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Phosphate conversion coating is a chemical treatment applied to steel parts that creates a thin adhering layer of
iron Iron () is a chemical element with Symbol (chemistry), symbol Fe (from la, Wikt:ferrum, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 element, group 8 of the periodic table. It is, Abundanc ...
,
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 ...
, or
manganese Manganese is a chemical element with the symbol Mn and atomic number 25. It is a hard, brittle, silvery metal, often found in minerals in combination with iron. Manganese is a transition metal with a multifaceted array of industrial alloy use ...
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 phosph ...
s, to achieve
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 ...
resistance, lubrication, or as a foundation for subsequent coatings or painting. It is one of the most common types of
conversion coating A conversion coating is a chemical or electro-chemical treatment applied to manufactured parts that superficially converts the material into a thin adhering coating of an insoluble compound. These coatings are commonly applied to protect the part ...
. The process is also called phosphate coating, phosphatization, phosphatizing, or phosphating. It is also known by the trade name
Parkerizing Phosphate conversion coating is a chemical treatment applied to steel parts that creates a thin adhering layer of iron, zinc, or manganese phosphates, to achieve corrosion resistance, lubrication, or as a foundation for subsequent coatings or p ...
, especially when applied to firearms and other military equipment. A phosphate coating is usually obtained by applying to the steel part a dilute solution of
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 ...
, possibly with soluble iron, zinc, and/or manganese salts. The solution may be applied by sponging, spraying, or immersion. Phosphate conversion coatings can also be used on
aluminium 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. I ...
,
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 ...
,
cadmium Cadmium is a chemical element with the symbol Cd and atomic number 48. This soft, silvery-white metal is chemically similar to the two other stable metals in group 12, zinc and mercury. Like zinc, it demonstrates oxidation state +2 in most of ...
,
silver Silver is a chemical element with the symbol Ag (from the Latin ', derived from the Proto-Indo-European ''h₂erǵ'': "shiny" or "white") and atomic number 47. A soft, white, lustrous transition metal, it exhibits the highest electrical ...
and
tin Tin is a chemical element with the symbol Sn (from la, stannum) and atomic number 50. Tin is a silvery-coloured metal. Tin is soft enough to be cut with little force and a bar of tin can be bent by hand with little effort. When bent, t ...
.


Types

The main types of phosphate coatings are manganese, iron, and zinc. * Manganese phosphate coatings are used both for corrosion resistance and lubricity and are applied only by immersion. *
Iron phosphate Iron phosphate may refer to: * Iron(II) phosphate * Iron(III) phosphate Iron(III) phosphate, also ferric phosphate, is the inorganic compound with the formula Fe PO4. Several related materials are known, including four polymorphs of FePO4 and ...
coatings are typically used as a base for further coatings or painting and are applied by immersion or by spraying. *
Zinc phosphate Zinc phosphate is an inorganic compound with the formula Zn3( PO4)2. This white powder is widely used as a corrosion resistant coating on metal surfaces either as part of an electroplating process or applied as a primer pigment (see also red lea ...
coatings are used for corrosion resistance, as a lubricant-holding layer, and as a paint/coating base and can also be applied by immersion or spraying. They can also be applied to
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 submerg ...
.


