THE ELECTRODEPOSITION OF ZINC FROM SULPHATE SOLUTIONS.

B Y ABRAHAM LINCOLN MARSHALL, PH.D.

( A Paper read h$ore the FARADAY SOCIETY, Mondaj, Jzdy 6fh, 1925, PROFESSOR F. G. DONNAN, C.B.E:, F.R.S., PRESIDENT in fh Chair.)

First Received, March IS< I 923.

Received in present form, June 3rd, I 9 2 5 ,

In the cathodic deposition of zinc from aqueous solutions it is important to consider carefully the conditions under which a good deposit can be obtained. Since zinc is far more electropositive than hydrogen one would expect the metal to dissolve readily in acids and that cathodic deposition would be impossible in acid solutions. Measurements of the discharge potential of hydrogen at a zinc electrode from normal sulphuric acid solutions show a value of 0.7 volt2 at !ow current densities. Conse- quent upon this high overvoltage for hydrogen on zinc the latter is pre- ferentially deposited at the cathode from a solution containing both zinc and hydrogen ions unless the zinc ion concentration is extremely low. If, however, the solution contains small amounts of other metal ions, which, when deposited at the cathode, have a lower hydrogen overvoltage than zinc, then hydrogen will be deposited from these solutions more readily than zinc.

Hansen3 has made a comprehensive study of the deposition of zinc from sulphate solutions of low acidity at a current density of 0 . 0 2 8

amps./crn2 Assuming there are two relevant factors in zinc electrolysis : (I) rate of deposition of zinc depending on current density ; (2) corrosion at rate independent of rate of deposition, he calculates from his data the rate of corrosion as the difference between the amount of zinc deposited and TOO per cent. current efficiency. H e also states that the rate of corrosion of cathode zinc doubles for each 22" rise in temperature. Acid concentration is the important variable in his work and the zinc concentra- tion is about 0.6 normal. H e gives some data for efficiency of deposition from solutions of low zinc content and high acidity which are reproduced in Table I. These are for a temperature of 28" C. and a current density of 0.028 amps./cm.:!

1 The standard potential of zinc referred to hydrogen as zero is Zn (s), Zn++ ; E= 0.76. Newberry, J. Chtm. SOL, 109, 1051 (1916).

3 Tram. Am. Iwst. Min. Eng., 60, 256 (1919). 297 2 0

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298

72'3 68.8 64'5 34'7

I

THE E1,ECTRODEPOSITION OF ZINC

TABLE I.

Zn. Conc.

0'37 N 0.3 I 0'2 3 0.15

Acid Conc. I Per Cent. Eficiency.

1 I I 1

2'94 N 3.02 3.20 3'39

Tainton 1 describes a process for depositing zinc with a current density of 0.06-0*33 arnps./cm.:! from solutions 4-28N zinc sulphate and from I-6N sulphuric acid. The zinc concentration is reduced to z'45N by electrolysis. Claims were made to obtain good deposits in the presence of electronegative impurities.

Pring and Tainton2 have studied the variation of current efficiency with current density.

The work described in this paper was carried out to obtain a closer insight into the effect of temperature on the characteristics of a zinc electrode at which zinc is being deposited and also to study the variation of current efficiency with temperature.

Experimental. The solutions were prepared from zinc oxide of a high degree of purity

and redistilled sulphuric acid. In the earlier experimental work four litres of solution of the desired concentration were circulated through a cell of 500 C.C. capacity at the rate of a litre every five minutes so that the com- position of the solution remained practically constant for the duration of the experiment. The arrangement employed consisted of a large aspirator placed about three feet above the cell from which the solution passed through a large glass coil immersed in a thermostat, to control the cell temperature, and thence to the cell in the same thermostat. The solution was returned to the bottle by an air lift which kept the solution level in the cell constant.

I n each experiment 3 to 4 gms. of zinc were removed from the solution by electrolysis and the composition of the electrolyte brought back to its original value by the addition of an amount of zinc oxide corresponding to the zinc extracted.

I n later experiments, working with impurities, it was not found feasible to use such a large quantity of solution for an experiment. Tests on the pure solution using a cell of 850 C.C. capacity, which was well stirred, gave practically the same results as with the larger volume of solution. In this case the change in composition of the solution with regard to its zinc content was 8 to 10 per cent.

The temperatures given in each case are those of the zinc cell, which were maintained by immersing it in a water thermostat. The work was carried out at temperatures ranging from ~ o ~ - g o ~ C. and it was found that in most cases there was a considerable difference between the working temperature of the cell and that of the thermostat.

