Surface Technology, 19 (1983) 45 - 57 45

EFFECT OF BATH CONSTITUENTS AND SOME PLATING VARIABLES ON THE ELECTRODEPOSITION OF CADMIUM FROM ACIDIC CHLORIDE BATHS

A. M. ABD EL-HALIM* and M. I. SOBAHI

Department of Chemistry, Faculty of Science, King Abdulaziz University, Jeddah (Saudi Arabia)

(Received January 4, 1983)

Summary

The electrodeposition of cadmium from acidic chloride electrolytes containing 0.3 M CdC12-(5/2)H20, 0.1 M HC1, 0.4 M H3BO 3 and 2.0 M NH4C1 (bath I) was investigated. Electrodeposition in the presence of 0.5 M sodium potassium tartrate and 5 g gelatin 1-1 as organic additives (bath II) was also studied. The effects of the bath constituents, the plating current density i, the duration t and the pH on the cathodic polarization, the effi- ciency and the throwing power of cadmium electroplating from each bath, as well as on the morphology and the microhardness of the as-plated cadmium electrodeposits, are discussed. It was found that the additive-containing bath II yields more compact finer-grained brighter and harder plates than the additive-free bath I. The opt imum conditions for cadmium electroplating from bath II at 25 °C are as follows: i, 1.0 - 1.6 A dm-2; t, 5 - 15 min; pH 4.5 - 3.8.

1. Introduct ion

The electroplating of cadmium from cyanide baths is a long-established process which produces uniform fine-grained and bright deposits. The bath has a wide range of plating current density, good throwing power and is easy to control [1, 2]. However, since both cadmium and cyanide are toxic substances, non-cyanide cadmium plating solutions have been widely sought. Several plating baths have emerged as alternatives to the cyanide-based cadmium plating bath. Of these, the fluoborate bath was the first to offer much promise from a commercial standpoint [3]. Recently, sulphate- fluoborate [4] and numerous sulphate [4- 6] baths that produce successful plating have been introduced.

*Permanent address: Department of Chemistry, Faculty of Science, Ain Shams University, Cairo, Egypt.

0376-4583/83/$3.00 © Elsevier Sequoia/Printed in The Netherlands

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In a previous paper [7] we r epo r t ed tha t cadmium elec t ropla tes f rom an acidic chlor ide-based bath were m o re adheren t , compac t , ha rder and br ighter than those f rom the cor responding sulphate bath. Moreover , the acidic chlor ide-based ba th was charac te r ized by its higher th rowing p o w er c ompa re d with tha t o f the sulphate bath. In fact , cadmium e lec t ropla t ing f rom chlor ide baths has no t received adequa te a t t en t ion . Very few studies have been publ ished in this respect [8, 9] ; however , a simple m e t h o d for c o m p l e t e analysis o f the chlor ide ba th has been descr ibed [10] .

The presen t work was t he re fo re u n d e r t ak en to t h r o w m o re light o n t o the e lec t ropla t ing o f cadmium f rom the acidic chlor ide bath in an a t t e m p t to f ind the o p t i m u m bath compos i t i on and e lec t ropla t ing condi t ions for this process.

2. Exper imen ta l details

The exper imenta l details have been descr ibed in a previous paper [7] . The e lec t ro ly tes were p repared in dou b ly distil led water using highly pu re chemicals (BDH); the i r compos i t ions are given in Table 1. The pH and the specific c o n d u c t a n c e (in reciprocal ohms per cen t ime t re ) o f these electro- lytes were measured when requi red with a Coming research mode l 12 pH me te r and a YSI mode l 32 c o n d u c t a n c e m e te r respect ively.

The exper imen ta l set-up for the m e a s u r e m e n t of the s ta t ionary cath- odic polar iza t ion ( i -E) curves and the ca thodic cur ren t e f f ic iency f (%) consis ted of a Perspex cell with a c o p p e r ca thode and two plane parallel cadmium anodes; the c a t h o d e was pos i t ioned m id w ay be tween the anodes and d ipped in to the appropr ia te e lec t ro ly te . The ca thode and b o th anodes were of equal area. The ca thodic potent ia ls were measured relative to the sa tura ted calomel e lec t rode (SCE) and the e f f ic iency was de t e rmined coulo- metr ical ly .

