LEXICONMaterials and finishes
Weathering steel
Also called: Corten, COR-TEN, ASTM A242, A588, EN 10025-5
A low-alloy structural steel with small additions of copper, chromium, nickel and phosphorus that forms a dense, adherent rust layer, slowing further corrosion so the steel can be left unpainted outdoors.
What it does and does not do
The alloying does not prevent oxidation; it changes the structure of the oxide that forms, producing a layer that slows further attack instead of flaking off and exposing fresh metal. The protective layer develops properly only under alternating wetting and drying. In permanently damp conditions — buried, submerged, sheltered but condensing — it never forms, and the steel corrodes at rates approaching ordinary carbon steel. Chlorides disrupt it, so unpainted weathering steel is generally advised against near the sea.
Indoors it has no advantage
Inside a heated building there is no rain, no wet-dry cycling and low humidity, so weathering steel never develops its designed patina and behaves essentially like mild steel with a decorative surface. The corrosion resistance the alloy premium paid for does nothing. It also stains: for the first two years, runoff carries oxide onto whatever sits below, and on porous substrates that stain is effectively permanent.