Microstructural evolution of 9Cr F/M steel aged at 550 °C up to 20,000 h and its impact on dual-mode corrosion in LBE at 500 °C with 1 × 10−7 wt% dissolved oxygen

• Thermal aging at 550 °C alters grain structure and precipitate morphology in 9Cr F/M steel. • Aging impairs oxide layer formation and promotes oxygen-enhanced dissolution in LBE. • Dual-mode corrosion—general dissolution and pitting—is intensified by aging duration. • Findings reveal a mechanistic link between microstructure evolution and corrosion behavior. In lead-cooled fast reactors (LFRs), structural materials such as 9Cr ferritic/martensitic (F/M) steel are exposed to long-term thermal exposure, which induces significant microstructural changes and affects corrosion resistance in oxygen-controlled Lead–Bismuth Eutectic (LBE) environments. This study examines the effects of thermal aging at 550 °C for 10,000 h and 20,000 h on the corrosion performance of 9Cr F/M steel in LBE at 500 °C with a dissolved oxygen concentration of 1 × 10 −7 wt %. Aging-induced microstructural evolution, characterized by prior austenite grain coarsening, a reduction in high-angle grain boundary density, and coarsening of Cr-rich M 23 C 6 precipitates, was found to compromise the nucleation and growth of protective oxide layers. As a result, aged specimens exhibited increased susceptibility to active dissolution corrosion. In parallel, coarsened M 23 C 6 precipitates promoted localized Cr enrichment at grain and lath boundaries, facilitating oxygen-enhanced dissolution and the accelerated formation of Fe(Fe,Cr) 2 O 4 spinel oxides. This dual degradation pathway led to intensified pitting corrosion and progressive loss of structural integrity. After 2,500 h of corrosion testing, pitting depths in the 10,000 h and 20,000 h aged specimens were approximately 85% and 72% greater, respectively, than in unaged samples. The findings elucidate the mechanistic link between aging-driven microstructural evolution and dual-mode corrosion, which encompasses both general dissolution and localized pitting—under reactor-relevant LBE conditions. These insights are critical for the long-term performance evaluation and lifetime assessment of F/M steels in advanced nuclear systems.