Materials Performance

DEC 2014

Materials Performance is the world's most widely circulated magazine dedicated to corrosion prevention and control. MP provides information about the latest corrosion control technologies and practical applications for every industry and environment.

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18 DECEMBER 2014 MATERIALS PERFORMANCE NACE INTERNATIONAL: VOL. 53, NO. 12 MATERIAL MATTERS NIST examines corrosion of storage tank components exposed to gasoline-ethanol blends A NIST study focused on corrosion of sump pump components for USTs, especially the pump casings, which typically are made of steel or cast iron. Image credit: U.S. Environmental Protection Agency. Optical micrographs of severe corrosion on steel alloy samples after 355, 643, and 932 h of exposure to ethanol and acetic acid vapors—conditions typical near sump pumps for USTs that store gasoline-ethanol blends. Image credit: NIST. C orrosion in parts of underground storage tanks (USTs) that could result in failures, leaks, and contamina- tion of groundwater, a source of drinking water for almost half of the American population, is a threat that lies beneath gas stations across the United States that pump gasoline-ethanol blends and ultra- low-sulfur diesel fuel. Reports in recent years from f ield inspectors in nine states indicate that many UST sump compo- nents, such as steel submersible turbine pumps and piping, copper tubing, and brass valves, are experiencing accelerated corrosion. A lso reported is a vinegar-like smell when sump components are examined. Ty pically, these incidents are connected with use of gasoline- ethanol blends and the presence of bacteria, Acetobacter aceti , which convert ethanol to acetic acid, and the rapidly occurring damage is suspected of being caused by microbiologically inf luenced corrosion (MIC). Because of the inspectors' f indings, researchers w ith the National Institute of Standards and Technolog y (N IST) (Gaithersburg, Mar yland) conducted a laborator y study. 1 The objectives of the study were to deter- mine if the rapid cor- rosion was caused by the presence of Aceto- bacter sp. and to mea- sure the corrosion rates of the UST mate- rials exposed to acetic acid produced by microorganisms, as well as develop a test to evaluate corrosion in biof uel systems that contain microbes and improve the methodolog y for mea- suring real-time corrosion. Much of the U.S. fuel infrastructure was designed for unblended gasoline. Now ethanol (an alcohol that can be made from corn) is widely used as a gasoline additive due to its oxygen content and octane rating, or antiknock index. A pre- vious NIST report 2 states that ethanol and bacteria are known to cause corro- sion and experimental evidence pre- sented by NIST researchers showed bac- teria that feed on ethanol and produce acid boost fatigue crack grow th rates by at least 25 times the levels occurring in air alone. The recent NIST study focused on materials used to fabricate sump pump components, which are located directly below access covers at gasoline stations, just above and connected to the USTs. The sump pumps move fuel from under- ground tanks to the fuel dispensers that pump gas into vehicles. The USTs and associated piping also may be made of steel and v ulnerable to MIC as well. "We know there are corrosion issues associ- ated with the inside of some tanks. We're not sure, at this point, if that ty pe of corrosion is caused by the bacteria," says Jeffrey Sowards, co-author of the NIST study. New test methods and equipment were developed by the NIST researchers for the study. In a controlled laboratory environment, both cold-worked copper and steel alloy samples that are charac- teristic of the metals used in tank compo- nents were either immersed in ethanol- water solutions inoculated with bacteria

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