The corrosion resistance of a new free-cutting soft-magnetic stainless steel containing Ti4C2S2 instead of MnS was examined by immersion corrosion tests in pure water and anodic polarization measurements in 0.1M Na2SO4. The Ti4C2S2 inclusions hardly dissolved in pure water at 353K and did not suffer anodic dissolution in the passivity region of stainless steel in 0.1M Na2SO4. In contrast, conventional free-cutting softmagnetic stainless steels with (Mn,Cr)S inclusions caused the significant release of Mn species during immersion in pure water, and showed a marked increase in the dissolution current of MnS under anodic polarization in 0.1M Na2SO4. The thermodynamic stability of Ti, Mn and Cr oxide films on the inclusions makes the difference in the corrosion resistance of Ti4C2S2 and (Mn,Cr)S, with no dissolution of Ti from Ti4C2S2 and the selective dissolution of Mn from (Mn,Cr)S.
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