## 4 Results and Discussion  
### Electrochemical Impedance Spectroscopy Measurements  
#### 4.1.1 Solar Salt (KNa)NO\({}_{3}\) Testing  
In a first instance, the different Nyquist diagrams are shown for solar salt (KNa)NO\({}_{3}\) in Figures 7-9 for the stainless steels AISI 304 and AISI 430 as well as for the nickel-based alloy HR-224, respectively. The measuring times were carried out after 2, 4, 6, 8, 24, 48, 72, and 96 h of exposure. The impedance spectra of the corrosion monitoring of AISI 304 and AISI 430 stainless steel and nickel alloy HR-224 shows a similar behavior after 24 h of measurement after the initial stages of the test, due to a typical behavior of the diffusion-controlled reactions of the ions at the metal/molten interface. The equivalent circuit for all the figures, was composed essentially of two capacitive-resistive circuits connected in parallel, in addition to both being connected in series to a resistor. According to the different molten salt corrosion models, the equivalent circuit corresponds to the formation of a protective layer [21]. According to the graphs of the impedance spectra for solar salt of the stainless steels AISI 304 and AISI 430 and the nickel alloy HR-224, they show that the corrosion process is controlled by the transfer of ions in the protective layer, which is consistent within the theoretical and chemical mechanism during the first hours of contact between the metal and the molten salt; the most significant hours of the corrosion process are found during the first hours of initiation of the process. The results obtained in terms of the equivalent electrical circuits obtained from the spectra shown in solar salt are \begin{table}
\begin{tabular}{c c c c c c c c c c c c} \hline \hline \multirow{2}{*}{**Samples**} & \multicolumn{6}{c}{**wt.\%**} \\ \cline{2-11}  & **Si** & **Al** & **Mn** & **Cr** & **Co** & **P** & **Mo** & **C** & **W** & **S** & **Ni** & **Fe** \\ \hline
**304** & 0.75 & & 2 & 18–20 & & 0.04 & - & 0.08 & & 0.03 & 8–12 & Balance \\
**430** & 1 & & 1–1.25 & 16–18 & & 0.04 & - & 0.12 & & 0.03 & 0.75 & Balance \\
**HR-224** & 0.3 & 3.8 & 0.3 & 20 & 0.4 & - & 0.2 & 0.05 & & - & 48.7 & 27.5 \\
**VM12** & 0.4–0.6 & & 0.15–0.451–12 & 1.4–1.8 & - & 0.2–0.4 & 0.1 & 1.3–1.7 & - & 0.1–0.4 & Balance \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Chemical composition of 304, 430 Stainless Steel, HR-224 alloy nickel, and ferritic steel VM12. Figure 6: Schematic of the electrochemical system. 1: Crucible furnace; 2: three-electrode arrangement (Re: reference electrode, CE: counter electrode, WE: working electrode or sample); 3: molten salt; 4: alumina crucible; 5: furnace controller; 6: potensiostat (**left**); predisposition of the high temperature furnace and welded and cemented electrodes in alumina tubes (**right**). represented in the Tables 3-5 in terms of the transfer resistance \(R_{t}\). This transfer resistance is related by the simplified Butler-Volmer equation (Equation (12)), and we can observe that this resistance has an inverse relationship to the current density exchanged. \[R_{t}=\frac{R\cdot T}{n\cdot F\cdot i_{o}} \tag{12}\]  
where, \(R\) is the gas constant, \(T\) is temperature, \(n\) is the number of electrons involved, \(F\) is the Faraday constant and \(i_{o}\) is the exchange current density. In the case of AISI 304 and AISI 430 stainless steels, a sustained trend of the low frequency semi-circle was observed in most measurement periods, but for AISI 430 steel a small high frequency semi-circle was observed at 96 h of immersion, which could mean a load transfer reaction at the layer/melt interface. For the HR-224 alloy showed a formation of the semicircle of high frequency values due to its high nickel content. Thus, the electrochemical impedance spectra of the materials evaluated in the first hours show a controlled ion diffusion reaction, mainly by the trend of the low frequency curve related to the capacitance of the metal oxide.