Method for forming chemical film on the surface of zinc or zinc alloy

PURPOSE: To form chemical conversion films having excellent corrosion resistance and adhesiveness by subjecting the surfaces of Zn metals to a surface treatment with a liquid contg. polyvalent metal ions and complex forming agent, then to washing the surfaces with an acidic post-washing liquid contg. Al, Zn and F and drying the surfaces. CONSTITUTION: The surfaces of a steel sheet having plating layers consisting of Zn or Zn alloy are subjected to degreasing and washing and are then surface treated by bringing the surfaces into contact with an alkaline soln. Which contains 0.3 to 3 g/l inorg. acid salt, such as nitrate of the polyvalent metals including Fe, Co, Ni, Cr, Al, etc., and 0.05 to 10 g/l org. chelating agent for maintaining the ions of the polyvalent metals in a dissolved state and complex forming agent consisting of an org. acid salt, such as salt of hexahydroxy hepanic acid and is adjusted in pH value to ≥11 by NaOH, etc. In succession, the surfaces are washed with the Al-Zn-fluoride- based post-washing liquid of 0.1 to 8.0 g/l in the total concn. of Al, Zn and F of aluminum fluorozirconate, (0.15 to 8.0):1:(5 to 52) in the molar ratio thereof and ≤5 in the pH value thereof. The chemical conversion films having the excellent corrosion resistance and coating material adhesion property are thus obtd. COPYRIGHT: (C)1992,JPO