Atomic-level mechanism of CO2-promoted oxidation of alumina-forming alloys

Abstract

The high-temperature oxidation of the NiAl alloy in the CO2 and O2 atmospheres is comparatively studied by a combination of electron microscopy characterization and first-principles calculations. It is shown that CO2 accelerates the oxidation of NiAl alloy by the formation of a highly porous γ-Al2O3 scale that is non-protective and follows the linear growth law. By contrast, the NiAl oxidation in O2 results in a double-layered oxide scale consisting of an outer columnar γ-Al2O3 layer and an inner equiaxed α-Al2O3 layer, which is protective and follows parabolic growth law. Our first-principles calculations show that this accelerated NiAl oxidation in the CO2 can be attributed to more ready decomposition of adsorbed CO2 than adsorbed O2 into atomic oxygen on defective γ-Al2O3 surfaces, which leads to the inward γ-Al2O3 growth by the infiltration of gaseous CO2 through the cracked oxide scale toward the exposed NiAl alloy. These results provide new insights into the atomic origin of CO2-promoted alloy oxidation.