Corrosion and Protection of Steel Structure in Seawater

The intensity of corrosion of an unprotected steel structure in seawater varies with position relative to the mean high and low tide level, as shown on the right figure.

An unprotected steel structure is most severely attacked at the spray and splash zone above the mean high tide level, due to continuous contact with highly aerated sea water and the erosive effects of spray, waves and tidal actions. 

Corrosion rates of bare steel pilings are often also very high at a position just below mean low tide in a region that is very anodic relative to the tidal zone, due to powerful differential aeration cells which form in the well aerated tidal region.

Protection of such steel structure can be achieved by various means; each corrosion zone must be separately considered. Three generally accepted methods are Cathodic Protection, Painting or Coating, and Sheathing.