Corrosion of 907 Steel Influenced by Sulfate-Reducing Bacteria

Coupons were cut from a piece of 907 steel sheet with the composition (wt.%) of 0.120 C, 0.790 Si, 1.010 Mn, 0.007 S, 0.420 Cu, 0.050 Ti, 0.640 Cr, 0.670 Ni, 0.016 P, and balance Fe. Cylinders with a diameter of 5.0 mm and height of 5.0 mm were used for conventional electrochemical measurements and surface analysis, and they were embedded in epoxy resin to leave only one end surface (19.6 mm²) exposed to aggressive electrolyte after copper wires were soldered. A WBE was prepared by the assembly of 112 steel wires (diameter 1.5 mm) into an array of 11 × 11, and the distance between each wire was 1 mm (Fig. 1a).

Prior to electrochemical measurements, the electrode surface was sequentially abraded with a series of silicon carbide papers with grit sizes of 400, 600, 800, and 1200 #, cleaned by ethanol in ultrasonication bath, dried by nitrogen, and sterilized by ultraviolet for 30 min.

The bacterium identified as Desulfovibrio sp. was isolated from marine sludge in Bohai Sea of China. The modified Postgate’s culture solution was used as the culture medium, and it consisted of 2.0 g magnesium sulfate, 0.5 g dipotassium hydrogen phosphate, 1.0 g ammonium chloride, 0.5 g sodium sulfate, 0.1 g calcium chloride, 1.0 g yeast extract, and 4.0 ml sodium lactate per liter of seawater. Culture medium was autoclaved at 121 °C for 20 min. After cooling, SRB seed culture with the 4-day old was inoculated, and then electrodes and rubber stopper were assembled rapidly. Figure 1(b) displays the schematic diagram of WBE setup. All the vessels were kept at 30 °C in a thermostatic incubator.

The quantity of active SRB (N_SRB) was detected by the most probable number (MPN) method according to the American Society for Testing and Materials (ASTM) Standard D4412-84 (Ref 18).

The surface morphology of coupons exposed to media for diverse time was examined with a scanning electron microscopy (SEM, JSM-6700F; JEOL Ltd.; Tokyo, Japan). After the samples were taken out from the media and rinsed with sterilized natural seawater, they were immersed in a phosphate buffer solution containing 2.5% glutaraldehyde for 2 h to fix the biofilm. And then coupons were dehydrated successively with an ethanol gradient (15 min each): 30, 50, 70, 90, and 100%. Subsequently, they were dried at critical point, sputter-coated with gold, and subjected to SEM analysis. To observe the corrosion morphology beneath corrosion products and biofilm, the coupons were cleaned with the Clark’s solution (ASTM G1-03) before SEM characterization.

Chemical composition of 907 steel surface immersed in sterile and SRB-containing media for 13 days was obtained by x-ray photoelectron spectroscopy (XPS, Thermo ESCALAB 250, Waltham, MA, USA). The C 1 s hydrocarbon peak was calibrated at a binding energy of 285.0 eV. Peak fitting was performed using software XPS Peak-fit 4.1.

Open-circuit potential (E_OC) and EIS experiments were performed on a CHI604D station (CH Instruments, Inc.; Texas, USA) with a three-electrode system, in which an Ag/AgCl (KCl-sat.) and a platinum wire were used as reference and counter electrodes, respectively. EIS studies were carried out at E_OC using a 5 mV amplitude sinusoidal signal with a frequency range of 10⁻² to 10⁵ Hz, and the data were fitted with ZSimpWin software.

Similar to those reported in the literature, the current distribution of WBE was measured using a test device (NI PXI-1042Q) consisting of NI PXI-8108 embedded controller and modular instruments: NI PXI-2535, PXI-4022, and PXI-4071 (Ref 16). After a WBE was immersed in media, all wire sensors were connected to allow electrons to move freely among wires. When current was recorded, each individual wire was separated temporarily from the wire array in sequence, and all the other wires were shorted together. The galvanic current between the temporarily separated wire and the residual wires was recorded by PXI-4071 and PXI-4022. All measurement processes were controlled via a self-designed program in LabVIEW environment. During the current measurements, the interval between two channels was 1 s. The current distribution maps were drawn with the Surfer 10.0 software.

