COR-TEN steel is trademark for weathering steel, and sometimes written without the hyphen as ‘CORTEN steel or even Corten steel’. It is a group of steel alloys which were developed to eliminate the need for painting, and form stable rust like appearance if exposed to the weather for several years. United States Steel Corporation (USS) holds the registered trademark for the name COR-TEN.
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.
Typical applications for COR-TEN structural steels include (i) external wall claddings of buildings, and weather strips, (ii) chimneys and other structures under flue gas conditions, (iii) transport tanks, (iv) freight containers, (v) bridges, (vi) heat exchangers, and (vii) similar other steel structures etc.
Types of COR-TEN 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.
Weather resistance of COR-TEN steels is based on the oxide layer, i.e. patina, which forms on the surface of the steel, and which, as a result of the alloying elements, is dense and nearly impervious to O2. Under normal weather conditions the patina forms in around 18 months to 36 months, if the surface is alternately wet and dry. At first, the patina is a reddish brown colour, but with time takes on a darker hue.
In industrial environments, the patina forms more quickly and is darker in colour than in rural areas. In marine environments, the chlorides hinder the formation of the protective patina and they can prevent the intrinsic behaviour of COR-TEN steel. So, COR-TEN steel is not to be used in the vicinity of sea or in areas where there is a high amount of chlorides in the air. For open air structures, the slow corrosion rate is taken into account by adding corrosion allowance to the nominal thickness
The birth of weathering steel can be traced back to the development of steels containing copper (Cu), known as copper steels. In 1910 Buck observed that steel sheets with 0.07 % Cu produced by US Steel and exposed in three environments of different corrositivities (rural, industrial, and marine) showed a 1.5 % to 2 % greater atmospheric corrosion resistance than the plain C (carbon) steel. Hence, in 1911, US Steel started to market steel sheets with a certain Cu content.
Buck subsequently reported that the improvement achieved with Cu concentrations in excess of 0.25 % was insignificant; noting that in most cases 0.15 % of Cu in the steel provided similar results as 0.25 % of Cu in the steel. Once this capacity of Cu steel had become known, further research led to the development of weathering steel and thus to ‘high strength low alloy’ (HSLA) steels. In the 1920s, US Steel produced a new family of HSLA steels intended primarily for the railway industry. Finally, in 1933 US Steel launched the first commercial weathering steel under the brand name USS COR‐TEN steel (Early COR-TEN steel or COR‐TEN B), a name which reflects the two properties which differentiate it from plain C steel, i.e. its corrosion resistance (COR); and from a Cu steel, i.e. its superior mechanical properties (tensile strength, TEN). This product was claimed to provide a 30 % improvement on the mechanical properties of conventional plain C steel, thus reducing the necessary thickness and accordingly the weight of the steel to be used for a given set of mechanical requirements.
Fig 1 shows the evolution of corrosion with time of the plain C, Cu, and COR-TEN steels in the industrial atmosphere. It can be seen, there is the lower corrosion rate experienced by the early COR‐TEN steel. Early versions of COR‐TEN steels were based on iron-copper-chromium-phosphorus (Fe‐Cu‐Cr‐P) systems, to which Ni (nickel) was later added in order to improve corrosion resistance in marine environments. USS COR‐TEN steels presented two specifications, A and B, whose main difference lay in the amount of P present in their composition. USS COR‐TEN A steel can be said to be the weathering steel with the highest P content (0.07 % to 0.15 %) and USS Cor‐Ten B is that steel with the lowest P content (less than 0.04 %).
Fig 1 Corrosion rate of different steels
Greater knowledge of the role played by the different alloying elements (Cu, Cr, Ni, P, etc.) in the atmospheric behaviour of weathering steel was achieved due to the two ambitious studies carried out in the United States, one began in 1941 by ASTM Committee A‐5 and another began in 1942 by US Steel Co. In the first, 71 low‐alloy steels were exposed to the industrial atmosphere and to the marine atmospheres. In the second study, 270 different steels were exposed in the semi‐rural, industrial) and marine atmospheres. The current composition of USS Cor‐Ten steels has altered to a certain extent, especially in the case of specification B, with the addition of Ni ( less than 0.40 % Ni), but they all continue to be marketed to the present day. In 1941, the first weathering steel was standardized by ASTM specification A‐242 which is steel roughly comparable to USS Cor‐Ten A steel. Its main characteristic is its high resistance to atmospheric corrosion, which is around 4 times higher than that of the plain C steel due to the presence of Cu, a high P content, and in general the presence of Ni (0.50 % to 0.65 %). However, it is now somewhat obsolete as a structural steel due to the fact that P can form iron phosphide (FeP3) during the welding process, decreasing its weldability and causing the steel to become brittle.
