Chromium-free rust inhibitive treatment method for metal products having zinc surface and metal products treated thereby

Abstract

Provided is a chromium-free surface treatment method which imparts practical rust inhibitive performance to a metal product having a zinc surface to which it was difficult to impart rust inhibitive performance of practical level. A conversion-coated film of zinc phosphate etc. is formed on the zinc surface of the metal product as pretreatment, and the silica-based film is formed by applying a chromium-free surface treatment agent which uses alcohol or mixture of water and alcohol as a solvent to the conversion-coated film. As a result, when an aqueous surface treatment agent containing alcohol is applied, it is possible to suppress the formation of white rusting for 72 hours or longer in the salt spray test.

1 . A chromium-free rust inhibitive treatment method for a metal product having a zinc surface comprising: 
 treating the zinc surface of the metal product with a chromium-free conversion coating liquid to form a conversion-coated film on the zinc surface and    applying a solution of a chromium-free surface treatment agent containing silica components or components that change into silica in an alcohol solvent or a mixed solvent of water and alcohol to the conversion-coated film on the zinc surface to form a silica-based film having an average thickness of 0.5 to 3 μm on the surface coated with the conversion coated film.    
     
     
         2 . The chromium-free rust inhibitive treatment method for a metal product having a zinc surface according to  claim 1 , wherein the solution of a chromium-free surface treatment agent contains the silica components or the components that change into silica of 10 to 25% by weight as an amount converted to silica.  
     
     
         3 . The chromium-free rust inhibitive treatment method for a metal product having a zinc surface according to  claim 2 , wherein the solution of a chromium-free surface treatment agent contains 0.3 to 2% by weight of a nano-sized powder of titanium dioxide subjected to dispersion treatment, which has an average primary particle size of not more than 40 nm.  
     
     
         4 . The chromium-free rust inhibitive treatment method for a metal product having a zinc surface according to  claim 2 , wherein the solution of a chromium-free surface treatment agent contains 4 to 16% by weight of a silane coupling agent.  
     
     
         5 . The chromium-free rust inhibitive treatment method for a metal product having a zinc surface according to  claim 2 , wherein the solution of a chromium-free surface treatment agent contains, as an amount converted to silica, 10 to 25% by weight of colloidal silica in the mixed solvent of water and alcohol.  
     
     
         6 . The chromium-free rust inhibitive treatment method for a metal product having a zinc surface according to  claim 2 , wherein the solution of a chromium-free surface treatment agent contains alkoxysilane oligomer, which is obtained by hydrolysis and condensation polymerization of an alkoxysilane monomer, as the component that changes into silica, of 10 to 25% by weight, as an amount converted to silica, in the alcohol solvent.  
     
     
         7 . The chromium-free rust inhibitive treatment method for a metal product having a zinc surface according to  claim 6 , wherein the weight averaged molecular weight of the alkoxysilane oligomer is 1000 to 10000.  
     
     
         8 . The chromium-free rust inhibitive treatment method for a metal product having a zinc surface according to  claim 1 , wherein the chromium-free conversion coating liquid is an aqueous solution which contains zinc phosphate.  
     
     
         9 . The chromium-free rust inhibitive treatment method for a metal product having a zinc surface according to  claim 2 , wherein the chromium-free conversion coating liquid is an aqueous solution which contains zinc phosphate.  
     
     
         10 . The chromium-free rust inhibitive treatment method for a metal product having a zinc surface according to  claim 5 , wherein the chromium-free conversion coating liquid is an aqueous solution which contains zinc phosphate.  
     
     
         11 . The chromium-free rust inhibitive treatment method for a metal product having a zinc surface according to  claim 6 , wherein the chromium-free conversion coating liquid is an aqueous solution which contains zinc phosphate.  
     
     
         12 . The chromium-free rust inhibitive treatment method for a metal product having a zinc surface according to  claim 6 , wherein the chromium-free conversion coating liquid is an aqueous solution which contains 0.5 to 5 g/liter of citric acid.  
     
     
         13 . The chromium-free rust inhibitive treatment method for metal products having a zinc surface according to  claim 12 , wherein the chromium-free conversion coating liquid contains 2 to 20 g/liter of aqueous silica sol, as an amount converted to silica, and 0.6 to 6 g/liter of zinc ions.  
     
     
         14 . The chromium-free rust inhibitive treatment method for a metal product having a zinc surface according to  claim 1 , wherein the metal product having the zinc surface is a galvanized metal product or a zinc alloy plated metal product or a cast zinc alloy product as a base component.  
     
     
         15 . A metal product having a zinc surface subjected to chromium-free rust inhibitive treatment, comprising a conversion coated film formed on the zinc surface and a silica-based film having a thickness of 0.5 to 3 μm formed on the surface of the conversion coated film, 
 the silica-based film containing 2 to 10% by weight of a nano-sized powder of titanium dioxide, subjected to dispersion treatment, having an average primary particle size of not more than 40 nm, and not less than 65% by weight of silica.    
     
     
         16 . The metal product having a zinc surface subjected to chromium-free rust inhibitive treatment according to  claim 15 , wherein the silica comes from alkoxysilane oligomer.  
     
     
         17 . The metal product having a zinc surface subjected to chromium-free rust inhibitive treatment according to  claim 15 , wherein the conversion-coated film is formed with zinc phosphate.  
     
     
         18 . The metal product having a zinc surface subjected to chromium-free rust inhibitive treatment according to  claim 15 , wherein the conversion-coated film has a dark color.  
     
     
         19 . The metal product having a zinc surface subjected to chromium-free rust inhibitive treatment according to  claim 15 , wherein the metal product is a small screw having a threaded portion with an outside diameter of not more than 3 mm, the small screw electrogalvanized by a barrel method.  
     
     
         20 . A metal product having a zinc surface subjected to chromium-free rust inhibitive treatment, comprising a conversion coated film formed on the zinc surface and a silica-based film having a thickness of 0.5 to 3 μm formed on the surface of the conversion coated film, 
 the silica-based film containing not less than 65% by weight of silica,    wherein the silica comes from alkoxysilane oligomer.    
     
     
         21 . The metal product having a zinc surface subjected to chromium-free rust inhibitive treatment according to  claim 20 , wherein the conversion-coated film is formed with zinc phosphate.  
     
     
         22 . The metal product having a zinc surface subjected to chromium-free rust inhibitive treatment according to  claim 20 , wherein the conversion-coated film has a dark color.  
     
     
         23 . The metal product having a zinc surface subjected to chromium-free rust inhibitive treatment according to  claim 20 , wherein the metal product is a small screw having a threaded portion with an outside diameter of not more than 3 mm, the small screw electrogalvanized by a barrel method.