An electrochemical study of the corrosion processes of Ni-based glassy metals in aqueous solutions

ARTICLE

An electrochemical study of the corrosion processes of Ni-based glassy metals in aqueous solutions

M.R. Barbosa^I; L.M. Gassa^II

^IFacultad de Ingeniería, Universidad Nacional del Centro de la Provincia de Buenos Aires, C.C. 12, 7400 Olavarría, Argentina

^IIInstituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas (INIFTA), Facultad de Ciencias Exactas, Universidad Nacional de La Plata, Sucursal 4, C.C. 16, 1900 La Plata, Argentina

ABSTRACT

The electrochemical behavior of amorphous Ni-Co alloys in carbonate/bicarbonate solutions was investigated using electrochemical impedance spectroscopy combined with voltammetry and stationary polarization techniques. The results indicate that the kinetics of the dissolution processes are dependent on the pH and ionic strength of the electrolyte. The composition of the glassy metal is an important factor in determining the kinetic parameters involved in the complex electrochemical behavior of these materials.

Keywords: impedance spectroscopy, glassy metals, Ni-Co alloys

RESUMO

O comportamento eletroquímico de ligas amorfas Ni-Co em soluções de carbonato/bicarbonato foi investigado usando espectroscopia de impedância eletroquímica combinada com voltametria e técnicas de polarização estacionárias. Os resultados indicam que a cinética do processo de dissolução é dependente do pH e da força iônica da solução eletrolítica. A composição do metal vítreo é um fator importante na determinação dos parâmetros cinéticos envolvidos no complexo comportamento eletroquímico desses materiais.

Introduction

Ni-Co amorphous alloys present an electrocatalytic activity on the OER in alkaline solution which is higher than that of their crystalline counterparts^{1-8, and reference therein}. The use of these materials requires a satisfactory resistance to general and localized corrosion. It has been found⁹ that the corrosion behavior of different polycrystalline Ni-Co alloys is a function of alloy composition.

A corrosion study of Ni-Co based glassy metals in carbonate-bicarbonate solutions at pH 8.9 showed that the dissolution processes are dependent on the ionic strength of the electrolyte¹⁰.

The present study was undertaken in order to evaluate the influence of the pH and ionic strength of the electrolyte, as well as the alloy composition, on the corrosion behavior of a glassy Ni-based alloy (Vitrovac 0080), and to compare these results with Ni-Co based alloys.

Experimental

Amorphous alloys (Vakuumschmelze GmbH, Hanau) with the compositions Vitrovac 0080 (Ni₇₈B₁₄Si₈), G15 (Ni₅₈Co₂₀B₁₂Si₁₀) and G16(Ni₂₅Co₅₀B₁₀Si₁₅), were employed as the working electrodes. A conventional three-compartment glass cell was used with a large area Pt counter electrode and a calomel reference electrode. Potentials in the text are referred to the normal hydrogen electrode (NHE) scale.

Electrochemical impedance spectroscopy (EIS) combined with steady-state current density/potential (j/E) curves, and single (STPS) and repetitive (RTPS) triangular potential sweeps, were performed under purified N₂ gas saturation in the following solutions: x M KHCO₃ + y M K₂CO₃ (0.75 ≤ x ≤ 2.5; 0.05 ≤ y ≤ 0.5), covering the 8.3-10.5 pH range at 25 °C.

Results and Discussion

For the characterization of the electrochemical behavior of the amorphous alloys, voltammograms were run between the potential regions of the hydrogen and oxygen evolution reactions. The current density/potential (j/E) profiles corresponding to the first forward potential scan of G15 alloy in the different electrolyte solutions are presented in Fig. 1. The voltammetric profile in 0.05 M K₂CO₃ + 0.75 M KHCO₃ (pH 8.9) shows a first anodic current peak (Peak I) at ca. -0.3 V, associated with Ni/Ni(II) hydroxide transformation¹¹, a second anodic contribution (Peak II) at ca. 0.05 V, related to the oxidation of Co(II) to Co(III) oxides or hydroxides¹², and a third anodic current peak (Peak III) at ca. 0.7 V, associated with the redox couple Ni(II)/Ni(III)¹¹. The current values of each peak are complex functions of the pH, ionic strength, and carbonate/bicarbonate ratio. j_pI increases and peak II appears less defined according to the ionic strength and pH. The influence of bicarbonate ions becomes evident at the top of peak III. The voltammograms of G16 (not shown here) also exhibit on the first anodic scan all of the current contributions described above. Furthermore, lower voltammetric charge densities of the peaks associated with Ni transformation are observed for G16 alloy in comparison with G15 alloy. It is interesting to note, that current peaks II related to the oxidation of Co(II) to Co(III) are the best defined, and that peak III appears at more negative potentials. This fact is associated with the increasing Co concentration in the alloy. This tendency is confirmed with data obtained under similar experimental conditions employing the non-Co containing alloy, Vitrovac 0080, in which case peak III shifts to 0.9 V. This Co effect has also been found by other authors^6,13.

The steady state polarization curves of Vitrovac 0080, G15, and G16 electrodes in 0.167 M K₂CO₃ + 2.5 M KHCO₃ (pH 8.9) and 0.5 M K₂CO₃ + 2.5 M KHCO₃ (pH = 9.5) are shown in Figs. 2a and 2b, where the dots indicate the potentials at which impedance spectra were recorded.

