At first, the steel begins to rust in the same fashion as any other carbon steel, when exposed to wet and dry weather conditions. However, as time passes, the rust layer transforms into a dense patina layer, sealing-off the steel surface, resisting further corrosion. The color changes from fresh, newly developed orange-brown, to light and eventually dark brown.
The speed of change and the final color of the steel depend on the atmospheric conditions of a site and the surrounding air quality. Sulfur dense atmospheres will speed up the oxidation process and result in a darker patina. Frequent cycles of wet and dry weather conditions will also speed up oxidation.