Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Zinc cyanide Sodium cyanide Sodium hydroxide This will give an analysis of: Copper (as metal) Zinc (as metal) Total sodium cyanide Sodium hydroxide Temperature of operation 3 oz/gal (22 g/L) 17 oz/gal (125 g/L) 6 oz/gal (45 g/L) 7 oz/gal (50 g/L) 2 oz/gal (15 g/L) 8 oz/gal (60 g/L) 6 oz/gal (45 g/L) 140-160° F Addition agents are necessary to give uniform color and throw in recesses. Higher temperatures can give higher speeds especially for wire and strip plating. The solution may also use potassium salts for higher efficiency and may also be diluted for flash plating. Impurities are a problem in all brass plating solutions. The most widely encountered impurity is lead. Amounts as little as 10 ppm can cause redness in recesses. The cause is most often lead in the anodes. All the metals in the tank and even the stamping lubricants on the work being plated should be suspect. Tin in larger amounts in the anodes will cause color problems. Both lead and tin should be less than 0.1%. Iron is seldom a problem, but if it is in the anode cir- cuit it may dissolve, forming ferrocyanides, which combine with zinc, forming insoluble zinc ferrocyanide and clouding the solution. Zinc ferrocyanides are fil- tered out either by batch filtration or continuous filtration. The normal analysis of brass anodes for yellow brass plating is 70% cop- per/30% zinc. The normal analysis of yellow brass plate is about 75% copper/25% zinc. This gives a slight excess of zinc in the anodes and makes for a more stable solution. For red brass plating, 85-15 or 90-10 anodes should be used, depend- ing on alloy being plated. Organic impurities, which include soaps and some wetting agents, may cause brown deposits. Carbon filtration either batch or continuous will eliminate this. Anodes may be cast, rolled, or extruded with good results for all types. Anode baskets should be of titanium to eliminate iron contamination. Equipment for brass plating may be of any conventional type. Barrels may be of conventional design as well as still tanks and strip or wire machines. Tanks should be lined or of plastic construction to avoid stray currents and iron con- tamination. Solutions may be used to plate alloys with higher copper content, such as 85- 15 (rich low brass) or 90-10 (architectural bronze), by slightly modifying the stan- dard solution, using higher copper content, lowering the zinc content, and increasing the cyanide content to give a ratio of total sodium cyanide to zinc from 7 to 9 times the zinc metal content. Ammonia is necessary but kept very low. A typical formula is as follows: Copper cyanide Zinc cyanide Sodium cyanide Sodium bicarbonate Sodium carbonate Ammonia 174 6 oz/gal (45 g/L) 1 oz/gal (7.5 g/L) 10 oz/gal (75 g/L) 1 oz/gal (7.5 g/L) 1 oz/gal (7.5 g/L) 0.5-1 pint/100 gal (60-120 ml/100 L)

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