Trivalent chromium-containing composition for use in corrosion resistant coatings on metal surfaces

Abstract

A low sludge trivalent chromium based conversion coating bath is provided which forms corrosion resistant coatings on aluminum and aluminum alloys by immersion in aqueous solutions containing trivalent chromium ions and fluorometallate ions followed by optional rinsing. Trivalent chromium coated aluminum also serves as an effective base for paint primers.

The invention claimed is: 
     
         1 . An article of manufacture having at least one portion that comprises a surface, the surface comprising at least one area of bare metal, at least one area of coating over an underlying metal substrate, or both of at least one area of bare metal and at least one area of coating over an underlying metal substrate; wherein the surface is coated according to a process comprising operations of:
 (I) covering a surface to be coated, touched up, or both coated and touched up with a layer of a liquid composition comprising water and:
 (A) a component of fluorometallate anions, each of the anions consisting of:
 (i) at least four fluorine atoms; and 
 (ii) at least one atom of an element selected from the group consisting of titanium, zirconium, hafnium, silicon, aluminum, and boron, and, optionally, one or both of (iii) at least one ionizable hydrogen atom; and 
 (iv) at least one oxygen atom; 
 
 (B) a component of water soluble chromium fluoride; 
 the composition being substantially free of hexavalent chromium; and 
   (II) drying the liquid layer formed in operation (I) to form a coated surface.   
     
     
         2 . The article according to  claim 1 , wherein the fluorometallate anions are selected from the group consisting of fluorosilicate, fluorotitanate, and fluorozirconate anions. 
     
     
         3 . The article according to  claim 1 , wherein the fluorometallate anions include fluorozirconate anions in a concentration within a range from about 0.4 to 100 g/kg; water soluble chromium fluoride in a concentration within a range from about 4 to 100 g/l and the liquid composition comprising not more than 0.06% of dispersed silica and silicates. 
     
     
         4 . The article according to  claim 1 , wherein trivalent chromium and zirconium are present in a ratio of trivalent chromium to zirconium in a range of 0.725:1 to 39:1. 
     
     
         5 . The article according to  claim 1 , wherein:
 the fluorometallate anions include fluorozirconate anions;   the concentration of fluorozirconate anions is within a range from about 1 to about 10 g/kg;   the concentration of chromium fluoride is within a range from about 4 g/l to about 100 g/l; and the ratio of trivalent chromium to zirconium is in the range of 7 to 17.   
     
     
         6 . An article of manufacture comprising a metal substrate surface having at least one portion that comprises a coating dried in place on the metal substrate surface thereby forming a coated surface, the coating comprising chromium in substantially only trivalent form and providing salt spray resistance to the metal substrate surface of at least 96 hours according to ASTM B-117. 
     
     
         7 . The article according to  claim 6 , wherein the metal substrate surface is predominantly comprised of iron, titanium, aluminum, magnesium, zinc and alloys thereof. 
     
     
         8 . The article according to  claim 6 , wherein the metal substrate surface is predominantly comprised of aluminum and alloys thereof. 
     
     
         9 . The article according to  claim 6 , wherein a total add-on mass after drying of the coating is at least 20 mg/ft 2  and not greater than 120 mg/ft 2 . 
     
     
         10 . The article according to  claim 6 , further comprising one or more subsequently applied protective layers, selected from paints, lacquers and other resin based coatings. 
     
     
         11 . A process for coating or touching up or both coating and touching up a surface, the surface comprising at least one area of bare metal, at least one area of coating over an underlying metal substrate, or both of at least one area of bare metal and at least one area of coating over an underlying metal substrate, the process comprising operations of:
 (I) covering the surface to be coated, touched up, or both coated and touched up with a layer of a liquid composition comprising water and:
 (A) a component of fluorometallate anions, each of the anions consisting of:
 (i) at least four fluorine atoms; and 
 (ii) at least one atom of an element selected from the group consisting of titanium, zirconium, hafnium, silicon, aluminum, and boron, and, optionally, one or both of 
 (iii) at least one ionizable hydrogen atom; and 
 (iv) at least one oxygen atom; 
 
 (B) a component of water soluble chromium fluoride; 
   the composition being substantially free of hexavalent chromium and storage stable at ambient temperature for at least 2 weeks; and   (II) drying the liquid layer formed in operation (I) to form a coated surface.   
     
     
         12 . The process according to  claim 11 , wherein the surface comprises at least one area of bare metal and at least one area of coating over an underlying metal substrate; and in operation (I), the liquid layer is formed over the at least one area of bare metal. 
     
     
         13 . The process according to  claim 11 , wherein, in the liquid composition used in operation (I): the fluorometallate anions are selected from the group consisting of fluorosilicate, fluorotitanate, and fluorozirconate anions. 
     
     
         14 . The process according to  claim 11 , wherein, in the liquid composition used in operation (I):
 the fluorometallate anions include fluorozirconate anions; the concentration of fluorozirconate anions is within a range from about 1 to about 10 g/kg; the concentration of chromium(III) ions is greater than 0 and not more than 100 g/l.   
     
     
         15 . The process according to  claim 14 , wherein:
 the surface comprises at least one area of bare metal adjacent to at least one area of coating over an underlying metal substrate,   the at least one area of coating over an underlying metal substrate comprising a first portion and a second portion,   in operation (I), the liquid layer is formed over both the area of bare metal and at least the first portion of the adjacent area of coating over an underlying metal substrate; and   the coating over an underlying metal substrate is selected from the group consisting of a phosphate conversion coating, a chromate conversion coating, and a conversion coating produced by contacting a surface consisting predominantly of iron, titanium, aluminum, magnesium and/or zinc and alloys thereof with an acidic treating solution comprising at least one of fluorosilicate, fluorotitanate, and fluorozirconate.   
     
     
         16 . The process according to  claim 13 , wherein, in the liquid composition used in operation (I):
 the fluorometallate anions include fluorozirconate anions;   the concentration of fluorozirconate anions is within a range from about 1 to about 8 g/kg;   the concentration of chromium fluoride is within a range from about 4 g/l to about 100 g/l; and   a concentration of hydrofluoric acid is present within a range from about 0.70 to about 1.3 ppt.   
     
     
         17 . The process according to  claim 11  further comprising a step of prewarming the surface before step (I) treatment. 
     
     
         18 . The process according to  claim 17  wherein temperature of the prewarming step is up to at least 65° C. 
     
     
         19 . The process according to  claim 11  wherein prior to step (I) the surface is cleaned, or cleaned and dried, and time between cleaning, or cleaning and drying, and coating the surface is no more than 48 hours. 
     
     
         20 . An article of manufacture having at least one portion that comprises the coated surface of  claim 11  wherein the bare metal substrate or underlying metal substrates is comprised of aluminum or aluminum alloy. 
     
     
         21 . The article of manufacture of  claim 20  wherein the coated surface is essentially free of hexavalent chromium and has corrosion resistance properties characterized by less than about 3 pits per 24 square inches when subjected to 5% neutral salt spray at 95° F. ASTMB-117 for at least 336 hours.