Last reviewed 30 Sept 2026 · 8 min read
The marine environment is severe
A structure in the sea is exposed to salt water, waves, tide, abrasion, biological growth and temperature changes. Chlorides in seawater (about 19 g/litre, 3.5 % total salts) are the main cause of the corrosion of reinforcing steel, and the salt-laden air attacks structures above the water too. The design life of a marine structure is typically 50 years or more, and durability is often the governing design consideration.
Exposure zones
A vertical structure such as a pile or a quay wall passes through several zones with different degrees of attack:
| Zone | Position | Conditions | Severity |
|---|---|---|---|
| Atmospheric zone | Above the reach of spray | Salt-laden air, humidity; chloride deposit | Moderate |
| Splash zone | Between the highest wave crests and the high-tide level | Frequent wetting and drying, oxygen and chlorides available, and wave impact | The most severe for concrete and steel |
| Tidal zone | Between low and high water | Alternate wetting and drying, marine growth, wave action | Severe |
| Submerged zone | Permanently under water | Little oxygen; corrosion is slow, but chemical attack on concrete and marine growth | Lower for steel, moderate for concrete |
| Buried zone | Below the mud-line | Little oxygen; sulphate-reducing bacteria possible | Low |
Steel piles corrode most quickly just below the low-water level (a concentration cell effect, where the aerated area above acts as a cathode and the lower part as an anode) and in the splash zone; the rate of corrosion of bare steel in the splash zone can reach about 0.3–0.5 mm/year per face or more; in the submerged zone, roughly 0.1 mm/year. (Indicative values; a corrosion allowance is added in design.)
Deterioration of concrete
| Cause | Effect |
|---|---|
| Chloride ingress | Penetrates the concrete and depassivates the steel, starting corrosion. Rust occupies about 2–6 times the volume of the steel, which cracks and spalls the cover |
| Sulphate attack | Sulphates in sea water react with the hydrated cement (tricalcium aluminate) and form ettringite and gypsum, causing expansion and cracking — but the chlorides limit the ettringite expansion in the sea, so it is less serious than in soil |
| Magnesium attack | Magnesium sulphate in sea water attacks the calcium silicate hydrate and weakens the paste (a softening from the surface) |
| Alkali–silica reaction | Reactive aggregates expand in the presence of moisture and alkalis |
| Freeze–thaw (cold climates) | Cycles of freezing in saturated concrete break the surface |
| Abrasion and erosion | By waves, sand, ice and ship contact |
| Carbonation | The carbon dioxide reduces the alkalinity, mostly in the atmospheric zone |
| Marine growth and biological attack | Barnacles, algae, borers; acid produced by bacteria |
| Thermal cracking | In massive sections, during the early hydration |
Corrosion of reinforcement
Steel in sound concrete is protected by a passive film produced by the high alkalinity (pH about 12.5–13) of the cement paste. Chlorides that reach the bar above a threshold (about 0.4 % by weight of cement, or 0.05–0.1 % by weight of concrete) break down the film. Corrosion is an electrochemical process:
- Anode:
- Cathode:
- The rust products form ferric and ferrous hydroxides and oxides.
Corrosion needs oxygen and moisture — which is why the splash and tidal zones are the worst, while in permanently submerged concrete there is little oxygen. The time to corrosion initiation is governed by the diffusion of chlorides (Fick's second law):
where is the chloride at the surface, the diffusion coefficient, the depth (the cover) and the time. Lower (a dense, low-permeability concrete) and a greater cover delay the initiation.