## 4 Conclusions  
A series of 100 h corrosion tests of various nickel-based alloys (Hastelloy\({}^{\text{\textregistered}}\) G-35\({}^{\text{\textregistered}}\), VDM\({}^{\text{\textregistered}}\) Alloy 59, KhN62M-VI, Hastelloy\({}^{\text{\textregistered}}\)B-3\({}^{\text{\textregistered}}\)) in FLiNaK and FLiNaK + UF\({}_{4}\) melts was performed at 550, 650, and 750 \({}^{\text{\textregistered}}\)C. Corrosion indicators such as corrosion rates, depths of corrosion, and \(k\)-parameters were determined based on comprehensive analysis.  
The key issue of nickel-based alloy corrosion in molten halides is discussed based on the obtained data. It was shown that the presence of oxidizer in the melt, namely UF\({}_{4}\), led to a decrease in material corrosion resistance. Increasing the temperature of the tests, as well as the chromium content in Ni-Cr-Mo alloys, also accelerated corrosion processes. It was demonstrated that a decrease of corrosion resistance with chromium content in nickel-based materials was associated with selective etching of more negative components from the alloy. Increasing corrosion rates with temperature, on the one hand is explained by the Arrhenius law, and on the other hand is caused by phase changes due to thermodynamic instability of Ni-based fcc solid solution that can affect both the corrosion rate and the mechanism of material degradation.  
The influence of microstructural changes in the studied alloys on the mechanism of their corrosion in molten salts is discussed in detail. It was established that the formation of anodic excess chain-type phases (enriched by electronegative components) is the most  
Figure 15: Regions of X-ray microanalysis of the microstructure of the Hastelloy\({}^{\text{\textregistered}}\) B-3\({}^{\text{\textregistered}}\) alloy after corrosion in FLiNaK + UF\({}_{4}\) at 750 \({}^{\text{\textregistered}}\)C.  
dangerous case when the material is held in contact with an electrolyte at high temperatures, and in particular with a salt melt, because these secondary phases can provoke intergranular corrosion.  
It should be understood that the presented data and proposed mechanisms concern short-time ampoule corrosion tests. However, this type of express analysis combining corrosion tests and microstructure investigations enabled selection of the most promising materials from a variety of steels and alloys. During long-term thermal exposure, the microstructure of the alloys can change further. Dynamic conditions also can have an effect on corrosion processes. Therefore, long-term dynamic tests should be carried out after preliminary selection of prospective candidate materials on the basis of short-term investigations to unambiguously establish the expected service life of the material and the characteristic corrosion mechanism for each specific alloy.  
Conceptualization, I.B.P. and V.A.V.; methodology, A.V.A.; software, R.R.A. and A.I.T., validation, R.R.A. and A.V.A.; formal analysis, I.B.P.; data curation, I.B.P.; writing--original draft preparation, A.V.A.; writing--review and editing, I.B.P. and V.A.V.; visualization, A.I.T.; supervision, V.A.V. and I.B.P.; project administration, I.B.P.; funding acquisition, I.B.P. All authors have read and agreed to the published version of the manuscript.  
Research funding from the Ministry of Science and Higher Education of the Russian Federation (Ural Federal University Program of Development within the Priority-2030 Program) is gratefully acknowledged.  
Not applicable.  
The original contributions presented in the study are included in the article; further inquiries can be directed to the corresponding author.  
The authors declare no conflicts of interest.