High-temperature oxidation of Zr-4 and Zr-1Nb-O alloys: Influencing factors, oxidation behaviors and mechanisms

Under loss-of-coolant accidents, the oxidation rate of zircaloy rapidly increases, which can cause cladding failure and pose serious safety risks. Thus, enhancing the oxidation resistance of zircaloy is of utmost importance to ensure the safe utilization of a nuclear power. This work provides a comprehensive review on influencing factors and mechanisms for high-temperature oxidation performance of Zr-4 and Zr-1Nb-O alloys. These factors mainly include alloying composition, oxidizing atmosphere, oxidation temperature, pre-oxidation, irradiation, and hydrogen absorption. Oxidation kinetics, behavior, and mechanisms in steam, O₂ and air are thoroughly discussed, and a comparative analysis of oxidation kinetics is presented. Overall, the addition of Nb enhances the oxidation resistance of zircaloy. In air, the oxidation rate of zircaloy is notably faster compared with that in steam and O₂ environments due to the formation of ZrN. At elevated temperatures, the critical size of zirconia increases, leading to a phase transition and a reduction in the volume fraction of monoclinic zirconia. The phase transition makes the zirconia oxide layer crack and less stable. Pre-oxidation at low temperatures in O₂ or steam significantly improves the oxidation resistance of samples. The formation of oxides during the oxidation process of zircaloy is controlled by O^2– diffusion. The breakaway oxidation of zircaloys occurs as a result of the transformation from tetragonal to monoclinic phase of zirconia, as well as stress relaxation of oxides and evolution of oxide morphology when reaching critical thickness.