Oxidation of FeCrAl alloy: influence of temperature and atmosphere on scale growth rate and mechanism

The oxidation behaviour of a FeCrAl alloy with little rare earth content (Y=0.01 wt.%) was investigated. Specimens of this alloy were submitted to long-term oxidation treatments (up to 30 days) at 900 and 1200°C, under gaseous atmospheres containing 21, 10 and 2 vol.% of O2. The weight gain for unit area was measured vs. oxidation time. The alumina scale growth was found to occur, at least during the first days of treatment, according to Wagnerʹs parabolic law. Afterwards, the layer rate growth decreases down to that expected on the basis of this law. The values of the parabolic rate constant for scale growth (Kp) chiefly depended on the treatment temperature, while only small variations of Kp resulted from significant changes in treatment of atmosphere composition. The morphology and the composition of surface layers were studied by SEM-EDS and XRD analyses. Whatever the treatment temperature, the surface layer contained α-Al2O3 and non-negligible amounts of Cr and Fe. The metal/scale interface was always flat, while the morphology of the scale/gas interface changed greatly with temperature. At 900°C an irregular scale/gas interface formed; this was characterised by the presence of long α-alumina whiskers protruding towards the gaseous atmosphere. Contrary, at 1200°C a flat scale/gas interface was observed. These different morphologies can be attributed to different mechanisms of layer growth.