Chemical and phase compositions of zinc + nickel alloys determined by stripping techniques

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Joumal of Electroanalytical Chemistry 404 (1996) 45-53

Chemical and phase compositions of zinc + nickel alloys determined by stripping techniques F. Elkhatabi, M. Sarret, C. Miiller * Departament de Qu(mica F[sica, Unioersitat de Barcelona, Martl i Franqu~s l, 08028 Barcelona, Spain

Received 6 January 1995; in revised form 23 June 1995

Abstract

Zinc-nickel alloys were obtained from a bath with a high concentration of ammonium chloride that was previously used to prepare the alloys under industrial conditions. These industrial alloys were characterized with the usual ex situ techniques, while in the present case potentiodynamic and galvanostatic stripping methods were used. Three substrates with different characteristics were selected and in all cases the stripping techniques were shown to be a reliable method for fast determination of both the chemical and phase composition of electrodeposited zinc-nickel alloys. With the plating conditions used in the present work, the oxidation potentials of or- and rl-phases coincided with those observed in other cases, while the ~/-phase was oxidized at a significantly more positive potential. This fact could be attributable to a very low value of the exchange current density of zinc from this phase. Moreover, the analysis of the composition of the alloys with different thicknesses revealed that the deposition of the nickel-rich a-phase was favoured in the initial stages of the process. Keywords: Zinc-nickel alloys; Chemical composition; Phase composition, Stripping technique

I. Introduction

In the last few years many efforts have been made to develop electroplating systems for zinc-nickel alloy coatings, because it is known that they improve the corrosion resistance and also the mechanical properties of pure zinc [1-11 ]. The purpose of most of these studies was to obtain Z n - N i alloys with a nickel content between 8 and 14%, the composition that seems to provide the best corrosion resistance, and to characterize the alloys with the usual ex situ techniques (atomic absorption, scanning electron microscopy, X-ray diffraction, etc.). It has also been demonstrated that electrochemical stripping techniques can be used to characterize an alloy deposition process and its products [12-17]. In particular, Swathirajan [12] established the current-potential relationships for the stripping of an alloy, depending on whether the deposited alloy was a eutectic, a solid solution or an intermediate phase, and he used this method to character-

* Corresponding author. 0022-0728/96/$15.00 © 1996 Elsevier Science S.A. All rights reserved SSDI 0022-0728(95)04359-4

ize zinc-nickel alloys obtained from different electrolytes. Moreover, he proposed a model to explain the phenomenon of anomalous codeposition that involved underpotentially deposited zinc [13]. Since galvanic industries at present use baths containing NH4C1 to deposit Z n - N i alloys, this group developed a simple chloride bath with a high ammonium concentration that gave uniform deposits with an Ni content between 10 and 15%. These alloys were obtained on iron plates under industrial conditions and the influences of different plating variables on the deposit characteristics were analyzed [18]. In the present study, this electrolyte was used to obtain Z n - N i alloys under controlled conditions, (pure chemicals and substrates) and stripping methods were used to relate the electrochemical results with those obtained in the previous study using ex situ techniques and also to obtain information about the deposition process. Three substrates with different characteristics were selected, iron, nickel and glassy carbon, to determine whether stripping techniques could be used to measure the chemical and phase compositions in all cases and, hence, to establish the influence of the nature of the substrate on the deposition process.

