control of microbiologically induced corrosion in

Arab Journal of Nuclear Sciences and Applications, 45(2)460-478(2012)

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Control of Microbiologically Induced Corrosion in Petroleum IndustryUsing Various Preventive Strategies

Abeer E. Zakaria*, H. M. Gebreil* * and Naglaa M. Abdelaal** Microbiology Department, National Center for Radiation Research and Technology, Egypt.

** Microbiology Department, Ain Shams University, Egypt.

ABSTRACT

Various inhibitive strategies were used to control Sulfate reducing bacteria (SRB).The traditional treatment of SRB by biocides was compared with other treatments suchas exposure to microwaves, ultraviolet, gamma radiation separately and addition ofeither nitrate or nitrite. Six commercial biocides were tested for SRB control.Champion- A was found to be the most efficient biocide. Addition of nitrate to SRBgrowth medium did not inhibit the growth at 10 mM/l whereas addition of 6 mM/Lnitrite completely inhibited the growth. On the other hand, physical treatments achievedsatisfactory results. the lethal doses required for complete inhibition of the growth onusing microwave, ultraviolet and gamma radiations were found to be at (50 second, 4hours and 0.9 KGy) respectively. On studying the effect of the lethal and sublethal dosesof different treatment on the growth and sulfide production rates of SRB, it was foundthat the lethal doses of all studied treatments except nitrate treatment achieved completeinhibition. Also the corrosion aspects and elemental analysis of metal coupons surfacesat such doses showed a clear variation in distribution and composition of the corrosionproducts adhered to their surfaces.

Key Words: Microbiologically Induced corrosion (MIC)/ Sulfate Reducing Bacteria(SRB)/ Corrosion Control.

INTRODUCTION

In aqueous environments, iron materials are corroded not only by purely chemical orelectrochemical reactions but also by microorganisms or the products of their metabolic activitiesincluding enzymes, exopolymers, organic and inorganic acids as well as volatile compounds such ashydrogen sulfide in a process termed Microbiologically Induced Corrosion (MIC) (Miranda et al.,2006). Booth (1964), in the UK, suggested that 50% of corrosion failures in pipelines involved MICand the replacement costs for biocorroded gas mains were recently reported to be 250 million perannum (Beech and Gaylarde, 1999). Bacterial activity and mainly Sulfate Reducing Bacteria (SRB)activity responsible for > 75% of the corrosion in productive oil wells and > 50% of the failures ofburied pipelines and cables. It has been reported to be responsible for extensive corrosion of drillingand pumping machinery and storage tanks (Javaherdashti, 2008). The presence of SRB in oilenvironments was rapidly recognized as responsible for the production of hydrogen sulfide, which is atoxic and corrosive gas responsible for a variety of environmental and economic problems includingreservoir souring (increase sulfur content) (Hubert et al.,2003). It is resulting in decrease the qualityand value of oil, contamination of natural gas and oil, corrosion of metal surfaces, and the plugging ofreservoirs due to the precipitation of metal sulfides in the fluid flow paths and the consequentreduction in oil recovery (Magot et al., 2000;Nemati et al.,2001; Davidova et al.,2001 and Hubert


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et al., 2005). So due to the detrimental effects of SRB in the oil industry, they have been the mostcommonly studied group (Miranda et al., 2006).

