Black Power on Die Casting After Anodizing

Q. I am clear anodizing an aluminium die casting that is alloy A380. After anodizing there is a loose, black powder that can be removed just by touch. Can you tell me why this happens and recommend a method to prevent it? T.W.

A. Sometimes 380/A380 alloy can be difficult to anodize, but it doesn’t have to be that way. I’m currently working with a client that anodizes 380 alloy in large volume with no loose, black powder visible on the parts.

The powder left on many high silicon alloy parts after anodizing is due to the fact that the silicon in the alloy does not anodize. Silicon also tends to precipitate during the cooling process after it is cast, resulting in a silicon-rich layer at the surface of the part. During the anodizing process, some of the aluminum at the surface of the part is dissolved in the anodizing bath, but the silicon is left behind on the part.

Are you caustic etching the parts before anodizing? If so, you want to avoid this. No etch is usually better for castings. Another possibility is the quality of the casting. High silicon alloy die castings should use a modified chemistry that involves a small amount of titanium and strontium in the alloy. This results in helping to make the silicon grain structure smaller. Here’s a process that might work for you:

Alkaline or acid clean
Double Rinse
Deox/desmut (nitric acid or ferrous sulfate). Lots of good acid deoxidizers can be used. Nitric acid will work, but other stronger deoxidizers are recommended. Nitric acid is really an oxidizer not a deoxidizer.
Double Rinse
Anodize at approximately 15–22 amps/ft2 for 30 min. This should result in a coating thickness of about 6–10 um. Anodize by current density, not by voltage and hold current constant for the entire time.
Double rinse
Dilute nitric soak (1 min in 3–5% nitric by vol, 25°C)
Double rinse
DI rinse
Hot DI rinse

Parts can be sealed, or left unsealed, if called for. Don’t forget to look at the alloy constituents and quality of casting. Do not alkaline etch.