(SRB) on the Corrosion of Buried Pipe Steel in Acidic Soil Solution

coatings

Article Effect of Sulfate-Reducing Bacteria (SRB) on the Corrosion of Buried Pipe Steel in Acidic Soil Solution

Lijuan Chen 1,2,* , Bo Wei 1,2 and Xianghong Xu 2

1 State Key Laboratory of Chemistry and Utilization of Carbon Based Energy Resources, Xinjiang University, Urumqi 830046, China; [email protected] 2 School of Chemical Engineering, Xinjiang University, Urumqi 830046, China; [email protected] * Correspondence: [email protected]

Abstract: The influence of sulfate-reducing bacteria (SRB) on the corrosion behaviors of X80 pipeline steel was investigated in a soil environment by electrochemical techniques and surface analysis. It was found that SRB grew well in the acidic soil environment and further attached to the coupon surface, resulting in microbiologically influenced corrosion (MIC) of the steel. The corrosion process of X80 steel was significantly affected by the SRB biofilm on the steel surface. Steel corrosion was inhibited by the highly bioactive SRB biofilm at the early stage of the experiment, while SRB can accelerate the corrosion of steel at the later stage of the experiment. The steel surface suffered severe pitting corrosion in the SRB-containing soil solution.

Keywords: X80 steel; sulfate-reducing bacteria; bioﬁlm; pitting; EIS

1. Introduction Citation: Chen, L.; Wei, B.; Xu, X. Corrosion has been acknowledged as the largest capital loss in oil/gas industries, Effect of Sulfate-Reducing Bacteria leading to various environmental and economic problems and even a fatal threat [1,2]. (SRB) on the Corrosion of Buried Pipe Microbiologically influenced corrosion (MIC) refers to material degradation that is influ- Steel in Acidic Soil Solution. Coatings enced by various microorganisms existing in soil, marine, and industrial environments, 2021, 11, 625. https://doi.org/ which is a serious threat to pipeline integrity [3]. In fact, MIC was also found in other 10.3390/coatings11060625 industrial equipment, such as heat exchangers, cooling systems, and storage tanks [4], estimated at about 20% of the annual corrosion in China [5]. Among several types of Academic Editor: Paweł Nowak microbes responsible for MIC in soils, sulfate-reducing bacteria (SRB) have been identified as the main culprit causing progressive MIC of pipes, further leading to buried pipeline Received: 22 April 2021 failure [6,7]. Accepted: 20 May 2021 ◦ Published: 24 May 2021 SRB can survive over a wide pH range of 4.0–8.0, at temperatures of 10–40 C, and at pressures up to 507 atm, which are active and can be a threat in anaerobic environments [8].

Publisher’s Note: MDPI stays neutral Enhanced corrosion of buried pipeline steel by SRB in soil environments has been reported. with regard to jurisdictional claims in Previous research has shown that the corrosion rate of steel increased six times in the published maps and institutional affil- presence of SRB compared with that in a sterile solution [9]. Bhat et al. [10] reported that a iations. new pipeline failed due to MIC in only 8 months. Abedi et al. [11] also reported an MIC failure of X52 pipeline in Iran. They demonstrated that the SRB intensified the corrosion of the pipeline steel and the further related stress cracking corrosion (SCC) process. Generally, buried pipelines are usually protected from external corrosion by coatings and cathodic protection (CP), but MIC still happens when the coating is disbonded and the CP current is Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. shielded after a long time of service [12]. Xu et al. [13] reported that SRB corrosion is severe This article is an open access article under a disbonded coating in a neutral soil solution. Wei et al. [14] also demonstrated distributed under the terms and that the corrosion of steel was accelerated by SRB under a disbonded coating in an acidic conditions of the Creative Commons soil solution. Various mechanisms of SRB corrosion have been proposed, e.g., cathodic Attribution (CC BY) license (https:// depolarization theory (CDT) [15], corrosive metabolites theory (CMT) [16], and direct creativecommons.org/licenses/by/ electron transfer theory (DETT) [17]. It was reported that the activities of SRB enhanced 4.0/). steel corrosion [18]. During the SRB corrosion process, a biofilm layer consisting of SRB

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cells, water, extracellular polymeric substances (EPS), and corrosion products is formed on the coupon, which has an effect on the dissolution of the steel [19]. Videla et al. [20] concluded that a biofilm containing iron sulfide is the main SRB corrosion product, which can accelerate steel corrosion by cathodic depolarization through film spalling or rupture by SRB metabolic actions. Dong et al. [21] also found that SRB biofilms seriously accelerate the corrosion of steel. In addition, one study proposed that SRB can acquire energy by directly harvesting electrons from iron under starvation and severely enhance corrosion [22]. Xu and Gu [23] also reported that SRB can utilize electrons released by anodic oxidation as an electron donor under the starvation condition, and that the biofilm is more corrosive against steel. Actually, the corrosion induced by SRB is a complex process, and the mechanism of SRB corrosion is not fully understood [24]. Furthermore, various studies have been carried out in neutral soil simulation solutions. An acidic soil located in southeast China with an average pH of 3.5–6.0 is distributed in several provinces. A number of oil/gas pipelines operate in this area. It has been found that acidic soils are extremely aggressive towards various metallic materials [25,26]. Yan et al. [27] reported that Fe oxides are enriched in this type of soil, which also stimulate soil corrosion in anaerobic conditions by acting as a cathodic depolarizer. Wei et al. [28] found that the stray current is a key factor leading to severe corrosion of pipeline steel in acidic soils. Thus far, the aggressiveness of acidic soils towards steel has not been fully understood. Can SRB grow in acidic soil environments? Are biotic factors such as SRB also an important factor in accelerating the corrosion of pipeline steel in red soil? However, SRB corrosion in acid soil environments is seldom reported. Therefore, it is necessary to investigate the effects of SRB on the corrosion of steel in acidic soils. In this work, electrochemical impedance spectroscopy (EIS) combined with scanning electron microscopy (SEM), energy-dispersive spectrometry (EDS) and X-ray photoelectron spectroscopy (XPS) techniques was used to investigate the effect of SRB on the corrosion behavior of X80 steel in acidic soils.

2. Experimental 2.1. Materials and Solutions In this work, the material used was API X80 steel pipe, and its composition is given in Table1. Figure1 shows the optical microstructure of the specimen after etching with 4% nital. Pearlite and ferrite are the major constituents, where the ferritic phase appeared white in color and in non-uniform, nonpolygenic shapes, and the pearlite region appeared dark in color. The specimens with dimensions of 10 mm × 10 mm × 3 mm were embedded in epoxy resin, leaving a working area of 10 mm × 10 mm. The working surface was progressively ground using silicon carbide papers (a series of 120, 240, 400, 600, and 800 grit), followed by degreasing with acetone and dehydration in 100% ethanol. Before use, all specimens were sanitized using an ultraviolet lamp for 20 min.

Table 1. Chemical composition of X80 steel (wt.%).

C Mn Si P S Cr Ni Cu 0.07 1.82 0.19 0.007 0.023 0.026 0.17 0.02 Al Mo Ti Nb V N B Fe 0.028 0.23 0.012 0.056 0.002 0.004 0.0001 Bal. Coatings 2021, 11, 625 3 of 14 Coatings 2021, 11, x FOR PEER REVIEW 3 of 14

FigureFigure 1. 1.Optical Optical microstructure microstructure of of API API X80 X80 steel steel used used in in this this work. work.

