Oxidation of FeCrAl alloys at 500–900°C in dry O2

Abstract

AbstractThis paper reports on the oxidation of a commercial FeCrAl alloy, Kanthal AF, in the temperature range 500–900°C. The samples are exposed isothermally in dry oxygen for up to 72 h using a thermo‐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 spectroscopy (AES).The rate of oxidation increases with temperature, the kinetics being parabolic in the range 700–900°C. 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°C 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‐Al2O3 already at 700°C. The low temperature of formation of α‐Al2O3 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, (Cr2O3). After exposure at 900°C AES shows large amounts of Mg in the outer part of the oxide, MgAl2O4 being detected by XRD together with γ‐ and γ‐Al2O3.