Carbon Steel Corrosionnext Term by Iron Oxidising and Sulphate Reducing Bacteria in a Freshwater Cooling System

Carbon steel corrosion by iron oxidising andsulphate reducing bacteria in a freshwater

cooling system

T.S. Raoa,*, T.N. Sairamb, B. Viswanathanb, K.V.K. Nairc

aWater and Steam Chemistry Laboratory, BARC Facilities, Marine Biology Programme, Kalpakkam,

Tamil Nadu, 603 102 IndiabMaterials Science Division, Indira Gandhi Centre for Atomic Research, Kalpakkam, 603 102 Tamil

Nadu, IndiacNational Institute of Ocean Technology, IIT Campus, Chennai, 600 036 India

Received 11 August 1999; accepted 15 October 1999

Abstract

Microbiologically in¯uenced corrosion of carbon steel has been investigated. Carbon steel

coupons were exposed online in the cooling water system of a nuclear test reactor to assessthe microbial growth on the coupons and the corrosion phenomena. Iron bacteria, sulphatereducing bacteria (SRB) and culturable aerobic heterotrophic bacteria (CAHB) were

monitored both on the coupons and in the cooling water. Corrosion rate was assayed byweight loss method and corrosion products analysed by XRD and Mossbauer spectroscopy.Extensive tuberculation of carbon steel coupons was noticed. SEM pictures revealed thepresence of ensheathed ®lamentous iron bacteria encrusted with corrosion products.

Beneath the tubercles signi®cant pitting and SRB induced corrosion in the form ofconcentric rings was observed. From the phase analysis, the following compounds werefound to be present: g-Fe2O3, Fe2PO5, FePS3, Fe(PO3)3 and BaFeO3 ÿ x. From the present

study it is inferred that iron bacteria (Leptothrix sp.) and SRB (Desulfovibrio sp.) areresponsible for the corrosion of carbon steel. The role of these bacteria in in¯uencing thecorrosion of carbon steel is highlighted. 7 2000 Elsevier Science Ltd. All rights reserved.

Keywords: Iron; Oxidation; Microbiological corrosion

0010-938X/00/$ - see front matter 7 2000 Elsevier Science Ltd. All rights reserved.

PII: S0010 -938X(99)00141 -9

Corrosion Science 42 (2000) 1417±1431

* Corresponding author. Fax: +91-4114-40396/40397.

E-mail address: [email protected] (T.S. Rao).

1. Introduction

The advances in bio®lm theory and techniques have allowed a betterunderstanding of the interactions between microorganisms, metal surfaces andcorrosion processes [1,2]. While penetration of metal matrix due to corrosion is acommon phenomenon with microbiologically in¯uenced corrosion (MIC) in manyindustries, an additional factor with carbon steel corrosion is the associatedhydraulic e�ects [1]. The cooling circuit of Fast Breeder Test Reactor (FBTR)situated at Kalpakkam had problems, which included ¯ow blockages of pipelinesdue to tubercle formation; choking of valves and strainers, pipe punctures andhigh corrosion rates of carbon steel pipelines [3]. The most troublesome group ofbacteria in connection with tuberculation are the ®lamentous sheathed members ofthe order Chlamydobacteriales. Leptothrix is the most widely distributed species ofthis order, even though Crenothrix and Sphaerotilus species are also responsiblefor problems arising from iron bacteria corrosion in cooling water systems [4].Tuberculation of the pipelines caused by these bacteria can reduce the available¯ow area signi®cantly leading to complete blockage of pipelines [5]. As tuberclesgrow, pressure drop increases and, if left unabated, can be so severe that coolingwater ¯ow to critical equipment will be insu�cient, resulting in safety problems,particularly in nuclear power plants [6]. Preliminary investigations carried out byRao et al. [3,7] showed that iron oxidizing, sulphate reducing bacteria(Desulfovibrio sp.) and exopolymer producing Pseudomonas aeruginosa wereprevalent in the FBTR cooling circuit. The present study was carried out to assessthe predominance of iron bacteria in the FBTR cooling system and also its role incorrosion of carbon steel. In addition, the role of sulphate reducing bacteria(SRB) in the corrosion of carbon steel underneath tubercles was also investigated.

