## 3 Results and Discussion
### The Results of EIS Tests  
The inhibition efficiency of the extracts was analyzed by electrochemical impedance spectroscopy. The Nyquist plots obtained by EIS are presented in Figures 4-6. The figures show the results of the EIS test after 24 h of immersion in the solution. The semicircles in the Nyquist diagram are due to the charge transfer reaction that occurs on the metal surface near the electrolyte. The simplest possible case is when the sample surface is not coated. According to the results, all the figures are semicircular, which indicates non-ideal capacitive behavior at the metal-solution interface. The inhibitor molecules must repel the solution molecules, which creates resistance in the solution, and then the inhibitor molecules must overcome a large barrier, which is the electrical double layer that occurs when the sample is immersed in the solution [1]. As can be seen from the figures, in the presence of the inhibitor, the charge transfer resistance increased, and, as a result, the diameter of the semicircle in the Nyquist diagram increased. Analysis of these results may indicate the inhibitory effect of the compound used on the corrosion rate of the steel surface. When an inhibitor is added to the environment, the resistance of the solution increases, meaning that when a corrosion inhibitor is added, it becomes more difficult for ions to reach the metal surface, which reduces the corrosion rate [4]. Figure 3: Quantitative values of the synergistic inhibitory effect of the mixture of BPE (40%) and RSE (60%) at 35 \({}^{\circ}\)C using the weight loss method data. Figure 4: Nyquist diagram of EIS tests in a blank case (0 ppm) and in the presence of BPE (**a**) and RSE (**b**) at various concentrations. Figure 5: Nyquist diagram of EIS tests in a blank case (0 ppm) and in the presence of the mixture of BPE and RSE at various mixing ratios at 750 ppm. Figure 6: Nyquist diagram of EIS tests in a blank case (0 ppm) and in the presence of green inhibitors (BPE, RSE, and the mixture of 40:60 BPE:RSE) at 750 ppm. If the resistance of the solution is high, in other words, the concentration of inhibitors is low and their passage through the solution molecules is difficult, then the controlling factor of the corrosion process is concentration polarization [40]. If the inhibitor concentration is high and the inhibitor molecules easily reach the sample surface, the molecules have to cross the electrical double layer, which means that the reaction on the metal surface is difficult. In this case, the corrosion process is controlled by polarization due to charge transfer. Therefore, it can be concluded that the corrosion process is essentially controlled by charge transfer. Figures 4 and 5 demonstrate that the system impedance changes dramatically with increasing extracts of BPE, RSE, and their mixture in comparison with a blank case. As the inhibitor concentration (C) increases, the capacity of the double electric layer decreases, which is due to the increase in adsorbed molecules with increasing inhibitor concentration. In this case, by decreasing the capacitance of the electrical double layer, fewer electrons are produced at the anode, and, as a result, fewer electrons are consumed at the cathode. As the inhibitor concentration increases, the surface coverage increases, and hence the effectiveness of the corrosion inhibitor increases [41]. Moreover, Figure 4 depicts that the diameter of the semicircle in the presence of BPE and RSE at 750 and 1000 ppm is almost the same, and no significant increase in diameter is observed with increasing concentration from 750 to 1000 ppm. Thus, the concentration of 750 ppm is the optimum value for both inhibitors. In addition, Figure 5 shows that the mixing ratio of 40% BPE and 60% RSE is the best value for the highest inhibition efficiency by the EIS test. This result is consistent with the value of the weight loss method. Figure 6 shows the Nyquist plot of BPE, RSE, and their mixture (40:60 BPE:RSE) at 750 ppm after 24 h of immersion.