Metal Finishing Guide Book


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Table III. A Selection of Typical Acid Gold Color Baths for Thick Deposits Hamilton Gold (g/L) Conducting salts (g/L) Nickel as nickel metal (g/L) Cobalt as cobalt metal (g/L) pH Temperature (°F) Current density (A/ft2) Agitation 22k Yellow 4-8 120 0.2 1 4.0-4.5 90-100 10-20 Required 24k Yellow 4-8 120 — 0.5 4.4-4.8 80-90 10-20 Required 1N 4-8 120 7-10 — 4.0-4.2 120-140 10-20 Required 2N 4-8 120 4-6 — 4.0-4.2 100-120 10-20 Required 3N 4-8 120 1-3 — 4.0-4.2 90-100 10-20 Required 5. Temperature. The effect is similar to current density. Low temperatures favor the gold yellows, and higher temperatures favor the alloy colors. Temperatures over 160OF should be avoided, except in the case of rose golds, because of the rapid breakdown of cyanide and the darkening of the color. 6. pH. It is rarely necessary to adjust the pH of a gold or gold alloy bath. They are usually buffered between pH 10 and 11. Only pink, rose, or red golds are favored by higher readings. BARREL PLATING (CLASSES A AND B) Gold as KAu(CN2), 0.4 g/L Free cyanide as NaCN, 30 g/L Disodium phosphate, 23 g/L Temperature, 100-120°F Anodes, stainless steel (1:1 or better) In typical jewelry barrel plating about 6 V is necessary. A decorative finish of 0.000002 in. is deposited in 3 to 4 min. If the parts are small and densely packed in the barrel it may be necessary to plate up to 8 min to get even coverage. The above formulation may be altered to achieve various colors. Champagne or light Hamilton colors may be achieved by adding 1.5 to 3 g/L of nickel. Lowering the temperature will also produce a lighter and more uniform color. ANTIQUE GOLDS (CLASSES A AND B) The art of the 19th-century platers was to produce a finish that looked as if it had been mercury gilt over silver or copper (vermeil) and buried or weathered by the elements for a century or so. Each master plater developed an antique finish that was his trademark. The basic modern method to achieve this effect is as follows: Table IV. Industrial/Electronic Gold Plating Plating Method Semiconductors Printed/etched circuits Contacts/connectors 288 Knoop Purity Hardness No. 99.95% 60-80 99.5-99.7% 120-180 99.5-99.7% 120-180 Surface Matte Bright Bright Rack Yes Yes Yes Barrel Yes No Yes Continuous Yes Yes Yes

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