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Zinc chromate conversion coating AND zinc rich primer before on steel

2005

I'm specifying a pre-treatment for a powder coat paint for steel electronic enclosures (and the steel poles they mount on) that will be used outdoors, throughout the US.

Does using a zinc-rich primer over a zinc phosphate conversion coating offer more protection than just the zinc phosphate alone?

Some of the things I have read state not to use them together since the conversion coat isn't conductive and isolates the steel from the zinc in the primer, but some companies actually offer this combination together.

The enclosures and the poles they mount on will probably see some abrasion during normal operation, so I expect the powder coat to scratch or chip.

Max Muraca
Engineering & manuf. services - Boston, Massachusetts

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There are three major techniques for protecting steel - Barrier Protection, Inhibitive Protection, and Sacrificial Protection. Powder coat is one example of barrier protection. Zinc phosphate is an inhibitor (though not as good as zinc chromate). It is often oiled to provide some barrier protection as well. Zinc is the most common sacrificial coating (and the most common use for zinc). If a zinc protective surface is scratched, it will protect the surrounding area by 'sacrificing' itself, setting up a small electrochemical cell. It is conventional to apply a zinc deposit directly to the steel surface, and to apply barrier protection as the outermost coating. Experts used to take the position that because intermediate coatings are non-conductive, the interposition of a non-conductive coating between the steel and the zinc was to be avoided. You hear that position less today, but that doesn't mean it is necessarily meritless. My recommendation is that the first coating on the steel should be zinc - electrochemical, hot dip, or zinc-rich paint. A zinc dust ⇦this on eBay or Amazon [affil links] can then be applied to the zinc, and finally, the powder coat.

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


2005