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Letter 42029
Can trivalent chromate finishes oxidize
to hexavalent? [California]
August 25, 2006
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
August 28, 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).
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Ken Vlach
- Goleta, California
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