Microbiologically Influenced Corrosion of a Pipeline in a Petrochemical

Article Microbiologically Inﬂuenced Corrosion of a Pipeline in a Petrochemical Plant

Mahdi Kiani Khouzani 1 , Abbas Bahrami 1, Afrouzossadat Hosseini-Abari 2, Meysam Khandouzi 1 and Peyman Taheri 3,*

1 Department of Materials Engineering, Isfahan University of Technology, Isfahan 84156-83111, Iran; [email protected] (M.K.K.); [email protected] (A.B.); [email protected] (M.K.) 2 Department of Biology, Faculty of Sciences, University of Isfahan, Isfahan 817463441, Iran; [email protected] 3 Delft University of Technology, Department of Materials Science and Engineering, Mekelweg 2, 2628 CD Delft, The Netherlands * Correspondence: [email protected]; Tel.: +31-15-278-2275

Received: 1 March 2019; Accepted: 16 April 2019; Published: 19 April 2019

Abstract: This paper investigates a severe microbiologically influenced failure in the elbows of a buried amine pipeline in a petrochemical plant. Pipelines can experience different corrosion mechanisms, including microbiologically influenced corrosion (MIC). MIC, a form of biodeterioration initiated by microorganisms, can have a devastating impact on the reliability and lifetime of buried installations. This paper provides a systematic investigation of a severe MIC-related failure in a buried amine pipeline and includes a detailed microstructural analysis, corrosion products/biofilm analyses, and monitoring of the presence of causative microorganisms. Conclusions were drawn based on experimental data, obtained from visual observations, optical/electron microscopy, and Energy-dispersive X-ray spectroscopy (EDS)/X-Ray Diffraction (XRD) analyses. Additionally, monitoring the presence of causative microorganisms, especially sulfate-reducing bacteria which play the main role in corrosion, was performed. The results confirmed that the failure, in this case, is attributable to sulfate-reducing bacteria (SRB), which is a long-known key group of microorganisms when it comes to microbial corrosion.

Keywords: microbiologically inﬂuenced corrosion (MIC); sulfate-reducing bacteria (SRB); failure; amine pipeline

1. Introduction Pipelines and fittings are considered to be strategic components in petrochemical plants, especially when it comes to sustainable and safe operation. Amongst different fittings, elbows, mostly with angles of 90◦ or 45◦, are the most used ones. Elbows can experience severe damage in the case of dramatic changes in the flow pattern. As far as materials are concerned, carbon steels are widely used for the transmission of petroleum products and water. On the grounds that carbon steel is not remarkable as a corrosion-resistant alloy, external corrosion of buried carbon steel pipelines/fittings is always considered a major issue in petrochemical plants. The consequences of failures in buried pipeline/fittings are severe in most cases, owing to the fact that repair and replacement of damaged pipes are costly and difficult, in the sense that the damaged area needs first to be reached by digging into the ground. This implies more time, needed for maintenance operations, associated with longer downtimes for an installation. Other than that, failure of pipes/fittings can cause environmental contaminations, imposing health risks to the public [1–4]. Pipeline corrosion is influenced by a large number of factors such as soil resistivity, soil chemistry, temperature, pH, aeration, and microorganisms.

