The mechanism of transpassive dissolution of Ni-Cr alloys in sulphate solutions

The processes in the potential region corresponding to transpassive dissolution of Ni–Cr alloys (10 and 20 wt.% Cr), pure Ni and Cr were studied by electrochemical techniques — ring-disk voltammetry, impedance spectroscopy and dc resistance measurements — in 1 M sulphate solutions (pH values 0 and 5). Both the electronic and the ionic conductivity of the anodic film formed on Ni and Ni–Cr alloys were found to be considerably higher than those of the passive film on pure Cr. The rate of transpassive dissolution of Cr from Ni–Cr alloys was found to be somewhat higher than that from pure Cr, probably due to the higher electronic conductivity of the film in the former case. A model for the transpassive dissolution of Ni–Cr alloys is proposed on the basis of an earlier model for transpassive dissolution of Cr and the present results. The effect of the transpassive dissolution of Ni from the oxide film is taken into account as well. The kinetic parameters of the transpassive dissolution process are thereby determined.