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Electrodeposition of Cu-Zn alloys from glucoheptonate baths
Surface and Coatings Technology, 35 (1988) 101
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ELECTRODEPOSITION OF Cu-Zn ALLOYS FROM GLUCOHEPTONATE BATHS YUTAKA FUJIWARA and HIDEHIKO ENOMOTO Osaka Municipal Technical Research Institute, 6-50, Morinomiya 1-chome, Joto-ku, Osaka 536 (Japan) (Received December 14, 1987)
Summary Cu—Zn alloy deposition processes from glucoheptonate baths have been studied by analysing the composition of deposits, measuring the current density—potential curves and measuring the absorption spectra of the bath. Alloying interactions during codeposition were discussed on the basis of partial current density—potential curves of each component and absorption spectra of the bath. Baths adjusted to pH 4.5 and 10.0 were studied. From the pH 4.5 bath, only copper was deposited at current densities below 0.5 A dm2. At current densities above 0.5 A dm2, however, the zinc content of deposits increased rapidly with increasing current density. The current density—potential curve of the pH 4.5 alloy bath was almost an algebraic sum of those of the individual baths and did not exhibit any alloying interactions. In contrast, the compositions of deposits from the pH 10.0 bath were almost constant over a wide range of current density from 0.5 to 2.0 A dm2, and zinc was codeposited even at low current densities below 0.2 A dm2. The partial current density—potential curve of copper exhibited a less noble (negative) shift with an addition of Zn2~.This less noble shift is attributed to the formation of a more stable Cu2~complex species, on the basis of the change in the absorption spectra. However, the partial current density— potential curve of zinc was essentially unchanged in the presence of Cu2~ except that the limiting current was increased. The difference in the limiting current was ascribed to the difference in the rate of mass transfer due to H 2 evolution.
1. Introduction Cu—Zn alloy (brass) deposits are widely used for decorative purposes [1]. Brass plating is also used to promote rubber adhesion to steel tyre cords [2]. Brass has been plated commercially only from cyanide baths in spite of their toxicity; although a number of studies have been made on non-cyanide brass plating baths, no non-cyanide baths have yet been operated commercially. 0257-8972/88/$3.50
© Elsevier Sequoia/Printed in The Netherlands
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Brenner [3] made an extensive literature survey on non-cyanide plating baths before 1963. Since then, several investigations have shown the possibility of Cu—Zn alloy deposition from non-cyanide baths. For example, Aotani et al. [4] obtained bright brass deposits from tartrate baths containing a small amount of cyanide as a brightener. Sakamoto and Ohta [5], Sakamoto et al. [6] and Abd El Rehim and El Ayashy [7] investigated Cu— Zn alloy deposition from polyphosphate baths, ethylenediamine baths and tartrate baths respectively. Vagramyan et al. [8] have shown that electrodeposition of brass is not possible from glycerol—zincate baths but may be possible from pyrophosphate baths at high current densities. We have reported the relationship between deposition potential and the composition of deposits from pyrophosphate baths and that an addition of N,N,N’,N’tetrakis-2( 2-hydroxypropyl)ethylenediamine (Quadrol) to the pyrophosphate baths suppressed the preferential deposition of copper [9, 101. However, these non-cyanide baths probably have some disadvantages for commercial use such as insufficient colour and/or appearance of the deposits, a narrow operating range, bath instability or the formation of an immersion deposit on steel. Recent studies of non-cyanide baths have been made only on baths containing complexing agents familiar to the plating industry. However, McCoy and Bohacek [11] proposed a non-cyanide bath containing a large quantity of glucoheptonate salts as a complexing agent. Glucoheptonates have not been used as a main constituent of plating baths. McCoy and Bohacek claimed that the glucoheptonate bath can be used commercially if appropriate brighteners were added. Nevertheless, the optimum bath composition and operating conditions are not given. Moreover, detailed studies such as (1) the effects of plating parameters on the composition and the properties of the deposits, and (2) the mechanism of codeposition have not been reported. The present study deals with the glucoheptonate bath as one of the non-cyanide Cu—Zn alloy plating baths with commercial potential. In this paper, we study the composition of the deposits, current density—potential curves and absorption spectra of the glucoheptonate baths in connection with each other and discuss the mechanism of alloy deposition. The nature of the deposits will be discussed in a subsequent paper [12].
2. Experimental details Cu—Zn alloys were deposited under galvanostatic conditions from glucoheptonate baths of the following composition: 0.1 M CuSO4, 0.1 M ZnSO4, 0.5 M sodium glucoheptonate (CH2OH(CHOH)5COONa), pH 4.5 or 10.0. The pH of the bath was adjusted with NaOH or H2S04. Reagent-grade CuSO45H2O, ZnSO4•7H20, NaOH (95%) and H2S04 (95%) were used. Sodium glucoheptonate was sodium heptonate dihydrate supplied by Croda Bowmans Chemicals Ltd. The bath temperature was 50 ±1 °Cand the bath
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was not stirred. Galvanostatic deposition was carried out using a Hokuto Denko HA-501 potentiostat—galvanostat and simultaneous monitoring of the deposition potential. A Hokuto Denko HF-201 coulomb meter was used to control the amount of charge during deposition. The cathode was a nickelplated stainless steel sheet, the anode was a platinized titanium sheet and the reference electrode was Ag—AgCl in a saturated KC1 solution connected to the cell by a Luggin capillary placed near the cathode. The total amount of copper and zinc deposited was determined by atomic absorption spectroscopy and the composition of deposits was calculated. The Cu—Zn alloy deposits as well as the nickel underlayer were dissolved simultaneously from the stainless steel substrate in a small amount of concentrated HNO3, diluted to a certain volume and analysed by atomic absorption spectroscopy. Galvanostatic deposition with simultaneous recording of deposition potentials, control of the total charge and subsequent analysis of the deposits gave the composition of the deposits vs. current density relationships, galvanostatic current density—potential curves (I—E curves) and partial current density—potential curves (partial I—E curves) of each component. Partial I—E curves of H2 evolution were determined, assuming the difference between the total current and the partial current of each component was the H2 evolution current. I—E curves under potentiodynamic conditions were also measured using a Hokuto Denko HA-501 potentiostat—galvanostat, an HB-104 function generator, an HG—104 logarithmic converter and a Riken model was D-52a 1. Denshi The cathode X—Y—t The potential sweep rate wasdeposition, 2 mV s whereas the anode copper recorder. sheet for copper deposition and alloy and the reference electrode were the same as those used in the galvanostatic deposition. For zinc deposition, a zinc sheet was used as the cathode. The absorption spectra of Cu2~—glucoheptonate solutions and Cu2+_ Zn2~—glucoheptonate solutions were measured between 400 and 900 nm with a Shimadzu UV-265FW spectrophotometer. The optical path length was 1 mm.
