Corrosion Crisis and Mechanism

The Corrosion Crisis

The increased prevalence of corrosion in the diesel infrastructure was first noticed in 2007, shortly after the introduction of ULSD (Ultra Low Sulfur Diesel) biodiesel blends. In 2016, the EPA published it's research results, which found 83% of storage tanks have moderate to severe corrosion. Corrosion has continued since 2016, do 100% of storage tanks now have moderate to severe corrosion?

 The main corrosion source is microbes in the fuel. Microbes produce acids as a byproduct of growth, acidifying the fuel, and corroding the diesel infrastructure.

The cost of corrosion is immense. The lifecycle of tanks is reduced, normally tanks last for 30 to 40 years, now tanks have been replaced in as little as 10 years. The cost of removing and replacing these storage tanks is huge. The increased corrosion is affecting all parts of the diesel infrastructure, piping, fittings and seals, so environmental damage, and cleanup liability, is increasing. 

Emergency generators are at risk, as acidified fuel corrodes the injectors


The Corrosion Mechanism

There  are two parts to the corrosion mechanism. In Part 1,  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, 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 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  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 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 water attached for convenience and enhanced growth.

Testing for Bonded Water  - ASTM D6304 Karl Fisher Titration

A key point is that in order to identify the presence of bonded water, a Karl Fisher titration (ASTM D6304) must be used.  Traditional test methods will not pick up  the presence of bonded water. For example, the visual “clear and bright”  test will continue to be “clear and bright” with substantial bonded  water, and no visible free water present. Water paste will not indicate  bonded 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 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  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.