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Electrochimica Acta 52 (2007) 7670–7680

Review article

The interaction of bacteria and metal surfaces

Florian MansfeldCorrosion and Environmental Effects Laboratory (CEEL), The Mork Family Department of Chemical Engineering

and Materials Science, University of Southern California, Los Angeles, CA 90089-0241, USA

Received 9 August 2006; received in revised form 7 May 2007; accepted 7 May 2007Available online 13 May 2007

bstract

This review discusses different examples for the interaction of bacteria and metal surfaces based on work reported previously by various authorsnd work performed by the author with colleagues at other institutions and with his graduate students at CEEL. Traditionally it has been assumed thathe interaction of bacteria with metal surfaces always causes increased corrosion rates (“microbiologically influenced corrosion” (MIC)). However,

ore recently it has been observed that many bacteria can reduce corrosion rates of different metals and alloys in many corrosive environments.or example, it has been found that certain strains of Shewanella can prevent pitting of Al 2024 in artificial seawater, tarnishing of brass and rustingf mild steel. It has been observed that corrosion started again when the biofilm was killed by adding antibiotics. The mechanism of corrosionrotection seems to be different for different bacteria since it has been found that the corrosion potential Ecorr became more negative in the presencef Shewanella ana and algae, but more positive in the presence of Bacillus subtilis. These findings have been used in an initial study of the bacterialattery in which Shewanella oneidensis MR-1 was added to a cell containing Al 2024 and Cu in a growth medium. It was found that the powerutput of this cell continuously increased with time. In the microbial fuel cell (MFC) bacteria oxidize the fuel and transfer electrons directly tohe anode. In initial studies EIS has been used to characterize the anode, cathode and membrane properties for different operating conditions of a

FC that contained Shewanella oneidensis MR-1. Cell voltage (V)—current density (i) curves were obtained using potentiodynamic sweeps. Theurrent output of a MFC has been monitored for different experimental conditions.

2007 Published by Elsevier Ltd.

eywords: Microbiologically influenced corrosion (MIC); Corrosion inhibition; Bacteria; Bacterial battery; Microbial fuel cell

ontents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76702. Examples for the interaction of bacteria and metal surfaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7671

2.1. Microbiologically influenced corrosion (MIC) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76712.2. Microbiologically influenced corrosion inhibition (MICI) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76722.3. The bacterial battery . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7676

2.4. The microbial fuel cell (MFC) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7678

3. Summary and conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7679. . .. . .

Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. Introduction

Traditionally it has been assumed that the interaction of bac-

eria and metal surfaces always results in increased corrosion

E-mail address: mansfeld@usc.edu.

013-4686/$ – see front matter © 2007 Published by Elsevier Ltd.oi:10.1016/j.electacta.2007.05.006

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7680

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activity. The term microbiologically influenced corrosion (MIC)is usually interpreted as to indicate an increase in corrosion ratesdue to the presence of bacteria that accelerate the rates of the

anodic and/or cathodic corrosion reaction, while leaving the cor-rosion mechanism more or less unchanged. The possibility ofcorrosion inhibition caused by microorganisms has rarely beenconsidered. Videla [1] has presented a short summary of the

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F. Mansfeld / Electrochim

iterature concerning corrosion inhibition by bacteria. In thiseview different examples for the interaction of bacteria andetal surfaces will be presented that are based on work reported

reviously by various authors and work performed by the authorith colleagues at other institutions and with his graduate stu-ents at CEEL.

Researchers at the University of Connecticut, University ofouthern California and the University of California at Irvineave evaluated the concept of corrosion control using regener-tive biofilms (CCURB) for a variety of materials such as Al024, mild steel and cartridge brass in laboratory tests [2–8]s well as field tests [9]. Recent results have shown that twotrains of Shewanella produced corrosion inhibition of Al 2024,rass and mild steel in artificial seawater (AS) [10]. Al 2024 isery susceptible to pitting corrosion in seawater, however, it haseen found that a number of microorganisms are able to preventitting of Al 2024 in AS. These results suggest that microbiolog-cally induced corrosion inhibition (MICI) is a more commonhenomenon than was previously assumed. It has been shownhat MICI occurs only in the presence of a live biofilm. The con-ept of the bacterial battery has been evaluated recently and itas been found that the power output of this battery increasedontinuously for several weeks [11]. Finally, various types oficrobial fuel cells (MFC) have been proposed in which bacte-

ia are assumed to oxidize the fuel and transport electrons to theuel cell anode [12].

