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Dependence of coating characteristics on deposition potential for electrodeposited Zn–Ni alloys
PERGAMON
Electrochimica Acta 44 (1999) 1645±1653
Dependence of coating characteristics on deposition potential for electrodeposited Zn±Ni alloys F. Elkhatabi, M. Benballa, M. Sarret, C. MuÈller * LCTEM, Department of Chemical Physics, University of Barcelona, MartõÂ i FranqueÁs 1, 08028 Barcelona, Spain Received 13 May 1998
Abstract The electrodeposition of zinc±nickel alloys from ammonium chloride baths was studied on dierent substrates under potentiostatic and galvanostatic conditions, from low potentials/current densities where the codeposition was normal to the conditions where anomalous deposition took place. Anodic linear sweep voltammetry (ALSV) and ex situ techniques were used to characterize the coatings obtained in each condition. The results indicated that three deposition potential zones could be de®ned corresponding to practically pure nickel, a- and g-phase domains. It was observed that the abrupt change in alloy composition and current eciency that occurred during potentiostatic deposition did not correspond to the transition from normal to anomalous codeposition. In our conditions, this change always coincided with the appearance of g-phase in the coatings. # 1999 Elsevier Science Ltd. All rights reserved. Keywords: Zinc±nickel alloys; Ammonium baths; Stripping techniques
1. Introduction It has been known for many years that zinc alloys can provide improved corrosion resistance for the protection of ferrous based metals and that, among others, Zn±Ni, Zn±Co and Zn±Fe are the most common systems [1±12]. According to Brenner [13], the electrodeposition of these alloys occurs anomalously, though normal codeposition can be obtained in particular plating conditions. In most cases, when the deposition of these alloys was studied in a wide interval of current densities/deposition potentials, at a given value of current density/potential an abrupt change in the composition of the alloys and in the current eciency of the process was observed. This change, in some cases, was attributed to the transition from normal to anomalous codeposition.
* Corresponding author. Fax: +349-3402-1231; E-mail: [email protected]
Since NH4C1 baths for obtaining Zn±Ni alloys are common in galvanic industries, this group developed an industrial ammonium bath that gave uniform deposits, with a nickel content between 10±15% and high corrosion resistance [14]. The same bath was used to obtain Zn±Ni alloys under controlled conditions (pure chemical and substrates) and stripping methods were used to characterize the coatings [15, 16]. By relating the electrochemical results with those obtained using ex situ techniques, it was demonstrated that stripping methods were useful to determine the chemical and phase compositions of the alloys. These ®rst studies were essentially designed to analyze the alloys and deposition process in industrial conditions, that is, high potentials and current densities. The aims of the present study were to analyze the deposition of zinc± nickel alloys at low current densities or low polarizations, to characterize the coatings obtained in these conditions and to analyze the possible causes of the sharp change in coating and deposition characteristics mentioned above. Therefore, ammonium baths with
0013-4686/99/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 3 - 4 6 8 6 ( 9 8 ) 0 0 2 8 6 - 2
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Table 1. Concentrations of Zn2+ and Ni2+ in the electrolyte Bath
Zn2+ (mol dm-3)
Ni2+ (mol dm-3)
Zn2+/Ni2+
A B C D E F
0.63 0.53 0.76 0.53 0.63 0.30
0.25 0.21 0.31 0.03 0.14 0.85
2.5 2.5 2.5 17.7 4.5 0.23
dierent cations content were selected: the industrial bath, baths with dierent concentrations but the same Ni2+/Zn2+ ratio and electrolytes with ratios both higher and lower than usual. Potentiostatic and galvanostatic polarization curves were obtained and the alloys under the dierent conditions were characterized by using ALSV (anodic linear sweep voltammetry) and ex situ techniques, such as SEM, X-ray diraction, XPS and EDS.
2. Experimental Experiments were carried out on a three-electrode cell with a capacity of 0.1 dm3. Dierent concentrations of ZnCl2 and NiCl26H20 were studied (Table 1) with a constant amount of NH4Cl, 4.11 mol dm ÿ 3, and pH adjusted to 5.6 by adding ammonia. All reagents were Merck pro analysis grade and the water was obtained from a MilliQ water puri®cation system. The alloys were obtained at 258C by depositing the metals potentiostatically or galvanostatically onto electrodes of dierent diameter, glassy carbon (GC, b= 3 mm), nickel (b= 5 min) and Armco iron (b= 3 and 7 min) mounted in a te¯on holder and rotating at 600 rpm, the minimum speed to avoid the blocking eect of hydrogen bubbles. The working electrode was polished before each run (GC using 3.75, 1.78 and 0.3 mm grades of alumina, and Ni and Fe with 1.00 and 0.25 mm diamond compound) before being rinsed and, ®nally, held in an ultrasonic bath for 1 min. Before each experiment, the solution was purged with high purity argon. The reference electrode was an Ag/AgCl/ KCl(sat) mounted in a Luggin capillary, and an Ni sheet was used as a counter-electrode. The zinc±nickel alloys were stripped under potentiodynamic conditions in the same electrolyte from which they were deposited. Previously, it was veri®ed that no replacement reaction between Ni(II) and Zn(II) took place [15]. Electrochemical measurements were performed using an EG&G 273 potentiostat controlled by a PC 486. Scanning electron microscopy (SEM) and X-ray diffraction (XRD) were used to study the morphology
and structure of the alloys. Energy dispersive analysis of X-rays (EDS) and X-ray photoelectron spectroscopy (XPS) were used to analyze the composition of the coatings.
