THE EFFECT OF SUPERPOSED ALTERNATING CUR- R E N T ON T H E DEPOSITION OF ZINC-NICKEL ALLOYS.

BY HERBERT CHARLES COCKS.

Received 8th Feebruary, I 9 2 8.

The electrodeposition of nickel from aqueous solution is always accompanied to some extent by hydrogen discharge. For a constant current density, the current efficiency for nickel deposition increases as the (H') of the electrolyte decreases, and at a fixed (H'), it increases with rise of current density within the range of 0.2 to 1.5 amps./dcm.2.1

The large polarisation which occurs during the deposition of nickel is due, mainly, to the irreversibility of its deposition and, to a less extent, to hydrogen overvoltage.2 Both the reaction-resistance to nickel deposition and the hydrogen overvoltage increase with rise of current density and become smaller with rise of temperature. The increase of the reaction- resistance to nickel deposition with increase of current density causes a reduction in the current efficiency for the deposition of the metal with rise of current density, while the increase of hydrogen overvoltage with current density has the opposite effect.l

If an agent, such as superposed alternating current reduces both the reaction-resistance and the hydrogen overvoltage, its effect on the former will tend to cause an increase in the current efficiency for nickel while its action on the latter will tend to the reverse effect. In both cases, however, it should make the deposition potential less negative.

Zinc, on the other hand, is a metal which exhi bits very little irreversibility of deposition, but for which the hydrogen overvoltage is high. The de- position of zinc from strongly acid solutions is dependent on the latter phen~menon .~ Increase of temperature lowers hydrogen overvoltage 5 nd thus favours hydrogen evolution, while zinc deposition is favoured by high current density and a large metal ion concentration.2 At a current density of 2 - 5 amps./dcm.2, the current efficiency for zinc only increases by about 2 per cent. with a change i n p ~ of the electrolyte of from I to 4.4

At a high temperature, and using solutions of identical metal ion and hydrogen ion concentrations, a low current density would be expected to favour the deposition of nickel relatively to that of zinc, whilst a high current density should have the opposite effect. This conclusion is co~i- firmed by the results of experiments on the electrolysis of feebly acid solutions of mixtures of nickel and zinc sulphate~.~

At low current densities the deposits consisted almost entirely of nickel and contained very little zinc, the cathode potentials approximating to the deposition potential of nickel. As the current density was raised, the zinc content of the deposit was found to increase very slowly, then, at a certain current density, it suddenly became greater than that of the nickel and the

1 M. R. Thompson, Trans. Amer. Electrochem. Soc., 41, 349 (1922). 2 Allmand and Ellingham, '( The Principles of Applied Electrochemistry," p. 335. ZTainton, Trans. Amer. Electrxhem. SOC., 41, 388 (1922 . 4 Frolich, Trans. Amer. Electrochem. SOC., 49, 285 (19261. 5 Foerster, Elektrochemie wdssseriger Losrcngen, p. 375 (new edn.), 2. Elektrochem.,

22, 96 (1916). 348

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H. C . COCKS 349

deposition potential approximated to that of zinc. Working at 80" C., with a solution of the composition, o-gN. ZnSO,, o-gN. NiS04, O-OIN. H2S04, the deposit contained 70-80 per cent. of nickel at a current density of 0.01 amps./cm.2, eH being - 0'5 volt, while it contained 20-30 per cent. of nickel at 0.02 amps./cm.', e H then being about - 0 - 7 volt.

