The Electropolishing Process
Stated simply, electropolishing is the controlled electrochemical removal of surface metal, resulting in a brilliant appearance and improved properties. Sometimes described as "reverse plating" or "super passivation," the process has a leveling effect, which produces smoothness and increased reflectivity. More importantly, the deformed, amorphous outer layer of the metal is removed, leaving a chromium-rich, passive surface free of imbedded contaminants and work-induced residual stresses. When properly performed, the process is completely safe, does not cause pitting or etching, and, in contrast to pickling or acidic cleaning, does not produce hydrogen embrittlement.
Electropolishing is accomplished by connecting the metal part to be processed to the positive terminal (the anode) of a DC power supply. The part is then immersed in a heated electrolytic bath that contains metal plates connected to the negative terminal (the cathode). The electrical reaction causes an ionic conduction resulting in the removal of particles of metal from the anode.
During the process, the products of this anodic metal dissolution react with the electrolyte to form a film at the surface of the metal. This film essentially conforms to the general contour of the surface of the metal and therefore is thinner over the micro-projections and thicker at the micro-depressions. The result is more rapid dissolution of the micro-projections causing micro-leveling at the surface.
The amount of metal removed is influenced by the composition of the metal part to be electropolished, the temperature and agitation of the electrolytic bath, the spatial and area relationships of the anode and cathode, the current, and the length of time the current is flowing.
The result of electropolishing is demonstrated in the following diagrams showing the effect on the surface of the metal:
Surface profile of workpiece before electropolishing.
Action of electropolishing demonstrating leveling effect and chrome-enriched surface layer.