Process

The process takes advantage of the low
solubility In chemistry, solubility is the ability of a substance, the solute, to form a solution with another substance, the solvent. Insolubility is the opposite property, the inability of the solute to form such a solution. The extent of the solub ...
of phosphates at medium or high pH. The bath is a solution of
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 ...
(), containing the desired iron, zinc or manganese cations and other additives. The acid reacts with the iron metal producing
hydrogen Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxic ...
and iron cations: : Fe + 2 → + + 2 The reaction consuming protons raises the pH of the solution in the immediate vicinity of the surface, until eventually the phosphates become insoluble and get deposited over it. The acid and metal reaction also creates
iron phosphate Iron phosphate may refer to: * Iron(II) phosphate * Iron(III) phosphate Iron(III) phosphate, also ferric phosphate, is the inorganic compound with the formula Fe PO4. Several related materials are known, including four polymorphs of FePO4 and ...
locally which may also be deposited. When depositing
zinc phosphate Zinc phosphate is an inorganic compound with the formula Zn3( PO4)2. This white powder is widely used as a corrosion resistant coating on metal surfaces either as part of an electroplating process or applied as a primer pigment (see also red lea ...
or manganese phosphate the additional iron phosphate may be an undesired impurity. The bath often includes an oxidizer, such as
sodium nitrite Sodium nitrite is an inorganic compound with the chemical formula NaNO2. It is a white to slightly yellowish crystalline powder that is very soluble in water and is hygroscopic. From an industrial perspective, it is the most important nitrite ...
(), to consume the hydrogen gas () — which otherwise would form a layer of tiny bubbles over the surface, slowing down the reaction. The main phosphating step can be preceded by an "activation" bath that creates tiny particles of
titanium Titanium is a chemical element with the Symbol (chemistry), 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, resista ...
compounds on the surface. The performance of a phosphate coating depends on its
crystal structure In crystallography, crystal structure is a description of the ordered arrangement of atoms, ions or molecules in a crystalline material. Ordered structures occur from the intrinsic nature of the constituent particles to form symmetric patterns ...
as well as its thickness. A dense
microcrystalline A microcrystalline material is a crystallized substance or rock that contains small crystals visible only through microscopic examination. There is little agreement on the range of crystal sizes that should be regarded as microcrystalline, but th ...
structure with a low
porosity Porosity or void fraction is a measure of the void (i.e. "empty") spaces in a material, and is a fraction of the volume of voids over the total volume, between 0 and 1, or as a percentage between 0% and 100%. Strictly speaking, some tests measur ...
is usually best for corrosion resistance or subsequent painting. A coarse grain structure impregnated with oil may be best for wear resistance. These factors can be controlled by varying the bath concentration, composition, temperature, and time.


Parkerizing

Parkerizing is a method of protecting a steel surface from
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 ...
and increasing its resistance to wear through the application of a chemical phosphate conversion coating. Parkerizing is usually considered to be an improved
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 ...
or
manganese Manganese is a chemical element with the symbol Mn and atomic number 25. It is a hard, brittle, silvery metal, often found in minerals in combination with iron. Manganese is a transition metal with a multifaceted array of industrial alloy use ...
phosphating process, and not to be an improved iron phosphating process, although some use the term ''parkerizing'' as a generic term for applying phosphating (or phosphatizing) coatings that does include the iron phosphating process. Bonderizing, phosphating, and phosphatizing are other terms associated with the Parkerizing process. It has also been known as
pickling Pickling is the process of preserving or extending the shelf life of food by either anaerobic fermentation in brine or immersion in vinegar. The pickling procedure typically affects the food's texture and flavor. The resulting food is cal ...
in the context of
wrought iron Wrought iron is an iron alloy with a very low carbon content (less than 0.08%) in contrast to that of cast iron (2.1% to 4%). It is a semi-fused mass of iron with fibrous slag inclusions (up to 2% by weight), which give it a wood-like "grain" ...
and steel. Parkerizing is commonly used on firearms as a more effective alternative to bluing, which is an earlier-developed chemical
conversion coating A conversion coating is a chemical or electro-chemical treatment applied to manufactured parts that superficially converts the material into a thin adhering coating of an insoluble compound. These coatings are commonly applied to protect the part ...
. It is also used extensively on automobiles to protect unfinished metal parts from corrosion. The Parkerizing process cannot be used on non-ferrous metals such as
aluminium 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. I ...
,
brass Brass is an alloy of copper (Cu) and zinc (Zn), in proportions which can be varied to achieve different mechanical, electrical, and chemical properties. It is a substitutional alloy: atoms of the two constituents may replace each other wit ...
, or
copper Copper is a chemical element with the symbol Cu (from la, cuprum) and atomic number 29. It is a soft, malleable, and ductile metal with very high thermal and electrical conductivity. A freshly exposed surface of pure copper has a pinkis ...
. It similarly cannot be applied to steels containing a large amount of
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 ...
, or on stainless steel. Passivation can be used for protecting other metals.


