Corten, also referred to as weathering steel, is a group of steel alloys which were developed to eliminate the need for painting. Instead, the surface forms a stable rust-like appearance when  exposed to weather. Weathering steel is quite popular due to its rustic look, and is commonly used in outdoor applications including sculptures, building facades and other types of architectural applications.
 
Note: weathering steel is sold in its original state. The rustic appearance occurs over time when exposed to the elements.