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Effect of bath constituents and superimposed sinusoidal A.C. on nickel electroplating from acidic acetate solutions
Surface and Coatings Technology, 29 (1986) 313
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324
313
EFFECT OF BATH CONSTITUENTS AND SUPERIMPOSED SINUSOIDAL A.C. ON NICKEL ELECTROPLATING FROM ACIDIC ACETATE SOLUTIONS S. M. ABD EL WAHAAB, A. M. ABD EL~HALlM*,S. S. ABD EL REHIM and E. A. ABD El MEGUID Department of Chemistry, Faculty of Science, Am Shams University, Cairo (Egypt) (Received October 21, 1985)
Summary The electroplating of nickel from acidic acetate solutions containing 0.25 mol (CH3COO)2Ni•4H20 F’ with the stepwise inclusion of 0.3 mol H3B03 F’, 0.2 mol CH3COOH F’ and 0.1 mol CH3COONa•3H20 F’ was investigated. Electroplating in the presence of 1 g gelatin F’ and/or 1 g toluene-4-sulphonic acid F’ as organic additives was also examined. The cathodic polarization and current efficiency for nickel electroplating were affected to different extents by the bath constituents. Structural studies with a scanning electron microscope proved that the most regular and coherent nickel plate was that obtained from the solution containing both of the organic additives. The operating conditions2,tot obtain = 10 mma minor-bright and pH 5.25 nickel plate from this solution were j = 2 A dm at 25 °C.It was found that sinusoidal a.c. superimposed on the direct plating current decreases both the cathodic polarization and current efficiency, but increases the brightness of the nickel plate.
1. Introduction Nickel electroplating is one of the most important industrial processes that finds a wide range of applications utiising the desirable decorative, protective, magnetic and mechanical properties of nickel [11. The selection of an electroplating bath depends primarily on the required characteristics of the nickel plate. These latter are, in turn, dependent on the nature of the anions of the salts employed in the bath. Accordingly, intensive studies were carried out to obtain nickel electroplates suitable for different purposes from, for example, chloride [21, suiphamate [3], fluoroborate [4] and chloride—acetate [5] baths. In addition, a wealth of data is available on the electroplating of bright nickel from Watts-type baths in presence of bathsoluble organic brighteners [6, 7]. However, a survey of the literature shows that the electroplating of nickel from acetate baths is not used at present. *
Author to whom correspondence should be addressed.
0257-8972/86/$3.50
© Elsevier Sequoia/Printed in The Netherlands
314
In contrast, several advantages have been claimed for the electroplating of nickel—cadmium alloys from an acetate bath [8]. Therefore, the present study was undertaken to throw more light on the electroplating of nickel from acetate baths. This study was also a continuation of our research on the influence of superimposed a.c. on the electroplating of some metals and alloys from various baths [9- 11]. 2. Experimental details All the plating solutions were freshly prepared in distilled water using pure chemicals (BDH). The compositions and symbols of these solutions are given in Table 1. The experimental setup for the electrodeposition consisted of a rectangular Perspex cell provided with a copper cathode positioned midway between two plane-parallel platinum anodes; all electrodes were of the same geometrical area. TABLE 1 Symbol, composition and pH of the nickel electroplating solutions and the corresponding current efficiency from each solution Solu-
Compound (mol 11) in solution
pH
f(%)
symbol (CH,COO) 2Ni. H,BO, CH,COOH CH3COONa~ Gelatina TSAa,b 4H20 3H20 Ni.1 Ni.2 Ni.3 Ni.4a Ni.4b Ni.4c Ni.5 Ni.6 Ni.7
0.25 0.25 0.25 0.25 0.15 0.10 0.25 0.25 0.25
—
0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3
—
—
—
—
—
—
—
—
—
—
—
7.00 6.50 5.35
—
—
5.45 77.7
—
—
—
—
0.2 0.2 0.2 0.2 0.2 0.2 0.2
0.1 0.1 0.1 0.1 0.1 0.1
— — —
5.45 68.3 5.45 58.8
1
—
—
1
5.35 78.3 5.25 80.7
1
1
5.25 78.5
aConcentration in units of g l~. bTSA, toluene-4-sulphonic acid.
In some experiments, a sinusoidal a.c. superimposed on the d.c. was supplied by a TRIO AG 203 CR oscillator and connected directly to the cathode and the anodes. To achieve separation of the external d.c. and a.c. circuits, an induction coil (2.5 H; 0.25 A) was introduced into the d.c. circuit and a condenser (100 ~iF;12 V) was joined to the a.c. circuit. In the absence and in the presence of a superimposed a.c., the cathodic polarization curves were measured relative to a saturated calomel electrode (SCE) using a potentiometer (type EIL 23 AUK). The percentage cathode current efficiency f(%) was determined with the aid of a standard copper coulometer. The surface morphology of the as-formed nickel plates was examined with a scanning electron microscope (JEOL Model JSM-T20). The
315
electroplating experiments were carried out at 25 °Cusing stationary electrodes. 3. Results and discussion Table 1 shows the compositions of the selected nickel electroplating solutions. It must be mentioned that the selected concentrations of each of the solution-constituents stemmed from a separate systematic study. In each step, only the optimum concentration of each constituent which enabled the formation of nickel electroplate of the best quality was considered. For example, preliminary experiments were carried out for the electroplating of nickel from pure nickel acetate solutions within the concentration range 0.05 0.35 M. The results proved that the concentration found in solution Ni.1 produces the most bright metallic nickel electroplate and the best coverage of the cathode. Therefore, this solution was selected as a basis to construct a suitable bath for nickel electroplating from acetate electrolytes. -
3.1. Polarization curves Figure 1 shows the cathodic polarization curves of nickel electroplating from solutions Ni. 1 Ni.4c. The data of curve a reveal that the electroplating of nickel from solution Ni.1 is accompanied by a relatively high cathodic polarization. Moreover, an arrest corresponding to the limiting diffusion is observed before the evolution of hydrogen. current Ni2~ion discharge Theofinclusion of 0.3 mol boric acid F’ in solution Ni.2 leads to a con-
siderable shift of the polarization curve to less negative potential values (curve b). However, it is well known that boric acid is recommended as an efficient buffer in nickel electroplating baths to maintain a constant pH at the cathode layer [1].
