A Link to the report is at the bottom of the post...
Corrosion in Systems Storing and Dispensing Ultra Low
Sulfur Diesel (ULSD), Hypotheses Investigation
Severe and rapid corrosion has been observed in systems storing and dispensing ultra low sulfur diesel (ULSD) since 2007. In addition, the corrosion is coating the majority of metallic equipment in both the wetted and unwetted portions of ULSD underground storage tanks USTs). To investigate the problem in an objective manner, multiple stakeholders in the diesel industry, through the Clean Diesel Fuel Alliance, funded this research project. The design included the identification of retail fueling sites and the development of an inspection and sampling protocol to ensure uniform and thorough inspections of USTs. Fuel, water bottoms,vapor, bottom sediments, and scrape samples were taken from six sites: one that was not supposed to have symptoms (but did to a much lesser degree) and five that were to have the severe corrosion. Then, samples from the inspections were analyzed for genetic material and chemical characteristics. These data, in combination with information on additives, have allowed Battelle to draw conclusions with respect to three working hypotheses.
Specifically, the hypotheses are:
1) Aerobic and anaerobic microbes are producing byproducts that are establishing a corrosive environment in ULSD systems;
2) Aggressive chemical specie(s) (e.g., acetic acid) present in ULSD systems is(are) facilitating aggressive corrosion; and
3) Additives in the fuel are contributing to the corrosive environment in ULSD systems.
All of the sites inspected contained microbes, although at different abundances. The dominant organism identified from three of the sites, Acetobacter, has characteristics pertinent to the corrosion observed in all of the sites, such as acetic acid production, ethanol utilization, low pH requirements, and oxygen. Although geographically on opposite sides of the country, with different fuel supplies, and from relatively new construction materials, the presence of the organisms was relatively uniform. The traditionally expected organisms, hydrocarbon degrading organism were found in insignificant abundances. This indicates that the inspected ULSD USTs
are selective environments for these specialized, acetic acid producing organisms.
Of note from the chemical analyses is that acetic acid was found to be ubiquitous (water bottoms, fuel, vapor, and scrapings) in all of the sites inspected. In addition, components necessary for the organisms identified to proliferate were analytically determined to be present: trace amounts of ethanol, low pH, oxygen, and water were present in the diesel USTs inspected.
Although additives could play a role in the corrosive environment, it has been determined that they are not a primary cause of the observed corrosion. Acetic acid is believed to be a prime contributor of the corrosion. The main circumstances around the production of the acid could be thriving Acetobacter microorganisms.
Finally, there needed to be a mechanism to disperse the acetic acid into the vapor portion of the USTs. Acetic acid has a higher vapor pressure than diesel fuel (0.5 psi compared to 0.1 psi). As a result of this and due to turbulent mixing during fuel deliveries, the acetic acid can be dispersed throughout the upper portions of the tank. Through the wetting and drying cycles (between deliveries), the acid concentrates on the USTs equipment causing severe and rapid corrosion.