Ethanol and Storage Tank Corrosion

Bob Greco
Posted September 14, 2012

A recent study suggested ethanol might be the source of corrosion in underground tanks used to store ultra low sulfur diesel fuel. Battelle, which conducted the study, checked a number of hypotheses, weighed the data and evidence and came to a conclusion pointing to ethanol. Not surprisingly, the Renewable Fuels Association (RFA) – the ethanol lobby – didn’t like the study’s result and went on the offensive this week with a post on its blog:

"Battelle’s report attempts to draw meaningful conclusions from surprisingly few data points which leave fuel experts with many questions. Quite frankly, there are numerous plausible scenarios that can cause severe and rapid corrosion, including the monumental shift away from diesel fuel containing high levels of sulfur that took place in 2006."

RFA’s post cites other possible reasons for storage tank corrosion, including water contamination, humidity, diesel fuel corrosiveness and others.

The short answer is that Battelle considered them. A third-party consultant with no stake in the question, Battelle was commissioned for the study by a diverse group that includes representatives from gasoline and diesel marketers, equipment manufacturers, the railroad industry and the auto industry.

Battelle followed sound science. It found that severe and rapid corrosion in underground storage tanks (UST) used for ultra low sulfur diesel (ULSD) probably resulted when trace amounts of ethanol were digested by bacteria, producing acetic acid at levels strong enough to corrode metal equipment. Battelle:

"The project final hypothesis for this investigation is that corrosion in systems storing and dispensing ULSD is likely due to the dispersal of acetic acid throughout (underground storage tanks). It is likely produced by Acetobacter bacteria feeding on low levels of ethanol contamination. Dispersed into the humid vapor space by the higher vapor pressure (0.5 psi compared to 0.1 psi for ULSD) and by disturbances during fuel deliveries, acetic acid is deposited throughout the system. This results in a cycle of wetting and drying of the equipment concentrating the acetic acid on the metallic equipment and corroding it quite severely and rapidly."

It’s possible that the trace levels of ethanol are coming from switch loading and/or tying together the vent lines of gasoline and diesel tanks, and Battelle sensibly recommended additional, more comprehensive research. From the study:

"Fuel distribution systems supply and handle other fuels in addition to diesel, such as gasoline, jet fuel, and ethanol. Diesel fuel is shipped in the same pipelines as gasoline and jet fuel. More importantly, ethanol is specifically kept separate from gasoline until blended in the tanker trucks. Since trucks may transport and switch between all fuels from 0 to 100 percent ethanol, it is possible that there be some cross contamination from the fuels and vapors. Because nearly all gasoline sold in the U.S. now contains 10 percent ethanol, it is not surprising that small amounts of ethanol were found in most of the diesel fuel and subsequent water bottom samples...However, further study is required to establish this causal link."

And:

"Another source of potential ethanol contamination is through common manifolded ventilation systems. At times, gasoline USTs are converted to diesel service with ventilation systems still connected to other gasoline USTs on site. Ethanol and gasoline have higher vapor pressures than diesel; therefore, vapors may collect in the ullage of the gasoline tanks and be forced back into the ULSD tank contaminating the system."

Back to RFA:

"If ethanol contamination were indeed the root cause of equipment corrosion that started appearing in 2007, then an epidemic of corrosion incidents would have plagued the storage and handling sectors since 2002 when ethanol (was used more by refiners)."

To be clear, the move to remove much of the sulfur from diesel fuel was a good idea, because the combination of lower sulfur levels and new automobile technology reduced emissions. This was phased in beginning in 2006 and finished in 2010. To RFA’s point: Corrosion problems in underground storage tanks wouldn’t necessarily have been seen in 2002, when much larger volumes of ethanol began being used, because there was still enough sulfur in the fuel then to prevent the bacterial growth that apparently is being seen today.

The ethanol industry no doubt has done significant research on bacteria and could actually help the problem by sharing more of what it knows with the transportation fuel industry. Instead, it points fingers at an industry that used more than 13 billion gallons of ethanol last year.

The Battelle report offers a scientific conclusion but doesn’t purport to be the final word on this problem. More study is recommended. RFA’s defensive rant does little to advance solutions to a potentially costly situation for distributors and dealers.