Bacteria crack ethanol pipeline steel

Dr. Neil Canter, Contributing Editor | TLT Tech Beat January 2012

These microbes present a major problem in the transportation of ethanol, but a solution is in the works.

KEY CONCEPTS
• There is concern that steel pipelines transporting ethanol may develop fatigue cracking.
• A recent study found that bacteria facilitate fatigue cracking in pipeline steel alloys exposed to ethanol.
• Preliminary testing with the biocide, glutaraldehyde shows promise in reducing the rate of fatigue cracking.

BACTERIA ARE ONE OF THE MAIN MICROBES THAT DEGRADE LUBRICANTS. Much attention has been paid to how bacteria can feed on the components in a metalworking fluid, which eventually leads to premature failure. In a previous TLT article, STLE Fellow Dr. Fred Passman outlined how bacteria can adversely impact the performance of a MWF (1).

One big concern about bacteria is the generation of acidic byproducts that reduce the pH of the MWF system, leading to the onset of corrosion. Early detection of corrosion has become a very important objective. In a previous TLT article, a new approach using a molecular probe was incorporated into an epoxy coating (2). The probe fluoresces reversibly when chelating ferric ions in a process called chelation-enhanced fluorescence.

One other area where corrosion can have an impact is biofuels. Jeffrey Sowards, metallurgist at NIST’s Materials Reliability Division in Boulder, Colo., says, “Ethanol, in particular, is expected to cause stress corrosion cracking when in contact with steel alloys.”

This issue becomes more significant because U.S. production of ethanol for use as a fuel is increasing quickly, reaching 13 billion gallons in 2010. The infrastructure needed to transport ethanol from its production sites through such distribution channels as rail, truck and barge is becoming strained.

One option that is a more efficient way to transport ethanol is a pipeline. Sowards says, “Pipelines could be a potential way to move ethanol, but the current literature suggests that problems will occur. Stress corrosion cracking has been found in steel storage tanks containing fuel-grade ethanol. The presence of water in ethanol leads to an increase in the fatigue crack growth rate of steel. Bacteria known to accelerate corrosion also have been identified in ethanol storage tanks.”

Sowards is concerned that pumping ethanol through steel pipelines will lead to the onset of stress corrosion cracking caused by bacteria. A just-conducted study confirms that bacteria will, in fact, facilitate the fatigue cracking of pipeline steel alloys that can be used to transport ethanol.

FATIGUE CRACK GROWTH RATE
Sowards and his associates conducted a study to determine the impact of bacteria in facilitating the growth of fatigue cracking in pipeline steel alloys. He says, “Our objective was to evaluate the extent of Stage II fatigue crack growth.”

In Stage II fatigue crack growth, the rate of the growth of a crack under a specific load cycle is evaluated under conditions of increasing load. This phase is in between Stage I, which is the initiation of fatigue crack, and the ultimate fracture of Stage III.

The researchers used the standard ASTM procedure (E647), which measures the fatigue crack growth rate versus the stress intensity factor range. Sowards says, “This latter parameter measures the stress imposed on a crack.”

An environmental chamber is used to evaluate the fatigue crack growth using the two steel alloys X52 and X70. Sowards says, “Both of these alloys are commonly found in oil and gas pipelines.”

The bacteria selected for use in this study were Acetobacter aceti, Desulfosporosinus sp. and Clostridium sp. The former is aerobic and converts ethanol into acetic acid. Both of the latter two bacteria are anaerobic, which means they reduce sulfate ions and oxidize an organic food source or molecular hydrogen.

Sowards says, “In collaboration with the Colorado School of Mines, we found that these three species of bacteria are present in ethanol storage tanks. We picked them because we felt they should create problems.”

Testing was done with simulated fuel-grade ethanol (98.5% active) and an aqueous solution containing 5% ethanol. Sowards says. “We chose to use water in combination with a carbon source in order to obtain the maximum response from the bacteria. At high levels of ethanol, bacteria tend to hibernate in a spore mode, thereby not affecting the growth of the fatigue cracks.”

Increased fatigue crack growth rates were observed in aqueous solutions of ethanol containing all three species of bacteria. Sowards says, “We determined that the bacteria increased the fatigue crack growth rate by 25 fold as compared to running the experiments in air.”

Figure 2 shows an optical microscope image of cracking occurring in X52 steel after the alloy was subjected to mechanical forces in an aqueous ethanol solution containing Acetobacter aceti for several days. As the load increased, the fatigue crack growth rate also increased.


Figure 2. This optical microscope image shows a crack forming in X52 pipeline steel after the alloy was exposed to an aqueous ethanol solution containing bacteria for several days. (Courtesy of NIST)

The acid and biofilm generated by the aerobic bacteria led to a large increase in stress corrosion cracking, which accelerated the fatigue cracking growth rate. But the largest fatigue cracking growth rate was seen with the anaerobic bacteria due to a large amount of hydrogen production and the presence of high concentrations of sulfides.

With bacteria contributing to an increase in fatigue crack growth rate, the researchers decided to evaluate the effectiveness of the biocide, glutaraldehyde, which is widely used in oil and gas operations. Sowards says, “We found in preliminary testing that glutaraldehyde prevented an increase in acidity, which means this biocide shows promise as a means to reduce fatigue crack growth rate.”

Sowards indicates that his facility has the unique ability to evaluate the degradation of materials through evaluation of their mechanical properties in biofuel and microbiological environments. He is looking to do further testing with industry relevant systems.

Further information can be found from a recent presentation (3) or by contacting Sowards at jeffrey.sowards@nist.gov.

REFERENCES
1. Passman, F. (2009), “Microbial Problems in Metalworking Fluids,” TLT, 65 (8), pp. 26-34.
2. Canter, N. (2010), “Early Corrosion Detector,” TLT, 66 (2), pp. 12-13.
3. Sowards, J., Weeks, T., McColskey, J. and Fekete, J. (2011), “Effect of Ethanol Fuel and Microbiologically Influenced Corrosion on the Fatigue Crack Growth Behavior of Pipeline Steels,” Presented at the DOD Corrosion Conference 2011, La Quinta, Calif., Aug. 1, 2011.


Neil Canter heads his own consulting company, Chemical Solutions, in Willow Grove, Pa. Ideas for Tech Beat items can be sent to him at neilcanter@comcast.net.