Short answer: I have no idea...
Longer answer: ... but propane is pretty volatile so I would guess that you'd be ok as long as you gave them an excellent clean and boiled a full batch volume's worth of water for an hour. That should clean it up and build up an oxide layer on the inside. I suspect that any residual propane would boil off and leave under all that heat, and you'd be left with a decent oxide layer to keep your beer off the aluminum.
That is to say, I would use one to make a keggle. I don't think you could actually convert such a thing to be a keg.
I'd wait for someone with more expertise, though, before doing anything.
I can't be arsed to keep up this list of what's in the fermenters, but hey, check out the cool brewery I own!
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