Phosphate conversion coating
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 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 H
The reaction 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 performance of a phosphate coating depends on its crystal structure as well as its thickness. A microcrystalline structure 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.
Phosphate coatings are also commonly used as an effective surface preparation for further coating and/or painting, providing excellent adhesion and electric isolation. 
Phosphate coatings are often used to protect steel parts against rusting 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 (electrically insulating) layer that can protect the part from electrochemical and under-paint corrosion.
While a zinc phosphate coating by itself is somewhat abrasve, it can be turned into a lubricating layer for cold forming operations by treatment with sodium stearate (soap). The soap reacts with the phosphate crystals forming a very thin insoluble and hydrophobic zinc stearate layer, that helps to hold the unreacted sodium stearate even under extreme deformation of the part, such as in wire drawing.
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