Available online at www.sciencedirect.com

www.elsevier.com/locate/corsci

Corrosion Science 50 (2008) 540–547

Electrochemical behaviour of stainless steels in media containingiron-oxidizing bacteria (IOB) by corrosion process modeling

J. Starosvetsky a,*, D. Starosvetsky b, B. Pokroy b, T. Hilel b, R. Armon a

a Division of Environmental, Water and Agricultural Engineering, Faculty of Civil and Environmental Engineering,

Technion – Israel Institute of Technology, Haifa 32000, Israelb Laboratory of Electrochemistry and Corrosion, Faculty of Materials Engineering, Technion – Israel Institute of Technology, Haifa 32000, Israel

Received 26 June 2007; accepted 27 July 2007Available online 23 August 2007

Abstract

Localized corrosion mechanism of stainless steel (SS) types UNS S30403 and UNS 31603 in the presence of iron-oxidizing bacteriaSphaerotilus spp. isolated from rust deposits was studied electrochemically. OCP transient, cyclic anodic and cathodic potentiodynamicpolarization curves were measured on steel electrodes through their exposure to 3% NaCl solution supplemented with Sphaerotilus cul-ture. The exposure period was composed of three parts: (a) 5 days incubation of steel electrodes in sterile 3% NaCl solution; (b) additionof 3 days-old Sphaerotilus culture to 3% NaCl at 3:2 v/v ratio with subsequent electrodes exposure for 11 days up to complete sedimen-tation of ferric oxides and (c) subsequent exposure of electrodes for 14 days in upper and bottom (sediments layer) fractions of the exper-imental medium. The results revealed an instantaneous gradual shift of the transient potential of both steels towards negative potentialsfrom steady-state value of �0.15 V to �0.35 to �0.42 V (SCE) during the whole exposure interval since IOB culture addition into sterile3% NaCl solution.

No evidence of pitting corrosion was found on SS samples subsequent to their exposure to sterile 3% NaCl solution, though in thepresence of IOB culture, numerous pits were revealed on 304 L steels specimens exposed to iron hydroxides sediments layer. Electro-chemical characteristics (OCP or corrosion potential – ECORR, breakdown potential – EBD, repassivation potential – ERP, passivationcurrent – iPASS) periodically measured by cyclic polarization method, allowed monitoring the electrochemical behavior changes of exper-imental SS and to establish the initiation of pitting corrosion in the presence of IOB, resulting in crevice effect caused by biogenic ferricoxides deposits precipitated on steel surface. Overall, steel 316L demonstrated higher resistance to pitting corrosion compared to 304L.� 2007 Published by Elsevier Ltd.

Keywords: Pitting corrosion; Stainless steels; Iron-oxidizing bacteria; Rust deposits; Electrochemical characteristics

1. Introduction

Numerous evidences obtained during the last two dec-ades show that biofouling plays a key role in microbiolog-ically influenced corrosion (MIC). The non-uniformcharacter of biofilms colonizing metal surface results inirregular physico-chemical conditions on different surfacesites, and consequently, in a non-equal distribution of cor-rosion rate over metal surface and localized corrosion

0010-938X/$ - see front matter � 2007 Published by Elsevier Ltd.

doi:10.1016/j.corsci.2007.07.008

* Corresponding author. Tel./fax: + 972 4 8293309.E-mail address: starjean@tx.technion.ac.il (J. Starosvetsky).

attack. Various scenarios of localized corrosion initiationand development under biofilms were described [1–4].Important role in the mechanisms proposed by these stud-ies was attributed to slime-forming microorganisms, whichin a short period of time produce large amounts of bio-mass. Metal-depositing microorganisms (iron/manganese-oxidizing or iron/manganese-precipitating bacteria) arethe best representative of this bacterial group. Thesemicroorganisms are capable to deposit iron/manganesehydroxides (rust) extracellularly at a rate of hundreds timehigher than the abiotic process [5,6]. Consequently, iron/manganese-oxidizing and iron/manganese-precipitatingbacteria are among the most dangerous microorganisms

