into the examination room. The final examination will be given on Friday. 

Non-communicating, battery-operated, silent non-printing calculators, including 
calculators with alphanumeric keypads, are permitted for use during the 
examination. Calculating and computing devices having a QWERTY keypad 
arrangement similar to a typewriter or keyboard are not permitted . Such devices 
include but are not limited to palmtop, laptop, handheld, and desktop computers, 
calculators, databanks, data collectors, and organizers. Also excluded for use during 
the examination are communication devices such as pagers and cell phones along 
with cameras and recorders. 



Certification Application 



Successful completion of the written examination and approval of the CP 3- 
Cathodic Protection Technologist certification application is required for 
certification. 



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J anuary 2007 



Cathodic Protection Certification Application 
CP3 - Catliodic Protection Teclinologist 

(Must be submitted to NACE Headquarters for certification to be processed) 

Please Note - This Is NotA Class Or Exam Registration 

NACE Member: DYes D No Member* 

Applicants Name: 



Home Address: 

Company Name: 
Company Address: 



Country: Zip/Postal Code: 



Phone: Home Business Fax 

E-mail: 

SIGNATURE: DATE: 

PRINTED NAME: 

REQUIREMENTS NECESSARY TO ACHIEVE CERTIFICATION 

To achieve this certification, qualified individuals have two options: 

CLASSROOM TRAINING - CP3-Cathodic Protection Teclinologist 

or 

EXAM ONLY OPTION - CP 3-Cathodic Protection Technologist Exam Only 

APPLICATION PROCEDURE 

It is strongly recommended thatExam Only candidates submita completed application for certification atleast60 
days in advance so that work experience requirements and education prerequisites can be verified. 

RECERTIFICATION REQUIREMENTS 

Every three (3) years. Recertification requires a minimum of 2 years work experience in cathodic protection and 8 
hours per year (24 hrs total) of professional development hours. 

SUBMIT COMPLETED APPLICATION AND SIGNATURE PAGES TO: 

MAIL TO: NACE INTERNATIONAL FAX TO: 281-228-6311 

CERTIFICATION DEPARTMENT 
1440 SOUTH CREEK DRIVE 
HOUSTON, TX 77084 

If you have any questions, please contactMarie Newton in our Certification departmentat281-228-6211orbye-mailat 
marie. newton@ nace.orq . 



AFFIRMATION 

I affirm that: 

1. I understand that I am solely responsible formal<ing sure that all necessary work experience documentation is 
complete and submitted in good orderto NACE Headquarters. 

2. I understand that if I knowingly provide, or cause to be provided, any false information in connection with my recognition 
under the NACE International Training and Certification Program, thatitwill be grounds foraction againstmy 
standing in the program. 

3. I understand that the names of the categories within the NACE International Cathodic Protection Training and Certification 
Program are as follows: 



Highest Level Successfully Completed 


Category Name 


CP 1 


CPl-Cathodic Protection Tester 


CP 2 


CP2-Cathodic Protection Technician 


CP 3 


C P 3-C athodic P rotection Technologist 


CP 4 


CP4-Cathodic Protection Specialist 



NACE has a firm policy regarding the use of its logos and certification numbers and tities. The certification number and 
category titie may be used only by NACE CP 1-Cathodic Protection Testers, NACE CP 2-Cathodic Protection Technicians, 
NACE CP 3-Catiiodic Protection Technologists, and NACE CP 4-Cathodic Protection Specialists, and may not be used by 
any other persons. 

All active CP card holders are permitted to use the term "NACE CP 1-Cathodic Protection Tester," "NACE CP 2-Cathodic 
Protection Technician," "NACE CP 3-Cathodic Protection Technologist," or "NACE CP 4-Cathodic Protection Specialist" 
(whichever level of recognition attained) and their certification number on business cards. 

This example illustrates how this information can be used by a NACE CP 1-Cathodic Protection Tester. 

J ohn Smith 

NACE CP 1-Cathodic Protection Tester, Cert No. 9650 

ACE Inspections, Inc., Knoxville, TN 

NACE CP 1-Cathodic Protection Testers, NACE CP 2-Cathodic Protection Technicians, NACE CP 3-Cathodic Protection 
Technologists, and NACE CP 4-Cathodic Protection Specialists who are members in good standing of NACE International 
may display the NACE Logo for the purpose of identifying the individual as having achieved a NACE Certification. 

I understand that violation of these rules will result in action againstmy standing in the program on the basis of violation of 
the NACE International Cathodic Protection Program Attestation. I understand that violation of these rules will result in 
action against my standing in the program on the basis of violation of the NACE International Cathodic Protection Program 
Attestation. 

I (re)affirm the NACE International Cathodic Protection Certification Program attestation and agree to abide by its provisions 
as long as I hold any level of recognition under the program. 



Signed: 



Date: 



THIS DOCUMENT MUST BE SIGNED AND RETURNED WITH APPLICATION 
RETAIN A COPY OF THIS DOCUMENT FOR YOUR RECORDS 



ATTESTATION 

I hereby: 

(1) Recognize and acknowledge that the pro per control of cathodic protection can be critical to the safety and welfare of 
the general public and industrial facilities. 

(2) Recognize and acl<now ledge that the control of cathodic protection is obligatory to maximize conservation of our 
material resources, to reduce economic losses, and to protectthe environment. 

(3) Recognize and acknowledge that the entire field of cathodic protection and its control encompasses the application of 
the knowledge and experience of many diverse disciplines and levels of technical competence which mustoften be 
consulted. 

(4) Recognize and acknowledge thatonly through continual association and cooperation with others in this field can the 
safestand mosteconomical solutions be found to the many cathodic protection problems. 

(5) Recognize and acknowledge thatthe quality of my work reflects on the entire profession of corrosion control. 
For these reasons I: 

(1) Agree to give first consideration in my cathodic protection work to public safety and welfare and to protection of the 
environment. 

(2) Agree to apply myself with diligence and responsibility to the cathodic protection work that lies within my area of 
competence. 

(3) Agree to pursue my work with fairness, honesty, integrity and courtesy, ever mindful of the best interests of the 
public, my employer, and of fellow workers. 

(4) Agree to not represent myself to be proficient or make recommendations in phases of cathodic protection work in 
which I am notqualified by knowledge and experience. 

(5) Agree to avoid and discourage untrue, sensational, exaggerated, and/or unwarranted statements regarding my 
work in oral presentations, written text, and/or advertising media. 

(6) Agree to treat as confidential my knowledge of the business affairs and/or technical process of clients, employers, 
or customers when their interests so require. 

(7) Agree to inform clients or employers of any business affiliations, interests, and/or connections which might 
influence my judgment. 

(8) Agree to uphold, foster and contribute to the achievementof the objectives of NACE International. 
I understand that my failure to comply with these requirements could result in disciplinary action. 

Signature: 

Printed Name: 

Date: 



THIS DOCUMENT MUST BE SIGNED AND RETURNED WITH APPLICATION 
RETAIN A COPY OF THIS DOCUMENT FOR YOUR RECORDS 



NAME: 

PLEASE CHECK APPROPRIATE BOX (choose only one): 



I AM APPLYING FOR CP TECHNOLOGIST CLASS & EXAM SCHEDULED FOR: 
(Please list city/dates of course you are planning to attend) 



-==A^ Click here for COURSE SCHEDULE 
OR 



I AM APPLYING FOR CP TECHNOLOGIST EXAM ONLY SCHEDULED FOR: 
(Please list city and date of exam you are planning to attend) 



<J> Click here for EXAM ONLY SCHEDULE 

REQUIREMENTS FOR CATHODIC PROTECTION TECHNOLOGIST 

Please provide the necessary CathodiC prOtSCtiOfl work experience in accordance with the following 
requirements for acceptance in class or exam checked. Cathodic protection is defined as a technique to reduce the 
corrosion of a metal surface by making that surface the cathode of an electrochemical cell. 

CP TECHNOLOGIST CLASSROOM TRAINING OR CP TECHNOLOGIST EXAM ONLY 

Path #1 

8 years CP work experience with progressively increasing technical responsibilities 

PLUS 

high school diploma orGED 

PLUS 

algebra and logarithm training 

CP Technician certification or equivalent training 

Path #2 

6 years CP work experience with progressively increasing technical responsibilities 

PLUS 

2 year post high school training from an approved math or science technical/trade school including 
algebra and logarithm training 

PLUS 

CP Technician certification or equivalent training 

Path #3 

3 years CP work experience with progressively increasing technical responsibilities PLUS 

4 year physical science or engineering degree PLUS 
CP Technician certification or equivalent training 

Please provide the necessary CattlOdiC prOtECtion work experience in accordance with above requirements for 
acceptance in class or exam checked. 



SAMPLE 

Form 1: Summary of Cathodic ProtfiCtion Related Work Experience 

Instructions: Make and use as many copies of this form as needed. Please provide all information requested. 

Forms must be printed legibly in black ink or typed. Illegible information can delay tiie application process. For 

assistance with this form, contact the Education Division atNACE International Headquarters. 

Applicant Information: 

Name: A. Sample Phone: 409/111-4321 



Company: ZZZ Coating Inspection Inc. 

Address: 987 Gage Avenue 



Fax: 



409/111-1234 



City: Millspec 

Zip/Postal Code: 77987 



State/Province: J[X 
Country: USA 



Please summarize below the information on each copy of Form 2, Individual Job Documentation. List your 
experience beginning with the most recent, followed by less recent experience. 



From 
Month/Year 


To 
Month/Year 


Number of 

Months in this 

job 


J Ob Tide 


Company Name 


1/92 


1/95 


36 


CP pipeline 
readings 


ZZZ Gas Co. 


12/89 


12/91 


24 


Installer 


AAA Tank Installers 


12/87 


12/89 


24 


Design Manager 


ABC CP Design 


/ 


/ 








/ 


/ 








/ 


/ 








/ 


/ 








/ 


/ 








/ 


/ 








/ 


/ 









Applicant Affidavit: I understand that if I knowingly provide false information in connection with my recognition 
under this program, it will be grounds for disciplinary procedures. 



Signed: 



XXX 



Date: 



Forml: Summary of CathodiC Protection Related Work Experience 

Instructions: Make and use as many copies of this form as needed. Please provide all information requested. 

Forms must be printed legibly in black ink or typed. Illegible information can delay the application process. For 

assistance with this form, contact the Education Division atNACE International Headquarters. 

Applicant Information: 

Name: Phone: 



Company: 
Address: 

City: 

Zip/Postal Code:_ 



Fax: 



State/Province: 
Country: 



Please summarize below the information on each copy of Form 2, Individual Job Documentation. List your 
experience beginning with the most recent, followed by less recent experience. 



From 
Month/Year 


To 
Month/Year 


Number of months 
in this job 


J Ob Tide 


Company Name 


/ 





































































































Applicant Affidavit: I understand that if I knowingly provide false information in connection with my recognition 
under this program, it will be grounds for disciplinary procedures. 



Signed: 



Date: 



CATHODIC PROTECTION WORK EXPERIENCE 

FORM 2: INDIVIDUAL J OB EXPERIENCE Page of 

Use one of these forms for each period of cathodic protection work experience ("job") you wisii to 
document. ivial<e and use as many copies of this form as you need. Please provide the information 
requested per the directions and definitions provided. 



J ob Information 

Applicant's Name: 




Who can NACE contact to verify this experience 


Job Title: 




Name: 


Company: 


Year 

Year 


Company: 


From: Month 
To: Month 


Address: 

State/Province: 


Phone: 


Zip/Postal Code: 


Fax: 




E-mail: 





C.2 CATHODIC PROTECTION WORK EXPERIENCE 

FORM 2: INDIVIDUAL J OB EXPERIENCE Page of 

Describe in detail what are/were your cathodic protection related duties in this job. (Do not write on the 
back of this form). You may attach additional single sided sheets) 

THIS SECTION MUST BE COMPLETED 
Your application will be returned if tliis space is left blank 



Signed: Date: 



Education and Training For CP 3 - CP Technologist 

NAME: 

Please check applicable statement of qualification: 
Path #1 



8 years CP work experience with progressively increasing technical responsibilities 

PLUS 

high school diploma orGED 

PLUS 

algebra and logarithm training 

CP Technician certification or equivalent training 

OR 

Path #2 

6 years CP work experience with progressively increasing technical responsibilities 

PLUS 

2 year post high school training from an approved math or science technical/trade 
school including algebra and logarithm training 
PLUS 
CP Technician certification or equivalent training 

OR 

Path #3 

3 years CP work experience with progressively increasing technical responsibilities 
PLUS 

4 year physical science or engineering degree 
PLUS 

CP Technician certification or equivalent training 

EQUIVALENT TRAINING OR TRADE/TECHNICAL SCHOOL INFORMATION 

Please list equivalent training OR trade/technical trade school information by providing name of school, 
course or training, company providing training, date of training, etc. 



DEGREE INFORMATION (please include certified copy of diploma with this application) 

Name of College or University Deqree Received Date Awarded StudentlD Number 



D. QUALIHCATION REFERENCE 

APPLICANT: Complete Items D.l, D.2, and D.3 then forward this form to the person you have listed in 
Item D.3. Ask this person to complete the remainder of the form and return it directly to NACE 
International. 

Qualification references may be submitted by the following, who have expertise in the CATHODIC 
PROTECTION field: 

■ Registered or chartered engineers 

■ Presentand/or previous supervisor(s) of the applicant 

■ Presentand/or previous professors/instructors of the applicant 

■ NACE International certificate holders. (ForCorrosion Specialistand Specialty Area applicants, one 
Qualification Reference must hold a NACE Certificate atthe level being applied for or higher, a P.E. 
registration, ICorr Professional Member or international equivalent.) 

D.l Applicant's full name: 



D.2 Certification category applied for: 



D.3 Name, titie, address, and phone #of person who is familiar with the work experience of the 
applicant: 



Phone: 



REFERENCE:The applicant is applying forCATHQDIC PRQTECTIQN recognition by NACE International. 
Applicants for certification must meet specific requirements; please see the list of these requirements on the 
following page. Evaluation of an applicant's qualifications also depends on assessment of professional 
CATHQDIC PRQTECTIQN experience by references. It is requested that you complete Items D. 4 through 
D.13 on this form and return it directly to NACE. 
D.4 I hold NACE Certification as a: 

Certified No. 

I am a Registered engineer (or equivalent) 



State/Province: Branch: No: 

Leave space blank if this section is not applicable 

D.5 I have known the applicantfor years. Whatis/was the nature of the association? 

Current Supervisor Previous Supervisor 

Current Client Previous Client 

Current Co-worker Previous Co-worker 

Current Professor/Instructor Previous Professor/Instructor 

Qther 

D.6 From personal knowledge, my assessment of the applicant's character and personal reputation is 
that it is 
Excellent |^^ Average |^^ Below average |^^ 

D.7 From personal knowledge, I know thatthe applicant has been engaged in cathodic protection work 

for years, ("cathodic protection" is defined as a technique to reduce the corrosion of a 

metal surface by making that surface the cathode of an electrochemical cell.) 



D.8 Based on this personal knowledge, I know thatthe quality of the applicant's work in the field of 
CATHODIC PROTECTION is 

Excellent I I Average FH Below average I I 

D.9 The applicant is proficient in the following phases of CATHODIC PROTECTION: 

D.IO Would you employ, or recommend the employment of, the applicantfor employment in the phase of 
CATHODIC PROTECTION work you have described in Item D. 9? 
Yes 1^ No I I 

D.ll Please describe any major projects /activities in which the applicanthas been involved in the field of 
CATHODIC PROTECTION work. Describe only those where you had personal knowledge of the 
applicant's work. Please indicate the degree of responsibility exercised by the applicant, the 
complexity of the project/activity, the degree of knowledge/skill required, etc. (Attach a separate 
sheet) 

D.12 Additional remarks or amplifying information: 



D.13 My signature below indicates that I have personal knowledge and expertise in the CATHODIC 
PROTECTION field upon which to evaluate the applicant's professional capabilities. 

Signaturej Date: 



Printed Name: 

Return completed form to: NACE International, Certification Department, Attn Marie Newton, 1440 South 
Creek Drive, Houston, TX 77084-4906 USA 

CP 2 - Cathodic Protection Technician must have: 

CP Tester Certification or equivalent training PLUS one of the following: 

1. 3 years CP work experience PLUS high school diploma orGED including algebra and 
logarithm training 

2. 1 year CP work experience PLUS 4-year physical science or engineering degree 

3. 2 years CP work experience PLUS 2-year post high school training from an approved math or 
science technical/trade school including algebra and logarithm training 

CP 3 - Cathodic Protection Technologist must have: 

CP Technician certification or equivalent training PLUS one of the following: 

1. 8 years C P work experience with progressively increasing technical responsibilities 
PLUS high school diploma or GED PLUS algebra and logarithm training 

2. 6 years CP work experience with progressively increasing technical responsibilities PLUS 
2 years post high school training from an approved math or science technical/trade school 
including algebra and logarithm training 

3. 3 years C P work experience with progressively increasing technical responsibilities PLUS 4 
year physical science or engineering degree 

CP4- Cathodic Protection Specialist must have: 

CP Technologist certification or equivalenttraining PLUS one of the following: 

1. 12 years CP work experience including 4 years in responsible charge PLUS 2 years post high 
school training in math or science from an approved technical/trade school 

2. 6 years CP work experience including 4 years in responsible charge PLUS 4-year engineering 
or physical science degree 

3. 4 years CP work experience in responsible charge PLUS a bachelors degree in engineering or 
physical sciences PLUS an advanced degree in engineering or physical science that required a 
qualification exam 



D. QUALIHCATION REFERENCE 

APPLICANT: Complete Items D.l, D.2, and D.3 then forward this form to the person you have listed in 
Item D.3. Ask this person to complete the remainder of the form and return it directly to NACE 
International. 

Qualification references may be submitted by the following, who have expertise in the CATHODIC 
PROTECTION field: 

■ Registered or chartered engineers 

■ Presentand/or previous supervisor(s) of the applicant 

■ Presentand/or previous professors/instructors of the applicant 

■ NACE International certificate holders. (ForCorrosion Specialistand Specialty Area applicants, one 
Qualification Reference must hold a NACE Certificate atthe level being applied for or higher, a P.E. 
registration, ICorr Professional Member or international equivalent.) 

D.l Applicant's full name: 



D.2 Certification category applied for: 



D.3 Name, titie, address, and phone #of person who is familiar with the work experience of the 
applicant: 



Phone: 



REFERENCE:The applicant is applying forCATHQDIC PRQTECTIQN recognition by NACE International. 
Applicants for certification must meet specific requirements; please see the list of these requirements on the 
following page. Evaluation of an applicant's qualifications also depends on assessment of professional 
CATHQDIC PRQTECTIQN experience by references. It is requested that you complete Items D. 4 through 
D.13 on this form and return it directly to NACE. 
D.4 I hold NACE Certification as a: 

Certified No. 

I am a Registered engineer (or equivalent) 



State/Province: Branch: No: 

Leave space blank if this section is not applicable 

D.5 I have known the applicantfor years. Whatis/was the nature of the association? 

Current Supervisor Previous Supervisor 

Current Client Previous Client 

Current Co-worker Previous Co-worker 

Current Professor/Instructor Previous Professor/Instructor 

Qther 

D.6 From personal knowledge, my assessment of the applicant's character and personal reputation is 
that it is 
Excellent |^^ Average |^^ Below average |^^ 

D.7 From personal knowledge, I know thatthe applicant has been engaged in cathodic protection work 

for years, ("cathodic protection" is defined as a technique to reduce the corrosion of a 

metal surface by making that surface the cathode of an electrochemical cell.) 



D.8 Based on this personal knowledge, I know thatthe quality of the applicant's work in the field of 
CATHODIC PROTECTION is 

Excellent I I Average FH Below average I I 

D.9 The applicant is proficient in the following phases of CATHODIC PROTECTION: 

D.IO Would you employ, or recommend the employment of, the applicantfor employment in the phase of 
CATHODIC PROTECTION work you have described in Item D. 9? 
Yes 1^ No I I 

D.ll Please describe any major projects /activities in which the applicanthas been involved in the field of 
CATHODIC PROTECTION work. Describe only those where you had personal knowledge of the 
applicant's work. Please indicate the degree of responsibility exercised by the applicant, the 
complexity of the project/activity, the degree of knowledge/skill required, etc. (Attach a separate 
sheet) 

D.12 Additional remarks or amplifying information: 



D.13 My signature below indicates that I have personal knowledge and expertise in the CATHODIC 
PROTECTION field upon which to evaluate the applicant's professional capabilities. 

Signaturej Date: 



Printed Name: 

Return completed form to: NACE International, Certification Department, Attn Marie Newton, 1440 South 
Creek Drive, Houston, TX 77084-4906 USA 

CP 2 - Cathodic Protection Technician must have: 

CP Tester Certification or equivalent training PLUS one of the following: 

1. 3 years CP work experience PLUS high school diploma orGED including algebra and 
logarithm training 

2. 1 year CP work experience PLUS 4-year physical science or engineering degree 

3. 2 years CP work experience PLUS 2-year post high school training from an approved math or 
science technical/trade school including algebra and logarithm training 

CP 3 - Cathodic Protection Technologist must have: 

CP Technician certification or equivalent training PLUS one of the following: 

1. 8 years C P work experience with progressively increasing technical responsibilities 
PLUS high school diploma or GED PLUS algebra and logarithm training 

2. 6 years CP work experience with progressively increasing technical responsibilities PLUS 
2 years post high school training from an approved math or science technical/trade school 
including algebra and logarithm training 

3. 3 years C P work experience with progressively increasing technical responsibilities PLUS 4 
year physical science or engineering degree 

CP4- Cathodic Protection Specialist must have: 

CP Technologist certification or equivalenttraining PLUS one of the following: 

1. 12 years CP work experience including 4 years in responsible charge PLUS 2 years post high 
school training in math or science from an approved technical/trade school 

2. 6 years CP work experience including 4 years in responsible charge PLUS 4-year engineering 
or physical science degree 

3. 4 years CP work experience in responsible charge PLUS a bachelors degree in engineering or 
physical sciences PLUS an advanced degree in engineering or physical science that required a 
qualification exam 



D. QUALIHCATION REFERENCE 

APPLICANT: Complete Items D.l, D.2, and D.3 then forward this form to the person you have listed in 
Item D.3. Ask this person to complete the remainder of the form and return it directly to NACE 
International. 

Qualification references may be submitted by the following, who have expertise in the CATHODIC 
PROTECTION field: 

■ Registered or chartered engineers 

■ Presentand/or previous supervisor(s) of the applicant 

■ Presentand/or previous professors/instructors of the applicant 

■ NACE International certificate holders. (ForCorrosion Specialistand Specialty Area applicants, one 
Qualification Reference must hold a NACE Certificate atthe level being applied for or higher, a P.E. 
registration, ICorr Professional Member or international equivalent.) 

D.l Applicant's full name: 



D.2 Certification category applied for: 



D.3 Name, titie, address, and phone #of person who is familiar with the work experience of the 
applicant: 



Phone: 



REFERENCE:The applicant is applying forCATHQDIC PRQTECTIQN recognition by NACE International. 
Applicants for certification must meet specific requirements; please see the list of these requirements on the 
following page. Evaluation of an applicant's qualifications also depends on assessment of professional 
CATHQDIC PRQTECTIQN experience by references. It is requested that you complete Items D. 4 through 
D.13 on this form and return it directly to NACE. 
D.4 I hold NACE Certification as a: 

Certified No. 

I am a Registered engineer (or equivalent) 



State/Province: Branch: No: 

Leave space blank if this section is not applicable 

D.5 I have known the applicantfor years. Whatis/was the nature of the association? 

Current Supervisor Previous Supervisor 

Current Client Previous Client 

Current Co-worker Previous Co-worker 

Current Professor/Instructor Previous Professor/Instructor 

Qther 

D.6 From personal knowledge, my assessment of the applicant's character and personal reputation is 
that it is 
Excellent |^^ Average |^^ Below average |^^ 

D.7 From personal knowledge, I know thatthe applicant has been engaged in cathodic protection work 

for years, ("cathodic protection" is defined as a technique to reduce the corrosion of a 

metal surface by making that surface the cathode of an electrochemical cell.) 



D.8 Based on this personal knowledge, I know thatthe quality of the applicant's work in the field of 
CATHODIC PROTECTION is 

Excellent I I Average FH Below average I I 

D.9 The applicant is proficient in the following phases of CATHODIC PROTECTION: 

D.IO Would you employ, or recommend the employment of, the applicantfor employment in the phase of 
CATHODIC PROTECTION work you have described in Item D. 9? 
Yes 1^ No I I 

D.ll Please describe any major projects /activities in which the applicanthas been involved in the field of 
CATHODIC PROTECTION work. Describe only those where you had personal knowledge of the 
applicant's work. Please indicate the degree of responsibility exercised by the applicant, the 
complexity of the project/activity, the degree of knowledge/skill required, etc. (Attach a separate 
sheet) 

D.12 Additional remarks or amplifying information: 



D.13 My signature below indicates that I have personal knowledge and expertise in the CATHODIC 
PROTECTION field upon which to evaluate the applicant's professional capabilities. 

Signaturej Date: 



Printed Name: 

Return completed form to: NACE International, Certification Department, Attn Marie Newton, 1440 South 
Creek Drive, Houston, TX 77084-4906 USA 

CP 2 - Cathodic Protection Technician must have: 

CP Tester Certification or equivalent training PLUS one of the following: 

1. 3 years CP work experience PLUS high school diploma orGED including algebra and 
logarithm training 

2. 1 year CP work experience PLUS 4-year physical science or engineering degree 

3. 2 years CP work experience PLUS 2-year post high school training from an approved math or 
science technical/trade school including algebra and logarithm training 

CP 3 - Cathodic Protection Technologist must have: 

CP Technician certification or equivalent training PLUS one of the following: 

1. 8 years C P work experience with progressively increasing technical responsibilities 
PLUS high school diploma or GED PLUS algebra and logarithm training 

2. 6 years CP work experience with progressively increasing technical responsibilities PLUS 
2 years post high school training from an approved math or science technical/trade school 
including algebra and logarithm training 

3. 3 years C P work experience with progressively increasing technical responsibilities PLUS 4 
year physical science or engineering degree 

CP4- Cathodic Protection Specialist must have: 

CP Technologist certification or equivalenttraining PLUS one of the following: 

1. 12 years CP work experience including 4 years in responsible charge PLUS 2 years post high 
school training in math or science from an approved technical/trade school 

2. 6 years CP work experience including 4 years in responsible charge PLUS 4-year engineering 
or physical science degree 

3. 4 years CP work experience in responsible charge PLUS a bachelors degree in engineering or 
physical sciences PLUS an advanced degree in engineering or physical science that required a 
qualification exam 



D. QUALIHCATION REFERENCE 

APPLICANT: Complete Items D.l, D.2, and D.3 then forward this form to the person you have listed in 
Item D.3. Ask this person to complete the remainder of the form and return it directly to NACE 
International. 

Qualification references may be submitted by the following, who have expertise in the CATHODIC 
PROTECTION field: 

■ Registered or chartered engineers 

■ Presentand/or previous supervisor(s) of the applicant 

■ Presentand/or previous professors/instructors of the applicant 

■ NACE International certificate holders. (ForCorrosion Specialistand Specialty Area applicants, one 
Qualification Reference must hold a NACE Certificate atthe level being applied for or higher, a P.E. 
registration, ICorr Professional Member or international equivalent.) 

D.l Applicant's full name: 



D.2 Certification category applied for: 



D.3 Name, titie, address, and phone #of person who is familiar with the work experience of the 
applicant: 



Phone: 



REFERENCE:The applicant is applying forCATHQDIC PRQTECTIQN recognition by NACE International. 
Applicants for certification must meet specific requirements; please see the list of these requirements on the 
following page. Evaluation of an applicant's qualifications also depends on assessment of professional 
CATHQDIC PRQTECTIQN experience by references. It is requested that you complete Items D. 4 through 
D.13 on this form and return it directly to NACE. 
D.4 I hold NACE Certification as a: 

Certified No. 

I am a Registered engineer (or equivalent) 



State/Province: Branch: No: 

Leave space blank if this section is not applicable 

D.5 I have known the applicantfor years. Whatis/was the nature of the association? 

Current Supervisor Previous Supervisor 

Current Client Previous Client 

Current Co-worker Previous Co-worker 

Current Professor/Instructor Previous Professor/Instructor 

Qther 

D.6 From personal knowledge, my assessment of the applicant's character and personal reputation is 
that it is 
Excellent |^^ Average |^^ Below average |^^ 

D.7 From personal knowledge, I know thatthe applicant has been engaged in cathodic protection work 

for years, ("cathodic protection" is defined as a technique to reduce the corrosion of a 

metal surface by making that surface the cathode of an electrochemical cell.) 



D.8 Based on this personal knowledge, I know thatthe quality of the applicant's work in the field of 
CATHODIC PROTECTION is 

Excellent I I Average FH Below average I I 

D.9 The applicant is proficient in the following phases of CATHODIC PROTECTION: 

D.IO Would you employ, or recommend the employment of, the applicantfor employment in the phase of 
CATHODIC PROTECTION work you have described in Item D. 9? 
Yes 1^ No I I 

D.ll Please describe any major projects /activities in which the applicanthas been involved in the field of 
CATHODIC PROTECTION work. Describe only those where you had personal knowledge of the 
applicant's work. Please indicate the degree of responsibility exercised by the applicant, the 
complexity of the project/activity, the degree of knowledge/skill required, etc. (Attach a separate 
sheet) 

D.12 Additional remarks or amplifying information: 



D.13 My signature below indicates that I have personal knowledge and expertise in the CATHODIC 
PROTECTION field upon which to evaluate the applicant's professional capabilities. 

Signaturej Date: 



Printed Name: 

Return completed form to: NACE International, Certification Department, Attn Marie Newton, 1440 South 
Creek Drive, Houston, TX 77084-4906 USA 

CP 2 - Cathodic Protection Technician must have: 

CP Tester Certification or equivalent training PLUS one of the following: 

1. 3 years CP work experience PLUS high school diploma orGED including algebra and 
logarithm training 

2. 1 year CP work experience PLUS 4-year physical science or engineering degree 

3. 2 years CP work experience PLUS 2-year post high school training from an approved math or 
science technical/trade school including algebra and logarithm training 

CP 3 - Cathodic Protection Technologist must have: 

CP Technician certification or equivalent training PLUS one of the following: 

1. 8 years C P work experience with progressively increasing technical responsibilities 
PLUS high school diploma or GED PLUS algebra and logarithm training 

2. 6 years CP work experience with progressively increasing technical responsibilities PLUS 
2 years post high school training from an approved math or science technical/trade school 
including algebra and logarithm training 

3. 3 years C P work experience with progressively increasing technical responsibilities PLUS 4 
year physical science or engineering degree 

CP4- Cathodic Protection Specialist must have: 

CP Technologist certification or equivalenttraining PLUS one of the following: 

1. 12 years CP work experience including 4 years in responsible charge PLUS 2 years post high 
school training in math or science from an approved technical/trade school 

2. 6 years CP work experience including 4 years in responsible charge PLUS 4-year engineering 
or physical science degree 

3. 4 years CP work experience in responsible charge PLUS a bachelors degree in engineering or 
physical sciences PLUS an advanced degree in engineering or physical science that required a 
qualification exam 



Qualification References 

A qualification reference is a person who will vouch for your technical competence. Two 
Qualification References are required, four are recommended. You are asked to give the names of 
persons (unrelated to you and not more than one from your company) who have personal 
knowledge of your cathodic protection experience and abilities or of your teaching in a cathodic 
protection-related field. Acceptable references are registered engineers, present supervisor, 
present clients, previous supervisors, previous clients, professors and instructors, and NACE 
International Certificate holders of atleastthe same category for which you are applying. 

NOTE: You are to send these individuals a Qualification Reference form (Item D), which they must 
complete and return directiy to NACE International. This is your responsibility. You should follow up with 
tiiese people to ensure thattiiey correctly complete and return tiie Qualification Reference forms in a timely 
manner. 

Name #1 

Name #2 

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CHAPTER 1 

MECHANISMS OF CORROSION 



1.1 Thermodynamic Considerations 

Corrosion is the deterioration of a material that resuhs from a reaction with its 
enviroimient. For a metal in contact with an aqueous solution, the reaction is an 
electrochemical one involving the transfer of electrical charge (electrons) across 
the metal/solution interface. 

The energy that exists in metals and causes them to corrode spontaneously results 
from the process of converting ore to metal. A measure of the energy available in 
a metal (Gibbs free energy) to power the corrosion reaction is given in Table 1-1 
listing the free energy of formation of some metal oxides. 



Table 1-1' 

Standard Free Energy (-AG °) of Formation of Some Oxides 

in kcal/mole of Oxide at 27°C 



Ag.0 2.55 

CU2O 34.6 

PbO 45.0 

NiO 51.4 

FeO 54.6 (at 227°C) 

ZnO 76.2 

MgO 136.5 

Al.Oa 377.6 



This energy, imparted on the metal during the refining process, is available as 
potential energy (-AG°) to power the corrosion reaction when the metal is placed 
in an aqueous enviroimient. This process for iron is illustrated in Figure 1-1. 



' Scully, J. C, Fundamentals of Corrosion, Pergamon Press, Oxford, 1966, p.7. 



''^NACE 



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1:2 




fflP 



^^^ 



IRON OXIDE + BLASTFURNACE + BESSEMER + PIPE MILL 








STEEL P IPE 
(-AG°) 



+ 



EARTH 




= IRON OXIDE 



Figure 1-1: Iron Oxide to Steel to Iron Oxide Cycle 



This illustrates that iron (or its alloys) will tend to transform to a lower energy 
state spontaneously. The transformation is produced by a change in free energy 
(-AG°). 

Metals are crystalline structures in which individual atoms are held together by 
the electrical attraction of each atom's bonding or conduction electrons (outer 
electrons) to the positive nucleus of adjacent atoms. The crystal lattice is 
composed of atoms arranged in repeating unit cells with characteristic 
arrangements such as body-centered cubic (BCC), face-centered cubic (FCC), or 
hexagonal close packed (HCP) as illustrated in Figure 1-2. 







body centered cubic 
(BCC) 



face-centered cubic 
(FCC) 



hexagonal close packed 
(HCP) 



Figure 1-2: Unit Cell Atomic Arrangement in Metal Crystal Structures 



- Moffatt, W.G.; Pearsall, G.W.; and WulffJ., Structure, John Wiley & Sons, Inc., New York, 1964, p.47. 



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1:3 



For a metal atom to leave the crystal structure, it must overcome the bonding 
energy with adjacent atoms in the crystal matrix. Metal atoms are not motionless 
but have vibrational energy depending on their temperature, since temperature is 
just an indication of atomic or molecular motion. As temperature increases above 
absolute zero (-273 °C or 0°K), the atomic motion increases and the interatomic 
bonds can be thought of as being elastic. Therefore, at the melting point, atomic 
motion is so great that the interatomic bonding forces can no longer maintain a 
crystal structure. 

Similarly, at ambient temperatures, surface atoms that have fewer interatomic 
bonds than internal atoms have a better chance of leaving the crystal structure. 
This is especially true for atoms at surface impurities, crystal dislocations, or slip 
planes where there are even fewer interatomic bonds and where the thermal 
vibrational energy of some poorly bonded atoms may be sufficient to escape the 
lattice structure. If a metal atom leaves the crystal structure, it leaves behind some 
of its bonding electrons (ne ) according to the following oxidation reaction: 



M° -^ M"+ + ne" 



[1-1] 



Oxidation is defined as a reaction in which an atom, ion, or molecule becomes 
more electropositive. The metal atom has now become a metal ion, with a net 
positive charge, taking with it most of the atomic mass residing primarily in the 
nucleus. It is also possible for the metal ion to return to the atom crystal structure, 
so the reaction can be reversible. 

When a metal is placed in an aqueous solution, other possibilities for the metal 
ion arise because of the presence of the polar water molecule and other cations as 
illustrated in Figure 1-3 for iron. Water, being a polar molecule, is attracted to the 
metal interface. A small number of water molecules (1 in 10 at pH 7) will ionize 
to produce a hydrogen ion (H ) and hydroxyl (OH ) ion. 



The metal can now react anodically in two ways other than Reaction 1-1, either to 
produce a metal hydroxide, as in Figure 1-4, or an aqueous ion as indicated in the 
following reactions. 



M" + nHjO -^ M(OH)„ + nH+ + ne 



M°+ mHzO -^ MO/"^"' + lirtr + ne 



[1-2] 
[1-3] 



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Mechanisms of Corrosion 



1:4 



^ej 


00 


X 


h 


oS© 0«% 


fFeJ 


rFeJ 


9 


m 


Kiron © 

jflon 


fy 


fPel 


fFeJ 


Uw 




|FeJ 


00 


X 


© 


A ^ 


to 


to 


X 


m 


^'" ® A © 


■ Fel 


|FeJ 


9. 


9 


© 



Figure 1-3: Metal/Aqueous Solution Interface 



(fJ 


^ 


00 


[pel c9"to 


©2<{) 


iFeJ 

rpej 


i 


fpej 

00 




O 


fel 


X 


[PeJ 




&^ 


fpel 


ft 


[PeJ 
[PeJ 


H^plon 





Figure 1-4: Corrosion Forming Ferrous Hydroxide 



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1:5 



In each case, the reactions produce positive charges on the solution side of the 
interface and leave behind negative charges in the metal. Clearly, then, a metal in 
contact with an aqueous solution will develop a potential difference (E) across the 
interface, as illustrated in Figure 1-5. 




Figure 1-5: Potential Difference Across MetalAVater Interface due to Corrosion 

1.2 The Pourbaix Diagram 

The potential E developed across the interface is a function of the metal involved 
and the pH (i.e., the relative concentration of H+ and OH- ions at the surface). 
The thermodynamic tendency for the metal to corrode by one or more of the three 
reactions above can be calculated using basic energy relationships. For iron, a 
potential-pH diagram (called a Pourbaix diagram) results as shown in Figure 1-6. 

■2 2 4 6 8 10 12 14 16 




10 12 14 16 



(assuming passivation by a film of Fe20 3) 



Figure 1-6: Theoretical Conditions of Corrosion, Immunity, and Passivation of Iron 
- Simplified pH Pourbaix Diagram for Iron in Water at 25-C 



^ Pourbaix, M., Atlas of Electrochemical Equilibria in Aqueous Solutions, National Association of 
Corrosion Engineers, Houston, TX, 1974, p.314. 

' *NACE 



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1:6 



Lines 1, 2, and 3 in Figure 1-6 relate to the following reactions: 
Line®: Fe" ^ Fe++ + 2e" 

Line®: Fe + 2H2O ^ Fe(0H)2 + 2H+ + 2e 

Line®: Fe++ + 3H2O ^ Fe(0H)3 + SlT + e 



[1-4] 
[1-5] 
[1-6] 



These lines define three distinct regions of relative stability of the ferrous ion 
(Fe^^)-corrosion, passivation, and immunity. In energy terms, an iron electrode 
with a potential/pH in the corrosion zone indicates corrosion can occur but 
doesn't necessarily mean it will happen. In the immunity zone, the ferrous ion is 
relatively insoluble and hence corrosion is unlikely. In the passivation zone, iron 
hydroxides and oxides are formed on the metal surface inhibiting further 
corrosion. Essentially, the hydroxide or oxide passive film forms a barrier 
between the substrate iron and the water. 

Lines @ and © represent the thermodynamic stability boundaries for a water 
molecule, which is potential and pH dependent. Between lines @and® water is 
considered thermodynamically stable but dissociates at these lines. 

Line © called the oxygen line, corresponds to the breakdown of a water molecule 
to produce oxygen gas and hydrogen ions resulting in the transfer of four 
electrons across the interface as in Reaction 1-7. 



+ 1 A + 



2H2O ^02 + 4H^+ 4e 



[1-7] 



At line@, called the hydrogen line, for every two molecules of water, a molecule 
of hydrogen gas and two hydroxyl ions are produced as in Reaction 1-8. 



2H2O + 2e ^ H2 + 20H 



[1-8] 



Both of these lines have important implications for operating cathodic protection 
systems, as will be seen in later chapters. 

Line ®, representing the stability of iron ions between corrosion and immunity, 
can be expanded regarding relative solubility of the ferrous ion as shown in 
Figure l-?."* 



'' Pourbaix, M., Atlas of Electrochemical Equilibria in Aqueous Solutions, National Association of 
Corrosion Engineers, Houston, TX, 1974, p.312. 

' *NACE 



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1:7 



0.2 

-0.2 
-0.4 
-0.6 
-0.8 



LU 



-1 

-1.2 
-1.4 
-1.6 
-1. 



corrosion^ 

n 


-^ 


\\,^^passivation 


-^ 




z:i^...[]\"-\ 


-fi 




^"""^---^ --« 


- 






- 


immunity 






Fe 




1 1 1 1 


1 1 1 


1 1 r 1 



■2-10 123456789 10 
PH 

Figure 1-7: Pourbaix Diagram for Iron in Water at 25^0 Showing Fe^^ Sdubility Lines 

Line © in the simplified Pourbaix diagram is replaced by a series of lines each 
representing a different solubility of the ferrous ion ranging from 10-10" of 
moles per liter of water. Hence as the iron potential becomes more electro- 
negative, the solubility of the ferrous ion decreases. Although theoretically it does 
not reach zero, for all practical purposes the corrosion rate is reduced to 
negligible values. The anticipated corrosion rate (mm/y) of iron in water at 25°C 
can be represented on the Pourbaix diagram as shown in Figure 1-8.^ The 
decreasing corrosion rate with increasingly negative potentials is a result of the 
decreasing solubility of the ferrous ion. 



10 12 14 16 



2 

1.6 

1.2 

0.8 

S 0.4 



> 

LU 



n I I I T" 



^(bX 



=<aX 



passivation 




immunity 



J I I I I L_ 



2 

1.6 
1.2 
0.8 

0.4 



-0.4 

-0.8 

-1.2 

-1.6 



10 12 14 16 



pH 



Figure 1-8: Pourbaix Diagram for Iron in Water at 25C Showing Corrosion Rates 



^ Pourbaix, M., Atlas of Electrochemical Equilibria in Aqueous Solutions, National Association of 
Corrosion Engineers, Houston, TX, 1974, p.316. 

' *NACE 



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1:8 



1.3 The E lectrode Potential 

The absolute potential across a metal/solution interface cannot be determined, by 
measurement, and can only be measured with respect to a second electrode called 
a reference electrode. For the Pourbaix diagram, a particularly stable electrode 
called a hydrogen electrode is used. 



reference 
electrode 




Figure 1-9: Measurement of Metal Potential Using a Reference Electrode 

The hydrogen reference electrode is composed of a platinum wire surrounded by 
a solution with 1 molar concentration of hydrogen ions (e.g., H2SO4 at pHO) 
through which hydrogen gas is bubbled. The solution is kept at 25°C. 

The platinum surface acts as a catalyst for the following reversible reaction: 



H+ + e <^ tf 



[1-9] 



Under the particular conditions sur- 
rounding the platinum, this results in the 
electrode maintaining a very stable 
potential. Because of this and the nature 
of the reversible reaction, the electrode is 
called a standard hydrogen electrode 
(SHE), illustrated in Figure 1-10. The 
she's potential Eho/h+ is used as the zero 
from which all other electrode potentials 
are measured. For instance, it is used to 
measure the standard potential of pure 
metals to produce the Electromotive 
Series (Table 1-2). 

Figure 1-10: 
Standard Hydrogen F lectrode 

Source: J ones, D., Principles and Prevention of 
Corrosion, MacMiiian Publishing Co. 1992, p. 64. 



Cofinecting wire 




Bubbler as 
almospherJc seal 



■a Elecifolyie bridge 



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1.3.1 The Electromotive Force Series 



Table 1-2'' 

The HedixMiDtive Series at 25°C (77°F) 

(also called EMF Series) 





Standard Potential, E" 




Electrode 


(Refared to the SHE) 
Volts 




k^Ik 


-2.92 


A 


Na+ Na 


-2.71 






Mg^lMg 


-2.34 






ai^Iai 


-1.67 






Zn^ 1 Zn 


-0.76 




Fe^lFe 


-0.44 






Pb^ Pb 


-0.13 




Fe^lFe 


-0.04 


Less Noble 


H^, H. 1 M* (SHE) 


0.00 


Potentials 


KCl (sat.), Hg.Cl. 1 Hg (SCE) 


(+0.245)** 


More Noble 


Cu^ 1 Cu 


+0.34 


Potentials 


r, 1. 1 M 


+0.53 






AglAg 


+0.80 




Br, Br2 M 


+1.07 






H^ H.O, 0. 1 M 


+1.23 






cr, Clo 1 M 


+1.36 






Au^ Au 


+1.50 




F", F. 1 M 


+2.87 


y 



*M in this table denotes an inert metal electrode that acts merely as a donor or 

acceptor of electrons. Platinum is often used for that purpose. 
**This is not a standard potential since the SCE is not a standard state electrode. The 
potential of the standard state calomel electrode is +0.268 volts. 



The standard potentials (E°) (often referred to as "redox" potentials) of various 
pure metals are measured only under specific (standard) conditions. These 
conditions are a temperature of 25°C, a solution of 1 molar concentration of the 
metal ions, and a measuring circuit where no current is drawn from the pure metal 
specimens. The resulting table not only indicates the standard potentials of the 
pure metal but relates the corrosion tendency of each — the more electro- 
negative the potential, the greater the tendency to corrode. The EMF series 
arrangement of metals is similar to that of Table 1-1, a reflection that the 
tendency to corrode corresponds to the energy available in a metal to produce an 
oxide. 



^ deBethune,A.J., "Fundamental Concepts of Electrode Potentials" , Corrosion, Vol. 9, Oct. 1953, p.339. 
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1.3.2 The Nernst Equation 

The need to maintain a 1 molar solution of metal ions indicates the importance of 
metal ion concentration in determining the magnitude of a metal electrode 
potential. When the ion concentration is not 1 molar (or unit activity) the metal 
potential will change according to the Nernst equation: 



where: 
Em 
R 
T 



<■(■ 



n 
F 



M"+ 



(m°) 



Em = Em + In 



(M" ) 



nF 



a 



(M°) 



[1-10] 



metal potential at standard conditions 
gas constant (8.31 J/mol - °K) 
absolute temperature 
number of electrons transferred 
Faraday's constant (96,500 coulomb) 

metal ion activity a = ym, where y is the activity coefficient 
(always <1) and m is the molar concentration of the metal ion 

metal activity (assumed to be 1) 
activity coefficient 



The Nernst equation indicates that a metal electrode potential is a function of the 
metal ion activity, which is related to the metal ion concentration. As the metal 
ion concentration (M"^) increases the metal electrode potential becomes more 
electropositive. 

1.3.3 Common Reference Electrodes 

The standard hydrogen electrode (SHE) is considered a primary reference 
electrode because it is used to determine the potential of other (secondary) 
reference electrodes that are better suited for field use. A number of secondary 
reference electrodes such as saturated copper-copper sulfate (CSE), saturated 
calomel (SCE), and silver-silver chloride (SSC) are used routinely in the 
corrosion and cathodic protection industry. As revealed by the Nernst equation, 
the metal ion concentration is important in maintaining the stability of the 
reference electrode potential. These secondary references have specific ion 
concentrations and corresponding potentials with respect to the standard 
hydrogen electrode as indicated in Table 1-3. 



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Table 1-3: Common Reference Electrodes and Their Potentials and Temperature Coefficients 



Reference 
Electrode 


Electrolyte 
Solution 


Potential 
@ 25°C 
(V/SHE) 


Temperature 

Co-efficient 

(mV/°C) 


Typical 
Usage 


CU/CUSO4 (CSE) 


Sat. CUSO4 


+0.316^^' 


0.9« 


soils, fresh water 


Ag/AgCl(SJ) (SSC) 


0.6M NaCl (3 'AVo) 


+0.256*^' 


0.33(^> 


sea water, brackish^''-' 


Ag/AgCl(LJ) (SSC) 


Sat. KCl 


+0.222*'^^ 


1.00P> 


— 


Ag/AgCl(LJ) (SSC) 


O.lNKCl 


+0.288*^' 


0.43P> 


— 


Sat. Calomel (SCE) 


Sat. KCl 


+0.24 1^''^ 


0.66^^' 


water, laboratory 


Zn (ZRE) 


Saline Solution 


-0.79±0.1^-^ 


— 


sea water 


Zn (ZRE) 


Soil 


-0.80±0.1^-^ 


— 


buried 



SJ - solid junction LJ - liquid junction 

*^' Peterson, M.H. and Groover, R.E., "Tests Indicate the Ag/AgCl Electrode Is Ideal Reference 

CeU in Sea Water", Materials Protection, Vol. 11 (5), May 1 972, p. 1 9-22. 
'^'^ von Baeckmann, W., Schwenk, W., and Prinz, W., Handbook of Cathodic Protection, Gulf 

Professional Publishing, 1997, p. 80. 
™ Uhlig, H.H., Corrosion and Corrosion Control, John Wiley & Sons, 2'"* Edition, 1971, p.33-36. 
'^''^ Potential becomes more electropositive with increasing resistivity. See monograph for 

correction in waters of varying resistivity in NACE SP0176, latest edition, or (1). 
'■^•' Ansuini, F., and Dimond, J., Factors Affecting the Accuracy of Reference Electrodes, Materials 

Performance, Vol. 33(11), Nov. 1994, p.14-17. 
*^' Jones, D., Principles and Prevention of Corrosion, MacMillan Publishing Co., 1992, p. 65. 



1.3.4 Effect of Temperature on Reference Electrode Potentials 

The effect of temperature on the reference potential can be expressed by the 
following equation: 



where: 



kt 



Et = E 25°c + kt(T-25°C) 
temperature coefficient 



[1-11] 



Et = reference potential at temperature (t) 
Therefore, for a copper-copper sulfate reference at 5°C, its potential would be: 



Ecse/she(§5°C = +0.316 VsHE + 0.0009V/°C(5°C-25°C) 

= +0.316 VsHE - 0.018 VsHE 

= +0.298 VsHE 
or at 40°C, its potential would be: 

EcsE/sHE (^40°C = +0.316 VsHE + 0.0009V/°C(40°C-25°C) 

= +0.316 VsHE + 0.014 VsHE 

= +0.330 VsHE 



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1.3.5 Converting Measured Potentials between Reference 
Electrodes 

In practice, it is often necessary to convert a potential measured with respect to one 
reference electrode to one with respect to another reference electrode. Such a 
conversion can be done either arithmetically or graphically. The graphical method 
is illustrated using the scale in Figure 1-11 for an original potential of -800 mV 
measured on a metal "X" with respect to a copper-copper sulfate reference 
electrode (CSE). 



CSE 

SSC(SJ) 
SCE 

SSC(LJ)-|- 0,222 



SHE -- 0,0 



x°/x"t 



0,316 



0.256 
0.241 



I 0,06V 



0,740V 



0,075V 



0,725V 



0,80V 



0,316V 



ZRE -- 0.80 



1.116V 



(SJ ) = only solid silver chloride (AgCI) 
over the silver wire. 

(LJ ) = a silver wire surrounded by a 
concentrated solution of KCI. 



Figure 1-11: Reference Electrode Conversion Scale 

For a potential of -0.800 Vcse, the measurement of an unknown metal electrode 
X^/X"^ with respect to a CSE electrode converts to the following potentials with 
respect to other secondary reference electrodes: 

X°/X"VscE is -0.80 Vcse - (-0.075 Vsce) = -0.725 VscE [1-12] 

XV X"V ssc is -0.80 Vcse - (-0.060 Vssc) = -0.740 Vssc (SJ) [MB] 
X°/X"VzRE is -0.80 Vcse -(-1.116 Vzre) = +0.316VzRE [1-14] 

This conversion process can be completed arithmetically by the following steps: 

1. Determine the potential difference between the first reference electrode 
and the second reference electrode (e.g., CSE is +75mV to SCE). 

2. Add the original potential difference to obtain the converted potential. 

e.g., X°/X"^/SCE = +75 mV + (-800m Vcse) 

= -725mVscE 



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1:13 



1.4 The Corrosion Cell 

An electrochemical reaction either produces or consumes electrons. Equations 1- 
1, 1-2, and 1-3 all produce positive charges on the solution side of the interface 
and leave behind negative charges (electrons). When hydroxides or oxides are 
formed, as in Reactions [2] and [3], electrical neutrality in the water and in the 
metal is disturbed. The formation of ferrous hydroxide leaves two excess positive 
charges in the water and two excess electrons in the metal. This results in the flow 
of electrons away from the corrosion site to an adjoining surface where there is a 
natural attraction to the positive hydrogen ions. The hydrogen ion will pick-up an 
electron to form an atom of hydrogen (H°), often called nascent hydrogen, as in 
the following reaction: 



H+ + e ^ H° 



[1-15] 



As both the corrosion reactions forming either hydroxides or oxides continues 
there will be a related reaction between the excess charges to maintain electrical 
neutrality as shown in Figure 1-12. 



^ 


(peY^ ©M© **^ 


/ Electron 


/Fe iFej Electrolyte x^^ 


^fJO®! 


/ Cathode ^^^ ^^ ^ \i0 

(0) ^ 



Figure 1-12: Iron Corrosion Cell 

The location on the iron surface where corrosion occurs is called the anode. 
Corrosion is defined as an oxidation reaction since the material being oxidized, 
iron atoms in this case, becomes more electropositive as a result of the corrosion 
process. Hence a corrosion reaction is an oxidation reaction. 



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1:14 



The location where excess electrons transfer across the surface to be picked up by 
the positive hydrogen ions is called the cathode. The cathode is defined as the 
surface where a reduction reaction occurs [1-15]. Reduction is a reaction in which 
the species being reduced, in this case hydrogen ions, becomes more 
electronegative. Thus the entire corrosion process involves both oxidation and 
reduction reactions that transfer charges across a metal/electrolyte interface. This 
process is called an electrochemical cell. 

As corrosion activity continues at the anode positive charges (e.g., hydrogen ions) 
flow in the water and negative charges flow in the metal towards the cathode site. 
Current typically flows in the direction of the positive charge (i.e., from the anode 
to the cathode in the water and from the cathode to the anode in the metal) as 
shown in Figure 1-13. 




Figure 1-13: Direction of Conventional Current 
(positive charge flow) 



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1:15 



1.4.1 Corrosion Cell Components 

All corrosion activity takes place in the context of a corrosion cell that has the 
following four essential components: 

• An anode (where the oxidation reaction occurs) 

• A cathode (where the reduction reaction occurs) 

• An electronic path that allows electrons to flow from the anode 
to the cathode (inside the metal) 

• An electrolytic path that allows ions to flow between the anode 
and cathode (in the electrolyte) 

Any solution containing ions is an electrolyte. Soils typically contain water that 
has many different ions in solution, not just hydrogen and hydroxyl. In deaerated 
or non-aerated soils and waters the typical reduction reaction is the reduction of 
hydrogen as in Reaction 1-15. In aerated soils and waters, where dissolved 
oxygen is present, the following reduction reaction also transfers electrons across 
the metal/electrolyte interface. 



O2 + 2H2O + 4e ^ 40H 



[1-16] 



Oxygen atoms have an affinity for electrons because their outer electron shell has 
only 6 electrons. A more stable condition would result if there were 8 electrons. 
Hence an atom of oxygen will pick up 2 electrons at a cathode to fill its outer 
electron shell. This reduction reaction produces hydroxyl ions that change the pH 
at the cathode site in the alkaline direction. 

When a corrosion cell is established, other ions in solution will tend to migrate 
toward either the anode or cathode. Positively charged ions, such as Na^, K^, 
Ca^^, and Mg^^, migrate toward the cathode as do hydrogen ions. Hence all 
positively charged ions are called cations because they move towards the cathode. 

Similarly, negatively charged ions, such as CI , SO4 , NO3 , CO3 , and HCO3 , 
migrate toward the anode as do hydroxyl ions. Therefore all negatively charged 
ions are called anions, because they move towards the anode. 

An operating corrosion cell showing direction of charge movement is shown in 
Figure 1-14. 



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1:16 




Electrolyte 
Figure 1-14: Charge Movement in a Corrosion Cell 



Charge flow as illustrated in Figure 1-14 produces a corrosion current. The 
international convention adopted for the direction of current (i.e., conventional 
current direction) assumes current direction is the direction in which positive 
charges flow. Hence the direction of corrosion current in a corrosion cell is from 
anode to cathode through the electrolyte and from cathode to anode in the 
external or metal paths as illustrated in Figure 1-15. 




^-Electrolyte 
Figure 1-15: Direction of Conventional Current (+ve Charges) in a Corrosion Cell 

1.4.2 Corrosion Cell Kinetics (Polarization) 

If the electronic path of the corrosion cell could be interrupted, the cell would be 
open-circuited and the corrosion current (Icon) would be zero. After allowing time 
for equilibrium to be established at the anode and cathode interfaces, their open 
circuit potentials, Ea,oc and Ec oc respectively, could be measured with respect to a 



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1:17 



reference electrode placed at the respective interfaces. This open circuit condition 
is illustrated in Figure 1-16. 



* Ecell 




A cJ"t 



© 



Figure 1-16: pen C ircuit C orrosion C ell 

The potential difference between the anode and the cathode is the cell EMF (Eceii) 
which equates to the Gibbs free energy (AG°) of the metal in the following 
relationship. 



"cell 



AG° 

nF 



[1-17] 



where: 



Ecell 

n 

F 

AG° 



corrosion cell potential (volts or joules/coulomb) 
number of charges transferred in the oxidation reaction 
Faraday's constant - 96,500 coulombs of charge 
Change in Gibbs free energy (joules) 



This cell voltage is the electrical energy source, which when the switch is closed, 
causes charges to flow in the corrosion cell circuit (i.e., the corrosion current). 
With the insertion of an ammeter and a variable resistor and with the switch 
closed as in Figure 1-17, the corrosion current (Icorr) can be measured as well as 
the potential at the anode (Ea,cc) and cathode (Eccc)- 



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1:18 



I® 
icorr 








Figure 1-17: Closed Circuit Corrosion Cell 

If the variable resistor is gradually reduced, the current will increase while the 
anode and cathode closed circuit potential will change. The change in potential 
can be expressed as follows: 

Ecc = Eoc ± AEp [1-18] 

where: AEp is the change in potential due to Icorr across the interface. 

The anode potential will become more electropositive (+AEp) while the cathode 
potential will become more electronegative. Hence the potential relationship can 
be expressed as follows: 



For the anode: 



■^a,cc 



+ AE 



p,a 



[1-19] 



For the cathode: Ec,cc = Ec,oc - AEp,c [1-20] 

The operation of the corrosion cell can be illustrated on an Evans diagram (named 
after U.R. Evans ), which compares the potential to the corrosion current. 



^ Evans, U.R., Metallic Corrosion, Passivity and Protection, Eduard Arnold & Co., 1948, p.343. 
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1:19 



Ecell - Polarization 




I2 I3 

Corrosion Current 



Figure 1-18: Evans Diagram for a Corrosion Cell Under Cathodic Control 

The change in potential AEpc and AEpa that occurs at the cathode and anode 
interface respectively is due to the electrical energy used up in the transfer of 
charge across the respective metal/electrolyte interfaces. This change is called 
polarization. Charge transfer processes, although described in a singular 
oxidation or reduction reaction, are usually multi-step processes. 

The slowest step in the process requires the most electrical energy and hence 
produces the most polarization. It can be seen from Figure 1-18 that there is 
greater polarization at the cathode than at the anode (AEp ^ > AEp a) even though 
the rate of charge transfer is the same (i.e., the corrosion current is the same). The 
slowest charge transfer step is, therefore, at the cathode site, and in these 
circumstances the corrosion cell is said to be under cathodic control (i.e., most 
polarization occurs at the cathode). 

A polarization analogy is the movement of people to and from work using public 
transit. The walk to the bus stop requires less energy per unit length traveled than 
does climbing up the steps into the bus. There may be an accumulation of people 
at the bus stop as well as inside the bus causing a wait to get on or off the bus. If 
the bus door is analogous to the metal/electrolyte interface, the slowest step in the 
flow of people to and from the office is at the bus stop. 

Furthermore, if people getting on the bus is analogous to cathodic polarization 
and people getting off the bus is analogous to anodic polarization, it is apparent 



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1:20 



that more energy is required to step up into the bus than down from the bus. Also 
as the flow rate of people increases, as at rush hour, the increased accumulation of 
people waiting to get on and off the bus is analogous to an increase in current in a 
corrosion cell. Hence as the flow rate increases, the change in potential (i.e., 
people arriving at the bus stop) increases. 

Most corrosion cells on iron or steel structures in contact with soil or water are 
under cathodic control. In some specific enviroimients, the corrosion cell can be 
under anodic or mixed control as illustrated in Figure 1-19. 



4 E 



c,oc 



-a,oc 



a,cc 




Corrosion Current 



4 E 



c,oc 



-a,oc 



c,cc 




Corrosion Current ► 

(b) Mixed Control 



(a) Anodic Control 

Figure 1-19: Evans Diagram for a Corrosioii Cell Under Anodic Control and Mixed Control 



The corrosion cell depicted in Figure 1-17 forms a series electrical circuit as 
illustrated in the following simple DC circuit (Figure 1-20). 




vi/iere: 

Rn 



resistance of metal path 
between anode and cathode 



Ec.cc 



Rg = resistance of electrolytic path 
between anode and cathode 

V^ = voltage drop in metal path 

Vg = voltage drop in electrolyte 



Figure 1-20: Simple DC Circuit Representing a Corrosion Cell 

In a series circuit the sum of the voltage drops must equal the sum of the 
electrical sources (e.g., from KirchhofP s law for series circuit). Since the two 
sources oppose each other, the difference in potential between the closed circuit 
anode and cathode potentials is equal to the total voltage drop in the circuit as 
shown in Equation 1-22 and Figure 1-21. 

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then: 

substituting: 

gives: 



F - F = V + V 

Ec,cc - Eaxc = Vt (total voltage drop) 

Equation 1-19 for Ea,cc and Equation 1-20 for Ec,c 



Vt + AEp.a + AEp,c 



[1-21] 




Ii h h 

Corrosion Current 



[1-22] 
[1-23] 



Figure 1-21: Polarizatioii Diagram for a C orrosion C ell 

This equation relates the original voltage (Eceii) arising from a change in Gibbs 
free energy to the total voltage drop in the circuit and the sum of the polarization 
potential changes occurring at the metal/electrolyte interface (i.e., the source 
energy is equal to the sum of the energy losses in the circuit). 



Experiment 1-1: 

To Demonstrate Polarization in a Corrosion Cell 



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1.5 Faraday's Law 



The amount of material lost at the anode or deposited at the cathode is a function 
of the atomic weight of the metal or substance, the number of charges transferred, 
and the corrosion current (Icorr)- This relationship (Equation 1-24) was developed 
by Michael Faraday while working as Sir Humphry Davy's assistant at the Royal 

o 

Institute in London, England in 1833. 



where: 



Wt = 

n = 

icorr ■ 

F = 
M = 



W, 



M_ 

nF 



tL 



[1-24] 



total weight loss at anode or weight of material produced at the 
cathode (g) 

number of charges transferred in the oxidation or reduction 
reaction 

the corrosion current (A) 

Faraday' s constant of approximately 96,500 coulombs y^ 

per equivalent weight of material (where equivalent weight = — ) 

the atomic weight of the metal which is corroding or the 
substance being produced at the cathode (g) 

the total time in which the corrosion cell has operated (s) 



If Equation 1-24 is multiplied by - then the expression becomes: 



W, 



M 

nF 



T 



K, T„ 



where: 



Km constant for each metal or substance. 



[1-25] 



This indicates that the weight loss per unit time (i.e., the consumption rate) for 
any metal is directly proportional to the corrosion current. Hence, the theoretical 
consumption rate of any metal can be calculated on an ampere-year (A-y) basis as 
listed in Table 1-4. 



Thomas, J ohn M., Michael Faraday and the Royal Institution, Institute of Physics Publishing, Bristol, 
^UK, 1991, p. 24. 

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M 


echanisms of Corrosion 








1:23 


Table 1-4: Theoretical Consumption Rates of Various Meta 


s and Substances 


» on an Ampere-Y ear Basis 




Reduced 
Species 


Oxidized 
Species 


Molecular 
Weight M 

(g) 


Electrons 

Transferred 

(n) 


Equivalent 
Weight, M/n 

(g) 


Theoretical 

Consumption Rate 

(Kg/A-y) 






Al 


Al^ 


26.98 


3 


8.99 


2.94 






Cd 


Cd^ 


112.4 


2 


56.2 


18.4 






Be 


Be^ 


9.01 


2 


4.51 


1.47 






Ca 


Ca^ 


40.08 


2 


20.04 


6.55 






Or 


Cr^ 


52.00 


3 


17.3 


5.65 






Cu 


Cu^ 


63.54 


2 


31.77 


10.38 






H. 


H^ 


2.00 


2 


1.00 


0.33 






Fe 


Fe^ 


55.85 


2 


27.93 


9.13 






Pb 


Pb^ 


207.19 


2 


103.6 


33.9 






Mg 


Mg^ 


24.31 


2 


12.16 


3.97 






Ni 


Ni^ 


58.71 


2 


29.36 


9.59 






OH" 


0. 


32.00 


4 


8.00 


2.61 






Zn 


Zn^ 


65.37 


2 


32.69 


10.7 





Example: Using Equation 1-24 to calculate the weight consumed by 1 ampere 
of stray DC current discharging from an iron structure in 1 year. 



where: 



M 

Wj = — X t X I^ 
nF 



t = 1 year = 60 s/min x 60 min/h x 8,760 h/y = 31.5 x 10 s 

M = 55.S5g( from Table 1-4) 

n = 2 

F = 96,500 coulombs 



then: 



W, 



55.85 g x 31.5 X lO's x 1 A 
2 X 96,500 coulombs 



9115 g = 9.12 kg 



Note that the weight of hydrogen produced at the cathode of a corrosion cell or 
the weight of oxygen produced at an inert impressed current anode can also be 
calculated. 

Electric current is the flow rate of charge which can be expressed as 



where: 



Q 

t 



Q charge in coulombs 

t time in seconds 

I electric current in amperes 



[1-26] 



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1:24 



substituting Equation 1 -26 into the Faraday relationship (Equation 1 -24) 
yields the following relationship: 



W, 



M 

nF 



^corr 



[1-27] 



This indicates that the total weight of material lost at the anode or produced at the 
cathode is directly proportional to the total charge passed in the corrosion cell. 

Similarly, if the Faraday equation is multiplied by the term: 

1 



where: 



Ax t 

A surface area of the anode or cathode 

t time in seconds 



[1-28] 



then the following relationship results: 



W MI 

A X t nF A 



[1-29] 



Because ^^ is equal to the corrosion current density (z'coir ) then: 
A 



Axt 



M . 



[1-30] 



Therefore the weight loss per unit time per unit area is directly proportional to 
corrosion current density, which is corrosion rate (rcorr) expressed in units such as 
mg/cm /day. Therefore, corrosion rates for metals are often expressed in terms of 
the corrosion current density. 

If Equation 1-30 is divided by the density (d) of the alloy, then the corrosion 
penetration (rcorr) can be expressed as mm/y as in Equation 1-31. 



M L 



nF d 



[1-31] 



where: 



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M = atomic weight (g) 
n = number of charges transferred in corrosion reaction 



lo 



corrosion current density (A/cm ) 



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1:25 



Example: 
where: 



d = density (g/cm ) 
fcorr = penetration rate in (cm/s) 

Using the Equation 1-31 to calculate the penetration rate for iron 
based on a current density of 1 A/m . 



M = 55.85 g 
n = 2 
F = 96,500 coulombs 



10"^A/cm^ 
7.87 g/cm^ 



then: 



55.85g X 10-^ A/cm' 



2 X 96,500 coulombs x 7.87g/cm" 



r,„,, =3.68x10- 



cm 



Now, convert the units to the more common form of mm/yr by muhiplying by the 
penetration rate by the number of seconds per year and by the number of mm per 
cm. 



-, ^o lA-gCm ,^^ ^^7 s ,^mm , ,^mm 
r^„,, =3.68x10 — x3. 15x10 — xlO = 1.16 



yr 



cm 



yr 



The penetration rate in mpy (0.001 in/y or 25x10" mm/y) is equivalent to a 
current density of l)j,A/cm for a number of common pure metals as given in 
Table 1-5. 

Table 1-5: Electrochemical and Current Density Equivalence with Corrosion Rate 



Metal/Alloy 


Element/ 

Oxidation 

State 


Density 
(g/cm^) 


Equivalent 
Weight 

(gm) 


Penetration Rate 
Equivalent to 1 |iA/cm^'^' 


(mpy) 


10^ imVs^^i 


PureMEtals 

Iron 


Fe/2 


7.87 


27.93 


0.463 


11.5 


Nickel 


Ni/2 


8.90 


29.36 


0.431 


10.8 


Copper 


Cu/2 


8.96 


31.77 


0.463 


11.6 


Aluminum 


Al/3 


2.70 


8.99 


0.435 


10.87 


Lead 


Pb/2 


11.3 


103.6 


1.20 


29.9 


Zinc 


Zn/2 


7.14 


32.7 


0.598 


14.95 


Tm 


Sn/2 


7.26 


59.35 


1.07 


26.7 


Titanium 


Ti/2 


4.51 


23.93 


0.69 


17.3 


Zirconium 


Zr/4 


6.52 


22.81 


0.457 


11.4 



Note: [1] A current density of 1 (.lA/cm^ is approximately = 1 mA/ft^ 
[2] 1 mm = 40 mils 



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1.6 Corrosion Potential 

If the potential measurement in the previous corrosion cell was made to a single 
reference electrode placed remote from either the anode or cathode site as shown 
in Figure 1-22, the potential recorded is considered a "corrosion" potential (Ecorr) 
or mixed potential. 




icorr 



Figure 1-22: Corrosion Potential Measurement to a Remote Reference Electrode 

The measured potential is a weighted geometric and electrical average of the 
anode polarized potential (Eap) and the cathode polarized potential (Ec.p). The 
value of this potential will depend on the relative size of the anodes and cathodes, 
the location of the reference electrode with respect to the anode and cathode, and 
the resistance of the electrolyte path between the reference electrode and the 
anode and cathode. For the example in Figure 1-22, assuming equal anode and 
cathode surface areas and a very remote reference, the corrosion potential (Ecorr) 
would be about midway between Ecp and Ea,p as illustrated in Figure 1-23. 



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tcorr 



Ea,o 




II I2 I3 

Corrosion Current 



Figure 1-23: Corrosion Potential Indicated on the Polarization Diagram 



If the reference electrode were moved closer to the anode site, the measured 
corrosion potential would become more electronegative. Conversely, if it were 
moved closer to the cathode site, the measured corrosion potential would become 
more electropositive. 

The difference between Ecp and Egp in a short circuited corrosion cell is simply 
the voltage drop in the electrolyte (Ve) since Vm = 0. Therefore the corrosion 
potential contains a voltage drop component. That is, the corrosion potential is 
more positive than the anode polarized potential by x millivolts and more 
negative than the cathode polarized potential by Ve - x millivolts. 

On an underground bare steel pipeline in the absence of a cathodic protection 
system, numerous corrosion cells will form, all having different anode and 
cathode polarized potentials. A potential measurement with the reference placed 
on grade will be a mixed potential of all the individual surface potentials within 
the influence of the reference electrode. The corrosion potential consists primarily 
(about 90%) of the polarized potentials on the pipe surface falling inside a 120° 
angle^ from the reference electrode as shown in Figure 1-24 plus a voltage drop 



^ Pearson, J. M., Concepts and Methods of Cathodic Protection Part 2, The Petroleum Engineer, April, 
1944,p.200. 

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1:28 



component due to corrosion currents in the earth between the reference and the 
respective anodes and cathodes. 




-550mV -490mV -eOOmV -SOOmV -550mV -SOOmV -585mV -450mV -620mV -450mV -590mV -SlOmV 

A ^* C A C A C A, c A C A C/ 



Figure 1-24: Measurement of Corrosion Potential (Mixed Potential) 

Measurement of the corrosion potential on a coated pipeHne with hoHdays as 
illustrated in Figure 1-25 presents a more complex situation. 



vy/AY/AV/Ay/A\y/AY//\y/AY// 




coated pipeline 
Figure 1-25: Measurement of Corrosion Potential on a Pipeline with Two Holidays 



Assuming the coating is a perfect insulator (which it is not), the corrosion 
potential is given by the following expression: 



E,„„ = E„ + — [r,,R,, + R„ (r,i +R,, +R. Jl 

corr n2 T> ' '- ' ' ^ ' ' ' ^ -' 



[1-32] 



10 



^^Bar\o, T.J. and Fessler, R.R., Investigation of Techniques to Determine the True Pipe-to-Soil Potential 
of a Buried PipeUne, American G as Association Catalog #151394, May 1980, p.1-63. 



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1:29 



where: AE = Ehi - Eh2 

Ehi = potential of holiday 1 
Eh2 = potential of holiday 2 

R' = (Rhl + Rh2 + Rej) (Re J + Re,2 + Rej) " Re,3 

Rh = resistance of soil in volume element of the holiday 
Re = resistance in the electrolyte between the holiday 
and reference electrode 
Re,3 = resistance in the electrolyte between the holidays 

For a situation where the reference electrode is midway between two holidays 
Equation 1-32 reduces to 



where: 





Ej^jR, + Ei^2Ri 




R, 


Ri = 


= Rhl + Rel 


R2 = 


= Rh2 + Re2 


Rx ~ 


= Rel + Re2 + Rhl + Rh2 



[1-33] 



If R2 = Ri, then the corrosion potential will be primarily influenced by the 
magnitude of the potential at each individual holiday. 

Example: 



hj - disbon ded co ating 



(No holiday) 



Given: 



Rn 



V/AY/Ay/AY/Ay/AVAy/A\/A//Ay/X\Ay/Ay/A7/A\'//V/AY/Ay/AV/Ay/AY/Ay 



Ehi - 


= -900mVcsE 




dhi = 


1 cm 


Eh2 = 


= -SOOmVcsE 




Ah2 


= Im^ = 10^ cm^ 


Psoil = 


= lO^Q-cm 




Pctg = 


= 10^*' Q-cm 




- Ei^iR, + Ei^2 


Ri 







J?l 



t, 



■ctg 

dre^2cm 



0.1cm 



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1:30 



where Rj = Rei + Re2 + Rhi + Ri 



h2 



2d 2cm 



R 



Pctgtctg 10'° Q-cm X 0.1 cm , 



h2 



"h2 



1 a4 2 

10 cm 



10 Q 



R,. = R.,=-P^= ^" ^-^"^ =2.5xlO-Q 
2djj.f 2x2 cm 



Ri = Ri^i +R^j = 5xlO'Q + 2.5xlO'Q = 7.5xlO'Q 



R2 = Rh2 +Re2 = 10'^ + 2.5xlO'Q= 102.5x10' Q 



, 900 X 102.5 Xl0-V500x7.5 X 10^ 

2.5x10^ + 2.5x10^+5x10^+100x10^ 



92.3x10^+3.75x10^ 96x10^ 



110 xlO- 



110x10- 



Ern = -873 mVcsE 

1.7 Factors Affecting the Operation of a Corrosion Cell 

The primary factors affecting the magnitude of the corrosion current, and hence 
the corrosion rate are: 

• polarization characteristics at the anode and cathode interface 

• circuit resistances 

• cell EMF (driving voltage); and 

• time 



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1:31 




icorr 

Con-Qsion Cun-ent 



Figure 1-26: Polarization Diagram Showing Charge Transfer Reactions 

As illustrated in Figure 1-26, the polarization that occurs at the corrosion cell 
anode results from a slow step in the oxidation reaction (M° -^ M"^ + ne^ and at 
the cathode, from a slow step in either of the two reduction reactions (H^ + e ^ 
H° or O2 + 2H2O + 4e -^ 40H ) that transfer charge across the interface. 

1.7.1 Depolarization in a Corrosion Cell 

If the charge transfer reactions are sped up, there will be less polarization for a 
given amount of current and therefore more driving voltage available to push 
charges through the circuit resistances, hence more current (I'corr)- This effect is 
termed depolarization and is illustrated in Figure 1-27. 



cathode depolarization 




- anode depolarization 



Con-Qsion Cun-ent 
Figure 1-27: Fffect of Depolarization at Botii Anode and Catiiode 



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1:32 



This figure shows the effect of depolarization at both the anode and cathode, 
ahhough they can occur independently. Depolarization at both the anode and 
cathode results in increased corrosion current (I'corr), a more electropositive 
cathode polarized potential (E'c,p), and a more electronegative anode polarized 
potential (E'a,p). Also, the open circuit potentials are often affected by the 
depolarization with a resultant increase in cell EMF. 

1.7.1(a) Cathode Depolarization 

In general, factors that increase the rate of charge transfer across the interface 
result in depolarization. An increase in the concentration of reactants at the 
cathode interface increases the rate of charge transfer. For instance, an increase 
in the dissolved oxygen concentration speed ups the oxygen reduction reaction 
(O2 + 2H2O + 4e -^ 40H ). Also, an increase in hydrogen ion concentration 
(H^) speeds up the hydrogen reduction reaction (H^ + e ^ H°). These effects are 
shown in Figure 1-28. 




increased [H"*"] 
increased aeration 
increased agitation 
increased temperature 
increased surface area 
^ increased iac 

a,P 



corr 



Corrosion Current 



Figure 1-28: Cathode Depolarization in a Corrosion Cell 



Cathode depolarization can also be caused by the removal of reduction reaction 
products (i.e., OH and H°) as would occur if there were increased agitation in the 
electrolyte. As with most chemical reactions, an increase in temperature increases 
the reaction rate and therefore the rate of depolarization increases. If the cathode 
surface area increases, perhaps due to coating failure, charges can transfer more 



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1:33 



readily (i.e., with less energy required). Cathodic depolarization results in an 
increased corrosion current, an electropositive shift in both the open circuit and 
polarized potential at the cathode, and an electropositive shift in the anode 
polarized potential. This is assuming there is no depolarization effect at the 
anode. A superimposed alternating current density on the cathode can also cause 
depolarization.'^ 

1.7.1(b) Anode Depolarization 

Anything that increases the rate of the oxidation reaction will result in anode 
depolarization. One cause of anode depolarization is the removal of the oxidation 
products that are the dilution of the metal ion concentration or removal of the 
oxidation products formed on the surface. 

For steel, a ferrous hydroxide corrosion product is formed over the anode surface. 
If the pH decreases, the ferrous hydroxide becomes more soluble thus exposing 
more surface. Similarly, chloride ions can attack corrosion films and depolarize 
anode surfaces. Increased temperature and agitation also will cause anode 
depolarization. The effect of anode depolarization is illustrated in Figure 1-29. 



-c,oc 



-a,oc 



El 



a,oc 




increased [H"'"] (for steel 
increased temperature 
increased [CI"] 
increased agitation 
increased surface area 
increased lac 



Corrosion Current 



Figure 1-29: Anode Depolarization in a Corrosion Cell 



" Bolzoni etal, Laboratory Testing on the Influence of Alternated Current on Steel Corrosion, 
CORROSION /2004, paper no. 208 (Houston, TX: NACE, 2004). 

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1:34 



Anode depolarization causes the corrosion current to increase to Fcoir, the anode 
polarized potential to shift electronegatively to E'ap, the anode open circuit 
potential to shift electronegatively to Eg oc and the cathode polarized potential to 
shift electronegatively to E'cp. Note that for an increase in temperature at the 
anode or an increase in anode surface area, the open circuit potential (Eaoc) of the 
anode could shift electropositively instead of electronegatively. 

1.7.2 Increased Polarization in a Corrosion Cell 

If conditions at the anode and cathode interface cause the rate of charge transfer 
to slow down, this will increase the polarization since it will require more 
electrical energy to transfer the charge. An increase in polarization of both the 
anode and cathode is illustrated in Figure 1-30. 




Corrosion Current 



Figure 1-30: Increased Polarization at Botii tiie Anode and Catiiode of a Corrosion Cell 

Increased polarization at both the anode and cathode results in less corrosion 

current, an electronegative shift in the cathode open circuit and polarized 

potential, and an electropositive shift in anode open circuit and polarized 
potential. 

A depletion of reactants, a lower temperature, less agitation, and a buildup of 
reaction products can cause this combined effect which results in less corrosion. 
Increased polarization can occur independently at the cathode or anode. 



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1:35 



1.7.2(a) Increased Polarization at the Cathode of a Corrosion Cell 

If only the speed of the reduction reaction is reduced, the resuhing polarization 
characteristics will be as shown in Figure 1-31. 




increased [OH"], increased pH 
decreased aeration 
decreased agitation 
decreased temperature 
decreased surface area 
increased film forming ions 



Corrosion Current 



Figure 1-31: Increased Polarization at the Cathode in a Corrosion Cell 



Such an increase in polarization can be due to a number of factors localized at the 
cathode such as an increase in the pH, decrease in aeration, decrease in agitation, 
decrease in temperature, and decrease in surface area. The result is less corrosion 
current, an electronegative shift in the cathode open circuit potential, and 
polarized potential, and the anode polarized potential. 

Note that an increase in pH would occur with time as the products of the oxygen 
reduction reaction (OH ions) build up. 

1.7.2(b) Increased Polarization at the Anode of a Corrosion Cell 

If the oxidation reaction at the anode is inhibited, it will require more electrical 
energy to transfer charge and thus a greater polarization shift in potential will be 
observed as depicted in Figure 1-32. 



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1:36 




increased [OH"], increased pH 
decreased agitation 
decreased temperature 
decreased surface area 
increased passivating ions 
increased |V1"+ concentrations 



Corrosion Current 



Figure 1-32: Increased Polarization at the Anode in a Corrosion Cell 



An increase in polarization at the anode can result from an increase in the pH of 
the electrolyte if the anode metal is steel, a decrease in agitation, a decrease in 
temperature, a decrease in anode surface area, or an increase in ions that will form 
a passive film on the anode surface (e.g., nitrates, phosphates, etc.). 

If the anode polarizes due to localized conditions, then the corrosion current is 
reduced and the anode open circuit potential and polarized potential both shift 
electropositively as does the cathode polarized potential. 

Increased polarization at either the anode or cathode of a corrosion cell results in 
less corrosion. 



1.7.3 Circuit Resistance Changes 

As shown in Equation 1-21 the difference in potential between the polarized 
potential of the anode and cathode [Ec.p - Eap] is equal to the sum of the voltage 
drops in the metallic and electrolytic current paths. The operating driving voltage 
(that is, the driving voltage remaining after subtracting anode and cathode 
polarization) is therefore equal to the total voltage drop in the corrosion cell. 



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The voltage drops are a product of the corrosion current muhiplied by the path 
resistance, sometimes called the IR drop. If the resistance of either the metallic or 
electrolytic current paths changes and the corrosion cell open circuit driving 
voltage remains unchanged, from Ohm's Law the corrosion current will change. 

1.7.3(a) Increase in Resistance in a Corrosion Cell 

Consider an increase in either the metallic or electrolytic path resistance on the 
corrosion cell operation as illustrated in Figure 1-33. 



:c,oc 



-a,oc 



decreased moisture 
decreased temperature 
increased metal path resistance 
decreased ion concentration 




Corrosion Current 



Figure 1-33: Increase in Resistance in a C orrosion C ell 



As would be expected, an increase in resistance decreases the corrosion current, 
which results in a more electropositive cathode polarized potential and a more 
electronegative anode polarized potential. 

An increase in resistance of a soil is typically due to either a decrease in moisture 
or freezing of the soil, both of which can appreciably change the soil resistivity as 
illustrated in Figure 1-34 and Figure 1-35. 



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1:38 



107 





















1 




















^ 


-Red Clay 






\ 




^ 


- Top Soil 






\ 




^^ 


- Sane 


y Loar 


n 


• 


\ 


\ 










\ 




\ 












\ 


\ 












\ 




\ 
















^ 








4 8 12 16 20 24 26 

jvioisture as Percentage by Weight of Dry Soli 

Figure 1-34: Soil Resistivity vs. 
Moisture Concentration 

Source: Earth Resistances, G.F.Tagg, Pitmann Publishing, 1964, p. 5 



106 - 



> 105 



104 



1 1 1 1 1 - 
\ \i Saturated Sand 


1 1 1 1 i^i" 
1 


|-- Clav 

1 1 1 1 1 



-15 -10 



-5 5 

Temperature, °C 



10 



15 



Figure 1-35: Typical Resistivity vs. Temperature 
for Three Soil Types 



As shown in Figure 1-34 the resistivity of red clay increases from 20,000 Q-cm to 
180,000 Q-cm (a factor of 8 times) with only a 4% reduction in moisture content 
(16 to 12%). Moisture content greater than about 16% does not significantly 
change the resistivity of any of the soils. 

A decrease in temperature from to -10°C results in at least an order of 
magnitude increase in resistivity for the three soil types shown in Figure 1-35. 
Yet above 0°C there is relatively little decrease in resistivity with increasing 
temperature. 



1.7.3(b) Decrease in Resistance in a Corrosion Cell 

A decrease in resistance of either the metallic or electrolytic path between the 
corrosion cell anode and cathode results in an increase of corrosion current as 
shown in Figure 1-36. 



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1:39 



-c,oc 



increased moisture 
increased temperature 
increased ion concentration 
decreased metal path resistance 




Corrosion Current 



Figure 1-36: Decrease in Resistance in a Corrosion Cell 



Even though the corrosion current increases, the total IR drop in the circuit is less. 
Both the cathode and anode polarized potentials have moved closer to one 
another resulting in more anodic and cathodic polarization because of the larger 
corrosion current. 

Changes in electrolyte resistance are common in corrosion cells in contact with 
the earth due primarily to seasonal variations in moisture or temperature. 



1.7.4 Effect of Driving Voltage on a Corrosion Cell 

From Ohm's Law it is clear that the greater the driving voltage, the greater the 
corrosion current will be, as illustrated in Figure 1-37. 



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1:40 




Corrosion Current 



Figure 1-37: Effect of Driving Voltage (EMF) on a Corrosion Cell 



Referring to Figure 1-24, the corrosion current would be greater in the -620 mV/ 
-450 mV corrosion cell than the -550 mV/-500 mV corrosion cell, since the 
operating EMF of the former is 170 mV compared to 50 mV for the latter. 



1.7.5 E ffect of Time on a C orros ion C ell 

As corrosion continues with time, where there is limited electrolyte agitation, 
oxidation and reduction reaction products will build up at their respective 
interfaces. In the case of the reduction reactions this means an increasing 
concentration of hydroxyl ions, and it is apparent from Figure 1-31 that an 
increase in pH will result in increased polarization. 

Similarly, as the metal corrodes at the anode, the metal ion concentration will 
increase anode polarization as indicated in Figure 1-32. Accordingly, polarization 
increases at both the anode and cathode and with time corrosion current will 
reduce as shown in Figure 1-38. 



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Corrosion Current 



Figure 1-38: Effect of Time on a C orrosion C ell 

Anything that increases polarization (i.e., slows down charge transfer reactions) 
in a corrosion cell will reduce the corrosion current and is therefore beneficial. 



1.7.6 Randle's Circuit Model for an Electrode Interface in a 
Corrosion Cell 

The electrode/electrolyte interface can be modeled electrically as shown in Figure 
1-39. 




v^iae: 



Cji = cknMe layer capacitance 
(1-200 nF/cn^) 

Rp = polarizatian resistance 
(l-lO^Q-cm?) 

Rg = resistance of steel surtace 
to remote earth 

E_- = potential difierence (volts) 



Figure 1-39: Randle's Flectrical Circuit Model of a Metal/Flectrolyte Interface 



This model shows that the interface is not simply a resistance but a parallel 
combination of the polarization resistance (Rp) and a capacitor (Cai) called the 



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1:42 



double layer capacitance. The potential E across the parallel combination can be 
considered the open circuit potential. Therefore the potential at more electro- 
negative (anodic) sites on a structure will be greater than at cathodic (more 
electropositive) sites. When coupled, as shown in Figure 1-40, charges move 
from the most highly charged capacitor to the least highly charged capacitor, 
changing the potential of each in the process. The anode capacitor is being 
discharged while the cathode capacitor is being charged. These changes in 
potential (AE) as a result of charge transfer is polarization, and the resulting 
potential across the metal/electrolyte interface is called the polarized potential 
(Ea,p and Ecp) for the anode and cathode site respectively. 




(Reduction: H+ + e" -► H°) 
(O2 + 2H2O + 4e- -► 40H" ) 



^ — + 

Icorr 



soil (electrolyte) 



Icorr 



— ' '++ 

ANODE (Oxidation: Fe** -► Fe++ + 2e-) 



Figure 1-40: Randle's Electrical Circuit Model for a Typical Corrosion Cell 
on an Unprotected Steel Surface 



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1:43 



1.7.7 Types of Corrosion 

Corrosion always occurs in the form of a corrosion cell where an anode site, a 
cathode site, and an electronic and electrolytic current path between the anode 
and cathode sites exist. However the conditions of the metal and its envirormient 
may vary widely leading to the characterization of different types of corrosion 
activity as listed below. 



Uniform Corrosion - anodes and 
^ cathodes change locations resulting in 
general metal loss (e.g., atmospheric 
corrosion). 




Figure l-41a 




Figure l-41b 



Figure l-41c 



copper service 



CAZJ 



iron 
pipe 



Pitting Corrosidi - the anode site 
remains fixed and corrosion is localized 
(e.g., stainless steels in the presence of 
chlorides). 




Crwice Corrosidi - the surface area 
in the crevice is oxygen starved but the 
surrounding surfaces have access to 
dissolved oxygen (e.g., overlapping 
seams on surface storage tank floors). 



Galvanic Conoaon - dissimilar metals 
are intercoimected and exposed to a 
common envirormient (e.g., cast iron 
water main with copper services). 



Figure 1-41(1 



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1:44 



Stress 
cracking 



fatigue 
cracking 




Figure l-41e 



Environmentally Induced Cracking 

- there is a brittle fracture of a ductile 
metal alloy in the presence of modest 
corrosion and a static or cyclic stress. 
This includes stress corrosion cracking 
(SCC), fatigue cracking, and hydrogen 
induced cracking (HIC). 

Typically the crack tips are anodic to 
the crack walls. 



graphite flakes & 
iron corrosion product 




Figure l-41f 



turbulent flow 




Figure l-41g 



Deollqying and DezindficaliGn - one 

of the alloying elements is more active 
than another resulting in the selective 
corrosion (sometimes called leaching) 
of the more active element (e.g., 
graphitic corrosion of gray cast iron). 



ErosiGn-CorrGgiGn and Fretting - 

corrosion product is removed from the 
metal surface by fluid flow or abrasion 
accelerating the corrosion reaction 
(e.g., pipelines transporting slurries). 



Except for hydrogen-induced cracking and fatigue cracking, cathodic protection 
can be effective in mitigating all of these forms of corrosion if the structure is 
buried or immersed. 



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1.7.7(a) Galvanic Corrosion 

Galvanic corrosion is one of the more common forms of corrosion because of the 
many different materials of construction available for use. As shown in Figure 1- 
42, not only can there be considerable corrosion potential difference between 
different alloys but also there is a range of possible corrosion potentials for each 
alloy, even in a relatively homogenous seawater environment. When any two 
metals are coupled, the more electronegative alloy will become the anode of a 
corrosion cell. Further, the greater the potential difference between these metals, 
the higher the corrosion current will be. 

A number of alloys such as the stainless steels, which depend on a passive film 
for their corrosion protection, have relatively noble (electropositive) potentials 
when passive but electronegative potentials if the passive film is damaged. 
Hence stainless steel is strongly cathodic to low-alloy steel when passive but only 
mildly cathodic when active. Magnesium, zinc, and aluminum are all more 
electronegative than low mild steel and cast iron, which makes them effective 
galvanic anode materials for the cathodic protection of ferrous structures. 



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1:46 



Volts versus saturatsd calomel reference electrode 



(Active) 
















(Noble) 


-1.6 -1.4 -1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.2 




















Graphite 






Platinum L 

1 \ 


^ 


Ni-Cr-M 

T 

Ni-Cr-Mo-C 


alloy C 1 1 

1 


Itanium 1 1 


u-Si alloy G D 






Stainlf 








Nickel-iron-chromium 

1 


alloy 825 | | 




Alloy 20 stainless steels, cast and 

1 1 


wrouaht 1 1 






Stainless steel-t/pes 316. 317 ^^M 


1 


ss steel - 1) 


1 1 
Nickel-copper alloys 400, K-5C 




1 1 


^pes 302,304,321,347 


^m 




□ 










Nickel-cl 


iromium all 




Silvf 
Nickel 200 


r n 






1 




Silver-bronze alloys 

3y600 ^^M 

1 


1 


□ 












Nickel-aluminum bronze 1 1 

1 h ' 


















70-30 copp 


sr nickel | | 
Lead 1 1 












c 


jtainless ste 


el- type 430 ^^| 


1 


















80-20 copper-nickel | | 


















90 


10 copper-nickel | | 
Nickel silver □ 














Stainle 


js steel - typ 


es 410, 416 ^H 1 ■ 


















Tin b 


ronzes (G &M) | | 
Silicon bronze Q 


















Mane 


1 
anese bronze \~^ 
















Admiralty brass, aluminum brass | || 


















SOP b-50Sn solder | | 




















Copper 1 1 














Na 






7 


in D 








val brass, yellow brass, red brass 


1 




Aluminun 






n bronze j | 






Austenitic nickel castiro 


n| 1 


Low-al 


oy steel F 


s 


Low-carbon steel, cast! 


ran 1 1 


Aluminum alloys 


Cadmiu 


m n 


1 




Beryllium 


■ 










Zinc 1 1 














D 


Magnesiu 


n 



















Figure 1-42: G alvanic Series for Seawater 
(Dark boxes indicate active behavior of active-passive alloys) 

Source: ASM Handbook, Vol.13, Corrosion, 9* Ed. of JVIetals Handbook, p.234 



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1:47 



Experiment 1-1 
To Demonstrate Polarization in a Corrosion Cell 




Experiment Schematic 



Procedure 



Step: 



A. Fill plastic tub with cold water to a depth of 5cm, add a cap full of salt, and 
place copper and steel sheets along one side of tub, making sure they are 
not touching. 

B. Measure open circuit potential of steel and copper. 

C. Coimect an ammeter and a 10,000 ohm resistor in series with the copper 
and steel sheet as shown in the schematic. 

D. With the switch in the open position, measure and record the potential of 
the copper and steel sheets with respect to a copper-copper sulfate 
reference electrode (CSE) placed next to the surface of each sheet. 

E. Measure and record the potential difference between the steel and copper 
sheet (Vm). 



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F. Measure and record the potential difference (Ve) between the two 
reference electrodes. 

G. Close the switch and wait 2 minutes before repeating steps C, D, E, and 
measuring and recording the corrosion cell current (Icorr) and direction. 

H. Reduce resistance to 1,000 ohms and repeat Step F. 

I. Reduce resistance to 100 ohms and repeat Step F. 

J. Reduce resistance to 10 ohms and repeat Step F. 

K. Reduce resistance to ohms and repeat Step F. 

L. Measure and record the potential of the corrosion cell (Ecorr) with reference 
placed at "x." 

M. Plot the cathodic and anodic polarization curves on the semi-log graph 
paper with potential on the ordinate and current on the abscissa (see Figure 
1-21). 

N. Leave current connected as in Step J in preparation for Experiment 2-1 and 

2-2. 



Results 



step 


Status 


Potentials 

Epe 


(mVcse) 

Ecu 


Voltage C 


>iiop (mV) 

Ve 


'corr 

(mA) 


Calc. 

Epe - Ecu 


Calc. 

V,.+Ve 


C,D,E 


OC 
















F 


10 kQ 
















G 


1 kQ 
















H 


100 Q 
















1 


10 Q 
















J 



















K 


t corr 

















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1:49 



Conclusions 



1. The difference between the open circuit (OC) potentials of the steel and 
copper electrode equals the corrosion cell initial EMF (Vm). 

2. The difference between the closed circuit potentials of the steel and copper 
electrodes equals the sum of external and electrolyte voltage drops since 
KirchhofP s voltage law applies to this series circuit (e.g., for Steps F through 
J). 

3 . The corrosion potential (Ecorr) recorded in Step K is about midway between 
the polarized potential of the steel and copper. 

4. More polarization occurs at the cathode (copper electrode) than at the anode 
(steel electrode), i.e., AEppe < A EpCu- 

5. Both the cathodic and anodic polarization curves are nonlinear. 

6. As corrosion current (Icorr) increases, the amount of polarization increases. 

7. The operating EMF (EpPe - Ep cu) is always less than the initial EMF and 
decreases with increasing current. 

8. The corrosion current (Icorr) direction is from copper to steel in the external 
circuit and from steel to copper in the electrolyte. 



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CHAPTER 2 

CATHODIC PROTECTION SYSTEMS 



2.1 Definition 

Although the concept of cathodic protection finds its roots in the experiments and 
works of Sir Humphrey Davy and Michael Faraday, it was not until 1938 that 
Mears and Brown offered the following theory of cathodic protection: 

...in cases where corrosion is entirely electrochemical in nature it is 
necessary to polarize the cathodes in the corrosion cell to the open 
circuit potential of the local anodes in order to obtain complete 
cathodic protection.^ 

The theory of cathodic protection is best understood by first considering a simple 
corrosion cell consisting of one anode and one cathode on a structure. The driving 
force for corrosion current is the difference in potential between the anode and 
cathode. The magnitude of the corrosion current, as determined by Ohm's law, is 
directly related to this potential difference and inversely related to the resistance of 
the current path. After the corrosion cell has reached a steady state condition, the 
driving potential is the potential difference between the polarized potential of the 
anode and the polarized potential of the cathode, as seen in Equation 2-1. 



c,p 



where: 



Ra 
Re 



R. + R. 



[2-1] 



Corrosion current (A) 
Potential of polarized anode (V) 
Potential of polarized cathode (V) 
Resistance of anode to electrolyte (Q) 
Resistance of cathode to electrolyte (Q) 



As seen in the electrical equivalent of this simple corrosion cell in Figure 2-1, the 
current discharged by the anode is exactly the same as the current collected at the 
cathode. Equation 2-2. 



la 



[2-2] 



^ R. B. Mears and R. H. Brown, "A Theory of Cathodic Protection," Transactions of the Electrochemical 
Society, presented October 15, 1938, vol. 74, pp. 519-531. 



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2:2 



where: 



Anode current (A) 
Cathode current (A) 





I. = L = I„ 



E - E 

a,p c,p 

R„ + R. 



Figure 2-1: Simple Corrosion Cell 

With the appHcation of cathodic protection, positive charge flows from an external 
source toward the structure (single corrosion cell), as seen in Figure 2-2. Now, the 
corrosion current or anodic current is no longer equal to the cathodic current. From 
KirchhofP s current law. Equation 2-3 indicates that the anodic or corrosion current 
is equal to the cathodic current from the corrosion cell minus the applied current. 



R, 



CP^^' 



H'N'-— t" 

III Icp 



-CP 



-a,p 



R. 



-c,P 



la = Ic - I 



c - iCP 



icp 



Figure 2-2: Application of CP Current 



la ~ Ic ~ IcP 



[2-3] 



where: Icp = cathodic protection current (A) 



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2:3 



The corrosion cell must reach a new steady state condition. Since the cathode on 
the structure is at a more electropositive potential than the anode, the current 
collects at the cathode site first, assuming equal resistance paths. This collection of 
current at the cathode site causes the cathode to polarize more in the 
electronegative direction (reduction). Because of the increased polarization of the 
cathode, the potential difference between the anode and cathode in the corrosion 
cell decreases. This causes a decrease in the magnitude of the corrosion current. 
With a smaller corrosion current discharging into the electrolyte at the anode, the 
anode depolarizes becoming less electropositive. A new steady state condition is 
reached with a smaller corrosion current due to the applied cathodic protection 
current. 

With each additional increment of applied cathodic protection, the corrosion 
current decreases further and the anode depolarizes more until the anode reaches 
its open circuit potential. At this point, the anode caimot depolarize further; 
therefore, the potential difference between the anode and cathode is zero. With 
zero potential difference, the anodic current becomes zero, the anode ceases to 
exist (i.e., ceases to function as an anode), and the cathodic current is equal to the 
applied cathodic protection current, as seen in Figure 2-3. Applying additional 
current serves only to polarize the two sites, which are now both cathodes, more 
electronegatively. 



Rcp ^; 




h = 




Ic = Icp when la = 



Figure 2-3: Corrosion Stopped 



Now consider a real corroding structure with many anodes and cathodes. As 
cathodic protection current is applied, the current enters the structure at the 
cathodes and causes polarization of the cathodes in the electronegative direction. 
As more and more cathodic protection current is applied, the potentials of the 
cathodes approach the potentials of the more active anodes. As the potential 

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2:4 



differences decrease, more and more anodes convert to cathodes. When the 
structure polarizes to the open circuit potential of the most negative (most active) 
anode, no further anodes exist on the structure, and complete cathodic protection is 
achieved. 

Therefore, the true criterion for complete cathodic protection of a structure is 
the polarization of the cathodes on the structure to the open circuit potential 
of the most active anode on the structure. 

Since corrosion current ceases at this point, further addition of protective current is 
uimecessary and wasteful. Although the true criterion is relatively easy to 
understand, application of this criterion to real corrosion problems is not possible 
because the open circuit potential of the most active anode on a structure caimot be 
accurately calculated or measured in the field. Therefore, surrogate criteria are 
necessary. 



2.2 Criteria 

The effectiveness of cathodic protection can be determined by methods that show 
corrosion on the structure is not occurring. Many of these methods involve 
physically inspecting for corrosion on the surface of the structure, reviewing the 
enviroimiental conditions and corrosion control operating parameters and/or a 
reduction in the rate of corrosion leaks. In most cases involving pipelines and other 
buried structures, it is not practical to routinely and frequently inspect the surface 
to verify that corrosion is not occurring. In the absence of this kind of specific data 
that adequate cathodic protection has been achieved, surrogate criteria based on 
cathodic polarization can be used. These criteria can be applied without waiting for 
evidence of corrosion. 

The purpose of a surrogate cathodic protection criterion is to provide a benchmark 
against which the level of cathodic protection applied to a specific structure can be 
compared. Some desirable characteristics of a good criterion include applicability 
to a wide range of structures and enviroimients, ease of application, sound 
scientific basis, high probability of mitigation of corrosion to an acceptable level, 
and low probability of excessive protection. 

Although several criteria have been recognized in the past, current versions of 
NACE Standards SP0169 (pipelines), RP0193 (on-grade tanks), and RP0285 
(underground storage tanks) recognize three primary criteria for steel exposed to 
soil environments: 1) the -850 mVcsE potential criterion with current applied, 2) 



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the -850 mVcsE polarized potential, and 3) the 100 mV polarization criterion. In 
this text we will refer to the first two criteria as the "potential criterion" and the 
third criterion as the "polarization shift criterion." Since all three of these standards 
provide essentially the same criteria with slight variations in order and wording, 
we will concentrate on SP0169 in the following discussions. 

2.2.1 Potential Criterion (- 850 mVcsE) 

The current version of NACE Standard SP0169 provides two different statements 
of the potential criterion. In paragraph 6.2.2. 1. 1, the criterion is stated as: 

A negative (cathodic) potential of at least 850 mV with the cathodic 
protection applied. This potential is measured with respect to a 
saturated copper/copper sulfate reference electrode contacting the 
electrolyte. Voltage drops other than those across the structure-to- 
electrolyte boundary must be considered for valid interpretation of 
this voltage measurement.^ 

This statement of the criterion goes on to indicate that "Consideration is 
understood to mean the application of sound engineering practice in determining 
the significance of voltage drops by methods such as: 6.2.2.1.1.1 Measuring or 
calculating the voltage drop(s)...." The standard contains other approaches to 
considering voltage drop significance. 

In paragraph 6.2.2.1.2, the potential criterion is stated as "A negative polarized 
potential (see definition in Section 2) of at least 850 mV relative to a saturated 
copper/copper sulfate reference electrode.'" In Section 2, the definition of a 
polarized potential is given as "...the sum of the corrosion potential and the 
cathodic polarization."^ 

In measuring the stmcture-to-soil potential in the field, the potential measured is 
the algebraic sum of the corrosion potential, the cathodic polarization (ric), and any 
IR drop present (Equation 2-4). 



^c +V 



IR 



[2-4] 



'NACE Standard SP0169, "Control of External Corrosion on Underground or Submerged Metallic Piping 

Systems," (Houston, TX: NACE, 2002), pp. 12-17. 
• Ihid.2 



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where: 



ViR 



Measured potential (V) 
Corrosion potential (V) 
Polarization (V) 
Voltage drop (V) 



If the IR drops are removed from the measurement, through interruption of the 
current or any other valid means, the remaining value represents the polarized 
potential of the structure as defined by Section 2 in the standard. Since the 
potential criterion, as stated in paragraph 6.2.2.1.1, requires "consideration of 
voltage drops other than those across the stmcture-to-electrolyte boundary" and 
"consideration" is further defined as "measuring or calculating the voltage 
drop(s)," the two statements of the potential criterion are exactly the same with one 
exception. The potential criterion, as stated in paragraph 6.2.2.1.1, specifically 
allows the engineer to evaluate IR drop by methods other than actual 
measurement or current interruption within the confines of "sound 
engineering practice." 



The -850 mVcsE criterion for the protection of steel in soil was first suggested by 
Kuhn in 1933 without scientific justification for this specific value."^ Over the next 
twelve years, the validity of this criterion was strengthened through actual field 
experience, as reported by Logan. ^ Through laboratory experimentation Ewing 
and, at about the same time, Schwerdtfeger and McDorman added further validity 
to the potential criterion showing that in some cases the -850 mVcsE value was 



conservative. 



6,7 



Schwerdtfeger and McDorman plotted the experimentally measured potentials of 
steel electrodes in different air free soils versus pH. By drawing a curve through 
this data and plotting the potential of a hydrogen electrode versus pH on the same 
graph, they observed that the intersection of the two curves occurred near a pH of 
9 and a potential of -770 mVscE (-846 mVcsE)- Based on this data and information 
from Holler, they concluded that since there would be no difference in potential 
between steel and a hydrogen electrode at this potential, corrosion would be 

Q q 

negligible. ' 



''Robert] .Kuhn, "Cathodic Protection of Underground Pipe Lines from Soil Condition," API 

Proceedings, vol. 14, sec. 4, November 1933, pp. 153-167. 
^ Kirk Logan, " Ur\derground Corrosion," Circular C450, NBS, November 1945, pp. 250-279. 
® ScottEwing, "Potential Measurements for Determining Cathodic Protection Requirements," 

CORROSION, vol. 7, no. 12, (1951): pp. 410-418. 
'' W.J. Schwerdtfeger and . N . McDorman, "Potential and Current Requirements for the Cathodic 

Protection of Steel in Soils," CORROSION, vol. 8, no. 11 (1952): pp. 391-400. 
^ Lbid. 7. 



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The potential criterion was further enhanced by more recent experimental 
confirmation that the -850 mVcsE value provided adequate protection in a wide 
range of soil types, even with varying moisture levels (from approximately 4 % to 
saturation) and oxygen levels (aerated and deaerated)/° This research was 
followed up by field testing in a variety of natural envirormients to confirm the 
laboratory results. The researcher selected eleven test sites in three different 
countries to test the validity of the criterion in varied field envirormients. Adequate 
corrosion mitigation assumed a corrosion rate of one mpy or less over the test 
period. Figure 2-4 shows the interim results for the eleven field test sites. As noted 
in Figure 2-4, this field testing further strengthened the validity of the -850 mVcsE 
criterion since all of the eleven sites tested were adequately protected according to 
the criterion. 



OLPE 






























m 














HOFFMAN 
































rH 




























































SHARON 




ELKINS 




























( 






































































MEADOW DN 




u 






















































MAPLE 




^^ 














































CARON 


■¥^ 










































BETHUNGRA 


^^^ 


























_, 
















MOOMBA 


■ 
















































TWIN FALL5 


1 
































































TILDEN 


T^ 




1 1 1 1 r 



0.2 



0.4 



0.6 



0.8 



1.2 



Eoff, V (CU/CUSO4) 



Figure 2-4: Range of Polarized Potentials for Protection at Each Site 

Source: Dr. Thomas .T. Barlo, "Field Testing the Criteria for Cathodic Protection," Research sponsored 
by Pipeline Research Committee of American Gas Association, SAIC Interim Report, Dec. 1987, Cat. No. L51546 
(Hoffman Estates, IL: AGA) 1988. 



^ H.D. Holler, "Studies on Galvanic Couples," Journal of Electrochemical Societ]i, vol. 97, (1950): pp. 

277-282. 
^° Thomas J. Barlo and Warren E. Berry, "An Assessment of the Current Criteria for Cathodic Protection 

of Buried Steel Pipeline," MP, vol. 23, no. 9 (1984): pp. 9-16. 
" Dr. Thomas J. Barlo, "Field Testing the Criteria for Cathodic Protection," Research sponsored by 

Pipeline Research Committee of American Gas Association, SAIC Interim Report, Dec. 1987, Cat No. 



L51546 (Hoffman Estates, IL: AGA) 1988. 



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Meanwhile, a number of investigators have attempted to calculate a specific value 
for the potential criterion based on thermodynamic considerations. Begirming with 
an assumption regarding what constituted "an acceptable corrosion rate," they used 
the Nemst equation with solubility data in an attempt to calculate a potential value. 
The values determined varied around the previously established value of 
-850 mVcsE depending on the specific assumptions. ^^'^'''^'^'^^'^^'^^'^^ 



As indicated in NACE Standard SP0169 and other standards, the data interpreter 
must consider IR drop when evaluating the protection level of a cathodically 
protected structure. In the practical application of cathodic protection, Pearson 
noted as early as 1944 that IR drop in the electrolyte did not significantly reduce 
corrosion. He concluded, "It is clear that any measurement of the polarization of a 
buried structure must be made to differentiate between the effects of purely IR 
drop and the electrochemical results of the current flow. Only the latter is of any 
use in controlling the rate of corrosion."^^ This concern over elimination of the IR 
drop error was reiterated by Logan, Ewing, Schwerdtfeger, and McDorman along 
with many other s.^°'^^'^^ 



Although the specific value of -850 mVcsE as a potential criterion for cathodic 
protection of steel in a soil or water environment is now widely accepted, this has 
not been the only value offered. Some argued that a more negative potential was 
required to mitigate corrosion fully. In fact, potential values as electronegative as 
-1000 mVcsE have been suggested. ' ' Also, a number of investigators report 



'-M. Pourhaix, CORROSION, vol. 5, no. 4 (1949): p. 125. 

