Zinc and Manganese Phosphating

Galvanic treatments

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Phosphating is a chemical process by which the surface of a metallic material is altered, forming crystals chemically bound to the substrate.

The phosphating treatment is carried out by immersion and uses phosphate solutions, usually zinc or manganese,  to coat the surface with a protective crystalline layer (thickness 5 - 10 µm) that confers high resistance to corrosion and improves the adhesion of the following layer of paint thanks to the increase in surface micro-roughness. The layer obtained must be uniform, compact and composed of small crystals.

The piece, at the end of treatment, must be covered with a uniform grey layer.

 
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The relative coatings

ZINC PHOSPHATING

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.

ZINC PHOSPHATING + POLYMERS
MANGANESE PHOSPHATING

Manganese phosphating (or phosphatisation) is a dark grey coating of phosphate material with anti-rust and anti-seize properties. The shade of the colour and the thickness of the phosphating vary according to the type of material and the parameters of the bath. Manganese phosphating has the advantage, compared to other surface treatments, of guaranteeing a constant thickness coating on the entire surface of the piece.

Phosphating ensures long protection against corrosion of the parts subject to sliding: unlike that with zinc phosphates, this version is less recommended as a pre-paint primer but has good anti-corrosion capabilities, especially if combined with protective oils.

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