Early stages of the oxidation of a FeCrAlRE alloy (Kanthal AF) at 900 °C: A detailed microstructural investigation

Early stages of the evolution of Al2O3 scales formed on a FeCrAlRE alloy (Kanthal AF) have been investigated by analytical TEM. The samples were oxidized isothermally at 900 °C in dry O2 or O2 + 40% H2O for 1 h or 24 h. All oxide scales exhibited a two-layered structure, with a continuous inward growing α-Al2O3 inner layer and an outward growing outer layer. After 1 h, the outer oxide layer consisted of γ-Al2O3 in both environments. After 24 h exposure in dry O2, the γ-Al2O3 in the outer oxide layer was partly transformed to α-Al2O3 and spinel oxide (Mg1−xFexAl2O4). In contrast, the γ-Al2O3 in the outer layer was not transformed after 24 h in O2 + 40% H2O, showing that water vapour stabilizes γ-Al2O3. All oxide scales contained a Cr-rich band, a product of the initial oxidation. The inner α-Al2O3 layer is suggested to nucleate on Cr2O3 or Cr2−xFexO3 in the initial oxide.