Today Cor-ten is one of the most famous trademarks among all atmosphere-resistant steels. The article describes the technical characteristics and peculiarities of application of Cor-ten atmosphere-resistant steel in construction.
Steels resistant to atmospheric corrosion, comprise normal structural steels alloyed with copper and phosphorus. Such steel creates a protective oxide layer, which slows the corrosion process in the conditions, when steel can freely become wet and dry in the open air. Earlier such steel grades were used mainly in industrial structures. However lately their popularity has considerably grown in facade solutions and landscape design, architects consider the Cor-ten colour to be very attractive. Apart from that, Cor-ten steel is a very sustainable material as it requires no additional anti-corrosive treatment.
History of Cor-ten creation
Atmosphere-resistant steels were developed in the USA in early 20th century, when it was noticed that a steel sheet alloyed with copper, is far more resistant to atmospheric corrosion, than an ordinary carbon steel sheet. U.S. Steel held extensive research, checking the physical properties and resistance to atmospheric corrosion of a great deal of metal sheets having various chemical composition, as the result of which Cor-ten steel was developed and patented in 1933.
Originally Cor-ten steel was used as a material for hopper structures in coal railway cars, because it is more resistant to mechanical loads and corrosion, than ordinary carbon steel.
Later Cor-ten steel gained wide usage in steel structures, where ordinary carbon steel would undergo premature corrosion due to joint influence of weather, water, and admixtures appearing during industrial production.
Today Cor-ten is used on transport containers, bridge structures, and various process equipment in chemical and petrochemical industry. Cor-ten steel is often used in power transmission line poles, lamp-posts or loading and unloading equipment, truck chassis structures, water tanks, chimneys, and construction industry.
Depending on application, Cor-ten steel can be used as it is or be painted. In case of correct usage, unprocessed Cor-ten steel quickly forms a dense and rigid oxide layer, which prevents from rust progressing. On the other hand, painted Cor-ten structures have more lengthy intervals of additional painting in the course of maintenance, because the rigid oxide layer appears on any damaged painted areas, however corrosion cannot progress under the paint, as with ordinary carbon steel.
Carbon steel is a highly active metal, compared to, for instance, stainless steel and copper. For this reason, any moisture or atmospheric oxygen unpainted carbon steel is exposed to, quickly causes oxidation and formation of ferric hydroxide. This process is usually called corrosion. As soon as the steel surface moistens multiple times, it becomes rusty, which may considerably worsen the steel structure properties. Cor-ten also oxidizes in the process of contact with air and moisture. However the oxidation mechanism in Cor-ten steel is different from rusting of structural steel. After multiple wetting and drying of Cor-ten steel, a dense and very rigid oxide layer is created on its surface. This layer prevents from corrosion development in normal weather conditions, therefore Cor-ten is called atmosphere-resistant steel.
After Cor-ten became patented, over 30,000 tests were held to optimize the chemical composition of steel and achievement of the best weather-resistance indicators. Depending on grade, Cor-ten may contain up to 10 alloying elements. Chrome, nickel, copper, and phosphorus improve steel resistance against atmosphere corrosion. Silicium, titan, molybdenum and vanadium increase the oxide layer density even more through interaction with copper and chrome. Today a series of steel grades exists, united with the overall patented name Cor-ten.
Chemical composition of Cor-ten steel, %
|Cor-Ten High temp||0,12||0,25-0,75||0,20-0,50||0,07-0,15||0,03||0,02-0,06||0,75-1,25||0,25-0,55||0,40||0,02|
The process of corrosion on the steel surface can be described with the following electrochemical reactions. When steel becomes moisturized, small local corrosion sites appear. Iron oxidation occurs in the anode point (-), while reduction – in the cathode point (+):
Fe -> Fe2 +2e (1)
½O2 + H2O + 2e -> 2OH (2)
The overall iron oxidation reaction comprises a sum of sub-reactions:
Fe + ½O2 + H2O -> Fe (OH)2 (3)
Ferric hydroxide further oxidates into ferric oxyhydroxide, which is considered to be the most widespread rust condition:
2Fe (OH)2 + ½O2 -> 2FeO(OH) + H2O (4)
In practice, the wet and dry phases alternate each other on the steel surface. During the wet phase, rust is reduced into ferric oxide (II, III), magnetite (5), and during the dry phase, magnetite is reduced into ferric oxyhydroxide, gothite, lepidocrocite (6):
Fe2 + 8FeO (OH) + 2e -> 3Fe3O4 + 4H2O (5)
2Fe3O4 + 3H2O + ½O2 -> 6FeO(OH) (6)
Reduction conditions prevailing on the steel surface, occur when the rust pores are filled with water. Oxidation takes place, when the rust external surface becomes dry. This means that rust created by atmospheric conditions, consists of several states of iron, mainly magnetite Fe3O4 and ferric oxyhydroxide FeO(OH).
