## 3 Results and Discussion  
### Films Formed by Cyclic Voltammetry  
The voltammograms obtained (Figure 2A) for AISI 304 and AISI 316 are comparable and are in agreement with the literature results [28; 34; 35; 36; 37]. For the two materials, there are clearly three zones:  
1. Zone I: extended from \(-1.4\) to \(-0.56\) V vs. SCE, called the pre-passivity zone, is the typical region of iron activity;
2. Zone II: extended from \(-0.56\) to 0 V vs. SCE is the region where a stable passive film covering the surface of the alloys is formed (passivation);
3. Zone III: extended from 0 to 0.6 V vs. SCE, called the zone of transpassivity, is the region where the oxidation-reduction processes of alloying elements occur (Cr, Ni, etc.). In this zone, water molecules also decompose, causing an strong increase in the anodic current density on the voltammograms, due to oxygen evolution. In Zone I, an oxidation peak is observed at about \(-0.8\) V vs. SCE due to the formation of the magnetite Fe\({}_{3}\)O\({}_{4}\)[37; 38; 39; 34], as shown in the following equations:  
\[3\mathrm{FeO}+2\mathrm{OH}^{-}\leftrightarrow\mathrm{Fe}_{3}\mathrm{O}_{4}+ \mathrm{H}_{2}\mathrm{O}+2\mathrm{e}^{-} \tag{2}\]  
\[3\mathrm{FeO}+2\mathrm{OH}^{-}\leftrightarrow\mathrm{Fe}_{3}\mathrm{O}_{4}+ \mathrm{H}_{2}\mathrm{O}+2\mathrm{e}^{-} \tag{3}\]  
Figure 2: Cyclic voltammograms (1 st and 12 th cycles) obtained in 0.1 M NaOH solution. Potential range: \(-1.4\) to 0.5 V vs. SCE. Scanning rate: dE/dt = 25 mV/s. (**A**) AISI 316 and AISI 304 samples. (**B**) Inconel 600. Magnetite is a mixed oxide mainly resulting from the oxidation of Fe\({}^{2+}\) to Fe\({}_{3}\)O\({}_{4}\) composed of Fe\({}^{2+}\) and Fe\({}^{3+}\) ions [36; 37]. As can be seen in Figure 2A, for both steels, the peak intensity increases with the increase in the number of cycles. In addition, the density of the anode current is greater for AISI 304 than for AISI 316 (975 uA/cm\({}^{2}\); 713 uA/cm\({}^{2}\); respectively). The presence of a less intense oxidation peak for AISI 316 is due to the presence of molybdenum in the chemical composition of the AISI 316 alloy. Mo promotes the inner layer enrichment by Cr of the chromium oxide film Cr\({}_{2}\)O\({}_{3}\) layer [37; 39; 40], which has an insulating behavior that blocks the movement of iron ions through the film. As indicated in their work, [PERSON] et al. [37] showed that a higher intensity of the oxidation peak also led to a thicker film. Therefore, the films formed on AISI 304 are thicker than those formed on AISI 316. Beyond the peak of magnetite formation, there is a sudden drop in anodic current density, which is characteristic of a transition from an active state of stainless steel dissolution to a state of passivation. The intensity of the reduction peaks at \(-\)0.97 and \(-\)1.12 V vs. SCE increases with the increase of the number of cycles. The processes that occur at these peaks are not well understood. Thus, some authors [19; 20] agree that these peaks correspond to the reduction of Fe\({}^{3+}\) to Fe\({}^{2+}\). Other authors [22] indicate that this would be the reduction of Ni\({}_{3}\)O\({}_{4}\) to NiO and NiO to Ni metal. In their studies on the two types of stainless steel (AISI 304 and AISI 316), [PERSON] et al. [34] suggested that there is an overlap between the reduction of nickel oxides (Ni\({}_{3}\)O\({}_{4}\) to NiO and NiO to Ni metal) and the reduction of iron oxides (Fe\({}_{3}\)O\({}_{4}\) to Fe\({}^{2+}\)). In Zone II, the current density is very small. This fact promotes the idea that in an alkaline medium, the films on the surface of the alloys act as a barrier separating the stainless steel from the electrolyte [38]. In Figure 2A, an oxidation peak is observed in the region of chromium activity at about 0.23 V vs. SCE that corresponds to the chromium oxidation process from Cr\({}^{3+}\) to Cr\({}^{6+}\)[34; 35; 36; 38; 41].