Composition changes around sulphide inclusions in stainless steels, and implications for the initiation of pitting corrosion

We report high-resolution SIMS microscopy measurements, which show that many ‘MnS’ inclusions have surrounding them a narrow ‘halo’ of half-width typically 100 nm that is strongly enriched in Fe, forcing consideration of the electrochemistry of FeS as a determinant of the behaviour of the inclusion boundary. We suggest that on exposure to water a very thin and porous metal-deficient polysulphide skin forms between the bulk of the inclusion and the steel, within which a pit can be triggered. The results resolve a controversy concerning the composition of the boundary region around inclusions in stainless steel and re-emphasise its potential significance.

Research highlights
► nm-Scale FeS-enriched ‘halo’ formed around MnS inclusions in stainless steels.
► Detailed review on stainless steel pitting and on sulphide electrochemistry.
► Hypothesis for pit initiation based on the presumed reactivity of the FeS ‘halo’.
► Processing to remove the ‘halo’ may significantly improve pitting resistance.