Investigation of microbiologically influenced corrosion of low alloy steels with different Cr contents in simulated oilfield produced water

Microbiologically influenced corrosion has become a predominant cause of pipeline and equipment failure in oil and gas fields. This research examines the corrosion behavior of steels with varying chromium contents in simulated shale gas formation water environments. Weight loss testing, Scanning Electron Microscopy (SEM), Energy Dispersive x-ray Spectroscopy (EDS), Focused Ion Beam (FIB), Transmission Electron Microscopy (TEM), and x-ray Photoelectron Spectroscopy (XPS) were employed. The findings suggest that an elevated Cr content in steel results in a gradual reduction in its uniform corrosion rate in a CO2-SRB coexistence environment. This can be primarily attributed to the significant distribution of Cr content throughout the entire product film, including both biofilm and corrosion products, thereby enhancing the product film’s protectiveness. Additionally, the competitive corrosion between CO2 and SRB, as well as the influence mechanism of Cr on corrosion, were discussed.


Introduction
As the national economy continues to expand rapidly, the demand for oil and natural gas in society is also increasing.The extraction methods of these resources have evolved from primary recovery techniques that relied on natural energy sources such as rock expansion, edge water drive, and natural gas expansion, to secondary recovery methods involving water or gas injection [1].However, during production operations like water injection and shale gas hydraulic fracturing, a significant number of microorganisms are inevitably introduced.This has led to corrosion of metal pipelines in oil and gas field casings and gathering systems, causing substantial economic losses [2] and garnering considerable attention from researchers [3][4][5][6].
Corrosive microorganisms, such as sulfate-reducing bacteria (SRB), contribute to metal corrosion either directly or indirectly through their metabolic activities.These organisms generate biofilms during their metabolic processes, altering the local environment between the metal surface and the biofilm in a manner distinct from the original solution.This alteration accelerates the corrosion process, leading to perforation failures in oil and gas field pipelines and facilities [7][8][9].SRB are non-strictly anaerobic bacteria that are prevalent in anaerobic environments including oil and gas field formation water, produced water, seawater, underground pipeline walls, and oil and gas wells.They not only induce pitting corrosion of metals but also facilitate stress corrosion cracking and crevice corrosion, resulting in blockages in oil and gas wells and leakage accidents in oil and gas pipelines [10][11][12][13], and then causing greater economic losses and ecological environment damage.The exact mechanism underlying microbial corrosion remains a topic of debate; however, the cathodic depolarization theory is currently the most accepted explanation for SRB corrosion.During this reaction process, SRB use sulfate as an electron acceptor, reducing sulfate ions to sulfur ions and producing hydrogen sulfide.This generates protons in the aqueous solution, accelerating the cathodic reaction and leading to steel corrosion and the formation of iron sulfides [14].Furthermore, bacterial metabolites can concurrently modify the surrounding microenvironment-including the concentration, pH, and dissolved oxygen content of corrosive ions-resulting in alterations to the corrosion process and further exacerbating localized corrosion [15].In industrial settings, the prevalence of bacteria such as SRB, coupled with the synergistic interaction of various microbial groups, complicates the understanding of microbial corrosion mechanisms.Consequently, elucidating the processes of microbial corrosion, particularly those involving SRB, and identifying their influencing factors is crucial for the detection, prevention, and control of industrial microbial corrosion.
In the context of oil and gas field production, the presence of microorganisms alongside corrosive media such as CO 2 and H 2 S often complicates the corrosion process.Chromium is a key component in the formation of protective passivation films; thus, its addition to steel can facilitate the creation of a Cr oxide film, thereby enhancing the steel's corrosion resistance [16,17].Low-Cr content steel, with concentrations of 1 wt% Cr, 3 wt% Cr, or 5 wt% Cr, has demonstrated a certain level of corrosion resistance and economic viability compared to carbon steel and stainless steel [18].This is largely due to the protection afforded by the amorphous chromium hydroxide film.Despite this, the corrosion behavior of low-Cr steel in the presence of CO 2 and microorganisms within oil and gas fields remains unclear.Furthermore, the resistance of low-concentration chromium films to localized microbial attack requires additional research.
In light of the preceding considerations, this study endeavors to investigate the corrosion behavior of low-Cr steel in a coexistent environment of CO 2 and SRB.This is achieved through an array of methodologies including weight-loss tests, three-dimensional imaging of pitting morphology, scanning electron microscopy (SEM), focused ion beam machining (FIB), and transmission electron microscopy (TEM).Furthermore, the synergistic corrosion mechanism engendered by the combined presence of CO 2 and SRB is thoroughly discussed.

