Protecting Those Workhorse Tanks

As the ethanol industry matures, potential corrosion issues call for a watchful eye
By Holly Jessen | October 18, 2011

Compared to the oil industry, the ethanol industry is relatively young. That puts the ethanol industry on a steep learning curve on matters already mastered by other industries. Jim Dooley, account executive for Corrpro Companies Inc., believes one of those areas is corrosion protection for tanks—including fermentation, water and fuel tanks.

The reality is, he says, some ethanol plants were built with little to no corrosion protection. “When the ethanol industry had designed a lot of these sites, they were unaware of a needs basis when it came to controlling [corrosion,]” he says.

As these plants age, ethanol producers are beginning to examine existing infrastructure to determine where corrosion protection is needed to prevent metal loss. That process starts with tank inspections. Corrpro, a corrosion solutions company with offices worldwide, has worked with ethanol plants that don’t have the special coatings or cathotic protection in place that they should have. In some cases, those ethanol producers are starting to have problems with leaks. “What they are seeing is that their structures are showing some signs of wear, with corrosion, things are starting to happen now, that maybe weren’t predesigned into their systems,” he says.

The bottom line is, corrosion might not be the sexiest subject, but it is one ethanol producers need to have on their radar. If not, it could mean costly repairs or possible contamination someday down the road, particularly for some older ethanol facilities.

Corrosion Experts

Steel tanks containing fuel-grade ethanol develop leaks due to stress corrosion cracking, says Oliver Moghissi, president of the National Association of Corrosion Engineers. Corrosion can be an issue near vents and any external appurtenances exposed to air, allowing ethanol to pick up moisture. Storing hydrous ethanol can produce even more corrosion due to higher water content.

There are a number of corrosion mitigation strategies, he adds, drawing on technical input from Narasi Sridhar, vice president of DNV USA, a classification society, and a fellow member of NACE. Galvanic sacrificial coatings, such as zinc or aluminum, will protect steel tanks containing ethanol. The main limitation to this system of corrosion mitigation is that it could have a negative effect on product quality. “Any dissolved metal species in the ethanol can render the ethanol unacceptable to the automotive industry, its main customer,” he tells EPM. “Any galvanic coating therefore must be tested rigorously by the end-user, the automotive community, which can be time consuming and expensive.”

When corrosion or stress-corrosion cracking is observed, stress-relief annealing, a heat treatment, can be used to reduce weld stresses. In addition, chemical inhibitors can be added, water can be minimized and the entrance of oxygen minimized. The American Petroleum Institute also recommends internal polymeric coating.

Water tanks need corrosion protection as well. Unlike tanks containing ethanol, cathodic protection can be used to protect tanks containing water. Cathotic protection involves either using sacrificial anodes that protect the tank by attracting corrosion to itself, or an impressed current from an external source. These same methods can be used to protect the bottoms and outsides of aboveground or underground tanks exposed to soil.

Cathodic protection, however, doesn’t work for the insides of metal structures containing ethanol, the corrosion engineers add. The fuel has low electrical conductivity when compared to water and, as a result, much of the applied voltage is insulated from the metal needing to be protected. “Therefore, impressed or sacrificial cathodic protection systems will not work in ethanol,” Moghissi says. “In ethanol-gasoline mixtures, the situation is even worse since gasoline has a lower electrical conductivity than ethanol.”

API offers two guidance documents detailing the inspection and maintenance of ethanol tanks and other infrastructure developed by an API task group: “Tank inspection, repair, alteration and reconstruction” and “Identification, repair and mitigation of cracking of steel equipment in fuel ethanol service.” Moghissi says they provide helpful information. “These documents provide a variety of inspection and mitigation methods as well as the latest results of the survey of industry on ethanol corrosion and stress corrosion cracking issues.” 

In addition, NACE has a technical exchange committee that meets annually to share information on corrosion mitigation methods for ethanol tanks, piping and pipelines. The committee also is working on a standard for stress corrosion cracking test methods for steel. Finally, Moghissi says, the Steel Tank Institute holds regular meetings on the issue of ethanol storage tanks, typically from the perspective of ethanol storage at gas stations. For corrosion mitigation services, ethanol producers should work with a NACE-certified service provider, he adds, and for more information, they can also attend a NACE biofuels committee meeting.

Bacteria Bad News

Ethanol’s impact on steel requires a closer look for another reason, as more consideration is given to moving ethanol via pipelines.  This summer, new experimental evidence came out, showing that ethanol contaminated with a common bacteria boosts fatigue crack rates in pipelines by 25 percent when compared to air. (Researchers didn’t look at crack rates for petroleum in this study.) National Institute of Standards and Technology researchers presented their findings at the Department of Defense Corrosion Conference held Aug. 1 in La Quinta, Calif. The findings are the first to emerge from NIST’s biofuel test facility.

Testing of tank materials is under way now, says NIST postdoctoral researcher Jeffrey Sowards. Although funding received from the Department of Transportation Pipeline and Hazardous Material Safety Administration was used up in the first half of the study, researchers believe there is more to learn. “The timeframe of the project is actually over, but we’re continuing it on our own,” he says.

The second half of the study will examine degradation of tank materials within the same parameters of the pipeline study, although fatigue loading isn’t as much of an issue for tanks as it is for pipelines. “I would say by the end of this calendar year we should have preliminary results,” he says, adding that researchers hope at some point to expand testing to other biofuels such as biodiesel and butanol.

The pipeline study involved testing two common pipeline steels, X52 and X70. For the study, the pipeline steels were installed in hydraulic test frames and subjected to mechanical forces while immersed in fuel. Researchers observed fatigue crack growth over a period of up to 10 days.

When immersed in fuel-grade ethanol, crack growth increased significantly at stress levels found in typical pipeline operating conditions, but not at low stress levels. Finer-grained X70 pipeline steel is known to better resist fatigue and had lower crack growth rates at all stress levels. The ethanol solution that contained bacteria promoted crack growth at stress intensity levels found in typical pipeline operating conditions. “We have shown that ethanol fuel can increase the rate of fatigue crack growth in pipelines,” Sowards says. “Substantial increases in crack growth rates were caused by the microbes. These are important data for pipeline engineers who want to safely and reliably transport ethanol fuel in repurposed oil and gas pipelines.”

The bacteria was isolated from industrial ethanol storage tanks. Samples were provided by the Colorado School of Mines, he says. Preliminary testing suggested that a biocide used in oil and gas operations, glutaraldehyde, could help control bacterial growth.

Cost Benefit

Dooley acknowledges planning corrosion mitigation is a challenge when tight margins mean ethanol producers need to carefully evaluate expenditures. “Everything comes down to a cost basis,” he says.

Take water tanks. With no danger of contamination, a water leak it may not seem concerning. However, Dooley points out, depending on plant design and water source, some plants use water tanks as backup systems, to provide a day or two of water supply for its ethanol production needs. If these water tanks need maintenance or even fail, this could disrupt production, something an ethanol plant can little afford in a time of tight margins.

Protecting metal structures such as water tanks from corrosion, whether at installation or retroactively, can help save money. For example, it may cost $5,000 to put in a system of cathotic protection, Dooley says, but compare that to $50,000 to repair a damaged tank or the cost of completely replacing that tank. “[Cathotic protection is] going to extend the life of their service infrastructure,” he says.

Author: Holly Jessen
Associate Editor, Ethanol Producer Magazine
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