The effect of superposed A.C. on the deposition of nickel has been investigated by Kohschutter and Schodl,6 who found that the polarisation observed during deposition with D.C. alone was greatly reduced but not to zero. They found no increase in the current efficiency for nickel deposition when low ratios were used, but a decrease with high ratios. Isgarischew and Berkrnann,7 have studied the effect of superposed A.C. on the polarisation curves for nickel deposition. With a solution of NiSO, of very low (He), A.C. of 5 0 cycles had very little effect. When the D.C. was kept constant and the A.C. was gradually increased during deposition from a solution of nickel ammonium sulphate of higher (He), they found that the polarisation first increased, passed through a maximum, and then decreased. Under the same conditions the polarisation was always decreased when a solution o much higher (H.), viz., N . NiSO,, o- IN . H,SO,, was used. They came to the conclusion that the chief effect of A.C. was on the hydrogen dis- charge accompanying the metal deposition. They attributed the increase of polarisation to a greater difficulty of hydrogen discharge during the cathodic pulses of the current and the reduction of polarisation to the depolarising action of oxygen liberated by the reverse current.

Hydrogen overvoltage is considerably reduced by the superposition of A.C. Goodwin and Knobel.8 observed that, with a sufficiently large ratio of A.C. : B.C., hydrogen was evolved at average potentials more positive than the reversible vaiue. Here, also, the depolarising effect of oxygen liberated during the anodic pulses of the current is considered to be the cause of the reduction of the irreversibility of the electrode proce~s.~

The small polarisation observed in zinc deposition is also reduced by the superposition of A.C. The decrease of the polarisation has been ascribed to a reduction in the concentration of atomic hydrogen on the surface layer of the cathode.1°

'The work described in this paper is an investigation of the effect of superposed A.C. in a case where Zn", Ni" and H' are discharged simul- taneously, i.e., the deposition of the alloy zinc-nickel.

The effect was larger the greater the ratio A.C. : D.C.

Experimental.

Experiments were first carried out with the electrolyte 0.5N. ZnSO,, o-gN. NiSO,, o * o ~ N . HzS04, at 80" C., using D.C. only, but although evi- dence of a break in the composition-current density curve was obtained, the results did not seem to be reproducible. Indications were obtained that the deposits were not wholly metallic. (I< during the deposition of zinc or nickel, the (H') of the electrolyte adjacent to the cathode becomes too low, hydroxide tends-at least at ordinary temperatures-to become included in the deposit.) It was found that a similar electrolyte containing ammonium sulphate gave more reproducible results.

About 5 litre of the electrolyte, o - 5 N . ZnSO,, o - g N . NiSO,,

6HeZv. Chim. Acta, 5 , 593 (1922).

lo Allmand and Cocks, Proc. Roy. SOL, A, 112, 259 (1926).

7 2 . EZektrocherrt., 31, 180 (1925). Trans. Amcr. Electrochtm. SOC., 37, 617 (1920). But see Jones, ibid. , 41, 151 (1922).

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350 DEPOSITION OF ZINC-NICKEL ALLOYS

0-5N. (NH,),SO,, O-OIN. H,S04, was used for each experiment. I t was maintained at 80 & 1' C., in a beaker which served as electrolysis vessel. The platinum cathode, to which a platinum wire was welded, had a total area of 35.6 cms3 In the experiments in which the composition of the alloy was determined, both sides of the cathode were used and it was sus- pended by means of the wire from a terminal which passed through a sup- port fitted on the beaker. The cathode could thus be removed conveniently for weighing. The deposits were dried at I 10' C. to constant weight and the nickel was determined gravimetrically by means of dimethyl glyoxime. Two zinc and two nickel anodes, placed in small porous pots filled with solutions of the respective sulphates, were used. They were all connected electrically and disposed symmetrically around the cathode. By this means, at least some of each metal which left the solution was replaced and the current should have been fairly evenly distributed at the cathode.

Only about 0-2 to 0.4 g. of alloy was deposited in each experiment and the electrolyte was very frequently replaced by an unused portion.