Atmospheric corrosion is also influenced by air admixtures, including sulphur dioxide and sulphur trioxide. When the air temperature is below the acid dew point, oxides are dissolved in the condensed water, thus producing sulphuric acid, H2SO4:
SO2 + H2O + ½ O2 -> H2SO4 (7)
SO3 + H2O -> H2SO4 (8)
Ferric oxide (rust) acts as catalyst for transformation of SO2 into sulphuric acid. Sulphuric acid reacts with steel, forming ferric sulphate (FeSO4), which can additionally oxidate to ferric sulphate Fe2(SO4)3 and ferric oxyhydroxide FeO(OH):
H2SO4 + Fe + ½O2 -> FeSO4 + H2O (9)
12 FeSO4 + 2H2O + 3O2 -> 4Fe2(SO4)3 + 4FeO(OH) (10)
Ferric sulphate can be additionally hydrolyzed into ferric oxide and sulphuric acid:
Fe2(SO4) + 4H2O -> 2FeO(OH) + 3H2SO4 (11)
In theory, sulphuric acid can appear again in oxidation reactions. This is also confirmed by the fact that in presence of acid-forming oxides, steel corrosion occurs relatively fast. Such conditions are the most probable in an industrial or urban environment.
Numerous corrosion resistance tests held throughout more than three decades, have shown that resistance of Cor-ten against atmosphere corrosion is far higher than that of ordinary carbon steels. Cor-ten demonstrates especially high corrosion resistance in industrial environment, where corrosion speed comprises only one-fifth of that of carbon steel, or even less. The considerable difference in corrosion speed is the most visible after 5-10 years of operation, when a dense and very rigid oxide layer appears on the surface of steel, protecting it from corrosion. Corrosion speed largely depends on the conditions of the environment. The following corrosion speeds calculated based on the change in samples’ weight after ten years of operation, can be used as reference values:
|Steel grade||Corrosion speed, nm/10 years|
|Cor-Ten А||20 – 30|
|Cor-Ten B||75 – 100|
|Carbon steel||150 – 200|
The oxide layer colour varies from reddish brown to dark violet, depending on the ambient conditions. Cor-ten steel colour changes from velvet bronze to saturated brown.
Cor-ten steel may be painted with any paint designed for steel anti-corrosion treatment. Research has shown that the durability of alkyd paint coating on Cor-ten steel is 1.5-2 times more than that of carbon steel. The figure below (left) shows the external appearance of painted Cor-ten and carbon steel samples after test use within 15 years in marine climate. In the right part of the figure, painted samples exposed to industrial environment within several years, are shown. Cor-ten steel does not show features of rust under the paint coat consisting of zinc-chromate primer and vinyl paint.
The longer life of paint coat on Cor-ten steel is explained by the fact that when scratches appear, a dense oxide layer is formed on the coat surface, which does not let rust penetrate under the paint, as it happens with ordinary carbon steel.
One has to be very careful with Cor-ten steel products It is necessary to avoid surface damage, and splashes from welding, like other surface pollution, have to be removed. Cor-ten steel can be stored in the open air, provided that free air circulation is maintained between sheets, so that the surfaces which can become wet, could get dry quickly. In case of long-term open-air storage of products, they have to be covered, while sheets and other components have to be separated from each other, to ensure even air circulation between them. This guarantees even patination and absence of corrosion stains.
