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

The high-temperature oxidation of the NiAl alloy in the CO ₂ and O ₂ atmospheres is comparatively studied by a combination of electron microscopy characterization and first-principles calculations. It is shown that CO ₂ accelerates the oxidation of NiAl alloy by the formation of a highly porous γ-Al ₂O ₃ scale that is non-protective and follows the linear growth law. By contrast, the NiAl oxidation in O ₂ results in a double-layered oxide scale consisting of an outer columnar γ-Al ₂O ₃ layer and an inner equiaxed α-Al ₂O ₃ layer, which is protective and follows parabolic growth law. Our first-principles calculations show that this accelerated NiAl oxidation in the CO ₂ can be attributed to more ready decomposition of adsorbed CO ₂ than adsorbed O ₂ into atomic oxygen on defective γ-Al ₂O ₃ surfaces, which leads to the inward γ-Al ₂O ₃ growth by the infiltration of gaseous CO ₂ through the cracked oxide scale toward the exposed NiAl alloy. These results provide new insights into the atomic origin of CO ₂-promoted alloy oxidation.