One particular group of microbes that frequently induce corrosion in pipelines in practice are sulfate reducing bacteria (SRB). [1] Although different strains of SRB exist that flourish under different conditions, this group of bacteria all perform similar chemical reactions involving the reduction of sulfate in the pipeline fluid. Due to the prevalence of these bacteria, many studies on microbial corrosion focus on SRB in detail, and some studies have been conducted solely on the corrosive abilities of SRB. [1] Through metabolic processes, SRB reduce this sulfate to sulfide, which corrodes the pipeline interior through a reduction/oxidation reaction. [2] Most sulfate reducing bacteria are anaerobic and thrive in oxygen-poor environments, such as within a biofilm. [2] In addition to reducing sulfate, different strains of this bacteria can also reduce other sulfur compounds such as sulfite or thiosulfate through different chemical reactions. [3]
Chemical Mechanism of Sulfate Reducing Bacteria
Although significant amounts of research have been performed on the corrosive effects SRB, the reaction mechanisms of SRB are still not fully understood. This lack of understanding is due to both the large number of different varieties of SRB as well as the fact that in a realistic environment, SRB will not be the only bacteria present and corrosion will be caused by all the microorganisms in the biofilm. [3] Thus, laboratory experiments cannot take these factors into account. Nevertheless, many theories have been proposed about SRB mechanisms and research is still being conducted to better understand SRB. Some of the most popular and influential proposed mechanisms are provided.
The Classical Theory
One the the first theories regarding the mechanism of SRB was proposed by VonWolzogen Kuhr and Van der Vlugt in 1934. In their paper, Kuhr and Vlugt provided evidence that a significant component of anaerobic corrosion was caused by SRB, and proposed a mechanism known as the Cathodic Depolarization Theory, which eventually was termed the Classical Theory of SRB. [3] Kuhr and Vlugt's research made significant advances on the role of SRB in microbially influenced corrosion, and many modern mechanisms are based upon this original theory. Although, the Classical Theory contains many flaws, it remains one of the most important mechanisms proposed for SRB due to its wide-reaching influence on modern theories. [2]
The Classical Theory states that SRB remove atomic hydrogen on the metal surface in the form of an enzyme called hydrogenase, in a progress termed depolarization by Kuhr and Vlugt. [3] The atomic hydrogen is created when water in the fluid briefly separates into its hydronium and hydroxide components. The hydronium ions then withdraw electrons from the metal and adsorb onto the metal surface. SRB then use the adsorbed atomic hydrogen is metabolic reactions through hydrogenase. The microorganisms reduce sulfate present in the solution using this hydrogenase to create sulfide and water. The produced sulfide then undergoes an electrochemical reaction with the metal, producing a metal sulfide. [4] This metal sulfide is structurally weaker, and reduces the strength of the pipe, causing sulfide stress cracking. [4] The metal sulfide can also splinter off the pipe. In addition to this reaction, the removal of hydrogen ions from solution also create hydroxide ions in solution. [2] These hydroxide ions can also undergo an electrochemical reaction with water to produce a weaker material. By removing adsorbed hydrogen from the system, SRB shift the equilibrium of the cathodic reaction toward producing more adsorbed hydrogen. If SRB was not present, the adsorbed hydrogen would remain inert, stopping the flow of electrons.
Figure 1 illustrates the various reactions in the Classical Theory, using iron as the reference metal.
Figure 1: Chemical Reactions Involved with the Classical Mechanism of SRB Corrosion [2]
The figure below shows a schematic of how and where these chemical reactions take place near a pipeline wall.