^^ C. Wagner,] Journal of Electrochemical Society, vol. 1, (1952). 

'"L. P. Sudrahin, CORROSION, vol. 13, no. 5 (1957): p. 87. 

'' M. H. Peterson, CORROSION, vol. 15, no. 9 (1959): p. 485t. 

^^H. H. Uhlig, Corrosion and Corrosion Control, 2"'' Ed. (]SfewYork,NY:Jdm VW/ey&Sans, 1971), p. 

224. 
" [/. R. Evans, The Corrosion and Oxidation of Metals (Edward Arnold Publishers, Ltd., 1960), p. 284. 
^^J.A. Davis and J. D. Kellner, "Electrochemical Principles Applied to Operating Pipelines,: 

CORROSION/89 paper no. 412(Houston, TX: NACE, 1989). 
^'J. M. Pearson, "Concepts and Methods of Cathodic Protection - Parti," The Petroleum Engineer, 

(March 1944): pp. 216, 218, 220. 
-° Kirk Logan, " Underground Corrosion," Circular C450, NBS, November 1945, pp. 250-279. 
"' ScottEviing, "Potential Measurements for D etermining Cathodic Protection Requirements," 
^ CORROSION, vol. 7, no. 12, (1951): pp. 410-418. 
'' y\I.J. Schwerdtfeger and O.N. McDorman, "Potential and Current Requirements for the Cathodic 

Protection of Steel in Soils," CORROSION, vol. 8, no. 11 (1952): pp. 391-400. 
'^ Robert]. Kuhn, "Cathodic Protection on the 840-Mile Texas Gas Transmission Corporation 26-Inch 

Pipe Line from Texas to Ohio," American Gas Association Annual Conference, Atlantic Cit]j, NJ ., Oct. 

1950. 
''^ A. C. Toncre, "A Review of Cathodic Protection Criteria," Proceedings of the 6" European Congress on 

MstaUic Corrosion, (London, England: Sbdety ofChenical Industry, 1977). 



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2:9 



that a more electropositive potential criterion will provide adequate protection in 
special cases. This was found to be especially true for aerated, dry soils?^'^^'^^'^^"^" 

Perhaps one of the primary reasons for the disagreement between various 
investigators regarding a specific potential criterion stems from the level of 
acceptable corrosion adopted by each. The acceptance of the -850 mVcsE potential 
is based on mitigating corrosion to an economically acceptable level, not stopping 
corrosion completely. 

2.2.2 Polarization Shift Criterion (100 mV) 

NACE Standard SP0169 also establishes a polarization shift criterion for 
determining adequate protection for steel structures in Section 6.2.2.1.3, as 
follows: 

A minimum of 100 mV of cathodic polarization between the structure 
surface and a stable reference electrode contacting the electrolyte. The 
formation or decay of polarization can be measured to satisfy this 
criterion.^^ 

The theoretical basis for understanding the polarization shift criterion begins with 
the assumption that the corrosion cell is operating under cathodic control. Further, 
it is assumed that the difference in the open circuit potential of the most active 
anode on the structure and the corrosion potential of the structure is 100 mV or 
less. Therefore, if the structure is cathodically polarized from an external source by 
at least 100 mV, there will be no potential difference between the anodes and 
cathodes on the structure, and corrosion will cease. 



'^Komei Kasahara, Taisaku Sato, and Haruhiko Adachi, "Results of Polarization Potential and Current 

D ensity Surveys on Existing Buried Pipelines," MP , vol. 19, no. 9 (1980): pp. 49-51. 
'^ Scott Ewing, "Potential Measurements for D etermining Cathodic Protection Requirements," 

CORROSION, vol. 7, no. 12, (1951): pp. 410-418. 
-'' Thomas J. Barlo and Warren E. Berry, "An Assessment of the Current Criteria for Cathodic Protection 

of Buried Steel Pipelines," MP, vol. 23, no. 9 (1984): pp. 9-16. 
-'^ Kirk H. Logan, "Comparisons of Cathodic Protection TestMethods," CORROSION, vol. 10, no. 7 

(1954): pp. 206-211. 
'^ T.J. Barlo et. ah, "Investigation of Techniques to Determine the True-Pipe-to-Soil Potential of a Buried 

Pipeline," Research sponsored by Pipeline Research Committee of American Gas Association, Battelle 

Annual Report, Feb. 1981. 
^° Standard DIN 30676, "Design and Application of Cathodic Protection of External Surfaces," (October 

1985). 
^' NACE Standard SPOl 69, " Control of External Corrosion on Underground or Submerged Metallic Piping 

Systems," (Houston, TX: NACE, 2002), pp. 12-1 7. 



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In addition to his confirmation of the -850 mVcsE criterion, Ewing's experimental 
work supported the 100 mV polarization shift criterion. He concluded "The change 
in potential or the polarization necessary for protection of pipe lines is probably 
always less than 0.1 volt."' von Baeckmann and Schwenk provided theoretical 
justification of the 100 mV polarization by applying the Nemst equation to 
experimentally determined corrosion rate potential plots. However, the 
comprehensive testing of various criteria by Barlo and Berry provided convincing 
support for the wide ranging validity of the 100 mV criterion. They concluded 
"The 100 mV polarization criterion appears to be the most generally valid and 
applicable criterion to prevent corrosion in various soils. "'^"^ Again, they further 
investigated the potential shift criterion through field testing at eleven field sites. 
As shown in Figure 2-5, the 100 mV criterion was found to be valid for all the 
field test sites. ^^ 



MOOMBA 




































1 1 ^^ 1 1 ' 


I I 






























BETHUNGRA 


1 ^^^^H^^^H 














1 1 1 1 


III 


TWIN FALLS 
































MEADOW DN 




t= 














































































































CARON 










— — — i — — 


1 1 1 1 1 1 


































TILDEN 










































































OPLE 






























































ELKINS 












MAPLE 


W 1 




















































HOFFMAN 


■ 1 




















































SHARON 


■ 1 

























































25 



50 



75 



100 



125 



150 



AE, mV 
Figure 2-5: Range of Polarization for Protection at Each Site 

Source: Dr. Thomas J. Barlo, "Field Testing the Criteria for Cathodic Protection," Research sponsored by Pipeline 
Research Committee of American Gas Association, SAIC Interim Report, Dec. 1987, Cat. No. L51546, 1988. 



^' ScottEwing, "Potential Measurements for Determining Cathodic Protection Requirements," 

CORROSION, vol. 7, no. 12, (1951): pp. 410-418. 
^^ W. von Baechmann and W. Schwenk, Handbook of Cathodic Protection, (Surrey, England: Portcullis 

Press Ltd., 1975), pp. 37-38, 58-59, 153-1 73, 185-198, and 208-21 7). 
^'' Thomas J. Barlo and Warren E. Berry, "An Assessment of the Current Criteria for Cathodic Protection 

of Buried Steel Pipelines," MP, vol. 23, no. 9 (1984): pp. 9-16. 
^^ Dr. Thomas J. Barlo, "Field Testing the Criteria for Cathodic Protection," Research sponsored by 

Pipeline Research Committee of American Gas Association, SAIC Interim Report, Dec. 1987, Cat. No. 

151546 (Hoffman Estates, II: AG A) 1988. 



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2.2.3 Factors A ffecting Validity of C riteria 

Both the potential criterion and the polarization shift criterion are based on soil and 
natural water environments at ambient temperatures. These criteria have 
decreasing validity as temperature increases or unusual conditions and chemistries 
are encountered. Some of the factors that can affect the validity and application of 
specific criteria include temperature, bacteria, alternating current (AC) 
interference, type of metal, presence of mixed metals, stress corrosion cracking 
(SCC), and disbonded coatings. 

2.2.3.1 Temperature 

The -850 mVcsE potential criterion and the 100 mV polarization shift criterion 
have been thoroughly investigated and validated under normal ambient 
temperature conditions (20 to 25 °C) for steel in soils and natural waters. However, 
a number of investigators have determined that these criteria become inadequate 
with increasing temperatures. Most agree that the potential criterion should 
be adjusted electronegatively to -950 mVcsE for temperatures above about 60 °C 
(140 °F). ' " According to results reported by Barlo and Berry, the polarization 
shift criterion should be adjusted from 100 mV to 150 - 250 mV at temperatures of 
60 °C (140 °F), while Morgan suggested an adjustment of 2 mV/°C for high 
temperatures.^'"'''^ 

2.2.3.2 Sulfate Reducing Bacteria 

The presence of sulfate reducing bacteria (SRB) can affect the criteria necessary 
for adequate cathodic protection. SRB are anaerobic microbes often found in 
marine sediments, wet marshy areas, and clay soils. Since these bacteria can use 
hydrogen adsorbed on steel surfaces to convert sulfates into sulfides and the 
resultant sulfides react with the iron ion to form iron sulfide, depolarization of the 
anodes and cathodes on the steel surface results. In the presence of SRB the 
potential criterion recommended is -950 mVcsE, and the polarization shift criterion 



^''Thomas J. Barlo and Warren E. Berry, "An Assessment of the Current Criteria for Cathodic Protection of 

Buried Steel Pipelines," MP, vol. 23, no. 9 (1984): pp. 9-16. 
^^ Standard DIN 30676, "Design and Application of Cathodic Protection of External Surfaces," (October 

1985). 
^^W. von Baechmann and W. Schwenk, Handbook of Cathodic Protection, (Surrey, England: Portcullis 

Press Ltd., 1975), pp. 37-38, 58-59, 153-1 73, 185-198, and 208-21 7. 
^^John Morgan, Cathodic Protection, 2"'' Ed. (Hoiiston, TK: mCE, 1987), pp. 37, 152-175, 205, and 254- 

258. 



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36,37,39,40 



becomes 200 mV. ' " ' Barlo and Berry further suggested the polarization shift 
criterion should be approximately 200 to 300 mV in the presence of SRB at 
temperatures of 60 °C (140 °F)/^^ 



2.2.3.3 Alternating Current (AC) Density 

The significance of AC current in the corrosion of steel has been investigated since 
the early 1900s. Several studies in the 1960s concluded that for steel "the 
corrosiveness of an induced AC current is equal to approximately 0.1% of an 
equivalent value DC current." Therefore, corrosion engineers ignored AC-induced 
corrosion as relatively insignificant for many years. However, the significance of 
AC corrosion re-emerged as the subject of numerous investigations into a pipeline 
failure in Germany in 1986 attributed to AC corrosion of a cathodically protected 
pipeline. Investigators have now determined that the presence of AC current at the 
metal/electrolyte interface of a steel structure can significantly affect the corrosion 
rate of the structure even with significant cathodic protection applied. ' 

German investigators concluded that the risk of AC corrosion on a cathodically 
protected structure was a function of the AC current density at the 

9 9 

metal/electrolyte interface. Up to an AC current density of 20 A/m (2 A/ft ), they 
concluded there was "probably no risk" of corrosion using conventional criteria for 

9 9 

cathodic protection. At AC current densities greater than 20 A/m (2 A/ft ) and less 

9 9 

than 100 A/m (10 A/ft ), they indicated conventional CP criteria were umeliable 
and corrosion was possible. However, with AC current densities greater than 100 

9 9 zf^ 

A/m (10 A/ft ), corrosion damage would be expected. 

The level of cathodic protection current density does have a mitigating effect on 
the AC corrosion up to a point. According to Helm, et al., cathodic protection 

9 9 

current densities up to 0.25 A/m (25 mA/ft ) had no mitigating effect; however, 

9 9 

noticeable effect occurred at densities of 4 A/m (400 mA/ft ). Devay, et al., 
reported that at AC current densities of 100 and 250 A/m^ (10 and 25 A/ft^) 



"^^Karl P. Fischer, "Cathodic Protection is Saline Mud Containing Sulfate Reducing Bacteria," MP, vol. 20, 

no. 10 (1981): pp. 41-46. 
""fi. A. Gummow, R. G. Wakelin, and S. M. Segall, "AC Corrosion - A New Challenge to Pipeline 

Integrit]!," NACE CORROSION/98, paper 566, (Houston, TX: NACE). 

''"J. Dabkowski and A. Taflove, "Mutual Design Considerations for Overhead AC Transmission Lines and 
Gas Transmission Pipelines - Volume I: Engineering Analysis," EPRI EL-904, (Chicago, IL: IIT 
Research Institute, 1978), pp. 7/1 - 7/10. 

''^G.Helm, H. Heinzen, and W. Schwenk, "Investigation of Corrosion of Cathodically Protected Steel 
Subjected to Alternating Currents," 3R International, 32, Issue 5, pp. 246-249 (German). 



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increasing DC current densities reduced the AC corrosion; however, AC corrosion 
was still significant with DC current densities as high as 10 A/m (1 A/ft ). 
Considering normal anticipated protective DC current densities of 10 to 30 mA/m 
(1 to 3 mA/ft ) in soil without the presence of AC, the DC current densities 
required in the presence of AC currents are quite high. ' 

Since AC current densities may not be known or easily measured, it is often 
desirable to convert to AC voltages, a more readily measurable quantity. This can 
be accomplished by considering the current at the interface of a circular disk 
holiday with a surface area of 1 cm , worst case according to Prinz. By combining 
Ohm's law with the equation for the resistance-to-earth of a circular disk and the 
equation for the surface area of the disk. Equation 2-5 results. ' 



V„ 



pTT 6. 



[2-5] 



^Alhere■. Vac = AC voltage pipe-to-earth (V) 
iac = AC current density (A/m ) 
p = soil resistivity (Q-m) 
d = diameter of disk holiday (m)* 

* (d = 0.01 13 m for disk with 1 cm surface area) 
Equation [2-5] is derived as follows: 
Ohm's Law 
V = IR 

Current Density 
I=iA 
Resistance to Earth of a Circular Disk 



R 



2d 



Area 



A 



TTd^ 



AC Pipe-to-Earth Vohage 

Vac = IR 

iacAp 



Vac 



2d 



^"^J. Devay, et. AL, " Electrolytic AC Corrosion of Iron," Acta Chimica, 52 (1967), pp. 65-66. 

^' Ibid 41 

"^W. Prinz, "AC Induced Corrosion on Cathodically Protected Pipelines," UK Corrosion 92, vol. 1, (1992). 



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Vac^ 



Va 



4(2d) 

kMp 



For an AC current of 100 AW (10 A/tf ), Figure 2-6 provides the threshold AC 
voltages for various soil resistivities and holiday sizes. This figure may be used to 
determine the likelihood of AC corrosion for a structure based on a specific AC 
voltage, soil resistivity, and holiday size."^^ 



1000 



100 



w 
o 

o 
n 




1 10 100 

Holiday Area (cm^) 

Figure 2-6: AC Voltage vs Holiday Size for Iac = 100 A/m^ 

Source: R. A. Gummow, R. G. Wakelin, and S. M. Segall, "AC Corrosion -A New Challenge to Pipeline Integrity," 
CORROSION/98, paper no. 566, (Houston, TX: NACE, 1998). 



2.2.3.4 Type of Metal 

Different metals exhibit different corrosion potentials. Since the corrosion 
potential of a metal is a mixed potential based on the weighted average of the 
polarized anodes and the polarized cathodes on the structure, the potential criterion 
must change depending on the specific metal involved. This is a result of the need 
to polarize the cathodes to the open circuit potential of the most active anode 
according to the true criterion for cathodic protection. Since the potential of the 
anodes is different for different metals, the potential criterion must be adjusted. 



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However, assuming cathodic control of the corrosion cell, the polarization shift 
criterion as established for steel may be sufficient for other metals. 

Although NACE Standard SP0169 does not provide a specific potential criterion 
for aluminum or copper, it does indicate that a cathodic polarization shift criterion 
of 100 mV would apply. However, a caution is given for excessive potentials 
(more electronegative than -1200 mVcsE) in the case of aluminum, an amphoteric 
metal"*^ In general, caution should be exercised with any amphoteric metal 
(aluminum, cadmium, lead, tin, zinc) since these metals can undergo significant 
corrosion at the high pH values encountered with excessive cathodic potentials. 

Other standards, such as British standard BS 7361 (Table 2-1) and German 
standard DIN 30676 (Table 2-2), do provide specific potential criteria for various 
types of metals. 

Table 2-1: Potential Criteria from British Standard BS 7361 



Material 


Potential, CSE 

Soils and 
Fresh Water 


Potential, 

Silver-silver Chloride 

Seawater 


Iron and Steel 
Aerobic environment 


-850 mV 


-800 mV 


Iron and Steel 
Anaerobic environment 


-950 mV 


-900 mV 


Lead 


-600 mV 


-550 mV 


Aluminum 
Not to exceed 


-950mV 
-1200 mV 


-900 mV 
-1 1 50 mV 


Copper Alloys 


-500 to -650 mV 


-450 to -600 mV 



^"^NACE Standard SP0169, "Control of External Corrosion on Underground or Submerged Metallic Piping 

Systems," (Houston, TX: NACE, 2002), pp. 12-17. 
''^C.F. Schrieber and Reece W. Murray, " Effect of Hostile Marine Environments on the Al-Zn-In-Si 

Sacrificial Anode," CORROSION/88, paper no. 32, (Houston, TX: NACE, 1988). 



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Table 2-2: Potential Criteria from German Standard DIN 30676 



Material 


Temperature, °C or 
Electrolyte 


Potential 

VcSE 


Unalloyed & low-alloy ferrous 
materials 


Below 40°C(104°F) 


-850 mV 


Same 


Greaterthan60°C(140°F) 


-950 mV 


Same 


Anaerobic media 


-950 mV 


Same 


Sandy soil, p>500 Q-m 


-750 mV 


Stainless steels with Cr>16% 


Soil or fresh water and less 
than 40°C 


-1 00 mV 


Same 


Soil or fresh water and 
higher than 40°C 


-300 mV 


Same 


Salt water 


-300 mV 


Copper, copper-nickel alloys 




-200 mV 


Lead 




-650 mV 


Aluminum 


Fresh water 


-800 mV 


Same 


Salt water 


-900 mV 


Steel in contact with concrete 




-750 mV 


Galvanized steel 




-1200 mV 



International Standard ISO 15589-1 
Petroleum and Natural Gas Industries- Cati\odic Protection of Pipeline 

Transportation Systems 
Part 1 On-land pipelines 

• Metal-to-electrolyte potential chosen for a corrosion rate less than 0.01 
mm/yr (0.39 mils/yr) 

• Polarized potential more negative than -850 mVcsE 

• Limiting critical potential not more negative than -1,200 mVcsE 

• Anaerobic soils or sulfate-reducing bacteria (SRB) more negative than 
-950 mVcsE 

• High soil resistivity 

750 mVcsE for 100 Q-m < p < 1,000 Q-m 

650 mVcsE for p > 1,000 Q-m 

• Cathodic polarization of 100 mV 
Precautions: 

Avoid using 100 mV under conditions of high temperatures, SRB, 
interference current, equalizing current, telluric current, mixed metals or 
sec conditions more positive than -850 mVcsE- 



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2.2.3.5 Mixed Metals 

When two different metals are electrically coupled, a macroscopic corrosion cell 
forms where one metal is anodic to the other. However, microscopic corrosion 
cells also exist on the surfaces of each of the individual metals. Although the 
corrosion cells on the surface of the cathodic metal may be suppressed by 
electrically coupling the metals, the corrosion cells on the anodic metal remain 
active. To mitigate all corrosion including the microscopic corrosion cells on the 
surface of the anodic metal, the metal couple must be cathodically polarized to the 
open circuit potential of the most anodic potential on the anodic metal. 



In other words, according to NACE Standard SP0169 for dissimilar metals the 
potential criterion becomes: "A negative voltage between all pipe surfaces and a 
stable reference electrode contacting the electrolyte equal to that required for the 
protection of the most anodic metal should be maintained. 



■ 49 



As indicated previously, the polarization shift criterion is based on cathodic control 
with 100 mV or less of potential difference between the structure corrosion 
potential and the open circuit potential of the most active anode. Although this 
assumption may be correct for an individual metal, when coupled to a more 
cathodic metal, the corrosion potential of the couple will shift electropositively. 
The magnitude of this potential shift is a function of the individual corrosion 
potentials and the surface areas of the two metals. Since validity of the polarization 
shift criterion requires shifting the potential of the structure from its corrosion 
potential, the criterion carmot be used unless the open circuit potential of the 
anodic metal is known, or the surface area of the cathodic metal is insignificant 
compared with the surface area for the anodic metal. That is, disconnecting the 
cathodic metal would not significantly change the measured corrosion potential. If 
this criterion is used, the polarization shift must be from the open circuit potential 
of the anodic metal. 

2.2.3.6 Stress Corrosion Craclcing (SCC) 

Two different forms of stress corrosion cracking (SCC) of pipeline steels have 
been identified: high-pH (classical) SCC first reported in the mid-1960s and near- 
neutral (low-pH) SCC identified in the 1980s.^'^'^^ High-pH SCC is intergranular 



"•^JVACE Standard SP0169, " Control of External Corrosion on Underground or Submerged Metallic Piping 

Sptems," (Houston, TX: NACE, 2002), p. 12-17. 
^°R. L. Wenk, "Field Investigation of Stress Corrosior\ Crackir\g," 5* Syirposiumon Line Pipe Research, 

Pipeline Research Cormittee of the American Gas Association, Catalog no. L30174, 1974, p. T-1. 



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and identified with a carbonate-bicarbonate environment existing at the structure 
surface. This environment can exist at a metal surface in soil or water due to the 
chemical reaction of dissolved carbon dioxide with natural carbonates in the higher 
pH ground water created by the application of cathodic protection. The 
susceptibility of steel to high-pH SCC is potential-dependent. The application of 
cathodic protection can increase the susceptibility of a structure to high-pH SCC if 
the protective potentials are insufficiently electronegative. The range of possible 
cracking potentials is larger and becomes more electronegative with increasing 
temperatures; however, cracking has not been observed at potentials more negative 
than -850 mVcsE (although the cracking range approaches this potential at 
temperatures near 75 °C). ' 

Near-neutral pH SCC, which occurs within the pH range of 6 to 8, is transgranular 
and linked with corrosion occurring at the crack faces. Near-neutral pH SCC is 
much less dependent on the interface potential. ' 

Due to the potential dependence and the enviroimient required for high-pH SCC, 
caution must be exercised in application of cathodic protection, especially with the 
application of the 100 mV polarization shift criterion. ' " Under precautionary 
notes NACE SP0169 warns: " Caution is advised against using polarized potentials less 
negative than -850 mV for cathodic protection of pipelines when operating pressures 
and conditions are conducive to stress corrosion cracking. . .."^"^ 

2.2.3.7 Disbonded Coatings 

Organic coatings adhere to a metallic substrate due to both mechanical and polar 
bonding. Mechanical bonding is a result of the anchoring pattern between the 
solidified coating and the irregular surface roughness of the metallic substrate. 
Polar bonding is due to the electrical attraction between polar molecules within the 
coating formulation and the metallic surface. 



^'j. T. Justice and ] . D . Maclemie, "Progress in the Control of Stress Corrosion Cracking in a 914 mm OD 
G as Transmission Pipeline," Proceedings of the NG-18/EPRG Seventh Biennial JointTechnical Meeting 
on Line Pipe Research, Pipeline Research Committee of the American Gas Association, Paper no. 28, 

J988. 

^-J. A. Beavers arid K. C. Garrity, "100 mV P olarizatior\ Criterior\ ar\cl External SCC of Underground 
Pipelines," CORROSION/2001, paper no. 1592, (Houston, TX: NACE, 2001). 

^^R.A. Gummow, "Cathodic Protection Potential Criterion for Underground Steel Structures," MP, 32, 11 
(1993): p. 21-30. 

^'^NACE Standard SP0169 (latest revision) , "Control of External Corrosion on Underground or Subn\erged 
Metallic Piping Systems," (Houston, TX: NACE, 2002), p. 12-1 7. 



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For purposes of this discussion, a disbonded coating will mean a coating that does 
not adhere to the metallic structure surface. A number of other factors such as soil 
stress, changing soil and/or pipe temperatures, high operating temperatures, 
tenting, blisters and cathodic (alkaline) disbondment may result in a disbonded 
coating. Cathodic disbondment is usually associated with areas having a holiday or 
damaged spot on the coating while blisters usually occur at holiday-free areas. 

Soil stress is typically a concern where the soil and pipe move in different 
directions and result in forces that try to shear the coating away from the pipe 
surface. This is not uncommon with clay soils that expand and contract when they 
go through wet-dry cycles. It can also happen due to typical seasonal changes in 
pipe and/or ground temperatures that cause the pipe to try to elongate or shrink as 
the temperature changes. High operating temperatures can soften some types of 
coating; especially those made from polyethylene and mastic, making them more 
susceptible to soil stress and water permeation. Tenting can occur at a longitudinal 
or circumferential weld and is a result of a coating that does not conform properly 
to the 'hump' in the steel surface. This results in a "tenf , or air gap, forming 
adjacent to the weld where dirt and/or water can collect. The problems described 
below that are associated with blisters and cathodic disbondment are also typical of 
these other causes of a disbonded coating. 

Blisters begin with poor coating adhesion caused by poor surface preparation, 
improper coating application, or poor coating properties. All organic coatings absorb 
and transmit moisture to some extent. If the coating does not adhere to the metallic 
substrate, the hydraulic gradient, osmotic pressure, and, in the case of a cathodically 
protected structure, electro-osmotic pressure will increase water transmission through 
the coating. When the coated structure is initially placed in water or water-saturated 
soil, a hydraulic gradient exists from the outer surface of the coating to the metal 
substrate. This gradient results in the transmission of water through the coating to the 
metal surface where it can accumulate between the coating and the metal surface. In 
addition, if the coating's lack of adhesion is due to poor surface preparation, 
contaminating ions are likely to be present on the surface of the metal. 

Once moisture reaches the metal surface due to the hydraulic gradient, an osmotic 
pressure can initiate due to the greater ionic concentration at the metal surface. This 
osmotic pressure will increase the water transmission rate through the coating. Finally, 
if the structure receives cathodic protection current, electro-osmosis will result. 
Elecfro-osmosis moves water molecules through semipermeable membranes in the 
direction of charge flow. Because of all of these forces, water continues to accumulate 
between the coating and the metal surface resulting in the formation of a blister. 



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Blister formation is minimized by proper surface preparation and coating 
application.^^'^^ 

Cathodic disbondment results from loss of adhesion between the coating and the 
metal substrate due to cathodic protection application. The cathodic reduction 
reactions generate hydroxyl ions at the metal surface. In this case, the hydroxyl 
ions (OH ) generated at pores or damaged spots in the coating can attack and 
destroy the polar bonds between the coating and the metal surface causing loss of 
coating adhesion. Some coatings are more susceptible to cathodic disbonding than 
others; therefore, proper coating selection is the first line of defense. Higher 
temperatures also promote cathodic disbondment. Finally, since the susceptibility 
to cathodic disbonding increases at higher polarization potentials due to the 
increased alkalinity, cathodic protection potential levels should be limited. ^^ 

The validity of both the potential and the polarization shift criteria are questionable 
for metallic surfaces shielded by a disbonded coating regardless of the cause. It is 
not the criteria themselves that are in question but the ability to make 
measurements that allow the application of the criteria. Especially in the case of 
unruptured blisters, the coating electrically shields the envirormient at the pipeline 
surface from potential measurements with the reference electrode at ground level 
above the structure. In fact, to determine the true potential, the reference electrode 
would have to be on the inside of the disbonded coating. Therefore, surface 
potential measurements do not provide a true measurement of the potential at the 
metal surface and a shielded surface can be inadequately protected and undetected 
by surface potential profiles. 

If both cathodic disbondment of the coating and SCC are concerns for a specific 
structure, what is the appropriate potential range for the steel structure? If 
susceptibility to SCC is a concern, the potential criterion (-850 mVcsE) should be 
applied in favor of the polarization shift criterion (100 mV) to avoid placing the 
structure within the potential range where susceptibility is increased. In applying 
the potential criterion to a structure, the corrosion engineer should avoid 
excessively negative potentials if cathodic disbonding of the coating is a concern. 
Therefore, what is an appropriate electronegative value for a steel structure? The 
answer to this question, as with many technical questions, is based on good 



W. von Baechmann and W. Schwenk, Handbook of Cathodic Protection, (Surrey, England: Portcullis Press 

Ltd., 1975), p. 34-38, 58-59, 153-173, 185-198, and 208-217. 
^^CP 4-Cathodic Protection Specialist Course Manual, (Houston, TX: NACE, 2002), p. 1:25-1:27, 3:4-3:14, 

3:18-3:33, and 8:34-8:35. 
^^T.J. Barlo and R. R. Fessler, "Interpretation of True Pipe-to-Soil Potentials on Coated PipeUnes with 

Holidays," CORROSION/83, paper no. 292, (Houston, TX: NACE, 1983). 



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engineering judgment balancing the needs of competing requirements. On one 
hand, more electronegative potentials near the power source offset the effects of 
attenuation resulting in fewer cathodic protection installations. But on the other 
hand, the maximum level of electronegative potentials must be limited to avoid 
excessive protection levels. 

To evaluate the most electronegative potential allowable, we must first determine 
what potential values are possible. As the potential becomes more electronegative, 
the most likely reduction reaction for structures buried in most soils becomes the 
electrolysis of water. As discussed later in this chapter, evidence suggests a 
theoretical limit of the most negative true polarized potential of a structure in the 
presence of sufficient moisture is approximately -1.15 Vcse- 

To polarize deep crevices created by cathodic disbondment, some investigators 
suggest limiting the potential below levels where hydrogen generation becomes 
significant. They suggest that as hydrogen bubbles form at the coating defect, 
current flow down the narrow passage under the disbonded coating is inhibited. 
Although the potential where hydrogen gas generation occurs is variable 
depending on envirormiental factors such as temperature, pH, and surface 
condition, the suggested limit is approximately -1.0 to -1.10 Vcse-^^"^^ 

2.3 Typical Cathodic Polarization Characteristics 
2.3.1 Cathodic Polarization Curve 

As suggested by the definition of cathodic protection, the polarization 
characteristics of the cathode and the specific potential of the most active anode 
determine the current requirement necessary to achieve cathodic protection. 
Polarization, as defined in Chapter 1, is the potential change due to a charge 
transfer (current) across a reacting interface. This potential change always occurs 
with a polarity that opposes the current causing it. Therefore, the potential of a 
cathode must change in the electronegative direction due to current from the 
electrolyte. 



^^R. R. Fessler, A.J. Markworth, and R. N. Parkins, " Cathodic-Protection Levels Under Disbonded 
Coatings," CORROSION/82, paper no. 118, (Houston, TX: NACE, 1982). 

^'F. M. Song et.al, "Corrosion Under Disbonded Coatings Having Catiiodic Protection," MP, 42, 9 
(2003): p. 24-26. 



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In Chapter 1, we illustrated the polarization or Evans diagrams plotting polarized 
potential versus current, E vs. I. Both the abscissa, X-axis, and the ordinate, Y- 
axis, were drawn on linear scales. However, Evans diagrams are commonly plotted 
with the potential on a linear scale but the current on a logarithmic scale (E log I). 
As we will discover when we discuss types of polarization, the polarization 
potential is a function of the logarithm of the current. If E log I plots are 
constructed, the Evans diagram will often be a straight line. However, when we 
plot Evans diagrams with current on a logarithmic scale an interesting feature of a 
logarithmic plot becomes obvious. There is no zero point on the logarithmic scale. 
As you move toward smaller numbers on the scale, the magnitude changes by a 
factor of 10 but never reaches zero. 

Investigating the types of polarization and the equations describing polarization, 
we will discover that as the corrosion current decreases a point is reached where it 
stops decreasing and reverses direction. At this point the Gibb's free energy for a 
specific reaction to occur in either direction (anodic or cathodic) is equal. 
Therefore, there is an equal probability the reaction will occur in either direction. 
Since all atoms, ions, and molecules are constantly in motion due to thermal 
energy, the thermal (vibrational) energy results in the electrochemical reaction 
occurring in both directions at once. In other words, at an anode every time an 
atom loses electrons and begins to go into solution as an ion, a metallic ion 
captures electrons at the metal interface and becomes a metal atom ("plates ouf ). 
Therefore, there is no net reaction and no net charge flow in a specific direction. 
Both anodic and cathodic reactions occur simultaneously, and charge flow is in 
both directions at once resulting in zero net current. 

This point is the equilibrium condition for the specific reaction involved. At the 
equilibrium point, the polarization potential is the equilibrium potential for the 
reaction, as determined by the Nemst equation, and the current is the exchange 
current at equilibrium conditions. Therefore, on an E log I plot for a specific 
reaction, the begiiming point is the equilibrium potential and the exchange current, 
as seen in Figure 2-7. In this figure polarization due to the oxidation and reduction 
reactions is shown for both hydrogen and iron emanating from their respective 
equilibrium points. The intersection of the oxidation curve for iron and the 
reduction curve for hydrogen establishes the corrosion potential and corrosion 
current for the corrosion of iron in a solution containing hydrogen ions (acid). On 
most polarization plots constructed to illustrate a specific corrosion cell, the 
polarization curve for oxidation of the reduced species and the polarization curve 
for the reduction of the oxidized species are not shown. Only the polarization 
curves determining the corrosion state are shown. However, each of the 
polarization curves begins at the equilibrium state. 



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■H/H-t 



■ Fe/Fe++" 




►E. 



log CP Current 



Figure 2-7: Polarization Curves for Iron Corrosion in Acid 

Considering a simplified corrosion cell involving one anode and one cathode on a 
reacting surface (Figure 2-8), the driving potential (Eceii) for current is the difference 
between the open circuit potentials of the cathode and anode (Ec oc - Eaoc) at the 
moment of immersion in an electrolyte. At this moment, a complete corrosion cell 
forms and charge begins to flow from anode to cathode within the electrolyte. As 
charge movement (current) begins, polarization of the reacting surfaces begins. This 
polarization results in opposing EMFs across the reacting interfaces thereby 
reducing the available driving potential. The decreased driving potential reduces the 
magnitude of current until finally a steady state condition is reached. The changes 
occurring at the anode and cathode stabilize through polarization to a specific, 
maintainable reaction rate. 



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Cathode Polarization 




log CP Current 



Figure 2-8: Evans Diagram for Corrosion Cell 

At this point the new driving potential (Ecc) for corrosion current is the difference 
between the polarized potentials of the cathode and the anode (Ec.p - Eap). The 
magnitude of current between the anode and cathode (Icorr) reflects the overall 
reaction rates determined at this steady state condition. This is the corrosion 
current for the cell. At this steady state operating point, the potential of the 
corrosion cell measured relative to a stable reference electrode is the corrosion 
potential (Ecorr)- As previously indicated, this potential will be a weighted 
(geometrical and electrical) average somewhere between the final polarized 
potential of the anode and the final polarized potential of the cathode. The Evans 
diagram (E log I diagram), shown in Figure 2-8, shows the steady state operating 
condition of the corrosion cell. 

When cathodic protection current is applied to the corrosion cell, the current 
collects at the cathodic site resulting in an increase in the cathodic reaction rate 
beyond the steady state rate previously achieved by the corrosion cell. This upset 
(perturbation) to the steady state condition causes changes in current magnitudes 
(reaction rates) and polarization levels. The charge transfer rate at the cathode 
increases resulting in a greater level of polarization. Since the driving voltage for 
corrosion current decreases, the anode reaction rate (corrosion current) must 
decrease. With time, the corrosion cell reaches a new steady state condition where 
the polarization levels of the cathode and the anode have changed. These changes 
result in a new operation point for the corrosion cell with a reduced corrosion rate 
(magnitude of anodic current, I'corr) compared to the original cell. This new 
operating condition is shown in Figure 2-9. 



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I 



corr "^corr 

log CP Current — 



Figure 2-9: Evans Diagram for Corrosion Cell with CP 

The extent of the reduction in corrosion current relates directly to the 
characteristics of the cathodic polarization curve for the structure and the open 
circuit potential of the most active anode. Since nothing has occurred to change the 
polarization characteristics of the structure, the polarization curves will not change. 
Therefore, the increased cathode polarization is shown by extending the cathodic 
polarization slope of the corrosion cell to the new operating point, and the 
decreased anodic polarization is indicated by sliding the anode operating point 
back down the anodic polarization curve. As previously suggested by Equation 2- 
3, the anodic current (I'corr) is now the total cathodic current (I'c) minus the 
cathodic protection current supplied (Icp)- From Figure 2-9, it is apparent that 
complete cathodic protection has not been achieved because a residual corrosion 
current (I'corr) exists. Supplying additional cathodic protection current causes the 
polarized potential of the cathode to reach the open circuit potential of the anode. 
Evans diagrams show steady state conditions only. They do not show the transient 
changes that occur to reach the new operating point. The polarization curves 
shown on an Evans diagram represent the locus of operating points established at 
steady state conditions. 

The objective of applying cathodic protection to a corrosion cell is to slow 
significantly or stop the oxidation reaction occurring at the anode. This oxidation 
reaction results in the loss of the metal through the familiar generalized reaction 
shown in Equation 2-6, 



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M° ^ M 



+11 



ne 



[2-6] 



or Equation 2-7 in the case of a steel structure. 



Fe° -^ Fe 



+2 



2e- 



[2-7] 



To accomplish this goal, the rate of the cathodic reactions must increase, as 
previously indicated. Although a number of different cathodic reactions are 
possible, the two most common reactions in soils and natural waters involve the 
reduction of oxygen or the evolution of hydrogen. For neutral or alkaline 
conditions, these cathodic reactions are as shown by Equations 2-8 and 2-9. 



Oxygen reduction: 



2H20 + 02 + 4e" -^ 40H" 



or 



[2-8] 

Electrolysis of water: 2H2O + 2e" -^ H2 + 20H" [2-9] 

For acid conditions, the reactions are as provided in Equations 2-10 and 2-11. 

Oxygen reduction: 4H^ + O2 + 4e" -^ 2H2O [2-10] 



or 



Hydrogen ion reduction: 2H^ + 2e ^ H2 



[2-11] 



The specific cathodic reaction dominating will depend on the pH, equilibrium 
potentials of the reactions, concentration of possible reactants, and exchange 
current densities for the possible reactions. Since the equilibrium potential for 
oxygen reduction is always more positive than the equilibrium potential for 
hydrogen evolution, oxygen reduction is always the favored reaction 
thermodynamically. However, the exchange current density (kinetics) must be 
considered. For low pH conditions, the exchange current density for the evolution 
of hydrogen is much higher than the exchange current density for oxygen 
reduction. However, this situation changes as the pH increases. Therefore, the pH 
will determine the dominant reaction due to the kinetics involved. For a low pH 
(acid) aerated environment, the hydrogen evolution reaction dominates. However, 
for a higher pH (neutral or alkaline) aerated enviroimient, oxygen reduction will be 
the dominant cathodic reaction up to its limiting current density.^^'^^'^^ 



^°]ohn O'M .Bockris and Amulya K.N.Reddy, Modern ElectTochemistry vol 2 (New York, NY: Plenum 

Press, 1970), p. 1298-1300. 
®' E. E. Star\shury ar\cl R.A. Buchar\ar\, Fundamentals of Electrochemical Corrosion, (Materials Park, OH: 

ASM International, 2000), p. 116-120 and 172-174. 



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The air in the earth's atmosphere suppHes the oxygen available for the reduction 
process (20%). Since the solubility of oxygen in water is relatively low and the 
dissolved oxygen must diffuse to the structure surface, the limiting current density 
for oxygen reduction can be reached at relatively low current densities. ""^'^^ 
However, with the increasingly negative potentials, the electrolysis of water 
producing hydrogen gas becomes possible. ^^ This reaction is observed, at least in 
the laboratory, as the controlling reaction for a steel structure under cathodic 
protection and may explain the apparent limit to true polarization potential of 
around 1.15 Vcse observed in the field.^'' 



2.3.2 Activation and Concentration Polarization 

As increments of cathodic protection current are applied, the structure polarizes in 
the electronegative direction due to the increasing rate of the cathodic reactions on 
the structure. At a specific applied current density, the change in polarization 
potential and the shape of the polarization curve for the structure depends on the 
slowest step in the reaction process at that point. Two different mechanisms can 
account for these effects: activation polarization and concentration polarization. 

Activation polarization is the result of the reaction steps at the structure/electrolyte 
interface including the actual transfer of charge. These are the reaction steps 
occurring after all of the necessary reactants are in place at the interface and are 
ready to take place. The charge transfer reaction involves moving an electron from 
the metal surface to the reactant on the electrolyte side of the interface. If the 
charge transfer reaction or any reaction step on the metal surface is the slowest 
step in the overall reaction process, the process is under activation polarization 
control. Equation (2-12) describes activation polarization. 



Va 



jB log 



^^ 



V^o J 



[2-12] 



where: 



ria = activation polarization (V) 
P = polarization slope (V) 
i = current density (mA/cm ) 



^'N. G. Thompson and T.J. Barlo, "Fundamental Processes of Cathodically Protecting Steel Pipelines," 
Gas Research Conference Proceedings, presented 1983 (Rockville, MD: G overnment Institutes, Inc.). 

^^T.J. Barlo and R. R. Fessler, "Interpretation of True Pipe-to-Soil Potentials on Coated PipeUnes with 
Holidays," CORROSION/83, paper no. 292, (Houston, TX: NACE, 1983). 

^"^MarsG. Fontana, Corrosion Engineering, Third ed. (New York, NY: McGraw-Hill, 1986), p. 19-21 and 
454-462. 



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io = exchange current density (mA/cm ) 

The activation polarization equation is also known as the Tafel equation. When 
plotted on an Evans (E log I) diagram, activation polarization appears as a straight 
line with the slope of the line equal to the Tafel constant, p. The polarization slope 
of a cathodic reaction is negative while the slope of an anodic reaction is positive. 
The Tafel slope is a function of the specific reaction, the electrolyte chemistry, 
and the reacting surface. The exchange current density, io, is the rate of the 
oxidation and reduction reactions at equilibrium and provides an indirect 
indication of the relative ease with which the reactions can take place. ^"^ 

Concentration polarization is the result of reaction steps involving the diffusion of 
reactants up to or the reaction products away from the reaction surface (structure 
interface). Anything that causes a depletion of available reactants or a buildup of 
reaction products results in a decrease in the reaction rate and an increase in 
concentration polarization. If the slowest step in the overall reaction process 
involves waiting for arrival of reactants or the removal of reaction products, the 
reaction is under concentration polarization control. Equation 2-13 describes 
concentration polarization. 



Vc 



23RT 
nF 



log 



[2-13] 



where: 



Tjc = concentration polarization (V) 

R = universal gas constant (8.3 145 J/mol °K) 

T = temperature (°K) 

n = number or electrons transferred 

F = Faraday's constant (96,500 coulombs/mol) 

11 = limiting current density (A/cm ) 

i = current density (A/cm ) 



A plot of the concentration polarization equation indicates very little change in 
polarization until the current density approaches the limiting current density. Since 
the limiting current density for a specific reaction caimot be exceeded, the 
polarization potential will become very large as this limit is approached. 
Concentration polarization is most commonly associated with cathodic reactions. 



''^MarsG. Fontana, Corrosion Engineering, Third ed. (New York, NY: McGraw-Hill, 1986), p. 19-21 and 
454-462. 



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The ability of the reactants to reach the reacting surface and the reaction products 
to move away from the reacting surface determines the Hmiting current density. 

As seen in Equation 2-14, the limiting current density is a function of the 
concentration of reactants, the diffusion coefficient, and the thickness of the 
diffusion layer. The thickness of the diffusion layer is a function of the system 
geometry, the shape of the reacting surface, and the solution agitation and must be 
determined experimentally for a specific case.*"^ 



DnFC, 



[2-14] 



where: 



11 = limiting current density (A/cm ) 

D = diffusion coefficient (cm /s) 

n = number of electrons transferred 

F = Faraday's constant (96,500 coulombs/mol) 

Cb = reactant concentration in bulk soln. (mol/cm ) 

X = thickness of diffusion layer (cm) 



The total polarization of a structure will be the sum of the activation and 
concentration polarization, as indicated in Equation 2-15. 



"H total 



^^ 



^c 



[2-15] 



Consider what happens with the application of the first increments of cathodic 
protection current assuming a single cathodic reaction. At first, the reaction rate is 
relatively slow, the reactants are plentiful, and the reaction products can move 
away with sufficient speed to avoid blocking the reacting surface. Therefore, the 
cathodic reaction occurring on the structure is most likely under activation control. 
However, with increasing increments of cathodic protection current, the reaction 
rate continues to increase until the availability of reactants at the interface begins 
to decrease, and the reaction products begin to accumulate. At this point, 
concentration polarization is begiiming to control, and the current approaches the 
limiting current density. As the current nears the limiting current density, the 
polarization potential at the interface becomes more negative very fast. Unless 
another cathodic reaction is possible at the more negative potentials, the reaction 
rate reaches the limit and the current density caimot increase further. 



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However, if another cathodic reaction becomes possible, the current density can 
continue to increase with the availability of new reactants. This is the most likely 
scenario for steel under cathodic protection in a neutral, aerated soil envirormient 
where oxygen reduction occurs at first up to the limiting current density for this 
reaction, and then water electrolysis occurs as the potentials become more 
negative. This polarization response is shown by the 20% O2 line in Figure 2-10. 




10"7 10"6 10"5 10"4 10"3 10"2 

Current Density, A/cm^ 

Figure 2-10: Polarization Curves in Aerated and Deaerated Solutions of pH 7 

Source: N. G. Thompson and T. J. Barlo, "Fundamental Processes of Cathodically Protecting Steel Pipelines," Gas 
Research Conference Proceedings, presented 1983 (Rockville, MD: Government Institutes, Inc.). 



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2.3.3 Factors Affecting Polarization 

Many factors can affect the polarization characteristics of a structure. To evaluate 
the polarization response to a specific change, we must evaluate the change based 
on its effect on the equilibrium potential, the exchange current density, the 
limiting current density, and the polarization slope, since these factors control 
the level of polarization (See Table 2-3). If a change involves variations in 
chemical concentrations, we first need to determine whether the species involved 
are reactants or reaction products for the type of reaction of interest (anodic or 
cathodic). 

Table 2-3: Factors Controlling Polarization Response 



Symbol 


Units 


Factor 


Em/m+ / Eoc 


Volts 


Equilibrium Potential 


lo 


mA/cm^ 


Exchange Current Density 


1l 


mA/cm^ 


Limiting Current Density 


P 


Volts/decade 


Tafel Slope 



The arrow in a chemical reaction formula points in the direction the reaction is 
proceeding. Standard form for a reaction is with the reaction arrow pointing from 
left to right. Reactants are the components required for the reaction to occur, and 
reaction products are the components produced by the reaction. Per standard form, 
reactants are all of the components on the left side of the reaction formula. If the 
reaction is a cathodic or reduction reaction, one of the necessary reactants is one or 
more electrons. The reaction products always appear on the right side of the 
reaction arrow, according to standard form. For an oxidation or anodic reaction, 
one of the necessary reaction products is one or more electrons. 

As indicated in Chapter 1, the Nemst equation (Equation 1-10) determines the 
equilibrium potential for a specific reaction. The equilibrium potential varies with 
changes in the specific reaction occurring, the concentration ratios of the reactants 
and reaction products, and the temperature. The equilibrium potential shifts in the 
noble direction as the concentration of oxidized species (reaction product for 
anode/reactant for cathode) increases, as indicated in Table 2-4. It also shifts in the 
noble direction as the concentration of reduced species (reactant for anode/reaction 
product for cathode) decreases as indicated in Table 2-4. 



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Table 2-4: Factors Controlling Equilibrium Potential, Em/m+ 



Factor 


Factor Change 


Direction of Potential 
Change 


Anodic 


Cathodic 


Specific reaction 


— 


— 


— 


Concentration of 
reactants 


increase 


active 


noble 


Concentration of 
reaction products 


increase 


noble 


active 


Temperature 


increase 


variable 


variable 



The exchange current density is a function of the type of metal electrode, the 
specific reaction occurring, the ratio of oxidized and reduced species present 
(concentrations), temperature, surface roughness, and the presence of surface 
films, as indicated in Table 2-5. For a cathodic reaction, the exchange current 
density increases for increasing concentration of reactants, decreasing 
concentration of reaction products, increasing temperature, the absence of surface 
films and greater surface roughness. 

Table 2-5: Factors Controlling Exchange Current Density, io 



Factor 


Factor Change 


Exchange Current 
Density Change 


Anodic 


Cathodic 


Type of metal 


— 


— 


— 


Specific reaction 


— 


— 


— 


Concentration of 
reactants 


increase 


increase 


increase 


Concentration of 
reaction products 


decrease 


increase 


increase 


Temperature 


increase 


increase 


increase 


Surface roughness 


increase 


increase 


increase 


Surface films 


increase 


decrease 


decrease 



As indicated by Equation 2-14, the limiting current density increases with greater 
reactant concentration in the bulk electrolyte. It also increases as the diffusion 
layer thickness, x, decreases. The diffusion layer thickness is at its maximum value 
under stagnant conditions and decreases with relative movement between the 
reaction interface and the electrolyte. Because the diffusion coefficient increases 
and the diffusion layer thickness decreases with increasing temperature, the 



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limiting current density increases with increasing temperature. The effect of these 
variables on the limiting current density is summarized in Table 2-6. 

Table 2-6: Factors Controlling Limiting Current Density, 11 



Factor 


Factor Change 


Limiting Current 
Density Change 


Anodic 


Cathodic 


Concentration of 
reactants 


increase 


increase 


increase 


Concentration of 
reaction products 


increase 


decrease 


decrease 


Diffusion coefficient 


increase 


increase 


increase 


Diffusion layer 
tliicl<ness 


increase 


decrease 


decrease 


Temperature 


increase 


increase 


increase 


Agitation 


increase 


increase 


increase 



As previously indicated and summarized in Table 2-7, the polarization slope (Tafel 
slope, P) depends on the specific reaction occurring and the temperature. The 
polarization slope is not a function of the concentration of the reactants or reaction 
products. The polarization slope is normally greater in the presence of surface 
films.^^^^^ 

Table 2-7: Factors Controlling Polarization Slope, p 



Factor 


Factor Change 


Polarization Slope 
Change 


Anodic 


Cathodic 


Specific reaction 


— 


— 


— 


Reacting surface 


— 


— 


— 


Surface films 


increase 


increase 


increase 


Temperature 


increase 


decrease 


decrease 



With the above thoughts in mind, we will investigate the effects various changes 
might have on the polarization response of a structure under cathodic protection. 
To better understand the results of a specific change, it is necessary to compare the 



^''John M. West, Basic Corrosion and Oxidation, Second ed. (Chichester, West Sussex, England: Ellis 

Norwood Ltd, 1986), p. 92-94. 
^'^R. L. Bianchetti, ed.. Control of Pipeline Corrosion, Second ed. (Houston, TX: NACE, 2001) p. 90, 166- 

173, 308-310, and 315-317. 



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polarization response of the structure before and after the change. Since the 
important polarization response in the case of the application of cathodic 
protection is the cathodic polarization response of the structure, we will 
concentrate on this response. Because the structure surface area is constant in the 
following discussions, we can plot the polarization potential versus the log of the 
total current (E log I), rather than the current density. 

2.3.3.1 Aeration (Oxygen) 

Consider changes to the amount of oxygen present in the electrolyte. Oxygen is a 
cathodic reactant; therefore, increases in the quantity of oxygen present result in a 
noble shift in the equilibrium (open circuit) potential of the cathodic reaction on 
the structure. A larger exchange current density is also probable due to the oxygen 
increase. Because the solubility of oxygen in water is relatively low, as the reaction 
rate increases due to cathodic protection, concentration polarization will dominate. 
Figure 2-11 compares the operating conditions of a cathodically protected structure 
before and after the amount of oxygen in the electrolyte is increased. The primed 
values represent the new values with an increase in oxygen concentration. The 
increase in oxygen results in an increase in the corrosion current and the amount of 
cathodic protection current necessary. In fact, as shown in Figure 2-11, although 
the level of cathodic protection current has increased, the structure is no longer 
polarized sufficiently to meet the potential criterion. 




Icorr ^^Plcorr ^^p 

log I ► 



Figure 2-11: Effect of Increasing Oxygen Concentration 



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2.3.3.2 Agitation (Velocity) 

As the level of agitation or movement at the metal/electrolyte interface increases, 
more reactants are swept to the reaction surface, and the reaction products are 
swept away. As a result, the effective diffusion layer thickness decreases, and the 
limiting current density increases. If the cathodic reaction is under concentration 
polarization control, the corrosion current will increase and the cathodic protection 
current will increase, but the overall protection level will decrease, as indicated in 
Figure 2-12. 

Agitation level is not a direct function of velocity, however. As the velocity is 
gradually increased, laminar flow conditions prevail until the fluid reaches some 
critical velocity. Beyond this critical velocity, the flow changes from laminar to 
turbulent flow. Turbulent flow conditions result in a significant increase in the 
agitation level at the reaction surface. This becomes more apparent if we consider 
some unit volume of the electrolyte flowing past the reaction surface. Figure 2-13. 

Under laminar flow conditions, the unit volume of electrolyte remains in contact 
with the reaction surface from one end of the flow stream to the other allowing 
time for depletion of the reactants within the unit volume. Under turbulent flow 
conditions, however, the unit volume would be swept up to the 
reaction surface momentarily, and then swept away only to be replaced by another 
unit volume. Under these conditions, the reactants near the reaction surface never 
have an opportunity to be depleted at all. Therefore, the agitation effect increases 
with increasing velocity up to the point of transition to turbulent flow conditions. 
At that point, the agitation effect increases significantly; however, beyond this 
point additional velocity has little effect. 



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Potential Criterion 



> \^s,cp 



Es.CD 1 



\ 



-S.cpT \ Turbulent \ 

1 >V ', »V * 



H 1— h 



icorr ^cp icp ^cp 



log I 



Figure 2-12: Effect of Increasing Agitation 



3 



Reacting^ 
Surface , 



Ti C 



Reacting 
Surface 







c 



Laminar 
Flow 



Turbulent 
Flow 



Figure 2-13: Laminar Flow Versus Turbulent Flow 

2.3.3.3 Temperature 

Temperature changes can have a number of different effects on the polarization 
response of a structure. Temperature effect on the equilibrium potential depends 
on the relative concentrations of the reactants/reaction products and the sign of the 
temperature coefficient of the equilibrium potential. Therefore, the change in the 

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equilibrium potential with temperature is variable; however, the exchange current 
density increases with increasing temperature. Because a temperature increase 
adds more energy to the system, the activation energy barrier, which must be 
overcome for the reactions to occur, is effectively lowered increasing the reaction 
rate for activation polarization control. In other words, the polarization slope 
decreases. Although the concentration polarization equation appears to indicate 
increasing polarization at higher temperatures, the temperature effect is more 
complicated. A temperature increase also results in a greater degree of ionization 
and increased mobility of the ions in the electrolyte causing an increase in the 
limiting current density for concentration polarization control. All of these factors 
result in a depolarization of the structure and an increase in the current requirement 
for adequate cathodic protection as indicated in Figure 2-14. 




increasing 
^ _ Temp. 

ts,p N 



■^E 



s,cp 



Potential Criterion 



4- 



-s,cp 



4- 



l 



corr 

log I 



lap Icorr 



Ir 



cp 



Figure 2-14: Effect of Increasing Temperature 

Increasing temperature can have the opposite results in some cases. As temperature 
increases, the solubility of oxygen in water decreases. Therefore, in cases where 
the concentration of oxygen in the bulk electrolyte is the controlling factor, 
increasing temperature can result in a decrease in the cathodic current requirement. 



2.3.3.4 pH 

As discussed in Chapter 1, one necessary condition for formation of a corrosion 
cell is the presence of an electrolyte. An electrolyte is a conductive medium 
(solution) where charge flow occurs by ion movement. Water is as an electrolyte; 



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however, pure water is not very conductive (resistivity of distilled water > 
1,000,000 Q-cm). Since pure water does not contain contaminating ions, water 
must dissociate (ionize) to a degree or no ions would be present for conduction to 
occur. However, since pure water is not a very good conductor, it is apparent that 
water does not ionize significantly. The ionization of water is described as follows: 



H2O <r> H+ + OH" 



[2-16] 



The ionization of water according to Equation 2-16 is a reversible reaction (reacts 
in both directions) that in time reaches a dynamic equilibrium where the rate of 
water dissociation is equal to the rate of ion recombination. Therefore, the law of 
chemical equilibrium. Equation 2-17, applies to this reaction. The law of chemical 
equilibrium states that at a constant temperature the ratio of the molar 
concentration of the reaction products to the molar concentration of the reactants, 
each raised to the power of the number of moles reacting, is a constant. Kg. 
Therefore, for water the equation for the equilibrium constant, Ke, is as follows 
with the number of moles reacting for each of the species equal to one, as indicated 
in Equation 2-17. 



K, 



[h][oh1 
[h,o] 



[2-17] 



where: 



Ke 

[0H-] 
[H2O] 



equilibrium constant 
hydrogen ion concentration (moles/liter) 
hydroxyl ion concentration (moles/liter) 
water molecule concentration (moles/liter) 



However, since the number of water molecules that dissociate is relatively small, 
the concentration of water molecules remains relatively unchanged. Therefore, we 
will define a new constant, Ki, by Equation 2-18. This constant is called the 
ionization constant or ion-product constant. 



K, = KJH2O] = [H"][OH] 



[2-18] 



The value of the ionization constant for water at 25° C is 1 x 10 ^'*. At a 

— 1 9 

temperature of 100° C, the ionization constant increases to 1 x 10 . This means 
that ten times the number of water molecules ionize at the higher temperature. 
Since for every water molecule that dissociates, one hydrogen ion and one 
hydroxyl ion are created, the concentration of hydrogen ions and hydroxyl ions 



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must be equal. From Equation 2-18, this means that at 25° C the concentration of 
hydrogen ions is equal to 1 x 10 . 

pH is defined as the negative logarithm of the hydrogen ion concentration (or more 
correctly, activity), as indicated in Equation 2-19. Therefore, the pH for pure water 
at 25° C is 7. This is neutral pH where the number of hydrogen ions is exactly 
equal to the number of hydroxyl ions. 



pH= -log[H^]= log 



H^ 



[2-19] 



The ionization (ion-product) constant of water at a given temperature always 
remains constant. Therefore, if we add hydrogen ions to water, the number of 
hydroxyl ions in the solution must decrease. The relationship between the 
concentration of hydrogen ions, hydroxyl ions, and pH is shown in Table 2-8. 
Remember, a pH change of one represents a concentration change often. 

Table 2-8: Relationship Between [H"^], [OH~], and pH 



pH 


[H1 


[OH] 







10°(1) 


10-^" 


strongly Acidic 

Neutral 
Strongly Basic 


1 


10-^ 


10" 


2 


10-2 


10-^2 


3 


10-^ 


10-^^ 


4 


10" 


10-^° 


5 


10-^ 


10-^ 


6 


10® 


10 -« 


7 


10-^ 


10-^ 


8 


10-^ 


10-® 


9 


10-^ 


10-= 


10 


10-^° 


10-" 


11 


10-^^ 


10-^ 


12 


10-^2 


10-2 


13 


10-^^ 


10-^ 


14 


10-^" 


10°(1) 



Hydrogen ions are cathodic reactants. Therefore, lowering the pH will increase the 
exchange current density and result in a noble shift in the equilibrium potential for 



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the structure cathodes. Decreasing the pH increases the cathodic protection current 
requirement as shown in Figure 2-15. 




■S,P ^s. 



•»Ei 



s,cp 



Potential Criterion 



• E: 



■s,cp 



+ 



-H- 



Icorr 

log I 



Icorr top 



•^cp 



Figure 2-15: Effect of Decreasing pH 



2.3.3.5 Surface Area 



For a given appHed cathodic protection current, increasing the surface area of the 
structure has the effect of reducing the current density on the structure. For all of 
the changes considered up to this point we have assumed that the surface area is 
constant and plotted the polarization potentials versus the logarithm of the total 
current rather than the current density as required by the Tafel equation (Equation 
2-12). To continue with our E log 1 plot analysis, we must consider how to show a 
change in surface area on this plot. Begin by replacing current density, i, with the 
total current, 1/A, in Equation 2-12 for a cathodic polarization slope. 



where: 



A = surface area 



A 

lo 



[2-20] 



Rearranging, we have 



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Tla 



-Plog 



^ I ^ 



vioAy 



[2-21] 



Taking the logarithm of the numerator and denominator separately and 
multiplying by P, we have 

Tl, = -piog(l)+piog(i„A) 



[2-22] 



Now, since everything in the last term on the right is a constant for a specific 
case. 



where: 



ri, = -piog(l)+k 
k = piog(i„A) 



[2-23] 
[2-24] 



From the above analysis, it is apparent that if the area remains constant, the E log I 
polarization plot will be exactly the same as a plot of Equation 2-12 in terms of 
current density with the curve shifted in the noble direction by the value of the 
constant k. However, if the surface area is increased, the constant k will also 
increase; therefore, the E log I plot must shift upward. Because nothing has 
changed in this analysis except the surface area, the specific polarization slope, P, 
and exchange current density, io, remain unchanged. However, since the exchange 
current density is constant, but the surface area has increased, the total exchange 
current, Iq, must also increase. (Since the plot is in terms of current, not current 
density, we must also plot the total exchange current, Iq, not the exchange current 
density, io.) Therefore, as the surface area changes, the E log I plot must shift 
upward and to the right by a constant amount as seen in Figure 2-16. As seen from 
the figure, an increase in surface area results in a greater current requirement for 
cathodic protection. 



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•v. E: 



Potential Criterion 



s,cp 



-s,cp 



H h 



•^corr 



Ir 



corr 



lop 



•^cp 



log I 



Figure 2-16: Effect of Increasing Surface Area 

Consider the polarization curve plotted versus current density and the effect of 
increasing the surface area. As seen in Figure 2-17, the effect of increasing the 
surface area is to slide the operating point back up the same polarization curve. 
Whether considering Figure 2-16 or Figure 2-17, the results are the same. 
Increasing the surface area results in decreased overall polarization and a higher 
cathodic protection current requirement. 



-s,oc 




-"» ^s,cp 



-t- 



-\ h 



icorr icorr icp icp 

log i ► 



Figure 2-17: Effect of Increasing Surface Area (Current Density) 



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2.3.3.6 EfFectofTime 



Polarization is an interface phenomenon driven by charge flow (current). Since the 
charge flow occurring within the electrolyte is ion movement and ions are massive 
particles, time is required for the mass movement to occur. Polarization requires 
time. Activation polarization requires time for charge separation at the interface 
(capacitance charging). Concentration polarization requires time for concentrations 
to adjust. In fact, a concentration gradient will establish itself from the reaction 
interface into the bulk solution. Therefore, whenever we switch a current on or 
change the level of current, the polarization level at the interface must change over 
time to a new steady state value. 

In addition to the corrosion rate reduction associated with cathodic activation 
polarization of the interface, a number of enviroimiental changes (concentration 
polarization) occur near the reaction surface also reducing the corrosion rate. First, 
the most common cathodic reactions result in a pH increase at the structure surface 
due to hydroxyl ion production. The pH at the metal/electrolyte interface can be 
increased substantially over the pH of the bulk electrolyte because of cathodic 
reactions. With time, the cathodic reactions establish a pH gradient (concentration 
gradient) from the metal surface into the bulk electrolyte. The distance from the 
metal surface into the bulk electrolyte involved in the pH gradient depends on the 
diffusion coefficient, pH, and buffering capacity of the bulk electrolyte and on the 
thickness of the diffusion layer. Kobayashi reported a very steep pH gradient at the 
interface of a steel surface cathodically polarized at 38 [lA/cm (35 mA/ft ) in an 
open water (3% NaCl solution) test. Figure 2-18 indicates the pH gradient in his 
test for five different solutions with various levels of bulk pH. It is interesting to 
note the magnitude of pH attained at the interface and the rate of gradient 
dissipation (within 1 mm of the steel surface) in this open container test. 



''^Toyoji Kobayashi, " Effect of EmironmenM Factors on the Protective Potential of Steel," Proceedings of 
the 5* International Congress on Metallic Corrosion, (Houston, TX: NACE, 1974), p. 627-630. 



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12 



11 



10 



X 



-^^v,^ 


I = 38 |aA/cm2 


\\ 




\ \ 




Nv \ 


's^ 


^ 









7 
6 

5 - 
4 - 
3 - 



c 
o 

o 

CO 
CD 



X 



0.1 1 10 

Distance from Steel Surface (mm) 

*Variation of pH with the distance from the steel surface cathodically 
polarized at 38 microamp/sq.cm in 3% NaCI solution with various 
bulkpH 

Figure 2-18: Variation of pH witli Distance 

Source: Toyoji Kobayashi, "Effect of Environmental Factors on the Protective Potential of Steel," Proceedings of the 
5"* International Congress on Metallic Corrosion, (Houston, TX: NACE, 1974), p. 627-630. 



Investigators have shown that pH values can reach values of 10 and higher even in 
low pH soils. ^^ In fact, some investigators have reported pH values as high as 14.^° 
An increase in pH at a steel interface increases the tendency for the steel surface to 
form passive films. The high pH values and associated passivation can 
significantly affect the anodic polarization behavior of the steel surface. ' 



®N. G. Thompson and T.J. Barlo, "Fundamental Processes of Cathodically Protecting Steel Pipelines," 
Gas Research Conference Proceedings, presented 1983 (Rockville, MD: G overnment Institutes, Inc.). 

^°Komei Kasahara, Taisaku Sato, and Haruhiko Adachi, "Results of Polarization Potential and Current 
Density Surveys on Existing Buried Pipelines," MP, 19, 9 (1980): p. 49-51. 

"i?. 1. Bianchetti, ed.. Control of Pipeline Corrosion, Second ed. (Houston, TX: NACE, 2001), p. 90, 166- 
173, 308-310, and 315-317. 



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In addition to the beneficial pH changes that occur at the structure interface, the 
reduction reactions consume oxygen and water. Since oxygen is a primary oxidizer 
and moisture is a necessary component for electrochemical corrosion, the 
enviroimient becomes less corrosive as these components decrease. The reduction 
in moisture content, especially in the case of a nonsaturated soil enviroimient, can 
significantly affect the structure polarization behavior. Also, due to the electric 
field direction, chloride ions and other potentially harmful anions are forced away 
from the structure surface. 

All of these changes occur over a significant time period until the environment 
near the structure reaches a new steady state condition. Because, unlike aqueous 
solutions, ionic movements in soils are more restricted soil enviroimients tend to 
be stagnant with reaction products maintained near the structure surface. Due to 
the restricted, stagnant enviroimient, the chemistry near the structure surface can 
be significantly different from the bulk electrolyte. As a result of the restricted 
movement, the final steady state chemical gradient can require weeks or even 

79 

months to stabilize. 

Another time-related, environmental change sometimes observed involves the 
deposition of calcareous films on steel surfaces. Although these calcareous 
deposits are commonly found in seawater environments, bicarbonates of calcium 
and magnesium may form deposits on steel surfaces in soil environments when 
cathodic protection increases the pH. Calcareous deposits increase oxygen 
concentration polarization by restricting oxygen access to the steel surface. The 
restriction of oxygen access is primarily due to a decrease in the diffusion 
coefficient, D, (See Equation 2-14) through the calcareous deposits. This reduces 
the limiting current density resulting in a significant reduction in the current 
required to maintain adequate polarization. In fact, designs of cathodic protection 
systems for seawater environments usually rely on the formation of these 
calcareous deposits to reduce the maintenance current requirement significantly. 
The primary constituents of the calcareous deposits found in seawater are calcium 
carbonate and magnesium hydroxide (CaCOa and Mg(0H)2). ' ' ' 



'''Neil G. Thompson, Kurt M. Lawson, and John A. Beavers, "Exploring the Complexity of the Mechanism 

of Cathodic Protection," CORROSION/94, paper no. 580, (Houston, TX: NACE, 1994). 
''^John Morgan, Cathodic Protection, 2"'' Ed. (Houston, TK: NACE, 1987), p. 37, 152-175, 205, ard254- 

258. 
^"R. L. Bianchetti, ed.. Control of Pipeline Corrosion, Second ed. (Houston, TX: NACE, 2001), p. 90, 166- 

173, 308-310, and 315-317. 
^^Francis L. LaQue, Marine Corrosion: Causes and Prevention, (New York, NY: John Wiley & Sons, 1975), 

p. 104-109. 



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Carbon dioxide dissolves in seawater to produce carbonic acid, which dissociates 
into bicarbonate and carbonate ions in a two-step process. Equations 2-25 through 
2-27 show these equihbrium reactions. 



C02 + H20 H2C03 


(carbonic acid) 


[2-25] 


H2CO3 H^ + HCOs" 


(bicarbonate ion) 


[2-26] 


HCO3" H^ + COb"' 


(carbonate ion) 


[2-27] 



This carbon dioxide system acts as a natural buffering system against pH changes 
in seawater. As cathodic reactions add hydroxyl ions to the system, equilibrium 
reactions (2-26) and (2-27) are shifted to the right opposing any pH change. The 
addition of hydroxyl ions results in the precipitation of calcium carbonate and 
magnesium hydroxyl according to Equations 2-28 through 2-30. A number of 
factors can influence the precipitation of calcareous deposits including 
temperature. Increased temperature increases the rate of precipitation. Therefore, 
calcareous deposits more readily form in warmer waters 



76 



OH +HCO3 -^ H2O 



CO3' 



(carbonate ion) 



[2-28] 



CO3 ^ + Ca^^ -^ CaC03 ^^ (calcium carbonate) [2-29] 

Mg^^ + OH~ -^ Mg(0H)2>l< (magnesium hydroxide) [2-30] 

2.4 Types of Cathodic Protection Systems 

We can use either galvanic (sacrificial) anodes or semi-inert anodes plus a power 
supply to provide cathodic protection current to a structure. For galvanic anodes to 
supply current, the electrochemical potential of the galvanic anode must be more 
electronegative than the structure to be protected. In fact, sufficient potential 
difference must exist between the galvanic anode and the structure to overcome the 
circuit resistance and supply adequate current to achieve polarization of the 
structure. Since galvanic anodes provide protective current through the process of 
electrochemical corrosion, practical galvanic anodes must also be low cost and 



^''William H. Hartt, Charles H. Culberson, and Samuel W. Smith, "Calcareous Deposits on Metal Surfaces 
in Seawater- A Critical Review," Corrosion, 40, 11 (1984): p. 609-618. 



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have sufficient current capacity to be cost effective. For the protection of steel, the 
three galvanic anodes commonly used are aluminum, magnesium, and zinc. 

Impressed current systems use semi-inert (semisoluble) anodes to supply 
protective current. Since these anodes are relatively inert, they exhibit relatively 
noble electrochemical potentials. To produce charge flow in the direction to 
cathodically polarize a steel structure, it is necessary to cormect an external power 
supply in series between the semi-inert anode and steel structure. The power 
supply must overcome the galvanic potential difference between the noble anode 
and steel structure before it can supply the first increments of protective current to 
the structure. The potential the power supply must first overcome is sometimes 
called "backvoltage." 

2.4.1 Galvanic Anodes 

2.4.1.1 Aluminum Anodes 

Pure aluminum cannot function as a galvanic anode because the formation of 
stable oxide films causes the electrochemical potential to shift to a very noble 
potential (passivation). However, adding activators to the alloy to disrupt the oxide 
film formation can maintain an active electrochemical potential for the aluminum 
alloy. Therefore, by proper alloying we can use aluminum anodes to supply 
cathodic protection current. The aluminum alloy consists of a combination of zinc 
plus cadmium, indium, mercury, or tin to keep the anode active. The zinc initially 
activates the anode with the cadmium, indium, mercury, or tin maintaining long- 
term activation. With the addition of these activators, the aluminum alloy 
experiences self-corrosion. Some formulations add manganese, silicon, or titanium 
to the alloy to optimize the balance between activation and self-corrosion. Both 
iron and copper can have detrimental effects on the current capacity and the 
driving potential of the anode. Small additions (up to 0.11%) of silicon can 
overcome some of the detrimental effects of iron. Table 2-9 provides information 

77 7R 7Q SO 81 

about two common aluminum anode alloys. ' ■ ■ ' 



"W. von Baechmann and W. Schwenk, Handbook of Cathodic Protection, (Surrey, England: PortcuUis 

Press Ltd., 1975), p. 37-38, 58-59, 153-173, 185-198, and 208-217. 
'"^CP 4-Cathodic Protection Specialist Course Manual, (Houston, TX: NACE, 2002), p. 1:25-1:27, 3:4- 

3:14, 3:18-3:33, and 8:34-8:35. 
™£d Lemieux, William H. Hartt, and Keith E. Lucas, "A Critical Review of Aluminum Anode Activation, 

Dissolution Mechanisms, and Performance," CORROSION/2001, paper no. 1509, (Houston, TX: NACE, 

2001). 
^°R. F. Crundwell, " Sacrificial Anodes ■ 

(Houston,TX: NACE, 1982). 



What is in Them and Why," CORROSION/82, paper no. 166, 



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Table 2-9: Aluminum Anode Alloys 



Element 


Alloy 1 


Alloy III 


Zn 


0.35 - 0.50% 


2.8 - 3.5% 


Si 


0.14 -0.21% max 


0.08 - 0.2% max 


Hg 


0.035 - 0.048% 


— 


In 


— 


0.01 - 0.02% 


Cu 


<0.01% 


<0.01% 


Fe 


<0.12% 


<0.12% 


Al 


remainder 


remainder 


Use 


open seawater 


seawater/mud 


Nominal Potential 


-1 .05 Vssc/sea 


-1.10 Vssc/sea 


Efficiency 


95% 


85% 


Capacity - sea 


2830 A-h/kg 
(1280A-h/lb) 


2530 A-h/kg 
(1150A-h/lb) 


Consumption - sea rate 


3.10kg/A-y 
(6.83 Ib/A-y) 


3.46 kg/A-y 
(7.63 Ib/A-y) 


Capacity - mud 


— 


2180 A-h/kg 
(990 A-h/lb) 


Consumption - mud rate 


— 


4.02 kg/A-y 
(8.87 Ib/A-y) 



Aluminum anodes generally require chloride ions in the electrolyte to function 
properly. As the quantity of chloride ions decreases below normal seawater 
concentrations (3.5% or 35,000 ppm), the current capacity of the anode decreases, 
and the anode potential becomes more noble. According to Schrieber and Murray, 
the type III aluminum alloy (indium activated) can function with a chloride 
concentration of only 1800-2000 ppm or about 5% of seawater concentration. 
However, the corrosion potential of the anode becomes more noble at chloride 
concentrations below about 33% of seawater, and at 12% seawater concentrations 
the potential reaches the minimally acceptable potential of -1.0 Vssc/sea- The 
capacity of the anode remains relatively constant down to the 12% seawater 
strength but deteriorates significantly below this level. Most investigators agree 
that the current capacity of aluminum anodes decreases with decreased current 



^' DNV RP B401 Appendix A, "Recommended Practice for Accelerated Laboratory Testing of Sacrificial 
Anode Materials with the Objective ofQualit]/ Control," DetNorske Veritas Industri Norge AS (1993), 
p.40-44. 



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2:49 



density. Figure 2-19 shows a plot of current capacity versus current density for 
aluminum anodes in seawater and saline mud. 



1200 



— 1100 



< 

o 

CO 
Q. 

CD 

o 

-I— < 

c 

CD 

t^ 
i_ 

O 

CD 
T3 
O 

C 
< 



1000 



900 



800 



700 



600 - 



Substitute seawater, 
37 weeks, 25-28°C 




Substitute seawater, 2°C 1 08 V 

-<^ ^ 1.100V 

'1.110V 

"I.IIOV 4^^>, 

1.09 V 



Saline mud, 
lab, 4 mos. 



Saline mud, 2 yr., 
' 1 .080 V free-running 



1.120V 



Conditions: 

1 . Potentials: negative volts to 
Ag-AgCI 

2. At low density, each point 
is average of 3 specimens 



X 



J- 



_L 



10 20 30 50 70 100 200 300 600 800 

Anode Current Density, mA/ft^ 

Figure 2-19: Current Capacity Versus Current Density for Aluminum 

Source: C. F. Schrieber and Reece W. Murray, "Effect of Hostile Marine Environments on the Al-Zn-In-Si Sacrificial 
Anode," CORROSION/88, paper no. 32, (Houston, TX: NACE, 1988). 



The potentials and current capacities in Table 2-9 are based on ambient 
temperatures of approximately 25°C. As the temperature increases, the current 
capacity increases up to temperatures of approximately 70°C. The current capacity 
begins to deteriorate rapidly beyond 70°C. Meanwhile, the corrosion potential of 
aluminum anodes become less negative as the temperature increases from 25°C to 
100°C.^' 

Due to the requirement for chloride ions to prevent passivation, aluminum anodes 
are used primarily in seawater applications. Because of their relatively high current 
capacity and light weight, aluminum anodes have virtually replaced zinc in 
seawater applications. Common applications for aluminum anodes include bracelet 
anodes for offshore pipelines, standoff anodes for platforms, eyebolt and threaded 



^'C.F. Schrieher and Reece W. Murray, " Effect of Hostile Marine Environments on the Al-Zn-In-Si 
Sacrificial Anode," CORROSION/88, paper no. 32, (Houston, TX: NACE, 1988). 



NACE 



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2:50 



Stud anodes for pier, piling, and heater-treater applications, and flush-mount 
anodes for hulls and special applications. 

2.4.1.2 Magnesium Anodes 

Pure magnesium exhibits a very electronegative potential, but the self-corrosion 
rate of unalloyed magnesium is excessive. The addition of about 6% aluminum and 
3% zinc results in a more electropositive corrosion potential but the self-corrosion 
rate is significantly improved. Zinc promotes a more uniform corrosion and 
reduces the sensitivity of the anode to other impurities. However, small 
concentrations of copper, nickel, silicon, and iron can cause significant reductions 
in both the current capacity and the electronegative potential of magnesium 
anodes. Nickel concentrations must be kept below 0.001% due to the rapid loss in 
capacity above this concentration. Copper concentrations should be held below 
about 0.05% for the same reason. The addition of about 0.3 % manganese can 
reduce the negative effects of iron by sequestering the iron within the alloy. With 
the sequestering benefit of manganese, iron concentrations up to about 0.01% can 
be tolerated.^-''^^'^^ 

The two most common alloys are the high-potential magnesium alloy (Ml) and the 
AZ-63 (HI) alloy as seen in Table 2-10. The high-potential alloy takes advantage 
of the very electronegative corrosion potential of pure magnesium by adding only 
a small percentage (about 1%) of manganese to improve the efficiency. The AZ-63 
(HI) alloy employs aluminum and zinc as primary alloying agents to improve the 
efficiency. Even after taking advantage of alloying improvements, the efficiency of 
magnesium is nominally around 50% under the best of enviroimiental conditions. 
However, efficiencies have been reported of about 10 to 60% using standard test 

1 83,84,86,87,88 

procedures. 



^^W. von Baechmann and W. Schwenk, Handbook of Cathodic Protection, (Surrey, England: PortcuUis 

Press Ltd., 1975), p. 37-38, 58-59, 153-173, 185-198, and 208-217. 
^'^R. F. Crundwell, "Sacrificial Anodes - What is in Them and Why," CORROSION/82, paper no. 166, 

(Houston,TX:NACE,1982). 
^^R. A. Gummow, "Performance Efficiency of High Potential Magnesium Anodes for Cathodically 

Protection Iron Watermains," Proceedings of Northern Area Eastern Conference, CD-ROM (Houston, 

TX: NACE, September 15-1 7, 2003). 
^''W. von Baechmann and W. Schwenk, Handbook of Cathodic Protection, (Surrey, England: Portcullis 

Press Ltd., 1975), p. 37-38, 58-59, 153-173, 185-198, and 208-217. 
^^CP 4-Cathodic Protection Specialist Course Manual, (Houston, TX: NACE, 2002), p. 1:25-1:27, 3:4- 

3:14, 3:18-3:33, and 8:34-8:35. 
^^ASTM G 97-97 (Reapproved 2002), " Standard Test Method for Laboratory Evaluation of Magnesium 

Sacrificial Anode Test Specimens for Underground Applications," (West Conshohocken, PA: ASTM, 

1997), p. 1-4. 



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2:51 



Table 2-10: Magnesium Anode Alloys 



Element 


High-potential 


AZ-63orHlAlloy 


Grade A 


Grade B 


Grade C 


Al 


0.010% max 


5.3 - 6.7% 


5.3-6.7% 


5.0 - 7.0% 


Mn 


0.50 - 1 .30% 


0.15% min 


0.15% min 


0.15% min 


Zn 


— 


2.5 - 3.5% 


2.5 - 3.5% 


2.0 - 4.0% 


Si 


0.05% max 


0.10% max 


0.30% max 


0.30% max 


Cu 


0.02% max 


0.02% max 


0.05% max 


0.10% max 


Ni 


0.001% max 


0.002% max 


0.003% max 


0.003% max 


Fe 


0.03% max 


0.003% max 


0.003% max 


0.003% max 


Otiier (total) 


0.30% max 


0.30% max 


0.30% max 


0.30% max 


iVIg 


remainder 


remainder 


remainder 


remainder 


Use 


soil/fresh water | 


Nominal potential 


-1 .75 VcsE 


-1 .55 VcsE 


Efficiency 


50% 


Capacity 


IIOOA-h/kg (500A-h/lb) 


Consumption Rate 


7.97kg/A-y (17.5lb/A-y) 



Not only does the alloy composition affect the efficiency of magnesium but also 
the composition of the surrounding environment. The efficiency (current capacity) 
rapidly decreases as the pH of the environment decreases to the point that below a 
pH of about 5 using magnesium as an anode is no longer practical. In addition, 
increasing concentrations of chloride ions can have a detrimental affect on 
magnesium anode efficiency. On the other hand, the presence of the sulfate ion can 
increase the efficiency. Finally, as with all galvanic anodes, the efficiency of the 
anode is a function of the current density. Increasing the current density causes an 
increase in efficiency, as seen in Figure 2-20 for the AZ-63 alloy. ^^'^°"^^ 



^^W. von Baechmann and W. Schwenk, Handbook of Cathodic Protection (Surrey, England: Portcullis 

Press Ltd., 1975), p. 37-38, 58-59, 153-1 73, 185-198, and 208-21 7. 
*R. F. Crundwell, "Sacrificial Anodes - What is in Them and Why," CORROSION/82, paper no. 166, 

(Houston,TX:NACE,1982). 
''K. a. Gummow, "Performance Efficiency of High Potential Magnesium Anodes for Cathodically 

Protection Iron Watermains," Proceedings of Northern Area Eastern Conference, CD-ROM (Houston, 



TX: NACE, September 15-1 7, 2003). 



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Cathodic Protection Systems 



2:52 



600 



500 



400 



o 

CO 

a. 
m 
O 

o 



300 



o 
o 

-*— < 
o 

LU 



200 



100 

































































/■ 


^- 


-— 

























/ 


























/ 




Mc 


)-6°/ 


, Al- 


3%Z 


n Alloy 














/ 


























/ 


/ 


























/ 




























/ 


























/ 




























/ 




























/ 





























1320 

1210 

1100 

990 

880 

770 

660 

550 

440 

330 

220 

110 






o 

OJ 
Q. 
CO 

O 

"to 
o 



o 
o 

o 

LU 



10 20 30 40 50 

Anode Current Density (approx. mA/sq.ft. or iiA/cm 



60 
2, 



80 



Figure 2-20: Current Capacity of AZ63 Magnesium Alloy vs. Current Density 

Source: R. A. Gummow, "Performance Efficiency of High Potential Magnesium Anodes for Cathodically 

Protection Iron Watermains," Proceedings of Northern Area Eastern Conference, CD-ROM 

(Houston, TX: NACE, Sept. 15-17, 2003). 

Due to the very electronegative corrosion potential of magnesium, it is normally 
the galvanic anode of choice for higher resistivity enviroimients such as soils and 
fresh water. For higher resistivity soil applications, we normally use long, 
cylindrical magnesium anodes because the increased length lowers the resistance- 
to-earth of the anode. In fact, using very long extruded magnesium ribbons or rods 
we can obtain minimum resistance. However, magnesium anodes are available in 
spheres or blocks for use in vessels. Magnesium anodes are also available in flush- 
mount designs. 

2.4.1.3 Zinc Anodes 

Zinc is one of the oldest galvanic anode materials, first used around 1824 by Sir 
Humphrey Davy. Early failures of zinc anodes were due to passivation of the zinc 
because of trace iron impurity. Researchers discovered that to prevent passivation 
in high-purity zinc anodes, they had to hold the iron content below 0.0014%. 
Alloys were subsequently developed containing about 0.5% aluminum 



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and up to 0.15% cadmium. This addition allowed zinc anodes to tolerate iron 
impurities up to 0.005%. The most common formulations of the two primary zinc 
alloys (high-purity and Al/Cd alloy) are listed in Table 2-11. Recent revisions to 
ASTM B-418 lowered the allowable cadmium content and raised the allowable 

1 J * * 92.93,94,95,96,97 

lead content. 

Table 2-11: Zinc Anode Alloys 



Element 


High 
Current 


Mil Spec 


ASTM B-418-01 


High-Purity 


A-18001K 


Type 1 


Type II 


Al 


0.1 -0.4% 


0.10-0.50% 


0.10-0.50% 


0.005% max 


— 


Cd 


0.025 - 0.06% 


0.025 - 0.07% 


0.025-0.07% 


0.003% max 


0.003% max 


Fe 


0.005% max 


0.005% max 


0.005% max 


0.001 4%max 


0.0014% max 


Pb 


0.006% max 


0.006% max 


0.006% max 


0.003% max 


0.003% max 


Cu 


— 


0.005% max 


0.005% max 


0.002% max 


— 


Si 


— 


0.125% max 


— 


— 


— 


Zn 


remainder 


remainder 


remainder 


remainder 


remainder 


Use 


seawater & bracl<isli water (T<50°C) [120°F] 


soil & fresh water 


Nominal potential 


-1.10 VcSE 


Efficiency 


90% 


Capacity 


738A-li/l<g (335A-li/lb) 


Consumption Rate 


1 1 .9 l<g/A-y (26.2 Ib/A-y) 



The addition of aluminum and cadmium to zinc resulted in an alloy with a smaller 
grain structure improving the uniformity of the corrosion pattern on the zinc anode. 
However, with the addition of the aluminum the zinc alloy was susceptible to 
intergranular corrosion with increasing temperature. The addition of cadmium 



"^'W. von Baechmann and W. Schwenk, Handbook of Cathodic Protection (Surrey, England: Portcullis 

Press Ltd., 1975), p. 37-38, 58-59, 153-1 73, 185-198, and 208-21 7. 
''^CP 4-Cathodic Protection Specialist Course Manual (Houston, TX: NACE, 2002), p. 1:25-1:27, 3:4-3:14, 

3:18-3:33, and 8:34-8:35. 
'^i?. F. Crundwell, " Sacrificial Anodes - What is in Them and Why," CORROSION/82, paper no. 166, 

(Houston,TX: NACE, 1982). 
^^George W. Kurr, " Zinc Anodes - Underground Uses for Cathodic Protection and Grounding," MP, 18, 4 

(1979): p. 34-41. 
'^''ASTM Standard B 418-01, "Standard Specification for Cast and Wrought Galvanic Zinc Anodes," (West 

Conshokocken, PA: ASTM, 2002). 
^^MIL-A-18001K, "Military Specification Anodes, Sacrificial Zinc Alloy," Department of Defense, 

December 16, 1991, p. 4. 



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further aggravated the susceptibiHty to intergranular corrosion and resuhed in a 
significantly decreased the anode efficiency. Use of the Al/Cd alloy is generally 
restricted to environmental temperatures below about 50°C (i20°F).^^"^^'^*'°''°^ 

Although zinc has a low current capacity compared with the other galvanic anodes, 
the capacity is relatively independent of current density. Even at a current density 
as low as 50 mA/m (5 mA/ft ) an efficiency of 90% or greater is anticipated. 
There is a slight decrease in current capacity at higher temperatures. " ' 

The enviroimient can adversely affect the corrosion potential of zinc. A noble shift 
in the corrosion potential of zinc occurs with increasing temperature. Also, in 
envirormients where carbonates, bicarbonates, or nitrates dominate, the potential of 
zinc can become very noble due to the presence of passivating surface films. 
Although passivation and the accompanying noble shift in potential can occur at 
room temperature in these envirormients, higher temperatures accelerate the 
passivation. Passivation of zinc does not occur when sulfates or chlorides 
predominate in the envirormient regardless of the temperature. In fact, it has been 
shown that zinc with passive potentials as noble as -0.50 Vcse established at room 
temperature in a bicarbonate-rich envirormient can be rapidly restored to the 
nominal corrosion potential (-1.1 Vcse) by adding sulfates in the form of 

98,100 

gypsum. 

We use high-purity zinc anodes in soil and fresh water envirormients in the form of 
ribbons and rods primarily. Grounding cells or structure grounding applications 
also commonly use high-purity zinc anodes. Applications requiring AC mitigation 
or safety grounding mats at test stations may use zinc ribbons. Zinc grounding 
cells employ two or four zinc rods spaced very close together, but insulated from 
physical contact, and placed in a special chemical backfill. The Al/Cd zinc alloy is 



"^^W. von Baechmann and W. Schwenk, Handbook of Cathodic Protection, (Surrey, England: PortculUs 

Press Ltd., 1975), p. 37-38, 58-59, 153-1 73, 185-198, and 208-21 7. 
*K. F. Crundwell, "Sacrificial Anodes - What is in Them and Why," CORROSION/82, paper no. 166, 

(Houston,TX:NACE,1982). 
^°°George W. Kurr, " Zinc Anodes - Underground Uses for Cathodic Protection and Grounding," MP, vol. 

18,no.4 (1979): p. 34-41. 
^°^ASTM Standard B 418-01, "Standard Specification for Cast and Wrought Galvanic Zinc Anodes," (West 

Conshokocken, PA: ASTM, 2002). 
^°'C. F. Schrieber and Reece W. Murray, "Effect of Hostile Marine Environments on the Al-Zn-In-Si 

Sacrificial Anode," CORROSION/88, paper no. 32, (Houston, TX: NACE, 1988). 
' "•' DNVRP B401 Appendix A, "Recommended PmcticeforAccdemted Labomtory Testing of Sacrificial 

Anode Materials with the Objective of Quality! Control" DetNDrske Veritas Indiistri M)rgeAS (1993), 

p.40-44. 



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used in seawater or chloride environments normally in the form of bracelets or 
flush-mounted shapes. 

2.4.1.4 Polarization Diagram 

When discussing polarization diagram for a cathodic protection system, we must 
take care not to confuse the anodes and cathodes of the corrosion cells on the 
structure and the cathodic protection anode with the macroscopic anodes and 
cathodes of the cathodic protection system. Therefore, there are three sets of 
corrosion cells each with their respective anodes and cathodes to be considered. 
The polarization diagram for a galvanic cathodic protection system in Figure 2-21 
illustrates this point. 



-sc.oc 



-s.corr 



-cpa.corr 




-cpa.oc 



^Es, 



cpa.p 



cp 



a,cp 



H — h 



icpa.corr ^s.corr 



•^cp 



log I 



Figure 2-21: Polarization Diagram for Galvanic Anode System 



In Figure 2-21, each of the three separate corrosion cells has an associated 
operating point described by a corrosion current and a corrosion potential. Often 
when our primary concern is the cathodic protection cell, we ignore the corrosion 
cells existing on the structure and the cathodic protection anode and show only the 
corrosion cell representing the cathodic protection system. Figure 2-22 illustrates 
the cathodic protection system. 



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-s.corr 



Ea.corr 



-s,p--A 

TEc( 
-a D - -' 



lis 



■cp 



lEs.corr 



\ 



\ 



\ 



\ 



^Es,p 
TlR = E 



cp 



-a,p 



-a.corr 



•^cp 



log I 



Figure 2-22: Polarization of Structure and CP Anode 



From Figure 2-22, we see that the difference between the corrosion potential for 
the structure, Eg con, and the polarized potential. Eg p, for the structure is equal to the 
polarization of the structure, tis- The difference between the corrosion potential for 
the cathodic protection anode, Eacom and the polarized potential of the cathodic 
protection anode, Ea,p, is equal to the polarization of the cathodic protection anode, 
r|A. The difference between the polarized potential of the structure, Es p, and the 
polarized potential of the cathodic protection anode, Eap, is the driving potential 
for cathodic protection current, Ecp. Since the IR drop within the cathodic 
protection circuit exactly balances the driving potential for cathodic protection 
current, the difference between the polarized potential of the structure and the 
polarized potential of the cathodic protection anode is also equal to the total IR 
drop in the cathodic protection circuit. 

2.4.1.5 Backfill 

A special chemical backfill is often used to surround galvanic anodes placed in a 
soil enviroimient. To take advantage of the chemical energy stored in a galvanic 
anode, the electrochemical reaction producing cathodic protection current must 
occur on the surface of the galvanic anode. This reaction, oxidation, results in the 
conversion of the galvanic metal to metallic ions according to the generalized 
corrosion reaction shown in Equation 2-6. Therefore, one chemical backfill 



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characteristic required for galvanic anodes is that it must be ionically, not 
electronically, conductive. The primary purposes of the chemical backfill are to 
provide 1) a homogeneous, favorable enviroimient to minimize self-corrosion of 
the anode, 2) a low resistivity fill to minimize earth contact resistance for the 
anode, 3) a chemical enviroimient that will minimize anode polarization, and 4) a 
backfill that will absorb and retain the moisture necessary for ionic conduction. 
Table 2-12 provides the most common chemical backfills for magnesium and zinc. 

Table 2-12: Galvanic Anode BacMills 



MAGNESIUM ANODE BACKFILL 


75% 


Ground hydrated gypsum 


CaS04 


20% 


Powdered bentonite 


clay 


5% 


Anhydrous sodium sulfate 


Na2S04 


ZINC ANODE BACKFILL 


50% 


Ground iiydrated gypsum 


CaS04 


50% 


Powdered bentonite 


clay 



The primary purpose of the gypsum (calcium sulfate) is to supply an abundance of 
sulfate ions. When the galvanic anode corrodes to produce cathodic protection 
current, the metal ions (cations) release into the electrolyte adjacent to the anode 
surface. However, these cations will further react with anions available in the 
electrolyte to form stable compounds. If the primary anions available are 
carbonates, bicarbonates, or phosphates, relatively insoluble compounds form, 
which deposit on the anode surface causing polarization of the anode. However, if 
sulfate ions are available in abundance, relatively soluble magnesium sulfate and 
zinc sulfate compounds form. Because these compounds are relatively soluble, 
they can harmlessly migrate away from the anode surface in the electrolyte. 

The bentonite is present in the backfill to retain moisture at the anode surface and 
to provide a source of conductive ions. Bentonite, sodium montmorillonite, is a 
form of clay, which readily absorbs and retains moisture. Sodium and other cations 
adsorb and loosely bind to the surface of the clay particles; therefore, when 
moisture is present, the cations can readily move providing the means for ionic 
conduction. 

Finally, the sodium sulfate will readily ionize providing a source of conductive 
ions. Sodium sulfate lowers the resistivity of the backfill and, therefore, lowers the 
earth contact resistance of the anode. Sodium sulfate is not generally added to the 



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zinc anode backfill because zinc anodes are primarily used in soils with low 
resistivity. 

2.4.1.6 Typical Uses 

Since galvanic anodes have relatively small, fixed driving potentials and metal 
consumption is a necessary result of CP current discharge, a designer typically 
considers galvanic anodes as the first choice in designs where the enviroimient 
resistivity is low and/or the current output is small. The higher the enviroimient 
resistivity for a particular situation, the larger (longer) the galvanic anode must be 
to obtain useable current outputs. Also, the larger the current requirement for a 
specific structure, the greater the galvanic anode weight must be. 

Typical situations where galvanic anodes may be the best choice include: 



Small isolated, coated structures 

Small, isolated fittings such as valves, risers, or couplings 

Structures where electrical continuity presents a problem 

Internal surfaces of small vessels 

Structures in seawater enviroimients 

Structures where geometry presents a current distribution problem 

Enviroimients where explosion hazards limit the use of power sources 

Mitigation of AC and DC interference 



2A.2 Impressed Current Anodes 

Since impressed current anodes are relatively inert, the anode material itself 
corrodes but at a very low rate. The metallic oxidation reaction shown in Equation 
2-6 represents corrosion of the anode. If we surround the impressed current anode 
with electronically conductive carbon backfill, the outer surface of the carbon 
backfill serves as the primary reaction surface. In this case, the anode functions 
simply as an electrical contact to the carbon backfill. The primary reactions 
occurring at the periphery of the carbon backfill are oxidation of the carbon or 
oxidation of species within the electrolyte. Other than metallic oxidation. 
Equations 2-3 1 to 2-34 are the most common oxidation reactions. 



Electrolysis of water: 2H2O -^ 4H^ + 02t + 4e" 

Oxidation of chloride ion: 2Cr -^ Cl2t + 2e" 



[2-31] 
[2-32] 



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Oxidation of carbon: C + 2H2O -^ 4H^ + C02t + 4e" [2-33] 

or 

C + H2O -^ 2H+ + CO t + 2e" [2-34] 

Notice from the above common impressed current oxidation reactions that in all 
cases gases are produced at the reaction surface. The only exception where gases 
are not produced involves the metallic consumption reaction. Also, the pH at the 
reaction surface decreases as hydrogen ions are produced. This is obvious from all 
of the anodic reactions except the reactions involving metallic consumption of the 
anode and oxidation of chloride ions. However, the pH also decreases in these two 
cases due to secondary reactions. In the case of metallic consumption of the anode, 
the metal cation reacts with water to produce a metallic hydroxide and hydrogen 
ions as seen in reaction Equation 2-35. In the case of the chloride ion oxidation, the 
chlorine gas reacts with water to produce hydrochloric and hypochlorous acids as 
seen in reaction Equation 2-36. 



M" + nH20 -^ M(OH)„ + nH 
Cl2t + H20 -^ HCl + HOCl 



[2-35] 
[2-36] 



Therefore, impressed current anode components and associated wiring must be 
resistant to oxidizing gases and acids for successful long-term operation. 

Impressed current anodes can be classified in two major groups based on corrosion 
behavior: massive anodes and dimensionally stable anodes (DSA). Massive anodes 
are large anodes such as graphite and high-silicon iron anodes that become shorter 
and smaller in diameter as they corrode. Dimensionally stable anodes such as 
mixed metal oxide, platinum, and polymer anodes do not change physical 
dimensions as they deteriorate. In fact, loss of the anode material is usually not 
obvious by visual inspection of DSAs. 

2.4.2.1 Massive (Large) Anodes 

The two primary types of massive (large) anodes are soluble and semisoluble. One 
of the first materials used as an impressed current anode, iron, is a soluble anode. 
This material consumes at a rate of approximately 9.1 kg/A-y (20 Ib/A-y). 
Therefore, a relatively large quantity of material is necessary for a reasonable 
output capacity. Aluminum is another soluble anode material sometimes used as an 
impressed current anode in potable water tanks. 



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July 2008 



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2:60 



Although soluble type anodes are not as commonly used today as in the past due to 
the need for large masses of material, one significant advantage to this type of 
anode can make it useful. Soluble anodes produce current through metal 
consumption reactions, not gas evolution reactions as with semisoluble anodes. 
This major difference can be useful in applications involving confined or restricted 
spacing. If the anode is very near the cathode, products of the anodic reaction, 
oxygen and hydrogen ions, can interfere with the polarization of the cathode 
(cathodic depolarizers). Using a soluble anode would minimize the production of 
these depolarizers. 

Although several other massive type impressed current anodes are used, by far the 
two most common semi-soluble anodes in this classification are graphite and high- 
silicon cast iron (HISI). Both anodes have a long history of use in the cathodic 
protection industry with graphite anodes first used in the 1940s and HISI anodes 
first introduced in 1954. Both anodes are generally available in long cylindrical 
shapes; HISI anodes are also commonly available in hollow, tubular forms. 
Diameters from 5-10 cm (2-4 in.) and lengths from 152 - 213 cm (60 - 84 in.) 
are the most common sizes for soil applications. Other sizes are available for 

• 1 r *• 104,105 

special applications. 

Graphite anodes perform well in environments where either oxygen or chlorine 
evolution occurs. However, the graphite consumption rate is higher when oxygen 
is evolved (fresh water and soil) due to the chemical reaction with oxygen 
producing carbon dioxide. In the case of chlorine evolution (sea or brackish water), 
hypochlorous acid is produced, which chemically reacts with graphite to produce 
carbon dioxide and hydrochloric acid. However, in this case the quantity of 
hypochlorous acid is generally small and easily moved away from the graphite 
surface without reacting. ^°^'^*^^ 

When high-silicon iron anodes are alloyed with a minimum of 14.5% silicon, the 
anode forms a protective oxide film on its surface when anodically polarized. This 
oxide film lowers the consumption rate of the anode. The protective film consists 
of hydrated, silicon dioxide. Although silicon dioxide is normally very high in 



^°^R. L. Bianchetti, ed., Control of Pipeline Corrosion, Second ed. (Houston, TX: NACE, 2001), p. 90, 166- 

173, 308-310, and 315-317. 
^°^NACE Publication 10A196, "Impressed Current Anodes for Underground Cathodic Protection Systems," 

(Houston, TX: NACE International, May 1996). 
^°^W. von Baechmann and W. Schwenk, Handbook of Cathodic Protection, (Surrey, England: Portcullis 

Press Ltd., 1975), p. 37-38, 58-59, 153-173, 185-198, and 208-217. 
^°^David H. Kroon and Charles F. Schrieber, "Performance of Impressed Current Anodes for Cathodic 

Protection Underground," CORROSION/84, paper no. 44, (Houston, TX: NACE, 1984). 



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2:61 



resistivity, when anodically formed under wet conditions the film becomes 
conductive. However, if the film is formed with insufficient moisture present, the 
film becomes high in resistivity and contact resistance of the anode-to-earth is 

high.l08,109ai0 

In 1959, chromium was added to the high-silicon iron alloy. The addition of 3 to 
5% chromium reduced pitting attack of high-silicon iron anodes in chloride 
envirormients, thus reducing the overall corrosion rate.^*"" 

Table 2-13 provides a summary of helpful application information for massive 
type impressed current anodes. Current density and consumption rate information 
for these anodes varies considerably from source to source, due in part to lack of 
consideration of variations in different envirormients. In the case of HISI anodes, 
some of the variation is due to differences in alloy composition. The information 
provided in Table 2-13 represents an integration of the recommendations from a 
number of sources with extreme values eliminated. The current densities provided 
represent nominal recommended values, not maximum values. ^°^'^*^^'^°^'^^^'^^^'^^"^ 



wsqp 4_cathoclic Protection Specialist Course Manual, (Houston, TX: NACE, 2002) p. 1:25-1:27, 3:4- 

3:14, 3:18-3:33, and 8:34-8:35. 
^'^NACE Publication 10A196, "Impressed Current Anodes for Underground Cathodic Protection Systems,' 

(Houston, TX: NACE International, May 1996). 
''%id.l04. 
"^W. von Baechmann and W. Schwenk, Handbook of Cathodic Protection, (Surrey, England: Portcullis 

Press Ltd., 1975), p. 37-38, 58-59, 153-1 73, 185-198, and 208-21 7. 
^^-John Morgan, Cathodic Protection, 2"'' Ed. (Houston, TK: NACE, 1987), p. 37, 152-1 75, 205, and 254-258. 
"^R. L. Bianchetti, ed., Control of Pipeline Corrosion, Second ed. (Houston, TX: NACE, 2001), p. 90, 166- 

173, 308-310, and 315-317. 



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July 2008 



Cathodic Protection Systems 



2:62 



Table 2-13: Massive Type Impressed Current Anodes 





Graphite 


HIS! 


Nominal Current Density: 






Soil/fresh water, A/m^ 


2- 10 


2- 5 


(A/ft^) 


(0.2-1) 


(0.2-0.5) 


Carbon baclcfill, A/m^ 


5- 10 


5- 10 


(A/ft^) 


(0.5-1) 


(0.5-1) 


Seawater A/m^ 


5- 10 


10- 50 


(A/ft^) 


(0.5-1) 


(1-5) 


Consumption Rate: 






Soil/fresh water, Icg/A-y 


0.5 - 0.9 


0.1 - 0.5 


(Ib/A-y) 


(1-2) 


(0.2 - 1 .2) 


Carbon baclcfill, l<g/A-y 


0.1 - 0.2 


0.05 - 0.3 


(Ib/A-y) 


(0.2-0.5) 


(0.1-0.7) 


Seawater, Icg/A-y 


0.1 - 0.3 


0.3 - 0.5 


(Ib/A-y) 


(0.2-0.7) 


(0.7-1) 


Comments / Limitations: 


Avoid: 

Low pH 
High sulfate 
Temp. > 50° C 
Consider: 

End effect 

Treatment 

Brittle 


Avoid: 

Dry soils 
High pH 
High sulfate 
Consider: 

End effect 

Brittle 

Chrome alloy - halides 



One major consideration in the application of impressed current anodes, especially 
massive type anodes, is end effect. End effect manifests itself as accelerated 
corrosion at the ends of long, cylindrical anodes often described as "penciling" of 
the anode. This accelerated corrosion is due to the increased current density 
discharged from the ends of long, cylindrically shaped anodes. The most 
significant result of end effect is the premature loss of electrical cormection to the 
anode since the electrical cormection is most often within about 15 cm (6 in.) of 
the end of the anode. One cost-effective solution is to place the electrical 
cormection at the center of the anode. This step is recommended to increase the 
operational life of deep and horizontally buried anodes. Center connection is 
urmecessary if the top of the anode is located near the surface of the earth since the 
current density from the top of the anode is reduced due to the nonconducting 
plane (air) near the anode end.^^"*'^^^'^^^ 



""•BP 4-Cathoclic Protection Specialist Course Manual, (Houston, TX: NACE, 2002) p. 1:25-1:27, 3:4- 
3:14, 3:18-3:33, and 8:34-8:35. 



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2:63 



2.4.2.2 Dimensionally Stable Anodes 

Platinum, mixed metal oxide, and polymer anodes represent some common DSA 
options. Platinum-coated anodes were first introduced to the cathodic protection 
field in the early 1960s for offshore applications. Mixed metal oxide anodes, on the 
other hand, arrived on the cathodic protection scene in Europe in the early 1980s. 
Both of these have active anode surfaces covering an inactive substrate metal such 
as titanium or niobium. These substrate metals, known as "valve metals," form thin, 
adherent, protective, self-healing, high-resistance oxide films when anodically 
polarized. These high-resistance surface films will not permit the passage of anodic 
current until a voltage sufficiently high to destroy the film is applied directly across 
the oxide interface. The magnitude of the breakdown voltage depends on the 
enviroimient. In fresh waters, where chloride concentrations are low, the breakdown 
voltage of titanium is greater than 60 volts. However, in high-chloride enviroimients 
the breakdown voltage is in the range of 8 to 10 volts. It is reduced even further in 
the presence of bromides or iodides, at higher temperatures, and with impurities 
present in the titanium, especially iron. The breakdown voltage for niobium, 
however, exceeds 100 volts even in the presence of high chloride concentrations. 
When the protective film is destroyed, the substrate metal corrodes and the anode 
surface is undercut.^^^'^^^'^^^ 

2.4.2.2(a) Platinum Anodes 

Platinum anodes are available with a very thin layer of platinum electroplated or 
clad onto the substrate metal. Common platinum thicknesses range from about 1.2 
to 7.5 microns (50 to 300 micro-inches). Platinum anodes are available in many 
different sizes and shapes, but wires, rods, and strips are the most common. The 
consumption rate for platinum in seawater, which is relatively constant at current 
densities from 540 to 5400 aW (50 to 500 A/ft^), is in the range of 2.4 to 12 
mg/A-y with 8 mg/A-y most often quoted. In fresh water, the consumption rate is 
approximately two to five times higher, and in brackish water is even higher. At 



^^^T. H. Lewis, Jr., Deep Anode Systems: Design, Installation, and Operation, (Houston, TX: NACE 

International, 2000), p. 28-32. 
^^^Thomas H. Lewis, Jr., "End Effect Phenomena," CORROSION/78, paper no. 163, (Houston, TX: NACE, 

1978). 
"'JVACE Publication 10A196, "Impressed Current Anodes for Underground Cathodic Protection Systems," 

(Houston, TX: NACE International, May 1996). 
^^^David H. Kroon and Charles F. Schrieher, "Performance of Impressed Current Anodes for Cathodic 

Protection Underground," CORROSION/84, paper no. 44, (Houston, TX: NACE, 1984). 
"^M. A. Warne, "Precious Metal Anodes 

no. 142, (Houston, TX: NACE, 1978). 



The Options for Cathodic Protection," CORROSION/78, paper 



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2:64 



chloride concentrations of approximately 25% of seawater, the consumption rate 
increases dramatically reaching values of ten times or more the consumption rate 
in undiluted seawater/i^-ii^-i^°'i^i'i^^ 

Some considerations and concerns when using platinum anodes are silting and 
deposits, abrasion damage, and attenuation problems. Platinum anodes do not 
perform well in enviroimients where silting or deposits can cover the anode 
surface. Under these conditions, the anode life can be shortened to only 10% of the 
anticipated life in open seawater. The significantly increased platinum 
consumption rate and corrosion of the substrate occur because of the low pH 
enviroimient created by the restricted mass flow. Abrasion can damage the active 
platinum surface; therefore, the installer must exercise care during installation and 
operation of these anodes. Attenuation of current along the length of the long, 
small diameter platinum wire anodes can prevent effective use of the entire anode 
surface. In these cases, copper-cored platinum anodes are often used to reduce 
attenuation effects. ^^^'^^'''^^^ 

Use of platinum anodes in soil enviroimients has had mixed results. Even at 
relatively low current densities, platinum anodes have failed prematurely in both 
shallow and deep groundbed applications. The failures have primarily manifested 
as loss of the electrical coimection due to corrosion of the titanium substrate, 
perhaps due to low pH. However, there are examples of good long-term 
performance of platinum when installed in a clean, conductive carbon backfill 
within a uniform, homogeneous soil stratum and operated at low current densities 
of 55 to 75 AW (5 to 7 A/ft^).^^"'^^^'^^'' 



^'"Cathodic Protection - Theory and Data Interpretation Course Manual, (Houston, TX: NACE, 1998), p. 
^ 6:25-6:28. 

