Metal Finishing Guide Book

2013

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Table II. Bath Parameters Acid Chloride Zinc Wetting agents, refining agents or carrier Alkaline Non-cyanide Zinc Cyanide Zinc 2.0–5.0% vol./vol. 1.0–3.0% vol./vol. Not applicable Brightener 0.05–0.2% vol./vol. 0.05–0.2% vol./vol. 0.2–0.5% vol./vol. Temperaturea 60–130°F (15–55°C)1 60–110°F (15–48°C) 60–110OF (15–43°C) a Positive and negative effects are observed when a bath is operated above room temperature (75°F): • Because of the solution evaporation, bath buildup problems can be minimized or eliminated. • At higher temperatures, higher conductivity means less power consumption. • Plating speed is increased at higher temperatures. This means less plating time is required. • Operating at higher temperatures means savings in refrigeration cost. • At elevated temperatures, brightener consumption may be higher than at room temperature. • The solubility of organic additives can become a negative factor. 1 New high-temperature chloride zinc systems for barrel work have reduced plating time 10–50% and increased production 30–100%. SAFETY CONCERNS The characteristics of zinc plating baths deserve consideration due to possible safety hazards. While acid chloride zinc and alkaline non-cyanide zinc result in no toxicity to humans, cyanide zinc is highly poisonous. Platers should remember that the formulations of acid chloride zinc baths make them corrosive to equipment. Alkaline non-cyanide and cyanide baths are noncorrosive to equipment. Exposure to the chemicals in alkaline non-cyanide zinc and cyanide zinc baths can be corrosive to living tissue, whereas acid chloride zinc appears to have little corrosive effect on tissue. Remember to follow all OSHA requirements, checking appropriate material safety data sheets prior to the handling and/or use of all chemicals, whether general or proprietary in nature. Table III. Miscellaneous Requirements and Properties Acid Chloride Zinc Anode polarization Seldom Alkaline Noncyanide Zinc Cyanide Zinc Yes Yes Conductivity of the bath solution (higher conductivity lowers energy costs) Excellent LC-Poor, HC-Good Fair Agitation in rack operations Required Not required Not required Heating or cooling required Yes Yes Yes Filtration required Yes Yes Not normally pH adjustment required Yes No No Purifier needed to treat impurities No Yes Yes Chromate receptivity Good LC-fair, HC-excellent Excellent Waste treatment Simple Simple Complex Iron treatment by oxidation1 Yes No No LC, low chloride; HC, high chloride. 1 30 - 35% Hydrogen peroxide is most commonly used. When necessary, 400 ml/1,000 gal (~100 ml/1,000L) of bath is a typical addition. The addition should be diluted with water to a 10% solution before adding. Potassium permanganate may also be used; however this generates a greater amount of sludge, possibly creating filtration problems and iron precipitation/filtration problems. 326

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