Role of sulphide inclusions in pitting and crevice corrosion of stainless steels

The role of sulphide inclusions in the pitting and crevice corrosion of stainless steels is reviewed. It is noted that manganese sulphides, particularly those containing a central oxide core, are the most favourable sites for pit initiation. Replacing the manganese in the sulphide by chromium renders pit initiation more difficult. However, as the severity of the environment increases, pitting occurs at the less active inclusions. It is concluded that changing the composition of sulphides can give a useful, if somewhat limited, improvement in pitting resistance and in crevice-corrosion resistance. Further studies are needed to define the mechanism by which molybdenum, or molybdenum and nitrogen in combination, improve pitting and crevice -corrosion resistance in the presence of sulphide inclusions.