Surface Technology, 17 (1982) 157 - 164 157

STUDIES OF THE EFFECTS OF ADDITION AGENTS ON THE ELECTRODEPOSITION OF Ni-Co-Zn ALLOY FROM A BORATE BATH

R. K. SHUKLA, S. K. JHA and S. C. SRIVASTAVA

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

(Received June 15, 1982)

Summary

The effects of glucose, saccharin, gelatine, glycine and ascorbic acid as addition agents in the electrodeposition of Ni-Co-Zn alloys from a borate bath containing 120 g NiS04 l-‘, 30 g CoSO, l-‘, 144 g ZnSO, l-l, 30 g boric acid 1-l and 2 g NH,&1 1-l were studied. Glucose, saccharin and ascorbic acid were found to produce brighter more compact largely uneven crystalline deposits with localized undeposited bare cathode surface regions. Gelatine and glycine gave smooth but cracked deposits. The nickel content was found to decrease with the addition of saccharin, ascorbic acid and glycine whereas in the presence of gelatine it increased. For zinc the behaviour was the reverse. The percentage of cobalt was noticed to increase with these agents, but the effect was more marked with gelatine. The deposit composition also varied with increasing concentration of the additives.

The general nature of the variation in the total cathode current effi- ciency with current density remained unchanged by the addition of these agents. The cathode current efficiency was found to be less at any particular current density in all cases except for glucose than when its value was deter- mined for the deposition of the alloy without an addition agent in the bath. The cathode overpotential became more negative on the addition of glucose, which also appreciably increased the throwing power.

1. Introduction

Certain organic substances known as addition agents [l, 21 when present in a particular plating bath in a small concentration relative to that of the metals are known to produce desirable effects on the character of the deposits. Sufficient literature is available to indicate the modifications intro- duced by these surfactants [3 - 51 but the exact mechanism by which the agents on reaching the cathode surface affect the nature and growth of the deposits by affecting charge transfer, surface diffusion, electrical migration and final inclusion on the cathode surface is still a subject of considerable interest and speculation.

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158

In an earlier communication [ 61 from our laboratory the electrodeposi- tion of Ni-Co-Zn alloys under various conditions of electroplating was investigated. In the present work, which is a continuation of these studies, the effects of glucose, saccharin, glycine, ascorbic acid and gelatine on the morphology, cathode current efficiency, composition of the alloy, cathode overpotential, Tafel slope and throwing power have been investigated. These agents have also been reported to influence the nature of other ternary alloys [ 7 - 181 containing nickel or cobalt.

2. Experimental details

The procedure adopted for electrodepositing the alloy in the presence of the above addition agents under various conditions was the same as that given in our previous communication [6]. The metals were also estimated in the way reported earlier [6]. Micrographs of the electroplates obtained in the presence of the various additives were taken in order to study the mor- phological changes.

The cathode potentials were measured to an accuracy of 50.000 02 V against a saturated calomel electrode. The steady value of the e.m.f. of the cathode-calomel electrode combination with and without a definite flow of current was recorded on a vernier potentiometer using a sensitive lamp and scale galvanometer. The difference between the potential attained with a definite flow of current and the Nernst potential gave the value of cathode overpotential q. The reported data correspond to the hydrogen scale.

The throwing power N was calculated from the specific resistance 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 [19] formula:

N = b/Sp

3. Results and discussion

Glucose, ascorbic acid and saccharin were found to produce brighter more compact largely uneven crystalline deposits with localized undeposited bare cathode surface regions whereas gelatine and glycine gave smooth but cracked deposits. However, in all cases the deposits were more smooth and bright, having a smaller grain size, than when they were obtained without an addition agent in the plating bath. This may be caused by the incorporation of the agent or one of its decomposition products in the deposit. As a result of this the nature of the cathode surface is changed. This in turn may pro- duce the polarization which is usually supposed to accompany the action of an addition agent. It is this increased polarization which is believed to bring about the improvement in the properties of the deposits, in accordance with the ideas expressed by Brenner [ 11. The exact nature of the morphological changes is summarized in Table 1.

