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.