World Applied Sciences Journal 17 (4): 524-531, 2012ISSN 1818-4952© IDOSI Publications, 2012

Corresponding Author: Saeid Kakooei, Centre for Corrosion Research, Department of Mechanical Engineering UniversitiTeknologi PETRONAS, Bandar Seri Iskandar, 31750, Tronoh, Perak, Malaysia.Tel: +601749581.

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Mechanisms of Microbiologically Influenced Corrosion: A Review

Saeid Kakooei, Mokhtar Che Ismail and Bambang Ariwahjoedi1 1 2

Centre for Corrosion Research, Department of Mechanical Engineering Universiti Teknologi PETRONAS,1

Bandar Seri Iskandar, 31750, Tronoh, Perak, MalaysiaDepartment of Fundamental and Applied Science, Universiti Teknologi PETRONAS, 2

Bandar Seri Iskandar, 31750, Tronoh, Perak, Malaysia

Abstract: The main problem of biogenic is the production of H S in the oil industry that can lead to corrosion2

and reservoir souring. Collection of bacteria called sulfate-reducing bacteria (SRB) is always the responsibleof problems such as reservoir souring, equipment and pipeline failures. The corrosion mechanismunderstanding of SRB is unavoidable. In this study, various mechanisms proposed for SRB induced corrosionare investigated.

Key words:Sulfate-reducing bacteria (SRB) Corrosion mechanism, H S Microbiologically influenced2

corrosion (MIC)

INTRODUCTION Cathodic Depolarization by Hydrogenise (1934): Von

The role of microorganisms on the metals corrosionhas been known since early 1900s [1-2]. Microbiologicallyinfluenced corrosion (MIC) is a big concern in the oil andgas industry. MIC pitting attacks tend to result inreservoir souring, equipment and pipeline failures that areof great problems in oil field. Collection of bacteria calledsulfate-reducing bacteria (SRB) is always the responsibleof these problems [3-5].

SRB are nonpathogenic and anaerobic bacteria, butSRB can act as a catalyst in the reduction reaction ofsulfate to sulfide [6]. It means they are able to make severecorrosion of metals in a water system by producingenzymes, which can accelerate the reduction of sulphatecompounds to H S [7-8]. However, to occur this2

reduction, three components namely SRB, sulphates,free electrons as an external energy source must bepresent and the water temperature must be less thanapproximately 65°C [9]. Mild steel, Stainless steel andcarbon steel are the most commonly exploited materialsin the petroleum realm which are known to undergo fromMIC.

To investigate SRB induced corrosion,understanding of its mechanism is necessary. Variousdifferent mechanisms have been proposed since 1934.Some of the mostly known mechanisms are presented inthis study.

Wolzogen Kuehr and Van der Vlugt in 1934 [1] proposedmechanism of corrosion induced by SRB which is adepolarization through oxidation of the cathodichydrogen as formulated in the cathodic depolarizationtheory. When metal expose to water, it becomes polarizedby losing positive metal ions (anodic reaction). The freeelectrons reduce water-derived protons (cathodicreaction) in the absence of oxygen, to produce hydrogenthat inhabits on the metal surface that will establish adynamic equilibrium. Sulfate-reducing bacteria areexpected to consume the formed hydrogen (according toreaction 4 indicated in Table (1), thus oxidation of Fehappens [10-11]. This mechanism increased the anodicmetal dissolution and consequently FeS and Fe(OH) as2

corrosion products are formed [12].

Table 1: Cathodic depolarization mechanism of metal corrosion by SRB.

