Effects of cathodic protection potential on microbiologically induced corrosion behavior of X70 steel in a near-neutral pH solution

Sulfate reducing bacteria (SRB) are considered as one of the main causes for the failures of buried metal pipes. Although many researchers reported that more negative cathodic protection potential was required in environments containing SRB, SRB would increase the concentration of hydrogen adsorbed on steel surface and thus lead to hydrogen embrittlement. In the study, the optimum cathodic protection (CP) potentials of X70 steel in bacterial and sterile media were evaluated with electrochemical impedance spectroscopy. The morphology and composition of corrosion products were characterized by a scanning electron microscope (SEM), an energy dispersion x-ray spectrometer (EDS), and an x-ray photoelectron spectrometer (XPS). The corrosion morphology of X70 steel in NS4 medium was pits and the corrosion in the bacterial medium was more serious than that in the sterile medium. The corrosion products of X70 steel were FeOOH and Fe2O3 in the sterile medium, whereas its corrosion products in the bacterial medium were FeOOH and FeS. When CP potential was −775 mV, SRB growth was promoted and the optimal protection effect on X70 steel was achieved in the bacterial NS4 medium. Pits were still observed under the biofilm and the corresponding corrosion mechanism was extracellular electron transfer (EET). When CP potential was −875 mV, X70 steel realized the optimal protection in the sterile NS4 solution. However, CO2 hydrolysis and SRB metabolism in the bacterial medium resulted in hydrogen-induced pits. When CP potential was −1025 mV, the growth of SRB was inhibited and severe hydrogen evolution corrosion occurred on X70 steel in bacterial and sterile NS4 media. The optimal CP potential for pipeline steel in the sterile medium may lead to hydrogen corrosion in the bacterial medium when H+ concentration was high.


