Page 170 - Challenges in Corrosion Costs Causes Consequences and Control(2015)

148 CORROSION CAUSES

3.10.2 Underground Fuel Storage Tanks
These are large tanks and form a major portion of the hazardous materials storage
sector. Corrosion is thought to be responsible for nearly 65% of tank failures while
35% is attributed to other causes such as third-party damage. It is generally agreed
that the vast majority of underground storage tanks (USTs) and piping failures are
because of external corrosion while a small percentage may be attributed to internal
corrosion.
One of the primary causes of external corrosion is exposure to corrosive soils.
The electrical and chemical characteristics of soil and water are closely related to
corrosivity. Variations in soil characteristics because of soil type, fill compaction,
amount of moisture, bacteria, chloride concentration help establish corrosion cells.
Over a period of time, if untreated, the corrosion process can result in wall thickness
reduction and can lead to leaks. The 6 o’clock position of the USTs is one of the
most critical locations because that is the rest point where the tank bottom touches
the bottom of the hole dug for the tank. At such a location, the layer of backfill is
relatively thin; therefore, the soil characteristics can be different from the adjacent
soil, setting up conditions for macrocell corrosion.
Analogous to the aboveground tank phenomena, internal corrosion can occur
because of contaminants that settle on the tank bottom, under the stored product.
Vapor-phase corrosion is usually limited because of the relatively constant tempera-
ture. A particular tank failure type, which is sometimes reported for gasoline service
stations, is localized internal corrosion at the location where the internal lining is
damaged. The inspector’s level stick may cause mechanical damages to the lining,
resulting in corrosion. Generally, a wooden pole is used to check the gas level in
the UST. Lining damage occurs at the location where the pole hits the bottom of
the UST.

3.11 CORROSION PROBLEMS IN AIRPORTS

The United States has the world’s most extensive airport system, which essentially
consists of national transportation, commerce, and defense. A typical airport infras-
tructure is relatively complex, and components that might be subject to corrosion
are natural gas distribution systems, jet fuel storage and distribution systems, vehicle
fueling systems, natural gas feeders, dry fire lines, parking garages, and runway light-
ing. Generally, each of these facilities is owned or operated by different organizations
and companies, and the impact of corrosion on an airport as a whole is not known or
documented; however, the airports do not have any specific corrosion-related prob-
lems that have not been described in other sectors such as corrosion in water and gas
distribution lines, corrosion of concrete structures, and corrosion in aboveground and
underground storage tanks.
Because of the diversity of airport facilities and different accountabilities, the cor-
rosion costs cannot be assessed in a simple manner. to reduce and control corrosion
costs, it is recommended that the airports establish databases that will allow engineers
to track corrosion and corrosion costs and increase awareness.