Effect of iron exposure in SRB media on pitting initiation

E�ect of iron exposure in SRB media onpitting initiation

D. Starosvetskya,*, O. Khaseleva, J. Starosvetskyb,R. Armonb, J. Yahaloma

aFaculty of Materials Engineering, Technion, Haifa 32000, IsraelbEnvironmental and Water Resources Engineering, Faculty of Civil Engineering, Technion, Haifa 32000,

Israel

Received 4 March 1999; accepted 30 June 1999

Abstract

Pitting corrosion of iron (99.9%) in Sulphate-reducing bacteria (SRB) media containing

0.09±0.30 g/l sulphides and 0.1±5.0 g/l [Clÿ] was studied. Iron electrode immersed in SRBmedia undergoes fast activation and numerous active sites formed on the surface.Depending on exposure time, pitting was initiated at di�erent potentials. After short period

(<2 h) in open circuit potential, when active sites were still exist on the surface, pittingoccurred at the potential range of ÿ0.60 to ÿ0.55 V (SCE). Extended exposures resulted inmore stable passivity of iron and the pitting potential shifted to positive values. Pit

initiation mechanism at the various exposure periods is discussed. # 2000 Elsevier ScienceLtd. All rights reserved.

Keywords: Iron; SRB medium; Pitting; Breakdown potential; Repassivation

1. Introduction

The pitting corrosion of iron and steel in media of sulphate-reducing bacteria(SRB) was shown by many investigators [1±12]. The main di�culties inunderstanding this phenomenon arise from the complexity of the chemical

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

PII: S0010 -938X(99)00088 -8

Corrosion Science 42 (2000) 345±359

* Corresponding author. Tel.: +972-4-8294-288; fax: +972-4-8321-978.

E-mail address: [email protected] (D. Starosvetsky)

composition of these media. Apart from chloride-ions SRB media containsulphides, which role in the mechanism of pitting formation is of particularimportance.

It is known that sulphides can induce pitting on steel in solutions free ofchloride-ions. Salvareza et al. [13] reported pitting of a mild steel at ÿ0.6 to ÿ0.7V (SCE) in a neutral bu�ered Na2S solution. They suggested that activation of themetal surface was caused by HSÿ ion and iron dissolution occurred through theformation of soluble FeHS+ species. The breakdown potential shifted to morenegative values with an increase in Na2S concentration.

In chloride containing solutions sulphides can promote steel activation.Salvarezza et al. [11] showed that the breakdown potential of mild steel in 0.5 MNaCl was reduced to values between ÿ0.6 and ÿ0.7 V (SCE) with very smalladdition of Na2S [0.001 M] or less.

In SRB medium pitting of mild steel was observed in the same potential rangeas in Na2S solution [1,5,9]. It is interesting to note that Clÿ content had no e�ecton a breakdown potential. Approximately the same values of breakdown potentialof mild steel were obtained in bacterial solutions containing both low [0.002 M][9] and higher chloride concentration [0.5 M] [1,5,11].

Nevertheless, the low values of pitting potential were observed only at the initialexposure stages of steel to sulphide solutions. It was shown that long termexposure in SRB media had a bene®cial e�ect on the passivity of steel and thepitting potential shifted to more positive values. Booth [1] observed a positive shiftin breakdown potential at longer exposure of mild steel to SRB media containing0.5 M Clÿ. A similar phenomenon was found by Ringas and Robinson [9] with anSRB medium containing only 0.002 M Clÿ. In this study, the breakdown potentialof steel shifted from ÿ0.64 to ÿ0.35 V (SCE) after 5 days in solution. This shiftrevealed that after long term exposure, pitting occurred at potentials whereassociated usually with chloride induced pitting [11]. Depending on exposure time,iron and steel passivation was also observed in sulphide and chloride containingsolutions under abiotic conditions [14,15]. Newman et al. [14] showed thatpassivation become more pronounced at higher sulphide concentrations.

