M A T E R I A L S C H A R A C T E R I Z A T I O N 5 9 ( 2 0 0 8 ) 2 4 5 – 2 5 5

Pitting corrosion behavior of 316L stainless steel in the mediaof sulphate-reducing and iron-oxidizing bacteria

Congmin Xua, Yaoheng Zhanga,b, Guangxu Chenga,⁎, Wensheng Zhub

aXi'an Jiaotong University, Xi'an 710049, ChinabResearch and Technology Center of Lanzhou Oil Refinery Factory, PetroChina Company limited, Lanzhou, 730060, China

A R T I C L E D A T A

⁎ Corresponding author. Tel.: +86 29 8266 557E-mail address: gxcheng@mail.xjtu.edu.cn

1044-5803/$ – see front matter © 2007 Elsevidoi:10.1016/j.matchar.2007.01.001

A B S T R A C T

Article history:Received 6 March 2006Received in revised form2 August 2006Accepted 2 January 2007

Pitting corrosion behavior of 316L SS was investigated in the presence of aerobic andanaerobic bacteria isolated from cooling water system in oil refinery using polarizationmeasurement, electrochemical impedance spectroscopy, scanning electron microscopyexaminations and energy dispersive spectrum analysis. The results show the corrosionpotential (Ecorr), pitting potential (Epit) and polarization resistance (RP) of 316L SS had adistinct decrease in the presence of bacteria, in comparison with those observed in thesterile medium for the same exposure time interval. Micrometer-scale pitting was observedon the 316L SS surface in the presence of bacteria. The combination of SRB and IOBdemonstrated higher corrosion rates than SRB or IOB alone. The synergy of 0.01 M NaCl+SRB+IOB yielded the highest corrosion rate. The synergies between the metal surface,abiotic corrosion products, chloride anion, and bacterial cells and their metabolic productsincreased the corrosion damage degree of the passive film and accelerated pittingpropagation.

© 2007 Elsevier Inc. All rights reserved.

Keywords:Iron-oxidizing bacteria (IOB)Sulphate-reducing bacteria (SRB)316L stainless steelPitting corrosionSynergy

1. Introduction

The microbiologically influenced corrosion (MIC) is a result ofinteractions, which are often synergistic, between the metalsurface, abiotic corrosion products (FeS, Fe(OH)3, Fe2O3 etc.),and bacterial cells and theirmetabolites [1]. The importance ofmicrobial synergy has recently been reconfirmed in studies ofcarbon steel corrosion in the presence of microorganismsisolated from rock samples obtained from the proposed high-level nuclear waste repository site at Yucca Mountain [2] andin the presence of thermophilic and thermotolerant bacteriafrom a hot spring in Mexico [3].

Microorganisms tend to colonize on solid surface in manyenvironments [4]. The biofilm forms a protective layer,reducing the exposure of the solid surface to the externalenvironment. However, it could also result in localizedcorrosion and deterioration of the substratum materials,such as metals, polymers and concrete [5–7]. The presence of

8; fax: +86 29 8323 7910.(G. Cheng).

er Inc. All rights reserved

microorganisms on a metal surface often leads to highlylocalized changes in the concentration of the electrolyteconstituents, pH and oxygen concentration [8,9]. Thesemicroorganisms and their metabolic activity have influencedseverely the corrosion process, they often stimulate localizedcorrosion, depending on the passive film forming and repair-ing capabilities of the metal or alloy [10,11].

Type 316L stainless steel has been used increasingly forcooling water service in the chemical, petrochemical and powerutility industries. The excellent corrosion resistance of stainlesssteel is due to the formation of a stable passive layer.Nevertheless stainless steel is susceptible to localized corrosionby chloride ions and reduced sulfur compounds [12]. In recentyears it has becomemore andmore common phenomenon thatmicrobiologically activity may play an important role in pittingcorrosionof stainless steel inmany industrial environments [13].

Nevertheless the importance of microbial synergy onstainless steel corrosion in the presence of anaerobic SRB

.