Materials
Three types of commercial 80ksi grade tubing, each with varying Cr content, were utilized as test materials.Their chemical compositions are detailed in table 1.Based on the Cr content, the three steels are designated as 1Cr steel, 3Cr steel, and 5Cr steel, respectively.The 1Cr steel is composed of tempered sorbite, while the 3Cr and 5Cr steels are characterized by their granular bainite, as shown in figure 1.
The weight loss test samples were machined from the as-received tubes with a sheet size of 50 mm × 10 mm × 3 mm.All samples were ground to 1000 # with sandpaper, and then ultrasonically cleaned with deionized water and anhydrous ethanol for 5 min each, followed by drying with cold air.The dimensions of the specimens were measured using a micrometer (accuracy of 0.01 mm), and their mass was measured using an electronic balance (accuracy of 0.1 mg).All samples were sterilized under a UV lamp for 30 min before the test.

Weight loss test
Weight loss tests were carried out in a self-made closed glass vessel.The composition of the test solution has been shown elsewhere [19], which was prepared using analytical grade chemical reagents and deionized water.The test bacteria were isolated and purified from a certain shale gas production water in China.Using 0.5 g l −1 potassium dihydrogen phosphate, 2.0 g l −1 magnesium sulfate, 0.1 g l −1 calcium chloride, 0.5 g l −1 sodium sulfate, 1.0 g l −1 ammonium chloride, 3.5 g l −1 sodium lactate and 1.0 g l −1 yeast extract, the culture medium was prepared.The medium was first sterilized at 121 °C for 20 min, then deoxygenated with high-purity sterilized nitrogen for 2.5 h.After that, the isolated SRB seeds were inoculated into the treated medium, and after constant temperature cultivation at below 35 °C for 7 days, the SRB enrichment liquid was obtained.A further enriched bacterial fluid was obtained by filtration, added to the sterilized test solution, and it was ensured that the initial number of SRB in the test solution reached 1.1 × 10 5 cells/mL.In an investigation centered on the operational conditions of a shale gas well in Western China, the experimental parameters were as follows: the temperature was maintained at 60 °C, atmospheric pressure served as the experimental pressure, and the solution under study was deoxygenated by the continuous introduction of sterilized high-purity nitrogen gas.Throughout the duration of the experiment, a mixture of sterilized CO 2 -N 2 (17% CO 2 -83% N 2 ) was consistently introduced.The test duration was 14 days.
In each test set, five parallel specimens were positioned.Following the examination, a chemical cleaning solution, comprising 100 ml/L hydrochloric acid and 10 g/l hexamethylenetetramine, was employed to eliminate corrosion products from the surfaces of three parallel specimens.The resulting weight loss was then used to calculate the uniform corrosion rate, as per equation (1).
where, CR is the corrosion rate (mm/y), ΔM is the weight loss (g), A is the surface area of the test specimen (cm 2 ), t is the test duration (h), and ρ is the density of the test specimen (g/cm 3 ).

Characterization techniques
To examine the properties of corrosion products on the surfaces of corroded specimens, samples that had not undergone chemical cleaning were immersed in a 2.5% (v/v) glutaraldehyde solution for a period of 8 h to preserve the biofilm.Following this, they were placed in a refrigerator with a temperature range of 2 °C for approximately 6 h.Subsequently, they were subjected to sequential dehydration in alcohol solutions at concentrations of 50%, 60%, 70%, 80%, 90%, 95%, and 100% (v/v), with each step lasting 10 min.The x-ray photoelectron spectrometer (XPS, ESCALAB 250Xi, Thermofisher) was used to detect the spectra of C, Cr, Fe, Mn, O, P, S elements.The data analysis was performed using Avantage software, with a calibration based on a binding energy of 284.8 eV for C1s.The scanning electron microscope (SEM, JEOL JSM 7200 F) and its accompanying energy dispersive spectrometer (EDS, Oxford X-Max) were used to analyze the morphology and chemical composition of corrosion products.For each sample, the composition of products with similar morphology was analyzed at several regions to ensure the validity of the EDS results.It should be noted that the accuracy of EDS in analyzing elements with low atomic number (such as C) is much worse than that of elements with higher atomic number than C, so the EDS results of C are only for reference.In order to observe the structure and composition of corrosion products from a micro-nano scale, a focused ion beam (FIB, FEI Helios NanoLab 600i) was employed in a Lift-Out processing method to machine thin slices on the vertical sample surface, which included features along the thickness direction of the corrosion product layer.High-resolution field emission transmission electron microscopy (TEM, FEI Talos F200X) was utilized to observe the morphological images and high angular annular dark-field images (HAADF) of the corrosion product layer.