^-^M. A. Warne and P. C. S. Hayfield, "Platinized Titanium Anodes for Use in Cathodic Protection," MP, 
^vol. 15, no. 3 (1976): p. 39-42. 
^''A. C. Toncre and P . C. S. Hayfield, " Consumption Rates and Operating Limits for Platinized Anodes in 

Brackish Waters," CORROSION/83, paper no. 148, (Houston, TX: NACE, 1983). 
'-^Ihid.115 

^"''M. a. Warne and P. C. S. Hayfield, "Platinized Titanium Anodes for Use in Cathodic Protection," MP, 
J5, 3 (1976): p. 39-42. 
^'^P. C. S. Hayfield and M.A. Warne, "Variables Affecting Platinized Anodes in Cathodic Protection 

Systems," CORROSION/82, paper no. 38, (Houston, TX: NACE, 1982). 
^'^NACE Publication 10A196, "Impressed Current Anodes for Underground Cathodic Protection Systems,' 

(Houston, TX: NACE International, May 1996). 



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2.4.2.2(b) Mixed Metal Oxide Anodes 

The active surface of mixed metal oxide anodes consists of a solid state solution of 
rare metal oxides (Group IV and Group VIII metals in the periodic table) with 
other nonprecious metal oxides. Some of the oxides often found in this application 
include iridium, ruthenium, tantalum, and titanium oxides. Since the active surface 
is preoxidized, the consumption rate of the surface coating is very low. In fact, 
anode failure is not attributed to consumption of the mixed metal oxide, but rather 
to the formation of a high-resistance, passive oxide film between the active surface 
coating and the titanium substrate blocking current between them. Formation of 
this nonconductive oxide film and, therefore, anode life are a function of the anode 

. , •. 123.127 

current density. 

Because major characteristics of the oxide film, such as specific oxide formulation, 
application technique, and film thickness, may vary for a specific anode or 
manufacturer, it is important to use design information (recommended 
enviroimient, current density, and anticipated life) supplied for a specific anode by 
the manufacturer. However, Table 2-14 provides general guidelines for current 
density ratings in various envirormients along with the specific design life based on 
information from several manufacturers. Mixed metal oxide anodes designed 
specifically for installation in carbon backfill generally have reduced film 
thickness to make the anode more economical. Consequently, the recommended 
current density for a specific design life is also reduced for this particular 

Table 2-14: Mixed Metal Oxide Anodes 





Carbon Baclcfill 


Fresh 
Water 


Braclcish 
Water 


Seawater 


Mud 
Saline 




High 
Current 


Special 


C urrent 
Density, A/m^ 
(A/ft^) 


83-140 
(7.7-13) 


35-40 
(3.3-3.8) 


83-170 
(7.7-16) 


83-260 
(7.7-24) 


480-610 
(45-57) 


83-240 
(7.7-22) 


Life, yrs 


20 


20 


20 


15 


15 


15 


Comments: 


Above ratings do not apply to expanded mesii anodes. 

Current densities must be dearated at temperatures below 5-10° C. 

Electrolyte impurities can affect ratings. 

Mixed metal oxide surface is susceptible to abrasion damage. 

Attenuation should be considered in long, thin wires & rods. 



^'^Richard A. Kus, "Advances in Coating Technology," World Pipelines (November/December, 2002). 
'-^Ibid. 123 

^-"^CerAnode Technologies International, Product Catalog, August, 1999. 
^^^'Eltech Systems Corporation, Lida Products Catalogue, 
http.//www. lidaproducts.com/cataloQue/catframe.htm (November 1 0, 2003). 



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2:66 



Mixed metal oxide anodes are available in a number of different sizes and shapes 
including tubes, wires, rods, meshes, and strips. Wires and rods may also include 
copper cores to improve conductivity, thus reducing attenuation. However, as for 
any long, small diameter anode, the designer should evaluate attenuation for each 
specific case and carefully evaluate the effects of differing soil resistivities in soil 
applications. Significant variation in envirormiental resistivities can cause 
excessive discharge within the low resistivity areas resulting in premature anode 
failure. 

2.4.2.2(c) Polymer Anodes 

Polymer anodes, introduced in the early 1980s, are manufactured by extruding a 
semiconductive polymer coating over a copper wire. The active anode material 
consists of a polymer matrix loaded with conductive carbon. This anode is 
designed for use in carbon backfill where the backfill surface provides the primary 
oxidation reaction site. The polymer anode primarily serves as an electrical contact 

1 ^ 1 

to the carbon backfill. 

Currently only one size polymer anode is commercially available. This anode has 
an outside diameter of 13 mm (0.5 in.) and contains a #6 AWG stranded, copper 
conductor. When used with a high quality carbon backfill, the anode is rated for a 
20-year life at a current output of 52 mA/m (16 mA/ft) of length. ' 

The primary design concerns with long, polymer anodes are attenuation and 
variations in current discharge along the anode due to soil resistivity variations. In 
addition, abrasion or penetration by sharp surfaces can damage the anode. Polymer 

1 9R 

anodes are available either bare or prepackaged with carbon backfill. When 
packaged in carbon backfill, the anode must be centered in the backfill to prevent 
premature failure. 

2.4.3 Polarization Diagram 

In constructing a polarization curve for an impressed current system, note that the 
corrosion potential (open CP circuit) of the anode is more noble than the corrosion 
potential for the cathode (structure). Of course, the anode potential must become 
even more electropositive as the anode polarizes (positive polarization slope); 



"^NACE Publication 10A196, "Impressed Current Anodes for Underground Cathodic Protection Systems," 

(Houston, TX: NACE International, May 1996). 
^^-Tyco Adhesives, Reychem AnodeFlex 500, http://tycoadhesives.com/site/pdf/DCANODEFLEX_500.pdf 

(November 10, 2003). 



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2:67 



therefore, the entire polarization curve for the anode must be more electropositive 
than the cathode curve. Ignoring the corrosion cells on the cathodic protection 
anode and structure, Figure 2-23 is the polarization graph for an impressed current 
system. 

It is apparent from the polarization plot that the rectifier output voltage must be 
equal to the sum of the galvanic potential difference between the CP anode and the 
structure (Egai), the polarization of the anode (ricpa), the IR drop at the anode 
(IRcpa), the polarization of the structure (ris), and the IR drop at the structure (IRs) 
as indicated in Equation 2-37. The polarization curves for both the anode and 
cathode are straight lines on the E log I plot due the logarithmic relationship 
between the current and polarization potential (Tafel equation). Since the IR drop 
associated with each electrode has a linear relationship with the current, this plot 
must be curvilinear on a logarithmic scale as seen in Figure 2-23. 



=cpa 



-cpa.p 



-cpa.corr 



-s.corr 



EsD-- 



-s,P 
Es 



cpa 



cpa 




Erect 



^cp 



log I 



Figure 2-23: Polarization Diagram for Impressed Current CP System 



-RECT 



Egai + ris + IRs + ricpa + IRc 



cpa 



[2-37] 



2.4.4 Carbon Backfill 

The previous section on impressed current anodes indicated that the oxidation 
reactions would occur on the periphery of the carbon backfill rather than at the 
anode surface. Further exploring the contact interface between the primary 
cathodic protection anode and the carbon backfill, we realize that this view is too 
simplistic since there are two possible conduction paths present at the anode 
interface. First, there is the electronic path from the anode surface through the 



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particles of carbon backfill; however, there is also an ionic conduction path from 
the anode surface into the pore water between the carbon particles, as indicated in 
Figure 2-24. 




Anode 
Surface/ 



■>• Ionic Current 

— ► Electronic Current 



Electrolyte 



Figure 2-24: Current Discharge Patlis from Anode Surface 

If current transfers from the anode surface through the carbon particles, no 
electrochemical reaction will occur at the anode surface. However, if current 
leaves the anode surface through the pore water, an electrochemical reaction must 
occur to allow the charge transportation to change from electronic to ionic. In the 
case of electronic transfer of current through the carbon, the periphery of the 
carbon backfill becomes the reaction surface . In the case of ionic transfer into the 
pore water, the reaction surface is the anode surface itself Since the carbon 
backfill surface is much larger than the anode surface, the current density at this 
surface must necessarily be much smaller than at the anode surface. Therefore, the 
opportunity for polarization is reduced significantly. 

Since the two paths for current discharge from the anode surface are in parallel, 
some charge must flow through each path. If the ionic discharge is minimized, the 
anode life is increased, and the electrochemical reactions occur primarily at the 
periphery of the backfill reducing the possibility of anode operational problems. 
Therefore, the ratio of the resistances of each path, one through the pore water and 
one through the carbon backfill, is the determining factor in improving anode life 
and long-term system operation. The resistivities of the water and the carbon 
backfill control the resistance through each path. Since the water resistivity is 
generally outside of our control, minimizing the carbon backfill resistivity 

^ ^NACE 



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becomes the critical control factor. The in-situ bulk resistivity of the carbon is the 
important factor. 

The resistivity of the carbon backfill is a function of the specific resistivity of the 
carbon particles, particle size, and the contact resistances between individual 
particles. Contaminants within the unprocessed carbon (green coke) and the carbon 
heat treatment determine the specific resistivity of the carbon. Selection of a very 
pure carbon (coke) and optimal calcination (heat treatment) of the carbon with 
electrical conduction as the desired characteristic minimize the specific resistivity 
of the final product. 

Carbon backfill for impressed current anodes begins as a form of hydrocarbon. The 
two primary raw hydrocarbons used to produce carbon backfill are coal, used to 
produce metallurgical coke, and petroleum (oil), used to produce petroleum coke. 
Metallurgical coke is produced by heating coal in the absence of oxygen in large 
coking ovens. This process drives gases and liquids out of the coal leaving a 
porous, solid carbon residue (coke). Due to the variability of the naturally 
produced coal and the lack of temperature controls with the coking ovens, the final 
product varies considerably. Petroleum cokes produced for their electrical 
characteristics begin with the selection of heavy crude oils carefully chosen for 
specific properties and heat-treated to exacting standards. 

Since coke has many different end uses, it is important to select cokes produced 
specifically for electrical conduction properties. The single largest use for coke is 
as a boiler fuel where BTU content is the important characteristic. Another major 
application of coke is chemical carbon raising for steel production. Unlike 
impressed current carbons, the electrical properties of the coke are not important in 
either of these major applications. 

The output of the coker for petroleum coke production is called "green" or "raw 
coke" and is not electrically conductive. The raw coke must be calcined to become 
conductive. As the raw petroleum coke is heated, the remaining hydrocarbon 
chains are broken burning off the gaseous hydrogen, increasing the density of the 
particles, reducing the resistivity of the particles, and begiiming the carbon 
crystallization process (graphitization). At the optimal point of calcination, the 
carbon particles have minimal specific resistivity while maintaining a high density. 
At this stage, the carbon is a semi-graphitized, surface-activated coke. 

Since the bulk, in-situ resistivity is the major concern, the particle-to-particle 
contact resistance must also be minimized. This contact resistance is a function of 
applied pressure and the carbon surface properties. The pressure pushing the 

' ^NACE 



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2:70 



carbon particles together is due to the weight of the particles within the carbon 
column after settling underwater. Therefore, to maximize this pressure we must 
enhance the weight of the particles underwater. By maximizing the specific gravity 
(weight of carbon particle relative to water) and releasing tiny air pockets trapped 
on the particle surface by treating with surface tension reducers (surfactants), the 
pressure forcing the particles together increases. The final reduction in in-situ 
resistivity is achieved through surface treatment of the carbon particles. ' 

Sizing of the carbon particles is also important. The average size of the carbon 
particles must be small when compared to the diameter of the anode to ensure 
maximum contact points per unit surface area of the anode. This will maximize the 
contact surface area to the anode and promote electronic current transfer from the 
anode surface. However, there is a limit to how small the carbon particles should 
be since particles less than about 75 microns become high in ash content, higher in 
resistivity, and dusty. 

Finally, in the application of carbon backfills it is important to ensure that the 
carbon surrounding the anode surface is as clean as possible. Any soil or mud 
contaminating the anode surface or carbon particles will hinder electronic current 
transfer. In addition, the installer should completely surround the anode with 
carbon on all sides to minimize the possibility of ionic current transfer from the 
anode surface. This is especially true for polymer-type anodes. If current transfers 
directly into the soil waters from a polymer anode, the polymer will become brittle 
and crack. This will result in attack of the internal copper conductor resulting in 
loss of anode continuity. 

2.4.5 Typical Uses 

Since the output current and voltage available from an impressed current system 
are limited only by the power supply and groundbed design ratings, designers 
typically select impressed current systems for high current requirements and/or 
high resistivity enviroimients. Because of the requirement for an external power 
supply, impressed current systems lend themselves to electronic monitoring and 
control. Within the limits of the specific design, output current and voltage can be 
adjusted as necessary to accommodate changes in the protection requirements of 
the structure. 



^^^ T. H. Lewis, Jr., Deep Anode Systems: Design, Installation, and Operation, (Houston, TX: NACE 
International, 2000), p. 28-32. 



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2:71 



Impressed current systems have the disadvantage of increased maintenance and 
power costs due to the required external power supply. Designers must carefully 
evaluate safety matters related to electrochemical oxidation products at the anode 
and any step and/or touch potentials associated with the groundbed or power 
supply. Also, because higher currents are normally associated with impressed 
current systems, electrical interference with other metallic structures becomes 
more important. 

2.4.6 Impressed Current Power Supplies 
2.4.6.1(a) Standard Transformer/Rectifiers 

The most common type of power supply used for impressed current cathodic 
protection is a transformer/rectifier, commonly referred to simply as a rectifier. 
The rectifier input is an AC voltage from the commercial electrical power grid. A 
transformer with tap adjustments in the secondary side provides a method to 
reduce and adjust the output voltage level and to isolate the DC circuit from the 
input power system. A rectifying circuit next converts the adjusted AC voltage to 
produce a DC voltage output. 

Rectifiers are available for either single-phase or three-phase input power. Although 
single-phase rectifiers are available in half-wave (1 diode), center-tapped (2 diodes), 
and full-wave (4 diodes) bridges, the full-wave bridge is the standard most ofi:en 
used, as shown in Figure 2-25. Three-phase rectifiers are available in wye (3 diodes) 
or full-wave (6 diodes) bridges with full-wave bridges (shown in Figure 2-26) most 
commonly used. Three-phase units are more efficient than single-phase units, but 
the initial investment costs are higher. The theoretical maximum efficiency for a 
three-phase, full-wave bridge rectifier is 96.5% compared to 81% for a single-phase, 
full-wave bridge rectifier. The actual operating efficiency depends on the specific 
output of the unit. The type of power available and the economic comparison of 
overall costs are the primary considerations in selecting a single-phase or three- 
phase rectifier. ^^"^ 



"'^ R. L. Bianchetti, ed., Control of Pipeline Corrosion, Second ed. (Houston, TX: NACE, 2001), p. 90, 166- 
173, 308-310, and 315-317. 



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Figure 2-25: Single-Phase Bridge Rectifier Circuit 

Source: CP 2-Cathodic Protection Technician Course Manual,, (Houston, TX: NACE, 2002), pp. 2:18 and 2:20. 



Grounded Shield 
Between Primary 
and Secondary 
Windings - ^ 



Step Down Transformer 
witli Voltage Adjusting 
Taps on Secondary 
Windings — 



Arrow head indicates 
direction of unidirectional 
current flow through 
element 



AC 3 Phase Supply 

i i I 



Ground 
Connection 
for Rectifier 
Cabinet ^ 




Bridge Connected 
Rectifier Stacl< 



To Pipeline or 
Protected Structure 



To Groundbed 



Figure 2-26: Tliree-Pliase Bridge Rectifier Circuit 

Source: CP 2-Cathodic Protection Technician Course Manual, 
(Houston, TX: NACE, 2002), p. 2:18 and 2:20. 



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Modem rectifiers employ either selenium or silicon diodes to provide the 
rectifying action. Diodes are electrical devices that allow current to pass in one 
direction but block current in the opposite direction. A PN junction accomplishes 
this with a semiconductor boundary with one side of the junction doped with 
positive charge carriers and the other side doped with negative charge carriers. If 
we apply a positive potential to the P or positive-doped semiconductor, the diode is 
forward biased and conduction can occur. However, if we apply a positive 
potential to the N junction or negative-doped semiconductor, the diode is reverse 
biased and current is blocked. In this manner, an AC voltage applied across a diode 
results in the diode alternating between being forward biased and conducting for 
one-half cycle and being reverse biased and blocking for the other half cycle. By 
proper intercoimection of four diodes in the case of a single-phase rectifier, a 
complete AC current cycle can pass through the bridge, but with the positive 
portion of the cycle directed to the positive DC output terminal and the negative 
portion of the cycle directed to the negative DC output terminal. The result is full 
rectification of the complete AC cycle. However, this is not true DC in the sense 
that the signal output is completely constant, but rather a varying, single- 
directional output is produced. 

We can illustrate the rectification process by considering the bridge circuit shown 
in Figure 2-27. The two AC input terminals to the rectifying bridge are II and 12. 
The two DC output terminals are 01 and 02. During the portion of the AC cycle 
when II becomes positive relative to 12, diodes 1 and 4 are forward biased 
(conducting). During this interval, the positive potential at II passes to output 
terminal 01, and the negative potential at 12 passes to output terminal 02. During 
the portion of the AC cycle when II becomes negative relative to 12, diodes 2 and 
3 are forward biased (conducting). During this interval, the negative potential at II 
passes to output terminal 02, and the positive potential at 12 passes to output 
terminal 01. As indicated, the positive potentials always transfer to output terminal 
01, and the negative potentials transfer to output terminal 02. Therefore, output 
terminal 01 is always positive relative to terminal 02. The top portion of Figure 2- 
28 shows the input signal between terminals II and 12, and the bottom portion of 
Figure 2-28 shows the resulting output signal between terminals 01 and 02. 



"^ R. L. Bianchetti, ed., Control of Pipeline Corrosion, Second ed. (Houston, TX: NACE, 2001), p. 90, 166- 
173, 308-310, and 315-317. 



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Figure 2-27: Single-Phase Diode Bridge 



Vorl 





-> t 



AC Input 



Vorl 




*- t 



DC Output 
Figure 2-28: Input and Output Signals from Single-Phase Diode Bridge 

Although the standard transformer/rectifier is by far the most common power 
supply used for impressed current cathodic protection systems due to economics, 
variations of the standard transformer/rectifier are available such as silicon- 
controlled rectifiers, switching-mode rectifiers, and pulse type rectifiers. Also, 
other alternative power supplies are available when the AC power grid is 
unavailable. The more common alternate power supplies include solar panels. 



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wind driven generators, batteries, thermoelectric generators, and more recently, 
fuel cells. 

2.4.6.1(b) Silicon Controlled Rectifiers (SCR) 

Silicon controlled rectifiers (SCR) are sometimes added to provide additional 
control of the rectification process for some types of cathodic protection rectifiers. 
SCRs are three-junction PN devices, as shown in the top of Figure 2-29. An SCR, 
much like a diode, is a rectifying device, which will permit current conduction in 
only one direction. However, unlike a diode, applying proper voltage polarity 
between the anode and cathode will not cause the SCR to conduct (fire). In order 
for the SCR to be forward biased and begin conducting, a positive potential (Vac) 
must be applied between anode (P side) and cathode (N side), and a positive 
voltage pulse must also be applied between the gate and the cathode (Vgc) as seen 
in the center of Figure 2-29. At the instant the gate pulse is applied, the SCR fires 
and begins to conduct current from the anode to the cathode. Regardless of the gate 
current, the SCR continues to conduct until the applied voltage (Vac) goes to zero 
and the current returns to zero. 



G 
Vac 



C 



SCR 









— |l 






A 




P 


N 


P 


N 




C 














I- 


— ► 




h 







Vgc SCR 

y^j. Forward Biased-Conducting 

H| 



i— P N P N 



C 



G Vgc SCR 

Reverse Biased-Non-conducting 
Figure 2-29: Silicon Controlled Rectifier (SCR) 



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If we apply an AC voltage across the SCR, we can control the output voltage 
waveform by controlling the timing for the gate pulse, which fires the SCR. For 
example, if the gate pulse arrives at the same time the applied voltage crossed the 
zero axis and begins to move in the positive direction, the SCR will conduct 
through a complete half cycle (180°) of the input waveform. If the gate pulse is 
delayed for one quarter of a cycle from the time the applied voltage crosses the 
zero axis and moves in the positive direction, one-half of the half cycle (90°) 
waveform will be allowed to pass. If the gate pulse is never applied, none of the 
half cycle waveform will pass (0°). 




• 02 



Figure 2-30: Single-Phase SCR Controlled Bridge 



If we construct a full-wave bridge by replacing diodes 1 and 3 with SCRs as 
shown in Figure 2-30, we can control the full-wave rectified output by controlling 
the timing of the gate pulses for the two SCRs. If the gate pulses are applied at the 
zero crossing of the input waveform (180° conduction angle), the output waveform 
is a fully rectified version of the input waveform, as shown in Figure 2-31. If we 
delay application of the gate pulse for one-third of the half-cycle (120° conduction 
angle), we block one-third of each half cycle at the output. The result is a reduction 
in the average DC voltage output. Therefore, by controlling the timing of the gate 
pulses we can continuously vary the level of the DC voltage output. However, the 
output waveform is not a continuous DC voltage, but rather a pulsing DC with 
ripple. The longer the control circuit delays the gate pulse, the greater the ripple in 



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the output waveform will be. To improve efficiency and reduce ripple, 
manufacturers normally add filters to the output terminals. 




Input Voltage 




Conduction Angle = 180 



► t 




-► t 



Conduction Angle = 120 



-► t 



Conduction Angle = 90° 
Figure 2-31: Full-Wave SCR Bridge Output Waveforms 



The above explanation of the full-wave SCR controlled rectifier operation assumes 
a pure resistance load at the output terminals. However, if the rectifier contains an 
output filter (energy storage), the SCRs may not be able to turn off when the input 
waveform crosses the zero axis due to the energy released from the filter. This 
condition is known as "latching" of the SCR. If we add a "free-wheeling" diode 
across the output terminals, the SCRs can again turn off normally at the end of 
each half-cycle. 



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2.4.6.1(c) Switching-Mode Rectifiers 

A different type of rectifier began to emerge in the late 1970s when switching- 
mode technology became commercially available. Rather than adjusting the output 
voltage level using a large, laminated steel core transformer, switching-mode 
rectifiers control the DC output voltage level by producing a series of high 
frequency (typically 50 to 500k Hz) DC pulses and adjusting the timing of DC 
pulses to produce the required output DC voltage level. Although switching-mode 
rectifiers use transformers for energy storage and for isolation of the output circuit, 
the transformer is a much smaller, high frequency, ferrite-core transformer. The 
block diagram for a typical switching-mode rectifier is shown in the bottom half of 
Figure 2-32. This diagram can be compared to the block diagram for a standard 
transformer/rectifier shown in the top of Figure 2-32. 



standard Transformer/Rectifier 



AC 
Input" 



Low 

Frequency 

Transformer 

w/TAPs 


- 


Rectifier 


- 


Filter 
(if used) 



DC 
Output 



Switching-Mode Rectifier 



AC_ 
Input" 



Primary 
Rectifier 




Transformer — ► 



Secondary 
Rectifier 



Sensing/ 

Switching 

Circuit 



_^ Secondary 
Filter 



Voltage Sense 



DC 
' Output 



Figure 2-32: Block Diagrams for Switcliing-Mode and Standard Rectifiers 

Source: Redrawn from Emerson Network Power, Switching Power Supplies, 
http://www.emersonnetworkpower-medical.coTn (January 7, 2004). 

As indicated in Figure 2-32, the switching-mode rectifier first converts and filters 
the input AC voltage to a DC voltage. The solid state switch next converts the DC 
to high frequency DC pulses. The DC pluses are fed through a high frequency 
transformer to isolate the output. Because the high frequency signal contains 
significant noise (positive and negative spikes), it is necessary to provide 
secondary rectification of the signal along with filtering. Finally, the sensing/ 



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switching control circuit provides feedback control to the solid state switch to 
control the "on" and "off timing (pulse width) of the switch. Therefore, using 
pulse width control, the magnitude of the output voltage is adjusted and 
controlled/^'' 

The primary advantages of a switching-mode rectifier over a standard transformer/ 
rectifier are: 

• Small size and weight 

• Output voltage regulation 

• High efficiency at low rated output 

• Current-limiting features available 

• Modular design for ease of repair 

• Multiple modes of operation possible (constant voltage, constant 
potential, constant current, IR free constant potential) 

The disadvantages are: 

• Can be significant source of high frequency noise (EMI/RFI) 

• Reliability less due to number of components 

• Prone to higher ripple (more filtering required) 

• Repair of individual modules not practical 

2.4.6.2 Solar Power Supplies 

Figure 2-33 shows a solar power supply consisting of a solar panel, a charge 
controller, and a battery system. Specially designed doped silicon semiconductors, 
which are photosensitive, convert solar energy to electrical energy. These 
semiconducting devices (photovoltaic cells) produce a voltage by absorbing energy 
from light photons striking the semiconductor and freeing electrons within the 
semiconductor. The conversion efficiencies for silicon-based photovoltaic cells are 
in the range of 8 to 14%. Research and development teams continue to improve the 
conversion efficiency and lower the production costs for photovoltaic technology. 
Recent irmovations in thin film modules using a fine layer of copper indium 
diselenide (CuInSe2 or CIS) on a glass backing can potentially lower productions 
costs significantly in the future. Research is currently under way combining the 



Lambda Power, Switch-mode Power Supplies, 
http://www.lambdapower.com/ftp/linera_versus_switching.pdf (.Tanuaiy 7, 2004) 



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amorphous silicon with CIS producing a double layer, thin film photovoltaic cell 
with a conversion efficiency of 15.6%. " " 

A single photovoltaic cell produces a very small voltage and current. By 
coimecting a number of cells in series, the output voltage available increases. By 
coimecting cells in parallel, the current output available increases. Therefore, 
manufacturers produce standard solar panels consisting of a number of 
photovoltaic cells coimected in a series/parallel arrangement to produce a specific 
output voltage and current. Solar panels are available in output voltages of 6, 12, 
and 24 volts with power outputs ranging from 5 to 160 watts. Designers can also 
coimect solar panels in series or parallel, as necessary, to produce an even larger 
output current or voltage. ^"^^ 

A backup battery system is necessary with a solar power supply to produce the 
required current output when solar energy is unavailable (night and overcast days). 
The designer must size the solar panel to produce the required current output plus 
additional current to charge the battery system when solar energy is available. 
Whenever the solar energy available is insufficient, the battery system supplies the 
current required for cathodic protection. A charge controller is an electronic 
monitor to determine the state-of-charge of the battery allowing charging current to 
the batteries when needed and preventing overcharging of the batteries. 



^^^Shell Solar, Solar Panels, http://www.shell.com (Novemherll, 2003). 
"^Siemens, Solar Panels, http://siemenssolar.co.uk (November 11, 2003). 
^^^Go Solar Company, Solar Panels, http://www .solarexpert.com (November 11,2003). 
^''°Go Solar Company, Solar Panels, http://www .solarexpert.com (November 11,2003). 



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Solar 
Panels 



Charge 
Controller 



C.R 
Regulator 



Batteries 



1 




Structure 



Anodes 



Figure 2-33: Solar Power Supply System 

2.4.6.3 Wind-Driven Generators 

If a sufficient, steady source of wind is available, wind-driven generators are 
another possible alternate energy source for cathodic protection. These DC 
generators generally begin producing useable current outputs at wind speeds of 
about 16 km/h (10 mph) with maximum output achieved at speeds of 40-55 km/h 
(25-35 mi/h). Since the output varies with wind speed, a battery system is required 
not only as a backup when the wind is flat, but also to provide a constant DC 
output for the cathodic protection system. The generator output charges a battery 
system, and the battery system supplies the output cathodic protection current. 

Due to the high maintenance requirements of wind-driven generators, these power 
sources are now used less frequently especially with the continuing development 
of other more cost competitive alternate power sources. Wind-driven generators 
are available in 400 to 3000 watt sizes with voltage outputs ranging from 12 to 240 
volts. 



^''^J ATS Alternative Power Company, Wind-driven Generators, http://www.jatSQreenpower.com/wind- 
power.html (November 11, 2003). 



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2.4.6.4 Batteries 

If the current requirement for a specific cathodic protection system is relatively 
small, it is possible to use batteries to supply the output current. A small, isolated, 
well coated structure in a high resistivity enviroimient might use a battery power 
supply, perhaps in conjunction with galvanic anodes, to supply the required current 
output. Batteries used in cathodic protection applications should be deep-cycle 
batteries designed for many charge/discharge cycles. 

Battery manufacturers rate batteries in terms of ampere-hour capacity. Simply 
stated, this is the amount of current in amps a battery can supply for a specific time 
interval, hours. Temperature can have a significant effect on output capacity of 
batteries, especially lead-acid batteries. At a temperature of 0° F, the capacity of a 
lead-acid battery drops to about 50% of rated output. A successfully designed 
battery system must be able to supply the output current required for a relatively 

1 49 

long time before battery replacement is necessary. 

Whether batteries are the primary power supply or a backup for another primary 
power source, regular maintenance is required to ensure the batteries operate 
successfully over the long term. If batteries are the primary power supply, they 
must be replaced with new fully charged batteries on a regular schedule. Due to the 
maintenance and regular replacement schedule requirement, the cost associated 
with battery systems can be relatively high. 

2.4.6.5 Thermoelectric Generators (TEG) 

Another alternative energy power supply available for cathodic protection 
applications is the thermoelectric generator (TEG). This power supply uses the 
Seebeck effect to generate small potentials across a dissimilar metal junction. 
Thomas Seebeck discovered that if a junction made up of two different metals is 
heated on one side and cooled on the other, a potential difference develops across 
the junction and charge flow will occur. Thermoelectric generators require only 
three primary components: a heat source, a thermopile, and cooling fins. 

Modem thermoelectric generators employ semiconducting PN junctions rather 
than dissimilar metal junctions. A burner applies heat to one side of the PN 
junction while a heat transfer system cools the other side of the junction. The 



^'''Go Solar Company, Solar Panels, http://www.solarexpert.com (November 11,2003). 

^''^Donald G.Fink and H. Wayne Beaty, ed.. Standard Handbook for Electrical Engineers, Eleventh ed. 



(New York, NY: McGraw-Hill, 1978), p. 2-3, and 11-71 to 11-81. 



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thermoelectric couple is a low voltage, high current power source. Although the 
potential generated across an individual PN junction is very small, approximately 
90 mV, the manufacturer increases the output voltage level by coimecting many 
junctions in series modules (thermopiles). They can also coimect several 
thermopiles in parallel to increase the current output. Burning of a fuel such as 
propane, butane, or natural gas produces the heat for one side of the junction. 
Temperatures on the hot side of the junction are approximately 535°C (1,000°F), 
while a heat transfer and elimination system maintains the temperatures of the cool 
side at about 165°C (325°F). The power produced by the unit is a direct function of 
the temperature difference across the junction. Some semi-conducting designs 
caimot be operated in the no-load condition since current output is necessary to 
cool the PN junction. ' 

Since thermoelectric generators use no moving parts, no significant maintenance is 
required. Normally, only aimual cleaning or replacing the fuel filter and fuel 
orifice is required. Units are available with output voltages of up to 48 volts and 
power ratings of over 500 watts. Manufacturers design and rate thermoelectric 
generators on a power output basis. For efficient application, TEGs should be 
matched to a specific load resistance (usually 1 ohm). The operator can make 
limited adjustment to the current output by adjusting the input fuel supply; 
however, a variable power resistor in series with the output or other type of voltage 
control is required to adjust output. 

2.4.6.6 Fuel Cells 

One of the newer and currently emerging alternative energy power supplies 
available is the fuel cell. Although NASA originally developed this technology for 
the space program, only recently have commercial applications exploited the 
technology. A fuel cell requires three parts: an anode, a cathode, and an electrolyte. 
The fuel, hydrogen, passes through a porous anode catalyst, which causes the 
hydrogen to release its electron into the metal electrode. The hydrogen ion moves 
through the electrolyte where it combines with oxygen gas passing through the 
porous cathode and the electrons from the anode to produce heat and water. ' ' 



^"'^CP 4-Cathodic Protection Specialist Course Manual (Houston, TX: NACE, 2002), p. 1:25-1:27, 3:4- 

3:11, 3:18-3:33, and 8:34-8:35. 
^"^R. L. Bianchetti, ed.. Control of Pipeline Corrosion, Second ed. (Houston, TX: NACE, 2001), p. 90, 166- 

173, 308-310, and 315-317. 
^''^Global Thermoelectric, 8550 Thermoelectric Generator Operating Manual (Calgary, Alberta, Canada: 

Global Thermoelectric, 2002). 
^'^^Online Fuel Cell Information Center, Fuel Cells, http://www.fuelcells.ora (November 11, 2003). 



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Depending on the specific design of the fuel cell, fuel may be in many forms 
including gaseous hydrogen, methane, propane, and even gasoline. Air from the 
atmosphere usually provides the oxygen required at the cathode. The fuel cell 
produces current electrochemically; therefore, no moving parts are required, and 
maintenance is minimal. Fuel cells are more efficient than any other form of 
energy conversion and free of polluting emissions. 



154.155 



2.4.6.7 Modes of Operation 

The power supply for a cathodic protection system can operate in any one of three 
possible modes: constant voltage, constant current, or constant potential. Constant 
voltage is the most common mode of operation since a standard rectifier is 
basically a constant voltage output device. For a standard rectifier the variation in 
output voltage with a change in the load resistance is relatively minor and caused 
by changes in IR voltage drops within the unit. If the external load resistance 
increases, the current output must decrease. The voltage output only changes when 
the transformer tap adjustment is manually changed. 

In the constant current mode of operation, the output voltage from the power 
supply automatically adjusts with changes in the external load resistance to supply 
a constant current output. This change in voltage to maintain constant current can 
only occur within the design limits of the unit. Either a saturable reactor, a silicon 
controlled rectifier, or a switch-mode design can achieve the constant current 
output. A designer normally selects a constant current mode for applications where 
the current requirement is not expected to change but the anode or structure 
resistance will vary. An example of this application is a dock structure where the 
chloride ion concentration varies enough to affect the dock and anode resistances, 
but the current requirement is relatively constant. 



The constant potential mode is really a constant structure potential mode of 
operation. In this mode of operation, the rectifier maintains the potential of the 
cathodically protected structure within specific limits. The power supply control 
circuit monitors the potential of the structure using a permanent reference cell. The 
control circuit compares this potential with internal set potential limits. When the 
structure potential is outside the set potential limits, the control circuit adjusts the 
rectifier output voltage until it is within the set limits. Either a silicon control 
rectifier or a switch-mode rectifier accomplishes the automatic output voltage 
adjustment necessary. An example of an application using this mode of operation 
is a coated, potable water storage tank. As the water level within the tank changes, 
the current requirement changes resulting in an automatic adjustment to the output 
voltage. 

^ ^NACE 

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Experiment 2-1 

Demonstrate the Use of a Galvanic Anode to Mitigate 

Corrosion in a Local-Action Cell 




Magnesium 
Anode 



iCP 



Figure 2-34: Experiment to Demonstrate Corrosion Mitigation of Local- 
Action Cells by Galvanic Anode Cathodic Protection 



PROCEDURE 

Part A 

1. This experiment is a continuation of Experiment 1-1 from Chapter 1. The 
copper and steel sheets should have been coimected directly together 
through an ammeter at the conclusion of Experiment 1-1. 

2. Using the data from Experiment 1-1, record the previously measured static 
potentials of the copper and steel in the following Results Table. 

3. Record the polarized potential of the copper and steel and the corrosion 
current. 



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Parte 

1. Place the magnesium anode in the tray on the opposite side from the steel 
and copper sheets. Coimect the magnesium anode to the copper sheet 
through an ammeter and a 10,000-ohm resistor. 

2. After stabilization of current (approx. 1-2 min.). Measure the corrosion 
current (Icorr)- 

3. Measure the cathodic protection current (Ic.p). 

4. Measure the polarized potentials of steel and copper with the reference cell 
in the same location as before. 

Parte 

1. Repeat Part B using resistors listed in Results Table. 

PartD 

1. Connect the magnesium anode directly through the ammeter to the copper 
sheet (zero resistance in series). 

2. Allow the CP current to polarize the corrosion cell overnight. 

PARTE 

1. Measure the CP current first thing in the morning. 

2. Measure pH at steel and copper surfaces (directly on surface) with CP 
current on. 

3. Measure the polarized potentials of the steel and copper with the reference 
cell in the same location as before with CP current on. 

4. Discormect the magnesium anode and record the potentials over time as the 
steel and copper depolarize. 

5. Note the condition of the steel and copper surfaces. 



RESULTS 



Circuit 
Conditions 


E steel 


^copper 


Corrosion Current 
Icorr (mA) 


CP Current 
IcP (mA) 


Static (OC) 










Polarized 










10,000 ohms 










1,000 ohms 










800 ohms 










600 ohms 










400 ohms 











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200 oinms 










100 oinms 










10 oinms 










oinms 











NEXT MORNING: 



Hours with CP current on 



Circuit 
Conditions 


E steel 


Ecopper 


CP Current 
IcP (mA) 


Polarized 
Condition 








After Disconnecting Magnesium Anode 


Time 


Esteel 


^copper 


X 


Instant Off 






X 


15 seconds 






X 


30 seconds 






X 


1 minute 






X 


5 minutes 






X 


10 minutes 






X 



CONCLUSIONS 



1. Corrosion current decreases as cathodic protection current increases. 

2. Corrosion current decreases as polarized potential of cathode is made more 
electronegative. 

3. Polarized potential of the structure shifts electronegatively with time. 

4. Cathodic protection current decreases with time. 

5. pH at the cathode increases with the application of CP and increasing time. 



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CHAPTERS 

INTERFERENCE 



3.1 Introduction 



In cathodic protection, the term interference refers to electrical interference as 
opposed to physical or chemical interference. Hence interference can be defined 
as any detectable electrical disturbance on a structure caused by a stray current. 
Stray current is defined as a current in an unintended path. 

Many electrical systems rely on the earth as a conducting medium, either for the 
main transmission of electrical energy as with cathodic protection systems or as 
an electrical ground. Still other systems such as electrified transit systems may 
not be adequately isolated from ground. Regardless, any electrical system in 
contact with the earth is a possible source of stray currents. As illustrated in 
Figure 3-1, a current entering the earth at point A has many parallel paths 
available to point B. 




B 



Figure 3- 1: Parallel C urrent Paths in the E arth 

The amount of current in each path is inversely proportional to the resistance of 
each path. It can be argued therefore that current will take all available paths. If 
point A is considered an impressed current groundbed coimected to the positive 
terminal of a transformer-rectifier and point B is a pipeline coimected to the 
negative terminal, then the parallel current paths may all be similar in resistance 
in which case all the currents are the same. This is only possible in homogeneous 
soil where points A and B are far apart and the pipe has no lineal resistance. 

However if the soil resistivity varies or the pipe has lineal resistance, then the 
current paths will have unequal resistances as illustrated in Figure 3-2. 



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Interference 



3:2 




drain point 
Figure 3-2: Parallel Current Paths in a Pipeline Cathodic Protection System 

It is apparent that each current path is composed of resistance through the earth 
(Re) plus a resistance through the pipe (Rp) from the point of current pick-up back 
to the drain point. Therefore the total resistance (R/i) of each parallel path is 
different and given by Equation 3-1. 



Ri - Ri,e + Ri,p 



[3-1] 



Because the length of each current path is different, both in the earth and in the 
pipe in any direction away from the drain point, the total resistance of each 
current path will increase with distance from the drain point. The amount of 
current in each path is given by Equation 3-2 



where: 



R, 



1 
R, 



R, 



R. 



1 
R. 



1 
R. 



1 
R. 



[3-2] 



1 
R„ 



and: 



h 



Ii + h 



In 



In stratified soil conditions where the soil resistivity or cross-sectional area of 
each stratum is different, even current paths of equal length will not have equal 
resistances as illustrated in Figures 3-3 and 3-4. 



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Interference 



3:3 



T/R 



y/Ay//v/AY/Ay/AY/Ay/AVAy/Ay/Ay/AV/A\y/Ay/Av/AV/x\y//o^x\y/AY/A\y/AY//\y/AY/ 




Figure 3-3: Parallel Current Paths in Vertically Stratified Soil Conditions 




drain point 
Figure 3-4: Parallel Current Paths in Horizontally Stratified Soil Conditions 



It is more common than not for soil geology to be stratified both vertically and 
horizontally and for the current in the low resistivity soils to be proportionately 
greater than in the high or moderate resistivity soils. The stratification need not be 
caused by different soils but can be due to similar soils with different moisture 
content. 

In the vertically stratified soils, the resistance of the current paths is not only a 
function of the soil resistivity but is also dependent on the cross-sectional area of 
the current path as in Equation 3-3. 

R.e = PsA^ [3-3] 



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Interference 3:4 



where: 

Rie = resistance of the current path (ohm) 

Ps = resistivity of the soil 

L = length of current path 

Axs = cross-sectional area of soil path 

From a point source electrode like a cathodic protection groundbed, the cross- 
sectional area of the soil increases exponentially with distance from the electrode, 
and therefore the resistance of each current path is not linear with distance from 
the source. 

Soil resistivities (ps) are typically in the range of 10"^ to 10*" Q-cm, whereas metal 
resistivities (pm) are in the range of 10"^ to 10"^ Q-cm. Hence the ratio of 
metal/soil resistivity can range from: 

p„ 10-' ^ 10-' 



to 



Ps 10' lO'^ 



^ = 10-' to 10-'' 



Ps 



Put in perspective, for high soil resistivity (e.g., 10^ ohm-cm) a metal object in the 

9 9 9 

earth having a cross-sectional area of 100 cm or 10" m is equivalent in 
resistance to a cross section of soil given by Equation 3-4. 







Pm _ 
P. 


^x,m 




K. 


substituting: 


Pm 
Ps 


= 10-'' 




then: 


Ks 


A.,. 




10-" 






K. 


10-' 
10-" 


= 10'° m' 



[3-4] 



That is, a metal conductor having a 0.01 m cross-sectional area is equal to a soil 
cross-sectional area of 10^° m^ if the soil resistivity is 10*" Q-cm. This means that 

© NACE International, 2005 C P 3-Cathodic P rotection Technologist 

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Interference 



3:5 



when a metallic structure is present in the earth, it can be a very attractive current 
path thus resulting in a stray current (Is) in the metallic structure as illustrated in 
Figure 3-5. 



metallic structure 




- drain point 
Figure 3-5: Stray Current in a Metallic Structure Parallel to a C athodically Protected Structure 

The stray current is picked up on the foreign metallic structure where it is being 
impacted by the groundbed anodic voltage gradient. If there is no direct electronic 
path between the foreign structure and the pipeline, the current will discharge 
from the metallic structure remote from the pick-up area. 

The amount of stray current in the metallic structure is a function of the resistance 
of the stray current paths and the driving voltage left at the location where the 
foreign metallic structure intersects the anodic voltage gradient. 

Currents from a single electrode, placed vertically in the earth, produce a voltage 
drop in the soil near the electrode forming equipotential surfaces perpendicular to 
the current paths as illustrated in Figure 3-6. 



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3:6 




Figure 3-6: Voltage vs. Distance from a Vertically Oriented Anode 

An equipotential surface has the same vohage difference between the anode and 
any place on its surface. Projection of each equipotential surface at grade and 
denoting its voltage and distance produces the voltage drop (Vax) profile in the 
earth with distance from the anode, as illustrated. 

The voltage rise (Vx,re) in the earth with respect to remote earth can be calculated 
using Equation 3-5. 



V. 



IP. 



27rL 



In 



Vi? 



TA 



[3-5] 



M'here: 



Vx,re = voltage rise in earth with respect to remote earth at 
a distance (x) from the anode 
I = anode current output (A) 
Ps = soil resistivity 



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3:7 



L = length of anode 

For example, for a 10 m long anode in 3000 Q-cm soil having an output of lOA, 
the voltage rise at 100 m is: 



V... 



lOA X 30 Q-m 



27il0m 



In 



10 m + 7(lOm)' + (lOOm)' 
100 m 



4.77 



In 



^lOm + 100.5m^ 
^ 100m J 



= 4.77 [In 1.105] 

Vx,re = 4.77 [0.1] 

= 0.48 V 

If a metallic structure was present 100 m from this anode, it would be subjected to 
about 0.5 V between that point and remote earth. This is the driving voltage that 
would produce a stray current in the structure. 

A similar voltage drop occurs in the earth around a bare pipeline as indicated in 
Figure 3-7. 








current path 



equipotential 
surface 






Figure 3-7: Voltage G radient in the E arth around a C athodically Protected Bare Pipeline 



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3:8 



The typical cathodic protection circuit can then be modeled as a series circuit 
shown in Figure 3-8. 




remote 
earth 



Rc,a & Rc,p 

Ra,re = anode resistance to remote earth 
Rp re = pipe resistance to remote earth 

Figure 3-8: Cathodic Protection Circuit Model 



If a metallic structure is located in the earth as shown in Figure 3-5, it will 
intercept the anode voltage gradient such that there will be a parallel path inserted 
into the model as illustrated in Figure 3-9. 




)| — j-nv-*'(b) 
Rs.re 




remote 
earth 



p,re 



uherB; 

Re, a ^ Rc,p = cable resistances 

Ra.re = anode resistance to remote earth 
Rp re = pipe resistance to remote earth 



R 



s,e 



resistance of foreign pipe to eartti in 
a stray current picl<-up area 



Rs re = foreign structure resistance to remote eartti 

Rs = longitudinal resistance of foreign structure 
between pick-up and discharge sites 



Figure 3-9: Cathodic Protection Circuit Model with Foreign Structure 
Intercepting the Anode Gradient 



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3:9 



The presence of the foreign structure has introduced a parallel circuit into the 
model where the voltage drop between point A and remote earth is applied to the 
foreign structure. This will lower the overall resistance of the anode to remote 
earth and diminish the cathodic protection current beyond point A to I'cp by the 
amount of Is. 

If the foreign structure also crosses the pipeline, as shown in Figure 3-10, then the 
foreign structure resistance to the pipeline will be lowered because of the close 
proximity of the two pipelines at the crossing. This would result in a larger stray 
current because the driving voltage between A and B will be greater (per Figure 
3-11). 



metallic structure 




drain point 



Figure 3-10: Stray Current in a Foreign Metallic Structure that Intercepts 
both the Anodic and Cathodic Voltage Gradient 



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3:10 



® Icf 




remote 
eartti 



where: 

Re, a ^ Rc.p = cable resistances 

Ra.re = anode resistance to remote eartti 

Rp re = pipe resistance to remote earth 

R5 e = resistance of foreign pipe to eartli 
in a stray current picl<-up area 



Rs,p = resistance of foreign pipe to cathodically 
protected pipe at discharge area 

Rs = longitudinal resistance of foreign structure 
between picl<-up and discharge sites 



Figure 3- IL Catticriic Protection CiixiiitMcrid with ForacpStnictiireli^ 
botii Anodic and Cattiodic Vcitage Gradiait 



A metallic foreign structure can also be subject to a stray current even if it only 
intersects the cathodic voltage gradient as illustrated in Figures 3-12 and 3-13. 



. , .Rn 




Figure 3-12: Stray Current in a Foreign Metallic Structure that Intercepts 
the Cathodic Protection Gradient 

In this situation the affected pipeline picks up stray current at remote earth A and 
transports it to the crossing where it discharges back to the interfering structure. 
This means that any pipeline protected by impressed current systems can cause 
interference on crossing metallic structures that are otherwise remote from the 
impressed current groundbeds. Also, the stray current discharge need not be to the 



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Interference 



3:11 



interfering structure but rather to a third party structure acting as an intermediate 
current path. 




a|— vwv-"(a) 



remote 
earth 



vdiere: 

Re, a ^ Rc,p = f^sble resistances 

Ra,re = anode resistance to remote earth 

Rp,re = pips resistance to remote earth 

Rs,re = foreign structure resistance to remote earth 

Rs,p = foreign structure resistance to cathodically protected 
structure at stray current discharge location 

Rs = longitudinal resistance of foreign pipe between 
remote earth and discharge location 

Figure 3-13: Cathodic Protection Circuit Model for Foreign Structure Intercepting 

the Cathodic Voltage Gradient 



As demonstrated, a stray current can occur in a foreign metalHc structure if it is 
impacted by either the anodic or cathodic vohage gradient produced by a pipeHne 
impressed current system. The magnitude of the stray current is directly 
proportional to the voltage between the current pick-up and discharge location 
and inversely proportional to the resistance of the interference current path. 

3.2 Detecting Stray Current 

There will be potential and current changes on and near a metallic structure due to 
any stray current. These electrical disturbances are as follows: 

• stmcture-to-soil potential changes at both stray current pick-up and 
discharge locations 

• current changes in the structure between the current pick-up and discharge 
locations 

• current changes in the earth near the structure at the current pick-up and 
discharge locations. 



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3:12 



If the output of the transformer-rectifier shown in Figure 3-10 is cycHcally 
interrupted and a close interval potential survey is conducted over the foreign 
structure from left to right, the potential profile as illustrated in Figure 3-14 would 
be typical. 



s/s 




Distance 



Figure 3-14: Typical Potential Profile on an Interfered-with Structure that Intersects both Anodic 
and Cathodic Voltage Gradient with the Current Source Interrupted 

Point A is the location on the structure immediately opposite the groundbed 
location and point B is at the pipeline crossing. When the current source is on 
there is a negative shift at the pick-up region (point A) and a positive shift at the 
discharge location (point B). 



Detection of current magnitude changes involved in the stray current situation in 
Figures 3-5, 3-10, and 3-12 are illustrated in Figure 3-15. 



® 



— @- 



Rs.l 



® 



w^A/v^^Vv' 




AVst 



Is Re 
IsRsl 




Figure 3-15: Current Changes In and Near an Interfered-with Structure 
Current changes are detected by measuring the voltage drop in the earth adjacent 
to the interfered-with structure and over a length of the structure with the stray 
current source cyclically interrupted. Therefore there is a change in the earth 

^NACE 



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Interference 



3:13 



voltage drop (Ve,g) at both A and B due to the stray current Ig. At A AVe g will be 
positive since the stray current is toward the structure, and at B it will be negative 
indicating a current away from the structure. The change in structure voltage drop 
will be in the positive direction for the meter polarity shown. 

3.2.1 E ffects of Stray C urrent on Metallic Structures 

The effects of stray current on metallic structures as illustrated in Figure 3-10 can 
be harmful, beneficial, or iimocuous depending on the magnitude of the current 
density, type of structure, and location of current pick-up and discharge areas. 

3.2.1(a) AtArea of Current Pic k-Up 

At the area of current pick-up, a negative shift will result in cathodic polarization, 
and if the foreign structure is mild steel then there is a beneficial effect as the 
structure is receiving some measure of cathodic protection. If the structure is 
coated and has its own cathodic protection system, the additional polarization 
from the stray current pick-up may result in cathodic blistering of the coating. 

If the foreign structure is not mild steel but is made of an amphoteric metal such 
as aluminum, lead, or zinc, then the high pH developed at the structure/earth 
interface due to the reduction reaction can cause "cathodic" corrosion. 
Amphoteric metals are susceptible to corrosion at both high and low pH as shown 
in Figure 3-16 for aluminum and zinc. 



I Al 



A. 



14 13 12 11 10 
Alkaline •* 







^^ 












>. 










70- 


CO 






1 


\ 


60- 


_ E 








\ 


50- 


- o 




Zn 




\ 




O 




J 




\ 


40- 


o 




j 




\ 




O 




/ 




■iO- 






/ 




\ 




o 




/ 




\ 


20- 


o 




/ 




\ 


10- 


E 
- < 

=LJ [ 


l_ d- 


/ 





pH 



3 2 1 
Acid 



Figure 3-16: Comparisonof Zn and Al for Corrosion Resistance as Functions of pH 

Aluminum is particularly sensitive to high pH attack. It is often used underground for 
water irrigation systems, gas distribution piping in rural areas, AC secondary 



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Interference 



3:14 



distribution conductors, and the sheathing on communication cables. Lead sheathing 
was commonly used on below ground AC power cables. 

Prestressed concrete cylinder pipe (PCCP) used for both water and sewage 
transmission is composed of a mild steel iimer cylinder over which a highly 
stressed steel wire is wound to give the concrete/steel cylinder strength. Typical cross 
sections of the two types of PCCP pipe are shown in Figures 3-17a and 3-17b. 



Prestressing Wire and Wire Fabric 
Around Bell or Thicker Bell Ring 
and Wire Fabric 



GroutJ oint 
After Installation 



Cement - Mortar Coating 

1— Prestressed Wire 



J- Steel Cylinder 




Concrete Core 

Steel Spigot Ring 



Steel Bell Ring 

Cement Mortar Placed 
in Field or Other Protection 



a. Lined Cylinder Pipe 



GroutJ oint 
After Installation 



Cement - Mortar Coating 

Prestressed Wire 




Concrete Core 

Steel Spigot Ring 



Cement Mortar Placed 
in Field or Other Protection 



Steel Cylinder 
Steel Bell Ring 



b. Embedded Cylinder Pipe 

Figure 3-17: Typical Section through a Joint in Two Types of PCCP Pipe 

Source: Prestressed Concrete Pressure Pipe-Steel Cylinder Type for Water and Other Liquids, AWWA Standard C301, 

American Waterworks Association, Denver, CO 

The prestressing wire in these pipes is normally cold drawn steel with a yield 
strength in the order of 200 ksi. The cold-worked hardened surface of the wire 



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Interference 



3:15 



makes it susceptible to hydrogen embrittlement. It is recommended that the 
polarized potential be limited to -970 mVcse or less negative to minimize the 
production of atomic hydrogen. If a stray current causes excessive cathodic 
polarization, a catastrophic failure could occur. 

If the foreign structure is coated at the stray current pick-up site, then coating 
blistering or disbondment can occur. Coating blistering is caused by the pressure 
buildup beneath the coating due to the electro-osmotic movement of water 
through the coating (Figure 3-18). The high pH produced by the reduction 
reaction at the metal surface can attack the coating adhesion bonds or a surface 
oxide layer resulting in coating disbondment. 




Figure 3-18: Cathodic Blistering/Disbondment of Protective Coating 



3.2.1(b) Along the Structure 

Stray current in a metallic structure does not usually cause damage between the 
stray current pick-up and discharge locations unless the current is very large or 
the structure is not electrically continuous. If the structure is electrically 
discontinuous, such as is often the case with cast iron water distribution piping or 
PCCP transmission piping, the structure resistance (Rs) is greater than if it were 
electrically continuous. This reduces the magnitude of Is, but creates a current 
discharge/current pick-up pattern at each electrical discontinuity as illustrated in 
Figure 3-19a and 3-19b. 

In many of these structures not every joint is discontinuous, but localized 
corrosion will occur on the discharge side of the discontinuous joints. On water 
and sewer piping, there is not only a soil path for the stray current but also an 
internal path through the aqueous medium as illustrated in Figure 3- 19b. 



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3:16 



^ electrically discontinuous joints -, 



C^ 



3 



I. 



^ 



3: 



^ 



I. 



I. 



I. 



Figure 3- 19a: Stray Current Discharge and Pick-Up Around an 
Electrically Discontinuous J oint Through the Earth 



rubber seal 




Eigure 3- 19b: Stray Current Discharge and Pick-Up Through the Internal Aqueous Medium 
Around an Electrically Discontinuous Bell and Spigot Joint on Cast Iron Piping 



Current in an AC distribution system can also affect the transformation 
characteristics in distribution transformers. At the AC distribution transformer 
which suppHes the AC service for an impressed current transformer-rectifier, a 
ground cable is normally run from the AC neutral to a ground rod at the base of 
the service pole. The ground rod, being relatively close to the groundbed, will 
pick up stray current which will be carried by the distribution neutral and the AC 
phase conductor to ground at remote transformers since DC does not see a high 
resistance through the primary winding. This circuit is illustrated schematically in 
Figure 3-20. 



Remote Distribution 
Transformer 



CP AC Distribution 
Transformer 




CP 
groundbed 



L 



Eigure 3-20: Stray Current Circuit in an AC Electrical Distribution System 



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3:17 



A DC current in the primary or secondary windings of a transformer will produce 
a magnetic flux in the transformer core that will tend to saturate the core and thus 
spoil its voltage transformation properties. This is a harmful effect in addition to 
the corrosion damage that results from the stray current discharging off the 
ground rod at the remote distribution transformer. 

3.2.1(c) Effects at the Stray Current Discharge Location 

Considerable attention is given to identifying the site of current discharge in stray 
current investigations because this is where corrosion damage is most likely to 
occur on all metallic structures. When a current transfers from a metallic structure 
to earth, as depicted in Figure 3-21, it must do so via an oxidation reaction which 
converts electronic current to ionic current. 



' / / / / 

/ / / / 

metal 



X 

1 


Is 

► 




structure 
(electrons). 


D 
A 

T 


I^. 


earth 
(ions) 


/ / / / 
/ / / / ^ 



N 


Is 

► 





Figure 3-21: Currait Discharge from a Metal Structure to Earth via an Oxidation Reaction 

The generic oxidation reaction is the corrosion of the metal as in Equation 3-6. 



M" ^ M"^ + ne 



[3-6] 



For steel, the oxidation reaction is 

Fe" ^ Fe' 



It 



[3-7] 



Stray current discharge from a metallic structure may not cause corrosion attack if 
the structure is receiving cathodic protection as in Figure 3-22. 



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July 2008 



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3:18 




Figure 3-22: Current Discharge from a Cathodically Protected Metal Structure to 
Earth via an Oxidation Reaction 

Cathodic protection current transfers across the metal/earth interface via a 
reduction reaction that produces hydroxyl ions in either of the two following 
reactions: 



O, 



2H2O + 4e -^ 40H 



[3-8] 



2H2O + 4e ^ H2^ + 20H 



[3-9] 



In the presence of a high concentration of hydroxyl ions a possible oxidation reaction 
is given in Equation 3-10 involving the oxidation of hydroxyl ions to oxygen and 
water. 

40H ^02 + 2H2O + 4e [3-10] 

This latter reaction does not consume metal atoms and therefore there is no corrosion 
damage. Hence as long as the polarized potential at the structure/electrolyte interface 
is not driven more electropositive than the cathodic protection criterion (e.g., 
-850 mVcse), then corrosion would not be expected. 

If the metal has a surface passive film or is a relatively inert material, as are some of 
the impressed current anode materials, not all the stray current need transfer through 
a corrosion reaction. If the stray current polarizes the metal surface electropositively 
to the oxygen line on the Pourbaix diagram, hydrolysis^ of water molecules by the 
reaction in Equation 3-1 1 is likely. 



2H2O ^ 4H+ + 02^ 



4e 



[3-11] 



Hydrolysis is defined as a double decomposition reaction involving the splitting of water into its ions 
and the formation of a weak acid or base or both. CRC Handbook of Chemistry and Physics, CRC 
Press, 53"^ Edition, 1972-1973, PF-83. 

^NACE 



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3:19 



This oxidation reaction does not result in the consumption of the metal surface, but it 
does produce an acidic pH because of the generation of hydrogen ions. 



3.2.2 Mitigation of Interference Effects from Impressed Current 
Catiiodic Protection Systems 

A number of methods can be used to lessen the harmful effects of cathodic 
protection system stray currents, as listed below: 

• Remove the source or reduce its output. 

• Install electrical isolating fittings in the interfered-with structure. 

• Bury a metallic shield parallel to the interfered-with structure at the stray 
current pick-up zone. 

• Install additional cathodic protection at current discharge locations on the 
interfered-with structure. 

• Install a bond between the interfered-with and interfering structure. 

• Apply a coating to the interfered-with structure in the area of stray current 
pick-up or to the interfering structure where it picks up the returning stray 
current. 

Before any mitigation activity can begin, conduct mutual interference tests where the 
output of the suspected source is cyclically interrupted and field measurements are 
taken in the presence of representatives of the interfering and interfered-with 
companies involved. Interference cases are often reported through local electrolysis 
committees, especially where there may be more than one interfered-with party. 

Presuming a need for mitigation is determined, the mutually acceptable 
mitigation technique(s) depend on the location and severity of the interference, on 
the cathodic protection operational preferences of each party, and on the relative 
capital and maintenance costs of the mitigation options. 



3.2.2(a) Source Removal or Output Reduction 

It is a difficult proposition to have a source removed if the interfering system was 
present before the interfered-with structure was installed. However, in the opposite 
situation, where the interfering source is newly installed, this method has greater 
appeal. 



NACE 



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Interference 



3:20 



If the interference is caused primarily by the proximity of the interfered-with 
structure to the interfering groundbed, it may not be necessary to remove the 
transformer-rectifier but simply relocate the groundbed or reduce the current output. 

Equation 3-5 or similar equations can be used to estimate how remote a particular 
groundbed needs to be from a foreign structure to minimize the interference effects. 

It should be noted however that the voltage rise at any point distance x from the 
groundbed is a percentage of the total voltage drop to remote earth (Vx,re/Vgb,re x 
100). It is a fiinction only of the geometry of the groundbed (i.e., its length, L) since 
the groundbed current output and soil resistivity would not change. Therefore, only 
the length parameter in the equation significantly affects the percentage. 

Reducing the current output of the source is also a viable option as long as there are 
safeguards to prevent the output from being raised inadvertently. 

3.2.2(b) Installation of Isolating Fittings 

Installation of isolating fittings as a stray current mitigation measure is an attempt to 
increase the path resistance (Rs) of the interfered-with structure thus decreasing the 
stray current (Is). This is seldom adequate as a standalone method. 

The stray current will certainly be reduced but the lesser amount of stray current will 
bypass each isolating fitting in the soil path thus creating several points of interference 
as previously shown in Figure 3- 19a. Consequently, additional cathodic protection 
may be needed at each isolating joint to compensate for the residual stray current. 

The installation of isolating fittings to elecfrically sectionalize piping systems as 
illusfrated in Figure 3-23 is a common practice. 



'\'on Baekmann, Schwenk, and Prinz, Cathodic Corrosion Protection, 3' Edition Gulf Publishing, 1997, 
p. 5 38-5 39. 

^NACE 



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Interference 



3:21 



isolating 
fitting 



w) 



t.^).^ 



isolating 
fitting 



T/R 



« + 



Figure 3-23: Stray Current Arising from Installation of Isolating Fittings 

Unfortunately, inserting electrical isolation often produces a stray current 
condition at the isolating fitting. Therefore, on piping networks protected with 
impressed current systems, electrical isolation should be used sparingly and, 
when used, facilities to mitigate the expected interference should be provided at 
each point of electrical isolation. 

3.2.2(c) Burying a Metallic Shield Next to the Affected Structure 

The intent of a buried metallic conductor is to intercept the stray current and thus 
provide an alternative low resistance path for the stray current compared to the 
metallic structure path. Cormecting the metallic shield, which could be a bare cable 
or pipe, directly to the negative terminal of the offending transformer-rectifier, as 
shown in Figure 3-24 and modeled in Figure 3-25, would be more effective than 
cormecting it to the interfered-with structure. 

Interfered-with Structure 



Bare Shield 



+ «' 



^cp 



T/R 



— u 



Cathodically Protected Structure 
Figure 3-24: Using a Buried Metallic C able or Pipe as a Shidd to Reduce Stray Current Interfa-ence 



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Interference 



3:22 




remote 
earth 



p,re 



vJiem: 



Re, a 5< Rc,p = cable resistances 

Rsh.e = shield resistance to earth 
Rsh = shield cable longitudinal resistance 

Figure 3-25: Cathodic Protection Current Model for a Buried Metallic Shidd Connected 
to the Negative Terminal of the Transf ormer-Rectifia' 



The alternative approach, which would be to connect the buried metallic shield to the 
interfered- with structure, would increase the stray current discharge at point B. 

This buried metallic shield method has the most merit where the interfered-with 
structure is either made of an amphoteric material or where there is a concern about 
coating blistering or cathodic disbondment. 

This technique puts the interfering system at a considerable disadvantage. It can 
seriously disrupt the current distribution pattern to the cathodically protected 
structure, perhaps even necessitating the installation of additional cathodic 
protection units to make up for the poorer current distribution. 

3.2.2(d) Installation of Galvanic Anodes on the Intetfered-with 
Structure at Point of Stray C urrent Discharge 

When the area of stray current discharge is very localized, such as at a crossing with 
the interfering structure and where the total stray current (Is) is typically less than an 
ampere, the installation of galvanic anodes as depicted in Figure 3-26 has 
considerable benefit. 



With the anodes placed alongside the interfering structure, the resistance through 
the anodes (Rp,g) is minimized and hence the stray current(Is') in this path is 
maximized. Unfortunately, the galvanic anodes may also more closely couple the 
interfering structure's cathodic potential gradient, thus increasing the magnitude 
of the interference current. The anodes can also be arranged parallel with the 

,& 

''^NACE 

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Interference 



3:23 



interfered-with structure which minimizes the anode circuit resistance and 
therefore maximizes the cathodic protection current (Icp.g) Less coupHng 
between the anodes and interfering structure also resuhs with this arrangement. 



Test station 




- Interferred-with Structure 
Figure 3-26: Interference Mitigation using G alvanic Anodes at Stray Current Discharge L ocation 

If the interfered-with structure is coated at the crossing, the path resistance (Ra.p) 
through the galvanic anodes will be substantially less than the interfered-with 
structure resistance (Rsip) (depicted in the electrical circuit model in Figure 3-27). 
Although there can still be a residual stray current (Is"), the total cathodic protection 
current (ZIcp) is expected to be greater, thus assuring total remediation of the 
interference. 



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Interference 



3:24 




remote 
earth 



where: 



Re, a ^ Rc,p = cable resistances 

Ra,p = anode(s) resistance to tlie interfering pipe 

Eg = galvanic anode driving voltage 
Icp.g = galvanic anode CP current 

Figure 3-27: Electrical Circuit Model for Mitigating Stray Current Interference at a 
Stray Current Discharge Site Using G alvanic Anodes 

Ideally, the galvanic anodes are distributed alongside the interfering structure to 
minimize the path resistance (Ra,p) so the stray current (I's) is a large percentage of 
the total stray current (Ist). The design life of the galvanic anodes must take into 
account the additional consumption by the stray current (I's) component of its total 
output. 

There are several advantages of this method as follows: 

• The interfered- with structure can maintain cathodic protection 
independence. 

• The galvanic anode cathodic protection current output boosts the level 
of protection at the crossing as an added buffer should the interference 
current (1st) increase. 

• Maintenance requirements are low compared to a direct bond. 

The disadvantages are that it is relatively expensive compared to a direct bond, and 
the interference current mitigation capacity is somewhat limited. To mitigate large 
interference currents an impressed current system can be used with the drain point at 
the crossing but the groundbed remote from both piping systems. 



NACE 



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Interference 



3:25 



3.2.2(e) Installation of an Impressed C urrent Distribution System 

on the Affected Structure at Point of Stray C urrent Discharge 

The stray current situation depicted in Figure 3-5 results in a typical potential 
profile along the interfered-with structure where the stray current discharge (+AE) 
occurs in an end- wise fashion as illustrated in Figure 3-28. 



■s/s 




Distance 



Figure 3-28: Potential Profile Changes on a Pipeline Where Stray Current 
is Discharging in an End-Wise Pattern 

Although the positive potential shift may be modest, the length of the discharge 
can be extensive. Under these conditions the installation of an impressed current 
system at the discharge locations (A and B) can be an effective means of 
compensating for the stray current interference. Care must be taken to ensure that 
the impressed current systems do not create interference on the original 
interfering structure. 



3.2.2(f) Installing a Bond Between the Interfered-with and 

Interfering Structures 

Perhaps the most common stray current mitigation method is installation of a bond, 
usually having some resistance between the two structures and usually at the point of 
maximum stray current discharge such as at a crossing as shown in Figure 3-29. 



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C P 3-Cathodic P ro tec tion Technologist 

July 2008 



Interference 



3:26 



I^ 



I Bond 
\ Cable 



pi)i-^vw^ te 



Variable 
Resistor 

(Rb) 




Test Station 



V/Ay/AWsWAy/AV/V/Ay//^y/A\7/^/AVXV/AVAV .VXV/AVXV/AVXV/A\//\>7Ay/y 



(pi) Interfered- with 
structure 




bond cable and 
test station 



^..^ Interfering 
@) structure 



burled reference 
electrode 



Figure 3-29: Interference Mitigation Using a Resistance Bond 

The electrical circuit model is similar to Figure 3-27 except with the bond resistance 
(Rb) replacing the galvanic anode resistance (Ra,p) in the circuit. Typically, the bond 
resistance is determined by monitoring the potential of the interfered-with structure 
while adjusting the resistance until the interfered-with structure is returned to its 
cathodic protection criterion or native potential on a structure having no cathodic 
protection. A zinc reference installed between the two structures at the crossing is an 
optional but worthwhile feature. 

A resistance bond will not eliminate all the current discharge at the crossing as there 
will still be a residual stray current discharge (Is") that must be countered by the 
interfered-with structure's cathodic protection system. 

The major advantages of resistance bonds over other mitigation methods are: 

• relatively inexpensive installation 

• easy to adjust if stray current magnitude changes 

• high current capacity 



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Interference 



3:27 



Disadvantages of resistance bonds include: 

• resistance bonds are vulnerable to AC fault current transients that can bum 
out the resistor unless protected with fault current devices 

• connecting two structures through a resistance bond means that cathodic 
protection changes on either structure will affect protection levels on the other 
structure 

• surveys to measure true polarized potentials on either structure may require 
the synchronous interruption of the bond or impressed current systems on 
both structures 

• resistance bonds are considered critical components and by regulation require 
frequent inspection. 

3.2.2(g) Using Coatings to {^litigate Interference Effects 

Applying a coating is an attempt to increase the resistance of the stray current path 
thus decreasing the stray current magnitude. As a standalone method, coating should 
only be applied at current pick-up locations. If the discharge area of a structure is 
coated, there is a risk of corrosion failure due to a high discharge current density at a 
holiday in the coating. 

There are two current pick-up regions — one on the interfered-with structure and one 
on the interfering structure in the vicinity of the stray current discharge as shown in 
Figure 3-30. 



coatsd pipe sections 



T/R 




Figure 3-30: Use of a Dielectric C oating to Mitigate Interference 



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July 2008 



Interference 3:28 



This technique is easy to implement on a new structure where a higher quality 
coating can be used in areas where stray current pick-up is anticipated, but it may be 
impractical for existing facilities. 

3.2.3 Other Sources of DC Stray Current 

Besides impressed current cathodic protection systems, other sources of DC stray 
current are as follows: 

• DC transit systems 

• DC welding equipment 

• high voltage DC transmission systems (HVDC) 

• DC rail systems in mines 

Due to the variable loading nature of these sources, the resulting stray current activity 
is dynamic (i.e., effects vary in magnitude and often location with time). Another 
source of dynamic stray current, telluric currents, is discussed in Section 3.4. 

3.2.3(a) DC Transit Systems 

The electrification of transit systems in the late 1800s throughout North America 
resulted in considerable interference corrosion on gray cast iron water mains. Much 
of the early attempts to mitigate this interference led eventually to the development 
of cathodic protection technology. 



Kuhn, R.J., Cathodic Protection of Underground Pipelines from Soil Corrosion, API Proceedings, Nov. 
1933, Vol 14, p. 164. 

© NACE International, 2005 C P 3-Cathodic P rotection Technologist 

July 2008 



Interference 



3:29 



0/H power conductor 




metallic structure 
(e.g. watermain) 



+ 


dc 
sub- 
station 








I^ 


- ground fl 


^discharge 

^ 1 



Figure 3-31: Typical Stray Current Paths Around a DC Transit System 

The load current (II), after passing through the trolley motor, divides into a number 
of current paths depending on the resistance of each path. 



therefore: 



It 



Ir + L 



L 



Despite the rails being a relatively low resistance path, the current leakage off the 
rails can be 5-10% of the load current. While this seems a small percentage, the stray 
currents can be substantial since the start-up load current can be several hundred 
amperes for a single trolley and several thousand amperes for a subway train. 

Not only will the magnitude of the stray current vary with time of day and whether 
or not the vehicle is accelerating or decelerating, but the location of stray current 
pick-up on the metallic structure will change as the trolley moves along the rail. 
Thus, a structure-to-soil potential recording will have a dynamic appearance as 
shown in Figure 3-32. 



NACE 



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C P 3-Cattiodic P ro tec tion Tec ti no legist 

July 2008 



Interference 



3:30 



1 

-500 




-1000 






u 


li 


IMJliJilJ 


m 


Mi 


i 


Iklilidii 


i 


lUJ 


llJLw.«J 


uiJi 


lii 


Lb 


1 


1 


1 


f 


1 


\ 


Pl| 


n 


n 


1 


T 


1 


^ 


n 


' 




f 


\ 


1 


'1 


1 






\ 


1 


It 
































1 


1 












1 




















II 




































-3500 






1 






■■ 










c 
c 
c 


11:00 - 
12:00 - 


o 
o 
&) 






D 
D 




O 
O 


o 
o 


o 
o 




o 
o 


19:00 - 
20:00 - 


o 
o 

fN 


o 
o 
rj 

fN 




o 
o 
m 

fN 




o 
o 
6 


o 
o 


2:00- 

3:00- 

4:00- 

5:00 

6:00 




o 
o 


o 
o 

CO 


o 
o 




o 
o 

c 



Time 



Figure 3-32: Typical Structure-to-Soil Potential Recording with Time C aused by 
Interference from a DC Transit System 



The potential-time recording of stray current effects from a DC transit system has 
a distinctive pattern. There are considerable potential fluctuations during the 
morning and evening rush-hour periods, light activity in the middle of the day and 
late evening, and virtually no changes during the early morning hours. 

Although the stray current pick-up locations change with time, the discharge sites 
are usually close to the substation ground. In urban areas, localized stray current 
can discharge from water piping around electrically discontinuous joints and from 
crossings with other utilities remote from the substation ground. 

Determining the impact of transit-caused stray current on metallic facilities in 
urban areas requires considerable potential and current recording, starting in the 
vicinity of the substation grounds and along the transit system route. 

A comprehensive method of analyzing dynamic stray current activity involves the 
construction of "beta" curves from current and potential measurements. This 
technique is not covered in this course but is discussed extensively in the NACE 
CP Interference Course. 



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Interference 



3:31 



3.2.3(a)(i) Mitigation of Transit System Stray Currents 

Mitigation methods for minimizing the harmful effects of DC transit system stray 
currents are similar to those used for mitigating cathodic protection stray currents 
and include: 

• electrical isolation of rails and substation 

• electrical bonds 

• reverse current switches 

• forced drainage bonds 

• cathodic protection 

On existing transit systems, stray current has been reduced significantly by 
improving the isolation between the rail and ballast. This is accomplished by 
installing insulating pads between the rail and ties, between the hold-down plates 
and the rail, and ensuring that the ballast is well drained. These measures coupled 
with disconnecting the negative rails from electrical grounds have proved 
relatively successful in many instances. Discoimecting the DC substation from 
electrical ground allows the rails and transit vehicles to electrically float. This 
then requires switching devices be installed coimecting the rails to earth if a 
specific rail voltage-to-ground potential is exceeded. The effectiveness of 
substation isolation in minimizing stray current activity is therefore lost when 
switches are activated. 

For new transit systems, it has become commonplace to electrically isolate the 
entire rail pocket if the rail is embedded in the road surface (Figure 3 -3 3 a) or to 
isolate the rail from ties (Figure 3-33b). 



Polyurettiane 
Sealant 



Flangeway 




Rail 



36 Mils of 
Coal Tar Epoxy 

Third Pour 
Fiber Concrete 

Second Pour 
Fiber Concrete 



Rail 


Clip 

)5 


If 


/ Elasto 


mer Pad 

r Concrete 

/ Tie 

Concrete 
Invert 


r 




1 


t 




J 


1 








r 


Figui 


'e3-33b: Typical Direct-F 
Isolating Fastener 


/ 

btation 



Source: FitzgeraldJ .H. and Lauber, M.D., 
Stray Current Control for the St. Louis Metrolink Rail 
SystEm, MP,34, l,(1995):p.22. 



Figure 3-33a: Typical Fmbedded Track Installation 

Source: Sidoriak, W., Rail Isolation on the Baltimore Central Light Rail Line, MP, 32, 7, (1993): p. 36. 



NACE 



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Interference 



3:32 



Some transit systems use a separate isolated rail (or "fourth rail") as a current return 
path which negates the need to isolate the running rails. 

The earliest attempts to mitigate the corrosive effects of transit stray currents simply 
involved running bonds from the utilities to the negative bus at each substation. This 
provided an elecfronic path for the stray current to return thus reducing the amount of 
stray current in the electrolytic path as shown in Figure 3-34. 



positive bus 

• f 



to Brd rail 



■.,9- 




negative bus 



rails 



shunt 



is,2 



TT 



^s,l 



metallic structures 



Figure 3-34: Typical Utilities Drainage System at a Transit Substation 

Facilities such as lead sheathed power cables, steel gas piping, telephone grounds, 
and iron water piping would be cormected in series with a switch and a shunt to the 
negative bus. The shunt provides a means of recording the stray current magnitude 
and direction. 

One weakness of this drainage arrangement is that providing a direct low resistance 
path for the sfray current results in the underground structures picking up more 
current than they would otherwise. For structures with electrical discontinuities, such 
as iron water mains, this can result in more severe corrosion at the isolating joints. 

A second disadvantage of the direct bond drainage system is that where there are 
multiple substations and many trains, the utilities represent an alternative path to the 
rails between substations and the stray currents can actually reverse. This situation is 
depicted in Figure 3-35. 



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Interference 



3:33 



— bus 



I" B i 



SS 
'A' 



+ bus 



3rd rail 

Il,a- 



+ bus 



Il,a + Il,b -=4-, 



SS 

'B' 



— bus 



i Ila 



^=i_l"A +1 



L,B 



!"[ 



running rails 



Il,b +I 



S,B 



I" A +1 



S,A 



utilities 



Il'a 



Figure 3-35: Schematic Showing Circulating Current between Transit Substations 
Through Direct Bonds to Utilities 

With the transit load located between substation A and B, it will draw some of the 
load current from each station. Each substation's load current has an alternative path 
through the utility bonds back to its respective source. 

To prevent circulating currents, reverse current switches can be installed in each 
bond. These devices present a high resistance in one direction (the reverse 
direction) and a low resistance in the other (direction of intended drainage). There 
are several types of reverse current switches/ as listed in Table 3-1, each with 
differing operational characteristics. 



Table 3-1: Types of Reverse Current Switches 



Type 


Characteristics 


Electromagnetic (relay) 


Requires AC power to operate the relay, relay must conduct 
all current, may be slow to open 


Diodes (germanium, 
silicon) 


Requires a minimum of 0.4V to conduct, have resistance, 
subject to surge failures and reverse voltage breakdown 


Hybrid (relay in parallel 
with diodes) 


Smaller relay required since diodes carry most current and are 
subject to reverse voltage breakdown. 


Potential Controlled 
Rectifier (Figure 3-36) 


Can drain all tiie stray current butare relatively expensive. 



Although cathodic protection is beneficial in mitigating transit system stray current, 
the stray currents are often so large that it precludes mitigation with galvanic anodes. 
Moreover, large capacity impressed current systems in an urban area are likely to 



Munro, J. I., Comparison and Optimization of Reverse Current Switches, CORROSION/80, paper no. 
142, (Houston, TX: NACE, 1980). 



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Interference 



3:34 



create interference on other facilities. Hence, cathodic protection has limited 
effectiveness. 

One the most successful measures is the use of a forced drainage bond. As shown in 
Figure 3-36, a forced drainage bond is a bond with a potential controlled rectifier 
coimected in series with the bond. 



Potential 

Controlled 

Rectifier 



V/AX/AX/AVAy/AV/C^ 



structure 



W: W7FT/ ^\/Ay/AV/XV/ NV/AX/AYA 



■1^=^ 



^ buried reference 
^ electrode 



Figure 3-36: Forced Drainage Bond Using a Potential C ontrolled Rectifier 

The voltage output of the auto-potential rectifier varies depending on the potential 
measured between the structure and a buried reference electrode. If the measured 
potential is more positive than the potential set on the controller, the rectifier output 
voltage increases to force more current through the bond. With a DC voltage source 
in series with the bond, the bond resistance is negative ensuring that all the stray 
current is drained from the structure and there is no residual stray current in the soil 
path. But the controller must be adjusted so there is no bond current during periods 
of no stray current activity, otherwise the transit system rails and grounding system 
will be corroded. 

To be completely effective, the forced drainage bond must be located at the point of 
maximum discharge. Just as with a resistance bond, if the structure is not electrically 
continuous a forced drainage system will aggravate corrosion at any isolating joints. 



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3:35 



3.2.3(b) High Voltage Direct Current (HVDC) Electrical Transmission 
Systems 

HVDC systems for transmitting large blocks of electrical power over long 
distances have operating cost advantages over high voltage alternating current 
(HVAC) transmission. Unlike HVAC systems, there are no inductive or 
capacitive losses on HVDC, and for lengths greater than about 800 km, the power 
savings easily justify the extra capital costs to build the AC/DC converter stations 
and their extensive electrical grounding systems. 

HVDC systems are built to operate in bipolar mode; that is, there is both a 
positive and negative circuit with large grounding electrodes at each terminus as 
illustrated in Figure 3-37. 



load 
end 



r 



T 



■Idc 



+ 

■«- - 



+ 

■4- - 



t_ 



positive 
AC /DC 
negative 



cables 



Converters 



n 



T 



supply 

end 



cables 



idc 



L > 800 km 



Figure 3-37: Electrical Schematic for a HVDC System 

The DC line currents are typically in the 1000 A range and imbalance currents, 
under normal operating conditions, are varying and about 1-2% of the line 
currents. Such small currents do not pose a significant stray current risk on 
underground metallic structures since the electrodes are intentionally located 
remote from other utilities. 

During emergency operating conditions where either the positive or negative 
cable networks are faulted or de-energized for maintenance, the line current 
passes through the earth via the grounding electrodes. Under these circumstances 
the system is operating in monopolar mode. 



HVDC grounding electrodes are large compared to impressed current 
groundbeds, although cathodic protection anode materials such as high silicon 
iron and coke are often used. The electrode is typically in the shape of a ring with 
about 100 m diameter and a depth of 1-2 m. Despite the large size and relative 
remoteness, the voltage gradient around the electrode can be appreciable even a 

^NACE 



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Interference 



3:36 



long distance away when the electrode is passing hundreds of amperes. For 
example, the voltage rise in earth at some distance x from such an electrode can 
be estimated using Equation 3-12. 



V„ 



27rx 



[3-12] 



where: 



V 



g.x 



Ps 
X 



voltage rise with respect to remote earth at a 

distance x from the electrode 

electrode current 

soil resistivity 

distance from the electrode 



given: 



Ps 
X 



500 A 
50Q-m 
1000 m 



then: 



V„ 



31km 



500 A X 50Q-m 
6.28 X 1000 m 



4V 



Hence a metallic structure located 1 km from the electrode would be exposed to 
4V during monopolar operation under these conditions. It is claimed that the 
HVDC system will operate in monopolar mode a small percentage of time. 
Nevertheless the rather large voltage gradients can present a serious corrosion 
risk on some structures on a cumulative basis. 

Also, the effect can be either a positive or negative potential shift on the structure 
as shown in Figure 3-38 depending on which of the power circuits has the outage. 



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Interference 



3:37 



Es/s -1.0 
(Vcse) 




+AE 



-AE 



Time 



Figure 3-38: Potential-Time Plot for a Metallic Structure being Interfered-with by a HVDC System 

Assume that the potential plot in this figure was from a structure located near the 
supply end groundbed. Then the negative shift from ti -^ t2 would result from a 
failure on the positive circuit and the positive shift from ts -^ U would result from 
a failure on the negative circuit. Note that the potential shifts are not necessarily 
equal, even if the stray current is the same, since the cathodic and anodic 
polarization characteristic can be different. 

Most structures would not extend the full 800 km nor be close enough to the 
electrode to make it economical to install a bond. Because of the large voltage 
shifts, galvanic anodes many not adequately compensate. The most practical 
mitigation method is to use an impressed current system powered by a potential 
controlled rectifier. Not only would the cathodic protection power supply be able 
to counteract the large positive potential shifts, but during the negative shift 
periods it would shut down, thus minimizing the stress on the coating if the 
structure was a coated steel pipeline. 

3.2.3(c) DC Welding Operations 

Welding operations on ships and barges have been known to create stray current 
interference, sometimes so severe that it has resulted in the sinking of the vessel. 
Interference arises where the negative of the welding generator is coimected to 
electrical ground on the dock and there is no electrical bond between the dock and 
the vessel. Under these circumstances, the welding current, which can be 
hundreds of amperes, discharges from the vessel to the dock as illustrated in 
Figure 3-39. 

,& 
^NACE 



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Interference 



3:38 




dc welding 
generator 



sheet 
steel piling 

Figure 3-39: Stray Current Caused by DC Welding Operations 

The interference is mitigated by bonding the vessel to the dock or by attaching the 
negative of the welding generator directly to the vessel. 



Experiment 3-1: 

To Demonstrate DC Interference and Its Mitigation 



3.3 AC Interference 

3.3.1 Introduction 

Electrical energy from an overhead power line can be transferred to a pipeline by 
three possible mechanisms — conductive coupling (during fault conditions), 
electrostatic or capacitive coupling, and electromagnetic or inductive coupling. 
How each of these affect a pipeline and how these effects can be predicted and 
mitigated is discussed in this chapter. 

It should be noted that predicting AC interference effects is a complex matter 
requiring fairly sophisticated mathematics. This course discusses methods of 
estimating the effects for a few very simple cases, but most problems can only be 
solved using either complicated analytical techniques^ or specialized software.^ 



"Power Line-Induced AC Potentials on Natural Gas Pipelines for Complex Rights-of-Way 
Configurations, " Electric Power Research Institute, EPRI Report N° EL-3 106. 

"AC Predictive and Mitigation Techniques, " Pipeline Research Council International, Inc., PRCI Report 
N°PR-200-9414, May 1999. 

^NACE 



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Interference 



3:39 



A pipeline can experience AC interference as a result of being near any AC 
power line. However, the vast majority of interference problems are created by 
three-phase power transmission systems, since these involve both high currents 
(steady-state and fault conditions) and high voltages and are more likely to 
parallel pipelines for long distances than are low voltage distribution systems, for 
instance. 

A three-phase (3^) power transmission system consists of three power lines, each 
having the same voltage to ground and each carrying approximately the same 
amount of current. One or two additional conductors, known as shield wires, may 
also be present, rurming between the tops of the power line support structures 
(Figure 3-40). Although their purpose is to protect the power line from lightning 
strikes rather than to transmit power, shield wires (as well as any other paralleling 
conductors) nevertheless affect how electrical energy is transferred to a pipeline. 

In a three-phase circuit, the AC waveforms for each of the three phases are 120 
degrees apart from one another (Figure 3-41). Waveforms that have the same 
frequency but start and end at different times are said to be "out of phase 
with one another." The point at which a waveform begins along the x-axis (in 
degrees) is known as the waveform's phase angle. When AC waveforms are 
discussed, phasor notation is used to indicate both the magnitude and the phase 
angle of the waveform. For instance, in Figure 3-41, the three voltage waveforms 
would be represented as 0.7Z0°, 0.7 Z120°, and 0.7 Z240° in phasor notation. 



\ 


\ 


/ 
4 1 




A>1\€^ 


^^/ 






T] 

1 




]T^^=cl2y 





Figure 3-40: Typical Three-Phase AC Power Line 
(Horizontal Configuration with Two Shield Wires) 



Note that the peak voltage shown in Figure 3-41 is 1.0 V, but the vohage magnitude shown in phasor 
notation is 0. 7V. This is because, when discussing the magnitudes of AC waveforms, "rms " (root mean 
square) values rather than peak values are generally used. For a sinusoidal waveform, the rms value of 

a voltage or current is l/V2 (approximately 0. 7) times its peak value. 



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Interference 



3:40 




180 

Angle (Degrees) 



360 



Figure 3-41: AC Vdtage Waveforms in a Three-Pliase Circuit 

3.3.2 Conductive Coupling Due to Faults 

3.3.2(a) Description 

Conductive coupling can occur when there is a Hne-to-ground short circuit or 
fauh on the power Hne. On high-vohage power Hues, fauhs are most Hkely to 
occur as the resuh of Hghtning, which can ionize the air in the vicinity of an 
insulator. Typically, a high-voltage transmission line experiences less than one 
lightning related fault per 100 km per year. Faults can also occur as the result of 
high winds, failure of the power line structures or insulators, or accidental 
contacts between the power line and other structures, such as cranes and other 
construction equipment. 

Under fault conditions, the current leaving the power line will return to its source 
using all paths available to it, including power line shield wires, the earth, and 
metallic structures in the earth such as pipelines. The amount of current 
transferred to a pipeline is dependant on the relative impedances of all parallel 
paths available to the fault current. It is also a function of the separation distance 
between the faulted structure and the pipeline, the available fault current, the 
impedance of the faulted structure to ground, and the impedance of the pipeline to 
ground. 

Fault current is conducted to the pipeline through its coating. The better the 
coating quality (i.e., the fewer the holidays) and the higher the coating's dielectric 
strength (i.e., breakdown voltage), the lower the current transfer to the pipeline. 



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Interference 



3:41 




^r^ ^ 





Figure 3-42: Conductive Coupling During Line-to-G round Fault C onditions 

3.3.2(b) Deleterious Effects 

Fault currents (a transient condition) are much greater in magnitude than steady- 
state power Hne currents, so conductive coupHng can resuh in very high pipeHne 
vohages; however, the length of time these voltages are present on the pipeline is 
limited to a fraction of a second (typically 0.1 second) due to power system 
protection devices. Even over such a short time period, large amounts of energy 
can be transferred to the pipeline, resulting in coating damage or even pipeline 
failure due to melting or cracking of the pipe wall. 

The high pipeline voltages resulting from conductive coupling represent a safety 
hazard to pipeline persormel and perhaps the general public in cases where test 
leads and pipeline appurtenances are accessible. While electric shocks can be 
painful and can result in the loss of muscular control at body currents of less than 
50 mA, the primary concern for short duration (transient) shocks resulting from 
fault currents is ventricular fibrillation, which may occur at body currents of 
greater than 50 mA and certainly occurs at body currents of greater than 100 mA. 
Ventricular fibrillation results in the total loss of coordination of the heart due to 
the disruption of its electrical signals and will result in death without 
defibrillation (i.e., a strong electrical pulse to restore the heart to its normal 
beating pattern). The maximum current that can be tolerated before the onset of 
ventricular fibrillation is a function of body weight and duration of current flow, 
as indicated by Equation 3-14. 



An electric shock can occur when a person touches an energized structure or even 
when a person is simply standing in the vicinity of an energized structure in 

© NACE International, 2005 C P 3-Cathodic P rotection Technologist 

July 2008 



Interference 



3:42 



contact with the earth. As an example, the structure in Figure 3-43 is energized to 
a vohage of 10 kV. The fault current Ip passes from the structure to the earth, 
creating a voltage gradient. A person touching the structure will be exposed to 2 
kV, since this is the potential difference between the structure and the point on 
the earth where the person is standing. A second person, who is not touching the 
structure, is exposed to 1 kV, since this is the potential difference between the 
two points on the earth where the person is standing. In general, "touch voltage" is 
defined as the potential difference between a grounded metallic structure and a 
point on the earth's surface separated by a distance equal to the normal maximum 
horizontal reach (approximately 1 m). "Step voltage" is the potential difference 
between two points on the earth's surface separated by a distance of one pace 
(approximately 1 m) in the direction of maximum potential gradient. 



Touch Potential = 2 kV 






"(^ Step Potential = 1 kV 



Figure 3-43: Example of Touch and Step Voltages at an Energized Grounded Structure 



3.3.2(c) Prediction and jviitigation 

The greatest concern regarding the transfer of fault current between a faulted 
power line structure and a pipeline is whether or not there is enough energy 
available to create an electric arc through the soil. If this should occur, the current 
path through the soil to the pipeline becomes ionized resulting in much higher 
currents and current densities than would be the case during normal conditions of 
soil conduction leading to a greater risk of pipeline damage. 

The most effective means of preventing arcs during fault conditions is to maintain 
a safe separation distance between the power line structures and the pipeline. 

^NACE 



© NACE International, 2005 



C P 3-Cathodic P ro tec tion Technologist 

July 2008 



Interference 3:43 



Minimum separation distances are usually specified by both the power company 
and the pipeline company; however, safe separation distances specifically to 
prevent arcing must either be calculated or determined from research reports. One 

o 

such calculation is provided by Sunde, who provided the following equations for 
the distance r (m) over which an arc could occur, based on soil resistivity p (Q-m) 
and fault current magnitude If (kA). 

r = 0.08^Ij- -p (p< 100 Q-m); r = 0.047^If -p (p> 1000 Q-m) [3-13] 

If safe separation distances are unattainable, then screening electrodes can be 
used to intercept the fault current. These would typically consist of either lengths 
of zinc ribbon or banks of packaged sacrificial anodes coimected directly to the 
pipeline, installed between the pipeline and the power line structure. While 
screening electrodes may prevent damage to the pipe at the location of fault 
current pick-up, they lower the resistance between the pipeline and the power line 
structure, thereby encouraging fault current to use the pipeline as a current path. 
Since this could possibly increase the risk of pipeline damage at locations of fault 
current discharge, screening electrodes should be used with caution. 

It is not possible to know when, where, or how a fault will occur, so it is difficult 
to predict the effects of a fault and the mitigation required to protect both the 
pipeline and persoimel. When the probability of a fault is much higher than 
normal (e.g., electrical storms, ice storms, high winds), it is common sense to 
avoid activities involving pipe contact (e.g., CP surveys, CP installations, pipeline 
maintenance) to minimize the chance of an electrical shock. Pipeline companies 
should also take extra precautions with pipelines located in power line corridors 
to ensure that the public does not have access to pipeline appurtenances and 
cathodic protection test leads where hazardous voltages could possibly be 
contacted. The use of lockable test stations (Figure 3-44) and test stations with 
dead-front construction (Figure 3-45) should be considered. 



Sunde, E.M., "Earth Conduction Effects, " Dover Publications, 1968, p. 296-298. 

© NACE International, 2005 C P 3-Cathodic P rotection Technologist 

July 2008 



Interference 



3:44 




^ 1 iNT^afiBoviNfiii;!. 

"Pi>t*Lim;f 





Figure 3-44: L ockable T est Station 



Figure 3-45: "Dead-Front" Test Station 



Should a transient electrical shock hazard occur, the goal of a mitigation system is 
to limit the current through a person's body to a value that can be tolerated (i.e., a 
value that will not result in ventricular fibrillation). This can be done by either 
minimizing the voltage to which a person might be exposed or by raising the 
person's body resistance. 

The maximum transient (short term) current Ib a human body can tolerate 
depends on shock duration ts (seconds) and body weight and is calculated as 
follows:^ 



0.157 



( for a 70 kg body) 



0.116 



(for a 50 kg body) [3-14] 



(The above equations have been developed and tested for a transient shock hazard 
within the time interval of from 0.03 to 3 seconds in duration.) 

Considering the resistance of a human body and the contact resistance between 
the feet and the earth (which depends on soil resistivity p in Q-m), equations have 
been developed for the electrical power industry to predict the maximum voltages 
that can be tolerated from hand to feet (touch voltage) or from foot to foot (step 
voltage) for various body weights as follows:^" 



"IEEE Guide for Safety in AC Substation Grounding, " Institute of Electrical and Electronics Engineers, 
ANSI/IEEE Std 80-1996. 

"IEEE Guide for Safety in AC Substation Grounding, " Institute of Electrical and Electronics Engineers, 
ANSI/IEEE Std 80-1996. 



NACE 



© NACE International, 2005 



C P 3-Cathodic P ro tec tion Technologist 

July 2008 



Interference 



3:45 



Y 



step 7, 



(l000 + 6p) 



0.157 



Y 



step 51 



(l000 + 6p) 



0.116 



[3-15] 



V, 



touch-^f 



(lOOO + 1.5p) 



0.157 



V, 



tOUC/7<;( 



(lOOO + 1.5p) 



0.116 



[3-16] 



These voltage limit equations are derived from Ohm's Law by multiplying the 
total resistance of the shock current flow path times the magnitude of the 
tolerable shock current. The resistance estimated for the feet in contact with 
the earth is a function of the soil resistivity, as seen in the above equations, 
and assumes homogeneous soil resistivity. Addition of a thin layer of high 
resistivity surface material, such as crushed stone (3000 Q-m), can 
significantly increase this contact resistance. According to IEEE standard 80, 
with the addition of a high resistivity surface layer the foot contact resistance 
is first calculated assuming the resistivity of the high resistivity surface layer 
and then multiplied by a derating factor to account for the lower resistivity 
soil underneath. The derating factor can be estimated using Equation 3-17 
below. ^° 



0.09 



C =1- 



P 



P: 



sj 



lh+0.09 



[3-17] 



Where: 



Cs = surface layer derating factor 
p = underlying soil resistivity, Q-m 
Ps = surface layer resistivity, Q-m 
hs = thickness of surface layer, m 



Example: 

A pipeline ruiming parallel to a power line may exhibit a 
maximum voltage to earth of 500 V for a duration of 1/2 second 
during a line-to-ground fault. Calculate the tolerable touch 
potential for a 50 kg person who is touching a pipeline 
appurtenance at the time of a fault while standing on 50 Q-m soil, 
and determine an appropriate mitigative measure. 

'NACE 



© NACE International, 2005 



C P 3-Cathodic P ro tec tion Technologist 

July 2008 



Interference 



3:46 



V. 



touch <;q 



(l 000 + 1. 5p) 



0.116 



V, 



touch t, 



(lOOO + 1.5x50) 



0.116 



176V 



The maximum touch potential the person could withstand is 176 V. 
To raise the tolerable touch voltage, a 150 mm thick layer of 
crushed stone having a resistivity of 3000 Q-m is placed around 
the appurtenance. The tolerable touch potential becomes 



^.„c. =(1000 + 1.5/7, Cj 



0.116 



Where: 



0.09 



C=\ 



50 
3000 



2(0.150)+0.09 



0.77 



0.116 



KucH,, =(1000 + 1.5x3000x0.77)-^ = 732F 



which exceeds the maximum pipe to earth voltage rise; and therefore, the pipeline 
is safe. 

The touch voltage to which a person is exposed during a fault can also be 
minimized by installing a gradient control loop around the pipeline appurtenance 
and coimecting it to the pipe. This loop is generally installed at a depth of 
between 300 and 500 mm and extends approximately 1,000 mm beyond the 
perimeter of the appurtenance. The loop raises the voltage of the earth during a 
fault and minimizes the voltage difference between a hand touching the pipeline 
and the feet. The loop is generally constructed of zinc ribbon, since copper wire, 
if directly coimected to the piping, can be detrimental to the cathodic protection 
system. 

The measures described above are illustrated in Figure 3-46. 



NACE 



© NACE International, 2005 



C P 3- Cathodic P ro tec tion Technologist 

July 2008 



Interference 



3:47 



Clean Crushed Stone 
Extending Beyond 
Gradient Control Loop 

Gradient Control Loop 

(Zinc Ribbon in Select Backfill) 




Figure 3:46: Mitigation of Hazardous Touch Potentials at Aboveground Appurtenance 



3.3.3 Electrostatic (Capacitive) Coupling 

3.3.3(a) Description 

With electrostatic coupling, energy is transferred through the electrical 
capacitance that exists between the power line and the pipeline. Any two 
conductors separated by a dielectric material can be considered a capacitor. 
Capacitance is a measure of the ability to store electrical charge between two 
conductors relative to the voltage between the conductors. Capacitance is 
proportional to the area of the conductors but is inversely proportional to the 
separation between the conductors (Figure 3-47). 

Consider the case in Figure 3-48 where a pipeline is under construction. Lengths 
of pipe have been strung out along the pipeline route and placed on wooden skids 
in preparation for welding. Although this may not look like a capacitor as 
previously discussed, the elements necessary for the construction of a capacitor 
are present — two conductive plates separated by a dielectric material. In this case, 
the power line is one conductive plate, the pipe is another, and these are separated 
by air that serves as a dielectric. Similarly, a second capacitor is formed between 
the pipe and the earth, since the earth (although noimietallic) is also a conductive 
plate. A section of pipe sitting on skids beneath an AC power line can therefore 
be represented as an electrical circuit consisting of two capacitors in series with 
an AC source, which forms a capacitive voltage divider. 



NACE 



© NACE International, 2005 



C P 3-Cathodic P ro tec tion Technologist 

July 2008 



Interference 



3:48 



I 



d 




J 


Conducting Plate — . 
^ (having area /A) \. 


} 


PI d 

T 




'/ 




Conducting Plate — ^ 







Figure 3-47: Elements of a C apacitor 




Air 
Dielectric 



TTT3^ \nl im 






3 



Figure 3-48: Pipeline During Construction Represented as a Capacitive Voltage Divider 

Recalling Kirchhoff s laws, the sum of the voltage drops across the resistors in a 
series circuit (Figure 3-49) will be equal to the sum of the voltage sources. 
Furthermore, these voltage drops are in direct proportion to the resistances that 
create them. Similarly, the voltage drops across the capacitors in an AC series 
circuit (Figure 3-50) will be in direct proportion to the respective capacitive 
reactances, and their sum will be equal to the sum of the voltage sources. 
Therefore, in the pipeline construction case of Figure 3-48, the line-to-ground 
voltage of the power line is divided between the two capacitors in inverse 
proportion to their capacitances. 



NACE 



© NACE International, 2005 



C P 3-Cathodic P ro tec tion Technologist 

July 2008 



Interference 



3:49 



d 




Q 



Vi ^ Ri 
V2 R2 

Figure 3-49: Resistive Voltage Divider 



Vi 



Xci 

Xc2 



ci: 



C2: 




C2 



Figure 3-50: C apacitive Voltage Divider 



3.3.3(b) Deleterious Effects and Mitigation 

Depending on the relative capacitance values and the power line voltage, very 
large voltages (on the order of thousands of volts) can be electrostatically 
generated on a single pipe joint, assuming it is well insulated from earth. 
However, because the capacitance between the power lines and the pipe is very 
small, typically in the picofarad range (10" farads), the capacitive reactance 
between them is very large, typically in the gigaohm range (10^ Q), so very little 
energy is transferred to the pipe by this mechanism. 

Although electrostatic coupling caimot generally produce enough body current to 
create an electrical shock hazard, it can result in nuisance voltages that produce a 
sensation similar to a shock from static electricity. This could conceivably create 
a secondary safety hazard if, for instance, someone quickly overreacted to the 
sensation of a voltage while working on a pipeline project. 

While the pipeline is up on skids and well insulated from ground, electrostatic 
voltages can be easily mitigated by coimecting the pipeline to earth, even through 
a very high resistance ground coimection, as long as the ground coimection has a 
much lower resistance than the pipe-to-earth capacitive reactance. As pipe 
sections are welded together, the surface area of the pipeline increases and the 
capacitive reactances associated with the pipeline decrease allowing more 
electrical energy to be transferred electrostatically. However, at this stage of 
pipeline construction, the pipe is also being taken off of the skids and lowered 
into the trench. By decreasing the separation between the pipeline steel and the 
earth, the capacitive reactance between the pipeline and earth decreases 
significantly. 



NACE 



© NACE International, 2005 



C P 3-Cathodic P ro tec tion Technologist 

July 2008 



Interference 



3:50 



For example, if the pipeline is 500 mm above the earth when it is on skids but is 
separated from the earth by only 1 mm of coating when it is in the trench, the 
capacitive reactance of the pipe to ground decreases by a factor of 500 as does the 
electrostatically generated voltage that appears on the pipe. Furthermore, the 
pipe-to-earth capacitance is now paralleled by a pipe-to-earth resistance due to 
contact between the coated pipe and the soil. This resistance is generally so low 
relative to the power line-to-pipeline capacitive reactance that no significant 
electrostatic voltage will remain once the pipeline comes into contact with the 
earth. 

3.3.4 Electromagnetic (Inductive) Coupling 
3.3.4(a) Description 

Voltages and currents are electromagnetically induced onto a pipeline in the same 
marmer that an inductive pipe locator induces an audio signal onto a pipeline or 
the primary winding of a transformer induces current to flow through the 
secondary winding. 

First consider the flow of electric current in a simple conductor, as shown in 
Figure 3-51. The flow of current creates an electromagnetic field around the 
conductor, indicated by the lines of magnetic flux O. The intensity of the 
magnetic field is directly proportional to the current magnitude and is inversely 
proportional to the distance from the conductor. Using a convention known as the 
right-hand rule, if a person were to place his right hand around the wire, with the 
thumb pointing in the direction of current flow, the fingers would indicate the 
direction of the magnetic flux. 

Electromagnetic induction occurs whenever there is a relative motion between an 
electrical conductor and a magnetic field. This motion may either result from the 
physical movement of a conductor through a stationary magnetic field or the 
movement of a magnetic field through a stationary conductor. The most obvious 
example of the first case is an electrical generator in which a rotating coil of wire 
passes through a stationary magnetic field to generate electric current. A less 
obvious example discussed later in this chapter deals with telluric current 
interference where the tidal movement of seawater (a conductor) passing through 
the earth's magnetic field creates geomagnetic earth currents. In the second case, 
where both the source of the magnetic field and the conductor are stationary, the 
magnetic field itself must be in motion to induce current in the conductor. This is 
done by using AC current to create a time-varying magnetic field that expands 



NACE 



© NACE International, 2005 



C P 3-Cathodic P ro tec tion Technologist 

July 2008 



Interference 



3:51 



and collapses around the conductor, thereby creating a relative motion. The best 
example of this is an electrical transformer as shown in Figure 3-52. 




Figure 3-51: Electromagnetic Field Created 
by Current Flow in a Wire 



I 



^ 



t^^ 



A 






i VVVVVVVV/ 






rffT^^^^ 



AAAAAAAA 



l2f I 



A4^4k 



^ 



Figure 3-52: Flectromagnetic Induction 
in a Multi-Turn, Iron Core Transformer 



An AC current (Ii) flows through the primary winding of the transformer. This 
creates a magnetic field around each turn of the winding, and these fields link 
together to create one large magnetic field. The magnetic field around this coil 
would normally tend to stray well outside the vicinity of the coil, but by 
introducing a transformer core made of iron or some other magnetic material, the 
magnetic field becomes primarily confined to the core. A secondary winding is 
also wound onto the iron core, and the magnetic field created by the primary 
winding is now expanding and collapsing around the turns of the secondary 
winding, which consequently induces a secondary current flow I2. 

To make transformers energy efficient, the windings and cores are designed to 
transfer as much energy as possible from the primary winding to the secondary 
winding. A transformer can be formed, however, simply by placing a conductor 
within a time-varying magnetic field around another conductor (Figure 3-53), 
although such a transformer would be highly inefficient. Note that in Figure 3-53 
the induced current is indicated to flow in the opposite direction of the primary 
current; however, this is not strictly true. Lenz' law states that the induced current 
flows in a direction that creates a secondary magnetic field that tends to oppose 
any change in the primary magnetic field. Therefore it is more accurate to say that 
the induced current is out of phase with the primary current, which is what the 
arrows are intended to show. 



NACE 



© NACE International, 2005 



C P 3-Cathodic P ro tec tion Technologist 

July 2008 



Interference 



3:52 



The case of the single-turn, air-core transformer in Figure 3-53 is representative 
of the electromagnetic coupling that occurs when a pipeline runs parallel to a 
power line as shown in Figure 3-54. While the voltages generated electrostatically 
are proportional to power line voltage, the voltages and currents that are 
electromagnetically induced are proportional to power line current. As the length 
of parallelism between the pipeline and power line increases, the electromagnetic 
coupling between them improves, just as increasing the number of turns on the 
primary and secondary windings of a transformer improves the efficiency of the 
transformer. As was the case with conductive coupling, electromagnetic coupling 
can produce voltages and currents that affect both the integrity of the pipeline and 
the safety of persormel. 





Figure 3-53: Electromagnetic Induction in a 
Single-Turn, Air-Core Transformer 



Figure 3-54: Electromagnetic Induction in a 
Pipeline due to an AC Power Line 



3.3.4(b) AC Corrosion 



11 



Since at least 1916 AC current discharge from steel has been known to cause 
corrosion but at a rate that is a small fraction of what would occur for an 
equivalent amount of DC current. It was also largely believed that AC corrosion 
effects could easily be overcome by the application of cathodic protection. 
However, in the 1990s corrosion failures occurring on cathodically protected 
pipelines began to be attributed to the discharge of steady state AC currents. 

While the mechanism of AC corrosion is still not completely understood, there 
appears to be a relationship between AC current density and corrosion rate, and 
there may be a current density threshold at which AC corrosion begins to occur, 
as follows: 



Giimmow, R.A., Wakelin, R.G., and Segall, S.M., "AC Corrosion -A New Challenge to Pipeline 
Integrity, " CORROSION/98, paper no. 566 (Houston, TX: NACE, 1998). 

^NACE 



© NACE International, 2005 



C P 3- Cathodic P ro tec tion Technologist 

July 2008 



Interference 



3:53 



i ac < 20 A/m No Corrosion 

20 A/m < i ac < 100 A/m Corrosion Unpredictable 

iac> 100 A/m Corrosion Expected 

The AC current density at a circular holiday is a function of the induced AC 
voltage on the pipeline (Vac), the soil resistivity (p [Q-cm]), and the holiday 
diameter {d [cm]), and can be calculated as follows: 

8V 

1,. = — ^ [3-18] 

iac - AC current density (A/m ) 
Vac - AC Vohs (V) 
p - Soil resistivity (Q-m) 
d- holiday diameter (m) 

The AC voltage required to produce a 100 A/m current density has been plotted 
against holiday area in Figure 3-55 for a variety of soil resistivities. As an 
example, for a holiday area of 1.5 cm , an induced AC voltage of 5.4 volts would 
produce an AC current density of 100 A/m in 1000 Q-cm soil. 



