Microbial corrosion, also called microbiologically influenced corrosion (MIC), microbially induced corrosion (MIC), or biocorrosion, is when microbes affect the electrochemical environment of the surface they are on. This usually involves building a biofilm, which can lead to either an increase in corrosion of the surface or, in a process called microbial corrosion inhibition, protect the surface from corrosion. Microbial corrosion is worth discussing because it causes trillions of dollars in damage around the globe annually and because every surface that is in some way exposed to the environment is also exposed to microbially induced corrosion. [1] Microbes can corrode in two ways. They can produce byproducts from their cellular processes that corrode metals, or they can keep normal corrosion inhibitors from functioning, leaving surfaces open to attack from other environmental factors. [2]
In presence of oxygen, aerobic bacteria like Acidithiobacillus thiooxidans, Thiobacillus thioparus, and Thiobacillus concretivorus, all three widely present in the environment, are the common corrosion-causing factors resulting in biogenic sulfide corrosion.
Without presence of oxygen, anaerobic bacteria, especially Desulfovibrio and Desulfotomaculum, are common. Desulfovibrio salixigens requires at least 2.5% concentration of sodium chloride, but D. vulgaris and D. desulfuricans can grow in both fresh and salt water. D. africanus is another common corrosion-causing microorganism. The genus Desulfotomaculum comprises sulfate-reducing spore-forming bacteria; Dtm. orientis and Dtm. nigrificans are involved in corrosion processes. Sulfate-reducers require a reducing environment; an electrode potential lower than -100 mV is required for them to thrive. However, even a small amount of produced hydrogen sulfide can achieve this shift, so the growth, once started, tends to accelerate.[citation needed]
Layers of anaerobic bacteria can exist in the inner parts of the corrosion deposits, while the outer parts are inhabited by aerobic bacteria.
Some bacteria are able to utilize hydrogen formed during cathodic corrosion processes.
Bacterial corrosion may appear in form of pitting corrosion, for example in pipelines of the oil and gas industry.[3] Anaerobic corrosion is evident as layers of metal sulfides and hydrogen sulfide smell. On cast iron, a graphitic corrosionselective leaching may be the result, with iron being consumed by the bacteria, leaving graphite matrix with low mechanical strength in place.
Microbial corrosion can also apply to plastics, concrete, and many other materials. Two examples are Nylon-eating bacteria and Plastic-eating bacteria.
Fungi are a substantial problem when it comes to microbial corrosion of concrete. With adequate environmental factors, such as humidity, temperature, and organic carbon sources, fungi will produce colonies on concrete. Some fungi can reproduce asexually. This common process among fungi allows many new fungal spores to quickly spread to new environments, developing entire colonies where nothing existed. These colonies and the new spores produced use hyphae to absorb environmental nutrients.
Hyphae are incredibly tiny and thin, growing only 2 to 6 micrometers in diameter. Fungal hyphae are used to reach deep into minuscule holes, cracks, and ravines in concrete. These areas contain moisture and nutrients the fungus survives on. As more hyphae force their way into these tiny cracks and crevices, the pressure causes those gaps to expand, similar to how water freezes in tiny holes and cracks, causing them to widen. The mechanical pressure enables cracks to expand, leading to more moisture getting inside, and thus, the fungi have more nutrients, allowing them to travel deeper into the concrete structure. By altering their environment, fungi break down concrete and its alkaline layer, thus providing ideal conditions for corrosion-causing bacteria to further degrade concrete structures.
Another way fungi cause corrosion on concrete is through organic acids naturally produced by the fungi. These organic acids chemically react with Calcium 2+ in the concrete which produces water-soluble salts as a product. The Calcium 2+ is then released, causing extensive damage over time to the structure. Due to the fact that fungi expel digestive juices to gain nutrients, the structure they grow on will begin to dissolve. This is no different for concrete when fungi such as Fusarium take root. An experiment compared the corrosion of the bacteria Tiobacillus to the corrosion of a fungus called Fusarium. In the experiment, both groups of organisms were provided with adequate conditions to grow, along with an equal piece of concrete in each experiment. After 147 days, the Tiobacillus bacterium caused an 18% mass reduction. However, the Fusarium fungus caused a 24% mass reduction in the same time frame, thus showcasing its corrosive abilities.