159

TABLE 1

Summary of the morphologies of the alloy deposits

Addition agent Current density (A dm-*)

PH Morphology of the deposit

Glucose 2.0 4.1 Fairly smooth; dark grey; crystalline even grain ; cracked deposit

Ascorbic acid

Gelatine

Glycine

5.0

2.0

4.1

3.2

Smooth; grey; crystalline uneven deposit with local- ized undeposited cathode surface regions

Uneven; blackish grey; fine grain deposit with local- ized undeposited cathode surface regions

5.0 3.2

2.0

5.0

3.8

3.8

Uneven; grey crystalline deposit with localized un- deposited cathode surface regions

Smooth; whitish grey (with a few black spots); uneven crystalline deposit

Fairly smooth; blackish grey (with a few black spots); cracked deposit

2.0 3.7 Fairly smooth; light grey; even cracked deposit

5.0 3.7 Smooth; bright grey; uneven deposit with local- ized undeposited cathode surface regions

Bath composition (g 1-l): NiS04, 120; CoS04, 30; ZnS04, 144; boric acid, 30; NH&l, 2; addition agent, 2.

The deposit composition is also changed in the presence of these agents as demonstrated by Table 2. It had been observed that the nickel content decreased on the addition of saccharin, ascorbic acid and glycine and increased with the addition of gelatine. Zinc, by contrast, showed the reverse behaviour. Further, the amount of cobalt increased with the addition of these agents and the increase was more marked with gelatine. These effects may be attributed to the preferential adsorption of one metal ion over the other on the cathode surface in the presence of these agents.

It should be mentioned here that the percentage composition of the alloy also varies with the concentration of the additives. Table 3 shows the effect of the concentration of these agents on the alloy composition. The

160

TABLE 2

Effect of addition agents on the deposit composition (30 Y!; current density, 5.0 A dme2)

Addition agenta

None 3.8 2.5 10.32 87.18 Saccharin 3.0 2.7 8.12 89.18 Gelatine 3.8 5.6 11.37 83.03 Ascorbic acid 3.5 2.7 5.45 91.85 Glycine 3.7 2.6 8.53 88.87 Glucose 4.1 2.5 10.38 87.12

PH Amount (%) of the following metals in the deposit

CO Ni Zn

Bath composition: as for Table 1 except for the addition agent Woncentration of addition agent, 1 g 1-l.

TABLE 3

Effect of addition agent concentration on the deposit composition (30 “C!; current den- sity, 5.0 A dm-‘)

_

Addition Concentration PH Amount f%) of the following I,.

agenf (g 1-l) metals in the deposit -__ --__ CO Ni

-

Nil 3.8 2.5 10.32

Saccharin 0.5 3.4 3.0 7.58 1.0 3.0 2.7 8.12

Gelatine 1.0 3.8 5.6 11.37 2.0 3.8 5.7 13.43 3.0 3.8 5.7 17.88

Ascorbic acid 1.0 3.5 2.7 5.45 2.0 3.2 2.5 4.25

Glycine 1.0 3.7 2.6 8.53 2.0 3.7 2.8 8.89

Glucose 1.0 4.1 2.5 10.38 2.0 4.1 3.0 8.52 3.0 4.2 3.0 7.15

Bath composition: as for Table 1 except for the addition agent.

Zn

87.18

89.42 89.18

83.03 80.87 76.42

91.85 93.25

88.87 88.31

87.12 88.48 89.85

nickel content of the alloy increases on increasing the concentration of sac- charin, gelatine and glycine but decreases with increasing concentration of ascorbic acid and glucose. However, the percentage of zinc behaved the opposite way. No appreciable change was noticed for cobalt when the con- centrations of the additives were increased.

