Phosphate coating is the treatment of Iron, steel, galvanized steel, or aluminum with a dilute solution of phosphoric acid and other chemicals in which the surface of the metal, relating chemically with the phosphoric acid media, is converted to an integral, mildly protective larger of insoluble crystalline phosphate.

These compounds are used primarily for removing mill oil, soil, forming lubricants, etc. from metal surfaces. Most modern cleaners in this group are made up of synthetic surfactants or detergents, water conditioning chemicals, and caustic or other alkaline materials. The synthetic surfactant emulsifies petroleum. Oils, so they can be removed. The alkaline portion reacts with animal or vegetable oils and converts to soap which is solvable. They also have a regenerating effect on some of the surfactants. The water conditioning material makes the rinsing easier. Heat is used to soften the various greases and oils, so they can be emulsified more rapidly. Synthetic surfactants have a definite temperature range where they work well, so follow the instructions on the data sheet.