Metal Finishing Guide Book


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plating processes, procedures & solutions TRIVALENT CHROME CONVERSION COATING FOR ZINC AND ZINC ALLOYS BY NABIL ZAKI SURTEC INTERNATIONAL, ZWINGENBERG, GERMANY; In the late 1990s the End-of-Life-Vehicle (ELV) directives were introduced in Europe, mandating the recycling of 85% of vehicles by weight by the year 2006. Recycled components should be free of compounds or elements known to be hazardous to man or the environment. Among the restricted items are carcinogenic hexavalent chromium compounds. Consequently, the directive imposed a limit of 2 g of Cr(6) per vehicle to be met by July 2003. Leachable hexavalent chrome can be found in the conversion coating layer used to passivate zinc and zinc alloy plated surfaces. The amount of Cr(6) varies from 5 to 400 mg/M2 depending on the type of passivation used, and is typically low for light blue coatings and heavier for the thicker yellow and olive green types. To insure compliance with the new regulation and eliminate the need for constant monitoring, the European car industry opted for the total removal of hexavalent chrome from their plated finishes. The U.S. car industry soon adopted the same restrictions and standardized finishes across their worldwide operations. The search for a viable Cr(6)-free conversion coating with similar functional properties led researchers ultimately to the selection of trivalent chrome passivation as the most adequate alternative to date. Availability of raw materials, safety, economical considerations, and ease of adaptability to existing finishing plants were important factors in the selection of this technology as a viable replacement process. TRIVALENT CHROME PASSIVATION TECHNOLOGY Trivalent chromium compounds are readily available, noncarcinogenic, and safe to handle. Some compounds are used extensively in many applications such as dyeing and waterproofing of fabrics, printing, wood preservation, chrome plating, and other industrial processes. Trivalent chrome conversion coating technology was introduced commercially in the late 1980s as an earlier attempt at replacing carcinogenic hexavalent chrome from as many processes in metal finishing as possible. Although there were no regulations or specifications requiring this change at the time, platers realized the environmental and safety advantages from such a substitution, along with improved and more reliable performance. The first generation of Cr(3) conversion coatings was limited to producing blue-bright passivation designed for light service conditions, meeting 12 to 24 hr of neutral salt spray (NSS) to white zinc corrosion. Further development was needed to meet the full range of automotive and other industry requirements for extended corrosion resistance. The mechanism of Cr6+ conversion coating and its corrosion resistance was used in developing a comparable substitute process. Upon immersion of zinc plated parts in hexavalent chromating solutions, zinc is oxidized at the interface by the Cr6+, which is reduced to Cr3+, dissolves at a controlled rate in the acidic solution, and reacts with Cr3+ to form zinc chromium oxide compounds. An increase in the pH at the interface causes the trivalent chrome compounds to precipitate on the surface, forming a gelatinous film 490

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