BULLETIN No. 6
DEPARTMENT OF SCIENTIFIC AND INDUSTRIAL RESEARCH
•
ON THE
ELECTRO=DEPOSITION OF IRON
BY
W. E. HUGHES, B.A. (Cantab.}
Late Chief Research Chemist, Electro- Metallurgical
Committee, Ministry of Munitions
\
LONDON:
PUBLISHED BY HIS MAJESTY'S STATIONERY OFFICE
1922
Price 6s. Qd. Net
BULLETIN No. G
DEPARTMENT OF SCIENTIFIC AND INDUSTRIAL RESEARCH
ON THE
ELECTRO=DEPOSltlON OF IRON
WITH AN APPENDIX CONTAINING A
BIBLIOGRAPHY OF THE SUBJECT
BY
W. E. HUGHES, B.A. (Cantab.}
Late Chief Research Chemist, Electro- Metallurgical
Committee, Ministry of Munitions
LONDON:
PUBLISHED BY HIS MAJESTY'S STATIONERY OFFICE
1922
Price 6s. 6d. J\et.
iii
PREFATORY NOTE
/
The research, the results of which are embodied in this report,
was carried out by Mr. W. E. Hughes, B.A. (Cantab.), with the
assistance of grants made by the Department on the recommenda-
tion of the Advisory Council for Scientific and Industrial Research.
In view of the extended use of Electro-Deposition of Iron for
many industrial purposes and the new applications which it is find-
ing, it was thought advisable, as an exceptional measure, to issue
Mr. Hughes' report as a bulletin of the Department. It will be
understood that the report is the work of Mr. Hughes, and that the
Department must not be taken as endorsing the statements therein
- contained.
At the Department's request, Mr. Hughes has included, in the
Appendix, a bibliography of the subject for which it is believed there
will be a general use.
Department of Scientific and Industrial Research,
16, Old Queen Street, Westminster,
London, S.W.I.
March, 1922.
49639?
85067
CONTENTS
PAGE
INTRODUCTION • 1
DIVISION l. — DESCRIPTIVE
General Note on the descriptions of the deposits ... 2
Series I.— On the effect of temperature 3
Series II. — On the effect of current density 11
Series III. — On the effect of mechanical movement ... 16
DIVISION 2.— THEORETICAL
Introduction 23
I. — The crystallisation of substances in general 23
II. — Application to electro-deposited metal 29
A consideration of the results of the experiments of
the series I, II and III ... 37
Some remarks on deposits —
(1) from other iron solutions 39
(2) of other metals ... "**!;.' 40
General conclusions . ••• 41
III. — Workshop application ... 42
APPENDIX
Bibliography comprising references to publications on ... 44
I. — The Electro-Deposition of Iron and Phenomena
connected therewith 44
II. — The Properties of Electrolytic Iron 48
III. — Works of Reference relating to the Electro-
Deposition of Iron 50
» ON THE
ELECTRO-DEPOSITION OF IRON
INTRODUCTION
This report includes, as Division 1, simple descriptions of the structures
of a number of samples of iron, electro-deposited from the chloride of
iron solution. The purpose is to show how the three factors of deposition,
namely, (1) temperature, (2) current density, and (3) movement (of
cathode or electrolyte), affect the structure of the iron deposited from
the chloride bath. Incidentally, the micrographs disclose variations of
structure produced by volume of solution (Figs. 22, 23, and 24), concen-
tration currents (e.g., Figs. 3 to 8), and some other factors.
Although the deposits considered were formed in the chloride of iron
solution, there is, it may be said, experimental evidence to show that the
conclusions drawn from the structures of deposits built up in that bath
may be extended to those formed in other solutions. This point is touched
on in Division 2; and, further, some evidence is adduced there to show
that extension may be made not only to deposits formed in different
solutions (solutions of different salts, that is) of the same metal, but also
to deposits of different metals. Indeed, a principal purpose of Division 2
is to show that, in general, this is the rational conclusion of the concentra-
tion hypothesis outlined in that Division and based on the assumption
that the crystallisation of electro-deposited metal is, essentially, in no
way different from crystallisation from rock magmas, molten metals, or
salt solutions. In Section III of Division 2 the importance of the study
of the structure of deposited metal to electro-deposition in the workshop
is briefly pointed out.
It may be mentioned that the author has made a critical survey of
such part of the literature upon the electro-deposition of iron as appears
to him of use or importance to those about to undertake work on that
subject. It has, however, been considered that that review would be
published most appropriately elsewhere,* and it has not, therefore, been
included in this report. The papers and researches referred to are,
however, detailed (in an Appendix hereto), together with others on the
properties of electrolytic iron, in a classification intended to indicate the
general subject matter of each, and, thereby, to facilitate the literary
labour required to be done by one who undertakes research, for this
purpose or that, on electrolytic iron. Patent specifications are, for the
most part, excluded; only such as appear to the author to contain subject
matter of real value are noted.
The author would be ungrateful if he did not acknowledge the great
kindness extended to him by Professor H. C. H. Carpenter, F.R.S., who
has exercised a general supervision over the work, and who has been
ever ready to afford the author the benefit of his advice and experience.
To Professor E. H. Lamb, D.S.C., M.Sc., of the East London College,
the author is indebted in many ways, and wishes to tender his sincere
thanks. Acknowledgment is also made of the permission of the Council
of the Iron and Steel Institute to reproduce the micrographs, marked with
an asterisk, that have appeared in the Journal of the Institute.
* v. Trans. Am. Electroch. Soc., 1921,
DIVISION 1— DESCRIPTIVE
GENERAL NOTE ON THE DESCRIPTIONS OF THE
DEPOSITS
It has been found by no means easy so to describe the structure of
the deposits as to produce even an approximately accurate picture of
them, and the variations that occur in them, in the mind of another.
The photographs help, of course. But it has to be remembered that
the diameter of the visible area of the deposit to be seen is only 2 mm.,
when using a Zeiss apochromat, 16 mm. objective (N.A. 0'30), to obtain,
with the x 8 ocular, a magnification of about 150 diameters. And
hence, the photograph represents only a small part of the deposit,
which may be, and often is, of varying structure. To reproduce the
whole area of a section of each specimen would be an altogether too
laborious and expensive task. And, moreover, no photograph, however
good it may be, ever conveys to the mind what can be seen by the
eye. While, therefore, the photographs have been relied on as aids,
the descriptions have been very carefully made. The method adopted,
in order to represent the truth as nearly as might be, was as follows : —
The structure was described in general outline from direct and
immediate observation through and at the microscope. The details were
then filled in. The memory was not trusted at all : that is, no appreciable
interval of time was allowed to transpire between observation and record.
After allowing four or more weeks to pass, another description of the
specimen was recorded without any reference being made to the former
one. The two descriptions were then compared; and, if necessary, as
it sometimes was, the descriptions were checked by renewed examination
of the specimen. In this way it was sought (1) to eliminate any bias
on the part of the observer — to avoid " seeing " what was not there,
and (2) to enable another to gain at least a fairly accurate picture of
the structure of the specimen. Further, the descriptions inserted in
the following are those finally " settled " and recorded in note-books.
No " amendments " have been introduced, except in some very few
instances, and then only after reference to the specimens concerned
yet once more.
For the purpose of condensing the descriptions, certain terms have
been used to denote various types of structure. These terms are, some
of them, already employed by the petrologist in descriptions of his rock
sections, e.g., columnar, fibrous, and others. Other terms used are names
of common objects which are readily pictured in the mind when the
eye sees the words by which they are known, e.g., cauliflower, poplar-tree,
and so on. The term most frequently, perhaps, made use of is the word
" normal." This will be defined here; but no definition of the other
terms used will be given; a list of them is drawn up, and a reference
given to one or more photographs showing clearly, in each case, the
type of structure to which the term is meant to apply.
Term (or Expression).
Reference.
Term (or Expression).
Reference.
Normal (approx.)
Columnar
Mosaic
Poplar-tree
Figs. 19, 20, &c.
Fig. 44.
Fig. 33.
Fig. 36.
Fibrous*
Fan
Cauliflower
Fountain
Figs. 52, 53, &c.
Fig. 32.
Figs. 34, 38, &c.
Fig. 59 especially.
* This term is employed by C. F. Burgess and O. P. Watts, Trans. Am. Electroch. Soc.
1906, vol.9, at p. 233,
To face p. 3.
FIG. 1. — Fractured Surface of an Iron Deposit. X 80.
(Oblique illumination.)
Change in
structure
occurs.
FIG. 2.— Fractured Surface of a Cobalt Deposit. X 140.
(Oblique illumination.)
The part of the deposit above the line indicated by the arrow is seen
to differ markedly in structure from the part below.
In none of these cases (except that of the term " normal ") does any
particular (individual) definition seem necessary.
The term NORMAL is given to that type of structure which is most
frequently of all approached in electro-deposited metal. It is that type
in which the grains (whether large or small) have the shape, more or
less, of the letter V, the angle of the V being directe'd inwards toward
the cathode. Such grains are well seen in Fig. 11, and, again, in
Fig. 20. As the term is intended to be interpreted, a strictly " normal "
type of structure would be wholly composed of grains (of various sizes)
having the V shape. As a fact, this is never observed in a deposit. The
grains are never, all of them, V-shaped; nor are the arms of the V
often straight lines. The ideal " normal " deposit is non-existent; but
the " practically." or " more or less " normal deposit is the one that
most frequently does occur. The deposits 9, lOa, and lla, show structures
that are more or less nearly " normal " : in deposit lla the normal
structure is closely approximated.
Deposits approaching the normal type in structure have been described
by several authors. C. F. Burgess and 0. P. Watts* describe and illustrate
the cone- or tooth-shaped pieces that they picked out from the fragments
of electro-deposited iron when it was broken up by smart hammer blows.
It is the longitudinal section of such a cone or tooth-shaped piece that one
sees when examining under the microscope a deposit that approaches the
normal type ill structure. Figs. 1 and 2 show fractured surfaces
of electrolytic iron and cobalt, respectively. In both figures the V-shaped
structure is very evident.
It may be added that this or that type of structure is not confined to any
particular metal, and hence the nomenclature here employed is applicable
not only to iron but also to other (electro-deposited) metals — copper, nickel,
cobalt, and so on. As it is hoped to show, structure does not depend upon
the particular metal deposited, but upon conditions of deposition.
SERIES I
ON THE EFFECT OF TEMPERATURE
i
A.— DEPOSITION
•
Constant current density : varying temperature.
The Conditions of Deposition were —
C.D •^ivi^I:-V>i';; '"•••'' 120 amp. /ft.*
Time ... ... ... • ...:>' ... 2 hours.
The cathode was either copper rod, or steel rod thinly coated with
copper in the cyanide bath.
The Cleaning was as follows (except where stated to be otherwise) : —
(a) // of copper, the rod was polished ; boiled in potash solution ;
brushed with Calais sand; swilled in clean running water;
treated in an electro-cleaner; swilled in water; dipped in 10 per
cent, (by volume) HC1 solution; swilled; brushed with Calais
sand; and then given a final rinse in running water.
(b) If of steel, the rod was treated as in (a); then given a 10-minute
coat of copper in the cyanide bath; swilled; dipped in the
dilute HC1; swilled; brushed with Calais sand; swilled again.
* Op. cit., pp. 233 and 234, and,especia,lly, Figs. 7 and 8,
Expt.
No.
Ref . to 1
Micro.
?emp.
Remarks.
Deposit.
A
tfo micro.)
70
Conditions, cathode, and all
Macroscopic. In the middle, the
It would
not have
else, as in Expt. 10 (v. post).
N.B. — This expt. was made,
deposit was dark and bright ;
at the ends, dark and powdery.
been pos-
at the end of the series,
The character of the deposit
sible to
prepare a
section of
with the object of fixing
the minimum temp, em-
ployable at the C.D. used
became worse and worse, and,
after some 20 mins., everywhere
split up and became useless.
this de-
(120 amp. per sq. ft.).
posit for
examina-
i
tion.
Pigs. 3 to 8
90
The electrolysis was con-
ducted in a 500 c.c. glass
Macroscopic. Good : smooth and
bright. Light grey. Outgrowth
beaker. Circular anodes.
(rather cindery) at both ends —
E.M.F. at 1£" rod distance
more at bottom end. The de-
= 0'9 volt. Cathode was
posit was split longitudinally
copper rod.
from the bottom upwards. On
The solution was filtered
saiving, it was ±ound to be hard
just before using.
and brittle ; it broke up and
came away from the base metal.
Adherence, therefore, poor.
Note : — Cathode was not dipped
in HC1 during cleaning.
9
Figs. 10
&11.
90
2^ gals, of solution (recently
filtered) was used. Swedish
iron anodes— 6" X 3" X %"
Macroscopic. Good. Light grey.
Crystalline on one side, particu-
larly toward the top end: smooth
— 2 on each anode rod : 4
on the other. On sawing, de-
in all.
posit did not split or break
E.M.F. 0-9 volt, at 5i". Cath-
away. Adherence, good.
3
Figs. 12 to
16
100 t(
86
ode, copper-ed steel rod.
Conditions as in 9, except
that solution was not
Macroscopic. Very good in colour
and smoothness. Thickening,
filtered before use.
but no outgrowth, at top :
rounded outgrowth at bottom
end. No spines or attachments.
On sawing, no breaking away.
Adherence, good.
4
Pig. 21
110
Conditions as in 3, except —
E.M. F. 0-8 volt. (i.e.} rather
Macroscopic, Good, light grey
colour. Close and even texture.
less than in 3).
Matt. Slight thickening at both
ends. No outgrowths : no lumps.
On sawing, no brittleness shown.
Adherence, good.
10
Fig. 18
112
Same solution as used for
Macroscopic. Good colour. Cry-
expt. 9, and same conditions.
Cathode, copper-ed steel rod.
stalline on one side : matt on
the other. Some thickening at
bottom end. On sawing, deposit
came away from the base metal,
but was not brittle. Adherence,
bad. (Note:— Not dipped in
HC1 during cleaning.)
lOa
Pigs. 19
&20.
112
Everything same as in 10, ex-
cepting (i) specimen dipped
Macroscopic. Good colour, and
generally, as cathode of No. 10 ;
in HC1 during the clean-
but crystalline all over, though
ing, and (ii) specimen was
rather leas coarsely crystalline
suspended vertically in the
on the sides facing the anodes.
bath. Cathode, as in 10.
On sawing, no Ureaking away
from the base metal. Adherence,
good. (Note : — Dipped in HC1
during cleaning.)
2
Figs. 22 to
116
Electrolysis conducted in
Macroscopic. Good colour :
24.
solution used in 1, and
bright. Floating particles cause
without filtering. General
loosely-adherent (sandy) layer
conditions as in 1, except
on the surface. Some cindery
1 E.M.F. 0-8 volt. Cathode,
outgrowth at top and bottom ;
as in 1.
otherwise deposit was similar to
that of 1, except (i) it was not
split, and (ii) it was more brittle.
On sawing, the deposit came
away in patches from the base
metal. Adherence, bad.
REMARKS ON THE DEPOSITS OB' THE SEVEN FOREGOING EXPERIMENTS
1. On Adhesion. — Experiments 10 and lOa show clearly the difference,
as regards adhesion, between a cathode that has been dipped in HC1 (dil.)
solution during the cleaning process and one that has not. Comparison
of experiments 1 and 9 shows the same difference. This difference is
probably due to the fact that swilling in water is not sufficient in itself
to remove alkalis, used in the cleaning process, from the metal — especially
from its pores; and, consequently, when the specimen is suspended in the
bath for deposition, a reaction occurs between the alkali and the salts of
the solution, causing a precipitation on the cathode surface.
2. On the Volume Of Electrolyte.— Experiments 1 and 9 show the
difference made by the volume of solution used for deposition. Where this
is small, the deposit is smooth (except for roughness caused by floating
matter), bright, and " hard-looking." Obviously, the texture is very
fine-grained. Further, the outgrowths that are formed have a cindery
appearance, though their colour remains the same as that of the rest of
the deposit. On the other hand, where the volume of solution is large,
as in experiment 9, the deposit is not bright but is dull, except where it
appears crystalline; it is of a coarser texture. Moreover, instead of out-
growths* one finds a thickening of the deposit at the ends (due to local
increase of current density), and the thickening is smooth and rounded.
In this case, too, the deposit is not so hard as it is where the volume of
electrolyte is small. It can be hit with a hammer without being broken
up. It is, in fact, malleable to some extent.
3. On Local Differences of Surface. — Many of the deposits show local
differences of macroscopic aspect, denoting differences of structure. These
are, no doubt, due to local differences of deposition conditions, especially
current density and concentration of electrolyte. As it is these differences
in conditions of deposition and their effects upon the structure of deposits
that are dealt with in Division 2, especially, of this Report, nothing more
will be said here on the matter.
4. On the Effect of Temperature Variation. — It may be remarked, in
the first place, that this series of experiments, which was conducted quite
independently of still another series on the same bath, carried out for
other purposes than the present and at an earlier date, confirm the con-
clusions arrived at before. Macroscopically, the character of the deposit
changes in the following way : — Working at a fixed current density
(120 amp. /ft.2), the deposit formed at 70° C. is dark and bright at first,
then becomes powdery, and finally splits up. As the temperature is raised,
the deposit becomes light in colour and smooth, still remaining bright, till
a temperature of (about) 90° C. is reached. As the temperature is still
further raised, the deposit becomes more coarse-grained and visually
crystalline. In general, then, the effect of rise of temperature upon the
macroscopic character of the deposit is, at constant current density, to
change the deposit from a fine-grained (visually, non-crystalline) one to a
coarse-grained and visually crystalline one.
B.— MICROSCOPIC EXAMINATION
Deposit of Experiment A
As already stated, no microscopic examination of this deposit was
possible.
* These outgrowths have nothing in common with what is called, in practical work
"burning," which is due to excessive current density.
6
Deposit of Experiment 1
Description of Structure, at a magnification = 125 (Obj. 16mm., Oc.x8).*
There is a gap in the deposit "due to a breaking away under the saw
when preparing the section. On one side of this gap the deposit is fibrous
in structure, which is uniform throughout the thickness of the deposit-
unless, perhaps, the fibres are slightly narrower near the base: the grains
are long, narrow, and fibre-like (Fig. 3). On the other side of the gap,
the strictly fibrous portion of the breadth of the deposit forms (approxi-
mately) one quarter of the whole— from within outwards (Fig. 4). 1
merges, radially, into an area in which the grains are wider and less
perpendicular to the axis of the specimen. They are here shorter, and
appear broken, and, further, become distinctly broader as the periphery
is approached. The distinction between the inner, fibrous, part and the
outer is marked by the more clear and regular appearance of the former,
which forms a sort of band. Continuing round the specimen, the same
type of dual structure continues for a considerable distance. In this
portion of the deposit the variation consists, especially, in the varying
widths of the two areas. These vary somewhat irregularly, but, on the
whole, the fibrous area decreases while the outer layer increases. Also, in
places, the merger of the fibrous into the outer layer is less marked and
distinct than elsewhere. And, again in places also, the grains in the
outer layer are distinctly wider (Fig. 5). In the portion of the deposit
situate at 90° from the gap, the structure merges (circumferentially) into
one in which there is nowhere a clear distinction to be seen be'tween the
strictly fibrous part and the outer. There is everywhere a gradual merger
of the one into the other. Moreover, the inner portion is less pro-
nouncedly fibrous, while the outer consists of considerably larger and
wider grains which are still less perpendicular to the axis (Fig. 6). Con-
tinuing round the circle, the difference between the two types of structure
again becomes more marked, the inner layer merging more quickly into
the outer; the fibrous band becomes more and more narrow, while the
grains in the outer layer become still broader and broader (Fig. 7).
