June 2016, Vol. 243, No. 6

Features

Treatment, Prevention of Microbially Induced Corrosion

Sometimes, the biggest problems are the ones you can’t see.

Microbially Induced Corrosion (MIC) is an enormous – and expensive -problem for the oil and gas industry, directly or indirectly costing billions of dollars each year in treatment, prevention and equipment repair. Emerging technologies may soon give companies more effective options of managing the microbial communities that eat away at the bottom line.

Where MIC Thrives

While not all corrosion is microbially induced, bacteria are implicated in a significant proportion of corrosion problems for pipelines, storage tanks, and offshore platforms and equipment. Oil and gas infrastructure often provide perfect conditions for corrosion-causing bacteria to thrive.

MIC-causing bacteria require:

• Water: Like all living things, microbes need water to live and reproduce. Bacterial colonies especially like stagnant sources of water, such as the pools that collect in the bottom of unused storage tanks or in low-lying areas along a pipeline.

• Metal: Metal acts as the host location for bacterial growth and provides a source of metal ions required for corrosion to take place. Some bacteria that cause corrosion depend on these metal ions to fuel their metabolic processes. For example, iron-related bacteria (IRB) use iron pulled out of the metal they are colonizing in their metabolism.

• An energy source: All bacteria require an energy source in order to grow and multiply. The specific source depends on the bacterial species. For example, sulfate-reducing bacteria (SRB), one of the most common types implicated in offshore MIC, requires sulfate and an electron donor such as molecular hydrogen or organic compounds.

• Oxygen (or a lack of it): Corrosion-causing bacteria can be broadly categorized into aerobic (oxygen requiring) and anaerobic (oxygen avoidant) species. Bacteria will only thrive in environments with the right oxygen levels for their species.

• A way to adhere to the surface: Cracks, crevices, weld seams, bolts, corrugation and other irregularities in the metal provide ideal spots for a microbial colony to get a toehold. Once established in these areas, bacteria can form biofilms that allow them to spread to smoother surfaces on the metal.

The interior of oil and gas pipelines can provide potential breeding grounds for both aerobic and anaerobic bacteria, depending on the environment. Exterior pipeline surfaces are also vulnerable to MIC, especially in low-lying areas where water pools and in moist, high-conductivity soils (such as clay-based soils).

Poor welds, damaged coatings and other surface imperfections can provide entry points for MIC in pipelines and other equipment. Storage tanks, pressure vessels and coiled tubing, especially if improperly drained and maintained, are also prime locations for development of an MIC problem.

In offshore environments, platforms, mooring chains and underwater pipelines and equipment are all vulnerable. MIC is especially prone to develop around the legs of oil platforms, where slag and drill cuttings tend to accumulate. These piles of irregular, uncoated waste metal and drilling mud provide ideal breeding grounds for anaerobic SRB colonies, which can then spread from the slag piles to valuable offshore equipment.

Growing Problem

Detection of an emerging MIC problem can be difficult through visual inspection alone. MIC often starts with very small, but localized, pinhole imperfections that are difficult for inline equipment to detect. Inspection equipment also may miss developing problems hidden in weld seams or crevices.

Periodic testing of soil, water and product samples, along with any solids collecting on the metal surfaces, to determine if corrosion-causing bacterial species are present, can provide a more accurate and reliable early warning system for developing MIC issues.

The presence of bacteria – even in large numbers – does not by itself indicate that a corrosion problem is microbially induced. Just as not all corrosion is caused by bacteria, not all bacteria are implicated in MIC. Many microbes are harmless hitchhikers on oil and gas infrastructure and may even provide a degree of protection against more destructive species.

To make the best treatment decisions, it’s important to determine what species of bacteria are present and potentially causing the problem. Lab tests – and soon, new hand-held field diagnostic kits – use DNA analysis to determine which species of MIC-causing bacteria are present.

Acute MIC Problem

Once corrosion-causing bacteria take hold, rapid treatment is paramount. The longer the colonies are allowed to reproduce unmolested, the more damage is likely to be done. As bacteria eat away at the metal they have colonized, they compromise the integrity of the structure and reduce its ability to hold weight or pressure, eventually leading to costly equipment failures.

There are several actions operators can take to prevent MIC from spreading:

• Drain and dry the equipment: For tanks and pressure vessels, this can be a good first line of defense. Removing water prevents bacteria from reproducing and may kill off the colony entirely. This can be done by heating equipment to evaporate pooling water or by using product additives. However, it’s important to be aware that some species of bacteria can survive dehydration and reanimate once they have a new water source. If drying equipment requires shutting down key production or transmission activities, it may not be a practical or economical choice – of course this is not an option at all for offshore infrastructure.