2. Material and methods

2.1. Description of FBTR cooling system

The FBTR cooling water system operates on an open recirculating basis withan induced draft cooling tower, drawing water from an open reservoir (thereservoir has a spread area of 130 m2 with a volume of 7.5 � 106 gallons and themaximum depth was 2.13 m. The reservoir receives water from a submerged riverand is used for intermediate storage before the water is used in the service andauxiliary systems of FBTR and other nuclear establishments at Kalpakkam. Thestructural material is mainly carbon steel. The cooling water treatment regimeconsists of (1) maintenance of a positive Langelier index (0.6 2 0.2); (2)intermittent chlorination (free chlorine residual range from 0.4 to 0.8 ppm; sixtimes a day, every four hours Chlorine dosage = 4 kg hÿ1 for 15 minutes) was inpractice to control microbial growth; (3) addition of commercial brandpolyphosphate based corrosion inhibitor (average ®ltered ortho phosphate rangedfrom 10±14 ppm) and a dispersant to prevent scale formation. The cooling circuit

T.S. Rao et al. / Corrosion Science 42 (2000) 1417±14311418

has a holdup volume of 500 m3, cycles of concentration range from 1.4 to 3.0 andblow down rates range from 80 to 100 m3 per day.

The cooling water is regularly monitored for iron bacteria, SRB andheterotrophic counts as per standard procedures detailed by Rao et al. [3,7].Selective assay of Pseudomonas sp.; Desulfovibrio sp.; iron bacteria and generalcounts of heterotrophic bacteria were made both in the ambient water and in thescrapings of the corrosion products from the carbon steel coupons [3]. Themicrobial counts in the cooling water are represented as cfu mlÿ1 and on thecoupon as cfu cmÿ2. The iron bacteria isolated from the pipelines was identi®ed,based on various biochemical and morphological characteristics as described byCullimore and Mcann; Mulder and Deinema and Mulder [8±10]. Pseudomonasspecies was identi®ed as per the methods detailed by Bergan [11]. King's Bmedium was used to isolate Pseudomonas sp. from the water sample. Mediumcomposition: Tryptone: 10.0 g; Proteose: 10.0 g; Dipotassium phosphate: 1.5 g;Magnesium sulphate: 1.5 g; Agar: 15.0 g. These chemicals were suspended in 1litre distilled water containing 10 ml glycerol and boiled and then sterilized byautoclaving at 15 lb. pressure for 15 minutes.

SRB were assayed using the Postgate medium. The inoculated plates wereincubated in anaerobic jar, SRB colonies were counted after 96 h of incubation[12±14]. Medium used for isolating of SRB: Tryptone: 10.0 g; Sodium sulphite: 1.0g; Sodium sulphite: 1.0 g; Ferric citrate: 0.5 g; Agar: 15.0 g. This above mediumminus agar was used for culturing the SRB in broth. SRB cultures weremaintained in Postgate medium for laboratory studies.

Carbon steel coupons (70 � 15 � 2 mm) were exposed online in the FBTRcooling circuit for 30 days (every month). After retrieving, the coupons wereimmediately brought to laboratory (immersed in cooling water). Later thecoupons were rinsed in ®lter sterilized cooling water (0.2 mm Millipore ®lter). Twosets of coupons were processed for corrosion rate measurement by weight lossmethod; one set was used for microscopic observation for description of themorphology of the micro¯ora colonised on the coupon (light and scanningelectron microscopy) and one set was used for microbiological assay. In order toaccess the corrosion rate in abiotic conditions, carbon steel coupons (a set of ®vecoupons 30� 20� 2 mm) were exposed to ®lter sterilised (0.2 mm Millipore ®lters)and autoclaved cooling water with a ¯ow rate of 7.5 ml minÿ1 in a Pedersendevice [15]. Corrosion rate was measured by weight loss method. The other studiescarried out include corrosion product analyses using X-ray di�raction (XRD) andMossbauer spectroscopy. The scraped out corrosion products from the couponswere ®nely powdered using a mortar and pestle. The powder was then dehydratedand compacted in to a circular disc using a press. XRD analysis was done using aD-500 Siemens powder X-ray di�ractometer ®tted with CuKa X-ray source.Mossbauer spectrum was recorded in the transmission geometry on a home builtconstant acceleration spectrometer [16], using a 5 mCi strong 57Co radioactivesource in Rh matrix. The Mossbauer data analysis was done using a curve ®ttingprogram based on Lavenberg±Marquardt method [17]. The corrosion product wasanalysed for Barium content using atomic absorption spectroscopy.