Due to creation of a homogeneous protective layer (patina) on the surface, Cor-ten steel can be left unpainted. For formation of even patina, when external appearance is of decisive value, it is necessary to perform thorough preliminary cleaning of surfaces – etching, sanding or polishing. In this case, lubricant and protective oil stains, slag, and scale have to be eliminated at first. After sanding, surface is considered to be clean enough, if its roughness is even.
If it is necessary to reduce the timing of patina formation, Cor-ten steel can be exposed to accelerated oxidation, which is performed in several stages:
The process of accelerated patination of Cor-ten allows obtaining the desired steel shade within only several days.
If it is required to fix the colour and stop the natural patination process, Cor-ten steel can be covered with glossy or matt protective varnish. In this case, one should note that the useful life of such varnish comprises only several years, thus its layer needs to be subsequently renewed from time to time.
Weld pollution such as scale and splashes, slows the patination process. The ISO 8501-1 standard determines the degree of rust for assessing surface purity. The degree of cleaning of ordinary carbon steel to be painted, shall comprise at least Sa 2 1/2, St 2, while for patination of Cor-ten steel, Sa 2 / St 2 is the sufficient degree of cleaning.
All ordinary welding methods can be applied for Cor-ten steel welding: arc welding with metal melting electrode or flux electrode, submerged arc welding, MIG/MAG welding, and resistance welding.
To ensure welded joint resistance to atmospheric impacts, it has to contain the same alloy as the main metal.
The most frequently used weld metal alloys contain nickel and copper. Non-alloyed weld metal can be also used, when a weld joint has the form of a gully or rounding and when, under submerged arc welding, the process of diffusion ensures high-quality alloying of the weld material with the main metal. Atmosphere-resistant weld metal has to be used in case of multi-run welds on surface areas of atmosphere-resistant steels.
Recommended welded materials for COR-TEN A and COR-TEN B steel
Welding in active gas environment (integral wire)
OK Autrod + gas
Elga-Matic + gas
Lincoln + gas
|OY UDDEHOLM AB|
Böhler Welding + gas
|13.26 + M21, CO2||140 + M21, CO2||LNM 28 + M21||Union Patinax + M21|
Arc welding with flux core
|ESAB OK Tubrod with steel core + gas||ESAB OK Tubrod with flux core + gas||ESAB Filarc with steel core + gas||ESAB Filarc with flux core + gas||ELGA with flux core + gas||LINCOLN ELECTRIC with flux core + gas||LINCOLN ELECTRIC self-screening wire||RETCO OY Trimark with flux core + gas||IMPOMET OY Oerlikon with flux core + gas|
|14.04 + M21||15.17 + M21||PZ 6104 + M21||PZ 6112 + M21, CO2||DW588 + CO2||OS 81 Ni 1-H + M21||IS NR 203 Ni 1||TM-81 W + M21||Fluxofil 18 + M21|
Arc welding with coated electrodes
|ESAB ordinary electrodes||ESAB Filarc|
ESAB Filarc high-efficiency electrodes
|LINCOLN ELECTRIC high-efficiency electrodes||OY UDDEHOLM AB|
|73.08||35Z||C75||P62 MR / P48 K||KRYO 1||KRYO 1-180||Böhler Welding FOX NiCuCr||Oerlikon|
COMET J 50 C
Submerged arc welding
wire + flux
wire + flux
|OY UDDEHOLM AB |
wire + flux
wire + flux
|OK Autrod + OK Flux|
13.36 + 10.71
|Lincoln + Lincoln|
LNS 163 + FX P 230
|Böhler Welding + Böhler Welding|
Union Patinax + UV 420 TT
|Oerlikon + Oerlikon|
FC 48 + OP 121TT
Bolt connections of atmosphere-resistant steel shall be rigid enough to prevent gap corrosion inside the joint and corrosion of connected materials. The distance between bolts along the joint edge shall not be more than 14 times the thickness of the thinnest connected element and shall not exceed 20 cm. The distance between a bolt and the joint edge shall not be more than 8 times the thickness of the thinnest connected element and shall not exceed 15 cm.
