Transpassive Behaviour of Iron–Chromium Alloys in Acidic Sulphate Solutions

Abstract

The mechanism of the transpassive behaviour of Fe–Cr alloys containing 5–30% Cr has been investigated using stationary and rotating disc electrodes in acidic sulphate solutions. At potentials > 1000 mV (NHE) the alloys undergo transpassive dissolution terminating in a plateau, at a c.d. the logarithm of which increases linearly with Cr content. This relationship, and its apparent independence of potential, lead to the assumption of limiting current behaviour, which should be proporiional to the rates of diffusion of the substances reacting at the electrode surface. On the basis of this assumption and the independence of the plateau c.d. for the 15% Cr alloy on rotation, it appears that surface processes are rate determining and that solution soluble intermediates do not participate. Restriciion by diffusion occurs only within the passivaiing fllm on the electrode.