Weatherproof structural steel
As weathering steel , a group of is structural steels referred to which is less by the alloying components of chromium, copper, nickel or a weather-resistant phosphorus patina forming.
Definition and classification
In terms of chemical composition, the weatherproof steels currently available on the market are similar to structural steels, which is why these are included in EN 10025-5 “Hot-rolled products made of structural steel”. The designation of the weatherproof structural steels, which, due to their low level of contamination with undesirable elements, such as sulfur, belongs to the stainless steels according to EN 10020 , is made there analogously to the unalloyed structural steels. The last letter is a W for weatherproof or WP for phosphorus-alloyed weatherproof steels: S235J2W is consequently a weatherproof structural steel with 235 N / mm² yield strength . In Germany, however, according to the building regulations list A, only those steels that are not phosphorus alloyed are approved by the building authorities. The phosphorus-alloyed steels are basically also suitable for welding, but special precautionary measures must be observed with them.
history
Weatherproof steel was first patented in 1926 by Vereinigte Stahlwerke AG in Düsseldorf, produced in the Dortmund plant and sold worldwide under the name Union-Stahl. During and immediately after the Second World War, construction with this structural steel was not pursued because the alloying elements copper and chromium were valuable and only available in insufficient quantities. The material was rediscovered in the USA at the beginning of the 1960s and was used again under the trade name "COR-TEN steel" and from there returned to the European market.
See main article: COR-TEN steel
After initial euphoria, its use in Europe came to a standstill in the early 1980s. In the first constructions, the limits and structural features of the material were not sufficiently taken into account. As a result, significant signs of corrosion were observed that were not known from - usually - much drier North America. Necessary material thickness surcharges due to corrosion were not sufficiently taken into account. In addition, doubts had arisen about the fatigue strength of the weatherproof steels, which practically brought the use of the material to a standstill in load-bearing parts, especially in steel bridge construction. These concerns have turned out to be unjustified, so that the material has been increasingly used again since the 1990s, taking into account its special features.
Use of weatherproof structural steels and economy
Weatherproof structural steels are used in all areas of building construction, in industry and especially in steel bridge construction. In building construction, the use as facade elements such as the outer cladding of buildings is often in the foreground. The rust-brown patina is often perceived as aesthetic and is often used as a prominent architectural feature of a building. A similar motivation can be assumed among artists, since a large number of art objects are made from weatherproof steels.
In the case of bridge structures, masts or industrial buildings such as tanks, the economic aspect is in the foreground. Weatherproof steels require no or only partial corrosion protection. As economic feasibility studies show, there is a clear cost advantage even during manufacture, despite the higher material value. The higher material costs arise from the higher basic material price and higher input masses, which are caused by the necessary material thickness surcharges. If the follow-up costs due to the corrosion protection obtained are taken into account, according to the state of the art 2009 economic benefits in the double-digit percentage range can result.
Corrosion mechanism in unalloyed steels
Unalloyed steels form rust within a short time under the influence of weather . Chemically, rust is the water-containing oxide of iron, which is formed by oxidation with the oxygen in the air while water is wetted. The influence of sulphurous or other acids, for example as a result of air pollution, accelerates rust formation.
See main article: Rust
With an increase in mass and a considerable increase in volume, rust forms a loose cover layer that flakes off the surface due to its internal stresses. The surface underneath is then exposed to the weather without any protection. Steel structures must therefore be protected from further corrosion with external protection.
Corrosion mechanism in weatherproof structural steels
With the help of sulfur oxides, a barrier layer of firmly adhering sulfates or phosphates forms under weathering. However, this layer is criss-crossed by micro-cracks. These micro-cracks remain electrochemically passive as long as the surface can dry again and again. However, with permanent moisture they become chemically active and the defects increase. Therefore, the alternation of moisture and dryness of the components is of the utmost importance. A permanent barrier layer only forms if the component is not permanently exposed to moisture. Even on surfaces that are not directly exposed to weathering, a passive layer forms due to the humidity and condensation on the surface. This is less dense, but the corrosion exposure is also lower due to the lack of weathering. However, here too the corrosion resistance depends on adequate ventilation so that no permanent moistening of the surfaces can occur.
Contrary to what was assumed in the first years of use, the corrosion does not come to a complete standstill, especially in the relatively humid Central and Northern European climate. Depending on the climate and the pollutant load in the atmosphere as well as the structural design of components, corrosion rates can be considerable in some cases, with corrosion being greatest in the first 10 years. This corrosion must be taken into account constructively by adding wall thicknesses. The information sheet MB434 from the Stahl-Informations-Zentrum classifies the corrosion exposure into a total of five corrosion categories C1 to C5. The assignment in
- C1: no significant corrosion exposure
- C5: very high corrosion exposure: the use of weatherproof structural steels is not recommended
In a predominantly humid climate, such as in Central Europe, with low to medium pollution of the air by sulfur oxides and chlorides, classification in corrosion category C4 with an assumed service life of 100 years results in a corrosion rate of around 1 mm per weathered side. The use of these steels near the sea is only recommended from a minimum distance of 1 km, as a classification in class C5 would be necessary due to the moisture and chloride pollution.
