How does redox potential (Eh) influence microbiologically influenced corrosion (MIC) in oil pipelines?

Redox potential (Eh), also known as oxidation-reduction potential, is a measure of the tendency of a chemical species to acquire electrons and be reduced. In the context of microbiologically influenced corrosion (MIC) in oil pipelines, Eh significantly influences the activity and types of microorganisms that can thrive and contribute to corrosion. A lower (more negative) Eh indicates a reducing environment, while a higher (more positive) Eh indicates an oxidizing environment. MIC is often associated with anaerobic (oxygen-free) conditions, which typically have a lower Eh. Sulfate-reducing bacteria (SRB), for example, are a common group of microorganisms involved in MIC and thrive in reducing environments. SRB reduce sulfate (SO42-) to hydrogen sulfide (H2S), which is highly corrosive to steel. A low Eh facilitates the activity of SRB by providing the necessary reducing conditions for their metabolic processes. Conversely, oxidizing conditions (higher Eh) may favor the growth of other types of microorganisms, such as aerobic bacteria or iron-oxidizing bacteria (IOB). While SRB are typically associated with anaerobic conditions, they can sometimes create micro-environments with very low Eh even within a generally aerobic system. Pitting corrosion, a common form of MIC, often occurs because the environment inside a pit becomes anaerobic and has a low Eh, allowing SRB to flourish. The Eh of the environment in the pipeline, therefore, dictates the types of microbial communities present and their potential to cause corrosion. Monitoring and controlling Eh can be a key strategy in mitigating MIC. For example, injecting oxidizing biocides can raise the Eh and inhibit the growth of SRB. However, it's crucial to consider the specific microbial ecology of the pipeline system, as altering the Eh might inadvertently promote the growth of other corrosive microorganisms.