Aluminum Workshop: Matching colors after anodizing

Q: I’m welding on 6061-T6 tubing which then gets anodized. I was advised to use 5356 filler metal to get a good color match between the tubing and the weld after anodizing. The color match is close, but the weld and the tubing are a slightly different color after anodizing. What can I do to get an exact color match?

A: Anodizing produces a satiny, shiny, clear finish on the weld and tubing. Usually this color matches well; however, the situation can change when an organic dye is infused into the anodized surface, which produces colored anodizing. When the anodizing is colored, the match between the weld and the base material is rarely exact.

The advice you were given to use 5356 was good, as it will provide the best color match of any available filler metal. But the weld and the base metal are usually slightly different colors. Why? The chemistries of 5356 and 6061 are different, so they anodize slightly differently (OK, so I’ll use a filler with the same chemistry as 6061, you say? No, you won’t. If you do that, assuming you could find such a filler metal, the weld would crack.)

The tubing has a fine wrought grain structure. The weld has a coarse cast grain structure. From an electrochemical standpoint, that means the weld and the base metal have a different galvanic potential even if they are the same chemistry. This is enough difference so that they will still anodize slightly differently.

So from a welding standpoint, there isn’t much you can do. But don’t give up. Discuss this with the company doing the anodizing. If it is being too aggressive with the acid etching used during the process, it will accentuate the color difference. It’s possible the anodizer can modify its parameters to minimize the color difference.