~
_______________
o
—800
—880
—960
—1040
—1120
Cathode potential (E), mV (SCS)
Fig. 1. Polarization curves for nickel electroplating from solutions: (a) Ni.3, (d) Ni.4a, (e) Ni.4b and (f) Ni.4c.
Ni.1, (b) Ni.2, (c)
316
On addition of 0.2 mol acetic acid F’ (solution Ni.3), to avoid the formation of nickel hydroxide and basic oxides at the cathode, a further depolarizing effect is achieved (curve c). This effect may be attributed to the decrease in the overpotential of H~ion discharge [12]. Consequently, an increase in the rate of hydrogen gas evolution in form of bubbles would be expected rather than the adsorption of the liberated hydrogen on the cathode which originally was responsible for the increase in the cathodic polarization [4]. A slight polarization-increasing effect is observed as a result of the addition of 0.1 mol sodium acetate F’, as a conducting salt in solution Ni.4a (curve d). The feature may be due to the formation of soluble Ni2~—acetate complex ions with a stability constant of log K, = 0.41 [13]. Such a complex formation gives rise to a decrease in the concentration of the free Ni2~ ions, especially in the cathode layer. This trend is enhanced by the continuous discharge of Ni2~ions which leaves behind an increasing concentration of the complexing ions. In fact, the composition of Ni.4a fulfils the requirements of a suitable bath for nickel electroplating with respect to the presence of a buffer, acid medium and conducting salt. A lowering of the concentration of Nj2~ions in solutions Ni.4b and Ni.4c results in a considerable increase in the cathodic polarization (curves e and f). In addition, the polarization curves lie in the region beyond the arrest corresponding to the limiting diffusion current of Ni2~ion discharge (curve a). This phenomenon could be accounted for by the depletion of Ni2~ions in the cathode layer and, accordingly, the preferential discharge of H~ions. Therefore, the latter data (curves e and f) confirm that the concentration of Ni2~ions in solution Ni.4a is the optimum for producing a nickel electroplate with relatively better characteristics. In an attempt to improve the quality of the nickel electroplates from solution Ni.4a, some addition agents were introduced into this solution (Table 1). Inspection of Fig. 2 reveals, in general, that the electroplating of nickel from the additive-containing solutions is attended by a greater polarization than from the additive-free solution (curve a). Moreover, the polarization-increasing effect created by the addition of gelatin in solution Ni.5 (curve b) is more remarkable than that exerted by the addition of toluene-4-sulphonic acid (TSA) in solution Ni.6 (curve c). However, the combined effect of the two addition agents in solution Ni.7 (curve d) appears to be not simply additive. The increase in cathodic polarization for gelatin could be referred to the formation of a very thin protein membrane in the cathode layer [14]. This membrane would result, to some extent, in hindrance of the migration and discharge of Ni2~ions. In contrast, the increase in cathodic polarization for TSA may be attributed to the adsorption and the consequent hydrogenolysis of this compound on the cathode yielding toluene and bisuiphite anion. The latter anion is reduced at the freshly forming nickel surface to sulphide ion which is incorporated into the nickel plate as nickel sulphide [6, 15]. The toluene formed may be adsorbed on the cathode; it is not hydrogenated, and in this sense it is
317 3.0 a
c
bd
2.5 Is
‘a
~
2.0
~ a
1.5
aa
1.0
aa ‘a aa
a
0.5
~-‘
0
—.———1I
0
—840
—880
I
—920
I
—960
—1000
Cathode potential (14), mV(SCE) Fig. 2. Polarization curves for nickel electroplating from solutions: (a) Ni.4a, (b) Ni.5, (c) Ni.6 and (d) Ni.7.
OH
14
///~~// ///t/5///
3C.()_S0,H —+143 C~~S03H—+ H3CrJ3o3H—~H3O~~)( .2_4. 143C.G>
/~//////~///
+ HS0~
p1/i
Cathode surface
Fig. 3. Mechanism for hydrogenolysis of TSA during nickel electroplating from solutions Ni.6 and Ni.7.
reversibly absorbed [16]. This suggested mechanism is shown schematically in Fig. 3. The cathodic polarization curves for nickel electroplating from solution Ni.7 were traced under the influence of sinusoidal superimposed a.c. Figure 4(A) shows that superimposed a.c. of a variable density, at a constant frequency of 50 Hz, shifts the cathodic polarization to less negative values. Furthermore, as the density of the a.c. j~.increases, the magnitude of the polarization shift also increases; but for the same j~.value the depolarizing effect decreases with increasing the d.c. density. Similar results have been reported for cobalt, nickel and cobalt—nickel alloys from Watts [10] and ammoniacal complex [11] baths. However, it was observed experimentally during the electroplating of nickel from solution Ni.7 that the anodic polarization is markedly greater than the cathodic polarization. This result may explain the data of Fig. 4 in terms of the expected distortion of the sine wave of the a.c. at the cathode surface [17]. Consequently, this leads to an asymmetric polarizabiity of the cathode [18 20], its potential fluctuating -
318 1 • 50
(A)
1.25
~
T
(B)
0.0
~
1.00
I
_________________
0.5
a d
c
b
./~2-’
_______________________
0
—500
—600
—700
—800
—900
—1000
Cathode pot~ntial (14), mV (SCE)
Fig. 4. Effect of superimposed a.c. density (w = 50 Hz) on (A) the polarization curves and (B) Tafel lines of nickel electroplating from solution Ni.7: (a) d.c. only, (b) j~..= 2. 0.20, (c)j~..= 0.50 and (d)j~..= 1.33 A dm
periodically, resulting in an average potential value which is less negative than the potential corresponding to a d.c. only. Similarly, at a constant j~.of 0.66 A dm”2, a depolarizing effect at the cathode could be observed and this effect decreases with increasing frequency (~)of the superimposed a.c. as shown in Fig. 5(A). These data could be related to the fact that, at a low frequency the time of a single oscifiation is enough for the rearrangement of the successive anodic and cathodic polarizations and thus the periodic fluctuation of the cathode potential is marked. This feature decreases as the frequency of the superimposed a.c. increases and its effect on the polarization is gradually diminished. Figures 4(b) and 5(b) show that superimposed a.c. of variable density and frequency, at low d.c. densities, creates shifts in the derived Tafel lines similar to those observed for the corresponding polarization curves. These shifts in the Tafel lines could be ascribed to changes in the concentration of Ni2~ions in the cathode layer [21]. However, it has been reported that passage of a.c. through an electrolyte of complex ions may produce periodical concentration changes which depend not only on the a.c. density and frequency, but also on the complex stability constants [22]. 3.2. Current efficiency The cathodic current efficiency f(%) of nickel electroplating was determined as a function of the plating solution-constituents, at a current density of 2 A dm2 and duration of 10 min, and the results are given in Table 1. The nickel electroplates from solutions Nil Ni.3 were nonadherent and, -
319 1.50 (A)
1.25
i1~
____________________ -
1
‘0
(B)
0.0.