Design and manufacture with weatherproof steels
In principle, weatherproof structural steels behave in terms of their processing technology like general structural steels. The EN 1993 (formerly in Germany DIN 18800-1) Steel Structures "design and construction" and EN 1090-2 (formerly in Germany DIN 18800-7) "execution and qualification" apply without exception to weathering structural steels. In the static design of the construction, the wall thickness allowances must be taken into account due to the corrosion. Especially in the case of thin-walled components, such as facade elements, this can also lead to a considerable increase in the weight of the component and in turn to reinforcement of fastening elements.
Constructive measures
The component must be designed in such a way that no water pockets can form. These would hinder drying due to the presence of water. Hollow profiles are to be constructed in such a way that any water that may penetrate can run off and the hollow body is well ventilated. Alternatively, a complete exclusion of air would have to be ensured, since no corrosive attack can occur here due to the absence of moisture. When standing in water, for example in hydraulic structures, weatherproof steels are not corrosion-resistant. If components are partially in water or in a damp environment, such as in the ground, additional corrosion protection must be provided on the surfaces that come into contact with moisture.
welding
It is recommended to remove the patina in the immediate welding area in order to counteract the risk of hot cracks caused by copper and chrome on the surface. Of course, the weld metal must also be made of weatherproof steel. However, since weatherproof and non-weatherproof structural steels can be welded without any problems, it is sufficient for multi-layer welds to weld only the outer layers that are accessible to weathering with the more expensive, weather-resistant material. The phosphorus-alloyed structural steels are not permitted in the building inspection area. Because of their tendency to become brittle, they could only be safely welded with additional technical welding measures.
Screw jacks
The distances between screws at screw joints must be kept small. It is recommended to choose the distances no smaller than seven times the drill hole diameter or no more than 14 times the sheet thickness. The reason for this is the possible rusting underneath the screw joints, as air or water could penetrate through the gaps between the joint plates. Since weatherproof structural steels can be coated with all common methods, corrosion protection of the contact points is recommended, unless this is already prescribed , as is the case with non-slip connections . Welded joints do not have these disadvantages and are therefore preferable. Weatherproof steel can be used for the screws. However, these are difficult to obtain on the market. Therefore, commercially available screws made of structural steel can also be used, provided that they are provided with corrosion protection. If there is direct wetting, the use of galvanized screws that are not additionally coated is not recommended. As with unalloyed structural steels, contact corrosion would result in the zinc being eroded and then rusting.
Construction in architectural building construction
Since the steel rusts permanently - especially in the first few years - it should be noted during the construction that rust-contaminated water can drip off without undesirable contamination of the structure, for example on painted surfaces, aluminum profiles or on window surfaces. Therefore, a targeted drainage of the water must be ensured. Buildings made of weatherproof steel are often surrounded by gravel beds or the like so that the rust streaks that run off the facade can be removed in a targeted and aesthetically justifiable manner. In order to achieve a homogeneous appearance through a uniform corrosion rate, it is recommended to remove impurities from the surface such as the mill skin, for example by sandblasting the surface once .
Optics and types of cladding
Architectural cladding can be constructed in three main ways: invisibly and visibly attached panels or as canted hook-in cassettes. The local corrosion exposure must be taken into account with an assumed service life as well as a dilation-free fastening of all elements with and with one another. Avoid water capillaries. An acid-free, artificially created rust patina within three to four hours is possible on request. With a coating system based on several layers for indoor + outdoor, you can seal or freeze this rusty surface.
Individual evidence
- ↑ Steel Information Center (PDF; 5.7 MB): Leaflet MB434, Weatherproof Structural Steel, Section 2.1.1: European Standard, page 6 uf
- ↑ Execution of steel structures - explanations on DIN 18800-7, Bär, L., Schmidt, H., Schulte, U., Zwätz, R., Beuth-Verlag, ISBN 978-3-410-15919-3
- ↑ Steel Information Center (PDF; 5.7 MB): Leaflet MB434, Weatherproof Structural Steel, Section 6: Economic Efficiency, page 31 uf, page 34, table 8
- ↑ Steel Information Center (PDF; 5.7 MB): Leaflet MB434, Weatherproof Structural Steel, Section 2.2: Surface Layer Formation, page 7 uf
- ↑ Steel Information Center (PDF; 5.7 MB): Leaflet MB434, Weatherproof Structural Steel, Section 2.4.2: Classification page 9 uf
- ↑ Steel Information Center (PDF; 5.7 MB): Leaflet MB434, Weatherproof structural steel, Section 3.2.2 Welding, page 12 uf
- ↑ Steel Information Center (PDF; 5.7 MB): Leaflet MB434, Weatherproof Structural Steel, Section 3.2.3 Bolts and Section 3.2.4 Connection Technology, page 13 uf
- ↑ Page no longer available , search in web archives: HSR Hochschule für Technik Rapperswil : Landscape architecture department, scripts: Material report - Corten and other weatherproof structural steels - Annalina Wegelin, Ursina Büchel
- ↑ Steel Information Center (PDF; 5.7 MB): Leaflet MB434, Weatherproof Structural Steel, Section 5: Inspection and Maintenance, page 30 uf
Web links
- WECOBIS * : Research project of the Bavarian Chamber of Architects on behalf of the Federal Ministry for Transport, Building and Urban Development (BMVBS)