# Recent Trends and Progress in Corrosion Inhibitors and Electrochemical Evaluation
## 2 Mechanism of Corrosion Inhibition  
Chemical corrosion occurs when there is a non-conductive liquid where there is the absence of current and electron flow, and it also occurs in dry gases. Electrochemical corrosion takes place in a solution when the surface potentials of the metallic materials and the electrolytes are different. The metal oxidizes and transforms into an ion at the anode. The mechanism of CIs chemisorbed on the metal surface to produce a corrosion protection layer through the inhibition effect or the combination of inhibitor ions with the metallic surface is described below. The inhibitor causes the metal's surface protection layer to form an oxide layer. The inhibitor leads to a complex product when it reacts with a potentially corrosive component in an aqueous environment [30]. Figure 2 depicts the scheme of electrochemical corrosion.  
The cathode is the other component, where depolarization occurs predominantly through the reduction of hydrogen cations and oxygen molecules.  
\[\text{O}_{2}+4\;\text{H}^{+}+4\;\text{e}\rightarrow 2\;\text{H}_{2}\text{O}\]  
\[2\;\text{H}^{+}+2\;\text{e}\rightarrow\text{H}_{2}\]  
Figure 1: Corrosion prevention methods.  
The development of a thick corrosion layer on the iron substrate in the atmosphere is explained by [PERSON] and [PERSON] with cyclic electrochemical mechanisms. This cyclic mechanism involves oxidizing iron metal to produce iron (II) compounds that fill the corrosion layer pores. When the metal is enclosed by a thick water film, it is oxidized by dissolved oxygen into iron (II) species and into iron (III) oxyhydroxide, and the iron is also oxidized into Fe (II). The phenomena that take place within a thick corrosion layer (CL) have only been briefly electrochemically characterized in a few investigations. Using the carbon paste electrode (CPE) technique, [PERSON] et al. [31] investigated the metal's electrochemical behavior/CL system in the presence and absence of corrosion inhibitor as well as the impact of corrosion inhibitor on the electrochemical activity of various iron (III) oxyhydroxide phases of CL. The oil and gas sectors have found that using CI is an efficient way to reduce corrosion because the fuel gas comprises a variety of contaminants, including gases, such as nitrogen, oxygen, carbon dioxide, and water vapor. When water evaporates from a stream of fuel gas to form an aqueous phase, corrosive species, like H\({}^{+}\), H\({}_{2}\)CO\({}_{3}\), and HCO\({}_{3}\), will be obtained when CO\({}_{2}\) dissolves into the water and leads to corrosion in metals and metal alloys [32]. In the presence of O\({}_{2}\), CO\({}_{2}\) inhibitors exhibit a significant decrease in inhibition performance. According to [PERSON] et al. [33] in the presence of 100 ppb O\({}_{2}\) at 40 \({}^{\circ}\)C (1 bar CO\({}_{2}\), pH 5.5), the inhibitory efficacies of all imidazoline-based, quaternary ammonium-based, and amine-based processes were significantly reduced because the N atoms interact with oxygen and the inhibitor volatilizes from the metal surface. According to [PERSON] et al. [34], in a coexisting O\({}_{2}\) and CO\({}_{2}\) environment, the corrosion prevention efficacy of 8 \(\times\) 10\({}^{-5}\) mol L\({}^{-1}\) amides was only 80.3%, whereas it was 98.3% in a CO\({}_{2}\)-saturated solution. Although it is evident that the presence of O\({}_{2}\) can considerably reduce the ability of CO\({}_{2}\) CIs to inhibit, the processes underlying this effect are not entirely understood.