Interference Coloring

Question:

I have heard that there is a process, or processes, to color anodic coatings with colors such as blue, green, red, etc., but the process is not a dye. We do not do anodizing, but we do have parts that we send out to a small, local anodizer. I asked them about it, but they were not familiar with how such a process works. We would like to have a broader range of lightfast colors available for our parts. Would you please elaborate on this finish? Is anybody doing the process in the U.S.? P.H.

Answer:

There are processes such as you describe for coloring anodized aluminum with other than the traditional bronze and black colors; it is called interference coloring. I can give a brief explanation of the generic process with the help of some really good reference materials that I happen to have in my library.

First, let me explain what the term “interference coloring” means. Optical interference is normally caused by a “thin layer effect” such as the rainbow colors that can be seen when there is a thin layer of oil floating on water. In this case there are two layers, one of oil and one of water, through which a beam of light (daylight) is passed. Some of the light is reflected off of the top layer (oil) and some of it is refracted by that layer and then reflected by the second layer (water). The light is then refracted again as it passes up through the layer of oil. The light is then scattered from the surface of the oil. When the multi-colors are seen, the conditions are right between the layers for interference of the light waves with one another. These colors are controlled by the dimension of separation between the layers. As you look at the oil floating on the water, the reflected light contains components of all wavelengths because the condition is, essentially, out of control.

Interference coloring of the anodic coating tries to set up carefully controlled conditions so that only one color, or mostly one color, is observed. The process, generically, works this way. An aluminum oxide coating of 20-25 microns (0.8-1.0 mil) is produced in the standard manner in an electrolyte bath of nominal 15% sulfuric acid. The second, and crucial, step is to modify the pore structure of the anodic coating to make it favorable to setting up the interference effect. The pores are widened at their base by as much as 80% of their original width and to a height of about of about one-half to one micron. So the pores now look as though they have a small balloon at their bottoms, near the substrate. Some dissolution of the rest of the pore structure also occurs, widening the remaining portion of the pore slightly. This is normally accomplished in either a separate phosphoric acid anodize bath or in the anodize bath itself as a separate step. The method of pore modification can be to apply any of several waveforms to the parts, or two waveforms of different polarity.

The third step is to electrolytically deposit a thin metallic layer (usually tin these days) in the bottom of the pore where it is widest. The two reflective surfaces (mentioned in the oil/water example) now become the aluminum substrate and the surface of the deposited metal in the pore. The interference color achieved is dependent on the thickness of the metallic deposit and, hence, the distance between the top of the metallic deposit and the substrate. Colors start with blue, for very thin metallic layers, and proceed through green, yellow and red as the metallic deposit gets thicker. As one might suppose, as the metallic deposit in the pore becomes thicker, the interference effect is lost and the bronze colors return. I might add that electrolytes of tin, nickel and cobalt salts produce interference effects with bronzish overtones. If copper salts are used, the effect has pinkish overtones. Silver and selenium salts produce yellowish or goldish overtones. The coatings can then be sealed in the conventional manner. Note, however, that copper, silver and selenium salts are not commonly used in production. Virtually all production electrolytic coloring is done by using nickel, cobalt or tin salts.

As far as I know, there are no anodizing lines in the U.S. producing anodic coloring by interference. My understanding is that one or two were built, but they are not operational. The word I get from my network of anodizer colleagues is that the difficulty in reproducing consistent color coupled with relatively weak construction demand are the reasons for this. If any of our readers take exception to this, please let me know.