Terminology

• COR-TEN: U.S. Steel trademark, held until 2016; A242 is COR-TEN A, A588 is COR-TEN B
• Patina: adherent rust layer that slows further corrosion
• 50W: weathering version of ASTM A709 Grade 50

History and demand

Invention

• 1910s: development began in the US; about 1926 phosphorus found to aid corrosion resistance
• 1933: U.S. Steel commercialized the steel

First uses in construction

• 1964: John Deere World Headquarters, first architectural use; Moorestown Interchange, New Jersey, believed the first highway application
• Barclays Center: 12,000 pre-weathered panels

Sourcing

• United States Steel holds the COR-TEN trademark; SSAB is the only licensed COR-TEN manufacturer in Europe, and Nippon Steel lists COR-TEN grades
• Generic weathering steel to ASTM A242 and A588 is made by many mills

Composition and production

Raw materials

• ASTM A242 (wt %): Cu 0.25 to 0.55; Cr 0.50 to 1.25; Ni 0.65 max; P 0.01 to 0.20; C 0.12; S 0.030 max
• ASTM A588 (wt %): Cu 0.25 to 0.40; Cr 0.40 to 0.65; Ni 0.40 max; P 0.030 max; C 0.16; S 0.030 max; V 0.02 to 0.10
• Copper is the most important alloying element for patina formation

Product forms

• Plate, shapes, and thin sheet (A606); pre-weathered panels

Grades and types

• ASTM A242 (COR-TEN A): original grade; 50 ksi (340 MPa) yield and 70 ksi (480 MPa) tensile up to 3/4 in. thick
• ASTM A588 (COR-TEN B): 50 ksi minimum yield and 70 ksi minimum tensile up to 4 in. thick
• ASTM A606: thin sheet
• ASTM A709 Grade 50W: bridge structural steel

Properties

Mechanical

• A242 yield: 50 ksi (340 MPa) up to 3/4 in.; 46 ksi (320 MPa) for 3/4 to 1 in.; 42 ksi (290 MPa) for 1-1/2 to 4 in.
• A588 yield: at least 50 ksi (340 MPa) up to 4 in.; 46 ksi (320 MPa) for 4 to 5 in.; 42 ksi (290 MPa) for 5 to 8 in.

Durability

• Patina typically forms in about six months; surface treatments can reduce this to about one hour
• Slower to form in rural areas than in cities with atmospheric pollutants
• Humid subtropical climates and sea-salt environments can prevent the patina from stabilizing

Standards

Material specifications

• ASTM A242: high-strength low-alloy structural steel
• ASTM A588: high-strength low-alloy structural steel up to 50 ksi yield with atmospheric corrosion resistance
• ASTM A606: thin sheet weathering steel (Type 4)
• ASTM A709: structural steel for bridges, Grades 50W, HPS 70W, 100W

Test methods

• ASTM G101: estimating atmospheric corrosion resistance of low-alloy weathering steels

Uses in buildings

Building elements

• Metal Wall Panels: pre-weathered facade panels
• Wide-Flange Steel Beams, Steel Columns and HSS: exposed weathering-steel shapes

Pros and cons

Pros

• Needs no paint once the patina forms
• Higher atmospheric corrosion resistance than carbon steel

Cons

• Rust runoff stains adjacent surfaces, including concrete
• Not suited to sea-salt or humid subtropical exposure
• Needs drainage detailing; lack of drainage was linked to the Fern Hollow Bridge collapse

Design considerations

Compatibility

• Keep runoff from draining onto concrete or other porous surfaces

Manufacturing and cost

Main manufacturers and suppliers

• U.S. Steel
• SSAB
• Nucor

Trade association: AISC.

Sources

• Wikipedia: Weathering steel, https://en.wikipedia.org/wiki/Weathering_steel
• Steel Technology: ASTM A588 shapes, https://www.steel-technology.com/products/central-steel-service-inc/corten-steel-shapes-a588
• ASTM: G101 listing, https://standards.globalspec.com/std/3805732/astm-g101-97
• Wikimedia Commons: Corten Stahl Cor-Ten Steel Museum history town Essen Germany.JPG, photo by ubor, ulf borutzky (CC BY-SA 3.0), https://commons.wikimedia.org/wiki/File:Corten_Stahl_Cor-Ten_Steel_Museum_history_town_Essen_Germany.JPG
• SSAB: the only approved COR-TEN manufacturer in Europe, https://www.ssab.com/fi-fi/uutiset/2017/01/ssab--the-only-approved-corten-manufacturer-in-europe
• Nippon Steel: COR-TEN plate, https://www-zam.nipponsteel.com/en/product/plate/list/04.html