Method for surface treatment of aluminum alloy

Abstract

A method for surface treatment of aluminum alloy to form a uniform and dense zinc phosphate coating, which has excellent corrosion resistance properties on the surface of the aluminum alloy. The method includes steps of surface adjusting the surface of the aluminum alloy by a surface adjustor containing a predetermined volume of zinc phosphate particles, copolymer containing carboxylic acid group, and natural hectorite and/or synthetic hectorite respectively, and chemical conversion treating the surface of the resultant aluminum alloy by a zinc phosphate treating agent containing a predetermined volume of chelating agent that can chelate bond with iron ion.

1 . A method for surface treatment of aluminum alloy comprising steps of: 
 degreasing and rinsing a surface of aluminum alloy; and    performing surface adjustment of the surface of aluminum alloy which is degreased and rinsed with a surface adjustor; and    performing chemical conversion treatment of the surface of the aluminum alloy which is surface-adjusted with a zinc phosphate chemical conversion treatment agent; and    wherein the surface adjustor has a pH of not less than 7 and not more than 12 and containing not less than 50 ppm and not more than 2000 ppm of zinc phosphate particles having a D 50  of not more than three micrometers, not less than 2 ppm and not more than 200 ppm of a copolymer containing carboxylic acid group that copolymerizes a monomeric composition containing less than 50% by weight of acrylic acid and more than 50% by weight of 2-acrylamide-2-methylpropanesulfonic acid, and not less than 3 ppm and not more than 200 ppm of natural hectorite and/or synthetic hectorite; and    the zinc phosphate chemical conversion treatment agent comprises an acid aqueous solution containing not less than 0.1 g/l and not more than 2.0 g/l of zinc ion, not less than 0.1 g/l and not more than 4.0 g/l of nickel ion, not less than 0.1 g/l and not more than 3.0 g/l of manganese ion, not less than 5 g/l and not more than 40 g/l of phosphate ion, not less than 0.5 g/l and not more than 1.0 g/l of fluoride in a complex fluoride, not less than 0.3 g/l and not more than 0.5 g/l of fluoride in a simple fluoride, and not less than 0.025 g/l and not more than 0.45 g/l of iron (Fe) chelating agent that can chelate bond with iron ion.    
   
   
       2 . A method for surface treatment of aluminum alloy according to  claim 1  wherein the content of copper in the aluminum alloy is not more than 0.2% by weight.