Role of Pseudomonas fluorescens FSYZ01 on the corrosion behavior of Q235B carbon steel in oilfield produced water

The corrosion behavior of Q235B carbon steel is investigated in water, LB medium, and oilfield produced water adding Pseudomonas fluorescens FSYZ01. After immersion at 30 °C for 13 days, the weight loss of carbon steel with this strain decreased by 32.23%, 54.07%, and 78.34%, respectively. X-ray diffraction (XRD) results show that P. fluorescens FSYZ01 inhibited conversion of iron oxides by hindering oxygen from approaching metal surface. Fourier transform infrared (FT-IR) and X-ray photoelectron spectrometer (XPS) results show that specific functional groups and bonds reacted with Fe(II/III) to form a dense and stable chelate-oxide protective layer, thereby inhibiting corrosion. Pyrolysis gas chromatography-mass spectrometer (Py-GCMS) results demonstrate the bacteria degraded C12 to C20 alkanes in oil. The inhibitory mechanism of crude oil-degrading bacteria P. fluorescens FSYZ01 on the carbon steel corrosion was proposed, so as to slow corrosion of oilfield produced water system pipeline and prolong its service life, helping to comprehend the microbial corrosion in the actual environment.


Introduction
Oilfield pipeline corrosion is a primary problem; in particular, oilfield produced water contains a lot of microorganisms, resulting in microbiologically influenced corrosion (MIC) (Eduok et al. 2016). MIC threatens the safety production in oil industry (Liu et al. 2018) and potentially causes destructive effects and huge economic losses (Benamor et al. 2018;Zhang et al. 2020). It was reported that MIC affects diverse processes ranging from water distribution in cast iron and sewers to natural gas transportation in steel pipelines. When microorganisms exist in the water environment, the corrosion process of metals will be more complicated (Liu et al. 2018).
Several studies have shown that EPS promotes corrosion; however, researchers also have shown beneficial microorganisms attached to metal surfaces interact with the liquid flow environment (Dong et al. 2016;Li et al. 2019a;Suma et al. 2019) forming a biofilm on the surface of the metal. This microbiologically influenced corrosion inhibition (MICI) may protect carbon steel surfaces by inhibiting corrosion and be used as an alternative method to combat corrosion . Reported previously, Marinobacter aquaeolei has high corrosion resistance for the corrosion of carbon steel (Saleem Khan et al. 2019). Pseudomonas putida has corrosion inhibition effect as a dominant bacterium under aerobic condition (Suma et al. 2019). The average corrosion inhibition rate of Pseudomonas fluorescens for carbon steel is 48.14% in the artificial water environment . P. fluorescens can inhibit the corrosion behavior of carbon steel in reclaimed water, and corrosion rate remains at 0.16-0.18 mm/a (Chu et al. 2020). It is found that Pseudomonas has great potential for corrosion inhibition in seawater, reclaimed water, and Responsible Editor: Philippe Garrigues * Lihua Liang lianglh@nwu.edu.cn 1 other environments, while seldom is used on carbon steel in the oilfield water. Oilfield produced water contains a large amount of aromatic, naphthenic, and paraffinic hydrocarbons. These components are not easy to volatile and will cause immeasurable harm when entering soil and water ecosystems (Sakthipriya et al. 2016;Totubaeva et al. 2019). Fortunately, some studies show that Pseudomonas can degrade hydrocarbons of crude oil. domonas and five isolates as P. fluorescens via microbiological methods and 16S rRNA sequencing (Goudarztalejerdi et al. 2015). Researchers isolated P. fluorescens W3 from oil polluted soil and found this bacterium quite successfully played the role of oil products decomposers (Totubaeva et al. 2019).
These studies show that Pseudomonas has the ability to inhibit corrosion or degrade oil. This makes it possible for Pseudomonas to inhibit metal corrosion and degrade oil simultaneously in a network of pipes and oilfield wastewater treatment systems. Despite the fact that the corrosion inhibition of EPS and the crude oil degradation effect of Pseudomonas have been investigated by many researchers, the study on the screen a strain with both functions focuses on evaluating the corrosion inhibition performance of this strain which is still scarce. In light of this, this work aims to isolate a suitable strain from activated sludge-treated crude oil wastewater on Luria-Bertani (LB) solid medium. The ability of Pseudomonas to degrade oilfield produced water, corrosion inhibition, and corrosion inhibition mechanism on carbon steel samples is examined by taking sterile deionized water (no food) and LB media (as rich medium) as control, so as to provide a theoretical basis for corrosion control and corrosion mechanism of pipelines and containers during the transportation, storage, and treatment of oilfield produced water.