The above results suggest that the pitting corrosion mechanism of steel in SRBmedia can alter at di�erent stages of the metal exposure to the solution. Thepresent work was undertaken to study the mechanism of pit initiation and the roleof sulphides at di�erent stages of iron exposure to SRB medium.

2. Experimental procedures

Pencil-type electrodes were made by mounting pure iron (99.9%) wire of 1 mmdiameter in an epoxy resin. The electrodes were freshly wet-abraded to a 600 grit®nish prior to each experiment. In order to explore the role of surface oxide iniron activation, electrochemical tests were conducted on pre-passivated electrodesas well as on electrodes with cathodically reduced surface oxide ®lm. Pre-passivation was achieved by exposure of the electrodes in borate bu�ered distilled

D. Starosvetsky et al. / Corrosion Science 42 (2000) 345±359346

water (pH 8), for 30 min at open circuit potential (OCP). Cathodic reduction ofthe oxide ®lm was performed at ÿ1.3 V for 5 min.

Mixed cultures of sulphate-reducing bacteria (SRB) were isolated from e�uentsamples collected at the Sewage Treatment Plant (Haifa, Israel) and usedthroughout this study. SRB medium preparation and experimental tests wereperformed as described by Ringas and Robinson [9].

SRB cultures were inoculated in a medium containing: 3.5 g/l sodium lactate;1.0 g/l beef extract; 2.0 g/l peptone; 2.0 g/l MgSO4�7H20; 0.5 g/l KH2PO4; 0.1 g/lCaCl2 and 0.1 g/l sodium ascorbate. The solution was autoclaved at 1208C for 15min. The culture was incubated at 308C. Bacteria growth was monitored bysulphide accumulation. The total sulphide concentration was measured byiodometric titration of 25 ml probe taken anaerobically (CO2 atmosphere) frombatch culture. Five days old culture medium (pH 7.8) was collected and usedexperimentally. The resulting sulphide concentration in these media rangedbetween 0.38 and 0.42 g/l. The chloride-ion concentration was adjusted from 0.1(basal medium) to 5 g/l by NaCl addition.

Electrochemical tests were conducted in 0.5 l well-capped three-electrode cellunder nitrogen atmosphere. Electrodes and gas bubblers were tightly inserted intothe cell through glass grinds sealed with a silicone rubber sealant. In order tostudy the e�ect of sulphide concentration on the corrosion behavior of iron theelectrochemical tests were conducted in solutions containing 0.30, 0.22 and 0.09 g/l of sulphides. Prior to experiment, to adjust the initial sulphide concentration tothe required values, the medium was purged with pure nitrogen. This enabled thevariations in sulphide content keeping constant the concentration of othermetabolic products. Once the required sulphide content was reached, the nitrogen¯ow was turned o�. Subsequently the working electrode that was positioned in thecell headspace during cell purging was immersed into the solution and theexperiment was started. The changes in the tested sulphide concentrations during5 h (the top time of corrosion test) are shown in Table 1. Potentials weremeasured using Luggin capillary. The resulting potential values were referred to asaturated calomel electrode (SCE). All the experiments were performed at roomtemperature (22218C).

Surface-associated bacterial distribution on iron was examined by confocal

Table 1

Total sulphide concentration in tested solutions during corrosion test

Initial sulphide concentration (g/l) Sulphide concentration after various exposures (g/l)

1 (h) 2 (h) 3 (h) 5 (h)

0.302 0.300 0.300 0.299 0.297

0.221 0.220 0.220 0.220 0.219

0.092 0.090 0.089 0.088 0.086

D. Starosvetsky et al. / Corrosion Science 42 (2000) 345±359 347

scanning laser microscopy (CSLM). Specimens for CSLM analysis were ®xed in4% glutaraldehyde and stained by 0.01% acridine orange (each stage for 15 min).

The morphology of corroded specimens was studied by scanning electronmicroscopy (JEOL 840 SEM). Prior to SEM observation the specimens werewashed in sterile distilled water with 0.1% Tween-80, which was added for betterremoving of the adsorbed microorganisms.

