One Material Family, Three Product Forms
Weathering steel reaches the market as heavy plate, as hot or cold rolled sheet, and as profiled panels or cassettes. The chemistry is similar in all three, but the governing standard changes with the form, and that is where most specification errors start. Heavy plate is normally ordered to ASTM A588/A588M with a minimum yield strength of 345 MPa and a minimum tensile strength of 485 MPa for the standard plate grades. Sheet and strip for cladding, roofing and containers are generally ordered to ASTM A606/A606M, where Type 4 is the weathering type supplied to a minimum yield strength of 345 MPa and a minimum tensile strength of 485 MPa. Architectural panels are simply sheet or plate that has been cut, folded and finished to a facade drawing.
Available Thickness and Delivery Condition
| Product form | Typical thickness | Typical standard |
|---|---|---|
| Hot rolled sheet and strip | 1.5–6 mm | ASTM A606/A606M Type 4, EN 10025-5 |
| Plate for structural and architectural use | 6–200 mm | ASTM A588/A588M, EN 10025-5, GB/T 4171 |
| Profiled and folded panels | 1.5–8 mm formed | Formed from sheet to project drawing |
| Length range, cut to order | 3,000–12,000 mm | Cut-to-length or mill length |
Plate is delivered hot rolled, edge trimmed and either mill finish, descaled or shot blasted; sheet is delivered hot rolled or cold rolled with an oiled or dry surface. Surface condition matters because blasting removes the mill scale that would otherwise flake unevenly during the first wetting cycles and delay the formation of an even patina.
How the Patina Forms, and When It Does Not
The protective layer is built by wetting and drying cycles over months, not by the alloy content alone. During each cycle the surface rust layer becomes denser and more chromium and copper enriched, until moisture and oxygen penetration is slowed dramatically. Three conditions prevent this: permanent wetting, where the surface never dries; burial or continuous contact with masonry or soil; and long periods of shelter where rainfall never washes the surface. The corrosion index described in ASTM G101 is the accepted way to screen a composition for expected atmospheric performance, and the corrosivity categories of ISO 9223, from C1 for very low corrosivity to CX for extreme, tell the designer what the environment will do to exposed steel. A coastal or heavily industrial site can shift a project by two categories, which changes the corrosion allowance rather than the grade.
Panels for Siding and Roofing
Weathering panels are usually detailed as a drained and back-ventilated rainscreen. Each panel should be isolated from the substrate by continuous standoff, fasteners should be stainless or galvanically compatible with the patina runoff, and horizontal surfaces should be laid to a fall so that water does not pond in the folded joints. Runoff is a real design item, not a cosmetic detail: the iron-rich water that drains off a new facade will stain adjacent stone, concrete and paving for the first years of service, and provision for it belongs in the architectural package. Where the patina cannot form, as on sheltered soffits or in constant splash zones, a coating system is the appropriate solution.
Cutting, Forming and Welding
Weathering plate and sheet cut and form like other low-alloy structural steels of the same strength class. Plasma, waterjet and laser cutting are all used, and the cut edge will rust slightly faster than the rolled face until the patina establishes itself. Bending radii follow the strength class rather than the corrosion-resistant chemistry. Welding at these thicknesses uses normal low-hydrogen consumables and controlled heat input; for exposed architectural work the weld should be dressed so that the patina develops evenly, because an undressed weld cap with different alloy content will weather to a visibly different colour.
Frequently Asked Questions
Q: What is the difference between weathering steel plate and weathering steel sheet?
A: The product form and the governing standard. Heavy plate is specified to ASTM A588/A588M or an equivalent structural grade, while sheet and strip are usually specified to ASTM A606/A606M, with Type 4 being the weathering type.
Q: How thick can weather resistant plate be supplied?
A: Hot rolled sheet starts from about 1.5 mm, and heavy plate is routinely produced from 6 mm up to about 200 mm, with cut-to-length service available in the 3,000 mm to 12,000 mm length range.
Q: How long does the patina take to stabilise?
A: It develops over months to a few years depending on rainfall, wetting frequency and air quality. Panels in a well-exposed facade with regular wetting and drying cycles stabilise faster than sheltered surfaces.
Q: Will weathering steel panels stain the facade below them?
A: Yes, in the early years. Runoff carrying iron oxide will streak concrete, stone and paving until the patina stabilises, so drainage paths, drip edges and splash zones should be detailed deliberately.
Q: Can weathering panels be used in a marine environment?
A: They will corrode faster and the patina will be less protective where chloride deposition is high. Corrosivity classification to ISO 9223 allows the environment to be assessed and the corrosion allowance adjusted accordingly.
Q: Do the panels need a protective coating on the back face?
A: Where the reverse face is enclosed, permanently wet or in contact with another material, a coating is advisable because the patina cannot form there and hidden corrosion would go unobserved.