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For buried underground carbon steel pipelines several measures are normally taken to minimize the corrosion risk, including applying coating and putting the cathodic protection system into service. Any damage in the coating or improper selection of cathodic protection settings results in corrosion progression over long-term service [5,6]. Amongst the different possible corrosion mechanisms, microbiologically induced corrosion (MIC), also named microbiologically influenced corrosion, is the one closely related to the activity of living microorganisms in the soil, including microalgae, bacteria, archaea, and fungi. The economic costs associated with microbiologically influenced corrosion in buried pipelines in oil, gas, petrochemical industries, power plants, and other chemical plants are enormous. Microbiologically influenced corrosion can take place in environments and working conditions where there is no other corrosion taking place, or it can take place in combination with other corrosion failures. More importantly, microorganisms can accelerate the kinetics of anodic/cathodic corrosion reactions, in such a way that they can be viewed as “catalytic” entities. Microorganisms are more known to induce a localized attack, including dealloying, pitting, localized galvanic corrosion, and stress corrosion cracking [1,7–9]. MIC-induced problems in buried pipelines/equipment have attracted lots of attention all around the world [1,9]. MIC, in any form, starts with the formation of a biofilm on the metal substrate [1], within which planktonic cells get attached to the substrate of steel, where they grow, reproduce, consume nutrients and produce an extracellular polymer substance (EPS) in addition to other metabolic products that generate a localized corrosion cell and collectively build the biofilm. It is also well known that microorganisms can use chemicals as nutrient sources and oxidize them [10–12]. Microbial corrosion of steel is divided into aerobic and anaerobic corrosions, with the latter known to induce a higher corrosion rate, while the former is associated with a lower corrosion rate [13]. Anaerobic microorganisms can also grow and control corrosion, in the absence of oxygen. When it comes to biocorrosion of steels, iron-reducing bacteria (IRB), iron- and manganese-oxidizing bacteria, acid-producing bacteria, and sulfate-reducing bacteria (SRB) are microorganisms which are recognized to have more detrimental effects. The latter is known to be one of the main culprits that cause severe biodegradation of the external surface of buried pipelines. As far as SRB-induced biocorrosion of the interior of pipes is concerned, the low content of oxygen, together with stagnant liquid inside the pipelines, provide desirable conditions for SRB to grow. In addition, SRB can appear under deposits of soil, water, hydrocarbons, chemicals, etc. The kinetics of corrosion in steels under the influence of microorganisms, including SRB, can be up to ten times faster than the kinetics in the absence of microorganisms [6]. It is reported that SRB could get electrons directly from Fe when there is no availability of organic carbon sources. This could occur when the soil acts as a barrier between the steel surface and the external environment [6]. Figure1 shows a schematic drawing of the SRB function in MIC. SRB can use elemental iron as an energy source or electron donor (anode reaction). Furthermore, in order to produce energy and maintain electroneutrality, SRB uses sulfate as a terminal electron acceptor in the reaction at the cathode (following reaction 1 as microorganism) [2,3,13]. Corrosion products which are formed by reactions are given here, in Reactions 2, 3 and 4:

2 + SO − + 9H + 8e− HS− + 4H O (1) 4 → 2 2+ + Fe + HS− FeS + H (2) → 2+ Fe + 2OH− Fe(OH) (3) → 2 Fe(OH) Fe O + H + 2H O. (4) 2 → 3 4 2 2 This research investigates a severe MIC-induced failure in a buried amine plant installation. The amine treatment process is, in fact, a “sweetening” process in which excess carbon dioxide and hydrogen sulﬁde are removed from sour or natural gas. Even though there are lots of studies on microbial corrosion, there are few ﬁeld investigations on buried pipelines available. This investigation studies MIC in a real industrial case. Although there are many microorganisms in the environment Metals 2019, 9, x FOR PEER REVIEW 3 of 14

hydrogen sulfide are removed from sour or natural gas. Even though there are lots of studies on Metals 2019, 9, 459 3 of 14 microbial corrosion, there are few field investigations on buried pipelines available. This investigation studies MIC in a real industrial case. Although there are many microorganisms in the sinceenvironment the major since failure the mechanism major failure is themechanism presence is of the SRB, presence the purpose of SRB, of this the paperpurpose is to of prove this paper the role is ofto SRB.prove The the ﬁndings role of of SRB. this studyThe findings could be of very this useful study when could itcomes be very to identifyinguseful when and it preventing comes to MICidentifying in buried and pipelines. preventing MIC in buried pipelines.

Figure 1.1. A schematic view of the function of sulfate-reducingsulfate-reducing bacteria (SRB) in microbiologicallymicrobiologically induced corrosioncorrosion (MIC).(MIC).

2. Materials and Methods 2.1. Case Background 2.1. Case Background This work studies the severe failure of carbon steel elbows in a buried amine pipeline of a This work studies the severe failure of carbon steel elbows in a buried amine pipeline of a petrochemical plant. Massive failure was observed after less than only five years of operation. The plant petrochemical plant. Massive failure was observed after less than only five years of operation. The underwent a major overhaul to replace damaged elbows. According to the design specifications, plant underwent a major overhaul to replace damaged elbows. According to the design the elbow was made of SA 234-WPB carbon steel and was placed underground. Table1 shows the specifications, the elbow was made of SA 234-WPB carbon steel and was placed underground. Table chemical–physical analysis and grain size of the soil. Results show that the soil contained 9.0% coarse 1 shows the chemical–physical analysis and grain size of the soil. Results show that the soil (>76.2 mm) particles. This is an important factor when it comes to the water retention capability of the contained 9.0% coarse (>76.2 mm) particles. This is an important factor when it comes to the water soil. In addition to that, the particle size distribution affects oxygen diffusion and solute transport, retention capability of the soil. In addition to that, the particle size distribution affects oxygen with both having significant implications for the corrosion rate. The pH of the soil was around diffusion and solute transport, with both having significant implications for the corrosion rate. The 8.5. Microbial diversity is noticeably affected by pH, with acidic soil harboring the least diverse pH of the soil was around 8.5. Microbial diversity is noticeably affected by pH, with acidic soil communities and neutral soil harboring the most diverse ones. pH values below 5.5 are generally a harboring the least diverse communities and neutral soil harboring the most diverse ones. pH sign of lower contents of calcium, magnesium, and phosphorus. The sulfate/sulfide content of the soil values below 5.5 are generally a sign of lower contents of calcium, magnesium, and phosphorus. is also important because sulfur stimulates the metabolism of SRB in the soil. The soil contained some The sulfate/sulfide content of the soil is also important because sulfur stimulates the metabolism of chloride ions as well. Chloride makes the environment more aggressive since it damages the passive SRB in the soil. The soil contained some chloride ions as well. Chloride makes the environment oxide film on the surface and causes localized corrosion [2,14]. more aggressive since it damages the passive oxide film on the surface and causes localized corrosion [2,14].

