1 Introduction

316L stainless steel (316LSS) is known for its excellent corrosion resistance and has been widely used in marine engineering (Ref 1). But 316LSS tends to suffer from local corrosion in seawater environment, especially crevice corrosion, which often causes unexpected harm. Compared with the open environment, the local micro-environment within the crevice is more unpredictable due to complex influencing factors, such as the geometric size of the crevice, pH value, temperature, and oxygen concentration (Ref 2,3,4,5). However, previous research has hardly paid attention to the influence of microbial factors on crevice corrosion, which cannot be ignored in the actual seawater environment.

In recent years, microbiologically influenced corrosion has been extensively studied in the open environment. As the most widespread corrosive bacteria, sulfate-reducing bacteria (SRB) has been widely reported (Ref 6,7,8,9,10,11). The polarization resistance of 316LSS immersed in the sterile solution was considerably higher than that immersed in the SRB-inoculated solution (Ref 12). Furthermore, it had been reported that stainless steel had a significantly higher corrosion rate in the SRB-inoculated solution due to the corrosive sulfide produced by the SRB (Ref 13,14,15). Several corrosion mechanisms have been proposed, such as the cathodic depolarization theory, the biogenic hydrogen sulfide theory, and the biocatalytic cathodic sulfate reduction theory, greatly advancing comprehension of SRB-induced corrosion (Ref 16,17,18,19,20). The oxygen-free environment within the crevice may be more conducive to the survival of anaerobic bacteria such as SRB. SRB is undoubtedly the best choice for studying the influence of microbial factors on crevice corrosion.

The wire beam electrode (WBE) technology can be used to measure the distribution and changing process of electrochemical information of the electrode surface (Ref 21). The application of WBE in localized corrosion, such as crevice corrosion and concrete localized corrosion, has made great research progress (Ref 22,23,24,25,26,27,28,29).

In this study, the potential–current distribution within the crevice was mapped by using the WBE, and the influence of the SRB’s distribution on the crevice corrosion of 316LSS was discussed.

2 Experimental Methods

2.1 Bacterium and Culture Medium

The SRB strains were obtained from the China General Microbiological Culture Collection Center (CGMCC). The preparation of the seawater culture medium required the mixing of two solutions. Solution A consisted of 0.5 g K2HPO4, 1.0 g NH4Cl, 1.0 g Na2SO4, 0.1 g CaCl2·2H2O, 2.0 g MgSO4·7H2O, 2.0 g DL-sodium lactate, 1.0 g yeast extract, 0.1 g sodium thioglycolate, and 0.1 g vitamin C into 0.5 L deionized water. Solution B was prepared by adding 24.53 g NaCl, 5.20 g MgCl2, 4.09 g Na2SO4, 1.16 g CaCl2, 0.695 g KCl, 0.201 g NaHCO3, and 0.101 g KBr into 0.5 L deionized water. The two solutions were adjusted to a pH of 7.0 ± 0.2 with 0.1 mol·L-1 NaOH and were autoclaved at 121 °C for 20 min, respectively. Finally, solutions A and B were mixed to 1 L to prepare the seawater culture medium in an anaerobic environment. The SRB was inoculated into the seawater culture medium and cultured in anaerobic bottles at 37 °C.

2.2 Materials and Crevice Former

316LSS was adopted as the experimental material. The chemical composition (wt.%) of 316LSS, determined by the spark discharge atomic emission spectrometric method, was C 0.03, Si 1.0, Mn 2.0, Cr 17.0, Mo 2.50, Ni 12.0, P 0.045, S 0.03, and Fe balanced.

The WBE consisted of 100 stainless steel wires and were regularly arranged as a 10 x 10 matrix and inserted in epoxy resin with an interval of 1mm from each other. The schematic diagram is shown in Fig. 1. The surface of the WBE was sequentially abraded by 200, 400, 800, and 1500 grit water sandpapers and was cleaned with ultrasonic waves in acetone and absolute ethanol. The crevice width was chosen to be 200 μm, which was based on the possibility of the SRB can just enter. A U-shaped PTFE gasket with a thickness of 200 μm and a square PMMA plate completely covered the WBE to form a crevice former.

Fig. 1
figure 1

Schematic diagram of the crevice former

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In addition, 316LSS was cut into a block sample with a size of 20 × 20 × 3 mm, equal to the effective working area of the WBE, combined with the same PTFE gasket and PMMA plate to form a crevice structure to observe the distribution of the SRB and corrosion morphology.

