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 O 2 or O 2 + 40% H 2 O. 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 a-Al 2 O 3 while the outer layer initially consists of outward-growing c-Al 2 O 3 . A straight and narrow Cr-enriched band is present at the top of the lower (a-Al 2 O 3 ) oxide, corresponding to the original sample surface. In dry O 2 , the top (c-Al 2 O 3 ) layer is converted into a mixture of c-Al 2Àx (Mg,Fe) x O 3À(x/2), MgAl 2 O 4 and a-Al 2 O 3 . This transformation does not occur in O 2 + H 2 O. The initial acceleration of oxidation by H 2 O is attributed to the stabilization of the outward-growing c-alumina layer by the hydroxylation of the c-Al 2 O 3 surface. A schematic mechanism of the early stages of oxidation of FeCrAl alloys is presented, emphasizing the influence of water vapor.

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The FeCrAlRE (where RE is reactive element) alloy Kanthal AF was exposed isothermally at 600 and 800°C for 72 h in dry normalO2 and in normalO2 with 10 vol % normalH2O . The mass gains were 3–5 times higher at the higher temperature. The presence of water vapor increased the oxidation rate at 800°C , while no significant effect was observed at 600°C . A thin two-layered oxide formed at 600°C : an outer (Fe,Cr)2normalO3 corundum-type oxide, containing some Al, and an inner, probably amorphous, Al-rich oxide. At 800°C a two-layered oxide formed in both environments. The inner layer consisted of inward grown α-Al2normalO3 . In dry normalO2 the originally formed outward grown γ-Al2normalO3 had transformed to α-Al2normalO3 after 72 h. Water vapor stabilized the outward grown γ-Al2normalO3 and hence no transformation occurred after 72 h in humid environment. RE-rich oxide particles with varying composition (Y, Zr, and Ti) were distributed in the base oxide at both temperatures and in both environments. The RE-rich particles were separated from the alloy substrate by a layer of Al-rich oxide. At 800°C the Y-rich RE particles were surrounded by thick oxide patches in both dry and humid normalO2 .

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