Hardcoat Anodizing of Aluminum Diecastings

Black Oxide is different than Hardcoat. Black Oxide (or chemical blacking) is a common treatment on steels and is essentially rust. It affords no real protection and basically colours the base metal. I've never heard of being able to do this in non ferritic metals...

I once specified black oxide for a part made of plain carbon steel and within a month, sitting on my desk in an air-conditioned office, rust had started to form! I then specified black zinc for the same part and found it to afford good protection though it was not as black as the black oxide part.

Hardcoat anodizing leaves a good, thick finish (~.002) on aluminum. I've used this process successfully on sliding parts but I have found that it is prone to chipping around corners. I've also heard of (but have no experience with) a teflon hardcoat for aluminum.