• June 21, 2024
  • Shatou, Changan, Dongguan, CN
Anodized aluminum

A complete guide on anodized aluminum parts

Anodized aluminum has the characteristics of anti-corrosion, wear resistance and aesthetics, and is widely used in automotive, construction, electronics, machinery and other fields. Here is a complete guide about anodized aluminum parts.

This paper introduces the principle, process and parameters of anodized aluminum parts in detail.

Oxidation process and principle

The oxidation process is the formation of an aluminum oxide film in an electrolyte. When a direct current is applied to the surface of the aluminum product, the anodic reaction will produce oxygen, while the cathodic reaction will produce hydrogen gas. 

These gases form bubbles on the surface of the electrode and propel the electrolyte flow. At the same time, an aluminum hydroxide layer is formed on the surface of the anode, and as time and current increase, the aluminum oxide film gradually thickens and forms a blocking layer.

Raw material selection

The selection of raw materials for anodic aluminum oxide is important. The commonly used aluminum alloys are 1000 series, 3000 series, 5000 series, 6000 series and 7000 series. 

Among them, 1000 series pure aluminum is softer and usually used for some parts that require a beautiful appearance and low corrosion resistance requirements, such as home decoration materials; while 5000 series and 6000 series aluminum alloys are stronger and difficult to process, and can be processed with high strength for some key parts.

Anodic oxidation process

The process of anodizing aluminum includes cleaning, pretreatment, anodizing, sealing treatment and post-treatment steps, etc. Among them, cleaning, pretreatment and sealing treatment are the key links to ensure the quality of oxide film.

1, Cleaning

The surface of aluminum parts will be cleaned of dust, grease, oxides and other substances, generally using an alkaline cleaning solution for cleaning, sometimes also using an acidic cleaning solution, at this time need to strictly control the acidity and alkalinity, so as to not to cause harm to aluminum parts.

2, Pre-treatment

The cleaned aluminum parts will be deburred, polished and deoxidized to improve the surface quality and facilitate the subsequent anodizing.

3, Anodizing

The aluminum parts are immersed in the electrolytic solution and connected to the anode and cathode to produce an oxide film on the surface of the aluminum parts through electrolytic action.

4, Seal treatment

After anodizing, an oxide film is produced on the surface of the aluminum part, but this film still has tiny pores and is easily eroded by air, moisture, and other factors. For this reason, the resulting oxide film is generally sealed.

5, Post-treatment

The purpose of post-treatment of anodized aluminum is to improve the corrosion resistance, wear resistance, appearance and color stability of its surface, containing treatments such as cleaning, drying and painting.

Process parameters control in the anodizing process

The main process parameters in the anodic oxidation process include voltage, current density, electrolyte composition, temperature and electrolysis time. These parameters affect the thickness, morphology and properties of the aluminum oxide film. By optimizing these parameters, aluminum oxide films with excellent properties can be obtained.

1, Voltage

Voltage is one of the most important parameters to control the thickness of alumina film. In general, the higher the voltage, the greater the thickness of the alumina film. However, if the voltage is too high, it will cause the electrolyte to heat up and the quality of the alumina film will be affected. Therefore, when performing anodizing, the appropriate voltage should be determined according to the desired film thickness and material properties.

The anodizing voltage is influenced by the type of aluminum alloy, the type of electrolyte, the concentration of the electrolytic bath solution, the concentration of aluminum ions, the temperature of the bath solution and the agitation. Under sulfuric acid bath solution, the voltage is about 13-14 V for 6063-T5 and 5052 aluminum alloys, 14-15 V for 6061-T6 and 3003 aluminum alloys, and 17-18 V for 2014-T6 and 2024-T3 aluminum alloys. 

under general conditions, the voltage is roughly 15 V for a sulfuric acid solution, about 20 V for a phosphoric acid solution, and more than 20 V for a chromic acid solution. The applied voltage for anodizing in oxalic acid bath solution is also higher, about 50V.

2, Current density

The current density also has an effect on the thickness and properties of the aluminum oxide film. For a given electrolyte and temperature, the higher the current density, the greater the thickness of the alumina film. However, if the current density is too high, it will cause the electrolyte to heat up and the quality of the alumina film will be affected. Therefore, when performing anodizing, the appropriate current density should be determined based on the desired film thickness and material properties.

Current density has an important effect on anodizing. A high current density increases film formation speed and productivity, but too high a current density can lead to workpiece burnout. In DC anodizing with sulfuric acid, a current density of 1.0 to 1.5 A/dm2 is often used. High-speed anodizing can be increased to 1.5~3.0A/dm2, but long continuous use should be avoided. Pulse anodizing is a solution. For hard anodizing, higher current densities of about 3.0~4.0A/dm^2 may be used.

