Corrosion Protection For Structural Steel

Structural steel has no inherent durability. Left unprotected in a humid, salty environment it loses section at a measurable rate, and the first place it matters is never the middle of a member.

Sydney makes this harder than most places, because a large share of the city is within the zone where airborne salt is a real factor.

How the environment is classified

Corrosion rate depends on time of wetness and on the contaminants present, mostly chloride from the sea.

The practical result is a set of atmospheric categories running from a dry interior through to severe marine, and the required protection changes sharply between them. A member in a conditioned interior may need very little. The same member on an exposed balcony two kilometres from the coast needs a substantially heavier system, which is the same gradient that drives [coastal exposure and concrete durability](/blog/coastal-exposure-and-concrete-durability).

Distance from the surf is the dominant variable, and it is not linear. The first few hundred metres are far more aggressive than the next few kilometres, and direct spray or breaking surf exposure is a different category again.

The protection options

Hot dip galvanizing. Zinc metallurgically bonded to the steel, applied by immersion. Long life, tolerant of handling, and it protects cut edges and recesses because zinc is sacrificial. Constrained by bath size, it distorts thin or asymmetric sections, and it requires vent and drain holes in hollow sections, which has to be designed rather than discovered at the galvanizer.

Paint systems, typically a zinc-rich primer, an intermediate build coat and a topcoat. Flexible, available in any colour, repairable, and entirely dependent on surface preparation. An excellent paint over poor preparation fails early.

Duplex systems, paint over galvanizing. The longest life available and the usual answer for severe exposure where appearance also matters.

Metal spray, for items that cannot be immersed.

Weathering steel, which forms a stable oxide layer and needs no coating, in the right conditions. It is not suited to marine exposure or to details that stay wet, and the runoff stains what is below it.

Stainless steel, for fixings and small components where replacement would be unacceptable. Grade matters, and in marine exposure the common grades are not sufficient.

Encasement in concrete, which is protection by cover and therefore the same mechanism as [concrete cover](/blog/concrete-cover-and-why-it-decides-durability).

Preparation is most of the result

Whatever the system, the failure is usually at the interface with the steel.

Mill scale left on the surface takes the coating off with it when it detaches. Soluble salts left after blasting draw moisture through the coating. An inadequate blast profile leaves the coating with nothing to key into. Coating applied outside the specified temperature and humidity window does not cure as intended.

Which is why the specification covers the preparation standard, the profile, the conditions and the inspection regime, not just the product. A coating specified by brand name alone has specified about a third of the system.

Detailing decides the life

The system matters. The detail matters more, because corrosion starts where water sits and where the coating is thinnest.

No water traps. Upturned angles, flat horizontal surfaces, and channels open side up all hold water. Turn them over or drain them.

Drainage and venting of hollow sections, so water cannot enter and sit inside where no coating exists and nobody can inspect.

Avoid narrow crevices and tight back-to-back angles, which cannot be coated properly and hold moisture by capillary action. A gap too small to coat is worse than no gap.

Separate dissimilar metals, because bimetallic contact in a wet, salty environment drives galvanic corrosion. Stainless fixings bearing directly on galvanized steel in marine exposure is a known problem and is solved with isolation.

Design for access, so the coating can be inspected and maintained. Steel that cannot be reached cannot be maintained, which means it needs a system rated for the full design life with no intervention.

Protect at connections, where bolt holes, cut edges, welds and site-drilled holes all break the shop-applied coating. Site touch-up is the weakest part of most systems and it needs a specified procedure.

Keep steel out of the ground and away from concrete interfaces, where the junction stays damp. A column base at ground level is the single most common place structural steel fails in Sydney.

Assessing existing steel

Section loss is measurable, by ultrasonic thickness testing and by direct measurement after cleaning. That gives a remaining capacity rather than an impression, and it is the basis for deciding whether a member is adequate, needs strengthening or needs replacement.

The important part is to check the connections and the base of columns rather than the mid-span, because that is where the loss concentrates and where the capacity consequence is highest. It is the same investigative logic as any [structural assessment of an existing building](/blog/assessing-an-existing-building-with-no-drawings), and the same reason [structural steel corrosion is assessed after a fire](/blog/structural-assessment-after-a-fire) alongside heat damage.

ACSES provides structural engineering and durability assessment across Sydney. Talk to us about a project.