Summary for chromate conversion coating Discussion
Chromate conversion coating on zinc alloys is discussed in the context of Zamak 5 parts that already have a 5–8 µm zinc-plated layer. Daniela Ciappara asked whether the intermediate zinc plate is really necessary or whether chromating the base alloy alone would provide the same corrosion protection. Tom Pullizzi replied that the parts will look quite different without the leveled electroplated zinc deposit, and that the cleaning required for electroplating helps ensure the whole part is properly cleaned; the leveling effect may also smooth surface imperfections and improve corrosion resistance. He also pointed readers to suppliers of hexavalent-chromium-free conversion coatings. In a later question, Cesar Guerrero said trivalent and yellow conversion on Zamak had caused problems, while hexavalent products had worked well. The response was that major suppliers offer reliable trivalent chromating processes, and the best choice is usually the vendor with the strongest local technical support.
We are applying a chromate conversion coating over a zinc based alloy – Zamak 5, with an intermediate zinc plated layer of thickness
5 – 8 um. Is the application of the intermediate layer really required? Or would the application of the chromate coating on the base metal give the same corrosion protection?
Are there hexavalent chromium free alternatives for this application?
Your reply to this query is most appreciated.
Thanks,
Daniela Ciappara
– Malta
The parts will look quite different without the leveled deposit of electroplated zinc. The surface cleaning required for the electroplating operation ensures that the entire part will be cleaned properly, and the leveling may smooth out some surface imperfections which may improve corrosion resistance.
See our Chemicals directory (button available on this page) for suppliers of hexavalent free conversion coatings.
Tom Pullizzi
Falls Township, Pennsylvania
Q. I am using a trivalent and a yellow converter for Zamak but I’m having a lot of problems; and when I used hexavalent there weren’t any problems. Can anybody guide me which new products can I use?
Thank you
Cesar Guerrero
plating – Brownsville, Texas, USA
June 7, 2012
A. Hi Cesar.
All of the major suppliers offer reliable trivalent chromating processes. We can’t suggest one particular brand over another brand in this forum, but a truism is that the supplier with the best tech service in your area offers the best trivalent chromating process. If you are not getting assistance from your vendor, find another vendor who will help you. Good luck.
Regards,
Ted Mooney, P.E.
Striving to live Aloha
finishing.com – Pine Beach, New Jersey
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June , 2012