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White rust on zinc plated parts with a trivalent chromate seal

We have a cast threaded nut which we send out for zinc plating and a subsequent trivalent chrome seal. the outside of the nut is "as cast" and has slight porosity.
we are experiencing a "white" powdery look. is this white rust?

Geo Wetzel
end user - Pennsylvania
April 25, 2008

April 28, 2008

Hi, George. It's hard to say, but e-mailing a photo to mooney@finishing.com for posting here may help.

Regards,

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

April 28, 2008

You may be seeing white rust, which is the zinc corrosion product (hydrated zinc oxide, etc.). It is also quite likely that you are seeing a phenomenon called efflorescence, which is a reaction between a silicate polymer leachant-sealant with the carbon dioxide in the air. Not all sealants exhibit this phenomenon, but most do. Have your plater contact their supplier.

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

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