Principles of Marine Corrosion 6.3 Marine Corrosion of Steel 113
Part A | 6.3
crobiological activity and (ii) the availability of essential nutrients to permit microbiological activity to occur.
As will be discussed further, nutrients and thus microbiological activity can also have an important effect on the
deterioration of metals in seawater and in freshwater.
The component of seawater most often associated
with its (deleterious) effect on materials is salinity.
Again, apart from the Baltic (where salinity tends to
be lower than average) and the Dead Sea (where it is
very high), salinity is remarkably similar in open ocean
waters. Typically, salinity is reduced in estuaries and
harbors, as a result of the inflow of river water. In chemistry, common salt (NaCl) is classed as one of the salts
supplying chloride ions (Cl
). It is one of the halides,
a small group of chemicals that usually act as aggressive
ions in chemical reactions. An often overlooked property of common salt is that it is hygroscopic, meaning
that it attracts water, as is easily verified by attempting
to dry out a seawater-soaked garment, such as a T-shirt
without washing-out the salt. In other than dehumidified
air, it will not dry. This means that once a metal object has been exposed to seawater, or has been sprayed
with salt particles, and then not washed, it will attract
moisture and thus tend to corrode at a much greater rate
than otherwise would be the case, simply because the
surface remains wet for longer. This is particularly important for the atmospheric corrosion of steel in coastal
environments.
The other important component of seawater is its
calcium carbonate content. It is supersaturated with it.
For the usual pH of seawater (about 8:2), it occurs as
calcium bicarbonate. Seashells (largely composed of
calcium carbonate) exist because they are able to maintain their pH at just a sufficiently high level to avoid
dissolution. The precise mechanisms involved are not
of concern here, but it is known that an increase in
carbon dioxide in the atmosphere will cause the pH of
seawater to drop slightly, thereby upsetting the calcium
carbonate balance in the sea, with, as is well known, potentially serious effects on seashells and molluscs. This
is one of the well-recognized concerns associated with
climate change. As will become clear in the following,
it also can have a significant effect on marine corrosion
of metals such as steels.
6.2 Materials Used in Marine Environments
Natural constructional materials are restricted largely
to rocks, some pure metals (e.g., gold), many oxidized
metals (e.g., iron ore), and various plant forms, including wood. However, there is an incredibly large
(and growing) range of man-made materials available for commercial and industrial use. They include
steels of various types including chromium steels, reinforced concrete, aluminum, glass and carbon fiber
composites, nylon and other man-made organic materials. To a greater or lesser extent all of these are
used in marine environments. Which material actually is used, or which combinations, depends largely
on the economics involved and often this includes
the first cost and, increasingly, also the anticipated
maintenance and replacement costs. In some cases,
it may include the consequential costs should failure
occur. In practice, the materials most used in major
marine infrastructure applications are steel and reinforced concrete. For mechanical equipment such as
harbor facilities and shipping steel is predominant, although aluminum and fiber composites also are used for
many applications. Herein attention is focused on steel
structures.
Good quality information and prediction tools for
likely long-term performance are scarce. Comprehensive compendia of the corrosion performance of many
metals and alloys and for many nonmetals are available [6.14, 15] but the approach is largely anecdotal.
Until recently this was also the case for steels, as described in more detail in the following. For reinforced
concrete, the principles governing the corrosion of steel
reinforcement and the performance of the concrete itself in marine environments are established and readily
available [6.16, 17], but there are some aspects not yet
fully explained, as also described in more detail in the
following.
6.3 Marine Corrosion of Steel
Steel is used extensively in industrial, institutional, and
commercial projects, and in structural systems, and has
a long history of such uses. Applications include offshore structures (floating and fixed), commercial ships,
defense force and commercial ships, mooring chain,
submerged pipelines, coastal jetties, sheet and other piling, cranes and other loading and offloading facilities,
storage tanks, bridges, multistory car-parks, electric-
Part A | 6.3
crobiological activity and (ii) the availability of essential nutrients to permit microbiological activity to occur.
As will be discussed further, nutrients and thus microbiological activity can also have an important effect on the
deterioration of metals in seawater and in freshwater.
The component of seawater most often associated
with its (deleterious) effect on materials is salinity.
Again, apart from the Baltic (where salinity tends to
be lower than average) and the Dead Sea (where it is
very high), salinity is remarkably similar in open ocean
waters. Typically, salinity is reduced in estuaries and
harbors, as a result of the inflow of river water. In chemistry, common salt (NaCl) is classed as one of the salts
supplying chloride ions (Cl
). It is one of the halides,
a small group of chemicals that usually act as aggressive
ions in chemical reactions. An often overlooked property of common salt is that it is hygroscopic, meaning
that it attracts water, as is easily verified by attempting
to dry out a seawater-soaked garment, such as a T-shirt
without washing-out the salt. In other than dehumidified
air, it will not dry. This means that once a metal object has been exposed to seawater, or has been sprayed
with salt particles, and then not washed, it will attract
moisture and thus tend to corrode at a much greater rate
than otherwise would be the case, simply because the
surface remains wet for longer. This is particularly important for the atmospheric corrosion of steel in coastal
environments.
The other important component of seawater is its
calcium carbonate content. It is supersaturated with it.
For the usual pH of seawater (about 8:2), it occurs as
calcium bicarbonate. Seashells (largely composed of
calcium carbonate) exist because they are able to maintain their pH at just a sufficiently high level to avoid
dissolution. The precise mechanisms involved are not
of concern here, but it is known that an increase in
carbon dioxide in the atmosphere will cause the pH of
seawater to drop slightly, thereby upsetting the calcium
carbonate balance in the sea, with, as is well known, potentially serious effects on seashells and molluscs. This
is one of the well-recognized concerns associated with
climate change. As will become clear in the following,
it also can have a significant effect on marine corrosion
of metals such as steels.
6.2 Materials Used in Marine Environments
Natural constructional materials are restricted largely
to rocks, some pure metals (e.g., gold), many oxidized
metals (e.g., iron ore), and various plant forms, including wood. However, there is an incredibly large
(and growing) range of man-made materials available for commercial and industrial use. They include
steels of various types including chromium steels, reinforced concrete, aluminum, glass and carbon fiber
composites, nylon and other man-made organic materials. To a greater or lesser extent all of these are
used in marine environments. Which material actually is used, or which combinations, depends largely
on the economics involved and often this includes
the first cost and, increasingly, also the anticipated
maintenance and replacement costs. In some cases,
it may include the consequential costs should failure
occur. In practice, the materials most used in major
marine infrastructure applications are steel and reinforced concrete. For mechanical equipment such as
harbor facilities and shipping steel is predominant, although aluminum and fiber composites also are used for
many applications. Herein attention is focused on steel
structures.
Good quality information and prediction tools for
likely long-term performance are scarce. Comprehensive compendia of the corrosion performance of many
metals and alloys and for many nonmetals are available [6.14, 15] but the approach is largely anecdotal.
Until recently this was also the case for steels, as described in more detail in the following. For reinforced
concrete, the principles governing the corrosion of steel
reinforcement and the performance of the concrete itself in marine environments are established and readily
available [6.16, 17], but there are some aspects not yet
fully explained, as also described in more detail in the
following.
6.3 Marine Corrosion of Steel
Steel is used extensively in industrial, institutional, and
commercial projects, and in structural systems, and has
a long history of such uses. Applications include offshore structures (floating and fixed), commercial ships,
defense force and commercial ships, mooring chain,
submerged pipelines, coastal jetties, sheet and other piling, cranes and other loading and offloading facilities,
storage tanks, bridges, multistory car-parks, electric-