1000 



100 



<u 

D) 

n 
< 




1 10 

Holiday Area (cm^) 



100 



Figure 3-55: AC Voltage Required to Produce 100 A/m^ Curratt Doisity for a 
Variety cf Hciid^ Sizes and Scil Resistivities 

Equation 3-18 indicates that AC current density increases as holiday diameter 
decreases, which suggests that AC corrosion is most likely to occur at pinholes 
rather than at large defects. It has indeed been found that corrosion rates increase 



NACE 



© NACE International, 2005 



C P 3-Cathodic P ro tec tion Technologist 

July 2008 



Interference 



3:54 



with decreasing holiday size; however, once the hoHday becomes smaller than 
approximately 1 cm in area, AC corrosion is generally not found to be a problem. 
Figure 3-56 shows an anomaly that occurred at a holiday on a cathodically 

1 ij 

protected pipeline. The site of the anomaly exhibited a polarized potential of 
-1170 mVcsE and a pH of greater than 12. When the site was cleaned, a corrosion 
pit was discovered, even though the site was apparently well protected (Figure 3- 
57). Based on the diameter of the holiday (0.01 m), the soil resistivity (16 Q-m), 
and the pipeline voltage (50 V), the AC current density was calculated to be 800 
A/m , and so the corrosion was concluded to be AC induced. 





Figure 3-56: Pipeline Anomaly Due to AC 
Corrosion (Before Cleaning) 



Figure 3-57: Pipeline Anomaly Due to AC 
Corrosion (After Cleaning) 



3.3.4(c) Electrical Shock Hazards 



13 



Electromagnetic coupling results in steady-state induced pipeline voltages, so the 
duration of a shock is not necessarily short as it is in conductive coupling. The 
tolerable voltage limits for exposure to steady state voltages are therefore much 
lower than for fault voltages. 

Various documents and standards have set the maximum allowable induced AC 
voltage to which a person should be exposed as 15 V.^"*'^^ This is based on the 



^'Wakelin, R.G., Sheldon, C, "Investigation and Mitigation of AC Corrosion on a 300 mm Diameter 

Natural. Gas Pipeline, " CORROSION/ 2 00 4, paper no. 4205 (Houston, TX: NACE, 2004). 
" "IEEE Guide for Safety in AC Substation Grounding, " Institute of Electrical and Electronics Engineers, 

ANSI/IEEE Std 80-1996. 

"Principles and Practices of Electrical Coordination Between Pipelines and Electrical Supply Lines, " 

National Standard of Canada (CAN/CSA-C22.3 N°. 6 - M91). 

^NACE 



© NACE International, 2005 



C P 3-Cathodic P ro tec tion Technologist 

July 2008 



Interference 



3:55 



assumptions that the average resistance of a human body is 1,000 ohms, and that 
an average person can withstand a maximum current of 15 mA for a prolonged 
period of time. Such a current may be painful and may in fact cause muscular 
contractions that would prevent a person from letting go of an energized structure 
but isn't expected to result in breathing difficulties. 



The 15 V exposure limit is conservative since it ignores the hand and foot contact 
resistances present in the actual shock current circuit. The actual voltage 
maximum based on the "let-go" current limit can be calculated using Equations 3- 
15 and 3-16 by replacing the transient current limit. Equation 3-14, with the 
steady-state let-go current limit of 15 mA. This is illustrated by the following 
equations. 



V,ep = (l000 + 6p)0.015 



[3-19] 



V.0,., = (l000 + 1.5p)0.015 



[3-20] 



3.3.4(d) Prediction 

A pipeline can be modeled for cathodic protection purposes as a network of series 
resistances representing the per unit longitudinal resistance of the pipe (Rl) and 
parallel resistances representing the pipe's per unit shunt resistance to earth (Rs). 
When determining the pipe's response to AC interference, two other factors must 
be considered — the pipe's longitudinal inductance (Ll) and its shunt capacitance 
(Cs) as shown in Figure 3-58. Because of these two additional factors, an AC 
current traveling along a pipeline sees a greater longitudinal pipeline impedance 
than does a DC current and also a lower shunt impedance to earth, which means 
that an AC signal attenuates more rapidly along a pipeline than a DC signal. For 
simplicity, the longitudinal resistance and the inductive reactance can be 
combined to form a longitudinal impedance (Zl). Similarly, the shunt resistance 
and capacitive reactance can be combined to form a shunt impedance (Zs). These 
simplifications are shown in Figure 3-59, but for reasons of symmetry, the shunt 
impedance (Zs) has been split into two shunt impedances of 2Zs in parallel. 



NACE Standard SPOl 77 (latest version), "Mitigation of Alternating Current and Lightning Effects on 
Metallic Structures and Corrosion Control Systems, " (Houston, TX: NACE). 



NACE 



© NACE International, 2005 



C P 3- Cathodic P ro tec tion Technologist 

July 2008 



Interference 



3:56 




Figure 3-58: AC Model of Pipe Section 




Figure 3-59: Simplified AC Model of Pipe Section 



To determine how voltages behave on a long pipeline, pipe sections as shown in 
Figure 3-58 can be joined and the voltages and currents can be determined using 
electrical network analysis techniques. As a simple example, consider the pipeline 
model in Figure 3-60 consisting of only two pipe sections. Assuming the 
electromagnetic field to which the pipeline is exposed is uniform along the length 
of the pipeline, then Vi = V2 . Furthermore, it can be shown using KirchhofP s 
laws that Ii = I2 , but this should also be apparent from the symmetry of this 
simple case. The network in Figure 3-60 can therefore be simplified to the one 
shown in Figure 3-61. 



NACE 



© NACE International, 2005 



C P 3-Cathodic P ro tec tion Technologist 

July 2008 



Interference 



3:57 



^ Vi Zl g V2 Zl q a ^ ^^ B ^ ^'^ C 



Figure 3-60: Network Analysis of 
Two-Section Pipe Model 



Figure 3-61: Network Analysis of Simplified 
Two-Section Pipe Model 



The voltages at each end of the pipeHne and at the midpoint can now be 
calculated. 



Va = - 2ZsI = -2ZsI 



Vr = + ZsI-ZsI = 



Vr 



+ 2ZsI = 2ZsI 



Plotting these voltages gives the AC voltage profile shown in Figure 3-62. Note 
that AC voltages do not really exhibit polarity as shown in the plot. What polarity 
indicates in this case is that the AC waveform at one end of the pipeline is 180° 
out of phase with the waveform at the other end of the pipeline. If an AC 
voltmeter was used to measure voltages to earth at points A, B, and C, the profile 
in Figure 3-63 would be observed, but keeping in mind the voltage at point A is 
180° out of phase with the voltage at point C, then the voltage between these two 
points would be twice the voltage at either point to ground. 



+2ZJ 



-2ZJ 





Figure 3-62: AC Voltage Profile Along Pipeline 
with 'Polarity" Indicated 



Figure 3-63: AC Voltage Profile Along Pipeline 
as Measured Using AC Voltmeter 



Figure 3-63 indicates the actual voltage profile that would be seen along a well 
coated pipeline where the electrical characteristics of the pipe (Zl , Zs), the soil 
resistivity, and the electromagnetic field strength are all uniform along the length 
of the pipeline. A key assumption here is that the pipeline is well coated. When a 
pipeline is well coated, the series combination of all of the longitudinal 
impedances is insignificant compared to the parallel combination of all of the 
shunt impedances. In other words, for a pipeline consisting of N identical 
sections, N'Zl « Zs/N. When this condition holds true, an AC signal entering the 
pipeline at one end will disperse to ground uniformly along the length of the 



NACE 



© NACE International, 2005 



C P 3-Cathodic P ro tec tion Technologist 

July 2008 



Interference 



3:58 



pipeline, since there would be no significant voltage drop along the pipeline; that 
is, the AC signal attenuates linearly. In this case, we say that the pipeline is 
"electrically short" (i.e., it behaves like a short piece of well coated pipe) rather 
than simply saying it is well coated. 

On the other hand, when a pipeline is either poorly coated or is well coated but so 
long that the series combination of longitudinal impedances can no longer be 
ignored, we say that the pipeline is either "lossy" or it is electrically long. In this 
case, if an AC signal is injected into one end of a pipeline, more current will 
discharge to earth at the injection end than at the far end due to the longitudinal 
impedance of the pipe. On an electrically long pipe, the AC signal would 
attenuate exponentially rather than linearly. This exponential signal attenuation is 
similar to what we see with cathodic protection currents, except that the AC 
attenuation constant that determines the shape of the attenuation curve is different 
from the DC attenuation constant, since it is a function of pipeline inductance and 
shunt capacitance and not just the longitudinal and shunt resistances. 
The effect of electrical length on the AC voltage profile is shown in Figure 3-64. 
Note that electrically long pipelines subjected to electromagnetic coupling can 
exhibit zero voltages over much of their length provided the electromagnetic field 
and the electrical characteristics of the pipeline and the soil are uniform along this 
length. 



Pipeline Becoming 
Increasingly Lossy 




Figure 3-64: Effect of Electrical Length of Pipeline on AC Voltage Profile 

The electromagnetic field responsible for inducing voltages and currents onto the 
pipeline is usually referred to as the longitudinal electric field (LEF), which has 
the units of volts per meter (V/m). The LEF is represented by the symbol E and is 
a complex number, meaning it has a magnitude and a phase angle. Calculating the 



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LEF the pipeline is exposed to is a complicated mathematical task. In general the 
LEF is proportional to power line current and inversely proportional to pipeline- 
to-power line separation. The LEF is also a function of how the power lines are 
arranged on the tower. A set of curves is available that provides the LEFs for 
standard power line configurations from which induced AC pipeline voltages can 
be calculated. ^"^ 

Alternatively, a procedure used for very simple pipeline-power line geometries 
can be used to estimate pipeline voltages. For the case shown in Figure 3-65, 
where a pipeline runs parallel to a power line at a fixed distance (d) and both ends 
of the pipeline are terminated with insulators, the LEF experienced by the 
pipeline can be estimated in the field using the procedure shown in Figure 3-66. 
An insulated conductor is laid out along the ground at a distance (d) from the 
centerline of the power line. If it is an existing pipeline, the wire should be placed 
on the opposite side of the power line from the pipeline so any fields created by 
the pipeline do not interfere with those of the power line. One end of the 
conductor is grounded, and the AC voltage at the other end of the wire is 
measured to earth. The induced voltage measured in the wire, divided by the 
length of the wire, is approximately equal to the magnitude of the LEF as seen by 
the pipeline. 




Figure 3-65: Simple Pipeline-Power Line Corridor 
(Plan View) 



Figure 3-66: Field Fstimation of LFF 



Current loading of the power lines can change significantly from hour to hour, 
from day to day, or seasonally. The time and date at which the LEF is measured 
should be recorded and, if possible, it should be correlated with the actual power 
line loading (available from the power company). For instance, if the power line 
was operating at 50% maximum load at the time of the measurement, it could be 



"Power Line-Induced AC Potentials on Natural Gas Pipelines for Complex Rights-of-Way 
Configurations, " Electric Power Research Institute, EPRI Report No. EL-3106. 



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assumed that the LEF would be double the measured value during periods of 
maximum loading. 

Once the magnitude of E has been measured, the voltage peaks on the pipeline 
can be calculated. As was shown in Figure 3-63, the voltage peaks for the simple 
pipeline-power line geometry shown in Figure 3-65 will always be at the 
insulators, with a zero voltage occurring at the midpoint. In the case of an 
electrically short pipeline, the voltages at the insulators can be calculated using 
Equation 3-21. The voltage profile for this ideal case is shown in Figure 3-67. 



Y 



E • L 



0,L 



[3-21] 



E - Longitudinal Electric Field [LEF] (V/m) 
L - Length (m) 

If the electrical characteristics of the pipeline are not symmetrical, the voltage 
peak at one end of the pipeline may be higher than that predicted by Equation 3- 
21, and the peak at the other end would be lower, as shown in Figure 3-68. The 
most extreme case occurs if one end of the pipeline is perfectly grounded (to a 
zero ohm ground) and the other end is insulated. In this case, the pipe voltage at 
the grounded end would be zero, but since the voltage induced from one end of 
the pipe to the other remains unchanged, the voltage at the opposite end will rise 
to twice what it was in the case of uniform conditions, as shown in Figure 3-69. It 
is important to remember that when mitigating induced voltages, the 
indiscriminate use of grounding electrodes may actually increase voltages. 

If grounds are applied to both ends of the pipeline or if grounds are uniformly 
distributed along the pipeline, then all voltages can be mitigated to less than those 
predicted by Equation 3-21, as shown in Figure 3-70. 



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Figure 3-67: AC Voltage Profile Along 

an Electrically Short Pipeline 
(Uniform Conditions - No Grounding) 




Figure 3-68: AC Voltage Profile Along 
an Electrically Short Pipeline 
(Non-Zero Resistance G round at Distance = 0) 




|E|.L 



-|E|-L 



Figure 3-69: AC Voltage Profile Along 

an Electrically Short Pipeline 
(Zero Resistance G round at Distance = 0) 



|E|-L- 



Mil. 

2 



|E|-L 

2 



-|E|-L- 




Figure 3-70: AC Voltage Profile Along 
an Electrically Short Pipeline 
(G rounds Evenly Distributed or at Both Ends) 



It is important to note that the value I E | • L represents an absolute limit of the 
maximum induced voltage that can appear on a pipeline, regardless of whether 
the pipeline is electrically long or short or where the grounds and insulators are 
installed. For example, if the field strength along the pipeline is 10 V/km under 
maximum loading conditions, and the pipeline parallels the power line for 5 km, 
then the maximum voltage that can appear on the pipeline is 50 V. However it is 
more likely that this voltage would be more evenly distributed with 
approximately 25 V appearing at each end and V appearing near the pipeline's 
midpoint. 

As the pipeline increases in length, it becomes more lossy and instead of seeing a 
linear voltage profile as shown in Figures 3-67 through 3-70, we begin to see an 
exponential change in voltages, as was shown in Figure 3-64. Furthermore, 

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Equation 3-21 predicts that as pipeline length increases, pipeline voltages 
increase proportionately and without limit, which is not the case when the 
pipeline is lossy. To calculate voltages on an electrically long pipeline, the 

pipeline's length — in Equation 3-21 is replaced by the pipeline's electrical 
length, 1/r, resulting in Equation 3-22. 



V, 



0,L 



r 

LEF (V/M) 



[3-22] 



1 



r - Propogation Constant ( — ) 

m 

The parameter Y is known as the pipeline propagation constant. It is a constant 

related to the electrical characteristics of the pipeline and is closely coimected to 

the pipeline's AC attenuation constant. The value of F can be calculated, however 

it is a complicated function of pipe depth, soil resistivity, AC frequency, and 

coating resistance, as well as pipe diameter, wall thickness, and material. 

1 7 

Alternatively, the value of Y can be obtained from tables or graphs. Figure 3-71 
shows the AC voltage profile along a pipeline for the simple pipeline-power line 
geometry shown in Figure 3-65, where the pipeline is electrically long or lossy. It 
was found in the case of an electrically short pipeline that grounding one end 
could actually increase voltages at the opposite end. In the case of an electrically 
long pipeline, however, grounding one end of the pipeline would have no effect 
on the other end, since the two points are electrically remote from one another 
(Figure 3-72). 



"Power Line-Induced AC Potentials on Natural Gas Pipelines for Complex Rights-of-Way 
Configurations, " Electric Power Research Institute, EPRI Report No. EL-3106. 



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0- 



M 
r 



L/2 
Distance 



L/2 
Distance 



Figure 3-71: AC Voltage Profile Along 
an Electrically Long or Lossy Pipeline 
(Uniform C onditions - No G rounding) 



Figure 3-72: AC Voltage Profile Along 

an Flectrically Long or Lossy Pipeline 

(Zero Resistance Ground at Distance = 0) 



As previously mentioned, electrically long pipelines may exhibit zero voltages 
over much of their length provided the electromagnetic field and the electrical 
characteristics of the pipeline and the soil are uniform along this length. If any 
one of these parameters changed at some point along the pipeline (referred to as 
an electrical discontinuity), an additional voltage peak would be introduced at 
that point. Although the magnitude of this voltage peak would depend on the 
severity and nature of the discontinuity, it could possibly create an additional 
peak of V = YiEYzG. This is an important aspect of electrically long pipelines. 

To better illustrate the difference between electrically long and electrically short 
pipelines, consider the case of a pipeline several hundred kilometers long running 
west across North America from the Atlantic coast. The pipeline is paralleled by a 
power line for the entire distance, and the electrical characteristics of the pipeline, 
power line, and the earth are uniform along the entire route. Because the pipeline 
is electrically long, voltage peaks would be created at each end of the pipeline 
having a voltage of V = I E | / F, whereas the majority of the pipeline would 
exhibit a zero voltage. Now consider what happens when the pipeline is extended 
to the Pacific coast, again assuming that all conditions remain uniform along the 
length of the pipeline. Even though the pipeline is now perhaps ten times longer, 
voltage peaks still exist only at the two ends of the pipeline, and the magnitude of 
these voltage peaks remains limited to V = I E | / P. 

The installation of insulators will introduce additional voltage peaks on the 
pipeline. On an electrically short pipeline, the installation of an insulator 
essentially creates two electrically separate pipelines having smaller voltage 
peaks than the original pipeline, since the voltages are proportional to the 
physical lengths of each pipe section (Figure 3-73). On an electrically long 

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pipeline, however, the magnitudes of the vohage peaks are unaffected, as long as 
any new pipe sections created by the insulators may still be considered 
electrically long (Figure 3-74). 





Figure 3-73: AC Voltage Profile Along 
Electrically Short Pipeline with 
Insulator at Midpoint 



Figure 3-74: AC Voltage Profile Along 

Flectrically Long Pipeline with 

Insulator at Midpoint 



There is one other important aspect regarding the effects of electrical insulators 

on induced AC pipeline voltages. As shown in Figure 3-73, an insulator installed 

at the midpoint of an electrically short pipeline reduces peak voltages to one-half 

their previous values. However, the voltage that appears across this insulator is 

double the voltage that appears between the pipeline and earth. This is because 

the voltages on either side of the insulator, while equal in magnitude, are of 

opposite polarity or, more accurately, are 180 degrees out of phase with one 

another. In the case of an electrically long pipeline, voltage peaks are not reduced 

by the installation of an insulator, so the voltage that occurs across the insulator 

2|e| 
will be double the peak value or -^. Therefore, as was the case with electrical 

grounds, electrical insulators must not be used indiscriminately on a pipeline 
affected by induced AC interference, since this could introduce new voltage 
peaks and could create voltage differences twice as severe as what could exist 
between the pipeline and ground. 

The prediction of induced AC voltages, as discussed above, requires knowing if 
the pipeline is electrically short or electrically long. If this is not known or if the 
pipeline falls somewhere in between these two cases, more complicated 
calculation procedures are required. Also, when there are changes in the power 
line circuit configuration or when the separation distance between the pipeline 
and power line changes significantly, the LEF will no longer be constant 



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along the pipeline. Equations 3-21 and 3-22 cannot be used under these 
conditions and the additional voltage peaks created cannot be accounted for. Such 
problems can be very difficult to solve and require either laborious analytical 
treatments or the use of specialized software packages. 

3.3.4(e) Mitigation 

To mitigate electromagnetically induced AC voltages to safe levels the pipeline 
must be grounded. Grounding the pipeline has the same effect as replacing a good 
coating with a poor coating; that is, by lowering the value of Zs in Figure 3-59 the 
voltages produced by the flow of AC current from the pipeline to earth are 
reduced. 

Ground electrodes may consist of either packaged sacrificial anodes, sacrificial 
anode ribbons installed in special backfill, or conventional grounding materials 
such as ground rods and cables. Materials that are not anodic to the pipeline, such 
as copper cables, would seriously affect the effectiveness of the cathodic 
protection system if directly coimected to the pipeline. Such materials should 
therefore only be used if they are DC decoupled from the pipeline using a suitable 
device, such as a polarization cell, or a solid state alternative to a polarization cell 
(Figure 3-75). 



Ground electrodes may be evenly 
distributed along the pipeline, such as by 
installing packaged sacrificial anodes at 
regular intervals; however, it is often 
more effective to concentrate the ground 
electrodes at electrical discontinuities 
where the voltage peaks tend to occur. 
The effects that various mitigation 
systems have on induced AC voltages is 
difficult to predict without complex 
calculations or the use of specialized 
software and, as was shown previously, 
the indiscriminate use of ground 
electrodes may actually increase induced 
voltages at some points along a pipeline. 




Figure 3-75: Solid State DC Decoupling Device 



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Grounding is also used to minimize the risk of AC corrosion damage to the 
pipeHne. By reducing induced AC vohages, the current densities at coating 
hoHdays as predicted by Equation 3-18 can be reduced. As a worst case, it is 
normally assumed that a coating holiday has a diameter of approximately 1 cm. 
Voltages are then mitigated to levels that would reduce AC current densities to 

9 9 

less than 100 A/m and preferably to levels less than 50 A/m . 



3.4 Telluric Current Interference 

Telluric currents are currents that are geomagnetically induced in the earth and in 
metallic structures on the earth, such as power lines and pipelines, as a result of 
the interaction of solar particles on the earth's magnetic field as shown in Figure 

3-76. 




Figure 3-76: Interaction of Solar Particles with the Earth's Magnetic Field 

Charged solar particles entering the earth's atmosphere are deflected by the 
earth's magnetic field creating current rings in the ionosphere centered around the 
north and south poles. These current rings typically contain more charges than are 
generated by man on earth. Because of the amplitude variation and directional 



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changes in this electrojet current, a changing magnetic field is produced that 
induces an electric field in any metallic conductor on or near the earth's surface 
as depicted in Figure 3-77. 



/' 



Varying Magnetic Field 



r 



I \ I 

ionospheric Current 



f 







\ 1 


\ 

\ 






I 




/ _ 


\ 












+ % 




\ 


► C \ 


^ Cr \ 






^ J 




N. 


J 







Figure 3-77: Schematic of G eomagnetic Induction Directiy into a Pipeline and the 
Resulting Change in Pipeline Potential Produced 

Source: Boteier, D.H., Gummow, R.A. and Rix, B.C., Evaluation of Telluric Current Effects on the Maritimes and 
Northeast Pipeline, NACE Northern Area Eastern Conference, Ottawa, October 1999, Paper No. 8A.3, p. 8 

This induction process is similar to that caused by AC power lines, except the 
frequency and amplitude vary considerably due to many factors such as 

• the solar cycle, which is a period of approximately 1 1 years between peaks 
of solar activity 

• the sun's rotational frequency - 27 days 

• the Earth's daily rotation - 24 hours (diurnal cycle) 

• tidal fluctuations (-12.5 hours) 

• direction of the magnetic field in the solar particle plasma 

3.4.1 Interference Effects 

The magnetic variations show up as current and potential fluctuations on 
pipelines. An example of the latter is shown in Figure 3-78. 



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3000 1 




Figure 3-78: Example of G eomagnetically Induced Potential Fluctuations on a Pipeline 

This stray current pattern reflects tidal related fluctuations, general randomness of 
the telluric activity, and a more energetic magnetic storm (days 7 and 8). 

Short-term positive excursions in potential do not represent a significant 
corrosion risk on a cathodically protected pipeline, but if the excursions occur 
daily, as with diurnal or tidal influences, serious corrosion can occur. The 
pipeline induced voltage versus distance profile is much the same as with AC 
induction. As illustrated in Figure 3-79, the longer the pipeline is and the better 
coated it is, the greater the induced voltage (Vp g) will be. 



Vp,g 0- 




poorly coated^ 



well coated 



V, 



p.g 



Figure 3-79: Typical Gecina^ieticaDy Induced Patentiainxfile en a Wdl Coated 
and Poorly Coaied Pipeiine 



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This shows that a pipeHne that is electrically lossy (i.e., leaks charges to earth) 
will have smaller amplitude fluctuations. Unfortunately, for cathodic protection 
purposes a very high resistance coating is desirable and improves cathodic 
protection current distribution. 

Geomagnetically induced voltage peaks appear on a pipeline: 

• at isolating joints 

• at changes in direction 

• as pipeline length increases 

• as pipeline coating resistance increases 

• as the pipeline location is closer to the earth's magnetic poles (but not at 
the poles) 

• at or near a sea coast 

• at or near sharp geological transitions or anomalies 

• in high resistivity soil 

Although one severe case of corrosion caused by telluric current has been 
reported, the major effect of telluric currents on pipelines is in the ability to 
accurately measure the pipeline polarized potential (Ep). The telluric current 
activity produces an additional voltage drop in the earth (Vt) between the 
reference and the pipe holidays as illustrated in Figure 3-80. 



test station 



grade 



pipe test iead 




Icp ±h 



-volt 



— portable reference 
electrode 



No Telluric Activity 


Vm = Ep + Ve 




With Telluric Activity 


Vm = Ep + Ve ± Vj 



Figure 3-80: Typical Pipe-to-Soil Potential Measurement Situation Where 
Telluric Current Activity is Present 



Brochu, B., NACE Northern Area Eastern Conference, Quebec City, August 2003. 

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Whereas the vohage drop (Ve) in the earth due to cathodic protection current can 
often be interrupted, the telluric voltage drop (Vt) cannot. This can introduce a 
significant error in the potential measurement (Vm) as shown in Figure 3-81. 



o 

CO 

3 

o 

"3 
o 

(0 

> 



c 

0) 

o 




E 
u 

< 

E 



c 

0) 

Q 

4-1 
C 

0) 

3 

O 



03:50 03:55 04:00 04:05 04:10 



04:15 04:20 04:25 04:30 04:35 04:40 04:' 

Time 



Figure 3-81: Current Flow and Calculated Off Potentials During a G IC Incident 

Source: Redrawn from Hesjevik, S.M. and Birketveit, 0., Telluric Current on ShortGas Pipelines in Norway 
- Risk of Corrosion on Buried Gas Pipelines, NACE Corrosion 2001, Houston, TX, Paper #313, p. 6 

With the calculated telluric voltage drop removed from the "On" potential 
measurement, during the positive potential excursions the calculated "Off 
potential is not more positive than about -0.25Vcse, whereas the measured"On" 
potential goes more positive than zero on two occasions. The measured coupon 
current direction also reverses on these two occasions for a total of about 20 
minutes during which one would expect corrosion to take place, although the 
corrosion rate is not as calculated by Faraday's law. 

As with most dynamic stray current interference, an investigation of the effects 
requires considerable potential and current recordings. But measuring an accurate 
polarized potential in the presence of an uninterruptible stray current requires 
special procedures. 



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Coupons facilities that do not require disconnection of the coupon to measure a 
true potential can be used at test station locations, an example of which is shown 
in Figure 3-82. 




cylindrical 
steel coupon 
53 cm2 



plastic 
sleeve in 
side port 



bottom 




t 




port 












6.0 cm 





removable 
plug 



Figure 3-82: Example Test Station in Which Coupon Does Not Require Disconnection to 
Minimize IR Drop Error in the Potential Measurement 

Nekoksa, G. and Turnipseed, S., "Potential Measurements on Integrated Salt Bridge and Steel Coupons" 
CORROSION/'96, paper no. 206 p.9 (Houston, TX: NACE, 1996). 



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The benefit of such facihties is shown in Figure 3-83. In this case it would be 
difficuh to evaluate the level of polarization based on the potential-time plot with 
respect to a copper-copper sulfate reference electrode placed on grade. 



-500 



-1000 



-1500 



-2000 



-2500 



-3000 4 




21:00 



23:24 



Figure 3-83: Example of Difference in Potential-Time Recording Between Reference Electrode 
at Grade and Inside a Soil Tube with a Coupon (e.g., Eigure 3-82) 

Despite the large potential fluctuations recorded by a surface reference electrode, 
the polarized potential remained relatively steady. 

Difficulty in minimizing the telluric current voltage drop is also a problem when 
conducting close interval surveys. One technique that has been used to account 
for the telluric IR drop error in the measurement, as shown in Figure 3-84, 
involves using two data loggers or a single data logger with two charmels. 
Potentials are recorded with respect to a fixed reference electrode (Vf) and a 
moving reference electrode (Vm). 



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Moving 
Datalogger 



Synchronized 



Fixed or 

Moving 

Datalogger 



■/A\'//v/Av/x\y/A\y/-o 

Moving Fixed 

Reference Reference 



Vps =Vm ±Wf 

where:^Vf,t= Vf,avg±Vf,t 



Survey Lengtti 



Figure 3-84: Pipe-to-Soil Potential Measurement Method to Compensate for Telluric Current 

Effects During a Close Interval CP Survey 



Before beginning the survey, potentials are recorded with respect to the fixed 
electrode to establish an average potential (Vf avg) of the structure at the fixed 
electrode position. Then the potential recorded by the moving reference at any 
time (t) is corrected by the change in potential (AVf) that occurs at the fixed 
electrode at the same time. Hence for any time (t): 



Vp,t = V^, ± AVf,t 



where: 



AVft = Vfav, ± V 



f,t 



[3-23] 
[3-24] 



the accuracy of this method diminishes as the distance between the moving 
reference and the fixed reference increases. Using a fixed reference at the start 
and finish of the close interval survey and extrapolating the changes in both fixed 
references (AVt i and AV£2) linearly with distance between the moving electrode 
and fixed electrode improves the accuracy. 



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3.4.2 Mitigation of Telluric C urrent E ffects 

Mitigating telluric interference is similar to mitigating AC interference since both 
are induced currents. Providing safe low resistance leakage paths to earth without 
compromising the level of cathodic protection is the principal objective. 

Galvanic anodes, distributed along the pipeline route, can be effective in 
minimizing telluric stray current corrosion effects provided the soil resistivity is 
low enough for the anode to produce a significant current. As depicted in Figure 
3-85, some of the telluric current (I't) will leak to ground via the galvanic anode. 




I jii residual telli 
T ^ current disc 



J 



telluric 
ischarge 

galvanic anode 



yi telluric current discharge 
^ from galvanic anode 



where: It = It + l" + It" 



Figure 3-85: Mitigatioii of Telluric Current Discharge Effects Using G alvanic Anodes 



The effectiveness of this method relies on the anode cathodic protection current 
(Icp) being greater than the residual telluric discharge current (It")- Typically 
distributed galvanic anodes can lower the pipe-to-earth resistance of a well coated 
pipeline by more than an order of magnitude. 

If impressed current cathodic protection is preferred, potential controlled 
transformer-rectifiers should be used as schematically illustrated in Figure 3-86. 



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Potentially Controlled 
DC Power Supply 




Icp 
and 

It 



Buried 
Reference 
E lectrode 



Remote 
Groundbed 



Figure 3-86: Schematic of Potentially Controlled Cathodic Protection System Used 
to Mitigate Telluric Current Effects 



When the buried reference electrode senses a positive shift in the pipe potential, 
the output voltage of the power supply is automatically increased, which makes 
the path through the rectifier to earth appear as a negative resistance to the telluric 
current. Thus, there should be no residual current discharge at this location during 
periods when the pipe tends to discharge the interference current unless the 
maximum current or voltage output of the power supply has been reached. The 
power supply and groundbed must be properly sized and rated to accommodate 
the anticipated telluric current activity. 

When the pipeline is subjected to the negative telluric half-cycle (current pick-up 
period), the power supply will shut down since the pipeline will be receiving free 
cathodic protection. The typical operating characteristics of the impressed current 
mitigation system are shown in Figure 3-87. 



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Figure 3-87: Pipe Potential and Rectifier Current Output vs. Time for an 
Impressed Current System Operating in Potential Control 

This graph shows the current output and pipe potential with respect to a buried 
zinc reference electrode as a function of time. Note that when the potential 
attempts to shift more positive than -lOOmVzRE the rectifier produces a current 
output, and when the potential is more negative than -lOOmVzRE the rectifier 
output is zero. The autopotential control effectively clips the positive telluric 
excursions at the set point of -lOOmVzRE- 



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Experiments-! 
To Demonstrate DC Inteiference and Its Mitigation 



steel 
rod — 




^ll^V\Ma>^ 



9V 10 ohm 
Experiment Schematic No. 1 



Procedure 



Step: 

A. Place bare steel rod along one end of the tub in 5 cm of water obtained from 
the cold water tap. Coimect the 9V battery, 10 ohm resistor, ammeter, and 
switch in series between the steel rod and magnesium anode. Close the 
switch and allow the CP system to operate for a minimum of 5 minutes. 

B. Measure and record the potential on the steel rod with the reference 
positioned at locations 1, 2, and 3. Record the current. 

C. Open the switch and insert the second steel rod (foreign structure) 
perpendicular to the first steel rod at location 2. 

D. Measure and record the foreign structure potential at reference locations 4, 
5, and 6 with switch remaining open. 



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E. Close the switch and allow the CP system to operate for a minimum of 5 
minutes. 

F. Measure and record structure potentials at all reference locations on both 
structures and record the CP current. 

G. Calculate the shift in potential at locations 4, 5, and 6 on the foreign 
structure and the change in CP current. 

Discussion Bmak 

H. Mitigate interference using a resistance bond connected between the 
cathodically protected structure and the foreign structure as in Schematic 
No. 2. Adjust resistance bond until the foreign structure potential at location 
4 is equal to or more negative than its native potential. 



resistance bond -- 



steel 
rod 




^ pL^^^vVKA)^-^ 



9V 10 ohm 
Experiment Schematic No. 2 



I. Measure and record potentials on both structures. At all reference locations, 
measure CP and mitigation current and record bond resistance. 

Discussion Bmak 

J. Disconnect the resistance bond and install a galvanic anode to mitigate the 
interference as shown in schematic No. 3. 



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steel 
rod 




9V 



W^H^ 



Experiment Schematic No. 3 

K. Measure and record all structure potentials, CP current, and galvanic 
interference current (Igaiv )• 



Discussion Bnsak 



Results 





Structure Potentials (iuVcse) 






STEP 


CPed Structure 


Fore gn Structure 


Icp 




1 


2 


3 


4 


5 


6 




B 








X 


X 


X 




CPed sti-ucture only 


D 


X 


X 


X 








X 


Foreign sti-ucture only 


F 
















Both structijres 


G 


X 


X 


X 










Shift calculations 


Mitigation 






1 
















lb = Rb = ohm 


K 
















J-galv 



NACE 



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C P 3-Cathodic P rotection Technologist 

July 2008 



Interference 



3:80 



Conclusions 



1 . Foreign structure potential shifts electropositively at the stray current 
discharge location (#4). 

2. Foreign structure potential shifts electronegatively at the stray current pick-up 
location (#6). 

3. The cathodic protection current distribution on the CPed structure is affected 
by the presence of the foreign structure. 

4. The cathodic protection current increases when the foreign structure is 
present. 

5. The resistance bond mitigates the stray current interference on the foreign 
structure. 

6. The cathodic protection current increases with the resistance bond inserted but 
the CPed structure is less well protected. 

7. A galvanic anode mitigation system can mitigate the interference problem and 
maintain Ecorr on the foreign structure. 

8. The stray current magnitude is greater for the resistance bond than for the 
galvanic mitigation system. 



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C P 3- Cathodic P ro tec tion Technologist 

July 2008 



CHAPTER 4 

CP DESIGN FUNDAMENTALS 



4.1 Design Objectives 

The principal cathodic protection design objectives are listed below: 

• Provide sufficient current density continuously to all parts of the 
structure to polarize the structure to an acceptable criterion. 

• Minimize interference effects to other metallic structures. 

• Provide sufficient operational flexibility to accommodate expected 
changes in the enviroimient, the coating, and the operation of the 
structure during the system service life. 

• Ensure the safety of the public and operating personnel and adhere to 
all applicable codes and standards. 

• Provide a system design life commensurate with the required life of 
the structure or system service life as stipulated by the owner or 
regulator. 

• Provide testing facilities and monitoring equipment so performance 
of the cathodic protection system can be tested and monitored with 
respect to industry standards and regulations. 

The cathodic protection design process for some structures can be very complex, 
requiring many iterations before arriving at an acceptable design. An example of 
a design procedure for a pipeline is outlined in the flow chart of Figure 4-1. 



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January 2007 



CP Design Fundamentals 



4:2 



START 



> 



Galvanic 



Choose Anode Type, 

Size, Weight, & 

Arrangement 



i 



Calculate Resistance 
of Anode or 
Groundbed 



i 



Calculate Number 

& Spacing of 

Anodes or Groundbeds 



i 



Calculate 
System Life 



i 



Estimate Installed 
Cost of System 



Evaluate All 

Structural and 

Environmental Factors 



Determine Current 

Requirements to 

Achieve Desired Criterion 



; 



Choose 

Between Galvanic & 

Impressed Current 

System 




NO 

_l_ 



Is Design 
Acceptable ? 



YES 



i 




Impressed 



Current 




DESIGN COMPLETE 



Choose Anode Type, 

Size, Weight, & 

Arrangement 



i 



Calculate Resistance 
of Anode or 
Groundbed 



i 



Choose Power Supply, 
Type & Rating 



i 



Calculate 
System Life 



I 



Estimate Installed 
Cost of System 



Figure 4-1: An Example of a Cathodic Protection Design Procedure 



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CP Design Fundamentals 



4:3 



The first step in the design process is to collect and evaluate pertinent data regarding 
the structure, its operational characteristics, and its environment. For a new pipeline 
cathodic protection project, the possible data set would include the following: 

• Pipeline Information 

— construction specifications 

— pipe material, diameter, wall thickness, and pressure rating 

— pipe and joint coating type, thickness, and conductance 

— temperature, conductivity, and operating pressure ranges inside of 
medium inside pipeline 

— pipe route, number, and location of interconnections to related piping 
systems and feasibility of installing electrical isolation 

— electrical continuity of the pipeline 

— AC power availability and road access to possible CP facilities 

— number, length, and location of pipe casings 

— number, location, and operation of mainline valves and coating type 

— number, location, and extent of stations (e.g., compressor, pumping, 
metering, etc.) 

— number, length, and location of directionally drilled sections 

• Environmental Information 

a) Soil Conditions 

— soil resistivity along the route at pipe depth 

— geotechnical information (soil types and depth) 

— moisture variability (seasonal) 

— temperature variability 

— pH variation 

b) Electrical Interference Considerations 

The presence, location, and operational characteristics of the 
following systems should be reviewed: 

— impressed current cathodic protection systems on foreign 
pipelines or other structures 

— electrical transit systems, high-voltage DC power lines, welding 
operations, and any other electrical system that uses the earth 
indirectly or directly as a current path 

— AC power transmission or distribution systems paralleling the 
pipeline or whose tower legs or electrical grounds are adjacent 
to the new pipeline 



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CP Design Fundamentals 



4:4 



— foreign metallic structures and their susceptibility to interference 
damage 

— susceptibility of the pipeline to telluric or ocean currents. 

Other Information Required 

— assess appropriate protection criteria 

— polarization characteristics to assess current requirement 

— structure attenuation characteristics 

— regulatory and permit requirements 

Typical Field Surveys 

— soil resistivity along pipeline route at pipeline depth and at a greater 
depth at prospective impressed current groundbed and galvanic anode 
locations 

— visual inspection of pipeline route for availability of electrical power 
for impressed current systems 

— induced AC test on an insulated wire along pipe route that parallels 
AC power lines 

— locating long-line corrosion "hot-spots" on bare piping. 



4.2 Determining Current Requirements 
4. 2. 1 Current Density 

As shown in Figure 4-2, the polarized potential for steel varies widely over a 
current density range of 0.1 to 200 mA/ft depending on soil conditions. Data in 
this figure were obtained by measuring the instant-off potential and current on 
bare steel coupons at 8 sites within the U.S. Note that the polarized potential at 
the Roswell site was less negative than -800 mVcse even at current densities 
approaching 100 [lA/cm (100 mA/ft ), whereas a polarized potential of about 
-850 mVcse was obtained at the Greenup test location at current densities as low 
as0.1)aA/cm^(0.1mA/ft^). 



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CP Design Fundamentals 



4:5 



A Elkins 
A Greenup 
n Plain City 
Victoriaville 

# Roswell 
O Saginaw 

■ Thomasville 

♦ Trinidad 




1000 



Current Density (mA/ft'^) {\iNcm' 



Figure 4-2: Coupon Current Density as a Function of Off -Potential 

Source: Tliompson, N.G. and Lawson, K. IVI., Development of Coupons for IVIonitoring Catliodic 
Protection Systems, PRCI Contract PR-1 86-9220 (Catalog No. L51887), Dec. 2001, p. 101, Fig. 93. 

One of the major factors that produce such a wide range of current density 
involves the degree of dissolved oxygen available at the steel/earth interface. For 
instance, the difference in polarization characteristics of steel in clay (a typically 
unaerated environment) and sandy well drained soil (an aerated enviroimient) is 
illustrated in Figure 4-3. 



1200 




O Carbon Steel-Sand 
^ Carbon Steel-Clay 



400 



0.01 



0.1 1 10 

Current Density (microAmps/sq.cm.) 



100 



Figure 4-3: Instant-off Potential vs. Current Density for Carbon Steel Electrode in 
Sand and Clay Soil from Long Term Polarization Test Results 

Source: Gummow, R.A., Qian, S. and Baldock, B., AC Grounding Effects on Cathodic Protection Performance 
in Pipeline Stations, PRCI Contract PR-262-9913 (Catalog No. L51908), Dec. 2001, p.27. Fig. 2-24. 



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CP Design Fundamentals 



4:6 



Part of the reason for the greater current density required in the aerated versus 
unaerated soils is that the corrosion potential for steel in high resistivity aerated 
sand is in the -300 to -400 mVcse range while in clay it is in the -700 to 
-800 mVcse range. Hence to polarize the steel to -850m Vcse requires only a 50 to 
150 mV polarized potential shift in clay versus a 450 to 550mV polarization shift 
in sand. 

Dissolved oxygen however is a cathode depolarizer because it speeds up the 
transfer of charges across the steel/earth interface through the reduction reaction. 



O2 (dissolved) + 2H2O + 4e ^ 40H 



[4-1] 



Thus, a higher current is needed to achieve a given amount of polarization as 
shown in Figure 4-4 that compares cathodic polarization for steel in saturated 
aerated and deaerated solutions. 



o 
w 

o 

O 
> 

c 


o 

CL 



aerated 



deaerated 



9 
- 
.1 
2 
3 



10" 



\ Oxygen 
\ Reduction 



Hydrogen 
Evolution 



10-6 -|o"5 10"4 

Current Density, A/cm^ 



10" 



Figure 4-4: Polarization Curves in Aerated and Deaerated Solutions of pH7 

Source: Fundamental Process of Cathodically Protecting Steel Pipelines, Thompson, N.G. and Barlo, T.J. 
1983 International Gas Research Conference, p. 278. 



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CP Design Fundamentals 



4:7 



Finally, low pH and elevated temperature can also increase the current density 
requirements due to depolarization effects. 

Because of the large variation in current density requirements, it is important to 
estimate the current density as accurately as possible. Once the current density 
requirements are determined, it is a simple matter of multiplying the required 
current density by the bare surface area to find the total current as follows: 



I 



T,cp 



cp,bare 



»-s,bare 



[4-2] 



Note that the total current calculated assumes the current can be distributed 
uniformly with no allowance for attenuation or non-uniform current density 
conditions. Also for well coated structures where the actual bare surface area may 
be quite small. Equation 4-2 may understate the actual current required because 
the conductance of the coating will let some current pass through it to the 
structure. 

Therefore Equation 4-2 could be modified to include the current density through 
the coating as follows: 



iT,cp 'cp,bare -**■ ■^s,bare """ 'cp,coated -^ -^s, coated 



[4-3] 



Sometimes on very well coated structures where the bare surface area is very 
small (i.e. Asbare ~^ 0) the current required for the bare surfaces is ignored and a 
conservative coated surface current density (icp,coated) is used. 

For some structures that are exposed to markedly different polarization 
characteristics, current requirements may need to be considered on a zone basis. 
For instance, offshore drilling platforms are exposed to aerated and agitated water 
near the water surface, calmer and less agitated water with depth, and mud at the 
base of the platform. Thus for current requirement calculations the offshore 
platform is often divided into three zones as illustrated in Figure 4-5. 



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CP Design Fundamentals 



4:8 



Areas 
sq. meters 

H _ y. _[ 

350 



Elevation (M) 



]_Wa_ter_Line_ 




Zone 1 

(icp = 150 mA/m2) 



-30 



-60 



Zone 2 
(icp = 60 mA/m2) 



-90 



-120 



-150 



Mud Line 

Zone 3 
(icp = 1 mA/m2) 



J 



'H' column is surface area of horizontal structural members 
"V column is surface area of vertical structural members 

Figure 4-5: Surface Areas and Current Densities for Different Zones 
on an Uncoated Jacket Offsliore Drilling Platform 

This zonal approach could also be used for poorly coated pipelines having 
sections exposed to markedly different soil conditions or on coated pipelines with 
distinctly different coating conductances. 

4.3 Current Requirement Estimating IVIethods 

Several methods of estimating the current required to achieve protection are in 
common use. 



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4:9 



4.3.1 Literature Sources 

Tables of current density ranges for steel in the presence of different 
environments are available from the literature such as the following tables: 

Table 4-1: Approximate Current Requirements for Cathodic Protection of Steel 



Environmental 
Conditions 


Current Density | 


mA/m'^ 


mA/ft^ 


Immersed in Seawater*^' 






Stationary 






Well coated 


1 to 2 


0.1 to 0.2 


Poor or old coating 


2 to 20 


0.2 to 2 


Uncoated 


20 to 30 


2 to 3 


Low Velocity*"' 






Well coated 


2 to 5 


0.2 to 0.5 


Poor coating 


5 to 20 


0.5 to 2 


Uncoated 


50 to 150 


5 to 15 


Medium Velocity"" 






Well coated 


5 to 7 


0.5 to 0.7 


Poor coating 


10 to 30 


1 to 3 


Uncoated 


150 to 300 


15 to 30 


High Velocity*^' 






Poor coating or uncoated 


250 to 1000 


25 to 100 


Buried Underground'®' 






Soil Resistivity 






0.5 to 5 n-m 


1 to 2 


0.1 to 0.2 


5 to 1 5 0-m 


0.5 to 1 


0.05 to 0.1 


15 to 40 n-m 


0.1 to 0.5 


0.01 to 0.05 



'"'structures or vessels 
*'0.3 to 1 m/s (1 to 3 ft/s) 
'■"h to 2 m/s (3 to 7 ft/s) 



•^"'Turbulent flow 

'-'^'pipelines or structures, coated or wrapped 



Source: NACE Corrosion Engineers Reference Bool<, NACE International, 
3''' Edition, Robert Baboian, Ed., 2002, p. 162 

The first section of this table lists current densities for bare and coated steel in 
seawater by water velocity with the current density increasing as the water 
velocity increases. These values could also be used for fresh waters since there is 
very little differences in polarization characteristics between fresh water and 
seawater. 



The second section of the table lists the current density for well coated steel with 
respect to soil resistivity. The current density is shown to decrease with 
increasing soil resistivity. If the higher resistivity soil is also more aerated, a 
higher current density, not lower, would be required. 



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4:10 



In both the seawater and soil environments, the current density for well coated 
steel is given as 1 to 2 mA/m (0. 1 to 0.2 mA/ft ). Also the current density range 
for bare steel under stagnant seawater conditions is 20 to 30 mA/m (2 to 3 
mA/ft ), which would also apply to soils and fresh water conditions. 
Typical current density ranges are listed in Table 4-2 for bare steel in contact with 
different enviroimients. 

Table 4-2: Typical Current Density Requirements for Cathodic Protection 



Environment 


Current Density | 


mA/ft' 


mklm^ 


Neutral soil 


0.4 to 1.5 


4.3 to 16.1 


Highly acidic soil 


3 to 15 


32.3 to 161 


Heated soil 


3 to 25 


32.3 to 269 


Moving fresh water 


3 to 6 


32.3 to 64.6 


Fresh water, turbulent, with dissolved oxygen 


3 to 15 


32.3 to 161 .4 


Hot water 


3 to 15 


32.3 to 161 .4 


Seawater 


3 to 15 


32.3 to 161 .4 


Chemicals, acid or alkaline solution in process tanks 


3 to 15 


32.3 to 161.4 


Wet concrete 


3 to 15 


32.3 to 161 .4 



*Based on entire surface area of structure. 
Source: Shrier, L.L., Corrosion, Vol. 2, Newness-Butterworths, Boston, 1976, p11;65 

Note that these values differ to some extent with those in Table 4-1, which illustrates 
the wide variation in the published literature. Certainly the current density range 
stated for steel in concrete is probably an order of magnitude greater than is being 
used in practice. 

Current requirements for other metals, such as in electrical grounding systems, 
are similar in nonaerated, low resistivity clay soil as indicated in Table 4-3 . For 
high resistivity, well aerated sandy soil, the current requirements vary 
considerably amongst the various types of grounding materials. 



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4:11 



Table 4-3: Typical Current Density Requirements to Cathodically Polarize 
Various Metals to -850 mV^,^ ^ 



Type of Grounding 
Electrode Material 


Clay'^t 
fiA/cm^ 


Sand '^' 
(jA/cm^ 


Carbon steel 


0.1 


20 


Copper 


0.1 


100 


Tinned copper 


0.1 


20 


High silicon iron 


0.1 


20 


Stainless steel 


0.1 


8 



'''' nonaerated, low resistivity (1800 Q-cm) 
"Veil drained, aerated, high resistivity (>50,000 Q-cm) 



To improve the accuracy of estimates quoted in the Hterature, it is wise to seek 
current requirement information for appHcations that are as close to the intended 
appHcation as possible. 



4.3.2 Experience on Similar Structures in Simiiar Conditions 

One of the most common methods of estimating current requirements is simply to 
use current density values that have proved successful in the past. This may take 
the form of a current requirement in terms of: 

• current density per unit area of coating surface (/cp,coated) 

• current density per unit area (/cpbare) multiplied by an estimate of the 
amount of bare surface area, or 

• current per unit area of an actual structure in similar environmental conditions. 

The weakness of all these methods arises when the application conditions differ 
significantly from the circumstances from which the experience was gained. 



4.3.3 Determining Current Requirements on a Coated 
Structure by Estimating tiie Percentage Bare 

For a coated structure, it is common to estimate the percentage that is bare of the 
coating and then multiply by a current density appropriate to the enviroimiental 
conditions. For example, if the structure is considered 1% bare (a common 



Giimmow, R.A., AC Grounding Effects on Cathodic Protection Performance in Pipeline Stations, PRCI 
Contract #PR-262-99 13, Final Report, Dec. 2001. 

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4:12 



assumption) and the bare area requires 20 mA/m (2 mA/ft ), then the equivalent 
current density required (icp) for the entire coated surface is: 

% bare 



cp 



100 



X /, 



cp,bare 



[4-4] 



= 0.01 X 20mA/m^ 

= 0.2mA/m^ (0.02 mA/ft^) 

This method ignores the amount of current that passes through the coating, but 
the allowance for bare area is so overstated for a well coated structure that the 
calculated current requirement is usually more than adequate. 

The percent of bare area on well coated pipelines (coating conductance < 100 
)j,S/m ) is at least 2 orders of magnitude less than 1% bare as shown in Figure 4-6 
for a fusion bonded epoxy coated steel. However, if a percentage bare of 0.001% 
were used in Equation 4-4, the current required would be significantly 
understated. This is because on extremely well coated structures, most of the 
current passes through the coating since the coated surface is many orders of 
magnitude greater than the holiday surface areas (see Section 4. 11.7). 



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July 2007 



CP Design Fundamentals 








































4:13 


100000. 


^+# 


h^++ 


^ 


^+++ 


^+++ 


^ 


^^ 


■-■-■,^ ^+# 










— 






— 










zz 






^ 






^ 








^ 


•' 







:: 










































/ 


















































/ 






























Soil Resistivity = 1000 ohm. cm 




/ 






























L 


T / 














10000 


_ 






_ 






_ 








— 






— 










— 






— 






— 


~~i 
















■"■ 










































































































































/ 


















poor 
































/ 




















? 1000 




^ 






^ 










^ 






^ 




/ 


^ 












^ 




" ~^~7^\. 


(A 




zz 






^ 










= 






= 


7^" 




= 












^ 




:: ,,. fa"^ 






























^ 






















good 

r 


























J 


/ 
























o 
























/ 


























E 

■r- 100 






















-/ 


/' 
























■^ 


a 




^ 






^ 










^ 


^^ 




^ 






^ 












^ 




:: 












































































/ 
































B 


















/ 


































3 


















/ 






























■^ 


r 


■a 




^ 






^ 






/ 


/ 


^ 






^ 






^ 












^ 




__ 






— 






— 






^ — : 




^ 






^ 






^ 

















--'- 
















/ 


















































/ 


















































/ 




































1 












/' 










































— 






^ 










— 






— 






— 

















-- 




























































/ 


















































f 




























































































0.1 



















































0.000001 0.00001 



0.0001 



0.001 



0.01 
Bare Area % 



0.1 



10 



100 



Figure 4-6: FBE Coating Conductance versus Bare Area Normalized for a 
Soil Resistivity of 1000 £2-cm as Related to General Coating Quality in Table 4-4 

Source: Gummow, R.A. and Segall, S.M., PRCI Contract PR-262-9738, In-Situ Evaluation of Directional 
Drill/Bore Coating Quality -Evaluation of Test Methods, Final Report, Oct. 1998, p.43 



4.3.4 Minimum Voitage Drop Mettiod 

Experience has shown that in mid-range soil resistivity conditions cathodic 
protection vohage of 300 mV appHed between remote earth and a structure is 
sufficient for polarization to meet industry criteria. In fact, in the original NACE 
Standard SP0169, a minimum potential shift of 300 mV was one of the accepted 
criteria. Even after it was removed as a criterion, it is sometimes used as a start- 
up criterion when energizing rectifiers to determine initial current output settings. 
The 300 mV shift will overestimate the current required in low resistivity 
conditions (e.g., seawater, brackish water) and underestimate current 
requirements in high resistivity soils. 

This method assumes that if a minimum voltage drop, such as 300 mV, is 
obtained between the structure and remote earth, the current required to do so is 
sufficient to provide adequate cathodic protection. This can be calculated using 

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4:14 



the estimated coating conductance or the resuhs from field tests as illustrated in 
Figure 4-7. 

4.3.4(a) Field Test to Determine Current Required on a Pipeline 
Based on Minimum Voltage Drop 

A DC test current (It) is applied from a remote earth location through an ammeter 
and intermptible switch. 



<y> 



^77^- 



W7. 



■<A)^F'^^ 



DC source 



^<Z<^ 



remote ground 




coated pipe 
Figure 4-7: Voltage Drop Method of Determining Current Requirements 



The test current is switched on and off by an interrupter set so that the "on" cycle 
time is short compared to the "off cycle time to avoid polarization effects. The 
pipe-to-soil potential with respect to a remote reference is measured when the 
current is on (Von) and off (Voff). The change in potential (AV) between Von and 
Voff is expressed using the equation: 



e-g; 



AV 



Vo 



V, 



off 



[4-5] 



The current required is then calculated as the amount needed to shift the potential 
300 mV. 



300mV 
I = X L 

AV 



[4-6] 



For a long structure, it is usual to measure on/off pipe-to-soil potentials at a 
number of locations using the minimum shift location (AVmm) to provide a 
conservative estimate of the required current. Also, where multiple cathodic 
protection current sources are anticipated, a lesser voltage drop criterion (e.g.. 



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4:15 



150 mV) could be used at the intersection between two sources. It is assumed that 
each source would contribute a 150 mV of voltage drop thereby achieving a total 
of 300 mV. Although the 300 mV minimum potential shift is often used in 
current requirement calculations, it is not an industry standard criterion. 



4.3.4(b) Using Pipe-to-Earth Resistance to Determine Current 
Required by the Voltage Drop IVIethod 

If an isolated section of pipe is tested per the test arrangement in Figure 4-7, the 
average change in potential (AVave) divided by the test current gives the average 
pipe resistance (Rp,re) to remote earth per Equation 4-7: 



R 



I. 



[4-7] 



The current required by the test section to obtain a 300 mV shift would be: 

300mV 



I 



R 



[4-8] 



The resistance of the pipe can also be calculated by other methods as will be 
covered later in this chapter. 

The disadvantages of this method are that it does not determine the polarization 
characteristics of the structure and will vary considerably with the earth resistivity 
unless the coating is of very high quality. Thus in high resistivity soils this 
technique will result in a higher voltage drop than in a low resistivity soil and 
therefore a lesser calculated required current. 



4.3.4(c) Calculation of Current Required to Achieve a Minimum 
Voltage Drop on a Coated Structure Based on Coating 
Resistance 

Assuming that most of the voltage drop is across the coating rather than in the 
electrolyte when a cathodic current is applied to a structure, the specific coating 
resistance (r Q can be used to determine the current required for a unit area of 
surface with respect to a minimum voltage drop criterion (e.g., 300 mV) by 
Equation 4-9: 



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4:16 



300mV 



cp 



[4-9] 



where: r ' is the specific coating resistance in Q-m or Q-ft 



The value of r ' can be obtained directly from Table 4-4 for typical pipe coating 
conductances of different quality coatings in 1000 Q-cm soil or indirectly from 
the specific coating conductance (g ') since 



[4-10] 



Table 4-4: Typical Specific Pipe to Earth Leakage Conductance for 
Dielectric Protective Coatings in 1000 £2-cm Soil 



Long Pipelines 

with 

Few Fittings 


Average Specific 
Coating Conductance 

g' 


Average Specific Coating 
Resistance 


Quality of Work 


Slemens/fi' 


Siemens/m^ 


Q-ft" 


O-m' 


Excellent 


<1x10-^ 


<1 X 10-" 


>10' 


>10" 


Good 


1 X lO'^toSxIO"^ 


1 xlO^toSx 10" 


2x 10" to 10^ 


2x10^ to 10" 


Fair 


5x10"^to1 xlO-^ 


5x10"to1x10-^ 


10"to2x10" 


10^to2x10^ 


Poor 


>1 X 10"" 


>1 xlO-^ 


<10" 


<10^ 


Bare Pipe 
(2 to 12 in.) 
(5 to 30cm) 


4x 10'^ to 2x10"^ 


4 X 10-2 to 2 X 10-^ 


50 to 250 


5 to 25 


1 


Gas or Water 

Distribution with 

Many Fittings 


Average Specific 
Coating Conductance 

g' 


Average Specific Coating 
Resistance 


Quality of Work 


Slemens/fi' 


Slemens/m^ 


Q-ft' 


Q-m' 


Excellent 


<5x 10"^ 


<5 X 10" 


>2x 10" 


>2x10^ 


Good 


5x10"^to1 xlO-" 


5x10"to1x10-^ 


10" to 2x10" 


10^ to 2x10^ 


Fair 


1 X lO'^toSxIO"* 


1 x10-^to5x 10-^ 


2x 10^ to 10" 


2x lO^tolO^ 


Poor 


>5x10-" 


>5x10-^ 


<2x10^ 


<2x102 


Bare Pipe 
(2 to 12 in.) 
(5 to 30 cm) 


4x10-^ to 2x10-2 


4x10-2 to 2x10"^ 


50 to 250 


5 to 25 



NACE 



© NACE International, 2005 



CP 3-Cathodic Protection Technologist 

July 2007 



CP Design Fundamentals 



4:17 



For example, for 1 m of excellent coating on a long pipeline in 1,000 Q-cm soil, 
the current density required to produce a 300 mV voltage drop would be 



0.3V 



cp@300mV 



lO^Q-m' 



[4-11] 



0.3 X 10"^A/m^ 
30)aAW (~3)aA/ft^) 



If the soil resistivity is not 1,000 Q-cm, then it is usual to linearly extrapolate the 
specific resistance to the actual resistivity. 



In the foregoing example for 10,000 Q-cm soil the specific resistance would be: 

10,000 Q-cm 



c (a) 10,000 ohm-cm 



r„ X 



1,000 Q-cm 



[4-12] 



= lO^Q-m^ X 10 

= lO^Q-m^(lO^Q-ft^) 

Therefore the current required by the voltage drop method would be: 



0.3V 



cp@300mV, 10,000 ohm-cm 



lO'Q-m' 



[4-13] 



3 |jA/m^ (-0.3 |iA/ft^) 



4.3.5 Polarization Test Metliod 

The polarization test method is similar in setup to the voltage drop method except 
the test current is increased in small increments from a low value, the test current 
remains on for an extended period of time, and the interrupter is adjusted to 
interrupt the current for a very short off cycle. The reference electrode does not 
need to be placed at remote earth unless the structure is inaccessible as in the case 
of a well casing. The structure polarized potential (Eott) is then plotted on semi- 
log graph paper versus the applied test current as depicted in Figure 4-8. 



NACE 



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July 2007 



CP Design Fundamentals 



4:18 



-0.50 



-0.55C3- 



o_ 

I 
"to 






-0.60 



-0.65 



-0.70 



Tafel Slope 



_L 



_L 




0.01 0.02 0.05 0.1 0.2 0.5 1 2 

Current (Amperes) 

Figure 4-8: Cathodic Polarization Plot and Determination of 
Cathodic Protection Current (Icp) Required 



On the graph, the Hnear portion of the polarization curve is identified and a 
straight Hne is drawn to intersect with the original corrosion potential (Ecorr) line. 
This line is called the Tafel line or slope if the data is collected in an oxygen free 
(anaerobic) environment. The point where this line intercepts the corrosion 
potential (pt.©) represents the magnitude of the corrosion current (Icorr)- The 
point where the line departs from the plotted data (pt.®) identifies the required 
cathodic protection current. 

In order to achieve Tafel behavior with short term polarization testing, the 
structure must be in an oxygen free (anaerobic) enviroimient. If anaerobic 
conditions exist, the cathode reduction reaction will be hydrogen ion reduction 
under activation control; however, if aerobic conditions exist, oxygen reduction 
under concentration control is more likely. The polarization curve will not exhibit 
a Tafel straight line segment, but rather, will be curvilinear in a shape depicting 
concentration effects. In this case, short term polarization testing caimot provide 

^*NACE 



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CP 3-Cathodic Protection Technologist 

July 2007 



CP Design Fundamentals 



4:19 



the current requirement adequate for cathodic protection. If Tafel behavior is 
observed, the current requirement for adequate cathodic protection is the current 
magnitude where the Tafel slope begins. 

If the enviroimient is aerobic, the polarization curve will change with time in a 
maimer similar to Figure 4-9. 



LU 



-I— • 

O 
Q. 




Log I 

Figure 4-9: Effect of Time on tlie Sliape of a Dynamic Catliodic Polarization Curve 

Source: Nisancioglu, K., Gartland, per Olav, Dahl, T., and Sander, E., The Role of Surface Structure and 
Flow Rate on the Polarization of Cathodically Protected Steel in Sea Water, Corrosion /86, Paper No. 296 

As the time interval increases, the current required to polarize steel to a specific 
potential decreases. The shape of the polarization curve changes from one 
exhibiting considerable oxygen concentration polarization (at time = 0) to a 
completely linear E log I relationship when t -^ oo. The latter behavior is probably 
due to the primary reduction reaction being the reduction of the hydrogen ion. 

Clearly, the E log I current requirement criterion applied to these graphs would 
produce dramatically different current requirements with the shorter time duration 
curve producing the most inaccurate but most conservative result. Also, as the 
oxygen concentration polarization increases, the more difficult it is to draw a true 
straight line because the Tafel slope approaches tangency to the curve rather than 
intersecting a number of defined points on the curve. 

Allowing long time intervals between incremental changes in the applied current 
imposes a time constraint as well as a cost penalty on this method as applied in 
the field. This technique has been used mainly for determining current 

"NACE 



© NACE International, 2005 



CP 3-Cathodic Protection Technologist 

July 2007 



CP Design Fundamentals 



4:20 



requirements on well casings, which because of their depth are generally exposed 
to unaerated soil conditions. Hence, the polarization characteristic is more linear 
and the polarized potential less time dependent. 

Conducting cathodic polarization scans either galvanostatically (setting a current 
and measuring potential as per the previous example) or potentiostatically (setting 
a potential and measuring current) has considerable merit in determining the 
current requirements for steel in aqueous process streams. The tests can easily be 
conducted on coupons either in the laboratory or in the vessel on site using 
automated potentiostats. 

4.3.6 Polarization Shift Method 

The polarization shift method involves applying a test current as illustrated in 
Figure 4-7 for a period of time until the change in polarized potential approaches 

zerof^^^oV The magnitude of the test current is selected to be a significant 

[At ) 

fraction of that obtained by a rough estimate of the current required. 



When the "on" potential has stabilized, the interrupter is adjusted to interrupt with 
a short off-cycle so the polarized potential at the structure-to-earth measurement 
location can be recorded. The polarization potential shift at each location is 
determined by subtracting the corrosion potential (Ecorr) from the instant-off 
(polarized) potential (Ep). 



e-g; 



AEn 



[4-14] 



The required current is then computed by dividing the desired polarization shift 
for protection (e.g., 100 mV) by the test polarization shift multiplied by the test 
current. 



e-g; 



I 



cp 



lOOmV 
AE„ 



X L 



[4-15] 



The advantage of this test over the E log I test is that less time is required on site 
and less test current capacity is needed. The disadvantage is that it assumes the 
relationship between polarized potential and applied current is linear, which it is 
not, as was illustrated in Figures 4-8 and 4-9. However, if the original estimate is 
close to the actual required current, this method gives good results. 



NACE 



© NACE International, 2005 



CP 3-Cathodic Protection Technologist 

July 2007 



CP Design Fundamentals 



4:21 



Current density requirements per 100 mV of polarization, based on test results on 
fusion bonded epoxy coated pipe with artificial holidays, is plotted against % bare 
as shown in Figure 4-10. 



0.0001 







^= 




^^ 


1 M 1 1 1 1 




























































































































i- 


<"' 


■^ 1000 - 






































_^ 


>■ 




w > 

Si E 




^ 






_,^ 


















j^-^i 






























.j^^ 












0) *- 
























L^^i^ 












*- 100 














! ' 




.J' 


i!^ 



















^_^^^ . 






£ E 
















\ \ 










r 


































" < 10^ 












II 1 II 






Lm 








































]M 


































' 




^x*^ 
























T3 O 




— 


— 


— 







^^'~ 




— 






— 


— 






— 


— 




0) 1- 
















: 1 — 
























.^ o 












i^\\ 
































0) 1 












^^^\ 




































^ 


^' 






, , 1 






















a. ' 









































— -^ 




































^^^ 





— 


— 









^ 











— 






— 


— 






— 


— 
































































































0.1 - 










_. 










_ 






_ 








_ 






_ 







0.001 



0.01 



1 



10 



100 



0.1 
Bare Area % 

Figure 4-10: Current Density in Clay Soil for 100 mV Polarization Shift vs. % Bare on Coated Steel 

Source: Gummow, R.A. and Segall, S.M., PRCI Contract PR-262-9738, In-Situ Evaluation of Directional 
Drill/Bore Coating Quality- Evaluation of Test Methods, Final Report, Oct. 1998, p.44 

This curve considers only the situation where the pipe is buried in an unaerated 
environment. 

For a well coated pipeline, the percentage bare should be 0.01% or less 
suggesting a current density requirement of 10 )j.A/m per 100 mV of 
polarization. 



4.4 Calculation of Cathodic Protection Circuit 
Resistances 



After the cathodic protection current requirements are determined by one or more 
of the foregoing methods, galvanic or impressed current must be chosen as 
indicated in the flow chart in Figure 4-1. Many factors affect this design choice 
but the magnitude of the current required and the resistivity of the enviroimient 
are the predominant factors that impact the cost of the system. Typically, as both 

"NACE 

© NACE International, 2005 CP 3-Cathodic Protection Technologist 

July 2007 



CP Design Fundamentals 



4:22 



the current and resistivity increase, as shown in Figure 4-11, impressed current 
becomes the most economical option. 



3 

(D 

a: 

■*-* 

o 

o 

'■4— ' 

O 

_Q) 

O 

i— 

CL 



3.5 
3.0 
2.5 
2.0 
1.5 
1.0 
0.5 























\ 






















\ 




In- 


pressed Cu 


rrent Anodes 






\ 




















\ 


\^ 




















"s 
















1 
Magnesium Anodes 















" 10 20 30 40 50 60 70 80 90 100 
Soil Resistivity (p in ohm-m) 

Figure 4-11: Relative Economic Range for Galvanic and Impressed Current Systems 
as a Function of Current Required and Soil Resistivity 

Source: von Baeckman, W., Schwenk, W., and Prinz, W., Handbook of Cathodic Corrosion Protection - 
Tlieory and Practice of Electrocliemical Protection Processes, Gulf Professional Publishing, 1997, p.495 



The cathodic protection circuit for a galvanic system can be illustrated by the 
electrical schematic in Figure 4-12. 



-p,s = polarized potential 
of structure (V) 

■■ resistance of structure 
to remote earth (ohm) 

■■ resistance of anode(s) 
to remote earth (ohm) 

■■ polarized potential of 
anode(s) (V) 

Re = cable resistance between 
anode & structure (ohm) 




R 



s,re 



Remote 
Earth 



R 



a, re 



-p,a 



Figure 4-12: Electrical Schematic for an Operating Galvanic Cathodic Protection System 



The electrical schematic for an impressed current system illustrated in Figure 4- 
13 is similar to Figure 4-12. 



NACE 



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CP 3-Cathodic Protection Technologist 

July 2007 



CP Design Fundamentals 



4:23 



DC Power 
Supply 




Remote 
Earth 



Rq s = resistance of negative 
cable from tlie structure 

Ep,s - polarized potential of 
structure (V) 



^s,re 



= resistance of structure 
to remote earth (ohm) 



Rgb.re = resistance of groundbed 
to remote earth (ohm) 

Ep a ~ polarized potential of 
the groundbed (V) 

Rc,a = resistance of positive 
cable to groundbed 



Figure 4-13: Electrical Schematic for an Operating Impressed Current Cathodic Protection System 

After choosing the type of system, anode materials, and arrangement, the cathodic 
protection circuit resistance Rep needs to be calculated as follows: 



for an impressed current system 



[4-16] 



for a simple galvanic system 



R 



cp 



Re + Rs re + R,i 



[4-17] 



4.4. 1 Resistance of a Single Rod Siiaped Anode Positioned 
Verticaiiy in tiie Eartii 

The resistance-to-remote earth (Ra,re) of a single anode positioned vertically in the 
earth with the top of the anode flush with grade as shown in Figures 4- 14a and 4- 
14b can be calculated using Dwight's modified equation as follows. 



R, 



2;rL 



V^ 



- 1 



[4-18] 



where: 



R 



v,re 
P 

L 
d 



resistance of vertical anode to remote earth (ohms) 
resistivity of soil 
length of anode 
diameter of anode 



NACE 



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July 2007 



CP Design Fundamentals 



4:24 



\\y/AvAy/AvAA' 



xA/V/AV/ 




AyAxy/XvASY/ 



x^yAAyxyAAA 

Rv,re 



(a) directly in earth contact (b) in low resistivity backfill 

Figure 4-14: Anode Placed Vertically in Earth at Grade 

This equation is applicable where L»d, soil resistivity is homogeneous, and the 
anode is not located well below grade. For galvanic anodes surrounded by low 
resistivity sulfate-rich select backfill (typically Pbackfiii < 100 Q-cm) the outside 
dimensions of the packaged anode can be used in the calculation rather than the 
actual casting dimensions. This also applies to impressed current anodes that are 
surrounded by coke backfill material. The outside dimensions of the coke column 
can be used in Equation 4-18 without appreciably affecting its accuracy. 

Example Calculation: 

Calculate using Dwight's equation the resistance of a 6 ft long by 2 in. 
diameter anode in 10,000 Q-cm soil. 



p = 10,000 Q-cm 
L = 6 ftx 30.5 cm/ft 



2 in. X 2.54 cm/in. = 5.1 cm 



183 cm 



R, 



10,000 Q-cm 



'', 8x183 cm 

In 

^ 5.1cm 



1 



2 71183 cm 
= 8.7 {(In 287)- 1} =8.7(5.65-1} = 40.5 Q 

For a deep well anode, where the depth (t) to the top of the anode or coke column 
is about the same order of magnitude as the length (L) of the anode or coke 
column. Equation 4-19 can be used. 



R, 



2;rL 



In 



4L 



[4-19] 



where: 



Rv 



P 
L 

d 



resistance of vertical anode to remote earth (ohms) 
resistivity of soil 
length of anode 
diameter of anode 



NACE 



© NACE International, 2005 



CP 3-Cathodic Protection Technologist 

July 2007 



CP Design Fundamentals 



4:25 



4.4.2 Resistance of Multiple Vertical Anodes Connected to a 
Common Header Cable or Structure 

When multiple anodes are connected in parallel to a common header cable, as 
illustrated in Figure 4-15, the resistance of the entire array of vertical anodes can 
be calculated as a simple parallel circuit if the anodes are sufficiently far apart 
(S»L) and the header cable has negligible resistance. 



xx/Ay/Ay/Ay//\y/Ay/Ay/Ay//v/Ay/Ay/Ay/Ay/Ay/A\y/Ay/Av/Ay/AV/Ay// 



<gb,v 



I 



Tl"l 



Ra,1 



Ra,2 



Ra,3 



^a,n 



Figure 4-15: Multiple Vertical Anodes Connected to a Common Header Cable 

That is, the parallel resistance of the groundbed (Rgb.v) is given by: 



J_ 
R„ 



R, 



R„ 



R. 



R. 



[4-20] 



R, 



N X 



R, 



therefore: R^^ 



N 



[4-21] 



Thus the resistance of the multiple anode array, assuming that Raj = Ra,2 = Ra,3, 
is simply the resistance of a single anode, calculated using Dwight's equation, 
divided by the number (N) of anodes. This also applies for galvanic anodes 
widely spaced and coimected directly to the structure. 

Since the anode spacing (S) is typically short (e.g., S < lOL), there is mutual 
interference (crowding effect due to the current from each anode competing for 
the same current path) causing the resistance to increase from that calculated in 
Equation 4-21. 



NACE 



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CP 3-Cathodic Protection Technologist 

July 2007 



CP Design Fundamentals 4:26 



The Sunde equation which follows includes a factor 



— ln(0.656N) 
s 



that calculates the added resistance due to the mutual interference. 



R. 



2;rNL 



In ^ I - 1 + — In (0.656N) I [4-22] 



where: 



Rv = resistance of multiple vertical anodes to remote earth, ohms 

p = soil resistivity 

L = length of anode 

d = diameter of anode 

s = anode spacing 

N = number of anodes 

Note that this equation is just Dwight's equation divided by N with the 
"crowding" correction factor added. 



Example Calculation: 

Calculate the resistance of 10 anodes (5.08 cm (2 in.) diameter by 182.9 cm 
(6 ft) long) connected to a header cable with a 152.4 cm (5 ft) spacing in 
10,000 Q-cm soil. 

diameter = 2 in. (5.08 cm) 

length = 6 ft. (182.9 cm) 

spacing = 5 ft. (152.4 cm) 

resistivity = 10,000 Q-cm soil 

Using Equation 4-22: 

10,000Q-cm If. 8 X 182.9cm^ ^ 2 x 182.9cm, ,^ ^^^ ,^ , J 

R = i In - 1 + In (0.656 x 10 anodes) 

2 ;r 10 anodes x 182.9 cm K 5.08 cm J 152.4 cm 



Therefore: 

Rv = 0.87 {(5.66-1) + 2.41(1.88)} 

= 0.87(4.66 + 4.51) 

= 0.87(9.17) 

= 7.98 Q 

*NACE 

© NACE International, 2005 CP 3-Cathodic Protection Technologist 

July 2007 



CP Design Fundamentals 



4:27 



4.4.3 Resistance of a Single Rod Stiaped Anode Positioned 
Horizontaiiy in tiie Eartii 

As with all anodes the resistance is a function of the soil resistivity but also a 
function of the depth (t) as illustrated in Figure 4-16. 



V/A\/AV/A4(/XV/AVXV/AY/XV//<\\// 



t 




q.---<e: 



3_d 



(a) directly in earth contact 



CL 



-q: 



i 
>>1 



(b) in low resistivity backfill 
Figure 4-16: Anode Placed Horizontally Below Grade 

When the depth is shallow (t « L) and (L » d), Equation 4-23 applies: 



where: 



R. 



P 



27iL 



In 



ytdj 



p = soil resistivity 

L = length of anode 

d = diameter of anode 

/ = depth below grade 

and when the depth is considerable (t » L), Equation 4-24 applies. 



[4-23] 



R„ 



p 



2;rL 



In 



^2L^ 



V d , 



[4-24] 



In both equations, it is assumed that the soil resistivity is uniform and the outside 
dimensions of packaged galvanic anodes and coke backfilled impressed current 
anodes can be used in the equations without introducing appreciable error. 

Equation 4-24 can also be used to calculate the resistance of a single vertical rod 
where / » L. 

"NACE 

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July 2007 



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4:28 



Equation 4-23 can also be used to calculate the resistance of multiple horizontal 
anodes if they are placed relatively close together (s < La) and surrounded by 
continuous coke as illustrated in Figure 4- 17a. 



\y/Ay//v/Ay//\y/Ay/Ay/AV/Ay/Ay/Ay/Av/Ay/Av/xv/Ay/AWAy/AV/AVAv/AV/A\)fA^^^^ 

Rgb.h - 



CL- 



- ^ 1 — V) 4 -" 



t 



— ^ 1 - ^) 4 — V^ -) M) -~ 



i 



id 



Figure 4-17a: Multiple Horizontal Anodes in a Coke Trench Connected 
to a Common Header Cable 



Class Group Exercise 



Calculate the resistance of the following anode positioned 
vertically and horizontally using Equations 4-18 and 4-23 
respectively. 



M'here: 



L 
D 



2 m 
20 cm 



t = 1 m 
p = lOQ-m 



NACE 



© NACE International, 2005 



CP 3-Cathodic Protection Technologist 

July 2007 



CP Design Fundamentals 



4:29 



4.4.4 Resistance of Multiple Horizontal Anodes Connected on 
a Common Header Cable 



For multiple horizontal anodes connected to a common header cable having 
negligible resistance as in Figure 4- 17b, the following equation, which includes a 
"crowding" factor F, can be used. 

V/AY/Ay/AV/V/Ay/AX/AY/Ay/Ay/XV/AY/XV/Ay/AV/Ay/AV/AY/AV/AY/AV/AY/AmY^^ 
f^tjh ► ~T \ \ t t 



(£■ 



^- 



3- 



^ L ► 






Figure 4-17b: Multiple Horizontal Anodes Connected to a Common Header Cable 



where: 



R 



R 



a.h 



gb.h 



N 



X F 



Rgb,h 
Ra.h 

N 
F 



resistance of N horizontal anodes 
resistance of a single anode 
number of anodes 
crowding factor 



[4-25] 



The crowding factor (F) can be calculated using Equation 4-26. 



TiSR 



ln0.656N 



a,h 



[4-26] 



where: 



F = multiple anode crowding factor 

p = soil resistivity 

S = center-to- center distance between anodes 

Ra,h = resistance of a single horizontal anode 

N = number of anodes connected in parallel 



NACE 



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CP Design Fundamentals 



4:30 



Example Calculation: 

Calculate the resistance of 10 - 2 in. diameter by 6 ft long horizontal 
anodes connected to a common header cable at 1 1 ft center-to-center 
spacing in 10,000 Q-cm soil using Equation 4-23. 



Length (L) = 


6 ft. X 30.5 cm/ft 


= 183 cm 


Diameter (D) = 


2 in. X 2.54 cm/in. 


= 5.1 cm 


Spacing (S) = 


11 ft. X 30.5cm/ft 


= 336 cm 


Soil Resistivity (p) = 


10,000 Q-cm 




Depth (0 = 


3 ft. X 30.5 cm/ft 


= 91.5 cm 



R 



a,h 



P 



2;rL 



In 



^L^^ 



ytdj 



10,000 Q-cm 
6.28 X 183 cm 



In 



^33489 cm'^ 
466.7 cm' 



K: 



a,h 



.7 In 71.8 
.7 X 4.27 



37 Q 



Substituting Rg h into Equation 4-26 to obtain the crowding factor (F). 

10,000 Q-cm 



1 + 



TT X 336 cm X 37 Q 



In (0.656 X 10) 



1 + 0.256 X 1.88 
1.48 



Substituting F into Equation 4-25 yields: 

37 Q 



R 



g/b,h 



10 



X 1.48 



5.5 Q 



This result compares favorably with the calculated resistance (Rgb.v) of 
7.9 Q or the vertical groundbed array having the same number of anodes, 
the same size anode, and the same spacing. 



NACE 



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CP 3-Cathodic Protection Technologist 

July 2007 



CP Design Fundamentals 



4:31 



4.4.5 Calculating Pipe Resistance to Remote Earth 



There are several different methods of determining the resistance of a coated pipe 
to remote earth. For any pipe, the potential shift (AV) that occurs between the 
pipe and a reference placed at remote earth (as in Figure 4-7) with a test current 
interrupted can be used in Ohm's law to calculate the pipe resistance (Rp.re) as 
follows: 



Rp.e = ^°" J ^°" (^) 



R 



p,re 



[4-27] 



AV 



(Q) 



To increase the reliability of the result, the change in voltage should be calculated 
at several points if the pipeline is extensive. For short sections of bare pipe, as 
illustrated in Figure 4-18, Equation 4-23 can be used to calculate pipe resistance. 
The section must be separated physically or electrically from adjoining or 
adjacent structures. 




Figure 4-18: Resistance of a Horizontal Coated Pipe Section 

For a coated pipeline, most of the resistance to remote earth will be across the 
coating. Accordingly, if a specific coating resistance (r'c) is assumed, the 
resistance of the pipe to remote earth (Rpre) can be calculated from Equation 4-28. 



R„ 



p,re 



A„ 



[4-28] 



where: 



r'c = specific coating resistance 
As = surface area of pipe 



NACE 



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CP 3-Cathodic Protection Technologist 

July 2007 



CP Design Fundamentals 4:32 



Example Calculation: 

Calculate the resistance of a 1,000 m long section of 21.9 cm 
diameter pipe in 5,000 Q-cm soil if the specific coating resistance 
(r'c) in 1,000 Q-cm soil is 10^ Q-ml 



r'^@5,00CQ-cm 



where: 

r;@5, 000 Q-cm = r;@l, 000 Q-cm x ^-^OOQ-cm 



LOOO Q-cm 



,„4 5,000 Q-cm 
1 X 10 X 



1,000 Q-cm 



5 X lO^Q-m^ 



and: 

As = TidL 



therefore: 



3.14 X 0.219 m x LOOOm = 688 m^ 



5 X 10' Q-m- ^.^ ^ 

Rpre = ^ = 72.7 Q 

^' 688 m - 



4.4.6 Calculation of Cable and Pipe Lineal Resistances 

For solid copper cable the resistance of a conductor having a length (L) and a 
diameter (d) is given by the following equation: 

Re = Pcu^ [4-29] 

where: 

L = length 

Ax = cross-sectional area 

Pcu = volume resistivity of copper (typically 1.72 |j,Q-cm) 

But typical cable resistances can be obtained from published tables as in Table 4-5. 

© NACE International, 2005 CP 3-Cathodic Protection Technologist 

July 2007 



CP Design Fundamentals 



4:33 



Table 4-5: Concentric Stranded Copper Single Conductors Direct Burial Service Suitably Insulated 



Size 
AWG 


Overall Diameter 

not Including 

Insulation 

(inches) 


Approx. Weight 

Not Including 

Insulation 

(lbs/1000 ft) 


Maximum 

Breaking 

Strength 

(lbs) 


Maximum 
DC Resistance 

@20°C 
(Ohms/1000 ft) 


Max. Allowable 

DC Current 

Capacity 

(Amperes) 


14 


0.0726 


12.68 


130 


2.5800 


15 


12 


0.0915 


20.16 


207 


1 .6200 


20 


10 


0.1160 


32.06 


329 


1.0200 


30 


8 


0.1460 


50.97 


525 


0.6400 


45 


6 


0.1840 


81.05 


832 


0.4030 


65 


4 


0.2320 


128.90 


1320 


0.2540 


85 


3 


0.2600 


162.50 


1670 


0.2010 


100 


2 


0.2920 


204.90 


2110 


0.1590 


115 


1 


0.3320 


258.40 


2660 


0.1260 


130 


1/0 


0.3730 


325.80 


3350 


0.1000 


150 


2/0 


0.4190 


410.90 


4230 


0.0795 


175 


3/0 


0.4700 


518.10 


5320 


0.0631 


200 


4/0 


0.5280 


653.30 


6453 


0.0500 


230 


250 MCM 


0.5750 


771.90 


7930 


0.0423 


255 



Note that the stated resistances are for direct current not ahemating current, since 
the resistance for ahemating current is greater due to skin effects. 

The longitudinal resistance (Rp) of a section of steel pipe having a length (L) and 
a resistivity (psteei) is inversely dependent on the cross-sectional area (Ax) of the 
pipe wall. Hence from Equation 4-29. 



R„ 



^ steel 



A„ 



[4-30] 



and 



A = -(OD^-ID^) 



[4-31] 



where: 



OD = pipe outside diameter 
ID = pipe inside diameter 



The pipe resistance for a length of standard pipe can be calculated using Table 
B-1 in Appendix B. 



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CP Design Fundamentals 4:34 



Example Calculation: 

Calculate the resistance of a 1 km length of 20 in. diameter schedule 
30 pipe. From Table B-1 for Steel Pipe Data (Appendix B). The pipe 
dimensions are as follows: 

where: OD = 50.8 cm 

ID = OD - 2 (wall thickness) 

= 50.8 cm -2 (1.27 cm) 

ID = 48.26 cm 

From Equation 4-3 1 : 

A^ = -[(50.8 cm)' -(48.26 cm)' 

= 0.785 [2581 cm^ - 2329 cm^] 
Ax = 0.785 [252 cm^] 
Ax = 198 cm^ 

Substituting for Ax in Equation 4-30 and assuming psteei = 18 x 10"^ Q-cm 
(a conservative value compared to Table B-1) gives: 



R„ 



18x10"'^ Q-cm X 10' cm 



" 198cm' 



Rp = 0.009 Q or 9 mQ per kilometer 



4.5 Calculating System Capacity and Life 

To determine the amount of anode material needed for a cathodic protection 
system it is usual to determine the electrochemical capacity required. 
Electrochemical capacity is simply the amount of cathodic protection current 
required multiplied by the anticipated service life of the system, as shown in 
Equation 4-32. 



Cep = lop X L [4-32] 



where: 



*NACE 



Icp = cathodic protection current required 
L = required service life 
Ccp = CP system electrochemical capacity 



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4:35 



For example, a cathodic protection system requiring 2 A of current for a service 
life of 15 years requires 30 A-y of electrochemical capacity. 

Once the electrochemical capacity is determined, the minimum weight of any 
anode material can be determined from either Equation 4-33 or 4-34. 



W„ 



W,, 



C X C 

cp r 

UxE 
C X U X E 



[4-33] 



[4-34] 



where: 



w, 



Cr = theoretical consumption rate of anode material 

Ca = theoretical capacity of anode material 

ta,min = minimum weight required 

U = utilization factor 

E = electrochemical efficiency 



Besides the theoretical values of consumption and capacity, the calculated weight 
must have an allowance for anode efficiency (E) and utilization factor (U). 

As an anode is consumed, it reaches a point when it can no longer provide the 
minimum cathodic protection current even though it is not yet fully consumed. 
This is addressed by the utilization factor, which typically ranges from 0.5 to 0.9 
depending on the type of system and its operating condition. 

Actual anode consumption rates and capacities are always less than the 
theoretical values as calculated from Faraday's law (see Chapter 1). Thus it is 
typical to assign an estimate of efficiency for each anode material depending on 
the application and operating conditions based on experience. 

Table 4-6 contains typical consumption rates and electrochemical capacities for 
various anode materials. Note that the capacity is the inverse of the consumption 
rate. 



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4:36 



Table 4-6: Typical Consumption Rate and Capacities of Different Anode Materials 

in Soils or Fresh Waters 





Theoretical 
Consumption Rate 


Theoretical 
Capacity 


Typical 

Efficiency!'! 




I^A-y 


Lb./A-y 


A-y/lcg 


A-y/lb 


% 


Galvanic 
Anode 
Material 


Magnesium 


3.98 


8.76 


0.250 


0.114 


50 


Zinc 


10.76 


23.50 


0.093 


0.042 


90 


Aiuminum 


2.94 


6.49 


0.340 


0.155 


85-95 


impressed 
Current 
Anode 


Graphite/CaiiDon 


0.1 to 1.0 


0.22 to 2.2 


10.1 to 1.0 


4.5 to 0.45 


(2) (3) 


High Silicon Iron 


0.25 to 1.0 


0.55 to 2.2 


4.0 to 1.0 


1.8 to 0.45 


(2)13) 


Steel 


9.1 


20 


0.11 


0.05 


90 



Note: Platinum clad and mixed metal oxide coated anodes are quantified by thickness of the 
surface film rather than by weight. 

(1) Efficiency of galvanic anodes is dependent on the anode current density. 

(2) Source: Jakobs, J. A., A Comparison of Anodes for Impressed Current Systems, NACE 
Canadian Region, Western Conference, Feb. 1980. 

(3) See Table 2-13 for consumption rate in different environments. 



Example Calculation 

Given: A required system capacity of 30 A-y. 

What is the minimum weight of magnesium anode alloy required? 
Assume a utilization factor of 0.85 and an efficiency of 50%. 



W,, 



w, 





Ccp 


xC, 






ta,min 


u 


xE 




30A- 


■y X 


3.98 kg/A 


■y 



0.85 X 0.5 



Wta,mi„ = 281kg 

4.6 Calculation of System Life 

By substituting Equation 4-32 for system capacity in Equations 4-33 and 4-34 and 
rearranging, system life can be calculated by the two resulting Equations 4-35 and 
4-36 using theoretical anode consumption rate and theoretical anode capacity 
respectively. 



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4:37 



Wt X U X E 



[4-35] 



Wj X C, X U X E 



op 



[4-36] 



where: 

L = anode life (yr) 

Icp = cathodic protection current (A) 

Wt= weight (kg) 

Cr = theoretical consumption rate (kg/A-yr) 

Ca = theoretical capacity (A - y/kg) 

U = utilization factor 

E = efficiency (%) 

Notes: 

1. Apply efficiency to Table 4-6. 

2. Efficiency included in Ca from Figure 2-20. 

3. 50% efficiency included in Table 2-10. 

Example Calculation 

Given: A magnesium anode weighing 17 lb has a 50 mA output. 
Assuming a utilization factor of 0.9 and an efficiency of 
50%, what will be its service life? 

Using Equation 4-35 and a theoretical consumption rate of 8.76 Ib./A-y 
(from Table 4-6). 



W, X U X E 



lop X C, 



171b. X 0.9 X 0.5 
0.05 A X 8.76 lb/A -y. 



17.5 y 



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4:38 



4.7 Calculating Number of Anodes 

Once the minimum total weight of anode material has been determined, the 
minimum number of anodes is calculated using Equation 4-37. 



W 

TVT t, mm 



W, 



[4-37] 



t, anode 



Example Calculation 

Given: From the previous example the minimum total weight of 
magnesium required was 281 kg. Choosing a #20D2 (20 
lb) from Table I (Appendix A), how many anodes are 
required? 



N., 



281kg X 2.2 lb/kg 
201b 



Nn 



31 anodes 



This is the minimum number of anodes required from a weight 
basis, but the current output of each anode must be enough to 
supply the total current of 2A. The minimum number of anodes 
required on a current basis, given that each anode will produce 
50 mA, is as follows: 



N. 



50 mA 



2000 mA 
50 mA 



40 anodes 



Hence the minimum number of 20 lb anodes required to satisfy 
both the current and life requirement is 40. 



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4:39 



4.8 Calculating System Driving Voltage 

4.8.1 Galvanic (Sacrificial) System 

As seen previously, a galvanic system can be depicted on a polarization diagram 
such as Figure 4-19. 




CP Current 



Figure 4-19: Polarization Diagram for a Galvanic Cathodic Protection System 

The initial driving voltage (i.e., the original EMF) is the difference between the 
galvanic anode open circuit potential (Eaoc) and the structure corrosion potential 
(Es,corr)- But the intended operating driving voltage is Ea.p - Escrit which is 
substantially different from the initial driving voltage due to the amount of the 
original voltage used up in polarization at the anode and cathode. 

For cathodic protection design purposes, the design driving voltage is the 
difference between the anode polarized potential (Ea,p) and the chosen cathodic 
protection criterion (Es,cnt)- 

For example, if the chosen criterion is -850 mVcse and the polarized potential for 
a high potential magnesium anode is -1700 mVcse (eg-, 50 mV allowance for 
polarization from an open circuit potential of -1750 mVcse), then the driving 
voltage (Ecp) will be: 

Ea np - anode polarization 



-a,p 



-cp 



-cp 



-cp 



^a,oc 



J-'a.p J-'s,crit 

(1700 mV - 
850 mV 



850 mV) 



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4.8.2 Impressed Current System 

The operation of an impressed current system can also be represented on a 
polarization diagram such as Figure 4-20. 




Voltage drop 
from anode to 
remote earth 



anode polarization voltage 



DC power 
supply output 
(E.) 



structure polarization 

voltage drop from structure 
A to remote earth 



cp, operating 



CP Current- 



Figure 4-20: Polarization Diagram for an Impressed Current Catliodic Protection System 

(cable resistances (RJ are ignored) 

Applying Ohm's law to the impressed current system, which is a series circuit, 
the required power supply voltage (Eq) is equal to the sum of the voltage drops 
(including polarization and back voltage) around the circuit as in Equation 4-38. 



Icp (Ra 



R. 



Re) 



[4-38] 



where: 



Re 

AEa,p 
AE: 

Eg 
Eb 



s,p 



required cathodic protection current 
resistance of the structure to remote earth 
resistance of anode or groundbed to remote earth 
sum of all cable resistances 
total polarization at anode 
total polarization at structure 
galvanic voltage 



AE 



a,p- 



AE 



s,P 



Galvanic voltage (Eg) is the difference in potential between the corrosion potential 
of the structure (Econ) and the corrosion potential of the anode or groundbed 
(Ecorr)- The semi-inert impressed current anodes have corrosion potentials that are 
more positive than steel structures (e.g., graphite could be up to 1.0 V positive to steel). 



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4:41 



The term "back voltage" is used to account for anticipated anode and structure 
polarization plus the galvanic voltage difference. 

4.9 Sample Cathodic Protection Designs 

The examples that follow demonstrate typical calculations used in designing both 
galvanic and impressed current systems. These examples illustrate concepts only 
and are not intended as complete design procedures. 

4.9.1 Galvanic Anodes 



4.9.1(a) Example No. 1 



Given the following information, design a galvanic anode system for a 30 
nominal by 6 km long FBE coated steel pipline. 

Soil resistivity is 5,500 Q-cm 

icp required is 30 fxA/m 

Pipeline is electrically isolated at both ends 

Specific coating resistance is 10 Q-m 

Minimum design life is 15 years 

Actual pipe OD is 32.4 cm (12.75 in) (See Table B-1) 



cm 



Step 1: Calculate Total current Required (Icp) 



^cp 

icp = 

At = 


Icp X At 
30 ^AW 
TidL 


= 


3.14 X 0.324 mx 6,000 m 


= 


6,107 m^ 


therefore: 

Icp = 


30xlO"^A/m^x6,107m^ 
0.183A= 183 mA 



Step 2: 



Choose a Magnesium Anode and Calculate its Resistance 



Choose a high potential #17D3 anode. 

Calculate resistance of the anode (Ra,h) placed horizontally at a depth of 

1.2 m 



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4:42 



R 



a.h 



P 



IttL 



^„2t 



where: 



L 
d 
R. 



30 inx 



6 inx 



2.54cm 

in 
2.54cm 

in 



76.2 cm 



15.2 cm 



5500 ohm -cm (, 152.4 cm '^ 
In 



6.28x76.2 cm 



15.2 cm 



Step 3: 



= 11.5 Q (In 10) 

= 11.5Qx2.30 = 26.5Q 

Calculate Anode Current Output 

tLp., a — tLp, s 



la 



Ra, h + Rp 



^Nhere: 



lip,a 


= -1.7Vcse 


lip,s 


= -0.85 Vcse 


Ra,h 


= 26.5 Q 


Rp 


= pipe resistance 



• Calculate pipe resistance 



Rn 



r 



specific coating resistance 
total pipe surface area 



lO^Q-m^ 



3.14 X 0.324m X 6,000m 



1.64 Q 



Calculate anode current output 

1.7V -(- 0.850V) _ 0.850V 



L 



L 



26.5Q + 1.64Q 



30 mA 



28.1Q 



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4:43 



Step 4: 



Calculate Number of Anodes Required Based Current Demand 



N-^ = i^^^ = 6.1-7anodes 



/„ 30 mA 



Step 5: Calculate Anode Life Based on Anode Current Density 

WxUxExCa 

J—/ 

la 

^Nhere: 



W 
U 
E 

la 

Ca 



17 1b 
0.85 
1.0 

30 mA 

actual capacity from Fig 2-20 based on anode current 
density 



Calculate anode current density 



la^ 



A 



where: 



Aa 
Aa 



where: 



therefore: 



surface area of anode 
2 (W + H) L 



w 

H 
L 


width of anode 
height of anode 
length of anode 


3.5 in 
3.75 in 
26 in 


therefore: 






Aa = 


2(3.5 + 3.75)26 





377 in = 2.6 ft^ 



NACE 



30mA 
2.6ft' 



11.5mA/ft' 



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4:44 



Determine magnesium capacity from Fig 2-20 for ia = 1 1.5 mA/ft 
Ca = 300 A-hr/lb 

Converting to capacity for a year 



Ca 

therefore: 



SOoAl^x y^ 



lb 



= 0.034 A - ^Xu 

8766 hr /lb 



17 lb X 0.85 X 1.0 X 0.034 A - yr/lb 

0.030 A 
16.4 yr 

Note to student: Repeat design to achieve a 20 year life. 



30.8 kg X 0.85 X 0.5 
0.1 A X 3.98 kg/A -y. 



32.9 y 



4.9.1(b) Example No. 2 

The pipe to be protected is 2,000 feet of 4-in. ID steel (OD = 4.500 in. = 11.43 
cm). It is coated and the coating is 98.5% effective. The pipe is electrically 
isolated and the estimated current requirement is 2 mA/ft (21.54 mA/m ) of 
exposed metal. Soil resistivity is 3,500 Q-cm. Assume the desired pipe-to-soil 
polarized potential = -1.00 Vcse- Life of CP system must be 20 years. Assume 
pipe resistance is negligible and the CP system utilization factor is 0.85. 

General steps: 

• Determine current required 

• Start with an anode size 

• Calculate resistance to ground of one anode 

• Determine the current output of one anode 

• Determine the number of high potential magnesium anodes required based 
on the current required 

• Determine the life of the anodes. Determine the weight of anode material 
needed based on the desired life. 

• Determine number of anodes required and locations 



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4:45 



1. Calculate Current Required 



In Metric Units: 



Ttl 1 43 cm 

Total Surface Area = n6L = ——- x 609.6m = 218.9 m^ 

100cm 

m 



In Imperial Units: 

Total Surface Area = 7rdL = ^ . ^" x 2000ft = 2356 ft^ 

12m 

ft 

Coating effectiveness = 98.5% or 1.5% bare = 0.015 
Surface Area to protect = 2356 ft^ x 0.015 = 35.3 ft^ 

= 218.9 X 0.015 = 3.28 m^ 

9 9 9 

therefore: At 21.5 mA/m , current required = 3.28 m x 21.5 mA/m =71 mA 



or 



9 9 9 

therefore: At 2 mA/ft , current required = 35.3 ft x 2 mA/ft =71 mA 



For a 7.7 kg (17 lb) high potential magnesium anode with dimensions of: 30 
in. 76.2 cm [2.5 ft] long, 15.2 cm [6 in] diameter. 



2. Calculate the Resistance of a Single Vertical Anode 



Using Equation 4-18: 
^ 8L 



R 



iTtL 



ln^^-1 
d 



3500 Q-cm 
2;r76.2 cm 



, I 8 X 76.2 cm , 

In I I - 1 

15.2 cm 



= 7.31 X 2.69 
Rv = 19.7 Q 



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4:46 



3. Calculate the Resistance of the Anode Lead Wire 

10 ft (3.048 m) of #12 lead wire ^ R = 1.62 Q/1000 ft 

= (5.32 Q/1000 m) 



Using Metric 



RwTRF = 3.05 mx— = 0.016Q 

^'"^ 1000 m 



Using Imperial 



R 



WIRE 



1 62Q 

lOft X- = 0.016Q 

1000 ft 



4. Total Resistance of Anode and Wire: 

Ranode = 19.7 Q + 0.016 Q = 19.7 Q 



5. Calculate the Current Output of a Single Anode 

Ep of magnesium anode = -1.70 Vcse 

Driving potential = AE = -1.70 V - (-1.00 V) =-0.7V 
therefore: 



AE 0.7 V 



anode 



R 19.7 Q 



0.036A = 36 mA 



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4:47 



6. Calculate Life and Number of Anodes Needed 



Number 



71 m A needed 



36 



mA/ 



2 anodes minimum required 



anode 



But we need to make sure the anode mass is available for the 20-year 
proposed design life. Assume the capacity is 0.250 A-y/kg (from Table 
4-6). 



therefore: 



W, X C^ X U X E 

7.7 kg X 0.250A-yr/kg x 0.85 x 0.5 



0.036 A 



L = 22.7 y 



This would be the life of the system at 50% efficiency, but for 50% efficiency the 
anode current density must exceed 20 [lA/cm , otherwise the life will be 
proportionately less. 



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4:48 



Group Activity: 

Calculate the anode current density assuming anode surface area is 
2.46 ft^ using Figure 2-20 and recalculate anode life. 



4.9.2 Impressed Current System 
4.9.2(a) Example No. 1 

Known from field data and calculations: 



Given: 

Current requirement (Icp) 

Pipe-to-earth resistance (Rp,re ) 

Soil resistivity testing 



15 A 
1 Q 



Layer 
(ft) 


Layer Resistivity 
(Q-cm) 


0-5 ft (0- 1.5m) 


30,000 


5- 10 ft (1.5 -3.0 m) 


20,000 


10 - 15 ft (3.0 - 4.5 m) 


5,000 


15 -20 ft (4.5 -6.0 m) 


10,000 



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4:49 



Design Calculations: 

Rectifier Option 1: Try an output voltage of 20 volts 

Driving Volts (Ecp) = Rectifier output - (Back voltage) 

= 20 V - 2 V 
= 18V 



Max.R, 



;b,re 



cp 



R 



p,re 



18V 
15A 



0.2 Q 



From experience, this very low groundbed resistance would be impractical to 
try to attain. It would be better to use a larger voltage rectifier. 



Rectifier Option 2: Try an output voltage of 40 volts 

Driving Volts (Ecp) = Rectifier output - Back voltage 
= 40V - 2V 
= 38 V 



Max.R. 



cp 



cp 



R. 



38V 
15A 



1.5Q 



This resistance is attainable with a reasonable anode system design. 

Anode System Option 1: 

Calculate the anode-to-remote earth resistance for a system consisting of 15 
impressed current anodes 20.3 cm (8 in.) by 2. 13 m (7 ft) in a low-resistivity 
carbon backfill column, spaced 4.57 m (15 ft) apart and placed vertically at a 
depth of approximately 3.0 - 4.6 m (10 - 15 ft). 

Using Equation 4-22, the vertical anode resistance would be: 



R., = 



2;zNL 



f, ST.^ 


f 


In 


-1 + 


I dj 


V 



2L 



X ln(0.656 N) 



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4:50 



5,000 Q-cm 
2(515 anodes x213cm 



, 8 X 213 cm 
m 



V 



/. 



20.3 ; 



- 1 + 



2 X 213 cm 
457 cm 



ln(0.656 xl5 anodes) 



0.248 {In (84) - 1 + 0.933 In (9.84)} 

0.249 {3.431 + 2.133} 

1.39Q 

This configuration would work, but using this many anodes may cost more than 
a deep anode system. The decision is made to investigate the economics for 
using a deep anode system instead of a surface system. 



Anode System Option 2: 

Local geological surveys indicate a good clay stratum at depths between 38 and 
61 m (125 and 200 ft). A carbon backfill column, 25.4 cm (10 in.) by 22.86 m 
(75 ft), containing the appropriate number of anodes for 15 A (based on the 
current density rating of the anode) will be used. Calculate the anode-to-remote 
earth resistance of this system assuming the clay stratum has a resistivity of 
2,000 Q-cm. 

A deep anode system-to-remote earth resistance is calculated as if the "active 
zone" were one long single anode. 

Using Equation 4-18 and metric units, the resistance of a single vertical 
groundbed is: 



R. 



liiL 



In 



8L 



-1 



2,000 Q - cm 
2712286 cm 



In 



8 X 2286 cm 
25.4 



-1 



Rv 



0.1392 {In (720)-!} 
0.78 Q 



Due to the low resistance, let's keep the 40-volt rectifier and shorten the 
length of carbon backfill column to 15.24 m (50 ft). 



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CP Design Fundamentals 4:51 

The anode-to-earth resistance using Equation 4-18 and metric now becomes: 

R., = 




2000 Q-cm If, 8 x 1524cm 
In 



-1 



2^524 cm K 25.4 J 



= 0.209 {In (480)- 1} 
Rv = 1.08 Q 



4.9.2(b) Example No. 2 

Design an impressed current cathodic protection system for 3048 meters (10,000 
feet) of 20.32 cm (8 in.) ID, 21.9 cm (8.625 in.) OD coated steel pipe. Coating 
effectiveness is 99%. Use a surface groundbed. Soil resistivity is 4,000 Q-cm and 
the estimated current requirement is 21.5 mA/m (2 mA/ft ) of bare pipe. Pipe-to- 
ground resistance was measured as 0.3 Q (Rpre)- 

General steps: 

Determine current requirement 

Determine anode resistance 

Select appropriate wire size 

Determine cable resistance. Re 

Determine pipe-to-soil resistance, Rp ^ 

Decide if resistance is reasonable. 

Determine number of anodes required if resistance is not reasonable 

Determine total resistance based on number of anodes, Rt 

Determine rectifier size - voltage and amperage 



1. Calculate Current Required 

Using metric units in meters 

Total surface area = TidL = 7i(0.219m)x 3048 m = 2097 m^ 

100-99 

Actual bare surface area = 2091 m^ = 20.91 m^ 

100 



9 9 

Current required = 20.97 m x 21.5 mA/m =451 mA 



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4:52 



Assume that over the hfe of the pipeline, the current required to protect the 
structure will increase by 5 times to 2.3 A. 

2. Calculate Minimum Anode Weight Required 

Anodes: Using 21 kg (48 lb) high silicon cast iron anodes having a 
consumption rate (Cr) of 1 kg/A-y 

2 1b 



A-yr 



where: 



Wt = Cr X I, 



cp 



Cr 

icp 

L 
Wt 



consumption rate (Ib/A-y) 

current (A) 

life(y) 

total weight of anode material required (lb) 



For 20 year life using metric: 

Wt = 1 kg/A/y x 2.3 A x 20 y = 46 kg 
N = 46 kg ^ 21 kg/anode = 2.2 = 3 anodes 

3. Calculate Groundbed Resistance 

The next step is to calculate the groundbed-to-remote earth resistance of multiple 
vertical anodes. 

Using Equation 4-22 



R> 



27iNL 



In— -1+ — xln0.656N 



where: 



p = 4000 Q-cm 




L = 213.4 cm 




d = 20.3 cm 




S = 609.6 cm 




N = 3 anodes (minimum) 




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R. 



4000Q-cm 



2 X ;r x3 X 213.4cm 



In 



8x213.4 cm 
20.3 cm 



2x213. 4cm 
609.6cm 



X ln0.656(3) 



Rn = 1.0 [4.43-1 + (0.7x0.68)] 
Rn = 3.9 Q 

For 2.3 A at 3.9 Q, the rectifier voltage will need to be: 

E = (It X Rt) 

= 2.3A(3.9Q) + 2V (back voltage) 
= 11 V 

Calculate the anode-to-remote earth resistance using 4 anodes at 609.6 cm 
(20 feet) spacing. 

Using Equation 4-22: 



R, 



4,000 Q-cm 



2 X 71 X 4 X 213.4 cm 



In 



8x213.4cm 
20.32 cm 



( 



■1 + 



2x213. 4cm 
609.6cm 



xln0.656(4) 



Rn = 0.746 [In (84) - 1 + (0.7 x In (2.624))] 
Rn = 0.746 [4.431 - 1 + 0.675] 
Rn = 3.06 Q 



Erect = It x Rn + 2V (back voltage) 
= (2.3 Ax 3.06 Q) + 2 
= 9.04 V 



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4. Calculate Cable Resistances 

Plan a surface single groundbed at the midpoint of the pipeline: 

P/L 





/ 


/- Rectifier 


60.96 m 


Anodes -\ 


\ 


15.24 m 






4 

- -4 








(50 ft.) 




(200 ft.) 







18.29 m 

* *1 

(60 ft.) 



Figure 4-21: Single Groundbed Design 

Experience has shown that No. 6 AWG cable works well for this area. The 
resistance of No. 6 AWG cable is 1.322 Q/1000 m (0.403 Q/1,000 ft). 



Cable resistance (Re) in Imperial: 

Negative cable = 

Positive cable = 

Cable in groundbed = 
TOTAL CABLE 

Cable resistance (Re): 



15.24 m (50 ft) 

60.96 m (200 ft) 

18.29 m -2 = 9.1m (60 ft -2 

85.3 m (280 ft) 



30 ft) 



Re = 85.3 X 



1.322 Q 
1,000 m 



Re = 0.11 Q 



or 



Re = 280 fl X 



0.403 Q 
1,000 ft 



Rf 



0.11 Q 



Do we need to consider pipe longitudinal resistance, Rp? 

PipeOD = 8 5/8 in = 8.625 in. (21.9 cm) 
Pipe ID = 8.000 in. (20.32 cm) 
Length = 10,000 ft. (3048 m) 



,-6, 



Psteei = 1 8 |aQ-cm = (18x10^ Q-cm) = 0.00001 8 Q-cm 



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The longitudinal resistance of the pipe can be calculated using the following 
formula: 



where: 



R 



pL 

A 



p = resistivity (Q-cm) 

L = length (cm) 

A = cross-sectional area (cm ) 



In this pipe example the cross-sectional area is: 



[4-39] 



where: 



71 



A = -(OD'- -ID') [4-40] 



A = cross-sectional area of the pipe (cm ) 
OD = pipe outside diameter, (cm) 
D = pipe inside diameter, (cm) 

Using Equations 4-39 and 4-40 in metric units for Vi of the total pipe length: 

pL 18xlO'Q-cmx 15.25x10 "cm 



R p {one direction ) 



-(OD ^ - ID ^ ) -[(21 .9 cm )' - (20 .32 cm f ] 



2.75 Q-cm 
52.4 cm' 



0.052 Q 



Since the two directions of pipe length are in parallel with the rectifier, the total 
longitudinal resistance of the pipe is one-half of this value (e.g. 0.026Q). 



