## 3 Results and Discussion
### Green Corrosion Inhibitors Derived from Plant Extracts
Some of the most common phytochemicals that have a corrosion inhibiting effect are flavonoids, glycosides, alkaloids, saponins, phytosterols, tannins, anthraquinones, phenolic compounds, triterpenenes and flobatanins [43; 44; 45]. These functional groups can donate electrons to empty d orbitals of iron atoms to form coordinated bonds and promote the adsorption of antioxidant molecules on the metal surface [46]. The complex chemical compositions of these plant extracts make it very difficult to attribute the inhibitory action to a particular component or group of components. Several authors propose that this inhibitory activity may occur due to the presence of flavonoids [47; 48; 49; 50], which are especially important antioxidants due to their high redox potential, which allows them to act as reducing agents, hydrogen donors and singlet oxygen inhibitors. In addition, they have metal chelating potential. In this respect, several authors have suggested that their chelating activity is due to their binding to the metal ion (Mn+) at two sites in the molecule: on the orthodiphenolic group of one ring and on another ring of the molecule, forming mainly bidentate metal chelates [51; 52; 53]. Among the studies analyzed, some described the use of species: _Endostemon tereticaulis_[54], _Tribulus terrestres_[55], _Ficus tikoua_[56], _Bagassa guianensis_[57], _Artiplex leucclada_[58] and _Asparagus racemosus_[59]. [PERSON] et al. [56] used _Ficus tikoua_ leaf extract (FTLE) to inhibit corrosion of carbon steel in HCl (1 M). The leaves were ground into powder. For extraction and processing, the leaf powder was added to boiling water. The filtrate was condensed and lyophilized to obtain FTLE powder. The extract was characterized by using Fourier transform infrared spectroscopy (FTIR), and the results revealed that the extract contained heteroatoms in functional groups or aromatic rings, universal characteristics of a traditional inhibitor. Electrochemical experiments showed that FTLE acted as a mixed-type inhibitor with an inhibition efficiency of up to 95.8% at 298 K, at a concentration of 200 mg\(\cdot\)L\({}^{-1}\). Surface analysis was performed by using scanning electron microscopy (SEM), demonstrating that samples submerged in this environment corroded seriously without the use of inhibitors. Metal and the acid medium as the corrosion inhibitor becomes adsorbed on the carbon steel surface, which effectively suppresses the dissolution reaction. The FTLE adsorption on the carbon steel surface followed the Langmuir isotherm. [PERSON] et al. [20] evaluated the performance of _Ginkgo_ leaf extract (GLE) in inhibiting corrosion of X70 steel in HCl (1 M). _Ginkgo_ leaves were ground into powder and its extract was prepared by extraction with ethanol. The solution was concentrated in a rotary evaporator and then dried in an oven. The extract was characterized by FTIR, demonstrating that GLE contains oxygen and nitrogen atoms in functional groups and an aromatic ring, which corresponds to the general characteristics of traditional corrosion inhibitors. Electrochemical studies revealed an inhibition efficiency of over 90% in the presence of 200 mg\(\cdot\)L\({}^{-1}\) of GLE at all tested temperatures, acting as a mixed type inhibitor. The excellent inhibition capacity, which was attributed to the formation of films resulting from the adsorption of the inhibitor on the surface of X70 steel, was confirmed by using SEM and atomic force microscopy (AFM). The adsorption of GLE on the steel surface followed the Langmuir adsorption model. The second most used part of the plants were bark (9.55%), interesting data, because the fruit peels are mostly considered residues, without an economic application [60]. This is added to the fact that they are already inhibitors from an organic source. Among the analyzed studies, some described the use of shells: coconut [61], rice [62], green beans [63], pomergnanate [64] and watermelon [65].