Microbe cultivation
P. fluorescens was isolated from activated sludge treated crude oil wastewater. The strain was classified as P. fluorescens by polymerase chain reaction (PCR) amplification of 16S rDNA. For the 16S rDNA analysis, genomic DNA was extracted with the alkaline lysis method, and 16S rDNA was amplified using the genomic DNA as template and bacterial universal primers 27F (5′-AGA GTT TGA TCC TGG CTC AG -3′). The 16S rDNA sequence was compared with sequences in the GenBank database using the Basic Local Alignment Search Tool (BLAST) program showing 99% homology. The strain is preserved in the China Center for Type Culture Collection (CCTCC) and named P. fluorescens FSYZ01 (M 2018924). The strain is preserved in the China Center for Type Culture Collection and named P. fluorescens FSYZ01. P. fluorescens FSYZ01 was activated at 30 °C in sterilized liquid LB medium under continuous shaking at a speed of 140 r/min for 1 day. After activation, P. fluorescens FSYZ01 was inoculated into each medium at a volume ratio of 2%.

Oil and COD degradation efficiency of oilfield produced water
The degradation efficiency of oil was determined using a NanoDrop 2000c ultraviolet spectrophotometer (Thermo Scientific). Oil in the oilfield produced water was extracted with petroleum ether at 60-90 °C, and the oil was scanned in the ultraviolet region to determine the maximum absorption peak (254 nm). P. fluorescens FSYZ01 cultivated for 48 h was inoculated into the oilfield produced water at 2% (V/V) for 5 days. After 5-day incubation, the degradation efficiency of oil was measured at 254.00 nm. Samples were tested three times and the results were averaged.
Chemical oxygen demand (COD) content of oilfield produced water was measured with a HACH DRB 200 digestion instrument and HACH DR 890 spectrophotometer (HACH Company, USA). Samples were tested three times, and the results were averaged.

Sample preparation
Q235B carbon steel coupons (50 mm × 25 mm × 2 mm, surface area 28 cm 2 ) were used as corrosion metal samples in corrosion effect experiment. Using Q235B carbon steel which the size of 10 mm × 10 mm × 2 mm (surface area of 2.8 cm 2 ) in electrochemical experiments, the nominal chemical composition of Q235B carbon steel is shown in Table 1.
The carbon steel samples were degreased in acetone, washed with anhydrous ethanol, and dried in the oven for 4 h at 105 °C, and then, all samples were sanitized under a UV lamp for 30 min before immersion into liquid medium "Liquid medium" section.

Liquid medium
In order to study carbon steel corrosion behavior with and without P. fluorescens FSYZ01 in different liquid mediums, experiments were conducted in 6 reaction systems: (a) sterile deionized water; (b) sterile deionized water with P. fluorescens and (f) oilfield produced water with P. fluorescens FSYZ01 (produced water was filtered through 15 ~ 20-μm filter paper before use). Table 2 presents the chemical composition of oilfield produced water determined by ion chromatography.
In addition, all liquids were autoclaved prior to experiments, and experiments were handled in an ultraclean workbench.

Weight loss experiments
Carbon steel samples were weighted before corrosion testing (W 0 ) using a digital balance with 0.0001 g precision. After corrosion testing at 30 °C for 3, 5, 7, 9, 11, and 13 days, samples were dried by a freeze dryer. After the corrosion test, according to Chinese national standard method for measuring metal corrosion by weight loss method (HG/T 2159-91), the coupons were taken out, and the corrosion products were stripped using an acid pickling solution containing hexamethylenetetramine corrosion inhibitor for several minutes (Lv et al. 2019;Xu et al. 2020), and the exposed sample surface was rinsed with sterile deionized water. The carbon steel samples were dried in an oven at 105 °C for 4 h prior to measuring the final weight (W 1 ). Each sample was weighed three times and the results were averaged. The corrosion rate where W 0 and W 1 are the original weight and final weight of samples (g), respectively, S (m 2 ) is the exposed surface area of samples, and t (h) is the immersion time.

