A conversion coating is a
chemical
A chemical substance is a form of matter having constant chemical composition and characteristic properties. Some references add that chemical substance cannot be separated into its constituent elements by physical separation methods, i.e., wi ...
or
electro-chemical treatment applied to manufactured parts that superficially converts the material into a thin adhering
coating
A coating is a covering that is applied to the surface of an object, usually referred to as the substrate. The purpose of applying the coating may be decorative, functional, or both. Coatings may be applied as liquids, gases or solids e.g. Pow ...
of an insoluble compound. These coatings are commonly applied to protect the part against
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 engine ...
, to improve the adherence of other coatings, for
lubrication
Lubrication is the process or technique of using a lubricant to reduce friction and wear and tear in a contact between two surfaces. The study of lubrication is a discipline in the field of tribology.
Lubrication mechanisms such as fluid-lubric ...
, or for aesthetic purposes.
[
]
Types
The most common conversion coating processes for metal parts with industrial use include
* Chromate (aluminum
Aluminium (aluminum in American and Canadian English) is a chemical element with the symbol Al and atomic number 13. Aluminium has a density lower than those of other common metals, at approximately one third that of steel. It has ...
, steel
Steel is an alloy made up of iron with added carbon to improve its strength and fracture resistance compared to other forms of iron. Many other elements may be present or added. Stainless steels that are corrosion- and oxidation-resistant ty ...
)
* Phosphate
In chemistry, a phosphate is an anion, salt, functional group or ester derived from a phosphoric acid. It most commonly means orthophosphate, a derivative of orthophosphoric acid .
The phosphate or orthophosphate ion is derived from phospho ...
(steel)
* Bluing (steel)
* Black oxide (steel)
* Anodizing
Anodizing is an electrolytic passivation process used to increase the thickness of the natural oxide layer on the surface of metal parts.
The process is called ''anodizing'' because the part to be treated forms the anode electrode of an electro ...
(aluminum)
* Stannate In chemistry the term stannate refers to compounds of tin (Sn). Stannic acid (Sn(OH)4), the formal precursor to stannates, does not exist and is actually a hydrate of SnO2. The term is also used in naming conventions as a suffix; for example the ...
(magnesium)[
* ]Molybdate
In chemistry a molybdate is a compound containing an oxoanion with molybdenum in its highest oxidation state of 6. Molybdenum can form a very large range of such oxoanions which can be discrete structures or polymeric extended structures, althou ...
(zinc
Zinc is a chemical element with the symbol Zn and atomic number 30. Zinc is a slightly brittle metal at room temperature and has a shiny-greyish appearance when oxidation is removed. It is the first element in group 12 (IIB) of the periodi ...
, zinc-nickel
Nickel is a chemical element with symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel is a hard and ductile transition metal. Pure nickel is chemically reactive but large pieces are slow to ...
)[
* ]Zirconate A zirconate is an oxyanion containing zirconium. Examples include Na2ZrO3, Ca2ZrO3 which can be prepared by fusing zirconium dioxide
Zirconium dioxide (), sometimes known as zirconia (not to be confused with zircon), is a white crystalline oxide ...
(steel, aluminum, magnesium
Magnesium is a chemical element with the symbol Mg and atomic number 12. It is a shiny gray metal having a low density, low melting point and high chemical reactivity. Like the other alkaline earth metals (group 2 of the periodic ta ...
, galvanized steel
Galvanization or galvanizing ( also spelled galvanisation or galvanising) is the process of applying a protective zinc coating to steel or iron, to prevent rusting. The most common method is hot-dip galvanizing, in which the parts are submerged ...
).[
* ]Titanate In chemistry, titanate usually refers to inorganic compounds composed of titanium oxides.
Together with niobate, titanate salts form the Perovskite group.
In some cases, the term is used more generally for any titanium-containing anion, e.g. iC ...
(steel, aluminum, magnesium).[
* Plasma electrolysis (aluminum, magnesium, ]titanium
Titanium is a chemical element with the symbol Ti and atomic number 22. Found in nature only as an oxide, it can be reduced to produce a lustrous transition metal with a silver color, low density, and high strength, resistant to corrosion in ...
