## 3 Results and Discussion
Thermal exposure led to the formation of secondary phases in the alloys. As a rule, this process intensifies at temperatures exceeding 600 \({}^{\circ}\)C, and this was confirmed by the results shown in Figures 4-8. The formation of excess phases led to heterogeneity in the alloy structure. The nature of the formation of secondary phases \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Alloy**} & \multirow{2}{*}{**Element**} & \multicolumn{6}{c}{**Melt and Temperature**} \\ \cline{3-8}  & & \multicolumn{3}{c}{**FLiNaK**} & \multicolumn{3}{c}{**FLiNaK + UF\({}_{4}\)**} \\ \cline{3-8}  & & 550 \({}^{\circ}\)C & 650 \({}^{\circ}\)C & 750 \({}^{\circ}\)C & 550 \({}^{\circ}\)C & 650 \({}^{\circ}\)C & 750 \({}^{\circ}\)C \\ \hline \multirow{3}{*}{Hastelloy\({}^{\circledR}\) G-35\({}^{\circledR}\)} & Ni & 24 \(\pm\) 3 & 51 \(\pm\) 4 & 103 \(\pm\) 3 & 23 \(\pm\) 3 & 61 \(\pm\) 4 & 140 \(\pm\) 8 \\  & Cr & 70 \(\pm\) 5 & 130 \(\pm\) 10 & 640 \(\pm\) 40 & 146 \(\pm\) 8 & 1190 \(\pm\) 80 & 1310 \(\pm\) 90 \\  & Fe & 25 \(\pm\) 3 & 26 \(\pm\) 4 & 85 \(\pm\) 5 & 59 \(\pm\) 4 & 142 \(\pm\) 10 & 150 \(\pm\) 10 \\ \hline \multirow{3}{*}{KhN62M-VI} & Ni & 25 \(\pm\) 3 & 24 \(\pm\) 3 & 54 \(\pm\) 4 & 22 \(\pm\) 3 & 24 \(\pm\) 3 & 65 \(\pm\) 3 \\  & Cr & 36 \(\pm\) 3 & 340 \(\pm\) 20 & 760 \(\pm\) 40 & 185 \(\pm\) 10 & 380 \(\pm\) 20 & 550 \(\pm\) 30 \\ \hline \multirow{3}{*}{VDM\({}^{\circledR}\) Alloy 59} & Ni & 24 \(\pm\) 3 & 23 \(\pm\) 2 & 88 \(\pm\) 5 & 55 \(\pm\) 5 & 58 \(\pm\) 5 & 58 \(\pm\) 5 \\  & Cr & 46 \(\pm\) 5 & 106 \(\pm\) 7 & 330 \(\pm\) 20 & 390 \(\pm\) 20 & 420 \(\pm\) 30 & 1630 \(\pm\) 90 \\ \cline{1-1}  & Fe & 27 \(\pm\) 3 & 28 \(\pm\) 3 & 97 \(\pm\) 7 & 106 \(\pm\) 8 & 122 \(\pm\) 8 & 141 \(\pm\) 9 \\ \hline \multirow{3}{*}{Hastelloy\({}^{\circledR}\) B-3\({}^{\circledR}\)} & Mo & – & 36 \(\pm\) 4 & – & 61 \(\pm\) 6 & 121 \(\pm\) 8 & 210 \(\pm\) 10 \\ \cline{1-1}  & Fe & – & 74 \(\pm\) 5 & – & 26 \(\pm\) 3 & 119 \(\pm\) 8 & 129 \(\pm\) 9 \\ \hline \end{tabular}
\end{table} 
Table 5: Content of the corrosion products in FLiNaK and FLiNaK + UF\({}_{4}\) after 100 h corrosion tests, ppm. can be different. The secondary phases can segregate into separate phases, and can form chain arrangements or streaks. Examples of the distribution of excess phases along the grain boundaries in nickel alloys are shown in Figure 9.  
Figure 4: Microstructure images of Hastelloy® G-35® alloy samples after 100 h corrosion tests in FLiNaK and FLiNaK + UF\({}_{4}\) melts at different temperatures. The formation of excess chain-type phases is the most dangerous case when the material is held in contact with an electrolyte, in particular with a salt melt. In this case, the excess phases are interconnected in a continuous grid. In the zone of contact of the alloy's surface with the electrolyte, microgalvanic couples are formed, where the alloy's grain and the excess phase act as microelectrodes. In reality, the system can be more complex, since, in addition to the excess phase and the alloy's grain, a depletion zone is formed, and the alloy's components diffuse from this zone to the grain boundary while the excess phases are formed. Thus, when excess phases are formed along the grain boundaries, a multielectrode system is formed, in which an anode and a cathode are present. The microanode is the phase with a lower electrode potential, and the more electropositive  
Figure 5: Microstructure images of KhN62M-VI alloy samples after 100 h corrosion tests in FLiNaK and FLiNaK + UF\({}_{4}\) melts at different temperatures. phase becomes the microachode. The electrode potential of the microphase is determined by a number of factors, such as chemical composition, phase structure, presence of defects, and deformations. In addition, the formation of excess phases can provoke the occurrence of mechanical stresses along the grain boundaries, which can cause the development of IGC and intergranular stress corrosion (IGSC).