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Pretreatment method of metal coating

Release time:2018-4-3

In industrial production, in the process of processing, transportation and storage, the surface of workpieces often contains rust, oxide skin, loose old paint film, oil and other dirt, which have adverse effects on metal coating.

(1)The existence of rust will reduce the contact area between the coating and the surface and the adhesion of the coating to the steel surface.At the same time, the impurities such as ferrous sulfate and sodium chloride in the corrosion products will also promote the corrosion of metals.Therefore, the primary work of metal surface pretreatment is rust removal. When there are salt impurities on the rusted surface, clean it with water before derusting.

(2)Oxide skin is a dense oxide layer formed at high temperature in the process of hot rolling or high-temperature forging and forming of iron and steel. It usually adheres firmly, but it is brittle compared with steel itself, and its expansion coefficient is smaller than that of steel, and it is easy to crack and fall off after cold and hot cycles.From the electrochemical analysis, the electrode potential of oxide skin is 0.15 ~ 0. 20V higher than that of iron. In the corrosive medium, oxide skin is the cathode, and iron is prior to oxide skin corrosion. Especially in the presence of a large number of oxide skin, a large number of oxide skin is the large cathode, and a small amount of exposed iron becomes a small anode, which will accelerate the local corrosion of iron.Due to the fact that there are numerous tiny cracks between the oxide skin and the steel surface, water vapor and corrosion medium are easy to penetrate into the loose oxide skin, resulting in crevice corrosion and concentration difference corrosion.Therefore, oxide scale must be completely removed before painting. If the steel surface is coated without completely removing the oxide skin, the coating quality will be poor and the service life will be greatly shortened.

(3)The free energy of the outer layer of the steel surface will be reduced (from 0.004n • cm to 0.0002n • cm), even lower than the free energy of the coating, as a result, the coating can not well wet the steel surface.It not only reduces the wetting adhesion of the coating to the surface, but also causes the shrinkage of the coating.

The investigation shows that the coating defects caused by pretreatment account for more than 50% of the whole coating defects.Therefore, metal materials usually need to be pretreated before coating, otherwise, it will not only affect the adhesion and corrosion resistance of the coating and the base metal, but also make the base metal continue to corrode even with the protection of the coating, so that the coating will peel off quickly.In a word, the surface treatment before coating is an important guarantee and measure to obtain a good protective layer and extend the service life of the product.

The workpiece can achieve the following goals after proper pretreatment.

(1)Increase the adhesion of the coating on the coating surface.Although the strength of adhesion is related to the variety and quality of paint, the quality of surface treatment is also a key factor. If the surface of workpiece before painting is not clean, there are water, oil, dust, oxide skin, rust and other dirt, or there is an old paint film that is not firm, the new paint film will not adhere firmly, resulting in blistering, cracking, falling off and other defects, making the metal and the surrounding environment The harmful medium is corroded and destroyed.The purpose of surface treatment is to remove these harmful substances.In addition, in order to enhance the adhesion between the coating film and the workpiece surface, it is required that the workpiece surface should have appropriate surface roughness before coating.The surface roughness of metal has a great influence on the coating quality. The proper surface roughness can improve the mechanical occluding effect of the coating on the surface and increase the binding force between the coating and the metal surface.However, the surface should not be too rough, otherwise the coating will be uneven on the surface, especially at the wave crest of the rough surface, which is easy to cause too thin coating and early corrosion. According to practical experience, the surface roughness of steel is suitable for 40~75μm.In addition, if a porous conversion membrane is formed in the pretreatment, the coating can penetrate into the pores of the conversion membrane, forming a "anchoring effect", and also enhancing the binding force.

(2)Improve the corrosion resistance of the surface and prolong the service life of the coating.If there are water, oil, rust and other dirt on the surface of steel, it will reduce the anti-corrosion performance of the coating. They exist between the coating and the surface of the coating, which will corrode the metal surface and destroy the coating.In addition, it is difficult to form non-conductive insoluble phosphate or oxide conversion film by pretreatment, which can greatly slow down the corrosion rate of the coating.It is generally believed that the corrosion resistance of the coating after phosphating will be increased by 2-3 times, so the phosphating treatment has attracted the attention of various industries and enterprises. At present, in the production of automobile coating, it is advocated to adopt the phosphating treatment 100%.

(3)Avoid the ill condition of coating film after coating and affect the appearance.If the metal surface is not treated thoroughly or not, there will be many pathological conditions after coating, such as oil pollution will make the film produce "fish eyes" and peeling; wax will make the film not dry, stick back and produce pinholes; rust and oxide skin will make the film blister, affect the appearance of the vehicle, and lose the decorative and protective functions of the paint.The surface roughness of the substrate directly affects the gloss of the coating. For example, the surface of iron castings is usually rough, and the coating after direct coating is dull. Generally, in order to obtain a good coating surface of the workpiece, the basic requirements for the surface of the workpiece before coating are as follows:

(1)No oil and moisture

(2)No rust and oxide

(3)No adhesive impurities

(4)No acid, alkali and other residues

The surface of workpiece has certain roughness.

With the rapid development of science and technology, laser technology has been more and more used in various fields of people's production and life.From the bar code of supermarket, laser printer to laser beauty, myopia treatment, and has been familiar to people. Laser equipment for industrial production of cutting, drilling and welding is also known to many people.But the application of laser equipment in cleaning industry is not familiar and understood.Laser cleaning technology is a new type of cleaning technology developed rapidly in recent 10 years. It has gradually replaced the traditional cleaning technology in many fields with its own advantages and irreplaceable.The application and research and development of Shanghai runqia Electronic Technology Co., Ltd. in the field of laser cleaning are in the forefront of domestic peers, and there are many successful cases, which have brought great benefits to customers.

Principle of laser surface cleaning

The process of pulse laser cleaning depends on the characteristics of the light pulse generated by the laser, which is based on the photophysical reaction caused by the interaction between high-intensity beam, short pulse laser and pollution layer.Its physical principle can be summarized as follows:

a)The laser beam is absorbed by the pollution layer on the surface to be treated.

b)The absorption of large energy forms a rapidly expanding plasma (highly ionized unstable gas), which produces shock wave.

c)The shock wave makes the pollutant fragment and be eliminated.

d)The optical pulse width must be short enough to avoid the heat accumulation which will damage the treated surface.

e)Experiments show that when there are oxides on the metal surface, the plasma is produced on the metal surface.

The plasma is produced only when the energy density is higher than the threshold value, which depends on the removed pollution layer or oxide layer.This threshold effect is very important for effective cleaning under the condition of ensuring the safety of substrate materials.There is a second threshold for the appearance of plasma.If the energy density exceeds this threshold, the substrate material will be destroyed.In order to ensure the safety of the substrate materials, the laser parameters must be adjusted according to the situation, so that the energy density of the light pulse is strictly between two thresholds.

Advantages of laser surface cleaning:

Compared with the traditional cleaning methods such as mechanical friction cleaning, chemical corrosion cleaning, liquid-solid strong impact cleaning, high-frequency ultrasonic cleaning, laser cleaning has obvious advantages.

3.1 Laser cleaning is a kind of "green" cleaning method, which does not need to use any chemical agent and cleaning liquid. The cleaned waste is basically solid powder, small size, easy to store, recyclable, and can easily solve the environmental pollution caused by chemical cleaning.

3.2 The traditional cleaning method is often contact cleaning, which has mechanical force on the surface of the cleaned object. The surface of the damaged object or the cleaning medium is attached to the surface of the cleaned object, which can not be removed, resulting in secondary pollution. The non grinding and non-contact of laser cleaning make these problems solved easily.