Cor-Ten (also known as weathering steel) is a trademark registered by the American company United States Steel Corporation which identified a particular metallic material developed in the 1930s. Its name derives from the physical properties of this type of steel: corrosion resistance (COR) and tensile strength or tenacity, (TEN).
Cor-Ten steel is mainly an iron and carbon alloy which includes, in small quantities and less than 5% of the total metallurgical composition, metals such as copper, chromium, nickel, phosphorus, silicon and manganese. The characteristics of strength and corrosion resistance, which make Cor-Ten such a popular choice. All possible thanks to its structural composition and the particular distribution and concentration of the alloy elements added to steel during the production process. These act to reduce the carbon content and give a greater load capacity, and above all assigning an exceptional resistance to corrosion.
Cor-Ten metal is designed to reach a stable oxidation point, in which any further alteration process does not affect the quality of the steel, although it changes its colour over time.
However, it is difficult to predict the final colour of the weathered Cor-Ten cladding precisely because of its extremely reactive surface. Indeed, there is no guarantee that the colouring will be homogeneous over time.
The colour evolution of the coating created on the panels varies and depends mainly on external environmental factors, such as:
- Local weather conditions: average temperature, orientation of prevailing winds, humidity
- Location: industrial, urban, maritime or countryside climate
- Orientation of the facade: exposure to the sun or shade
- Corrosive character of the atmosphere, i.e. concentration of sulphur dioxide, SO2 and chlorides
To obtain an efficient anti-rust cladding, that is to have a homogeneous and uniformly distributed self-protective coat, the metal surface requires an alternating cycle of wet and dry conditions.
Rainwater and humidity activate the oxidation process. As it dries, the oxide coat begins to release water particles, forming an adherent and compact layer with low permeability. The amount of oxide produced following exposure to the elements tends to decrease over time and, even if the process does not almost completely stop, the coating could eventually lose its corrosion protection capacity.
Hence, when using Cor-Ten, especially in environments with highly corrosive atmospheric characteristics, (industrial areas or seaside locations), it is necessary to take into account the loss of material caused by corrosion and, if necessary, compensate for these factors by increasing the thickness of the sheet based on durability requirements.