Total circuit resistance: 



Rt 



where: 



Rn + Rp,re + Re + Rp 



Rt = Total circuit resistance (Q) 

Rn = Multiple anode resistance (Q) 

Rp,re = Pipe-to- electrolyte contact resistance (Q) 

Re = Cable resistance (Q) 

Rp = Pipe resistance (Q) 

Rt = 3.06 + 0.30 + 0.11 + 0.026 

= 3.496 Q (use 4 Q) 



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5. Determining Rectifier Output Rating 

Rectifier voltage output (Eq): 

Eo = (I xRt) + 2.0 V (Back voltage) 

= (2.3 A X 4.0 Q) + 2.0 V (Back voltage) 
= 11.2 Volts 

Note: Spare voltage capacity allowance for anode deterioration still needs to 
be considered. 

4.10 Design of Performance Monitoring Facilities 

Test facilities are an integral part of a complete cathodic protection design so that 
the system performance can be adequately monitored for industry regulations and 
practices. For pipelines, test leads to accommodate the surveys outlined in 
Chapter 5 are generally included: 

a) at frequent intervals (e.g., < 2 km) 

b) at crossings with foreign structures 

c) at points of electrical isolation 

d) at some galvanic anode locations 

e) at casings 

f) at current spans upstream and downstream of DC power supplies 

g) near sources of electrical interference 

h) at location of stray current discharge to earth 



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4.11 Current Distribution 

4.11.1 Introduction 

The amount of polarization and therefore the level of cathodic protection depend 
on the current density applied across the electrolyte/structure interface. To 
achieve uniform protection at all locations on the structure under constant 
conditions requires uniform, or ideal, current density on all protected surfaces. 
However, ideal current distribution is virtually impossible to obtain in practice 
due to non-uniformity of the electrolyte, characteristics of the structure, and 
placement of the cathodic protection anodes. 

The relative resistance of each current path in the cathodic protection circuit 
determines the current in that path and the resulting current density at the 
structure surface. The resistance of a current path is related to the path length (L), 
cross-sectional area (A), and material resistivity (p) according to the expression in 
Equation 4-41. 



R 



pL 
A 



[4-41] 



where: 



R 

P 
L 

A 



resistance (Q) 
resistivity (Q-cm) 
length of current path (cm) 
cross-sectional area (cm ) 



In Equation 4-41 the cross-sectional area is the area perpendicular to the current 
direction. This equation is only valid for a system where the cross-sectional area 
is constant in the current direction. For electrodes in the earth, the cross-sectional 
area is constantly increasing as the current approaches remote earth; therefore, the 
simple relationship of Equation 4-41 is not valid. 

For a cylindrical anode placed vertically in a homogeneous soil, the current 
distributes radially from the anode as shown in Figure 4-22. The total anode 
current (Ia) is equal to the sum of an infinite number of radial currents (la). 



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equipotential 
surfaces 




Figure 4-22: Typical Current Distribution witli a Vertical Cylindrical Anode 

All of the current paths have equal lengths and cross-sectional areas to remote 
earth; thus, all paths have equal resistances if the soil resistivity is homogeneous. 
Equation 4-42 gives the total resistance (Rt) to remote earth. 



1 



1 



1 



1 



Rn 



R, 



R 



a2 



R 



a3 



R, 



[4-42] 



where: 



Rt = total resistance of anode (Q) 
Ran = resistance of individual path (Q) 
n = path number 



Figure 4-23 depicts the current paths from the anode to remote earth and the 
distribution of current to the structure from remote earth. 



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In + l3 



R„ 




@ 



® 



-> In + l3 + l2 



-► In + l3 + l2 + Il ► 



R, 



R, 



Drain 
Point 





< L 



anode at remote earth 



Figure 4-23: Current Path Resistances for Ideal Current Distribution 

To complete the path for ideal current distribution, the current must enter the 
structure from remote earth. The resistance of the structure to remote earth is 
composed of an infinite number of individual parallel leakage resistances (Rl) 
that are equal in value in the ideal case. Assuming the internal resistance of the 
structure is zero, all current paths between the anode and cathode (structure) have 
equal resistances and therefore equal currents. This ideal circuit produces uniform 
(ideal) current density on the structure. Under ideal current distribution, the 
magnitude of cathodic protection current in the structure increases linearly toward 
the drain point (i.e., the point of negative coimection to the structure) as 
illustrated in Figure 4-24. 



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® 
Location in Structure Drain Point 

Figure 4-24: Current in Structure under Ideal Conditions 

Because the assumptions made to produce ideal current distribution are 
umealistic, plus the fact that CP equipment caimot be located at remote earth, 
ideal current distribution caimot be achieved in practice. 



Table 4-7: Some Factors That Affect Relative Current Distribution 



Factors Affecting Current Distribution 



Current attenuation (structure lias finite resistance) 
Variations in electrolyte resistivity 
Anode-to-structure and anode-to-anode spacing 
Variations in electrolyte geometry and resistivity 
Protective coatings 
Polarization 



4.11.2 Attenuation 

In Figure 4-23, ideal current distribution is obtained in homogeneous soil with 
anodes remote from the structure and a structure resistance equal to zero. 
Structure resistance alters the ideal linear current distribution shown in Figure 4- 
24 so that it is no longer linear with distance but is logarithmic in nature. For 
structures such as pipelines and cables, the structure resistance can be significant 



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as modeled in Figure 4-25. Figure 4-26 illustrates the nonlinear tapering of the 
current and voltage along the structure known as attenuation. 
When the structure resistance is included, the individual path resistance increases 
with distance from the drain point. 



r^s n ^^ / '^s,4 



Rs,3 Rs,2 



Rs,i 

AW 



V, 



cp Rin- 



Rl,4- 



Rl,3 



Rl,2 



Rl,i- 




f 



remote earth 
Figure 4-25: Current Path Resistance Including Resistance of Pipeline 



icp 



Even in homogeneous soil, the current density remote from the drain point will be 
less than the current density near the drain point. It follows then that the structure 
potential will also attenuate away from the drain point. 



-corr 



Distance from drain point 




Figure 4-26: Current and Voltage Attenuation Away from the Drain Point 



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The shape of the attenuation curve is governed by the relative values of the 
structure longitudinal resistance (Rs) in ohms per unit length and the structure 
leakage resistance to remote earth (Rl) in ohms per unit length. More specifically, 
these two parameters determine the structure attenuation constant (a) that governs 
the rate of attenuation. 

The attenuation constant is related to the two structure resistance parameters as 
indicated in Equation 4-43. 



a 



I 



[4-43] 



-where: a = attenuation constant 

Rs = longitudinal resistance of structure (Q) 
Rl = leakage resistance of structure (Q) 

The attenuation constant controls the rate at which the current and voltage are 
reduced with distance from the drain point. As shown in Figure 4-27, a small 
attenuation constant (a) will result in a small amount of attenuation with distance, 
whereas a large attenuation constant will result in much greater attenuation with 
distance from the drain point. 



-corr 



Distance from drain point 




Figure 4-27: Effect of the Attenuation Constant a on Attenuation Cliaracteristics 



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When Rs is large with respect to Rl, a will be relatively large. When Rs is small 
with respect to Rl, a will be relatively small. The attenuation constant will be 
large if, for example, the coating is poor, the resistivity of the enviroimient is low, 
or the longitudinal resistance of the pipe is high. 

Assuming the structure is infinitely long, the current (Ix) in the structure at any 
distance (x) from the structure drain point is related to total cathodic protection 
current (lo) as seen in Equation 4-44. 



I. 



IqC 



[4-44] 



where: l^ = current at point x (A) 

lo = current at drain point (A) 

a = attenuation constant 

X = distance from drain point (unit lengths) 



The structure voltage (Vx) with respect to remote earth at any distance (x) from 
the drain point in a structure that is infinitely long is related to the drain point 
voltage (Vo) as seen in Equation 4-45. 



V,. 



VoC- 



[4-45] 



where: V^ = voltage-to-remote earth at point x (V) 

Vo = voltage-to-remote earth at drain point (V) 



Since the cross-sectional area (A) of a structure such as a pipeline or cable is 
constant along the length of the structure, the longitudinal structure resistance can 
be calculated using Equations 4-30 and 4-31 (see Section 4.4.6). The resistivity 
(p) is the resistivity of the structure metal (e.g., steel is 18 x 10"*" Q-cm) and the 
length (L) is a unit length along the structure. 

The leakage resistance (Rl) per unit length for a bare structure can be estimated 
by Equation 4-23. 



Rt 



27t:L 



In 



^L^^ 



td 



[4-46] 



where: p = soil resistivity (Q-cm) 

t = depth of burial (cm) 

D = diameter of pipe (cm) 

L = unit length of pipe (cm) 



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4.11.3 Effect of Coating on Current Distribution 

Properly selected and applied protective coatings have a dramatically beneficial 
effect on current distribution and reduce the total current requirements. The 
principal effect of the coating involves its high electrical resistivity, which 
increases the structure leakage resistance (Rl) thereby minimizing current 
attenuation. 

The Rl as shown in Figure 4-28 is the sum of the leakage resistance of the coating 
(Rl,c) and the leakage resistance to remote earth of the steel structure (Rl,e)- 



That is: Rt 



R 



L,c 



R 



L,e 




Coating 



WA/vWVWWV^<- 



Remote Earth 



^L,e 



-► t ^ 



Figure 4-28: Leakage Resistance to Remote Eartli of a Coated Structure 



The leakage resistance of the coating is a function of its electrical resistivity, 
thickness, and surface area exposed to the electrolyte. Therefore, modifying 
Equation 4-41 by changing the length (L) to thickness (t) produces Equation 4-47, 
which can be used to calculate the resistance of the coating for a given surface 
area (As). 



R 



L,C 



A„ 



[4-47] 



where: Rl,c = leakage resistance of coating (Q) 

Pc = coating electrical resistivity 

t = coating thickness 

A, = coated surface area 



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Typical coating resistivities are on the order of 10^° to 10^^ Q-cm, and coating 
thickness are on the order of 0.005 to 0.015 cm. Accordingly, the coating leakage 
resistance (Rl,c) for one square meter of a coating from Equation 4-47 would be 
as follows: 



R 



(lO'° Q-cm) (0.005 cm) 



L,C 



10^ cm' 



5000 Q 



The value of the leakage resistance of the structure (Rl,e) to remote earth can be 
approximated by assuming the surface is a disk with an area of one square meter. 
The resistance-to-remote earth of the soil side of the disk is given by Equation 4- 
48. In this case, the leakage resistance for one side of a bare metal disk (Rl,e) can 
be calculated using Equation 4-48. 



R 



L,E 



4r 



[4-48] 



where: 



soil resistivity 
radius of disk 



cm (i.e., 1 m ), the radius can be 



Given a disk with a surface area of 10 

determined by rearranging the surface area equation for a circle and solving for 

radius (r). This is given in Equation 4-49 below. 




[4-49] 



where: 



K 



radius 
surface area 



Using the value for Ag, we find that the radius is as given in Equation 4-49. 



1 a4 2 

10 cm 



56.4cm 



7t 



Therefore, for a soil resistivity of 1,000 Q-cm solving Equation 4-48, we have: 



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R 



lOOOQ-cm 



L.E 



4 (56.4 cm) 
The total leakage resistance (Rl) is therefore: 



4.4Q 



where: 



Rl - Rl,c + Rl.b 

= 5000 Q + 4.4 Q 
= 5004.4 Q 



This example shows that the coating leakage resistance (Rl,c) is the dominant 
component of the structure leakage resistance (Rl). A good coating significantly 
increases the leakage resistance resulting in a small a and therefore less 
attenuation and better current distribution. 

The leakage resistance of most coatings can be expected to decrease in service 
because coatings deteriorate with time. For this reason, it is important to measure 
the leakage resistance structure periodically. Equation 4-50 determines the 
average leakage resistance to remote earth of all or a portion of the structure. 



R. 






[4-50] 



where: Rl = average leakage resistance (Q) 

AVs = average potential change (V) 
Altest = average test current change (A) 



The average potential change of the structure to remote earth (AVs) is measured 
with a reference electrode placed remote from the structure with the test current 
switched on and off The average change in test current (Altest) is the difference 
between the applied test current as it changes from one value to another. 
Normally, the test current is simply switched on and off; therefore, the change in 
test current is equal to the test current value with the switch on. Both of these 
values, AEs and Altest must be instantaneous values obtained immediately after the 
switch is opened or closed. 

The specific leakage resistance (r') for the coating is related to the average total 
leakage resistance as shown by Equation 4-5 1 . 



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RlAs 



[4-51] 



where: 



r' 

Rl 

As 



specific leakage resistance 
average total leakage resistance 
total surface area tested 



The specific leakage resistance, therefore, is typically expressed in units of Q-m 
(Q-ft ). Coating quality can be rated on the basis of comparing the specific 
coating resistance or conductance in 1000 Q-cm soil to ranges established from 
experience, as shown previously in Table 4-8. 



Table 4-8: Specific Lealiage Resistances and Conductances 



Long Pipelines 

with 

Few Fittings 


Average Specific 
Coating Conductance 

g' 


Average Specific Coating 
Resistance 


Quality of Work 


Siemens/ft' 


Siemens/m^ 


Q-ft' 


n-m" 


Excellent 


<1 X 10"^ 


<1 X lO"* 


>10^ 


>10" 


Good 


1x10"^to5x10-^ 


1 x10-"to5x10-" 


2x10" to 10^ 


2x10^ to 10" 


Fair 


5x 10'^tol xlO"* 


SxlO^tol X 10"^ 


10"to2x10" 


10^ to 2 X 10^ 


Poor 


>1x10-^ 


>1 xlO-^ 


<10" 


<10^ 


Bare Pipe 
(2 to 12 in.) 
(5 to 30cm) 


4x10'^ to 2x10-2 


4x10-2 to 2x10"^ 


50 to 250 


5 to 25 


1 


Gas or Water 

Distribution with 

Many Fittings 


Average Specific 
Coating Conductance 

g' 


Average Specific Coating 

Resistance 

r'c 


Quality of Work 


Siemens/ft' 


Siemens/m 


n-ft" 


Q-m^ 


Excellent 


<5x10-^ 


<5 X 10"" 


>2x10" 


>2x10^ 


Good 


5x 10"^to1 xlO" 


SxlO^tol X 10-^ 


10" to 2x10" 


10^ to 2 X 10^ 


Fair 


IxlO'^toSxIO"^ 


1 x10-^to5x10-^ 


2x10^ to 10" 


2x102to10^ 


Poor 


>5x 10" 


>5x10-^ 


<2x 10^ 


<2x102 


Bare Pipe 
(2 to 12 in.) 
(5 to 30 cm) 


4x10-^ to 2x10-2 


4x10-2 to 2x10-^ 


50 to 250 


5 to 25 



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4.11.4 Effect of Anode-to-Structure Spacing on Current 
Distribution 

In homogeneous soil, the shorter current paths between the anode and structure 
carry larger currents than the longer current paths. For example in Figure 4-29, 




Figure 4-29: Current Distribution witli a Close Anode-to-Structure Spacing 

the current path A-1, between a vertical anode and a vertical steel sheet, is shorter 
than current path A-2. When the angle between A-1 and A-2 is 60°, then the 
length of the current path A-2 is given by Equation 4-52. 



cosG 



L. 



[4-52] 



and 



cos 60 



0.5 



^A-2 



^A-2 



■^A-2 



2d 



The current path La-2 is twice the perpendicular distance (d), and therefore the 
current density (i^) at location © would be expected to be double that at location 

©. 



In an attempt to achieve uniform current distribution, a second anode can be 
located at a distance 2Li.2 from the first anode as shown in Figure 4-30. The 
current density (i ) will now be doubled, each anode contributing equal currents. 



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Figure 4-30: Anode-to-Anode Spacing to Acliieve Relatively Uniform Current 
Distribution with Close Anode-to-Structure Spacing 



Distance (L) can be calculated from Equation 4-53 



dtan0 



[4-53] 



Therefore the anode-to-anode separation distance (La- a) will be: 

La-a = 2 d tan 6 

For example, if the anode-to-stmcture spacing (d) is 1 m, the anode-to-anode 
spacing La-a will be: 

La-a = 2 X 1 m x tan 60 

= 2 X 1 m X 1.73 

= 3.5 m 

Close distributed anode systems are often used to protect specific piping runs 
such as in petrochemical and chemical plants where the piping caimot be 
electrically isolated form other structures located in the immediate vicinity of the 
piping. 

Close distributed impressed current anodes are widely used to protect existing 
underground storage tanks as shown in Figure 4-31. 



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PlanViart/ 



8.4m 



II 



12.2 m' 



1 





3.7 m ^: 

0-* 






r 




V/AV/V/AV// • W/AY// o 

BevationVlew 



W/AY// 



xV/AY// 



- vertic^ 
anodes 



horizontal 
anodes 



77?^3^77 

horizontal 
anodes 

vertical 
anodes 






Figure 4-31: Distributed Impressed Current Anodes around Underground Storage Tanks 

In this anode arrangement both horizontally and vertically installed anodes are 
used to provide relatively uniform current distribution. 

4.11.5 Effect of Structure Arrangement on Current Distribution 

One of the current distribution problems presented by the array of underground 
storage tanks in Figure 4-3 1 is the difficulty of providing sufficient current to the 
facing surfaces of the tanks. Because the soil current paths between tanks are 
constricted, these paths have a higher resistance than the current paths to the 
outside tank surfaces. This problem can also exist in a run of multiple parallel 
pipelines. The relative current density to the facing surfaces is a function of the 
diameter of the structures and the separation distance between the structures as 
shown in Figure 4-32. 



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1 


1 






'r^' 


8^-=^v2 




3 


- 3|f )|7 


7l( j|3 






Kr4 


6W^4 


o 




\o 5 


o 5 


_z 




\ 




^ 




X 


8 cm Model 


z 




\ 


6000 ohm-cm 


— 


2 


- \ 




o" 




\ 












ro 




^v 




cr 




^~-— — _____^ 




•*-> 






— ■ — 


c 






— — — o 


9^ 








L_ 








Z5 


1 


_ 




o 




1 1 


1 1 1 



10 



20 



30 



40 



50 



Separation, cm 



Figure 4-32: Current Max/Min Ratio for Two Parallel Structures 

Source: Status of Catliodic Protection of Pipelines Under Some Particular Situations, 
Keije Nunomura, Katsumi Masamura and Iwao Matsushima, NACE Corrosion '81 Conference, 

Paper #81, Apr. 1981. 

Note that the Imax/Imin ratio only drops below 2 when the separation distance 
between the structures is equal to or exceeds the diameter of the structures. 



4.11.6 Effect of Electrolyte Resistivity Variation on Current 
Distribution 

When the electrolyte resistivity is uniform and the structure resistance is not 
negligible, the highest current density will occur at the point on the structure 
closest to the anode. Conversely, the lowest current density will be at the most 
remote structure surface. Most soil resistivities, however, are seldom uniform, 
and when there are extreme variations in electrical resistivity along the structure, 
the current distribution can be seriously affected. For example, in Figure 4-33 
where the electrical resistivity of swamp (psw) is much less than the electrical 
resistivity of the adjacent soil (ps), a disproportionate amount of the cathodic 
protection current will follow the swamp path. This results in a higher current 
density on the structure surfaces exposed to the swamp and a lower current 
density on the remaining structure surfaces. In extreme cases like this, where 
Ps » Psw, better current distribution can be obtained by installing the groundbed 



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in the higher resistivity soil, ahhough the groundbed resistance will be much 
greater. 

f Swamp / 

/ / Pipeline 



-900 mV 



-800 mV 



High Resistivity Soil 
(Ps) 




Figure 4-33: Localized Low Resistivity Patli in a Higli Resistivity Soil 

A similar variation in electrolyte resistivity occurs due to soil or moisture 
stratification. Any structure, such as the well casing in Figure 4-34, that traverses 
strata of various resistivities will receive non-uniform current distribution. The 
largest current density need not appear on the surfaces exposed to the lowest 
resistivity enviroimient, which in this case is brine, because the path resistance is 
also a function of the path length and cross-sectional area. The relative proximity 
of the anode to the fresh water and clay, each with similar electrical resistivities, 
means that the current densities would be similar, differing because the clay strata 
are farther from the anode. The current density on the structure exposed to the 
sandy loam above the water table may not be high despite the anode proximity 
because the sandy loam will have a low moisture content and relatively high 
electrical resistivity. Furthermore, the sandy loam would be reasonably well 
aerated, and its polarization level would be less than for a similar current density 
in the clay or fresh water strata. 



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4:73 



Well Casing 



v/A^xv/Ay/Ay/ 

P/S = -800 mV 



T/R 



A/Xv/A^AV/Ay/AV/Ay/AV/AV/xVA.-A/\y/A\/AV/ 
Layer of Sandy Loam ^Anode 



I 



SL p = 2,500 ohm-cm 




Figure 4-34: Current Distribution on a Well Casing with Variable Resistivities 

The current density in the brine layer is as large as that in the low moisture con- 
tent sandy loam because the current path through the limestone layer is short and 
the cross-sectional area large resulting in a relatively low resistance path. The 
polarized potential is greater in the brine layer than in the dry sandy loam because 
the brine envirormient is relatively deaerated. 

Where the current density on a structure in a high resistivity electrolyte is 
insufficient for complete protection, current distribution can be improved by 
installing additional anodes. Positioning of the anodes is also a critical factor in 
improving the current distribution. 



4.11.7 Effect on Current Distribution of Hoiidays on a Coated 
Structure 

Although the path resistance from remote earth to a holiday or to the coated 
surface adjacent to the holiday is essentially the same, the current densities 
through the coating are orders of magnitude less than at the holiday. Nevertheless 
on well coated pipelines, most of the cathodic protection current passes through 
the coating rather than the holidays. 

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Consider a 30 cm diameter pipeline that is 20 m long and has a dielectric coating 
with a specific resistance of 10 Q-m in 4000 Q-cm soil. Assuming there is a 
single 1 cm diameter circular holiday as shown in Figure 4-35, the current to both 
the holiday and through the coated surface can be calculated. Note a 1 cm 
diameter holiday is 0.0004% of the total surface area of the 20 m length of pipe 
that is realistic for new coated piping (see Figure 4-6). 



20 m 



well ooated pipe 



1 cmdia 

r .„4 I 2 T T holiday 

re =10 ohm-m^ lcp,dd ^op,h 

Figure 4-35: Cathodic Protection Current Distribution to a Well Coated Pipe with a Holiday 

The leakage resistance (Rl) of the coated pipe to remote earth is given by 
Equation 4-54. 



Rr 



A, 



[4-54] 



where the surface area of the pipe: 



A 



s,p 



TidL 



3.14 X .3 m X 20 m 



18.84 m^ 



therefore: 



R 



lO^Q-m' 



^ 18.84m' 



0.53 X lO'Q 



Assuming an applied CP voltage of 300 mV between the pipe and remote earth, 
then from Ohm's law and Equation 4-8: 



/ 



300mV 



cp^ctd 



0.53 X 10' Q 
and the current density through the coated surface 



0.565 mA 



cp,ctd 



0.565 mA 
18.84m' 



0.033 



mA 



m 



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4:75 



= 33 laA/m^ = 33 x 10"^ )aA/cm^ 
The holiday resistance to remote earth can be calculated using Equation 4-55 for 
a circular disk. 



R. 



_P_ 
2d 



[4-55] 



then: 



Rv 



4000 Q- cm 
2 cm 



2000 Q 



and from Ohm 's law: 



cp,h 



300 mV 
R. 



300 mV 
2000 Q 



0.15 mA 



then: 



cp.h 



0.15 mA 



0.15mA 



0.15 mA X 4 
3.14 cm' 



0.191 



mA 



cm 



= 191 laA/cm^ 

The ratio of the holiday current density (icph) and coating current density (icp,ctd) is 
therefore: 



cp.h 



cp.ctd 



191|jA/cm' 



33 X 10-^ |iA/cm' 



5.8x10' 



Even though the current density at the holiday is over 4 orders of magnitude 
greater than through the coating, the total current through the coating (0.565 mA) 
is almost 4 times greater than at the holiday. Thus on well coated pipelines most 
of the cathodic protection current passes through the coating not through the 
holidays. 



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4.11.8 Effect of Polarization (Time) on Current Distribution 

Calculations of current distribution do not normally incorporate polarization 
effects because they are difficult to calculate. When current is initially applied in 
the absence of significant polarization, the distribution of cathodic protection 
current is solely determined by the relative resistances of the current paths. This 
initial distribution of current is referred to as primary current distribution. 

With time and the buildup of cathodic reaction products such as calcareous 
deposits, the polarized potential becomes more negative resulting in less 
attenuation and improved current distribution as indicated in Figure 4-36. 



drain point 




primary current distribution 
secondary current distribution 



Figure 4-36: Effect of Polarization witli Time on Attenuation Profile 

Polarization is enhanced on structures in some soils and in seawater because of 
the formation of calcium and magnesium deposits at the holidays in the coating. 
These deposits reduce the diffusion of oxygen to the surface, help maintain the 
high surface pH, and, in low resistivity electrolytes, increase the resistance of the 
holiday current path. Thus when a cathodic protection system is initially turned 
on there is a primary distribution current based on electrical circuit resistances, 
but as polarization increases with time there is a secondary distribution of 
cathodic protection current due to the polarization back voltage (see Figure 4-20). 



4.11.9 Summary of Current Distribution Factors 

The most difficult aspect of a cathodic protection design is achieving relatively 
uniform current distribution on the structure being protected, due to the many 
factors affecting current distribution as summarized in Table 4-9. 



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Table 4-9: Summary of the Effect of Various Factors on Current Distribution 



Factor 


Effect on Current Distribution 


Electrolyte Resistivity 

Increase 

Decrease 

Variable 


Improves 
Diminishes 
Diminishes 


Structure Resistivity 

Increase 

Decrease 

Variable 


Diminishes 

Improves 

Diminishes 


Coating Quality 

Poor 
Excellent 


Diminishes 
Improves 


Distance between Anode and Structure 

Small 
Large 


Diminishes 
Improves 


Polarization 

Large 
Small 


Improves 
Diminishes 



Many of these factors can be addressed in designing a cathodic protection system, 
particularly in the choice of system, anode-to-stmcture spacing, and placement of 
the anodes. As illustrated in Figure 4-37, there are many options from which to 
choose. 



Galvanic 



Distributed Remote 



Surface 



Current Source 

and 

Anode Configurations 



1 



Surface 



Impressed Current 



Distributed 



Remote 



Surface 



I 



Su rface 



Deep 



1 



Semi-deep 



Figure 4-37: Typical Anode Arrangements 



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Team Project 

Design a cathodic protection system with a 20 year minimum life for a 30 cm 
diameter coated high pressure natural gas pipeline located in a semi-urban area, 
given the following conditions: 

• pipe is electrically isolated at both ends 

• average soil resistivity is 4,700 Q-cm to a depth of 4 m then 21,000 
Q-cm 

• specific coating resistance in 4,700 Q-cm soil is 6 x 10 Q-m 

• a minimum cathodic protection voltage of 300 mV must be applied 
to the pipe. 

Produce a design sketch showing cathodic protection current source(s), type of 
anode, etc. as per the design flow chart of Figure 4-1. Omit calculation of system 
cost. State advantages and disadvantages of your design. The instructor will 
choose the pipe length. 



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CHAPTER 5 

EVALUATION OF CP SYSTEM PERFORMANCE 



5.0 Introduction 

The effectiveness of a cathodic protection system ultimately is confirmed by 
whether or not it controls corrosion adequately. Although corrosion can be 
identified on some pipelines using smart pigs, determining corrosion rate directly 
is generally not a simple process. Hence indirect methods of assessing the 
adequacy of a cathodic protection system must be relied upon. The principal 
method is the measurement of the structure potential for comparison to the 
selected criterion. System currents are also measured as an additional 
performance parameter. Indeed, for oil and gas pipelines, potentials must be 
measured routinely for compliance with government regulations. 



5.1 The Potential Measurement 



A stmcture-to-electrolyte potential measurement involves using a high resistance 
input meter coimected between the structure or structure test lead and a portable 
reference electrode placed in contact with the electrolyte, as illustrated for a 
pipeline in Figure 5-1. 



high input 
resistance meter 



t 




portable reference electrode ' 



Figure 5-1: Illustration of a Typical Pipe-to-Soil Potential Measurement 

It is assumed that that the potential of the reference electrode does not change 
while moving from location to location, otherwise an error is introduced into the 
measurement. To ensure measurement accuracy and reproducibility, the reference 
electrode must be maintained in good condition. 



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5.1.1 Copper-Copper Sulfate R eference Electrode 

Table 5-1 lists maintenance items for the saturated copper-copper sulfate electrode 
(CSE) shown in Figure 5-2. 



Removal 
Cap 



Clear 
Window 



Undissolved 

Copper Sulfate 

Crystals 




Connection 
for Test Lead 



Copper Rod 



Saturated 
Copper Sulfate 
Solution 



Porous Plug 



Figure 5-2: Copper-Copper Sulfate Reference Electrode 



Table 5-1: List of Copper-Copper Sulfate Maintenance Items 



Use and Care of CSE 



Keep clean 

Cap when not in use 

Clean porous plug 

Keep free of contamination, especially by CI" 

Keep spares in field 

Keep in calibration (versus SCE) 

Record temperature when in use 

Shield from direct sunlight 

Coppersulfate crystals must be present to 

ensure a saturated solution 



Two important items on the maintenance list are the need to prevent 
contamination especially by chloride and the requirement for the solution to be 
saturated with copper ions. Variations in these conditions can significantly 
change the reference electrode potential as shown in Figures 5-3a and 5-3b. As 
the concentration of chloride ions increases the reference electrode potential shifts 
in the negative direction. As the concentration of copper sulfate decreases, the 

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5:3 



reference electrode potential also shifts electronegatively. In both cases the 
measured structure potential would be more electropositive. Typically a silver- 
silver chloride reference is used to measure potentials on structures in seawater or 
brine solutions. 




5 10 

CI Concentration, ppt 



150 

100 
> 

E 

E 50 

SI 

if) 



B 
o 

^ -50 



-100 



^35*^ 




0.01 0.1 1 10 100 1,000 

Copper Sulfate Concentration, ppt 



Figure 5-3a: Effect of CI Concentration 
on CSE Potential^ 



Fig-5-3b: Effect of Copper-Sulfate 
Concentration on CSE Potential^ 



In addition, another important factor is the reference electrode temperature. The 
copper sulfate reference electrode has a temperature coefficient of about 0.9 
mV/°C as indicated in Table 1-3. Thus the expression for the copper sulfate 
electrode potential with respect to a standard hydrogen electrode (SHE) at any 
temperature (Ecse,t) is given in Equation 5-1. 



^cse,t 



Ecse,25"c + 0.9mV/°C(T-25°C) 



[5-1] 



Centigrade degrees can be converted to degrees Fahrenheit using the following 
relationship: 



°Cx- + 32 



[5-2] 



Therefore for 25°C: 



r 



)^ 



25°Cx- 

V 5y 



+ 32 



45 + 32 



^ Ansmni, F.J. and DimondJ.R., Factors Affecting the Accuracy of Reference Electrodes, MP, Vol. 33, 
No.ll,Nov.l994,p.l6. 

- Ibid. 1 

a 

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5:4 



°F 



77°F 



Since the equivalent temperature coefficient is 0.5 mV/°F, the temperature 
compensation in Equation 5-1 can be written as follows: 



■^cse,t 



-cse,77°F 



0.5 mV/°F (T - 77 °F) 



[5-3] 



For example at 25°C, the copper-copper sulfate reference electrode potential is 
+0.3 16 Vshe. At 5°C the potential would be: 

Ecse,5°c = +316mVshe + 0.9 mV/°C [5°C - 25°C] 
= +316mVshe + 0.9 mV/°C [-20°C] 
= +316mVshe - 18 mV 

Ese,5°C = +298mVshe 

This means a -850 mVcse structure potential measured at 25°C would be equal to 
a -832 mVcse potential measured with the reference electrode temperature at 5°C. 
Conversely, a -850 mVcse structure potential measured at 25°C would be equal to 
a -868 mVcse with the reference electrode temperature at 45°C. Accordingly, 
when potentials are measured the temperature of the reference electrode should 
also be noted. 



5.1.2 B uried R eference E lectrodes 

In some situations, it is desirable to use a buried or permanently immersed 
reference electrode such as at a crossing between two cathodically protected 
pipelines, between underground storage tanks beneath a reinforced concrete slab, 
inside a water storage tank, etc. These references, sometimes labeled "permanenf 
reference electrodes have a finite design life normally stipulated by the 
manufacturer. 

Reference electrodes intended for burial or immersion are commercially available 
from a number of manufacturers. For underground use CSE, SSC, and zinc 
reference electrodes are available. For extended service life, these electrodes are 
normally prepackaged in a bentonite/sulfate rich backfill similar to that used with 
magnesium and zinc galvanic anodes. 



a 



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5.1.3 Polarity Considerations 

There are two common practices for coimecting the voltmeter between the 
reference electrode and the structure whose potential is being measured as shown 
in Figures 5 -4 a and 5 -4b. 



Voltmeter - 



■900 mV 



+ - 



Meter display shows 
a negative polarity sign. 
Record a negative 
structure potential. 



Reference 
Cell 




-y/Ay//V/AV/Ay/. 



Electrolyte 



Structure 



Voltmeter 




Meter display Is a 
positive reading. 
Record a negative 
structure potential. 



7v/AV/v/Ay/A>'/AVAH/Ay/Ay/Ay// 

Electrolyte 



Structure 



(a) (b) 

Figure 5-4: Structure-to-Soil Potential Measurements 



As long as the meter polarity sign, which appears in the display, is interpreted 
properly it does not matter which method is used. The polarity sign should be 
interpreted as follows. For Figure 5 -4a, the negative polarity indicator means the 
polarity of the pipe with respect to the reference is opposite to the polarity 
markings on the meter terminals. Therefore the pipe, which is cormected to the 
positive terminal, is not positive but negative and the structure potential is written 
as: 



V„ 



-900 mV/ref 



For Figure 5-4b, the lack of a polarity indicator in the display (on some meters a 
positive sign will appear) means the polarity of the pipe agrees with the polarity 
markings on the meter. Since the pipe is cormected to the negative terminal the 
pipe potential is written as: 



Vn 



-900 mV/ref 



The structure potential measurement recorded above must have four distinct 
components: a polarity sign, a numerical magnitude, a unit of measurement, and 
the reference used. This notation is a short form for the statement: " The structure 



a 



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potential is negative 900 miUivolts witi] respect to the reference electrode." 
Hence if one component is missing from the notation, the statement is incoherent. 

5.1.4 The Potential Measurement C ircuit and Measurement 
Error 

The intent of the potential measurement is to determine the pipe potential (Ep) 
accurately at the test location. The measurement circuit can be approximated by 
the following electrical circuit. 




R|^ = \olttrEter input resistance 
Rfl = test lead resistance 
Rp g = pipe-to-earth resistance 
R = reference-to-earth resistance 
I|-p = meter curreait 

Etrue = Ep - Eref 

but Eref isassurrEdtobezero 



Figure 5-5: Electrical Schematic of the Pipe-to-Soil Measurement Circuit 



It is the true potential difference (Etrue) between the pipe and reference electrode 
that ideally should appear across the meter terminals. Because the meter circuit is 
a series circuit, the magnitude of the voltage drop that appears across the meter 
will be proportional to the ratio of the meter resistance to the total meter circuit 
resistance. 

For the measurement circuit, KirchhofP s voltage law applies and the true 
potential difference is equal to the sum of the voltage drops around the series 
circuit. 



Etrue = Im [RfU + Rtl.2 + RtL3 + Rp,e + Rr,e + Rm] 

Etr.e = VtU + Vtl.2 + VtU + Vp,e + V,e + V,, 



[5-4] 



V„. = Etrue - [Vtu + Ytia + Vtu + Vp.e + Vr.e] 



a 



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Let Vcirc equal all voltage drops in the circuit except for the meter voltage drop 



Vn 



^true * circ 



then: 


Divide both sides by Etrue 






v„ 


E - V 

true circ 




'^true 


^true 


but: 


Etrue = ImRt O^nd Vcirc 


^m^circ 



and after substitution: 



v„ 



^iric 



iJRt) 



and: Rt - Re 



R„ 



V,. 



R„ 



R. 



[5-5] 



Hence, the amount of voltage (Vm) that appears across the meter compared to the 
true potential difference (Etme) is proportional to the ratio of the meter resistance 
(Rm) compared to the total resistance. 

Forexanjie. Consider a true potential Etme = 1,000 mV, each test lead 
resistance of 0.01 ohm, a pipe-to-earth resistance (Rpe) of 10 ohms, a 
reference electrode resistance to earth (Rr,e) of 100 kQ, and a meter 
resistance of 1 MQ. Calculate the voltage that would appear across the 
voltmeter. 



Rt 



Rt 



3Rti + R 



P,e 



Rr,e + Rm 



3(0.01) + 10 + 10^ + 10^ 

1.1 MQ 



FwmEquation 5-5 



v„ 



R, 



R, 



X E, 



a 



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V. = 
This is an error of: 



l.OMQ 
I.IMQ 



X 1000 mV 



1000 - 909 
1000 



X 100 



909 mV 



9% 



If the meter input resistance in the foregoing example is increased to 
10 MQ, the voltmeter would read 990 mV which would reduce the error 
to 1%. 

The voltage drop across the voltmeter (Vm) will approach the true 
potential difference between the reference and pipe as the ratio of the 
voltmeter resistance to total measurement circuit resistance approaches 
one (i.e., Rm/Rt ~^ !)• That is, the voltage across the voltmeter 
approaches the true potential as the meter resistance becomes much 
greater than the other resistances in the measuring circuit. 

High resistances in the measuring circuit other than across the voltmeter should 
therefore be avoided. Reference electrode contact resistance can be a source of 
error when the reference is placed on dry soil, well drained gravel, crushed stone, 
frozen ground, asphalt, or concrete. To minimize this error, the contact 
conductance can be improved by wetting the area around the reference. In 
extreme cases, a hole can be drilled from the surface to a depth of permanent 
moisture and the reference placed in the hole or an electrolytic bridge can be 
created between the reference and earth (Figures 5-6a and 5-6b). 



PVC tube 




drysoil 
or ■ 
frozen ground 




small diameter 
hole filled with 
a soapy water 



^/AYAxX/AVAy/AyTTy/AV/A^/AV/Ay/AV/ 



a) dry soil or frozen ground b) asphalt or concrete 

Figure 5-6: Methods of Minimizing Reference Electrode Contact Resistance 



In Figure 5-6a, the depth from grade to clay must be below the frost line in frozen 
soil and to the depth of permanent moisture in dry soil. For asphalt or concrete, a 



a 



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Evaluation of CP System Performance 5:9 



soapy water solution will usually provide sufficient electrolytic contact even if 
the water level in the hole drops. 

High measurement circuit resistance can also occur as a result of broken test 
leads, test lead coimection resistances, and pipe resistance to earth if the pipeline 
is short and well coated. 

When measuring a pipe-to-soil potential, it may not be immediately apparent that 
a high circuit resistance is present. If the voltmeter has an input resistance selector 
switch the existence of a high resistance in the measurement circuit can be 
identified by switching to a lower or higher input resistance. If the potential 
indicated by the voltmeter differs significantly (i.e., more than 10%) between the 
two input impedances then there is a high resistance in the measurement circuit. 
Further by knowing the two input resistances and their corresponding measured 
voltage, the true potential can be calculated using Equation 5-6. 



[5-6] 





V,(l-K) 


true 


V, 



where: 

Etrue = true potential (V) 

K = input resistance ratio R;/Rh 

Ri = lowest input resistance 

Rh = highest input resistance 

V| = voltage measured with lowest input resistance 

Vh = voltage measured with highest input resistance 

For example: If a potential difference (Vi) of -650 mVcse was measured 
with an input resistance (R;) of 1.0 MQ and a potential difference of - 
800m Vcse (Vh) was measured with an input resistance (Rh) of 10 MQ, 
then the true potential (Etrue) would be calculated as follows: 



E 



■800 mV(l - 0.1) 
'™^ " ^-800 mV^ 



0.1 

-650 mV 

-720 mV -720 mV 



J 



= -821mV„3 
0.123 0.877 



a 



Etrue is the same as the polarized potential. 
MACE 



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5:10 



In addition, the total circuit resistance (Rt) can be determined using Equation 

5-5. 



R. 






10 MQ X 821 mV 
800 mV 



R = 10.3 MQ 

This means that the resistance in the measuring circuit, excluding the 
meter resistance is: 



Kcirc 



Rt - Rm 

10.3 MQ -lOMQ 
0.3 MQ or 300,000 Q 



5.2 Voltage Drop Errors External to the Metering Circuit 

5.2.1 Voltage Drop Errors in the Potential Measurement due to 
Current in the Earth 



As charges flow in the earth to or from the pipe and with earth's resistance, 
voltage drops occur in the earth creating a voltage gradient around the pipe as 
illustrated for a bare pipe in Figure 5-7. 



equipotential 
line (surface)- 



J 



MVmH- 



4a. 



current 
line 




Figure 5-7: Voltage and Current Lines around a Bare Pipeline Receiving 
Cathodic Protection Current 

Source: Parker, Marshall and Peattie, Edward, Pipeline Corrosion and Cathodic Protection, 3^'' Edition, Gulf Publisliing 

Co., Houston, TX, p. 25 



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The radial lines denote the current paths while the lines perpendicular to the 
current lines represent the equipotential surfaces created by the current. The 
equipotential surfaces, which are perpendicular to the current paths, are not 
evenly spaced but increase with distance away from the pipe because each 
successive shell of earth has a larger surface area and hence a lower resistance. 

If a potential measurement is taken with the reference electrode located at A and 
the current direction is toward the pipe (as would be the case in cathodic 
protection), then there is a voltage drop (Vg) in the soil between the reference 
electrode and the pipe surface. The soil at point A is more positive than the soil 
immediately adjacent to the pipe surface. If the potential difference between 
adjacent equipotential surfaces is 10 mV, the voltage drop in the soil between the 
pipe surface and the reference location would be 10 lines x 10 mV =100 mV. 
The soil at the pipe surface is -100 mV with respect to the soil at the reference 
electrode. 



structure 
drain point 



-MV, 



reference electrode 



Ve I + 



Ep - 



CP 
Source 



+ 



icp 



-groundbed 



Figure 5-8: Electrical Schematic Illustrating Soil Voltage Drop in the Potential Measurement 



For the pipe-to-soil measurement shown in Figure 5-7 and illustrated in Figure 5- 
8, the potential difference (Vm) indicated on the voltmeter will be given by the 
Equation 5-7. 



V. 



Ep + Ve 



[5-7] 



where: Ve = voltage drop in the earth 



If the current direction was away from the pipe, then Equation 5-8 would apply. 



V. 



Ep- Ve 



[5-8] 



a 



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5:12 



For example, if the polarized potential (Ep) of the pipe is -790 mVcse the 
voltmeter will read: 

V,„ = -790mVcse + (-100 mV) 
V,, = -890 mVose 

Thus there is a 100 mV error in the measurement that makes it appear as if the 
pipe is better protected than it is. 

For a well coated pipeline, the equipotential field forms in close proximity to 
the holidays as shown in Figures 5-9 and 5-10. 



"X 



-x 



X 




Figure 5-9: Current and Voltage Lines around a Holiday on a Coated Pipeline 




bli 






-7^Kd>^<: 




Figure 5-10: Current and Voltage Lines in Immediate Vicinity of a Holiday 

On a coated pipeline, most of the voltage drop is concentrated in the immediate 
vicinity of the holiday. Typically 95% of the total voltage drop between the 
reference and the steel exposed at the holiday is found within about 10 diameters 



a 



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5:13 



of the holiday (i.e., 10 d). For a 1 cm diameter holiday, 95% of the voltage occurs 
within a radius of 10 cm from the holiday. 

5.2.2 Voltage Drop Errors in the Potential Measurement due to 
Current in the Pipeline 

Voltage drops also occur in current-carrying metal paths and if the coimection to 
the pipe is remote from the location of the reference electrode, as shown in Figure 
5-11, there will be an IR drop error (Vp) in the potential measurement. 




Figure 5-11: Voltage Drop in a Pipeline Carrying Current 

When a coimection to a current-carrying conductor is made as illustrated, the 
voltage drop error will be additive or subtractive depending on the direction of 
the current. This situation is illustrated in the electrical schematic of Figure 5-12. 



■<5H 



® 



Q- 



-MV,)-* 



® 



\y/W-W 



VWV\/^y\AA/^A^V^rvVVNAA''^^vV^WW'/^vW\A 



icp- 



-► + 



■Vp 



->+ -* 1, 



cp 



CP current 
source 



n + 



Figure 5-12: Electrical Schematic to Illustrate Potential Measurement Error 
due to CP Current in a Pipeline 



Gummow, R.A., The Cathodic Protection Potential Criterion for Underground Steel Structures, NACE 
International, CORROSION/93, Paper No. 564, p. 5. 

''NACE 



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J anuary 2007 



Evaluation of CP System Performance 



5:14 



At location © the potential difference (Vm) measured by the voltmeter will be the 
pipe potential (Ep) less the pipe voltage drop (Vp) as follows: 



V„ 



Ep- Vp 



[5-9] 



But for location @ with the current in the opposite direction, the Vp error will be 
additive rather than subtractive and: 



V,, = Ep + Vp 



[5-10] 



5.3 Methods of Minimizing Voltage Drop Errors in the 
Potential Measurement 

A typical potential measurement on a pipeline can include both earth and pipe 
voltage drops so that the potential difference between a pipe and a reference 
electrode can be expressed as in Equation 5-11: 



Vn 



Ve ± Vn 



[5-11] 



Whether the error is additive or subtractive depends on the direction of current. 
Equation 5-11 can also be written in terms of current and resistance according to 
Ohm's law as follows: 



V„ 



Ie(Re)± Ip(Rp) 



[5-12] 



Examination of either equation indicates it is the polarized potential of the pipe 
(Ep) that is required for comparison to industry potential criteria (e.g., 
-850 mVcse). The ideal situation is to have V^ = Ep with no IR drop error. 

5.3.1 C urrent Interruption Method 

If the only current producing the voltage drops is the cathodic protection current. 
Equation 5-12 can be rewritten to 

V^ = Ep ± Icp*Re+ Icp'Rp [5-13] 

Then it is apparent that if Icp = 0, Vm would equal Ep since the IR drop terms go to 
zero. 



a 



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5:15 



Vn 



Ep ±J^, ±1^^ 



[5-14] 



In this method, the cathodic protection current is interrupted momentarily and a 
potential recorded immediately after interruption, which is referred to as an 
instant-off potential or just off potential. The interruption for convenience is 
accomplished using a cyclic interrupter inserted in the cathodic protection circuit 
and adjusted so that the ON half-cycle time is at least twice the OFF half-cycle 
time (e.g., 10 seconds ON and 5 seconds OFF). It is advisable to keep the OFF 
half-cycle time to a minimum so the structure doesn't depolarize significantly 
during the course of the survey. This potential response is shown in Figure 5-13. 



-I- 



o 

Q. 



'OFF' Potential 



IR drop-< 



'ON' Potential 



Time - 



Figure 5-13: Graphical Illustration of the Current Interruption Method of 
Minimizing Voltage Drop Error in the Potential Measurement 

The interrupter is started at ti and interrupts the current for ti-t2 and turns the 
current on at ^2- The ON half-cycle ^2 to t^ should be at least twice the ti-t2 half- 
cycle. The sudden change in potential from ON to OFF is the disappearance of 
the IR drop in the measurement. This IR drop reappears when the current is 
switched back on. 

The instant-off potential is considered equal to the polarized potential because it 
is assumed the potential across the structure-electrolyte interface is stored 
momentarily in the double-layer capacitance. 



The current interruption technique is widely used when conducting close interval 
potential surveys. The example data in Figure 5-14 indicates that subcriterion 
potentials are located in sections A and B. In section A the instant-off potentials 
are subcriterion but the on-potential is more negative than -850 mVcse, whereas in 

''NACE 

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5:16 



section B both the on and instant-off potentials are subcriterion. After a close 
interval survey has been completed, on-potentials at each location where the 
instant-off potential is more negative than the -850 mVcse criterion can be used as 
a second-hand criterion for future surveys providing there are no changes in the 
soil conditions such as moisture, aeration, pH, etc. or in the pipe operating 
temperature. 



0.6- 




-•- on-potential 

-A- instant-off potential 


0.7- 




A^ 


0.8- 


-850mVcse j^- 


.-^--A- -A. A/\! A 




. criterion^- - A' ' JS— 


-^r-- ■ ■ \ :^- ^ ■ ■ ^»>. 


0.9- 


-^ •— ^^A--A^ *j;^ 


1.0- 


- ti^ m^^^'*'^ ! 


A k < R » V ^ 


-1.1- 


¥ 1"^ 




1.7- 


\ 1 \ h- 


— 1 1 1 1 1 1 1 1 1 1 1 — 



Distance 



Figure 5-14: Example of Close Interval Potential Survey Data Plotted Versus Distance 
for Both ON and Instant-Off Potentials 

Using the Randle's circuit model of electrode interface in Figure 5-15, it can be 
seen that the potential across the interface (Ep) is across the parallel combination 
of the double layer capacitance and the polarization resistance. Before the current 
is interrupted, the voltmeter measures the polarized potential plus the voltage 
drop across the electrolyte resistance (Re). When the current is zero, IcpRe will be 
zero and the voltmeter will measure the polarized potential (Ep) 




® — T 



•< Icp 

soil (electrolyte) 



Cjji = double la^«r capacitance 
(1-200 |xF/crr?) 

R = polarization resistance 
(1-10* Q-cn^) 

Rg = resistance of steel surface 
to remote earth 

E = potential dilierence (volts] 



Figure 5-15: Illustration of Current Interruption Technique for Minimizing Voltage Drop Errors 
Using Randle's Model of the Electrode/Electrolyte Interface 



a 



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5:17 



If there is significant inductance in the cathodic protection circuit or if the current 
is large, a positive spike may appear in the potential upon interruption as shown 
in Figure 5-16. 



c 

0) 

o 

D. 



positive spike 



'OFF'Potentiai 



'ON' Potentiai 



Time (m sec) 



Figure 5-16: Illustration of Positive Spike in Potential When CP Current is Interrupted 

Typically the spike time lasts less than 300 milliseconds'* after which the off- 
potential can be recorded without incorporating this transient spike error in the 
potential measurement. Therefore, if ^2 is the time when the off-potential is being 
measured, then ti-t2 > 300 milliseconds. If a shorter measurement time period is 
contemplated, the potential-time waveform should be captured on an electronic 
oscilloscope to determine the magnitude and time duration of any switching 
transient. 

When there are multiple current sources, simultaneous interruption of all the 
current sources achieved using synchronized interrupters is used for convenience, 
but it is not essential. Individual sources can be interrupted and the IR drop 
contribution of each can be recorded separately and the totaled IR drop subtracted 
from the on-potential. This is quite an arduous procedure for a long pipeline with 
many widely spaced rectifiers and many test point locations, but is more practical 
in a process plant were travel time is not an issue or where simultaneous 
interruption of rectifiers may be impractical. 



As indicated in Equation 5-14, the current interruption technique reduces both the 
soil and structure IR drop components to zero, but for accuracy all currents must 



"* Thompson, N.G., and Lawson, K.M., Causes and Effects of the Spiking Phenomenon, PRCI, Report 
#]PR186-006, .Tan 1992, p 94. 

'^NACE 



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Evaluation of CP System Performance 



5:18 



be interrupted, including stray currents. This is not possible however in some 
cases if telluric currents or transit stray currents are present. 

Also, the accuracy of this technique is compromised if there are recirculating 
currents present. A recirculating current is a post-interruption current between 
highly polarized and lesser polarized areas on a structure as depicted in Figure 5- 
17 or between parallel pipelines where one pipeline is more highly polarized than 
the other. 



recirculating current (Ir) 



highly polarized area 



lesser polarized area 



T/R 



Figure 5-17: Illustration of Recirculating Current Activity after the Interruption of CP Current 

As would be expected, the piping closest to the transformer-rectifier and 
groundbed will be polarized to a greater extent than remote piping surfaces. Off 
potentials measured along the pipeline in the highly polarized areas will be more 
electropositive than the true potential. This is due to the IR drop created by the 
recirculating current (Ir) flowing away from the pipe (i.e., V^ = Ep- IfRe). 
Conversely, in the recirculating current pick-up region, the off-potential will be 
more negative than its true potential (i.e., Vm = Ep + IrRe), which might mask an 
otherwise subcriterion condition. Potential shifts due to recirculating currents are 
typically in the order of to ±150 mV. 



5.3.2 Stepwise C urrent Reduction Method of Determining the 
Amount of SoillR Drop in the On-Potential 

The stepwise current reduction technique involves recording the potential shift in 
the pipe-to-soil potential measurement and a side drain potential measurement as 



^ Thompson, N.G. and Lawson, KM., Most Accurate Method for Measuring an Off-Potential, PRCI Final 
Reporton Contract PR-186-9203, March, 1994, p.ix. 



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5:19 



the cathodic protection current is reduced. The test arrangement is illustrated in 
Figure 5-18. 



to pipe test lead 5 (V 




Figure 5-18: Field Test Arrangement for the Stepwise Current Reduction Method 
of Determining the Amount of IR drop in the On-Potential 



It is understood that the on-potential recorded on the voltmeter (Vm) is 



Vn 



I • R 



And that if the Icp current was reduced the on-potential would decrease because 
the earth voltage drop (IcpRe) would decrease. If the stepwise reduction in 
cathodic protection current was continued until Icp = 0, then V^ = Ep and the IR 
drop would also be zero. Further, the side drain potentials Vs,c and Vsa should 
also approach zero as Icp approaches zero. Assuming the earth voltage drop IcpRe 
and the side drain voltages obey Ohm's law, then these parameters should be 
linearly related. 

By reducing the cathodic protection current in increments and measuring Von, 
Vs,c, and Vsa, the data can then be used to construct the graph of Figure 5-19. 



a 



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5:20 















> A( 






/ 


extrapolation 




' ' 


\ 












\ 








>'^ 




^x-^ 








c 




\ 


\ 






Q. 
O 






\ 


""^ 




Q 












a: 








t\ 




S 
o 








AVon,2\ 


NP 


1- 










"P^® 










!^AV3,2 


-!^AV3,i^| 



Side Drain Potential (mV) 
Figure 5-19: Data Plot for Stepwise Current Reduction Technique 

Step © Plot Vsa or Vs c on the abscissa with full current 

Step © Reduce Icp, calculate AVsi, and AVon,i; 
plot AVsi and AVon,i to obtain point © 

Step ® Repeat Step © and plot AVs2 and AVon.2 to obtain point ® 

Step ® Draw a best fit straight line through the data points and 
extrapolate the line until it intersects the ordinate at A. 
A is then the amount of IR drop in the original on-potential 
measurement. 



The polarized potential (Ep) is then determined by subtracting the IR drop at A 
from the on-potential measured at location B. 



I.e., 



V 



m,b 



A 



Two lines could be plotted for both side drain measurements that should give the 
same intercept A if the current and soil resistivity is symmetrical around the pipe. 

Current reduction time intervals should be kept as short as possible, otherwise the 
polarized potential (Ep) will reduce as well as the earth IR drop and result in a 
larger IR drop indication. If the potential changes are recorded quickly, this 
technique can be used in the presence of dynamic stray currents. 



a 



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5:21 



The technique is time consuming and seldom used since if it is possible to reduce 
the cathodic protection current it is possible to interrupt the current and use the 
current interruption technique. 

Theoretically, this method could be used on piping with attached galvanic anodes 
by applying a test current in increments. This would then be a stepwise current 
increase technique. 

On very well coated pipelines, the side drain potential may be small which would 
increase the angle of the line such that a small error in plotting would result in a 
large change in the point of interception. Hence, its accuracy for well coated 
pipelines may be compromised. 



5.3.3 Reference Electrode Placement Close to the Structure 

The voltage drop (IRe) included in the on-potential measurement is the voltage 
drop in the earth between the surface of the structure and the reference electrode 
position as previously illustrated in Figure 5-7. If the reference electrode could be 
moved closer to the pipe, as shown in Figure 5-20, the on-potential (Von) would 
approach the polarized potential (Ep) in value since the IRg drop would approach 
zero. 



to pipe test lead 




reference electrode 
placed close to 
pipe surface 






Figure 5-20: Reference Electrode Placed Close to Pipe Surface to Minimize IR Drop Error in 

Potential Measurement 



a 



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5:22 



The reference should not be placed too close to the surface or the surface will be 
shielded from cathodic protection current. It is usually recommended that the 
reference be no closer than two diameters of the reference. 

This technique is not very practical for buried pipelines but is applicable to 
underwater structures and to piping appurtenances such as valves and risers as 
illustrated in Figure 5-21. 







/<7A 




T 




C 

f 


-v^ 






— - ^ 






V 




\ 








Figure 5-21: Reference Electrode Placed Close to a Bare Riser Pipe 

If the reference is placed close to a coated pipeline the amount of IR drop 
reduction will be minimal as depicted in Figure 5-22. 



>; 



X 



X 




Figure 5-22: Reference Electrode Placed Close to a Coated Pipe Surface 



The reference electrode would have to be placed extremely close to a sizable 
coating holiday before the IRg drop reduction would be significant, which is 
generally impractical. 



NACE 



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5:23 



An alternative to placing the reference close to the structure is to install a plastic 
tube filled with soil from grade next to the pipe surface as shown in Figure 5-23. 



to pipe , 
test lead ^ 



plastic soil tube 







Figure 5-23: Using a Soil Tube to Minimize IRe Drop in a Potential Measurement on Bare Pipe 

As there can be no cathodic protection current in the soil tube, there can be no IRe 
drop between the reference and the bottom of the tube, thus minimizing the IRg 
drop error in the measurement. 



5.3.4 Using Coupons to Minimize Voltage Drop Errors in the 
Potential Measurement 

A cathodic protection coupon is intended to simulate a small portion of the pipe 
surface, which on a coated pipe would be a holiday. Coupons are made from an 
alloy similar to that of the structure. They are typically 10 to 100 cm in surface 
area and are installed in conditions similar to those a coating holiday would 
experience. A coupon is placed near the structure with its test lead routed into a 
test station and coimected to a pipe test lead as shown in Figure 5-24. 



a 



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Evaluation of CP System Performance 



5:24 




steel coupon 



Figure 5-24: Using a Steel Coupon to Simulate a Holiday on a Pipeline 

To measure the polarized potential of the coupon, the coupon is momentarily 
discoimected from the pipeline and its instant-discormect potential is recorded 
with respect to the portable reference electrode placed in the soil tube positioned 
above the coupon. The measured coupon polarized potential (Ep cpj is considered 
to be the same as a nearby holiday having the same surface area as the coupon 
and exposed to the same soil conditions. It should be noted that the coupon 
polarized potential will not necessarily be the same as the pipe polarized potential 
at this location since the pipe potential represents a number of holidays of 
differing surface areas rather than a single holiday. Nevertheless, it is assumed 
that if the coupon polarized potential (Ep cpn) is equal to or more negative than 
-850 mVcse, any holiday of similar surface area or smaller will be equally as well 
protected. 

The ability of a coupon to monitor the effectiveness of a cathodic protection 
system was evaluated in a series of field tests at eight pipeline field sites by the 
Pipeline Research Council International Inc. (PRCI).^ The cathodic protection 
coupon arrangement used exclusively in the testing program consists of two steel 
rods, each having a surface area of 9 cm (1.4 in ) integrated into a plastic test 
station as shown in Figure 5-25. 



'' Thompson, N.G. and Lawson, K M ., D evelopment of Coupons for Monitoring Cathodic Protection 
Systems, PRCI Contract PR-186-9220, Catalog No. L51888, Final Report, Dec. 2001. 

''NACE 



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5:25 



Coupons/ 
Conduit 




Reference Tube 

(Filled w/soil to 

ground level) 



Reference Electrode 

Duplicate Leads 

Plastic Conduit 

^—Ground Level 



Soil 



i^ 



Plastic Conduit 

Coupons 
(placed near pipe) 



Figure 5-25: Schematic of an Integrated Coupon Test Station 

Source: Thompson, N.G. and Lawson, K.M., Development of Coupons for Monitoring Cathodic Protection Systems, 
PRCI Contract PR-186-9220, Catalog No. L51888, Final Report, Dec. 2001. 

Tests were conducted that compared the coupon instant-disconnect potential with 
the instant-off potential with the coupon and ICCP system interrupted. The vast 
majority of these two potential measurements were within ±25 mV as indicated in 
Figure 5-26. 



<«60 

CD 



= 40 

o 

o 

O 30 



Z 20 
f 10 







% 



^o. 



^cf. 



^^, 



^■^s. 



^^o. 



% 



^o 



"^^ 



'^o 



^% 



Coupon-to-Soil AEoff (mV) 



Figure 5-26: Difference between Coupon Disconnected Potential and 
Coupon-Pipe Potential with ICCP Interrupted 

Source: Thompson, N.G. and Lawson, K.M., Development of Coupons for M onltoring Cathodic P rotectlon Systems, 
PRCI ContractPR-186-9220, Catalog No. L51888, Final Report, Dec. 2001. 

The merits of placing the portable electrode in the soil tube versus on grade were 
also explored. As shown in Figure 5-27, the potentials with the reference on grade 



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Evaluation of CP System Performance 



5:26 



were often more negative, indicating a measurement error due to the IR drop 
between the coupon and the on-grade reference. 



zs 
o 
o 
O 



a> 

E 

3 



25 
20 
15 
10 

5 



n 



ri 



^^ 



<e- 



~-^•^ 









'o 'o 'o 'o ^o 

•^ V?<. ^&^ V^ 



Coupon Potential atGrade minus Tube (mV) 

Figure 5-27: Difference between Coupon Disconnected Potential Measured with 
Reference on Grade versus in tlie Soil Tube 

Source: Thompson, N.G. and Lawson, K.M., Development of Coupons for Monitoring Cathodic Protection Systems, 
PRCI Contract PR-186-9220, Catalog No. L51888, Finai Report, Dec. 2001. 

Ahhough it is understood that a coupon does not represent the pipe but rather a 
holiday of similar surface area in similar soil conditions, the correlation between 
the coupon instant-discoimect potential and the pipe instant-off potential, as 
shown in Figure 5-28, was very strong even though there was considerable scatter 
in the data. Hence, it can be assumed that if a coupon is adequately polarized the 
pipe exposed at a holiday, having a surface area equal to or less than the coupon, 
will be as equally well protected. 




-1.2 -1.0 -0.8 -0.6 
Coupon Eoff {V,CSE) 



0.0 



Figure 5-28: Pipe versus Coupon Off-Potential 

Source: Thompson, N.G. and Lawson, K.M., Development of Coupons for Monitoring Cathodic Protection Systems, 
PRCI Contract PR-186-9220, Catalog No. L51888, Final Report, Dec. 2001. 



&. 



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Evaluation of CP System Performance 



5:27 



The number of coupons used for monitoring the level of cathodic protection on 
pipelines on a worldwide basis is probably around 10,000. Typically these 
cathodic protection coupons have been installed for one or more of the following 
reasons: 

• To monitor a polarized potential with a minimum of IR drop in the 
measurement 

• On pipelines subject to transient or static stray currents. Note that if 
a continuous recording of a polarized potential is required, an 
integrated coupon reference electrode probe may be required. 

• To eliminate errors due to long line recirculating currents when the 
cathodic protection current is interrupted. 

• To measure a representative polarized potential on pipelines with 
direct connected galvanic anodes or with impressed current systems 
whose outputs carmot easily be synchronously interrupted. 

• On parallel intercormected pipelines to avoid measurement error 
caused by the proximity of the paralleling pipeline. 

• On structures where use of an on-grade reference is likely to produce 
a measurement error such as under reinforced concrete slabs, at a 
crossing with other pipelines, at the bottom elevation of closely 
spaced underground storage tanks, etc. 

• On structures exhibiting potentials that are marginally protected to 
assess the merits of installing additional cathodic protection. 

• To measure cathodic protection current magnitude and density. 

• To measure AC interference current density. 

A soil tube is often included with the coupon arrangement so the portable 
reference electrode can be placed inside to avoid any voltage drop error in the 
coupon-to-soil potential measurement when the coupon is discormected. Coupon 
test stations are commercially available where the coupon and the soil tube are an 
integral part of the test station. 

In dynamic stray current locations where a recording of the polarized pipe-to-soil 
potential with time is required, discormecting the coupon to measure its polarized 
potential is impractical. For these measurements, vertical IR drop-free coupons 



a 



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5:28 



are available and indispensable in determining the true impact of stray current 
activity. This is a photo of one such arrangement where the coupon does not need 
to be repeatedly interrupted to record a polarized potential. 





Removable cap to hold 
— factory installed backfill 
in place during shipping 
and Installation 



2" (nom.) PVC pipe 



1/8" wide wood/polymer 
laminate membrane 



Split steel coupon with 1/8" 
slot in center exposed area = 
10 sq. cm. 



2" (nom.) PVC end cap 



Figure 5-29a: Photo of a Vertical Figure 5-29b: VerticallR Drop Coupon 

IR Drop Coupon Scliematic 

Source: Photo and schematic courtesy of Electrochemical Devices Inc. 

The coupon is embedded in the face of the test station post with a vertical porous 
plug slot in the middle of the coupon. With a portable reference electrode placed 
inside the post, there is no IR drop between it and the coupon. Therefore there is 
no need to discoimect the coupon or interrupt the cathodic protection current to 
obtain a reasonably accurate coupon polarized potential. 



Experiment 5-1 

To demonstrate various methods of minimizing 
IR drop error in a potential measurement. 



a 



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5:29 



5.4 Measurement of Polarization Potential Shift 

One of the cathodic protection criteria involves polarizing a steel structure by at 
least 100 mV from its corrosion potential (Ecorr)- Since polarization is a function 
of time, the amount of polarization can be determined either in the formation of 
polarization or the decay of polarization as shown in Figure 5-30. 




TIME 



Figure 5-30: Potential versus Time Plot for Determining Polarization Potential Shift 

Polarization formation can be determined in two ways. First, the difference 
between the instant-on potential measured at ti is compared to the on-potential 
(Eon) measured at ^2- If the difference is equal to or exceeds 100 mV then the 
structure is considered cathodically protected. Second, when the cathodic 
protection current is interrupted at ^2 and the instant-off potential (e.g., polarized 
potential) is measured, if the difference between the instant-off potential and the 
corrosion potential (Ecorr) is equal to or exceeds 100 mV then the structure is 
considered cathodically protected. These two cases can be summarized 
respectively as follows: 



^instant-on 



> 100 mV 



[5-15] 



^instant-off 



F > 

J-^corr — 



100 mV 



[5-16] 



a 



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5:30 



Polarization formation methods can only be used at the time a system has been 
energized since the corrosion potential is likely to change with time and soil 
conditions. Also, if the instant-on potential is going to be used to determine the 
amount of polarization shift the system must be energized through an interrupter 
where the ON half-cycle is very short compared to the OFF half-cycle to avoid 
polarization being included in the instant-on potential. 

To determine the amount of polarization by the polarization decay method, the 
current when turned off at t2 must remain off for a period of time until the 
difference between the instant-off potential and the decayed off potential equals 
or exceeds 100 mV or until the change in potential (AE) approaches zero (i.e., 
AE/At -^ 0). Therefore, Equation 5-17 must be satisfied for protection to be 
confirmed. 



Einstant-off " Edecayed-off ^ lOOmV 



[5-17] 



If the 100 mV criterion is met at a location, then the on-potential at that location 
can be used as a proxy criterion as long as the soil conditions, pipe operating 
temperature, and coating conditions remain the same. 

The decay method is often applied when the structure instant-off potential is not 
equal to or more negative than the potential criterion (e.g., -850 mVcse) since in 
many soils the potential criterion is overly conservative. The fact that the cathodic 
protection system is turned off for several days or weeks is a significant 
disadvantage although not particularly detrimental to pipeline integrity. If 
coupons are being used, however, it is only necessary to discoimect the coupon 
rather than the cathodic protection system. 

Verifying adequate cathodic protection using the 100 mV polarization shift 
criterion usually results in a lesser current requirement than for the 
-850 mVcse polarized potential criterion and often avoids expensive remedial action 
that would normally be required to restore the structure to the potential criterion. 

Note that where there are significant recirculating currents upon interruption of 
the cathodic protection current, IR drop errors in the "decayed-off ' potential 
measurement will result. At the least negative locations, the time to full 
depolarization will be extended until the recirculating current being picked up in 

Q 

these locations has reduced to zero. This can take up to 175 hours. 



^ Thompson, N.G. and Lawson, KM., Impact of Short-Term Depolarization on Pipelines, PRCI Contract 

#PR-186-9611, Catalog #151801, Final Report, Feb. 1999. 
'Ibid. 5. 

'^NACE 



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Evaluation of CP System Performance 



5:31 



5.5 Current Measurement 

Measuring current in the cathodic protection circuit is a necessary procedure in 
evaluating system performance. Typical current measurements are: 

• galvanic anode current 

• impressed current system output currents 

• current in the structure 

• bond current 

Both direct and indirect methods of current measurement are available. A direct 
measurement involves inserting an ammeter into the cathodic protection circuit as 
illustrated in Figure 5-31. 



5.5.1 Using an Ammeter to Measure Current 

'^\/^y\j' 



Vd.cpi 



kp 



®^^^ 



Figure 5-31: Measurement of CP Current Using an Ammeter 

An electronic ammeter is typically composed of a voltage measuring device that 
measures the voltage drop across a low resistance internal shunt. Ideally, an 
ammeter should have a low input resistance compared to the circuit resistance 
(i.e., Rm « Rep) to prevent measurement error. 

For example, in Figure 5-3 1 from Ohm's law: 



V, 



d,cp 



R 



[5-18] 



cp 



but with the ammeter inserted in series in the series circuit, the current 
measured on the ammeter (!„) is given by: 



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5:32 



V, 



d,cp 



Rep + R. 



[5-19] 



Hence, the measured current (Im) will be less than I^p, depending on the 
resistance of the ammeter. 

In many digital multimeters when the milliampere scale is selected, the ammeter 
circuit has an input resistance of several ohms. This can lead to significant errors 
if the ammeter is used to measure the current from a galvanic anode. 

Even if a 10 A or 20 A scale is chosen, the input resistance, which may be as low 
as O.lohm, may still be too high to produce an accurate current measurement in 
some circumstances. For instance, if the ammeter is placed in series with a 
negative drain cable in a parallel set of drain cables, as shown in Figure 5-32, an 
appreciable error can occur. 



(A>: 



rrrr 



V/ANy/V/AY/v: 



negative bonding 
station 



- + 



transformer- 
rectifier 



negative 

bonding station 

O 



Rammeter^ 
= .01otim . 



ti. 



Rdrain cable 
= ,01 ohm 



y 



Vd =Ix Rdrain cable 
= lx.01 =10mV 



Figure 5-32: Current Measurement in Parallel Drain Conductors 

If the shunt resistance inside the ammeter is 0.01 ohm and the resistance of the 
negative return cable is 0.01 ohm, insertion of the ammeter has doubled the 
negative return resistance and possibly reduced the return current (Ii) by half 

In both the above examples a more accurate method is to install an appropriately 
rated shunt permanently in each circuit and simply measure the voltage drop 
across the shunt and calculate the current. (See shunt table in Appendix D.) 



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5.5.2 Using a Shunt to Determine Current Magnitude 



In the parallel negative drain cable example, a shunt of the same rating, hence the 
same resistance, should be installed in series with each negative drain cable as 
illustrated in Figure 5-33. 



equally rated shunts 




Figure 5-33: Use of Shunts for Current Measurements in Parallel Conductors 

When selecting a shunt, its current rating must exceed the anticipated circuit 
current, and the millivolt drop at the anticipated operating current should be 
easily measurable on a standard digital multimeter. 

For instance, if a shunt rated at 5 A, 50 mV is placed in series with a galvanic 
anode having an output of 5 mA, the voltage drop across the shunt will be: 



V 



shunt 



icp ^ ^shunt 

V, 
5mA X 



[5-20] 



rating 



I 



5mA X 



rating 

050V 

5A 



Vshunt = 0.05 mV 

This small shunt voltage drop is below the resolution of most digital voltmeters 
used in the field. For a 5 mA current, a shunt resistance of at least 1 ohm is more 
appropriate. (See shunt table in Appendix D.) 



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5.5.3 Zero R es is tanceA m meter 

Sometimes the currents are so small (e.g., < 0.1 mA) they caimot be measured 
accurately without using very high resistance shunts, which can alter the current 
magnitude because of their resistance. An example is the measurement of coupon 
current as illustrated in Figure 5-34. 




steel coupon 



Figure 5-34: Current Measurement Using a Zero Resistance Ammeter (ZRA) 



9 9 

If the coupon has a surface area of 10 cm and a current density of 10 )aA/cm the 
coupon current (Icp) would be: 



■^cpn 



10 |j,A/cm X 10 cm 



I 



cpn 



100 nA or 0.1 mA 



Measurement of such a small current with an ammeter would introduce several 
ohms of resistance into the circuit as would a shunt since a resistance of 100 
ohms is required to measure in the 10 mV range. Under these circumstances, a 
zero resistance ammeter should be used. 



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5.5.4 C lamp-on A m meter 

A relatively noninvasive method of measuring current in a conductor is by using 
a clamp-on ammeter as illustrated in Figure 5-35. 



magnetic field 




Figure 5-35: Using a Clamp-on Ammeter to Measure Current 



The clamp-on ammeter contains a "Hall effect" device that produces a voltage 
output proportional to the strength of the magnetic field, which is proportional to 
the magnitude of the current in the conductor. 

The Hall effect is illustrated in Figure 5-36 for a fixed meter current (Im). As 
electrical charges move perpendicular to the magnetic field (B), a lateral force is 
exerted on the charges causing a potential difference to appear across the sides of 
the copper plate. 




copperplate 



Figure 5-36: Scliematic of tlie Hall Effect for Conventional Current Direction 



The magnitude of this voltage is proportional to the magnetic field (B), which in 
turn is dependent on the magnitude of the current (Idc) in the conductor. 

Accuracy of the clamp-on ammeter diminishes at currents of a few milliamperes. 
When there are multiple current-carrying conductors in a congested area, the 
accuracy is reduced if there is magnetic interference from adjacent conductors. 



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5.5.5 Pipeline C urrent Measurements 

Pipeline currents can be measured with a clamp made up of a number of turns of 
wire as shown in Figure 5-37. 



sea 

clamp 



sea 

clamp 



^^ 



'iT 



earth 
current/ 



-K).9A 



-K).4A 



indicator 



Earth current leaving subject pipeline is 0.9 - 0.4 =0.5 A. 

Figure 5-37: Pipe Current Measurement Using Sensing Loop and Swain Meter 

Source: Swain, W.H., Clamp-On Ammeters Can Watch Cathodic Protection CurrentFlow, 
Pipe Line & Gas Industry, iviarch 1998, p. 38 



Pipeline current can also be measured using the four-wire span illustrated in 
Figure 5-38. For accurate measurement, the span is calibrated by injecting a 
known DC test current through the pipe using the outside test leads © and ® and 
measuring the resulting voltage drop across test leads © and ®. 




3) (4 



Figure 5-38: Calibrating a Pipeline Current Span 



The resistance of the pipe between test leads © and © is calculated from Ohm's 
law: 



R, 



AV. 



2-3 



AI, 



[5-21] 



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The result can be anticipated prior to the test by referring to the pipe table in 
Appendix B. 

Attention to polarity is important in this measurement since there will probably be 
a residual current during the test, and the test current may cause a reversal in the 
voltage drop polarity. 



For example, 



V2-3 = +21 mV (before test current applied) 
V2-3 = -19 mV (after test current applied) 
It = lOA 



The resistance (Rp) of the pipe section being tested is: 



R 



R 



+ 21mV-(-19mV) 



lOA 
+ 40mV 



lOA 



4mQ 



Therefore the current calibration factor is: 



lOA 
40 mv 



: 0.25A/mV 



and the residual current magnitude is: 



isidual 



21 mV X 0.25 A/mV = 5.25 A 



with a direction from 2 to 3. 



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5.6 Close Interval Potential Survey 



When measuring a potential at one location, the amount of pipe sampled in the 
measurement is considered the length of pipe encompassed by a 120° arc centered 



on 9 



on the reference electrode as shown in Figure 5-39. 




Figure 5-39: Length of Pipe Sampled in a Pipe-to-Soil Potential Measurement 

To determine the potential over the entire pipeline surface requires that the 
reference electrode be moved along the centerline of the pipe route and placed at 
regular intervals. For bare pipe, the interval distance is a function of pipe depth 
(d) to top of pipe as shown in Figure 5-40. 




2 3 

Depth (feet) 



Figure 5-40: Length of Bare Pipe Over Which Potentials are Averaged 
as a Function of Burial Depth 

Source: Thompson, N.G. and Lawson, K.M., Improved Pipe-to-Soil Potential Survey Methods, 
PRCI Final ReportPR-186-807, April, 1991, p. 4-2 



Pearson, .T.M., Concepts and Methods of Cathodic Protection, Part 2, The Petroleum Engineer, April, 
1994, p. 200. 

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This figure shows that field test data on a bare 24 in. pipeline verified the 
predictions of a finite element model. Hence, the length of pipeline sampled (Ls) 
in a potential measurement is given by the equation 



3.5d + 1 



[5-22] 



where: 



d = pipe depth 



Therefore for a pipeline buried at a 3 ft (0.92 m) depth, the length of pipe 
surveyed would be: 

U = 3.5 X 3 + 1 
= 11 ft (3.35 m) 

Accordingly, the reference electrode spacing interval should be no greater 
than 11 ft (3.35 m). 

The percentage of the circumferential area sampled on a bare pipe is not only a 
function of pipe depth but also of pipe diameter as shown in Figure 5-41. 




0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90 1.00 

Diameter/Depth Ratio 



E (top pipe surface) E(ground level] 

E{top pipe surface) ~ E(average) 



X 100% 



Figure 5-41: Relative Circumferential Sampling Distance as a 
Function of Pipe Diameter-to-Pipe Depth Ratio 

Source: Thompson, N.G. and Lawson, K.M., Improved Pipe-to-Soil Potential Survey Methods, 
PRCI Final Report PR-186-807, April 1991, p 4-4. 

This figure illustrates that the percentage of circumferential surface sampled on a 
bare pipe increases as the pipe diameter/pipe depth ratio decreases. 



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For coated pipe, the length of pipe sampled depends on the size and location of 
the holidays in the coating. A finite element analysis^" suggests the important 
parameter in determining the potential measured with respect to a reference 
electrode at grade is the ratio of coating resistivity to earth resistivity (pctg/pe)- 
This model predicted that only relatively large holidays (20 to 200 in.^ or 130 to 
1300 cm ) are detectible at ground level on a pipeline with a Pctg/pe ratio equal to 
40. It also predicted that potential changes on a pipeline located beneath a high 
resistivity environment can be shielded from ground level potential measurements 
(e.g., coated piping directionally drilled through bedrock). 

When conducting a close-interval potential survey, it is good practice to keep the 
reference electrode lead as short and as well insulated as possible. If there is a 
break in the reference electrode lead insulation and the test lead comes in contact 
with groundwater, as illustrated in Figure 5-42, an error in the potential 
measurement will result. 



reference electrode 
test lead 




Figure 5-42: Error in Potential Measurement Introduced by Reference Electrode 
Lead Conductor Contacting the Earth 



Essentially, the meter senses a reference potential that is a combination of the 
actual reference electrode potential and the test lead conductor potential. Both 
contribute metering current. Thus the reference electrode lead should be kept 
short and well insulated. A break in the insulation on the voltmeter to pipe wire 
will not affect the potential measurement significantly because of the relative low 
resistance of the pipeline to earth compared to the test lead. 

Potential measurement errors can accumulate in close-interval survey data due to 
current in the pipeline. As shown in Figure 5-43, the measured potential at each 



10 



Thompson, N.G. and Lawson, KM., Improved Pipe-to-Soil Potential Survey Methods, PRCI Final 
Report PR-186-807, April 1991, p4-4. 



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placement location becomes more electropositive when the current direction is 
the same as the survey direction. 



50 mV error 




WAV/,'^fNy/A//Av://v/Ay/Ay/AVAy/AyAV/A\/AWAy//xWAy/xv/AVXV/^ 



icp 



50 mV 



Figure 5-43: Error in Potential Measurement Introduced by Current in the Pipeline 

The amount of error accumulated can be determined by switching the point of 
coimection at the last reference electrode position (i.e., at test station #2) and 
observing the potential. The difference in the two potentials should be linearly 
distributed with distance back to test station #1. For instance, the potential 
measurement at the halfway point between the two test stations would be 
increased in the negative direction by 25 mV. The voltage drop in the pipeline 
can also be determined by simply coimecting the voltmeter leads to the two test 
stations. 

If the pipe current is not steady state but is fluctuating with time, as could be the 
situation with telluric or other dynamic stray currents, a more complex method of 
identifying and correcting for this measurement error is required (see Chapter 3). 

The presence of parallel intercoimected pipelines as illustrated in Figure 5-44 can 
also produce errors in potential measurements. 



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Figure 5-44: Error in Potential Measurement due to Interaction witli a 
Parallel Interconnected Pipeline 

The proximity of an adjacent pipeline can introduce errors in the potential 
measurement if there is a significant difference in the coating quality of each 
pipeline or in the level of polarization. First, there is the possibility of a 
recirculating current upon interruption of the cathodic protection current. Second 
the potential measurement with the reference located over Pi will also be 
influenced by the potential on P2. The magnitude of the error will increase as 
depth (t) increases and the pipeline separation distance (s) decreases. 

The relative impact of a paralleling pipeline on the potential measurement is 
summarized in Table 5-2.^^ 

Table 5-2: Summary of Relative Interaction for Two Paralleling Pipelines on 

Potential Measurements 



Pipeline Coating Quality 
on Pipe of Interest 


Relative Interaction 
on-potential 


From Other Pipeline 
off -potential 


bare 
bare 


significant 
significant 


not significant 
not significant 


bare 
coated 


not significant 
marginally significant 


not significant 
marginally significant 


poorly coated 
well coated 


not significant 
significant 


not significant 
significant 



Where the interaction is significant or unknown, the inter-pipeline bonds should 
be interrupted synchronously along with the rectifiers. 



" Thompson, N.G ., Lawson, KM., Multiple Pipelines in Right-of-Way: Improved Pipe-to-Soil Potential 
^Survey Methods, PRCI Conti-actPR-lSS-giOS, Final Report, Oct. 1993, p.94. 

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5.7 Coating Condition Surveys 

Since a close-interval potential survey is unlikely to identify coating holidays 
having a surface area less than about 20 cm , other means of identifying coating 
holidays and general coating conditions are used. One such survey technique 
relies on detection of the voltage gradient produced at a holiday by a signal 
current. 

5.7.1 Voltage Gradient Method of Detecting Holidays in a Pipe 
Coating 

The principle of this technique involves detecting a voltage difference on the 
earth surface using two or more portable electrodes separated by a short distance 
as illustrated in Figure 5-45. 



High Resistance 
Voltmeter 



Copper/Copper Sulfate 
Reference Electrodes 




Coating Defect 



Current to Defect 



Potential Gradient 



Coated Pipeline 



Figure 5-45: Coating Holiday Detection Using Voltage Gradient Method 

A signal generator is connected between the pipe and ground at a test point 
location. When the signal current passes from the pipeline to earth at a holiday, a 
voltage gradient is created in the soil radially from the holiday. Separated surface 
electrodes located either transverse to the pipe or along the pipe axis will detect a 
change in potential between them as they pass over the gradient. 

The magnitude of the voltage gradient at any point in the earth is given by 
Equation 5-23. 



V, 



Pel 



[5-23] 



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where: 



Vg 

Pe 

z 



voltage gradient (V/unit length) 

earth resistivity 

current density at point of interest 



The magnitude of the voltage difference depends on the signal current magnitude, 
the soil resistivity, and the holiday size. 

Larger holidays will produce a larger potential difference (Vgi - Vg2) between the 
surface electrodes. Because of this, the surface area of the holiday can be 
estimated. The signal can be either DC (e.g., DCVG) or AC (Pearson, C-scan, 
etc.). 

5.7.2 Coating Conductance Method of Evaluating Coating 
Quality 

The quality of a coating can be determined by evaluating the coating conductance 
(Gc) or the specific coating resistance (r'c) of a pipeline coating. The test method 
consists of applying an interrupt DC current between the pipe and earth, using 
either an existing rectifier or a temporary test current. (See TMO 102-2002 in 
Appendix F.) With the current interrupted, the pipe-to-soil potential and pipeline 
current are measured at two different locations (i.e., opposite ends of the pipe 
section of interest) as shown in Figure 5-46. Current would normally be measured 
with a current span or sometimes through a bond around an isolating fitting. 




& 



Es 



a 



•^sect 



Figure 5-46: Arrangement for a Pipeline Coating Resistance (Conductance) Test 



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An example of a data set and coating resistance calculations are as follows: 





Von 


Voff 


AV 


Aon 


loff 


AI 


TS #1 


-2.00 


-0.90 


1.10 


2.8 


0.10 


2.70 


TS #2 


-1.65 


-0.85 


0.80 


3.0 


0.20 


2.80 



The average resistance (Rsect) of the section of pipe is determined by Ohm's law 
as follows: 



R„ 



AV„ 



[5-24] 



where: 



AY 
I 



sect 



AVi + AV2 



AI2 - All 



Substituting field data into Equation 5-24 gives: 



R. 



I.IOV + 0.80V 



2.80 A - 2.70 A 



0.95 V 
O.IA 



R, 



sect 



9.5 Q 



To determine the coating quality for the section of pipe tested, the specific 
coating resistance (r'c) must be calculated based on a soil resistivity of 1,000 
Q-cm using Equation 5-25. 



Rsect@1000a-cm X ^s 



[5-25] 



where: 



R 



sect®! 000 n-( 



Rsect X 



1000 
ptest 



A, 



TidL, 



the surface area of the pipe section 



For a 60 cm diameter pipe, 2 km long in 6500 Q-cm soil, the specific coating 
resistance in 1000 Q-cm soil will be: 



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9.5Qx ^^ — ~-^ X 3.14 X 0.6m x 2000m 
6500 Q- cm 



r'c = 1.46 Q X 3768 m^ 

r'c = 5507 qW 
The specific coating conductance (g'c) can then be calculated from Equation 
5-26. 



1 



[5-26] 



therefore: 



1 



5507 ohm -m^ 



1.82 X 10-^S/m' 



The resulting specific coating conductance can be compared to Table 5-3 to 
estimate coating quality. For a specific coating conductance of 1.82 x 10""^ 
S/m , the coating on the test section would be rated "good" if the pipeline has 
few fittings in the test section or "excellenf if there are valves or fittings 
within the test section. 



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5:47 



Table 5-3: Typical Specific Leakage Conductance for Dielectric Protective Coatings 

in 1000 £l-cm Soil 



Long Pipelines 

with 

Few Fittings 


Average Specific 
Coating Conductance 


Average Specific Coating 

Resistance 

r'c 


Quality of Work 


9' 2 

Siemans/ft 


9' 2 

Siemans/m 


Q-ft^ 


Q-m^ 


Excellent 


<1 X 10"^ 


<1 X 10* 


>10^ 


>10' 


Good 


1 X 10^ to 5 X 10^ 


1 X 10-^ to 5 X 10* 


2 X 10* to 10^ 


2 X 10' to 10^ 


Fair 


5 X 10^ to 1 X 10 * 


5 X lO""* to 1 X 10^ 


10^ to 2x10* 


10^ to 2x10^ 


Poor 


>1 X 10* 


>1 X 10^ 


<L0* 


<10^ 


Bare Pipe 

(2 to 12") 

(5 to 30 cm) 


4x10^ to 2x10^ 


4 X 10-^ to 2 X 10' 


50 to 250 


5 to 25 


1 


Gas or Water 

Distribution with 

Many Fittings 


Average Specific 
Coating Conductance 


Average Specific Coating 

Resistance 

r'c 


Quality of Work 


9' 2 

Siemans/ft 


9' 2 

Siemans/m 


Q-ft^ 


Q-m^ 


Excellent 


<5 X 10^ 


<5 X 10-^ 


>2 X 10* 


>2 X 10^ 


Good 


5 X 10^ to 1 X 10 * 


5 X 10-^ to 1 X 10-^ 


10^ to 2 X 10* 


10^ to 2 X 10^ 


Fair 


1 X 10* to 5 X 10 * 


1 X 10^ to 5 X 10^ 


10^ to 2x10* 


2 X 10^ to 10^ 


Poor 


>5 X 10* 


>5 X 10^ 


<2 X 10' 


<2xlO^ 


Bare Pipe 

(2 to 12") 

(5 to 30 cm) 


4x10^ to 2x10^ 


4 X 10-^ to 2 X 10-^ 


50 to 250 


5 to 25 



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5:48 



5.8 Troubleshooting Cathodic Protection Systems 

When subcriterion polarized potentials are measured on a structure, the remedial 
solution may simply be increasing the output of the cathodic protection system or 
adding additional current sources. Such a measure ultimately costs money 
whether it is for increased consumption of the anodes or for purchasing and 
installing additional materials. The exact cause of subcriterion potentials should 
be determined before arbitrary current additions are implemented. 

There are many methods of identifying malfunctions in a cathodic protection 
system. For discussion purposes, the troubleshooting procedure has been applied 
independently to four fundamental factors of a cathodic protection system: 

• polarization changes 

• resistance changes 

• power supply changes 

• stray current effects 



5.8.1 Polarization Changes 

5.8.1(a) Structure Depolarization 

Depolarization of the structure is one of the most common causes of loss of 
protection because so many possible factors result in depolarization. Any 
circumstance that speeds up the charge transfer across the structure electrolyte 
interface will result in cathode depolarization as depicted in Figure 5-47 for a 
galvanic cathodic protection system. 



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increased surface area 

- coating deterioration 

- sliort 

- failed electrical isolation 
increased aeration 
increased agitation 
increased temperature 
increased acidity 
increased lac 



cp 

CP Current- 



Figure 5-47: Structure Depolarization in a Galvanic CP System 

The key symptom of depolarization is the cathodic protection current increase 
from Icp to I'cp along with an electropositive shift in structure potential. This effect 
will be more apparent on a galvanic protection system than on an impressed 
current system because most of the driving voltage in the latter is used up in the 
earth near the anode, not in polarization. 

Coating deterioration, a metallic contact with a foreign structure, or a failed 
isolating fitting provide more structure surface resulting in less energy needed to 
transfer charge and therefore less polarization. 

Increased temperature increases the rate of the two predominant reduction 
reactions: 

H+ + e ^ H° 
and, 

O2 + 2H2O + 4e ^ 40H 

Increased aeration, agitation, and acidity provide more reactants to the reduction 
reactions increasing the reaction rate. Agitation also sweeps away the products of 
the reduction reaction allowing the reaction to proceed more readily. 



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5:50 



5.8.2 Anode Polarization 

Anode polarization is a malfunction that is more common on impressed current 
systems than on galvanic systems, but in either case, as illustrated in Figures 5-48 
and 5-49, the symptoms are the same. 




a.P 



passivation 
anode consumption 
gas biocking 
ioss of anodes 
poor backfiii 
low temperature 



P.s 



cp 

CP Current 



Figure 5-48: Increased Anode Polarization in an Impressed Current System 



-a,oc 



Ea,o 




increased [OH"], increased pH 
decreased agitation 
decreased temperature 
decreased surface area 
increased passivating ions 
increased M'^+ concentrations 



Corrosion Current 



Figure 5-49: Increased Anode Polarization in a Galvanic System 

Anode polarization is characterized by a positive shift in the anode polarized 
potential (Ea,p) and usually a positive shift in the anode's open circuit potential 
(Ea,oc)- This causes a reduction in current output. The key symptom is the positive 



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5:51 



shift in anode potential, which for identification purposes requires that the anode 
potential be measured routinely as part of survey procedures. 

Any factor that slows the oxidation reaction will cause anode polarization. 
Anode polarization can occur gradually as the anode is consumed resulting in a 
smaller surface area for charge transfer in the oxidation reaction or as the 
products of the oxidation reaction build up around the anode. 

In impressed current systems anode polarization can occur abruptly if one or 
more anodes in a group are lost due to cable or splice failure or if a vent becomes 
plugged resulting in gas blockage. 



Inadequate or poor quality backfill around the anode can cause anode polarization 
since without the proper backfill or the proper quantity, passive films may form 
on the anode surface. 



A lower temperature, especially below freezing conditions, will 
oxidation reaction and produce more anode polarization. 



slow the 



5.8.3 Increased Resistance 

Any increase in resistance in a cathodic protection circuit will result in less 
current and hence less polarization at both the anode and structure, as illustrated 
in Figures 5-50 and 5-51. 



a 



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Evaluation of CP System Performance 



5:52 




CP Current 



Figure 5-50: Increased Resistance in a Galvanic CP System 




CP Current 



Figure 5-51: Increased Resistance in an Impressed Current System 



Besides a decrease in current, an increase in resistance results in a more negative 
polarized anode potential (E'a,p). This is the key symptom and distinguishes the 
cause of subcriterion potentials from anode polarization where the polarized 
potential of the anode shifts in the positive direction. The increase in resistance 
can be in either the earth or metal current paths. The resistance increase can occur 

'''■'NACE 

© N AC E International, 2005 C P 3- C a thodic P rotection Technologist 

J anuary 2007 



Evaluation of CP System Performance 



5:53 



in the bulk soil or can be localized at the anode or structure. Bulk earth resistivity 
increases with decreasing moisture and lower temperature, especially if the 
temperature drops below freezing (see Figures 1-35 and 1-36). 

Gradual increase in circuit resistance will occur as anode consumption progresses 
due to decreasing anode geometry. 

Sudden changes in resistance are usually due to broken and corroded cables or 
corroded splices. Cables and splices on the positive side of an impressed current 
system are particularly vulnerable to corrosion when and where the insulation has 
been damaged or where the splice has not been insulated properly. A localized 
resistance increase around a pipeline can result if the pipe backfill provides a 
drainage path for water as might occur on a hillside. The high temperature of 
piping downstream of a compressor station can result in localized drying of the 
soil adjacent to the pipe surface. 

Localized drying of the soil around an impressed current anode can occur if the 
anode current density is too high. Typically, anode current densities should be 

9 9 19 

limited to 50 [lA/cm (50 mA/ft ) in low permeability soils such as clay . 

Although the above events have been treated independently, it is not uncommon 
for a combination of these circumstances to be present. Very permeable well 
drained soils change in resistivity seasonally and allow more dissolved oxygen to 
contact structure surfaces creating a depolarization-increased resistance 
combination as illustrated in Figure 5-52. Similarly, high temperature can both 
dry out the soil and cause depolarization. 



'- Haanestead, .T.W. and Stapp, J., HVDC Ground Electrode Design, EPRI, Report #1467-1, August 1981, 
''NACE 



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5:54 



depolarization 




CP Current 



Figure 5-52: Seasonal Effects on a Galvanic CP System due to Drying of the Soil 
Resulting in Increased Resistance and Depolarization 

When soil dries, its resistance increases because the moisture between soil 
particles constitutes the principle ionic current paths. Furthermore, as the soil 
dries out, the earth becomes better aerated resulting in a reduction in polarization. 
Note that the key symptoms in this situation are the more electronegative 
polarized potentials of the galvanic anode and a more electropositive open circuit 
potential (Ecorr,s) of the structure. 

5.8.4 Power Supply Changes 

A decrease in the driving voltage in a cathodic protection system will result in a 
decrease in current and structure polarization. This is more common in an 
impressed current system than in a galvanic system due to the vulnerability of the 
power supplies to damage and outages. 

One of the more serious problems associated with DC power supplies is the 
reverse connection of the output terminals such that the positive terminal is 
coimected to the structure. This is not uncommon since electricians are taught that 
for DC circuits the line side is positive and negative is ground. On well coated 
pipelines corrosion leaks can occur in a matter of months. It is therefore advisable 
when any work at a transformer-rectifier involves discoimecting cables, either 
internally or externally, that a pipe-to-soil potential be measured at the pipeline 



a 



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Evaluation of CP System Performance 



5:55 



test lead nearest to the power supply location immediately after the work has been 
completed. 

Often a DC power supply is turned off for a variety of reasons and not re- 
energized. Transformer-rectifiers should not be coimected to an AC circuit that is 
routinely interrupted. 

Failure of a single phase transformer-rectifier as schematically illustrated in 
Figure 5-53 can occur for any one of the following reasons: 



Loss of input power 

Blown fuses 

Failed diodes 

Failed transformer windings 

Short circuit lightning arrestors 

Direct short in CP circuit 

Open connections and broken cables 



a 



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Evaluation of CP System Performance 



5:56 



ightning ,,1 ^ 

arrester I ^-^ 



Line Neutral 

AC INPUT 



-) circuit breal<er 



Ju^AXXXXXXjJ primary winding 
transformer 



coarse taps 



secondary winding 
4 fine taps 

staci< 
sliunt 



t> fuse 

iightning arrester 
- <i DC OUTPUT i'H- 

Figure 5-53: Typical Single Phase Tap Set Transformer-Rectifier 




When checking rectifier outputs on a routine basis, four basic situations require 
investigation: 

• Zero current and vohage outputs 

• Zero current output with unchanged output vohage 

• Significant current change with unchanged vohage 

• Significant changes in both voltage and current outputs 

Note that output vohage and currents may not be precisely zero. There may be 
back voltage due to the potential difference between the pipe and groundbed or 
residual currents due to stray current activity. 



a 



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J anuary 2007 



Evaluation of CP System Performance 



5:57 



5.8.4(a) Zero Current and Voltage Outputs 

For the case of zero output for both current and vohage, either there is no input 
power to the unit or an open circuit within the rectifier is indicated. First, 
determine if input AC vohage is present. If not, the problem is external to the 
rectifier. If AC voltage is present at the input terminals, an open circuit exists 
within the rectifier. However, the open circuit may be due to a tripped circuit 
breaker at the rectifier input. 

The component causing the open circuit can be located by realizing that the 
rectifier voltage must exist across the open circuit element. If it is determined that 
the input circuit breaker has tripped, a high current or overload has occurred. 
This high current could have been a temporary problem, perhaps due to a 
lightning surge, or a permanent short circuit. The best method of proceeding is to 
reduce the voltage output tap to a low level and reset the circuit breaker. If the 
circuit breaker does not trip again, the problem was probably temporary and full 
output voltage can be restored. If the circuit breaker does trip, a permanent short 
circuit is indicated. 

To determine if the short circuit is external to the rectifier, disconnect one of the 
DC output connection leads and reset the breaker. If the short circuit is external to 
the rectifier, the circuit breaker will not trip. If the short circuit is internal to the 
rectifier, the circuit breaker will again trip. Next, the best approach involves 
isolating the problem to a particular section of the rectifier by beginning at the 
input terminals and adding one component at a time to the circuit until the circuit 
breaker trips. The short circuit must be the last component connected when the 
circuit breaker trips. For example, the transformer can be connected to the input 
circuit breaker with the tap adjustment bars removed. 

The sequence of short circuit location is shown in Figure 5-54. 



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5:58 




Yes 



Yes 



Lower Voltage 

Taps & Reset 

Breaker 




No 



Problem external 

to rectifier 
on the input side 



Locate open 
circuit by finding 
component with 

voltage drop 




Disconnect 
Output Lead 6< 
Reset Breaker 



No 



Problem external 

to rectifier 
on the input side 




No 



Short is in 
output circuit 



Isolate shorted 

component 

by adding one 

component ata time 



Figure 5-54: Locating a Sliort Circuit in an ICCP System 

5.8.4(b) Zero Current Output with Unchanged Voltage Output 

If the DC voltage output of the rectifier is relatively unchanged but the current 
output is zero, an open output circuit is indicated. This could be caused by: 

• An open fuse in the output circuit 

• An open positive or negative lead wire 

• A failed groundbed 

If an open fuse in the output circuit is found, a short exists (or has existed) in the 
output circuit. 

5.8.4(c) Significant Current Change with Unchanged Voltage 



If the DC current output significantly changes with no change in the output 
voltage, the output circuit resistance has changed. If the current output has 
significantly increased, a lower circuit resistance is indicated. This could be due 
to system additions, shorts to other underground structures, or major coating 
damage. If the current output significantly decreased, a higher circuit resistance is 
indicated. Some of the possible causes might include installation of inline 
isolators, groundbed deterioration, discontinuity due to discoimection of a system 



a 



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Evaluation of CP System Performance 



5:59 



component, or anode gas blockage. Seasonal variations in soil conditions, such as 
drying or frost, can also increase the current resistance. 

5.8.4(d) Significant Changes in Both Voltage and Current Outputs 

Sometimes both the voltage and current outputs will decrease significantly. If the 
voltage and current outputs are approximately one-half of the normal values, the 
most probable cause is partial failure of the rectifier stacks ("half waving"). If the 
rectifier stacks are found to be operating properly, the transformer should be 
investigated for possible winding-to-winding shorts. 

5.8.4(e) Transformer-Rectifier Efficiency 

A transformer-rectifier with selenium stacks may lose efficiency as the rectifier 
elements age. Transformer-rectifier efficiency should be determined routinely by 
comparing input and output power as per Equations 5-27 and 5-28. 



T/R efficiency 



" dc, out 



X 100 



[5-27] 



where: 



dc,out 



Edc ^ tdc 



and: 



3600 NK 
t 



[5-28] 



where: N = number of revolutions of wattmeter disc 
K = wattmeter constant on meter nameplate 
t = time in which revolutions were counted (sec.) 

The efficiency can be used to determine the average AC power consumption over 
a specific time period using Equation 5-29. 



W 



Idc X Edo X t 

1000 X Eff. 



[5-29] 



where: W = total ac power consumption (kW-h) 
Idc = average dc current output (A) 
Edc = average dc voltage output (V) 
t = length of period (h) 
If the calculated input power read on the wattmeter differs from that calculated, a 
temporary power supply outage during the period would be suspected. 



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Evaluation of CP System Performance 



5:60 



Furthermore the total time (t) the rectifier has been operating can be determined 
by Equation 5-30, which is just a rearrangement of Equation 5-29. 



t 



W X 1000 X Eff 
L X E. 



[5-30] 



Much of the power lost in the transformer-rectifiers is across the diodes and the 
transformer. Therefore a transformer-rectifier with a center-tapped secondary and 
only two diodes will have a higher efficiency than a bridge coimected rectifying 
element with four diodes. 

Also three phase power supplies, although more expensive, are much more 
efficient than single phase. The extra expense is often justified where three phase 
power is available once the CP system power requirements exceed about 2 
kilowatts. 



5.8.5 Cathodic Protection Troubleshooting Flow Chart 

The sequence of steps involve in troubleshooting the operation of a cathodic 
protection system can be summarized in a flow chart such as Figure 5-55. Prior to 
the potential measurement, it is assumed that both the meter and reference 
electrode have been calibrated and the potential previously recorded at the 
measurement location was equal to or more electronegative than the criterion. 



a 



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5:61 



Investigate 

•Ea,p 

•Ea,oc 

• rectifier output 



Less 





No / Is rneasured 

potentiai sub-criterion? 





Is tliere a liigli 

resistance in measuring 

circuit? 




Yes 



No 




No 




Has Rep clianged? 



same 




Is there stray 
current interference? 



No 




Is additional 
CP required? 



No 



FINISH 



, greater 
Has Icp changed? y ^ 




orsame 




greater 




Yes 




Yes 



Investigate 



•reference electrode 
contact res. 

•continuity of test leads 
•all connections 



Investigate for 
structure depolarization 

• increase aeration? 

• increase agitation? 

• increase temperature? 

• decrease pH? 

• coating deterioration? 

• electrical short? 



Investigate 



• soil res. increase 

• cable breaks 

• anode consumption 
•trouble-shoot rectifier 

(5.9.4) 
•anode polarization 



Investigate 



•source of interference 

• conduct interference 
tests 

• mitigate (Chapt. 3) 



Install CP as required 



Figure 5-55: Cathodic Protection Troublesliooting Flow Cliart 



a 



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Evaluation of CP System Performance 



5:62 



Class Exercise 5-1 
Evaluating Cathodic Protection Performance 



The following problems are presented for the student to exercise his performance- 
evaluation skills. Plot the data on semi-log graph paper. 

Given the following cathodic protection data, state the reason(s) the design 
objective was not achieved. 



Problem 1: 



Potentials (mV) 



Current (mA) 



t^con, structure 


= -550 




Design: 






J^corr, anode 


= -1,500 


= 100 


J^structure 


= -1,000 




Ji anode 


= -1,300 




Actual: 






J^structure 


= -700 


= 1,000 


Ji anode 


= -1,200 




Problem 2: 






Potentials (mV) 




Current (mA) 


iicorr, structure 


= -550 




Design: 






J^con-, anode 


= -1,500 


= 100 


J^structure 


= -1,000 




Ji anode 


= -1,300 




Actual: 






listructure 


= -600 


= 15 


li anode 


= -650 




J^con-, anode 


= -1,100 




'^NACE 







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C P 3- Cathodic P ro tec tion Technologist 

J anuary 2007 



Evaluation of CP System Performance 



5:63 



Problem 3: 




Potentials (mV) 




J^corr structure 


= -550 


Design: 




J^oc, anode 


= -1,500 


iistructure 


= -1,000 


J^ anode 


= -1,300 


Actual: 




J^structure 


= -700 


J^ anode 


= -1,450 


J^oc anode 


= -1,500 


Problem 4: 





Current (mA) 



100 



20 



The corrosion potential of a steel structure is -500 mV. When the cathodic 
protection is applied using an impressed current system, the structure potential 
shifts to -960 mV. Upon interrupting the current, the instant-off potential is 
-610 mV. If all potentials are measured with respect to a copper/copper sulfate 
reference electrode, which cathodic protection criteria are satisfied? 



Problem 5: 

A fixed- voltage rectifier is set to operate at an output of 30 V and 15 A. During 
routine surveillance the rectifier is found to be 30 V and 7 A. What malfunction(s) 
could have occurred? 



Group Case Study: 

The figure below shows the plan and polarized potential profile for a portion of a 
coated and cathodically protected pipeline. List at least seven possible causes for 
the apparent lack of protection in the vicinity of the road crossing. 



a 



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Evaluation of CP System Performance 



5:64 



Pipeline 



+ 



Vp/s 

(Vcse) 



■0.7 
■0.8 
■0.9 
■1.0 



Road 




^ 



"t 



+ 
Anode Bed 




Exercise Schematic 5-1: 
Plan and Potential Profile for an Underground Cathodically 
Protected Pipeline 



a 



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Evaluation of CP System Performance 



5:65 



Experiment 5-1 

To Demonstrate Four Methods of Minimizing 

IR Drop Error in a Potential Measurement 



1 cm 

T 



fl 



(!) 

® 

® 



® 



-mag 
anode 



<E>^^^^^ 



Experiment Schematic 5-1 



Procedure 



Step A: Setup 

1 . Fill tub to a depth of 5 cm with water from a cold water tap. 

2. Tape the steel rod with plastic tape leaving 3 uncoated sections, each 1 cm 
long, as shown on the schematic. 

3. Place rod in tub and connect the rod to the magnesium anode through an 
ammeter and switch. 

4. Close the switch and wait for 5 minutes for the structure to reach a steady 
state polarization level. 

Step B: Current Interruption Method 

1 . Measure potential of steel rod with top of reference electrode touching the 
top of the water at locations © through ® with the current on and 
momentarily interrupted. Do not allow the interruption time to exceed 5 
seconds. 

2. Record the potentials in Results Table. 

3 . Calculate the IR drop at each measurement location. 



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5:66 



Step C: Reference Placed Close to Structure Holiday 



1. 



2. 
3. 



With switch closed, measure the structure potential with reference placed 

close to each of the 3 holidays. Close means a distance equal to 2 

diameters of the reference (approximately 1 cm). 

Record the potentials in Table 5-2. 

Compare results to the instant-off potentials measured in Step A. 



Step D: Stepwise Current Reduction Method 



I® 

h^^^y. r 


^— mag 
» anode 


1 





Experiment Schematic 5-2 

1. Place reference electrodes at locations © and ®. 

2. Measure and record structure potential with respect to the reference 
touching the top of the water at location ©. 

3. Measure and record side drain voltage drop (Vj) between © and ®. 

4. Insert 10 ohm resistor in series with the ammeter and repeat steps D2 and D3. 

5. Insert 100 ohm resistor in series with the ammeter and repeat steps D2 and D3. 

6. Calculate the change in the structure potentials AVs and side drain (Va) IR 
drops after step 4 and 5. 

7. Plot data on quad paper with the change in structure potential (AVs) on the 
ordinate and the change in side drain potential (AV2,4) on the abscissa (see 
Figure 5-19). 

8. Linearly extrapolate the curve to intersect the ordinate and estimate the 
total IR drop. 

9. Subtract the IR drop from the full current on-potential to obtain a 
calculated off-potential. 

10. Compare with potential at © in steps B and C. 



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5:67 



Step E: Use of a Cathodic Protection Coupon and Reference Tube 





Steel Rod i > 



5/8" dia. PVC reference tube 

coated coupon rod 




Magnesium Anode 



bare end of coupon rod (1/8" di 




Experiment Schematic 5-3 

1. Insert integrated coupon/reference cube as per schematic 5-3 and connect 
the coupon to the steel rod through a switch. Allow several minutes to 
polarize. 

2. Place reference electrode touching top of water adjacent to the tube. 
Measure and record the potential of the coupon with the coupon coimected 
and discoimected. 

3. Place reference electrode inside reference tube so reference touches top of 
water inside the tube. Measure and record the potential of the coupon with 
the coupon coimected to the structure and discoimected. 

Step F: Compare the Following IR Drop Mitigated Data 

1. B instant-off potentials at locations © to ®. 

2. C on-potentials at locations ® to ®. 

3 . D-9 calculated polarized potential at © . 

