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AISC | Steel Interchange
WEATHERING STEEL
Question
01/01/2004

What are some design considerations for using ASTM A588 weathering steel? Should the steel be painted?

Question sent to the AISC Steel Solutions Center

 
Answer(s)

United States Steel (USS) Corporation has a product called USS COR-TEN B that is available in ASTM A588. The following is an excerpt from some literature from USS regarding design considerations for USS COR-TEN:

Design Considerations

*    Surfaces of COR-TEN Steel that are wet for prolonged periods of time will corrode at an unacceptably rapid rate. Therefore, the detailing of members and assemblies should avoid pockets, crevices, faying surfaces or locations that can collect and retain liquid water, damp debris and moisture. Damp debris on COR-TEN Steel surfaces will cause accelerated corrosion. In addition to these precautions, all interior surfaces, including faying surfaces, not boldly exposed to the weather must be protected by paint.

*    Surfaces of COR-TEN Steel members and assemblies that are not boldly exposed to the atmosphere are subject to moisture accumulation from numerous sources including capillarity and condensation. The designer must, therefore, exercise extreme care in the detailing of such elements, to assure absolutely no possibility of moisture entrapment. All such unexposed surfaces, including faying surfaces, are to be treated as if they were carbon steel and must be protected by paint.

*    Hollow steel members should be sealed to prevent entry of moisture. If this is not possible, provisions must be made to insure adequate drainage and ventilation so that the potential for entrapped moisture and accelerated corrosion is eliminated. Furthermore, if the member or structure is inaccessible for inspection and maintenance, protection of the interior surfaces should be considered. Those structures which are accessible may be left bare but the areas must be periodically inspected for evidence of corrosion. The frequency of inspection is to be established by the designer due to the many variables which influence the protective oxide. Should excessive corrosion become apparent, the steel must be cleaned and protected with paint.

*    All COR-TEN Steel surfaces which are to be covered by caulking or gaskets must be painted before being covered. This is to insure a positive seal and to provide adequate protection against corrosion of the steel that is covered. If this is not done the COR-TEN Steel will corrode at an unacceptably rapid rate. See “Painting” section.

*    To minimize “oil canning” in large, slat assemblies, the minimum recommended thickness for bare COR-TEN Steel application is 18 gage (0.0478 inches, or 1.2 mm).

*    For structural joints where high-strength bolts are desired, ASTM A325, Type 3 bolts (COR-TEN X) must be used. Where lower strength bolts are satisfactory, bolts of either COR-TEN A Steel or stainless steel are suitable. Galvanized steel nuts and bolts are not suitable for use in time, the zinc coating will be consumed, leaving an exposed carbon-steel fastener which is less resistant to atmospheric corrosion than the COR-TEN Steel.

Painting

The paint requirements for the COR-TEN steels do not differ from those for carbon steels. In locations where the outdoor environment is aggressive, the same quality paint systems found effective in protecting carbon steel are recommended for the COR-TEN steels. As noted earlier, environments which prevent proper oxide formation require the same surface protection against the effects of moisture as do carbon steel surfaces. Similar requirements apply where faying surfaces and bolted joints are involved, and where COR-TEN steel surfaces are likely to be in contact with other structural materials. To achieve the effectiveness of the protective coating selected, the stringent rules of surface preparation must be adhered to. While the designer must specify the paint system for the particular environment, the following are two examples of paint systems which have been found readily available and generally acceptable under most environmental conditions.

*    A one-coat system consisting of a high-quality air-drying rust-inhibitive shop-primer which is applied to a nominal dry film thickness of 1.5 to 2 mils.

*    A two-coat baked system consisting of 0.2 to 0.25 mils of a rust-inhibitive primer such as epoxy chromate followed by a top coat of synthetic-resin paint such as a polyester, acrylic or alkyd applied to a total dry film-thickness of 1.0 mils.

However, for prolonged exposure to water, the coating system should be upgraded to that of a tank lining.

More information can be found at www.ussteel.com. Alternatively, information about weathering steel from Bethlehem Steel can be found at www.intlsteel.com/PDFs/products/weather.pdf

Keith Mueller, Ph.D.
AISC Steel Solutions Center

 
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