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topic 1685

Removing dye from anodized aluminum


A discussion started in 1998 but continuing through 2019

1998

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

Dennis Nolan


adv.
"Surface Treatment & Finishing of Aluminium and Its Alloys"
by Wernick, Pinner & Sheasby
from Abe Books
or
info on Amazon

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, although it is very high in caustic so it will attack the anodizing and the dye. 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.
finishing.com - Pine Beach, New Jersey
Striving to live Aloha


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

Ariel Medina


Yes, Ariel, it doesn't attack the aluminum base metal but...

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

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


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



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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


adv.
probert book
Aluminum How-To

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

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.
finishing.com - Pine Beach, New Jersey
Striving to live Aloha


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



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Changing Anodizing color?

2005

Q. Hello, my name is Steve. I am an Aerospace Engineer Major at Embry Riddle Aeronautical University in my Junior year. In my spare time a play paintball. I have been looking into anodizing some parts on my gun myself instead of spending a few hundred dollars to have someone else do it.

My problem is that the parts I want to anodize already have a black anodize on them. My question is if I can simply anodize a new color over the black or if I will have to remove the old anodizing first? If I did need to remove it, I could use the chem lab on campus, but I am not exactly sure of the best way to do so without damaging the aluminum parts.

I would also consider powder coating over the existing black, but I am not exactly sure of the process. Whatever I do, I plan on using the chem lab and practicing on some spare parts first. Thanks.

Steve Short
student/hobbyist - Holly Hill, Florida, United States


2005

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 [paid link to product info at Amazon] 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 [paid link to product info at Amazon] 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.

James Watts
- Navarre, Florida



September 14, 2016 -- this entry appended to this thread by editor in lieu of spawning a duplicative thread

Q. I need to remove ink from one of our Anodized T2C2 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


December 2019

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,

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



December 18, 2019 -- this entry appended to this thread by editor in lieu of spawning a duplicative thread

Q. Hi all, I have a piece of 7075-t6 aluminum that was anodized with a type 3 class 2 finish and dyed what was supposed to be a dark green. It looks more like a booger green to me though. Is it possible to remove the sealant and re-dye or darken the color of this part? Or preferably maybe do something to darken the appearance without removing the seal coat?

Francis Savage
- Michigan


December 2019

adv.
Prismacolor Markers

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,

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

adv.    CMLSbanner

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