Corten A and Corten B: The Two Weathering Grades
Corten steel is the trade name for a family of weathering steels whose surface forms a protective rust-like layer when exposed to the elements, minimising corrosion and the need for painting. The family has two well-known members. Corten A is the high-phosphorus grade, alloyed with copper, chromium and nickel plus a deliberate phosphorus addition, which gives it the highest atmospheric corrosion resistance in the family and a distinctive aesthetic; it is used for gas flues, facades and architectural cladding. Corten B is the structural grade, with lower phosphorus and higher strength and toughness, used for bridges, containers and heavy-load applications. Both are weldable, and both rely on the same patina mechanism for protection.
Chemical Composition Differences
Typical composition of the two grades (commonly cited values):
| Element (%) | Corten A | Corten B |
|---|---|---|
| Carbon, max | 0.12 | 0.16 |
| Silicon, max | 0.75 | 0.50 |
| Manganese, max | 1.00 | 1.50 |
| Phosphorus | 0.06–0.15 | 0.030 max |
| Sulfur, max | 0.030 | 0.030 |
| Chromium | 0.30–1.25 | 0.40–0.80 |
| Copper | 0.25–0.55 | 0.25–0.55 |
| Nickel, max | 0.65 | 0.65 |
The phosphorus range is the key difference: Corten A uses it for accelerated, uniform patina formation, while Corten B keeps phosphorus low for toughness and weldability in structural members.
Mechanical Properties
In the common thickness range, Corten B delivers a minimum yield strength of about 345–355 MPa with a tensile range of 470–630 MPa, while Corten A sits in the same yield tier with slightly lower toughness. Elongation is typically 20% or higher. The structural grades such as Corten B are the correct choice for load-bearing members; Corten A is specified where corrosion resistance and appearance dominate and loads are light.
Thickness and Product Range
Weathering steel plate is commonly available in thicknesses from 3 mm to 40 mm in hot-rolled supply, with the 3 mm, 5 mm and 8 mm gauges covering most architectural cladding, edging and light structural applications. Thin sheet from about 0.5 mm to 2 mm is supplied cold rolled, and thicker plates up to 100 mm or more are available from plate mills. Length is typically 4–12 m and width 0.6–3 m, cut to requirement. The surface is supplied clean, or blasted and painted where the design calls for a coated finish.
Applications: Architecture and Structure
Corten plate is used for facades, cladding, edging, planters and landscape features, where the rusted appearance is an aesthetic feature, and for structural applications such as bridges, containers, transmission towers and industrial equipment, where the protective patina removes the need for painting. Because the patina reduces maintenance over decades, the material is also used for noise barriers, retaining structures and outdoor sculptures.
Frequently Asked Questions
What is the difference between Corten A and Corten B?
Corten A has a high phosphorus content (0.06–0.15%) for maximum corrosion resistance in mild atmospheres but lower toughness; Corten B has low phosphorus, higher strength and better weldability for structural use.
Which thickness should I choose for cladding?
For architectural cladding and edging, 3 mm, 5 mm and 8 mm plates are the most common choices; heavier gauges are used for structural members and retaining structures.
Can Corten plate be welded?
Yes, with standard arc processes. Weathering-grade filler metals are recommended so the weld zone matches the corrosion behaviour of the base plate, and low-hydrogen practice is used for thicker sections.
Does Corten steel need painting?
No. The material is designed to be used bare so the patina develops; painting cancels the weathering benefit and is only used in marine or permanently wet environments.
What standard covers Corten A and Corten B?
There is no single ASTM grade called Corten A or B. The family is covered by standards such as EN 10025-5 (S355J0WP for the A-type chemistry, S355J2W for the B-type), ASTM A588 and A242, and GB/T 4171 (09CuPCrNi-A, Q355NH).
How long does the patina take to form?
Under normal outdoor wet-dry cycling, a stable patina develops over about six to twelve months, deepening in colour with continued exposure.
