Evaluation of an Organic Corrosion Inhibitor 2007

8/6/2019 Evaluation of an Organic Corrosion Inhibitor 2007

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Evaluation of an Organic Corrosion Inhibitor on Abiotic Corrosion and

Microbiologically Influenced Corrosion of Mild Steel

Xiaoxia Sheng, Yen-Peng Ting,* and Simo Olavi Pehkonen

Department of Chemical and Biomolecular Engineering, National UniVersity of Singapore

Inhibition of mild steel corrosion by 2-methylbenzimidazole (MBI) in sterile enriched artificial seawater, andseawater with sulfate-reducing bacteria (SRB), was investigated using direct current polarization, electrochemi-cal impedance spectroscopy (EIS), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy(XPS), and atomic force microscopy (AFM). MBI was shown to be an effective inhibitor in controlling abioticcorrosion, and as well as microbiologically influenced corrosion (MIC) by two strains of SRB: DesulfoVibriodesulfuricans and a local marine isolate (DesulfoVibrio singaporenus). Tafel plots reveal that MBI predominantlycontrols the cathodic reaction. MBI is more effective in the inhibition of corrosion caused by D. desulfuricansthan that caused by D. singaporenus. The corrosion inhibition effect of MBI on MIC is partially due to theinhibition of the bacterial activity. The adsorption of MBI on the mild steel surface follows a Langmuiradsorption isotherm model.

1. Introduction

Corrosion is a perennial problem in many industrial systems.

Apart from abiotic corrosion, microbiologically influencedcorrosion (MIC) also reduces the lifetime of various industrialmaterials and equipment. It is estimated that20% of corrosionis due to MIC.1 Microorganisms have a tendency to formbiofilms on metals,2 which consist of bacterial cells embeddedin a highly hydrated, extracellular polymeric matrix.3 Amongthe anaerobic microorganisms, sulfate-reducing bacteria (SRB)are one of the most important groups commonly associated withmicrobial corrosion.4

The effective control of corrosion extends the life of industrialequipment. Corrosion inhibitors recently have become moreimportant, because of their application in inhibiting corrosionunder a wide range of environments. Organic corrosion inhibi-tors are generally more environmentally friendly than inorganic

corrosion inhibitors. Organic corrosion inhibitors adsorb ontometal surfaces through heterocyclic atoms (such as nitrogen,oxygen, sulfur, and phosphorus), multiple bonds, or aromaticrings and block the active sites, thus decreasing the corrosionrate.5,6 Nitrogen-containing heterocyclic substances, such asazole-type compounds, have been reported to be effectivecorrosion inhibitors.7-11 The effectiveness of numerous organicazole-type compounds (e.g., 2-mercapto-benzimidazole (MBI),imidazole (IMD), benzimidazole (BIA), and pyrazole) has beenreported.9-12

To control MIC, the traditional strategy is the application ofbiocides to kill the microorganisms in the aqueous environment.However, it is now recognized that the effectiveness of biocidesis much lower when bacteria are incorporated into a biofilmthan when they are suspended. The exopolymeric matrixconstitutes a diffusion barrier that hinders biocide penetrationinto the biofilm.13-16 Indeed, recent research has shown thatMIC control is more successfully accomplished using a corro-sion inhibitor.17,18

2-Mercapto-benzimidazole has been shown to possess goodinhibition characteristics against steel and copper corrosion.9,11

Substituent groups, which enhance the electron-donating or

electron-withdrawing properties of the active N atom on theheterocyclic ring, would strengthen or weaken the interactionwith the metal surface.19 It has been shown that the presence

of the mercapto group in 2-mercapto-benzimidazole enhancedcorrosion inhibition, as compared to benzimidazole. Thus, theinhibition mechanism is likely to be related to the substituentgroup in benzimidazole.11 In the present study, a new organiccompound, 2-methyl-benzimidazole (MBI), which substitutesthe mercapto group in 2-mercapto-benzimidazole with anelectron-donating methyl group (Figure 1), was investigated forits inhibitive effect on both abiotic corrosion and microbiologi-cally influenced corrosion by two strains of SRB.

2. Experimental Materials and Methods

2.1. Coupon Preparation. Mild steel coupons were used inthis study; they are composed of 0.16% carbon, 0.37% silicon,

1.24% manganese, 0.027% phosphorus, 0.026% sulfur, 0.19%copper, 0.007% nitrogen, 0.02% aluminum, and 97.96% iron.Mild steel coupons were polished using diamond paste withparticle sizes of 6, 3, and 0.5 m successively on a polishingcloth purchased from Kemet International, Ltd. The couponswere subsequently cleaned with a 70% ethanol solution andstored in a vacuum desiccator before use. Individual couponsfor each experimental condition were used for each analysis.