Bhattacharyya[4] did a study on the three separate types of fungi that are known to cause concrete corrosion: Aspergillus tamarii, Aspergillus niger, and Fusarium.Aspergillus tamarii was the most destructive of the three fungi. It causes cracks to widen and deepen, quickly and efficiently takes root, and promotes calcium oxalate. By causing calcium oxalate, there is an increase in the speed of calcium ion leaching, which lowers the overall strength of concrete. In 90 days, exposure to the fungus resulted in a mass reduction of 7.2% in the concrete. Aspergillus niger was the second worst offender out of the three, followed by Fusarium, which can lower the mass of concrete by 6.2 grams in a single year, as well as cause the pH to down from 12 to 8 in the same time frame.[5]
Aviation fuel
Hydrocarbon utilizing microorganisms, mostly Cladosporium resinae and Pseudomonas aeruginosa and sulfate reducing bacteria, colloquially known as "HUM bugs", are commonly present in jet fuel. They live in the water-fuel interface of the water droplets, form dark black/brown/green, gel-like mats, and cause microbial corrosion to plastic and rubber parts of the aircraft fuel system by consuming them, and to the metal parts by the means of their acidic metabolic products. They are also incorrectly called algae due to their appearance. FSII, which is added to the fuel, acts as a growth retardant for them. There are about 250 kinds of bacteria that can live in jet fuel, but fewer than a dozen are meaningfully harmful.[6]
Multiple factors produced by the environment stimulate the corrosion and deterioration of concrete, such as freezing conditions, radiation exposure, and extensive heat cycles or freeze-thaw and wet-dry cycles. Cycles that cause mechanical breakdowns of concrete, such as freeze-thaw cycles, are incredibly ruinous. All these provide ways for microbes to take over, further eroding and weakening structures made of concrete. An uptick in damages on urbanized sewer systems and cities that line the coast has forced people to look further in-depth at how to preserve concrete from microbes.
To halt the damage done by microbes, a complete comprehension of corrosion-causing microbes must be undertaken. This includes knowing what the specific microbes and their community are made up of and how they break down structural concrete. Environmental stressors on structures often promote microbial corrosion caused by bacteria, Archaea, algae, and fungi. These microorganisms depend on their environment to provide proper moisture, pH levels, and resources that allow reproduction.
The pH level of concrete greatly influences what microbes can reproduce and how much damage is done to the concrete. A concrete surface is alkaline, making it difficult for microbes to germinate. However, chemical processes by the environment and microorganisms themselves cause changes in the concrete. Environmental conditions combined with carbonization caused by select microbes fabricate negative changes in the pH of the concrete. These few microbes can excrete metabolites that change the pH from 12 to 8. With a lower pH level, more microorganisms can survive on the concrete, thus quickening the corrosion rate. This becomes an extreme problem, as many microbes that attack concrete survive in anaerobic conditions. Sewers, for example, have low oxygen levels and are high in nitrogen and sulfuric gas, making them perfect for microbes that metabolize those gases.[4]
Sewerage
Sewer network structures are prone to biodeterioration of materials due to the action of some microorganisms associated to the sulfur cycle. It can be a severely damaging phenomenon which was firstly described by Olmstead and Hamlin in 1900[7] for a brick sewer located in Los Angeles. Jointed mortar between the bricks disintegrated and ironwork was heavily rusted. The mortar joint had ballooned to two to three times its original volume, leading to the destruction or the loosening of some bricks.
Around 9% of damages described in sewer networks can be ascribed to the successive action of two kinds of microorganisms.[8]Sulfate-reducing bacteria (SRB) can grow in relatively thick layers of sedimentary sludge and sand (typically 1mm thick) accumulating at the bottom of the pipes and characterized by anoxic conditions. They can grow using oxidized sulfur compounds present in the effluent as electron acceptor and excrete hydrogen sulfide (H2S). This gas is then emitted in the aerial part of the pipe and can impact the structure in two ways: either directly by reacting with the material and leading to a decrease in pH, or indirectly through its use as a nutrient by sulfur-oxidizing bacteria (SOB), growing in oxic conditions, which produce biogenic sulfuric acid.[9] The structure is then submitted to a biogenic sulfuric acid attack. Materials like calcium aluminate cements, PVC or vitrified clay pipe may be substituted for ordinary concrete or steel sewers that are not resistant in these environments. Mild steel corrosion reduction in water by uptake of dissolved oxygen is carried out by Rhodotorula mucilaginosa(7).
