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Yes a compact protective salt film forms on carbon steel in con sulphuric acid. As long as there's no water, the film doesn't dissolve. There is a corrosion rate of ~20 mils/year though. I was just assuming a 308 stainless would be cheap enough to relieve headaches.

They probably have some sea water in there. I'd hate to be nearby if that thing dumps. There will be quite a boil over. A long time ago I worked for a company with a division that had a 2500gal carbon steel tank holding con sulfuric inside the plant and in a hallway with offices along it. Idiots ran it. It ruptured, because of poor maintainance and contamination...pits and tunnels along welded seams. ~ 1000 gals of con acid ran across the floor, into all the sales and accounting offices in addition to the plant area. no one was killed, or seriously injured. They repaired it and within one year they did it again. Needless to say, except for the idiots, they had a high turnover rate and of course product rarely went out the door.

My experience with the H2SO4 has been in alkylation units in refineries. Carbon steel is used for piping as long as the velocity is under ~1 foot per second. After that there are erosion problems, where Alloy 20 is required (consider piping elbows). Alloy 20 is also used on valve trim, although I have also used some Hastelloy C valves on oleum tank car loading. Another thing with oleum is the viscosity is up there (possibly contributing to the erosion issue), around 25 centipoise.