In the past decade, clean air concerns have resulted in numerous changes to today’s motor vehicle fuels. Steel Tank Institute personnel are involved in fuel quality concerns with the storage and dispensing of motor vehicle fuels that did not exist ten years ago. I have attended many conferences, both learning from speakers there and presenting my own perspectives.

I want to share some of my observations.

Ultra Low Sulfur Diesel

The US EPA promulgated its first rule limiting sulfur in diesel vehicle fuel in 2006. Since then, several more related rules have been implemented. Sulfur has been removed from diesel to encourage cleaner air from vehicle exhaust. But with removal of much of the sulfur, gas station and c-store operators are experiencing a new problem in their underground storage tank system that did not exist before.

The metal components within both fiberglass reinforced plastic (FRP) and steel underground storage tanks are experiencing corrosion. Submersible turbine pump columns, ball float valves, overfill prevention/shut-off valves, and tank gauging sensors are deteriorating from a corrosion process over their entire surface, especially in the ullage area. This phenomenon did not exist 10 years ago.

Equipment in or under dispensers is also impacted, such as under-dispenser shear valves and metering equipment. Filters and strainers are rapidly becoming clogged with particulates that have a “coffee ground” appearance.

Initially, many of the phone calls STI’s office received about corrosion of metallic equipment (particularly from 2007-2013) came from sites with non-metallic (FRP) tanks. But since then, the corrosion has been found to occur inside steel tanks as

The corrosion phenomenon is quite random, but reports seem to be increasing, perhaps because the petroleum equipment service industry has a much greater awareness today of the problem’s existence. A whole new industry of service providers now offers tank cleaning, internal tank inspections, fuel polishing, tank bottom water and sludge/slime/particulates removal, and biocide treatment.

Studying the problem

In 2008, the Clean Diesel Fuel Alliance, an industry interest group, hired Battelle Laboratories to perform a study on ULSD corrosion, which was subsequently published in 2012.

Six FRP tank sites were selected. One was expected to be clean and free of corrosion, but all six sites were discovered to have corrosion.

One unusual finding was that ethanol, although in small amounts, was present in tank bottoms with water. Many readers understand that ethanol and gasoline are considered flammable liquids, while diesel is slightly less hazardous as a combustible liquid. The fire service does not want Class I flammable liquids to be stored in the same container with combustible liquids, regardless of the amount, for safety reasons.

An hypothesis was formed that the ethanol was present due to switch loading practices by fuel trucks delivering their loads to the gas station. Bacteria were feeding on the ethanol and the oxygen in the water to create an acidic environment. Since ethanol blended fuels have been around for decades and this

type of corrosion only started recently, an additional focus was placed on the removal of sulfur from diesel as a key, recently-occurring change in the fuel. Perhaps the sulfur that used to be in diesel prevented bacteria from thriving in a UST environment.

More recently, the EPA Office of Underground Storage Tanks has become concerned that USTs with potential equipment failures from this corrosion (overfill prevention valves, in-tank inventory sensors, leak detection equipment, ball float vent valves, shear valves, metering, etc.) pose a greater risk of release and an unwanted environmental event.

EPA published their findings on 42 underground storage tanks in the summer of 2016. Here’s a link to the study: Notice Of Corrosion Risks In Underground Storage Tanks Storing Diesel Fuel US Environmental Protection Agency, Office Of Underground Storage Tanks.

Severe or moderate corrosion was discovered in 83% of the tanks. Of even greater concern to EPA was that less than 25% of the tank owners were aware that corrosion was occurring. EPA estimates that approximately 100,000 ULSD USTs exist in the US today, many of which are single wall containment.

Tank water bottoms were tested for content, with ethanol, methanol, and gasoline present in nearly all samples. Acetic acid, formic acid, and propionic acid were also identified in the water bottoms and in the ullage space, helping to explain why the entire surface area of hanging metal hardware was experiencing corrosion. In my 30-plus years of experience with underground storage tank systems, I’ve never seen corrosion in the ullage or vapor space.

