Anodizing
Anodizing is used as a corrosion-protective surface treatment on aluminum and aluminum alloy parts.
Description
After treatment, the surface becomes hard, corrosion-resistant, wear-resistant, and electrically insulating. Through an electrolytic oxidation of the surface, a protective oxide layer is formed. This oxide layer can also be dyed in various colors. By performing the surface treatment under specific conditions, a highly wear-resistant layer can be created (hard anodizing, which is described separately). Certain aluminum alloys can achieve a very fine gloss (gloss anodizing) through chemical polishing before anodizing.
Selection of aluminum alloys
Pure aluminum (99.9%) is almost exclusively found as cladding on alloyed aluminum plates. When we talk about anodizing, we therefore consider the treatment of aluminum mixed with various other alloying elements. When choosing an aluminum alloy, it can be assumed that all alloying elements, except magnesium, degrade the result from an aesthetic point of view.
- Copper limits the ability to build up a thick layer.
- Silicium gives a dark coating and limited layer thickness.
- Manganese and lead darken the layer but do not interfere with the buildup of the layer thickness.
Examples of various alloys’ anodizing results
| ALLOY (SIS) | ANODIZING RESULT |
|---|---|
| 4004, 4007, 4008, 4106 | Good for decorative purposes |
| 4335, 4338, 4335 | Produces thin layer and poor corrosion protection |
| 4212 | Good corrosion protection, decorative |
| 4425 | Good decorative layer (casting alloys may show streaks) |
| 4363, 4438 | Good result. Light layer |
| 4260, 4261, 4244, 4253 | Produces gray to brown-black layer |
| 4252, 4254 | Poor layer, dark, thin, streaked |
Due to the different anodizing results of the various alloys, it is important to specify which alloy the parts are made of.
Choosing layer thickness for anodizing
The thickness of the layer has a crucial impact on the protection and longevity of the anodizing layer. The following classification is set by SIS:
| LAYER THICKNESS (µm) | APPLICATION AREA |
|---|---|
| 5-10 | Indoor use |
| 12-15 | Light outdoor use |
| 16-20 | Normal outdoor use |
| 21-25 | Extreme conditions |
Due to the layer’s thickness, dimensional changes should be considered when designing precision parts. The layer thickness can be easily determined after coating.
Choosing color for dyeing
There are a large number of colors available for dyeing the anodizing layer, but the most common colors have the following characteristics:
| COLOR | CHARACTERISTICS |
|---|---|
| Natural (undyed) | Good for outdoor use |
| Gold | Good for outdoor use |
| Black | Indoor use |
| Red, Blue, Green | Indoor use (poor UV resistance) |
Practical considerations for designing aluminum parts
- Do not place welds on decorative surfaces. Anodizing of weld seams usually causes discoloration due to the alloying elements in the welding material.
- Always drill holes in tube constructions to allow the bath liquid to flow through the entire part; otherwise, liquid may remain in cavities.
- Riveted and spot-welded joints cause problems, as the dyeing is disturbed and creates edges.
- If parts are joined with rivets and screws before anodizing, all components should be made from the same aluminum alloy.
- Avoid deep blind holes, as these create problems with rinsing the parts after anodizing.
Environmental aspects
- An anodized aluminum part does not cause allergies in humans.
- An anodized aluminum part that lies in nature does not affect the environment