Study On Corrosion Behaviors Of Metallic Materials In Molten LiCl-Li₂O

A lithium reduction technique has been developed as an effective method for reducing the volume and radiation of the spent nuclear fuel, which can benefit to the disposal and management of the spent nuclear fuel. In this process, structural materials used in the technique undertake serious corrosion in molten LiCl-Li2O, which delayed the application of the new technique. To date, there have been few studies on the corrosion behavior of materials in molten LiCl-Li2O. In this paper, immersion experiments were used to simulate service environment of structural material in the lithium reduction process. X-ray diffraction(XRD), optical microscopy(OM), Scanning electron microscopy with coupled with energy dispersive X-ray microanalysis (SEM/EDAX), and Electron probe microanalysis (EPMA) were used to investigate the corrosion behaviors of pure metal Fe,Cr and Ni, Fe-Ni-Cr alloys with different Cr content, several intermetallics, Ni-B compound coating in molten LiCl-Li2O. It primarily discussed the corrosion mechanism and provided useful results and testing data for selecting materials and protective coatings under these conditions.Corrosion rates of pure Fe and Cr in molten LiCl-Li2O increase with the increase of concentration of Li2O in the melt. The corrosion products of pure Fe and Cr also changed with the increase of concentration of Li2O in the melt: corrosion products of pure Fe changed from LiFe5O8 to LiFeO2; corrosion products of pure Cr changed from solid LiCrO2 to liquid Li2CrO4. Based on the comparison of this experiment environments with the traditional basic fluxing-reprecipitation, it is primarily discusses the corrosion mechanism of materials in molten LiCl-Li2O. According to both the establishment of the Li-Fe-0 and Li-Cr-0 phase stability diagrams and the experimental results, the effect of activity of Li2O and pressure of O2 on the corrosion of material in molten LiCl-Li2O were discussed. Nickel shows very fast corrosion in the melt under formation of layer of NiO (5h) after short corrosion time (5h), of NiO and Li2Ni8O10 after corroded for 15h, and of NiO and LiNiO2 after longer times (25h-50h). The weigh loss curve shows linear, which is mainly related to harmful reaction between the oxides and melt at the interface of melt/scale accompanied by the formation of non-protective scale.The corrosion products of Fe-Cr-Ni alloys after corroded in molten LiCl and LiCl-Li2O are the same: LiCrO2 (or LiFeO2). The alloys suffered more severe attack in the molten LiCl-Li2O mixture than in the molten LiCl at temperature higher than 750℃. In Fe-Ni-Cr alloys containing more than 7%Cr, Cr was selectively corroded, and the corrosion rates decrease with increase of Cr content in the alloy in the molten salts; but for the alloys containing less than 7wt%Cr, Fewas selectively corroded, and the corrosion rates increased with increase Fe content.The intermetallics investigated in this paper all exhibited accelerated corrosion in molten LiCl-Li2O, and their corrosion products of LiAlO2 were non-protective. Serious spallations of corrosion scale were observed on the surface of FeAl and tow NiAl intermetallics. Only inner layer of titanium oxides formed on TiAl-5Nb was protective, but the out layer of TiAl-5Nb was non-protective.The pack bonding process of pure nickel produced a dense and continuous borided layer which is composed of Ni-B compound. The effect of borided hot treatment on the corrosion resistance of pure nickel in molten LiCl-Li2O at 750℃ was studied. The experimental results indicate that the borided coating can greatly improve the corrosion resistance of pure nickel in molten LiCl-Li2O, which is ascribed to the preferential corrosion of B in the borided layer that prevent the severe corrosion of nickel.