What about "corten" steel (what they make the "rusty" guard-rails and bridge beams out of)?
Wonder what the composition of that stuff is. Supposed to never need painting--as the rust forms a protective barrier. Sounds like the same process as what "misawite" is.
One of the major buildings in downtown Pittsburgh was made of something like this. It was born rusted but wouldn't rust further. Particles of rust flaked off and got in the eyes of pedestrians, so there were or are complaints.