Anodic reaction (1) 4Fe 4Fe + 8e2+ -

Water dissociation (2) 8H O 8H + 8OH2+ -

Cathodic reaction (3) 8H + 8e 8H + 4H+ -2

Hydrogen oxidation (4) SO + 4H H S + 2H O + 2OH4 2 2 22- -

Precipitation (5) Fe + H S FeS + 2H2+ +2

Precipitation (6) 3Fe + 6OH 3Fe (OH)2+ -2

Total Reaction: 4Fe + SO + 4H O FeS + 3Fe (OH) + 2OH4 2 22- -

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Fig. 1: Scheme of iron corrosion by SRB based on revealed oxidation of cathodic hydrogen with sulfate inreactions as suggested by the cathodic different growing cultures of hydrogenase-positivedepolarization theory [13]. I, iron dissolution; II, Desulfovibrio species. They demonstrated that thewater dissociation; III, proton reduction; IV, process took place only when lactate as an organicbacterial sulfate reduction and V, sulfide electron donor was present. It was believed that aprecipitation. simultaneous consumption of H and the organic

Fig. 1 shows iron corrosion mechanism that is based The corrosion rate was reported to depend largely onon CDT. In real condition, SRB are attached to metal the total activity of hydrogenase within the biofilm rathersurface, but for convenience, the bacterial cells are shown on the bacterial population size [21]. The biofilms withseparately. At the cathodic site, reducing agents SRB in pipelines with intense corrosion had lowerdesignated as [H] from the iron flow to the bacteria and cell numbers but much higher total hydrogenase activityare used for reduction of sulfate (SO ) to sulfide (H S). [22]. In contrast, biofilm including SRB in a non-corroding4 2

2-

At the anodic site, only one fourth of the dissolved Fe pipeline had higher cell densities but low hydrogenase2+

reacts stoichiometrically with H S to form FeS [13]. In the activity and showed a low corrosion rate. Often,2

presence of CO and bicarbonate, as common in marine hydrogenase genes have been subject to investigations2

environments, the remaining Fe precipitates as FeCO ; with the goal to monitor corrosion under field conditions2+3

in the absence of bicarbonate, the more soluble Fe(OH) [22]. However, the primers used for the polymerase chain2

is formed [14]. The total reaction of corrosion is as reaction (PCR) amplification or the probes applied tofollows: detect hydrogenase genes in situ covered merely

4Fe + SO + 3HCO + 5H FeS + 3FeCO + 4H O (1) cannot yield a complete picture of SRB associated with4 3 3 22- - +

Iverson showed that this theory is valid for SRB FeS formed by SRB activity is feasible in terms ofcorrosion of other metals such as aluminum alloys. He thermodynamically aspect. These corrosion products canpresented direct documentation for the cathodic speed up corrosion considering on the environmental anddepolarization theory via benzyl viologen (an electron physicochemical conditions of the FeS ?lm. Formation ofacceptor) and a cell suspension of a hydrogenase- FeS on metal surface can induce a local decrease in pHpositive SRB [15]. Experimental results with Desulfovibrio that enhances the breakdown of passive film, which canspecies for supporting the cathodic depolarization theory lead activation of corrosion cells between the steelhave demonstrated frequently that they use H very surface as anode and the FeS as cathode [24]. Several2

effectively [16]. Also Cypionka and Dilling in 1986 important factors are not considered in the classicalillustrated that the hydrogenase-negative depolarization theory mention as following:Desulfotomaculum orientis could not depolarize the (a) The effects of sulfide, bisulfide and hydrogencathode, whereas hydrogenase-positive Desulfovibrio sulfide produced from the sulfate reduction on the anodicvulgaris could [17]. The current density at a given reaction; (b) The effect of hydrogen sulfide on theelectrode potential in the presence of Desulfovibrio cathodic reaction; (c) the effect of elemental sulfur fromvulgaris cells was always higher than in their absence the biotic or abiotic oxidation of sulfur; (d) Fluctuations in[16]. Booth and Tiller in 1968 utilized electrochemical the environmental conditions between anaerobic andtechniques to investigate cathodic depolarization of steel aerobic conditions; (e) the production of other corrosivewith cell suspensions of different SRB [18]. metabolites [10].

Da Silva et al. demonstrated a new mechanism ofcathodic depolarization. They explained the improvementof the charges exchanged must relate to a cathodicdepolarization that happens by using a direct electrontransfer from stainless steel to hydrogenase [19]. It wasdemonstrated that the corrosion rate with added FeS wassignificantly higher than without FeS. This resultprovide proof for the influence of FeS in cathodicdepolarization [18]. Cord-Ruwisch and Widdel (1986) also

2

substrate occurred [20].

