Surface Technology, 21 (1984) 11 - 17 11

STUDIES OF ELECTRODEPOSITION OF N i - C o - Z n ALLOYS

R. K. SHUKLA and S. C. SRIVASTAVA

Department of Chemistry, University of Lucknow, Lucknow 226007 (India)

(Received May 3, 1983)

Summary

The electrodeposit ion of N i - C o - Z n alloys from a citrate bath contain- ing NiSO 4 (120 - 144 g l-l), COSO4 (30 - 46 g 1-1), ZnSO4 (144 - 168 g 1-1), citric acid (30 g 1-1) and NH4C1 (2 g 1-1) was investigated. Whitish grey depos- its were obtained which gradually became black when the concentrat ions of cobalt and zinc were increased. The brightness was improved when the nickel concentra t ion was increased or when the pH of the solution was decreased. The total cathode current efficiency for alloy deposition increases continuously with decreasing current density and temperature but increases with increasing pH of solution. The cathode overpotential and the throwing power of the bath are both reduced when the temperature of the bath is increased.

1. In t roduct ion

The electrodeposit ion of alloys from aqueous media has been gaining in importance in recent years. The use of binary and higher alloys appre- ciably increases the possibility of satisfying a wide variety of practical requirements which cannot be achieved by the application of pure metals. Alloy plates can be relatively harder, more corrosion resistant and more useful for antifr ict ion purposes than are pure metals. Some work on the electrodeposit ion of ternary alloys containing nickel as one component using various complexing agents [1 - 15] has been carried out. The deposi- tion of these alloys from a borate bath has been reported [16]. Recently efforts have been made to prepare magnetic alloys [17 - 20] which have important engineering applications.

In the present study it was thought desirable to determine the composi- t ion of the alloys under various conditions of electrolysis. The cathode cur- rent efficiency, the cathode overpotential , the Tafel slope and the throwing power of the bath were also calculated.

0376-4583/84/$3.00 © Elsevier Sequoia/Printed in The Netherlands

12

2. Exper imenta l p rocedure

The detailed p rocedure has been given elsewhere [16] . The bath con- sisted o f sulphates o f three metals toge ther with 30 g of citric acid per litre for buffer ing and 2 g NH4C1 1-1 to act as a brightener. In each test, 225 ml of fresh solut ion was electrolysed fo r 30 min. Af ter each depos i t ion the ca thode was washed with distilled water and dried. The films were then peeled of f carefully, weighed and then dissolved in sulphuric acid; they were then subjected to e lemental analysis. Nickel was es t imated gravimetrically as a d ime thy lg lyox ime com plex and zinc was de te rmined as zinc a m m o n i u m phospha te . Cobal t was es t imated color imetr ical ly [21] . The mos t dense un i fo rm bright grey deposi ts were ob ta ined under the fol lowing condi t ions : cur ren t densi ty , 1.0 - 4.0 A dm-2; pH 2.0 - 5.8; t empera tu re , 20 - 40 °C.

The ca thode cur ren t eff ic iency for the alloy depos i t ion was calculated on the basis o f the elemental analysis by the usual m e t h o d [22] . The pH of the solut ion was measured with a glass e lectrode and was adjusted to the desired value with ei ther sulphuric acid or sodium acetate. The ca thode potent ia ls were measured to an accuracy o f + 0 .000 01 V against a sa tura ted calomel e lec t rode using an agar-agar bridge which was drawn into a capillary approx ima te ly 0.1 cm in d iameter to ensure tha t a p roper con t ac t with the

6.0

5.0 7

E ,<

~ 4 . 0

z

o 3 .0

z

~: 2 .0

' f .O

1 2 6 3 4 5

2/ I I I I

1,0 .1 1.2 1.3 1.4

CATHODE P O T E N T I A L ~ . V

I

1 . 5

Fig. 1. Cathode potential-current density curves for nickel, zinc and cobalt and their binary alloys (pH 2; temperature, 30 °C): curve 1, nickel (bath composition: NiSO4, 120 g 1-1; citric acid, 30 g 1-1; NH+CI, 2 g l-t); curve 2, zinc (bath composition: ZnSO4, 144 g 1-1; citric acid, 30 g l-l; NH4C1, 2 g l-a); curve 3, cobalt (bath composition: COSO4, 30 g 1-1; citric acid, 30 g l-t; NH4C1, 2 g l 1); curve 4, Ni-Co (bath composition: NiSO4, 120 g 1-1; COSO4, 30 g 1 1; citric acid, 30 g 1-1; NH4C1, 2 g 1-1); curve 5, Zn-Co (bath composition: ZnSO4, 144 g l-t; COSO4, 30 g l-t; citric acid, 30 g 1-1; NH4CI , 2 g 1-1); curve 6, Ni-Zn (bath composition: NiSO4, 120 g 1-1; ZnSO4, 144 g 1-1; citric acid, 30 g 1-1; NH4CI , 2 g 1-1).

