## 2 Methodology  
A ternary chloride molten salt composed of 20.4 wt.% KCl + 55.1 wt.% MgCl\({}_{2}\) + 24.5 wt.% NaCl (Sigma Aldrich 99%, Missouri, USA) was selected as thermal energy storage material. Experimental tests were carried out at 720 \({}^{\circ}\)C under inert atmosphere (N\({}_{2}\)). The salts were dried and purified following stepwise heating for the purification process, previously reported in [10], was carried out. After this process, the electrodes were immersed in the chloride salt at 720 \({}^{\circ}\)C, including 10 wt.% Al metal (100% bare Al metal, Randrade, Spain). The electrochemical impedance spectroscopy (EIS) cell was composed of a working electrode (WE, with the alloy to be tested), a quasi-reference (RE, platinum wire), and a counter (CE, platinum mesh) electrode. For the corrosion test, the electrodes were introduced in the molten salt, acting as electrolyte, and the electrochemical parameters were measured using the EIS equipment (Gamry 1010E, Gamry, Philadelphia, PA, USA). The experimental set up is shown in Figure 1.  
EIS tests were carried out at 3, 5, 24, and 50 h of immersion in order to monitor the corrosion mechanism and the frequency range for EIS tests was between 10\({}^{5}\) and 0.01 Hz. Finally, linear polarization resistance (LPR) tests were performed from a potential of \(-\)0.6 to 0.4 V were carried out in two different alloys, with a chemical composition shown in Table 1.  
After the corrosion tests, the samples were cleaned with hot distilled water and dried. For SEM characterization, a polymer resin was prepared by mixing HardRock 554 resin (Phase, Firenze, Italy) and catalyst, in a 2:1 proportion. After the resin cured, the samples were ground and polished using different granulometry of SiC abrasive papers, using diamond power (1 micron) to polish the samples. Quanta 200 FEG SEM operated in high vacuum mode at 20 kV equipped with a backscattered electron detector was used for the microstructural analysis, including an EDX analyzer with a recommended working distance of around 10 mm. For XRD tests, a PANalytical X'Pert PRO powder diffractometer (Perkin-Elmer, Waltham, MA, USA) was used with a 0/20 incidence from 10 to 100\({}^{\circ}\). Using electrochemical impedance spectroscopy (EIS), different authors [11, 12, 13] have proposed equivalent circuits corresponding to the main corrosion mechanisms in molten salts at high temperatures, obtaining protective, porous, or localized corrosion models. For linear polarization resistance tests, it is important to quantify the polarization resistance, \(R_{p}\), related to the potential drop attributed to the molten salt, acting as electrolyte. The corrosion density current \((\;i_{corr})\) can be obtained using Equation (1):  
\[i_{corr}=\frac{B}{R_{p}} \tag{1}\] \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline \multirow{2}{*}{**Alloys**} & \multicolumn{8}{c}{**wt.\%**} \\ \cline{2-10}  & **Nb** & **Mn** & **Cr** & **Ti** & **Co** & **Mo** & **Ni** & **Al** & **Fe** \\ \hline
**OCT** & **3** & **-** & **14** & **2** & **-** & **-** & **35** & **3** & **Balance** \\ \hline
**HR224** & **-** & **0.5** & **20** & **0.3** & **2** & **0.5** & **47** & **3.8** & **27.5** \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Chemical composition of alloys tested. Figure 1: Electrochemical impedance spectroscopy (EIS) experimental setup test: 1: furnace; 2: reactor; 3: counter electrode, 4: thermocouple, 5: working electrode, 6: quasi-reference electrode, 7: gas inlet, 8: sealed reactor, 9: molten salt, 10: carrier gas (N\({}_{2}\)), 11: MgO trap; 12: security trap; 13: NaOH [1M] and 14: gas outlet.