Processing solution for forming hexavalent chromium free, black conversion film on zinc or zinc alloy plating layers, and method for forming hexavalent chromium free, black conversion film on zinc or zinc alloy plating layers

Abstract

It is an object of the present invention to provide a processing solution used for forming a hexavalent chromium free, black conversion film, which is applied onto the surface of zinc or zinc alloy plating layers, and which has corrosion resistance identical to or higher than that achieved by the conventional hexavalent chromium-containing conversion film. According to an aspect of the present invention, there is provided a processing solution for forming a hexavalent chromium free, black conversion film on zinc or zinc alloy plating layers, the processing solution comprising: nitrate ions and trivalent chromium in a mole ratio (NO 3− /Cr 3+ ) of less than 0.5/1, wherein the trivalent chromium is present in the form of a water-soluble complex with a chelating agent; and cobalt ions and/or nickel ions, wherein the cobalt ions and/or nickel ions are stably present in the processing solution without causing any precipitation by forming a hardly soluble metal salt with the chelating agent; wherein the solution reacts with zinc when it is brought into contact with the zinc or zinc alloy plating to form a hexavalent chromium free, black conversion film containing zinc, chromium, cobalt and/or nickel, and the chelating agent on the plating.

1 . A processing solution for forming a hexavalent chromium free, black conversion film on a zinc or zinc alloy plating layer(s), the processing solution comprising: 
 nitrate ions and trivalent chromium in a mole ratio (NO 3− /Cr 3+ ) of less than 0.5/1, wherein the trivalent chromium is present in the form of a water-soluble complex with a chelating agent; and    cobalt ions and/or nickel ions, wherein cobalt ions and/or nickel ions are stably present in the processing solution without causing any precipitation by forming a hardly soluble metal salt with the chelating agent;    wherein the solution reacts with zinc when it is brought into contact with the zinc or zinc alloy plating to form a hexavalent chromium free, black conversion film containing zinc, chromium, cobalt and/or nickel, and the chelating agent on the plating layer(s).    
     
     
         2 . The processing solution according to  claim 1  wherein the trivalent chromium concentration ranges from 0.5 to 10 g/L and a molar ratio of the chelating agent to the trivalent chromium (the chelating/Cr 3+ ) ranges from 0.2/1 to 4/1.  
     
     
         3 . The processing solution according to  claim 1  or  2  wherein the trivalent chromium source is chromium phosphate (Cr(H n PO 4 ) (3/(3-n)) ).  
     
     
         4 . The processing solution according to  claim 1  wherein the chelating agent is one or more members selected from the group consisting of monocarboxylic acids, dicarboxylic acids, tricarboxylic acids, hydroxycarboxylic acids, aminocarboxylic acids and salts thereof.  
     
     
         5 . The processing solution according to  claim 4  wherein the chelating agent is one or more members selected from the group consisting of oxalic acid, malonic acid, succinic acid and a salt thereof.  
     
     
         6 . The processing solution according to  claim 1 , which further comprises one or more metal ions selected from the group consisting of silicon, iron, titan, zirconium, tungsten, vanadium, molybdenum, strontium, niobium, tantalum, manganese, calcium, barium, magnesium and aluminum.  
     
     
         7 . The processing solution according to  claim 1  wherein the nitrate ion concentration ranges from 0.1 to 1 g/L.  
     
     
         8 . The processing solution according to  claim 1 , which further comprises phosphate ions.  
     
     
         9 . The processing solution according to  claim 1 , which further comprises one or more ions selected form the group consisting of a sulfate ion, a halogen ion and a borate ion.  
     
     
         10 . A method for forming a hexavalent chromium free, black conversion film, which comprises the step of bringing a zinc or zinc alloy plating layer(s) into contact with the processing solution of  claim 1 .  
     
     
         11 . The method according to  claim 10 , which comprises the step of subjecting a hexavalent chromium free, black conversion film to an aging treatment at 100 to 250° C. for 30 to 300 minutes.  
     
     
         12 . The method according to  claim 11  wherein the aging treatment is applied at 200° C. for 60 to 300 minutes.  
     
     
         13 . A method for rust-proofing zinc and zinc alloy plating, which comprises the step of subjecting a hexavalent chromium free, black conversion film formed by the method according to any one of  claims 10  to  12  to an overcoating treatment.