Monitoring of anaerobic microbially influenced corrosion via electrochemical frequency modulation

Electrochemical frequency modulation (EFM) is a rather novel technique to monitor corrosion rates in situ, providing direct access to Tafel slopes with only minor polarization. So far EFM has been applied mainly for electrochemical corrosion monitoring in purely chemical corrosion systems with predominant uniform corrosion, but it has also great potential for monitoring microbially influenced corrosion (MIC). Thus, in this study MIC induced by different cultures of sulfate-reducing bacteria (SRB) was monitored with EFM, and compared to results from linear polarization resistance (LPR) and measurements of the free corrosion potential Ecorr. The electrochemical results obtained in small scale bioreactors with a three electrode setup were complemented by chemical analysis and SEM observations. SRB featuring high corrosion rates were compared to non-corrosive but still H2-consuming control SRB, sterile controls and abiotic sulfide corrosion controls. Corrosion rates determined with EFM and LPR were accurate and precise in all cases as long as the system was based on uniform corrosion. However with the onset of localized corrosion both methods fail to predict accurate rates. Unlike other electrochemical corrosion rate measurements, EFM has the advantage to provide indicating ratios in the current response at different frequencies, so-called causality factors, which allow identifying changes in the corrosion system and therefore assessing the accurateness of the obtained results. As a consequence EFM is a promising electrochemical corrosion monitoring technique for MIC in fundamental and applied studies.