Mechanism of MnS-mediated pit initiation and propagation in carbon steel in an anaerobic sulfidogenic media

•In carbon steel, pits are initiated in the immediate surroundings of MnS inclusions.•Unlike stainless steel, MnS inclusions do not dissolve during pit initiation.•The presence of biofilms accelerates pit growth and development. In a saline anaerobic sulfidogenic environment, pitting on 1018 carbon steel was initiated within a 20–30nm zone at the MnS inclusion boundary. Nanoscale analysis was performed using scanning electron microscopy and a scanning Auger nanoprobe. The pitting was more pronounced in the presence of a biofilm of sulfate-reducing bacteria than in abiotic sulfide medium. It is proposed that initiation of an anodic reaction leading to dissolution of Fe matrix and subsequent pitting of steel in MnS inclusion boundary regions is due to disorder and strain exerted on the Fe matrix by MnS contamination of the interface from metallurgical processes.