Microbial corrosion can occur and advance through two main mechanisms. The first method of microbially enhanced corrosion occurs from microorganisms producing acidic metabolic by-products or from microorganisms participating directly in the electrochemical corrosion of the pipe [1]. These mechanisms directly cause or promote the corrosion process [2]. Microbially influenced corrosion can also occur from the presence of biofilms [2]. Biofilms are colonies of adherent microorganisms and their secreted matrix [3]; these biofilms create a micro-environment which differs from the surrounding environment, forming an corrosion cell and initiating the corrosion process [1,2].
Microbial corrosion commonly occurs from fungi, acid-producing bacteria (APB), sulfate-reducing bacteria (SRB), iron reducing bacteria (IRB). In oil and gas pipelines, acid producing bacteria and sulfate reducing bacteria are commonly found [1].
Fungi
Fungi which are commonly involved in the corrosion of metals are mold and yeast. Fungi release organic acids as metabolites, products of the organism's metabolic process, which induce or enhance corrosion [1].
Acid-producing bacteria generate acetic acid or sulfuric acid (sulfur oxidizing bacteria), which are both compounds highly corrosive to pipelines [1]. These acids likewise contribute to the acid corrosion mechanism.
The overall corrosion mechanisms which occur from sulfuric and acetic acid are listed below [4]:
Sulfuric acid:
Acetic acid:
Sulfate Reducing Bacteria
Many theories regarding the mechanism of corrosion due to sulfate reducing bacteria focus on hydrogenase. Hydrogenase is an enzyme contained by sulfate reducing bacteria, which allows for the oxidation of hydrogen generated at cathodic corrosion sites [1]. The removal of hydrogen depolarizes the cathode; this process maintains or increases the cell potential for corrosion [5]. Due to the significance of sulphate reducing bacteria and the extensive research conducted about sulphate reducing bacteria, a more detailed analysis can be found here.
Iron Reducing Bacteria
Iron reducing bacteria reduce ferric ions (Fe3+) to ferrous ions (Fe2+). Ferric salts are highly insoluble and are chemically inactive, which protects the metal surface from corrosion; in contrast, ferrous salts are soluble and do not offer protection from corrosion [1]. The continual removal of the protective layer of ferric compounds exposes the metal surface, allowing for further corrosion to occur [6].
Biofilms
When adherent bacteria proliferate on a surface and secrete an extracellular matrix, a biofilm is formed [3]. On sites affected by microbially influenced corrosion, it is found that the sites contain a combination of bacteria species rather than a single species [7]. These biofilms contribute to corrosion by altering the local environment [8]. The secreted extracellular matrix is commonly composed of polysaccharides, proteins, nucleic acids, and lipids [7].
Biofilm formation often follows a few critical steps: initial rapid and reversible bacterial attachment to the surface (reversible attachment), a more stable and longer term attachment (irreversible attachment), bacterial growth and matrix secretion (microcolony formation), biofilm maturation (macrocolony formation), and bacterial migration (bacterial dispersal) [7,9].
Figure 1. The developmental model of biofilm formation. The stages (i) planktonic (in fluid), (ii) attachment, (iii) microcolony formation, (iv) macrocolony formation, (v) dispersal [9]
The biofilm can act as a chemical barrier. This chemical barrier provides resistance to the diffusion of chemical species into and out of the biofilm. Thus, the chemical barrier creates localized chemical environments which differ from the surrounding environment. By inhibiting the diffusion of chemical species out of the biofilm, a greater concentration of chemical species could be in contact with the metal surface. An example of this would be the hydrogen sulfide generated by sulfate reducing bacteria, which is highly corrosive [7].
The physical structure of the biofilm can also contribute to the corrosion of metals. Since biofilms are heterogeneous, being patchy and varying in thickness, isolated oxygen pockets can be found. The oxygen pockets can initiate the corrosion process by creating corrosion cells. Alternatively, biofilms can form regions which are depleted of oxygen compared to the non-colonized surroundings. The different oxygen concentrations create a difference in electric potential and form corrosion cells [10].
Figure 2. Heterogeneous biofilms resulting in a difference of aeration causing corrosion cells, the absence of oxygen within the biofilm results in the corrosion of the metal surface [10]
In addition, the biofilm offers more protective conditions that foster the growth of different microorganisms [11]. For instance, the biofilm may contain an anaerobic zone that promotes the growth of anaerobic bacteria, such as sulfate reducing bacteria. Other mechanisms involve the enzymes contained within the extracellular matrix, and their corrosion enhancing effects [7].
