A controlled laboratory study conducted by the Coordinating Research Council (CRC), intended to provide clarity on the cause of corrosion of metallic components in tanks storing ultralow sulfur diesel fuel, was released August 2, 2021. This is what we learned from the 402-page study, available here.
Background. Beginning in 2007, a considerable amount of anecdotal evidence indicated that the incidence of corrosion-related diesel fuel underground storage tank (UST) component failures had increased substantially. Results and conclusions from two previous studies, using data obtained from a limited number of retail sites, had been inconclusive. Both studies had suggested that diesel fuel sulfur content, free-water, microbial contamination and ethanol were the primary factors contributing to UST system corrosion.
The Test. The CRC test plan included eleven independent (controlled) variables:
- Sulfur concentration—500 parts per million (ppm) low-sulfur diesel (LSD) versus 15 ppm ultra-low-sulfur diesel (ULSD)
- Microbial contamination
- Cold flow improver
- Conductivity additive
- Corrosion inhibitor
- Lubricity additive
- Fiber-reinforced polymer
Several dependent (uncontrolled) variables were observed, with a sharp eye on corrosion coupon corrosion ratings. The test plan was designed to determine whether one or more of the dependant variables—corrosion rating of the coupon, in particular—co-varied with one or more of the independent variables to affect fuel corrosivity. The plan was NOT to test causes and effects.
What did the researchers observe? There was corrosion. The most severe corrosion was observed at the aqueous fuel interface in microorganisms that contained fuel over the aqueous-phase. In fact, substantial corrosion was visible within a week.
What caused the corrosion? The study concluded: “The results from this 12-week study confirmed that the presence of free-water was essential to corrosion. The data also suggested that microbiologically influenced corrosions—including biogenic oxidation of ethanol to acetic acid—were important corrosion mechanisms.”
Where do we go from here? Additional study is recommended. Researchers recommended: “Based on the results of this study, future studies should focus on the relationships between microbial contamination, FAME (Fatty Acid Methyl Ester), ethanol, and water as factors contributing to diesel fuel system component corrosion.”