Metal Finishing Guide Book

2013

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SUMMARY OF UNIQUE QUALITIES AND BENEFITS Stress relief of surface Removes oxide Passivation of stainless steel, brass, and copper Superior corrosion resistance Hygienically clean surfaces Decarbonization of metals No hydrogen embrittlement No direction lines Low-resistance welding surface Reduces friction Both polishes and deburrs odd-shaped parts Radiuses or sharpens edges depending upon rack position Reduces annealing steps SIMPLICITY OF THE SYSTEM Practically speaking, three major process steps are necessary to electropolish most metal surfaces successfully: 1. Metal preparation and cleaning 2. Electropolish (electropolish drag-out rinse) 3. Posttreatment (rinse, 30% by volume of 42¡ Baumé nitric acid, rinse, deionized hot water rinse) EQUIPMENT NEEDED FOR ELECTROPOLISHING Electropolishing Tank The electropolishing tank is generally constructed of 316L stainless steel, double welded inside and out. Stainless steel can withstand high temperatures, which are needed if too much water enters the electrolyte. Polypropylene usually 3/4 to 1-in. thick, is another tank choice. This tank can withstand temperatures of 180-190¡F. Power Supply The direct current source is called a rectifier. The rectifier is generally matched to the size of the electropolish tank. If the tank is to be cooled by tap water through a plate coil, no more than 5.0 A/gal should be used, therefore, in a 500-gal tank, the capacity of the rectifier should not be more than 2,500 A. If 3,500 A are 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 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. 86

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