CultureCulture media media were were used used to investigate to investigate SRB SRB corrosion corrosion in most in research most research works. works How-. ever,However, soil environments soil environments are more are complex.more complex. Therefore, Therefore, the test the solution test solution in this studyin this was study a soil-extractedwas a soil-extracted solution solution for an acidicfor an soil acidic located soil located in Ying-Tan in Ying of Jiang-Xi-Tan of Jiang Province,-Xi Province, China. SoilsChina. were Soils taken were from taken 1.2 tofrom 1.5 1.2 m underground.to 1.5 m underground. The composition The composition of the soil isof shownthe soil in is Tableshown2. Firstly, in Table the 2. soil Firstly, and distilledthe soil waterand distilled were mixed water in were a ratio mixed of 1:1 in to a obtain ratio aof saturated 1:1 to ob- soiltain liquid. a saturated The soil soil liquids liquid were. The stirredsoil liquids for 1 hwere and stirred then kept for stationary1 h and then for kept 1 day. stationary Finally, thefor soil-extracted 1 day. Finally, solution the soil was-extracted obtained solution by filtering was theobtained supernatant by filtering with qualitative the supernatant filter paperswith q (0.22ualitativeµm). Beforefilter papers testing, (0.22 high-purity μm). Before N2 gas testing, was purged high-purity from theN2 testgas solutionwas purged for 2from h to achievethe test ansolution anaerobic for 2 condition, h to achieve and an the anaerobic gas flow wascondition, maintained and the throughout gas flow thewas test.maintained Prior to use,throughout the soil solutionthe test. wasPrior autoclaved to use, the at soil 121 solution◦C for 20 was min autoclaved and stored at at 121 4 ◦C. °C for 20 min and stored at 4 °C. Table 2. Chemical composition of the soil in this work (mg/kg soil). Table 2. Chemical composition of the soil in this work (mg/kg soil). 2+ 2+ + + − − 2− − pH Ca Mg K Na NO3 Cl SO4 HCO3 2+ 2+ + + − − 2− − 4.5pH Ca 5 Mg 2 4K Na 8 NO 43 9Cl SO 104 HCO 11 3 4.5 5 2 4 8 4 9 10 11 2.2. SRB Culturing and Inoculation 2.2. SRB Culturing and Inoculation The strain of SRB used in this work is Desulfovibrio desulfuricans, which was isolated from soils.The strain The SRB of SRB were used cultured in this in work API RP-38 is Desulfovibrio medium (g/L) desulfuricans [29]. Before, which inoculation, was isolated the strainfrom wassoils kept. The in SRB an incubatorwere cultured for 12 in h toAPI activate RP-38 its medium biological (g/L) activity. [29]. Before A 5%(vol.%) inoculation, SRB culturingthe strain medium was kept was in an added incubator to the for soil 12 solution h to activate to obtain its biological the test solution. activity. Additionally, A 5% (vol.%) equalSRB culturing normal sterilized medium culturewas added medium to the was soil added solution to the to soilobtain solution the test as abioticsolution. control Addi- fortionally, comparison. equal normal sterilized culture medium was added to the soil solution as abiotic control for comparison. 2.3. Electrochemical Measurements 2.3. Electrochemical Measurements impedance spectroscopy (EIS) measurements were carried out using a three-electrodeElectrochemical system impe by andance EG&G spectroscopy PAR 2273 (AMETEK (EIS) measurements Instruments, Princeton,were carried NJ, out USA). us- Theing testa three specimen-electrode was system used as by a workingan EG&G electrode PAR 2273 (WE). (AMETEK A saturated Instruments, calomel electrodePrinceton, (SCE)NJ, USA). and a The platinum test specimen plate served was used as reference as a working electrode electrode (RE) and (W counterE). A saturated electrode calomel (CE), respectively.electrode (SCE) All and EIS tests a platinum were conducted plate served at the as reference open-circuit electrode potential (RE) (OCP) and withcounter a sinusoidalelectrode (CE),alternating respectively. amplitude All signalEIS test ofs 10 were mV conducted over the frequency at the open range-circuit from potential 105 to − 10(OCP)2 Hz. with The a curvesinusoidal fitting alternating was performed amplitude with signal ZSimpWin of 10 mV software over the (version frequency 3.21). range All ◦ measurementsfrom 105 to 10 were−2 Hz. conducted The curve atfitting a room was temperature performed of with 25 ± ZSimpWin2 C, in triplicate. software (version 3.21). All measurements were conducted at a room temperature of 25 ± 2 °C , in triplicate. 2.4. Surface Analysis 2.4. AfterSurface testing, Analysis the specimens were removed from the cell, immersed in a 4% (w/w) glutaraldehydeAfter testing, solution the specimens for 2 h, sequentially were removed dehydrated from the with cell, alcohol immerse for 5d min in a at 4% various (w/w) concentrations (25%, 50%, 75%, 95%, and 100% (w/w)), and then dried. SEM (FEI Quanta glutaraldehyde solution for 2 h, sequentially dehydrated with alcohol for 5 min at vari- 450, Hillsboro, OR, USA) was used to observe corrosion products’ morphologies on the ous concentrations (25%, 50%, 75%, 95%, and 100% (w/w)), and then dried. SEM (FEI Quanta 450, Hillsboro, OR, USA) was used to observe corrosion products’ morphologies

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Coatings 2021, 11, 625 4 of 14 on the specimen surface, and the elemental compositions of the products were analyzed via equipped energy-dispersive spectrometry (EDS). The live/dead cells were characterized by a confocal laser scanning microscope specimen surface, and the elemental compositions of the products were analyzed via (CLSM, C2 Plus, Nikon, Tokyo, Japan). Before characterization, the specimens were dyed equipped energy-dispersive spectrometry (EDS). using the Live/Dead® BacLight™ Bacterial Viability Kit L7012 (Invitrogen, Molecular The live/dead cells were characterized by a confocal laser scanning microscope (CLSM, Probes, Inc., Eugene, OR, USA) in darkness for 20 min. C2 Plus, Nikon, Tokyo, Japan). Before characterization, the specimens were dyed using the XPS (ESCALAB250, Thermo VG, Waltham, MA, USA) was performed employing an Live/Dead® BacLight™ Bacterial Viability Kit L7012 (Invitrogen, Molecular Probes, Inc., Eugene,amonochromatic OR, USA) X in-ray darkness source for(Al 20Ka min. line of 15 kV and 150 W) with a pass energy of 50 eV andXPS a (ESCALAB250, step size of 0.1 Thermo eV, and VG, XPS Waltham, PEAK software MA, USA) (version was performed 41) was used employing to fit thean amonochromaticcurve. X-ray source (Al Ka line of 15 kV and 150 W) with a pass energy of 50 eV and aThe step corrosion size of 0.1 products eV, and XPSon specimen PEAK softwares were removed (version 41)using was a usedchemical to fit method the curve. with a descalingThe corrosion solution, products and then on specimens the morphologies were removed of corroded using a specimen chemical surface methods withwere aobserved descaling by solution, SEM. T andhe thenpit depth the morphologies on the specimen of corroded surface specimen was measured surfaces were by a observedthree-dimensional by SEM. (3D) The pit surface depth profilometer on the specimen (MicroXAM, surface Milpitas was measured, CA, USA by). a During three- dimensionalthe process of (3D) the surface measurement, profilometer the position (MicroXAM, of the Milpitas, deepest CA, pits USA). on the During specimen the processsurface ofwas the located measurement, under an the optical position microscope of the deepest at 10× pits magnification on the specimen and surfacethen zoomed was located in at 100× un- dermagnification an optical microscope to further measure at 10× magnification the depth. and then zoomed in at 100× magnification to further measure the depth. 3. Result 3.3.1. Result Live/Dead Staining 3.1. Live/Dead Staining During the corrosion process, cells adhered on the surface of the specimens gradually toDuring form thean SRB corrosion biofilm. process, Figure cells 2 shows adhered the onSRB the biofilm surface morphologies of the specimens after graduallylive/dead tostaining form anunder SRB CLSM, biofilm. where Figure the2 shows green theand SRB red biofilmcolors indicate morphologies the live after and live/deaddead cells, stainingrespectively. under Clearly, CLSM, it where can be the see greenn that andthe redSRB colorsbiofilm indicate formed the on livethe entire and dead specimen cells, respectively.surface, full of Clearly, live sessile it can cells be seen after that 14 days the SRBin the biofilm SRB-containing formed on soil the solution, entire specimen and the surface,number full of liv ofe live cells sessile on the cells surface after 14 is days much in more the SRB-containing than that of dead soil solution,cells. The and biofilm the numberthickness of measured live cells on is the20 μm. surface Additionally, is much more the thanSRB biofilm that ofdead observed cells. is The sparse, biofilm which thick- is µ nessmainly measured attributed is 20 to them. Additionally,depletion of nutrients the SRB biofilm in the solution observed after is sparse, 14 days which of immersion is mainly attributed[12,30]. to the depletion of nutrients in the solution after 14 days of immersion [12,30].

