It is a chemical treatment that enables you to obtain a protective phosphate layer on ferrous alloys.
It is carried out by immersing in acid solutions of appropriate composition at a temperature of about 70°C.
The chemical reaction that is triggered by the immersion of the pieces produces a layer of controlled thickness, composed mainly of zinc phosphate.
The dimensional increase is minimal, but it must be taken into account on precision parts.
Zinc phosphate has an acicular and porous appearance.
Dry it has moderate corrosion resistance that is increased by impregnating the porosity of the layer with protective oils. With the best protective or polymeric coatings the corrosion resistance is very high.
Contrary to popular belief, zinc phosphate does not have a higher corrosion resistance than manganese phosphate, because zinc is not in the metallic state, but combined.
In addition, due to the acicular shape of the layer, it is not suitable for hydraulic components that have metal sealing seats, because the shape of the phosphate crystals does not guarantee perfect sealing.
The appearance of the pieces ranges from light grey to black grey, and depends on the chemical composition of the treated alloys.
Zinc phosphating can be carried out on iron, carbon steel, alloy steel and cast iron.