Figure 2: Schematic of the Classical SRB Mechanism [2]
Although the theory was groundbreaking and catapulted further investigation into SRB mechanisms, the Classical theory is now regarded as inaccurate due to a number of flaws. Firstly, hydrogenase has been shown to not be effective at removing hydrogen ions from solution. [3] Thus, the SRB cannot use the adsorbed hydrogen on the metal for its metabolic reactions. In addition, the ratio of corroded iron to iron sulfide in studies differs significantly from the 1:4 ratio suggested by Kuhr and Vlugt. [2]
Alternative Theories
Later theories of SRB mechanisms place more importance on corrosion caused by the byproducts of SRB, rather than reaction caused by SRB itself. [2]
One theory proposed by Miller and King in 1971 states that the iron sulfide produced by the SRB and the iron pipe material produced a galvanic cell, caused by localized corrosion, that also contributed to the rate of corrosion. [5] The proposed mechanism stated that depolarization was caused by the iron sulfide rather than the SRB. Hydrogen would be adsorbed onto FeS rather than Fe, and electrons transferred to the FeS would depolarize the atomic hydrogen into molecular hydrogen, thus allowing the SRB to use it via hydrogenase. [5] Thus, by associating the Fe/FeS electrochemical reaction as the main mechanism for SRB corrosion, Miller and King avoided the issue of hydrogenase's ineffectiveness with regards to atomic hydrogen. A diagram of the mechanism proposed by Miller and King can be found below. In addition to circumventing the issue with hydrogenase, the main reaction in this mechanism is the electrochemical reaction, which operates at a 1-to-1 ratio for Fe and FeS. In the mechanism proposed by Miller and King, the role of SRB was mainly to facilitate the regeneration of FeS, rather than the cathodic reaction itself. [2]
Figure 3: Schematic Representation of Miller and King's Mechanism [5]
The theory proposed by Miller and King was later slightly revised by Costello in 1975. While Costello considered that the cathodic depolarization of hydrogen through the Fe/FeS galvanic cell was correct, he proposed an alternative cathodic reaction. Instead of the dissociation of water producing hydrogen ions, Costello suggested that the cathodic reaction is: [2]
Where the hydrogen sulfide is produced from the reaction of sulfide produced by the SRB and water: [3]
Costello suggested that this cathodic reaction is more likely in the neutral pH, sulfide rich environments encountered in pipelines when SRB is present. [3] In this case, the production of hydrogen sulfide is only favoured in environments where high concentrations of sulfide are present. In these environments however, the amount of hydrogen sulfide is solution is much larger than the amount of hydrogen ions produced by the dissociation of water.
More recently, Hang proposed a radically different mechanism for SRB corrosion in 2003. Hang proposed that SRB extremely close to the pipe surface may withdraw electrons directly from the metal, in a process called electron pick-up. [2] These electrons are then used to reduce sulfates in the solution. In this case, SRB play a much more direct role in the corrosion process compared to the mechanisms proposed by Miller & King and Costello. In Hang's process, the SRB oxidize the pipe material directly. While the proposed mechanism is interesting and represents a significant shift in the role of SRB, Hang does not provide a detailed mechanism for the proposed electron pickup. In addition, unlike the cathodic depolarization theories, Hang's mechanism has not yet been scrutinized to a significant degree. [2]
In addition to the proposed mechanisms above, theories for the corrosion mechanism of SRB have also considered the role of elemental sulfur and acidic hydrogen sulfide in solution. In the former case, elemental sulfur can be formed by the SRB. A concentration cell with the elemental sulfur can then be formed. In the latter case, the acidic products generated by the reaction of sulfide with water can react directly with the pipe material. However, in both cases, the contribution towards the total rate of corrosion is minimal compared to the contribution of cathodic depolarization. [3]
[1] W. A. Hamilton. "Sulphate-reducing Bacteria and Anaerobic Corrosion," Annual Review of Microbiology, vol. 39, pp. 195-217, 1985
[2] R Javaherdashti, Microbiologically Influenced Corrosion: An Engineering Insight. London, United Kingdom: Springer London, 2008, pp. 49-57.
[3] P. Marcus, Corrosion Mechanisms in Theory and Practice, 2nd ed. New York, NY: CRC Press, 2002, pp. 750-754.
