## 3 Results and Discussion
### The Effects of CS Immersion in Fire Extinguishing Solution
The main crust had the most complex composition, and the elements identified in this area were components absorbed from the fire extinguishing solution (oxygen, carbon, sulfur) and the corrosion products of the alloying elements. \begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \multirow{2}{*}{**Immersion Time**} & \multirow{2}{*}{\(\mathbf{R_{s}}\) \(\mathbf{\Omega\cdot cm^{2}}\)} & \multirow{2}{*}{\(\mathbf{C_{ext}}\) \(\mathbf{R_{ext}}\) \(\mathbf{\Omega\cdot cm^{2}}\)} & \multicolumn{3}{c}{**CPE**} & \multirow{2}{*}{\(\mathbf{R_{ct}}\) \(\mathbf{\Omega\cdot cm^{2}}\)} & \multirow{2}{*}{\(\mathbf{W}\)} & \multirow{2}{*}{\(\mathbf{10^{3}\chi^{2}}\)} & \multirow{2}{*}{\(\mathbf{\epsilon z}\)} \\ \cline{4-4} \cline{6-10}  & & & & & & & & & \\ \hline
1 h & 4.64 & 1.713 & 6.42 & \(1.43\times 10^{-3}\) & 0.646 & 154.4 & 0.277 & 0.556 & 2.30 \\
36 days & 34.39 & 0.771 & 15.10 & \(6.31\times 10^{-3}\) & 0.783 & 134.6 & 0.105 & 2.31 & 4.81 \\
78 days & 43.59 & 15.89 & 1459 & \(12.47\times 10^{-3}\) & 0.743 & 129.5 & 0.066 & 0.090 & 0.95 \\ \hline \end{tabular}
\end{table} 
Table 2: Equivalent circuit element values for the CS immersed in FES. Figure 4: Chemical compositions of the crust formed on the surface of the CS immersed for 78 days in FES. Figure 5: Energy spectra for zones A and B of the CS immersed for 78 days in FES. Figure 3: SEM micrographs of the CS immersed for 78 days in the fire extinguishing solution. In zone B, the number of identified elements was smaller. Compared to area A, the percentage of iron was lower and the percentage of carbon was higher. Ca appeared instead, therefore, in correlation with the increase in carbon, the presence of calcium carbonate can be considered. Punctual microanalysis performed on a small agglomeration of light color (point 1 in Figure 4) showed that in these agglomerations, the corrosion product was in very small quantities, instead oxygenated compounds of fire extinguishing solution, calcium carbonate, and sodium chloride predominate. The energy spectra and the elemental mapping of the two areas on the sample surface are shown in Figures 5-7. In A zone, the distribution of elements was more uniform, while in zone B, some elements were preferentially distributed (Fe, O, C, S). The resistance of the solution at the initial moment was identical to that found for the CS/FES system, proof that both the double-electric layer and the phosphate layer were porous, at least in the initial moments. The values of the parameters that characterize constant phase elements (Q and n) indicate that both the outer layer and the double-electric layer are characterized by non-ideal capacitors. In the case of the double-electric layer, the constant phase element is very close to a Warburg impedance, therefore, the corrosion process is influenced by the diffusion of active species. To adapt the EIS data recorded in the case of samples immersed for longer periods in the solution, an equivalent circuit proposed by [PERSON] [23] and by [PERSON] et al. [24] for a nodular cast iron/water system was used. The circuit describes a three-layer physical model: the double-electric layer represented by \(R_{\text{ct}}\) (charge transfer resistance) and \(C_{\text{dl}}\) (electrical capacity); an inner layer characterized by the resistance of \(R_{\text{in}}\) and CDE (the constant phase element related to the electrical capacity of this layer (\(C_{\text{in}}\))); and an outer layer characterized by \(R_{\text{ext}}\) and \(C_{\text{ext}}\). The inner layer was the phosphate film, porous, with free pores, or partially clogged with reaction products, and the outer layer consisted of adsorbed substances from the fire extinguishing solution and corrosion products. The values of the parameters of the circuit elements for the two immersion periods are presented in Table 4.  
The analysis of the data in the table led to the following observations: