Okay. I probably missed the part where you said it was buried. When the return lines run along the floor they usually corrode at the threaded fittings unless there's something else going on. In my experience they rust through from the outside in when they're partially buried. I had one that rusted through from being buried in the sand from an old concrete foundation leaking and leaching over the years.
If you ever have call to do it again go with black iron. You won't need any dielectric fittings and you can use a union or a left/right threaded coupling and nipple in lieu of the union.
R