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Why is Corten left to rust?

The natural patina of Corten steel has gained popularity in recent years, as its subtle hues of orange and brown complement a more naturalistic approach to landscaping and garden sculpture. Maybe the best-known example of this is Antony Gormley’s Angel of the North at Gateshead, although it is widely used in many less ambitious surroundings such as private and public gardens, parks, and terraces.

This patina is formed by oxidation of the steel, creating a fine layer of rust. Conventional mild steel will form a light, crumbly layer of rust that retains moisture and allows oxygen to reach the non-corroded metal, so corrosion will continue until the steel is completely eroded.

Due to the alloy composition of Corten steel, this layer is denser and acts as a barrier to oxygen and moisture that hinders the corrosion process.

What causes Corten to oxidise?

Corten is conventionally supplied and installed before the oxidation process begins, and will generally have a dark grey finish. Following installation, exposure to water, oxygen, sunlight, and airborne pollution begins the rusting process.

These factors all influence how quickly Corten will acquire its protective oxide film and even the appearance of the film. In the northern hemisphere, surfaces facing south or west will tend to be warmed and dried by the sun more frequently and quickly develop a smoother and more uniform finish than those facing north and west, whose finishes will be slower to develop and more granular.

Urban and industrial areas typically have more airborne pollution which – particularly sulphur – will cause the oxidation to be darker than that in rural areas.

Pre-weathered Corten steel planter at Tailors Corner

How can I protect my paving?

Unfortunately even the fine layer of rust on Corten can stain run-off water, and while it is attractive on the steel, it can quickly disfigure stone and concrete paving. There are, however, ways in which this can be prevented.

If a Corten planter is being installed next to paving, the most common solution is to leave a five-to-ten milimetre cement gap between the planter and the paving. If the installation is on a pedestal terrace system, a spacer will have the same result. This allows any water run-off to fall below the finished floor level (FFL) and bypass the paving.

If a gap is not viable for any reason, a deeper, gravel border can run along the outside edge of the planter wall. This can be an attractive feature and assist with drainage also the space can be infilled with gravel. ExcelEdge AluExcel can also be used to define a neat edge that follows the curvature of paving and planters.

Where a Corten product overhangs paving, at Kinley we can powder-coat the product’s underside and fixings with a finish that looks like Corten but won’t develop the oxidation that causes unsightly stains.

Corten steel planter at North West Cambridge