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topic 1685

Removing dye from anodized aluminum



A discussion started in 1998 & continuing through 2017

(1998)

Q. Is there a way to remove dye from anodized parts without damaging the anodized surface?

Dennis Nolan



Surface Treatment & Finishing of Aluminium and Its Alloys
Wernick, Pinner & Sheasby

(1998)

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. What do you intend to do with the parts . . . just leave them natural aluminum color?

Ted Mooney, finishing.com Teds signature
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey


(1998)

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 August 2014: Elsevier closed down Metal Finishing in December 2013] , Products Finishing, etc.) for more info.

Ariel Medina


Yes, Ariel, but...

That removes the anodizing as well as the dye; just so Dennis realizes that.

Ted Mooney, finishing.com
Teds signature
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey


(1998)

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 then 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.

Ward Barcafer, CEF
aerospace - Wichita, Kansas



To minimize searching and offer multiple viewpoints, we've combined multiple threads into the dialog you're viewing. Please forgive any resultant repetition or failures of chronological order.



Selective stripping of anodizing

(1999)

Q. I am looking for an acidic stripper of colored anodizing to selectively remove areas using photoresist masking to protect the other areas. If it just strips the color and leaves the clear anodizing, better yet.

Ron Squibbs
engraving company - Easton, Pennsylvania


probert book
Aluminum How-To

by Robert Probert
$89 New
The Chromating - Anodizing - Hardcoating Handbook

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 letter #2776.

Ted Mooney, finishing.com
Teds signature
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey


(1999)

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.

James Watts
- Navarre, Florida


(1999)

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

Phil Johnson
- Madison Heights, Michigan


(2002)

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,

Gene Small
Retired - Albany, Oregon, USA



Removing anodize dye before sealing

(2005)

Q. We are a small anodizing shop that has recently started to delve into the world of multi-color anodizing. We are having some issues in removing dye after the parts have been dyed. For instance, we will dye a part black, then remove some dye and then over dye with a lighter color such as orange. We have been using a mild bleach solution to remove the dye, but it hasn't been able to remove all of the dye to bring it back to the clear oxide. Typically it just becomes a brownish color, so when the lighter color is applied it turns out a darker shade than we want it to be. Also with this method, there has been some issues with the pores that have had the dye removed not taking the second dye color. I have seen some of this multi-color done where it has been dyed black then parts have been washed out and retain the clear/silver color without the silver tinted at all. It looks like silver clouds in the black dye. What can I do to duplicate this?

Randy Fairbank
- Lewiston, Idaho, USA


(2005)

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.

James Watts
- Navarre, Florida


(2005)

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.

Jim Gorsich
Accurate Anodizing Inc.
supporting advertiser
Compton, California, USA
accurate anodizing banner


(2005)

thumbs up signThank 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.

Randy Fairbank [returning]
- Lewiston, Idaho, USA



September 14, 2016

Q. I need to remove ink from one of our Anodized T2 C2 parts, what solvents can I use to remove the ink? and to cure the ink what temperature is recommended so I don't ruin the anodized coating?

Carlos J. Martinez
- Paso robles California



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