Others' comments are on the money- this is not an NEC problem, but a violation of the ASME Boiler & Pressure vessel code (and you thought the NEC was expensive!) From an engineering viewpoint, the cadweld process is hot enough to affect the temper/strength of the metal. The depth of the weld also introduces a stress riser/weak spot in the tank wall. It is also located in possibly the most vulnerable spot of the tank. Last but not least, if the tank was filled with anything -even water- at the time, the cadweld isn't even electrically sound. Another fine Darwin candidate.
