Corrosion-resistant trivalent-chromium chemical conversion coating and solution for trivalent-chromium chemical treatment

A method for reducing an amount of hexavalent chromium eluted from a trivalent chromium chemical conversion coating film left for 30 days in a constant temperature and humidity chamber at a temperature of 80° C. and at a humidity of 95% (an amount eluted by immersion of the coating film into hot water at a temperature of 100° C. for 10 minutes) to less than 0.05 μg/cm2, the method comprising the steps of:

• preparing chemical conversion treatment liquid for forming a trivalent chromium chemical conversion coating film, and
• immersing a zinc or zinc alloy plated substrate into the chemical conversion treatment liquid, wherein
• a content of trivalent chromium ions in the chemical conversion treatment liquid is 0.002 to 0.5 mol/1,
• a concentration of hexavalent chromium ions in the chemical conversion treatment liquid is 1 ppm or less,
• a content of cobalt ions in the chemical conversion treatment liquid is 0.1 mol/1 or less,
• the chemical conversion treatment liquid further contains a hexavalent chromium generation suppressing agent for suppressing generation of hexavalent chromium which is generated in the trivalent chromium chemical conversion coating film, wherein the hexavalent chromium generation suppressing agent is selected from the group consisting of ascorbic acid and salts thereof, tannic acid and salts thereof, and gallic acid and salts thereof,
• a content of the hexavalent chromium generation suppressing agent in the chemical conversion treatment liquid is 0.1 to 5 g/l,
• a pH of the chemical conversion treatment liquid is 0.5 to 5, and
• a concentration of hexavalent chromium in terms of metal atoms in a chemical conversion coating film formed by using the same chemical conversion treatment liquid as the above chemical conversion treatment liquid excluding the hexavalent chromium generation suppressing agent is less than 0.01 μg/cm2.