The role of Mo alloying in the pit initiation process at MnS inclusions in stainless steels was investigated by measuring polarization curves, depassivation pH, and scanning electron microscopy. Molybdenum existed in the steel matrix but was not detected in the MnS inclusions in Type 316 stainless steel. While Mo alloying did not inhibit the dissolution of the MnS inclusions in a NaCl solution, neither a metastable nor a stable pit occurred in the small areas with the MnS inclusions in the Mo-added specimen. The MnS/steel matrix boundary preferentially dissolved in the Mo-free specimen, with deep trenches formed. However, no deep trench was observed in the Mo-added specimen at low potentials. The depassivation pH of the Mo-added specimen in a 0.1 M NaCl-1 mM Na2S2O3 solution was lower than that of the Mo-free specimen, suggesting that Mo alloying prevents the depassivation of the steel matrix at the boundary and inhibits the formation of trenches. In a 1 M HCl-1 mM Na2S2O3 solution, the active dissolution rate of the steel was suppressed by Mo alloying. This suggests that, even after trenching at high potentials, Mo alloying inhibits the initiation of pitting inside the trenches.
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