Corrosion behaviour of low-manganese stainless steels

Electrochemical measurements have shown that low-manganese (approximately 0.2% Mn) stainless steels with low (&le1.2% Mo) or no molybdenum exhibit a higher pitting resistance in neutral sodium chloride (0.005-2 M) than the analogous commercial steels containing 1.4 to approximately 2% Mn and approximately 2.8% Mo, or the special laboratory heats of similar composition, but, with a high S content, up to approximately 0.2%. Microscopic examinations, X-ray microprobe analysis and SAM investigations have revealed a correlation between pitting resistance of the steels, the composition of the non-metallic inclusions, and the composition of the passive film formed at the inclusions and at the adjacent matrix. Electrochemical measurements in acidic and neutral sulphates have shown that the low-manganese steels generally exhibit higher ability to passivate than the commercial ones. In contrast to the behavior in neutral solutions, also in those containing chlorides, the passivity of a modified steel containing neither Mo nor Ti is less stable in acidic media than that of the other novel steels.