~-0.5• ~
0.75
—1.0-
b~
0,7
I -
‘0 ,
aa
I
I
-500 -700 14, mV (SCE)
—900
0.50
b
a ~‘
a
d
I
c
d
a
0.25
0
~j
0
I
—500
I
—600
I
—700
Cathode potential
—800
I
—900
I
—1000
(14), mV (SCE)
Fig. 5. Effect of superimposed a.c. frequency (j_ 0.66 A dm’) on (A) the polarization curves and (B) Tafel lines of nickel electroplating from solution Ni.7: (a) d.c. only, (b) = 50, (c) w = 250 and (d) c~ = 1000 Hz.
therefore, it was difficult to determine f from these solutions with enough accuracy. 2~’ions On the inother hand, Ni.4a f decreases with a decrease in the solutions c. This trend of decrease in concentraf could be tion of Nito the corresponding increase in the cathodic polarization (Fig. 1) referred and the preferential discharge of H~’ions. A slight improvement in f of nickel electroplating is achieved as a result of the inclusion of either gelatin (solution Ni. 5) or TSA (solution Ni.6), in comparison to f for the additive-free solution Ni.4. Also a similar increase in f is observed for a combination of both additives in solution Ni.7. However, these results contradict the experimental finding that electroplating of nickel from the additive-containing solutions is accompanied by greater polarization than from the additive-free solution (Fig. 2). These results may be correlated with the expected incorporation of small amounts of these addition agents and/or their reduction products into the nickel deposit, as discussed in Section 3.1. Nevertheless, f of nickel electroplating from the acetate solutions examined, whether in the absence or the presence of addition agents, is relatively lower than the values of f observed with other acidic baths [1, 23]. Table 2 presents the influence of superimposed a.c., of variable density and frequency, on f of nickel electroplating from solution Ni.7. A notable feature is that a superimposed a.c. leads to a considerable decrease in f of nickel electroplating. In addition, the higher the superimposed a.c. density and the lower its frequency the more pronounced is the decrease in f. This -
320 TABLE 2 Effect of superimposed a.c. on percentage current efficiency of nickel electroplating from solution Ni.7 (j = 2 A ~—2, t = 10 mm)
f(%)
Superimposed a.c.
j-~(A dm’)
~ (Hz)
0.00 0.20
0 50
77.7
0.46
50
57.2
1.33 0.66 0.66 0.66
50 1000 250 50
56.4 60.2 59.2 56.8
58.4
feature may be ascribed to the possibility of dissolution of some of the nickel deposit during the anodic half-cycle of the a.c. and the additional polarization created during the cathodic half-cycle of the a.c. However, these two factors are more effective the higher is the superimposed a.c. density and the lower its frequency [9]. 3.3. Surface morphology Figure 6 shows the electron micrographs taken for the as-deposited nickel froi~some of the solutions under test, at a current density of 2 A din2 and duration of 10 mm. The nickel electroplate from solution Ni.4a consists of a disperse, fine-grained deposit leaving some bare areas on the cathode surface (Fig. 6(a)). The formation of such a pitted dendritic shaped nickel plate may be referred to the relatively low cathode efficiency for nickel deposition (Table 1); thus sufficient H~ions are discharged which slowly form hydrogen gas bubbles clinging to the growing nickel deposit. Figure 6(b) shows that the inclusion of gelatin in solution Ni.5 results in compact irregular flat layers of nickel characterized by complete suppression of dendritic growth and better coverage of the cathode, although some cracks are observed. Taking into consideration that gelatin is cationic in acid baths and aqueous gelatin solutions are coagulated by salts but solubiized by simple acids, it has been postulated [14] that a slightly thicker protein, membrane-like, film is formed in the higher current density areas, than in the lower current density areas, of the cathode diffusion layer. This film would be formed not only by cataphoresis, but also as a result of the decreased H~ ion concentration in the cathode layer due to cathodic discharge. Consequently, the permeability of this membrane would be decreased at the higher current density areas for the Ni2~ions, but not appreciably for the H4’ ions, which would be the conditions for obtaining the above-mentioned morphological changes. The inclusion of TSA in solution Ni.6 leads to the formation of a nodular, finer-grained texture completely covering the cathode surface
321 •
. ,.
