Effect of Niobium on the Defect Chemistry and Oxidation Kinetics of Tetragonal ZrO2

Abstract

Zirconium–niobium alloys are currently proposed for applications in water-cooled nuclear reactors. However, the mechanisms by which Nb impacts the corrosion resistance of these alloys are yet to be clarified. In this work, we utilize a thermodynamic framework informed by density functional theory calculations to predict the effect of Nb on the equilibria of charged defects in tetragonal ZrO2, and discuss how the changes in the defect concentrations affect the protectiveness of this oxide that grows natively on Zr alloys during oxidation. Our analysis shows that Nb dissolves predominantly in the oxidation state 5+ as a substitutional defect on the Zr sublattice, with charge compensation achieved by the negatively charged Zr vacancies. Moreover, Nb dissolution is limited to the oxygen-rich conditions, i.e., in the oxide surface facing oxidizing environment. We validate this finding by performing X-ray photoelectron spectroscopy on oxidized Zr–Nb alloys. The introduction of Nb in tetragonal ZrO2 is found to enhance the concentration of Zr vacancies and of free electrons and to decrease the concentration of oxygen vacancies. We conclude that the net effect of Nb on the corrosion kinetics of Zr alloys is favorable if the rate limiting process is oxygen transport, while Nb would be detrimental if electron transport limits the oxidation kinetics.