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Phosphating

The prerequisite for producing a phosphate coating is providing a metallically clean surface. All grease residues (drilling oils, corrosion protection agents, fingerprints, etc.) and other impurities on the surface of the parts to be treated must, therefore, be reliably removed prior to the phosphatising process.


A phosphate layer is chemically generated by immersing the workpieces into a heated and acidic zinc, manganese, zinc-calcium or tri-cation phosphate solution. A chemical reaction occurs through the pickling attack of the metal surfaces with the substrate. This is followed by hydrogen-formation and a shift in pH value on the metal surface. Low solubility phosphates are formed, uniformly distributed across the metal surface as permanently anchored crystallites.

 

Properties of a phosphate coating:

  • Light to dark grey manganese, zinc or zinc/calcium phosphate coating consisting of secondary and tertiary manganese, zinc or zinc/calcium phosphates.
  • Tightly fixed on the base material.
  • Fine to coarse crystalline appearance.
  • Coating-forming layer (3–20 µm), has to be taken into account for fitting pieces.
  • Corrosion protection oils and varnishes can be absorbed very well due to the presence of many capillaries in the coating, resulting in a very high corrosion protection.
  • Phosphate coatings are electrically non-conductive, i. e. the insulation resistance is high.
  • The phosphate coating can be applied as a corrosion protection or adhesion base for subsequent coating treatments, or varnishes or for improvement of sliding properties to reduce the sliding friction (non-cutting deformation).
  • Damaged coatings will hardly be tunnelled by rust.