## 3 Results and Discussion
### Corrosion Resistance Evaluation
#### 3.2.2 The Equivalent Circuit R(C(R(QR)))  
The equivalent circuit shown in Figure 4 was the most suitable for fitting the data (empirical curve fitting) obtained for the C45/rainwater system, for all three phosphate samples in seawater (I-Zr/BSW, II-Zn/Fe/BSW and III-Mn/BSW) and for the oil sample/rainwater system (OPS/RW). The circuit describes a double-layer model, in which the pair (R\({}_{\text{ext}}\), C\({}_{\text{ext}}\)) represents the properties of the coating layer, while the pair (CPE, R\({}_{\text{ct}}\)) represents the properties of the double-electric layer, which is responsible for the kinetics of the corrosion process. R\({}_{\text{s}}\) is the uncompensated resistance of the electrolyte column between the reference electrode and the working electrode. In the case of C45 immersed in rainwater, the surface layer is very thin and probably non-conductive, this being characterized by an extremely low electrical capacity (2.54 nF) and high electrical resistance (1014 \(\Omega\cdot\)cm\({}^{2}\)), as can be seen in Table 3. \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline \multirow{2}{*}{**System**} & **R\({}_{\omega}\)** & **C\({}_{\text{ext}}\)**, & **R\({}_{\text{ext}}\)**, & **CPE** & & & **R\({}_{\text{ct}}\)**, & **\(\Omega\cdot\)cm\({}^{2}\)** & **10\({}^{3}\chi^{2}\)** & \(\epsilon_{\text{Z}}\)** \\  & **\(\Omega\cdot\)cm\({}^{2}\)** & **\(\mu\)F/cm\({}^{2}\)** & **\(\Omega\cdot\)cm\({}^{2}\)** & **Q** & **Q** & **Q** & **Q\(\Omega\cdot\)cm\({}^{2}\)** & **10\({}^{3}\chi^{2}\)** & \(\epsilon_{\text{Z}}\)** \\  & & & & & & **S-s”/cm\({}^{2}\)** & **n** & & & \\ \hline C45/RW & 1386 & \(2.54\times 10^{-3}\) & 1014 & \(1.00\times 10^{-4}\) & 0.65 & 3596 & 0.932 & 3.05 \\ \hline I-Zr/BSW & 36.3 & 0.106 & 30.82 & \(6.20\times 10^{-5}\) & 0.51 & 1982 & 5.77 & 7.59 \\ \hline II-Zn/Fe/BSW & 40.4 & 2.870 & 11.04 & \(2.51\times 10^{-4}\) & 0.61 & 883 & 0.694 & 2.44 \\ \hline III-Mn/BSW & 37.9 & 4.46 & 2.76 & \(5.31\times 10^{-2}\) & 0.73 & 463 & 0.109 & 1.02 \\ \hline OPS/RW & 1969 & \(2.06\times 10^{-6}\) & 1393 & \(1.48\times 10^{-5}\) & 0.49 & \(2.19\times 10^{4}\) & 1.89 & 4.68 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Equivalent circuit parameters for the systems analyzed with the circuit presented in Figure 4.  
Figure 4: The equivalent circuit for the corrosion of a coated metal. EDL, electric double layer. For both C45 and the three phosphate samples, the surface layer is uniform, both in terms of composition and microstructure, so that the capacity of this layer is represented by an ideal capacitor. In the case of the C45/rainwater system, the electric capacity of the surface layer is extremely low (2.54 pF). This is explained by the fact that although the thickness of the product layer formed in the initial moments is very small, it is impregnated with rainwater whose conductivity is low (\(\varepsilon_{r}\) = 81; \(\varepsilon=\varepsilon_{r}\cdot\varepsilon_{0}=81\cdot 8^{1}854.10^{-12}\) F/m). For the same reason, the strength of this layer is high. The resistance of the electric double layer (R\({}_{\text{ct}}\)--load transfer resistance) is also very high so that the polarization resistance here is: \(\text{R}_{\text{P}}\) = R\({}_{\text{ct}}\) + R\({}_{\text{ext}}\) = 4980 \(\Omega\cdot\)cm\({}^{2}\). For phosphate samples, the capacity is in the range of microfarads, these values are characteristic for porous layers. Because the pores are filled with the corrosion environment solution (in this case seawater) and their resistance is very low. For the I-Zn sample immersed in seawater, the charge transfer resistance is the highest; the phosphate layer deposited with the I-Zn solution provides better corrosion protection compared to the other two phosphate solutions. The frequency exponent value in this case (n = 0.51) indicates that the constant phase element that replaces the capacity of the electric double layer is, in fact, a Warburg element, which also indicates the influence of diffusion on the corrosion rate.