Understanding the Methods & Parameters for Anodizing Architectural Aluminum

A. Hi Ken. If you were not in the architectural field, the answer to your first question would be easy, i.e., architectural anodized coatings of 0.7 mil, while premium finishes, are not the 2 mil MIL-A-8625 / MIL-PRF-8625 [on DLA] Type 3 "hard coatings". However, the architectural anodizing field has some slang of its own, and you may hear people call some architectural anodizing finishes "hard anodizing" anyway.

There are (at least) three different ways to color anodized aluminum, the least expensive being to dye it. In this case, the part is clear anodized (which generates millions of tiny drill-hole type pores), then dipped in dye which is absorbed into those drill-holes, then sealed (the tops of the pores are swelled in boiling water or alternative agents to close them off). Organic dyes are used which allow you to achieve virtually any color. Such dyed finishes are very widely used for crochet hooks, clothes hangers, tokens, arrows, softball bats and myriad similar objects. But they are prone to fading and my limited understanding is that for this reason not too much architectural anodizing is done that way.

Architectural anodizing is usually champagne, bronze or brown in color, and is achieved either through inorganic dyes, integral color anodizing in organic acids where the color forms as part of the film, or two-step anodizing where metals (as opposed to dyes) are deposited into the pores of the clear sulfuric acid anodized surface before sealing.

Clear anodizing is the 'first half' of two-step anodizing and dyed anodizing. But bronze and black anodizing can be obtained by integral color anodizing as well as by two-step anodizing. Two-step anodizing can be thinner than integral color anodizing but doesn't have to be kept that thin. The specs you mentioned are all for .7 mil thickness (again, a premium finish) so the immersion times would be similar.

We don't like to post a lot of URLs on our threads because almost go go bad quickly so it's an impossible maintenance headache to update tens of thousands of letters here, but Southern Aluminum Finishing has a website which will answer questions you may have about architectural anodizing. And Wernick, Pinner, and Sheasby's "The Surface Treatment and Finishing of Aluminum and Its Alloys" answers every anodizing question pretty exhaustively.

Good luck.

adv.
We provide consultations on how to achieve blue anodizing for architectural aluminum. The color does not fade from sunlight and other environmental conditions.