13

cathode was made. All the measurements were made with unstirred solu- tions. The cathode overpotential was determined from the difference be- tween the cathode potential at the given current density and the cathode potential when no current was flowing through the electrolytic bath. The throwing power N was calculated by using Gardam's formula [23] :

b N -

2p

where p is the resistivity of the electrolyte and b is the Tafel slope. The Tafel slope was calculated from a plot of the cathode overpotential against the logarithm of the current density.

3. Experimental results

Figure I represents the cathode potent ia l -cur rent density curves for nickel, cobalt and zinc and for Ni-Co, Ni-Zn and Co-Zn binary alloys. Their relative positions show that the deposition of ternary alloys of these metals is feasible. The electrolytic bath was found to be fairly stable under various plating conditions but on prolonged electrolysis its composit ion was difficult to control.

3.1. Alloy composition It was observed that thin films were deposited with composit ions in

the range 2.78% - 7.52% Ni, 0.5% - 3.0% Co and 89.48% - 96.72% Zn under various plating conditions. At low current densities, bright grey deposits were obtained but, as the current density was increased, smooth whitish grey films were deposited. An increase in the temperature appeared to effect no significant change in the nature of the deposits. However, thin bright films were usually obtained in electrolytic solutions with comparatively low pH values. When the cobalt and zinc concentrat ions in the bath were in- creased, the films gradually became black as the current density was increas- ed. In contrast, the brightness of the films was improved when the nickel concentra t ion was increased. The percentages of all the metals in the depos- its were always greater than those originally present in the bath. The effects of various parameters on the alloy composi t ion are shown in Tables 1 -4 . The effect of the metal concentra t ion on the alloy composit ion is shown in Table 3 which indicates that the inclusion of a metal in the alloy is favoured when the concentra t ion of the metal is increased in the electrolyte. The percentages of nickel and cobalt increase with increases in the current density, the pH of the solution and the temperature, whereas that of zinc decreases under these conditions.

3.2. Cathode efficiency The cathode efficiency was always found to be less than 100% owing

to the simultaneous discharge of hydrogen ions with other metallic ions.

14

TABLE 1

Effect of the current density on the composition of the deposit at pH 2.0 and 30 °C

Current Amounts (%) o f the following Cathode density metals in the deposit current

(A dm 2) Ni Co Zn efficiency (%)

Percentage of the current utilized for the deposition o f the following elements

Ni Co Zn t t 2

1.0 2.78 0.5 96.72 93.85 2.90 0.52 90.43 6.15 1.5 3.05 0.6 96.35 92.33 3.14 0.62 88.57 7.67 2.0 3.25 0.7 96.05 91.67 3.32 0.71 87.64 8.33 2.5 3.35 0.8 95.85 91.13 3.37 0.80 86.96 8.87 3.0 3.50 0.9 95.60 90.31 3.50 0.90 85.91 9.69 3.5 3.66 1.0 95.34 90.14 3.64 0.99 85.51 9.86 4.0 3.86 1.2 94.94 90.12 3.86 1.20 85.06 9.88

Bath composition: NiSO4, 120 g 1 1; COSO4, 30 g 1-1; ZnSO4, 144 g l- l ; citric acid, 30 g 1-1; NH4C1, 2 g 1-1.

TABLE 2

Effect of pH on the composition of the deposit at 30 °C and a current density of 4.0 A dm -2

pH Amounts (%) o f the following metals in the deposit

Ni Co Zn

Cathode current efficiency (%)

Percentage of the current utilized for the deposition o f the following elements

Ni Co Zn H 2

2.0 3.86 1.2 94.94 90.12 3.86 1.20 85.06 9.88 3.0 5.37 1.7 92.93 91.69 5.42 1.71 84.56 8.31 4.0 6.67 2.3 91.03 91.86 6.71 2.31 82.84 8.14 5.0 7.12 2.7 90.18 91.96 7.18 2.72 82.06 8.04 5.8 7.52 3.0 89.48 92.02 7.58 3.02 81.42 7.98

Bath composition: as for Table 1.