3. Results Fig. 1 shows the variation in the composition of the alloys with current density for three Zn2+Ni2+ ratios used in this study, together with the composition reference lines (CRL) corresponding to these baths. The in¯uence of current density on the composition of the alloys obtained with our electrolytes was normal in anomalous codeposition: a wide interval of current densities where the composition of the alloy was quite constant and a region at low current densities where the % Nidep increased abruptly. The transition current density, jT (value of j at which the composition of the alloy coincided with that of the electrolyte), was aected by electrolyte composition and was practically independent of the substrate. As observed in Fig. 1, for electrolyte D the Ni content of the alloys always remained above the CRL and a sharp increase in Nidep was observed at low current densities, although normal to anomalous transition did not take place. Fig. 2 shows the polarization curves obtained under potentiostatic conditions for baths containing dierent zinc/nickel molar ratios. In all conditions N-shaped polarization curves were observed, although with the ammonium baths used in this study the current density maximum was less pronounced than the maximum
Fig. 1. Eect of current density on nickel content of deposits and current reference lines for three baths of Table 1: (r) bath A, (*) bath D, (w) bath F.
F. Elkhatabi et al. / Electrochimica Acta 44 (1999) 1645±1653
Fig. 2. Potentiostatic polarization curves on a Ni substrate and for three baths of Table 1: (*) bath A, (w) bath D, (r) bath F.
observed in other cases [17±19]. According to other studies performed for Zn±Ni deposition, the polarization curves were found to be practically independent of the type of electrode used [20]. On examining the characteristics of the deposits and the stripping response obtained during their oxidation, three potential zones could be de®ned for Zn±Ni deposition. Fig. 3 shows the zones obtained with bath A, the industrial electrolyte, of Table 1. Zone I corresponded to the most positive potentials in the polariz-
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ation curve (ÿ700/ÿ 800 mV), where the deposit grew at low current densities (i < 0.5 mA cm ÿ 2) and with a current eciency higher than 75% (with some electrolytes current eciencies of about 90% were obtained), unlikewhat occurred with other electrolytes [18, 19]. In this zone, the potentiodynamic stripping response showed only one peak at potentials corresponding to nickel oxidation (Fig. 4(a)). EDS analysis of the deposits obtained in this zone indicated that they consisted of practically pure nickel with a maximum zinc content of about 5%. The diractograms of the deposits obtained in these conditions corroborated the electrochemical results: they showed only Ni peaks (Fig. 5(a)) and the deposits were formed of aggregates of various sizes, similar to nickel deposits (Fig. 6(a)). It is interesting to note that this low zinc content led to important changes in the electrochemical behaviour of the deposits. Fig. 4 also includes the oxidation response of a pure nickel deposit: under the same deposition conditions, the main oxidation peak was obtained at very positive potentials. In this case, the alloy obtained in this zone of potentials, although practically all nickel, had a suciently dierent structure to be easier oxidized and with a high current eciency. In this zone the deposition of nickel was inhibited by underpotentially deposited zinc, as was suggested in other cases [5, 21]. When the deposition of pure zinc was studied on a nickel electrode, it was observed that no deposition took place at these potentials, which indicated that zinc was only deposited on the nickel clusters and not on the nickel substrate. Zone II corresponded to the interval of potentials prior to the maximum of the polarization curve (ÿ850/
Fig. 3. Variation of the nickel content and current eciency with deposition potential for alloys obtained on a Ni electrode from bath A.
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Fig. 4. Potentiodynamic stripping response obtained on a nickel substrate after deposition at ÿ750 mV for 1500 s: curve (a) Zn±Ni alloy from bath A, (b) nickel deposit from a bath with the same nickel content. Scan rate 5 mV s ÿ 1.
