## 3 Results and Discussion  
### The Results of Weight Loss Tests  
Studies have shown that the greatest protective effect, due to the presence of a synergistic effect, is provided by mixtures of corrosion inhibitors [37; 38; 39]. The results of weight loss tests (inhibition efficiency) in the presence of BPE (100% banana peel), RSE (100% rice straw), and their mixture in different mixing ratios are shown in Figure 2. The results were obtained at a constant temperature of 35 \({}^{\circ}\)C in HCl. The figure demonstrates that the content of the extracts in the acid affects the control of the corrosion process. It should be noted that if the extracts used are not used in sufficient quantities, the corrosion rate will be high and the inhibition process will be useless. The concentrations of green corrosion inhibitors ranged from 250 to 1000 ppm. The figure shows that significant increases in efficiency occur with increasing concentrations of BPE, RSE, and their mixture in all mixing ratios up to approximately 750 ppm. At higher inhibitor dosages (above 750 ppm), the changes in efficiency were not significant. Thus, Figure 2 demonstrates that the optimal concentration of the extract is 750 ppm. Such an increase in the effectiveness of the studied extracts as green corrosion inhibitors is associated with the formation of a layer on the metal surface. Moreover, the highest inhibition efficiency was obtained when mixing 40% BPE and 60% RSE (according to experimental results). The inhibition efficiency of 100% BPE, 100% RSE, and 40:60 (BPE:RSE) at 750 ppm after 24 h was 77.87, 95.03, and 96.36%, respectively. It should be noted that the details of the data obtained by the weight loss method (mass reduction, corrosion rate, and inhibition efficiency) are shown in the Supplementary Materials in Table S1. case, the inhibition efficiency was determined at 35 \({}^{\circ}\)C. As can be seen from the graphs, the inhibition efficiency increases with increasing immersion time. Thus, over a longer period of time, a stronger protective layer of inhibitor against the corrosion process is formed. It should be noted that the corrosion process of steel samples in an HCl solution without an inhibitor or at low inhibitor concentrations occurs through anodic and cathodic reactions and leads to the formation of chloride products as follows:  
\[\text{Fe (s)}\rightarrow\text{Fe}^{2+}\text{ (aq)}+2\text{e}^{-}\] \[2\text{H}^{+}\text{ (aq)}+2\text{e}^{-}\rightarrow\text{ H}_{2}\text{ (g)}\] \[\text{Fe (s)}+2\text{HCl (aq)}\rightarrow\text{ Fe}^{2+}\text{ (aq)}+\text{H}_{2}\text{ (g)}+2\text{Cl}^{-}\text{ (aq)}\rightarrow\text{ FeCl}_{2}\text{ (aq)}+\text{H}_{2}\text{ (g)}\]  
Other side reactions also occur due to the presence of dissolved oxygen (1 ppm) and produce oxide products as follows:  
\[\text{O}_{2}+4\text{H}^{+}+4\text{e}^{-}\rightarrow\text{ 2H}_{2}\text{O (l)}\] \[2\text{Fe (s)}+\text{O}_{2}\text{ (g)}+2\text{H}_{2}\text{O (l)}\rightarrow\text{ 2 Fe}^{2+}\text{ (aq)}+4\text{OH}^{-}\text{ (aq)}\rightarrow\text{ 2 Fe(OH)}_{2}\text{ (s)}\] \[2\text{Fe(OH)}_{2}\text{ (s)}+0.5\text{O}_{2}\text{ (g)}+\text{H}_{2}\text{O (l)}\rightarrow\text{ 2 Fe(OH)}_{3}\text{ (s)}\] \[2\text{Fe(OH)}_{3}\text{ (s)}\rightarrow\text{ Fe}_{2}\text{O}_{3}\text{ (s)}+3\text{H}_{2}\text{O (l)}\]  
Figure 2: The dependence of corrosion inhibition efficiency on the mixing ratio of BPE and RSE after 6 h (a), 24 h (b), and 48 h (c) at various concentrations and at 35 \({}^{\circ}\)C in HCl solution using the weight loss method. At a sufficient concentration of the inhibitor, the above reactions do not occur due to the prevention of contact of the metal with the acidic environment and the inhibition of the formation of electrons. Figure 3 depicts the results of the synergistic inhibitory effect between BPE and RSE for corrosion control of a carbon steel sample using weight loss data. These results are related to the mixture of 40% BPE and 60% RSE as the best ratio based on the weight loss test results (Figure 2).