Is water a crucial factor in the oxidation of FeCrAl alloys?

Licentiatavhandling, 2005

FeCrAl alloys are much used at high temperatures because the formation of a dense and slow-growing alumina scale provides resistance towards oxidation and high temperature corrosion. While the FeCrAl alloys are primarily intended for temperatures above 950ºC, they are sometimes used at lower temperature, for example as supports in automotive catalysts. This study investigates the oxidation behaviour of the FeCrAl alloy Kanthal AF at 500-900ºC. In addition, the influence of water vapour on oxidation is studied. Alloy Kanthal AF was isothermally oxidized in dry O2 and in O2+40% H2O using tube furnaces. Thermobalance exposures were carried out to investigate oxidation kinetics. The oxide scale was investigated by XRD, SEM, EDX, AES, FIB and TEM techniques. Kanthal AF follows parabolic oxidation kinetics in dry O2 >700°C. The parabolic rate constant increases monotonically with temperature. There is no indication of a maximum kp value at 850-975°C as reported in the literature. At all temperatures the initial oxidation rate was higher in O2/H2O compared to dry O2. At 900°C a duplex scale forms in both environments. The bottom part of the scale is dense and consists of columnar α-Al2O3 grains and grows inward. In dry O2 the top layer consists of a mixture of MgAl2O4 and α-Al2O3. In O2 + H2O, the top layer consists of γ-Al2O3 containing some magnesium and iron. In both environments the scale contains a straight and narrow Cr-rich band which is situated between the top and the bottom layer. This band corresponds to the original sample surface. The stabilization of γ-Al2O3 in O2+H2O is attributed to the hydroxylation of the γ-Al2O3 surface. Accordingly, the initial acceleration of oxidation by H2O is attributed to the stabilization of the fast-growing γ-Al2O3. At 900°C RE-rich oxide protrusions form. Patches with thicker oxide (mainly consisting of γ-Al2O3) form around the Y-enriched oxide protrusions. FeCrAl forms α-Al2O3 already at 700°C. This is in contrast to the commonly held view that γ-Al2O3 forms below 950°C. The concentration of Fe and Cr in the scale was below 5at.%. The formation of α-Al2O3 is attributed to the presence of Cr2O3 in the initial oxide. It is argued that corundum nucleates on the isostructural Cr2O3. At 500 and 600°C significant amounts of Fe and Cr are present in the oxide, indicating a mixture of oxides. Water vapour does not influence the amount and distribution of Fe and Cr in the oxide scale.