Corrosion of Stainless Steel 316L in Molten LiCl-Li2o-Li

Abstract

Material interactions with molten solutions of salt and metal are of interest to a wide verity of commercial electrochemical processes. Specifically, molten solutions of LiCl-Li2O containing dissolved elemental Li are applicable to the electrolytic reduction process used to incorporate used oxide nuclear fuels into pyroprocessing based fuel cycles [1, 2]. Additionally, electrolytic reduction of TiO2 [3, 4], SiO2 [5], Ta2O5 [6] and Nb2O5 [7] has been achieved in molten LiCl-Li2O electrolytes. While these processes have been shown to be highly effective, the co-reduction of Li at potentials more anodic than the Li+|Li standard potential is also well documented [8]. As a result of the formation of Li during electrochemical processing in molten LiCl-Li2O, and the known miscibility of Li in molten LiCl, a complex ternary molten solution of LiCl-Li2O-Li forms as these processes are conducted. Material interactions with molten LiCl-Li2O-Li are complicated by the simultaneous presence of dissolved oxidizing (Li2O) and reducing (Li) species in the melt. Coupons of stainless steel 316L (SS316L) were exposed to molten LiCl-Li2O-Li for periods of 20, 50 and 100 hours. Post exposure surface analysis was conducted using Raman and X-ray photoelectron (XPS) spectroscopies, as well as by X-ray diffraction and electron microscopy. Additionally, the leaching of alloying elements that occurred during the exposures was quantified using ICP-OES. Additional studies evaluated the effect of varying crucible materials and the presence of trace quantities of H2O on the corrosion behavior of SS316L in molten LiCl-Li2O-Li. Cr and Mo were observed to be preferentially leached from SS316L exposed to molten LiCl-Li2O containing high concentrations of Li. Alternatively; no material leaching was detected from SS316L exposed to molten LiCl-1wt%Li2O containing 0, 0.1 or 0.3wt%Li for 50 or 100 hours. The surface of SS316L was observed to be hardened as a result of the exposure; however the cause of the hardening is currently unknown. No alteration to the austenite crystal lattice perimeter, or trend in ferrite phase transformation was observed by XRD analysis of the samples. EDS analysis demonstrated a preferential enrichment of Fe and Ni, accompanied by a depletion in Cr and Mo, on the surface of SS316L following exposure. XPS analysis confirmed the presence of LiCrO2 on SS316L exposed to melts containing low concentrations of Li, however, the surfaces of samples exposed to melts containing excess quantities of Li were observed to be metallic. A mechanistic understanding of material interactions with molten LiCl-Li2O-Li will be discussed based on these and other observations obtained from this study. Acknowledgements: This work was performed under the auspices of the Department of Energy (DOE) under contracts DE-NE0008262, and DE-NE0008236 as well as the US Nuclear Regulatory Commission (USNRC) under contracts NRCHQ-11-G-38-0039 and NRC-38-10-949. A.M. acknowledges the Fellowship Award from the USNRC. Dr. Kenny Osborne serves as the program manager for the DOE award and Ms. Nancy Hebron-Isreal serves as the grants program officer for the NRC awards.