3. Results and discussion
3.1. Effects of current density on the composition of the deposits Figure 1 shows the effects of current density on the composition of deposits. The composition—reference line (CRL) [13] is also shown in Fig. 1. The composition—reference line is an auxiliary line where the composition of the deposits is the same as the metal ion concentration ratio in the bath. For the pH 4.5 (as prepared) bath, there were plots far above the composition—reference line at current densities 1.0 A dm2 or below. Thus, preferential deposition of copper occurred at low current densities and, in particular, only copper was deposited at current densities below 0.5 A dm~2.However, at current densities above 0.5 A dm2, the copper content of the deposits rapidly decreased with increasing current density and approached the
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composition—reference line. The composition of the deposits obtained at high current densities was almost the same as the metal ion concentration ratio in the bath. Also, from the bath adjusted to pH 10.0, copper was preferentially deposited at current densities 2.0 A dm2 or below. The preferential deposition was, however, not as pronounced in the pH 4.5 bath. Furthermore, the composition of the deposits was almost constant (Cu60%—Zn40%) at current densities between 0.5 and 2.0 A dm2 and zinc was codeposited even at current densities below 0.2 A dm2. At current densities greater than 4.0 A dm2, the composition of the deposits approached the bath composition in a similar manner to the pH 4.5 bath. For the commercial use of an alloy plating system, the most basic factor is whether or not the composition of deposits is constant over a wide range of current density. The glucoheptonate bath adjusted to pH 10.0 may be a potential alternative to the commercial cyanide brass plating baths, because the composition of deposits is almost constant over a wide current density range. The difference in the features of the deposit composition vs. current density relationship between the pH 4.5 bath and the pH 10.0 bath suggests that discharging species and alloy deposition processes change with the bath pH. To confirm this, total I—E curves of the pH 4.5 bath and the pH 10.0 bath were measured and the deposition potentials of alloy deposition were compared with those of the individual copper and zinc deposition. The individual deposition was carried out from the bath of the same composition as the alloy bath, except that one of the parent metal ions was absent. (Individual baths will be referred to as “the copper bath” and “the zinc bath”.) Furthermore, partial I—E curves of copper and zinc deposition from the alloy baths were compared with partial I—E curves of those from the individual baths.
3.2. Current density—potential curves of the pH 4.5 bath Figure 2 shows the total I—E curves of the pH 4.5 baths. Plots show the results of the galvanostatic measurements and curves were measured by the potential sweep method (sweep rate, 2 mV s~1). Since the sweep rate was slow enough to attain a quasi-stationary condition, the results of the potential sweep measurements were in good agreement with those of the galvanostatic measurements. In the galvanostatic measurements, deposition potentials were immediately stabilized. However, in some cases at high current densities, deposition potentials gradually shifted to more noble (positive) values from the initial values which were stable for a while. The deposition potential shifts during 10 C cm2 electrolysis are shown in Fig. 2 by arrows. The deposits obtained with the noble shift of the deposition potential had a rough surface or a powdery form. Therefore, the noble shift of the deposition potential is probably attributed to the decrease in net current density caused by the increase in cathode surface area. The total I—E curves of the copper bath gave a well-defined limiting current plateau of 0.5 A dm~2at potentials between —0.7 and —1.0 V. A
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well-defined limiting current was also obtained in the total I—E curve of the zinc2.bath at potentials between —1.2of and where thetwo value was 1.0 A However, the total I—E curve the—1.4 alloyV,bath gave limiting curdm rent plateaux at around —0.7 V and —1.2 V, whereas these plateaux were not as clear as those of the individual baths. Thus, the total I—E curve of the alloy bath was composed of two waves which were not sharply distinguished. The first wave is attributed to copper deposition and the second is attributed to zinc deposition, because of the agreement of the potentials where limiting currents are attained. Cu—Zn alloys were deposited only at the second wave region under the diffusion control of the Cu2~species. However, only copper was deposited at the potentials of the first wave region, i.e. at current densities below 0.5 A dm2, as shown in Fig. 1. This situation can be seen more explicitly by comparing each partial I—E relationship of the alloy bath with that of the individual baths. Figures 3 and 4 show the partial I—E relationship of copper deposition and that of zinc deposition respectively. The total I—E curves of the individual deposition measured by a potential sweep method are also shown by solid curves. The partial I—E relationship of both copper and zinc deposition from the alloy bath coincided with those from the individual baths. Moreover, the partial I—E plots were in good agreement with the total I—E curves of the individual baths at current densities below the limiting current
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plateau, i.e. the current efficiencies were almost 100% at low current densities. However, the partial current of copper exceeded the limiting current plateau at the less noble potentials where H2 evolution occurred. This is probably because of the enhancement of mass transfer by the evolved H2 gas bubbles [14]. Consequently, for the pH 4.5 bath, the I—E curve of the alloy bath was almost an algebraic sum of the curves of the individual baths and there was little effect of alloying interactions on the rate of deposition of each component. Furthermore, zinc can be codeposited only at higher current densi2); ties than the limiting current density ofunder copper deposition (0.5 and A dm therefore, Cu—Zn alloys were deposited diffusion control, hence the Cu—Zn alloy deposition from the pH 4.5 bath can be classified as a regular plating system [15]. Similar situations were encountered in the Cu—Zn alloy deposition from pyrophosphate baths without additives [10]. 3.3. Current density—potential curves of the pH 10.0 bath Figure 5 shows the total I—E curves of the pH 10.0 bath. The symbols and curves are the same as for Fig. 2 except that the baths are adjusted to pH 10.0. Well-defined limiting current plateaux were not observed in the I—E curves of each individual bath as well as the alloy bath in contrast to the pH 4.5 bath. The total I—E curves of both individual baths were shifted to less noble potentials compared with those of the pH 4.5 baths. The less noble shifts of the total I—E curves with the rise in bath pH was considerably larger
107
in the copper bath than in the zinc bath, and hence the deposition potentials of the individual copper and zinc deposition approached each other. The total I—E curve of the alloy bath was located at less noble potentials than that of the copper bath and, in contrast to the pH 4.5 bath, was not an algebraic sum of the total I—E curves of the individual baths. Both in the individual baths and in the alloy bath, H2 evolution was observed even at the lowest current density examined. Since the significant current due to the simultaneous H2 evolution concealed the features of the I—F relationship of copper, zinc and Cu—Zn alloy deposition, the total I—F curves did not exhibit any limiting current plateaux but a monotonous increase. Therefore, the partial I—F relationship must be investigated to elucidate the effects of alloying interactions on the deposition potentials or rate of each component. Figures 6, 7 and 8 show the partial I—E relationship of copper deposition, zinc deposition and H2 evolution respectively. From the pH 10.0 alloy bath, “unalloyed” zincwill in an stateinwas codeposited at current densities 2 this be ionic reported a subsequent paper [12]. Thus, the below 1 A dm partial currents of zinc deposition from the alloy bath shown in Fig. 7 are not calculated from the total zinc content in deposits but from “metallic” zinc content. The amount of “metallic” zinc in deposits was estimated on the basis of the composition vs. lattice constant relationships [12, 16]. Partial I—F curves of copper deposition from both the alloy bath and the copper bath gave a limiting current at less noble potentials than —1.6 V.
