Microstructure, corrosion and mechanical properties characterization of AISI type 316L(N) stainless steel and modified 9Cr-1Mo steel after 40,000 h of dynamic sodium exposure at 525 °C

Abstract

This paper presents results of a comprehensive study on 316L(N) stainless steel (SS) and modified 9Cr-1Mo steel exposed to flowing sodium at 525 °C for 40,000 h in a bi-metallic (BIM) loop. A non-uniform ferrite layer of 8–14 μm thickness was formed on the surface of 316L(N) SS due to elemental leaching of Ni and Cr. Higher carbon activity (aC) in sodium resulted in carburization of 316L(N) SS with a surface carbon content of 0.5 wt.%. The effective carbon diffusion coefficient, DeffC was estimated to be 3.64 × 10 −19 m2/s. Consequent to carbon transport in 316L(N) SS, extensive sensitization was detected and the charge density, Pa, values for sodium exposed, thermally aged and carburized zone were found to be 23.3, 18.5 and 4.8 C/cm2, respectively. 42 and 60% decrease in percent total elongation (%TE) and percent reduction in area (%RA), respectively were observed in sodium-exposed 316L(N) SS, as compared to thermally aged specimen, due to surface carburization. Unlike 316L(N) SS, modified 9Cr-1Mo steel did not show carburization, however, it showed a decrease in the impact energy values due to the presence of Laves phase and M23C6 precipitates in sodium-exposed and thermally aged specimens.