“above the fuel level” corrosion has
been reported in ULSD systems.
Unfortunately, the more common
tests for detecting corrosion, weak
acid presence or microbial growth
don’t always indicate a problem.
Bugs can be present in a diesel
tank—and the fuel and tank water
bottoms can still pass NACE, be
“normal” for pH, and look good on
Though fuel is typically treated with sufficient pipeline corrosion inhibitor at the refinery, some of the inhibitor may get used
up as it travels through the distribution system and the fuel continues to make contact with metal surfaces. This is entirely normal.
Though less frequent, some inhibitors also may react with caustics or salts in tank water bottoms. The caustics and salts often are
traceable to the pipeline. Most pipelines require a “B+” NACE
specification (TM-0172) or better upon entry of the fuel. However, this is the last steel corrosion specification that the fuel must
meet. There are currently no downstream steel corrosion specifications for gasoline or diesel fuel in ASTM D 4814 (gasoline) or
D 975 (diesel fuel) standard specifications.
When added at the terminal level, other surface active lubricity additives may slightly improve the NACE rating of the fuel.
However, this only protects end users and cannot be counted
on to keep pipelines and terminal tanks from rusting. Having a
“retail-level” NACE corrosion specification within ASTM D 975
along with regularly checking for microbials would help ensure
that the customer always receives non-corrosive fuel.
Corrosion By-product/Microbials Adhered to Tank Walls, etc. Aluminum Drop Tube Carbon Steel Shaft Aluminum Shaft TLS (some rust and corrosion by-products) Submersible Tank Pump (aluminum housing) Average ULSD Level
uSt showing corrosion and possible microbial corrosion spots.
With all of these changes, the propensity for bug growth in
ULSD fuel systems has increased dramatically. The bugs can be
either aerobic (needing oxygen) or anaerobic (needing no oxygen)
and can be found in both the fuel and water phases. In some
cases, bugs have been found even in corrosion “barnacles” in the
Typically, the anaerobes generate the damaging acids. These
weak acids have higher vapor pressure than the diesel fuel and
thus rise to the top of the storage tanks, lines and dispensing
equipment. This appears to be one reason why so much
WHat to Do
Experts say that the most important thing that can be done to
prevent microbial growth is to eliminate the water. Remember:
no water, no bug growth. Draining water regularly and thoroughly
from any fuel holding storage tank will go a long way in reducing
the possibility of acquiring or supporting a bug infection.
Water also is an excellent solvent for many impurities, so
removing it also removes many of these potentially fuel filter and
fuel system clogging compounds. Using an appropriate fuel-soluble biocide at the correct treat rate may be necessary to kill
any lingering infections and ward off future ones. As a word of
precaution, using a biocide inappropriate for the circumstance or