Microelectrochemical Aspects of Cu Effects on Pit Initiation at MnS Inclusions in Tempered Martensitic Steel

This study investigates the effects of copper (Cu) on pit initiation at manganese sulfide (MnS) inclusions in tempered martensitic steel. The tempered martensitic microstructures and MnS inclusions in Cu-free and Cu-added steels exhibited similar morphologies and compositions. Cu was uniformly distributed within the steel matrix of Cu-added steel, and no Cu enrichment was detected within MnS inclusions or at the inclusion/matrix interface. The Cu-added steel exhibited higher pitting corrosion resistance than the Cu-free steel in a NaCl solution. Polarization conducted using microscale electrode areas revealed that partial dissolution of MnS inclusions occurred within a similar potential range for both steels, indicating that the dissolution behavior was largely unaffected by Cu addition. Although the composition and thickness of surface oxide films on the steel matrix did not substantially differ between Cu-free and Cu-added steels, Cu addition reduced the active dissolution rate of the steel matrix.