## 3 Results and Discussion  
### The Effects of CS Immersion in Fire Extinguishing Solution  
To process the EIS data obtained for the CS samples in the initial and after 36 days or 78 days of immersion in fire extinguishing solution, the circuit shown in Figure 2 was used. The equivalent circuit is characteristic of a corrodible metal covered with a porous non-conductive film in which the corrosion process takes place under a mixed, kinetic, and diffusion control. In this circuit, R\({}_{\text{s}}\) is the resistance of the solution, and R\({}_{\text{ext}}\) and C\({}_{\text{ext}}\) are the resistance and the electrical capacity of the coating, respectively. Moreover, R\({}_{\text{ct}}\) and CPE are the resistance and the constant phase element, which takes the place of the double-electric kinetic. Additionally, W is the characteristic constant in the expression of \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Corrosion Environment** & **Immersion Time (Days)** & **CS, pH** & **ZnPS, pH** & **EPS, pH** \\ \hline \multirow{3}{*}{**FES**} & Initial & 6.41 & 6.41 & 6.41 \\  & 36 & 6.52 & 6.43 & 6.41 \\  & 78 & 6.60 & 6.60 & 6.50 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Variation of the fire extinguishing solution pH with the immersion time of the samples. Figure 2: The equivalent circuit used in the analysis of EIS data for the CS immersed in FES. the diffusion impedance, a quantity that controls the reaction rate by the influence of the diffusion of oxidizing and reducing species to and from the metal. The parameter values of the circuit elements are presented in Table 2. The Nyquist (a) and Bode (b) diagram for CS sample is presented in the Figures S1-S3. Following the analysis of the data in this table, the following aspects can be observed:  
* For the CS sample immersed in FES, it is necessary, even from the initial moments, to use a bi-layer model and it is needed to take into account the diffusion, although the alloy used has been freshly ground. This phenomenon can be explained if it is considered that the fire extinguishing solution or some of its components adhere to the surface of the metal, even when it is introduced into the liquid and forms a very thin film. The film is porous, and as a result, its resistance is very low and does not affect the resistance of the solution or the charge transfer resistance;
* By increasing the thickness of the outer layer (i.e., more products from the solution are adsorbed to the formed layer), there is an increase in R\({}_{\text{s}}\) and the resistance of the outer layer, however, the charge transfer resistance decreases;
* The constant W decreases with the immersion time so that the Warburg impedance increases significantly and the effect that the diffusion has on the reaction speed intensifies (the speed decreases); and
* The double-electric layer is represented by a non-ideal capacitor, and its capacity varies randomly, probably due to the changes that take place at the metal/solution interface at various moments of immersion. The surface microstructure of the sample immersed for 78 days in fire extinguishing solution, after drying, is presented in Figure 3 for various areas and at various magnifications. As can be seen, a crust, apparently uniform, formed on the entire surface of the sample (zone A), and in the center of the sample, there was an additional deposit with a thickness of 0.5-0.6 mm (zone B). In both areas, the deposits were formed by small agglomerations that formed an irregular porous mass, and over them were distributed, from place to place, lighter agglomerations. In zone B, the number of these agglomerates was much higher than in zone A; for this reason, zone B is lighter in color. EDS measurements indicate the complexity of the compositions in these two areas, and the agglomerations distributed locally on the surface (Figures 4 and 5).