Metal Finishing Guide Book


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Conversion Coating Pretreatment % Passed % Failed Enhanced Etched 81 19 Standard Etched 31 69 Enhanced Non-etched 90 10 Standard Non-etched 53 47 Table 1: Effect of an Additive on Corrosion Resistance lems. Chrome sores, which are severe damage to mucous membranes and skin lesions, occur from exposure to the ever-present chrome-mists and aerosols in job shops. Environmental guidelines and regulations are in place that restrict and prohibit its usage. The finishing industry is developing less toxic alternatives in order to comply with substance restriction legislation and directives from the European Union. The most significant directive is RoHS, signed on Jan. 27, 2003, which went into effect July 1, 2006. The restriction covers six hazardous substances: lead, mercury, cadmium, Cr+6, polybrominated biphenyls (PBB), and polybrominated diphenyl ether (PBDE). Another European Union legislative action, the second edict that also contains Cr6+, is the End of Life Vehicle (ELV) directive, which went into effect on July 1, 2007. Four heavy metals included in ELV directive include: cadmium, lead, mercury, and Cr+6 (approximately 70% of total heavy metals is Cr+6). Industry has been actively following any new development to replace Cr+6. The most common alternative is trivalent chromium, which is environmentally friendly. However, there are still some weaknesses with trivalent chromate coatings. In order to achieve equal or better corrosion resistance compared with hexavalent chromate, in most cases a sealer or a topcoat is required. Some chemical manufacturers now offer better salt spray performance without any sealers or topcoats. Trivalent chromates do not have self-healing properties. Their bath life is shorter than a hexavalent chromate bath, they require a 140��F operating temperature, and they do not offer identical colors. In recent years there have been new developments in trivalent chemistries. More colors are now available and coating performance has significantly improved, especially with respect to corrosion resistance. Typical trivalent chromate film has a pale greenish color. Trivalent chromate deposits are electrically non-conductive (unless applied over a zinc alloy or a metallic substrate). The most significant development for the replacement of hexavalent chromates is the trivalent chromium pretreatment (or post-treatment), developed by the United States Navy, Naval Air Systems Command (NAVAIR). This is a unique chemistry, specially formulated and developed for aluminum. This formulation contains <1% trivalent chromium and operates at ambient temperatures (65���85��F). It does not contain any restricted or hazardous substances and, as a result, does not require any exhaust systems. Most importantly, it complies with all European Union edicts, including RoHS, ELV, and the Waste Electric and Electronic Equipment (WEEE) directives. This novel chemistry has shown outstanding performance when compared with other conventional trivalent chromates. This trivalent chromate chem film is hard397

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