The aspect of weathering steels changes over time. It evolves from its dark grey mill finish to an orange patina in just a few weeks. The patina continues to evolve, reaching its final dark brown colour after a few years – the exact time depends on local weather conditions. Due to its brownish, non-uniform patina, and rough texture, weathering steels fit perfectly into both urban and natural environments. It is impossible to predict the final colour of weathering steel because of its extremely reactive surface. The evolution of the patina is strongly influenced by the complex chemical relationship between the weathering steels surface, exposure to the sun, orientation to prevailing winds, and atmosphere. The patina’s appearance also depends on time, the average temperature, and on exposure to moisture. Appearance is also influenced by the concentration of sulfur dioxide (SO2) and chlorides. For example, in an industrial environment the patina tends to develop a darker colour compared to weathering steels used in a rural area. Surfaces directly exposed to the weather will have a finer grain than sheltered surfaces. It is essential that the patina develops at a steady pace to ensure effective corrosion protection. For example, in exposed marine environments the patina develops faster than in a rural environment. However, it does not adhere as well to the steel substrate and may not protect the steel from corrosion. The best results are obtained where the steel is exposed to a succession of wet/dry cycles, and has no permanent contact with stagnant water.
Weathering steels are suitable for use in most European locations. However, certain environments can limit the durability of weathering steels. Major problems can occur in environments with constant humidity, an aggressive atmosphere, or high levels of atmospheric pollution.