All forms of metal, both ferrous and nonferrous, develop oxide films when exposed to the atmosphere. Oxide development is a natural occurrence by which a metal tends toward its natural density. All metals have this entropy. When exposed to the atmosphere, they all seek to return to the more natural, energy-balanced state.

In most common carbon steels, the oxide develops to the point that total decay may occur, as signaled by the powdering of red rust. The oxide film on most steels is porous and allows moisture and oxygen to continue corroding the surface. Some high-strength steel and copper-bearing steels develop dense adherent oxide films that slow the rate of corrosion, similarly to the oxides on copper and aluminum. These steels are the "weathering" steels. Cor-Ten is the trade name for one of the more common of these weathering steels developed by US Steel Corporation. 

These copper-bearing weathering steels are characterized by a reddish brown, coarse rust film that rapidly develops on the surface. Initially the surface is a bluish black color. As moisture, even slight condensation, collects on the surface, streaks of reddish brown rust develop. Further exposure leads to a coarse fine layer of rust eventually covering the entire surface of the metal. The runoff of the rust film quickly stains adjacent materials such as concrete wainscots and sidewalks below the weathering steel installation. Designing with this material requires special attention to runoff and moisture control.

The rust film on a metal surface is very adherent. The impervious iron oxide film will work to resist further corrosion of the base metal. Eventually, the runoff slows down to a point where further staining is minimal.

Copper-bearing weathering steel is a high-strength, low-carbon steel alloy containing small amounts of chromium and copper. The nominal range of alloying elements is as follows:

The alloys are very hard and have a yield strength of 50 ksi (345 MPa). The addition of chromium provides corrosion resistance characteristic of a hard, impervious surface film.

History: The Rise, Fall, and Resurgence of Weathering Steel

In the 1960s and 1970s, weathering steel came into its own. Cor-Ten, originally developed for railcar, began to see wide use as an architectural metal. One of the first major examples of weathering steel used in an architectural sheathing context is the John Deere Headquarters. Designed by Eero Saarinen, the facade used Cor-Ten weathering steel. Saarinen had noticed the interesting dark brown steel surfaces on railway cars used to transport grain. An unfinished weathering steel was used for grain cars because of the abrasive nature of the grain kernels on painted steel cars.

Over the next decade, many more designers would specify the industrial metal for architectural purposes. However, the designers were specifying the material far too thin for the sustainability of the material. Problems developed when the corrosion of the inside surface of the steel occurred at accelerated rates in comparison with the outer surface. 

When sheet metal thicknesses less than 0.032 inches (0.813 mm) were used, areas began to experience premature perforation of the metal. In applications where water was allowed to erode the surface, such as on leaders and scuppers, the protective layer of rust was removed. Corrosion of the surface continued until portions of the metal dissolved completely. For a number of years, weathering steel was discontinued as an architectural metal in sheet form because of premature perforation of the sheet steel. 

Today, weathering steel is experiencing a resurgence as a result of correct thickness specification, and modern detailing which allows for proper drainage. In addition, many successful installations of the metal are typified by painting the inner side of the sheet metal with a thick protective paint coating. Most importantly, the thicknesses of the metal used in exterior applications are 16ga or thicker, double the thickness of previous uses. 

Works featuring Weathering Steel