ABSTRACT

A newly developed multielectrode array sensor (MASS) was used to conduct a series of abiotic and biotic tests to determine if the probe can detect corrosion induced by microbial activity. The probe was able to determine the maximum corrosion rate in the presence of sulfate reducing bacteria (SRB) and showed that this rate was at least a factor of 10 greater than in the absence of SRBs. In addition, the corrosion rates obtained using the probe were much higher than those determined using linear polarization resistance further demonstrating its inherent better sensitivity to localized corrosion.

INTRODUCTION

Microbiologically influenced corrosion (MIC)is a problem in many engineering applications, including seacraft, process equipment and cooling water systems. MIC is usually manifested in the form of localized corrosion, and like abiotic localized corrosion, it tends to be catastrophic in effect. Of concern in many applications (e.g., process streams, cooling water systems, pipelines) is monitoring for corrosion and in particular MIC. Monitoring enables the operator to determine if the mitigation schemes, such as corrosion inhibitors and biocide applications, are effective. Furthermore, monitoring can provide an indicator to establish the dosing schedule and importantly can serve as a gage of system integrity. That is, the monitoring scheme can focus on microbial activity as well as corrosion activity. The ideal situation would be to have a method capable of monitoring the corrosion rate and corrosion mode (particularly localized corrosion) that could sensitively monitor system integrity and the net effects of inhibitors and biocides.

Several methods have been employed to monitor corrosion processes in various process stream-type applications, including electrochemical noise (EN), linear polarization resistance (LPR), and electrical resistivity probes (ER). Each of these methods has been utilized to some degree of success in various situations. EN offers an advantage over LPR and ER in that it may be sensitive to localized corrosion, as might occur in the presence 1 of microorganisms. However, EN has been shown in some cases to not detect pitting. This observation can be easily explained by considering two pits that form nearly simultaneously on each electrode such that there is no net current exchange between them. A slightly different approach to monitoring specifically for MIC is used in the BioGeorge probe and has shown the potential for determining the presence of a biofilm and the effects of 24 subsequent antimicrobial remedial actions -. Though this approach provides valuable information, just because a biofilm is detected, however, does not mean that MIC is occurring or that MIC is even a risk. Considering the ubiquitous nature of microbes, it is easy to misinterpret corrosion processes as MIC based solely on the detection of microbial activity as demonstrated recently for double-hulled transport barges. 5

To mitigate some of the possible limitations of currently available localized corrosion monitoring tools, we have developmented a Multielectrode Array Sensor System (MASS). This sensor has been successfully used to monitor corrosion processes in both laboratory tests and industrial process stream applications. 6-9 The MASS probe consists of multiple miniature electrodes made of metals to be studied (Figure 1). The miniature electrodes were coupled together by connecting each of them to a common joint through independent resistors, with each electrode simulating an area of a corroding metal. In a localized corrosion environment, anodic currents flow into the more corroding electrode, and cathodic currents flow out of the less or non-corroding electrodes. These cu

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