161

The current density within the range studied did not appear to affect significantly the quality of the electroplates obtained in the presence of the agents. However, the percentages of the metals change, e.g. nickel and cobalt always tend to increase whereas zinc decreases continously with increasing current density, as shown in Table 4.

TABLE 4

Effect of current density on deposit composition (30 “C) in the presence of addition agents

Metal pH Amounts (%) of the metals in the deposit at the following current densities (A dmv2)

Addition agenta

2.0 2.5

Ni 3.8

co 3.8 Zn 3.8

Ni 3.0 co 3.0

Zn 3.0

Ni 3.8 co 3.8 Zn 3.8

Ni 3.5

co 3.5 Zn 3.5

Ni 3.1

co 3.1 Zn 3.7

Ni 4.1 co 4.1

Zn 4.1

5.28

0.9 93.82

6.34

1.8 91.86

6.18 2.8

91.02

3.15

1.6 95.25

4.06 1.3

94.64

7.31

0.9 91.79

5.55

1.1 93.35

6.75

2.0 91.25

7.35

3.4 89.25

3.32

1.7 94.98

4.45

1.5 94.05

7.53

1.0 91.47

- 3.0

5.76 1.5

92.14

7.15 2.2

90.65

8.94 4.0

87.06

3.58 1.9

94.52

4.88

1.8

93.32

7.72

1.5

90.78

- 3.5 4.0

6.03 6.36

1.8 2.1 92.17 91.54

- 7.65 - 2.5

89.85

9.25 9.75

4.5 5.1

86.25 85.15

3.75 4.05

2.1 2.4 94.15 93.55

5.35 5.95

2.0 2.3 92.65 91.75

7.96 8.12

1.8 2.3 90.24 89.58

None

Saccharin

Gelatine

Ascorbic acid

Glycine

Glucose

Bath composition: as for Table 1 except for the addition agent. aConcentration of addition agent, 1 g 1-l.

The cathode current efficiency was observed to be greatly dependent on the current density as shown in Fig. 1. The efficiency was higher at com- paratively lower current densities. However, in all such cases the efficiency first decreases with increasing current density and then begins to increase beyond a current density of 4 A dm-‘. A maximum value of 98.14% is obtained with glucose at a current density of 2 A dm-‘. However, in each case except for glucose the efficiency is always less at a given current density than when it was determined by depositing the alloy without an addition agent.

It is significant that the general nature of the variation in the total cathode current efficiency with current density remains unaltered by the

162

G--- 6

Fig, 1. The effect of various addition agents on the cathode current efficiency as a func- tion of the current density (conditions as given in Table 2): curve 1, no addition agent; curve 2, glucose; curve 3, saccharin; curve 4, ascorbic acid; curve 5, gelatine; curve 6, glycine.

addition of these agents. Further, an abnormal decrease is noticed in the case of glycine where its value drops by about 10% at 5.0 A dmP2.

The variation in the cathode overpotential with current density during

electrodeposition, in both the presence and the absence of an addition agent in the electrolyte, is given in Table 5. It is seen that comparatively less nega- tive values are obtained with saccharin, glycine, gelatine and ascorbic acid

at all current densities. Glucose, by contrast, caused the overpotential to shift to more negative values beyond 2.5 A dmP2. This is consistent with the observed increase in the cathode current efficiency in the presence of glucose

at current densities greater than 2.5 A drn- 2. A gradual shifting in the cath- ode overpotential to more negative values with increasing current density in each case may be caused by increased surface diffusion of the metal ions under these conditions.

The Tafel relation was found to be satisfied in the presence of these additives and hence Gardam’s relationship was applied to calculate the throwing power N (Table 6). N was found to be higher in the presence of ad- ditives than in their absence. This suggests that a relatively more uniform deposit should be obtained in the presence of these agents and in fact this has been found particularly for glucose where an appreciably higher value for N was obtained.

Acknowledgment

One of the authors (R.K.S.) is grateful to the Council of Scientific and Industrial Research, New Delhi, for providing the financial assistance.

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