Another item I found interesting in this recent EPA study is that severe or moderate corrosion was evident with older steel tanks, but conversely, also with newer FRP tanks. The metal pipe flanges in FRP tanks were also found to have severe corrosion. This might be a coincidence, as EPA notes that their tests are not statistically significant enough to form ironclad conclusions. Or perhaps, since the steel tank itself adds significantly more surface area, the corrosion on steel tank walls has not been found by my contacts to have the same severity of corrosion as in FRP tanks, in most cases.

Attempting to solve the problem

Industry groups and research teams such as STI, Petroleum Equipment Institute (PEI), and Coordinating Research Council (CRC) are publishing tank maintenance guidelines with a greater emphasis on water removal. Maintenance and inspections performed via tank cleaning, fuel polishing, in-tank camera visual inspections, and equipment removal inspections are becoming commonplace, even though such services did not exist five years ago. Here are a few examples:

Some petroleum equipment suppliers have developed new solutions to prevent the corrosion problem from developing. For example, nitrogen inerting systems are being promoted and installed to eliminate and replace the oxygen needed to cause corrosion within USTs. Others market a vapor inhibitor (think of an object that works like your car’s air freshener) that covers the wall of the metal and prevents corrosion. The fuel quality experts offer services to test the water bottoms for bacteria and to treat the tanks with biocides to kill the instigating bacteria.

Some service providers have reported a greater amount of sludge and particulates accumulating on tank bottoms, and hence, a concern that microbial corrosion will develop on the steel tank bottom. The EPA has also expressed concern about this possibility. Of course, STI and steel tank manufacturers have strongly reiterated that internal corrosion incidents can be avoided by good maintenance practices:

  • Remove accumulated
  • Clean tank bottoms to remove sludge and
  • Treat microbial activity with

Let me offer one example of what was found because a tank owner did not follow good maintenance practices. A nine year old tank had never been cleaned, nor water removed. The walls of the diesel storage tank showed this unique type of corrosion. The walls of the tank showed no pitting except at the 6:00 line of the tank bottom. Sludge and particulates were allegedly over 2” thick in some places along the tank bottom.

This is an extreme example, but demonstrates the importance of good fuel storage tank maintenance practices. Just think about the poor fuel quality that customers were dispensing into their vehicles!

Even though a few steel tanks have been found to have corrosion on the tank surface in the ullage or vapor space, no corrosion penetration through the entire wall in the ullage space has been reported to STI. Nonetheless, there are concerns that steel tanks will become more susceptible to corrosion from ULSD as time goes on.

Even the steel bungs and manholes fabricated into FRP tanks are rapidly corroding, and stories of these metal components falling into the tank are being told.

Plastic tanks aren’t the answer

But don’t limit your thinking to only metallic corrosion with today’s ULSD systems. Plastics and elastomerics can degrade over time when exposed to various environments. For example, the November issue of Fuel Marketer News Magazine includes an article entitled “An Ounce of (Water) Prevention,” by Katherine Vanover of Clean Fuels National, that also discusses the potential harm from bacteria in FRP USTs. “Fiberglass tanks aren’t safe either,” Vanover says. “ Some fungi and bacteria have the ability to break down the resin that holds fiberglass together, resulting in a loss of integrity of the tank wall and increasing the chances of cracks and leaks.”

The New Hampshire Materials Laboratory wrote that various polyester resins are subject to deterioration by hydrolysis, when water is present. “In the hydrolysis reaction, water molecules break up the resin molecules, leaving an organic acid of varying acidity, depending on the particular resin, and a mixture of molecules of water, alcohols, and glycols…” Resins used in FRP tanks have changed over time, so this statement may or may not apply to today’s fabricated FRP tanks.

