The Role of Oxide Films on TiS and Ti4C2S2Inclusions in the Pitting Corrosion Resistance of Stainless Steels

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.