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Corrosion in a STP Sump. (Subtitle: What Causes It and What Can Be Done About It?)
Wilson, J., C. Adair, C. Paul, J. Skender, A. Barbery, L. Thompson, E. Fowler, R. Strauss, K. West, J. Hickey, R. Hansen, C. French, J. Owens, B. Kowalski, E. Humlie, AND B. Jergenson. Corrosion in a STP Sump. (Subtitle: What Causes It and What Can Be Done About It?). The PEI Journal. Petroleum Equipment Institute, Tulsa, OK, , 26-34, (2013).
Submission to the Petroleum Equipment Institute Journal. The journal is not normally peer reviewed. A peer review was conducted as if the article were an EPA report.
State regulators have noticed extensive corrosion in the sumps for the submersible turbine pump (STP) of an underground storage tanks storing gasoline and E85. Acetic acid produced by biodegradation of ethanol that found its way into the sump is one plausible explanation. Research staff of the United States Environmental Protection Agency (EPA) set out to evaluate the association between ethanol vapors in sumps and the production of acetic acid and, correspondingly, the association of acetic acid to corrosion in sumps. To make this evaluation, EPA staff asked state regulators to collect samples from sumps at sites across the United States. To facilitate the work, EPA provided a sampling kit that allowed sampling of water from the sumps and vapors within the sumps. Participants were asked to sample sumps that showed evidence of corrosion and neighboring sumps that did not show corrosion. A passive diffusion sampler was installed in the sump to sample the concentrations of ethanol and acetic acid in air. Then the sampler was recovered and sent to the Ada lab for analysis. In general, corrosion was associated with the higher concentrations of ethanol or acetic acid in air. The concentration of ethanol and acetic acid air in the sumps was roughly one-hundred fold higher in sumps that had rust tubercles or a blue corrosion product.For ethanol to cause corrosion in a sump, three components are necessary: ethanol, bacteria to degrade the ethanol to acetic acid and water. Water provides the environment for the bacteria to live in and is also required for the actual chemical corrosion event. Corrosion can be managed by stopping any leak that is allowing ethanol into the sump. It is more difficult to manage water in a sump. The seals and covers for the sump should be maintained in good condition and the sump elevated above grade to prevent the entry of rain water or snow melt. Water may also enter a sump through groundwater leaks. A sump testing and inspection program can help to identify and correct leaks in sumps and sump lids. Biocides are designed to kill bacteria that might grow in a “water bottom” below the fuel in a tank. However, adding a biocide to the sump will only affect the bacteria that might grow in the water in the bottom of the sump. The biocide will not have access to bacteria growing in water that is condensing on the pump head or distribution lines in the sump. Using biocides, therefore, will probably not be as effective as stopping the leak of fuel or vapors that provides the ethanol to the acetic acid bacteria.