. Examples for the interaction of bacteria and metalurfaces

.1. Microbiologically influenced corrosion (MIC)

The interaction of bacteria and metal surfaces results in theormation of biofilms in a process known as biofouling. MICas been reported in the chemical processing, oil and gas, andower generation industries in a wide variety of environments.cid producing bacteria have been found to be the main causef MIC of carbon steels. One of the first studies of MIC involvedulfate-reducing bacteria (SRB) that thrive only under anaerobiconditions and are found widespread in many waters and soils.RBs easily reduce inorganic sulfates to sulfides in the pres-nce of hydrogen or organic matter and are aided in the processy the presence of an iron surface. von Wolzogen Kuehr [13] in923 proposed the so-called cathodic depolarization mechanismhich assumes that the SRBs remove atomic hydrogen from the

ron surface which causes accelerated corrosion of iron. Thealidity of the mechanism seems questionable since the concen-ration of atomic hydrogen is extremely small in the pH range of.5–8.5. Little et al. [14] have given examples for MIC and alsoave described the use of different experimental techniques forhe evaluation of biofilm formation and MIC.

No other phenomenon has probably fascinated those study-ng MIC more than ennoblement, i.e. the increase of the

pen-circuit potential Ecorr due to the formation of a biofilm.nnoblement has been mainly observed for stainless steels (SS)xposed to natural seawater (NS). Mansfeld and Little [15] haveeviewed experimental results and various attempts to explain

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ta 52 (2007) 7670–7680 7671

he observed ennoblement phenomena. Early explanations usedhermodynamic arguments that suggested that the reversibleotential E0 of the oxygen electrode increased in the presencef biofilms either due to an increase in the partial pressure ofxygen or a decrease of the surface pH. Since E0 increases onlyery slightly with an increase of oxygen pressure and since it isnlikely that acidification would increase passivity, these expla-ations need to be rejected. Formation of H2O2 has also beenuggested as causing an increase of E0 [16].

The possibility that biofilms cause an increase of thexchange current density for oxygen reduction has apparentlyot been considered. Johnsen and Bardal suggested that enno-lement was due to a change of the cathodic properties of thetainless steels as a result of microbiological activity on theurface [17]. The very interesting explanation of ennoblementbserved for SSs in river water by Lewandowski et al. [18,19]nvolving formation of MnO2, which has an E0 close to thebserved ennobled Ecorr of SSs, deals with a special case of enno-lement. Linhardt [20,21] found large amounts of Mn mineralsmainly MnOOH and MnO2) on severely pitted stainless steelurbine blade runners in a hydroelectric plant and suggested thatitting was due to ennoblement caused by biomineralized Mnxide. The possibility that formation of a biofilm can decreasehe passive current density which would also lead to ennoble-

ent has not been considered [22]. One of the few exceptionss the suggestion by Eashwar et al. [23] that ennoblement oftainless steels in seawater is due to the production of inhibitorsy bacteria that are retained in the biofilm matrix. An impor-ant observation, which has not been explained, is the fact thatcorr for SSs exposed to NS can exceed the pitting potential Epiteasured in sterile seawater without pitting to occur. This result

ould be due to formation of inhibiting species by the biofilm asuggested by Eashwar et al. [23] or to a reduction of the chlorideoncentration at the SS surface that is covered by the biofilm.

Little and Mansfeld [22] have discussed the general topic ofassivity of stainless steel (SS) in natural seawater with empha-is on the various attempts to formulate a mechanism for thebserved phenomenon of ennoblement. They suggested that theo content of stainless steel may play a role since in previous

tudies [17] SSs with higher Mo content showed more posi-ive values of Ecorr. Little and Mansfeld observed that Ecorr for54 SMO stainless steel with 6% Mo was about 300 mV moreositive than that of Ti grade 2 after exposure the Pacific Oceanater for 1 year [24]. They observed that the impedance for these

wo materials did not change with time despite formation of aiofilm and suggested that the porous and water-like structuref the biofilm did not produce the characteristic changes in thempedance spectra that result from the application of protectiveolymer coatings [25].

Mansfeld [26] exposed samples of SS 316L in NS at Keyest, FL for periods up to 6 months. Samples were exposed

t three different times of the year in the time period betweenay 1995 and August 1996 to determine whether seasonal vari-

tions affect the degree of ennoblement. As shown in Fig. 1corr reached values between 300 and 400 mV versus Ag/AgCl

n about 2 months. Despite the ennoblement the impedancepectra did not change with time with more or less constant

7672 F. Mansfeld / Electrochimica Acta 52 (2007) 7670–7680

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ig. 1. Time dependence of Ecorr for SS 316L exposed to NS at Key West, FL.

alues of the polarization resistance Rp and electrode capaci-ance Cdl (Fig. 2). These results can be explained by assuminghat while Ecorr increased, it remained in the passive region withconstant value of the passive c.d. ipass. Ennoblement was con-

idered to be due to an increase of the rate of oxygen reductiony a yet unknown mechanism. Chemical analysis with variousurface analytical techniques did not show any evidence of theeposition of manganese oxide compounds.