T.S. Rao et al. / Corrosion Science 42 (2000) 1417±1431 1419

3. Results

The iron bacteria species isolated from the carbon steel coupons exposed in theFBTR cooling circuit was identi®ed as Leptothrix sp.; other bacteria identi®ed arePseudomonas aeruginosa and SRB isolate Desulfovibrio sp. Fig. 1 providesinformation on the iron bacteria population on carbon steel coupons exposedonline in the cooling circuit. The data presented here cover the period fromOctober 1994 to July 1996. The signi®cant observation from the graph is that thepeaks in carbon steel corrosion rates coincide with the peaks in the iron bacteriapopulation on the carbon steel coupons. The corrosion rates ranged from 3±13.5mpy. The iron bacteria in water has not shown much variation during the courseof this study. The iron bacteria population showed peaks during summer and NEmonsoon months. Carbon steel coupons exposed in sterile conditions (no bacterialgrowth) showed corrosion rates in the range 1.7520.62 mpy (44.45 m yearÿ1) fora set of ®ve coupons. Fig. 2 illustrates the variation in SRB numbers on thecarbon steel coupons and cooling water, the SRB counts varied from 7� 102 to 9� 103 cfu cmÿ2 during the period October 1994 to June 1996. SRB population inthe cooling water ranged from 10 to 35 cfu mlÿ1. Fig. 3 provides data onculturable aerobic heterotrophic bacteria (CAHB) of the source water (MAPSopen reservoir) and the FBTR cooling water from 1992 to 1996. The bacterialpopulation ranged from 105 to 108 cfu mlÿ1. Although the heterotrophic countswere almost similar during the year 1992 to 1993, a variation of 1 to 2 orders ofmagnitude was observed from 1994 to 1995.

Table 1 provides details on composition of the corrosion products. In thescrapings of the carbon steel coupon, iron amounting to 65% and organic content(loss on ignition) to 25% are the major constituents. Other signi®cant componentspresent are phosphate 4%; copper 1% and reactive silica 1%. However, sediment(deposit collected from inter-cooler air compressor) composition showed anorganic content of 16%; phosphate Ð 10%; iron Ð 40% and copper, calcium

Table 1

Composition of the fouling deposit on carbon steel coupon and sediment collected from the FBTR

cooling circuit

Sno Species Carbon steel (% w/w) Sedimenta (% w/w)

1 Organics 25 16

2 Phosphate 4 10

3 Calcium 0.8 6.4

4 Chloride 0.2 3.6

5 Silica 2 24

6 Iron 65 40

7 Copper 1 2

8 Nickel Traces Traces

9 Miscellaneous 2 ±

a Sample from intercooler air compressor.


and chloride amounting to 2.5%, 9.4% and 3.6%, respectively. Silica content wasabout 24%. Barium (ca. 4 mg lÿ1) was found to be present in the Palar river waterwhich is the source for FBTR cooling water.

The X-ray di�raction pattern (Fig. 4) of the carbon steel corrosion productgives qualitative information about the possible phases present. The phaseidenti®cation, carried out using a search-and-match ®t of the XRD data to thepatterns in the ICDD database, revealed that Fe(PO3)3 is the predominant phase.The other phases identi®ed are g-Fe2O3, Fe2PO5 and BaFeO3 ÿ x. Apart from

Fig. 1. Iron bacteria counts both in cooling water and on the carbon steel coupon and corrosion rate

of carbon steel in FBTR cooling circuit monitored during the period: October' 94 to July' 96.


peaks corresponding to various crystalline phases, features corresponding topoorly crystallised phase are also observed.

