When weathering steel is exposed to the ambient atmosphere it develops an initial layer of iron oxide in the same way as carbon steel. The rate of oxidisation depends on how much oxygen, moisture, and atmospheric contaminants can access the surface of the metal. In the initial stages, a complex mix of iron oxides covers the surface to create a layer of rust. As the process progresses, the rust layer forms a barrier against the corrosive agents and the rate of corrosion slows. On a low alloy carbon steel, the iron oxide layer is porous. Over time that layer detaches from the surface of the metal and the corrosion process starts again. The oxidation rate progresses in increments which depend on the chemical and mechanical aggressiveness of the environment. It can end with the complete destruction of the metal. To increase the resistance of weathering steels to corrosion, alloying elements such as copper, phosphorous, nickel, or chromium are incorporated. These alloys create an oxide layer which remains stable and adheres to the metal’s surface. A rust ‘patina’ develops as the weathering steel is exposed to alternate wetting and drying cycles. As well as being aesthetically pleasing, the patina creates a protective barrier which impedes the access of oxygen, moisture, and pollutants to the metal’s surface. This results in a much lower corrosion rate than that of untreated structural steels. Influence of alloying elements The different alloying elements added to weathering steels influence the properties of the steel in the following ways: • Copper increases the adherence, compactness, and elasticity of the patina. • Phosphorous acts as a catalyst for copper and increases the initial reactivity of the weathering steels. It leads to evenly spread corrosion without spots, giving the patina a more homogeneous look. It also accelerates the healing process if the oxide layer is accidentally damaged • Silicon has a positive effect on corrosion resistance • Chromium and nickel help to form insoluble basic sulphates which reduce the porosity of the oxide layer, ensuring the underlying metal surface is protected against water and oxygen • Chromium, nickel and silicon boost the mechanical characteristics of the steel substrate Alternate wet and dry cycles are required to form the patina. Moisture helps to create the oxide layer. As it dries, the oxide layer starts to dehydrate, resulting in a compact adherent layer with low-permeability: the protective patina. During patina formation, some of the oxides are washed out by rain. This is important, especially during the first two to six years as the oxide layer stabilises. The amount of oxide leached out by the rain diminishes over time, but never stops completely. This can stain neighbouring materials. Careful design of the structure is needed to ensure that the brownish rainwater is collected and directed away from other materials to eliminate staining.