## 2 Corrosion Mechanism for Nitrates Based on Chemical Models
### Model for Oxyanionic Salts  
According to Figure 2, we can indicate that nitrate salts can dissociate and form ion species (ions of the form \(A{O_{y}}^{-2}\)) by the effect of temperature increase. Then, the oxyanionic salt (\(A{O_{y}}^{-2}\)) is formed, as well as \(R^{+z}\), representing the alkaline metal in cation species. This step of the process will be represented by step 0 (s0). Stage 1 (s1) is considered to be the initial reaction between atmospheric oxygen \(O_{2}\) and the transition metals of the steel, generating the first layer of oxide of the \(M_{x}O_{y}\) type. This reaction is supposed to be slow and depends on the atmospheric conditions of the site. Subsequently, it is necessary to concatenate the different ion species obtained from other reactions to give a clear idea of the effect of the oxide ion \(O^{-2}\) that is the cause of corrosion. Stage 2 (s2) is the most important, since the metal oxides formed in stage 1 (s1) are dissociated and generate the first evidence of oxide ion \(O^{-2}\), which in contact with the metal oxides compose the passivation layer. This reaction is likely to be fast.  
The abundant oxide ion \(O^{-2}\), found in the ion cloud formed by the dissolution of the passive layer, reacts again with the oxides of the passive layer, forming a species of the form \(M_{x}O_{y+z}^{-2}\). This stage (s3) is also supposed to be fast, like (s2). Finally, stage 4 (s4) is the slowest and is controlled by the diffusion of the oxide ion \(O^{-2}\), formed by the dissociation of the oxyanionic salts (\(A{O_{y}}^{-2}\)) in the electrolyte. The mass transfer is affected by the ion cloud in contact with the metal oxide.  
Figures 3-5 show the proposed corrosion mechanisms for Fe, Cr, and Ni metallic elements during the corrosion process, and are related to Figure 2. Iron metal reacts with O\({}_{2}\) generating oxidized products on the metal surface, shown in Equations (3), (5), and (6), generating the oxide metal layer. The dissolution of the metal oxide can occur via the acid and basic pathways. When the \(O^{-2}\) ion activity is low in the melt, the metal oxide will dissociate, resulting in oxides ions and potentially soluble metal ions (Equation (4)). Meanwhile, increase of \(O^{-2}\) ions in the boundary layer create metal oxide anions soluble in the salt generating oxide ion \(O^{-2}\) in contact with the metal surface, thus, these processes could be identified by a change in electrochemical impedance as a function of time.  
\[2 Fe+O_{2}\leftrightarrow 2 FeO \tag{3}\]  
\[FeO+O^{-2}\leftrightarrow FeO_{2}^{-2} \tag{4}\]  
\[3 FeO+O^{-2}\leftrightarrow Fe_{3}O_{4}+2e^{-} \tag{5}\]  
\[2 Fe_{3}O_{4}+O^{-2}\leftrightarrow 3 Fe_{2}O_{3}+2e^{-} \tag{6}\]  
Figure 2: General scheme of corrosion mechanisms on surface metal.  
The species generated by the reaction with \(O^{-2}\) ions due to the dissociation \(NO_{3}^{-}\) ions in the bulk electrolyte are showed in the Equations (4), (9), and (11) respectively.  
\[4 Cr+3O_{2}\leftrightarrow 2 Cr_{2}O_{3} \tag{7}\]  
\[Cr_{2}O_{3}+2e^{-}\leftrightarrow 2 Cr^{+2}+3O^{-2} \tag{8}\]  
\[Cr_{2}O_{3}+3O^{-2}\leftrightarrow Cr_{2}O_{4}^{-2} \tag{9}\]  
Some additional reactions related with nickel ions were identified:  
\[2 Ni+O_{2}\leftrightarrow 2 NiO \tag{10}\]  
\[NiO+O^{-2}\leftrightarrow NiO_{2}^{-2} \tag{11}\]  
Figure 4: Corrosion mechanisms of Cr in contact with Oxyanionic Salts.  
Figure 3: Corrosion mechanisms of Fe in contact with Oxyanionic Salts.  
According to the reaction mechanism of nickel in Figure 5, NiO can be dissociated in acid/basic medium through the activity of the oxide ion \(O^{-2}\), resulting in potentially soluble nickel oxide ions. Therefore, the formation of different nickel anions is clearly seen as \(NiO_{2}^{-2}\), \(NiO_{2}^{-}\) y \(NiO_{3}^{2-}\).  
These reactions can help us to understand the chemistry involved in the molten salt corrosion process, as well as to identify the proper equivalent circuit to fit the impedance signals obtained in this research.