I just went outside and looked at the pipe it clearly says sch 40 on the pipe. But I didn't notice it before and it says it not intended for high pressure. The pressure is pretty strong from that gravity system I built.
But if you are saying that anything that is not sch 40 it will rot you are 100 percent right. It will crumble like egg shells after some time.
I used sched 40 for lawn irrigation systems and it will sustain high pressure. Sched 20 is crap and is okay for drain systems underground.