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Fig. 7. Galvanostatic partial current density—potential relationship of zinc deposition from pH 10.0 baths; •, pH 10.0 alloy bath; Y, pH 10.0 zinc bath; bath compositions are the same as for Fig. 5. Fig. 8. Galvanostatic partial current density—potential relationship of H 2 evolution from pH 10.0 baths; ‘pH 10.0 alloy bath; A, pH 10.0 copper bath; V. pH 10.0 zinc bath; bath compositions are the same as for Fig. 5.
The limiting current density of the2.alloy bath andatthat of the copperbelow bath In contrast, current densities werelimiting almost current, equal: about 2.5 A dm the deposition potentials from the alloy bath were apparently less noble than those from the copper bath. In other words, the deposition potentials of copper at lower current densities than the limiting current were shifted to less noble values on adding Zn2~to the bath. This can be explained as follows: (1) species taking place directly in copper deposition were altered in the presence of Zn2’~and a larger overvoltage for copper deposition was required, whereas the diffusion coefficient of the discharging species was not significantly changed, or (2) zinc-containing species were adsorbed on and covered the cathode surface and inhibited copper deposition only at potentials more noble than —1.6 V. In contrast to the partial copper deposition, partial I—F curves of zinc deposition were virtually unchanged in the presence of Cu2~in the bath except that the limiting current was increased. Thus, the presence of Cu2~in the bath may not change the discharging species or processes of zinc deposition. The difference in the limiting current between the alloy bath and the zinc bath is probably due to the difference in the rate of H 2 evolution (Fig. 8), which enhances the mass transfer in the cathode diffusion layer. Thus, the higher rate of H2 evolution in the alloy bath caused the higher rate of mass transfer in the cathode diffusion layer, and hence resulted in the larger limiting current.
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The H2 evolution rate in the alloy bath and that in the copper bath were comparable, whereas that in the zinc bath was considerably smaller. The difference in the H2 evolution rate can be explained on the basis of the difference in the hydrogen overpotential of the cathode material, i.e. the dominant surface component of the deposits. The hydrogen overpotential of copper is known to be smaller than that of zinc [17]. The higher rate of H2 evolution in the copper bath than in the zinc bath is due to the smaller hydrogen overpotential of copper than that of zinc. Similar rates of H2 evolution in the alloy bath and the copper bath suggest that most of the surface of alloy deposits is essentially copper rather than zinc. Therefore, the codeposition mechanism by which the adsorbed species containing zinc covered the cathode surface and inhibited the copper deposition should be excluded. Thus, the2~should less noblebeshift of the copper potentials with attributed to the deposition change of the discharging the addition of Zn species of copper. Consequently, in the pH 10.0 bath, cathode potential plays an important role in alloy deposition, whereas the more noble component copper is still preferentially deposited. Therefore, the Cu—Zn alloy deposition from the pH 10.0 bath can be classified as an irregular plating system [151. 3.4. Absorption spectra Figure 9 shows the typical absorption spectra of Cu21’—glucoheptonate solutions and Cu2~—Zn2~—glucoheptonate solutions in the visible and near infrared region. The absorption curve of 0.1 M CuSO 4 solution is also shown 2~—glucoheptonate solutions had a in Fig.absorption 9. The absorption Cu larger peak at a spectra shorter of wavelength than those of Cu2’~solution; this indicates the formation of Cu2~—glucoheptonatecomplexes. Moreover, the spectral difference between the pH 4.5 solution and the pH 10.0 solution indicates the formation of different complex species in these solutions. Cu2~ complex species formed in the pH 10.0 bath were probably more stable or
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Fig. 9. Absorption spectra of Cu2’~—glucoheptonate solutions and Cu2~—Zn2’~—glucoheptonate solutions: (1) 0.1 M CuSO 4 (2) 0.1 M CuSO4, 0.5 M sodium glucoheptonate, pH 4.6 (as prepared); (3) 0.1 M CuSO4, 0.1 M Zn504, 0.5 M sodium glucoheptonate, pH 4.5 (as prepared); (4) 0.1 M CuSO4, 0.5 M sodium glucoheptonate, pH 10.0 (adjusted with NaOH); (5) 0.1 M CuSO4, 0.1 M ZnSO4, 0.5 M sodium glucoheptonate, pH 10.0 (adjusted with NaOH).
