Evolution of He bubbles during the steam oxidation in He-implanted ferritic-martensitic steels

• Bubble evolution and outward shift of bubble band are activated by steam oxidation. • The position relationship between bubbles and oxide layer plays a decisive role. • Bubble surface oxidation and outward diffusion of Fe induce bubble growth/shrink/coalescence. • The evolutionary behavior of He bubbles affects the oxidation rate of steels. • The results are applicable to other oxidative environments such as lead–bismuth eutectic. The evolution of He bubbles during the oxidation process was investigated in He-implanted ferritic-martensitic steels exposed to high-temperature steam. Similar He bubble bands formed in the SIMP and T91 steels after 500 keV He 2+ irradiation at 450 °C with a fluence of 1.0 × 10 17 ions/cm 2 , and duplex oxide films composed porous (Fe, Cr) 3 O 4 inner layer and dense Fe 3 O 4 outer layer formed on the both steels after 450 °C steam oxidation for 100 h. He bubbles displayed obviously different evolution between the oxide layer not reaching the bubble band and the oxide layer exceeding the bubble band. The significant size increase and density decrease of the visible bubbles and outward shift of bubble band occurred in the latter case, while the bubbles size/density and depth distribution were almost unchanged in the former case, indicating that the He bubbles were largely affected by the local oxidation of the alloy matrix in the He bubbles distribution area. As the increase of He bubbles size not only has obvious effects on material properties such as embrittlement and swelling, but also causes the degradation of the protective performance of oxide films, the interaction effect of corrosion and He bubbles should be paid more attention in advanced nuclear systems such as the high He yield accelerator-driven system.