METHOD FOR SURFACE-TREATING METAL MATERIAL

摘要

PROBLEM TO BE SOLVED: To provide a method for surface-treating a metal material represented by an automobile body and components before cationic electrodeposition coating, and means for suppressing occurrence of poor electro-coating appearance.SOLUTION: The method for surface-treating a metal material before cationic electrodeposition coating comprises: performing at least one stage of degreasing treatment; at least one stage of washing treatment; performing film chemical conversion treatment; and performing at least one stage of washing treatment. The metal material is supplied to a cationic electrodeposition coating step without compulsively drying water on the metal material. A period of time until immersing the metal material into a cationic electrodeposition coating bath from the completion time of the final washing in the surface-treating step is 1-10 minutes. A chemical conversion treatment bath used for the film conversion treatment step includes Zr of 0.1 - 5.5 mmol/L and F, Al and Mg having at least 6 times mol concentration of Zr. An aqueous solution has a free F ion concentration of 30 ppm or less in the chemical conversion treatment bath.