2019 年 105 巻 2 号 p. 207-214
This paper presents an introduction to the electrochemical properties at the boundary between MnS and steel matrix of Type 304 stainless steel. It starts with the information about the corrosion mechanism from trenching to pitting at the boundary. The proposed mechanism is as follows: 1) MnS dissolution leads to the deposition of elemental sulfur on and around the inclusions; 2) the coexistence of S and Cl– ions results in the dissolution of the steel matrix side of the boundary, forming the trenches along the periphery of the inclusions, 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 ohmic drop. The prevention of MnS dissolution is effective to inhibit the pit initiation. The oxide film was found to be generated on the MnS inclusions by the exposure to humidified air for 30 and 90 days, and it was confirmed that the oxide films effectively inhibit the inclusion dissolution and the trench formation at the boundaries, making the inclusions less active as the initiation sites for pitting. The suppression of the active dissolution of the steel matrix is also effective to prevent the pitting. No pit initiation occurred at the MnS inclusions in low-temperature carburized stainless steels. It was clarified that the interstitial carbon sufficiently inhibits the active dissolution rate of the steel matrix.