2.2. Microorganisms. The SRB used in this study, Des-ulfoVibrio desulfuricans ATCC 27774 (DesulfoVibrio desulfu-ricans subsp. desulfuricans), was obtained from American TypeCulture Collection (ATCC), USA. The bacterium was culturedat 37 C under anaerobic conditions in an anaerobic workstation(Don Whitley, model MASC MG500) that contained 10% H2,10% CO2, and 80% N2, in modified Baars medium : MgSO4,2.0 g/L; sodium citrate, 5.0 g/L; CaSO4, 1.0 g/L; NH4Cl, 1.0g/L; K2HPO4, 0.5 g/L; sodium lactate, 3.5 g/L; yeast extract,1.0 g/L; and Fe(NH4)2(SO4)2, 1.0 g/L. A marine SRB strain(DesulfoVibrio singaporenus) that was isolated from localseawater also was used in this investigation.

D. desulfuricans was first cultured in modified Baarsmedium, and D. singaporenus was incubated in the marinePostgate medium B20 in a batch culture under an N2 atmosphereat 37 C for 2 days. A 10-mL aliquot of the culture wassubsequently transferred to 500 mL of a sterilized enriched

* To whom correspondence should be addressed. Tel: +65-65162190. Fax: +65-67791936. E-mail address: [email protected].

7117 Ind. Eng. Chem. Res. 2007, 46, 7117-7125

10.1021/ie070669f CCC: $37.00 2007 American Chemical SocietyPublished on Web 09/19/2007


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artificial seawater (EASW) medium: NaCl, 23.476 g/L; Na2-SO4, 3.917 g/L; NaHCO3, 0.192 g/L; KCl, 0.664 g/L; KBr,0.096 g/L; H3BO3, 0.026 g/L; MgCl26H2O, 10.610 g/L; SrCl26H2O, 0.040 g/L; CaCl22H2O, 1.469 g/L; sodium lactate, 3.5g/L; yeast extract, 1 g/L; tri-sodium citrate, 0.5 g/L; MgSO4H2O, 0.4 g/L; CaSO4, 0.1 g/L; NH4Cl, 0.1 g/L; K2HPO4, 0.05g/L; Fe(NH4)2(SO4)2, 0.1 g/L. The D. desulfuricans and D.singaporenus were cultured individually. After 2 days of

incubation, the biocorrosion experiments were initiated byhanging polished mild steel coupons on a nylon string in themedium with the bacteria in a 500-mL Duran bottle. MBI at aconcentration of 0.1-2.5 mM was added to the medium. Thispart of the experiment was performed in the anaerobic worksta-tion.

2.3. Electrochemical Impedance Spectroscopy (EIS) Analy-

sis. EIS was used to investigate the electrochemical propertiesof the corroded surface after 24 h of immersion in an enrichedartificial seawater with D. desulfuricans and D. singaporenus.All experiments were performed in a three-electrode electro-chemical cell, with a platinum electrode as the counterelectrode,and an Ag/AgCl electrode as the reference electrode. The EISmeasurements (using duplicates coupons) were performed ex

situ; the coupons that were removed from the enriched artificial

seawater served as the working electrode by embedding themin a sample holder of the corrosion cell (which was purchasedfrom Metrohm Pte, Ltd). The working electrode had an exposedsurface area of 0.785 cm2. An aliquot of 500 mL of the mediawas transferred into the magnetically stirred electrochemical cellto serve as the electrolyte for the EIS analysis. The analysiswas performed using the Autolab Version 4.9 (Metrohm)software. The frequency range was 5 mHz to 100 kHz, and theamplitude of the sinusoidal voltage signal was 10 mV. The EISresults were modeled and simulated using the EQUVRTsoftware.21 Tafel plots were measured with a scan rate of 2 mV/s, and the potentiodynamic scanning curves were measured witha scan rate of 10 mV/s.

2.4. Scanning Electron Microscopy (SEM) Analysis. In the

scanning electron microscopy (SEM) measurements, the biofilmattached onto the mild steel in enriched artificial seawater wasvisualized after preparation using the procedure previouslymentioned:22 Samples were fixed with 3% glutaraldehyde in aphosphate buffer solution (PBS, pH 7.3-7.4) for more than 4h, and then washed with PBS for two changes (5 min each),rinsed with distilled water for another two changes (again, 5min each), and dehydrated using an ethanol gradient (at 50%,75%, 95%, and 99% for 10 min) before being finally stored ina desiccator. An SEM microscope (JEOL, model JSM-5600)with a beam voltage of 15 kV was used to visualize themorphology of the biofilm.

2.5. Atomic Force Microscopy (AFM) Analysis. The mildsteel coupons, after 24 h of immersion, were removed from the

Figure 1. Structure of 2-methylbenzimidazole (MBI).