Figure 2 displays the growth curve of SRB, which can be divided into three stages: an exponential phase, a death phase, and a residual phase. In the first 3 days, N_SRB increased exponentially, and then decayed quickly. Bacteria are highly active in the exponential stage (Ref 19-21), and there is a high rate of nutrient consumption and metabolite accumulation. For the present bath culture model, the consumption of nutrients and accumulation of metabolites restricted the further increase in SRB quantity and promoted their decay, which occurred from the third day. After the 11th day, the nutrients in the medium were almost exhausted, and the growth process reached the third stage. During the residual phase, N_SRB went back to the original level, and the active cells almost disappeared.

SEM images of 907 steel exposed to sterile and SRB-containing media for different times are shown in Fig. 3. In sterile medium, a small amount of corrosion products dispersed randomly on the surface at the time of 2 h, and scratch lines from the pretreatment were clearly observed. When the exposure time extended to 1 day, the morphology did not change significantly except for a slight increase in the number and size of corrosion product particles. Subsequently, a thin film began to cover the surface at the third day, and the scratch lines became blurred. After 13 days of immersion, a dense layer with several small humps was formed on the 907 steel surface.

Distinct from the samples in sterile medium, bacteria and their EPS accumulated gradually on the surface with increasing the exposure time in SRB-containing medium. Small loose aggregates mainly composed of EPS appeared on the surface after 2 h of immersion and quite few bacterial cells were observed. The size of aggregates increased when the exposure time rose to 1 day, and in the meanwhile, the quantity of attached SRB cells grew. EPS accumulated rapidly on the surface during the exponential stage from day 1 to day 3 due to the vigorous metabolic activity of SRB, which facilitated the adherence of bacterial cells. The attached SRB cells, in turn, benefited the further accumulation of EPS. As a result, 907 steel was totally covered by a thick film consisting of EPS and bacterial cells on the third day. Although the planktonic SRB decayed from the third day, there were still a few cells present in the film on the 13th day. Long-term survival of SRB on steel surface is in good agreement with the report by Chen et al. (Ref 22).

Figure 4 exhibits SEM images of 907 steel substrates after the removal of biofilm and corrosion products. After 13 days of immersion, samples in sterile medium suffered slight and uniform damage, while the corrosion of coupons in the medium containing SRB was worse and severe pits with different diameters were distributed unevenly on the surface. Consequently, the presence of SRB resulted in localized corrosion of 907 steel.

The presence of SRB altered the corrosion morphology of 907 steel, and their influence on the chemical composition of corrosion products was further investigated by XPS, and the results are displayed in Fig. 5. As shown, characteristic peaks for Fe, S, C, N, and O elements were observed on the wide spectra of coupons exposed to sterile and SRB-containing media. In comparison with the case in sterile medium, the proportion of element S in the sample from SRB-containing medium was much higher, indicating that sulfide from metabolism of SRB participated in the corrosion process of 907 steel.

Figure 6 depicts the high-resolution Fe 2p_3/2 and S 2p spectra of samples after 13 days of immersion in both media, and their deconvolution was achieved by the curve-fitting procedure using the Peak-Fit 4.1 software. The Fe 2p_3/2 spectrum in the sterile medium could be divided into three peaks at the position of 710.4, 711.8, and 713.0 eV, corresponding to FeO, Fe₃O₄, and FeSO₄, respectively (Ref 23-26). Fe₃O₄ generated via Schikorr reaction is common in the anaerobic corrosion of iron and carbon steel (Ref 27). The relative content of each component is summarized in Table 1, and the proportions of FeO, Fe₃O₄, and FeSO₄ were 60.02, 32.45, and 7.53%, respectively. Therefore, iron oxides predominated the corrosion products in sterile medium. The Fe 2p_3/2 spectrum in SRB-containing medium also consisted of three peaks, and the peak at 710.6 eV could be assigned to FeS, a typical corrosion product induced by SRB (Ref 28). Furthermore, the high proportion of FeS (46.91%) indicated that the metabolic activity of SRB resulted in a transformation from iron oxides to iron sulfides.