The th rowing power TP (%) was measured in a Har ing -Blum cell with a dis tance ra t io o f 3.

TABLE 1

Composition of the cadmium electroplating solutions

Constituent Compositions for the following solution numbers

1 2 3 4 a 5 6 7 b

CdC12-(5/2)H20 (mol 1-1) 0.3 0.3 HC1 (mol 1-1) -- 0.1 H3BO3 (tool 1 - l ) -- -- NH4C1 (tool 1-1) -- -- Sodium potassium tartrate (mol 1-1) _ _

G e l a t i n (g 1-1) _ _

0.3 0.3 0.3 0.3 0.3 0.1 0.1 0.1 0.1 0.1 0.4 0.4 0.4 0.4 0.4 - - 2.0 2.0 2.0 2.0 - - - - 0 . 5 - - 0 . 5

- - - - - - 5 5

aThis solution is denoted bath I. bThis solution is denoted bath II.

47

Using the same set-up as that used for the measurement of the effi- ciency, the morphology of the as-plated cadmium electrodeposits was examined with a scanning electron microscope (JEOL model JSM 35). The magnification is indicated on the photomicrographs. The microhardness Hv (in kilogrammes-force per square millimetre) of these plates was measured by a Leitz-Wetzlar microhardness tester. The corresponding average thickness d (in micrometres) of each plate was also calculated.

The electroplating experiments were carried out at various current densities i (A dm-2), durations t (min) and pH values. One variable was changed while the other two were held constant. All measurements were made at 25 °C.

3. Results and discussion

3.1. Polar izat ion curves Figure 1 shows the cathodic polarization curves for cadmium electro-

deposition from solutions 1 - 4 with the compositions given in Table 1. The data of curve a reveal that the deposition of cadmium metal from pure CdC12 (solution 1) is accompanied by a slight overpotential. On addition of HC1 (solution 2) in order to avoid metal hydroxide and basic oxide forma- tion at the cathode, a further slight shift in the polarization curve to a more negative value is achieved (curve b). This effect can be attributed to the hydrogen overpotential [11]. The inclusion of H3BO3 in solution 3, as a

1.75

1.5o I

"~ 1.25

g ~ o.75

O.50

0.25

O. O0

o b c d

J - - / ~ - - - ~ ' - ' -- I/" ' - ' / , ~ ~ ' ' - ~ '

-130 -150 -- 300 -320 -7oo -72o -7ao -76o -780 Cathode "po%an%lal mV (SCE)

Fig. 1. Polarization curves for cadmium electrodeposition from solution 1 (curve a), solu- tion 2 (curve b), solution 3 (curve c) and solution 4 (curve d).

48

buffering agent to maintain a constant pH in the cathode layer, leads to an appreciable polarization-increasing effect (curve c). When a large excess of NH4C1 is added as an inert salt (solution 4) a corresponding overpotential of more than 70 mV is created even at low current densities (curve d). This is largely due to the activation overpotential arising from complex formation between Cd 2+ ions and excess Cl- anions [7]. This complex is characterized by a stability constant log K 1 of 1.2 [12]. The slope of the polarization curve increases only slightly with further increases in current density.

The significance of the overpotential in practice lies in the fact that it often influences some characteristics of the plating bath and the structure of the deposit, as discussed in the following sections. Therefore the composi- tion corresponding to that of solution 4 was chosen to fulfil the require- ments for the cadmium electrodeposition electrolyte; this is denoted chloride bath I.

In order to improve the quality of the cadmium electroplates from bath I, some additives were introduced into this bath. Figure 2 shows the cathodic polarization curves for solutions 4 - 7 with compositions as given in Table 1. The addition of either sodium potassium tartrate (solution 5) or gelatin {solution 6) to bath I caused a slight shift in the cathodic polarization to more negative values (curves b and c). The shift in cathodic polarization for sodium potassium tartrate is thought to be due to the formation of a weak complex with a stability constant log K1 of 1.1 [12]. In contrast, the shift

1.75

1.50

oJ I

"~ i .oo g

i 0.75

0.50

0.25

0.00 --'qt ' 0 --~I0

. I

-:5~0

acb d

-?90 - 9 6 o - 9 9 0 - 9 8 o -790 Cathode potentlal mF (SCE)

Fig. 2. Polarizat ion curves for cadmium elec t rodeposi t ion f rom bath I (curve a), so lut ion 5 (curve b), solut ion 6 (curve c) and bath II (curve d).