FigureFigure 2. CLSMCLSM images images of of biofilm bioﬁlm formed formed on onspecimen specimen surface surface after after 14 days 14 daysof immersion of immersion in in SRB-inoculatedSRB-inoculated soilsoil solution.solution. 3.2. Characterization of Corrosion Products on Specimen Surface 3.2. Characterization of Corrosion Products on Specimen Surface Figure3 shows the morphologies of the corrosion products formed on the specimen Figure 3 shows the morphologies of the corrosion products formed on the specimen surface in the abiotic and SRB-containing soil solutions after 14 days, and the corresponding surface in the abiotic and SRB-containing soil solutions after 14 days, and the corre- EDS analysis results are given in Table3. In the absence of SRB, some cluster-like products sponding EDS analysis results are given in Table 3. In the absence of SRB, some clus- can be observed on the specimen surface. In the presence of SRB, different corrosion ter-like products can be observed on the specimen surface. In the presence of SRB, dif- morphologies can be observed on the specimen surface. Apparently, mushroom-like ferent corrosion morphologies can be observed on the specimen surface. Apparently, corrosion products can be observed on the specimen surface, with a large number of sessile mushroom-like corrosion products can be observed on the specimen surface, with a large SRB cells presenting a rod shape with a length of 1–4 µm and overlapping with the corrosion products. Additionally, the corresponding EDS analysis results clearly demonstrate that the element of S is detected, which is related to the SRB activities [31]. The elements Al and

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number of sessile SRB cells presenting a rod shape with a length of 1–4 μm and overlapping with the corrosion products. Additionally, the corresponding EDS analysis results clearly demonstrate that the element of S is detected, which is related to the SRB activities [31]. The elements Al and Si should be ascribed to the soil solution. Furthermore, the C content (7.41%) in the abiotic soil solution is much lower than that in the inoculated soil solution (17.53%), which is due to the fact that the element C is the main element in the SRB biofilm [32]. Coatings 2021, 11, 625 5 of 14 Table 3. Results of EDS of the corrosion products on the steel surface.

Element (wt.%) Positions Si should be ascribed to the soilC solution.O Furthermore, Al the C contentSi (7.41%)P in the abioticS Fe soil solution is muchA lower than7.41 that in the23.45 inoculated 0.12 soil solution0.68 (17.53%),6.53 which is due– 61.71 to the fact that the elementB C17.53 is the main26.94 element in0.03 the SRB bioﬁlm0.51 [32]. 13.09 1.83 40.38

Figure 3. SEMFigure image 3.sSEM of the images corrosion of the products corrosion on products the specimen on the surface specimen in surfacesterile solution in sterile (a solution,b) and (SRBa,b)-containing and solution (c,d) afterSRB-containing 14 days of immersion solution (c., d) after 14 days of immersion.

The composition of corrosion products formed on the specimen surface in the Table 3. Results of EDS of the corrosion products on the steel surface. SRB-containing solution was further analyzed by XPS. Figure 4 shows the high-resolution XPS spectra ofElement Fe 2p for (wt.%) the specimen in the SRB-containing soil solution Positions afterC 14 days. O The Fe 2p Alspectra were Si curve fitted P as Fe3O4 S (711.4 eV), Fe FeS (713.6 eV), and FeOOH (725.3 eV) [33,34], which are the major corrosion products formed on the speci- A 7.41 23.45 0.12 0.68 6.53 – 61.71 Bmen. 17.53 Additionally, 26.94 the 0.03XPS results 0.51 further confirm 13.09 the existence 1.83 of 40.38 sulfides in corrosion products, which is consistent with the EDS results. FeS is the typical metabolic sulfide generated by SRB activities, which can be embedded in an SRB biofilm [14,35,36]. The composition of corrosion products formed on the specimen surface in the SRB- containing solution was further analyzed by XPS. Figure4 shows the high-resolution XPS spectra of Fe 2p for the specimen in the SRB-containing soil solution after 14 days. The Fe 2p spectra were curve fitted as Fe3O4 (711.4 eV), FeS (713.6 eV), and FeOOH (725.3 eV) [33,34], which are the major corrosion products formed on the specimen. Additionally, the XPS results further confirm the existence of sulfides in corrosion products, which is consistent with the EDS results. FeS is the typical metabolic sulfide generated by SRB activities, which can be embedded in an SRB biofilm [14,35,36].

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24,00024,0002400024000 FeOOHFeOOH Fe O Fe22O33 FeS 20,00020,0002000020000 FeS

16,00016,0001600016000

Intensity (a.u.) Intensity

Intensity (a.u.) Intensity OriginalOriginal CurveCurve 12,00012,0001200012000 FittingFitting CurveCurve BackgroundBackground LineLine

80008000 735735 730730 725725 720720 715715 710710 705705 700700 BindingBinding EnergyEnergy (eV)(eV)

FigureFigureFigure 4. 4.4. High-resolutionHighHigh--resolutionresolution XPS XPS spectraspectra ofof FeFe Fe 2p2p 2p forfor for specimenspecimen specimen inin in thethe the SRBSRB SRB-containing--containingcontaining solutionsolution solution afterafter after 141414 days. ddaysays..