‘
I
(a)
(b)
(c)
(d)
Fig. 6. Electron micrographs of nickel electroplated from: (a) solution Ni.4a, (b)2solution and t Ni.5, (c) solution Ni.6 and (d) solution Ni.7. Magnification x760, j = 2 A dm’ 10 mm.
without any visible pits (Fig. 6(c)). The disappearance of pits relates to the fact that TSA, which is classified as a Class I brightener [1], also acts as an anionic wetting agent and therefore prevents the formation of pits caused by clinging hydrogen bubbles. The striking effects achieved by TSA indicate its adsorption on growth sites, points and edges of crystals producing an inhibiting effect on the most active growing sites [24, 25]. This indication is in agreement with the observed increase in the cathodic polarization (Fig. 2) and the corresponding expected increase in the nucleation density [26]. The growth morphology of the nickel electroplate from solution Ni.7 including both gelatin and TSA (Fig. 6(d)) is very similar to that obtained from solution Ni.5 containing gelatin alone, except for the occurrence of
322
small pores instead of the cracks. It seems reasonable that these pores are not formed by actual pitting of the deposited nickel layer but probably result from local inhibition of outward growth of the deactivated growth sites by the addition agents. Inspection of Figs. 7(a) (d) reveals that a superimposed a.c. during the electroplating of nickel from solution Ni.7 gives rise to a more regular deposit exhibiting more or less porosity than in the absence of a super. imposed a.c. (Fig. 6(d)). Furthermore, Figs. 7(a) (c) show that both the pore size and density increase with an increase of the a.c. density, at a constant a.c. frequency of 50 Hz. This feature may be correlated with the -
-
(a)
(b)
(c) (d) Fig. 7. Electron micrographs of nickel electroplated solution under the influ2, w =from 50 Hz; (b)j~_Ni.7 = 0.66 A dm”, ~ = ence of superimposed p.c.: (a) j_ = 0.20 A dm 50 Hz; (c)j~-=1.33Adnf2, w=5OHz; (d)J-~=0.66Adm2, w=l000Hz. Magnification X760;J 2 Adm2 and t 10mm.
323
expected redissolution (during the anodic half-cycle of the a.c.) of the thin layer of nickel formed at the most inhibited sites of the cathode surface and its consequent deposition (during the cathodic half-cycle of the a.c.) at the less inhibited cathode areas. Therefore, the repetitive anodic—cathodic oscillations of the a.c. lead to pore coalescence. In contrast, a comparison between Figs. 7(b) and (d) shows that an increase in the a.c. frequency, at a constant a.c. density of 0.66 A dm2, results in a flat electroplate with an obvious decrease in both the pore size and density. Such a morphological modification may be related to the decrease in the depolarizing effect of the a.c. with an increase of its frequency (Fig. 5).
4. Conclusions It would be feasible to obtain a satisfactory nickel electroplate from acidic acetate solutions by careful control of the bath composition. Solution Ni.4a is the most promising and produces a yellowish white semibright nickel plate with a percentage cathodic current efficiency around 78%. Both gelatin and toluene-4-sulphonic acid proved to be efficient levelling agents and brighteners in nickel electroplating from acetate solutions. Practically, a yellowish-white mirror bright nickel plate is formed from solution Ni.7. A superimposed a.c., depending on its density and frequency, could improve the brightness of the nickel electroplate but unfortunately this is accompanied by a considerable decrease in the current efficiency of about 20%. Therefore it seems that, in future, this latter factor might be used in controlling the percentage of nickel in some electrodeposited nickel-base alloys. References 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
F. A. Lowenheim, Modern Electroplating, Wiley, New York, 1974, pp. 287 - 323. R. J. Kendric, Trans. Inst. Met. Finish., 44 (1966) 78. A. J. Dill, Plating, 61 (1974) 1001. Y. M. F. Marikar and K. I. Vasu, Chem. Era, 6 (1970) 5. G. B. Hogaboom, U.S. Patent 2,351,966 (1944). H. Brown, Plating, 55 (1968) 1047. H. Brown, Trans. Inst. Met. Finish., 47(1969) 63. D. Singh and V. B. Singh, Indian J. Technol., 14 (1976) 189. A. M. Abd El-Halim, A. 0. Baghiaf and M. I. Sobahi, Surf. Technol., 22 (1984) 143. 5. 5. Abd El Rehim, A. M. Abd El-Halim and M. M. Osman, Surf. Technol., 22 (1984) 337. S. S. Abd El Rehim, A. M. Abd El-Halim and M. M. Osman, Acta Chim. Acad. Sci. Hung., 117(1984)393. S. S. Abd El Rehim, Acta Chim. Acad. Sci. Hung., 82 (1974) 535. L. G. Sillén and A. E. Martell, Stability Constants of Metal Ion Complexes, Chemical Society, London, 1964, p. 366. C. A. Hampel, Encyclopedia of Electrochemistry, Reinhold, London, 1964, p. 125. J. Matulis, A. Bodnevas and M. Vainilaviciene, Plating, 56 (1969) 1147.
324 16 A. H. Du Rose, Trans. Inst. Met. Finish., 38 (1961) 27. 17 J. Devay and L. Meszaros, Acta Chirn. Acad. Sci. Hung., 43 (1965) 17. 18 S. 5. Abd El Rehim and A. M. Abd El-Halim, Acta Chim. Acad. Sci. Hung., 80 (1974) 65. 19 S. S. Abd El Rehim and M. G. Helmy, Acta Chim. Acad. Sci. Hung., 89 (1976) 215. 20 S. S. Abd El Rehim, Acta Chim. Acad. Sci. Hung., 99 (1979) 289. 21 R. J. Walter, Tech. Proc. Am. Electroplat. Soc., 66 (1979) E-3. 22 Z. Kovac, J. Electrochem. Soc., 118 (1971) 51. 23 S. S. Abd El Rehim, A. M. Abd El-Halim and M. M. Osman, J. AppI. Electrochem., 15 (1985) 107. 24 A. T. Vagramyan and L. A. Uvarov, Trans. Inst. Met. Finish., 39 (1962) 56. 25 M. H. Jones and M. G. Kenez, Plating, 53 (1966) 995. 26 A. M. Abd El-Halim, J. App!. Electrochem., 14 (1985) 587.
