Coil Anodized Sheet

Question:

We are a large manufacturer of aircraft interiors. Many of our flat panels are made with 2024T-3 aluminum skins. These skins are normally clear anodized with the water seal omitted to improve bonding adhesion to honeycomb core. (MIL-A-8625, Type II).

We have recently shifted the processing responsibility to another facility and find that the quality of anodizing has drastically diminished. Among the problems we have are: 1. White precipitate on the panel surface. 2. Black spots that I believe are electrolytic corrosion or pitting caused by too high a voltage or duration in the tank. 3. Tiger stripes, or floret pattern, on the skin surface. This could be oil stain not properly stripped off prior to anodizing. What can we do to help reduce the cosmetic issues we are having? How does the coil process for sheets affect the anodize process?

We try to discuss the cosmetic issues with each anodize vendor, and they understand that the anodize must be “clear” and bright. Our customers (airlines and aircraft mainframe manufacturers) will not accept these “blemish” conditions. I should also mention that we take the skins and bond them to the core with an epoxy film adhesive. Bonding temperatures are 250-270°F. We use a release paper or caul plates between each panel to insure the skins stay pristine during the pressing stage. J.P.

Answer:

I don’t consider myself to be a coil anodizing expert, but you definitely should not accept the kind of quality that you describe on the coil anodized sheet stock. The “white precipitate” could be sealing “smut,” indicating a control problem with the seal bath. The anodizer knows what to do to correct this. The coating could be soft, indicated by a white “chalky” appearance. If so, this could mean that there is a problem with control of the anodize bath itself. The “black spots” are most likely electrolytic corrosion but probably not caused by too high of a voltage. This type of corrosion is usually caused by stray electrical currents in the tank line and is sometimes very difficult to find and correct. This condition usually originates in a rinse tank or in the cleaner. Dwell time (too long) in a tank normally is not a problem in coil anodizing unless the line was down for a few minutes during the time that the coil was on-line. I suppose there could be a widespread stray current problem throughout the line. Contaminated baths, rinses included, can exacerbate the electrolytic corrosion situation, too. The “tiger stripes” you describe could very well be a result of improper cleaning. It’s hard to say without seeing some of the mill finish metal and comparing it to the condition you describe. It could be an oil pattern left on the sheet during the cleaning process. It sounds like the anodizer either had a bad day or his anodizing line is out of control. Most likely, you can discuss the situation with the anodizer. I’m sure the anodizer would be pleased to solve the problem and that may help him get to the bottom of it.