Characterization of Long-Term, In-Reactor Zircaloy-4 Corrosion Coupons and the Impact of Flux, Fluence, and Temperature on Oxide Growth, Stress Development, Phase Formation, and Grain Size

Eleven Zircaloy-4 samples were irradiated in the Advanced Test Reactor at a variety of temperatures and neutron flux levels for up to 6.5 years. Subsequently, the coupons were characterized with complementary techniques to understand the mechanisms behind oxide growth as a function of different corrosion environments. Samples were examined using synchrotron X-ray diffraction/ fluorescence, traditional X-ray diffraction, focused ion beam/scanning electron microscopy serial sectioning, and three-dimensional reconstruction to develop an improved understanding of the influence of the underlying oxide microstructure on oxide growth. The oxide microstructure formed under irradiation was compared to that in samples corroded in an autoclave to discern the impact of neutron irradiation and temperature on corrosion rate, oxide kinetic transition, irradiation-induced breakaway corrosion, stress development, phase formation, and oxide grain size. The microstructure of the oxide changed with the corrosion temperature, with larger crack spacing (characteristic of kinetic transition) and larger monoclinic oxide grains formed during higher temperature corrosion. The specimens that were exposed to a neutron flux exhibited larger oxide grains and an increase in the fraction of tetragonal phase at the metal-oxide interface (but less tetragonal phase in the bulk oxide) compared to those exposed in autoclave. Data obtained from electron microscopy demonstrated the effect of irradiation and corrosion temperature on oxide morphology. One specimen underwent an irradiated-induced breakaway oxidation that was characterized by a sharp change in the corrosion rate and a decrease in the spacing between adjacent crack layers in the oxide film. Stress is hypothesized to be a key driver in the oxide growth formation, with samples nearer transition having more plastic deformation in the metal and increased elastic strain. These observations lead to a theory of oxide growth on zirconium alloys that attempts to connect and integrate the effects of stress, irradiation, temperature, phase formation, crystal orientation, porosity, and precipitate amorphization.