## 3 Results and Discussion
It was also observed that as the Na\({}_{2}\)CO\({}_{3}\) concentration decreased, CR decreased from 1.81 (assay 7) to 1.29 mmpy (assay 9), even though the pH decreased, which confirmed that a lower availability of Na\({}^{+}\) to create vacancies led to the decrease in CR. Although there was a decrease in the corrosion rate when the Cl\({}^{-}\) concentration decreased (CR assay 1 \(>\) CR assay 2 \(>\) CR assay 5; CR assay 6 \(>\) CR assay 8), at the same concentration of Cl\({}^{-}\), we observed that CR was higher in NaCl than in CaCl\({}_{2}\), as shown by assays 5 and 6. This reinforces the fact that siderite crystals are unstable in the presence of NaCl due to the incorporation of Na\({}^{+}\) into the deposition and the subsequent vacancy generation; but, in presence of Ca\({}^{2+}\), the formation of ankerite becomes possible, which is much more stable and, is thus better for protecting against corrosion. Following our reasoning and applying it to ankerite, we can argue that the increase in the corrosion rate is due to Na\({}^{+}\) ions incorporating into the crystal system of CaFe(CO\({}_{3}\))\({}_{2}\), creating new crystal defects; the Na\({}^{+}\) ions place themselves in interstitial positions and create vacancies of Ca\({}^{2+}\) and/or Fe\({}^{2+}\) to keep the electroneutrality of the deposition. In line with the above:  
\[(1-x)\ \text{FeCO}_{3}+x\text{Na}_{2}\text{CO}_{3}+\ \text{yCaCO}_{3}\to\text{Fe}_{(1-x)} \square_{\text{x}}\big{[}\text{Ca}_{y}\text{Na}_{2x}\big{]}\text{(CO}_{3})_{1+y} \tag{7}\]  
\[(1-x)\ \text{CaCO}_{3}+x\text{Na}_{2}\text{CO}_{3}+\ \text{yFeCO}_{3}\to\text{Ca}_{(1-x)} \square_{\text{x}}\big{[}\text{Fe}_{y}\text{Na}_{2x}\big{]}\text{(CO}_{3})_{1+y} \tag{8}\]  
where Na\({}_{2}\)CO\({}_{3}\) is obtained from the reaction shown by Equation (5), \(\square\) are the Ca\({}^{2+}\) or Fe\({}^{2+}\) vacancies, and [ ] are interstitial ions. This can be observed by comparing assay 10, with a CR of 0.97 mmpy due to the formation of ankerite, which, by adding NaCl (assay 4) increases its CV to 4.06 mmpy because it leads to the formation of corrosion products, such as the ones indicated by Equation (7) or Equation (8), and continues to increase up to 5.01 mmpy with assay 2, even though it has the lowest total salt concentration. These results coincide with the ones obtained by [PERSON] et al. [6], who studied the effects of Ca\({}^{2+}\) ions on the CO\({}_{2}\) corrosion of API 5L-Gr B steel by measuring it in two mediums saturated by NaCl and by CO\({}_{2}\) in the presence and absence of CaCO\({}_{3}\). Therefore, the Ca\({}^{2+}\) cation also has a passivizing effect on corrosion in the absence of NaCl. If we observe the results in Table 1, we can see that the role that the Ca\({}^{2+}\) cation plays against corrosion is influenced by the associated anion. In the absence of NaCl, the corrosion rate of AISI 1040, under the conditions that are tested herein, is higher when the Ca\({}^{2+}\) cation has Cl\({}^{-}\) as its anion instead of CO\({}_{3}\)\({}^{2-}\). This can be verified by comparing assays 8 and 10, which have the same Ca\({}^{2+}\) concentrations in solution and a CR of 1.41 mmpy when working with CaCl\({}_{2}\), while it decreases to 0.97 mmpy when working with CaCO\({}_{3}\). This is because chloride ions, similar to an acidic pH, can destroy the passivation layer [39]. To sum up, the absence of chlorides improves the encrustation protectiveness against corrosion, and ankerite is formed when working with CaCO\({}_{3}\).