ÿ 1040 mV). In this zone the deposition process took place at intermediate current densities (0.5±1.5 mA cm ÿ 2) and was accompanied by a high hydrogen evolution. The potentiodynamic stripping response showed a separate peak for zinc oxidation (Fig. 7), although the response was complex, with the peaks not clearly separated, and deconvolution was needed to calculate the composition of the alloys. After this deconvolution the composition determined electrochemically was consistent with the EDS and XPS results. For the more positive potentials, the percentage of nickel in the alloys was fairly constant at about 75% and current eciency gradually decreased as polarization increased. The morphology and structure of the coatings in this zone also evolved with potential; for the most positive potentials, nodular grains could be distinguished at all deposition stages (Fig. 6(b)) and the diractograms were similar to those obtained in zone I, but on increasing potential the grains were more dicult to distinguish and the diractometric peaks became broader and less intense. For the most negative potentials in this zone the nickel content of the alloys was about 65%, which corresponded to the equilibrium composition of a-phase. The X-ray diractograms obtained in these conditions veri®ed the presence of this phase (Fig. 5(b)). SEM pictures also showed the non-structured morphology characteristic of the aphase, with evidence of hydrogen bubbles (Fig. 6(c)). Since this phase tended to be amorphous and structureless, thick deposits were needed. Therefore, in zone II, as the deposition potential was increased, more zinc was deposited and an a-phase was gradually formed. The structural change from a nickel-like deposit to an a-phase structure was accompanied by an increase in hydrogen evolution,
which had its maximum value near the maximum of the polarization curve. XPS analysis performed at dierent sputtering times on coatings obtained in this II region showed the presence of oxygen in the inner part of the coatings and also indicated that the amount of oxygen increased at more negative potentials. Therefore, the deposits in this zone must contain some oxygen-containing species, probably zinc oxide/hydroxide as indicated in other cases [22, 23]. Finally, it must be mentioned that the results obtained with the dierent electrolytes indicated that the current densities obtained in the potential zones I and II were mainly dependent on the Ni2+ concentration. At about ÿ1045 mV (the exact potential depended on the substrate and bath composition) and parallel to the decrease in current in the polarization curve, a sudden increase in current eciency was observed (zone III). After this change, the composition of the alloy was maintained during a short interval of potentials between 40±45% Ni and then, as polarization increased, the nickel decreased to values less than 15%. The potentiodynamic stripping response in this zone indicated that this sharp change coincided with the appearance of the oxidation peak corresponding to g-phase: the shape of the Ni oxidation peak changed and it shifted 30±40 mV to more positive potentials (Fig. 8). As seen in this ®gure, at low polarizations in this zone the peak caused by the oxidation of zinc from g-phase appeared as a shoulder of the a-phase peak (Fig. 8(a)). In line with composition results, the coatings were mainly formed of nickel-rich a-phase with small amounts of g-phase. Previously [15, 16], it was demonstrated that the stripping peak observed in this zone of potentials corresponded to the g 0 phase [10]. As the deposition potential was made more negative, the Ni content of the alloys decreased and the oxidation peaks of g- and Z-phases were observed in the stripping voltammograms at more negative potentials (Fig. 8(b)). X-ray diractograms veri®ed the stripping results, showing the presence of g-phase with the highest re¯ection intensity corresponding to the (330) preferential orientation (Fig. 5(c)). The shapes of the coatings in these conditions were also very dierent, as they showed the pyramidal crystallites (Fig. 6(d)) observed in other cases of zinc±nickel alloys with the same composition [2, 14, 15]. Zone III could be considered as the g-phase domain, as the appearance of this phase entailed an abrupt change in alloy composition and deposition current eciency. It was observed that the appearance of a gphase coincided with the minimum of current density in the polarization curves. By plotting the partial current densities for zinc, nickel and hydrogen during deposition (Fig. 9), it could be seen that this minimum and the following increase in current density were due to a big drop in hydrogen current density, in line with
F. Elkhatabi et al. / Electrochimica Acta 44 (1999) 1645±1653
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Fig. 5. X-ray diractograms for Zn±Ni deposits obtained potentiostatically from bath A on iron substrates: (a) at ÿ750 mV for 3 104 s; (b) at ÿ1000 mV for 104 s; (c) at ÿ1080 mV for 103 s.
the fact that hydrogen evolution is very low when a gphase is deposited [24]. XPS analysis indicated that oxygen was not present in the inner part of coatings obtained at polarizations higher than the minimum. Moreover, it was observed that the potential corresponding to this current density minimum was deter-
mined by the zinc concentration at dissolution. As observed in Fig. 2, the potential of the minimum shifted to more negative values as the zinc concentration decreased, and was independent of the nickel concentration in the electrolyte (the same value was obtained for baths A and E as for B and D).
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Fig. 6.
Continued overleaf
F. Elkhatabi et al. / Electrochimica Acta 44 (1999) 1645±1653
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Fig. 6. SEM micrographs of Zn±Ni deposits obtained from bath A on GC electrodes: (a) at ÿ750 mV for 1500 s; (b) at ÿ850 mV for 1500 s; (c) at ÿ950 mV for 1500 s; (d) at ÿ1080 mV for 1000 s.
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Fig. 7. Stripping response obtained on a nickel substrate, after deposition of alloys from bath A: (a) at ÿ850 mV for 1500 s; (b) at ÿ1000 mV for 1500 s. Scan rate 5 mV s ÿ 1.
4. Discussion The potential zones described above for solution A were observed with all the electrolytes used in this study. In all cases the limit of each zone corresponded to the appearance of a new phase. However, the transition from normal to anomalous codeposition was obtained in dierent zones, which depended on the composition of the electrolyte. As Fig. 3 shows, for bath A the transition from normal to anomalous codeposition took place in zone III, after the minimum of the polarization curve and this was the same for baths B, C and E (medium zinc/nickel ratios). However, for the lowest Zn2+Ni2+ ratio (bath F) the transition
Fig. 8. Stripping response obtained on a nickel substrate, after deposition of alloys from bath A: (a) at ÿ1060 mV for 1000 s; (b) at ÿ1100 mV for 500 s. Scan rate 5 mV s ÿ 1.
occurred in zone II, at much more positive potentials than the minimum. For bath D (high zinc/nickel ratio), the process was normal in all the intervals of conditions analyzed and no transition could be de®ned. Although for zinc±nickel alloys the minimum in the potentiostatic polarization curves was usually associated with the transition normal to anomalous codeposition, there are some examples in the literature for both Zn±Ni and Zn±Co alloys where the transition does not coincide with the minimum. For alloys deposited from sulfate baths [18, 19] or from H3BO3 containing electrolytes [24], the transition occurred at the maximum of the polarization curves or did not occur,
Fig. 9. Eect of the deposition potential on the partial current densities for the Zn±Ni alloy (q), zinc (w), nickel (r) and hydrogen (*). Bath A and Ni electrode.
