
Weathering steel, often referred to by the
genericised trademark COR-TEN steel and sometimes written without the hyphen as corten steel, is a group of
steel alloy
Alloy steel is steel that is alloyed with a variety of elements in amounts between 1.0% and 50% by weight, typically to improve its mechanical properties.
Types
Alloy steels divide into two groups: low and high alloy. The boundary between the ...
s that form a stable external layer of
rust
Rust is an iron oxide, a usually reddish-brown oxide formed by the reaction of iron and oxygen in the catalytic presence of water or air moisture. Rust consists of hydrous iron(III) oxides (Fe2O3·nH2O) and iron(III) oxide-hydroxide (FeO(OH) ...
that eliminates the need for painting.
U.S. Steel (USS) holds the
registered trademark
A trademark (also written trade mark or trade-mark) is a form of intellectual property that consists of a word, phrase, symbol, design, or a combination that identifies a product or service from a particular source and distinguishes it from ot ...
on the name COR-TEN. The name COR-TEN refers to the two distinguishing properties of this type of
steel
Steel is an alloy of iron and carbon that demonstrates improved mechanical properties compared to the pure form of iron. Due to steel's high Young's modulus, elastic modulus, Yield (engineering), yield strength, Fracture, fracture strength a ...
:
corrosion
Corrosion is a natural process that converts a refined metal into a more chemically stable oxide. It is the gradual deterioration of materials (usually a metal) by chemical or electrochemical reaction with their environment. Corrosion engine ...
resistance and
tensile strength
Ultimate tensile strength (also called UTS, tensile strength, TS, ultimate strength or F_\text in notation) is the maximum stress that a material can withstand while being stretched or pulled before breaking. In brittle materials, the ultimate ...
. Although USS sold its discrete plate business to
International Steel Group
International Steel Group Inc. (ISG), headquartered in Cleveland, Ohio, was one the largest integrated steel producers in North America. It had annual raw steel production capability of 23 million net tons. In April 2005, the company was acquired ...
(now
ArcelorMittal
ArcelorMittal S.A. is a Luxembourg-based multinational steel manufacturing corporation, headquartered in Luxembourg City. It is ranked second on the list of steel producers behind Baowu, and had an annual crude steel production of 58 millio ...
) in 2003, it makes COR-TEN branded material in
strip mill
The strip mill was a major innovation in steelmaking, with the first being erected at Ashland, Kentucky in 1923. This provided a continuous process, cutting out the need to pass the plates over the rolls and to double them, as in a pack mill. At t ...
plate and sheet forms.
The original COR-TEN received the standard designation A242 (COR-TEN A) from the
ASTM International
ASTM International, formerly known as American Society for Testing and Materials, is a standards organization that develops and publishes voluntary consensus technical international standards for a wide range of materials, products, systems and s ...
standards group. Newer ASTM grades are A588 (COR-TEN B) and A606 for thin sheet. All of the alloys are in common production and use.
The surface oxidation generally takes six months to develop, although surface treatments can accelerate this to as little as one hour.
History
The history of weathering steels began in the US in 1910s, when steels alloyed with different amounts of copper were exposed to the elements; the research continued into the 1920s and it was discovered that phosphorus content also helps with the corrosion resistance.
In 1933 the
United States Steel Corporation
The United States Steel Corporation is an American steel company based in Pittsburgh, Pennsylvania. It maintains production facilities at several additional locations in the U.S. and Central Europe.
The company produces and sells steel products, ...
decided to commercialize the results of their studies and patented a steel with exceptional mechanical resistance, primarily for use in railroad
hopper car
A hopper car () or hopper wagon () is a type of railroad freight car that has opening doors or gates on the underside or on the sides to discharge its cargo. They are used to transport loose solid bulk commodities such as coal, ore, grain, a ...
s, for the handling of heavy
bulk loads including coal, metal
ore
Ore is natural rock or sediment that contains one or more valuable minerals, typically including metals, concentrated above background levels, and that is economically viable to mine and process. The grade of ore refers to the concentration ...
s, other mineral products and
grain
A grain is a small, hard, dry fruit (caryopsis) – with or without an attached husk, hull layer – harvested for human or animal consumption. A grain crop is a grain-producing plant. The two main types of commercial grain crops are cereals and ...
. The controlled corrosion for which this material is now best known was a welcome benefit discovered soon after, prompting USS to apply the trademarked name Cor-Ten. Because of its inherent toughness, this steel is still used extensively for bulk transport,
intermodal shipping containers and bulk storage.