Finally, on the opposite side of the specimen to that where the gap is,
the structure has approached the normal type. There is no fibrous area
at all (Fig. 8). The segment over which this structure extends subtends
an angle of some 30° ; and as the eye travels on round the circle back to
the gap a series of changes in structure similar to those described is
visible, but the order of the changes is reversed.
Some other features of the deposit are : —
(i) Very small holes.
(ii) Oxide inclusions (Figs. 7 and 9).
(iii) A few cracks. Some of these can be traced from the base metal
to (well-nigh) the periphery. They run perpendicularly to the
axis of the specimen.
Deposit of Experiment 9
Description of Structure, at a magnification = 125.
This specimen is free from numerous oxide inclusions. The peripheral
outline is smooth over about half its length, and undulating over the other
half. The undulations of the latter are, however, not extreme. (See
* In all cases Zeiss lenses were used. For a magnification of 125 diameters, which was
that used for most descriptions, the objective used was the 16 mm. apochromat in com-
bination with the X 8 compensating ocular. The magnifications of the photomicrographs
are usually 150 or 200 (obj. = 16 mm., Oc. = X 8, plus camera extension).
To face p. 6.
Fia. 3.:
FIG. 4.*
FIG. 5.«
STRUCTURES OF DEPOSIT 1.
(All x 150.)
* Photographs marked with an asterisk were used, in whole or part, to
illustrate a paper entitled " Some Defects in Electro-deposited Iron." Jour.
Iron and Steel Inst., 1920, Yol. 101, p. 321.
35067
FIG. 6.*
FIG. 7.<
FIG. 8.*
STRUCTURES OF DEPOSIT 1. (All x 150.)
Inclu-
sion of
Oxide.
PART OF
DEPOSIT 1.
Showing Oxide
Inclusions.
FIG. 9. x 150.
nw
fr^TO^
•&&:*•• jliim
¥!KP !W:;';:I.' * &i[W$
£ I* ; ^i;; 'M J;
DEPOSIT 9.
Smooth Periphery.
FIG. 10. x 200.
y wimmm%s —•
S • fj '\ *r •' • '^ .- I Vv,H ',', ,/.; ^ ,'/^ Undulating
1 •' !w^' V/1 ../-lr '' .'V \ ' liJLi \J. A \ i PorirkVior-
Periphery.
FIG. 11. x 200.
Smooth Periphery. Difficult to etch.
FIG. 12. x 160.
Undulating Periphery. Easy to etch.
FIG. 13. x 160.
Area 90° from those shown in Figs. 12 and 13.
FIG. 14. x 100.
DEPOSIT 3.
To face p. 1.
1, .." ••- ••» ••
f* x
_
Unetched. Area as Fig. 13.
FIG. 15. x 200.
Area near base of deposit.
FIG. 16. x 1,000.
DEPOSIT 3.
Fig. 10, an'd cf. Fig. 11.) The deposit has a dual structure correspond-
ing to the peripheral outline. Where this is undulating, the grains are,
comparatively, much larger and more pronouncedly V-shaped; that is,
the structure approaches the normal type. Where the peripheral outline
is smooth, the structure approaches the normal type much less closely :
the grains are more columnar and narrower, and they often appear as
though broken up. Owing to the specimen having been cut obliquely to
the central axis of the specimen, the approach to the normal structure is
not, in many places, so evident as it otherwise would be (Fig. 10). The
large grains of the deposit are remarkably clear and free from holes and
inclusions. There are two or three radial cracks; and in one place — in
the area of smaller grain — the deposit has broken away over a small arc.
Deposit of Experiment 3
Description, at a magnification = 125.
The deposit contains numerous small holes and inclusions of solid matter
(oxide). This makes it difficult to so etch the specimen that one. can see
the crystal boundaries well. One side of the specimen was much more
difficult to etch* (satisfactorily) than the other. The side that etched the
more satisfactorily is that which consists of the larger grains. The
structure appears to approach the normal type over most of the cross-
section of the specimen; but the crystal grains are more columnar, that
is, less divergent, and also more broken than in the true normal type
of deposit.
The structure of this specimen may be considered as twofold : Over one
half of the whole annulus (the better-etching half) the grains are much
larger (broader and longer) than those in the other half. In the former
the crystal boundaries 'are more easy to see clearly even at 125 magnifica-
tions; while, on account of the inclusions and holes, the boundaries of the
grains in the latter half are not easy to see at this magnification. Figures
12 and 13 show the difference in structure between the two halves of the
annulus more clearly than does Fig. 14, which shows the structure at a
lower magnification. The difference in the character of the peripheral
outline of the two parts of the deposit is very marked. In brief, it may
be said that that part of the deposit which etches the more easily and
satisfactorily : (1) consists of larger grains ; (2) has an irregular peri-
pheral outline, resulting from the rough surface; and (3) is less full,
perhaps, of inclusions and holes.
Fig. 16 shows a portion of the deposit near the base. In this figure the
minuteness and number of the small holes (appearing in the photographs
as bright spots) can be well seen. In this figure, too, the difference
between the holes (bright spots) and the inclusions of oxide (dark irregular
patches) is very marked. Fig. 15 shows the surface of the specimen,
polished but unetched, at 200 diameters.
Deposit of Experiment 10
Description of structure, at 125 magnifications.!
The deposit extends unbroken over only about two-thirds of the annulus
(as in Fig. 17). A central piece (6) lies in the gap. The structure is,
* All the specimens of which photomicographs are shown in this report were etched
with a 2 per cent, solution of pure nitric acid in absolute alcohol.
t No photographs of this specimen — to show the general structure at ordinary
magnifications — are inserted, since those of Nos. 9 and lOa show the types of structure
that occur in it.
8
in general, similar to that of the deposit of experiment 9, being of a
dual character. The part of the deposit to the right of the dotted line,
xy, corresponds to that part of 9 that has an undulating peripheral
outline with large-grained internal structure. The part to the left of xy
corresponds to the portion of 9 that has a smooth periphery. The metal
in 10, where the outline is smooth, seems, however, more broken than the
corresponding part of 9. It was much more difficult to etch satisfactorily
FIG. 17.
than was 9; and it was very much more difficult to etch than that portion
of 10 to the right of xy. In the area b the grains are very large a.nd
clear. They very frequently start at the base of the deposit, and continue
throughout the layer. This occurs in the corresponding part of 9, but
only exceptionally; the reason is, probably, that the section of 10 is. cut
more perpendicular to the central axis than is 9.
» At a magnification of 1,000 diameters the metal appears to be over-
etched. A curious feature (seen in Fig. 18) is the presence of what look
like holes filled with liquid. They are greenish in colour, and more or
less rounded in shape; and they are characterised by showing concentric
ring markings which follow closely the shapes of the holes. Another
matter of note is that these (?) holes are arranged in rows, which
frequently run in parallel lines.
•'; ' • i
Deposit o! Experiment lOa
Description of Structure, at 125 magnifications.
The structure is, throughout the section, of a type closely approaching
the normal. The two-fold character of deposits 9 and 10 is present in
this one also, but it is very much less marked. Where the periphery
ift most uneven, the grains below are largest and most divergent
(V-shaped) from within outwards; and, often, one grain extends over
the whole width of the deposit. Where the periphery is more even,
the grains are less divergent and narrower ; and the structure appears
more broken. Nowhere is the periphery of this specimen smooth; it is
always more or less uneven. The surfaces of the largest grains appear
to contain numbers of small holes, which become bright or dark as
one focusses down or up — that is, towards or away from the surface of
the specimen. In this deposit a curious dotted appearance of the surface
To face p. 8.
Showing holes filled
with greenish liquid.
(Cy.J.C.W.Humfrey,
Carnegie Mem., 1912,
vol. 4, p. 82.)
FIG. 18. x 1,000.
DEPOSIT 10.
Smooth Periphery.
The dotted appearance
of the surface of
some of the grains is
notable.
FIG. 19.* x 150.
Undulating
Periphery.
(Rough to the
touch.)
FIG. 20. X 150.
DEPOSIT 10A.
To face p. 9.
FIG. 21. x 200
DEPOSIT 4.
of some of the grains is very noticeable (v. especially Fig. 19). The
clean 'division line between the copper and steel on the one hand, and
the copper and iron deposit on the other, is to be noted.* (N.B. This
cathode was dipped in HC1, v. p. 4.) The deposit, too, is a clean one —
free, that is, from numerous oxide incisions.
Deposit of Experiment 4
Description of Structure, at 125 magnifications.
The periphery is smooth over the greater part of the specimen. The
structure is columnar, the columns narrowing quickly towards the base
metal. It is often possible to trace a grain running through the entire
thickness of the deposit from base to circumference, the grain, while
mostly of even breadth, widening somewhat as the circumference is
approached. The shape of the grains is well seen in Fig. 21. The
surfaces of the grains are bespeckled with minute holes or inclusions;
but larger patches, such as are found in No. 3, are absent almost entirely.
The same bath was used in the two cases. While, however, No. 3 was
being deposited there was considerable oxide in suspension; when No. 4
was introduced, all but the finest suspended matter had settled down.
The structure of this specimen is more truly representative of the con-
ditions of formation that is No. 3, during the formation of which the
temperature varied during the last quarter of the time from 100° C.
to 86° C. It is likely, too, that the large amount of floatirig matter
affected the structure of No. 3. The grains of this specimen (No. 4)
are considerably smaller than those of Nos. 9, 10, and lOa (cf. the
photographs and respective magnifications).
Deposit of Experiment 2
Description of Structure, at 125 diameters.
No such variation of structure occurs, when passing, in a circular
direction, round the annulus, as occurs in the case of the deposit of
experiment 1. Other notable differences between the two deposits are: —
(i) There is no completely fibrous area, such as is seen in Fig. 3. (ii)
There is a distinct thinning of the deposit on one side, forming a, so
to say, wide syncline or valley, (iii) Annular wavy rings are seen in
the deposit, especially in the half near the base metal. These can be
seen to run completely round the circle, but they are more defin-ed in
some places than in others, (iv) No radial cracks are to be observed,
though (v) the structure is that of a more broken and strained metal,
(vi) The deposit is obviously thinner, as can be seen by comparing the
photographs of the specimens, which are seen at the same magnification,
namely, 150 diameters.
Traverse Of the Annulus. — Starting observations from a definite place —
where, i'n fact, some small lumps occur on the surface — the deposit
approaches the normal type. The grains quickly become considerably wider
from within outwards, and continue to widen somewhat as the periphery is
approached. The shape of the grains is long and, in general, columnar, or,
perhaps, roughly lenticular. Proceeding round the circle, the grains i'n the
outer part of the deposit are seen to be much larger and to, most of them,
resemble columns with irregular sides; some of the grains are, relatively,
* The layer of copper itself is too thin to be seen at most places.
10
very large and V-shaped (Fig. 22). But the arc of the area in which euch
large grains are located subtends a central angle of some 10° only. The
structure of the next following portion of the deposit vari'es rather quickly
to that first described, and this type of structure then merges, at about 90°
from the starting point, into a typ* in which the grains of the inner portion
of the deposit approach the fibrous form, becoming somewhat wider and
lenticular outwards as the periphery is approached. This last type extends
over about one-quarter of the circle, but, at the siame time, the deposit
gradually narrows until a minimum width is reached at about 180° from
the starting point of observation. The syncline subtends a central angle
of some 30°. The structure of the remaining half of the deposit
goes through a series of changes similar to those already described, but in
a reversed order of sequence.
Minute holes are visible on the surface of some of the grains; and some
more or less rounded patches (probably of oxide) are to be seen.
CORRELATION OF MACROSCOPIC AND MICROSCOPIC
FEATURES.
The remarks to be made upon the correlation of features seen in the
above-described deposits of Series I will be confined to such features as
may be influenced by temperature. For the sake of brevity and clearness,
the correlation may, perhaps, best be made in tabular form. Thus : —
Macroscopic feature Microscopic aspect
1. Smooth, bri'ght or matt, non- Fine-grained, fibrous, or only
crystalline surface (Nos. 1, 2, 3 slightly divergent V-shaped
and 4). grains.
2. Smooth, crystalline surface (e.g., Coarse-grained, non-fibrous, large
No. lOa). (roughly) V-shaped grains.
3. Smooth, matt, or finely crystal- As 1, on one side (or on two
line on one side (or two opposite opposite sides), and as 2, on the
sides); coarsely crystalline on other (or remaining two opposite
the other (or two remaining sides). From 2, especially, it
opposite sides (Nos. 9 and 10) ). seems fai*r to conclude that the
deposit is coarse-grained where
it is more coarsely crystalline on
the surface.
CONCLUSIONS— ON THE EFFECT OF TEMPERATURE
It is suggested that one may deduce the general conclusion from the
above, that where two deposits are formed under otherwise similar condi-
tions of deposition, that which is formed at the higher temperature will be
the coarser-grained in structure.* It would seem, further, that the coarser
the grain, the less brittle and hard the deposit isf, and hence, tempera-
ture affects the physical character of a deposit in this respect also. No. 4
does not, at first sight, seem to support this latter conclusion; but it is
noticeable that, though even and matt on the surface, and finer-grained
(than 9, 10 and lOa), microscopically, they are not so fine-grained as Nos.
1 and 2, and they are not so brittle. The larger (somewhat columnar)
structure of Nos. 3 and 4 may possibly be due to the volume of electrolyte
used (v. p. 4).
* Cf. W. Blum, Trans. Am. Electroch. Soc., 1919, vol. 36, at p. 221.
t Cf. H. S. Rawdon and B. J. Gil, Bur. Standards Sci. Paper, No. 397 ; Jour. Franklin
Inst., 1020, vol. 190, p. 731.
To face p. 10.
Structure more
open in outer half
of the deposit.
FIG. 22. x 150.
Grains narrower in
outer half of
deposit ; more
fibrous in inner
half, especially near
the base.
*- Note : Wavy
lines.
Wavy lines near base are well seen.
(v. Jour. Iron and Steel Inst., 1921 (No. 1), vol. 103, p. 355).
FIG. 24. x 100.
DEPOSIT 2.
(35067)
11
SERIES II
ON THE EFFECT OF CURRENT DENSITY
A.— DEPOSITION
Constant temperature : varying current density.
The Conditions of Deposition were — *.
Temperature
Time ...
110° C.
This was vari'ed in such a way as to give
. approximately the same thickness of
deposit in all cases.
The bath used was the 2£ gallon one ; and the process of cleaning and the
type (shape, &c.) of cathodes were the same as in Series I.
Expt.
No.
Ref . to
micro.
C.D.
a/ft.2
E.M.F.
volts
at 6£".
Time
his.
Remarks.
Deposit.
5 Figs. 25 to
(50
0-4
4
_
Macroscopic. Good, even : light
27.
colour. Very finely crystalline.
Slight thickening at lower end.
No lumps or outgrowth. On
sawing, broke away in one or
two places — not badly, however.
Rather more brittle than 10 and
lOa. Adherence, very fair.
4
Fig. 21
120
0-8
2
—
See Table, Series I.
10 and
Figs. 18 to
120
—
2
—
lOa
20.
6
Figs. 28 to
31.
160
0-8
H
—
Macroscopic. Light grey: smooth.
Finally crystalline. Some thick-
ening — especially at lower end.
On sawing, no chipping off.
Adherence, very good.
7
Figs. 32, 33
&34.
200
0-9
1-2
—
Macroscopic. As 6 in colour, &c.,
but surface somewhat more
coarsely crystalline. On sawing,
no breaking away. Adherence,
very good. The deposit was
found to be quite malleable
under hammer blows.
8
Figs. 35 &
240
1-2
1
Not dipped
Macroscopic. Light grey: Rather
36.
in HC1 in
rough — partly from little hemi-
cleaning.
spherical lumps. More coarsely
crystalline than Nos. 6 and 7.
No sign of " burning."f On
sawing, comes away cleanly from
the base metal. Chips under
hammer blows and powders.
N.B.— HC1 dip not used. Ad-
herence, poor.
8a
Figs. 37 to
41.
240
1-2
1
Dipped in
HC1.*
Macroscopic. Light grey : smooth.
Crystalline. Slight thickening
round bottom edge : slightly
cindery here. On sawing, no
chipping or breaking. Adherence,
very good.
* In experiment 8a a siphon arrangement with a narrow outlet jet was used for intro-
ducing water into the bath to replace that lost by evaporation. This prevented stirring up
the sludge at the bottom of the tank, which happens when water is introduced in bulk.
f v. note, p. 5. •
12
REMARKS ON THE FOREGOING DEPOSITS
1. On Adhesion. — The same result of not dipping the cathode in dilute
HC1, before suspending it in the depositing solution is seen in the cases
of Nos. 8 and 8a as was seen in the cases of Nos. 10 and lOa of Series I.
The deposit on the dipped cathode, 8a, adhered well, while that on the
other, 8, did not.
2. On Local Differences Of Surface.— This is again noticeable in the
deposits of Series II, just as it was in those of Series I.
3. Effect of Current Density. — The deposits of Series II show that, at
constant temperature, and under otherwise similar conditions, increase of
current density causes an increase in the coarseness of the crystalline
surface. The variation is from matt at lower current densities, through
finely crystalline at intermediate current densities, to coarsely crystalline
at the highest intensities employed. It may be conveniently stated here,
with respect to the observations made, that although the terms " coarse "
and " fine " are relative, and not absolute, in meaning (and hence, the
scale of coarseness and fineness may be a different one in the mind of one
observer to that present to the mind of another) yet, in the case of thfe
deposits of Series I, II, and III, the author has had th*e advantage of con-
sulting the note-book of his (one time) assistant,* who aided him in the
deposition. These notes were made independently, and are found to agree
with the author's in general, and in particular where the terms " coarse "
and " fine " are used. It thus appears that increase of temperature at
constant current density and increase of current density at constant
temperature have much the same effect upon the macroscopic appearance
of the deposited iron. But the effect is much less marked in the former
case than in the latter.
B. MICROSCOPIC EXAMINATION
Deposit of Experiment 5
Description of Structure, at 125 magnifications.
\
(1) Before etching. — The polished surface shows many holes and some
irregular patches of oxide. The periphery is, in general, fairly smooth.
Several cracks can be seen, some of which extend from periphery to base
metal. Four or five of these cracks can be easily seen with a pocket
magnifying glass, and at least two with the naked eye.
(2) After etching (with a 2 per cent, solution of HNO3 in absolute
alcohol).
The deposit has a structure which resembles the normal type fairly
closely. The grains are moderately large — not so large as those in
Nos. 9, 10, and lOa. The structure varies round the circle only in the
shape of the grains being more lenticular and irregular in some places :
the structure, as a whole, has a more broken look in some places than
elsewhere (cf. Figs. 25 and 26). The V-shape of the grains is less
apparent in such areas of broken structure. No annular markings are
visible at this magnification (125 diameters), but numerous holes are
visible, as also are cracks that often run irregularly through the whole
thickness of the deposit. It is to be noted that the surfaces of some of
the grains have a speckled appearance. The periphery is, in general,
. J.W. Gardom.
To face p. 12.