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less labile than those in the pH 4.5 bath, and hence the partial I—E curve of copper in the pH 10.0 bath was shifted to less noble potentials compared with the I—E curve of copper in the pH 4.5 bath. The absorption spectrum of the pH 4.5 Cu2~—Zn2~—glucoheptonate solution almost coincided with that of the Cu2’F_glucoheptonate solution. Since the solution free from Cu2~gave no characteristic absorption peak and has its absorbance near zero over the whole wavelength range examined, the absorption spectra of the Cu2~—Zn2~—glucoheptonate solutions reflect only the Cu2~complex species in solutions. Therefore, the coincidence of the spectra of the solutions with and without Zn2~ indicates that the Cu2~’ complex species formed in the pH 4.5 solution are not changed in the presence of Zn2’4’. However, the absorption spectrum of the pH 10.0 Cu2~—glucoheptonate solution was apparently changed with the addition of Zn2’4’ the absorbance at wavelengths longer than 660 nm, where the Cu2’4’—glucoheptonate solution gave an absorption peak, was significantly decreased whereas absorbance at wavelengths shorter than 620 nm was slightly increased. The wavelength of maximal absorption was consequently shifted from 660 to 645 nm. Furthermore, the absorbance increase in the wavelength region of 480 nm or less became more rapid. These spectral changes indicate that Cu2’4 in the pH 10.0 solution forms new complex species with the addition of Zn2~. Glucoheptonate anion has one carboxyl group and six hydroxyl groups. These seven groups would be dissociated successively and coordinated to metal ions with increasing pH. Thus, the Cu2’4’—glucoheptonate—Zn2’4’ binuclear complex may conceivably be formed in the pH 10.0 solution. Simultaneous H 2 evolution in the pH 10.0 bath would cause the rise of the pH at the cathode surface from that of the bulk, and different complex species from those present in the bulk may be formed in the high pH region near the cathode if glucoheptonates did not have the ability of surface buffering. Since we have no data of the effectiveness of glucoheptonates in 2~combuffering against surface pH rise, it is difficult to see whether the Cu plex species present in the bulk of the pH 10.0 baths are identical with or different from copper discharging species near the cathode. Nevertheless, both the spectral change of the Cu2’4’—glucoheptonate solution and the less noble shift of the partial I—E curve of copper deposition were observed with the addition of Zn2~’.Therefore, Zn2’~’in the pH 10.0 bath altered both the Cu2~ complex species in the bulk and copper discharging species on the cathode surface, and lowered the rate of copper deposition. This decrease in copper deposition rate or the less noble shift in copper deposition potentials resulted in the higher zinc content of the deposits at lower current densities than that of the deposits obtained from the pH 4.5 bath. 4. Conclusions The current density vs. deposit composition relationships of Cu—Zn alloy deposits from glucoheptonate baths depended greatly on the pH of the
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bath. The I—F curve of the Cu—Zn alloy deposition from the pH 4.5 bath was expressed simply as an algebraic sum of the I—E curves of the individual copper and zinc depositions without alloying interactions. Zinc was deposited only at current densities above the limiting current density of copper deposition (0.5 A dm2), whereas only copper was deposited at current densities below 0.5 A dm2. At current densities above 0.5 A dm2, the zinc content of deposits increased rapidly with increasing current densities. The effects of Zn2’4 on the absorption spectra of Cu2~—glucoheptonatesolutions revealed that the predominant Cu2~—glucoheptonate complex species were not changed in the presence of Zn2’4. In contrast, the composition of the deposits from the pH 10.0 bath was almost constant over a wide current density range from 0.5 to 2.0 A dm2, and zinc was codeposited even at current densities below 0.2 A dm2. Therefore, the pH 10.0 glucoheptonate bath could be put to commercial use. Partial I—F curves of each component exhibited alloying interactions; deposition potentials of copper were shifted to less noble values and the limiting current density of zinc was increased in the alloy bath. The deposition potential shifts of copper were attributed to the change of the copper discharging species in the presence of Zn2~. Predominant Cu2~complex species in the bulk of the bath were also changed with the addition of Zn2~,where the new species may conceivably be a Cu2~—glucoheptonate—Zn2’4’ binuclear complex. The increase in limiting current density of zinc was attributed to the enhancement of mass transfer due to a high H 2 evolution rate in the alloy bath. References 1 Staff Report, PlatingSurf. Finish., 69 (2) (1984) 38. 2 W. J. van Ooij, Rubber Chem. Technol., 57 (1984) 421. 3 A. Brenner, Electrodeposition of Alloys, Vol. 1, Academic Press, New York, 1963, p. 457. 4 K. Aotani, I. Kaneko, S. Takahashi and M. Oiwa, J. Met. Finish. Soc. Jpn., 22 (1970) 496. 5 Y. Sakamoto and M. Ohta, Denki Kagaku, 44 (1976) 472. 6 Y. Sakamoto, F. Yamashita and K. Takao, Denki Kagaku, 44 (1976) 524. 7 S. S. Abd El Rehim and M. E. El Ayashy, J. Appl. Electrochem., 8 (1978) 33. 8 T. Vagramyan, 3. S. L. Leach and J. R. Moon, Electrochim. Acta, 24 (1979) 234. 9 Y. Fujiwara and H. Enomoto, J. Met. Finish. Soc. Jpn., 36 (1985) 33. 10 Y. Fujiwara and H. Enomoto, J. Met. Finish. Soc. Jpn., 37 (1986) 411. 11 E. H. McCoy and J. F, Bohacek, in Proc. Am. Electroplators’Soc., 7lstAnnu. Tech. Conf., 1984, N-67. 12 Y. Fujiwara and H. Enomoto, Surf. Coat. Technol., 35 (1988) 113. 13 A. Brenner, Electrodeposition of Alloys, Vol. 1, Academic Press, New York, 1963, p. 80. 14 V. A. Ettel, in I. H. Warren (ed), Application of Polarization Measurements in the Control of Metal Deposition, Elsevier, Amsterdam, 1984, p. 12. 15 A. Brenner, Electrodeposition of Alloys, Vol. 1, Academic Press, New York, 1963, p.76. 16 W. B. Pearson, Handbook of Lattice Spacings and Structure of Metals and Alloys, Pergamon Press, New York, 1958, p. 619. 17 H. Kita, J. Electrochem, Soc., 113 (1966) 1095.
