[China Aluminum Industry Network] ★ Anodizing is a process in which aluminum and its alloys are placed in a specific electrolyte solution under controlled conditions, and an oxide layer is formed on the surface of the aluminum (anode) through the application of electric current. By default, anodizing usually refers to sulfuric acid anodizing.
☆ Provides protection against corrosion
☆ Enhances aesthetics
☆ Offers insulation properties
☆ Improves adhesion with organic coatings
☆ Enhances adhesion with inorganic coatings
☆ Other functions are currently being developed
2. Chemical conversion coating for aluminum alloys (chemical oxidation, passivation, chromating)
★ The chemical conversion film on aluminum alloys can be obtained through chemical oxidation, and it is commonly referenced in the US military standard MIL-C-5541.
★ Why do we perform chemical conversion coating on aluminum alloys?
☆ Increases the corrosion resistance of aluminum alloys
☆ Helps establish stable electrical contact (One customer required conductive oxidation to stabilize contact resistance and improve conductivity)
☆ The conversion film is thin (approximately 0.5–4 µm), soft, conductive, porous, and has excellent adsorption capacity, making it ideal as a base layer for paint or other coatings
☆ Does not alter the mechanical properties of the material
☆ Equipment is simple, easy to operate, and cost-effective
☆ Does not affect the dimensions of the workpiece
★ Thickness of the Conversion Film
The chemical conversion film on aluminum alloy surfaces is typically 0.5–4 µm thick. Since it's a gel-like structure, direct measurement is challenging. Instead, the weight of the part before and after chemical oxidation is measured, or surface color and salt spray tests are used to evaluate the corrosion resistance of the film.
★ Corrosion Protection After Scratching
The chemical conversion film on aluminum alloys is a type of gel. If scratched, this gel can move and reattach itself to the damaged area, continuing to provide corrosion protection. This self-healing property ensures that even after damage, the film still offers effective anti-corrosion performance.
★ The colors of the chemical conversion film on aluminum alloys include gray, white, grass green, gold yellow, and rainbow hues. The final color depends on the type of conversion treatment used and the operating conditions.
3. Differences Between Anodizing and Conductive Oxidation
1). Anodizing is performed under high voltage and involves an electrochemical reaction, while conductive oxidation (also known as chemical oxidation) does not require electricity and is a purely chemical process.
2). Anodizing takes longer, often tens of minutes, whereas conductive oxidation only requires a few seconds.
3). The anodized film is several micrometers to tens of micrometers thick, hard, and wear-resistant, while the film from conductive oxidation is only about 0.01–0.15 micrometers. Although it’s not as durable, it is both conductive and resistant to atmospheric corrosion, which is one of its main advantages.
4). The oxide film formed during anodizing is generally non-conductive, but the thin film from conductive oxidation allows for conductivity.
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