4. E coupon instant-disconnect potential. 

Which is the most effective technique? 
NACE 



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5:68 



Results Table 





Structure-to-Electrolyte Potentials (mV/csE) 






Reference Electrode Location 




step 


© 


® 


0) 


® 


Other 


Icp 


Remarks 


B 

ON 


















OFF 
















IRcalc. 
















c 

ON 


















D 

Fulllcp 


















(1) 






10 Q 
















Calc.AVs 






Calc. AV2,4 










100 Q 




^^■1 








Calc. AVs 






Calc. AV2,4 










E 


Water Surface 


Inside Ref. Tube 


1 








P 


Coupon 


^1[ Coupon 








Cpn connected 
















Cpn disconnected [, 















a 



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J anuary 2007 



APPENDIX A 

ANODE SPECIFICATIONS 



Table I 

Packaged Magnesium Anodes 



*The first number in tine 
anode designation refers to 
tlie weiglit in pounds. Tine 
letter indicates tine anode 
sliape. Tine final number 
represents the approximate 
width in inches. 




ANODE 
TYPE* 


MAGNESIUM CASTING DIMENSIONS (in.) 


PACKAGE DIMENSIONS (in.) & WEIGHT (lb.) 


A 


B 


C 


D 


Length 


Width 


Backfill Wt. 


Total Wt. 


3D3 


3-1/2" 


3-3/4" 


5" 


1-1/2" 


6-1/2" 


6" 


6# 


9# 


5D3 


3-12" 


3-3/4" 


8-1/2" 


1-1/2" 


13-1/2" 


6" 


9# 


14# 


9D3 


3-1/2" 


3-3/4" 


13-5/16" 


1-1/2" 


17" 


6" 


15# 


24# 


9D2 


2-3/4" 


3" 


27" 


1-1/8" 


33" 


6" 


15# 


24# 


17D3 


3-1/2" 


3-3/4" 


25-7/8" 


1-1/2" 


30" 


6" 


25# 


42# 


17D2 


2-3/4" 


3" 


50" 


1-1/8" 


60" 


6" 


25# 


42# 


20D2 


2-3/4" 


3" 


59-3/4" 


1-1/8" 


62" 


5" 


50# 


70# 


24D4 


4-3/4" 


4-1/2" 


22-1/4" 


2" 










32D3 


3-1/2" 


3-3/4" 


45-1/4" 


1-1/2" 


48" 


6" 


40# 


72# 


32D5 


5-1/2" 


5-3/4" 


20-9/16" 


2-7/16" 


30" 


8" 


38# 


70# 


40D3 


3-1/2" 


3-3/4" 


59-3/4" 


1-1/2" 


62" 


6" 


50# 


90# 


48D5 


5-1/2" 


5-3/4" 


31-1/16" 


2-7/16" 


34" 


8" 


48# 


96# 



^NACE 



CP 3-Cathodic Protection Technologist Course Manual 
© NACE International, 2005 



Appendix A - Anode Specifications 



A:2 



Table II 

Zinc Packaged Ground Anodes 



TYPE 


WEIGHT 


SIZE 


ZA5 


5 


1.4" X 1.4" X 110" 


ZA6 


6 


1.4" X 1.4" X 112" 


ZA9 


9 


1.4" X 1.4" X 118" 


ZA12 


12 


1.4" X 1.4" X 224" 


ZA18 


18 


1.4" X 1.4" X 336" 


ZA24 


24 


2"x 2"x24" 


ZA30 


30 


1.4" X 1.4" X 660" 


ZA48 


48 


2"x 2"x48" 


ZA60 


60 


2"x2"x60" 




^NACE 



CP 3-Cathodic Protection Techno legist Course JVlanua! 
© NACE International, 2005 
J anuary 2007 



Appendix A - Anode Specifications 



A:3 



Table IMA 

High Silicon Iron Tubular Anodes 



TYPE 


DIMENSIONS (Inches) 


WEIGHT 
(lbs.) 


AREA 
(sq.ft.) 


D 


L 


A 


H 


2284 


2.2 


84 


2.6 


8 


46 


4.2 


2684 


2.6 


84 


3.0 


8 


64 


4.9 


3884 


3.8 


84 


4.2 


8 


87 


7.0 


4884L 


4.8 


84 


5.2 


8 


112 


8.8 


4884H 


4.8 


84 


5.2 


8 


177 


8.8 



L 



D0 



^ 



A0 



T 



H — ► 
L 



.S 



When ordering Compression Wedges Specify: Style 1 (Crimp) or Style 2 (Solder) 

SHIPPING INFORMATION 





TUBE ANODES PER CRATE 


GROSS WEIGHT (lbs.) | 


Anode Type 


Small Crate 


Large Crate 


Small Crate 


Large Crate 


2284 


45 


63 


2190 


3018 


2684 


40 


56 


2800 


3872 


3084 


21 


35 


1947 


3165 


4884L 


15 


20 


1845 


2420 


4884H 


15 


20 


2745 


3620 



^NACE 



CP 3-Cathodic Protection Techno legist Course JVlanua! 
© NACE International, 2005 
J anuary 2007 



Appendix A - Anode Specifications 



A:4 



Table NIB 

High Silicon Iron Solid Chill Cast Anodes 



TYPE 



STYLE 



DIMENSIONS 



WEIGHT 

(lbs.) 



AREA 

(sq.ft.) 



EHA 



1 (PIN) 

2 (LEAD) 



|3"dia. 



|2"dia. 



60" 



IP 



44 



2.6 



EHK 



2 (LEAD) 



|2" dia. 

-< 



|l-l/2"dia. 



60" 



26 



2.0 



EHM 



1 (PIN) 

2 (LEAD) 



|3"d ia. 

rr 



|2"dia. 



|3"dia . 



60" 



60 



2.7 



EHR 



1 (PIN) 

2 (LEAD) 



I 4" dia. 



ri 



1 3" dia. 



50" 



110 



4.0 



SHA 



I 2" dia. 



2 (LEAD) 



60" 



43 



2.6 



SHIPPING INFORMATION 





SOLID ANODES PER CRATE 


GROSS WEIGHT (lbs.) | 


Anode Type 


Small Crate 


Large Crate 


Small Crate 


Large Crate 


EHA, SHA 


50 


80 


2350 


3720 


EHJVl 


45 


-- 


2800 


-- 


EHK 


70 


126 


1990 


3502 


EHR 


24 


-- 


2740 


-- 



^NACE 



CP 3-Cathodic Protection Techno legist Course Manual 
© NACE International, 2005 
J anuary 2007 



Appendix A - Anode Specifications 



A:5 



Table IV 

Concentric Stranded Copper Single Conductors 

Direct Burial Service Suitably Insulated 



Size 
AWG 


Overall Diameter 

Not including 

Insulation 

(inches) 


Approx. weight 

Not Including 

Insulation 

(lbs./M ft.) 


Maximum 

Breaking 

Strength 

(lbs.) 


Maximum 
D.C. Resistance 

@205C 
Ohms/1000 ft. 


Max. Allowable 

D.C. Current 

Capacity 

(Amperes) 


14 


0.0726 


12.68 


130 


2.5800 


15 


12 


0.0915 


20.16 


207 


1.6200 


20 


10 


0.1160 


32.06 


329 


1.0200 


30 


8 


0.1460 


50.97 


525 


0.6400 


45 


6 


0.1840 


81.05 


832 


0.4030 


65 


4 


0.2320 


128.90 


1320 


0.2540 


85 


3 


0.2600 


162.50 


1670 


0.2010 


100 


2 


0.2920 


204.90 


2110 


0.1590 


115 


1 


0.3320 


258.40 


2660 


0.1260 


130 


1/0 


0.3730 


325.80 


3350 


0.1000 


150 


2/0 


0.4190 


410.90 


4230 


0.0795 


175 


3/0 


0.4700 


518.10 


5320 


0.0631 


200 


4/0 


0.5280 


653.30 


6453 


0.0500 


230 


250IV1CIV1 


0.5750 


771.90 


7930 


0.0423 


255 



Table V 

Typical Platinum Clad Niobium (Copper-cored) Anode Specification 



Diameter 
inches 


% 
Nb 


Nb Thickness 
inches 


Resistance 
microhm/ft 


Pt Thickness 
H-in. (2X)** 


Anode Life* 
AY/ft. (2X) 


.750 


20 


.038 


22 


300 


373 


.500 


20 


.025 


50 


200 


166 


.375 


20 


.019 


89 


150 (300) 


93 (186) 


.250 


20 


.013 


201 


100 (200) 


41 (82) 


.188 


20 


.009 


356 


75 (150) 


22 (44) 


.125 


20 


.006 


806 


50 (100) 


10 (20) 


.125 


40 


.013 


1025 


50 (100) 


10 (20) 


.093 


40 


.010 


1822 


38 (75) 


5.5 (11) 


.063 


40 


.007 


4102 


25 (50) 


2.5 (5) 



*Basedon0.08g/Amp-Year 
^Double Platinum Thickness 



^NACE 



CP 3-Cathodic Protection Techno legist Course JVlanua! 
© NACE International, 2005 
J anuary 2007 



Appendix A - Anode Specifications 



A:6 



Table VI 

Graphite Cylindrical Anodes 



Anode Size 


Anode Weight 


3"x30" 


14 1b 


3"x60" 


27 1b 


4"x80" 


68 1b 



^NACE 



CP 3-Cathodic Protection Techno legist Course JVlanua! 
© NACE International, 2005 
J anuary 2007 



APPENDIX B 

PIPE DATA TABLE 



Table B-1: Steel Pipe Data for Corrosion Calculations 

Resistivity of steel = 5.292E-06ohm5-in/1.344E-05 ohms-cm 



Nominal 
Size 


Nominal 
Size 


Schedule 
Number 


OD 


Wall 
Thickness 


Weight 
Per ft Perm 


Linear Res 
Per ft Perm 


Inch 


Cm 




Inch 


Cm 


Inch 


Cm 


lbs 


kg 


ohms 


ohms 


4 


10 


40 


4.500 


11.4 


0.237 


0.602 


10.79 


4.90 


2.001E-05 


6.582E-05 


6 


15 




6.625 


16.8 


0.188 


0.476 


12.89 


5.85 


1.675E-05 


5.493E-05 


6 


15 




6.625 


16.8 


0.219 


0.556 


14.97 


6.80 


1.442E-05 


4.731E-05 


6 


15 




6.625 


16.8 


0.250 


0.635 


17.02 


7.73 


1.268E-05 


4.160E-05 


6 


15 


40 


6.625 


16.8 


0.280 


0.711 


18.98 


8.62 


1.138E-05 


3.732E-05 


6 


15 


80 


6.625 


16.8 


0.432 


1.097 


28.58 


12.97 


7.556E-06 


2.478E-05 


6 


15 


120 


6.625 


16.8 


0.562 


1.427 


36.40 


16.52 


5.932E-06 


1.946E-05 


8 


20 


20 


8.625 


21.9 


0.250 


0.635 


22.37 


10.15 


9.654E-06 


3.167E-05 


8 


20 


30 


8.625 


21.9 


0.277 


0.704 


24.70 


11.21 


8.742E-06 


2.867E-05 


8 


20 


40 


8.625 


21.9 


0.322 


0.616 


28.58 


12.97 


7.561E-0 


2.480E-05 


8 


20 


60 


8.625 


21.9 


0.406 


1.031 


35.64 


16.18 


6.058E-06 


1.987E-05 


10 


25 


20 


10.750 


27.3 


0.250 


0.635 


28.04 


12.73 


7.701E-06 


2.526E-05 


10 


25 


30 


10.750 


27.3 


0.307 


0.780 


34.25 


15.55 


6.305E-06 


2.068E-05 


10 


25 


40 


10.750 


27.3 


0.365 


0.927 


40.49 


18.38 


5.333E-06 


1.749E-05 


10 


25 


60 


10.750 


27.3 


0.500 


1.270 


54.74 


24.85 


3.944E-06 


1.294E-05 


12 


30 


20 


12.750 


32.4 


0.250 


0.635 


33.38 


15.15 


6.468E-06 


2.122E-05 


12 


30 


30 


12.750 


32.4 


0.330 


0.638 


43.78 


19.88 


4.932E-06 


1.618E-05 


12 


30 




12.750 


32.4 


0.375 


0.953 


49.57 


22.51 


4.356E-06 


1.429E-05 


12 


30 


40 


12.750 


32.4 


0.406 


1.031 


53.53 


24.30 


4.033E-06 


1.323E-05 


12 


30 




12.750 


32.4 


0.500 


1.270 


65.43 


29.70 


3.300E-06 


1.082E-05 


14 


36 


10 


14.000 


35.6 


0.250 


0.635 


36.72 


16.67 


5.880E-06 


1.929E-05 


14 


36 


20 


14.000 


35.6 


0.312 


0.792 


45.62 


20.71 


4.733E-06 


1.552E-05 


14 


36 


30 


14.000 


35.6 


0.375 


0.953 


54.58 


24.78 


3.956E-06 


1.298E-05 


16 


41 


10 


16.000 


40.6 


0.250 


0.635 


42.06 


19.10 


5.134E-06 


1.684E-05 


16 


41 


20 


16.000 


40.6 


0.312 


0.792 


52.28 


23.74 


4.130E-06 


1.355E-05 


16 


41 


30 


16.000 


40.6 


0.375 


0.953 


62.59 


28.42 


3.450E-06 


1.132E-05 


16 


41 


40 


16.000 


40.6 


0.500 


1.270 


82.78 


37.58 


2.608E-06 


8.555E-06 



QNACE 



CP 3-Cathodic ProtsctionTechnologistCourse Manual 
© NACE Intsmational, 2004 



Appendix B - Pipe Data Table 



B:2 



Table B-1, cont'd: Steel Pipe Data for Corrosion Calculations 

Resistivity of steel = 5.292E-06ohms-in/1.344E-05 ohms-cm 



Nominal 
Size 


Nominal 
Size 


Schedule 
Number 


OD 


Wall 
Thickness 


Weight 
Per ft Perm 


Linear Res 
Per ft Perm 


Inch 


Cm 




Inch 


Cm 


Inch 


Cm 


lbs 


kg 


ohms 


ohms 


18 


46 


10 


18.000 


45.7 


0.250 


0.635 


47.40 


21.52 


4.555E-06 


1.494E-05 


18 


46 


20 


18.000 


45.7 


0.312 


0.792 


56.95 


26.76 


3.663E-06 


1.201E-05 


18 


46 


30 


18.000 


45.7 


0.437 


1.110 


61.98 


37.22 


2.634E-06 


8.639E-06 


18 


46 




18.000 


45.7 


0.500 


1.270 


93.47 


42.43 


2.310E-06 


7.577E-06 


20 


51 


10 


20.000 


50.8 


0.250 


0.635 


52.74 


23.94 


4.094E-06 


1.343E-06 


20 


51 


20 


20.000 


50.8 


0.375 


0.953 


78.61 


35.69 


2.747E-06 


9.009E-06 


20 


51 


30 


20.000 


50.8 


0.500 


1.270 


104.15 


47.28 


2.073E-06 


6.800E-06 


24 


61 


10 


24.000 


61.0 


0.250 


0.635 


63.42 


28.79 


3.404E-06 


1.117E-05 


24 


61 


20 


24.000 


61.0 


0.375 


0.953 


94.64 


42.96 


2.282E-06 


7.484E-06 


24 


61 




24.000 


61.0 


0.500 


1.270 


125.51 


56.98 


1.720E-06 


5.643E-06 


30 


76 




30.000 


76.2 


0.250 


0.635 


79.45 


36.07 


2.718E-06 


8.915E-06 


30 


76 




30.000 


76.2 


0.375 


0.953 


118.67 


53.88 


1.820E-06 


5.968E-06 


30 


76 




30.000 


76.2 


0.500 


1.270 


157.56 


71.53 


1.370E-06 


4.495E-06 


36 


91 




36.000 


91.4 


0.250 


0.635 


95.47 


43.34 


2.262E-06 


7.418E-06 


36 


91 




36.000 


91.4 


0.375 


0.953 


142.70 


64.79 


1.513E-06 


4.963E-06 


36 


91 




36.000 


91.4 


0.500 


1.270 


189.60 


86.08 


1.139E-06 


3.735E-06 


42 


107 




42.000 


106.7 


0.250 


0.635 


111.49 


50.62 


1.937E-06 


6.352E-06 


42 


107 




42.000 


106.7 


0.375 


0.953 


166.74 


75.70 


1.295E-06 


4.248E-06 


42 


107 




42.000 


106.7 


0.500 


1.270 


221.65 


100.63 


9.742E-06 


3.195E-06 


46 


122 




48.000 


121.9 


0.250 


0.635 


127.52 


57.89 


1.693E-06 


5.554E-06 


46 


122 




48.000 


121.9 


0.375 


0.953 


190.77 


86.61 


1.132E-06 


3.712E-06 


46 


122 




48.000 


121.9 


0.500 


1.270 


253.70 


115.18 


8.511E-06 


2.792E-06 


54 


137 




54.000 


137.2 


0.250 


0.635 


143.54 


65.17 


1.504E-06 


4.934E-06 


54 


137 




54.000 


137.2 


0.375 


0.953 


214.81 


97.52 


1.005E-06 


3.297E-06 


54 


137 




54.000 


137.2 


0.500 


1.270 


285.74 


129.73 


7.557E-06 


2.479E-06 


60 


152 




60.000 


152.4 


0.250 


0.635 


159.56 


72.44 


1.353E-06 


4.439E-06 


60 


152 




60.000 


152.4 


0.375 


0.953 


238.84 


108.43 


9.040E-06 


2.965E-06 



QNACE 



CP 3-Cathodic Protection TecinnologistCourse JVlanual 
© NACE International, 2004 



APPENDIX C 

METRIC CONVERSION TABLE 



U.S. Customary/Metric Conversions 
for Units Commonly Used in Corrosion-Related Literature 



1 A/ft' 


= 10.76 A/nf 


1 acre 


= 4,047 m'= 0.4047 ha 


1 A-h/lb 


= 2.205 A-h/kg 


1 Angstrom 


= 10Vm = 10'"m=0.1rmi 


1 atm 


= 101.325 kPa 


1 bar 


= 100kPa 


1 bbl, oil (U.S.) 


= 159.0 L 


Ibpd(oil) 


= 159L/d 


IBTU 


= 1,055 J 


1 BTU/rf 


= ll,360J/m' 


1 BTU/rf/h 


= 3.152 W/m^(K-factDr) 


1 BTU/rf/h/F 


= 5.674 W/m'-K 


1 BTU/rf/h/F/in. 


= 0.144 W/m-K 


1 BTU/h 


= 0.2931 W 


1 cfm 


= 28.3 L/min = 0.0283 mVmin = 


1 cup 


= 236.6 mL 


1 cycle/s 


= lHz 


1ft 


= 0.3048 m 


1ft? 


=0.0929 m'= 929 cm' 


If^ 


=0.02832 m' =28.32 L 


1 ft-lb (force) 


= 1.356 J 


1 ft-lb (torque) 


= 1.356 N-m 


Iftys 


= 0.3048 m/s 


1 gal (Imp.) 


= 4.546 L= 0.004546 m^ 


1 gal (U.S.) 


= 3.785 L= 0.003785 m' 


1 gal/bag (U.S.) 


= 89 mL/kg (water/cement ratio) 


1 grain 


= 0.06480 g = 64.80 mg 


1 grain/ft* 


= 2.212 g/m' 


1 grain/100 ft? 


= 22.12 mg/m 


Ihp 


= 0.7457 kW 


1 microinch 


= 0.0254 |xm= 25.4 nm 


lin. 


= 2.54 cm =25.4 mm 


lin.^ 


= 6.452 cm' 


lin.^ 


= 16.387 cm' =0.01639 L 


1 in.-lb (torque) 


= 0.113 N-m 


1 in. mercury 


= 3.387 kPa 


1 in. water 


= 248.8 Pa 



Ikgfam' 


=9.807 MPa 


1 kilocalcirie 


= 4.184 kj 


Iknot 


= 0.51 5 m/s 


Iksi 


= 6.895 MPa 


lib 


= 453.6 g = 0.4536 kg 


1 lb/ft? 


= 47.88 Pa 


1 lb/ft? 


=0.01602 g/cm' 


1 lb/100 U.S. gal 


= 1.1981 g/L 


1 lb/1,000 bbl 


= 2.852 mg/L 


1 mA/in' 


=0.155 mA/cm' 


1 mA/ft? 


= 10.76 mA/m' 


1 MBPD (dl) 


= 159kL/d 


1 mile 


= 1.609 km 


1 sq. mile 


= 2.590 km' 


1 ni. (naut) 


= 1.852 km 


Imil 


= 0.0254 mm = 25.4 |xm 


1 MMCFD 


= 2.83xlO*mVd 


1 mm mercury 


= 0.1333kPa 


In^ 


= 1.609 km/h 


1 mpy 


= 0.0254 mm/y = 25.4 |xm/y 


1 N/mm' 


= lPa 


1 oz 


= 28.35 g 


1 oz fluid (Imp.) 


= 28.41 ml 


1 oz fluid (U.S.) 


= 29.57 ml 


1 oz/ft' 


= 2.992 Pa 


1 oz/U.S. gal 


= 7.49 g/L 


lpartyi,OOObbl 


= 2.32 mg/L 


Ipsi 


= 0.006895 MPa = 6.895 kPa 


1 qt (Imp.) 


= 1.1365 L 


lqt(U.S.) 


= 0.9464 L 


1 teaspoon (tsp) 


= 4.929 mL 


1 ton (short) 


= 907.2 kg 


Itorr 


= 133.3 Pa 


1 U.S. bag cement 


= 42.63 kg (94 lb) 


1yd 


= 0.914 m 


1yd' 


= 0.8361 m' 


1yd' 


= 0.7646 m' 



Do Nat Use 


Use Instead 




MisoeUaneaus IMts Nat to Be Used 


are 


1 are 


= ldam'=100rrf 


calorie 


candle 


1 candle 


= lcd 


conventional millimeter of mercury 


candlepower 


1 candlepower 


= 1 cd 


grade (1 grade = (71/2OO) rad) 


fermi 


1 fermi 


= lfm = 10"m 


kilogram-force 


gamma 


1 gamma 


= lnT 


langley ( 1 langley = 1 cal/cm ) 


micron 


1 micron 


= 1 |xm = 10'm 


metric carat 


millimicron 


1 millimicron 


= 1 nm=10"'m 


metric hausqxjwa: 


mho 


1 mho 


= 1S 


millimeter, centimeter, or meter of water 


7 (mass) 


ly 


= 1 W 


standard atmosphere (atm = 1 01 .325 kPa) 


X (volume) 


1 X 


= mm' =1 |xL =lmm' 


technical atmosphae (1 at =98.0665 kPa) 


%IACE 









CP 3-Cathodic Protection Technologist Course Manual 
© NACE International, 2005 



APPENDIX D 

SHUNT TABLE 



Table D-1: Shunt Types and Values 





Shunt Rating 


Shunt Value 


Shunt Factor 




Amps 


mV 


Ohms 


A/mV 


HollowayType 




RS 


5 


50 


.01 


.1 


SS 


25 


25 


.001 


1 


SO 


50 


50 


.001 


1 


SWorCP 


1 


50 


.05 


.02 


SWorCP 


2 


50 


.025 


.04 


SWorCP 


3 


50 


.017 


.06 


SWorCP 


4 


50 


.0125 


.08 


SWorCP 


5 


50 


.01 


.1 


SWorCP 


10 


50 


.005 


.2 


SW 


15 


50 


.0033 


.3 


SW 


20 


50 


.0025 


.4 


SW 


25 


50 


.002 


.5 


SW 


30 


50 


.0017 


.6 


SW 


50 


50 


.001 


1 


SW 


60 


50 


.0008 


1.2 


SW 


75 


50 


0.00067 


1.5 


SW 


100 


50 


.0005 


2 


J. B. Type 




Agra-Mesa 


5 


50 


.01 


.1 


CottorMCM 




Red(MCM) 


2 


200 


.1 


.01 


Red (Cott) 


.5 


50 


.1 


.01 


Yellow(MCM) 


8 


80 


.01 


.1 


Orange (MCM) 


25 


25 


.001 


1 



NACE 

CP 3-Cathodic ProtBction Technologist Course Manual 
© NACE International, 2005 



NACE GLOSSARY OF CORROSION-RELATED TERMS 



@ 



ABRASIVE 

Small particles of material that 
are propelled at high velocity to 
impact a surface during abrasive 
blastcleaning. 

ABRASIVE BLAST 

CLEANING 

Cleaning and roughening of a 
surface produced by the high- 
velocity impact of an abrasive 
that is propelled by the discharge 
of pressurized fluid from a blast 
nozzle or by a mechanical device 
such as a centrifugal blasting 
wheel. (Also referred to as 
Abrasive Blasting.) 

ACCELERATOR 

A chemical substance that 
increases the rate at which a 
chemical reaction (e.g., curing) 
would otherwise occur. 

ACRYLIC 

Type of resin polymerized from 
acrylic acid, methacrylic acid, 
esters of these acids, or 
acrylonitrile. 

ACTIVATOR 

A chemical substance that 
initiates and accelerates a 
chemical reaction (e.g., curing). 
Heatand radiation may also 
serve as activators for some 
chemical reactions. 

ACTIVE 

(1) The negative direction of 
electrode potential. (2) A state 
of a metal that is corroding 
without significant influence of 
reaction product. 



AERATION CELL 

[See Differential Aeration Cell.] 

AIR DRYING 

Process by which an applied wet 
coat converts to a dry coating 
film by evaporation of solvent or 
reaction with oxygen as a result 
of simple exposure to air without 
intentional addition of heat or a 
curing agent. 

AIRLESS SPRAYING 

Process of spraying coating 
liquids using hydraulic pressure, 
not air pressure, to atomize. 

ALKYD 

Type of resin formed by the 
reaction of polyhydric alcohols 
and polybasic acids, part of 
which is derived from saturated 
or unsaturated oils or fats. 

ALLIGATORING 

Pronounced wide cracking over 
the surface of a coating, which 
has the appearance of alligator 
hide. 

AlvPhHOTERIC IVETAL 

A metal that is susceptible to 
corrosion in both acid and 
alkaline environments. 

ANAEROBIC 

Free of air or uncombined 
oxygen. 

ANION 

A negatively charged ion that 
migrates through the electrolyte 
toward the anode under the 
influence of a potential gradient. 

ANODE 

The electrode of an 
electrochemical cell at which 
oxidation occurs. Electrons flow 
away from the anode in the 



external circuit. Corrosion 
usually occurs and metal ions 
enterthe solution atthe anode. 

ANODE CAP 

An electrical insulating material 
placed over the end of the anode 
atthe lead wire connection. 

ANODE CORROSION 
EFFICIENCY 

The ratio of the actual corrosion 
(mass loss) of an anode to the 
theoretical corrosion (mass loss) 
calculated from the quantity of 
electricity that has passed 
between the anode and cathode 
using Faraday's law. 

ANODIC INHIBrrOR 

A chemical substance that 
prevents or reduces the rate of 
the anodic or oxidation reaction. 

ANODIC POLARIZATION 

The change of the electrode 
potential in the noble (positive) 
direction caused by current 
across the electrode/electrolyte 
interface. [See Polarization.] 

ANODIC PROTECTION 

Polarization to a more oxidizing 
potential to achieve a reduced 
corrosion rate by the promotion 
of passivity. 

ANODIZING 

Oxide coating formed on a metal 
surface (generally aluminum) by 
an electrolytic process. 

ANOLYTE 

The electrolyte adjacent to the 
anode of an electrochemical cell. 

ANTIFOULING 

Preventing fouling. [See 
Fouling.] 



© 2002, NACE International. This publication may not be reprinted without the written consent of NACE International. 

Page 1 of 18 



NACE GLOSSARY OF CORROSION-RELATED TERMS 



ATTENUATION 

Electrical losses in a conductor 
caused by current flow in the 
conductor. 

AUGER ELECTRON 
SPECTROSCOPY 

Analytical technique in which the 
sample surface is irradiated with 
low-energy electrons and the 
energy spectrum of electrons 
emitted from the surface is 
measured. 

AUSTENmC STEEL 

A steel whose microstructure at 
room temperature consists 
predominantly of austenite. 

AUXILIARY ELECTRODE 

An electrode, usually made from 
a noncorroding material, which is 
commonly used in polarization 
studies to pass current to or from 
a test electrode. 



B 



BACKFILL 

Material placed in a hole to fill the 
space around the anodes, vent 
pipe, and buried components of a 
cathodic protection system. 

BARRIER COATING 

(1) A coating that has a high 
resistance to permeation of 
liquids and/or gases. (2) A 
coating that is applied over a 
previously coated surface to 
prevent damage to the 
underlying coating during 
subsequent handling 

BEACH MARKS 

The characteristic markings on 
the fracture surfaces produced 
by fatigue crack propagation 



(also known as clamshell marks, 
conchoidal marks, and arrest 
marks). 

BETA CURVE 

A plot of dynamic (fluctuating) 
interference current or related 
proportional voltage (ordinate) 
versus the corresponding 
structure-to-electrolyte potentials 
at a selected location on the 
affected structure (abscissa). 

BINDER 

The nonvolatile portion of the 
vehicle of a formulated coating 
material. 

BrruMiNOus coating 

An asphalt orcoal-tarcompound 
used to provide a protective 
coating for a surface. 

BLAST ANGLE 

(1) The angle of the blast nozzle 
with reference to the surface 
during abrasive blast cleaning. 

(2) The angle of the abrasive 
particles propelled from a 
centrifugal blasting wheel with 
reference to the surface being 
abrasive blast cleaned. 

BLOWDOWN 

(1) Injection of air or water under 
high pressure through a tube to 
the anode area for the purpose of 
purging the annular space and 
possibly correcting high 
resistance caused by gas 
blockage. (2) In conjunction with 
boilers or cooling towers, the 
process of discharging a 
significant portion of the aqueous 
solution in order to remove 
accumulated salts, deposits, and 
other impurities. 

BLUSHING 

Whitening and loss of gloss of a 
coating, usually organic, caused 



by moisture (also known as 
blooming). 

BRACELET ANODES 

Galvanic anodes with geometry 
suitable for direct attachment 
around the circumference of a 
pipeline. These may be half- 
shell bracelets consisting of two 
semi-circular sections or 
segmented bracelets consisting 
of a large number of individual 
anodes. 

BRRTLE FRACTURE 

Fracture with littie or no plastic 
deformation. 

BRUSH-OFF BLAST 
CLEANED SURFACE 

A brush-off blast cleaned 
surface, when viewed without 
magnification, shall be free of all 
visible oil, grease, dirt, dust, 
loose mill scale, loose rust, and 
loose coating. Tightiy adherent 
mill scale, rust, and coating may 
remain on the surface. Mill 
scale, rust, and coating are 
considered tightiy adherent if 
they cannot be removed by lifting 
with a dull putty knife. [See 
NACE No. 4/SSPC-SP 7.] 



c 



CALCAREOUS COATING 

A layer consisting of calcium 
carbonate and other salts 
deposited on the surface. When 
the surface is cathodically 
polarized as in cathodic 
protection, this layer is the result 
of the increased pH adjacent to 
the protected surface. 

CALCAREOUS DEPOSFT 

[See Calcareous Coating.] 



© 2002, NACE International. This publication may not be reprinted without the written consent of NACE International. 

Page 2 of 18 



NACE GLOSSARY OF CORROSION-RELATED TERMS 



CASE hHARDENING 

Hardening a ferrous alloy so that 
the outer portion, or case, is 
made substantially harder than 
the inner portion, or core. 
Typical processes are 
carburizing, cyaniding, carbo- 
nitriding, nitriding, induction 
hardening, and flame hardening. 

CASEIN PAiisrr 

Water-thinned paint with vehicle 
derived from milk. 

CATALYST 

A chemical substance, usually 
present in small amounts relative 
to the reactants, that increases 
the rate at which a chemical 
reaction (e.g., curing) would 
otherwise occur, but is not 
consumed in the reaction. 

CATHODE 

The electrode of an 
electrochemical cell at which 
reduction is the principal 
reaction. Electrons flow toward 
the cathode in the external 
circuit. 

CAThHODIC CORROSION 

Corrosion resulting from a 
cathodic condition of a structure, 
usually caused by the reaction of 
an amphoteric metal with the 
alkaline products of electrolysis. 

CATHODIC 
DISBONDME NT 

The destruction of adhesion 
between a coating and the 
coated surface caused by 
products of a cathodic reaction. 

CAThHODIC INHIBrrOR 

A chemical substance that 
prevents or reduces the rate of 
the cathodic or reduction 
reaction. 



CATHODIC 
POLARIZATION 

The change of the electrode 
potential in the active (negative) 
direction caused by current 
across the electrode/electrolyte 
interface. [See Polarization.] 

CATHODIC PROTECTION 

A technique to reduce the 
corrosion of a metal surface by 
making that surface the cathode 
of an electrochemical cell. 
CATHOLYTE 
The electrolyte adjacent to the 
cathode of an electrochemical 
cell. 

CATION 

A positively charged ion that 
migrates through the electrolyte 
toward the cathode under the 
influence of a potential gradient. 

CAVTTATION 

The formation and rapid collapse 
of cavities or bubbles within a 
liquid which often results in 
damage to a material atthe 
solid/liquid interface under 
conditions of severe turbulent 
flow. 

CELL 

[See Electroctiemical Cell.] 

CEMENTATION 

The introduction of one or more 
elements into the surface layer of 
a metal by diffusion at high 
temperature. (Examples of 
cementation include carburizing 
[introduction of carbon], nitriding 
[introduction of nitrogen], and 
chromizing [introduction of 
chromium].) 

ChHALKING 

The development of loose, 
removable powder (pigment) at 



the surface of an organic coating, 
usually caused by weathering. 

CHECKING 

The development of slight breaks 
in a coating which do not 
penetrate to the underlying 
surface. 

CHEMICAL CONVERSION 

COATING 

An adherent reaction product 
layer on a metal surface formed 
by reaction with a suitable 
chemical to provide greater 
corrosion resistance to the metal 
and increase adhesion of 
coatings applied to the metal. 
(Example is an iron phosphate 
coating on steel, developed by 
reaction with phosphoric acid.) 

CHEVRON PATTERN 

A V-shaped pattern on a fatigue 
or brittle-fracture surface. The 
pattern can also be one of 
straight radial lines on cylindrical 
specimens. 

CHLORIDE STRESS 
CORROSION CRACKING 

Cracking of a metal underthe 
combined action of tensile stress 
and corrosion in the presence of 
chlorides and an electrolyte 
(usually water). 

COAT 

One layer of a coating applied to 
a surface in a single continuous 
application to form a uniform film 
when dry. 

COATING 

A liquid, liquefiable, or mastic 
composition that, after 
application to a surface, is 
converted into a solid protective, 
decorative, or functional adherent 
film. 



© 2002, NACE International. This publication may not be reprinted without the written consent of NACE International. 

Page 3 of 18 



NACE GLOSSARY OF CORROSION-RELATED TERMS 



COATING SYSTEM 

The complete number and types 
of coats applied to a substrate in 
a predetermined order. (When 
used in a broader sense, surface 
preparation, pretreatments, dry 
film thickness, and manner of 
application are included.) 

COLD LAP 

(1) Discontinuity caused by 
solidification of the meniscus of a 
partially cast anode as a result of 
interrupted flow of the casting 
stream. The solidified meniscus 
is covered with metal when the 
flow resumes. Cold laps can 
occur along the length of an 
anode. (2) A protective film 
consisting of one or more coats, 
applied in a predetermined order 
by prescribed methods to an as- 
specified dry film thickness, 
including any reinforcing material 
that may be specified. 

COLD SHUT 

Horizontal surface discontinuity 
caused by solidification of a 
portion of a meniscus during the 
progressive filling of a mold, 
which is later covered with more 
solidifying metal as the molten 
metal level rises. Cold shuts 
generally occur at corners 
remote from the point of pour. 

COMMERCIAL BLAST 
CLEANED SURFACE 

A commercial blastcleaned 
surface, when viewed without 
magnification, shall be free of all 
visible oil, grease, dust, dirt, mill 
scale, rust, coating, oxides, 
corrosion products, and other 
foreign matter. Random staining 
shall be limited to no more than 
33 percent of each unit area 
(approximately 58 cm^ [9.0 in.^]) 
of surface and may consist of 
light shadows, slight streaks, or 



minor discolorations caused by 
stains of rust stains of mill scale, 
or stains of previously applied 
coating. [See NACE No. 
3/SSPC-SP 6.] 

CONCENTRATION CELL 

An electrochemical cell, the 
electromotive force of which is 
caused by a difference in 
concentration of some 
component in the electrolyte. 
(This difference leads to the 
formation of discrete cathodic 
and anodic regions.) 

CONCENTRATION 
POLARIZATION 

That portion of polarization of a 
cell produced by concentration 
changes resulting from passage 
of current though the electrolyte. 

CONDUCTIVE COATING 

(1) A coating thatconducts 
electricity. (2) An electrically 
conductive, mastic-like material 
used as an impressed current 
anode on reinforced concrete 
surfaces. 

CONDUCTIVE 
CONCRETE 

A highly conductive cement- 
based mixture containing coarse 
and fine coke and other material 
used as an impressed current 
anode on reinforced concrete 
surfaces. 

CONDUCTIVFTY 

(1) A measure of the ability of a 
material to conduct an electric 
charge. It is the reciprocal of 
resistivity. (2) The current 
transferred across a material 
(e.g., coating) per unit potential 
gradient. 



CONTACT CORROSION 

[See Galvanic Corrosion.] 

CONTINUFTY BOND 

A connection, usually metallic, 
that provides electrical continuity 
between structures that can 
conduct electricity. 

CONTINUOUS ANODE 

A single anode with no electrical 
discontinuities. 

CONVERSION COATING 

[See Chemicai Conversion 
Coating.] 

CORROSION 

The deterioration of a material, 
usually a metal, that results from 
a reaction with its environment. 

CORROSION FATIGUE 

Fatigue-type cracking of metal 
caused by repeated or fluctuating 
stresses in a corrosive 
environment characterized by 
shorter life than would be 
encountered as a result of either 
the repeated or fluctuating stress 
alone or the corrosive 
environment alone. 

CORROSION INHIBFTOR 

A chemical substance or 
combination of substances that, 
when present in the environment, 
prevents or reduces corrosion. 

CORROSION POTENTIAL 

ICcorrJ 

The potential of a corroding 
surface in an electrolyte relative 
to a reference electrode under 
open-circuit conditions (also 
known as rest potential, open- 
circuit potential, or freely 
corroding potential). 



© 2002, NACE International. This publication may not be reprinted without the written consent of NACE International. 

Page 4 of 18 



NACE GLOSSARY OF CORROSION-RELATED TERMS 



CORROSION RATE 

The rate at which corrosion 
proceeds. 

CORROSION 
RESISTANCE 

Ability of a material, usually a 
metal, to withstand corrosion in a 
given system. 

CORROSIVENESS 

The tendency of an environment 
to cause corrosion. 

COUNTER ELECTRODE 

[S ee A uxiliary Electrode. ] 

COUNTERPOISE 

A conductor or system of 
conductors arranged beneath a 
power line, located on, above, or 
most frequently, below the 
surface of the earth and 
connected to the footings of the 
towers or poles supporting the 
power line. 

COUPLE 

[See Galvanic Couple.] 

CRACKING (OF 
COATING) 

Breaks in a coating that extend 
through to the substrate. 

CRAZING 

A network of checks or cracks 
appearing on the surface of a 
coating. 

CREEP 

Time-dependent strain occurring 
under stress. 

CREVICE CORROSION 

Localized corrosion of a metal 
surface at, or immediately 
adjacentto, an area that is 
shielded from full exposure to the 



environment because of close 
proximity of the metal to the 
surface of another material. 

CRFTICAL HUMIDFTY 

The relative humidity above 
which the atmospheric corrosion 
rate of some metals increases 
sharply. 

CRFTICAL PFTTING 
POTENTIAL (Ep, Epp) 

The lowest value of oxidizing 
potential (voltage) at which pits 
nucleate and grow. The value 
depends on the test method 
used. 

CURING 

Chemical process of developing 
the intended properties of a 
coating or other material (e.g., 
resin) over a period of time. 

CURING AGENT 

A chemical substance used for 
curing a coating or other material 
(e.g., resin). [Also referred to as 
Hardener.] 

CURRENT 

(1) A flow of electric charge. (2) 
The amount of electric charge 
flowing pasta specified circuit 
point per unit time, measured in 
the direction of net transport of 
positive charges. (In a metallic 
conductor, this is the opposite 
direction of the electron flow.) 

CURRENT DENSmr 

The current to or from a unit area 
of an electrode surface. 

CURRENT EFFICIENCY 

The ratio of the electrochemical 
equivalent current density for a 
specific reaction to the total 
applied current density. 



D 



DC DECOUPLING DEVICE 

A device used in electrical 
circuits that allows the flow of 
alternating current (AC) in both 
directions and stops or 
substantially reduces the flow of 
direct current (DC). 

DEALLOYING 

The selective corrosion of one or 
more components of a solid 
solution alloy (also known as 
parting or selective dissolution). 

DECOMPOSFTION 
POTENTIAL 

The potential (voltage) on a 
metal surface necessary to 
decompose the electrolyte of an 
electrochemical cell or a 
component thereof. 

DECOMPOSFTION 
VOLTAGE 

[See Decomposition Potential.] 

DEEP GROUNDBED 

One or more anodes installed 
vertically at a nominal depth of 
15 m (50 ft) or more below the 
earth's surface in a drilled hole 
for the purpose of supplying 
cathodic protection. 

DEPOLARIZATION 

The removal of factors resisting 
the current in an electrochemical 
cell. 

DEPOSFT ATTACK 

Corrosion occurring under or 
around a discontinuous deposit 
on a metallic surface (also known 
as poultice corrosion). 



© 2002, NACE International. This publication may not be reprinted without the written consent of NACE International. 

Page 5 of 18 



NACE GLOSSARY OF CORROSION-RELATED TERMS 



DEZINCIFICATION 

A corrosion phenomenon 
resulting in the selective removal 
of zinc from copper-zinc alloys. 
(This phenomenon is one of the 
more common forms of 
dealloying.) 

DIELECTRIC COATING 

A coating that does not conduct 
electricity. 

DIELECTRIC SHIELD 

An electrically nonconductive 
material, such as a coating, 
sheet or pipe, that is placed 
between an anode and an 
adjacent cathode, usually on the 
cathode, to improve current 
distribution in a cathodic 
protection system. 

DIFFEREISrriAL 
AERATION CELL 

An electrochemical cell, the 
electromotive force of which is 
due to a difference in air 
(oxygen) concentration atone 
electrode as compared with that 
at another electrode of the same 
material. 

DIFFUSION-LIMrrED 
CURRENT DENSmr 

The current density that 
corresponds to the maximum 
transfer rate that a particular 
species can sustain because of 
the limitation of diffusion (often 
referred to as limiting current 
density). 

DISBONDME NT 

The loss of adhesion between a 
coating and the substrate. 

DISSIMILAR METALS 

Different metals that could form 
an anode-cathode relationship in 
an electrolyte when connected by 



a metallic path. 

DOUBLE LAYER 

The interface between an 
electrode or a suspended particle 
and an electrolyte created by 
charge-charge interaction leading 
to an alignment of oppositely 
charged ions atthe surface of the 
electrode or particle. The 
simplest model is represented by 
a parallel plate condenser. 

DOUBLER PLATE 

An additional plate or thickness 
of steel used to provide extra 
strength atthe point of anode 
attachment to an offshore 
platform. 

DRAINAGE 

Conduction of electric current 
from an underground or 
submerged metallic structure by 
means of a metallic conductor. 

DRIVING POTENTIAL 

Difference in potential between 
the anode and the steel 
structure. 

DRYING OIL 

An oil capable of conversion from 
a liquid to a solid by slow 
reaction with oxygen in the air. 



E 



ELASTIC DEFORMATION 

Changes of dimensions of a 
material upon the application of a 
stress within the elastic range. 
Following the release of an 
elastic stress, the material 
returns to its original dimensions 
without any permanent 
deformation. 



ELASTIC LirTT 

The maximum stress to which a 
material may be subjected 
without retention of any 
permanent deformation after the 
stress is removed. 

ELASTIC FTY 

The property of a material that 
allows it to recover its original 
dimensions following deformation 
by a stress below its elastic limit. 

ELECTRICAL 
INTERFERENCE 

Any electrical disturbance on a 
metallic structure in contact with 
an electrolyte caused by stray 
current(s). 

ELECTRICAL ISOLATION 

The condition of being electrically 
separated from other metallic 
structures or the environment. 

ELECTRO-OSMOSIS 

The migration of water through a 
semipermeable membrane as a 
result of a potential difference 
caused by the flow of electric 
charge through the membrane. 

ELECTROCHEMICAL 
CELL 

A system consisting of an anode 
and a cathode immersed in an 
electrolyte so as to create an 
electrical circuit. The anode and 
cathode may be different metals 
or dissimilar areas on the same 
metal surface. 

ELECTROCHEMICAL 
EQUIVALENT 

The mass of an element or group 
of elements oxidized or reduced 
at 100% efficiency by the 
passage of a unit quantity of 
electricity. 



© 2002, NACE International. This publication may not be reprinted without the written consent of NACE International. 

Page 6 of 18 



NACE GLOSSARY OF CORROSION-RELATED TERMS 



ELECTROCHEMICAL 
POTEISrriAL 

The partial derivative of the total 
electrochemical free energy of a 
constituent with respect to the 
number of moles of this 
constituent where all other 
factors are kept constant. Itis 
analogous to the chemical 
potential of a constituent except 
that it includes the electrical as 
well as chemical contributions to 
the free energy. 

ELECTRODE 

A conductor used to establish 
contact with an electrolyte and 
through which current is 
transferred to or from an 
electrolyte. 

ELECTRODE POTEISTTIAL 

The potential of an electrode in 
an electrolyte as measured 
against a reference electrode. 
(The electrode potential does not 
include any resistance losses in 
potential in either the electrolyte 
or the external circuit. It 
represents the reversible work to 
move a unit of charge from the 
electrode surface through the 
electrolyte to the reference 
electrode.) 

ELECTROKINETIC 
POTEISTTIAL 

A potential difference in a 
solution caused by residual, 
unbalanced charge distribution in 
the adjoining solution, producing 
a double layer. The 
electrokinetic potential is different 
from the electrode potential in 
that it occurs exclusively in the 
solution phase. This potential 
represents the reversible work 
necessary to bring a unit charge 
from infinity in the solution up to 
the interface in question but not 
through the interface (also known 



as zeta potential). 

ELECTROLYTE 

A chemical substance containing 
ions that migrate in an electric 
field. 

ELECTROLYTIC 
CLEANING 

A process for removing soil, 
scale, or corrosion products from 
a metal surface by subjecting the 
metal as an electrode to an 
electric current in an electrolytic 
bath. 

ELECTROMOTIVE FORCE 
SERIES 

A list of elements arranged 
according to their standard 
electrode potentials, the sign 
being positive for elements 
whose potentials are cathodic to 
hydrogen and negative for those 
anodic to hydrogen. 

ELLIPSOMETRY 

An optical analytical technique 
employing plane-polarized light 
to study films. 

EIVBRRTLEIVE NT 

Loss of ductility of a material 
resulting from a chemical or 
physical change. 

EIVF SERIES 

[See Electromotive Force 
Series.] 

ENAIVEL 

(1) A paint that dries to a hard, 
glossy surface. (2) A coating that 
is characterized by an ability to 
form a smooth, durable film. 

END EFFECT 

The more rapid loss of anode 
material atthe end of an anode, 
compared with other surfaces of 



the anode, resulting from higher 
current density. 

ENDURANCE LIMFT 

The maximum stress that a 
material can withstand for an 
infinitely large number of fatigue 
cycles. 

ENVIRONMENT 

The surroundings or conditions 
(physical, chemical, mechanical) 
in which a material exists. 

ENVIRONMENTAL 
CRACKING 

Brittle fracture of a normally 
ductile material in which the 
corrosive effect of the 
environment is a causative 
factor. 

Environmental cracking is a 
general term that includes all of 
the terms listed below. The 
definitions of these terms are 
listed elsewhere in the Glossary: 
Corrosion Fatigue 
Hydrogen Embrittlement 
Hydrogen-Induced Cracking — 
(Stepwise Cracking) 
Hydrogen Stress Cracking 
Liquid Metal Cracking 
Stress Corrosion Cracking 
Sulfide Stress Cracking 

The following terms have been 

used in the past in connection 

with environmental cracking but 

are now obsolete and should not 

be used: 

Caustic Embrittlement 

Delayed Cracking 

Liquid Metal Embrittlement 

Season Cracking 

Static Fatigue 

Sulfide Corrosion Cracking 

Sulfide Stress Corrosion 

Cracking 



© 2002, NACE International. This publication may not be reprinted without the written consent of NACE International. 

Page 7 of 18 



NACE GLOSSARY OF CORROSION-RELATED TERMS 



EPOXY 

Type of resin formed by the 
reaction of aliphatic or aromatic 
polyols (lil<e bisphenol) with 
epichlorohydrin and 
characterized by the presence of 
reactive oxirane end groups. 

EQUILIBRIUM 
POTEISrriAL 

The potential of an electrode in 
an electrolyte at which the 
forward rate of a given reaction is 
exactly equal to the reverse rate; 
the electrode potential with 
reference to a standard 
equilibrium, as defined by the 
Nernst equation. 

EROSION 

The progressive loss of material 
from a solid surface due to 
mechanical interaction between 
that surface and a fluid, a 
multicomponentfluid, or solid 
particles carried with the fluid. 

EROSION-CORROSION 

A conjoint action involving 
corrosion and erosion in the 
presence of a moving corrosive 
fluid or a material moving through 
the fluid, leading to accelerated 
loss of material. 

EXChHANGE CURRENT 

The rate at which either positive 
or negative charges are entering 
or leaving the surface when an 
electrode reaches dynamic 
equilibrium in an electrolyte. 

EXFOLIATION 
CORROSION 

Localized subsurface corrosion in 
zones parallel to the surface that 
result in thin layers of uncorroded 
metal resembling the pages of a 
book. 



EXTERNAL CIRCUFT 

The wires, connectors, 
measuring devices, current 
sources, etc., that are used to 
bring about or measure the 
desired electrical conditions 
within an electrochemical cell, 
is this portion of the cell through 
which electrons travel. 



It 



F 



FATIGUE 

The phenomenon leading to 
fracture of a material under 
repeated or fluctuating stresses 
having a maximum value less 
than the tensile strength of the 
material. 

FATIGUE STRENGTH 

The maximum stress that can be 
sustained for a specified number 
of cycles without failure. 

FAULT CURRENT 

A current that flows from one 
conductor to ground or to another 
conductor due to an abnormal 
connection (including an arc) 
between the two. A faultcurrent 
flowing to ground may be called 
a ground faultcurrent. 

FERRFTE 

The body-centered cubic 
crystalline phase of iron-based 
alloys. 

FERRmC STEEL 

A steel whose microstructure at 
room temperature consists 
predominantly of ferrite. 

FILIFORM CORROSION 

Corrosion that occurs under a 
coating in the form of randomly 
distributed thread-like filaments. 



FILM 

A thin, not necessarily visible 
layer of material. 

FINISH COAT 

[See Topcoat] 

FORCED DRAINAGE 

Drainage applied to underground 
or submerged metallic structures 
by means of an applied 
electromotive force or sacrificial 
anode. 

FOREIGN STRUCTURE 

Any metallic structure that is not 
intended as a part of a system 
under cathodic protection. 

FOULING 

An accumulation of deposits. 
This includes accumulation and 
growth of marine organisms on a 
submerged metal surface and 
the accumulation of deposits 
(usually inorganic) on heat 
exchanger tubing. 

FRACTOGRAPHY 

Descriptive treatment of fracture, 
especially in metals, with specific 
reference to photographs of the 
fracture surface. 

FRACTURE MEChHANICS 

A quantitative analysis for 
evaluating structural reliability in 
terms of applied stress, crack 
length, and specimen geometry. 

FREE MACHINING 

The machining characteristics of 
an alloy to which an ingredient 
has been introduced to give 
small broken chips, lower power 
consumption, better surface 
finish, and longer tool life. 

FRETTING CORROSION 

Deterioration at the interface of 



© 2002, NACE International. This publication may not be reprinted without the written consent of NACE International. 

Page 8 of 18 



NACE GLOSSARY OF CORROSION-RELATED TERMS 



two contacting surfaces under 
load which is accelerated by their 
relative motion. 

FURAN 

Type of resin formed by the 
polymerization or 
polycondensation of furfuryl, 
furfuryl alcohol, or other 
compounds containing a furan 
ring. 



G 



GALVANIC ANODE 

A metal that provides sacrificial 
protection to another metal that is 
more noble when electrically 
coupled in an electrolyte. This 
type of anode is the electron 
source in one type of cathodic 
protection. 

GALVANIC CORROSION 

Accelerated corrosion of a metal 
because of an electrical contact 
with a more noble metal or 
nonmetallic conductor in a 
corrosive electrolyte. 

GALVANIC COUPLE 

A pair of dissimilar conductors, 
commonly metals, in electrical 
contact in an electrolyte. 

GALVANIC CURRENT 

The electric current between 
metals or conductive nonmetals 
in a galvanic couple. 

GALVANIC SERIES 

A list of metals and alloys 
arranged according to their 
corrosion potentials in a given 
environment. 

GALVANOSTATIC 

Refers to an experimental 



technique whereby an electrode 
is maintained ata constant 
current in an electrolyte. 

GENERAL CORROSION 

Corrosion that is distributed more 
or less uniformly over the surface 
of a material. 

GRAPHmC CORROSION 

Deterioration of gray cast iron in 
which the metallic constituents 
are selectively leached or 
converted to corrosion products, 
leaving the graphite intact. 

GRAPHmZATION 

The formation of graphite in iron 
or steel, usually from 
decomposition of iron carbide at 
elevated temperatures. [Should 
not be used as a term to describe 
graphitic corrosion.] 

GRrr 

Small particles of hard material 
(e.g., iron, steel, or mineral) with 
irregular shapes that are 
commonly used as an abrasive in 
abrasive blast cleaning. 

GRrr BLASTING 

Abrasive blastcleaning using grit 
as the abrasive. 

GROUNDBED 

One or more anodes installed 
below the earth's surface for the 
purpose of supplying cathodic 
protection. 



H 



hHALF-CELL 

A pure metal in contact with a 
solution of known concentration 
of its own ion, ata specific 
temperature, develops a potential 



that is characteristic and 
reproducible; when coupled with 
another half-cell, an overall 
potential that is the sum of both 
half-cells develops. 

hHALF-CELL POTENTIAL 

The potential in a given 
electrolyte of one electrode of a 
pair relative to a standard state 
or a reference state. Potentials 
can only be measured and 
expressed as the difference 
between the half-cell potentials of 
a pair of electrodes. 

hHANDTOOL CLEANING 

Removal of loose rust, loose mill 
scale, and loose paint to degree 
specified, by hand chipping, 
scraping, sanding, and wire 
brushing. [See SSPC-SP 2.] 

hHARDENER 

[See Curing Agent] 

HEAT-AFFECTED ZONE 

That portion of the base metal 
that is not melted during brazing, 
cutting, or welding, but whose 
microstructure and properties are 
altered by the heat of these 
processes. 

HEAT TREATMENT 

Heating and cooling a solid metal 
or alloy in such a way as to 
obtain desired properties. 
Heating for the sole purpose of 
hot working is not considered 
heat treatment. 

HIGH-PRESSURE WATER 

CLEANING 

Water cleaning performed at 
pressures from 34 to 70 M Pa 
(5,000 to 10,000 psig). 



© 2002, NACE International. This publication may not be reprinted without the written consent of NACE International. 

Page 9 of 18 



NACE GLOSSARY OF CORROSION-RELATED TERMS 



HIGH-PRESSURE WATER 

JETTING 

Water jetting performed at 
pressures from 70 to 170 MPa 
(10,000 to 25,000 psig). 

HIGH-TEMPERATURE 

HYDROG E N ATTAC K 

A loss of strength and ductility of 
steel by high-temperature 
reaction of absorbed hydrogen 
with carbides in tiie steel, 
resulting in decarburization and 
internal fissuring. 

hHOLIDAY 

A discontinuity in a protective 
coating that exposes unprotected 
surface to the environment. 

hHYDROGEN BLISTERING 

The formation of subsurface 
planar cavities, called hydrogen 
blisters, in a metal resulting from 
excessive internal hydrogen 
pressure. G rowth of near- 
surface blisters in low-strength 
metals usually results in surface 
bulges. 

hHYDROGEN 
ElVBRnTLEIVE NT 

A loss of ductility of a metal 
resulting from absorption of 
hydrogen. 

hHYDROG E N-INDUC E D 
CRACKING 

Stepwise internal cracks that 
connect adjacent hydrogen 
blisters on different planes in the 
metal, or to the metal surface 
(also known as stepwise 
cracking). 

hHYDROGEN 
OVERVOLTAGE 

Overvoltage associated witii the 
liberation of hydrogen gas. 



hHYDROGEN STRESS 

CRACKING 

Cracking that results from tiie 
presence of hydrogen in a metal 
in combination with tensile 
stress. It occurs most frequently 
with high-strength alloys. 



I 



IMPINGEMENT 
CORROSION 

A form of erosion-corrosion 
generally associated with tiie 
local impingement of a high- 
velocity, flowing fluid against a 
solid surface. 

IMPRESSED CURRENT 

An electric current supplied by a 
device employing a power source 
tiiat is external to the electrode 
system. (An example is direct 
current for cathodic protection.) 

IMPRESSED CURRENT 
ANODE 

An electrode, suitable for use as 
an anode when connected to a 
source of impressed current, 
which is generally composed of a 
substantially inert material that 
conducts by oxidation of the 
electrolyte and, for tills reason, is 
not corroded appreciably. 

IMPULSE DIELECTRIC 
TEST 

A metiiod of applying voltage to 
an insulated wire tiirough the use 
of electric pulses (usually 170 to 
250 pulses per second) to 
determine the integrity of the 
wire's insulation. 

INCLUSION 

A nonmetallic phase such as an 



oxide, sulfide, or silicate particle 
in a metal. 

INORGANIC ZINC -RICH 
COATING 

Coating containing a metallic zinc 
pigment (typically 75 wt% zinc or 
more in the dry film) in an 
inorganic vehicle. 

INSTANT-OFF 
POTENTIAL 

The polarized half-cell potential 
of an electrode taken 
immediately after the cathodic 
protection current is stopped, 
which closely approximates the 
potential without IR drop (i.e., tiie 
polarized potential) when the 
current was on. 

INTERCRYSTALLINE 
CORROSION 

[See Intergranular Corrosion.] 

INTERDENDRmC 
CORROSION 

Corrosive attack of cast metals 
tiiat progresses preferentially 
along paths between dendrites. 

INTERFERENCE BOND 

An intentional metallic 
connection, between metallic 
systems in contact with a 
common electrolyte, designed to 
control electrical current 
interchange between the 
systems. 

INTERFERENCE 
CURRENT 

[See Stray Current] 

INTERGRANULAR 
CORROSION 

Preferential corrosion at or along 
tiie grain boundaries of a metal 
(also known as intercrystalline 



© 2002, NACE International. This publication may not be reprinted without the written consent of NACE International. 

Page 10 of 18 



NACE GLOSSARY OF CORROSION-RELATED TERMS 



corrosion). 

IISTTERGRANULAR 
STRESS CORROSION 
CRACKING 

Stress corrosion cracking in 
wiiicii the cracl<ing occurs along 
grain boundaries. 

INTERNAL OXIDATION 

The formation of isolated 
particles of oxidation products 
beneath the metal surface. 

INTUMESCENCE 

The swelling or bubbling of a 
coating usually caused by 
heating. [The term is commonly 
used in aerospace and fire- 
protection applications.] 

ION 

An electrically charged atom or 
group of atoms. 

IR DROP 

The voltage across a resistance 
in accordance with Ohm's Law. 

IRON ROT 

Deterioration of wood in contact 
with iron-based alloys. 



L 



i 



KNIFE-LINE ATTACK 

Intergranular corrosion of an 
alloy along a line adjoining or in 
contact with a weld after heating 
into the sensitization temperature 
range. 



LAMELLAR CORROSION 

[See Exfoliation Corrosion.] 

LANGELIER INDEX 

A calculated saturation index for 
calcium carbonate that is useful 
in predicting scaling behavior of 
natural water. 

LINE CURRENT 

The direct current flowing on a 
pipeline. 

LINING 

A coating or layer of sheet 
material adhered to or in intimate 
contact with the interior surface 
of a container used to protect the 
container against corrosion by its 
contents and/or to protect the 
contents of the containerfrom 
contamination by the container 
material. 

LIQUID IVETAL 
CRACKING 

Cracking of a metal caused by 
contact with a liquid metal. 

LONG-LINE CURRENT 

Current though the earth 
between an anodic and a 
cathodic area that returns along 
an underground metallic 
structure. 

LOW-CARBON STEEL 

Steel having less than 0.30% 
carbon and no intentional 
alloying additions. 



LOW-PRESSURE WATER 

CLEANING 

Water cleaning performed at 
pressures less than 34 MPa 
(5,000 psig). 

LUGGIN PROBE 

A small tube or capillary filled 
with electrolyte, terminating close 
to the metal surface of an 
electrode understudy, which is 
used to provide an ion- 
conducting path without diffusion 
between the electrode under 
study and a reference electrode. 



M 



MARTENS FTE 

A hard supersaturated solid 
solution of carbon in iron 
characterized by an acicular 
(needle-like) microstructure. 

IVETAL DUSTING 

The catastrophic deterioration of 
a metal exposed to a 
carbonaceous gas at elevated 
temperature. 

IVETALLIZING 

The coating of a surface with a 
thin metal layer by spraying, hot 
dipping, or vacuum deposition. 

IVILL SCALE 

The oxide layer formed during 
hot fabrication or heat treatment 
of metals. 

IVIXED POTENTIAL 

A potential resulting from two or 
more electrochemical reactions 
occurring simultaneously on one 
metal surface. 



© 2002, NACE International. This publication may not be reprinted without the written consent of NACE International. 

Page 11 of 18 



NACE GLOSSARY OF CORROSION-RELATED TERMS 



MODULUS OF 

ELASTIC mr 

A measure of the stiffness or 
rigidity of a material. Itis actually 
the ratio of stress to strain in the 
elastic region of a material. If 
determined by a tension or 
compression test, it is also called 
Young's Modulus orthe 
coefficient of elasticity. 



N 



NATURAL DRAINAGE 

Drainage from an underground or 
submerged metallic structure to a 
more negative (more anodic) 
structure, such as the negative 
bus of a trolley substation. 

NEAR-WHrrE BLAST 
CLEANED SURFACE 

A near-white blast cleaned 
surface, when viewed without 
magnification, shall be free of all 
visible oil, grease, dust, dirt, mill 
scale, rust, coating, oxides, 
corrosion products, and other 
foreign matter. Random staining 
shall be limited to not more than 
5% of each unit area of surface 
(approximately 58 cm^ [9.0 in.^]), 
and may consist of light 
shadows, slight streaks, or minor 
discolorations caused by stains 
of rust stains of mill scale, or 
stains of previously applied 
coating. [See NACE No. 
2/SSPC-SP 10.] 

NEGATIVE RETURN 

A point of connection between 
the cathodic protection negative 
cable and the protected 
structure. 

NERNST EQUATION 

An equation that expresses the 



exact electromotive force of an 
electrochemical cell in terms of 
the activities of products and 
reactants of the cell. 

NERNST LAYER 

The diffusion layer atthe surface 
of an electrode in which the 
concentration of a chemical 
species is assumed to vary 
linearly from the value in tiie bulk 
solution to the value atthe 
electrode surface. 

NOBLE 

The positive direction of 
electrode potential, thus 
resembling noble metals such as 
gold and platinum. 

NOBLE METAL 

(1) A metal that occurs 
commonly in nature in the free 
state. (2) A metal or alloy whose 
corrosion products are formed 
with a small negative or a 
positive free-energy change. 

NOBLE POTENTIAL 

A potential more cathodic 
(positive) than the standard 
hydrogen potential. 

NORMALIZING 

Heating a ferrous alloy to a 
suitable temperature above the 
transformation range 
(austenitizing), holding at 
temperature for a suitable time, 
and then cooling in still air to a 
temperature substantially below 
the transformation range. 



o 



measured with respect to a 
reference electrode or another 
electrode in the absence of 
current 

ORGANIC ZINC-RICH 

COATING 

Coating containing a metallic zinc 
pigment (typically 75 vj\% zinc or 
more in the dry film) in an organic 
resin. 

OVERVOLTAGE 

The change in potential of an 
electrode from its equilibrium or 
steady-state value when current 
is applied. 

OXIDATION 

(1) Loss of electrons by a 
constituent of a chemical 
reaction. (2) Corrosion of a 
metal that is exposed to an 
oxidizing gas at elevated 
temperatures. 

OXIDATION-RE DUCTION 
POTENTIAL 

The potential of a reversible 
oxidation-reduction electrode 
measured with respect to a 
reference electrode, corrected to 
the hydrogen electrode, in a 
given electrolyte. 

OXYGEN 
CONCENTRATION CELL 

[See Differential Aeration Cell.] 



OPEN-CIRCUrT 
POTENTIAL 

The potential of an electrode 



PAINT 

A pigmented liquid or resin 
applied to a substrate as a thin 
layer that is converted to an 
opaque solid film after 
application. Itis commonly used 



© 2002, NACE International. This publication may not be reprinted without the written consent of NACE International. 

Page 12 of 18 



NACE GLOSSARY OF CORROSION-RELATED TERMS 



as a decorative or protective 
coating. 

PAIISrr SYSTEM 

[See Coating System.] 

PARTING 

[See Dealloying.] 

PASSIVATION 

A reduction of the anodic 
reaction rate of an electrode 
involved in corrosion. 

PASSIVATION 
POTENTIAL 

[See Primary Passive Potential.] 

PASSIVE 

(1) The positive direction of 
electrode potential. (2) A state 
of a metal in which a surface 
reaction product causes a 
marked decrease in the corrosion 
rate relative to tiiat in tiie 
absence of the product. 

PASSIVE-ACTIVE CELL 

An electrochemical cell, the 
electromotive force of which is 
caused by the potential 
difference between a metal in an 
active state and the same metal 
in a passive state. 

PASSIVFTY 

The state of being passive. 

PATINA 

A thin layer of corrosion product, 
usually green, thatforms on the 
surface of metals such as copper 
and copper-based alloys 
exposed to the atmosphere. 



PH 

The negative logarithm of the 
hydrogen ion activity written " 



as: 



pH =-logio(aH') 



where an* = hydrogen ion activity 
=ttie molar concentration of 
hydrogen ions multiplied by tine 
mean ion-activity coefficient. 

PICKLING 

(1) Treating a metal in a chemical 
bath to remove scale and oxides 
(e.g., rust) from the surface. (2) 
Complete removal of rustand 
mill scale by acid pickling, duplex 
pickling, or electrolytic pickling. 
[SeeSSPC-SP 8.] 

PICKLING SOLm'ION 

A chemical bath, usually an acid 
solution, used for pickling. 

PIGMENT 

A solid substance, generally in 
fine powder form, that is 
Insoluble in the vehicle of a 
formulated coating material. Itis 
used to impartcolor or other 
specific physical or chemical 
properties to the coating. 

PIPE-TO-ELECTROLYTE 
POTENTIAL 

[See Structure-to-Electrolyte 
Potential.] 

PIPE-TO-SOIL 
POTENTIAL 

[See Structure-to-Electrolyte 
Potential.] 

PFTTING 

Localized corrosion of a metal 
surface that is confined to a small 
area and takes the form of 
cavities called pits. 

PFTTING FACTOR 

The ratio of the depth of the 
deepest pit resulting from 
corrosion divided by the average 
penetration as calculated from 
mass loss. 



PLASTIC DEFORMATION 

Permanent deformation caused 
by stressing beyond the elastic 
limit. 

PLASTIC FTY 

The ability of a material to deform 
permanently (nonelastically) 
without fracturing. 

POLARIZATION 

The change from the open-circuit 
potential as a result of current 
across the electrode/electrolyte 
interface. 

POLARIZATION 
ADIvrTTANCE 

The reciprocal of polarization 
resistance. 

POLARIZATION CELL 

A DC decoupling device 
consisting of two or more pairs of 
inert metallic plates immersed in 
an aqueous electrolyte. The 
electrical characteristics of the 
polarization cell are high 
resistance to DC potentials and 
low impedance of AC. 

POLARIZATION CURVE 

A plot of current density versus 
electrode potential for a specific 
electrode/electrolyte 
combination. 

POLARIZATION DECAY 

The decrease in electrode 
potential with time resulting from 
the interruption of applied 
current. 

POLARIZATION 
RESISTANCE 

The slope (dE/di) atthe corrosion 
potential of a potential (E)-current 
density (i) curve. (The measured 
slope is usually in good 
agreement with the true value of 



© 2002, NACE International. This publication may not be reprinted without the written consent of NACE International. 

Page 13 of 18 



NACE GLOSSARY OF CORROSION-RELATED TERMS 



the polarization resistance when 
the scan rate is low and any 
uncompensated resistance is 
small relative to the polarization 
resistance.) 

POLARIZED POTEISTTIAL 

The potential across the 
structure/electrolyte interface that 
is the sum of the corrosion 
potential and the cathodic 
polarization. 

POLYESTER 

Type of resin formed by the 
condensation of polybasic and 
monobasic acids with polyhydric 
alcohols. 

POS7WELD HEAT 
TREATMEISrr 

Heating and cooling a weldment 
in such a way as to obtain 
desired properties. 

POTEISrriAL-pH DIAGRAM 

A graphical method of 
representing the regions of 
thermodynamic stability of 
species for metal/electrolyte 
systems (also known as Pourbaix 
diagram). 

POTEISrriODYNAIVIC 

Refers to a technique wherein 
the potential of an electrode with 
respect to a reference electrode 
is varied at a selected rate by 
application of a current through 
the electrolyte. 

POTEISrriOKINETIC 

[See Potentiodynamic] 

POTEISrriOSTAT 

An instrumentfor automatically 
maintaining a constant electrode 
potential. 



POTEISrriOSTATIC 

Refers to a technique for 
maintaining a constant electrode 
potential. 

POT LIFE 

The elapsed time within which a 
coating can be effectively applied 
after all components of the 
coating have been thoroughly 
mixed. 

POULTICE CORROSION 

[See Deposit Attack.] 

POURBAIX DIAGRAM 

[See Potential-pl-i Diagram.] 

POWER TOOL CLEANING 

Removal of loose rust, loose mill 
scale, and loose paint to degree 
specified by power tool chipping, 
descaling, sanding, wire 
brushing, and grinding. [See 
SSPC-SP 3.] 

PRECIPFTATION 

hHARDENING 

Hardening caused by the 
precipitation of a constituent from 
a supersaturated solid solution. 

PRIMARY PASSIVE 
POTENTIAL 

The potential corresponding to 
the maximum active current 
density (critical anodic current 
density) of an electrode that 
exhibits active-passive corrosion 
behavior. 
PRIIVE COAT 
[See Primer.] 

PRIIVER 

A coating material intended to be 
applied as the first coat on an 
uncoated surface. The coating is 
specifically formulated to adhere 
to and protect the surface as well 
as to produce a suitable surface 



for subsequent coats. [Also 
referred to as Prime Coat] 

PROFILE 

Anchor pattern on a surface 
produced by abrasive blasting or 
acid treatment. 

PROTECTIVE COATING 

A coating applied to a surface to 
protect the substrate from 
corrosion. 



R 



REDUCTION 

Gain of electrons by a constituent 
of a chemical reaction. 

REFERENCE 
ELECTRODE 

An electrode whose open-circuit 
potential is constant under similar 
conditions of measurement, 
which is used for measuring the 
relative potentials of other 
electrodes. 

REFERENCE hHALF-CELL 

[See Reference Electrode.] 

RELATIVE HUMIDFIY 

The ratio, expressed as a 
percentage, of the amount of 
water vapor present in a given 
volume of air at a given 
temperature to the amount 
required to saturate the air at that 
temperature. 

REMOTE EARTH 

A location on the earth far 
enough from the affected 
structure that the soil potential 
gradients associated with 
currents entering the earth from 
the affected structure are 



© 2002, NACE International. This publication may not be reprinted without the written consent of NACE International. 

Page 14 of 18 



NACE GLOSSARY OF CORROSION-RELATED TERMS 



insignificant. 

RESISTIVrTY 

(1) The resistance per unit lengtii 
of a substance with uniform cross 
section. (2) A measure of the 
ability of an electrolyte (e.g., soil) 
to resist the flow of electric 
charge (e.g., cathodic protection 
current). Resistivity data are 
used to design a groundbed for a 
cathodic protection system. 

REST POTEISrriAL 

[See Corrosion Potential.] 

REVERSIBLE POTEISTTIAL 

[See Equilibrium Potential.] 

RIMMED STEEL 

An incompletely deoxidized steel. 
[Also called Rimming Steel.] 

RISER 

(1) That section of pipeline 
extending from the ocean floor 
up to an offshore platform. (2) 
The vertical tube in a steam 
generator convection bank that 
circulates water and steam 
upward. 

RUST 

Corrosion product consisting of 
various iron oxides and hydrated 
iron oxides. (This term properly 
applies only to iron and ferrous 
alloys.) 

RUST BLOOM 

Discoloration indicating the 
beginning of rusting. 



S 



SACKING 

Scrubbing a mixture of a cement 
mortar over the concrete surface 



using a cementsack, gunny 
sack, or sponge rubberfloat. 

SACRIFICIAL ANODE 

[See Galvanic Anode.] 

SACRIFICIAL 
PROTECTION 

Reduction of corrosion of a metal 
in an electrolyte by galvanically 
coupling it to a more anodic 
metal (a form of cathodic 
protection). 

SCALING 

(1) The formation at high 
temperatures of thick corrosion- 
product layers on a metal 
surface. (2) The deposition of 
water-insoluble constituents on a 
metal surface. 

SCANNING ELECTRON 
MCROSCOPE 

An electron optical device that 
images topographical details with 
maximum contrast and depth of 
field by the detection, 
amplification, and display of 
secondary electrons. 

SENSmZING HEAT 
TREATMENT 

A heat treatment, whether 
accidental, intentional, or 
incidental (as during welding), 
that causes precipitation of 
constituents (usually carbides) at 
grain boundaries, often causing 
the alloy to become susceptible 
to intergranular corrosion or 
intergranular stress corrosion 
cracking. 

ShHALLOW GROUNDBED 

One or more anodes installed 
either vertically or horizontally at 
a nominal depth of less than 15 
m (50 ft) for the purpose of 
supplying cathodic protection. 



SHIELDING 

(1) Protecting; protective cover 
againstmechanical damage. (2) 
Preventing or diverting cathodic 
protection current from its natural 
path. 

SHOP COAT 

One or more coats applied in a 
shop or plant prior to shipment to 
the site of erection or fabrication. 

ShHOT BLASTING 

Abrasive blast cleaning using 
metallic (usually steel) shot as 
the abrasive. 

ShHOT PEENING 

Inducing compressive stresses in 
the surface layer of a material by 
bombarding itwith a selected 
medium (usually steel shot) 
under controlled conditions. 

SIGMA PhHASE 

An extremely brittle Fe-Cr phase 
that can form at elevated 
temperatures in Fe-Cr-Ni and Ni- 
Cr-Fe alloys. 

SLIP 

A deformation process involving 
shear motion of a specific set of 
crystallographic planes. 

SLOW STRAIN RATE 
TECHNIQUE 

An experimental technique for 
evaluating susceptibility to 
environmental cracking. It 
involves pulling the specimen to 
failure in uniaxial tension at a 
controlled slow strain rate while 
the specimen is in the test 
environment and examining the 
specimen for evidence of 
environmental cracking. 

SLUSHING COMPOUND 

Oil or grease coatings used to 



© 2002, NACE International. This publication may not be reprinted without the written consent of NACE International. 

Page 15 of 18 



NACE GLOSSARY OF CORROSION-RELATED TERMS 



provide temporary protection 
against atmospheric corrosion. 

SOLm'ION HEAT 
TREATMEISrr 

Heating a metal to a suitable 
temperature and holding atthat 
temperature long enough for one 
or more constituents to enter into 
solid solution, then cooling 
rapidly enough to retain the 
constituents in solution. 

SOLVE ISrr CLEANING 

Removal of oil, grease, dirt, soil, 
salts, and contaminants by 
cleaning with solvent, vapor, 
alkali, emulsion, or steam. [See 
SSPC-SP 1.] 

SPALLING 

The spontaneous chipping, 
fragmentation, or separation of a 
surface or surface coating. 

STANDARD ELECTRODE 
POTENTIAL 

The reversible potential for an 
electrode process when all 
products and reactions are at unit 
activity on a scale in which tiie 
potential for the standard 
hydrogen reference electrode is 
zero. 

STANDARD J ETTING 
WATER 

Water of sufficient purity and 
quality tiiat it does not impose 
additional contaminants on the 
surface being cleaned and does 
notcontain sediments or other 
impurities that are destructive to 
tiie proper functioning of water 
jetting equipment. 

STEEL SHOT 

Small particles of steel with 
spherical shape that are 
commonly used as an abrasive in 



abrasive blastcleaning oras a 
selected medium for shot 
peening. 

STEP POTENTIAL 

The potential difference between 
two points on the earth's surface 
separated by a distance of one 
human step, which is defined as 
one meter, determined in the 
direction of maximum potential 
gradient. 

STEPWISE CRACKING 

[See Hydrogen-Induced 
Cracking.] 

STRAY CURRENT 

Current through paths other than 
the intended circuit. 

STRAY-CURRENT 
CORROSION 

Corrosion resulting from current 
through paths other than the 
intended circuit, e.g., by any 
extraneous current in the earth. 

STRESS CORROSION 

CRACKING 

Cracking of a material produced 
by the combined action of 
corrosion and tensile stress 
(residual or applied). 

STRESS RELIEVING 
aHERMAL) 

Heating a metal to a suitable 
temperature, holding at that 
temperature long enough to 
reduce residual stresses, and 
then cooling slowly enough to 
minimize the development of new 
residual stresses. 



STRUCTURE-TO- 
ELECTROLYTE 
POTENTIAL 

The potential difference between 
the surface of a buried or 
submerged metallic structure and 
the electrolyte that is measured 
with reference to an electrode in 
contact with the electrolyte. 

STRUCTURE-TO-SOIL 
POTENTIAL 

[See Structure-to -Electrolyte 
Potential.] 

STRUCTURE-TO- 
STRUCTURE POTENTIAL 

The potential difference between 
metallic structures, or sections of 
the same structure, in a common 
electrolyte. 

SUBSURFACE 
CORROSION 

[See Internal Oxidation.] 

SULFIDATION 

The reaction of a metal or alloy 
with a sulfur-containing species 
to produce a sulfur compound 
thatforms on or beneatii the 
surface of the metal or alloy. 

SULFIDE STRESS 

CRACKING 

Cracking of a metal underthe 
combined action of tensile stress 
and corrosion in the presence of 
water and hydrogen sulfide (a 
form of hydrogen stress 
cracking). 

SURFACE POTENTIAL 
GRADIENT 

Change in the potential on the 
surface of the ground with 
respect to distance. 



© 2002, NACE International. This publication may not be reprinted without the written consent of NACE International. 

Page 16 of 18 



NACE GLOSSARY OF CORROSION-RELATED TERMS 



TACK COAT 

A thin wet coat applied to the 
surface that is allowed to dry just 
until it is tacky before application 
of a thicker wet coat. (Use of a 
tack coat allows application of 
thicker coats without sagging or 
runs.) 

TAFEL PLOT 

A plot of the relationship between 
the change in potential (E) and 
the logarithm of the current 
density (log /) of an electrode 
when it is polarized in both the 
anodic and cathodic directions 
from its open-circuit potential. 

TAFEL SLOPE 

The slope of the straight-line 
portion of the E log /curve on a 
Tafel plot. (The straight-line 
portion usually occurs at more 
than 50 mV from the open-circuit 
potential.) 

TARNISH 

Surface discoloration of a metal 
resulting from formation of a film 
of corrosion product. 

THERMAL SPRAYING 

A group of processes by which 
finely divided metallic or 
nonmetallic materials are 
deposited in a molten or 
semimolten condition to form a 
coating. 

THERMOGALVANIC 
CORROSION 

Corrosion resulting from an 
electrochemical cell caused by a 
thermal gradient. 



THROWING POWER 

The relationship between the 
current density at a point on a 
surface and its distance from the 
counterelectrode. The greater 
the ratio of the surface resistivity 
shown by the electrode reaction 
to the volume resistivity of the 
electrolyte, the better is the 
throwing power of the process. 

TOPCOAT 

The final coat of a coating 
system. [Also referred to as 
Finish Coat] 

TOUCH POTENTIAL 

The potential difference between 
a metallic structure and a point 
on the earth's surface separated 
by a distance equal to the normal 
maximum horizontal reach of a 
human (approximately 1.0 m [3.3 
ft]). 

TRANSPASSIVE 

The noble region of potential 
where an electrode exhibits a 
higher-than-passive current 
density. 

TUBERCULATION 

The formation of localized 
corrosion products scattered over 
the surface in the form of knob- 
like mounds called tubercles. 



U-V-W 



ULTIMATE STRENGTH 

The maximum stress that a 
material can sustain. 

ULTRAHIGH-PRESSURE 

WATER JETTING 

Water jetting performed at 
pressures above 170 MPa 
(25,000 psig.) 



UNDERFILM CORROSION 

[See Filiform Corrosion.] 

VEHICLE 

The liquid portion of a formulated 
coating material. 

VOID 

(1) A holiday, hole, or skip in a 
coating. (2) A hole in a casting 
or weld deposit usually resulting 
from shrinkage during cooling. 

WASH PRIIVER 

A thin, inhibiting primer, usually 
chromate pigmented, with a 
polyvinyl butyral binder. 

WATER CLEANING 

Use of pressurized water 
discharged from a nozzle to 
remove unwanted matter (e.g., 
dirt, scale, rust, coatings) from a 
surface. 

WATER JETTING 

Use of standard jetting water 
discharged from a nozzle at 
pressures of 70 MPa (10,000 
psig) or greater to prepare a 
surface for coating or inspection. 

WEIGhTT COATING 

An external coating applied to a 
pipeline to counteract buoyancy. 

WHFTE IVETAL BLAST 
CLEANED SURFACE 

A white metal blast cleaned 
surface, when viewed without 
magnification, shall be free of all 
visible oil, grease, dust, dirt, mill 
scale, rust, coating, oxides, 
corrosion products, and other 
foreign matter. [See NACE No. 
1/SSPC-SP 5.] 

WELD DECAY 

Intergranular corrosion, usually of 



© 2002, NACE International. This publication may not be reprinted without the written consent of NACE International. 

Page 17 of 18 



NACE GLOSSARY OF CORROSION-RELATED TERMS 



stainless steel or certain nickel- 
base alloys, that occurs as the 
result of sensitization in the heat- 
affected zone during the welding 
operation. [This is not a preferred 
term.] 

WET FILM GAUGE 

Device for measuring wetfilm 
thickness of a coating. 

V\»RKING ELECTRODE 

The test or specimen electrode in 
an electrochemical cell. 

WROUGhrr 

Metal in the solid condition that is 
formed to a desired shape by 
working (rolling, extruding, 
forging, etc.), usually at an 
elevated temperature. 




YIELD poiisrr 

The stress on a material at which 
the first significant permanent or 
plastic deformation occurs 
without an increase in stress. In 
some materials, particularly 
annealed low-carbon steels, 
there is a well-defined yield point 
from the straight line defining the 
modulus of elasticity. 

YIELD STRENGTH 

The stress at which a material 
exhibits a specified deviation 
from the proportionality of stress 
to strain. The deviation is 
expressed in terms of strain by 
either the offset method (usually 
at a strain of 0.2%) or the total- 
extension-under-load method 
(usually at a strain of 0.5%.) 



© 2002, NACE International. This publication may not be reprinted without the written consent of NACE International. 

Page 18 of 18 



C"***.. 



t'X"''; N ACE SP01 69-2007 

W<..*^ I j^ ^^^^ (formerly RP0169-2002) 

Item No. 21001 



NACE 

INTERNATIONAL 



Standard Practice 



Control of External Corrosion on Underground or 
Submerged Metallic Piping Systems 

This NACE International standard represents a consensus of those individual members who have 
reviewed this document, its scope, and provisions. Its acceptance does not in any respect 
preclude anyone, whether he or she has adopted the standard or not, from manufacturing, 
marketing, purchasing, or using products, processes, or procedures not in conformance with this 
standard. Nothing contained in this NACE International standard is to be construed as granting 
any right, by implication or otherwise, to manufacture, sell, or use in connection with any method, 
apparatus, or product covered by Letters Patent, or as indemnifying or protecting anyone against 
liability for infringement of Letters Patent. This standard represents minimum requirements and 
should in no way be interpreted as a restriction on the use of better procedures or materials. 
Neither is this standard intended to apply in all cases relating to the subject. Unpredictable 
circumstances may negate the usefulness of this standard in specific instances. NACE 
International assumes no responsibility for the interpretation or use of this standard by other 
parties and accepts responsibility for only those official NACE International interpretations issued 
by NACE International in accordance with its governing procedures and policies which preclude 
the issuance of interpretations by individual volunteers. 

Users of this NACE International standard are responsible for reviewing appropriate health, 
safety, environmental, and regulatory documents and for determining their applicability in relation 
to this standard prior to its use. This NACE International standard may not necessarily address 
all potential health and safety problems or environmental hazards associated with the use of 
materials, equipment, and/or operations detailed or referred to within this standard. Users of this 
NACE International standard are also responsible for establishing appropriate health, safety, and 
environmental protection practices, in consultation with appropriate regulatory authorities if 
necessary, to achieve compliance with any existing applicable regulatory requirements prior to 
the use of this standard. 

CAUTIONARY NOTICE: NACE International standards are subject to periodic review, and may 
be revised or withdrawn at any time in accordance with NACE technical committee procedures. 
NACE International requires that action be taken to reaffirm, revise, or withdraw this standard no 
later than five years from the date of initial publication. The user is cautioned to obtain the latest 
edition. Purchasers of NACE International standards may receive current information on all 
standards and other NACE International publications by contacting the NACE International 
FirstService Department, 1440 South Creek Drive, Houston, Texas 77084-4906 (telephone +1 
[281)228-6200). 

Reaffirmed 2007-03-15 
Reaffirmed 2002-04-11 
Reaffirmed 1996-09-13 

Revised April 1992 

Revised January 1983 

Revised September 1976 

Revised January 1972 

Approved April 1969 

NACE International 

1440 South Creek Drive 

Houston, Texas 77084-4906 

+1 281/228-6200 

ISBN 1-57590-035-1 

©2007, NACE International 



SP01 69-2007 



Foreword 

This standard practice presents procedures and practices for aciiieving effective control of 
external corrosion on buried or submerged metallic piping systems. These recommendations are 
also applicable to many other buried or submerged metallic structures. It is intended for use by 
corrosion control personnel concerned with the corrosion of buried or submerged piping systems, 
including oil, gas, water, and similar structures. This standard describes the use of electrically 
insulating coatings, electrical isolation, and cathodic protection (CP) as external corrosion control 
methods. It contains specific provisions for the application of CP to existing bare, existing coated, 
and new piping systems. Also included are procedures for control of interference currents on 
pipelines. 

This standard should be used in conjunction with the practices described in the following NACE 
standards and publications, when appropriate (use latest revisions): 



SP0572^ 
TPC 11 ' 



RP0177' 
TM0497^ 



RP0285' 



SP0186' 



SP0286 



SP0387'' 



SP0188 



For accurate and correct application of this standard, the standard must be used in its entirety. 
Using or citing only specific paragraphs or sections can lead to misinterpretation and 
misapplication of the recommendations and practices contained in this standard. 

This standard does not designate practices for every specific situation because of the complexity 
of conditions to which buried or submerged piping systems are exposed. 

This standard was originally published in 1969, and was revised by NACE Task Group (TG) T-10- 
1 in 1972, 1976, 1983, and 1992. It was reaffirmed in 1996 by NACE Unit Committee T-1 OA on 
Cathodic Protection, and in 2002 and 2007 by Specific Technology Group (STG) 35 on Pipelines, 
Tanks, and Well Casings. This standard is issued by NACE International under the auspices of 
STG 35, which is composed of corrosion control personnel from oil and gas transmission 
companies, gas distribution companies, power companies, corrosion consultants, and others 
concerned with external corrosion control of buried or submerged metallic piping systems. 



In NACE standards, the terms shall, must, should, and may are used in accordance with the 
definitions of these terms in the NACE Publications Style Manual, 4th ed.. Paragraph 7.4.1 .9. Shall 
and must are used to state mandatory requirements. The term should is used to state something 
considered good and is recommended but is not mandatory. The term may is used to state 
something considered optional. 



NACE International 



SP01 69-2007 



NACE International 
Standard Practice 

Control of External Corrosion on Underground or Submerged 

Metallic Piping Systems 

Contents 

1. General 1 

2. Definitions 1 

3. Determination of Need for External Corrosion Control 3 

4. Piping Systems Design 4 

5. External Coatings 6 

6. Criteria and Other Considerations for CP 12 

7. Design of Cathodic protection Systems 17 

8. Installation of CP Systems 20 

9. Control of Interference Currents 22 

10. Operationa and Maintenance of CP Systems 24 

11. External Corrosion Control Records 25 

References 26 

Table 1 8 

Table 2 8 

Table 3 9 

Table 4 10 

Tables 11 

Bibliography for Section 6 14 

Bibliography for Section 7 20 

Appendix A 28 

Appendix B 28 

Appendix C 28 

Appendix D 29 



NACE International 



SP01 69-2007 



Section 1 : General 



1.1 This standard presents acknowledged practices for tiie 
control of external corrosion on buried or submerged steel, 
cast iron, ductile iron, copper, and aluminum piping 
systems. 

1.2 This standard is intended to serve as a guide for 
establishing minimum requirements for control of external 
corrosion on the following systems: 

1.2.1 New piping systems: Corrosion control by a 
coating supplemented with CP, or by some other 
proven method, should be provided in the initial design 
and maintained during the service life of the piping 
system, unless investigations indicate that corrosion 
control is not required. Consideration should be given 
to the construction of pipelines in a manner that 
facilitates the use of in-line inspection tools. 

1.2.2 Existing coated piping systems: CP should be 
provided and maintained, unless investigations indicate 
that CP is not required. 



1.3 The provisions of this standard should be applied 
under the direction of competent persons who, by reason of 
knowledge of the physical sciences and the principles of 
engineering and mathematics, acquired by education and 
related practical experience, are qualified to engage in the 
practice of corrosion control on buried or submerged 
metallic piping systems. Such persons may be registered 
professional engineers or persons recognized as corrosion 
specialists or CP specialists by NACE if their professional 
activities include suitable experience in external corrosion 
control of buried or submerged metallic piping systems. 

1.4 Special conditions in which CP is ineffective or only 
partially effective sometimes exist. Such conditions may 
include elevated temperatures, disbonded coatings, thermal 
insulating coatings, shielding, bacterial attack, and unusual 
contaminants in the electrolyte. Deviation from this 
standard may be warranted in specific situations provided 
that corrosion control personnel in responsible charge are 
able to demonstrate that the objectives expressed in this 
standard have been achieved. 



1.2.3 Existing bare piping systems: Studies should be 
made to determine the extent and rate of corrosion on 
existing bare piping systems. When these studies 
indicate that corrosion will affect the safe or economic 
operation of the system, adequate corrosion control 
measures shall be taken. 



1.5 This standard does not include corrosion control 
methods based on chemical control of the environment, on 
the use of electrically conductive coatings, or on control of 
internal corrosion. 



Section 2: Definitions 



(1) 



Amphoteric Metal: A metal that is susceptible to corrosion 
in both acid and alkaline environments. 

Anode: The electrode of an electrochemical cell at which 
oxidation occurs. Electrons flow away from the anode in the 
external circuit. Corrosion usually occurs and metal ions 
enter solution at the anode. 

Anodic Polarization: The change of the electrode 
potential in the noble (positive) direction caused by current 
across the electrode/electrolyte interface. (See 
Polarization.) 



Beta Curve: A plot of dynamic (fluctuating) interference 
current or related proportional voltage (ordinate) versus the 
corresponding structure-to-electrolyte potentials at a 
selected location on the affected structure (abscissa) (see 
Appendix A [nonmandatory]). 

Cabie: One conductor or multiple conductors insulated 
from one another. 

Cathiode: The electrode of an electrochemical cell at which 
reduction is the principal reaction. Electrons flow toward the 
cathode in the external circuit. 



Bacl<fiii: Material placed in a hole to fill the space around 
the anodes, vent pipe, and buried components of a cathodic 
protection system. 



Catliodic Disbondment: The destruction of adhesion 
between a coating and the coated surface caused by 
products of a cathodic reaction. 



'^' Definitions in this section reflect common usage among practicing corrosion control personnel and apply specifically to how 
the terms are used in this standard. In many cases, in the interests of brevity and practical usefulness, the scientific 
definitions are abbreviated or paraphrased. 



NACE International 



SP01 69-2007 



Cathodic Polarization: The change of electrode potential 
in the active (negative) direction caused by current across 
the electrode/electrolyte interface. See Polarization. 

Cathodic Protection: A technique to reduce the corrosion 
of a metal surface by making that surface the cathode of an 
electrochemical cell. 

Coating: A liquid, liquefiable, or mastic composition that, 
after application to a surface, is converted into a solid 
protective, decorative, or functional adherent film. 

Coating Disbondment: The loss of adhesion between a 
coating and the pipe surface. 

Conductor: A material suitable for carrying an electric 
current. It may be bare or insulated. 

Continuity Bond: A connection, usually metallic, that 
provides electrical continuity between structures that can 
conduct electricity. 

Corrosion: The deterioration of a material, usually a metal, 
that results from a reaction with its environment. 

Corrosion Potential (Ecorr): The potential of a corroding 
surface in an electrolyte relative to a reference electrode 
under open-circuit conditions (also known as rest potential, 
open-circuit potential, or freely corroding potential). 

Corrosion Rate: The rate at which corrosion proceeds. 

Criterion: Standard for assessment of the effectiveness of 
a cathodic protection system. 

Current Density: The current to or from a unit area of an 
electrode surface. 

Diode: A bipolar semiconducting device having a low 
resistance in one direction and a high resistance in the 
other. 

Distributed-Anode impressed Current System: An 

impressed current anode configuration in which the anodes 
are "distributed" along the structure at relatively close 
intervals such that the structure is within each anode's 
voltage gradient. This anode configuration causes the 
electrolyte around the structure to become positive with 
respect to remote earth. 

Electrical Isolation: The condition of being electrically 
separated from other metallic structures or the environment. 

Electrical Survey: Any technique that involves coordinated 
electrical measurements taken to provide a basis for 
deduction concerning a particular electrochemical condition 
relating to corrosion or corrosion control. 



Electrode: A conductor used to establish contact with an 
electrolyte and through which current is transferred to or 
from an electrolyte. 

Electroosmotic Effect: Passage of a charged particle 
through a membrane under the influence of a voltage. Soil 
or coatings may act as the membrane. 

Electrolyte: A chemical substance containing ions that 
migrate in an electric field. For the purpose of this standard, 
electrolyte refers to the soil or liquid adjacent to and in 
contact with a buried or submerged metallic piping system, 
including the moisture and other chemicals contained 
therein. 

Foreign Structure: Any metallic structure that is not 
intended as a part of a system under cathodic protection. 

Galvanic Anode: A metal that provides sacrificial 
protection to another metal that is more noble when 
electrically coupled in an electrolyte. This type of anode is 
the electron source in one type of cathodic protection. 

Galvanic Series: A list of metals and alloys arranged 
according to their corrosion potentials in a given 
environment. 

Groundbed: One or more anodes installed below the 
earth's surface for the purpose of supplying cathodic 
protection. 

Holiday: A discontinuity in a protective coating that 
exposes unprotected surface to the environment. 

Impressed Current: An electric current supplied by a 
device employing a power source that is external to the 
electrode system. (An example is direct current for cathodic 
protection.) 

in-Llne Inspection: The inspection of a steel pipeline 
using an electronic instrument or tool that travels along the 
interior of the pipeline. 

Insulating Coating System: All components of the 
protective coating, the sum of which provides effective 
electrical isolation of the coated structure. 

Interference: Any electrical disturbance on a metallic 
structure as a result of stray current. 

Interference Bond: An intentional metallic connection, 
between metallic systems in contact with a common 
electrolyte, designed to control electrical current 
interchange between the systems. 

IR Drop: The voltage across a resistance in accordance 
with Ohm's Law. 



NACE International 



SP01 69-2007 



Isolation: See Electrical Isolation. 

Line Current: The direct current flowing on a pipeline. 

Long-Line Current: Current through the earth between an 
anodic and a cathodic area that returns along an 
underground metallic structure. 

Mixed Potential: A potential resulting from two or more 
electrochemical reactions occurring simultaneously on one 
metal surface. 



Shorted Pipeline Casing: A casing that is in direct metallic 
contact with the carrier pipe. 

Sound Engineering Practices: Reasoning exhibited or 
based on thorough knowledge and experience, logically 
valid and having technically correct premises that 
demonstrate good judgment or sense in the application of 
science. 



Stray Current: 

intended circuit. 



Current through paths other than the 



PIpe-to-Electrolyte Potential: 

Potential. 



See Structure-to-Electrolyte 



Polarization: The change from the open-circuit potential as 
a result of current across the electrode/electrolyte interface. 

Polarized Potential: The potential across the 

structure/electrolyte interface that is the sum of the 
corrosion potential and the cathodic polarization. 

Reference Electrode: An electrode whose open-circuit 
potential is constant under similar conditions of 
measurement, which is used for measuring the relative 
potentials of other electrodes. 

Reverse-Current Switch: A device that prevents the 
reversal of direct current through a metallic conductor. 

Shielding: (1) Protecting; protective cover against 
mechanical damage. (2) Preventing or diverting the 
cathodic protection current from its intended path. 



Stray-Current Corrosion: Corrosion resulting from current 
through paths other than the intended circuit, e.g., by any 
extraneous current in the earth. 

Structure-to-Electrolyte Potential: The potential 

difference between the surface of a buried or submerged 
metallic structure and electrolyte that is measured with 
reference to an electrode in contact with the electrolyte. 

Telluric Current: Current in the earth as a result of 
geomagnetic fluctuations. 

Voltage: An electromotive force or a difference in electrode 
potentials expressed in volts. 

Wire: A slender rod or filament of drawn metal. In practice, 
the term is also used for smaller-gauge conductors (6 mm^ 
[No. 10 AWG'^'] or smaller). 



Section 3: Determination of Need for External Corrosion Control 



3.1 Introduction 

3.1 .1 This section recommends practices for 
determining when an underground or submerged 
metallic piping system requires external corrosion 
control. 

3.1 .2 IVIetallic structures, buried or submerged, are 
subject to corrosion. Adequate corrosion control 
procedures should be adopted to ensure metal integrity 
for safe and economical operation. 

3.2 The need for external corrosion control should be 
based on data obtained from one or more of the following: 
corrosion surveys, operating records, visual observations, 
test results from similar systems in similar environments, in- 
line inspections, engineering and design specifications, and 



operating, safety, and economic requirements. The 
absence of leaks alone is insufficient evidence that 
corrosion control is not required. 

3.2.1 Environmental and physical factors include the 
following: 

3.2.1.1 Corrosion rate of the particular metallic 
piping system in a specific environment (see 
Appendix B [nonmandatory]); 

3.2.1.2 Nature of the product being transported, 
the working temperature, temperature differentials 
within the pipeline causing thermal expansion and 
contraction, tendency of backfill to produce soil 
stress, and working pressure of the piping system 
as related to design specification; 



(2) 



American Wire Gauge. 



NACE International 



SP01 69-2007 



3.2.1 .3 Location of the piping system as related to 
population density and frequency of visits by 
personnel; 

3.2.1 .4 Location of the piping system as related to 
other facilities; and 

3.2.1.5 Stray current sources foreign to the 
system. 



3.2.2 Economic factors include the following: 

3.2.2.1 Costs of maintaining the piping system in 
service for its expected life (see Appendix B 
[nonmandatory]) 

3.2.2.2 Contingent costs of corrosion (see 
Appendix C [nonmandatory]); and 

3.2.2.3 Costs of corrosion control (see Appendix 
D [nonmandatory]). 



Section 4: Piping System Design 



4.1 Introduction 



4.1 .1 This section provides accepted corrosion control 
practices in the design of an underground or 
submerged piping system. A person qualified to 
engage in the practice of corrosion control should be 
consulted during all phases of pipeline design and 
construction (see Paragraph 1.3). These 

recommendations should not be construed as taking 
precedence over recognized electrical safety practices. 

4.2 External Corrosion Control 

4.2.1 External corrosion control must be a primary 
consideration during the design of a piping system. 
IVIaterials selection and coatings are the first line of 
defense against external corrosion. Because perfect 
coatings are not feasible, CP must be used in 
conjunction with coatings. For additional information, 
see Sections 5 and 6. 

4.2.2 New piping systems should be externally coated 
unless thorough investigation indicates that coatings 
are not required (see Section 5). 

4.2.3 Materials and construction practices that create 
electrical shielding should not be used on the pipeline. 
Pipelines should be installed at locations where 
proximity to other structures and subsurface formations 
do not cause shielding. 

4.3 Electrical Isolation 

4.3.1 Isolation devices such as flange assemblies, 
prefabricated joint unions, or couplings should be 
installed within piping systems in which electrical 
isolation of portions of the system is required to 
facilitate the application of external corrosion control. 
These devices should be properly selected for 
temperature, pressure, chemical resistance, dielectric 
resistance, and mechanical strength. Installation of 
isolation devices should be avoided or safeguarded in 
areas in which combustible atmospheres are likely to 
be present. Locations at which electrical isolating 
devices should be considered include, but are not 
limited to, the following: 



4.3.1.1 Points at which facilities change 
ownership, such as meter stations and well heads; 

4.3.1.2 Connections to mainline piping systems, 
such as gathering or distribution system laterals; 

4.3.1.3 Inlet and outlet piping of in-line measuring 
and pressure regulating stations; 

4.3.1 .4 Compressor or pumping stations, either in 
the suction and discharge piping or in the main 
line immediately upstream and downstream from 
the station; 

4.3.1.5 Stray current areas; 

4.3.1 .6 The junction of dissimilar metals; 

4.3.1.7 The termination of service line 
connections and entrance piping; 

4.3.1.8 The junction of a coated pipe and a bare 
pipe; and 

4.3.1.9 Locations at which electrical grounding is 
used, such as motorized valves and 
instrumentation. 

4.3.2 The need for lightning and fault current 
protection at isolating devices should be considered. 
Cable connections from isolating devices to arresters 
should be short, direct, and of a size suitable for short- 
term high-current loading. 

4.3.3 When metallic casings are required as part of 
the underground piping system, the pipeline should be 
electrically isolated from such casings. Casing 
insulators must be properly sized and spaced and be 
tightened securely on the pipeline to withstand insertion 
stresses without sliding on the pipe. Inspection should 
be made to verify that the leading insulator has 
remained in position. Concrete coatings on the carrier 
pipe could preclude the use of casing insulators. 
Consideration should be given to the use of support 
under the pipeline at each end of the casing to 
minimize settlement. The type of support selected 



NACE International 



SP01 69-2007 



should not cause damage to the pipe coating or act as 
a shield to CP current. 

4.3.4 Casing seals should be installed to resist the 
entry of foreign matter into the casing. 

4.3.5 When electrical contact would adversely affect 
CP, piping systems should be electrically isolated from 
supporting pipe stanchions, bridge structures, tunnel 
enclosures, pilings, offshore structures, or reinforcing 
steel in concrete. However, piping can be attached 
directly to a bridge without isolation if isolating devices 
are installed in the pipe system on each side of the 
bridge to isolate the bridge piping electrically from 
adjacent underground piping. 

4.3.6 When an isolating joint is required, a device 
manufactured to perform this function should be used, 
or, if permissible, a section of nonconductive pipe, such 
as plastic pipe, may be installed. In either case, these 
should be properly rated and installed in accordance 
with the manufacturer's instructions. 

4.3.7 River weights, pipeline anchors, and metallic 
reinforcement in weight coatings should be electrically 
isolated from the carrier pipe and designed and 
installed so that coating damage does not occur and 
the carrier pipe is not electrically shielded. 

4.3.8 IVIetallic curb boxes and valve enclosures should 
be designed, fabricated, and installed in such a manner 
that electrical isolation from the piping system is 
maintained. 

4.3.9 Insulating spacing materials should be used 
when it is intended to maintain electrical isolation 
between a metallic wall sleeve and the pipe. 

4.3.10 Underground piping systems should be 
installed so that they are physically separated from all 
foreign underground metallic structures at crossings 
and parallel installations and in such a way that 
electrical isolation could be maintained if desired. 

4.3.11 Based on voltage rating of alternating current 
(AC) transmission lines, adequate separation should 
be maintained between pipelines and electric 
transmission tower footings, ground cables, and 
counterpoise. Regardless of separation, consideration 
should always be given to lightning and fault current 
protection of pipeline(s) and personnel safety (see 
NACE Standard RP0177^). 

4.4 Electrical Continuity 

4.4.1 Nonwelded pipe joints may not be electrically 
continuous. Electrical continuity can be ensured by the 
use of fittings manufactured for this purpose or by 
bonding across and to the mechanical joints in an 
effective manner. 

4.5 Corrosion Control Test Stations 



4.5.1 Test stations for potential, current, or resistance 
measurements should be provided at sufficient 
locations to facilitate CP testing. Such locations may 
include, but are not limited to, the following: 

4.5.1.1 Pipe casing installations, 

4.5.1 .2 IVIetallic structure crossings, 

4.5.1.3 Isolating joints, 

4.5.1.4 Waten/vay crossings, 

4.5.1.5 Bridge crossings, 

4.5.1.6 Valve stations, 

4.5.1 .7 Galvanic anode installations, 

4.5.1.8 Road crossings, 

4.5.1 .9 Stray-current areas, and 

4.5.1 .1 Rectifier installations. 

4.5.2 The span of pipe used for line current test 
stations should exclude: 

4.5.2.1 Foreign metallic structure crossings; 

4.5.2.2 Lateral connections; 

4.5.2.3 Mechanical couplings or connections such 
as screwed joints, transition pieces, valves, 
flanges, anode or rectifier attachments, or metallic 
bonds; and 

4.5.2.4 Changes in pipe wall thickness and 
diameter. 

4.5.3 Attachment of Copper Test Lead Wires to Steel 
and Other Ferrous Pipes 

4.5.3.1 Test lead wires may be used both for 
periodic testing and for current-carrying purposes. 
As such, the wire/pipe attachment should be 
mechanically strong and electrically conductive. 

4.5.3.2 IVIethods of attaching wires to the pipe 
include (a) thermit welding process, (b) soldering, 
and (c) mechanical means. 

4.5.3.3 Particular attention must be given to the 
attachment method to avoid (a) damaging or 
penetrating the pipe, (b) sensitizing or altering of 
pipe properties, (c) weakening the test lead wire, 
(d) damaging internal or external pipe coatings, 
and (e) creating hazardous conditions in explosive 
environments. 

4.5.3.4 Attachment by mechanical means is the 
least desirable method. Such a connection may 



NACE International 



SP01 69-2007 



loosen, become highly resistant, or lose electrical 
continuity. 

4.5.3.5 The connection should be tested for 
mechanical strength and electrical continuity. All 
exposed portions of the connection should be 
thoroughly cleaned of all welding slag, dirt, oils, 
etc.; primed, if needed; and coated with materials 
compatible with the cable insulation, pipe coating, 
and environment. 

4.5.4 Attachment of Aluminum Test Lead Wire to 
Aluminum Pipes 

4.5.4.1 Aluminum test lead wire, or aluminum 
tabs attached to aluminum wire, may be welded to 
aluminum pipe using the tungsten inert-gas 
shielded arc (TIG) or metal inert-gas shielded arc 
(IVIIG) process. Welded attachments should be 
made to flanges or at butt weld joints. Attachment 
at other sites may adversely affect the mechanical 
properties of the pipe because of the heat of 
welding. 

4.5.4.2 Test lead wire may be attached to 
aluminum pipe by soldering. If low-melting-point 
soft solders are used, a flux is required. Flux 
residues may cause corrosion unless removed. 

NOTE: The use of copper test lead wire may 
cause preferential galvanic attack on the 
aluminum pipe. When copper wire or flux is used, 
care must be taken to seal the attachment areas 
against moisture. In the presence of moisture, the 
connection may disbond and be damaged by 
corrosion. 

4.5.4.3 Aluminum tabs to which test lead wires 
have been TIG welded can be attached by an 



explosive bonding technique called high-energy 
joining. 

4.5.4.4 Mechanical connections that remain 
secure and electrically conductive may be used. 

4.5.5 Attachment of Copper Test Lead Wire to Copper 
Pipe. 

4.5.5.1 Copper test lead wire, or copper tabs 
attached to copper wire, may be attached to 
copper pipe by one of the following methods. The 
relative thickness of the wire and the pipe wall 
dictates, in part, which of the methods can be 
used. 

4.5.5.1.1 Arc welding (TIG, IVIIG, or shielded 
metal); 

4.5.5.1 .2 Electrical resistance (spot) welding; 

4.5.5.1.3 Brazing; 

4.5.5.1.4 Soldering; or 

4.5.5.1 .5 Mechanical connection. 

4.5.5.2 Attention should be given to proper joining 
procedures to avoid possible embrittlement or loss 
of mechanical properties of the metals from the 
heat of welding or brazing. 

4.5.5.3 A flux may be required, or self-produced, 
when brazing with some filler metals or soldering 
with some low-melting-point soft solders. Because 
flux residues may cause corrosion, they should be 
removed. 



Section 5: External Coatings 



5.1 Introduction 

5.1 .1 This section recommends practices for selecting, 
testing and evaluating, handling, storing, inspecting, 
and installing external coating systems for external 
corrosion control on piping systems. 

The function of external coatings is to control corrosion 
by isolating the external surface of the underground or 
submerged piping from the environment, to reduce CP 
current requirements, and to improve current 
distribution. 

5.1 .2 External coatings must be properly selected and 
applied and the coated piping carefully handled and 
installed to fulfill these functions. Various types of 
external coatings can accomplish the desired functions. 



