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Effect of Impurities in Chromic Acid Anodizing Solutions

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We currently use a chromic acid anodizing process, and it has come to our attention that we should be analyzing for sulfates as an impurity. My question is, what effect does sulfates have on aluminum and/or aluminum alloys in this process?

Jennifer Friesen
Chemical Technologist - Winnipeg, Canada

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First of two simultaneous responses -- +++++

I have seen companies that did not test for sulfate. They bought certified low sulfate chromic acid, used DI water in all tanks, did not use any chemicals contaaining sulfate and rinsed well. The tank will die from aluminum buildup before sulfates get to a level of causing a problem.
One reason that sulfate is a problem is the chrome will plate out on the cathodes.

James Watts
- Navarre, Florida

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Second of two simultaneous responses -- +++++

Sulfates cause the coating to be less opaque. Sulfates may come in through grag-in if there are any upstream or as a common impurity in commodity flake chromic acid. You may remove sulfates from the anodizing solution with barium carbonate or barium hydroxide. One part barium hydroxide removes 1/5 part Sulfate by weight. Graham expresses it as,"One part Barium Hydroxide will remove 0.3 parts SO4/weight". Boeing limits sulfates to 500 mg/L. All of the above taken from "Aluminum How-To" by Robert H Probert, publication pending.

Robert H Probert Robert H Probert Technical Services Garner, North Carolina Editor's note:        Mr. Probert is the    author of Aluminum How-To / Aluminio El Como