Corrosion resistant steel - Weather resistant steel grade

Weathering steel, often referred to by the genericized trademark COR-TEN steel and sometimes written without the hyphen as corten steel, is a group of steelalloys which were developed to eliminate the need for painting, and form a stable rust-like appearance after several years exposure to weather.

U.S. Steel holds the registered trademark on the name COR-TEN. The name COR-TEN refers to the two distinguishing properties of this type of steel: corrosion resistance and tensile strength. Although USS sold its discrete plate business to International Steel Group (now Arcelor-Mittal) in 2003, it still sells COR-TEN branded material in strip-mill plate and sheet forms.

The original COR-TEN received the standard designation A242 (COR-TEN A) from the ASTM International standards group. Newer ASTM grades are A588 (COR-TEN B) and A606 for thin sheet. All alloys are in common production and use.

Properties

Weathering refers to the chemical composition of these steels, allowing them to exhibit increased resistance to atmospheric corrosion compared to other steels. This is because the steel 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 particular distribution and concentration of alloying elements in it. The layer protecting the surface develops and regenerates continuously when subjected to the influence of the weather. In other words, the steel is allowed to rust in order to form the protective coating.

Chemical composition of Weathering steel grades (%, besides iron)

Grade

C

Si

Mn

P

S

Cr

Cu

V

Ni

ASTM A242

0.12

0.25–0.75

0.20–0.50

0.01–0.20

0.030

0.50–1.25

0.25–0.55

0.65

ASTM A588

0.16

0.30–0.50

0.80–1.25

0.030

0.030

0.40–0.65

0.25–0.40

0.02–0.10

0.40

The mechanical properties of weathering steels depend on which alloy and how thick the material is.

ASTM A242

The original A242 alloy has a yield strength of 50 kilopounds per square inch (340 MPa) and ultimate tensile strength of 70 ksi (480 MPa) for light-medium rolled shapes and plates up to 0.75 inches (19 mm) thick. It has yield strength of 46 ksi (320 MPa) and ultimate strength of 67 ksi (460 MPa) for medium weight rolled shapes and plates from 0.75–1 inch (19–25 mm) thick. The thickest rolled sections and plates – from 1.5–4 in (38–102 mm) thick have yield strength of 42 ksi (290 MPa) and ultimate strength of 63 ksi (430 MPa). ASTM A 242 is available in Type 1 and Type 2. Both have different applications based on the thickness. Type 1 is often used in housing structures, construction industry and freight cars. The Type 2 steel which is also called Corten B is used majorly in urban furnishing, passenger ships or cranes. 

ASTM A588

A588 has a yield strength of at least 50 ksi (340 MPa), and ultimate tensile strength of 70 ksi (480 MPa) for all rolled shapes and plate thicknesses up to 4 in (100 mm) thick. Plates from 4–5 in (102–127 mm) have yield strength at least 46 ksi (320 MPa) and ultimate tensile strength at least 67 ksi (460 MPa), and plates from 5–8 in (127–203 mm) thick have yield strength at least 42 ksi (290 MPa) and ultimate tensile strength at least 63 ksi (430 MPa).

Uses

 

Weathering steel is popularly used in outdoor sculptures, such as in the large Chicago Picasso sculpture, which stands in the plaza of the Daley Center Courthouse in Chicago, which is also constructed of the same weathering steel and as exterior facades, for its rustic antique appearance. Examples include Barclays CenterBrooklynNew York, The Angel of the NorthGateshead, UK and the Humanities and Arts complex at Leeds Metropolitan University – Broadcasting Place – Leeds, UK

It is also used in bridge and other large structural applications such as the New River Gorge Bridge, the second span of the Newburgh–Beacon Bridge (1980), and the creation of the Australian Centre for Contemporary Art (ACCA) and MONA.

It is very widely used in marine transportation, in the construction of intermodal containers[12] as well as visible sheet piling along recently widened sections of London's M25 motorway.

The first use of weathering steel for architectural applications was the John Deere World Headquarters in Moline, Illinois. The building was designed by architect Eero Saarinen, and completed in 1964. The main buildings of Odense University, designed by Knud Holscher and Jørgen Vesterholt and built 1971–1976, are clad in weathering steel, earning them the nickname Rustenborg. In 1977, Robert Indiana created a Hebrew version of the Love sculpture made from weathering steel using the four-letter word ahava (אהבה, "love" in Hebrew) for the Israel Museum Art Garden in Jerusalem, Israel. In Denmark, all masts for supporting the catenary on electrified railways are made of weathering steel for aesthetic reasons.

