Corten, also known as weathering steel, is a steel alloy initially developed by the US Steel Corporation in 1933. ASTM A606-4, often sold under the name Corten, forms a protective oxide film when exposed to alternating wet and dry conditions. Unlike untreated steel, Cor-Ten steel is resistant to corrosion due to its alloy composition, preventing bubbling, flaking, or peeling commonly seen with raw steel.
The lifespan of weathering steel depends on factors such as environmental conditions, wet/dry cycles, and steel thickness. For example, in urban areas with moderate pollution and low salinity, Corten steel may corrode approximately 0.5mm over 50 years. To accommodate unexpected corrosion, designers often choose steel that is 0.5mm to 1.0mm thicker than required.
Due to the nature of the rusting process, variations in colour, hue and texture should be expected. Cor-Ten steel is sold un-weathered (grey/blue steel) and will produce a rust run-off which will may stain stone, concrete, and decking surfaces if not allowed to weather first. To limit staining to surrounding surfaces, place your planter in grass or soil during the initial weathering period.
When comparing Corten to mild steel corrosion, the nominal thickness varies depending on the location and atmospheric conditions. For rural, urban, or industrial areas with different pollutant levels, the recommended thickness changes over time to ensure longevity.
To fasten Corten weathering steel, it's advised to use stainless steel TEK screws to avoid bimetallic corrosion caused by dissimilar metals. Direct contact between Corten and galvanized steel is not permitted; isolation pads should be used to prevent corrosion. Similarly, zinc flashings should not directly touch Corten to prevent dissimilar metallic corrosion. Installing a non-conductive insulation layer between dissimilar metals can mitigate this issue.
For applications where Corten steel cannot undergo wet/dry cycles to develop the protective oxide layer, such as the inside of planters, it's recommended to seal the unseen steel with a bitumen damp proof membrane.
There's no warranty with unprotected raw steel; the specifier must match the expected environment conditions with the selected steel thickness to estimate the lifespan. While domestic landscaping applications may accept a 30-year lifespan, bridge engineers often require 120 years or more.