High-Temperature Oxidation of FeCrAl Alloys: The Effect of Mg Incorporation into the Alumina Scale

Apart from reactive element additions, commercial FeCrAl alloys contain a number of poorly controlled or unavoidable impurities, which might significantly affect the material's oxidation behaviour. In the present study it was found that magnesium, if present in a concentration of only 80 ppm, can substantially affect composition and morphology of the alumina surface scales. This was found for scales formed after short (a few minutes) as well as very long (up to 1000 h) oxidation treatments. Magnesium appeared to diffuse outward through the alumina scale and it became enriched in the outer part of the oxide scale, forming a layer of MgAl₂O₄ spinel. An important effect associated with the Mg enrichment and spinel formation was the development of porosity within the scale which likely affects the scale growth rate and adherence, however, did not fundamentally change the sub-parabolic character of the scale thickening rate, which has previously frequently been reported for pure α-alumina scales.