Effect of microstructural features on initial stages of corrosion of 316L in liquid lead-bismuth eutectic.

Heavy liquid metal (HLM) is one of the coolant technologies foreseen for the next generation of nuclear reactors. In MYRRHA (Multi-purpose hYbrid Research Reactor for High-tech Applications) accelerator-driven system liquid lead-bismuth eutectic (LBE) was selected as a coolant and spallation source, however its compatibility with the structural materials is a key challenge to be addressed in order to ensure the safety and viability of HLM cooled nuclear reactors. In general, the interaction between the structural material surface and the liquid metal at high temperatures leads to physicochemical degradation known as liquid metal corrosion (LMC). Previous studies have shown that microstructure features (grain boundaries, deformed surface layers, twinning, etc.) and surface preparation significantly influence LMC development in AISI 316L [1-4]. Therefore, this study focused on the effect superficial microstructural features in the initial stages of AISI 316L corrosion when exposed to liquid LBE at 500 °C with low dissolved oxygen concentration. Dissolved oxygen concentration and temperature during the exposure in LBE was controlled with a potentiometric oxygen sensors and a K-type thermocouple. The surface of the samples was prepared to make possible thoughtful microstructural examination before and after exposure Microscope techniques as light optical microscopy (LOM), scanning electron microscopy (SEM), SEM-EDS and SEM-EBSD were performed to find and to characterize the preferential sites of corrosion. The experiments showed that corrosion starts at grain, evidenced by the local transformation of austenite to ferrite, due to preferential dissolution of Ni.