Figure 2. N 1s spectra for (a) bare mild steel and (b) mild steel with MBI.

Figure 3. Fe 2p spectra for (a) bare mild steel and (b) mild steel withMBI.

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medium for AFM analysis. To obtain biofilm images, thecoupons were lightly rinsed in sterile distilled water and thendried in air. To reveal the extent of underlying steel biocorrosion,the biofilm on the coupons was removed by immersing thecoupons in an ultrasonic bath for 5 min. The surfaces of theexposed coupons were finally rinsed with distilled water, cleanedin 100% ethanol, and dried under a N2 flow.23

A Nanoscope III AFM (Digital Instruments, Santa Barbara,CA) in tapping mode was used to image the biofilm and the

pits on the metal surface. Nanoprobe silicon nitride (Si3N4)cantilevers with a spring constant ofk) 0.06 N/m were obtainedfrom Digital Instruments.

2.6. X-ray Photoelectron Spectroscopy (XPS) Analysis. Thenature of the surface film was examined using a commercialX-ray photoelectron spectroscopy (XPS) system (Kratos Axis165). The excitation source was Al KR radiation (photoelectronenergy ) 1486.71 eV). Binding energies for the componentsof interest were referenced to the binding energy of C 1s at284.6 eV.

3. Results and Discussion

3.1. XPS Analysis. The attachment of MBI on the mild steelwas examined using XPS N 1s and Fe 2p spectra. XPS analysiswas conducted at two stages of this investigation: (i) on thebare mild steel immersed in the seawater without MBI, and (ii)

on the mild steel in the seawater with the addition of 1 mMMBI. Figures 2 and 3 show the respective spectra of N 1s andFe 2p from these coupons.

3.1.1. N 1s Spectra. XPS analysis revealed whether MBIhad attached onto the steel surface during exposure to theseawater in the presence of MBI. Figure 2 shows the nitrogen(N 1s) spectra for both the bare mild steel and the mild steeldeposited with MBI. The total peak area for N 1s spectra ofthe bare mild steel is only 112.9, whereas that for the MBI-deposited mild steel is 2081.9. The large peak at 398.9 eV wasattributed to the amide group (R-NH-R), and the smaller peakat 401.5 eV was attributed to the -CdN-C group in MBI.The significant increase of the nitrogen intensity and theappearance of-CdN-C group imply that MBI had become

attached onto the steel surface.3.1.2. Fe 2p Spectra. From the XPS analysis, Figure 3 shows

that the chemical states of iron on the surface are very differentfor the coupons with and without MBI. Three states of ironwere found: Fe0 (at a binding energy of 706.8 eV), Fe2+ (at abinding energy of 709.0 eV), and Fe3+ (at a binding energy of710.7 eV). The Fe2+ cation is attributed to the presence of FeO,whereas the Fe3+ cation is attributed to Fe2O3 or FeOOH. Thecoupon with MBI contains more metallic iron (Fe0), whereas,for the bare coupon, more oxidized iron and very little metalliciron contribute to the proportion of iron compounds on the steelsurface. This is because when the coupon was immersed in theseawater in the presence of MBI, the steel surface was depositedwith MBI molecules, which blocked the interaction of iron

Figure 4. Nyquist plots for mild steel in enriched artificial seawater(EASW) for 24 h (a) without bacteria, (b) with D. singaporenus, and (c)with D. desulfuricans.

Figure 5. Equivalent circuit for the metal/liquid interface. (Legend: Rs,solution resistance; Rct, charge-transfer resistance; and Qdl, constant phaseelement of electrical double layer.)

Table 1. Charge-Transfer Resistance (Rct) and Corrosion InhibitionEfficiency (IE) Parameters for the Corrosion of Mild Steel in EASW

with or without MBI

Mild SteelMild Steel +

D. singaporenusMild Steel +

D. desulfuricansMBI conc.

(mM) Rct () IE (%) Rct () IE (%) Rct () IE (%)

0 841 389 3630.1 1075 21.70.5 1904 55.81 2071 59.4 479 18.7 590 38.52.5 2608 67.8 715 45.6 1291 71.9

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with water and chloride molecules. On the other hand, in the

absence of MBI, the steel surface was easily oxidized. Theintensity of the metallic Fe peak increased for the coupon withMBI, and is therefore indicative of the protection offered bythe inhibitor.