Inhibition of Microbial Corrosion
Many methods have been developed for the restriction of microbial corrosion. The primary challenge has been finding ways to prevent or stop microbial growth without negatively impacting the surrounding environment. The list below provides an overview of some of the tactics that have been used or that are in development.
Using biocide (any chemical that inhibits life) to kill microorganisms. Because biofilms are so resistant, a lot of biocide must be used. This is expensive, has negative effects on the surrounding environment, and can actually cause more corrosion of the surface due to its toxicity. Biocides and other chemical treatments against microbes also tend to be dangerous for the people preparing and applying them. [10]
Rao and Mulky[2] developed an extensive list of methods to limit the growth of microbes and therefore microbial corrosion.
Plant products could aid in restricting microbial growth. These would be biodegradable and safe for the people applying them, but have not yet been widely tested.
Surfactants, specifically ones generated by organisms as secondary metabolites. They’re useful because they get between the corrosive liquid and the surface and keep them apart.
Putting a superhydrophobic coating on a surface. This keeps a biofilm from being able to develop, but is sensitive and can easily lose its superhydrophobic qualities.
Using self-healing surfaces can prevent corrosion in cracks or faults. This could be used with a superhydrophobic surface, to mitigate its sensitivity.
Using hydrophilic surfaces to create a region that deters the development of proteins into a film covering a surface.
Using synthetically-created substances that deter corrosion because of their chemical structures. This may have a smaller negative effect on the environment than other options.
Using biofilms that are grown intentionally to inhibit microbial corrosion. This is done by growing a biofilm on a surface made of a bacteria that can release compounds that kill other microbes and that prevent corrosion.
Using essential oils. The effectiveness of essential oils against microbial corrosion has not been widely tested.
Coating a surface with various nanomaterials or ozone to prevent microbial corrosion.
Microbes Acting to Inhibit Corrosion
Though microorganisms are often responsible for corrosion, they can also protect surfaces from corrosion.[11] For example, oxidization is a common cause of corrosion. If a susceptible surface has a biofilm covering it that takes in and uses oxygen, then that surface will be protected from corrosion due to oxidization. Biofilms can also release antimicrobial compounds, which is helpful if the biofilm is not corrosive and can deter microbes that would be. Biofilms provide a barrier between a surface and the ecosystem surrounding it, so as long as the biofilm has no adverse effects, it can serve as protection from corrosion as well.[10] Because biofilms don’t negatively impact the ecosystem, they are potentially one of the best mechanisms for corrosion inhibition. They can also alter the conditions on the surface of a metal so that the metal is less likely to be damaged, preventing corrosion.[2]
Corrosion is a natural process that converts a refined metal into a more chemically stable oxide. It is the gradual deterioration of materials by chemical or electrochemical reaction with their environment. Corrosion engineering is the field dedicated to controlling and preventing corrosion.
Geomicrobiology is the scientific field at the intersection of geology and microbiology and is a major subfield of geobiology. It concerns the role of microbes on geological and geochemical processes and effects of minerals and metals to microbial growth, activity and survival. Such interactions occur in the geosphere, the atmosphere and the hydrosphere. Geomicrobiology studies microorganisms that are driving the Earth's biogeochemical cycles, mediating mineral precipitation and dissolution, and sorbing and concentrating metals. The applications include for example bioremediation, mining, climate change mitigation and public drinking water supplies.
Sulfide (British English also sulphide) is an inorganic anion of sulfur with the chemical formula S2− or a compound containing one or more S2− ions. Solutions of sulfide salts are corrosive. Sulfide also refers to large families of inorganic and organic compounds, e.g. lead sulfide and dimethyl sulfide. Hydrogen sulfide (H2S) and bisulfide (SH−) are the conjugate acids of sulfide.
Anaerobic respiration is respiration using electron acceptors other than molecular oxygen (O2). Although oxygen is not the final electron acceptor, the process still uses a respiratory electron transport chain.
