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Bachelors Theses and Reports, 1928 - 1970 Student Scholarship

5-7-1937

The Electroplating of Cadmium from SulfateSolutionsLewis S. Prater

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Recommended CitationPrater, Lewis S., "The Electroplating of Cadmium from Sulfate Solutions" (1937). Bachelors Theses and Reports, 1928 - 1970. Paper 73.

THE ELECTROPLAT ING OP CADMIUM

}i'ROM

S~:ATE SOLUTIONS

by

Lewis S. Prater

A ThesisSubmitted to the Department of Metallurgy

in Partial Fulfillment ofthe Requirements for the Degree of

Bachelor of Science in Metallurgical Engineering

Montana School of MinesButt e, MontanaIJay 7, 1937

THE ELECTROPLATING OF CADMIUMl1'ROM

SULFATE SOLUTIONS

by

Lewis S. Prater

A ThesisSubmitted to the Department of Metallurgy

in Partial Fulfillment ofthe Requirements for the Degree of

Bachelor of Science in Metallurgical Engineering

Montana School of MinesBLl tte, MontanaM~y 7, 193717973

TABLE OF CONTENTS

Item I?age

Introduction ------------------------------------- 1

History --------~------------~-------------------- 3

Experimentation ---------------------------------- 5A. Current Density Tests ---------------------- 6.

Current Density - Current Efficiency Tables- 10B. Concentration Tests ------------------------ 14C. Temperature Tests -------------------------- 15

Concentration - Current Efficiency-Table --- 17Temperature - Current Erficiency Tables ---- 17

Conclusions -------------------------------------- 20

Di~am ----------------------~-------------------21

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THE ELECTROPLATll'JG OF CADMIUM FROM SULFATE SOLUTIONS

INTRODUCT IONIt has been proven that cadmium forms a very

satisfactory plate on steel, which has to withstandsevere corrosion, especially the corrosion of seawater and spray. The metal is now successfullyplated electrolytically from cyanide solutions, butwherever work is carried on with cyanide, there isgreat danger of poisoning to the workers. This isespecially true, if acids are used in the vicinityof the cyanide tanks. A few drops of acid spilledon a floor on which cyanide has been previouslyspilled would generate the deadly gas, hydrogencyanide. Besides the danger of poisonous gas,solutions of cyanide salts are also deadly if takeninwardly.

Vith these dangers in mind it can be readilyseen that if a satisfactory substitute could befound for the cyanide electrolyte in cadmium plating,it would be of great value. Of all the salts ofcadmium, cadmium sulfate is the most commom and alsothe cheapest. It would, therefore, be the logicalelectrolyte for plating, if a satisfactory operationcould be worked out. Cadmium sulfate solutions arenow widely used in the electro-winning of cadmium,but so far have a very limited application in the

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electroplating industry. It is the purpose of thisthesis to test the possibilities of a cadmiumsulfate solution for plating purposes.

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HISTORYCadmium is a common companion of zinc, and

traces of the metal can be found in practicallyall zinc ores. It was discovered by F. Stromeyerof Goettingen in 1817.1) Stromeyer named the newelement cadmium after cadmia fornacum.

As early as 1876 Wrightson 2) found that cadmiumwas not satisfactorily deposited from sulfatesolutions. In 1905 Holtz 3) also endeavored to platecadmium from sulfate solutions, but his experimentas well as those of others showed that a spongydeposit com19ined with excessive "treeing" was theusual result. Many other electrolytes such as thefluosilicate, oxalate, and chloride have beeninvestigated but none have proven entirely satisfactory.

Several patents 4) have been granted for theprocess of plating from the sulfate bath but none ofthem have been used to any extent. 'I'herevolvingcathode has also bee~~experimented with and has had--------------------1) J.W. Mellor - ."Comprehensive Treatise on Inorganic

Chemistry" - Vol. IV2) Fink - Trans. Electrochem. Soc. - 673) Wernick - Trans. Electrochem. Soc.-= 624) A.W. Young and G. Stimson - British Pat. 304,668

Humphries - British Pat. 330,289328,574309,0:y'1-2

A.H. Young - U.S. Pat. 1,833,450

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some practical application, but it can be readilyseen that its use would be limited for platingpurposes.

-s-

EXPERIMENTATIONFor making the electrolyte crystallized cadmium

sulfate (3CdS04 • 8H20) ,C.P. was used. Other chemicalswere added as needed in order to produce the desiredconditions, as explained in the test. Electrodes ofvarious metals were tried, all being as nearly 100%pure as possible.

