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

Abstract

A trivalent-chromium chemical conversion coating from which substantially no hexavalent chromium is released. The trivalent-chromium chemical conversion coating is one formed on the surface of a zinc or zinc-alloy deposit. In a brine spray test, the chemical conversion coating has unsusceptibility to corrosion (time required for white-rust formation) of 96 hours or longer. The chemical conversion coating has a hexavalent-chromium concentration less than 0.01 μg/cm in terms of metal atom amount. The amount of hexavalent chromium released after 30-day standing in a thermo-hygrostatic chamber at a temperature of 80° C. and a humidity of 95% (amount of hexavalent chromium released when the coating is immersed in 100° C. water for 10 minutes) is smaller than 0.05 μg/cm 2 .

1 - 19 . (canceled) 
     
     
         20 - 23 . (canceled) 
     
     
         24 . A chemical conversion treatment liquid for forming a trivalent chromium chemical conversion coating film on zinc or zinc alloy plating, characterized in that
 a content of trivalent chromium ions in the treatment liquid is 0.002 to 0.5 mol/l,   a concentration of hexavalent chromium ions is 1 ppm or less,   a content of cobalt ions is 0.1 mol/1 or less,   a hexavalent chromium generation suppressing agent that can suppress generation of hexavalent chromium which is generated in the trivalent chromium chemical conversion coating film is contained in said liquid, and   a pH of the chemical conversion treatment liquid is 0.5 to 5.   
     
     
         25 . The chemical conversion treatment liquid according to  claim 24 , wherein the hexavalent chromium generation suppressing agent is within the range of 0.1 to 5 g/l. 
     
     
         26 . The chemical conversion treatment liquid according to  claim 24 , wherein the hexavalent chromium generation suppressing agent is a reducing compound. 
     
     
         27 . The chemical conversion treatment liquid according to  claim 24 , wherein the hexavalent chromium generation suppressing agent is selected from the group consisting of a tannic acid and a salt thereof, gallic acid and a salt thereof, tartaric acid and a salt thereof, citric acid and a salt thereof, ascorbic acid and a salt thereof, a vanadium compound, a titanium compound, phosphoric acid and a salt thereof, and chromium phosphate. 
     
     
         28 . The chemical conversion treatment liquid according to  claim 24 , further comprising a chelating agent in the range of 0.2 to 2 mole per mole of Cr3+. 
     
     
         29 . The chemical conversion treatment liquid according to  claim 24 , wherein a content of nitrogen is 500 ppm or less in terms of nitrogen atoms. 
     
     
         30 . The chemical conversion treatment liquid according to  claim 24 , further comprising a quinoline-based compound or a derivative thereof. 
     
     
         31 . Washing water or a finishing liquid for a trivalent chromium chemical conversion coating film formed on zinc plating or zinc alloy plating, characterized by comprising
 a hexavalent chromium generation suppressing agent that can suppress generation of hexavalent chromium which is generated in the trivalent chromium chemical conversion coating film at 0.1 to 10 g/l, wherein   a pH of the washing water or the finishing liquid is 2 to 10.   
     
     
         32 . A chemical conversion treatment liquid for forming a trivalent chromium chemical conversion coating film on zinc or zinc alloy plating, characterized in that
 a content of trivalent chromium ions in the treatment liquid is 0.002 to 0.5 mol/l,   a concentration of hexavalent chromium ions is 1 ppm or less,   a content of cobalt ions is 250 ppm or less, and   a sulfur compound is contained in the range of 100 to 1500 ppm in terms of sulfur atoms.   
     
     
         33 . The chemical conversion treatment liquid according to  claim 32 , wherein a content of nitrogen is 500 ppm or less in terms of nitrogen atoms. 
     
     
         34 . The chemical conversion treatment liquid according to  claim 32 , further comprising a silicon compound. 
     
     
         35 . The chemical conversion treatment liquid according to  claim 32 , further comprising a quinoline-based compound or a derivative thereof. 
     
     
         36 - 37 . (canceled) 
     
     
         38 . The trivalent chromium chemical conversion coating film, wherein
 a corrosion resistance (a time required for the formation of white rust) in a salt spray test is 96 hours or more,   a concentration of hexavalent chromium in terms of metal atoms in the chemical conversion coating film is less than 0.01 μg/cm2, and   an amount of hexavalent chromium eluted from the 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) is less than 0.05 μg/cm2, wherein   the trivalent chromium chemical conversion coating film is formed on zinc or zinc alloy plating, and   after the formation thereof, the coating film is treated with the washing water or the finishing liquid; comprising:   a hexavalent chromium generation suppressing agent that can suppress generation of hexavalent chromium which is generated in the trivalent chromium chemical conversion coating film at 0.1 to 10 g/l, wherein   a pH of the washing water or the finishing liquid is 2 to 10.