Considerable efforts have been directed toward controlling SRB growth and inhibitionof corrosion induced by its activity. Corrosion inhibition is the slowdown of the corrosionreaction usually performed by substances which when added in small amounts, theydecrease the rate of attack by these bacteria on a metal (Ash and Ash, 2001). A number ofmethods for controlling SRB sulfide production in different oil production facilitiesincluding the use of biocides (Gardner and Stewart, 2002). Although biocide treatmentsare widely used to decrease biofouling and MIC in steel pipes and in closed systems, theresults are far from satisfactory. This is because biocides are much less effective againstsessile microorganisms with biofilms compared to their effectiveness against planktonicpopulations. In addition, biocide resistance may be developed and biocidal action reducedby dilution (Gardenr and Stewart, 2002). An alternative approach for the control of SRBsulfide production in water- oil systems is the use of repeated injection of nitrate(Thorstenson et al., 2002 and Kjellerup et al., 2005). Also the promise of nitrite additionas an effective sulfide control strategy was reported by Mohanakrishnan et al. (2008). Theuse of nitrate and nitrite was proven to be very effective and was originally attributed to theinhibition of SRB (Zhang et al., 2008). Also Cirne et al. (2008) reported that addition oflimited amounts of nitrate or nitrite is a simple strategy to control anaerobic sulfideproduction by SRB. On the other hand the physical treatment strategy such as ultra violet(UV), microwave, and radiations provides an alternative method to control SRB as both ofthese methods are non intrusive and do not require the injection of any foreign material(Bjorndalen et al., 2003). The aim of this study is comparing between different strategiesfor controlling SRB growth and prevention of corrosion induced by it and selection of themost efficient strategy among them.

MATERIALS AND METHODS

Sampling and preparation of SRB inoculum

A water sample from Agiba oilfield located in Egypt was collected in sterile and anaerobicpolypropylene bottles, then stored in ice coolers upon collection and analyzed within 24 hours. Ten mlof water sample was inoculated into a tightly closed anaerobic 500 ml bottle containing a liquidStarkey growth medium (Starkey, 1948). The inoculum of SRB culture used in all experiments wasprepared by taking 1 ml of stock culture solution and inoculated into a vial containing 9 ml ofanaerobic Starkey liquid medium for 3 days at 33 C.

Enumeration and estimation of sulfide productivity of SRB

The Enumeration of SRB in all experiments was done using the Most Probable Number (MPN)technique according to ASTM D-4412 (1990).The MPN count of SRB was compared with thestatistical table of Cochran (Cochran, 1950). Differences were considered significant at the 95%confidence interval. The total dissolved sulfide in the medium was determined spectrophotometricallyaccording to (Cord- Ruwish, 1985) method.

Evaluation of different biocide efficacy

Six different biocides (Champion A, Champion B, E.C.C. A, E.C.C. B, biocide Z-A and biocideX-B) obtained from the Egyptian Petroleum Research Institute (EPRI)were evaluated for inhibition of


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SRB. Each biocide was added to sterile anaerobic vials containing 100 ml of Agiba oilfield watersample inoculated with 1 ml of SRB culture to give final biocide concentration 100 ppm. Each vialwas inoculated with 1ml of SRB inoculum. The control vial was left untreated to be the control vial.Each biocide was kept in contact with SRB for 2 hours at room temperature (the contact time). Thenone ml aliquots of each vial were withdrawn by a sterile syringe and inoculated into free biocideStarkey medium vial and incubated at 33C for 21 days. The MPN values of SRB were recorded. Thebiocide which inhibited the SRB growth at 100 ppm will be selected for evaluation of itsefficacy at concentration less than 100 ppm in the next experiment. The selected biocides willbe tested in concentrations (10, 30, 50, 80& 100 ppm). Each biocide was added to sterileanaerobic vials containing 100 ml of Agiba oilfield water sample inoculated with 1 ml of SRBculture to give the desired final biocide concentration. One vial was left untreated to be thecontrol vial. After 2- hours contact time, one ml aliquots of each vial were withdrawn bysyringe and inoculated into free biocide Starkey medium vial and incubated at 33C for 21days. The MPN values of SRB were recorded.

Evaluation of nitrate and nitrite addition on SRB growthOne ml of SRB enumerated in Starkey medium containing different nitrate (NO3) - and nitrite

(NO2)- concentrations (0, 2, 4, 6, 8& 10 mM/l) separately. The inoculated media incubated at 33C for21 days. The MPN values of SRB and the amount of produced sulfide in each medium were recorded.

