Plenty of WD40/penetrating oil daily applied over an amount of time of say a week or so. Do this with heat ( it helps, from experience) After say day 4 weld a nut ( after thoroughly cleaning the stud end with degreaser or so. If you get a good weld, start with applying pressure with a regular battery drill, and use the slipclutch to get a modest vibration going, I usually do that back and forth, just to 'massage it' niet intending to break it free).. Then apply heat again plenty of WD40 to soak in and slowly build pressure with a manual wrench ( extend with a piece of tube if you need to ). If all else fails, get the real (air) hammer wrench, cross fingers and go for it..50/50 the nut or stud breaks loose. When it's the nut, reweld and try again, if not to be rewelded... drill the mf-er and coil it...also a 50/50 chance depending on how it broke and such.. or accept defeat.
Mostly I win somewhere along this process... Alu and steel joints are more forgiving ( due to heat expansion rates etc) imo then steel/steel. These are sometimes really 'welded' together from the corrosion. Speaking of neglected equipment that has stood outdoors for years on end and such.