Removing dye from anodized parts

Letter 1685

Dye removal from anodized parts

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

Dennis Nolan

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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. Then, what would you do with the parts . . . just leave them natural aluminum color?

Ted Mooney, P.E.
finishing.com
Brick, New Jersey

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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,
A.Medina

P.S. Do check with the different publications (Metal Finishing , Product Finishing, ETC. ) for more info.

Ariel Medina

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Yes, Mr. Medina, but...

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

P.S.: On second thought, maybe it doesn't?

Ted Mooney
finishing.com
Brick, New Jersey

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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 redying 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 reanodized 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 reccomend it.

Ward Barcafer, CEF
aerospace - Wichita, Kansas

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