## 3 Results and Discussion
### Corrosion Resistance Evaluation
#### 3.2.1 The Equivalent Circuit R(QR(LR))  
The electrical equivalent circuit presented in Figure 3a describes the best experimental data for the C45/seawater system. The Nyquist diagram (Figure 3b) for this sample, after one hour of immersion in seawater, shows a depressed semicircle in the high-frequency region and an inductive loop in the low-frequency region. This inductive loop can be attributed to adsorption of ions, neutral molecules, or even of some particles (insoluble corrosion products) from the solution, but it may also indicate the existence of intermediate reactions related to the adsorption of aggressive chlorine ions, which produce instability of the electrode surface [25; 26; 27; 28]. However, a better adjustment of the data was obtained by replacing the capacity of the double layer with a constant phase element, respectively constant phase element (CPE), which indicates the non-ideal behavior of the capacity of the double-electric layer (changing the capacity with frequency). The equivalent circuit R(QR(LR)) consists of a resistor (\(\mathrm{R_{s}}\)) in series with a parallel combination of a CPE, a resistor (\(\mathrm{R_{ct}}\)), and a resistor-inductance series combination (\(\mathrm{R_{L}}\), \(\mathrm{L}\)). The values of the parameters that best satisfy the parameter data are presented in Table 2.  
The \(\chi^{2}\) parameter values and the \(Z\) percentage errors of the elements of this circuit show a satisfactory adjustment of the data with this circuit. As can be seen in Table 2, the charge-transfer resistance (\(\mathrm{R_{ct}}\)) has a relatively low value indicating a high corrosion rate. Also, by corrosion, the surface of the sample becomes rough, which makes the capacity of the electric double layer unimportant, thus it is necessary to introduce the constant phase element. The deviation of the frequency exponent (n) from the value 1 is a measure of the deviation from the ideality of the capacitor that represents the capacity of the double-electric layer. The polarization resistance, in this case, is calculated with the relation: \(\mathrm{R_{p}}=(\mathrm{R_{ct}}\cdot\mathrm{R_{L}})/(\mathrm{R_{ct}}+\mathrm{R_ {L}})\). On the surface of the sample, no continuous layer was formed, but only insoluble ions and/or corrosion products were adsorbed. Due to this fact in the equivalent circuit, the \(\mathrm{R_{L}}\)-L series group was introduced in parallel with the time constant characteristic for the double layer (CPE-\(\mathrm{R_{ct}}\)). The circuit element \(\mathrm{R_{L}}\) represents the resistance due to the components adsorbed on the surface and \(\mathrm{L}\) is an inductor (coil), which in EIS describes the inductive loops in the low-frequency region. \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline  & \multicolumn{3}{c}{**CPE**} & \multicolumn{3}{c}{**R\({}_{\mathrm{ct}}\)**} & \multicolumn{3}{c}{**R\({}_{\mathrm{L}}\)**} & \multicolumn{3}{c}{**L\({}_{\mathrm{r}}\)**} & \multicolumn{3}{c}{**10\({}^{3}\chi^{2}\)**} & \(\varepsilon_{\mathrm{z}}\)** * \\ \cline{2-9}  & \multicolumn{1}{c}{**Q,**} & \multicolumn{1}{c}{**S\({}^{\mathrm{n}}\)/cm\({}^{2}\)**} & \multicolumn{1}{c}{**n**} & \multicolumn{1}{c}{**\(\Omega\) cm\({}^{2}\)**} & \multicolumn{1}{c}{**\(\Omega\) cm\({}^{2}\)**} & \multicolumn{1}{c}{**H cm\({}^{2}\)**} & \multicolumn{1}{c}{**10\({}^{3}\chi^{2}\)**} & \(\varepsilon_{\mathrm{z}}\)** * \\ \cline{2-9}  & \multicolumn{1}{c}{**39.6**} & \multicolumn{1}{c}{**3.418 \(\times\) 10\({}^{-4}\)**} & \multicolumn{1}{c}{**0.780**} & \multicolumn{1}{c}{**588**} & \multicolumn{1}{c}{**1007**} & \multicolumn{1}{c}{**496**} & \multicolumn{1}{c}{**5.92**} & \multicolumn{1}{c}{**6.34**} \\ \hline \(\varepsilon_{\mathrm{EC}}\)** **\%** & \% & 1.22 & 5.6 & 1.59 & 3.95 & 5.4 & 2.87 & - & - \\ \hline \hline \end{tabular}  
* \(\varepsilon_{\mathrm{z}}\)—inductance measurement error; ** \(\varepsilon_{\mathrm{EC}}\)—relative error.  
\end{table} 
Table 2: Equivalent circuit parameters for C45 after one hour of immersion in Black Sea water.