Studies on the electrodeposition of NiCdZn alloys from a borate bath

Surface Technology, 18 (1983) 107 - 115

107

STUDIES ON THE ELECTRODEPOSITION OF N i - C d - Z n ALLOYS FROM A BORATE BATH S. N. SRIVASTAVA and S. C. SRIVASTAVA Department of Chemistry, University of Lucknow, Lucknow 226007 (India)

(Received September 13, 1982)

Summary Ternary alloys of N i - C d - Z n were electrodeposited from a borate bath containing NiSO4 (60 - 66 g dm-3), CdSO4 (15 - 18 g dm-3), ZnSO4 (80 - 86 g dm -3) and boric acid (30 g dm -3) under various plating conditions. In general, semibright blackish grey thin films were obtained at low values of pH, temperature and current density. The cathode current efficiency increases slowly with increasing pH value and with decreasing temperature; however, at any particular temperature and pH value of the bath its value increases slowly to begin with and then increases rapidly as the current density is raised. The throwing power of the electrolytic bath was calculated from the values of the Tafel slope and the resistivity. Photomicrographs of the alloy plates were taken under various plating conditions to study their morphological behaviour.

1. Introduction The electrodeposition of alloys of nickel and cadmium with other metals has become technologically important because of their potential application as protective coatings against corrosion which have an attractive appearance on various base metals and attractive magnetic characteristics. Ternary thin films containing nickel and cadmium as major components have been electrodeposited mainly from sulphate [1, 2] and acetate [3, 4] solutions using different complexing agents. In several other papers, the codeposition of cadmium with A u - C u [5 - 7] and F e - C o [8] has been reported. The details of plating conditions for fluoride [9] and fluoroborate [10] baths have also been reported. Attempts have been made to codeposit C d - Z n with copper [11] and tin [12 - 16] from such baths. Recently the use of borate baths [17 - 23] has become widely accepted for the electrodeposition of these films. In the present study therefore the electrochemical deposition of a ternary alloy N i - C d - Z n from a borate bath was investigated under various plating conditions. 0376-4583/83/0000-0000/$03.00

© Elsevier Sequoia/Printed in The Netherlands

108

2. Experimental procedure Thin alloy films were obtained by electrolysing 225 ml of a solution containing sulphates of the three metals together with 30 g dm -3 of boric acid in a rectangular electrolytic cell (a detailed description of which has been given elsewhere [24] ) for 20 min. A fresh solution was used each time. The deposits were washed with distilled water, dried and then peeled off carefully. A fixed amount of the alloy was dissolved in sulphuric acid for analysing its various metal constituents. Nickel and cadmium were estimated gravimetrically using dimethylglyoxime and ~-naphthaquinoline complexes respectively and zinc was estimated using zinc ammonium phosphate. The deposit composition and hence the cathode current efficiency under various plating conditions were calculated by the usual m e t h o d [25]. The pH of the electrolytic solution was measured using a glass electrode and was adjusted to the desired value using either sulphuric acid or ammonia. The cathode potentials with and without a definite flow of current were measured to an accuracy of +0.000 01 V against a standard saturated calomel electrode. The difference between the potential attained with and witho u t a definite flow of current gave the value of the cathode overpotential ~?. The throwing power N was calculated from the resistivity p of the electrolyte and the Tafel slope b of the plot of the cathode overpotential against the logarithm of various current densities by using Gardam's formula [26] : N-

b 2p

Photomicrographs of the electroplates obtained under various plating conditions were taken in order to study their morphological behaviour.

