Microbial corrosion is a major concern for both aboveground and underground fuel storage tanks.
BP Americas chairman/president Bob Malone created a stir in 2006 when he made a comment on a CNN newscast regarding a leaking BP transit line in Prudhoe Bay, Alaska:
“We have found that there is something we did not anticipate finding, which is a bacterium that is growing in the pipeline. It eats the oil and secretes a substance which is acidic that has caused a hole in the pipeline.”
Edward W English II, vice-president and technical director for Fuel Quality Services Inc, said Malone was probably referring to sulfate-reducing bacteria. English discussed microbial corrosion during the Independent Liquid Terminals Association's 31st Annual International Operating Conference June 6-8 in Houston, Texas.
“That's not a bacterium that just showed up on the market a couple of weeks ago or a couple of years ago,” English said. “That's been around for decades. It's kind of the bane of the oil industry when you're out in the field because it can get into piping and destroy metal rapidly.
“When that comment was made, it was kind of a little shock. I'm sure their people knew about sulfate-reducing bacteria (SRB). They just weren't controlling it in the right way. It has the potential of going through metal at a very significant rate. If you have SRBs in your tank, you may have a potential problem on the horizon. But they can be controlled effectively with certain classes of biocides.”
In his presentation “Microbial Contamination and The Impact on Tanks,” he said that a century of investigations, decades of research, and numerous peer-reviewed studies have conclusively substantiated the causal links between fuel, water, and microorganisms that cause Microbially Influenced Corrosion (MIC), which is responsible for both exterior and interior corrosion of metal fuel systems, including underground storage tanks — resulting in the release of hydrocarbons that endanger the environment and the nation's drinking water supply.
MIC was first found in 1910. And in 1986, offshore pipeline failures due to MIC were costly to the oil industry and the environment.
The costs associated with MIC are not isolated to one or two industries, but are experienced across the entire spectrum of industries that store, distribute and utilize petroleum-based energy and biomass-based energy.
With hazardous materials storage in above-ground storage tanks (AST), the greatest issue is external corrosion of tank bottoms sitting on grade.
“The most common problem related to internal corrosion is the water at the bottom of tank, which is not ameliorated by corrosion-protection systems internal or external to the AST,” he said. “Corrosion rates are accelerated at the liquid/fuel interface due to oxygen gradients and varies with depth. Even though the fuel is benign, aqueous-phase corrosion can occur when contamination and settling results in water bottoms that contain sludge deposits along with microbial contamination.”
It is estimated that 20% to 30% of all corrosion on gas and liquid transmission pipelines is MIC-related, and can affect internal and external surfaces of a pipeline. He said microbes do not create a new or novel form of corrosion but rather accelerate known forms of corrosion by interacting with product to prevent natural mechanisms that inhibit corrosion, and by providing mechanisms to accelerate the corrosion process.
The presence of microbes can lead to various corrosion mechanisms such as: SRB (hydrogen sulfide and pitting corrosion), metal-reducing bacteria (MRB, dissolution of corrosion-resistant oxide films), metal-deposition bacteria (MDB facilitate cathodically reactive metal oxides), and acid-producing bacteria (APB) and fungi (produces organic and inorganic acids).
Global standards and organizations that recognize the deleterious effects of MIC in ground transportation include ASTM D975, ASTM D6751, ASTM D7467, and ASTM D4814; in aviation, ASTM D1655, ASTM Manual 5, and IATA, and airframe (Boeing/Airbus) OEMs and aircraft maintenance manuals; in the military, Australian Army, Canadian Navy, Czech Army, French Army & Navy, Italian Navy, Hungarian Army, RAF; and in industry, API, CSA, DFOG, DOT, NBB, Oil Heat, PEI, STI, USEPA-OUST, and NACE.
“MIC has no boundaries or political affiliations,” English said. “MIC is present throughout the life cycle of petroleum and biomass fuels with one exception. The presence of microbes in crude oil is eliminated by the sterilizing temperatures associated with refining petroleum crude. However, microorganisms re-contaminate the newly finished fuel products, adapting to their environment to grow and facilitate MIC during storage, bulk distribution, and retail distribution. The same post-production issues are true for biomass fuels.
“Microbes enter the crude from the field. Refining sterilizes the products. Moisture and microbial spores enter the refinery storage tank via ventilation with insufficient stay time to remove water and microbes. After refining, fuel cools along the distribution system, condensing water and microorganisms to form active pools/biofilms distributing microorganisms and their corrosive metabolites downstream to ASTs and USTs. Poor ballast stripping sends water and microbes with offloaded fuel. Pipelines and storage tanks are contaminated from upstream product tenders.”