Other Corrosion Mechanisms
Some bacteria can contribute to microbially enhanced corrosion by consuming corrosion by-products. Sulfate reducing bacteria for example consume hydrogen, which is a byproduct in the standard corrosion process. The removal of hydrogen causes an increase in the rate of corrosion to reach chemical equilibrium by Le Chatelier's principle [12].
The co-culture of symbiotic microorganisms can contribute to microbially influenced corrosion. Manganese oxidizing bacteria and sulfate reducing bacteria are an example of how the co-culture of symbiotic bacteria allows for the corrosion mechanism to initiate and advance. Manganese oxidizing bacteria deposit MnO2; this creates an anaerobic region which promotes the growth of sulfate reducing bacteria. The manganese oxidizing bacteria initiate the corrosion mechanism and promotes the growth of sulfate reducing bacteria which enhances the rate of corrosion [1].
[11] W.H. Dickinson and Z. Lewandowski. (1996, Feb). Manganese biofouling and the corrosion behavior of stainless steel. The Journal or Bioadhesion and Biofilm Research. 10. 79-93. [Online]. Available: http://www.tandfonline.com/doi/pdf/10.1080/08927019609386272
[12] C. I. Ossai. (2012, Aug). Advances in asset management techniques: an overview of corrosion mechanisms and mitigation strategies for oil and gas pipelines.ISRN Corrosion [Online]. Available: http://www.hindawi.com/journals/isrn/2012/570143/abs/
Microbial Corrosion Mechanisms
Microbial corrosion can occur and advance through two main mechanisms. The first method of microbially enhanced corrosion occurs from microorganisms producing acidic metabolic by-products or from microorganisms participating directly in the electrochemical corrosion of the pipe [1]. These mechanisms directly cause or promote the corrosion process [2]. Microbially influenced corrosion can also occur from the presence of biofilms [2]. Biofilms are colonies of adherent microorganisms and their secreted matrix [3]; these biofilms create a micro-environment which differs from the surrounding environment, forming an corrosion cell and initiating the corrosion process [1,2].
Microbial corrosion commonly occurs from fungi, acid-producing bacteria (APB), sulfate-reducing bacteria (SRB), iron reducing bacteria (IRB). In oil and gas pipelines, acid producing bacteria and sulfate reducing bacteria are commonly found [1].
Fungi
Fungi which are commonly involved in the corrosion of metals are mold and yeast. Fungi release organic acids as metabolites, products of the organism's metabolic process, which induce or enhance corrosion [1].
The organic acids contribute to the acid corrosion mechanism.
Acid Producing Bacteria
Acid-producing bacteria generate acetic acid or sulfuric acid (sulfur oxidizing bacteria), which are both compounds highly corrosive to pipelines [1]. These acids likewise contribute to the acid corrosion mechanism.
The overall corrosion mechanisms which occur from sulfuric and acetic acid are listed below [4]:
Sulfuric acid:
Acetic acid:
Sulfate Reducing Bacteria
Many theories regarding the mechanism of corrosion due to sulfate reducing bacteria focus on hydrogenase. Hydrogenase is an enzyme contained by sulfate reducing bacteria, which allows for the oxidation of hydrogen generated at cathodic corrosion sites [1]. The removal of hydrogen depolarizes the cathode; this process maintains or increases the cell potential for corrosion [5]. Due to the significance of sulphate reducing bacteria and the extensive research conducted about sulphate reducing bacteria, a more detailed analysis can be found here.
Iron Reducing Bacteria
Iron reducing bacteria reduce ferric ions (Fe3+) to ferrous ions (Fe2+). Ferric salts are highly insoluble and are chemically inactive, which protects the metal surface from corrosion; in contrast, ferrous salts are soluble and do not offer protection from corrosion [1]. The continual removal of the protective layer of ferric compounds exposes the metal surface, allowing for further corrosion to occur [6].
Biofilms
When adherent bacteria proliferate on a surface and secrete an extracellular matrix, a biofilm is formed [3]. On sites affected by microbially influenced corrosion, it is found that the sites contain a combination of bacteria species rather than a single species [7]. These biofilms contribute to corrosion by altering the local environment [8]. The secreted extracellular matrix is commonly composed of polysaccharides, proteins, nucleic acids, and lipids [7].
Biofilm formation often follows a few critical steps: initial rapid and reversible bacterial attachment to the surface (reversible attachment), a more stable and longer term attachment (irreversible attachment), bacterial growth and matrix secretion (microcolony formation), biofilm maturation (macrocolony formation), and bacterial migration (bacterial dispersal) [7,9].
