Influence of long term Sodium Exposure on the Corrosion and Tensile Properties of AISI Type 316LN stainless steel and modified 9Cr-1Mo steel

AISI Type 316LN stainless steel (SS) and modified 9Cr-1Mo (P91) steel are used in the intermediate heat exchanger and steam generator (SG), respectively of prototype fast breeder reactor (PFBR). This paper presents the effect of long-term sodium exposure (50000 h) on the corrosion and tensile properties of 316LN SS and P91 steel. The results showed that the prolonged sodium exposure of 316LN SS can lead to selective leaching of alloying elements (depletion of Cr, Ni, and Mo), enrichment of Fe, formation of ferrite layer and carburized zones with increased hardness at the surface (272 HV), and a reduction in ductility by nearly 40 %. Double-loop electrochemical potentiokinetic reactivation (DLEPR) results of sodium exposed 316LN SS showed a 25% higher degree of sensitization (DOS) as compared to thermally aged one. The microstructural examination of sodium exposed 316LN SS revealed a ditch structure near the surface and carbide precipitates along the grain boundaries indicating the diffusion of carbon through grain boundaries and surface carburization. XRD studies of sodium exposed 316LN SS revealed predominant phases of ferrite and Cr23C6 phases, which further corroborated the results of DLEPR, microstructure, and hardness data. The sodium exposed P91 steel showed no significant microstructural, microchemical changes and tensile properties after sodium exposure. The carbon depth profiles carried out using SIMS in sodium exposed 316LN SS and P91 steel showed a surface carbon concentration of 0.85 and 0.32 wt.%, respectively. Fractography of sodium exposed 316LN SS showed intergranular fractures whereas the fractographs of P91steel showed fibrous dimple and cleavage type of fractures indicating the growth of carbides. The present results confirm that P91 steel can sustain the reactor life in flowing sodium containing 25 ppm carbon and less than 2 ppm oxygen.