Synergistic roles of Cr and Mo in low temperature steam oxidation of FeCrAl alloys

The objective of this study is to understand the synergistic role of Cr and Mo on low temperature steam oxidation behaviour of two commercial FeCrAl alloys. FeCrAl alloys are leading candidates for accident-tolerant nuclear fuel claddings owing to their resistance to high temperature steam oxidation in accident scenarios. But low temperature steam oxidation of FeCrAl alloys, representative of top-of-core normal operation of boiling water reactors, must also be understood before these claddings can be safely deployed in reactors. This study evaluates FA-SMT (Fe-21Cr-5Al-3Mo) and PM-C26M (Fe-12Cr-6Al-2Mo) in 400 ºC steam for up to 1000 hr. The higher bulk Cr content in FA-SMT accelerates all reaction kinetics, leading to rapid growth of an outer Cr₂O₃ layer, which then enables formation of a passivating inner Al₂O₃ layer that hinders further oxidation. Meanwhile, Mo replaces Cr along grain boundaries, preventing additional Cr and Fe outward diffusion, and further hindering oxidation. By contrast, PM-C26M is more susceptible to steam oxidation as Mo remains in solid solution, allowing uncontrolled Fe oxidation. The beneficial effects of Mo in the presence of high Cr content may be considered a “fourth element effect” that can guide future design of high-strength, oxidation-resistant FeCrAl alloys.