## 3 Results and Discussion
### Green Corrosion Inhibitors Derived from Plant Extracts
[PERSON], [PERSON] and [PERSON] [66] evaluated the corrosion inhibition of carbon steel in H\({}_{2}\)SO\({}_{4}\) solution (0.5 M) by lychee (_Litchi chinensis_) peel extract. The peels were ground into powder. For extraction and processing, the peel powder was added to water. The filtrate was evaporated and condensed in a water bath and dried in a desiccator to obtain a solid extract. The results show that the lychee bark extract acted as a mixed type inhibitor with an inhibition efficiency of 95.7% at 298 K at a concentration of 3.0 g\(\cdot\)L\({}^{-1}\); this behavior was confirmed by mass loss method and electrochemical tests. The corrosion inhibition is due to the adsorption of the extract on the metal surface, which is in accordance with the Langmuir adsorption isotherm. UV-Vis and FTIR spectroscopy studies demonstrated the presence of functional groups such as OH- and C=O, scanning electron microscopy (SEM) revealed that the metal surface showed roughness on the surface of the iron that was exposed only to the acid solution while there was a smooth surface on the surface of the metal that was exposed to acid solution with the extract, confirming that corrosion inhibition of steel occurs through adsorption of inhibitor molecules. [PERSON], [PERSON] and [PERSON] [36] investigated the aqueous extracts of mango (CME) and orange (COE) peels as inhibitors of corrosion on carbon steel in HCl solution (1 M). The electrochemical tests showed the action of the extracts as inhibitors of mixed type, with maximum efficiencies of 96% (CME) and 91% (COE), at a concentration of 600 mg\(\cdot\)L\({}^{-1}\). The complex chemical compositions of these extracts made it very difficult to attribute the inhibitory action to a particular constituent or group of constituents, but other studies point to these peels as rich sources of antioxidant compounds, such as polyphenols, carotenoids and vitamins C and E [47; 67]. Phenolic compounds, particularly flavonoids, have been shown to possess significant antioxidant activity, which is mainly based on their structural characteristics (number and position of phenolic hydroxyls, other groups, conjugation) [47]. Thus, the inhibitory effect was promoted through the adsorption of the compounds present in the fruit peel extracts on the steel surface. Regarding the type of metal to be protected against corrosion, carbon steel appears in 83.33% of the researches (Figure 6b). Being the most used steel in the world (in construction, chemical and oil industries, means of transport and communication, among others), its world production in 2020 was estimated at 1864.0 million tons [3]. This fact is due to its versatility and peculiar mechanical properties (promoted by the addition of carbon to the metal alloy), although the technology of these steels is well developed, representing an economic choice for many applications. However, these materials have some disadvantages, such as poor generalized corrosion performance, oxidation at low temperatures and rapid formation of FeO (or high oxides) at high temperatures [68]. Regarding the corrosive medium tested, 79.54% of the studies used acidic medium (Figure 6c), with more than 85% using hydrochloric acid (HCl). This result is justified since acidic solutions are used for various routine procedures in industries (chemical cleaning, decalcification and pickling), a fact that promotes corrosive acid attack [69]. Specifically, the use of HCl that is associated with the great damage that the oil industry suffers from corrosion. These costs are usually related to corrosive fouling, mud and dust, which cause failures in pipes and other industrial plant structures by metal corrosion. Moreover, the cleaning of these pipes is performed with the injection of HCl 15-28% solution to clear channels near the lumen, in the formation and increase in the oil flow [70; 71; 72]. Regarding the activity of the new inhibitors, most acted as mixed type (Figure 6d).