46

F. Elkhatabi et a l . / Journal of Electroanalytical Chemistry 404 (1996) 45-53 20.00

2. Experimental Experiments were carried out in a three-electrode cell with a capacity of 0.1 dm 3. The electroplating solution was 0.25 mol dm -3 NiC12 - 6 H 2 0 + 0.63 mol dm -3 ZnC12 + 4.11 mol dm -3 NHaC1 with pH adjusted to 5.6 by adding ammonia. The deposition of the pure metals was performed from solutions containing only NiC12 or ZnCI 2 at the same concentrations and with the same NHaCI concentration and pH. All reagents were of Merck pro analysis grade and the water was obtained from a MilliQ water purification system. The alloys were obtained by depositing the metal potentiostatically or galvanostatically onto electrodes of different area (glassy carbon, • = 3 mm2; nickel, • = 2 mm2; iron ARMCO, • = 3 and 7 mm 2) mounted in a teflon or resin holder which could be mounted on a rotating disc arrangement. The working electrode was polished before each run (GC with 3.75, 1.78 and 0.3 /xm alumina and Ni and Fe with 1.00 and 0.25 /~m diamond compound), then rinsed and finally held in an ultrasonic bath for 1 min. Before each experiment, the solution was purged with high purity argon. The reference electrode was Ag IAgCl mounted in a Luggin capillary, but all potentials in the text are quoted vs. SCE. A nickel sheet was used as a counterelectrode. In some cases, the zinc-nickel alloys were stripped under potentiodynamic or galvanostatic conditions in the same electrolyte from which they were deposited, and also in a solution containing only zinc ions and ammonium chloride with a Pt counter-electrode. The electrochemical measurements were performed using an E G & G 273 potentiostat controlled by a 386 IBM PC. A Tacussel rotating disc electrode setup was used in some cases, which permitted rotation speeds up to 5000 rev min-1. The deposit morphology was examined by scanning electron microscopy (SEM) and the deposit phases were analyzed by X-ray diffraction (XRD).

3. Results and discussion

3.1. Dependence of the stripping response on different experimental conditions The typical voltammetric stripping response obtained with this alloy is shown in Fig. l, together with that of pure zinc and nickel. As seen in the figure, the oxidation voltammogram of pure zinc consisted of a single peak at - 9 5 0 mV, while that of nickel showed irreversible behaviour with small peaks at about - 3 0 0 and 0 mV with the main dissolution peak at very positive potentials. The oxidation of the zinc-nickel alloys always took place at more negative potentials than the oxidation of pure nickel. To characterize the deposits, a series of Z n - N i electrodeposition experiments were carried out on the different

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substrates, under potentiostatic or galvanostatic conditions, and each deposition was followed by potentiodynamic or galvanostatic stripping. The potentiodynamic deposition and dissolution curves of the Z n - N i alloys on different electrodes (Fig. 2(a)) showed that the response was similar in all substrates; they consisted of a sharp oxidation peak at about - 3 5 0 mV and other anodic peaks at more negative potentials. By combining electrochemical measurements with ex situ techniques, the assignment of the oxidation peaks made by Swathirajan [12,13] was verified [19]: the peak at - 3 5 0 mV was due to the dissolution of nickel while those at more negative potentials corresponded to the dissolution of zinc from the different phases of zinc-nickel alloys. However, the oxidation responses obtained in the present study were more complex than those obtained by Swathirajan. The peak corresponding to the Ni oxidation and one or two peaks in the potential region of about - 950 mV coincided with those observed by him, but in between two oxidation peaks were obtained under most conditions. With the iron electrode in the supporting electrolyte, an abrupt increase of current was observed at about - 4 0 0 mV (curve 1 in Fig. 2(b)) while, when zinc and nickel ions were present in solution, a peak appeared due to surface oxidation [20] (curve 2 in Fig. 2(b)) which overlapped with that corresponding to the nickel dissolution. At a given scan rate, the charge corresponding to this iron peak was constant, and hence it could be subtracted to evaluate the charge corresponding to the Ni oxidation; with thick deposits it was negligible compared with that of zinc-nickel oxidation. Jovic and coworkers [15,16] suggested that the stripping of an alloy should be performed in a solution that does not contain the more noble metal to avoid the replacement reaction NimZn . + n Ni 2+= ( m + n ) N i + n Zn 2+ which can take place at more negative potentials than that of nickel. Taking into account that in galvanostatic experiments at low current densities, nickel deposition was ob-

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(1) GC (area 7.1X 10 -2 cruZ), (2) Ni (area 3.14X 10 -z cm2), (3) Fe (area 3.14× l0 -2 cm2). Final potential - l l 2 0 mV. Scala rate 10 mV s - i. (b) Cyclic voltammetry of: iron in the supporting electrolyte (curve l), iron in the working solution (curve 2, final potential - 9 2 5 mV) nickel in the supporting electrolyte (curve 3). Scan rate l0 mV s - ~.

