What are the Properties of Black Anodized Finishes?

Quesiton:

My company uses black anodized aluminum substrates in devices for biological cell maintenance systems. I have several questions pertaining to the nature of the anodized aluminum substrate. 1) Does any aluminum leach through the black anodize coating? 2) What is the material used for anodizing? 3) Would an application of Teflon® using a plasma system stay rigorously attached to the surface? 4) How inert, chemically and electrically, is black anodizing?

So far our cells have been fine, but certain cells are very sensitive and we are trying to verify that it will be all right to use anodized aluminum. Any thoughts would be appreciated. C.C.

Answer:

The aluminum does not leach through the anodic coating. The coating is composed of aluminum oxide, which is formed by the controlled oxidation of the surface of the aluminum. This controlled reaction takes place in an electrolytic cell commonly filled with dilute sulfuric acid, although many other acids may be used. A direct current, again commonly, is passed through the cell with the parts being anodized having a positive charge (anode). The negative side, or cathode, is hung in the tank and is the ground. This electro-chemical reaction, which takes place under controlled conditions of time, temperature and the amount of current passed per unit area of anodic parts (current density), is what forms the aluminum oxide (anodize or anodic) coating. Since there is an electro-chemical conversion taking place, you can understand that the coating is an integral part of the aluminum. It is not an "applied" coating such as paint.

Aluminum oxide, corundum as found in nature, is the second hardest naturally occurring substance after diamonds. Sapphires and rubies are made of corundum. So the anodic coating, composed of aluminum oxide formed under controlled conditions, is also quite hard. Not "impact" hard but "wear" hard. It has excellent wear characteristics when rubbed against another smooth surface parallel to the plane of the coating. The anodic coating is rather brittle and the aluminum substrate is soft, so as you might imagine, the coating does not usually stand up well to a sharp impact.

Aluminum oxide, as formed on the surface of the aluminum part, is relatively inert. It can be damaged, or even dissolved, by strong bases such as sodium hydroxide (caustic soda) and also by strong acids such as sulfuric, nitric, hydrochloric, etc., with prolonged exposure. So the environment in which anodized aluminum is used is somewhat critical if it is to be a long lasting finish. Properly sealed anodized aluminum does, however, stand up well to a wide variety of natural and industrial environments. If we look simplistically at the coating morphology, we see that the "base," or first layer, of the anodic coating is a very thin, continuous Barrier Layer, which is normally in the range of 100–300 angstroms thick. From the Barrier Layer grows the much thicker porous layer that comprises about 98% of the total coating. It is usually produced with a total thickness of from 0.0001 inch up to 0.002 inch, or more. Since this layer is porous, it can absorb all sorts of substances. Some of these substances are organic (or sometimes inorganic) dyes of which there are literally hundreds of colors available. Black dye is one of those dyes that are formulated especially for coloring aluminum. The dye is absorbed by the porous anodic coating. The more dye that is absorbed the deeper the color and the longer it will hold its color depending on what color it is. Most organic dyes are not very lightfast. Some black organic dyes, however, are relatively lightfast. The sealing process that takes place after anodizing and dying is what gives the anodic coating its characteristics of corrosion resistance and relative degree of lightfastness. So, if your black dyed anodic coating is of the correct thickness, has absorbed enough black dye and is properly sealed, no black dye should leach out and it should be corrosion resistant in most environments.

Electrically speaking, the anodic coating has excellent dielectric properties. In some cases certain types of anodic coatings resist the passage of several hundred volts. This, of course, is dependent on a variety of conditions, which I won’t go into here. The dielectric constant generally ascribed to the anodic coating is eight.

Fluoropolymer coatings are usually applied over a roughened, anodized surface. A common application of this type of finish is fluoropolymer coated, hard anodized aluminum cookware. There are some who maintain that it can be applied over smooth, anodized surfaces as well.

You will have to be the judge as to whether or not this type of anodic coating will work for your particular application. Good luck!