Little and Mansfeld [27] exposed polymer coated steel sam-les with intentional defects to NS and observed that a largeumber of bacteria were present on the rust layers of sampleshat were exposed at their natural Ecorr, while no bacteria wereound for samples that were cathodically protected by connec-ion to a piece of zinc. The effect of applied potential on biofilmormation is being evaluated in more detail at present becausef its potential significance in the prevention of MIC as wells corrosion of metallic implants and resulting infection in theuman body.

.2. Microbiologically influenced corrosion inhibitionMICI)

Corrosion inhibition due to the formation of biofilms has beenbserved for different materials exposed to corrosive environ-ents in the presence of different bacteria [2–8]. In the following

he concept of MICI will be illustrated for Al 2024, mild steelnd brass exposed to artificial seawater (AS) [28].

Al 2024-T3 (UNS A92024) was exposed to AS prepared asataanen nine salts solution ((VNSS), pH 7.5) with and withoutgrowth medium that was a mixture of peptone, starch, glu-

ose and yeast extract. Electrochemical impedance spectroscopyEIS) data were used to evaluate the corrosion behavior of Al024 exposed to AS with and without bacteria. Impedance mea-

urements were made with a model PCI4/300 system (Gamrynstruments) using a three-electrode cell. For tests with S. algaer S. ana the electrochemical cell consisted of two identicallectrodes with an exposed area of 4.0 cm2 for each electrode

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ig. 2. Time dependence of Rp (a) and Cdl (b) for SS 316L exposed to NS atey West, FL.

nd a saturated calomel electrode (SCE) as a reference elec-rode. A sinusoidal voltage signal of 10 mV was applied in arequency range of 105 to 10−3 Hz. For tests with S. algae anodicnd cathodic polarization curves were obtained at a scan rate of.6 V/h after recording of EIS data for 7 days. The two electrodessed for collecting EIS data were used separately to obtain thenodic and cathodic polarization curves, respectively.

The tests in sterile AS and in the presence of Bacillus subtilisere performed by Prof. T. K. Wood and co-workers at the Uni-ersity of Connecticut. Their experimental approach has beenescribed elsewhere [2–8]. For tests in abiotic AS the impedanceata for Al 2024 had the typical shape observed previously whenitting occurred [2–5]. Fig. 3 shows experimental impedancepectra obtained for Al 2024 during exposure to AS for 30 days.nly four of the spectra collected during this time are shown

n the Bode plots of Fig. 3. The spectra suggest that pitting

ccurred during the entire test period as evidenced by the typi-al low-frequency minimum of the phase angle which is partiallyasked by the scatter of the data points below 0.01 Hz. Never-

heless, the spectra in Fig. 3 are in agreement with the pitting

F. Mansfeld / Electrochimica Acta 52 (2007) 7670–7680 7673

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Fig. 5. Bode plots for Al 2024 exposed to AS containing S. ana; addition ofkanamycin after 7 days.

Fig. 3. Bode plots for Al 2024 exposed to AS for 30 days.

odel proposed by Mansfeld et al. [29,30]. Qualitatively it cane observed that the polarization resistance of active pits Rpit,hich is close to the impedance value |Z| at the frequency min-

mum at low frequencies, increased with increasing exposureime as the pit growth rate decreased.

S. algae and S. ana are classified under the group knowns iron reducing bacteria. Both strains are able to grow in sea-ater under aerobic and anaerobic conditions. S. algae or anaere added to VNSS containing growth medium and EIS dataere collected for 1 week. The spectra in Fig. 4a and b which

re typical for those found for passive metals demonstrate thatoth strains provided excellent corrosion protection for Al 2024n AS. After 1 week exposure time 200 �g/ml kanamycin wasdded to the solution containing S. ana to kill the bacteria andIS data were obtained for another week or longer. After addition

f kanamycin the impedance spectra did not change immedi-tely (Fig. 5). Significant changes in the spectra indicating thatitting had been initiated were observed in the low-frequencyegion only after total exposure for 15 days (Fig. 5).

Fig. 4. Bode plots for Al 2024 exposed to AS containing Shewanella algae (a) and Shewanella ana (b).

7674 F. Mansfeld / Electrochimica Acta 52 (2007) 7670–7680

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ig. 6. Time dependence of Ecorr (a), Rp and Rpit (b) for Al 2024 exposed to ASontaining S. algae or S. ana.