T h e c a t h o d e e f f i c i e n c y fo r t he d e p o s i t i o n o f p u r e m e t a l s is 2 .90% - 7 .58% for n i cke l , 0 .52% - 3 .02% for c o b a l t a n d 8 1 . 4 2 % - 9 0 . 4 3 % for z inc a n d for t he d e p o s i t i o n o f a l loys u n d e r s imi la r p l a t i n g c o n d i t i o n s i t is 8 9 . 4 7 % - 93 .85%. T h e t o t a l c a t h o d e c u r r e n t e f f i c i e n c y increases c o n t i n u o u s l y w i th dec rea s ing c u r r e n t d e n s i t y ( T a b l e 1) a n d t e m p e r a t u r e (Tab le 4). Howeve r , it decreases s lowly a t c o m p a r a t i v e l y h ighe r c u r r e n t dens i t i es . In c o n t r a s t , t he t o t a l c u r r e n t e f f i c i e n c y increases w h e n t he p H of t he s o l u t i o n is i nc r e a se d (Tab le 2) b e c a u s e , as t he h y d r o g e n i o n c o n c e n t r a t i o n decreases , c o m p a r a - t ive ly less c u r r e n t is r e q u i r e d to d i scharge t he h y d r o g e n ions , a n d h e n c e t he c u r r e n t e f f i c i e n c y o f t h e a l loy d e p o s i t i o n increases .

15

TABLE 3

Effect of the metal concentration in the bath on the composition of the deposit at 30 °C, pH 2.0 and a current density of 4.0 A dm -2

Concentration Amounts (%) o f the following Bath composition of metal sulphate metals in the deposit (g 1 -I)

in bath (g 1-1) Ni Co Zn

Ni 120 3.86 1.2 94.94 128 4.23 1.0 94.77 136 4.45 0.9 94.65 144 4.60 0.8 94.60

Co 30 3.86 1.2 94.94 38 3.73 1.4 94.86 42 3.73 1.5 94.76 46 3.73 1.5 94.76

Zn 144 3.86 1.2 94.94 152 3.67 1.1 95.23 160 3.50 1.0 95.50 168 3.45 1.0 95.55

COSO4, 30; ZnSO4, 144; citric acid, 30; NHaC1, 2

NiSO4, 120; ZnSO4, 144; citric acid, 30; NH4C1, 2

NiSO4, 120; COSO4, 30; citric acid, 30; NH4CI, 2

TABLE 4

Effect of the temperature on the composition of density of 4.0 A dm -2

the deposit at pH 2.0 and a current

Temper- Amounts (%) o f the following Cathode ature metals in the deposit current

(°C) Ni Co Zn efficiency (%)

Percentage of the current utilized for the deposition o f the following elements

Ni Co Zn H 2

20 3.15 1.00 95.85 92.02 3.20 1.02 87.85 7.98 25 3.54 1.10 95.36 91.47 3.58 1.11 86.78 8.53 30 3.86 1.20 94.94 90.12 3.86 1.20 85.06 9.88 35 4.65 1.30 94.05 89.76 4.60 1.29 83.87 10.24 40 5.37 1.30 93.33 89.47 5.30 1.29 82.88 10.53

Bath composition: as for Table 1.

3.3. C a t h o d e o v e r p o t e n t i a l

A n o v e r p o t e n t i a l r e su l t s f r o m t h e s l o w n e s s o f an e l e c t r o d e p roces s ,

a n d a c a t h o d e o v e r p o t e n t i a l u sua l l y ex i s t s f o r t h e d e p o s i t i o n o f a l l oys o n t o t h e c a t h o d e . I t is m a i n l y a c o n c e n t r a t i o n p o l a r i z a t i o n [ 2 4 ] p r o d u c e d by

16

decreases in the ca t ion concen t r a t ion in the ca thode c o m p a r t m e n t . As the cur rent densi ty is increased, the rate of discharge of the ions at the ca thode is enhanced, result ing in a reduc t ion in the cat ion concen t r a t ion near the ca thode , which increases the overpotent ia l . The thickness of the diffusion layer adjacent to the ca thode surface is appreciably decreased by an increase in the tempera ture . Thus the diffusion of meta l ions to the ca thode increases with increases in the t empera tu re ; consequen t ly , the overpotent ia l is de- creased (Table 5).

TABLE 5

Variation in the cathode overpotential with temperature at various current densities

Current Cathode overpotential 7? (V) for the following temperatures density (A dm -2) 20 °C 25 °C 30 °C 35 °C 40 °C

1.0 0.952 0.935 0.915 0.899 0.872 1.5 0.976 0.950 0.932 0.918 0.900 2.0 1.013 0.978 0.960 0.949 0.928 2.5 1.032 1.006 0.978 0.960 0.939 3.0 1.050 1.022 0.993 0.972 0.950 3.5 1.065 1.036 1.013 0.988 0.962 4.0 1.072 1.042 1.026 0.995 0.975

Bath composition: as for Table 1.