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ELECTRODEPOSITION OF Cu-Zn ALLOYS FROM GLUCOHEPTONATE BATHS YUTAKA FUJIWARA and HIDEHIKO ENOMOTO Osaka Municipal Technical Research Institute, 6-50, Morinomiya 1-chome, Joto-ku, Osaka 536 (Japan) (Received December 14, 1987)
Summary Cu—Zn alloy deposition processes from glucoheptonate baths have been studied by analysing the composition of deposits, measuring the current density—potential curves and measuring the absorption spectra of the bath. Alloying interactions during codeposition were discussed on the basis of partial current density—potential curves of each component and absorption spectra of the bath. Baths adjusted to pH 4.5 and 10.0 were studied. From the pH 4.5 bath, only copper was deposited at current densities below 0.5 A dm2. At current densities above 0.5 A dm2, however, the zinc content of deposits increased rapidly with increasing current density. The current density—potential curve of the pH 4.5 alloy bath was almost an algebraic sum of those of the individual baths and did not exhibit any alloying interactions. In contrast, the compositions of deposits from the pH 10.0 bath were almost constant over a wide range of current density from 0.5 to 2.0 A dm2, and zinc was codeposited even at low current densities below 0.2 A dm2. The partial current density—potential curve of copper exhibited a less noble (negative) shift with an addition of Zn2~.This less noble shift is attributed to the formation of a more stable Cu2~complex species, on the basis of the change in the absorption spectra. However, the partial current density— potential curve of zinc was essentially unchanged in the presence of Cu2~ except that the limiting current was increased. The difference in the limiting current was ascribed to the difference in the rate of mass transfer due to H 2 evolution.
1. Introduction Cu—Zn alloy (brass) deposits are widely used for decorative purposes [1]. Brass plating is also used to promote rubber adhesion to steel tyre cords [2]. Brass has been plated commercially only from cyanide baths in spite of their toxicity; although a number of studies have been made on non-cyanide brass plating baths, no non-cyanide baths have yet been operated commercially. 0257-8972/88/$3.50
© Elsevier Sequoia/Printed in The Netherlands
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Brenner [3] made an extensive literature survey on non-cyanide plating baths before 1963. Since then, several investigations have shown the possibility of Cu—Zn alloy deposition from non-cyanide baths. For example, Aotani et al. [4] obtained bright brass deposits from tartrate baths containing a small amount of cyanide as a brightener. Sakamoto and Ohta [5], Sakamoto et al. [6] and Abd El Rehim and El Ayashy [7] investigated Cu— Zn alloy deposition from polyphosphate baths, ethylenediamine baths and tartrate baths respectively. Vagramyan et al. [8] have shown that electrodeposition of brass is not possible from glycerol—zincate baths but may be possible from pyrophosphate baths at high current densities. We have reported the relationship between deposition potential and the composition of deposits from pyrophosphate baths and that an addition of N,N,N’,N’tetrakis-2( 2-hydroxypropyl)ethylenediamine (Quadrol) to the pyrophosphate baths suppressed the preferential deposition of copper [9, 101. However, these non-cyanide baths probably have some disadvantages for commercial use such as insufficient colour and/or appearance of the deposits, a narrow operating range, bath instability or the formation of an immersion deposit on steel. Recent studies of non-cyanide baths have been made only on baths containing complexing agents familiar to the plating industry. However, McCoy and Bohacek [11] proposed a non-cyanide bath containing a large quantity of glucoheptonate salts as a complexing agent. Glucoheptonates have not been used as a main constituent of plating baths. McCoy and Bohacek claimed that the glucoheptonate bath can be used commercially if appropriate brighteners were added. Nevertheless, the optimum bath composition and operating conditions are not given. Moreover, detailed studies such as (1) the effects of plating parameters on the composition and the properties of the deposits, and (2) the mechanism of codeposition have not been reported. The present study deals with the glucoheptonate bath as one of the non-cyanide Cu—Zn alloy plating baths with commercial potential. In this paper, we study the composition of the deposits, current density—potential curves and absorption spectra of the glucoheptonate baths in connection with each other and discuss the mechanism of alloy deposition. The nature of the deposits will be discussed in a subsequent paper [12].
2. Experimental details Cu—Zn alloys were deposited under galvanostatic conditions from glucoheptonate baths of the following composition: 0.1 M CuSO4, 0.1 M ZnSO4, 0.5 M sodium glucoheptonate (CH2OH(CHOH)5COONa), pH 4.5 or 10.0. The pH of the bath was adjusted with NaOH or H2S04. Reagent-grade CuSO45H2O, ZnSO4•7H20, NaOH (95%) and H2S04 (95%) were used. Sodium glucoheptonate was sodium heptonate dihydrate supplied by Croda Bowmans Chemicals Ltd. The bath temperature was 50 ±1 °Cand the bath
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was not stirred. Galvanostatic deposition was carried out using a Hokuto Denko HA-501 potentiostat—galvanostat and simultaneous monitoring of the deposition potential. A Hokuto Denko HF-201 coulomb meter was used to control the amount of charge during deposition. The cathode was a nickelplated stainless steel sheet, the anode was a platinized titanium sheet and the reference electrode was Ag—AgCl in a saturated KC1 solution connected to the cell by a Luggin capillary placed near the cathode. The total amount of copper and zinc deposited was determined by atomic absorption spectroscopy and the composition of deposits was calculated. The Cu—Zn alloy deposits as well as the nickel underlayer were dissolved simultaneously from the stainless steel substrate in a small amount of concentrated HNO3, diluted to a certain volume and analysed by atomic absorption spectroscopy. Galvanostatic deposition with simultaneous recording of deposition potentials, control of the total charge and subsequent analysis of the deposits gave the composition of the deposits vs. current density relationships, galvanostatic current density—potential curves (I—E curves) and partial current density—potential curves (partial I—E curves) of each component. Partial I—E curves of H2 evolution were determined, assuming the difference between the total current and the partial current of each component was the H2 evolution current. I—E curves under potentiodynamic conditions were also measured using a Hokuto Denko HA-501 potentiostat—galvanostat, an HB-104 function generator, an HG—104 logarithmic converter and a Riken model was D-52a 1. Denshi The cathode X—Y—t The potential sweep rate wasdeposition, 2 mV s whereas the anode copper recorder. sheet for copper deposition and alloy and the reference electrode were the same as those used in the galvanostatic deposition. For zinc deposition, a zinc sheet was used as the cathode. The absorption spectra of Cu2~—glucoheptonate solutions and Cu2+_ Zn2~—glucoheptonate solutions were measured between 400 and 900 nm with a Shimadzu UV-265FW spectrophotometer. The optical path length was 1 mm.
