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
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 New Zealand 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 Sika Blackseal 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.