J. Starosvetsky et al. / Corrosion Science 50 (2008) 540–547 541

from biofouling and corrosion point of view. Metal-depos-iting organisms create conditions favorable to localizedcorrosion, especially in the case of metals passivity (i.e. instainless steels case). Pitting corrosion of SS initiated bymetal-oxidizing bacteria in natural fresh waters had anextremely high rate, as already published [7–9]. Variousexplanations of this phenomenon were suggested[3,4,8,10–13]. One of them is based on the principle of cre-vice corrosion. According to this approach, [4,11] densedeposits of biomass layer produced by metal-oxidizingbacteria on metal surface provide conditions for the devel-opment of crevice effect on metal/biomass interface initiat-ing pitting phenomenon. This hypothesis is based on thetheory of localized corrosion proposed by Sato [14]. Itwas suggested that pitting is favored by a deposit withanion-selective membrane properties, while passivity is sta-bilized by cation selectivity. Ferric hydroxides membranesare anion-selective at neutral pH values, and thereforeshould be aggressive towards stainless steel. Such mem-brane may stabilize pitting by acting as a super crevice,combining anion-selectivity with a relatively low ionicresistance. Unfortunately little direct experimental evi-dences are available to prove this elegant theory [11,15].

In the present study different aspects of stainless steelscorrosion initiated by IOB in chlorine-containing waterwas electrochemically examined simulating different corro-sion stages and conditions.

2. Methods and materials

The experiments were conducted with disc shapespecimens and pencil-type electrodes made of 304L (UNSS30403) and 316L (UNS S31603) stainless steels. Chemicalcompositions of the experimental steels are shown inTable 1. Disc shape specimens with a diameter of 15 mmand 2 mm thickness, were used for scanning electronmicroscopy (SEM) analysis. Pencil-type electrodes for elec-trochemical measurements were made of the same steels bymounting 5 mm-diameter steel rode in an epoxy resin. Theexperimental specimens were abraded through 600-grit sil-icon carbide metallurgical paper, degreased in acetone,washed with sterile three-distilled water and dried in adesiccator.

Experimental iron bacteria were isolated from rustdeposits of clogged carbon steel heat exchanger from oil-refinery plant (Haifa, Israel) [16]. The primary identifiedstrain was Sphaerotilus sp. The culture of iron-oxidizingbacteria was grown on Winogradsky nutrient medium(g l�1): 0.5 K2HPO4, 0.5 NaNO3, 0.2 CaCl2, 0.5 MgSO4 �7H2O, 0.5 NH4NO3, 6.0 ammonium iron citrate (pH 4.8).

Table 1Chemical composition (w/w %) of tested SS detected by atomic adsorptionspectroscopy

Steel Fe Cr Ni Mn Si Mo P C S

304L Bal 17.48 8.18 0.798 0.787 0.55 0.039 0.046 0.016316L Bal 17.42 8.65 1.687 0.59 2.36 0.037 0.035 0.029

