Metal Finishing Guide Book


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Page 90 of 843

When building a rack, remember that 1 in.2 of copper carries 1,000 A; therefore, if you use two spines of 1 +/÷ in., this rack will carry 500 A. When large volumes of parts are to be processed, a specially constructed barrel may be used, or a tray. Agitation An air line is usually placed diagonally on the bottom of the electropolishing tank to stir up the solution, preventing temperature stratification. Air is not used directly under the parts to be electropolished because "white wash" can occur. Mechanical agitation is the optimum method for part agitation. This brings fresh solution to the surface of the part for faster electropolishing. Other methods of agitation are mixer, filter-pump, or separate pump. Filtration is used on many electropolishing systems. The solution lasts longer and the tank does not have to be cleaned as often. In high-technology operations this may be a requirement. Temperature Most electropolishing solutions must be heated and cooled during the operating period. Heating is accomplished by using quartz or Teflon-coated stainless steel electric heaters with controls. If steam is used, Teflon coils are used. Lead is no longer used because it is toxic. Cooling is accomplished with 316L stainless steel plate coils. Stainless steel cannot be used for steam heating as most baths contain sulfuric acid, which attacks stainless steel at the high-temperature surface of the plate coil. Tank construction of 316L stainless steel is all right because excessively high temperatures (above 250OF) are not present. Chillers are used when the tank solution will have 10-15 A/gal from the rectifier. Heat exchangers are used when input amperage is above 5 A/gal. TYPICAL SOLUTIONS There are organic electropolishing baths, inorganic baths, and organic/inorganic baths. Some typical formulas are shown below. Aluminum Because it is amphoteric in nature, aluminum can be electropolished in both acid and alkaline electrolytes. The brightening process involves low rate of attack, use of high-purity aluminum, and requires prefinishing. Alzac Process: First Stage (Brightening) Fluoboric acid, 2.5% Temperature, 85¡F Voltage, 15-30 V Current density, 10-20 A/ft2 The polarized film is stripped in hot alkaline solution. Anodizing, as usual, in the sulfuric acid bath follows. Only superpurity alloys (99.95%) should be used. Polishing and brightening are obtained in concentrated acid-type solutions that feature greater stock removal and greater smoothing. Battelle Sulfuric acid, 4.7% 87

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