3. Results

Fig. 1 shows typical corrosion potential transients of pre-passivated iron in SRBmedium containing 0.1 g/l Clÿ and sulphide amounts of 0.09, 0.22 and 0.30 g/l.Immediately after immersion the corrosion potential droped to approximatelyÿ0.75 V indicating an activation of the initially passive iron. The potential dropwas more pronounced under aerated conditions, as shown in Fig. 1(inset). Theactivated sample repassivated with time and the corrosion potential shifted towardthe positive direction. The repassivation occurred more rapidly in solutions withhigher sulphide concentration. An iron electrode activated by cathodic reductionof the initial surface oxide, showed an initial corrosion potential of ÿ1.05 V. Thecorrosion potential shifted rapidly to ÿ0.8 V in the ®rst 10 min and thengradually continued to rise and reached ÿ0.5 V after more than 200 min. Incomparison, Fig. 1 inset shows the corrosion potential transient of pre-passivatediron in a NaCl solution with a similar Clÿ concentration (0.1 g/l) but withoutsulphide. The initial corrosion potential shifted gradually from approximatelyÿ0.4 V in the positive direction indicating a slow iron passivation.

Fig. 1. Corrosion potential transients for pre-passivated and cathodically activated (ÿ1.3 V for 5 min)

iron in SRB media containing 0.1 g/l Clÿ and di�erent sulphide concentrations. (Inset: Corrosion

potential transients for pre-passivated iron in aerated SRB medium with 0.3 g/l sulphides and 0.1 g/l

Clÿ and in NaCl (0.1 g/l Clÿ)).


Fig. 2. SEM micrographs of iron sites containing large pits (a � 7000) and small pits (b � 12,000)

developed on pre-passivated surface, after 10 min exposure at OCP in SRB medium containing 0.3 g/l

sulphides and 0.1 g/l Clÿ.


The surface of the pre-passivated iron electrode was examined after 10 minexposure at OCP in a medium containing 0.30 g/l sulphides. SEM micrographs(Fig. 2a and b) revealed localized character of iron activation. Numerous activesites were present on the entire surface. Further exposure produced localprecipitation of iron sulphide corrosion products, which apparently causedrepassivation of the electrode.

The distribution of adsorbed SRB cells over iron surface were studied by CSLM(Fig. 3). After 4 h exposure at OCP only scattered single cells and small colonieswere observed on the iron surface.

Fig. 4 shows potentiodynamic curves of a pre-passivated iron electrode in SRBmedium containing di�erent amount of sulphides. Potential was appliedimmediately upon immersion of the electrode in order to reveal the anodicbehavior of the activated iron electrode. In a 0.09 g/l sulphides solution, thecurrent density increased sharply with the application of the potential and reached4 mA/cm2 at ÿ0.65 V. At higher potentials the current density slightly decreaseddue, presumably, to iron passivation induced by precipitation of ferrous sulphidecorrosion products. The initial peak of iron dissolution became much lower athigher sulphide concentration. It was 0.06 mA/cm2 in 0.22 g/l sulphides and only0.01 mA/cm2 in 0.30 g/l sulphides. After passivation all curves showed a currentincrease at potentials of 0ÿ 0:55 V with the onset of pitting phenomenon. The

Fig. 3. CSLM micrograph of the distribution of adsorbed SRB cells over iron surface after 4 h

exposure at OCP SRB medium containing 0.22 g/l sulphides and 0.10 g/l Clÿ.


pitting potential was better de®ned at higher concentrations of sulphides. In thesolution containing 0.22 g/l sulphides, the current density sharply increased toapprox. 2 mA/cm2 at ÿ0.57 V. The sharp increase in current density was haltedabove ÿ0.55 V and then slightly increased. Equally, in a solution with 0.30 g/lsulphides the current density started to increase at ÿ0.55 V, but pitting wasunstable. The current density then increased slowly with potential and at approx.ÿ0.42 V, a second passivation occurred. At potentials around ÿ0.15 V, activepitting has developed.