Electrochemical experiments
Corrosion potentials were performed using a Chen-Hua 660e Electrochemical Workstation (Chen-Hua, China). A conventional three-electrode system was used. The counter electrode was a Pt wire, and all potentials were measured against a saturated calomel electrode (SCE). Potentiodynamic polarization curves were performed in the potential region of −1 to −0.4 V with a 0.005 V/s scan rate.

Surface analysis and corrosion product analysis
An optical microscope (OM) was used to estimate the thickness of corrosion products on carbon steel surface. The thickness of the corrosion products was roughly (1) measured by randomly selecting points on the bottom layer and the corrosion products on the surface layer. Each sample was tested at least six times, and the results were averaged. A scanning electron microscope (SEM; Hitachi, Japan) was used to observe microbial morphology and analyze metal corrosion morphology with the beam voltage at 15 kV.
Chemical composition of the corrosion products after immersion for 13 days was examined by X-ray diffraction (XRD). The XRD measurements were performed using a Bruker D8 diffractometer (Germany). Spectra were obtained in the range of 10˚ < 2θ < 90˚.
Organic ingredients of the corrosion products after immersion for 13 days were analyzed by Bruker TENSOR 27 Fourier transform infrared (FT-IR) spectroscopy (Germany) in IR-Reflectance mode from 400 to 4000 cm −1 .
The composition of corrosion products on the coupon was analyzed by X-ray photoelectron spectrometer (XPS) (PHI 5000 VersaProbeIII, ULVAC-PHI). The X-ray source (Al Kα) micro-aggregation monochromator excitation source energy was 1486.68 eV, the depth analysis was 10 mA, and the power was 150 W.
A Frontier Lab EGA 2020 pyrolyzer equipped with an AS-1020E auto-shot sampler coupled with GCMS (Shimadzu QP 2010 Ultra) was used for characterize degradation of P. fluorescens FSYZ01 on the organic matter in oilfield produced water and LB media. GC separations were achieved using a SLB-5MS (Supelco) capillary column (30 mm long, 0.25 mm i.d., 0.25 μm phase thickness). The injector temperature was set to 250 °C with a 50/1 split ratio and a helium flow set to 1 mL/min. The column temperature was programmed from 50 to 300 °C with a rate of 5 °C/min and held at 300 °C for 14 min. The ionization mode was electron impact (70 eV), and the source temperature was 220 °C. The acquisition was realized in full scan mode from m/z 50 to 600 at 0.2 uma/s.

Oilfield produced water composition analysis and pH, DO content analysis after corrosion
The pH and anion and cation components of oilfield produced water from Changqing Oilfield Oil Production Plant (No. 5) were measured by pH meter and ion chromatography. The contents of various pollutants are shown in Table 2. It can be seen from Table 2 that the pH of oilfield produced water is 5.23, which is weakly acidic. Additionally, oilfield produced water has a very high salt content, and it can be seen that anion and cation test results and the content of Ca 2+ , Na + , and Cl − in soluble substances were highest and are 1222.47 mg/L, 1755.46 mg/L, and 5457.24 mg/L, respectively. The higher content of Ca 2+ and Mg 2+ will increase the scaling tendency of the inner wall of water pipeline (Xin and Li 2014). Due to Cl − has higher penetration and polarity, it can preferentially attach to the surface of metal materials, thereby reducing the formation probability of passivation films on metal surfaces and causing electrochemical corrosion of metal materials. COD of 2408 mg/L indicates a high concentration of organic matter in the produced water. Figure 1 illustrates the pH and dissolved oxygen (DO) changes of different environments. The pH increased 13 days after the corrosion. After adding bacteria, the pH with a little shift to the higher value, which may be due to Fe oxides, is reduced and OH − as by-products is formed. The concentration of DO decreased as corrosion occurred, because corrosion of carbon steel consumes little oxygen in sterile medium, and the metabolism of P. fluorescens FSYZ01 results in lower concentration of DO than that in sterile medium.

