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Investigation of the Corrosion Mechanism for Sulfate Reducing Bacteria (SRB) Using a Split-Chamber Zero Resistance Ammetry Technique

Microbiologically influenced corrosion (MIC) is one of the most aggressive forms of corrosion leasing to infrastructure and equipment damage in various industries, including oil and gas, water systems, medical devices, marine environments, nuclear waste storage facilities, and aviation fuel systems and storage. During the last 10 year, PHMSA estimates that MIC has caused 503 internal corrosion incidents at a reported property damage of $188 million and a loss of 53,000 barrels of oil. Some common bacteria associated with MIC are sulfate-reducing bacteria (SRB), iron and CO2 reducing bacteria and iron and manganese oxidizing bacteria. SRB are generally considered the most aggressive group of bacteria in pipeline systems that causes MIC and pitting, especially of carbon steel in the oil and gas industry. SRB are facultative anaerobes and thrive in anoxic environments, using sulfate as a terminal electron acceptor and producing hydrogen sulfide (H2S) as a metabolic byproduct. Furthermore, SRBs also can reduce both nitrate and thiosulfate and obtain their energy from organic nutrients, such as lactate.
 Electrochemical techniques to monitor for MIC focus on studying the electrochemical characteristics of the interface or mass transport properties of a system that are modified by the microbiological activities. Polarization sensors, such as the BIOX system or BioGeorge, use polarization based on a galvanic couple between a stainless steel electrode and a sacrificial anode. The measured galvanic current is proportional to biofilm that has grown on the electrode surface. Other electrochemical sensors use electrochemical impedance spectroscopy (EIS) or amperometry to measure biofilm thickness by comparing the electrochemical signatures of a reference channel (without bacteria) to a measurement channel (exposed to bacteria); while sensors based on electrochemical resistance use linear polarization measurements to determine the amount of biofilm on an electrode surface. Electro hydrodynamical impedance has also been used to measure the diffusion coefficient in a biofilm and correlate to biofilm growth and thickness. While these approaches can accurately predict the presence and/or thickness of a biofilm on a metal surface, they cannot determine the risk of MIC associated with biofilm formation, as the presence of a biofilm does not necessarily mean that a surface experiences MIC. Additionally, many of the techniques are destructive or require visual examination of the surface after analysis.
 This work presents a split-chamber zero resistance ammetry (SC-ZRA)-based approach to overcome the limitations to MIC monitoring described above and serve as a screening system to determine the risk of MIC associated with certain microorganisms or groups of microorganisms. Previous work using a split-chamber approach to assess MIC was used by Daumus for the study of stainless steel corrosion in the presence of sulfate reducing bacteria, and subsequently used by Miller et al to evaluate MIC under aerobic, Fe (III)- and nitrate-reducing conditions. In this approach, two identical electrochemical cells (chambers) are separated by an ion-transport membrane. Each chamber contains an identical electrode of the same material which are electrically connected through a zero-resistance ammeter. When one of the chambers is inoculated with microorganisms, the galvanic current between the two electrodes is measured through the zero ammeter. This configuration mimics the microbiologically induced development of localized anodic and cathodic patches on a metal surface that leads to corrosion. The flow of electrons (difference in corrosion current between the two chambers) depends solely on the microbiological activities of bacteria in one of the two chambers, so the influence of the microorganisms on MIC, as well as the extent of corrosion, can be quantified.
 Using the SC-ZRA technique we were able to characterize the mechanism and electrochemical signatures of SRB corrosion. Specifically, we found that in split-chamber incubations containing an electron donor, electrons flow from the inoculated to the uninoculated chambers. While the current direction could be interpreted as electron transfer such as in a microbial fuel cell, these systems deploy inert graphite electrodes. When used for MIC characterization, the SC-ZRA uses reactive carbon steel electrodes. Indeed, when positive current was detected, greater corrosion was detected on WE1, which is consistent with redox couples, as well as previous work to characterize MIC using SC-ZRA measurements. After depletion of an electron donor, SRB uses electrons from the metals surface as an electron donor reversing the flow of electrons from the uninoculated to the inoculated chamber. In future work, this technique can be used to provide a mechanistic understanding and a monitoring tool for corrosion of metals that are exposed to SRB under a variety of redox regimes.