(179n) Microbial Inhibition of Corrosion-the Role of Biofilms

In the United States alone, the cost of corrosion (both money spent to stop it, and money spent to repair it) is estimated to be as much as $364 billion per year. To put this in perspective, the amount spent each year is roughly equivalent to the cost of the damage done by Hurricane Katrina. The problem is widespread, affecting systems ranging from water sprinklers to oil well rigs, airplanes, and fuel storage tanks. It is commonly believed microorganisms increase the corrosion rate, but recent work has shown that microorganisms can inhibit corrosion in various metals, including iron, copper, and aluminum. Recent investigations in microbiologically influenced corrosion inhibition (MICI) have produced models: bacteria neutralizing the corrosive reactants and bacteria forming a protective film on the metal surface.

The mechanism through which the bacteria alter the corrosion kinetics in decreasing the corrosion rate is not well understood. The thermodynamics of MICI is determined by the microenvironments on the metal surfaces, biofilm dynamics, and media interfaces. That is, because of microbial activity changing pH, Eh, and other important variables, the microbes can greatly alter the kinetics of the system. Understanding how each variable is microbially affected requires a clear knowledge of the metabolic and electron transfer activities of relevant microbes. Hence, in this study a well characterized model organism Shewanella oneidensis MR-1 was used to understand relevant mechanisms, especially the role of biofilm formation on the metal surface.

A galvanic cell was set up with an aluminum rod as the anode and a copper rod as the cathode. An Ag/AgCl was used as the reference electrode was used for all the experiments. The electrolytes used were PIPES buffered (50mM, pH 7.0) Luria-Bertani media or PIPES buffer (50mM, pH 7.0) with lactate as carbon source. The cells were maintained under aerobic or anaerobic conditions with air or nitrogen being flushed to maintain conditions. To understand the importance of biofilm in corrosion inhibition, MR-1 a good biofilm forming bacteria and a poor biofilm forming mutant variant of MR-1, ΔpilD, were used. Non-destructive electrochemical techniques such as potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) were employed to study corrosion reaction kinetics. Planktonic cell counts were performed at the beginning and at the end of the experiments. The exposed metal surfaces were analyzed using scanning electron microscopy (SEM) and deep ultra-violet microscopy, a non destructive method eliminating the need for sample processing or fixing.

The power output of the cell increased significantly under aerobic conditions when compared to the anaerobic condition. Under aerobic conditions, the polarization resistance (Rp) i.e. corrosion resistance of the MR-1 cell increased. Unexpectedly running the cell under anaerobic conditions had the opposite effect on Rp.

Bacterial-conditioned media of MR-1 (spent media, bacteria removed by centrifugation) also exhibited some corrosion inhibition properties compared to sterile media indicating that compounds synthesized by the bacteria present in the media might offer protection to the metal electrodes, even in the absence of biofilms.

Also in order to identify the compounds that potentially offer protection to the metals, experiments are being carried out with PIPES minimal media, instead of a rich PIPES buffered LB media. Experiments with the ΔpilD (biofilm formation mutants) are also being carried out and these experiments should help us understand the importance of biofilms in protecting the metal from corrosion.