Revealing the long-term oxidation and carburization mechanism of 310S SS and Alloy 800H exposed to supercritical carbon dioxide

Abstract

The oxidation/carburization behavior and failure mechanism of 310S stainless steel (SS) and Alloy 800H exposed to supercritical carbon dioxide at 650 °C/20 MPa were investigated. The results show that the oxidation kinetics of 310S SS and Alloy 800H both approximately follow the parabolic oxidation law. The weight gain of Alloy 800H is higher than 310S SS for the first 2000 h exposure. While with the increase of exposure time to 3000 h, the weight gain of Alloy 800H decreases, and becomes lower than 310S SS. Cr2O3 is formed on the surface of 310S SS, while a multilayer structure mainly composed of outer MnCr2O4-Cr2O3 layer and inner MnCr2O4-SiO2 layer is formed on Alloy 800H. Carburization happens in the oxidation process of 310S SS and Alloy 800H. However, the carburization performances of the two kinds of materials are different. For Alloy 800H, a C layer is deposited nearby the O/M interface, which is because that the inner MnCr2O4 and SiO2 layer in the oxide scale acts as a barrier for the inward diffusion of C-containing substances. For 310S SS, a carburized region with more and bigger carbides is formed in the matrix alloy beneath the Cr2O3 layer. The oxide spallation occurs on Alloy 800H after 2000 h exposure, which explains the reason that the weight gain of Alloy 800H decreases with the further increment of testing time to 3000 h. The spallation of oxide film on Alloy 800H is likely to be attributed to the presence of the inner MnCr2O4 layer, continuous SiO2 layer and the deposited amorphous C nearby the O/M interface.