Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 301 of 707

DESIRABLE CHARACTERISTICS OF HEXAVALENT CHROMATE PASSIVATES • Prevents oxide formation • Provides color • Slow corrosion in prototypic tests (e.g., salt spray, rooftop, etc.) • Provides adhesion for organics (e.g. paint) • Prevents corrosion of painted surfaces • Conductive • Thin • Flexible • Lubricious • Easily applied • Stable for weeks or months • Durable • Resilient (self healing) • Coats in recesses • Easy to strip • Inexpensive equipment • Single tank • Inexpensive (charge-up cost) The successful application of this conversion process requires the aluminum to be clean and free of organic soils, oxides, and corrosion products. Therefore, a pretreatment process is required that can be applied to aluminum and provides a suitable basis for subsequent coatings. Conversion coatings that can be used on aluminum alloys and are compatible with most paint systems have been developed. The name "conversion coating" describes a process of chemical reaction that results in a surface film. As a result of this reaction and conversion, the film becomes an integral part of the metal sur- face, which exhibits excellent adhesion properties. Chromate conversion coatings are a thin chemical film, usually less than 0.25 microns in thickness and are elec- trically conductive. HEXAVALENT CHROMATES Historically, hexavalent chemistry has been used to process aluminum chro- mate conversion parts. Chromate passivation systems containing Cr+6 com- pounds are an extremely versatile group of aqueous chemistries that are exten- sively used in a diverse range of electroplating and metal treatment processes. They impart many beneficial and essential characteristics to metallic substrates and deposits obtained from a number of techniques, such as zinc electroplating. Chromate conversion coatings on alloys are formed by the reduction of chromate ions and the development of a hydrated Cr2 O3 barrier layer, which provides cor- rosion resistance and further protection due to residual chromate ions. Hexavalent-based passivation (Cr+6) exhibits a number of desirable char- acteristics. The process will passivate the surface of zinc and zinc alloy elec- trodeposits with a thin film that provides end-user benefits such as color, abrasion resistance, and increased corrosion protection. When damaged, these hexavalent chromates possess a unique "self-healing" property. This means that soluble Cr+6 compounds contained within the passivation films will re-passivate any exposed areas. Hexavalent chromate has wet, gelatinous film drying at the surface. Subsurface moisture (dehydrating in approximately 48–72 hours) provides self-healing and lubricity characteristics. The deposits are harder than conventional trivalent chromate film, and they offer torque and tension to meet the finishing require- ments of fasteners. Unfortunately, the Cr+6 used in generating cheap and very effective coatings poses serious health hazards as well as waste treatment prob- 300

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