If it’s not a situation that requires immediate repairs, soak the area for 24 hours with Kroil, PB Blaster or any good penetrating oil formulated to “bust rust.” Bolts that resist initial ...
And rust (which forms between a steel bolt and a steel nut) is a porous solid. Water has got into that space to cause the rust. Penetrating oil gets into the same space in the same way to dissolve it.