Fig. 6 shows the time dependence of Ecorr, the polarizationesistance Rp of passive surfaces and the polarization resistancepit of growing pits for Al 2024 in AS and AS containing S.lgae or ana. Also shown are results obtained after addition ofanamycin which is an antibiotic. In the presence of the bacte-ial strains Ecorr was more negative than in the abiotic solutionFig. 6a). After addition of kanamycin an immediate increase ofcorr was observed and the values of Ecorr observed in the abi-tic solution were reached after about 10 days exposure time.ue to passivation in the presence of the bacteria Rp was much

arger than Rpit (Fig. 6). Despite the immediate increase of EcorrFig. 6a) Rp remained at the levels observed before the additionf kanamycin for several days (Fig. 6b). Rpit reached similaralues as those determined in abiotic AS only after 1 week fol-owing the addition of kanamycin, indicating that active pits hadtarted to grow again (Fig. 6b). As can be seen from Fig. 6 theharp drop of Rp occurred at the time when Ecorr reached orxceeded the values observed in the sterile solution, where Ecorrquals the pitting potential Epit.

Anodic and cathodic polarization curves were determinedsing the two electrodes for which the impedance spectra inS containing S. algae were obtained [10]. As shown in Fig. 7

he rate of oxygen reduction was drastically reduced, while noignificant changes in the rate of the anodic reaction were found.hese findings indicate that the observed prevention of pitting ofl 2024 in the presence of Shewanella was due to the decrease of

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ig. 7. Potentiodynamic polarization curves for Al 2024 after 7 days of exposureo AS containing S. algae or S. ana.

corr below Epit as a result of the decrease of the rate of oxygeneduction.

Similar tests were performed for cartridge brass and mild steel10]. As shown in Fig. 8 S. ana produced a larger decrease of EcorrFig. 8a) and a much larger increase of Rp (Fig. 8b) than S. algaeor brass. After addition of kanamycin Ecorr increased rapidlyFig. 8a), while Rp remained unchanged for at least another 3ays before it dropped to values similar to those found in abi-tic AS (Fig. 8b). Contrary to the results obtained for Al 2024nd brass Ecorr for mild steel was more positive in AS con-aining the bacteria than in sterile AS (Fig. 9a). No significanthanges of Ecorr for mild steel were observed after the additionf kanamycin to AS containing S. ana. Rp remained unchangedor at least 3 days and then decreased to values comparable tohose found in sterile AS (Fig. 9b).

The polarization curves for brass were similar to thosebtained for Al 2024 (Fig. 7), however, for mild steel the rate ofhe metal dissolution was decreased and only a small decreasef the rate of the cathodic reaction was found [10]. As also dis-ussed by Dubiel et al. [31] the decrease of the corrosion rate ofild steel in the presence of Shewanella is due to a reduction of

he rate of both the anodic and the cathodic reaction.In the presence of B. subtilis pitting occurred in the first

days of exposure, however, after 3 days the spectra agreedith those for a passive surface, i.e. a simple one-time-constant

F. Mansfeld / Electrochimica Acta 52 (2007) 7670–7680 7675

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oEcbwpiformation of a stable biofilm is needed to stop growth of activepits.

The importance of live biofilms has been evaluated in moredetail for Al 2024 exposed to sterile AS or sterile AS con-

ig. 8. Time dependence of Ecorr (a) and Rp (b) for brass in AS containing S.lgae or S. ana.

odel in which Rp is in parallel with the electrode capacitance(Fig. 10). The fairly high values of Rp, which approached

he M� cm2 range, suggest that pits formed in the initial stagesf exposure have become passivated. Very similar results werebtained in the presence of B. subtilis producing polygluta-ate or polyaspartate [2–5]. The increased Rp values suggest

hat the inhibitors produced by the bacteria provided additionalorrosion protection.

Fig. 11 illustrates the time dependence of the relative cor-osion rates expressed as 1/Ro

pit for the tests in the absence ofacteria (test #45) and 1/Ro

p for the tests in the presence of. subtilis (tests #42–44). These values have been obtained byormalizing the experimental Rp values with the total exposedrea and the Rpit values with the time dependent values of theitted area Apit determined by analysis of the impedance spec-ra as explained elsewhere [32]. For the tests in the absence ofacteria Rp could not be determined due to the lack of sufficientow-frequency data (Fig. 10). Fig. 11 clearly demonstrates thenhibition of pitting corrosion in the presence of B. subtilis. Theowest corrosion rates were observed for the biofilm producingolyaspartate (test #44).

The inhibition of pitting in the presence of B. subtilis could beue to exclusion of oxygen from the metal surface which wouldeduce the rate of the cathodic reduction reaction resulting in aecrease of Ecorr below Epit as observed for tests in the presence F

ig. 9. Time dependence of Ecorr (a) and Rp (b) for mild steel in AS containing. algae or S. ana.

f Shewanella (Fig. 6). However, the experimental values ofcorr had the lowest values in the absence of bacteria, while aertain degree of ennoblement was observed in the presence ofacteria (Fig. 12) [6]. This beneficial effect is apparent evenhen the biofilm contains bacteria that were not engineered toroduce inhibitors. Indeed, the observation that pitting occurredn all cases in the first 2 days of exposure clearly suggests that

ig. 10. Bode plots for Al 2024 exposed to AS containing Bacillus subtilis.

