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What is metal chromating used for?

Chromation – surface treatment with a thin layer of chromium phosphate.

Typically, chromating is used before applying powder paint to aluminum

This type of aluminum preparation makes it possible to increase the durability and adhesion of powder paint with metal by 2-3 times. But besides this there are additional functions of this type processing which use metal chromating.

хроматирование металла

Metal chromating is used in several cases:

For protective function. Chromation perfectly protects the product from corrosion. In addition, it gives an attractive appearance to the metal. Usually, parts of cars, bicycles, appliances, etc. are exposed to chrome plating.

To increase wear resistance. Chrome plating helps give metal products extra durability. For example, chromating is used for increasing the durability of the rubbing surfaces of the walls of the cylinders and piston rings of internal combustion engines.

To restore worn out sizes. Increasing a layer of chromium on the worn surface of bushings heat-treated shafts allows you to restore the dimensions of the parts and thereby increase the life of the products.

Chromation type

Metal chromation is divided into the following types:

  • ● green
  • ● yellow

These two chromating methods are distinguished from each other by the layer thickness, the stage of formation and composition.

Green chromation is usually thicker than yellow.

Yellow chromation forms an inhibitory effect in the layer as a result of exposure to soluble Cr6 + ions.

Green chromate

Green chromate layers are obtained in solutions of chromic acid, fluorides and phosphoric acid.

They consist mainly of chromium and aluminum phosphates and do not contain toxic Cr6 + compounds, therefore they can be used in the food industry.

Yellow chromate

Yellow chromate layers are formed in acidic solutions containing chromic acid, activating salt complexes and fluorides or fluoride complexes. As a result of the aluminum dissolution  hydrogen gas is formed. In this case, the pH increases in the diffusion layer that leads to the formation of an Al or Cr (OOH) precipitate. Depending on the reaction conditions and reaction time, a colorless to golden yellow layer forms, what is explained by the presence of an additional amount of Cr6 ions in the chromate layer. Therefore, appling of this surface preparation method is unacceptable in food industry.

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