Aluminum Workshop: The ABCs of anodizing

Q: What is anodizing? Is it the same as hardcoating, and can I do it myself? Can you weld on anodized parts?

A: Anodizing is a process in which a thin, uniform coating of aluminum oxide is formed on the surface of aluminum. Any aluminum alloy can be anodized, but different alloys form anodized coatings of slightly different colors.

The part to be anodized becomes the anode in an electrolytic cell. A carbon or metal rod is the other electrode. The electrolyte is most commonly 15 percent sulfuric acid. A thin coating of aluminum oxide forms on the aluminum anode when a current is passed through the cell.

Standard anodizing forms a relatively clear and colorless layer of aluminum oxide that is 0.00001 to 0.001 in. thick. Its purpose is either decorative or to form a uniform coating to protect the aluminum from atmospheric corrosion.

Hardcoating produces a thicker oxide layer of 0.001 to 0.005 in. thick. It’s usually gray to black in color and is used to provide aluminum with wear resistance.

Can you anodize parts yourself? Yes, it is fairly easy to anodize small aluminum parts. The process is a bit too long to describe here, but if you perform an Internet search on “home anodizing,” you will find a number of helpful articles.

I mentioned that anodizing produces a clear, shiny coating that could be used for decorative purposes. The coating is porous enough to absorb organic dyes. When you look at a bright red, green, or blue anodized part, the color isn’t from the anodizing, it comes from a dye that is diffused into the coating after anodizing.

Can you weld on anodized parts? You need to remember that the anodizing is aluminum oxide, which is a very good electrical insulator. It is usually very difficult to strike an arc on an anodized surface, and it is often impossible to strike one on a hardcoated surface. If you succeed in making a weld, it is usually very ugly with lots of oxide and porosity in it.

The easiest way to weld on anodized parts is to grind or sand off the anodized coating in the weld area. A few industries routinely weld anodized parts, but that requires specialized techniques that are beyond the scope of our discussion today.