Introduction
Microbiologically induced corrosion (MIC) is one of the main failure modes of buried oil and gas pipelines. About 20%-30% of external corrosion failures of buried oil and gas transmission pipelines are caused by MIC [1]. Bacteria lead to pipe MIC in the forms of pits, crevice corrosion, and stress corrosion cracking [2][3][4]. Cathodic protection, coating and corrosion inhibitors are the most common methods to prevent buried pipeline corrosion. The effectiveness of CP depends on the range of the potential applied onto a pipeline [5][6][7][8]. According to the international engineering design standard [9], the minimum CP potential applied on pipelines is −850 mV (CSE). However, in the presence of microorganisms, minimum CP potential cannot completely inhibit the corrosion on metal surface. In some standards such as NACE RP0169-02 and DNV-RP-B401-05, the minimum CP potential of −0.950 V (CSE) is recommended in steel pipes in the presence of microorganisms. Some researchers reported that the cathodic potential of −0.950 V (CSE) could not provide effective protection for steel pipelines [10] and some scholars even proposed that the CP potential of −1.35 V (CSE) achieved the ideal protection effect [11]. However, in some studies [12][13][14][15], the biofilm promoted hydrogen absorption in high-strength steel under the synergistic action of cathode polarization and SRB and greatly increased the hydrogen embrittlement sensitivity of steel. In addition, excessive CP for the purpose of realizing zero corrosion inevitably led to hydrogen evolution rapidly, which resulted in hydrogen embrittlement of pipeline steel.
CP affects the metabolism of SRB [15][16][17]. The metabolic activity and metabolites of cells affect the electrochemical corrosion process of material surface and then directly or indirectly participate in the corrosion process of pipeline steel. In addition, SRB changes the corrosion environment of material surface by generating a biofilm, which in turn affects CP effect. Sun et al [18] studied the effect of SRB on the cathodic protection on Q235 steel in soils and found that with the negative shift of potential, the number of SRB decreased and that the cathodic protection efficiency increased. Saravia et al [19] also found that when CP was applied in the initial formation stage of biofilm, the number of sessile bacteria on steel surface decreased. However, Shirtliff et al [20] applied a cathodic current on a stainless steel electrode and found that the current even increased the number of bacteria. Guan et al [21] found that the direct electron transfer from an electrode enhanced the metabolic activity of SRB under the SCE potential of −0.85 V, but the too high negative CP potential led to the conversion of sulfide rust into carbonate rust at the SCE potential of −1.05 V because it inhibited the direct electron transfer and thus reduced the number of SRB. Lv et al [22] demonstrated that SRB activity was promoted under the CP potential of −0.80 V (SCE), but CP potential of −1.05 V (SCE) effectively inhibited SRB-induced pits. Teng et al [23] found that the reproduction of SRB was promoted under CP potential of −0.85 V (SCE), but the death of bacteria was accelerated under the CP potential of −1.0 V (SCE). Olivares et al [24] also investigated the interaction between cathode polarization and biofilm formation and found that the number of SRB under CP potential of −0.80 V (SCE) was two times of that under OCP potential. Liu et al [25] argued that CP had little effect on the growth of planktic bacteria, but CP promoted the adhesion of bacteria to steel and the formation of a layer of biofilm on the cathodic protection steel and thus decreased CP effect. When CP further negatively shifted to −1.0 V (SCE), uniform corrosion was effectively curbed, but pits were still inevitable. In summary, the interaction mechanism between CP and microbial activity was controversial and consistent corrosion features were seldom reported in different studies.
In addition, the interaction mechanism between CP and MIC has not been well elaborated. At present, the proposed interaction mechanisms between CP and MIC mainly involve electrostatic repulsion [26], redox products [27], and direct utilization of CP electrons by SRB [28,29]. Wilson et al [30] proposed that bacterial surface was usually electronegative and that the electrostatic force generated by the external cathode current stimulated the desorption of sessile bacteria and thus mitigated MIC. According to the redox product mechanism, partial reduction of oxygen caused by CP led to the production of H 2 O 2 , which could prevent the growth of biofilm on metal surface [27,31]. CP increased the concentration of OH-and interfacial pH [17] and resulted in the precipitation of CaCO 3 and Mg(OH) 2 on metal surface [32], the decrease in the concentrations of Ca 2+ and Mg 2+ , and the inhibition on bacterial proliferation [33] However, some researchers reported [25,34] that if CP potential was negative enough, SRB would directly utilize electrons from CP as electron donors and trigger pits. Although CP has been widely accepted as an effective means to prevent MIC, the effect of CP on microbial metabolism and the interaction mechanism between CP and MIC are obscure and relevant reports are also controversial.
The effect of CP potential on SRB metabolism in a near-neutral pH solution (NS4), a typical electrolyte trapped under a disbonded coating [35], was investigated in the study. The morphology and composition of corrosion products of X70 pipeline steel in bacterial and sterile near-neutral pH media were characterized by SEM, EDS, and XPS. The optimal CP potential range of X70 steel in bacterial and sterile media was evaluated with electrochemical impedance spectroscopy. The mechanism of SRB corrosion of X70 steel in NS4 solution under different CP potentials was discussed. Although many researchers reported that more negative cathodic protection potential was required in environments containing SRB, it was found that the optimal CP potential for sterile system might lead to hydrogen corrosion in bacterial systems when H + concentration in the solution was high. The results are significant for mitigating and preventing MIC.

Materials and solutions
The experimental material was X70 pipeline steel (table 1). X70 steel was used in China's The First West-East Gas Pipeline because of its high strength, high toughness, and good welding performance. X70 steel was cut into specimens with a size of 10 × 10 × 2 mm. The back of the specimens was spot-welded with Cu wire and sealed into epoxy resin so that a working surface with a size of 10 mm × 10 mm was reserved. The working surface of specimens was polished with coarse-grit to fine-grit wet sandpaper (80 grit to 2000 Grit), so that the surface was smooth without obvious scratch. Then, specimen surface was wiped with acetone and anhydrous ethanol to remove oil, cleaned with deionized water, and dried. All the specimens were placed in a glove box and disinfected with ultraviolet light for subsequent experiments.
The experimental solution was a near-neutral pH solution, NS4 solution composed of 0.137 g l −1 CaCl 2 , 0.122 g l −1 KCl, 0.131 g l −1 MgSO 4 •7H 2 O, and 0.483 g l −1 NaHCO 3 . Before the experiment, the solution was sterilized at 121°C for 15 min in a pressurized steam sterilizer. After the solution was cooled to 30°C, the mixture gas of 5% CO 2 and 95% N 2 (vol%) was injected into the solution for 2 h to remove oxygen in the solution. The pH of the solution was adjusted to 7.0 ± 0.2 with 5% (mass fraction) NaOH or 10% (mass fraction) acetic acid and stored at 4°C.

