Oxidation process
The oxidation process of the steel surface that starts with the help of weather and wind. This process makes the corrosion of the steel to stop over time. The rust layer that forms thus becomes protective of the Corten steel and adheres to the metal surface which prevents moisture from penetrating. The insulating patina resists corrosion, requires no painting or weather insulation. It does not compromise structural strength.
Early in the process, COR-TEN acquires a light reddish-brown color. The longer the steel is exposed to weather and wind, the darker shade of brown is achieved. The above retaining wall shows the color of Corten steel after about 3 years of outdoor use.
The oxidation process of Corten steel becomes noticeable already after 6 months of outdoor use. It takes about 2 years for the steel to achieve an evenly rusty surface. That unless you speed up the process by, for example, brushing on salt water. If you want to speed up the process, a good mixture is about 2-4 tablespoons of common table salt per liter of water.