Corrosion of Metallic Materials in 3LiCl-2KCl and 3LiCl-2KCl-UCl3

Abstract

High-temperature molten salts are widely used in production and refining of non-ferrous and rare metals, in nuclear technology and solar storage systems. However, application of such technologies is limited by the problem of finding suitable corrosion resistant materials capable of withstanding a prolonged contact with molten salts.Current study is focused on the corrosion of metallic materials in 3LiCl-2KCl-based melts.Metallic tantalum and molybdenum, nickel-chromium-molybdenum alloys (VDM Alloy C-4, Hastelloy G-35 and specially designed KhN62M alloy) were chosen as the objects of the investigation. Corrosion tests were performed in 3LiCl-2KCl and 3LiCl-2KCl-UCl3 (5 wt. % U) melts. The corrosion tests run for 100 h, and the temperature was varied from 450 to 750 °C. The corrosion rates were determined by both gravimetric technique and chemical analysis of quenched melts. The last technique is very useful in case of alloys formation on the surface of tested samples.It was found that all the above metallic materials can be used in 3LiCl-2KCl melts at 450 and 550 °C. Increasing working temperature led to intensification of corrosion processes. This effect was more pronounced for nickel-based alloys where increasing temperature to 650 °C also resulted in changing character of the corrosion. The undesirable intergranuluar corrosion was noticed for the nickel-based alloys at 750 °C and this was induced by the formation of secondary phases at the grain boundaries.Metallic tantalum and molybdenum were subjected to only gradual etching even at 750 °C. Their corrosion rates did not exceed 0.01 mm/year and these materials can be recommended for the application in contact with molten chlorides at 750 °C.Addition of uranium(III) chloride to the melt led to increasing corrosion rate for all the materials studied. The corrosion mechanism, however, did not change much with temperature. The presence of oxidant (U3+) resulted in acceleration of corrosion etching. The rate of the exchange reaction between uranium chlorides and components of the materials can be accelerated by the formation of U-Ni alloys. Formation of metallic uranium-containing layers on the surface of nickel-based samples deserves a separate attention as an element of possible currentless protection coatings.The corrosion resistance of tantalum and molybdenum is also sufficient at 750 °C in 3LiCl-2KCl-UCl3 (5 wt. % U). However, it should be remembered that their mechanical properties, weldability, and high-temperature resistance in air are noticeably worse compare to the nickel-based alloys.