Mention the term electropolishing and most people get an image of a bright, reflective metal surface—whether it is found on a diner wall, behind the scenes of an operating room or in the form of a laboratory instrument. While it is true that electropolishing has long been used to shine up many metals, especially stainless steel, the reason why may be much more than just appearance.
Electropolishing (also known as electrochemical polishing or electrolytic polishing) is critical to reducing contaminants in equipment used in industries that touch peoples’ health. These industries include, food and beverage, cosmetics and—perhaps most critical—pharmaceutical and biotechnology. During the normal course of alloy fabrication, bending, grinding, machining and other normal processes change the metal surfaces significantly. These alterations take the form of burrs, scales and tooling marks. Without treatment, these surface imperfections can be a focal point for infection or metal contamination.
Electropolishing removes this surface material from metal. It deburrs and polishes metal parts. This is particularly useful when applied to objects of complex shape, such as the storage and pressurized tanks used in the pharmaceutical and biotechnology industries. Electropolishing may also be used in place of the abrasive fine polishing in microstructural preparation.
Smoother, Cleaner Surfaces
Electropolishing occurs through the electrolytic removal of metal in a highly ionic solution by means of an electric potential and current. In this process, the work piece is typically immersed in a temperature-controlled bath of electrolyte. It is then connected to the positive terminal of a DC power supply, while the negative terminal is 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.
A particular advantage of electropolishing can be found among the chemical interactions that occur on the surface of the metal. Electropolishing levels the grain boundaries of the metal; this, in turn, removes sites for potentially contaminating dirt, microorganisms and chemicals to accumulate.
Another benefit is the reduction of surface area of the grain boundaries (this step is also known as microfinishing). Grain boundaries are often prone to corrosion. During microfinishing, surface metal on the work piece is greatly reduced, leaving the part much smoother both visually and measurably by a profilometer.
A Beneficial Process
Highly sophisticated surface analysis reveals the many benefits that are derived from the fine surface finishing achieved through electropolishing. In general, it is much better to restore a metal surface than to coat a defect. Electropolishing has proven itself to be a critical step in alloy fabrication, a step that is best performed by superior crafts people employed by superior vendors and service providers.