Microbial and corrosion morphology
Cell morphology of P. fluorescens FSYZ01 was observed by SEM at 3000 × and 5000 × . In previous studies, P. fluorescens FSYZ01 has good biofilm formation ability (Chu et al. 2020;Xu et al. 2020). Mature biofilms are complex extracellular polymeric substances (EPS). It shows that the P. fluorescens FSYZ01 had strong growth and reproduction ability; bacterial distribution was regular, and obvious boundaries were observed. These images in Fig. S1 are evidence that the bacteria were able to form dense biofilms on the samples.
Visible morphology of corrosion products on Q235B carbon steel samples after a 13-day immersion in different liquid mediums is shown in Fig. 2. The water environment were sterile deionized water (a), sterile deionized water with P. fluorescens FSYZ01 (b), sterilized LB medium (c), sterilized LB medium with P. fluorescens FSYZ01 (d), oilfield produced water (e), and oilfield produced water with P. fluorescens FSYZ01 (f). It can be seen that the extent of corrosion and the mass of corrosion products were different in the different liquid mediums. Figure 2 shows in all liquid mediums that less corrosion scales were produced and slighter corrosion extent was observed on carbon steel coupons when liquid media were added with P. fluorescens FSYZ01. The thicknesses of the corrosion products after a 13-day immersion in (a), (b), (c), (d), (e), and (f) were about 198 μm, 122 μm, 59 μm, 34 μm, 106 μm, and 87 μm, respectively, which also indicate that corrosion scales on coupons with P. fluorescens FSYZ01 were thinner than without P. fluorescens FSYZ01.
The morphology of the carbon steel samples immersed for 13 days and removed the corrosion scales which are observed under SEM. The coupons immersed in pure water, and produced water were corroded to some extent, while those coupons immersed in LB medium both without (c) and with bacteria (d) were less disturbed, and their original lines on steel surface were still inerratic. Little pits were observed on (c) along the direction of the lines, but there are fewer corrosion pits in (d), which supports the result that P. fluorescens FSYZ01 has good potential of corrosion inhibition. In oilfield produced water, it was observed that the steel surface without bacteria (e) was corroded more significantly with many big pits than that with bacteria (f) with only small cracks. In conclusion, corrosion was inhibited by P. fluorescens FSYZ01 effectively.

Weight loss
Weight loss of samples in water environment with and without bacteria was carried out after 3, 5, 7, 9, 11, and 13 days (Fig. 3a). As immersion time increased, sample weight loss gradually increases. Weight loss in samples without bacteria was significantly higher than samples with bacteria, which indicates the bacteria inhibit carbon steel corrosion, in agreement with the SEM results. After 13-day immersion, all systems containing bacteria showed lower weight loss than their counterparts without bacteria. Specifically for P. fluorescens FSYZ01 in samples with water, LB medium, and oilfield produced water, the weight loss of carbon steel decreased by 32.23%, 54.07%, and 78.34%, respectively, compared to without P. fluorescens FSYZ01. The corrosion rates of carbon steel samples exposed to different liquid environments are depicted in Fig. 3b. After 13 days, corrosion degree of oilfield produced water was the highest, and the maximum corrosion rate could reach about 0.08 g/ (m 2 ·h), while LB with bacteria was the least likely to be corroded, and the minimum corrosion rate could reach about 0.005 g/(m 2 ·h). The corrosion rate in the initial immersion stage gradually decreases with the immersion time. The steel coupons just immersed in liquid medium were directly corroded, and corrosion rate was relatively high. The corrosion product films that accumulate on steel surface began to play a role of blocking protection after 3-5 days.
Compared with the oilfield produced water after adding bacteria, the corrosion rate of pure water with bacteria had a rising trend, which may be related to P. fluorescens FSYZ01 in pure water and may gradually die due to lack of nutrients, and biofilm would collapse, but the oilfield produced water environment is relatively complex and riches in organic matter. The corrosion rates gradually decreased except in pure water, and then gradually increased except in LB medium with bacteria and oilfield produced water with bacteria. After the biofilm was disintegrated, the local corrosion aggravated due to incompleteness of corrosion product layer within 9-13 days. LB medium provided a good environment for P. fluorescens FSYZ01 cultivation; this sample is the most stable, and the corrosion rate of carbon steel in the LB medium with bacteria was low and constant. In contrast, P. fluorescens FSYZ01 needed to adapt to the complex environment of oilfield produced water, so the corrosion rate initially decreased slowly and decreased faster over time. The corrosion rates of samples with P. fluorescens FSYZ01 were consistently lower than those environments without P. fluorescens FSYZ01.

