Electropolishing stainless steel is a process that involves controlled removal of the metal from its surface: contaminations are eliminated by acting deeper than with pickling, and the surfaces are passivated simultaneously with electropolishing thanks to the anode current.
Electropolishing is the opposite of a galvanic deposition process: instead of depositing a coating of another material on a surface, the action of electropolishing is to remove a surface layer, typically between 10 and 40 microns in the case of stainless steel. In this way all contaminants are removed by acting deeper than with pickling. Surface passivation occurs simultaneously with electropolishing (ASTM B912)
The quality of passivation will depend on the quality of the stainless steels - AISI 304, AISI 316 etc. -, the formulation of the electrolyte and the operating conditions. The free iron on the surface of the stainless steel is removed giving the surface a higher corrosion resistance. Microscopic smoothing achieved with electropolishing will also help improve corrosion resistance. An electropolished product is therefore passive as reported in ASTM B912/02.
Electropolishing requires a source of electric current and a rectifier in order to convert it from alternating to direct. By means of metal bars, the direct current is transmitted in order pass through an anode and cathodes suspended above a tank containing an electrolyte solution that is normally acid-based. The workpiece to be treated (positive pole) is suspended from the anode facing the cathode (negative pole). When the current is started, the material is removed from the part to be processed, with preferential removal of the micro-peaks of material. This microscopically smooths the surface, while its macro-profile remains unaltered.
Carefully choosing the electrolyte used allows the electropolishing technique to be applied to different materials including aluminium, titanium and copper, although the application that has piqued the most interest commercially is electropolishing of stainless steel. Hastelloy©, Inconel© and other nickel-chromium alloys have been successfully electropolished. In most cases, electropolished stainless steel surfaces are bright and shiny, as a result of the removal of the often contaminated surface layer and the electrochemical action of micro-deburring. The advantages of such a shiny and smooth surface are many. By reducing the total surface, as a result of micro-deburring, any products that come into contact contact adhere less easily to an electropolished stainless steel surface. For the same reason, surfaces can be cleaned and kept clean more easily.
The friction of an electropolished surface is lower thanks to the smoothing effect that makes electropolishing an excellent technique to eliminate machining burrs that are difficult to remove mechanically. The glossy and shiny finish is ideal for a huge variety of aesthetic applications, in particular where mechanical polishing is difficult. The applications that benefit from the characteristics of electropolishing are many, and continue to grow every day.
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