Phosphating is the treatment of metallic surfaces with aqueous and acid phosphate-containing solutions. The metal phosphate conversion layer resulting from the chemical reaction is slightly soluble and adheres firmly to the substrate.
Phosphating can be applied to steel, ferrous materials, zinc and aluminium. First, the base material is attacked by pickling so that pH values of the solution associated with near-surface areas shift. This leads to precipitation of poorly soluble phosphates, which form a layer. The metal cations dissolved from the base material are stored in this layer again. However, there are processes in which metal cations of the solution are incorporated into the layer. This conversion layer is used industrially as a temporary corrosion protection and/or adhesion primer for e.g. oiling or cathodic dip painting.
Essentially, a distinction is made between:
The process is usually carried out by dipping, but can also be applied by spraying. Depending on the application, medium to fine-crystalline zinc manganese or mixed iron phosphate layers are applied to screws or nuts. The layer thicknesses are between 2 μm and 20 μm.