• Mechanical cleaning: In some cases, an emerging MIC problem can be contained by simply flushing or brushing away the problem. Pipelines can be cleaned by pushing large volumes of slurries through or by using special pipeline pigs that push out debris that harbors bacterial growth. Cleaning up drill cuts residing at the base of platform legs may prevent developing bacterial colonies from spreading to the equipment.

• Biocides: In most cases, some form of biocide will be needed to ensure that MIC-causing bacteria are truly gone. Biocides can be applied in different ways. Some biocides can be mixed into tanks or injected directly into pipelines with the product itself. In other cases, pipeline operators may choose to send a “slug” of biocide through the pipeline using a batch pig system that holds concentrated biocide in between multiple pigs.

Not all bacteria respond equally to individual biocides, so proper biocide selection is critical. It’s also important to get the right loading, or dosage, for the selected biocide. Biocides need to be targeted so they kill the MIC-causing bacteria while minimizing damage to the environment and to equipment. More is not always better: biocides can themselves be corrosive to the equipment they are meant to protect. Biocides may also present health, safety and environmental concerns.

In addition, improper use of biocides can lead to development of resistant bacteria, much like improper use of medical antibiotics. Matthew Henderson, research scientist at Fusion Technologies, Inc. explained, “If you have the right biocide and the right loading, you’ll see the bacterial population go way down and stay down. But sometimes a biocide will cause an immediate drop in bacterial populations but lead to a major rebound later.”

At Fusion Technologies, scientists perform “kill studies” to determine the right biocide selection and dosing. Samples from the affected area are exposed to different chemicals and concentrations over a two-week period to select the right remedy. In the future, data from rapid field detection kits may allow operators to accurately diagnose MIC and select appropriate biocide treatments in the field.

Prevention Options

The best option for combating MIC is prevention. Proper equipment maintenance, such as draining and drying pipes and storage tanks when not in use, can make oil and gas infrastructure less hospitable to bacterial colonization. Protective coatings can also protect metal surfaces from MIC. Regular inspection of exposed surfaces and coatings is critical to early detection.

Companies should also look at equipment design and infrastructure layout. Taking steps such as eliminating low points where stagnant water can gather will reduce development of MIC. Operation changes can be utilized to re-entrain settled water, for example, by increasing turbulence, velocity or product density.

Improving weld seam quality and streamlining equipment design to avoid unnecessary cracks and crevices will also help. Finally, companies should look at materials selection when designing and manufacturing oil and gas infrastructure and equipment. Some metals and alloys are less attractive to corrosion-causing bacteria.

Material science may soon provide new options for oil and gas companies.

• Smart coatings: Self-healing coatings release healing chemicals when they are damaged to slow the spread of corrosion. One example already on the market is the Battelle Smart Corrosion Detector Bead™, which provides corrosion detection and mitigation in one step. The beads detect the markers of corrosion at the microscopic level and automatically release a self-healing payload. They also fluoresce under a black light if corrosion is present to assist with rapid manual inspection.

• Antimicrobial wraps: Protecting mooring chains from MIC is a particular challenge; the nooks and crannies in the chain provide ideal conditions for bacterial adhesion, but chemical biocides cannot be applied in the open ocean. Battelle is testing antimicrobial wraps that slow down the growth of bacteria on mooring chains.

• Functional materials: Research is underway on novel advanced materials to protect against MIC that may one day offer more options for equipment designers. Scientists are developing functionalized surfaces that are able to respond to specific triggers such as the presence of bacteria.

In a healthcare setting, these materials can be designed to detect the presence of a particular bacterial species, such as E. coli, and release a targeted biocide in response. A similar approach could one day be used against MIC.

Because these materials release biocides only when the target bacteria are present, they reduce the buildup of biocides in the environment. Other materials use nano-structures to prevent bacteria from adhering. Conductive coatings could also be used to develop self-sterilizing surfaces that heat up to kill bacteria when an electric current is applied.

With appropriate treatment and prevention strategies, oil and gas companies can get MIC under control. Early treatment and effective prevention can go a long way toward reducing the costs and risks of corrosion-related failures.

Author: Craig Bartling, principal research scientist at Battelle, is part of the company’s Applied Genomics and Biology group. He has more than 12 years of laboratory and project management experience and is focused on characterization of environmental samples for microbial DNA and proteins.

Contributions were made by Jennifer O’Brian, a research scientist in Battelle’s Energy Resources group, who manages Battelle’s Pipeline Simulation Facility and is focused on managing and supporting pipeline integrity programs.

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