* Current was obtained from a T O volt 144 ampere-hour battery which was connected across the D.C. power line and helped to smooth out the voltage fluctuations.

A copper coulometer was used to determine the amount of current.

B.P., 11335 (1915). '7. Chcm. SOC., 105, 717 (1914).

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FROM. SULPHATE SOLUTIONS

Weight Copper (Found). Time. Current.

299

Weight Copper (Calculated).

It consisted of a circular platinum gauze cathode which was rotated at about 1000 r.p.m. and a heavy sheet copper anode. The latter was ring- shaped from a sheet of copper 4 inches x 18 inches ; with a current of I 2 amps. the anodic current density was 0.03 amps./cm.2 The coulometer solution consisted of 125 gms. hydrated copper sulphate, 50 gms. con- centrated sulphuric acid and 50 C.C. alcohol in a liter of water.

Tests made on this coulometer using a standard Weston ammeter to measure the current gave the following results :-

10 mins 9 9

3 9

TABLE 11.

I j amps. 2'964 2'964 9 9

12 &ps. 1 2.369 2'371

The copper deposit was beautifully hard and coherent. The results show that this is a very convenient method for measuring even comparatively large currents.

In it was placed an aluminium cathode placed centrally between two peroxidised lead anodes. In the working cell at the current densities and acidities employed it was found that, at 20' C., the anodes disintegrated rapidly and large flakes of lead peroxide were observed floating about in the solution after continued electrolysis. Some of these would adhere to the cathode and zinc would plate over them in a leaf-like form having only firm contact with the cathode at one place. This of course caused a great roughening of the surface with continued electrolysis although the main deposit was very massive. At 40° C. and higher this phenomenon with the lead peroxide was not observed.

The zinc cell consisted of a rectangular-shaped battery jar.

Analytical Methods. The zinc in solution was determined by titration with standard ferro-

cyanide solution using a dilute solution of ammonium molybdate as an external indicator.

The acid was titrated with standard alkali using methyl orange as an indicator to a colour obtained with the same amount of indicator in a neutral zinc sulphate solution.

Check analysis agreed within 2 per cent. I t was always found necessary to filter the solution to remove suspended lead peroxide.

Manganese was determined by titrating a hot solution made neutral with an emulsion of zinc oxide with standard permanganate.

Ferrous iron was titrated with permanganate, and ferric iron by reduction with stannous chloride and titration with bichromate.

Antimony was determined by an adaptation of the Gutzeit arsenic method. Test papers sensitised with silver nitrate were used in place of the mercuric chloride papers for arsenic and it was unnecessary to add iron and stannous chloride, the small amount of antimony being sufficient to start very active corrosion of the zinc at the temperature (60" C.) used.

Copper was determined colorometrically with ammonia.

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300 THE ELEC'I'RODEPOSITION OF ZINC

Effect of Temperature, Composition and Current Density on Current Efficiency.

A series of measurements was made at 80" C. with a n acid zinc sulphate so!ution containing 0.98N zinc sulphate and 3'51N sulphuric acid, and the following efficiencies were obtained with increased time of deposition :-

TABLE 111.

Time in Amt. Zn. Deposited Minutes. ' in Time Interval.

1

11 1 2'456 2x 31 4 1 61 81 101 I21

141

2.267

2-281

4'502 4.601 4'5 54 4'478

2'221

4'607

Per Cent. Efficiency for Interval.

9I.5 92.8 93'5 93 '4 93'4 92'2 9 2.0

92'9 91.8

Total Amt. Zinc Deposited.

2'456 4'723 6.944 9'225 13'832 18.334 22Y35 27'489 3 1.967

Overall Efficiency.

91'5 92.1 92'5 92-8 92'9 92'7 92-6 92% 92'4

.____

The area of the cathode used was IOO sq. cm. and the current density 0.1 2 amps./cm2.

The deposit obtained was hard, bright and of a shiny silver white colour ; it was slightly rough with small hard nodules scattered over it which grew very slowly in size with increased time of deposition.

Similar results were obtained at 20' C. but the deposition was dis- continued after 80 minutes on account of the tendency to form flakes due to lead peroxide sticking to the electrode and zinc depositing over it.

After completing the above electrolysis the cathode was allowed to stand in the solution for 35 minutes and the loss of weight due to corrosion was 0.026 gm. A zinc surface which had been obtained by 2 0 minutes deposition from the same solution lost 0.009 gm. while standing for 55 minutes in the solution.