3. Results and discussion
3.1. Effects of current density on the composition of the deposits Figure 1 shows the effects of current density on the composition of deposits. The composition—reference line (CRL) [13] is also shown in Fig. 1. The composition—reference line is an auxiliary line where the composition of the deposits is the same as the metal ion concentration ratio in the bath. For the pH 4.5 (as prepared) bath, there were plots far above the composition—reference line at current densities 1.0 A dm2 or below. Thus, preferential deposition of copper occurred at low current densities and, in particular, only copper was deposited at current densities below 0.5 A dm~2.However, at current densities above 0.5 A dm2, the copper content of the deposits rapidly decreased with increasing current density and approached the
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composition—reference line. The composition of the deposits obtained at high current densities was almost the same as the metal ion concentration ratio in the bath. Also, from the bath adjusted to pH 10.0, copper was preferentially deposited at current densities 2.0 A dm2 or below. The preferential deposition was, however, not as pronounced in the pH 4.5 bath. Furthermore, the composition of the deposits was almost constant (Cu60%—Zn40%) at current densities between 0.5 and 2.0 A dm2 and zinc was codeposited even at current densities below 0.2 A dm2. At current densities greater than 4.0 A dm2, the composition of the deposits approached the bath composition in a similar manner to the pH 4.5 bath. For the commercial use of an alloy plating system, the most basic factor is whether or not the composition of deposits is constant over a wide range of current density. The glucoheptonate bath adjusted to pH 10.0 may be a potential alternative to the commercial cyanide brass plating baths, because the composition of deposits is almost constant over a wide current density range. The difference in the features of the deposit composition vs. current density relationship between the pH 4.5 bath and the pH 10.0 bath suggests that discharging species and alloy deposition processes change with the bath pH. To confirm this, total I—E curves of the pH 4.5 bath and the pH 10.0 bath were measured and the deposition potentials of alloy deposition were compared with those of the individual copper and zinc deposition. The individual deposition was carried out from the bath of the same composition as the alloy bath, except that one of the parent metal ions was absent. (Individual baths will be referred to as “the copper bath” and “the zinc bath”.) Furthermore, partial I—E curves of copper and zinc deposition from the alloy baths were compared with partial I—E curves of those from the individual baths.
3.2. Current density—potential curves of the pH 4.5 bath Figure 2 shows the total I—E curves of the pH 4.5 baths. Plots show the results of the galvanostatic measurements and curves were measured by the potential sweep method (sweep rate, 2 mV s~1). Since the sweep rate was slow enough to attain a quasi-stationary condition, the results of the potential sweep measurements were in good agreement with those of the galvanostatic measurements. In the galvanostatic measurements, deposition potentials were immediately stabilized. However, in some cases at high current densities, deposition potentials gradually shifted to more noble (positive) values from the initial values which were stable for a while. The deposition potential shifts during 10 C cm2 electrolysis are shown in Fig. 2 by arrows. The deposits obtained with the noble shift of the deposition potential had a rough surface or a powdery form. Therefore, the noble shift of the deposition potential is probably attributed to the decrease in net current density caused by the increase in cathode surface area. The total I—E curves of the copper bath gave a well-defined limiting current plateau of 0.5 A dm~2at potentials between —0.7 and —1.0 V. A
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well-defined limiting current was also obtained in the total I—E curve of the zinc2.bath at potentials between —1.2of and where thetwo value was 1.0 A However, the total I—E curve the—1.4 alloyV,bath gave limiting curdm rent plateaux at around —0.7 V and —1.2 V, whereas these plateaux were not as clear as those of the individual baths. Thus, the total I—E curve of the alloy bath was composed of two waves which were not sharply distinguished. The first wave is attributed to copper deposition and the second is attributed to zinc deposition, because of the agreement of the potentials where limiting currents are attained. Cu—Zn alloys were deposited only at the second wave region under the diffusion control of the Cu2~species. However, only copper was deposited at the potentials of the first wave region, i.e. at current densities below 0.5 A dm2, as shown in Fig. 1. This situation can be seen more explicitly by comparing each partial I—E relationship of the alloy bath with that of the individual baths. Figures 3 and 4 show the partial I—E relationship of copper deposition and that of zinc deposition respectively. The total I—E curves of the individual deposition measured by a potential sweep method are also shown by solid curves. The partial I—E relationship of both copper and zinc deposition from the alloy bath coincided with those from the individual baths. Moreover, the partial I—E plots were in good agreement with the total I—E curves of the individual baths at current densities below the limiting current
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plateau, i.e. the current efficiencies were almost 100% at low current densities. However, the partial current of copper exceeded the limiting current plateau at the less noble potentials where H2 evolution occurred. This is probably because of the enhancement of mass transfer by the evolved H2 gas bubbles [14]. Consequently, for the pH 4.5 bath, the I—E curve of the alloy bath was almost an algebraic sum of the curves of the individual baths and there was little effect of alloying interactions on the rate of deposition of each component. Furthermore, zinc can be codeposited only at higher current densi2); ties than the limiting current density ofunder copper deposition (0.5 and A dm therefore, Cu—Zn alloys were deposited diffusion control, hence the Cu—Zn alloy deposition from the pH 4.5 bath can be classified as a regular plating system [15]. Similar situations were encountered in the Cu—Zn alloy deposition from pyrophosphate baths without additives [10]. 3.3. Current density—potential curves of the pH 10.0 bath Figure 5 shows the total I—E curves of the pH 10.0 bath. The symbols and curves are the same as for Fig. 2 except that the baths are adjusted to pH 10.0. Well-defined limiting current plateaux were not observed in the I—E curves of each individual bath as well as the alloy bath in contrast to the pH 4.5 bath. The total I—E curves of both individual baths were shifted to less noble potentials compared with those of the pH 4.5 baths. The less noble shifts of the total I—E curves with the rise in bath pH was considerably larger
107
in the copper bath than in the zinc bath, and hence the deposition potentials of the individual copper and zinc deposition approached each other. The total I—E curve of the alloy bath was located at less noble potentials than that of the copper bath and, in contrast to the pH 4.5 bath, was not an algebraic sum of the total I—E curves of the individual baths. Both in the individual baths and in the alloy bath, H2 evolution was observed even at the lowest current density examined. Since the significant current due to the simultaneous H2 evolution concealed the features of the I—F relationship of copper, zinc and Cu—Zn alloy deposition, the total I—F curves did not exhibit any limiting current plateaux but a monotonous increase. Therefore, the partial I—F relationship must be investigated to elucidate the effects of alloying interactions on the deposition potentials or rate of each component. Figures 6, 7 and 8 show the partial I—E relationship of copper deposition, zinc deposition and H2 evolution respectively. From the pH 10.0 alloy bath, “unalloyed” zincwill in an stateinwas codeposited at current densities 2 this be ionic reported a subsequent paper [12]. Thus, the below 1 A dm partial currents of zinc deposition from the alloy bath shown in Fig. 7 are not calculated from the total zinc content in deposits but from “metallic” zinc content. The amount of “metallic” zinc in deposits was estimated on the basis of the composition vs. lattice constant relationships [12, 16]. Partial I—F curves of copper deposition from both the alloy bath and the copper bath gave a limiting current at less noble potentials than —1.6 V.
