Processing solution for forming hexavalent chromium free and corrosion resistant conversion film on zinc or zinc alloy plating layers, hexavalent chromium free and corrosion resistant conversion film and method for forming the same

A hexavalent chromium free, corrosion resistant, trivalent chromate conversion film containing zinc, trivalent chromium, cobalt, oxalic acid and SiO2 and formed on zinc or zinc alloy plating layers, wherein the SiO2 content thereof ranges from 1 to 10 mg/dm2, the mass ratio of trivalent chromium to (trivalent chromium+zinc) [Cr/(Cr+Zn)] is not less than 15/100, the mass ratio of cobalt to (trivalent chromium+cobalt) [Co/(Cr+Co)] ranges from 1/100 to 40/100 and the mass ratio of the oxalic acid to (trivalent chromium+oxalic acid) [oxalic acid/(Cr+oxalic acid)] ranges from 5/100 to 50/100.

A hexavalent chromium free, corrosion resistant, trivalent chromate conversion film containing zinc, trivalent chromium, cobalt, oxalic acid and SiO2 and formed on zinc or zinc alloy plating layers, wherein the SiO2 content thereof ranges from 1 to 5 mg/dm2, the mass ratio of trivalent chromium to (trivalent chromium+zinc) [Cr/(Cr+Zn)] is not less than 20/100 to 60/100, the mass ratio of cobalt to (trivalent chromium+cobalt) [Co/(Cr+Co)] ranges from 4/100 to 40/100 and the mass ratio of the oxalic acid to (trivalent chromium+oxalic acid) [oxalic acid/(Cr+oxalic acid)] ranges from 15/100 to 50/100.

A method for forming a hexavalent chromium free, corrosion resistant, trivalent chromate conversion film comprising:

• the step of bringing zinc or zinc alloy plating layers into contact with a processing solution comprising a silicon compound, trivalent chromium and oxalic acid in a molar ratio ranging from 0.5/1 to 1.5/1, wherein the trivalent chromium is present in the form of a water-soluble complex with oxalic acid, and cobalt ions, which are stably present in the processing solution without causing any precipitation due to formation a hardly soluble metal salt with oxalic acid;
• wherein the solution reacts with zinc to form a hexavalent chromium free, corrosion resistant, trivalent chromate conversion film containing zinc, trivalent chromium, cobalt, oxalic acid and SiO2 on the plating.