## 4 Conclusions  
In this work, banana peel extract, rice straw extract, and their mixture were used as green corrosion inhibitors for carbon steel in the acidic environment. The following findings were drawn:  
1. The inhibition efficiency of the mixture at a ratio of 40:60 (banana peel to rice straw) was higher than that of the individual extracts at all concentrations and immersion times based on the weight loss method.
2. The inhibition efficiency was increased by increasing the concentration of the green corrosion inhibitors. Weight loss and electrochemical experiments showed that the optimum concentration of individual inhibitors and their mixture was 750 ppm, and the performance did not change significantly with further increase of the extract content in the HCl solution. Moreover, increasing the immersion time increased the corrosion inhibition efficiency, but this effect was weaker than the effect of inhibitor concentration.
3. For all immersion times and inhibitor concentrations, a positive synergistic effect was observed at a mixing ratio of banana peel and rice straw of 40:60. The highest synergistic value was 1.65, indicating an average increase of 65% in inhibition efficiency for the mixture compared to the extract alone.
4. The electrochemical tests confirmed the higher efficiency of the mixture (40:60 banana peel and rice straw) compared to 100% banana peel and 100% rice straw. When extracts were added to the corrosive environment, the resistance of the solution increased, and it became more difficult for ions to reach the metal surface, which led to an increase in the effectiveness of corrosion inhibition.
5. Scanning electron microscope images clearly showed the formation of a stable film on the metal surface. The film formed in the presence of the banana peel and rice straw mixture was more stable and uniform than that in the presence of the individual extract.
6. The Langmuir isotherm model most accurately describes the adsorption characteristics of the used extracts and their mixture. Both physical and chemical adsorption occurred for the banana peel and rice straw extracts, as well as for their mixture.  
The following supporting information can be downloaded at: [[https://www.mdpi.com/article/10.3390/app15095026/s1](https://www.mdpi.com/article/10.3390/app15095026/s1)]([https://www.mdpi.com/article/10.3390/app15095026/s1](https://www.mdpi.com/article/10.3390/app15095026/s1)), Table S1. The details of data obtained by weight loss method (mass reduction, corrosion rate and inhibition efficiency).  
Methodology, A.K.; Validation, A.T.; Formal analysis, A.T.; Investigation, M.D.; Writing--original draft, A.K.; Writing--review & editing, A.K.; Supervision, A.K.; Project administration, A.K. All authors have read and agreed to the published version of the manuscript.  
This work was supported by Gonbad Kavous University (master's thesis of [PERSON]).  
The original contributions presented in this study are included in the article and Supplementary Materials. Further inquiries can be directed to the corresponding author.  
Figure 13: Structure of gallocatechin and glycoside molecules that were identified as major components of banana peel and rice straw extracts, respectively.