The substrate surface treatment is all the preparation work for the surface of the coated object before the coating of materials, which is the first process of coating construction and also a very basic work. The treatment of substrate surface before coating is directly related to the anti-corrosion performance and protection life of the entire coating system.

Through long-term practice, it has been proved that more than 70% of the early failure of many protection systems is caused by improper pre-treatment. British Iron and Steel Research Association has conducted exposure test on the coating system of two layers of red lead oily primer and two layers of oily finish in industrial atmosphere environment, and obtained the relationship between various relevant factors and the protection life of the coating system,

Role of substrate treatment

1. Improve the adhesion of coating to material surface

According to the adsorption theory, the physical adsorption strength is inversely proportional to the sixth power of the distance, so the coating should have sufficient infiltration with the substrate to form a good film adhesion. Wetting occurs on the contact surface and is related to the different cohesive energy and surface tension between different objects. When the contact angle is smaller, the paint and the substrate can be fully soaked to produce good adhesion. Generally, the surface tension of clean steel surface is higher than that of any coating, so it can be well wetted by coating. When some oil stains and grease are attached to the substrate, these pollutants will make the surface tension very low, so that the coating can not fully wet the substrate, resulting in poor adhesion, which will result in the whole film falling off or various appearance defects. Therefore, the pollutants on the substrate surface must be completely removed before coating.

2. Improve the anti-corrosion protection ability of the coating on the metal substrate

After steel rusting, the rust product contains very unstable ferric acid (α - FeOOH), which will still cause corrosion to expand and spread under the coating, resulting in rapid destruction of the coating and loss of protection function. If the coating is applied after rust removal, the reliability of coating protection will be greatly improved. In recent years, although there are some rusty coatings that do not need to remove rust from the substrate, so far the long-term reliability of rusty coatings is still lacking.

3. Improve the flatness of the substrate surface

The molding sand, welding slag and rust on the casting surface will seriously affect the appearance of the coating and must be removed by sand blasting, polishing and other methods; After the rough surface is painted, the gloss of the coating will become dull due to the impact of the uneven surface. Therefore, the coated objects with high decorative requirements must first be smoothed by surface treatment.

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