Analytical-grade reactant and distilled water were usedfor aqueous solutions preparation. The experimental cul-tures were grown by vigorous shaking for 3 days at roomtemperature (22 ± 2 �C). Bacterial growth was detectedby medium color shift from green to red. Iron bacteria enu-meration was performed by membrane filtration method asalready described [17], followed by 5 days incubation at25 �C on Winogradsky nutrient medium supplementedwith 1.6 % bacto-agar. Iron bacteria (mixed culture) con-centration after three days of incubation reached1.3 � 1010 CFU ml�1. Liquid Winogradsky nutrient med-ium had an initial yellow color. The 3 days-old culture ofSphaerotilus sp. bacteria had light orange color due to for-mation of soluble ferric-ions produced by these bacteria.As an outcome of bacterial activity, culture medium colorgradually shifts with time from yellow to intense red–brown (5–6 days), whereupon visual sedimentation of ferrichydroxides developed during next 4–7 days. Finally, theiron bacteria culture medium formed two fractions: thebottom part - a thick layer (2–3 cm) of red–brownprecipitates, and the upper part - colorless, turbid liquid.According to chemical and microbiological analysis itwas found that major fraction of iron bacteria cells and fer-ric-ion compounds precipitated on the 11 to 13th day ofgrowth [18]. The pH of tested media was measured withpH-meter PHM210-Meterlab (Radiometer Copenhagen).The electrochemical measurements were performed withpotentiostat M273 (EG&G, USA). Corrosion test and elec-trochemical measurements were conducted in 2 liters-flaskequipped with a saturated calomel reference electrode(SCE), which was connected to flask through a Luggin–Habber capillary tip assembly, and Pt-wire counter elec-trode. Test commenced by immersion of coupons andpencil-type electrodes in flask containing 800 ml of sterile3% NaCl solution. After 5 days-exposure in the 3 days-old Sphaerotilus sp. culture (1.2 L) was added to 3% NaCl(3:2 v/v ratio). Further exposure of experimental specimenswas conducted in bacterial solution up to complete sedi-mentation of ferric oxides and hydroxides. The overall testduration was 30 days. In order to maintain the initial con-centration of chlorides along the experimental time period,prior to culture addition the concentration of NaCl solu-tion was increased so that after bacterial medium augmen-tation, it remained at 3%. Since the experimental culturemedium following bacterial growth decay by sedimentationonto two phases (ferric hydroxides precipitates and upperliquid) tested coupons and pencil-type electrodes wereexposed in upper and bottom parts of tested flask, in orderto determine the variations in electrochemical behavior of‘‘top” and ‘‘bottom” specimens. Chemical and microbio-logical characteristics of tested medium (pH, Fe2+/Fe3+

ratio, bacterial concentration) were reported elsewhere[18]. During specimens exposure the ECORR transient andpolarization characteristics of tested steel were measuredwith pencil-type electrodes. Some coupons of tested stain-less steel were periodically removed from solution forfollowing surface examination by optical and scanning

542 J. Starosvetsky et al. / Corrosion Science 50 (2008) 540–547

electron microscopy (SEM). For comparison, similarexperiments with the same stainless steel coupons and pen-cil-type electrodes were conducted for 30 days in sterile 3%NaCl solution only and in 3% NaCl solution supplementedwith sterile Winogradsky nutrient medium (2:3 v/v accord-ingly) without bacteria. After nutrient culture augmenta-tion, final concentration of chloride was also adjusted to3%. Prior to supplementation, the pH of Winogradskynutrient medium was adjusted to 7.2 with NaOH solution.

3. Results and discussion

ECORR transient of 304L steel measured by open circuitpotential, OCP, during exposure in 3% NaCl solution,prior to and after addition of iron-oxidizing bacteria cul-ture is shown in Fig. 1. As can be seen, ECORR of steel insterile NaCl solution (without culture medium augmenta-tion) was relatively stable and slightly increased in therange between �0.1 and �0.2 V throughout the 5 days ofexposure. Upon culture medium addition the ECORR

decreased in the range between �0.1 and �0.4 V. Thedecrease in ECORR can be attributed either to weakeningof steel passivity or surface activation, or to decrease ofthe cathodic process rate, for example due to decrease inthe oxygen concentration of dissolved in tested medium.Marked decrease of ECORR was detected on the 2nd dayfollowing culture augmentation (approx. five days beforevisual sedimentation). During further exposure up to ironhydroxides sedimentation this tendency persisted. After fulliron hydroxide sedimentation the ECORR becomes stable. Itshould be noted that ECORR transients measured with spec-imens exposed in upper and bottom parts of solution werealmost similar. In contrast to culture medium, no markedchanges in ECORR of 304L SS specimens were observedduring their exposure for 30 days in control media: NaClsolution with addition of sterile Winogradsky nutrientmedium (3:2 v/v ratio).

-5 0 5 10 15 20 25 30

-0.4

-0.3

-0.2

-0.1SS 304L

Culture medium

Sedimentation

BOTTOM TOP

Pote

ntia

l (V SC

E)

Time (day)

Fig. 1. Corrosion potential transients of 304L stainless steel in 3% NaClsolution before and after augmentation with culture medium. Zero pointon the time scale corresponds to augmentation of NaCl solution in whichall specimens were previously exposed for 5 days.