Table 1 – Analytical results for cooling water sampled from oil refinery

Cl− CO32− HCO3

− Ca2+ Mg2+ SO42− pH Total hardness TDS SAL

242.66 19.246 435.504 112.30 200.15 240.24 8.16 1105.35 932 0.9

246 M A T E R I A L S C H A R A C T E R I Z A T I O N 5 9 ( 2 0 0 7 ) 2 4 5 – 2 5 5

and aerobic IOB is still unknown. The purpose of this study isto investigate the influence of microbial synergy in thepresence of SRB and IOB, separated from cooling water systemof oil refinery, on 316L stainless steel about the corrosionprocess characteristics and the electrochemical behaviorusing open circuit potential measure, potentiodynamic scan-ning, cyclic polarization measure, scanning electron micros-copy (SEM) and energy dispersive spectrum (EDS) techniques.

1 Princeton Applied Research (PAR) is a global leader in themanufacture of electrochemical instrumentation including po-tentiostats and galvanostats for research electrochemistry, ap-plied corrosion, battery and fuel cell research, biomedicalresearch, plating and sensor applications. It is part of AdvancedMeasurement Technology, Inc, a division of AMETEK, Inc.

2. Materials and Methods

2.1. Metal Samples Preparation

The corrosion specimens were cut from Type 316L stainlesssteel sheet, the nominal elemental composition (wt.%) of 316LSS specimens was : C 0.029, Cr 16.97, Ni 10.11, Mo 2.04 Mn 1.38,Si 0.39, P 0.031, S 0.005. Disc shape specimens with a diameterof 18 mm and thickness of 2 mm were used for electrochem-ical measurements, rectangle specimens with dimensions30×25×2 mm were used for biofilm observation. To createworking electrodes, an electrical contact to each sample wasprovided by a length of copper wire connected to the back ofeach specimen mounted in an epoxy resin, then the speci-mens were abraded through 240, 400 and 600-grit siliconcarbide metallurgical paper, degreased in acetone, washedwith sterile distilled water and dried in a desiccator until use.

2.2. Medium

All tests were conducted using a nutrient-rich medium. Themedium consists of 4.12 g MgSO4·7H2O, 5 g sodium citrate,1.26 g CaSO4 2H2O, 1.0 g NH4Cl, 1.0 g K2HPO4, 3.5 g sodiumlactate, and 1.0 g yeast extract. The pH of the medium wasadjusted to 7.2. Sealed 2-l flasks containing 1.5 l of nutrientmedium were autoclaved at 121 °C for 20 min and stored atroom temperature until use.

2.3. Microbiological Cultivation and Inoculation

Experimental SRBand IOBwere isolated fromcyclic coolingwatersystem of an oil refinery plant, the chemical composition ofcooling water is provided in Table 1. SRB and IOBwere cultivatedseparately in appropriate media. SRB culture was grown incorrected Postgate C medium (g/L): 0.5 KH2PO4, 1.0 NH4Cl, 4.5Na2SO4, 0.06 CaCl2·2H2O, 0.06 MgSO4·7H2O, 6 sodium lactate, 1.0Yeast extract, 0.004 FeSO4·7H2O, 0.3 sodium citrate (pH 7.2) underanaerobic conditions. IOB culture was grown in Winogradskinutrient medium (g/L): 0.5 K2HPO4, 0.5 NaNO3, 0.2 CaCl2, 0.5MgSO4·7H2O, 0.5 NH4NO3, 6.0 ammonium iron citrate (pH 6.8)underaerobicchamber.Thesesolutionswereautoclavedat121 °Cfor 20 min. These cultures were incubated at 30 °C. Enrichmentcultureswere used as the corrosion cell inoculum. Test cellswereinoculated with 5%(V/V) of each of the selected cultures.

