Weathering Steel (Corten) for Facades and Screens

What it is

Low-alloy steel containing copper, chromium, nickel and phosphorus, which forms a dense oxide layer that adheres to the surface and slows further corrosion — instead of the flaking rust of ordinary steel.

It is specified for its appearance: a deep orange-brown that changes with weather and age.

Specification at a glance

The conditions it needs

Weathering steel works only where the surface goes through genuine wet then fully dry cycles. That rules out:

• Permanently shaded or sheltered positions
• Details that trap or pond water
• Buried or submerged elements
• Coastal and chloride-laden atmospheres
• Continuously humid enclosed spaces

Where those conditions apply, it simply keeps corroding.

Designing for run-off

The staining is the practical problem on most projects. Plan for it:

• Drip details that throw water clear of surfaces below
• Avoid light-coloured stone, concrete or paving directly beneath
• Protect or plan to clean adjacent surfaces during the stabilising period
• Consider factory pre-patinated material where the programme cannot absorb it

Where projects go wrong

• Coastal specification. The patina never stabilises.
• No run-off strategy, so the plaza below is permanently stained.
• Water-trapping details — flat horizontal ledges, closed sections with no drainage.
• Carbon steel fixings, which corrode at a different rate and streak.
• Contact with other metals without isolation, setting up galvanic corrosion.

Standards referenced

Weather-resistant structural steel under IS 11587 and ASTM A588. Wind loading on cladding under IS 875 (Part 3).