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

摘要

This study investigates corrosion of Fe-Ni-based alloy 800H that were exposed to supercritical CO2 (sCO(2)), ambient air and argon gas at pressures up to 20MPa, at 650 and 750 degrees C for up to 1000h. This alloy and other comparable metal alloys are expected to be used in sCO(2) 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 sCO(2) have been evaluated and compared with exposures in air and Ar. The 800H alloy showed similar oxide scale thicknesses in sCO(2) as in air. A recrystallized zone was observed beneath the oxide formed in air and sCO(2). 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 sCO(2) than in air. The effect of air and sCO(2) on internal oxidation and carburization was investigated as well, showing that air led to more internal oxidation but less internal carburization than sCO(2). It was concluded that the carbon species provided by the sCO(2) 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.