The alternating current was obtained from a I 6-pole alternator. With the arrangement shown (Fig. I) , although a leak of A. C. into the D. C. part

of the circuit and vice versa occurs, Moving Coil Hot Wire the readings of the hot-wire and

moving-coil ammeters in series with the cathode only indicate the current which passes through the electrolysis cell. The moving-coil instrument gives the value of the D. C. only, while the reading of the hot-wire ammeter is the square root of the sum of the squares of the A.C. (in r.m.s. amperes) and

In the experiments in which the average cathode potential was measured, the wire on the cathode was sealed into a glass tube and

connection made by mercury. Only one side of the cathode was used, the back being insulated by baked shellac varnish : it was also pressed up against a glass plate. These changes were made so that a Luggin tube could be pressed up against the cathode and so that a good electrical connection could be made to the latter. The electrical connections were similar to those shown in Fig. I, but only one zinc and one nickel anode were used and a lead was taken directly from the cathode to the potentiometer. One end of the Luggin tube was pressed against the cathode and the other dipped into coZd electrolyte contained in a small vessel. This was conp.lected (just before taking readings), by an inverted U-tube filled with the solutions, to another small vessel filled with 3N. KC1. Into the latter dipped the side-tube of a normal calomel electrode. The purpose of this arrangement was to prevent diffusion of the hot electrolyte into the 3N. KC1, and to avoid a junction of a hot electrolyte with a cold, dissimilar one.

In all cases the alloy deposited in one experiment was dissolved off the cathode before it was used for the next. The volume of the electrolyte was always maintained by the addition of hot water. Throughout all the ex- periments small quantities of N. H,S04 were added at intervals of 5 or 10 mins., the quantity added being that estimated to maintain the acidity at O*OIN In some cases this was successful but in others it was not. In

Ammeter. Ammeter.

FIG. I.

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H. C. COCKS 351

extreme cases the acidity at the end of an electrolysis was found to be O-OZN; or had dropped to zero as estimated by titration using congo red as

I. D.C.

11. A.C. : D.C. = 0.46 : I.

111. A.C. : D.C. = 0.75 : I.

IV. A.C. : D.C. = 1-61 : I.

.V. A.C. : D.C. = 2.83 : I.

VI . I L I I A.C.:D.C.

0.025 o.030 = 3-87 : I. 0'010 0~0150 0'020

Direct Current Density, Amps./Cm.2

FIG. 2.

indicator. I t would, no doubt, have been much better to have used a well- buffered electrolyte similar to that employed by Glasstone and Symes in

90 Io0 I

their work on the electrode- position of iron- nickel alloys.ll 'The electrolyte was well stirred in all experi- ments.

The visual phenomena ob- served during e l e c t r o l y s i s may now be b r i e f l y d e - scribed. When the currents w e r e f i r s t switched on, the cathode became com- pletely covered with bubbles and then hy- drogen w a s

0'010 0.0150 0'020 0.025 0.030 Direct Current Density, Amps./Crna2

FIG. NICKEL (Curves LVI. as in Fig. 2).

evolved in quantities which varied according to the D.C. density and the

l1 Trans. Farad. SOL, 23, 213 (1927).

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352 DEPOSITION OF ZINC-NICKEL ALLOYS

ratio A.C. : D.C. Usually the number of bubbles which adhered to the cathode gradually decreased as the experiment proceeded; in a few cases, none remained at the end. I t is of interest that in one case (composition experiment, D.C. only, 0.02 amps./cm.2) the deposit appeared to be quite

0'010 0.0150 0'020 0.025 0.030

Direct Current Density, Amps./Cm.2 FIG. ZINC (Curves I.-VI. as in Fig. 2.)

bright diring the first 2 0 mins. of the electrolysis and then suddenly became dull- white and re- mainedso. As a rule, the de- posits obtained at potentials below - 0.7 volt ( z k , be- low a D.C. density of 0.02 amps./cm.2, for D. C. alone and for A.C. : D.C. = 0.46 : I, 0.75 : I, and 1-61 : I, and in all cases for A.C. :D.C =

2.83 : I and 3-87 : I) were bright, or, in a few cases, light grey. obtained above - 0.7 volt were usually dull-white.

Those

&sults of Current Efficiency Experiments.