-
324
313
EFFECT OF BATH CONSTITUENTS AND SUPERIMPOSED SINUSOIDAL A.C. ON NICKEL ELECTROPLATING FROM ACIDIC ACETATE SOLUTIONS S. M. ABD EL WAHAAB, A. M. ABD EL~HALlM*,S. S. ABD EL REHIM and E. A. ABD El MEGUID Department of Chemistry, Faculty of Science, Am Shams University, Cairo (Egypt) (Received October 21, 1985)
Summary The electroplating of nickel from acidic acetate solutions containing 0.25 mol (CH3COO)2Ni•4H20 F’ with the stepwise inclusion of 0.3 mol H3B03 F’, 0.2 mol CH3COOH F’ and 0.1 mol CH3COONa•3H20 F’ was investigated. Electroplating in the presence of 1 g gelatin F’ and/or 1 g toluene-4-sulphonic acid F’ as organic additives was also examined. The cathodic polarization and current efficiency for nickel electroplating were affected to different extents by the bath constituents. Structural studies with a scanning electron microscope proved that the most regular and coherent nickel plate was that obtained from the solution containing both of the organic additives. The operating conditions2,tot obtain = 10 mma minor-bright and pH 5.25 nickel plate from this solution were j = 2 A dm at 25 °C.It was found that sinusoidal a.c. superimposed on the direct plating current decreases both the cathodic polarization and current efficiency, but increases the brightness of the nickel plate.
1. Introduction Nickel electroplating is one of the most important industrial processes that finds a wide range of applications utiising the desirable decorative, protective, magnetic and mechanical properties of nickel [11. The selection of an electroplating bath depends primarily on the required characteristics of the nickel plate. These latter are, in turn, dependent on the nature of the anions of the salts employed in the bath. Accordingly, intensive studies were carried out to obtain nickel electroplates suitable for different purposes from, for example, chloride [21, suiphamate [3], fluoroborate [4] and chloride—acetate [5] baths. In addition, a wealth of data is available on the electroplating of bright nickel from Watts-type baths in presence of bathsoluble organic brighteners [6, 7]. However, a survey of the literature shows that the electroplating of nickel from acetate baths is not used at present. *
Author to whom correspondence should be addressed.
0257-8972/86/$3.50
© Elsevier Sequoia/Printed in The Netherlands
314
In contrast, several advantages have been claimed for the electroplating of nickel—cadmium alloys from an acetate bath [8]. Therefore, the present study was undertaken to throw more light on the electroplating of nickel from acetate baths. This study was also a continuation of our research on the influence of superimposed a.c. on the electroplating of some metals and alloys from various baths [9- 11]. 2. Experimental details All the plating solutions were freshly prepared in distilled water using pure chemicals (BDH). The compositions and symbols of these solutions are given in Table 1. The experimental setup for the electrodeposition consisted of a rectangular Perspex cell provided with a copper cathode positioned midway between two plane-parallel platinum anodes; all electrodes were of the same geometrical area. TABLE 1 Symbol, composition and pH of the nickel electroplating solutions and the corresponding current efficiency from each solution Solu-
Compound (mol 11) in solution
pH
f(%)
symbol (CH,COO) 2Ni. H,BO, CH,COOH CH3COONa~ Gelatina TSAa,b 4H20 3H20 Ni.1 Ni.2 Ni.3 Ni.4a Ni.4b Ni.4c Ni.5 Ni.6 Ni.7
0.25 0.25 0.25 0.25 0.15 0.10 0.25 0.25 0.25
—
0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3
—
—
—
—
—
—
—
—
—
—
—
7.00 6.50 5.35
—
—
5.45 77.7
—
—
—
—
0.2 0.2 0.2 0.2 0.2 0.2 0.2
0.1 0.1 0.1 0.1 0.1 0.1
— — —
5.45 68.3 5.45 58.8
1
—
—
1
5.35 78.3 5.25 80.7
1
1
5.25 78.5
aConcentration in units of g l~. bTSA, toluene-4-sulphonic acid.
In some experiments, a sinusoidal a.c. superimposed on the d.c. was supplied by a TRIO AG 203 CR oscillator and connected directly to the cathode and the anodes. To achieve separation of the external d.c. and a.c. circuits, an induction coil (2.5 H; 0.25 A) was introduced into the d.c. circuit and a condenser (100 ~iF;12 V) was joined to the a.c. circuit. In the absence and in the presence of a superimposed a.c., the cathodic polarization curves were measured relative to a saturated calomel electrode (SCE) using a potentiometer (type EIL 23 AUK). The percentage cathode current efficiency f(%) was determined with the aid of a standard copper coulometer. The surface morphology of the as-formed nickel plates was examined with a scanning electron microscope (JEOL Model JSM-T20). The
315
electroplating experiments were carried out at 25 °Cusing stationary electrodes. 3. Results and discussion Table 1 shows the compositions of the selected nickel electroplating solutions. It must be mentioned that the selected concentrations of each of the solution-constituents stemmed from a separate systematic study. In each step, only the optimum concentration of each constituent which enabled the formation of nickel electroplate of the best quality was considered. For example, preliminary experiments were carried out for the electroplating of nickel from pure nickel acetate solutions within the concentration range 0.05 0.35 M. The results proved that the concentration found in solution Ni.1 produces the most bright metallic nickel electroplate and the best coverage of the cathode. Therefore, this solution was selected as a basis to construct a suitable bath for nickel electroplating from acetate electrolytes. -
3.1. Polarization curves Figure 1 shows the cathodic polarization curves of nickel electroplating from solutions Ni. 1 Ni.4c. The data of curve a reveal that the electroplating of nickel from solution Ni.1 is accompanied by a relatively high cathodic polarization. Moreover, an arrest corresponding to the limiting diffusion is observed before the evolution of hydrogen. current Ni2~ion discharge Theofinclusion of 0.3 mol boric acid F’ in solution Ni.2 leads to a con-
siderable shift of the polarization curve to less negative potential values (curve b). However, it is well known that boric acid is recommended as an efficient buffer in nickel electroplating baths to maintain a constant pH at the cathode layer [1].
