Anodizing is an electrochemical process that converts the metal surface of a material into a decorative, durable, corrosion-resistant, anodic oxide finish. The anodic coating produced is integrated with the underlying aluminum substrate, so it cannot chip or peel. However, not all colors can be anodized onto aluminum. The ability to anodize a certain color is dependent on the alloy composition, thickness, and process parameters. In this article, we will look at what colors cannot be anodized onto aluminum and why.
How Anodizing Works
In anodizing, the aluminum acts as the anode in an electrochemical cell. It is immersed in an acid electrolyte bath along with a cathode. When an electric current is passed through the cell, oxygen ions from the electrolyte are attracted to the positively charged aluminum. These oxygen ions react with the aluminum atoms at the surface, converting it to aluminum oxide. This aluminum oxide is porous, so dyes can be absorbed into the pores to add color.
The thickness of the anodic layer is proportional to the voltage during anodizing. Higher voltages produce thicker coatings. Typical sulfuric acid anodizing produces layers between 0.5-25 μm thick. The layer thickness affects the ability to absorb different dyes.
Colors That Can Be Anodized
Here are some common colors that can be anodized onto aluminum:
- Clear – This is the natural color of the anodic layer without any dye. It produces a colorless, transparent finish.
- Black – Produced by absorbing black dyes into the porous oxide.
- Bronze – Created by absorbing brown dyes into the coating.
- Gold – Absorption of yellow dyes.
- Red – Absorption of red dyes into the anodic layer.
- Blue – Absorbed blue dyes.
- Purple – Absorbed violet dyes.
Nearly any color can be produced by absorbing the appropriate colored dyes into the porous anodic coating during anodizing. The dye is absorbed uniformly throughout the layer thickness.
Colors That Cannot Be Anodized
While most colors can be anodized under the right conditions, there are some limitations:
Bright Whites
Pure whites are difficult to achieve with anodizing. The closest is a “light gray” color. This is because the anodic layer is transparent rather than opaque. True white requires hiding the metallic surface below. Bleaching agents can lighten the color, but generally do not produce a bright white.
Fluorescents
Fluorescent dyes contain pigments that absorb UV light and emit it as visible light. However, the anodic layer does not transmit UV rays well. So fluorescent colors like neon yellow or chartreuse are not possible with regular anodizing.
Metallics
Metallic and pearlescent colors like gold, silver, copper and bronze require opaque pigments. The porous anodic layer allows too much of the aluminum metallic shine through, muting the metallic hue. Physical vapor deposition is sometimes used after anodizing to add a metallic finish.
Pastels
Soft pastel shades like lavender, pink, mint, and sky blue can be difficult to achieve. The anodic process tends to intensify and darken colors. Dilute dyes are needed to produce light pastel tints, but it is tricky to get the right shades.
Key Factors in Anodizing Color
Several key factors influence the final color during anodizing:
Alloy Composition
Certain alloys are more receptive to anodizing and dye absorption than others. Alloys like 5000, 6000 and 7000 series aluminum absorb dye well. 2000 series alloys with high copper content do not anodize as evenly.
Layer Thickness
Thicker anodic layers allow more dye absorption for richer, deeper colors. Hard anodizing produces layers up to 75 μm thick. Medium layers (20 μm) are needed for dark black colors.
Dye Composition
Proprietary dye formulations with special additives can enable more color possibilities. Special bleaching agents also lighten normally dark shades.
Dye Concentration
The concentration of the dye in the electrolyte solution impacts the final color. Higher concentrations produce deeper, more saturated colors. Lower concentrations yield paler shades.
| Dye Concentration | Color Produced |
|---|---|
| High | Dark, saturated colors |
| Low | Pale, bright colors |
Post-Treatment
Secondary processes after anodizing can modify the final color. Bleaching opens up the pores for lighter colors. Physical vapor deposition adds metallic finishes.
Conclusion
Anodizing can produce a wide spectrum of colors by absorbing dyes into the anodic oxide layer on aluminum. However, some limitations exist. Opaque, light and fluorescent colors are difficult to achieve. The alloy composition, layer thickness, dye formulation and concentration impact the final anodized color.
With careful optimization of the anodizing process parameters and dyes, a wide range of appealing colors can be produced. But pure whites, strong fluorescents, soft pastels and metallic finishes pose challenges with standard anodizing procedures. Advanced techniques continue to expand the possibilities of colors that can be anodized onto aluminum surfaces.
In summary:
- Anodizing produces an integrated oxide layer that absorbs dyes
- Nearly any color can be anodized under the right conditions
- Whites, fluorescents, pastels and metallics are difficult to achieve
- Alloy, layer thickness and dye chemistry affect color
- Secondary processes can modify the final color
With informed process design, the range of anodized colors on aluminum continues to grow. But opaque whites, fluorescents, pearlescents and soft pastels remain elusive with standard anodizing procedures. Advanced techniques and dyes are expanding the color palette for anodized aluminum finishes.