Evaluation of different irradiation strategiesOne ml of SRB culture was added separately to 9 ml Agiba oilfield water sample in different

sterile anaerobic vials to create the test samples. The test samples were then exposed to microwaveand ultra violet and gamma irradiation separately at doses (20, 30, 40 & 50 second), (1, 2, 3 & 4hours) and (0.1, 0.3, 0.5, 0.7 & 0.9 KGy) respectively. After irradiation, 1 ml of each sample wasinoculated into Starkey medium. The inoculated medium incubated at 33C for 21 days. The MPNvalues of SRB of each sample were recorded.

Preparation of metal coupons for pre and post- test examinationPrior to use, the surfaces of metal coupons were metallographically polished according to

(ASTM G 1-72, 1993). The metal coupons were serially polished with 80, 120, 220 and 500 gritsilicon- carbide papers, degreased in acetone, washed with sterile distilled water. Then coupons weredried in a current air and sterilized with ethanol before exposure to the experimental media (Gonzalez-Rodriguez et al., 2008). After metal coupons use (at the end of each experiment), the metal couponsurfaces were cleaned under a stream of tap water and scrubbed vigorously with rubber stopper (Zuoet al., 2004).

Studying the effect of lethal and sublethal doses of different treatment on both the growth andactivity of SRB and corrosion aspects of metal coupons

One ml of SRB inoculum was inoculated separately into vails containing 9 ml Agiba oilfieldwater sample and exposed to the sublethal and lethal doses of champion A biocide, microwave,ultraviolet and irradiations. An inoculated untreated vial was kept as a positive control. Aftertreatments directly, one ml aliquots of each treated vial and the control one were withdrawn by syringeand enumerated in the modified medium at 33C for 21 days to obtain the count at zero time aftertreatment. On the other hand, one ml aliquots of each treated vial and the control one was inoculatedinto the modified medium vial containing a metal coupon for studying the corrosion aspects of themetal after exposing SRB to each treatment and incubated at 33C for 30 days. One medium vialcontaining a metal coupon was left uninoculated to study the corrosion aspect of the metal coupon in


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the medium in the absence of SRB (negative control). At regular time intervals upon 30 days, one mlof the inoculated vials containing a metal coupon was withdrawn and enumerated in another freshmedium. The MPN values of SRB and the amount of produced sulfide was estimated upon 30days.For studying the corrosion aspects of the metal coupons surfaces at nitrate and nitrite treatmentstrategy, one ml of SRB inoculum was cultured and enumerated separately into two sets of 9 mlmedium vials with a metal coupon, one set containing (4& 6 mM/l) of nitrite and the other setcontaining (10 mM/l) of nitrate, then incubated at 33C for 30 days. The MPN values of SRB and theamount of produced sulfide were estimated upon 30 days at different time intervals. At the end ofexperiment (after 30 days), the metal coupons were taken from the vials. Finally the corrosion aspectand elemental analysis of the metal coupons surfaces were examined by Scanning ElectronMicroscope (SEM) coupled with Energy Dispersive X- ray (EDX).

RESULTS AND DISCUSSION

It is concluded from Figure (1) that biocide treatment for two hours contact time sharply decreasedthe count of SRB comparing to the control. Biocide X-B exhibited less efficiency than Champion B butboth biocides exhibited reduced count than the control whereas other biocides stopped the growth. Sobiocides Champion B and X-B were excluded from the following evaluation studies. Also it was observedthat the all tested biocides of aldhydic (Champion A, E.C.C.A &Z-A) and only one of amine type biocides(E.C.C.B) exhibited complete inhibition of the growth. Hence, it can be concluded that the aldhydicbiocides were more effective than the amine type biocides for SRB inhibition. These data goes parallel towhat has been achieved by other investigators. Von Reg and Sand (1998) evaluated different biocideefficacy for SRB treatment. The samples were treated with the biocide formaldehyde,tetramethylammoniumhydroxide, 1,8 dihydroxyanthraquinone and a commercial biocide named Dilurt atvarying concentrations It was found that formaldyde exhibited the best effect. Only 3% of the originalmicrobial activity remained and reduction in SRB cell numbers of five orders of magnitude. In contrast,tetramethylammoniumhydroxide had only slight effect. Microbial activity was reduced only to 20% and thecell numbers did not decrease at all. The other biocide exhibited intermediate effects. Also Gardner andStewart (2002) reported that 50 ppm of glutaraldehyde retarted the SRB growth to 143 hour in Postgate Cmedium due to its ability to cross- link proteins.