2.4. Dual Culture Experiments

Anaerobic sulphate-reducing bacteria (SRB) were added insome experiments to already growing aerobic iron-oxidizingbacteria (IOB) to examine the combined effect of IOB with thatfor SRB. We observed that SRB can be active in an anaerobicbottom layer when the bulk solution contains oxygen.Formation and maintenance of such anaerobic conditionsare due to the presence of aerobic bacteria. The respiration ofaerobic IOB scavenges oxygen and favors growth conditionsfor SRB. In these experiments, the SRB were added from 3 to5 days after the inoculation of the medium with the aerobicbacteria.

2.5. Electrochemical Measurements

All electrochemical testswere carried out in a 2 l corrosion cell,with three-electrode system, the measurements were donewith M 263A potentiostat and M 5210 phase lock-in amplifierof PAR, Inc., Oak Ridge, USA1. Working electrode potentialswere referred to a saturated calomel electrode (SCE). Thecounter electrode was a Pt-plate. Polarization curves weredetermined potentiodynamically with a scan rate of 0.5 mV/s.EIS measurements were made at the open circuit potentialusing a 10 mV amplitude sinusoidal signal over frequenciesranging from 5mHz to 20 kHz. All measurements were carriedout at 30 °C for optimum bacteria growth.

2.6. Surface Analysis

The test coupons were examined for their surface biofilm andcorrosion features using SEM and EDS. The coupons withbiofilm were immersed for 15 min in a 4% glutaraldehydesolution in order to fix the biofilm to the stainless steelsurface, and then become dehydrated using four ethanolsolutions (15 min each): 25, 50, 75 and 100% successively. Afterthat, the samples were taken to the SEM and EDS for theirsurface analysis.

3. Results and Discussion

3.1. Corrosion Potential vs. Time

The iron bacteria species isolated from cyclic cooling watersystem was identified as Leptothrix sp.; SRB identified wasDesulfovibrio sp. Fig. 1 shows the variations of the corrosionpotential (Ecorr) with immersion time for stainless steel in

Fig. 3 –Cyclic polarization curves for stainless steel in fourbiological solutions after 15 days immersion at 30 °C.

Fig. 1 –Variation of corrosion potential with time for stainlesssteel in different solutions.

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sterile medium, IOB, SRB, IOB+SRB and 0.01 M NaCl+IOB+SRBsolutions at 30 °C. In the sterile medium, no significantchanges in Ecorr occur, then observed the electrode surface anddidn't find the clear corrosion after the termination of test,indicating that the steel specimens were in a passive stateduring the whole test session. In the presence of only IOB, Ecorrwas reduced by about −0.23 V (from −0.06 to −0.29 V vs. SCE).Inthe presence of only SRB, Ecorr was reduced by about −0.37 V(from −0.06 to −0.43 V vs. SCE). The combination of SRB andIOB make Ecorr values drop at a rate faster than that observedin the presence of IOB or SRB alone, the decrease of Ecorr couldbe attributed to the synergy between microbial cells and theirmetabolites in the presence of SRB and IOB [2]. In 0.01MNaCl+IOB+SRB solutions, Ecorr values dropped to −0.53 V at thefastest rate, the decrease of Ecorr indicated that the addition ofcorrosive chloride ions leads to the further drop of Ecorr thanthat observed in the combination of IOB and SRB.

3.2. Polarization Curves

Fig. 2 gives the potentiodynamic polarization curves forstainless steel electrodes in four biological solutions after3 days immersion at 30 °C. It was found that there was a widerpassive region for stainless steel in the presence of only SRB orIOB, which is at a potential of −0.2 to+1.1 V and −0.2 to+1.0 V(versus SCE) respectively. In contrast, there was a narrower

Fig. 2 –Anodic polarization curves for stainless steel in fourbiological solutions after 3 days immersion at 30 °C.

passive region in the combination of IOB and SRB, which is at apotential of −0.08 to 0.75 V (versus SCE). The much narrowerpassive region was observed in 0.01 M NaCl+ IOB+SRBsolution, which is at a potential of +0.05 to +0.7 V (versusSCE). In addition, in four biological solutions, the corrosioncurrent density (icorr) for stainless steel increased from 10 to1000 μA·cm−2, and Ecorr decreased from −0.3 to −0.55 V andpitting corrosion potential (Epit) also decreased from +1.1 to+0.7 V (versus SCE), indicating the passivity of metals weredeteriorated.

