## 2 Experimental
\begin{table}
\begin{tabular}{c c c} \hline
**Component** & **FLiNaK** & **FLiNaK + UF\({}_{4}\)** \\ \hline LiF & 29.2 \(\pm\) 0.6 & 27.3 \(\pm\) 0.6 \\ NaF & 11.7 \(\pm\) 0.2 & 10.6 \(\pm\) 0.2 \\ KF & 59.1 \(\pm\) 1.2 & 55.2 \(\pm\) 1.1 \\ UF\({}_{4}\) & — & 6.6 \(\pm\) 0.1 \\ \hline \end{tabular}
\end{table} 
Table 1: Chemical composition of electrolytes, wt. %. \begin{table}
\begin{tabular}{c c c} \hline
**Analyte** & **FLiNaK** & **FLiNaK + UF\({}_{4}\)** \\ \hline Al & 74 \(\pm\) 4 & 75 \(\pm\) 4 \\ Fe & 26 \(\pm\) 3 & 54 \(\pm\) 4 \\ Ca & 62 \(\pm\) 6 & 60 \(\pm\) 5 \\ O & 132 \(\pm\) 15 & 187 \(\pm\) 18 \\ Si & 28 \(\pm\) 4 & 75 \(\pm\) 6 \\ Mg & 69 \(\pm\) 6 & 68 \(\pm\) 6 \\ Cu & 14 \(\pm\) 2 & 13 \(\pm\) 2 \\ Mo & 11 \(\pm\) 2 & 12 \(\pm\) 2 \\ Ni & 23 \(\pm\) 2 & 25 \(\pm\) 3 \\ Pb & \(<\)5 & \(<\)5 \\ S & \(<\)10 & \(<\)10 \\ Cl & 47 \(\pm\) 4 & 63 \(\pm\) 4 \\ Cr & 18 \(\pm\) 3 & 15 \(\pm\) 2 \\ \hline \end{tabular}
\end{table} 
Table 2: Initial content of impurities in the salt electrolytes, ppm. Each assembled cell was evacuated, filled with high-purity argon (99.9998%), and placed in a vertical tube furnace. A continuous argon flow of approximately 45 mL/min was maintained through the upper part of the cell throughout the experiment. The cells were heated to 650 \({}^{\circ}\)C, with the temperature monitored by a thermocouple positioned near the crucible (Figure 1), and held at this temperature for 100 h. This was sufficient for assessment of the corrosion processes and estimation of their mechanisms. After the experiment, the cells were cooled to room temperature and opened in the glovebox. The crucibles containing the samples were retrieved, and the alloy specimens were carefully separated from the molten salt. The samples were then washed sequentially with 1 M Al(NO\({}_{3}\))\({}_{3}\) solution and deionized water, dried, and weighed to determine mass loss. The weight change was used to calculate the corrosion rate using this gravimetric method (GM). The concentrations of micro- and macro-components and the contents of impurities and corrosion products in the salt phase were determined using X-ray fluorescence analysis (ARL ADVANT'X 4200, ThermoFisher Scientific, Basel, Switzerland) and atomic emission spectrometry with inductively coupled plasma (Optima 2100 DV, PerkinElmer, Wallingford, CT, USA). The oxygen content of the salt was determined by the method of carrier gas hot extraction (Horiba EMGA 620W/C, Horiba, Kyoto, Japan). The results from the chemical analysis allowed independent calculation of the corrosion rate values based on the mass fraction of corrosion products in the salt system (subsequently referred to as the chemical method, ChM). For metallographic analysis, the specimens were mounted in phenol hot-mounting resin with carbon filler. Mounted samples underwent grinding in six stages using SiC papers of progressively finer grades (80 \(\rightarrow\) 220 \(\rightarrow\) 320 \(\rightarrow\) 1200 \(\rightarrow\) 2400 \(\rightarrow\) 4000 grit), followed by polishing with a water-based 1 \(\upmu\)m diamond suspension (DiADuo-2, [PERSON],  
Figure 1: Experimental setup for corrosion experiments: (1) heating elements; (2) test sample; (3) gas inlet; (4) furnace; (5) titanium sponge getter; (6) thermocouple; (7) molten salt; (8) nickel heat reflecting screens; (9) steel cell; (10) crucible; (11) gas outlet. Copenhagen, Denmark) and a final polish with colloidal silica suspension (OP-S, Struers). No etching of samples was performed. The microstructure of the samples before and after the tests was examined by scanning electron microscopy (Zeiss Auriga CrossBeam (Carl Zeiss NTS, Oberkochen, Germany) and JSM 6490 (Tokyo, Japan) with an Oxford Inca X-Ray microanalysis setup) and optical microscopy (Olympus GX-71F, Tokyo, Japan) to assess the presence of inclusions and excess phases, as well as the nature and depth of corrosion. After the corrosion tests, the degree of intercrystalline damage, characterized by the value of the \"\(k\)-parameter\", was also determined.