How does corrugated galvanized pipe work for a dam drain ?

Galvanized pipe eventually rusts through and has to be replaced. If you are looking for longevity use schedule 40 pvc.

Is this used as a spillway drain? I used 2 24” corrugated black plastic pipe for mine and it’s holding up

Double wall corrugated pipe (wavy on the outside - smooth on the inside) gets my vote for simple through-the-dam drains. PVC is great too for ponds that don't need large diameter drain pipes.

I'd give corrugated galvanized pipe 30 to 50 years depending on the amount of moisture is sees (inside and out).