Letter 42029

Can trivalent chromate finishes oxidize to hexavalent?  

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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, CA, USA

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 - Goleta, California

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