Fig. 2 presents cyclic anodic potentiodynamic curves of304L stainless steel measured after 9 days-exposure, includ-ing 5 days-pre-exposure in NaCl and 4 days-exposure aftereither nutrient or culture medium augmentation. It may beseen that there are no marked differences in anodic curvesmeasured in these media on the 9th day of exposure. Theonset of anodic current was slightly above �0.2 V in bothtested solutions, i.e. it was slightly above ECORR valuesmeasured by OCP at 9th exposure day. The wide regionof steel passivity approx. up to 0.4 V can be clearly seenin potentiodynamic curve measured at positive potentialscan. Breakdown at potentials above 0.4 V was detectedin both curves. During further positive shift in appliedpotential an anodic current sharply increased. Marked hys-teresis, which was detected between anodic curves mea-sured at positive scan and back scan both in bacterialand model solutions, can be attributed to pitting phenom-enon. It should be also noted that no marked differences inanodic behavior of tested electrodes exposed at ‘‘top” and‘‘bottom” parts of electrochemical cell were detected at thistest stage. This may be considered as the fact that sedimen-tation process did not start so far after 9 days-exposure andchemical composition of solutions at top and bottom partsof cell was the same.

Significant changes in electrochemical behavior of testedelectrodes exposed at ‘‘top” and ‘‘bottom” parts of electro-chemical cell was found after initiation of sedimentationprocess. Cyclic polarization curves of 304L stainless steelmeasured with ‘‘top” electrodes at 4th day after bacteriaaugmentation and at 4th day of sedimentation (total expo-sure time after bacterial medium augmentation was 11days) are shown in Fig. 3. As can be seen, the onset of ano-dic current in the curve following sedimentation occurredat more negative potential value (�0.34 V) compared tothat detected by measurement before sedimentation(�0.2 V). Significant (approx. two times) increase of anodic

-0.2

0.0

0.2

0.4

0.6

10-6 10-5 10-4 10-3 10-2

NaCl+Nutrient medium NaCl+Culture medium

Current (A/c m2)

Pote

ntia

l (V

SCE)

Fig. 2. Cyclic anodic potentiodinamic curves of 304L stainless steelmeasured after OCP exposure: (M) 5days in 3% NaCl solution andadditional 4 days-exposure after nutrient medium addition; (s) 5 days inNaCl solution and further 4 days after bacterial culture addition.

10-6 10-5 10-4 10-3 10-2

-0.4

-0.2

0.0

0.2

0.4

4 days of augmentation 4 days of sedimentation

Pote

ntia

l (V

SCE)

Current (A/cm2)

Fig. 3. Cyclic anodic polarization curves of 304L stainless steel measuredon 4th day after iron bacteria culture augmentation and 4th day after theculture sedimentation (total 11 days); ‘‘top” electrodes.

-0.4

-0.2

0.0

0.2

0.4

0.6

10-6 10-5 10-4 10-3 10-2

Time, days: 1 4 9 21

Current (A/cm2)

Pote

ntia

l (V

SCE)

-0.4

-0.2

0.0

0.2

0.4

0.6

10-6 10-5 10-4 10-3 10-2

Time, days: 1 4 9 21

Current (A/cm2)

Pote

ntia

l (V

SCE)

Fig. 4. Cyclic anodic polarization curves of 304L stainless steel measuredon 1, 4, 9, 15 and 21st day after sedimentation beginning; (a) ‘‘top”

electrodes, (b) ‘‘bottom” electrodes.

J. Starosvetsky et al. / Corrosion Science 50 (2008) 540–547 543

current values was detected in the region of passivity whenan anodic curve was measured at 4th day of sedimentationcompared to the values obtained by measurements prior tosedimentation. Subsequent to sedimentation, breakdown(EBD) and repassivation (ERP) potentials measured in cyclicpolarization curves were much more negative (EBD was0.32 V and ERP was 0.05 V) than those detected before sed-imentation (0.45 V and 0.15 V respectively). In accordancewith these results it can be suggested that initiated sedimen-tation causes markedly weaker passivation characteristicsof tested steel, even though the tested specimens were notexposed within the precipitated mass.

