Corrosion behaviour of U-bend 316 stainless steel in static liquid lead at 550 and 700°C

The compatibility of material structural with liquid lead (Pb) is a critical factor for the longevity of high-temperature nuclear systems. The corrosion behaviour and mechanism of 316 stainless steel (316SS) U-bend specimens subjected to 150-hour static immersion tests at 550°C and 700°C, respectively. This study systematically investigates the local corrosion behaviour of U-bend 316SS specimens under high-temperature liquid Pb exposure, representing realistic stress conditions experienced by U-tube heat exchanger structures in the coolant circulation circuit of LFR. The results shows oxidation was identified as the primary degradation mode at 550°C, leading to the development of a characteristic duplex scale composed of an outer Fe-O layer and an inner Fe-Cr spinel. However, at 700°C, a transition to dissolution corrosion occurred, driven by the increased solubility of Ni, Cr, and Fe in liquid Pb. Dissolution corrosion causes thinning of the specimen at a temperature of 700°C. The oxygen saturation limit in liquid Pb corresponds to solubilities of 1.27 × 10−3 wt% and 1.11 × 10−2 wt% at 550°C and 700°C, respectively. At 550°C, oxygen availability is insufficient for the development of a continuous protective Cr-oxide scale, leading to negligible oxidation. At 700°C, however, the markedly higher oxygen solubility promotes the formation of an Fe-Cr-O oxide system. Then, the impact of height reduction variations (0.9 cm and 1.0 cm) indicating that minor differences in local strain do not significantly exacerbate the corrosive attack. This indicates that the variation in deformation was insufficient to substantially alter Pb diffusion pathways.