served at - 7 5 0 mV, some stripping experiments were carded out without removing the electrode from the deposition bath and others in solutions not containing nickel ions. The stripping response obtained in both cases was the same, showing that in the time interval of our experiments nickel deposition did not occur. The composition of the Zn-Ni alloys was determined by integrating the charge corresponding to the nickel and zinc dissolution on the potentiodynamic stripping or by determining the same charges from the plateau regions of galvanostatic strippings. Alloy layers deposited on the glassy carbon electrode at thicknesses greater than 0.5/xm could not be dissolved successfully; all the deposited material fell off the electrode surface immediately after the onset of dissolution due to the low cohesion between glassy carbon and the alloy. In the case of iron and nickel electrodes, the cohesion between the alloy and the substrate was high and the dissolution was completed without difficulty, favoured by the high chloride concentration in the electrolytic bath [21 ]. Since the charge of the peak due to the iron oxidation depended on the scan rate, the effect of this parameter on the potentiodynamic response was tested to select the best conditions for analyzing the voltammograms. As usual, the peak currents increased with scan rate, peak potentials shifted in the positive direction and the best peak resolution was obtained at very low sweep rates. However, up to 10 mV s-t the response was qualitatively the same, the current efficiencies were constant and the ratio between the integrated charges corresponding to the nickel and zinc oxidation was also constant. However, with the iron substrate, as the scan rate was increased the charge of the iron oxidation relative to that of nickel was smaller, which facilitated the determination of the nickel dissolution charge. Hence, a scan rate of 10 mV s-I was selected to carry out most of the potentiodynamic stripping.

47

With respect to the influence of agitation, the fundamental differences in the oxidation voltammograms appeared between the alloys obtained with or without agitation (with rotation the charge corresponding to the zinc oxidation was slightly higher but the number and positions of the different peaks were the same), while the rotation during the oxidation process did not markedly affect either the voltammetric or the galvanostatic stripping response. Because the Zn-Ni alloys obtained under industrial conditions were deposited on stationary electrodes, in the present case the deposition and oxidation processes were performed under the same conditions, except when controlled mass-transfer conditions were needed to calculate some parameter or to compare with other authors. Finally, the influence of the resting time of the alloys on the stripping response was also analyzed. It was observed that when the deposit was taken out of the solution, washed, dried and left in air for some time, no essential changes were observed in the stripping response, indicating that no solid-state reaction occurred. This fact ensured that the results obtained with the ex situ methods (SEM, XRD) corresponded to the same deposits analyzed electrochemically.

3.2. Identification of stripping peaks Electrodeposited Zn-Ni alloys exhibit three major phases: the a-phase which is a solid solution of zinc in nickel with an equilibrium solubility of about 30% Zn; the y-phase which is an intermediate phase with a composition NisZn2]; and the ~-phase which is a solid solution of nickel in zinc with less than 1% nickel. The/3- (NiZn) and 3- (Ni3Zn22) phases have not been readily obtained by electrodeposition [ 1]. As mentioned above, the stripping response obtained in the present study was more complex than that obtained by Swathirajan and it was more difficult to establish the

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F. Elkhatabi

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Zn from the r/- and 3,-phases of the Zn-Ni deposits (peak A in Fig. 3) and that at - 3 7 0 mV (peak D) as the oxidation of nickel; but in between two anodic peaks were observed (peaks B, C) at potentials corresponding to the dissolution of zinc from the a-phase. A double peak in this potential region has been observed in other cases [10] and has been interpreted as the dissolution of zinc from the a-

relationship between the stripping response and the different Zn-Ni alloy phases. When conditions led to deposits with good properties from the industrial point of view (12%-18% Ni), the stripping voltammograms consisted of three peaks for zinc dissolution and nickel oxidation (Fig. 3). According to Swathirajan, the anodic multi-peak at about - 8 5 0 mV may be interpreted as the dissolution of

