## 3 Results and Discussion
### Corrosion Resistance Evaluation
#### 3.2.4 The Equivalent Circuit R(C(R(Q(RW))))  
The experimental data for the C45/fire extinguishing solution system were processed with the equivalent circuit shown in Figure 6. This circuit is characteristic of a corrosive metal coated with a porous non-conducting layer in which the corrosion process is carried out under a mixed kinetic and diffusion control. In this case, the coating layer is formed by adsorption of the solution components on the surface of the freshly polished alloy. The deposited layer is compact enough to act as a barrier for the electrons transfer, which is taking place by diffusion.  
\begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline \multirow{3}{*}{**System**} & \multirow{3}{*}{\(\mathbf{R_{s}}\), \(\mathbf{\Omega}\)**cm\({}^{2}\)**} & \multicolumn{3}{c}{**CPE 1**} & \multicolumn{3}{c}{**CPE 2**} & \multirow{3}{*}{\(\mathbf{R_{ct}}\)**} \\  & & \(\mathbf{Q_{1}}\) & & & \(\mathbf{R_{ct}}\) & & \(\mathbf{Q_{2}}\) & & \(\mathbf{\Omega\)**cm\({}^{2}\)**} & \\  & & \(\mathbf{S^{n}/cm^{2}}\) & & & & \(\mathbf{S^{n}/cm^{2}}\) & & & \\ \hline I-Zn/RW & 2913 & \(7.78\times 10^{-10}\) & 0.90 & 4894 & \(9.53\times 10^{-6}\) & 0.52 & \(3.91\times 10^{4}\) & 0.25 & 1.57 \\ \hline II-Zn/Fe/RW & 2328 & \(1.63\times 10^{-9}\) & 0.95 & 1987 & \(1.58\times 10^{-4}\) & 0.68 & 1858 & 0.21 & 1.44 \\ \hline III-Mn/RW & 2159 & \(2.45\times 10^{-9}\) & 0.95 & 1507 & \(1.51\times 10^{-3}\) & 0.72 & 1357 & 5.52 & 7.5 \\ \hline I-Zn/FES & 4.59 & \(1.14\times 10^{-3}\) & 0.79 & 115 & \(7.22\times 10^{-3}\) & 0.64 & 326 & 0.88 & 2.97 \\ \hline II-Zn/Fe/FES & 4.64 & \(3.80\times 10^{-4}\) & 0.63 & 283 & \(2.03\times 10^{-3}\) & 0.53 & 989 & 2.26 & 4.75 \\ \hline III-Mn/FES & 4.96 & \(3.47\times 10^{-2}\) & 0.80 & 130 & \(3.62\times 10^{-2}\) & 0.78 & 2.97 & 0.21 & 1.46 \\ \hline \hline \end{tabular}
\end{table}
Table 4: Parameter values of equivalent circuit elements R(Q(R(QR))) for phosphate C45 samples, after one hour of immersion in rainwater and fire extinguishing solution respectively.  
In this equivalent circuit, \(\rm R_{s}\) is the resistance of the solution, \(\rm R_{ext}\) and \(\rm C_{ext}\) are the resistance and the electric capacity of the coating layer, \(\rm R_{ct}\) and CPE are the resistance and the constant phase element that takes place from the capacity of the electric double layer (these control the reaction rate from the kinetic point of view), and \(\rm W\) is the characteristic constant from the diffusion impedance expression, this is the parameter that controls the reaction rate through the influence of the diffusion of oxidizing and reducing species to and from the metal. The experimental values of the parameter for the elements of this circuit are presented in Table 5.  
In the fire extinguishing solution, even if the surface of the alloy is freshly polished, because the solution or one of its components adhere to the surface of the metal, from the initial moments it is necessary to use a double-layer model. Also, the diffusion must be taken into account because on the metal surface a very thin layer is formed. Due to the fact that the film is porous and therefore its resistance is very low, it does not influence the resistance of the solution or the resistance of the charge transfer. Moreover, this surface layer is uniform so that its capacity is represented by an ideal capacitor. The resistance of the electric double layer is relatively high, much higher than the value of the polarization resistance evaluated from the linear polarization curve, in the present case. However, the EIS method underestimates the conductive properties of the electric double layer.