Electropolishing

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Electropolishing, also referred to as electrochemical polishing, is an electrochemical process that removes material from a metallic workpiece. It is used to polish, passivate, and deburr metal parts. It is often described as the reverse of electroplating. It differs from anodizing in that the purpose of anodizing is to grow a thick, protective oxide layer on the surface of a material (usually aluminum) rather than polish. Also known as electrolytic polishing in metallography, it may be used in lieu of abrasive fine polishing in microstructural preparation.^[1]

Mechanism

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). A current passes from the anode where metal on the surface is oxidized and dissolved in the electrolyte. At the cathode, 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.

To achieve electropolishing of a rough metal surface, the protruding parts of a surface profile must 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.

Applications

Electropolishing has many applications in the metal finishing industry because of its simplicity and it can be applied to objects of complex shape. Typical examples are electropolished stainless steel drums of washing machines and stainless steel surgical devices. 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 deburrs metal objects due to increased current density on corners and burrs. Ultra high vacuum (UHV) components are typically electropolished in order to have a smoother surface for improved vacuum pressures, outgassing rates, and pumping speed.

Standards

• ASME BPE Standards for Electropolishing Bioprocessing Equipment
• SEMI F19, Electropolishing Specifications for Semiconductor Applications
• ASTM B 912-00, Passivation of Stainless Steels Using Electropolishing
• ASTM E1558, Standard Guide for Electrolytic Polishing of Metallographic Specimens

References

External links

• Electropolishing Users Guide