## 3 Results and Discussion
### Spontaneous Growth of Passive Films on Stainless Steels and Inconel 600
#### 3.2.1 Electrochemical Impedance Spectroscopy
At low frequencies, the second time constant is associated with the oxidation-reduction process, which takes place in the passive film. C\({}_{2}\) is associated with the capacitive behavior of the film, and R\({}_{2}\) is linked to the resistance of the oxidation-reduction process [1; 26]. Figure 5: Equivalent circuits with two characteristic times constants used to represent the impedance behavior of the film–electrolyte interface as a function of immersion time. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline  & **1 h** & **1 h 30** & **2 h** & **3 h 30** & **24 h** & **72 h** \\ \hline CPE\({}_{1}\) & 34.0 & 18.2 & 15.1 & 13.8 & 10.5 & 9.2 \\ (\(10^{-6}\)\(\Omega^{-1}\) cm\({}^{-2}\) s\({}^{-n}\)) & 0.36 & 1.90 & 1.43 & 1.75 & 0.89 & 1.11 \\ \(\alpha_{1}\) & 0.94 & 0.93 & 0.93 & 0.93 & 0.92 & 0.93 \\ Error \% & 0.25 & 2.40 & 0.27 & 0.21 & 2.14 & 0.28 \\ R\({}_{1}\) (k\(\Omega\)/cm\({}^{2}\)) & 17.3 & 39.2 & 35.4 & 41.3 & 48.1 & 69.7 \\ Error \% & 4.78 & 3.25 & 0.73 & 4.52 & 6.14 & 4.22 \\ CPE\({}_{2}\) & 87.1 & 78.0 & 61.4 & 32.2 & 25.4 & 28.6 \\ (\(10^{-6}\)\(\Omega^{-1}\) cm\({}^{-2}\) s\({}^{-n}\)) & & & & & \\ Error \% & 3.06 & 5.21 & 2.04 & 4.75 & 3.93 & 0.48 \\ \(\alpha_{2}\) & 0.84 & 0.84 & 0.83 & 0.84 & 0.82 & 0.84 \\ Error \% & 1.93 & 1.26 & 2.39 & 4.5 & 5.2 & 2.89 \\ R\({}_{2}\) & 459.1 & 568.2 & 698.0 & 754.8 & 878.4 & 993.9 \\ (k\(\Omega\)/cm\({}^{2}\)) & & & & & & \\ Error \% & 5.25 & 4.11 & 3.70 & 1.39 & 3.24 & 5.01 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Best fit parameters for the impedance spectra obtained on AISI 316 in NaOH solutions after OCP stabilization. Tables 3 and 4 group together the values of the equivalent circuit parameters in the case of stainless steel. For AISI 316, the values of \(\alpha_{1}\) vary between 0.94 and 0.92, while those of \(\alpha_{2}\) vary between 0.84 and 0.82. For AISI 304, \(\alpha_{1}\) varies between 0.95 and 0.94 and \(\alpha_{2}\) between 0.87 and 0.86. The values obtained for \(\alpha\) are very close to 1 for both types of stainless steel, in agreement with the literature [22]; therefore, in our case, CPE can be assimilated into pure capacitors (\(\mathrm{CPE}\approx\mathrm{C}\)). Like stainless steel, in the case of Inconel 600, CPE can also be assimilated to pure capacitors (\(\alpha_{1}\) and \(\alpha_{2}\) are close to 1, as shown in Table 5). For all three alloys, the values obtained by fitting \(\mathrm{CPE}_{1}\) (between 34.0 and 7.9\(\times 10^{-6}\)\(\mathrm{\Omega}^{-1}\mathrm{cm}^{-2}\)\(\mathrm{s}^{-\mathrm{n}}\)) indicate that it can be assimilated to the capacity of the double layer. The increase for both types of stainless steel of the charge transfer resistance R\({}_{1}\) from 20.4 to 69.7 \(\mathrm{k\Omega}/\mathrm{cm}^{2}\) for AISI 316 and from 28.6 to 81.1 \(\mathrm{k\Omega}/\mathrm{cm}^{2}\) for AISI 304 with the increase of the immersion time, shows the increasing difficulty that the charge transfer process can encounter to be achieved. Thus, this hypothesis seems to be relatively in line with the increase in resistance R\({}_{2}\) from 459.1 to 993.9 \(\mathrm{k\Omega}/\mathrm{cm}^{2}\) for AISI 316 and from 251.2 to 749.9 \(\mathrm{k\Omega}/\mathrm{cm}^{2}\) for AISI 304 (Figure 6). The \(\mathrm{CPE}_{2}\) and \(\mathrm{R}_{2}\) parameters associated with the