Weathering steel

Weathering steel carries small additions of copper, chromium, nickel and phosphorus, and the effect is that its rust behaves differently. Ordinary rust is porous and flakes off, exposing fresh steel and continuing indefinitely. This alloy's rust forms a dense adherent patina that seals the surface and slows further corrosion to a small fraction of the initial rate.

The practical consequence is a structure that needs no paint, which removes not only the initial cost but a repainting cycle every fifteen or twenty years over a hundred-year life. For a bridge over a river or a motorway, where access for painting is the expensive part, that is a very large saving.

The conditions are specific and are routinely ignored. The patina requires alternating wet and dry cycles to form — permanently damp steel does not develop it and corrodes normally — and chlorides destroy it, so weathering steel is wrong near the coast and wrong where road salt is thrown at it. Detailing matters: water must not be allowed to sit anywhere, and the run-off stains everything below it for the first few years.

Rolled, welded and fabricated as ordinary structural steel, with matching weathering-grade filler so that the weld patinates at the same rate as the parent metal. Detailing is where the design effort goes: no water traps, no crevices, adequate clearance above ground, and provision for the run-off.

The patina takes between eighteen months and six years to stabilise depending on climate, and the colour continues to change for years after that.

Bridges, which is the largest structural use and where the maintenance argument is strongest. Building facades and cladding. Sculpture — Richard Serra's work is the best-known case, and the material's colour and its continuing change are part of what is being looked at. Shipping containers, historically. Transmission towers and highway structures.

Developed by US Steel and introduced as Cor-Ten in 1933, initially for railway hopper wagons, where abrasion resistance rather than the patina was the selling point. Its architectural career begins with Eero Saarinen's John Deere headquarters in 1964, which made the material's appearance the reason for choosing it.

Its limitations were learned expensively. A number of early bridges in chloride-rich environments corroded far faster than predicted, and modern guidance is correspondingly specific about where it may and may not be used.