3.2. Bacteria Concentration. To examine the influence ofMBI on bacterial activity, the bacteria concentration was countedusing the most probable number (MPN) method. The initialbacteria concentration was 108 cells/mL for both SRB strains.After the SRBs were cultured for 7 days, the concentration inEASW was 9.3 108 and 4.6 108 cells/mL for D.singaporenus and D. desulfuricans, respectively, while, inEASW, with the addition of 1 mM MBI, the concentrations for

D. singaporenus and D. desulfuricans turned out to be 1.5 108 and 4.3 104, respectively. It is evident that MBI was

effective in inhibiting the growth of the SRBs. This effect isnot unexpected, because benzimidazole derivatives are knownto be highly inhibitory against certain microorganisms,24

although it is noteworthy that 1 mM MBI was more effective

in inhibiting the growth of D. desulfuricans, compared to D.singaporenus.

3.3. Electrochemical Impedance Spectroscopy (EIS) Analy-

sis. The corrosion behavior of mild steel in EASW in thepresence and absence of MBI, after an immersion for 24 h, wasinvestigated using EIS at 37 C. Nyquist plots of mild steel inseawater are shown in Figure 4a. It is easily observed that theaddition of MBI in EASW increases the polarization resistanceof the metal (Figure 4a). In the presence of SRB, the MIC isreduced in the system with the organic inhibitor (see Figures4b and 4c). The EIS results can be interpreted in terms of theequivalent circuit models of the electrical double layer,19 asshown in Figure 5. As can be seen from Table 1, an increase inthe concentration of MBI also increases the charge-transfer

Figure 6. Tafel polarization curves of bare mild steel and inhibited mild steel in EASW for 24 h (a) without bacteria, (b) with D. desulfuricans, and (c) withD. singaporenus.

Figure 7. Potentiodynamic scanning (PDS) curves of mild steel exposed to EASW for 24 h (a) without bacteria, (b) with D. desulfuricans, and (c) with D.singaporenus.

Table 2. Electrochemical Polarization Parameters for Bare MildSteel and Inhibited Mild Steel Calculated from Tafel Plots

MBI conc.(mM) Ecorr (V) icorr (A/cm2)

corrosion rate(mm/yr) IE (%)

Mild Steel with No Bacteria0 -0.677 2.06 10-5 0.2190.1 -0.684 1.22 10-5 0.129 41.00.5 -0.697 1.13 10-5 0.120 45.31 -0.691 7.56 10-6 0.081 63.22.5 -0.682 5.16 10-6 0.055 75.0

Mild Steel with D. singaporenus0 -0.605 2.83 10-4 3.2401 -0.598 2.37 10-4 2.720 16.12.5 -0.597 1.04 10-4 1.190 52.6

Mild Steel with D. desulfuricans0 -0.616 9.28 10-5 1.0701 -0.677 2.06 10-5 0.219 77.82.5 -0.687 1.39 10-5 0.148 85.0

Table 3. Atomic Force Microscopy (AFM) Study of Biofilm SurfaceRoughness and Pit Depth

coupon roughness (nm)a pit depth (nm)

mild steel +D. desulfuricans 104.9 ( 10.4 59.3 ( 11.2

mild steel +D. desulfuricans +MBI (1 mM) 54.8 ( 8.6 40.3 ( 9.8mild steel +D. desulfuricans +

MBI (2.5 mM)46.7 ( 4.1 29.4 ( 10.6

mild steel +D. singaporenus 232.7 ( 23.5 42.0 ( 16.7mild steel +D. singaporenus +

MBI (1 mM)172.2 ( 20.1 37.5 ( 12.4

mild steel +D. singaporenus +MBI (2.5 mM)

113.9 ( 19.3 15.6 ( 5.8

a Including the root mean square (rms) deviation.



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resistance. The inhibition efficiency (IE) is calculated using thecharge-transfer resistance:

where Rct is the charge-transfer resistance of the mild steel inthe presence of the inhibitor, and Rct is the resistance of themild steel in the absence of the inhibitor. The IE value increasedas the MBI concentration increased, reaching a maximum of67.8% at 2.5 mM in EASW.

The corrosion of mild steel by the SRBs (in the presenceand absence of the inhibitor) was also examined. The Nyquistplots are shown in Figures 4b and 4c, and the parametersafter fitting with the equivalent circuit model are presented inTable 1. Results from the control experiments (i.e., without theinhibitor) show that the corrosion resistance of the metal in thepresence of D. desulfuricans and D. singaporenus are notsignificantly dissimilar after immersion in the enrichedartificial seawater for 1 week, but they are lower in comparison

to the abiotic system. In the presence of MBI, the inhibitionefficiency was determined to be greater in seawater with D.desulfuricans than with D. singaporenus. The results showedthat MBI inhibits MIC by D. desulfuricans with an IE of 38.5%and 71.9.9% at 1 mM and 2.5 mM, respectively, compared to18.7% and 45.6%, resepctively, for D. singaporenus. The

difference in the inhibition efficiency of MBI for these bioticsystems may lie in the physical properties of the bacteria.Significant slime was observed in the bottle with the D.singaporenus culture, which indicated that D. singaporenusexcreted more extracellular polymeric substances (EPS). A lowerdecrease in the bacterial number for D. singaporenus, whichexcrete more EPS, was observed, in comparison to D. desulfu-ricans. Therefore, it is speculated that EPS may confer protectionagainst the MBI by combining with the inhibitor molecule. D.desulfuricans, on the other hand, excretes much less EPS, whichthus confers less protection. This observation is consistent withthe previous discussion: D. singaporenus has a higher celldensity after the addition of MBI in EASW than D. desulfuri-cans.