7676 F. Mansfeld / Electrochimica Acta 52 (2007) 7670–7680

Fig. 11. Time dependence of Ropit and Ro

p for Al 2024 exposed to sterile AS(p

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test #45) and to AS containing B. subtilis (test #42), B. subtilis producingolyglutamate (test #43) or B. subtilis producing polyaspartate (test #44).

aining B. subtilis [33]. The spectra obtained in the sterile ASndicated that pitting occurred as shown in Fig. 3, while pas-ive behavior was observed in the presence of the bacteria ashown in Fig. 10. In another experiment, the live biofilm wasilled by adding 2.5 mg/ml penicillin G and 1 mg/ml neomycinimultaneously after exposure of the Al 2024 sample to ASontaining the bacteria for 90.5 h. The effect of adding the antibi-tics is shown in Fig. 13. The impedance spectrum recorded.5 h after addition of the antibiotics indicated that Al 2024 wastill passive, however, the spectrum obtained 7 h later showedndication of pitting (Fig. 13). The spectra recorded after aotal exposure time of 113.5 h and 124 h were unstable at theowest frequencies, however, the following five tests resultedn stable spectra similar to the ones recorded in sterile ASFig. 3). The impedance spectra obtained in the sterile solu-

ion did not change when the antibiotics were added after 90.5 h33].

ig. 12. Time dependence of Ecorr for Al 2024 exposed to sterile AS (test #45)nd AS containing B. subtilis (test #42), B. subtilis producing polyglutamatetest #43) or B. subtilis producing polyaspartate (test #44).

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ig. 13. Bode plots for Al 2024 exposed to AS containing B. subtilis, additionf antibiotics after 90.5 h.

.3. The bacterial battery

As discussed above, a number of different bacteria are ableo reduce corrosion rates of different materials in several corro-ive media. The difference between the mechanism of corrosionnhibition of brass and Al 2024 produced by Shewanella (S. anand algae) and B. subtilis was that the corrosion potential Ecorrecame more negative in the presence of Shewanella, but becameore positive in the presence of B. subtilis. The observation that

ne type of bacteria can shift Ecorr of one metal in the positiveirection, while another type can shift Ecorr of some other metaln the negative direction suggests that it might be possible toonstruct a bacterial battery that has a larger cell voltage thanhe same battery that does not contain bacteria. Some prelim-nary results are discussed below including a new observationoncerning the formation of the biofilm on copper [11].

A galvanic cell with pure copper and Al 2024 and an elec-rolyte containing S. oneidensis MR-1 in Luria Bertani (LB)

edium has been prepared. The changes in electrode surfaceroperties have been monitored by means of EIS and mea-urement of Ecorr of both electrodes. A control cell, whichid not contain the bacteria, has also been tested. Poten-

iodynamic polarization experiments were used to determineurrent–voltage (I–V) curves as a function of time. From the–V curves current–power (I–P = I × V) curves were calculated.

F. Mansfeld / Electrochimica Ac

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ig. 14. i–V and i–P curves for the cell containing sterile LB medium (a) andB + MR-1 (b).

he current density i–V and i–P curves for the cell without bac-eria shown in Fig. 14a demonstrates that the maximum powerutput was obtained in the first day and dropped drastically inhe following days. For the cell with MR-1 the power values

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Fig. 15. Impedance spectra for copper in ster

ta 52 (2007) 7670–7680 7677

ncreased with time (Fig. 14b). Although the change was slow,t was continuous for about 100 days and it remained at similaralues for up to 200 days.

Impedance spectra were recorded for each electrode beforeach i–V measurement. In sterile LB medium there was a largencrease in the total capacitance of the Al 2024 with exposureime, which suggests that active pitting occurred. Pitting wasccelerated by the deposition of metallic Cu resulting from theopper corrosion reaction. Visual observation at the end of theest in LB without MR-1 showed that the Al 2024 surface wasitted and covered by metallic Cu.

The spectra for the Cu electrode in sterile LB medium showedhe typical behavior for an actively corroding metal. On the otherand, the impedance of Al 2024 in the LB medium containingR-1 remained at high levels in the first week of exposure and

hen decreased continuously. Visual observation showed thathe Al 2024 surface remained shiny and unattacked. In addi-ion, epifluorescence microscopy of the Al 2024 and Cu platesmmersed in the solution containing MR-1 revealed the presencef a patchy biofilm. Important changes have been observed inhe impedance spectra of the Cu electrode in the LB + MR-1 cell.lthough the impedance for Cu in the first days of exposure was

ypical for that of a corroding metal (similar to the spectra for Cun LB (Fig. 15a)), after 4 days of exposure a significant changen impedance was observed and a second time constant appearedFig. 15b). The impedance increased continuously and the sec-nd time constant became more significant with time. Sincehese spectra were very similar to those usually observed for met-ls covered by polymer films, they can be attributed to biofilm

ormation on the Cu surface. Epifluorescence microscopy ofhe copper plate immersed in the solution containing MR-1evealed the presence of a dense biofilm covering the surface.he changes of the impedance were accompanied by an enno-

ile LB medium (a) and LB + MR-1 (b).

