Rainbow effect / defect on anodized aluminum?

A. Rainbows sometimes appears if you have a high pH in your sealing tank. But only (as I know of)when sealing dark colors. I have had this problem when sealing deep black MLW (10 g/l), bronze 2LW (3 g/l) the alloy was 6063-T6.

A. Rainbow (or iridescence) on clear anodize is sometimes due to thin coating thickness. A coating thickness check on rainbow vs.non-rainbow should tell you if you're heading in the right direction.If it is coating thickness differences, there are many possible causes. Verify first, then we'll discuss further later.

A. Shankar

Check your coating thickness. My guess is that you do not want a very thick coating, but by going to the other extreme, you could be getting the rainbow.

The only time I have seen rainbow on parts is when they lost contact early in the anodize process, and had almost no coating.

A. Hi, Sandrine.

Although it is difficult to troubleshoot from many miles away, rainbow 'iridescence' is a phenomenon that is sought in such fields as carnival glass and the anodizing of titanium jewelry, and it is achieved by applying a transparent/translucent coating of partial wavelength thickness. In such cases, part of the light bounces off the outer surface of the coating, and part bounces off the inner surface, and it interferes, canceling out some wavelengths. So two possible explanations include: 1). The anodizing is very thin, only partial wavelength thickness. 2). Somewhere in the post-anodizing steps, an oily film has been deposited.

If you can attach some photos someone may recognize the situation. Good luck.

Regards,

A. Current/voltage and anodizing solutions being proper, there could be two possible reasons.
The jigging of the parts is probably causing them to loosen during the process and hence get improper electrical contact resulting in poor anodizing.

The time for anodizing is insufficient. Increasing the time could give better results.

Hope this helps.

A. Make sure the temperature of your tank is within the range.

A. I think that the original post wanted to have a rainbow effect. That is normally a result of a very thin anodize which will not take color very well For dye and probably the same for two step.

Here is what I learned on anodizing.org:

Why do aluminum parts I send to be black anodized sometimes come back with a purple hue, and how can this be avoided?

Thank you for your responses.

The plan is to switch to a nickel acetate sealer, it is just slow going getting there. As far as the 1 mil, it was a measured 1 mil. We will lower the pH of the DI water with acetic and see what we come up with.

Upon further investigation, when we run 6061 vs 7075 same procedure and same thickness, we only get the sheen on the 6061 parts.

Thanks!

A. Hi David. Rainbow coloration -- whether it be on a puddle of water, or in carnival glass, or on anodized aluminum -- is almost always actually due to a transparent or translucent film which is so thin and variable in thickness that light bouncing off the top of the film and the bottom of the film "interferes", causing the various colors. It is possible that the film is a dye-smut "oil" on top of the anodizing, or that the film is the anodizing itself being too thin in that area. I think it's easily checked with an eddy current thickness tester. Good luck.

Regards,

A. Hi cousin Nawaz. We appended your inquiry to a thread which discusses the possibilities in good detail. Most likely, either the anodizing is extremely thin due to a bad connection or other issue, or an "oil film" from a dye smut or other post-anodizing tank has been deposited onto the parts. Good luck, and forward a photo of the problem to if you can.

Regards,

A. Hi Karima,
My first guess is that it's just a partial-wavelength interference film as discussed on this thread. But the same film on anode and cathode sounds a bit strange. Are you sure your power supply is okay? BTW, giving the amperage without the area of the part doesn't help much, since we can't determine the current density without the area :-)

Luck and Regards,

A. Rainbow major cause is low micron and loose jigging or loose tying of parts. Just make sure your parts are tightly held is anodising process or try to increase the micron with help of increasing current density or time in anodising tank.

Hope it serves.

A. Neville

Your rainbow appearance might be from localized overheating, typically a result of inadequate anodize solution agitation.

A. Check/measure your coating thickness after anodizing. First reason for rainbow appearance after anodizing: low micron thickness (under 4).

A. Hi Frits. What actually causes rainbow coloration, whether it be carnival glass, an oil-slick on a puddle of water, a DVD-ROM, anodized titanium, or your anodized aluminum, is a very thin (partial wavelength) transparent or translucent coating. Incident light reflects off the outside of the coating and off the substrate (or some would say the inside of the coating) and, since one path is longer than the other, the two reflections are out of phase and interfere, augmenting one wavelength (color) and attenuating another.

The coating which is causing the interference can be either a very thin anodized film, or a film of oil. In your case it is apparently a thin anodizing layer rather than an oil.

Potential solutions which may or may not be inapplicable for you include using a different alloy which produces a clearer anodized layer so that you can go thicker; with slight simplification, the purer the aluminum and the finer the grain, the thicker you can go. Electropolishing or chemical polishing (bright dipping) may allow a thicker anodized coating without dulling than buffing.

Hopefully a reader can relate anecdotes of rainbow coloration being a problem or not being a problem in your industry. Sorry, I can't offer any assurance about that myself and it seems that there will always be at least one customer who will complain :-)

Regards,

A. Hi Frits. Type 6xxx aluminum is generally quite acceptable for anodizing. But when we want the anodized layer to be as transparent as possible, for reflectors and mirrors for example, very pure 1xxx aluminum is better. 5 to 10 minutes of anodizing at 5 to 10 ASF is not uncommon for such applications, but I don't have any personal experience regarding whether, for example, 5 minutes at 10 ASF or 10 minutes at 5 ASF is better for your requirement.

Regards,

A. Hi Deepu. This thread, plus four or five others on this site, explain what generates the rainbow, but generally discuss the rainbow look on anodized aluminum more as a defect more than as something sought after. However, I would think that if you mirror polish the aluminum and then apply a very thin anodizing or clearcoat, you may be able to achieve a rainbow finish that pleases you.

I doubt, though, that it will ever be as impressive as a titanium rainbow finish because titanium is a harder metal with a more consistent oxide film.

Regards,

A. Hi Deepu. Although it's possible, it may not be practical or cost competitive depending on what the article is, its size, thickness of material, etc., because color anodizing of titanium is very easy and low cost.

Why try to imitate titanium with aluminum? Have you considered multicolor anodizing (beuatiful examples on Thread 48706), "splash anodizing", or other decorative aluminum effects?

Regards,