Fig. 3 shows the cyclic polarization curves for stainless steelelectrodes in four biological solutions after 15 days immersionat 30 °C with a scan rate of 0.5 mV·s−1. A wide region ofpassivity for stainless steel in IOB or SRB solution wasobserved before Epit was reached. In contrast, there wasa narrower passive region in IOB+SRB solution. The 0.01MNaCl+IOB+SRB combination exhibited the much narrowerpassive region before Epit was reached. The anodic currentdensity increased steeply after pitting potential (Epit) wasimposed, indicating passive film breakdown on stainless steelsurface. The Epit can be defined as the potential where acontinuous increase in the anodic current occurs. The repassi-vation potential (Erp) can be also defined as the potential atwhich the reverse scan intersects the forward scan andcompletes the hysteresis loop [14–16]. Following passivitybreakdown and reversing the potential direction, a pronouncedhysteresiswas observed in four different biological solutions. Atpotentialsmore negative than Erp, pit initiationwould not occurand existing pitswould cease to propagate and repassivate. The

Table 2 – The values of Epit and Erp in four biologicalsolutions after 15 days immersion with a scan rate of0.5 mV·s−1

Solutions Parameters

Epit (V) Erp (V)

IOB 0.92 −0.18SRB 0.88 −0.10SRB+IOB 0.82 −0.180.01 M NaCl+SRB+IOB 0.62 −0.20

Fig. 4 –K–K transforms of real and imaginary components of the EIS data for 316L SS in different solutions after 7 daysimmersion at 30 °C (a) sterile; (b) IOB; (c) SRB; (d) IOB+SRB; (e) 0.01MNaCl+IOB+SRB.Msd: experimental data; Calc: fitted values.

248 M A T E R I A L S C H A R A C T E R I Z A T I O N 5 9 ( 2 0 0 7 ) 2 4 5 – 2 5 5

local environment chemistry affects both Epit and Erp. There-fore, the presence of microorganisms can significantly affectboth potentials. The values of Epit and Erp for stainless steelelectrodes after 15 days immersion at 30 °C in four biologicalsolutions are shown inTable 2, it shows that the combination ofSRB and IOB caused Epit and Erp to drop than that observed inthe presence of IOB or SRB alone. The further drop of Epit and Erpwas observed in 0.01 M NaCl+IOB+SRB combinative solutionthan that observed in SRB+IOB solution.

These above analysis results revealed that the iron-oxidizing bacteria produced the lowest corrosion rate, as themetabolism of these microorganisms is very slow, especiallyunder non-optimum, circumneutral pH (7.2) conditions [2],SRB takes second place. The combination of IOB and SRBexhibited higher corrosion rate than those with a single IOB orSRB. These results illustrate how important bacterial consortia

are to MIC [17–19]. Consortia appear to be particularlyimportance to anaerobic corrosion processes (i.e., SRB). IOBcan provide anaerobic microniches through biofilm formationon the metal surface and promote corrosion by SRB. Thehighest corrosion rate was observed in 0.01 M NaCl+IOB+SRBsolution, indicating the addition of corrosive chloride ionsfurther enhance pitting corrosion. The increment of corrosionrate is attributed to the synergies between the metal surface,abiotic corrosion products, and bacterial cells and theirmetabolites [1].

3.3. EIS Spectra

Simple inspection of the electrochemical impedance data doesnot necessarily reveal whether or not the data are valid orhave been distorted by some experimental artifact. But the

Fig. 6 –Electrochemical impedance spectra for stainless steel

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validity of the data can be assessed using the Kramers–Kronig(K–K) transforms. Any system that satisfies the a prioriconditions of linearity, stability, and causality must satisfythe K–K transforms[20,21]. The K–K technique transforms thereal component into the imaginary component and vice versa,so that the transformed quantities may be compared directlywith experimental values for the same parameters. As can beseen from Fig. 4, the high fidelity between the experimentaland transformed impedance data for both real and imaginarycomponents show that the system under investigationcomply with the linearity, causality, and stability constraintsof linear system theory and thereby validates the EIS data.