3. Results and discussion The relative positions of the cathode potential-current density curves given in Fig. 1 for nickel, cadmium, zinc and binary Ni-Cd, N i - Z n and Z n - C d alloys suggest that a ternary alloy deposition of these metals is feasible. Deposits of various compositions were obtained from baths of different compositions under the following conditions: current density, 2 . 0 - 5.0 A dm-2; pH, 2.8 - 5.8; temperature, 20 - 35 °C. In general uniform semibright blackish grey thin coatings were obtained at comparatively low values of pH, temperature and current density. The deposits became partially powdery as the temperature and the pH of the electrolyte increased and as the current density decreased below 2.0 A dm -2. The brightness improved when the pH and the temperature of the bath were lowered and when higher current densities were used. The percentages of the metals in the deposits were always greater than those originally present in the electrolytic bath, which was stable under the various experimental conditions.

109 5.5 1

LE

3.5

g o c: !_ t. u

1.5

0.5

0.8

I

l

i

10 1.2 1.4 Cathode Potentials (Volts)

I ~.6

Fig. 1. Cathode potential curves for nickel, cadmium and zinc and their binary alloys (pH, 3.8; temperature, 30 °C): curve 1, Ni-Cd (bath composition: NiSO4, 60 g din-3; CdSO4, 15 g din-3; boric acid, 30 g din-3); curve 2, zinc (bath composition: ZnSO4, 80 g din-3; boric acid, 30 g din-3); curve 3, nickel (bath composition: NiSO4, 60 g din-3; boric acid, 30 g din-3); curve 4, Zn-Cd (bath composition: ZnSO4, 80 g din-3; CdSO4, 15 g din-3; boric acid, 30 g din-3); curve 5, Ni-Zn (bath composition: NiSO4, 60 g dm-3; ZnSO4, 80 g din-3; boric acid, 30 g din-3); curve 6, cadmium (bath composition: CdSO4, 15 g din-3; boric acid, 30 g din-3).

4. Alloy composition Tables 1 - 3 show the effects of the plating variables on the alloy composition. As the current density is increased, the nickel and zinc contents of the deposits gradually increase; this effect is more marked beyond 3.0 A dm -2. In contrast, the cadmium c o n t e n t decreases continuously. The percentage of nickel in the alloy rises with increasing temperature and increasing pH of the electrolyte, whereas the percentage of zinc exhibits the reverse behaviour. In contrast, the percentage of cadmium increases as the temperature is increased from 20 to 35 °C but decreases with increasing pH. These results suggest the noble behaviour of cadmium. Furthermore, it was noticed (see Table 3) that if the concentration of nickel in the bath is increased, the a m o u n t of nickel in the deposits increases, whereas the amounts of the other two constituents, namely cadmium and zinc, decrease. A similar situation is found with cadmium. However, when the concentration of zinc in the electrolyte is increased, the concentrations of both zinc and nickel in the alloy rise slowly, whereas the concentration of cadmium shows a downward trend.

5. Cathode efficiency The cathode efficiency for the deposition of the single metals is 10.71 30.74 for nickel, 19.17- 30.75 for cadmium, 3 4 . 0 0 - 5 2 . 7 5 for zinc and 78.31 - 94.50 for alloy deposition under similar conditions. The variations in

110 TABLE 1 Effect of temperature and current density on the deposit composition at a pH of 3.8 Temperature

Metal

(°c)

Amounts (%) o f the metals in the deposit at the following current densities (A dm -2) 2.0

2.5

3.0

4.0

5.0

20

Ni Cd Zn

10.02 45.97 44.01

11.23 43.54 45.23

12.63 41.27 46.10

15.58 36.25 48.17

18.29 31.75 49.96

25

Ni Cd Zn

10.97 48.38 40.65

12.22 45.83 41.95

13.67 43.58 42.75

16.42 38.83 44.75

19.77 34.08 46.15

30

Ni Cd Zn

11.77 50.74 37.49

13.21 48.64 38.15

14.46 46.09 39.45

17.48 41.46 41.06

20.32 36.78 42.90

35

Ni Cd Zn

12.89 53.32 33.79

14.55 50.71 34.74

15.42 48.29 36.29

18.28 43.56 38.16

21.07 38.76 46.17

Bath composition: NiSO4, 60 g dm-s;CdSO4, 15 g din-3; ZnSO4, 80 g dm-S; boric acid, 30 g dm -s.

