Fundamental Understanding of Nb Effect on Corrosion Mechanisms of Irradiated Zr-Nb Alloys

Abstract

Although zirconium-niobium (Zr-Nb) alloys are commonly used over Zircaloys as fuel claddings in light-water reactors, the fundamental understanding of Nb effects on oxidation and hydrogen pickup mechanisms, especially under irradiation, is still lacking. This study aims at filling this knowledge gap by coupling state-of-the-art experimental and modeling approaches on a set of Zr-xNb (x = 0.5/1.0) model alloys, with different heat treatments and irradiation dose levels to understand the effect of Nb redistribution on corrosion kinetics. To investigate the effect of irradiation on Nb distribution in a Zr-Nb metal substrate, Zr-Nb alloys have been irradiated by 2-MeV protons up to 1 dpa (flat region, about 1.3 × 1019 ions/cm2) at 350°C at the University of Wisconsin Ion Beam Laboratory. Nb redistribution after irradiation has been characterized by scanning transmission electron microscopy and atom probe tomography with a focus on radiation-induced platelets and solute concentrations. Results from atom probe tomography show that indeed solute concentration in solid solution decreases with irradiation dose, which may explain the enhanced corrosion resistance of irradiated Zr-Nb alloys via space-charge effects in the oxide. To understand the correlation between oxide space charge and corrosion kinetics, X-ray absorption near-edge spectroscopy was performed to measure the Nb oxidation states as a function of exposure time and oxide depth in two different annealed Zr-1.0Nb samples, corroded in high-temperature water up to 240 days. X-ray absorption near-edge spectroscopy results show that Zr-1.0Nb with lower, sub-parabolic, corrosion kinetics contains less Nb concentration in solid solution. Lastly, the observed corrosion kinetics are rationalized based on the coupled current charge compensation model in terms of Nb redistribution and its compensation effect on oxide space charges. The effect of irradiation on corrosion kinetics of Zr-Nb alloys is discussed in the light of experimental and modeling results.