As early as 2007 some distributors began noticing corrosion issues that they believed were connected to the newly mandated ultra low sulfur diesel. The corrosion was in many cases severe, but sporadic and inconsistent and there was no obvious "smoking gun," such as a specific supplier, to provide an easy explanation. The corrosion occurred in both liquid and vapor areas and metallic wetted in unwanted areas. Further, similar corrosion could not be traced to refineries or pipelines or tied down to specific geographical regions. When found, the impact was noticed on virtually any metallic component in the fuel dispensing infrastructure from the tank to the nozzle.
To pursue the matter further, the Clean Diesel Fuel Alliance, which includes the Association of American Railroads; American Petroleum Institute; Ford Motor Company; National Association of Convenience Stores; National Association of Truck Stop Operators; Petroleum Equipment Institute; Petroleum Marketers Association of America; and the Steel Tank Institute, commissioned the Columbus, Ohio-based research firm Battelle in conjunction with the underground storage tank testing and compliance services provider Tanknology, based in Austin, Texas, to study the matter.
The results of the completed study were presented at the 2012 PEI/NACS show on Oct. 8 in Las Vegas, Nevada and raised some controversy. In general, the corrosion was tied to microbial action relative to the presence of ethanol in the tanks from cross contamination. As the conclusion states:
<i>“…the project final hypothesis is that corrosion in systems storing and dispensing ULSD is likely due to the dispersal of acetic acid throughout USTs. It is likely produced by Acetobacter bacteria feeding on low levels of ethanol contamination. Dispersed into the humid vapor space by the higher vapor pressure and by disturbances during fuel deliveries, acetic acid is deposited throughout the system. This results in a cycle of wetting and drying of the equipment concentrating the acetic acid on the metallic equipment and corroding it quite severely and rapidly.”</i>
The investigation was broken down into stages. The first phase focused on some of the more subjective observations and Tanknology's inspection database. That generated 12 potential hypotheses that were narrowed down to three working hypotheses:
1. Aerobic and/or anaerobic microbes are producing by-products that are establishing a corrosive environment in ULSD systems. To test this hypothesis, genetic sequencing was used to
definitively determine whether microbes are present, which microbes are in the samples from
inspected sites, and whether the microbes have metabolites that could contribute to the corrosion.
2. Aggressive chemical specie(s) (e.g., acetic acid) present in ULSD systems is(are) facilitating aggressive corrosion. Testing this hypothesis involved analysis of the chemical constituents present in the fuel, water, and headspace vapor within the USTs. These chemical constituents may be corrosive in nature or may contribute to the production of corrosive species, more specifically, acetic acid.
3. Additives in the fuel are contributing to the corrosive environment in ULSD systems. The approach for testing this hypothesis focused on gathering information from additives manufacturers, refineries, terminals, stations, and published literature to understand the potential effect of additives on the overall chemical characteristics of the fuel and headspace vapor within USTs.
The second phase involved a detailed investigation of six sites (culled from 12) that included field sampling. The site inspections entailed documenting the extent of corrosion in the UST systems and the fuel circumstances (inventory volume, water bottom height, temperature, etc.). One site was from North Carolina; two sites were from New York; and three sites were from California. While the sample was small, the sites selected were seen as providing a fair degree of continuity in tight material, fuel throughput and tank size while maximizing the variety in age, prior fuel service history, and geographic location. Five of the sites have experienced rapid and severe corrosion and one site was believed to be corrosion free.
Those results were analyzed and a final hypothesis was presented. Specifically, microbiological activity was found in all six sites and acetic acid was found in all vapor samples, all water samples and in four out of six fuel samples. Acetic acid was identified in 75 percent of the scrape samples and from all sites.
Additionally, additives and the fuel were seen as contributing to the corrosive environment in ULSD systems by enhancing the microbial action and, thus, the production of the acetic acid. This last point is the most controversial, as the additive in question, is ethanol, which was identified and measured in liquid samples suggesting that it is present as a cross-contaminant or that it was forming in the fuel.
The Acetobacter process requires oxygen, water, low pH, and ethanol, which are all present in the tested underground storage tanks. The ethanol contamination would likely be linked to switch loading and/or manifold and ventilation systems. The formation of ethanol could be linked to microbes or fungi producing ethanol.
The American Coalition for Ethanol is highly dubious of the report. “If it is traces of ethanol in the fuel tanks causing this problem, many have delivered both petroleum fuels and ethanol in the same infrastructure for over 20 years, and we would have seen this all over the place,” said Ron Lamberty, ACE Senior Vice President. “If the difference is the ULSD, then why isn’t this happening everywhere you now have ULSD? The test raises some interesting points, but it doesn’t prove anything. In the ethanol industry, we are not interested in covering up something if a problem does exist. If there is an issue, we work to find out what it is and then find a solution to correct it. But this doesn’t seem like an ethanol problem that is real.”
It’s generally acknowledged at PEI and among the Clean Diesel Fuel Alliance that given the implications of the study to the industry (basically requiring a dedicated logistics infrastructure specifically for ULSD), far more definitive research needs to be done to confirm that the corrosion firmly exists and that this is the source of the corrosion. Battelle itself recommends further research on this issue. For example, it suggests that a larger and more diverse sample set be used, where the sites would be sampled multiple times over a period of time. In particular, Battelle proposes that steel USTs and tanks without corrosion problems be investigated. Furthermore, Battelle advises that the source and magnitude of the ethanol contamination should be determined.