Characterisation studies of chromia formation on commercial FeCrAlRE alloy foils following chemically induced failure at 900°C

Abstract The chemical failure of commercial FeCrAlRE (where RE is a reactive element) alloys, such as Aluchrom YHf and Kanthal AF foils (30–50 μm thick), is induced by oxidation, in air, at 800–950°C. This is also the life limiting process of this family of alloys at higher temperatures (≥1100°C) and occurs when the Al activity in the alloy becomes depleted below a critical level, as a result of oxidation. At this junction, the integrity of the protective alumina scale is not sustainable but continued ‘pseudo-protection’ can be provided by chromia formation beneath the alumina layer. Oxidation continues at a slow linear rate controlled by transport through the alumina scale. The duration of the pseudo-protection region can extend to several hundred hours and has considerable technological implications, if it could also be used in defining component lifetimes. Greater understanding is, therefore, required concerning the formation and growth of the chromia subscale, as this has received only limited attention to date. Of particular possible significance is the mode of formation of the subscale. At temperatures above 1000°C, this forms a continuous, essentially uniform layer. In contrast, at ∼900°C scalloped shaped pits of chromia develop beneath the outer alumina scale. Hence a detailed characterisation study has been undertaken of the formation and development of this chromia attack; the results of which form the basis of this paper. The study has involved the deployment of a range of microscopy and analytical techniques, including optical and scanning electron microscopy, electron microprobe analysis (EPMA) and energy dispersive X-ray analysis (EDX).