## 3 Results and Discussion
Thus, the occurrence of heterogeneity along the grain boundaries of nickel alloys is a destabilizing factor that provokes more intense destruction of the material. Figure 6: Microstructure images of VDM® Alloy 59 alloy samples after 100 h corrosion tests in FLiNaK and FLiNaK + UF\({}_{4}\) melts at different temperatures. The chemical composition of excess phases can be used to roughly predict the role of the phase in the resulting microgalvanic system if we extrapolate the values of the corrosion  
Figure 8: Microstructure images of Hastelloy® B-3® alloy samples after 100 h corrosion tests in FLiNaK + UF\({}_{4}\) melts at 550 and 750 \({}^{\circ}\)C.  
Figure 7: Microstructure images of Hastelloy® B-3® alloy samples after 100 h corrosion tests in FLiNaK and FLiNaK + UF\({}_{4}\) melts at 650 \({}^{\circ}\)C.  
Figure 9: Types of distribution of excess phases along grain boundaries. rates of individual metals to the chemical composition of the excess phases. A separate series of corrosion tests was conducted with various individual metals (over 99% purity) in molten FLiNaK at 650 \({}^{\circ}\)C. The experiments were performed under the same conditions used for assessment of corrosion performance of the alloys. As a result, the corrosion resistance of individual metals was determined (Figure 10). Here, the corrosion rates were determined by gravimetric and chemical analysis methods; averaged values of the corrosion rates are presented in the figure. The results confirmed our previous conclusion that chromium and iron are the main corroding elements in the studied alloys. However, in case of alloys, rather than individual metals, the activity of components in the solid solutions (alloys), the presence of defects in metallic phases, and the formation of intermetallic compounds can affect the corrosion resistance of the elements. As a result, the corrosion behavior of the alloys' components may in some instances deviate from the corrosion resistance row shown in Figure 10.  
Under the experimental conditions, the best corrosion resistance was exhibited by electropositive metals like nickel and molybdenum; the most electronegative metals were chromium, zirconium, and iron. Thus, it can be expected that the formation of excess phases enriched in chromium will be accompanied by their intense destruction, since they will act as microanodes in the microgalvanic system (Figure 11). An opposite phenomenon can also be expected. Enrichment of excess phases in molybdenum and nickel will contribute to the formation of microachodes. This process does not lead to intergranular corrosion; however, in some cases IGC may develop due to selective dissolution of depleted zones from which molybdenum and nickel have diffused into the excess phases. These zones will be depleted in the most electropositive metals, and they will also have an increased concentration of defects. This will provoke the formation of a microgalvanic system in which the alloy's grain and excess microphase will act as microachodes, and the depleted zone will play a role of a microanode. Such behavior (formation of microachodes) was observed in the present study for the Hastelloy\({}^{\circledR}\) B-3\({}^{\circledR}\) alloy and will be considered in more detail later. Figure 12 and Table 6 show the chemical composition of secondary phases (Point 2) in the Hastelloy\({}^{\circledR}\) G-35\({}^{\circledR}\) alloy and grain composition (Point 1), which corresponds to the content of main components in the bulk of the material. Excess phases in this case are significantly enriched in chromium, which points to the anodic nature of these microphases. Figure 4 shows the nature of the destruction of Hastelloy\({}^{\circledR}\) G-35\({}^{\circledR}\) alloy samples after 100 h of contact with FLiNaK and FLiNaK + UF\({}_{4}\) melts. The alloy samples experienced pronounced intercrystalline damage, especially at 750 \({}^{\circ}\)C, which indicates the localization of corrosion processes along grain boundaries.