Fig. 5 illustrates the Mossbauer spectrum of the corrosion product. Thespectrum is seen to be quite complex with many overlapping peaks particularly inthe region of ÿ3 mm/s to +3 mm/s. The single sextet observed corresponds to g-Fe2O3. The e�ective ®eld of 492 kOe obtained from this sextet is less than that forthe bulk (506 kOe). This gives an indication that the g-Fe2O3 phase in thecorrosion product might be micro-crystalline. Mossbauer spectra of Fe2O3 as afunction of particle size are reported by Kundig et al. [18]. Taking this intoconsideration, a systematic ®tting to the Mossbauer spectrum was carried out. The®nal analysis of the Mossbauer data revealed subspectra corresponding toFe(PO3)3, g-Fe2O3, Fe2PO5, BaFeO3 ÿ x and FePS3, which are shown as stickdiagrams in the ®gure. The ®nal ®t to the spectrum is shown as a solid line inFig. 5 and the ®tted parameters for the component phases are listed in Table 2.

From the Mossbauer data an estimate of the particle size for the g-Fe2O3 phasewas made as follows:

The collective spin-relaxation time, t, of the super-paramagnetic particles isgiven by Vertes et al. [19]

t � 1

f0exp

�2KV

kT

��1�

where f0 and K are the frequency and anisotropy factors, V is the volume of thesuperparamagnetic particle; k is the Boltzmann constant and T is the absolutetemperature. Morup and Topsoe [20] proposed the following equation relating thee�ective magnetic ®eld, H, with the volume and hence the size of the particles:

H � Hbulk

�1ÿ kT=2KV

� �2�

Fig. 2. SRB counts on carbon steel coupon and cooling water in FBTR cooling system.


Fig. 3. Culturable heterotrophic bacteria counts in FBTR cooling water and the source water (open

reservoir).

Table 2

Mossbauer parameters of corrosion product of Carbon steela

Component Compound IS (with respect to a-Fe)mm/s QS (mm/s) H (kOe) RA (%)

(a) BaFe2O3ÿ x (i) 0.36 ± ± 6.8

(ii) ÿ0.3 ± ±

(b) FePS3 1.04 1.53 ± 0.7

(c) Fe2PO5 (i) 0.425 1.03 ± 1.1

(ii) 1.09 2.53 ±

(d) g-Fe2O3 0.45 ÿ0.03 492 0.7

(e) Fe(PO3)3 0.23 1.0 ± 16

(f) Amorphous phase 0.7 2.44 ± 74

a IS: Isomer shift; QS: Quadrupolar splitting; H: Hyper®ne ®eld; RA: Relative area.


Values of K19 range from 1� 104 to ÿ4� 104 erg cmÿ3 and those of f range from2� 108 to 5� 108 sÿ1. Using the average values of K (25000) and f (3.5� 108 sÿ1)and the observed hyper®ne ®eld of 492 kOe for the sextet, the average particle sizeis found to be about 18 nm.

Figs. 6(a) and 6(b) are the scanning electron micrographs, showing thecolonisation of iron bacteria ®laments on carbon steel surface. The typicalencrustation of corrosion products in the ®laments could be seen. Fig. 6(a) alsoshows the hold fast, through which the iron bacteria ®rmly adheres to the metalsubstratum. Fig. 6(c) is stereo-zoom photograph, showing the tubercle induced byiron bacteria on carbon steel coupon and Fig. 6(d), also a stereo-zoomphotograph, shows the SRB induced concentric ring patterns on carbon steelsurface (underneath the tubercle).

4. Discussion

The common iron oxidizing bacteria viz. Gallionella, Sphaerotilus, Crenothrix,and Leptothrix species oxidize ferrous ions to ferric ions to obtain their energy.Iron bacteria's ability to metabolize ferrous ions to ferric and then forming a lowdensity hydrated iron oxide in the tubercles is the key factor for corrosion of steel[21]. Thus, metal depositing bacteria create environments that are conducive to

Fig. 4. XRD pattern of the corrosion product of carbon steel coupon exposed online in FBTR cooling

circuit for 30 days.


sustaining their growth and subsequently promote corrosion. During the course ofthis study, the iron bacteria species which infested the FBTR cooling circuit, wasidenti®ed as Leptothrix sp. This is the most common iron storing ensheathedbacterium apparently occurring in slow running, ferrous iron-containing waters,poor in decomposable organic material [10]. The sheaths of Leptothrix sp. assist inthe formation of a membrane that is relatively impervious to oxygen, and in theprocess decrease the quantity of oxygen in the tubercle vicinity, thus establishing amicro-electrochemical cell. With increasing thickness in growth, the inside of thetubercle becomes more anaerobic and favours SRB growth. The di�erence in thepotential between the iron surface underneath and outside the tubercle increases,and thus corrosion gets accelerated. Di�erential aeration cells formed due tomicrobial colonisation leads to corrosion by dissolution of ionized material oroxides at the grain boundaries [1,4]. Under these conditions there will be apronounced growth and activity of the iron bacteria giving rise to theaccumulation and sedimentation of large quantity of ferric hydroxide [22].