The results for D.C. and for a superposed A.C. of 40 to 5 0 cycles per The percentage current efficien- sec., are given graphically in Figs. z to 6.

cies are calcu- lated on the basis of the 6o D.C. compon- a ent of the 2 50 current as mea- .$ sured by the 40 moving - coil c,

30 meter. Those for nickel are 5 2o calculated from the weight of I. nickel deter- mined by ana- 0

lysis and those 0'010 0.0 I 50 0'020 0.025 0.030

for zinc from Direct Current Density, Amps./Cm.a

zinc obtained by subtracting the weight of nickel from the total weight of the deposit. I t is recognised, however, that the deposits probably contained traces of

the weight of FIG. 5.-HYDROGEN (Curves I.-vI. as in Fig. 2).

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H. C. COCKS 353

hydroxide. The difference between the sum of the percentage current efficiencies for nickel and zinc thus arrived at and IOO is taken as that for hydrogen.

Deposition Potentials. The current was allowed to flow from 15 to 30 mins, (according to the

current density) before the first reading of the average cathode potential was taken. The potentials given below, all of which are on the hydrogen scale, are each the mean of these seven readings. In some cases the potentials measured during the 4 hour were constant but in other cases they varied. The fre- quency was from 40 to 50 cycles per sec., and the tem- perature 80

Then seven readings were taken at intervals of 5 mins.

0.500

IO C. in all o.560 cases. T h e r e s u l t s are 4 shown graphi- $ callyin Fig. 6. $ O n l y o n e $0'620 curve is drawn '2 for A. C. : B. C. = 0.46 : I and 0.75 : I, 2 as the points % 0.680 lie close to- Q)

As an ex- > ample of an 4 experiment in 0.740 w h i c h t h e readings were very constant, one in which D.C. only was 0.800 used, at a den- sity of 0.025 Direct Current Density, Amps/Cm.2 a m p s ./c m. 2. FIG. 6 (Curves I.-VI. as in Fig. 2).

may be given : here the potential only varied from - 0'775 to - 0.773 volt. As one in which they were variable, the experiment in which the D.C. density was 0.0125 amps./cm.2, and the ratio of A.C. : D.C was 0.75 : I where the potential varied from - 0.593 to - 0.559 volt, may be taken.

The following experiments were made in order to try to confirm the small differences in potential (indicated by the curves in Fig. 6 ) , observed with the different A. C. : D. C. ratios at low D. C. densities and with ratios of 1-61 : I and under at D.C. densities approaching 0.03 amps./cm.2. In these experiments the deposit was not removed from the cathode between the various steps.

(a) EZectro&!yte, 0.5 N . NiS04, 0.5 N . ZnS04, 0 - 5 N . (NH4)aS04,

o : I, 0.46 : I etc., to 3-87 : I.

a"

u

gether. h

0'010 0.0150 0'020 0.025 0.030

0.01 N . H,SO,. A.C.:B.C., B.C. density, 0.010 amps./cm.2

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354 DEPOSITION OF ZINC-NICKEL ALLOYS

0'010 '010 '012 j *or25 -0125 '015 '015 '0175 '0175

.a0225 -0225 '025 so25 '025 -03 0 -03 0 0.030

'020

D.C. only.

- 0-520 '531 '546

'555

368 ,668 '683 '767 -762 '774 '773 '751 '773 - 0'783

- - -

-

0.46 : I. 0.75 : I.

Ratio A.C. : D.C.

Id1 : I. 2-83 : I.

- 0'526 '507 '547 '529 -528 '583 '540 '575

'595

'602

- - - - -

'585 '635 - 0'655

3'87 : I.

With D.C. only the potentials were :- after 5 mins. - 0.522 volt eH

9 , 1 0 ,, '524 9 ,

9 , I 5 ? 9 '526 9 ,

The A.C. was then superposed for 15 mins. at each ratio and readings taken at 5 mini intervals. No definite change in potential could be observed at any ratio-the potential only fluctuated about the mean value of - 0.524 volt. The extreme values were observed with A.C. : D.C. = 0.46 : I, these being - 0-5 18 volt after 5 mins., and - 0.534 volt after 10 mins. After the A.C. had been switched off for 15 mins., the D.C. potential was - 0.525 volt.

