Weathering steel, widely known by the trade name COR-TEN, is a group of low-alloy steels designed to form a stable, protective rust layer when exposed to the atmosphere. Ordinary carbon steel also rusts, but its rust is porous and keeps attacking the base metal, causing progressive section loss. Identifying weathering steel correctly is important for structural safety, maintenance planning, and aesthetics.
Colour and Texture of the Patina
Freshly rolled weathering steel looks similar to ordinary steel: grey-blue with a mill scale surface. After several months of wet-dry cycling, weathering steel develops a dense, even patina in warm brown to dark chocolate tones with a slightly granular, velvety texture. Ordinary steel, by contrast, forms a loose, flaky, orange-brown rust that rubs off on the fingers and shows visible pitting underneath. In most climates the protective patina begins to form within 6-9 months, but in dry or very cold climates it can take one to two years or longer.
Mill Markings, Certificates, and Traceability
Structural weathering steel is normally delivered with mill test certificates that state the standard (for example EN 10025-5, ASTM A588/A242, or JIS G3114) and the grade (S355J2W, A588 Grade A, SMA490AP, and so on). Plates and beams often carry painted grade markings. If a piece has no certification and no markings, treat it as ordinary steel; most weathering steel is supplied with full traceability because it is a controlled low-alloy product.
Simple Field Checks
Scratch test: on a fresh cut edge, weathering steel shows a brighter, finer-grained surface than ordinary hot-rolled steel, and its rust layer stays thin and tightly bonded. Magnet and spark tests cannot distinguish the two reliably. The only definitive check is chemical analysis: weathering steel contains copper, chromium, nickel, and sometimes phosphorus in measurable amounts, while mild steel is essentially iron and carbon with trace residuals. A portable X-ray fluorescence (XRF) analyser will show the copper and chromium peaks typical of weathering steel.
Composition Differences
Mild steel is mostly iron with up to about 0.25% carbon. Weathering steel adds small amounts of copper (0.25-0.55% typical), chromium (0.40-1.25% depending on grade), nickel (0.25-0.65%), and in some grades phosphorus (about 0.07-0.15%). These elements make the rust layer dense and adherent instead of porous and flaky, which is the whole principle behind the material's long unpainted life.
Behaviour and Life-Cycle Cost
Weathering steel costs more than mild steel at purchase, but in suitable climates it eliminates repainting cycles, so the life-cycle cost is often lower. Mild steel left unpainted rusts faster and needs frequent maintenance. Both materials are fully recyclable; weathering steel avoids the environmental burden of paint systems. However, weathering steel is not suitable for permanently wet conditions, buried service, or severe marine splash zones, where even its protective patina cannot prevent accelerated attack.
Frequently Asked Questions
Q1: How long does corten steel take to rust?
A1: The patina typically begins forming within several months to a year, depending on climate and exposure; full stabilization can take one to three years.
Q2: Is rust on corten steel a sign of damage?
A2: No. The stable patina is protective. Damage is indicated by loose, flaky rust, pitting, or section loss, which suggest the steel is not genuine weathering steel or is used in the wrong environment.
Q3: Can I grow vegetables in corten planters?
A3: Weathering steel is generally considered safe for garden use; small amounts of iron and alloying elements can leach into soil. For edible crops, follow local guidance and avoid acidic soil conditions that accelerate corrosion.
Q4: What is the difference between corten steel and galvanized steel?
A4: Galvanized steel is coated with zinc for sacrificial protection; corten steel relies on its own alloyed patina. They have different appearances, corrosion mechanisms, and cost profiles.
Q5: How can I verify a steel is really weathering grade?
A5: Check the mill certificate for an EN/ASTM/JIS weathering grade, look for alloying elements with XRF analysis, and confirm the patina stays thin and even rather than flaking.
