Aluminum alloy, as a lightweight, high-strength metal material with good corrosion resistance, has found extensive applications in numerous fields such as aerospace, automotive manufacturing, architectural decoration, and electronic products. However, to further enhance the performance, aesthetic quality, and service life of aluminum alloys, surface treatment has become an indispensable and crucial step. This article will delve into the common surface treatment methods for aluminum alloys and their characteristics.
I. Anodizing
Anodizing is one of the most frequently used surface treatment technologies for aluminum alloys. The principle involves using the aluminum alloy as the anode and placing it in an electrolyte. By applying a direct current, a dense layer of aluminum oxide film is formed on the surface of the aluminum alloy. This oxide film possesses several excellent properties.
In terms of protective performance, the oxide film can significantly improve the corrosion resistance of aluminum alloys. It effectively prevents external moisture, oxygen, and other corrosive media from coming into contact with the aluminum alloy substrate, thereby slowing down the corrosion rate of the aluminum alloy and extending its service life. In highly corrosive environments such as marine settings and the chemical industry, aluminum alloy products treated with anodizing exhibit outstanding anti-corrosion capabilities.
Regarding decorative aspects, the anodized film has good coloring properties. By adding different colorants to the electrolyte or using methods such as electrolytic coloring and chemical dyeing, the surface of the aluminum alloy can be presented in a rich variety of colors to meet the aesthetic needs of different customers. For example, in the field of architectural decoration, colored aluminum alloy doors, windows, and curtain walls are not only visually appealing but also able to harmonize with the surrounding environment.
In addition, the anodized film has high hardness and wear resistance. It can enhance the scratch resistance of the aluminum alloy surface, reducing damage caused by friction during use and maintaining the flatness and smoothness of the product surface. This is of great significance for some aluminum alloy components that frequently come into contact or experience friction, such as the outer shells of electronic products and transmission parts of mechanical equipment.
However, anodizing treatment also has some limitations. For instance, the thickness of the oxide film is relatively limited, generally ranging from a few micrometers to several tens of micrometers, which may not meet the requirements for some applications with extremely high wear resistance needs. Moreover, the anodizing process has strict requirements on the material and pre-treatment of the aluminum alloy. If the aluminum alloy contains too many impurities or the pre-treatment is improper, it may affect the quality and performance of the oxide film.
II. Electroplating
Electroplating is a process of depositing a layer of metal or alloy on the surface of an aluminum alloy using the principles of electrolysis. Through electroplating, coatings with different properties can be obtained on the aluminum alloy surface, such as chrome coatings, nickel coatings, and zinc coatings.
Chrome coatings have extremely high hardness and wear resistance, as well as good luster and corrosion resistance. In the automotive components and hardware tool industries, electroplated chrome aluminum alloy products can not only enhance the appearance quality of the products but also increase their service life. For example, after chrome electroplating, automobile rims have a mirror-like surface, which is not only aesthetically pleasing but also able to effectively resist the impact of road gravel and corrosion.
Nickel coatings have good electrical conductivity, corrosion resistance, and solderability. In the electronics industry, many aluminum alloy electronic components require nickel electroplating to ensure their good electrical performance and reliability. In addition, nickel coatings can also serve as an underlayer for other coatings to improve the adhesion between the coatings.
Zinc coatings are a commonly used protective coating that can provide sacrificial anode protection for aluminum alloys. When an aluminum alloy product is in a corrosive environment, the zinc coating will be preferentially corroded, thereby protecting the aluminum alloy substrate from damage. In the fields of architectural hardware and fasteners, electroplated zinc aluminum alloy products are widely used in outdoor environments and can effectively extend the service life of the products.
However, electroplating treatment also has some problems. A large amount of wastewater, exhaust gas, and waste residue are generated during the electroplating process, causing certain pollution to the environment. Therefore, electroplating enterprises need to take effective environmental protection measures to treat the wastewater, exhaust gas, and waste residue to meet national environmental protection standards. In addition, the adhesion of electroplated coatings may be slightly worse compared to that of anodized films. If the surface treatment of the aluminum alloy is improper or the electroplating process parameters are not reasonably controlled, problems such as peeling and delamination of the coating may occur.