Cor-ten X steel is recommended as the material for bolts, because its corrosion resistance equals to that of connected steels, while the patinated colour of bolts would be identical to the main material. Zinc or cadmium-coated bolts can also be used, however usually they are not recommended, because such coating would wear off relatively fast as the result of electrochemical reactions between bolt coating and Cor-ten steel. Small fixing elements, screws etc. can be made of Cor-Ten A steel or non-oxidative metals, such as brass or bronze. In such a case, electrochemical corrosion would not appear, because the noble metal area will be far smaller than the Cor-ten steel area.
Cor-ten steel as a unique material is excellently suitable for application in conditions when atmosphere resistance or expressive external appearance are of prime importance. The dense, self-renewing oxide patina on Cor-ten steel ensures long life and unique design of structures. Cor-ten steel is excellent for facade cladding or erection of building frames, architectural compositions, fences, construction of bridges, containers, tanks, chimneys etc.
Cor-ten steel is relatively resistant to abrasive wear caused by loading and unloading. However long-term abrasive wear of the material can reduce its life.
Excellent corrosion resistance and strength of Cor-ten steel are also preserved under high working temperatures. In this case, Cor-ten ensures better properties, for instance, higher scale resistance, than ordinary structural steels. Cor-ten steel can be used in less critical conditions for high-pressure vessels, when usage of hardened Cr-Mo steels is not required. The maximum working temperature of Cor-ten A steel comprises +540°C. Under higher temperatures, the flowability and tear strength, as well as the impact hardness of this steel grade become considerably worse. Cor-ten B steel grade is not recommended for application in bearing structures, if the working temperature exceeds +425°C.
The excellent property of paint on Cor-ten steel allows substantial cost-saving on operation, because the painting intervals for Cor-ten maintenance are longer than those for ordinary carbon steel.
This is achieved due to the ability of Cor-ten steel to create a dense and rigid oxide layer above any paint damage sites, thus preventing steel corrosion under the finish paint layer. For instance, according to a major container lease company, the costs for maintenance and repair of painted carbon steel containers have become four times higher than those of Cor-ten steel containers.
When selecting Cor-ten as a cladding material, it is necessary to consider that as the result of patination, the water contacting Cor-ten steel, will become rusty within the first two years. The ‘rusty’ water will need to be collected and drained so that it does not stain other materials used. The following materials can be easily cleaned in case of weak staining by “rusty” water:
Certain materials are easily coloured, therefore their cleaning is difficult or impossible. For this reason, it is important to carefully consider advisability of using the following materials together with Cor-ten steel:
When using Cor-ten with other materials, it is necessary to ensure absence of gap corrosion or water or dirt accumulation in metal connection locations.
Galvanized steel sheets or other galvanized materials must not be in direct contact with non-painted Cor-ten steel, because zinc as a more active metal, would be exposed to electrochemical corrosion.
Butt joints between various materials and Cor-ten steel have to be sealed. Special attention has to be paid to the fact that many sealers, such as polyurethane foams and other substances containing fire-retardant agents, absorb water. Therefore usage of such substances together with Cor-ten steel can lead to serious corrosion damage.
Cor-ten steel is a natural material, which gets its colour and protective surface layer as the result of oxidation. Changes on the surface of Cor-ten due to light, air humidity, and time, make it an even more interesting material. Patina gives Cor-ten steel an expressive bronze shade, which ideally complements brick surfaces.
Cor-ten is characterized with matt surface, which reduces reflections typical for other metals, and hides the possible surface unevenness.
Cor-ten is a cost-effective material and is only 20-25% more expensive than ordinary carbon steel. As far as Cor-ten does not require surface finishing, the overall cost of Cor-ten products is extremely attractive.
Apart from that, Cor-ten is sustainable, as it is fully re-usable and does not require coating. Therefore its overall environmental impact is extremely low throughout the whole useful life.
Cor-ten steel creates on its surface a rigid oxide layer, which prevents from corrosion. Local climatic conditions, as well as duration of patination period affect the surface colour. The colour changes from orange-brown to reddish brown and, finally, dark brown.
