Metal Finishing Guide Book

2013

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specially modified trivalent chrome passivating solutions containing metals, such as cobalt or iron, but are difficult to control. Deep uniform black finishes are best produced on zinc-iron alloys. Other alternatives for black finishes over trivalent chrome coatings include organic topcoats applied by the dip-spin process, suitable for bulk-processed parts such as fasteners and other barrel-plated work. After passivation and rinsing in such a process, parts are transferred into baskets, immersed in the blackening solution, spin-dried in a centrifuge under controlled rotational speed to remove excess liquid, then dried at a specified curing temperature. POSTPASSIVATION TREATMENT The use of postpassivation may be necessary in order to accomplish one or more of the following functions: 1. Modify the color of the conversion coating film 2. Provide specific lubrication or coefficient of friction for threaded components 3. Extend the corrosion resistance particularly for barrel and bulk processed parts. There are several commercially available types of postpassivation treatments. Thorough testing of the selected system under simulated end use conditions is recommended in order to qualify the process for the intended application. This may include heat treatment prior to accelerated corrosion or torque-tension testing on components for near engine exposure. Sealers These are types of products that will react with the conversion coating film and result in more durable resistant finishes. Examples of sealers are silicatebased products applied at either room or elevated temperatures. The latter will leach out a small portion of the passivation film and leave a heavier protective layer of silicate reaction products on the surface. This type of sealers may not be adequate for high-temperature exposure depending on the type and degree of alkalinity of the residual silicate film and their resistance to cracking. Aged residual film alkalinity may represent hazardous handling problems and should be adequately investigated. Other types of sealers may contain phosphates, silanes, and transition metals. Topcoats These may be organic lacquers, polymers, lubricants, waxes, oils, and oil emulsions with suspended particles and coloring dyes. They may be applied by immersion, spray, or dip-spinning. A topcoat used commercially for zinc plate consists of coatings containing zinc or aluminum flakes to provide protection against galvanic corrosion between the fasteners and magnesium or aluminum surfaces. The dip-spin process has also been used to apply black finishes with torque modification properties for fasteners and washers. Generally, drying prior to the application of topcoats produces more uniform, heavier films with better corrosion protection. 442

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