硫化物系介在物のマイクロ電気化学特性とステンレス鋼の孔食発生機構

抄録

Stainless steels suffer from pitting corrosion in chloride-containing environments. Sulfide inclusions, such as MnS, are known to act as the initiation sites of pitting. The pit initiation mechanism at MnS inclusions in chloride-containing environments is as follows: 1) dissolution of MnS inclusions leads to the deposition of elemental sulfur on and around the inclusions; 2) the coexistence of elemental sulfur and chloride ions results in the dissolution of the steel matrix, forming the trenches at the boundaries between the inclusions and the steel matrix, 3) rapid active dissolution occurs locally at the bottom of the trenches, because of the decrease of pH due to the hydrolysis reaction of Cr³⁺ and the electrode potential decrease at the bottom of the trench due to the solution resistance. Dissolution behavior of sulfide inclusions is of key importance in pit initiation process. Insoluble sulfide inclusions, such as CrS, TiS, and Ti₄C₂S₂, are unlikely to act as the pit initiation sites. Cr-or Ti-oxide films on the inclusions act as a barrier against inclusion dissolution. An applied stress causes micro-cracks on MnS inclusions, which promotes pit initiation of stainless steels. The micro-crack initiation is closely related to oxide film formation on MnS inclusions.