Figure 6(c) exhibits the high-resolution S 2p spectrum for the coupon in sterile medium. Peaks at 162.6 and 163.7 eV were ascribed to the adsorbed organic sulfur-containing compounds (Org-S) from culture solution, and the other at 167.2 eV corresponds to SO ₄ ²⁻ (Ref 29, 30). According to the result in Table 1, most sulfur was present in the form of Org-S (76.06%). When SRB cells were inoculated, besides the two peaks for Org-S, another two appeared at the position of 161.2 and 162.2 eV. These two peaks could be attributed to FeS (Ref 31), which is in accordance with the result displayed in Fig. 6(b). Meanwhile, FeS gave a proportion of 67.30% in the total sulfur.

Thus, the adherence of SRB on 907 steel surface altered the corrosion type and chemical composition of corrosion products, leading to severe localized corrosion and the generation of FeS.

Figure 7 illustrates the variation of E_OC with respect to time in sterile and SRB-containing media. It could be seen that the value of E_OC shifted positively at the beginning of the experiment, and then started to stabilize from the fourth day in sterile medium. The gradual accumulation of corrosion products on the surface hampered the diffusion of corrosive species toward the steel substrate, and accordingly protected the coupon against further corrosion. In SRB-containing medium, E_OC of 907 steel was more negative than that in sterile medium, and it varied with the growth states of SRB. In the exponential growth period (1-3 days), E_OC decreased rapidly and reached a minimum value. This was closely related to the sulfide from bacterial metabolism, which accelerated the metal dissolution process (Ref 32). There was a slight positive shift of E_OC during the death period, and the formation of biofilm on 907 steel played a key role in the ennoblement (Ref 33-35). E_OC keeps stable in the residual period, which might be associated with the reduction in the biological activity of bacteria and the deposition of corrosion products.

Figure 8 shows Nyquist plots of 907 steel in sterile and SRB-containing media for different times. In the sterile medium, there were only impedance loops in the plots, and the diameter increased with time. This demonstrated that the corrosion rate was substantially controlled by charge transfer process, and it decreased with time. In combination with the SEM results, the diameter increase in the Nyquist plot was considered to be caused by the gradual formation of a dense corrosion product film (Ref 36). In SRB-containing medium, diameters of impedance loops decreased in the exponential growth phase and increased slightly during the death and residual phases, implying that corrosion of 907 steel was accelerated during the exponential growth phase and inhibited in the other two phases.

The impedance spectra were further analyzed by fitting with proper equivalent circuits shown in Fig. 9. In the sterile medium, it was observed from SEM images that a uniform corrosion product film covered 907 steel surface gradually, and consequently a two-time constant model was adopted, which consisted of the corrosion product layer and the electrical double layer (Ref 37). In SRB-containing medium, a three-constant time model was selected based on the contribution of each phenomenon, such as the adsorption of biofilm, the formation of corrosion product film, and the presence of electrical double layer (Ref 16).

In these electrical analog circuits, Q, constant phase element (CPE), was applied instead of capacitance due to the dispersion effects from heterogeneous surfaces, and it is given by:where Y₀ is a parameter related to capacitance, ω is angular frequency, j is the imaginary number, and n is an exponential term associated with the roughness of electrode surface (Ref 38). Q_bf, Q_pf, and Q_dl are CPEs of biofilm, corrosion product film, and electrical double layer, respectively. R_s, R_bf, R_pf, and R_ct correspond to the resistance of electrolyte solution, biofilm, corrosion product film, and charge transfer, respectively.

According to Stern–Geary equation, R_ct is inversely proportional to the corrosion rate, and its variation with time in different media is shown in Fig. 10. R_ct increased with time in the sterile medium, which was attributed to the gradual formation of protective corrosion product film that reduced the corrosion rate of 907 steel. The value of R_ct in SRB-containing medium was larger than that in sterile medium on the first day of exposure, which might be ascribed to the protection from EPS (Ref 39). R_ct decreased sharply from day 1 to day 3, when SRB cells were in exponential growth phase and their quantity increased dramatically, and it was believed that the high concentration of corrosive metabolites (sulfide and organic acids) was responsible for the increased corrosion rate. After the slight change from day 3 to day 7, R_ct increased when the residual phase came, which could be ascribed to the low metabolic activity of SRB and the increase in the thickness of biofilm and corrosion product layer (Ref 40). EIS results confirmed the important role of SRB in the corrosion of 907 steel again, and the influence was metabolic activity-dependent.