49

in cathodic polarization for gelatin may be attr ibuted to the formation of a very thin protein membrane in the cathode layer [1, 13]. This membrane would result in hindrance to some extent of the migration and discharge of the Cd 2÷ ions.

The combined effects of both sodium potassium tartrate and gelatin (solution 7) on the cathodic polarization of cadmium electroplating seem to be additive (curve d). Since solution 7 yields a more compact deposit with better coverage and brightness, it was selected for further investigations and was denoted bath II. The polarization difference between the additive-free bath I and the additive-containing bath II is about 20 mV.

3.2. Cathodic current efficiency The effects of the plating variables i, t and pH on the cathodic effi-

ciency from baths I and II were examined and the results are given in Tables 2, 3 and 4 respectively.

Increases in the current density within the range 0.1666- 2.000 A dm -2 increase the efficiency of each bath gradually until a maximum is reached at 1.666 A dm-2; the efficiency then starts to decrease (Table 2). A similar trend of increasing efficiency with current density was observed in several acidic electrolytes [14, 15]. The decrease in the efficiency above

T A B L E 2

Effec t of i on the charac ter i s t ics of the ba th s and the e lec t rop la tes

i f (%) TP (%) H v (kgf m m -2) d (/am)

(A dm -2) Bath I Bath II Bath I Bath H Bath I Bath II Bath I Bath l I

0 .166 94 .0 93.8 . . . . 0 ,95 0.95 0 .500 95 .0 94 .2 . . . . 2 ,88 2.85 1 .000 98 .9 97 .0 36.4 37.3 99.7 138.9 5.99 5.88 1 .666 99 .3 97 .0 36.0 37.1 102.1 139.9 10 .04 9 .80 2 .000 97.2 94.5 36.8 38.2 106.5 138.6 11 .78 11.45

t = 15 m i n ; p H 1.81 ( b a t h I ) ; p H 4 .94 ( b a t h II).

T A B L E 3

Effec t of t on the characteristics of the baths and the e lec t rop la tes

t f (%) TP (%) H V (kgf m m -2) d (pro) ( ra in)

Bath I Bath H Bath I Bath H Bath I Bath II Bath I Bath II

3 96 .2 95.8 . . . . 1 .94 1.93 5 98.7 96.1 . . . . 4 .27 3.24

10 99.1 96.7 36.4 37 .0 103 .0 124.5 6.67 6 .51 15 99.3 97 .0 36.0 37.1 102.1 139 .9 10 .04 9 .80 30 99.8 98.1 36.3 37.0 117.6 148 ,2 20 .16 19 .82

i = 1 .666 A d in -2 ; pH 1.81 ( b a t h I ) ; pH 4 .94 ( b a t h II).

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TABLE 4

Effect of pH on the characteristics of the baths and the electroplates

Bath number [HC1] (M) pH f(%) TP (%) H V (kgf mm -2) d (pro)

I 0.0 4.00 98.3 18.0 88.9 6.62 0.1 1.81 99.1 36.4 103.0 6.67 0.2 1.56 99.1 36.9 117.9 6.67 0.4 1.40 96.8 38.6 127.2 6.52

II 0.0 5.25 98.1 21.0 162.3 6.61 0.1 4.94 96.7 37.0 124.5 6.51 0.2 4.59 94.8 37.7 155.5 6.38 0.4 3.83 95.5 41.1 159.3 6.43

t = 1 0 m i n ; i = 1 . 6 6 6 A dm -2.

a cer ta in cur ren t dens i ty l imit (i = 1 .666 A d m -2) m a y be a t t r i b u t e d to the pre fe ren t ia l increase in h y d r o g e n evolu t ion . Under ident ical cu r ren t dens i ty condi t ions , ba th I gives sl ightly higher e f f i c iency values t han ba th I I does. This could be due to the add i t iona l ove rpo ten t i a l c rea ted as a resul t o f the add i t ion of s od i um po ta s s ium t a r t r a t e and gelatin to ba th II.