Fio. 25. x 150
FIG. 26. x 150.
C/. W. Austin's photo-
graph showing effect of
oxygen in pure iron.
FIG. 27. X 1,000.
In focus. Numerous holes and inclusions camouflage the structure.
Fia. 28. x 150.
«
"&&nr
Out of focus. Structure better shown.
FIG. 29. x 150.
DEPOSIT 6.
To face p. 13.
-Structure of area opposite that shown in Fig. 28.
FIG. 30. * 150.
•
Area at 90° from areas of Figs. 28 and 30.
FIG. 31. x 150.
DEPOSIT 6,
18
regular; where a slight lump occurs, the structure is of the cauliflower
variety.
At 500 diameters the holes are seen to be irregular in shape, and appear
coloured. Other very small holes become apparent, which change from
black to slightly grey or colourless (and bright) as one focusses up and
down. The speckled surfaces (before alluded to), which are dark
(brownish) at low magnification, are seen to have a structure that reminds
one of the section of an oxide inclusion illustrated by W. Austin*, and
contained in pure iron to which oxygen has been added. Austin describes
this as an eutectic structure. Fig. 27 shows the speckled surface at high
magnification.
Deposit of Experiment 6
Description of Structure, at 125 diameters.
In general the structure does not differ very much from that of No. 5;
but the holes, which are very small and very numerous, cause it to appear
to be of smaller grain than No. 5. A comparison of Figs. 28 and 29 shows,
however, that this is not so. Fig. 28 was taken with the specimen in
focus : Fig. 29 shows the same area somewhat out of focus. From these
it is seen that it is the tiny holes (dark in Fig. 28) that cause the struc-
ture to, at first sight, seem of finer grain. Comparing Fig. 25 (speci-
men 5) with Fig. 29 (specimen 6), it is seen that there is not much
difference in grain size, though the grains near the base metal are, in
No. 5, more V-«haped than those near the base metal in No. 6. Inclusions
of oxide are few only ; and no radial cracks that traverse the whole thick-
ness of the deposit are to be found. The structure varies round the
circle. It varies between that shown in Figs. 28 and 31 and the structure
shown in Fig. 30. In the former the periphery is smooth; in the latter
it is lumpy, and the structure is somewhat of the cauliflower type. It
was noted that the latter part of the specimen was much more easy to
etch satisfactorily than the former. The area shown in Fig. 31 is located
between, and at approximately 90° from, those shown in Figs. 28 and 30.
It extended over about onei-third the circle, and its type of structure
gradually merged, on each side, into those shown in Figs. 28 and 30.
Deposit of Experiment 7
The surface of the polished specimen, as seen before etching, showed
numerous holes and inclusions. Where the periphery is undulating, t the
locus of inclusions (of oxide) is often, in shape, a curve having a curvature
inverse to that of the corresponding undulation of the periphery. In
this specimen, the periphery is, for the most part; of an undulating
(lumpy) character.
Description of Structure, at 125 diameters.
Very light etching (5 seconds) shows the inclusions to often lie along
the grain boundaries. In general, the grains are large — much larger
than those of Nos. 5 and 6. They resemble (broadly) those of Nos. 9
and. 10 ; but they are, in this specimen, characterised (i) by the boundaries
between them being (as before stated) the loci of inclusions, and (ii) by
their fan-shaped form, especially towards the periphery (Fig. 32). The
* Jour. Iron and Steel Inst., 1915 (No. 2), vol. 92, p. 157.
f One has to remember that a periphery which appears lumpy or undulating in a micro-'
graph, at a magnification of 150, will very often appear smooth when examined with the
naked eye.
35067 B
14
general structure approximates closely to the normal type : the angle of
the V is, often, situate among the small grains near the base metal and
at or close to this. The surface of the grains is, frequently, much pitted,
as though containing very numerous minute inclusions. There seems, in
places, to be interlocking or intergrowth between the grains, marked by
a difference in the pitting of the surfaces, which is emphasised by the
etching, and caused by the comparatively clear surface of one grain,
having the V shape, being broken by a pitted area (v. especially, Fig. 33).
This is due to the section being cut across the plane parallel to the current
lines, with the result that the structure appears different in one area
than elsewhere. It has a mosaic appearance. Apart from this apparent
difference, the structure is uniform over the annulus, except that in
places (i) the V-angle of some grains is more acute than in others, and
(ii) the individual large grains commence farther from the base metal
(cf. Figs. 32 and 34).
Deposit of Experiment 8
An examination of the polished surface before etching discloses lines
that cause the specimen to be full of cracks. The lines often extend
throughout the deposit, and, frequently, branch. Usually, they are not
radial in direction, but run across the radii. Etching shows most of
these lines to be division lines between the grains, the lines being often
marked by oxMe inclusions (cf. No. 7). To distinguish such lines from
true cracks, they have been called " quaei-cracks ".* The periphery of
this specimen is undulating or lumpy (cf. No. 7); small holes and
inclusions are present, but do not appear numerous on the unetched
surface. The thickness varies.
Description of the Structure, at 125 diameters.
The structure is fairly uniform all round the annulus. It consists at the
base of a layer, of varying width, of small grains, followed by large
grains that originate among the small ones at varying distances from the
base metal. The outer layer has the cauliflower qr " poplar-tree " type
of structure, and is similar to part of No. 7. In general, the size of
grain is not markedly different from that of No. 7; the grains may,
perhaps, be rather narrower. There is some interlocking of grains, but
this is nowhere so marked as in No. 7.
The line between the base metal and the deposit is often far from clear,
and the grains of the deposit near the base metal are frequently confused.
It may be recalled in this connection that the cathode was not, in this
instance, dipped in dilute HC1 during the final part of the cleaning
process.
Deposit of Experiment 8a
The deposit of this specimen does not vary much in thickness ; but it is,
in general, thinner than No. 8. The peripheral outline is everywhere
undulating; in places it is very irregular. Figs. 37 to 40 show the
unetched surfaces and the effects of varying amounts of etching.
Description of the Structure, at 125 diameters.
The structure is, in general, similar to the structures of Nos. 7 and 8;
it Varies between the cauliflower and the poplar-tree types. The lines
of inclusions (of oxide), situate on the boundaries between the grains,
* See u Some Defects of Electro-deposited Iron " (Jour. Iron and Steel Inst 1920 vol.
101, p. 321).
Fan-shape
grain.
Showing general
type of
structure.
FIG. 32. X 150.
Shows
inter-locking
of grains.
FIG. 33. x 150.
FIG. 34. x 150.
Copper/iron line ->
This specimen was Section unetched showing " quasi-crack*
not dipped in HC1.
FIG. 35.* x 150.
Etched section. " Poplar-tree " structure.
FJG. 36.* x 150.
DEPOSIT 8.
«< . *#'..
.
:'-v,-' y
Unetched.
FIG. 37.* X 150.
Clean copper/
iron line.
Etched — 15 seconds.
FJG. 38. x 150.
DEPOSIT 8A.
Tofacep.\15.
Etched—
(15 + 10) sees.
FIG. 39.* x 150.
Etched—
(15 + 10 + 10) sees.
FIG. 40. x 150.
FIG. 41. x
DEPOSIT SA.
15
are at places very marked (see Figs. 37 and 38). The surfaces of the
grains are often very speckled. The mosaic type of structure (Fig. 41)
is seen in one part especially; and in this area the surfaces of many of
the grains are clearer. The cauliflower (or poplar-tree) and the mosaic
types of structure seem sometimes to be confused or combined, the one
being, as it were, imposed upon the other.
CORRELATION OF MACROSCOPIC AND MICROSCOPIC
FEATURES
All the deposits of this series (except No. 4) appear crystalline to the
naked eye. Observations made and noted at the time of removal of the
deposits from the depositing solution were as follows : —
No. 5 .... very minutely crystalline.
No. 4 .... matt.
No. 10 .... crystalline on one side, matt on the other.
No. lOa . . . crystalline — rather less coarse on the parts facing the
anodes.
No. 6 .... minutely crystalline.
No. 7 .... somewhat more coarsely crystalline than No. 6.
No. 8 .... more coarsely crystalline than Nos. 6 and 7.
No. 8a . . . crystalline.
An examination of the photomicrographs shows that, in size of grain,
the internal structure corresponds quite well with the macroscopic aspect
in the case of each of the deposits except No. 4. The more coarsely
crystalline the surface is, the coarser (or larger) is the grain of the
interior. This fact indicates that observation of the surface of a deposit
enables a conclusion to be formed with some certainty as to its internal
structure — a fact of considerable importance in the control of deposition.
As regards No. 4, all that can be said is that it is an exceptional case
for which the only explanation that can be offered is that possibly some
change in the deposition conditions occurred just before the removal of
the cathode from the 'bath. Though forming an exception, it was
considered proper, nevertheless, to include it.
CONCLUSIONS— ON THE EFFECT OF CURRENT DENSITY
Whereas the deposit of experiment 5 is certainly of smaller grain than
those of Nos. 10 and lOa, on the one hand, and Nos. 7 to 8a, on the other,
it is not easy to decide as to the relative size of grain of Nos. 10 and lOa,
on the one hand, and Nos. 7 to 8a, on the other. This seems to depend
upon whether each " cauliflower " or " poplar-tree " of Nos. 7, 8, and 8a
(Figs. 32 to 41), is to be regarded as one grain or as a conglomerate of
.small grains. The author's view is that the former view is the correct
one. Figs. 33 and 41, in which the (apparent) interlocking of the grains
is seen, as well as the starting of the poplar-trees at or near the base
metal and their gradual widening outwards, show, it is suggested, that
this view is correct. Visual examination of the specimens, which is, of
course, much more determinative than photographs, confirms it. If, on
the 'one hand, each " tree " is not one grai'n but a conglomerate of
separately formed grains, then undoubtedly the structure of Nos. 10 and
lOa is much larger than those of Nos. 7, 8, and 8a. If, on the other
hand, each " tree " forms one grain, then it cannot be said, with any
35067 B 2
Iti
certainty (either from the micrographs or the specimens themselves),
which group of deposits has the larger structure. The deposit of experi-
ment 6 must be considered. This, in grain size, certainly comes between
No. 10 (and lOa) and No. 7.
Macroscopic and microscopic observations of the deposits of Series II
seem, therefore, to warrant the following conclusions : —
1. As the current density rises from 60 amp. /ft.2 so does the size of
grain until —
2. A maximum grain size is reached somewhere in the region of
120 amp. /ft.2
3. The size of grain then diminishes to a minimum which lies some-
where between 120 and 200 amp. /ft.2
4. It again becomes larger at the highest current densities used.*
Note on the Plane Of the Sections. — It may be conveniently remarked
here that it seemed preferable to cut and examine sections that lay in
planes parallel to the lines of flow of the current rather than sections
at right angles to the current lines. If the latter method is adopted,
that is, if sections perpendicula'r to the current lines are made (as is
done by Sieverts and Wippelmann),t the size of the grains, and hence the
number of grains per unit area, will depend upon the position of the
plane of the section relative to base metal and periphery. For instance,
in the case of a deposit of normal structure, the farther the plane of the
section is from the base metal, the larger the grains appear, and hence
the fewer will be the number of grains appearing in unit area. Hence,
unless two deposits, which it is necessary to compare, have the same thick-
ness, and the planes of the sections made of them are at the same distance
from the base metal, wrong conclusions as to grain size will, most probably,
be drawn.
SERIES III
ON THE EFFECT OF MECHANICAL MOVEMENT
In experiments 12, 12a, and 13, 13a, it was sought to ascertain the effect
on structure of mechanical movement. Nos. 11 and lla were both con-
ducted with moving cathodes, but the cathode was attached to the rotating
epi'ndle (used in these cases) in a different way in each case, so that one
obtained in each case a different type of movement of the electrolyte against
the cathode.
Nos. 11 and lla can be compared with Nos. 10 and lOa, the deposits of
which were formed under similar conditions as were those of Nos. 11 and
lla, except that these were moved while the cathodes of Nos. 10 and lOa
were stationary. J
* Similar alternations in grain-size as the C.D. is progressively increased have been
observed in the case of copper also. A. Sieverts and W. Wippelmann, Zeit. f. anorg. Ch.,
1915, vol. 91 (Zusammenfassung von Teil I). On the other hand. Dr. W. Blum claims
that "... increase in current density up to a certain point decreases the size of the
crystals
t Op. cit., and, sometimes, by Blum.
jThe "
difference in the C.D. employed in the two cases (namely, 114 amp./ft.2 and
120 amp./ft.2) is negligible.
17
A.— DEPOSITION
Cleaning. — As in Series I and II.
No. of
Ref . to
Micro.
C.D.
Amp/ft.2
Time
hrs.
Temp
°C.
Remarks.
De
11
Figs. 42
&43.
120
2
—
Dipped in HC1 dil. After
a few minutes the
Macroscopic. Very good. Light
blue-grey, almost " metallic "
motor stopped for a min-
appearance. Very close grain.
ute or two.
Very finely crystalline down
Cathode was attached with
one side. Very slight thicken-
its axis parallel to that
ing round the bottom edge. A
of the rotating spindle,
so that it did not revolve
few pits caused (probably) by
sediment. On sawing, adher-
on its own axis (cf. lla).
ence was found very good.
Rotation of spindle was
very slow — only a few
revolutions per minute.*
lla
Figs. 44,
120
2
110
The bath was freed from
Macroscopic. Very similar to
45 & 46.
all but some fine sedi-
No. 11 in appearance. A few
ment. Cathode dipped
pits, elongated in the direction
in dil. HC1.
of rotation. Very slightly
Cathode was attached to
crystalline on one side. On
spindle so as to form a
sawing, deposit broke away
continuation of the latter.
from the steel base-metal.
I
Hence, it rotated on its
carrying the slight coat of
4
own axis. R.P.M. = 100.
copper with it. Poor adher-
ence, probably due, therefore,
to defective cleaning.
12af
Figs. 52,
120
i|
100-
Cathode movement, to and
Macroscopic. Very fine-grained:
53 & 54.
105
fro (suspended from —
smooth. Amorphous ap-
rod) parallel to anodes.
pearance. Light grey colour.
Strokes per minute =
No outgrowth: very slight
100. After f hour move-
thickening at bottom end.
ment stopped for a minute
On sawing, found that deposit
or two. HC1 dip was
used. One gal. of solu-
tion only was used.
was remarkably hard. Adher-
ence, fair ; deposit was in-
clined to chip.
12
Figs. 47 to
120
1|
100-
No movement : cathode
Macroscopic. Good colour.
50.
105
stationary. 1 gal. of
Smooth and even. Finely
solution only.
crystalline. Some cindery
outgrowth at bottom end.
On sawing, deposit was found
to be much softer than that
of 12aJ, and it did not chip off
the base-metal at all.
13
Fig's. 55
&56.
144-6
2
98-100
No movement. Copper
cathode, cleaning as
Macroscopic. Good light-grey
colour. Finely crystalline.
usual.
Slight (smooth) thickening at
bottom end. Some small pits.
13a
Figs. 57
144-6
2
98-100
Movement, as in 12a. No.
On sawing, adhered well.
Macroscopic. As No. 13 ; but
Ito61.
of strokes was 72 jper
could only just (and only in
minute.
places) discern the crystalline
nature of the deposit. On
sawing, adherence was found
to be good.
* Unfortunately, through an oversight, the R.P.M. were not counted ; but they certainly
did not exceed 30.
f Newly made solution, 1 gall. only. (Nos. 11 and lla were deposited in tha 2i-gall.
vat).
£ Both Mr. Gardom and the author make special remark, in their notes, on the hardness
of the deposit of 12a as compared with that of 12.
Eemarks on the Foregoing Deposits. — It is very apparent on actual
observation of the specimens themselves that the motion of the cathode
during deposition causes a distinct effect upon the character of the deposit.
Where two deposits are formed under precisely the same conditions, except
that one cathode is moved mechanically while the other is maintained in
a stationary position, the deposit on the moved cathode will be of finer
18
grain than that on the stationary one. This is very apparent, even to the
naked eye. The above-described deposits (Nos. 12, 12a, 13, 13a) are typical.
Many other samples have been prepared for the purpose of making
a comparison between deposits formed under the respective conditions of
movement and stationary position*; the result was always the same. It is,
therefore, suggested that —
The General Effect Of Movement upon the macroscopic character of
a deposit of iron is — To diminish the size of grain of the deposit.
It is certain that such an effect is caused by movement in the case of iron
deposited from the chloride (neutral) bath.
B.— MICROSCOPIC EXAMINATION
Deposit of Experiment 11
Description of Structure, at 125 diameters.
The deposit is >a clean one, i.e., free from numerous oxi'de inclusions; it
is, too, free from numerous holes. Fig. 42 shows the unetched polished
surface. It shows the effect of the stoppage of the rotation that occurred
during the formation of the deposit. The whole deposit is divided into two
broad layers (an inner and an outer), with a narrow layer between them.
This narrow layer was formed during the stoppage of the spindle; it con-
tains numerous inclusions of solid matter (oxide) that settled down upon
the deposit during the rest period. The micrograph shows clearly also how
the narrow layer forms a source of weakness in the deposit; one part of the
outer layer has broken away from the remainder. The thickness of the
deposit varies round the circle. This is to be expected, considering the way
in which the cathode was attached to the rotating spindle.
Fig. 43 shows the etched surface of the section. In structure the
deposit is of the approximately normal type; and it is very much the
same at all parts. The structure resembles that of No. lla at one
part of the latter (see Fig. 45); and it is not unlike some of the deposits
of the two former series, e.g., No. 2 (Fig. 23) and No. 5 (Figs. 25 and 26).
Deposit of Experiment lla
Description of Structure, at 125 diameters.
The deposit contains several radial cracks; one is visible with the naked
eye. It is " clean," and contains but few holes. It was noticed that
the deposit etched differently at different parts. t The part that required
longer time to etch extended over an arc subtending a central angle
of 45°. Its structure (Fig. 44) was more columnar than that of the
remainder of the deposit, which was of the approximately normal type
(Fig. 45), and very like that of No. 11. In one part the structure
approached very close to the normal (see Fig. 46), and resembles that
of No. lOa (Fig. 19). The difference, however, between the structure
of this deposit at the part seen in Fig. 46 and that of No. lOa, seen
in Fig. 19, is that the V-shaped grains of the former are much narrower
^ * It is considered unnecessary to insert particulars of all these experiments. Those
introduced are typical of all.
f This had frequently been noticed in other specimens, but the difference was particu-
larly marked in this one.
To face p. 18.
<- Part
of deposit
formed
during
stoppage
of
spindle.
Deposit — before etching.
FIG. 42. X 150.
a Plf'l
- Vt.y^''
|fvf:V.r%
Deposit — after etching.
FIG. 43.* x 150.
The clear inter-
mediate layer of
copper.
(Cf. Fig. 35.)
DEPOSIT 11.
.v -' f
x 150.
FIG. 45.
X 150.
FIG. 46.
x 200.
To face p. 19.
Unetched.
FIG 47. X 150,
.'»•<**•••.<
**H^
^^1^J|^^^|
^«/£:^ic'' i^k^>*lw* 1 1
Etched.
FIG. 48. X 150.
*r -« . ^!Ti
i^. ^^MU^ :^5iv ,.;^^>*
^&
r'.f55?4;*
\j -^*#*i'i
Unetched.
FIG. 49.* x 150.
DEPOSIT 12.
