(Keynote) Corrosion of 1018 Plain Carbon Steel in Petroleum-Based and Renewable Diesel Fuel/Seawater Mixtures
Authors: Kealoha Jan , Li Shengxi , Hihara Lloyd H. 
Published: December 2020

The substitution of biofuels for conventional fuels can result in unexpected issues and forms of corrosion. A study was conducted on corrosion rates and the identification of corrosion products that formed on plain-carbon 1018 steel (UNS G10180) exposed in seawater/fuel mixtures of petroleum-based F-76; a 50%:50% by volume (50:50 v/v) blend of F-76 and hydro-treated renewable diesel derived from algae (HRD-76); or HRD-76 for various time durations (i.e., 3 days, 10 days, 1 month, 6 months, and 1 year). The 1018 coupons were immersed in bottles with the bottom half of the sample submerged in the seawater layer and the top half in the fuel layer. The 1018 steel has a composition which is within specifications for steels typically used for pipelines and storage vessels and tanks. Energy dispersive X-ray analyses (EDXA), X-ray diffraction (XRD), Raman spectroscopy, and FTIR spectroscopy were used to identify the corrosion products. To differentiate between abiotic electrochemical corrosion and microbiologically influenced corrosion (MIC), various permutations of filtered and unfiltered fuel and seawater combinations were used as the exposure media. A 0.22 micron filter was used to produce sterile seawater and fuel. Both aerobic and anaerobic conditions were examined. Natural, off-shore, surface seawater and synthetic seawater according to ASTM International specifications were used in the study. Hence, the test conditions were aerobic or anaerobic of the following seawater-fuel mixtures: 1) filtered seawater/filtered fuel, 2) filtered seawater/non-filtered fuel, 3) non-filtered seawater/filtered fuel, and 4) non-filtered seawater/non-filtered fuel.
Corrosion rates of 1018 plain-carbon steel in the seawater-fuel mixtures were driven primarily by oxygen reduction and were highest in HRD-76, followed by the F-76/HRD-76 blend, and then F-76. This ordering corresponded to that of the dissolved-oxygen diffusion coefficients which were also ordered from highest to lowest as HRD-76, F-76/HRD-76 blend, and F-76.
The region of steel in the seawater layer was most severely corroded due to the high corrosivity induced by chlorides. Corrosion was dominated by abiotic mechanisms and overshadowed any effects of MIC. At the fuel/water interface and zones submerged in the seawater layer, rust formation showed an inner black layer identified as magnetite and an outer red/yellow layer identified as lepidocrocite and goethite. The amount of magnetite in the inner layer increased with increasing content of HRD-76 in the fuels. Hence, the highest amount of magnetite was observed for the coupons exposed in HRD-76 followed by those in the HRD-76/F-76 50v:50v blend and least in the F-76. White precipitates identified as Mg(OH)2 (magnesium hydroxide), MgO (magnesium oxide), Na2CO3 (sodium carbonate), Na2CO3•H2O (thermonatrite), Na3H(CO3)2(H2O)2 (trona), CaCO3 (calcite and aragonite) formed in the fuel zone and was shown to be a consistent visual indicator for a high corrosion rate of steel. Precipitation was likely induced by a high amount of oxygen reduction (in the fuel layer) that generated OH-, which precipitated the carbonates and hydroxides. The presence of the white precipitates was most abundant on the coupons exposed to the HRD-76 and to a lesser degree on those exposed to the HRD-76/F-76 50v:50v blend, which is consistent with higher rates of oxygen diffusion and hence corrosion in the HRD-76 fuel.

Acknowledgements:

This work was supported under the Hawaii Natural Energy Institute’s Asia Pacific Research Initiative for Sustainable Energy Systems (APRISES) program, University of Hawaii at Manoa, Office of Naval Research (Grant Award Number N00014-13-1-0463).