3.3.3.3.3.3. PittingPittingPitting Morphologies MorphologiesMorphologies FigureFigFigureure5 55 shows showsshows thethethe surfacesurface morphologies ofof of thethe the specimenspecimen specimen surfacesurface surface afterafter after 1414 14 ddaysays days inin inthethe theabioticabiotic abiotic andand and SRB SRB SRB-containing--containingcontaining soilsoil soil solutions solutions solutions after after after thethe the corrosioncorrosion corrosion products products products were removed. removed removed.. LocalizedLocalizedLocalized corrosion corrosioncorrosion is isis observed observedobserved in inin the thethe absence absenceabsence and andand presence presencepresence of ofof SRB, SRB,SRB, as asas shownshownshown ininin FigureFigFigureure5 5.5.. ItItIt isis seen seen thatthat that thethe the surfacesurface surface ofof of thethe the steelsteel steel inin the inthe the abioticabiotic abiotic conditioncondition condition isis relativelyrelatively is relatively flatterflatter flatter withwith a witha fewfew alocalizedlocalized few localized corrosioncorrosion corrosion pits,pits, whilewhile pits, moremore while pitpit more holesholes pit cancan holes bebe observedobserved can be observed onon thethe specimenspecimen on the specimen surfacesurface inin surfacethethe inoculatedinoculated in the inoculated conditioncondition..condition. TheThe amountsamounts The andand amounts sizessizes of andof corrosioncorrosion sizes of corrosionpitspits forformedmed pits onon formed thethe speci-speci- on themenmen specimen sursurfaceface inin surface thethe presencepresence in the presenceofof SRBSRB significantlysignificantly of SRB significantly inincreasecrease,, increase,demonstratingdemonstrating demonstrating thethe enhancedenhanced the enhancedpittingpitting corrosion. corrosion. pitting corrosion. Additionally, Additionally, Additionally, some some pit pit some holes holes pit are are holes connected connected are connected with with each each with other other each to to other form form tolargelarge form pitting pitting large holes,pitting holes, and and holes, the the and surface surface the surface of of the the specimenof specimen the specimen becomes becomes becomes rougher rougher rougher thanthan that that than in in that the the in the abiotic control. The maximum pit morphologies of specimens observed by the 3D abioticabiotic controlcontrol.. TheThe maximummaximum pitpit morphologiesmorphologies ofof specimensspecimens observedobserved byby thethe 3D3D surfacesurface surface profilometer are presented in Figure6. In the absence SRB, the maximum pitting profilometerprofilometer areare presentedpresented inin FigFigureure 6.6. InIn thethe absenceabsence SRB,SRB, thethe maximummaximum pittingpitting depthdepth depth on the specimen surface measured is only 2.32 ± 0.2 µm. However, the maximum onon thethe specimenspecimen surfacesurface measuredmeasured isis onlyonly 2.322.32 ±± 0.20.2 μm.μm. However,However, thethe maximummaximum pittingpitting pitting depth on the specimen in the inoculated solution is 6.01 ± 0.6 µm, much larger than depthdepth onon thethe specimenspecimen inin thethe inoculatedinoculated solutionsolution isis 6.016.01 ±± 0.60.6 μm,μm, muchmuch largerlarger thanthan thatthat that in the abiotic solution. The above results clearly suggest that pitting corrosion was inin thethe abioticabiotic solutionsolution.. TheThe aboveabove resultsresults clearlyclearly suggestsuggest thatthat pittingpitting corrosioncorrosion waswas con-con- considerably enhanced by SRB. siderablysiderably enhancedenhanced byby SRB.SRB.

FigureFigureFigure 5. 5.5. SEM SEMSEM images imageimagess ofof specimenspecispecimenmen surfacesurface with with corrosion corrosion products products removed removed inin thethe abioticabiotic (( aaa))) and and inoculatedinoculatedinoculated solutions solutionsolutionss ( (b(bb))) after afterafter 14 1414 days. ddaysays..

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Figure 6. Maximum pit depth measured on specimen in the abiotic (a–c) and inoculated solutionssolutions ((d–f)) afterafter 1414 days.days.

3.4. OCP OCP Measurements Measurements The variation variationss in OCP values with time for specimens in the abiotic and inoculated soil solutions are shown in FigureFigure 77.. TheThe OCPOCP of of the the specimenspecimen inin the the abioticabiotic solutionsolution shifts in a negative direction at the beginning and then reaches a relatively steady value with time. For For the the inoculated conditioncondition,, however, the OCP of the specimen shifts in a positive direction at the beginning and then also reaches a relatively steady value. After 11 days, the OCP of the steel further decreases in both abiotic and inoculated conditions conditions.. Additionally, thethe OCPOCP ofof thethe specimenspecimen in in the the presence presence of of SRB SRB is is more more positive positive than than that that in inthe the absence absence of SRBof SRB during during the 14the days 14 days of immersion, of immersion, which which is mainly is mainly due to due the to fact the that fact a thatbioﬁlm a biofilm and corrosion and corro productssion products were formed were formed on the specimen,on the specimen, leading toleading a more topositive a more positivepotential potential [14,28]. [14,28].

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-0.60 Sterile SRB-inoculated

-0.63

-0.66

-0.69

Potential (Vvs.SCE)Potential

-0.72

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 TimeTime ((day)day)

FigureFigure 7. 7.Time Time dependence dependence of of the the potential potential for for specimens specimens in in the the abiotic abiotic and and inoculated inoculated soil soil solutions solutions over 14 days. over 14 days.

3.5.3.5. EIS EIS Results Results Analysis Analysis TheThe Nyquist Nyquist plots plots of of specimens specimens during during 14 14 days days of immersionof immersion in the in abioticthe abiotic and and inocu- in- latedoculated soil solutionssoil solutions are shown are shown in Figure in Fig8. Allure measured8. All measured impedance impedance curves curves(Figure (Fig8a,bure) show8a,b) ashow semicircle a semicircle over the over whole the whole frequency frequency range, range, and anyand diffusionany diffusion process process cannot cannot be identifiedbe identified from from the Nyquist the Nyquist plots. Asplots. shown As shown in Figure in8 a,Fig theure size 8a, of the the size semicircle of the becomessemicircle smallerbecomes with smaller time inwith the time abiotic in solution,the abiotic which solution, indicates which that indicates the electrochemical that the electrochem- corrosion processical corrosion of steel process is accelerated of steel with is accelerated time. For thewith SRB-inoculated time. For the SRB solution,-inoculated the semicircle solution, sizethe alsosemicircle reduces size with also time reduces and becomes with time smaller and becomes and smaller smaller during and the smaller 14 days during of testing. the 14 Additionally,days of testing. it is Additionally, also seen that it theis also diameters seen that of thethe semicirclesdiameters of in the the semicircles SRB-containing in the solutionSRB-containing (Figure8 b)solution are bigger (Figure than 8b) those are measuredbigger than in those the abiotic measure solutiond in the (Figure abiotic8a) solu- in thetion first (Fig 4ure days 8a) of in testing, the first which 4 days suggests of testing, that thewhich corrosion suggests of thethat steel the corrosion is suppressed of the in steel the presenceis suppressed SRB in in the the first presence 4 days SRB of testing. in the first The increased4 days of testing. diameters The of increased the semicircle diameters are as- of sociatedthe semicircle with the are corrosion associated of the with steel the initially. corrosi Inon the of presencethe steel ofinitially. SRB, a thickIn the biofilm presence layer of formedSRB, a onthick the biofilm steel. Thelayer living formed bacteria on the have steel. a significantThe living bacteria effect on have the corrosion a significant process effect ofon steel the [37 corrosion,38]. After process 7 days, of the steel diameters [37,38]. Afterof the 7 semicircle days, the measured diameters in of the the inoculated semicircle solutionmeasured are in smaller the inoculated than those solution measured are smaller in the abiotic than th solution,ose measured suggesting in thecorrosion abiotic solu- is enhancedtion, suggesting by SRB. corrosion The above is results enhanced indicate by thatSRB. the The SRB above activity results and indicate metabolite that influence the SRB theactivity corrosion and metabolite behavior of influence the steel, the which corrosion inhibited behavior the corrosion of the steel, of the which steel inhibited in the first the 4corrosion days and of accelerated the steel in it the in the first rest. 4 days and accelerated it in the rest.

