How is Aluminum Anodized?

The purpose of anodizing aluminum is to form a layer of aluminum oxide that will protect the aluminum beneath it. Aluminum is a soft metal, and anodization is an electrochemical process that gives a metal surface like aluminum a decorative, durable, and corrosion-resistant finish, according to the Aluminum Anodizers Council (AAC). The aluminum oxide layer has much higher corrosion and abrasion resistance than the aluminum by itself. Anodized aluminum helps improve a product’s lifespan, durability, and aesthetics, among other benefits.

Anodized aluminum is used in a variety of consumer and commercial products and equipment.  Anodized aluminum can be found in a wide range of end products, such as:

• Building exteriors, such as storefronts, window frames, mailboxes, and roofing systems
• Appliances such as refrigerators, dryers, coffee brewers, ranges, televisions, and microwave equipment
• Equipment for the food (display cases) and leisure industries (golf carts, boats, and camping/fishing equipment), and
• Everyday items such as clocks, fire extinguishers, solar panels, telephones, picture frames, and bathroom accessories.

Aluminum is ideally suited to anodizing, although other nonferrous metals, such as magnesium and titanium, also can be anodized.

How is Aluminum Anodized?

According to the AAC, anodizing is accomplished by immersing the aluminum into an acid electrolyte bath and passing an electric current through the medium. A cathode is mounted to the inside of the anodizing tank, and the aluminum acts as an anode. Oxygen ions are released from the electrolyte to combine with the aluminum atoms at the surface of the part being anodized. A process of highly controlled oxidation, anodizing is the enhancement of a naturally occurring phenomenon.

The anodizing step takes place in a tank that contains a solution of sulfuric acid and water. After anodizing is complete, the parts can be immersed in an optional coloring tank, to achieve a deep black tone instead of the standard clear or silver finish.

The 4 Types of Anodization

There are generally four methods for anodizing aluminum.

1. Chromic Acid Anodize (CAA). Chromic acid anodizing results in the thinnest anodic coat of all anodization types. It appears gray in color and absorbs less color when dyed. Therefore, it can’t be used for a decorative finish, but can be dyed black as a non-reflective, protective coating.
2. Boric-Sulfuric Acid Anodize (BSAA). BSAA is an alternative to chromic acid, mostly due to environmental, worker safety, and health concerns. Paint adhesion is equal or superior to chromic acid, and the process is more energy-efficient than chrome-based processes. It’s commonly found in precision machined components, such as aircraft components.
3. Sulfuric Acid Anodize. This is the most common method for anodizing aluminum. It is particularly suited for applications where hardness and resistance to abrasion are required. However, where parts are subjected to considerable stress, (such as aircraft parts), the possible presence of the corrosive acid residue is undesirable.
4. Hard Anodize (Hardcoat). Hardcoat anodization is much thicker and denser than sulfuric anodization. Hardcoat is appropriate for aluminum components that need protection from extreme wear, abrasion, and corrosion. It can also be valuable where enhanced electrical insulation is required and for salvaging worn or mis-machined components.

Benefits of Anodized Aluminum

When a high-performance aluminum finish is needed, anodization fits the bill. Here are just a few of anodized aluminum’s benefits, according to the AAC:

• Durability and Cost Efficiency. Most anodized products have an extremely long lifespan. Their durability results in lower maintenance costs, creating greater long-term value and significant economic advantages.
• Color Stability and Aesthetics. Exterior anodic coatings are stable against ultraviolet rays, do not chip or peel, and are easily repeatable. Anodizing offers a variety of gloss and color alternatives and minimizes or eliminates color variations. Unlike other finishes, anodizing allows the aluminum to maintain its metallic appearance.
• Ease of Maintenance. Anodized aluminum is impervious to scars and wear from fabrication, handling, installation, and frequent surface dirt cleaning and usage. Mild cleaning usually restores an anodized surface to its original appearance.
• Health and Safety. Anodizing is a safe process that is not harmful to human health. Because the anodizing process is a reinforcement of a naturally occurring oxide process, it is non-hazardous and produces no harmful or dangerous by-products.

What Colors are Available in Anodized Aluminum?

The porous nature of aluminum oxide allows it to absorb dyes well, and subsequent sealing helps to prevent color loss in service. Some of the colors that can be applied to anodized aluminum include Black, Red, Blue, Green, Urban Grey, Coyote Brown, and Gold. Parts can be treated prior to anodizing to achieve a matte (non-reflective) finish.

Aluminum Anodized Color Chart

Aluminum service providers will be able to provide you with a full aluminum anodized color chart but, in general, you will find a full array of anodized colors are available.

What Types of Aluminum Can Be Anodized?

The following alloys of aluminum plate, aluminum tube, aluminum sheet, and aluminum bar are among those suitable for the anodizing process:

5XXX Series

This series’ alloying constituent is manganese. When anodized, the alloys in the 5XXX series, such as 5052, have a resulting oxide layer that is strong and clear. Common products in this series include architectural, welding, and lighting applications.

6XXX Series

The 6XXX series’ alloying constituents are magnesium and silicon. These alloys are excellent candidates for anodizing. The oxide layer that follows the anodizing process is transparent and offers excellent protection. Because the 6XXX series alloys, including 6061 (bar), 6061 (tube), 6061 (plate), 6063 (bar), and 6063 (tube), offer great mechanical properties and are readily anodized, they are frequently used for architectural and structural applications.

7XXX Series

This series of alloyed aluminum uses zinc as its primary alloying element. It takes to the anodizing process very well. The subsequent oxide layer is clear and offers great protection. It’s commonly used in automotive applications.

Anodized aluminum is a highly specialized product with a lot of custom characteristics. Kloeckner Metals is proud to have trusted sources with whom we partner to create customized solutions for our customers who need this type of service.