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Difference between Zinc Gold Dichromate and Yellow Trivalent




Q. Hi, I am Betty Koons and I would like someone to answer a question for me. We received a print from a customer that stated under plating "Zinc Gold Dichromate (Trivalent)". Our plater plated it Zinc Gold Dichromate. Our customer asked about trivalent and we told them we sent the print to the plater and this is what they plated the parts. I talked to the customer and they said they wanted Trivalent. I called the plater and they said you cannot have Zinc Gold Dichromate and Trivalent in the same plating requirement. So my question is, what is the difference between Zinc Gold Dichromate and Trivalent? Your help will be appreciated.

Thank you,

Betty Koons
- Bucyrus, Ohio
2006


A. Trivalent was specified, so trivalent must be provided -- although the spec invites trouble like this :-)

This is probably the situation: European RohS and EOLV and other standards now require that many components be free of hexavalent chromium. Also some manufacturers are being pro-active on that issue even if hexavalent chromium is not forbidden by law on their components. The components in question presumably must be free of hexavalent chromium, so trivalent was specified.

A problem is that 'dichromate' has a specific chemical meaning; it means Cr2O7, and the chrome would have to be hexavalent, not trivalent for that formula to hold. Similarly, 'chromate' also has a specific chemical meaning; it means CrO3, and the chrome would have to be hexavalent for that formula to hold as well. But as slang and a colloquialism the trivalent formulas are widely called 'trivalent chromates' even though it is actually an oxymoron to a chemist.

Yet another problem is that dichromate is naturally gold in color whereas trivalent formulations are naturally clear, and some people question whether it is possible to get a good gold color from a trivalent formulation even with dye.

The bottom line is that it is probably essential that the parts be free of hexavalent chromium, so they probably have to be stripped and redone. The spec is perhaps poorly written on the one hand, but the plater should not have applied hexavalent chromate when the spec said trivalent. Plenty of blame to go around; but from this distance, without any real investigation of the facts, I'd say it's the plater's job to fix this. Thanks for an interesting and timely question!

Ted Mooney, finishing.com
Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
2006


A. The following web page could be an example of the apparent contradiction in "trivalent dichromate":
www.codyzincplating.com/Zinc+Plating/Zinc+Trivalent+Dichromate+Plating/default.aspx

A close reading of this manufacturer's process description indicates that it is a trivalent coating.

It appears a marketer, not an engineer, is the origin of this confusion.

David Ings
- LaGrange, Illinois, USA
April 30, 2015



thumbs up signThanks David. We should probably call such coatings "RoHS compliant trivalent chromium conversion coatings" or "RoHS compliant hexavalent-free conversion coatings" since there can be no such thing as "trivalent chromate". Another shortcoming of the original callout was that it may not have included a Class, Type, and Thickness according to an ASTM or other recognized specification.

Regards,

pic of Ted Mooney
Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
May 2015




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