F. Elkhatabi et al. / Electrochimica Acta 44 (1999) 1645±1653
depending on the pH of the bath. With Zn±Co alloys obtained from dierent composition baths, the transition was observed at dierent points of the polarization curve, depending on the electrolyte and operating conditions [25]. In all these cases there existed a point at which the deposit and deposition characteristics changed abruptly, but this point did not necessarily coincide with the point at which deposit and bath had the same composition. As mentioned above, for our ammonium electrolytes, the normal to anomalous transition was also observed at dierent points of the polarization curves, but in all cases the current density minimum on these curves could be linked to the appearance of g-phase. Since XPS analysis indicated the presence of some oxygen-containing species at potentials prior to the minimum, the presence of these species could favor the transition between hydrogen evolution and the deposition of the zinc-rich g-phase, but more work is required to con®rm this hypothesis.
5. Conclusions The electrodeposition of zinc±nickel alloys from ammonium chloride baths was studied using dierent techniques. From an analysis of the process under a wide range of conditions, three deposition zones were de®ned: at very low polarizations/current densities an alloy that was practically pure nickel, yet did not have the electrochemical characteristics of nickel, was obtained. In this zone the process had high current eciency, unlike what was observed with other plating baths. As the deposition potential became more negative, the deposition of a-phase was observed accompanied by a high hydrogen evolution. XPS pro®les indicated that there were some oxygen-containing species in the coatings in this zone. Finally, coinciding with the current density minimum of the polarization curves, g-phase was deposited and the hydrogen evolution fell abruptly, which caused a decrease in the total deposition current density. The appearance of gphase was responsible for the sharp change in alloy composition and current eciency observed at this potential, it did not necessarily coincide with the transition from normal to anomalous codeposition. With these ammonium electrolytes, this transition was observed at dierent points of the polarization curves, which depended mainly on the composition of the electrolytes.
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Acknowledgements The authors are grateful to the CICYT (Project No. NMT 97-0379) and to the Generalitat de Catalunya (SGR-67, 1996) for ®nancial assistance and to the Serveis Cienti®co-TeÁcnics of the University of Barcelona for SEM, XPS and EDS measurements. References [1] D.E. Hall, Plat. Surf. Finish. 70 (1983) 59. [2] L. Felloni, R. Fratesi, E. Quadrini, G. Roventi, J. Appl. Electrochem. 17 (1987) 574. [3] M.F. Mathias, T.W. Chapman, J. Electrochem. Soc. 134 (1987) 1408. [4] S. Swathirajan, J. Electrochem. Soc. 133 (1986) 671. [5] S. Swathirajan, J. Electroanal. Chem. 221 (1987) 211. [6] R. Albalat, E. GoÂmez, C. Miiller, J. Pregonas, M. Sarret, E. ValleÂs, J. Appl. Electrochem. 20 (1990) 635. [7] R. Albalat, E. GoÂmez, C. MuÈller, J. Pregonas, M. Sarret, E. ValleÂs, J. Appl. Electrochem. 21 (1991) 44. [8] E. GoÂmez, C. MuÈller, M. Sarret, E. ValleÂs, J. Pregonas, Metal Finish. 90 (1992) 87. [9] G.W. Loar, K.R. Romer, T.J. Aoe, Surf. Finish. 78 (1991) 74. [10] J. Giridhar, W.J. Ooji, Surf. Coat. Technol. 52 (1992) 17. [11] G.D. Wilcox, D.R. Gabe, Corros. Sci. 35 (1993) 1251. [12] E. GoÂmez, E. ValleÂs, J. Electroanal. Chem. 421 (1997) 157. [13] A. Brenner, Electrodeposition of Alloys, Academic Press, Vol. 1, New York and London, 1963, p. 77. [14] G. BarceloÂ, J. Garcia, M. Sarret, C. MuÈller, J. Pregonas, J. Appl. Electrochem. 24 (1994) 1249. [15] F. Elkhatabi, M. Sarret, C. MuÈller, J. Electroanal. Chem. 404 (1996) 45. [16] F. Elkhatabi, G. BarceloÂ, M. Sarret, C. MuÈller, J. Electroanal. Chem. 419 (1996) 71. [17] S.S. Abd El Rehim, E.E. Fouad, S.M. Abd El Wahab, H.H. Hassan, Electrochim. Acta 41 (1996) 1413. [18] F.J. Fabri Miranda, O.E. Barcia, S.L. Diaz, O.R. Mattos, R. Wiart, Electrochim. Acta 41 (1996) 1049. [19] F.J. Fabri Miranda, O.E. Barcia, O.R. Mattos, R. Wiart, J. Electrochem. Soc. 144 (1997) 3441. [20] E. Chassaing, R. Wiart, Electrochim. Acta 37 (1992) 545. [21] M.I. Nicol, H.I. Philip, J. Electroanal. Chem. 70 (1976) 233. [22] H. Dahms, I.M. Croll, J. Electrochem. Soc. 112 (1965) 771. [23] J. Mindowicz, C. Capel-Boute, C. Decroly, Electrochim. Acta 10 (1965) 901. [24] J. Balej, J. Divisek, H. Schmitz, J. Mergel, J. Appl. Electrochem. 22 (1992) 705. [25] R. Fratesi, G. Roventi, G. Giulani, C.R. Tomachuk, J. Appl. Electrochem. 27 (1997) 1088.
