Rust is the self-protecting process whereby steel forms a layer of oxidization over itself. It is a beneficial process, less costly than painting.
Andrew Muzi, owner of Yellow Jersey in Madison, Wisconsin, about once a year publishes a photo of a bike he sold forty or so years ago which is entirely encrusted in rust, not a speck of paint remaining. It works very well and the owner rides it daily. (And, to address another current topic, who would steal a fully, completely, thoroughly rust-decorated steel bike?)
Andre Jute