In this experimental study, the impact of B100 biodiesel corrosion on the 15 years old ASME SA 516 Grade 70 carbon steel storage tank was investigated. This study provides novel insights into the material’s performance and reliability of 15 years old ASME SA 516 Grade 70 carbon steel plate material under prolonged B100 biodiesel storage. Various analytical techniques, including ultrasonic thickness gauge measurements, scanning electron microscopy (SEM), and energy-dispersive X-ray spectroscopy (EDS), were used to assess corrosion behaviour and morphology. Tensile tests were conducted to evaluate the mechanical properties. The findings showed significant deterioration in both mechanical properties and corrosion resistance due to B100 biodiesel corrosion beyond the API 653 tolerance; hence, replacement is required to ensure mechanical integrity. The study also examines surface characteristics using SEM and EDS analyses, showing significant alterations in the metal’s morphology and composition due to B100 biodiesel exposure. Additionally, the paper discusses corrosion measurement and the remaining life of tanks, indicating a need for regular maintenance and repair. The mechanical performance of the steel is also assessed, showing a decrease in tensile strength and ductility. This indicates the potential risk B100 biodiesel corrosion poses to the carbon steel storage tank’s performance and reliability.