The ca thod ic e f f ic iency of b o t h ba ths increases slightly wi th increasing t ime (Table 3); increases o f 3.6% in ba th I and 2.6% in ba th I I are obse rved wi th in the range 3 - 30 min . This behav iou r can be ascr ibed to the d i f fe rence b e t w e e n the anodic e f f ic iency which is close to 100% and the ca thod ic effi- c iency which is less than 100% [1] . As t increases, this d i f fe rence leads to increases in the c o n c e n t r a t i o n o f Cd 2+ ions in the ba th ; hence the ca thod ic po la r i za t ion decreases and the e f f ic iency increases. In con t ras t , if the differ- ence is apprec iab le it will cause the c o n c e n t r a t i o n of Cd 2+ ions in the ba th to increase above the o p t i m u m range, resul t ing in an undes i rab le e l ec t rop la t ed depos i t ; this is cons ide red to be a d r a w b a c k o f the ba th . To avoid such effects , it is r e c o m m e n d e d tha t the ba th be ope ra t ed a t ca thod ic cu r r en t densi t ies in the range f r o m a b o u t 1 to 1.6 A dm -2 where the ca thod ic effi- c iency is m a x i m u m (greater than or equal to 97%) and n o t t oo m u c h lower than 100%.

As can be seen f r o m Table 4, the ca thod ic e f f i c iency o f each ba th decreases on ly sl ightly as the p H o f the ba th decreases on add i t ion o f HC1. This relates to the fac t t h a t s imple meta l l ic ions are on ly slightly sensi t ive to var ia t ions in the p H o f the so lu t ion [16] . These resul ts are also in good a g r e e m e n t wi th those ob t a ined b y o the r worke r s w h o s tudied the e lect ro- depos i t ion o f coba l t f r o m acidic chlor ide so lu t ions [ 17 ].

3.3. Throwing p o w e r The ef fec ts o f the p la t ing variables i, t and p H on the t h rowing p o w e r

of c a d m i u m e lec t rop la t ing f r o m baths I and I I were inves t iga ted and the resul ts are given in Tables 2, 3 and 4 respect ively . These resul ts are discussed on the basis o f the t w o main fac tors t ha t con t ro l the t h rowing power ,

51

namely the cathodic polarization [18] and the conductivity of the electro- lyte [14].

The lack of dependence of the throwing power on the current density (Table 2) relates to the negligible increase in cathodic polarization with increases in current density in each bath (Fig. 2).

The lack of dependence of the throwing power on the time (Table 3 } gives an indication of the stability of the bath and shows the accumulation of Cd 2÷ ions in each bath; the difference between the anodic and cathodic efficiencies does not attain the undesirable range. This is because, if the reverse were true, it would lead to a decrease in the cathodic polarization to a considerable extent and consequently to a decrease in the throwing power. It must also be considered that, if there were any increase in the Cd 2÷ ion concentrat ion in the bath with increasing time, it would result in a corre- sponding increase in the electrolytic conductance, which in turn would com- pensate for the decrease in the cathodic polarization and maintain the throwing power nearly constant.

The slight improvement in the throwing power with a lowering of the pH of the baths (Table 4) can be at tr ibuted to the increase both in the cathodic polarization [11] and in the conductivity of each bath as a result of the additional H ÷ ions present.

Under identical conditions, the throwing power from bath II is slightly higher than that from bath I (Tables 2 -4) . This may be caused firstly by the increased polarization of cadmium electrodeposition from bath II com- pared with that from bath I (Fig. 2) and secondly by the increased specific conductance of bath II (0.158 ~-1 cm-1) relative to that of bath I (0.151 ~-1 cm-1) as detected from the experimental measurements. However, it must be ment ioned that the throwing power range (36%- 41%) of the present CdC12 baths is comparable with that of the Cd(CN)2 bath (40%- 45%), which was also based on Haring cell measurements but with a distance ratio of 5 [1 ].

