Black hexavalent chromium-free plating treatment system

Abstract

The present invention provides a black hexavalent chromium-free plating treatment system in which the corrosion resistance of a surface of a metal part is improved by activating the surface of a zinc-plated coating of the metal part in a treatment solution in a dilute nitric acid activation treatment step, subsequently forming a black coating in an inorganic salt solution containing trivalent chromium and iron components as main ingredients in a black chromate treatment step, forming a conversion coating in a solution of inorganic salt and organic acid containing trivalent chromium and silica as main ingredients in a finish treatment step, and drying the resulting metal part. Therefore, even in a black trivalent chromate coating, sufficient corrosion resistance and sufficient strength are obtained. Moreover, a black trivalent chromate coating with heavy feeling is obtained for the first time. In addition, because the coating is one which is made mainly of trivalent chromium, less penetration into the ground occurs and the probability of occurrence of soil pollution caused by penetration of harmful substances into the ground is reduced.

What is claimed is:  
     
         1 . A black hexavalent chromium-free plating treatment system which is designed so that the corrosion resistance of a surface of a metal part is improved by 
 plating the surface of the metal part as a base with zinc in a zinc plating treatment step ( 10 ),    activating a surface of the zinc-plated coating in a treatment solution in a dilute nitric acid activation treatment step ( 11 ),    then rinsing the activated metal part to remove nitric acid components,    subsequently forming a black coating on the rinsed metal part in an inorganic salt solution containing trivalent chromium and iron components as main ingredients in a black chromate treatment step ( 20 ),    rerinsing the black coated metal part,    then subjecting the rerinsed metal part to a finish treatment by forming a conversion coating on the rerinsed metal part in a solution of inorganic salt and organic acid containing trivalent chromium and silica as main ingredients in a finish treatment step ( 30 ), and    drying the finished metal part in a drying step ( 34 ).    
     
     
         2 . A black hexavalent chromium-free plating treatment system which is designed so that the corrosion resistance of a surface of a metal part is improved by 
 plating the surface of the metal part as a base with zinc in a zinc plating treatment step,    activating a surface of the zinc-plated coating in a treatment solution in a dilute nitric acid activation treatment step,    then rinsing the activated metal part to remove nitric acid components,    forming a black regulation coating on the rinsed metal part in solution of inorganic salt and organic acid containing trivalent chromium and silica as main ingredients in a conversion treatment step ( 13 ) which is arranged next to the rinsing step,    rinsing the coated metal part,    subsequently forming a black coating on the rinsed coated metal part in an inorganic salt solution containing trivalent chromium and iron components as main ingredients in a black chromate treatment step,    rerinsing the black coated metal part,    then subjecting the rerinsed metal part to a finish treatment in a finish treatment step by forming a conversion coating on the rerinsed metal part in a solution of inorganic salt and organic acid which contains trivalent chromium and silica as main ingredients and is less concentrated than the solution used in the foregoing conversion treatment step, and    drying the finished metal part in a drying step.    
     
     
         3 . The black hexavalent chromium-free plating treatment system according to  claim 1  or  2  wherein the finish treatment step comprises an initial finish treatment step ( 31 ) in which a conversion coating is formed in a solution of inorganic salt and organic acid which contains trivalent chromium and silica as main ingredients and a final finish treatment step ( 33 ) in which after rinsing following the initial finish treatment, the rinsed metal part is immersed in any one of a overcoat treatment solution containing silica and cobalt as main ingredients and a water-soluble anti-corrosive solution.