Corrosion of a stainless steel and nickel-based alloys in high temperature supercritical carbon dioxide environment

Corrosion of four alloys has been studied in supercritical carbon dioxide at 650 °C and 20 MPa, specifically AL-6XN stainless steel and three nickel-based alloys, PE-16, Haynes 230, and Alloy 625. The tests were performed for exposure durations of up to 3000 h with samples being removed for analyses at 500 h intervals. The corrosion performance of the alloys was evaluated by weight change measurements, and the surface oxide layers were characterized by scanning electron microscopy, X-ray diffraction and X-ray photoelectron spectroscopy. Weight gain measurements showed that the Al-6XN stainless steel exhibited the least corrosion resistance while the weight gains were nearly similar for the other alloys. The oxide layer in AL-6XN stainless steel was composed of large equiaxed grained outer layer of Fe₃O₄ (magnetite) and an inner layer of FeCr₂O₄. Oxide spallation was observed in this stainless steel even after 500 h exposure. In all alloys, Cr-rich oxides phases of Cr₂O₃ and Cr_1.4Fe_0.7O₃ were identified as the protective layers. In alloy PE-16 a thin layer of aluminum oxide formed that promoted the corrosion resistance of the alloy. Cr₂O₃ was identified as the main protective oxide layer in nickel base alloys Haynes 230 and 625.