# Recent Trends and Progress in Corrosion Inhibitors and Electrochemical Evaluation
## 3 Types of Corrosion Inhibitors
### Organic Corrosion Inhibitors  
Organic corrosion inhibitors (OCIs) are commonly employed in industry due to their efficiency over a wide temperature range, compatibility with protected materials, relatively low toxicity, and superior solubility. These substances have cathodic and anodic inhibitory effects. Cathodic corrosion inhibitors reduce the corrosion potential and impede or postpone the reactions that take place at the cathode (oxygen reduction and hydrogen evolution). Anode corrosion inhibitors work with the metal cation to form an insoluble hydroxide, obstruct the active sites on the metal surface, and shift the corrosion potential toward positive values. This reduces the rate of corrosion by stopping the metal's further oxidation process (dissolution) [40]. By moving water molecules away from the metal surface and forming a pressurized barrier, the bulk of organic inhibitors are absorbed onto the metal surface. The transit of the electron from the inhibitor to the metal is facilitated by the presence of p electrons and non-bonding electrons (a single pair) in the inhibitor molecule [41]. Heteroatoms with lone electron pairs are necessary for efficient organic corrosion inhibitors, such as those found in nitrogen, oxygen, sulfur, and phosphorus, as well as moiety with electrons (aromatic rings and multiple bonds), which can interact with free orbital d metal and facilitate the adsorption process [42; 43]. The annual corrosion rate employing an organic inhibitor is shown in Figure 4 as a function of time. Some inorganic inhibitors are only partially soluble in some conditions. As a result, protection may be less effective if the inhibitor is less soluble, and it may be distributed unevenly. The number and location of inhibitor containers, as well as the coating matrix, have been presented as factors influencing anticorrosion efficiency [44]. OCIs will protect the metal by forming a hydrophobic layer on its surface. The quality of an OCI is determined by its molecular structure (electronic structure), nature of metal surface affinities, electrolyte, and polar substituents, and chemical composition. However, this type of organic barrier protection is susceptible to damage from mechanical impacts that create holes during application, exposing the metal substrate to the environment. Additionally, when water and hostile species penetrate into the coating and make their way to the metal substrate, they can cause corrosion products to occur which can lead to the coating delamination or blistering and the material losing its barrier protection [45]. In contrast, metal surfaces can be guarded by using hybrid or organic corrosion inhibitors. In particular, functional groups on amines with an aliphatic tail are known to create an adsorption-derived barrier that prevents the entry of species that are corrosive, such as active anions or water molecules. It  
Figure 3: Applications of corrosion inhibitors in various areas. is very desirable for the inhibitor to have a longer release because it can provide long-term resistance and increase its viability for practical applications. Heterocyclic compounds are found to be good CIs because of their high inhibition effectiveness, cost-effectiveness, and the possibility of recyclization. Due to the electron-withdrawing nature of the -CN and NO\({}_{2}\) substituents in OCIs, it was anticipated that the efficiency would decrease due to the +R (the effect is referred to as the +R effect if the replacement group contains a lone pair of electrons to donate to the associated pi bond or conjugate system of pi bonds) and -I (the -I effect, also known as the negative inductive effect, happens when the substituent pulls electrons out) effects. Note that the highest occupied molecular orbital (HOMO) is associated with a metal inhibitor, whereas the lowest occupied molecular orbital (LUMO) is associated with a metal to restrict electron transfer. Organic molecules interact with metallic surfaces via donor-acceptor interactions.