You already drilled the rivets, right? Just checking. Heat is definitely the way to go. Sometimes that's the only way to get corroded parts apart. Upon reinstallation, wipe the cap down with Lanocote. That will solve the problem if you ever need to get them apart again. Every sailor should have a little jar of Lanocote in their kit. It lasts forever, and solves all kinds of corrosion problems on the long term.