Manufacturing: Surface Finishing

Anodizing Process

Why do you anodize aluminum?

Exposed to the earth's atmosphere, aluminum combines with oxygen to form a protective surface film which inhibits further oxidation of the aluminum. Unlike steel or iron alloys, aluminum will not continue to oxidize (rust) once this protective layer is formed. This natural oxide is extremely thin and loosely adhered to the aluminum surface, however, and is easily removed by handling. Anodizing is a process which thickens the natural oxide film resulting in a heavy aluminum oxide film of controlled thickness having the hardness similar to that of a ruby or sapphire.

Aluminum anodizing is the electrochemical process by which aluminum is converted into aluminum oxide on the surface of a part. This coating is desirable in specific applications due to the following properties:

• Increased corrosion resistance
• Increased durability / wear resistance
• Ability to be colored through dying
• Electrical insulation
• Excellent base or primer for secondary coatings

During the anodizing process, several controls are critical to assure the specified film thickness, its abrasion resistance and density. These controls include a precise combination of chemical concentration, temperature and current density. In the production of quality anodized products, there is no alternative to having sophisticated monitoring equipment and highly-trained, experienced personnel. The company you choose for your anodizing projects must be able to demonstrate these qualities.

The Process

When aluminum is anodized conventionally, direct electrical current (DC) is passed through a bath of sulfuric acid -- the electrolyte -- while the aluminum being treated serves as the anode. This produces a clear film of aluminum oxide on the aluminum's surface. Electron microscopy indicates that this layer is mostly porous with a very thin barrier layer at the base. This structure lends itself very well to electrolytic coloring or absorptive dying.

1. Pre-Treatment: Cleaning is done in a non-etching, alkaline detergent heated to approximately 145 degrees Fahrenheit. This process removes accumulated contaminants and light oils.
   
   
2. Rinsing: Multiple rinses, some using strictly de-ionized water, follow each process step.
   
   
3. Etching (Chemical Milling): Etching in caustic soda (sodium hydroxide) prepares the aluminum for anodizing by chemically removing a thin layer of aluminum. This alkaline bath gives the aluminum surface a matte appearance.
   
   
4. Desmutting: Rinsing in an acidic solution removes unwanted surface alloy constituent particles not removed by the etching process.
   
   
5. Anodizing: Aluminum is immersed in a tank containing an electrolyte having a 15% sulfuric acid concentration. Electric current is passed through the electrolyte and the aluminum is made the anode in this electrolytic cell; the tank is the cathode. Voltage applied across the anode and cathode causes negatively charged anions to migrate to the anode where the oxygen in the anions combines with the aluminum to form aluminum oxide (Al2O3). View our anodizing video.
   
   
6. Coloring: Anodic films are well suited to a variety of coloring methods including absorptive dyeing, both organic and inorganic dyestuffs, and electrolytic coloring, both the Sandocolor® and Anolok® processes.
   
   
7. Sealing: In all the anodizing process, the proper sealing of the porous oxide coating is absolutely essential to the satisfactory performance of the coating. The pores must be rendered nonabsorbent to provide maximum resistance to corrosion and stains. This is accomplished through a hydrothermal treatment in proprietary chemical baths or by capping the pores via the precipitation of metal salts in the pore openings.