Fig. 5. Mossbauer pattern of the corrosion product sample showing the presence of following phases:

(a) g-Fe2O3 (b) Fe2PO5 (c) FePS3 (d) Fe(PO3)3 (e) BaFeO3ÿ x and (f) amorphous phase.


Fig. 6. (a) Scanning electron micrograph showing the hold fast structure by which the iron bacteria

®laments attach to the metal surface and colonise. The typical Fig. 6(b) schematic of the attachment of

the ®laments could be seen (Magni®cation 1225X, 15 kV). (b) SEM picture showing the iron bacteria

®laments running parallel on the carbon steel surface. The ®laments are encrusted with corrosion

products which are mainly iron oxides and hydroxides (Magni®cation 1750X, 15 kV). (c) Photograph

showing iron bacteria tubercle, on carbon steel coupon exposed in FBTR cooling circuit. (d) Stereo-

zoom photograph (Magni®cation 40X) of carbon steel coupon showing typical SRB induced shallow

concentric rings.


The mode of attachment of iron bacteria ®laments to the metal surface, which®nally results in the formation of tubercle is shown in Figs. 6(a) to (c). The modeof attachment is distinctly seen in the SEM pictures (Figs. 6(a) and (b)), whichclearly show the colonisation of iron bacteria ®laments on the carbon steelsurface. The typical long ®laments encrusted with iron oxide/hydroxide corrosionproducts could be observed clearly in Fig. 6(b). Subsequent to colonisation andpropagation of the ®laments of iron bacteria has resulted in tubercle formation(Fig. 6(c)).

Clean carbon steel pipes are subjected to rapid corrosion during the initialstages of tubercle formation. The bacteria thrive at the edge of the tubercle whereoxygen and iron are readily available, since iron bacteria require oxygen for theirgrowth. Water velocity is a major parameter in predicting the selective growth ofiron bacteria. Stagnant and low velocity waters appear to deprive the iron bacteriaof required oxygen which reduces its growth (we have noticed very low corrosionrates 1.75 mpy in sterile conditions). Moderate velocities (2 to 7 ft sÿ1) encourageiron bacteria growth with signi®cant corrosion rates (carbon steel couponsexposed online in FBTR cooling circuit resulted in corrosion rates as high as 13.5mpy). Iron bacteria form ®lamentous sheaths and layers of oxides (see Figs. 6(a)and (b)). This action chemically and physically traps water and e�ectively reducesthe oxide density. According to Mettel [4], the oxides consist of 70% hydratediron oxide, 20 to 25% silica (silt) and 5 to 10% other oxides. Sulphur content willbe less than 1%. XRD analysis of the carbon steel corrosion products reported byRao et al. [3] showed the presence of U-FeOOH, FeO, Fe2O3 and FeSi andsulphur. The production of ferric iron from the oxidation of ferrous iron atneutral pH leads to precipitation of the iron. Since ferric iron is insoluble, it willtend to settle out of suspension or crystallise and dehydrate forming compoundssuch as [23], Lepidocrocite (U-FeOOH), geothite (Fe2O3 H2O), or hematite(Fe2O3)3.

Iron can exist in three oxidation states (Fe0, Fe2+ and Fe3+), metallic iron(Fe0) oxidises to form the stable Fe2+ but at pH >5 it is chemically oxidized toFe3+. Further Fe3+ is reduced under acidic conditions. Therefore, in naturalsystems such as bio®lms Fe2+ is microbially chelated to increase its availability. Inoxygenated environments, the area beneath the deposits gets deprived of oxygenand becomes small anodes compared with the surrounding large oxygenatedcathodes. Cathodic reduction of oxygen may result in an increase in pH of thesolution in the vicinity of the metal. The metal will form metal cations at anodicsites [1]. In cooling waters containing chloride, the ions can migrate to the anodeto neutralise. Any buildup of charge results in the formation of heavy metalchlorides that are extremely corrosive. Under such circumstances the conventionalfeature of di�erential aeration, a large cathode to anode surface area, and thedevelopment of acidic conditions and formation of metallic chlorides inducepitting in carbon steel [4].