Fig. 4. Potentiodynamic curves (scan rate 1 mV/s) of pre-passivated iron in SRB media containing 0.1

g/l Clÿ and di�erent sulphide concentrations.

Fig. 5. Potentiodynamic curves (scan rate 1 mV/s) of pre-passivated iron in SRB media containing 0.3

g/l sulphides and di�erent chloride ion concentrations (Inset: Corrosion potential transients for pre-

passivated iron recorded in the same solutions).


To elucidate the role of chloride-ions in the pitting process of iron electrode inSRB medium, corrosion potential transient and potentiodynamic tests wereconducted in the medium containing 0.30 g/l sulphides and chloride-ions of 0.5and 5 g/l (Fig. 5). The corrosion potential transients in the inset of Fig. 5 show anincrease of corrosion potential in all solutions after initial electrode activation,indicating electrode repassivation. In 0.1 and 0.5 g/l Clÿ solutions the corrosionpotential reached values above ÿ0.5 V and then remain almost constant with timeimplying full iron passivation. On the contrary, in solutions containing 5 g/lchloride-ions the corrosion potential increased up to approximately ÿ0.55 V andthen started declining indicating surface activation.

The chloride-ion concentration strongly a�ect the anodic behavior in potentialregion above ÿ0.6 V where pitting attack occurs. The breakdown potential of pre-passivated iron electrode as measured by potentiodynamic tests slightly decreasedwith Clÿ concentration. Pitting initiated in solutions containing 0.5 and 5.0 g/lClÿ caused a rapid current increase and no pit repassivation occurred as observedat the same scan rate in SRB medium with a lower chloride concentration (curve3). No e�ect of chloride-ion concentration on the anodic behavior of iron atpotentials below ÿ0.6 V was detected.

Fig. 6 shows the anodic behavior of pre-passivated iron electrode followingdi�erent exposure time in SRB medium (0.22 g/l sulphides, 0.1 g/l Clÿ, and scanrate 1 mV/s). The anodic curve, after exposure of 1 h, showed essentially similarbehavior as the anodic curve obtained directly upon immersion. However, thecurrent density was lower after 1 h over the same potential domain, apparentlydue to partial passivation of active sites. After 5 h exposure iron passivation wascompleted and the polarization curve revealed a wide region of passivity thatbroke down at approximately ÿ0.17 V. As is seen (Fig. 6), the potentiodynamic

Fig. 6. Potentiodynamic curves (scan rate 1 mV/s) of pre-passivated iron in SRB medium containing

0.22 g/l sulphides and 0.10 g/l Clÿ after di�erent exposure times at OCP and in NaCl (0.1 g/l Clÿ)upon immersion.


curve obtained after long term exposure at OCP is similar to that measured insulphide free sodium chloride solution.

Fig. 7 presents the e�ect of the potential scan rate on the anodic behavior ofiron in SRB medium (0.30 g/l sulphides and 0.1 g/l Clÿ). The potential sweep wasapplied immediately after the electrode immersion (while active sites were presenton the electrode surface). At scan rates of 2 and 5 mV/s stable pitting appeared atpotentials just above ÿ0.6 V. At a scan rate of 1 mV/s the slight increase incurrent that initiated at ÿ0.55 V was unstable compared to higher scan rates. Theiron repassivated at higher potentials and remained passive up to breakdownpotential of approx. ÿ0.15 V. At a scan rate of 0.5 mV/s, no increase of currentdensity occurred at ÿ0.55 V and iron was passive up to approx. ÿ0.15 V.

