02 January 2019

What is weathering steel?

Weathering steel is a generic term for grades of steel that have been developed to form a protective rust that inhibits further corrosion and extends the steels design life. These atmospheric corrosion resistant steels were originally developed in the United States and given the trade name Cor-ten.

Cor-ten weathering steels are high strength, low alloy steels with small chemical additions, usually copper and chromium, that provide an enhanced resistance to corrosion compared to ordinary structural steel.

Weathering steel corrosion


In the presence of moisture and air, steel will corrode, or rust and the rate of corrosion depends on the access of oxygen, moisture and atmospheric contaminants to the metal surface. Corrosion forms a layer of rust and this, to some extent, forms a barrier to further corrosion.


The rust layers in most structural steels don’t adhere well to the underlying metal, so they detach and the corrosion cycle starts again. Corrosion of regular carbon steel progresses as a series of incremental curves approximating to a straight line while the overall rate depends on the details of the environment.

With weathering steel, corrosion starts in the same way, but the initial rust layer is much more stable and dense thus adhering much better to the underlying metal. This dense rust layer, called a ‘patina’ protects the underlying steel from further corrosion. The overall result significantly slows down the rate of corrosion.

For weathering steels to function properly it is essential that the patina is given a good chance to develop, this usually takes many wet/dry cycles over the first year or more of external exposure.

At Tata Steel, we have now made our popular  of true hot-finished structural hollow sections available in a weathering grade steel, matching the maintenance-free long-life corrosion protection of weathering steel with our premium hollow sections. This new combination will help unlock further possibilities for long-life, maintenance-free steel structures. 

To find out more about this product visit our Celsius® Weathering Grade page where you can find key product features, technical data and brochure downloads. 

For further information