0-0 I amps./cm.2 (6) Electrolyte, N. NiSO, 0-5 N . (NH4)2S04. 0.01 N. H2S04. D. C. density, Readings were taken at 5 min. intervals at each ratio. Inlexperi-

ment ( I ) the potentials became more positive by about 5 millivolts during the 15 mins. for each ratio, but in (2) they did not-they more or less varied about a mean value. In (I) the ratios were altered in the order 0.46 : I, 1-61 : I, 0.75 : I, but in (2) they were changed in the order shown below :-

A.C. : D.C.

0.46 : I G'75 : I + 42 ,, 1-61 : I + 7 5 ;,

0.75 : I + 23 9 ,

1-61 : I + 85 9 ,

9 ) final 9 , Y , - 0'473 9 ,

Change from mean initial D.C. value.

+ 13 millivolts. (1)

(2) 0.46 : I + 9 Y J

- 0'505 volt. (1)

( 2 )

Mean initial D.C. value

- 0.502 ,, > 2 final 3, 3 , - 0.481 ,,

Mean initial D.C. value

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H. C. COCKS 355

These results under (b) agree qualitatively with those of Isgarischew The and Berkmann

superposed current has a depolarising action. for the electrolyte N. NiS04, 0-1 N. H2S04.

(c) EZectrodyte, N . ZnS04, 0.5 N . (NH4)&304, 0.01 N. H2S04. D. C. density, 0.03 amps./cm.2

Here the cathode potential more or less varied about a mean value during the 1 5 mins. for each ratio.

A.C. : D.C. 0-46 : I 0.75 : I + 1 ,Y

1.61 : I + 9 J Y

Change from mean initial D.C. value. - z millivolts.

Mean initial D.C. value - 0.839 volt. Y , final J , Y , - 0.828 ),

The contrast between the results under (b) and (6) and those for the mixed electrolyte, under (a), is interesting.

Discussion.

With regard to the results with D.C. alone, attention is directed to the large increase in the current efficiency for hydrogen at the “critical current density” of about O - O I ~ amps./cm.2 I t appears that, as the current density is raised, the reaction-resistance to nickel deposition is increased and hydrogen evolution becomes greater. Then, as the current density is still further increased, the hydrogen overvoltage rises and zinc deposition is proportionately increased until it becomes the main process.

The explanation which will be given of the influence of superposed A. C. on the deposition of the alloy is based upon :-

(a) The known depolarising effect of the superposed current on irre- versible electrode processes when its strength relative to that of the B.C. component is such that reverse current flows.

(b) An assumption as to the nett result of the periodic increases of current density above that of the D.C. component. This assumption is primarily made ad hoc in respect of the present phenomena. I t may, perhaps how- ever apply to other cases in which there is a sudden change in the electrode process with a small change of current density.

The data for A.C. : D.C. = 0.46 : I, where the current is a pulsating direct one, will be considered first. I t is upon the results obtained with this current, rather than with B.C. alone, that the elucidation of those obtained with other ratios of A.C. : D.C. is based. When the D.C. density is 0.01 amps./cm.2, the current density at the minima of the pulses is 0.004 amps./cm.2, and that at the maxima 0.016 amps./cm.2 At the “ critical direct current density ” of 0.015 amps./cm.2, the cathode process is in a state of instability. I t is assumed that the periodic rise of current density above this critical value when pulsating current of D.C. component 0.01 amps./cm.2 is used, results in the production of a state of affairs similar to that due to an unvarying D.C. at a density of 0.016 amps./cm.2, but that the cyclic decreases of current density below the “ critical current density” do not cause a return to the conditions obtaining below the latter value.

