Multifunctional anodized layer

Abstract

A method of anodizing includes immersing an aluminum alloy workpiece in a phosphoric acid anodizing solution and applying a voltage to form a porous oxide layer on the workpiece. The workpiece is then removed from the phosphoric acid anodizing solution and immersed in a controlled anodizing solution. A voltage is applied to form a dense oxide layer under the porous oxide layer. Dissolution of the porous oxide layer is controlled during the formation of the dense oxide layer by using tartaric acid in the controlled acid solution such that the thickness of the porous oxide layer is substantially equivalent before and after the formation of the dense oxide layer. The duplex anodized layer can be further sealed by soaking in a sealing solution to protect the porous oxide layer from hydrolytic decomposition, to improve corrosion protection, and to enhance the bonding with other structural components through adhesives.

What is claimed is: 
     
         1 . A method of anodizing comprising:
 immersing an aluminum alloy workpiece in a phosphoric acid anodizing solution;   applying a voltage to the aluminum alloy workpiece in the phosphoric acid anodizing solution, the phosphoric acid anodizing solution and the voltage acting to form a porous oxide layer on the aluminum alloy workpiece;   removing the aluminum alloy workpiece from the phosphoric acid anodizing solution and then immersing the aluminum alloy workpiece in a controlled anodizing solution;   applying a voltage to the aluminum alloy workpiece in the controlled anodizing solution, the controlled anodizing solution and the voltage acting to form a dense oxide layer on the aluminum alloy workpiece under the porous oxide layer; and   controlling dissolution of the porous oxide layer during the formation of the dense oxide layer by using tartaric acid in the controlled acid solution such that the thickness of the porous oxide layer is substantially equivalent before and after the formation of the dense oxide layer.   
     
     
         2 . The method as recited in  claim 1 , wherein the controlled anodizing solution includes the tartaric acid and sulfuric acid. 
     
     
         3 . The method as recited in  claim 1 , wherein the applying of the voltage to the aluminum alloy workpiece in the controlled anodizing solution includes ramping the voltage to a predetermined hold voltage within three minutes, and then holding at the predetermined hold voltage for no more than 30 minutes. 
     
     
         4 . The method as recited in  claim 1 , wherein the controlled anodizing solution has a temperature of 20-35° C. during the applying of the voltage. 
     
     
         5 . The method as recited as  claim 1 , wherein the tartaric acid has a concentration in the controlled acid solution of 60-100 gram/L. 
     
     
         6 . The method as recited in  claim 1 , wherein the controlled anodizing solution consists essentially of the tartaric acid and sulfuric acid. 
     
     
         7 . The method as recited in  claim 6 , wherein the controlled anodizing solution has a ratio of the tartaric acid to the sulfuric acid from 1:1 to 4:1. 
     
     
         8 . The method as recited in  claim 6 , wherein the controlled anodizing solution has a ratio of the tartaric acid to the sulfuric acid of approximately 2:1. 
     
     
         9 . The method as recited in  claim 1 , wherein the phosphoric acid anodizing solution is a 7.5 volume % phosphoric acid aqueous solution, and the phosphoric acid anodizing solution is at room temperature of 20-25° C. during the applying of the voltage to the aluminum alloy workpiece in the phosphoric acid anodizing solution. 
     
     
         10 . The method as recited in  claim 1 , wherein the phosphoric acid anodizing solution consists essentially of an aqueous phosphoric acid solution, and the controlled anodizing solution consists essentially of the tartaric acid and sulfuric acid. 
     
     
         11 . The method as recited in  claim 1 , further comprising immersing the aluminum alloy workpiece that has the porous oxide layer and the dense oxide layer in a nitrilotrismethylene solution. 
     
     
         12 . The method as recited in  claim 1 , further comprising immersing the aluminum alloy workpiece that has the porous oxide layer and the dense oxide layer in an aqueous trivalent chromium-containing sealing solution to deposit a chromium compound in the dense oxide layer. 
     
     
         13 . An anodized article comprising:
 an aluminum alloy substrate with a surface portion that is converted to a porous oxide layer of aluminum oxides/phosphates;   a dense oxide layer under the surface portion, wherein the porous oxide layer of aluminum oxides/phosphates and the dense oxide layer together are a duplex coating that has an electric resistance of at least 10 9  Ohms; and   an electrically conductive material adjacent the duplex coating, the electrically conductive material being different in composition from the aluminum alloy, and the electric resistance of the duplex coating providing a galvanic corrosion bather between the aluminum alloy substrate and the electrically conductive material.   
     
     
         14 . The anodized article as recited in  claim 13 , wherein the dense oxide layer includes residual tartaric acid and sulfate ions. 
     
     
         15 . The anodized article as recited in  claim 13 , wherein the dense oxide layer is sealed with a chromium compound. 
     
     
         16 . The anodized article as recited in  claim 13 , wherein the dense oxide layer is thicker than the porous oxide layer. 
     
     
         17 . An anodized airfoil comprising:
 an aluminum alloy airfoil extending between a leading end and a trailing end, with at least a surface portion of the leading end being converted to a porous oxide layer of aluminum oxides/phosphates;   a dense oxide layer under the surface portion, wherein the porous oxide layer of aluminum oxides/phosphates and the dense oxide layer together are a duplex coating that has an electric resistance of at least 10 9  Ohms; and   a sheath formed of an electrically conductive material and mounted adjacent the duplex coating at the leading end of the aluminum alloy airfoil, the electrically conductive material being different in composition from the aluminum alloy, and the electric resistance of the duplex coating providing a galvanic corrosion barrier between the aluminum alloy airfoil and the electrically conductive material of the sheath.   
     
     
         18 . The anodized airfoil as recited in  claim 17 , wherein the dense oxide layer includes residual tartaric acid and sulfate ions. 
     
     
         19 . The anodized airfoil as recited in  claim 17 , wherein the dense oxide layer is sealed with a chromium compound. 
     
     
         20 . The anodized airfoil as recited in  claim 17 , wherein the dense oxide layer is thicker than the porous oxide layer.