Composition and method for surface treatment of oxidized metal

Abstract

The present invention provides an electrically conductive element for a proton exchange membrane fuel cell having low electrical contact resistance and high corrosion resistance. The conductive element comprises a corrosion susceptible metal substrate with a surface, which is preferably treated to activate the surface (i.e. to remove a passivation layer of oxides from the surface) with an acidic treatment solution. The treated surface is then overlaid with an electrically conductive, corrosion-resistant, protective coating to protect the substrate re-forming a passivation layer while exposed to the corrosive environment of the fuel cell. The present invention also provides methods of preparing an electrically conductive element to have low electrical contact resistance and high corrosion resistance.

1 . A composition for treating a surface of a metallic substrate having metal oxide at the surface, said composition comprising: 
 a solution comprising a solvent and solute, said solution having a pH equal to or less than about 4, wherein said solute comprises an anionic species of one or more halogen ions present in said solution at a concentration of from about 1×10 −1  to about 1×10 −6  molarity.    
     
     
         2 . The composition according to  claim 1 , wherein said concentration of halogen ions in solution is from about 1×10 −3  to about 1×10 −5  molarity.  
     
     
         3 . The composition according to  claim 1 , wherein said solvent comprises water.  
     
     
         4 . The composition according to  claim 1 , wherein said solution comprises sulfuric acid.  
     
     
         5 . The composition according to  claim 1 , wherein said solution comprises nitric acid.  
     
     
         6 . The composition according to  claim 1 , wherein said halogen ion is selected from the group consisting of: F − , Cl − , Br − , I − , and mixtures thereof.  
     
     
         7 . The composition according to  claim 1 , wherein said halogen ion comprises fluorine.  
     
     
         8 . The composition according to  claim 7 , wherein said solution comprises hydrofluoric acid.  
     
     
         9 . The composition according to  claim 1 , wherein said solution has a pH of from about 2 to about 4.  
     
     
         10 . The composition according to  claim 1 , wherein said solution comprises water, sulfuric acid, and fluoride ions at a concentration of between about 1×10 −3  and about 1×10 −5 .  
     
     
         11 . A method of treating a surface of a metallic substrate having metal oxide at the surface, the method comprising: 
 contacting the surface with an acidic solution, having a pH of from 0 to about 4, for reaction with the metal oxide, without an impressed electrical current, to form a metal halide species soluble in said solution thereby reducing the amount of the metal oxide; and,    separating said metal halide and said acidic solution from the surface.    
     
     
         12 . The method of  claim 11 , wherein said contacting is conducted at a temperature of from about 25° C. to about 100° C.  
     
     
         13 . The method of  claim 11 , wherein said contacting is conducted for a duration less than 600 seconds.  
     
     
         14 . The method of  claim 11 , wherein said separating comprises rinsing the surface with a rinse solution, thereby removing said acidic solution and said metal halide from the surface.  
     
     
         15 . A method of treating a surface of a metallic substrate having metal oxide at the surface, the method comprising: 
 contacting the surface with an acidic solution having an anionic species of one or more halogen ions present at a concentration of from about 1×10 −1  to about 1×10 −6  molarity for reaction with the metal oxide, without an impressed electrical current, to form a metal halide species soluble in said solution; and separating said metal halide and said acidic solution from the surface.    
     
     
         16 . The method of  claim 15 , wherein said contacting is conducted at a temperature of from about 25° C. to about 100° C.  
     
     
         17 . The method of  claim 15 , wherein said contacting is conducted for a duration less than 600 seconds.  
     
     
         18 . The method of  claim 15  wherein said separating comprises rinsing the surface with a rinse solution, thereby removing said acid solution and said metal halide from the surface.  
     
     
         19 . The method of  claim 15  wherein said metal oxide provides the source of said metal in said metal halide and said halogen ions provide the source of said halide in said metal halide, whereby the amount of said metal oxide at the surface is lessened.