## 3 Results and Discussion
### Corrosion Inhibition Mechanism
#### 3.5.3 The Synergistic Effect  
Table 5 presents the calculated values of the synergistic parameter (\(S_{i}\)) using Equation (2). In general, the synergistic effect can be described as the trend, whereas the combination of two or more compounds results in greater action than of those of the individual compounds. Regarding the corrosion inhibition, the synergistic effect refers to the effective interaction of two or more substances in respect of the protection of a material against corrosion. The synergistic effect can be considered as an effective method to enhance the inhibitive properties of a compound corrosive media. A value of (\(S_{i}\)) \(>\)1 indicates the existence of inhibition synergism between the two compounds, whereas (\(S_{i}\)) = 1 means that no interaction between the two compounds exists; when (\(S_{i}\)) \(<\) 1, it denotes toward an antagonistic effect. The synergistic effect of the corrosion inhibition behavior of MBT and Na\({}_{2}\)HPO\({}_{4}\) in a molar ratio of 1:1 revealed that the admixture performed effectively with an inhibition efficiency above 90%, \(R_{\mathrm{ct}}\) up to 6.7 kohm cm\({}^{2}\) and with a synergistic parameter above 22. The synergistic effect of the corrosion inhibition behavior of MBT and Na\({}_{2}\)HPO\({}_{4}\) is increased in the presence of NaCl due to the CI\({}^{-}\) interaction. In synergism with the organic inhibitor, phosphate was capable of converting appropriately the corroding surface and in parallel to enhancing the local positive electronic cloud, above the inner layer, encountering the negative abundance of the organic voluminous inhibitor, accelerating and boosting in that way the corrosion inhibitive action of both substances. By the means of electrochemical, spectroscopic, microscopic, and image analyzing techniques of characterization, it is excluded that the acceleration and enhancement of the highly efficient Na\({}_{2}\)HPO\({}_{4}\) inhibition was achieved, in combination with the mitigation of the disadvantages of MBT, especially in near-neutral aquatic solutions of mild and low-alloyed steel. The synergistic effect resulted in the formation of more compact and more durable film, which significantly retards the corrosion process. Surface hardening was achieved as well as a considerable increase in the coherence of corrosion product layer, by developing microstructures with ameliorated resistance against the wear effects of mild steel at the micro and nanoscale, originating from fine-grained phases with powerful bonding not only inside the various chemical species but also between them [124]. Achieving the creation of \(\alpha\)-crystals, as goethite and hematite, and the prolonged protection of the converted magnetite layer, it seems reasonable that we have the most thermodynamically stable phases due to the face-centered cubic (fcc)and hexagonal close-packed (hcp)systems, which are the crystalline structures with the highest density, and due to their connectivity provided by phosphorus [126]. The unique chemical \begin{table}
\begin{tabular}{c c c} \hline \hline  & **Time of Exposure (h)** & **Synergistic Parameter (\(S_{i}\))** \\ \hline _Steel–WSS–Na\({}_{2}\)HPO\({}_{4}\)–MBT_ & 24 & 4.76 \\ _Steel–NaCl–Na\({}_{2}\)HPO\({}_{4}\)–MBT_ & 24 & 22.6 \\ _Steel–WSS–Na\({}_{2}\)HPO\({}_{4}\)–MBT_ & 96 & 1.99 \\ _Steel–NaCl–Na\({}_{2}\)HPO\({}_{4}\)–MBT_ & 96 & 4.22 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Tabulated values of the synergistic parameter (\(S_{i}\)) calculated using Equation (2). connection of the phosphated mild steel surface, by means of the electronegative, non-polar -SH edge of the organic planar part of the layer, is able to rotate, conferring to the bonding flexibility, as it facilitates ions mobility, which confers the most powerful chemical bonding to the metal (Figure 25). The complex interacts with the outer iron oxides and hydroxides, thus maintaining relatively electrical surface neutrality.