Fig. 5 shows the electrochemical impedance spectra forstainless steel specimens in the sterile medium for anexposure period of 7 days. Impedance data for all sterilecontrol samples remained practically unchanged with expo-sure time, indicating that localized corrosion did not occur. Nosigns of localized corrosion were detected visually, micro-scopically or electrochemically over the entire experimentperiod. SEM micrographs also have validated that localizedcorrosion did not occur under sterile control conditions.

The impedance plots for stainless steel in IOB solution for7 days exposure is shown in Fig. 6. Aerobic biofilm formationin IOB solution on several stainless steel samples at OCPproduces an apparent decrease in RP, in comparison withthose obtained in sterile medium, indicating IOB enhancedcorrosion. This decline in EIS could be attributed to thinning ofthe passive layer due to the presence of aerobic IOB. Activity of

Fig. 5 –Electrochemical impedance spectra for stainless steelafter 7 days immersion in sterilemedium at 30 °C. (a) Nyquistplot; (b) Bode plot. Msd: experimental data.

after 7 days immersion in IOB solution at 30 °C. (a) Nyquistplot; (b) Bode plot. Msd: experimental data.

IOB decreases the concentration of oxygen reaching theelectrode surface and restricts access of fresh oxygen to themetal surface. This reduces the formation of fresh oxide byreaction of the metal with dissolved oxygen, and promotesstainless steel corrosion.

Fig. 7 shows the electrochemical impedance spectra forstainless steel specimens in SRB medium at 7 days ofexposure. The EIS suggest a distinct reduction in the polari-zation resistance, RP, in comparison with those obtained inthe sterile mediums. This decline in EIS could be attributed tothe corrosive H2S metabolically generated by SRB initiatespitting corrosion [12].

Fig. 8 shows the impedance plots for stainless steelspecimens in the combination of SRB and IOB at 7 days ofexposure. The EIS suggest a greater reduction in RP, incomparison with those obtained in sterile medium, SRB orIOB solution, indicating the combination of SRB and IOBaccelerate the corrosion process of stainless steel. This declinein RP could be attributed to the presence of the reduced sulfurcompounds by SRB that can promote the formation ofmicrocolonies and induce pitting corrosion of stainless steel[22]. The formation of an imperfect oxide layer on stainlesssteel in the presence of sulfur has been reported [23].Therefore, it is reasonable to assume that the addition ofSRB not only reduces the passive layer thickness by formingmetal sulfide but also initiates pitting corrosion. Thus, thepresence of heterogeneous biofilm distribution induced by the

Fig. 7 –Electrochemical impedance spectra for stainless steelafter 7 days immersion in SRB solution at 30 °C. (a) Nyquistplot; (b) Bode plot. Msd: experimental data.

Fig. 8 –Electrochemical impedance spectra for stainless steelafter 7 days immersion in IOB+SRB solution at 30 °C. (a)Nyquist plot; (b) Bode plot. Msd: experimental data.

250 M A T E R I A L S C H A R A C T E R I Z A T I O N 5 9 ( 2 0 0 7 ) 2 4 5 – 2 5 5

synergy of SRB and IOB induces the formation ofmicropits andaccelerates corrosion.

The electrochemical impedance spectra obtained fromstainless steel specimens in 0.01 M NaCl+IOB+SRB solutionat 7 days of exposure reveal the lowest RP values (Fig. 9),compared with those obtained in sterile medium, SRB, IOB,IOB+SRB solutions. This further decline in RP could be due tothe addition of another aggressive chloride anion that altersthe oxide layer structure, breaks down the passive film, flakesoff of the biofilm, facilitates the formation of corrosion andaccelerates pitting propagation on stainless steel surface [24].