Development of SRB in surface microbial ®lms can be expected, wheneverenvironmental conditions such as redox potential or oxygen tension and nutrientsare suitable for SRB growth. SRB exists in aerobic waters (such as cooling water),


anaerobic microniches and in the anoxic micro layers of bio®lms [5]. In case ofSRB corrosion when oxygen is present, local anodes and cathodes are expected toexist, acidi®cation will occur at the local anode. Under anaerobic conditionsnormal electrochemical corrosion does not occur because the cathode becomespolarised by the buildup of a layer of atomic hydrogen. This cathodic polarisationsti¯es the dissolution of iron at any anodic site [24]. The activity of SRB isthought to stimulate the normal electrochemical corrosion mechanism by theremoval of cathodic hydrogen or by formation of iron sulphide which itself iscathodic to steels [25]. It is well established in the case of SRB that the majorcorrosion e�ect is due to the biogenic production of sulphide. Rapid corrosionoccurs when sulphide is added or when SRB growth is stimulated by addition ofnutrients [25].

The initial corrosion product formed as far as SRB is concerned ismackinawite, an iron rich iron sulphide, that forms a poorly protective layer onthe metal surface [26]. It has been reported that the corrosion rate of mild steelin the presence of iron sulphide increases with iron sulphide concentration. Ithas been reported that more the FeS formation, the higher is the corrosion rateof carbon steel [27]. In presence of sul®des and other sulphur anions. Videla [24]has reported that presence of sulphide could explain the pit morphology. SRBinduces pitting in the form of large radial growth patterns on carbon steelsurface which are shallow in nature. In our case, the distinctive corrosionpattern on carbon steel is the presence of pits as disk-shaped concentric rings,which are similar to those obtained in the presence of sul®des due to fast radialpit growth. Fig. 6(d) shows the typical SRB induced concentric ring patterns oncarbon steel surface.

Mossbauer spectroscopy has proved to be a very useful technique for studyingcorrosion processes and corrosion products [28]. The main advantages ofMossbauer technique in corrosion research are (1) the non destructive character ofthe technique, (2) possibility of qualitative and quantitative phase analysis, and (3)identi®cation of poorly crystallized or amorphous corrosion products. In thepresent study, the phase analysis of the Mossbauer data of the corrosion productshowed the presence of the phases listed in Table 2 among which, g-Fe2O3 wasfound to be nano crystalline. In complex mixtures such as corrosion products ofiron, it is di�cult to determine the particle size distribution for the g-Fe2O3 phasealone. In the absence of such an information, therefore, it is prudent to suggestthat component ( f ), tabulated in Table 2, consists of doublets due to nanosized g-Fe2O3 phase as well as a signature due to an unknown amorphous phase.Presence of iron-bearing colloidal phases, which are known to occur [29] wheneverSRB are present, may well contribute to doublet f (Fig. 5) in the Mossbauerspectrum of the corrosion sample. This can also be corroborated with the broadbackground in the XRD spectrum (Fig. 4). The observation of phosphate basedcompounds in the corrosion deposit could be the result of a phosphate basedwater treatment programme. Phosphate was maintained at 10±15 ppm (as orthophosphate) in the cooling circuit as per the requirement of the commercial ®rmwhich was treating the cooling water of FBTR with proprietary formulations.


Microbial degradation of the polyphosphate might have resulted in the formationof the phosphate based oxides and hydroxides.

