Application of new pretreatment technology for painting?

What are the specific contents of the application of the new pretreatment process before painting? The following Zhong Da consults the bidding teacher to answer your questions for your reference.

1, oil and rust removal

High-quality phosphating film is only formed on the surface of the workpiece, and impurities such as oil, rust and scale on the surface of the workpiece have been completely removed. Because the residual oil, rust and scale on the surface of the workpiece will seriously hinder the growth of phosphating film. In addition, it will also affect the adhesion, drying performance, decorative performance and corrosion resistance of the coating, and thorough removal of these foreign bodies is a necessary condition for phosphating.

Oil removal and rust removal are two basic processes before phosphating. Relatively speaking, oil is more harmful than rust, and the workpiece with oil directly affects the speed of rust removal. Therefore, derusting should be carried out on the basis of oil removal, but for workpieces with less oil and more rust, the two basic processes can also be combined, and the oil removal and derusting processes can be completed in one tank at the same time, which can shorten the production line and reduce the investment cost of equipment and workshop, but the treatment quality is not as good as that of a separate tank, so it can be used in occasions with low requirements. After adopting this process, the degreasing agent should also be used in conjunction with the acid agent and rust remover. Hydrochloric acid is still used for rust removal, which is fast, clean and thorough, has a good effect of removing oxide scale, and is used at room temperature. The disadvantage is that hydrochloric acid rust removal acid mist is large, which is harmful to health and serious environmental pollution. With the development of industry, environmental protection and improvement of working conditions have become a common concern. Therefore, the need of environmental protection should be considered when selecting chemicals. Therefore, the selection of degreasing agent requires simple preparation, strong decontamination ability, no sodium hydroxide, silicate, OP emulsifier and other components that are difficult to clean at room temperature, easy to clean at room temperature, no toxic substances, no harmful gases and good working conditions; When selecting rust remover, it is required to contain accelerant, corrosion inhibitor and inhibitor, which can improve the speed of rust removal, prevent excessive corrosion and hydrogen embrittlement of workpiece and better inhibit acid mist. Particularly noteworthy is the inhibition of acid fog. The acid mist produced in the process of pickling and rust removal not only corrodes equipment and factories, pollutes the environment, but also causes symptoms such as tooth corrosion, conjunctival redness, tears, pain, dry throat and cough. Therefore, effectively suppressing acid fog is not only the need of environmental protection, but also the need of the unit itself.

Considering the above requirements, after screening, we adopted XH- 16C oil and rust removal additive from Xianghe Phosphating Company, and made it through checking calculation. It can be used at room temperature 10-30min, which can improve the rust removal speed, prevent excessive corrosion and hydrogen embrittlement of the workpiece, and better inhibit acid mist.

Formula: XH- 16C 4% hydrochloric acid (35%)60% H2O.

Step 2: Wash

Although washing after degreasing and rust removal is an auxiliary process before painting, it also needs to be paid enough attention. After degreasing and rust removal, some nonionic surfactants and CL- are easy to attach to the surface of the workpiece. If these substances are not thoroughly cleaned, the phosphating film may become thinner, resulting in linear defects, or even unable to phosphatize. Therefore, it is necessary to improve the washing quality after degreasing and rust removal. Be sure to rinse it several times, twice 1-2min, and change the clean water frequently to ensure the PH value of the clean water is between 5 and 7.

3. Phosphate antirust treatment on metal surface

The so-called phosphating refers to the method of treating metal workpiece with acidic solution containing dihydrogen phosphate and forming a stable insoluble phosphating film on its surface through chemical reaction. The obtained film is called phosphating film. The main purpose of phosphating film is to increase the adhesion and corrosion resistance of the coating. There are many phosphating methods, which can be divided into high-temperature phosphating (90-98℃), medium-temperature phosphating (60-75℃), low-temperature phosphating (35-55℃) and normal-temperature phosphating according to phosphating temperature.

In order to provide a good coating substrate, it is required that the thickness of phosphating film is appropriate and the crystal is dense and fine.

Although the phosphating process at medium and high temperature is fast in phosphating speed and good in corrosion resistance, the phosphating film is coarse in crystallization, heavy in ash hanging, fast in liquid surface volatilization, unstable in bath solution and rich in sediments, while the phosphating film formed by the phosphating process at low temperature and normal temperature is fine in crystallization, suitable in thickness, less in sediment inclusion between films, less in paint absorption and good in coating luster, which can greatly improve the adhesion, flexibility and impact resistance of the coating and can better meet the requirements. It is worth noting that in the past, it was always considered that the thickness of phosphating film has high corrosion resistance of the coating after painting. Phosphating film itself does not bear too much corrosion resistance in the whole painting system, mainly because it makes the paint film have strong adhesion, and the corrosion resistance of the whole painting system mainly depends on the corrosion resistance of the paint film and the strong adhesion formed by the good cooperation between the paint film and phosphating film.

Phosphating solution generally consists of main salt, accelerator and neutralizer. In the past, sodium nitrite was mostly used as an accelerator in phosphating solution, and the effect was obvious in ten years, but it was very harmful in phosphating solution: First, it affected the stability of phosphating solution, was extremely unstable under acidic conditions and decomposed in a very short time. So you have to add it often. This characteristic of nano-2-2 often leads to unstable main salt of phosphating solution, more precipitation of phosphating solution, serious scaling of phosphating film, difficult control of bath solution and unstable phosphating quality. Second, NaNO2 is recognized as a carcinogen in the world. Long-term exposure is harmful to human health and seriously pollutes the environment.

4. Passivation

Passivation technology of phosphating film is widely used in North America and European countries. Passivation technology is based on the characteristics of phosphating film. The phosphating film is thin, generally at 1-4g/m2, and the maximum is not more than 10g/m2. Its free pore area is large and the corrosion resistance of the film itself is limited. Some even develop yellow rust rapidly in the drying process. After phosphating, the exposed metal in the pores of phosphating film can be further oxidized or form a passivation layer, which can be filled with oxidized phosphating film to make the phosphating film stable in the atmosphere.

5. Drying of phosphating film

Drying phosphating film can play two roles. On the one hand, it prepares for the next working procedure, removes the moisture on the surface of phosphating film, on the other hand, it further improves the corrosion resistance of coating film.

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