Corten Life Span Information

Weathering steel alloys, are often refereed to as Corten,

Corten steel, was a steel alloy that was first developed in 1933 by the US steel Corporation.

Weathering steel  ASTM A606-4 often sold as Corten,  develops a protective oxide film, when exposed to repeated wet-dry cycles.

Unlike raw steel, Cor-Ten steel is corrosion resistant due to its alloy composition, it does not show bubbling, flaking or peeling which normally occurs with raw steel

This oxide bonds with the base metal and continues to corrode over time at a much much slower rate.

The inherent nature of the rusting process, is that variations in colour, hue and texture should be expected, these variations are not considered flaws or defects
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.

corten vs mild steel corrosion

Fixings in direct contact with corten steel should be stainless steel to avoid bimetallic corrosion issues

Zinc coated rivets, bolts and screws are not recommended. Galvanic corrosion of the zinc coated fixings, leads to a rapid failure of the screws

Zinc flashings should not be in direct contact with corten, to avoid dissimilar metallic corrosion . A solution can be to install a non conductive permanent insulation layer between the two dissimilar metals

Increased thickness = increased life span

Adding 1mm to the exposed face of corten steel adds approx 100yrs to life span

The life expectancy of weathering steel depends on the following

1/ the environment

2/ the steel has repeated wet/dry cycles to form and maintain the protective oxide coating

3/ the thickness of the steel specified

For example

In urban areas of moderate pollution, and low salinity, over a 50yr period one may expect 0.5mm of corten steel to corrode away

For an exposed steel beam this would mean adding 1mm to the flange thickness at year zero, to ensure that at year 50 the flange thickness is still at the design thickness.

For longer required life spans and more corrosive environments additional thickness would be added.

The NZ building code requires that roofing and cladding have a minimum 15 years resistance to water penetration.

For structural elements the requirement may be 50yrs

Common thicknesses of Colorsteel and other Zincalume products is 0.4mm or 0.55mm

In conclusion 2mm thick corten weathering steel in moderate urban environments should be approx 1mm thick after 50yrs weathering from both sides.

The weathering process is not linear, with approx 80% of the decay occurring in the first years of life, most specifiers therefore tend to use steel that is 0.5mm to 1.0mm thicker than required to allow for unexpected corrosion.

In applications where the corten steel can not maintain the wet/dry cycles to establish the protective oxide, such the inside of planters. It is recommended to seal inside/unseen steel with a bitumen damp proof membrane such as Fostoc Mulseal Plus or equivalent

There is no warranty with unprotected raw steel, the specifier matches the expected environment conditions and the selected thickness of steel, to arrive at a prediction of the expected life span. Domestic landscaping applications may be happy with an expected life span of 30yrs plus, while bridge engineers often require 120yrs plus

Shipping container are made from corten steel, and are then painted to extend there economic life.

Q355NH Weathering steel

Chemical composition(%)

C:≤0.16 Si≤0.5 Mn:0.5-1.5 P≤0.03 Cr:0.4-0.8 Cu:0.25-0.55

Mechanical property

Yield strength/Mpa:≥355 Tensile strength/Mpa:490-630 Elongation/%≥22