Its alloying elements, principally copper, chromium, nickel and phosphorus, change the nature of the corrosion product. Instead of porous rust that flakes and exposes fresh metal, it forms a dense, tightly adherent patina that acts as a barrier and slows further corrosion to a small fraction of the initial rate. The steel therefore reaches a near-stable condition rather than continuing to lose section at a constant rate.

Where the wet and dry cycling that forms the patina does not occur. Permanently wet surfaces, buried or partly buried steelwork, enclosed spaces without ventilation and areas kept damp by debris will corrode broadly as ordinary steel does. Marine atmospheres and locations exposed to de-icing salt are also unsuitable, because chlorides prevent a protective patina forming and can produce accelerated attack instead.

Before the patina stabilises, which takes a period of years, rainwater running off the steel carries rust particles and stains whatever it lands on. Concrete, paving, masonry and glass mark permanently. It is designed around with drip details that throw water clear, sacrificial protection to surfaces below during the early years, collection and drainage details, or by selecting materials below that tolerate staining.

To shed water and avoid retaining it. Avoid horizontal surfaces and upward-facing ledges that collect water and debris, avoid crevices and unsealed lapped joints where moisture is held against the steel, provide drainage from box sections and ventilation behind cladding panels. Poor detailing is the usual reason a weathering steel structure performs worse than expected, not the material itself.

Yes. The patina reduces the rate of loss but does not stop it, so design standards include a sacrificial thickness allowance appropriate to the exposure and the design life, applied to each exposed face. That allowance is additional to the structural thickness required. Where the environment is aggressive or the element is critical and inaccessible, painting or a different material may still be the better choice.