Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 379 of 707

needed, then the tank size must be increased to compensate for the increased wattage going into the tank (amps volts = watts). Voltage is also determined by the number of amperes needed to electropolish the part. Generally, 600-3,000 A requires an 18-V DC output, and 3,500-10,000 requires a 24-V rectifier. Optimum running voltage is 9-13 V for stainless steel. Aluminum requires a 30-40 V rectifier. Aluminum is run by voltage rather than amperage. Racks 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 bar- of copper carries 1,000 A; therefore, 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 direct- ly 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 meth- ods of agitation are mixer, filter-pump, or separate pump. Filtration is used on many electropolishing systems. The solution lasts longer rel may be used, or a tray. Agitation and the tank does not have to be cleaned as often. In high-technology operations this may be a requirement. Most electropolishing solutions must be heated and cooled during the operat- ing 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 can- Temperature not 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 construc- tion of 316L stainless steel is all right because excessively high temperatures (above 250O F) 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/inor- ganic 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. 378 Electropolish racks for most metals are made of copper spines and crosspieces, which have been pressed in a thin skin of titanium. Copper, phosphor-bronze, or titanium clips are used and can be bolted on with titanium nuts and bolts. Some racks are made of copper and copper spines and are coated with PVC. These racks are generally for electropolishing of aluminum, copper, brass, and bronze, although titanium can be used here instead. When building a rack, remember that 1 in.2

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