**11.3 Corrosion inhibitors**

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

**15**

*Corrosion Inhibitors*

inhibitors [30, 31].

hydroxide.

**11.4 Role of inhibitors**

*DOI: http://dx.doi.org/10.5772/intechopen.80542*

tive. This is very common with anodic inhibitors.

tory analysis/synthesis [33].

*11.5.1 Based on electrode process*

*11.5.1.1 Anodic inhibitors*

industries [29]. In the oil extraction, processing and chemical industries, corrosion inhibitors have always been considered to be the first line of defense. A great number of scientific studies have been devoted to the subject of corrosion

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

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 effec-

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 labora-

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,

An anodic inhibitor increases the anodic polarization and hence moves the corrosion potential to the cathodic direction and hence also called as passivating inhibitors. Anodic inhibitors such as chromates, phosphates, tungstates and other ions of transition elements with high oxygen content are those that stifle the

thereby increasing adsorption of the cationic organic inhibitor.

Corrosion inhibitors are briefly classified **(Figure 13**) as follows,

**11.5 Classification of corrosion inhibitors**

**Figure 12.** *Schematic representation of inhibition process.*

industries [29]. In the oil extraction, processing and chemical industries, corrosion inhibitors have always been considered to be the first line of defense. A great number of scientific studies have been devoted to the subject of corrosion inhibitors [30, 31].