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Table 1. Speciﬁcations of the onsite soil (taken from the laboratory of the plant according to ASTM D4318-17e1 and ASTM D4972-18).

Percentage of Salt SO Plasticity Liquid Parameters Coarse Particles pH (%) 3 Cl (%) Content (%) Index Limit (%) (>76.2 mm) (%) Sample result 9.0 8.5 0.14 0.03 0.05 N.A. 17 Speciﬁcations limit - - - - - 6% 25% ≤ ≤

2.2. Experiment The failure analysis, in this case, is mostly based on a microstructural investigation, using routine metallographic techniques, including optical microscopy (OM, Nikon, Tokyo, Japan) and scanning electron microscopy (SEM), coupled with energy dispersive X-ray spectroscopy (EDS, Philips, Eindhoven, The Netherlands). Failed elbows were studied in this investigation. Optical emission spectroscopy (OES, Quantometer 34000-ICP-OES System, Thermo Scientific, Arun, UK), was used to determine the elemental composition of failed samples. Macroimages were taken using a Stereo Microscope Nikon SMZ 800 (Nikon, Tokyo, Japan). For metallography, samples were prepared by grinding the surface with 180 to 1200 grinding papers, followed by polishing with diamond paste (3 and 1 µm). Etching was performed with Nital 60% (the solution contained 1–10 mL nitric acid with 100 mL ethanol). Phase identification was conducted using an X-ray diffractometer (XRD, Philips, Eindhoven, The Netherlands), using Cu K-alpha radiation. To study the presence and effects of SRB, some surface cuts of the corroded tube and the soil from the damaged area were inoculated in an SRB-specific medium (the modified form of API RP-38 medium), which contained Na lactate 3.5 g, peptone meat extract 1.2 g, MgSO4 0.9 g, NaSO4 0.3 g, K2HPO4 0.3 g, CaCl2 0.06 g, Fe(NH4)2(SO4)2-6H2O 0.39 g, and ascorbic acid 0.1 g in 1000 mL distilled water [15]. In order to prevent O2 penetration, liquid paraffin was added after inoculation and incubated at 25 ◦C for 20 days in anaerobic conditions. Soil samples were collected from the vicinity of the damaged area. Samples were collected from ten different spots with an approximate weight of 10 g each. Collected soil samples were stored in plastic bottles. In order to assess the microbial activity, the redox potential of the as-received soil before inoculation in an SRB-specific medium and that after inoculation was measured using a multi-parameter analyzer (Consort C 535 model, Belgium).

3. Result and Discussion

3.1. Visual Observations Figure2 shows a typical failed elbow, taken out of the service. Both the inner and outer surfaces were covered with red/yellow–brown and, in some spots, black rust layers. The elbows were heavily damaged at the outer surface. Large pits and perforations were seen on the surface. Pits on the exterior of the failed elbows were at diﬀerent stages, with some at the early stages, while a few have turned into big holes during service. While the outside part of the elbows had undergone severe localized corrosion, the inside part hardly showed any pitting, see Figure3, indicating that the pitting had started from the outer surface. There were hardly any signs of surface irregularities (e.g., erosion or surface deformation) other than severe surface pitting. Metals 2019, 9, 459 5 of 14 Metals 2019, 9, x FOR PEER REVIEW 5 of 14

Figure 2. An example of the failed elbows.

Figure 3. An inner surface of the failed elbows.

