Surface Technology, 21 (1984) 155 - 160 155

STUDY OF THE VARIATION IN THE CATHODE POTENTIAL WITH TEMPERATURE IN Ni-Cd ALLOY PLATING FROM A SULPHATE BATH

0. P. GUPTA, M. CHAUHAN and R. LOOMBA Department of Chemistry, Lucknow University, Lucknow (India)

(Received March 15, 1983)

Summary

The effect of temperature on the cathode potential during Ni-Cd alloy plating was studied for various current densities ranging from 1.093 to 4.115 A dm-2 and at temperatures from 20 to 80 °C in a bath containing nickel sulphate, cadmium sulphate, ammonium sulphate, boric acid and gelatin at pH 4. The relation between cathode potential V and temperature T is linear. The extrapolated V - T curves converge at about 0 V and 130 °C. An empir- ical equation fitting the data is V = (0.0026iw~+ 0.0065)(T--TO). The observed change in cathode potential with current density is at tr ibuted to the hydrogen overvoltage, which is controlled by the diffusion of H ÷ ions through a layer of alloy metal ions at the cathode surface. Deviations from the empirical equation are at tr ibuted to hydrogen adsorption.

1. Introduction

Studies of the electrodeposit ion of Ni-Cd alloy from a sulphate bath and of the changes in its physical properties produced by varying the deposi- t ion parameters have already been made [1, 2] but the effect of temperature on the cathode potential has not been explored hitherto. The character of the deposit in this alloy plating process is controlled primarily by the nature of the electrolyte immediately adjacent to the cathode surface. For op t imum results for a particular plating process, it is necessary to adjust the concentra- tion of the bath constituents, the pH level, the current density and the tem- perature to some specific values. Each of these factors affects the cathode potential.

The concentrat ion of alloy metal ions at the electrode surface, which take part in the cathode reaction, tends to decrease during plating. This tendency is opposed by transport of ions to the cathode surface by diffu- sion, convection and migration [3].

Alloy ions reach the cathode through a diffusion layer adjacent to the cathode surface [4]. The thickness of the diffusion layer in unstirred

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156

aqueous solution at room temperature has a constant value. It is appreciably decreased by an increase in temperature. Solution agitation is also very effec- tive in decreasing the thickness of the diffusion layer. Therefore both an increased temperature and agitation of the solution increase the rate of dif- fusion of ions to the cathode and permit higher plating rates.

The purpose of this investigation was to make a detailed study of the effect of temperature on the cathode potential during Ni-Cd alloy plating.

2. Experimental details

The process of electroplating to determine the cathode potential was carried out in a Hating cell made of Perspex [5]. The chemicals used were of analytical reagent grade and their solution was prepared according to the following concentrations: 0.02 M 3CdSO4-8H20; 0.12 M NiSO4.7H20; 0.48 M (NH4)2SO4; 1% H3BOa; 1% gelatin.

Solutions were prepared in pure conductivity water. Stainless steel plates were used as electrodes. The cathode potential at various temperatures was measured using a saturated calomel electrode and an agar-agar bridge, drawn into a capillary approximately 1 mm in diameter and pressed tightly against the electrode to minimize the error due to the potential drop across the electrolyte. Following the experimental technique recommended by Bockris [6], the temperature-cathode potential data were obtained in two ways: (a) at a constant temperature, in intervals of 10 °C, the cathode poten- tial was measured at several current densities (Table 1); (b) at each constant current density the cathode potential was measured with continually chang- ing temperature within the limit of experimental reproducibility. The results were the same by both methods. In obtaining data by the first method the electrode potential was allowed to decay to its unpolarized value before a new current density was applied. This technique was used to obtain more reproducible results. The cathode potentials were measured from the given solution at pH 4 in the temperature range 20 - 80 °C for the current density range 1.093 - 4.115 A dm -2.

3. Results and discussion

The cathode potentials measured are reported in Table 1, for various temperatures and current densities. These data may be represented graphical- ly in two ways: (a) a plot of current density against cathode potential to ob- tain a curve at each temperature (Fig. l(a)); (b) a plot of cathode potential against temperature to obtain a curve at each current density (Fig. l(b)). Deviations from straight line relations are observed at both ends of the curves. These deviations from the straight lines coincide with the evolution of large quantities of hydrogen.

