Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 173 of 707

electroplating solutions BRASS AND BRONZE PLATING BY HENRY STROW OXYPHEN PRODUCTS, OAKVILLE, CONN. Brass plating is one of the oldest cyanide plating solutions. Records of it go back to 1841 in Russia by a German chemist. All of the alloys, from pure copper to pure zinc, can be plated and useful alloys cover most of the entire range. Much of the literature is incomplete and contains false conclusions. The range of solutions that can be used is extremely wide, so only some of the most use- ful and stable are given. The uses of brass plate are largely decorative, with thin plates (flash plates) over bright nickel or other bright plates predominating. Heavier plates can be buffed or burnished to a bright smooth finish or can be treated to produce antique fin- ishes usually brown or black. These finishes can be relieved by brushing or tum- bling to produce highlights. Although decorative finishes predominate, engineering uses are also impor- tant. Brass plating can be used on zincated aluminum prior to nickel plating. A very important use is for rubber bonding. The greatest use is in the plating of steel tire cord wire, where the bond of the rubber to the steel depends on brass plat- ing the steel wire. The brass plating is done on heavier wire, which can be drawn after plating to the final size without destroying the integrity to the brass plate. The usual analysis of the brass is 65-68% copper. A wide variety of formulations can be used. The cyanide complexes of cop- per and zinc are so stable and of the right order that the copper and zinc deposition potentials are very close. All of the useful brass plating solutions are based on cyanide. Noncyanide brass plating is possible and has been used commercially, but with limited success. Noteworthy are the pyrophosphate- based solutions and solutions based on hydroxylaliphatic acid salts. Because of the stable cyanide complexes, the ratio of copper to zinc in the solutions has no relationship to the plated alloy. There are three basic types of solutions used. First is the conventional low tem- perature (70-100O is a similar solution operated at a temperature above 110O with a hydroxide content above 1 oz/gal and usually with higher metal content. The conventional solution has a wide range of compositions, depending on use. The following composition is typical and may be varied widely, subject to certain rules. Copper cyanide Zinc cyanide Sodium cyanide Sodium bicarbonate Sodium carbonate Ammonia This gives an analysis as follows: Copper (as metal) Zinc (as metal) Sodium cyanide 172 4.2 oz/gal (32 g/L) 1.3 oz/gal (10 g/L) 6.5 oz/gal (50 g/L) 1.5 oz/gal (10 g/L) 2.5 oz/gal (18 g/L) 0.25-1 gal/100 gal 3.0 oz/gal (22 g/L) 0.7 oz/gal (5 g/L) 3.0 oz/gal (22 g/L) F), with a wide range of compositions and alloys plated. Second F. Third is a solution

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