## 3 Results and Discussion
### Corrosion Inhibition Mechanism
#### 3.5.2 Corrosion Inhibitors  
According to the literature, two main corrosion inhibition mechanisms for mild steel protection have proved to be more reliable [77]. The first one is through chemisoption onto the mild steel surface, limiting the metal dissolution rate by forming an adherent passive film, as MBT does for lightly acidic and near-neutral aqueous systems [78]. The second one is by the formation of a new phase onto the metallic surface, creating a conversion coating, as during the phosphating process. Mercapto functional azide compounds present enhanced inhibitive efficiency to mild steel. The coexistence of sulfur and nitrogen atoms in the heterocyclic ring of the inhibitor molecule provides increased binding capability for the inhibitor attachment [98; 99]; the aromatic ring of the molecule is known for its contribution to increase electronic density [100], whilst the higher electronegativity of sulfur than that of nitrogen renders the thiol group an especially efficient electron donor [29; 99]. Particularly, according to the works of [PERSON] et al. [29] and [PERSON] et al. [101], it can be claimed that of the two tautomers of MBT, thiol (MBT-thiol) or thione (MBT-thione) (Figure 24a), coexisting usually in ionic solutions and ambient temperatures, the second seems to be more stable, especially in solid state, as in rusts. The latter group of researchers mentioned by the means of chemical hardness measurements that MBT-thione has increased electronegativity compared to MBT-thiol, which confers to the former a denser electronic cloud around the functional mercapto group of the molecule, and hence enhanced inhibition ability. Considering that electronegativity is proportionate to the chemisorption of the inhibitor and therefore to the level of its inhibition ability [100], they found that the heterocyclic ring could bond to the metallic surface through the endocyclic nitrogen, as the most electronegative of the three more possible donors. Furthermore, and at the same time, the heterocyclic ring could also bond with the exocyclic sulfur, chelating in that manner the iron cations and attaching, with its planar aromatic ring parallel to the metallic surface. In our study, the most electronegative positions would be the two atoms of sulfur. In spite of the electrostatic interaction preference, due to steric hindrance, the more stable coordination geometry of the MBT-thione with mild steel was commented to realize by means of the exocyclic sulfur, with the aromatic plane perpendicular to the metallic surface. The two tautomeric types of MBT usually coexist  
Figure 24: Schematic representation of (**a**) 2-mercaptoethanolhorazole (thiol form) and 2(3H)-benzothiozolethion (thion form); (**b**) sodium phosphate dibasic [Na\({}_{2}\)HPO\({}_{4}\)]. in circumneutral aquatic and ionic solutions [102], but be that as it may, a supplementary clue in favor of the thione mode, as predominant in the rust layer, emerges from a characteristic Raman shift in the _Steel-WSS-Na2 HPO4-MBT_ sample. Taking into account that MBT-thione inhibition efficiency upon time depends on the rate of the attachment of the exocyclic sulfur to mild steel, two stages of the inhibitor adsorption were distinguished [29]. The first stage is meant to be a physisorption process of the organic hydrolyzed molecule to the solid metallic surface, leading to the creation of a primary monomolecular layer. Physical binding of the inhibitor needs relatively low quantities of exchangeable energy, takes place to a limited extent as a front reaction, and is the prominent and necessary process for the inhibitor's attaching and hence to initiate the adsorption of the inhibitor [99]. The second stage, chemisorption, is also quickly initiated, as Fe-S chemical bonding is considered to be prompt and more powerful. The two adsorption processes were identified as similar systems to MBT-thione and iron by [PERSON] et al. in 2018.