Process and composition for treating metal surfaces using trivalent chromium compounds

Abstract

Aqueous compositions useful as pretreatments prior to painting to reduce the formation of rust in the uncoated condition. The compositions consist essentially of water, a trivalent chromium compound with the formula Cr(HxPO4)3, where x can be 1.5 or 2, a silica compound, and optionally hydrogen peroxide. The composition may have a pH between about 1 and about 4. 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;   silica;   a trivalent chromium compound with the formula Cr(H x PO 4 ) 3 , where x can be 1.5 or 2.   
     
     
         2 . The aqueous pretreatment composition of  claim 1 , wherein the pretreatment composition is acidic. 
     
     
         3 . The aqueous pretreatment composition of  claim 1 , wherein the pretreatment composition has a pH of between about 1 and about 4. 
     
     
         4 . The aqueous pretreatment composition of  claim 1 , further comprising hydrogen peroxide. 
     
     
         5 . The aqueous pretreatment composition of  claim 1 , wherein the trivalent chromium compound is present in an amount of between about 10% wt. % to about 0.5% wt. %. 
     
     
         6 . The aqueous pretreatment composition of  claim 1 , wherein x is 1.5. 
     
     
         7 . The aqueous pretreatment composition of  claim 1 , wherein x is 2. 
     
     
         8 . An aqueous pretreatment composition for treating a metal surface, the composition consisting of:
 water;   silica;   a trivalent chromium compound with the formula Cr(H x PO 4 ) 3 , where x can be 1.5 or 2.   
     
     
         9 . The aqueous pretreatment composition of  claim 8 , wherein the pretreatment composition is acidic. 
     
     
         10 . The aqueous pretreatment composition of  claim 8 , wherein the pretreatment composition has a pH of between about 1 and about 4. 
     
     
         11 . The aqueous pretreatment composition of  claim 8 , wherein the pretreatment composition is acidic. 
     
     
         12 . The aqueous pretreatment composition of  claim 8 , wherein the trivalent chromium compound is present in an amount of between about 10% wt. % to about 5% wt. %. 
     
     
         13 . The aqueous pretreatment composition of  claim 8 , wherein x is 1.5. 
     
     
         14 . The aqueous pretreatment composition of  claim 8 , wherein x is 2. 
     
     
         15 . A method for preparing a metal pretreatment composition consisting of:
 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 can be 1.5 or 2, at a pH of between about 1 and about 4; and   adding silica after the hexavalent chromium compound has been fully reduced.   
     
     
         16 . The method of  claim 15 , wherein the silica is Aerodisp W7512S. 
     
     
         17 . 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 with the composition of  claim 1 .   
     
     
         18 . The method of  claim 17  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.