Corrosion morphology revealed in Fig. 4 demonstrated localized corrosion induced by SRB, and the WBE technique was further employed to give deeper comprehension of the localized corrosion process. Figure 11 and 12 depicts the temporal evolution of current distribution maps of WBE in sterile and SRB-containing media, respectively. In the sterile medium, the maximum current density (i_max) was 9.222 μA cm⁻² after 1 day of immersion, and it decreased to 3.755 μA cm⁻² at the time of day 3. Subsequently, the values of i_max were smaller than 1.000 μA cm⁻², and decreased gradually. When the fresh 907 steel WBE was introduced into the electrolyte, it was susceptible to corrosion attack since it was naked without any protection, resulting in a high i_max. Corrosion products generated and accumulated with time on the electrode surface, retarding the corrosion process, and lower i_max was expected. Furthermore, the distribution of anodic and cathodic sites changed with time. At the time of day 1, anodes concentrated on the area with the corners of (2, 9), (6, 9), (6, 7), and (1, 6), and these sites behaved as cathodes at day 3 with the new typical anodes at locations of (11, 10), (11, 8), and (1, 1). The initial anodic sites might be due to their lower activation energy from material feature and polishing treatment, and their polarity reversal was closely related to the accumulation of corrosion products. After day 5, sporadic cathodic sites among large anodic area turned to anodes gradually [such as sites of (8, 4) and (8, 8)], and cathodes concentrated on limited sites, demonstrating the development of stable galvanic corrosion.

Different from that in sterile medium, i_max was just 0.9234 μA cm⁻² at the first day, and increased to 1.378 μA cm⁻² at the time of day 3. Subsequently, it stabilized at ca. 0.7 μA cm⁻² during 5-9 days, and decreased to ca. 0.6 μA cm⁻² at the end of the test. This evolution trend was also positively correlated with the growth curve of SRB. Similar to the case without SRB, the distribution pattern of anodic and cathodic sites changed greatly before day 3, and then stabilized. The initial anodic current peaks located at the position of (10, 1) and (2, 1) vanished after 3 days of exposure, and new peaks with the position of (8, 5) and (5, 7) appeared on the fifth day in the death period, which could still be observed in the residual period. This demonstrated that the electrochemically active sites turned up at the beginning were not stable, and some stable sites formed gradually with the immersion time extended. The heterogeneous biofilm composed of SRB bacterial cells and their metabolites caused local gradient differences and the enlargement of active sites (Ref 41), resulting in the localized corrosion of 907 steel.

To clarify the relationship between localized corrosion of 907 steel and metabolic activity of SRB, the variations of the ratio between cathodic and anodic areas (r) as a function of immersion time are presented in Fig. 13. Different from the case in sterile medium, where r decreased in the first 7 days and then kept stable, r in SRB-containing medium was evidently related to SRB growth as it increased in the exponential growth period, decreased in the death period, and maintained stable in the residual period. Besides, r in SRB-containing medium was much higher than that in sterile medium from the third day, suggesting that there was a larger cathodic area and a smaller anodic area on the current distribution map of 907 steel, which benefited the development of localized corrosion. According to the classical cathodic depolarization theory (Ref 3), hydrogen evolution reaction is promoted by SRB via the utilization of atomic hydrogen, and the efficiency of cathodic depolarization highly depends on the activity of SRB. During the exponential phase, cathodic hydrogen was consumed rapidly owing to the vigorous growth and high metabolic activity, thereby promoting cathodic reaction and leading to the appearance of a large cathodic area in the current distribution map. In death and residual phases, the metabolic activity of SRB toward cathodic hydrogen utilization was reduced, and accordingly, the cathodic depolarization was weakened. These results are in good agreement with those from SEM, and the localized corrosion of 907 steel induced by SRB is confirmed.