Desulfovibrio species [22-23]. Thus, such approaches

corroding iron in situ.

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Fig. 2: King’s proposed mechanism of corrosion by thesulfate reducing bacteria [25]

Iron Sulfides (King’s Mechanism) (1971): King andMiller proposed that the solid FeS formed on the metalsurface played the role of absorber of molecular hydrogenthen iron sulphide can be reproduced [25]. In thissituation where the area covered by iron sulfide becomescathode, while biofilm area behaves as anode and thecorrosion rate of metal will remain in high level [25-27].An increase of Fe concentration in the solution leads to2+

the crack-up of the protective Mackinawite film, when thefilm was ruptured, the corrosion rate is accelerated interms of the Fe concentration [28]. Figure 22+

schematically illustrates this proposed mechanism. Itseems that after sufficient formation of FeS for starting agalvanic cell between FeS and Fe, no sulfide film areformed while a high rate corrosion rate was recorded bygalvanic corrosion [25].

Lee and Characklis in 1993 studied the impact ofsuspended FeS on the corrosion of mild steel in ananaerobic biofilm reactor where the Fe concentration2+

rose from 0 to 60 mg/L. When the increase of Fe2+

concentration reached 60 mg/L, FeS particles were able topenetrate through the protective iron sulfide film,consequently the protective film was ruptured.Intergranular corrosion were also detected on the metalsurface by SEM imaging [29].

A Volatile Phosphorous Compound (1983): Iverson andOlson in 1983 demonstrated that a volatile material, aphosphorous compound was the responsible of corrosionevent. They supposed that Sulfate reducers canaccelerate corrosion through production of extremelycorrosive phosphorous compounds such as phosphine(H P), which can lead to iron phosphide (Fe P) production3 2

[30-31]. The result shows that phosphorus compound wasseen in yeast extract that seems to be a precursor to thecorrosive phosphorous compound [32].

Anodic Depolarization (1984): Anodic depolarization byiron-reducing bacteria has been the subject of extensiveinvestigation. According to Table 2, Corrosion of iron in

Table 2: Anodic depolarization mechanism of metal corrosion by SRBAnodic reaction (1) 4Fe 4Fe + 8e2+ -

Water dissociation (2) 8H O 8H + 8OH2+ -

Cathodic reaction (3) 8H + 8e 8H + 4H+ -2

Anodic depolarization (4) 3Fe + 6OH 3Fe (OH)2+ -2

Hydrogen oxidation (5) SO + 4H H S + 2H O + 2OH4 2 2 22- -

Dissociation of hydrogen sulfide (6) H S S + 2H22- +

Anodic depolarization (7) Fe + S FeS2+ 2-

Total Reaction: 4Fe + SO + 4H O FeS +4 22-

3Fe (OH) + 2OH2-

an aqueous anaerobic environment is an electrochemicalevent that electrons generated through anodic reaction ofmetal (reaction1) and H generated during dissociation of+

water (reaction 2). With combination of these tworeactions molecular H form (reaction 3). This process is2

called cathodic polarization that a layer of H can protect2

metal. The main corrosion product is Fe(OH) . When2

SRB come and reduce sulfate to sulfide by consumptionof H , reaction 5 will take place. Then dissociation of2

H S (reaction 6) increases the H concentration in2+

cathodic area, changing the kinetics of reaction 3. Withanodic depolarization in reaction 7, a new corrosionproduct FeS forms [33-37].

Wang, H. and C.H. Liang demonstrated that SRBaccelerates anodic active dissolution of 1OCrMoAl steelin seawater through anodic depolarization process ofsulfide. The S ions from SRB activity reacts with Fe2- 2+

ions to form FeS that will accelerate anodic activedissolution [38].

Fe-Binding Exopolymers (1995): Generally, biologicaldeposits on any surface are called Biofilm and biofouling.In a better description, biofilms are a community ofmicrobes and their extracellular polymer substance (EPS),usually polysaccharides. The protection of microbes isthe main purpose of biofilm, although biofilm can trapnutrient for microbe growing. Many review articles areconducted on the influence of biofilm in differentindustries such as drinking water system [39], dairy andfood [9] and oil field industries [5].

