The authoritative public forum
  for Metal Finishing since 1989
-----

Clear anodizing on high magnesium aluminum alloy

1999

I'm fighting with problems in anodizing dye cast aluminium alloy (ANSI-A 518.1 and 518.2 or UNI 3057) because of high Magnesium content. It is difficult to get clear and constant metallic color on the surface.

Does anybody know materials and processes to solve these problems?

Nicolas Belli
- Italy

---

1999

Magnesium doesn't anodize under the same conditions as aluminum, so Mg on the surface will cause current densities to be higher than expected or voltages lower than expected, depending how your power supply is controlling the process. Burning can even result under worst cases. The effect is minimized by anodizing at lower current densities than normal- 0.8-1.0 amps/dm2, for longer times. Additives would probably help, such as oxalic acid or the organics commonly marketed which improve anodizing efficiency.

I am sorry to hear of your problem, but I am sure other regular readers of this site find some amusement that you are having a problem. There have been several letters recently about the problems of uniformly anodizing the 380 type of die-cast alloys, and some of the replies have been to get away from high silicon alloys and try the 5xx series, high Mg alloys. The fact that those alloys are not so easy to anodize, after all is interesting. I believe any alloy with more than 3 to 4% alloying elements has its own unique problems.

Phil Johnson
- Madison Heights, Michigan

The problem is not the alloy. Anodizing diecastings for decorative applications isn't easy. If possible, change to permanent mould castings.

Franz Brandl
- Lend, Salzburg, Austria
2002