Chapter 1
Why Tanks Fail
Why is it some storage tanks have lasted for
over fifty years while others fail in less than
one year? Are some individual tanks that much
better or that much worse than similar tanks
made by the same manufacturer, in the same
plant, at the same time?
Research indicates that most tank failures
can be attributed to one of four reasons. In
order from the most likely to the least likely:
1. Corrosion
2. On-site damage
3. Transportation damage
4. Manufacturing defects
1. Corrosion
Internal corrosion is the main cause of
AST tank failure. Regular tank maintenance,
including checking the tank for the presence
of water, should be recommended to all
customers. The presence of water in the tank
can contribute to corrosion in several ways.
Fuel storage experts report that bacteria can
live in the water and feed on the fuel. The
bacterial by-products can be a catalyst for
the creation of a biologically active sludge
that can accelerate the rate of corrosion of
4 National Oilheat Research Alliance
the tank and other components of the system.
This is known as microbial induced corrosion.
The liquid in the tank will typically separate
into three layers:
• The top layer is the clean fuel
• The second layer is a mixture
containing bacteria, water and fuel
• The third is sludge and water from
condensation, delivery, loose fittings,
loose fill or vent caps, etc.
clean (dyed) No. 2 heating fuel
mixture of bacteria, water, fuel
sludge and water from condensation,
delivery, loose fittings, loose fill
or vent caps, etc.
Results of recent studies suggest the bacteria
in the second level often excrete substances
that are acidic. These substances can combine
with moisture and form compounds that
negatively affect fuel quality, the steel tank
bottom and system components.
Keeping water out of above ground heating
oil storage tanks will help maintain fuel quality,
reduce the frequency of service issues and help
prevent corrosion related component failures.