Weathering steel was used in 1971 for the Highliner electric cars built by the St. Louis Car Company for Illinois Central Railroad. The use of weathering steel was seen as a cost-cutting move in comparison with the contemporary railcar standard of stainless steel. A subsequent order in 1979 was built to similar specs, including weathering steel bodies, by Bombardier. The cars were painted, a standard practice for weathering steel railcars. The durability of weathering steel did not live up to expectations, with rust holes appearing in the railcars. Painting may have contributed to the problem, as painted weathering steel is no more corrosion-resistant than conventional steel, because the protective patina will not form in time to prevent corrosion over a localized area of attack such as a small paint failure. These cars have been retired by 2016.

Weathering steel was used to build the exterior of Barclays Center, made up of 12,000 pre-weathered steel panels engineered by ASI Limited & SHoP Construction. The New York Times says of the material, "While it can look suspiciously unfinished to the casual observer, it has many fans in the world of art and architecture."

Disadvantages

Using weathering steel in construction presents several challenges. Ensuring that weld-points weather at the same rate as the other materials may require special welding techniques or material. Weathering steel is not rustproof in itself. If water is allowed to accumulate in pockets, those areas will experience higher corrosion rates, so provision for drainage must be made. Weathering steel is sensitive to humid subtropical climates, and in such environments it is possible that the protective patina may not stabilize but instead continue to corrode. For example, the former Omni Coliseum, built in 1972 in Atlanta, never stopped rusting, and eventually large holes appeared in the structure. This was a major factor in the decision to demolish it just 25 years after construction. The same thing can happen in environments laden with sea salt. Hawaii's Aloha Stadium, built in 1975, is one example of this. Weathering steel's normal surface weathering can also lead to rust stains on nearby surfaces.

 

US Steel Tower

The rate at which some weathering steels form the desired patina varies strongly with the presence of atmospheric pollutants which catalyze corrosion. While the process is generally successful in large urban centers, the weathering rate is much slower in more rural environments. Uris Hall, a social sciences building on Cornell University's main campus in Ithaca, a small town in Upstate New York, did not achieve the predicted surface finish on its Bethlehem Steel Mayari-R weathering steel framing within the predicted time. Rainwater runoff from the slowly rusting steel stained the numerous large windows and increased maintenance costs. Corrosion without the formation of a protective layer apparently led to the need for emergency structural reinforcement and galvanizing in 1974, less than two years after opening.

The U.S. Steel Tower in Pittsburgh, Pennsylvania was constructed by U.S. Steel in part to showcase COR-TEN steel. The initial weathering of the material resulted in a discoloration of the surrounding city sidewalks which is known as "bleeding" or "runoff", as well as other nearby buildings. A cleanup effort was orchestrated by the corporation once weathering was complete to clean the markings. A few of the nearby sidewalks were left uncleaned, and remain a rust color. This problem has been reduced in newer formulations of weathering steel. Staining can be prevented if the structure can be designed so that water does not drain from the steel onto concrete where stains would be visible.

                                                                                                     source:    Wikipedia

 

Scope
COR-TEN A applies to plates up to 12.5mm in thickness, COR-TEN B applies to plates up to 50mm in thickness.

Definition
Weathering means that due to their chemical compositions COR-TEN A and COR-TEN B steels, when utilised unprotected, exhibits increased resistance to atmospheric corrosion compared to unalloyed 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 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 general weather condition (e.g. continental) macroclimate (e.g. industrial, urban, maritime or countryside 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 countryside climate. 

When utilising this steel in unprotected condition it is up to the designer to take into account the expected loss of thickness due to 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 recommended. 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 undercreepage by rust is less in the case of weathering steel than in the case of comparable non-weathering steel.

Applications
Corrosion resistant steel - Weather resistant steel grade 
The steel is used for various types of welded, bolted and riveted constructions e.g. steel frame structures, bridges, tanks and containers, exhaust systems, vehicles and equipment constructions. 

Basic guidles for the use of this steel in the unprotected condition are described in EN 10025-5 and DASt rule 007. 

