Electropolishing is the process of smoothing and / or brightening a metal surface anodically in a concentrated acid or alkaline solution. Typically, the workpiece is immersed in a temperature-controlled bath of electrolyte and serves as the anode; it is connected to the positive terminal of a DC power supply, the negative terminal being attached to the cathode.
A current passes from the anode, where metal on the surface is oxidized and dissolved in the electrolyte, to the cathode. At the cathode, a reduction reaction occurs, which normally produces hydrogen.
Electrolytes used for electropolishing are most often concentrated acid solutions having a high viscosity, such as mixtures of sulphuric acid and phosphoric acid. Other electropolishing electrolytes reported in the literature include mixtures of perchlorates with acetic anhydride and methanolic solutions of sulphuric acid.
While it can be done on many base metals as a pre-plate operation, it is commonly done on Stainless Steel as a final finish.
It provides a chemically and physically clean surface and removes any mechanical surface asperities which may be detrimental to the production of uniform and pit-free electroplated surfaces or the future performance and appearance of a Stainless Steel product.
It helps to deburr machined edges and holes as well as removes any embedded iron from the manufacturing process.
The current is greatest on the outside edges and outside corners of parts, which are left especially smooth.
The longer the process is applied, the greater the amount of metal that is removed: holes can be enlarged,threads can be rounded, and sharp edges can be smoothed. 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.
Nickel alloys possess excellent corrosion resistant properties. They are chosen for their superior performance at elevated temperatures. Nickel alloys, including Hastelloy, Inconel and Monel require special consideration when electropolishing.
Aluminum can be divided into two major categories: wrought and casting alloys. Casting alloys, known by three number designations (1xx, 2xx, 3xx, etc.), contain a high level of silicon. The silicon gives the casting alloys low melting points and good fluidity, yet prevents a desirable finish from aluminum electropolishing.
Wrought alloys, known by four number designations (1xxx, 2xxx, 3xxx, etc.), are shaped by plastic deformation and have significantly different compositions and microstructures from casting alloys. The electropolishing of wrought alloys produces desirable results. An aluminum electropolished wrought surface will exhibit an improved micro-surface and will be free of surface contaminates. The surface will also exhibit a lustrous finish, however, not to the same degree as stainless steel.