Case study: alternatives to chromate conversion coatings for corrosion protection of zinc plated electronic shelves

Abstract

Hexavalent chromium oxide has been used for at least 60 years as a corrosion preventative for zinc and aluminum surfaces. This coating has been a very successful corrosion preventative for both painted and unpainted aluminum, zinc and other metal surfaces. In the electronics industry, chromate is used extensively to coat equipment shelves and other zinc plated steel and aluminum surfaces. Up to now, the electromagnetic interference shielding requirements of the electronic shelves were such that the contact and surface resistivity of the chromate coating provided no problem to EMI protection but now signal speed and proper amplitudes dictate that the coatings have higher electrical conductivity. The shelf is essentially a Faraday cage and the electronics within emanate a broad spectrum of EMI and in turn is sensitive to EMI, even between shelves. There is increasing pressure in the environmental and work safety agencies of the various governments to eliminate hexavalent chrome from processes and products because it is a toxic pollutant and uses a hazardous application process. Nortel became increasingly aware of the environmental and electrical issues for the use of hexavalent chrome and decided to search for alternatives in 1994. A program was initiated to explore conductive paints, zinc alloys and new conversion coatings to provide an alternative to the hexavalent chromate coating as a metal protectant and EMI shielding gasket interface.