It was also established that the longer exposure of testedspecimens under sedimentation process in culture medium,a stronger reduction in protective characteristics of steelpassivity. This was true for tested electrodes exposed bothin upper and bottom parts of the cell. This phenomenon isillustrated by cyclic polarization curves obtained with‘‘top” (Fig. 4a) and ‘‘bottom” (Fig. 4b) specimens of tested304L stainless steel after different time intervals of exposureunder sedimentation process. Fig. 4 shows significantdecrease in protective characteristics of steel passivity(EBD decrease, increase of anodic current in the region ofpassivity, iPASS) along exposure time under sedimentationprocess in iron bacteria containing medium. It should benoted that the electrochemical behaviour of 304 L speci-mens in control media practically did not changed duringwhole test period of time.

According to electrochemical characteristics obtainedwith ‘‘top’ and ‘‘bottom” electrodes we can conclude thatcorrosion resistance of ‘‘bottom” electrodes, exposed withinthe thick deposit layer of biologically formed ironhydroxides, reduces much more rapidly compared with‘‘top” electrodes. ‘‘Bottom” electrodes exposed within bio-precipitates undergone activation during OCP exposure atthe 21st test day (Fig. 4b), while exposure of steel specimens

in the upper part during the same time interval, did notbring specimen surface to activation (Fig. 4a).

The different stages of steel exposure subsequent toNaCl augmentation with culture medium were also ana-lyzed by examination of surface morphology of tested cou-pons following different exposure periods. Fig. 5 presentsSEM micrographs obtained from surface of ‘‘bottom” cou-pons after 4 days followed by culture augmentation and 7days of sedimentation. A small amount of iron bacteriacells adsorbed to steel surface together with organics andinorganic metabolites (dark spot) are seen on this SEMmicrograph obtained at 4th day followed augmentation(Fig. 5a). Practically whole electrode surface was coveredwith a dense deposit layer at 7th day of sedimentation pro-cess (Fig. 5b). It was established by energy dispersive spec-troscopy (EDS) that these deposits are evidently Fe2O3.The SEM micrographs obtained with surface of couponsexposed in the ‘‘top” part of flask both 4 days after aug-mentation and 7 days after sedimentation were similar tothat presented in Fig. 5a.

Fig. 5. SEM micrographs of 304 L stainless steel surface pre-exposed to 3% NaCl solution for 5 days and additionally exposed for 4 days in augmentedculture (a); and 7 days after sedimentation initiation (b).

544 J. Starosvetsky et al. / Corrosion Science 50 (2008) 540–547

It should be noted that the structure of deposits detectedon specimen surface of stainless steel significantly differsfrom that detected on the surface of carbon steel specimensafter expose in the same solution [18]. Deposit covered thecarbon steel surface in iron bacteria solution had a layeredstructure. The upper layer of deposit contained spongy andneedle-shaped aggregates. Quantitative EDS analysisshowed that these aggregates are evidently Fe2O3 crystals.Among the crystalloid structures of the upper layer, accu-mulation of filamentous bacteria identified as Sphaerotilus

sp. was observed [18]. Unlike carbon steel no filamentousbacteria were detected among deposits covered stainlesssteel specimens.

Fig. 6 illustrates micrographs of 304L steel surfaceexposed for 30 days in iron bacteria culture at the upper(a) and bottom (b) flask parts. Prior to SEM examinationthe deposits precipitated on electrode surface during expo-sure to culture medium were completely removed. No evi-dence of pitting corrosion was detected on the surface

specimens, which were exposed at the upper part of flaskfor 30 days (Fig. 6a). In contrast to ‘‘upper” specimens,numerous dip pits were found on the surface of specimensafter 30 days-exposure at the bottom part of iron bacteriaculture medium (Fig. 6b). It should be also mentioned thatno pitting phenomenon was detected with specimensexposed to control media: NaCl solution and NaCl solu-tion augmented with sterile nutrient medium.

