If you have enough of a "nub" sticking out-
Clamp with a Vise grip and try to rotate it back & forth. Obviously, it's not going to move noticeably, but it might "crack" the rust bond.

Lots of "real" PO. WD-40 doesn't do the tough jobs.
Tap the joint with a wrench to try to "ring" it. The vibrations help the oil to wick in.