finishing.com logo ips1
HOME FAQs BOOKS JOBS: Help Wanted Suggestions      you are here: Hotline/Forum => Letter 25003

Where does the Cr go?

+++

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, Minnesota, USA


+++

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".

pic of Ted Mooney Teds signature
Ted Mooney, P.E.
finishing.com
Brick, New Jersey


ANSWER or FOLLOW UP POST an unrelated QUESTION HOT Topics

Disclaimer: It is not possible to diagnose a finishing problem or the hazards of an operation via these pages. All information
presented is for general reference and does not represent a professional opinion nor the policy of an author's employer. The
internet is largely anonymous; some names may be fictitious and some recommendations may be deliberately harmful.

If you are seeking a product or service related to metal finishing, please check these Directories:

Jobshops Capital Equip. & Install'n Chemicals & Consumables Consult'g, Train'g, Software Environmental Compliance Testing Svcs. & Devices Used & Surplus


Home    -    Contact    -    ©1995-2012 finishing.com     -    Privacy    -    Search