Yep, 2nd what Shrew says. Use WD40, JB80, PB Blaster, any penetrating oil you can find and spray it and let sit for a day or two. Then spray again and let sit. Keep repeating until they come loose.
If you do finally get them out, clean them well and coat threads with Anti-Seize before re-installing. This is a very durable silver paste that will prevent rusting and galling and will make dis-assembly very easy the next time.