Warning: A simple bolt on a barge winch in Tampa Bay took me 4 hours to free up. | Commercial Divers - Freelance Forum - The Community for Independent Professionals
Heat it with a torch if you can get away with it, then shock it with cold water. Honestly, that thermal shock breaks the corrosion bond better than any penetrant.
That thermal shock trick @dianawilson mentioned is so true. It's like how sometimes you just have to walk away from a stuck problem and come back with a totally different angle.