3.4. T h r o w i n g p o w e r The calculated values of the th rowing power at various tempera tures

are listed in Table 6. The th rowing power , i.e. the capaci ty to p roduce a un i fo rm coat ing on both the accessible and the inaccessible ca thode areas, seems to be of practical value for a commerc ia l e lect roplat ing bath. The th rowing power was observed to decrease with increases in the tempera ture .

TABLE 6

Data for the resistivity and the throwing power at various temperatures

Temperature (°C) Resistivity p ( ~ cm) Tafel slope (V) Throwing power

20 23.75 0.4332 0,0091 25 22.65 0.4106 0.0091 30 21.25 0.3820 0.0090 35 20.18 0.3568 0,0086 40 18.86 0.3115 0,0082

Bath composition: as for Table 1.

17

R e f e r e n c e s

1 V. B. Singh, L. C. Singh and P. K. Tikoo, J. Electrochem. Soc., 127 (3) ( 1 9 8 0 ) 590 - 596.

2 G. Dunk ic and S. Djordjevic, Chem. Ind. (London), 31 (8) ( 1 9 7 7 ) 395 - 400. 3 Y. N. Fa ruq and K. I. Vasu, J. Electrochem. Soc. India, 18 (4) ( 1 9 6 9 ) 181 - 185. 4 V. B. Singh, J. Appl. Electrochem., 9 (3) ( 1 9 7 9 ) 285 - 289. 5 S. Ar imura and T. Anada , Jpn. Patent 7,646,528, April 21, 1976; Jpn. Patent Appl.

74/120,840, O c t o b e r 18, 1974. 6 R. P. Damba l and T. L. Ramacha r , Electroplat. Met. Finish., 25 (4) ( 1 9 7 2 ) 10 - 18. 7 L. Domnikov , Met. Finish., 71 (12) ( 1 9 7 3 ) 47 - 49. 8 R. S ivakumar and T. L. Ramacha r , Met. Finish., 18 (206) ( 1 9 7 2 ) 65 - 70. 9 T. S. Vanaja and T. L. Ramacha r , Galvanotechnik, 66 (2) (1975) 108 - 115.

10 K. Matsuoka , Jpn. Patent 7,902,608, F e b r u a r y 9, 1979 ; Jpn. Patent Appl. 71/74746, S e p t e m b e r 25, 1971.

11 M. S a r o j a m m a and T. L. R a m a c h a r , Plating (East Orange, NJ), 57 (6) {1970) 619 - 625.

12 B. Bulo ta i te , V. Skominas and J. Matulis , Liet. TSR Mokslu Akad. Darb., Ser. B, 5 ( 1 9 7 7 ) 3.12.

13 W. Machu, Metalloberfliiche, 30 (10) ( 1 9 7 6 ) 460 - 465. 14 V. V. Pove tk in , Elektrokhimiya, 15 (5) ( 1 9 7 9 ) 761 - 763. 15 S. K. Jha, R. K. Shukla and S. C. Srivastava, Surf. Technol., 14 ( 1 9 8 1 ) 53 - 63. 16 R. K. Shukla , S. K. J h a and S. C. Srivastava, J. Appl. Electrochem., 11 ( 1 9 8 1 ) 697 -

701. 17 I. I. Toropova , I. B. Murashova and A. V. Pomosov , Izv. Vyssh. Uchebn. Zaved.,

Khim. Khim. Tekhnol., 20 (1) ( 1 9 7 7 ) 100 - 103. 18 S. V. Afanaslev, Spraw. Spektrotekh. Meter. Vtroe. Pererab. Iz., 3 ( 1 9 7 6 ) 45 - 63. 19 H. Ueno , H. Hayashi , S. Takagi , T. Shimizll , K. Sasegawa, S. Miwa and T. Hayashi ,

Kinzoku Hyomen Gijutsu, 24 (7) ( 1 9 7 3 ) 364 - 368. 20 N. I. Shklovshaya , I. B. Murashiva, O. F. B o n d a r e n k o and N. A. Marchenkov , Zashch.

Met., 13 (2) ( 1 9 7 7 ) 240 - 243. 21 R. S. Y o u n g and A. J. Hall, Ind. Eng. Chem., Anal. Edn., 18 ( 1 9 5 6 ) 246 - 266. 22 F. A. L o w e n h e i m (ed.), Modern Electroplating, Wiley, New York, 2nd edn. , 1963,

p. 470. 23 G. E. Gardam, Trans. Faraday Soc., 34 ( 1 9 3 8 ) 698. 24 J. N. Agar and F. P. Bowden , Proc. R. Soc. London, Set. A, 169 ( 1 9 3 8 ) 206.