Correlating nanoporosity, Nb oxidation states and deuterium transport in the oxide layer formed on a Zr-2.5 Nb alloy

In Canada Deuterium Uranium reactors, Zr-2.5 Nb alloys are used as a pressure boundary material and understanding corrosion and deuterium transport is critical. The objective of this study is to characterize the oxide layer formed on Zr-2.5 Nb alloy at the nanoscale and understand the diffusion of deuterium from the water-oxide interface to the base metal. Results indicate that defects, nanoporosity and microcracks in the oxide govern the oxidation tendency of Nb, which has +5, +4, +2, or partially oxidized states. Deuterium transport through the oxide layer is also directly correlated with defect density. The concentration of deuterium is highest at the water-oxide interface and lowest at the metal-oxide interface. This correlates with a change in defect density and Nb oxidation state, which also decreases between these two interfaces. The present findings provide a mechanistic understanding of oxidation and deuterium ingress in Zr-2.5 Nb alloys, helping to ensure the long-term performance of the material for reactor applications.