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Weathering & Copper-Bearing Steels

Weathering steel, is a group of steel alloys that form a stable rust-like appearance after several years' exposure to weather.  

Weathering steel, copper-bearing steels, containing from 0.15% to 0.25% copper, develop increased resistance to atmospheric corrosion by forming a protective oxide coating witha uniform deep brown color. The chemical composition of these steels increases resistance to rusting by forming a protective layer on its surface.

The layer protecting the surface develops and regenerates continuously.  The steel is allowed to rust in order to form the protective coating.

ASTM grades are A242, A588, and A606 for thin sheet.

ASTM A242

A242 alloy has a yield strength of 50 ksi (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.  ASTM A 242 is available in Type 1 and Type 2. Both have different uses based on the thickness. Type 1 is often used in housing structures, construction industry and freight cars.  Type 2  is used mainly in passenger ships and 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 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).

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Weathering steel is popularly used in outdoor sculptures for its rustic 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.

It is also used in bridge and other large structures. It is very widely used in marine transportation, in the construction of intermodal containers as well as visible sheet piling along recently widened sections of London's M25 motorway.

Disadvantages

Ensuring that weld-points weather at the same rate as the other materials may require special welding methods.  Weathering steel is not rustproof in itself., so provision for drainage must be made. In humid subtropical climates, the protective patina may continue to corrode.

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. 

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