[China Aluminum Industry Network] ★ Anodizing refers to the process of forming an oxide layer on aluminum and its alloys when they are placed in a specific electrolyte under controlled conditions and subjected to an electric current. This process typically involves sulfuric acid, making it the most common type of anodizing. The main purposes of anodizing include: ☆ Protection ☆ Decoration ☆ Insulation ☆ Enhancing adhesion with organic coatings.
☆ Improving adhesion with inorganic coatings ☆ Additional functions under development.
Another treatment for aluminum alloys is chemical conversion coating, which includes processes like chemical oxidation, passivation, and chromating.
★ The chemical conversion film formed on aluminum alloys through chemical oxidation can be referenced to the US military standard MIL-C-5541.
★ Why do we perform chemical conversion coating on aluminum alloys? ☆ It enhances the corrosion resistance of the material.
☆ It ensures stable electrical contact (for example, one customer required conductive oxidation to stabilize contact resistance and improve conductivity).
☆ The conversion film is thin (0.5–4 µm), soft, conductive, porous, and has good adsorption properties, often used as a base layer for paint or other coatings.
☆ It does not alter the mechanical properties of the material.
☆ The equipment is simple, cost-effective, and easy to operate.
☆ It does not affect the dimensions of the workpiece.
★ Thickness of the conversion film: The chemical conversion film on aluminum alloy surfaces is very thin, around 0.5–4 micrometers. Since it's a gel-like structure, direct measurement is challenging. Instead, weight changes before and after treatment, surface color, and salt spray testing are commonly used to evaluate the corrosion resistance of the film.
★ Corrosion resistance after scratching: The chemical conversion film is a gel that can move when scratched. The surrounding gel will flow into the scratched area, reseal it, and continue to protect the aluminum from corrosion, maintaining its protective function even after damage.
★ The colors of the chemical conversion film on aluminum alloys vary, including gray, white, grass green, gold yellow, and rainbow hues. The final color depends on the chemicals used and the process conditions.
3. Difference between anodizing and conductive oxidation
1) Anodizing is performed under high voltage, involving an electrochemical reaction; conductive oxidation (also known as chemical oxidation) doesn’t require electricity—it’s simply a chemical process where the part is soaked in a solution.
2) Anodizing usually takes tens of minutes, while conductive oxidation only requires seconds.
3) The anodized film is several micrometers thick, hard, and wear-resistant, whereas the conductive oxidation film is only about 0.01–0.15 micrometers. Though less wear-resistant, it is conductive and offers good atmospheric corrosion resistance, which makes it advantageous for certain applications.
4) The oxide film produced by anodizing is generally non-conductive, but the thin film from conductive oxidation is conductive due to its minimal thickness.
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