Practical Welding Today - March/April 2009

Mastering GTAW of

anodized aluminum

A welder’s guide

useful material in the first place. It is relatively inexpensive, visually appealing, lightweight, and corrosion-resistant, making it suitable for high-salinity environments, such as coastal areas. It is used in the manufacture of tuna towers, rod holders, chairs, and T-tops on sport fishing boats, piers, bow rails, and wakeboard towers.

Without any treatment, aluminum naturally forms a very thin layer of oxides. The process of anodizing aluminum uses an electrolytic chemical application, usually with sulfuric acid, to create a layer of oxides several times thicker than would naturally form —0.0002 to 0.001 inch thick.

GTAW on anodized aluminum requires different equipment, preparation, and technique than on standard aluminum. Its corrosion resistance makes it ideal for boating accessories, such as this tuna tower, that are exposed to high-salinity environments.

By Jack Fulcer

Performing gas tungsten arc welding (GTAW) on anodized aluminum is a bit like trying to poke a hole in a sheet of ice without disturbing the water underneath. In this case, the ice is the hard layer of oxide created by the anodization process and the water is the soft aluminum underneath.

Anodized aluminum is simply standard aluminum that has been treated to produce

a thick layer of oxides on its surface. The aluminum base is a very soft material and melts at approximately 1,100 degrees F, but the oxide layer is extremely hard (some types approach the hardness of diamonds) and melts at 3,600 degrees F. The difficulty in welding anodized aluminum lies in removing the oxide layer without burning through the aluminum base.

Interestingly, it is these properties of the oxide layer that make anodized aluminum a

Anodized Aluminum Types

The four types of anodized aluminum are standard, bright-finish, colored, and hardened.

Bright-finish anodized aluminum can be visually distinguished from standard anodized aluminum by its shiny, chrome-like finish. This type of material has a thicker layer of oxides than standard anodized aluminum, making it more difficult to GTAW, and is used primarily for cosmetic reasons.

Colored anodized aluminum is also used for cosmetic purposes. This material uses dyes in the anodization process, which allow the material to take on different hues, but also introduce potential contaminants into the weld.

Hardened anodized aluminum is almost as hard as a diamond and is very difficult to weld. This type of material is usually used only in highly specialized industrial applications.

Bright-finish, colored, and standard anodized aluminums all use much the same equipment, preparation, and technique as standard aluminum, but they have some unique requirements in order to be welded successfully.

Selecting the Right Equipment

Selecting the right equipment is the first step in welding anodized aluminum successfully. Equipment unique to the process includes a torch with a fingertip control and a 5356-class filler metal. An air-cooled torch is suitable for less than 200 amps, but a water-cooled torch should be used for applications requiring more amperage.