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(a) 10 1day 4days 8 7days 10days

) 2 6 14days 0.19 Hz Fitting line

kΩ·cm

( 4

¢¢

− 2 0. 01 Hz

0 0. 01 Hz 0 2 4 6 8 10 2 Z¢ (kΩ·cm ) 0.01 Hz (b) 12 1day 10 4days 7days 10days 8

)

2 14days Fitting line 6

kΩ·cm

(

0.19 Hz ¢¢ 4

− 0. 01 Hz 2 0. 01 Hz 0

0 2 4 6 8 10 12 2 Z¢ (kΩ·cm ) 0.01 Hz FigureFigure 8. 8. NyquistNyquist plots plots of of specimen specimen in in the the abiotic abiotic ( (aa)) and and SRB SRB-inoculated-inoculated soil soil solution solutionss ( b))..

AnAn electrical equivalent circuit (EEC) (Figure(Figure9 9),), i.e., i.e., RRss((QQdldlRct),), w wasas used used to analyze thethe EIS EIS measured measured results results ( (FigureFigure 88),), and and the the ﬁtted fitted impedance impedance kinetics kinetics parameters parameters are are listedlisted in in Table Table 44.. InIn thethe EEC,EEC, Rs andand Rctct areare the the solution solution resistance resistance and the charge transfer resistanceresistance,, respectively;respectively; Qdldl isis the the electric double layer capacitance. The impedanceimpedance ofof QQdldl isis determined determined by [2 [211]] 1 Z (w) = 1 (1) Q Zw( )= − n Q Y0(jw)n (1) Y0 () jw 2 where ZQ is the impedance of Qdl, j is the imaginary unit, j = −1, w is the angular where ZQ is the impedance of Qdl, j is the imaginary unit, j2 = −1, w is the angular fre- frequency, Y0 is the CPE parameter, and n is the dispersion coefﬁcient related to the surface quency, Y0 is the CPE parameter, and n is the dispersion coefficient related to the surface inhomogeneity. inhomogeneity. A higher Rct value indicates a lower corrosion rate of the steel [21]. On day 1, the A higher Rct value indicates a lower corrosion rate of the steel [21]. On day 1, the Rct R value of steel in the absence of SRB is 9318 Ω·cm2 and decreases gradually with time. ct 2 value of steel in the absence of SRB is 9318 Ω·cm and decreases gradually2 with time. After 14 days, the Rct of steel reaches the smallest value, 4640 Ω·cm , which decreases After 14 days, the Rct of steel reaches the smallest value, 4640 Ω·cm2, which decreases over over two times, indicating a higher corrosion rate. In the presence of SRB, the Rct slightly two times, indicating a higher corrosion rate. In the presence of SRB, the Rct slightly in- increases to 13950 Ω·cm2 compared with that in the abiotic condition after 1 d and then creases to 13950 Ω·cm2 compared with that in the abiotic condition after 1 d and then also decreases gradually with time. After 4 days, the Rct of steel also slightly increases

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also decreases gradually with time. After 4 days, the Rct of steel also slightly increases comparedcompared with with that that in in the the abiotic abiotic solution. solution With. With the the time time further further increasing, increasing, however, however, the the RRctctvalues values of of steel steel are are much much lowerlower thanthan thosethose inin thethe control.control. TheThe RRctct is the smallest with with a avalue value of of 1467 ΩΩ·cm·cm2 after 14 days inin thethe inoculatedinoculated solution,solution, whichwhichis is over over three three times times smallersmaller than than that that in in the the control. control.

FigureFigure 9. 9.Electrochemical Electrochemical equivalent equivalent circuits circuits used used to to ﬁt fit the the EIS EIS data data in in this this work. work.

TableTable 4. 4.Fitted Fitted parameters parameters for for the the Nyquist Nyquist plots plots in in the the abiotic abiotic and and inoculated inoculated solutions. solutions.

SterileSterile SRB SRB-Inoculated-Inoculated Time Time Rs Qdl Rct Rs Qdl Rct (day)R s Qdl Rct Rs Qdl Rct (day) 2 2 n −2 2 2 22 n −2 2 2 (Ω·cm ) (Ω·cm ) nY0/S− s2 ·cm n (Ω(Ω·cm·cm ) ) ((Ω·cmΩ·cm ) Y0/S s ·cmn −2 n (Ω·cm(Ω·)cm ) Y0/S s ·cm n Y0/S s ·cm n 1 137.2 1.017 × 10−4 0.913 9318 129.8 4.862 × 10−4 0.791 13950 1 137.2 1.017 × 10−4 0.913 9318 129.8 4.862 × 10−4 0.791 13,950 4 137.7 1.423 × 10−4 0.903 6754 126.7 9.03 × 10−4 0.902 10810 4 137.7 1.423 × 10−4 0.903 6754 126.7 9.03 × 10−4 0.902 10,810 −4 −4 77 101.3 101.32.964 ×2.96410−4 × 10 0.7860.786 66996699 110.1 3.1733.173 × 10× 10 −4 0.8660.866 3836 3836 1010 217.4 217.41.582 ×1.58210−4 × 10−4 0.7890.789 55335533 146 1.7821.782 × 10× −104 −4 0.81 0.812678 2678 1414 147.3 147.31.425 ×1.42510−4 × 10−4 0.8510.851 46404640 190.9 2.4092.409 × 10× −104 −4 0.7460.746 1467 1467

4. Discussion 4. Discussion In the soils, the corrosion of steel was governed by the electrochemical activation In the soils, the corrosion of steel was governed by the electrochemical activation reaction. Research [27,39,40] has shown that the anodic process of corrosion of X80 steel is reaction. Research [27,39,40] has shown that the anodic process of corrosion of X80 steel is the dissolution of Fe (Equation (2)), and the cathodic reaction of corrosion is mainly the the dissolution of Fe (Equation (2)), and the cathodic reaction of corrosion is mainly the depolarization of H+ (Equation (3)): depolarization of H+ (Equation (3)): Anodic reaction (metal dissolution): Anodic reaction (metal dissolution): Fe0 → Fe2+ + 2e (2) Fe0 → Fe2+ + 2e (2) Cathode reaction:

Cathode reaction: + +2H + 2e → H2 (3) 2H + 2e → H2 (3) Total reaction: Total reaction: 0 + 2+ Fe +Fe 2H0 + 2H→+ Fe→ Fe+2+ H+ 2H2 (4)(4) InIn the the presence presence of ofSRB, SRB, the the mechanism mechanism model model of the of electrochemical the electrochemical reactions reactions is pre- is sentedpresented in Figure in Fig 10ure. 10. SRB SRB can can shuttle shuttle electrons electrons from from extracellular extracellular iron iron oxidation oxidation across across thethe cell cell wall wall toto reach reach the the cytoplasmcytoplasm where where sulfate sulfate reduction occurs occurs under under biocatalysis biocatalysis[22] [22. S].essile Sessile SRB SRB cells cells under under an SRB an biofilm SRB bioﬁlm harvest harvest energy energy from the from reduction the reduction of SO42−of for 2− 2− − SOmetabolic4 for metabolic activities [ activities41]. Therefore [41]., Therefore, SO42− ions were SO4 reducedions were by SRB reduced to HS by− ions SRB (Eq touation HS 2+ − ions(5)). ( EquationFe2+ ions generated (5)). Fe ionsby the generated anodic process by the further anodic reacted process with further HS− reacted based on with Equation HS based(6). on Equation (6). SulfateSulfate reduction reduction by by SRB: SRB:

