CorTen’s unique properties first emerged in 1930s America. Manufacturers of railroad coal wagons discovered steel alloys which developed a protective layer of rust when they were exposed to the atmosphere. This oxidation process gives CorTen its distinctive earthy-brown patina, and it’s this which provides a protective layer over the steel and lets it withstand all types of weather corrosion, in a way that carbon steel just can’t match.
Architects quickly adopted CorTen, featuring it in iconic buildings in Chicago and New York and in many other cities across the world.
CorTen’s highest-profile use in the UK is probably Sir Antony Gormley’s 1998 Angel of the North at Gateshead - at 20m tall and 54m across, the largest sculpture in the UK. Built with CorTen to signify the industrial heritage of the North-east, the Angel was designed to withstand winds of over 100mph, showing how versatile and resilient CorTen is. The funky, post-industrial vibe it gives off has also added to its appeal
CorTen – an abbreviation from CORrosion resistance-TENsile strength - has come to be known as the generic term for weathering steel but the name is actually registered as a trademark by the United States Steel Corporation, which no longer manufactures it. That notwithstanding, the terms have become synonymous today.
Multiple landscaping uses
As well as being strong, CorTen is also thin and flexible. This makes it ideal for bordering, retaining walls and raised flowerbeds, as well as water features and art installations.
It can be cut, fabricated and welded in the same way as other mild steels and, like other steels, there’s no danger of it harming adjacent plantings or surroundings. In fact, there are environmental benefits from using CorTen as it absorbs high levels of Co2.
Not surprisingly, Kinley CorTen planters and edging products have become increasingly popular. At Handyside Gardens, a new inner-city park garden near King’s Cross, North London, for example, the pathways were created with curved plant and tree beds that were defined with raised planters made of CorTen steel (see case study) provided by Kinley.
Similarly, in the public square in front of the Oldham Leisure Centre, 12 bespoke planters of heights ranging from 0.6m to 2.9m were created using our CorTen steel (see case study) products.
Many landscape architects also love the way CorTen’s appearance alters over time as the patina blocks off oxygen and moisture, creating lots of different surface tones and textures.
Speed of change is influenced by the surrounding environment and exposure to the elements. In an industrial atmosphere the weathering process is quicker (particularly in the presence of sulphur) and the patina will be darker than in a rural atmosphere.
Thanks to the sun, south- and west-facing surfaces in the Northern Hemisphere develop a smoother, more uniform patina than those facing east and north. Higher temperatures bring on more rapid surface alteration, whereas surfaces less exposed to the sun react more slowly, giving a granular texture to the patina.
A blot on the landscape – and how to avoid it
CorTen’s rusting process does, however, have one potential drawback – it gradually leaches rust-coloured water onto nearby surfaces and stains them permanently. If the adjoining surface is lightly coloured, the leaching can make it look pretty unsightly.
To minimise leakage, Kinley recommends applying oxidation accelerators to shorten surface-maturing time. This significantly reduces the staining effect once the CorTen is in place. Kinley’s CorTen products are treated in this way.
To speed up the weathering process once in place, Kinley suggests using a standard garden spray canister to apply either salt water or, preferably, a chemical solution that Kinley provides onto the surface of the edging systems after they have been cleaned with water. After letting the solution soak in for 90 minutes, the process is repeated. Then, after approximately 45 minutes, the edging is again cleaned with water.
Ideally, you should do this when the edging is cool. During summer months, the edging will be warm during the day, and if you do it then the solution will evaporate and be far less effective. During the summer months the solution should be applied earlier in the morning, or later at night when the weather is cooler.
Doing this once or twice a week won’t fully oxidise the material but it will achieve a clean rusting finish on the face of the product and speed up the remaining weathering process.