Rustproofing agent for zinc plated steel sheet

Abstract

A rustproofing agent for a zinc plated steel sheet is provided which contains (A) a chromium aqueous solution containing an organic substance and (B) an acid metal such as a metal salt of nitric acid or phosphoric acid. All of the chromium in the chromium aqueous solution of component. A is trivalent chromium. It is preferable if the organic substance in component A contains at least one type of oxy acid or oxide thereof. The metal in the acid metal salt of component B is an alkaline earth metal, cobalt, nickel, iron, zirconium, titanium, etc. It is also preferable if (C) a water-dispersible silica is contained as an additional component. The present invention also provides a zinc plated steel sheet, in which at least one side of the surface of a zinc plated steel sheet is coated with the zinc plated steel sheet rustproofing agent and the rustproofing agent dried to form a coating with a dry coating weight of 0.01 to 3 g/m 2 .

What is claimed is:  
     
         1 . A rustproofing agent for zinc plated steel sheet, comprising 
 (A) an aqueous solution comprising chromium and an organic substance, wherein all of the chromium in the aqueous solution is trivalent chromium; and    (B) at least one acid metal salt selected from the group consisting of nitric acid metal salts and phosphoric acid metal salts.    
     
     
         2 . The rustproofing agent of  claim 1  wherein the organic substance is selected from. the group consisting of oxy acids, oxides of oxy acids and mixtures thereof.  
     
     
         3 . The rustproofing agent according to  claim 1  or  2 , wherein chromium (calculated or metallic chromium) is present in (A) in an amount from 1 to 30 wt % relative to a total solids content in (A) and (B) combined of 100 wt %.  
     
     
         4 . The rustproofing agent of any of claims  1 ,  2  or  3 , wherein the metal in the acid metal salt is selected from the group consisting of alkaline earth metals, cobalt, nickel, iron, zirconium, and titanium.  
     
     
         5 . The rustproofing agent of any of claims  1 ,  2 ,  3  or  4 , additionally comprising a water-dispersible silica.  
     
     
         6 . The rustproofing agent of  claim 5 , wherein said water-dispersible silica is present in an amount of from 5 to 50 wt % relative to a total solids content in (A) and (B) combined of 100 wt %.  
     
     
         7 . A method of rustproofing a zinc plated steel sheet comprising contacting at least one surface of said zinc plated steel sheet with the rustproofing agent according to any of  claims 1  to  6  to form a wet coating of the rustproofing agent on said surface and drying said wet coating to form a dry coating with a coating weight of weight of 0.01 to 3 g/m 2 .