Electropolishing is an electrochemical process that preferentially etches microprojections (peaks) on a metal surface, and minimally etches microdepressions (valleys), resulting in a metal surface that is smooth on an atomic scale, typically producing a mirror-like appearance. In addition to improving the surface finish, certain metals, such as stainless steel, have their surface chemistry modified by selective removal of iron. This produces a chromium-rich oxide passivation film which significantly improves corrosion resistance and surface cleanliness, reduces moisture absorption and friction or surface drag, and stress-relieves the surface. These properties are important to the chemical processing, medical, nuclear, and semiconductor industries.

Traditional electropolishing of stainless steels, the largest commercial user of this process, consists of heated baths primarily of concentrated sulfuric and phosphoric acids. In addition to being toxic, corrosive, and hazardous, large quantities of hydrogen gas are evolved under high electrical current densities, which result in corrosive acid mists.

Various ionic liquids provide a �green� alternative electropolish medium. They are generally of much lower toxicity, can operate at room temperature, exhibit greater electrical current efficiency, and consequently produce less evolved hydrogen gas. Test results of traditional acid-based electropolish are compared with ionic liquids composed of choline chloride (vitamin B4) and ethylene glycol (�antifreeze�) on various stainless steels. Additional tests substituting propylene glycol produce comparable results, while being even more environmentally friendly.