Microstructure evolution during oxidation of 304L in breakaway conditions (850 °C, O2). Elemental and structural mapping techniques and thermodynamic calculations. (Fe–Ni alloy + Fe–Cr spinel oxide) mixture and Cr2O3 or FeCr2O4 layer. Cr2O3 healing layer at the oxidation affected zone border depending on steel GB location. Oxidation front propagation related to a local conversion of Cr2O3 in less protective FeCr2O4.