When selecting Corten Steel for outdoor and structural use, designers and buyers often compare ASTM A606 Type 2 and Type 4. While both fall under the same standard and offer weathering performance, they differ in alloy design, corrosion resistance, and ideal use cases.
The core difference: Type 2 relies on minimum copper for basic weathering; Type 4 adds extra alloys for stronger corrosion resistance and meets a formal corrosion index requirement.
Alloy & Corrosion Resistance
ASTM A606 Type 2Requires a minimum 0.20% copper (heat analysis) to deliver improved atmospheric corrosion resistance over carbon steel. Verification is done simply by checking copper content.
ASTM A606 Type 4Includes additional alloying elements (chromium, nickel, etc.) beyond copper. It must meet a minimum corrosion index of 6.0 per ASTM G101, giving it substantially better weather resistance than Type 2.
Strength & Structural Performance
Both are high‑strength low‑alloy (HSLA) Corten steels, but Type 4 typically provides higher tensile strength, making it suitable for lighter‑gauge high‑load parts.
Typical Applications
Type 2: General outdoor structures, simple cladding, frames, and non‑critical architectural parts.
Type 4: Long‑life exterior panels, facades, coastal‑friendly components, and projects where bare, unpainted service is preferred.
Quick Summary
Type 2: Cost‑effective, copper‑based basic weathering.
Type 4: Higher corrosion resistance, alloy‑enhanced, index‑certified.
Choose Type 2 for standard outdoor use; select Type 4 when you need stronger, verified long‑term weather protection.
