Effect of alloying elements on hydrogen pickup in zirconium alloys

Although the optimization of zirconium-based alloys has led to significant improvements in hydrogen pickup and corrosion resistance, the mechanisms by which such alloy improvements occur are still not well understood. In an effort to understand such mechanisms, we conducted a systematic study of the alloy effect on hydrogen pickup, using advanced characterization techniques to rationalize precise measurements of hydrogen pickup. The hydrogen pickup fraction was accurately measured for a specially designed set of commercial and model alloys to investigate the effects of alloying elements, microstructure, and corrosion kinetics on hydrogen uptake. Two different techniques for measuring hydrogen concentrations were used: a destructive technique, vacuum hot extraction, and a non-destructive one, cold neutron prompt gamma activation analysis. The results indicate that hydrogen pickup varies not only from alloy to alloy, but also during the corrosion process for a given alloy. These variations result from the process of charge balance during the corrosion reaction, such that the pickup of hydrogen decreases when the rate of electron transport or oxide electronic conductivity (σ ox/e) through the protective oxide increases. According to this hypothesis, alloying elements (either in solid solution or in precipitates) would affect the hydrogen pickup fraction by modifying σ ox/e. Because the mechanism whereby these alloying elements are incorporated into the oxide layer is critical to changing electron conductivity, the evolution of the oxidation state of two common alloying elements, Fe and Nb, when incorporated into the growing oxide layers of two commercial zirconium alloys (Zircaloy-4 and ZIRLO) and model alloys (Zr-0.4Fe-0.2Cr and Zr-2.5Nb) was investigated using x-ray absorption near-edge spectroscopy with microbeam synchrotron radiation on cross-sectional oxide samples. The results show that the oxidation of both Fe and Nb is delayed in the oxide layer relative to that of Zr, and that this oxidation delay is related to the variations of the instantaneous hydrogen pickup fraction with exposure time.