7678 F. Mansfeld / Electrochimica Acta 52 (2007) 7670–7680

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ig. 16. Time dependence of cell voltage in sterile LB medium and LB + MR-1.

lement of Cu in LB medium containing MR-1. While the Ecorralues of Cu in sterile LB medium remained between −0.3 and0.2 V versus SCE, they exceeded −0.1 V after about 300 h

n LB containing MR-1. For Al 2024 Ecorr remained at around0.55 V in both media. The cell voltage reflected these trends

Fig. 16).

.4. The microbial fuel cell (MFC)

It has been suggested that in a microbial fuel cell (MFC) thehemical energy of the fuel is converted into electrical energyy the catalytic actions of microorganisms [12]. Many factorsay affect the overall performance of these MFCs including

he type of the utilized microorganism and the fuel as well ashe exposed areas of the anode and cathode and the nature ofhe ion exchange membrane. Other factors that may also playsignificant role include the microorganism growth conditions,

he anolyte/catholyte substances and their respective pH val-es. The present studies examine these factors and their effectsn the power density generation in a MFC as well as how thelectrode surface properties are affected by the presence of aicroorganism. Some preliminary results are discussed below.A dual compartment MFC was assembled using a pretreated

afion 117 ion exchange membrane and graphite electrodes1 cm2 surface area). The electrodes were constructed fromraphite felt bonded to platinum wire with carbon conductiveement adhesive. The assembled MFC was sterilized in an auto-lave for 10 min at 121 ◦C. Two sets of measurements werebtained using the sterile MFC. The first set of measurementsas recorded without microorganisms added to the MFC. The

node compartment was injected with 8 ml of sterile Shewanellaederation Minimal Media (un-inoculated) which contained lac-

ate as the fuel. The cathode compartment was injected with 6 mlf sterile Minimal Media plus 2 ml of potassium ferricyanide40 mM).

Impedance spectra for the anode and cathode were recordedn a frequency range of 100 KHz to 5 mHz by applying an acmplitude of 10 mV at Ecorr. Ag/AgCl reference electrodes werelaced in the anode or cathode compartment. Polarization curvesere recorded following the impedance measurements. Polar-

zation started at the open-circuit potential of the MFC at whichhe current I = 0 and ended at the short-circuit potential at whichhe cell voltage V = 0 and I = Imax. A scan rate of 0.1 mV/s wassed. From these measurements I–V and I–P curves were con-

t3cm

ig. 17. I–V and I–P curves for the un-inoculated (1–5, 2–29 h) and inoculated3–53, 4–77 h, 5–101 h) MFC.

tructed. The first impedance and polarization measurements forhe un-inoculated MFC were performed after 5 h and repeatedfter a 29 h exposure period.

The measurements in the presence of bacteria were obtainedith the same MFC after the anode compartment was injectedith S. oneidensis MR-1. Two milliliters of anolyte were

emoved from the MFC and replaced by 2 ml of dense cul-ure MR-1 in fresh Minimal Media. The catholyte remainednchanged. The inoculated MFC was exposed for 24 h beforeny measurements were conducted. Impedance spectra andolarization curves were then recorded for the inoculated MFCvery 24 h over a period of 4 days.

Preliminary results for I–V and I–P curves are presented inig. 17 for the un-inoculated and the inoculated MFC. Theenerated power increased slightly between the two measure-ents performed in the un-inoculated solution, but dramatically

ecreased upon the initial addition of MR-1. Each subsequent–P curve for the inoculated MFC showed a substantial increaserom the initial power generation and also an increase rela-ive to the un-inoculated case. Ultimately, the power generatedy the inoculated MFC was greater than that generated by then-inoculated MFC.

Ecorr of the inoculated anode became more negative with time.he initial value was 104 mV versus Ag/AgCl and the final Ecorralue after 92 h of exposure was 11 mV. Impedance spectra forhe anode and cathode are given in Fig. 18a and b, respectively.