Experiments were made with this same solution at the same temperature and current density including a mercury interruptor in the circuit which broke the circuit about 2 0 0 0 times per minute, the current being on half the time ; with this running, the ammeter read 6 amps. while the steady current in use was 12 amps. With the interruptor running, the average of a number of experiments gave an efficiency of deposition of 94.1 per cent. From these results and the actual rate of corrosion of the zinc on standing in the solution it seems very improbable indeed that the current is wholly employed in depositing zinc and that the hydrogen formed is due to an after corrosion effect. I t would seem much more probable that the hydrogen and zinc are actually deposited together in amounts depending on their relative concentrations in the surface film of electrolyte and on the over- voltage.

The maximum variation in the zinc content of the above solution was 10 per cent. and the solution was kept at approximately constant composition by addition of zinc oxide from time to time in amounts equivalent to the zinc removed.

The results given in Figs. I and 2 are for depositions for 2 0 minutes from pure solutions and show the variations in current efficiency with temperature and current density. For Figs. I and 2 the composition of the solution was, zinc sulphate I -93N, sulphuric acid 3.5 IN. The results, however, are only

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FROM SULPHATE SOLUTIONS 301

in approximate tion with rising

agreement, but show the rise in current efficiency of deposi- temperature for the whole range of current density studied.

97 1 95 i

I 93 I /

20 30 40 50 60 70 80 go Temperature.

FIG. 1.--Composition of solutions : 1.g3N zinc sulphate.

Current density : A - 0.48 amps.lcm3. 3-5 I I” sulphuric acid.

x -0.24 ,, -0.12 ,,

0 - 0 - 0 6 ,,

At the stage of the work when these experiments were made large stocks of solution had not been made up and the solutions actually used were

m a

20 30 40 50 60 70 80 Temperature.

FIG. 2.-Composition of solution : x*gN zinc sulphate. 3.5 N sulphuric acid.

Current density : - 0’24 amps./cm2. x -0‘12 ))

0 - 0.06 9 ?

made from different specimens of zinc oxide. From the marked effect of small quantities of impurities in the electrolyte on current efficiency (vide znfra) it would seem that a probable explanation of these discrepancies lay

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302 THE ELECTRODEPOSITION OF ZINC

no0 40° 60° 80"

in the presence of a trace of some impurity in one of the solutions. In all the later work where large quantities of solution were kept in stock and where a large supply of zinc oxide was at hand the results of any series of experiments could be duplicated within the range of experimental error allowed.

The increase in current efficiency with rising temperature is much more marked with solutions of lower zinc content as will be seen from the following table :-

TABLE IV.

79'9 44'4 35'4 58'9 90'5 72'7 92.1 76'5

Temp.

1 Metal. 1 Conc. Acid.

! Ni ' 0.01 N

0.03 N E: 1 0'0075 N

I Composition.

Per Cent. Current Efficiency.

I

75O. 95O.

87

10. ~ ZOO.

I ~-

- 21 i 40 80 84 - 94 60 I 80 86

I I

97

--. -- 20 40 60 80

Temperature.

FIG. composition of solution : I - ~ N zinc sulphate. 0 - r6gN sulphuric acid. rn -3'51N 9 )

x -5'16N Y Y

Current density-0.12 amps./cm2.

The current density was 0.1 2 amps./cm.2 and the results are reproducible to about 0.3 per cent.

Foerster' gives the following table for the effect of temperature on the efficiency of deposition of nickel, cobalt and iron from normal sulphate solutions with a current density of o*oog amps./cm.2

Elektrochemie, p. 315.

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FROM SULPHATE SOLUTIONS 303

From these results it will be seen that these metals show this behaviour much more markedly than zinc.

95

75 I

20 40 60 80

Temperature.

FIG. 4.-Composition of solution : 0'95 N zinc sulphate. @ - 1.69 N sulphuric acid.

-5.16 N ,, x -3'55 N 9 ,

Current density-o*rz amps./cm2.

A study was made of the effect of varying both acid and zinc con- centration on the temperature-current efficiency relationship, and the results are given in Figs. 3 to 5 .

20 40 60 80

Temperature.

FIG. 5.-Composition of solution : 0.305 N zinc sulphate.

Current density-0.12 amps./cm?.

