Page 307 - Challenges in Corrosion Costs Causes Consequences and Control(2015)

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more room for cargo. High-strength steel is more sensitive to corrosion than carbon
steel. In fact, many failures of structural components have been reported in late 1970s
and early 1980s.
Double-hulled tankers are designed to have a ship inside a ship to reduce the risk
of sinking of the ship and loss of cargo. The space between the inner and outer hulls is
often used for ballast water to balance the tanks. These areas often have coating dam-
age followed by corrosion because of the conditions in the tanks. These are difficult
to inspect, and corrosion occurs because of the thermos effect.
In a single-hull tanker, the surrounding seawater keeps it at a lower temperature,
which is not the case with double-hulled tankers. On double-hulled tankers, the outer
hull acts as an insulator and the ballast (outer) tanks stay at a higher temperature
resulting in a higher corrosion rate. The resulting higher temperature will cause the
degradation of coating and in particular tar epoxy coatings.
The bottom of crude cargo tanks show pitting corrosion. A small amount of water
at the bottom of a crude oil bearing tank and the hydrogen sulfide in the crude oil will
result in corrosion. Pitting corrosion occurs at the bottom of the tanks.
The water at the bottom of the tank can cause microbiological corrosion because of
the presence of acid-producing and sulfate-reducing bacteria (SRB). Under suitable
conditions, the SRB damage steel by reducing sulfates in the crude oil to sulfides,
leading to pitting corrosion in the areas under the SRB colonies.
The top of the tank or ullage showed general corrosion. These areas are difficult
to inspect and repair. The top portion of the tank experiences general corrosion rather
than pitting. Inspection and repairing in these areas is a major concern. The top por-
tion of the crude tanks is made of HT steel, and the flexing/descaling problem is of
concern.
Two different mechanisms affect the top of tanks depending on cargo or emptiness.
When the tank is filled with crude, the ullage space is filled with inert gas that should
limit corrosion. The crude oil will emit corrosive gases such as hydrogen sulfide that
can combine with moisture and oxygen to form sulfuric and sulfurous acids that can
attack the steel and cause general corrosion.
Although the tank in ballast voyage is filled with inert gas saturated with water
vapor, the water vapor on condensation absorbs oxides of sulfur, carbon, and nitrogen
to form various acids that attack the steel.
The first double-hulled tanks were protected in the same manner as the
single-hulled tankers. This involved no coating to line the crude oil cargo tanks but
using only a single-layer tar epoxy coating on the water ballast tanks. The corrosion
rate in the single-hulled tank was constant, and repairs and steel replacement were
done when the ship was 12 years old. However, a significant amount of corrosion in
double-hulled tankers was observed in 5 years.
To prevent corrosion of double-hulled tanks, coatings with corrosion resistance at
∘
temperatures of 70–90 C and resistance to MIC as well as resistance to acid attack in
the ullage were found desirable. Because of the flexing of HT steel, the coating needs
to be flexible and does not become brittle and break off over time as the ship flexes.
The coating should last for 20 years. Solvent-free epoxies satisfied these require-
ments.