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49

F. Elkhatabi et al. /Journal of Electroanalytical Chemistry 404 (1996) 45-53

(peak C) and fl- (peak B) phases, the latter due to a phase transition 3' ~ fl during the oxidation of zinc from the 3,-phase. However, this peak identification does not agree with our experimental results. If the deposition current or potential was very low, so that the deposit composition corresponded to the a-phase, galvanostatic stripping showed the plateau of nickel oxidation and that corresponding to zinc dissolution between - 6 0 0 and - 7 0 0 mY (Fig. 4(b)). The X-ray diffractograms of deposits obtained under these conditions showed the a-phase structure (Fig. 4(a)), although thick deposits were needed to detect this phase due to its poor crystallinity. For plating conditions which led to deposits with a composition between 15% and 20% Ni, the predominant peak for zinc oxidation was peak C which could not correspond to a nickel-rich phase because the nickel content of the deposits decreased as this peak increased. The diffractograms of these deposits always corresponded to the 3"-phase, with a (330) preferential orientation (Fig. 5(a)) and, therefore, peak C must also correspond to the oxidation of zinc from the 3'-phase. However, the r/-phase was not observed in the diffractograms, indicating that the percentage of this phase present in the deposits was small and it was not possible to determine this percentage from the stripping results because poorly defined multi-peak or potential plateaus were always obtained during the dissolution process. Additional experiments were performed to

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confirm the identification of the stripping peak C: deposits obtained under galvanostatic or potentiostatic conditions were oxidized at potentials below peak C and diffractommetric analysis of the remaining deposits was made. The diffractograms obtained after these partial oxidations were the same as those in Fig. 5, showing the sole presence of the y-phase. Thus, the zinc-nickel alloys obtained in these experimental conditions consisted essentially of y-phase with small amounts of r/- and a-phases. Although it seems that in the present case the a-phase was really formed during the deposition process, some charge under the stripping peak B could also correspond to a phase transition 3' ~ a during the oxidation of zinc from the 3'-phase, as suggested by Swathirajan [12]. Thus, the most surprising fact of the stripping results presented here was the presence of two oxidation peaks for the dissolution of zinc from the 3'-phase, the first at the same potentials observed by Swathirajan and the second at about 270 mV more positive. One of the electrolytes used by Swathirajan was reproduced with our electrodes and the X-ray diffractommetric analysis of these 10% Ni deposits was performed (Fig. 6). In the stripping voltammogram the peak at - 870 mV corresponds to the zinc dissolution from 3"- and r/-phases and that at - 5 0 0 mV to the zinc oxidation from a-phase, due to the phase transition mentioned above. The diffractograms of these deposits were in agreement with this peak assignment, showing the presence of ~/- and 3,-phases with a (330) preferential orientation.

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F. Elkhatabi et al./ Journal of Electroanalytical Chemistry 404 (1996) 45-53

50

Therefore, there was no difference between the y-phases observed in Figs. 5 and 6, although in our case this phase was oxidized at two different potentials. With thick deposits, the stripping potential Ei, J of a component i from a phase j at a stripping current l, in the absence of diffusion effects is given by [12]:

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Taking these considerations into account, if two stripping potentials are observed for the dissolution of zinc from the y-phase, they must correspond to two different values of iJ.i . Usually, the exchange current density for the dissolution of an alloy component can be considered independent of the phase from which the component is dissolving, especially when there are no morphological differences between the bulk metal and the component in various phases. Thus, with the Jo and a values corresponding to bulk zinc and the stripping plateau potentials, the equilibrium potentials of zinc in the different phases could be calculated using Eq. (1). As mentioned above, it was not possible to separate the contribution of the ~?-phase to the potential plateau observed at about - 8 5 0 mV in the galvanostatic stripping, and hence it was not possible to calculate the different equilibrium potentials exactly. However, with an approximate calculation, using an equilibrium potential of the y-phase similar to that determined in other cases [13] and with the Jo and ct values corresponding to bulk zinc, the most negative potential for the