Railroad passenger cars were also being built with Cor-Ten, albeit painted, by
Pullman-Standard
The Pullman Company, founded by George Pullman, was a manufacturer of railroad cars in the mid-to-late 19th century through the first half of the 20th century, during the boom of railroads in the United States. Through rapid late-19th century d ...
for the
Southern Pacific from 1936, continuing through commuter coaches for the
Rock Island Line
"Rock Island Line" ( Roud 15211) was originally sung as a spiritual by slaves on the plantations of the Mississippi River Valley, and was first transcribed as a folk song in 1929. The first recording was made by John Lomax, who was traveling amo ...
in 1949.
[“Railroad News and Editorial Comment,”'']Trains
A train (from Old French , from Latin">-4; we might wonder whether there's a point at which it's appropriate to talk of the beginnings of French, that is, when it wa ... , from Latin , "to pull, to draw") is a series of connected vehicles th ...
'' magazine, January 1950
In 1964, the
Moorestown Interchange was built over
New Jersey Turnpike
The New Jersey Turnpike (NJTP) is a system of controlled-access highway, controlled-access toll roads in the U.S. state of New Jersey. The turnpike is maintained by the New Jersey Turnpike Authority (NJTA).The Garden State Parkway, although ma ...
at milepost 37.02. This overpass is believed to be the first highway structure application of weathering steel.
Other states including Iowa, Ohio, and Michigan followed soon after. Those were followed by
University of York
The University of York (abbreviated as or ''York'' for Post-nominal letters, post-nominals) is a public Collegiate university, collegiate research university in York, England. Established in 1963, the university has expanded to more than thir ...
Footbridge in the United Kingdom in 1967. Since then, the practice of using weathering steel in bridges has expanded to many countries.
Properties
''
Weathering
Weathering is the deterioration of rocks, soils and minerals (as well as wood and artificial materials) through contact with water, atmospheric gases, sunlight, and biological organisms. It occurs '' in situ'' (on-site, with little or no move ...
'' 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. It is not yet clear how exactly the patina formation differs from usual rusting, but it's established that drying of the wetted surface is necessary and that copper is the most important alloying element.
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.
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
In materials science and engineering, the yield point is the point on a stress–strain curve that indicates the limit of elastic behavior and the beginning of plastic behavior. Below the yield point, a material will deform elastically and w ...
of and
ultimate tensile strength
Ultimate tensile strength (also called UTS, tensile strength, TS, ultimate strength or F_\text in notation) is the maximum stress that a material can withstand while being stretched or pulled before breaking. In brittle materials, the ultimate t ...
of for light-medium rolled shapes and plates up to thick. It has yield strength of and ultimate strength of for medium weight rolled shapes and plates from thick. The thickest rolled sections and plates – from thick have yield strength of and ultimate strength of . ASTM A242 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 primarily in urban furnishing, passenger ships or cranes.
ASTM A588
A588 has a yield strength of at least , and ultimate tensile strength of for all rolled
shapes and plate thicknesses up to thick. Plates from have yield strength at least and ultimate tensile strength at least , and plates from thick have yield strength at least and ultimate tensile strength at least .
Uses

Weathering steel is popularly used in outdoor sculptures for its distressed antique appearance. One example is the large
Chicago Picasso sculpture, which stands in the plaza of the
Daley Center Courthouse in Chicago, which is also constructed of weathering steel. Other examples include
Barnett Newman's ''
Broken Obelisk''; several of
Robert Indiana's ''Numbers'' sculptures and his original ''
Love
Love is a feeling of strong attraction and emotional attachment (psychology), attachment to a person, animal, or thing. It is expressed in many forms, encompassing a range of strong and positive emotional and mental states, from the most su ...
'' sculpture; numerous works by
Richard Serra
Richard Serra (November 2, 1938 – March 26, 2024) was an American artist known for his large-scale Abstract art, abstract sculptures made for Site-specific art, site-specific landscape, urban, and Architecture, architectural settings, a ...
; the ''
Alamo
The Battle of the Alamo (February 23 – March 6, 1836) was a pivotal event and military engagement in the Texas Revolution. Following a 13-day siege, Mexican troops under President General Antonio López de Santa Anna reclaimed the Alam ...
