Phosphate conversion coating

Phosphating is a widely used process in surface technology, in which chemical reactions of metal surfaces with aqueous phosphate solutions called a conversion layer is formed of firmly adhering metal phosphates. Phosphating is usually applied to steel, but may also be used for zinc or cadmium-plated steel and aluminum. Main fields of application are corrosion protection, adhesion, friction and wear reduction as well as electrical insulation. Phosphating is also known under various brand names such as bonders or Parkerisieren.

In the phosphating first a pickling attack on the base material, go with the metal cations with evolution of hydrogen in solution. Then, the film formation is carried out by precipitation of sparingly soluble phosphates. Depending on the nature of the solution, this may be be iron, zinc, or manganese phosphate layers. In the non- film-forming phosphating, the metal cations involved in the film formation are taken from the base metal, the metal cations of the phosphate solution are not involved in the layer structure. Wherein layer-forming phosphating is carried out by the layer structure of metal cations from the phosphate solution, the metal cations may be involved in addition of the base material. The layer thicknesses range from a few hundred nanometers at the iron phosphating of up to two microns, and more at the zinc and Manganphospatierungen.

The importance of the phosphating because of the different properties of the phosphate coating ago. The phosphate layer adheres well to the substrate and allowed through the microporous layer structure or microcapillary a good anchoring of subsequent coatings. Therefore, phosphate coatings are very often used as a base for paint. In addition, it complicates the rusting of damaged areas of the coating. Phosphate layers alone provide a serviceable temporary corrosion protection, which is often sufficient for the storage prior to a subsequent processing step. Zinc phosphate coatings are superior to iron phosphate layers in the protective effect. The corrosion protection of the phosphating may be improved by oiling or waxing. The phosphate layers have good sliding properties. This is utilized during the cold forming of steel. Here zinc phosphate coatings are used because of the easy Abscherbarkeit. The latter may react with the alkali soaps commonly used as a lubricant to zinc soaps, thereby forming a firmly adhering and thermally higher load carrying capacity lubricant system. Manganese phosphate coatings are used to reduce wear of highly loaded sliding surfaces. They should keep the wear during running low and prevent seizure on intermittent lack of lubricant. The high electrical resistivity of phosphate layers is used in electrical panels for the magnetic cores, to isolate it with a thin separating layer relative to one another.

• Surface Technology