3.2. Chemical Chemical Analysis Analysis OES, oror Quantometer,Quantometer, was was used used to to measure measure the the chemical chemical compositions compositions of the of alloy.the alloy. Results Results were werecompared compared with standard with standard ASTM A234ASTM WPB A234 to check WPB the to conformity check theof conformity the chemical of composition the chemical of compositionthe alloy with of thethe standardalloy with values. the standard Table2 showsvalues. theTable chemical 2 shows composition the chemical of thecomposition sample and of the its samplecorresponding and its ASTM corresponding standard values. ASTM The standard results showvalues. that The the chemicalresults show composition that the of thechemical failed compositionelbow is almost of the in accordancefailed elbow with is almost standard in accordance values. with standard values.

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Table 2. The chemical composition of the sample and its corresponding ASTM standard values.

MetalsElements 2019, 9, x FOR PEER C REVIEW Mn P S Si Cr Mo Ni Cu V6 of Nb14 Sample 0.20 0.64 0.02 0.01 0.23 0.03 0.05 0.01 0.01 - - ≤ ASTM-A234Table WPB2. The chemical 0.3 0.29–1.06composition0.05 of the 0.058sample an 0.1d minits corresponding 0.4 0.15 ASTM 0.4standard 0.4 values. 0.08 0.02 Elements C Mn P S Si Cr Mo Ni Cu V Nb 3.3. MorphologySample of Pits0.20 0.64 0.02 0.01 0.23 0.03 ≤0.05 0.01 0.01 - - ASTM-A234 WPB 0.3 0.29–1.06 0.05 0.058 0.1 min 0.4 0.15 0.4 0.4 0.08 0.02 Figure4a shows the microstructure of the failed elbow at the inner surface. The observed elongated ferritic–pearlitic microstructure is an indication that the alloy was in the as-rolled state. As mentioned 3.3. Morphology of Pits before, there was no indication of pitting at the inner surface. Figure4b shows the microstructure of a cross-sectionFigure 4a at theshows outer the surface. microstructure Severe perforation of the faile andd elbow deep pitsat the were inner observed surface. at The the outerobserved surface. Furthermore,elongated ferritic–pearlitic pits were covered microstructure with corrosion is an indi products,cation that while the in alloy some was cases, in the corrosion as-rolled products state. As had amentioned layered structure. before, there was no indication of pitting at the inner surface. Figure 4b shows the microstructure of a cross-section at the outer surface. Severe perforation and deep pits were 3.4.observed Analysis at ofthe Corrosion outer surface. Products Fu/Biofilmrthermore, at the pits Outer were Surface covered with corrosion products, while in some cases, corrosion products had a layered structure. Figure5 shows the typical features found inside pits at the outer surface. This figure clearly shows that3.4. theAnalysis surface of Corrosion was covered Products/Biofilm with a biofilm. at the Outer Biofilms, Surface consisting of complex communities/colonies of microorganisms and EPS, can dissolve oxygen and create gradients of pH, resulting in localized Figure 5 shows the typical features found inside pits at the outer surface. This figure clearly forms of corrosion, such as crevice corrosion and pitting [10]. Three noticeable features were seen in shows that the surface was covered with a biofilm. Biofilms, consisting of complex the biofilm: communities/colonies of microorganisms and EPS, can dissolve oxygen and create gradients of pH, (i)resultingRod-shaped in localized products, forms withof corrosion, approximate such as length crevice of corrosion 50 µm; and pitting [10]. Three noticeable (ii)featuresColonies were ofseen spheres, in the biofilm: with their size ranging from 10 to 100 µm; (i) Rod-shaped products, with approximate length of 50 μm; (iii) Flower-like surface features, all shown in Figure5. (ii) Colonies of spheres, with their size ranging from 10 to 100 μm; Rod-like(iii) Flower-like features surface were, features, statistically, all shown more in frequentlyFigure 5. seen than the other two morphologies. TheRod-like distributions features ofwere, all the statistically, mentioned more morphologies frequently wereseen ratherthan the homogeneous, other two morphologies. i.e., all features The were seendistributions in each and of all every the pit.mentioned morphologies were rather homogeneous, i.e., all features were seen in each and every pit.

FigureFigure 4. 4.The The optical optical microscopy microscopy (OM) (OM) microstructures microstructures of of the the (a )( innera) inner surface surface and and (b) the(b) outerthe outer surface. surface.

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FigureFigure 5.5. Scanning Electron Microscope (SEM) photomicrographsphotomicrographs showing biofilmsbiofilms (or(or surfacesurface deposit)deposit) and features features inside inside biofilms, biofilms, i.e., i.e., (a) and (a) and(b) rod-like (b) rod-like features, features, (c) colonies (c) colonies of spheres, of spheres, and (d) andflower-like (d) flower-like features. features.

