8/9/2019 20060621

1/6

Chinese J.Chem.Eng., 14(6) 829834 (2006)

RESEARCH NOTES

Corrosion and Electrochemical Behavior of 316L Stainless Steel in

Sulfate-reducing and Iron-oxidizing Bacteria Solutions*

XU Congmin()a, ZHANG Yaoheng()a, CHENG Guangxu()a,**andZHU Wensheng()baDepartment of Chemical Engineering, Xian Jiaotong University, Xian 710049, China

b Research and Technology Center of Lanzhou Oil Refinery Factory, PetroChina Company Ltd., Lanzhou 730060,China

Abstract Corrosion and electrochemical behavior of 316L stainless steel was investigated in the presence ofaerobic iron-oxidizing bacteria (IOB) and anaerobic sulfate-reducing bacteria (SRB) isolated from cooling watersystems in an oil refinery using electrochemical measurement, scanning electron microscopy (SEM) and energydispersive atom X-ray analysis(EDAX). The results show the corrosion potential and pitting potential of 316Lstainless steel decrease distinctly in the presence of bacteria, in comparison with those observed in sterile mediumunder the same exposure time. SEM morphologies have shown that 316L stainless steel reveals no signs of pittingattack in the sterile medium. However, micrometer-scale corrosion pits were observed on 316L stainless steel sur-face in the presence of bacteria. The presence of SRB leads to higher corrosion rates than IOB. The interactionsbetween the stainless steel surface, abiotic corrosion products, and bacterial cells and their metabolic products in-

creased the corrosion damage degree of the passive film and accelerated pitting propagation.Keywords sulfate-reducing bacteria (SRB), iron-oxidizing bacteria (IOB), 316L stainless steel, pitting corrosion,electrochemical behavior

1 INTRODUCTION

Type 316L stainless steel has good corrosion re-sistance and has been used increasingly for coolingwater service in the chemical, petrochemical andpower utility industries. However stainless steel issusceptible to localized corrosion by chloride ions andreduced sulfur compounds

[1]. The presence of micro-

organisms on a metal surface often leads to highly

localized damages in the concentration of the electro-lyte constituents, pH and oxygen levels[2]

. These mi-croorganisms and their metabolic activity have influ-enced severely the corrosion process, and oftenstimulated localized forms of corrosion

[3]. During the

last several years, various cases of corrosion damagecaused by bacteria were observed in cooling watersystem of oil refinery, sulfate-reducing bacteria (SRB)and iron-oxidizing bacteria (IOB) were the most trou-blesome group of bacteria on tubercular corrosion andinduced microbiologically influenced corrosion incooling circuit, caused poor water quality and equip-ment clogging, pipe punctures and high corrosion

rates, resulted in the serious pitting corrosion of car-bon steel equipment

[47]. The biological corrosion of

steels has received increasing consideration in the lastfew decades. Similar studies by Duan et al.

[8]showed

the characteristics of sulfide corrosion products on316L stainless steel surfaces with the presence of SRBin seawater and soil environment, the study of Romeroet al.

[9] indicated that attack morphology of carbon

steel generated by SRB in systems for secondary re-

covery of crude oil is characterized by rounded holesin chains or groups, Chamritski et al.

[10]founded that

IOB could give rise to minor localized corrosiondamage of 304L stainless steel by under-deposit (i.e.,crivice) corrosion in natural spring water. However,few studies were reported about the corrosion andelectrochemical behavior of stainless steel in coolingwater system of oil refinery.

The present study is aimed to gain better under-standing of corrosion process characteristics and elec-trochemical behavior of 316L stainless steel in themedia of anaerobic SRB and aerobic IOB separatedfrom cooling water systems of oil refinery using opencircuit potential measurement, electrochemical meas-urement, scanning electron microscopy (SEM) andenergy dispersive atomi X-ray analysis (EDAX) tech-niques.