Early history

Development of the process was started in England and continued by the Parker family in the
United States The United States of America (U.S.A. or USA), commonly known as the United States (U.S. or US) or America, is a country primarily located in North America. It consists of 50 states, a federal district, five major unincorporated territori ...
. The terms ''Parkerizing'', ''Parkerize'', and ''Parkerized'' are all technically registered U.S.
trademark A trademark (also written trade mark or trade-mark) is a type of intellectual property consisting of a recognizable sign, design, or expression that identifies products or services from a particular source and distinguishes them from othe ...
s of Henkel Adhesives Technologies, although the terminology has largely passed into generic use for many years. The process was first used on a large scale in the manufacture of firearms for the United States military during
World War II World War II or the Second World War, often abbreviated as WWII or WW2, was a world war that lasted from 1939 to 1945. It involved the vast majority of the world's countries—including all of the great powers—forming two opposing ...
. The earliest work on phosphating processes was developed by British inventors William Alexander Ross, British patent 3119, in 1869, and by Thomas Watts Coslett, British patent 8667, in 1906. Coslett, of
Birmingham, England Birmingham ( ) is a city and metropolitan borough in the metropolitan county of West Midlands in England. It is the second-largest city in the United Kingdom with a population of 1.145 million in the city proper, 2.92 million in the We ...
, subsequently filed a patent based on this same process in America in 1907, which was granted in 1907. It essentially provided an iron phosphating process, using
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 ...
. An improved patent application for manganese phosphating based in large part on this early British iron phosphating process was filed in the US in 1912, and issued in 1913 to Frank Rupert Granville Richards as . Clark W. Parker acquired the rights to Coslett's and Richards' U.S. patents, and experimented in the family kitchen with these and other rust-resisting formulations. The ultimate result was that Clark W. Parker, along with his son Wyman C. Parker, working together, set up the Parker Rust-Proof Phosphating Company of America in 1915. R. D. Colquhoun of the Parker Rust-Proof Phosphating Company of America then filed another improved phosphating patent application in 1919. This patent was issued in 1919 as , for an improved manganese phosphating (Parkerizing) technique. Similarly, Baker and Dingman of the Parker Rust-Proof Company filed an improved manganese phosphating (Parkerizing) process patent in 1928 that reduced the processing time to of the original time that had been required through heating the solution to a temperature in the precisely controlled range of . This patent was issued as in 1930. Manganese phosphating, even with these process improvements, still required the use of expensive and difficult-to-obtain manganese compounds. Subsequently, an alternative technique was developed by the Parker Company to use easier-to-obtain compounds at less expense through using zinc phosphating in place of manganese phosphating. The patent for this zinc phosphating process (using strategic compounds that would remain available in America during a war) was granted to inventor Romig of the American Chemical Paint Company in 1938 as , just prior to the loss of easy access to manganese compounds that occurred during
World War II World War II or the Second World War, often abbreviated as WWII or WW2, was a world war that lasted from 1939 to 1945. It involved the vast majority of the world's countries—including all of the great powers—forming two opposing ...
. Somewhat analogous to the improved manganese phosphating process improvements discovered by Baker and Dingman, a similarly improved method was found for an improved zinc phosphating process as well. This improvement was discovered by Darsey of the Parker Rust Proof Company, who filed a patent in February 1941, which was granted in August 1942, , that improved upon the zinc phosphatizing (Parkerizing) process further. He discovered that adding copper reduced the acidity requirement over what had been required, and that also adding a chlorate to the nitrates that were already used would additionally permit running the process at a much lower temperature in the range of , reducing the cost of running the process further. With these process improvements, the end result was that a low-temperature (energy-efficient) zinc phosphating (Parkerizing) process, using strategic materials to which the United States had ready access, became the most common phosphating process used during World War II to protect American war materials such as firearms and planes from rust and corrosion.


