Weathering steel refers to a range of steels with a chemical composition that makes them more resistant to atmospheric corrosion compared to other, mild steels. The steel forms a protective layer on its surface under the influence of outdoor weather conditions. CorTen is an example of a weathering steel.
The origins of CorTen
The original weathering steel was developed and patented in 1933 by U.S. Steel under the brand name Cor-Ten®, deriving it’s name from ‘CORrosion Resistance’ and ‘TENsile Strength’.
It was standardised as a product by the American Society for Testing and Materials (ASTM).
In more recent years, ‘CorTen’ has become a standard term covering both Cor-Ten® and weathering steels. In general, all weathering steel grades have equivalent corrosion resistant properties, as well as a similar look and feel.
CorTen was originally developed for railroad coal wagons but soon became popular in construction due to being structurally strong and requiring little or no maintenance.
By the 1950s and 60s, it’s strength and industrial beauty made CorTen popular for artistic and sculptural projects. Today, one of the largest users of CorTen is the shipping container industry.
CorTen A and B
The original CorTen was ASTM A242 which is widely known as Type 1 and Type 2, or CorTen A and CorTen B.
Type 1 (CorTen A) is available in 2mm – 12mm thick sheets or plates.
Type 2 (CorTen B) has the addition of vanadium, making it suited to heavy structural applications. It is generally available in 2mm – 40mm thick sheets or plates.
In Europe, Steel Grade S355JOWP is equivalent to CorTen A and S355JOW is equivalent to CorTen B. Another option, S355J2W, which is also equivalent to CorTen B, is more economical but also fully recyclable.
At Kinley, we use all types of Cor-Ten and weathering steel at our in-house Innovation Centre in East Sussex to fabricate our edging systems.
Manufacturing and weathering
Weathering steel undergoes a hot rolling method during production, which sees the steel being passed between rollers at very high temperatures. During this process, the iron waste material (known as ‘mill scale’) moves to the surface. This leads to rust run off and staining during the early stages of the weathering process.
Once the outer iron waste layer has rusted away, the material has a stable surface with minimal shedding or staining. The process can be accelerated, or the raw material can be ‘pickled’ where an acid treatment is used to remove the mill scale prior to delivery.
CorTen steel embraces its surroundings and its final finish depends on the environment where it is installed.
For example, it will appear different in coastal areas as opposed to inland or under tree cover. The beauty of CorTen is the natural inconsistencies in the finish.
However, water pooling and puddling will affect appearance and reduce the life of the material, so this needs consideration during design and installation.
As the steel ages, it will darken in colour.
Example of how the colour can change over time: (Source - The Nippon Steel Corporation).
Benefits of using CorTen
All weathering steels have the rust layer which protects the steel beneath, making them much more corrosion resistant and long-lasting than standard mild steel alloys.
When the initial rusting process is complete, the steel requires little or no maintenance, making it ideal for large scale and high profile projects.
Its make up ensures the material’s strength, yet it remains flexible enabling it to be formed to create various shaped planters, among other products.
CorTen’s versatility goes beyond its strength and flexibility. It can also be powder-coated in any RAL colour to fit seamlessly into a landscape design.
We carefully choose the materials that we use in all our landscape products to make sure that you have the best quality, hardwearing design, as well as being aesthetically beautiful.
At Kinley, we’ve got you covered.
We can offer our edging products in either untreated steel (ideal for if you are working to a tighter budget) as well as genuine Cor-ten if you are looking for that premium finish to your project.
Unless you are a metallurgist it is very difficult to see any visible differences between mild and weathering steel in its unweathered state.
All CorTen and weathering steel can be supplied with a material certificate that confirms it is genuine. Any steel stockholder who is CE (BS EN 1090-2) approved will be able to supply this documentation.