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Methods of identifying Trivalent chrome passivation on Zinc plating.

We are receiving Zinc plated components with both Trivalent and Hexavalent chrome passivation. We would like to know the simplest method to differentiate the +3 and +6
Chrome passivation.

Selvaraj Venkatesalu
Quality and Testing - Coimbatore, Tamil Nadu, INDIA
July 1, 2009

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My advice would be to rely on the representations of your suppliers. If you are uncomfortable relying on your supplier, you should get another supplier. That being said, testing for hexavalent chromium in deposits is problematical. You should read some of the literature on the subject. In particular, I recommend the article that Zachary Kennedy and I wrote in Metal Finishing, October 2007, pp 14 - 18. We followed up with a paper that I presented in Louisville last month. You can test for hex chrome with the ISO 3613 procedure or the GMW 3034 testing procedure. Spot tests can give false negatives, as I reported; that is also why the preferred test procedures incorporate leaching the hex chrome (if any) out of the coating. But a positive test result in either of these tests does not necessarily mean that the passivation process is hexavalent. As I said in Louisville, "Trivalent passivates protect zinc surfaces by generating hexavalent chromium during the corrosion process." So if the corrosion process has commenced, then trivalent-passivated parts can test positive for hex chrome. That's just the way they work. So, returning to the beginning of my response, the best alternative is to rely on the representations of a trusted supplier. (I should point out that these opinions are my own and there are some in the finishing industry that disagree with my position. Most platers know I'm right; most suppliers hope I'm wrong.)

Tom Rochester
CTO - Jackson, Michigan, USA
Plating Systems & Technologies, Inc.


July 2, 2009