Extracellular polymeric substances (EPS) producedby SRB have the ability to accelerate corrosion by bindingwith metal ions [40-43]. SRB with EPS of differentcomposition were shown to cause different corrosionrates. In EPS released by a relatively aggressiveDesulfovibrio strain, uronic acid was detected [44-45].Fang et al. in another study showed that the increasedproduction of EPS in the presence of Cr also accelerated3+

the corrosion of mild steel in seawater [46]. Chan et al. in2002 demonstrated that EPS alone is a metal corrosion

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agent. They prepared two solution with 1% EPS and strength, but at similar cathodic polarization, this steelwithout EPS and the result showed that EPS enhanced absorbs the highest amount of hydrogen and reveals thecorrosion [47]. It also supposed that EPS can trap metal most pronounced degradation [53].ions leading to stimulation of the anodic reaction [48].

Videla in 2001 [49] presented four steps for biofilm Sulfide (1998): Little et al. in 1998 investigated the role offormation on the metal surface as following: biomineralization in microbiologically induced corrosion.

1) Transportation of organic material to metal surface; They demonstrated biomineral dissolution reactions by2) Transportation of microbial cells from bulk to surface, metal-reducing bacteria remove oxide layers or force3) Attachment of microbial cells; 4) Growth within the mineral replacement reactions that promote decompositionbiofilm. of metal. Biomineralization that results in mineral

These mechanisms can modify the interface structure deposition on a metal surface can shift the corrosionby biofilm accumulation that should be considered as the potential in either a positive or a negative direction,main reason of MIC [49-50]. depending on the nature of the mineral. Bioprecipitated

Sulfide and Hydrogen -Induced Stress Corrosion active direction, resulting in accelerated corrosion of someCracking (SCC) (1995): Undoubtedly, we can say SRB metals and alloys. Iron oxide formation can begin aactivity can enhance the corrosion-fatigue crack growth sequence of events that results in under depositand hydrogen embrittlement [51-52]. The corrosion fatigue corrosion of susceptible metals [8, 54].crack growth of the high strength steel notablyaccelerates by the biologically produced hydrogen Three Stages Mechanism (Romero Mechanism) (2005):sulphide in natural sea water [53]. Recently, Romero proposed a mechanism for the SRB

Biologically active environment change with the induced corrosion of iron. As it is clear from Figure 3, thisactivities of the bacteria and their interactions with other mechanism is definited by three stages [55]. In the firstcomponents of the environment such as interactions of stage (Table.3), the adsorption of bacterial cells and ironbacteria and the metal, production of EPS, degradation by sulfide products were taken place. A micro galvaniclarge fouling organisms. There are two different local corrosion cells form through iron sulfide productsenvironments surrounding the metal surface with and (Mackinawite and Pyrite) and the metallic surface thatwithout bacteria, even with the same levels of sulphide. generated a hydrogen permeation peak [55-56].Furthermore, sour environments are specifically There is equilibrium between bacterial and inorganicscorrosive because of high levels of hydrogen accessible in second stage (Tacle.4). The metal was slightlyat the metal surface or in a crack because of sulfide ennobled in this stage by the development of aactivation at the cathode [26]. The influence of hydrogen combination film of a more dense iron sulfide film and EPScan be modified by the presence of organic molecules on [55-56]. Bacterial corrosion and film stabilization are twothe metal surface and EPS matrix, which can describe the major occurrences at this stage. The third stage (Table.5)differences between crack tip effects (as measured by was controlled by a local pH decrease that caused by SRBcrack growth) and general embrittlement effects (as activity on the steel in the presence of HS . SRB reducemeasured by hydrogen flux) [51]. The differences between locally Pyrite to Mackinawite, which produce a severe,abiotic and biotic solution including similar levels of localized corrosive process configured into groups ofcorrosive compounds such as iron sulfides can be deep, rounded holes, then subsequent detachment tookassociated with to the presence of EPS and to the place. It supposed that a galvanic corrosion generatedheterogeneities produce at the metal surface by the between the anodic metal and the cathodic iron sulfideformation of a biofilm [26]. products, composed of Mackinawite, Pyrite, Esmitite,