0 - 1-73 N sulphuric acid. El -3'57 N 9 9

Using the interruptor mentioned before at 2000 r.p.m. it was found that with a solution of 0.1 gN ZnS04 and 5 . IN H,SOI the efficiency of deposition was considerably increased.

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304 THE ELECTRODEPOSITION OF ZINC

TABLE V.

CURKENT DENSITY-3-12 AMPS. SQ. CM.

Per Cent. Efficiency. Temp.

I

I Steady. ] Interrupted.

I I 1 38'5 20"

5 I '0

_ _ I 40°

To obtain some insight into the cause of this increase in current efficiency with rising temperature, measurements were undertaken of the variation of the single potential of a zinc electrode with current density and temperature in sulphuric acid and acid zinc sulphate solutions.

Potential of the Zinc Electrode. The electrode used for the potential measurements was a cylindrical

rod cast from pure electrolytic zinc and plated with a smooth coherent deposit of zinc which gave it a silvery matt surface. A circular lead peroxide anode was used which was 5 inches in diameter and I + inch wide. The zinc electrode was 1 .5 cm. in diameter and 2 0 cm.2 area was exposed, the remainder being covered with rubber tubing which had previously been treated with hot acid.

A hydrogen electrode containing zN H,SO, was used as a comparison electrode, and it was checked from time to time against a standard mercurous sulphate electrode. The tube connecting the hydrogen electrode to the zinc one terminated in an extremely fine capillary, the end of which was bent upwards at an angle of 30' to the horizontal to prevent bubbles from the electrode collecting in the end. This capilIary was placed so that its end touched the zinc electrode tangentially, and being flexible it was kept closely pressed to the electrode by allowing it to support a portion of its weight. A side tube on the connecting tube was always filled with zN H2S04 and this maintained a very slow flow from the end of the capillary preventing any back diffusion of zinc to the electrode. It was found with this arrangement that very reproducible results could be obtained.

The measurements of potential were made using a slide wire bridge as a potentiometer and a galvanometer as a zero indicator.

A polished zinc surface was obtained on the electrode and a smooth adherent layer of electrolytic zinc was deposited on this from a solution containing o'g5N ZnSO, and 3*5N H,S04 at zoo and a current density of o* L z amps./cni,z, electrolysing for ten minutes.

Table VI. gives the potentials obtained in a 3N H,S04 solution with varying current density and temperature. All the values are negative, the potential being regarded as that of the electrode minus that of the solution and taking the hydrogen electrode as zero. No attention has been paid to liquid : liquid potentials.

Fig. 6 illustrates these results and compares them with those for a solution containing o*30N ZnS04 in the solution together with the acid.

I t was found that the potential of the zinc electrode decreased with increasing time of zinc deposition, probably due to an increase in the surface area. I t was always found in the electrolytic work that the bubbles of hydrogen evolved at the higher temperatures were much larger than those at the lower and tended to cling much more tenaciously to the zinc.

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FROM SULPHATE SOLUTIONS 305

The surface condition of the zinc also showed this as that deposited at 80" had little craters all over the surface where the zinc had deposited around the bubbles of hydrogen.

TABLE VI. POTENTIAL OF ZINC ELECTRODB.

Current Density Amps.lCm.2

0*400 0.300

0.150

0.075 0.050 0.040 0-030 0'020 0.015 0'010 0.005 0.0025 0.00 I 3

0'200

0'100

0'000

200.

1-36 1'33 1-29 1.26 1-23

1.18 1-16 1-14

1'2 I

1-12 1-10

1-02 1-07

0.98 0'94 0'79

Temperature.

4x0.

1-25 1-23 1'21 1'20 1-18 1-16 1-14 1-13 1'12 1-10 I '09 1-06 1-03 0.98 0'94 0.80

0-300

0'200

... 5 !z 13

z 2

U

i) 0'100

0.080

0.060 0.040 0'020

0'000

59'. I _ _ ~

1.25 1.23

1.18 1-16 1.15 1.13

1'20

1-12 1-10 1-09 1.07 1.05

0.97 0.92 0.81

1'01

40/r i: J i

7 8 O .

1'20 1-19 1-17 1-16 1-14 1-13 1'1 I 1-10 1-09 1-07 1-06 1-04

0.95 0'90 0.80

1'00

4007 i I 2oo I / I i

-0.8 -0.9 - 1-0 - 1.1 - 1'2 - 1-3

Potential of Zinc Electrode.

FIG. 6.-Composition of solutions : 0, x - 0 3 1 N zinc sulphate; 3.j1 N sulphuric acid. 8 , a - 3-51 N sulphuric acid.

