• Adsorbed inhibitors reaction

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 reduced to sulfides which are more efficient inhibitors.

#### • Diffusion barrier formation

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 inhibitors affects both anodic and cathodic reactions.

#### • Blocking of reaction sites

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 slopes of the polarization curves remain unaffected [65].

### • Electrode reactions

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, benzaldehyde derivatives and pyridinium salts [67].

• Electrical double layer alteration

The adsorption of ions or species that can form ions on metal surfaces will change the electrical double layer at the metal/solution interface, and this will affect the rates of the electrochemical reactions [68]. The adsorption of cations such as quaternary ammonium ions and protonated amines makes the potential more positive in the plane of the closest approach to the metal ions from the solution. This positive potential shift hinders the discharge of the positively charged hydrogen ions. These effects have been observed with sulfosalicylate ions and the benzoate ions [69, 70].