19
than those of No. lOa. This corresponds to a less crystalline appearance
macroscopically, or, in other words, to a finer grain.
Polished, unetched section. Enlarged.
To show how inclusions vary over the
area of the deposit.
Diagrammatic.
FIG. 51.
Deposit of Experiment 12
Description of the Structure, at 125 diameters.
The appearance of the polished and unetched surface of the section
of this deposit is seen in Figs. 47 and 49. On one side, a (Figs. 51 and
49), the deposit is .eeen to be crowded with holes or inclusions of oxide ;
while on the other side, b (shown in Fig. 47), there appears a layer
of them near the base, but otherwise the deposit seems comparatively
clear. Between a and b, as extremes, there are areas, c and d (Fig. 51),
in which the appearance of the surface is intermediate between that of
a and that of b, as regards the number of holes or inclusions contained
in it. There is, in fact, a gradation from a to b through c and d.
When the surface of the section is etched there remains the same
difference in general appearance. Fig. 50 shows the appearance, when
etched, of the surface seen in Fig 49, and similarly, Figs. 47 and 48,
are the unetched and etched surfaces, respectively, of the same part
of the specimen — a part situate in area b.
The specimen was very difficult to etch at all satisfactorily, and so
as to show the grain boundaries. Especially was this the case as regards
area a. Fig. 48 (area b), however, shows the boundaries of the grains
with fair distinctness. The structure is fibrous for some distance from
the base metal, the fibres then broadening as the periphery is approached;
in general, it may be said that the structure, so far as can be seen,
varies, radially, from fibrous to columnar. In Fig. 50 the fibrous
structure of the deposit can only here an'd there be seen in the photo-
graph; but it is more obvious on actual visual examination of the
specimen.
The periphery is smooth all round ; no radial cracks are visible, and
the thickness does not vary much.
A principal interest of this deposit is the effect of inclusions upon
structure that is disclosed in it. Obviously, what has happened is as
20
follows: — The solution was not a clean one,* and the cathode was
suspended with the side, a, facing upwards. The suspended matter in
the solution has gradually settled down upon the deposit during its
formation and become enclosed in it. The upturned face, a, of the
deposit received the bulk of the settling matter, c and d much less but
more than b.
Another observation made in connection with this specimen is the
following : — The section was polished and etched several times before
even an approximately satisfactory etching could be obtained. It was
noticed when following the extent of the etching under the microscope,
that during re-polishing the holes or inclusions gradually became
covered with a film, so that finally an appearance was obtained as seen,
for example, in Fig. 47, in which the filmed-over holes can be seen in
the outside half of the section — especially, close to the periphery. It was
much more difficult to arrive at this condition in the case of area, a,
than in that of area, b — a consequence, no 'doubt, of the presence of
more holes and inclusions. This is, of course, nothing else than an
instance of the effect of polishing first observed and explained by Sir
George Beilby.f.
Deposit of Experiment 12a
Description of the Structure) at 125 diameters.
The deposit (on steel rod, coppered, as base metal) of this specimen is
very brittle. Only a little more than one-third part of the circumference
of the base rod is covered with deposit, the remainder having broken
away when the sample was sawn. The deposit remaining is piecemeal;
one part is separated completely from the rest (the greater piece), and
this greater piece is divided up into parts by straight and clean radial
cracks (See Fig. 53). The deposit is uniform in thickness, and has an
even periphery. As compared with No. 12, the number of holes and
inclusions is few. Where the deposit has not come away from the base
metal on which it was deposited, the copper layer is clear. But in general
it has come away, and is separated from the base metal by a gap which
has become filled up with casting metalt that has flowed over into and
filled up the gap during the polishing process.
The structure is uniform throughout so much of the deposit as is present.
It is of the fibrous type (Figs. 52 and 53) : there is no banding, as, for
instance, .in the deposit of Experiment 1, Series I. There is, indeed, a
thin (but very noticeable) central layer between wide inner and outer
layers. This is due to the stoppage of mechanical movement that occurred
after continuance of- the deposition for about three-quarters of an hour.
This central layer can be seen in all the micrographs of the specimen;
but it is especially well seen in Fig. 54, as also are some circumferential
lines. §
* The solution used for 12 had previously been used for 12a — the moving cathode, and
sufficient time had not been allowed for the oxide formed during the deposition of 12a to
settle down. The solution used for 12a was a newly made and clean one ; but, being only
1 gal. in bulk, the solution became badly oxidised during the deposition of 12a, and thus
was full of suspended matter.
t Brit. Assoc. Rep., 1901, p. 604 ; Proc. Roy. Soc., 1903-1904, vol. 72a, p. 218 ; Proc. Roy.
Soc.. 1913-1914, A., vol. 89, p. 593 ; Nature, 1914 (Feb. 19), vol 92. See also Nature, 1913,
p. 322,
J This had not been removed.
§ The nature and origin of these lines, seen in many other specimens also, has been dis-
cussed elsewhere, v. Jour. Iron and Steel Inst., 1921, vol. 103, p. 355.
Radial crack.
FIG. 52. x 150.
Area formed during
stoppage.
FIG. 53.* x 150.
I Annular
rings
Area formed
during stop-
page.
FIG. 54. x 750.
DEPOSIT 12A.
Dentate periphery — crystalline macroscopically.
FIG. 55. x 160.
Smooth periphery — matt macroscopically.
FIG. 56. x 160.
DEPOSIT 13.
35067
To face p. 21,
Area opposite that of Fig. 59.
FIG. 57. x 160.
Banding is shown with particular clearness.
FIG. 58. x 100.
DEPOSIT 13A.
21
A comparison of the structures of Nos. 12 and 12a can be conveniently
made thus : —
Deposit No. 12 Deposit No. 12a
(Cathode — stationary) (Cathode — moving)
1. Full of holes and inclusions. Few holes and inclusions.
2. No radial cracks apparent. Many clean radial cracks.
Deposit complete. About two-thirds of deposit absent.
3. Fibrous to columnar : fibres Fibrous throughout : fibres are
broaden towards the periphery. narrow and remain of uniform
4. No circumferential lines seen. width.
A large number of lines can be seen
running circumferentially round
the specimen*. They continue
throughout that part of the de-
posit which remains.
NOTE. — Attention is particularly drawn to the association of brittleness
and clean radial cracks with a deposit of completely fibrous structure.
This association is frequently to be observed (Cf. Fig. 54, which shows a
gap in the fibrous area of the deposit). Reference has before been made
to this association t.
1 , , r " ,t * : ., •..,,.'..*,. . .*
Deposit of Experiment 13
Description of the Structure, at 125 diameters.
The general structure of this deposit appears to consist of a mass of
allotriomorphic crystals, elongated in a direction more or less perpen-
dicular to the surface of deposition. Figs. 55 and 56 illustrate the two
types of structure that exist in the deposit, namely, (1) where the
periphery is dentate, a coarser type, consisting of broader, but broken,
grains, and (2) where the outline is regular, a finer type of structure,
consisting of grains, narrow, almost fibrous, at base, but which very
gradually broaden towards the periphery. The structure of this latter
type seems to be dual, as if a fibrous structure is imposed upon on© of a
more or less normal type. A characteristic of this deposit is a herring-
bone appearance seen on individual grains at high magnification.
Deposit of Experiment 13a
Description of the Structure, at 125 diameters.
Two distinct types of structure occur in this deposit. On two opposite
sides there occurs one type, and on the other two opposite sides (at right
angles to the former two) occurs another type of structure. In the one
type (illustrated in Fig. 57) the greater portion of the deposit is, from
within outwards, strictly fibrous, with the direction of the fibres normal
to the surface of deposition, while the remainder — the peripheral portion —
consists of fairly large and clear (but narrow) V-shaped grains of often
rather irregular outline. Fig. 58 shows the consequent banded arrange-
ment very clearly. The boundary between the two circumferential layers
is, however, not always sharp. The fibres of the inner layer can often be
seen continuing for some distance into the outer layer. A wave-like
* The nature and origin of these lines, seen in many other specimens also, has been dis-
cussed in Jour. Iron and Steel Inst., 1921, vol. 103, p. 355.
f e.g., in the paper on " Some Defects in Electro-deposited Iron."
22
appearance can be seen running circumferentialiy through the crystals
of the outer band : this is more clear in some places than in others. In
the other type of structure (shown in Fig. 59) there is usually some fibrous
layer, which varies in width in different parts but is always narrow. This
layer merges radially and gradually into an outer layer which, though
it seems essentially fibrous, is composed of, as it were, bundles of fibres
that lie at an angle to one another, forming an arch-like appearance, as
shown in Fig. 60. This middle area merges in turn into an area where the
wavy appearance apparent in the outer layer of the former type of structure
Fia. 60.
can be well seen (Fig. 59). This wave-like appearance proceeds inwards to
various depths at different places. It may, perhaps, be described as a
" herring-bone " structure: it resembles the lamellar twinning often seen
in the orthoclase felspars of some igneous rocks.* The outer edge is not
so clear and smooth in this type as in the former. The grains in the outer-
most layer (where the wave-like appearance is most marked) are broader —
broadening from within outwards ; but the grain boundaries are not definite
and clear. The general appearance of these parts of the specimen is
somewhat similar to that of a foliated schistose (igneous) rock.t
It may be remarked finally that, in the four regions of the deposit
intermediate between the area where the two types of structure above-
described are found, the structure is of an intermediate type.
A comparison of the structures of Nos. 13 and 13a may (as for Nos. 12
and 12a) be usefully made thus: —
Deposit No. 13 Deposit No. 13a
(Cathode — stationary) (Cathode — moving)
1. Larger grain : nowhere of the Smaller grain : often fibrous
fibrous type of structure, with throughout almost the whole of
the direction of the fibres the thickness of the deposit,
normal to the surface of deposi-
tion.:!:
2. No circumferential banding. Banding always present, usually
very marked.
3. Some holes and inclusions — more Some holes and inclusions.
numerous than in No. 13a.
CONCLUSIONS— ON THE EFFECT OF MECHANICAL MOVEMENT
The great and striking difference between two deposits formed in the
chloride bath — one in a still vat, the other on a mechanically moved
cathode — is that, as was noted in the remarks on the macroscopic examina-
tion of the samples, the structure of the deposit formed in the still vat
is larger than that of the one formed on a moving cathode. And further,
* See Petrology for Students, by A. Barker, 1895, p. 61 (Fig. 13).
f This expression (schistose) is used to help make clear a type of structure. It is not
intended to imply any similarity in mode of origin or development in the two cases.
£ The portion of No. 13, shown in Fig. 56, was found very difficult to etch." It appears to
show (even when viewed directly through the microscope) two structures imposed the one
upon the other, and the structure is not clear enough to enable deductions to be made.
To face p. 22.
^
Area opposite that of Fig. 57.
FIG. 59. x 160.
Intermediate area.
FIG. 61. x 160.
DEPOSIT 13A.
23
the predominating type of structure of the latter is the fibrous. This
difference is very apparent "upon both macroscopic and microscopic
examination.
DIVISION 2— THEORETICAL
INTRODUCTION
Before attempting to explain the structure of electro-deposited iron,
especially as exemplified in the results contained in Division 1, some
consideration will be given to the facts and theories that relate to the
crystallisation of substances other than electro-deposited metal. This
will be done even though it involves a re-statement (which, to some, may
appear superfluous) of those facts and theories. An endeavour will then
be made to show that the ascertained laws of crystallisation applicable
to salts in solution and to fused masses or melts, including rock magmas,
are applicable also to electro-deposited metal*; and that such extended
application enables one to understand and' interpret what one sees when
electro-deposited metal is subjected to microscopic examination. The
result of such understanding, from the practical point of view, will then
be briefly discussed, namely, that the knowledge acquired can be put to
practical application in the workshop, enabling the depositor to more
certainly control the operation which is to give him the result he wishes
to obtain. Accordingly, the following sub-division of the subject may
be made : —
I. The crystallisation of substances in general.
II. The application of the theories of crystallisation to electro-
deposited metal.
III. The practical application in the workshop of present-day know
ledge of the laws and phenomena of crystallisation.
THE CRYSTALLISATION OF SUBSTANCES IN GENERAL
The Conditions of the Birth and Growth of Crystals.— A hot,
saturated solution of, say, copper sulphate will, if quickly cooled, yield
a cloud of very small crystals. It is the same with saturated solutions of
numerous otljer salts. It is common knowledge of the laboratory that: —
" In order to obtain as good crystals as possible, the solution is allowed
to cool slowly without being disturbed. If a substance, on slow cooling,
separates out in very coarse crystals, it is expedient, in case a sample of
the substance for analysis is desired, to accelerate the crystallisation by
artificial cooling, so that smaller crystals will separate out. ... If a
deposit of crystals as abundant as possible is desired, the vessel is put in a
cool place — in a cellar or ice-chest, if practicable.' 't
With these facts in mind, R. W. MooreJ has recently produced perfect
crystals of potassium sodium tartrate nearly 10 cms. long by cooling,
extremely slowly, a saturated solution of the salt. The rate of cooling
was regulated to QO-1 per day for the first day, and then gradually in-
creased to 0°'6 per day as the crystals grew. Sir H. Miers' experiments §
* O, Lehmann, in 1877, pointed out that many of the phenomena of crystal formation
by electrolytic means are comparable with those of ordinary crystal formation (gewohnliche
Krystallbildung). Zeit. fur Krystallog., 1877, vol. 1, p. 453. And see also O. Lehmann
Zeit. fur Krystallog., 1890, vol. 17, p. 274.
t Practical Methods of Organic Chemistry, L. Gattermann (Trans, by W. B Schober),
1905, p. 8, and of, " Crystals," by A. E. H. Tutton, Chaps. XV. and XVI.
j Jour. Amer. Chem. Soc., 1919, Vol. 41, p, 1060.
§ Proc. Roy. Soc., 1907, A, Vol. 79, p. 322 ; Jour. Chem. Soc., 1906, Vol. 89, p. 413.
24
have shown that similar rules hold for the production of crystals of salts
from the liquid state, by cooling below the melting point, to those applying
to crystallisation from solution.
It is the same with the rock magmas of the petrologist. " The dis-
tinctive features of these " (the Plutonic) " rocks of deep-seated consolida-
tion are those which point to slow cooling (not necessarily slow consolida-
tion) and great pressure. The rocks are without exception holo crystalline.
The texture of plutonic rocks may be comparatively coarse, i.e.,
the individual crystals of the essential minerals may attain considerable
dimensions. The typical structure is that known as hypidiomorphic, only
a minor proportion of the crystals being ' idiomorphic ' (i.e., developing
their external forms freely), while the majority, owing to mutual inter-
ference, are more or less ' allotriomorphic ' (taking their shape from their
surroundings). "* In another place Harker says: — "It is evident from
the foregoing considerations that slow cooling will cause larger crystals
and more rapid cooling smaller crystals. "t Professor Iddings't observa-
tions with respect to crystallisation in general and to the magmas of
igneous rocks in particular are very explicit. " The size f)f crystals is
dependent on the molecular concentration, or amount of substance in
solution, since the rate of separation of solid is proportional to molecular
concentration. . . . When crystallisation commences at a certain
number of points in a given volume of liquid1, the greater the amount of
a separating substance in the solution, the greater the amount of material
crystallised in a given time. . . . The size of crystals depends also
on the degree of supersaturation taken in conjunction with the molecular
concentration, for . . . the number of centres of crystallisation varies
with the degree of supersaturation for a given concentration of a sub-
stance, the fewest occurring when crystallisation starts near the saturation
point in the metastable condition, the greatest when it starts in the labile
condition. The growth of a few crystals in a given volume of solution
must lead to larger individuals than the growth of many crystals in the
same volume of liquid. If the degree of supersaturation increases during
crystallisation, by reason of the rate of cooling, the number of crystals
may increase from time to time, resulting in different sized crystals of the
same substance in some cases; the largest being fewest, and the smallest
most numerous in most cases." And further§ : — " The relation between
size of crystal and' the rate of cooling is seen upon comparing parts of the
same body of magma that have cooled at different rates, though it is
difficult to eliminate the effects due to viscosity that may have existed
in the two cases. It is found that those parts of a magma that have
cooled rapidly consist of smaller crystals than parts that have cooled
slower. A definite expression of the relation between the two has been
attempted by Lanej] and also Queneau."U
In metallurgy, too, the same general relationship between size of grain
or crystal and rate of cooling is recognised. " Whether crystallisation
occurs in solidifying from the liquid or during the cooling of an already
solid piece, it results in the formation of an aggregate of grains, each one
of which is a true crystal. Their size may be large or small. In general,
quick cooling means that crystallisation starts from many nuclei, and the
resulting grains are consequently small ; with very slow cooling, you get
* A. Harker, Petrology for Students, 18s)5, p. 22.
f Natural History of Igneous Rocks, 1909, p. 218.
+ Igneous Rocks, Vol. 1, p. 190.
§ Op. cit., p. 188.
|1 A. C. Lane, Bull. Geol. Soc. Amer., 1897, Vol. 8, p. 403 ; Rep.Geol. Surv., Mich., Vol.6,
part 1 ; Ann. Rep. Geol. Soc. Mich., 1903 and 1904.
t A. L. Queneau, Sch. of Mines Quart., 1902, Vol. 23, p. 181.
25
a gross structure made up of grains of a much larger size."* And again
Sauveur, also in regard to metals, says: — "The number of crystalline
grains of which a certain mass of metal is composed must depend . . ''1
upon the number of centers or nuclei at which crystallisation begins, and
this in turn probably depends upon the metal itself, its rate of cooling,
and, according to some, its purity ; the slower its solidification and the
greater its purity the fewer the nuclei and therefore the larger the
crystalline grains. "f
Conclusions from the above considerations. —Thus, it is recognised
that in the process of crystallisation, whether from solutions or from melts
of salts, or from molten rock magmas, or, again, from molten metal : —
(1) There is a direct relation between rate of cooling and size of
grain or crystal;
(2) Quicker cooling produces a greater number of centres of
crystallisation or crystal nuclei, resulting in smaller crystals;
and
(3) Crystals already in being grow where the rate of cooling is slow.
The Conditions that determine whether Birth or Growth will occur.
— There is nothing determinate, in the above-cited remarks, upon the
answer to the double question : When will a crystal grow? When will a
new nucleus be formed? The question may be put into a unified form
thus : A number of atoms are, it is supposed, arranged in the regular
orientation known as the crystalline, forming a crystal ; and the periphery
of that crystal is the locus of free metal atoms. Will those free metal
atoms join up to the others, and thus form part of a whole, or will they
join to one another to form separate entities? In the former case the
crystal already formed will grow ; in the latter, new nuclei will be formed.
In the former case, there will be operative some force or tractation be-
tween the atoms of the already formed crystal and the free atoms; in
the latter case, the force or tractation that operates acts between the
free atoms, the atoms of the crystal being, pro tanto, inert.
The matter seems to resolve itself into this : —
Is—
ATOM <~ > ATOM > ATOM <— > CRYSTAL ?
(A) (B)
The scheme represents that the tractation between free atom and free
atom may be greater or less than that between free atom and crystal. If
(A) is greater than (B), new nuclei will be formed ; if (A) is less than (B),
then the already formed crystal will grow.
