Steel Corten (Cor-Ten) is a weather resistant steel that can more appropriately be described as an atmospheric corrosion resistant steel. It is a copper chromium alloy steel - this alloy exhibits a greater level of resistance to atmospheric corrosion than other unalloyed steels.
As for corten steels, the rusting process occurs in the same way, but instead results in a more stable rust layer known as a patina, which adheres to the base metal and is less porous. When exposed to the weather, this naturally-developed patina regenerates continuously and creates a protective barrier that hinders the entry of oxygen, moisture and pollutants.
Does this make corten steel rustproof? Not at all. Corten steel is left to rust in order to form a protective patina coating. While this results in a much lower corrosion rate compared to other steels, it does NOT make corten steel corrosion-proof.