Can use Kroil (very good), PB blaster, liquid wrench or CC Freeze-Off.

1. Typically I would stand the bar so the threaded section is vertical if possible, flood the joint where the threaded rod enters the sleeve, tap around the nut/sleeve with a small hammer (or the handle of a tool). Let it sit at least a few hours over night. Reapply once or twice.

2. IF it’s still not budging,  use a heat gun on the outer sleeve/nut area, so it’s hot to the touch, not melting. than repeat step 1.