The biofilm can act as a chemical barrier. This chemical barrier provides resistance to the diffusion of chemical species into and out of the biofilm. Thus, the chemical barrier creates localized chemical environments which differ from the surrounding environment. By inhibiting the diffusion of chemical species out of the biofilm, a greater concentration of chemical species could be in contact with the metal surface. An example of this would be the hydrogen sulfide generated by sulfate reducing bacteria, which is highly corrosive [7].
The physical structure of the biofilm can also contribute to the corrosion of metals. Since biofilms are heterogeneous, being patchy and varying in thickness, isolated oxygen pockets can be found. The oxygen pockets can initiate the corrosion process by creating corrosion cells. Alternatively, biofilms can form regions which are depleted of oxygen compared to the non-colonized surroundings. The different oxygen concentrations create a difference in electric potential and form corrosion cells [10].
In addition, the biofilm offers more protective conditions that foster the growth of different microorganisms [11]. For instance, the biofilm may contain an anaerobic zone that promotes the growth of anaerobic bacteria, such as sulfate reducing bacteria. Other mechanisms involve the enzymes contained within the extracellular matrix, and their corrosion enhancing effects [7].
Other Corrosion Mechanisms
Some bacteria can contribute to microbially enhanced corrosion by consuming corrosion by-products. Sulfate reducing bacteria for example consume hydrogen, which is a byproduct in the standard corrosion process. The removal of hydrogen causes an increase in the rate of corrosion to reach chemical equilibrium by Le Chatelier's principle [12].
The co-culture of symbiotic microorganisms can contribute to microbially influenced corrosion. Manganese oxidizing bacteria and sulfate reducing bacteria are an example of how the co-culture of symbiotic bacteria allows for the corrosion mechanism to initiate and advance. Manganese oxidizing bacteria deposit MnO2; this creates an anaerobic region which promotes the growth of sulfate reducing bacteria. The manganese oxidizing bacteria initiate the corrosion mechanism and promotes the growth of sulfate reducing bacteria which enhances the rate of corrosion [1].
[1] N. Muthukumar. et al. (2013, Sept). Microbiologically Influenced corrosion in petroleum product pipelines - A review. Indian Journal of Experimental Biology [Online]. 41. 1012-1022. Available: http://nopr.niscair.res.in/bitstream/123456789/17162/1/IJEB%2041%289%29%201012-1022.pdf
[2] Rob. (2010, Jun). Biological Corrosion of Metals. [Online]. Available: http://failure-analysis.info/2010/06/biological-corrosion-of-metals/
[3] Colgate. What Is Biofilm?. [Online]. Available: http://www.colgateprofessional.com/patient-education/articles/what-is-biofilm
[4] J. Wright. Inhibiting Rust and Corrosion to Prevent Machine Failures. [Online]. Available: http://www.machinerylubrication.com/Read/29116/inhibiting-rust-corrosion
[5] Corrosionpedia. Cathodic Polarization. [Online]. Available: http://www.corrosionpedia.com/definition/231/cathodic-polarization
[6] D.T. Hang. (2003). Microbiological study of the anaerobic corrosion of iron. [Online]. Available: http://elib.suub.uni-bremen.de/publications/dissertations/E-Diss725_Hang.pdf?origin=publication_detail
[7] DKL Engineering. (2011). Corrosion. [Online]. Available: http://www.sulphuric-acid.com/techmanual/Corrosion/corrosion.htm
[8] T.S. Rao. (2011, Nov). Microbial fouling and corrosion: fundamentals and mechanisms. Operational and Environmental Consequences of Lager Industrial Cooling Water Systems. [Online]. 95-126. Available: http://link.springer.com/chapter/10.1007%2F978-1-4614-1698-2_6#page-1
[9] R.D. Monds. G.A. O'Toole. (2009, Feb). The developmental model of microbial biofilms: ten years of a paradigm up for review. Trends in Microbiology. 17(2). 73-87. [Online]. Available: http://www.sciencedirect.com/science/article/pii/S0966842X09000055
[10] CLI Houston. Mechanism of microbiologically influenced corrosion, caused by bacteria. [Online]. Available: http://www.clihouston.com/news/mechanism-of-mic-caused-by-bacteria.html
[11] W.H. Dickinson and Z. Lewandowski. (1996, Feb). Manganese biofouling and the corrosion behavior of stainless steel. The Journal or Bioadhesion and Biofilm Research. 10. 79-93. [Online]. Available: http://www.tandfonline.com/doi/pdf/10.1080/08927019609386272
[12] C. I. Ossai. (2012, Aug). Advances in asset management techniques: an overview of corrosion mechanisms and mitigation strategies for oil and gas pipelines.ISRN Corrosion [Online]. Available: http://www.hindawi.com/journals/isrn/2012/570143/abs/