Later developments

Glock Ges.m.b.H., an
Austria Austria, , bar, Östareich officially the Republic of Austria, is a country in the southern part of Central Europe, lying in the Eastern Alps. It is a federation of nine states, one of which is the capital, Vienna, the most populous ...
n firearms manufacturer, uses a black Parkerizing process as a topcoat to a
Tenifer Ferritic nitrocarburizing or FNC, also known by the proprietary names Tenifer, Tufftride and Melonite as well as ARCOR,Other trade names include Tuffride/ Tuffrider, QPQ, Sulfinuz, Sursulf, Meli 1, and Nitride, among others is a range of proprie ...
process to protect the slides of the
pistols A pistol is a handgun, more specifically one with the chamber integral to its gun barrel, though in common usage the two terms are often used interchangeably. The English word was introduced in , when early handguns were produced in Europe, ...
they manufacture. After applying the Tenifer process, a black Parkerized finish is applied and the slide is protected even if the Parkerized finish were to wear off. Used this way, Parkerizing is thus becoming a protective and decorative finishing technique that is used over other underlying improved techniques of metal protection. Traditional iron phosphate, zinc phosphate, and manganese phosphate chemical conversion coatings, including Parkerizing variations, have all been criticized''U.S. Environmental Protection Agency Recommendations''
/ref> in recent years for introducing phosphates into surface water systems, encouraging the rapid growth of algae (
eutrophication Eutrophication is the process by which an entire body of water, or parts of it, becomes progressively enriched with minerals and nutrients, particularly nitrogen and phosphorus. It has also been defined as "nutrient-induced increase in phytopla ...
). As a result, in recent years, new, emerging technology alternatives to traditional phosphate coatings have started to see limited use, for replacing all phosphating coatings, including Parkerizing. The majority of these newer conversion coatings are fluorozirconium-based. The most popular of these fluorozirconium-based conversion coatings, introduced in 2005, incorporates the transition metal vanadium. This new, more environmentally friendly coating is referred to as a
vanadate In chemistry, a vanadate is an anionic coordination complex of vanadium. Often vanadate refers to oxoanions of vanadium, most of which exist in its highest oxidation state of +5. The complexes and are referred to as hexacyanovanadate(III) and no ...
conversion coating. Besides vanadate coatings,
arsenate The arsenate ion is . An arsenate (compound) is any compound that contains this ion. Arsenates are salts or esters of arsenic acid. The arsenic atom in arsenate has a valency of 5 and is also known as pentavalent arsenic or As(V). Arsenate res ...
coatings may theoretically provide similar protection, at the risk of being a health hazard to humans and animals. It remains to be seen if these, or other new chemical conversion coatings, will ultimately replace traditional phosphating and Parkerizing. Various of similar recipes for stovetop kitchen Parkerizing circulate in gun publications at times, and Parkerizing kits are sold by major gun-parts distributors such as Brownells.


Uses


Painting primer

Phosphate coatings are also commonly used as an effective surface preparation for further coating and/or painting, providing excellent adhesion and electric isolation.


Corrosion resistance

Phosphate coatings are often used to protect steel parts against
rusting Rust is an iron oxide, a usually reddish-brown oxide formed by the reaction of iron and oxygen in the catalytic presence of water or air moisture. Rust consists of hydrous iron(III) oxides (Fe2O3·nH2O) and iron(III) oxide-hydroxide (FeO(OH) ...
and other types of corrosion. However, they are somewhat porous, so this use requires impregnating the coating with oil, paint, or some other sealing substance. The result is a tightly adhering
dielectric In electromagnetism, a dielectric (or dielectric medium) is an electrical insulator that can be polarised by an applied electric field. When a dielectric material is placed in an electric field, electric charges do not flow through the mate ...
(electrically insulating) layer that can protect the part from
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 ...
and under-paint corrosion.


Wear resistance

Zinc and manganese coatings are used to help break in components subject to wear and help prevent galling.


