Phoenix Handcraft It's hard to tell from the photo- oxidized corten is very similar to mild steel. The main difference (besides cost) is that corten will only patina to a certain level then stop, while mild steel will continue to oxidize till it rusts out- not a problem if the water running off the wall goes into a flower bed filled with plants that like a high-iron content soil. My guess is this is corten because there isn't any visible staining on the patio which the water running off of mild steel would cause.