Phosphate coating is produced on ferrous and non-ferrous metal surface and is composed of a tiny crystal of iron, zinc, or manganese phosphate. These inorganic coatings produced on metal surface retard corrosion and promote better paint bonding, phosphate coating is produced after precleaning or is formed in a combination bath known as a cleaner phosphate. Phosphate coating are generally used by the metal finishing industry for the following reasons :
1. To provide a base for bonding organic finishes such as paints, lacquers, plastics, rubber, adhesive, etc;
2. To provide a base of oils, wax and rust prevention to reduce metal corrosion;
3. To provide a base for lubricants on bearing surface to reduce friction and
4. To aid in drawing and forming of metal. the primary use of phosphate coating however is for bonding of paint. The coating produced on metal is not only stable and chemically inert towards organic finishes, but they are also absorptive and bind organic finishes to the metal. the most important reason to use a phosphate coating is to prevent or retard the spread of corrosion under the paint.