Metallic and non-metallic coatings

Metallic and non-metallic coatings are used for corrosion protection in metals. The major types of metallic coatings are hot dipping, spraying, electroplating and diffusion coating. In hot dipping, the metal part which is to be protected against corrosion is dipped in a molten bath of the coat metal. In spraying method, the coat is melted into small drops of liquid and propelled by a blast of air on to the metal surface. Aluminium coatings are obtained by hot dipping or spraying. In electroplating method, the metal part which is to be coated is made cathode and immersed in an electrolytic bath with the electrolyte containing the ions of the coating material. Nickel coating and tin coating are usually done by employing electroplating technique. In diffusion coating, the diffusion of the coat metal into the part through the surface is achieved at an elevated temperature. Metal coating can be classified according to their electrochemical behavior into two-noble coatings and sacrificial coatings. The coat is noble with respect to the underlying metal in noble coatings. The underlying metal is protected by the sacrificial action of the coating in sacrificial coatings. Zinc coatings on iron are sacrificial. There are two categories in nonmetallic coatings- organic and inorganic. Organic coatings refer mainly to paints applied to metals. Paints are normally mixture of particles of a pigment in a continuous organic medium. Examples of pigments are red lead and zinc chromate. Examples of inorganic nonmetallic coatings are enamel and porcelain.