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The protection system is chosen together with the structural design. It depends on the corrosivity of the site, the durability the owner requires before the first major maintenance, access for future maintenance, appearance and the transport and erection method.
1. Classify the environment
Atmospheric corrosivity is classified in categories C1 (very low) to C5 (very high) and CX (extreme) in ISO 9223 and ISO 12944-2. ISO 12944-2 also defines categories Im1 to Im4 for steel that is immersed in water or buried in soil. Local factors matter: distance from the sea, de-icing salts on or below the bridge, industrial pollution, splash zones and sheltered areas where moisture and salts collect.
2. Define the required durability
ISO 12944-1 describes durability as the expected time to the first major maintenance: low (up to 7 years), medium (7 to 15 years), high (15 to 25 years) and very high (more than 25 years). Durability is a technical planning parameter, not a guarantee period. Owners and national road or rail authorities often set their own minimum requirements.
3. Compare the main protection systems
| System | Typical reference standards | Points to consider |
|---|---|---|
| Protective paint systems | ISO 12944-4 and -5, ISO 8501-1 (surface preparation grade, for example Sa 2½); SSPC / AMPP specifications in North America | Free choice of colour; performance depends on surface preparation, film thickness and site repair of transport damage; periodic maintenance painting. |
| Hot-dip galvanizing | ISO 1461, ASTM A123/A123M, AS/NZS 4680 | Metallurgically bonded zinc coating on all surfaces, including inside hollow sections; member size is limited by the galvanizing bath; vent and drain holes and steel chemistry (silicon and phosphorus content) must be considered at design stage. |
| Duplex system (galvanizing plus paint) | ISO 1461 or ASTM A123 with ISO 12944-5; ASTM D6386 for preparing galvanized surfaces for painting | Combines the two protection mechanisms for aggressive sites or long durability, and allows a colour finish; the galvanized surface needs correct preparation for paint adhesion. |
| Thermal spray zinc or aluminium (metallizing) | ISO 2063-1 and -2; AWS C2.23M / SSPC-CS 23.00 | No bath size limit; usually sealed and often over-coated with paint; requires blast cleaning to a high standard. |
| Uncoated weathering steel | See the separate solution page on weathering steel | Forms a protective patina in suitable climates; not suitable for every site. |
| Stainless steel components | EN 1993-1-4 (design); grade selection by exposure | Often used for railings, fixings and other parts touched by users; dissimilar metals must be isolated to avoid galvanic corrosion. |
4. Detail for durability
- Avoid details that trap water and debris; provide drainage holes and slopes.
- Seal box sections completely, or make them accessible and ventilated for inspection.
- Prepare welds, edges and surface imperfections to the grade required by ISO 8501-3 for the corrosivity category.
- Plan access for inspection and maintenance painting over the life of the bridge.
- Agree how bolted connections, faying surfaces and site welds are protected after erection.
5. Inspection and records
Typical checks include surface cleanliness and profile before coating, dry film thickness (for example by ISO 19840), adhesion tests where specified (ISO 4624 pull-off or ISO 2409 cross-cut), and zinc coating thickness for galvanized parts. The checks, acceptance criteria and hold points are listed in the inspection and test plan, and the results are delivered with the quality documentation.
Information we need
- Project location and known environmental conditions (coast, de-icing salts, industry, water crossing).
- Required durability or design life and the owner's maintenance strategy.
- The coating specification, if one exists, and colour requirements (for example a RAL reference).
- Transport and erection method, so that touch-up and site coating can be planned.
Final member sizes, details and specifications are determined by the engineer of record for each project. This page describes general practice and does not replace project-specific engineering.
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