Corrosion rate
Figure 2 illustrates the uniform corrosion rate of various steels containing different amounts of chromium in a coexistence environment with CO 2 and SRB over a period of 14 days.The figure reveals that an increase in Cr content corresponds to a gradual decrease in the corrosion rate.Specifically, when the Cr content rises from 1% to 3%, the uniform corrosion rate experiences a decrease of 16.24%; and the Cr content continues to escalate to 5%, the uniform corrosion rate will be reduced by an additional 37.01%.
An attempt was made to observe the pitting morphology on the surfaces of the de-filmed samples using an optical microscope with great depth of field.Relatively shallow, localized corrosion features were found on the surface of 1Cr and 3Cr steels, but the depth of these 'pits' was all relatively shallow (less than 10 μm).In contrast, no obvious, measurable pits were found on the surface of 5Cr steel.It can be seen that the 3 steels with low Cr content are not susceptible to pitting under simulated test conditions.Nevertheless, as the Cr content increases, the susceptibility of the steel to pitting tends to decrease.

Surface morphology of the corrosion products
Figure 3 shows the microscopic morphology of corrosion products on the surface of three low-Cr steels after 14 days of exposure in the CO 2 and SRB co-existing environment.At low magnification, obvious products (with possible presence of biofilm) can be observed on the surface of 1Cr steel, the product film on the surface of 3Cr steel appears obvious cracking and falling off, and the product on the surface of 5Cr steel seems to be less (the traces of grinding of the specimen can be seen).At high magnification, SRB cells can be observed on the surface of 1Cr steel, a large number of cracks exist on the surface of 3Cr steel, while the surface of 5Cr steel is relatively flat with some visible cracking features.EDS analysis (see table 2) indicates that common elements of EPS such as  C, P, O, and S are present in the products on the surface of three low-Cr steels, indicating the formation of biofilm on the surface.The presence of phosphorus enrichment was observed in the cell wall, capsule, and cell membrane [20].The content of Cr in the product on the surface of 1Cr steel is low.When the content of Cr in the steel increases to 3%, the content of Cr in the outer surface product increases significantly, which makes the metal surface exposed after local falling off.When the content of Cr in the steel rises to 5%, the content of Cr in the outer surface product further increases, and the content of Fe further decreases.This shows that increasing the content of Cr in the steel is beneficial to increase the content of Cr in the product film, so as to improve the protectivity of the product film.
Table 3 presents the atomic percentage of major elements in the surface products detected by XPS.The results are consistent with the EDS analysis.Figure 4 shows high-resolution XPS spectra of C 1s, O 1s, Fe 2p, Cr 2p, and S 2p for the surface products on steels with various concentration of chromium, and table 4 summarizes the XPS parameters including binding energy and corresponding components.In C 1s spectra, peaks at 284.80, 286.20, and 288.50 eV correspond to organic components like C-C, C=O, C-H, and C-N.These chemical bonds are usually components of nucleic acids and cell walls.In O 1s spectra, peaks at 530.10, 531.20, and 532.50 eV correspond to O 2− , organic O, and OH − [21].In Fe 2p spectra, peaks at 710.30, 711.30, 712.30, 718.80, and 725.30eV correspond to FeS x [22], Fe 2 O 3 , FeOOH, Fe, and FeCO 3 [23], respectively.FeS x is the typical products of SRB corrosion, and FeCO 3 is the typical products of CO 2 corrosion [24,25].In Cr 2p spectra, peaks at 577.20 and 586.80 eV correspond to Cr 2 O 3 and Cr(OH) 3 respectively.In S 2p spectra, peaks at 163.20 ∼ 163.60 eV correspond to FeS x and C=S which is from the EPS in SRB biofilms [26].)

