Metal finishing Q&As since 1989

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Alkaline cleaners and chromate sealers

January 10, 2011

I am trying to get to the bottom of a question posed to me. A colleague of mine insists that if zinc plated parts (with trivalent chrome sealer) are cleaned in alkaline cleaner prior to e-coat that metallic soaps will form in the cleaning solution preventing adhesion of the e-coat to the zinc. This does not make sense to me, but in my research I can not find anything to prove or disprove this claim. Any ideas?

Sincerely,
A Wilson

Aaron Wilson
Manufacturing Engineer - Jamestown, New York, USA

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Hi, Aaron. I have definitely heard of companies that had had no previous problems with cleaning and electropainting hexavalent chromate, and then lost adhesion when they switched to trivalent chromating. But I don't believe that this problem occurs with all trivalent chromates. Unfortunately, trivalent chromating is still exceptionally proprietary, with many different approaches: thin film vs. thick film, natural coloration vs. dye, topcoats of a variety of types vs. no topcoats. I think we're still in the phase of choosing the best trivalent chromate for the particular situation, rather than having reached the point where we must resign ourselves to suffering problems and trying to account for the reason :-)

Regards,

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
January 11, 2011

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March 23, 2011

Often trivalent chrome passivates do contain a polymer, where hexavalents chromates are usually fully inorganic.
The polymer is needed to improve the salt spray resistance to the level of hex chrome.
It's however difficult (if not impossible) to wash it off...
Some polymers however give good adhesion to paints. The galvanizer should know if his passivation is suitable for post-painting or not.

Barry Groeneveld
chemicals - Heijningen, Zuid Holland, The Netherlands