Metal Finishing Guide Book


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Page 349 of 843

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) 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 surface, which exhibits excellent adhesion properties. Chromate conversion coatings are a thin chemical film, usually less than 0.25 microns in thickness and are electrically conductive. HEXAVALENT CHROMATES Historically, hexavalent chemistry has been used to process aluminum chromate conversion parts. Chromate passivation systems containing Cr+6 compounds are an extremely versatile group of aqueous chemistries that are extensively 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 Cr2O3 barrier layer, which provides corrosion resistance and further protection due to residual chromate ions. Hexavalent-based passivation (Cr+6) exhibits a number of desirable characteristics. The process will passivate the surface of zinc and zinc alloy electrodeposits 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 repassivate any exposed areas. Hexavalent chromate has wet, gelatinous film drying at the surface. Subsurface moisture (dehydrating in approximately 48–72 hours) provides selfhealing and lubricity characteristics. The deposits are harder than conventional trivalent chromate film, and they offer torque and tension to meet the finishing requirements of fasteners. Unfortunately, the Cr+6 used in generating cheap and very effective coatings poses serious health hazards as well as waste treatment problems. Chrome sores, which are severe damage to mucous membranes and 342

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