Page 305 - Challenges in Corrosion Costs Causes Consequences and Control(2015)

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and grit-blasted before the application of coating, and such a job on a larger tanker
can cost as much as $3 million.
CP or other protection systems should be incorporated, if necessary, in the
design phase of the ship. CP system is a secondary defense against corrosion when
holidays or cracks form in the coating. CP systems use either sacrificial zinc anodes
or impressed-current systems to mitigate corrosion. Other corrosion prevention
equipment and materials are inert gases to drive out corrosive gases. Corrosion
inhibitors are also used.
There are several elements of ship fabrication and manufacturing that influence
the corrosion performance of a ship. The ship classification societies such as Lloyd’s
Register of Shipping, the American Bureau of Shipping, and the Nippon Kaiji Kyokai
have published tolerance standards with which the ships must comply. The tolerances
permit a gap of up to 3 mm width and misalignments up to one-half of the plate
thickness (53). Keeping the gaps and the misalignments under this level will reduce
the possibility of stress concentrations and other possible causes of structural failure.
Good painting practices in terms of application and curing, not adherence to the least
proper temperatures to ensure a good solvent release in the wet stage, will ensure
coatings with low internal stress and hence a longer service life.
Surface preparation before coating is very important. Almost all the coatings used
in the marine industry adhere to the metal by mechanical adhesion; thus it is important
to have a clean metal surface that readily bonds with the coating to protect the metal.
There are two elements in the surface preparation. It is important to clean the metal
surface thoroughly by removing any salt, dirt, and chemicals present on the surface.
The second step involves the creation of a textured or anchor pattern surface so that
the coating can mechanically adhere to the surface. The preferred method of sur-
face preparation is grit blasting. The coating manufacturer will provide information
regarding the degree of surface profile that is needed by surface blasting.
The coating is applied after surface preparation of the sample metal or alloy. The
quality of coating has a profound effect on the corrosion performance of a ship and
hence the directions of the coating manufacturer must be closely followed. One of
the most important steps in coating application is hand-finishing, where the painter
coats the corners, angles, and edges with a brush by hand. This must be done because
surface tension causes drying coatings to draw away from sharp edges. Because of
this, coatings tend to be thinner in the corners, angles, and edges; therefore, extra
coating must be applied by stripping to ensure proper coating thickness. The coating
should not be too thick that can result in solvent and thinner retention, film cracks,
and gas pockets.
Human actions such as the actions of owner and crew during the operation of the
ship are the last element of corrosion control. Coating represents the most important
part of corrosion control on a ship and hence maintaining the integrity of the coating
during operation is vital to corrosion control.
Damage to coatings can be caused in many ways such as the following:

1. Wear caused by crew members and equipment moving through the tank.
2. Wear caused by water sloshing in partially filled ballast tanks.