Electrochimica Acta 44 (1999) 1645±1653
Dependence of coating characteristics on deposition potential for electrodeposited Zn±Ni alloys F. Elkhatabi, M. Benballa, M. Sarret, C. MuÈller * LCTEM, Department of Chemical Physics, University of Barcelona, MartõÂ i FranqueÁs 1, 08028 Barcelona, Spain Received 13 May 1998
Abstract The electrodeposition of zinc±nickel alloys from ammonium chloride baths was studied on dierent substrates under potentiostatic and galvanostatic conditions, from low potentials/current densities where the codeposition was normal to the conditions where anomalous deposition took place. Anodic linear sweep voltammetry (ALSV) and ex situ techniques were used to characterize the coatings obtained in each condition. The results indicated that three deposition potential zones could be de®ned corresponding to practically pure nickel, a- and g-phase domains. It was observed that the abrupt change in alloy composition and current eciency that occurred during potentiostatic deposition did not correspond to the transition from normal to anomalous codeposition. In our conditions, this change always coincided with the appearance of g-phase in the coatings. # 1999 Elsevier Science Ltd. All rights reserved. Keywords: Zinc±nickel alloys; Ammonium baths; Stripping techniques
1. Introduction It has been known for many years that zinc alloys can provide improved corrosion resistance for the protection of ferrous based metals and that, among others, Zn±Ni, Zn±Co and Zn±Fe are the most common systems [1±12]. According to Brenner [13], the electrodeposition of these alloys occurs anomalously, though normal codeposition can be obtained in particular plating conditions. In most cases, when the deposition of these alloys was studied in a wide interval of current densities/deposition potentials, at a given value of current density/potential an abrupt change in the composition of the alloys and in the current eciency of the process was observed. This change, in some cases, was attributed to the transition from normal to anomalous codeposition.
* Corresponding author. Fax: +349-3402-1231; E-mail: [email protected]
Since NH4C1 baths for obtaining Zn±Ni alloys are common in galvanic industries, this group developed an industrial ammonium bath that gave uniform deposits, with a nickel content between 10±15% and high corrosion resistance [14]. The same bath was used to obtain Zn±Ni alloys under controlled conditions (pure chemical and substrates) and stripping methods were used to characterize the coatings [15, 16]. By relating the electrochemical results with those obtained using ex situ techniques, it was demonstrated that stripping methods were useful to determine the chemical and phase compositions of the alloys. These ®rst studies were essentially designed to analyze the alloys and deposition process in industrial conditions, that is, high potentials and current densities. The aims of the present study were to analyze the deposition of zinc± nickel alloys at low current densities or low polarizations, to characterize the coatings obtained in these conditions and to analyze the possible causes of the sharp change in coating and deposition characteristics mentioned above. Therefore, ammonium baths with
0013-4686/99/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 3 - 4 6 8 6 ( 9 8 ) 0 0 2 8 6 - 2
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Table 1. Concentrations of Zn2+ and Ni2+ in the electrolyte Bath
Zn2+ (mol dm-3)
Ni2+ (mol dm-3)
Zn2+/Ni2+
A B C D E F
0.63 0.53 0.76 0.53 0.63 0.30
0.25 0.21 0.31 0.03 0.14 0.85
2.5 2.5 2.5 17.7 4.5 0.23
dierent cations content were selected: the industrial bath, baths with dierent concentrations but the same Ni2+/Zn2+ ratio and electrolytes with ratios both higher and lower than usual. Potentiostatic and galvanostatic polarization curves were obtained and the alloys under the dierent conditions were characterized by using ALSV (anodic linear sweep voltammetry) and ex situ techniques, such as SEM, X-ray diraction, XPS and EDS.
2. Experimental Experiments were carried out on a three-electrode cell with a capacity of 0.1 dm3. Dierent concentrations of ZnCl2 and NiCl26H20 were studied (Table 1) with a constant amount of NH4Cl, 4.11 mol dm ÿ 3, and pH adjusted to 5.6 by adding ammonia. All reagents were Merck pro analysis grade and the water was obtained from a MilliQ water puri®cation system. The alloys were obtained at 258C by depositing the metals potentiostatically or galvanostatically onto electrodes of dierent diameter, glassy carbon (GC, b= 3 mm), nickel (b= 5 min) and Armco iron (b= 3 and 7 min) mounted in a te¯on holder and rotating at 600 rpm, the minimum speed to avoid the blocking eect of hydrogen bubbles. The working electrode was polished before each run (GC using 3.75, 1.78 and 0.3 mm grades of alumina, and Ni and Fe with 1.00 and 0.25 mm diamond compound) before being rinsed and, ®nally, held in an ultrasonic bath for 1 min. Before each experiment, the solution was purged with high purity argon. The reference electrode was an Ag/AgCl/ KCl(sat) mounted in a Luggin capillary, and an Ni sheet was used as a counter-electrode. The zinc±nickel alloys were stripped under potentiodynamic conditions in the same electrolyte from which they were deposited. Previously, it was veri®ed that no replacement reaction between Ni(II) and Zn(II) took place [15]. Electrochemical measurements were performed using an EG&G 273 potentiostat controlled by a PC 486. Scanning electron microscopy (SEM) and X-ray diffraction (XRD) were used to study the morphology
and structure of the alloys. Energy dispersive analysis of X-rays (EDS) and X-ray photoelectron spectroscopy (XPS) were used to analyze the composition of the coatings.