Figure 8. Scanning electron microscopy (SEM) images of mild steel in EASW for 24 h (a) without MBI, (b) with MBI at 0.1 mM, (c) with MBI at 0.5 mM,and (d) with MBI at 1 mM. (Magnification for each image ) 1000).

Figure 9. SEM images of mild steel in EASW with D. desulfuricans for 24 h (a) without MBI, (b) with MBI at 1 mM, and (c) with MBI at 2.5 mM.(Magnification for each image ) 1000).

IE )Rct - Rct

Rct



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3.4. Linear Polarization Curves and Potentiodynamic

Scanning Curves. Figure 6 shows Tafel polarization curves ofthe bare mild steel electrode and the MBI-treated mild steel inthe sterile seawater and seawater in the presence of the SRBs.

The deposition of the inhibitor MBI on the metal substratemarginally shifts the corrosion potential of mild steel positivelyor negatively, while the inhibitor strongly reduces the cathodiccurrent density. In particular, for the sterile mild steel, thecathodic reaction is inhibited to a larger extent than the anodicreaction. The inhibitor is adsorbed on mild steel and acts as acathodic inhibitor by retarding the transfer of hydrogen andchloride from the bulk solution to the mild steel/solutioninterface. Because amine inhibitors function as adsorbates, theirinhibition performance primarily relies on the adsorption bondbetween the atoms in the metal and the inhibitor molecules.25

The N atoms in the MBI molecule have an affinity toward mildsteel and anchor on the metal surface via the amine bond.Nitrogen-containing organic heterocyclic compounds are con-

sidered to be excellent chelate-forming substances with several

transition metals.18,19,26

Table 2 shows the corrosion current density, corrosionpotentials, and corrosion rates, which are calculated from the

Tafel plots in Figure 6. The corrosion rate can be calculatedfrom the corrosion current, using the following equation:19

where icorr is the corrosion current density, F the specimendensity, and M the atomic mass of the metal. The inhibition

effect of MBI can be calculated from the icorr value. It is obviousthat the icorr values of mild steel coupons in the presence of

MBI are lower than those without MBI for the seawater both

with and without the SRBs (Table 3). Accordingly, the IE valueof MBI on mild steel can be calculated from the icorr value:27

Figure 10. SEM images of mild steel in EASW with D. singaporenus for 24 h (a) without MBI, (b) with MBI at 1 mM, and (c) with MBI at 2.5 mM.(Magnification for each image ) 1000).

Figure 11. Biofilm on mild steel: (a) D. singaporenus without MBI, (b) D. singaporenus with MBI at 1 mM, (c) D. desulfuricans without MBI, and (d)D. desulfuricans with MBI at 1 mM.

corrosion rate (mm/yr) ) 0.00328icorr( M(g)F(g/cm3)) (1)



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In the aforementioned equation, i is the corrosion current densityof the inhibitor-containing mild steel and i is that of the baremild steel. With an increasing MBI concentration, the corrosioncurrent density and the corrosion rate decrease, and the inhibitionefficiency increases. These results show a consistent trend withthat calculated from Rct (see Table 1). MBI is more effective in

inhibiting biocorrosion caused by D. desulfuricans (with aninhibition efficiency of 85% in the presence of 2.5 mM of MBI).Generally, a higher concentration of SRB would lead to more-severe corrosion on the steel surface, because the bacteriaproduces the deleterious hydrogen sulfide (H2S) and directlyaccept electrons from the metal substrate to accelerate theanodic/cathodic reaction, thereby enhancing corrosion. Thehigher inhibition efficiency may be due to the higher bacteriainhibition effect of MBI on D. desulfuricans; as mentionedpreviously, the cell density ofD. desulfuricans showed a greaterdecrease than that ofD. singaporenus after the addition of MBI.

Potentiodynamic scanning (PDS) curves were plotted toinvestigate the metal surface properties. Similar to Figure 6a,the cathodic slopes of the PDS curves are constant and the

cathodic currents gradually decrease with the addition of MBI,which indicates that MBI predominantly controls the cathodicreaction (Figure 7a). For the mild steel exposed to sterile EASW,the current density in the passivity region for the coupon with2.5 mM of MBI is clearly visible, indicating a slight decreasein the anodic current. The passive film breakdown occurred at-0.2 V and was followed by a gradual increase in the current.Its passive region is much larger than the region without theaddition of MBI, implying that it suffers less corrosion withthe protection of MBI.