The ability to change colours in the course of time makes Cor-ten steel one of the most unique and memorable materials.
Prior to delivery to a construction site, it is recommended to keep steel for patination, when its surface becomes of homogeneous colour without rusty stains.
After the desired shade is achieved, it is possible to stop the change in Cor-ten colour with the help of surface chemical treatment or varnish application. In this case, it is necessary to note that after varnishing, the surface can lose its natural matt appearance.
Cor-ten ventilated facades can also be assembled prior to patination process. Tight construction schedules often make this the only possible option, because materials are selected on such a late stage of the project, that there remains no time for patination in advance.
When using non-patinated Cor-ten facades, one has to be ready for the initial stained surface and rusty appearance of the building.
Cor-ten is a material of active nature. One should note that the patination process makes it thinner, thus it is desirable to avoid applying this material with less than 0.5 m thickness.
In this case, to ensure high quality of profiles manufacturing, highly precise dimensions are of extreme importance.
Cyclic surface moisturizing and drying is an obligatory requirement for the patination process. If the surface remains moisturized for a long time, it will become rusty. Horizontal surfaces of structures, as well as surfaces located too close to each other, are the most vulnerable parts. At the beginning of the process, patinated water flowing down on the structure, can accumulate on horizontal surfaces. In this case, rust contained in water, will keep the surface moisturized, and the corrosion process will not stop. If two surfaces are located too close to each other, moisture remains between the materials, which may cause gap corrosion.
Therefore, when designing facades of Cor-ten steel, it is necessary to ensure controlled water drainage, as well as sufficient ventilation of structures. Water must in no way linger on the surface. The ventilation gap has to be sufficiently wide, at least 30 mm. All abutments to plinth, uneven surfaces, as well as adjacent structures (balconies, canopies etc.) shall be arranged so as to leave the ventilation gap open. Sloped long battens on abutments must not let water flowing on the facade structure, penetrate to the surfaces of other materials. Water from the cornice and roof must be drained in a centralized way to the extent possible, preferably through a hidden water drainage system, to prevent from staining caused by flow water.
In the course of patination, it is necessary to avoid arranging wide overhanging structures, because after facade installation, patination is slower in the shadow.
Electrochemical incompatibility as well as peculiarities related to flow water, are the factors, which limit the choice of materials to be used together with Cor-ten steel. In this case, stainless steel is the recommended material of fixings. If various materials are combined, joints between them always have to be thoroughly isolated. The safest choice for adjacent materials is materials with smooth and hard surface, as well as electrochemically compatible materials and materials not stained with the colour of water flowing from Cor-ten steel.
In case of application of Cor-ten steel facade with external glazing or in absence of the possibility of cyclic moisturizing and drying, preliminary patination is recommended.
Preliminary patination allows getting a material with aesthetically finished appearance, as well as avoiding most difficulties related to water flow from Cor-ten structures onto the adjacent materials.
If it is impossible to organize preliminary patination, Cor-ten steel has to be thoroughly cleaned from scale, oil, and other impurities, and after installation Cor-ten should be washed to enable even commencement of patination.
The environmental value of Cor-ten steel will be stressed even more in future, because the environmental classification criteria will become stricter. Certain coatings may be fully forbidden as construction materials, thus causing problems when surface treatment requires renewal or maintenance. This risk does not apply to Cor-ten. As far as Cor-ten does not require coating, such steel can be remelted and reused.
Special properties of Cor-ten steel can be used in long life structures, which require almost no maintenance. However, it is necessary to take account of the special requirements to this steel in design. Attention should be paid to the structure of joints and units to ease the Cor-ten patination process and to avoid electrochemical and gap corrosion. It is also important to correctly select a particular Cor-ten steel grade, most suitable for the application under consideration, because there exist substantial differences between various steel grades.
It is necessary to avoid horizontal surfaces during design and installation, because peeling oxidation products, especially from the back side of steel sheets, are easily accumulated in bended locations, where they keep the moisture, which can lead to uncontrolled corrosion. If it appears impossible to avoid horizontal surfaces, efficient water drainage needs to be provided, for instance, with the help of drainage openings. Connection units also need to be designed so as not to form surfaces, which can collect water.
