Page 183 - Challenges in Corrosion Costs Causes Consequences and Control(2015)

SHIPS 161

3.18 SHIPS

The size of the shipping industry can be measured by the number of miles that ships
sail and the tons of cargo they haul. Corrosion of ships involves several different
types of corrosion. The most common form of corrosion is general corrosion or
wall thinning of the hull because of seawater attack. Studies have shown that this
form of corrosion is approximately 0.1 mm (4 miles) per year (21). At this corrosion
rate, it would take approximately 62 years to have a reduction of 6.4 mm (0.25 in.).
Because of this slow rate, general corrosion is normally not a consideration in a ship’s
design life.
Galvanic corrosion occurs between two metals with dissimilar electrochemical
potentials. In this form of corrosion, one of the metals is more electrochemically
active and corrodes, while the second metal is protected by the corroding metal. The
metals can even be of the same material if the electrochemical potential of one of
the materials has been charged because of stresses or differential aeration. Previous
studies have indicated that most hull corrosion is galvanic in nature (22).
Salt spray and atmospheric corrosion can severely attack external ship compo-
nents. Coatings provide the primary corrosion control, and maintenance of these
coatings is required at regular intervals.
Direct chemical corrosion attack occurs when certain chemicals are present in the
internal holds and tanks of transport ships. Elements such as chlorine and sulfur can
readily attack the steel and cause accelerated corrosion and pitting.
Corrosion in ships can also be caused by MIC. In this type of corrosion, microbial
organisms present in the environment can accelerate corrosion. For example, SRB,
which are present in stagnant water of many harbors, can build up on the hulls of
ships. Other corrosion-causing bacteria, such as acid-producing and anaerobic bacte-
ria, are also present in ballast tanks as well as in the liquid products that some tankers
carry. The microbes cause a localized change in the environment, which can promote
aggressive pitting and other types of corrosion.
Table 3.5 shows the percentage of the world’s fleet by class of ship on the basis of
number.
TABLE 3.5 Percentage of the World’s Fleet
by Class of Ship
Class Percent
Refrigerated cargo 2
Chemical tankers 3
Bulk dry 7
Passenger/ferry 7
Oil tankers 8
Supply/tugs 15
Cargo/roll on/roll off 22
Fishing 27
Others/unknown 9