Looking for an authentic, weathered outdoor metal planter? Start with this bare steel planter and let nature slowly do the rest. Corten steel was originally developed for use in railroad cars. It is considered “weathering steel” and is made of a group of alloys that together have exceptional tensile strength and corrosion-resistant properties. When exposed to the weather, the steel forms a layer of rust on its surface that serves as a protective coating. This controlled corrosion creates the rustic, antiquated appearance for which Corten steel has best become known.