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Selective stripping of anodizing
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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 Squibbsengraving company - Easton, Pennsylvania
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.
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 Ted Mooney, P.E. finishing.com Brick, New Jersey |
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Years ago we tried a strip that was basically a fine silica and nitric acid to decolor dyed anodize. Required a little manual agitation to work well. I have no idea who we bought it from.
James Watts- Navarre, Florida
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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.
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Phil Johnson |
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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 SmallRetired - Albany, Oregon, USA
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We use the chromic- phosphoric acid solution for anodizing & Alodine coating stripping. But I search throughout and can not find any method to analyze the components of tank solution. Could anybody give any information or help?
hong huming- xiamen china