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

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 degrees C for 40,000 h in a bi-metallic (BIM) loop. A non-uniform ferrite layer of 8-14 mu m thickness was formed on the surface of 316L(N) SS due to elemental leaching of Ni and Cr. Higher carbon activity (a(C)) in sodium resulted in carburization of 316L(N) SS with a surface carbon content of 0.5 wt.%. The effective carbon diffusion coefficient, D-C(eff) was estimated to be 3.64 x 10(-19) m(2)/s. Consequent to carbon transport in 316L(N) SS, extensive sensitization was detected and the charge density, P-a, values for sodium exposed, thermally aged and carburized zone were found to be 23.3, 18.5 and 4.8 C/cm(2), 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. (C) 2019 Elsevier B.V. All rights reserved.