Electropolishing
Encyclopedia
Electropolishing, also known as electrochemical polishing or electrolytic polishing (especially in the metallography
field), 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 may be used in lieu of abrasive
fine polishing in microstructural
preparation.
and serves as the anode
as 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 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 surface, the protruding parts of a surface profile must dissolve faster than the recesses. This process, referred to as anodic leveling, is achieved by a mass transport limited dissolution reaction. Anodic dissolution under electropolishing conditions deburrs metal objects due to increased current density on corners and burrs.
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. Ultra high vacuum (UHV) components are typically electropolished in order to have a smoother surface for improved vacuum pressures, outgassing rates, and pumping speed.
Metallography
Metallography is the study of the physical structure and components of metals, typically using microscopy.Ceramic and polymeric materials may also be prepared using metallographic techniques, hence the terms ceramography, plastography and, collectively, materialography.-Preparing metallographic...
field), is an electrochemical process that removes material from a metallic workpiece. It is used to polish
Polishing
Polishing is the process of creating a smooth and shiny surface by rubbing it or using a chemical action, leaving a surface with a significant specular reflection In some materials polishing is also able to reduce diffuse reflection to...
, passivate
Passivation
Passivation is the process of making a material "passive", and thus less reactive with surrounding air, water, or other gases or liquids. The goal is to inhibit corrosion, whether for structural or cosmetic reasons. Passivation of metals is usually achieved by the deposition of a layer of oxide...
, and deburr metal parts. It is often described as the reverse of electroplating
Electroplating
Electroplating is a plating process in which metal ions in a solution are moved by an electric field to coat an electrode. The process uses electrical current to reduce cations of a desired material from a solution and coat a conductive object with a thin layer of the material, such as a metal...
. It may be used in lieu of abrasive
Abrasive
An abrasive is a material, often a mineral, that is used to shape or finish a workpiece through rubbing which leads to part of the workpiece being worn away...
fine polishing in microstructural
Microstructure
Microstructure is defined as the structure of a prepared surface or thin foil of material as revealed by a microscope above 25× magnification...
preparation.
Mechanism
Typically, the workpiece is immersed in a temperature controlled bath of electrolyteElectrolyte
In chemistry, an electrolyte is any substance containing free ions that make the substance electrically conductive. The most typical electrolyte is an ionic solution, but molten electrolytes and solid electrolytes are also possible....
and serves as the anode
Anode
An anode is an electrode through which electric current flows into a polarized electrical device. Mnemonic: ACID ....
as it is connected to the positive terminal of a DC power supply, the negative terminal being attached to the cathode
Cathode
A cathode is an electrode through which electric current flows out of a polarized electrical device. Mnemonic: CCD .Cathode polarity is not always negative...
. A current
Electric current
Electric current is a flow of electric charge through a medium.This charge is typically carried by moving electrons in a conductor such as wire...
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
Viscosity
Viscosity is a measure of the resistance of a fluid which is being deformed by either shear or tensile stress. In everyday terms , viscosity is "thickness" or "internal friction". Thus, water is "thin", having a lower viscosity, while honey is "thick", having a higher viscosity...
, such as mixtures of sulfuric acid
Sulfuric acid
Sulfuric acid is a strong mineral acid with the molecular formula . Its historical name is oil of vitriol. Pure sulfuric acid is a highly corrosive, colorless, viscous liquid. The salts of sulfuric acid are called sulfates...
and phosphoric acid
Phosphoric acid
Phosphoric acid, also known as orthophosphoric acid or phosphoric acid, is a mineral acid having the chemical formula H3PO4. Orthophosphoric acid molecules can combine with themselves to form a variety of compounds which are also referred to as phosphoric acids, but in a more general way...
. Other electropolishing electrolytes reported in the literature include mixtures of perchlorates with acetic anhydride
Acetic anhydride
Acetic anhydride, or ethanoic anhydride, is the chemical compound with the formula 2O. Commonly abbreviated Ac2O, it is the simplest isolatable acid anhydride and is a widely used reagent in organic synthesis...
and methanolic solutions of sulfuric acid.
To achieve electropolishing of a rough surface, the protruding parts of a surface profile must dissolve faster than the recesses. This process, referred to as anodic leveling, is achieved by a mass transport limited dissolution reaction. Anodic dissolution under electropolishing conditions deburrs metal objects due to increased current density on corners and burrs.
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 steelStainless steel
In metallurgy, stainless steel, also known as inox steel or inox from French "inoxydable", is defined as a steel alloy with a minimum of 10.5 or 11% chromium content by mass....
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. 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