Corrosion protection of steel bridges: coatings and weathering steel

How bridges get built · Steel erection · Corrosion protection of steel bridges: coatings and weathering steel

Also called: bridge painting, weathering steel, metallising, duplex coating

Corrosion protection of steel bridges: coatings and weathering steel — concept diagram — Concept diagram (TheBridgeEng)

How it is done

Classify the environment (ISO 9223 / ISO 12944-2 corrosivity C1–CX) and the durability required, then choose a paint system, metallising with sealer and topcoat (duplex), hot-dip galvanizing for smaller members, or uncoated weathering steel.
Shop surface preparation: remove soluble salts and contaminants, blast to Sa 2½ with the specified profile, and work within humidity and dew-point limits.
Apply the shop system in a controlled environment: zinc-rich primer, epoxy intermediate, polyurethane or polysiloxane finish, measuring dry film thickness coat by coat; mask the faying surfaces of slip-critical joints or use a primer qualified for the slip class.
Weathering steel: detail out water and debris traps, seal or drain expansion joints, keep the steel out of salt spray and continuous wetness, and protect substructure concrete from run-off staining in the early years.
Protect coated steel in transport and erection (padded slings, designated lifting points) and record damage.
After erection, clean and coat the splices and touch up damage with compatible materials under the same humidity and temperature limits.
Inspect at handover (DFT, adhesion and holiday tests where specified) and set the maintenance cycle: periodic inspection, spot repair, and full recoating when breakdown reaches the specified level.

Key numbers

Blast cleaningSa 2½ (ISO 8501-1), profile typically 50–100 µm
High-durability paint system for C4–C5total DFT about 240–360 µm in three coats
Thermal spray zinc or zinc-aluminiumabout 100–250 µm, sealed and topcoated in marine exposure
ISO 12944 durability classeslow up to 7 years, medium 7–15, high 15–25, very high over 25 years to first major maintenance
Weathering steelcorrosion allowance commonly 1–1.5 mm per exposed face for a 120-year life (UK practice)
Shop coatingUSD 40–80 per m² (indicative); bridge girders carry roughly 10–25 m² of surface per tonne

Where it fits

All steel bridges; weathering steel for rural dry climates; metallising for marine sites.

Where it does not

Weathering steel in the Gulf splash zone or under leaking joints; site-only coating of large areas.

Choosing it

Choose uncoated weathering steel where the site lets the patina dry out and stay free of salt (inland, well drained, no de-icing spray onto the steel); it takes repainting out of the life cycle.
Prefer a high-durability paint or duplex system in marine, coastal (including the Gulf), industrial or de-icing salt environments, and under leaking joints.
Prefer metallising with a sealer for C5 and CX exposure or where first maintenance must be beyond 25 years.
Weigh access cost: where repainting needs traffic closures, rail possessions or containment over water, pay more for a longer-life first system.

Plant, pace and money

PlantBlasting and coating facilities, inspection instruments.

ProductivityShop-driven.

CostCoatings USD 40–80 per m² (bridge girders carry roughly 10–25 m² of surface per tonne, so the shop system is typically about 7–15 % of the fabricated steel cost); repainting every 20–30 years, more often in severe exposure.

Risks and controls

What goes wrong

Coating failure at edges and splices, patina not forming in humid marine air, salt contamination under the coating.

Quality assurance

Blast profile and salt tests, dry film thickness, holiday tests, inspection at handover.

Origins

US Steel introduced Cor-Ten weathering steel in 1933 and it entered bridge use in the US in the 1960s. Bridge paints moved from lead-based systems to zinc-rich primers with epoxy and polyurethane topcoats from the 1970s and 1980s as lead was phased out.

Examples

Korean coastal steel bridges are metallised; US rural steel bridges use weathering steel; the game's marine-grade durability option includes coatings.

Case studies

New River Gorge BridgeUSA · 1977518 m steel arch built of uncoated weathering (Cor-Ten) steel.

Forth Bridge restorationUK · 2002–2011The steel was blasted back and recoated with a glass-flake epoxy system expected to last about 25 years, ending the tradition of continuous repainting.

Michigan DOT uncoated weathering steel bridgesUSA · 1980After severe corrosion of weathering steel girders under leaking joints and in de-icing salt spray, Michigan placed a moratorium on uncoated weathering steel. Experience of this kind shaped the FHWA's 1989 technical advisory on where and how to use it.