Steam Oxidation Evaluation of Fe–Cr Alloys for Accident Tolerant Nuclear Fuel Cladding

New nuclear fuel cladding materials are being evaluated that can withstand steam environments ≥1200 °C for short (≤4 h) periods in case of a beyond design basis accident. This study focused on commercial and model Fe–Cr alloys, where there is considerable experience in fabricating and joining. Exposures in 1 bar steam and air for 4 h at 800–1300 °C showed that the commercial Fe–Cr alloys were very sensitive to composition and only Fe-25.8%Cr-1%Mo formed a protective chromia scale at 1200 and 1300 °C in steam. A model Fe-22.5%Cr + Mn,Si,Y alloy also formed a protective scale at 1200 °C in steam. Analytical transmission electron microscopy of the reaction products revealed that (1) nominally equiaxed Cr₂O₃ formed at 1000–1200 °C; (2) at 1000 °C, there was a Mn inner and outer layer but at 1100 and 1200 °C only an outer layer was observed; (3) an amorphous SiO₂ inner layer was observed at 1000 and 1100 °C, but the SiO₂ was crystalline on the 22.5%Cr model alloy at 1200 °C, which was confirmed by electron and X-ray diffraction; and (4) Fe was found throughout the Cr₂O₃ formed on alloys without Mn at 1200 °C in steam and air, Fe-rich oxide near the gas interface and Fe-rich metal precipitates near the metal–oxide interface. A few Fe-rich precipitates were detected in oxides formed at 1100 °C and none at 1000 °C. The incorporation of Fe and crystallization of SiO₂ at 1200 °C may be detrimental to the formation of a protective chromia scale in steam at ≥1200 °C for this application and explain why such high Cr contents are needed for protective behavior.