Effect of Rare Earth Metal Alloying on Pitting Corrosion Resistance and Corrosion Layer Formation of High-Strength Oil Casing Steels

Pitting corrosion significantly affects the lifespan of oil-casting steel. This study investigated the pitting corrosion behavior and corrosion layer characteristics of cerium-treated oil casing steel in a 4 pct NaCl solution containing 0.01 mol·L⁻¹ Na₂S at pH 4 and 25 °C. It was found that the pitting corrosion induced by CaO·2Al₂O₃–CaO·6Al₂O₃, Ce₂O₃, and Ce₂O₂S inclusions resulted from both chemical dissolution and the formation of micro-crevices at the inclusion/matrix interfaces. The low self-dissolution tendency of CeAlO₃ inclusions exhibited an inhibitory effect on the pitting propagation. Furthermore, Ce promoted the formation of a thin and dense corrosion layer. During corrosion layer formation, the outward diffusion of Cr and Mo ions was suppressed by Ce diffusion occupying the active sites, which led to the enrichment of Cr and Mo in the corrosion layer. In this study, the optimum Ce content for corrosion resistance in oil casing steel should be in the range of 61 to 120 ppm.