"Corten" or "weathering steel"
Corten was a US Steel trade name for self-weathering steel -- a steel which is designed to lightly rust, but for the rust layer to be relatively corrosion resistant, as opposed to normal rust which is powdery, non-adherent, and hydroscopic (attracting moisture, worsening the problem).
If you see lightly rusty guardrails along the highway, or lightly rusted shipping containers, most of this is Corten weathering steel.
