Correlated chromium carbide dissociation and phase transformation in liquid lead-bismuth eutectic corroded T91 steel

Gen IV nuclear reactors promise better safety, economic benefits and a potential path to reducing carbon emissions. The liquid lead-bismuth eutectic (LBE) cooled fast reactor is a key Gen IV reactor concept. This study investigates the corrosion of T91 steel, a candidate structural material for Gen IV reactors, exposed to liquid LBE. Specifically, we focus on the dissociation of Cr carbides and the overall Cr depletion adjacent to the corrosion interface. Our results suggest a correlation between Cr depletion and phase transformation in the steel, most likely ferritization. We have analysed the corrosion-induced changes from the micro- to atomic-scale by combining complementary characterisation techniques: SEM, EDX, (HR)EBSD, DAXM, (S)TEM, EELS, and APT. Based on these investigations, we propose a potential mechanism of Cr depletion, Cr carbides dissociation, and phase transformation in T91 induced by LBE exposure. These microstructural changes may alter the mechanical properties, and are thus of direct importance to reactor designs, safety, and lifetime estimation. Correlated Chromium Carbide Dissociation and Phase Changes in Liquid Lead-Bismuth Eutectic Corroded T91 Steel. • Cr content near the T91 surface drops after LBE contact. • Cr carbides become unstable and dissociate in this region. • Dissociated Cr diffuses into LBE, while C moves away, forming or enlarging carbides. • Cr depletion lowers T91's recrystallization temperature. • Retained C does not hinder ferritization, staying below ferrite’s solubility limit.