A similar corrosion and electrochemical tests were con-ducted with 316L SS. Fig. 7 presents ECORR transient of‘‘upper” and ‘‘bottom” specimens made of 316L steel dur-ing their exposure to 3% NaCl solution before and afterculture medium addition. Concomitantly to iron bacterialculture addition the ECORR of 316L SS of ‘‘upper” and‘‘bottom” specimens gradually decreased during the wholetest period, except a short period between 5th and 7th days,when small ennoblement of corrosion potential wasobserved. It should be noted that similar corrosion poten-tial ennoblement at the same exposure period was also

Fig. 6. SEM micrographs of 304 L stainless steel surface exposed for 30 days to sterile 3% NaCl solution with addition of nutrient medium (a) and ironbacteria-containing medium (b).

J. Starosvetsky et al. / Corrosion Science 50 (2008) 540–547 545

detected in the case of 304L SS. The difference in ECORR

between ‘‘top” and ‘‘bottom” 316L SS specimens was verysmall, except last period exposure (>20 days) when sedi-mentation was finalized. At the end of corrosion test thedifference in ECORR values of ‘‘bottom” and ‘‘upper” elec-trodes was approximately 0.17 V. The decrease of ECORR

during 316L SS exposure in iron bacteria solution can bealso attributed either to reduction in protective characteris-tics of steel passivity or to decrease of oxygen concentra-tion at steel/solution interface, as in the case of 304L SS.

Electrochemical measurements with ‘‘top” and ‘‘bot-tom” 316L SS specimens were conducted after differenttime expose both in 3% NaCl solution before and afternutrient and cultural media supplementation. No markeddifference in anodic behaviour of 316L SS steel wasrevealed in these solutions during long term exposure.Small alterations in anodic behaviour were detected onlyon later stages of sedimentation process. Fig. 8 representscyclic polarization curves obtained with ‘‘top” (8a) and

‘‘bottom” (8b) electrodes of 316L SS at 1st and 21stsedimentation days. As can be seen (Fig. 8a), a smalldecrease in protective characteristics of passivity wasdetermined with ‘‘top” specimens by cyclic polarizationexperiments after 21 days exposure under sedimentation:the onset of anodic current at 21st precipitation dayoccurred at �0.52 V (�0.38 at 1st day), EBD was 0.596 V(0.66 V at 1st day) and iPASS slightly increased at potentialsabove 0.5 V. Similar results were obtained with ‘‘bottom”specimens (Fig. 8b). Small decrease in specimen passivitywas detected by increase of exposure time under sedimenta-tion: the onset of anodic current decreased from �0.362 Vat 1st day of sedimentation to �0.505 V at 21st day andEBD decreased from 0.608 V to 0.52 V. The values of iPASS

at 1st and 21st day of sedimentation were very close toeach other. Pits appearance was not detected on 316L SSby SEM examination of both ‘‘top” and ‘‘bottom” speci-men surface after 21 days of exposure to experimentalmedia.

-5 0 5 10 15 20 25 30

-0.4

-0.3

-0.2

-0.1

0.0

End

Oneset

Sedimintation

Calture addition

BOTTOM TOP

Pot

entia

l (V

SC

E)

Time (h)

Fig. 7. Corrosion potential transients of 316L stainless steel in 3% NaClsolution before and after augmentation with culture medium. Zero pointon the time scale corresponds to augmentation of NaCl solution in whichall specimens were previously exposed for 5 days.

-0.5

0.0

0.5

1.0

10-6 10-5 10 10-3 10-2

Time, day: 1 21

Current (A/cm2)

Pote

ntia

l (V

SCE)

Top

-0.5

0.0

0.5

1.0

10-6 10-5 10-4 10-3 10-2

Time, day: 121

Current (A/cm2)

Pote

ntia

l(V

SCE)

Bottom

-4

Fig. 8. Cyclic anodic polarization curves of 316L stainless steel measuredat 1st and 12th day after sedimentation beginning: (a) ‘‘top” electrodes,(b)‘‘bottom” electrodes.

546 J. Starosvetsky et al. / Corrosion Science 50 (2008) 540–547

According to polarization characteristics obtained with304L and 316L SS specimens during exposure in bacterialmedium before and after sedimentation one can concludethat 316L SS is characterized by stronger passivity and,consequently, by enhanced resistance to pitting attack inthe presence of iron bacteria compared with 304L SS (seeFigs. 6 and 8). All the breakdown parameters detected incyclic polarization curves of 316L SS, such as: EBD, ERP

and iPASS were markedly superior to that obtained with304L SS.

