What type of chromium is in the conversion coating applied to zinc plating?
A. – The substance of concern with regard to chromate conversion coatings on zinc plating and RoHS compliance is hexavalent chromium. The RoHS Directive restricts hexavalent chromium, but does not restrict less hazardous trivalent chromium. Historically, both clear and yellow chromate conversion coatings contained hexavalent chromium compounds. In recent years suppliers of chromate conversion coating materials have been working hard to develop chromate conversion coatings that contain only trivalent chromium compounds and provide acceptable corrosion resistance. Presently there are a number of such products available in clear. Yellow chromates have been more difficult to develop because the corrosion resistance requirements are greater. For example, ASTM B 633 requires 12 hours of neutral salt spray to white corrosion for clear chromate, but 96 hours for yellow chromate. With regard to our standard zinc plated PEM® brand steel fasteners, we began using a trivalent clear chromate in Jun
