Effect of Cu Addition on Localized Corrosion Originating from MnS Inclusions for Low-alloy Steel in a 3% NaCl Solution

The effect of MnS inclusions on the localized corrosion of low-alloy steel in a 0.5 mol/kg (3%) NaCl solution was investigated to propose countermeasures for inhibiting localized corrosion initiated by MnS inclusions. Low alloy steels without MnS inclusions did not corrode in a 0.5 mol/kg (3%) NaCl solution, regardless of the addition of Cu. That is, no matrix improvement effect due to solute Cu was confirmed. Slight corrosion occurred in the Cu containing steel with MnS inclusions; however, the MnS inclusions remained. Cu7.2S4 precipitated on the MnS in contact with a 0.5 mol/kg (3%) NaCl solution. Therefore, Cu7.2S4 precipitates on the MnS inclusions during immersion, which could suppress the localized corrosion initiated by the MnS inclusions because Cu sulfide is not dissolved based on potential-pH diagram. The addition of Cu in a 0.5 mol/kg (3%) NaCl solution does not improve the corrosion resistance of the matrix due to solute Cu but does suppress localized corrosion initiating from MnS by precipitating Cu sulfide on MnS.