EPA: Corrosion in UST Systems Could be Common



EPA reports that 83 percent of USTs studied exhibited moderate or severe corrosion, but less than 25 percent of owners were aware of it

EPA announced the results of its study of enhanced corrosion in storage systems with ultra-low sulfur diesel. Here is an excerpt from the executive summary:

This research focused on better understanding a type of rapid and severe corrosion of metal components in underground storage tanks (USTs) storing diesel fuel. UST owners first began reporting this corrosion to UST industry servicing companies in 2007. Several changes to the national fuel supply and fuel storage practices have occurred since the mid-2000s. To address the potential for corrosion problems, the U.S. Environmental Protection Agency’s (EPA) Office of Underground Storage Tanks began working on this research in 2014 to understand how serious and widespread the metal corrosion problem could be. In addition, to help identify the cause or solutions, we wanted to identify predictive factors between UST systems with corrosion issues and UST systems relatively free of the problem. EPA’s objective for the research was to develop a better understanding of potential risks to human health and the environment caused by the evolving corrosion problem in USTs storing diesel fuel.

In 2014, EPA held discussions with UST industry experts and worked collaboratively to develop field-based research that would further the understanding of corrosion inside USTs storing diesel.


EPA designed our research to examine many factors on a diverse population of 42 UST systems in order to find potential predictive factors among them. We thought any predictive factors identified in our research would help focus the search of potential causes for the next phase of follow-on research.

In January and February 2015, EPA conducted on-site inspections of 42 diverse, operating UST systems at 40 sites across the country. Of these UST systems, 24 had fiberglass tanks, and 18 had steel tanks. Field teams documented the conditions of the UST systems with in-tank video cameras and photos so they could later assign a category of corrosion coverage to each system. The field teams also collected samples of vapor, fuel, and aqueous phase (also known as water bottom), if present, from each of the tanks. Field teams used a detailed questionnaire to gather information from each owner about the storage history, operation, and maintenance practices of each UST system.

EPA chemically analyzed the vapor, fuel, and aqueous phase samples. Three assessors reviewed the videos of each UST and categorized the USTs by the extent of the corrosion judged to be present: minimal, moderate, or severe. In an attempt to identify corrosion predicting factors among UST systems experiencing either minimal corrosion or severe corrosion, we then statistically evaluated the analytical results and responses from the questionnaires against the corrosion categories.

The major finding from our research is that moderate or severe corrosion on metal components in UST systems storing diesel fuel in the United States could be a very common occurrence. Observations suggest that corrosion may be commonly severe on metal surfaces in the upper vapor space of UST systems, an area that before 2007 was not known to be prone to corrosion. Furthermore, it appears many owners may not be aware of the corrosion nor are they aware that corrosion, which could affect the operability of their UST systems, could already be at an advanced stage. We observed 83 percent of the inspected tanks had moderate or severe metal corrosion. Prior to our research inspections, less than 25 percent of owners reported knowledge of corrosion in their UST systems.

It appears from our research that corrosion inside of UST systems could result in an increased chance of releases of fuel to the environment and subsequent groundwater contamination. Across the sample population, EPA observed corrosion occurring on all types of UST system metal components, including submersible turbine pump shafts, automatic tank gauge probe shafts, risers, overfill equipment like flapper valves and ball valves, bungs around tank penetrations, inner walls of tanks, and fuel suction tubes. Many of these UST system components are designed to prevent overfilling the tank or to identify leaks, and the components must be able to move and function as designed. Corrosion of some metal components could hinder their proper operation and possibly allow a release of fuel to occur or continue unnoticed. Anecdotal reports since EPA began our research suggest that other metal components in UST systems, such as tank walls, could also eventually fail by corroding completely through the metal if corrosion is not stopped. This would most likely occur in the bottom of an UST where aqueous phase and tank sludge collect. Corrosion through the bottom or wall of a tank could potentially allow fuel to leak into secondary containment areas or release to the environment.