III. Spray Coating
Spray coating is a surface treatment method that evenly sprays a coating onto the surface of an aluminum alloy. According to the type of coating, spray coating can be divided into powder spray coating and liquid spray coating.
Powder spray coating involves uniformly adsorbing powder coating onto the surface of an aluminum alloy through an electrostatic spray gun and then curing it at high temperatures to form a uniform and dense coating. Powder spray coating has many advantages. Firstly, the thickness of the coating can be adjusted according to needs, generally reaching several tens of micrometers to over a hundred micrometers, providing good protective performance. Secondly, the utilization rate of powder coatings is high, and the over-sprayed powder can be recycled and reused, reducing production costs. In addition, the powder-sprayed coating has good weather resistance, corrosion resistance, and wear resistance, and can adapt to various harsh environmental conditions. In the fields of architectural doors, windows, and curtain walls, powder-sprayed aluminum alloy products are widely used.
Liquid spray coating involves spraying liquid coating onto the surface of an aluminum alloy through a spray gun and then curing it into a film through natural drying or low-temperature baking. The advantage of liquid spray coating is that there is a rich variety of liquid coatings and a wide range of color choices, which can meet the personalized needs of different customers. Moreover, the construction process of liquid spray coating is relatively simple, and the equipment investment is small. However, the thickness of the liquid-sprayed coating is relatively thin, and its protective performance may be slightly inferior to that of powder-sprayed coatings. At the same time, liquid coatings contain organic solvents, which will produce volatile organic compounds (VOCs) during the spraying process, causing certain impacts on the environment and human health. Therefore, effective ventilation and protective measures need to be taken to reduce the emission of VOCs.
IV. Micro-Arc Oxidation
Micro-arc oxidation is a new type of surface treatment technology developed on the basis of anodizing. Under the action of a high-voltage electric field, micro-arc discharge is generated on the surface of the aluminum alloy. Under the action of high temperature, high pressure, and plasma, a ceramic-like oxide film is grown in situ on the surface of the aluminum alloy.
The micro-arc oxidation film has extremely high hardness and wear resistance, with a hardness that can reach above HV1000, far exceeding that of an anodized film. This makes micro-arc-oxidized aluminum alloy products have broad application prospects in the field of wear resistance, such as engine pistons, cylinders, and other components. In addition, the micro-arc oxidation film also has good corrosion resistance, high-temperature resistance, and insulating properties. It can maintain stable performance in high-temperature, strongly corrosive, and other harsh environments and is suitable for applications in the aerospace, petrochemical, and other industries.
However, the micro-arc oxidation technology also has some shortcomings. The investment in micro-arc oxidation equipment is relatively large, the control of process parameters is complex, and high technical requirements are placed on operators. Moreover, the micro-arc oxidation process consumes a large amount of energy, and the processing cost is relatively high.
V. Chemical Conversion Treatment
Chemical conversion treatment involves forming a chemical conversion film on the surface of an aluminum alloy through a chemical reaction. Common chemical conversion treatments include chromate conversion treatment and phosphate conversion treatment.
The conversion film formed by chromate conversion treatment has good corrosion resistance and coating adhesion. It can provide a certain degree of protection for the aluminum alloy and, at the same time, serve as an underlayer for coating to improve the adhesion between the coating and the aluminum alloy surface. However, chromate contains hexavalent chromium, which is highly toxic and poses great harm to the environment and human health. With increasingly strict environmental protection requirements, chromate conversion treatment is gradually being replaced by chrome-free chemical conversion treatment.
Phosphate conversion treatment is an environmentally friendly chemical conversion treatment method. The phosphate conversion film it forms also has good corrosion resistance and coating adhesion and is environmentally friendly. In the automotive manufacturing, home appliance, and other industries, phosphate conversion treatment is widely used.