Though not expressed in the above terms, something of this kind would
seem to have been in the minds of the authorities cited above. It can
be inferred often from what they say. For instance, Iddings states
that : " The number of centers of crystallisation varies with the degree
of supersaturation for a given concentration." When, therefore, the de-
gree of supersaturation is, for a given concentration, exceedingly small,
the number of centres of crystallisation is exceedingly small also. But it
cannot be intended that the relation holds indefinitely, that is to say
until the supersaturation becomes nil and, hence, no nuclei are formed,
so that all the while there is any supersaturation, so long will nuclei be
formed. If this were so, there would be no growth, but only formation
of nuclei. The inference must, therefore, be allowed that there is a
* J. A. Ewing, The Molecular Structure of Metals, Phil. Mag., 1906, Vol. 12, p. 256.
f The Metallography and Heat Treatment of Iron and Steel, 1916, p. 88.
26
point — a certain supersaturation in the neighbourhood of a crystal — at
which the excess of atoms (or molecules) is insufficient, for some reason
or other, to form fresh nuclei. It is only such as to enable addition to be
made to an already formed crystal. Or, put otherwise, the supersatura-
tion is not sufficient to enable the inter-atomic (or inter-molecular) tracta-
tions to come into play; these are displaced by crystal — free atom (or
molecule) tractations.
While one can draw inferences like the above from what the authorities
say upon the matter of the general conditions 'of crystal formation and
growth, the fact is that very little is known of the act of crystallisation
itself; and this is, no doubt, the reason why so little direct statement
about it is to be found in the literature. Even the experimental con-
ditions necessary to effect nucleus formation, on the one hand, or crystal
growth, on the other, are not too precisely known. One finds instances
given to show that the unexpected may happen — where, in fact, the general
and usual effect from a given cause does not result. J. C. Hostetter* has
recently made some interesting observations on this point, in connexion
with the formation and growth of crystals from salt solutions. He says : —
" The degree of supersaturation in the mother liquor at any time deter-
mines the increment of growth; consequently, the conditions affecting
supersaturation — primarily, temperature and evaporation — must be under
definite control " (if, that is, large, well-formed crystals are to be
obtained). " Of lesser importance — but, nevertheless, essential — are the
direction of concentration currents, and the number of crystals which
serve as nuclei for growth. When the variables are controlled, it is not
a difficult task to grow very perfect crystals of large size." But, on
p. 93 (op. cit.) — and this is more germane to the matter of immediate
discussion — Hostetter says: — "The mere fact that essential conditions
are under control in the crystal-growing apparatus described above is
not, in itself, a guarantee that any salt can be made to form large
crystals under the conditions obtaining therein. Some salts may be
readily enough crystallised in large, well-formed crystals — other salts
under the same condifions will yield a multitude of small crystals rather
than a few large ones. Potassium alum and sodium chlorate were grown
successfully in this apparatus; but experiments with ammonium chloride
yielded only a mass of fine, fern-like crystals instead of growth on certain
crystals which had been introduced as nuclei. In this case the effect was
not caused by incorrect adjustment of conditions, for these fine crystals
appeared and increased in size in the crystalliser, thus showing that con-
ditions were optimum. On several occasions all crystals except one were
carefully removed from the crystallizing chamber and the circulation of
the liquid continued, but here again, instead of deposition taking place on
the remaining crystal, other nuclei were formed, developing later into
the usual feather-like growths."
Hostetter offers no explanation of the phenomena he describes; but one
learns that even though the conditions may be optimum for growth, yet,
in some cases, nucleus formation results.
Despite the fact that very little is, in truth, known of the act of
crystallisation, the theories of Miers, Wulff, and others, developed of
recent years upon the basis of a large number of extraordinarily interesting
and beautiful facts, enable one to picture in the mind an image, helpful,
although, it may be, untrue, of the mechanism of that act. Miers assumes
that the supersaturated liquid in contact with the growing crystal consists
* " An apparatus for growing crystals under controlled conditions," Jour. Wash. Acad.
Sci., 1919, vol. 9, p. 85.
27
of molecules of salt uniformly mingled with t'hose of the solvent, and that
the act of crystallisation consists of the escape of the solvent molecules
and solidification of the salt. Where the solution is only feebly super-
saturated (where, that is, experiment in general shows growth to be slow),
more time is afforded for the escape of the solvent molecules, and for the
salt molecules to deposit themselves as directed by the molecular guiding
force of crystallisation*— the " Richtkraft der Krystallisation," of Wulff.
On the other hand, where the supersaturation is, at any instant, great,
then there may result a labile or metastable shower of crystals, according
to the degree of supersaturation, or, in other words, nuclei are formed.
As has been already said, a mental picture of the act of crystallisation
can be formed from such explanations as that which Miers gives of it,
but one derives no help in solving the problem as to why in some cases
nuclei are formed, and in others crystals, already in being, grow. Miers
leaves one in the position of picturing the escape of the solvent molecules ;
there is no guidance in forming pictures of the resultant molecules forming
nuclei on the one hand, or increasing the size of already present crystals
on the other.
It is suggested that one can picture the two cases of crystallisation of a
salt from its solution somewhat as follows: —
(1) // no crystal surface is present. — The solution, at or near
saturation, consists, for the greater part, of salt molecules and
solvent (say, water) molecules. The concentration would seem
then to be such that the salt molecules are not able to effect
any permanent union; that is to say, the inter-molecular
tractations are not able to bring about any permament result.
But as soon as the solution becomes supersaturated, as, ftSr
instance, by slow evaporation or by a lowering of temperature,
then those molecules which come within sufficiently close range
of one another effect permanent union. In the case of slow
evaporation, the difference between the saturated and super-
saturated solutions is one of concentration. There are fewer
molecules of solvent per unit volume in the supersaturated
solution to separate and keep apart the salt molecules, and
one can picture the tractation of these to one another with
permanent result. If the supersaturation occurs from sudden
lowering of temperature, one can suppose either the kinetic
energies of the salt molecules and the solvent molecules to be
differently affected, so that the latter are unable to prevent
the former effecting permanent union, or that, though the
energies of the molecules are equally affected, the energy of the
solvent molecules is too small to keep the salt molecules apart,
or, again, that some aggregation occurs that allows of the con-
gregation of the salt molecules, so that these can then unite
inter se permanently. Perhaps there is more than one cause
operative. In any case, the determining factor is concentra-
tion of the salt molecules, and one can picture their tractations.
(2) If a crystal of the salt in solution is present. — In this case one
can imagine that, in a saturated solution, an exchange of
molecules occurs as between solution and crystal. Upon very
slight supersaturation, water molecules make their escape from
* Cf. Tutton, op. cit.j pp. 250-252, and the references there given.
28
the salt molecules (according to Miers' theory), and solidification
occurs at the crystal surface. This takes place slowly, pro-
ducing growth of the crystal. The supersaturation being
supposed very feeble, very few salt molecules are available for
solidification. Their concentration is very small, consequently
so small that they are not sufficiently numerous or close
together to enable inter-molecular tractation to occur, and hence
nucleus formation is not possible. Crystal-molecule tractation
is dominant under the conditions: "the directive force of
crystallisation " is able to operate. . If the layer of liquid round
the crystal present in the solution suddenly, from one cause
or another, became considerably supersaturated, then the avail-
able molecules would become more numerous and closer together,
and a value of the concentration might be attained at which
. they would be able to tractate — their tractation inter se over-
coming that between crystal and molecule.
Molecule <— > crystal > molecule •^~> molecule
represents the phenomena at slight supersaturation ;
Molecule •<— > crystal < molecule <— >• molecule
represents them at considerable supersaturation.
The molecular theory and Miers' theory, together, enable one to form
a mental picture of the phenomena of crystallisation — a picture of the
very act of crystallisation; and it is possible to distinguish between when
nuclei will be formed and when growth will occur. The dominant factor
is always concentration — number of molecules per unit volume. Ostwald*
puts the matter very clearly for the case of supersaturation from over-
cooling. " If, now, the liquid is allowed to cool " (he says), " no crystal
being present, the molecules receive no impulse to assume a regular
arrangement, t and there is ' over-cooling.' The kinetic energy of the
molecules decreases, they get nearer and nearer together, and it may
happen that amongst the many encounters one may occur so that the
molecules will just be in the specially stable regular arrangement that
determines the crystal form. The circumstances are then given under
which the liquid crystallises spontaneously; the molecules add themselves
gradually to the crystalline nucleus, those being retained that approach
, in a suitable way " This is said of a super-cooled " melt"
or fused mass; but " the spontaneous crystallisation of a supersaturated
solution depends on exactly the same circumstances as the solidification of
an over-cooled fused mass " (op. cit., p. 152). Indeed, Ostwald generalises
in the following words: — "The application of these relations is by no
means limited to aqueous solutions ; they hold for solutions with other
solvents as well as for fusions of all kinds. In particular they play a
decisive part — one hitherto far too much neglected^ — in the crystallisation
of melted silicates, as in the case of lavas and other eruptive rocks, and
form the foundation for the right comprehension of these exceedingly
important geological phenomena " (op. cit., p. 152).
* W. Ostwald, Outlines of General Chemistry (trans. J. Walker), 1895, p. 151 and p. 147.
f Because-^ crystal is present ta exert any such influence -- - - - - - - ~- - -
J Petrologists, prominent among whom are J. P. Iddings and A. Harker, have done much
during the last twenty years to remedy this neglect.
29
II
APPLICATION TO ELECTRO-DEPOSITED METAL
An endeavour will be made in this section to show that the ascertained
(general) laws or relations of crystallisation, applicable to other substances
hold also in the case of electro-deposited metal.
If the laws governing the phenomena are the same in the two cases,
cause and effect may be expected to correspond. That is to say: —
(1) There will be a direct relation between (not rate of cooling, but
its equivalent for deposited metal, namely,) rate of deposition
and size of grain or crystal;
(2) Quicker deposition will produce a greater number of centres of
crystallisation or crystal nuclei, resulting in smaller crystals ;
and
(3) Crystals already in being will grow where the rate of deposition is
slow.
Again, if the correspondence holds, the dominant factor here also will
be concentration : slight concentration will lead to coarse structure, great
concentration to fine structure. To show that these relations hold for
electro-deposited metal, the coarse and fine crystalline structures (together
with lamination and the relation of base metal and deposit), and the
conditions under which they are formed, will be considered; and an
endeavour will be made to show that these result from differences of con-
centration. This is, of course, the same as showing that propositions (2)
(3) hold good, and, hence, that the direct general relation (1) obtains.
If one may expect to get coarse structures when the concentration is
small, and fine structures when it is great, these results should have been
obtained in the deposits of the experiments of Series I, II, and III. The
effects of temperature, current density, and movement, respectively, upon
concentration should be apparent in the structure of the deposits. These
will be considered, and it is hoped to show that the effects are, in general,
such as might be expected. A few cases of other solutions and one other
metal — copper — will be briefly considered.
It is suggested that, in the discussion outlined above, the general rela-
tionship between rate of deposition and size of grain will be established.
Such correspondence between cause and effect can more easily, perhaps, be
shown (for the case of deposited metal) if the nature of the act of deposition
has been in the first place, briefly, considered.
A close and detailed examination of a large number of specimens of
electro-deposited iron has suggested the conclusion —
That its crystal structure depends upon the rate at which the ions are
discharged and the resultant atoms are able to combine to form crystal
grains. Or, one can say — and it amounts to the same — the structure
depends upon the concentration of the crystal- forming atoms in the
immediate neighbourhood of the cathode.
If this be true, the type of structure obtained in any given case is the
product of two factors, namely : —
(i) The rate of discharge of the metal ions, and
(ii) The availability of the resultant atoms for grain formation.
As -regards (ii), it has to be remembered that a resultant metal atom
may not be able to unite with others to form part of a crystal grain. It
may, instead, have to enter into chemical reaction. The alkali and alkaline
earth metals are well-known examples. Aluminium and magnesium
35067 C
30
cannot be deposited — alone — from aqueous solutions of their salts.*
Chromium also is difficult to deposit, but it certainly can be deposited. In
the cases of all these metals one cannot but suppose that metal ions are
discharged ; yet, except in the case of chromium, no metal deposits appear
on the cathode. Hydrogen, in equivalent amount, appears instead; and
the explanation is, that the metal atoms, at the instant following dis-
charge, attack some substance in their neighbourhood, for example, the
water, and hydrogen results as the product of the reaction. Nickel, zinc,
and iron are, also, more or less difficult to deposit from solutions of their
salts containing any considerable amount of free acid.
It is clear, therefore, that the rate of discharge of the metal ions cannot
in et per se result in the formation of metal deposit : the liberated atoms
must be available for the formation of crystal grains. It is the two
factors together that determines the concentration of metal-forming
atoms present in the cathode neighbourhood at any instant ; and it is upon
this concentration that, it is suggested, the crystalline structure of the
electro-deposited metal depends.
TRANSITION:— ION TO CRYSTAL GRAIN
The steps, and their sequence, from ion to crystal have been well set out
by H. Freundlich and J. Fischerf in the following way: —
(1) (2) (3)
Discharge. Dehydration Crystallisation.
Metal ion -»• discharged hydrate -> metal atom -> metal crystal
(charged, hydrated).
This scheme excludes those cases where, as mentioned above, the liberated
atoms enter into secondary reactions with other substances. It assumes
the crystalline structure of the solid metal; and this assumption that
electro-deposited metal is crystalline is certainly well founded in genera l.J
It may be that the presence of colloidal substances in the electrolyte will,
in some cases, so far modify the form of the deposited metal that it is no
longer crystalline; but the general case is that which is considered here.
Of the three steps of the scheme of Freundlich and Fischer the third is
the important one for present purposes, and it will now be discussed at
length.
TRANSITION:— ATOM TO CRYSTAL
As has been already indicated in Section I, the processes of crystal origin
and crystal growth are phenomena of several branches of natural science.
Chemists, petrologists, and metallurgists have, all of them, to study these
phenomena. The electro-metallurgist does not appear to have concerned
himself much with them up to the present ; yet the processes of crystallisa-
tion do, in fact, closely concern his work. Why is the metal deposited
from the acid copper bath sometimes of obviously crystalline character,
sometimes plastic in appearance and without visually apparent crystalline
* It is claimed that magnesium can be deposited together with nickel. Coehn and Sie-
mens, Zeit. f. Elektroch, 3901-1902, vol. 8, p. 249; and H. B. Patten and W. R. Mott
state that aluminium can be deposited from acetone solutions. Trans. Amer. Electroch
Soc., 1909, vol. 15, p. 529.
f Zeit. f. Elektroch., 1912, vol. 18, p. 886.
t S. Cowper-Coles states that, "the structure of electrolytic iron varies considerably, and in
4some cases it is found to be amorphous." Jour. Iron and Steel Inst., 1908 (No. 3), p. 147.
The former part of this statement is certainly true; but no evidence in support of the latter
is produced.
31
structure?* Why is it impossible to deposit some metals from certain of
their salts in other than loosely adherent crystal masses, while in the cases
of other salts a reguline, adhesive deposit is obtained? These and many
other questions of like kind await answers from the electro-metallurgist.
The formation of the crystal embryo or nucleus may or may not be a
single, simple process. A nucleus may — it is conceivable — be the end result
of a number of operative causes which may be consecutive or may co-exist.
In other words, the coming together or tractation of atom to atom to form
an embryo may be a resultant process, helped or hindered by others. It
is likely that, for many purposes, the possible existence of such operative
causes must be considered. But it is suggested that for the purposes of
the present study — the crystalline structure of electro-deposited metal in
general, and of deposited iron in particular — this is not necessary.
According to modem views on the subject of electrolysis, the ion becomes
atom upon discharge. Observation shows that solid metal results, and
that that solid metal is crystalline. According to Nernst (and others)
metals in the solid state are constituted of atoms. Thus, in some way or
other atoms must have come together and arranged themselves in the fixed,
regular orientation of the crystal form. Accepting the modern views on
electrolysis and also Nernst 's view on the constitution of a solid metal, it
would seem that, for present purposes, one can leave all else out of account,
and consider the atoms at that instant when two or more are on the point
of coming together (as the result of whatever causes) to form part of a
crystal structure.
The actual transition process (or processes) of the stage — Atom to
Crystal — has been, to some extent, considered by other writers. Freund-
lich and Fischer, in their work on the " Influence of colloids on the
electrolytic separation of lead,"t have dealt with the problem that is con-
cerned with the reasons why the presence of colloids in the electrolyte
prevents the discharged atoms arranging themselves in the order necessary
to crystal form, determining experimentally what minimum amount of
this or that colloid will prevent such arrangement. They appear to con-
clude, from the results of their experiments, that the colloids are adsorbed
by the metal, and, in consequence, the velocity of crystallisation is
diminished and the denseness and compactness of the metal is favoured.
V. Kohlschiitter,t however, believes that the colloids give rise to colloidal
metal (e.g., in the case of silver), and that, therefore, the form of the
deposited metal is non-crystalline. The most recent research on the
transition stage is that lately published by V. Kohlschiitter and E. Vuil-
leumier.§ They conclude from their experiments that the formation of a
deposit from the atoms is not an immediate, single and simple, process,
but that there is an intermediate stage, consisting of the formation of
compound bodies containing the metal atoms, and that it is after passing
through this stage that the atoms come to be available for deposit forma-
tion. ||
In general, those workers who have dealt with the formation of electro-
deposited metal have proceeded with a view to determining what sub-
stances will, if present in the solution, prevent the deposition of spongy,
loosely-adherent deposits, and conduce to the formation of smooth, reguline,
* i.e apart from microscopic observation.
f Op. cit. " tJber den Einfluss von Kolloiden auf die elektrolytische Abscheidung des
"'
j".' A series of researches, by V. Kohlschutter and collaborators, Zeit. f. Elektroch.,
19ZeTt! f9'EPle^rocnri'918, vol. 24, p. 300. Of. also E. Marc Zeit. f. Elektroch, 1913,
. 19, p. 431, and H. Stager, Helv. Chim. Acta, 1920, vol. 3, p. 584.
Cf. J. A. Nussbaum, U.S. Pat, No. 832,024 (1906).
35067 ° 2
§
vol.
32
and adherent metal. For the most part the work has been experimental
only : causes have remained unconsidered. In other words, the processes
of the transition stage — Atom to Crystal — have not been, of themselves, of
immediate interest to the majority of workers. Nor can it be claimed
that the present research is immediately concerned with them. The posi-
tion taken as starting point is as follows : — The atoms have been dis-
charged and are ready to form part of crystal grains. The question is —
Will they initiate new crystals, or will they aid in the growth of grains
already existing? The answer suggested is that — It depends upon the
concentration of the atoms, and on that only (that is to say, in the case
of deposited metal). That answer is, from our knowledge of the crystalli-
sation of salt solutions, rock magmas, and molten metal, to be expected.
The birth and growth of crystals of deposited metal.— Assuming, then,
that there are a number of available metal atoms in the neighbourhood
of the cathode, will they form new crystal embryos, or will they take their
ordered places in grains already formed? The argument is that results
show that just the same will happen in the case of the metal atoms as
would occur in the case of molecules of a salt. That is to say, whether
new crystals are originated or whether growth will occur depends upon
the concentration of the available atoms. Metal atoms (derived from
.ions) and salt molecules give rise to the same effects, namely, crystalline
bodies. Like effects suggest like causes. The causes of the formation
of crystal bodies from salt molecules have been considered, and the
dominant factor seems to be concentration. Does concentration operate
as the dominant factor in the formation of crystals of deposited metal?