2 EXPERIMENTAL

2.1 Preparation of specimen

The corrosion specimens were cut from type

316L stainless steel sheet, the nominal elementalcomposition (%, by mass) of 316L SS specimens was:C 0.029, Cr 16.97, Ni 10.11, Mo 2.04, Mn 1.38, Si0.39, P 0.031, S 0.005. Disc shape specimens with adiameter of 18mm and thickness of 2mm were usedfor electrochemical measurements, rectangular speci-mens with dimensions 30mm25mm2mm wereused for biofilm observation. To create working elec-trodes, an electrical contact to each sample was

Received 2006-03-20, accepted 2006-09-18.* Supported by the National Natural Science Foundation of China (No.20576108).

** To whom correspondence should be addressed. E-mail: gxcheng@mail.xjtu.edu.cn

8/9/2019 20060621

2/6

Chinese J. Ch. E. (Vol. 14, No.6)

December, 2006

830

provided by a long copper wire connected to the backof each specimen mounted in an epoxy resin, then the

specimens were abraded through 240, 400 and600-grit silicon carbide metallurgical paper, degreasedin acetone, washed with sterile distilled water anddried in a desiccator until use.

2.2 Microbiological cultivation and inoculation

Experimental SRB and IOB were isolated fromcyclic the cooling water system of an oil refinery plant,the chemical composition of cooling water is providedin Table 1. IOB species isolated from cyclic coolingwater system was identified as Leptothrix sp. This isthe most common iron storing ensheathed bacterium

apparently occurring in slow running, ferrousiron-containing waters, poor in decomposable organicmaterial. Leptothrix species oxidize ferrous ions toferric ions to obtain their energy; SRB identified was

Desulfovibrio sp. This is documented to show aggres-sive corrosion with many metals under anaerobic con-ditions. SRB and IOB were cultivated separately inappropriate media. SRB culture was grown in correctedPostgateC medium: 0.5gL

1 KH2PO4, 1.0gL

1

NH4Cl, 4.5gL 1

Na2SO4, 0.06gL 1

CaCl22H2O,0.06gL

1 MgSO47H2O, 6gL

1 sodium lactate,

1.0gL 1

Yeast extract, 0.004gL 1

FeSO47H2O,0.3gL

1 sodium citratepH 7.2under anaerobic

conditions. IOB culture was grown in Winogradskinutrient medium: 0.5gL

1K2HPO4, 0.5gL

1NaNO3,

0.2gL1

CaCl2, 0.5gL1

MgSO47H2O, 0.5gL 1

NH4NO3, 6.0gL

1 ammonium iron citrate (pH 6.8)

under aerobic chamber. These solutions were auto-claved at 121 for 20min. Enriched cultures wasincubated at 30. Enriched cultures were used as thecorrosion cell inoculum. Test cells were inoculatedwith 5% (by volume) at approximately 10

8cellsml

1

of each of the selected cultures.

2.3 Electrochemical measurements

All electrochemical tests were carried out in a 2Lcorrosion cell, with a three-electrode system, themeasurements were done with M263A potentiostatand phase lock-in amplifier (EG & G, USA). Workingelectrode potentials were referred to a saturated calo-mel electrode (SCE). The counter electrode was aPt-plate. Polarization curves were determined poten-tiodynamically with a scan rate of 0.5mVs

1. EIS

measurements were made at the open circuit potentialusing a 10mV amplitude sinusoidal signal over fre-quencies ranging from 5mHz to 100kHz. All meas-urements were carried out at 30for optimum bacte-

ria growth.

2.4 Surface analysis

The test coupons were examined for surface

biofilm and corrosion features using SEM and EDAX.The coupons with biofilm were immersed for 15minin a 4% glutaraldehyde solution in order to fix thebiofilm to the stainless steel surface, and then becomedehydrated using four ethanol solutions (15min each):25%, 50%, 75% and 100% successively. After that,the samples were taken to the SEM and EDAX fortheir surface analysis.

3 RESULTS AND DISCUSSION

3.1 Corrosion potential vs. timeEach test was allowed to run until the corrosion

potential (Ecorr) and the polarization resistance(Rp)reached their asymptotic values. Fig.1 shows thevariations of the corrosion potential (Ecorr) with theimmersion time for stainless steel in sterile medium,IOB and SRB solutions at 30. In the sterile medium,no significant changes in Ecorr occur, indicating thatthe specimen was in a passive state during the wholetest session. Ecorrof both electrodes were reduced byabout 0.36V (from 0.06 to 0.42V vs. SCE)and 0.46V (from 0.06 to 0.52V vs. SCE) inIOB solution and SRB solution respectively. More-over SRB make Ecorrvalues drop at a rate faster thanthat observed in the presence of IOB. After about 10dof immersion, Ecorrof both electrodes becomes stabi-lized. The decrease ofEcorrwas related to the dissolu-tion of electrode surface passive film induced bymetabolic activity of SRB and IOB during the immer-sion.

