Letter 25003

Where does the Cr go?

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Okay, I've been in sales too long. I understand the chemical concepts behind electroplating well enough, but evidently not well enough because switching the problem around i.e with anodizing (let's say in a chromic acid solution), I lose it. Besides chromium emissions due to mist (and some due to tank cleaning I'd think, but at low levels with proper precautions), is there an actual oxidation-reduction reaction occurring, and is the Cr +6 reduced to + 3 chromium that subsequently bonds to the tank shields and is removed as a shiny solid for clean up? In other words, in a chromic acid batch, where does the chromium element go after the oxygen is provided to the aluminum anode?

Thanks,

Brad Carpenter
Environmental Management Software - Maple Grove, MN, USA

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By far the biggest loss of chromium is drag out by the racks and parts. Chrome 3 in the solution should be converted to chrome 6 by the high voltage used.

James Watts
- Navarre, Florida

Yes, in a way, anodizing is the opposite of plating because the polarity is reversed. But it is not electroplating in the opposite direction (onto the anode). Rather the anodization is a compound of aluminum from the substrate and oxygen from the water in solution. There is no chrome plating involved except incidentally; the chromic acid is simply the electrolyte that the oxidation process takes place in. As Jim says, dragout is the major way chrome leaves the tank.

For more on this topic, you might want to get the book "The Surface Treatment and Finishing of Aluminum and its Alloys".

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

Dear Reader, please --

Answer or follow-up on this subject (in non-commercial fashion).