For a pre-passivated iron electrode the electrochemical behavior in SRB mediais determined by the formation and repassivation of active surface sites. In orderto study the anodic processes occurring at a single active site, a simulation wasperformed by cathodic reduction of initial oxide ®lm at ÿ1.3 V for 5 min. Fig. 8presents potentiodynamic curves of the iron electrode measured at di�erent scanrates in an SRB medium containing 0.30 g/l sulphides and 0.1 g/l Clÿ immediatelyfollowing the cathodic pre-treatment of the oxide ®lm at ÿ1.3 V (SCE) for 5 min.At 100 mV/s the anodic current started at approx. ÿ1.0 V and it rapidly increasedwith potential indicating anodic dissolution of bare iron surface, presumably sinceprecipitation of corrosion products was insigni®cant at this scan rate. At lowerscan rates, a current reduction and distinctive passivation was noticed. At scanrates of 1 and 0.5 mV/s the current peak was observed at about ÿ0.65 V. As such,this peak appeared at about the same potential as for pre-passivated iron (Fig. 5),although corresponding to higher anodic current density, evidently due to a largerarea of the activated surface. Pitting was detected on the cathodically activatediron electrode at approx. ÿ0.6 V, i.e. in the same region of potentials as observed

Fig. 7. Potentiodynamic curves of pre-passivated iron in SRB medium containing 0.3 g/l sulphides and

0.1 g/l Clÿ at di�erent scan rates.


Fig. 8. Potentiodynamic curves of cathodically activated (ÿ1.3 V for 5 min) iron in SRB medium

containing 0.3 g/l sulphides and 0.1 g/l Clÿ at di�erent scan rates.

Fig. 9. Current density transients of iron in SRB medium containing 0.22 g/l sulphides and 0.10 g/l Clÿ

at di�erent potentials: (a) upon immersion; (b) after 5 h exposure at OCP.


on the pre-passive electrode. However, on cathodically activated electrode onceinitiated, pitting developed rapidly as indicated by a continuously increasingcurrent. No current restrain was observed on anodic curves of pitting corrosionpotential region as observed on pre-passivated electrode. The portions of anodiccurves indicating pitting practically merge at all the tested scan rates. Whencathodically activated iron electrode was exposed for 4 h in SRB medium at OCPthe full iron passivation was achieved. The potentiodynamic curve revealed a wideregion of passivity that brakes down at approx. ÿ0.15 V (curve 6).

As follows from the results (Figs. 6±8) the activation of iron can occur at twodi�erent values of breakdown potential depending on both the exposure at OCPand the potential scan rate. After short exposure when active sites exist on theiron surface, the activation was detected at potentials just above ÿ0.6 V, that is atthe initial breakdown potential. After long term exposure (when iron was passive)the pit initiation was observed at potentials around ÿ0.15 V, that is at the ®nalbreakdown potential. In order to compare the mechanism of pit initiation in thesetwo cases, the current density transients at various potentials were reordered inSRB medium (0.22 g/l sulphides and 0.1 g/l Clÿ) for di�erent exposure times. Fig.9(a) depicts the current transient at the speci®c values of potential (ÿ0.6, ÿ0.55

Fig. 10. Cyclic polarization curves of iron in SRB medium containing 0.30 g/l sulphides and 0.1 g/l Clÿ

after di�erent exposure times at OCP: (a) pre-passivated iron; (b) cathodically activated (ÿ1.3 V for 5

min) iron.


and ÿ0.54 V) applied after 1 h pre-exposure at OCP when active sites still exist onthe iron surface. At ÿ0.6 V (a value below the initial breakdown potential) theanodic current reached a maximum of 0.5 mA/cm2 immediately after theapplication of potential and then decreased with time indicating iron passivation.At a potential of ÿ0.54 V (above the initial breakdown potential), the anodiccurrent reached an instant maximum of 3.6 mA/cm2 followed by a decrease for 10min up to 1.5 mA/cm2 and then the current density remained constant (1.7 mA/cm2) throughout the experiment. Application of ÿ0.55 V (slightly below the initialbreakdown potential) caused full repassivation of the already initiated pitting. Thepassivity was maintained throughout the 20 min exposure. Hence, for short-termexposures when the active sites still remain on the electrode surface, the anodiccurrent reached fairly high values without any incubation just after the applicationof potentials higher than Ecorr. Upon following exposure at the applied potentialthe anodic current could either fall up to approx. zero indicating the fullpassivation of active sites on the electrode surface or slightly decrease, which isattributed to iron dissolution in the active sites.