The main peculiarity of elements’ connection is thorough prevention of gap and electrochemical corrosion. A gap at least 1 mm wide between the connected panels prevents from gap corrosion caused by capillarity. In case of narrower gap, capillarity can draw water into it, thus causing corrosion. The easiest way to prevent formation of electrochemical couples lies in usage of distance plates between parts of connected elements.
If Cor-ten steel structures are used under facade cladding, sufficient ventilation needs to be ensured for patination process based on cyclic moisturizing and drying of steel. Ventilated gap has to be arranged along the whole length of the facade and be at least 30 mm wide.
The nature of steel destruction is an important factor in bearing structures, especially in case of their exterior usage. Sufficient impact hardness of the material ensures ductile fracture rather than brittle fracture.
Usage of phosphorus as an alloying element, enhances the steel weather resistance, but high impact hardness cannot be achieved, if phosphorus content exceeds 0.025% or sulphur content is over 0.020%.
Phosphorus content in all Cor-ten steel grades, except for Cor-ten B-D, comprises 0.07-0.15%. For this reason, Cor-ten B-D meets the minimal impact hardness requirements as per EN 10025.
The designer determines the steel impact hardness (quality class) in accordance with Eurocode ENV 1993-1-1:1992, Annex C. D (J2) grade steel is normally used in exterior bearing structures. Factors facilitating brittle fracture, include low temperatures and shock load.
Impact hardness of welded connections in external bearing structures must be confirmed.
The yield limit of Cor-ten steels does not exceed 350 N/mm2, thus special measures are not required to ensure welding capacity. However, if the thickness of a welded sheet exceeds 25 mm, check is needed.
The welded capacity of Cor-ten steels is virtually similar to that of structural steels of the same strength class. All ordinary methods can be used for welding Cor-ten steel (whether Cor-ten with Cor-ten or Cor-ten with other structural steels): metal arc welding using coated or flux electrode, submerged arc welding, MIG/MAG welding, and contact welding.
Recommended welding rod and electrodes (ESAB)
|Rod / Electrode||Gas / Flux|
MAG, continuous wire
|OK Autrod 13.26||M21+C02|
|MAG, metal-cored electrode||OK Tubrod 14.04*||M21|
|MAG, flux-cored electrode||OK Tubrod 15.17*||M21|
|Universal electrode||OK 73.08||–|
|High recovery electrodes||OK 73.58*||–|
|Submerged arc welding||OK Autrod 13.36||OK Flux 10.71|
* does not contain copper
Prior to welding, it is necessary to remove from the surface of a sheet the oxide coating for the width of 10-12 mm. In case continuous and point welding are used, the joint has to be filled with varnish filler.
Corrosion resistance is ensured due to usage of atmosphere-resistant base for welded metal, the composition of which is similar to that of the main metal, for instance, wire and electrodes containing copper and nickel alloy. The yield limit of welded metal normally has to be 5% higher than that of the main metal. In corner welded connections with joint leg of up to 4 mm, as well as butt weld joints less than 4 mm wide, the welded joint metal normally becomes sufficiently alloyed with the help of the main metal, thus alloying additions are not required. Multi-run welds can be partially made of carbon steel welding materials and completed from low-alloy steel electrodes having atmosphere resistance characteristics.
Atmosphere-resistant steels contain alloying elements, such as chrome, copper, and nickel, which enhances steel strength. For this reason, larger sheet thickness requires slightly more preliminary heating, compared to other structural steels. The working temperature and the need for preliminary heating is set based on the overall sheet thickness, which is determined as the overall thickness of a composite sheet. Weld joints shall be checked in accordance with EN 3834, in accordance with the requirements indicated in drawings.
