**4. Corrosion inhibitors**

A corrosion inhibitor is known as a chemical constituent that can diminish or prevent and control corrosion when added in small amount to the metal environment. Corrosion inhibitors are considered as the first line of defense against oil and chemical industry corrosion [27]. Corrosion inhibitors are sought after giving metals temporary protection during transportation and storage as well as localized protection to prevent corrosion that may have resulted from accumulation of small amounts of an aggressive phase. An effective corrosion inhibitor should be cost-effective, compatible with the corrosive medium, and produce desired effect when present in small concentrations [28]. Corrosion inhibitors act by (i) forming a film that is adsorbed on the metal surface, (ii) producing corrosion products, for example, iron sulfide (FeS) that acts as a passivator, and (iii) yielding precipitates that can eliminate or inactivate an aggressive constituent [29].

Depending on which electrochemical reactions are being blocked, these film-forming or interface inhibitors can be classified into anodic, cathodic, or mixed-type [28, 30]. Anodic inhibitors, alternately known as passivation inhibitors, suppress the rate of anodic reactions by producing sparingly soluble deposits, such as hydroxides, oxides, or salts in close to neutral conditions. On the other hand, cathodic inhibitors function by reducing the rate of cathodic or reduction reactions by producing a protective layer on cathodic areas against hydrogen in acidic conditions and oxygen in alkaline conditions. Mixed inhibitors influence both the anodic and cathodic reaction sites by forming an adsorptive film on the metal surface. About 80% of organic inhibitors fall into this category. Based on the chemical nature of the inhibitors, they can be divided into organic and inorganic [31]. Organic and inorganic inhibitors, based on their compositions and mechanism of actions, can be further classified into neutralizing, scavenging, barrier or filmforming, and other miscellaneous inhibitors [32].
