Alodine and Anodizing

1.The following is a specific case of the aluminum conductive process.

1) Pretreatment

Before the conductive oxidation treatment, the aluminum materials need to undergo pretreatment such as cleaning, degreasing, and deoiling. First, immerse the aluminum materials in an alkaline cleaning solution to remove the surface oil stains, then place them in an acidic cleaning solution for pickling, and finally soak them in deionized water to ensure that the aluminum surface is clean and free of dirt.

2) Oxidation Treatment

Inject the prepared aluminum materials into an electrolytic cell containing sulfuric acid, oxidants, and additives through electrolysis. Through the electrolytic reaction, a uniform and dense oxide film with an average thickness is formed on the surface of the aluminum materials. During the oxidation process, parameters such as the temperature, concentration, current density, and oxidation time of the oxidation solution need to be controlled to ensure the quality and thickness of the oxide film.

3) Sealing Treatment

The formed oxide film has a non - porous structure, which is not conducive to surface coating and use. Therefore, it is necessary to seal part of the surface of the oxide film to prevent the appearance of holes and contamination on the oxide film surface. Usually, hot water or steam is used for sealing treatment. Immerse the aluminum materials in a steam box for sealing.

4) Post - treatment

After the sealing treatment, the aluminum materials need to be rinsed and inspected. If any defects are found in the oxide film, re - treatment is required. Then, through post - treatment such as drying and packaging, the aluminum materials are made into finished products.

2.The anodizing surface treatment process :

Anodizing:

Using aluminum or aluminum alloy products as the anode, place them in an electrolyte solution, and utilize electrolysis to form an aluminum oxide film on the surface. When an electric current passes through, hydrogen is released at the cathode; the oxygen at the anode includes not only molecular oxygen but also atomic oxygen and ionic oxygen. Usually, molecular oxygen is used to represent the reaction. The aluminum as the anode is oxidized by the oxygen to form an anhydrous aluminum oxide film. The pore diameter of the oxide film is approximately 0.01 - 0.03 um. Not all of the generated oxygen reacts with aluminum, and a part is released in a gaseous form. There are mainly three types of anodizing methods: sulfuric acid anodizing, hard anodizing, and porcelain anodizing. The sulfuric acid anodizing process is mainly used.

Dyeing: 

Place the oxide film on the surface of the aluminum part after anodizing in a dyeing solution to allow color molecules to be adsorbed in the pores of the anodized film. The pore diameter of the anodized film is approximately 0.01 - 0.03 um, and the dyes separate into single molecules in water with a diameter of approximately 0.0015 - 0.0030 um. Dyeing methods are divided into hot dyeing and cold dyeing.

Sealing treatment:

 After the oxide film is treated with hot water, high - temperature steam, or nickel salts for sealing, its corrosion resistance and wear resistance can be further improved.

3.Differences between Alodine and Anodizing:

Requirement of electricity:

Anodizing is carried out under high - voltage electricity. It is an electrochemical reaction process. Chemical oxidation (also known as chemical conversion coating) does not require electricity. It only needs to be immersed in the chemical solution, and it is a pure chemical reaction.

Time consumption:

Anodizing takes a long time, often dozens of minutes. Chemical oxidation only takes a short time of dozens of seconds.

Wear resistance: 

The film formed by anodizing ranges from a few micrometers to dozens of micrometers in thickness and is hard and wear - resistant. The film formed by chemical oxidation is only about 0.01 - 0.15 micrometers thick. Its wear resistance is not very good, but it can conduct electricity and resist atmospheric corrosion, which is its advantage.

Electrical conductivity:

Oxide films are generally non - conductive. The film formed by anodizing has good insulation properties. The film formed by chemical oxidation is extremely thin, so it can conduct electricity. After chemical conductive oxidation of aluminum parts, they have certain corrosion resistance and electrical conductivity. In electronic equipment, chemically conductively oxidized aluminum parts can prevent electromagnetic signal interference.

Application scope:

Anodizing is suitable for cast aluminum, but it has poor results for aluminum products with complex shapes. Chemical conductive oxidation is suitable for deformed aluminum electrical parts, as well as larger parts or assemblies that are not suitable for anodizing.

4.Summary

Ordinary anodizing (2.54-25.4µm): Available in a variety of colors. Hard anodizing is generally natural or black, with a film thickness of 25.4µm or greater. Because of the thick film, the pores are densely packed, making coloring more difficult.

Anodizing Benefits:

Corrosion resistance, wear resistance, hardness, aesthetics, insulation, and easy cleaning.

Alodine: Natural (non-toxic) or yellow (toxic).

Benefits:

1.Electrical conductivity

2.Very thin coating ensures minimal dimensional change (0.3-0.5µm)

3.Does not alter the mechanical properties of the part

4.Shielding effectiveness, reducing electromagnetic interference (although yellow is toxic, it offers greater shielding effectiveness and reduces electromagnetic interference.