2− 2− + + − − SO4 SO+4 9H + 9H+ 8e+ 8e→ →HS HS++ 4H 4H2O2O (5)(5)

2+ − + Fe Fe+2+ HS + HS→− →FeS FeS + H+ H+ (6)(6) FeSFeS is is considered considered the the typical typical corrosion corrosion product product when when steels steels areexposed are exposed to an SRB-to an containingSRB-containing environment environment [42]. [4 The2]. The accumulation accumulation of iron of iron sulfide sulfide at theat the early early stage stage may may have protective effects. However, sulfide films are unstable [43], and the protective ability

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have protective effects. However, sulfide films are unstable [43], and the protective abil- ityof theof the films film cans can be be degraded degraded by by bacteria’s bacteria’s metabolic metabolic actions. actions. Hence,Hence, galvanic couple activeactive corrosion corrosion cells cells occur occur between between the the sulfide sulfide and and the the exposed matrix (anode), further acceleratingaccelerating the corrosioncorrosion raterate significantly. significantly. The The reduced reduced sulfur sulfur compounds compounds (ferrous (ferrous sulfide) sulfide)thus enhancethus enhance localized localized corrosion corrosi byon inducing by inducing pitting pitting corrosion. corrosion. Furthermore, Furthermore, some cations, some cations,e.g., H+ ,e.g., are cagedH+, are beneath caged beneath the biofilm the and biofilm corrosion and corrosion product due product to the due anion to selectivitythe anion selectivityof the biofilm, of the which biofilm, cause which acidification cause acidification underneath underneath the biofilm, the further biofilm, leading further to severe lead- ingpitting to severe corrosion. pitting corrosion.

FigureFigure 10. 10. MechanismMechanism model model to to illustrate illustrate the the SRB SRB corrosion corrosion process process of X80 steel steel..

InIn addition, addition, EIS EIS testing testing can can provide provide insights insights into into corrosion corrosion kinetic kinetic parameters parameters by by reflectingreflecting the the electrochemical electrochemical characteristics characteristics of a of corrosion a corrosion system system [44,45 [4].4 Figure,45]. Fig 11ur showse 11 the variation in 1/R values as a function of time. In general, at high frequencies, the shows the variation inct 1/Rct values as a function of time. In general, at high frequencies, resistance of the steel is related to the solution resistance (R ), while that at low frequencies the resistance of the steel is related to the solution resistances (Rs), while that at low fre- is relative to the charge transfer resistance (Rct), which is inversely proportional to the quencies is relative to the charge transfer resistance (Rct), which is inversely proportional tocorrosion the corrosion current current density density and reflects and reflects the corrosion the corrosion rate of rate steel. of steel. As shown As shown in Figure in Fig- 11, the Rct of the specimen in the presence of SRB is lower than that in the abiotic solution in ure 11, the Rct of the specimen in the presence of SRB is lower than that in the abiotic so- the first 4 days, which indicates that the corrosion of steel is inhibited in the first 4 days of lution in the first 4 days, which indicates that the corrosion of steel is inhibited in the first testing in the presence of SRB. This inhibition is relevant to the iron sulfide and the activity 4 days of testing in the presence of SRB. This inhibition is relevant to the iron sulfide and of the SRB biofilm. The inhibition effect is attributed to the living cells. In the presence the activity of the SRB biofilm. The inhibition effect is attributed to the living cells. In the of SRB, the SRB biofilm layer on the specimen surface with FeS particles, which can be presence of SRB, the SRB biofilm layer on the specimen2− surface with FeS particles, which an obstacle to the migration of ions, e.g., SO4 , into the interface of the metal, further can be an obstacle to the migration of ions, e.g., SO42−, into the interface of the metal, decreases the corrosion of steel [28]. Moreover, it is well known that the SRB biofilm, with further decreases the corrosion of steel [28]. Moreover, it is well known that the SRB bio- negative charges, is highly bioactive in the early stage of this experiment, which also has a film, with negative charges, is highly bioactive in the early stage of this experiment, repulsion effect on corrosive ions. However, with the increase in the immersion time, the which also has a repulsion effect on corrosive ions. However, with the increase in the activity of the biofilm would decrease. Therefore, the corrosion rate continues to increase immersion time, the activity of the biofilm would decrease. Therefore, the corrosion rate rapidly in the inoculated condition and shows a higher value compared to that in the continuesabiotic solution. to increase The above rapidly results in the show inoculated that SRB activitiescondition have and a showssignificant a higher effect on value the comparedcorrosion ofto that steel. in Steel the abiotic corrosion solution. is inhibited The above by the results highly show bioactive that SRB SRB activit biofilmies ha atve the a significantearly stage effect of the on experiment, the corrosion while of SRBsteel. can Steel accelerate corrosion corrosion is inhibited at the by later the stagehighly of bio- the activeexperiment. SRB biofilm at the early stage of the experiment, while SRB can accelerate corrosion at the later stage of the experiment.

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7 Sterile SRB-inoculated 6

)

2 – 5

4 4

–

( 3

1/ 2

0 0 2 4 6 8 10 12 14 16 TimeTime (day) (day)

Figure 11. 1/Rct value of specimen in the abiotic and inoculated soil solutions as a function of time. Figure 11. 1/Rct value of specimen in the abiotic and inoculated soil solutions as a function of time. 5. 5.Conclusion Conclusion TheThe effect effect of ofSRB SRB on on the the corrosion corrosion of ofX80 X80 steel steel was was investigated investigated inin an an SRB SRB-containing-containing soilsoil solution. solution. The The conclusions conclusions are are provided provided below: below: (1) (1)The The biofilm biofilm distributed distributed on on the the specimen specimen surface, surface, and and the the biofilm biofilm thickness thickness was was 20 20µm μm afterafter 14 days. 14 days. (2) (2)Both Both the the SRB SRB activity activity and and metabolite metabolite significantly significantly affect affect the the corrosion corrosion behavior behavior of of X80 X80 steel.steel. SRB SRB can can form form a biofilm a biofilm on on the the steel steel surface surface in inthe the inoculated inoculated soil soil solution solution and and play a vitalplay role a vital in the role corrosion in the corrosion of X80 ofsteel. X80 steel. (3) (3)EIS EIS results results indicate indicate that that the the corrosion corrosion of of steel steel is is inhibited inhibited by by the the activity activity of of SRB SRB in in the the firstfirst 4 days 4 days and and enhanced enhanced in the in therest rest of th ofe theexperiment. experiment. (4) (4)Localized Localizedcorrosion corrosion dominatesdominates inin thethe soilsoil solution,solution, andand pittingpitting corrosioncorrosion ofof steelsteel is is severesevere in the in inoculated the inoculated soil solution. soil solution.

