## 3 Results and Discussion
### Spontaneous Growth of Passive Films on Stainless Steels and Inconel 600
#### 3.2.1 Electrochemical Impedance Spectroscopy
\begin{table}
\begin{tabular}{c c c c c c c} \hline  & **1 h** & **1 h 30** & **2 h** & **3 h 30** & **24 h** & **72 h** \\ \hline \(\mathrm{CPE}_{1}\) & & & & & & \\ (\(10^{-4}\)\(\mathrm{\Omega}^{-1}\)\(\mathrm{cm}^{-2}\)\(\mathrm{s}^{-\mathrm{n}}\)) & 22.1 & 18.1 & 17.9 & 16.8 & 10.1 & 7.9 \\ \(\mathrm{Error}\)\% & 3.10 & 0.97 & 2.80 & 0.93 & 0.82 & 2.71 \\ \(\alpha_{1}\) & 0.95 & 0.95 & 0.95 & 0.94 & 0.94 & 0.94 \\ Error \% & 0.13 & 025 & 0.14 & 0.21 & 0.29 & 0.98 \\ \(\mathrm{R}_{1}\) & & & & & & \\ (\(\mathrm{k\Omega}/\mathrm{cm}^{2}\)) & & & & & & \\ Error \% & 6.30 & 2.91 & 0.27 & 4.94 & 1.97 & 2.39 \\ \(\mathrm{CPE}_{2}\) & & & & & & \\ (\(10^{-4}\)\(\mathrm{\Omega}^{-1}\)\(\mathrm{cm}^{-2}\)\(\mathrm{s}^{-\mathrm{n}}\)) & 42.2 & 21.9 & 15.2 & 13.4 & 11.4 & 9.67 \\ Error \% & 5.87 & 6.01 & 1.35 & 0.90 & 231 & 1.74 \\ \(\mathrm{\alpha}_{2}\) & & & & & & \\ Error \% & 1.32 & 0.90 & 1.7 & 2.3 & 0.1 & 0.4 \\ \(\mathrm{R}_{2}\) & & & & & & \\ (\(\mathrm{k\Omega}/\mathrm{cm}^{2}\)) & & & & & & \\ Error \% & 0.8 & 5.12 & 1.84 & 3.30 & 2.24 & 2.62 \\ \hline \end{tabular}
\end{table} 
Table 4: Best fit parameters for the impedance spectra obtained on AISI 304 in NaOH solutions after OCP stabilization. \begin{table}
\begin{tabular}{c c c c c c c} \hline  & **1 h** & **1 h 30** & **2 h** & **3 h 30** & **24 h** & **72 h** \\ \hline \(\mathrm{CPE}_{1}\) & & & & & & \\ (\(10^{-6}\)\(\mathrm{\Omega}^{-1}\)\(\mathrm{cm}^{-2}\)\(\mathrm{s}^{-\mathrm{n}}\)) & 32.4 & 28.2 & 26 & 25.4 & 23 & 23.0 \\ Error \% & 1.51 & 2.4 redox process that can take place within the film are affected by the immersion time. One of the possible reactions that contributes to this process is represented by the following equation [26]:  
\[\text{Fe}_{3}\text{O}_{4}+\text{OH}^{-}+\text{H}_{2}\text{O}\leftrightarrow 3 \text{FeOOH}+\text{e}^{-}\]  
As expected, the barrier properties of passive films increase with immersion time [9,26]. For stainless steel, it is recognized in the literature that the increase in film resistance is due to the enrichment of chromium oxide. The inner layer of the film adhering to the substrate is composed of Cr\({}_{2}\)O\({}_{3}\) oxide, which has the properties of an insulator. The outer layer is essentially composed of iron oxide. As shown by [PERSON] et al. [9], the aging of the films in alkaline environments gives rise to a predominance of the oxide Fe\({}_{2}\)O\({}_{3}\). Compared to AISI 304, for example, Inconel 600 does not show much difference between the spectra acquired at t = 1 h and t = 72 h. The linear character of the capacitive loop at low frequencies at t = 24 h and t = 72 h is less clear than for the other two alloys, suggesting that the films formed naturally on Inconel 600 in alkaline media would be less effective. This is because the value of the polarization resistance R\({}_{\text{P}}\) (which is obtained by extrapolation of the Nyquist curve) at t = 72 h for Inconel 600 is lower than for stainless steel. As for the stainless steel, the film formed on the Inconel 600 also has a bilayer structure with the Cr\({}_{2}\)O\({}_{3}\)[14] enriched inner layer and the outer layer formed essentially by Fe\({}_{2}\)O\({}_{3}\) oxide and NiO [37]. Other studies suggest that nickel oxide would be present in that structure as Ni(OH)\({}_{2}\)[37,47]. With the increase of the immersion time, the diameter of the capacitive arc of the Nyquist diagrams (Figure 4A,B) is increased for both types of stainless steel. However, the values of the resistors R\({}_{2}\) greater for AISI 316 represented in Tables 3 and 4 show that the films formed on AISI 316 are more protective than those formed on AISI 304 in alkaline media. This difference between the two types of stainless steel is due to the presence of molybdenum in the chemical composition of AISI 316 stainless steel, which promotes a more stable and protective chromium oxide enrichment [38]. It can be seen in Figure 6 that the resistance R\({}_{2}\) attributed to the film increases continuously up to 72 h of immersion for both types of stainless steel.