Year 2003 and its importance in understanding Microbial Corrosion (MIC)

Microbiologically influenced corrosion (MIC) as a phenomenon was known since mid-19th century. However, the first attempt to explain it in terms of electrochemistry came in 1930s:?this attempt was named "Classical Theory"" or "Cathodic depolarisation theory-CDT" and it was proposed by VonWolzogen Kuhr and Van der Vlugt in 1934. Main features of CDT were (1) according to this theory, the Bacteria (SRB) use the cathodic hydrogen through consumption of an enzyme called hydrogenase. Thus, main probable effect of SRB on corrosion is the removal of hydrogen from the metal surface by means of hydrogenase and catalysing the reversible activation of hydrogen. In addition to not being able to explain the corrosion rates observed in the field, it was- at the?best-suitable to explain corrosion by SRB. The classical theory had a lot of drawbacks that made it unfit to explain MIC. Alternative theories then emerged: Galvanic Corrosion between the iron sulphide film generated via SRB and underlying steel (Stumper, 1923), mixed effect of hydrogenase and the FeS-steel galvanic effect (Miller and King, 1971), replacing FeS with hydrogen sulphide as the cathodic reactant (Costello, Mid-1970s) and the existence of a corrosive volatile phosphorous metabolite leading to observed high corrosion (Iverson, 1998).

?We need to also explain Gu–Xu Model that was proposed by these Chinese researchers in their paper ““Demystifying MIC mechanisms.” Paper No. 10213, Corrosion 2010, NACE International, Houston, TX, 2010. The heart of this model is a new theory about MIC that has been branded by these authors as “biocatalytic cathodic sulphate reduction” (BCSR) theory.

In their 2009 paper, Gu et al., they describe their mechanistic model based on features such as charge transfer resistance and mass transfer resistance. In addition, it should also be noted that this model is a “bacteria-specific” model and thus cannot explain all various types of MIC. In addition, the model’s main factor is hydrogenase-positive SRB, and therefore, the possible role of hydrogenase-negative SRB in enhancing corrosion (such as hydrogenase-negative Desulfococcus multivorans is overlooked.

?All the above try to put the blame on the chemistry initiated and sustained by the bacteria and not directly the bacteria itself, we may brand these theories as Chemical MIC (CMIC).

Nonetheless, it was in 2003 that a PhD student, D. T. Hang, in Germany along with a research team consisting of him and F. Widdel and H. Cypionka, found the first evidence for direct interaction of SRB and the metal to facilitate direct electron pick-up (Hang et al.,2003). This theory explains the very high corrosion rates observed in practice by electron exchange between the bacteria (not just SRB) and the metal. In fact, it postulates that the bacteria take electron directly from the metal via developing nanowires and through three types of mechanisms collectively known as “External Electron Transfer”. This and similar theories are referred to as Electrical MIC (EMIC) and if MIC is happening via EMIC, very high corrosion rates observed in the field can be explained.

Year 2003 marked a leap forward in the mechanistic description of MIC. We have explained and reviewed these theories in two of our publications below:

Reza Javaherdashti, Chikezie Nwaoha, Henry Tan “Corrosion and Materials in Oil and Gas Industries”, published by CRC Press/Taylor&Francis, USA, 2013.

Reza Javaherdashti, “Microbiologically influenced corrosion-an engineering insight”, Springer-Verlag, UK, 2nd Edition, 2017.

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