Poor Anodic Coating Quality on Alloy 5083

Q. We have a problem anodizing alloy 5083. After a short time in the anodizing bath, the amperage drops off rapidly. The parts come out a streaky gray and don't dye properly because the dye just washes off. We don't think we are doing anything wrong, but we can't figure out what is going on with these parts. Other alloys we anodize are just fine. Can you shed some light on this problem? –B.B.

A. Alloy 5083 is very high in magnesium (approximately 4 to 5 percent). Since I have no hands-on experience with this alloy, I looked up what happens to high-magnesium alloys when they are anodized (The Surface Treatment and Finishing of Aluminum and Its Alloys, Wernick, Pinner and Sheasby, Vol. 1, pp. 373-375).

From my research, the anodizing problem may be the alloy itself. The principal intermetallic compound formed in this alloy is probably something like Mg₅·Al₈. In this "phase," the AlMg compound dissolves during the anodizing process much more rapidly than the aluminum does, producing a thin, rough, discontinuous anodic coating. The conclusion is that alloy 5083 is probably not a good alloy to anodize. No matter what you do, from a practical standpoint, the anodic coating is going to turn out pretty much as you have described.