If so, variety of concentration may be expected to produce variety of
crystal — large or small, nuclear or growing ; and, further, one may expect
it to account for a number of features observed in connexion with deposited
metal. A number of definite cases will, therefore, be considered, and
an endeavour made to ascertain whether these can be explained by means
of what one may call " the concentration factor."
The following definite cases will be discussed: —
(a) The coarsely crystalline structure;
(6) The finely crystalline structure;
(c) Laminated structure; and
(d) The relation between base metal and deposit.
(a) The coarsely crystalline structure.
It is matter of common observation, in the cases of many metals electro-
deposited from a solution that is not mechanically agitated, that the
surface of the deposit becomes more visibly crystalline as the deposition
process goes on. This is so in the cases of zinc, lead, copper, cobalt,
cadmium, and other metals ; and it is so in the case of iron. The deposit
may take longer or shorter time in which to become coarsely crystalline
to the naked eye. This, as a fact, depends upon the current density
employed and upon the amount of metal salt in solution. The greater
the current density is, the more quickly the deposit becomes obviously
crystalline; the stronger the solution, the slower it does so. The fore-
going statements are statements of experimental facts. It is suggested
that the explanation is contained in the following considerations: —
Deposition of metal naturally tends to exhaust the cathode layer of
the electrolyte of its metal content. Unless the loss of metal in that
layer is made good by input in some way or other, and to the same extent
as output (in deposition) proceeds, the content of metal in the cathode
layer will become less and less. In a still vat, and at ordinary
33
temperature, such ingress of metal into the cathode layer must occur
through either " concentration currents," liquid diffusion, or ion
migration. The second and third means are exceedingly feeble, and
certainly would not suffice to maintain constant th6 content of metal in
the cathode layer, except at very small current density. Concentration
currents are more effective: they can be seen to cause movement in the
cathode neighbourhood. But it is suggested that these cannot suffice to
maintain the indicated equilibrium. Consequently, although the current
density remains constant, less and less current is carried by metal as the
electrolysis proceeds — more and more is carried by other cathions — fewer
and fewer metal ions are discharged, and, consequently, metal atoms
available for crystal formation become continually less in number. In
short, the concentration of available metal atoms in the cathode neighbour-
hood becomes continually less as deposition proceeds, until a point is
reached at which input does approximately equal output, and the metal
content of the cathode layer becomes approximately constant. A very
simple experiment visibly illustrates how the electrolyte in the cathode
area gradually becomes weaker in metal content, where constancy is not
maintained artificially. If copper sulphate (electro-typing) solution be
poured into a long glass cylinder, an anode, connected with an insulated
conducting wire, placed at the bottom, and a cathode immersed in the
liquid at the top, it will be noticed that after electrolysis has proceeded
for some while the electrolyte in the cathode area grows pale, and it
becomes more and more pale as electrolysis proceeds. Owing to the
experimental arrangements the operation of " concentration currents "
is obviously almost negatived. Hence, little or no mixing, caused by such
currents, is effected, and liquid diffusion and migration are far from
sufficient to maintain equilibrium of metal content in the cathode area.
The result is that the metal content gradually diminishes, the diminution
being indicated by the gradual decoloration of the liquid. One has, then,
ill a still vat (i) the deposit becoming more coarsely crystalline as deposition
proceeds, and (ii) at the same time, the catholyte becoming weaker in
metal. One knows that the solidification of electro-deposited metal is a
crystallisation process,* and that in a crystallisation process (in general),
the slower it proceeds, that is, the less the amount of material available
for solidification, the coarser the resulting crystals are. Analogy strongly
suggests, therefore, that the increasing coarseness of the electro-deposited
metal is the consequence of diminishing availability of metal.
(b) The finely crystalline structure.
If small availability of metal gives rise to a coarse structure, then, per
contra, great availability should cause the structure to be fine. Experi-
mental results show that where a high current density is employed, together
with movement of either cathode or solution (or both), the structure of the
deposit is small. In this connexion the results obtained by J. G. Zimmer-
man are of interest. t He found that for copper, " The fineness of the
grain is dependent upon the current density, other things being equal,
and the fineness increases with the current density until, at a critical
value, a powdery deposit will occur. The increase in the number of
revolutions per minute increases the critical current density, although
whether it is exactly proportional I have been unable to determine. The
* Cf. F. Foerster, Elektrochemie wasser. Losungen, 1910, pp. 200 and 250, and else-
where (Knapp, Halle) ; also M. Schlotter, Galvanostegie, Teil. 1, p. 36 (Knapp, Halle).
f Trans. Amer. Electroch. Soc., 1903, vol. 3, p. 245. Zimmerman deposited metal on
small cylinders of base metal as cathodes. These were rotated on a vertical axis during
deposition. See also " The structure of metal electro-deposited upon rotating cathodes,"
Jour. Phys. Chem., 1921. vol. 25, p. 495.
u
highest speed which I used was 2,500 revolutions, corresponding to 573
feet per minute, and my observations tend to confirm the statement of
Mr. Cowper-Coles that if a peripheral velocity of about 1,000 feet per
minute and a current density of about 200 amperes per square foot be used
the copper will plate out with a high polish and to any desired thick-
ness." Similar results were obtained with nickel and zinc. The results
obtained were certain; but Zimmerman, despite his view as to the
connexion between fineness of grain and current density (above cited),
seemed to think that the " high polish " was, in a measure, due to some
burnishing action of the deposit by the electrolyte through the high
speed of revolution. Professor C. F. Burgess, also, seemed to incline to
this opinion; but other contributors to the discussion on Zimmerman's
paper (including such authorities as Carl Hering and C. J. Reed) differed
from it. For instance, C. Hering says: — " I think the explanations given
in the paper why the deposit is so much better are not the correct ones.
I think that the correct explanation is that with high current densities
the molecular layer of liquid next to and in molecular contact with the
cathode is exhausted of its metal before fresh liquid can get there, hence
hydrogen or other things will be set free, spoiling the deposit, unless,
that is, rotation (or other means of agitation) is used." C. J. Reed
says: — " I entirely agree with that " (Hering's) " view, that neither the
rotation nor the friction have anything to do with the smoothness of
the deposit. It is simply the supplying of the ions of the copper in
sufficient quantities to transmit the total current It is the
deficiency of the copper and deposition of hydrogen that causes roughness.
I do not think that the friction of the liquid has anything to do with
it." The present author's view is that the only effect of rotation (or
other means of agitation) is renewal of ions at the cathode surface, that
rotation replaces quickly those discharged by the high current density,
and that the rapid formation of metal atoms results in a deposit of a
very fine structure — so fine that, under certain conditions, it may have
a burnished appearance.
The connexion between fineness of deposit and current density has been
studied and commented upon by Faust,* Sieverts and Wippelmann,t and
others. + Faust states that he found that the crystallites were smaller the
higher the current density. Sieverts and Wippelmann confirm this up to
a point, but they say that their experiments show that the connexion holds
only up to a certain current density which in each case depends upon the
experimental conditions. At a particular current density, differing as
stated, a " minimum of crystal size " is reached.
In the present instance, then, one has (i) a fine-grained deposit, and
(ii) at the same time, high current density (plus agitation). High
current combined with sufficiently great agitation will effect and maintain
a high concentration of metal ions, and, hence, of metal atoms, at the
cathode surface. In the crystallisation process (from solutions of salts
and from melts) great concentration leads to small crystals or crystal
structure. Here, again, analogy suggests the fineness of grain of electro-
deposited metal to be due to great concentration.
(c) Laminated structure.
By " laminated structure " is meant one, often seen in electro-deposited
metal and very often in deposited iron, that causes the deposit to appear
to be made up of a number of separate layers. It resembles, broadly, the
* Zeit. f. anorg. Ch., 1912, vol. 78, p. 201.
t Zeit. f. anorg. Ch., 1915, vol. 91, p. 1, and vol. 93, p. 287.
j e.g., the present author, Jour. Phys. Cheni., loc. cit.
35
structure often seen in certain argillaceous rocks,* and particularly in
the sand-stones. This type of structure has been noticed in deposited
metals by several observers. C. F. Burgess and O. P. Watts refer to what
they call " surfaces of cleavages."! As to their origin, these investi-
gators remark : " These surfaces of cleavages seem to be produced when
any marked change in or interruption of the deposition process occurs.
If the current be interrupted for a time, or if the cathodes are removed
from the tank and exposed to the air, a cleavage surface may be produced.
It is also believed that a sudden change in the current density may have
the same effect." It is the break in the continuity of the deposit, denoted
in it by a line of demarcation, and caused by the stopping of the current
or the removal of the cathodes from the solution, to which reference is
here made. O. W. StoreyJ refers to the lines of lamination here alluded
to. He states that he found that the deposit (iron) could be split by a
knife along these lines (or planes), and he, again, attributes their
formation to momentary stoppage of the current, as occurs when a
cathode is temporarily removed from the solution. An instance of such
a structure, formed in a deposit prepared by the present author, is shown
in photo-micrograph, Fig. 53. In this case, the times of formation of the
lines are known to correspond with temporary removal of the cathode
from the solution. An extreme case is shown in photo-micrograph, Fig.
42. In this instance, the deposit was formed upon a rotating cathode.
When about half the whole period of deposition had elapsed, the rotation
was stopped for some minutes, the current flowing the while, and then
continued. Two distinct lines of demarcation indicate the stoppage and
the re-continuance of the rotation ; and they can be definitely said1 to be
due to those causes. The metal shown in Figures 42 and 53 was iron;
but the laminated structure here being considered is not confined to that
metal. A similar line is seen in photo-micrograph, Fig. 2, which shows
the fractured surface of a cobalt deposit. The same structure often
occurs in nickel-plating. In this case, one or more of the outer layers
will split and peel away from the cathode, leaving the inner (or first
formed) deposit intact,
As above indicated, the present author believes the lamina of the
structure under consideration are caused by either definite, temporary,
but complete, stoppages of the deposition process, such as occur when a
cathode is temporarily removed from the solution, or when a definite and
marked change in the current conditions (including herein the relation of
current density and movement) occurs. A line of demarcation can be seen
in most deposits (with the aid of the microscope) to be the locus of differ-
ence of structure, the deposit on one side of it having one structure, whilst
that on the other side has another. It is considered that an analogy can
be found in the crystallisation of other substances. If a saturated solution
of (say) copper sulphate is made, saturated at KXPC. or thereabout, the
containing vessel be set in water at the same temperature, and the whole
be allowed to cool slowly, a layer of crystals of copper sulphate will be
slowly formed on the bottom of the vessel. If, when this layer has been
formed, the hot water in the outer vessel be removed and substituted by
ice-cold water, a shower of minute crystals will be formed which will form
* Though the lamination in rocks is due to different causes than those that would operate
during the electro-deposition of metal, v. A. Geikie, Text Book of Geology, 1893, p. 499.
f Trans. Amer. Electroch. Soc., 1906, vol. 9, at p. 233 ; and see op. cit., figs. 5, (5, and 7r
p. 231. Qftf also C. F. Burgess and C. Hambuechen, Jour. Phys. Chem., 1903, vol. 7,
p. 409.
J Trans. Amer. Electroch. Sec , 1914, vol. 25, p. 489.
36
a very marked and distinct layer upon the layer of larger crystals first
formed. The line of demarcation between the layers corresponds to a
line between two lamina of a deposit. Further, the lines of division are,
in the author's opinion, due to the same cause in the two cases, namely,
to change in supersaturation, in the one case of salt molecules, and in
the other of metal atoms, though, of course, this change is brought about
by different means in the two cases. It is suggested, too, that such a line
of demarcation between two layers of one and the same deposit is due to
the same cause as the clear line of demarcation which, so far as the
microscope discloses, always exists between base metal and deposit.
Reference will now be made to this.
(d) Eelation between base metal and deposit.
Despite the use of the highest powers of the microscope there always
appears to be a line of demarcation between base metal and deposit.*
This line is more or less clear according as the surface of the base metal
was clean or not clean at the time when the cathode was put into the
depositing solution. t It may possibly be that the metal first deposited
forms an alloy with the base metal, and that at the junction or, rather,
forming a junction between base metal and deposit, is a thin (ultra-
microscopic or irresolvable) layer of alloy which appears, under the micro-
scope, as a very thin line. There are authorities to support the view that
such an alloy is formed]; ; but it would lead to too long a digression to
discuss the subject in detail here. Reference to the micrographs enables
one to see that, in every case, the deposit adjacent the base metal is
small in structure. It either remains small and approximately uniform
throughout the width of the deposit, or it gradually becomes coarser
from within outwards, according to whether the deposit was formed in
an agitated or still solution. § One may call this the microscopic equivalent
of the macroscopic observation before referred to. If a piece of metal
be taken, say, copper, that has an obviously crystalline structure, and
deposits of the same metal, copper, be formed upon it, the crystals of
the base metal do not grow. No instance of such growth has ever been
observed.]] The deposit is always of the finely crystalline type at first,
and then, in a still vat, becomes coarser and coarser, from within out-
wards, as deposition proceeds. If, after deposition has proceeded for
some time — an hour or two, the process is interrupted and then continued
after a short interval, the second deposit will be finely crystalline to
commence with, becoming coarser as the second period of deposition
proceeds ; and the point of interruption of the deposition is marked by a
line, so that the two deposits form laminae. The explanation suggested
is that the concentration of available atoms at the cathode surface is
greater at the commencement of each period of deposition than at any
time afterwards during each period as a consequence of the gradual
exhaustion of the metal in the cathode layer of solution as deposition
goes on, and that it is this variation of concentration that effects the
variation in the structure of the deposit.
* Cf. O. Faust, op. cit.
f A base metal that is liable to chemical attack by the electrolyte may cause the line to
be confused in appearance, instead of definite and clear.
J E.g., M. Schlotter, Chem. Ztg., 1914, vol. 38, p. 289, and C. H. Desch, Brit. Assoc.
Rep., 1912 (sect. 3) ; also see G. Gore, Electro-metallurgy (Longmans).
§ This statement refers to deposits formed in neutral solutions only. In acid solutions,
the deposit frequently shows the small and uniform structure, whether agitation is used or
not. This, however, is not always so.
|| Cf. O. Faust, op. cit.
37
A CONSIDERATION OF THE RESULTS OF THE EXPERIMENTS
OF THE SERIES I, II, and III
Attention may now be redirected to the experiments on the effect of
temperature, current density, and movement, respectively, upon the
structure of iron deposited from the (neutral) chloride bath, with a view
to considering how far the explanation advanced in regard to the structure
of electro-deposited metal in the foregoing pages is justified by the results
of those experiments.
(i) Effect of Temperature.— It is stated on p. 10 and elsewhere that
the results of both macroscopic and microscopic investigations show that,
ceteris paribus, increase of temperature causes increased coarseness of
deposit. In an earlier series of experiments made on the Fischer-
Langbein bath, it was found that a series of changes occurs in
the physical character of the deposit as the temperature is gradually
raised from (about) 45° C. (a temperature considerably below the working
temperature of the bath), while the current density is maintained
constant. Below land at 45° C. the deposit is dark and metallic,
extremely hard and brittle; it may even be cindery and friable. There is,
too, considerable gas evolution (hydrogen). As the temperature rises the
deposit becomes lighter in colour : it becomes at first light grey and
bright-metallic in appearance, and still hard and brittle; but as the
temperature continues to rise the deposit becomes successively dull, matt,
and finally crystalline. The temperature is then 90° to 100° C. In the
same series of experiments, it was noted that two effects of hydrogen in
the cathode neighbourhood were (1) that the deposit was bright and
metallic in appearence, and hard and brittle, and (2) that there was
considerable gas evolution. It may also be added that, as a fact, a
bright, hard metallic deposit connotes a finely crystalline internal
structure. There is one other experimental fact about the ferrous calcium
chloride bath that may be conveniently mentioned here. When the
solution cools after use a pale green salt crystallises out. The same salt
separates after a solution, that has been made (but not used for deposition
purposes), has stood idle for some while. Analysis of this salt showed*
that it contained chlorine and iron in approximately the percentages that
would be contained in a double (or complex) chloride having the formula,
CaFe(Cl)2 2H20. The actual figures were: —
Fe Cl
(found) (found)
Analysis I ... "U " ... 26-4 % 35-45%
Analysis 2 ... : ...' ' ... 27-05% 35-45%
The precentages contained in CaFe(Cl)2 2H2O, supposing it to exist, t
would be: —
Fe 27-5 %
Cl 35-0 %
It is, therefore, suggested that this salt is largely present in the solution
at lower temperatures, and that when the solution is electrolysed at low
temperature calcium ions carry the greatest portion of the current and
are discharged at the cathode. Upon discharge the calcium reacts with
the water of the solution, giving rise to hydrogen evolution, and the
calcium hydrate formed at the same time causes the catholyte to be
* The analyses were carried out for the author by Mr. J. W. Gardom.
f Salts of the same type known to exist are: 2 KC1. FeCl2. 2H2O, 2NH4 Cl. FeCl2,
2CdCl2. FeCl2. 12H20, aud HgCl2. Fe.Cl2. 4 H2O. See Dammer's Handbuch, 1893, vol. 3,
p. 309, and Gmelin-Kraut Handbuch, 1875, vol. 3.
3$
alkaline. It is believed that the result on the deposit is to cause it to
contain compounds (perhaps oxides), and that the dark colour is the
result. At the lowest temperatures (ordinary and temperatures not much
above this), another result of the basic condition of the solution around
the cathode is that the deposit, after a short time of deposition, is not
reguline, bright, and metallic, but dark, powdery, and loosely adherent.
But it is suggested that according as the temperature rises the double
salt becomes destroyed, and the resulting ferrous chloride supplies more
and more ions to act as carriers of the current. In the result, with
increasing temperature the discharge of ferrous ions increases, while the
discharge of calcium ions decreases, and the deposit contains a continually
increasing percentage of iron, until, finally, at a temperature of (about)
90° C., normal deposition of iron occurs. It must be further remarked
that as the discharge of calcium ions decreases so will the evolution of
hydrogen. The known effect of hydrogen upon the structure of deposited
metal is to cause it to be very small in size of grain and often bright in
appearance. The experiments of Sieverts and Wippelmann with copper*
have shown that acidity causes smallness of grain, and the present author's
experiments with iron show it to be so in the case of this metal; while it
is the author's experience with many metals — iron, nickel, zinc, and
others — that acidity causes the deposit to be bright. It is, therefore,
considered that it is the hydrogen which causes the deposit of iron, in the
case under consideration, to be bright and metallic, at first dark on
account of the admixed iron compounds, and then light. No explanation
is here offered as to how the hydrogen produces its effect. Pfanhauser
has, indeed, given an explanation, but this is dissented from by Sieverts
and Wippelmann. In fact, the cause has not been experimentally deter-
mined, and one has, therefore, only conjecture to rely upon.t
The matter that it is wished to make clear is that as the temperature
approaches the working temperature less and less hydrogen will be
evolved (in consequence of diminishing discharge of calcium ions), and
the deposit becomes more normal; and, further, that the reason why a
deposit formed at a higher working temperature is more coarse in
structure than one formed at a lower one is because at the higher tempera-
ture the catholyte (or the cathode layer of it, at least) becomes the weaker in
metal at a greater rate, and this diminution of concentration more
quickly makes itself apparent. The author is perfectly well aware that
the foregoing explanation involves some assumptions which (although not
unreasonable yet) require to be justified by experiment. At the same
time it is suggested that, if the assumptions were justified by experiment,
the explanation would afford an understanding of the phenomena on the
most rational basis, namely, that the structure of a deposit is dependent
upon concentration of the ions of the metal deposited. J
(ii) The Effect Of Current Density. — The observations made upon the
effect of current density on the size of grain of a deposit are summarised
on page 16. The size of grain appears to alternate. As the current density
increases, the size of grain at first increases also, then diminishes, and
then seems to increase again. The explanation is as follows : The cathode
* Op. cit.
f The remarkable change that acid may cause in the structure of a deposit (v. W. B.