If the conditions with the pulsating current were the same as when using a D.C. density of 0.016 amps./cm.2, the current efficiency for nickel would be reduced from 79 to 28 per cent., that for zinc would be increased

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356 DEPOSITION O F ZINC-NICKEL ALLOYS

from 4 to 33 per cent. and that for hydrogen raised from 17 to 37 per cent. The observed current efficiencies with pulsating current were :-Ni 26 per cent., Zn I 7 per cent. and H, 57 per cent. There is thus some hydrogen evolution at the expense of zinc deposition, hence the percentage of nickel in the deposit is greater than that obtained with an unvarying D.C. of 0.016 amps./cm.2

As the density of the D.C. component of the pulsating current is increased, still keeping the same A.C. : D.C. ratio, the conditions become more and more like those for steady D. C. densities above 0-01 5 amps./cm.2, and when the D.C. component is at a density above the “critical value,” the periodic variation of current density has no longer an appreciable effect. The current efficiency for nickel is hardly affected by a further increase of steady D. C. density, hence pulsating current gives the same current efficiency for nickel as the unvarying current. For zinc, the current efficiency rises according to the value of the density of the D.C. component in almost complete agreement with its rise with the density of D.C. alone. (Only one curve is drawn in Fig. 4 for D.C. alone, A.C. : D.C = 0.46 : I and 0.75 : I.) (Only one curve is drawn for the three currents in Fig. 5.) I t follows, therefore, that above the “ critical direct current density,” the composition of the deposits .obtained with pulsating current follows the curve for D.C. alone (Fig. 2 ) .

The slight increase in polarisation above the D.C. value, observed with A.C. : D.C. = 0.46 : I and 0.75 : I (see Fig. 6 ; one curve is drawn for both ratios), below the ‘( critical current density ’’ is qualitatively in agree- ment with the higher current efficiency for zinc, but there is no apparent reason for the slight increase observed above the ‘‘ critical current density.”

As the strength of the A.C. relative to the D.C. is increased, the periodic increases of current density at the maxima occur in greater degree. I n the case of A.C. : D.C. = 2.83 : I and 3-87 : I, the average current density at the cathodic pulses is always above the “ critical current density,” hence the assumption made under (b) above, is even more justified. When A.C. : D.C. > I / Ji : I, ie., > 0.707 : I, the depolarising effect of A.C. (see (a) abovej will take place. This is attributed, as has been done by other investigators, to oxygen discharge during the anodic pulses of the current. As was mentioned above, it has been shown that superposed A.C. largely reduces the polarisation for nickel deposition, that with a sufficiently high ratio of A. C. : D. C., it completely eliminates hydrogen overvoltage, and that it has a slight depolarising effect in zinc deposition.

In the deposition of the alloy, when A.C.1: D.C. = 0-75 : I, there is a slight depolarising effect due to the small anodic pulses. When the D.C. component has a density of 0.01 amps./cm.2, the current efficiency for nickel is slightly increased above that for A.C. : D.C. = 0.46 : I while the zinc and hydrogen current efficiencies are barely affected (see Figs. 4 and 5 ; one curve is drawn for A.C. : D.C. = 0.46 : I and 0-75 : I). The deposit thus contains a little more nickel than with A.C. : D.C. = 0.46 : I but on account of effect (b) above, less nickel than with D.C. alone. Above the ‘‘ critical current density,” zinc deposition is the main process, hence the effect of the slight depolarising action of the anodic pulses is inappreciable. The composition of the alloy thus almost follows the D.C. curve.

With A. C. : D. C. = 2 ‘83 : I , the depolarising effect is much greater than with A.C. : D.C. = 0.75 : I and the retardation to nickel deposition is slightly reduced, especially below the ‘( critical current density,” where hydrogen overvoltage is low. Above the “ critical current density,” hydrogen overvoltage which with A.C. : D.C. = 0.75 : I was high, is de-

That for hydrogen decreases in a similar manner.