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

2.8 3.8 4.8 5.8

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

Cd

Zn

14.93 17.48 20.57 22.47

43.18 41.46 39.34 38.19

41.89 41.06 40.09 39.34

Bath composition: as for Table 1.

t h e t o t a l c u r r e n t e f f i c i e n c y w i t h t e m p e r a t u r e a n d p H are r e p r e s e n t e d i n Figs. 2 a n d 3. I t is f o u n d t o i n c r e a s e s l o w l y w i t h i n c r e a s i n g p H b u t d e c r e a s e s e q u a l l y s l o w l y w h e n t h e t e m p e r a t u r e is i n c r e a s e d f r o m 2 0 t o 3 5 °C. F u r t h e r m o r e , i t is s e e n t h a t a t a n y p a r t i c u l a r t e m p e r a t u r e a n d p H o f t h e b a t h t h e total current efficiency increases slowly to begin with and then increases r a p i d l y as t h e c u r r e n t d e n s i t y is r a i s e d .

111 TABLE 3 Effect of varying the concentration of the constituents of the bath on the deposit composition at a temperature of 30 °C, a pH of 3.8 and a current density of 4.0 A dm -2 Concentration o f the following bath constituents (g dm -3)

A m o u n t s (%) o f the following metals in the deposit

NiS04

Boric acid

Ni

Cd

Zn

30

17.48 20.73 23.60 26.62

41.46 39.81 38.38 37.16

41.06 39.46 38.02 36.22

30

17.48 16.05 14.87 13.95

41.46 44.21 46.65 48.63

41.06 39.74 38.48 37.42

30

17.48 18.71 20.26 21.11

41.46 38.56 35.46 33.28

41.06 42.73 44.28 45.61

CdS04

ZnS04

N i S 0 4 concentration varying 60 15 80 62 64 66 CdS0 4 concentration varying 6O 15 80 16 17 18 ZnSO 4 concentration varying 6O 15 8O 82 84 86

95

1

1 2

95 c o

"5 90

3

"~ 9 0 Lu

4

bd c

85

Q 85 L

L

o

ao

o ~ 80

•~

75

I

1.0

2.0 Current

I

31o

,.o

Density

( A d m -2)

s.~

'~

75 1.0

2.0 Current

3.0

4.0

5.0

D e n s i t y ( A d m -2)

Fig. 2. Effect of temperature on the cathode efficiency (bath composition: NiSO4, 60 g din-3; CdSO4, 15 g dm-3; ZnSO4, 80 g din-3; boric acid, 30 g din-3; pH, 3.8): curve 1, 20 °C; curve 2, 25 °C, curve 3, 30 °C; curve 4, 35 °C. Fig. 3. Effect of pH on the cathode efficiency (bath composition: as given in Fig. 2; temperature, 30 °C): curve 1, pH 5.8; curve 2, pH 4.8; curve 3, pH 3.8; curve 4, pH 2.8.

6. Cathode overpotential The variation in the cathode overpotential with current density during e l e c t r o d e p o s i t i o n a t p H v a l u e s o f 2 . 8 - 5 . 8 a n d t e m p e r a t u r e s o f 2 0 - 3 5 °C is s u m m a r i z e d i n T a b l e s 4 a n d 5. I t c a n b e o b s e r v e d t h a t t h e o v e r p o t e n t i a l

112 TABLE 4 Effect of pH and current density on the cathode overpotential at a temperature of 30 °C Current density (A dm -2)

Logarithm o f the current density

1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0

0.0000 0.1761 0.3010 0.3979 0.4771 0.5441 0.6021 0.6532 0.6990

Cathode overpotential ~ (V) at the following p H values 2.8

3.8

4.8

5.8

--0.97745 --1.02849 --1.07046 --1.10849 --1.13552 --1.16056 --1.17560 --1.19357 --1.21850

--1.01620 --1.05025 --1.09136 --1.13533 --1.16830 --1.19528 --1.22033 --1.24235 --1.25821

--1.02727 --1.05832 --1.11035 --1.15235 --1.19040 --1.22047 --1.25547 --1.27542 --1.30044

--1.04555 --1.07060 --1.12061 --1.17558 --1.22063 --1.25069 --1.27571 --1.31067 --1.33563

Bath composition: as for Table 1.

