## 3 Results and Discussion  
The employment of nickel alloys in fluoride melts, in contrast to aqueous media, inevitably introduces the problem of high temperatures, normally around 550-750 \({}^{\circ}\)C. In this case, the key role in the corrosion processes is played not by the increase in the kinetics of chemical reactions between the alloy components and the salt melt but by phase changes that occur in the bulk of the alloys at high rates at these temperatures. Under thermal influence, the studied alloys with initially homogeneous structure are transformed into heterogeneous systems. Such transformation can lead to the precipitation of excess phases of various nature along grain boundaries or long-range ordered structures [46; 52; 53; 54; 55; 56; 57; 58; 59; 60; 61; 62]. The tendency to form grain boundary precipitation increases with as the temperature increases from 550 to 750 \({}^{\circ}\)C [46; 52; 55; 56; 58; 59; 61; 62]. For example, no secondary chain phases were observed below 550 \({}^{\circ}\)C in Hastelloy\({}^{\circledR}\) G-35\({}^{\circledR}\) subjected to up to 1000 h of thermal exposure [46]. Increasing the temperature to 650 \({}^{\circ}\)C can lead to the formation of various secondary phases (\(\sigma\), P, \(\alpha\)-chromium, etc.) at the grain boundaries of the studied alloys [46; 52; 58; 60; 61], and the time required for the development of such phases in Hastelloy\({}^{\circledR}\) G-35\({}^{\circledR}\) is up to 100 h [46]. At 750 \({}^{\circ}\)C, the chain precipitates are formed  
most electronegative element in the alloys, determined the corrosion rates and the degree of corrosion damage. The relatively high content of iron, a minor alloy component, in the corrosion products also confirmed the electrochemical nature of the corrosion processes in molten halides. The corrosion rate measurements revealed three patterns common to all of the alloys: (a) the corrosion rate increased with increasing oxidizing capacity of the medium; (b) the corrosion rate increased with increasing chromium content in the alloys; and (c) the corrosion rate increased with increasing test temperature. In the first instance, uranium tetrafluoride increased the oxidizing ability of FLiNaK. Uranium is a polyvalent metal and capable of forming ions in different oxidation states in fluoride melts. Here, U(IV) can be reduced to U(III) in the presence of metals or alloy components. The redox potential of the U(III)/U(IV) couple in systems containing predominantly U(IV) is shifted to the region of positive values; therefore, the FLiNaK + UF\({}_{4}\) salt system is more aggressive than pure FLiNaK. The second pattern is associated with the phenomenon of selective etching of more negative components from the alloy. In Ni-Cr-Mo systems, chromium is oxidized first [62; 63; 64; 65; 66; 67]. On the other hand, increasing the temperature leads both to growth of the corrosion rate and formation of secondary phases at the grain boundaries, inducing IGC processes. To analyze the microstructural changes in the alloys, the samples were subjected to SEM analysis after the corrosion tests. Figures 4-8 show images of the materials' microstructure after exposure in FLiNaK-based melts. Analysis of the images shown in Figures 4-8 confirms previous conclusions that the degree of destruction of the materials increased with increasing temperature and oxidizing ability of the salt environment. After tests conducted in pure FLiNaK at 550 \({}^{\circ}\)C, all samples exhibited continuous uniform surface damage. In the FLiNaK + UF\({}_{4}\) salt system at the same temperature, the onset of localization of corrosion processes was observed for all alloys except Hastelloy\({}^{\circledR}\) B-3\({}^{\circledR}\); the corrosion penetration depth (\(d_{max}\)), however, had low values. At 750 \({}^{\circ}\)C, intergranular corrosion was observed on all alloys except Hastelloy\({}^{\circledR}\) B-3\({}^{\circledR}\). The main reason for the development of IGC was the precipitation of excess phases along the grain boundaries.