Figure 1 Variation of corrosion potential with time forstainless steel in different solutionssterile medium;IOB;SRB

3.2 Electrochemical test

Figure 2 illustrates the potentiodynamic polariza-tion curves of stainless steel electrodes in sterile me-dium, IOB and SRB solutions after 4d immersion at30. The width of the passive range in sterile me-dium is the largest, that is 1.40V (0.15 to 1.25VSCE),

and the value of pitting potential (Epit) is the highest,

Table 1 Analytical results for cooling water sampled from oil refinery (mgL1)

Cl 23CO

3HCO Ca

2+ Mg2+ 24SO pH Total hardness Total dissolved solid Salt

242.66 19.246 435.504 112.30 200.15 240.24 8.16 1105.35 932 0.9

8/9/2019 20060621

3/6

Corrosion and Electrochemical Behavior of 316L Stainless Steel in SRB and IOB Solutions

Chinese J. Ch. E. 14(6) 829 (2006)

831

about +1.25V, which shows a good pitting resistance.Extended passive region, from 0.3 to +0.48V, isfound for stainless steel in IOB solution, and unstablepitting was initiated at 0.48V. A second passivationoccurred at 0.52V, but the passive region was verynarrow. At around 0.82V, stable pitting was developed.In contrast, a narrower passive region (from 0.53 to

0.29V) in SRB solution was observed before unsta-ble pittingpotential was reached, indicating decreasein passive state stability. At potentials around 0.88V,stable pitting corrosion occurred. There is a tendencyfor polarization curves to shift right (to higher cur-rent densities) in sterile medium, IOB and SRB solu-tions respectively. In addition, Fig.2 also show that theanodic polarization current density increases signifi-cantly and that the width of the passive range ofpotentials and corrosion potential decrease in sterile

medium, IOB and SRB solutions in turn, indicatingthe corrosion rate (corrosion current density) was thehighest in SRB solution, higher in IOB solution andthe lowest in sterile medium.

Figure 3 presents the cyclic polarization curvesfor stainless steel electrodes in sterile medium, SRBand IOB solutions after 20d immersion at 30with a

scan rate of 0.5mVs

1. The potential where the loopwas closed is referred to as the repassivated potential(Erp). Pits grow between Epitand Erp, but the nuclea-tion of new pits only take place above Epit. BelowErppit growth is not observed. The local environmentchemistry affects both Epit and Erp

[11]. Therefore, the

presence of microorganisms can significantly affectboth potentials. As can be seen from Fig. 3, during thereverse scan period in sterile medium, smaller currentdensities are recorded for the same values of potential

Figure 2 Anodic polarization curves for stainless steel in three different solutions after 4d of immersion at 30

1sterile medium; 2SRB; 3IOB

Figure 3 Cyclic polarization curves for stainless steel in three different solutions after 20d of immersion at 30

1

sterile medium; 2

SRB; 3

IOB

8/9/2019 20060621

4/6

Chinese J. Ch. E. (Vol. 14, No.6)

December, 2006

832

and the loop has a small area. The finalEcorrat +0.88Vis much more noble than the startingEcorrat 0.05V,indicating stainless steel specimen is efficiently pas-sivated from the moment of its immersion. In two bio-logical solutions, following passivity breakdown andreversing the potential direction, a pronounced hys-teresis was observed. In the presence of IOB, pitting

corrosion was initiated at 0.8V(positive scan) and Erpat 0.1V. In SRB solution, Epit and Erp were muchlower than that caused by IOB, which is at a potentialof 0.45V and 0.08V respectively. Compared withFig.2, Epitvalues of two electrodes in IOB and SRBsolutions decreased with increasing exposure time,which shows the pitting corrosion of stainless steelwas further enhanced with exposure time.

