Page 306 - Challenges in Corrosion Costs Causes Consequences and Control(2015)

284 CORROSION CONTROL AND PREVENTION

3. Wear caused by mud silt and other debris that accumulate in the tanks.
4. Aggressive corrosion caused by high-temperature cargos.
5. Abrasion of the ballast tanks caused by sloshing sand.

To ensure that a ship operates through its design life, it is necessary that the
operator does everything possible to keep the coatings intact and inspect the coatings
periodically to enable making timely repairs while the damage is minimal.
Several major changes in corrosion control technologies, environmental legisla-
tion, and ship design have led to significant changes in corrosion control approaches
in the marine industry during 1975–2000. The most important change is in coatings
such as limiting the use of lead, chromates, and certain VOC and the formulation of
better performing multipart epoxies and other coatings. This led to high solid epoxies
of different types as the primary choice of coatings. These coatings are more effective
and more expensive than the coatings used in the past. These coatings require more
extensive surface preparation than earlier coatings and increased costs for application
and repair.
Another major change is the switch to high-strength steel and other materials with
higher strength-to-weight and thickness ratios than the standard carbon steel. The
transition to use of high-strength steel resulted in thinner structural elements of ships
and allowing the ship hull to have more internal room to hold cargo. The negative
aspect is that a small degree of corrosion will affect the structural integrity of the
high-strength steel compared to carbon steel. Unprotected high-strength steels are
prone to failure even when the corrosion rates are small. This is borne out by several
failures of structural components in the late 1970s and early 1980s.
Double-hulled tankers are in use over the single-hulled variety in coastal waters.
The space between the inner and outer hulls of the tankers is often used for ballast
water, and the coating damage occurs here and hence corrosion. Greater corrosion
damage in oil tanks because of the thermos effect has been observed. In double-hulled
∘
tankers the temperature is 46–55 C, which leads to a higher corrosion rate.
One of the elements of corrosion is the actions of the owner and the crew during the
operation of the ship. As the coating represents the most important part of corrosion
control on a ship, it is vital to maintain the integrity of the coating during operation
of the ship. Damage to coating can occur in many different ways including: wear
caused by crew members and equipment moving through the tank; wear caused by
sloshing in partially filled ballast tanks; wear caused by mud silt and other debris that
accumulate in the tanks; aggressive corrosion caused by high-temperature cargos;
abrasion of the ballast tanks caused by sloshing sand. To ensure the operation of the
ship through its design life, the coating has to be intact along with periodic inspection
so that the necessary repairs can be made when the damage is minimal.
From 1975 to 2000, environmental regulations led to avoiding the use of lead,
chromates, and VOC. Thus coatings comprised multipart epoxies and high-solid
epoxies. The use of epoxies requires extensive surface preparation, which results in
higher costs of application and repair.
Another change is the use of high strength steel in the place of carbon steel that
allows the structural elements to be thinner. The hull size remains the same, but has