The ®nal breakdown potential was con®rmed by the results in Fig. 9(b). Itshows that after exposure at OCP for 5 h at applied potential of ÿ0.18 and ÿ0.20V (under the ®nal breakdown potential, curve 3 Fig. 6) the current densityremained very low and decreased slowly indicating electrode passivity. The ironelectrode was activated at ÿ0.17 V (above the breakdown potential) afterincubation of approx. 20 min.

To distinguish between these pitting processes occuring at two di�erentbreakdown potentials (ÿ0.6 and ÿ0.15 V) depending on the extent of ironpassivity pitting repassivation potentials were checked by cyclic polarization. Theexperiments were performed in SRB medium containing 0.30 g/l sulphides and 0.1g/l chloride-ions at the scan rates of 0.5 and 2 mV/s (Fig. 10). For a pre-passivated iron electrode the polarization sweep was applied both upon immersion(2 mV/s) and after 2 h exposure at OCP (0.5 mV/s). A scan rate of 2 mV/s forpotentiodynamic measurements upon immersion was chosen because at lower scanrate pits initiated at ÿ0.55 V are unstable in the tested medium (see Fig. 7). After2 h exposure, pitting was initiated at ÿ0.15 V (Fig. 10(a)). The repassivationpotential was detected at approx. ÿ0.58 V (backscan part of the curve). Whenapplying the potential sweep upon immersion breakdown appeared at approx.ÿ0.55 V, though the backscan part of the curve revealed the repassivationpotential of ÿ0.6 V.

For the cathodically activated iron electrode the polarization sweep of 0.5 mV/swas applied both immediately upon cathodic pre-treatment and after 5 h at OCP.The results obtained (Fig. 10(b)) are similar to those shown in Fig. 9(a). Thecurve obtained right upon immersion did not show the characteristic loop forpitting corrosion and both values of breakdown and repassivation potentials werepractically the same, approx. ÿ0.6 V. After 5 h exposure the breakdown andrepassivation potentials were found to be ÿ0.17 and ÿ0.6 V, respectively.

Comparison of experimental data of pitting corrosion that took place in NaClsolutions and in SRB medium (0.22 g/l sulphides) with identical chloride-ion


concentrations for long term exposure at OCP is shown in Table 2. Table 2 showsthat breakdown and repassivation potentials values obtained with SRB media areclose to those measured with NaCl solution.

4. Discussion

The results of this study demonstrate that the pitting phenomena on iron inSRB media can emerge at di�erent potentials depending on iron-solution exposuretime. After short exposure at OCP, pitting occurred around ÿ0.6 V while long-term exposure shifted the breakdown potential to higher values of ÿ0.35 andÿ0.15 V according to the chloride-ion concentration. In both cases, the pittingprocess is characterized by the same value of repassivation potential ofapproximately ÿ0.6 V. Since the repassivation potential separates the range ofpitting corrosion from that of the stable passivity, these results suggest that pittingin both cases occurs at the same pitting corrosion region. The observed di�erencein the breakdown potentials is related to the extent of surface passivity of the ironelectrode.

When the pre-passivated iron electrode was immersed in SRB media fastactivation occurred, which resulted in formation of numerous active sites on theelectrode surface. At OCP, the active sites on the metal surface are slowlyrepassivated.

Following short exposure periods of the electrode surface containing active sitesin solution, the potentiodynamic test revealed iron dissolution at potentials belowÿ0.6 V. The iron dissolution rate slowed down due to the precipitation of ironsulphide corrosion products in the active sites. The process particularly slowsdown in solutions with the high concentration of sulphides. In the case where theactive sites were not fully passivated (short pre-exposure at OCP, high scan rates,etc.) the current increased at potentials right above the repassivation potential ofÿ0.6 V, as already described by Janic-Czachor [15]. Ferrous sulphide precipitatedduring pitting restrained the rate of current increase, followed by stabilizationover a wide potential region. In some cases, previously initiated pitting can berepassivated by ferrous sulphide precipitation (Fig. 5, curve 3).

