The oxidation behaviour of an Fe,AI, Si alloy containing uranium as an impurity, oxidised for short times at high temperatures

Abstract

The short term oxidation behaviour of an Fe,Al,Si alloy, oxidised at 1000°C in unpurified air, has been characterised using a range of techniques including transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM). It was observed that this alloy formed a flat adherent alumina scale, even though analysis did not detect any conventional reactive elements. The research showed that the scale developed during the early stages of oxidation was similar to that formed at longer times (up to 24 hours) and that the tendency for the scale to remain flat and adherent was due to the presence of an impurity, uranium. Upon initial oxidation, uranium was found to segregate strongly to the metal/oxide interface, but was not detected at the oxide grain boundaries until oxidation times of approximately 20 minutes. Sulphur was observed within the scale during the initial stages of oxidation but was not associated with the uranium, the oxide grain boundaries or the metal/oxide interface. Therefore, uranium is not acting as a sulphur getter, reducing sulphur activity in the scale, but instead it appears to either block the sites at the oxide/metal interface where sulphur would normally segregate or change the chemistry of the interface in such a way as to make the segregation of sulphur thermodynamically unfavourable.