What UST Inspectors Should Know About
Corrosion Flakes & Metal Loss In UST Sumps
By Dan Jenkins, Iron Ox
When operating and inspecting mechanical equipment, there is an old adage that states “small
problems turns into big problems.” Whether it is knocking, leaking, smoking, vibrating or
squeeling, it makes sense to have a deeper look into the problems so owners can make better
decisions to repair or replace equipment before a major accident occurs. In many cases, the
federal government or and industry association will provide guidelines to determine repair or
replacement. A good example of this is the Federal Motor Carrier Safety Administration
(FMCSA) regulations for truck tires. The minimum tread depth for a steer tire is 4/32 of an inch
and that no tire can be showing exposed belts or ply. No one wants to see a truck slide off the
road due to faulty tires. The same concept can be said about Underground Storage Tanks
(USTs), no one wants to learn of fuel leaking into groundwater due to corrosion.
Background & Problem
UST inspectors all across the country have been witnessing corrosion
building in UST Sumps for the last 7 years, ever since changes in
EPA regulations mandated increased production of
biofuels. The corrosion can take on many characteristics; everything
from wet corn flakes, barnacles on a bridge piling to dry layers of rust
like a flakey pastry.
When the problems were first observed 5 to 6 years ago, most in the industry shrugged the
corrosion off as cosmetic and paid little attention. As the corrosion began to lead to costly
repairs and equipment replacement, most UST technicians tried scraping and painting sumps.
Unfortunately, the paint made poor attempts to smother the corrosion and made matters worse.
As time went on and corrosion continued to increase, inspectors and regulators began to notice
more than just surface corrosion, they began to notice “metal loss” as evidenced my metal
chips laying on the floor of the sump or different colored corn flakes attached to components.
So as corrosion levels increased, the question for inspectors and regulators began to take shape:
How much metal loss from corrosion can there be on UST Sump components before there is a
component failure which contributes to a leak of fuel? We investigated this question and the
first placed we looked was the 40 CFR Parts 280 and 281 from the EPA Office of Under
Ground Storage Tanks that was published in July of 2015. While the authors of this standard
did a great job under difficult circumstances, they failed to address the corrosion levels in the
sumps (Cathodic Protection does not apply.) The closest the authors arrived to this issue is revealed in section: Subpart E—
280.50 Reporting of suspected releases.
E—-280.50
It states:
(2) Any defective system equipment or component is immediately repaired or replaced;
Unfortunately, the authors did not define what is the term “defective” when it comes to
corrosion like the FMCSA did with tires. Therefore, a UST sump can have a STP, Hardware or
Leak Detector that is completely covered with corrosion and suffering from significant metal
loss, yet still operate and pass inspection. Since this federal standard took 20 years to revise
and complete, we don’t expect any time soon that the standard will be changed to reflect
increased levels of corrosion found in sumps. We contacted an met with over 20 states and
could not find any current guidelines on acceptable corrosion levels in sumps.
Therefore, since the FMCSA measure tires for tread thickness and exposed belts to prevent
major accidents, we would like suggest some common sense guidelines regarding metal loss on
UST component to prevent major accidents that could cause fuel to leaking into the
groundwater.
Before we get to suggested guidelines, let’s better understand the problem of Exfoliation
Corrosion that is one of several types found in in sumps today.
Exfoliation corrosion proceeds laterally from the site of initiation along planes parallel to the
surface forming iron oxides. The rate of expansion of iron oxides is about 3 times that of the
base metal, therefore, the buildup of the iron oxides (rust) is exactly like driving a wedge
between the grain boundaries of the base metal. The result can be seen in the photograph below
and please note that the bolt is exfoliating large flakes of (what used to be) metal. Besides bolts,
other parts in sumps that corrode are piping, housings and hardware. The thickness of these
flakes averages 0.055 inches which is about the thickness of a penny.
Typically, an engineer selects the appropriate steel components based on a calculation (using
Barlow’s Formula) to determine pipe working pressures. The wall thickness value is in the
numerator, therefore, when the wall thickness is reduced (by exfoliation) the maximum working
pressure is also reduced. In other words, as the wall thickness decreases the strength of the pipe
also decreases.
So if the standard wall thickness of a 4 inch outside diameter of a pipe is 0.226 inches and one
(1) exfoliation flake is 0.055 inches thick, the bursting pressure of that pipe is reduced by
approximately 27%. With this formula, it would be easy to predict the “remaining life” of the
pipe in the sump. A standard ASTM handbook lists the wall thickness for pipes. If an
inexpensive micrometer were used to measure the outside diameter of the pipe and this
“number” subtracted from the original wall thickness. By using a chart like the one included,
the life (of the pipe) remaining could be forecast.
Bursting Pressure of Pipe
Bursting Pressure of Pipe is Reduced As Wall Thickness is Reduced
Measurement of Inches in Thousandths
Bottom Line
As more inspectors encounter more corrosion in sumps, a simple way to help predict
component failure and prevent accidents is to measure corrosion flakes. If flakes on the floor
or hanging on components are thicker than a penny, the time to act in now.
For more information , contact Dan Jenkins
Iron Ox—813-877-4665 [email protected]