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