3. Results Fig. 1 shows the variation in the composition of the alloys with current density for three Zn2+Ni2+ ratios used in this study, together with the composition reference lines (CRL) corresponding to these baths. The in¯uence of current density on the composition of the alloys obtained with our electrolytes was normal in anomalous codeposition: a wide interval of current densities where the composition of the alloy was quite constant and a region at low current densities where the % Nidep increased abruptly. The transition current density, jT (value of j at which the composition of the alloy coincided with that of the electrolyte), was aected by electrolyte composition and was practically independent of the substrate. As observed in Fig. 1, for electrolyte D the Ni content of the alloys always remained above the CRL and a sharp increase in Nidep was observed at low current densities, although normal to anomalous transition did not take place. Fig. 2 shows the polarization curves obtained under potentiostatic conditions for baths containing dierent zinc/nickel molar ratios. In all conditions N-shaped polarization curves were observed, although with the ammonium baths used in this study the current density maximum was less pronounced than the maximum
Fig. 1. Eect of current density on nickel content of deposits and current reference lines for three baths of Table 1: (r) bath A, (*) bath D, (w) bath F.
F. Elkhatabi et al. / Electrochimica Acta 44 (1999) 1645±1653
Fig. 2. Potentiostatic polarization curves on a Ni substrate and for three baths of Table 1: (*) bath A, (w) bath D, (r) bath F.
observed in other cases [17±19]. According to other studies performed for Zn±Ni deposition, the polarization curves were found to be practically independent of the type of electrode used [20]. On examining the characteristics of the deposits and the stripping response obtained during their oxidation, three potential zones could be de®ned for Zn±Ni deposition. Fig. 3 shows the zones obtained with bath A, the industrial electrolyte, of Table 1. Zone I corresponded to the most positive potentials in the polariz-
1647
ation curve (ÿ700/ÿ 800 mV), where the deposit grew at low current densities (i < 0.5 mA cm ÿ 2) and with a current eciency higher than 75% (with some electrolytes current eciencies of about 90% were obtained), unlikewhat occurred with other electrolytes [18, 19]. In this zone, the potentiodynamic stripping response showed only one peak at potentials corresponding to nickel oxidation (Fig. 4(a)). EDS analysis of the deposits obtained in this zone indicated that they consisted of practically pure nickel with a maximum zinc content of about 5%. The diractograms of the deposits obtained in these conditions corroborated the electrochemical results: they showed only Ni peaks (Fig. 5(a)) and the deposits were formed of aggregates of various sizes, similar to nickel deposits (Fig. 6(a)). It is interesting to note that this low zinc content led to important changes in the electrochemical behaviour of the deposits. Fig. 4 also includes the oxidation response of a pure nickel deposit: under the same deposition conditions, the main oxidation peak was obtained at very positive potentials. In this case, the alloy obtained in this zone of potentials, although practically all nickel, had a suciently dierent structure to be easier oxidized and with a high current eciency. In this zone the deposition of nickel was inhibited by underpotentially deposited zinc, as was suggested in other cases [5, 21]. When the deposition of pure zinc was studied on a nickel electrode, it was observed that no deposition took place at these potentials, which indicated that zinc was only deposited on the nickel clusters and not on the nickel substrate. Zone II corresponded to the interval of potentials prior to the maximum of the polarization curve (ÿ850/
Fig. 3. Variation of the nickel content and current eciency with deposition potential for alloys obtained on a Ni electrode from bath A.
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Fig. 4. Potentiodynamic stripping response obtained on a nickel substrate after deposition at ÿ750 mV for 1500 s: curve (a) Zn±Ni alloy from bath A, (b) nickel deposit from a bath with the same nickel content. Scan rate 5 mV s ÿ 1.
