Black pipe used for sprinkler system dry is rotting inside out

Then they need replaced. The system I deal with is a preaction system that have the pipes charged with compressed air until needed and flood the pipes with water awaiting heat to trigger the sprinkler heads. Pipes won't rot out.

You used black iron pipe to supply the sprinklers? WHY? Replace with PVC. Good luck, Tom

That would be normal for black iron pipe. That is typically used for gas lines, not buried.

Good point, are we talking fire sprinklers or lawn sprinklers? I was assuming fire sprinklers which do use black iron pipe. But they never sit with water in them at least not after the large diameter risers. At least in commerical applications.

Wet AS systems have water in them, dry and deluge don't. I used to test them all the time.