Cross-section morphology of the corrosion products
The examination of cross-sectional morphology and compositional distribution of biofilms and their products is crucial for comprehending the MIC process.Traditional mechanical polishing methods, however, often result in damage to both the biofilm and bacterial cell wall.To circumvent this issue, the FIB technique was employed to minimize potential damage at the interface.Figures 5 and 6 present FIB-SEM images of surface products derived from 1Cr and 5Cr steel respectively.SEM morphology of the selected surface area is shown in figures 5(a) and 6(a).After platinum deposition, the protected area was milled into a slice for TEM observation, as shown in figures 5(b) and 6(b).The cross-sectional view (as shown in figures 5(c) and 6(c)) reveals a thin surface film, with small, round holes of dispersed distribution on its exterior.These are usually identified as bacterial cells, strategically positioned to facilitate exchange with external substances for energy.Additionally, the film contains several large pores situated in its center.These pores may serve as channels for both internal and external corrosion media to access the steel surface during the corrosion process, thereby promoting ongoing corrosion progression [21].
Figure 7 illustrates cross-sectional morphology and EDS mapping of the product film on the surface of 1Cr steel.As observed through BF and HAADF images, the film layer at the outward convexity is loose and porous, and exhibits the thickest consistency, approximately 900 nm.Combined with the EDS mapping, it can be seen that the outer convex part possesses elevated concentrations of C, O, P, which are the main components of extracellular polymers in biofilms.The film layer near the steel surface comprises Fe, Cr, O, C, and S, which are corrosion products, aligning with XPS findings.Cr within the inner layer does not establish a comprehensive film but rather forms dispersed Cr-rich particles.This suggests that the Cr present in 1Cr steel is inadequate for the formation and preservation of Cr-rich film layers.Figure 8 presents the cross-sectional morphology and EDS mapping of the product film on the 5Cr steel.Analogous to the 1Cr steel, both BF and HAADF images reveal loose and porous films with convex protrusions on the outer layer and corrosion products within the inner layer.However, the film's thickness is marginally thinner than that of 1Cr steel.The EDS mapping further suggests that the outer layer comprises extracellular polymeric substances in the biofilm, while the inner layer consists of corrosion products.Notably, compared to 1Cr steel, Cr is more prominently distributed across the entire film layer on the 5Cr steel, whereas Fe is primarily found in the inner layer.This observation implies that the higher concentration of Cr in the 5Cr steel may enhance the formation of Cr-rich film.The composition of the corrosion products is similar to that of 1Cr steel and aligns well with XPS results.

Discussion
Biofilms, compounds of Fe (i.e.carbonates, oxides, hydroxides, and sulfides), as well as compounds of Cr (encompassing hydroxides and oxides) were identified on the surface of the low-Cr steels in the CO 2 -SRB coexistence environment.This suggests that Fe and Cr within these low-Cr steels interact with CO 2 and SRB.Unlike carbon steel, corrosion of low-Cr steel in a CO 2 -SRB coexistence environment exhibits a diverse range of   competitive reactions.These include the distinct activities of Fe and Cr, which compete to form a film, and the varying activities of the same element when interacting with CO 2 and SRB, leading to two types of media competition reactions.

Competitive corrosion between CO 2 and SRB
In the initial phase, when low-Cr steel is exposed to a medium containing both CO 2 and SRB, competitive corrosion occurs between the two.The process involves the ionization of water by CO 2 (equations ( 2)-( 4)) [25,27], resulting in the formation of CO 3 2-ions.Concurrently, SRB reduces sulfate anions to produce H 2 S and its corresponding ionization products (equations ( 5)-( 6)) [28].Simultaneously, Fe present on the surface of the low alloy steel lose electrons, leading to the formation of metal ions as depicted in equation (7).These iron ions subsequently interact with two types of anions, forming insoluble salts such as FeCO 3 and FeS x , as shown in equations (8)- (9).The results from the XPS (as shown in figure 4) and EDS (as shown in figures 7-8) analyses reveal that, in addition to FeCO 3 and FeS x , the products contain two types of iron oxides (FeOOH and Fe 2 O 3 ).This is common in aqueous solution systems.The formation of Cr 2 O 3 and Cr(OH) However, there are discernible differences between the two.The carbonate film, formed by CO 2 corrosion, exhibits a relatively denser structure which can decelerate the subsequent corrosion process to a certain degree [29].On the other hand, SRB tends to form biofilms, subsequently leading to the formation of loose films.This results in a significant acceleration of the ensuing corrosion process [26].In the present work, the structure of outer loose biofilm and inner corrosion products was observed from the cross-sectional morphology (as shown in figures 5-8).
When the concentration of SRB in solution is high and the corrosion period extends, the proliferation of SRB under biofilm becomes more pronounced, thereby accelerating steel corrosion, particularly localized corrosion.Moreover, due to the significantly larger solubility K sp of ferric carbonate compared to ferrous sulfide, ferric carbonate may react with HS − produced by SRB, resulting in the formation of FeS x .As the corrosion duration increases, the presence of ferric carbonate in the corrosion products diminishes progressively.Consequently, the competitive corrosion between CO 2 and SRB was initially dominated by CO 2 .However, once a biofilm formed, SRB emerged as the primary mechanism of corrosion.

