Corten is a structural steel. This makes it an extremely long lasting material when using it as a planter. The slow rate at which corten oxidizes gives it a unique beauty that will change over time, and thrive in any weather condition. If you are planning for a large Corten Steel Planter installation, it is important to know the rust runoff will bleed onto the surface where you place them. Steel rusts, it is what steel does, and itis what makes it a beautiful material.