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.

Essentially, weathering steel is known as atmospheric corrosion-resistant steel, and it is mainly used in the construction industry. It has a nominal tensile yield of 50 ksi and a tensile strength of 70 ksi. This steel is made of low carbon steel, with a weight of less than 0.3% carbon, and alloying elements. It is the alloying elements that increase its strength and durability to maintain its structure during bad weather.

The other main elements found in this steel are:

Nickel
Chromium
Copper

One of the main reasons this type of steel is used is its protective rust properties. Basically, when the steel oxidises due to the pollutants in the air, this causes it to rust. This rust creates an additional, regenerative layer of protection to stop the steel from corroding. Therefore, painting or external treatment is not needed in the future.

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?

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.

What is meant by weathering steel?

Weathering steel, often referred to by the genericised trademark COR-TEN steel and sometimes written without the hyphen as corten steel, is a group of steel alloys which were developed to eliminate the need for painting, and form a stable rust-like appearance after several years’ exposure to weather.

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What is Corten Steel?

Atmospheric corrosion-resistant steel, also known as Corten steel, is a copper chromium alloy steel that is another form of weathering steel. It is far superior to other unalloyed steels because it has a chemical composition that creates rust to protect it against pollutants.

Corten A is best used for gas flue applications and projects that need an aesthetic appeal, but it should not be used for applications that need to carry heavy loads. If you need a steel for that type of project, you should use Corten B. Corten B will corrode to protect the steel underneath it, therefore it is stronger and more cost-effective.

What are the Advantages of Using Weathering Steel?

There are a variety of industries that can benefit from using weathering steel, including agriculture, infrastructure and construction. Because weathering steel has high durability in outdoor environments, it is great to use in steel structures. It does not need to be constantly repaired. The rust acts as protection to slow down the corrosion rates.

Additional weathering steel benefits can also include:

Low maintenance costs
The chance to create a protective barrier against oxygen and moisture
That it is an environmentally-friendly material
Tough and ductile strength
It being perfect for structures that are hard to maintain due to its corrosion resistance

Why is Weathering Steel Better than Galvanized Steel?

If you are undertaking a project that involves using steel in construction, it is often recommended that you use weathering steel over galvanized steel. Weathering steel naturally protects itself from corrosion through rusting. However, galvanizing means that the steel is dipped in a molten zinc coating to prevent rust.

Galvanized steel does come with its disadvantages compared to weathering steel. These include:

Its high costs, especially in projects that are large-scale or have a budget
That it can develop white rust if it is in contact with water or moisture
The likelihood that it will experience internal steel corrosion
The galvanized coating potentially creates an uneven surface because it is likely to chip
A lack of flexibility

10 Facts about Weathering Steel

Weathering steel is corrosion-resistant and extremely strong. Compared to other steels, weathering steel has a higher resistance against atmospheric corrosion as its protective coating inhibits further corrosion.
Weathering steel has a longer life-span than bare cold rolled steel.
Weathering steel also is more cost-effective as it saves money on painting and rust-prevention maintenance. There is no need to paint or treat weathering steel.
When weathering steel is first installed it is black and shiny. Once it has been exposed to some weathering, it turns streaky yellow, then orange and then a dark-brown color can be seen after 10+ years.
Weathering steel continuously rusts for an extremely long time. The process is fast to begin with but slowly decreases over time.
The mixing of a specific blend of steels and alloys produces weathering steel. It is this combination that improves the rusting properties to provide the well-known oxidized dark brown look.
Regeneration of weathering steel ends when the pores at the rust and steel interface are clogged by the alloying elements in the steel and their production of insoluble compounds.
Although weathering steel is only ¼ inch thick when produced in sheets, it is both durable and sturdy.
Weathering steel should not be used for roofing or siding when used in architectural or building applications