.

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H. C. COCKS 35 7

polarised and hence the current efficiency for hydrogen is increased and that for zinc decreased. The percentage of nickel in the deposit is thus greater than with A.C. : D.C. = 0-75 : I both below and above the '' critical current density."

The results for A.C. : D.C. = 1-61 : I are intermediate between those for A.C. :D.C. = 0-75 : I and 2.83 : I. At A.C. :D.C. = 2.83 : I, the depolar- ising effect of A.C. has apparently almost reached its limit, hence a further increase of A.C., i.e. when A.C. : D.C. = 3-87 : I only results in a slightly greater depolarisation, which is, however, distinctly noticeable when the density of the D.C. component is above about 0.025 amps./cm.2

The deposition potential-current density curves (Fig. 6) indicate a slight increase of polarisation for a change of A.C. : D.C. of from 2.83 : I to 3-87 : I. This may be due to experimental error. I t will be noticed however, that while the composition-current density curves for these two ratios diverge as the density of the D.C. component is increased, those for the deposition potential-current density, converge.

A comparison of the curves in Fig. 2 with those in Fig. 6 , will show that alloys of the same composition have been produced at different deposi- tion potentials according to the type and strength of the current used in their deposition, e.g., the potential of the alloy deposited at a D.C. density of 0.02 amps./cm2, with A.C. : D.C. = 1-61 : I is about 0.1 volt more negative than that of the one deposited at a D.C. density of 0.03 amps./cm2., with A.C. : D.C. = 2-83 : I, although both had the composition, Ni 36 per cent., Zn 64 per cent. The alloy deposited at the more negative potential was dull-white while the other was mainly bright.

Before concluding, it may be mentioned that three experiments were made with superposed A.C. of higher frequency with the following results :-

FREQUENCY 400-450 CYCLESISEC. A.C. : D.C. = 2-83 : I.

I I Current Efficiencies, Per Cent. D.C. Density Wt. of Nickel in amDs/cma. I Deposit, Per Cent. I I

I I I Ni. Zn. - . _ .

I I I 0'010 0'020 0.030

60'3 33'8 27.1

32.8 23'5 26'4

19'4 41'4 64'5

Goodwin and Knobel,s working between the limits of 2 and roo cycles/sec., found that the effect of A. C. in reducing hydrogen overvoltage fell off with rise of frequency. The depolarising effect is less at 430 than at 43 cycles/sec., in the case of the deposition of nickel alone (Unpublished results of the Author), while the effect of A.C. on the polarisation for zinc deposition is hardly affected by a change of frequency of from 50 to 450 cycles/sec. (Allmand and Cocks).lo As a consequence of these observa- tions, the results for the deposition of the alloy with superposed A.C. of 400 to 450 cycles appear to fall into line if it is assumed that the results for A.C. : D.C. = 0-46 : I at 4 0 0 to 450 cycles are the same as those at 40 to 50 cycles. As the data for this ratio at the higher frequency are not available, the effect of frequency will not be discussed further.

The Author wishes to express his thanks to Professor A. J. Allmand for suggesting this investigation and for his help and interest in all stages of the work. His thanks are also due to Mr. R. H. D. Barklie, M.Sc., for

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358 DISCUSSION

suggestions which led to the development of the explanation of the results.

Summary. (I) An outline of the phenomena attending the cathodic deposition of

zinc and nickel separately and together is given. (2) An investigation of the deposition of the alloy, zinc-nickel from

an acid sulphate electrolyte using D.C. and superposed A.C. is described. (3) The variation of composition of the alloy and the average deposition

potential with D. C. density and the ratio A. C. : D. C is recorded. (4) The results are discussed, and an explanation of the effect of super-

posed A. C. is given, based upon :- (a) The known depolarising action of the superposed current on

(6) The assumed nett result of the periodic increases of current irreversible electrode processes.

density above that of the D.C. component.

University of London, King's CoZZege.

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