The curves in Fig. 7 are all for 20' C. For each series of measurements the eiectrode was stripped down to the original zinc surface and then a coating of electrolytic zinc plated on as previously described so that the results are strictly comparable. They deal with the effects of adding

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306 THE ELECTRODEPOSITION O F ZINC

varying amounts of zinc sulphate to the 3'5N acid and also show the effect of the addition of a small amount of antimony.

Two possible explanations suggest themselves for the shape of the current density-potential curves. The governing factor in determining the potential of the zinc electrode is the concentration of zinc ions at the surface of the electrode. These are being continually removed by deposition on the electrode and replaced either by diffusion from the main body of solution or by a slow ionisation of zinc sulphate molecules. If the effect is due to diffusion, increased stirring, which cuts down the thickness of the diffusion layer, should decrease the electrode potential. This has been shown to hold in the electrolysis of copper sulphate solutions.1 I n these experiments there is twice as much hydrogen evolved during the electrolysis of a 0.30N ZnSO, solution as during that of a o-gsN one (both containing 3-4N H,S04). One would expect the increased gas evolution

0'300

0'200 Y .- fi n

0.080

0.060

0.040 0'020

0'000

-0.8 -0.9 - 1.0 - 1.1 - 1'2 - 1.3 -1'4

FIG. 7.-Composition of solutions : 3.51 N sulphuric acid. - 0-31 N zinc sulphate ; 0~00004 N Sb.

Q - I - ~ I N ,, + -0*031 N ,, x -0'93 N 7 9 -pure acid.

A - 0.31 N zinc sulphate.

to decrease the thickness of the diffusion layer and give a steeper current density-potential curve; this is not the case.

Le Blanc and Schick2 have shown by superimposing an alternating current on the direct one that, in the case of complex cyanides such as copper and zinc, the chemical reaction affording the metallic ions does not take place to bring about equilibrium instantaneously, and that there is an enhanced polarisation at the electrode due to this retarded reaction. Foerster has made a study of the current density-potential relationships for the iron group from sulphate solutions, and has found that the curves obtained were very similar to those for the complex cyanides. The re- tardation in the formation of ions may be due to the presence of some hydrated ion which only slowly decomposes to bring about equilibrium with the metallic ion Fe++(H,O),, = Fe++ + nH,O.

1 Sprent, Dissevtation, Dresden, 1910. Elektrociienait, 2nd Edition, p. 305.

22. phys. Chcnz., 46, 213 (1903).

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FROM SULPHATE SOLUTIONS 307

In the case of zinc the reaction (whether it be the actual ionisation of the zinc sulphate or the breaking up of a hydrated ion) proceeds much more rapidly than in the case of the iron group. There is, however, evidence from the curves of its existence and of an increase in the chemical polarisation with a decrease in the zinc content of the solution. The chemical reaction affording zinc ions proceeds much more rapidly with an increase of temperature, and in this way the supply of zinc ions at the electrode can be increased at high temperatures. This means an increased value of the ratio Zn++ : H+ at the electrode surface, and hence an in- creased efficiency in zinc deposition for any given current density.

(Nate.--AddedAzgust 4ih, I 925.) Lash Miller working with an oscillo- graph has recently studied the current-voltage curves in the electrolysis of copper salts. This work seems to establish fairly definitely that the over- voltage depends on the previous history of the cathode and not to some slow reaction in the solution.

Power Characteristics of Zinc Cell. For a consideration of the power characteristics of the cell a series of

e.m.f. measurements were made at various temperatures. The electrode spacing was 5 cm. between the centres of the two anodes and the solutions

0'120

3 0.116 I" g * 0'112

s U - .g 0.108

0.104

5 0'100 2

rr

M \

u Y

0-096 - iz

0.092

0.088 20 40 60 80

Temperature. FIG. &--Composition of solution : 0.96 N zinc sulphate.

a - 5-2 N sulphuric acid.

x - 1-70 N 9 9

0 - 3'55 N 9 9

used contained 2N zinc sulphate. I t was found in making these measure- ments that the potential required for any given current density was always much higher when the current was first switched on and fell to a constant

lr. Frank. Inst., 1g9, 773 (1925).

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308 THE ELECTRODEPOSITION OF ZINC

lower value in a few minutes. Care was taken in making the measurements to avoid all contact resistances.

From these e.m.f. measurements a n d the current efficiency-temperature curves for various concentrations and current densities one can obtain some idea of the power characteristics of the zinc cells studied. The data obtained are illustrated in Fig. 8.