Even though FRP tank manufacturers market that their tanks do not rust, keep in mind that non-metallic tanks are vulnerable to degradation. It is important to keep water out of nonmetallic tanks, just as with steel tanks, to assure optimal fuel quality. Further, even the FRP tank warranties can be voided when tanks are damaged by servicing or lack thereof, and by maintenance or lack thereof.

Protecting fuel quality

ASTM fuel quality standards limit the amount of particulates in the fuel departing the refinery. The EPA study noted earlier identified water and particulates as a major concern. Engine manufacturers demand cleaner fuels for today’s fuel injection systems.

One item that surprised many in the petroleum marketing industry is how additives can be introduced into the fuel as the fuel changes hands, from the refinery, to the terminal, to the pipeline operator, to the regional bulk plant, and eventually to the gas station tank owner. Additives might include lubricity,
conductivity, biodiesel, cold flow improvers, and corrosion inhibitors. There are no ASTM fuel quality standards that limit or regulate the additives industry, as most ASTM standards establish fuel quality parameters at the refinery.

Let me offer a few examples. I heard an owner of a terminal operation in the northeast speak optimistically of how much cleaner heating oil has become with the removal of sulfur to 500 ppm. In his terminal operations, the following additives are made, and surprisingly, the operator was no longer certain why all the additives were still being introduced:

  • Sludge dispersant
  • Antioxidant and thermal stabilizer
  • Corrosion inhibitor
  • Metal deactivator
  • Water dispersant
  • De-foamer

At a gas station in the Southeast, a customer noticed a sign explaining why the station owner’s fuel should be considered so much better because of the as additives included:

  • Fuel detergent #1
  • Fuel detergent #2
  • Anti-adhesion compound
  • Corrosion inhibitor
  • Demulsifier
  • Solvent fluid
  • Marker molecules

A recent article in a new publication, Commercial Fuel Buyer, included an excellent article about “Diesel Fuel’s Quality Problems” by a major participant in the fuel distribution and dispensing network. The author mentions the new “engines with a daisy chain of emissions reduction equipment on the exhaust” (DPF, DOC, SCR) that introduces “new maintenance challenges and burdens on the fuel and the engine.”

“With sulfur out of the fuel, diesel tanks simply require a much more stringent maintenance and cleanliness regime than they had in the past,” the author continues. He notes that it’s possible there might be 3.5 gallons of water in every truckload delivered to a UST. The author was pleased that suppliers were starting to offer “advanced additive treatment programs, tank cleaning and maintenance solutions,” and open communications are occurring.

A fuels test lab in Georgia recently discovered that ULSD as it is transported and delivered into gas station USTs has a much lower flash point than when the fuel left the terminal, dangerously close, in fact, to the flash point of Class I flammable liquids (100° F) such as gasoline and ethanol. (The “flash point” is the lowest temperature at which vapors of a volatile material will ignite.)

The Coordinating Research Committee, a group of technical experts in the fuels and motor vehicle industry, are considering a thorough laboratory research effort to identify the cause of this significant problem.

Nitrous Oxide Removal

Nitrous oxides are also being removed from the exhaust of newer diesel truck engines with a special liquid injected into the exhaust. This “diesel exhaust fluid” (DEF) is stored in tanks. DEF is made from urea and water. The agricultural industry knows that urea has been used to fertilize crops. Dispensing of DEF is now common at truck stops across America, and is typically consumed in a 50:1 ratio versus the ULSD.

The DEF requires a higher level of cleanliness than was ever required with the actual motor fuels. It can be stored in polyethylene totes, FRP tanks, stainless steel tanks, or lined carbon steel tanks that meet certain ISO (international standards) requirements. New standards have been developed for DEF by industry groups, including PEI for installing DEF systems, and STI for fabricating DEF steel tanks.

Ethanol Blended Gasoline

Gasoline with 10% ethanol has been available routinely and without major incident in the Midwest farm belt of the UnitedStates for several decades. But in other parts of the USA, it has only become the norm in the past five years or so.