From above results, the impedance spectra obtainedexperimentally were analyzed using ZSimpWin2 equivalentcircuit software, two time constant equivalent circuits in Fig.10 were proposed as the models for the corrosion systemsstainless steel/solution. Data for stainless steel in sterilemediumwere analyzed using the equivalent circuit illustratedin Fig. 10a, the impedance expression is written as Formula (1).Data for stainless steel in four biological solutions wereanalyzed using the equivalent circuit shown in Fig. 10b,where a Warburg impedance (Zw) was introduced to accountfor a diffusion process within the metal heterogeneity surfacelayer, the impedance expression is written as Formula (2). The

2 ZSimpWin is an Electrochemical Impedance Spectroscopy(EIS) Data Analysis Software utilizing the fast computation speedof recent computers.

Fig. 9 –Electrochemical impedance spectra for stainless steelafter 7 days immersion in 0.01 M NaCl+IOB+SRB solution at30 °C. (a) Nyquist plot; (b) Bode plot. Msd: experimental data.

Fig. 10 –Two equivalent circuits used in fitting impedancedata of stainless steel at different conditions.

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two time constant equivalent circuit fitted the impedance atall the frequencies, as shown in Figs. 5–9.

Z ¼ RS þ 1jwCf þ 1

Rfþ 1

jwCdlþ 1Rp

ð1Þ

Z ¼ RS þ 1jwCf þ 1

Rfþ 1

jwCdlþ 1RpþZw

ð2Þ

where Rs represents the electrolyte resistance, Rf and Cf

represent the resistance and capacitance of heterogeneitysurface layer, RP and Cdl represent the polarization resistanceand double layer capacitance, respectively. ZW represents thediffusion impedance through the porous film layer. Inaddition, both Cf and Cdl were replaced with constant phaseelement (CPE) in the fitting procedure due to the non-idealcapacitive response of the interface stainless steel/solution[25]. The impedance of CPE is written as ZCPE=Y0

−1(jω)−α, whereω is angular frequency in rad/s, Y0 is the admittancemagnitude of CPE which can be approximately convertedinto a capacitance, and α is the exponential term which canvary between 1 for pure capacitance, and 0 for pure resistor[26]. In this context, α can be used as the measure of surfaceinhomogeneity, its decrease being connected with certainincrease in surface metal roughening [27].

The fitted results of EIS spectra for stainless steel in sterilemedium and four different biological solutions are given inTable 3. The value of αf for stainless steel in sterile mediumwas found to be less than one, which reflects some degree of

Table 3 – Fitted results for EIS spectra after different times of ex

Time/day RS/Ω cm2 Y0− f/sa Ω−1 cm−2 α

In sterile medium 9.19 6.78×10−5 0.9In IOB solution 11.33 4.15×10−5 0.9In SRB solution 10.53 3.76×10−5 0.9In SRB+ IOB solution 12.24 12.93×10−5 0.8In 0.01 M NaCl+SRB+IOB solution 9.57 1.09×10−5 0.7

passive film heterogeneity. A lower value of αf was observed inthe presence of IOB alone and in the presence of IOB and SRB.The lowest αf value was found in the presence of 0.01 M NaCl+IOB+SRB. The continuous decrease of αf values indicates theincrement of the surface inhomogeneity due to corrosion. Theheterogeneity distribution of the biofilms and the initiation oflocalized corrosion would expect to increase the passive filmheterogeneity [21]. The values of Rf and RP at same exposuretime for stainless steel in sterile medium and four differentbiological solutions are sort by size, R (in sterile medium) NR(in IOB solution) NR (in SRB solution) NR (in SRB+IOB solution)NR (in 0.01 M NaCl+IOB+SRB solution). This continuousdecrease of Rf and RP values observed in different solutionsindicates the increment of corrosion rate, which could be dueto the synergies between the metal surface, abiotic corrosionproducts, and bacterial cells and their metabolic productswhich are produced and remained into the biofilm at themetal surface, increasing the corrosion process [1,28]. Theseresults agree with the polarization analysis results.

