Analytical microscopy of corroded zirconium alloys

Zirconium alloys are widely used as structural and fuel cladding alloys in current nuclear pressurised water reactors. The MUZIC-2 programme, which this project has been part of, is a collaboration between an international team of universities and industry in furthering the understanding of oxidation and hydrogen pickup of these alloys. This project has contributed to the programme by analysing a large number of oxidised zirconium samples by scanning transmission electron microscopy (STEM), scanning electron microscopy (SEM), 3D focused ion beam (FIB) sectioning and secondary ion mass spectrometry (NanoSIMS). By STEM observations, ZIRLO and Zircaloy-4 have been analysed at different stages of oxidation, comparing oxide thickness, crack content, porosity and second-phase particle content. Utilising state-of-the-art fast low-loss electron energy loss spectroscopy (EELS), the oxygen-saturated metal region has been mapped. The band of oxygen-saturated metal below the metal-oxide interface is found to be thinner for samples that have recently finished the fast stage of oxidation (early pre-transition and early post-transition) than samples in the slower stages of oxidation (late pre-transition). Core-loss EELS has been fitted using a low-loss convolution curve fitting approach and has been used to measure the energy position and intensity of the Zr L edges and white lines across the metal-oxide interface. As the Zr oxidises, the white lines are found to shift to higher energy loss, and the ratio between the L3 and L2 white lines increases. The metastable ZrO suboxide phase is found to follow this trend. Novel hydrogen and deuterium measurements made using NanoSIMS have been performed on deuterium-traced alloys in both cross-section and as three-dimensional depth profiles to understand the ingress of hydrogen through the oxide barrier. The pre-transition oxide is found to act as an adequate barrier to hydrogen ingress whilst the post-transition oxide is found to be porous and only protective close to the metal-oxide interface. A hydrogen uptake mechanism hypothesis is proposed. Energy-dispersive X-ray spectroscopy suggests that second phase particles may constitute a possible route for hydrogen ingress when close to the metal-oxide interface. Together, these observations contribute to improving the understanding of the corrosion of zirconium alloys.