**4. Conclusions**

The electrochemical characterizations demonstrated that MBT and Na2HPO4 can be considered as corrosion inhibitors of steel. The initial mixture of corrosion products constitutes a primary viscous layer on the mild steel surface, which afterwards undergoes gradation and is separated into two distinguishable layers: an adherent rust layer and a loosely adherent anodic oxide film. At an early stage, amorphous masses of oxides with spongy appearance are created that transform chiefly to lepidocrocite, part of which is further transformed in goethite. During submission, the reduction of lepidocrocite to hydrated Fe2<sup>+</sup> intermediate takes place due to metal anodic dissolution, especially near the steel surface, where magnetite appears. The chloride ions are considered to accelerate the corrosion process as aggressive anions, penetrating the protective layer and hence attacking the fresh unharmed metal surface beneath the layer, triggering pitting corrosion.

Adsorbed phosphate is assumed to act as a precursor for hematite creation and often for goethite nucleation and growth; at different concentrations and especially at locally more basic areas, phosphate anions increase the negative electronic charge of ferrihydrite (present at earlier stages) between particles, and therefore, the formation of hematite retards or is limited. The interaction of ions or neutral molecules at the electrical double layer changes its properties and structures. The water molecules pre-adsorbed at the metal surface in contact with the aqueous solution are involved in the successive adsorption processes. A thin multimolecular adsorption layer via the self-assembly process was formed (phosphonate layer formation) due to the intermolecular interaction between phosphonate groups. The inhibition mechanism is anodic type, hindering the active iron dissolution to a large extent.

Since MBT contains polar groups with an atom of nitrogen and sulfur, the inhibiting properties were determined by the electron density at the reaction center. With an increase in the electron density at the reaction center, the chemisorption bonds between the MBT and the metal are strengthened. Therefore, MBT is adsorbed on the surface of mild steel, formatting stable produced self-assembled monolayers of MBT compounds on the surface of mild steel, protecting from aggressive ions. The inhibitive action of MBT takes place through the adsorption of its molecules onto the mild steel surface via the lone pair of electrons of the N and S atoms. The presence of both aforementioned inhibitors into the corrosive environment exhibited the highest impedance modulus and *R*ct value as the exposure time elapsed. The synergistic e ffect of the corrosion inhibition behavior of MBT and Na2HPO4 in a molar ratio of 1:1 revealed that the admixture performed e ffectively with inhibition efficiency above 90% and with a synergistic parameter above 22. The synergistic e ffect of the corrosion inhibition behavior of MBT and Na2HPO4 is increased in the presence of NaCl due to the Cl− interaction. Considering the electrochemical, spectroscopy, and morphology characterizations, the corrosion protection mechanisms of steel can be attributed to the protective layers formed onto the metal surface due to the presence of the inhibitors, which prevent chloride's penetration.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/2076-3417/10/1/290/s1, Table S1. Classification of the Raman spectra peaks. Table S2. Fitting parameters according to equivalent circuits of Figure 21 obtained for the mild steel immersed in WSS solution. Table S3. Fitting parameters according to equivalent circuits of Figure 21 obtained for the mild steel immersed 3.5 wt % NaCl solution.

**Author Contributions:** I.A.K., P.S., and C.A.C. conceived and designed the experiments. P.S. and I.A.K. conducted the experiments. I.A.K., P.S., and E.K.K. performed the characterization and evaluation of the data. I.A.K., P.S., and C.A.C. discussed the data and wrote the paper. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Conflicts of Interest:** The authors declare no conflict of interest.
