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.
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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