In 1968 ASTM standard A‐242 presented two specifications, one with high P content ( less than 0.15 %) and the other with a lower phosphorus content (0.04 % maximum). The latter was ultimately replaced by ASTM standard A‐588 weathering steel (Tab 1), which is roughly comparable to USS Cor‐Ten B steel. This steel possesses less resistance to atmospheric corrosion due to its lower P content, but for this same reason it has better weldability.
|Tab 1 Chemical composition of commonly used weathering steels|
|Elements / Steel type||Unit||ASTM A-242 (COR-TEN A)||ASTM A-588 Gr. A (COR-TEN B)|
|Typical concentration||Typical concentration|
|Carbon (C)||%||0.15 maximum||0.019 maximum|
|Manganese (Mn)||%||1.0 maximum||0.8-1.25|
|Phosphorus (P)||%||0.015 maximum||0.15 maximum||0.04 maximum||0.04 maximum|
|Sulphur (S)||%||Less than 0.05||Less than 0.05|
|Copper (Cu)||%||0.2 minimum||0.25-0.4||0.25-0.4||0.3-0.4|
|Nickel (Ni)||%||0.5-0.65||0.4 maximum||0.02-0.3|
Weathering means that due to their chemical compositions COR-TEN A and COR-TEN B steels, when used unprotected, shows increased resistance to atmospheric corrosion compared to plain C steels. This is because it forms a protective layer on its surface under the influence of the weather. The corrosion retarding effect of the protective layer is produced by the nature of its structure components and the particular distribution and concentration of the alloying elements in it. The layer protecting the surface develops and regenerates continuously when subjected to the influence of the weather. Formation, duration of development and protective effect of the covering layer on weathering steels depend largely upon the corrosive character of the atmosphere. Its influence varies and depends mainly upon the general weather conditions (e.g. continental), macro-climate (e.g. industrial, urban, maritime, or rural climate) and the orientation of the structure components (e.g. exposed to or shaded from the weather, vertical or horizontal position). The amount of aggressive agents in the air has to be taken into account. In general the covering layer offers protection against atmospheric corrosion in industrial, urban and rural climate. When using this steel in unprotected condition, it is upto the designer to take into account the expected loss of thickness due to the corrosion and as far as necessary, compensate for it by increasing the thickness of the material.
In cases of particular air pollution by aggressive agents conventional surface protection is desired. Coating is absolutely necessary in cases of contact with water for long periods, when permanently exposed to moisture, or if it is to be used in the vicinity of the sea. The susceptibility of paint coats to under creepage by rust is less in the case of weathering steel than in the case of comparable non-weathering steel.
Requirements for the formation of protective rust layers
The enhanced corrosion resistance of COR-TEN steel is due to the formation of a dense and well‐adhering corrosion product layer. Experiments carried out in 1969 with low alloy steel (Cor‐Ten A) indicated that the texture of the oxide layer was dependent upon the washing action of rain water and the drying action of the sun. Surfaces sheltered from the sun and rain tended to form a loose and non‐compact oxide while surfaces openly exposed to the sun and rain produced strongly adherent layers. The surfaces of the protective layer receiving less sunlight developed oxide layer somewhat more slowly as a result.
The role of a large number of environmental and design variables in the behaviour of COR-TEN steel in architectural applications has been studied, verifying the decisive influence on the formation of the protective patina of whether or not the metallic surface was exposed to the rain, or whether or not areas where moisture was liable to accumulate were drained. These effects were more intense in atmospheres with higher pollution levels, in which case the protective patina may not fully form.
Extensive research studies have thrown light on the requisites for the protective rust layer to form. It is now well accepted that wet/dry cycling is necessary to form a dense and adherent rust layer, with rainwater washing the steel surface well, accumulated moisture draining easily, and a fast drying action (absence of very long wetness times). Structures are to be free of interstices, crevices, cavities and other places where water can collect, as corrosion can progress without the formation of a protective patina. It is also not advisable to use bare COR-TEN steel in continuously moist exposure conditions or in marine atmospheres where the protective patina does not form.
Hence, the ability of COR-TEN steels to fully develop their anticorrosive action is dependent on the climate and exposure conditions of the metallic surface. It is also to be taken into account that a truly protective oxide film can never develop on certain areas, or that their evolution can be excessively slow.
Mechanical properties of COR-TEN steels
COR-TEN A steel has the best weathering properties. Because of the P used as an alloying element, however, the impact strength of this steel grade is lower than that of CORTEN B steel. COR-TEN A steel is best suited for sheet steel structures which are not under the risk of brittle fracture.