Plating tests were run varying the condit.ions.of' deposition. The type of'plate f'cr-medin each casewas observed, and tne current ef'f'icienciesweredetermined. 'l'hetests were made with 100 c ,c , ofelectrolyte.in a small beaker. The electrodes werecut l~'inches wide and immersed 2 inches in thesolution. With this size a variation of 0.1 ampereis equivalent to a variation in current density of5 amperes per square foot. The electrodes weresupported by a wooden holder consisting of severalslats ~ inch thick, bolted together at each end.The electrodes were placed between the desired piecesand supported by resting the holder on the edge ofthe beaker. All the tests were run with the anodeand cathode ~ inch apart in order that the electricalenergy consumed would be a minimum. The current drawnfor each test was measure~ by means of an anmeterand a copper coulometer. The cathode area was alsomeasured for each test, and the current density was

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.;.

calculated. On the small scale it was impossibleto vary the current density in exact intervalsof 5 amperes per square foot, but with this set-upthe apverage deviation was of about this m~itude.

It is very essential that a clean surface beused for plating. Experimentation showed that sheetcopper made a very satisfactory cathode. Beforeelectrolysis the cathode was cleaned by dippinginto a dilute solution of potassium cyanide toremove the oxide film. It was washed off in a stream

.,

of running water and then with ethyl alcohol. Inorder to dry the plate for weighing) the alcohol wasignited. Current efficiencies were measured with acopper coulometer assuming its efficiency to be99.58 per cent.l)

A. Current Density 'restsFor preliminary tests the conditions used in

electro-winning were duplicated as closely as possible.A molar cadmium'sulfate solution was made up for theelectrolyte. A lead anode and aluminum cathode wereus ed, and the solution was maintained at roomtemperature for electrolysis.

In the first two tests it was found that theplate was almost white, but it was very granular.

1) Thompson - "Theoretical and Applied Chemistry"

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The deposited cadmium could be rubbed off thecathode with the finger, and it was thereforeimpossible to make any accurrate current efficiencydeterminations.

It is well known that zinc can be satisfactorilydeposited from zinc sulfate solutions under similarconditions. Two tests were therefore run with zincto determine whether or not the results of practicecould be duplicated on a small scale in thelaboratory. The results proved to be very satisfactory.A good deposit which could be stripped from thecathode was obtained both times, and the currentefficiencies could be determined.

For the second series of tests with cadmiumplating, a nickel cathode was used with the otherconditions the same as before. Nickel is easilyplated on practically all other metals and wouldtherefore make a satisfactory base for a cadmiumplate, if a good cadmium plate could be made on it.

This series of tests was run at room temperaturewith increasing current density in each succeedingtest. The results obtained are tabulated in Table Iand plotted in Fig. I. The variation in currentdensity between successive tests was made as smallas possible in order that any large variations incurrent efficiency might be observed. At a current

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density of 40 amperes per square foot the plateformed was very granular and non-adherant whichmade it impossible to determine the currentefficiency. At current densities below this theplate was satisfactory f'or- stripping, but itwas not adherant enough for plating purposes.

With an insoluble anode it is impossibleto maintain a bath of constant compositionbecause on continued electrolyses the acid contentis increased, and the cadmium content is decreased.In order to overcome this difficulty a cadmiumanode was cast from pure electrolytic cadmium. Itwas made about ~8 inch thick and of the samedimensions as the cathode. A solution of molarcadmium sulfate buffered with 30 grams per literof boric acid and decinormal sodium chloride wasused for an electrolyte. With a soluble anode theelectrolyte, buffered in this manner, was foundto remain fairly constant both in acid contentand in cadmium content. These tests were also runat room temperature, and it was found that withthe soluble anode the current efficiency was slightlyhigher than before.

With the soluble anode it was found thatsatisfactory deposits could be made on nickel withcurrent densities below 25 amperes per square foot,

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but above this the plate tended to form blistersover the surface o~ the cathode. This would notmake any difference for electrowinning cadmiumbecause the plate was fairly adherant with theexception of' the blistered places. For electroplating,however, such a deposit is of little use.

