John,

Thanks for your comments. I am amazed to read that colorimetric
determination of Cr6 can detect it below 1 ppm. This should be of some
concern for those who have switched to trivalent chromium for corrosion
protection. The coating may contain Cr6 as an impurity at more than 0.1
percent, and the test is certainly simple enough for many labs to
perform. The reasonable doubt defense for those using a trivalent
chromium coating would probably prevail in court as long as the amount
of Cr3 far exceeded the amount of hexavalent chromium, but I don't see
how one could claim reasonable doubt when Cr6 is found as the
predominant oxidation state. Maybe there is a forum participant who has
performed colorimetric determinations on trivalent and hexavalent
chromium coatings who would care to comment.

XPS would never make it to a factory floor, but it doesn't need to. Its
application would not be for a factory but forensic - to prove that the
chromium in a chromium-containing coating on a challenged electronic
product was Cr6. The fact that the technique is matrix-sensitive is
important to establish calibration for quantitative analysis. But
quantitative analysis is not needed to answer the critical question: is
there a measurable amount of Cr6 in this coating? If there is, then
because the XPS detection limit for Cr6 - in any matrix - is almost
certainly well above 0.1 percent, the abundance of Cr6 must be well
above 0.1 percent, too. Particularly if there's more Cr6 than Cr3.
That's all. No uncertainty.

 

Gordon Davy 




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