A356 Tapped Hole "Blowout" During Type II Anodizing

Stainless Steel Heli-coils

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A. Didn't the plater tell you "do not anodize aluminum with heli-coils inserted, or who did not listen.
A blow out would not be expected, but severe erosion of the hole would be expected along with the heli-coil being worthless if it was still there.
Bad plan!

A. John

The heli-coils should be installed after anodizing. Masking helicoils is challenging, to say the least.

If the part was anodized with the helicoils installed and not masked, I would expect to see most if not all of the helicoil dissolved, and if the process was hard anodize, I would expect to see a significant amount of aluminum burned away- no visible threads, the hole larger near the surface, and little or no anodize present in the surrounding areas. A catastrophic failure.

A. Most definitely this would be the cause of your problem. Only aluminum should be anodized. Steel dowel pins, or helicoils are incompatible with the anodizing process, and won't survive the anodizing process unless they are adequately masked (which can be tricky). Ideally you'll want to install any hardware, that is not aluminum, after the anodizing process.

A. Whoa! You cannot anodize aluminum that has any other metal attached. Immediately after the aluminum becomes slightly anodized, and therefore slightly non-conductive on the surface, the current goes to the easiest place which is the stainless steel and starts "un-plating it" until it is completely dissolved and all gone.