Localized Corrosion of Stainless Steel Triggered by Typical Inclusions in NaCl Solution: Oxy-Sulfide and MnS.

The localized corrosion behavior of stainless steel (SS) induced by typical inclusion such as MnS and oxy-sulfide in NaCl solution was investigated by immersion tests and microelectrochemical tests. Oxy-sulfide consists of an internal polygonal oxide part and an external sulfide part. The surface Volta potential of the sulfide part is always lower than that of the surrounding matrix, as in the case of individual MnS, while the potential of the oxide part is indistinguishable from that of the surrounding matrix. Sulfides are soluble, while oxides are almost insoluble. Oxy-sulfide exhibits a complex electrochemical behavior in the passive region, which can be attributed to its complex composition and multi-interface coupling effects. It was found that both MnS and oxy-sulfide increase the susceptibility of the local area to pitting corrosion.