Oxidation of typical Zr-1 Nb alloy in high-temperature air under 700–900 °C

In order to promote the understanding of Zirconium alloy cladding failure under loss of coolant accidents (LOCA), air oxidation behavior of Zr-1 Nb-0.12 O (M5) alloy at 700–900 °C for 1–2 h was investigated. At different temperatures and times, m-ZrO₂ was always the dominant phase of surface oxides, whereas oxide shapes, the integrity and roughness of oxide films differed noticeably. At 900 °C, M5 alloy exhibited accelerated kinetics and breakaway oxidation after approximately 80 min air exposure. The presence of ZrO and ZrO_0.35 sub-oxides accounts for the high reaction activation energy (149.6 kJ/mol). During the kinetics acceleration, some intergranular cracks turned transgranular. The oxide/metal (O/M) interface became wavy, massive long transverse cracks and small radial cracks appeared in the oxide film, and pores distributed on the oxide layer and O/M interface. Air ingress through the oxide layer to initiate the matrix oxidation is jointly affected by air transport and ion diffusion.