FigureFigure6 6shows shows the the results results of EDS of analysesEDS analyses of rod-like, of rod-like, spherical, spherical, and flowerlike and flowerlike corrosion products.corrosion Inproducts. terms of In the terms chemistry, of the there chemistry, was no meaningfulthere was no diff meaningfulerence between difference these features, between inferring these features, that the diinferringfference that between the difference these features between is only these morphological. features is only The morphological. presence of sulfur The in presence all the EDS of sulfur spectra in isall an the indication EDS spectra that the is observedan indication features that are the SRB, observed which isfeatures an anaerobic are SRB, bacterium. which SRBis an is knownanaerobic to bebacterium. the primary SRB factor is known of MIC. to be Organic the primary compounds factor attracted of MIC. toOrganic the surface compounds of steel makeattracted it an to ideal the environmentsurface of steel for make the colonization it an ideal environment of microorganisms for the [ 2colonization]. Amongst diofff mierentcroorganisms microorganisms, [2]. Amongst SRB is knowndifferent to havemicroorganisms, excellent adaptability SRB is known to extreme to conditions.have excellent Biochemical adaptability energy to forextreme the growth conditions. of SRB 2 2 isBiochemical obtained by energy reducing for sulfatethe growth (SO4 −of) SRB to sulfide is obtained (S− ) in by the reducing presence sulfate of natural (SO4 organic−2) to sulfide compounds. (S−2) in Atthe the presence same time, of natural with the organic absorption compounds. of atomic hydrogenAt the same (H) ontime, the with surface the of absorption iron, it will graduallyof atomic disappearhydrogen (H) [16]. on Overall, the surface the majorof iron, detrimental it will gradually metabolic disappear activities [16]. of Overall, SRB are the mostly major related detrimental to its abilitymetabolic to: activities of SRB are mostly related to its ability to: (i) use natural organic compounds as electron donors, (i) use natural organic compounds as electron donors, (ii) oxidize hydrogen, (ii) oxidize hydrogen, (iii) utilize aromatic and aliphatic hydrocarbons, and (iii) (iv)utilize reduce aromatic sulfate and to aliphaticsulfide [17]. hydrocarbons, and (iv) Somereduce sulfatesulfate-reducers to sulfide [17can]. oxidize methane in anaerobic reactions. Another unique characteristicSome sulfate-reducers of SRB is its canability oxidize to survive methane wh inen anaerobic exposed reactions. to oxygen, Another even though unique characteristicits growth is ofinhibited SRB is itsin abilitysuch a tocase. survive It is wo whenrth exposedmentioning to oxygen,that the evenspherical though geometry its growth of particles is inhibited might in be such an aindication case. It is of worth the presence mentioning of iron that (II) the carbonate spherical (FeCO geometry3) surrounded of particles by might a mixture be an of indication iron oxides of and the iron sulfide (FeS). From an engineering point of view, the importance of microorganisms’ nature is presence of iron (II) carbonate (FeCO3) surrounded by a mixture of iron oxides and iron sulfide (FeS). Fromthat they an engineering have quite a point large of variation view, the in importance size. Theyof are microorganisms’ also homogeneously nature distributed, is that they potentially have quite avery large fast-growing, variation in size.and Theythey arehave also significantly homogeneously grown distributed, with perceivable potentially restraints very fast-growing, in nutrient andavailability they have induced significantly by an overpopu grown withlating perceivable of microor restraintsganisms [11,17]. in nutrient availability induced by an overpopulating of microorganisms [11,17].

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Figure 6.6. EnergyEnergy dispersive dispersive X-ray X-ray spectroscopy spectroscopy (EDS) analyses(EDS) analyses of areas of shown areas in shown Figure 5in, relatedFigure to5, rod-like,related to spherical, rod-like, andspherical, ﬂower-like and flower-like features (x -axis:features Energy (x-axis: (KeV), Energyy-axis: (KeV), a.u.). y-axis: a.u.).

