Corten Steel has a chemical composition in which the rust forms a protective coating that protects the part from atmospheric corrosion.
The degree of exposure has an important effect on the formation of the protective coat. When placed in a site that is exposed to the rain, sun and wind, the rust layer will form more quickly than if it is protected. In the latter case, the rust will be rougher, less dense and less even.
Frequent changes in humidity and drying are necessary, to accelerate the weathering process. Atmospheric contamination also plays a part, and for this reason, in industrial environments the tones are darker. In rural environments, the process is slower and the rust takes on a lighter tone.