Research on electrostatic powder coating pretreatment process of aluminum alloy materials

Process requirements for aluminum alloy materials in coating

1.1 Coating process

Aluminum alloy material coating is a systematic project, mainly including the surface treatment before coating, material coating and curing and leveling three basic processes, it is necessary to select the appropriate process conditions, ensure good working environment conditions and ensure quality supervision, technology in place, and consider the economy and other important links.

The process flow is: washing→ degreasing→ washing→ washing→ pure water washing→ passivation treatment→ washing → washing→ pure water washing→ water drying→ hanging material → spraying→ curing → unloading → inspection packaging.

1.2 Pretreatment process

Surface pretreatment is the preparation work on the surface of aluminum alloy materials, to remove foreign objects (such as dust, oil, aluminum chips, etc.) on the surface of the material, to ensure that the resulting film layer has good adhesion, corrosion resistance, decoration, and to provide a good coating material suitable for coating requirements.

The biggest advantage of this process is that it is environmentally friendly, free of polluting substances, and has high efficiency, low cost and stable quality of passivation layer.

The pretreatment process index control is not up to standard or the operation is not standardized, which will cause many quality problems, and the more serious thing is that the adhesion of the coating is unqualified, and the surface paint film will gradually fall off after a period of use. There are five main causes of such defects.

1.2.1 Skim is not clean

Improper degreasing will lead to problems such as oil stains, too low etching amount or incomplete removal of the natural oxide film on the surface of the aluminum alloy material, which cannot be passivated normally. The experiment shows that the degreasing and dust removal are not clean, and the coating will fall off the surface after spraying.

1.2.2 Passivation cell Al3+ concentration

The effect of Al3+ concentration in the passivation tank on the adhesion of the coating to boiling water is shown in Figure 1. As can be seen from Figure 1, in order to ensure 100% compliance with the coating adhesion, the concentration of Al3+ in the passivation tank must be controlled within 0.2g/l.

In order to better verify the results of this experiment, we conducted several strong impact tests in the laboratory. The results show that when the concentration of Al3+ in the passivation tank exceeds 0.2g/l (0.3~0.4g/l), the surface coating begins to fall off after the passivated aluminum matrix is coated and the impact resistance test is done, and the adhesion is unqualified.

The crystalline phase observation of the passivated matrix with different Al3+ concentrations showed that the surface of the treated matrix showed a regular hexagonal structure and was dense at low concentrations. On the contrary, although it also has a hexagonal structure, it is sparse.

1.2.3 Conductivity and pH of the last pure water wash

The effect of the conductivity of the last pure water wash on the adhesion of the boiling water of the coating is shown in Figure 2, and the effect of the pH value of the last pure water wash on the adhesion of the boiling water of the coating is shown in Figure 3. As can be seen from Figure 2 and Figure 3, in order to ensure that the adhesion of the coating is 100% qualified, the conductivity of 9# pure water washing must be < 80μs/cm and the pH value must be > 4.5.

In order to better verify the results of this experiment, we conducted several strong impact tests in the laboratory.

The results show that after the last pure water washing with different conductivity and pH, the impact resistance test of the cleaned aluminum matrix is carried out, and the degree of peeling off of the surface coating is different, which verifies the accuracy of the results of this project.

1.2.4 Storage environment and storage time after passivation

Because the passivation film is thinner and the corrosion resistance is much lower than that of the hexavalent chromium passivation layer, it should be sprayed in time after passivation to prevent the passivation film from corroding caused by being placed in the plant.

Especially in the case of small plant or poor ventilation, the corrosive gas in the plant will accelerate the corrosion of the passivation film.

Practice shows that the passivated aluminum is best sprayed on duty, and the placement time under special circumstances cannot exceed 24 hours.

Problems that should be paid attention to in aluminum alloy coating

To meet the reasonable matching of various properties, it is necessary to adopt advanced forming technology, optimized pretreatment process and advanced extrusion process through reasonable composition design, including composition optimization and fine-tuning;

The integration of many technologies such as reasonable structure, grain size, uniform distribution of the second phase and size control can produce automotive aluminum alloy materials with good performance.

The development direction of aluminum alloy materials is mainly to improve strength, improve formability and reduce costs.

conclusion

Through the research of this subject, we have mastered the common powder coating (including pretreatment) process technology of aluminum alloy materials, and improved it on the basis of traditional technology, which is more environmentally friendly, green, efficient and high-quality, and has a great role in promoting the research and development, promotion and product transformation of new aluminum alloy materials.