What is Weathering Steel?
Weathering steels are a family of ASTM specifications, as well as several trademarked specifications exhibiting similar corrosion-resistant properties. The ASTM specifications are A588, A242, A606-4, A847, A871-65 and A709-50W. The primary trademarked product is Cor-ten®. The selection of a particular ASTM weathering steel specification is dependent on the type of product desired. ASTM A588 covers structural shapes, plate, and bar. ASTM A242 covers steel plate through one-half inch in thickness. ASTM A606-4 covers steel sheet and coil and ASTM A847 is for weathering steel tube and pipe. ASTM A871-65 also cover plate, but with a 65ksi minimum yield strength. The corrosion-retarding effect of the protective layer is produced by the particular distribution and concentration of alloying elements within it. The naturally-developed patina protecting the surface 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. This protective coating guards against future corrosion.
Benefits of Weathering Steels:
- Extensive Appeal: The atmospheric corrosion resistance of weather resistant steel enables it to be used unpainted in many structural and architectural applications for structures, such as bridges, open-frame buildings, transmission towers and sculptures. There are also high temperature benefits, which make weathering steel suitable for flues, chimneys and ducting.
- Practical Benefits: Weathering steel offers significant advantages over other metals for structures that are exposed to the elements:
- Very low maintenance: Periodic inspection and cleaning should be the only maintenance required to ensure the structure continues to perform well. Weathering steel is ideal for bridges and other structures where access is difficult or dangerous, and where future disruption needs to be minimized.
- Initial cost benefits: Cost savings from the elimination of any protective paint system may outweigh the additional material costs. For example, the cost of weathering steel has been shown to be approximately 5 percent lower than conventional painted steel alternatives in bridges using a HA Type 2 paint system.
- Whole-life cost benefits: The minimal maintenance requirements of weathering steel structures greatly reduce both the direct costs of maintenance operations and the indirect costs of traffic delays or rail possessions.
- Speed of construction: Overall construction times are reduced as both shop and site painting operations are eliminated.
- Attractive appearance: The attractive appearance of mature weathering steel often blends pleasingly with the environment. Its appearance changes and improves with age.
- Environmental benefits: The environmental problems associated with volatile organic compounds (VOC) from paint and the disposal of blast-cleaning debris from future maintenance work are avoided.
- Safety benefits: Health and safety issues relating to initial painting are eliminated, and the risks are associated with future maintenance are minimized.
- High temperature benefits: Oxidation loss of steel at temperatures above 400 degrees Celsius can be decreased by using weathering steels. At these temperatures, a patina forms even in the absence of water. The precise performance improvement is dependent on the heating cycle and environmental conditions. A typical improvement would be an increase of 50 degrees Celsius over equivalent loss in carbon manganese steels.
Please note, weathering steels are not suitable for use in significant load-bearing members above 450 degrees Celsius.