High temperature oxidation of Fe-Cr-Al alloys containing reactive elements

A study has been carried out into the oxidation performance of high purity, low sulphur-containing alumina-forming alloys, with and without reactive elements. Alloys containing 0.1% Y or 0.5% mischmetal (Ce and La) were tested under thermal cycling and isothermal conditions at 1300 degrees C in air. Beneficial effects of removing sulphur were confirmed and the role of reactive elements and their effectiveness were examined. Stress generation during scale growth was evident but stress relief occurred by two different mechanisms. In some areas, adhesion was good and the substrate deformed to relieve stress. Conversely, in other areas, the scale buckled and detached, leading to development of a highly convoluted configuration. A difference in behaviour of the various reactive elements was noted while the presence of second phase precipitates in the 0.5% mischmetal-containing alloy appeared to facilitate the preferential inward growth of the scale, eventually leading to depletion of aluminium and formation of less protective scales. Small additions of reactive elements appeared to reduce plastic deformation of the alloy substrate.