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So what is Corten steel?

Corten, a material originally developed in the 1930’s by the United States Steel Corporation, was once primarily used for railway coal wagons. This new material was in high demand thanks to its  inherent toughness and ability to withstand weather conditions, which are possible due to the controlled corrosion that makes Corten unique.

The features of robustness and improved resistance to corrosion that make Corten such a popular choice for specification is possible thanks to the careful manipulation of the alloying elements added to the steel during the production process, and works by reducing the carbon content of the material to therefore give it a higher capacity for carrying load.

One of the most beneficial properties of Corten is it’s high level of weather resistance. This works by the material being able to control the rate at which oxygen in the atmosphere reacts with the surface of the metal, which slows down the progress of corrosion and makes the material self-protecting. This not only have obvious benefits in regard to longevity, strength and protection against the elements, but is also economical in that it reduces the need for surface treatment costs.

The visual appearance of Corten is also something that sets it, and your design, apart from others; it is designed to reach a stable oxidation point where any further oxidation doesn’t affect the quality of the steel, and this allows the material to weather to a natural brown colour which then darkens gradually.

Barbados National Heroes manufacturing the shackles
Barbados National Heroes shackles installation
Barbados National Heroes shackles in Monument Square

The technical bit...

If normal low alloy steel is exposed to moisture and air then rust forms on the surface, this rust is non-coherent and falls away allowing the layers below to rust until eventual failure occurs. With Cor-ten steel the surface oxidises in the normal way but due to its metallurgical formulation it forms a coherent patina that adheres to the surface and reduces further
corrosion.

The patina begins its formation with a reddish brown hue and slowly darkens. In normal weather conditions the patina will take 18 to 36 months to stabilise.

During this patina formation process, any run-off water may contain soluble iron salts which can cause discolouration of adjacent materials especially if they are slightly porous.

The layers develop and regenerate continuously when subjected to weathering influences. The rate of formation and protective effect of the covering layer depend largely upon the corrosive nature of the atmosphere and the likelihood and type of aggressive agents in the air should be taken into account. Due to the constant regeneration of the surface layer, when incorporating Corten in its unprotected condition, designers must also take account of the anticipated loss of thickness due to corrosion. As far as necessary, compensation must be made for this by increasing the thickness of the original material.

Uneven patina formation can be caused by surface contamination, handling marks and welding during fabrication.

Corten surface stages over time
Corten surface stages over time
Corten surface stages over time

Corten Steel is a weathering steel facade that naturally rusts over time...

It is a copper chromium alloy steel - strong, durable and sustainable, this alloy displays a greater level of resistance to atmospheric weathering when compared to other unalloyed steels.

Corten is used for many external applications, either on it's own, or to compliment more traditional materials. Used as a more sustainable solution, Corten will last longer than alternatives, and the material evolves organically in its environment.

Specification Suitability

Corten is not advised for public realm application in coastal and harsh-weather environments or for areas which will remain permanently damp. The build up of the patina relies on the surface being regularly dried and so careful design must be employed allowing good ventilation and uniform exposure to the weather. Areas in contact with the ground, such as a lighting column root, must be protected with a barrier such as a glass flake paint.

If Cor-ten is specified for a planter then the inner surface will be continually exposed to damp soil. This will not generally cause any structural issues and the run-off water will pass through the soil or bedding material in the planter. However, care should be taken to provide for adequate drainage from planters and to ensure that this water cannot flow over easily stained surfaces.

Cor-ten has also been used for public seating. During the patina formation process, staining to clothing could occur if the seat is not completely dry. Paint finishes are available which closely mimic the surface of Cor-ten and these may be more suitable for metal likely to be in close contact with clothing.

corten screen facade for an outdoor bar

Protection

The protective layer should offer sufficient protection against atmospheric corrosion in most urban conditions but, where particularly aggressive air pollution exists, surface protection is recommended. When installed in a maritime location, in regular contact with water or permanently in a damp environment (for instance light columns installed below ground
level), a secondary coating is essential.

Protection can be achieved using the coating of the raw COR-TEN steel with a product such as Owatrol Oil, but we find in our climate that the best results come from the raw finish.

Using an oil stabilises the rust and seals the surface, however it does change the appearance of the surface from a flat non reflective finish to that similar to burnished antique leather.

Speeding up the finishing process

The protective layer should offer sufficient protection against atmospheric corrosion in most urban conditions but, where particularly aggressive air pollution exists, surface protection is recommended. When installed in a maritime location, in regular contact with water or permanently in a damp environment (for instance light columns installed below ground
level), a secondary coating is essential.

Protection can be achieved using the coating of the raw COR-TEN steel with a product such as Owatrol Oil, but we find in our climate that the best results come from the raw finish.

Using an oil stabilises the rust and seals the surface, however it does change the appearance of the surface from a flat non reflective finish to that similar to burnished antique leather.

What is the lifetime of Corten?

There is no definitive answer to this question. It depends on the environment and the way in which it was installed. Many projects using bare cold rolled and Corten products were installed in excess of forty years ago and there are no problems. Corten will have a longer life-span than bare cold rolled steel. One of the most common problems is using a light gauge steel. Using a heavier gauge steel is the most important thing you can do to increase the longevity of Corten steel.