)[
]
Non-metallic substrates
Conversion coatings have been studied for non-metallic substrates, such as for protection of fluorozirconate glass
Fluoride glass is a class of non-oxide optical glasses composed of fluorides of various metals. They can contain heavy metals such as zirconium, or be combined with lighter elements like aluminum and beryllium. These heavier elements cause the gla ...
es used in optoelectronics
Optoelectronics (or optronics) is the study and application of electronic devices and systems that find, detect and control light, usually considered a sub-field of photonics. In this context, ''light'' often includes invisible forms of radiatio ...
.[
]
Regulations
US military specifications for conversion coatings include MIL-DTL-5541
The military specification referred to as MIL-DTL-5541, "Military Specification, Chemical Conversion Coatings on Aluminum and Aluminum Alloys" covers chemical conversion coatings formed by the reaction of chemical conversion materials with the sur ...
, MIL-DTL-81706, and MIL-DTL-5574, all dealing with aluminum.
See also
* Rust converter
Rust converters are chemical solutions or primers that can be applied directly to an iron or iron alloy surface to convert iron oxides (rust) into a protective chemical barrier. These compounds interact with iron oxides, especially iron(III) oxi ...
– tannate conversion coating
References
[A. A. O. Magalhães, I. C. P. Margarita, O. R. Mattos (2004): "Molybdate conversion coatings on zinc surfaces". ''Journal of Electroanalytical Chemistry'', volume 572, issue 2, pages 433-440. ]
[M. A. Gonzalez-Nunez, C. A. Nunez-Lopez, P. Skeldon, G. E. Thompson, H. Karimzadeh, P. Lyon, and T.E.Wilks (1995): "A non-chromate conversion coating for magnesium alloys and magnesium-based metal matrix composites". ''Corrosion Science'', volume 37, issue 11, pages 1763-1772. ]
[Fastener Surface Conversion]
, In: FastenerData.co.uk
[A. P. Rizzato, C. V. Santilli, S. H. Pulcinelli, Y. Messaddeq, and P. Hammer (2004): "XPS Study of the Corrosion Protection of Fluorozirconate Glasses Dip-Coated with SnO2 Transparent Thin Films". ''Journal of Sol-Gel Science and Technology'', volume 32, pages 155–160. ]
[I. Milošev and G. S. Frankel (2018): "Review—Conversion Coatings Based on Zirconium and/or Titanium". ''Journal of the Electrochemical Society'', volume 165, issue 3,pages C127-C144. ]
[Lishi Wang, Tao Feng, Shanwen Yu, Yihang Cheng, Zhixiang Bu and Xinbin Hu (2020): "Fluoride effect on plasma electrolytic oxidation coating formed on Mg-Al alloy in alkaline electrolytes". ''Materials Research Express'', volume 7, issue 1, article 016412. ]
[Raoling Wang (2018): "Chromium-free aluminum conversion coating agent, aluminum material, and surface conversion coating treatment method". Paten]
WO2018006270
filed on 2016-07-05, granted on 2018-01-11.
[Charles E. Tomlinson (1998): "Chromate-free conversion coatings for metals".]
US Patent 5759244
filed on 1996-10-09, granted on 1998-06-02, expired on 2016-10-09.
[Liu Feng, Shan Dayong, Song Yingwei, Liu Xianbin, and Han Enhou (2012): "Highly corrosion-resistant ceramic coating solution prepared through magnesium alloy surface micro-arc oxidation and application thereof"]
Chinese patent 101994145
filed on 2009-08-19, granted on 2012-05-23.
[J. A. Wharton, G. D. Wilcox, and K. R Baldwin (1996): "Non-Chromate Conversion Coating Treatments For Electrodeposited Zinc-Nickel Alloys". ''Transactions of the IMF - The International Journal of Surface Engineering and Coatings'', volume 74, issue 6, pages 210-213. ]
Coatings
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