## 3 Results and Discussion  
EIS signals (Nyquist plots) obtained for OCT alloy (Figure 2) were fitted to a protective layer model during the corrosion process at the different testing times. The equivalent circuit element values for this material are shown in Table 2.  
R\({}_{3}\) represents the molten salt (electrolyte) resistance and R\({}_{5}\) is the electrochemical transfer resistance or polarization resistance of the tested material. Q\({}_{1}\) is the double layer at the metal/salt interface and Q\({}_{2}\) represents the oxide capacitance. The R\({}_{7}\) electrochemical parameter is related to the transfer resistance of ions in the scale. In this case, it is important to point out that the electrolyte resistance (R3) \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Element**} & **R3** & **Q1** & **R5** & **Q2** & **R7** & \multirow{2}{*}{**Equivalent Circuit**} \\  & **(Ohm)** & **(f/s)** & **(Ohm)** & **(f/s)** & **(Ohm)** & \\ \hline
3 h & 1.507 & 0.325 & 36.21 & 0.183 & 59.45 & \\
5 h & 1.561 & 0.243 & 15.56 & 0.289 & 232.07 & \\
24 h & 0.853 & 0.007 & 0.06 & 0.056 & 63.56 & \\
50 h & 0.855 & 0.079 & 32.9 & 0.477 & 167.8 & \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Electrochemical parameters obtained for electrochemical impedance spectroscopy (EIS) tests in OCT alloy. Figure 2: Nyquist plots of OCT immersed chloride molten salt at 3, 5, 24, and 50 h.  
decreases during the experimental time, so electronic conductivity may be favored as well as corrosion potential processes. These results were confirmed by SEM analysis (Figure 3), obtaining a protective layer with Fe-Cr-Ni-Al-O content. OCT alloy was previously tested [14] for 8 h of immersion in the same chloride molten salt. In this case, no internal oxidation was detected after 50 h using Al metal as cathodic protection. Results obtained for HR224 alloy (Figure 4) were fitted to a protective corrosion model during all the testing times. Electrochemical parameters obtained for the equivalent circuits are shown in Table 3.  
Figure 4: Nyquist plots of HR224 immersed in chloride molten salt at 3, 5, 24, and 50 h.  
Figure 3: Top view (**left**) and cross-section; (**right**) images for OCT immersed in chloride molten salt for 50 h at 720 \({}^{\circ}\)C including energy dispersive X-ray (EDX) analysis (A–B). SEM analysis (Figure 5) showed internal oxidation of 28.35 microns of thickness (Figure 5 right) with higher Ni content. This internal oxidation was reported [15] as a potential problem since it can cause mechanical failures during thermal cycles in the CSP plant operation. The superficial image (Figure 5 left) showed some needle-shaped particles in the alloy surface, with content in aluminum. Similar particles were obtained by [PERSON] et al. [16] adding Al\({}_{2}\)O\({}_{3}\) nanoparticles in nitrate molten salts, reporting a higher corrosion resistance behavior due to this protective barrier formed in the alloy/salt interphase. In order to confirm the corrosion layer composition obtained, XRD analysis was carried out in the materials tested. Protective layers composed of Al\({}_{2}\)O\({}_{3}\) and NiO were obtained for HR224 (Figure 6) and Al\({}_{2}\)O\({}_{3}\) for OCT (Figure 7), confirming the protective mechanism detected by EIS and SEM. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Element**} & \multirow{2}{*}{**R3**} & **Q1** & **R5** & **Q2** & **R7** & \multirow{2}{*}{**Equivalent Circuit**} \\  & **(Ohm)** & **(F/s)** & **(Ohm)** & **(F/s)** & **(Ohm)** & \\ \hline
3 h & 1.762 & 0.272 & 2.39 & 0.098 & 37.89 & \\
5 h & 1.896 & 2.508 & 15.55 & 0.072 & 27.39 & \\
24 h & 1.968 & 0.073 & 24.98 & 1.536 & 18.04 & \\
50 h & 2.051 & 0.171 & 63.14 & 0.130 & 8.101 & \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Electrochemical parameters obtained for EIS tests in HR224 alloy. Figure 5: Top view (**left**) and cross-section; (**right**) images for HR224 alloy immersed in chloride molten salt for 50 h at 720 \({}^{\circ}\)C including EDX analysis (A–B).