Fig. (1): Final SRB counts at 100 ppm concentration of different biocides after 21 days ofincubation.


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From Figure (2), it is clear that Champion A is the most effective biocide among the others. Asat all its concentrations, it exhibited high efficiency for reducing SRB count contrary to others biocidesconcentrations. Also it exhibited the lowest minimal inhibitory concentration at (80 ppm) whereas forothers, it was at (100 ppm). So it will be selected as the most efficient biocide among the others. Themodes of action of these biocides may be attributed to their electrochemically active properties as theyreact with the nucleophilic components of the microbial cell or their ability to form chelates with metalcations necessary for cell metabolism (Heitz, 1996). Other mode of action is that the ingredients ofbiocide are membrane active which coat the cell wall of the microbes adsorptively. This processcauses changes in the outer membrane. These outer barriers loose their integrity with the results thatthe biocide molecules are allowed access to cytoplasmic membrane, so that they can release theirlethal effects, resulted in inhibition of the enzymes localized in the cytoplasmic membrane, escape ofessential components from the cytoplasm (Heitz, 1996).

Fig. (2): The final SRB counts after 21 days of incubation at different concentrations of the fourtested biocides.

The overall effect of nitrate and nitrite addition on the final counts and amounts of producedsulfide by SRB after 21 days of incubation is illustrated in Figure (3). The data shows that the growthand sulfide production activity of SRB were completely inhibited at 6 mM/l nitrite whereas at thesame concentration or even at the higher concentration up to (10 mM/l) of nitrate, the count andsulfide production activity were only reduced without complete inhibition. From this observation, itcan be concluded that nitrite is more effective than nitrate for controlling SRB growth. This conclusionagreed with some worker's studies. Sturman and Goeres (1999) suggested that the use of nitrite aloneas an alternative to nitrate may promote higher reactivity and more rapid scavenging of sulfide.Kjellerup et al. (2005) demonstrated that nitrite alone reduced the SRB production of sulfide, whilenitrate alone had no effect. Also Garcia De-lomas et al. (2007) reported that sulfate reduction activitywas not fully inhibited by nitrate addition.


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On the other hand there were other researches disagreed with this conclusion. Jenneman et al.(1986) found that the addition of (59 mM/l) nitrate completely inhibited the sulfide production and thenumber of SRB decreased with prolonged incubation period. Also Myhr et al. (2002) found thatinjection on 0.5 mM nitrate for 2.5- 3.5 months led to complete elimination of H2S.

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Fig. (3): Comparison between the effect of nitrate and nitrite addition on both the final countsand amounts of produced sulfide by SRB.

Figures (4) represent rates of SRB growth when exposed to different doses of microwaveradiation, the results show that with increasing microwave exposure period, the SRB growth sharplydecreased. It was observed that SRB growth was stimulated by very low exposure doses of microwavewhereas the high doses inhibited it. Also it was cleared up that the lethal dose required for completegrowth inhibition is (50 sec.) of exposure period.

