Micro-electrochemical properties of CeS inclusions in stainless steel and inhibiting effects of Ce³⁺ ions on pitting

Micro-scale polarization data from small surface areas of stainless steels with either CeS or MnS inclusions was measured in NaCl and Na₂SO₄ solutions to elucidate the dissolution and pit initiation behavior of both types of inclusions. Stable pitting was initiated at the CeS inclusion at potentials which exceeded the dissolution potential region of the CeS inclusion, whereas a stable pit at the MnS inclusion was initiated in the dissolution potential range of the MnS inclusion. Thermodynamic calculations indicated that the Ce³⁺ ions are likely to be produced by the dissolution of the CeS inclusion. In the micro-scale polarization of a small area with a MnS inclusion, stable pitting at the MnS inclusion was inhibited in NaCl solutions containing Ce³⁺. The formation of a shallow trench was observed at the MnS inclusion/steel matrix boundary in the Ce³⁺-containing solution, whereas a deep trench was formed in the Ce³⁺-free solution. It is suggested that the Ce³⁺ ions inhibit trench formation at the MnS/steel matrix boundary, resulting in improved pitting corrosion resistance at sulfide inclusions.