Figure7 7 representsrepresents a closer look look at at the the biofilm biofilm in in the the outer outer surface surface in inwhich which microorganisms microorganisms are aredistributed. distributed. Biofilm Biofilm is a is self-produced a self-produced firmly-attached firmly-attached EPS EPS surface surface layer. layer. In In fact, fact, the the stability of microorganisms is dependent on the biofilm,biofilm, given that the biofilmbiofilm provides structural support for microorganisms and, to a greatgreat extent,extent, protectsprotects themthem fromfrom environmental,environmental, physical,physical, and chemicalchemical stresses. BiofilmsBiofilms could could host host a variety a variety of diff erentof diff microorganismserent microorganisms including including fungi, bacteria, fungi, eukaryotes, bacteria, andeukaryotes, archaea, and with archaea, each having with aeach diff erenthaving metabolic a different ability metabolic [2]. The ability SRB, in[2]. particular, The SRB, has in aparticular, vigorous metabolism,has a vigorous producing metabolism, a certain producing amount a ofcertain EPS (inamou fact,nt oftenof EPS there (in arefact, also often other there microorganisms are also other presentmicroorganisms that are mainlypresent responsible that are mainly for EPS responsibl production),e for EPS which production), rapidly adhere which to rapidly the surface adhere of the to metalthe surface and build of the a biofilm metal [and14,18 build]. The a biofilm,biofilm in[14,18]. this case, The has biofilm, a porous in nature,this case, making has a the porous ion/nutrition nature, exchangemaking the much ion/nutrition easier. EPS exchange are composed much of easier. charged EPS polymers are composed and can of significantly charged polymers contribute and to can the accumulationsignificantly contribute of different to chemical the accumulation species inside of EPS.different In the chemical case where species organic inside acids EPS. are produced In the case by microbialwhere organic species, acids mineral are produced dissolution by rates microbial are expected species, to increasemineral [19dissolution,20]. The resultsrates are of EDSexpected analysis to showincrease that [19,20]. elements The oxygen, results sulfur,of EDS calcium, analysischloride, show that and elements iron were oxygen, present sulfur, in the calcium, biofilm. chloride, Figure8 showsand iron typical were XRDpresent patterns in the of biofilm. corrosion Figure products, 8 shows taken typical from XRD the surfacepatterns of of the corrosion pipe. The products, results confirmedtaken from the the formation surface of of the a significantpipe. The results amount confirmed of oxide-hydroxide the formation (FeOOH), of a significant iron oxide, amount and iron of sulfide.oxide-hydroxide The latter (FeOOH), is a typical iron corrosion oxide, productand iron when sulfide. SRB The is the latter controlling is a typical microorganism. corrosion product when SRB is the controlling microorganism.

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Figure 7. Examples of SEM images of biofilms bioﬁlms together with EDS analysis.

Figure 8. An X-ray diffraction diﬀraction analysis of the surface of the pipe. 3.5. Analysis of Corrosion Products at the Inner Surface 3.5. Analysis of Corrosion Products at the Inner Surface Figure9 shows SEM images of corrosion products at the inner surface. As mentioned earlier, there Figure 9 shows SEM images of corrosion products at the inner surface. As mentioned earlier, was hardly any sign of localized corrosion. The inner surface in some areas was covered with a porous there was hardly any sign of localized corrosion. The inner surface in some areas was covered with a scale-like layer intermingled with some globular α-FeOOH particles, see Figure9a,b. Other observed porous scale-like layer intermingled with some globular α-FeOOH particles, see Figure 9a,b. Other morphologies of corrosion products were plate-like micaceous crystalline features, see Figure9c, observed morphologies of corrosion products were plate-like micaceous crystalline features, see and honeycomb-like structures, see Figure9d, with both being typical morphologies of γ-FeOOH. Figure 9c, and honeycomb-like structures, see Figure 9d, with both being typical morphologies of Therefore, overall, the inner surface was covered with diﬀerent iron oxide-hydroxides and possibly γ-FeOOH. Therefore, overall, the inner surface was covered with different iron oxide-hydroxides other oxide products. Figure 10 shows the EDS analysis of the areas shown in Figure9. There was no and possibly other oxide products. Figure 10 shows the EDS analysis of the areas shown in Figure 9. sign of sulfur and chloride, unlike what was seen at the outer surface. It appears that the failure has There was no sign of sulfur and chloride, unlike what was seen at the outer surface. It appears that started from the outer pipe surface, and corrosion products at the inner surface were caused by the the failure has started from the outer pipe surface, and corrosion products at the inner surface were liquid inside the pipeline. caused by the liquid inside the pipeline.

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Figure 9. Different morphologies of the corrosion products at the inner surface: (a) and (b) scale-like Figure 9. DifferentDiﬀerent morphologies of the corrosion products at the inner surface: ( a) and ( b) scale-like layer intermingled with some globular α-FeOOH particles, (c) plate-like micaceous crystalline layer intermingledintermingled with with some some globular globularα-FeOOH α-FeOOH particles, particles, (c) plate-like (c) plate-like micaceous micaceous crystalline crystalline features, features, and (d) honeycomb-like structures. features,and (d) honeycomb-like and (d) honeycomb-like structures. structures.

