## 3 Results and Discussion
### The Results of Analysis of Inhibitor Adsorption on Metal Surface  
It is generally accepted that the main stage of inhibitor action in acidic solutions is adsorption on the metal surface [12]. In other words, the action of inhibitors is mainly associated with a change in the state of the protected surface due to adsorption, which leads to a decrease in the active surface of the metal and an increase in the activation energy of corrosion, and is of an electrochemical nature [4]. Moreover, the basic information about the adsorption of inhibitor molecules on the metal surface can be obtained from the adsorption isotherm. There are several adsorption isotherms, such as Langmuir, Flory-Huggins, Temkin, Freundlich, and Frumkin, which explain the mechanism of corrosion inhibition. In this study, Temkin, Freundlich, and Langmuir models were used for adsorption analysis of inhibition data of banana peel, rice straw, and their mixture at three immersion periods. The average value of the coefficient of determination for the linear fitting of experimental data for Temkin, Freundlich, and Langmuir was 0.832, 0.891, and 0.955, respectively. Thus, the Langmuir model was selected for the adsorption analysis since it could better describe the adsorption of the used extracts as green corrosion inhibitors. Figure 11 shows the linear fit of C/\(\theta\) at different inhibitor concentrations and times using the Langmuir model. The figures show that the used model can fully describe the adsorption behavior of banana peel extract, rice straw extract, and their mixture on a metal surface. As can be seen from Figure 11, the slope of all graphs is close to unity, which indicates the high accuracy of the used adsorption isotherm for the studied inhibitors. The value of the adsorption constant (\(K_{ads}\)) is obtained from the reciprocal of the y-intercept of the graph line. This value was used in all cases of immersion time to determine the standard calculation of the adsorption energy. The adsorption constant value represents the degree of adsorption. The calculated values of adsorption free energy at times 6, 24, and 48 for all three inhibitors are given in Table 3 (at a constant temperature of 35 \({}^{\circ}\)C). In this table, the values of \(K_{ads}\) in molar were obtained by dividing \(K_{ads}\) in ppm by 1000 \(\times\) MW (molecular weight). According to the literature [25; 30], gallocatechin (306 g/mol) and glycosides (584 g/mol) were the main components for banana peel and rice straw, respectively. For the mixture, the average molecular weight of gallocatechin and glycosides (472.8 = 0.4 \(\times\) 306 + 0.6 \(\times\) 584) was used. A negative value of the Gibbs free energy of adsorption indicates the spontaneity of the process, and the extract molecules are adsorbed on the metal. Energy values less than 20 kJ-mol\({}^{-1}\) indicate physical adsorption of the inhibitor molecule on the metal surface. In addition, the free energy of adsorption of more  
than 40 kJ-mol\({}^{-1}\) indicates chemical adsorption and the establishment of coordination interactions between the inhibitor compounds and iron atoms [42]. Therefore, the molecular structure of the inhibitors is of particular importance. In this study, the results show that the Gibbs free energy values range from \(-36.7\) to \(-38.4\) kJ/mol\({}^{-1}\), indicating that both types of adsorption occurred.