Abstract: In lead-cooled fast reactor (LFR) systems, the liquid lead-bismuth eutectic (LBE) coolant provides a corrosive environment that damages the steel components during high-temperature operation. This study investigated the microstructural deterioration of 9Cr ferritic/martensitic (F/M) steel under thermal aging at 550 °C for 2,000, 10,000, or 20,000 h and its effect on oxidation corrosion in an LBE environment using multiscale characterization techniques. The results indicated that the thickness of the internal oxidation zone (IOZ) increased significantly with extended thermal aging, whereas that of the spinel layer remained relatively constant. The abundant subgrain boundaries that emerged during extensive thermal aging facilitated Fe diffusion, and the enlarged Cr-rich M₂₃C₆ carbides contributed to the formation of preferential oxidation regions, accelerating IOZ layer growth. The spinel layer formed from the IOZ was influenced by microstructural defects within the IOZ. A theoretical model describing the accelerated oxide layer growth due to thermal aging was developed. These findings support the advancement of LFR technology.

Key words: Thermal aging treatment, Lead-bismuth eutectic, Internal oxidation zone, Preferential oxidation region