Corrosion electrochemical analysis of Q235B carbon steel samples
By electrochemical analysis of Q235B carbon steel immersed in different liquid mediums after 13 days, the corrosion inhibition ability of P. fluorescens FSYZ01 was further determined to carbon steel.
The corresponding potentiodynamic polarization curves and EIS Nyquist of the samples are shown in Fig. 4. It can be seen that all the basic shapes of the potentiodynamic polarization curves are similar, showing that the P. fluorescens FSYZ01 does not change the properties of the electrode in corrosion process but only the corrosion rate . And Table 3 represents the corresponding electrochemical parameters. It is clear that the corrosion current density (I corr ) is less than the abiotic system, and the high Fig. 2 Visible morphology of corrosion scales and SEM images of coupons surfaces (corrosion scales were removed) after 13-day immersion in a water, b water + bacteria, c LB, d LB + bacteria, e oilfield produced water, f oilfield produced water + bacteria impedance is observable in P. fluorescens FSYZ01 systems, indicating the mitigation of corrosion. Due to I corr changes, the most in oilfield produced water before and after adding bacteria, P. fluorescens FSYZ01 has distinct corrosion inhibition effect on carbon steel in oilfield produced water. It may be related to the fact that the bacteria can adapt to the complex environment of oilfield produced water and exhibit excellent corrosion inhibition performance. I corr of LB medium with bacteria was the smallest as with oilfield produced water adding bacteria, which indicates the carbon steel was not prone to corrosion in this environment compared to other systems. The heteroatoms of amino acid contained in LB medium interact chemically or/and electrostatically with metal surface to form adsorbed molecular layer and thus has a corrosion mitigation on metals. This may be due to the heteroatoms of amino acid contained in LB medium, interact chemically or/and electrostatically with carbon steel surface to form adsorbed molecular layer, and thus has a corrosion mitigation on metals.
In condition of six types, anode slope (β a ) is greater than the cathode slope, showing that carbon steel initially contacts with the liquid environment and the anode reaction occurs easily. After adding bacteria, β a was raised in oilfield produced water, illustrating that the addition of bacteria effectively restrain the dissolved iron anode. In LB medium, I corr decreases in the presence of bacteria, and the OCP slightly shifts to negative direction. This indicates that cathodic reaction process is inhibited in this environment, which may be due to the decrease of DO concentration in P. fluorescens FSYZ01 media (Fig. 1). However, in oilfield produced water environment, the OCP change is more significant in the positive direction (0.026 V), which is due to the faster formation of a stable film on the carbon steel surface and faster access to the steady state (Khayatkashani et al. 2022).
The E corr of sample in oilfield produced water shifted 0.026 V positive; in water, the potential shifted 0.004 V positive; and in LB, the media shifted 0.004 V negative. The offset of the corrosion potential was less than 0.085 V, indicating the bacteria acted as a mixed corrosion inhibitor in these systems (Lgaz et al. 2020;Tan et al. 2020).

