Oxidation of FeCrAl alloys at 500-900 ℃ in dry O{sub}2

摘要

This paper reports on the oxidation of a commercial FeCrAI alloy, Kanthal AF, in the temperature range 500-900 ℃. The samples are exposed isothermally in dry oxygen for up to 72 h using a ther-mo-balance. In addition, 168 h exposures are carried out in a tube furnace. The exposed samples are investigated by grazing angle X-ray diffraction (XRD), scanning electron microscopy/energy dispersive X-ray analysis (SEM/EDX), and auger electron spectro-scopy (AES). The rate of oxidation increases with temperature, the kinetics being parabolic in the range 700-900 ℃. At all exposure temperatures, most of the sample surface is covered by a thin smooth base oxide. In addition, RE-rich particles, with a typical size of 1 -3 μm form. At 800 and 900 ℃patches of thick oxide appear, featuring needle-formed crystallites situated on top of the base oxide. The thick oxide usually forms around Y-rich oxide particles. The concentration of iron and chromium in the oxide decreases with increasing temperature. XRD proves the formation of α-Al{sub}2O{sub}3 already at 700 ℃. The low temperature of formation of α-Al{sub}2O{sub}3 is attributed to the presence of chromium in the initial oxide. It is proposed that corundum nucleation is facilitated on a surface consisting of the isostructural escolaite, (Cr{sub}2O{sub}3). After exposure at 900 ℃ AES shows large amounts of Mg in the outer part of the oxide, MgAl{sub}2O{sub}4 being detected by XRD together with γ- and α-Al{sub}2O{sub}3.