5.1 .2.1 Desirable characteristics of external 
coatings include the following: 

5.1 .2.1 .1 Effective electrical insulator; 

5.1 .2.1 .2 Effective moisture barrier; 

5.1 .2.1 .3 Application to pipe by a method that 
does not adversely affect the properties of the 
pipe; 

5.1.2.1.4 Application to pipe with a minimum 
of defects; 

5.1 .2.1 .5 Good adhesion to pipe surface; 



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5.1 .2.1 .6 Ability to resist development of 
holidays with time; 

5.1 .2.1 .7 Ability to resist damage during 
handling, storage, and installation; 

5.1 .2.1 .8 Ability to maintain substantially 
constant electrical resistivity with time; 

5.1 .2.1 .9 Resistance to disbonding; 

5.1.2.1.10 Resistance to chemical 
degradation; 

5.1.2.1.11 Ease of repair; 

5.1.2.1.12 Retention of physical 
characteristics; 

5.1 .2.1 .1 3 Nontoxic to the environment; and 

5.1.2.1.14 Resistance to changes and 
deterioration during aboveground storage and 
long-distance transportation. 

5.1 .2.2 Typical factors to consider when selecting 
an external pipe coating include: 

5.1.2.2.1 Type of environment; 

5.1 .2.2.2 Accessibility of piping system; 

5.1 .2.2.3 Operating temperature of piping 
system; 

5.1.2.2.4 Ambient temperatures during 
application, shipping, storage, construction, 
installation, and pressure testing; 

5.1 .2.2.5 Geographical and physical location; 

5.1 .2.2.6 Type of external coating on existing 
pipe in the system; 

5.1.2.2.7 Handling and storage; 

5.1 .2.2.8 Pipeline installation methods; 

5.1.2.2.9 Costs; and 



5.1.2.2.10 Pipe 
requirements. 



surface 



preparation 



5.1 .2.3 Pipeline external coating systems shall be 
properly selected and applied to ensure that 
adequate bonding is obtained. Unbonded 
coatings can create electrical shielding of the 
pipeline that could jeopardize the effectiveness of 
the CP system. 

5.1.3 Information in this section is primarily by 
reference to other documents. It is important that the 
latest revision of the pertinent reference be used. 

5.1 .3.1 Table 1 is a listing of types of external 
coating systems, showing the appropriate 
references for material specifications and 
recommended practices for application. 

5.1 .3.2 Table 2 is a grouping of references for 
general use during installation and inspection, 
regardless of coating type. 

5.1 .3.3 Table 3 is a list of external coating system 
characteristics related to environmental conditions 
containing suggested laboratory test references for 
various properties. 

5.1 .3.4 Table 4 is a list of external coating system 
characteristics related to design and construction, 
with recommended laboratory tests for evaluating 
these properties. 

5.1 .3.5 Table 5 lists the references that are useful 
in field evaluation of external coating systems after 
the pipeline has been installed. 

5.2 Storage, Handling, Inspection, and Installation 

5.2.1 Storage and Handling 

5.2.1.1 Coated pipe to be stored should be 
protected internally and externally from 
atmospheric corrosion and coating deterioration. 

5.2.1.2 Damage to coating can be minimized by 
careful handling and by using proper pads and 
slings. 



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TABLE 1 
Generic External Coating Systems witli IVIaterial Requirements 



and Recommended Practices for Application 



(A) 



Generic External Coating System 



Reference 



Coal Tar 

Wax 

Prefabricated Films 

Fusion-Bonded Epoxy Coatings 



Polyolefin Coatings 



ANSI<^VaWWA"^'C203'° 
NACE Standard RP0375' 



ANSI/AWWAC214'^ 
ANSI/AWWAC209'' 

Peabody's Control of Pipeline Corrosion^ ' 

ANSI/AWWAC213^* 

APl'°' RP5L7'^ 

CSA'^'Z245.20M^' 

NACE Standard RP0394'' 

NACE Standard RP01 85 
DIN'^'30 670^° 
ANSI/AWWAC215^ 



19 



''^' NOTE: Many other references are available, and this table is not comprehensive. Listing does not constitute 

endorsement of any external coating system in preference to another. Omission of a system may be due to unavailability of 

reference standards or lack of data. 

'"' American National Standards Institute (ANSI), 1819 L St. NW, Washington, DC 20036. 

"^' American Water Works Association (AWWA), 6666 West Ouincy Ave., Denver, CO 80235. 

'"' American Petroleum Institute (API), 1220 L St. NW, Washington, DC 20005-4070. 

'^' CSA International, 178 Rexdale Blvd., Toronto, Ontario, Canada IVleW 1 R3. 

' ' Deutsches Institut fur Normung (DIN), Burggrafenstrasse 6, D-10787 Berlin, Germany. 



TABLE 2 
References for General Use in the Installation and Inspection of External Coating Systems 

for Underground Piping 



Subject 



Reference 



Application of Organic Pipeline Coatings 



Film Thickness of Pipeline Coatings 
Inspection of Pipeline Coatings 



ANSI/AWWAC203'° 

NACE Standard RP0375" 

Peabody's Control of Pipeline Corrosion^' 

ANSI/AWWAC213' = 

API RP5L7'' 

CSAZ245.20M" 

ASTM'*'G 128^^ 

NACE Standard RP0274" 



(A) 



ASTM, 100 Barr Harbor Dr., West Conshohocken, PA 19428-2959. 



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TABLE 3 
External Coating System Characteristics Relative to Environmental Conditions''^^ 



Environmental Factor 



Recommended Test Methods 



w 



General underground exposure with or without CP 



Peabody's Control of Pipeline Corrosion ' 
ANSI/AWWAC213^^ 



API RP5L7'^ 
CSA Z245.20IVI 
ASTM G 8^* 
ASTMG19" 
ASTM G 42^^ 
ASTM G 95" 



17 



Resistance to water penetration and its effect on choice 
of coating thickness 

Resistance to penetration by stones in backfill 



ASTM G 9' 



28 



29 



ASTM G 1 7 
ASTM D 2240 
ASTM G 1 3^ 
ASTMG14'' 



30 



Soil stress 

Resistance to specific liquid not normally encountered 
in virgin soil 

Resistance to thermal effects 



Suitability of supplementary materials for joint coating 
and field repairs 



Resistance to microorganisms 



Underground Corrosion' 
ASTM D 427' ' 



.3 4 



ASTM D 543' 

Federal Test Standard'' 

ASTM G 20 



37 



NO.406A, Method 701 r 



ASTM D 2304' 
ASTM D 2454^ 
ASTM D 2485* 



24 



ASTM G 8' 
ASTM G 19" 
ASTM G 42^ 
ASTM G 95" 
ASTM G 9^' 
ASTM G 18*^ 
ASTM G 55*^ 

ASTM G 21 " 

Federal Test Standard No. 406A, Method 6091* 



Jl^J NOTE: Apply only those factors pertinent to the installation. 

No specific criteria are available. Comparative tests are recommended for use and evaluation as supplementary information only. 



(c 



Available from General Services Administration, Business Service Center, Washington, DC 20025. 



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TABLE 4 
External Coating System Characteristics Related to Design and Construction 



Design and Construction Factor 



Recommended Test iUletliods*'*' 



Yard Storage, Weathering 

Yard Storage, Penetration Under Load 

Handling Resistance, Abrasion 
Handling Resistance, Impact 

Field Bending Ability 

Driving Ability (Resistance to Sliding Abrasion) 

Special Requirements for Mill-Applied Coating 



Special Requirements for Application of Coating Over the 
Ditch 



ASTIVI G 1 r° 

ASTMG 17" 
ASTM D 2240^ 



32 



ASTM G 6"^ 

ASTMG 13^' 
ASTMG 14 

ASTMG 10" 

ASTM G 6" 
ASTM D2197" 

ANSI/AWWAC203'° 
NACE Standard RP0375' 



ANSI/AWWAC214 

ANSI/AWWAC209 

Peabody's Control of Pipeline Corrosion 

ANSI/AWWAC213" 



15 



16 



API RP5L7 

CSAZ245.20M" 

NACE Standard RP01 85' 



21 



10 



DIN 30 670" 
ANSI/AWWAC215' 

ANSI/AWWAC203 

NACE Standard RP0375" 

ANSI/AWWAC214'^ 

ANSI/AWWAC209'^ 

Peabody's Control of Pipeline Corrosion^ ' 

ANSI/AWWAC213'^ 

API RP5L7"" 



CSA Z245.20M 



17 



Backfill Resistance 



Resistance to Thermal Effects 



ASTMG 13' 
ASTMG 14 

ASTM G 8^" 
ASTMG 19" 
ASTM G 42" 
ASTM G 95" 
ASTM D 2304^ 
ASTM D 2454^ 
ASTM D 2485^' 



32 



Suitability of Joint Coatings and Field Repairs 



Peabody's Control of Pipeline Corrosion ' 

ANSI/AWWAC213'* 

API RP5L7" 

CSAZ245.20M" 



ASTM G 8^" 
ASTMG 19" 
ASTM G 42" 
ASTM G 95" 
ASTM G 9" 
ASTMG 18*' 
ASTM G 55*' 



(A) 



No specific criteria are available. Comparative tests are recommended for use and evaluation as supplementary information only. 



10 



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TABLE 5 
Methods for Evaluating In-Service Field Performance of External Coatings 



Title or Subject of Method Reference 



Basis for Rating 



(1 ) Rate of Change in Current Underground Corrosion ' 
Required for CP 



(2) Inspection of Pipeline 
Coating 

(3) Cathodic Disbondment 



NACE Standard RP0274' 



ASTIVI G 8" 
ASTM G 1 9^ 
ASTM G 42^ 
ASTM G 95^ 



Comparison of initial current requirement with 
subsequent periodic determination of current 
requirement 

(a) With CP: no active corrosion found 

(b) Without CP: no new holidays showing active 
corrosion 

Purpose is to obtain data relative to specific 
conditions for comparison with laboratory data 



5.2.2 Inspection 

5.2.2.1 Qualified personnel should keep every 
phase of the coating operation and piping 
installation under surveillance. 

5.2.2.2 Surface preparation, primer application, 
coating thickness, temperature, bonding, and 
other specific requirements should be checked 
periodically, using suitable test procedures, for 
conformance to specifications. 

5.2.2.3 The use of holiday detectors is 
recommended to detect coating flaws that would 
not be observed visually. The holiday detector 
should be operated in accordance with the 
manufacturer's instructions and at a voltage level 
appropriate to the electrical characteristics of the 
coating system. 

5.2.3 Installation 

5.2.3.1 Joints, fittings, and tie-ins must be coated 
with a material compatible with the existing 
coating. 

5.2.3.2 Coating defects should be repaired. 

5.2.3.3 Materials used to repair coatings must be 
compatible with the existing pipe coating. 

5.2.3.4 The ditch bottom should be graded and 
free of rock or other foreign matter that could 
damage the external coating or cause electrical 
shielding. Under difficult conditions, consideration 
should be given to padding the pipe or the ditch 
bottom. 

5.2.3.5 Pipe should be lowered carefully into the 
ditch to avoid external coating damage. 

5.2.3.6 Care should be taken during backfilling so 
that rocks and debris do not strike and damage 
the pipe coating. 



5.2.3.7 Care shall be exercised when using 
materials such as loose wrappers, nonconducting 
urethane foam, and rock shield around pipelines 
as protection against physical damage or for other 
purposes, because these materials may create an 
electrical shielding effect that would be detrimental 
to the effectiveness of CP. 

5.2.3.8 When a pipeline comes above ground, it 
must be cleaned and externally coated, or 
jacketed with a suitable material, for the prevention 
of atmospheric corrosion. 

5.3 Methods for Evaluating External Coating Systems 

5.3.1 Established Systems Proven by Successful Use 

5.3.1.1 Visual and electrical inspection of 
in-service pipeline coatings should be used to 
evaluate the performance of an external coating 
system. These inspections can be conducted 
wherever the pipeline is excavated or at bell holes 
made for inspection purposes. 



5.3.2 Established 
Environments 



or Modified Systems for New 



5.3.2.1 This method is intended for use when 
external coating systems will continue to be used 
and are qualified under Paragraph 5.3.1, but when 
application will be extended to new environments 
or when it is desired to revise a system to make 
use of new developments, one of the following 
should be used: 

5.3.2.1.1 The use of applicable material 
requirements, material specifications, 
standards, and recommended practices for 
application, as given in Table 1 , is 
recommended. 

5.3.2.1 .2 The use of applicable references in 
Table 2 is recommended unless previously 
covered in applicable references in Table 1 . 



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5.3.3 New External Coating System Qualification 

5.3.3.1 The purpose of this method is to qualify a 
new external coating material by subjecting it to 
laboratory tests appropriate for the intended 
service. After laboratory tests have been 
conducted and indicate that the external coating 
system appears to be suitable, application and 
installation are conducted in accordance with 
recommended practices. In-service field 

performance tests are made to confirm the 
success of the previous steps. The steps of the 
method are (1) laboratory tests, (2) application 
under recommended practices, (3) installation 
under recommended practices, and (4) in-service 
field performance tests. If good results are 
obtained after five years, only Steps 2 and 3 are 
required thereafter. 

5.3.3.1 .1 Applicable sections of Tables 3 and 
4 are recommended for the initial laboratory 
test methods. 



5.3.3.1 .3 During a period of five years or 
more, the use of the evaluation methods 
given in Table 5, Item 1 or 2 are 
recommended. The test method in Item 3 
may be used as a supplementary means for 
obtaining data for correlation with laboratory 
tests. 

5.3.4 Method for Evaluating an External Coating 
System by In-Service Field Performance Only 

5.3.4.1 The purpose of this method is to qualify an 
external coating system when none of the first 
three methods given in Paragraph 5.3 has been or 
will be used. It is intended that this method should 
be limited to minor pilot installations. 

5.3.4.1.1 The use of at least one of the first 
two methods given in Table 5 is 
recommended on the basis of at least one 
investigation per year for five consecutive 
years. 



5.3.3.1 .2 Applicable sections of Tables 1 and 
2 are recommended for conditional use during 
Steps 2 and 3. 



Section 6: Criteria and Otiier Considerations for CP 



6.1 Introduction 

6.1 .1 This section lists criteria and other 
considerations for CP that indicate, when used either 
separately or in combination, whether adequate CP of 
a metallic piping system has been achieved (see also 
Section 1 , Paragraphs 1 .2 and 1 .4). 



6.1.5 Corrosion leak history is valuable in assessing 
the effectiveness of CP. Corrosion leak history by 
itself, however, shall not be used to determine whether 
adequate levels of CP have been achieved unless it is 
impractical to make electrical surveys. 



6.2 Criteria 



6.1 .2 The effectiveness of CP or other external 
corrosion control measures can be confirmed by visual 
observation, by measurements of pipe wall thickness, 
or by use of internal inspection devices. Because such 
methods sometimes are not practical, meeting any 
criterion or combination of criteria in this section is 
evidence that adequate CP has been achieved. When 
excavations are made for any purpose, the pipe should 
be inspected for evidence of corrosion and coating 
condition. 

6.1 .3 The criteria in this section have been developed 
through laboratory experiments or verified by 
evaluating data obtained from successfully operated 
CP systems. Situations in which a single criterion for 
evaluating the effectiveness of CP may not be 
satisfactory for all conditions may exist. Often a 
combination of criteria is needed for a single structure. 

6.1 .4 Sound engineering practices shall be used to 
determine the methods and frequency of testing 
required to satisfy these criteria. 



6.2.1 It is not intended that persons responsible for 
external corrosion control be limited to the criteria listed 
below. Criteria that have been successfully applied on 
existing piping systems can continue to be used on 
those piping systems. Any other criteria used must 
achieve corrosion control comparable to that attained 
with the criteria herein. 

6.2.2 Steel and Cast Iron Piping 

6.2.2.1 External corrosion control can be 
achieved at various levels of cathodic polarization 
depending on the environmental conditions. 
However, in the absence of specific data that 
demonstrate that adequate CP has been 
achieved, one or more of the following shall apply: 

6.2.2.1 .1 A negative (cathodic) potential of at 
least 850 mV with the CP applied. This 
potential is measured with respect to a 
saturated copper/copper sulfate reference 
electrode contacting the electrolyte. Voltage 



12 



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drops other than those across the structure- 
to-electrolyte boundary must be considered 
for valid interpretation of this voltage 
measurement. 

NOTE: Consideration is understood to mean 
the application of sound engineering practice 
in determining the significance of voltage 
drops by methods such as: 

6.2.2.1.1.1 Measuring or calculating the 
voltage drop(s); 



6.2.2.1.1.2 Reviewing the 
performance of the CP system; 



historical 



6.2.2.1.1.3 Evaluating the physical and 
electrical characteristics of the pipe and 
its environment; and 

6.2.2.1.1.4 Determining whether or not 
there is physical evidence of corrosion. 

6.2.2.1 .2 A negative polarized potential (see 
definition in Section 2) of at least 850 mV 
relative to a saturated copper/copper sulfate 
reference electrode. 

6.2.2.1.3 A minimum of 100 mV of cathodic 
polarization between the structure surface 
and a stable reference electrode contacting 
the electrolyte. The formation or decay of 
polarization can be measured to satisfy this 
criterion. 

6.2.2.2 Special Conditions 

6.2.2.2.1 On bare or ineffectively coated 
pipelines when long-line corrosion activity is of 
primary concern, the measurement of a net 
protective current at predetermined current 
discharge points from the electrolyte to the 
pipe surface, as measured by an earth current 
technique, may be sufficient. 

6.2.2.2.2 In some situations, such as the 
presence of sulfides, bacteria, elevated 
temperatures, acid environments, and 
dissimilar metals, the criteria in Paragraph 
6.2.2.1 may not be sufficient. 



pipe, in stray-current areas, or where local 
corrosion cell action predominates. 

6.2.2.3.2 Caution is advised against using 
polarized potentials less negative than -850 
mV for CP of pipelines when operating 
pressures and conditions are conducive to 
stress corrosion cracking (see references on 
stress corrosion cracking at the end of this 
section). 

6.2.2.3.3 The use of excessive polarized 
potentials on externally coated pipelines 
should be avoided to minimize cathodic 
disbondment of the coating. 

6.2.2.3.4 Polarized potentials that result in 
excessive generation of hydrogen should be 
avoided on all metals, particularly higher- 
strength steel, certain grades of stainless 
steel, titanium, aluminum alloys, and 
prestressed concrete pipe. 

6.2.3 Aluminum Piping 

6.2.3.1 The following criterion shall apply: a 
minimum of 100 mV of cathodic polarization 
between the structure surface and a stable 
reference electrode contacting the electrolyte. The 
formation or decay of this polarization can be used 
in this criterion. 

6.2.3.2 PRECAUTIONARY NOTES 

6.2.3.2.1 Excessive Voltages: 

Notwithstanding the minimum criterion in 
Paragraph 6.2.3.1, if aluminum is cathodically 
protected at voltages more negative than - 
1 ,200 mV measured between the pipe 
surface and a saturated copper/copper sulfate 
reference electrode contacting the electrolyte 
and compensation is made for the voltage 
drops other than those across the pipe- 
electrolyte boundary, it may suffer corrosion 
as the result of the buildup of alkali on the 
metal surface. A polarized potential more 
negative than -1,200 mV should not be used 
unless previous test results indicate that no 
appreciable corrosion will occur in the 
particular environment. 



6.2.2.2.3 When a pipeline is encased in 
concrete or buried in dry or aerated high- 
resistivity soil, values less negative than the 
criteria listed in Paragraph 6.2.2.1 may be 
sufficient. 

6.2.2.3 PRECAUTIONARY NOTES 

6.2.2.3.1 The earth current technique is often 
meaningless in multiple pipe rights-of-way, in 
high-resistivity surface soil, for deeply buried 



6.2.3.2.2 Alkaline Conditions: Aluminum 
may suffer from corrosion under high-pH 
conditions, and application of CP tends to 
increase the pH at the metal surface. 
Therefore, careful investigation or testing 
should be done before applying CP to stop 
pitting attack on aluminum in environments 
with a natural pH in excess of 8.0. 



6.2.4 Copper Piping 



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6.2.4.1 The following criterion shall apply: a 
minimum of 100 mV of cathodic polarization 
between the structure surface and a stable 
reference electrode contacting the electrolyte. The 
formation or decay of this polarization can be used 
in this criterion. 

6.2.5 Dissimilar Metal Piping 

6.2.5.1 A negative voltage between all pipe 
surfaces and a stable reference electrode 
contacting the electrolyte equal to that required for 
the protection of the most anodic metal should be 
maintained. 

6.2.5.2 PRECAUTIONARY NOTE 

6.2.5.2.1 Amphoteric materials that could be 
damaged by high alkalinity created by CP 
should be electrically isolated and separately 
protected. 

6.3 Other Considerations 

6.3.1 IVIethods for determining voltage drop(s) shall be 
selected and applied using sound engineering 
practices. Once determined, the voltage drop(s) may 
be used for correcting future measurements at the 
same location, provided conditions such as pipe and 
CP system operating conditions, soil characteristics, 
and external coating quality remain similar. (Note: 
Placing the reference electrode next to the pipe surface 
may not be at the pipe-electrolyte interface. A 
reference electrode placed at an externally coated pipe 
surface may not significantly reduce soil voltage drop in 
the measurement if the nearest coating holiday is 
remote from the reference electrode location.) 

6.3.2 When it is impractical or considered 
unnecessary to disconnect all current sources to 
correct for voltage drop(s) in the structure-to-electrolyte 
potential measurements, sound engineering practices 
should be used to ensure that adequate CP has been 
achieved. 



6.3.3 When feasible and practicable, in-line inspection 
of pipelines may be helpful in determining the presence 
or absence of pitting corrosion damage. Absence of 
external corrosion damage or the halting of its growth 
may indicate adequate external corrosion control. The 
in-line inspection technique, however, may not be 
capable of detecting all types of external corrosion 
damage, has limitations in its accuracy, and may report 
as anomalies items that are not external corrosion. For 
example, longitudinal seam corrosion and general 
corrosion may not be readily detected by in-line 
inspection. Also, possible thickness variations, dents, 
gouges, and external ferrous objects may be detected 
as corrosion. The appropriate use of in-line inspection 
must be carefully considered. 

6.3.4 Situations involving stray currents and stray 
electrical gradients that require special analysis may 
exist. For additional information, see Section 9, 
"Control of Interference Currents." 

6.4 Alternative Reference Electrodes 

6.4.1 Other standard reference electrodes may be 
substituted for the saturated copper/copper sulfate 
reference electrode. Two commonly used reference 
electrodes are listed below along with their voltage 
equivalent (at 25°C [77°F]) to -850 mV referred to a 
saturated copper/copper sulfate reference electrode: 



6.4.1.1 Saturated KCI 
electrode: -780 mV; and 



calomel reference 



6.4.1.2 Saturated silver/silver chloride reference 
electrode used in 25 ohm-cm seawater: -800 mV. 

6.4.2 In addition to these standard reference 
electrodes, an alternative metallic material or structure 
may be used in place of the saturated copper/copper 
sulfate reference electrode if the stability of its 
electrode potential is ensured and if its voltage 
equivalent referred to a saturated copper/copper 
sulfate reference electrode is established. 



Bibliography for Section 6 



Criteria for Copper 



Criteria for Aluminum 



Schwerdtfeger, W.J. "Criteria for Cathodic Protection — 

Highly Resistant Copper Deteriorates in Severely 

Corrosive Soil." Materials Protection 57, 9 (1968): p. 
43. 



BS CP 1021 (latest revision). "Code of Practice for 



Cathodic Protection." London, England: BSI 



(3) 



DIN30 676 (latest revision). "Design and Application of 
Cathodic Protection of External Surfaces." Berlin, 
Germany: DIN 



(3) 



British Standards Institution (BSI), British Standards House, 389 Chiswick High Road, London W4 4AL, United Kingdom. 



14 



NACE International 



SP01 69-2007 



NACE Publication 2IVI363 (withdrawn). "Recommended 
Practice for Catiiodic Protection of Aluminum Pipe 
Buried in Soil or Immersed in Water." Houston, TX: 
NACE. 

Schwerdtfeger, W.J. "Effects of Cathodic Current on the 
Corrosion of An Aluminum Alloy." National Bureau of 



Standards 
p. 283 



(4) 



Journal of Research 68c (Oct. -Dec. 1 964) : 



Criteria for Steel and Cast Iron 

Doremus, E.P., and T.L. Canfield. "The Surface Potential 
Survey Can Detect Pipeline Corrosion Damage." 
Materials Protection 6, 9 (1967): p. 33. 

Ewing, S.P. "Potential IVIeasurements for Determination of 
Cathodic Protection Requirements." Corrosion 7, 12 
(1951): p. 410. 

Haycock, E.W. "Current Requirements for Cathodic 
Protection of Oil Well Casing." Corrosion 13, 11 
(1957): p. 767. 

Kuhn, R.C. "Cathodic Protection of Underground Pipelines 
Against Soil Corrosion." American Petroleum Institute 
Proceedings IV, 14 (1953): p. 153. 

McCollum, B., and K.H. Logan. National Bureau of 
Standards Technical Paper No. 351 , 1927. 



Romanoff, M. Underground Corrosion. 
NACE, 1989. 



Houston, TX: 



Protection of Steel Buried in Soils. Ninth International 
Congress on Metallic Corrosion 4, (1984): June 7. 
National Research Council Canada.'*' 

Barlo, T.J., and W.E. Berry. "An Assessment of the Current 
Criteria for Cathodic Protection of Buried Steel Pipes." 
MP 23, 9 (1984). 

Barlo, T.J., and R.R. Fessler. "Interpretation of True Pipe-to- 
Soil Potentials on Coated Pipelines with Holidays." 



CORROSION/83, paper no. 292. 
1983. 



Houston, TX: NACE, 



Barlo, T.J., and R.R. Fessler. "Investigation of Techniques 
to Determine the True Pipe-to-Soil Potential of a Buried 
Pipeline." AGA'*' Project PR-3-93, 1979 Annual 
Report, IVIay, 1980. 



Cathodic Protection Criteria- 
TX: NACE, 1989. 



-A Literature Survey. Houston, 



Comeaux, R.V. "The Role of Oxygen in Corrosion and 
Cathodic Protection." Corrosions, 9 (1952): pp. 305- 
309. 

Compton, K.G. "Criteria and Their Application for Cathodic 
Protection of Underground Structures." Materials 
Protection 4, 8 (1965): pp. 93-96. 

Dabkowski, J. "Assessing the Cathodic Protection Levels of 
Well Casings." AGA Project 151-106, Final Report, 
January 1983: pp. 3-92. 



Pearson, J.M. "Electrical Instruments and Measurement in 
Cathodic Protection." Corrosions, 11 (1947): p. 549. 

Pearson, J.IVI. "Null Methods Applied to Corrosion 
Measurements." Transactions of the Electrochemical 
Society 8^ (1942): p. 485. 



Dexter, S.C, L.N. Moettus, and K.E. Lucas. "On the 
Mechanism of Cathodic Protection." Corrosion 41, 10 
(1985). 



"Field Testing the Criteria for Cathodic Protection." 
Interim Report PR-1 51 -163, December, 1987. 



AGA 



Schwerdtfeger, W.J., and O.N. McDorman. "Potential and 
Current Requirements for the Cathodic Protection of 
Steel in Soils." Corrosions, ^^ (1952): p. 391 . 



Fischer, K.P. "Cathodic Protection in Saline Mud 
Containing Sulfate Reducing Bacteria." MP 20, 10 
(1981): pp. 41-46. 



Sudrabin, L.P., and F.W. Ringer. "Some Observations on 
Cathodic Protection Criteria." Corrosion 13, 5 (1957) p. 
351 1. Discussion on this paper Corrosion 13, 12 
(1957):p. 835t. 

Additional References 

Barlo, T.J., and W.E. Berry. "A Reassessment of the -0.85 
V and 100 mV Polarization Criteria for Cathodic 



Holler, H.D. "Studies on Galvanic Couples ll-Some 
Potential-Current Relations in Galvanic Corrosion." 
Journal of the Electrochemical Society September 
(1950): pp. 277-282. 

Gummow, R.A. "Cathodic Protection Criteria — A Critical 
Review of NACE Standard RP0169." MP 25, 9 (1986): 
pp. 9-16. 



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National Institute of Standards and Technology (NIST) (formerly National Bureau of Standards), 100 Bureau Dr., Gaithersburg, MD 20899. 
' National Research Council Canada (NRC), 1200 Montreal Road, Ottawa, Ontario K1 A 0R6, CANADA. 
' American Gas Association (AGA), 400 North Capitol St. NW, Suite 400, Washington, DC 20001. 



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Hoey, G.R., and M. Cohen. "Cathodic Protection of Iron in 
tiie Temperature Range 25-92°C." Corrosion 14, 4 
(1958):pp. 200t-202t. 

Howell, R.P. "Potential Measurements in Cathodic 
Protection Designs." Corrosions, 9 (1952). 

Jones, D. "Electrochemical Fundamentals of Cathodic 
Protection." CORROSION/87, paper no. 317. 
Houston, TX: NACE, 1987. 

Kasahara, K., T. Sato, and H. Adachi. "Results of 
Polarization Potential and Current DensitySurveys on 
Existing Buried Pipelines." MP 19, 9 (1980): pp. 45- 
51. 

Kehn, G.R., and E.J. Wilhelm. "Current Requirements for 
the Cathodic Protection of Steel in Dilute Aqueous 
Solutions." Corrosion!, 5 (1951): pp. 156-160. 

Koybayaski, T. "Effect of Environmental Factors on the 
Protective Potential of Steel." Proceedings of the Fifth 
International Congress on Metallic Corrosion. Houston, 
TX:NACE, 1980. 

Krivian, L. "Application of the Theory of Cathodic Protection 
to Practical Corrosion Systems." British Corrosion 
JournahB, 1 (1984). 

Kuhn, R.J. "Cathodic Protection on Texas Gas Systems." 
AGA Annual Conference. Held Detroit, Ml, April 1950. 

Lattin, B.C. "The Errors of Your Ways (Fourteen Pitfalls for 
Corrosion Engineers and Technicians to Avoid)." MP 
20, 3 (1981): p. 30. 

Logan, K.H. "Comparison of Cathodic Protection Test 
Methods." Corrosion 1 0, 7 (1954). 

Logan, K.H. "Underground Corrosion." National Bureau of 
Standards Circular C450, November 1945, pp. 249- 
278. 

Logan, K.H. "The Determination of the Current Required for 
Cathodic Protection." National Bureau of Standards 
Soil Corrosion Conference, March 1943. 

Martin, B.A. "Cathodic Protection: The Ohmic Component 
of Potential Measurements — Laboratory Determination 
with a Polarization Probe in Aqueous Environments." 
MP 20, 1 (1981): p. 52. 

Martin, B.A., and J. A. Huckson. "New Developments in 
Interference Testing." Industrial Corrosion 4, 6 (1986): 
pp. 26-31 . 

Mears and Brown. "A Theory of Cathodic Protection." 
Transactions of the Electrochemical Society 74 (1938): 
p. 527. 



NACE Technical Committee T-2C Report (withdrawn). 
"Criteria for Adequate Cathodic Protection of Coated, 
Buried, or Submerged Steel Pipe Lines and Similar 
Steel Structures." Houston, TX: NACE. 

Pearson, J.M. "Concepts and Methods of Cathodic 
Protection." The Petroleum Engineer ^5, 6 (1944): p. 
218; and 15, 7 (1944): p. 199. 

Pourbaix, M. Atlas of Electrochemical Equilibria in Aqueous 
Solutions. Houston, TX: NACE, 1974, p. 319. 

Prinz, W. "Close Interval Potential Survey of Buried 
Pipelines, Methods and Experience." UK Corrosion 
'86, p. 67. 

Riordan, M.A. "The Electrical Survey — What It Won't Do." 
MP 17, 11 (1978): pp. 38-41. 

Riordan, M.A., and R.P. Sterk. "Well Casing as an 
Electrochemical Network in Cathodic Protection 
Design." Materials Protection 2, 7 (1 963): pp. 58-68. 

SchaschI, E., and G.A. Marsh. "Placement of Reference 
Electrode and Impressed Current Anode Effect on 
Cathodic Protection of Steel in a Long Cell." MP 13, 6 
(1974): pp. 9-11. 

Stern, M. "Fundamentals of Electrode Processes in 
Corrosion." Corrosion 13,11 (1957): p. 97. 

CEA 54277 (withdrawn). "State-of-the-Art Report, 

Specialized Surveys for Buried Pipelines." Houston, 
TX: NACE. 

Thompson, N.G., and T.J. Barlo. "Fundamental Process of 
Cathodically Protecting Steel Pipelines." International 
Gas Research Conference, 1983. 

Toncre, A.C. "A Review of Cathodic Protection Criteria." 
Proceeding of Sixth European Congress on Metallic 
Corrosion. Held London, England, September 1977, 
pp. 365-372. 

Van Nouhuys, H.C. "Cathodic Protection and High 
Resistivity Soil." Corrosion^, 12 (1953): pp. 448-458. 

Van Nouhuys, H.C. "Cathodic Protection and High 
Resistivity Soil— A Sequel." Corrosion 14, 12 (1958): 
p. 55. 

Von Baekmann, W., A. Ballest, and W. Prinz. "New 
Development in Measuring the Effectiveness of 
Cathodic Protection." Corrosion Australasia, February, 
1983. 

Von Baekmann, W., and W. Schwenk. Handbook of 
Cathodic Protection. Portellis Press, 1975, Chapter 2. 



Morgan, J. Cathodic Protection. 2" 
NACE, 1987. 



Ed. Houston, TX: 



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Webster, R.D. "Compensating for the IR Drop Component 
in Pipe-to-Soil Potential IVIeasurements." MP 26, 10 
(1987): pp. 38-41. 

Wyatt, B.S., and K.C. Lax. "Close Interval Overline 
Polarized Potential Surveys of Buried Pipelines." UK 
Corrosion Conference, 1985. 

Stress Corrosion Cracking 



Parkins, R.N., and R.R. Fessler. "Stress Corrosion 
Cracking of High-Pressure Gas Transmission 
Pipelines." Materials in Engineering Appiications 1 , 2 
(1978) pp. 80-96. 

Parkins, R.N., and R.R. Fessler. "Line Pipe Stress 
Corrosion Cracking — IVIechanisms and Remedies." 
CORROSION/86 paper no. 320. Houston, TX: NACE, 
1986. 



Barlo, T.J., et al. "An Assessment of the Criteria for 
Cathodic Protection of Buried Pipelines." AGA Final 
Report, Project PR-3-129, 1983. 

Barlo, T.J., et al. "Controlling Stress-Corrosion Cracking by 
Cathodic Protection." AGA Annual Report, Project-3- 
164, 1984. 

Parkins, R.N., A.J. Markworth, J.H. Holbrook, and R.R. 
Fessler. "Hydrogen Gas Evolution From Cathodically 
Protected Surfaces." Corrosion 4^ ,7 (1985): pp. 389- 



Parkins, R.N., A.J. IVIarkworth, and J.H. Holbrook. 
"Hydrogen Gas Evolution From Cathodically Protected 
Pipeline Steel Surfaces Exposed to Chloride-Sulfate 
Solutions." CorrosionAA, 8 (1988): pp. 572-580. 

IVIcCaffrey, W.R. "Effect of Overprotection on Pipeline 
Coatings." Materials Protection and Performance 1 2, 2 
(1973): p. 10. 

PR-1 5-427. "An Assessment of Stress Corrosion Cracking 
(SCC) Research for Line Pipe Steels." AGA, 1985. 



Section 7: Design of Catliodic Protection Systems 



7.1 Introduction 

7.1 .1 This section recommends procedures for 
designing CP systems that will provide effective 
external corrosion control by satisfying one or more of 
the criteria listed in Section 6 and exhibiting maximum 
reliability over the intended operating life of the 
systems. 

7.1 .2 In the design of a CP system, the following 
should be considered: 

7.1 .2.1 Recognition of hazardous conditions 
prevailing at the proposed installation site(s) and 
the selection and specification of materials and 
installation practices that ensure safe installation 
and operation. 

7.1 .2.2 Specification of materials and installation 
practices to conform to the latest editions of 
applicable codes. National Electrical 
Manufacturers Association (NEMA)'''' standards. 
National Electrical Code (NEC),' ' appropriate 
international standards, and NACE standards. 



7.1 .2.4 Selection of locations for proposed 
installations to minimize currents or earth potential 
gradients, which can cause detrimental effects on 
foreign buried or submerged metallic structures. 

7.1 .2.5 Cooperative investigations to determine 
mutually satisfactory solution(s) of interference 
problems (see Section 9). 

7.1 .2.6 Special consideration should be given to 
the presence of sulfides, bacteria, disbonded 
coatings, thermal insulating coatings, elevated 
temperatures, shielding, acid environments, and 
dissimilar metals. 

7.1 .2.7 Excessive levels of CP that can cause 
external coating disbondment and possible 
damage to high-strength steels as a result of 
hydrogen evolution should be avoided. 

7.1 .2.8 When amphoteric metals are involved, 
care should be taken so that high-pH conditions 
that could cause cathodic corrosion of the metal 
are not established. 



7.1 .2.3 Selection and specification of materials 
and installation practices that ensure dependable 
and economical operation throughout the intended 
operating life. 



7.2 Major objectives of CP system design include the 
following: 

7.2.1 To provide sufficient current to the structure to 
be protected and distribute this current so that the 
selected criteria for CP are effectively attained; 



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National Electrical Manufacturers Association (NEMA), 1300 North 17th St., Suite 1752, Rosslyn, Virginia 22209. 
' National Fire Protection Association, Batterymarch Park, Quincy, MA 02269. 



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7.2.2 To minimize tiie interference currents on 
neighboring underground structures (see Section 9); 

7.2.3 To provide a design life of tiie anode system 
commensurate with the required life of the protected 
structure, or to provide for periodic rehabilitation of the 
anode system; 

7.2.4 To provide adequate allowance for anticipated 
changes in current requirements with time; 

7.2.5 To install anodes when the possibility of 
disturbance or damage is minimal; and 

7.2.6 To provide adequate monitoring facilities to test 
and evaluate the system performance. 

7.3 Information Useful for Design 

7.3.1 Useful piping system specifications and 
information include the following: 

7.3.1 .1 Route maps and atlas sheets; 

7.3.1.2 Construction dates; 

7.3.1 .3 Pipe, fittings, and other appurtenances; 

7.3.1.4 External coatings; 

7.3.1.5 Casings; 

7.3.1 .6 Corrosion control test stations; 

7.3.1 .7 Electrically isolating devices; 

7.3.1 .8 Electrical bonds; and 

7.3.1 .9 Aerial, bridge, and unden/vater crossings. 

7.3.2 Useful information on piping system site 
conditions includes the following: 

7.3.2.1 Existing and proposed CP systems; 

7.3.2.2 Possible interference sources (see 
Section 9); 

7.3.2.3 Special environmental conditions; 

7.3.2.4 Neighboring buried metallic structures 
(including location, ownership, and corrosion 
control practices); 

7.3.2.5 Structure accessibility; 

7.3.2.6 Power availability; and 

7.3.2.7 Feasibility of electrical isolation from 
foreign structures. 



7.3.3 Useful information from field surveys, corrosion 
test data, and operating experience includes the 
following: 

7.3.3.1 Protective current requirements to meet 
applicable criteria; 

7.3.3.2 Electrical resistivity of the electrolyte; 

7.3.3.3 Electrical continuity; 

7.3.3.4 Electrical isolation; 

7.3.3.5 External coating integrity; 

7.3.3.6 Cumulative leak history; 

7.3.3.7 Interference currents; 



7.3.3.8 Deviation 
specifications; and 



from 



construction 



7.3.3.9 Other maintenance and operating data. 

7.3.4 Field survey work prior to actual application of 
CP is not always required if prior experience or test 
data are available to estimate current requirements, 
electrical resistivity of the electrolyte, and other design 
factors. 

7.4 Types of CP Systems 

7.4.1 Galvanic Anode Systems 

7.4.1.1 Galvanic anodes can be made of 
materials such as alloys of magnesium, zinc, or 
aluminum. The anodes are connected to the pipe, 
either individually or in groups. Galvanic anodes 
are limited in current output by the anode-to-pipe 
driving voltage and the electrolyte resistivity. 

7.4.2 Impressed Current Anode Systems 

7.4.2.1 Impressed current anodes can be of 
materials such as graphite, high-silicon cast iron, 
lead-silver alloy, precious metals, or steel. They 
are connected with an insulated cable, either 
individually or in groups, to the positive terminal of 
a direct-current (DC) source, such as a rectifier or 
generator. The pipeline is connected to the 
negative terminal of the DC source. 

7.5 Considerations influencing selection of the type of CP 
system include the following: 

7.5.1 IVIagnitude of protective current required; 

7.5.2 Stray currents causing significant potential 
fluctuations between the pipeline and earth that may 
preclude the use of galvanic anodes; 



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7.5.3 Effects of CP interference currents on adjacent 
structures that may limit tiie use of impressed current 
CP systems; 

7.5.4 Availability of electrical power; 

7.5.5 Physical space available, proximity of foreign 
structures, easement procurement, surface conditions, 
presence of streets and buildings, river crossings, and 
other construction and maintenance concerns. 

7.5.6 Future development of the right-of-way area and 
future extensions to the pipeline system; 

7.5.7 Costs of installation, operation, and 
maintenance; and 

7.5.8 Electrical resistivity of the environment. 
7.6 Factors Influencing Design of CP Systems 

7.6.1 Various anode materials have different rates of 
deterioration when discharging a given current density 
from the anode surface in a specific environment. 
Therefore, for a given current output, the anode life 
depends on the environment and anode material, as 
well as the anode weight and the number of anodes in 
the CP system. Established anode performance data 
may be used to calculate the probable deterioration 
rate. 

7.6.2 Data on the dimensions, depth, and 
configuration of the anodes and the electrolyte 
resistivity may be used to calculate the resultant 
resistance to electrolyte of the anode system. 
Formulas and graphs relating to these factors are 
available in the bibliography literature and from most 
anode manufacturers. 

7.6.3 Design of galvanic anode systems should 
consider anode-to-pipe potential, electrolyte resisivity, 
current output, and in special cases, anode lead-wire 
resistance. A separate design for each anode or 
anode system may not be necessary. 

7.6.4 Galvanic anode performance in most soils can 
be improved by using special backfill material. 
IVIixtures of gypsum, bentonite, and anhydrous sodium 
sulfate are most commonly used. 

7.6.5 The number of impressed current anodes 
required can be reduced and their useful life 
lengthened by the use of special backfill around the 
anodes. The most common materials are coal coke. 



calcined petroleum coke, and natural or manufactured 
graphite. 

7.6.6 In the design of an extensive distributed-anode 
impressed current system, the voltage and current 
attenuation along the anode-connecting (header) cable 
should be considered. In such cases, the design 
objective is to optimize anode system length, anode 
spacing and size, and cable size in order to achieve 
efficient external corrosion control at the extremities of 
the protected structure. 

7.6.7 When it is anticipated that entrapment of gas 
generated by anodic reactions could impair the ability 
of the impressed current groundbed to deliver the 
required current, suitable provisions should be made 
for venting the anodes. For the same current output of 
the system, an increase in the surface area of the 
special backfill material or an increase in the number of 
anodes may reduce gas blockage. 

7.6.8 When it is anticipated that electroosmotic effects 
could impair the ability of the impressed current 
groundbed to deliver the required current output, 
suitable provisions should be made to ensure adequate 
soil moisture around the anodes. Increasing the 
number of impressed current anodes or increasing the 
surface area of the special backfill materials may 
further reduce the electroosmotic effect. 

7.7 Design Drawings and Specifications 

7.7.1 Suitable drawings should be prepared to 
designate the overall layout of the piping to be 
protected and the location of significant items of 
structure hardware, corrosion control test stations, 
electrical bonds, electrical isolation devices, and 
neighboring buried or submerged metallic structures. 

7.7.2 Layout drawings should be prepared for each 
impressed current CP installation, showing the details 
and location of the components of the CP system with 
respect to the protected structure(s) and to major 
physical landmarks. These drawings should include 
right-of-way information. 

7.7.3 The locations of galvanic anode installations 
should be recorded on drawings or in tabular form, with 
appropriate notes on anode type, weight, spacing, 
depth, and backfill. 

7.7.4 Specifications should be prepared for all 
materials and installation practices that are to be 
incorporated in construction of the CP system. 



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Bibliography for Section 7 



Benedict. R.L., ed. Anode Resistance Fundamentals and 
Applications — Classic Papers and Reviews. Houston, 
TX: NACE, 1986. 



Kurr, G.W. "Zinc Anodes — Underground Uses for Cathodic 
Protection and Grounding." MP 18, 4 (1979): pp. 34- 
41. 



Baboian, R., P.P. Drew, and K. Kawate. "Design of 
Platinum Clad Wire Anodes for Impressed Current 
Protection." Materials Performance 23, 9 (1984): pp. 
31-35. 

Collected Papers on Cathodic Protection Current 
Distribution. Houston, TX: NACE, 1989. 

Doremus, G., and J.G. Davis. "IVIarine Anodes: The Old 
and New — Cathodic Protection for Offshore 
Structures." Materials Performance 6, 1 (1967): p. 30. 

Dwight, H.B. "Calculations for Resistance to Ground." 
Electrical Engineering 55 (1936): p. 1319. 

George P.P., J.J. Newport, and J.L. Nichols. "A High 
Potential Magnesium Anode." Corrosion ^2, 12 (1956): 
p. 51. 

Jacobs, J. A. "A Comparison of Anodes for Impressed 
Current Systems." NACE Canadian Region Western 
Conference, Edmonton, Alberta, Canada, February 
1980. 



NACE Publication 2B160 (withdrawn). "Use of High Silicon 
Cast Iron for Anodes." Houston, TX: NACE. 

NACE Publication 2B156 (withdrawn). "Final Report on 
Four Annual Anode Inspections." Houston, TX: NACE. 

Parker, IVI.E. Pipe Line Corrosion and Cathodic 
Protection — A Field IVIanual. Houston, TX: Gulf 
Publishing Company, 1962. 

Robinson, H.A., and P.P. George. "Effect of Alloying and 
Impurity Elements in IVIagnesium Cast Alloy Anodes." 
Corrosion ^0, 6 (1954): p. 182. 

Rudenberg, R. "Grounding Principles and Practices." 
Electrical Engineering 64 (1 945): p. 1 . 

Schreiber, C.F., and G.L. IVIussinelli. "Characteristics and 
Performance of the LIDA Impressed-Current System in 
Natural Waters and Saline Muds." CORROSION/86, 
paper no. 287. Houston, TX: NACE, 1986. 

Sunde, E.D.. Earth Conduction Effects in Transmission 
Systems. New York, NY: Dover Publications, 1968. 



Section 8: Installation of CP Systems 



8.1 Introduction 

8.1 .1 This section recommends procedures that will 
result in the installation of CP systems that achieve 
protection of the structure. The design considerations 
recommended in Sections 4 and 7 should be followed. 

8.2 Construction Specifications 

8.2.1 All construction work on CP systems should be 
performed in accordance with construction drawings 
and specifications. The construction specifications 
should be in accordance with recommended practices 
in Sections 4 and 7. 

8.3 Construction Supervision 

8.3.1 All construction work on CP systems should be 
performed under the surveillance of trained and 
qualified personnel to verify that the installation is in 
strict accordance with the drawings and specifications. 
Exceptions may be made only with the approval of 
qualified personnel responsible for external corrosion 
control. 



8.3.2 All deviations from construction specifications 
should be noted on as-built drawings. 

8.4 Galvanic Anodes 

8.4.1 Inspection, Handling, and Storage 

8.4.1.1 Packaged anodes should be inspected 
and steps taken to ensure that backfill material 
completely surrounds the anode. The individual 
container for the backfill material and anode 
should be intact. If individually packaged anodes 
are supplied in waterproof containers, the 
containers must be removed before installation. 
Packaged anodes should be kept dry during 
storage. 

8.4.1.2 Lead wire must be securely connected to 
the anode. Lead wire should be inspected for 
assurance that it is not damaged. 

8.4.1.3 Other galvanic anodes, such as the 
unpackaged "bracelet" or ribbon type, should be 
inspected to ensure that dimensions conform to 



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design specifications and that any damage during 
iiandling does not affect application. If a coating is 
used on bands and the inner side of bracelet 
anode segments, it should be inspected and, if 
damaged, repaired before the anodes are 
installed. 

8.4.2 Installing Anodes 

8.4.2.1 Anodes should be installed according to 
construction specifications. 

8.4.2.2 Packaged galvanic anodes should be 
backfilled with appropriately compacted material. 
When anodes and special chemical backfill are 
provided separately, anodes should be centered in 
special backfill, which should be compacted prior 
to backfilling. Care should be exercised during all 
operations so that lead wires and connections are 
not damaged. Sufficient slack should exist in lead 
wires to avoid strain. 

8.4.2.3 When anodes in bracelet form are used, 
external pipe coating beneath the anode should be 
free of holidays. Care should be taken to prevent 
damage to the external coating when bracelet 
anodes are installed. After application of concrete 
(if used) to pipe, all coating and concrete should 
be removed from the anode surface. If reinforced 
concrete is used, there must be no metallic 
contact between the anode and the reinforcing 
mesh or between the reinforcing mesh and the 
pipe. 

8.4.2.4 When a ribbon-type anode is used, it can 
be trenched or plowed in, with or without special 
chemical backfill as required, generally parallel to 
the section of pipeline to be protected. 

8.5 Impressed Current Systems 

8.5.1 Inspection and Handling 

8.5.1 .1 The rectifier or other power source should 
be inspected to ensure that internal connections 
are mechanically secure and that the unit is free of 
damage. Rating of the DC power source should 
comply with the construction specification. Care 
should be exercised in handling and installing the 
power source. 

8.5.1.2 Impressed current anodes should be 
inspected for conformance to specifications 
concerning anode material, size, length of lead 
cable, anode lead connection, and integrity of seal. 
Care should be exercised to avoid cracking or 
damaging anodes during handling and installation. 

8.5.1.3 All cables should be carefully inspected to 
detect defects in insulation. Care should be taken 
to avoid damage to cable insulation. Defects in 
the cable insulation must be repaired. 



8.5.1.4 Anode backfill material should conform to 
specifications. 

8.5.2 Installation Provisions 

8.5.2.1 A rectifier or other power source should be 
installed so that the possibility of damage or 
vandalism is minimized. 

8.5.2.2 Wiring to rectifiers shall comply with local 
and national electrical codes and requirements of 
the utility supplying power. An external disconnect 
switch should be provided in the AC circuit. A 
rectifier case shall be properly grounded. 

8.5.2.3 On thermoelectric generators, a reverse 
current device should be installed to prevent 
galvanic action between the anode bed and the 
pipe if the flame is extinguished. 

8.5.2.4 Impressed current anodes can be buried 
vertically, horizontally, or in deep holes (see NACE 
Standard RP0572^) as indicated in construction 
specifications. Backfill material should be installed 
to ensure that there are no voids around anodes. 
Care should be exercised during backfilling to 
avoid damage to the anode and cable. 

8.5.2.5 The cable from the rectifier negative 
terminal to the pipe should be connected to the 
pipe as described in Paragraph 8.6. Cable 
connections to the rectifier must be mechanically 
secure and electrically conductive. Before the 
power source is energized, it must be verified that 
the negative cable is connected to the structure to 
be protected and that the positive cable is 
connected to the anodes. After the DC power 
source has been energized, suitable 
measurements should be made to verify that these 
connections are correct. 

8.5.2.6 Underground splices on the header 
(positive) cable to the groundbed should be kept to 
a minimum. Connections between the header and 
anode cables should be mechanically secure and 
electrically conductive. If buried or submerged, 
these connections must be sealed to prevent 
moisture penetration so that electrical isolation 
from the environment is ensured. 

8.5.2.7 Care must be taken during installation of 
direct-burial cable to the anodes (positive cable) to 
avoid damage to insulation. Sufficient slack 
should be left to avoid strain on all cables. Backfill 
material around the cable should be free of rocks 
and foreign matter that might cause damage to the 
insulation when the cable is installed in a trench. 
Cable can be installed by plowing if proper 
precautions are taken. 

8.5.2.8 If insulation integrity on the buried or 
submerged header cable, including splices, is not 



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maintained, this cable may fail because of 
corrosion. 



8.6 Corrosion Control Test Stations, 
Bonds (see Paragraph 4.5) 



Connections, and 



8.6.1 Pipe and test lead wires should be clean, dry, 
and free of foreign materials at points of connection 
when the connections are made. Connections of test 
lead wires to the pipe must be installed so they will 
remain mechanically secure and electrically 
conductive. 

8.6.2 All buried or submerged lead-wire attachments 
should be coated with an electrically insulating 
material, compatible with the external pipe coating and 
wire insulation. 

8.6.3 Test lead wires should be color coded or 
othen/vise permanently identified. Wires should be 



installed with slack. Damage to insulation should be 
avoided and repairs made if damage occurs. Test 
leads should not be exposed to excessive heat and 
sunlight. Aboveground test stations are preferred. If 
test stations are flush with the ground, adequate slack 
should be provided within the test station to facilitate 
test connections. 

8.6.4 Cable connections at bonds to other structures 
or across isolating joints should be mechanically 
secure, electrically conductive, and suitably coated. 
Bond connections should be accessible for testing. 

8.7 Electrical Isolation 

8.7.1 Inspection and electrical measurements should 
ensure that electrical isolation is adequate (see NACE 
SP0286^). 



Section 9: Control of Interference Currents 



9.1 Introduction 

9.1 .1 This section recommends practices for the 
detection and control of interference currents. The 
mechanism and its detrimental effects are described. 

9.2 IVIechanism of Interference-Current Corrosion (Stray- 
Current Corrosion) 

9.2.1 Interference-current corrosion on buried or 
submerged metallic structures differs from other 
causes of corrosion damage in that the direct current, 
which causes the corrosion, has a source foreign to the 
affected structure. Usually the interfering current is 
collected from the electrolyte by the affected structure 
from a DC source not metallically bonded to the 
affected structure. 

9.2.1.1 Detrimental effects of interference 
currents usually occur at locations where the 
currents transfer between the affected structures 
and the electrolyte. 

9.2.1.2 Structures made of amphoteric metals 
such as aluminum and lead may be subject to 
corrosion damage from a buildup of alkalinity at or 
near the metal surface collecting interference 
currents. 

9.2.1 .3 Coatings may become disbonded at areas 
where voltage gradients in the electrolyte force 
current onto the affected structure. However, as 
the external coating becomes disbonded, a larger 
area of metal may be exposed, which would 
increase the demand for a CP current. This 
disbondment may create shielding problems. 



9.2.2 The severity of external corrosion resulting from 
interference currents depends on several factors: 

9.2.2.1 Separation and routing of the interfering 
and affected structures and location of the 
interfering current source; 

9.2.2.2 IVIagnitude and density of the current; 

9.2.2.3 Quality of the external coating or absence 
of an external coating on the structures involved; 
and 

9.2.2.4 Presence and location of mechanical 
joints having high electrical resistance. 

9.2.3 Typical sources of interference currents include 
the following: 

9.2.3.1 Direct current: CP rectifiers, 
thermoelectric generators, DC electrified railway 
and transit systems, coal mine haulage systems 
and pumps, welding machines, and other DC 
power systems; 

9.2.3.2 Alternating current: AC power systems 
and AC electrified railway systems; and 

9.2.3.3 Telluric current. 

9.3 Detection of Interference Currents 

9.3.1 During external corrosion control surveys, 
personnel should be alert for electrical or physical 
observations that could indicate interference from a 
foreign source such as the following: 



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9.3.1.1 Pipe-electrolyte potential changes on the 
affected structure caused by the foreign DC 
source; 

9.3.1.2 Changes in the line current magnitude or 
direction caused by the foreign DC source; 

9.3.1.3 Localized pitting in areas near or 
immediately adjacent to a foreign structure; and 

9.3.1 .4 Damage to external coatings in a localized 
area near an anode bed or near any other source 
of stray direct current. 

9.3.2 In areas in which interference currents are 
suspected, appropriate tests should be conducted. All 
affected parties shall be notified before tests are 
conducted. Notification should be channeled through 
corrosion control coordinating committees, when they 
exist (see NACE Publication TPC 11^). Any one or a 
combination of the following test methods can be used. 

9.3.2.1 Measurement of structure-electrolyte 
potentials with recording or indicating instruments; 

9.3.2.2 Measurement of current flowing on the 
structure with recording or indicating instruments; 

9.3.2.3 Development of beta curves to locate the 
area of maximum current discharge from the 
affected structure (see Appendix A); and 

9.3.2.4 Measurement of the variations in current 
output of the suspected source of interference 
current and correlations with measurements 
obtained in Paragraphs 9.3.2.1 and 9.3.2.2. 

9.4 Methods for Mitigating Interference Corrosion Problems 

9.4.1 Interference problems are individual in nature 
and the solution should be mutually satisfactory to the 
parties involved. These methods may be used 
individually or in combination. 

9.4.2 Design and installation of electrical bonds of 
proper resistance between the affected structures is a 
technique for interference control. The bond electrically 
conducts interference current from an affected 
structure to the interfering structure or current source. 

9.4.2.1 Unidirectional control devices, such as 
diodes or reverse current switches, may be 
required in conjunction with electrical bonds if 



fluctuating currents are present, 
prevent reversal of current flow. 



These devices 



9.4.2.2 A resistor may be necessary in the bond 
circuit to control the flow of electrical current from 
the affected structure to the interfering structure. 

9.4.2.3 The attachment of electrical bonds can 
reduce the level of CP on the interfering structure. 
Supplementary CP may then be required on the 
interfering structure to compensate for this effect. 

9.4.2.4 A bond may not effectively mitigate the 
interference problem in the case of a cathodically 
protected bare or poorly externally coated pipeline 
that is causing interference on an externally 
coated pipeline. 

9.4.3 CP current can be applied to the affected 
structure at those locations at which the interfering 
current is being discharged. The source of CP current 
may be galvanic or impressed current anodes. 

9.4.4 Adjustment of the current output from interfering 
CP rectifiers may resolve interference problems. 

9.4.5 Relocation of the groundbeds of cathodic 
protection rectifiers can reduce or eliminate the pickup 
of interference currents on nearby structures. 

9.4.6 Rerouting of proposed pipelines may avoid 
sources of interference current. 

9.4.7 Properly located isolating fittings in the affected 
structure may reduce or resolve interference problems. 

9.4.8 Application of external coating to current pick-up 
area(s) may reduce or resolve interference problems. 

9.5 Indications of Resolved Interference Problems 

9.5.1 Restoration of the structure-electrolyte potentials 
on the affected structure to those values that existed 
prior to the interference. 

9.5.2 Measured line currents on the affected structure 
that show that the interference current is not being 
discharged to the electrolyte. 

9.5.3 Adjustment of the slope of the beta curve to 
show that current discharge has been eliminated at the 
location of maximum exposure (see Appendix A). 



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SP01 69-2007 



Section 10: Operation and l\/laintenance of CP Systems 



10.1 Introduction 

10.1.1 This section recommends procedures and 
practices for energizing and maintaining continuous, 
effective, and efficient operation of CP systems. 

10.1.1.1 Electrical measurements and inspection 
are necessary to determine that protection has 
been established according to applicable criteria 
and that each part of the CP system is operating 
properly. Conditions that affect protection are 
subject to change. Correspondingly, changes may 
be required in the CP system to maintain 
protection. Periodic measurements and 
inspections are necessary to detect changes in the 
CP system. Conditions in which operating 
experience indicates that testing and inspections 
need to be made more frequently than 
recommended herein may exist. 

10.1.1.2 Care should be exercised in selecting the 
location, number, and type of electrical 
measurements used to determine the adequacy of 
CP. 

10.1.1.3 When practicable and determined 
necessary by sound engineering practice, a 
detailed (close-interval) potential survey should be 
conducted to: 

(a) assess the effectiveness of the CP system; 

(b) provide base line operating data; 

(c) locate areas of inadequate protection levels; 

(d) identify locations likely to be adversely affected 
by construction, stray currents, or other unusual 
environmental conditions; or 

(e) select areas to be monitored periodically. 

10.1.1.4 Adjustments to a CP system should be 
accompanied by sufficient testing to assure the 
criteria remain satisfied and to reassess 
interference to other structures or isolation points. 

10.2 A survey should be conducted after each CP system 
is energized or adjusted to determine whether the 
applicable criterion or criteria from Section 6 have been 
satisfied. 

10.3 The effectiveness of the CP system should be 
monitored annually. Longer or shorter intervals for 
monitoring may be appropriate, depending on the variability 
of CP factors, safety considerations, and economics of 
monitoring. 



10.4 Inspection and tests of CP facilities should be made 
to ensure their proper operation and maintenance as 
follows: 

10.4.1 All sources of impressed current should be 
checked at intervals of two months. Longer or shorter 
intervals for monitoring may be appropriate. Evidence 
of proper functioning may be current output, normal 
power consumption, a signal indicating normal 
operation, or satisfactory CP levels on the pipe. 

10.4.2 All impressed current protective facilities 
should be inspected annually as part of a preventive 
maintenance program to minimize in-service failure. 
Longer or shorter intervals for monitoring may be 
appropriate. Inspections may include a check for 
electrical malfunctions, safety ground connections, 
meter accuracy, efficiency, and circuit resistance. 

10.4.3 Reverse current switches, diodes, interference 
bonds, and other protective devices, whose failures 
would jeopardize structure protection, should be 
inspected for proper functioning at intervals of two 
months. Longer or shorter intervals for monitoring may 
be appropriate. 

10.4.4 The effectiveness of isolating fittings, continuity 
bonds, and casing isolation should be evaluated during 
the periodic surveys. This may be accomplished by 
electrical measurements. 

10.5 When pipe has been uncovered, it should be 
examined for evidence of external corrosion and, if 
externally coated, for condition of the external coating. 

10.6 The test equipment used for obtaining each electrical 
value should be of an appropriate type. Instruments and 
related equipment should be maintained in good operating 
condition and checked for accuracy. 

10.7 Remedial measures should be taken when periodic 
tests and inspections indicate that CP is no longer 
adequate. These measures may include the following: 

10.7.1 Repair, replace, or adjust components of CP 
systems; 

10.7.2 Provide supplementary facilities in which 
additional CP is necessary; 

10.7.3 Thoroughly clean and properly coat bare 
structures if required to attain CP; 

10.7.4 Repair, replace, or adjust continuity and 
interference bonds; 

1 0.7.5 Remove accidental metallic contacts; and 



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SP01 69-2007 



1 0.7.6 Repair defective isolating devices. 

1 0.8 An electrical short circuit between a casing and carrier 
pipe can result in inadequate CP of the pipeline outside the 
casing due to reduction of protective current to the pipeline. 

10.8.1 When a short results in inadequate CP of the 
pipeline outside the casing, steps must be taken to 
restore CP to a level required to meet the CP criterion. 
These steps may include eliminating the short between 
the casing and carrier pipe, supplementing CP, or 



improving the quality of the external coating on the 
pipeline outside the casing. None of these steps will 
ensure that external corrosion will not occur on the 
carrier pipe inside the casing; however, a shorted 
casing does not necessarily result in external corrosion 
of the carrier pipe inside the casing. 

10.9 When the effects of electrical shielding of CP current 
are detected, the situation should be evaluated and 
appropriate action taken. 



Section 1 1 : External Corrosion Control Records 



11.1 Introduction 

11.1.1 This section describes external corrosion 
control records that will document in a clear, concise, 
workable manner data that are pertinent to the design, 
installation, operation, maintenance, and effectiveness 
of external corrosion control measures. 

11.2 Relative to the determination of the need for external 
corrosion control, the following should be recorded: 

11.2.1 Corrosion leaks, breaks, and pipe 
replacements; and 

11.2.2 Pipe and external coating condition observed 
when a buried structure is exposed. 

11.3 Relative to structure design, the following should be 
recorded: 

11.3.1 External coating material and application 
specifications; and 

11.3.2 Design and location of isolating devices, test 
leads and other test facilities, and details of other 
special external corrosion control measures taken. 

11.4 Relative to the design of external corrosion control 
facilities, the following should be recorded: 

1 1 .4.1 Results of current requirement tests; 

1 1 .4.2 Results of soil resistivity surveys; 

1 1 .4.3 Location of foreign structures; and 

11.4.4 Interference tests and design of interference 
bonds and reverse current switch installations. 



11.4.4.2 Record of interference tests conducted, 
including location of tests, name of company 
involved, and results. 

1 1 .5 Relative to the installation of external corrosion control 
facilities, the following should be recorded: 

1 1 .5.1 Installation of CP facilities: 

1 1 .5.1 .1 Impressed current systems: 

11.5.1.1.1 Location and date placed in 
service; 

1 1 .5.1 .1 .2 Number, type, size, depth, backfill, 
and spacing of anodes; 

11.5.1.1.3 Specifications of rectifier or other 
energy source; and 

1 1 .5.1 .1 .4 Cable size and type of insulation. 

11.5.1.2 Galvanic anode systems: 

11.5.1.2.1 Location and date placed in 
service; 

11.5.1.2.2 Number, type, size, backfill, and 
spacing of anodes; and 

1 1 .5.1 .2.3 Wire size and type of insulation. 

1 1 .5.2 Installation of interference mitigation facilities: 

1 1 .5.2.1 Details of interference bond installation: 

11.5.2.1.1 Location and name of company 
involved; 



11.4.4.1 Scheduling of interference tests, 
correspondence with corrosion control 
coordinating committees, and direct 

communication with the concerned companies. 



11.5.2.1.2 Resistance value or other 
pertinent information; and 

11.5.2.1.3 Magnitude and polarity of 
drainage current. 



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SP01 69-2007 



1 1 .5.2.2 Details of reverse current switch: 

1 1 .5.2.2.1 Location and name of companies; 

11.5.2.2.2 Type of switcii or equivalent 
device; and 



11.7.1.2 Repair or replacement of anodes, 
connections, wires, and cables. 

1 1 .7.2 IVIaintenance of interference bonds and reverse 
current switches: 



11.5.2.2.3 Data showing effective operating 
adjustment. 

1 1 .5.2.3 Details of other remedial measures. 

11.6 Records of surveys, inspections, and tests should be 
maintained to demonstrate that applicable criteria for 
interference control and CP have been satisfied. 



1 1 .7.2.1 Repair of interference bonds; and 

11.7.2.2 Repair of reverse current switches or 
equivalent devices. 

11.7.3 IVIaintenance, repair, and replacement of 
external coating, isolating devices, test leads, and other 
test facilities. 



11.7 Relative to the maintenance of external corrosion 
control facilities, the following information should be 
recorded: 

1 1 .7.1 IVIaintenance of CP facilities: 

11.7.1.1 Repair of rectifiers and other DC power 
sources; and 



11.8 Records sufficient to demonstrate the evaluation of 
the need for and the effectiveness of external corrosion 
control measures should be maintained as long as the 
facility involved remains in service. Other related external 
corrosion control records should be retained for such a 
period that satisfies individual company needs. 



References 



1. NACE SP0572 (latest revision), "Design, Installation, 
Operation, and IVIaintenance of Impressed Current Deep 
Anode Beds" (Houston, TX: NACE). 

2. NACE Standard RP0177 (latest revision), "IVIitigation of 
Alternating Current and Lightning Effects on IVIetallic 
Structures and Corrosion Control Systems" (Houston, TX: 
NACE). 

3. NACE Standard RP0285 (latest revision), "Corrosion 
Control of Underground Storage Tank Systems by Cathodic 
Protection" (Houston, TX: NACE). 

4. NACE SP0186 (latest revision), "Application of 
Cathodic Protection for Well Casings" (Houston, TX: 
NACE). 

5. NACE SP0286 (latest revision), "The Electrical 
Isolation of Cathodically Protected Pipelines" (Houston, TX: 
NACE). 

6. NACE SP0387 (latest revision), "IVIetallurgical and 
Inspection Requirements for Cast Galvanic Anodes for 
Offshore Applications" (Houston, TX: NACE). 

7. NACE SP0188 (latest revision), "Discontinuity (Holiday) 
Testing of Protective Coatings" (Houston, TX: NACE). 

8. NACE Publication TPC 1 1 (latest revision), "A Guide to 
the Organization of Underground Corrosion Control 
Coordinating Committees" (Houston, TX: NACE). 



9. NACE Standard TIVI0497 (latest revision), 
"IVIeasurement Techniques Related to Criteria for Cathodic 
Protection on Underground or Submerged Metallic Piping 
Systems" (Houston, TX: NACE). 

10. ANSI/AWWA C 203 (latest revision), "Standard for 
Coal-Tar Protective Coatings and Linings for Steel Water 
Pipelines — Enamel and Tape — Hot Applied" (Washington, 
DC: ANSI and Denver, CO: AWWA). 

11. NACE Standard RP0375 (latest revision), "Field- 
Applied Underground Coating Systems for Underground 
Pipelines: Application, Performance, and Ouality Control" 
(Houston, TX: NACE). 

12. ANSI/AWWA C 214 (latest revision), "Tape Coating 
Systems for the Exterior of Steel Water Pipelines" 
(Washington, DC: ANSI and Denver, CO: AWWA). 

13. ANSI/AWWA C 209 (latest revision), "Cold-Applied 
Tape Coatings for the Exterior of Special Sections, 
Connections, and Fittings for Steel Water Pipelines" 
(Washington, DC: ANSI and Denver: CO: AWWA). 

14. Ronald Bianchetti, ed., Peabody's Control of Pipeline 
Corrosion, 2nd ed. (Houston, TX: NACE, 2001). 

15. ANSI/AWWA C 213 (latest revision), "Fusion-Bonded 
Epoxy Coating for the Interior and Exterior of Steel Water 
Pipelines" (Washington, DC: ANSI and Denver: CO: 
AWWA). 



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SP01 69-2007 



16. API RP 5L7 (latest revision), "Recommended Practices 
for Unprimed Internal Fusion Bonded Epoxy Coating of Line 
Pipe" (Washington, DC: API). 

17. CSA Z245.20M (latest revision), "External Fusion Bond 
Epoxy Coated Steel Pipe" (Toronto, ON: CSA). 

18. NACE Standard RP0394 (latest revision), "Application, 
Performance, and Quality Control of Plant-Applied, Fusion- 
Bonded Epoxy External Pipe Coating" (Houston, TX: 
NACE). 

19. NACE Standard RP0185 (latest revision), "Extruded 
Polyolefin Resin Coating Systems with Soft Adhesives for 
Underground or Submerged Pipe" (Houston, TX: NACE). 

20. DIN 30 670 (latest revision), "Polyethylene-Coatings for 
Steel Pipes and Fittings Requirements and Testing" (Berlin, 
Germany: DIN). 

21. ANSI/AWWA C 215 (latest revision), "Extruded 
Polyolefin Coatings for the Exterior of Steel Water Pipe 
Lines" (Washington, DC: ANSI and Denver, CO: AWWA). 

22. ASTM G 128 (latest revision), "Standard Guide for 
Control Of Hazards And Risks In Oxygen Enriched 
Systems" (West Conshohocken, PA: ASTM). 

23. NACE Standard RP0274 (latest revision), "High- 
Voltage Electrical Inspection of Pipeline Coatings Prior to 
Installation" (Houston, TX: NACE). 

24. ASTIVI G 8 (latest revision), "Standard Test IVIethod for 
Cathodic Disbonding of Pipeline Coatings" (West 
Conshohocken, PA: ASTIVI). 

25. ASTM G 19 (latest revision), "Standard Test Method for 
Disbonding Characteristics of Pipeline Coatings by Direct 
Soil Burial" (West Conshohocken, PA: ASTM). 

26. ASTM G 42 (latest revision), "Standard Test Method for 
Cathodic Disbonding of Pipeline Coatings Subjected to 
Elevated Temperatures" (West Conshohocken, PA: 
ASTM). 

27. ASTM G 95 (latest revision), "Test Method for Cathodic 
Disbondment Test of Pipeline Coatings (Attached Cell 
Method)" (West Conshohocken, PA: ASTM). 

28. ASTM G 9 (latest revision), "Standard Test Method for 
Water Penetration into Pipeline Coatings" (West 
Conshohocken, PA: ASTM). 

29. ASTM G 1 7 (latest revision), "Standard Test Method for 
Penetration Resistance of Pipeline Coatings (Blunt Rod)" 
(West Conshohocken, PA: ASTM). 

30. ASTM D 2240 (latest revision), "Standard Test Method 
for Rubber Property — Durometer Hardness" (West 
Conshohocken, PA: ASTM). 



31 . ASTM G 1 3 (latest revision), "Standard Test Method for 
Impact Resistance of Pipeline Coatings (Limestone Drop 
Test)" (West Conshohocken, PA: ASTM). 

32. ASTM G 14 (latest revision), "Standard Test Method for 
Impact Resistance of Pipeline Coatings (Falling Weight 
Test)" (West Conshohocken, PA: ASTM). 

33. M. Romanoff, Underground Corrosion (Houston, TX: 
NACE, 1989). 

34. ASTM D 427 (latest revision), "Standard Test Method 
for Shrinkage Factors of Soils by the Mercury Method" 
(West Conshohocken, PA: ASTM). 

35. ASTM D 543 (latest revision), "Standard Practices for 
Evaluating the Resistance of Plastics to Chemical 
Reagents" (West Conshohocken, PA: ASTM). 

36. Federal Test Standard No. 406A, Method 701 1 (latest 
revision), "Test Method for Resistance of Plastics to 
Chemical Reagents" (Washington, DC: GSA). 

37. ASTM G 20 (latest revision), "Standard Test Method for 
Chemical Resistance of Pipeline Coatings" (West 
Conshohocken, PA: ASTM). 

38. ASTM D 2304 (latest revision), "Standard Test Method 
for Thermal Endurance of Rigid Electrical Insulating 
Materials" (West Conshohocken, PA: ASTM). 

39. ASTM D 2454 (latest revision), "Standard Practice for 
Determining the Effect of Overbaking on Organic Coatings" 
(West Conshohocken, PA: ASTM). 

40. ASTM D 2485 (latest revision), "Standard Test Methods 
for Evaluating Coatings for High-Temperature Service" 
(West Conshohocken, PA: ASTM). 

41 . ASTM G 1 8 (latest revision), "Standard Test Method for 
Joints, Fittings, and Patches in Coated Pipelines" (West 
Conshohocken, PA: ASTM). 

42. ASTM G 55 (latest revision), "Standard Test Method for 
Evaluating Pipeline Coating Patch Materials" (West 
Conshohocken, PA: ASTM). 

43. ASTM G 21 (latest revision), "Standard Practice for 
Determining Resistance of Synthetic Polymetric Materials 
To Fungi" (West Conshohocken, PA: ASTM). 

44. Federal Test Standard No. 406A, Method 6091 (latest 
revision), "Test Method for Mildew Resistance of Plastics by 
Mixed Culture Method (Agar Medium)" (Washington, DC: 
GSA). 

45. ASTM G 1 1 (latest revision), "Standard Test Method for 
Effects of Outdoor Weathering on Pipeline Coatings" (West 
Conshohocken, PA: ASTM). 



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SP01 69-2007 

46. ASTM G 6 (latest revision), "Standard Test IVIethod for 
Abrasion Resistance of Pipeline Coatings" (West 
Conshohocken, PA: ASTM). 



47. ASTM G 1 (latest revision), "Standard Test Method for 
Specific Bendability of Pipeline Coatings" (West 
Conshohocken, PA: ASTM). 

48. ASTM D 2197 (latest revision), "Test Method for 
Adhesion of Organic Coatings by Scrape Adhesion" (West 
Conshohocken, PA: ASTM). 



Appendix A — Interference Testing 



A beta curve is a plot of dynamic (fluctuating) interference 
current or related proportional voltage (ordinate) versus 
values of corresponding structure-to-soil potentials at a 
selected location on the affected structure (abscissa). If the 
correlation is reasonably linear, the plot will indicate whether 
the affected structure is receiving or discharging current at 
the location where the structure-to-soil potential was 
measured. Dynamic interference investigation involves 



many beta curve plots to search for the point of maximum 
interference-current discharge. Interference is resolved 
when the correlation of maximum current discharge has 
been changed to a correlation that shows that current 
pickup is being achieved in the exposure area by the 
corrective measures taken. These corrective measures 
may be accomplished by metallic bonding or other 
interference control techniques. 



Appendix B — l\/letliod for Determining Probable Corrosion Rate and Costs of Maintaining Service 



Maintenance of a piping system may include repairing 
corrosion leaks and reconditioning or replacing all or 
portions of the system. 

In order to make estimates of the costs involved, it is 
necessary to determine the probability of corrosion or the 
rate at which corrosion is proceeding. The usual methods 
of predicting the probability or rate of corrosion are as 
follows: 

(a) Study of corrosion history on the piping system in 
question or on other systems of the same material in the 
same general area or in similar environments. Cumulative 
leak-frequency curves are valuable in this respect. 

(b) Study of the environment surrounding a piping system: 
resistivity, pH, and composition. Redox potential tests may 
also be used to a limited extent. Once the nature of the 
environment has been determined, the probable 
corrosiveness is estimated by reference to actual corrosion 
experience on similar metallic structures, when 
environmental conditions are similar. Consideration of 



possible environmental changes such as might result from 
irrigation, spillage of corrosive substances, pollution, and 
seasonal changes in soil moisture content should be 
included in such a study. 

(c) Investigation for corrosion on a piping system by visual 
inspection of the pipe or by instruments that mechanically or 
electrically inspect the condition of the pipe. Condition of 
the piping system should be carefully determined and 
recorded each time a portion of the line is excavated for any 
reason. 

(d) Maintenance records detailing leak locations, soil 
studies, structure-to-electrolyte potential surveys, surface 
potential surveys, line current studies, and wall thickness 
surveys used as a guide for locating areas of maximum 
corrosion. 

(e) Statistical treatment of available data. 

(f) Results of pressure testing. Under certain conditions, 
this may help to determine the existence of corrosion. 



Appendix C— Contingent Costs of Corrosion 



In addition to the direct costs that result from corrosion, 
contingent costs include: 

(a) Public liability claims; 

(b) Property damage claims; 



(c) Damage to natural facilities, such as municipal or 
irrigation water supplies, forests, parks, and scenic areas; 

(d) Cleanup of product lost to surroundings; 

(e) Plant shutdown and startup costs; 



28 



NACE International 



(f) Cost of lost product; 

(g) Loss of revenue through interruption of service; 



SP01 69-2007 

(h) Loss of contract or goodwill through interruption of 
service; and 

(i) Loss of reclaim or salvage value of piping system. 



Appendix D— Costs of Corrosion Control 



The usual costs for protecting buried or submerged metallic 
structures are for complete or partial CP or for external 
coatings supplemented with cathodic protection. Other 
corrosion control costs Include: 

(a) Relocation of piping to avoid known corrosive 
conditions (this may include installing lines above ground); 

(b) Reconditioning and externally coating the piping 
system ; 



(c) Use of CO rros ion -resistant materials; 

(d) Use of selected or Inhibited backfill; 

(e) Electrical Isolation to limit possible galvanic action; and 

(f) Correction of conditions in or on the pipe that might 
accelerate corrosion. 



NACE International 



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NACE SP01 77-2007 

j>*«. (formerly RP0177) 

/■T% Item No. 21021 



V|%' 



NACE 

INTERNATIONAL 



Standard Practice 

Mitigation of Alternating Current and Lightning Effects 
on Metallic Structures and Corrosion Control Systems 

This NACE International standard represents a consensus of those individual members who have 
reviewed this document, its scope, and provisions. Its acceptance does not in any respect 
preclude anyone, whether he or she has adopted the standard or not, from manufacturing, 
marketing, purchasing, or using products, processes, or procedures not in conformance with this 
standard. Nothing contained in this NACE International standard is to be construed as granting 
any right, by implication or othen/vise, to manufacture, sell, or use in connection with any method, 
apparatus, or product covered by Letters Patent, or as indemnifying or protecting anyone against 
liability for infringement of Letters Patent. This standard represents minimum requirements and 
should in no way be interpreted as a restriction on the use of better procedures or materials. 
Neither is this standard intended to apply in all cases relating to the subject. Unpredictable 
circumstances may negate the usefulness of this standard in specific instances. NACE 
International assumes no responsibility for the interpretation or use of this standard by other 
parties and accepts responsibility for only those official NACE International interpretations issued 
by NACE International in accordance with its governing procedures and policies which preclude 
the issuance of interpretations by individual volunteers. 

Users of this NACE International standard are responsible for reviewing appropriate health, safety, 
environmental, and regulatory documents and for determining their applicability in relation to this 
standard prior to its use. This NACE International standard may not necessarily address all 
potential health and safety problems or environmental hazards associated with the use of 
materials, equipment, and/or operations detailed or referred to within this standard. Users of this 
NACE International standard are also responsible for establishing appropriate health, safety, and 
environmental protection practices, in consultation with appropriate regulatory authorities if 
necessary, to achieve compliance with any existing applicable regulatory requirements prior to the 
use of this standard. 

CAUTIONARY NOTICE: NACE International standards are subject to periodic review, and may be 
revised or withdrawn at any time in accordance with NACE technical committee procedures. 
NACE International requires that action be taken to reaffirm, revise, or withdraw this standard no 
later than five years from the date of initial publication and subsequently from the date of each 
reaffirmation or revision. The user is cautioned to obtain the latest edition. Purchasers of NACE 
International standards may receive current information on all standards and other NACE 
International publications by contacting the NACE International First Service Department, 1440 
South Creek Dr., Houston, Texas 77084-4906 (telephone +1 281/228-6200). 

Revised 2007-06-22 

Reaffirmed 2000-09-1 9 

Revised March 1995 

Revised July 1983 

Approved July 1977 

NACE International 

1440 South Creek Drive 

Houston, Texas 77084-4906 

-1-1 (281)228-6200 

ISBN 1-57590-116-1 
©2007, NACE International 



SP01 77-2007 



Foreword 

This standard practice presents guidelines and procedures for use during design, construction, 
operation, and maintenance of metallic structures and corrosion control systems used to mitigate 
the effects of lightning and alternating current (AC) power transmission systems. This standard is 
not intended to supersede or replace existing electrical safety standards. As shared right-of-way 
and utility corridor practices become more common, AC influence on adjacent metallic structures 
has greater significance, and personnel safety becomes of greater concern. This standard 
addresses problems primarily caused by proximity of metallic structures to AC-powered 
transmission systems. 

The hazards of lightning and AC effects on aboveground pipelines, while strung along the right-of- 
way prior to installation in the ground, are of particular importance to pipeline construction crews. 
The effects of AC power lines on buried pipelines are of particular concern to operators of 
aboveground appurtenances and cathodic protection (CP) testers, CP designers, safety engineers, 
as well as maintenance personnel working on the pipeline. 

Some controversy arose in the 1995 issue of this standard regarding the shock hazard stated in 
Section 5, Paragraph 5.2.1.1 and elsewhere in this standard. The reason for a more conservative 
value is that early work by George Bodier^ at Columbia University and by other investigators has 
shown that the average hand-to-hand or hand-to-foot resistance for an adult male human body can 
range between 600 ohms and 10,000 ohms. A reasonable safe value for the purpose of 
estimating body currents is 1,500 ohms hand-to-hand or hand-to-foot. In other work by C.F. 
Dalziel^ on muscular contraction, the inability to release contact occurs in the range of 6 to 20 mA 
for adult males. Ten mA hand-to-hand or hand-to-foot is generally established as the absolute 
maximum safe let-go current. Conservative design uses an even lower value. Fifteen volts of AC 
impressed across a 1,500-ohm load would yield a current flow of 10 mA; thus, the criterion within 
this standard is set at 15 volts. Prudent design would suggest an even lower value under certain 
circumstances. 

IVIany are now concerned with AC corrosion on buried pipelines adjacent to or near overhead 
electric transmission towers. This subject is not quite fully understood, nor is there an industry 
consensus on this subject. There are reported incidents of AC corrosion on buried pipelines under 
specific conditions, and there are also many case histories of pipelines operating under the 
influence of induced AC for many years without any reports of AC corrosion. The members of 
NACE Task Group (TG) 025 agreed that any criteria for AC corrosion control should not be 
included in this standard. However, the mitigation measures implemented for safety and system 
protection, as outlined in this standard, can also be used for AC corrosion control. 

This standard was originally published in July 1977 by Unit Committee T-10B on Interference 
Problems and was technically revised in 1983, 1995, 2000, and 2007. NACE continues to 
recognize the need for a standard on this subject. Future development and field experience 
should provide additional information, procedures, and devices for Specific Technology Group 
(STG) 05 on Cathodic/Anodic Protection to consider in future revisions of this standard. This 
standard was revised in 2007 by TG 025 on Alternating Current (AC) Power Systems, Adjacent: 
Corrosion Control and Related Safety Procedures to IVIitigate the Effects. This standard is issued 
by NACE under the auspices of STG 05. 



In NACE standards, the terms shall, must, should, and may are used in accordance with the 
definitions of these terms in the NACE Publications Style Manual, 4th ed.. Paragraph 7.4.1.9. Shall 
and must are used to state mandatory requirements. The term should is used to state something 
good and is recommended but is not mandatory. The term may is used to state something 
considered optional. 



NACE International 



SP01 77-2007 



NACE International 
Standard Practice 

Mitigation of Alternating Current and 

Lightning Effects on Metallic Structures 

and Corrosion Control Systems 

Contents 

1 . General 1 

2. Definitions 1 

3. Exposures and Effects of Alternating Current and Lightning 3 

4. Design Considerations for Protective Devices 5 

5. Personnel Protection 14 

6. AC and Corrosion Control Considerations 17 

7. Special Consideration in Operation and Maintenance of Cathodic Protection and Safety 

Systems 19 

8. References 20 

9. Bibliography 21 

10. Appendix A: Wire Gauge Conversions 22 

1 1 . Figure 1 : Approximate Current Required to Raise the Temperature of Stranded 
Annealed Soft-Drawn Copper Cable 684 °C (1 ,232°F) Above an Ambient Temperature 
of20°C (68 °F) 8 

1 2. Figure 2: Allowable Short-Circuit Currents for Insulated Copper Conductors 1 

13. Figure 3: Allowable Short-Circuit Currents for Insulated Copper Conductors 1 1 

14. Figure 4: Zinc Ribbon Ampacity 12 



NACE International 



SP01 77-2007 



Section 1 : General 



1.1 This standard presents acknowledged practices for the 
mitigation of AC and lightning effects on metallic structures 
and corrosion control systems. 

1 .2 This standard covers some of the basic procedures for 
determining the level of AC influence and lightning effects to 
which an existing metallic structure may be subjected and 
outlines design, installation, maintenance, and testing 
procedures for CP systems on structures subject to AC 
influence, primarily caused by proximity of metallic 
structures to AC power transmission systems. However, 
this standard is not intended to be a design guide or a "how- 
to" engineering manual to perform AC interference studies 
or mitigation designs. 



1.3 This standard does not designate procedures for any 
specific situation. The provisions of this standard should be 
applied under the direction of competent persons, who, by 
reason of knowledge of the physical sciences and the 
principles of engineering and mathematics, acquired by 
professional education and related practical experience, are 
qualified to engage in the practice of corrosion control on 
metallic structures. Such persons may be registered 
professional engineers or persons recognized as being 
qualified and certified as corrosion specialists by NACE, if 
their professional activities include suitable experience in 
corrosion control on metallic structures and AC interference 
and mitigation. 

1.4 This standard should be used in conjunction with the 
references contained herein. 



Section 2: Definitions 



2.1 Definitions presented in this standard pertain to the 
application of this standard only. Reference should be 
made to other industry standards when appropriate. 



Coupling: The association of two or more circuits or 
systems in such a way that energy may be transferred from 
one to another. 



AC Exposure: Alternating voltages and currents induced 
on a structure because of the AC power system. 



AC Power Structures: 

power systems. 



The structures associated with AC 



AC Power System: The components associated with the 
generation, transmission, and distribution of AC. 



Dead-Front Construction: A type of construction in which 
the energized components are recessed or covered to 
preclude the possibility of accidental contact with elements 
having electrical potential. 

Direct Current (DC) Decoupling Device: A device used 
in electrical circuits that allows the flow of AC in both 
directions and stops or substantially reduces the flow of DC. 



Affected Structure: Pipes, cables, conduits, or other 
metallic structures exposed to the effects of AC or lightning. 

Bond: A low-impedance connection (usually metallic) 
provided for electrical continuity. 



Eartli Current: Electric current flowing in the earth. 

Electric Field: One of the elementary energy fields in 
nature. It occurs in the vicinity of an electrically charged 
body. 



Breakdown Voltage: A voltage in excess of the rated 
voltage that causes the destruction of a barrier film, coating, 
or other electrically isolating material. 

Capacitive Coupling: The influence of two or more circuits 
upon one another, through a dielectric medium such as air, 
by means of the electric field acting between them. 

Circular IVIil: A unit of area of round wire or cable equal to 
the square of the diameter in mils (1 mil = 0.001 inch). 



Electric Potential: The voltage between a given point and 
a remote reference point. 

Electrolytic Grounding Cell: A device consisting of two or 
more buried electrodes installed at a fixed spacing, 
commonly made of zinc, and resistively coupled through a 
prepared backfill mixture. The electrical characteristics of a 
grounding cell include a small degree of resistance and a 
subsequent reduced voltage drop across the cell during a 
fault condition. 



Coating Stress Voltage: Potential difference between the 
metallic surface of a coated structure and the earth in 
contact with the outer surface of the coating. 



Fault Current: A current that flows from one conductor to 
ground or to another conductor due to an abnormal 
connection (including an arc) between the two. A fault 
current flowing to ground may be called a ground fault 
current. 



NACE International 



SP01 77-2007 



Fault Shield: Shallow grounding conductors connected to 
the affected structure adjacent to overhead electrical 
transmission towers, poles, substations, etc., to provide 
localized protection to the structure and coating during a 
fault event from nearby electric transmission power 
systems. 

Ground: An electrical connection to earth. 



Ground Current: 

grounding circuit. 



Current flowing to or from earth in a 



Grounded: Connected to earth or to some extensive 
conducting body that serves instead of the earth, whether 
the connection is intentional or accidental. 



Lumped Grounding: Localized grounding conductors, 
either shallow or deep, connected to the affected structure 
at strategic locations to provide protection to the structure 
and coating during steady-state and fault AC conditions 
from nearby electric transmission power systems. 

Magnetic Field: One of the elementary energy fields in 
nature. It occurs in the vicinity of a magnetic body or 
current-carrying medium. 

Over-Voltage Protector (Surge Arrester): A device that 
provides high resistance to DC and high impedance to AC 
under normal conditions within the specified DC and AC 
threshold rating and "closes" or has a very low resistance 
and impedance during upset conditions. 



Ground Electrode Resistance: The ohmic resistance 
between a grounding electrode and remote earth. 

Gradient Control Mat: A system of bare conductors 
connected to the affected structure and placed on or below 
the surface of the earth, usually at above grade or exposed 
appurtenances, arranged and interconnected to provide 
localized touch-and-step voltage protection. IVIetallic plates 
and grating of suitable area are common forms of ground 
mats, as well as conventional bare conductors closely 
spaced. 

Gradient Control Wire: A continuous and long grounding 
conductor or conductors installed horizontally and parallel to 
the affected structure at strategic lengths and connected at 
regular intervals to provide protection to the structure and 
coating during steady-state and fault AC conditions from 
nearby electric transmission power systems. 

Grounding Grid: A system of grounding electrodes 
consisting of interconnected bare conductors buried in the 
earth to provide a common electrical ground. 

Guarded: Covered, fenced, enclosed, or othen/vise 
protected by means of suitable covers or casings, barrier 
rails or screens, mats, or platforms; designed to limit the 
likelihood, under normal conditions, of dangerous approach 
or accidental contact by persons or objects. ^ 

Inductive Coupling: The influence of two or more circuits 
upon one another by means of changing magnetic flux 
linking them together. 

Lightning: An electric discharge that occurs in the 
atmosphere between clouds or between clouds and the 
earth. 

Load Current: The current in an AC power system under 
normal operating conditions. 



Polarization Cell: A DC decoupling device consisting of 
two or more pairs of inert metallic plates immersed in an 
aqueous electrolyte. The electrical characteristics of the 
polarization cell are high resistance to DC potentials and 
low impedance of AC. 

Potential: See Electric Potential. 

Potential Gradient: Change in the potential with respect to 
distance. 

Reclosing Procedure: A procedure that normally takes 
place automatically whereby the circuit breaker system 
protecting a transmission line, generator, etc., recloses one 
or more times after it has tripped because of abnormal 
conditions such as surges, faults, lightning strikes, etc. 

Reference Electrode: An electrode whose open-circuit 
potential is constant under similar conditions of 
measurement, which is used for measuring the relative 
potentials of other electrodes. 

Remote Earth: A location on the earth far enough from the 
affected structure that the soil potential gradients 
associated with currents entering the earth from the affected 
structure are insignificant. 

Resistive Coupling: The influence of two or more circuits 
on one another by means of conductive paths (metallic, 
semi-conductive, or electrolytic) between the circuits. 

Shock Hazard: A condition considered to exist at an 
accessible part in a circuit between the part and ground or 
other accessible part if the steady-state open-circuit AC 
voltage is 15 V or more (root mean square [rms]). For 
capacitive build-up situations, a source capacity of 5 mA or 
more is recognized as a hazardous condition. For short- 
circuit conditions, the permissible touch-and-step voltages 
should be determined in accordance with the methodology 
specified in accordance with lEEE'^' Standard 80.'' 



''' Institute of Electrical and Electronics Engineers (IEEE), Three Park Avenue, 1?"' Floor, New York: NY 10016-5997. 



NACE International 



SP01 77-2007 



Solid-State DC Decoupler: A dry type of DC decoupling 
device comprising solid-state electronics. The electrical 
characteristics of a solid-state decoupler are high resistance 
to low voltage DC and low impedance to AC. 

Step Potential or Voltage: The potential difference 
between two points on the earth's surface separated by a 
distance of one human step, which is defined as one meter, 
determined in the direction of maximum potential gradient. 



Stray Current: 

intended circuit. 



Current through paths other than the 



Surface Potential Gradient: Change in the potential on 
the surface of the ground with respect to distance. 



Switching Surge: The transient wave of potential and 
current in an electric system that results from the sudden 
change of current flow caused by a switching operation, 
such as the opening or closing of a circuit breaker. 

Touch Potential or Voltage: The potential difference 
between a metallic structure and a point on the earth's 
surface separated by a distance equal to the normal 
maximum horizontal reach of a human (approximately 1.0 
m [3.3 ft]). 

Voltage: The difference in electrical potential between two 
points. 



Section 3: Exposures and Effects of Alternating Current and Liglitning 



3.1 Introduction 

3.1 .1 This section outlines the physical phenomena by 
which AC, AC power systems, and lightning may affect 
metallic structures. 

3.2 Resistive Coupling (Electrolytic) 

3.2.1 Grounded structures of an AC power system 
share an electrolytic environment with other 
underground or submerged structures. Coupling 
effects may transfer AC energy to a metallic structure 
in the earth in the form of alternating current or 
potential. Whenever a power system with a grounded 
neutral has unbalanced conditions, current may flow in 
the earth. Substantial currents in the earth may result 
from phase-to-phase-to-ground or phase-to-ground 
faults. A metallic structure in the earth may carry part 
of this current. Also, a structure in the earth coated 
with a dielectric material may develop a significant AC 
potential across the coating. 

3.2.2 Resistive coupling is primarily a concern during a 
short-circuit condition on a power system, for example, 
when a large part of the current in a live conductor 
flows into the earth by means of the foundations and 
grounding system of a tower, pole, or substation. This 
current flow raises the electric potential of the earth 
near the structure, often to thousands of volts with 
respect to remote earth, and can result in a 
considerable stress voltage across the coating (see 
Paragraph 4.13) of a long metallic structure, such as a 
pipeline. This can lead to arcing that damages the 
coating, or even the structure itself. This difference in 
potential between the earth and the structure can 
represent an electric shock hazard. The effect of 
resistive coupling is usually concentrated in the vicinity 
of each of the first few power system poles or towers 
nearest the short-circuit location and near any 
substations involved in the short circuit. Under some 
circumstances, the electric potential of the structure 



may be raised enough to transfer hazardous potentials 
over considerable distances, particularly if the structure 
is well coated. Resistive coupling effects are strongly 
dependent on a number of factors, the most important 
of which are: 

(a) The total short-circuit current; 

(b) The power line overhead ground wire type and 
length back to the source; 

(c) The size of the foundations and grounding systems 
of the poles, towers, or substations through which the 
current is flowing; 

(d) The electrical resistivity of the soil as a function of 
depth; and 

(e) Separation distance between power systems and 
the affected metallic structure. 

The electrical layering of the soil alone can easily 
change resistive coupling effects by an order of 
magnitude or more. 

3.3 Capacitive Coupling 

3.3.1 The electric field associated with power 
conductors cause a well defined current to flow 
continuously between a nearby aboveground metallic 
structure and the earth, whether that aboveground 
structure is grounded or simply suspended in the air. 
This current flows from the structure to the earth 
partially through the air as a displacement current and 
partially through conductive or semi-conductive paths 
such as deliberate grounds, wooden supports, or 
human beings touching the structure. The magnitude 
of the total current flowing from the structure is a 
function of the size of the structure, its proximity to the 
power conductors, the voltage level of the power 
conductors, and their geometrical arrangement. The 
total current flowing between the metallic structure and 



NACE International 



SP01 77-2007 



earth distributes itself between tiie different available 
paths to earth in direct proportion to the relative 
conductivity of each path. For example, a 100-ohm 
ground rod would carry 10 times as much current to 
earth as a 1 ,000-ohm human being, thus reducing the 
magnitude of the available shock current by a factor of 
10. Capacitive coupling is typically a hazard during 
construction with respect to electric shock or arcing 
when the structure is on insulating supports prior to 
lowering in or connecting to an adjacent section. 
Ground rods and bonding often provide sufficient 
protection. The need for additional grounding can be 
verified with a simple voltmeter test. 

3.4 Inductive Coupling 

3.4.1 AC flow in power conductors produces an 
alternating magnetic field around these conductors, 
thereby inducing AC potentials and current flow in an 
adjacent structure. The magnitude of the induced 
potential depends on many factors. The most 
important are: 

(a) The overall separation distance between the 
structure and the power line; 

(b) The length of exposure and the power line 
current magnitude; 

(c) Changes in the arrangement of power line 
conductors or in separation distance; 

(d) The degree to which current flowing in one 
power line conductor is balanced by the currents 
flowing in the others due to conductor arrangement and 
current distribution; 

(e) The type of conductor used for the lightning 
shield wires on the power line; 

(f) The coating resistance of the structure; 

(g) The grounding present on the structure; and 

(h) The soil resistivity as a function of depth. 

Grounding is usually present to some degree because 
of leakage across the coating or anodes of the CP 
system connected to the structure. Induced voltages 
increase in magnitude during fault conditions. The 
coating stress voltages caused by the inductive 
coupling near a short-circuit location tend to reinforce 
those caused by resistive coupling; therefore, both 
factors must be considered. The same is true for 
touch-and-step voltages. Hazardous induced 

potentials can easily extend over distances of many 
kilometers (miles), both within a power line corridor and 



beyond the extremities of the corridor. Considerable 
power may be transferred to a structure by means of 
inductive coupling and can result in currents of tens or 
even hundreds of amperes flowing in the structure 
during peak power system operating conditions, and 
thousands of amperes during short-circuit conditions. 
Potential peaks tend to occur at locations in which 
there are abrupt changes in the parameters. These 
are usually locations where power lines and structures 
deviate away from or cross one another at substations 
or at power line phase transposition locations. 
Installing grounding at one location can make matters 
significantly worse elsewhere; therefore, it is important 
to consider the whole system carefully when designing 
mitigation. 

3.5 Power Arc 

3.5.1 During a fault-to-ground on an AC power 
system, the AC power structures and surrounding earth 
may develop a high potential with reference to remote 
earth. A long metallic structure, whether coated or 
bare, tends to remain at remote earth potential if not 
running parallel to the AC power lines. Worse still, if 
the structure runs parallel to the AC power lines, the 
induced potential on the structure tends to be opposite 
in polarity to the earth potential near the fault location at 
any given instance in time. Either way, if the resulting 
voltage to which the structure is subjected exceeds the 
breakdown voltage of any circuit element, a power arc 
can occur, damaging the circuit elements. Elements of 
specific concern include coatings, isolating fittings, 
bonds, lightning arresters, and CP facilities. If the 
potential gradient in the earth is large enough to ionize 
the soil for a finite distance, a direct arc from the power 
system ground to the structure can occur within that 
distance and result in coating damage, arc burn, or 
puncture/failure of the structure. 

3.6 Lightning 

3.6.1 Lightning strikes to the power system can initiate 
fault current conditions. Lightning strikes to a structure 
or to earth in the vicinity of a structure can produce 
electrical effects similar to those caused by AC fault 
currents. Lightning may strike a metallic structure at 
some point remote from AC power systems, also with 
deleterious effects. 

3.7 Switching Surges or Other Transients 

3.7.1 A switching surge or other transient may 
generate abnormally high currents or potentials on a 
power system, causing a momentary increase in 
inductive and capacitive coupling on the affected 
structures. 



NACE International 



SP01 77-2007 



Section 4: Design Considerations for Protective Devices 



4.1 Introduction 

4.1 .1 This section describes various protective devices 
used to iielp mitigate AC effects on metallic structures 
subject to hazardous AC conditions, minimize damage 
to the structures, and reduce the electrical hazard to 
people coming in contact with these structures. 

4.1 .2 The methods listed can be used to mitigate the 
problems of power arcing, lightning arcing, resistive 
coupling, inductive coupling, and capacitive 
coupling.^'*'^ These methods may also be used to 
mitigate AC corrosion. 



grounding locations may be required for complete 
protection. 

4.2.3 Gradient control wires consist of a continuous 
and long grounding conductor or conductors installed 
horizontally and parallel to a structure (e.g., pipeline 
section) at strategic lengths and connected at regular 
intervals. They are intended to provide protection to 
the structure and coating during steady-state and fault 
AC conditions from nearby electric transmission power 
systems. Gradient control wires can reduce the 
steady-state voltages and the possibility of puncturing 
the coating or structure under fault conditions. 



4.1 .3 Design considerations should include steady- 
state conditions (including touch voltage and maximum 
pipe potentials during normal, emergency, and future 
loads) and fault conditions (including touch-and-step 
voltage, avoidance of pipe wall puncture and arc burns, 
and tolerable coating stress voltages). 

4.1 .4 Design mitigation objectives should be clearly 
defined. As a minimum, the mitigation objectives 
should include the maximum steady-state voltage at 
above-grade portions and appurtenances, maximum 
pipe potential (ground potential rise [GPR]) for the 
normally buried and inaccessible portions, touch-and- 
step voltage criteria at above-grade portions and 
appurtenances during fault conditions, and the 
maximum coating stress voltage during fault 
conditions. 



4.2 Fault Shields, 
Control Wires 



Lumped Grounding, and Gradient 



4.2.1 Fault shields consist of shallow grounding 
conductors (i.e., electrodes) connected to the affected 
structure adjacent to overhead electrical transmission 
towers, poles, substations, etc. They are intended to 
provide localized protection to the structure and coating 
during a fault event from a nearby electric transmission 
power system. Fault shields can reduce the possibility 
of puncturing the coating or structure under fault 
conditions. 

4.2.2 Lumped grounding consists of a localized 
conductor or conductors connected to the affected 
structure at strategic locations (e.g., at discontinuities). 
It is intended to protect the structure from both steady- 
state and fault AC conditions. Lumped grounding 
systems may be installed in shallow or deep 
configurations, depending on the site-specific 
parameters. Lumped grounding can reduce the 
steady-state touch voltages and the possibility of 
puncturing the coating or structure under fault 
conditions; however, grounding between the lumped 



4.2.4 Among the factors that influence mitigation 
design is the extent to which the structure is affected 
and the magnitude of the electrical potential between 
the structure and earth. These factors vary from one 
location to another and must be calculated or 
determined for each specific location. A combination of 
the above methods may be utilized, depending on the 
specific AC mitigation requirements. 

4.2.5 Electrodes constructed of materials that are 
cathodic to the protected structure must be connected 
to the structure through a DC decoupling device, 
unless both the structure and electrode are cathodically 
protected as a single unit. Electrodes constructed of 
materials that are anodic to the protected structure may 
be connected directly to the structure; however, the CP 
design must be verified to be compatible with this type 
of circuitry. 

4.2.6 Other types of systems can be designed for 
protection against faults on miscellaneous underground 
or aboveground structures. 

4.3 Gradient Control Mats 

4.3.1 Gradient control mats, bonded to the structure, 
are used to reduce electrical touch-and-step voltages 
in areas where people may come in contact with a 
structure subject to hazardous potentials. Permanent 
mats bonded to the structure may be used at valves, 
metallic vents, CP test stations, and other aboveground 
metallic and nonmetallic appurtenances in which 
electrical contact with the affected structure is possible. 

4.3.2 Gradient control mats should be large enough to 
extend through and beyond the entire area on which 
people may be standing when contacting the affected 
structure. They should be installed close enough to the 
surface to adequately reduce touch-and-step voltages 
for individuals coming in contact with the structure.* 
Gradient control mats should be engineered to provide 
acceptable touch-and-step voltages during both load 



NACE International 



SP01 77-2007 



and fault conditions, accounting for tiie local soil 
conditions. 

4.3.3 Gradient control mats, regardless of materials of 
construction, must be bonded to the structure, 
preferably at more than one point. If CP of the 
structure becomes difficult because of shielding, a DC 
decoupling device may be installed. Connections to 
the structure should be made aboveground to allow a 
means of testing for effect of the gradient control mat in 
reducing AC potentials and its effectiveness on the CP 
system. Care should be taken to prevent the possible 
establishment of detrimental galvanic cells between the 
gradient control mat and structures that are not 
cathodically protected. 

4.3.4 A bed of clean, well-drained gravel can reduce 
the shock hazard associated with touch-and-step 
voltages. Although an excellent practice, if hazardous 
conditions exist for pipeline applications, increasing the 
surface resistance should be used to augment the 
grounding system and not as a sole protection 
measure, as it may not be well maintained and kept 
clean. The thickness of the bed should be no less than 
76 mm (3.0 in.). Gravel should be a minimum of 13 
mm (0.50 in.) in diameter. The hazards of step 
voltages at the edge of a mat may be mitigated by 
extending the gravel beyond the perimeter of the 
grounding mat. 

4.4 Independent Structure Grounds 

4.4.1 Wherever a metallic structure subject to 
hazardous AC that is not electrically connected to an 
existing grounded structure is installed, it shall have an 
independent grounding system. This grounding 
system may consist of one or more ground rods and 
interconnecting wires. Care shall be taken to 
interconnect all components of the structure to be 
grounded properly. Factors considered in the design of 
the grounding system of an independent structure 
include the resistivity of the soil and the magnitude of 
the induced potential and current that the designer 
expects the structure to encounter under all possible 
conditions. 

4.4.2 When an independent metallic structure or its 
grounding system is in close proximity to an existing 
grounded structure, an electrical hazard may develop 
for any person contacting both structures or their 
grounds simultaneously. In such cases, both 
grounding systems should be connected, either directly 
or through a DC decoupling device, unless it is 
determined that such a connection is undesirable. The 
electrical and CP designers should both be involved 
with this evaluation. For more details on designing 
systems for independent structures, see IEEE 
Standard 80.'* 



4.5 Bonding to Existing Structures 

4.5.1 One available means of reducing induced AC 
potentials on a structure involves bonding the structure 
to the power system ground through adequately sized 
cables and decoupling devices. Such bonds may, 
under fault conditions, contribute to increased 
potentials and currents on the affected structure for the 
duration of the fault. If the bonded structure is 
aboveground, or well-insulated from earth, elevated 
potentials may be created and exist temporarily along 
the entire length of the bonded structure. In such 
instances, additional protective devices may be 
required outside the immediate area of the origin of 
electrical effects. Close coordination should be 
maintained with all other utilities in the area, especially 
with those utilities to which bond connections are 
proposed. The corresponding utilities shall be notified 
in advance of the need to bond to their structures and 
shall be furnished with details of the proposed bonding 
arrangements. A utility may prefer to have the 
connection to its structures made by its own personnel. 
Other methods of reducing AC potentials should be 
considered before committing to bonding. The 
increased hazards during fault conditions and extra 
installation requirements may make this method 
questionable from safety and economic perspectives. 

4.5.2 Whenever such a bond is installed, full 
consideration must be given to mitigation of hazardous 
AC transferred to the influenced structure. 

4.6 Distributed Anodes 

4.6.1 Whenever distributed galvanic anodes are used 
as part of the grounding system to reduce the AC 
potential between a structure and earth, they should be 
installed in close proximity to the protected structure 
and away from power system grounds. Connecting 
anodes directly to the affected structure, without test 
connections, may be desirable. Direct connection 
reduces the number of points at which people can 
come in contact with the structure and offers the 
shortest path to ground. Should it be desirable for 
measurement purposes to open the circuit between the 
distributed grounding system and the structure, the test 
lead connection should be made in an accessible 
dead-front test box. When galvanic anodes are used 
as part of a grounding system, the useful life of the 
electrode material should be considered. Normal 
deterioration and consumption of the anode material 
increases the grounding system resistance. 

4.7 Casings 

4.7.1 Bare or poorly coated casings may be 
deliberately connected to a coated structure through a 
DC decoupling device to lower the impedance of the 
structure to earth during surge conditions and to avoid 
arcing between the structure and the casing. 



NACE International 



SP01 77-2007 



4.8 Connector (Electrical and Mechanical) and Conductor 
Sizes 

4.8.1 All anodes, bonds, grounding devices, and 
jumpers must have secure, reliable, low-resistance 
connections to themselves and to the devices to which 
they are attached. Structure members with rigid bolted, 
riveted, or welded connections may be used in lieu of a 
bonding cable for part or all of the circuit. Steady-state 



conductor sizing should consider the AC load with the 
mitigation applied. For adequate fault sizing of 
conductors, refer to Table 1 and Figures 1,2,3, and 4. 
For wire gauge conversions, refer to Table A1 in 
Appendix A (nonmandatory). All cables, connections, 
and structural members should be capable of 
withstanding the maximum anticipated magnitude and 
duration of the surge or fault currents with mitigation 
applied. 



