New Insights into Sulfide Inclusions in 1018 Carbon Steels

The role of heterogeneous phases in the localized corrosion of materials is an emerging area of research. This work addresses the detrimental role MnS inclusions play in the localized corrosion of carbon steels. We report the results of surface and bulk characterization of MnS inclusions in 1018 carbon steel, using a high-resolution integrated Auger nanoprobe. It is shown that the surface morphology and composition of the inclusions are highly heterogeneous. MnS inclusions are found to be nonstoichiometric and to contain a highly O-enriched surface layer. Some inclusions are covered with a thin (approximately 5 nm) layer of Cu2S. The bulk composition of “MnS” inclusions is found to include 5–7% Fe and O, and these inclusions are believed to occupy Mn and S positions within the “MnS” lattice. Interfaces between “MnS” and host ferric grains are highly disordered. We hypothesize that pitting initiates and develops at these interfaces through a galvanic coupling between the strained and the unstrained ferrite grains.