Compare Corten A and Corten B weathering steel to choose the right grade for atmospheric corrosion resistance in structures and cladding, and learn what to specify when ordering.
Corten weathering steel develops a stable rust patina that protects the surface, which lets it be used unpainted in many atmospheric environments. The two main grades, Corten A and Corten B, are not identical: Corten A is a phosphorus-bearing grade optimized for thin sheet and cladding, while Corten B is a higher-strength grade for plate and structural members. Choosing the wrong one affects both formability and strength, and supplying ordinary structural plate in its place destroys the whole point of specifying weathering steel. This guide explains how to match the grade to the application.
How Corten A and Corten B Differ
Corten A is the original atmospheric corrosion-resistant grade, with higher phosphorus and a chemistry tuned for forming into thin sheet and light plate, giving excellent weathering performance in moderate atmospheres. Corten B carries higher carbon and is supplied as plate with higher minimum yield strength, making it the structural version suited to thicker sections and load-bearing members. Both rely on the same copper-chromium-nickel alloying to form the protective patina, but their strength and formability targets differ, so they are chosen for different parts of a structure.
When Corten A Is the Right Choice
For architectural cladding, roofing, decorative panels, planters, and light-gauge formed parts where appearance and atmospheric corrosion resistance matter more than high strength, Corten A is the appropriate grade. Its formability suits the thin gauges used in facades, and its stable patina gives the characteristic weathered look without painting. Because it is supplied in thinner sheet and light plate, it is the grade most often seen in architectural and landscaping applications.
When Corten B Is the Right Choice
For structural members, bridge components, freight containers, heavy plates, and any weathering part that must carry load, Corten B is the correct grade. Its higher yield strength suits thicker plate, and its weathering chemistry protects the structure without coating where the patina can form properly. For welded structural assemblies, Corten B is matched with compatible welding consumables, since the phosphorus and alloy content must be accounted for in the weld to preserve corrosion performance.
What to Specify on the RFQ
Name the grade explicitly, Corten A or Corten B, by the relevant standard such as ASTM A588 or A242, or EN 10025 S355J0W or S355J2W, rather than writing only weathering steel or Corten. Specify the thickness range, since Corten A is typically thin and Corten B is typically plate, and request the full chemistry including copper, chromium, and phosphorus on the mill test certificate. If the part will be welded, state the welding consumable requirement, and confirm that ordinary structural plate such as A36 or Q235 is not an acceptable substitute, since it lacks the alloying needed to form the protective patina.