Multi-modal analysis of oxidation on Fe-Cr-Ni austenitic stainless steel 304 exposed to beyond-design basis temperatures

Stainless Steel 304 is being investigated for accident tolerant claddings. In this investigation, we use multi-modal characterization to evaluate oxidation under laboratory air versus 100% steam at 1200 ºC for 2 h. Air oxidation features Fe(NiMn)O4, Fe3O4, Fe(3−x)CrxO4, and Cr2O3. Steam oxidation produces a dual layer, containing Fe2O3-enriched porous outer layer and sub-surface matrix of Fe3O4/Fe(3−x)CrxO4 with SiO2/Fe(3−x)SixO4. From these findings, we postulate the oxidation mechanism. Arrangement of oxidation products in air corroborates with anticipated diffusion behavior, except Cr2O3. However, arrangement in steam deviates due to presumably intergranular interactions with H2O, resulting in the sub-surface matrix.