The coupon in seawater with D. desulfuricans does not revealan obvious passivity region (see Figure 7b). The breakdownoccurred suddenly and appeared as an eruption in the anodiccurrent. Such an instant increase (10-fold in the anodic current,from 3.6 10-5 to 5.8 10-4 A at 0.15 V) may possibly beassociated with a situation in which the inner surface of well-developed pits located beneath the corrosion products and thebiofilm are exposed to the bulk solution after the biofilm layerwas disintegrated.28 However, the coupon with protection fromMBI has a much higher breakdown potential, and the anodiccurrent increases more slowly, compared to that without MBI.The corrosion inhibition for the coupon with D. singaporenusis not so obvious; the addition of MBI decreases the anodiccurrent only marginally.

3.5. SEM Metal Surface Analysis. The protection of mildsteel afforded by MBI against corrosion that was caused by bothabiotic corrosion and microbiologically influenced corrosion was

corroborated by SEM analysis. Figures 8-10 show the appear-ance of polished mild steel at a magnification of 1000 after24 h of immersion in EASW. Figure 8 shows that the surfaceof the mild steel coupon is protected by MBI under sterileconditions; the coupon without the addition of MBI is full ofcorrosion products, whereas the coupon with MBI is smootherand contains fewer corrosion products. The surface of thecoupons with D. desulfuricans and D. singaporenus, whichcontains both corrosion products and the biofilm, suffered moresevere corrosion (Figures 9a and 10a). The addition of MBI inthe bacterial solution successfully reduced the extent of corrosion(see Figures 9 and 10).

The bacteria with and without the MBI was also observedusing SEM at a greater magnification (see Figure 11). In the

absence of MBI, the cell surface of D. singaporenus was smooth(see Figure 11a), whereas, in the presence of MBI in thesolution, the bacteria surface became much rougher and seemedto be flakelike (see Figure 11b). This flake distribution is alsoobserved on the steel in the presence of 600 ppm (4 mM)2-mercaptobenzoimidazole.11 A possible reason is that theextracellular polymeric substances (EPS) that were excreted by

D. singaporenus combine with the inhibitor to protect the cellsfrom the toxic inhibitor. On the other hand, no flakelike structurewas observed on the cell surface ofD. desulfuricans (see Figures11c and 11d).

IE )(i - i)

i 100 (2)

Figure 12. Atomic force microscopy (AFM) images of mild steel in EASWwith D. desulfuricans for 24 h (a) without MBI, (b) with MBI at 1 mM,and (c) with MBI at 2.5 mM.



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3.6. AFM Analysis. AFM was used to image the biofilmtopography and to quantify the surface roughness using the root-mean-square (rms) deviation (i.e., the standard deviation of Zvalues within the image) after analyzing five biofilm imagesfor each coupon. It is evident that the addition of MBI resultedin a decrease in the surface roughness (see Figures 12 and 13).The roughness of the mild steel coupon with D. desulfuricansand D. singaporenus was 104.9 ( 10.4 and 232.7 ( 23.5 nm,respectively, and the addition of 2.5 mM MBI reduced thesurface roughness to almost half of the original value (i.e., 46.7( 4.1 and 113.9 ( 19.3 nm, respectively) (see Table 3). The

AFM images and the roughness quantification are consistentwith the SEM images shown in Figures 9 and 10. Furthermore,the pit depth on the mild steel was quantified by the AFMsection analysis of 10 images for each coupon. It showed areduction with the addition of MBI (from 59.3 ( 11.2 nm to20.4 ( 10.6 nm for D. desulfuricans, and from 42.0 ( 16.7nm to 15.6 ( 5.8 nm for D. singaporenus). These results indicatethat the corrosion of mild steel by the two strains of SRB isinhibited by the addition of MBI.

3.7. Adsorption Isotherm. To understand the mechanism ofcorrosion inhibition, it is necessary to know the adsorptionbehavior of the organic adsorbate on the metal surface. Thedegree of the surface coverage () for different concentrationsof MBI has been evaluated from electrochemical data. Thefractional surface coverage () is represented by the followingequation:11

where i is the corrosion current density of the inhibited mildsteel and i is that of bare mild steel. Different adsorption modelswere used to determine empirically which adsorption isothermbest fits the surface coverage data. The equations pertaining tothe adsorption models are given as follows:

where Kad is the adsorption equilibrium constant, C theconcentration of inhibitor, and f the molecular interactionconstant. The surface coverage values () were determinedgraphically by fitting a suitable adsorption isotherm to explainthe behavior associated with the experimental results. Amongthese models, only the Langmuir model (Figure 14) provides agood fit. The MBI molecule is adsorbed on the mild steel surfacedue to the free electron pairs on the N atom. MBI was adsorbedonto the metal surface via the amine group in the heterocycliccompound to occupy the free surface sites on the metal. The

Figure 13. AFM images of mild steel in EASW with D. singaporenus for24 h (a) without MBI, (b) with MBI at 1 mM, and (c) with MBI at 2.5mM.

