Microbes accelerate corrosion in ethanol-carrying fuel tanks

Above ground fuel tanks and steel pipelines may be at risk of corrosion due to new fuel grade ethanol (FGE). Doctorate student Luke Jain has been investigating how fuel tanks corrode with new and ever more common ethanol-based automotive fuels. In Jain’s study, it was found that the ethanol in the new fuels is providing an environment where microbes can corrode the metal. Initially, it was thought that ethanol-gasoline fuels would circumvent the problem by locking up the ethanol in the gasoline. In the field, however, it has been noted that the ethanol will fall out of the gasoline and dissolve in any water that may be present in the storage tank. This mixture of ethanol and water provides an environment capable of producing microbes that “eat” the metal.

Jain’s work is primarily focused on determining the reliability of FGE pipelines and storage facilities. This involves identifying the microbes present in the FGE/water mixture. Jain said, “Our first task was to go out into the field to these [affected sites] and test them for the microbes and materials present.” In the most notable cases of corrosion on storage tanks, Jain found that there were “Common acid-producing microbes oxidizing the ethanol to produce acetic acid.” The gasoline in ethanol-gasoline automotive fuels is not preventing the ethanol from “falling out” and being available to the acid-producing microbes.

Examination of the corroded metals found in fuel storage tanks using a Scanning Electron Microscope(SEM) showed scaling, pitting, and cracking of the metal surface. Along the edges of the cracks, sulfur deposits indicated the presence of microbes that were responsible for the corrosion. This further confirmed Jain’s suspicions that the corrosion had to be due to a specific type of microbe.

To understand and quantify the exact mechanism and material that is causing this corrosion, Jain set up a testing apparatus to precisely control the conditions under which the corrosion takes place. The apparatus consisted of a series of four-point clamps that would subject a plate of steel to an exact and precisely controlled strain. The apparatus was designed to accommodate five samples at any given time. Each sample was held in the clamp and submerged in a bath of FGE. The samples could be left indefinitely to allow for long-term testing. This test system was designed to bend the samples at a frequency of 0.1 Hz, or once every ten seconds. While this loading is not equivalent to the strain experienced in the field, Jain’s methodology was to examine the corrosion at a maximum strain and then back the apparatus down to levels more consistent with the real world.

Jain designed this apparatus to allow for very precise control of the strain on each sample, to ensure that each one was subjected to the exact same stress and could then be analyzed simultaneously. For his initial testing, Jain put the samples in an E10 (10% ethanol, 90% gasoline) bath and put them under a stress for two weeks. After the testing period was complete, the samples showed corrosion and pitting that was consistent with what Jain was expecting. Upon close examination of the pitting under an SEM, Jain found that cracks were beginning to propagate through the material with their origin at the mouth of the pits. The cracks followed the crystalline structure of the metal, as Jain expected.

This research into how FGE affects our existing fuel storage delivery infrastructure will be very helpful in preventing fuel spillage that could cause damage and potentially loss of life. Jain will continue his work by finding various ways to prevent this corrosion from taking place, and also determining to what extent our current infrastructure must be modified to allow for these new fuels.