XRD
The XRD spectra of the corrosion products on the steel surface in different environments after 13 days are presented in Fig. 5. It can be seen that the corrosion products covered on the steel are mainly γ-FeOOH, Fe 3 O 4 , and a little Fe 2 O 3 . In pure water without bacteria, Fe lost electrons and became Fe 2+ via Eq. (2) at the anode and H 2 O and O 2 got electrons to generate OH − , via Eq. (3) at the cathode; the Fe 2+ and OH − corrosion products generated Fe(OH) 2 , which was oxidized by O 2 to FeOOH. Fe(OH) 2 may be oxidized directly to γ-FeOOH when corrosion is rapid, and Fe(OH) 2 is produced faster than it is consumed (Wang et al. 2021). Fe(OH) 2 was further oxidized by O 2 to Fe 3 O 4 via Eq. (7); FeOOH can directly generate Fe 2 O 3 via Eq. (9).
The process of iron oxidation can be expressed as follows (Liu et al. 2015): γ-FeOOH is a corrosion product with high oxidation activity; moreover, the feathery loose structure of FeOOH was beneficial to the dissolution of dissolved oxygen and corrosive ions (Chu et al. 2020), which may accelerate corrosion of carbon steel to a certain extent (Lair et al. 2006). The content of γ-FeOOH in oilfield produced water samples was less than pure water environment, and the percentage of γ-FeOOH when adding bacteria was less than that of sterile  (7), compared with the formation of γ-FeOOH via Eq. (8). Additionally, the formation of Fe 3 O 4 is favored in the anaerobic environment (Hernandez et al. 1994;Tanaka et al. 2014;Wang et al. 2013Wang et al. , 2021, FeOOH can be converted to Fe 3 O 4 via Eq. (10), and a dense corrosion product film was formed, which protected the carbon steel surface from further corrosion. Due to corrosive medium, O 2 was consumed under respiration of P. fluorescens FSYZ01, and the content of corrosion products was less in oilfield produced water adding bacteria compared with the oilfield produced water without bacteria. After adding bacteria, the content of Fe 3 O 4 in oilfield produced water was higher than pure water, and it is probably because in the initial phase of corrosion, the areas beneath biofilms formed by P. fluorescens FSYZ01 existing in oilfield produced water were prone to local anaerobic and promoted the dissolution of Fe, and corrosion products were gradually converted to Fe 3 O 4 . However, the large peak that contain Fe 3 O 4 at 2 theta = 31 degrees was missing when oilfield produced water with bacteria and CK, it maybe because the EPS is protective resulting in less iron dissolution and less further corrosion products Fe 3 O 4 .
Overall, in pure water and oilfield produced water, no matter with and without P. fluorescens FSYZ01, the main component of corrosion product was γ-FeOOH which was loose, rather than dense Fe 3 O 4 or Fe 2 O 3 , and corrosion mitigation effect is limited.