As a complementary technique, electrochemicalimpedance spectrum (EIS) was conducted to confirmthe trends of the corrosion rates determined by poten-tiodynamic polarization method. Nyquist diagrams of

stainless steel in sterile medium, SRB solution andIOB solution are given in Fig.4. The Nyquist plot insterile medium shows an open capacitance arc. It isevident that in the frequency range of the measure-ment, two diagrams in two biological solutions revealqualitatively similar features. The effect of SRB is todecrease the magnitude of the impedance value com-pared to that of IOB. The impedance induced by SRBis the smallest, followed by IOB and sterile medium.

Figure 4 Niquist plots for 316L SS in two biological solu-tions

sterile medium (exp.);IOB (exp.);SRB (exp.);fitted values

3.3 Surface analysis

SEM was carried to validate the adhesion of themicroorganisms to the stainless steel surface and alsoto analysis microbial diversity. Fig.5 shows a detail ofbiofilm developed on stainless steel surface exposedto SRB solution and IOB solution for 15d. As seen inFig.5(a), there was a predominance of rod-shapedSRB cells in the presence of SRB, where a high celldensity with the typical morphology of the genus used(Desulfovibrio between 1.5m to 2.0m) can be ob-served. In the presence of IOB, there was a dominanceof spherical IOB cells (Leptothrixbetween 1.0m to1.5m), where big colonies were observed [Fig.5(b)].

To analyze the effects of corrosion product layer,

SEM coupled with EDAX analysis patterns weretaken to identify the constituents of corrosion productlayer formation on stainless steel surface. SEM andEDAX analysis on the samples after 15d of exposure

in SRB solution and IOB solution are shown in Figs.6and 7. As can be seen from Fig.6(a), specimens werecovered with black corrosion products which weredense, brittle and lumpy deposits, the cracks eventu-ally were clearly seen on the surface, the characteristiccolor of iron sulfide was observed in the cell withSRB, produced by the reaction of the sulfides gener-ated by the SRB with the ferrous salts present in theculture medium

[12]. High sulfur content and small iso-

lated heaps were observed in Fig.6(b). It is importantto point out that, in accordance with the pH of me-dium (7.2), this compound could be kansite (Fe9S8),which has very poor protective properties

[13]. In the

presence of IOB, the corrosion products were porousbrownie deposits [Fig.7(a)], EDAX analysis showedthat these deposits evidently are iron oxides.

Typical SEM micrographs of corrosion pits forstainless steel coupons are presented in Fig.8 for 30d.The SEM analysis on the stainless steel surface indi-cated that the metal exposed to sterile medium, exhib-ited basically no localized corrosion observed on themetal surface under open circuit conditions for 30d[Fig.8(a)]. Once the corrosion products film wasremoved from the steel exposed to two biologicalsolutions, localized corrosion was observed on the

metal surface, as shown in Figs.8(b) and 8(c). In SRB

(a) SRB

(b) IOB

Figure 5 SEM micrograph of biofilms on electrodesurface after 15d of exposure

8/9/2019 20060621

5/6

Corrosion and Electrochemical Behavior of 316L Stainless Steel in SRB and IOB Solutions

Chinese J. Ch. E. 14(6) 829 (2006)

833

solution, the presence of SRB apparently initiates pit-ting corrosion of the exposed specimens as indicatedby the presence of large and deep corrosion pits asshown in Fig.8(b). As SRB have been documented to

show aggressive corrosion with many metals under

anaerobic conditions. SRB are ubiquitous and easy toculture, they produce hydrogen sulfide that can reactwith stainless steel in sulfate-containing environments.Sulfide produced by the SRB then migrates to edges

of the deposits where it is oxidized to thiosulfate,which is a well-known activator of pitting corrosion

[14].

