Biodiesel is an alternative fuel produced from renewable resources such as vegetable oils and animal fats. The conventional biodiesel is produced using a transesterification process and consists of long-chain esters. While a new generation of biodiesel (or the renewable/green diesel) is chemically not esters but the similar as petrodiesel. HRD-76 is a hydrotreated renewable diesel (HRD) derived from extracted algal oil. The HRD has been hydrotreated to remove oxygen leaving predominantly linear and branched chained hydro-carbons in the fuel.A mixture of 50% algal HRD-76 and 50% petroleum diesel F-76 has been successfully used in warships as a demonstration by the U. S. Navy. During the operation of the ships seawater was drawn into the fuel tank from the bottom to balance the ships. The seawater (bottom)/diesel (up) environments could pose severe corrosion damage to the carbon steel fuel tanks. The aim of this study is to access the corrosion damage of plain carbon steel (1018) caused by the seawater/diesel environments. Three types of fuels namely 100% F-76 50% HRD-76/50% F-76 and 100% HRD-76 were used to form the mixtures. In addition to the original seawater and diesel samples they were also filtered in order to study the effects of biological microorganisms on the corrosion of carbon steel. Therefore totally 6 seawater/diesel environments were evaluated and compared. In each environment 3 conditions namely open cap vented cap and tight cap were tested to simulate the aerobic selective aerobic and anaerobic conditions. The 1018 carbon steel samples were immersed for time periods of 3 days 10 days 30 days 6 months and 12 months. The corrosion products after immersion were characterized using scanning electron microscopy Raman spectroscopy Fourier transform infrared spectroscopy and X-ray diffraction. All the steel immersed in sea water was severely corroded due to the extremely strong corrosivity of NaCl. The inner rust layer (black) was identified as magnetite while the outer layer (yellow) was identified as lepidocrocite. The magnetite layer on steel immersed in seawater/100% HRD-76 appeared thicker than those formed in the other two mixtures indicating more severe corrosion. The appearance of the steel immersed in the diesel layer varied in different immersion conditions which is more apparent after longer time periods of immersion. Generally red rust formed on steel immersed in 100% F-76 and 50% HRD-76/50% F-76 while white deposits formed on steel immersed in 100% HRD-76. The red rust formation indicates that corrosion had occurred on steel immersed in 100% F-76 and 50% HRD-76/50% F-76 possibly due to the fact that F-76 is more acidic (with higher acid number) than HRD-76. The large amount of white deposits on steel immersed in 100% HRD-76 suggests strong cathodic activity on steel in the fuel layer and therefore severe corrosion on steel in the seawater layer.