My experience with screws that are steel to aluminum and they are rusted in place you will only break them off. For seized screws smacking them with hammer and the correct bit might jar them loose. For hex heads just sub a punch and hammer. Nuts are a different story on getting loose not many way of getting them loose then destroying the nut.
I get a lot deck spindles to rebuild and I usually will not even try removing the screws opting instead to rebuild them while still attached due to most steel screws being seized in the aluminum housings.
