Effect of H2O on Corrosion of Ni in Molten Eutectic LiCl-KCl

Abstract

In the absence of water and oxygen, it is known that molten chloride salts are relatively non-corrosive to a wide range of metals. Corrosion rates are expected to increase when water is present, however. To understand the threshold at which very small concentrations of water can cause corrosion of metals such as nickel and iron, a series of electrochemical measurements were made in molten eutectic LiCl-KCl and LiCl-KCl-UCl3. The presence of UCl3 is relevant for the use of this electrolyte in nuclear fuel electrorefining systems. Such systems are normally operated in inert atmosphere glove boxes or hot cells, where water concentration ranges from 1 to 50 ppm. In this study, argon gas containing up to 900 ppm H2O was bubbled into molten LiCl-KCl or LiCl-KCl-UCl3. Either a nickel or iron rod was inserted into the salt along with a Ag/AgCl reference electrode encased in a close-ended ceramic tube. The open circuit potential (OCP) was measured between the metal rod and the reference electrode as the gaseous Ar+H2O was bubbled into the salt. The temperature was held constant at 773 K. This is expected to result in formation of HCl, which will corrode the metal and cause a metal chloride to form and partition into the salt phase. Based on the Nernst Equation, this should cause a positive rise in the OCP. Based on changing OCP as a function of time, the rates of corrosion were compared for different concentrations of H2O in the gas stream. Cyclic voltammetry scans and samples of the salt were taken to verify the concentration of nickel and iron chloride in the salt at the end of each experiment. Noticeable corrosion was achieved in a time frame of 4 hours with water concentration as low as 120 ppm. This result demonstrates the importance of achieving a very high level of dryness in the salt prior to heating and melting it. Water concentration in the contacting atmosphere should also be well below 100 ppm to ensure long-term stability of the steels and similar metal alloys in molten LiCl-KCl or LiCl-KCl-UCl3. Implications and strategies for long term operation of electrorefiners using LiCl-KCl will be discussed.