**11.6 Mechanism of corrosion inhibition**

The mechanism of the inhibition process of the corrosion inhibitors under consideration is mainly due to the adsorption. The phenomenon of adsorption is influenced by the nature and surface charge of the metal and by chemical structure of inhibitors. The surface charge of the metal is due to the electrical field which emerges at the interface on immersion in the electrolyte [62–64].

Inhibition usually results from one or more of the following mechanisms

• Adsorption of corrosion inhibitors onto metals

The inhibitive performance is usually depends on the fraction of the surface covered, θ with adsorbed inhibitor. But, at low surface coverage (θ < 0.1), the effectiveness of adsorbed inhibitor species in retarding the corrosion reactions may be greater than at high surface coverage.

• Presence of surface charge on the metal

Adsorption of inhibitor on the metal surface may be due to dipoles of the adsorbed species or electrostatic force of attraction between ionic charges and the electric charge on the metal at the metal/solution interface.

• Effect of functional group and structure

Usually, when the metal contains vacant electron orbitals of low energy such as transition metals. Inhibitors can also bond to metal surfaces by electron transfer to the metal to form a coordinate type of bond. Electron transfer from the adsorbed

**21**

*Corrosion Inhibitors*

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

tive efficiency in a series of related compounds.

• Inhibitor and water molecules interaction

• Interaction between adsorbed inhibitor species

reduced to sulfides which are more efficient inhibitors.

slopes of the polarization curves remain unaffected [65].

tors affects both anodic and cathodic reactions.

hyde derivatives and pyridinium salts [67].

• Adsorbed inhibitors reaction

• Diffusion barrier formation

• Blocking of reaction sites

• Electrode reactions

adjacent molecules leads to stronger adsorption at high coverage.

species is favored by the presence of relatively loosely bound electrons. Example: Anions and neutral organic molecules containing lone pair of electrons or electron systems associated with multiple bonds especially triple bonds or aromatic rings. The electron density at the functional group is directly proportional to the inhibi-

Adsorbed water molecule are removed from the metal surface due to displacement reaction of adsorbed inhibitor molecules and increases the size of hydrocarbon part of inhibitor, which leads to decreasing solubility and increasing adsorption ability. This is consistent with the increasing inhibitive efficiency observed at constant concentrations with increasing molecular size in a series of related compounds.

Lateral interactions between adsorbed inhibitor species may become significantly increases the surface coverage and the adsorbed species. These interactions either attractive or repulsive. If attractive interactions occur between molecules containing large hydrocarbon components (e.g., n-alkyl chains), may the chain length increases. Then the increasing Van der Waals attractive force between the

The adsorbed corrosion inhibitor may react usually by chemical or electrochemical reduction to form a product that may exhibit inhibitive action. A process of added small quantity of substance is called as primary inhibition and that due to the reaction product is secondary inhibition. In these cases, the inhibitive efficiency may increase or decrease with time, it depends on the extent of secondary inhibition is more effective than the primary inhibition. For example, sulfoxides can be

The absorbed inhibitor molecules may form a surface layer that acts as a physical barrier to the diffusion of ions or molecules and to or from the metal surface, and hence retard the rate of corrosion reactions. A surface film of these types of inhibi-

The blocking decreases the number of metal atoms at which corrosion reactions can occur. During this, mechanisms of the reactions are not affected, and the Tafel

Corrosion reactions involve the formation of adsorbed intermediate molecules with surface metal atoms. The adsorbed inhibitors will forbid the formation of these adsorbed intermediates, but the electrode processes may proceed by alternative paths through intermediates containing the inhibitor. In this process, the inhibitor act as catalyst and remain unchanged. Such reactions of inhibitor are characterized by an increase in the Tafel slope of the anodic dissolution of the metal. Inhibitors may also retard the rate of hydrogen evolution on the metals by affecting the mechanism of the reaction [66]. This effect has been observed on iron in the presence of inhibitors such as phenylthiourea, aniline derivatives, benzalde-

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

*Corrosion Inhibitors*

**Figure 15.**

*11.5.4.8 Green corrosion inhibitors*

*Schematic representation of volatile inhibitors.*

**11.6 Mechanism of corrosion inhibition**

be greater than at high surface coverage.

• Presence of surface charge on the metal

• Effect of functional group and structure

There is no clear and accepted definition of "environmentally friendly" or "green" corrosion inhibitors. In practice, corrosion inhibition studies have become oriented towards human health and safety considerations. For this purpose recently, the researchers have been focused on the use of eco-friendly compounds such as plant extracts, expired nontoxic medicines, etc. which contains many organic compounds [59–61]. Amino acids, alkaloids, pigments and tannins are used as green alternatives for the toxic and hazardous compounds. Due to biodegradability, eco-friendliness, low cost and easy availability and the extracts of some common plants and medicinal plant and its products have been studied as corrosion inhibitors for various metals and alloys under different environmental conditions.

The mechanism of the inhibition process of the corrosion inhibitors under consideration is mainly due to the adsorption. The phenomenon of adsorption is influenced by the nature and surface charge of the metal and by chemical structure of inhibitors. The surface charge of the metal is due to the electrical field which

Inhibition usually results from one or more of the following mechanisms

The inhibitive performance is usually depends on the fraction of the surface covered, θ with adsorbed inhibitor. But, at low surface coverage (θ < 0.1), the effectiveness of adsorbed inhibitor species in retarding the corrosion reactions may

Adsorption of inhibitor on the metal surface may be due to dipoles of the adsorbed species or electrostatic force of attraction between ionic charges and the

Usually, when the metal contains vacant electron orbitals of low energy such as transition metals. Inhibitors can also bond to metal surfaces by electron transfer to the metal to form a coordinate type of bond. Electron transfer from the adsorbed

emerges at the interface on immersion in the electrolyte [62–64].

• Adsorption of corrosion inhibitors onto metals

electric charge on the metal at the metal/solution interface.

**20**

species is favored by the presence of relatively loosely bound electrons. Example: Anions and neutral organic molecules containing lone pair of electrons or electron systems associated with multiple bonds especially triple bonds or aromatic rings. The electron density at the functional group is directly proportional to the inhibitive efficiency in a series of related compounds.
