Process and composition for passivating metal surfaces

Abstract

Aqueous compositions useful as a passivation treatment prior to painting to reduce corrosion. The compositions includes water, a trivalent chromium salt with the formula Cr(H x PO 4 ) 3 , where x is 1.5 or 2, a polymer system having a plurality of carboxylic functional groups, an organo-functional silane, and an organopolyphosphonic acid. A process for treating a metal surface includes contacting the surface with such aqueous compositions. The compositions and the processes provide benefits in comparison to the zinc phosphate metal pretreatment thought to be the standard in the industry.

What is claimed is: 
     
         1 . An aqueous pretreatment composition for treating a metal surface, the composition comprising:
 water;   a trivalent chromium salt with the formula Cr(H x PO 4 ) 3 , where x is 1.5 or 2;   a polymer system having a plurality of carboxylic functional groups;   an organo-functional silane; and   an organopolyphosphonic acid.   
     
     
         2 . The aqueous pretreatment composition of  claim 1 , wherein x is 2. 
     
     
         3 . The aqueous pretreatment composition of  claim 1 , wherein the polymer system comprises carboxylate functional polymer or a modified carboxylate functional polymer. 
     
     
         4 . The aqueous pretreatment composition of  claim 1 , wherein the polymer system is selected from the group consisting of: a polyacrylic acid (PAA) homopolymer, an acrylic/maleic copolymer, and polyacrylates having a molecular weight of from about 1,000 to about 400,000. 
     
     
         5 . The aqueous pretreatment composition of  claim 1  wherein the polymer system comprises acrylic acid combined with at least one additional moiety. 
     
     
         6 . The aqueous pretreatment composition of  claim 5  where at least one additional moiety is selected from the group consisting of: maleic acid; methacrylic acid; phosphonate; maleic acid and vinyl acetate; acrylamide; sulfophenol methallyl ether; 2-acrylamido-2-methylpropane sulfonic acid; 2-acrylamido-2-methylpropane sulfonic acid and sodium styrene sulfonate; methyl methacrylate, sodium methallyl sulfonate, and sulfophenol methallyl ether; polymaleates; polymethacrylates; polyaspartates; ethylenediamine disuccinate; and organo polyphosphonic acids and their salts. 
     
     
         7 . The aqueous pretreatment composition of  claim 1 , wherein the polymer system is a polymer blend comprising a first polymer having carboxylic functional groups and a second polymer having hydroxyl groups. 
     
     
         8 . The aqueous pretreatment composition of  claim 7  wherein the first polymer is comprised of a polyacrylic acid or a polymethylvinylether-co-maleic acid, and the second polymer is comprised of a polyvinyl alcohol. 
     
     
         9 . The aqueous pretreatment composition of  claim 7  wherein the ratios of carboxylic functional groups to hydroxyl functional group is from about 0.3:1 to about 3.5:1. 
     
     
         10 . The aqueous pretreatment composition of  claim 1  wherein the organofunctional silane is selected from the group consisting of aminopropyltriethoxy silanes, mercapto silanes, epoxy silanes, and mixtures thereof. 
     
     
         11 . The aqueous pretreatment composition of  claim 1  wherein the organopolyphosphonic acid is a bisphosphonic acid, etidronic acid, or their salts. 
     
     
         12 . The aqueous pretreatment composition of  claim 1  wherein the organopolyphosphonic acid is selected from the group consisting of alendronic acid, ibandronic acid, incadronic acid, pamidronic acid, risedronic acid, zoledronic acid, clodronic acid, tiludronic acid, and etidronic acid. 
     
     
         13 . The aqueous pretreatment composition of  claim 1  wherein the organopolyphosphonic acid is a polymethylenephosphonic acid of Formula I
   (HO) 2 P(O)CH 2 —R 1 —CH 2 (O)P(OH) 2   (Formula I)
 
 
       wherein R 1  is a divalent organic radical comprising additional phosphonic acid groups. 
     
     
         14 . The aqueous pretreatment composition of  claim 1  wherein the organopolyphosphonic acid is a bisphosphonic acid of Formula II
   H 2 PO 3 —CR 1 R 2 —PO 3 H 2   (Formula II)
 
 
       wherein R 1  and R 2  are each independently a hydrogen, a hydroxyl group, an alkyl group, an alkylamine, an aryl group, a substituted aryl group, or a nitrogen-containing heterocyclic group. 
     
     
         15 . The aqueous composition of  claim 1 , further comprising a wetting agent, a thickener, a biocide, or a wax. 
     
     
         16 . The aqueous composition of  claim 1 , further comprising a fluorosurfactant 
     
     
         17 . The aqueous composition of  claim 1 , wherein the composition is halide ion free and has a pH from about 1.5 to about 3.0. 
     
     
         18 . A method for preparing a metal pretreatment composition comprising:
 reducing a hexavalent chromium compound, with hydrogen peroxide, in the presence of phosphoric acid to create a trivalent chromium compound with the formula Cr(H x PO 4 ) 3 , where x is 1.5 or 2, at a pH of between about 1 and about 4; and   after the hexavalent chromium compound has been fully reduced, adding: (1) a polymer system having a plurality of carboxylic functional groups, (2) an organo-functional silane, and (3) an organopolyphosphonic acid.   
     
     
         19 . A method for coating a metal surface comprising:
 roll coating the metal surface with the composition of  claim 1 ;   spraying the metal surface for a period of up to about 300 seconds in the composition of  claim 1 ; or   dipping the metal surface for a period of up to about 300 seconds in the composition of  claim 1 .   
     
     
         20 . The method of  claim 19  wherein the metal surface is selected from the group consisting of zinc, zinc alloys, aluminum, aluminum alloys, steel, zinc coated steel, and zinc with aluminum alloy.