*2.4.1.4 Chemical metallic coating*

Chemical metallic coating is a coating technique where the protective coat is chemically bounded with the material to be protected. There are two principal techniques for chemical coat materials. The nonelectric plating and hot-dip galvanization (**Figure 14**).

**Figure 14.** *Chemical metallic coating.*

#### *2.4.1.5 Nonelectric plating*

Nonelectric or electroless plating is the process where the metal is deposited on the material *via* a chemical reaction in the presence of a catalyst. The most common nonelectric plating is electroless nickel plating. Electroless nickel plating is generally achieved by depositing the nickel ions with sodium hypophosphite as the reducing agent to oxidize and form a negative charge on the metal surface for the deposition of nickel ions. In this process, the sodium hypophosphite will release the hydrogen as a hydride ion. The overall reaction of electroless nickel plating is as follows.

$$\text{NiLx}^{(2+)} + 4\text{H}\_2\text{PO}\_2^{(-)} + \text{H}\_2\text{O} \rightarrow \text{Ni} + \text{P} + \text{3H}\_2\text{PO}\_3^{(-)} + \text{H}^{(+)} + \frac{\text{3}}{2}\text{H}\_2\text{\uparrow} + \text{xL} \tag{1}$$

As electroless plating chemically generates the charge on the surface of workpiece, it is highly recommended for irregularly shaped objects which are difficult to plate evenly with electroplating. Moreover, nonelectric plating is also used as a pretreatment unit to deposit a conductive surface on nonconductive surfaces, such as polymers, to electroplate the nonconductive materials.

#### *2.4.1.6 Hot-dip galvanization*

The hot-dip galvanization process has been around for more than 250 years. The hot-dip galvanization process is used for the corrosion protection of artistic sculptures. The hot-dip galvanization corrosion prevention method involves dipping the material into molten metal. The material reacts with the metal to create a tightly bonded coating. This chemically bonded coating acts as corrosion protection.

The generally used molten metal is zinc. When the steel is immersed in the molten bath of zinc, the zinc adheres to the steel. When the adhered zinc comes in contact with the air, it immediately reacts with the oxygen present in the air and forms a very strong zinc oxide layer, preventing corrosion. The zinc and steel form a metallurgical bond. Hence, the applied coating will not flake off. The hot or cold rolled coils are supplied with the metallic coating applied by either electroplating or hot dipping. The generally applied coating includes zinc, aluminum, tin, and lead.

#### *2.4.1.7 Mechanical metallic coating*

The mechanical metallic coating is the process of cold welding the fine metal powder on the material to be protected by tumbling the material with metal powder and a media in an aqueous solution. The mechanical metallic coating is generally used to apply zinc or cadmium to small parts as fasteners (**Figure 15**).

#### *2.4.1.8 Organic coating*

The organic coating technique is a process of coating material by utilizing carbonrich compounds to get a monolithic or multilayer protection coat. Such compounds are generally obtained from vegetables or animals. The organic coating thickness of it *Corrosion Protection and Modern Infrastructure DOI: http://dx.doi.org/10.5772/intechopen.111547*

**Figure 15.** *Mechanical metallic coating.*

depends on time and temperature. Initially, the organic coating deposition is rapid but slows down as the coating begins to build or get mature, that is, the rate of coating declines with time and thickness. The coating thickness from 15 to 25 μm is achievable in the organic coating technique.
