Modern Zinc Phosphate Films on Zinc Coated Steels

Recently, many kinds of zinc coated steels have been developed and applied to improve corrosion resistant properties of automobile bodies. Cationic electrodeposition coating method is popular, and to ensure adhesion of steel and the electrodeposited films, a zinc phosphating treatment is performed before electrodeposition (1,2,3,4,5)*. However, chemical properties of zinc phosphate films such as adhesiveness and chemical stability depended on their chemical composition. The action of the heavy metal components in zinc phosphating solution was studied for zinc coated steels. Nickel and manganese ions in the solution entered into the films and modified the properties of zinc phosphate films. The action of manganese ion was remarkable. This action was due to the chemical structure of zinc phosphate films, and it was though that modern zinc phosphate films have a structure of Zn_3-xMe_x(PO₄)₂·4H₂O (Me: Mn and or Ni). The characterization of these films was revealed by EXAFS method.