METHOD FOR FORMING MULTILAYER COATING FILM

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PROBLEM TO BE SOLVED : To provide a method for forming a multilayer coating film excellent in economic efficiency and environmental conservation by eliminating a conventional complicated and expensive rust preventing chemical conversion treatment process, for example zinc phosphate treatment process, and further as an alternative thereto, capable of providing an excellent rust preventing property, equal to or above that in the conventional rust preventing chemical conversion treatment process, and further the rust preventing property is suitable for real corrosive environment. SOLUTION : The method for forming the multilayer coating film includes : (a) a process for forming an electrolytic film containing a rare earth metal (A), zinc (B), and a rare earth metal (C) on a metallic base material by immersing an untreated metallic base material in an aqueous solution containing a nitrate of the rare earth metal (A) with precipitation pH lower than that of zinc, a zinc (B) salt, and a nitrate of the rare earth metal (C) with precipitation pH higher than that of zinc, and carrying out cathodic electrolysis; and (b) a process for forming an electrodeposition film by immersing the metallic base material on which the electrolytic film is formed in the process (a) in an electrodeposition coating material to carry out the electrodeposition coating, wherein mass of the electrolytic film after the process (b) is 30-200 mg/m2, and the electrolytic film includes 20-65 pts.mass of the rare earth metal (A), 20-60 pts.mass of zinc (B), and 15-50 pts.mass of the rare earth metal (C) with respect to 100 pts.mass of the sum total of the rare earth metal (A), zinc (B), and the rare earth metal (C). COPYRIGHT : (C)2010, JPO&INPIT