Yummy for microbes, that is.  Who turn the methyl esters in biodiesel -- with some intermediate steps -- into hydrogen sulfide that corrodes carbon steel.

This according to a paper last month in Energy & Fuels, Aktas et al explore "Anaerobic Metabolism of Biodiesel and Its Impact on Metal Corrosion".  The authors observe that "Despite the global acceptance of biodiesel, the impact of integrating this alternate fuel with the existing infrastructure has not been fully explored."

Here is a paragraph from the paper, full of interesting tidbits:

The chemical stability characteristics of biodiesel are well-documented,(3, 4) but the susceptibility of this fuel to biodegradation is not well-known. Biodiesel methyl esters are sparingly soluble in seawater, with a saturation concentration of 7 ppm at 17 °C.(5) Several studies showed that aerobic microorganisms readily degrade biodiesel.(6-8) The half-life for the biodegradation of the vegetable methyl esters in agitated San Francisco Bay water was less than 4 days at 17 °C.(9) However, anaerobic conditions prevail whenever heterotrophic microbial respiration consumes oxygen at a rate that exceeds diffusion. This is typically the case in subsurface environments, including oil reservoirs,(10-12) oil-contaminated habitats,(13) refineries, storage vessels, pipelines, oil−water separators, and ballast tanks.

In particular, it is interesting that biodiesel spills might be metabolized by bugs in the environment at a much greater rate than petrodiesel.  Next, it is interesting that our steel infrastructure might be susceptible to more rapid degradation with the inclusion of bio-products.  Plastics, anyone?

The paper concludes:

Our studies suggest that biodiesel can be quite easily hydrolyzed and converted to a variety of fatty acid intermediates by anaerobic microorganisms, regardless of their previous hydrocarbon- or biodiesel-exposure history. The acidic nature of these intermediates accelerates the pitting corrosion process of the most common metal alloy used throughout the fuel infrastructure.(39) The corrosion of pipelines, tanks, storage units, and associated equipment increases the risk of the release of hazardous materials to the environment, with concomitant pollution issues. With the widespread use of biodiesel as an additive to fuel supplies, it is at least prudent to consider how best to avoid the negative consequences associated with the microbial metabolism of these labile fuel components.

Something to watch, obviously.