'' sculpture in Manhattan, NY; the
Barclays Center
Barclays Center ( ) is a multi-purpose list of indoor arenas, indoor arena in the New York City Boroughs of New York City, borough of Brooklyn. The arena is home to the Brooklyn Nets of the National Basketball Association and the New York Liber ...
,
Brooklyn
Brooklyn is a Boroughs of New York City, borough of New York City located at the westernmost end of Long Island in the New York (state), State of New York. Formerly an independent city, the borough is coextensive with Kings County, one of twelv ...
,
New York
New York most commonly refers to:
* New York (state), a state in the northeastern United States
* New York City, the most populous city in the United States, located in the state of New York
New York may also refer to:
Places United Kingdom
* ...
; the ''
Angel of the North
The ''Angel of the North'' is a contemporary sculpture by Antony Gormley, located in Gateshead, Tyne and Wear, England. Completed in 1998, it is seen by an estimated 33 million people every year due to its proximity to the A1 road (Great Bri ...
'',
Gateshead
Gateshead () is a town in the Gateshead Metropolitan Borough of Tyne and Wear, England. It is on the River Tyne's southern bank. The town's attractions include the twenty metre tall Angel of the North sculpture on the town's southern outskirts, ...
; and ''
Ribbons'', a sculpture by
Pippa Hale, celebrating women in Leeds;
and
Broadcasting Tower at
Leeds Beckett University
Leeds Beckett University (LBU), formerly known as Leeds Metropolitan University (LMU) and before that as Leeds Polytechnic, is a public university in Leeds, West Yorkshire, England. It has campuses in the Leeds city centre, city centre and Hea ...
.
It is also used in bridge and other large structural applications such as the
New River Gorge Bridge
The New River Gorge Bridge is a steel arch bridge long over the New River Gorge near Fayetteville, West Virginia, in the Appalachian Mountains of the eastern United States. With an arch long, the New River Gorge Bridge was the world's longest ...
, 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 container
An intermodal container, often called a shipping container, or a freight container, (or simply "container") is a large metal crate designed and built for intermodal freight transport, meaning these containers can be used across different Mode ...
s as well as visible sheet piling along recently widened sections of London's
M25 motorway
The M25 or London Orbital Motorway is a major ring road encircling most of Greater London. The motorway is one of the most important roads in the UK and one of the busiest. Margaret Thatcher opened the final section in 1986, making the M25 th ...
.
The first use of weathering steel for architectural applications was the
John Deere World Headquarters in
Moline, Illinois
Moline ( ) is a city in Rock Island County, Illinois, United States. With a population of 42,985 in the 2020 census, it is the largest city in Rock Island County and the List of municipalities in Illinois, ninth-most populous in Illinois outside ...
. The building was designed by architect
Eero Saarinen
Eero Saarinen (, ; August 20, 1910 – September 1, 1961) was a Finnish-American architect and industrial designer who created a wide array of innovative designs for buildings and monuments, including the General Motors Technical Center; the pa ...
, and completed in 1964. The main buildings of
Odense University (built 1971–1976), designed by
Knud Holscher and Jørgen Vesterholt, are clad in weathering steel, earning them the nickname ''Rustenborg'' (Danish for "rusty fortress"). In 1977,
Robert Indiana created a
Hebrew
Hebrew (; ''ʿÎbrit'') is a Northwest Semitic languages, Northwest Semitic language within the Afroasiatic languages, Afroasiatic language family. A regional dialect of the Canaanite languages, it was natively spoken by the Israelites and ...
version of the
''Love'' sculpture made from weathering steel using the four-letter word ahava (אהבה, "love" in Hebrew) for the
Israel Museum
The Israel Museum (, ''Muze'on Yisrael'', ) is an Art museum, art and archaeology museum in Jerusalem. It was established in 1965 as Israel's largest and foremost cultural institution, and one of the world's leading Encyclopedic museum, encyclopa ...
Art Garden in
Jerusalem
Jerusalem is a city in the Southern Levant, on a plateau in the Judaean Mountains between the Mediterranean Sea, Mediterranean and the Dead Sea. It is one of the List of oldest continuously inhabited cities, oldest cities in the world, and ...
, 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
Stainless steel, also known as inox, corrosion-resistant steel (CRES), or rustless steel, is an iron-based alloy that contains chromium, making it resistant to rust and corrosion. Stainless steel's resistance to corrosion comes from its chromi ...