Figure 14. Application of the Langmuir isotherm model to the corrosionprotection behavior.

)

i - ii (3)

Temkin model: KadC) ef

(4)

Langmuir model: KadC)

1 - (5)

Freundlich model: KadC1/n ) (6)

Frumkin model: KadC) (

1 - )ef (7)



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smaller interference by Cl- ions may lead to lower adsorptionand the inhibition of corrosion (see Tables 1 and 2). Theadsorption of some other inhibitors, such as 2-mercaptoben-zoimidazole, 2,5-bis(3-pyridyl)-1,3,4-thiadiazole, 2,5- bis(3-pyridyl)-1,2,4-thiadiazole, and 3,5- bis(2-thienyl)-4-amino-1,2,4-triazoles, has also been reported to follow the Langmuir isothermmodel when being adsorbed onto mild steel.8,11,30

The standard free energy of adsorption (Gads) can beevaluated using the following expression:11

The MBI adsorption parameters were calculated from the fittingprocedure of the Langmuir adsorption equation to the experi-mental data. The free energy of adsorption is calculated to be-17 kJ/mol for MBI, which indicates that the inhibitor caneasily adsorb onto the metal surface. Usually the adsorption freeenergy involved in a chemisorption process is more negativethan -25 kJ/mol.12 This means that MBI protects mild steelthrough physical adsorption to compete with the aggressivespecies (i.e., chloride, hydrogen).

4. Conclusions

The conclusions drawn from the results may be given asfollows:

(1) From the corrosion point of view, 2-methylbenzimidazole(MBI) is an effective inhibitor in the control of corrosion ofmild steel in seawater and in the presence of sulfate-reducingbacteria (SRB).

(2) MBI predominantly inhibits the cathodic reaction.(3) The corrosion inhibition by MBI is more effective for

corrosion caused by D. desulfuricans than that caused by D.singaporenus.

(4) For microbiologically influenced corrosion (MIC) that iscaused by SRB, corrosion inhibition is partly due to the

inhibition of bacterial activity.(5) The adsorption of MBI on the mild steel surface obeysthe Langmuir adsorption isotherm.

Acknowledgment

The authors are thankful for the financial support from theNational University of Singapore (NUS) and the TropicalMarine Science Institute (TMSI) in Singapore.

Literature Cited

(1) Flemming, H.-C. Economical and technical overview. In MicrobiallyInfluenced Corrosion of Materials: Scientific and Engineering Aspects;Heitz, E., Flemming, H.-C., Sand, W., Eds.; Springer-Verlag, Berlin, 1996.

(2) Sheng, X.; Ting, Y. P.; Pehkonen, S. O. Force measurements of

bacterial adhesion on metals using a cell probe atomic force microscope. J. Colloid Interface Sci. 2007, 310, 661-669.

(3) Costerton, J. W.; Irvin, R. T.; Cheng, K. J. The bacterial glycocalyxin nature and disease. Annu. ReV. Microbiol. 1981, 35, 399-424.

(4) Jones, D. A.; Amy, P. S. Related Electrochemical Characteristics ofMicrobial Metabolism and Iron Corrosion. Ind. Eng. Chem. Res. 2000, 39,575-582.

(5) Banerjee, G.; Malhotra, S. N. Contribution to adsorption of aromatic-amines on mild-steel surface from HCl solutions by impedance, UV, andraman-spectroscopy. Corrosion 1992, 48, 10.

(6) El-Sayed, A. Phenothiazine as inhibitor of the corrosion of cadmiumin acidic solutions. J. Appl. Electrochem. 1997, 27, 193.

(7) El Azhar, M.; Mernari, B.; Traisnel, M.; Bentiss, F.; Lagrenee, M.Corrosion inhibition of mild steel by the new class of inhibitors [2,5-bis-(n-pyridyl)-1,3,4-thiadiazoles] in acidic media. Corros. Sci. 2001, 43, 2229-2238.

(8) Bentiss, F.; Lebrini, M.; Vezin, H.; Lagrenee, M. Experimental andtheoretical study of 3-pyridyl-substituted 1,2,4-thiadiazole and 1,3,4-thiadiazole as corrosion inhibitors of mild steel in acidic media. Mater.Chem. Phys. 2004, 87, 18-23.

