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Metal phosphating before painting

Metal phosphating is used to improve surface adhesion, further protect the product against rust formation, and to make it more rigid and durable. The phosphate coating is applied to already cleaned steel, cast iron and non-ferrous metals. This is one of the best priming for paint application.

Such a coating can be used independently as a protective coating. But it is rarely used because it is very prone to destruction by alkaline and acid solutions.

Phosphate coating: characteristics

Phosphating is the creation of a layer of iron, zinc or manganese phosphates on a product. The thickness of the layer can be from 2 to 50 microns, and it depends on the structure of the coating. For example, the fine crystalline layer will be up to 5 microns thick and with the best protective properties. That is why it is used most often. This coating is obtained through zinc phosphate solutions with accelerators. However, it can only be used if the product is painted after phosphating. It is not used alone as a protective coating.

Manganese-phosphate solutions form a thick layer of large crystalline coating. It is not necessary to paint the metal thus treated.

The phosphate coating can have different shades from light gray to almost black. Light shades of coating are formed on non-ferrous metals. But after phosphating ferrous metals, the product has a darker color. The color of nickel and chromium alloy steel takes on a greenish tint after this procedure.

Metal phosphating before painting

Metal phosphating: technology

Phosphates precipitate during phosphating and partially dissolve the parent metal.

The phosphate coating on ferrous metals is formed by chemical or electrochemical method. The first is cold, normal and accelerated.

During cold phosphating the product is applied “Majef” and monosubstituted zinc phosphate. The solutions are not heated. Processing takes 15 to 40 minutes. This method is rarely used. The fact is that these solutions are rapidly hydrolyzed. And because of this, the coating becomes porous and has low protective properties.

The same “Majef” is used for normal phosphating. The tool is heated to 98 ° C. Processing takes up to 10 minutes. High-alloy steels need to be treated for about an hour at a solution temperature of 100 ° C.

The most common method of applying phosphate coating is accelerated. In addition to processing time, it has other advantages over the two previous ones. The duration of such phosphating is 8 to 15 minutes. Processing is carried out by the same “Majef”. It is heated to 45-65 ° C. Oxidizers are added to the solution. The phosphate layer is of a small thickness and is great for subsequent painting of the product.

Improved coating structure can be achieved by adding zinc oxalates to the agent.

For electrochemical phosphating, the same agent is used, only alternately or alternating current is used. Products are hung on rods that serve as a cathode. Anode – plates of zinc or steel. Such processing takes up to 20 minutes. This tool is not suitable for products of complex shape.

Metal phosphating before painting

To reduce the porosity of the films, the phosphated products are subjected to additional processing – passivation in a dilute solution having dihydrohexafluoro zirconate and aminoalkylsilanes. Passivation allows to increase in 2-2,5 times the protective property of powder paint coating. A distinctive feature of modern preparations for passivation is the complete absence of compounds of 6-valent chromium, which has a positive effect on process safety and the environment.

Non-ferrous metals are phosphated in orthophosphoric acid with fluorides.

For phosphating, the product is either immersed in a bath with the necessary means, or sprayed into a workpiece.

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