[4] S. Kakooei, M. C. Ismail and B. Ariwahjoedi. "Mechanisms of Microbiologically Influenced Corrosion: A Review," World Applied Sciences Journal, vol. 17, pp. 524-531, 2012
[5] R A. King and J. D. A. Miller. "Corrosion by the Sulphate-reducing Bacteria," Nature, vol. 233, pp. 491-492, Oct. 1971
Sulfate Reducing Bacteria
One particular group of microbes that frequently induce corrosion in pipelines in practice are sulfate reducing bacteria (SRB). [1] Although different strains of SRB exist that flourish under different conditions, this group of bacteria all perform similar chemical reactions involving the reduction of sulfate in the pipeline fluid. Due to the prevalence of these bacteria, many studies on microbial corrosion focus on SRB in detail, and some studies have been conducted solely on the corrosive abilities of SRB. [1] Through metabolic processes, SRB reduce this sulfate to sulfide, which corrodes the pipeline interior through a reduction/oxidation reaction. [2] Most sulfate reducing bacteria are anaerobic and thrive in oxygen-poor environments, such as within a biofilm. [2] In addition to reducing sulfate, different strains of this bacteria can also reduce other sulfur compounds such as sulfite or thiosulfate through different chemical reactions. [3]
Chemical Mechanism of Sulfate Reducing Bacteria
Although significant amounts of research have been performed on the corrosive effects SRB, the reaction mechanisms of SRB are still not fully understood. This lack of understanding is due to both the large number of different varieties of SRB as well as the fact that in a realistic environment, SRB will not be the only bacteria present and corrosion will be caused by all the microorganisms in the biofilm. [3] Thus, laboratory experiments cannot take these factors into account. Nevertheless, many theories have been proposed about SRB mechanisms and research is still being conducted to better understand SRB. Some of the most popular and influential proposed mechanisms are provided.
The Classical Theory
One the the first theories regarding the mechanism of SRB was proposed by VonWolzogen Kuhr and Van der Vlugt in 1934. In their paper, Kuhr and Vlugt provided evidence that a significant component of anaerobic corrosion was caused by SRB, and proposed a mechanism known as the Cathodic Depolarization Theory, which eventually was termed the Classical Theory of SRB. [3] Kuhr and Vlugt's research made significant advances on the role of SRB in microbially influenced corrosion, and many modern mechanisms are based upon this original theory. Although, the Classical Theory contains many flaws, it remains one of the most important mechanisms proposed for SRB due to its wide-reaching influence on modern theories. [2]
The Classical Theory states that SRB remove atomic hydrogen on the metal surface in the form of an enzyme called hydrogenase, in a progress termed depolarization by Kuhr and Vlugt. [3] The atomic hydrogen is created when water in the fluid briefly separates into its hydronium and hydroxide components. The hydronium ions then withdraw electrons from the metal and adsorb onto the metal surface. SRB then use the adsorbed atomic hydrogen is metabolic reactions through hydrogenase. The microorganisms reduce sulfate present in the solution using this hydrogenase to create sulfide and water. The produced sulfide then undergoes an electrochemical reaction with the metal, producing a metal sulfide. [4] This metal sulfide is structurally weaker, and reduces the strength of the pipe, causing sulfide stress cracking. [4] The metal sulfide can also splinter off the pipe. In addition to this reaction, the removal of hydrogen ions from solution also create hydroxide ions in solution. [2] These hydroxide ions can also undergo an electrochemical reaction with water to produce a weaker material. By removing adsorbed hydrogen from the system, SRB shift the equilibrium of the cathodic reaction toward producing more adsorbed hydrogen. If SRB was not present, the adsorbed hydrogen would remain inert, stopping the flow of electrons.
Figure 1 illustrates the various reactions in the Classical Theory, using iron as the reference metal.
The figure below shows a schematic of how and where these chemical reactions take place near a pipeline wall.
