Effects of Corrosion in Supercritical CO2 on the Microstructural Evolution in 800H Alloy

Abstract

This study investigates corrosion of Fe–Ni-based alloy 800H that were exposed to supercritical CO2 (sCO2), ambient air and argon gas at pressures up to 20 MPa, at 650 and 750 °C for up to 1000 h. This alloy and other comparable metal alloys are expected to be used in sCO2 heat exchanger cycles as proposed in the DOE Advanced Ultra-Supercritical program. Alloy 800H is considered for this application, because it meets the high-temperature strength and creep rupture requirements and is a lower cost alternative to other Ni-based alloys. The oxidation performance and microstructural changes due to exposure in sCO2 have been evaluated and compared with exposures in air and Ar. The 800H alloy showed similar oxide scale thicknesses in sCO2 as in air. A recrystallized zone was observed beneath the oxide formed in air and sCO2. No such zone was observed after exposure to Ar, suggesting this recrystallization was associated with the oxidation process and not simply an effect of surface finishing. A wider recrystallized zone was observed underneath the oxide formed in sCO2 than in air. The effect of air and sCO2 on internal oxidation and carburization was investigated as well, showing that air led to more internal oxidation but less internal carburization than sCO2. It was concluded that the carbon species provided by the sCO2 atmosphere in conjunction with the increased grain boundary density in the recrystallized zone allowed for more ingress of carbon into the base metal, which resulted in a higher densities of carbides beneath the oxide scale.