Powder coating of aluminum profiles: pretreatment - the cornerstone of coating adhesion

In modern industrial manufacturing, aluminum profiles have become an indispensable material in many fields such as construction, automobiles, aerospace, etc. due to their light weight, high strength, corrosion resistance and easy processing. However, in order to further improve the performance and aesthetics of aluminum profiles, powder coating technology is widely used. In this technology, the pretreatment link plays a vital role. It is a key step to ensure that the coating is closely bonded to the substrate.

Pretreatment before powder coating is the core link of aluminum profile surface treatment. Its purpose is to thoroughly remove grease, oxides and other pollutants on the surface of aluminum, and at the same time form a layer of microstructure that is conducive to coating adhesion. This process is not only related to the aesthetics and durability of the coating, but also directly affects the bonding strength between the coating and the substrate. It is the basis for ensuring that the coating does not fall off for a long time and maintains excellent performance.

During the production, transportation and storage of aluminum profiles, various pollutants such as grease and dust are often attached to the surface. These pollutants will hinder the direct contact between the coating and the substrate and reduce the adhesion of the coating. Therefore, degreasing is the first step of pretreatment and a crucial step.

There are many methods for degreasing, including solvent cleaning, alkaline cleaning, and electrochemical cleaning. Among them, solvent cleaning uses the principle of organic solvents dissolving grease, but attention should be paid to the volatility and toxicity of the solvent; alkaline cleaning uses the saponification reaction of alkaline solution and grease to convert grease into water-soluble substances, but attention should be paid to the control of alkali concentration and temperature to avoid corrosion to aluminum; electrochemical cleaning uses the principle of electrolysis to decompose grease through the action of electric current, but the equipment is complex and the cost is high.

In practical applications, the choice of degreasing method needs to be considered comprehensively based on the degree of oil pollution, material characteristics and subsequent process requirements of aluminum profiles.
Pickling (or alkaline cleaning): remove oxides and expose fresh metal surfaces
Aluminum is very easy to react with oxygen in the natural environment to generate aluminum oxide, forming a dense oxide film. Although this film has a certain protective effect, it will hinder the direct bonding of the coating and the substrate in powder spraying. Therefore, pickling (or alkaline cleaning) becomes a necessary step to remove oxides and expose fresh metal surfaces.

Pickling usually uses strong acid solutions such as sulfuric acid, hydrochloric acid or nitric acid to remove oxides on the surface of aluminum through chemical reactions. However, the acid concentration, temperature and time must be strictly controlled during the pickling process to avoid excessive corrosion of the aluminum. In addition, neutralization treatment is required after pickling to remove the acid remaining on the surface of the aluminum to prevent adverse reactions in subsequent processes.

Compared with pickling, alkaline cleaning is more suitable in some cases. Alkaline cleaning uses alkaline solutions such as sodium hydroxide to remove oxides and oil stains on the surface of aluminum through chemical reactions. The advantage of alkaline cleaning is that it has less corrosion to aluminum and can remove oil stains and oxides at the same time, but attention must be paid to the control of the concentration and temperature of the alkaline solution to avoid damage to the aluminum.

After pickling (or alkaline cleaning), a loose oxide layer will form on the surface of the aluminum. Although this oxide layer is thin, it will still have an adverse effect on the bonding of the coating and the substrate. Therefore, passivation becomes a key step in pretreatment, aiming to form a dense microstructure on the surface of the aluminum through chemical reactions that is conducive to coating adhesion.

There are many passivation methods, including chemical passivation and electrochemical passivation. Among them, chemical passivation uses the reaction between the oxidant and the surface of the aluminum material to form a dense oxide film; electrochemical passivation uses the principle of electrolysis to form a uniform, protective oxide film on the surface of the aluminum material.

During the passivation process, the reaction conditions, such as the type and concentration of the oxidant, the reaction temperature and time, must be strictly controlled to ensure that the formed oxide film has both a protective effect and is conducive to the adhesion of the coating. In addition, cleaning treatment is required after passivation to remove the passivation liquid remaining on the surface of the aluminum material to prevent adverse effects on subsequent coatings.

The quality of pretreatment directly affects the bonding strength between the subsequent coating and the substrate. If the pretreatment is not thorough, pollutants such as grease and oxides will still remain on the surface of the aluminum material, which will hinder the direct contact between the coating and the substrate, resulting in reduced adhesion of the coating, and easy to fall off and crack. On the contrary, if the pretreatment is done properly, the aluminum surface is clean and free of pollutants, and a layer of microstructure that is conducive to the adhesion of the coating is formed, which will greatly improve the bonding strength between the coating and the substrate, making the coating more solid and not easy to fall off.

In practical applications, in order to ensure the quality of pretreatment, the pretreatment process needs to be strictly controlled. This includes selecting a suitable pretreatment process, strictly controlling the reaction conditions, and strengthening quality monitoring. At the same time, the pretreatment process needs to be flexibly adjusted according to factors such as the material characteristics of the aluminum profile, the degree of oil pollution, and the subsequent process requirements to ensure the best pretreatment effect.

Pretreatment before powder spraying of aluminum profiles is a key step to ensure the bonding strength of the coating. Through careful treatment of steps such as degreasing, pickling (or alkali washing), and passivation, the grease, oxides and other pollutants on the surface of the aluminum can be completely removed, and a layer of microstructure that is conducive to the adhesion of the coating can be formed. This process is not only related to the aesthetics and durability of the coating, but also directly affects the bonding strength between the coating and the substrate. Therefore, in the powder coating of aluminum profiles, it is important to pay attention to the pretreatment link to ensure that the pretreatment quality is optimal and lay a solid foundation for the close combination of the coating and the substrate.

With the continuous advancement of science and technology and the continuous optimization of processes, the pretreatment technology before aluminum profile powder coating will be more efficient, environmentally friendly and intelligent in the future. For example, more advanced oil removal technology, more environmentally friendly pickling (or alkali cleaning) agents, more precise passivation processes, etc. are used to further improve the quality of pretreatment, enhance the bonding strength between the coating and the substrate, and inject new vitality into the development of aluminum profile powder coating technology.