Although the theory was groundbreaking and catapulted further investigation into SRB mechanisms, the Classical theory is now regarded as inaccurate due to a number of flaws. Firstly, hydrogenase has been shown to not be effective at removing hydrogen ions from solution. [3] Thus, the SRB cannot use the adsorbed hydrogen on the metal for its metabolic reactions. In addition, the ratio of corroded iron to iron sulfide in studies differs significantly from the 1:4 ratio suggested by Kuhr and Vlugt. [2]
Alternative Theories
Later theories of SRB mechanisms place more importance on corrosion caused by the byproducts of SRB, rather than reaction caused by SRB itself. [2]
One theory proposed by Miller and King in 1971 states that the iron sulfide produced by the SRB and the iron pipe material produced a galvanic cell, caused by localized corrosion, that also contributed to the rate of corrosion. [5] The proposed mechanism stated that depolarization was caused by the iron sulfide rather than the SRB. Hydrogen would be adsorbed onto FeS rather than Fe, and electrons transferred to the FeS would depolarize the atomic hydrogen into molecular hydrogen, thus allowing the SRB to use it via hydrogenase. [5] Thus, by associating the Fe/FeS electrochemical reaction as the main mechanism for SRB corrosion, Miller and King avoided the issue of hydrogenase's ineffectiveness with regards to atomic hydrogen. A diagram of the mechanism proposed by Miller and King can be found below. In addition to circumventing the issue with hydrogenase, the main reaction in this mechanism is the electrochemical reaction, which operates at a 1-to-1 ratio for Fe and FeS. In the mechanism proposed by Miller and King, the role of SRB was mainly to facilitate the regeneration of FeS, rather than the cathodic reaction itself. [2]
The theory proposed by Miller and King was later slightly revised by Costello in 1975. While Costello considered that the cathodic depolarization of hydrogen through the Fe/FeS galvanic cell was correct, he proposed an alternative cathodic reaction. Instead of the dissociation of water producing hydrogen ions, Costello suggested that the cathodic reaction is: [2]
Where the hydrogen sulfide is produced from the reaction of sulfide produced by the SRB and water: [3]
Costello suggested that this cathodic reaction is more likely in the neutral pH, sulfide rich environments encountered in pipelines when SRB is present. [3] In this case, the production of hydrogen sulfide is only favoured in environments where high concentrations of sulfide are present. In these environments however, the amount of hydrogen sulfide is solution is much larger than the amount of hydrogen ions produced by the dissociation of water.
More recently, Hang proposed a radically different mechanism for SRB corrosion in 2003. Hang proposed that SRB extremely close to the pipe surface may withdraw electrons directly from the metal, in a process called electron pick-up. [2] These electrons are then used to reduce sulfates in the solution. In this case, SRB play a much more direct role in the corrosion process compared to the mechanisms proposed by Miller & King and Costello. In Hang's process, the SRB oxidize the pipe material directly. While the proposed mechanism is interesting and represents a significant shift in the role of SRB, Hang does not provide a detailed mechanism for the proposed electron pickup. In addition, unlike the cathodic depolarization theories, Hang's mechanism has not yet been scrutinized to a significant degree. [2]
In addition to the proposed mechanisms above, theories for the corrosion mechanism of SRB have also considered the role of elemental sulfur and acidic hydrogen sulfide in solution. In the former case, elemental sulfur can be formed by the SRB. A concentration cell with the elemental sulfur can then be formed. In the latter case, the acidic products generated by the reaction of sulfide with water can react directly with the pipe material. However, in both cases, the contribution towards the total rate of corrosion is minimal compared to the contribution of cathodic depolarization. [3]
[1] W. A. Hamilton. "Sulphate-reducing Bacteria and Anaerobic Corrosion," Annual Review of Microbiology, vol. 39, pp. 195-217, 1985
[2] R Javaherdashti, Microbiologically Influenced Corrosion: An Engineering Insight. London, United Kingdom: Springer London, 2008, pp. 49-57.
[3] P. Marcus, Corrosion Mechanisms in Theory and Practice, 2nd ed. New York, NY: CRC Press, 2002, pp. 750-754.
[4] S. Kakooei, M. C. Ismail and B. Ariwahjoedi. "Mechanisms of Microbiologically Influenced Corrosion: A Review," World Applied Sciences Journal, vol. 17, pp. 524-531, 2012
[5] R A. King and J. D. A. Miller. "Corrosion by the Sulphate-reducing Bacteria," Nature, vol. 233, pp. 491-492, Oct. 1971