Fig. 8. SEM picture of a Zn-Ni alloy obtained on an Fe electrode (area 7.1 × 10 -2 cm 2) at 20 mA cm -2 for 300 s from the bath used in the present work ([Ni 2+ ]/[Zn 2+ ] = 0.41, pH 5.6).

dissolution of zinc from the y-phase (peak A) can be justified, but to obtain a stripping potential of about - 550 mV (peak C), a much lower value of Jo must be postulated. At present, other Zn-Ni alloys were obtained with electrolytes with different [Ni2+]/[Zn 2÷ ] ratios at the same pH (5.6) and it was observed that at low [Ni2+]/[Zn2+], the stripping voltammograms and X-ray diffractograms were similar to those of Fig. 6 with the oxidation of zinc from the y-phase close that of the 7/-phase. The photomicrographs of these alloys show a nodular and not very compact deposit that includes a columnar structure (Fig. 7) and they were quite different from those obtained with the electrolyte used in the present work. In this case, alloys consisting essentially of y-phase and with the stripping peak C in the voltammograms showed a compact, uniform structure (Fig. 8) and after the partial oxidation process mentioned above they were cracked but the main structure was maintained. Thus, it seems that when the deposit contains high amounts of T/-phase this is dispersed among the intermediate y-phase, and after the stripping of zinc from the solid solution the remaining deposit is porous with a high exchange current density. However, in the absence of, or with low amounts of, the solid solution, the y-phase forms essentially a more compact structure with a lower value of Jo, causing a positive shift in the stripping potential of zinc from this phase.

3.3. Dependence of the chemical and phase composition on the current density: Composition profiles

Fig. 7. SEM picture of a Zn-Ni alloy obtained on an Fe electrode (area 7.1×10 -2 cm 2) at 20 mA cm -2 for 200 s from a bath with [Ni 2+ ]/[Zn 2+ ] = 0.06, p n 5.6.

With the different substrates, the dependence of the alloy composition and deposition potential on the current density was similar to that obtained in other cases: the percentage of nickel was approximately constant over a wide range of current densities and increased strongly at

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the lowest current densities. This increase in the nickel content coincided with a sharp decrease in the deposition potential (Fig. 9). The transition current density JT where the transition from normal (low current densities) to anomalous codeposition (high current densities) occurred was 0.7 mA cm -2 for the nickel electrode and 1.1 mA cm -2 for the iron substrate. The current efficiency was very low below i T and increased up to 95-97% in the anomalous codeposition region. Fig. 10(a) shows the potential-time dependence for the deposition of zinc-nickel alloys on a glassy carbon substrate at different current densities. At very low current densities, the deposition of zinc occurred by the interaction of zinc atoms with the growing clusters of nickel leading to the formation of the solid solution a-phase, as was observed in the stripping response of Fig. 4(b). At higher current densities, just below the iT, a potential plateau at about - 1 0 7 5 mV was observed and then the potential

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increased to a more positive value. The composition of thick deposits obtained in these conditions corresponded to a normal codeposition, but the stripping response showed the presence of some "),-phase (Fig. 10(b)). It seems that in the first stages of the process the zinc inhibited the deposition of nickel and thus shifted the deposition potential to negative values, and after the formation of the first nuclei the deposit could grow at less negative overpotentials. Although this potential plateau was observed with all substrates, it was small with the iron electrode and increased with nickel and glassy carbon substrates. When the deposition current density was higher than iT, the electrode potential shifted to more negative values, all phases could be deposited and, due to the higher value of Jo, zinc was preferentially deposited, leading to anomalous codeposition. Thus, with the plating conditions used in the present work, in the normal codeposition region, the zinc-nickel alloys were formed of a-phase, sometimes with small amounts of T-phase, while in the anomalous region they were mostly formed of T-phase with some r/- and a-phases. In contrast, in the characterization of alloys obtained under industrial conditions [18], it was observed that the composition-depth profiles of the deposits analyzed by XPS were not uniform, with a higher nickel content in the first deposit layers. As the XPS studies presented preferential sputtering of zinc, a stripping analysis was performed with variable deposition time and the alloy composition at different deposit thicknesses was calculated to check whether anodic stripping was useful to study these composition profiles. Fig. 11 shows the voltammetric stripping of alloys obtained on the iron substrates for different deposition times. The charges corresponding to the nickel and zinc oxidation were integrated and the composition of each alloy was calculated. Fig. 12 shows the dependence of the percentage of nickel on the deposition time, for alloys of similar final composition obtained under potentiostatic or galvanostatic conditions on the three electrodes. As shown in the figure, with the iron electrode the composition of the

