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.

Accommodation of tin in tetragonal ZrO2

“…Niobium is generally assumed to exist in the Nb 5+ state within the oxide layer. If we assume the 5+ state, this results in a predominant Nb • Zr defect, which agrees with DFT simulations by Otgonbaatar et al [54]. This would lead to the injection of positive charge into the oxide layer.…”

“…As discussed in Section 1.3, tetragonal ZrO 2 is stabilised in the undoped oxide layer by a combination of grain size and compressive stress. In this work, stress was not applied and so the Brouwer diagrams were plotted at 1500 K, the approximate temperature at which tetragonal phase is stable under standard conditions, rather than at a normal reactor operation temperature of around 600 K. In this regard we follow the approach used in previous DFT studies on the tetragonal system [48,52].…”

The influence of alloying elements on the corrosion of Zr alloys

“…O suppression associated with Nb • Zr ; the Nb defect in ZrO 2 as predicted by previous DFT simulations [22]. However, experimental work [23,24] has consistently suggested Nb in ZrO 2 in fact occupies a range of states between 0 and 5+, with one X-ray absorption near edge structure (XANES) study by Froideval et al [25] unable to identify any 5+ Nb at all, thus casting doubt on the previous assumptions regarding the method by which Nb improves corrosion resistance and HPUF.…”

Doping in the Valley of Hydrogen Solubility: A Route to Designing Hydrogen-Resistant Zirconium Alloys

“…The details of the approach are described in our recent work [13,21], and here we outline the notions relevant to the current study.…”