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

The processes in the potential region corresponding to transpassive dissolu tion of Ni-Cr alloys (10 and 20 wt,% Cr), pure Ni and Cr were studied by el ectrochemical techniques - ring-disk voltammetry, impedance spectroscopy an d de resistance measurements - in 1 hi sulphate solutions (pH values 0 and 5). Both the electronic and the ionic conductivity of the anodic film forme d 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 fr om Ni-Cr alloys was found to be somewhat higher than that from pure Cr, pro bably due to the higher electronic conductivity of the film in the former c ase. A model for the transpassive dissolution of Ni-Cr alloys is proposed o n 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 tra nspassive dissolution process are thereby determined. (C) 2000 Elsevier Sci ence Ltd. All rights reserved.