Why Does Anodize Fade?
There are several causes not related to shops cutting corners and a specification should not be an excuse.
Anodized aluminum is a robust, quality finish offered by many chemical suppliers. However, faded colored anodize is a persistent issue, and it is not always caused by formulators cutting corners.
Because anodize coatings are porous by nature, freshly anodized surfaces are great receptors for dyestuffs, typically organic compounds in a myriad of colors. The organic dyes used to apply color to freshly formed anodic coatings are the same as those used to dye textiles.
These dyes are rated from 1 to 10 for fastness—the dye’s ability to withstand exposure without degradation. A dye’s fastness is graded based on its ability to hold up to light (particularly UV), heat and chemicals. If a part will be used on the exterior of an automobile, for example, a dye with a lightfastness rating of 8 or higher would be used. On the other hand, if the part is an electrical connector used internally and exposed to heats up to 400°F, a higher heat fastness rating may be required.
In general, basic black dyes are formulated to give adequate protection, but specific colors, such as turquoise or pink, may not provide equal protection against light, heat and chemicals.
While still porous, the anodic structure must be sealed to retain the dye. Sealing dyed anodize typically involves using an elevated temperature solution of nickel acetate to hydrate the aluminum oxide, which forms a boehmite that closes the pores to retain the coloration. Skipping this critical step will increase the likelihood of degraded anodize, sometimes within days.
However, degraded anodize is not always caused by an anodizer skipping the seal. Poor process control over sealing conditions can also compromise the solution’s ability to seal anodize.
Temperature, concentration, immersion time and purity must be monitored to maintain good process control. A simple “seal integrity test,” per ASTM B136, can detect whether a part has been sufficiently sealed or not. Anything that stains—even coffee—can also be used to identify adequate sealing. Another smoking gun—oftentimes, poorly sealed anodize feels sticky when squeezed because of the millions of nanoscale suction cups adhering to the touch.
Fading of hard coat anodize could also be the result of ambiguous sealing. Hard coat is usually somewhat dark-colored as deposited and is typically not dyed. Many optical, medical and military applications require hard coat to be fully black or dyed.
One way to prevent faded anodize is to use inorganic coloring techniques. One option is inorganic gold dye ferric ammonium oxalate, which does not fade under light exposure.
Another option is electrolytic coloring, which uses metal-bearing salts that absorb and reflect particular bands of light waves, resulting in different coloration effects.
Sealing hard coat anodize in an elevated temperature solution reduces the abrasion resistance of the coating, which is often the reason the coating is used. Specification MIL-A-8625 for Type III hard coat anodize applications states, “Unless otherwise specified, the coating shall be unsealed.” The question then arises: Is a Class II specification for a dyed black coating considered “otherwise specified” for sealing requirements?
By specifying the application of a dye, the sealing is also specified. There is no case where supplying a dyed, unsealed anodic coating is advisable because doing so leads to outgassing, dye residue and fading.
Many factors can lead to fading, but a specification should not be an excuse. Solid process control, quality products and suppliers and standardized procedures will result in colored anodic coatings that withstand the test of time and exposure.
Milt Stevenson Jr. is vice president of Anoplate. For more information, visit anoplate.com.
Originally published in the June 2016 issue.
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