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Problem with electrical conductivity of electroplated zinc

We manufacture medical equipment and our products are required to pass EMI testing. Recently we have had problems passing these tests because of poor electrical properties of the surface finish. Because our products are painted, we call out mild steel with Paintlok finish. We use EMI gaskets on masked off regions of metal, but would like to avoid grinding off the finish.

Some of the materials we have received have better contact impedance than others, which makes me suspect that we are not always getting the same product from the manufacturer.  Could anyone tell me which electroplated zinc products offer the best electrical properties so I can specify a material that will give us consistent performance?  Also, I would also appreciate any other suggestions to get good surface contact between an electroplated zinc finish and EMI gaskets.

John Berndt
Medical equipment manufacturer - Deerfield, Wisconsin
2004

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2004

It probably isn't the zinc itself, but the chromate (and possible topcoat). You need to specify the finish by starting with a Mil spec or ASTM or similar spec. Then you need to specify the topcoat, if any, and probably a conductivity test.

There are at least two possible problems. First, some shops will apply a water soluble lacquer or wax, that wasn't specified or intended, in order to give better corrosion protection. Second, with the worldwide and industry-wide move away from hexavalent chromates to trivalent, topcoats are being added to assist in corrosion resistance, and they may not meet your conductivity requirement.

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