3.4. Surface morphology Figures 3 - 6 show photomicrographs of as-plated cadmium electro-

deposits obtained from baths I and II under various conditions of current density, t ime and pH.

The electrodeposition of cadmium from bath I at a low current density (i = 0.166 A dm -2) shows an outward growth of well-developed spiral hexagonal crystals, in the form of islands, sparsely covering the substrate (Fig. 3(a)). This may be at tr ibuted to adsorption of C1- anions on the sub- strate surface; hence the cathode is expected to be relatively inhibited. Under such conditions plating takes place only by some random deposition [19]. Therefore the deposit grows on some isolated centres where the local current density is strong enough to overcome the inhibition [20]. At a high plating current density (i = 1.666 A dm -2) a polycrystalline cadmium deposit with a pseudohexagonal structure is observed (Fig. 3(b)). Moreover the coverage of the substrate becomes much better with polycrystals of finer

52

(a) (b)

(c)

Fig. 3. Photomicrographs of cadmium electrodeposited from bath I (pH 1.81): (a) i = 0.166 A dm -2, t = 15 rain; (b) i = 1.666 A dm -2, t = 15 rain; (c) i = 1.666 A dm -2, t = 3 rain.

size. This ef fec t cou ld be due to the increased nuclea t ion densi ty with increases in the cur ren t densi ty because o f the cor responding increase in the overpotent ia l [ 21] .

For shor t plat ing t imes (t = 3 min), fine well-separated polycrys ta ls are obta ined f rom bath I (Fig. 3(c)). An increase in the plat ing t ime (t = 15 min) leads to an increase in the grain size and be t te r coverage o f the substrate (Fig. 3(b)). This seems to be reasonable since larger grains are m o r e stable t he rmodynamica l ly , and if the system has enough t ime it will reach its m o s t

53

(a) (b)

(c)

Fig. 4. Photomicrographs of cadmium electrodeposited from bath II (pH 4.94): (a) i = 0.166 A dm -2, t = 15 min; (b) i = 1.666 A dm -2, t = 15 rain; (c) i = 1.666 A dm -2, t = 3 rain.

stable state [20] . This p h e n o m e n o n m a y result f rom the redissolut ion and the recrystal l izat ion of deposits , t h rough the fo rma t ion of local cells be tween them, which occur s imul taneous ly with the e lec t rodepos i t ion [21] .

Figures 5(a) and 5(b) show the ef fec t of lowering the pH of bath I by the addi t ion o f 0.0 - 0.4 M HC1; at a lower pH value grain r e f inement and be t te r coverage of the substrate can be achieved. This is an expec ted result because the addi t ional H + ions present result in an increased overpotent ia l ,

54

(a) (b)

Fig. 5. Photomicrographs of cadmium electrodeposited from bath I (i = 1.666 A din-2; t = 10 min): (a) pH 4.00; (b) pH 1.40.

(a) (b)

Fig. 6. Photomicrographs of cadmium electrodeposited from bath II (i = 1.666 A dm -2 ; t = 10 rain): (a) pH 5.25;(b) pH 3.83.

as discussed in Sect ion 3.1, which consequen t ly enhances the nuclea t ion density.

A compar i son be tween Figs. 7(a) and 3(b) shows tha t the addi t ion o f 0.5 M sodium potass ium tar t ra te to ba th I (solut ion 5) gives rise to a finer- grained deposi t and an increased nucleat ion density. This cou ld be related to the fo rma t ion of a relatively weak c a d m i u m - t a r t r a t e complex , as discussed in Sect ion 3.1, and to the crea t ion o f a cor responding overpotent ia l . In con-

55

(a) (b)

Fig. 7. Photomicrographs of cadmium electrodeposited from (a) solution 5 and (b) solu- tion 6 (i -- 1.666 A dm -2 ; t = 15 rain).

trast, a comparison between Figs. 7(b) and 3(b) indicates that the addition of 5 g gelatin 1-1 into bath I (solution 6) is responsible for suppression of the growth of the isolated crystals and the production of a more coherent finer deposit which spreads over the substrate. Since gelatin solutions are coagu- lated by neutral salts and dissolved by acids, the effect of gelatin can be explained by the formation of membrane-like films in the cathode layer. These films are thicker in the higher current density areas (because of dis- charge of H + ions) than they are in the lower current density areas [1, 13]. They cause hindrance of the migration and discharge of Cd 2+ ions compared with H + ions and thus produce the above-mentioned effects.

