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Concept
Phosphate conversion coating
Summary
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 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 name 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.
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 1940s it was very popular in the USA until more modern but similar methods of metal finishes were introduced.
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.
The process takes advantage of the low solubility of phosphates at medium or high pH. The bath is a solution of phosphoric acid (), 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.
Official source
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