Effect of Cerium on Mechanisms of Pitting Corrosion Induced by Inclusions in a 304 Stainless Steel

Laboratory experiments were performed to study the pitting process induced by different kinds of inclusions in stainless steels with and without cerium. When the content of total cerium in the stainless steel increased from 0 to 430 ppm, the evolution path of inclusions in the stainless steel was Al2O3-SiO2-MnO(-CaO)-MnS -> Ce-Al-Si-Ca-O-S -> homogeneous Ce-O-S -> heterogeneous Ce-O-S. Immersion test was performed and morphologies of pits induced by different types of inclusions were observed. The dissolution sequence was Ce-O phase in heterogeneous Ce-O-S and MnS -> homogeneous Ce-O-S, Ce-S phase in heterogeneous Ce-O-S and steel matrix -> Ce-Al-Si-Ca-O-S -> Al2O3-SiO2-MnO(-CaO). A corrosion index was defined to compare the corrosion resistance of the steel matrix around inclusions. Besides, first-principles calculations of related inclusions were performed to clarify the mechanism of pitting corrosion induced by different kinds of inclusions in the stainless steel.