Factors that contribute to MIC include water, ionic contaminants, fuel, and microorganisms (bacteria, yeast, and mold).
“Most people think of fuel products distributed or stored in tanks as pristine and pure product,” he said. “However, what really occurs inside a storage tank includes accumulation of water, biofilm formation, interior system corrosion, and loss of system integrity, resulting in leaking storage tanks.”
Microbial contamination pathways are present in the environment soil, vents (air, water, dust), tanks with floating roofs that leak, ships' ballast/seepage water, transfer piping, and cross-contamination between fuel system tankage.
He said classic galvanic corrosion involves two dissimilar metals, a conductive path between the two metals, and a conductive path between the metals so electrons will flow from the anode to the cathode.
The conditions for a differential aeration cell: water in contact with the metal surface will normally contain dissolved oxygen; a differential aeration cell can develop at any point where the oxygen in the solution is not allowed to diffuse uniformly onto the surface; the zone of low oxygen thereby creates a difference in oxygen concentration between the anode and cathode; and corrosion will occur at a relatively fast rate, releasing metal ions into the biofilm.
The conditions for pitting corrosion by SRB: the anode is the site of metal wastage, electrons flow to the cathode; the cathode is depolarized by the hydrogenase enzyme, which continues the flow of electrons from the anode to the cathode; the sulfate anion (SO4-2) is used as the electron acceptor and is reduced to sulfide (S-); and iron sulfide (FeS) precipitates to form a cathode and continue metal loss at the anode. Dihydrogen sulfide (H2S) is also produced and creates a zone of low pH which facilitates rapid dissolution of the metal at the anode.
He said there have been a number of problems associated with accelerated corrosion of mild carbon steel in the fuel system several years after the introduction of ULSD in September 2006. This investigation is now under the auspices of the Clean Diesel Fuel Alliance (CDFA) chaired by API.
Electrolysis from a lack of proper grounding for submersible turbine pumps (STP).
The presence of hydroperoxides, excess corrosion inhibitor, and a lack of corrosion inhibitor.
Low levels of sulfur level, and aerobic versus anaerobic microorganisms.
“Traditional growth techniques involve a fuel sample that is filtered on a sterile disc, placed on growth media, and incubated for four to eight days to express results microbes as colonies,” he said. “There is good repeatability but longer incubation times can produce results not representative of original sample. The test complies with IP 385 and ASTM D6974.
“Modified growth techniques involve inoculating the dip slide with water samples only, and incubating for four to eight days to express colonies as cfu/mlH2O. Long incubation times can produce results not representative of the original sample. Fuel contamination of the dip slide will ruin test. The test is portable but results take four to eight days.
“A liquid tester involves injecting the fuel/water sample into a bottle with growth media and color indicator sensitive to pH changes, and incubating the sample 30-72 hours to produce color change express activity. Acidity in fuels can produce false positive results. The test is portable but results take 30-72 hours.”
A few novel techniques:
Introduce prepared fuel or water sample onto the paddles. Microbial antibodies combine with antigens fixed on paddle. Paddles read after 10 minutes. Test is species-specific. Test is portable and results are immediate.
Fuel/water samples are prepared and introduced to test pen and measured (five minutes). Test is portable and analysis is immediate. Test complies with ASTM D7463.
With remediation, considerations when selecting a biocide include: local, state, or country regulatory issues; anti-microbial type (biocide, biostat); method of action (kill mechanism); product solubility, partitioning capability; dose rate (lethal versus sub-lethal, resistance versus tolerance), frequency of use; and response time.
“Antibiotic is a chemical agent that kills by targeting a single, very specific critical site in the organism,” he said. “Biocide is a chemical agent that kills microbes by targeting multiple critical sites of the organism. Biostat is a chemical agent that stops microorganisms from reproducing, while not necessarily harming them. Upon removal of the agent, the microorganism usually starts to grow again.”
“A lethal dose is a dose rate of biocide sufficient to kill or eliminate a microbial population by a minimum of three to four orders of magnitude. A sub-lethal dose is an insufficient dose rate of biocide that results in population reduction of less than three orders of magnitude. Resistance is the ability of the organism to lower its susceptibility to the actions of a chemical agent. Tolerance is the ability of an organism to tolerant the presence of a chemical agent.” ♦
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