Method of anodizing a part made of aluminum alloy

Abstract

The invention provides a method of anodizing a part made of aluminum alloy. According to the invention, the method comprises the following steps in succession: providing an aqueous anodizing bath essentially comprising sulfuric acid at a concentration lying in the range 55 g/l to 85 g/l, and excluding the presence of any phosphoric acid or any boric acid; maintaining said bath at a constant temperature essentially lying in the range 15° C. to 27° C.; dipping said part into said bath; applying a voltage to said part dipped in said bath, the voltage lying essentially in the range 5 V to 30 V, with low current density on said part; and maintaining said part in said bath until a coating of desired thickness has been obtained, which thickness lies substantially in the range 1 μm to 3 μm.

What is claimed is:  
     
         1 / A method of anodizing an aluminum alloy part, the method comprising the following steps in succession: 
 providing an aqueous anodizing bath essentially comprising sulfuric acid at a concentration lying in the range 55 g/l to 85 g/l, and excluding the presence of any phosphoric acid or any boric acid;    maintaining said bath at a constant temperature essentially lying in the range  15 ° C. to 27° C.;    dipping said part into said bath;    applying a voltage to said part dipped in said bath, the voltage lying essentially in the range 5 V to 30 V, with low current density on said part; and    maintaining said part in said bath until a coating of desired thickness has been obtained, which thickness lies substantially in the range 1 μm to 3 μm.    
     
     
         2 / A method according to  claim 1 , in which the concentration of sulfuric acid in the bath lies essentially in the range 57 g/l to 67 g/l, and is preferably close to 62 g/l.  
     
     
         3 / A method according to  claim 1 , in which the bath is maintained at a constant temperature close to 22° C.  
     
     
         4 / A method according to  claim 1 , in which the voltage applied to the part dipped in the bath is fixed on a constant value throughout the duration of the anodizing treatment, said constant value lying in the range 5 V to 30 V.  
     
     
         5 / A method according to  claim 4 , in which the constant value of the voltage lies in the range 7 V to 20 V.  
     
     
         6 / A method according to  claim 1 , in which the voltage applied to the part dipped in the bath is initially fixed to a first constant value, and then after a predetermined duration, to a second constant value that is higher than the first, said first and second constant values both lying in the range 5 V to 30 V.  
     
     
         7 / A method according to  claim 6 , in which the first constant voltage value lies in the range 5 V to 11 V, and the second constant voltage value lies in the range 15 V to 30 V.  
     
     
         8 / A method according to  claim 1 , in which the current density on the part dipped in the bath remains well below 100 A/m 2 .  
     
     
         9 / A method according to  claim 8 , in which the current density is essentially less than 80 A/m 2 , preferably lying in the range 30 A/m 2  to 70 A/m 2 .  
     
     
         10 / A method according to  claim 1 , in which the aqueous anodizing bath also contains an acid alcohol having one to three acid functions, at a concentration lying in the range 12 g/l to 22 g/l.  
     
     
         11 / A method according to  claim 10 , in which the acid alcohol used in the that is tartaric acid or citric acid, and the concentration of said acid alcohol lies essentially in the range 12 g/l to 17 g/l.  
     
     
         12 / A method according to  claim 1 , in which the part to be treated is subjected to preliminary degreasing/pickling treatment prior to being dipped in the bath.  
     
     
         13 / A method according to  claim 1 , in which the treated part is subjected to subsequent treatment of sealing the coating.