Growth rates of alumina scales on Fe-Cr-Al alloys

The growth rates of alumina scales formed on high-temperature alloys often show a strong deviation from classical parabolic kinetics, and frequently scale growth rates near to a cubic time dependence are observed. This is e. g. the case for most RE doped Fe - Cr - Al- alloys oxidized at temperatures in the range of 1000 to 1300degreesC. For a number of components made of Fe - Cr - Al alloys, which e. g. prevail in car catalyst carriers, gas burners or hot-gas filters, the detailed knowledge of the rate law governing scaling kinetics is an absolutely necessary requirement for a reliable prediction of the component lifetime and/or the materials application limits. It seems that for ideally gastight alpha-Al2O3 scales a near-cubic time dependence is more a rule than an exception, although the frequently used methods for evaluating measured oxidation data might lead to the erroneous conclusion that the scale growth is governed by a transient-oxidation stage followed by parabolic oxidation. Deviations from an ideal, near-cubic time dependence of the oxidation kinetics of the Fe - Cr - Al alloys are caused by scale cracking during thermal cycling and/or local inhomogenities in the alloy and/or the scale.