Phosphate coating of ferrous alloys

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Features of galvanic treatments, Phosphate coating of ferrous alloys

The phosphating treatment is the formation of a surface layer of zinc phosphate or manganese and iron, by immersion in appropriate solutions, without electricity application: one can therefore speak of a chemical treatment or chemical conversion that produces a uniform coating thickness on the order of 5-12μ and color from light gray to dark gray: the thickness can vary according to immersion time of the parts in the bath but also in function of the treated steel type, its degree of finish and of the thermal treatment to which it has been subjected: the quality of the steel affects on the final color of the treatment.
The phosphate coating is carried out on all ferrous alloys, except stainless steels, when you want to give a good protection against corrosion, to improve the flow characteristics (friction) or the anchoring of subsequent coatings (paint, Teflon, ...).


You have two types of phosphate:

  • Manganese phosphating

The manganese phosphating is particularly suitable on metal (ferrous) in reciprocal movement since it facilitates the sliding reducing friction and exerting an anti-seizure action: after a short running-in time the crystals of the phosphate get rounded layer, and very smooth surfaces are obtained, with a low coefficient of friction and good wear resistance. The structure of the covering, a bit porous, promotes the absorption of any lubricants, enhancing their function.
This type of phosphate, as well as offering a good protection against corrosion, provides a good basis for subsequent painting treatments and coatings like the Teflon-covering.

  • The zinc phosphating

Zinc phosphating is chosen when the main requirement is the protection against corrosion of the base material, and to have a good anchor for paints and rubber coatings: the slightly porous structure of the phosphate layer provides an excellent adhesion of subsequent applications.