Method for anodizing aluminum materials

Abstract

Structural components made of aluminum and aluminum alloys are anodized in two sequential steps in two different electrolytes. The first electrolyte is a mixed acid of two inorganic acids such as a phosphoric-sulfuric acid mixture. The second electrolyte is a further acid mixture of an organic acid and an inorganic acid to form a tartaric sulfuric acid mixture. The two sequential anodizing steps result in a surface texture that has three excellent surface characteristics simultaneously, namely: a corrosion resistance, a coating acceptance for lacquer coatings and an adhesive bonding with other aluminum material components.

1 . A method for anodizing a surface of structural components made of aluminum or aluminum alloys, said method comprising the following steps: 
 a) first preparing an inorganic acid mixture of at least two inorganic acids as a first electrolyte,    b) first exposing said surface to said first electrolyte to provide an anodized first surface coating,    c) second preparing a further acid mixture of an organic acid and an inorganic acid as a second electrolyte, and    d) second exposing said surface to said second electrolyte to provide a second surface coating.    
     
     
         2 . The method of  claim 1 , wherein said first preparing step is performed by mixing a phosphoric acid and a sulfuric acid to form said inorganic acid mixture as said first electrolyte.  
     
     
         3 . The method of  claim 2 , wherein said inorganic acid mixture contains between 50 to 250 g/l (gram per liter) phosphoric acid (H 3 PO 4 ) and 50 to 150 g/l of sulfuric acid (H 2 SO 4 ).  
     
     
         4 . The method of  claim 3 , wherein said inorganic acid mixture contains a mixture of 125 g/l of phosphoric acid (H 3 PO 4 ) and 75 g/l of sulfuric acid (H 2 SO 4 ).  
     
     
         5 . The method of  claim 1 , wherein said second preparing step comprises mixing a tartaric acid and a sulfuric acid to form said further acid mixture as an organic and inorganic acid mixture forming said second electrolyte.  
     
     
         6 . The method of  claim 5 , wherein said further acid mixture forming said second electrolyte contains 20 to 150 g/l (L(+)tartaric acid, and 20 to 150 g/l of sulfuric acid (H 2 SO 4 .)  
     
     
         7 . The method of  claim 6 , wherein said second electrolyte contains 80 g/l of sulfuric acid (H 2 SO 4 ).  
     
     
         8 . The method of  claim 1 , further comprising performing said first and second exposing steps each at an anodizing DC voltage profile, and controlling said DC voltage profile such that an initial rising DC voltage forming a ramp voltage as a function of time, is followed by a constant DC voltage forming a plateau as a function of time.  
     
     
         9 . The method of  claim 8 , wherein said ramp voltage reaches said plateau voltage at a DC voltage within the range of 3 to 25 V DC.  
     
     
         10 . The method of  claim 8 , wherein said ramp voltage has a rise time within the range of 0.5 to 10 minutes.  
     
     
         11 . The method of  claim 8 , wherein said plateau voltage is maintained for a duration within the range of 5 to 90 minutes.  
     
     
         12 . The method of  claim 8 , further comprising performing said first exposing step at an anodizing DC voltage profile by controlling said anodizing DC voltage to increase from 0 to 15 V DC within 5 minutes and then holding said DC voltage at 15 V DC for 15 minutes.  
     
     
         13 . The method of  claim 8 , further comprising performing said second exposing step at an anodizing DC voltage, and controlling said anodizing DC voltage to increase from 0 to 13 V DC within 3.5 minutes and then holding said DC voltage at 13 V DC for 25 minutes.  
     
     
         14 . The method of  claim 8 , further comprising performing said first and second exposing step at a temperature within the range of 20 to 70° C. for anodizing.  
     
     
         15 . The method of  claim 8 , further comprising performing said first exposing step at room temperature for anodizing.  
     
     
         16 . The method of  claim 8 , further comprising performing said second exposing step at a temperature of 35° C. for anodizing.  
     
     
         17 . The method of  claim 8 , further comprising continuing said exposing steps until an anodized layer thickness is achieved within the range of 1 to 10 μm for each layer formed by said first and second exposing steps.  
     
     
         18 . The method of  claim 8 , wherein said inorganic acid mixture for said first exposing step contains between 50 to 250 g/l of phosphoric acid (H 3 PO 4 ).  
     
     
         19 . The method of  claim 8 , wherein said further acid mixture of said inorganic acid and of said organic acid for said second exposing step contains between 20 to 150 g/l of L(+)tartaric acid.  
     
     
         20 . A structural component made of any one of aluminum and aluminum alloys comprising an anodized surface texture produced according to the steps of  claim 1 .  
     
     
         21 . A method for anodizing a surface of structural components made of aluminum or aluminum alloys, said method comprising the following steps: 
 a) first preparing an acid mixture of an organic acid and an inorganic acid as a first electrolyte,    b) first exposing said surface to said first electrolyte to provide a first surface coating,    c) second preparing an inorganic acid mixture of at least two inorganic acids as a second electrolyte, and    d) second exposing said surface to said second electrolyte to provide an anodized second surface coating.