The application of surface coating is the common method to protect the surface of the metal from the corroding environment. These surface coatings exhibit chemical inertness to corrosive environment, adhesive properties and impermeable.
a. Organic surface coatings
· Organic surface coatings are applied over the metallic surfaces to prevent from the corrosion.
· Properties of Organic surface coatings.
· Chemical inertness to the corrosive environment
· Good surface adhesion
· Impermeability to water, gases and salts Eg. Paints
· Paint is a mechanical dispersion mixture of several constituents in a vehicle oil or drying oil.
· The following are the constituents of paints and their functions.
Constituent Functions Examples
1. Pigment
· It is a major constituent of the paint.
· Provides desired colour to the paint
· It protects the paint by reflecting harmful U.V radiation.
· Gives strength and increases weather resistance of the film
· White- white lead, ZnO
· Red- Red lead,
· Ferric oxide
2. Vehicle oil/ Drying oil
· It forms the film forming constituent of the paint.
· It acts as medium for the dispersion of various constituents.
· It gives durability, adhesion and water proofness to the paint.
· Sunflower oil, Mustard oil, Soya bean oil.
3. Thinners
· Reduces the viscosity and increases the elasticity of the
· Paint film.
· Turpentine, Kerosene, Naphtha.
4. Driers
· Driers are oxygen carrying catalysts.
· They accelerate the drying of the paint film through oxidation, polymerization and condensation. Tunstates and nahthalates of Pb, Zn and Co.
5. Extenders/ Fillers
· Low refractive indices materials.
· They reduce the cost and cracking nature of the paint film.
· BaSO4, gypsum,
6. Plasticizers
· They provide elasticity to the film and minimize cracking.
· Tributyl phosphate,
· triphenyl phosphate
7. Anti-skinning agents
· They prevent the gelling nature the paint film.
· Polyhydroxy phenols