According to the different distances to the crevice mouth, the specimen surfaces were divided into three regions, namely, crevice mouth (region A), crevice inner (region B), and crevice bottom (region C).

2.3 Electrochemical Measurements

The potential and current distribution was tested by the potential–current analyzer (Corrtest Company, CST520). Electrochemical impedance spectroscopy (EIS) was conducted on a three-electrode cell in the SRB-inoculated artificial seawater using an electrochemical station (Autolab, PGSTAT302N), with the 316LSS wires samples as the working electrode, platinum foil as the counter electrode, and a saturated calomel electrode (SCE) as the reference electrode. EIS was performed by applying a voltage signal of 10 mV in the range of 100 kHz-10 mHz, and the number of points per frequency decade was 10.

2.4 Surface Analysis

The simulated samples were put into the SRB-inoculated artificial seawater to conduct immersion tests. In order to ensure that the concentration of SRB maintained a high level, half of the original SRB-inoculated solution was replaced with a new artificial seawater culture medium every 7 d.

The simulated samples were removed from the SRB-inoculated artificial seawater after the immersion tests and were cleaned in phosphate buffered saline solution to wash down the planktonic bacteria. Then, the samples were immersed in a 2.5% glutaraldehyde solution for 12 h and were then dehydrated with 50, 70, 80, 90, and 95 ethanol for 10 min and 100% ethanol for 30 min. The surface morphology was observed by scanning electron microscopy (SEM) (FEI, Quanta250).

After immersion for 30 d, the composition of the corrosion products formed in the SRB-inoculated solution was analyzed by x-ray photoelectron spectroscopy (XPS). XPS (ESCLAB 250) was conducted with an Mg-Kα x-ray source at 15 kV and 150 W. The charging shifts were corrected with the C 1s peak at 284.8 eV as a reference. XPS-PEAK 4.1 software was employed to process and fit the photoelectron peaks. The spot size of the x-ray beam was 650 μm, and the pressure in the analysis chamber was maintained at 10−7 − 10−9 torr during each measure.

The three-dimensional morphology of the 316LSS surface was obtained by a confocal laser scanning microscopy (CLSM) (KENYENCE, VKX), and the numbers and depths of corrosion pits on the stainless steel surfaces were characterized by CLSM.

3 Results and Discussion

3.1 Distribution of the SRB within the Crevice

Figure 2 depicts the SEM images of different regions within the crevice after immersion for 30 d. In the crevice mouth, the SRB was aggregated. A small number of the SRB were distributed in the crevice inner, and almost no bacteria were distributed in the crevice bottom, implying that SRB attachment and survival were more difficult with increasing distance from the crevice mouth.

Fig. 2
figure 2

SEM images of different regions in the crevice after immersion for 30 days: (a) crevice mouth; (b) crevice inner; (c) crevice bottom

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3.2 Distribution of the Current and Potential in the Crevice

Figure 3 shows the corrosion potential–current distribution of the WBE after immersion for different times in the SRB-inoculated artificial seawater. It was found that the corrosion potential and current were unevenly distributed during the immersion test and changed with time. On the 3 d, a more negative potential area and an anode current appeared in the crevice bottom, indicating that the corrosion initially occurred in the crevice bottom. According to the critical crevice solution theory (CCST), the deepest part in the crevice first reaches the critical crevice solution composition, and local corrosion occurs (Ref 30). At the same time, in the early stage of complete biofilm formation, the potential will shift positively (Ref 31). On the 10 d, the potential became negative and the anode current increased in the crevice inner, indicating that the corrosion occurred in these regions. Subsequently, the corrosion in the crevice inner was more serious, as shown in Fig. 3(c). At this stage, due to the presence of SRB in the crevice inner, the destruction of the passive film had been accelerated. When immersed for 30 d, the potential in the crevice bottom showed the most negative position again, implying that the environment in the crevice bottom became harsher, with the prolonging of the immersion time. It is worth mentioning that the corrosion current increases in the crevice mouth, which may be due to the corrosion products produced by SRB increased with the extension of time, which accelerated the corrosion in the crevice mouth.

