Investigation of oxide layers formed on Zircaloy-4 coarse-grained specimens corroded at 360 °C in lithiated aqueous solution

•The microstructure of oxide on a grain surface near (1 0 −1 0) orientation was studied.•Oxygen-rich regions and zirconium sub-oxide were detected at oxide/metal interface.•The sub-oxide with body-centred cubic structure has a coherent relationship with α-Zr.•New insight into the migration of the oxide/metal interface was provided.

Zircaloy-4 coarse-grained specimens were used to investigate the microstructure and crystal structure of oxide layers formed on (1 0 −1 0) crystal plane by autoclave tests at 360 °C in lithated aqueous solution. Oxygen-rich regions, of which oxygen content was close to the level of a saturated solid solution in α-Zr, were only distributed at the wave valley of the O/M interface. A new kind of zirconium sub-oxide about 10 nm thick formed between α-Zr matrix and monoclinic ZrO2 was detected, which had a bcc structure (lattice parameter a = 0.37 nm) in the coherent relationship with α-Zr matrix and with monoclinic ZrO2.