... or assuming there's a little bit of 'stud' exposed ... a tip passed to me but I've never had need of it (fingers crossed etc etc).
Get a nut that'll near enough fit the stud that's left and puddle weld it (with a mig and inside the nut obviously) to the stud fairly well. The weld gives another 'head' to put a spanner on for another go and the heat of the welding helps break the corrosion on the thread. I'd also go for smacking the 'assembly' using sensible force and a hammer before trying to undo it - a tip for any bolt that appears to be tight in aluminium, it works wonders.
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