Removing dye from anodized aluminum; changing the color

A. Hi Luke. I think there is no realistic chance to open the pores and bleach out the color. I think you'd need to strip the anodizing and then re-anodize, re-dye, and re-seal.

Type 6xxx alloys are easier to anodize than type 2xxx or type 7xxx, but that doesn't mean anodizing is easy. The next step is either to find an anodizing shop who will strip and recolor this for you, or to start practicing on scrap if you intend to try to anodize it yourself.

Luck & Regards,

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A. Hi Dennis. Bleach readily destroys organic dyes but, assuming the parts have been sealed, I don't think the bleach will be able to get at the dye effectively enough, although it is very high in caustic so it will attack the anodizing and the dye. What would you intend to do with the parts . . . just leave them natural aluminum color?

A. You can try a stripping solution made of the following:

85 % Phosphoric acid
20 % Chromic acid (200 grams/liter)
Top with Water

I believe we got this formula from one of the "Metal Finishing" publications.

It has proved effective for us and it doesn't harm the metal.

Hoping this will help you.

Regards,

P.S. Do check with the different publications ( Metal Finishing   [ed. note: unfortunately, Elsevier closed down Metal Finishing in December 2013], Products Finishing, etc.) for more info.

---

Thanks, Ariel, that doesn't attack the aluminum base metal but...

It removes the anodizing as well as the dye; wgich is fine as long as Dennis realizes it.

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

A. Depending upon what the purpose is, and for what period of time the parts have been dyed and sealed, it is possible to remove dye without damaging the coating. We are talking about organic dye impregnation of Sulfuric Acid produced anodic films as the dying process. One has to "break" the seal with a deox or 10% Nitric Acid solution. A deox solution works better if the parts have been sealed for very long, or if sealed in Hot Nickel Acetate. Monitoring of the part while breaking the seal will show a slight leaching out of the dye at the part surface. If using a deox solution to break the seal, as soon as the seal is broken immediately rinse, and then place the part in the Nitric Acid solution until the dye completely leaches out. This usually works if one is redyeing with the same color, or one is dying a darker color over a lighter one. Nickel Fluoride (cold seals), and Hot Water DI seals are easier to break and leach out the dye than parts sealed in Hot Nickel Acetate. This process sometimes leaves slight residues or light color tints. In an emergency, I have broken the seal, right after sealing in a cold seal, and re-anodized for another 15 minutes, and redyed which turned out fine. The parts remained wet and were never unracked. However, we all thought we were pretty lucky that one time. I would not ever recommend it.

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by Robert Probert

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A. Typical organic dyes for anodizing are little different than fabric dyes and ought to be readily bleachable (before sealing), although I'm only talking ideas here not actual experience. Whether it can be done in a reasonable time, with bleach that is weak enough to not harm your photoresist or the anodizing, I don't know. Once the surface is sealed, you would actually have to strip the anodized finish using the chromic acid-phosphoric acid stripper described in thread 2776.

A. Years ago we tried a strip that was basically a fine silica and nitric acid to de-color dyed anodize. Required a little manual agitation to work well. I have no idea who we bought it from.

A. It depends on the nature of the dye and the condition of the anodize. If the anodize is sealed, the only way to remove any dye is to remove the anodize with the chromic/phosphoric stripping solution that Ted mentions, or with an alkaline etch if you don't mind attacking the metal. If the anodize is unsealed and relatively fresh, 10-15% nitric acid will remove most dyes, but not all. Keep immersion times less than 15 minutes, as a slight acid attack will occur, and the film will become more porous and harder to seal, if required. Even if the anodize is supposedly unsealed, this may not work if the oxide is exposed to the atmosphere for days or weeks, as air and humidity will partially seal the film. If you intend to re-dye with a different color, the new dye will absorb faster after the acid treatment. I never learned why, but not all dyes will strip this way, and we used to sometimes be able to tell which dyes were actually mixtures of other colors, as sometimes one color would strip, but leave another color behind. I like Ted's idea about bleaching out the colors, but I believe that peroxide should be used for bleaching rather than hypochlorite, as any residual chlorides in the pores would be bad for corrosion resistance. Again, bleaching would not work if the anodize is sealed.

Phil Johnson
- Madison Heights, Michigan

A. The proper way to strip anodizing is as follows: 30% phosphoric acid to 70% DI water. If you stay on top of the striping tank, you should be able to test when the nickel acetate is dissolved , and the dye is starting to dissolve. Test by rinsing in running water. The above solution is designed to attack only the anodic coating, and not the substrate material.

Best of luck,

A. You are basically talking about what is commonly referred to as splash anodizing. You will find that the few people that can do it well are not going to talk. Why should they? If all of the finesse and witchcraft became public it would cost them dearly on the bottom line. Add to that the few companies that can do it very well hold their employees to a very ridged non-disclosure and non competition agreements.
All of that said, it is my understanding that most people do it the opposite of you. Put the light color on first and the darker colors on last. That hides the off color parts.

A. Another thing to consider is that using bleach will quite possibly accelerate corrosion after anodize. You may want to use Nitric acid instead.

Good luck.

Thank you for all the help. I have the splash down where you dye the lighter color, then mask and remove then dye the darker color. What I am having problems with is an acid wash finish. There are no defined lines, and the only way I can get it to work is if I lay the darker color, then wash some out by dabbing it with a sponge with the bleach. I haven't tried to use nitric, but will try that next. So I thank all of you for your help.

A. To do a good job, the equipment will cost several hundred dollars, and then you have the haz waste to dispose at a very high cost.
You can do an internet search and find all kinds of ways people have tried to do garage anodizing. some have good luck (so they say) and others have terrible luck. The main problems, is you need a controllable power source. A 12 volt battery charger simply will not do as good a job as something that you can ramp up to 15 volts and has enough amperage to not blow it up on the initial surge and has the amperage to get the part anodizing rapidly enough that you can raise the power to the final voltage in a matter of about 3 minutes.
Next, it generates a lot of heat. Decorative anodizing needs a constant temperature of about 72 °F, so you need some method of regulating the temperature. No, you cannot put ice cubes in the tank.
The dye step requires a given concentration, temperature and pH. Rit Dye [affil links] may work in some cases, but do not expect a permanent color.
Boiling DI water is a decent seal, but a nickel acetate seal is more permanent.
NO, you can not anodize over the existing anodize. It has to be stripped. When you strip the old anodize, you will remove about a thousandth of an inch from the existing size.This is going to affect the fit and function of the weapon, some. Your gun may be clear coated or waxed. This gives several more challenges to be overcome.
Spend the money and have more time to enjoy your paintball hobby. On second thought, the beautiful gun will not work a bit better. Spend the time and money on practice and become a winner.

A. Hi Carlos. The dye was 'sealed' into the pores of the anodized coating by closing the tops of those pores to lock it in. How 'ruinous' the process is depends not only on how it is done, but upon what standards you apply in saying whether the anodizing has been ruined :-)

You can carefully consider the previous replies on this thread which we appended your question to, and decide whether carefully following the prescribed plan 'ruins' the anodizing in your view. Best of luck!

Regards,

A. Hi Francis. If this item is just for your own personal and casual use, and it will be kept indoors, it might be possible to just darken it with a dark green permanent marker (that stuff does work pretty well on aluminum). If you are selling it or really want to actually fix it, the least you must do is break the seal, leach the dye, apply the darker dye, and re-seal.

Regards,

A. Hi Praveen. I don't think it is at all possible, at least once it's sealed, but we've posted your inquiry and we'll see if anyone can offer encouragement.

Luck & Regards,