Metal Finishing Guide Book


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in service since, like all chromium deposits, they are hard and brittle. Because the appearance of the part after corrosion is very important, in North America most specifications have eliminated microcracked deposits from use because they tend to lose their reflective appearance much faster than microporous deposits. DECORATIVE BLACK CHROMIUM Thin black chromium deposits are used for functional and decorative applications. Important functional applications include solar energy collectors for heat production and anti-glare surfaces. Decorative functions include furniture, plumbing fixtures, optical equipment, boat equipment, and automotive and builders��� hardware. The decorative jet black finish enhances users��� appeal for the product and so its popularity depends upon the customers��� changing desire for black finishes. As plated, black hexavalent chromium deposits have a decrease in wear and corrosion resistance. However, black chromium deposits have a greater degree of microporosity, which helps absorb oil, waxes, and paint. Along with other posttreatments, this property might be used to improve its corrosion resistance, wearability, and appearance. Dark trivalent chromium deposits have properties similar to standard chromium deposits. If extended corrosion protection is required, some post-treatments might be required. These deposits have become very desirable to designers for interior and exterior automotive parts. Most black chromium processes are proprietary because of the difficulty of obtaining consistent plating characteristics and deposit properties. In many applications, plating black hexavalent chromium over regular chromium is recommended. For all decorative and most functional black chromium deposits, the typical thickness is [approximate] 0.25 microns. Three general formulations for black hexavalent chromium processes are given in Table III. Low carbon steel anodes can be used with plating times of up to 10 min. Most formulations will only produce a maximum deposit thickness and then stop plating due to the nonconductive nature of the deposit. To produce black deposits, barium salts are typically added to remove any traces of sulfate. Dark trivalent chromium process are all proprietary due to their complex chemistry. BULK CHROMIUM PLATING Plating racks are typically used to hold parts, transfer parts to and from the plating solution, and to carry the direct current to the part. Very small parts such as eyelets, screws, nuts, and bolts have high labor costs when placed individually on racks so they are sometimes bulk plated in barrels or trays; however, even under the best of plating conditions, the chromium reject rate can be very high. This is due to incomplete coverage and black/gray deposits mostly resulting from poor cathode contact and current interruption. Under special conditions, both hexavalent and trivalent chromium processes have been used for bulk plating of parts. Table IV contains simple barrel hexavalent chromium formulations. Hexavalent chromium has been used primarily and special proprietary formulations have been developed that can produce near 100% coverage if precise control is used. Since current interruptions are not avoidable in barrels and trays, highly complexed-fluoride-containing solutions are normally used. It is extremely important that the bright nickel used prior to chromium not be passive. This is typically accomplished by plating a minimum of 0.1 mil of nickel in a barrel and, with a few rinses in between, transferring the 302

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