NIST study: Ethanol blends cause severe corrosion in gasoline tanks

30 July 2014

Laboratory tests conducted by the US National Institute of Standards and Technology (NIST) suggest that ethanol may be responsible for corrosion in parts of underground gasoline storage tanks—corrosion that could result in failures, leaks and contamination of groundwater, a source of drinking water. In recent years, field inspectors in nine US states have reported many rapidly corroding gas storage tank components such as sump pumps. These incidents are generally associated with use of gasoline-ethanol blends and the presence of bacteria, Acetobacter aceti, which convert ethanol to acetic acid, said NIST.

Following up on the inspectors’ findings, NIST conducted a laboratory study that demonstrated severe corrosion—at rates as high as 1 millimeter of wall thickness per year—on steel alloy samples exposed to ethanol and acetic acid vapors. Based on this finding, NIST researchers suggest gas stations may need to replace submersible pump casings, typically made of steel or cast iron, sooner than expected. Such retrofits could cost an estimated $1,500 to $2,500 each, and there are more than 500,000 underground gas storage tanks in the United States.

The NIST study focused only on sump pump components, located directly below access covers at filling stations, just above and connected to underground gas storage tanks. The sump pumps move fuel from underground tanks to the fuel dispensers that pump gas into cars. These underground tanks and pipes also may be made of steel and could be vulnerable, 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,” said NIST co-author Jeffrey Sowards.

Much of the US fuel infrastructure was designed for unblended gasoline. Ethanol, an alcohol that can be made from corn, is now widely used as a gasoline additive to satisfy the requirements of the US Renewable Fuel Standard (RFS2). A previous NIST study found that ethanol-loving bacteria accelerated pipeline cracking.

For the latest study, NIST researchers developed new test methods and equipment to study copper and steel alloy samples either immersed in ethanol-water solutions inoculated with bacteria, or exposed to the vapors above the medium—conditions mimicking those around sump pumps. Corrosion rates were measured over about 30 days.

The NIST study confirmed damage similar to that seen on sump pumps by field inspectors. The worst damage, with flaky iron oxide products covering corrosion, was found on steel exposed to the vapors. Copper in both the liquid and vapor environments also sustained damage, but corrosion rates were slower. Steel corroded very slowly while immersed in the liquid mixture; the NIST paper suggests bacteria may have created a biofilm that was protective in this case.

Although copper corroded slowly—it would take about 15 years for 1.2-millimeter-thick copper tube walls to develop holes—localized corrosion was observed on cold-worked copper, the material used in sump pump tubing. Therefore, stress-corrosion cracking is a concern for bent copper tubing because it would greatly reduce tube lifetime and result in leaks.

A 2012 study by Battelle suggested that biocorrosion triggered by ethanol contamination may be also responsible for corrosion observed in ultra-low sulfur diesel (ULSD) fuel tanks in the United States.

In July 2013, NIST held a workshop on biocorrosion associated with alternative fuels.

Source: NIST