Iverson [29] suggested that in addition to FeS, SRB also produces a highlycorrosive metabolite (iron phosphide), a soluble compound containingphosphorous which enhances the dissolution of iron under anaerobic conditions atneutral pH. He further reported that the high corrosion rates observed in the ironsystem could be due to the formation of a soluble compound which containsphosphorous. However, during the course of this study many phosphatecompounds were observed (see Figs. 4 and 5). The XRD and Mossbauer spectrashowed signi®cant peak for BaFeO3 ÿ x. On further probing the occurrence of thiscompound, we could arrive at the following conclusion; it is reported that speci®ctypes of bacterial activity are believed to produce soluble barium in certain sub-surface waters [30]. Earlier, Weintritt and Cowan [31] conducted an elegant studyto access the ability of SRB to produce hydrogen sulphide in a medium containingbarium sulphate as the only sulphate source. To their surprise the SRB culture notonly utilized insoluble barium sulphate but in the process also producedsigni®cant amount of soluble barium ion (120 mg lÿ1). This could be one of thereasons for the presence of barium (up to 4 mg lÿ1) in the subsurface waters of thePalar river which is the source of cooling water for all the freshwater cooledsystems at Kalpakkam. In view of the signi®cant presence of SRB population inthe cooling water we attribute the detection of barium compound to be due tomicrobial activity [30]. The presence of BaFeO3 ÿ x compound could be the resultof the interaction of iron oxide with barium.

Similarly, the presence of FePS3 compound could be due to two possiblereasons; (1) Iverson [29] reported that SRB produced colloidal iron phosphate.Seed [32] further con®rmed that phosphate increased the rate of corrosion ofcarbon steel in the presence of SRB. Because reduced phosphorus is highlyreactive, it could contribute to corrosion of iron. Furthermore, in an environmentof sulphide the iron phosphorus formed could have reacted with sulphide to formthe FePS3 compound. (2) It is well known that ferrous sulphide layer is formed onmetal surface by Fe2+ reacting with hydrogen sulphide produced by SRB [24].The crystalline nature of iron sulphide has profound in¯uence on iron corrosion[25]. The phosphate based water treatment programme in operation at FBTRcould have resulted in the phosphorus ions reacting with the iron sulphide,thereby leading to the formation of the FePS3 compound.

Presently, there is no one complete solution to control iron bacteria corrosiononce the system is infested with it. Chlorination is known to e�ectively controlbacteria in cooling circuits [33,34]. However, laboratory studies carried out bySatpathy et al. [35] showed that chlorination had very little biocidal action on ironbacteria (It was observed that, up to 1.8 ppm free chlorine residual chlorine hadno e�ect on iron bacteria). Moreover, Pope et al. [36] and Mettel [4] reported thatchlorination would increase crevice corrosion attack in tuberculated pipelines bycreating chloride concentration cells within the porous oxide layer. The chlorideion would increase chemical corrosion potential and therefore attribute to netincrease in corrosion rates. Thus iron bacteria can create an environment in which


local crevice corrosion can become signi®cant, even in waters that are notnormally considered corrosive [4]. This can be coupled with under depositcorrosion (galvanic corrosion). The present study reveals that the mechanism ofcarbon steel corrosion is not a simple process but a combination of several factorswhich chie¯y include bacteria. Based on the observations from this study it mayconcluded that the presence of Leptothrix sp. and Desulfovibrio sp. is responsiblefor carbon steel corrosion.

5. Conclusions

Microbially induced corrosion of carbon steel has been monitored by assayingiron bacteria and sulphate reducing bacteria (SRB) in cooling water and alsocarbon steel coupons exposed online in the cooling circuit of a nuclear reactor.SEM examination has shown ®lamentous growth of iron bacteria impregnatedwith corrosion products. Typical SRB induced pitting in the form of large radialgrowth patterns on carbon steel has been observed. Detailed phase analysis ofthe corrosion products by XRD and Mossbauer Spectroscopy have shown thepresence of following compounds: g-Fe2O3, Fe2PO5, FePS3, Fe(PO3)3 andBaFeO3 ÿ x. It is suggested from the present study that iron bacteria (Leptothrixsp.) and sulphate reducing bacteria (Desulfovibrio sp.) are responsible for thecorrosion of carbon steel.

Acknowledgements

The ®rst author wishes to record his gratitude to Prof. Kuntala Jayaraman,Dean (Technology), Anna University, Chennai for her encouragement and keeninterest during the course of this study. The authors are also grateful to Dr.Baldev Raj, Director, Metallurgy and Materials Group and Dr. R.P. Kapoor,Associate Director, ROMG, Indira Gandhi Centre for Atomic Research, for theirconstant support during the course of this study.

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Carbon Steel Corrosionnext Term by Iron Oxidising and Sulphate Reducing Bacteria in a Freshwater Cooling System