**2. Materials and processes**

These coating materials are applied in various sectors to prevent corrosion and to retain the integrity of the metal performance. Generally two main strategies have been considered to achieve the required functionalities into coatings: (a) encapsulation or loading of the active species in host carriers and (b) manipulation in composition of the coating matrix to include bulk or surface functional groups. The former one involves the intermixing of carriers of active agents (polymeric capsules, nanotubes, mesoporous inorganic particles, clays, etc.) along with the functional species (cerium ions, benzotriazole, nitrates, silyl esters, etc.) with the coating matrix. These carriers act as the reservoirs that store the active material and release at the time of requirement. It is necessary to maintain the compatibility and to check the long-term stability of the carrier material with the coating matrix. A number of carriers, functional species, and coating matrix have been summarized in **Table 1**.

In addition to the above-discussed processes, an alternative route is also explored and demonstrated successfully for corrosion protection. It constitutes the functionalization of the coating surface or bulk matrix by manipulating its molecular structure and composition [11–16]. Out of others, the process involving the manipulation of cross linking/molecular structure of the polymeric matrixes by integrating different additives (like cyclic carbonates, amines, and siloxanes) is considered as the greener route to surface functionalization. As a result a thin and dense cost-effective coating is formed showing improved protective action with better durability. Some examples on the coating materials based on surface modification is presented in **Table 2**.


#### **Table 1.**

*Examples of different active agents, carriers, and coating matrixes used for coatings to protect the metallic surface from corrosion.*


#### **Table 2.**

*Examples of different active agents used in manipulation of the coating matrix to make effective coatings for corrosion protection.*
