Minor alloying additions and alloy impurities can be of crucial importance for the protective properties of oxide scales on high temperature alloys. An example of an impurity is magnesium. In the present study, the effect of Mg impurity (concentration 80 ppm) on the oxidation behaviour of a commercial FeCrAl alloy at 1200°C was investigated. By a combination of various analysis techniques, such as optical metallography, SEM/EDX/WDX, X-ray diffraction and TEM, it was found that Mg appeared to become easily incorporated in the surface oxide already after relatively short exposure times.
Comparison of the oxidation products showed a change in the scale growth mechanism to occur in course of the oxidation treatment. Magnesium was found to migrate through the alumina scale and it became enriched in the outer part of the oxide scale forming a layer of a (Mg,Mn)Al2O4 spinel. An important effect associated with the Mg-enrichment and spinel formation was the development of porosity within the scale which may be of great importance for the growth rate and adherence of the protective alumina based scale.