TABLE 5 Effect of temperature and current density on the cathode overpotential at a pH of 3.8 Current density (A dm -2)

Logarithm o f the current density

1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0

0.0000 0.1761 0.3010 0.3979 0.4771 0.5441 0.6021 0.6532 0.6990

Cathode overpotentia177 (V) at the following temperatures (°C) 20

25

30

35

--1.05065 --1.09177 --1.13777 --1.17079 --1.20380 --1.22581 --1.24583 --1.26783 --1.28095

--1.03330 --1.08134 --1.12033 --1.15532 --1.18634 --1.21440 --1.23450 --1.25447 --1.26555

--1.01620 --1.05025 --1.09136 --1.13533 --1.16830 --1.19528 --1.22033 --1.24235 --1.25821

--0.96558 --1.01560 --1.07055 --1.10560 --1.15058 --1.18153 --1.20551 --1.22856 --1.24558

Bath composition: as for Table 1.

shifts to m o r e negative values w h e n the c u r r e n t density and the pH value of t h e s o l u t i o n are i n c r e a s e d . T h i s m e a n s t h a t c o m p a r a t i v e l y m o r e c u r r e n t is utilized for the deposition of the alloy than for the discharge of the hydrogen ions provided by the ionization of the solvent; this accounts for the gradual increase in current efficiency under these conditions. In contrast,

113 with increasing t e m p e r a t u r e t h e o v e r p o t e n t i a l b e c o m e s less negative at a c o n s t a n t c u r r e n t density. H e n c e the e f f i c i e n c y decreases with increasing temperature.

7. T h r o w i n g p o w e r T h e c a t h o d e o v e r p o t e n t i a l was f o u n d t o vary linearly with the logar i t h m o f the c u r r e n t d e n s i t y ; t h u s the Tafel relationship holds. T h e t h r o w i n g p o w e r , which is a measure o f the t e n d e n c y o f the bath to give a u n i f o r m deposit, was calculated f r o m the values o f the Tafel slope and the resistivity;

TABLE 6 Effect of temperature and pH on the resistivity, the Tafel slope and the throwing power of electrodeposits Temperature

pH

Resistivity

Tafel slope

Throwing power

3.8 3.8 3.8 3.8 2.8 4.8 5.8

44.2295 42.3875 40.0002 38.6808 41.2440 37.4840 35.5221

2.9032 2.6250 2.4286 2.2377 2.6923 2.2222 1.8750

0.0328 0.0309 0.0303 0.0289 0.0326 0.0296 0.0264

(°C) 20 25 30 35 30 30 30

Bath composition: as for Table 1.

the results are given in Table 6. Its value was f o u n d t o decrease with increasing t e m p e r a t u r e and with increasing p H values. This indicates t h a t relatively low values o f t e m p e r a t u r e and pH s h o u l d f a v o u r m o r e u n i f o r m deposits, and in f a c t t h e y d o so.

8. M o r p h ~ o ~ T h e micrographs o f the alloy films o b t a i n e d u n d e r d i f f e r e n t plating c o n d i t i o n s indicate t h a t u n e v e n fine grain deposits are f a v o u r e d b y increasing the pH value, t h e t e m p e r a t u r e and the c u r r e n t density. In c o n t r a s t , an increase in t h e c o n c e n t r a t i o n o f nickel o r c a d m i u m leads to m o r e even and finer grain deposits. T h e m o r p h o l o g i c a l results are s u m m a r i z e d in Table 7.

