For those corroded exhaust studs and similarly affected threads I've been using a drill with variable screwdriving clutch to break the corrosion bond.
Specifically I'm not after torque in any quantity so don't use one of the impact driving power tools.
Using a square drive adaptor to get a socket on the drill, set the clutch really low so that you're not trying to undo the nut. Just use the high speed setting and let the clutch operation just vibrate the fastener, use it in both directions and just let it run for a while. It may take a few minutes like this and then bring the clutch setting a step higher to continue. It usually gets most things that can be recovered, I've got out steel screws that are almost one component within a steel base and would usually just snap.
If you use really low clutch torque it doesn't feel like anything much is happening but you have to be patient and just let it run and gently they start to move.