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Fig. 7. Galvanostatic partial current density—potential relationship of zinc deposition from pH 10.0 baths; •, pH 10.0 alloy bath; Y, pH 10.0 zinc bath; bath compositions are the same as for Fig. 5. Fig. 8. Galvanostatic partial current density—potential relationship of H 2 evolution from pH 10.0 baths; ‘pH 10.0 alloy bath; A, pH 10.0 copper bath; V. pH 10.0 zinc bath; bath compositions are the same as for Fig. 5.
The limiting current density of the2.alloy bath andatthat of the copperbelow bath In contrast, current densities werelimiting almost current, equal: about 2.5 A dm the deposition potentials from the alloy bath were apparently less noble than those from the copper bath. In other words, the deposition potentials of copper at lower current densities than the limiting current were shifted to less noble values on adding Zn2~to the bath. This can be explained as follows: (1) species taking place directly in copper deposition were altered in the presence of Zn2’~and a larger overvoltage for copper deposition was required, whereas the diffusion coefficient of the discharging species was not significantly changed, or (2) zinc-containing species were adsorbed on and covered the cathode surface and inhibited copper deposition only at potentials more noble than —1.6 V. In contrast to the partial copper deposition, partial I—F curves of zinc deposition were virtually unchanged in the presence of Cu2~in the bath except that the limiting current was increased. Thus, the presence of Cu2~in the bath may not change the discharging species or processes of zinc deposition. The difference in the limiting current between the alloy bath and the zinc bath is probably due to the difference in the rate of H 2 evolution (Fig. 8), which enhances the mass transfer in the cathode diffusion layer. Thus, the higher rate of H2 evolution in the alloy bath caused the higher rate of mass transfer in the cathode diffusion layer, and hence resulted in the larger limiting current.
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The H2 evolution rate in the alloy bath and that in the copper bath were comparable, whereas that in the zinc bath was considerably smaller. The difference in the H2 evolution rate can be explained on the basis of the difference in the hydrogen overpotential of the cathode material, i.e. the dominant surface component of the deposits. The hydrogen overpotential of copper is known to be smaller than that of zinc [17]. The higher rate of H2 evolution in the copper bath than in the zinc bath is due to the smaller hydrogen overpotential of copper than that of zinc. Similar rates of H2 evolution in the alloy bath and the copper bath suggest that most of the surface of alloy deposits is essentially copper rather than zinc. Therefore, the codeposition mechanism by which the adsorbed species containing zinc covered the cathode surface and inhibited the copper deposition should be excluded. Thus, the2~should less noblebeshift of the copper potentials with attributed to the deposition change of the discharging the addition of Zn species of copper. Consequently, in the pH 10.0 bath, cathode potential plays an important role in alloy deposition, whereas the more noble component copper is still preferentially deposited. Therefore, the Cu—Zn alloy deposition from the pH 10.0 bath can be classified as an irregular plating system [151. 3.4. Absorption spectra Figure 9 shows the typical absorption spectra of Cu21’—glucoheptonate solutions and Cu2~—Zn2~—glucoheptonate solutions in the visible and near infrared region. The absorption curve of 0.1 M CuSO 4 solution is also shown 2~—glucoheptonate solutions had a in Fig.absorption 9. The absorption Cu larger peak at a spectra shorter of wavelength than those of Cu2’~solution; this indicates the formation of Cu2~—glucoheptonatecomplexes. Moreover, the spectral difference between the pH 4.5 solution and the pH 10.0 solution indicates the formation of different complex species in these solutions. Cu2~ complex species formed in the pH 10.0 bath were probably more stable or
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Fig. 9. Absorption spectra of Cu2’~—glucoheptonate solutions and Cu2~—Zn2’~—glucoheptonate solutions: (1) 0.1 M CuSO 4 (2) 0.1 M CuSO4, 0.5 M sodium glucoheptonate, pH 4.6 (as prepared); (3) 0.1 M CuSO4, 0.1 M Zn504, 0.5 M sodium glucoheptonate, pH 4.5 (as prepared); (4) 0.1 M CuSO4, 0.5 M sodium glucoheptonate, pH 10.0 (adjusted with NaOH); (5) 0.1 M CuSO4, 0.1 M ZnSO4, 0.5 M sodium glucoheptonate, pH 10.0 (adjusted with NaOH).
