Surface Analysis of Corrosion Pits Initiated at MnS Inclusions in 304 Stainless Steel

Formation of pits near MnS inclusions on 304 stainless steel in NaCl (0.1 and 0.3M) and Na₂S₂O₃ (0 and 2 mM) solutions was investigated by a combination of surface analyses (Auger electron spectroscopy, scanning electron microscopy, and energy dispersive x-ray analyses) and electrochemical methods. Microlithography techniques were developed to monitor a single corrosion pit initiated at a single preselected inclusion. Pits were observed to form primarily at MnS inclusions. It was found that chemical dissolution of MnS occurred at the rest potential. Sulfur species were found to spread onto the adjacent passive surface over an area having a diameter about four times the size of the inclusion. Subsequent dissolution of metal was observed to occur only at MnS inclusions larger than about 1 μm. The surface of the microcavity adjacent to the inclusion was found to be enriched in S and, in later stages of dissolution, by Cr. These observations indicated that S-species play a key role in the processes that lead to pit initiation in this system.