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What is the black dip we are applying to our zinc plated fasteners?

I am in Plating industry of Fasteners, and I'm very curious on the answer that I'm reading on letter 23742 because it seems different from how we do our black finish.

First, we zinc plate the fastener in barrel, then dip in proprietary black solution, and put some oil so that it will not rub off. Often times, we received complaints of the black color rubbing off in their hands. Is our process correct? In the many treads that I have read, cold black oxide would not work on plated items -- is our process cold black oxide or black plating? Please share the correct process and if possible the composition of the black solution because it is very expensive to buy the proprietary solution.

Maria Cristina Reyes
fastener plating - Manila, Philippines
July 9, 2010

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A. Hi, Maria. I would be very surprised if someone in your shop did not know exactly what you are doing, and would thus be in a better position to help you than we, who must simply guess :-)

The right process, which may be what you are doing, would probably be to chromate conversion coat the fasteners after zinc plating. The conversion coating can be clear, yellow, or -- in your case -- black. It could well have a silicate or similar topcoat on it that might look a bit like oil.

These are indeed proprietary coatings, especially if they are trivalent chromate in order to be RoHS compatible. You can shop around for other brands which may be somewhat less expensive, but no one will share their development knowledge, which cost them a great deal of money, and which they are earning back a gallon at a time rather than asking you to pay for up front like they did. Good luck.

Regards,

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
July 12, 2010