The estimated contribution of SRB to the total bacte-rial biomass of the systems always exceeded that ofthe IOB consortium, which was consistently the low-est

[15]. In IOB solution, the corrosion damage ob-

served on coupons surface illustrates only a minor andshallow circular corrosion pits [Fig.8(c)]. This is at-tributed to iron bacterias slow metabolic ability tooxidize ferrous ions to ferric and then form a low den-sity hydrated iron oxide in the corrosion tubercles.The metabolic ability is key factor for corrosion of

steel, as the metabolism of these microorganisms is

(a) SEM

(b) EDAX (spectrum 1)

Figure 6 SEM micrographs and EDAX analysis ofcorrosion products induced by SRB on sample

surface after 15d of exposure

(a) SEM

(b) EDAX (spectrum 1)

Figure 7 SEM micrographs and EDAX analysis ofcorrosion products induced by IOB on sample

surface after 15d of exposure

(a) Sterile medium

(b) SRB solution

(c) IOB solution

Figure 8 SEM micrographs of corrosion pits on sample

surface after 30d of exposure

8/9/2019 20060621

6/6

Chinese J. Ch. E. (Vol. 14, No.6)

December, 2006

834

very slow, especially under non-optimum, circumneu-tral pH (7.2) conditions

[15]. The results indicate that

pitting corrosions of 316L SS in SRB solution is themost severe, followed by IOB and sterile medium.This is consistent with the result of the electrochemi-cal measurement.

NOMENCLATUREEcorr corrosion potential, VEpit pitting potential, VErp repassivated potential, VZ real resistance impedance, cm2Z imaginative resistance impedance, cm2

REFERENCES1 Ismail, Kh.M., Jayaraman, A., Wood, T.K., Earthman,

J.C., The influence of bacteria on the passive film sta-bility of 304 stainless steel, Electrochim. Acta, 44,46854692(1999).

2 Costerton, J.W., Lewandowski, Z., Caldwell, D.E.,Korber, D.R., Lappin-Scott, H.M., Microbial biofilms,

Annual Review of Microbiology, 49, 711(1995).3 Dexter, S.C., Duquette, D.J., Siebert, O.W., Videla, H.A.,Use and limitations of electrochemical techniques forinvestigating microbiological corrosion, Corrosion,47(4), 308318(1991).

4 Rosenfeld, I.L., Corrosion Inhibitors, Chemistry, Pub-lishing Ltd., Moscow (1977).

5 Starosvetsky, J.O., Armon, R., Groysman, Fouling ofcarbon steel heat exchanger caused by iron bacteria,Mater. Performance, 38(1), 5561(1999).

6

Rao, T.S., Sairam, T.N., Viswanathan, B., Nair, K.V.K.,

Carbon steel corrosion by iron oxidising and sulphatereducing bacteria in a freshwater cooling system, Cor-ros.Sci., 42, 14171431(2000).

7

Starosvetsky, D., Armon, R., Pitting corrosion of carbonsteel caused by iron bacteria,Int. Biodeter. Biodegr., 47,7987(2001).

8 Duan, J.Z., Hou, B.R., Yu, Z.G., Characteristics of sul-fide corrosion products on 316L stainless steel surfaces

in the presence of sulfate-reducing bacteria, Mat. Sci.Eng. C, 26, 624629(2006).9

de Romero, M., Duque, Z., Rodrguez, L., de Rincn, O.,Araujo, I., A study of microbiologically induced corro-sion by sulfate-reducing bacteria on carbon steel usinghydrogen permeation, Corrosion, 61, 6874(2005).

10 Chamritski, I.G., Burns, G.R., Webster, B.J., Effect ofiron-oxidizing bacteria on pitting of stainless steel,Corrosion, 60(7), 658669(2004).

11 Khan, M.A., Williams, R.L., Williams, D.F., Conjointcorrosion and wear in titanium alloys,Biomaterials, 20,765772(1999).

12 de Mele, M.F.L., Moreno, D.A., Ibars, J.R., Videla, H.A.,Effect of inorganic and biogenic sulfide on localizedcorrosion of heat-treated type 304 stainless steel, Cor-

rosion, 47, 2430(1991).13

Scott, P.J.B., Michael, D., Survey of field kits for sul-fate-reducing bacteria,Mater. Performance, 31, 6468(1992).

14 Felder, C.M., Stein, A.A., Microbiologically influencedcorrosion of stainless steel weld and base metal-four-yearfield test results, Corrosion, 50, 275(1994).

15 Pitonzo, B.J., Castro, P., Amy, P.S., Southam, G., Jones,D.A., Ringelbery, D., Microbiologically influencedcorrosion capability of bacteria isolated from yuccamountain, Corrosion, 60, 6474(2004).