Letter 42062

Hex. or tri.?  

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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
Otscon Inc. - Columbia, MO, USA

This is the hundredth time for regular readers, but newbies are always welcome, Jeff :-)

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

But in response to RoHS 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 greater cost, approximately match the corrosion resistance of hexavalent chromates. Consequently there has been an industry-wide switch over the last few years to where trivalent chromating is by now probably at least as popular as hexavalent chromating.

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

I would be fearful of being caught at "musical chairs". Some time in the not too distant future, as less and less hexavalent chrmate is used, it may be considered socially unacceptable to use hexavalent chromates -- and the small percentage of people still using it may receive a lot more attention than they want :-)

The last ones still using it when he music stops? "They're called 'boobs', Ed".

Ted Mooney, P.E.  finishing.com Brick, New Jersey

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