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Aluminum Workshop: Welding anodized aluminum

Q: I bought some anodized 6061-T6 tubing to make a structural framework and I’m having trouble welding it. The welds are black and ugly. What am I doing wrong?

A: You probably aren’t doing anything wrong. It is very difficult to weld over anodizing. First, let me explain what anodizing is. Anodizing is an artificial aluminum oxide coating of sorts that forms after the aluminum has been placed in an electrochemical bath with a current applied. The coating is very thick—about 1,000 times thicker than a natural oxide coating—and usually a clear satin color, although various alloys can give different colors to the anodized coating.

The coating is also fairly porous, so that it can accept various dyes, which are what you are looking at when you see bright red or blue baseball bats. The anodized coating gives the aluminum an attractive, uniform, hard surface which is also very corrosion-resistant.

The thick aluminum oxide coating that forms after anodizing makes it very arduous to weld over. First of all, aluminum oxide is an electrical insulator, so starting and maintaining a stable arc is difficult. If the anodized layer is quite thick, such as that produced by hardcoating, you may not even be able to strike an arc at all.

Second, although aluminum melts at 1,260 degrees F, aluminum oxide melts at around 3,700 degrees F. This means the oxide doesn’t melt during welding, which causes the weld puddle to be very gummy and not fluid like it is normally.

Last, as I’ve said previously, the oxide is porous, so it will absorb water or water vapor from the air. If you try to weld over it, the water is liberated and produces a very porous weld. At the end of it all, you usually produce an extremely porous, poorly flowing weld.

Advising Against Welding Anodized Aluminum

As a general recommendation, I advise you not to weld over anodized coatings. Instead, use a sanding or grinding disc to remove the anodizing in the area you want to weld. Or you can use a rather specialized technique that can produce good results in some cases. It requires quite a lot of practice to perfect and is used only in a very few industries, so I won’t go into detail here. If any of you are really interested in exploring this technique, you can reach me by phone or email to discuss it.

About the Author
Aluminum Consulting Inc.

Frank Armao

President

Aluminum Consulting Inc.

440-479-0239

Frank Armao was an active member of the AWS D1 Committee, chairman of the AWS D1 Aluminum Subcommittee, and member of the Aluminum Association Committee on Welding and Joining. He also was the author of The WELDER's "Aluminum Workshop" column from 2001 to 2020.