Phosphate conversion coating is a chemical treatment applied tosteel parts that creates a thin adhering layer ofiron,zinc, ormanganesephosphates to improvecorrosion resistance orlubrication or as a foundation for subsequent coatings or painting.[1][2][3] It is one of the most common types ofconversion coating. The process is also calledphosphate coating,phosphatization,[4]phosphatizing, orphosphating. It is also known by the trade nameParkerizing, especially when applied tofirearms and othermilitary equipment.[5]: 393
A phosphate coating is usually obtained by applying to the steel part a dilute solution ofphosphoric acid, possibly with soluble iron, zinc, and/or manganese salts. The solution may be applied by sponging, spraying, or immersion.[6] Phosphate conversion coatings can also be used onaluminium,zinc,cadmium,silver andtin.[7][8]
The phosphatizing of firearms was discovered around 1910, when it was found that the surface of steel, if changed to a phosphate, acquires significant corrosion resistance.[5]: 393 Until the 1940s it was very popular in the USA until more modern but similar methods of metal finishes were introduced.[5]: 393
The main types of phosphate coatings are manganese, iron, and zinc.[9]
The process takes advantage of the lowsolubility of phosphates at medium or highpH. The bath is a solution ofphosphoric acid (H3PO4), containing the desired iron, zinc or manganese cations and other additives.[10] The acid reacts with the iron metal producinghydrogen and iron cations:
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 createsiron phosphate locally which may also be deposited. When depositingzinc phosphate ormanganese phosphate the additional iron phosphate may be an undesired impurity.
The bath often includes an oxidizer, such assodium nitrite (NaNO2), to consume the hydrogen gas (H
2) — which otherwise would form a layer of tiny bubbles over the surface, slowing down the reaction.[10]
The main phosphating step can be preceded by an "activation" bath that creates tiny particles oftitanium compounds on the surface.[10]
The performance of a phosphate coating depends on itscrystal structure as well as its thickness. A densemicrocrystalline structure with a lowporosity 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.[6]
Parkerizing is a method of protecting asteel surface fromcorrosion and increasing its resistance to wear through the application of a chemical phosphate conversion coating. It was usually applied to firearms.[5]: 393 Parkerizing is usually considered to be an improvedzinc ormanganese phosphating process, and not to be an improved iron phosphating process, although some use the termparkerizing as a generic term for applying phosphating (or phosphatizing) coatings that do include the iron phosphating process.
Bonderizing,phosphating, andphosphatizing are other terms associated with the Parkerizing process but were often used for finishes of car parts as it gave finer grain on the surface.[5]: 394 It has also been known aspickling in the context ofwrought iron andsteel.[11]
Parkerizing is commonly used onfirearms as a more effective alternative tobluing, which is an earlier-developed chemicalconversion coating. It is also used extensively on automobiles to protect unfinished metal parts from corrosion.
The Parkerizing process cannot be used to protect non-ferrous metals such asaluminium,brass, orcopper but can be used for chemical polishing or etching instead. It similarly cannot be applied tosteels containing a large amount ofnickel, or onstainless steel.Passivation can be used for protecting other metals.
Development of the process was started in Britain and continued by the Parker family in theUnited States. The termsParkerizing,Parkerize, andParkerized are all registered U.S.trademarks ofHenkel Adhesives Technologies, although the terminology has largely passed intogeneric use for many years. The process was first used on a large scale in the manufacture of firearms for the United States military duringWorld War II.[12]
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, ofBirmingham, England, subsequently filed a patent based on this same process in America in 1907, which was grantedU.S. patent 870,937 in 1907. It essentially provided an iron phosphating process, usingphosphoric acid.
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 asU.S. patent 1,069,903.
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 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 asU.S. patent 1,311,319, 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 to1⁄3 of the original time that had been required through heating the solution to a temperature in the precisely controlled range of 500 to 550 °F (260 to 288 °C). This patent was issued asU.S. patent 1,761,186 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 (usingstrategic compounds that would remain available in America during a war) was granted to inventor Romig of the American Chemical Paint Company in 1938 asU.S. patent 2,132,883, just prior to the loss of easy access to manganese compounds that occurred duringWorld War II.
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,U.S. patent 2,293,716, 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 115 to 130 °F (46 to 54 °C), 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.
Glock Ges.m.b.H., anAustrian firearms manufacturer, uses a black Parkerizing process as a topcoat to aTenifer process to protect theslides of thepistols 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.
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.
Phosphate coatings are also commonly used as an effective surface preparation for further coating and/or painting, providing excellent adhesion and electric isolation.[6]
Phosphate coatings are often used to protect steel parts againstrusting 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 adheringdielectric (electrically insulating) layer that can protect the part fromelectrochemical and under-paint corrosion.[6]
Zinc and manganese coatings are used to help break in components subject to wear[1] and help preventgalling.[6]
While a zinc phosphate coating by itself is somewhatabrasive, it can be turned into alubricating layer forcold forming operations by treatment withsodium stearate (soap). The soap reacts with the phosphate crystals forming a very thin insoluble andhydrophobiczinc stearate layer, that helps to hold the unreacted sodium stearate even under extreme deformation of the part, such as inwire drawing.[1][13]