The Effect of Water Vapor on the Initial Stages of Oxidation of the FeCrAl Alloy Kanthal AF at 900 °C

The effect of water vapor on the initial stages of oxidation of the FeCrAl alloy Kanthal AF is reported. Polished samples were exposed isothermally at 900 °C for 1, 24, 72 and 168 h in a well-controlled environment consisting of dry O₂ or O₂ + 40% H₂O. The samples were investigated using a combination of gravimetry and several surface-analytical techniques, including XRD, SEM, EDX, FIB, AES and TEM. The presence of water vapor significantly accelerates oxidation during the first 72 h. A two-layered oxide forms in both the dry and wet environments. The bottom layer consists of inward-growing α-Al₂O₃ while the outer layer initially consists of outward-growing γ-Al₂O₃. A straight and narrow Cr-enriched band is present at the top of the lower (α-Al₂O₃) oxide, corresponding to the original sample surface. In dry O₂, the top (γ-Al₂O₃) layer is converted into a mixture of γ-Al_2−x (Mg,Fe) _x O_3−(x/2), MgAl₂O₄ and α-Al₂O₃. This transformation does not occur in O₂ + H₂O. The initial acceleration of oxidation by H₂O is attributed to the stabilization of the outward-growing γ-alumina layer by the hydroxylation of the γ-Al₂O₃ surface. A schematic mechanism of the early stages of oxidation of FeCrAl alloys is presented, emphasizing the influence of water vapor.