(a) (b)

(a)Figure 10. EDS analyses of (a) area 1 and(b) (b) area 2, shown in Figure9. Figure 10. EDS analyses of (a) area 1 and (b) area 2, shown in Figure 9. 3.6. SRB MonitoringFigure at the 10. Corroded EDS analyses Site of (a) area 1 and (b) area 2, shown in Figure 9. 3.6. SRB Monitoring at the Corroded Site Surface scratches from the corroded elbows and samples of soil from the corrosion area were 3.6.inoculated SRB SurfaceMonitoring in SRB-specific scratches at the fromCorroded media the andcorrodedSite incubated elbows in and anaerobic samples conditions of soil from for the 20 corrosion days. As area shown were in inoculated in SRB-specific media and incubated in anaerobic conditions for 20 days. As shown in FigureSurface 11, the scratches black precipitation from the corroded is created elbows due to and SRB samples growth of and soil H from2S and the FeS corrosion production area inwere the Figure 11, the black precipitation is created due to SRB growth and H2S and FeS production in the inoculatedmedia. It is in noticeable SRB-specific that media the SRB-induced and incubated black in precipitationanaerobic conditions was only for created 20 days. on theAs shown corroded in media. It is noticeable that the SRB-induced black precipitation was only created on the corroded Figuresurface 11, of thethe failed black elbow, precipitation and there is wascreated no sign due of to black SRB precipitation growth and onH2 theS and cut FeS surface, production see Figure in 11theb. surface of the failed elbow, and there was no sign of black precipitation on the cut surface, see Themedia. formation It is noticeable of SRB cells that was the confirmed SRB-induced by SEM black images, precipitation see Figure was 12, only and EDScreated spectra, on the see Figurecorroded 13. Figure 11b. The formation of SRB cells was confirmed by SEM images, see Figure 12, and EDS SRB is resistant to environmental stresses such as pH and temperature variations, chloride concentration, surfacespectra, of thesee failedFigure elbow, 13. SRB and is thereresistant was to no en signvironmental of black stresses precipitation such as on pH the and cut temperature surface, see and high values of pressure. They form biofilms and create massive corrosion products by deteriorative Figurevariations, 11b. The chloride formation concentration, of SRB cells and was high confirmed values of bypressure. SEM images, They form see biofilmsFigure 12, and and create EDS spectra,mechanismsmassive see corrosion [Figure21,22]. 13. products SRB is by resistant deteriorative to en mechanismsvironmental [21,22]. stresses such as pH and temperature variations, chloride concentration, and high values of pressure. They form biofilms and create massive corrosion products by deteriorative mechanisms [21,22]. Metals 2019, 9, x; doi: FOR PEER REVIEW www.mdpi.com/journal/metals

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FigureFigure 11. SRB11. SRB growth growth monitoring monitoring in in specificspecific specific mediamediamedia afterafter 20 2020 days. days.days. Control: Control:Control: The The specific specificspecific medium medium withoutwithout inoculation. inoculation. ( (aa)) (Surfacea Surface) Surface scratch scratch scratch from fromfrom corrosion corrosioncorrosion tube,tube, tube, ( b( (b)b )the) the the corrosion corrosion tube tube tube cuts cuts (c) the ( c) soil the soil samplessamples of corrosion of corrosion area area afterafter after fivefive five days,days, days, andand and (( d(dd))) afterafterafter 20 20 days. days.days.

FigureFigure 12. SEM12. SEM images images of of SRB SRB growth growth in in specificspecific media inoculated inoculated by by the the surface surface scratches scratches of the of the corroded sample (all pictures are from similar features with different magnifications). corroded sample (all pictures are from similar features with different magnifications). FigureMetals 2019 12., 9 SEM, x; doi: images FOR PEER of REVIEWSRB growth in specific media inoculated by thewww.mdpi.com/j surface scratchesournal/metals of the corroded sample (all pictures are from similar features with different magnifications).

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FigureFigure 13.13. An13. An EDS EDS analysis analysis of of area area shown shown in Figure Figure 12 1212 ((x (-axis:xx-axis:-axis: Energy Energy (KeV), (KeV),(KeV), y-axis: y-axis:-axis: a.u.). a.u.).

