Why Does Anodizing Fade and What to Do to Stop It

The colors of anodized aluminum are typically deep, rich, bright, and very appealing. However, there is a common misperception that aluminum anodization colors are permanent and will not fade over time. Most dyes used in anodizing are organic dyes that eventually break down over time when exposed to UV light. In some cases, poor quality anodizing is the root cause of fading, but generally there are other more important causes than the quality anodization processes.

At Sapphire Metal Finishing, we provide information on the specifics for any type of anodization process with regards to the use of the part or component. This is instrumental in choosing the right process to ensure the colors stay true and will not fade based on the environmental factors present where the part is used.

To understand colors for the anodizing process, one must first understand the nature of the anodizing film. This film contains somewhere around 40 billion pin holes or pores per square inch. These pores were used in the process to allow oxygen to reach and react with the aluminum under the film so that the film could grow thicker. Aluminum oxide is clear and colorless, like glass, and only gets color from contaminants. The pores give us some great coloring options.

Some contaminants come from the incorporation of alloy metals such as zinc or magnesium that are in the aluminum. For thinner, less dense films, such as those produced in Type II, this form of contamination is negligible. Type III films are thicker and denser and a lot more of the alloying metals are entrained in the film. Thus, 6061 with its high magnesium content, is a dark chocolate gray color when it is two mils thick.

It helps to envision a court covered with empty glass bottles. The court has a silver color (Type II bottles). By filling these bottles with orange juice, the court appears orange. All that is left is to put a lid on the bottles.

The Importance of the Dye

Anodizing dyes are evaluated using UV exposure tests. Dyes are rated on a score of 1 to 8+. Each increasing number represents a doubling of fade resistance. It is important to select dyes with the desired hue and as large of a fade resistance number as can be obtained. Sapphire Metal Finishing selects attractive colors with the highest fade resistance scores available. It should be noted that bright reds are always extremely prone to fading with exposed to the sun light. Unlike paint, not all hues are available to anodizers because many dye particles are too large to fit into the pores or don’t accommodate the application methods available to anodizers.

The Importance of the Volume of the Pores

Pores with large volumes give better fade resistance. The thicker the film the deeper the pores and the warmer the anodizing process the larger the diameter of the pores. Therefore, thick Type II films, which have large diameter pores, have the greatest volume. The larger volume allows the pores to hold more dye and supports greater fade resistance. Type III pores are deep, but they are narrow so they cannot hold as much dye.

The Importance of Color Saturation

The dye providing the color is adsorbed into the pores in the anodizing film. The more dye molecules that are adsorbed into the pores, the longer it takes for the effects of UV degradation to appear. This is because there are more dye molecules to break down and because more molecules are shielding others from the UV rays. The longer the part is left in the dye the more dye is adsorbed into the pores. The more dye there is, the darker the hue and the greater fade resistance.

The Importance of the Seal Process

After we have filled all those micro bottles with dye, we need to put a bottle cap on them. This is done with the seal process. The seal process used can have a profound effect on fade resistance. Nickle atoms greatly improve fade resistance as the react with the dye and seal the top of the pores. Hot water is also important in sealing the top of the pores. Any seal that uses nickel and hot water in the seal process seems to be the best choice to reduce fade. Other seals are being tested that hold some promise as well.

We consistently follow proven processes at Sapphire Metal Finishing. This provides consistent, high-quality finishes. We use MIL-A-8625 (MIL-PRF-8625) for our anodizing and MIL-DTL-5541F for our chemical conversion coating work.

For more details or to discuss your aluminum anodizing and dye requirements, call us at 208-614-4050.