The rust layers formed on most ordinary structural steels are porous and detach from the metal surface after a certain time, and the corrosion cycle commences again. Hence, the rusting rate progresses as a series of incremental curves approximating to a straight line, the slope of which depends on the aggressiveness of the environment.
The color changes during the oxidation process: from an initial orange color to a brown-reddish as the patina stabilizes. Under normal weather conditions, the oxidation process is completed in 18-36 months, with a thickness decrease of 0.05 mm.
Research has shown repeated wet-dry cycling is essential to form the optimum dense and adherent rust layer, with rainwater washing the steel surface well, accumulated moisture draining easily, and a fast drying action.
In industrial areas, the patina appears in a shorter time and with a darker tone; this is due to the pollutant agents and the lower presence of oxygen in comparison with the rural areas.