Corrosion Resistance of a Free-Cutting Soft-Magnetic Stainless Steel in Pure Water

Abstract

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 0.1 M Na₂SO₄. The Ti₄C₂S₂ inclusions hardly dissolved in pure water at 353 K and did not suffer anodic dissolution in the passivity region of stainless steel in 0.1 M Na₂SO₄. In contrast, conventional free-cutting soft-magnetic 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 0.1 M 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.