What is Anodizing?

One of the greatest advantages of aluminum is its natural resistance to corrosion. Ferrous metals such as steel are highly vulnerable to corrosion due to their iron content; iron atoms react with oxygen to produce rust, which wears away and damages the metal. Aluminum atoms react even more quickly to oxygen, but its oxide doesn’t flake off as iron oxide does. Aluminum oxide is quite hard and difficult to remove from the metal’s surface, so it become a natural layer of protection from oxygen permeating the metal underneath.
However, this natural oxidation takes some time to occur – and manufacturers understandably want faster results to produce corrosion-resistant aluminum for sale. This is where anodizing comes into play: this electrochemical process enables oxide to form much more quickly than it would on its own. Unlike paint or plating, which can chip or wear away, the anodized surface layer is fully integrated with the metal itself to provide long-lasting protection.

The Anodizing Process
Anodizing is a multi-step process, but at its most basic it can be described as highly controlled oxidation. By treating the metal and applying an electric current, the aluminum atoms are prompted into an accelerated interaction with oxygen. The end result is a metal surface that is evenly and completely anodized, with a high degree of corrosion resistance.
These steps include:
1. Cleaning: A thorough cleaning must be done to prepare the aluminum for the processing. Either an alkaline or acid-based cleaner is used to remove any surface grease or dirt from the metal.

2. Pre-Treatment: The benefits of anodizing can be both functional and aesthetic. During processing, dyes and other treatments can be applied to create a more attractive appearance in the final product. To modify the metal’s finish, two procedures can be done as pre-treatment:

a. Etching: To achieve a matte finish, heated sodium hydroxide solution is applied to the aluminum. Sodium hydroxide, also known as lye, is an extremely caustic substance capable of corroding metal. The solution strips away any minor surface imperfections, leaving it smooth and matte.

b. Brightening: Mirror-finish aluminum is created by treating the aluminum with a concentrated mixture of phosphoric and nitric acids. Similar to the lye solution, the acids corrode away the metal’s top layer along with minor imperfections. Once complete, the result is smooth, shiny metal.

3. Anodizing: The cleaned and pre-treated aluminum is then submerged in an electrolyte solution. Anodizing requires electricity, and an acid solution provides a boost in conductivity. As the electric current passes through the tank containing the aluminum and solution, oxygen ions are released from the electrolytes and interact with the aluminum to form a layer of aluminum oxide. The process is carefully monitored to achieve the degree of oxide thickness for the desired product.

4. Coloring: To produce colored anodized aluminum, four different methods can be used. Since this is done during the anodizing process, the color will be highly resistant against fading, scratches, and other wear. Generally this is done through these four methods:

a. Dye: The anodized metal is immersed in a dye and lye solution.

b. Electrolytic coloring: An electric current sent through an electrolyte solution the dye to the aluminum’s surface.

c. Integral coloring: This process combines anodizing and coloring into one step to anodize, color, and seal the aluminum.

d. Interference coloring: The aluminum is treated with sulfuric acid and then soaked in dye.

5. Sealing: Although freshly anodized aluminum has the corrosion resistance of aluminum oxide, sealing offers an additional layer of protection. During the processing, the various chemicals used will leave microscopic “pores” open on the metal’s surface. Left unsealed, contaminants can be absorbed more easily onto the surface during storage, risking corrosion or discoloring. Once the pores in the anodic film are closed by applying a final seal, anodized aluminum will be highly resistant to scratches, abrasions, and color fading.