Weatherproof structural steel, which is particularly known under the brand name Corten steel (actually: COR-TEN steel), is an alloyed structural steel that has a significantly higher corrosion resistance under weather conditions than conventional structural steel due to certain alloy additives, in particular chromium, nickel and copper. In contrast to stainless steel, which is also corrosion-resistant, Corten steel forms a continuous layer of rust on all surfaces. The steel is protected from destruction by this rust layer, but above all by a barrier layer that forms within the first three years below the rust layer. The barrier layer consists of sulphates or phosphates that are formed under the influence of the weather and the alloy additives. After completion of the superficial corrosion process, the natural colouring is rusty-earth-coloured.
Image: Cortenstahl Documentation Center of the SS Memorial - Hinzert Special Camp - Norbert Miguletz
The strengths of weatherproof structural steel lie in its unique rough "archaic" appearance and in its durability with low maintenance/repair costs, provided that possible risks of increased corrosion (see below) can be largely excluded. In building construction, it is mainly used as a cladding material for rear-ventilated façades, usually in the form of prefabricated cassettes.
However, if weatherproof structural steel is exposed to permanent moisture, e.g. by backwater or insufficient rear ventilation, it corrodes in a similar way to unprotected unalloyed structural steel, i.e. until it completely dissolves. Elements made of weatherproof structural steel must therefore always be installed at a distance from adjacent components, i.e. with only punctual contact, otherwise the permanent moisture at the connection/overlap points would lead to corrosion damage. This decoupling is important for the load-bearing substructure, but also for adjacent panel areas in a cladding.
Electrolytic interactions with other metals (e.g. fastening screws, parapet sheeting, window sheets) can also cause corrosion damage to components made of weatherproof structural steel, as can acid corrosion, e.g. due to animal urine or excrement, fingerprints on regularly touched components, or acid rain, e.g. in industrial areas.
The rust plumes that occur as a result of weathering, especially in the first few years below constructions or cladding with weatherproof structural steel, must be taken into account in building planning in such a way that no disturbing effects arise, e.g. through the arrangement of gutters or an earth-coloured gravel/slab covering below the façades.
Source: bauwion