The diffusion and convection of alloy metal ions (nickel and cadmium) to the cathode surface in this region are insufficient to maintain the major

157

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Current density (a) (b)

lo ~to 6= 8o loe t ll~° ~ l io

Temperature ( 'c)

Fig. 1. (a) Plot of cathode potential vs. current density (the numerals on the curves indicate the temperatures). (b) Plot of cathode potential vs. temperature: o, 1.093 A dm-2; e, 1.541 A dm-2; A, 2.018 A din-2; i , 2.535 A din-2; ~, 3.103 A din-2; i , 3.521 A dm-2;ff, 4.115 A dm -2.

p o r t i o n o f the cu r ren t and h y d r o g e n ions are d ischarged in grea ter quant i t ies . This results in m o r e negat ive c a t h o d e poten t ia l s . The devia t ions a t t he high t e m p e r a t u r e end appea r to s tar t at a b o u t 0 .79 V as t he s lope o f t he line changes ab rup t ly . I f t he original lines are e x t r a p o l a t e d t h e y all a p p e a r to converge a t one p o i n t near 0 V and 130 °C.

The change d V / d T in c a t h o d e po t en t i a l wi th t e m p e r a t u r e was f o u n d to be p r o p o r t i o n a l t o the square r o o t o f the cu r ren t dens i ty (Fig. 2). F r o m the s lope and in t e rcep t o f this line it was possible to der ive an empir ica l equa t i on fo r m a n y o f the da ta . The e q u a t i o n o f the s t ra ight line is

d V - 0 .0026 i l i 2 - 0 .0065 (1)

d T

where V is in volts , T is in degrees Celsius and i is the cu r ren t dens i ty in am- peres per square dec ime t re . On in tegra t ing we ob ta in

V = (0 .0026i I n + 0 . 0 0 6 5 ) T + K (2)

where K is t he in tegra t ion cons tan t . R e a r r a n g e m e n t and fac to r ing gives

V K T = - - (3)

0 .0026 i li2 + 0 .0065 0 .0026 i li2 + 0 .0065

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.ot~.

.009.

~ .oo7.

.oos

(current dcnshy)

Fig. 2. Plot of dV/dT vs. the square root of the current density (slope, 0.0026; intercept, 0.0067).

At V= 0 and T = 1 3 0 °C,

K T O = 130 = -- (4)

0.0026i 1/2 + 0.0065

V T = + 130 (5)

0.0026i 1/2 + 0.0065

V = (0.0026i 1/2 + 0 . 0 0 6 5 ) ( T - T °) (6)

where T o = 130 °C. Plots of this equation and of the experimental points given in Table 1

are shown in Fig. 3. The empirical equation agrees well with the experimen- tal data.

The cathode potential during electrolysis is generally considered to be proportional to the logarithm of the current density. The empirical equation developed from the data of Table 1 relates the cathode potential to the square root of the current density. This indicates a diffusion~ontrol led pro- cess in a fixed layer of alloy metal ions on the electrode which may have semiconducting properties [ 7 ].

The potential break at 0.79 V is interpreted as the point at which the available cathode surface energy is equal to the energy required to desorb hydrogen from the alloy metal ions. An increase in surface atomic hydrogen due to adsorption should produce a change in the voltage towards more negative potentials. This is observed with the change in the slope of the V - T

curves. The observed change in cathode potential with current density is at tr ibuted to a hydrogen overvoltage which is controlled by diffusion of hydrogen ions through a layer of alloy metal ions at the cathode surface.

A sudden change in the slope of the V - T curves at 0.79 V is attributed to adsorption of atomic hydrogen at the cathode.

The temperature coefficient of the cathode potential during alloy plat- ing is positive in sign as indicated in eqn. (6); in the static condition the elec- trode potential becomes more negative with increasing temperature.

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o-9

o. 8

I-o I-Sf~, t r e n t 2.0 , 2.5 dens i t y~ z Fig. 3. Plot of cathode potential vs. the square root of the current density (values ob- tained from the equation) (the numerals on the curves indicate the temperatures).

A c k n o w l e d g m e n t s

G r a t e f u l t h a n k s are e x t e n d e d to t he Counci l o f Sc ience and Techno l - ogy , U t t a r Pradesh , and the Univers i ty Gran t s Commiss ion , New Delhi , fo r award ing fe l lowsh ips to M. Chauhan and R. L o o m b a .

Refe rences

1 T. L. Rama Char, K. G. Sheth and T. S. Kannan, J. Electrochem. Soc. India, 13 (1966) 102.

2 O.P. Gupta, M. Chauhan and R. Loomba, Metalloberfl6"che, 8 (1982) 383. 3 J. N. Agar and F. P. Bowden, Proc. R. Soc. London, Set. A, 169 (1938) 206. 4 A. Hickling and W. H. Wilson, Nature (London), 162 (1948) 489. 5 I. Mendel, J. Electrochem. Soc., 84 (1952) 49. 6 J. O'M. Bockris, Trans. Faraday Soc., 43 (1947) 413. 7 J. Rubin, Chem. Phys., 19 (1951) 803.