Microbeam X-ray Absorption Near-Edge Spectroscopy study of the oxidation of Fe and Nb in zirconium alloy oxide layers

Alloy optimization of zirconium based alloys used for nuclear fuel cladding is key to increasing corrosion resistance and reducing hydrogen pickup. The mechanism by which alloying elements influence these processes is investigated by focusing on the chemical state evolution of two alloying elements, Fe and Nb, when incorporated into the growing oxide layers of various production zirconium alloys - Zircaloy-4, ZIRLO® and Zr-2.5Nb - as well as a model alloy - Zr-0.4Fe-0.2Cr. X-ray Absorption Near-Edge Spectroscopy (XANES) measurements to determine the evolution of their oxidation states is performed using micro-beam synchrotron radiation on cross sectional oxide samples. A thin (∼12 μm) cross-sectional sample of Zircaloy-4 oxide was also designed and fabricated to differentiate the signal coming from the Fe in solid solution from the signal coming from the Fe in precipitates. The XANES spectra were fitted using a combination of standards, to determine the evolution of the oxidized fractions of Fe and Nb in the oxide as function of distance from the oxide/metal interface. The results show that the oxidation of Fe and Nb in the oxide layer is delayed relative to that of Zr. Both the second phase precipitates and solid solution Fe atoms were initially incorporated in metallic form into the oxide layer, although it appears that Fe in solid solution oxidizes first. It is shown that after a given distance from the metal/oxide interface (which is alloy dependent), the alloying elements start to oxidize. Qualitative TEM examinations of precipitates embedded in zirconium oxide layers correlate well with the quantitative XANES results. These results allow a discussion of a qualitative oxidation model of Fe and Nb in Zr alloys.