There are two parts to the corrosion mechanism. In Part 1, emulsified water moves permanently from tank bottoms into the ULSD biodiesel fuel column and bonds to the biodiesel using a hydrogen bond, and in Part 2, microbes follow the bonded emulsified water into the fuel column; this greatly increases the microbe’s growth volume; more microbes, more acids, more corrosion.
Part 1: For over a century, free standing water was mainly on the diesel tank bottom. Now, emulsified water has permanently migrated into the fuel column and bonded to the biodiesel component. We know from the research paper "Moisture Absorption in Biodiesel and its Petrodiesel Blends" that 100% biodiesel can hold 15 to 25 times more emulsified water compared to 100% diesel, and hits saturation quickly. This is because both water and biodiesel molecules are polar (they have a positive and negative parts to the molecule), so the water molecules are attracted to, and bond quickly to, the biodiesel.
The actual bonding mechanism of emulsified water to biodiesel is by a hydrogen bond. While weaker than a covalent or ionic bond, it still attaches multiple water molecules to a biodiesel molecule. The hydrogen atoms (positive charge) in the biodiesel bonds to the oxygen atom of water (negative charge). As seen in the biodiesel molecule diagram to the right, there are many hydrogen attachment points. This is why pure biodiesel can hold 15 to 25 times more water than pure diesel, and why bonded emulsified water is so difficult to remove with traditional filters. As well, additional water molecules continue to bond to the already attached water molecules, so the number of water molecules attached to the biodiesel will continue to grow.
Note: 100% pure ULSD and 100% pure gasoline are not polar, so do not bond to water. This is why, prior to the introduction of biofuels (biodiesel and ethanol), water was found mainly on the tank bottom. Microbial growth was mainly limited to the thin fuel/water interface.
Part 2: Microbes follow the water into the fuel column to greatly increase their growth volume, and produce more acids
Microbes permanently migrate into the fuel column, as they now have the water they need to live there. This migration has greatly increased the microbe’s growth volume, resulting in more microbes, more acids, acidified fuel, and more corrosion. In addition, the microbes can create biofilms on surfaces that the diesel touches.
These factors have led to the corrosion crisis we now have.
Biodiesel itself is an easy source of nutrients for microbes. Think “fast food”, with the bonded emulsified water attached for convenience and enhanced growth.
Testing for Bonded Emulsified Water ASTM D6304
A key point is that in order to identify the presence of bonded emulsified water, a Karl Fisher titration (ASTM D6304) must be used. Traditional test methods will not pick up the presence of bonded emulsified water. For example, the visual “clear and bright” test will continue to be “clear and bright” with substantial bonded emulsified water, and no visible free water present. Water paste will not indicate bonded emulsified water.
Different Tank Corrosion Scenarios:
Storage tanks are used in different situations, and some are at a much greater risk to corrosion. Major factors affecting corrosion are, in addition to the level of bonded emulsified water: age of the stored fuel, how often the fuel is turned over, and exposure of the tanks to the elements.
Above Ground Storage Tanks (AST) or Underground Ground Storage Tanks (UST): the corrosion risk applies to both, and a case can be made that ASTs are more at risk due to the wider swings in temperature and humidity during the day/night cycle, thereby supplying more emulsified water to be bonded into the stored ULSD biodiesel blends, and hence more corrosion.
Emergency Backup Generator Storage: the highest corrosion risk is for emergency backup generator storage tanks. Please see the next section "Emergency Generators".
Before 2006, microbes were limited to the fuel/water interface, close to the tank bottom. This interface is typically very thin, being described as the thickness of a sheet of paper. While microbes produced acids, their numbers were so low that acid production was also low.