(9) Zhang, D.; Gao, L.; Zhou, G. Inhibition of copper corrosion in aeratedhydrochloric acid solution by heterocyclic compounds containing a mercaptogroup. Corros. Sci. 2004, 46, 3031-3040.

(10) Tan, Y. S.; Srinivasan, M. P.; Pehkonen, S. O. Self-assembledorganic thin films on electroplated copper for prevention of corrosion. J.Vac. Sci. Technol. A 2004, 22, 1917-1925.

(11) Morales-Gil, P.; Negron-Silva, G.; Romero-Romo, M.; A ngeles-Chavez, C.; Palomar-Pardave, M. Corrosion inhibition of pipeline steel gradeAPI 5L X52 immersed in a 1 M H2SO4 aqueous solution using heterocyclicorganic molecules. Electrochim. Acta 2004, 49, 4733-4741.

(12) Geler, E.; Azambuja, D. S. Corrosion inhibition of copper inchloride solutions by pyrazole. Corros. Sci. 2000, 42, 631-643.

(13) Boulange, I.; Petermann, L. Processes of bioadhesion on stainlesssteel surfaces and cleanability: a review with special reference to the foodindustry. Biofouling 1996, 10, 275-300.

(14) OToole, G.; Kaplan, H. B.; Kolter, R. Biofilm formation asmicrobial development. Annu. ReV. Microbiol. 2000, 54, 49-79.

(15) Boyd, A.; Chakrabarty, A. M. Pseudomonas aeruginosa biofilms:role of the alginate exopolysaccharide. J. Ind. Microbiol. 1995, 15, 162-168.

(16) Allison, D. G. The biofilm matrix. Biofouling 2003, 19, 139-150.(17) Batista, J. F.; Pereira, R. F. C.; Lopes, J. M.; Carvalho, M. F. M.;Feio, M. J.; Reis, M. A. M. In situ corrosion control in industrial watersystems. Biodegradation 2000, 11, 441-448.

(18) Ramesh, S.; Rajeswari, S. Evaluation of inhibitors and biocide onthe corrosion control of copper in neutral aqueous environment. Corros.Sci. 2005, 47, 151-169.

(19) Tan, Y. S.; Srinivasan, M. P.; Pehkonen, S. O.; Chooi, S. Y. M.Effects of ring substituents on the protective properties of self-assembledbenzenethiols on copper. Corros. Sci. 2006, 48, 840-862.

(20) Postgate, J. R. The Sulphate Reducing Bacteria, 2nd Edition;Cambridge University Press: New York, 1984.

(21) Boukamp, B. A. A nonlinear least-squares fit procedure for analysisof immittance data of electrochemical systems. Solid State Ionics 1986,20, 31.

(22) Sheng, X.; Ting, Y. P.; Pehkonen, S. O. The influence of sulphate-reducing bacteria biofilm on the corrosion of stainless steel AISI 316.

Corros. Sci. 2007, 49, 2159-2176.(23) Xu, L. C.; Chan, K. Y.; Fang, H. H. P. Application of atomic forcemicroscopy in the study of microbiologically influenced corrosion. Mater.Charact. 2002, 48, 195.

(24) Bartlett, M. S.; Edlind, T. D.; Durkin, M. M.; Shaw, M. M.;Queener, S. F.; Smith, J. W. Antimicrotubule benzimidazoles inhibit in vitrogrowth of Pneumocystis carinii. Antimicrob. Agents Chemother. 1992, 36,779-782.

(25) Sastri, V. S. Corrosion Inhibitors: Principles and Applications;Wiley: New York, 1998.

(26) Patel, N. K. Corrosion Inhibitors for 63/37 brass in trichloroaceticacid solution. J. Electrochem. Soc. 1972, 21, 136.

(27) Ouyang, X.; Qiu, X.; Lou, H.; Yang, D. Corrosion and ScaleInhibition Properties of Sodium Lignosulfonate and Its Potential Applicationin Recirculating Cooling Water System. Ind. Eng. Chem. Res. 2006, 45,5716-5721.

(28) Starosvetsky, D.; Armon, R.; Yahalom, J.; Starovetsky, J. Pitting

corrosion of carbon steel caused by iron bacteria. Int. Biodeterior. Biodegrad.2001, 47, 79-87.(29) Beech, I. B.; Gaylarde, C. C. Recent advances in the study of

biocorrosionsan overview. ReV. Microbiol. 1999, 30, 177-190.(30) Bentiss, F.; Lagrenee, M.; Traisnel, M.; Hornez, J. C. The corrosion

inhibition of mild steel in acidic media by a new triazole derivative. Corros.Sci. 1999, 41, 789-803.

ReceiVed for reView May 11, 2007ReVised manuscript receiVed July 14, 2007

Accepted July 30, 2007

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