The corrosion resistance characteristics of Ni, Mn, and Zn phosphates in automotive body panel coatings

The phosphates of three elements, Nickel, Manganese and Zinc, were investigated for use in automotive body panel coatings in order to observe the adherence of the metallic elements through their reaction time and to study the change in corrosion resistance depending on the metal ion concentration. The phosphate reaction was found to be almost complete at about 3 s. At the very early reaction stage, the phosphate structures developed horizontally in regards to the Zn layer. As the reaction time increased, it was confirmed that the structures then developed vertically. This may be due to epitaxial growth, which requires further study. It is thought that among the three elements in the phosphate solution, the Ni reacts first and adheres to the Zn layer, while inducing competitive reaction with the Mn all the while making no direct contribution to the Mn phosphate reaction. The corrosion resistance increased as the Mn concentration increased, but decreased when the Ni concentration increased. In conclusion, it was found that an excessive amount of Ni included in the three elements phosphate solution induced a decline in the Mn phosphate reaction, decreasing the corrosion resistance for the Mn phosphate below its requisite level.