Fretting corrosion of steels for lead alloys cooled ADS

Fretting is a particular type of wear that is expected to occur in a molten lead alloy cooled ADS due to the flow induce vibrations and that mainly affects fuel claddings and heat exchanger tubes. A dedicated facility (i.e. FRETHME) was designed and realized to investigate for the first time fretting in liquid lead alloys at reactor relevant conditions. Several fretting tests, were performed on candidate steels such as the f/m T91 steel, the austenitic 15-15Ti steel and Al surface alloyed T91 (GESA-T91). The experimental outcomes highlighted that the fretting damage increases with the increasing number of cycles/time and temperature. Fretting interacts with the corrosion mechanisms occurring in liquid Pb alloys (fretting corrosion) and destabilizes the corrosion barriers, favoring e.g. dissolution attacks. Due to the favorable wear and corrosion resistance properties of the surface alloyed layer, GESA-T91 steel showed the best fretting corrosion behavior up to 550 °C. On the contrary, due to the high Ni content, the 15-15Ti steel is affected by dissolution enhanced fretting; while oxidation enhanced fretting characterizes T91 steel at temperatures higher than 500 °C. In this respect, dedicated tests suggested that besides the use of aluminized steels, possible countermeasures to mitigate the fretting impact are the use of pre-oxidized components and Ni-enriched liquid Pb.