The typical mechanical properties of COR-TEN steels are given in Tab 2. The mechanical properties are dependent on the type of the processing of steels (hot rolling or cold rolling). The cold rolled sheets of COR-TEN steels have better cold formability but have slightly different mechanical properties. The typical minimum elongation of cold rolled sheet steel is 25 %.
|Tab 2 Typical mechanical properties of COR-TEN steels|
|Steel grade||Thickness||Mechanical properties|
|Yield strength||Tensile strength||Elongation|
|mm||Newton/sq mm||Newton/sq mm||% minimum|
Conceptually, corrosion refers to the chemical or electro-chemical degeneration of construction materials under environmental impacts. Corrosion is a characteristic of each material. In the case of steel, it results into rusting of steel. The rusting behaviour of steel depends on its chemical composition, the environment, structural considerations, and an appropriate design of the construction. In order to control the corrosion behaviour, weathering steels are alloyed with small amounts of Cr, Ni, Cu, and P. Comparison of the corrosion rates of COR-TEN steel and plain C steel is shown in Fig 2.
Fig 2 Comparison of the corrosion rates of COR-TEN steels and plain C steels
The environment also has a considerable influence on the corrosion behaviour of the steels. Corrosion rates of COR-TEN steels in different environments are shown in Fig 3.
Fig 3 Corrosion rates of COR-TEN steels
When the surface of COR-TEN steel is exposed to outdoor air, the O2 and moisture in the air produce a compact oxide layer, patina, developing on the surface, thus preventing the further propagation of corrosion. Fig 4 illustrates the difference between the corrosion rates of general structural steels and COR-TEN steels. There is difference between the corrosion rates of general structural steel and COR-TEN steel. Cyclical corrosion loss represents the change in weight/ reduction of thickness which occurs when a protective oxide layer fails to develop. The corrosion cycle of steel consists of steel oxidizes – the rust layer protects – wears off – the steel surface oxidizes again, etc. In COR-TEN steel, the initial corrosion rate remains high until the protective oxide layer prevents further oxidation. The actual corrosion loss is low.
Fig 4 Comparison of corrosion rates of COR-TEN steel and structural steel
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.
The CE values are slightly higher in COR-TEN steels than in plain C structural steels, which increase the preheating requirement correspondingly. In practice this difference applies only to COR-TEN B and the corresponding steels because, thanks to their lower material thickness, steel grades alloyed with P do not normally need an elevated working temperature. When welding these steels, it is desired that for plates with thickness higher than 15 mm, the working temperature is to be increased to 100 deg C to 200 deg C. In multi-pass welding, the temperature between different passes is not to exceed 200 deg C in order that the toughness of the heat affected zone (HAZ) remains good.
The choice of the welding consumables depends on several aspects as described here. The weather resistance of welded joints can be ensured by using filler materials corresponding to the alloying of the base material. The mechanical properties of the welded joint have to be at least equivalent to those of the base material. Unnecessary over strength is to be avoided as an increase in strength also increases residual stress. The impact strength of the welded joint is to meet the set requirements, which are normally the same as those for the normal structural steels. If the base and filler materials mix sufficiently to provide good weathering resistance, ordinary non-alloyed consumables can be used. Sufficient mixing is achieved in single run welding of less than 4 mm thick plates for butt joints, and for fillet welds with a design throat thickness of upto around 4 mm. Generally there is a small colour difference between a non-alloyed consumable material used in a weld and the weathering steel base material. In the multi run welding of thick plates at least the final runs are to be made using weathering consumables if the weld metal is also intended to be weather resistant. Welding consumables of sufficient deformation capacity are to be used for the sealing and root runs. Low H2 consumables are to be used, stored and dried in accordance with the instructions of the supplier.
COR-TEN steel applications do not normally require post weld heat treatment. In case, it is to be carried out due to any reason then usually stress relieving or normalizing is carried out.
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. Slowing down the cutting speed and increasing the working temperature have a similar effect on cutting i.e. the cooling rate of the cutting point decreases and so does the risk of thermal cracks.
Advantages of COR-TEN Steels
COR-TEN 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. COR-TEN 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 COR-TEN steel gives it major advantages over other metals for structures which are exposed to the outside environment. These are given below.
Low maintenance – COR-TEN 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 COR-TEN 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 COR-TEN steel structures significantly reduce the costs of maintenance operations and the potential indirect costs of traffic delays in case of bridges.
Construction Speed – Since COR-TEN 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 COR-TEN steel often blends pleasingly with the environment. Its appearance changes and improves with age.
Environmental benefits – Use of COR-TEN 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 COR-TEN steels. At temperatures above 400 deg C COR-TEN steels form a protective patina. A typical improvement is to be an increase of 50 deg C over equivalent loss in cc-Mn steels. COR-TEN steels are not suitable for use in significant load bearing members above 450 deg C.