Table 1 : Maximum 60 Hz Fault Currents — Grounding Cables''^' 



Cable Size 


Fault Time 
Cycles 


rms'=' 


Amperes 


Cable Size 


Fault Time 
Cycles 


rms'^' 


Amperes 


AWG'»' 


Copper 


Aluminum 


Copper 


Aluminum 


1 


15 


10,550 


6,500 


3/0 


15 


26,500 


16,500 




30 


7,500 


4,600 




30 


18,500 


16,500 




60 


5,300 


3,200 




60 


13,000 


8,000 


1/0 


15 


16,500 


10,500 


4/0 


15 


30,000 


21,000 




30 


11,500 


7,500 




30 


21,000 


15,000 




60 


8,000 


5,300 




60 


15,000 


10,000 


2/0 


15 


21,000 


13,000 


250 

MCM<°' 


15 


35,000 


25,000 




30 


15,000 


9,000 




30 


25,000 


17,500 




60 


10,000 


6,500 











(A) 



Based on 30^ (86°F) ambient and a total temperature of 175^ (347°F) established by the Insulated Cable Engineers 
Association (ICEA)' for short-circuit characteristic calculations for power cables. Values are approximately 58% of fusing currents. 
'"' American Wire Gauge (AWG) 
' Root mean square 
'"' MCM = 1,000 circular mils 



4.8.2 Mechanical connections for the installation of 
permanent protective devices should be avoided when 
practical except when they can be inspected, tested, 
and maintained in approved aboveground enclosures. 
When practical, field connections to the structure or 
grounding device should be made by the exothermic 
welding process. However, compression-type 

connectors may be used for splices on connecting 
wires. Mechanical connectors may be used for 
temporary protective measures, but extreme care 
should be taken to avoid high-resistance contacts. 
Soft-soldered connections are not acceptable in 
grounding circuits. 

Figure 1 is based on the assumption that no heat is 
radiated or conducted from the cable to the 
surrounding media during a fault period. Electrical 
energy released in the cable equals the heat energy 
absorbed by the cable. This is illustrated in Equation 
(1): 



I^Rt: 



CFQ 



(1) 



Where: 

I = fault current in amperes 

R = average AC resistance (in ohms) of conductor over 

temperature range Ti to T2 in °C (°F) 

t = fault duration in seconds 

Q = heat energy in kJ (BTU) 

CF = conversion factor = 1 ,000 for SI units (1 W=J/s 

and 1,000 J/kJ) and 1,055 for U.S. units (1,055 W- 

s/BTU) 

Q is calculated using Equation (2): 



Q = CM (T2 - Ti) [Thermodynamics] 



(2) 



Where: 



C = average specific heat in kJ/kg °C (BTU/lb °F) of 

annealed soft-drawn copper over the temperature 

range Ti to T2 

M = mass of copper in kg (lb) 

Ti and T2 = initial and final temperatures respectively in 

°C (°F). 



Insulated Cable Engineers Association (ICEA), P.O. Box 440, Carrollton, GA 301 1 2. 



NACE International 



SP01 77-2007 

Figure 1 was developed using C = 0.104 BTU/lb °F, T1 = 68°F, and T2 = 1,300°F Typical resistance values are shown in 
Table 2. 



(0 

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Fault Duration (seconds) 



29 



FIGURE 1 : Approximate Current Required to Raise tlie Temperature of Stranded Annealed Soft-Drawn 
Copper Cable 6842C (1,2322F) Above an Ambient Temperature of 202C (eS^Ff* 



(3) 

allowed its publication in the original standard. 



This figure was developed by Ebasco Services, Inc. (now Raytheon Company, 870 Winter St., Waltham, MA 02451), who graciously 



NACE International 



SP01 77-2007 



TABLE 2: Average Impedance for Various Conductor Sizes''^' 



Conductor" 



Average 60-Hz Impedance 
(Ohms/1 ,000 ft) 



Average 60-Hz Impedance 
(Ohms/km) 



"W 



#6 AWG 

#2 AWG 

#1/0 AWG 

#4/0 AWG 

250 MCM 

500 MCM 

1,000 MCM 

2,000 MCM 

4,000 MCM 



0.923 
0.366 
0.2295 
0.1097 
0.0968 
0.0492 
0.0259 
0.0151 
0.00972 



3.03 
1.20 
0.753 
0.360 
0.318 
0.161 
0.0850 
0.0495 
0.0319 



Fusing current is 10% liigliertlian current for 684 °C (1,232°F) temperature rise. 
For cable sizes in metric units, see Appendix A (nonmandatory). 



NACE International 



SP01 77-2007 



100 
•0 


















y 


1 


^ 


^ 




















r^ 


7 


X J 


y ^ 


















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40 






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1 


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1. T <a* 
1 _-J 


.6 


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LT, -I- 234 J 


.6 


/ 








Where; 

■ 1 = Short-circuit current in amperes ~*~ 

■ A = Conductor area in circular mils — 
. t = Time of short-circuit in seconds 


A 




















Ji 










A 










T 


1 = Maximum operating temperature 

of 75»C ^=. 












; [ 


i = Maximum short-circuit 












lemperaiure or i3u-*j 




































.1 












1__ 


i_ 


i 


J — 


J- 


J 


J J 



10 



2 1 1/D 2n 3/0 «0 AWQ 

250 MCM 500 



1000 



(A) 



CONDUCTOR SIZE 

FIGURE 2: Allowable Short-Clrcult Currents for Insulated Copper Conductors^ 

To calculate this formula when the conductor sizes are in metric units, change metric values to circular mils for A as indicated in Table A1 , 



Appendix A (nonmandatory). 



10 



NACE International 



SP01 77-2007 



too 








N= 




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= 


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1 


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r 1 T J r T- + »i T 




/ 








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■ 1 = Short-circuit current in amperes ***" 
• A = Conductor area In circular mils 

, t = TimR r>f •thnrt-rJrcuit in <ieconds 












A 










J 










Ti 


= Maximum operating temperature 
ofSO'C = 












: T: 


= Maximum short-circuit ^=: 












temoerature or i:&o"u i^^j 














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1 


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["1 I 1 1 LJ 1 1 



10 



4 2 1 1/0 2m M 4n AWO 

280 MCM 600 
CONDUCTOR SIZE 



1000 



FIGURE 3: Allowable Short-Clrcult Currents for Insulated Copper Conductors' 



(A) 



Appendix A (nonmandatory). 



To caicuiate tiiis formuia wiien tiie conductor sizes are in metric units, ctiange metric values to circular mils for A as indicated in Table A1 , 



NACE International 



11 



SP01 77-2007 




0.1 



1 
Time (Seconds) 



10 



FIGURE 4: Zinc Ribbon Ampacity 



(4) 



Experimental results for 60-Hz current required to raise the temperature of three sizes of cast- and rolled-zinc 

ribbon anode from 20 °C (68 T) to 250 "C (482 °F).' 



Super: 25.4 mm x 31 .75 mm (1 in. x 1 .25 in.) 
Plus: 1 5.88 mm x 22.22 mm (0.625 in. x 0.875 in.) 
Standard: 12.7 mm x 14.22 mm (0.50 in x 0.56 in.) 

Note: A design reduction factor sliould be determined by 
the user and applied in conjunction with the data in Figure 
4, and connections to the ribbon steel core must be 
equivalent to the design ampacity requirement. 

4.9 Isolation Joints 

4.9.1 Although isolation joints (including flanges and 
fittings) can be installed to divide a structure into 
shorter electrical sections or to isolate a section 
adjacent to an AC power system from the remainder of 
the structure, this practice must be considered carefully 
for the specific application. When used to reduce the 
length of pipeline exposed to AC at the entrance and 
exit of AC right-of-ways, the beneficial grounding effect 
from the remaining pipeline is lost, and AC potentials 
can increase on the isolated section. The voltage will 



be reduced in proportion to the length of the sections if 
used in the joint corridor; however, a potential hazard 
may exist across the isolation joint (IJ), and it will as a 
minimum require fault protection. Hazardous 

conditions may be transferred to the other side of 
isolating devices, where mitigation and protection 
measures may not be present during a fault even 
without protection devices. Therefore, the AC 
interference study and mitigation design should not 
ignore pipe sections and appurtenances that are 
normally DC isolated. DC decouplers or other devices 
that continuously pass AC should be utilized in most 
cases of steady-state AC interference. In cases in 
which the steady-state concerns are low, but faults are 
a possibility, over-voltage protection devices that close 
during an electrical disturbance should be provided. 
The breakdown voltage for a typical IJ is in the range of 
3 kV; however, arcing can occur at much lower 
voltages without dielectric breakdown. Arcing 
conditions must also be avoided in hazardous 
(classified) locations. 



(4) 



Courtesy of the Piatt Bros, and Company, Inc., 2760 S. Main St., Waterbury, CT 06721 . 



12 



NACE International 



SP01 77-2007 



4.10 Electrolytic Grounding Cells, Solid-State DC 
Decouplers, Polarization Cells, and Other Devices 

4.10.1 The coordinated selection and installation of 
electrolytic grounding cells, solid-state DC decouplers, 
polarization cells (2.5-V DC maximum threshold), or 
other devices between the affected structure and 
suitable grounds should be considered where arcing 
and induced AC potentials could develop. These 
devices may eliminate or greatly reduce the induced 
potentials resulting during normal operation or surge 
conditions and also reduce the possibility of arcing and 
structure puncture. Polarization cells and solid-state 
DC decouplers should be considered for steady-state 
AC interference applications, as these devices pass 
AC continuously. The device and installation must be 
rated for the area classification, when installed in a 
hazardous location. 

4.10.2 When electrolytic grounding cells, solid-state 
DC decouplers, polarization cells (2.5-V DC maximum 
threshold), or other devices are used, they must be 
properly sized, located, connected, and physically 
secured in a manner that safely conducts the maximum 
amount of anticipated surge current. Cables 
connecting these devices to the structures shall be 
properly sized, as described in Paragraph 4.8.1 . 
Cables should be kept as short and straight as 
possible. An adequately sized bypass circuit should be 
provided prior to any electrical isolation of the 
grounding device during testing and maintenance. 

4.1 1 Over-Voltage Protectors 

4.11.1 Surge arresters are available in many different 
types and for many applications. These include 
lightning arresters, spark gaps, solid-state electronic 
devices, and metal oxide varistors (MOV). These 
devices may be used between structures and across 
pipeline electrical isolating devices, generally when 
steady-state interference is not a problem. However, 
one restriction to the use of arresters is that a potential 
difference has to develop before the arrester conducts. 
With certain types of arresters, this potential may be 
high enough to become hazardous to people coming in 
contact with the arrester. When arresters are used, 
they must be connected to the structure through 
adequately sized cables, as described in Paragraph 
4.8.1. Arresters require a reliable, low-resistance 
ground connection. They should be located close to 
the structure being protected and have a short, straight 
ground path. Short lead length is especially important 
for lightning protection when the voltage build-up 
caused by lead induction can be significant. An 
adequately sized bypass circuit should be provided 
prior to any isolation of the grounding device during 
testing or maintenance. 



4.11.2 Certain types of sealed, explosion-proof, 
enclosed, or repetitive transient arresters may be used 
in locations where a combustible atmosphere is 
anticipated, but only if it can be determined that the 
maximum possible power fault current does not exceed 
the design rating of the arrester. Open spark gaps 
shall not be used in these locations. The device and 
installation must be rated for the area classification, 
when installed in a hazardous location. 

4.1 2 Stray Direct Current Areas 

4.12.1 Galvanic anodes (including those in electrolytic 
grounding cells), grounding grids, or grounds directly 
connected to the structure may pick up stray DC in 
areas where stray direct currents are present. This 
current could possibly discharge directly to earth from 
the structure at other locations, resulting in corrosion of 
the structure at those points. Also, DC pickup by the 
structure could lead to DC discharge to earth through 
the galvanic anodes or grounding devices, resulting in 
increased consumption of the anode material or 
corrosion of grounding rods and an increase in their 
effective resistance to earth. The use of DC 
decoupling devices should be considered in these 
cases. 

4.1 3 Coating Stress Voltage 

4.13.1 External pipeline coatings can be subjected to 
stress voltages during a fault event on a nearby high- 
voltage power system. Both conductive and inductive 
components of a fault contribute to the stress voltage, 
with the conductive component acting on soil potentials 
and the inductive component acting on the pipeline 
steel potential. Typically, the pipeline steel potential 
tends to be of opposite polarity to that of the earth 
potential so that the total coating stress voltage is on 
the same order as the sum of the magnitudes of the 
inductive and conductive components. Properly 
designed mitigation (i.e., grounding) can reduce the 
coating stress voltage to protect the coating from 
disbondment or puncture. This, in effect, also protects 
the pipeline wall from arc burns and puncture, as the 
tolerable coating stress voltages are lower than the 
conditions reported to cause damage to steel. 

4.13.2 Limiting the coating stress voltage should be a 
mitigation objective. Expected threshold values for 
coatings differ with type and are generally considered 
to be in the range of up to 2 kV for tape wraps and coal 
tar enamels and 3 to 5 kV for fusion-bonded epoxy 
(FBE) and polyethylene coatings for a short duration 
fault. 



NACE International 



13 



SP01 77-2007 



Section 5: Personnel Protection 



5.1 Introduction 

5.1 .1 This section recommends practices tiiat 
contribute to tiie safety of people who, during 
construction, system operation, corrosion survey, or CP 
maintenance of metallic structures, may be exposed to 
the hazards of AC potentials on those structures. The 
possibility of hazards to personnel during construction 
and system operation because of contact with metallic 
structures exposed to AC electrical or lightning effects 
must be recognized and provisions made to alleviate 
such hazards. The severity of the personnel hazard is 
usually proportional to the magnitude of the potential 
difference between the structure and the earth or 
between separate structures. The severity also 
depends on the duration of the exposure. Before 
construction work is started, coordination with the 
appropriate utilities in the area must be made so that 
proper work procedures are established and the 
construction does not damage or interfere with other 
utilities' equipment or operations.*^* 

5.1 .2 Each utility should be aware of the others' 
facilities and cooperate in the mitigation of the electrical 
effects of one installation on the other. The mitigation 
required for a specific situation must be based on 
safety considerations with good engineering judgment. 



5.1 .3 Increasing the separation distance between 
facilities is generally effective in reducing the electrical 
effects of one installation on another. 

5.2 Recognition of Shock Hazards to Personnel 

5.2.1 AC potentials on structures must be reduced to 
and maintained at safe levels to prevent shock hazards 
to personnel. The degree of shock hazard and the 
threshold levels of current that can be tolerated by 
human beings depend on many factors. The possibility 
of shock from lower voltages is the most difficult to 
assess. The degree of shock hazard depends on 
factors such as the voltage level and duration of human 
exposure, human body and skin conditions, and the 
path and magnitude of any current conducted by the 
human body. The magnitude of current conducted by 
the human body is a function of the internal impedance 
of the voltage source, the voltage impressed across the 
human body, and the electrical resistance of the body 
path. This resistance also depends on the contact 
resistance (e.g., wet or dry skin, standing on dry land or 
in water), and on the current path through the body 
(e.g., hand-to-foot, hand-to-hand, etc.). Tables 3 and 4 
indicate the probable human resistance to electrical 
current and current values affecting human beings. 



TABLE 3: IHuman Resistance to Electrical Current^ 



Dry skin 

Wet skin 

Internal body — hand to foot 

Ear to ear 



100,000 to 600,000 ohms 
1,000 ohms 
400 to 600 ohms 
about 100 ohms 



14 



NACE International 



SP01 77-2007 



TABLE 4: Approximate 60-Hz Alternating Current Values Affecting Human Beings 



Current 



Effects 



1 mA or less No sensation — not felt. 

1 to 8 mA Sensation of shock — not painful; individual can let go at will; muscular control not lost. 

8 to 15 mA Painful shock — individual can let go at will; muscular control not lost. 

1 5 to 20 mA Painful shock — muscular control lost; cannot let go. 

20 to 50 mA Painful shock — severe muscular contractions; breathing difficult. 

50 to 1 00 mA Ventricular fibrillation — Death results if prompt cardiac massage not administered. 

100 to 200 mA Defibrillator shock must be applied to restore normal heartbeat. Breathing probably 

stopped. 
200 mA and Severe burns — severe muscular contractions; chest muscles clamp heart and stop it 
over during shock. Breathing stopped — heart may start following shock, or cardiac massage 
may be required. 



Source: Typical industry values 

5.2.1.1 The safe limits must be determined by 
qualified personnel based on anticipated exposure 
conditions. For the purpose of this standard, a 
steady-state touch voltage of 15 V or more with 
respect to local earth at above-grade or exposed 
sections and appurtenances is considered to 
constitute a shock hazard. 

5.2.1.2 It must be recognized that when touch 
voltages are below 15 V, the current may be 
dangerously high in the structure and continuity 
provisions and other procedures are mandatory 
prior to separating affected sections. All 
precautions must be implemented to eliminate the 
possibility of a person being placed in series with 
this current. 

5.2.1.3 During short-circuit conditions, the 
permissible touch-and-step voltages at above- 
grade portions of the structure and appurtenances 
should be determined in accordance with the 
methodology specified in IEEE Standard 80* or 
other analogous methodologies, such as the 
International Electrotechnical Commission (lEC).'^* 

5.2.1.4 In areas (such as urban residential zones 
or school zones) in which a high probability exists 
that children (who are more sensitive to shock 
hazard than are adults) can come in contact with a 
structure under the influence of induced AC 
voltage, a lower touch voltage shall be considered. 

5.2.1.5 The beginning sensation of shock, which 
may occur at 1 to 8 mA, may not be painful or 
harmful to a human being, but may lead to an 
accident by causing rapid involuntary movement of 
a person. 



5.2.2 In areas of AC influence, measured AC voltages 
between a structure and either an adjacent structure, a 
ground or an electrolyte are considered an indication 
that further investigation is needed to determine 
whether AC mitigation is required. 

5.2.3 When the touch voltage on a structure presents 
a shock hazard, the voltage must be reduced to safe 
levels by taking remedial measures. In those cases in 
which the voltage level cannot be practically reduced to 
a safe level on aboveground appurtenances by fault 
shields, gradient control wires, lumped grounding, AC 
continuity, etc., other safety measures shall be 
implemented to prevent shock to operating and 
maintenance personnel and to the public (see 
Paragraph 4.3) to satisfy the requirements in 
Paragraph 5.2.1 . The use of dead-front construction 
may be utilized in lieu of gradient control mats for test 
stations and other CP equipment enclosures when 
approved by the owner; however, caution is advised, 
and it must be recognized that this does not reduce 
any hazardous voltage present. 

5.3 Construction 

5.3.1 Severe hazards may exist during construction of 
facilities adjacent to AC power systems. A competent 
person shall be in charge of electrical safety. This 
person shall be fully aware of proper grounding 
procedures and of the dangers associated with 
inductive and capacitive couplings, fault current, 
lightning, etc., on aboveground and underground 
structures. The person must also know the hazards of 
the construction equipment being used as related to 
the "limit-of-the-approach" regulations governing them.* 
The person shall be furnished with the instrumentation, 
equipment, and authority required to implement and 
maintain safe working conditions. 



International Electrotechnical Commission (lEC), 3, rue de Varembe, P.O. Box 131 , CH -1211 , Geneva, Switzerland. 



NACE International 



15 



SP01 77-2007 



5.3.2 The AC potential difference between a structure 
and the earth can be substantially reduced by 
appropriate grounding procedures. The AC potential 
difference between structures can be reduced by 
appropriate bonding procedures. The AC potential 
difference between separate points in the earth can be 
reduced through the use of appropriate grounding 
grids. The grounding or bonding procedure for safe 
construction activities depends upon the type, 
magnitude, and duration of the AC exposure. Each 
situation shall be analyzed by a competent person, and 
safe operating procedures shall be employed during 
the entire construction operation. 

5.3.3 During the construction of metallic structures in 
areas of AC influence, the following minimum 
protective requirements are prescribed: 

(a) On long metallic structures paralleling AC power 
systems, temporary electrical grounds shall be used at 
intervals not greater than 300 m (1 ,000 ft), with the first 
ground installed at the beginning of the section. Under 
certain conditions, a ground may be required on 
individual structure joints or sections before handling. 

(b) All temporary grounding connections shall be left 
in place until immediately prior to backfilling. Sufficient 
temporary grounds shall be maintained on each portion 
of the structure until adequate permanent grounding 
connections have been made. 

5.3.4 Temporary grounding connections may be made 
to ground rods, bare pipe casing, or other appropriate 
grounds. These temporary grounding facilities are 
intended to reduce AC potentials. Direct connections 
made to the electrical utility's grounding system during 
construction could increase the probability of a hazard 
during switching surges, lightning strikes, or fault 
conditions, and may intensify normal steady-state 
effects if the grounding system is carrying AC; such 
connections should be avoided when possible. 

5.3.5 Cables used for bonding or for connections to 
grounding facilities shall have good mechanical 
strength and adequate conductivity. As a minimum, 
copper conductor 35-mm^ (0.054-in.^) (No. 2 AWG) 
stranded welding cable or equivalent is recommended. 
See Table 1 and Figures 1, 2, and 3 for cable sizes 
adequate to conduct the anticipated fault current safely. 

5.3.6 Temporary cable connections to the affected 
structure and to the grounding facilities shall be 
securely made with clamps that apply firm pressure 
and have a current-carrying capacity equal to or 
greater than that of the grounding conductor. Clamps 
shall be installed so that they cannot be accidentally 
dislodged. 

5.3.7 All permanent cable connections shall be 
thoroughly checked to ensure that they are 
mechanically and electrically sound and properly 
coated prior to backfilling. 



5.3.8 The grounding cable shall first be attached to the 
grounding facilities and then securely attached to the 
affected structure. Removal shall be in reverse order. 
Properly insulated tools or electrical safety gloves shall 
also be used to minimize the shock hazards. THE 
END CONNECTED TO THE GROUND SHALL BE 
REMOVED LAST. 

5.3.8.1 In those instances in which high power 
levels are anticipated in the grounding cable, the 
following procedure is recommended to prevent 
electrical arc burns or physical damage to the 
coating or metal on the structure. 

(a) The grounding clamp shall be connected to 
the structure without the ground lead. 

(b) The grounding cable shall first be connected 
to the grounding facility. 

(c) Next, the grounding cable shall be connected 
to the grounding clamp on the structure. 

5.3.9 All grounding attachments and removals shall be 
made by, or under the supervision of, the person in 
charge of electrical safety. 

5.3.10 If hazardous AC potentials are measured 
across an isolating joint or flange, both sides of the joint 
or flange shall be grounded and/or bonded across. If 
required, a permanent bond shall be made before the 
temporary bond is removed. 

5.3.11 Before the temporary grounding facilities are 
removed, provisions must be made to permanently 
control the effects of AC potentials on the affected 
structure. These provisions depend on the type of CP, 
the type of structure, and the anticipated magnitude of 
AC potentials. 

5.3.12 Vehicles and other construction equipment are 
subject to existing electrical safety regulations, when 
operated in the vicinity of high-voltage AC lines. ^ 

5.3.12.1 IVIetallic construction sheds or trailers, 
fences, or other temporary structures shall be 
grounded if subject to hazardous AC influence. 

5.3.13 The person in charge of electrical safety shall 
communicate at least daily with the utility controlling the 
involved power lines to ascertain any switching that 
might be expected during each work period. This 
person may request that reclosing procedures be 
suspended during construction hours and may explore 
the possibility of taking the power line out of service. 
The person shall also keep informed of any electrical 
storm activity that might affect safety on the work site. 
The person shall order a discontinuation of 
construction during local electrical storms or when 
thunder is heard. 



16 



NACE International 



5.3.14 The use of electrically isolating materials for 
aboveground appurtenances such as vent pipes, 
conduits, and test boxes may reduce shock hazards in 
specific instances. However, electrical wires 

permanently attached to the pipeline, such as CP test 
wires, may have a high possibility of a shock hazard 
because they cannot be isolated from the pipe (see 
Paragraph 7.2.6). 

5.4 Operations and Maintenance 

5.4.1 Maintenance of structures and CP facilities 
under conditions that include AC potentials may require 
special precautions. Warning signs shall be used as a 
minimum precaution. All maintenance shall be 
performed by or under the supervision of a person 
familiar with the possible hazards involved. Personnel 
must be informed of these hazards and of the safety 
procedures to follow. 

5.4.2 Testing of devices intended to limit AC potentials 
shall be in accordance with manufacturer's 
recommendations and performed under the 
supervision of a person familiar with the possible 
hazards involved. In those areas in which the 
presence of combustible vapors is suspected, tests 
must be conducted before connections are made or 
broken to determine that the combustible vapor level is 
within safe limits. No more than one device intended to 
limit the AC potential should be disconnected at any 
one time. When a single protective device is to be 
installed, a temporary shunt bond, with or without 
another decoupling device, must be established prior to 
removing the unit for service. 

5.4.3 Testing of CP systems under the influence of AC 
potentials must be performed by or under the 
supervision of a qualified person. In all cases, tests to 
detect AC potentials shall be performed first, and the 



SP01 77-2007 

structure shall be treated as a live electrical conductor 
until proven otherwise. CP records should include the 
results of these tests. 

5.4.4 Test stations for CP systems on structures that 
may be subject to AC potentials shall have dead-front 
construction to reduce the possibility of contacting 
energized test leads. Test stations employing metallic 
pipes for support must be of dead-front construction. 

5.4.5 Safe work practices must include attaching all 
test leads to the instruments first and then to the 
structure to be tested. Leads must be removed from 
the structure first and from the instruments last. 

5.4.6 When structures subject to AC influence are 
exposed for the purpose of cutting, tapping, or 
separating, tests shall be made to determine AC 
potentials or current to ground. In the event that 
potentials or currents greater than those permitted by 
Paragraph 5.2 are found, appropriate remedial 
measures shall be taken to reduce the AC effects to a 
safe level. In the event this cannot be achieved, the 
structure shall be regarded as a live electrical 
conductor and treated accordingly. Solid bonding 
across the point to be cut or the section to be removed 
shall be established prior to separation, using as a 
minimum the cable and clamps outlined in Paragraphs 
5.3.5 and 5.3.6. 

5.4.7 On facilities carrying combustible liquids or 
gases, safe operating procedures require that bonding 
across the sections to be separated precede structure 
separation, regardless of the presence of AC. 



Section 6: AC and Corrosion Control Considerations 



6.1 Introduction 

6.1 .1 This section recommends practices for 
determining the level of AC influence and lightning 
effects to which an existing metallic structure may be 
subjected. This section also outlines several points for 
consideration regarding the effects these potentials 
may have on corrosion control systems and associated 
equipment. 

6.2 Determination of AC Influence and Lightning Effects 

6.2.1 A CP system design should include an 
evaluation to estimate the level of AC potentials and 
currents under normal conditions, fault conditions, and 
lightning surges. Because significant AC potentials 
may be encountered during field surveys, all personnel 



shall follow proper electrical safety procedures and 
treat the structure as a live electrical conductor until 
proven otherwise. 

6.2.2 Tests and investigations to estimate the extent of 
AC influence should include the following: 

(a) Meeting with electric utility personnel to determine 
peak load conditions and maximum fault currents and 
to discuss test procedures to be used in the survey. 

(b) Electrical measurement of induced AC potentials 
between the affected structure and ground. 

(c) Electrical measurement of induced AC current on 
the structure. 



NACE International 



17 



SP01 77-2007 



(d) Calculations of the potentials and currents to 
which the structure may be subjected under normal 
and fault conditions.'" 



6.2.3 A survey should be conducted over those 
portions of the affected structure in which AC exposure 
has been noted or is suspected. The location and time 
that each measurement was taken should be recorded. 

6.2.3.1 The potential survey should be conducted 
using a suitable AC voltmeter of proper range. 
Contact resistance of connections should be 
sufficiently low to preclude measurement errors 
because of the relationship between external 
circuit impedance and meter impedance. Suitable 
references for measurements are: 

(a) A metal rod."' 

(b) Bare pipeline casings, if adequately isolated 
from the carrier pipe. 

(c) Tower legs or power system neutrals, if in 
close proximity to the affected structure. (Meter 
connections made to tower legs or power system 
neutrals may present a hazard during switching 
surges, lightning strikes, or fault conditions.) 

6.2.3.2 The presence of AC on a structure may 
be determined using a suitable AC voltmeter to 
measure voltage (IR) drop at the line current test 
stations. This method, however, provides only an 
indication of current flow, and cannot be readily 
converted to amperes because of the AC 
impedance characteristics of ferromagnetic 
materials. A clamp-on AC ammeter may be used 
to measure current in temporary or permanent 
bond and ground connections. Instrumentation 
with sufficient resolution may be used to measure 
current at buried coupons that are connected to 
the structure to provide an indication of the local 
AC leakage current density. 

6.2.3.3 Indications of AC power levels on affected 
structures may be obtained by temporarily bonding 
the structure to an adequate ground and 
measuring the resulting current flow with a clamp- 
on AC ammeter while measuring the AC potential. 
Suitable temporary grounds may be obtained by 
bonding to tower legs, power system neutral, bare 
pipeline casings, or across an isolating joint to a 
well grounded system. DC drainage bonds 



existing on the structure under investigation should 
also be checked for AC power levels. 

6.2.3.4 Locations indicating maximum AC 
potential and current flow values during the survey 
discussed in Paragraphs 6.2.3 through 6.2.3.2 
should be surveyed with recording instruments for 
a period of 24 hours or until the variation with 
power line load levels has been established.''' 

6.2.4 To facilitate AC interference studies and to 
design mitigation measures, the following data are 
typically required: 

(a) Power line cross-sectional dimensions, phasing, 
conductor types, and static wire bonding information; 

(b) Power line structure grounding details (including 
footings) and substation ground resistances; 

(c) Substation and power plant grounding system 
dimensions, if close to pipelines; 

(d) Single line diagrams for power lines within 
interference corridor; 

(e) Single phase-to-ground currents for representative 
faults on all power lines; 

(f) Load current details for all power lines, including 
maximum load unbalance and system operating 
frequency; 

(g) IVIaximum fault clearing time for each power line; 

(h) Details on nearby power plants fed by any of the 
pipelines in the interference corridor; 

(i) Alignment drawings of pipelines and 
appurtenances, power lines and structures, and power 
line installations (substations and power plants) 
throughout the common corridor and up to extremities 
of pipelines and power lines; 

(j) Pipeline characteristics, dimensions, and design 
information; 

(k) Soil resistivity measurements up to spacings of 
100 m (328 ft) or more at representative locations 
throughout the common corridor; 

(I) Drawings and locations of exposed appurtenances 
(scraper traps, valves, metering stations, etc.); 



(m) Pipeline coating resistance and coating 
characteristics and thickness; and 



'^' Following meter hookup, the reference rod should be inserted deeper into the earth until no further potential increase is noted. This 
reduces the possibility of high-resistance contact errors in the measurement. 

' Survey data gathered in accordance with Paragraphs 6.2.3 through 6.2.3.4 should be reviewed with electric utility personnel for the 
purpose of correlating with the power line operating conditions at the time of the survey. 



18 



NACE International 



SP01 77-2007 



(n) CP anode bed locations, dimensions, and 
design information. 

6.3 Special Considerations in CP Design 

6.3.1 AC influence on the affected structure and its 
associated CP system should be considered. 

6.3.2 CP survey instruments should have sufficient AC 
rejection to provide accurate DC data. 



6.3.3.2 Resistance bonds for the purpose of DC 
interference mitigation should be designed for the 
maximum normal AC and DC current flow in order 
to prevent damage to the bond. Installation of 
solid state DC decouplers, polarization cells, or 
other devices in parallel with DC resistance bonds 
may prevent damage to bonds. Installation of 
semiconductors in DC interference bonds between 
cathodically protected structures may result in 
undesirable rectification. 



6.3.3 The AC in the structure to be protected may flow 
to ground through CP equipment. Current flowing in 
the CP circuits under normal AC power system 
operating conditions may cause sufficient heating to 
damage or destroy the equipment. Heating may be 
significantly reduced by the use of properly designed 
series inductive reactances or shunt capacitive 
reactances in the CP circuits. 

6.3.3.1 Rectifiers should be equipped with 
lightning and surge protection at the AC input and 
DC output connections. 



6.3.3.3 When bonds to other structures or 
grounds are used for AC considerations, the 
requirements as described in Paragraph 4.2.5 
apply in order to maintain effective levels of CP. 

6.3.3.4 Semiconductor drain switches (reverse 
current) for the mitigation of stray DC from transit 
systems should be provided with surge current 
protection devices. 

6.3.4 In DC stray current areas, the grounding 
methods should be chosen to avoid creating 
interference problems. 



Section 7: Special Considerations in Operation and l\/laintenance of 
Cathodic Protection and Safety Systems 



7.1 Introduction 

7.1 .1 This section outlines safe maintenance and 
testing procedures for CP systems on structures 
subject to AC influence. 

7.2 Safety Measures for Operation and IVIaintenance of CP 
Systems 

7.2.1 CP rectifiers that are subject to damage by 
adjacent electric utility systems should be checked for 
proper operation at more frequent intervals than 
rectifiers not subject to electric system influence. 

7.2.2 CP testing or work of similar nature must not be 
performed on a structure subject to influence by an 
adjacent electric utility system during a period of 
thunderstorm activity in the area. 

7.2.3 Positive measures must be taken to maintain 
continuous rectifier operation when repeated outages 
can be attributed to adjacent electric utility system 
influences. One or more of the following mitigation 
measures may be employed: 

(a) Repetitive transient lightning arresters across the 
AC input and DC output terminals. 

(b) Heavy-duty choke coils installed in the AC and/or 
DC leads. 



7.2.4 If galvanic anodes are used for CP in an area of 
AC influence, and if test stations are available, the 
following tests should be conducted during each 
structure survey using suitable instrumentation: 

(a) Measure and record both the AC and DC from the 
anodes. 

(b) Measure and record both the AC and DC 
structure-to-electrolyte potentials. 

7.2.5 At all aboveground pipeline metallic 
appurtenances, devices used to keep the general 
public or livestock from coming into direct contact with 
the structure shall be examined for effectiveness. If the 
devices are found to be ineffective, they shall be 
replaced or repaired immediately. 

7.2.6 In making test connections for electrical 
measurements, all test leads, clips, and terminals must 
be properly insulated. Leads shall be connected to the 
test instruments before making connections to the 
structure. When each test is completed, the 
connections shall be removed from the structure before 
removing the lead connection from the instrument. All 
test connections must be made on a step-by-step 
basis, one at a time. 

7.2.7 When long test leads are laid out near a power 
line, significant potentials may be induced in these 



NACE International 



19 



SP01 77-2007 



leads. The hazards associated with this situation may 
be reduced by using the following procedures: 

(a) Properly insulate all test lead clips, terminals, and 
wires. 

(b) Avoid direct contact with bare test lead terminals. 

(c) Place the reference electrode in position for 
measurement prior to making any test connections. 

(d) Connect the lead to the reference electrode, and 
reel the wire back to the test location. 

(e) Connect the other test lead to the instrument and 
then to the structure. 

(f) Connect the reference electrode lead to the 
instrument. 



7.2.8 Tools, instruments, or other implements shall not 
be handed at any time between a person standing over 
a ground mat or grounding grid and a person who is 
not over the mat or grid. 

7.2.9 Grounding facilities for the purpose of mitigating 
AC effects should be carefully tested at regular 
intervals to ascertain the integrity of the grounding 
system. 

7.2.9.1 No disconnection or reconnection shall be 
allowed when a flammable or explosive 
atmosphere is suspected without first testing to 
ensure a safe atmosphere. 

7.2.9.2 No one shall make contact with the 
structure, either directly or through a test wire, 
while a grounding grid is disconnected for test 
purposes. 



(g) When the tests are complete, disconnect in 
reverse order. 

NOTE: Close interval pipe-to-electrolyte surveys using 
long lead wires require special procedures and 
precautions. 



7.2.9.3 Measurement of the resistance to earth of 
disconnected grounds shall be made promptly to 
minimize personnel hazards. 

7.2.10 All interference mitigation devices and test 
equipment should be maintained in accordance with 
the manufacturer's instructions. 



References 



1. G. Bodier, Bulletin de la Societe Francaise Des 
Electriciens, October 1947. 

2. C.F. Dalziel, "The Effects of Electrical Shock on Man," 
Transactions on Medical Electronics, PGME-5, Institute of 
Radio Engineers,'^* 1956. (Available from IEEE.) 



6. OSHA'^^' Standard 2207, Part 1926 (latest revision), 
"Construction, Safety, and Health Regulations" 
(Washington, DC: OSHA). 

7. ICEA P-32-382 (latest revision), "Short-Circuit 
Characteristics of Insulated Cable," (Carrollton, GA: ICEA). 



3. ANSI/IEEE Standard C2 (latest revision), "National 
Electrical Safety Code (NESC)" (New York, NY: 
ANSI/IEEE). 



8. J.H. Michel, "Ampacity Characteristics of Zinc Ribbon," 
CORROSION/2005, paper no. 621 (Houston, TX: NACE, 
2005). 



4. IEEE Standard 80 (latest revision), "Guide for Safety in 
AC Substation Grounding" (New York, NY: IEEE). 

5. NFPA'"' Standard 70 (latest revision), "National 
Electrical Code" (Ouincy, MA: National Fire Protection 
Association). Also available from the American National 
Standards Institute (ANSI),'"' New York, NY. 



9. Accident Prevention Manual for Business & Industry: 
Engineering & Technology, 12th ed. (Itasca, IL: National 
Safety Council,"^' 1992). 



''' The Institute of Radio Engineers (IRE) and the American Institute of Electrical Engineers (AIEE) merged in 1963 to form the Institute of 

Electrical and Electronics Engineers (IEEE). 

''"' National Fire Protection Agency (NFPA), 1 Batterymarch Park, Quincy, MA 02169-9101 . 

'"' American National Standards Institute (ANSI), 1819 L St. NW, 6th Floor, Washington, DC 20036. 



U.S. Occupational Safety and Health Administration (OSHA), 200 Constitution Ave. NW, Washington, DC 20210. 
National Safety Council (NSC), 1121 Spring Lake Drive, Itasca, IL 60143-3201. 



20 



NACE International 



SP01 77-2007 



10. Mutual Design Considerations for Overhead AC 
Transmission Lines and Gas Transmission Pipelines, 
Volume 1 : Engineering Analysis, and Volume 2: Prediction 
and IVIitigation Procedures, AGA*^*' Catalog No. L51278 
(Arlington, VA: AGA, 1978). Published in conjunction with 



The Electric Power Research Institute (EPRI) 



(15^ 



11. D.G. Fink, J.M. Carroll, Standard Handbook for 
Electrical Engineers, 10th ed. (New York, NY: McGraw-Hill, 
1968). 



Bibliography 



CAN/CSA-C22.3 No. 6-M91 (latest revision). "Principles 
and Practices of Electrical Coordination Between 
Pipelines and Electric Supply Lines." Etobicoke, ON: 
CSA.'^^* 



J. A. Lichtenstein. "Interference and Grounding Problems 
on Metallic Structures Paralleling Power Lines." Proc. 
Western States Corrosion Seminar. Houston, TX: 
NACE, 1982. 



CGA"" Standard OCC-3-1981 (latest revision). 
"Recommended Practice OCC-3-1981 for the Mitigation 
of Alternating Current and Lightning Effects on Pipelines, 
Metallic Structures, and Corrosion Control Systems." 
Toronto, ON: CGA. 

CIGRE"" Working Group 36.02 Guide. "Guide on the 
Influence of High Voltage AC Power Systems on Metallic 
Pipelines." 1995. 

Gilroy, D.E. "AC Interference — Important Issues for Cross 
Country Pipelines." CORROSION/2003, paper no. 699. 
Houston, TX. NACE, 2003. 

Gummow, R.A., R.G. Wakelin, and S.M. Segall. "AC 
Corrosion — A New Challenge to Pipeline Integrity." 
CORROSION/98, paper no. 566. Houston, TX: NACE, 
1998. 

Inductive Interference Engineering Guide. Murray Hills, NJ: 
Bell Telephone Laboratories, March, 1974. (Available 
through local Bell System Inductive Coordinator.) 

J. A. Lichtenstein. "Alternating Current and Lightning 
Hazards on Pipelines." Materials Performance 31, 12 
(1992): pp. 19-21. 



Marne, D., McGraw - Hill's National Electrical Safety Code 
(NESC) Handbook, New York, NY: IEEE, 2002. 

"Powerline Ground Fault Effects on Pipelines." Canadian 
Electricity Association" " Report No. 239 T 81 7, 1 994. 

"Some Safety Considerations for Pipelines Near Overhead 
Power Lines." NACE Audio/Visual Presentation. 
Prepared by Work Group WG 025a. Houston, TX: 
NACE, 2004. 

Southey, R.D., and F.P. Dawalibi, "Advances in Interference 
Analysis and Mitigation on Pipelines." In NACE 
International Canadian Region International Conference, 
Corrosion Prevention '95, held October 31, 1995. 
Houston, TX: NACE, 1995. 

Wakelin, R.G., R.A. Gummow, and S.M. Segall. "AC 
Corrosion — Case Histories, Test Procedures, and 
Mitigation." CORROSION/98, paper no. 565. Houston, 
TX: NACE, 1998. 

Westinghouse Transmission and Distribution Handbook. 
Newark, NJ: Westinghouse Electric Corp. Relay- 
Instrument Division, 1950. 



' American Gas Association (AGA), 1515 Wilson Blvd., Arlington, VA 22209. 
^' Electric Power Research Institute (EPRI), 3420 Hillview Ave., Palo Alto, CA 94304. 
*' CSA International (CSA), 178 Rexdale Blvd., Etobicoke, Ontario M9W IR3, Canada. 
'' Canadian Gas Association (CGA), 350 Sparks St., Suite 809, Ottawa, Ontario K1 R 7S8, Canada. 
^' International Council on Large Electronic Systems (CIGRE), 21 rue d'Artois, 75008 Paris, France. 
" Canadian Electricity Association (CEA), 66 Slater St., Suite 1210, Ottawa, Ontario K1P 5H1, Canada. 



NACE International 



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SP01 77-2007 

Appendix A: Wire Gauge Conversions 
(Nonmandatory) 

Table A1 provides the nearest metric size for the conductor sizes mentioned in this standard. 

TABLE A1 : Wire Gauge Conversions^^ 



Conductor Size 


Diameter in miis 


Nearest metric size (mm^) 


Diameter in mm of nearest 
metric size 


4,000 IVICIVI 


2,000 


2,000 


50.5 


2,000 IVICIVI 


1,410 


1,000 


35.7 


1,000 MCM 


1,000 


500 


25.2 


500 MCM 


707 


240 


17.5 


250 MCM 


500 


120 


12.4 


4/0 AWG 


460 


120 


12.4 


3/0 AWG 


410 


80 


10.01 


2/0 AWG 


365 


70 


9.44 


1/0 AWG 


325 


50 


7.98 


1 AWG 


290 


50 


7.98 


2 AWG 


258 


35 


6.68 


4 AWG 


204 


25 


5.64 


6 AWG 


162 


16 


4.51 


8 AWG 


128 


10 


3.57 


10 AWG 


102 


6 


2.76 



22 



NACE International 



^NACE 



ERNATIONAL 



THE CORROSION SOCIETY 



NACE Standard TM0497-2002 
Item No. 21231 



Standard 
Test Method 

Measurement Techniques Related to Criteria for 

Cathodic Protection on Underground or 

Submerged Metallic Piping Systems 

This NACE International standard represents a consensus of those individual members who have 
reviewed this document, its scope, and provisions. Its acceptance does not in any respect 
preclude anyone, whether he has adopted the standard or not, from manufacturing, marketing, 
purchasing, or using products, processes, or procedures not in conformance with this standard. 
Nothing contained in this NACE International standard is to be construed as granting any right, by 
implication or otherwise, to manufacture, sell, or use in connection with any method, apparatus, or 
product covered by Letters Patent, or as indemnifying or protecting anyone against liability for 
infringement of Letters Patent. This standard represents minimum requirements and should in no 
way be interpreted as a restriction on the use of better procedures or materials. Neither is this 
standard intended to apply in all cases relating to the subject. Unpredictable circumstances may 
negate the usefulness of this standard in specific instances. NACE International assumes no 
responsibility for the interpretation or use of this standard by other parties and accepts 
responsibility for only those official NACE International interpretations issued by NACE 
International in accordance with its governing procedures and policies which preclude the 
issuance of interpretations by individual volunteers. 

Users of this NACE International standard are responsible for reviewing appropriate health, safety, 
environmental, and regulatory documents and for determining their applicability in relation to this 
standard prior to its use. This NACE International standard may not necessarily address all 
potential health and safety problems or environmental hazards associated with the use of 
materials, equipment, and/or operations detailed or referred to within this standard. Users of this 
NACE International standard are also responsible for establishing appropriate health, safety, and 
environmental protection practices, in consultation with appropriate regulatory authorities if 
necessary, to achieve compliance with any existing applicable regulatory requirements prior to the 
use of this standard. 

CAUTIONARY NOTICE: NACE International standards are subject to periodic review, and may be 
revised or withdrawn at any time without prior notice. NACE International requires that action be 
taken to reaffirm, revise, or withdraw this standard no later than five years from the date of initial 
publication. The user is cautioned to obtain the latest edition. Purchasers of NACE International 
standards may receive current information on all standards and other NACE International 
publications by contacting the NACE International IVIembership Services Department, 1440 South 
Creek Drive, Houston, Texas 77084-4906 (telephone +1 281/228-6200). 

Reaffirmed 2002-04-1 1 
Approved 1997-12-22 

NACE International 

1440 South Creek Drive 

Houston, Texas 77084-4906 

-1-1 281/228-6200 

ISBN 1-57590-047-5 
©2002, NACE International 



TM0497-2002 



Foreword 

This NACE International standard test method provides descriptions of the measurement 
techniques and cautionary measures most commonly used on underground piping to determine 
whether a specific criterion has been complied with at a test site. This test method includes only 
those measurement techniques that relate to the criteria or special conditions, such as a net 
protective current, contained in NACE Standard RP0169.^ This test method is intended for use by 
corrosion control personnel concerned with the corrosion of buried underground or submerged 
piping systems, including oil, gas, water, and similar structures. 

The measurement techniques described require that the measurements be made in the field. 
Because the measurements are obtained under widely varying circumstances of field conditions 
and pipeline design, this standard is not as prescriptive as those NACE standard test methods that 
use laboratory measurements. Instead, this standard gives the user latitude to make testing 
decisions in the field based on the technical facts available. 

This standard contains instrumentation and general measurement guidelines. It includes methods 
for voltage drop considerations when making pipe-to-electrolyte potential measurements and 
provides guidance to prevent incorrect data from being collected and used. 

The measurement techniques provided in this standard were compiled from information submitted 
by committee members and others with expertise on the subject. Variations or other techniques 
not included may be equally effective. The complexity and diversity of environmental conditions 
may require the use of other techniques. 

Appendix A contains information on the common types, use, and maintenance of reference 
electrodes. Appendix B contains information for the net protective current technique, which, while 
not a criterion, is a useful technique to reduce corrosion. Appendix C contains information 
regarding the use of coupons to evaluate cathodic protection. While some engineers use these 
techniques, they are not universally accepted practices. However, there is ongoing research into 
their use. 

The test methods in this standard were originally prepared by NACE Task Group T-10A-3 on Test 
Methods and Measurement Techniques Related to Cathodic Protection Criteria, a component of 
Unit Committee T-1 OA on Cathodic Protection. It was reviewed by Task Group 020 and reaffirmed 
in 2002 by Specific Technology Group (STG) 35 on Pipelines, Tanks, and Well Casings. This 
standard is issued by NACE under the auspices of STG 35. 



In NACE standards, the terms shall, must, should, and may are used in accordance with the 
definitions of these terms in the NACE Publications Style Manual, 4th ed.. Paragraph 7.4.1.9. Shall 
and must are used to state mandatory requirements. Should \s used to state that which is considered 
good and is recommended but is not absolutely mandatory. May is used to state that which is 
considered optional. 



NACE International 



TM0497-2002 



NACE International 

Standard 

Test Method 

Measurement Techniques Related to Criteria 

for Cathodic Protection on Underground 

or Submerged Metallic Piping Systems 

Contents 

1 . General 1 

2. Definitions 1 

3. Safety Considerations 3 

4. Instrumentation and Measurement Guidelines 3 

5. Pipe-to-Electrolyte Potential Measurements 4 

6. Causes of Measurement Errors 7 

7. Voltage Drops Other Than Across the Pipe Metal/Electrolyte Interface 8 

8. Test Method 1— Negative 850 mV Pipe-to-Electrolyte Potential 

of Steel and Cast Iron Piping With Cathodic Protection Applied 1 

9. Test Method 2— Negative 850 mV Polarized Pipe-to-Electrolyte 

Potential of Steel and Cast Iron Piping 1 1 

10. Test Method 3—100 mV Cathodic Polarization 

of Steel, Cast Iron, Aluminum, and Copper Piping 13 

References 17 

Bibliography 17 

Appendix A: Reference Electrodes 18 

Appendix B: Net Protective Current 19 

Appendix C: Using Coupons to Determine Adequacy of Cathodic Protection 25 

Figures 

Figure 1 : Instrument Connections 6 

Figure 2: Pipe-to-Electrolyte Potential Corrections for Pipeline Current Flow 9 

Figure 3: Cathodic Polarization Curves 14 

Figure B1 : Surface Potential Survey 23 

Figure B2: Pipe-to-Electrolyte Potential Survey of a 

Noncathodically Protected Pipeline 24 



NACE International 



TM0497-2002 



Section 1 : General 



1.1 This standard provides testing procedures to comply 
witii tiie requirements of a criterion at a test site on a buried 
or submerged steel, cast iron, copper, or aluminum pipeline. 

1.2 The provisions of this standard shall be applied by 
personnel who have acquired by education and related 
practical experience the principles of cathodic protection of 
buried and submerged metallic piping systems. 



1.3 Special conditions in which a given test technique is 
ineffective or only partially effective sometimes exist. Such 
conditions may include elevated temperatures, disbonded 
dielectric or thermally insulating coatings, shielding, 
bacterial attack, and unusual contaminants in the 
electrolyte. Deviation from this standard may be warranted 
in specific situations. In such situations corrosion control 
personnel should be able to demonstrate that adequate 
cathodic protection has been achieved. 



Section 2: Definitions 



(1) 



Anode: The electrode of an electrochemical cell at which 
oxidation occurs. Electrons flow away from the anode in the 
external circuit. Corrosion usually occurs and metal ions 
enter the solution at the anode. 

Cable: A bound or sheathed group of insulated conductors. 

Cathode: The electrode of an electrochemical cell at which 
reduction is the principal reaction. Electrons flow toward the 
cathode in the external circuit. 

Cathodic Disbondment: The destruction of adhesion 
between a coating and the coated surface caused by 
products of a cathodic reaction. 

Cathodic Poiarization: The change of electrode potential 
in the active (negative) direction caused by current across 
the electrode/electrolyte interface. See also Polarization. 

Cathodic Protection: A technique to reduce the corrosion 
of a metal surface by making that surface the cathode of an 
electrochemical cell. 

Cathodic Protection Coupon: A metal sample 
representing the pipeline at the test site, used for cathodic 
protection testing, and having a chemical composition 
approximating that of the pipe. The coupon size should be 
small to avoid excessive current drain on the cathodic 
protection system. 

Coating: A liquid, liquefiable, or mastic composition that, 
after application to a surface, is converted into a solid 
protective, decorative, or functional adherent film. 

Conductor: A bare or insulated material suitable for 
carrying electric current. 



Corrosion: The deterioration of a material, usually a metal, 
that results from a reaction with its environment. 

Corrosion Potential (Ecorr): The potential of a corroding 
surface in an electrolyte relative to a reference electrode 
under open-circuit conditions (also known as rest potential, 
open-circuit potential, or freely corroding potential). 

Criterion: A standard for assessment of the effectiveness 
of a cathodic protection system. 

Current Density: The current to or from a unit area of an 
electrode surface. 

Electrical Isolation: The condition of being electrically 
separated from other metallic structures or the environment. 

Electrode: A conductor used to establish contact with an 
electrolyte and through which current is transferred to or 
from an electrolyte. 

Electrode Potential: The potential of an electrode in an 
electrolyte as measured against a reference electrode. 
(The electrode potential does not include any resistance 
losses in potential in either the electrolyte or the external 
circuit. It represents the reversible work to move a unit 
charge from the electrode surface through the electrolyte to 
the reference electrode.) 

Electrolyte: A chemical substance containing ions that 
migrate in an electric field. (For the purpose of this 
standard, electrolyte refers to the soil or liquid, including 
contained moisture and other chemicals, next to and in 
contact with a buried or submerged metallic piping system.) 

Foreign Structure: Any metallic structure that is not 
intended as part of a system under cathodic protection. 



''' Definitions in this section reflect common usage among practicing corrosion control personnel and apply specifically to how terms are used 
in this standard. As much as possible, these definitions are in accord with those in the "NACE Glossary of Corrosion-Related Terms" 
(Houston, TX: NACE). 



NACE International 



TM0497-2002 



Free Corrosion Potential: See Corrosion Potential. 

Gaivanic Anode: A metal that provides sacrificial 
protection to another metal that is more noble when 
electrically coupled in an electrolyte. This type of anode is 
the current source in one type of cathodic protection. 

hHoiiday: A discontinuity in a protective coating that 
exposes unprotected surface to the environment. 

impressed Current: An electric current supplied by a 
device employing a power source that is external to the 
electrode system. (An example is direct current for cathodic 
protection.) 

"instant Off" Potential: A measurement of a pipe-to- 
electrolyte potential made without perceptible delay 
following the interruption of cathodic protection. 

interference: Any electrical disturbance on a metallic 
structure as a result of stray current. 

isolation: See Electrical Isolation. 

Long-Line Current: Current through the earth between an 
anodic and a cathodic area that returns along an 
underground metallic structure. 

Long-Line Current Voltage Drop Error: That voltage drop 
error in the "off" potential that is caused by current flow in 
the soil due to potential gradients along the pipe surface. 

"Off" or "On": A condition whereby cathodic protection 
current is either turned off or on. 

Pipe-to-Electrolyte Potential: The potential difference 
between the pipe metallic surface and electrolyte that is 
measured with reference to an electrode in contact with the 
electrolyte. This measurement is commonly termed pipe-to- 
soil (P/S). 

Pipe-to-Soil: See Pipe-to-Electrolyte Potential. 

Polarization: The change from the open-circuit potential as 
a result of current across the electrode/electrolyte interface. 

Polarized Potential: The potential across the 
structure/electrolyte interface that is the sum of the 
corrosion potential and the cathodic polarization. 

Potential Gradient: A change in the potential with respect 
to distance, expressed in millivolts per unit of distance. 

Protection Potential: A measured potential meeting the 
requirements of a cathodic protection criterion. 

Reference Electrode: An electrode whose open-circuit 
potential is constant under similar conditions of 



measurement, which is used for measuring the relative 
potentials of other electrodes. 

Resistance to Electrolyte: The resistance of a structure to 
the surrounding electrolyte. 

Reverse-Current Switcli: A device that prevents the 
reversal of direct current through a metallic conductor. 

Shielding: Preventing or diverting the cathodic protection 
current from its intended path to the structure to be 
protected. 

Sliorted Pipeline Casing: A casing that is in metallic 
contact with the carrier pipe. 

Side Drain Potential: A potential gradient measured 
between two reference electrodes, one located over the 
pipeline and the other located a specified distance lateral to 
the direction of the pipe. 

Sound Engineering Practices: Reasoning exhibited or 
based on thorough knowledge and experience, logically 
valid, and having true premises showing good judgment or 
sense in the application of science. 

Stray Current: Current through paths other than the 
intended circuit. 

Telluric Current: Current in the earth that results from 
geomagnetic fluctuations. 

Test Lead: A wire or cable attached to a structure for 
connection of a test instrument to make cathodic protection 
potential or current measurements. 

Voltage: An electromotive force or a difference in electrode 
potentials expressed in volts. 

Voltage Drop: The voltage across a resistance according 
to Ohm's Law. 

Voltage Splicing: A momentary surging of potential that 
occurs on a pipeline when the protective current flow from 
an operating cathodic protection device is interrupted or 
applied. This phenomenon is the result of inductive and 
capacitive electrical characteristics of the system and may 
be incorrectly recorded as an "off" or "on" pipe-to-electrolyte 
potential measurement. This effect may last for several 
hundred milliseconds and is usually larger in magnitude 
near the connection of the cathodic protection device to the 
pipeline. An oscilloscope or similar instrument may be 
necessary to identify the magnitude and duration of the 
spiking. 

Wire: A slender rod or filament of drawn metal. In practice, 
the term is also used for smaller gauge conductors (size 6 
mm^ [No. 1 AWG'^'] or smaller). 



(2) 



American Wire Gauge (AWG). 



NACE International 



TM0497-2002 



Section 3: Safety Considerations 



3.1 Appropriate safety precautions, including tiie following, 
shall be observed when making electrical measurements. 

3.1 .1 Be knowledgeable and qualified in electrical 
safety precautions before installing, adjusting, 
repairing, removing, or testing impressed current 
cathodic protection equipment. 

3.1 .2 Use properly insulated test lead clips and 
terminals to avoid contact with unanticipated high 
voltage (HV). Attach test clips one at a time using a 
single-hand technique for each connection. 

3.1 .3 Use caution when long test leads are extended 
near overhead high-voltage alternating current (HVAC) 
power lines, which can induce hazardous voltages onto 
the test leads. High-voltage direct current (HVDC) 
power lines do not induce voltages under normal 
operation, but transient conditions may cause 
hazardous voltages. 



3.1 .4 Use caution when making tests at electrical 
isolation devices. Before proceeding with further tests, 
use appropriate voltage detection instruments or 
voltmeters with insulated test leads to determine 
whether hazardous voltages may exist. 

3.1 .5 Avoid testing when thunderstorms are in the 
area. Remote lightning strikes can create hazardous 
voltage surges that travel along the pipe under test. 

3.1 .6 Use caution when stringing test leads across 
streets, roads, and other locations subject to vehicular 
and pedestrian traffic. When conditions warrant, use 
appropriate barricades, flagging, and/or flag persons. 

3.1 .7 Before entering, inspect excavations and 
confined spaces to determine that they are safe. 
Inspections may include shoring requirements for 
excavations and testing for hazardous atmospheres in 
confined spaces. 



3.1.3.1 Refer to NACE Standard RP01772 
additional information about electrical safety. 



for 



3.1 .8 Observe appropriate 
applicable safety regulations. 



electrical codes and 



Section 4: Instrumentation and l\/leasurement Guidelines 



4.1 Cathodic protection electrical measurements require 
proper selection and use of instruments. Pipe-to-electrolyte 
potential, voltage drop, potential difference, and similar 
measurements require instruments that have appropriate 
voltage ranges. The user should know the capabilities and 
limitations of the equipment, follow the manufacturer's 
instruction manual, and be skilled in the use of electrical 
instruments. Failure to select and use instruments correctly 
causes errors in cathodic protection measurements. 

4.1 .1 Analog instruments are usually specified in 
terms of input resistance or internal resistance. This is 
usually expressed as ohms per volt of full meter scale 
deflection. 

4.1 .2 Digital instruments are usually specified in terms 
of input impedance expressed as megaohms. 

4.2 Factors that may influence instrument selection for field 
testing include: 

(a) Input impedance (digital instruments); 

(b) Input resistance or internal resistance (analog 
instruments); 

(c) Sensitivity; 

(d) Conversion speed of analog-to-digital converters used 
in digital or data logging instruments; 

(e) Accuracy; 



(f) Instrument resolution; 

(g) Ruggedness; 

(h) Alternating current (AC) and radio frequency (RF) 

signal rejection; and 

(i) Temperature and/or climate limitations. 

4.2.1 Some instruments are capable of measuring and 
processing voltage readings many times per second. 
Evaluation of the input wave-form processing may be 
required if an instrument does not give consistent 
results. 

4.2.2 IVIeasurement of pipe-to-electrolyte potentials on 
pipelines affected by dynamic stray currents may 
require the use of recording or analog instruments to 
improve measurement accuracy. Dynamic stray 
currents include those from electric railway systems, 
HVDC transmission systems, mining equipment, and 
telluric currents. 

4.3 Instrument Effects on Voltage Measurements 

4.3.1 To measure pipe-to-electrolyte potentials 
accurately, a digital voltmeter must have a high input 
impedance (high internal resistance, for an analog 
instrument) compared with the total resistance of the 
measurement circuit. 



NACE International 



TM0497-2002 



4.3.1.1 An input impedance of 10 megaolims or 
more stiould be sufficient for a digital meter. An 
instrument with a lower input impedance may 
produce valid data if circuit contact errors are 
considered. One means of making accurate 
measurements is to use a potentiometer circuit in 
an analog meter. 

4.3.1 .2 A voltmeter measures the potential across 
its terminals within its design accuracy. However, 
current flowing through the instrument creates 
measurement errors due to voltage drops that 
occur in all resistive components of a 
measurement circuit. 

4.3.2 Some analog-to-digital converters used in digital 
and data logging instruments operate so fast that the 
instrument may indicate only a portion of the input 
waveform and thus provide incorrect voltage 
indications. 

4.3.3 Parallax errors on an analog instrument can be 
minimized by viewing the needle perpendicular to the 



face of the instrument on the centerline projected from 
the needle point. 

4.3.4 The accuracy of potential measurements should 
be verified by using an instrument having two or more 
input impedances (internal resistance, for analog 
instruments) and comparing potential values measured 
using different input impedances. If the measured 
values are virtually the same, the accuracy is 
acceptable. Corrections need to be made if measured 
values are not virtually identical. Digital voltmeters that 
have a constant input impedance do not indicate a 
measurement error by changing voltage ranges. An 
alternative is to use a meter with a potentiometer 
circuit. 

4.4 Instrument Accuracy 

4.4.1 Instruments shall be checked for accuracy 
before use by comparing readings to a standard 
voltage cell, to another acceptable voltage source, or to 
another appropriate instrument known to be accurate. 



Section 5: Pipe-to-Electrolyte Potential l\/leasurements 



5.1 Instruments used to measure AC voltage, direct current 
(DC) voltage, or other electrical functions usually have one 
terminal designated "Common" (COM). This terminal either 
is black in color or has a negative (-) symbol. The positive 
terminal either is red in color or has a positive (+) symbol. 
The positive and negative symbols in the meter display 
indicate the current flow direction through the instrument 
(Figure la). For example, a positive symbol in the meter 
display indicates current flowing from the positive terminal 
through the meter to the negative terminal. One instrument 
test lead is usually black in color and the other red. The 
black test lead is connected to the negative terminal of the 
instrument and the red lead to the positive terminal. 

5.2 Voltage measurements should be made using the 
lowest practicable range on the instrument. A voltage 
measurement is more accurate when it is measured in the 
upper two-thirds of a range selected for a particular 
instrument. Errors can occur, for example, when an 
instrument with a 2-V range is used to measure a voltage of 
15 mV. Such a value might be a voltage drop caused by 
current flowing in a metal pipeline or through a calibrated 
shunt. A much more accurate measurement would be 
made using an instrument having a 20-mV range. 

5.3 The usual technique to determine the DC voltage 
across battery terminals, pipeline metal/electrolyte interface, 
or other DC system is to connect the black test lead to the 
negative side of the circuit and the red test lead to the 
positive side of the circuit. When connected in this manner, 
an analog instrument needle moves in an upscale 



(clockwise) direction indicating a positive value with relation 
to the negative terminal. A digital instrument connected in 
the same manner displays a digital value, usually preceded 
by a positive symbol. In each situation the measured 
voltage is positive with respect to the instrument's negative 
terminal. (See instrument connections in Figure la.) 

5.4 The voltage present between a reference electrode and 
a metal pipe can be measured with a voltmeter. The 
reference electrode potential is normally positive with 
respect to ferrous pipe; conversely the ferrous pipe is 
negative with respect to the reference electrode. 

5.5 A pipe-to-electrolyte potential is measured using a DC 
voltmeter having an appropriate input impedance (or 
internal resistance, for an analog instrument), voltage 
range(s), test leads, and a stable reference electrode, such 
as a saturated copper/copper sulfate (CSE), silver/silver 
chloride (Ag/AgCI), or saturated potassium chloride (KCI) 
calomel reference electrode. The CSE is usually used for 
measurements when the electrolyte is soil or fresh water 
and less often for salt water. When a CSE is used in a 
high-chloride environment, the stability (lack of 
contamination) of the CSE must be determined before the 
readings may be considered valid. The Ag/AgCI reference 
electrode is usually used in seawater environments. The 
saturated KCI calomel electrode is used more often for 
laboratory work. However, more-rugged, polymer-body, 
gel-filled saturated KCI calomel electrodes are available, 
though modifications may be necessary to increase contact 
area with the environment. 



NACE International 



TM0497-2002 



5.6 Meter Polarity 

5.6.1 Pipe-to-electrolyte potentials are usually 
measured by connecting the instrument negative 
terminal to the pipe and the positive terminal to the 
reference electrode, which is in contact with the pipe 
electrolyte. With this connection the instrument 
indicates that the reference electrode is positive with 
respect to the pipe. Because the reference electrode 
has a positive value with respect to the pipe, the pipe 
voltage is negative with respect to the reference 
electrode (see Figure 1a). This negative pipe-to- 
electrolyte potential is the value used for NACE criteria. 

5.6.2 Pipe-to-electrolyte potential measurements are 
sometimes made with the reference electrode 
connected to the instrument negative terminal and the 
pipeline to the positive terminal. Figure lb illustrates 
this connection. 

5.6.2.1 If the instrument is a data logging device, 
the recorded data may be printed out with a 
negative symbol unless a polarity reversal occurs. 

5.7 The pipe-to-electrolyte potential measurement of a 
buried pipe should be made with the reference electrode 
placed close to the metal/electrolyte interface of the pipe. 
The common practice, however, is to place the reference 
electrode as close to the pipe as practicable, which is 
usually at the surface of the earth above the centerline of 
the pipe. (See Figure la.) This measurement includes a 
combination of the voltage drops associated with the: 



(a) 


Voltmeter; 


(b) 


Test leads; 


(c) 


Reference electrode; 


(d) 


Electrolyte; 


(e) 


Coating, if applied; 


(f) 


Pipe; and 


(g) 


Pipe metal/electrolyte interface. 



5.8 The pipe-to-electrolyte potential measurement as 
described above is a resultant of the: 

(a) Voltage drop created by current flowing through the 
electrical resistances of the items listed in Paragraph 5.7; 
and 

(b) For coated pipe, the influence of coating holidays, 
depending on their location, number, and size. 

5.9 Pipe-to-electrolyte potential measurements made to 
determine the level of cathodic protection at the test site 
should consider the following: 

(a) Effectiveness of coatings, particularly those known or 
suspected to be deteriorated or damaged; 

(b) Bare sections of pipe; 

(c) Bonds to mitigate interference; 

(d) Parallel coated pipelines, electrically connected and 
polarized to different potentials; 

(e) Shielding; 



(f) Effects of other structures on the measurements; 

(g) History of corrosion leaks and repairs; 
(h) Location of impressed current anodes; 

(i) Unknown, inaccessible, or direct-connected galvanic 

anodes; 

(j) Location of isolation devices, including high-resistance 

pipe connections and compression couplings; 

(k) Presence of electrolytes, such as unusual corrosives, 

chemical spills, extreme soil resistivity changes, acidic 

waters, and contamination from sewer spills; 

(I) Location of shorted or isolated casings; 

(m) DC interference currents, such as HVDC, telluric, 

welding equipment, foreign rectifier, mining equipment, and 

electric railway or transit systems; 

(n) Contacts with other metals or structures; 

(o) Locations where the pipe enters and leaves the 

electrolyte; 

(p) Areas of construction activity during the pipeline 

history; 

(q) Underground metallic structures close to or crossing 

the pipeline; 

(r) Valves and other appurtenances; and 

(s) HVAC overhead power lines. 

5.10 Voltage drops other than those across the pipe 
metal/electrolyte interface shall be considered for valid 
interpretation of pipe-to-electrolyte voltage measurements 
made to satisfy a criterion. Measurement errors should be 
minimized to ensure reliable pipe-to-electrolyte potential 
measurements. 

5.1 1 The effect of voltage drops on a pipe-to-electrolyte 
potential measurement can be determined by interrupting 
all significant current sources and then making the 
measurement. This measurement is referred to as an 
"instant-off" potential. The measurement must be made 
without perceptible delay after current interruption to avoid 
loss of polarization. The voltage value measured is 
considered to be the "polarized potential" of the pipe at that 
location. Because the current interruption may cause a 
voltage spike, recording the spike as the "instant-off 
potential" must be avoided. The magnitude and duration of 
the voltage spike can vary; however, the duration is usually 
within 0.5 second. The following are examples of when it 
may not be practical to interrupt all current sources to make 
the "instant-off potential" measurement. 

5.11.1 Galvanic Anodes 

5.11.1.1 Galvanic anodes connected directly to 
the pipe without benefit of aboveground test 
stations or connections. Interruption requires 
excavation of the connections. 

5.1 1 .2 Impressed Current Systems 

5.11.2.1 Galvanic anodes directly connected to 
piping protected using an impressed current 
system; 

5.1 1 .2.2 Multiple impressed current sources; 



NACE International 



TM0497-2002 



Voltmeters 



f ^ 


+0.850 


(g) vTlt®- 




Pipe Test Lead 




Direction of meter current 



Reference 
Electrode 

Electrode potential 
does not vary 



Pipe potential 
is the variable 



Figure 1a 
Instrument Connection 



Voltmeter 



0.850 



0^ 



Direction of meter current 



Reference 
Electrode 



Pipe Test Lead 




Electrode potential 
does not vary 



Pipe potential 
is the variable 



Figure 1b 
Alternative Instrument Connection 

FIGURE 1 
Instrument Connections 



NACE International 



TM0497-2002 



5.11.2.3 Impressed current devices on foreign 
piping; and 

5.11.2.4 Numerous cross bonds to parallel 
pipelines. 

5.1 1 .3 Natural and Manmade Stray Currents 

5.11.3.1 Telluric currents; and 



5.11.3.2 IVIanmade DC stray currents, such 
those from mass transit and mining operations. 



as 



5.12 When voltage drops have been evaluated at a test 
location and the pipe-to-electrolyte potential found to be 
satisfactory, the "on" pipe-to-electrolyte potential value may 
be used for monitoring until significant environmental, 
structural, or cathodic protection system parameters 
change. 



5.12.1 Significant environmental, structural, or 
cathodic protection system parameter changes may 
include: 

(a) Replacement or addition of piping; 

(b) Addition, relocation, or deterioration of cathodic 
protection systems; 

(c) Failure of electrical isolating devices; 

(d) Effectiveness of coatings; and 

(e) Influence of foreign structures. 

5.13 After a cathodic protection system is operating, time 
may be required for the pipe to polarize. This should be 
considered when measuring the potential at a test site on a 
newly protected pipe or after reenergizing a cathodic 
protection device. 



Section 6: Causes of Measurement Errors 



6.1 Factors that contribute 
measurements include: 



to faulty potential 



6.1.1 Pipe and instrument test leads 

(a) Broken or frayed wire strands (may not be visible 
inside the insulation); 

(b) Damaged or defective test lead insulation that 
allows the conductor to contact wet vegetation, the 
electrolyte, or other objects; 

(c) Loose, broken, or faulty pipe or instrument 
connections; and 

(d) Dirty or corroded connection points. 

6.1 .2 Reference electrode condition and placement 

(a) Contaminated reference electrode solution or 
rod, and solutions of insufficient quantity or saturation 
(only laboratory-grade chemicals and distilled water, if 
water is required, should be used in a reference 
electrode); 

(b) Reference electrode plug not sufficiently porous 
to provide a conductive contact to the electrolyte; 

(c) Porous plug contaminated by asphalt, oil, or 
other foreign materials; 

(d) High-resistance contact between reference 
electrode and dry or frozen soil, rock, gravel, 
vegetation, or paving material; 

(e) Reference electrode placed in the potential 
gradient of an anode; 

(f) Reference electrode positioned in the potential 
gradient of a metallic structure other than the one with 
the potential being measured; 

(g) Electrolyte between pipe and disbonded coating 
causing error due to electrode placement in electrolyte 
on opposite side of coating; 



(h) Defective permanently installed reference 

electrode; 

(i) Temperature correction not applied when 

needed; and 

(j) Photo-sensitive measurement error (in CSE with 

a clear-view window) due to light striking the electrode 

electrolyte solution (photovoltaic effect). 

6.1.3 Unknown isolating devices, such as unbonded 
tubing or pipe compression fittings, causing the pipe to 
be electrically discontinuous between the test 
connection and the reference electrode location. 

6.1 .4 Parallel path inadvertently established by test 
personnel contacting instrument terminals or metallic 
parts of the test lead circuit, such as test lead clips and 
reference electrodes, while a potential measurement is 
being made. 

6.1 .5 Defective or inappropriate instrument, incorrect 
voltage range selection, instrument not calibrated or 
zeroed, or a damp instrument sitting on wet earth. 

6.1 .6 Instrument having an analog-to-digital converter 
operating at such a fast speed that the voltage spikes 
produced by current interruption are indicated instead 
of the actual "on" and "off" values. 

6.1 .7 Polarity of the measured value incorrectly 
observed. 

6.1 .8 Cathodic protection current-carrying conductor 
used as a test lead for a pipe potential measurement. 



NACE International 



TM0497-2002 



6.1 .9 Interference 



6.1 .9.1 Electromagnetic interference or induction 
resulting from AC power lines or radio frequency 
transmitters inducing test lead and/or instrument 
errors. This condition is often indicated by a fuzzy, 
fluctuating, or blurred pointer movement on an 
analog instrument or erratic displays on digital 
voltmeters. A DC voltmeter must have sufficient 
AC rejection capability, which can be determined 
by referring to the manufacturer's specification. 



6.1.9.2 Telluric or stray DC 
through the earth and piping. 



currents flowing 



6.2 Reference electrode contact resistance is reduced by: 

6.2.1 Soil moisture — If the surface soil is so dry that 
the electrical contact of the reference electrode with the 



electrolyte is impaired, the soil around the electrode 
may be moistened with water until the contact is 
adequate. 

6.2.2 Contact surface area — Contact resistance may 
be reduced by using a reference electrode with a larger 
contact surface area. 

6.2.3 Frozen soil — Contact resistance may be reduced 
by removing the frozen soil to permit electrode contact 
with unfrozen soil. 

6.2.4 Concrete or asphalt-paved areas — Contact 
resistance may be reduced by drilling through the 
paving to permit electrode contact with the soil. 



Section 7: Voltage Drops Other Than Across the Pipe l\/! eta I/Electrolyte Interface 



7.1 Voltage drops that are present when pipe-to-electrolyte 
potential measurements are made occur in the following: 

7.1 .1 IVIeasurement Circuit — The voltage drop other 
than across the pipe metal/electrolyte interface in the 
measurement circuit is the sum of the individual 
voltage drops caused by the meter current flow through 
individual resistances that include: 

(a) Instrument test lead and connection resistances; 

(b) Reference electrode internal resistance; 

(c) Reference electrode-to-electrolyte contact 
resistance; 

(d) Coating resistance; 

(e) Pipe metallic resistance; 

(f) Electrolyte resistance; 

(g) Analog meter internal resistance; and 
(h) Digital meter internal impedance. 

A measurement error occurs if the analog meter 
internal resistance or the digital meter internal 
impedance is not several orders of magnitude higher 
than the sum of the other resistances in the 
measurement circuit. 

7.1 .2 Pipe — Current flowing within the pipe wall 
creates a voltage drop. This voltage drop and the 
direction of the current shall be considered when the 
reference electrode is not near the pipe connection and 
significant current is conducted by the pipe. 
Consideration is needed because an error in the pipe- 
to-electrolyte potential measurement will occur if the 
pipe current causes a significant voltage drop. Current 
directed to the pipe connection from the reference 
electrode causes the measured potential to be more 
negative by the amount of the pipe current voltage drop 
(see Figure 2a). Conversely, the potential is less 
negative by that amount if the pipe current direction is 



from the pipe connection to the reference electrode 
(see Figure 2b). 

7.1 .3 Electrolyte — When a pipe-to-electrolyte potential 
is measured with cathodic protection current applied, 
the voltage drop in the electrolyte between the 
reference electrode and the metal/electrolyte interface 
shall be considered. IVIeasurements taken close to 
sacrificial or impressed current anodes can contain a 
large voltage drop. Such a voltage drop can consist of, 
but is not limited to, the following: 

(a) A voltage drop caused by current flowing to 
coating holidays when the line is coated; and 

(b) A voltage drop caused by large voltage gradients 
in the electrolyte that occur near operating anodes 
(sometimes termed "raised earth effect"). 

7.1 .3.1 Testing to locate galvanic anodes by 
moving the reference electrode along the 
centerline of the line may be necessary when the 
locations are not known. 

7.1 .4 Coatings — IVIost coatings provide protection to 
the pipe by reducing the pipe surface contact with the 
environment. Due to the relative ionic impermeability 
of coatings, they resist current flow. While the 
insulating ability of coatings reduces the current 
required for cathodic protection, coatings are not 
impervious to current flowing through them. Current 
flow through the coating causes a voltage drop that is 
greater than when the pipe is bare, under the same 
environmental conditions. 

7.2 Specialized equipment that uses various techniques to 
measure the impressed current wave form and to calculate 
a pipe-to-electrolyte potential free of voltage drop is 



NACE International 



available. This equipment may minimize problems resulting 
from spiking effects, drifting of interrupters, and current from 
other DC sources. 



TM0497-2002 



VOLTMETER 



PIPE TEST 
CONNECTION 



REFERENCE 
ELECTRODE 




-PIPE METAL VOLTAGE DROP 



PIPELINE CURRENT 



ADD PIPE METAL VOLTAGE DROP TO PIPE-TO-ELECTROLYTE 
MEASUREMENT WHEN CURRENT IS TOWARD PIPE CONTACT 

Figure 2a 
Correction When Pipeline Current Flows Toward Pipe Test Connection 



T t 



VOLTMETER 



PIPE TEST 
CONNECTION 



REFERENCE 
ELECTRODE 



-PIPE METAL VOLTAGE DROP 



PIPELINE CURRENT 



SUBTRACT PIPE METAL VOLTAGE DROP FROM PIPE-TO-ELECTROLYTE 
MEASUREMENT WHEN CURRENT IS AWAY FROM PIPE CONTACT 

Figure 2b 
Correction When Pipeline Current Flows Away from Pipe Test Connection 

FIGURE 2 
Pipe-to-Electrolyte Potential Corrections for Pipeline Current Flow 



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Section 8: Test Method 1— Negative 850 mV Pipe-to-Electrolyte Potential 
of Steel and Cast Iron Piping with Cathodic Protection Applied 



8.1 Scope 

Test Method 1 describes a procedure to assess the 
adequacy of cathodic protection on a steel or cast iron 
pipeline according to the criterion stated in NACE Standard 
RP01 69,^ Paragraph 6.2.2.1 .1 : 

A negative (cathodic) potential of at least 850 mV with 
the cathodic protection applied. This potential is 
measured with respect to a saturated copper/copper 
sulfate reference electrode (CSE) contacting the 
electrolyte. Voltage drops other than those across the 
structure-to-electrolyte boundary must be considered 
for valid interpretation of this voltage measurement. 

NOTE: Consideration is understood to mean the 
application of sound engineering practice in 
determining the significance of voltage drops by 
methods such as: 

(a) IVIeasuring or calculating the voltage drop(s); 

(b) Reviewing the historical performance of the 
cathodic protection system; 

(c) Evaluating the physical and electrical 
characteristics of the pipe and its environment; and 

(d) Determining whether there is physical evidence of 
corrosion. 

8.2 General 

8.2.1 Cathodic protection current shall remain "on" 
during the measurement process. This potential is 
commonly referred to as the "on" potential. 

8.2.2 Test IVIethod 1 measures the pipe-to-electrolyte 
potential as the sum of the polarized potential and any 
voltage drops in the circuit. These voltage drops 
include those through the electrolyte and pipeline 
coating from current sources such as impressed 
current, galvanic anodes, and telluric effects. 

8.2.3 Because voltage drops other than those across 
the pipe metal/electrolyte interface may be included in 
this measurement, these drops shall be considered, as 
discussed in Paragraph 8.6. 

8.3 Comparison with Other IVIethods 
8.3.1 Advantages 

(a) Minimal equipment, personnel, and vehicles are 
required; and 

(b) Less time is required to make measurements. 



8.3.2 Disadvantages 

(a) Potential measured includes voltage drops other 
than those across the pipe metal/electrolyte interface; 
and 

(b) Meeting the requirements for considering the 
significance of voltage drops (see Paragraph 8.6) can 
result in added time to assess adequacy of cathodic 
protection at the test site. 

8.4 Basic Test Equipment 

8.4.1 Voltmeter with adequate input impedance. 
Commonly used digital instruments have a nominal 
impedance of 10 megaohms. An analog instrument 
with an internal resistance of 100,000 ohms per volt 
may be adequate in certain circumstances in which the 
circuit resistance is low. A potentiometer circuit may be 
necessary in other instances. 

8.4.2 Two color-coded meter leads with clips for 
connection to the pipeline and reference electrode. 

8.4.3 Reference Electrode 

8.4.3.1 CSE. 

8.4.3.2 Other standard reference electrodes may 
be substituted for the CSE. These reference 
electrodes are described in Appendix A, 
Paragraph A2. 

8.5 Procedure 

8.5.1 Before the test, verify that cathodic protection 
equipment has been installed and is operating 
properly. Time should be allowed for the pipeline 
potentials to reach polarized values. 

8.5.2 Determine the location of the site to be tested. 
Selection of a site may be based on: 

(a) Location accessible for future monitoring; 

(b) Other protection systems, structures, and anodes 
that may influence the pipe-to-electrolyte potential; 

(c) Electrical midpoints between protective devices; 

(d) Known location of an ineffective coating if the line 
is coated; and 

(e) Location of a known or suspected corrosive 
environment. 

8.5.3 Make electrical contact between the reference 
electrode and the electrolyte at the test site, directly 
over the centerline of the pipeline or as close to it as is 
practicable. 



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8.5.4 Connect the voltmeter to the pipeline and 
reference electrode as described in Paragraph 5.6. 

8.5.5 Record the pipe-to-electrolyte potential and its 
polarity with respect to the reference electrode. 

8.6 Considering the Significance of Voltage Drops for Valid 
Interpretation of the Criterion 



8.6.1 The significance 
considered by: 



of voltage drops can be 



8.6.1.1 Comparing historical levels of cathodic 
protection with physical evidence from the pipeline 
to determine whether corrosion has occurred. 

8.6.1.2 Comparing soil corrosiveness with 
physical evidence from the pipeline to determine 
whether corrosion has occurred. 

8.6.2 Physical evidence of corrosion is determined by 
evaluating items such as: 

(a) Leak history data; 



(b) Buried pipeline inspection report data regarding 
locations of coating failures, localized conditions of 
more-corrosive electrolyte, or substandard cathodic 
protection levels have been experienced; and/or 

(c) Verification of in-line inspection-tool metal loss 
indications by follow-up excavation of anomalies and 
inspection of the pipe external surface. 

8.6.3 Cathodic protection shall be judged adequate at 
the test site if: 

(a) The pipe-to-electrolyte potential measurement is 
negative 850 mV, or more negative, with respect to a 
CSE; and 

(b) The significance of voltage drops has been 
considered by applying the principles described in 
Paragraphs 8.6.1 or 8.6.2. 

8.7 Monitoring 

When the significance of a voltage drop has been 
considered at the test site, the measured potentials may be 
used for monitoring unless significant environmental, 
structural, coating integrity, or cathodic protection system 
parameters have changed. 



Section 9: Test Method 2— Negative 850 mV Polarized Pipe-to-Electrolyte 
Potential of Steel and Cast Iron Piping 



9.1 Test Method 2 describes the most commonly used test 
method to satisfy this criterion (see Paragraph 9.2). This 
method uses current interruption to determine whether 
cathodic protection is adequate at the test site according to 
the criterion. 

9.2 Scope 

This method uses an interrupter(s) to eliminate the cathodic 
protection system voltage drop from the pipe-to-electrolyte 
potential measurement for comparison with the criterion 
stated in NACE Standard RP0169,^ Paragraph 6.2.2.1.2: 

A negative polarized potential of at least 850 mV 
relative to a saturated copper/copper sulfate reference 
electrode (CSE). 

9.3 General 

9.3.1 Interrupting the known cathodic protection 
current source(s) eliminates voltage drops associated 
with the protective currents being interrupted. 
However, significant voltage drops may also occur 
because of currents from other sources, as discussed 
in Section 7. 

9.3.2 To avoid significant depolarization of the pipe, 
the "off" period should be limited to the time necessary 



to make an accurate potential measurement. The "off" 
period is typically less than 3 seconds. 

9.3.3 The magnitude and duration of a voltage spike 
caused by current interruption can vary, but the 
duration is typically within 0.5 second. After the current 
is interrupted, the time elapsed until the measurement 
is recorded should be long enough to avoid errors 
caused by voltage spiking. On-site measurements with 
appropriate instruments may be necessary to 
determine the duration and magnitude of the spiking. 

9.3.4 Current sources that can affect the accuracy of 
this test method include the following: 

(a) Unknown, inaccessible, or direct-connected 
galvanic anodes; 

(b) Cathodic protection systems on associated 
piping or foreign structures; 

(c) Electric railway systems; 

(d) HVDC electric power systems; 

(e) Telluric currents; 

(f) Galvanic, or bimetallic, cells; 

(g) DC mining equipment; 

(h) Parallel coated pipelines, electrically connected 

and polarized to different potentials; 

(i) Uninterrupted current sources; 

(j) Unintentional connections to other structures or 

bonds to mitigate interference; and 

(k) Long-line currents. 



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9.4 Comparison with Other IVIethods 

9.4.1 Advantages 

(a) Voltage drops associated with the protective 
currents being interrupted are eliminated. 

9.4.2 Disadvantages 

(a) Additional equipment is required; 

(b) Additional time, personnel, and vehicles may be 
required to set up equipment and to make pipe-to- 
electrolyte potential measurements; and 

(c) Test results are difficult or impossible to analyze 
when stray currents are present or direct-connected 
galvanic anodes or foreign impressed current devices 
are present and cannot be interrupted. 

9.5 Basic Test Equipment 

9.5.1 Voltmeter with adequate input impedance. 
Commonly used digital instruments have a nominal 
impedance of 10 megaohms. An analog instrument 
with an internal resistance of 100,000 ohms per volt 
may be adequate in certain circumstances in which the 
circuit resistance is low. A potentiometer circuit may be 
necessary in other instances. 



9.5.2 Two color-coded meter leads with clips 
connection to the pipeline and reference electrode. 



for 



9.5.3 Sufficient current interrupters to interrupt 
influential cathodic protection current sources 
simultaneously. 

9.5.4 Reference electrode 



protecting the pipe at the test site, and place in 
operation with a synchronized and/or known "off" and 
"on" cycle. The "off" cycle should be kept as short as 
possible but still long enough to read a polarized pipe- 
to-electrolyte potential after any "spike" as shown in 
Figure 3a has collapsed. 

9.6.3 Determine the location of the site to be tested. 
Selection of a site may be based on: 

(a) Location accessible for future monitoring; 

(b) Other protection systems, structures, and anodes 
that may influence the pipe-to-electrolyte potential; 

(c) Electrical midpoints between protection devices; 

(d) Known location of an ineffective coating when 
the pipeline is coated; and 

(e) Location of a known or suspected corrosive 
environment. 

9.6.4 Make electrical contact between the reference 
electrode and the electrolyte at the test site, directly 
over the centerline of the pipeline or as close to it as is 
practicable. 

9.6.5 Connect voltmeter to the pipeline and reference 
electrode as described in Paragraph 5.6. 

9.6.5.1 If spiking may be present, use an 
appropriate instrument, such as an oscilloscope or 
high-speed recording device, to verify that the 
measured values are not influenced by a voltage 
spike. 

9.6.6 Record the pipe-to-electrolyte "on" and "off" 
potentials and their polarities with respect to the 
reference electrode. 



9.5.4.1 CSE. 



9.7 Evaluation of Data 



9.5.4.2 Other standard reference electrodes may 
be substituted for the CSE. 
electrodes are described 
Paragraph A2. 



These reference 
in Appendix A, 



9.6 Procedure 

9.6.1 Before the test, verify that cathodic protection 
equipment has been installed and is operating 
properly. Time should be allowed for the pipeline 
potentials to reach polarized values. 



Cathodic protection shall be judged adequate at the test site 
if the polarized pipe-to-electrolyte potential is negative 850 
mV, or more negative, with respect to a CSE. 

9.8 Monitoring 

When the polarized pipe-to-electrolyte potential has been 
determined to equal or exceed a negative 850 mV, the 
pipeline "on" potential may be used for monitoring unless 
significant environmental, structural, coating integrity, or 
cathodic protection system parameters have changed. 



9.6.2 Install and place in operation necessary 
interrupter equipment in all significant DC sources 



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Section 10: Test Method 3—100 mV Cathodic Polarization 
of Steel, Cast Iron, Aluminum, and Copper Piping 



10.1 Test Method 3 describes the use of either pipeline 
polarization decay or pipeline polarization formation to 
determine whether cathodic protection is adequate at the 
test site according to the criterion. Consequently, this test 
method consists of two mutually independent parts, Test 
Methods 3a and 3b, that describe the procedures for 
testing. Cathodic polarization curves for Test Methods 3a 
and 3b are shown in Figure 3. These are schematic 
drawings of generic polarization decay and formation. 



Use of Pipeline Polarization Decay 



10.2 Test Method 3a ■ 
(Figure 3a) 

10.2.1 Scope 



This method uses pipeline polarization decay to assess 
the adequacy of cathodic protection on a steel, cast 
iron, aluminum, or copper pipeline according to the 
criterion stated in NACE Standard RP0169,^ Paragraph 
6.2.2.1.3, 6.2.3.1, or 6.2.4.1 (depending on the pipe 
metal). The paragraph below states Paragraph 
6.2.2.1.3: 

The following criterion shall apply: A minimum of 
100 mV of cathodic polarization between the 
structure surface and a stable reference electrode 
contacting the electrolyte. The formation or decay 
of polarization can be measured to satisfy this 
criterion. 

10.2.2 General 



10.2.2.1 Interrupting 
protection source(s) 
associated with the 
interrupted. 



the known cathodic 

eliminates voltage drops 

protective current(s) being 



10.2.2.2 Other current sources that can affect the 
accuracy of this test method include the following: 

(a) Unknown, inaccessible, or direct-connected 
galvanic anodes; 

(b) Cathodic protection systems on associated 
piping or foreign structures; 

(c) Electric railway systems; 

(d) HVDC electric power systems; 

(e) Telluric currents; 

(f) Galvanic, or bimetallic, cells; 

(g) DC mining equipment; 

(h) Parallel coated pipelines, electrically 

connected and polarized to different potentials; 

(i) Uninterrupted current sources; 

(j) Unintentional connections to other structures 

or bonds to mitigate interference; and 

(k) Long-line currents. 



10.2.2.3 The magnitude and duration of a voltage 
spike caused by current interruption can vary, but 
the duration is typically within 0.5 second. After 
the current is interrupted, the time elapsed until the 
measurement is recorded should be long enough 
to avoid errors caused by voltage spiking. On-site 
measurements with appropriate instruments may 
be necessary to determine the duration and 
magnitude of the spiking. 

1 0.2.3 Comparison with Other Methods 

10.2.3.1 Advantages 

(a) This method is especially useful for bare or 
ineffectively coated pipe; and 

(b) This method is advantageous when 
corrosion potentials may be low (for example, 500 
mV or less negative) and/or the current required to 
meet a negative 850 mV polarized potential 
criterion would be considered excessive. 

10.2.3.2 Disadvantages 

(a) Additional equipment is required; 

(b) Additional time, personnel, and vehicles may 
be required to set up equipment and to make pipe- 
to-electrolyte potential measurements; and 

(c) Test results are difficult or impossible to 
analyze when direct-connected galvanic anodes or 
foreign impressed current devices are present and 
cannot be interrupted, or when stray currents are 
present. 

10.2.4 Basic Test Equipment 

10.2.4.1 Voltmeter with adequate input 
impedance. Commonly used digital instruments 
have a nominal impedance of 10 megaohms. An 
analog instrument with an internal resistance of 
1 00,000 ohms per volt may be adequate in certain 
circumstances in which the circuit resistance is 
low. A potentiometer circuit may be necessary in 
other instances. 



10.2.4.1.1 Recording voltmeters 
useful to record polarization decay. 



can be 



10.2.4.2 Two color-coded meter leads with clips 
for connection to the pipeline and reference 
electrode. 

10.2.4.3 Sufficient current interrupters to interrupt 
influential cathodic protection current sources 
simultaneously. 



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1,100 

5" 1,000 

1 900 





800 


^ 




Q 




■^ 


/()() 


V 




V 




LU 




4^ 


600 



i 



500 



400 



1,200 



? 




1 




TS 


1,000 






c 




(D 




5 


900 


•D 




"5. 




D 




13 


800 


S 




^ 


700 



^ 



600 



500 



"On" Potential 




ThME PERJOD 



Figure 3a 
Polarization Decay 



Current Interruption 



Polarizing Line 



Instant-Off" 
Potential 




■ Cathodic Protection Applied 
Corrosion Potential 



TIHEPEFtOD 



Figure 3b 
Polarization Formation 

FIGURES 
Cathodic Polarization Curves 



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10.2.4.4 Reference electrode 

10.2.4.4.1 CSE. 

10.2.4.4.2 Other standard reference 
electrodes may be substituted for the CSE. 
These reference electrodes are described in 
Appendix A, Paragraph A2. 

10.2.5 Procedure 

10.2.5.1 Before the test, verify that cathodic 
protection equipment has been installed and is 
operating properly. Time should be allowed for the 
pipeline potentials to reach polarized values. 

10.2.5.2 Install and place in operation necessary 
interrupter equipment in all significant DC sources 
protecting the pipe at the test site, and place in 
operation with a synchronized and/or known "off" 
and "on" cycle. The "off" cycle should be kept as 
short as possible but still long enough to read a 
polarized pipe-to-electrolyte potential after any 
"spike" as shown in Figure 3a has collapsed. 

10.2.5.3 Determine the location of the site to be 
tested. Selection of a site may be based on: 

(a) Location accessible for future monitoring; 

(b) Other protection systems, structures, and 
anodes that may influence the pipe-to-electrolyte 
potential; 

(c) Electrical midpoints between protection 
devices; 

(d) Known location of an ineffective coating if 
the pipeline is coated; and 

(e) Location of a known or suspected corrosive 
environment. 

10.2.5.4 Make electrical contact between the 
reference electrode and the electrolyte at the test 
site, directly over the centerline of the pipeline or 
as close to it as is practicable. 

10.2.5.4.1 Identify the location of the 
electrode to allow it to be returned to the 
same location for subsequent tests. 

1 0.2.5.5 Connect the voltmeter to the pipeline and 
reference electrode as described in Paragraph 
5.6. 

10.2.5.5.1 If spiking may be present, use an 
appropriate instrument, such as an 
oscilloscope or high-speed recording device, 
to verify that the measured values are not 
influenced by a voltage spike. 

10.2.5.6 IVIeasure and record the pipe-to- 
electrolyte "on" and "instant off" potentials and their 
polarities with respect to the reference electrode. 



10.2.5.6.1 The "instant off" pipe-to-electrolyte 
potential is the "baseline" potential from which 
the polarization decay is calculated. 

10.2.5.7 Turn off sufficient cathodic protection 
current sources that influence the pipe at the test 
site until at least 100 mV cathodic polarization 
decay has been attained. 

10.2.5.7.1 Continue to measure and record 
the pipe-to-electrolyte potential until it either: 

(a) Has become at least 100 mV less 
negative than the "off" potential; or 

(b) Has reached a stable depolarized level. 

10.2.5.7.2 IVIeasurements shall be made at 
sufficiently frequent intervals to avoid attaining 
and remaining at a corrosion potential for an 
unnecessarily extended period. 

10.2.5.7.3 When extended polarization 
decay time periods are anticipated, it may be 
desirable to use recording voltmeters to 
determine when adequate polarization decay 
or a corrosion potential has been attained. 

1 0.2.6 Evaluation of Data 

Cathodic protection shall be judged adequate at the 

test site if 100 mV or more of polarization decay is 

measured with respect to a standard reference 
electrode. 

10.2.7 Monitoring 

When at least 100 mV or more of polarization decay 
has been measured, the pipeline "on" potential at the 
test site may be used for monitoring unless significant 
environmental, structural, coating integrity, or cathodic 
protection system parameters have changed. 

10.3 Test Method 3b — Use of Pipeline Polarization 
Formation (Figure 3b) 

10.3.1 Scope 

This method provides a procedure using the formation 
of polarization to assess the adequacy of cathodic 
protection at a test site on steel, cast iron, aluminum, or 
copper piping according to the criteria stated in NACE 
Standard RP0169,^ Paragraphs 6.2.2.1.3, 6.2.3.1, or 
6.2.4.1 (depending on the pipe metal). The paragraph 
below states Paragraph 6.2.2.1 .3: 

The following criterion shall apply: A minimum of 
100 mV of cathodic polarization between the 
structure surface and a stable reference electrode 
contacting the electrolyte. The formation or decay 
of polarization can be measured to satisfy this 
criterion. 



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10.3.2 General 

Ferrous, aluminum, and copper pipelines may be 
adequately cathodically protected if applying cathodic 
protection causes a polarization change of 100 mV or 
more with respect to a reference potential. 

10.3.2.1 Current sources that can affect the 
accuracy of this test method include the following: 

(a) Unknown, inaccessible, or direct-connected 
galvanic anodes; 

(b) Cathodic protection systems on associated 
piping or foreign structures; 

(c) Electric railway systems; 

(d) HVDC electric power systems; 

(e) Telluric currents; 

(f) Galvanic, or bimetallic, cells; 

(g) DC mining equipment; 

(h) Parallel coated pipelines, electrically 

connected and polarized to different potentials; 

(i) Uninterrupted current sources; 

(j) Unintentional connections to other structures 

or bonds to mitigate interference; and 

(k) Long-line currents. 

1 0.3.3 Comparison with Other Methods 

10.3.3.1 Advantages 

(a) This method is especially useful for bare or 
ineffectively coated pipe; and 

(b) This method is advantageous when 
corrosion potentials may be low (for example, 500 
mV or less negative) and/or the current required to 
meet a negative 850 mV potential criterion would 
be considered excessive. 

10.3.3.2 Disadvantages 

(a) Additional equipment is required; 

(b) Additional time, personnel, and vehicles may 
be required to set up equipment and to make the 
pipe-to-electrolyte potential measurements; and 

(c) Test results are difficult or impossible to 
analyze when stray currents are present or when 
direct-connected galvanic anodes or foreign 
impressed currents are present and cannot be 
interrupted. 

10.3.4 Basic Test Equipment 

10.3.4.1 Voltmeter with adequate input 
impedance. Commonly used digital instruments 
have a nominal impedance of 10 megaohms. An 
analog instrument with an internal resistance of 
100,000 ohms per volt may be adequate in certain 
circumstances in which the circuit resistance is 
low. A potentiometer circuit may be necessary in 
other instances. 



10.3.4.2 Two color-coded meter leads with clips 
for connection to the pipeline and reference 
electrode. 

10.3.4.3 Sufficient current interrupters to interrupt 
influential cathodic protection current sources 
simultaneously. 

10.3.4.4 Reference electrode 

10.3.4.4.1 CSE. 

10.3.4.4.2 Other standard reference 
electrodes may be substituted for the CSE. 
These reference electrodes are described in 
Appendix A, Paragraph A2. 

10.3.5 Procedure 

10.3.5.1 Before the test, verify that cathodic 
protection equipment has been installed but is not 
operating. 

10.3.5.2 Determine the location of the site to be 
tested. Selection of a site may be based on: 

(a) Location accessible for future monitoring; 

(b) Other protection systems, structures, and 
anodes that may influence the pipe-to-electrolyte 
potential; 

(c) Electrical midpoints between protection 
devices; 

(d) Known location of an ineffective coating if 
the line is coated; and 

(e) Location of a known or suspected corrosive 
environment. 

10.3.5.3 IVIake electrical contact between the 
reference electrode and the electrolyte at the test 
site, directly over the centerline of the pipeline or 
as close to it as is practicable. 

10.3.5.3.1 Identify the location of the 
electrode to allow it to be returned to the 
same location for subsequent tests. 

10.3.5.4 Connect the voltmeter to the pipeline and 
reference electrode as described in Paragraph 
5.6. 

10.3.5.5 Measure and record the pipe-to- 
electrolyte corrosion potential and its polarity with 
respect to the reference electrode. 

10.3.5.5.1 This potential is the value from 
which the polarization formation is calculated. 

10.3.5.6 Apply the cathodic protection current. 
Time should be allowed for the pipeline potentials 
to reach polarized values. 



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10.3.5.7 Install and place in operation necessary 
interrupter equipment in all significant DC sources 
protecting the pipe at the test site, and place in 
operation with a synchronized and/or known "off" 
and "on" cycle. The "off" cycle should be kept as 
short as possible but still long enough to read a 
polarized pipe-to-electrolyte potential after any 
"spike" as shown in Figure 3a has collapsed. 

10.3.5.8 Measure and record the pipe-to- 
electrolyte "on" and "off" potentials and their 
polarities with respect to the reference electrode. 
The difference between the "off" potential and the 
corrosion potential is the amount of polarization 
formation. 

10.3.5.8.1 If spiking may be present, use an 
appropriate instrument, such as an 



oscilloscope or high-speed recording device, 
to verify that the measured values are not 
influenced by a voltage spike. 

1 0.3.6 Evaluation of Data 

Cathodic protection shall be judged adequate if 
100 mV or more of polarization formation is 
measured with respect to a standard reference 
electrode. 

10.3.7 IVIonitoring 

When at least 100 mV or more of polarization 
formation has been measured, the pipeline "on" 
potential may be used for monitoring unless 
significant environmental, structural, coating 
integrity, or cathodic protection system parameters 
have changed. 



References 



1. NACE Standard RP0169 (latest revision), "Control of 
External Corrosion on Underground or Submerged Metallic 
Piping Systems" (Houston, TX: NACE). 

2. NACE Standard RP0177 (latest revision), "Mitigation of 
Alternating Current and Lightning Effects on Metallic 
Structures and Corrosion Control Systems" (Houston, TX: 
NACE). 



3. F.J. Ansuini, J.R. Dimond, "Factors Affecting the 
Accuracy of Reference Electrodes," MP 33, 11 (1994), p. 
14. 

4. NACE Publication 35201 (latest revision), "Technical 
Report on the Application and Interpretation of Data from 
External Coupons Used in the Evaluation of Cathodically 
Protected Metallic Structures" (Houston, TX: NACE). 



Bibliography 



Ansuini, F.L., and J.R. Dimond. "Factors Affecting the 
Accuracy of Reference Electrodes." MP 33, 11 (1994): 
pp. 14-17. 



Applegate, L.M. Cathodic Protection. 
McGraw-Hill, 1960. 



New York, NY: 



Bushman, J.B., and F.E. Rizzo. "IR Drop in Cathodic 
Protection Measurements." A//P17, 7 (1978): pp. 9-13. 



DeBethune, A.J. "Fundamental Concepts of Electrode 
Potentials." Corros/on 9, 10 (1953): pp. 336-344. 

Escalante, E., ed. Underground Corrosion, ASTM STP 741. 
Philadelphia, PA: ASTM, 1981. 

Ewing, S.P. "Potential Measurements for Determining 
Cathodic Protection Requirements." Corrosion 7, 12 
(1951): pp. 410-418. 



Cathodic Protection Criteria — A Literature Survey. Ed. 
coord. R.A. Gummow. Houston, TX: NACE, 1989. 

Corrosion Control/System Protection, Bool< TS-1, Gas 
Engineering and Operating Practices Series. Arlington, 
VA: American Gas Association, 1986. 



Gummow, R.A. "Cathodic Protection Potential Criterion for 
Underground Steel Structures." MP 32, 11 (1993): pp. 
21-30. 

Jones, D.A. "Analysis of Cathodic Protection Criteria." 
Corrosion 28, ^^ (1972): pp. 421-423. 



Dabkowski, J., and T. Hamilton. "A Review of Instant-Off 
Polarized Potential Measurement Errors." 

CORROSION/93, paper no. 561. Houston, TX: NACE, 
1993. 

Dearing, B.M. "The 100-mV Polarization Criterion." MP 33, 
9(1994): pp. 23-27. 



NACE Publication 2C154. "Some Observations on 
Cathodic Protection Potential Criteria in Localized 
Pitting." Houston, TX: NACE, 1954. 

NACE Publication 2C157. "Some Observations on 
Cathodic Protection Criteria." Houston, TX: NACE, 
1957. 



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NACE Publication 35201 (latest revision). "Technical Report 
on the Application and Interpretation of Data from 
External Coupons Used in the Evaluation of 
Cathodically Protected Metallic Structures." Houston, 
TX: NACE, 2001. 

NACE Publication 54276. "Cathodic Protection IVIonitoring 
for Buried Pipelines." Houston, TX: NACE, 1990. 



Peabody's Control of Pipeline Corrosion. 
Bianchetti, ed. Houston, TX: NACE, 2001 



2"' ed. 



R. 



Parker, M.E. Pipeline Corrosion and Cathodic Protection. 
2nded. Houston, TX: Gulf Publishing, 1962. 

Stephens, R.W. "Surface Potential Survey Procedure and 
Interpretation of Data," in Proceedings of the 
Appalachian Corrosion Short Course, held May 1980. 
Morgantown, WV: University of West Virginia, 1980. 

West, L.H. "Fundamental Field Practices Associated with 
Electrical Measurements," in Proceedings of the 
Appalachian Corrosion Short Course, held May 1980. 
Morgantown, WV: University of West Virginia, 1980. 



Appendix A: Reference Electrodes 



A1 Pipeline metals have unstable electrical potentials 
when placed in an electrolyte such as soil or water. 
However, a half-cell that has a stable, electrochemically 
reversible potential characterized by a single, identifiable 
half-cell reaction is a reference electrode. The stability of a 
reference electrode makes it useful as an electrical 
reference point or benchmark for measuring the potential of 
another metal in soil or water. When connected by a 
voltmeter to another metal in soil or water, the reference 
electrode becomes one half of a corrosion cell. The 
reference electrodes used for measuring potentials on 
buried or submerged pipelines have voltage values that are 
normally positive with respect to steel. 

A2 Pipeline potentials are usually measured using either a 
saturated copper/copper sulfate (CSE), a silver/silver 
chloride (Ag/AgCI), or a saturated potassium chloride (KCI) 
calomel reference electrode. CSEs are usually used for 
measurements when the electrolyte is soil or fresh water, 
and less often for salt water. When a CSE is used in a 
high-chloride environment, the stability (i.e., lack of 
contamination) of the electrode must be determined before 
the readings may be considered valid. Ag/AgCI reference 
electrodes are usually used for seawater environments. 
The KCI calomel electrodes are more often used for 
laboratory work because they are generally less rugged, 
unless specially constructed, than the other two reference 
electrodes. 

A2.1 The voltage equivalents (at 25°C [77°F]) to 
negative 850 mV referred to a CSE are: 

A2.1.1 Ag/AgCI seawater reference electrode 
used in 25 ohm-cm seawater: -800 mV,' and 



A2.1.2 Saturated 
electrode: -780 mV. 



KCI calomel reference 



A2.2 A CSE is composed of a pure copper rod 
immersed in a saturated solution of distilled water and 
copper sulfate (CUSO4). The pure copper rod extends 
from one end of the reference electrode, providing a 
means of connection to a voltmeter. The other end of 
the reference electrode has a porous plug that is used 



to make an electrical contact with the pipeline 
electrolyte. Undissolved CUSO4 crystals in the 
reference electrode should always be visible to ensure 
the solution is saturated. The reference is reasonably 
accurate (within 5 mV when measured against a 
reference electrode known to be free of contamination). 
The advantages of this reference electrode are low 
cost and ruggedness. 

A2.3 Ag/AgCI reference electrodes are used in marine 
and soil environments. The construction and the 
electrode potential vary with the application and with 
relation to the potential of a CSE reference electrode. 
The electrolytes involved may be natural seawater, 
saturated KCI, or other concentrations of KCI. The 
user shall utilize the manufacturer's recommendations 
and potential values for the type of Ag/AgCI cell used. 
The Ag/AgCI reference electrode has a high accuracy 
(typically less than 2 mV when handled and maintained 
correctly) and is very durable. 

A2.4 A saturated KCI calomel reference electrode for 
laboratory use is composed of a platinum wire in 
contact with a mercury/mercurous chloride mixture 
contacting a saturated KCI solution-enclosed in a glass 
container, a voltmeter connection on one end, and a 
porous plug on the other end for contact with the 
pipeline electrolyte. For field use a more-rugged, 
polymer-body, gel-filled KCI calomel electrode is 
available, though modifications may be necessary to 
increase contact area with the environment. The 
presence of mercury in this electrode makes it 
environmentally less desirable for field use. 

A2.5 In addition to these standard reference 
electrodes, an alternative metallic material or structure 
may be used in place of the saturated CSE if the 
stability of its electrode potential is ensured and if its 
voltage equivalent referred to a CSE is established. 

A2.6 A permanently installed reference electrode may 
be used; however, whether it is still accurate should be 
determined. 



18 



NACE International 



TM0497-2002 



A3 It is good practice to verify the accuracy of reference 
electrodes used in the field by comparing them with a 
carefully prepared master reference electrode that, to avoid 
contamination, is never used for field measurements. The 
accuracy of a field reference electrode can be verified by 
placing it along with the master reference electrode in a 
common solution, such as fresh water, and measuring the 



voltage difference between the two electrodes. A maximum 
voltage difference of 5 mV between a master reference 
electrode and another reference electrode of the same type 
is usually satisfactory for pipeline potential measurements. 
When reference electrode-to-reference electrode potential 
measurements are made in the field, it is necessary that 
electrodes with matching potentials be used. 



Appendix B: Net Protective Current 



B1 NACE Standard RP0169,^ Paragraph 6.2.2.2.1, states 
that measuring the net protective current from the 
electrolyte to the pipe surface by an earth current technique 
at predetermined current discharge points may be sufficient 
on bare or ineffectively coated pipelines when long-line 
corrosion activity is of primary concern. 

B1 .1 This technique is a measure of the net protective 
current from the electrolyte onto the pipe surface and is 
most practicable for use on bare pipelines. 

B1 .2 The e