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less labile than those in the pH 4.5 bath, and hence the partial I—E curve of copper in the pH 10.0 bath was shifted to less noble potentials compared with the I—E curve of copper in the pH 4.5 bath. The absorption spectrum of the pH 4.5 Cu2~—Zn2~—glucoheptonate solution almost coincided with that of the Cu2’F_glucoheptonate solution. Since the solution free from Cu2~gave no characteristic absorption peak and has its absorbance near zero over the whole wavelength range examined, the absorption spectra of the Cu2~—Zn2~—glucoheptonate solutions reflect only the Cu2~complex species in solutions. Therefore, the coincidence of the spectra of the solutions with and without Zn2~ indicates that the Cu2~’ complex species formed in the pH 4.5 solution are not changed in the presence of Zn2’4’. However, the absorption spectrum of the pH 10.0 Cu2~—glucoheptonate solution was apparently changed with the addition of Zn2’4’ the absorbance at wavelengths longer than 660 nm, where the Cu2’4’—glucoheptonate solution gave an absorption peak, was significantly decreased whereas absorbance at wavelengths shorter than 620 nm was slightly increased. The wavelength of maximal absorption was consequently shifted from 660 to 645 nm. Furthermore, the absorbance increase in the wavelength region of 480 nm or less became more rapid. These spectral changes indicate that Cu2’4 in the pH 10.0 solution forms new complex species with the addition of Zn2~. Glucoheptonate anion has one carboxyl group and six hydroxyl groups. These seven groups would be dissociated successively and coordinated to metal ions with increasing pH. Thus, the Cu2’4’—glucoheptonate—Zn2’4’ binuclear complex may conceivably be formed in the pH 10.0 solution. Simultaneous H 2 evolution in the pH 10.0 bath would cause the rise of the pH at the cathode surface from that of the bulk, and different complex species from those present in the bulk may be formed in the high pH region near the cathode if glucoheptonates did not have the ability of surface buffering. Since we have no data of the effectiveness of glucoheptonates in 2~combuffering against surface pH rise, it is difficult to see whether the Cu plex species present in the bulk of the pH 10.0 baths are identical with or different from copper discharging species near the cathode. Nevertheless, both the spectral change of the Cu2’4’—glucoheptonate solution and the less noble shift of the partial I—E curve of copper deposition were observed with the addition of Zn2~’.Therefore, Zn2’~’in the pH 10.0 bath altered both the Cu2~ complex species in the bulk and copper discharging species on the cathode surface, and lowered the rate of copper deposition. This decrease in copper deposition rate or the less noble shift in copper deposition potentials resulted in the higher zinc content of the deposits at lower current densities than that of the deposits obtained from the pH 4.5 bath. 4. Conclusions The current density vs. deposit composition relationships of Cu—Zn alloy deposits from glucoheptonate baths depended greatly on the pH of the
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bath. The I—F curve of the Cu—Zn alloy deposition from the pH 4.5 bath was expressed simply as an algebraic sum of the I—E curves of the individual copper and zinc depositions without alloying interactions. Zinc was deposited only at current densities above the limiting current density of copper deposition (0.5 A dm2), whereas only copper was deposited at current densities below 0.5 A dm2. At current densities above 0.5 A dm2, the zinc content of deposits increased rapidly with increasing current densities. The effects of Zn2’4 on the absorption spectra of Cu2~—glucoheptonatesolutions revealed that the predominant Cu2~—glucoheptonate complex species were not changed in the presence of Zn2’4. In contrast, the composition of the deposits from the pH 10.0 bath was almost constant over a wide current density range from 0.5 to 2.0 A dm2, and zinc was codeposited even at current densities below 0.2 A dm2. Therefore, the pH 10.0 glucoheptonate bath could be put to commercial use. Partial I—F curves of each component exhibited alloying interactions; deposition potentials of copper were shifted to less noble values and the limiting current density of zinc was increased in the alloy bath. The deposition potential shifts of copper were attributed to the change of the copper discharging species in the presence of Zn2~. Predominant Cu2~complex species in the bulk of the bath were also changed with the addition of Zn2~,where the new species may conceivably be a Cu2~—glucoheptonate—Zn2’4’ binuclear complex. The increase in limiting current density of zinc was attributed to the enhancement of mass transfer due to a high H 2 evolution rate in the alloy bath. References 1 Staff Report, PlatingSurf. Finish., 69 (2) (1984) 38. 2 W. J. van Ooij, Rubber Chem. Technol., 57 (1984) 421. 3 A. Brenner, Electrodeposition of Alloys, Vol. 1, Academic Press, New York, 1963, p. 457. 4 K. Aotani, I. Kaneko, S. Takahashi and M. Oiwa, J. Met. Finish. Soc. Jpn., 22 (1970) 496. 5 Y. Sakamoto and M. Ohta, Denki Kagaku, 44 (1976) 472. 6 Y. Sakamoto, F. Yamashita and K. Takao, Denki Kagaku, 44 (1976) 524. 7 S. S. Abd El Rehim and M. E. El Ayashy, J. Appl. Electrochem., 8 (1978) 33. 8 T. Vagramyan, 3. S. L. Leach and J. R. Moon, Electrochim. Acta, 24 (1979) 234. 9 Y. Fujiwara and H. Enomoto, J. Met. Finish. Soc. Jpn., 36 (1985) 33. 10 Y. Fujiwara and H. Enomoto, J. Met. Finish. Soc. Jpn., 37 (1986) 411. 11 E. H. McCoy and J. F, Bohacek, in Proc. Am. Electroplators’Soc., 7lstAnnu. Tech. Conf., 1984, N-67. 12 Y. Fujiwara and H. Enomoto, Surf. Coat. Technol., 35 (1988) 113. 13 A. Brenner, Electrodeposition of Alloys, Vol. 1, Academic Press, New York, 1963, p. 80. 14 V. A. Ettel, in I. H. Warren (ed), Application of Polarization Measurements in the Control of Metal Deposition, Elsevier, Amsterdam, 1984, p. 12. 15 A. Brenner, Electrodeposition of Alloys, Vol. 1, Academic Press, New York, 1963, p.76. 16 W. B. Pearson, Handbook of Lattice Spacings and Structure of Metals and Alloys, Pergamon Press, New York, 1958, p. 619. 17 H. Kita, J. Electrochem, Soc., 113 (1966) 1095.