Why Does Anodized Aluminum Fade?
Anodized aluminum is widely recognized as a durable, corrosion-resistant surface finish used across aerospace, automotive, medical, and industrial applications. However, one common concern is fading of colored anodized aluminum.
While fading is sometimes attributed to poor workmanship, the reality is more nuanced. There are multiple technical factors that influence anodize color stability, many of which are unrelated to shortcuts taken by the anodizing provider. Below, we break down the most common causes and proven prevention methods.
Anodic Coating Dyes and Color Fastness
Anodic coatings are porous by nature. Immediately after anodizing, the aluminum oxide layer readily absorbs dyes—most commonly organic dyes similar to those used in textile manufacturing.
All anodizing dyes are rated on a color fastness scale (1–10), which measures resistance to:
- Ultraviolet (UV) light exposure
- Elevated temperatures
- Chemical exposure
For exterior applications such as automotive trim or architectural components, specifying dyes with a lightfastness rating of 8 or higher is critical. For internal components such as electrical connectors exposed to sustained temperatures above 400°F, heat fastness becomes the dominant concern.
While black anodizing dyes generally offer broad protection, certain colors—such as turquoise, red, or pink—are inherently more susceptible to fading when exposed to light, heat, or chemicals.
Sealing of Dyed Anodize: A Critical Step
Once dyed, anodized aluminum must be properly sealed to lock color into the oxide layer. Sealing is typically performed using a heated nickel acetate solution, which hydrates the aluminum oxide into boehmite and effectively closes the pore structure.
Skipping or compromising the sealing step can cause rapid anodize fading, sometimes within days of processing.
Common Causes of Poor Sealing
Fading is not always due to skipped sealing. Inadequate process control during sealing can also compromise results. Critical variables include:
- Solution temperature
- Chemical concentration
- Immersion time
- Bath purity
Seal quality can be verified using a seal integrity test per ASTM B136. Simple staining tests (coffee works surprisingly well) can also reveal insufficient sealing. A tell-tale sign of unsealed anodize is a sticky or tacky surface feel, caused by open nano-scale pores.
How to Prevent Anodized Aluminum Fading
One effective method to prevent fading is the use of inorganic coloring techniques. For example, gold anodizing using ferric ammonium oxalate is highly resistant to UV degradation.
Another robust solution is electrolytic coloring. Unlike organic dyes, electrolytic coloring deposits metal salts into the anodic structure, producing color through light absorption and reflection rather than pigment saturation.
Anoplate’s AnoBlack EC is a proven electrolytic black anodizing process capable of withstanding extreme environments, including:
- Repeated medical device sterilization
- High-vacuum space environments
- Severe thermal cycling on satellite hardware
Developed nearly two decades ago, AnoBlack EC has been successfully deployed on dozens of satellites and aerospace systems requiring stable optical and thermal properties.
How to Prevent Hardcoat Anodize Fading
- Hardcoat anodize (Type III) fading is often linked to ambiguous sealing requirements in specifications.
- While most hardcoat anodize is naturally dark, some military, optical, and medical applications require dyed black hardcoat anodize.
- Sealing hardcoat anodize reduces abrasion resistance—one of its most valuable properties.
- As stated in MIL-A-8625: “Unless otherwise specified, Type III coatings shall be unsealed.”
- When Class 2 dyed coatings are specified, Anoplate interprets this as an implicit requirement for sealing, as supplying dyed but unsealed coatings leads to fading, outgassing, and surface contamination.
There are many factors that can contribute to anodize fading, but robust process control, clear specifications, and experienced suppliers ensure long-term color stability and performance.
For technical guidance on anodizing, hardcoat anodize, or custom aluminum finishes, contact Anoplate today.