Hughes, Trans. Faraday Soc., 1921) has not, it is suggested, been sufficiently closely con-
sidered by W. Blum in his work on " Factors Governing the Structure of Electro-deposited
Metals." Trans. Am. Electroch. Soc., 191P, vol. 36, p. 213.
J Compare W. Blum's suggestions as to the cause of increased coarseness of grain in
deposits formed at higher current density. Trans. Am. Electroch. Soc., 1919, vol. 36,
p. 221.
layer of solution is diminished in metal content by deposition. This will
occur more quickly, the greater the current density employed; and since
the lower the concentration, the greater the grain size, it follows that the
greater the current density is, the greater the size of grain will be. It is
suggested that the change from larger to smaller grain when a certain
region of current density is reached (which will depend on the tempera-
ture) is due to increased discharge of hydrogen ions. The effect of acid
(i.e., hydrogen ions) is, as has been already said, to diminish the size of
grain of the deposit. The uncertainty of the possible further change from
smaller to larger grain as the current density goes on increasing is such
as to render discussion unprofitable. If it occurs, it would be difficult to
explain; but, it is suggested, the conditions at (about) 200 amperes per
square foot and a temperature of 110° to 120° C. are such that any change
in size of grain may be due to quite other causes than increase of current
density in and by itself.* For instance, at 120° C. (approx.) the boiling
point of the solution is either nearly or quite reached (this depending
upon the concentration of the solution), and the mechanical disturbances
produced might well of themselves be sufficient to vitiate conclusions on
the effect of current density.
(iii) The Effect of Mechanical Movement. — The effect of movement of
the cathode upon the structure of the deposits from the chloride of iron
bath is certain. The effect is that movement causes diminution of grain
size; microscopic examination confirms visual observation. This is what
is to be expected. Movement of the cathode helps to maintain constant
the concentration of metal in the cathode area and the supply of ions at
the cathode. The diminution in metal content of the cathode layer is,
if not entirely, largely prevented. Hence ,conditions favourable to growth
of large crystals do not exist ;t and either crystals must grow out from
the large number of nuclei formed on the cathode surface at the commence-
ment of deposition, or fresh nuclei be formed. Very probably both events
occur; but the fibrous structure of the deposits formed on the moving
cathodes points to the former occurring to, at least, a very considerable
extent.
SOME REMARKS ON DEPOSITS : — (1) FROM OTHER IRON
SOLUTIONS, AND (2) OF OTHER METALS
The deposits particularly considered in the foregoing discussion were
formed in the chloride of iron bath — neutral, in regard to the presence of
free acid. General conclusions cannot, of course, be drawn from what
happens in the case of one metal deposited from one type of solution only.
But there is evidence to show that, i'n general, the same relations between
structure of deposit and conditions of deposition hold good.
(l) Deposits from Other Iron Solutions. — In order to obtain some
information as to the above-mentioned relation in the case of another and
quite different type of iron solution, some deposits were formed in the
Klein-Maximowitsch iron bath.J This is a solution of an entirely different
constitution to that of the ferrous calcium chloride bath. It is compara-
tively dilute, and is worked either at the ordinary temperature, or at tem-
* Compare similar suggestions respecting the marked effect by a factor not under study
on any change produced by one that is being considered. W. Bluin (op. cit.,p. 219.)
f Cf. Sieverts and Wippelmann, op. cit., and M. von Schwarz, Internat. Zeit. f. Metallo-
graphie, 1915, vol. 7, p. 124.
J v. Zeit. fur Elektroch., 1905, vol. 11, pp. 52 and 91.
40
pera+ures not much above the ordinary. It is, too, a solution that can be
operated at a very low current density only (0'3 to 0'5 amp./dcm.2).*
The solution from which the deposits were formed contained : —
1| Ibe FeS047H20.
4 ozs. MgS04aq.
dissolved in one gallon of water.
It was treated with bicarbonate of soda in the way described by
Maximowitsch.* The solution was shown to be saturated with respect to
the iron salt by the fact that, upon cooling to ordinary temperature from
that of deposition (v. inf.), some of the iron salt separated out.
The conditions of deposition were as follows: —
Current density 4*6 amp. /ft.2 (0*5 amp./dcm.2 approx.).
Temperature 34° G.
Time of deposition ... 24 hours.
The Deposit was of a beautiful silver-grey colour, lustrous and glittering.
It was obviously crystalline. A photograph of the cathode with the deposit
upon it is shown in Fig 62 at three-quarters actual size. In Fig. 63 is seen
the internal structure of the deposit. The result in this case is just what
would be expected, and, indeed, is what was anticipated. Low-current
density, stationary cathode, and neutral solution should give a structure
of the " normal " type and large grain. It is seen that this is the struc-
ture that was obtained. The experiment was repeated several times,
using cathodes of different sizes and composition (copper, steel, and iron)
with, in all cases, a like result.
(2) Deposits of Other Metals
(a) Copper. — Reference has already several times been made to the work
of Faustf and of Sieverts and Wippelmann.J Their results seem to be in
general harmony with those obtained by the author, they working with
copper and the author with iron. Reference has also been made to
Zimmerman's results, § which were obtained by depositing copper on
rotating cathodes, and which support some results obtained by Cowper-
Coles.H Further, the work of M. von SchwarzK on idiomorphs of electro-
deposited copper, and, later, that of the present author** on the same sub-
ject, show that a similar relation between structure and conditions of
deposition holds for copper as obtains for iron, and one and the same
explanation appears applicable in the two cases.
Mention must also be made of some recently published work
by W. Blum, H. D. Holler, and H. S. Rawdon,tt which, so far as it
goes, appears to support the general conclusions formed and the
observations made by the present author. The conclusions of Blum and
his co-workers (which they illustrate by photo-micrographs) are : — That
" with low current density, especially at higher temperatures, the copper
possesses a relatively coarse structure except at the surface where ths
initial deposit is made. By increasing the current density, particularly
at lower temperatures, the structure assumes a columnar appearance,
the crystals being long and finger-like. With still further increase in
* Op. cit.
t Zeit. f. anorg. Ch., 1912, vol. 78, p. 201.
J Ibid., 1915, vol. 91, p. 1, and vol. 93, p. 287.
§ Trans. Amer. Blectrochem. Soc., 1903, vol. 3, p. 245.
|| Trans. Far. Soc., 1905, vol. 1, p. 215.
t Internat. Zeit. f. Metallog., 1915, vol. 7, p. 124.
** Jour. Inst. Metals, 1920, vol. 23, p 525.
ft Trans. Am. Elect.roch. Soc., 1916, Vol. 30, p. 159.
To face p. 40.
Cathode with deposit from Klein's bath.
FIG. 62. x l.f
j/ir^'- ^%
Iron deposit.
Copper deposit.
Base metal.
Section (etched) of deposit shown in Fig. 62.
FIG. 63. x 150.
f This photograph was taken for the author by Dr. G. D. West, D.Sc.
(Lond.), lately of the Physics Department, East London College.
.
the current density, the structure is much broken up, and numerous
evidences of twinning are found. In every case the direction of growth
of the crystals is perpendicular to the surface of deposition." It is to
be noted that agitation was used in the experiments of these investigators.*
(b) Other Metals.— So far as the author can ascertain, little or no
systematic and sufficiently thorough work appears to have been done in
connection with the crystalline structure of other metals than copper
and iron.f Some few photo-micrographs of electro-deposited zinc are,
indee'd, to be found in the literature, and, in those cases where the
conditions of deposition are given at the same time, these certainly
seem to show that the relation between structure and conditions of
deposition which, it is suggested, obtains for iron and copper, holds
also for zinc, and that, therefore, the concentration hypothesis is valid
for that metal. But, as indicated, a sufficient amount of work (ad hoc)
has not been done — or, at any rate, published — to enable one to form
a positive opinion in regard to zinc.
GENERAL CONCLUSIONS
It is suggested that sufficient evidence has been adduced in the fore-
going pages to show : —
1. That the general theories entertained in regard to the crystallisa-
tion of other substances hold also for the case of deposited
metal.
2. That the dominant factor governing the structure of the
crystallised substances is, in all cases, concentration — of
molecules or atoms.
3. That, in the case of electro-deposited iron and copper (and, perhaps,
other metals), concentration of .available metal atoms at the
cathode surface is the dominant factor, other factors of
deposition, such as temperature, current density, and so on,
being contributors ;J and that, consequently, there is a direct
relation between conditions of deposition and structure of
deposited metal.
4. That, specifically, great concentration (plus agitation) leads to
small grain-size, and, conversely, small concentration (with or
without agitation) leads to large grain-size.
5. That recognition of the relation stated in 3 — the deduction from
the concentration hypothesis — will enable one to explain the
history of an electro-deposit.
* Blum's work (op. cit. supra) leads him to different conclusions on several points from
those reached by the present author, who, however, reserves criticism of Blum's work to
another occasion.
f Dr. Blum frankly admits the incompleteness (to date) of his experiments and obser-
vations. Op. cit., p. 214.
J It has been recently suggested anew by authors whose views are entitled to respectful
consideration that the cathode potential exercises an important influence on the form and
structure of deposited metal. (H. Stager, op. cit., A. H. W. Aten et M. Boerlage, Rec. des
Travaux Chim. des Pays-Bas, 1920, vol. 39 p. 720). Prof. W. D. Bancroft appears to have
expressed a similar view, thus :— " Increasing the current density, increasing the potential
difference at the cathode decreases the size of the crystals " (v. Trans. Am.
Electroch. Soc., 1904, vol. 6, p. 27 ; Journ. Phys. Chem., 1905, vol. 9, p. 277, and Trans. Am.
Blectroch. Soc., 1913, vol. 23, p. 266. And cf. also W. Blum, Trans. Am. Electroch. Soc.,
1919, vol. 36, at p. 223. The present author has considered this view. He has, however,
come to the conclusion that cathode potential takes its place with current density, tempera-
ture, and the other factors of deposition, which, as he suggests, operate only in that and in
so far as they affect concentration, which is the dominant causa causans.
42
The converse of the statement contained in 5 will now be very shortly
considered, that is to say, that one can, by the aid of the concentration
hypothesis, build up a deposit of this or that structure. It enables one
to adopt those conditions of 'deposition which will lead to the formation
of a deposit having the desired structure. In other words, the
hypothesis becomes of practical utility in the workshop.
Ill
WORKSHOP APPLICATION
The application of laboratory results to works practice must
always be cautiously made. This is true in general : it is true in par-
ticular of investigations carried out on the structure of electro-deposited
metals. Nevertheless, laboratory results, applied with due caution, can
often be used, either to the betterment of the products of the workshop or
to effect some improvement in the general control of the processes of pro-
duction. The practice of the electro-deposition of metals has been long
established; but workshop practice has not, for the most part, been based
on scientific knowledge, which has been conspicuous chiefly from its absence.
Freundlich and Fischer state the position, as it existed in 1912, quite
truly in the following words* : — " The form in which the metal is separated
during electrolysis is, in the majority of electrolytic processes of decisive
importance. Yet, regarding the conditions which favour the formation of
a coarse or a finely crystalline form, or, again, an adherent or loose
deposit, there is still, at the present time, very little known. It is known
that metals are separated in an adherent and finely crystalline form when
deposited from solutions in which the metals are contained in complex ions ;
it is, further, known that the addition of quite small amounts of foreign
substances will often influence the form of the deposited metal to an
extraordinary degree. But it is found that the statements concerning
the most advantageous composition of solutions for electro-plating ....
and other objects have, for the most part, the character of purely
empirical recipes." Since 1912, however, the results of several investiga-
tions relating to the effect of the various factors of deposition upon the
structure of deposited metals, have been published. The papers containing
these results have been frequently referred to in either the text or notes
of this Report. It appears clear from them, as well as from the results
contained in Division 1, that current density, concentration of metal,
temperature, and other factors influence, often in a very marked manner,
the structure of the deposited metal. But the work so far done has had
relation to individual cases, and the results cannot be said to have much
general application. It is, indeed, still too early to generalise with any
great confidence : much must be done before the general influence of this
or that factor can be said to be known. Hence, it is necessary to use with
caution such information as has been obtained in regard to any one metal.
What has been found to be true for one metal may not hold for another,
and thus, any attempt to apply in the workshop the information gained as
to the former may lead to trouble if that knowledge be applied to another
different metal. Then again, factors such as the presence of free acid or of
some colloid, may cause usual results to be modified in a greater or less
degree. For instance, the acid copper bath contains a considerable per-
centage of free sulphuric acid; yet, as Blum, Holler, and Rawdon have
shown, the deposits obtained at low current densities have a typical coarse
* Zeit. f. Bloktroch., 1912, Vol. 18, p. 885.
43
structure.* In the case of iron, however, there are indications that free
acid causes the structure to be fine-grained, f even when the acid is present
in small quantity only and a low current density is used. Some of the
general conclusions that have been stated in Division 2 (e.g., Nos. 3 and 4,
p. 41 ante) appear to hold as well for copper (deposited from the acid
copper bath), iron (from the sulphate bath), and, probably for zinc, as for
iron deposited from the chloride solution, in the absence of such factors as,
for instance, the presence of colloids. But there is, at present, no certainty
about this. It can, however, be said that, as a consequence of work on
individual metals, results obtained in the case of a metal that has been
thoroughly investigated can be put to good use in the workshop ; one has
only to take care that these results are not hastily applied in connection
with the deposition of another metal. One important fact is becoming more
and more clear, namely, that one can, with the necessary knowledge of the
relation between structure and conditions of deposition, build up a deposit
having any desired structure, and hence, possessing certain desired physical
properties.
It will be useful to illustrate the present position by an example or two.
(1) The author has several times been asked how a deposit of iron that
will not be hard and brittle can be obtained. Electrolytic iron is commonly
believed to be both hard and brittle, and it often is. The answer to the
question just stated is contained in the following remarks. Experience
shows that iron can be deposited (from the chloride bath, at any rate)
with structures that vary with and depend upon the conditions of deposi-
tion. If acid be present in the solution, the structure will be fibrous ;J it
will, again, be fibrous, if agitation of the electrolyte or movement of the
cathode be employed during the deposition. § Experience also shows that
a deposit from the chloride bath possessing a fibrous structure is hard
and brittle. On the other hand, if no free acid is present in the solution,
and if " still vat " conditions are maintained, then a deposit from the
chloride bath will possess a normal structure, || and experience has shown
the author that a deposit from the chloride bath possessing that structure
is soft and malleable — so much so that the iron can be flattened out under
hammer blows.
(2) Let it be assumed that a thick, smooth deposit of iron is required.
Experience has shown that where the deposit is of fine and fibrous grain
within, the surface is smooth; but that if the structure be of the normal
type, the surface is irregular. In other words, there is a correlation
between internal structure and surface features; these can be observed
continuously as deposition proceeds, and hence, control of the structure
of a deposit maintained. To obtain a fine and fibrous structure, one
must employ a solution that contains free acid, or one must agitate
the electrolyte or move the cathode during deposition. To secure a
thick and smooth deposit, it is necessary, in addition, to maintain approxi-
mately constant the concentration of metal in the solution ; for concentra-
tion is always an important — the author believes it to be the dominant
factor determining the structure of a deposit, the other factors
affecting the structure, if not, perhaps, wholly, then, at any rate,
•especially, in consequence of their effect upon the concentration. To
* " Preliminary Studies in the Deposition of Copper in Electro-typing Baths," Trans.
Am. Electroch. Soc., 1916, Vol. 30, p. 159.
f Cf. Trans. Faraday Soc., 1921.
£ Cf. " The Forms of Electro -Deposited Iron, and the Effect of Acid upon its Struc-
ture," Trans. Faraday Soc., 1921.
§ v. Figs. 52 et alia, Division 1.
|| v. especially, Figs. 11, 19, & 20, which are typical of the "normal" structure.
44
obtain a thick, smooth, deposit of iron, one must therefore use a bath
containing free acid or must agitate the electrolyte or move the cathode
during the deposition, and one must, in addition, see to it that the metal
concentration of the bath does not become appreciably diminished.
The author does not, of course, pretend to be either the first or the only
student of the structure of electro-deposited metal to point out the value of
such studies to works practice. Among workers in this field of research,
Dr. W. Blum is, perhaps, the most prominent. He and his collaborators,
working at the Bureau of Standards, Washington, U.S.A., have carried
out much research in connection with the structure of electro-deposited
metal, and much of the work has been specially directed to the practical
application of their observations. In regard to this matter, Dr. Blum
writes as follows* : — " From the point of view of the plater we may usually
define a ' good deposit ' as one which is fine grained, since fineness of
grain is in general accompanied by a high lustre, relative freedom from
porosity, and comparative hardness t. In electro-typing, the structure of
the metal may be even more important since it largely determines the
physical properties upon which the durability of the product depends.
It is therefore highly desirable to define, if possible, conditions of opera-
tion which will produce a fine structure (or which in some cases will permit
any change in structure required to produce the desired finish), whicH
conditions should be susceptible of control and should permit favourable
operation over a considerable range."
APPENDIX
BIBLIOGRAPHY | COMPRISING REFERENCES TO PUBLICATIONS
ON—
I. The Electro-deposition of Iron and Phenomena connected therewith.
II. The Properties of Electrolytic Iron.
III. Works of Reference relating to Electro-deposition of Iron.
I. IRON DEPOSITION
The publications contained in Sections 1 and 2 are less concerned with
the deposition of iron generally, than with its deposition from particular
types of solution — the sulphate or the chloride. In several instances, e.g.,
those of Pfaff's work and the Langbein-Pfanhauser patents, the optimum
conditions for working such solutions are worked out or given. This
enables a comparison as to the relative merits of the two types to be
made, and it renders the easier the choosing of this or that solution for
a particular practical purpose.
1. Sulphate Solutions
C. HOEPFNER and KLIE. v. a paper by K. Arndt, Zeit. fur Elektroch.,
1912, vol. 18, p. 233, entitled: — " Zur Geschichte des Elektrolyteisens."
E. KLEIN. — Klein worked with a mixed, chloride-sulphate, bath also;
but the solution usually associated with his name is the sulphate bath,
developed and improved by Maximowitsch (q.v., infra). See the
following : —
Eng. Pat. 1869, No. 2456.
* Trans. Am. Electroch. Soc., 1919, Vol. 36, at p. 215.
f The present author can, as the result of independent work and work on a metal (iron)
that Dr. Blum has not especially examined, confirm this general statement.
J This bibliography is not exhaustive ; but it is believed to contain all the most
important literature.
45
M. II. von Jacobi. — Bull, de 1'Acad. Imper. des Sci. de St. Peters-
burg, 1869, vol. 13, p. 40.
" Note sur la production des depots de fer."
(This contains an account of some of his work by Klein himself.)
M. H. von Jacobi. — Brit. Assoc. Rep., 1869, p. 67.
" On the electro-deposition of iron."
