This, I believe, is Corten steel. It is a steel alloy that turns this reddish/orange from weathering. The rust appearance actually strengthens the material. It becomes this color after a few years of exposure.

Just to add some more information about cor-ten, by dad just sent this to me:

It's what we call a high strength, low alloy steel. It can be had in all shapes ( plate, bar and tube ). One of the things it's alloyed with is copper so it will rust on the outside but the rust forms a barrier that preserves the steel underneath. It will never need painting and during the summer it is perfectly colored for it's backdrop. Cor-ten is used a lot for bridges and in ship building.

Anniekendall, Thanks so much for all the information!

No problem. If you haven't already, make sure you check out John Hill's ideabook on cor-ten steel.

https://www.houzz.com/magazine/interesting-industrial-materials-cor-ten-steel-stsetivw-vs~358346