2.2.
Strains, media, and experimental procedures SRB (Desulfovibrio desulfuricans 1.5189) was purchased from China General Microbiological Culture Collection Center (CGMCC) and the medium recommended by CGMCC was used in this study. Desulfovibrio desulfuricans was cultured in the medium for three generations and a third-generation strain with a high activity was selected for the experiment. The recommended medium was prepared with the following two media: . Medium A was sterilized in a pressurized steam sterilizer at 121°C for 15 min After cooling, the pH of Medium A was adjusted to 7.0 ± 0.1 with 5% (mass fraction) NaOH solution in a sterile operation box. Then, Medium B was added after being filtered with a cylindrical filter and treated with ultraviolet sterilization. The sterilized NS4 solution was used as the sterile experimental medium. Media A and B and sterilized NS4 solution were mixed at a ratio of 1:1:2 to prepare a mixed solution, which was then mixed with SRB at a ratio of 1:50 for inoculation experiments [36]. The solution temperature was kept at 30 ± 1°C in the experiment.

SRB growth curves
The growth curves of SRB under different CP conditions in 14 days were measured with an UV-2550 ultraviolet spectrophotometer according to optical density (OD) method. The concentration of bacteria is inversely proportional to light absorbance [37], so the curve of bacterial concentration, namely, growth curve, could be drawn with measured OD values.
The most probable number (MPN) method was used to measure the number of plankton SRB at corresponding time when maximum OD value was measured. The measurement was carried out according to GB/T 14643. . The number of plankton SRB was reported in cells/mL.

Surface morphology and composition characterization
X70 steel specimens were soaked in bacterial and sterile media under different CP potentials (OPC, -−775 mV/ SCE, −875 mV/SCE, and −1025 mV/SCE) for 4 days and then taken out. The specimens taken out from the sterile solution were gently rinsed with deionized water and then dried. The specimens taken out from the bacterial medium were firstly fixed on the surface with 5% glutaraldehyde for 4 h, and then respectively dehydrated with 30%, 50%, 70%, and 100% alcohol. Finally, the specimens were taken out and dried. The morphology of corrosion products was characterized by a scanning electron microscope (SEM, SU-8010). The composition of corrosion products was determined by an energy dispersion x-ray spectrometer (EDS, Q500MW) and an x-ray photoelectron spectrometer (XPS, hermoFisher K-Alpha) under the following conditions: an Al-Kα x-ray emission source with an incident energy of 2 keV, the sputtering size of Φ2 mm, and the sputtering rate of 0.1 nm s −1 .
Corrosion products and biofilms were removed with a rust remover (500 ml of hydrochloric acid (36% wt%), 3.5 g hexamethylenetetramine, and 500 ml of deionized water) and then pits were observed under the same scanning electron microscope.
2.5. Electrochemical impedance spectroscopy X70 steel specimens were immersed for 4 days under different CP potentials (OCP, −775 mV/SCE, −875 mV/ SCE, and −1025 mV/SCE) in bacterial and sterile media for electrochemical impedance spectroscopy (EIS) measurements. The EIS was measured on the PAR STAT2273 electrochemical workstation. In the used classic three-electrode system, platinum electrode and saturated calomel electrode (SCE) were respectively used as Hz and the amplitude of sinusoidal AC voltage signal was 10 mV. The measurement was repeated three times to obtain the best reproducible results. EIS results were fitted and analyzed with ZSimWin software. In this paper, the electrode potential was relative to saturated calomel electrode.