The experiments with the nickel cathode provedthat it was not very satisfactory for platingpurposes. A third series of tests was therefore runwith conditions the same as before, but this time

.using a copper cathode. Previous tests had shown thatthe curren~.density did not have an appreciableeffect on the current efficiency, so the interval ofvariation in current between successive tests wasmade approximately twice that of the previous ones.The results of these tests are recorded in Table II

and plotted in Fig. II. INith a current densityhigher than 40 amperes per square foot the plateforms "trees" which grow across to short circuitthe cell. The plate is also a dark gray color andis spongy whereas at lower current densities thecharacteristic silvery white color is obtained. Theresults showed that the best deposit was obtainedwith a low current density but that no abrupt changein·the type of plate occurred until the maximumcurrent denSity of 40 amperes per square foot was

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TABLE I~ffect of Current Density on Current Efficiency

Insoluble Anode

C.D.-~ps. Wgt Cu Vfgt Cd C. E. Type of Plateper ft. deposited deposited

5.3 0.1220 0.2162 99.8% 'i']hite - granular10.2 0.2347 0.4149 99.61& It tI

17.5 0.4054 0.7135 99.1% If tI

21.6 0.4830 0.8498 99 1'" n n• /0

30.1 0.6317 1.1073 98.70/; II n

35.1 0.6323 1.1122 99.1% " II

41.1 ------ ------- Spongy Dark

TABLE IIEffect of Current Density on Current ~fficiency

Soluble Anode

C.D.-~ps. ~{gteu "Wgt Cd C. E. Type of Plateper ft. deposited deposited

11.6 0.:2689 0.4763 99.7~; Easily strippedn

19.5 0.2397 0.4263 100.0;:-0 " fI

31.0 0.5732 0.9501 93.3~~ " If

38.4 0.5035 0.8868 99.25& 'ore granular47.0 -- ..--- ------ Spongy - "Treed"

..;11-

100

99 0

~

0 gc,

~\l..... 97~...~'i..~ go'\-....~ 95\)

~

0 CIt!../ ...-

10 2(~ :::0 <10Curr,nt Density - Amps./sq. ft.

Fig. I - Effect of Current Density on Current EfficiencyConditions of Plating

1. Insoluble lead anode2. 1:'::olarcadmium S rl.rat e electrolyte3. Nickel cathode4. lJlectrolyte at room t empe ratn re

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100 0

~99

~~\I 98.....\J"'""'- 97~

, 96~"t~ 95\J

940

10 20 30 40Current Density - Amps./sq. :ft.

Fig. II - E:frect of Current Density on Current E:friciencyConditions o:f Plating

1. Soluble cadmium anode2. Buffered molar cadmium sulfate electrolyte3. Copper cathode4. Electrolyte at room temperature

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reached.Copper proved to be much more satisfactory

for a cathode than nickel. The plate was moreuniform and seemed to be somewhat finer in texture,but not to any appreciable extent. In all the teststhe cadmium seemed to adhere to the copper betterthan it did to the nickel, but it could still bestripped ort quite readily. In stripping, the platewas removed in one sheet showing that it was moreuni:t"ormthan in the previous tests, where it wasquite difr1cult to keep the plate 1ntact during thestripping operation.

There was no difficulty of oxidation in dryingthe plated cathode as with the nickel cathodebecause the plate did not build up on the edgesquite as much. The current efficiency is slightlylower at high current densities than at lowercurrent densities, but the difference is small andwould make no practical difference.

Tests with an iron cathode were run to see i~cadmium· could be plated directly onto iron. It wasfound that the plate was not as satisfactory aswith copper, so further investigation was suspended.

-. '.,

i

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B.Variation in the Concentration of the ElectrolyteFrom the previous tests it is concluded that the

best results are obtained with a copper cathode anda current density of approximately 10 amperes persquare foot. Keeping these conditions constant andusing a soluble cadmium anode the concentration ofthe electrolyte with respect to the ~admium ion wasvaried. A solution of cadmium sulfate containing150 grams of cadmium per liter and buffered with 40grams per liter of boric acid and 7.8 grams per literof sodium chloride was used for the electrolyte. Bydiluting a measured volume of this solution with theproper amount of water a solution whose concentrationwas 125 grams per liter was obtained. The weakersolutions used were made by diluting the next strongersolution with water.

In appearance the plates obtained from the bathscontaining 75 grams of cadmium per liter or more yerevery similar to those obtained in the previous testswith the copper cathode. With a concentration of 50grams per liter the plate was dark around the edges,and with a concentration of 37.5 grams per liter theplate was dark and mushy allover. The currentefficiency of this test could not be accuratelydetermined. Table III shows that in general thecurrent efficiencies decreased as the concentration

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of the solution decreased, but there was no markedchange until the solution was below 75 grams ofcadmium per liter. The gradual decrease in currentefficiency may be due to increased solubility ofthe cadmium plate as the concentration of thesolution is decreased. The abrupt change below 75degrees C. is probably due to the character of theplate and to the fact that particles dropped offbefore the current efficiency could be determined.