~
_______________
o
—800
—880
—960
—1040
—1120
Cathode potential (E), mV (SCS)
Fig. 1. Polarization curves for nickel electroplating from solutions: (a) Ni.3, (d) Ni.4a, (e) Ni.4b and (f) Ni.4c.
Ni.1, (b) Ni.2, (c)
316
On addition of 0.2 mol acetic acid F’ (solution Ni.3), to avoid the formation of nickel hydroxide and basic oxides at the cathode, a further depolarizing effect is achieved (curve c). This effect may be attributed to the decrease in the overpotential of H~ion discharge [12]. Consequently, an increase in the rate of hydrogen gas evolution in form of bubbles would be expected rather than the adsorption of the liberated hydrogen on the cathode which originally was responsible for the increase in the cathodic polarization [4]. A slight polarization-increasing effect is observed as a result of the addition of 0.1 mol sodium acetate F’, as a conducting salt in solution Ni.4a (curve d). The feature may be due to the formation of soluble Ni2~—acetate complex ions with a stability constant of log K, = 0.41 [13]. Such a complex formation gives rise to a decrease in the concentration of the free Ni2~ ions, especially in the cathode layer. This trend is enhanced by the continuous discharge of Ni2~ions which leaves behind an increasing concentration of the complexing ions. In fact, the composition of Ni.4a fulfils the requirements of a suitable bath for nickel electroplating with respect to the presence of a buffer, acid medium and conducting salt. A lowering of the concentration of Nj2~ions in solutions Ni.4b and Ni.4c results in a considerable increase in the cathodic polarization (curves e and f). In addition, the polarization curves lie in the region beyond the arrest corresponding to the limiting diffusion current of Ni2~ion discharge (curve a). This phenomenon could be accounted for by the depletion of Ni2~ions in the cathode layer and, accordingly, the preferential discharge of H~ions. Therefore, the latter data (curves e and f) confirm that the concentration of Ni2~ions in solution Ni.4a is the optimum for producing a nickel electroplate with relatively better characteristics. In an attempt to improve the quality of the nickel electroplates from solution Ni.4a, some addition agents were introduced into this solution (Table 1). Inspection of Fig. 2 reveals, in general, that the electroplating of nickel from the additive-containing solutions is attended by a greater polarization than from the additive-free solution (curve a). Moreover, the polarization-increasing effect created by the addition of gelatin in solution Ni.5 (curve b) is more remarkable than that exerted by the addition of toluene-4-sulphonic acid (TSA) in solution Ni.6 (curve c). However, the combined effect of the two addition agents in solution Ni.7 (curve d) appears to be not simply additive. The increase in cathodic polarization for gelatin could be referred to the formation of a very thin protein membrane in the cathode layer [14]. This membrane would result, to some extent, in hindrance of the migration and discharge of Ni2~ions. In contrast, the increase in cathodic polarization for TSA may be attributed to the adsorption and the consequent hydrogenolysis of this compound on the cathode yielding toluene and bisuiphite anion. The latter anion is reduced at the freshly forming nickel surface to sulphide ion which is incorporated into the nickel plate as nickel sulphide [6, 15]. The toluene formed may be adsorbed on the cathode; it is not hydrogenated, and in this sense it is
317 3.0 a
c
bd
2.5 Is
‘a
~
2.0
~ a
1.5
aa
1.0
aa ‘a aa
a
0.5
~-‘
0
—.———1I
0
—840
—880
I
—920
I
—960
—1000
Cathode potential (14), mV(SCE) Fig. 2. Polarization curves for nickel electroplating from solutions: (a) Ni.4a, (b) Ni.5, (c) Ni.6 and (d) Ni.7.
OH
14
///~~// ///t/5///
3C.()_S0,H —+143 C~~S03H—+ H3CrJ3o3H—~H3O~~)( .2_4. 143C.G>
/~//////~///
+ HS0~
p1/i
Cathode surface
Fig. 3. Mechanism for hydrogenolysis of TSA during nickel electroplating from solutions Ni.6 and Ni.7.
reversibly absorbed [16]. This suggested mechanism is shown schematically in Fig. 3. The cathodic polarization curves for nickel electroplating from solution Ni.7 were traced under the influence of sinusoidal superimposed a.c. Figure 4(A) shows that superimposed a.c. of a variable density, at a constant frequency of 50 Hz, shifts the cathodic polarization to less negative values. Furthermore, as the density of the a.c. j~.increases, the magnitude of the polarization shift also increases; but for the same j~.value the depolarizing effect decreases with increasing the d.c. density. Similar results have been reported for cobalt, nickel and cobalt—nickel alloys from Watts [10] and ammoniacal complex [11] baths. However, it was observed experimentally during the electroplating of nickel from solution Ni.7 that the anodic polarization is markedly greater than the cathodic polarization. This result may explain the data of Fig. 4 in terms of the expected distortion of the sine wave of the a.c. at the cathode surface [17]. Consequently, this leads to an asymmetric polarizabiity of the cathode [18 20], its potential fluctuating -
318 1 • 50
(A)
1.25
~
T
(B)
0.0
~
1.00
I
_________________
0.5
a d
c
b
./~2-’
_______________________
0
—500
—600
—700
—800
—900
—1000
Cathode pot~ntial (14), mV (SCE)
Fig. 4. Effect of superimposed a.c. density (w = 50 Hz) on (A) the polarization curves and (B) Tafel lines of nickel electroplating from solution Ni.7: (a) d.c. only, (b) j~..= 2. 0.20, (c)j~..= 0.50 and (d)j~..= 1.33 A dm
periodically, resulting in an average potential value which is less negative than the potential corresponding to a d.c. only. Similarly, at a constant j~.of 0.66 A dm”2, a depolarizing effect at the cathode could be observed and this effect decreases with increasing frequency (~)of the superimposed a.c. as shown in Fig. 5(A). These data could be related to the fact that, at a low frequency the time of a single oscifiation is enough for the rearrangement of the successive anodic and cathodic polarizations and thus the periodic fluctuation of the cathode potential is marked. This feature decreases as the frequency of the superimposed a.c. increases and its effect on the polarization is gradually diminished. Figures 4(b) and 5(b) show that superimposed a.c. of variable density and frequency, at low d.c. densities, creates shifts in the derived Tafel lines similar to those observed for the corresponding polarization curves. These shifts in the Tafel lines could be ascribed to changes in the concentration of Ni2~ions in the cathode layer [21]. However, it has been reported that passage of a.c. through an electrolyte of complex ions may produce periodical concentration changes which depend not only on the a.c. density and frequency, but also on the complex stability constants [22]. 3.2. Current efficiency The cathodic current efficiency f(%) of nickel electroplating was determined as a function of the plating solution-constituents, at a current density of 2 A dm2 and duration of 10 min, and the results are given in Table 1. The nickel electroplates from solutions Nil Ni.3 were nonadherent and, -
319 1.50 (A)
1.25
i1~
____________________ -
1
‘0
(B)
0.0.
