Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 377 of 707

surface treatments ELECTROPOLISHING BY KENNETH B. HENSEL ELECTRO POLISH SYSTEMS INC., MILWAUKEE; The electropolishing system smoothens, polishes, deburrs, and cleans steel, stain- less steel, copper alloys, and aluminum alloys in an electrolytic bath. The process selectively removes high points on metal surfaces, giving the surface a high luster. HOW IT WORKS The metal part is immersed in a liquid media (electrolyte) and subjected to direct current. The metal part is made anodic (+) and a metal cathode (-), usually 316L stainless steel or copper, is used. The direct current then flows from the anode, which becomes polarized, allowing metal ions to diffuse through the film to the cathode, removing metal at a controlled rate. The amount of metal removed depends on the specific bath, temperature, current density, and the par- ticular alloy being electropolished. Generally, on stainless steel, 0.0005 in. is removed in 1,500 amp-minutes per square foot. Current and time are two vari- ables that can be controlled to reach the same surface finish. For example, 100 A/ft2 electropolished for 5 min is 500 amp-minutes; 200 A/ft2 for 21 /2 min is 500 amp-minutes. Both pieces of metal would have about the same surface profile. Current densities of 90 to 800 A/ft2 are used in this process depending upon the part to be polished and other parameters. Electropolishing times vary from about 1-15 minutes. ADVANTAGES Conventional mechanical finishing systems tend to smear, bend, stress, and even fracture the crystalline metal surface to achieve smoothness or luster. Electropolishing offers the advantages of removing metal from the surface pro- ducing a unidirectional pattern that is both stress- and occlusion-free, micro- scopically smooth, and often highly reflective. Additionally, improved corro- sion resistance and passivity are achieved on many ferrous and nonferrous alloys. The process micro- and macro-polishes the metal part. Micro-polishing accounts for the brightness and macro-polishing accounts for the smoothness of the metal part. Deburring is accomplished quickly because of the higher current density on the burr, and because oxygen shields the valleys, enabling the constant exposure of the tip of the burr. Because the metal part is bathed in oxygen, there is no hydrogen embrittlement to the part. In fact, electropolishing is like a stress-relieve anneal. It will remove hydrogen from the surface. This is important to parts placed under torque. Another benefit is that bacteria cannot successfully multiply on a surface devoid of hydrogen, therefore, electropolishing is ideal for medical, pharma- ceutical, semiconductor, and food-processing equipment and parts. The com- bination of no directional lines due to mechanical finishing, plus a surface rel- atively devoid of hydrogen, results in a hygienically clean surface where no bacteria or dirt can multiply or accumulate. 376

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