The Pretreatment of Steel and Galvanized Steel for Cathodic Electrodeposition Paint Systems

The advent of cathodic electrodeposition in the automotive industry has brought about substantial changes in the pretreatment of steel and galvanized steel surfaces and the testing thereof. Cyclic scab blistering tests, water soak tests, coating solubility, coating porosity and ESCA/SEM/Auger surface analysis techniques have provided significant insights into the factors that optimize the performance of zinc phosphate coatings for cationic paint systems. Specifically, the crystal structure and iron content of the phosphate coating and a chromium based post-rinse have been shown to make a significant effect upon the subsequent corrosion resistance properties of the phosphate/metal/CED paint system. Further, ESCA/SEM surface analysis and accelerated testing have shown that the processing method substantially affects both the crystal structure and composition of the coating.