Lubrication

While a zinc phosphate coating by itself is somewhat
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 ...
, it can be turned into a
lubricating 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-lubrica ...
layer for
cold forming In metallurgy, cold forming or cold working is any metalworking process in which metal is shaped below its recrystallization temperature, usually at the ambient temperature. Such processes are contrasted with hot working techniques like hot r ...
operations by treatment with
sodium stearate Sodium stearate is the sodium salt of stearic acid. This white solid is the most common soap. It is found in many types of solid deodorants, rubbers, latex paints, and inks. It is also a component of some food additives and food flavorings.Klaus ...
(
soap Soap is a salt of a fatty acid used in a variety of cleansing and lubricating products. In a domestic setting, soaps are surfactants usually used for washing, bathing, and other types of housekeeping. In industrial settings, soaps are use ...
). The soap reacts with the phosphate crystals forming a very thin insoluble and
hydrophobic In chemistry, hydrophobicity is the physical property of a molecule that is seemingly repelled from a mass of water (known as a hydrophobe). In contrast, hydrophiles are attracted to water. Hydrophobic molecules tend to be nonpolar and, t ...
zinc stearate Zinc stearate is a "zinc soap" that is widely used industrially. In this context, soap is used in its formal sense, a metal salt of a fatty acid: in this case stearic acid. It is a white solid that repels water. It is insoluble in polar solvents ...
layer, that helps to hold the unreacted sodium stearate even under extreme deformation of the part, such as in
wire drawing Wire drawing is a metalworking process used to reduce the cross-section of a wire by pulling the wire through a single, or series of, drawing die(s). There are many applications for wire drawing, including electrical wiring, cables, tension-loa ...
.


See also

*
Chromate conversion coating Chromate conversion coating or alodine coating is a type of conversion coating used to passivate steel, aluminium, zinc, cadmium, copper, silver, titanium, magnesium, and tin alloys. The coating serves as a corrosion inhibitor, as a primer ...
*
Iron pillar of Delhi The iron pillar of Delhi is a structure high with a diameter that was constructed by Chandragupta II (reigned c. 375–415 AD), and now stands in the Qutb complex at Mehrauli in Delhi, India.Finbarr Barry Flood, 2003"Pillar, palimpsets, and pr ...


References

W. Meisel (1986): "Studies of the Phosphatization of Steel and its Corrosion Products". Chapter of ''Industrial Applications of the Mössbauer Effect''. Jim Dufour (2006): ''An Introduction to Metallurgy'', 5th edition, pages IX 11–12. J. Skar, M. Walter, and D. Albright (1997): "Non-Chromate Conversion Coatings for Magnesium Die Castings". ', https://www.sae.org/publications/technical-papers/content/970324/ DOI: https://doi.org/10.4271/970324 Citation: Skar, J., Walter, M., and Albright, D., "," SAE International, Technical Paper 970324 Joseph Edwards (1997): ''Coating and Surface Treatment Systems for Metals''. Finishing Publications Ltd. T.S.N. Sankara Narayanan (2005): " urface pretreatment by phosphate conversion coatings - A review Surface pretreatment by phosphate conversion coatings - A review ''Rev.Adv.Mater.Sci'', volume 9, pages 130-177.


Sources

* MIL-HDBK-205, ''Phosphate & Black Oxide Coating of Ferrous Metals'': a standard overview on phosphate and black oxide (bluing) coatings * * {{Citation , surname=Brimi , given=Marjorie A. , title=Electrofinishing , publisher=American Elsevier Publishing Company, Inc. , place= New York, New York , year=1965 , pages=62–63 .


External links


Henkel Surface Technologies
€”Current owner of Parco-Lubrite (a manganese phosphating process) and other Parkerizing rust-prevention coatings. (Parco is a registered trademark of Henkel Surface Technologies.)
Coral Chemical Company
€”Current owner of Coral Eco Treat (vanadium conversion coating process)
Parker Rust-Proof of Cleveland
€”Last remaining of the four original job shop licensees of Parker Chemical, currently offers phosphating services Coatings Corrosion prevention