Influence of the dissolution of MnS inclusions under free corrosion and potentiostatic conditions on the composition of passive films and the electrochemical behaviour of stainless steels

The influence of the dissolution of MnS inclusions at the OCP value and 400 mV versus SCE on the chemical composition of passive films and the electrochemical behaviour of resulfurized austenitic stainless steel was studied in 1 M NaClO4, pH 3, solution using SIMS, XPS, electrochemical impedance spectroscopy and the electrochemical microcell technique. The electrochemical dissolution (at 400 mV versus SCE) of inclusions occurred uniformly along the interface and this process was almost complete after 25 min immersion. A small amount of sulfur was detected on the matrix that exhibited a wide passive range. By contrast, the dissolution process under free corrosion was not uniform along the interface and numerous inclusions were partly dissolved. Grains close to the completely dissolved inclusions showed an active behaviour. This was connected with the adsorption of a large number of sulfur on the surface. In the two cases, the presence of sulfur species on the surface was also found to enhance the kinetics of cathodic reactions.