The chemical processes involved in the Phosphating process are quite complex but in brief terms the phosphating process converts the surface layer of certain metals into strong, adherent, insoluble metal phosphates. These can either be crystalline or amorphous in structure. Phosphating forms both a rust resisting layer in itself and also gives an excellent key to the subsequent coating. If scratched, the underlying metal surface does not corrode as rapidly as it would without the phosphate layer.
Phosphating solutions are usually based on iron or zinc acid phosphates, dissolved in dilute phosphoric acid. The products to be phosphated are either dipped or more usually sprayed with the phosphating solution, the concentration and ingredients of which are carefully balanced so that they are just held in the solution. The free acid present reacts with the metal surface liberating hydrogen and causing the pH of the solution immediately adjacent to the metal to rise. This unbalances the solution and the metal phosphate begins to precipitate. The careful balance of the solution confines this precipitation to the metal surface, so that the phosphate film becomes a chemically bonded, integral part of the surface.