[PSUBS-MAILIST] tanks

I am taking welding. A guy in the class ask me why I was taking welding and I told him of my interest in submarines. He works in the oil fields and says he can get me any size tank I need for a good price if not free. My question is this. There is no telling what will have been in the tanks but I am sure it is some sort of hydrocarbon. Can these tanks be cleaned sufficiently (at a reasonable cost and amount of labor) or should I say thanks but no thanks. Anyone? Thanks, Ken