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Keeping the Navy’s Green Fleet from Rusting

The Department of Defense consumes more energy than any other entity in the United States. Thirty percent of that energy is used by the Navy, primarily to power ships and submarines.

As a result, the Navy has been very active in developing and implementing energy conservation efforts, energy-efficiency standards, and renewable energy technologies. Since three-quarters of the energy used by the Navy is used to power its seagoing vessels, the service has made it a priority to develop, test, and apply alternative fuels, such as biodiesel.

Sailors stand their post aboard the Riverine Command Boat
Sailors stand their post (top and bottom) aboard the Riverine Command Boat as it conducts test runs at Naval Station Norfolk in October 2010. The boat is powered by an alternative fuel blend of 50 percent algae-based and 50 percent NATO F-76 fuels, designed to reduce total energy consumption on naval ships. Photo by Navy Mass Communication Specialist Clifford L. H. Davis.
Sailors stand their post aboard the Riverine Command Boat

Jason S. Lee, a materials engineer at the Naval Research Laboratory (NRL), and a team of scientists at NRL recently conducted studies to assess how the use of biodiesel may affect corrosion in existing infrastructure such as pipelines, fuel tanks, storage units, and associated equipment in the absence of chlorides. The Office of Naval Research Alternative Fuels Program funded their research.

“Biodiesel is the common description of first-generation biofuel made from plant oils or animal fats,” said Lee, principal investigator of the studies. Biodiesel is produced through a process called transesterification, where the plant/animal products are reacted with an alcohol to form fatty acid methyl esters (FAME).

While traditional diesel is derived from naturally occurring petroleum deposits, it is not a renewable resource—meaning, the supply is not endless. However, renewable sources of naturally occurring oils and fatty acids (e.g., algae, coconut, rapeseed, soy, peanut, sunflower) can be used to produce fuel that is functionally similar to traditional diesel. Biodiesel can be used neat, in the version known as B100, but is often blended with traditional diesel, such as the B20 alternative, which is 20 percent biodiesel and 80 percent traditional diesel.

Although the immediate economic benefits of using an alternative fuel may be obvious, the Navy must ensure that employing such an alternative fuel will have no significant negative consequences on the existing infrastructure, such as increased mechanical wear on an engine or an increased rate of corrosion in fuel containment and distribution systems.

NRL researchers discovered through their experiments that alloys used to construct structures that store and transport ship fuel may exhibit increased microbiologically influenced corrosion (MIC) when exposed to first-generation biodiesel compared to traditional diesel.

The Navy is looking at the possibility of using biofuels for powering its sea-going vessels. To do this, they must ensure that the future green fleet will not be subjected to more corrosive conditions when renewable fuels are used.

Both traditional diesel and biodiesel are often contaminated with microorganisms such as bacteria. In addition, diesel fuel invariably contains small amounts of dissolved water. If left stagnant in a pipe or storage tank, the heavier water condenses and collects at the bottom of the structure, providing a suitable environment for microbial growth. “Previous studies involving traditional petroleum-based fuels have highlighted the occurrence of accelerated corrosion at or below the fuel/water interface. The blending of biodiesel with traditional diesel (B20) resulted in the first known demonstration of localized corrosion of aluminum in the fuel layer itself,” said Lee. Since water is more soluble in biodiesel, NRL researchers suggest that biodiesel will be more susceptible to biofouling and MIC. It should be noted that while localized corrosion (pitting) was observed in aluminum, stainless steel exhibited passive behavior in all exposures.

Combining Two Already Severe Environments Leads to Microscopic Challenges

Many naval vessels use compensated fuel ballast systems. These systems help maintain the vessel’s stability by replacing consumed fuel with seawater. Although this method helps maintain the vessel’s ballast, it exposes the fuel tank to marine microorganisms and chlorides, leading to accelerated corrosion and fuel degradation.

Magnifications of 250X Magnifications of 4000X Localized corrosion after acid cleaning
Carbon steel (UNS C10200) after 60 days’ exposure to natural seawater with biodiesel addition illustrating biofilm formation at magnifications of a) 250X, b) 4000X, and c) localized corrosion after acid cleaning. Photos courtesy of the Naval Research Laboratory.

MIC has been a challenge for sea-based petroleum operations for years and is said to cause millions of dollars in damage to equipment and infrastructure that support the exploration, production, transportation, and storage of oil.

The combination of fuel and seawater results in an ideal environment for microbiological activity, which has been shown to frequently lead to corrosion of certain structures. Biodiesel has been shown to be especially susceptible to degradation by certain microbial species and may accelerate MIC.

Though MIC associated with conventional petroleum-seawater environments has been investigated, the NRL researchers’ investigation of corrosion in biodiesel-seawater environments is novel. “The addition of marine microorganisms and chloride to alternative fuels changes both the biofouling and electrochemical characteristics of the fuels,” Lee explained.

So what do these results mean? These laboratory results may indicate the effect of first-generation biofuel in Navy applications. Lee and his NRL team are already engaged in continuing the work to make sure the Navy remains at the forefront of efforts to apply renewable energy sources while mitigating potential corrosion problems.

“The Navy is also examining newer alternative fuels such as hydro-refined biofuels and algae-produced biofuels for use in Navy systems,” said Lee. At present, Lee plans to take a closer look at the impact of adding seawater to alternative fuels and investigate more state-of-the-art biofuels.

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