Phosphating is a chemical method of surface treatment in which a metallic surface reacts with an aqueous phosphate solution. This creates a hardly soluble conversion layer made of metal phosphates. For this purpose, the material is first cleaned with acid and then the phosphate layer is formed.
Phosphating is a chemical/electro-chemical process in which thin, fine-crystalline and water insoluble phosphates are generated on metal surfaces from phosphoric acid solutions in a dipping or spraying procedure.
As they are the result of a chemical reaction with the base metal, the pale or dark grey colored metal phosphate layers are securely anchored in the metal surface and include many cavities and capillaries. This characteristic gives the phosphate layers an optimum absorption capacity for oils, waxes, color pigments and lacquers, so they have proven their value as corrosion protection and as an adhesive substrate for paint and varnishes. A further characteristic of the phosphate layer is its ability to dissipate friction forces (as generated in deformation, drawing or sliding processes). So, in addition to an improvement of the surface quality of the pieces treated, the processing speed can be considerably increased with prolonged service life of the tools.
A distinction is made between layer-forming and non-layer-forming phosphating. In the latter, the metal cations that form the layer come only from the base material itself and not from the solution. In the layer-forming phosphating variant, these cations come from the phosphate solution.
characteristics of phosphating include:
Pale to dark grey manganese, zinc or zinc/calcium phosphate layer consisting of secondary and tertiary manganese, zinc or zinc/calcium phosphates.
Securely anchored on the base metal
Fine-crystalline to coarse-crystalline appearance (according to process)
The additional layer (3–20 micrometers) – must be taken into account for fitting parts.
Thanks to the existence of many capillaries in the layer, corrosion protection oils and lacquers are very well absorbed, providing very good corrosion protection.
Phosphate layers are electrically non-conductive, i.e. their insulating resistance is relatively high.
Damaged layers are hardly penetrated by rust.
fields of application
Phosphating is used particularly for components made of steel and iron. Elements made of zinc or other non-ferrous metals can also be phosphated. The areas of application of this process are corrosion protection, reduction of wear, adhesion promotion and electrical insulation. Since the phosphate layer produced adheres very well, it is often used as a substrate for coatings.