Seawater is an aggressive, complex fluid that affects nearly all common structural materials to some extent. There are two competing processes that operate simultaneously in seawater environments" (1) the chloride ion activity tends to destroy the passive film, and (2) dissolved oxygen, which acts to promote and repair the passive film on metallic materials of construction. Metals and alloys that develop protective films by the formation of thin metal oxides can have the formation of passive films be delayed or have films destroyed by chloride ions in seawater.
The type of corrosion testing required depends on the extent and type of information desired and the environmental conditions expected in service for the alloy component. Seawater corrosion is dependent on a number of factors such as alloy composition, water chemistry, pH, biofouling, microbiological organisms, pollution and contamination, alloy surface films, geometry and surface roughness, galvanic interactions, fluid velocity characteristics and mode, oxygen content, heat transfer rate, and temperature. Understanding how these factors may affect experimental results can help the design of seawater corrosion testing to minimize experimental variations and best simulate service conditions.
The Seawater Environment
Seawater, the Earth's most abundant resource, covers 71% of the Earth's surface. This electrolyte approximates a 3.5 weight percent sodium chloride solution but it is much more complex, containing almost all naturally occurring elements [1,2]. The major chemical constituents of seawater, listed in Table 1, are consistent throughout the world . The minor constituents, including dissolved trace elements and gases, can vary substantially. Biological organisms, which, combined with the minor constituents, often control the rate of the corrosion reactions occurring on metals in seawater [2-4]. In addition, the concentration of seawater can vary from full strength ocean water to coastal seawater or brackish conditions. The type of corrosion and the corrosion rate occurring on metals and alloys in seawater is highly dependent on the specific seawater composition, its salinity, and the environmental zone to which the material is exposed.
Seawater is a unique environment that cannot be duplicated in the laboratory. Throughout the world, seawater can vary widely in terms of its specific chemical composition, oxygen content, temperature, salinity, pH, and biological activity. At a given location, seawater is also prone to variations from seasonal influences. All of these variables, combined with the long-term time dependence of many metal reactions, will affect the corrosion occurring on metals and alloys in seawater. In addition, the composition of corrosion products and of calcareous deposits formed under cathodic polarization conditions will influence metal corrosion.