**11.4 Role of inhibitors**

*Corrosion Inhibitors*

and velocity).

drastically.

**11.2 Definition of inhibition**

**11.3 Corrosion inhibitors**

metal [12].

cooling system must satisfy the following criteria

In order to avoid or reduce the corrosion of metallic materials, inhibitors used in

• It must give good corrosion protection at a very low concentration of inhibitor.

• It must remain efficient in extreme operating conditions (higher temperature

• In case of an under or over dosage of inhibitor, corrosion rate should not increase

• The inhibitor or reaction products of the inhibitor should not form any deposits on the metal surface particularly at locations where heat transfer takes place.

One of the extensively studied topics in the field of corrosion is inhibition. Inhibition is a process of preventive measure against corrosive attack on metallic materials. Chemical compounds may be used which, when added in small concentrations to an aggressive environment, are able to decrease corrosion of the exposed

Corrosion inhibitors can be any forms (solids, liquids and gases). Based on the solubility or dispersibility in fluids corrosion inhibitors are selected which are to be inhibited. Corrosion inhibitors have been found to be effective and flexible means of corrosion mitigation. The use of chemical inhibitors to decrease the rate of corrosion processes is quite variable. Corrosion inhibitors are used in oil and gas exploration and production, petroleum refineries, chemical manufacturing, heavy manufacturing, water treatment and product additive

• It must protect all exposed materials from the attack of corrosion.

• It should suppress both uniform and localized corrosion.

• It should not cause toxicity and pollution problems.

• It should have long range effectiveness.

**14**

**Figure 12.**

*Schematic representation of inhibition process.*

Organic and inorganic compounds constitute a large class of corrosion inhibitors, which as a general rule; affect the entire surface of a corroding metal when present in sufficient concentration. Most of the organic/inorganic compounds containing elements of groups V B, VI B or functional groups of the type amine, carbonyl and alcoholic groups are more effective corrosion inhibitors. The inhibitor is adsorbed on the entire metal surface of the corroding metal and so prevents attack from the corrosion. Organic inhibitors are adsorbed on the metals surface [32]. The inhibitors may be considered as two fundamental types, they are, those which form a protective barrier film on anodes or cathodes by reaction between the metal and the environment. This type of inhibitors function in neutral or in some cases, alkaline solution in which the main cathodic reaction is an oxygen reduction reaction in which the corroding metal surface is covered by a film oxide or hydroxide.

Another type is initially adsorbed directly onto the metal surface by interaction between surface charges and ionic and/or molecular dipole charges. This division of inhibitor types results principally from the pH of the solution where they operate. Inhibitors must be present in a minimum concentration for them to be fully effective. This is very common with anodic inhibitors.

The efficiency of organic inhibitors can be improved in the presence of certain halogen ions. Halogen ions are also known to inhibit corrosion to some extent in acid solutions. The efficiency of the corrosion inhibition is in the order; I- > Br- > Cl-. Fluoride does not show inhibition characteristics. Synergism of halogen ions can be attributed to the fact that the metal adsorbs halogen ions whose charge shifts the surface in a negative direction, thereby increasing adsorption of the cationic organic inhibitor. Being able to discover possible compounds that can be used as corrosion inhibitors requires a lot of hard work, innovation and laboratory analysis/synthesis [33].

Certain halogen ions present in the organic inhibitors are known to inhibit corrosion to some extent in acid solutions. The efficiency of the corrosion inhibition is following in the order of I<sup>−</sup> > Br<sup>−</sup> > Cl<sup>−</sup>. Fluoride does not shows any inhibition characteristics. Synergism of halogen ions can be attributed to the fact that the metal adsorbs halogen ions whose charge shifts the surface in a negative direction, thereby increasing adsorption of the cationic organic inhibitor.
