Steel will not age, weaken, and embrittle, leaving 55 gals of oil dumped out in the floor. Ask my Uncle how he knows about that. BUT on the flip side I went and picked up 110 gals of oil from a guy the other day and his metal drums were just a few months from doing the very same thing but because they were sitting directly on the ground for 10 years. One of them had already dumped about 5 or so gallons out til it had created enough suction to stop leaking from the rust. It dropped another gallon or so out in my bed before I got it pumped out. To heat and process I use metal because it doesn't require a 1000 dollar machine to weld it. On top of being heat resistant. I just lay my heater element in the bottom of the drums. I'm currently building a water separator from 30 gal plastic totes. And will also build a receiving system from plastic cubies and totes.