FT-IR results
The FT-IR is recorded for the samples and it is shown in Fig. 6. This is a significant trapping way to identify the functional groups present in the samples. The absorption peaks at 3422 ~ 3387 cm −1 can be consigned to the stretching vibration of hydroxyl (-OH) groups and the presence of H 2 O due to the fact that iron oxide form retains certain adsorbed water (Sangaiya andJayaprakash 2020, Tadic et al. 2014). In addition, 2362 cm −1 spectrum in CK, 2372 cm −1 spectrum in LB, 2365 cm −1 spectrum in oilfield produced water, and 2363 cm −1 spectrum in oilfield produced water with bacteria belong to atmospheric presence of CO 2 (Sangaiya and Jayaprakash 2020). The characteristic absorption peak of γ-FeOOH appears near 1024 cm −1 , and it can be seen in water, CK, oilfield produced water, and oilfield produced water with bacteria, but not in any LB medium. The results are consistent with the XRD results.
The peaks at 1198 ~ 1096 cm −1 were attributed to -C-O-C-and -C-O, the stretching vibration for polysaccharide (Wang et al. 2014). The absorption peak at 1153 cm −1 in oilfield produced water disappeared after adding P. fluorescens FSYZ01, while it was not found in LB medium with or without bacteria, indicating that -C-O-C-and -C-O in oilfield produced water were involved in the formation of the EPS-iron protective layer. The metal-EPS system can inhibit corrosion by reducing the amount of electron acceptors at the interface by binding Fe(II) and Fe(III) (Finkenstadt et al. 2011). The carboxyl groups of polysaccharides in EPS contain -C-O and -C = O-bonds, which are complex with metal ions such as Fe(II/III) to form a dense protective layer (Ghafari et al. 2013).
Both the amino acid -COO antisymmetric stretching in 1543 ~ 1442 cm −1 and the amino acid -COO symmetric stretching with ionic groups in 1411 ~ 1340 cm −1 (Wang et al. 2014) after adding P. fluorescens FSYZ01 significantly shifted or merged into strong vibrational peaks, indicating that -COO acts as a major role in the formation of EPS-iron protective layer and the adsorption of the protein to the carbon steel surface. Carboxylic acid in EPS is important for corrosion resistance of carbon steel (Jin et al. 2014). Functional groups such as carboxyl and hydroxyl groups in proteins contain solitary electrons and easily reacted with metal ions (Liu et al. 2017), which strengthened the chemical adsorption of EPS on the surface of carbon steel.
There are enough amino acids sufficient in LB medium, such as aspartate (C 4 H 7 NO 4 , 5.1%) (Zerfaoui et al. 2004) and glutamate (C 5 H 9 NO 4 , 10.6%) (Kumar et al. 2020;Ziemba et al. 2019), which can be spontaneously adsorbed on metal surface with strong and stable chemical bonds. Apart from physical electrostatic interactions to form adsorbed molecular layer, adsorption modes for amino acids on metal surface have (1) "donor-acceptor" interactions of heterocyclic ring (Amin et al. 2010;Ashassi-Sorkhabi et al. 2004;Gong et al. 2019), (2) role of functional groups, and (3) role of hydrogen bond (Amin et al. 2009). Indeed, hydrogen bond will mainly work in the presence of the oxide film and corrosion intermediates on the substrate (Qiu et al. 2017). The peaks in both amino acid -COO symmetric stretching in 1411 ~ 1340 cm −1 and antisymmetric stretching and in 1543 ~ 1442 cm −1 for LB medium with or without bacteria are wider and stronger, which indicates the carbon steel in LB medium is less affected by corrosion. Additionally, weight loss experiment has proved the corrosion rate of carbon steel in the LB medium with bacteria was low, and this result means the amino acid inhibitor in LB medium was better than the EPS-iron complex protector in oilfield produced water with P. fluorescens FSYZ01.
In short, EPS produced by P. fluorescens FSYZ01 contains related functional groups such as -C = O, C-O, -COO, and C-OH that can react with metal Fe (chemical adsorption), and the viscosity of EPS adhered corrosion products simultaneously; hence, the surface of Q235B carbon steel formed a protective layer of chelate-oxide barriers which reduced the damage of corrosion media to carbon steel. EPS attachment to the material surface controlled biocorrosion by bonding with the binding sites at the metal surface which would otherwise be available for the attachment of corrosive bacteria (Saleem Khan et al. 2019). Moreover, LB medium rich in amino acids can form protective layer which more stable and denser environment. It is a barrier that prevents oxygen from approaching the metal surface to delay metal corrosion.

XPS
In order to further corroborate chemical state of corrosion products, we studied XPS for corrosion products on Q235B carbon steel immersed in different environments. C (1 s) XPS scan spectra are shown in Fig. S2. The C 1 s peak was mainly resolved into three component peaks: C-C or C-H, C-O, and O-C = O. Furthermore, the peak at 284.73 eV in water group, the peak at 284.78 eV in CK group, the peak at 284.70 eV in LB medium, the peak at 284.53 eV in LB medium with P. fluorescens FSYZ01, the peak at 284.82 eV in oilfield produced water, the peak at 284.75 eV in oilfield produced water with bacteria, these peaks all corresponding to the C-C(C-H) from lipids or amino acid side chains (Li et al. 2019b). And the peak at 286.13 eV in water group, 286.96 eV in CK group, 286.09 eV in oilfield produced water, and 286.44 eV in oilfield produced water with bacteria correspond to C-OH. The peak at 288.45 eV in water group, 288.15 eV in CK group, 287.39 eV in LB medium, 288.07 eV in LB medium with bacteria, 287.50 eV in oilfield produced water, and 288.03 eV in oilfield produced water with P. fluorescens FSYZ01 correspond to the O-C = O from carboxylic acid, carboxylate, or ester. In addition, the peak at 285.97 eV in LB medium and 285.57 eV after bacterial inoculation was assigned to C-N bonds (Durainatarajan et al. 2018).
These results indicate that EPS-iron complex can be adsorbed on the surface of carbon steel through C-C or C-H, C-O and C = O bonds in oilfield produced water, whereas amino acid protector layer in LB medium via C-N bonds, C-C, or C-H and C = O bonds. C-N bonds show the role of amino acids for corrosion inhibitory effect in LB media, which confirms FT-IR results. The addition of the bacteria acted as a barrier with corrosion scales blocking oxygen from approaching the metal surface.

