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

Abstract

Corrosion of four alloys has been studied in supercritical carbon dioxide at 650°C and 20MPa, 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 3000h with samples being removed for analyses at 500h 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 500h exposure. In all alloys, Cr-rich oxides phases of Cr2O3 and Cr1.4Fe0.7O3 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.