In recent years, the natural sheen of weathering steel has become increasingly popular as its subtle orange and brown hues complement a more natural approach to landscaping and garden sculpture. Perhaps the most famous example is Antony Gormley's Angel of the North at Gateshead, although it is widely used in many less grandiose Settings such as private and public gardens, parks and terraces.
This patina is formed by the oxidation of steel, forming a fine layer of rust. Conventional mild steel forms a light and brittle rust layer that retains moisture and allows oxygen to reach the uncorroded metal, so corrosion will continue until the steel is completely corroded.
This layer is denser due to the alloying composition of weathering steel and acts as a barrier to oxygen and moisture from the corrosion process.
Weathering steel is usually supplied and installed prior to the start of the oxidation process and usually has a dark grey finish. After installation, exposure to water, oxygen, sunlight and air pollution begins the rusting process.
All these factors affect the speed of obtaining protective oxide film and even the appearance of oxide film. In the Northern Hemisphere, surfaces facing south or west tend to be heated and dried by the sun more frequently, resulting in smoother, more uniform surfaces than surfaces facing north and west, which develop more slowly and become more granular.
Cities and industrial areas usually have more air pollution, especially sulfur, which will lead to oxidation deeper than in rural areas.
Unfortunately, even the fine rust coating on weathering steel can contaminate runoff water, and while it is attractive to steel, it can quickly damage stone and concrete pavements. However, there are ways to prevent this from happening.
If a corten steel drill is installed next to the pavement, the most common solution is to leave a cement gap of 5 to 10 mm between the drill and the pavement. If installed on a pedestal platform system, the gasket will have the same result. This allows any moisture to run off below the finished floor (FFL) and around the paving.
If for any reason a gap is not feasible, a deeper, gravelly boundary can run along the outer edge of the planting wall. This is an attractive feature that helps with drainage and can also fill the space with gravel.
Where the weathering steel product hangs over the road surface, at AHL we can coat the underside and accessories of the product with powder to make it look like weathering steel, but without the oxidation that leads to unsightly stains.
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