The spectra for the anode followed a one-time-constantodel and did not change significantly upon addition of MR-1

Fig. 18a). The spectra for the cathode seem to be influenced bydiffusion effect as indicated by the frequency dependence of

he low-frequency impedance (Fig. 18b).Another set of experiments was conducted using different

ual compartment MFC’s. Each MFC was assembled usingafion 424 proton exchange membranes and graphite felt elec-

rodes (18 cm2 surface area) bonded to Pt wire with carbononductive epoxy. The cathode electrodes were coated with Pto catalyze the oxygen reduction reaction, while the anode elec-rodes remained bare graphite. Each MFC compartment held

0 ml of electrolyte (50 mM sodium phosphate, 100 mM sodiumhloride) and nitrogen was passed through the anode compart-ent to maintain anaerobic conditions. Additionally, air was

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ig. 18. Impedance spectra for the un-inoculated (1–5, 2–29 h) and inoculated3–53, 4–77 h) MFC (a) anode and (b) cathode.

assed through the cathode compartment to enhance the ratef the oxygen reduction reaction. A 10 � resistor was placedetween the anode and the cathode and the current generatedy the MFC was monitored as a function of time for differentxperimental conditions. Fig. 19 shows results for three MCFshat had been prepared in the same way. When only electrolyte

as present in the cathode department, a very small current was

ecorded. A small initial increase in current was observed whenR-1 was added to the anode compartment (Fig. 19). Upon addi-

ion of fuel (1.5 mM lactate) a sharp rise of the current occurred

ig. 19. Current–time curves for three MFCs for different experimental condi-ions.

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ollowed by a more gradual increase. After reaching a maximumhe current decreased again. The fuel concentrations were mon-tored using a high pressure liquid chromatography techniquend these results showed that all of the fuel was consumed whenhe current reached a minimum. When additional fuel (1.5 mMactate) was injected, the current showed a larger immediatencrease relative to the first fuel injection. This steep rise wasollowed by a more gradual increase until a second maximumas reached. The current then decreased to values that werebserved before the first addition of fuel (Fig. 19). The higherurrent production after the second fuel injection may be due tolarger amount of MR-1, i.e. a thicker biofilm, being attached

o the anode after the 24 exposure.

. Summary and conclusions

Corrosion inhibition caused by bacteria has been observed forl 2024, mild steel and brass exposed to AS containing growthedium in the presence of S. algae or S. ana. Ecorr decreased

n the presence of bacteria suggesting that the observed preven-ion of pitting for Al 2024 was due to the creation of anaerobiconditions on the metal surface as a result of which Ecorr isaintained at values that are lower than Epit which in abioticS is equal to Ecorr. The increase in the corrosion resistance ofrass was also considered to be due to reduction of the oxygenoncentration at the electrode surface. Potentiodynamic polar-zation curves obtained in AS containing S. algae after exposureor 7 days showed indeed a significant reduction of the cathodicurrents for Al 2024 and brass. For mild steel an increase in theorrosion resistance was accompanied by an increase of Ecorr.n this case anodic polarization curves demonstrated a smallecrease of the anodic current in AS containing S. algae withittle change of the cathodic polarization curve. The observedecrease of corrosion rates accompanied by an increase of Ecorrould be due to production of an inhibiting species and/or – asuggested recently by Dubiel et al. [31] – due to microbial res-iration involving reduction of Fe3+ to Fe2+ accompanied by aeduction of the oxygen concentration at the electrode surface.n the other hand, the results obtained in the CCURB projectave consistently shown that corrosion inhibition of Al 2024 andrass in AS by B. subtilis, B. licheniformis or E. coli was alwaysccompanied by an ennoblement of Ecorr [2–8]. In these caseshe main cause of prevention of pitting of Al 2024 and reduc-ion of corrosion rates of brass apparently was the production ofnhibitors by living cells in the biofilm.

It is important to note that the cases discussed here, where theame bacteria inhibit corrosion of a number of different metals,iffer from the cases of corrosion inhibition by chemical com-ounds, where inhibition normally is observed only for verypecific combinations of metal/compound/environment.

A notable difference in the effect of the additions of biocidesas been observed for Shewanella on the one hand and B. sub-ilis on the other hand. In the first case Ecorr of Al 2024 and brass

ncreased slowly, but Rp remained more or less constant for sev-ral days. In the second case Ecorr decreased almost immediatelynd pitting of Al 2024 initiated after a few hours. These findingsemonstrate that live biofilms formed by different bacteria can

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rovide significant corrosion protection by different – at presentot well understood – mechanisms.

A preliminary evaluation of the bacterial battery has shownhat the power output of a cell with Cu and Al 2024 electrodesncreased continuously with exposure time. While this result isery promising, it has to be realized that similar to the microbialuel cell the power output is very low. Present work is concentrat-ng on increasing the power output of both devices by evaluatinghe effects of different electrode and electrolyte materials as wells the use of different strains of Shewanella. One interestingspect of the MFC seems to be that – contrary to conventionaluel cells – a large number of fuels can be used. This possibilityill be addressed in future research.

cknowledgements

The work on the bacterial battery and on the microbial fuelell is being carried out by Esra Kus and Orianna Bretschger,espectively, who are Ph.D. students in the Mork Family Depart-ent of Chemical Engineering and Materials Science at USC.

eferences

[1] H.A. Videla, Manual of Biocorrosion, CRC Press, 1996.[2] F. Mansfeld, C.H. Hsu, D. Ornek, T.K. Wood, B.C. Syrett, Electrochem.

