Abstract Oxidation of Fe–10Cr in dry and wet O2 was studied at 600°C for up to 168h. Oxide microstructure was investigated by STEM/EDX, FIB/SEM and TEM. Oxidation in dry O2 gives a Cr-rich protective (Fe1−xCrx)2O3 scale. The same protective oxide initially forms in O2+H2O environment, but after an incubation period scale breakdown is triggered by CrO2(OH)2 evaporation that depletes the substrate in Cr and converts (Fe1−xCrx)2O3 to FeCr spinel oxide. Internal oxidation occurs after breakaway. Alternating external and internal oxidation result in the inward-growing scale showing a characteristic banded morphology.