Phosphate coating is a treatment for metals, where a layer of phosphate from another metal is applied to provide corrosion protection. The coating provides a stable base for other coatings and treatments. Zinc, manganese, and iron are used for phosphate coatings, and oil is often applied for additional protection. Phosphate coating is useful as an underlying layer for paint to adhere to.
Phosphate coating is a type of treatment for metals, especially iron and steel, in which a layer of phosphate from another metal, such as zinc, is applied to provide corrosion protection. The phosphate coating of a steel part provides an inert layer that helps prevent rust and provides a stable base for other coatings such as paint to be applied. Phosphates are dissolved in an acid solution and when the metal part is immersed in the phosphate acid bath, a chemical reaction occurs which deposits the phosphate on the surface.
A phosphoric acid solution is used as the basis for the phosphating bath. The desired phosphate for the coating is dissolved in the solution. When the piece to be coated is immersed in the solution, the acid reacts with the metal. This causes the pH of the solution immediately around the piece of metal to rise, which causes phosphate to precipitate out of the solution and onto the metal. The phosphate bonds to the metal in a fairly uniform layer thickness regardless of the shape of the part.
The primary metals used in phosphate coatings are zinc, manganese and iron. Zinc manganese phosphates provide greater corrosion resistance than iron phosphate, but because a phosphate layer is somewhat porous, none of the three phosphates are a perfect barrier to rust or other types of corrosion. Phosphating on the other hand provides a basis for further coatings or treatments.
The oil is often applied over a phosphate coating for additional corrosion protection. This works well to prevent rust because the porous phosphate coating absorbs the oil, and together, the two create an effective barrier. This type of coating is often known as a P&O, phosphate and oil coating. Other coatings are often applied over phosphate coatings, especially sealants and lubricants. These types of treatments are effective in preventing corrosion and reducing wear on moving parts by reducing friction.
Metal parts that are to be painted are often phosphated in preparation for applying the paint. In these cases the corrosion resistance is secondary to the characteristics of the coating, making it particularly useful as an underlying layer for paint to adhere to. The porous phosphate coating absorbs and bonds to the paint much more effectively than it would with bare metal.
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