EPA has heard anecdotes of functionality failures of release prevention equipment and leak detectors, as well as failures of metal walls resulting in leaks into secondary containment areas. Outside of anecdotes, however, very little verifiable data exists about how equipment functionality and integrity are being affected by corrosion in USTs storing diesel fuel. However, that information should become more available as owners become more aware of the findings of our research and corrosion in USTs storing diesel becomes more visible.

Even absent a release of fuel to the environment, severe corrosion poses concerns for owners. Corrosion increases servicing and equipment maintenance costs for UST system owners. Anecdotes suggest that dispenser filters may become clogged with corrosion debris that resembles coffee grounds, resulting in filters needing to be changed more frequently. Other equipment may need to be repaired more often and sometimes may need to be prematurely replaced.

The data and analyses could not pinpoint a cause of corrosion that UST owners began reporting in 2007. It appears multiple underlying factors and corrosion mechanisms could be contributing to the corrosion; one such mechanism is microbiologically-influenced corrosion (MIC). Previous research on the recent corrosion phenomenon is limited, but suggests that the reduced sulfur in diesel could be allowing microbial life to proliferate in ultra-low sulfur diesel tanks and, through MIC, cause corrosive conditions that were less possible in USTs storing low sulfur diesel. Several independent organizations have produced publicly available resources that suggest following certain enhanced maintenance practices when storing diesel fuel in USTs. If followed, these practices can likely minimize MIC risks by reducing bacterial populations or preventing an environment where microbial life can thrive.

EPA’s research builds on industry’s first study about rapid and severe corrosion in USTs storing diesel, which the Clean Diesel Fuel Alliance (CDFA) completed in 2012. The objective of CDFA’s study was to produce an initial hypothesis about the mechanism of corrosion from data collected on six UST systems. EPA’s Office of Research and Development (ORD) completed research shortly after CDFA.2 Both the CDFA and ORD research hypothesized that biofuel components in diesel, such as ethanol and biodiesel, could be providing the energy source for microbial populations of bacteria like acetobacter in USTs. This genus of bacteria was the most abundant in samples that underwent DNA sequencing in CDFA’s study. EPA’s research plan to identify any predictive factors by default included checking the plausibility of the hypotheses previously suggested. However, there are numerous other types of bacteria that could also be consuming chemical components of the fuel or fuel contaminants found in USTs. In addition to bacteria, there are also a number of other microorganisms that could cause or contribute to the corrosion attacks, including fungi, archaea, and eukaryotic organisms. A combination of one or more of these factors could also be responsible, but we did not test for those factors in our research.

Because only limited scientific research was available, EPA assumed from the beginning of our research that, within our research scope, it was not feasible to definitively pinpoint a cause of the corrosion. Further, most familiarity with the extent and geographic spread of the problem was anecdotal. Therefore, an exploratory approach was most helpful in expanding the knowledge around the issue. Our research suggests that MIC is likely involved in the moderate or severe internal corrosion in USTs storing diesel. However, further identification of specific bacteria was not possible within the scope of our research. Therefore, while previous research hypotheses about the role of specific species of a genus oxidizing biofuel components were not disproven by the results of our research, validation would be speculative. EPA’s research population of 42 USTs was geographically, materially, and operationally diverse and was the largest field research of this issue to date. However, the population is a small percentage of diesel USTs across the United States, and the types of USTs and maintenance practices by owners in the research population may differ from those in the national population of USTs storing diesel. Therefore, EPA cannot predict if the presence of moderate or severe corrosion in diesel USTs across the United States will be higher or lower than identified in our research.

EPA recommends owners check their diesel UST systems for corrosion and take steps to ensure the proper operability of their UST systems. EPA is recommending this because 83 percent of USTs in the study affected by moderate or severe corrosion is very high, most of the owners were not aware of the extent of the corrosion in their USTs, and it appears that corrosion could potentially affect equipment functionality and potentially lead to a release of fuel to the environment.


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