3, Electrolyte composition

The anodizing process requires a specific electrolyte. The composition of the electrolyte has a significant impact on the morphology and properties of the aluminum oxide film. Common electrolyte compositions include sulfuric acid, ethylene glycol, phosphoric acid, fluoride, etc. Depending on the material and application requirements, choosing the right electrolyte composition can result in superior alumina films.

Electrolyte bath solutions used to generate porous aluminum anodic oxide films mainly use inorganic acid electrolytes, such as sulfuric acid, chromic acid and phosphoric acid. Sulfuric acid is the most widely used to produce colorless and transparent anodic oxide films with good overall performance, coloring and sealing. Chromic acid can generate corrosion-resistant anodic oxide film but is not transparent. The anodic oxide film of phosphoric acid has a larger pore size and is suitable as a base layer for painting or plating. 

The anodic oxide film generated by alkaline electrolytes has a rough surface, large pores and poor wear resistance, and has limited application except for use as a coating base layer. Although alkaline electrolytes are often used in high voltage anodizing.

4, Temperature

The effect of temperature on anodizing mainly affects the morphology and density of the aluminum oxide film. Within a certain range, as the temperature increases, the density of the alumina film gradually increases and the surface smoothness improves. However, if the temperature is too high, it may cause the electrolyte to heat up and affect the quality of the alumina film. Therefore, when anodizing, the appropriate temperature should be determined according to the desired film thickness and material properties.

Anodizing requires control of the bath temperature to ensure the film’s properties. The optimum temperature range varies depending on the type of electrolytic bath, the type of aluminum alloy, the anodizing conditions and the performance requirements of the anodized film. Generally speaking, the temperature of the sulfuric acid solution should be selected below 20°C, while the phosphoric acid solution is about 25°C and the chromic acid solution can reach even higher than 30°C. 

If the temperature of the electrolytic bath solution increases, the hardness, corrosion resistance and abrasion resistance of the anodic oxide film will decrease, and even scorching or chalking of the anodic oxide film will occur; if the operating temperature decreases, the transparency and staining will decrease, but the hardness will increase.

5, Electrolysis time

Electrolysis time has a great influence on the thickness and nature of the aluminum oxide film. Generally, the thickness of alumina film will gradually increase with the extension of electrolysis time. However, if the electrolysis time is too long, it may lead to the depletion of the components in the electrolyte and affect the quality of the alumina film. Therefore, when performing anodizing, the appropriate electrolysis time should be determined according to the desired film thickness and material properties.

Application areas of the anodizing process

Anodized aluminum parts have a wide range of application areas, mainly including aircraft, automotive, electronics, construction, furniture, machinery and other industry fields. Common anodized aluminum parts include automobile hoods, luggage racks, boxes, fenders, door handles, watch cases, etc. In the construction industry, anodized aluminum can be used as material for curtain walls, windows, exterior walls, etc. 

In the electronic field, anodized aluminum can be used as the shell of electronic components, heat sinks and other parts.

Physical and chemical properties and testing after anodizing

The anodized aluminum oxide film has excellent corrosion resistance, hardness, wear resistance and aesthetic properties. Testing methods include electron microscope scanning, transmittance test, natural oxidation test, acid and alkaline test, etc. The physical and chemical properties and testing methods of aluminum oxide films are also relevant to their application areas.

Surface treatment methods after anodic oxidation treatment

The surface after anodizing can be treated by various methods, such as positive electrophoretic coating, dyeing, hard anodizing, sandblasting and wire drawing. These methods can further improve the appearance and wear resistance of the aluminum oxide film.

1, Positive electrophoretic coating treatment

The positive electrophoretic coating is a surface treatment method in which the metal surface is electrochemically pretreated for better adhesion of the coating. The coating is applied to the anode by electrodes placed in the coating solution and a voltage is applied in an electric field. This method is very effective in improving the corrosion resistance and appearance of the material.

2, Dyeing treatment

Dyeing is the method of diffusion of pigments such as mouth stones into the aluminum oxide film. The anodized surface can be dyed with a variety of stains, which not only changes the color but also increases the thickness of the surface coating, and this coating can enhance its wear resistance to a certain extent.

3, Hard Anodizing

Hard anodizing is a method of passivating the surface, which has higher strength and wear resistance than ordinary aluminum oxide film. Hard anodizing is commonly used to make wear-resistant parts such as engine components and hydraulic valves.

4, Sandblasting

Sandblasting is also a surface treatment that improves the slip resistance of a surface by blasting abrasives on the surface to create a textured layer. In addition, sandblasting can eliminate surface bubbles and reduce surface electrical problems.

5, Brushing

Brushing is a process designed to improve the surface gloss and produce additional texture. It is achieved by applying force to the metal surface using a wire brush or other type of tool. This process is particularly useful in creating a decorative appearance, texture and sense of habit.

Conclusion

In summary, anodized aluminum is a widely used surface treatment technology for different application scenarios in many industry sectors. By controlling the process parameters and surface treatment methods, it is possible to obtain high-quality aluminum oxide films that enhance their performance and meet market demands.

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