After extended exposure of iron in the SRB solution, the electrode becomes

Table 2

Breakdown potential (Ebd) and repassivation potential (Erp) of pitting corrosion for iron electrode in

SRB medium and NaCl solutions

Clÿ (g/l) SRB (5 h exposure) NaCl

Ebd (V) Erp (V) Ebd (V) Erp (V)

0.1 ÿ 0.15 ÿ 0.6 ÿ 0.1 ÿ 0.54

0.5 ÿ 0.35 ÿ 0.6 ÿ 0.35 ÿ 0.58


passive and pitting was initiated at much higher than ÿ0.6 V potentials. Theperiod necessary to reach full passivation of the electrode strongly depends on thesulphide concentration, i.e. 4±5 h at 0.22 g/l and only 0.5±1 h at 0.30 g/l.

Pit initiation and propagation at di�erent stages of iron exposure to SRB mediaoccurred by di�erent ways. After short exposure, when the active sites are presenton the iron surface, pit nucleation occurred instantly and the anodic currentreached its maximum immediately upon potential application (Fig. 7(a)).Precipitation of ferrous sulphide corrosion products suppressed pit propagationand the current gradually decreased with time. In certain experiments, fullrepassivation of already initiated pitting was observed. In contrast, in cases whenfull passivity of iron was achieved the pit nucleation occurred within a wellde®ned incubation period and the current increased constantly with time.

The Clÿ concentration had no e�ect on iron dissolution at potentials belowÿ0.6 V. This situation suggests that chloride-ions do not a�ect iron dissolution inthis potential region. The breakdown potential measured after short exposure toSRB solution was also slightly a�ected by Clÿ content. It was approximately thesame up to 5 g/l [Clÿ] (highest concentration tested). However, the rate of irondissolution at potentials higher than the breakdown potential, clearly increasedwith Clÿ content. This fact indicates the direct role of Clÿ in the pitting attack. Inthe solution containing 0.1 g/l [Clÿ], pitting development can be suppressed byprecipitation of iron sulphides up to full repassivation (Fig. 5, curve c), while insolutions containing 0.5 and 5 g/l [Clÿ] pitting repassivation was not observed.

The chloride concentration had a marked e�ect on the pitting potential of fullypassivated electrode. The breakdown potential shifted to more negative values asthe chloride content increased in the solution. A comparison of the resultsobtained in SRB media and those in NaCl solutions, suggests that iron pitting isinduced by the chloride-ions presence.

5. Conclusions

1. Iron electrodes exposed to SRB media undergo fast activation, which results inthe formation of numerous active sites on the electrode surface. At the OCP,the metal surface active sites are slowly repassivated by local precipitation ofiron sulphides.

2. Following short exposure at OCP, the electrode surface active sites are not yetfully passivated and pitting occurs at the potential range between ÿ0.6 andÿ0.55 V. Pit nucleation occurs without an incubation period and the anodiccurrent reaches its maximum immediately upon application of potential.Precipitation of ferrous sulphide corrosion products suppresses the propagationof pits and causes the current to decrease slowly with time. In this case thechloride-ions concentration has only a slight e�ect on the breakdown potential.

3. Long-term exposure results in repassivation of iron and the breakdown


potential shifts to much higher potentials. Pit nucleation occurs after incubationperiod and the current increases progressively with time. An increase inchloride-ion concentration shifts the breakdown potential to more negativevalues.

4. Under all conditions the pitting process is characterized by the same value ofrepassivation potential, which suggests the same mechanism of pittingcorrosion.

Acknowledgements

This study was supported by Grant No. 015-022 from the Water ResearchInstitute (WRI), Technion, Haifa.

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Effect of iron exposure in SRB media on pitting initiation

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