# Weathering Steel (Corten) for Facades and Screens

> Weathering steel (Corten) is a low-alloy steel containing copper, chromium, nickel and phosphorus that forms a dense, adherent oxide layer which slows further corrosion. It works only where the surface goes through repeated wet-then-dry cycles, so it does not suit coastal, permanently damp or water-trapping positions, and its run-off stains surfaces below.

## 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

| Property | Typical specification |
| --- | --- |
| Standard | IS 11587, or ASTM A588 |
| Thickness | 3mm — 8mm for panels and screens |
| Finish | Natural weathered, or accelerated patina |
| Fixing | Stainless or matching weathering steel |
| Drainage | Free-draining; no water traps anywhere |
| Fire class | Non-combustible |

## 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)**.

## Frequently asked questions

### Does weathering steel stop rusting?

It forms a dense, adherent oxide layer that slows further corrosion dramatically, but only under the right conditions: it must get wet and then dry out fully in repeated cycles. Where it stays permanently damp — buried, submerged, under constant shade, or in a detail that traps water — the protective layer never stabilises and the steel keeps corroding like ordinary steel.

### What is the run-off problem?

While the patina is stabilising, which takes a season or more, rainwater carries oxide off the steel and stains everything below it. Concrete, stone, paving and glass downstream can be permanently marked. This has to be designed for from the outset with drip details, sacrificial protection during weathering, and careful choice of what sits beneath the panels.

### Is Corten suitable for Indian coastal sites?

Generally not. Chloride exposure prevents the protective layer from stabilising, so the steel continues to corrode at a high rate rather than slowing down. Weathering steel suits inland atmospheres with clear wet and dry cycles. Near the coast it is the wrong specification regardless of how good it looks in the render.

## Sources

- [IS 11587](https://standards.bis.gov.in/website/published-standards/department-wise) — Structural weather resistant steel
- [ASTM A588](https://www.astm.org/products-services/standards-and-publications.html) — High-strength low-alloy structural steel with atmospheric corrosion resistance
- [IS 875 (Part 3)](https://standards.bis.gov.in/website/published-standards/department-wise) — Wind loads on buildings

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Source: https://hagerstone.com/materials/weathering-steel-corten