Relationships between pitting corrosion potentials and MnS dissolution of 5–18 mass% Cr steels

With the increase in the Cr content in the steels, the Cr and Fe concentrations in the inclusions increased and decreased, respectively. The MnS inclusions were found to dissolve in 0.1 M Na₂SO₄ around 0 V vs. Ag/AgCl, (3.33 M KCl). Even though the dissolution potential was independent of Cr concentration, the dissolution morphology was remarkable in that the inclusions in Fe-5Cr dissolved and disappeared. For Fe-12Cr, the Cr-enriched portions remained. In the case of Fe-18Cr, Cr-oxides formed on the inclusion surfaces. The micro-scale polarization was performed in 0.1 M NaCl. At approximately 0.05 V, a large pit was generated at the inclusion of Fe-5Cr, and the inclusion disappeared after polarization. For Fe-12Cr, a stable pit was initiated at 0.13 V, and most of the inclusion remained. No stable pit occurred on Fe-18Cr; however, a meta-stable pitting event was observed in the dissolution potential region of the MnS. Cr and O were detected on the entire surface on the inclusion after polarization. It was suggested that the pitting corrosion resistance at the MnS inclusions of Fe-Cr alloys was related to the inclusion dissolution morphology originating from the difference in the Cr concentration of the inclusions.