Lab tests have shown that underground gas tanks at over 150,000 US gas stations pose a potential water contamination hazard. Many of the underground gas tanks have corroded parts that could lead to failure or leaks that may contaminate groundwater, from where much of US drinking water supplies originate. Field inspections conducted across 9 states have revealed that key components of gas storage tanks, such as sump-pumps, are commonly corroded. In most of the reported cases the corroded tanks stored a mixture of gasoline-ethanol fuels, together with the existence of Acetobacter aceti -- a strain of bacteria that is able to convert ethanol to acetic acid, a highly corrosive agent.
Upon conducting a follow up laboratory study, the National Institute of Standards and Technology (NIST) found that corrosion of steel alloy components was severe when exposed to the combination of ethanol and vaporized acetic acid, often eating 1mm of steel per annum. Considering these findings, researchers from the NIST recommend that gas stations should consider replacing casings of submersible pumps constructed from cast iron or steel at much shorter intervals. However, at a cost of anywhere between $1500 - $2500 per storage tank, these retrofits could be costly, especially tanking into account that there are over 500,000 underground gas storage tanks dotted around the US.
The NIST assessment only looked at components of sump-pumps, which are found immediately below tank access covers and thus were readily accessible. The sump-pumps pump fuel from the gas storage tanks below, to the gas pumps at filling stations. While only the sump-pumps were tested, researchers are concerned that pipes, and even the gas storage tanks themselves, could be constructed from steel, which would make them vulnerable to corrosion too.
"We know there are corrosion issues associated with the inside of some tanks. We're not sure, at this point, if that type of corrosion is caused by the bacteria," says NIST co-author, Jeffrey Sowards.
Most of the gas storage facilities across the US were designed to contain unblended gasoline. Now, ethanol, a fuel derived from corn, is being widely used as a fuel additive due to the benefits it offers. An earlier NIST study showed that pipeline cracking was accelerated when ethanol-loving bacteria were present.
For their latest study, the NIST research team studied the effect on samples made of steel and copper alloys exposed to conditions similar to those found at sump-pumps -- samples were either placed in a solution of ethanol and water to which bacteria was added, or exposed to vapors above this mixture. They then measured the rate of corrosion over a 30-day period. Their results echoed the findings reported by field inspectors.
Steel components exposed to vapors experienced the most damage, with copper components experiencing some damage, albeit slower, both when immersed in the fuel and when exposed to vapors. Immersed steel corroded slowly, which the authors suggest may be attributed to a protective biofilm coating produced by the bacteria.
Even though corrosion on copper was slow and could take approximately 15 years for holes to form in copper tube with a wall thickness of 1.2mm, the researchers observed localized corrosion on cold-worked copper, as used in tubing for sump-pumps. Consequently, stress-corrosion cracking is an area of concern where copper tubing is bent, as it would substantially reduce the lifetime of the tube and lead to leaks.
J.W. Sowards and E. Mansfield. Corrosion of copper and steel alloys in a simulated underground storage tank sump environment containing acid producing bacteria. Corrosion Science. July, 2014. In press, corrected proof available online. DOI: 10.1016/j.corsci.2014.07.009.