ÿ 1040 mV). In this zone the deposition process took place at intermediate current densities (0.5±1.5 mA cm ÿ 2) and was accompanied by a high hydrogen evolution. The potentiodynamic stripping response showed a separate peak for zinc oxidation (Fig. 7), although the response was complex, with the peaks not clearly separated, and deconvolution was needed to calculate the composition of the alloys. After this deconvolution the composition determined electrochemically was consistent with the EDS and XPS results. For the more positive potentials, the percentage of nickel in the alloys was fairly constant at about 75% and current eciency gradually decreased as polarization increased. The morphology and structure of the coatings in this zone also evolved with potential; for the most positive potentials, nodular grains could be distinguished at all deposition stages (Fig. 6(b)) and the diractograms were similar to those obtained in zone I, but on increasing potential the grains were more dicult to distinguish and the diractometric peaks became broader and less intense. For the most negative potentials in this zone the nickel content of the alloys was about 65%, which corresponded to the equilibrium composition of a-phase. The X-ray diractograms obtained in these conditions veri®ed the presence of this phase (Fig. 5(b)). SEM pictures also showed the non-structured morphology characteristic of the aphase, with evidence of hydrogen bubbles (Fig. 6(c)). Since this phase tended to be amorphous and structureless, thick deposits were needed. Therefore, in zone II, as the deposition potential was increased, more zinc was deposited and an a-phase was gradually formed. The structural change from a nickel-like deposit to an a-phase structure was accompanied by an increase in hydrogen evolution,
which had its maximum value near the maximum of the polarization curve. XPS analysis performed at dierent sputtering times on coatings obtained in this II region showed the presence of oxygen in the inner part of the coatings and also indicated that the amount of oxygen increased at more negative potentials. Therefore, the deposits in this zone must contain some oxygen-containing species, probably zinc oxide/hydroxide as indicated in other cases [22, 23]. Finally, it must be mentioned that the results obtained with the dierent electrolytes indicated that the current densities obtained in the potential zones I and II were mainly dependent on the Ni2+ concentration. At about ÿ1045 mV (the exact potential depended on the substrate and bath composition) and parallel to the decrease in current in the polarization curve, a sudden increase in current eciency was observed (zone III). After this change, the composition of the alloy was maintained during a short interval of potentials between 40±45% Ni and then, as polarization increased, the nickel decreased to values less than 15%. The potentiodynamic stripping response in this zone indicated that this sharp change coincided with the appearance of the oxidation peak corresponding to g-phase: the shape of the Ni oxidation peak changed and it shifted 30±40 mV to more positive potentials (Fig. 8). As seen in this ®gure, at low polarizations in this zone the peak caused by the oxidation of zinc from g-phase appeared as a shoulder of the a-phase peak (Fig. 8(a)). In line with composition results, the coatings were mainly formed of nickel-rich a-phase with small amounts of g-phase. Previously [15, 16], it was demonstrated that the stripping peak observed in this zone of potentials corresponded to the g 0 phase [10]. As the deposition potential was made more negative, the Ni content of the alloys decreased and the oxidation peaks of g- and Z-phases were observed in the stripping voltammograms at more negative potentials (Fig. 8(b)). X-ray diractograms veri®ed the stripping results, showing the presence of g-phase with the highest re¯ection intensity corresponding to the (330) preferential orientation (Fig. 5(c)). The shapes of the coatings in these conditions were also very dierent, as they showed the pyramidal crystallites (Fig. 6(d)) observed in other cases of zinc±nickel alloys with the same composition [2, 14, 15]. Zone III could be considered as the g-phase domain, as the appearance of this phase entailed an abrupt change in alloy composition and deposition current eciency. It was observed that the appearance of a gphase coincided with the minimum of current density in the polarization curves. By plotting the partial current densities for zinc, nickel and hydrogen during deposition (Fig. 9), it could be seen that this minimum and the following increase in current density were due to a big drop in hydrogen current density, in line with
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Fig. 5. X-ray diractograms for Zn±Ni deposits obtained potentiostatically from bath A on iron substrates: (a) at ÿ750 mV for 3 104 s; (b) at ÿ1000 mV for 104 s; (c) at ÿ1080 mV for 103 s.
the fact that hydrogen evolution is very low when a gphase is deposited [24]. XPS analysis indicated that oxygen was not present in the inner part of coatings obtained at polarizations higher than the minimum. Moreover, it was observed that the potential corresponding to this current density minimum was deter-
mined by the zinc concentration at dissolution. As observed in Fig. 2, the potential of the minimum shifted to more negative values as the zinc concentration decreased, and was independent of the nickel concentration in the electrolyte (the same value was obtained for baths A and E as for B and D).
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Fig. 6.
Continued overleaf
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Fig. 6. SEM micrographs of Zn±Ni deposits obtained from bath A on GC electrodes: (a) at ÿ750 mV for 1500 s; (b) at ÿ850 mV for 1500 s; (c) at ÿ950 mV for 1500 s; (d) at ÿ1080 mV for 1000 s.
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Fig. 7. Stripping response obtained on a nickel substrate, after deposition of alloys from bath A: (a) at ÿ850 mV for 1500 s; (b) at ÿ1000 mV for 1500 s. Scan rate 5 mV s ÿ 1.
4. Discussion The potential zones described above for solution A were observed with all the electrolytes used in this study. In all cases the limit of each zone corresponded to the appearance of a new phase. However, the transition from normal to anomalous codeposition was obtained in dierent zones, which depended on the composition of the electrolyte. As Fig. 3 shows, for bath A the transition from normal to anomalous codeposition took place in zone III, after the minimum of the polarization curve and this was the same for baths B, C and E (medium zinc/nickel ratios). However, for the lowest Zn2+Ni2+ ratio (bath F) the transition
Fig. 8. Stripping response obtained on a nickel substrate, after deposition of alloys from bath A: (a) at ÿ1060 mV for 1000 s; (b) at ÿ1100 mV for 500 s. Scan rate 5 mV s ÿ 1.
occurred in zone II, at much more positive potentials than the minimum. For bath D (high zinc/nickel ratio), the process was normal in all the intervals of conditions analyzed and no transition could be de®ned. Although for zinc±nickel alloys the minimum in the potentiostatic polarization curves was usually associated with the transition normal to anomalous codeposition, there are some examples in the literature for both Zn±Ni and Zn±Co alloys where the transition does not coincide with the minimum. For alloys deposited from sulfate baths [18, 19] or from H3BO3 containing electrolytes [24], the transition occurred at the maximum of the polarization curves or did not occur,
Fig. 9. Eect of the deposition potential on the partial current densities for the Zn±Ni alloy (q), zinc (w), nickel (r) and hydrogen (*). Bath A and Ni electrode.