Dom alicki et al. described that SRB can produce Marcasite, Greigite, Troilite and Pyrrhotite. In this morehydrogen sulphide at low and medium cathodic aggressive phase, there was no hydrogen permeation duepolarization. Hydrogen sulphides decrease pH near to the barrier or anti-diffusive effect of the EPS. At thiselectrode electrolyte and inhibit the deposits formation stage, the bacteria grow exponentially about 10 UFC/cmand thus encourage the hydrogen charging and the that can generate enough H S. So the corrosionplasticity loss. These effects are a function of studied accelerates without the absorption of atomic hydrogensteels. The same amount of hydrogen makes a less [55-56]. Reactions could be occurring as shown indetrimental effect on the sorbite steel of increased Table 5.

sul?des move the corrosion potential in a negative, more

-

8 2

2

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Fig. 3: Mechanism for the SRB induced corrosion of iron proposed by F. de Romero[56].

Table 3: The first stage reactions of Romero mechanism (3-9h)H S HS + H (1)2

+

Fe + HS FeS + H (2)++ +

FeS + HS FeS + H + 2e (3)2+ -

3FeS + HS Fe S + H+ + 2e (4)-3 4

2H + 2e H (5)+ -2

Fe Fe + 2e (galvanic) (6)++ -

Fe + HS FeS + H + 2e (microbial) (7)+ -

Table 4: The second stage reactions of Romero mechanism (9-15 h)FeS (C) FeS (O) (1)2 2

Fe S (R) Fe S © (2)3 4 3 4

Fe + HS FeS + H + 2e (microbial) (3)- + -

2H + 2e H (4)+ -2

Table 5: The third stage reactions of Romero mechanism (>15 h)FeS + H + 2e FeS + HS (1)2

+ - -

Fe Fe + 2e (galvanic) (2)++ -

Fe + HS FeS + H +2e (microbial) (3)+ -

H S + e ½H (4)2 2-

7FeS + HS Fe S + H + 2e (5)- + -7 8

Biocatalytic Cathodic Sulfate Reduction (BCSR) (2009):In the BCSR theory, MIC takes place since the sulfatereduction at the cathode will consume the electronsreleased by iron dissolution at the anode with the help ofbiocatalyst and the interface of biofilm and the metal area place for both anodic and cathodic sites [57-58].

It assumes that a corrosive SRB biofilm is formed onan iron surface causing the following reactions to goforward due to biocatalysis.

Anodic: 4Fe 4Fe + 8e (Iron dissolution)2+ -

Cathodic: SO + 8H + 8e HS + OH + 3H O4 22- + - - -

(BCSR)

Cathodic reaction shows the half reaction of sulfatereduction from sulfate to sulfide due to biofilm catalysis.Some species were added solely to balance the charges

and elements in order to be consistent with otherreactions. One should not interpret cathodic reactionstrictly as converting proton to hydroxide because theactual sulfate reduction in SRB is coupled with otherbiochemical reactions. An increase in sessile SRBpopulation may be observed due to externally suppliedelectrons in impressed current cathodic protectionsituation. Another factor may be that SRB cell walls carrycharges that are attracted to the surface. If there is astoppage of the electron supply, the sessile SRB cells mayturn to attacking iron to get electrons for BCSR. To assurea steady supply of externally supplied electrons, acontinuous impressed current is desired. A more negativevoltage (i.e., a larger driving force) is needed to deliver thecurrent due to the increased ohmic resistance exerted bythe biofilm [59].

CONCLUSION

Nowadays different methods are used to inhibitMIC. Understanding of microbial corrosion mechanismis one of primary tools to get better result in corrosioninhibition. We have considered many possiblemechanisms that propose for SRB activity insteel-water interface. Depending on the environmentalcondition, one mechanism or a combination ofseveral mechanisms can happen. Further studiesmust be done to find out how bacteria can grow andprotect themselves from biocides and other inhibitionmethods.

ACKNOWLEDGMENTS

The authors want to gratefully acknowledgeUniversiti Teknologi PETRONAS for supporting of thiswork.

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