Oxidative dissolution and anion-assisted solubilisation in the transpassive state of nickel-chromium alloys

The transpassive state of pure Ni, Cr and two Ni–Cr alloys (10 and 20 wt.% Cr) in 14.8 M H₃PO₄ is studied by voltammetric, rotating ring-disc electrode and impedance spectroscopic measurements. The results indicate that the rate of transpassive dissolution of the alloys in a major part of the transpassive range is higher than the rate of this process for both pure metals. A kinetic model retaining the essential features of the corresponding models for the pure metals proposed in the literature has been found to reproduce both the steady-state current versus potential curves and the ac impedance spectra in the transpassive range. The parameters of the model are determined by a fitting procedure and the relevance of their values for the relative importance of two possible pathways in the transpassive state—oxidative dissolution and anion-assisted solubilisation—are discussed.