Discussions on some Properties of Alumina Scales and their Protectiveness

Abstract

This small review deals mainly with three issues regarding the nature and protectiveness of alumina scales grown during high-temperature oxidation: (1) sequences of phase transportation of alumina scales formed on Fe-Cr-Al and NiAl alloys, and a few aluminides, (2) combined additions of reactive element (RE) and (3) convolution of α-Al2O3 scales. Though the general phase transformation sequence of alumina scales is γ to θ to α phases at intermediate temperatures, variations have been reported. Directional growth of transient aluminas such as γ-Al2O3 and θ-Al2O3 is discussed with a particular emphasis on its driving force. Parabolic rate constants for the growth of α-Al2O3scales are smaller when the period of transient alumina is longer because of larger α-Al2O3grains. The effect of RE in slowing the parabolic oxidation saturates at a certain concentration, however combined addition further decreases the oxidation rate. The α-Al2O3 scales on Fe-Cr-Al alloys without RE are highly convoluted, however those on NiAl and other aluminides are not so convoluted. The α-Al2O3 layer beneath the outer NiO layer or NiAl2O4 layer is flat in the oxidation of Ni3Al. Directions for future work are proposed.