The presented results demonstrate corrosion behaviourof two SS in the presence of slime-forming iron bacteriaat different stages of their development. The initiation oflocalized corrosion attack by metal-precipitated microor-ganisms commonly occurs in fresh waters with low chlo-rides concentration (0.1–0.4 ppm). In order to accelerateinitiation and development of localized corrosion ourexperiments were conducted in solutions with much higherchloride content (3% NaCl). Essentially, 304L and 316Lstainless steels are characterized by relatively strong passiv-ity and could remained passive in chloride solution duringlong periods of time. Pitting corrosion of these steels inchloride containing solutions (without oxidants) is usuallyassociated either with structure irregularities or with cre-vice effect occurrence during operation. The experimentsconducted in control media (sterile 3% NaCl solution andthis solution augmented with sterile Winogradsky nutrientmedium) showed that both tested steels remained passiveduring the whole exposure time despite of elevated chlorideconcentration. The ECORR of tested steels slightly increased(ennobled) with exposure time in examined control media.

However, the augmentation of 3% NaCl solution withiron bacterial culture, significantly affected electrochemicaland corrosion behaviour of tested steels. The strongdecrease of ECORR detected during steels exposure in ironbacteria culture (Figs. 2 and 8) was completely oppositeto the results obtained by deposition of abiotic or bioticmanganese/iron hydroxides on SS surface [19,20], whichcaused substantial ennoblement of corrosion potential.

The decrease of ECORR measured during 304L and 316Lstainless steels exposure in iron bacteria culture mediumcan be attributed to two different phenomena: (a) decreasein protective characteristics of its passive film and/or steelactivation following culture medium addition and (b)decrease of soluble oxygen concentration in solution dueto the respiratory activity of iron bacteria during their pro-liferation (iron-oxidizing bacteria are obligate aerobes).The occurrence of first scenario, namely, weakness of steelpassivity and its activation presumably can be expectedonly at final stages of exposure especially for the ‘‘bottom”

specimens situated into a thick, dense layer of ferrichydroxides precipitates. At early stages of exposure thedecrease of ECORR is rather associated with oxygen defi-ciency. The activity of strictly aerobic iron bacteria, whichaccumulate on metal surface with biofilm formation, maylead to significant decrease of oxygen concentration atsteel/solution interface. Since SS corrosion in neutral solu-

J. Starosvetsky et al. / Corrosion Science 50 (2008) 540–547 547

tions occurs with oxygen depolarization it is reasonable tosuggest that decrease of oxygen concentration results in arate reduction of cathodic process and a shift of ECORR

to negative values during specimen OCP exposure. Sedi-mentation of ferric oxides/hydroxides that took place aftera certain time of specimen exposure may substantiallyaggravate the protective characteristics of passive film.Deposition over SS specimen surface results in initiationof crevice effect on the steel/deposit interface affecting thesteel passivity and even activating it during long term expo-sure under sedimentation. The sedimentation effect wasvery important for 304L SS specimens exposed to the flaskbottom part. The negative role of this phenomenonwas clearly seen by comparison of electrochemical charac-teristic measured with ‘‘top” and ‘‘bottom” specimens atdifferent exposure periods under sedimentation. It was con-firmed that electrodes located into biological slime,undergo stronger decrease in protective characteristics ofpassive film than those located in the upper zone of theexperimental flask. This phenomenon is highly reflectedin marked changes of EBD and iPASS measured with ‘‘bot-tom” electrodes of 304L SS. After extended exposure intothe sediments, numerous pits were detected on 304 SS. Itshould be noticed, that similar tendencies were observedin electrochemical behaviour of ‘‘top” specimens, althoughin this case the sedimentation effect was less pronounced.The obtained results are in good agreement with earlierinvestigations conducted in [11,15], which substantiatedthe hypothesis of crevice corrosion mechanism for micro-bial corrosion phenomenon in potable water.

