# Review on Corrosion Inhibitors for Oil and Gas Corrosion Issues
## 2 Corrosion
### Sources of Corrosion  
Mostly, corrosion is thought to occur in the water phase, except when the water is restricted in the middle of the stream or surrounded by oil, in which there will be no occurrence of corrosion. The major sources of corrosion in the oil industries are hydrochloric acid and its aqueous solutions, hydrogen sulfide, corrosion of steel at hydrocarbon-electrolyte interfaces and in emulsified two-phase environments, oxygen, naphthenic acids, carbon dioxide, as well as water cut. The sources and respective roles are described below:  
Hydrogen sulfide is known to be very harmful in the corrosion of metals or alloys, regardless of its application in oil and gas. In a matter of fact, hydrogen sulfide can be the root of sulfide stress corrosion cracking (SSCC) failure in pipelines. However, hydrogen sulfide is only corrosive when it is dissolved in water, where its solubility is relatively higher than that of carbon dioxide and oxygen. Moreover, hydrogen sulfide can cause danger in sour oil and gas fields due to its abundance in oil and production processes. The issues associated with the corrosion by the hydrogen sulfide acid are becoming more significant, as the sulfur availability in crude oil is growing proportionally with the reduction of existing sweet oil [28]. Iron sulfide and hydrogen are the products of the corrosion by sulfide. The hydrogen crack is created by the internal stresses that are caused by the molecular hydrogen. The outcomes of hydrogen cracking are unexpected and disastrous most of the time, since there are not any visible signs shown during the early phase [29]. Therefore, it is vital to select the optimal material for well completions, particularly in fields containing sour oil and gas, as the presence of hydrogen sulfide and tensile strength can possibly be the root of sulfide stress cracking (SSC) that might result in equipment loss. Chloride, which can be found in the mineralized water at the well bottom is known as another vital substance that causes severe corrosion at high temperatures. This can lead to failure that results from intergranular corrosion and chloride stress corrosion cracking (CSCC). Both the existence of sulfide and chloride have been detected in deep sour gas production that consists mainly of methane without liquid hydrocarbons under high pressure [30]. Normally, the corrosion that results from carbon dioxide is labelled as \"sweet corrosion\". The carbon dioxide that is produced along with oil and gas dissolves in water to procedure carbonic  
Figure 1: Electrochemical cell [27]. acid, ensuing a decrease in pH. Moreover, using an injection method for enhanced oil recovery can lead to further corrosion that is caused by CO\({}_{2}\). This will eventually lead to downhole and surface equipment corrosion. One of the characteristics of CO\({}_{2}\)-based corrosion is pitting, which is imaged as deep, sharp-edged pits. In the oil and gas industries, one of the most acidic surroundings found is the aqueous system that has higher concentrations of carbon dioxide. The partial pressure, temperature, speed, and pH are the examples of factors that contribute to corrosion in the existence of carbon dioxide [31; 32]. Oxygen  
Oxygen plays a vital role as the corrosive agent in the secondary recovery by water flooding. The corrosion caused by the dissolved oxygen causes pits in the drill pipe, where propagated fatigue cracks occur due to the stress. Additionally, oxygen can be detrimental to water injection equipment like pumps, piping, and water storage tanks, and the byproduct of corrosion might plug the formation. Figure 2 shows a comparison of the rusting rates of oxygen, hydrogen sulfide, and carbon dioxide on carbon steel in a water solution containing 2 to 5 g/L sodium chloride at the temperature of 25 \({}^{\circ}\)C. From this, it can be derived that oxygen ought to be the source for higher rusting rates at much lesser concentrations than carbon dioxide and hydrogen sulfide.