The Mechanism of Oxide Film Formation on AISI 316 Stainless Steel in Sulphate Solutions

Abstract

Oxide film formation on AISI 316 stainless steel (a material often used for building nuclear reprocessing plants) in acidic and neutral sodium sulphate solutions is investigated using electrochemical and surface analysis techniques. It is found that the corrosion potential varies with pH in the amount of -55 mV / pH unit and critical currents increase with decreasing pH and increasing temperature in neutral solutions. The activation energy for the reaction in the active region is 45.6 kJ mol-1. Potentiostatic experiments indicate that three different film building processes are involved in film formation: the initial active-passive transition, the logarithmic growth of the film and the parabolic growth of the film. AES shows chromium enrichment at the surface, and ESCA gives little difference for the relative amount of Fe2+ and Fe3+ for 0.2 and 0.6 V. A theoretical model is being developed to account for the observed kinetics.