316L SS revealed stronger corrosion resistance in iron-oxidizing bacteria media compared to 304L SS. Only smalldifferences in anodic characteristics of 316L SS weredetected during its exposure in bacterial medium beforeand after sedimentation and between ‘‘upper” and ‘‘bot-tom” specimens.

4. Conclusions

1. Electrochemical behaviour of 304L and 316L SS pre-exposed to 3% NaCl solution for 5 days and thenadditionally exposed for 30 days in this solution supple-mented with 3 days-old culture of Sphaerotilus sp. gen-era was investigated.

2. Low corrosion resistance of 304L SS to localized corro-sion attack was detected in 3% NaCl solution containingiron bacteria culture. Strong decrease in corrosion resis-tance of 304L SS as a function of exposure time, wasdetected when steel specimens were located at the flask

bottom and remained prolonged period of time into alayer of biogenic hydroxide sediments. Pitting corrosionattack occurred on the surface of ‘‘bottom” specimens.No pitting corrosion attack was detected on the surfaceof ‘‘top” specimens.

3. 316L SS showed higher resistance to pitting corrosion iniron bacteria-containing medium with 3% NaCl com-pared with 304L SS.

4. According to the present experimental results it can besuggested that MIC of stainless steels media of iron-oxi-dizing bacteria can be explained by the mechanism ofcrevice corrosion.

Acknowledgements

We would like to thank KAMEA Foundation, Ministryof Immigration and Adsorption, Israel for the financialsupport to two of the authors, D. Starosvetsky and J.Starosvetsky.

References

[1] H-C. Fleming, G.G. Geesey, Biofouling and Biocorrosion in Indus-trial Waters, Springer–Verlag, New York, 1991.

[2] H.A. Videla, W.G. Characklis, Inter. Biodeter. & Biodegrad. 29(1992) 195.

[3] S.W. Borenstein, Microbiologically Influenced Corrosion Handbook,Woodhead, Cambridge, London, 1994.

[4] B. Little, P. Wagner, Mater. Perform. 36 (6) (1997) 40.[5] H.I. Ehrlich, Geomicrobiology, in: M. Dekker (Ed.), New York,

1996.[6] D.R. Cullimor, Microbiology of well biofouling, Lewis Publishers,

Boca Raton-London-New York-Washington-DC, 1999.[7] J.F.D. Stott, in: L.L. Shreir, R.A. Jarman, G.T. Burstein (Eds.),

Corrosion, third ed., Butterworth– Heinemann, UK, 1994.[8] G. Kobrin, Mater. Perform. 33 (4) (1994) 62.[9] J.T. Titz, Stainless Steel World 2 (1999).

[10] R.E. Tatnall, Mater. Perform. 20 (9) (1981) 32.[11] M.I. Suleiman, I. Ragault, R.C. Newman, Corros. sci. 36 (1994) 479.[12] X. Shi, R. Avci, M. Geiser, Z. Lewandowski, Corros. Sci. 45 (2003)

2577.[13] P. Linhardt, Mater. Corros. 55 (2004) 158.[14] N. Sato, Corros. Sci. 27 (1987) 421.[15] I.G. Chamritski, G.R. Burns, B.J. Webster, N.J. Laycock, Corrosion

60 (2004) 658.[16] J.O. Starosvetsky, R. Armon, J. Yahalom, D. Starosvetsky, Inter.

Biodeter. & Biodegrad. 47 (2) (2001) 79.[17] D.R. Cullimor, A.E. McCann, Aquatic Microbiology, in: F.A.

Skinner, J.M. Shewan (Eds.), Symposium Series – Society for AppliedBacteriology, vol. 6, Academic Press, 1977.

[18] J.O. Starosvetsky, R. Armon, A. Groysman, D. Starosvetsky, Mater.Perform. 38 (1) (1999) 55.

[19] W.H. Dickinson, F. Caccavo, B. Olesen, Z. Lewandowski, Appl. andEnviron. Microbiol. 7 (1997) 2502.

[20] X. Shi, R. Avci, Z. Lewandowski, Corros. Sci. 44 (2002) 1027.