Anodizing Process Overview

Anodizing is a process for finishing aluminum that employs electrolytic oxidation of the metal surface to produce a clear, protective coating. The metal itself is the raw material for the coating, thus anodizing is said to be a conversion coating.

Anodizing is accomplished by immersing the work in a series of solutions where various operations are performed. The solutions are held in open-top tanks and the work passes through each in turn. The work is rinsed thoroughly after each operation in order to avoid contamination and interference in the next solution.

The steps are as follows:

Cleaning – work is soaked for several minutes in a water-based solution containing detergents and mild alkalies or acids.

Pre-Treatment – an operation used to improve the appearance of the metal prior to the application of the coating. The most common pre-treatments are Etch and Bright Dip

Etching – a solution of caustic soda (sodium hydroxide). Etching imparts a satin, matte finish by dissolving aluminum from the surface in a microscopically irregular pattern, causing reflected light to be scattered. The Etch bath is fairly aggressive in dissolving aluminum. An etched surface may loose a thousandth of an inch (a mill).

Desmut – after Etch or Bright Dip the work is immersed in a “desmut” or “deoxidizer” to remove residues of alloying agents (smut) left on the surface by these operations. Dilute nitric acid is a commonly used desmutting agent.

Anodizing – the step that produces the actual coating. The metal becomes the positive electrode (anode) in an electrolytic cell. In conventional anodizing (Type II), sulfuric acid is the electrolyte. When direct current is passed through the cell, water in the electrolyte is decomposed into hydrogen and oxygen. The reaction is complex, but the net result is that oxygen is liberated at the anode, and combines with the aluminum metal to form aluminum oxide. This process is allowed to continue until the desired coating thickness is achieved. The freshly formed anodic coating is transparent and colorless, and therefore almost invisible…Clear Anodize.

Coloring / Dye - color is absorbed into the pores by soaking in a dye bath, as you might color cloth. A large variety of dyestuffs and colors are available, but they are not all equally stable over time, and in the presence of sunlight. Thus they are not well suited for outdoor application.

Sealing / Conventional - accomplished by soaking the freshly anodized (or colored) work in a solution of metal salts, such as nickel acetate in hot water.

Sealing / Hot Water - requires a longer immersion time than nickel acetate, and has a narrow operating range for purity, temperature, pH, etc.

Some applications require welding after anodizing, and unlike nickel sealed coatings, hot water only sealed coatings will not discolor (turn yellow) under the heat of welding. Hot water only seal will also allow for painting of the anodized metal.