Influence of aluminum depletion effects on the calculation of the oxidation lifetimes of FeCrAl alloys

The oxidation behavior of FeCrAl-based alloys at temperatures in the range 1000 to 1300°C can be well described in terms that are readily quantified and so may be used for predicting their oxidation-limited lifetimes. These alloys rapidly establish a scale that is exclusively αAl₂O ₃, which forms as a single, uniformly thick layer; when scale spallation occurs, the alumina layer is reformed with minimal formation of base-metal/transient oxide. This behavior is maintained until essentially all of the Al content of the alloy has been consumed, after which rapid ('breakaway') oxidation ensues. This well controlled oxidation behavior allows a straightforward approach for calculating oxidation lifetime by equating the total (available) alloy Al content with the rate of its consumption by oxidation. A simplifying assumption is that a flat Al depletion profile is maintained throughout, so that the minimum Al content at which breakaway occurs (C_Bb) is essentially zero. This paper presents experimental measurements of C_Bb for different specimen shapes, and explores possible reasons for the apparent effects of specimen shape on oxidation lifetime. The above assumptions appear reasonable for disc-shaped specimens, and values of Ce_Bb = 0.001 mass fraction were measured on specimens taken to failure. However, specimens of the same alloy with the same thickness (and/or same volume/surface area ratio) but different shapes were found to exhibit different oxidation lifetimes.