Effect of Cr on corrosion
Cr serves as the primary corrosion-resistant alloying element, designed to form a protective passivation film rich in chromium (e.g.Cr 2 O 3 ).It is worth noting that chromium exhibits some degree of cytotoxicity towards SRB.In stainless steel, Cr has a significant effect on the attachment of SRB biofilms and elevated Cr content tends to decrease the number of attached bacteria [30].
Contrary to stainless steel, the addition of a small amount of Cr in steel was initially employed to enhance its resistance to CO 2 corrosion.This is attributed to the superior reactivity of Cr compared to Fe, which results in the loss of electrons and subsequent formation of metal ions in an aqueous solution (equation ( 10)).These metal ions further react to yield an amorphous Cr(OH) 3 film (equation ( 11)), or even protective Cr 2 O 3 (equation ( 12)).In this study, Cr(OH) 3 and Cr 2 O 3 were identified on all three steels' surfaces (figure 4).The surface EDS analysis (tables 2 and 3) revealed that the Cr content in the surface products increased proportionally with the increase of Cr content in the steels.Furthermore, the cross-sectional morphology (figures 7 and 8) indicated a more pronounced distribution of Cr throughout the products as the Cr content in the steels increased.However, a small amount of Cr does not appear to form a comprehensive, protective passivation film, but rather results in the dispersion of Cr-rich particles.The degree of protection escalates with an increase in Cr content, consequently leading to a decrease in weight loss rate (as shown in figure 2).

Cr
In the context of CO 2 -SRB coexistence, the formation of a biofilm triggers a change in the local environment beneath it, thereby perpetuating corrosion.However, when the Cr content in steel is elevated, the surface layer of the substrate retains a certain degree of protection from the passivation film (as shown in figure 8), consequently decelerating the rate of corrosion.

Conclusions
The corrosion behavior of low-Cr steel with different Cr contents in simulated shale gas formation water environment was investigated in this work.Based on the above results and discussions, the following conclusions are summarized.
(1) An increase in Cr content of low-Cr steels leads to a gradual decrease in the corrosion rate.When the Cr content rises from 1% to 3%, the uniform corrosion rate experiences a decrease of 16.24%, and 5%Cr reduces the uniform corrosion rate by an additional 37.01%.
(2) The cross-sectional morphology and elemental distribution of surface products from steel with varying Cr contents were examined using FIB-SEM/TEM.The findings revealed the presence of external convex, loosely bound biofilms and inner thinner corrosion product structures.As the Cr content in the steel increased, the distribution of Cr throughout the entire product film became more pronounced, suggesting enhanced corrosion resistance to CO 2 /SRB.

Figure 2 .
Figure 2. The corrosion rate of various low-chromium steels in the CO 2 and SRB co-existing environment.

Figure 3 .
Figure 3. Surface morphology at various magnifications of the corroded specimens after 14 days of exposure in the CO 2 and SRB coexisting environment: (a-c) 1Cr steel,(d-f) 3Cr steel, and (g-i) 5Cr steel.In each row, the magnification gradually increases from left to right.

Figure 5 .
Figure 5. FIB-SEM images of the surface products formed on 1Cr steel.(a-b) SEM image of the position where FIB milling was performed.(c) cross section SEM tomography, and (d) SEM image of the final slice for TEM characterization.

Table 4 .Figure 6 .
Figure 6.FIB-SEM images of the surface products formed on 5Cr steel.(a-b) SEM image of the position where FIB milling was performed.(c) cross section SEM tomography, and (d) SEM image of the final slice for TEM characterization.

Figure 7 .
Figure 7. BF image, HAADF image, and STEM-EDS mapping of the slice milled from the corroded 1Cr steel.

Figure 8 .
Figure 8. BF image, HAADF image, and STEM-EDS mapping of the slice milled from the corroded 5Cr steel.

Table 2 .
EDS analysis of surface products on the corroded specimens after 14 days of exposure in the CO 2 and SRB co-existing environment (at.%).

Table 3 .
Atomic percentage of major elements on the surface products detected by XPS (%).
3 will discussed later.