Black Finish for Aluminum

Question:

The original question from G.B. was, “I have instrument display bezels machined from 6061 aluminum plate. They are then engraved with a logo and lettering, then painted black. Because of the operating atmosphere, being a drilling rig in offshore waters, the paint gets scratched or damaged. I have tried black anodizing and found that it fades when exposed to sun and sea mist. I am asking for suggestions for a black finish that is tough and durable.”

My answer to G.B. was, “. . . polyurethane coating can be formulated to be extremely durable. Your best bet for paint may be black polyurethane. As an alternative, architectural anodized aluminum panels that have been coated with clear acrylic enamel have been used for many years . . . The building panels look as good today as they did when they were erected. I suggest you investigate this option by contacting aluminum architectural products suppliers.”

R.M. sent the following comments regarding my answer, “Carl, I just got the March 2000 PF and was reading your column. There was a question about having a black finish for aluminum. The person was trying to coat a 6061 instrument panel that was exposed to sun and a sea environment. First, 6061 is one of the hardest alloys to get a good black color for dyed anodizing. The next problem is that the dyes that could give good black color may not be UV resistant. This issue should be investigated with a good dye supplier. The bottom line here is all black dyes are not created equal.

“The reason that I did answer is the person should try hard anodizing. Alloy 6061 will have a good dark gray color with a 0.002 mil thick coating. The coating would be extremely scratch resistant. The color could be enhanced by dyeing with a UV-resistant black (it will take dye better than regular anodizing) and/or polishing. With the sea environment the panel should also be sealed.”

Answer:

Thank you for the information R.M. My solution to the problem involved organic coatings. Yours involved dyes for anodized aluminum, which are outside the realm of this column. Perhaps this question should have been posed to the Aluminum Finishing Forum. However, we all learned something. I feel cheated if I don’t learn something new everyday.