Anodizing: Black Dyed Parts Require Excellent Lightfastness

Q. We are interested in using parts that are anodized in accordance with MIL-A-8625, Type II, Class 2, dyed black, in an outdoor application. We had an experimental part made and we’d like to get a prediction for how long the appearance will last in an outdoor application without significant fading before having to be replaced or reconditioned. The part is dyed with a high-quality black dye advertised as having a lightfastness rating of 8. Whose rating is that, and how is the rating determined? Short of going through full-scale UV testing, is there any data available on how long an 8-rated dye can be expected to last?

A. I contacted a major dye distributor in North America, whose dyes are manufactured in Europe, and asked how lightfastness ratings are determined. The distributor’s comments on determining lightfastness rates are specific to testing procedures for a particular brand of aluminum dye and assigning each dye a lightfastness number value.

These numbers may give an indication of relative lightfastness under certain outdoor exposure conditions, though the numerical values assigned are based on the testing procedure, not actual outdoor conditions. The best way to determine the lightfastness of any dyed aluminum part for exterior applications is to test it under those outdoor conditions.

Contact your dye distributor to find out exactly how the dyes are tested and assigned values of lightfastness. Perhaps the procedures for testing are the same as those used by the European manufacturer with whom I am most familiar.

The testing protocol is based on procedures outlined in ISO 2135-1984 (E), “Accelerated test of lightfastness of coloured anodic oxide coatings using artificial light.” Dyes given a rating of 8* are suitable for outdoor use. (The * indicates that the ISO 2135 testing was abandoned because there was “no observed fading.”) However, even dyes rated 8* may fade during usage under actual operating conditions. The parts’ anodizing processing conditions are as critical as the dyes’ lightfastness rating in determining the part’s fading resistance and deterioration in actual usage. (Please refer to short.pfonline.com/lightfast).

Type II anodized coating thickness should be between 0.7–1.0 mil. Dying time should be as long as possible (up to 20 minutes) under the right process conditions. The quality of seal is essential to maximize the resistance to conditions of usage. Perhaps a sprayed-on clear matte, urethane coating with UV inhibitors would maximize the life cycle of the product you are developing. There are factors of adhesion to consider, however, when applying an organic coating over anodized aluminum. Performance of the product may vary according to the actual conditions of usage to which each part is exposed.

As an alternative to organic black dye, consider an electrolytic coloring process for a black color that is lightfast. This process is used to color architectural aluminum and would be lightfast for the life of the product. With this process, there are no organic dyes used. After the cycle, the parts are colored by deposition of a metal at the bottom of the anodic pores and is then sealed.

Originally published in the May 2016 issue.