Fig. 3
figure 3

The corrosion potential–current distribution of the WBE after immersion for different times: (a)3 d; (b)10 d; (c)20 d; (d)30 d

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3.3 EIS Results

EIS technology was used to further study the electrochemical behavior of 316LSS in different regions within the crevice. The WBE was immersed in the SRB-inoculated solution, and the impedance spectra were tested on the 3, 10, 20, and 30 d, respectively. Figure 4 shows the Nyquist plots of the WBE after immersion in the SRB-inoculated solution for different times. It was seen that the measured impedance plots were featured differently, depending on the region and the testing time. The larger the radius of the Nyquist semicircle, the better the corrosion resistance of the material. On the 3 d, the Nyquist diagram in the crevice bottom shows a semicircle in the high frequency region and a straight line in the low frequency region. The radius of the Nyquist semicircle in the crevice bottom was the smallest in Fig. 4(a). From days 10 to 20, the radius of the Nyquist semicircle in the crevice inner gradually decreased. During the whole corrosion process, the impedance in the crevice mouth always kept the maximum in all regions. This result was consistent with the corrosion potential–current distribution of the WEB.

Fig. 4
figure 4

Nyquist diagrams of the WBE after immersion for different times: (a) 3 d; (b) 10 d; (c) 20 d; (d) 30 d

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To obtain impedance parameters, appropriate equivalent circuits should be used to fit the impedance data. Figure 5(a) shows the electrochemical equivalent circuits, where Rs is solution resistance, Rf and CPEf are the resistance and capacitance of the biofilm, respectively, Rct and CPEdl are charge transfer resistance and double layer capacitance, respectively. The impedance of CPEdl can be calculated by the Eq 1 (Ref 32):

$$ {\text{Z}}_{{{\text{CPE}}}} \left( {\text{w}} \right) = {\text{Y}}^{{ - {1}}} \left( {{\text{jw}}} \right)^{{ - {\text{n}}}} $$
(1)
Fig. 5
figure 5

(a) The electrical equivalent circuit (EEC) and (b) Rp values of the WBE immersed in the SRB-inoculated solution

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where Y is the parameter similar to capacitance and n is the dispersion coefficient related to the surface inhomogeneity.

As shown in Table 1, the impedance of the passive film was similar for the three conditions (mouth, inner, and bottom) and decreased from 3 to 10 d, remained constant, and decreased again for 30 d. According to the critical crevice solution theory (CCST), the pH of the solution in the crevice will drop over time. When a certain critical value was reached, the passive film was broken. From 3 to 20 days, the pH of the solution was not low enough to break the passive film. Therefore, the impedance of the passive film remained constant. When the pH value dropped to the critical value, the impedance of the passive film decreased again. The charge transfer resistance was higher for the steel at the mouth all the time owing to the protective effect of the complete biofilm. After 3 and 10 d, the charge transfer resistance of the steel at the inner section was higher than at the bottom. After 20 d of immersion, the charge transfer resistance of steel was the same at the inner part and at the bottom, which was the result of the combined effect of microbial factors and crevice factors.

Table 1. EIS parameters of the WBE after immersion for different immersion times
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Generally, the resistance Rp, i.e., the sum of Rf and Rct, can be used to evaluate the corrosion resistance of a material in corrosive environments. The Rp is inversely proportional to the corrosion rate. Figure 5(b) shows the time dependence of the fitted Rp values for the 316LSS in the SRB-inoculated solution. On the 3 d, the Rp values of the crevice bottom were the smallest, implying that the corrosion initially occurred in the crevice bottom. It was seen that the Rp of 316LSS reduced gradually with time. When WBE was immersed for 30 d, the Rp values in the crevice inner and crevice bottom were similar and significantly less than that in the crevice mouth, indicating that the corrosion mainly occurred in the crevice inner and bottom.

3.4 XPS Analysis

The spectra of Fe, Cr, and O were fitted by XPS-PEAK according to the Cui (Ref 13) and the information given in the NIST XPS database.

Figure 6 shows the XPS spectrum of Fe 2p, Cr 2p, and O 1s of the corrosion products formed in different regions after immersing for 30 d. In Fig. 6(a1), the Fe 2p spectrum of the crevice mouth comprised Fe(OH)3 (712.9 eV), Fe2O3 (711 eV), FeO (709.5 eV), Fe3O4 (708.6 eV), FeS2(707.9 eV), and Fe0 (706.8 eV). The content percentages obtained by calculation were 10.4, 14.2, 15.4, 25.1, 18.5, and 16.4%, respectively (Table 2). In Fig. 6(a2), the Fe 2p spectrum of the crevice inner consisted of peak components with BEs at 712.7, 710.9, 709.4, 708.1, 707.2, and 706.5, which were attributed to Fe(OH)3, Fe2O3, FeO, Fe3O4, FeS2, and Fe0. The content percentages were 9.9, 15.2, 13.1, 21, 21.7, and 19.1%, respectively. In Fig. 6(a3), the Fe 2p spectrum of the crevice bottom comprised Fe2+ sat 2p (714.3 eV), Fe(OH)3 (712.8 eV), Fe2O3 (710.9 eV), FeO (709.4 eV), Fe3O4 (708 eV), and Fe0 (706.9 eV). The content percentages were 6.7, 10.4, 15.1, 16.3, 15.9, and 35.5%. FeS2 only existed in the crevice mouth and inner, indicating that the SRB had adhered to in these regions and produced H2S through the metabolism, which corresponded to the result in Fig. 2.

