Corten works because it forms a dense, tightly adherent oxide layer that seals the surface and slows further corrosion. That process depends on wet–dry cycling. In marine air the surface never fully dries and chloride ions disrupt the oxide as it forms, so the patina never stabilises — the steel simply keeps corroding, at a rate that will visibly thin a four-millimetre plate within a couple of decades.
We use five kilometres as a working threshold, but the real variable is exposure rather than distance. A piece behind a building on a sheltered estuary at two kilometres may be fine; a piece on an exposed promenade at eight kilometres facing prevailing onshore wind may not be. Where it is marginal, we ask for a corrosivity category assessment to ISO 9223 rather than guessing.
The alternative is 316L stainless. The two to three per cent molybdenum is what resists chloride pitting, and it is the reason we will not specify 304 in a coastal setting regardless of the cost saving. Expect light tea staining in aggressive exposure — a brown surface discolouration that is cosmetic rather than structural and removable with a proprietary cleaner. Clients experience it as failure, so we say it will happen before it happens.
Fiberglass is the third option and it is genuinely good in marine air, because there is nothing there to corrode. The trade-off is service life: fifteen to twenty-five years against a century for stainless. For a temporary installation, a resort refurbishment on a ten-year cycle, or a weight-constrained roof terrace, that is the right trade. For a civic memorial it is not.
If a client insists on the corten look at the coast, the honest options are a corten-effect coating over galvanised steel, or accepting a shorter life and a maintenance commitment. We will quote either, with the consequence written down.