SRB growth curves under different CP potentials
SRB growth curves under different CP potentials are shown in figure 1. Under the open circuit potential (OCP, −675 mV) and CP potentials of −775 mV and −875 mV, the obtained SRB growth curves can be roughly divided into two phases. In the first five days, OD value rose rapidly, indicating that SRB was in logarithmic growth phase. In subsequent 8 days, OD value decreased rapidly, indicating that SRB entered death phase. Under the CP potential of −1025 mV, SRB growth curve was divided into three phases: logarithmic growth phase (Day 1 to Day 3), stable growth phase (Day 4 to Day 9), and death phase (Day 10 to Day 14). In addition, due to the negative shift of CP potential, maximum OD value increased firstly and then decreased. Maximum OD value was reached under CP potential of −775 mV. MPN method was used to measure the number of plankton SRB corresponding to maximum OD value under different CP potentials (figure 2). Due to the negative shift of CP potential, the number of SRB increased firstly and then decreased. The maximum number of SRB reached 1.8 × 10 5 cells/mL under CP potential of −775 mV. The numbers of SRB under CP potentials of −775 mV and −850 mV were higher than that under OCP. Under CP potential of −1025 mV, the number of bacteria decreased significantly to 7.9 × 10 4 cells/mL, indicating that SRB could still survive. SRB growth was promoted within a certain CP potential range (−775 mV to −825 mV), but it was inhibited when CP potential was excessively negative (−1025 mV). Figure 3 shows the SEM images of X70 steel with/without removing corrosion products after soaking for 4 days in sterile NS4 solution under different CP potentials. Red boxes in figure 3 represent EDS detection positions and corresponding EDS results are provided in table 2. The corrosion morphology of X70 steel in sterile NS4 solution under different CP potentials showed significant differences. When there was no applied potential (OCP), corrosion products were thick and plate-like and many cracks and the large-area spalling phenomenon were observed ( figure 3(a1)). Large and deep corrosion pits were found after removing corrosion products. (figure 3(a2)). When CP potential was −775 mV ( figure 3(b1)), surface corrosion products presented small scattered clusters, which were significantly reduced compared with those under OCP. After the removal of corrosion products ( figure 3(b2)), the specimen surface roughness increased and shallow and wide corrosion pits were observed. When CP potential was −875 mV ( figure 3(c1)), there were few corrosion products on the specimen surface. After the removal of corrosion products ( figure 3(c2)), mechanical scratches and small pits were clearly observed. When CP potential was −1025 mV ( figure 3(d1)), rod-like crystalline particles appeared on the surface, but no corrosion product was observed. After the removal of corrosion products ( figure 3(d2)), there were a large number of white spots on the specimen surface, namely, hydrogen bubbling. The results showed that the potential of −1025 mV yielded over-protection and resulted in severe hydrogen evolution. In addition, a large area of small and dense pits were observed. Figure 4 shows the SEM images of X70 steel with/without removing corrosion products after soaking for 4 days in the bacterial NS4 medium under different CP potentials and the corresponding EDS results are also provided in table 2. Compared with the sterile environment, the bacterial medium increased significantly corrosion products on steel surface under the same CP potential. Under OCP ( figure 4(a1)), the corrosion product film was thick, but it was not bound closely with the substrate and fell off, thus exposing large pits. After the removal of corrosion products ( figure 4(a2)), the specimen experienced severe uneven general corrosion with large and deep pits. When CP potential was −775 mV ( figure 4(b1)), the corrosion product film was thicker and many small cracks were distributed on metal surface. After the removal of corrosion products (figure 4(b2)), the corrosion was slight and shallow and wide pits were observed. When CP potential was −875 mV, corrosion products appeared as bubbling protrusions with cracks. (figure 4(c1)). After the removal of corrosion products ( figure 4(c2)), a large number of small and shallow pits were observed to be widely distributed on specimen surface. When CP potential was −1025 mV (figure 4(d1)), corrosion products were thick and plate-like and large cracking bubbles were observed. After the removal of corrosion products (figure 4(d2)), pits were found and the pit size was larger and deeper than that under CP potential of −875 mV. In addition, a large number of small pits were observed.

