## 2 Material and Methods
### Weight Loss Method  
The weight loss method is one of the most common and effective methods for determining the corrosion rate of metals under laboratory conditions and determining the inhibitory efficiency of corrosion inhibitors under static conditions. In this case, the metal being tested is determined by the difference in mass loss in the solution with and without the addition of the inhibitor [32]. All samples were preliminarily degreased and weighed on high-precision analytical scales. The experiments were conducted in a hydrochloric acid solution and in the presence of corrosion inhibitor additives at different concentrations (0-blank, 250, 500, 750, and 1000 ppm). The tests were carried out in three immersion periods: 6, 24, and 48 h at a constant temperature of 35 \({}^{\circ}\)C. In this study, 100% banana peel extract (BPE), 100% rice straw extract (RSE), 50:50 BPE:RSE, 40:60 BPE:RSE, and 60:40 BPE:RSE were used in weight loss tests. After completion of each test, the mass of the steel specimens was measured again, and the mass reduction during immersion was determined. Three parallel measurements were performed at each inhibitor concentration, and the average mass loss values were used to evaluate the inhibition efficiency. The corrosion rate and inhibition efficiency of the green inhibitors were determined using the following relationships [33]:  
\[CR=\frac{8760\ \Delta m}{A\rho t} \tag{1}\]  
\[Inhibition\ efficiency\ (IE)=\frac{CR_{0}-CR_{1}}{CR_{0}}\times 100 \tag{2}\]  
Figure 1: The flowchart of conducted experiments for evaluation of inhibition performance of extracts.  
where \(CR\) is the corrosion rate of the carbon steel samples in the studied corrosive medium (mm\(\cdot\)y\({}^{-1}\)); 8760 is a constant obtained by converting an hour to a year (7860 = 24 \(\times\) 365); \(\Delta m\) is the mass reduction of the metal samples after test completion (mg); \(A\) is the surface area of the metal samples (mm\({}^{2}\)); \(\rho\) is the density of the examined steel samples (kg\(\cdot\)m\({}^{-3}\)); \(t\) is the time (h); and \(CR_{0}\) and \(CR_{1}\) are the corrosion rates without and with the addition of the extracts as green corrosion inhibitors to the solution (mm\(\cdot\)y\({}^{-1}\)).  
The values of the synergistic inhibitory effect of the mixture of BPE (40%) and RSE (60%) at 35 \({}^{\circ}\)C were determined. For this purpose, the inhibitory efficiency values of the mixture at different concentrations (250, 500, 750, 1000 ppm) were used for test durations of 6, 24, and 48 h. The following formula was used to quantify the synergistic inhibition effect [34]:  
\[S\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\! \!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!\!  
times. Surface coverage values were used to test how they fit different isotherm models. The obtained data showed the best agreement with the Langmuir adsorption model. In the Langmuir adsorption isotherm, the adsorbed layers do not interact with each other, and the surface is uniform and homogeneous. The Langmuir adsorption isotherm is expressed by the following relationship [34]:  
\[\frac{C}{\theta}=\frac{1}{K_{ads}}+C \tag{4}\]  
where \(C\) is the molar concentration of green corrosion inhibitor (M), \(\theta\) is the surface coverage value (IE/100), and \(K_{ads}\) is the adsorption constant. By plotting C/\(\theta\) versus C and obtaining the y-intercept, the adsorption constant is determined. The \(K_{ads}\) values were used to determine the Gibbs free energy using the following relationship [36]:  
\[\Delta G_{ads}=-RTln(55.5K_{ads}) \tag{5}\]  
where \(\Delta G_{ads}\) is the Gibbs free energy, which is used for evaluating the extract adsorption on the metal surface (kJ\(\cdot\)mol\({}^{-1}\)); \(R\) is the gas constant in SI units; and \(T\) is the absolute temperature (308.15 K in this study). The Gibbs free energy values were determined for 100% BPE, 100% RSE, and their mixture (40:60 BPE:RSE) for immersion durations of 6, 24, and 48 h.