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Effect of Impurities in Chromic Acid Anodizing Solutions

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

Jennifer Friesen
Chemical Technologist - Winnipeg, Canada
2005

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First of two simultaneous responses --

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

James Watts
- Navarre, Florida
2005

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Second of two simultaneous responses --

sulphates cause the coating to be less opaque. sulphates may come in through drag-in if there are any upstream or as a common impurity in commodity flake chromic acid. You may remove sulphates from the anodizing solution with barium carbonate or barium hydroxide. One part barium hydroxide removes 1/5 part sulphate by weight. Graham expresses it as,"One part Barium Hydroxide will remove 0.3 parts SO4/weight". Boeing limits sulphates 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

2005