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These results are supported by various examinations of other microorganisms exposed tomicrowave irradiation and the inhibitory action of microwaves on SRB growth was reported by(Fujikawa and Ohta, 1994; Sato et al., 1996 and Hamid et al., 2001). It has also been previouslyreported that the destructive effect of microwave irradiation is not only due to temperature change butalso the irradiation effect itself (Sato et al., 1996). Also Bjorndalen et al. (2003) studied the effect ofmicrowave irradiation on SRB and concluded that with increasing microwave time, the growth of thebacteria is inhibited. Also he deduced that with slight microwave irradiation, SRB is activated. Thismay be attributed to slight molecular agitation of certain molecules such as water produced by theaction of microwave irradiation.

On studying the effect of U.V exposure on SRB growth, it was found from Figure (5) that onincreasing U.V exposure period, there was a significant decrease in the count of SRB. Also thepresented data pointed out that the 4 hours exposure period was the lethal dose which was sufficientfor complete inhibition of SRB growth. The inhibitory effect of ultraviolet radiation can be attributedto the production of a limited of specific types of chemical changes. The aromatic rings of the purinesand pyrimidine bases selectively absorb ultraviolet radiation where two adjacent pyrimidine in thesame chain (T-T, C-C or T-C) become covalently bonded together; these unpleasant pyrimidinedimmers disrupt the local structure of the DNA (Eckardt-Schupp and Klause, 1999).

It has been proved that the U.V. radiation method is effective for SRB elimination from groundwater. It was found that doses below 40 mJ/cm2 were ineffective in SRB elimination. In doses rangingfrom 40 to 75 mJ/cm2 the effectiveness of SRB removal varied from 55 to 86%. The highest removaleffectiveness (about 100%) was observed when UV doses above 77 mJ/cm2 were applied (Wargin etal., 2007). Although the efficiency of U.V in a 99% reduction in viable bacterial numbers, its effectwas efficient only at very long exposure time. This may be due to the poor penetrating power ofultraviolet light (Mittelman, 1990). Some investigators have addressed U.V application as analternative to biocides (Saiz-jimenez, 2001).

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The results presented in Figure (6) indicated that on increasing gamma radiation doses, thegrowth sharply decreases. The control (non-irradiated) sample exhibited the highest SRB count(2.4x105). At low dose of gamma radiation (0.1 KGy), the growth less final count (2.1x103) than at thecontrol sample. At doses (0.3, 0.5& 0.7 KGy), the growth exhibited final counts (2.0x10, 7.8& 2.0)respectively showing a dramatic decrease in the count. Also the given data pointed out that the lethaldose of gamma radiation required for complete inhibition of SRB growth is at (0.9 KGy). Among themethods which can be used for changing metabolic activities of living cells is gamma radiation(Alabostro et al., 1987). Several previous studies recorded that the low doses of gamma radiation maystimulate the microbial metabolic activities (El-Batal and Khalaf, 2003). Meanwhile, high doses ofgamma radiation were proved to be inhibitory of microbial activities (Meleigy, 2009).

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Fig. (6): Effect of different gamma irradiation doses on the MPN values of SRB after 21 days ofincubation.

The growth rate of SRB upon 30 days was determined in the modified medium containing ametal coupon after the exposure to the lethal and sublethal doses of champion A biocide, microwave,ultraviolet, gamma radiation, nitrite and the most efficient nitrate concentration for controlling SRB inorder to study the effect of such treatment doses on the growth rate of SRB. All samples wereinoculated with initial count (1.0x 104) of SRB. Figure (7) revealed that the control sample (untreated)exhibited the highest growth rate and count comparing to the treated samples. The lethal doses of allthe studied treatments as well as the lethal and sublethal doses of Champion A biocide (50& 80 ppm)respectively exhibited complete inhibition of SRB growth without growth recovery upon 30 days ofincubation.