Acidithiobacillus is a genus of the Acidithiobacillia in the phylum "Pseudomonadota". This genus includes ten species of acidophilic microorganisms capable of sulfur and/or iron oxidation: Acidithiobacillus albertensis, Acidithiobacillus caldus, Acidithiobacillus cuprithermicus, Acidithiobacillus ferrianus, Acidithiobacillus ferridurans, Acidithiobacillus ferriphilus, Acidithiobacillus ferrivorans, Acidithiobacillus ferrooxidans, Acidithiobacillus sulfuriphilus, and Acidithiobacillus thiooxidans.A. ferooxidans is the most widely studied of the genus, but A. caldus and A. thiooxidans are also significant in research. Like all "Pseudomonadota", Acidithiobacillus spp. are Gram-negative and non-spore forming. They also play a significant role in the generation of acid mine drainage; a major global environmental challenge within the mining industry. Some species of Acidithiobacillus are utilized in bioleaching and biomining. A portion of the genes that support the survival of these bacteria in acidic environments are presumed to have been obtained by horizontal gene transfer.
Sulfate-reducing microorganisms (SRM) or sulfate-reducing prokaryotes (SRP) are a group composed of sulfate-reducing bacteria (SRB) and sulfate-reducing archaea (SRA), both of which can perform anaerobic respiration utilizing sulfate (SO2− 4) as terminal electron acceptor, reducing it to hydrogen sulfide (H2S). Therefore, these sulfidogenic microorganisms "breathe" sulfate rather than molecular oxygen (O2), which is the terminal electron acceptor reduced to water (H2O) in aerobic respiration.
Iron-oxidizing bacteria are chemotrophic bacteria that derive energy by oxidizing dissolved iron. They are known to grow and proliferate in waters containing iron concentrations as low as 0.1 mg/L. However, at least 0.3 ppm of dissolved oxygen is needed to carry out the oxidation.
Biogenic sulfide corrosion is a bacterially mediated process of forming hydrogen sulfide gas and the subsequent conversion to sulfuric acid that attacks concrete and steel within wastewater environments. The hydrogen sulfide gas is biochemically oxidized in the presence of moisture to form sulfuric acid. The effect of sulfuric acid on concrete and steel surfaces exposed to severe wastewater environments can be devastating. In the USA alone, corrosion is causing sewer asset losses estimated at $14 billion per year. This cost is expected to increase as the aging infrastructure continues to fail.
Biomining is the technique of extracting metals from ores and other solid materials typically using prokaryotes, fungi or plants. These organisms secrete different organic compounds that chelate metals from the environment and bring it back to the cell where they are typically used to coordinate electrons. It was discovered in the mid 1900s that microorganisms use metals in the cell. Some microbes can use stable metals such as iron, copper, zinc, and gold as well as unstable atoms such as uranium and thorium. Large chemostats of microbes can be grown to leach metals from their media. These vats of culture can then be transformed into many marketable metal compounds. Biomining is an environmentally friendly technique compared to typical mining. Mining releases many pollutants while the only chemicals released from biomining is any metabolites or gasses that the bacteria secrete. The same concept can be used for bioremediation models. Bacteria can be inoculated into environments contaminated with metals, oils, or other toxic compounds. The bacteria can clean the environment by absorbing these toxic compounds to create energy in the cell. Bacteria can mine for metals, clean oil spills, purify gold, and use radioactive elements for energy.
Oral microbiology is the study of the microorganisms (microbiota) of the oral cavity and their interactions between oral microorganisms or with the host. The environment present in the human mouth is suited to the growth of characteristic microorganisms found there. It provides a source of water and nutrients, as well as a moderate temperature. Resident microbes of the mouth adhere to the teeth and gums to resist mechanical flushing from the mouth to stomach where acid-sensitive microbes are destroyed by hydrochloric acid.
Phototrophic biofilms are microbial communities generally comprising both phototrophic microorganisms, which use light as their energy source, and chemoheterotrophs. Thick laminated multilayered phototrophic biofilms are usually referred to as microbial mats or phototrophic mats. These organisms, which can be prokaryotic or eukaryotic organisms like bacteria, cyanobacteria, fungi, and microalgae, make up diverse microbial communities that are affixed in a mucous matrix, or film. These biofilms occur on contact surfaces in a range of terrestrial and aquatic environments. The formation of biofilms is a complex process and is dependent upon the availability of light as well as the relationships between the microorganisms. Biofilms serve a variety of roles in aquatic, terrestrial, and extreme environments; these roles include functions which are both beneficial and detrimental to the environment. In addition to these natural roles, phototrophic biofilms have also been adapted for applications such as crop production and protection, bioremediation, and wastewater treatment.
