Corrosion of intermetallic compound Fe40Al in molten LiCl and LiCl-Li2O

Abstract

A lithium reduction technique has been developed for the spent nuclear fuel management [1]. In this process the oxide (UO2) is reduced to metallic form by the reaction with lithium dissolved in the molten LiCl at 750 ◦C. The strong basic Li2O formed during the reduction process is soluble in salt. Thus, the accelerated corrosion of alloys resulting form the Li2O in molten LiCl cause a serious problem in containment materials, resulting in the delay of the application of the new technique. To date, only a few reports have been published on the corrosion of materials in molten LiCl-Li2O, with the main objective to find a material that can be used to improve the vessel longevity for this new technology [1– 3]. In these studies, corrosion behaviors of many ironbase and nickel-base alloys with good high-temperature oxidation resistance have been investigated in molten LiCl-Li2O under air. Unfortunately, all these alloys experience serious corrosion in the melt. Fe40(at%)Al-base intermetallics in presence of molten salts present better corrosion resistance when comparing them with common alloys [4]. Accordingly, this work was conducted to investigate the corrosion behavior of Fe40Al in molten LiCl and LiClLi2O mixture by immersion experiments. Fe40Al was prepared by vacuum induction melting and casting into an ingot. The ingot was cut into specimens of 10 mm × 8 mm × 2 mm. All the specimens were finely ground and polished. Immersion experiments were carried out at 750 ◦C in molten LiCl or LiCl-3 wt%Li2O. The concentration of Li2O in the molten mixture was chosen to be 3 wt% which corresponded to approximately 100% theoretical UO2 reduction [1]. LiCl and the mixture of LiCl and Li2O, contained in alumina crucibles were dried at 350 ◦C for 24 h, and then held in the molten state at 750 ◦C. For every group of experiments, 25 g salt was contained in an alumina crucible of 25 ml. After a selected period of time, the corroded specimens were rinsed with distilled water, dipped in a solution containing hydrochloric acid and tetrabutylammonium iodide to strip corrosion products, dried and weighted by microbalance. Fig. 1 shows the weight loss curves for the corrosion of Fe40Al in molten LiCl and LiCl-Li2O at 750 ◦C in air. The presence of Li2O significantly accelerates the corrosion of Fe40Al. In molten LiCl, the weight loss increased linearly with test time in the initial stage, and then increased parabolically after about 5 h. Similarly, in molten LiCl-Li2O, a large weight loss is first obtained at short exposure times (about 1 h), and then the 0 5 10 15 20 25 0 20 40 60 80 100 120