~-0.5• ~
0.75
—1.0-
b~
0,7
I -
‘0 ,
aa
I
I
-500 -700 14, mV (SCE)
—900
0.50
b
a ~‘
a
d
I
c
d
a
0.25
0
~j
0
I
—500
I
—600
I
—700
Cathode potential
—800
I
—900
I
—1000
(14), mV (SCE)
Fig. 5. Effect of superimposed a.c. frequency (j_ 0.66 A dm’) on (A) the polarization curves and (B) Tafel lines of nickel electroplating from solution Ni.7: (a) d.c. only, (b) = 50, (c) w = 250 and (d) c~ = 1000 Hz.
therefore, it was difficult to determine f from these solutions with enough accuracy. 2~’ions On the inother hand, Ni.4a f decreases with a decrease in the solutions c. This trend of decrease in concentraf could be tion of Nito the corresponding increase in the cathodic polarization (Fig. 1) referred and the preferential discharge of H~’ions. A slight improvement in f of nickel electroplating is achieved as a result of the inclusion of either gelatin (solution Ni. 5) or TSA (solution Ni.6), in comparison to f for the additive-free solution Ni.4. Also a similar increase in f is observed for a combination of both additives in solution Ni.7. However, these results contradict the experimental finding that electroplating of nickel from the additive-containing solutions is accompanied by greater polarization than from the additive-free solution (Fig. 2). These results may be correlated with the expected incorporation of small amounts of these addition agents and/or their reduction products into the nickel deposit, as discussed in Section 3.1. Nevertheless, f of nickel electroplating from the acetate solutions examined, whether in the absence or the presence of addition agents, is relatively lower than the values of f observed with other acidic baths [1, 23]. Table 2 presents the influence of superimposed a.c., of variable density and frequency, on f of nickel electroplating from solution Ni.7. A notable feature is that a superimposed a.c. leads to a considerable decrease in f of nickel electroplating. In addition, the higher the superimposed a.c. density and the lower its frequency the more pronounced is the decrease in f. This -
320 TABLE 2 Effect of superimposed a.c. on percentage current efficiency of nickel electroplating from solution Ni.7 (j = 2 A ~—2, t = 10 mm)
f(%)
Superimposed a.c.
j-~(A dm’)
~ (Hz)
0.00 0.20
0 50
77.7
0.46
50
57.2
1.33 0.66 0.66 0.66
50 1000 250 50
56.4 60.2 59.2 56.8
58.4
feature may be ascribed to the possibility of dissolution of some of the nickel deposit during the anodic half-cycle of the a.c. and the additional polarization created during the cathodic half-cycle of the a.c. However, these two factors are more effective the higher is the superimposed a.c. density and the lower its frequency [9]. 3.3. Surface morphology Figure 6 shows the electron micrographs taken for the as-deposited nickel froi~some of the solutions under test, at a current density of 2 A din2 and duration of 10 mm. The nickel electroplate from solution Ni.4a consists of a disperse, fine-grained deposit leaving some bare areas on the cathode surface (Fig. 6(a)). The formation of such a pitted dendritic shaped nickel plate may be referred to the relatively low cathode efficiency for nickel deposition (Table 1); thus sufficient H~ions are discharged which slowly form hydrogen gas bubbles clinging to the growing nickel deposit. Figure 6(b) shows that the inclusion of gelatin in solution Ni.5 results in compact irregular flat layers of nickel characterized by complete suppression of dendritic growth and better coverage of the cathode, although some cracks are observed. Taking into consideration that gelatin is cationic in acid baths and aqueous gelatin solutions are coagulated by salts but solubiized by simple acids, it has been postulated [14] that a slightly thicker protein, membrane-like, film is formed in the higher current density areas, than in the lower current density areas, of the cathode diffusion layer. This film would be formed not only by cataphoresis, but also as a result of the decreased H~ ion concentration in the cathode layer due to cathodic discharge. Consequently, the permeability of this membrane would be decreased at the higher current density areas for the Ni2~ions, but not appreciably for the H4’ ions, which would be the conditions for obtaining the above-mentioned morphological changes. The inclusion of TSA in solution Ni.6 leads to the formation of a nodular, finer-grained texture completely covering the cathode surface
321 •
. ,.
