Page 306 - Corrosion Engineering Principles and Practice

276 C h a p t e r 8 C o r r o s i o n b y W a t e r 277

Brackish Water
Brackish water contains between 0.1 and 2.5 percent sodium chloride
(1000 and 25,000 ppm), either from natural sources (e.g., salt deposits)
around otherwise fresh water or by dilution of seawater (e.g., tidal
rivers). Brackish water differs from open seawater in other respects.
The biological activity, for example, can be significantly modified
by higher concentrations of nutrients and fouling is likely to be
more severe as a consequence of the greater availability of these
nutrients.
Within harbors, bays, and other estuaries, marked differences
can exist in the amount and type of fouling agents present in the
water. The main environmental factors responsible, singly or in
combination, for these differences are the salinity, the degree of
pollution, and the prevalence of silt. Moreover, the influence of these
factors can be very specific to the type of organism involved. Apart
from differences that can develop between different parts of the same
estuary, there can also be differences between fouling in enclosed
waters and on the open coast. In this respect, the extent of offshore
coastal fouling is strongly determined by the accessibility to a natural
source of infection. Local currents, average temperature, seasonal
effects, depth, and penetration of light are operative factors. The
presence of pollutants can also be quite important and highly variable
in coastal areas.
Depending on composition brackish waters can be more
aggressive than seawater. In tidal estuaries, the highest corrosion rate
of carbon steel is just below the tidal zone, while in open seawater the
highest corrosion rates are in the splash zone [8].

Seawater
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 firefighting, oilfield water injection, and desalination plants.
The corrosion problems in these systems have been well-studied over
many decades, 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, 11 of the constituents
account for 99.95 percent of the total solutes, as indicated in Table 8.7,
with chloride ions being by far the largest constituent. Seawater
typically contains about 3.5 percent sodium chloride, although the
salinity may be weakened in some areas by dilution with fresh water
or concentrated by solar evaporation in others.
A large part of the dissolved components of seawater is present as
ion pairs or in complexes, rather than as simple ions. While the major
cations are largely uncomplexed, the anions other than chloride are to