On the other hand the nitrate treatment (10 mM/l) exhibited relatively reduction in SRB count.At the second day of incubation, the SRB growth at the control sample (untreated) increased by onelog cycle comparing to the initial count and continued the increase to reach the highest final count(1.1x106) at day (12), then entered the stationary phase up to day (20). The decline phase appeared atday (24) at which the count decreased to be (1.8x105). On the other hand at day (2) of incubation, thenitrate dose (10 mM/l) and the sublethal doses of nitrite (4 mM/l), microwave (40 sec.), U.V. (3 hours)and gamma radiation (0.7 KGy) sharply reduced the counts by ratio (79, 93, 98, 97& 99%) of theinitial count respectively, whereas the growth at such treatments started to recover and the counts


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gradually increased to reach final counts (6.6x103, 1.8x103, 3.6x103, 8.1x102& 1.0x102) at days (12,16, 12, 12& 8) respectively after that the growth entered the stationary phase. Then the counts startedto decrease at the decline phase to be (3.8x10, x& 2.1x10) at the sublethal doses of microwave,U.V. and gamma radiation respectively at day (30). Whereas at the nitrate dose (10 mM/l) and thesublethal dose of nitrite (4 mM/l) the growth remained constant. Hubert et al. (2005) found that thetreatment with 17.5 mM nitrate lowered the planktonic SRB by 3-5 log units, whereas, direct additionof 20 mM nitrite reduced SRB populations by 3-7 log units.

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Control Campion-A(50 ppm) Campion-A(100 ppm)Nitrate(10mM/l) U.V.(3hours) Nitrite (6mM/l)Microwave (40 sec.) Microwave (50 sec.) Nitrite (4mM/l)U.V. (4 hours) Radiation (0.7KGy) Radiation (0.9KGy)

Fig. (7): Effect of the lethal and sublethal doses of different treatments on the growth rates ofSRB.

The sulfide production rate of SRB upon 30 days was determined during its growth in themodified medium containing a metal coupon after the exposure to the lethal and sublethal doses ofchampion A biocide, microwave, ultraviolet, gamma radiation, nitrite and the most efficient nitrateconcentration for controlling SRB (10 mM/l) in order to study the effect of such treatment doses onthe sulfide production rate of SRB. Data given in Figure (7) indicated that the lethal doses of all thestudied treatments as well as the sublethal dose of Champion A biocide (50 ppm) exhibited completeinhibition of the SRB sulfide productivity as the sulfide was absent in the medium upon 30 days ofincubation.The sulfide was immediately detected in the medium at the second day of incubation forthe control sample, whereas for the treated samples with the sublethal doses of nitrite (4 mM/l),microwave (40 sec.), U.V. (3 hours) and nitrate dose (4 mM/l), it was detectable at day (5). Also it wasobserved that sublethal dose of gamma radiation resulted in suppression of the sulfide productionactivity for (8) days. Also the presented data revealed that the control sample (untreated) exhibited thehighest sulfide productivity (0.48 mM/l) comparing to the all treated samples. The sulfide productivitywas dramatically reduced by approximately ratio (56, 66, 73, 75& 81%) at the nitrate dose (10 mM/l)as well as the sublethal doses of nitrite (4 mM/l), U.V. (3 hours) microwave (40 sec.), and gammaradiation (0.7 KGy) respectively comparing to the control sample. These obtained data agree withearlier scientific results. The response to nitrate treatment was a rapid reduction in number and activityof SRB in the water injection system. The activity of SRB has remained low at < 0.3 and < 0.9 g H2Scm2/ day at Veslefrikk and Gulfaks oil fields respectively (Bodtker et al., 2008).


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Fig. (7): Effect of lethal and sublethal doses of different treatments on the sulfide productionrates of SRB.

From the EDX analysis and SEM images presented at Table (1) and Figure (8), a clear variationcan be noticed between the positive control sample (a coupon immersed in medium inoculated withuntreated SRB) and the negative control (a coupon immersed in medium without SRB) as well as thesamples treated at lethal doses of nitrite, microwave, ultraviolet and gamma radiation (couponsimmersed in medium inoculated with SRB exposed to the applied treatments separately). The SEMimages showed many corrosion pits on the metal surface of the positive control coupon and noncontentious deposits of bacteria adhered to the surface were clearly observed (Fig. 8-B). Whereas, thenegative control showed a clear surface without pits or any deposits on the surface (Fig. 8-A).