. 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
Patina ( or ) is a thin layer that variously forms on the surface of copper, brass, bronze, and similar metals and metal alloys ( tarnish produced by oxidation or other chemical processes), or certain stones and wooden furniture (sheen prod ...
will not form in time to prevent corrosion over a localized area of attack such as a small paint failure. These cars were retired by 2016.
Weathering steel was used to build the exterior of
Barclays Center
Barclays Center ( ) is a multi-purpose list of indoor arenas, indoor arena in the New York City Boroughs of New York City, borough of Brooklyn. The arena is home to the Brooklyn Nets of the National Basketball Association and the New York Liber ...
, 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." In 2015, a new building for the
KTH Royal Institute of Technology
KTH Royal Institute of Technology (), abbreviated KTH, is a Public university, public research university in Stockholm, Sweden. KTH conducts research and education in Institute of technology, engineering and technology and is Sweden's largest te ...
School of Architecture was completed on its campus. The use of weathering steel helped the futuristic shapes of the facade fit in well with its much older surroundings and in 2015 it was awarded the
Kasper Salin Prize.
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
Rustproofing is the prevention or delay of rusting of iron and steel objects, or the permanent wikt:protection, protection against corrosion. Typically, the protection is achieved by a process of surface finishing or treatment. Depending on mec ...
in itself: if water is allowed to accumulate on the surface of the steel, it will experience a higher corrosion rate, so provision for drainage must be made. According to the
NTSB
The National Transportation Safety Board (NTSB) is an independent U.S. government investigative agency responsible for civil transportation accident investigation. In this role, the NTSB investigates and reports on aviation accidents and inc ...
, lack of drainage is what ultimately led to the
Collapse of the Fern Hollow Bridge. 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
Omni Coliseum (often called The Omni) was an indoor arena in Atlanta, Georgia, United States. Completed in 1972, the arena seated 16,378 for basketball and 15,278 for ice hockey, hockey. It was part of the Omni Complex, now known as the CNN Cente ...
, built in 1972 in
Atlanta
Atlanta ( ) is the List of capitals in the United States, capital and List of municipalities in Georgia (U.S. state), most populous city in the U.S. state of Georgia (U.S. state), Georgia. It is the county seat, seat of Fulton County, Georg ...
, 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.

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
Cornell University is a Private university, private Ivy League research university based in Ithaca, New York, United States. The university was co-founded by American philanthropist Ezra Cornell and historian and educator Andrew Dickson W ...
's main campus in
Ithaca, a small city in
Upstate New York
Upstate New York is a geographic region of New York (state), New York that lies north and northwest of the New York metropolitan area, New York City metropolitan area of downstate New York. Upstate includes the middle and upper Hudson Valley, ...
, did not achieve the predicted surface finish on its
Bethlehem Steel
The Bethlehem Steel Corporation was an American steelmaking company headquartered in Bethlehem, Pennsylvania. Until its closure in 2003, it was one of the world's largest steel-producing and shipbuilding companies. At the height of its success ...
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
The U.S. Steel Tower, also known as the Steel Building, or USX Tower (1988–2001), is a 64-story skyscraper at 600 Grant Street in downtown Pittsburgh, Pennsylvania. The interior has of leasable space. At tall, it is the tallest building i ...
in
Pittsburgh, Pennsylvania
Pittsburgh ( ) is a city in Allegheny County, Pennsylvania, United States, and its county seat. It is the List of municipalities in Pennsylvania#Municipalities, second-most populous city in Pennsylvania (after Philadelphia) and the List of Un ...
, was constructed by
U.S. Steel in part to showcase COR-TEN steel. The initial weathering of the material resulted in a discoloration, known as "bleeding" or "runoff", of the surrounding city sidewalks and 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.
See also
*
Iron pillar of Delhi and
Dhar iron pillar; ancient metal monuments with some characteristics similar to weathering steel
References
External links
Report on Weathering Steel in TxDOT Bridgesfrom the
Texas Department of Transportation
The Texas Department of Transportation (TxDOT ) is a Texas state government agency responsible for construction and maintenance of the state's immense Texas state highway system, state highway system and the support of the state's maritime trans ...
(4464 KB). Contains recommended details to avoid staining. ''Note: wrapping of piers was later found not to be cost-effective.''
A Primer on Weathering Steelfrom ''STRUCTURE'' magazine (2005)
from Colorado and London (2020)
{{Authority control
Structural steel
Steels
Sculpture materials
U.S. Steel