W. Roberts-Austen. — Jour. Iron and Steel lust., 1887 (No. 1), p. 71.
" The electro-deposition of iron."
The following papers contain information concerning Klein's solution : —
F. Haber.—Zeit. fur Elektroch., 1898, vol. 4, at p. 413.
" tiber galvanisch gef allies Eisen."
S. Maximowitsch. — Zeit. fur Elektroch., 1905, vol. 11, p. 52.
" Ein neues Verfahren zur Herstellung des Elektrolyteisens."
A. Buss and A. Bogomolny. — Zeit. fur Elektroch., 1906, vol. 12,
pp. 701 and 702.
" Studien iiber die elektrolytische Abscheidung des Eisens aus
wasserigen Losungen seines Chloriirs und Sulfats."
C. F. BURGESS and C. HAMBTJECHEX. — Trans. Am. Electroch. Soc., 1904,
vol. 5, p. 201; Electro-chem. Ind., 1904, vol. 2, p. 184.
" Electrolytic Iron."
The following papers contain considerations of the work of Burgess and
Hambuechen : —
A. Neuburger.—Elektrocb. Zeits., 1904-1905, vol. 11, p. 77.
" Einige Bemerkungen zu dem Vortrag von Burgess und
Hambuechen, iiber elektrolytisches Eisen."
E. Amberg.—Zeit. fur Elektroch., 1908, vol. 4, p. 326, and Ib., 1910,
vol. 16, p. 125.
" Notiz zur Darstellung von Elektrolyteisen."
H. Lee. — Abhandl. der deutsch. Bunsengesellschaft, No. 2, 1909.
" Das Elektrolyteisen."
A. M'ufter.— Metallurgie, 1909, vol. 6, p. 145.
" Uher die Darstellung des, Elektrolyteisens, dessen' Zusammen-
setzung und thermische JEigenschaften."
A. Pfaff.—Zeit. fiir Elektxoch., 1910, vol. .16, p. 220.
" Uber die elektrolytische Abscheidung von Eisen."
A. Russ and A. BOGOMOLNY: — Op. cit. supra.
R; .AMBERG.— Op. cit. sup.
H. LEE. — Dp. cit. .sup.
A. MTJLLER.— - X)p. cit. sup.
A. BFAFF.— -.Op. cit. .sup.
W. A. MACEAnyEN.— Trans. Ear. .Soc.; 1920, vol. 15, p. 98.
".An Aspect , of Electrolytic Iron Deposition."
M. SCHLOTTER.— D.R.P., 309271.
2. Chloride Solutions
W. M. HICKS and L. T. O'SHEA.— The Electrician, 1895, vol. 35, p. 843;
Brit. Assoc. Rep., 1895, p. 634.
" Some Points connected with the Preparation of Pure Iron by
Electrolysis."
E. MERCK.— D.R.P., No. 126839, 1900. Pfaff has studied and com-
mented upon .Merck's method for iron deposition. (Op., cit.)
35067 D
46
THE LANGBEIN-PFANHAUSER PATENTS. — The most important are: —
D.R.P., No. 212994 (1908, issued 4th Sept., 1909). " Verfahren zur
Herstellung von geschmiedigem Elektrolyteisen."
D.R.P., No. 228893 (1909, issued 24th Nov., 1910). Same title.
D.R.P., No. 230876 (1909, issued 7th Feb., 1911). " Gewinnung
von reiiiem Eisem aus gewohnlichem Eisen auf elektrolytischem Wege."
Eng. Pat., No. 24841 (1909). Corresponds to D.R.P., No. 212994.
Eng. Pat., No. 25092 (1910), sub. nom. E. C. R. MARKS. " Improve-
ments in the Manufacture of detachable Electrolytic Iron Deposits."
Eng. Pat., No. 25969 (1910). Corresponds to D.R.P., No. 228893.
The author has tested the workshop efficiency of the ferrous calcium
chloride bath patented by the Langbein-Pfanhauser A.-G. It may be
stated that much of the work done in elaborating the solution was
carried out by A. Fischer. Consequently, the bath has come to be
known as the Fischer-Langbein solution : the product is sometimes referred
to as •" Fischer's iron."
The following are some papers containing information respecting the
properties of iron deposited, from this bath: —
Anon. Zeit. fur Elektroch., 1909, vol. 15, p. 595.
W. Pfanhauser, Junr., Galvanotechnik, 1910, p. 750.
C. Duislerg. Internat. Cong. App. Chem., 1912, vol. 28, p. 60.
J. ESCARD. — Le Genie Civil, 1919, vol. 75, Nos. 8, 9, and 10, pp. 165,
199, and 225; v. also Stahl and Eisen, 1919, at p. 805. Abstracted in
The Electrical Review, 1920, vol. 86, p. 490.
" Fabrication, proprietes, et utilisation industrielle du fer electro-
lytique."
Rusa and BOGOMOLNY, and MTJLLER, also give the results obtained
by them when working with chloride baths. Miiller's paper contains an
excellent comparison of the results he obtained from chloride and sul-
phate solutions.
3. Sulphate-Chloride Solutions
The work that has been done on baths containing both ferrous sulphate
and ferrous chloride, as original components, is small. It is, too, unim-
portant, except for one notable exception, namely, that of O. P. Watts
and L.H.Li (v. inf. Sub-sect. 4). The work of these investigators was,
however, directed to determining the effect of the presence of " addition
agents " in solutions intended for iron deposition. Ammonium chloride
has been sometimes used in sulphate solutions : in these the ammonia salt
is, usually, intended to act as a " conducting salt." No advantage has
been claimed as resulting from the use of such mixed baths : on the other
hand, many complications and difficulties in matters of control are intro-
duced. References to them are, therefore, omitted, since the author
wishes this bibliography to be of practical use, and not merely an indis-
criminate collection of miscellaneous references.
F. VARRENTRAPP. Ding. Polytech. Jour., 1868, vol. 187, p. 152.
" Galvanische Fallung von Eisen in coharenter Form."
This paper is given as being of historical interest.
4. General Researches on Iron Deposition
In this sub-section there are included references to researches of a more
general character on the electro-deposition of iron, while Sub-section 5
contains references to publications of a more purely scientific kind.
47
A. WATT. — The Electrician, 1887, Nov. 11 and 25, Dec. 16 and 30; Ib.,
1888, Jan. 13.
" Electrolysis of iron salts."
A. Russ and A. BOGOMOLNY. (Op. cit., sup.)
E. F. KERN. — Trans. Am. Electroch. Soc., 1908, vol. 13, p. 103.
" Electrolytic refining of iron."
S. COWPER COLES. — Jour. Iron & Steel Inet., 1908 (No. 3), p. 134.
" The Production of finished Iron Sheets & Tubes in one operation."
The following references containing criticisms of Cowper Coles' work
will be useful : —
W. PALMAER and J. A. BRINELL.— Met. & Chem. Eng., 1913, vol. 11,
p. 197.
W. PALMAER.— Chem. Zeitg., 1913, vol. 37, p. 393.
L. GUILLET. — Jour. Iron & Steel Inst., 1914, vol. 90, at p. 67. (v. sub.
Applications.)
A. MTJLLER. — Metallurgie, 1909, vol. 6, p. 145. (v. sup.)
S. A. TUCKER and E. SCHRAMM. — Jour. Ind. & Eng. Chem., 1910, vol. 2,
p. 236.
" A Comparison of the Methods for depositing Iron electrolytically."
0. P. WATTS and L.H.Li.— Trans. Am. Electroch. Soc., 1914, vol. 25,
p. 529.
" The Effect of Addition Agents in the Electro-deposition of Iron.'*
E. H. ARCHIBALD and L. A. PIGUET. — Trans. Roy. Soc., Canada. 1917-
1918, vol. 11 (series III), p. 107.
" The Electro-deposition of Iron from Organic Solvents."
R. KREMANN and his collaborators. — The work of Kremann .and his co-
workers relates more to the electro-deposition of alloys of iron with nickel
and magnesium than to the deposition of iron alone. The following
papers contain interesting results : —
R. KREMANN, C. TH. STJCHY, and R. MAAS. — Monatsh. fur Chem., 1913,
vol. 34, p. 1757.
" Die bei gewohnlicher Temperatur abgeschiedenen Nichel-eisen-
legierungen."
R. KREMANN and J. LORBER. — Ibid., 1914, vol. 35, p. 603.
" Uber versuche zur Abscheidung von Eisen-magnesiumlegierungen
aus wasserigen Losungen."
R. KREMANN and R. MAAS.— Ibid., 1914, vol. 35, p. 731.
" Die bei hoherer Temperatur aus Sulfatbadern abgeschiedenen
Nickeleisenlegierungen . ' '
R. KREMANN and H. BREYMESSER.— Ibid., 1917, vol. 38, p. 359.
" Uber die bie gewohnlicher Temperatur unter hoheren Wasserstoff-
drucken erhaltenen kathodischen Abscheidungen von Eisen und
Eisen-nickellegierungen. ' '
5. Phenomena Connected with Iron Deposition, together with th*
Preparation of Pure Iron
*F. VARRENTRAPP. Op. cit. (Sub-sect. 3).
*J. THIELE.— Lteb. Annal., 1891, vol. 265, at p. 58.
" Zum Nachweis des Arsens."
*W. M. HICKS and L. T. O'SHEA. Op. cit. (Sub-sect. 2).
*R. AMBERG. Op. cit. (Subject. 1).
An asterisk denotes publications containing work on the preparation of
/ pure iron.
F. FOERSTER. — Abhandl. der deutsch. Bunsengesellschaft, No. 2, 1909.
35067 R
48
This symposium contains the following : —
F. Foerster and F. Harold. — " Das elektromotorische Verfahren des
Eisens."
O. Mustad. — (Diss. Dres., 1909): " Die Abscheidungspotentiale des
Eisens aus Ferrosulfat-und Ferrochloridlb'sungen."
H. Lee.— (Diss. Dres., 1906): " Das Elektrolyteisen."
R. KEEMANN and J. LORBER. — Monatsh. fiir Chem., 1914, vol. 35, p.
1387.
" Tiber die kathodischen, funkenden Abscheidungen aus gemischten
eisensulfat-magnesiumchlorid, glycerinhaltigen Badern."
*J. R. KAIN, E. SCHRAMM, and H. E. CLEAVES. — Bur. of Stand., Sci.
paper, No 266, 1916, vol. 13, No. 1, p. 1.
11 Preparation of pure iron and iron-carbon alloys."
(This monograph contains a bibliography on the preparation of elec^ro-
lytic iron.)
R. KREMANN, R. SCHADINGER, and R. KROPSCH. — Monatsh. fiir Chem.,
1917, vol., 38, p. 91.
" Versuche zur Darstollung kathodischer, funkender Abscheidungen
aus glycerinhaltigen Eisensalzlosungen bei Zusatz anderer Salze, im
besondern von Cerochlorid."
W. A. NOYES, Junr.— Compt. rend., 1919, vol. 164, p. 971.
" Sur le potentiel necessaire pour electrolyser les solutions de fer."
N. R. DHAR and G. URBAIN. — Compt. rend., 1919, vol. 164, p. 1395.
" Tensions de polarisation du fer dans les solutions de ses sels com-
plexes. Relations entre ces tensions et la dissimulation des caracteres
analytiques des ions ferriques."
II. ELECTROLYTIC IRON
1. Composition and General Properties
M. H. VON JACOBI.— Bull, de 1'Acad. Imp. de St. Petersburg, 1868-1869,
vol. 14, p. 252.
" Notiz iiber die Wasserstoff absorption des galvanischen Eisens."
R'. LENZ. — Jour, fiir prakt. Chem., 1869, vol. 108, p. 438; Pogg. Annal..,
1871, p. 242. (5th Erganzungsband.)
" Uber einige Eigenschaften des auf galvanischem Wege nieder-
geschlagenen Eisens."
L. CAILLETET.— Compt. rend., 1875, vol. 80, p. 309.
" Sur le fer hydrogene."
F. WINTELER.— Zeit. fiir Elektroch., 1897, vol. 4, p. 338.
" Einiges iiber Metallniederschlage."
T. HABER.— Zeit. fiir Elektroch., 1898, vol. 4, p. 410.
11 Uber galvanisch gefalltes Eisen."
R. ABEGG. — Stahl u. Eisen, 1901, p. 736.
" Uber die Elektrochemie des Eisens."
A. SKRABAL.— Zeit. fiir Elektroch., 1904, vol. 10, p. 749.
" Uber das Elektrolyteisen."
I?. LEE. — Dise Dres., 1906 (v. F. Foerster, I, 5, sup.).
" Uber den Wasserstoffgehalt des Elektrolyteisens."
ANON.— Zeit. fiir Elektroch., 1909, vol. 15, p. 595.
(Properties of Fischer-Langbein Iron.)
A. PFAFF.— Zeit. fiir Elektroch., 1909, vol. 15, p. 703.
" "Uber den Schwefelgehalt des Elektrolyteisens."
C. F. BURGESS. — Trans. Am. Electroch. Soc., 1911, vol. 19, p. 181.
" Electrolytic refining as a step in the production of steel."
49
L. GUILLET and A. POETEVIN. Compt. rend., 1913, vol. 156, p. 702.
" Sur quelques proprietes d'un.fer electrolytique industriel."
J. ESCABD. — Le Genie Civil. Loc. cit.
This author's articles in Le Genie Civil contain an excellent exposition
•of the properties of electro-deposited iron.
2. Individual Properties
The number of publications in which information is to be found on
some property of electrolytic iron is very great. Those cited below are
such as will, it is hoped, facilitate research into the literature, since the
researches referred to are, mostly, by eminent authorities and the papers
containing them provide, often, numerous references to work connected
with the subject matter dealt with by them in each case.
(a) Crystallising Properties.
J. E. STEAD and H. C. H. CARPENTER. — Jour. Iron & Steel Inst., 1913,
vol. 88, p. 119.
" The Crystallising Properties of Electro-deposited Iron."
(b) Mechanical Properties.
B. NEUMANN. (V. Infra.)
A. MTJLLER. Op. cit.
L. GUILLET. Op. cit.
J. ESCARD. Op. cit.
•(c) Thermal Properties.
A. MTJLLER. Op. cit,
L. GUILLET and A. PORTEVIN. Op. cit.
W. BRONIEWSKI. Compt. rend., 1913, vol. 156, p. 699.
" Sur les points critiques du fer."
L. GUILLET. Op. cit.
C. BENEDICTS.— Jour. Iron & Steel Inst., 1914, vol. 89, p. 407.
" Experiments on the Allotropy of Iron : Behaviour of Ferro-
magnetic Mixtures : Dilatation of pure Iron."
O. W. STOREY. — Trans. Am. Electroch. Soc., 1914, vol. 25, p. 489.
" A Microscopic Study of Electrolytic Iron."
J. COURNOT.— Compt. rend., 1920, vol. 171, p. 170.
" Sur le recuit du fer electrolytique."
(For other references, v Sub-sect, e.)
(d) Electrical and Magnetic Properties.
C. F. BURGESS and A. H. TAYLOR.— Trans. Am. Inst. Electrical' Eng ,
1906, vol. 25, p. 459; Electroch. and Met. Ind., 1906, vol. 4, p. 208.
''' The Magnetic Properties of Electrolytic Iron."
B. NEUMANN. — Stahl u. Eisen, 1914, vol. 34, p. 1637.
" Magnetische und mechanische Eigenschaften reinsten Elektro-
lyteisens."
G. K. BURGESS and I. N. KELOBERG. — Jour. Wash. Acad., 1914 vol 4
p. 436.
^f Electrical Resistance and Transformation Points of Pure Iron."
E. GUMLICH. — Stahl u. Eisen, 1921, vol. 41, p. 1249.
" Die magnetische Eigenschaften von Elektrolyteisen."
(e) Thermo-Electric Properties.
C. BENEDICTS.— Jour. Iron and Steel Inst., 1916, vol. 93, p. 211.
" A New Thermo-electric Method of Studying Allotropic Changes in
Iron and other Metals."
G. K. BURGESS and H. SCOTT.— Jour. Iron anl Steel Inst., 1916 vol 94
p. 258.
" The Thermo-electric Measurement of the Critical Ranges of Pure
Iron."
50
3. Structure
C. F. BURGESS and 0. P. WATTS. — Trans. Am. Electroch. Soc., 1906,
vol. 9, p. 229.
" A Microscopic Study of Electro-deposits."
0. W. STOREY.— Trans. Am. Electroch. Soc., 1914, vol. 25, p. 489.
" A Microscopic Study of Electrolytic Iron."
W. E. HUGHES.— Jour. Iron and Steel Inst., 1920, vol. 101, p. 321.
" Some Defects in Electro-deposited Iron."
W. E. HUGHES.— Trans. Far. Soc., 1921.
" The Forms of Electro-deposited Iron and the Effect of Acid upon
its Structure."
W. E. HUGHES. — Jour. Iron and Steel Inst., 1921, vol. 103, p. 355.
" Slip-lines and Twinning in Electro-deposited Iron."
Cowper Coles, J. Escard, and J. R. Cain and his co-workers show micro-
graphs of electrolytic iron, but the structure of the deposited iron is not
made the subject of study.
4. Applications
S. COWPER COLES.— Op. cit. (v. I. 4, sup.).
C. F. BURGESS.— Trans. Am. Electroch. Soc., 1911, vol. 19, p. 181.
" Electrolytic Refining as a Step in the Production of Steel."
L. GUILLET. — Jour. Iron and Steel Inst., 1914, vol. 90, p. 66.
" Electrolytic Iron, its Manufacture, Properties, and Uses."
J. ESCARD. — Op. cit. (v. I, 2, sup.).
W. E. HUGHES. — The Engineer, 1920, vol. 130, p. 350.
" Electro-deposited Iron : Its Value for Engineering Purposes."
W. E. HUGHES. — The Electrician, 1920, vol. 85, p. 530.
" The Industrial Future of Electro-deposited Iron."
C. P. PERIN and D. BELCHER.— Mining and Metallurgy, 1921 (Dec.),
p. 17. " Commercial Production of Electrolytic Iron."
The publications of von Jacobi, R. Lenz, and W. Roberts-Austen, all
citfe-d above, should be consulted. The deposition of iron for repair work
is described in Machinery, 1920, vol. 27, p. 381.
III. WORKS OF REFERENCE
The following works of reference contain useful general information
on the electro-deposition of iron. It is unfortunately the case, however,
that they are, most of them, out of date.
E. JORDIS.
" Die Elektrolyse wassriger Metallsalzlosungen," pp. 63 to 66.
(1901, Knapp-Halle).
A. WATT (Ed. A. PHILIP).
" Electro-plating and Electro-refining of Metals," p. 348 et sq.
(1902, Crosby, Lockwood & Son.)
B. BLOUNT.
" Practical Electro-chemistry," pp. 281 to 283. (1906, Constable.)
M. SCHLOTTER.
" Galvanostegie, 1 Teil. Uber elektrolytische Metallniederschlage,"
pp. 81 to 101. (1910, Knapp-Halle.)
W. G. MCMILLAN (Ed. W. R. COOPER).
" A Treatise on Electro-metallurgy," pp. 230 to 236, and p. 289.
(1910, Griffins.)
A. J. ALLMAND.
" The Principles of Applied Electrochemistry," pp. 121, 229 et sq.,
and 316. (1912, Arnold.)
F. FOERSTER.
" Elektrochemie waeseriger Losungen." (1915, Barth-Leipsic.)
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