AuthorAuthor Contributions: Contributions: ConceptualizationConceptualization,, L.C L.C.;.; methodology, methodology, L.C L.C.;.; software, software, L.C L.C.;.; validation, validation, L.C L.C.,., B.WB.W.., and and X X.X.;.X.; formal formal analysis, analysis, L L.C.;.C.; investigation, L.C.;L.C.; resources,resources, L.C.; L.C. data; data curation, curation, L.C.; L.C writing—.; writ- ingoriginal—original draft draft preparation, preparation, L.C.; L.C writing—review.; writing—review and and editing, editing, L.C. L. andC. and B.W.; B.W visualization,.; visualization, X.X.; X.Xsupervision,.; supervision, X.X.; X.X project.; project administration, administration, L.C.; L.C funding.; funding acquisition, acquisition, L.C. L. andC. and B.W. B. AllW. All authors authors have haveread read and and agreed agreed to the to the published published version version of the of the manuscript. manuscript. Funding:Funding: WeWe are are grateful grateful for for the the financial financial supported supported by by the the National Natural Science FoundationFoundation of of ChinaChina (No.(No. 51966017)51966017) andand thethe TeachingTeaching ReformReform ProjectProjectof ofXinjiang Xinjiang(XJJG201911). (XJJG201911). InstitutionalInstitutional Review Review Board Board Statement: Statement: NotNot applicable. applicable. InformedInformed Consent Consent Statement: Statement: NotNot applicable. applicable. DataData Availability Availability Statement: Statement: TheThe data data presented presented in inthis this study study are are available available on on request request from from the the correspondingcorresponding author. author. ConflictsConflicts of of Interest: Interest: TheThe authors authors declare declare no no conflict conflict of ofinterest. interest.

ReferenceReferencess 1. 1. Feng,Feng, Y.; Y.; Cheng, Cheng, Y.F. Y.F. AnAn intelligent intelligent coating coating doped doped with with inhibitor-encapsulated inhibitor-encapsulated nanocontainers nanocontainers for corrosion for corrosion protection protection of pipeline of pipelinesteel. Chem. steel. Eng.Chem. J. Eng.2017 ,J.315 2017, 537–551., 315, 537 [–CrossRef551. ] 2. 2. Wei,Wei, B.X.; B.X.; Pang, Pang, J.Y.; J.Y.; Xu, Xu, J.; Sun, J.; Sun, C.; Zhang, C.; Zhang, H.W H.W.;.; Wang, Wang, Z.Y.; Z.Y.;Ke, W. Ke, Microbiologically W. Microbiologically influenced inﬂuenced corrosion corrosion of TiZrNb of TiZrNb me- diummedium-entropy-entropy alloys alloys by Desulfovibrio by Desulfovibrio desulfuricans desulfuricans. J. Alloys. J. Alloys Compd Compd.. 2021,2021 875,, 160020875, 160020.. [CrossRef] 3. 3. Xu,Xu, D.K.; D.K.; Huang, Huang, W.; W.; Ruschau, Ruschau, G.; G.; Hornemann, Hornemann, J.; Wen,Wen, J.;J.; Gu,Gu, T.Y. T.Y. Laboratory Laboratory investigation investigation of MICof MIC threat threat due due to hydrotest to hydrotest using usinguntreated untreated seawater seawater and subsequentand subsequent exposure exposure to pipeline to pipeline ﬂuids fluids with andwith without and without SRB spiking. SRB spikingEng. Fail.. Eng. Anal. Fail.2013 Anal,.28 2013, 149–159., 28, 149[CrossRef–159. ]

Coatings 2021, 11, 625 13 of 14

4. Starosvetsky, J.; Starosvetsky, D.; Armon, R. Identification of microbiologically influenced corrosion (MIC) in industrial equipment failures. Eng. Fail. Anal. 2007, 14, 1500–1511. [CrossRef] 5. Hou, B.R. The Cost of Corrosion in China; Science Press: Beijing, China, 2019. 6. Huang, L.Y.; Huang, Y.; Lou, Y.T.; Qian, H.C.; Xu, D.K.; Ma, L.W.; Jiang, C.Y.; Zhang, D.W. Pyocyanin-modifying genes phzM and phzS regulated the extracellular electron transfer in microbiologically-influenced corrosion of X80 carbon steel by Pseudomonas aeruginosa. Corros. Sci. 2020, 164, 108355. [CrossRef] 7. Brooks, W. Microbiologically–influenced corrosion Riviera Park case study. In Corrosion; NACE International: Houston, TX, USA, 2013. 8. Tambe, S.P.; Jagtap, S.D.; Chaurasiya, A.K.; Joshi, K.K. Evaluation of microbial corrosion of epoxy coating by using sulphate reducing bacteria. Prog. Org. Coat. 2016, 94, 49–55. [CrossRef] 9. Sarioglu,ˇ F.; Javaherdashti, R.; Aksöz, N. Corrosion of a drilling pipe steel in an environment containing sulphate-reducing bacteria. Int. J. Pres. Ves. Pip. 1997, 73, 127–131. [CrossRef] 10. Bhat, S.; Sharma, V.K.; Thomas, S.; Anto, P.F.; Singh, S.K. 8-in pipeline from group gathering station to Central Tank Farm. Mater Perform. 2011, 50, 50–53. 11. Abedi, S.S.; Abdolmaleki, A.; Adibi, N. Failure analysis of SCC and SRB induced cracking of a transmission oil products pipeline. Eng. Fail. Anal. 2007, 14, 250–261. 12. Liu, H.W.; Cheng, Y.F. Mechanistic aspects of microbially influenced corrosion of X52 pipeline steel in a thin layer of soil solution containing sulphate-reducing bacteria under various gassing conditions. Corros. Sci. 2018, 133, 178–189. [CrossRef] 13. Xu, J.; Wang, K.X.; Sun, C.; Wang, F.H.; Li, X.M.; Yang, J.; Yu, C.K. The effects of sulfate reducing bacteria on corrosion of carbon steel Q235 under simulated disbonded coating by using electrochemical impedance spectroscopy. Corros. Sci. 2011, 53, 1554–1562. [CrossRef] 14. Wei, B.X.; Xu, J.; Fu, Q.; Qin, Q.Y.; Bai, Y.L.; Sun, C.; Wang, C.; Wang, Z.Y.; Ke, W. Effect of sulfate-reducing bacteria on corrosion of X80 pipeline steel under disbonded coating in a red soil solution. J. Mater. Sci. Technol. 2021, 87, 1–17. [CrossRef] 15. Von Wolzogen Khur, C.A.H.; van der Vlugt, L.S. De grafiteering van gietijzeralselectro biochemichproces in anaerobe Gronden. Water 1934, 18, 147–165. 16. AlAbbas, F.M.; Bhola, R.; Spear, J.R.; Olson, D.L.; Mishra, B. Electrochemical characterization of microbiologically influenced corrosion on linepipe steel exposed to facultative anaerobic Desulfovibrio sp. Int. J. Electrochem. Sci. 2013, 8, 859–871. 17. Venzlaff, H.; Enning, D.; Srinivasan, J.; Mayrhofer, K.J.J.; Hassel, A.W.; Widdel, F.; Stratmann, M. Accelerated cathodic reaction in microbial corrosion of iron due to direct electron uptake by sulfate-reducing bacteria. Corros. Sci. 2013, 66, 88–96. [CrossRef] 18. Iverson, W.P. Biological corrosion. In Advances in Corrosion Science and Technology; Fontana, M.G., Staehle, R.W., Eds.; Springer: Boston, MA, USA, 1972; pp. 1–42. 19. Dong, Z.H.; Liu, T.; Liu, H.F. Influence of EPS isolated from thermophilic sulphate-reducing bacteria on carbon steel corrosion. Biofouling 2011, 27, 487–495. [CrossRef] 20. Videla, H.A.; Herrera, L.K. Understanding microbial inhibition of corrosion. A comprehensive overview. Int. Biodeterior. Biodegrad. 2009, 63, 896–900. [CrossRef] 21. ZDong, H.; Shi, W.; Ruan, H.M.; Zhang, G.A. Heterogeneous corrosion of mild steel under SRB-biofilm characterised by electrochemical mapping technique. Corros. Sci. 2011, 53, 2978–2987. 22. Gu, T.Y.; Jia, R.; Unsal, T.; Xu, D.K. Toward a better understanding of microbiologically influenced corrosion caused by sulfate reducing bacteria. J. Mater. Sci. Technol. 2019, 35, 631–636. [CrossRef] 23. Xu, D.K.; Gu, T. Carbon source starvation triggered more aggressive corrosion against carbon steel by the Desulfovibrio vulgaris biofilm. Int. Biodeterior. Biodegrad. 2014, 91, 74–81. [CrossRef] 24. Liu, H.W.; Cheng, Y.F. Mechanism of microbiologically influenced corrosion of X52 pipeline steel in a wet soil containing sulfate-reduced bacteria. Electrochim. Acta 2017, 253, 368–378. [CrossRef] 25. Daia, M.J.; Liu, J.; Huang, F.; Zhang, Y.H.; Cheng, Y.F. Effect of cathodic protection potential fluctuations on pitting corrosion of X100 pipeline steel in acidic soil environment. Corros. Sci. 2018, 143, 428–437. [CrossRef] 26. Yan, M.C.; Sun, C.; Xu, J.; Ke, W. Anoxic corrosion behavior of pipeline steel in acidic soils. Ind. Eng. Chem. Res. 2014, 53, 17615–17624. [CrossRef] 27. Yan, M.C.; Sun, C.; Xu, J.; Dong, J.H.; Ke, W. Role of Fe oxides in corrosion of pipeline steel in a red clay soil. Corros. Sci. 2014, 80, 309–317. [CrossRef] 28. Wei, B.X.; Qin, Q.Y.; Fu, Q.; Bai, Y.L.; Xu, J.; Yu, C.; Sun, C.; Ke, W. X80 steel corrosion induced by alternating current in water-saturated acidic soil. Corrosion 2020, 76, 248–267. [CrossRef] 29. Sun, C.; Xu, J.; Wang, F.H.; Yu, C.K. Effect of sulfate reducing bacteria on corrosion of stainless steel 1Cr18Ni9Ti in soils containing chloride ions. Mater Chem. Phys. 2011, 126, 330–336. [CrossRef] 30. Wu, T.Q.; Yan, M.C.; Yu, L.B.; Zhao, H.; Sun, C.; Yin, F.C.; Ke, W. Stress corrosion of pipeline steel under disbonded coating in a SRB-containing environment. Corros. Sci. 2019, 157, 518–530. [CrossRef] 31. Liu, H.W.; Xu, D.; Yang, K.; Liu, H.F.; Cheng, Y.F. Corrosion of antibacterial Cu-bearing 316L stainless steels in the presence of sulfate reducing bacteria. Corros. Sci. 2018, 132, 46–55. [CrossRef] 32. Zhou, E.Z.; Li, H.; Yang, C.; Wang, J.; Xu, D.K.; Zhang, D.W.; Gu, T.Y. Accelerated corrosion of 2304 duplex stainless steel by marine Pseudomonas aeruginosa biofilm. Int. Biodeterior. Biodegrad. 2018, 127, 1–9. [CrossRef] Coatings 2021, 11, 625 14 of 14