3.4. Surface analysis

SEM was carried to validate the adhesion of the microorgan-isms to the stainless steel surface and also to analysismicrobial diversity. EDS was taken to verify the mechanismof corrosion products film formation on stainless steel surface.Fig. 11 shows a detail of corrosion products and biofilmsdeveloped on stainless steel surface exposed to: (a), (b) IOBsolution; (c), (d) SRB solution; (e), (f) IOB+SRB solution; (g), (h)0.01 M NaCl+SRB+IOB solution for 20 days respectively. In thepresence of IOB, the corrosion products were spongy browniedeposits (Fig. 11a), EDS analysis showed that these depositsevidently are iron oxides (Fig. 12a). Fig. 11b shows the biofilmformed on the metal surface, where typical ellipsoidal IOBcells, approximately 1 μm size, are observed. In the solution ofSRB, specimens were covered with black corrosion productswhich were dense, brittle and lumpy deposits, the crackseventually were clearly seen on the surface (Fig. 11c), Fig. 11dshows the biofilm formed on the metal surface, where typicalrod-shaped SRB cells, about 1 μm size, are observed. Thecharacteristic color of iron sulfidewas observed in the cell withSRB, produced by the reaction of the sulfides generated by theSRB with the ferrous salts present in the medium [24]. Highsulfur content and small isolated heaps were observed in Fig.12b. It is important to point out that, in accordancewith the pHofmedium (7.2), this compound could be kansite (Fe9S8), whichhas very poor protective properties[22,23].

In the solution of SRB and IOB combination, stainless steelspecimenswere coveredwith rough and cluster black corrosionproducts (Fig. 11e), EDS analysis showed that these corrosion

posure

f Rf/Ω cm2 Y0−dl/sa Ω−1 cm−2 αdl Rp/Ω cm2 Zw/Ω−1

4 240.7 2.93×10−5 0.70 2.19×106 –1 115.5 2.25×10−5 0.71 2.50×105 2.79×10−5

0 110.1 2.33×10−5 0.72 8.50×104 5.28×10−5

8 68.41 2.32×10−5 0.73 3.48×104 10.45×10−5

6 7.24 2.29×10−5 0.75 717.6 15.22×10−5

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products are iron sulfides and iron oxides (Fig. 12c). Fig. 11fillustrates the biofilm formed on the metal surface, where therod-shaped SRB of approximately 2 μm size and sphericalfilamentous IOB of approximately 1 μm size, are observed.

In the solution of 0.01M NaCl+SRB+IOB, specimens werecovered with black corrosion products which were dense,cluster and thick film layer (Fig. 11g), EDS analysis illustratedthat the film layer is made up of iron sulfides, iron oxides and

Fig. 11 –SEMphotomicrograph showing biofilms and bacteria on sIOB (a), (b); SRB (c), (d); SRB+IOB (e), (f) and 0.01 M NaCl+SRB+IOB

iron chlorides (Fig. 12d). Fig. 11h shows the biofilm formed onthemetal surface, where the rod-shaped SRB of approximately2.5 μm size and spherical filamentous IOB of about 1 μm sizeare observed, and a high cell density can be observed, wherebig colonies were observed on the metal surface.

Typical SEM micrographs are presented in Fig. 13 forstainless steel coupons at the end of the experimental period(20 days). The SEM analysis on the stainless steel surface

tainless steel surface after 20 days exposure, with presence of(g), (h).

Fig. 12 –EDS analysis showing corrosion products formed on stainless steel surface after 20 days exposure, with presence of IOB(a), SRB (b), SRB+IOB (c) and 0.01 M NaCl+SRB+IOB (d).