The entire application technology is of fundamental importance for the performance of the products made from this steel. It must be taken into account that not only general climate conditions but also specific unfavourable local climate conditions in the broadcast sense as well as details of a construction may affect the corrosion behaviour of unprotected weathering steel. The dependency on these facts makes it understandable that no warranty can be given. It is recommended to control the corrosion progress of protected parts out of weathering steel exposed to the influence of weather in reasonable time intervals. A minimum thickness of 5mm is recommended when exposed to the weather in the unprotected condition. 

To use the benefits of the higher atmospheric corrosion resistance of COR-TEN in comparison to unalloyed steel it is necessary that design and execution of structures as well as the performance of maintenance works allow an impeded formation and regeneration of the protective rust layer. The methods must meet the latest requirements of technical progress and must be suited for the proposed application. Due consideration must be given to relevant construction specifications. 

The selection of the material is up to the purchaser.

Chemical Composition (heat analysis, %)

Grade

C

Si

Mn

P

S

Cr

Cu

V

Ni

COR-TEN A

0.12

0.25-0.75

0.20-0.50

0.07-0.15

0.030

0.50-1.25

0.25-0.55

 

0.65

COR-TEN B

0.16

0.30-0.50

0.80-1.25

0.030

0.030

0.40-0.65

0.25-0.40

0.02-0.10

0.40

In order to obtain fine grain structure a sufficient amount of nitrogen absorbing elements is added (e.g. ≥0.02% Al).

Mechanical Properties, in the state of delivery condition

At room temperature for plates ≥3mm in thickness (transverse test specimans, according to EN 10002). Requirements to hot rolled plates ≤3mm in thickness according to EN 10025-5.

Grade

Minimum
yield point
(ReH Mpa *)

Tensile
strength
Rm MPa

Minimum
elongation
A (Lo=5.65
√So) %

COR-TEN
A

355

470-630

20

*) 1 Mpa = 1N/mm2 

In case of cold rolled material the yield point is min. 310 Mpa and the tensile strength min. 445 MPa. Furthermore cold rolled sheets ≤3mm in thickness made of steel grade COR-TEN A-F for increased demand to the cold formability is available. Mechanical properties: Yield point min. 275 Mpa; Tensile strength min. 410 Mpa; elongation min. 25%. Tolerances on dimensions and shape according to EN 10131. 

Mechanical Properties, in the state of delivery condition

At room temperature for plates ≥3mm in thickness (transverse test specimans, according to EN 10002). Requirements to hot rolled plates ≤3mm in thickness according to EN 10025-5.

Grade

Material
thickness
mm

Minimum
yield point
(ReH Mpa *)

Tensile
strength
Rm MPa

Minimum
elongation
A (Lo=5.65
√So) %

COR-TEN
B

≤16

355

470-630

20

> 16 ≤50

345

*) 1 Mpa = 1N/mm2 

The notched-bar impact energy is determined on ISO-V longitudinal test specimans at a temperature of - 20°C as an average of three tests. For product thicknesses ≥10mm the average value is at least 27 J. For thicknesses between 10mm and 6mm, the minimum impact value is reduced proportionally to the speciman width (product thickness). 

No impact test is performed on products below 6mm in thickness. 

General Processing Information
Corrosion resistant steel - Weather resistant steel grade
The information given below can only deal with some important points.

Forming
The conditions for hot forming are in accordance with those stated in EN 10025-5. For cold forming the statements according to table 6 of EN 10025-5 are valid. If the mechanical properties have undergone changes due to cold forming, the properties indicated in the table can be substantially restores by stress relieving - at least 30 minutes at 530°C - 580°C. For higher degrees of cold forming subsequent normalising is recommended.

Flame Cutting
COR-TEN is suitable for flame cutting provided proper operating methods are used. At temperatures below 5°C a sufficiently wide zone on either side of the intended cut should be preheated. If flame cut edges are to undergo cold forming, the hardening effect should be prevented by preheating - as in the case of S355J2 or the hardened zones must be worked off e.g. by appropriate grinding.

Welding
COR-TEN can be welded both manually and mechanically, provided the general rules of welding practices are observed. 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 appropraite welding conditions. To consider are EN 10025-5 - Technical delivery conditions for structural steels with improved atmospheric corrosion resistance.

Recommendations for welding are also given in EN 1011 part 1 and part 2 - Welding, Recommendations for welding of metallic materials-. 

Lime basic electrodes, inert-gas welding wire, and wire/power combinations equivalent to the tensile strength of S355 are used as welding consumables. 

For unprotected use care must be taken that the welded joint is also weather resistant. This is possible by using welding consumables matching the base material. 