The current contribution at about 0.6 V can be associated with the redox couple Ni(II)/Ni(III)¹¹. It is worth noting that this current peak appears at more positive potentials for the non Co-containing alloy. This effect was also found by Lian et al.⁶, who suggested that cobalt additions to Ni(OH)₂ stabilize Ni hydroxide, improve the conductivity and charge efficiency of Ni(OH)₂, and increase the oxygen overpotential. Furthermore, Folquer et al.¹³ found a catalytic effect of the Co(II)/Co(III) reaction on the Ni(II)/Ni(III) reaction in co-precipitated Ni-Co alloys through hydroxide ions inserted into the metal hydroxide layers.

At a constant pH, the high value of the current peak (j_p) increases with increasing Ni concentration in the alloys and the ionic strength of the electrolyte. These effects hold for all of the tested alloys. On the other hand, as pH increases (Fig. 2b) the value of the j_p becomes higher for the Co-containing alloy and remains practically constant in the case of Vitrovac 0080. At a constant concentration of bicarbonate ions, linear log j_pvs. pH relationships are obtained with a slope of 0.42 ± 0.05 for G15 and 0.4 ± 0.02 for G16. Accordingly, the data revealed a complex effect of carbonate/bicarbonate ions on the corrosion behavior of Ni-Co based glassy metals.

The frequency response of each investigated amorphous alloy was found to be remarkably dependent on the applied potential, on the alloy composition, and on the electrolyte concentration. Nyquist plots recorded with Vitrovac 0080 in 0.167 M K₂CO₃ + 2.5 M KHCO₃ (Fig. 3) exhibit a capacitive semicircle at potential values close to the corrosion potential (Fig. 3a), and at more positive polarizations an inductive contribution appears at the intermediate frequencies, where a new time constant at low frequencies can be observed (Figs. 3b-e). As the anodic polarization increases, the resistance values associated with the low frequency contribution become negative (Fig. 3f). The above-described influence of the electrolyte composition on the steady-state measurement is also observed in the impedance data.

Otherwise, the impedance response obtained in the case of the Co-containing alloys (G15 and G16) in carbonate/bicarbonate solutions presents the same general features as the Vitrovac 0080 one, but involves higher inductance, which in turn hinders the appearance of a well-defined inductive behavior at intermediate frequencies and a non-complete capacitive semicircle at low frequencies (see, for instance, Figs. 4 and 5). The higher impedance values obtained employing these alloys in comparison with Vitrovac 0080 are consistent with the lower current densities under steady-state conditions for the alloy with a higher Co content.

The whole set of experimental impedance spectra can be discussed according to the following total transfer function:

with

where R^W is the electrolyte resistance contribution, w = 2pf, [CPE] denotes the constant phase element given by [CPE] = [C_dl(jw)^a]^-1, C_dl is the Helmholtz layer capacitance, a takes into account the distribution of time constants due to surface unhomogeneities in the amorphous alloys, R_ct represents the charge transfer resistance defined as Z(w = ¥), L and R_ic are associated with the inductive contribution which can be interpreted by assuming a strong potential dependence of the active site concentration causing surface relaxation phenomena, and R_ad and C_ad can be related, in principle, to a Faradaic pseudo-capacitive contribution of an adsorbed reaction intermediate at the lowest frequencies.

The good agreement between the experimental and simulated data according to transfer function (1) analysis using non-linear least-square fit routines is demonstrated in Figs. 6 and 7 in 0.167 M K₂CO₃ + 2.5 M KHCO₃ (pH = 8.9) and 0.5 M K₂CO₃ + 2.5 M KHCO₃ (pH = 9.5) solutions, respectively.

The values of C_dl and R_ct were determined from the optimum fitting according to Eq. 1. In the potential range -0.4 V ≤ E ≤ 0 V, it was possible to estimate mean values by C_dl = 30 ± 10 μF cm^-2 and α = 0.95 ± 0.05, while the value of R_ct decreases linearly. At positive applied potentials the R_ct values are nearly constant and lower capacitance values, C = 9 ± 2 μF cm^-2, are obtained. These low C values are typical of a metal covered with a passive film¹⁴, and may be considered the passive film and double layer capacitance connected in series. The potential independence of the capacitance indicates either insulating or semiconducting properties of the passive films. At E ± 0 V, R_ct = 0.434 b/j_ss for a charge transfer controlled reaction, where j_ss denotes the steady-state current density and b = ∂E/∂logIj_ss I is the Tafel slope of the anodic reaction, b » 120 mV was found, corresponding to a charge transfer coefficient of 0.5.

The analysis of the impedance parameters associated with the time constant contribution at the lowest frequencies is difficult in the case of alloys G15 and G16 due to an incomplete capacitive profile, although it was possible to estimate a value of C_ad » 860 ± 60 μF/cm². The values of resistance and capacitance associated with the second capacitive loop for Vitrovac 0080 were close to 3.5 ± 0.2 kW cm² and 4 ± 0.2 mF/cm², respectively.

It is possible to conclude, in principle, that the fit parameters related to the inductive loop observed at intermediate frequencies fail to obey a direct function of the Ni/Co concentration ratio in the alloy matrix. This fact is reaffirmed by the experimental results obtained with Vitrovac 0080, which present a clear inductive contribution at intermediate frequencies.

The impedance results clearly show that the dissolution processes are taking place through a complex reaction mechanism, and it is necessary to determine the composition of the surface films formed on amorphous alloys under different polarization conditions. Consequently, XPS studies are in progress.

Acknowledgments

This research project was financially supported by the Consejo Nacional de Investigaciones Científicas y Técnicas, the Comision de Investigaciones de la Provincia de Buenos Aires, and the Fundación Antorchas. Part of the equipment used in the present work was provided by DAAD and the Alexander von Humboldt-Stiftung.

Received: June 30, 1996; January 20, 1997

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Publication Dates

• Publication in this collection
  30 Oct 2012
• Date of issue
  1997

History

• Accepted
  20 Jan 1997
• Received
  30 June 1996