β Scribed by R.D. Armstrong; M. Henderson
No coin nor oath required. For personal study only.
Recent work has shown that for electrochemical reactions with an absorbed intermediate ~ or an adsorbed inhibitor 2, useful information can be obtained from the electrode impedance plotted in the complex plane. In the specific instances of the active-passive transition of chromium 3 and titanium 4 in sulphuric acid, the mechanism of passivation has been interpreted from impedance spectra as that in which the electrode is passivated by adsorbed oxygen which inhibits the dissolution reaction, the surface coverage of O-species varying with potential.
A dissolution-precipitation model has been proposed for the passivation of nickel 5-7. However, in some of this work 5'6 an erroneous equation s'9 was used and iz Β½ behaviour which would result from a solid state reaction in parallel with metal dissolution was not considered. Other workers 1Β°-12 have reported a mechanism, similar to that proposed for chromium and titanium, in which a submonolayer of oxygen is chemisorbed on the nickel surface. The present work reports evidence for the latter mechanism, based on the impedance spectra of a polycrystalline nickel electrode in sulphuric acid at fixed potentials over the active-passive region.
The nickel electrode used was in the form of a disc (~ = 5 mm) cut from a high purity rod (Metals Research Ltd., purity 4N8). The impedance of the electrode in the frequency range 10 kHz-10-2 Hz was measured as described earlier 3'13. All measurements were carried out at room temperature (25 + 2Β°C) in deoxygenated 0.5 M H2SO 4 (triply distilled water and B. D. H. AristaR H2SO4). All potentials are referred to a hydrogen electrode in the same solution.
The nickel electrode was mechanically polished with 7-alumina and thoroughly rinsed prior to immersion in the cell. The steady-state current-potential curve is shown in Fig. , and was found to be independent of disc rotation speed as in earlier work ~2. As can be seen, the electrode showed the expected active-passive transition but this was obtained only after the application of repeated linear potential sweeps over the region. Prior to sweeping over the active-passive branch, the current-potential curve increased exponentially far beyond the peak potential expected. Also shown (Fig. ) is the series capacitance (C~) measured at 5 kHz at each potential. The only notable feature of this curve is the maximum, close to the maximum in the i-E curve.
The observation that the i-E curve is independent of the rotation speed suggests that :
π SIMILAR VOLUMES