C. Varying TemperaturePrevious tests have shown that a bright uniform

deposit of cadmium can be obtained from cadmiumsuLt'at.ebut none of the deposits have been adherantenough for plating purposes. In the last series oftest the temperature of the electrolyte was variedleaving the other conditions constant. A solublecadmium anode, copper cathode, and buffered molarcadmium sulfate solution were used ror electrolysis.

The first test was run at room temperature andthe last at 50 degrees C. with intervals of 10degrees C. between successive tests. The results(Table IV and Fig. IV) showed that up to 40 degreesC. the type of plate gradually impDoved. At 20degrees the plate could be entirely stripped, at 30degrees only the edges could be stripped, and at 40degrees none of it eQuId be removed. It was also

-26-

noted that the plate became finer in texture asthe temperature was raised up to about 40 degTeesC., but at 50 degTees C. the plate was not assatisfactory. It was more granular and could bescraped off in patches. This may have been due toincreased solubility of cadmium at elevatedtemperatures.

The current efficiency tests showed that thetemperature had practically no effect on theefficiency.

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r:rABLEIII

Effect of the Cadmiur~ Concentrationof the Electrolyte on Current Efficiency

Conc. Wgt Cu 'lgt Cd C. E. Type of Plategm , cdlL deposited deposited

150 0.2930 0.5186 99.7% ~Jlhite - granular125 0.2767 0.4860 98.95'; n rr

100 0.4364 0.7716 99.6$6 II H

75 0.3063 0.5412 99.5~S n ff

50 0.3549 , 0.4957 78. 7~6 Dark edges37.5 ------ ------ Spongy - Dark

Ef'fect of Tempera.ture on Current Efficiency

Tsmp. Vlgt Cu 1, gt Cd C. E. 'llypeof PlateC. deposi ted. deposited

20 0.2689 0.4763 99.77b ~Nhite - granular31 0.2766 0.4889 99.55'; Fairly adherant40 0.4053 0.7163 99. 5~S Good Plate49 0.4900 0.8765 99.7~~ Less adherant

than at 40°C.

-1E6-

100 -0 -e(j)-

Il~ 99~I::: 98~-,\J.;:\... 94.J

" 96~\J

'-'-~ 9~

9

50 75 100 125 150Concentration - grams Cd/liter

Fig. III - Et"f'ectof'the Cadmium Concentration of'the Electroly~e on Current EfficiencyCondiLions of Plating

1. Soluble cadmium anode2. Buffered cadmium sulfate electrolyte3. eopper cathode4. Electrolyte at room temperature

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0 0 0 0

9~

~ 9t:::~......

9v"

~~ 9'\-.....t:\) 9"-"-~\)

9

10 20 30Temperature - degrees C.

5040

Fig. N- E:ffect o:fTemperature on Current Ef:fiuiencyConditions o:fPlating

1. Soluble cadmium anode2. Burfered molar cadmium sul:tate electrolyte3. Copper cathode

MONTA:{.:1SC}~COL OF ~.f!'\msLIBRARYBUT'.I.'E

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CONCLUSIONSThe experiments indicate that the most

sat.t st'act.or-yde.posits are obtained by platingwith a soluble cadmium anode. O:fthe metals tested:for a cathod~ copper was :fonnd to give the bestdeposit. Bu:ff'eredmolar cadmium sul:fate solutionwas :found to be satis:factory :for an electrolyte.In the current density tests the results showedthat satis:factory deposits are obtained at allcurrent densities below 35 amperes per square f'oot ,but above this the plate is spongy and dark. Thebest plates were obtained at current densities o:fabout 10 amperes per square :foot. As long as theconcentration o:fthe solution is over 75 grams o:fcadmium per liter a good plate can be obtained.Below this concentration the deposit is dark, andthe current e:f:ficiencyis greatly reduced. Withrespect ~o the temperature of plating it was :foundthat at 40 degrees C. the most adherant platewas :formed.

;,

17973

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DIAGRAM OF THE APPARATUS

Thermostat

E.M.F.11

Voltmeter

RheostatCopper Coulometer

Acknowledgments

I wish to thank Dr. Curtis L. Wilson,Professor of Metallurgy, Montana School of Mines,under whose super-vfs.Ionthis work was conducted;also Dr. Ettore A. Peretti of the MetallurgyDepartment for his many helpful suggestions.