Need in preliminary heating / working temperature of Cor-ten B and B-D steels
|Welding method||Composite sheet thickness, mm|
|MIG / MAG continuous wire and cored electrode||20||20||20||75||100||125|
|Covered electrode (main)||20||20||100||150||150||150|
|Submerged arc welding||20||20||100||125||125||150|
Cor-ten steel can be cut and bent similarly to ordinary structural steels. Cor-ten AF is the atmosphere-resistant steel with the best bendability. The bending radius of open profiles comprises (2-3) x sheet thickness, depending on thickness. The minimum with of a bendable workpiece comprises 50 mm. The tubular profiles’ bending radius comprises 2.5 x wall thickness. However, it is worth noting that complications can arise during tubes production, if the ratio of the diameter to the wall thickness is small (D / t < 10).
Minimum acceptable internal bending radius for Cor-ten steels, depending on steel thickness, mm
|Steel grade||Cor-ten sheet thickness, mm|
Mechanical properties of Cor-ten steel
|Steel grade||Thickness, mm||Yield limit Rel, min., N/mm2||Bending limit Rm, min., N/mm2||Linear strain Aε, min., %||Impact hardness class|
|Cor-Ten High temp||2-13||345||485||18||–||–||–|
The lower yield limit of Cor-ten steels equals to Rel = 345 N/mm2, while the resistance to rupture comprises Rm = 485 N/mm2. In this case, corrosion may considerably influence metal sheets of small thickness. In industrial environment, corrosion reduces the Cor-ten B sheet thickness approximately by 0.16 mm within ten years, while that of Cor-ten A – by 0.12 mm. This means that the strength of thin sheets is considerably reduced, especially if both sides of a sheet are exposed to corrosion.
For this reason, it is recommended to take account of the corrosion allowance of the nominal thickness of a material. Good atmosphere resistance of Cor-ten steel is ensured with the material itself, upon its alternate moisturizing and drying. If it appears impossible, the structure is in need for anti-corrosion treatment, for instance, painting.
Projected corrosion allowance of Cor-ten steel in case of exterior application
|Operation conditions||Corrosion allowance per one side per each 10 years of useful life, (mm)|
|First 10 years||Subsequent 10-year periods|
|Urban environment||0.2 1)||0.05 1)|
|Industrial environment||0,2 2)||0.1 2)|
1) Main pollutant — SO.
2) Chlorine content in air, combined with SO.
|Also in areas close to the sea.|
Certificates for materials from atmosphere-resistant steels are prepared in accordance with the requirements of EN 10204.
As a rule, Cor-ten steel does not require additional thermal processing after welding. In case of manufacturing especially responsible bearing structures from thick sheets, when thermal processing is initiated by the client, it is recommended to perform:
As a rule, acid-resistant steel is the most reliable material for fixing Cor-ten steel. AISI 304 grade stainless steel can also be used for self-tapping screws, provided that a rubber seal is used. It is possible to use fixings with coatings, which reduce friction and corrosion, for instance, Ruspert coating. The Cor-ten X steel grade was also developed for bolt connections.
In bolt connections, one should avoid gaps between the bolt and the connected element. Air-tightness of connection can be ensured through usage of a suitable seal. It is recommended to use neoprene with hardness of at least 65 units Shore scale A and rupture limit of at least 6 N/mm2. Neoprene is highly resistant to ozone, UV irradiation, chemicals, and wear. Neoprene sheets are usually available with thickness of 0.5-30 mm, as well as with possibility of cutting-to-length, and with self-adhesive coating. If it is necessary to ensure gas-tightness, butyl rubber should be used as seal.
On connections, it is necessary to use teflon tape (polytetrafluoride, PTFE).
For small fixings, such as self-tapping screws, EPDM rubber gaskets are used between the head and the washer. Sleeves can be used as a distancing element for filling the space between the sheet and the fixing element. Such sleeves are pressed from both sides of an opening drilled in advance, as well as prevent turning of a fixing element.
Distancing elements should be also used with other materials, because all metals are equally exposed to gap corrosion. Apart from that, when various metals are connected, the probability of electrochemical corrosion appears. In this case, the recommended seal thickness shall comprise at least 1.0 mm.
