Weathering steel was developed in the United States at the beginning of the 20th century when was noticed that steel plate alloyed with copper resisted atmospheric corrosion far better than normal carbon steel plates. Weathering steel was developed as result of long tests 1933. The original chemical composition is Phosphorus 0.07%, Silicon 0.5%, Copper 0.3, Nickel 0.2% and Chromium 0.7%

COR-TEN® is the U.S. Steel’s trade name for atmospheric, corrosion resistant steel while weathering steel and corrosion-resistant steel are the generic terms. U.S. Steel holds the registered trademark on the name COR-TEN. The name COR-TEN refers to the two distinguishing properties of this type of steel: CORrosion resistance and TENsile strength.

Weathering steel is used to build shipping containers, bridges and processing equipment in the chemical and petrochemical industry.. It is also frequently used in power line poles, lamp posts or handling equipment, chassis structures of trucks, water tanks, chimneys as well as in the construction industry.

Weathering steel has traditionally been used for industrial applications. Its popularity has increased over the years and migrated into architectural applications. The natural colour, variation and longevity of weathering steel makes it interesting to the design community.

Weathering steel is characterized by a matte surface, which reduces reflections that are typical of other metals and also reduces any unevenness of the surface .The patina developed over time gives weathering steel a distinguished, brass-like appearance. It is also a long lasting material that can be expected to last up to 100 years inland and up to 60 years in coastal areas.

Under normal weather conditions the patina will form in 3 months depending on the geographical location. Initially the patina is a reddish brown colour. Over time with will develop darker hue. Most weathering steel types need cycles of wet/dry weather to develop and oxidize.

During the initial patination process rust will leach off the panels. Care should be taken when designing with weathering steel to collect the runoff so that it does not stain neighbouring materials.

In the presence of moisture, air and other elements, most low alloy steels have the tendency to rust. Overtime, the rust layer becomes porous and detaches from the metal surface. With weathering steel, the rusting process occurs in the same way, but the steel produces a stable rust layer called the “patina,” which adheres to the base metal and is much less porous. This naturally-developed patina regenerates continuously as it is exposed to weather and produces a protective barrier that impedes further access of oxygen, moisture and pollutants.

While it is available it can be quite costly. It is often recommended to spray the sheets with a mixture of peroxide, vinegar and salt. This significantly accelerates the initial stage of patina and reduces the amount of run off.