Specifying Zinc Alloy Coatings for Improved Galvanic Corrosion Performance

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Specifying Zinc Alloy Coatings for Improved Galvanic Corrosion Performance

O'Grady, James / Commander, John / Wing, Linda M. / Koga, T.

Zinc alloy coatings have been used commercially since the early 1980's. Its origin is credited to both the Japanese and Europeans, where improved corrosion protection was observed over steed components. The coatings gained acceptance among American automotive companies in response to two industry-wide directives. The first was the mandate to restrict the level of cadmium on vehicles, beginning with the 1995 model year. The other was the challenge to extend the service life of vehicles to ten years, or in effect, to develop the “ten-year” car.Alternative finishes were sought as potential replacements for cadmium. Although a single surface finish has not been identified to have all of the functional characteristics of cadmium, zinc alloy coatings have many of the desirable properties. Among them are corrosion resistance and lubricity. Also, an added benefit is the extended corrosion protection that qualifies the zinc alloy coatings for the ten-year vehicle.Standard laboratory test results such as torque tension are presented. The use of topcoats over the zinc alloy finish has been found to be an acceptable practice of achieving the torque tension properties of cadmium. The outcome of both a four-year environmental exposure evaluation, and a cyclic corrosion test support the corrosion performance gained with zinc alloys coatings.Finally, zinc alloy coatings not only have been accepted as a viable replacement for cadmium, but the coatings have become an option for improving galvanic corrosion protection on vehicular components. Specified applications including fasteners, fuel components, brake assemblies, and fluid transfer systems are reviewed.