The oxidation mechanisms of the Xe20+ ion-irradiated Cr coatings on Zr alloy coupons: Accelerated diffusion and internal oxidation

Protective coatings of fuel claddings inevitably suffer from irradiation damage under actual nuclear operating condition. Here, effect of ion irradiation on the high-temperature oxidation behavior was studied for the Cr coatings on Zr alloy coupons that had already experienced the irradiation of 5 MeV Xe20+ ions at room temperature with the fluences of 1 × 1013 - 1 × 1015 ions/cm2. Although the irradiation damage concentrated in the ~1 µm thick surface layer, the oxidation resistance of the coatings was drastically degraded after the irradiation. It was observed that the irradiation-induced structural defects migrated and aggregated into cavities in large columns, which greatly accelerated the inward-diffusion of O and the outward-diffusion of Cr and Zr. The oxides rapidly nucleated within the irradiated area in the low-oxygen-partial-pressure conditions. During the 1200 °C steam oxidation, the oxides scales could not remain structural integrity, because the formation of Zr(O) phases along the oxide-metal interfaces resulted into bubbles and cracks. Therefore, the oxidation process was very sensitive to the irradiation-induced structural defects, and the irradiated coatings mainly proceeded the internal oxidation mechanism.