## 3 Results and Discussion
### Electrochemical Studies
Figure 15: The evolution of open circuit potential (OCP) versus time in case of the mild steel panels after submission in WSS for 23 h: (**filled square**) _Steel–WSS–blank_, (**filled circle**) _Steel–WSS–Na\({}_{2}\)HPO\({}_{4}\)_, (**filled triangle**) _Steel–WSS–MBT_, (**filled triangle**) _Steel–WSS–MBT_, (**filled triangle**) _Steel–WSS–Na\({}_{2}\)HPO\({}_{4}\)–MBT_. The polarization curves recorded for mild steel panels in the presence or absence of inhibitors after subjection in WSS for 24 h are demonstrated in Figure 17. Considering the cathodic branches of the diagram (Figure 17a), it can be seen that the presence of MBT into the solution (_Steel-WSS-MBT_) results to the appearance of lower cathodic current densities compared to the _Steel-WSS-blank_, indicating a possible reduction of the cathodic reactions. On the other hand, according to the same diagram, there is no evidence that the other two systems (_Steel-WSS-Na2 HPO4_, _Steel-WSS-Na2 HPO4-MBT_) affect the cathodic reactions. Taking into account the anodic branches of the polarization curves (Figure 17b), it may be remarked that the addition of Na2 HPO4 into the commensurate solution retards the anodic reactions effectively, since the corresponding anodic current densities are decreased compared to those of the _Steel-WSS-blank_. On the contrary, there is no indication that the _Steel-WSS-MBT_ and _Steel-WSS-Na2 HPO4-MBT_ systems influence the anodic reactions. Furthermore, studying the cathodic branches of the polarization curves recorded for mild steel panels in the presence or absence of inhibitors after subjection in 3.5 wt % NaCl for 24 h, it is mentioned that the addition of Na2 HPO4 either alone or in combination with MBT affected the cathodic reactions by lowering the corresponding cathodic current densities (Figure 18a). Therefore, there is an indication of a possible reduction of the cathodic reactions for both the _Steel-NaCl-Na2 HPO4_, and _Steel-NaCl-Na2 HPO4-MBT_ systems. Considering the anodic branches, it could be documented that the introduction of the inhibitors suppresses the anodic reactions because of the reduction of the correlative anodic corrosion current densities (Figure 18b). This outcome can be attributed to the formation of corresponding protective layers. Taking into account the Pourbaix diagram of iron [79], it may be remarked that in all the PP characterizations, reductions of both water and oxygen take place because the PP measurements were conducted in a pH range from 7.33 to 7.97 (Table 2) as well as the cathodic and anodic branches of polarization curves being recorded in a potential range between \(-\)1.6 V and 0 V versus Ag/AgCl,KCl(sat) electrode. Furthermore, in the aforementioned ranges of pH and potential, dissolution of iron exists but, be that as it may, the oxidation of iron does not involve the oxidation of water. In addition, it should be taken into consideration that the PP characterization technique was used only for qualitative interpretations, because the calculation of both the corrosion current (\(i_{\text{corr}}\)) and \(R_{\text{P}}\) values via the linear fitting of the polarization curves according to the Tafel method is not feasible, due to the existence of two reduction processes in the system: the reduction of water and the reduction of oxygen [80]. Moreover, the corrosion current density cannot be determined from the anodic branches, as there is no clear linear region. Finally, under the aforementioned experimental conditions, steel is also corroded via localized corrosion with the formation of pits [81; 82]. Therefore, the Tafel method is not appropriate  
Figure 17: The polarization curves (**a**) cathodic, (**b**) anodic, recorded for mild steel panels after submission in WSS for 24 h: (**filled square**) _Steel–WSS–blank_, (**filled circle**) _Steel–WSS–Na2 HPO4_, (**filled triangle**) _Steel–WSS–MBT_, (**filled triangle**) _Steel–WSS–Na2 HPO4–MBT_. to be used for localized corrosion, such as pitting corrosion, as it only yields an average uniform corrosion rate [83].