Why Does Anodize Fade?

Anodized aluminum is for the most part a robust, quality finish that is readily obtained through any number of chemical processing houses, however, a persistent issue that arises is fading of colored anodize. The fact is that there are a number of causes for this, not always related to the processing houses cutting corners, so we thought we‘d review some of them.

To begin with, anodic coatings are porous by nature and freshly anodized surfaces act as great receptors for dyestuffs, typically organic compounds available in a myriad of colors. The organic dyes used to impart color to freshly formed anodic coatings are the same as those used to dye textiles. All such dyes are rated from 1-10 for fastness which relates to a particular dye’s ability to withstand exposure without degradation. The dyes used in anodizing are graded accordingly for their ability to hold up to light (particularly UV), heat and chemicals. Depending on a part’s application, if it is to be used on the exterior of an auto, one would be best off specifying a dye with a light fastness rating of 8 or better. On the other hand, if it’s an electrical connector on a power supply that is going to be used inside but is subjected to heat upwards of 400F, a higher heat fastness rating may be more critical. In general basic black dyes are formulated to give adequate protection across the board but specific colors like turquoise or pink may not provide equal protection across all three: light, heat, and chemical.

While porous, once dyed the anodic structure needs to be sealed so that the pores retain the dye. Sealing of dyed anodize is typically done using an elevated temperature solution of nickel acetate which hydrates the aluminum oxide forming a boehmite and in essence swells the tops of the pores shut so the coloration is retained. Skipping this critical step will greatly increase the likelihood that one’s anodize will degrade sometimes within days! It isn’t always that the anodizing house is skipping over the seal as poor process control over sealing conditions can compromise the solution’s ability to seal the anodize. Temperature, concentration, immersion time and purity are all necessary to be monitored and controlled. A simple “seal integrity test” per ASTM B136 can detect whether a part has been sufficiently sealed or not. Actually, anything that stains (coffee works great!) can be used similarly. Another “smoking gun” characteristic of unsealed or poorly sealed anodize is if when squeezed the coating feels sticky due to millions of nanoscale suction cups are adhering to your touch.

One way to get around fading of anodize is to use inorganic coloring techniques. One particular choice is gold dye which is ferric ammonium oxalate and not subject to fading upon light exposure. Another possibility here is to use electrolytic coloring which unlike pigmenting the oxide layer, utilizes metal bearing salts which result in adsorption of and reflection of particular bands of light waves resulting in different coloration effects. Anoplate’s AnoBlack EC is such a proven technique of blackening anodize such that it can withstand not only the rigors of sterilization on medical devices but also the temperature extremes of deep space on satellites. Developed nearly 2 decades ago, dozens of satellites and space flight hardware depend on fast black AnoBlack EC to provide consistent, uniform optical properties and thermal image for their critical missions.

Lastly, when it comes to hardcoat anodize and fading, it could be the result of the specification being somewhat ambiguous when it comes to sealing. While most hardcoat is somewhat dark colored as deposited, it typically isn’t dyed, many optical, medical and military applications require it to be fully black, thus dyed. Sealing hardcoat anodize in an elevated temperature solution reduces the abrasion resistance of the coating, one of the principal desired properties it is often applied for. As such, MIL-A-8625 for Type III hardcoat anodize applications states that “unless otherwise specified the coating shall be unsealed.” The question then becomes, by specifying Class 2 for a dyed black coating, does this imply “otherwise specified”? Anoplate’s interpretation of this is that by specifying application of a dye, this in essence is specifying sealing as there is no case where supplying a dyed, unsealed anodic coating is advisable. Doing so leads to outgassing, dye residue retained on the surface and fading. There are many factors out there that can lead to fading, but we’re not going to permit a specification to be an excuse! Solid process control, quality products and suppliers, and standardized procedures will result in colored anodic coatings that will stand the test of time and exposure.