A microbial mat is a multi-layered sheet of microorganisms, mainly bacteria and archaea, or bacteria alone. Microbial mats grow at interfaces between different types of material, mostly on submerged or moist surfaces, but a few survive in deserts. A few are found as endosymbionts of animals.
Sulfur is metabolized by all organisms, from bacteria and archaea to plants and animals. Sulfur can have an oxidation state from -2 to +6 and is reduced or oxidized by a diverse range of organisms. The element is present in proteins, sulfate esters of polysaccharides, steroids, phenols, and sulfur-containing coenzymes.
Bacterial anaerobic corrosion is the bacterially-induced oxidation of metals. Corrosion of metals typically alters the metal to a form that is more stable. Thus, bacterial anaerobic corrosion typically occurs in conditions favorable to the corrosion of the underlying substrate. In humid, anoxic conditions the corrosion of metals occurs as a result of a redox reaction. This redox reaction generates molecular hydrogen from local hydrogen ions. Conversely, anaerobic corrosion occurs spontaneously. Anaerobic corrosion primarily occurs on metallic substrates but may also occur on concrete.
Anaerobic corrosion is a form of metal corrosion occurring in anoxic water. Typically following aerobic corrosion, anaerobic corrosion involves a redox reaction that reduces hydrogen ions and oxidizes a solid metal. This process can occur in either abiotic conditions through a thermodynamically spontaneous reaction or biotic conditions through a process known as bacterial anaerobic corrosion. Along with other forms of corrosion, anaerobic corrosion is significant when considering the safe, permanent storage of chemical waste.
Oral ecology is the microbial ecology of the microorganisms found in mouths. Oral ecology, like all forms of ecology, involves the study of the living things found in oral cavities as well as their interactions with each other and with their environment. Oral ecology is frequently investigated from the perspective of oral disease prevention, often focusing on conditions such as dental caries, candidiasis ("thrush"), gingivitis, periodontal disease, and others. However, many of the interactions between the microbiota and oral environment protect from disease and support a healthy oral cavity. Interactions between microbes and their environment can result in the stabilization or destabilization of the oral microbiome, with destabilization believed to result in disease states. Destabilization of the microbiome can be influenced by several factors, including diet changes, drugs or immune system disorders.
Concrete degradation may have many different causes. Concrete is mostly damaged by the corrosion of reinforcement bars due to the carbonatation of hardened cement paste or chloride attack under wet conditions. Chemical damages are caused by the formation of expansive products produced by various chemical reactions, by aggressive chemical species present in groundwater and seawater, or by microorganisms. Other damaging processes can also involve calcium leaching by water infiltration and different physical phenomena initiating cracks formation and propagation. All these detrimental processes and damaging agents adversely affects the concrete mechanical strength and its durability.
Sulfur concrete, sometimes named thioconcrete or sulfurcrete, is a composite construction material, composed mainly of sulfur and aggregate. Cement and water, important compounds in normal concrete, are not part of sulfur concrete. The concrete is heated above the melting point of elemental sulfur at ca. 140 °C (284 °F) in a ratio of between 12% and 25% sulfur, the rest being aggregate.
Acidithiobacillus thiooxidans, formerly known as Thiobacillus thiooxidans until its reclassification into the newly designated genus Acidithiobacillus of the Acidithiobacillia subclass of Pseudomonadota, is a Gram-negative, rod-shaped bacterium that uses sulfur as its primary energy source. It is mesophilic, with a temperature optimum of 28 °C. This bacterium is commonly found in soil, sewer pipes, and cave biofilms called snottites. A. thiooxidans is used in the mining technique known as bioleaching, where metals are extracted from their ores through the action of microbes.
The hydrothermal vent microbial community includes all unicellular organisms that live and reproduce in a chemically distinct area around hydrothermal vents. These include organisms in the microbial mat, free floating cells, or bacteria in an endosymbiotic relationship with animals. Chemolithoautotrophic bacteria derive nutrients and energy from the geological activity at Hydrothermal vents to fix carbon into organic forms. Viruses are also a part of the hydrothermal vent microbial community and their influence on the microbial ecology in these ecosystems is a burgeoning field of research.
References
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Islander, R.L., Devinny, J.S., Mansfeld, F., Postyn, A., Shih, H., 1991. Microbial ecology of crown corrosion in sewers. Journal of Environmental Engineering 117, 751–770.
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