Researchers have determined that a combination of water, ethanol, and a specific bacterium caused unusually accelerated corrosion in certain components of underground sump pumps for gas stations. The researchers hope the findings can prevent similar corrosion in pipelines and bridges. Courtesy of NIST
A team investigating rapid corrosion in fuel tank sump pumps develops a way to better understand corrosion in vaporous environments.
August 12, 2014—Researchers from the National Institute of Standards and Technology (NIST) have developed a test to better understand the corrosion caused by bacteria in a vapor-filled, enclosed environment. The test was developed to address specific concerns affecting the sump pump wells at gas stations across the country, but could have applications in pipelines and bridges as well.
The research originated when inspectors began to notice rapid corrosion of components in the underground sump wells of gas stations. These are the wells that are capped by the metal discs visible in gas station parking lots. Inspectors were reporting an unusual and unexpected pattern.
“They see the same features. When they raise the covers up, they smell a vinegar [odor] and see this corrosion product all over the pump and the tubing,” says Jeffrey Sowards, Ph.D., a researcher for NIST. “This corrosion issue pops up within a matter of months. They will put in a new installation and they see corrosion four to six months later.”
That’s a corrosion level that that is much faster and more pervasive than is predicted by conventional models. It also has troubling implications. Accelerated corrosion rates could lead to perforations and leakage of fuel into groundwater. In examining the issue further, the researchers realized the issue isn’t affecting the tanks, but only the sump pump assemblies and wells. The corrosion was found in correlation with high levels of acetate.
The researchers discovered that the corrosion was the product of a “perfect storm” of water, the ethanol commonly found in gasoline today, and an exceptionally common bacteria—Acetobacter sp.
“[Acetobacter sp.] are ubiquitous,” Sowards says. “They are everywhere. They are floating in dust particles. They are in dirt. They are really common. It’s very easy for a system to become contaminated.” But contamination isn’t enough to create the corrosive atmosphere, he notes.
“You need three things for the bacteria. You need a food source, which in this case is ethanol, you need water, and then you have to have an electron acceptor, another compound that the bacteria can give electrons to.” In this case, the acceptor is the copper and steel.
When these elements are all present, the bacteria thrive, creating organic acetic acid, a component of vinegar. This organic acid has a low pH and permeates a vapor in the sump pump assembly. Steel is especially vulnerable in low pH conditions.
To test the process, the team used steel and copper rods to form small coupons that were placed in various places in a simulated sump assembly. The corrosion rates of coupons submerged in liquid and those exposed to vapors were similar. The corrosion rates of steel were greatly accelerated when exposed to vapor with acetate.
“The bacteria were producing pretty high concentrations,” says Elisabeth Mansfield, Ph.D., a research chemical engineer at NIST. “It was like a concentrated version of your home white vinegar. You can imagine if you are put steel or copper into a vinegar solution that it would cause corrosion. This particular species of bacteria creates that vinegar as a result of their metabolic process.”
Sowards and Mansfield recently published their findings, “Corrosion of Copper and Steel Alloys in a Simulated Underground Storage-Tank Sump Environment Containing Acid-Producing Bacteria,” in the journal Corrosion Science.
The researchers next plan to examine the best way to mitigate this problem, which will likely take the form of either a resistant coating to protect the steel from acetate or a biocide to kill the bacteria. In the meantime, gas stations across the country might have a corrosion issue in sump pumps well before it would be expected, they note.
Beyond this specific issue, the researchers believe the test they have developed has applications in other environments in which water, bacteria, and a food source create a corrosive vaporous atmosphere, such as pipelines and some locations beneath bridges.
“I would say the test is set up to look at vapor corrosion. Bridges are especially important in terms of the materials used and the exposure they get to the environment. We could potentially extend this out to other material systems and other conditions that they may be observing in the environment,” Mansfield says.
“Anywhere you can think of where you have water, with open space above the water that is contaminated,” Sowards says, there is the potential for this type of corrosion. He says that bridges, pipelines, pipe piles, and sewage lines are all potential candidates for this testing method, from which the researchers hope to learn more about the potential corrosion mechanisms at work.