Cyclic oxidation of thin FeCrAl(RE) foils in air:: SEM and EPMA investigations

Research has been carried out on the cyclic oxidation behaviour of thin commercial FeCrAl(RE) foils in air. The influence of cycle time (5-, 20-, 50-h) and temperature (1100, 1200, 1300 degreesC) on the mass change has been investigated. The mechanism of oxidation and the morphology of the scale have been examined by SEM and EDX/WDX at different stages of the oxidation process. Preliminary conclusions can be drawn on the breakaway process in thin foils. The results show that breakaway appears in different stages: formation of a chromia layer underneath the still adherent alumina scale, slight changes in the growth law and spallation of the scale followed by rapid oxidation of the iron-rich matrix. This is different from the case of thicker sheet material, where breakaway can be established by rapid mass change solely. The main cause of breakaway - the depletion of aluminium in the foil - has been examined by EPMA measurements of the remaining aluminium level in the alloy. These results are consistent with aluminium revels calculated from the mass change.