253M A T E R I A L S C H A R A C T E R I Z A T I O N 5 9 ( 2 0 0 8 ) 2 4 5 – 2 5 5

indicated that the metal exposed to sterile medium, exhibitedbasically no localized corrosion observed on the metal surfaceunder open circuit conditions after 20 days exposure (Fig. 13a),impedance data also provide further evidence that localizedcorrosion did not occur under sterile conditions. Once thecorrosion products film was removed from the steel exposedto four biological solutions, localized corrosion process wasobserved at the metal surface, as shown in Fig. 13b–e. In IOBsolution, the corrosion damage observed on coupons surfaceillustrates only a minor corrosion pits (Fig. 13b), this isattributed to iron bacteria's ability to metabolize ferrous ionsto ferric and then forming a low density hydrated iron oxide inthe corrosion tubercles is key factor for corrosion of steel [29].Thus, IOB create environments that are conducive to sustain-ing their growth and subsequently promote corrosion. In SRBsolution, the presence of SRB apparently initiates pittingcorrosion of the exposed specimens as indicated by thepresence of large and deep pits as shown in Fig. 13c.

However, in the combination solution of SRB+IOB, thecorrosion damage was larger and deeper than that observed inSRB or IOB alone, this is due to the inside of corrosion tuberclesproduced by IOB becomes more anaerobic and favors SRBgrowth, the presence of SRB and IOB in the solution could besynergizing in the corrosionprocess,whichproduceand remainthe corrosivemetabolic products of iron sulfide into the biofilmat the metal surface, accelerating the corrosion process thanthat observed in the presence of SRB or IOB alone [30], as shownin Fig. 13d. At the same time, the corrosion damage observed on

stainless steel surface exposed to 0.01 M NaCl+SRB+IOBsolution was more severe than the damage observed in SRB+IOB solution, this is due to the presence of chloride furtheraccelerates the pits progression, as shown in Fig. 13e.

The above results indicate that the degree of pittingcorrosion for stainless steel in the combination of 0.01 MNaCl+SRB+IOB is the most severe, it takes second place inSRB+IOB solution, it is the least in IOB solution, and thespecific conditions of the cooling water system in oil refineryplant favored biofilm formation and allowed the growth ofboth anaerobic SRB and aerobic IOB. These results agree withthe electrochemical analysis results.

4. Conclusions

• Electrochemical analysis indicates that Ecorr and RP of 316LSS in the sterilemedium remained virtually unchangedwithexposure time, indicating that localized corrosion did notoccur, and stainless steel is relatively stable in the sterilemedium. The combination of SRB and IOB leads to a distinctdecrease in Ecorr, Epit and RP, but the addition of chlorideions further accelerates the decline range in the values ofEcorr, Epit and RP, and induces localized corrosion.

• SEM morphologies have also shown that 316L SS reveals nosigns of pitting attack in the sterile medium. However,micrometer-scale pitting corrosion was observed on thestainless steel surface in four different biological solutions.

Fig. 13 –SEMmicrographs showing corrosion pits on stainless steel surface after 30 days exposure, in sterile medium (a); in thepresence of IOB (b); SRB (c); SRB+IOB (d) and 0.01 M NaCl+SRB+IOB (e).

254 M A T E R I A L S C H A R A C T E R I Z A T I O N 5 9 ( 2 0 0 7 ) 2 4 5 – 2 5 5

• The synergies between the metal surface, abiotic corrosionproducts, and bacterial cells and their metabolic productsinfluenced the pitting corrosion process of stainless steel,increased the corrosion damage degree of the passive filmand accelerated the pitting corrosion process.

• Two bacterial types tested demonstrated the ability tocorrode the 316L SS specimens at rates above that seen inthe sterile medium. The combination of SRB and IOBdemonstrated higher corrosion rates than that observed inthe presence of SRB or IOB alone. Furthermore, the synergyof 0.01 M NaCl+SRB+IOB yielded the highest corrosion rate,indicating the participation of chloride anion furtheraccelerated pitting propagation.

Acknowledgement

This work was supported by National Natural ScienceFoundation of China, No. 20576108.

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