Passivation is applied at the last stage of phosphate baths. There are two types, with or without chrome. It is used at room temperature in concentrations between 0.1-0.3%. The phosphate film coming out of the phosphating bath is not homogeneous on the surface. Passivation will make the phosphate homogeneous and minimize the amount of air that can remain under the paint. Chromic acid reaching the layer below the phosphate provides an extra corrosion resistance. In addition, when the anions and cations from the water combine with the humidity of the air, they form salt. Since the paint is a permeable layer, these salts will dissolve with the humidity of the air, causing tearing and swelling in the paint. Passivation prevents this formation by cleaning anions and cations from the water. Phosphated metals may be damaged if they sit too long in the passivation bath. The reason is that chromic acid erodes the phosphate crystals.
