We have a water heater that's covered with rust, that's literally crumbling with rust. Rivers of rust. There are even holes in the side. Yet, no pipe has sprung a leak. No plumbing shows any sign of ever having dripped onto this tank, nor caused this rust.

What, then? We can only surmise. Sometimes the automatic vent dampers malfunction, stopping in the mostly closed position, causing hot gases to condense inside the shell and rusting it from the inside out. Yet, this damper was in the open position.

But the water heater was in a swimming pool room. That's a nasty place to put a water heater. Why? Because that's where the water for the pool is chlorinated, so you have extremely corrosive fumes in the air, and a water heater has to breathe to combust gas and heat water. Then, too, often there is diatomaceous earth, used for filtering, scattered around the floor, which can be sucked into the burner, clogging it and causing the tank to soot up.

We don't know, but we think this was a case of corrosive fumes.