A conversion coating is a
chemical
A chemical substance is a unique form of matter with constant chemical composition and characteristic properties. Chemical substances may take the form of a single element or chemical compounds. If two or more chemical substances can be combin ...
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, or substrate. The purpose of applying the coating may be decorative, functional, or both. Coatings may be applied as liquids, gases or solids e.g. powder coatings.
Paints ...
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-lubr ...
, or for aesthetic purposes.
[
]
Types
The most common conversion coating processes for metal parts with industrial use include
* Chromate (aluminum
Aluminium (or aluminum in North American English) is a chemical element; it has chemical symbol, symbol Al and atomic number 13. It has a density lower than that of other common metals, about one-third that of steel. Aluminium has ...
, steel
Steel is an alloy of iron and carbon that demonstrates improved mechanical properties compared to the pure form of iron. Due to steel's high Young's modulus, elastic modulus, Yield (engineering), yield strength, Fracture, fracture strength a ...
)
* Phosphate
Phosphates are the naturally occurring form of the element phosphorus.
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 orthop ...
(steel)
* Bluing (steel)
* Black oxide (steel)
* Anodizing (aluminum)
* Stannate (magnesium)[
* Molybdate (]zinc
Zinc is a chemical element; it has symbol Zn and atomic number 30. It 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 periodic tabl ...
, zinc-nickel
Nickel is a chemical element; it has 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 slo ...
)[
* Zirconate (steel, aluminum, ]magnesium
Magnesium is a chemical element; it has Symbol (chemistry), 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 ...
, galvanized steel).[
* Titanate (steel, aluminum, magnesium).][
* Plasma electrolysis (aluminum, magnesium, ]titanium
Titanium is a chemical element; it has 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 glasses 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 radi ...
.[
]
Regulations
US military specifications for conversion coatings include MIL-DTL-5541, MIL-DTL-81706, and MIL-DTL-5574, all dealing with aluminum.
See also
* Rust converter
Rust converters are chemical solutions or Primer (paint)#Primers for metal, primers that can be applied directly to an iron or steel, iron alloy surface to convert iron oxides (rust) into a protective chemical barrier. These compounds interact with ...
– 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
{{industry-stub