The corrosion you'd get all depends on what the water itself has dissolved in it and what stray currents you have running through the piping. Last water heater I replaced, the new one had dielectric couplings built in, was a Sears. Rigid copper runs from tank to iron in the wall.
The thing with Sears is you can usually get parts, that gas valve SHOULD be replaceable, I can remember replacing my dad's whole heater when I was a kid, ran about $79, this one was $379. This was slightly fancier than that one, has a pushbutton piezo ignitor, but basically the same type of unit.
The one he replaced that old one with was one of those condensing units with fans and a digital controller, no chimney needed, just a small hose for a vent. Ran several grand installed, wasn't significantly better at heating water than the oldtimer, lots more noisy and took a lot longer firing up due to all the fans having to run in sequence before lighting off. You can run a pilot light for years on the cost difference.
Stan