Where does the organic orange colour come from?

The orange-brown finished colour in A606-4 comes mainly from the copper content. With 5% copper in the alloy mix, the copper immediately comes to the top as the patina process begins. Additionally, the copper along with manganese, silicon and nickel content in A606-4 creates that protective layer as the material continues to patina. Standard carbon steel will rust but it will not have the beautiful colors that come from A606-4.

COR-TEN steel gets its properties from a careful manipulation of the alloying elements added to steels during the production process. It has a combination of Cr (chromium), Cu (copper), Si (silicon), and P (phosphorus), the amounts depending on the properties required. COR-TEN steel works by controlling the rate at which O2 (oxygen) in the atmosphere can react with the surface of the steel. The rusting of a steel takes place in the presence of air and water,
resulting in the product of corrosion which is the iron oxide.

Non-weather resisting steels have a relatively porous oxide layer, which can hold moisture and promote further corrosion. After a certain time (dependent on conditions), this rust layer delaminates from the surface of the steel, exposing the surface and causing more damage.

Types of Corten Steels

Two types of weathering steel are normally produced. These are sometimes referred to as COR-TEN A and COR-TEN B. The types differ primarily in the amounts of P alloyed in the composition. COR-TEN A type is typically produced as sheet or coil and has applications in cladding and ductwork. The COR-TEN B type is more commonly produced as plate, structural sections, or tubes.

Forming
COR-TEN steel can be cold formed in the same way as the general structural steels of the corresponding grades. Successful forming requires good workshop technology from the producer of the steel product. Worn tools, insufficient lubrication, surface defects on plates, and cutting burrs may all reduce the quality. Shot blasting can also be unfavourable.

For strip rolled products, bending with the axis transverse to the major rolling direction is preferred. The suggested minimum bending radii for bending longitudinal to the rolling direction are given in exacting forming operations are easier to carry out either by warm forming at a temperature under 600 deg C, or by hot forming at 800 deg C to 1050 deg C.

Welding of COR-TEN steels
A prerequisite for obtaining identical mechanical properties in the weld and in the base material is the application of suitable welding consumables and the choice of appropriate welding conditions. The welding of structural COR-TEN steels is similar to that of conventional structural steels, but COR-TEN such steels generally have higher carbon equivalent (CE) values which can increase the likelihood of hydrogen-induced cracking of the welds which need to be considered when defining preheat levels. One aspect to consider on welded connections is that all joints, including fillet welds, are to be continuously welded to avoid moisture and corrosion traps such as crevices.

COR-TEN steels can be welded under workshop conditions using all the common welding processes. Low H2 welding procedures and consumables are recommended. Before welding, the patina is to be removed, down to the bare steel, from the steel surface over a band of around 10 mm to 20 mm wide along the welded joint. It is also equally important to remove any moisture, grease, oil, and other impurities from the surface.

Cutting
COR-TEN steels can be cut thermally and mechanically in almost the same manner as the plain C structural steels. When flame cutting of thick plates then the working temperature used for the welding can be used as a guideline. Due to thin plate thickness COR-TEN A and corresponding steels do not normally need an elevated working temperature for thermal cutting. We can cut stunning patterns on our lasercutter, both decorative and technical details.

Advantages of Corten Steels

CORTEN steel has atmospheric corrosion resistance and this enables it to be used without paint for many structural / architectural applications, which include structures like bridges, some open-frame buildings, transmission poles and sculptures. CORTEN steel also has high temperature advantages which make it a good choice of material for many flues, chimneys and high temperature ducting. The corrosion resistance of CORTEN steel gives it major advantages over other metals for structures which are exposed to the outside environment. These are given below.

Low maintenance – CORTEN steel is ideal for bridges and other structures where access is difficult or dangerous, and where future disruption needs to be minimized. Inspection and cleaning are to be the only maintenance required to ensure the structure continues to perform well.

Start up cost benefits – The saving of not needing to use any protective coating / paints compensates for the incremental material cost of the CORTEN steel. As an example, weathering steels cost is around 5 % lower than conventional painted steel alternatives in bridges.

Project life cost benefits – Nominal maintenance needs of the CORTEN steel structures significantly reduce the costs of maintenance operations and the potential indirect costs of traffic delays in case of bridges.

Construction Speed – Since CORTEN steels do not need paint both on site as well as in the fabrication shop, the construction activity gets streamlined.

Aesthetic appeal – The attractive appearance of mature CORTEN steel often blends pleasingly with the environment. Its appearance changes and improves with age.

Environmental benefits – Use of CORTEN steel eliminates the need for blast cleaning and VOC laden paints.

High temperature benefits – Steel can suffer oxidation at temperatures above 400 deg C. This can be decreased by using CORTEN steels. At temperatures above 400 deg C CORTEN steels form a protective patina. A typical improvement is to be an increase of 50 deg C over equivalent loss in cc-Mn steels. CORTEN steels are not suitable for use in significant load bearing members above 450 deg C.

Corten large garden planter
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