Process for brightening aluminum, and use of same

Abstract

The present invention relates to a process for treatment of the surface of a workpiece of aluminum or of an aluminum alloy, which is characterized in that the workpiece is brightened in an aqueous electrolyte by application of an electric d.c. voltage, the electrolyte having the following composition per liter of water: H 2 SO 4 (sulfuric acid) 85 to 340 g/l H 3 PO 4 (phosphoric acid) 850 to 1360 g/l Al (aluminum) 8.5 to 43 g/l.

We claim:  
     
         1 . A process for treatment of the surface of a workpiece of aluminum or of an aluminum alloy, comprising the steps of brightening the workpiece in an aqueous electrolyte by applying an electric d.c. voltage, the electrolyte having the following composition per liter of water:  
       
         
           
                 
                 
                 
                 
               
                     
                     
                 
                     
                     
                 
                     
                   H 2 SO 4  (sulfuric acid) 
                   85 to 340 
                   g/l 
                 
                     
                   H 3 PO 4  (phosphoric acid) 
                   850 to 1360 
                   g/l 
                 
                     
                   Al (aluminum) 
                   8.5 to 43 
                   g/l. 
                 
                     
                     
                 
                     
                     
                 
             
                
                
               
               
                
                
                
                
                
               
            
           
         
       
     
     
         2 . A process according to  claim 1 , wherein the concentration of H 2 SO 4  (sulfuric acid) in the electrolyte ranges from 85 to 255 g/l.  
     
     
         3 . A process according to  claim 1 , wherein the concentration of H 3 PO 4  (phosphoric acid) ranges from 1020 to 1360 g/l in the electrolyte.  
     
     
         4 . A process according to  claim 1 , wherein the concentration of Al (aluminum) ranges from 17 to 43 g/l in the electrolyte.  
     
     
         5 . A process according to  claim 1 , wherein the product of the concentrations of sulfate ions and aluminum ions divided by the square of the concentration of water and multiplied by 100 has a value of between 8 and 15.  
     
     
         6 . A process according to  claim 5 , wherein the product of the concentrations of sulfate ions and aluminum ions divided by the square of the concentration of water and multiplied by 100 has a value of between 9 and 12.  
     
     
         7 . A process according to  claim 1 , wherein the temperature of the aqueous electrolyte during brightening of the workpiece ranges from 60 to 80° C.  
     
     
         8 . A process according to  claim 1 , wherein the d.c. voltage for electrobrightening of the workpiece ranges from 12 to 45 V.  
     
     
         9 . A process according to  claim 1 , wherein the treatment time for electrobrightening of the workpiece ranges from 3 to 20 minutes.  
     
     
         10 . A process according to  claim 1 , wherein the workpiece surfaces to be brightened are mechanically polished and then chemically cleaned before electrobrightening.  
     
     
         11 . A process according to  claim 10 , wherein the workpiece surfaces to be brightened are chemically cleaned in a bath containing an aqueous solution of a mixture of phosphoric acid and surfactant.  
     
     
         12 . A process according to  claim 1 , wherein the brightened surfaces of the workpiece are pickled, passivated, anodized and then sealed after electrobrightening.  
     
     
         13 . A process according to  claim 12 , wherein the brightened surfaces of the workpiece are pickled in a solution of caustic soda (NaOH) and if necessary aluminum (Al) in water.  
     
     
         14 . A process according to  claim 13 , wherein the sodium hydroxide (NaOH) concentration ranges from 30 to 40 g/l and the aluminum (Al) concentration ranges from 0 to 15 g/l in the pickling solution.  
     
     
         15 . A process according to  claim 12 , wherein the pickled surfaces of the workpiece are passivated in a solution of nitric acid in water.  
     
     
         16 . A process according to  claim 15 , wherein the nitric acid concentration ranges from 160 to 190 g/l.  
     
     
         17 . A process according to  claim 12 , wherein the brightened surfaces of the workpiece are anodized in an aqueous electrolyte containing sulfuric acid (H 2 SO 4 ) and aluminum (Al).  
     
     
         18 . A process according to  claim 17 , wherein the sulfuric acid concentration ranges from 100 to 400 g/l and the aluminum concentration ranges from 7 to 15 g/l.  
     
     
         19 . A process according to  claim 12 , wherein during anodizing, the aqueous electrolyte has a temperature of 5 to 25° C.  
     
     
         20 . A process according to  claim 12 , wherein the anodic current density during anodizing ranges from 0.5 to 2.5 A/dm 2 .  
     
     
         21 . A process according to  claim 12 , wherein the workpiece is anodized for a period of 5 to 90 minutes.  
     
     
         22 . A process according to  claim 12 , wherein the thickness of the anodized aluminum layer ranges from 3 to 15 μm.  
     
     
         23 . A process according to  claim 12 , wherein the anodized surfaces of the workpiece are sealed by cold impregnation and hot-water compaction.  
     
     
         24 . A process according to  claim 12 , wherein the brightened surfaces of the workpiece are colored after anodizing.  
     
     
         25 . A process according to  claim 1 , wherein the brightened surfaces of the workpiece are coated directly with a clear lacquer after electrobrightening.  
     
     
         26 . A process according to  claim 1 , wherein the brightened surfaces of the workpiece are pickled, passivated, provided with an adhesion promoter and then coated with a clear lacquer after electrobrightening.  
     
     
         27 . A process according to  claim 1 , wherein the concentration of H 2 SO 4  (sulfuric acid) in the electrolyte is approximately 170 g/l.  
     
     
         28 . A process according to  claim 1 , wherein the concentration of H 3 PO 4  (phosphoric acid) is approximately 1088 g/l in the electrolyte.  
     
     
         29 . A process according to  claim 5 , wherein the product of the concentrations of sulfate ions and aluminum ions divided by the square of the concentration of water and multiplied by 100 has a value of approximately 9.4.  
     
     
         30 . A process according to  claim 1 , wherein the temperature of the aqueous electrolyte during brightening of the workpiece ranges from 62 to 75° C.  
     
     
         31 . A process according to  claim 1 , wherein the d.c. voltage for electrobrightening of the workpiece ranges from 25 to 40 V.  
     
     
         32 . A process according to  claim 1 , wherein the d.c. voltage for electrobrightening of the workpiece ranges from 30 to 38 V.  
     
     
         33 . A process according to  claim 1 , wherein the treatment time for electrobrightening of the workpiece is about 10 minutes.  
     
     
         34 . A process according to  claim 17 , wherein the sulfuric acid concentration ranges from 160 to 200 g/l, and the aluminum concentration ranges from 9 to 13 g/l.  
     
     
         35 . A process according to  claim 17 , wherein the sulfuric acid concentration is 180 g/l and the aluminum concentration is 10 g/l.  
     
     
         36 . A process according to  claim 12 , wherein during anodizing, the aqueous electrolyte has a temperature of about 18 to 19° C.  
     
     
         37 . A process according to  claim 12 , wherein the anodic current density during anodizing is about 0.6 A/dm 2 .  
     
     
         38 . A process according to  claim 12 , wherein the workpiece is anodized for a period of 5 to 25 minutes.  
     
     
         39 . A process according to  claim 12 , wherein the thickness of the anodized aluminum layer ranges from 5 to 10 μm.