Sorry Plasmech,
Every pressure vessel code in the world has developed a set of rules which allow safe and lasting use of the pressure vessels. If the train carriage made of Corten plate has developed a hole on the side, too bad, they'll patch it up. A vessel can be fabricated of Corten, but nobody will guarantee you that it will perform like the "approved" materials, you won't be able to use it on a plant with safety regulations. Would you stay next to it during the hydrotest after being told the vessel cannot be guaranteed to retain pressure? Would you use it on your plant, hoping that it won't hapen today, maybe not even tomorrow...Just look up to the ASME VIII history, takes only 5 minutes reading..
cheers,
gr2vessels