Below are links to more information about weathering steel and the data behind the protective rust layer that develops:
Weathering Steel: ArcelorMittal USA
Formation of Corrosion Protective Rust on Steel Exposed to the Atmosphere
Atmospheric Corrosion Data of Weathering Steels (Review)
RTD Fact Sheet – Weathering Steel
Nanostructure of Protective Rust Layer on Weathering Steel Examined