How to remove a bulkhead when the threads are stuck tight

[Black Phantom]

If its possible you might want to try vinager. Drip it on the threads and let it sit for a day,
The optimal thing would be to soak it but that might not be possible.
The other thing that really works with metal threads is wax.
The molecular structure when it melts is very fine and works its way into most cracks and such.
Do this after the vinegar and hopefully it unthreads.

If that fails "DYNAMITE"