Pit Initiation Mechanism at MnS Inclusions in Stainless Steel: Synergistic Effect of Elemental Sulfür and Chloride Ions

Microscopic polarization, scanning transmission electron microscopy, and Raman spectroscopy were performed to ascertain the pit initiation mechanism at MnS inclusions in stainless steels. While the inclusion surfaces dissolved under anodic polarization in 0.1MNa₂SO₄as well as0.1 and 3 M NaCl solutions, the boundaries between the inclusions and the steel matrix dissolved selectively only in the NaCl solutions. This selective dissolution resulted in the formation of trenches, in which metastable and stable pits were initiated. The trenches were shown to be formed by the active dissolution of the steel sides of the boundaries, where no anomalous phase and no compositionally altered zone was observed. It was found that elemental sulfür was deposited on the inclusions and at the boundaries after anodic polarization in 3 M NaCl. The active dissolution of the steel matrix occurred in solutions in which chloride ions and elemental sulfür coexist. The synergistic effect of the elemental sulfür produced by MnS inclusion and chloride ions is likely to cause the trenches, and the decrease in both pH and potential inside the trenches results in pit initiation.