The following table shows the effect of spacing on the K.W.H. required per gram equivalent of zinc and also gives a comparison with Hansen's figure for 10 cm. spacing.

TABLE VII.

K.W.H. PER GRAM EQUIVALENT.

Acid.

8 per cent. 17 9 ,

25 9 ,

I 5 cm. Spacing. 1 10 cm. Spacing. 1 Hansen's Value.

0.103 ' 0'137

0'10 ' 0'117 0-104 0.124

i

0-141

The agreement with Hansen is very good indeed as he took no pains to overcome contact resistances in his e. m.f. measurements.

Effect of Impurities on Efficiency of Zinc Deposition. Hansenl gives a considerable discussion on the effect of the presence

of small quantities of metallic salts other than zinc in the electrolyte on the cathode efficiency of zinc deposition.

Arsenic and antimony in concentrations up to 10 mg. per litre are not deposited with the zinc. Their effect on the current efficiency increases rapidly with increasing acidity. A combination of antimony and manganese is particularly harmful.

H e thinks that the presence of iron in the electrolyte, as well as bringing about depolarisation of the cathode, may also cause re-solution of zinc already deposited. Zinc deposited from a solution containing I so g./L. iron contains no more iron than that from a solution containing only

H e found that, when manganese was present in the zinc solution, if the acidity were below 60-70 g./L., the MnO, formed adhered to the anode; above this acidity it fell to the bottom as a finely divided sludge.

Laist, Frick, Elton and Caples a give an account of the effect of some of the commoner impurities in large scale electrolytic zinc practice. Copper in a concentration smaller than xo mg. per litre does not affect deposition, but above this concentration pitting of the zinc surface commences. Less than one part per million of antimony gives poor current efficiency and badly-sprouted deposits, Moderate amounts of iron have no effect. It is necessary to remove cadmium from the electrolyte in order to make high grade zinc since cadmium is deposited along with zinc.

Field3 states that a few parts per million of nickel or cobalt reduce markedly the current efficiency.

Schol14 gives a very interesting account of the effect of various impurities on the character of the zinc deposit.

0'02 g./L.

'Trans. A m . I m t . Miri. Eng., 60, 206 (1919). Trar~ls. Far. soc., 17. 400 (1922).

JChetn. Met . Eng., 26, 595 (1922).

2 Ibid., 699 (1921).

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FROM SULPHATE SOLUTIONS 309

‘raintoill gives a very complete account of the effects of hydrogen overvoltage and current density on the electrodeposition of zinc.

I t is probable that there is some minimum size which a spot of impurity deposited on the zinc surface must attain before active corrosion sets in. Probably a crystal or segregation of molecules must reach a certain size at least before beconiing effective and this would account for the fact that there seems to be a certain critical concentration for each impurity. The deposition of solid metallic zinc therefore resolves itself into a problem of maintaining at all points on the cathode surface a high hydrogen over- voltage.

He finds that for every metal there is a critical current density (for any given solution) at which hydrogen overvoltage for the metal is raised to a point where zinc can deposit. The point is marked very distinctly as hydrogen evolution ceases and a clear bright deposit of zinc appears.

A zinc surface which had been corroded with acid had a lower hydrogen overvoltage than the fresh surface. So long as the zinc surface in the solution containing the impurity remained bright and uiicorroded there was no appreciable fall in overvoltage. If, however, corrosion once began the overvoltage of the corroded metal was so low that no zinc could deposit. On this theory the worst impurities are those which show the least increase in hydrogen overvoltage with current density.

Two small ingots of pure zinc were made and in one 0.01 per cent. antimony was mixed. There was not much difference in their rate of corro- sion, till the bright surface of the latter turned black-it then corroded 1000 times faster than the pure zinc. Whether the effect is due to the low overvoltage of the antimony or to a modification of the molecular structure is doubtful. The important point is that the presence of an impurity does not greatly lower the overvoltage of a bright zinc surface, it is only after being attacked by acid that the bad effects are noticed.

A comprehensive discussion is made of some of the conditions affecting the zinc deposit, and the following conditions are given as favourable for good deposits :-

( I ) High current density giving a high hydrogen overvoltage. ( 2 ) High electrolytic conductivity giving a low potential gradient in

the solution and restraining the growth of trees which lower the current density.

(3) Diminished interfacial tension at contact surface of cathode and electrolyte which will allow hydrogen bubbles to leave cathode surface more easily.

