# Review on Corrosion Inhibitors for Oil and Gas Corrosion Issues
## 4 Green-Based Corrosion Inhibitors  
Green-based inhibitors that are nontoxic in nature, such as plant extracts, have higher demand compared to commercial inhibitors [57; 58; 59]. This is because plant extracts are observed to be green and sustainable materials due to their natural and biological properties and will be able to inhibit the metals and alloys from corroding [60]. The leaf, out of all parts of the plant, has the utmost preference for its abundance of phytochemicals (active components) produced through synthesis, that act similarly to commercial inhibitors. It is also vital to acknowledge that the extract of other parts of a plant such as root, bark, flower, fruit, wood, seed and peel have contributed to the inhibition efficiency [61; 62; 63]. Furthermore, the phytochemical synthesis uses carbon dioxide, which is known as the highly poisonous greenhouse gas, to undergo the photosynthesis, contributing to the green chemistry theory as well. The green corrosion inhibitors function when used in a very low concentration to inhibit the metal surface from a corrosive medium. The rate of corrosion by adsorption process on the metal surface is affected by plant extracts via influencing either the anodic or cathodic reaction kinetics and then affecting the rate of diffusion of aggressive ions from interacting with the metal surface. Consecutively, a layer of film can be established by increasing the electrical resistance of the metal surface [64]. Besides, green corrosion inhibitors are well known for their adsorptive properties (site-blocking elements), enabling the active molecules from the plant extract to adsorb on exposed metal surfaces [65]. Equation (2) shows the working mechanism of the inhibitor molecules in the form of neutral molecules adsorbing on the metal surface instead of the hydrogen ions [66]:  
\[\mathrm{Inhibitor}+\mathrm{nH_{ads}}\rightarrow\mathrm{Inhibitor_{ads}}+ \mathrm{H_{2}} \tag{2}\]  
where \(\mathrm{nH_{ads}}\) is adsorbed hydrogen ions sourced from water and \(\mathrm{Inhibitor_{ads}}\) is adsorbed neutral molecules sourced from plant extract. In conjunction with this, there are a few limitations that must be examined during the preparation of plant extracts. Normally, the solvents used for extraction will diffuse into plant tissue, solubilize, and extract the available phytochemicals [67; 68]. Hence, it is essential to select the right solvent for better results. One of the solvents that is readily available, cheap, and safe is water [69; 70], yet ethanol and methanol are still in demand for selective plant extracts [71; 72]. Next, the temperature creates a noticeable result when it comes to extract preparation. The solubility of the phytochemicals will be hindered at relatively low temperature, whereas at high temperatures, the phytochemicals result in decomposition. The recommended temperature for an ideal extraction falls between the range of 60-80 \({}^{\circ}\)C [73; 74]. As for the drying temperature, oven drying is advisable, since drying at room temperature can take up to months to accomplish. Green-based corrosion inhibitors can be divided into two classes: organic and inorganic [75; 76; 77]. The organic class of green-based corrosion inhibitors consist of synthetic substances that are nontoxic for the environment. Flavonoids, alkaloids, and byproducts of plants are example of organic inhibitors [78]. The inorganic class of inhibitors are vastly utilized in aqueous systems due to their high productivity [79]. The chromates exhibit a toxic nature in which the employment of this inorganic inhibitor for industrial use is limited. Concerning this issue, lanthanide salts were studied as an eco-friendly inhibitor substitute [80]. Correspondingly, the fruit wastes, e.g., seeds and peels, have gained noticeable attention for their natural antioxidant properties.