Electrochemical Corrosion Behavior of Refractory Metals in LiCl-Li_2O Molten Salt

摘要

In a typical electrochemical cell, platinum wire is used as an anode for electrolytic reduction of UO_2 spent LWR fuel in a molten salt system consisting of LiCl + 2-8 wt% Li_2O. During the electrolysis, the metallic lithium migrates to the anode and attacks the platinum wire. Stability of the anode material is critical for sustained operation of the electrolytic UO_2 reduction cell for reprocessing spent nuclear oxide fuels. This investigation aims at developing dimensionally stable anode materials for sustained operation under aggressive conditions. The investigated materials are: tungsten, molybdenum, hafnium, and tantalum, and stainless steels type 304 and 316. Corrosion properties of the candidate materials are compared with that of platinum using electrochemical polarization and impedance spectroscopy results. In order to simulate the lithium attack on the anode material, lithium was cathodically deposited and equilibrated for about one hour before starting the anodic polarization test in LiCl + 2 wt% Li_2O molten salt at 650°C. Platinum samples showed an extensive surface cracking due to penetration of lithium. Type 316 stainless steel samples showed higher impedance and smaller passive current density than that of platinum at lower anodic potentials. Tungsten samples showed higher electrochemical impedance modulus and better passivity than any other materials tested in this study.