‘
I
(a)
(b)
(c)
(d)
Fig. 6. Electron micrographs of nickel electroplated from: (a) solution Ni.4a, (b)2solution and t Ni.5, (c) solution Ni.6 and (d) solution Ni.7. Magnification x760, j = 2 A dm’ 10 mm.
without any visible pits (Fig. 6(c)). The disappearance of pits relates to the fact that TSA, which is classified as a Class I brightener [1], also acts as an anionic wetting agent and therefore prevents the formation of pits caused by clinging hydrogen bubbles. The striking effects achieved by TSA indicate its adsorption on growth sites, points and edges of crystals producing an inhibiting effect on the most active growing sites [24, 25]. This indication is in agreement with the observed increase in the cathodic polarization (Fig. 2) and the corresponding expected increase in the nucleation density [26]. The growth morphology of the nickel electroplate from solution Ni.7 including both gelatin and TSA (Fig. 6(d)) is very similar to that obtained from solution Ni.5 containing gelatin alone, except for the occurrence of
322
small pores instead of the cracks. It seems reasonable that these pores are not formed by actual pitting of the deposited nickel layer but probably result from local inhibition of outward growth of the deactivated growth sites by the addition agents. Inspection of Figs. 7(a) (d) reveals that a superimposed a.c. during the electroplating of nickel from solution Ni.7 gives rise to a more regular deposit exhibiting more or less porosity than in the absence of a super. imposed a.c. (Fig. 6(d)). Furthermore, Figs. 7(a) (c) show that both the pore size and density increase with an increase of the a.c. density, at a constant a.c. frequency of 50 Hz. This feature may be correlated with the -
-
(a)
(b)
(c) (d) Fig. 7. Electron micrographs of nickel electroplated solution under the influ2, w =from 50 Hz; (b)j~_Ni.7 = 0.66 A dm”, ~ = ence of superimposed p.c.: (a) j_ = 0.20 A dm 50 Hz; (c)j~-=1.33Adnf2, w=5OHz; (d)J-~=0.66Adm2, w=l000Hz. Magnification X760;J 2 Adm2 and t 10mm.
323
expected redissolution (during the anodic half-cycle of the a.c.) of the thin layer of nickel formed at the most inhibited sites of the cathode surface and its consequent deposition (during the cathodic half-cycle of the a.c.) at the less inhibited cathode areas. Therefore, the repetitive anodic—cathodic oscillations of the a.c. lead to pore coalescence. In contrast, a comparison between Figs. 7(b) and (d) shows that an increase in the a.c. frequency, at a constant a.c. density of 0.66 A dm2, results in a flat electroplate with an obvious decrease in both the pore size and density. Such a morphological modification may be related to the decrease in the depolarizing effect of the a.c. with an increase of its frequency (Fig. 5).
4. Conclusions It would be feasible to obtain a satisfactory nickel electroplate from acidic acetate solutions by careful control of the bath composition. Solution Ni.4a is the most promising and produces a yellowish white semibright nickel plate with a percentage cathodic current efficiency around 78%. Both gelatin and toluene-4-sulphonic acid proved to be efficient levelling agents and brighteners in nickel electroplating from acetate solutions. Practically, a yellowish-white mirror bright nickel plate is formed from solution Ni.7. A superimposed a.c., depending on its density and frequency, could improve the brightness of the nickel electroplate but unfortunately this is accompanied by a considerable decrease in the current efficiency of about 20%. Therefore it seems that, in future, this latter factor might be used in controlling the percentage of nickel in some electrodeposited nickel-base alloys. References 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
F. A. Lowenheim, Modern Electroplating, Wiley, New York, 1974, pp. 287 - 323. R. J. Kendric, Trans. Inst. Met. Finish., 44 (1966) 78. A. J. Dill, Plating, 61 (1974) 1001. Y. M. F. Marikar and K. I. Vasu, Chem. Era, 6 (1970) 5. G. B. Hogaboom, U.S. Patent 2,351,966 (1944). H. Brown, Plating, 55 (1968) 1047. H. Brown, Trans. Inst. Met. Finish., 47(1969) 63. D. Singh and V. B. Singh, Indian J. Technol., 14 (1976) 189. A. M. Abd El-Halim, A. 0. Baghiaf and M. I. Sobahi, Surf. Technol., 22 (1984) 143. 5. 5. Abd El Rehim, A. M. Abd El-Halim and M. M. Osman, Surf. Technol., 22 (1984) 337. S. S. Abd El Rehim, A. M. Abd El-Halim and M. M. Osman, Acta Chim. Acad. Sci. Hung., 117(1984)393. S. S. Abd El Rehim, Acta Chim. Acad. Sci. Hung., 82 (1974) 535. L. G. Sillén and A. E. Martell, Stability Constants of Metal Ion Complexes, Chemical Society, London, 1964, p. 366. C. A. Hampel, Encyclopedia of Electrochemistry, Reinhold, London, 1964, p. 125. J. Matulis, A. Bodnevas and M. Vainilaviciene, Plating, 56 (1969) 1147.
324 16 A. H. Du Rose, Trans. Inst. Met. Finish., 38 (1961) 27. 17 J. Devay and L. Meszaros, Acta Chirn. Acad. Sci. Hung., 43 (1965) 17. 18 S. 5. Abd El Rehim and A. M. Abd El-Halim, Acta Chim. Acad. Sci. Hung., 80 (1974) 65. 19 S. S. Abd El Rehim and M. G. Helmy, Acta Chim. Acad. Sci. Hung., 89 (1976) 215. 20 S. S. Abd El Rehim, Acta Chim. Acad. Sci. Hung., 99 (1979) 289. 21 R. J. Walter, Tech. Proc. Am. Electroplat. Soc., 66 (1979) E-3. 22 Z. Kovac, J. Electrochem. Soc., 118 (1971) 51. 23 S. S. Abd El Rehim, A. M. Abd El-Halim and M. M. Osman, J. AppI. Electrochem., 15 (1985) 107. 24 A. T. Vagramyan and L. A. Uvarov, Trans. Inst. Met. Finish., 39 (1962) 56. 25 M. H. Jones and M. G. Kenez, Plating, 53 (1966) 995. 26 A. M. Abd El-Halim, J. App!. Electrochem., 14 (1985) 587.