Investigating the effect of zirconium oxide microstructure on corrosion performance: a comparison between neutron, proton and non-irradiated oxides

Automated crystal orientation mapping in the transmission electron microscope has been used to simultaneously map the phase, orientation and grain morphology of oxides formed on Zircaloy-2 after 3 and 6 cycles in a BWR reactor in unprecedented detail. For comparison, a region of a pre-oxidised autoclave-formed oxide was also proton irradiated at the Dalton Cumbrian Facility. The proton irradiation was observed to cause additional stabilisation of the tetragonal phase, attributed to the stabilising effect of irradiation-induced defects in the oxide. In the reactor-formed oxides, no extra stabilisation of the tetragonal grains was observed under neutron irradiation, as indicated by the similar tetragonal phase fraction and transformation twin boundary distributions between the non-irradiated and reactor-formed oxides. It is suggested that the damage rate is too low in the newly formed oxide to cause significant stabilisation of the tetragonal phase. This technique also reveals the oxide formed under reactor conditions has a more heterogeneous microstructure and the growth of well-oriented columnar monoclinic grains is significantly reduced when compared to a non-irradiated oxide. High angle annular dark field scanning transmission electron microscopy (HAADF STEM) also revealed the development of extensive networks of intergranular porosity and eventually grain decohesion in thereactor formed oxides. The current results suggest that the tetragonal-monoclinic transformation is not responsible for the accelerated corrosion exhibited under reactor conditions. It is proposed that the usual out-of-reactor oxide growth and nucleation processes are significantly modified under reactor conditions, resulting in a more heterogeneous and randomly oriented oxide microstructure with reduced columnar grain growth. It is suggested that this disordered oxide microstructure allows for the formation of extensive intergranular porosity which could lead to accelerated inreactor corrosion.