Zirconium-Vanadium Conversion Coating Compositions For Ferrous Metals And A Method For Providing Conversion Coatings

Abstract

The invention provides a method and composition for coating a ferrous metal surface with a zirconium/vanadium conversion coating which is substantially free of an organic film forming composition and tannins. The method is a low temperature method which contemplates an aqueous conversion coating composition which is low in phosphates and which comprises zirconium, vanadium, fluoride, as well as phosphate ions in a ratio and a concentration effective for providing a conversion-coated ferrous metal surface.

1 - 20 . (canceled) 
     
     
         21 . A process for forming a conversion coating on a ferrous metal surface, the process comprising:
 exposing the ferrous metal surface to an aqueous composition comprising zirconium as ZrF 6   2−  ions, vanadium as a part of vanadium/oxygen ions, fluoride as a part of the ZrF 6   2−  ions, and phosphate ions in a ratio and a concentration effective for providing the ferrous metal surface with a protective conversion coating, the aqueous composition having a pH in the range of from 2.0 to 5.0, the aqueous composition forming the protective conversion coating which includes zirconium and vanadium throughout the pH range and forming a protective conversion coating on the metal surface which includes zirconium, vanadium and phosphate at a pH of from about 4 to about 5, and the ratio of fluoride to zirconium in the aqueous composition at least 6 fluoride to one zirconium to provide the aqueous coating composition consisting essentially of a hydrogen ion source, zirconium, vanadium, fluoride, oxygen and phosphate to provide a conversion coating on the ferrous metal surface, the conversion coating substantially free of an organic film forming composition and tannins and, the conversion coating including at least vanadium and zirconium.   
     
     
         22 . The process according to  claim 21 , wherein the ferrous metal surface is exposed to the aqueous composition at a temperature of from 70° F. to 90° F. and the aqueous composition includes a source of zirconium and fluoride which comprises hydrofluorozirconic acid and fluoroboric acid, a source of phosphate ions which comprises phosphoric acid, a source for vanadium which comprises ammonium metavanadate and the hydrogen ion source comprises nitric acid. 
     
     
         23 . The process according to  claim 21  wherein the aqueous composition comprises from 90 to 185 ppm zirconium, from 178 to 360 ppm fluoride, from 50 to 110 ppm vanadium, and from 50 to 180 ppm phosphate ions. 
     
     
         24 . The process according to  claim 23  wherein the ferrous metal surface is exposed to the aqueous composition at a temperature of from 70° F. to 90° F. 
     
     
         25 . The process according to  claim 24  wherein the ferrous metal surface is exposed to the aqueous composition for 30 seconds to two minutes. 
     
     
         26 . The process according to  claim 21 , wherein the aqueous composition provides a conversion coating consisting essentially of zirconium, vanadium, fluoride and oxygen on the ferrous metal surface when the ferrous surface is exposed to the aqueous composition at a pH of from about 2.0 to about 3.5, and wherein the aqueous composition provides a conversion coating consisting essentially of zirconium, vanadium, oxygen and phosphate on the ferrous metal surface when the ferrous surface is exposed to the aqueous composition at a pH of from about 4.0 to about 5. 
     
     
         27 . The process according to  claim 26 , wherein the aqueous composition provides a conversion coating consisting essentially of ZrF 4 , VO 2  and oxygen when the ferrous metal surface is exposed at a pH of from about 2.0 to about 3.5.