Corten (weathering) steel

What is Corten steel?

Corten steel is the common name for weathering steel, a low-alloy structural steel designed to rust in a controlled way. Instead of being painted or coated, its surface oxidises into a dense brown-orange layer called patina, and that layer slows further corrosion. COR-TEN is a registered trademark, so in specifications and in Slovak trade talk the generic term cortenová oceľ (weathering steel) is the safer one to write.

The patina is not there on delivery. During the first years of exposure the surface keeps changing colour and sheds loose oxide, so the finished look is a stage in the steel's life rather than its starting point. Any specification should say which stage the client is actually buying.

How does the patina form, and why do wet-dry cycles matter?

The protective layer forms when the surface gets wet, dries out, and gets wet again. Each drying phase lets the oxide consolidate, and each wet phase keeps the chemistry moving. A surface that stays wet all the time never settles, while one that rarely gets wet barely develops colour, so the patina stays thin and blotchy. Water that pools on a ledge or sits against a foundation creates the constant-damp condition that defeats the process.

Table 1 summarises how common exposures on a house behave and what the design response should be.

Location	Typical behaviour	Design response
Vertical facade open to sun and rain	Regular wet-dry cycles, patina forms evenly over time	Ventilated build-up that lets the cladding weather freely
Under an eave or balcony	Dries slowly, colour develops later and unevenly	Accept the lag, or shed water from the sheltered area onto open ones
Horizontal ledges, cappings, pooling areas	Water sits, the layer stays soft and wet	Falls and drip edges so water leaves quickly
Ground-level splash zone or planter base	Constant damp and corrosion from below	Keep the steel off direct soil contact and allow drainage
Next to a salted driveway or pavement	Salt keeps the surface damp and aggressive	Limit exposure, rinse where practical, avoid low fixings in the spray zone

Why does Corten stain paving and render below it?

During the early years, rain washes fine oxide particles off the surface. These particles settle on whatever lies below: porous paving, natural stone, concrete kerbs, and mineral or silicate render. The rust-coloured stain is hard to lift from porous materials. The run-off is normal and it reduces as the patina matures, but the timing depends on weather and orientation, so a client should not assume it will stop quickly.

Three details manage most of the problem. A projecting cap or drip edge throws water clear of the wall before it reaches the render. A gravel strip or drainage channel at the base catches run-off before it reaches a patio. Planters need particular care, because water passing through soil can carry salts and iron that stain adjacent concrete as well as the steel itself.

How is Corten steel different from galvanized steel?

Both are steel, but they protect themselves in different ways. Galvanized steel carries a zinc coating that sacrifices itself to protect the iron beneath, and it dulls to a fairly uniform grey. Corten has no coating. It relies on its alloy and a patina that keeps changing colour, which is why every sheet looks slightly different and why it can stain its surroundings. Galvanized steel is the better choice where a clean, uniform, stain-free surface matters more than the rusted look.

Table 2 sets the two side by side for typical domestic use.

Factor	Corten (weathering steel)	Galvanized steel
Surface	Uncoated, forms a brown-orange patina	Zinc coating, matte grey finish
Look over time	Changes for years, unevenly	Dulls gradually and fairly uniformly
Early run-off	Rust-coloured, can stain paving and render	Little staining in normal use
Cut and welded areas	Cut edges weather like the face	Cut or welded areas need zinc-rich repair
Typical domestic use	Facade cladding, edging, planters, gates	Structural parts, fixings, roofing details

What happens when Corten touches other metals or de-icing salt?

Corten is most vulnerable at two places: where it meets a different metal, and where salt builds up. Where a dissimilar metal is in contact with it, the two can corrode at the junction. Choose fixings, brackets and cladding screws in a compatible material, and keep copper gutters or copper flashings from discharging their run-off onto the steel.

De-icing salt is an accelerant in winter. It keeps surfaces damp, disrupts the layer as it tries to form, and carries chloride into joints and fixings. Gates and fences beside a gritted driveway, and cladding at pavement level, need far more attention than cladding high on a sheltered wall.

Where does Corten suit a Slovak home?

In residential work, Corten is most often specified as facade cladding, garden edging, planters, fences and gates. On a facade it is usually run as a rainscreen element on a ventilated facade, with a rainscreen cavity behind it, because the steel is not a weathertight barrier in its own right. The cold winters, freeze-thaw cycles and winter road salt typical of Slovakia mean that the lower parts of a house and any side facing a gritted road need the most careful detailing. Use Corten where the patina can develop in open, drying conditions, and avoid it in permanently damp corners or where it will sit in soil or salted pavement.

What do homeowners most often get wrong about Corten?

• Sealing it stops the rust. Clear coatings are unreliable outdoors and trap moisture where they fail. The honest approach is to design for the patina.
• It is stainless. It is a corrosion-resistant steel that changes colour and stains its surroundings.
• It is maintenance-free. It needs no repainting, but it does need drainage checked, fixings inspected and run-off cleaned up.
• Dark wet patches mean failure. Wet patina looks darker than dry patina, and it lightens again as it dries.
• Every sheet looks the same. Colour varies with sun, position and age, so mock up a sample on site before committing.

How should you specify Corten for a home?

Ask the supplier for the steel grade and its documented weathering behaviour, and request a sample panel exposed on site before ordering the full run. Specify the surface condition as well: some steel is delivered in its as-rolled state and some is pre-weathered. Pre-weathering can reduce early run-off, but it takes time and adds cost. Then check the detailing:

• drip edges and falls on every horizontal and projecting edge;
• a clear gap at the base of cladding, with no contact with soil;
• a gravel strip or channel to catch run-off before it reaches paving;
• fixings and flashings in compatible metals;
• a ventilated cavity behind any facade cladding.