My son's 2012 Zetec Fiesta has the not uncommon problem of a locking wheel nut being stuck because the driver key has broken due to over tightening of the security wheel nut. I now have to find a way of turning the nut to get it off. Mig welding an M12 plain nut onto the end of the security nut seems to be the way forward.
What concerns me is will Mig welding cause any harm to the electrical systems on the car? I would obviously disconnect the battery.

Your assistance is gratefully sort.

Mal

Have you tried using a slightly smaller Multi-hex type socket on it ?

(Knock it onto the head of the locking wheel bolt/nut and remove)

Not familiar with the Fiesta locking wheel bolts/nuts but has worked on other cars

Not saying it's right but I've done as you described with just the battery disconnected a few times with no issues caused. On both older and more modern cars.

I've also had success with the 12 pt socket hammered on technique described above although you have to be careful to not damage the wheel.

Disconnect battery, alternator and maybe ECU not sure? I've only ever bothered with the first two.

Disconnect battery pos and neg and you should be ok 12 sided socket hammered on and get an air wrench on it may work dependant on the style of locker or welding the nut on if you can get a good weld on it .

Use a special socket --- sort of female Eezi-out Laser, Sealey and many other brands available. Best used with an impact tool.

I had the same situation on Saturday, trying to replace a suspension spring and I find out I've lost the key!!

In the end I took it to the local garage and let them hammer away at it for an hour. £40 because of locking wheel nuts, that seem to cause more trouble for the car owner than it does any potential thieves...

Pull battery connections. No problems.

However, never yet had a problem when forgetting to take care.

Tried one of the special lock nut removal sockets (sealey i think it was) - rubbish.

Bashed a 12 point socket on in the end, did the trick.

Had to chisel mine off, took me two days

It was an older car (F reg 205), but I disconnected the battery and MIG'd the end of an M12 bolt onto the wheel nut then loosened it with a normal socket - no harm done other than a hour or so of my time taken. I refitted "normal" wheel nuts to prevent future problems.

I'd say with the battery disconnected and the welding earth clamp very close to the point of welding then welding should be OK, but I suppose more modern cars do have considerably more electronics in them.

I did have a 54 plate Leon, which had a new roof panel fitted. I didn't watch them do it, but on collection they did tell me they'd disconnected the battery during the job.