UST Sumps and Corrosion

Today’s UST system sumps are installed to house and access from grade the submersible turbine pump housing, leak detection equipment and controls, and overfill spill buckets. The sump itself has had issues with maintaining a dry environment when storing ethanol-blended gasoline, and has gone through several generations of improvements.

New issues caught the attention of state environmental UST regulators about six years ago. Corrosion was discovered upon the entire surface of metallic components within sumps mounted on ethanol blended gasoline underground storage tanks. Just like ULSD, the corrosion seemed to occur on a random basis. The hypothesis was that ethanol vapors, humidity or condensation, coupled with bacteria, was causing the corrosion. EPA’s Office of Research and the National Institute of Standards and Technology (NIST) studied the phenomenon and to verify the cause.

Compatibility and ethanol

The compatibility of non-metallic storage tank components with ethanol blended gasoline has also become a major concern. The US Department of Energy (DOE) test laboratories, such as National Research Energy Lab (NREL) and Oak Ridge Labs (ORL), provide ongoing research efforts and some best practice manuals for E15 and E85 storage and dispensing systems. ORL identified elastomerics and plastics as a concern with storage of various ethanol blended fuels. The materials either lose their flexibility, hardness, or strength over time when exposed to non-compatible ethanol blended gasoline fuels. Rubber O-rings can lose their ability to seal. Plastics can soften and lose their strength, especially if the fuel can permeate through or into the material.

In 2014, the EPA revised its UST regulations and expanded requirements for tank owners regarding compatibility of fuels that exceed 10% ethanol and 20% biodiesel. An association of state environmental regulators, ASTSWMO, has published a document on compatibility, providing more than 20 examples of equipment failures. This included ten examples of FRP UST failures and two steel tank internal corrosion failures:

Stories of more frequent FRP tank failures, caused either by incompatibility or aging, seem to be coming out of the Southwest. Tanks have been discovered to have delamination, exhibit resin-starved laminates, cracking, blistering, loss of roundness and structural integrity, and more. The industry points out that older FRP tanks were built and listed for gasoline only, and never intended for ethanol blended fuels.

Some single wall FRP tanks that were fabricated as recently as a dozen or so years ago are listed for ethanol blended fuels only up to 10%, whereas double wall tanks were UL listed for ethanol blends as high as E85 since 1990. Most of the industry already knows that FRP tanks fabricated before the early to mid-1980’s were not made to be compatible with E10, nor listed for such storage. Some state regulatory agencies track these time frames and make them available online.

Internal Tank Inspection and Tank Lining and Repairs

Many petroleum equipment service providers now offer internal tank inspection services. One company uses a camera to inspect tanks and internal equipment for compatibility and corrosion, has web pages devoted to aging FRP tank compatibility and ULSD tank component corrosion.

Other petroleum equipment service providers have offered to interior line and coat the tank, and some will even add a new wall within the existing tank and provide a UL label for this work.

Some state regulators are concerned about tank linings. In the 1980’s and 1990’s, old steel tanks without external corrosion protection could be upgraded to comply with corrosion prevention regulations by lining the inside of the tank. Normally, these systems would be re-inspected after ten years; some of them are still in operation. But there were problems. Some contractors provided a high-quality liner product, but many did not and were in the industry for only a short time. Lower-quality lining systems failed, with sheets of the lining product peeling off the tank. A state fire marshal spoke at a tank conference a couple years ago in the Southwest and expressed concern that repairs and tank linings were made under cover of night at FRP tank facilities, without going through the required reporting and permit process through his office.

Today, all installed tanks must meet the 1988 EPA requirement for external corrosion protection. In addition, they may be lined for two reasons:

  • FRP tanks may be lined to provide compatibility with today’s fuels. At least one service provider offers installation of a second tank inside the existing tank, and provides a UL label for their work.
  • Steel tanks that have internal corrosion may also be lined. In some cases, service providers will repair a steel tank by welding a patch plate as described in STI’s RP131 for repair of USTs.