The previous data were confirmed by the EDX elemental analysis of the metal surface andrepresented in Table (1) and Fig. (8-A, right column); it shows a clear variation in distribution andcomposition of the corrosion products between the positive control and the negative one. There was anintense accumulation of sulfur based compound (may be iron sulfide) on the surface of metal couponof the positive control, whereas in case of the negative control, sulfur element and other metabolicproducts were absent. High corrosion rate of steel in de-aerated SRB medium containing high Fe+2concentration was detected by Booth et al. (1967). The corrosion rate of N-80 steel in SRB containingtest medium was six times more than that in abiotic medium (Sorioglu et al., 1997). Also in thepresence of SRB, there is accumulation of sulfur-based compounds whereas in its absence, there is not(Castaneda and Benetton, 2008). Li et al. (2009) reported that pitting corrosion of steels is a verycomplex process in the media inoculated with SRB. On the other hand, there was no clear differencein the SEM images and EDX analysis between the negative control coupon sample (Fig. 8-A) andcoupons samples treated at the lethal doses of nitrite, microwave, ultraviolet and gamma radiation asall of them exhibited clear non corroded surfaces (Fig. 8- F, H, J, L) and the absence of sulfur elementdemonstrating the absence of bacterial activity at such treatment doses. So they can de used efficientlyfor controlling the corrosive effect of SRB. Hubert et al. (2005) observed absence of corrosion duringthe 64 day treatment with 20 mM nitrite. Videla and Herrera (2009) reported that inorganic anionssuch as nitrite form an ionically bonded surface compound which produces a barrier to corrosionreaction.


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Table (1): The EDX analysis of metal coupons surfaces different immersed at the modified medium inoculated with SRB ofdifferent treatments.

Element (Wt %)- Total= 100Sample Fe S Cr Al Si Mn Ca Cl O N P MgNegative control sample: (Without SRB) 98.5 - 0.07 0.34 0.25 0.65 0.07 0.07 - - - -

Positive control sample: (With untreated SRB) 92.21 1.61 - 0.12 1.0 0.61 0.21 0.13 4 - - -

Treated samples DoseCampion A Sublethal:(50 ppm) 77.02 - 0.07 0.2 0.72 0.56 2.11 0.06 11.87 3.58 2.62 1.19

Nitrate (10 mM/l) 98.11 0.22 0.07 0.13 0.26 0.68 0.11 - - - 0.34 0.07Sublethal: (4 mM/l) 97.28 0.28 0.07 0.14 0.84 0.72 0.26 - - - 0.28 0.12Nitrite

Lethal: (6 mM/l) 96.46 - 0.08 0.29 1.15 0.75 0.35 - - - 0.38 0.54Sublethal: (40 sec.) 96.25 0.12 0.06 0.23 0.74 0.70 0.73 - - - 0.88 0.28Microwave

Lethal: (50 sec.) 97.20 - 0.10 0.06 0.62 0.66 0.48 - - - 0.60 0.29Sublethal: (3hours) 94.32 0.96 0.05 0.12 1.1 0.73 1.02 - - - 1.07 0.64U.V.

Lethal: (4 hours) 98.24 - 0.08 0.15 0.53 0.66 0.23 0.11 - - - -Sublethal:(0.7 KGy) 89.09 0.79 0.06 0.08 1.13 0.58 0.47 - 3.86 2.99 0.61 0.34Gamma radiationLethal: (0.9 KGy) 97.41 - 0.08 0.40 0.41 0.63 0.25 - - - 0.71 0.12


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(SEM images) (EDX analysis charts)

A- Negative control sample (without SRB).