The corroded tube was also cultivated in the media. The results revealed that due to The corrodedcorroded tube tube was was also also cultivated cultivated in the in media. the media. The results The revealed results thatrevealed due to that reactivation due to reactivation and reproduction of SRB, the amount of corrosion products increased. The corrosive andreactivation reproduction and reproduction of SRB, the amount of SRB, of the corrosion amount products of corrosion increased. products The increased. corrosive The eﬀects corrosive of SRB effects of SRB are shown in Figure 14. The redox potentials of as-received soil before inoculation in are shown in Figure 14. The redox potentials of as-received soil before inoculation in an SRB-speciﬁc effectsan ofSRB-specific SRB are shown medium in and Figure that after14. The inoculation redox pote werentials 19 and of − as-received315 mV, respectively. soil before The inoculation decrease in medium and that after inoculation were 19 and 315 mV, respectively. The decrease in the redox an SRB-specificin the redox mediumpotential canand be that attributed after inoculation to the enhanc− wereed activity 19 and of − SRB,315 mV, which respectively. is in accordance The with decrease potentialin thethe redox presented can potential be attributed results can of beFigures to theattributed enhanced 11–13 [23,24].to the activity enhanc ofed SRB, activity which of is SRB, in accordance which is in with accordance the presented with resultsthe presented of Figures results 11– 13of[ 23Figures,24]. 11–13 [23,24].

Figure 14. Corrosion products development after reactivation of SRB due to inoculation of the corroded tube cuts in the SRB-specific media (Images are from similar features with different magnifications) Figure 14.14.Corrosion Corrosion products products development development after after reactivation reactivation of SRB of due SRB to due inoculation to inoculation of the corroded of the tube4.corroded Conclusions cuts intube the SRB-specificcuts in the mediaSRB-specific (Images media are from (Images similar are features from withsimilar di fffeatureserent magnifications). with different magnifications) 4. ConclusionsThis paper investigates an MIC-induced severe failure of carbon steel elbows in an underground amine pipeline. The following conclusions can be drawn: 4. ConclusionsThis paper investigates an MIC-induced severe failure of carbon steel elbows in an underground amine(i) pipeline. The observed The following failure was conclusions in the form canof loca belized drawn: pitting corrosion, which originated from the Thisouter paper surface investigates of the elbow. an Observed MIC-induced pits had asevere different failure size and of morphology, carbon steel with someelbows being in an underground amine pipeline. The following conclusions can be drawn: (i) Thevery observed small, failurewhile a few was were in the deep form and of had localized already pittingturned into corrosion, a hole penetrating which originated the pipe wall. from the (i) (ii) Theouter Theobserved surface corrosion offailure the at the elbow. was inner in Observed thesurface form was pitsof predloca hadominantlylized a diff pittingerent in sizethe corrosion, form and morphology,of awh homogeneousich originated with general some from being the corrosion. It appears that the corrosion products at the outer surface have three distinctive outervery small,surface while of the a fewelbow. were Observed deep and pits had had already a different turned size into and a holemorphology, penetrating with the some pipe being wall. morphologies: Rod-like morphologies, colonies of spheres, and flower-like morphologies. (ii) veryThe corrosionsmall, while at thea few inner were surface deep and was had predominantly already turned in into the forma hole of penetrating a homogeneous the pipe general wall. These features in some cases were as large as a few hundred micrometers. (ii) Thecorrosion. corrosion It appearsat the inner that surface the corrosion was pred productsominantly at the in outerthe form surface of a havehomogeneous three distinctive general

Metalscorrosion.morphologies: 2019, 9, x;It doi: appears FOR Rod-like PEER that REVIEW morphologies,the corrosion products colonies ofat the spheres, outer andsurfacewww.mdpi.com/j ﬂower-like have threeournal/metals morphologies. distinctive morphologies:These features inRod-like some cases morphologies, were as large colonies as a few of hundred spheres, micrometers. and flower-like morphologies. (iii) TheseThe EDS features results in showed some cases a high were concentration as large as of a sulfurfew hundred in the chemistry micrometers. of these products, inferring that these features are SRB-related products. This was conﬁrmed by SRB inoculation experiments. Metals 2019, 9, x; doi: FOR PEER REVIEW www.mdpi.com/journal/metals Metals 2019, 9, 459 13 of 14

(iv) In order to prove the presence of SRB, corroded parts of elbows and samples of soil close to the damaged areas were inoculated in SRB-specific media and incubated in anaerobic conditions. The black precipitation was created due to SRB growth, confirming the existence of SRB and that it had a corrosion-inducing role in this failure. This was confirmed by microscopy results.

Author Contributions: Conceptualization, M.K.K. and A.B.; methodology, M.K.K., A.B. and A.H.-A; formal analysis, M.K.K., A.B. and A.H.-A.; resources, M.K.K., A.H.-A and A.B.; data curation, M.K.K., A.B., A.H.-A. and M.K.; writing—original draft preparation, M.K.K.; writing—review and editing, A.B., M.K.K., A.H.-A., P.T. and M.K.; supervision, A.B. and P.T.; project administration, A.B. Funding: This research received no external funding. Conﬂicts of Interest: The authors declare no conﬂicts of interest.

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