Towards a better understanding of localised corrosion induced by typical non-metallic inclusions in low-alloy steels

Detailed understanding of localised corrosion phenomena induced by inclusions in low-alloy steel is extremely critical as any corrosion-induced premature failure can potentially lead to a catastrophe. In particular, there is still ambiguity as to whether the localised corrosion initiation induced by non-metallic inclusions is an electrochemical process or a chemical process. Herein, the localised heterogeneous electrochemical mechanism of typical non-metallic inclusions–Al₂O₃, ZrO₂-Ti₂O₃-Al₂O₃, and (RE)AlO₃-(RE)₂O₂S-(RE)_xS_{y –} was investigated using scanning Kelvin probe force microscopy and current-sensing atomic force microscopy. The chemical stability of typical non-metallic inclusions was characterized by Pourbaix diagrams. The results indicate that the localised corrosion behaviour shows no correlation to the Volta potential difference between the non-metallic inclusions and the matrix. Moreover, the localised corrosion initiation process is mainly a chemical corrosion process rather than an electrochemical corrosion process. Thus, the applicability of Volta potential difference in predicating the preferential corrosion area in different system (conductive/nonconductive system) should be given full attention.