Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 176 of 707

Temperature 80-110° F Control of this solution is more critical since the alloy must be held within 1% to maintain color. The ratio of cyanide to zinc is the major controlling factor in color. Excess ammonia will give a yellow color or two-toned plate. The pH is the same as in yellow brass and is not a major factor in color. WHITE BRASS Alloys of brass with more than 50% zinc are white. They are also brittle limiting them to thin deposits to avoid cracking. The deposits with addition agents are fully bright and may be directly chromium plated. They have been used as an inter- mediate layer under nickel and chromium plate to secure better corrosion resis- tance. They have been used on articles such as shopping carts. These coatings are not often used today, with their major uses back in the 1950s. Several formulations are used to secure varying zinc contents. A typical for- mulation is as follows: Copper cyanide Zinc cyanide Sodium cyanide Sodium hydroxide Current density Alloy composition 2.3 oz/gal (17 g/L) 8.5 oz/gal (64 g/L) 11.5 oz/gal (85 g/L) 8.0 oz/gal (60 g/L) 10 to 40 A/ft2 75% zinc/25% copper Anodes are usually balls or chunks since bars tend to crack. Composition of the plate with this formulation is about 28% copper. Alloy composition may be varied by varying cyanide and hydroxide content. Raising the amounts increas- es the zinc content. BRONZE PLATING Copper-tin alloys are used for decorative fittings, for corrosion protection of builders' hardware, and for bearing surfaces. Alloys with 10-12% tin have a gold- en color, which can closely resemble gold and is sometimes used for costume jew- elry. A typical formulation is as follows: Sodium cyanide Copper cyanide Sodium stannate Sodium hydroxide Rochelle salt Temperature 8.6 oz/gal (64 g/L) 4.0 oz/gal (30 g/L) 4.6 oz/gal (35 g/L) 1.5 oz/gal (10 g/L) 160° 6.0 oz/gal (45 g/L) (or equivalent) F (70° C) Anodes are usually pure copper. Addition of tin as stannate is necessary to maintain the tin content. Use of bronze anodes will produce a black color in the solution and result in poor performance. Addition agents improve the range and can produce a bright plate by the use of a small amount of lead. Since copper forms only a cyanide complex, and tin forms only an alkaline complex, control is simple by varying metal content and controlling the cyanide and hydroxide content. A variation of the bronze solution, which produces a plate that is about 40% 175

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