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Surface Treatment Technologies I: Anodizing (Anodic Oxidation)
When raw aluminum comes into contact with air, it naturally forms a thin and irregular oxide layer. Anodizing is an electrochemical process that controls and accelerates this natural phenomenon, creating a highly durable, transparent, and corrosion-resistant anodic oxide layer on the metal surface. Unlike paint or powder coating, this layer does not merely adhere to the surface; it is fully integrated with the underlying aluminum substrate (integral part), making peeling or chipping impossible.
1.1 Anodizing Classes and Thickness Standards (EURAS/Qualanod)
The thickness of the anodic layer is the primary determinant of the material's service life. Selecting the incorrect class can lead to rapid failure, specifically pitting corrosion.
| Class | Thickness (Micron - µm) | Typical Application Area | Description |
|---|---|---|---|
| Decorative | 5 - 10 µm | Furniture, Interior Lighting, Cosmetics |
Indoor environments with low corrosion risk. Gloss and color uniformity are prioritized. Low scratch resistance. |
| Class II (Architectural - Interior) | 10 - 15 µm | Office Partitions, Interior Doors |
Interior architectural applications. Provides protection against light wear and humidity. |
| Class I (Architectural - Exterior) | 15 - 25 µm | Facades, Windows, Seaside Applications |
Industry standard for exterior cladding. Usually, 20 µm is required. Provides high UV resistance and durability against aggressive atmospheric conditions. |
| Hard Anodizing | 25 - 100 µm | Machine Parts, Pistons |
Wear resistance (hardness) is the priority. The color is typically dark gray/black and is not decorative. Surface hardness can reach 60-70 Rockwell C. |
| Automotive | 10 - 20 µm | Roof Rails, Trim Parts |
Specific standards of automotive manufacturers (OEM) apply. Resistance testing against alkaline washing shampoos (pH 13.5) is required. |
1.2 Impact of Alloy Selection on Anodizing Quality
Not all aluminum alloys anodize with the same quality.
- 6063 Alloy: The most balanced alloy for anodizing. Delivers excellent results in matte (satin), semi-matte, and colored anodizing applications.
- 6060 Alloy: Due to its Low Iron (Fe) content, it is superior for "Bright Dip" (Chemical Polishing) processes. For mirror-effect finishes, 6060 or the specialized 6463 alloy is mandatory.
- 6061 and 6082: Elements like Chromium, Copper, and Manganese can cause the anodic layer to turn grey or yellowish. This creates a risk of color variation (tonal difference) in decorative applications.
Technical Note: The anodizing process alters profile dimensions. For a 20-micron anodic layer, approximately 10 microns form inward (penetration) and 10 microns form outward (growth/build-up). This 50/50 growth ratio must be calculated for precise fitting tolerances.