Sequential oxidation and hydrothermal corrosion of FeCrAl alloys at BWR top-of-core conditions

FeCrAl alloys are a promising accident-tolerant cladding material for nuclear reactors. Previous studies have evaluated either oxidation or hydrothermal corrosion of FeCrAl alloys, though never in sequence. But in a boiling water reactor (BWR), fuel rods toward the top of the core can be exposed to both steam and water, underscoring the need to test FeCrAl in such conditions. Possible reuse of FeCrAl cladding after low-severity accidents also necessitates study of hydrothermal corrosion on previously oxidized material. In this work, surface chemistry of FA-SMT (Fe-21Cr) and PM-C26M (Fe-12Cr) alloys is investigated under steam oxidation and steam with subsequent hydrothermal (BWR hydrogen and normal water chemistries) corrosion. Mass change is inversely related to Cr content, providing greater corrosion resistance in FA-SMT than in PM-C26M alloys over all conditions. However, the corrosion mechanisms are identical in both alloys. Steam oxidation creates stable Cr-rich and Al-rich oxide layers through grain boundary depletion and segregation of Cr and Al. During subsequent hydrothermal corrosion, Fe drives unstable oxide growth and dissolution to a greater extent in hydrogen water chemistry than normal water chemistry. Understanding these corrosion mechanisms helps establish operating windows for FeCrAl claddings in BWRs and supports their potential reuse after minor accidents. • Effect of Cr content on oxidation mechanism is examined in latest generation FeCrAl alloys. • A high Cr content provides greater corrosion resistance in FA-SMT (21Cr) than in PM-C26M (12Cr). • Steam oxidation results in stable Cr- and Al-rich oxide layers. • After steam oxidation, subsequent hydrothermal corrosion results in unstable Fe oxidation and spallation.