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Confusion over hexavalent vs. trivalent chromating on zinc plating


Q. I am in the automotive industry and a new Quality Engineer so therefore I am rather new to plating but willing to learn. There seems to be quite a bit of confusion at the plant that I work at as to the proper callout between hexavalent and trivalent. The callout that is in question is MFZn8-C. I know that one of our suppliers is claiming MFZn8-C to be a trivalent coating. But yet another customer is claiming MFZn5-C to be a hexavalent coating. The only difference between the two callouts that I know is the 5 and 8 which to my understanding is the difference of the coating thickness. Can this coating callout be for both hexavalent and trivalent? If so is there a specific book that would explain the breakdown of the coating callout so that I can understand it better? Or what is the proper callout for trivalent coating? If you could shed any light on these questions it would be greatly appreciated.

Thank you.

Jeff Marshall
- Columbia, Missouri, USA


A. Although this is probably the umpteenth time for regular readers, newbies are always very welcome, Jeff!

Traditionally, trivalent chromates exhibited far less corrosion resistance than hexavalent chromates; so although they have existed for many decades, they were rarely specified. Chromates were "de facto" hexavalent and nobody even bothered specifying it.

But in response to RoHS pressures and other requirements to get hexavalent coatings off of parts, new generations of trivalent chromates were developed in the last few years which can often, but at somewhat greater cost, approximately match the corrosion resistance of hexavalent chromates. Consequently there has been an industry-wide switch over the last few years to the point where trivalent chromating is by now probably more popular than hexavalent chromating.

To keep up with the curve, and stay ahead of it, you should specify trivalent chromating unless there is a compelling (but temporary) reason not to. Some specs don't yet incorporate the distinction, so you must add as a note the requirement that the coating be "trivalent and RoHS-compliant".

If you don't move to trivalents, I would be fearful of being caught at "musical chairs". In the not too distant future, as less and less hexavalent chromate is used, it may suddenly become socially unacceptable to use hexavalent chromates -- and the small percentage of people still using it may receive much more attention than they want :-)

What do you call the people still using it when he music stops? "They're called 'boobs', Ed" in the immortal words of Julia Roberts from "Erin Brockovich", the movie about hexavalent chrome pollution :-)

Ted Mooney   Teds signature
Ted Mooney, P.E. RET
Pine Beach, New Jersey

May 8, 2012

A. MFZn5-C or MFZn8-C are codes that come from a Japanese specification, likely Nissan M4040 from the coding, but it could be from others. You need to find out from the drawing or the customer what specification the code is taken from. The specification should then tell you whether hexavalent chromates are allowed, for the most part they are not. As it happens, M4040 has grandfathered older codes for hexavalent materials with newer codes that are specific for trivalent chromates. MFZn5-TC is an example where the T means trivalent. Practically, I would not recommend using a hexavalent chromate on an automotive part without a clear understanding from the customer that they will accept a non-RoHS compliant finish.

Don Troy
- Monee, Illinois, USA

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