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depending on the pH of the bath. With Zn±Co alloys obtained from dierent composition baths, the transition was observed at dierent points of the polarization curve, depending on the electrolyte and operating conditions [25]. In all these cases there existed a point at which the deposit and deposition characteristics changed abruptly, but this point did not necessarily coincide with the point at which deposit and bath had the same composition. As mentioned above, for our ammonium electrolytes, the normal to anomalous transition was also observed at dierent points of the polarization curves, but in all cases the current density minimum on these curves could be linked to the appearance of g-phase. Since XPS analysis indicated the presence of some oxygen-containing species at potentials prior to the minimum, the presence of these species could favor the transition between hydrogen evolution and the deposition of the zinc-rich g-phase, but more work is required to con®rm this hypothesis.
5. Conclusions The electrodeposition of zinc±nickel alloys from ammonium chloride baths was studied using dierent techniques. From an analysis of the process under a wide range of conditions, three deposition zones were de®ned: at very low polarizations/current densities an alloy that was practically pure nickel, yet did not have the electrochemical characteristics of nickel, was obtained. In this zone the process had high current eciency, unlike what was observed with other plating baths. As the deposition potential became more negative, the deposition of a-phase was observed accompanied by a high hydrogen evolution. XPS pro®les indicated that there were some oxygen-containing species in the coatings in this zone. Finally, coinciding with the current density minimum of the polarization curves, g-phase was deposited and the hydrogen evolution fell abruptly, which caused a decrease in the total deposition current density. The appearance of gphase was responsible for the sharp change in alloy composition and current eciency observed at this potential, it did not necessarily coincide with the transition from normal to anomalous codeposition. With these ammonium electrolytes, this transition was observed at dierent points of the polarization curves, which depended mainly on the composition of the electrolytes.
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Acknowledgements The authors are grateful to the CICYT (Project No. NMT 97-0379) and to the Generalitat de Catalunya (SGR-67, 1996) for ®nancial assistance and to the Serveis Cienti®co-TeÁcnics of the University of Barcelona for SEM, XPS and EDS measurements. References [1] D.E. Hall, Plat. Surf. Finish. 70 (1983) 59. [2] L. Felloni, R. Fratesi, E. Quadrini, G. Roventi, J. Appl. Electrochem. 17 (1987) 574. [3] M.F. Mathias, T.W. Chapman, J. Electrochem. Soc. 134 (1987) 1408. [4] S. Swathirajan, J. Electrochem. Soc. 133 (1986) 671. [5] S. Swathirajan, J. Electroanal. Chem. 221 (1987) 211. [6] R. Albalat, E. GoÂmez, C. Miiller, J. Pregonas, M. Sarret, E. ValleÂs, J. Appl. Electrochem. 20 (1990) 635. [7] R. Albalat, E. GoÂmez, C. MuÈller, J. Pregonas, M. Sarret, E. ValleÂs, J. Appl. Electrochem. 21 (1991) 44. [8] E. GoÂmez, C. MuÈller, M. Sarret, E. ValleÂs, J. Pregonas, Metal Finish. 90 (1992) 87. [9] G.W. Loar, K.R. Romer, T.J. Aoe, Surf. Finish. 78 (1991) 74. [10] J. Giridhar, W.J. Ooji, Surf. Coat. Technol. 52 (1992) 17. [11] G.D. Wilcox, D.R. Gabe, Corros. Sci. 35 (1993) 1251. [12] E. GoÂmez, E. ValleÂs, J. Electroanal. Chem. 421 (1997) 157. [13] A. Brenner, Electrodeposition of Alloys, Academic Press, Vol. 1, New York and London, 1963, p. 77. [14] G. BarceloÂ, J. Garcia, M. Sarret, C. MuÈller, J. Pregonas, J. Appl. Electrochem. 24 (1994) 1249. [15] F. Elkhatabi, M. Sarret, C. MuÈller, J. Electroanal. Chem. 404 (1996) 45. [16] F. Elkhatabi, G. BarceloÂ, M. Sarret, C. MuÈller, J. Electroanal. Chem. 419 (1996) 71. [17] S.S. Abd El Rehim, E.E. Fouad, S.M. Abd El Wahab, H.H. Hassan, Electrochim. Acta 41 (1996) 1413. [18] F.J. Fabri Miranda, O.E. Barcia, S.L. Diaz, O.R. Mattos, R. Wiart, Electrochim. Acta 41 (1996) 1049. [19] F.J. Fabri Miranda, O.E. Barcia, O.R. Mattos, R. Wiart, J. Electrochem. Soc. 144 (1997) 3441. [20] E. Chassaing, R. Wiart, Electrochim. Acta 37 (1992) 545. [21] M.I. Nicol, H.I. Philip, J. Electroanal. Chem. 70 (1976) 233. [22] H. Dahms, I.M. Croll, J. Electrochem. Soc. 112 (1965) 771. [23] J. Mindowicz, C. Capel-Boute, C. Decroly, Electrochim. Acta 10 (1965) 901. [24] J. Balej, J. Divisek, H. Schmitz, J. Mergel, J. Appl. Electrochem. 22 (1992) 705. [25] R. Fratesi, G. Roventi, G. Giulani, C.R. Tomachuk, J. Appl. Electrochem. 27 (1997) 1088.