Electropolishing

Electropolishing, also referred to as electrochemical polishing, is an electrochemical process which is used for smoothing and polishing of metal surfaces by removing a microscopic amount of material from the work piece. Typically, the metal work piece is immersed in a temperature controlled bath of electrolyte and connected to the positive terminal (anode) of a DC power supply, the negative terminal being attached to an auxiliary electrode (cathode). An ionic current passes from the anode where metal is oxidized to the cathode where a reduction reaction, normally hydrogen evolution, takes place. Electrolytes used for electropolishing are most often concentrated acid solutions having a high viscosity such as mixtures of sulfuric acid and phosphoric acid. Other electropolishing electrolytes reported in the literature include mixtures of perchlorates with acetic anhydride and methanolic solutions of sulfuric acid.

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To achieve electropolishing of a rough metal surface, the protruding parts of a surface profile should dissolve faster than the recesses. This behavior (referred to as anodic leveling) is achieved by applying a specific electrochemical condition, most often involving a mass transport limited dissolution reaction. A second condition for achieving polishing is that surface heterogeneities due to crystal orientation in a polycrystalline material are suppressed and that no pitting occurs. These conditions, often associated with surface brightening, are usually fulfilled with the above mentioned polishing electrolytes and with proper process control.

Electropolishing finds many applications in the metal finishing industry because of its simplicity and because it can be applied to objects of complex shape. Typical examples are electropolished stainless steel drums of washing machines. Electropolishing is also commonly applied to the preparation of thin metal samples for transmission electron microscopy because electropolishing does not cause mechanical deformation of surface layers usually observed when mechanical polishing is used. Anodic dissolution under electropolishing conditions also serves for deburring of metal objects.