We performed isothermal oxidation tests on a 95mum thin commercial FeCrAl foil at around 900degreesC in ambient and synthetic air in order to examine the effects of microstructural and morphological evolution of metastable aluminas on the overall growth kinetics by mean of TGA, AET, FEG-SEM, TEM, EPMA and GIXRD techniques. The origin of platelet-like morphology formation at an early stage of oxidation is examined and discussed. Further heating produces phase transformation and significant structural changes of this morphology. Formation of a corundum-alumina-rich compact layer from an outer metastable layer by a coupling phenomenon between grain growth and the sintering of the platelets was identified as responsible of the discontinuous increase in growth rate at around 900degreesC widely reported in the literature.