Owing to their low CO₂ emissions the use of bio-fuels and their blends in the Navy has become not only increasingly attractive but also mandatory. Onboard Navy ships, fuel is stored in seawater compensated fuel ballast tanks (SWCFBT) that are connected in series using copper alloy sluice pipes. Majority of Navy ships have several groups of such tanks which are typically made of carbon steel. Seawater is drawn into the tanks to compensate for fuel reduction during operations. While petro-fuels in contact with seawater are known to suffer from microbial contamination problems, bio-fuels, in particular methyl-ester (ME)-based fuels, are susceptible to biodegradation, which compromises the fuel quality, equipment performance and can lead to microbially-influenced corrosion (MIC) of SWCFBT. ME-based biofuels are, therefore, not recommended for use during naval operations. Of concern to the Navy is the possible impact of marine microbial proliferation in the presence of a second generation, methyl ester-free biofuels on corrosion of SWCFBT. Commonly, preliminary risk assessment of MIC is undertaken through laboratory investigations. The difficulty in designing and performing studies that can accurately simulate real life operational conditions in SWCFBT onboard Navy ships is one of the major drawbacks to diagnostics and mitigation of MIC in these systems. This problem is further compounded by the fact that military ship’s transit data and operational conditions are mostly classified and not easily accessible. As a result, corrosion rates observed in most laboratory experiments seldom reflect those observed in real service life environments.

Here presented laboratory investigation attempted to simulate the operational condition of a SWCFBT that is adjoining to the overflow/expansion tank onboard a Navy ship and to compare marine corrosion of 1018 carbon steel in the presence of a conventional and a 50/50 blend of an alternative Navy fuel in fully oxygenated and oxygen-limited North-Pacific seawater.

Studies were carried out employing six independently operated batch reactors containing as-received and 0.1 µm filter-sterilised San Diego Bay seawater (SDBSW). These reactors were augmented with either a conventional Navy fuel (petro-F76), a 50/50 blend of petro-F76 fuel with an alternative algal FT-F76 biofuel and with corresponding fuel-free controls. Linear polarization resistance (LPR) and corrosion potential (E_corr) measurements were conducted using 312 mm² cylindrical electrodes manufactured from 1018 carbon steel. For each electrode, LPR data were recorded every 20 s by scanning through a narrow potential range within ±5 mV of the corrosion potential (E_corr) at a scan rate of 0.125 mV/s for 8 weeks at 23^oC.

Reactors were sampled at regular time intervals. Aliquots of bulk fluids were collected to (i) confirm the sterility of the bulk phase in the control reactor and (ii) for genomic and metabolomic analyses. Upon reactor decommissioning, corrosion deposits were aseptically recovered from the electrode surfaces and subjected to comprehensive chemical and microbiological characterization. Metabolomics analysis of recovered deposits and of liquid samples collected from the planktonic phase was performed using Agilent 1290 binary HPLC coupled with Agilent 6538 UHD Accurate Mass Q-ToF mass spectrometer. Characterization of crystalline phases in the corrosion deposits, imaging and elemental analysis were carried out using powder X-ray diffraction (XRD) and field emission scanning electron microscopy coupled with energy dispersive X-ray analysis (FEM/EDX), respectively. Biofilm and planktonic prokaryotic community structures were determined through DNA extraction from corrosion deposits and from liquid samples retrieved from reactors bulk phases, followed with lllumina sequencing of 16S rDNA.

 Electrochemical measurements, metabolomics and mineralogical analyses demonstrated marked differences between the corrosion of 1018 carbon steel exposed in the reactors. Corrosion rates recorded in the reactors varied with fuel type and oxygen concentration. The highest level of corrosion was recorded for 1018 carbon steel exposed to as-received SDB seawater augmented with conventional petro-F76 fuel. Extensive surface pitting damage was observed on all carbon steel electrodes exposed in the reactors. The severity of pitting damage decreased in the following order: conventional Navy fuel > 50/50 blend Navy fuel > SDB seawater. Metabolomics data analysis, revealed considerable differences in chemical signatures between deposits recovered from the surfaces of 1018 carbon steel electrodes exposed in the conventional Navy fuel compared to the 50/50 fuel blend. XRD patterns demonstrated that magnetite (Fe₃O₄) and Goethite (αFeOOH) were the dominant crystalline phases detected in the corrosion deposits recovered from the surfaces of all carbon steel electrodes.

DNA profiling of biofilm communities in fuel augmented and fuel-free reactors is in progress. It is anticipated that the outcome of this multidisciplinary study will aid in elucidating the risk of MIC of carbon steel in compensated fuel tanks containing blends of a second generation naval petro- and bio-fuels.