Microbially induced corrosion (MIC) is a complex problem that affects various industries. Several techniques have been developed to monitor corrosion and elucidate corrosion mechanisms, including microbiological processes that induce metal deterioration. We used zero resistance ammetry (ZRA) in a split chamber configuration to evaluate the effects of the facultatively anaerobic Fe(III) reducing bacterium Shewanella oneidensis MR-1 on the corrosion of UNS G10180 carbon steel. We show that activities of S. oneidensis inhibit corrosion of steel with which that organism has direct contact. However, when a carbon steel coupon in contact with S. oneidensis was electrically connected to a second coupon that was free of biofilm (in separate chambers of the split chamber assembly), ZRA-based measurements indicated that current moved from the S. oneidensis-containing chamber to the cell-free chamber. This electron transfer enhanced the O2 reduction reaction on the coupon deployed in the cell free chamber, and consequently, enhanced oxidation and corrosion of that electrode. Our results illustrate a novel mechanism for MIC in cases where metal surfaces are heterogeneously covered by biofilms.

Minimizing contamination of control treatments in microbiologically influenced corrosion (MIC) studies is of critical importance. Metal sterilization procedures should not alter the surface nor affect the inherent susceptibility of the metal to corrosion while adequately deactivating biological activity. However, there is no consensus in the literature regarding such procedures due to, in part, the lack of a universally accepted methodology. This investigation evaluates various sterilization methods for carbon steel concerning practicality, efficacy, and effects on the electrochemical response of the metal. Three sterilization procedures using i) dry heat, ii) ethanol, or iii) glutaraldehyde as sterilizing agents were evaluated. Even though all sterilization approaches were equally effective in eliminating microorganisms and spores from the metal surface, dry heating at 170 • C in an inert atmosphere was identified as the most convenient sterilization method regarding practicality and consistency in the electrochemical response of the metal. Sterilization of carbon steels in 75 vol% ethanol and glutaraldehyde, as well as alcohol followed by flaming, is discouraged given the large dispersion in corrosion response caused by the exposure to the sterilization media. Microbiologically influenced corrosion (MIC) studies deal with the role microorganisms have on electrochemical processes leading to corrosion.1-3 As bacteria and fungi interact with the metal surface and its environment, they are able "to initiate, facilitate or accelerate the corrosion reaction without changing its electrochemical nature."4 The ubiquitous nature of some microorganisms can present a challenge to MIC studies, where contamination with species of diverse metabolic capabilities can alter the result of the experiments. Therefore, minimizing contamination from foreign microorganisms to maintain a microbial community that properly reflects the desired microbial composition (i.e. these being an environmental sample or a specifically defined community) is of critical importance in MIC research.Most of the components used in an MIC study (i.e. glassware, reactors or electrochemical cells, and solutions) can be sterilized following standard sterilization procedures 5 used by microbiologists; however, there is no consensus regarding sterilization procedures for metal samples in the literature. In the ideal situation, the sterilization methodology should kill all microorganisms and spores on the metal, but it should not alter its surface nor affect the inherent susceptibility of the metal on corrosion.In a literature survey of over 200 papers dealing with MIC of carbon and low alloy steels, there were over 20 different procedures to sterilize metal samples, Table I. Some of the methods described in the literature range from autoclaving steel stubs inside of watertight containers, submerging the stubs in 70 vol% ethyl alcohol and flaming them before inoculation, 6 to immersion in a 2000 ppm chlorine solution (i.e. approximately 4000 times the residual...

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Crevice corrosion affects the integrity of stainless steels used in components exposed to seawater. Traditionally, crevice corrosion testing involves the use of artificial crevice formers to obtain a critical crevice potential, which is a measure of the crevice corrosion resistance of the alloy. The critical acidification model proposed by Prof. J.R Galvele predicts that the critical crevice potential is the minimum potential required to maintain an acidic solution with a critical pH inside either a pit or a crevice. Application of Galvele's model requires an estimation of both the diffusion length and the i vs. E behavior of the metal in the solution inside the crevice. In this work, the crevice corrosion resistance of a 22% Cr duplex stainless steel (UNS S31803) and a 25% Cr super duplex stainless steels (UNS S32750) was investigated. The i vs. E response of the two stainless steels was determined in acidified solutions of various chloride concentrations, which simulate those found in an active crevice. Critical potentials predicted by the critical acidification model were compared with critical crevice potentials measured in simulated seawater. Results showed that despite the various assumptions and simplifications made by Galvele, the model correctly predicted the occurrence of crevice corrosion of both UNS S32750 and UNS S31803 close to room temperature in a 3.5 wt.% NaCl environment. Critical potentials obtained by Galvele's model were similar if assuming that the chloride concentration of the simulated crevice solutions was between 7 M and 12 M acidified to a pH of 0.