(4) Rapid upward movement of electrolyte past cathode surface. (5) Presence of some addition agent which will restrain crystal growth

of cathode deposit and raise overvoltage of impurities. The whole problem is to maintain a smooth surface while the zinc is

depositing so that the current density remains high and uniform at all points on the surface.

A series of measurements was made by the author to determine the effect of various impurities on the efficiency of zinc deposition. The results with copper sulphate are shown in Figs. g and 10. The solutions contained 1.76N zinc sulphate and 3’55N sulphuric acid together with varying amounts of copper sulphate. A current density of 0-12 amps./cm.2 was used.

The results were reproducible to within about 0. j per cent. and those

1 Trans. Anr. Electrochem. Soc., 1% (1922).

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3 1 0 T H E ELECTRODEPOSITION OF ZINC

given are the average of a number of experiments. The amounts of copper present are given in the figure as the number of grams of copper (added

1 -- 20 40 60 So

Temperature.

FIG. 9.-Effect of copper sulphate :- @ - pure solution. x -0-10 gm. per litre copper. El -0.25 9 , 9 ) 9 1 Current density-Iz amps per A-O0.50 9 9 9 9 9 9 [sq. cm.

Composition of solution :- 1.76 N zinc sulphate. 3-55 N sulphuric acid.

as copper sulphate) in 850 C.C. solution in the cell. The cell solution was analysed after each experiment and amounts of copper added equivalent to those removed during the electrolysis. The electrolysis was carried on

97 1

95

0.06 0.12 0.24 0'48

Current Density.

FIG. 10.-Effect of copper sulphate :- 0 - pure solution. x - 0.1 gm. per litre copper.

Composition of solution :- 1-76 N zinc sulphate. 3*?5 N sulphuric acid.

remperature 80".

for 2 0 minutes. In this time from 2 5 to 40 per cent. of the copper was re- moved from the solution. At 80" C., with the solution containing 0-1 gram copper, the zinc surface was covered with a blackened spongy layer which

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FROM SULPHATE SOLUTIONS 311

rubbed off on the hand but was of very light weight. When the cathode was allowed to stand in its electrolyte for 1 3 minutes it lost 0.23 gram in weight (8 per cent.) and the surface became very black.

At 20' C., with 0-75 gram copper present, a very rough blistered deposit was obtained which was commencing to corrode badly at the top of the deposit. With the smaller amounts of copper recorded and also at 4oC and 60" C. very excellent deposits were obtained, those from solutions having most copper having a brassy sheen. All the deposits contained copper.

A series of measurements was made at 80" on the effect of current density on the efficiency of deposition from solutions containing copper and the results are reproduced in Fig. 10. With a current density of 0.48 amps./cm.2 and 0-5 gram per litre of copper the deposit commenced well and then the zinc started to corrode badly from the surface downwards.

The results on the effect of iron salts in the solution are given in

20 40 60 80

Temperature.

FIG. 11.-Effect of ferrous sulphate :- @ - pure solution. x - 0.52 gm. per litre of iron.

Composition of solution :- 1-83 N zinc sulphate. 3-51 N sulphuric acid.

EI -3.0 1 9 9 1 9 9 Current Density - 0.12 amps per + -6.0 9, 9 , 9 , [sq. cm.

Fig. 11. The iron was added to the electrolyte in the form of ferrous sulphate and one notices as electrolysis continues that the amount of ferrous sulphate gradually decreases until an equilibrium concentration is reached. I t was found by analysis that the iron content of the solution remained , practically constant during electrolysis and there was only a trace of iron in the zinc deposited.

The results with manganese in the solution are given in Figs. I 2 and I 3. I t was at first thought to work with a solution of constant content of manganese sulphate, but it was found that the manganese dioxide formed by the anodic reactions which remained partially suspended in the solution exercised a very considerable effect on the efficiency of deposition of the zinc. As more manganese solution was added to keep the content of the zinc cell constant the efficiency of deposition gradually fell, but if the manganese dioxide sludge were removed, the efficiency rose again to its original value. At the higher temperatures there is practically no permanganate present in the cell, and the greater part of the depolarising

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312 THE ELECTRODEPOSITION OF ZINC

reaction must be due to the suspended MnO,. At the lower temperatures, however, there is considerable permanganate present due to the fact that the reaction between it and the manganese sulphate to form the dioxide does not go to completion in cold strongly acid solutions.