Py-GCMS
In order to study the degradation of P. fluorescens FSYZ01 on the organic matter in oilfield produced water and LB medium, Py-GCMS was used to analyze the corrosion products (Fig. S3).
Comparing the components of the corrosion products after pyrolysis of LB medium before and after inoculation, the content of heterocyclics decreased after inoculation, indicating that P. fluorescens FSYZ01 degrades the protein in LB medium. Comparing the pyrolysis components of the oilfield produced water corrosion products before and after inoculation and the pyrolysis components of CK, it was found that the content of alkanes decreased and the content of both heterocyclics and aromatics increased, indicating the alkanes were degraded by the bacteria, and the increased heterocyclics and aromatics may come from pyrolysis of the bacterial themselves.
After pyrolysis of corrosion products on Q235B carbon steel immersed in oilfield produced water, from the carbon number distribution of alkanes, we can find that the corrosion product from oilfield produced water without bacteria released short even chained alkanes in the C 12 -C 20 range. In the presence of P. fluorescens FSYZ01, a series of long chained alkanes in the C 22 -C 32 range were observed. A small amount (3.6%) of oxidation products such as ketones was released as well as nitrogen containing compounds and aromatics, and alkanes in the C 12 -C 20 range were greatly reduced. Combined with results of percentage corrosion products, the addition of P. fluorescens FSYZ01 reduced the content of aliphatic compounds in oilfield produced water by half, demonstrating that P. fluorescens FSYZ01 has good degradability of alkanes in the C 12 -C 20 range in oilfield produced water.

Discussion
The Q235B carbon steel corrosion inhibition mechanism of P. fluorescens FSYZ01 is shown in Fig. 7. The mechanism of corrosion inhibition is speculated as follows: (1) Before EPS-corrosion products layer works: the corrosion evolution in the initial phase tightly depends on the composition of the rust (Wang et al. 2021). The steel surface can be covered with corrosion scales upon contact with solution. These corrosion products are composed of a mixture of iron oxides and hydroxides (Qiu et al. 2017). The first corrosion product of carbon steel in the water environment is γ-FeOOH, which will con- Fig. 7 The mechanism of P. fluorescens FSYZ01 inhibited carbon steel corrosion vert to Fe 3 O 4 and α-FeOOH, while the Fe 3 O 4 process is the main reaction process. (2) Respiration: P. fluorescens FSYZ01 is an aerobic bacterium. Bacteria adhere to the surface of the metal material and reduce the oxygen concentration on the surface of the material by aerobic respiration. (3) Products secreted by the bacteria decrease contact of corrosive agents (O 2 , H + , etc.) with metal surface. (4) Coordination bond formation with the metal surface: After corrosion products cover the surface of the carbon steel, -C = O, C-O, -COO, and C-H bonds or functional groups react with Fe(II/III) of the carbon steel to form a dense and stable chelate-oxide protective layer. Due to the stability of Fe 3+ , the chelateoxide protective layer firmly adheres to the surface of the carbon steel, and corrosive ions such as oxygen in the water are blocked from reaching the surface of the carbon steel, thereby inhibiting the corrosion of the carbon steel. At the same time, due to the reduction of oxygen, the corrosion crystals are not easily converted, and the structure of the fouling layer is more stable and compact, thereby slowing down the corrosion rate of carbon steel. Thus, the biofilm developed on samples acts as a barrier against corrosion.

Conclusions
P. fluorescens FSYZ01 what we isolated has dual functions that both inhibits carbon steel corrosion and degrades oil in oilfield produced water. Weight loss of samples for 13 days in different liquid mediums with bacteria was reduced by 32.23%, 54.07%, and 78.34%, respectively. I corr is lower and impedance is higher than the abiotic system. The main corrosion inhibition mechanism of P. fluorescens FSYZ01 on carbon steel was a synergistic effect of corrosion products and secreted EPS, these substances with metal surface to form chelate-oxide barriers. In-depth study microbial anticorrosion mechanism of Pseudomonas with dual functions is conducive to synergistically solve various common problems in oilfield produced water system.