Metal finishing Q&As since 1989

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Can trivalent chromate finishes oxidize to hexavalent?

I have been told, from certain suppliers, that the trivalent chromate as applied to a product can oxidize over a period of time to a hexavalent condition. If this is true, under what conditions does this occur?
Gary Hein
job shop plating - Santa Fe Springs, California, USA
2006

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2006

Not spontaneously.
It may occur under some oxidizing conditions:
Alkaline oxidation with hypochlorite, ozone, permanganate or peroxide.

The presence of chlorine in oxidizing flames can lead to formation of the oxychloride CrO₂Cl₂(g) in welding fumes; one reason for OSHA's crackdown on hex. Cr.

"A Cl₂ [Cl-sub2, free chlorine] residual in a drinking water distribution system may oxidize any soluble Cr(III) to Cr(VI) because of the long contact time"
-- 'Chromium Redox Chemistry in Drinking Water Systems,' Han Lai and Laurie S. McNeill, J. Envir. Engrg., Vol. 132, pp. 842-851 (August 2006).
Ken Vlach [deceased]
- Goleta, California
Finishing.com honored Ken for his countless carefully researched responses. He passed away May 14, 2015.
Rest in peace, Ken. Thank you for your hard work which the finishing world, and we at finishing.com, continue to benefit from.