114

~

.~ .~ ~ ? .~: ~ .~.~ .~ ~ .~o~

G~1

Cxl

C~I

Cxl

C~1

Cq

C~1

q

o.

q

o

o

o

o

C" I

0

O

0

O

O

0

0

"0

o,~ o,~ o.~ o .~ o.~ o,~ o,~ 0 ~U

O0

O0

115

Acknowledgment O n e o f t h e a u t h o r s ( S . N . S . ) is g r a t e f u l t o t h e U n i v e r s i t y G r a n t s C o m mission, New Delhi, for supporting this work with a grant.

References 1 2 3 4 5 6 7 8 9

10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26

R. P. Dambal and T. L. Ramachar, Electroplat. Met. Finish., 25 (4) (1972) 10 - 18. R. Sivakumar and T. L. Ramachar, Met. Finish. J., 18 (206) (1972) 65 - 70. V. B. Singh and P. K. Tikoo, Electrochim. Acta, 23 (4) (1978) 393 - 396. V. B. Singh, J. Appl. Electrochem., 9 (3) (1979) 285 - 289. M. Dettke, Oberflaeehe-Surf., 18 (4) (1977) 101 - 102. F. Zuntini, W. Morrens and P. Laude, F.R.G. Patent 2,829,979 (C1.C25D3/56), January 18, 1979;Swiss Patent Application 77/8,552, July 8, 1977. V. George, H. Merk and A. F. Bogenschnetz, Galvanotechnik, 69 (10) (1978) 884 892. S. K. Narang and T. L. Ramachar, Met. Finish., 69 (9) (1971) 52 - 53. G . Dunkic and S. Djordjevic, Hem. Ind., 31 (8) (1977) 395 - 400. S. Djordjevic and G. Dunkic, Hem. Ind., 29 (2) (1975) 59 - 65. J. A. Harrison, D. A. Sandbach and D. J. Spohach, Electrochim. Acta, 24 (2) (1979) 179 - 189. H. J. Steeg, Jahrb. Oberflaechentech., 37 (1981) 65 - 73. H. J. Steeg, Jahrb. Oberflaechentech., 35 (1979) 86 - 91. H . J . Steeg, Jahrb. Oberflaechentech., 36 (1980) 67 - 76. H . J . Steeg, Jahrb. Oberflaechentech., 33 (1977) 94 - 101. H . J . Steeg, Jahrb. Oberflaechentech., 34 (1978) 81 - 89. A. N. Yagubets and V. N. Sherstkina, Elektron. Obrab. Mater., 5 (1974) 31 (in Russian). G. A. Volyanuk and E. V. Guseva, Zh. Prikl. Khim. (Leningrad), 48 (1975) 648. W. Machu, Metalloberfla'che, 30 (1976) 460. V. Racinskas and V. Charlusiene, U.S.S.R. Patent 370274 (C1.C23b), 1973, cited in Otkryt., Izobret., Prom. Obrazcy, Tovar. Znaki, 50 (1973) 86. L. V. Orlovskaya, V. Racinskas and O. Dzilbute, Issled. Obl. Osazhd. Met. Mater. Respub. Konf. Elektrokhim. Litov., S.S.R., 16 (1978) 175 - 179. V. V. Povetkin, Elektrokhimiya, 15 (1979) 761. S. V. Afanaslev, Spray. Elektrotekh. Mater. Vtoroe. Pererab. Izd., 3 (1976) 45. R. K. Shukla, S. K. Jha and S. C. Srivastava, J. Appl. Electrochem., 11 (1981) 697 701. F. A. Lowenheim (ed.), Modern Electroplating, Wiley, New York, 2nd edn., 1963, p. 470. G. E. Gardam, Trans. Faraday Soc., 34 (1938) 698.