Types of materials to be used between Cor-ten steels
|Connected product||Cassette||Sheet (thickness > 3 mm)||Span (thickness 0.5-2.0 mm)||Self-tapping screw (A2)|
|Cassette (thickness 1-2 mm)||Neoprene or EPDM gasket||Neoprene or EPDM gasket||EPDM distancing sleeve||EPDM|
|Sheet (thickness > 3 mm)||Neoprene or EPDM gasket||Teflon, neoprene||Teflon, neoprene||Teflon, neoprene|
In case of fire, Cor-ten steel behaves the same way as ordinary structural steel. Normally Cor-ten does not require fire protection, because atmosphere-resistant steel is mainly used in exterior structures.
If Cor-ten is applied in bearing columns, the most advisable fire protection method is usage of a composite structure made of a steel tube filled with reinforced concrete. The structure dimensions have to be selected so as to exclude the need to apply any type of fire protection. Columns and beams can be used without any fire protection, if they are located at a sufficient distance from windows or are in some other way protected from heating. However, such cases often require a separate check.
It is recommended to paint Cor-ten steel, if the operation conditions of a structure may somehow obstruct natural patination, or if it remains moisturized for a long period of time. The useful life of painted Cor-ten steels is approximately twice longer than that of ordinary carbon steels.
In this chapter, the example of thin Cor-ten sheets usage in facade structures is shown – Baltic Square Office Building in Finland.
The main design principle is ensuring free consequential moisturization and drying process.
If the surface remains moisturized for a long period of time, corrosion will start progressing, and the material can finally rust through. Water penetrating the joint between the connected surfaces, can cause gap corrosion.
Drying of Cor-ten surfaces and minimization of painting caused by rusty water flowing from the external cladding, were ensured due to application of the following design principles:
Figures 1-3 show the solutions used for external rainscreen panel wall cladding to ensure drying of metal surfaces. Figure 4 shows solutions on application of design profiles on a ventilated facade. The double facade of Baltic Square Office Building was not glazed until partial patination of Cor-ten steel was achieved. Figure 5 shows incorrect solutions, which do not allow steel to get dry, which may result in through corrosion on a thin steel sheet.
1. Rainscreen panel of 1.5 mm thick Cor-ten A steel grade. Bends exclude emergence of horizontal surfaces.
2. Substructure of 1.0 mm thick Cor-ten A steel grade. The bearing substructure of an open joint forms a water drainage gully. Junctures of gullies are sealed, for instance, with a butyl sealer, and water is freely drained in the lower part of a wall structure.
3A. AISI 316 fixing screw (for instance, Spedec Sx 3/10 — S16 — 5.5×28) + chloroprene rubber washer.
3B. AISI 316 fixing screw (for instance, Spedec Sx 3/15 — S16 — 5.5×38) + chloroprene rubber washer.
4. Sleeve under the screw (for instance, Teknikum 378 720, black). In case of its absence, free ventilation stops.
5. If required, butyl tape is used as a distancing layer between the rail and the bearing substructure.
6. Substructure (for instance, wind protection plasterboard sheet).
7. Top flashing of 1.0 mm thick Cor-ten steel.
8. Support elements from a steel sheet with Hiarc coating and butyl tape (RR 32 colour, dark brown) are installed on joints under the top flashing.
9. The dark colour of the plinth hides stains from rusty water flowing. Water drainage has to be arranged in an organized way.
10. Attic flashing of 1.0 mm thick Cor-ten steel, 1:20 slope. Attic flashing is fixed to moisture-resistant veneer with the help of self-tapping screws into vertical plane. Cornice boards are isolated with butyl tape.
11. Drip edge of Hiarc-coated steel.
12. Cor-ten steel ventilation grille.
13. Design profile of 1.0 mm thick Cor-ten A steel grade. Design profiles overlap with insulation.
14. Substructure of 1.0 mm thick Cor-ten A steel grade.
15. AISI 316 fixing screw (for instance, Spedec Sx 3/10 — S16 — 5.5×28) + chloroprene rubber washer.
16. If required, distancing butyl tape is used between the substructure and the bearing wall.
17. Substructure (for instance, wind protection plasterboard sheet).
18. Chloroprene rubber tape between the design profile and the substructure.