Weathering steel is usually referred to by its trademark name COR-TEN  

Known for its superior corrosion resistance and thickness. This is because the steel forms a protective layer on its surface under the influence of the weather. This protective surface continues to regenerate over years of exposure, making it ideal for outdoor use. Over time, the steel forms a stable rust-like appearance. This iconic weathered look and rusty orange colouring make it popularly used in outdoor applications