A singular substance creates a visible dialogue between the past and the present. It changes every day. It flows onto the pavement like a river delta leaving behind striations cut in the color of orange. Massive yet graceful, Cor-Ten speaks in echoes and pattering of rain drops.
Cor-Ten (or weathering steel) is a steel product that oxidizes very rapidly, then maintains a stable, dark-brown, rusted surface. Alloyed with copper (Cu) and chromium (Cr), Cor-Ten resists atmospheric corrosion by developing and continuously regenerating a protective layer of rust on its surface that inhibits deeper penetration by atmospheric agents when subjected to the weather. Made primarily of steel, or iron oxide (Fe2O3) with a little bit of carbon (C), Cor-Ten has high tensile strength and compressive strength, allowing it to be used as a structural component for buildings, sculptures, and bridges. The addition of chromium increases hardness and melting temperature and prevents corrosion by forming a hard oxide layer on the surface. Consequently, Cor-Ten exhibits much greater resistance to atmospheric corrosion than unalloyed steels.1
Richard Serra, Torqued Ellipse, UCLA
The surface texture of Cor-Ten depends largely on the nature of its surrounding atmosphere. The macroclimate (industrial, urban, or maritime) drastically changes the effectiveness of the material against corrosion. Although Cor-Ten resists weathering by rain, snow, fog, and ice, it cannot stand up to a constantly submerged environment or the presence of salt. The readiness of salt to take up moisture maintains a constantly damp environment on the metal surface, causing it to rust beyond its ability to regenerate a protective layer. As a result, Cor-Ten should not be used within 2km of the coast line. In a marine environment, applying conventional coating and performing maintenance works on the Cor-Ten by removing salt buildup quickly gives it the chance to regenerate its protective rust layer.
Cor Ten
The orientation of the Cor-Ten (exposed to or shaded from weathering, vertical or horizontal position) also affects how the material stands up to the climate. Cor-Ten surfaces facing south and west and those exposed to frequent wet and dry cycles develop a smoother fine-grained texture. North and east facing surfaces and those that are shaded develop a coarser texture.2
Lastly, the connection points between panels of Cor-Ten must be detailed such that weld-points and bolts weather at the same rate as the Cor-Ten. Using welded consumables matching the base material ensures that the welded joint also resists corrosion. Threading bolts made of weathering steel into the Cor-Ten material prevents the formation of localized electrochemical cells. (When a metal loses electrons, it becomes positively charged and quickly reacts with oxygen to create iron oxide, or rust. If this process happens too rapidly, the Cor-Ten cannot regenerate its protective layer in time.)3 In addition, applying sealants around the joints stops capillary action from inflicting permanent moisture damage by corrosion.
Although a finicky material to deal with, Cor-Ten expresses every architect’s desire to design the building to express the passing of time, poetically, beautifully.
1. Thyssen Krupp. COR-TEN.
2. Finishing.com. “Marine environment effects on corten steel in relation to public art”
3. NASA. Corrosion Technology Laboratory.
Tags: climate, cor, environment, erosion, evolution, materials, structure, ten, time, weather