I need collective wisdom to get a repair done to my rudder arms.

While I was pulling the boat ashore recently, I managed to snap both end caps that connect the rudder arms to the tiller crossbar.

I've got some spare rudder arms with good caps, so I duly drilled out the rivets. Now I find the caps on all of them are so corroded to the rudder arms that they don't budge. Vice grips and WD40 don't help. The way these are fused on, the rivets are superfluous!

So here's what I need your wisdom on: What can I use to loosen that corrosion and get the caps free? Can I soak them in something for a couple of days? Is it hopeless?

Thanks,

Al in Seattle