## 4 Results and Discussion
### Electrochemical Impedance Spectroscopy Measurements
#### 4.1.2 Mixture with Lithium Content (LiNaK)NO\({}_{3}\) Testing

Table 6: Fitting results of the impedance spectra of stainless steel AISI 304 during corrosion in the presence of LiNaKNO\({}_{3}\) (ternary salt with lithium content) at 550 \({}^{\circ}\)C.  
Figure 12: Nysquist diagram of Nickel alloy HR224 layer in (LiNaK)NO\({}_{3}\) (ternary salt with lithium content) at 2 h, 4 h, 24 h, 48 h, 72 h, and 96 h at 550 \({}^{\circ}\)C.  
The relationship between the corrosion mechanisms from both chemical and electrochemical models suggests that the general situation of how ions interact within the metal-melted system is primarily governed by oxidation, which in this case is the oxide ion \(O^{-2}\). Therefore, the resistance (impedance) of the system is directly related to the different layers that are formed in the time of exposure of the samples. Therefore, the charge transfer in the system between the metals and the electrolyte makes the corrosion process slower/fast, leading to the change in impedance. For this reason, the equivalent circuit for the formation of the protective layer is shown in Figure 13, and it should be conceptualized based on the reaction mechanisms already represented in this article. For the protective layer model shown, ion transport in the protective layer can limit the speed of corrosion. The elements that compose the equivalent circuit of the protective layer are represented by the following elements: \(R_{s}\) is the molten salt resistance; \(C_{dl}\) is the double layer capacitance at the metal/molten interface; and \(R_{t}\) is the electrochemical charge transfer resistance. Often the behaviour of the capacitors in EIS tests are not the most ideal, so a constant phase element acts instead for experimental data settings. \(R_{ox}\) and \(C_{ox}\), both represent the resistance and the capacitance of the transfer of the ions of the metal oxide layer. Therefore, this equivalent protective layer circuit is expressed in Equation (13). \[Z=R_{s}+\frac{1}{jwC_{dl}+wC_{dl}\cot\left(\frac{\beta_{dl}\pi}{2}\right)+ \frac{1}{R_{t}}}+\frac{1}{jwC_{ox}+wC_{ox}\cot\left(\frac{\beta_{ox}\pi}{2} \right)+\frac{1}{R_{ox}}} \tag{13}\]  
where, \(\beta_{dl}\) and \(\beta_{ox}\) represent the dispersion coefficient of the first and second capacitance of the circuit, respectively. Therefore, \(wC_{dl}\cot\left(\frac{\beta_{dl}\pi}{2}\right)\) and \(wC_{ox}\cot\left(\frac{\beta_{ox}\pi}{2}\right)\) are the elements of the impedance caused by the dispersion effect. Since corrosion is controlled by the transport of species in the protective layer, the radius of the second capacitance circuit must be greater than that of the first cycle. In addition, in this case, the corrosion resistance of the metals in the molten salts can be represented by the \(R_{ox}\) parameter [21]. The charge transfer resistance values for the ternary mixture with Li content obtained for the materials AISI 304 and AISI 430 and the alloy HR-224 were higher with respect to the solar salt of the  
Figure 13: Equivalent circuit representing the corrosion of metals forming a protective scale in molten salts. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Time** & \(R_{s}\) & \(C_{dl}\) & \(\beta_{dl}\) & \(R_{t}\) & \(C_{ox}\) & \(\beta_{ox}\) & \(R_{ox}\) \\ \hline  & \(\Omega\)**-cm\({}^{2}\)** & F \(\mathbf{s}^{(\beta_{dl}-1)}\) & & \(\Omega\)**-cm\({}^{2}\)** & F \(\mathbf{s}^{(\beta_{ox}-1)}\) & & \(\Omega\)**-cm\({}^{2}\)** \\
2 h & 1.617 & 0.  
same materials, and therefore the corrosion rate in the lithium content salt is lower compared to the values obtained in the solar salt. With the background of the results obtained in the stainless steels AISI 304 and AISI 430, and the nickel-based alloy HR-224, in addition to the improvement in the corrosion process by the ternary mixture with lithium content, we proceeded to evaluate a new ferritic steel (VM12), a promising material to be use as structural material in CSP plants, in the same way.