B- Positive control sample (with untreated SRB).

C- Champion-A biocide (50ppm).


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D- Nitrate (10 mM/l).

E- Nitrite (4 mM/l).

F- Nitrite (6 mM/l).


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Continued

G- Microwave (40 sec.)

H- Microwave (50 sec.).

I- Ultraviolet (3 hours).

J- Ultraviolet (4hours).


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K- radiation (0.7KGy).

L- radiation (0.9KGy).Fig. (50): Scanning electron microscopy (SEM) micrograph (Left column) and EDX analysis

(Right column) of metal coupons surfaces immersed at the modified medium:A- without SRB, B- with untreated SRB, C- with SRB pretreated with Champion-A biocide(50ppm), D- containing nitrate (10 mM/l) and inoculated with SRB, E- containing nitrite (4mM/l) and inoculated with SRB, F- containing nitrite (6 mM/l) and inoculated with SRB, G-with SRB pre-exposed to microwave for(40 sec.), H- with SRB pre-exposed to microwavefor(50 sec.), I- with SRB pre-exposed to ultraviolet for(3 hours), J- with SRB pre-exposed toultraviolet for (4 hours), K- with SRB pre-exposed to gamma radiation dose (0.7 KGy), L- withSRB pre-exposed to gamma radiation dose (0.9 KGy).

Also Fig. (8) and Table (1) show a clear variation between the positive control sample and allthe treated samples at the sublethal doses of (nitrite, microwave, U.V. and gamma radiation), wherethe pit density and amount of sulfur element precipitated on the metal surface of positive controlsample as shown in Fig. (50-B) were more intensive and much greater than other sublethal dosetreated samples (Fig. 8- E, G, I, K). The localized damage to the metallic surface of control sample andthe higher concentration of sulfur based compound and other potentially biotical generated corrosioninfluencing compounds are likely to be enhanced due to SRB metabolism and biofilm formation(Castaneda and Benetton, 2008).


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The SEM images and EDX analysis presented at Fig. (8- B, D) and Table (1) show the presenceof pitting corrosion and sulfur element on the coupon surface of nitrate treated sample but still muchless than what was detected on coupon surface of the positive control sample. Bodtker et al. (2008)found that long term nitrate treatment provided efficient inhibition of SRB activity at Gulfaks oil field,the reduction in activity was followed by a significant reduction in corrosion up to 40%. Also thecorrosion rated were less than 1.5 mpy when coupons were incubated in medium containing sulfateand nitrate than sulfate only. Further more the occurrence of pitting corrosion was fairly low under allcircumstances (Dunsmore et al., 2004). On the other hand some researchers have observed anincrease in corrosion rates related to nitrate mediated souring control (Nemati et al., 2001; Rempel etal., 2006 and Schwermer et al., 2008).

It was observed that the applied biocide (Champion- A) of the dose (50 ppm) showed the worsttreatment as it exhibited an intense localized corrosion of the coupon surface although the EDXanalysis presented in Table (45) and Fig. (50-C) did not show any presence of sulfur indicatingcomplete suppression of SRB activity at such treatment. This corroded aspect of the coupon surface ofbiocide treated sample can be attributed to the corrosive action of the used biocide itself or killing themicrobial community members that offer protection against corrosion. This was previously reportedby Zuo et al. (2004). Also the tendency of increasing corrosion rate with biocide treatment has alsobeen observed in water injection systems at the Veslefrikk and Gulfaks field (North Sea)(Thorstenson et al., 2002 and Sunde et al., 2004). The mean corrosion rate observed at the well headof a studied oil field during nitrate treatment was lower than observed during biocide treatment, butthe reduction was not significant (Bodtker et al., 2008). These results impose the use of other controltreatments rather than the traditional and common use of chemical biocides which resulted corrosionaspects on the surfaces of the studied metal coupons.

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