Clarifying the mitigation effect of proton irradiation on the intergranular oxidation of 316L stainless steel in high temperature water

The role of proton irradiation in the intergranular oxidation of 316L stainless steel after long-term immersion in simulated PWR primary water was clarified by comparing the microstructural and microchemical features of oxide between the irradiated grain boundary (GB) and its un-irradiated counterpart. As such, the interference from difference in GB structure can be ruled out. Surprisingly, the results reveal that the intergranular oxide penetration in the irradiated region is shallower than that in the un-irradiated region. It is found that Si segregated at the irradiated GB diffuses outwards preferentially and gets oxidized due to its high diffusivity and affinity to oxygen. The Si-enriched oxide in the intergranular oxide tip of irradiated GB can act as a temporary diffusion barrier for oxygen, although it tends to dissolve near the sample surface. Meanwhile, the efficiency of elements (especially Cr) transport along irradiated GB to the oxidation front is promoted mainly due to the vacancies created by preferential diffusion of Si, resulting in higher Cr content at the intergranular oxide tip. The combination of Si and Cr enrichments in the intergranular oxide tip can enhance the resistance to oxidation and eventually leads to a lower oxidation rate for the irradiated GB. For the first time, our study determines that irradiation can enhance the intergranular oxidation resistance of stainless steel after long-term immersion in simulated PWR primary water when the sample is not stressed.