Oxidation behaviour of Kanthal APM and Kanthal AF at 1173 K: Effect of yttrium alloying addition

The oxidation behaviour of two commercial FeCrAl alloys, Kanthal APM (produced by powder metallurgy) and Kanthal AF (produced by melting and containing alloying additions of yttrium), has been investigated during isothermal exposures in air at 1173 K Mass gains were measured by thermogravimetry and identification of the oxidation products determined by in situ high temperature X-ray diffraction. The morphology of the oxide layer surfaces was studied by SEM, the elemental distribution inside the scales being carried out by EDXS analysis. After an initial transient stage, a diffusional process appears to predominantly control the oxidation kinetics of both alloys. During the transient stage, relatively important mass gains have been revealed, and the presence of yttrium does not seem to have a significant effect on the oxidation rate. In the parabolic oxidation stage however, yttrium showed a marked influence on the composition and growth rate of the oxide scale. The present study indicates that the reactive element drastically reduces the parabolic rate constant and promotes the transformation of transition alumina into alpha-Al2O3, which leads to the growth of a more protective oxide scale. It has also revealed that an enrichment of chromium (as chromium carbide precipitates) near the scale/metal interface occurred in the oxide layer formed on Kanthal APM.