Recent Advances in the Study of Microbiologically Influenced Corrosion
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RECENT ADVANCES IN THE STUDY OF MICROBIOLOGICALLY INFLUENCED CORROSION BRENDA LITTLE AND PATRICIA WAGNER Naval Research Laboratory Stennis Space Center, MS 39529-5004
ABSTRACT The study of microbiologically influenced corrosion (MIC) has progressed from phenomenological case histories to a mature interdisciplinary science including electrochemical, metallurgical, surface analytical, microbiological, biotechnological and biophysical techniques. With gene probes and microelectrodes it is now possible to measure interfacial dissolved oxygen, dissolved sulfide and pH and to further determine the microbial species responsible for the localized chemistry. Biofilms can be tailored to contain consortia of specific microorganisms and naturally occurring biofilms can be dissected into cellular and extracellular constituents. Scanning vibrating electrodes can be used to map the distribution of anodes and cathodes so that localized corrosion can be correlated with the location of microorganisms. The development of environmental scanning electron, atomic force, and laser confocal microscopy makes it possible to image cells on surfaces and to accurately determine the spatial relationship between microorganisms and corrosion. Transport of nutrients through biofilms is being modeled using techniques including optical density measurements to precisely locate the water/ biofilm interface and nuclear magnetic resonance imaging to visualize flow characteristics near surfaces colonized with microorganisms. The ways in which these new techniques can be used to understand fundamental mechanisms and to discriminate critical issues of MIC will be discussed.
Introduction Corrosion associated with microorganisms has been recognized for over 50 years, yet the study of MIC is a relatively new, multidisciplinary field [1, 2]. Several traditionally disparate disciplines have been combined to elucidate the complexities of microbial interactions with metal substrata. Some of the most important developments in MIC research have resulted from nondestructive surface analytical and electrochemical methods to characterize biofilms and electrochemical reactions.
Techniques for Determining Cellular Constituents Within Biofilms Culture techniques For many years, the standard for evaluating MIC has been the enumeration of sulfatereducing bacteria (SRB), either in bulk liquids or in surface deposits, using a liquid medium with sodium lactate as the carbon source [3, 4]. When SRB are present in the sample, sulfate is reduced to sulfide, which reacts with iron in solution to produce black ferrous sulfide. Blackening of the medium over a 28-day period signals the presence of SRB. Usually, 1 ml samples are injected by syringe into media bottles for 10-fold Mat. Res. Soc. Symp. Proc. Vol. 294. c)1993 Materials Research Society
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dilutions. It is assumed that only a single living bacterium is required to blacken a bottle. The simplest interpretation of test results is to consider that if one bottle is blackened, the sample contained at least one organism;
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