From the results in Figs. 1 2 and 13 it would appear that at 20' C., I I

20 40 60 Eo

Temperature.

FIG. 12.-Effect of manganese sulphate :- Composition of solution :-

Current density-0.12 amps per [sq. cm.

the presence of manganese actually retards the formation of hydrogen, thus increasing the efficiency. These results were obtained keeping the manganese content of the cell constant.

The results obtained with antimony are very interesting and show the particularly baleful effect of this impurity in the zinc cell. The deposit obtained at 80. with 10 mg. antimony was very badly treed at the edges

0 - pure solution. x - I gm. per litre manganese.

1-83 N zinc sulphate. 3.51 N sulphuric acid.

94 I I

Y

E 2 82 '3

78 20 40 60 80

Temperature.

FIG. 13.-Effect of manganese sulphate :- 0 - Fure solution. x - 0-10 gm. per litre manganese.

Composition of solution :- 0.97 N zinc sulphate. 3-51 N sulphuric acid.

El - 1-00 9 , 11 11 Current Density-0.12 amps per [sq. cm.

and very fluffy in appearance, being rubbed off easily with the fingers. From the cell containing 5 mg. antimony a coherent film of zinc was obtained, which, however, was very fluffy at the edges, and on holding up to the light was lace like and transparent and crumpled to pieces on being crushed in the hand. From the cell with I mg. antimony a much better deposit was obtained and the zinc sheet was more elastic and did not crumple up so badly. The deposits at 40° and 60" from

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FROM SULPHATE SOLUTIONS 3' 3

95

91

Y E 79 5

75

7 1

20 40 60 80

FIG. ~+-Effect of antimony tartrate Composition of solution :- 1-95 N zinc sulphate. 3-55 N sulphuric acid.

@ - pure solution. x - 1.0 mg. per litre antimony. r 3 - 5 ' 0 9 7 9 9 9 9 Current density-*12 amps per

[sq. cm.

90

86

66

62

20 40 60 80

Temperature.

FIS. q.-Effect of antimony tartrate. Composition of solution :- 0.97 N zinc sulphate. 3-51 N sulphuric acid.

0 - pure solution. x -0.33 mg. per litre antimony. tJ-r-0 9 9 9 9 9 9 Current density-0.12 amps per A-YO Y Y 9 9 Y 9 [sq. cm.

21

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314 THE ELEC~RODEYOSITION OF ZINC

the cell with 5 mg. antimony formed trees badly and gave verj- poor zinc. At 40° with I mg. antimony a good deposit was obtained which was just commencing to tree at the edges; at zoo from the same cell there was no sign of tree formation.

Hansenl remarks that no antimony is deposited with the zinc and Elton confirms the statement. (These remarks are for cells of low acidity and c.d. 0.03 amps./sq. cm.) I was unable to detect any antimony in the cathode zinc even in the worst deposits obtained. I t will be noticed in the above table for the cell containing 5 mg./L. antimony that in each case the efficiency rises with continued deposition and much faster at the higher temperatures. In these cases three consecutive experiments were made renewing the zinc removed by electrolysis. The results suggest that antimony is being eliminated from the cell in the form of stibine and much more rapidly at the high than the low temperatures. No attempt was made to detect stibine in the cell gases. The markec! increase in efficiency at 20' with 0-33 mg./L. antimony present was found to be readily reproducible.

Summary.

A convenient copper coulometer has been developed for measuring currents up to 15 amperes or greater.

Measurements have been made of the effects of varying temperature, current density, and the concentrations of zinc and acid on the efficiency of zinc deposition. I t has been found that with pure solutions the efficiency always increases with rising temperature ; more rapidly the lower the zinc and the higher the acid content of the solution.

A study has been made of the effects of temperature and concentration changes on the single potential of the zinc electrode with varying current densities. An explanation has been given of these variations and also of the increase in efficiency of zinc deposition with rising temperature based on chemical polarisation.

The effect of varying temperature and concentration of zinc and acid on the power consumption of the zinc cell has been investigated.

Curves are given to show the effects of the presence of small quantities of copper, iron, manganese and antimony salts in the electrolyte on the efficiency and nature of the zinc deposit.

I wish to express my indebtedness to Professor F. G. Donnan for suggesting this research and for kindly advice during the progress of the work.

1 Tram. Am. Inst. Min. Er,g., 60, 206 (1919). CIbid. , 699 (1921).

Sir WiZZiam RamsalJ Laboratory of Physical Chnzistry, Univenity College, London.

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