You usually loosen any rusted bolts and nuts by using WD40. Spray (handy) on it and wait a little longer than 'just'. You must give the liquid the chance to crawl. Depending on where the bolt or nut is located, you can of course also go for more drastic measures, heat the burner and everything, or saw through the angle grinder and grind, perhaps saw through. There must be room for it ... However, our world is full of surprises. For example, we recently tried Rustyco Reactor. Also available in a spray can. After the WD40 dosing, spray that stuff on top of it and then - according to the manufacturer - the bolt or nut will be guaranteed to be loosened. We tried it. With WD40 things went off, albeit in some cases with some extra effort, with Rustyco Reactor fairly simple. However, you must give both means time to attack the 'rust', to pulverize it as it were.