In general, the morphology of the cadmium deposits from bath II (containing both sodium potassium tartrate and gelatin) is affected by the plating variables i, t and pH (Figs. 4, 5 and 6) in a similar manner to that discussed previously for bath I.

Figure 4(a) shows the earlier stages of cadmium deposition where suppression of the growth of the isolated single crystals can be seen. How- ever, at higher current densities and for short plating times (t = 3 rain) the typical features of the crystalline structure are still observed (Fig. 4(c)). As a result of continued deposition (t = 15 min) the metal forms a coherent accumulation of crystallites without any preferred grain orientation, but bud-like excrescences can be distinguished (Fig. 4(b)). On lowering the pH of the bath (Fig. 6(b)) complete coverage of the substrate with the finest deposit could be attained.

3.5. Microhardness The microhardness H v (kgf mm -2) of the as-plated cadmium electro-

deposits from baths I and II was measured under varying conditions of

56

current density, t ime and pH and the results are given in Tables 2, 3 and 4 respectively.

On increasing the plating current density in the range 1 .00- 2.00 A dm -2, there is no marked change in the microhardness of the cadmium electroplates from either bath (Table 2). Since the factors that lead to polarization-increasing effects produce deposits of finer grain size and higher microhardness [7], the lack of dependence of the microhardness on the current density can be ascribed to the very slight variation in the cathodic polarization with current density in both baths (Fig. 2).

The microhardness of cadmium electroplates from bath I increases only slightly with increasing plating time in the range 10 - 30 min (Table 3). This behaviour may be at tr ibuted to the increasing thermodynamic stability of the cadmium deposits with increasing time via a redissolution-recrystalliza- tion mechanism [21].

At constant current density and time, the lowering of the pH of either bath, by addition of 0.1 - 0.4 M HC1, results in a considerable improvement in the microhardness of the cadmium electroplates (Table 4). This can be explained by the additional polarization created by the added H + ions which produces an increased nucleation density and grain refinement (Figs. 5 and 6). This interpretation agrees with the common knowledge that finer-grained deposits are harder than coarser-grained deposits [7, 14]. The extra high microhardness of the cadmium plates from the non-acidified bath II (Table 4) may stem from the codeposit ion of some basic compounds which are expected to be formed preferentially at the relatively higher pH value of 5.25. As pointed out by Macnaughtan and Hothersall [22] and SchlStter [23], the inclusion of basic materials and impurities may be an important factor in increasing the hardness. By reference to Figs. 6(a) and 6(b) it can be concluded that the grain size is not the sole factor that controls the hardness.

As a general feature, under similar conditions the cadmium electro- plates from bath II are characterized by their higher microhardness compared with that of tl~e corresponding electroplates from bath I. Such a large difference in microhardness can be correlated not only to the polariza- tion-increasing effects arising from the addition of sodium potassium tartrate and gelatin (Fig. 2) and to the suppression of crystal growth and the produc- tion of finer-grained compact deposits by gelatin (Figs. 4, 6 and 7(b)) but also to the inclusion of basic compounds in the deposit (from bath II). This suggestion originates from the weak response of the pH of bath II towards successive additions of HC1 (Table 4).

4. Conclusions

Cadmium electroplating from the acidic chloride bath I was unsatisfac- tory because the deposits were coarsely grained and the substrate was not completely covered. From the acidic chloride bath II, containing tartrate and

57

gelatin, the covering power was much better and the grains much finer. Bath II could be easily controlled and was characterized by a high cathodic effi- ciency (97%), a moderate throwing power range (37%- 41%) and by the production of plates of good hardness reaching 159 kgf mm -2. According to the electrolysis conditions, the electroplates varied from pale grey to silverish semibright.

It is concluded that this bath seems very promising and a search for appropriate brighteners is required.

References

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