Causes and solutions to the problem of color spots on anodized aluminum plates

As the core material for building curtain walls, electronic equipment and high-end home furnishings, anodized aluminum plates are favored for their advantages such as lightweight and corrosion resistance. However, the problem of color spots has long plagued the industry – uneven oxide film, local corrosion, sealing defects, etc., directly lowering the product grade and even causing customer complaints.

The formation of color spots on anodized aluminum plates may involve multiple aspects such as materials, processes, equipment and environment.

◆ First, in terms of materials, uneven aluminum alloy composition will lead to different oxidation reaction rates in different areas, resulting in color spots.

◆ Secondly, incomplete surface pretreatment, such as oil, oxide scale, dust, etc., will cause uneven growth of the oxide film and form color spots.

◆ In addition, improper control of process parameters such as oxidation tank liquid composition, temperature, current density, oxidation time, etc. will also lead to the appearance of color spots.

What should be paid attention to when selecting and processing aluminum alloy materials?

First of all, aluminum alloy materials with uniform composition should be selected. When purchasing, the quality inspection report of aluminum plates and aluminum coils should be checked to ensure uniform distribution of alloy elements. For materials with large composition deviations, homogenization treatment can be performed, such as heat treatment to make the distribution of alloy elements more uniform.

In addition, surface pretreatment is also very important. Pollutants such as oil, scale and dust on the surface of aluminum must be thoroughly removed to ensure surface cleanliness. Common pretreatment methods include degreasing, degreasing, alkali washing and pickling.

What effect do process parameters have on the formation of color spots? How to optimize process parameters to reduce color spots?

The composition of the oxidation bath must be strictly controlled. Too high or too low sulfuric acid concentration, too high aluminum ion content, insufficient or excessive additives will affect the formation of oxide film. The composition of the bath should be regularly tested and adjusted, and the reagents should be added or supplemented according to the process requirements.

The oxidation temperature needs to be stably controlled between 18-22℃. Too high a temperature will cause the oxide film to dissolve too quickly and the film layer to be loose; too low a temperature will cause the oxide film to grow slowly and local uneven oxidation will occur.

The current density also needs to be reasonably adjusted, generally 1-2A/dm². Too large or too small will cause uneven growth of the oxide film. The oxidation time should be determined by testing according to the aluminum alloy material and the shape of the workpiece, generally 20-60 minutes.

Finally, the dyeing process also needs to be strictly controlled to ensure that the dyeing solution concentration is uniform, the pH value is appropriate, and the dyeing time and temperature are properly controlled.

How to optimize the equipment to reduce color spots?

The design of the anodizing tank must be reasonable. The uneven flow of the solution in the tank will cause different degrees of contact between the workpiece and the solution, and the growth of the oxide film will be inconsistent. The solution can be made to flow evenly by optimizing the design of the oxidation tank, adding a stirring device or arranging the inlet and outlet of the liquid reasonably.

The contact of the conductive device must be good. Poor contact between the hanger and the workpiece, and the electrode and the hanger will cause uneven current distribution and oxidation spots. Therefore, it is necessary to check the conductive device regularly to ensure good contact and replace damaged hangers and electrodes in time.

How to control the environment to reduce color spots?

The humidity of the workshop environment is high. The high humidity environment will cause the surface of the oxidized workpiece to absorb a large amount of water vapor, affecting the drying and curing of the oxide film, resulting in color spots. Therefore, the humidity of the workshop should be controlled between 40%-60%, and dehumidification equipment can be installed.

There are a lot of dust and impurities in the workshop. Dust and impurities fall on the surface of the oxidized workpiece, which will affect the appearance of the oxide film and form color spots. Therefore, it is necessary to keep the workshop clean, clean it regularly, install air purification equipment, and reduce dust and impurities.

Original Source:Anodized Aluminum Sheet Coil