Microelectrochemical polarization, thermodynamic calculations, and Auger electron spectroscopy were performed to determine the predominant factors affecting the pit initiation resistance at sulfide and carbosulfide inclusions in stainless steels. TiS and Ti4C2S2 inclusions did not dissolve in the passive or the trans-passive regions of stainless steels in NaCl solutions, and no pits were initiated at these inclusions in NaCl solutions. The calculations made from the potential-pH diagrams for Ti-S-TiS-H2O and Ti-S-Ti4C2S2-H2O systems indicated a high likelihood that Ti-oxide enriched surface films would form, and this was confirmed by surface analysis. The Ti-oxide enriched layers on the inclusions inhibited the dissolution of the TiS and Ti4C2S2 inclusions in the passive and trans-passive regions of stainless steels. In contrast, MnS inclusions dissolved in NaCl solutions, and pits were initiated at the inclusions. While MnS inclusion surfaces were covered by Mn-oxides, the corrosion resistance of Mn-oxides was not sufficient to prevent inclusion dissolution and subsequent pit initiation. The oxide enriched surface layers on the sulfide and carbosulfide inclusions were shown to significant enhance pit initiation resistance at the inclusions.

Microelectrochemical measurements were carried out to ascertain the effects of applied stress on pit initiation behavior at (Mn,Cr)S inclusions, which contained around 15 at% chromium. During anodic polarization in 1.5 mol kg −1 MgCl 2, the MnS selectively dissolved and the composition changed from (Mn,Cr)S to CrS covered with a surface oxide film. Stable pitting did not always occur when no stress was applied, even when the MnS dissolved and meta-stable pits formed. Under applied stress, stable pitting was consistently initiated in the potential range of the MnS dissolution, and micro-cracks were generated perpendicular to the tensile direction. The cracks were thought to be stress corrosion cracking induced by the formation and rupture of the oxide film on the inclusion under applied stress. The inclusion was perforated by the crack, and the steel matrix under the inclusion was then exposed to the solutions, and stable pitting was initiated.

Microelectrochemical measurements were carried out to ascertain the effect of stress on the dissolution morphology and pit initiation behavior of (Mn,Cr)S inclusions, which contained around 15 at. % chromium. Under anodic polarization, the selective dissolution of MnS and the composition change from the (Mn,Cr)S to CrS occurred in 1.5 mol kg -1 MgCl 2 . No stable pit was initiated under no applied stress, even though the MnS dissolution and the formation of meta-stable pits were observed. Under applied stress, a stable pit was initiated in the potential range of the MnS dissolution, and small cracks that were perpendicular to a tensile direction were generated. The cracks were thought to be induced by a mechano-chemical reaction, and propagated to the steel under the inclusion. The steel matrix was then exposed to the solutions, and stable pitting was initiated at the inclusions under applied stress.

Microelectrochemical polarization, thermodynamic calculations, and Auger electron spectroscopy were performed to elucidate the predominant factor affecting the pit initiation resistance at sulfide and carbosulfide inclusions in stainless steels. The Ti-oxide enriched layers on the inclusions inhibit the dissolution of the TiS and Ti 4 C 2 S 2 inclusions in the passive and trans-passive regions of stainless steels. While the MnS inclusion surfaces are covered by Mn-oxides, the corrosion resistance of Mn-oxides is not sufficient to prevent the inclusion dissolution and pit initiation. The oxide enriched surface layers on the sulfide and the carbosulfide inclusions greatly influence the pit initiation resistance at the inclusions.