It’s interesting, though, that there is no federal regulatory requirement to inspect these linings after a specific period of operation. Most states do not even have a reporting mechanism in place when such work takes place. One state regulatory representative at a recent meeting was worried that tank owners were becoming confused by the warranties being offered by tank liners, believing the clock started anew for 30 more years, even if the tank had been in the ground for 20 years.

E15, E25, E30, and E85

E85 is available at over 2500 gas stations and only flex-fueled vehicles have the option to use this fuel alternative. However, now that E10 is available across the US in nearly all gasoline, the ethanol industry and some government agencies encourage higher ethanol blends. Tank owners are reluctant because some of their equipment is not listed or compatible with higher ethanol blended gasoline.

Nonetheless, we are seeing the Midwest again take the lead to make E15 more available. Dispensers are available that will pump the desired blend on site at the gas station. At least one dispenser manufacturer now makes all dispensers rated for E25 and lower blends. NREL has published a listing of equipment that is rated for higher blends.

Petroleum marketers look for ways to offer fuels that competitors are not offering. Some promote the ethanol fuel as “made in the USA”, while others say it is a “green” fuel, or a “lower pollution causing alternative” fuel. Meanwhile, the petroleum fuel industry disagrees that ethanol is made or used with less pollution consequences, and the fracking revolution has enabled the USA to become less dependent on foreign imported oil.

Future Fuels

Optimum Octane Fuel

Where are we going from here? Many industry groups have their own ideas or desires, but no one can say for sure. New associations, such as the Fuels Institute, have formed to explore that subject. With the advent of electric cars, hydrogen fuel, fuel cells, isobutane fuels, propane, and liquefied natural gas (LNG) and compressed natural gas (CNG), the future is wide open.

One very serious effort is worthy of mention. The Department of Energy works with over a dozen different laboratories across the United States. The DOE is researching development and use of an “ultimate fuel”, to be fully in place by 2035 so that all vehicles on the road would be fueling with it by 2050. The ultimate fuel would have high mileage per gallon, low harmful emissions from nitrous oxides and carbon dioxide, and power more efficient engines.

DOE’s project will develop a liquid fuel, using existing or slightly modified infrastructure and technology that will maximize engine efficiency. Today’s engines waste about 50% of the available energy in fuels when the fuel is ignited to drive the engine. By maximizing engine efficiency, preventing engine knock, vehicles’ mileage per gallon will increase. The US government has set a goal of over 50 mpg as the total vehicular average in the not so distant future.

To increase engine efficiency while preventing engine knock, the amount of octane of the fuel must increase. One fuel being closely looked at is a gasoline blend with approximately 30% ethanol.

Changing Gasoline

As if these changes are not enough, there is another mandate to reduce the sulfur in gasoline in 2017. Many old timers may remember when lead had to be removed from gasoline, and dispensers were labeled as having “unleaded fuel.” Will the removal of additional sulfur from gasoline create the same challenges occurring with ultra low sulfur diesel? Let’s hope not.

In conclusion, you may be asking what can be done today to ensure safe storage and retained quality of these newer fuels. Nearly all industry groups, including STI, recommend continued diligence in tank maintenance and internal fuel conditions to circumvent premature service disruptions:

  • Remove water and particulates from tank bottoms regularly
  • Inspect your tank system and the internal
  • Clean the tank, filter or polish the fuel, treat microbial activity with biocides.
  • Explore new methods to control the environment, such as nitrogen blanketing and vapor
  • Be aware of fuel additives and make sure they’re sensible and compatible with the fuel.

Stay tuned folks. I think this is just the beginning of a sea change in the fueling industry.

Published Date

March 1, 2017

Resource Type

  • Guidance


  • Alternative Fuels