## 1 Introduction  
Molten salts exhibit a broad spectrum of properties, rendering them highly suitable for application in advanced energy generation and storage systems. The benefits of molten salt-based systems, particularly for nuclear industry applications, arise from their physical and chemical properties: high boiling point and, as a consequence, low vapor pressure; good heat transfer capacity; wide electrochemical stability window; resistance to ionizing radiation; absence of neutron moderators; and good solubility of actinide compounds [1, 2, 3, 4]. Currently, two primary technological frontiers are advancing towards the application of molten salts in power engineering on a semi-industrial scale: in concentrated solar power (CSP) systems and in the nuclear industry as primary or secondary coolants in both fission and fusion systems, as well as the media for spent nuclear fuel reprocessing [5, 6, 7, 8, 9, 10, 11, 12, 13, 14]. Molten salt reactor (MSR) technology was initially demonstrated in the 1950s to 1960s at ORNL [15]; significant renewed interest and research efforts were notably intensified from the 2010s [16]. The gap between the initial historical demonstration of the technology and the current renaissance is attributed to a number of factors, including unresolved issues with insufficient corrosion resistance of the construction materials, the complexity of operating molten salt reactors, and the substantial level of funding required for implementingsuch nuclear power installations. The comprehensive spectrum of challenges associated with the deployment of MSRs is described in the IAEA Technical Report 489 [17]. One of the critical issues is the lack of suitable structural materials capable of withstanding prolonged operation under the severe conditions inherent to MSR systems. Two types of molten salt systems are considered for use in MSRs, i.e., based on fluorides, which were tested in the past, and chlorides, which are considered as prospective working media. Among fluorides, there are two principal compositions of the solvent melt--a mixture of lithium and beryllium fluorides (FLiBe) and the ternary eutectic mixture of lithium, sodium, and potassium fluorides (FLiNaK) [18]. The latter salt is more promising because it is characterized by higher solubility of actinide fluorides [19; 20; 21]. The current research focused on studying structural material compatibility with FliNaK-based molten fluoride salts. Possible causes of corrosion in molten halide media have been extensively investigated worldwide [22; 23; 24; 25; 26; 27; 28; 29; 30; 31; 32; 33; 34; 35; 36; 37; 38; 39]. The primary factors influencing corrosion are generally considered to be the exposure temperature, the nature of the fused electrolytes, the type and content of impurities in the salt, the purity of the surrounding atmosphere (presence of moisture and oxidants), and the composition of the structural material itself. Despite numerous studies, insufficient attention has been devoted so far to the effect of the microstructure of the materials on their corrosion resistance. Existing data concerning corrosion phenomena in molten halides and material structure are somewhat controversial and do not allow a sensible connection between an alloy's internal structure changes and its corrosion performance in a molten salt environment to be established. For example, aging the Ni-based alloy VDM\({}^{\otimes}\) Alloy C-4 at 550 \({}^{\circ}\)C for 2500 h resulted in the formation of a Ni\({}_{2}\)(Cr,Mo) phase, which showed some positive effect on the alloy's corrosion resistance in an acidic KCl-AlCl\({}_{3}\) melt [39]. Improvement in the corrosion resistance of a C-4 type alloy in the same melt aligned well with the results of [PERSON] and [PERSON] [40] for aqueous systems. They also observed that the rate of corrosion of a C-4 type alloy with a long-range ordered (LRO) structure was lower compared to its initial annealed state.