[en] Highlights: • Synchrotron based methods provide a powerful approach to understand the oxidation of Alloy 33. • Superheated steam environment was used to assess Alloy 33 as an accident tolerant cladding. • Continuous oxide scale consisted of two layers, chromium oxide and spinel phase (FeCr₂O₄) oxides. • Concentration of the spinel phase decreased from the oxide surface to the alloy-oxide interface. • Increase in temperature from 800 °C to 1000 °C led to increase in surface oxidation. - Abstract: Alloys of iron-chromium-nickel are being explored as alternative cladding materials to improve safety margins under severe accident conditions. Our research focuses on non-destructively investigating the oxidation behavior of the FeCrNi alloy “Alloy 33” using synchrotron-based methods. The evolution and structure of oxide layer formed in steam environments were characterized using X-ray diffraction, hard X-ray photoelectron spectroscopy, X-ray fluorescence methods and scanning electron microscopy. Our results demonstrate that a compact and continuous oxide scale was formed consisting of two layers, chromium oxide and spinel phase (FeCr₂O₄) oxides, wherein the concentration of the FeCr₂O₄ phase decreased from the surface to the bulk-oxide interface.