Seawater systems are used by many industries such as shipping, offshore oil and gas production, power plants and coastal industrial plants. The main use of seawater is for cooling purposes but it is also used for fire fighting, oil field water injection and for desalination plants. The corrosion problems in these systems have been well studied over many years, but despite published information on materials behavior in seawater, failures still occur.
Most of the elements that can be found on earth are present in seawater, at least in trace amounts. However, eleven of the constituents alone account for 99.95% of the total solutes, chloride ions being by far the largest constituent. The concentration of dissolved materials in the sea varies greatly with location and time because rivers dilute seawater, rain, or melting ice, or is concentrated by evaporation. The most important properties of seawater are:
The ratios of the concentrations of the major constituents are remarkably constant worldwide
High salt concentration, mainly sodium chloride;
High electrical conductivity
Relatively high and constant pH
Solubility for gases, of which oxygen and carbon dioxide in particular are of importance in the context of corrosion
The presence of a myriad of organic compounds
The existence of biological life either as microfouling (e.g. bacteria, slime) or macrofouling (e.g. seaweed, mussels, barnacles and many kinds of animals or fish).
The U.S. flag fleet can be divided into several categories as follows: the Great Lakes with 737 vessels at 62 billion ton-miles, inland with 33,668 vessels at 294 billion ton-miles, ocean with 7,014 vessels at 350 billion ton-miles, recreational with 12.3 million boats, and cruise ship with 122 boats serving North American ports (5.4 million passengers). The total annual direct cost of corrosion to the U.S. shipping industry is estimated at $2.7 billion. This cost is divided into costs associated with new construction ($1.1 billion), with maintenance and repairs ($0.8 billion), and with corrosion-related downtime ($0.8 billion). (Internet reference)