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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, collect ...
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 Surface roughness, often shortened to roughness, is a component of surface finish (surface texture). It is quantified by the deviations in the direction of the normal vector of a real surface from its ideal form. If these deviations are large, ...
by levelling micro-peaks and valleys, improving the
surface finish Surface finish, also known as surface texture or surface topography, is the nature of a surface as defined by the three characteristics of lay, surface roughness, and waviness.. It comprises the small, local deviations of a surface from the p ...
. Electropolishing is often compared to, but distinctly different from,
electrochemical machining Electrochemical machining (ECM) is a method of removing metal by an electrochemical process. It is normally used for mass production and is used for working extremely hard materials or materials that are difficult to machine using conventional me ...
. It is used to polish, passivate, and deburr metal parts. It is often described as the reverse of
electroplating Electroplating, also known as electrochemical deposition or electrodeposition, is a process for producing a metal coating on a solid substrate through the reduction of cations of that metal by means of a direct electric current. The part to be ...
. 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 An electrolyte is a medium containing ions that is electrically conducting through the movement of those ions, but not conducting electrons. This includes most soluble salts, acids, and bases dissolved in a polar solvent, such as water. Upon ...
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 is ...
; 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 int ...
, such as mixtures of
sulfuric acid Sulfuric acid (American spelling and the preferred IUPAC name) or sulphuric acid ( Commonwealth spelling), known in antiquity as oil of vitriol, is a mineral acid composed of the elements sulfur, oxygen and hydrogen, with the molecular fo ...
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 aqueo ...
with
acetic anhydride Acetic anhydride, or ethanoic anhydride, is the chemical compound with the formula (CH3CO)2O. Commonly abbreviated Ac2O, it is the simplest isolable anhydride of a carboxylic acid and is widely used as a reagent in organic synthesis. It is a co ...
(which has caused fatal explosions), and
methanol Methanol (also called methyl alcohol and wood spirit, amongst other names) is an organic chemical and the simplest aliphatic alcohol, with the formula C H3 O H (a methyl group linked to a hydroxyl group, often abbreviated as MeOH). It is ...
ic 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 Stainless steel is an alloy of iron that is resistant to rusting and corrosion. It contains at least 11% chromium and may contain elements such as carbon, other nonmetals and metals to obtain other desired properties. Stainless steel's r ...
, 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 approximatel ...
(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 ...
and
atom probe The atom probe was introduced at th14th Field Emission Symposium in 1967by Erwin Wilhelm Müller and J. A. Panitz. It combined a field ion microscope with a mass spectrometer having a single particle detection capability and, for the first time ...
tomography because the process does not mechanically deform surface layers like mechanical polishing does.


Standards

* 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 ASTM International, formerly known as American Society for Testing and Materials, is an international standards organization that develops and publishes voluntary consensus technical standards for a wide range of materials, products, systems, an ...
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 (American and British English spelling differences#-re, -er, international spelling as used by the International Bureau of Weights and Measures; SI symbol: μm) or micrometer (American and British English spelling differences# ...
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.


See also

*
Corrosion Corrosion is a natural process that converts a refined metal into a more chemically stable oxide. It is the gradual deterioration of materials (usually a metal) by chemical or electrochemical reaction with their environment. Corrosion engi ...
*
Electrochemistry 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 out ...
*
Electroetching Electroetching is a metal etching process that involves the use of a solution of an electrolyte, an anode, and a cathode. The metal piece to be etched is connected to the positive pole of a source of direct electric current. A piece of the same m ...
* Electro-Glo Distribution *
Electroplating Electroplating, also known as electrochemical deposition or electrodeposition, is a process for producing a metal coating on a solid substrate through the reduction of cations of that metal by means of a direct electric current. The part to be ...
*
Passivation (chemistry) Passivation, in physical chemistry and engineering, refers to coating a material so it becomes "passive", that is, less readily affected or corroded by the environment. Passivation involves creation of an outer layer of shield material that is a ...
*
Polishing (metalworking) Polishing and buffing are finishing processes for smoothing a workpiece's surface using an abrasive and a work wheel or a leather strop. Technically, ''polishing'' refers to processes that uses an abrasive that is glued to the work wheel, while ...
*
Stainless steel Stainless steel is an alloy of iron that is resistant to rusting and corrosion. It contains at least 11% chromium and may contain elements such as carbon, other nonmetals and metals to obtain other desired properties. Stainless steel's r ...
* Surface finishing * Electro-Max Inc


References

{{Reflist


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