Fig. 6
figure 6

XPS spectrum of Fe 2p3/2 (a), Cr 2p3/2 (b), and O 1s (c) of the corrosion products formed in different regions after immersion for 30 d

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Table 2. The relative content (%) of Fe 2p in the passive film fitted by the XPS results
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In Fig. 6(b), the Cr 2p spectrum of the 316LSS consisted of peak components with BEs at 577.7, 576.2 and 574 eV, which were attributed to Cr(OH)3, Cr2O3, and Cr0. The content percentage of the crevice mouth were 51.4, 37.4, and 11.2%, respectively. The content percentage of the crevice inner were 52.8, 34.9, and 12.2%, respectively. In addition, the content percentage of the crevice bottom were 54.3, 36.5, and 9.2, respectively. It could be seen that there was no obvious difference in the existence and content of the Cr element in different regions.

Figure 6(c) shows the results of the O 1s spectrum of the 316LSS. The O element mainly existed in three forms in the passivation film, namely H2O (532.5 eV), OH- (531 eV), and O2- (530.1 eV). The percentage content of each component in different regions was roughly the same.

3.5 Corrosion Morphology

The CLSM images and pit depths of 316LSS in the crevice after immersion for 30 d are shown in Fig. 7. In the crevice mouth, the number and size of pits on the surface were small, and the largest pit depth was only 0.963 μm. Comparatively, the number and size of pits both increased in the crevice inner. In the crevice bottom, pits were densest, and the largest pit depth was up to 3.416 μm. Therefore, according to the above results, the samples suffered obvious corrosion in the crevice inner and bottom, and corrosion in the crevice bottom was more serious.

Fig. 7
figure 7

CLSM images and pit depth of 316L in different regions in the crevice after immersion for 30 days: (a) crevice mouth; (b) crevice inner; (c) crevice bottom

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According to CCST, the corrosion occurred preferentially in the crevice bottom because the innermost solution changed greatly and firstly reached the critical state (Ref 30). With the addition of microbial factors, a complete biofilm was formed in the crevice mouth and an incomplete biofilm was formed in the crevice inner. The reason for the lack of biofilm formation in the crevice bottom may be that the composition of the solution was not suitable for microorganisms’ survival or the existence of the crevice hindered the entry of microorganisms. Biofilms provided a protective effect in the early stages of corrosion (Ref 31). Therefore, the most serious corrosion priority occurred in the crevice bottom. With the extension of time, the production of FeS2 under the biofilm and the change in the composition of the passive film caused the corrosion resistance of 316LSS to decrease. Galvanic effect under incomplete biofilm made it corrode faster than under complete biofilm (Ref 33). Therefore, corrosion is equally serious in the crevice inner and bottom under the synergistic effect of the microbial factors and crevice factors. The schematic diagram of the crevice corrosion in the SRB-inoculated artificial seawater is shown in Fig. 8.

Fig. 8
figure 8

Schematic diagram of the crevice corrosion in the SRB-inoculated artificial seawater

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4 Conclusion

The corrosion behavior of 316L stainless steel in the SRB-inoculated artificial seawater was investigated. The main conclusions are summarized as follows.

The distribution of the SRB within the crevice was uneven. The SRB was aggregated in the crevice mouth and became a complete biofilm. With increasing distance from the crevice mouth, SRB’s attachment and survival were more difficult.

In the SRB-inoculated artificial seawater, corrosion preferentially occurred in the crevice bottom. And due to the existence of the SRB, corrosion began to extent to the regions near the crevice mouth. Subsequently, corrosion moved to the crevice bottom again.

The corrosion products near the crevice mouth contained FeS2, indicating that the protective performance of the passive film was reduced. Due to the synergistic effect of the microbial factors and crevice factors, corrosion is equally serious in the crevice inner and bottom.