33. Wu, T.Q.; Jin, X.; Yan, M.C.; Cheng, S.; Yu, C.K.; Wei, K. Synergistic effect of sulfate-reducing bacteria and elastic stress on corrosion of X80 steel in soil solution. Corros. Sci. 2014, 83, 38–47. [CrossRef] 34. Tan, B.J.; Klabunde, K.J.; Sherwood, P.M.A. X-ray photoelectron spectroscopy studies of solvated metal atom dispersed catalysts. Monometallic iron and bimetallic iron-cobalt particles on alumina. Chem. Mater. 1990, 2, 186–191. [CrossRef] 35. Zheng, B.; Li, K.; Liu, H.; Gu, T. Effects of magnetic fields on microbiologically influenced corrosion of 304 stainless steel. Ind. Eng. Chem. Res. 2013, 53, 48–54. [CrossRef] 36. Li, Y.; Feng, S.; Liu, H.; Tian, X.; Xia, Y.; Li, M.; Xu, K.; Yu, H.; Liu, Q.; Chen, C. Bacterial distribution in SRB biofilm affects MIC pitting of carbon steel studied using FIB-SEM. Corros. Sci. 2020, 167, 108512. [CrossRef] 37. Zuo, R.; Kus, E.; Mansfeld, F.; Wood, T.K. The importance of live biofilms in corrosion protection. Corros. Sci. 2005, 47, 279–287. [CrossRef] 38. Michael, D.W. Bacterial adhesion: Seen any good biofilms lately? Clin. Microbiol. Rev. 2002, 15, 155–166. 39. Yan, M.C.; Sun, C.; Xu, J.; Wu, T.Q.; Yang, S.; Ke, W. Stress corrosion of pipeline steel under occluded coating disbondment in a red soil environment. Corros. Sci. 2015, 93, 27–38. [CrossRef] 40. Xu, R.; Zhao, A.; Li, Q.; Kong, X.; Ji, G. Acidity regime of the red soils in a subtropical region of southern China under field conditions. Geoderma 2003, 115, 75–84. [CrossRef] 41. Li, H.W.; Dake, X.K.; Yingchao, L.; Hao, F.; Zhiyong, L.; Xiaogang, L.; Tingyue, G.; Ke, Y.; Zhang, W. Extracellular electron transfer is a bottleneck in the microbiologically influenced corrosion of C1018 carbon steel by the biofilm of sulfate-reducing bacterium Desulfovibrio vulgaris. PLoS ONE 2015, 10, e0136183. 42. Fatah, M.C.; Ismail, M.C.; Wahjoedi, B.A. Effects of sulphide ion on corrosion behaviour of X52 steel in simulated solution containing metabolic products species: A study pertaining to microbiologically influenced corrosion (MIC). Corros. Eng. Sci. Techn. 2013, 48, 211–220. [CrossRef] 43. Castaneda, H.; Benetton, X.D. SRB-biofilm influence in active corrosion sites formed at the steel-electrolyte interface when exposed to artificial seawater conditions. Corros. Sci. 2008, 50, 1169–1183. [CrossRef] 44. Wei, B.X.; Qin, Q.Y.; Bai, Y.L.; Yu, C.K.; Xu, J.; Sun, C.; Ke, W. Short-period corrosion of X80 pipeline steel induced by AC current in acidic red soil. Eng. Fail. Anal. 2019, 105, 156–175. [CrossRef] 45. Wu, T.Q.; Yan, M.C.; Zeng, D.C.; Xu, J.; Yu, C.K.; Sun, C.; Ke, W. Microbiologically induced corrosion of X80 pipeline steel in a near-neutral pH soil solution. Acta. Metall. Sin. Engl. 2014, 28, 93–102. [CrossRef]