Morphology and composition of corrosion products
EDS results of corrosion products of X70 steel in bacterial and sterile NS4 media under different CP potentials are provided in table 2. In the sterile NS4 solution, corrosion products were mainly oxides and carbides of Fe in the CP potential range of OCP to −875 mV. Under CP potential of −1025 mV, Ca and Mg particles were deposited. In the bacterial medium, in addition to Fe oxides and carbides, P and S elements were detected in corrosion products. P was derived from phosphorus compounds in the medium and S was produced by SRB metabolism.
In order to further determine the composition of corrosion products, XPS analysis was carried out. XPSpeak4.1 software was used to fit XPS spectra. With the obtained binding energy, the composition of corrosion products in bacterial and sterile media under different cathodic protection potentials was determined. Figure 5 shows the XPS spectra of Fe 2p3/2 on specimen surface soaked in the sterile solution under different CP potentials. Corrosion products detected under all CP potentials except −875 mV were FeOOH (711.3 eV) and Fe 2 O 3 (709.8 eV) [38]. Under CP potential of −875 mV, in addition to FeOOH and Fe 2 O 3 , FeO (709.0 eV) and Fe 0 (706.9 eV) [37,38] were also detected in corrosion products. Figure 6 shows the XPS spectra of Fe 2p3/2 on the specimen surface soaked in the bacterial medium under different CP potentials. Corrosion products detected under all CP potentials except −875 mV were FeOOH, Fe 2 O 3 , and FeS (713.6 eV) [38,39]. Under CP potential of −875 mV, corrosion products were FeOOH and FeS. Figure 7 shows the EIS results of X70 steel specimens soaked in the sterile NS4 solution under different CP potentials. In sterile solution, the Nyquist plots under all CP potentials showed a single capacitive loop. Corrosion product film and electrode potential affected electrochemical reactions. The radius of capacitive loop increased firstly and then decreased due to the negative shift of the applied CP potential. The radius of capacitive loop indicates the resistance and substrate protection performance of the corrosion product film. Generally, the larger the radius is, the better the corrosion resistance of the electrode system is. When CP potential was −875 mV, the radius of capacitive loop was the largest, indicating the best corrosion resistance. Therefore, the potential of −875 mV was the optimal CP potential of X70 steel in the sterile NS4 solution. When the potential was −1025 mV, the radius of capacitive loop was the smallest, indicating that a new corrosion reaction, namely, hydrogen evolution reaction, occurred on metal surface. In Bode plots ( figure 7(b)), phase angles are all less than 50°, suggesting a low coverage of corrosion products on electrode surface [40]. In addition, Bode plots involve two time constants. The time constant in the high frequency region was ascribed to the formation of the corrosion product film, whereas the time constant in the low frequency region was ascribed to electrochemical reactions at the interface [41]. Figure 8 shows the EIS results of X70 steel in the bacterial NS4 medium under different CP potentials. In the presence of bacteria, the Nyquist plot also presented a single loop, but the impedance spectroscopy in the bacterial medium was different from that in the sterile solution. The radius of capacitive loop was the largest under CP potential of −775 mV and the smallest under CP potential of −1025 mV, suggesting that the optimal CP potential of X70 steel in the bacterial NS4 medium was −775 mV and that the corrosion resistance of X70 steel was the worst under CP potential of −1025 mV. The impedance magnitude in the bacterial medium was larger than that in the sterile solution (figures 7 and 8), indicating that the corrosion product film coverage on specimen in the bacterial medium was larger.

EIS results
The equivalent circuit in figure 9 was used to fit the EIS data in both bacterial and sterile media. In the equivalent circuit, R s represents the solution resistance; Q f and R f respectively represent the capacitance and resistance associated with the corrosion product film/biofilm; Q dl and R ct respectively represent the capacitance of double electrical layer and charge transfer resistance. The fitting results are shown in table 3. In the sterile solution, R ct reached its maximum value when CP potential was −775 mV. In the presence of bacteria, R ct reached the maximum value when CP potential was −875 mV. The larger the R ct was, the lower the corrosion reaction rate was. Due to the further negative shift of potential, R ct decreased, indicating that the cathode hydrogen evolution reaction was promoted. Figure 10 shows R ct under different potentials in bacteria and sterile solutions. Under the conditions of OCP and CP potential of −775 mV, R ct in the bacterial medium was larger than those in the sterile solution, indicating that the biofilm/corrosion product film in the bacterial medium had the better protection effect. When CP potential was −875 mV or −1025 mV, R ct in the bacterial medium was smaller than that in the sterile solution, indicating that the presence of SRB reduced the corrosion resistance of X70 steel.

Effect of CP on the corrosion behavior of X70 steel in sterile NS4 solution
Before the experiment, the mixture gas of 5% CO 2 and 95% N 2 (vol%) was injected into NS4 solution. When CO 2 is dissolved in water, equilibrium reactions are provided as follows [42]: In the electrochemical corrosion process, anode dissolution and cathode depolarization occurred simultaneously. In the absence of CP, the corrosion of X70 steel in NS4 solution proceeded under self-corrosion potential and the corrosion process was only affected by the solution environment. The following electrochemical reactions occurred on the surface of X70 steel: Anode reactions:         EDS results revealed the deposition of magnesium and calcium. Due to low CO 2 concentration and low temperature, the quantity of FeCO 3 generated was small and FeCO 3 film could not be formed on the surface of X70 steel. The XPS results further proved that the surface products of X70 steel were FeOOH and Fe 2 O 3 . When CP potential was applied, anodic dissolution of X70 was inhibited and the electrochemical reaction at the interface between steel and solution changed accordingly. According to the mixed potential theory, the charge transfer resistance R ct of the electrode under the mixed potential is expressed as [23]:

