Phosphate conversion coating is a chemical treatment applied to steel parts that creates a thin adhering layer of iron, zinc, or manganese phosphates to improve corrosion resistance or lubrication or as a foundation for subsequent coatings or painting. It is one of the most common types of conversion coating. The process is also called phosphate coating, phosphatization, phosphatizing, or phosphating. It is also known by the trade term Parkerizing, 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, 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, zinc, cadmium, silver and tin.
History 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. Until the 1940s it was very popular in the USA until more modern but similar methods of metal finishes were introduced.
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 coatings are typically used as a base for further coatings or painting and are applied by immersion or by spraying. Zinc phosphate 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.
Process The process takes advantage of the low solubility of phosphates at medium or high pH. The bath is a solution of phosphoric acid (H3PO4), containing the desired iron, zinc or manganese cations and other additives. The acid reacts with the iron metal producing hydrogen and iron cations:
Fe + 2 H3O+ → Fe2+ + H2 + 2 H2O 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 locally which may also be deposited. When depositing zinc phosphate or manganese phosphate the additional iron phosphate may be an undesired impurity. The bath often includes an oxidizer, such as sodium nitrite (NaNO2), to consume the hydrogen gas (H2) — 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 compounds on the surface. The performance of a phosphate coating depends on its crystal structure as well as its thickness. A dense microcrystalline structure with a low porosity 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 and increasing its resistance to wear through the application of a chemical phosphate conversion coating. It was usually applied to firearms. Parkerizing is usually considered to be an improved zinc or manganese phosphating process, not an improved iron phosphating process, although some use the term parkerizing as a generic term for applying phosphating (or phosphatizing) coatings that do include the iron phosphating process. Bonderizing, phosphating, and phosphatizing 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. It has also been known as pickling in the context of wrought iron and steel. Parkerizing is commonly used on firearms as a more effective alternative to bluing, which is an earlier-developed chemical conversion 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 as aluminium, brass, or copper but can be used for chemical polishing or etching instead. It similarly cannot be applied to steels containing a large amount of nickel, or on stainless steel. Passivation can be used for protecting other metals.
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