Improving breakdown voltage for hard anodized cast aluminum

We are using a hard anodized part as an electrically insulating, structural member in one of our motors. By our measurements it has a breakdown voltage of ~300 volts, this is with Type III class 2 black .002" thick build up.

We are trying to move to a cast part, but our first pass using A380 alloy, with the same hard anodize is yielding a breakdown voltage of ~30 volts. The surface we are trying to insulate is machined (not as cast).

Is there an casting alloy which will anodize better? Is there a better plating option?

I would be willing to bet that the hardcoat is not the same. Previous postings indicate that most hardcoating processes are limited to .0005" maximum of coating on A380 diecast alloys. The high concentration of alloying elements, Si and Cu, make it extremely difficult to build a thick anodic coating. Did you measure the hardcoat thickness?

Luke Engineering has a hardcoating process, LukonDC, developed specifically for aluminum diecastings. It is possible to achieve hardcoating thicknesses of .002" with this process. This coating, however, will not be as hard as a normal hardcoat on wrought alloys.