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 degC 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 degC 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 degC in steam. A model Fe 22.5%Cr Mn,Si,Y alloy also formed a protective scale at 1200 degC in steam. Analytical transmission electron microscopy of the reaction products revealed that (1) nominally equiaxed Cr2O3 formed at 1000 1200 degC; (2) at 1000 degC, there was a Mn inner and outer layer but at 1100 and 1200 degC only an outer layer was observed; (3) an amorphous SiO2 inner layer was observed at 1000 and 1100 degC, but the SiO2 was crystalline on the 22.5%Cr model alloy at 1200 degC, which was confirmed by electron and X ray diffraction; and (4) Fe was found throughout the Cr2O3 formed on alloys without Mn at 1200 degC 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 degC and none at 1000 degC. The incorporation of Fe and crystallization of SiO2 at 1200 degC may be detrimental to the formation of a protective chromia scale in steam at =1200 degC for this application and explain why such high Cr contents are needed for protective behavior.
Oxidation of Metals