Soc. Proc. 2000–24 (2001) 99.[3] F. Mansfeld, C.H. Hsu, D. Ornek, T.K. Wood, B.C. Syrett, J. Electrochem.

Soc. 149 (2002) B130.[4] D. Ornek, A. Jayaraman, T.K. Wood, Z. Sun, C.H. Hsu, F. Mansfeld, Corros.

Sci. 43 (2001) 2121.[5] D. Ornek, T.K. Wood, C.H. Hsu, Z. Sun, F. Mansfeld, Corrosion 58 (2002)

761.[6] F. Mansfeld, C.H. Hsu, Z. Sun, D. Ornek, T.K. Wood, Corrosion 58 (2002)

187.

[7] D. Ornek, A. Jayaraman, B.C. Syrett, C.H. Hsu, F. Mansfeld, T.K. Wood,

Appl. Microbiol. Biotechnol. 58 (2002) 651.[8] D. Ornek, T.K. Wood, C.H. Hsu, F. Mansfeld, Corros. Sci. 44 (2002) 2291.[9] J.C. Earthman, P. Arps, Z.S. Farhangrazi, K. Trandem, T. Wood, Corro-

sion/2001 Paper no. 1271, NACE, 2001.

[

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10] A. Nagiub, F. Mansfeld, Electrochim. Acta 47 (2002) 2319.11] E. Kus, R. Abboud, R. Popa, K.H. Nealson, F. Mansfeld, Corros. Sci. 47

(2005) 1063.12] H.J. Kim, H.S. Park, M.S. Hyun, I.S. Chang, M. Kim, B.H. Kim, Enzyme

Microb. Technol. 30 (2002) 145.13] C. von Wolzogen Kuehr, Water Gas 7 (26) (1923) 277.14] B.J. Little, P.A. Wagner, F. Mansfeld, Corrosion Testing Made Easy, vol.

5, Microbiologically influenced corrosion, NACE, 1997.15] F. Mansfeld, B. Little, Corros. Sci. 32 (1991) 247.16] P. Chandrasekaran, S.C. Dexter, “Mechanism of Potential Ennoblement

of Passive Metals by Seawater Biofilms”, Paper no. 493, Corrosion/93,NACE, 1993.

17] R. Johnsen, E. Bardal, Corrosion 41 (1985) 296.18] W.H. Dickinson, Z. Lewandowski, Biodegradation 9 (1998) 11.19] B.H. Olesen, R. Avci, Z. Lewandowski, Corros. Sci. 42 (2000) 211.20] P. Linhardt, Werkstoffe und Korosion 45 (1994) 79.21] P. Linhardt, Failure of chromium–nickel steel in a hydroelectric power plant

by manganese oxidizing bacteria, in: E. Heitz, -C.H. Fleming, W. Sand(Eds.), Microbially Influenced Corrosion of Materials, Springer Verlag,1996, p. 221.

22] B. Little, F. Mansfeld, Proceedings of the H.H. Uhlig Symposium on Pas-sivity of Stainless Steels in Natural Seawater, vol. 94-26, The Electrochem.Soc., 1994, p. 42.

23] M. Eashwar, S. Maruthamutu, S. Sathyanarayanan, K. Balakrishnan, Pro-ceedings of the 12th International Corrosion Congress, vol. 5b, September1993, Houston, TX, NACE, 1993, p. 3708.

24] B.J. Little, P.A. Wagner, R.I. Ray, Corrosion/93 Paper no. 308, NACE,1993.

25] F. Mansfeld, J. Appl. Electrochem. 25 (1995) 187.26] F. Mansfeld (Unpublished results).27] F. Mansfeld, C.C. Lee, L.T. Han, G. Zhang, B.J. Little, P. Wagner, R. Ray, J.

Jones-Meehan, The Impact of Microbiologically Influenced Corrosion onProtective Polymer Coatings, Final Report to the Office of Naval Research,Contract No. N00014-94-2-0026, August 1998.

28] F. Mansfeld, A. Nagiub, D. Oernek, T.K. Wood, Electrochem. Soc. Proc.4 (2002) 522.

29] F. Mansfeld, H. Shih, ASTM STP 1134 (1992) 141.30] F. Mansfeld, Y. Wang, S.H. Lin, H. Xiao, H. Shih, ASTM STP 1182 (1993)

297.31] M. Dubiel, C.C. Chien, C.H. Hsu, F. Mansfeld, D.K. Newman, Appl.

Environ. Mirobiol. 68 (2002) 1449.32] F. Mansfeld, C.H. Tsai, H. Shih, ASTM STP 1154 (1992) 186.33] R. Zuo, T.K. Wood, E. Kus, F. Mansfeld, Corros. Sci. 47 (2005) 279.