Electropolishing, also known as electrochemical polishing, anodic polishing, or electrolytic polishing (especially in the

metallography Metallography is the study of the physical structure and components of metals, by using microscopy. Ceramic and polymeric materials may also be prepared using metallographic techniques, hence the terms ceramography, plastography and, collecti ...

field), is an

electrochemical Electrochemistry is the branch of physical chemistry concerned with the relationship between electrical potential difference, as a measurable and quantitative phenomenon, and identifiable chemical change, with the potential difference as an outc ...

process that removes material from a metallic workpiece, reducing the surface roughness by levelling micro-peaks and valleys, improving the surface finish. Electropolishing is often compared to, but distinctly different from, electrochemical machining. It is used to

polish Polish may refer to: * Anything from or related to Poland, a country in Europe * Polish language * Poles Poles,, ; singular masculine: ''Polak'', singular feminine: ''Polka'' or Polish people, are a West Slavic nation and ethnic group, w ...

, passivate, and

deburr A burr is a raised edge or small piece of material that remains attached to a workpiece after a modification process. It is usually an unwanted piece of material and is removed with a deburring tool in a process called 'deburring'. Burrs are mo ...

metal parts. It is often described as the reverse of electroplating. It may be used in lieu of

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 by friction. While finishing a material often means polishing it to gain a smooth, reflec ...

fine polishing in microstructural preparation.

Mechanism

Typically, the work-piece is immersed in a temperature-controlled bath of electrolyte and serves as the

anode An anode is an electrode of a polarized electrical device through which conventional current enters the device. This contrasts with a cathode, an electrode of the device through which conventional current leaves the device. A common mnemonic ...

; it is connected to the positive terminal of a DC power supply, the negative terminal being attached to the cathode. A

current Currents, Current or The Current may refer to: Science and technology * Current (fluid), the flow of a liquid or a gas ** Air current, a flow of air ** Ocean current, a current in the ocean *** Rip current, a kind of water current ** Current (stre ...

passes from the anode, where metal on the surface is oxidised 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 The viscosity of a fluid is a measure of its resistance to deformation at a given rate. For liquids, it corresponds to the informal concept of "thickness": for example, syrup has a higher viscosity than water. Viscosity quantifies the inte ...

, such as mixtures of sulfuric acid and

phosphoric acid Phosphoric acid (orthophosphoric acid, monophosphoric acid or phosphoric(V) acid) is a colorless, odorless phosphorus-containing solid, and inorganic compound with the chemical formula . It is commonly encountered as an 85% aqueous solution, w ...

. Other electropolishing electrolytes reported in the literature include mixtures of

perchloric acid Perchloric acid is a mineral acid with the formula H Cl O4. Usually found as an aqueous solution, this colorless compound is a stronger acid than sulfuric acid, nitric acid and hydrochloric acid. It is a powerful oxidizer when hot, but aqueous s ...

with acetic anhydride (which has caused fatal explosions), and methanolic solutions of sulfuric acid. To electropolish a rough surface, the protruding parts of a surface profile must dissolve faster than the recesses. This process, referred to as ''anodic levelling'', can be subject to incorrect analysis when measuring the surface topography. Anodic dissolution under electropolishing conditions deburrs metal objects due to increased current density on corners and burrs. Most importantly, successful electropolishing should operate under diffusion limited constant current plateau, achieved by following current dependence on voltage (polarisation curve), under constant temperature and stirring conditions.

Applications

Due to its ease of operation and its usefulness in polishing irregularly-shaped objects, electropolishing has become a common process in the production of semiconductors. As electropolishing can also be used to sterilize workpieces, the process plays an essential role in the food, medical, and pharmaceutical industries. It is commonly used in the post-production of large metal pieces such as those used in drums of washing machines, bodies of ocean vessels and aircraft, and automobiles. While nearly any metal may be electropolished, the most-commonly polished metals are 300- and 400-series stainless steel, aluminum, copper, titanium, and nickel- and copper-alloys.

Ultra-high vacuum Ultra-high vacuum (UHV) is the vacuum regime characterised by pressures lower than about . UHV conditions are created by pumping the gas out of a UHV chamber. At these low pressures the mean free path of a gas molecule is greater than approximately ...

(UHV) components are typically electropolished in order to have a smoother surface for improved vacuum pressures, out-gassing rates, and pumping speed. Electropolishing is commonly used to prepare thin metal samples for

transmission electron microscopy Transmission electron microscopy (TEM) is a microscopy technique in which a beam of electrons is transmitted through a specimen to form an image. The specimen is most often an ultrathin section less than 100 nm thick or a suspension on a g ...

and atom probe tomography because the process does not mechanically deform surface layers like mechanical polishing does.

Standards

ISO ISO is the most common abbreviation for the International Organization for Standardization. ISO or Iso may also refer to: Business and finance * Iso (supermarket), a chain of Danish supermarkets incorporated into the SuperBest chain in 2007 * Iso ...

.15730:2000 Metallic and other Inorganic Coatings - Electropolishing as a Means of Smoothing and Passivating Stainless Steel *

ASME The American Society of Mechanical Engineers (ASME) is an American professional association that, in its own words, "promotes the art, science, and practice of multidisciplinary engineering and allied sciences around the globe" via "continuing ...

BPE Standards for Electropolishing Bioprocessing Equipment *SEMI F19, Electropolishing Specifications for Semiconductor Applications * ASTM B 912-02 (2008), Passivation of Stainless Steels Using Electropolishing *ASTM E1558, Standard Guide for Electrolytic Polishing of Metallographic Specimens

Benefits

* The results are considered to be aesthetically pleasing by many people. * Creates a clean, smooth surface that is easier to sterilise. * Can polish areas that are inaccessible by other polishing methods. * Removes a small amount of material (typically 20-40

micrometre The micrometre ( international spelling as used by the International Bureau of Weights and Measures; SI symbol: μm) or micrometer (American spelling), also commonly known as a micron, is a unit of length in the International System of Unit ...

in depth in the case of stainless steel) from the surface of the parts, while also removing small burrs or high spots. It can be used to reduce the size of parts when necessary. * Stainless steel preferentially removes iron from the surface and enhances the chromium/nickel content for the most superior form of passivation for stainless steel.

References

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External links

ElectropolishingThe "Then & Now" of Electropolishing
by Anopol Limited/Surface World. Published January 2010.
Electropolishing of Stainless Steels
by British Stainless Steel Association
Electropolishing Users GuideElectropolishing: A Space Age Process
Aircraft Engineering and Aerospace Technology, Volume 67 Issue 3

Chemical processes Metallurgical processes Metalworking