You’ve probably seen COR-TEN steel on buildings with its attractive (and fashionable) orange patina.
What is it?
COR-TEN steel is a steel alloy that was originally developed for applications where corrosion resistance was important (originally for hopper cars for transporting bulk products along the railway in the US in the 1930s). Since its invention, however, it has been used by architects to achieve an industrial aesthetic. For instance, the Angel of the North, and Leeds Broadcasting Tower. It weathers to a characteristic rich red/brown colour over a number of months or years. As a result of its use in architecture, the term COR-TEN has become synonymous with all types of weathering steel. COR-TEN is actually a brand name, and is derived from the steels CORrosion resistance, and high TENsile strength.
What makes it special?
Weathering steel is very durable and designed to last well against abrasion. In addition to its toughness, it is also very strong. The most common grade used for structural work is S355J2W, which has a yield strength of at least 355MPa.
As an aside, the yield strength of a material is the maximum stress a material can take before permanent deformation occurs. At or above the yield stress, very large deformations can occur with only modest increases in stress. In the case of buildings, this would probably be signified by collapse. Most engineered parts are designed to keep the maximum stress in operation below the yield stress of the material with an additional safety factor. There are always exceptions, for instance stretch bolts in cylinder heads. These are designed to be stretched to the yield strength of the bolt material. This allows the bolts to cope with the changes in length due to the thermal expansion of the cylinder head without greatly increasing the stress in the bolt, or the cylinder head. It keeps the clamp load on the cylinder head more constant than non-stretch bolts, which provides a more even seal across the face of the head gasket. Some examples of yield strengths of other materials are:
- Human bone: 115MPa (Tensile), 182MPa (Compressive)
- Pine Wood: 59-100MPa
- Oak Wood: 71-127MPa
- Tungsten Carbide: 300-1000MPa
Weathering steel is resistant to atmospheric corrosion due to the Nickel, Chromium and Copper content. Due to its chemical constituents, it forms a semi-protective layer on the surface of the steel, which drastically reduces the rate at which material is lost due to corrosion. Additionally, if the layer is scraped off the surface, a new layer will form in its place, thereby preventing moisture and air getting to the steel underneath.
Where is it used?
- Bridge construction (Structural elements of bridges)
- Marine Transport (Shipping containers, Ships etc)
- Outdoor Sculptures (Angel of the North)
- Building Cladding (Leeds broadcasting tower)
There are limitations to where CORTEN/weathering steel can be used as structural elements in construction:
- Exposure to chloride ions (from sea water or otherwise) at levels above 300mg/sqm/day in the air would require the steel to be covered by a barrier layer to prevent excessive corrosion.
- Exposure to excessive levels of corrosive pollutants, such as Sulphur Dioxide, can lead to a shortened lifespan of the structure. As a result, the use of weathering steel is discouraged if atmospheric Sulphur Dioxide levels are above 80mg/sqm/day.
- Continuously wet environments cause the steel to corrode at a rate that is similar to normal steel because a stable, adherent patina cannot form on the steel. As a result, a barrier layer should be used to protect the steel form the moisture. Additionally, care has to be taken when detailing weathering steel to avoid moisture traps.
- When detailing connections between weathering steel and other metal elements of the structure, special effort must be given to preventing direct electrical contact with other types of metal. Failure to do so will cause premature corrosion of the weathering steel due to galvanic action, similar in principle to a battery.
How do we use it?
We use it for:
- Fencing/Shading/Trellis panels – In particular, we produce panels from photos or hand drawings supplied by clients. In this way we can offer a genuinely personal service.
- Planters/raised beds
- Animal Silhouettes
The uses are only limited by imagination. If you want something specific, get in touch.