If due to design or building specification stress relieving is required, it should be performed in the range temperature from about 530°C to 580°C.

Bolting and Riveting 
Joining elements such as bolts, rivets and their accessories (nuts and washers) must be so selected that the formation of local electro-chemical cells are avoided. The joining elements should preferably consist of weathering steel. 

At these joints capillary action can lead to permanent moisture resulting in increased corrosion. Critical zones should therefore be protected by painting, sealing or other protective measures. 

In the case of high-strength connections (HV) the conditions for non-weathering structural steels as given in DIN 18800 part 1 apply.

 

Frequently Asked Questions

COR-TEN® is a trademarked name owned by U.S. Steel. 

COR-TEN® steel is becoming more popular by roll formed product end-users. Its unique look and naturally oxidizing finish make it especially desirable for many architectural projects. Weathering steel, best-known under the trademark COR-TEN® steel, is a group of steel alloys which were developed to obviate the need for painting, and form a stable rust-like appearance if exposed to the weather for several years. Weathering steel has increased resistance to atmospheric corrosion when compared to other steels. COR-TEN® resists the corrosive effects of rain, snow, ice, fog, and other meteorological conditions by forming a coating of dark brown oxidation over the metal, which inhibits deeper penetration and negates the need for painting and costly rust-prevention maintenance over the years. In simple terms the steel is allowed to rust and that rust forms a protective coating that slows the rate of future corrosion. 

CAN I BUY COR-TEN®?

No, COR-TEN® is not available. What is available is steel in an A606-4 or an A588 finish. If somebody tells you that they are supplying you COR-TEN® they are almost certainly unaware of their own product. Simply ask them if their steel was purchased from U.S. Steel. If the answer is " No " or " I don't know " it's almost certainly NOT COR-TEN®. When someone needs COR-TEN® they are normally referring to A606 Type 4 or A588. If you want COR-TEN® this will be the best available option. Weathering steels, such as ASTM A588, A242, A606 and COR-TEN® exhibit superior corrosion resistance over regular carbon steel as a result of the development of a protective oxide film on the metals surface which slows down further corrosion. A606-4 steel is available in metal roofing/siding panels, standing seam panels, flat sheets, and coil form. 

WHAT IS A606 TYPE 4?

The original COR-TEN® received the standard designation A 242 ("COR-TEN® A") from the ASTM International standards group. Newer ASTM grades are A 588 ("COR-TEN® B") and A606 for thin sheet. All alloys are in common production and use. 

CLIMATES NOT TO USE COR-TEN®:

COR-TEN® steel requires alternating wet and dry cycles to form a properly adhered protective layer. Areas that have salt laden air, high rainfall, humidity, or persistent fog are typically not the proper environment for COR-TEN®. 

DOES IT COME PRE-RUSTED?

A606-4 and A588 steel arrives UNRUSTED and will rust with naturally exposure to the weather. 

WHAT DO I HAVE TO DO TO GET COR-TEN® TO RUST?

Nothing. This material will rust on its own if left exposed to the weather. Getting the material wet will increase the speed in which it rust. 

WILL COR-TEN® CONTINUE TO RUST?

Weathering Steel never completely stops rusting. The weathering process occurs after the panel is installed. It eventually decreases and from that point on the finish acts as a protective coating for the base metal. 

This protective coating essentially stops the corrosion. Laboratory analysis of the rust film have shown that the alloying elements in the steel, particularly copper, chromium, and nickel produced insoluble compounds that clogged the pores at the rust/steel interface, thereby ending the regeneration. 

WILL THE RUST STAIN THE SURROUNDING AREAS?

Yes, rust runoff will likely stain the surrounding areas. 

IS THERE A WARRANTY?

There are no warranties for A606-4 or A588 Steel. 

DISSIMILAR METALS AND COR-TEN®:

In general do NOT use dissimilar metals with COR-TEN®. 
Galvanized, galvalume®®®®, and zinc should NOT come into contact with COR-TEN®. 
Stainless steel is compatible with COR-TEN®. 

WHAT ABOUT COLOR MATCHING THE SCREWS?

Screws are furnished in a Stainless Steel/Painted Koko Brown 

IS THERE A SPECIAL TYPE OF UNDERLAYMENT FOR USE WITH COR-TEN®?

It is recommended to use a high temperature underlayment. We do not sell underlayment. Contact an expert in underlayment for further details.