( )
where R cta and R ctc are respectively the charge transfer resistances of anode reaction and cathode reaction; R ct varies with CP potential. With the increase in CP degree, cathode reaction resistance R ctc decreases, but anode reaction resistance R cta increases. When R cta increases to a certain value, 1/R cta can be ignored so that R ct is fully determined by R ctc . CP can provide electrons to replace lost metal electrons for anode reaction, so the corrosion degree of metal is weakened and the metal protection effect is obvious [43]. Due to the negative shift of CP, the corrosion rate decreases and R ct reaches its extreme value. The optimum CP potential is close to that of the maximum value of R ct [44,45]. The minimum CP potential should be no higher than the potential corresponding to the maximum value of R ct , so that anodic dissolution is completely inhibited [46]. Therefore, the optimum CP potential can be determined by measuring a series of R ct under different potentials and the potential corresponding to the maximum value of R ct is the optimum CP potential. In theory, when the metal is polarized to its reversible potential of anodic dissolution, anodic dissolution and deposition are in dynamic equilibrium. In other words, anodic dissolution rate is equal to anodic deposition rate and there is no metal loss.
In electrochemical experiments, R ct reached its maximum value under CP potential of −775 mV in the sterile solution, indicating the maximum reaction resistance of anode. In SEM results, uniform corrosion still occurred on the surface of X70 steel, indicating that anode reaction was not completely inhibited. When CP potential was −875 mV, R ct increased significantly because the applied current met the condition of cathode reaction and anode reaction was completely suppressed. SEM results also showed that the uniform corrosion was completely inhibited under this potential. When CP potential was −1025 mV, R ct decreased sharply. White spots of hydrogen corrosion appeared, indicating that strong hydrogen evolution reactions occurred on the surface of X70 steel. Therefore, the optimal CP potential of X70 steel in the sterile NS4 solution was −875 mV. surface. Although SRB consumed some hydrogen through reactions (19) and (23), a large quantity of residual hydrogen exited on the surface of X70 steel under CP potential of −875 mV. Lunarska et al [15] and Zhu et al [55] reported that the hydrogen concentration on electrode surface in the solution containing SRB was 4-5 times higher than that in the sterile environment under the same CP potential. The biofilm prevented hydrogen diffusion and led to the preferential adsorption of hydrogen onto steel surface where the surface activity was higher. However, the positions of these active sites were unstable and their number and size gradually changed due to hydrogen adsorption and hydrogen permeation [57]. The structure and composition of X70 pipeline steel were relatively uniform so that corrosion pits were uniformly distributed under the biofilm.
When the potential negatively moved to −1025 mV, hydrogen evolution reaction was further intensified, thus leading to bubbling rupture. The rupture and falling of biofilm resulted in the decrease in the quantity of hydrogen adsorbed on steel surface. The increased pH on the steel surface under the cathodic current significantly decreased biofouling attachment [58]. It is widely believed that electronegative SRB can utilize cations such as Ca 2+ and Mg 2+ as the bridge to attach to metal surfaces [22]. The deposited Ca 2+ and Mg 2+ could not been utilized by bacteria. In addition, when FeS film on steel surface was incomplete, active corrosion cells between FeS film (cathode) and metal substrate (anode) accelerated the corrosion rate significantly. Therefore, smaller corrosion pits on X70 steel surface were caused by hydrogen evolution, whereas the larger pits were ascribed to unstable falling of FeS.

Conclusion
In NS4 solution, cathodic polarization enhancement firstly promoted SRB growth and then inhibited it. When CP potential was −775 mV, CP promoted the mitosis of SRB and the number of SRB reached its maximum value. When CP potential was −1025 mV, excessive current damaged SRB and inhibited its growth. At OCP, the corrosion degree of X70 steel in bacterial NS4 medium was much greater than that in sterile solution and MIC mechanism involved cathode depolarization and EET. When CP potential was −775 mV, the protection effect on X70 steel in bacterial NS4 medium was the best, but pits were still observed under the biofilm and MIC mechanism was EET. When CP potential was −875 mV, the protection effect on X70 steel in sterile NS4 solution X70 steel was the best. However, the hydrolysis of HCO 3 − and CO 2 and SRB metabolism in bacterial NS4 medium led to the increase in hydrogen evolution potential and hydrogen evolution reactions occurred on the surface of X70 steel, thus resulting in hydrogen-induced pits. When CP potential moved negatively to −1025 mV, hydrogen evolution reactions occurred in both bacterial and sterile NS4 media.