Corrosion of Carbon Steels in Seawater

Typical steel structures exposed to marine environments are (i) marine piles (ii) offshore structures (iii) vessels and (iv) others. The corrosive zones encountered in marine environment are (i) atmospheric (ii) splash (iii) tidal (iv) submerged and (v) seabed-embedded zone. 

The environmental factors that influence the corrosion rate vary with the depth in seawater as detailed8 in Table 4.3. The variation of the factors in seawater at different global locations has been documented in the literature.9 

The overall corrosion rates obtained for periods 1-40 yr range from 0.02 to 0.37 mm/yr with an average value of 1 mm/yr. The corrosion rate of steel was found to decrease with time as is evident from the data obtained by Southwell and Alexander.10 

Salinity 1. Open sea variation with horizontal location is small, 32-36 ppt. 2. Near river outlets lower. 3. Variation with depth very small. 

Dissolved oxygen concentration 1. Surface water (1) near the equilibrium saturation concentration with atmospheric oxygen at a given temperature [6 ppm (in the tropics) 11 ppm (in the Arctic) (2) can be supersatured due to photosynthesis by microscopic plants (up to 200%) and entrainment of air bubbles (up to 10%) 2. Variation with depth (1) tends to be under saturated due to consumption by the biochemical oxidation of organic matter (2) goes though a minimum at intermediate depths (400-2400 m) 

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