What is Electropolishing?
In electropolishing, the metal is removed ion by ion from the surface of the metal object being polished. Electrochemistry and the fundamental principles of electrolysis (Faraday’s Law) replace traditional mechanical finishing techniques, including grinding, milling, blasting and buffing as the final finish. In basic terms, the metal object to be electropolished is immersed in an electrolyte and subjected to a direct electrical current. The object is maintained anodic, with the cathodic connection being made to a nearby metal conductor.Electropolishing is an electrochemical process similar to, but the reverse of, electroplating. The electropolishing process smooths and streamlines the microscopic surface of a metal object such as 304, 316, and the 400 series stainless steel. As a result, the surface of the metal is microscopically featureless, with not even the smallest speck of a torn surface remaining.
During electropolishing, the polarized surface film is subjected to the combined effects of gassing (oxygen), which occurs with electrochemical metal removal, saturation of the surface with dissolved metal and the agitation and temperature of the electrolyte.
Smoothness of the metal surface is one of the primary and most advantageous effects of electropolishing. During the process, a film of varying thickness covers the surfaces of the metal. This film is thickest over microdepressions and thinnest over microprojections. Electrical resistance is at a minimum wherever the film is thinnest, resulting in the greatest rate of metallic dissolution. Electropolishing selectively removes microscopic high points or “peaks” faster than the rate of attack on the corresponding micro-depressions or “valleys.” Stock is removed as metallic salt. Metal removal under certain circumstances is controllable and can be held to 0.0001 to 0.0025 inch.
In summary, an electropolished object has had the metal removed. The process does not move it or wipe it. As a result, the surface of the metal is microscopically featureless, with not even the smallest speck of a torn surface remaining. The basic metal surface is subsequently revealed — bright, clean and microscopically smooth. By contrast, even a very fine mechanically finished surface will continue to show smears and other directionally oriented patterns or effects.
Most stainless steel can be successfully electropolished. Electropolishing of sulphurised free-machining grades, however, does not give a high standard of surface finish. The anodic dissolution of a thin layer of the surface is similar in principal to electropolishing that can be done on other metals. Around 20 to 40 microns of the surface is removed, leaving a smoothed surface that optimises the corrosion resistance of the steel in any given environment.
The electropolishing process for stainless steel
The process uses relatively low voltages of between 12 and 18 volts, but with large currents of between 750 and 3000 amperes. This gives anode current densities around 20 to 40 amps/dm2. The stainless steel item that is being electropolished is the anode in this direct current cell. Electrolytes used are usually mixtures of phosphoric acids and sulphuric acids.
The process takes around 10-20 minutes.
The process induces preferential dissolution of the ‘peaks’ or high spots on the surface of the work piece. This results in a net smoothing of the surface, which is also beneficial in removing surface stresses left over from mechanical polishing pre-treatments. Contamination and debris left from mechanical surface treatments is also removed by electropolishing. However, scratches and visible surface irregularities are not likely to be removed by electropolishing. Non-metallic inclusions at the surface of the steel may also be more visible after electropolishing, compared to the finish after mechanical polishing methods. Electropolishing can be used on castings to check the surface soundness.
The design of holding jigs is critical, especially on complex shapes, as it influences the consistency of the polished surface and reduces the risk of gas streaking. Both hydrogen and oxygen are generated as a bi-product of the process, with the oxygen coming from the stainless steel ‘anode’. This means that there is no risk of any hydrogen embrittlement to the stainless steel from the electropolishing process.
Benefits of electropolished finishes on stainless steel
Can be used on complex shapes eg wire radiator grilles, where mechanical polishing is difficult or impossible.
Improves surface reflectivity.
Removes machining burrs on small components with a lower risk of entrapped surface contamination from prior mechanical polishing. This confers the added benefit of easier and more efficient in-service cleaning of electropolished items.
Lower tendency for contact substances to adhere (cake-on) to component surfaces and for fibres to ‘snag’ in paper and textile processing applications.
Improves surface cleansability compared to mechanically finished surfaces.
Lower rate of bacterial growth in food industry applications.
Lower surface stresses in machined components. Improved fatigue life has resulted where stainless steel springs have been electropolished, especially compared to normal shot peening treatments.
Elimination of occluded surface gases from items operating under high vacuum conditions.
There is a wide range of product and industry applications for electropolished stainless steel items.
These include :-
Gates, door furniture, floor (durbar) plating, handrails, lampposts, sculptures, glass panel fixings (wrought or cast) etc.
Radiator grilles, bezels, bull-bars, safety equipment frames, tyre making plant vessels etc
Food and Beverages
Brewing vessels, food mixing blades, vending machine water tanks, confectionery moulds etc
Swimming pool building furniture such as ladders, and disabled lift frames
Surgical implants, vein stents, surgical instruments
Process tanks, pipes and valves, powder hoppers etc
Pulp and paper
Semiconductor and high vacuum plant
Pipework, valves, pump parts, clean room process equipment and furniture
Specifying the finish
Electropolished finishes are not caterogised in stainless steel standards for flat products (BS EN 10088:2) or long products (BS EN 10088:3). The subject is however covered in BS ISO 15730:2000-Metallic and other inorganic coatings. Electropolishing as a means of smoothing and passivating stainless steel.
The BSSA Buyers Guide can be used to find member companies who can help with the specification and supply of electropolished finishes. At the ‘Member Products and Services’ page type ‘Electropolishing’ in the ‘Find What’ box with the ‘Service Supplier’ button selected. Click on the ‘Search’ button with ‘Electropolishing-Surface Finishing’ highlighted to view a contacts list.
Source: Zhejiang Yaang Pipe Industry Co., Limited (www.yaang.com)