What Is Weathering Steel?

Weathering steel is a family of low carbon steels that have additional alloying elements mixed in with the carbon and iron atoms. These alloying elements give weathering steel better strength and more corrosion resistance than typical low carbon steel grades. As a result, weathering steel is commonly used in outdoor applications or environments where normal steel would be inclined to rust.

The Composition of Weathering Steel

As a low carbon steel, weathering steel typically has less than 0.3% carbon by weight. This low amount of carbon allows it to remain ductile and tough. Weathering steel also includes other alloying elements that help to increase the strength, but more importantly, the corrosion resistance. There are many different alloying elements used, but the key three elements in weathering steel are nickel, copper, and chromium.

How Does it Work?

Weathering steel is not like other corrosion resistant steels, such as austenitic stainless steel, that resist rust formation. Weathering steel will rust, however, it will only rust on the outer surface. The rust does not penetrate deeper into weathering steel once the outer layer of rust has formed. The surface layer of rust acts as a barrier and actually protects the steel from further corrosion. In plain carbon steel, the rust layer that forms is porous and will break off which allows another layer to form deeper into the steel. This cycle repeats until the steel is rendered useless. In weathering steel, the alloying elements cause the initial rust layer that forms to adhere much better to the steel, preventing the rust from penetrating deeper and weakening the steel. This protective rust layer eliminates the need to coat weathering steel.

Weathering Steel Grades

Weathering steel is a family of low carbon alloy steels that consists of a variety of grades. Some grades are proprietary, such as COR-TEN A or COR-TEN B. The Patinax weathering steels are another group of proprietary grades. All of these proprietary grades are similar to the ASTM classifications A 242 and A 588.

When is Weathering Steel Used?

abetxuko bridge weathering steel
The Abetxuko Bridge in Spain – made of Weathering Steel

Because weathering steel can outlast plain carbon steel in outdoor conditions, it is frequently used for exposed steel structures. This removes the need for constant repainting and recoating of the steel. Examples include building and bridge construction. The protective rust coating slows the corrosion rate enough that by the time the amount of corrosion would be considered unsafe, the structure would have already exceeded its design life for other reasons.

There are environments where weathering steel should not be used because the corrosion resistance is not capable of withstanding the conditions. Weathering steel should not be used in environments with high amounts of chlorine, as the protective rust layer will not be able to withstand the high amount of corrosives present in chlorine rich environments. This can lead to premature failure. Applications that could result in galvanic corrosion or corrosion induced by extreme pH levels should also not utilize weathering steel.

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