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F. Elkhatabi et al./ Journal of Electroanalytical Chemistry 404 (1996) 45-53

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t/s

Fig. 12. Dependence of the Ni content of the alloys on the deposition time for Z n - N i deposits obtained under different conditions: Fe at 10 mA cm -2 (O), Fe at - 1100 mV, ( * ) GC at - 1195 mV (area 7.1 × 10 -2 cm 2) (~), Ni at 14 m A c m -2 (area 3.14× 10 -2 cm 2) (O).

alloy showed great variation with deposit thickness; the nickel content was very high in the first deposit layers and decreased with deposition time up to 12-15% at 400 s. This alloy composition was in agreement with that determined by atomic absorption for alloys of 10/xm thickness obtained under industrial conditions. However, the cathodic efficiency was low in the initial stages and increased up to 95% at long times. The alloys obtained on the nickel substrate showed much less pronounced profiles and in the case of glassy carbon the composition was almost constant after the first 15 s of deposition. These experimental results indicate that, with the electrolyte used in the present work and with plating conditions where the composition of thick deposits corresponded to an anomalous deposition, the first stages of the process included the deposition of a-phase joined with hydrogen codeposition. At longer times, the deposition of the zinc-rich r/- and y-phases was favoured, the hydrogen codeposition was also reduced and the current efficiency increased. Although the general behaviour was similar for all substrates, the time needed to observe the formation of the y-phase was different: iron required the longest time and glassy carbon the shortest. With this substrate, even at very low deposition times, some y-phase was always observed in the stripping response and, consequently, the nickel content of the alloy was not so high and its composition was almost constant. Therefore, the nature of the cathode surface determined to some extent the initial stages of the deposition process. More data is required to determine whether this influence was due to its chemical nature, to some special orientation of the surface or to the presence of a thin film, such as an oxide, covering the surface. In any case, the characteristics of the electrodes used in the present work were not reproduced by the deposit, since the steady-state alloy composition and the deposit morphology observed by SEM were the same in all cases.

The purpose of this study was to use stripping techniques to characterize zinc-nickel alloys obtained from a bath with a high ammonium content. Three substrates with different characteristics were used, and in all cases this electrochemical method enabled an accurate determination of the chemical and phase compositions of the alloys. The nickel content of thick alloys determined with this method was in agreement with that reported previously, obtained using atomic absorption. Moreover, the stripping results confirmed the composition profiles pointed out by previous XPS studies and also demonstrated some substrate influence on the alloy composition. With plating conditions corresponding to the anomalous codeposition region, by varying the deposition time it was observed that the composition through the deposit depth was not uniform: the Ni content of the first layers was higher than that of the remainder. The experimental results indicated that the deposition of a-phase was favoured initially together with hydrogen codeposition and, as the deposit thickness increased, y-phase and also small amounts of B-phase were deposited. The composition profile was more pronounced with the iron electrode because the formation of y-phase was more difficult, while on glassy carbon the 3,-phase was formed almost from the beginning of the deposition process. Since the properties of thick deposits were the same in all substrates and the time needed to reach the steady-state composition was lower with glassy carbon, this substrate could be used for fast characterization of the chemical and phase composition of zinc-nickel alloys in the industrial bath. The oxidation potential of zinc from y-phase was more positive that that observed in other cases, and to explain this potential shift a very low value for the exchange current density of zinc from this phase must be postulated. This low value of Jo would be in agreement with the microstructure of these alloys, which was much more compact than that observed when the deposit was formed by a mixture of ~- and y-phases and both phases were oxidized at similar potentials. This fact demonstrated the clear dependence of the kinetic parameters on the morphology of the deposits. Acknowledgements

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