Introduction

Natural seawater with unique physicochemical and biological factors has been considered to be one of the harshest and most complex corrosive environment1. Marine engineering materials face serious corrosion failure problems in seawater, which extremely affects their service life in the ocean2. Therefore, materials with good corrosion resistance are preferentially chosen for use in marine engineering. Stainless steel has the advantages of excellent corrosion resistance, relatively low cost, and great mechanical properties, and is widely used in offshore platforms and shipbuilding3,4. Although stainless steel has a lower corrosion rate in seawater, it is also more prone to localized corrosion, which is less noticeable but often causes sudden failure5,6. The crevice configurations in the ocean can be divided into the artificial crevice and the biological crevice7,8. The closed or semi-closed environment formed by the above two crevice configurations is not easy to monitor and is often more corrosive than the traditional open seawater9. Therefore, research on crevice corrosion of stainless steel in natural seawater faces many challenges.

The artificial crevice structure mainly refers to the crevice formed between the same or dissimilar materials with a distance of <500 μm, such as bolts and flanges in marine engineering. As well known, the narrow crevice could significantly reduce the diffusion processes of the substances, which makes the oxygen consumed by the cathode reaction inside the crevice cannot be replenished in time, thus forming the oxygen concentration cell. The oxygen-deficient interior region plays as the anode due to its more negative potential, while the exterior region acts as the cathode. At the same time, metal cations are released inside the crevice. In order to balance the charge, the negative ions Cl- outside the crevice migrate to the inside, and the generated metal chlorides are hydrolyzed to form hydroxides and free acids, which reduce the pH value inside the crevice and accelerate the crevice corrosion10,11. Cai studied the corrosion behavior of three kinds of crevices (316L-to-polytetrafluoroethylene, 316L-to-fluoroelastomeric and 316L-to-316L) in artificial seawater, and the results showed that the 316L-to-316L specimen was the most susceptible to crevice corrosion12. Zhang found that crevice corrosion of stainless steel was a more serious form of corrosion than pitting corrosion through long-term immersion tests in the Western Pacific Ocean13. Especially, crevice corrosion involving fouling organisms have not been explored, which is a common issue in the real marine environment.

Marine fouling organisms, including soft foulers and hard foulers, prefer attaching to the bare metals to form crevice structures14,15. A sufficient magnitude of fouling adhesion results in the formation of a physical barrier, which can reduce the corrosion rate of the materials16,17. However, its accelerating effect on localized corrosion cannot be ignored. The biofoulings attachment to the materials can create suitable conditions for the initiation of crevice corrosion. Meanwhile, the occlusion area created by the biofoulings can support various bacterial growth and promote microbiologically influenced corrosion18,19. Regarding crevice corrosion induced by fouling organisms, researchers have mainly focused on organisms with hard lime shells. Blackwood found that in tropical marine environments, the propensity for corrosion by shellfish of UNS S31603 is ranked as oyster » barnacles » green mussels20. Due to their strong adhesion and wider distribution, barnacles have caused more extensive corrosion problems and have attracted more attention from researchers. Eashwar believed that the death of barnacles is a prerequisite for the initiation of crevice corrosion. After the barnacle dies, spoilage bacteria break down the barnacle meat, lowering the pH in the barnacle shell, and allowing crevice corrosion of stainless steel21. However, Sangeetha found that the initiation of crevice corrosion was not related to the death of barnacles, and crevice corrosion also occurred under living barnacles22. Some researchers have cultivated barnacles in the laboratory to study their effect on the corrosion behavior of stainless steel, but the results are still limited by the test period and cannot represent the actual marine environment23. Therefore, it is necessary to carry out a long-term immersion test in natural seawater on crevice corrosion including fouling organisms.

In this study, three types of stainless steels are immersed in natural seawater for 36 months. The test site was selected in the sea area where fouling organisms thrived. Based on the observations on corrosion morphologies and the analysis of corrosion products, the crevice corrosion behavior of three stainless steels in natural seawater was compared. Furthermore, the effect of large organisms on artificial crevice corrosion and barnacle-induced crevice corrosion are discussed.

Results

Macroscopic corrosion morphology

Figure 1 shows the macroscopic corrosion morphologies of three stainless steels after being immersed in natural seawater for different time. The images clearly present that all three stainless steel samples were fully covered with biofoulings. Types and numbers of large fouling organisms on the surfaces of the stainless steel were recorded. It was found that Amphibalanus reticulatus, Perna viridis and Tubularia mesembryanthemum Allman were the dominant species, although the attachment frequency and density were distinct in different stainless steels. The ratio of the area of contaminated organisms remaining on the surface to the total area was used as the biofouling coverage. This evaluation of biofouling coverage on metal surfaces has also been reported in previous papers24. As shown in Fig. 1, the biofouling coverage on 316 L SS surface was about 50% for 6 months, and increased over 80% after 12 months. Comparatively, the biofouling coverage on 2205 DSS and 2507 SDSS were consistently below 50%. Barnacles were easily detached under water pressure on the 2507 SDSS surface. The formation of biofilms on metal surfaces provided a prerequisite for the colonization of large fouling organisms25. The interaction between the biofilms and the stainless steel interface may be the factor leading to the difference in the biofouling coverage.

Figure 2 gives the macroscopic corrosion morphologies of three stainless steels after removing biofoulings and corrosion products. According to the difference in induced factor, the type of corrosion was divided into artificial crevice corrosion and biofouling-induced crevice corrosion. It was worth noting that artificial crevice corrosion did not occur under all 24 teeth of both spacers. Corrosion occurred preferentially under certain teeth at random. Artificial crevice corrosion of 316L SS initiated before 6 months. In contrast, similar phenomenon on 2205 DSS and 2507 SDSS could be observed at 12 months and 24 months, respectively. Further, much more serious corrosion occurred under almost all artificial teeth on the 316L SS surface. Moreover, biofouling-induced crevice corrosion on the 316L SS surface first appeared after 24 months of immersion. As shown in Fig. 2, the obvious biofouling-induced crevice corrosion firstly appeared on 316L SS surface after immersing for 12 months. Subsequently, the biofouling-induced corrosion was gradually severed. However, the biofouling-induced crevice corrosion was hardly found on 2205 DSS and 2507 SDSS, which may be related to the low biofouling coverage.

Weight loss and corrosion rate

Figure 3 presents the weight loss and corrosion rate of three stainless steels after immersing for 36 months. As shown in Fig. 3a, the weight loss of the three stainless steel gradually increased with time. After immersing for 36 months, the weight loss of 316L SS, 2205 DSS and 2507 SDSS was 323.56 g·m−2, 8.78 g·m−2 and 2.15 g·m−2, respectively. Variations in the corrosion rate of three stainless steels are shown in Fig. 3b. It was obvious that the corrosion rate of 316L SS was two orders of magnitude greater than that of 2205 DSS and 2507 SDSS. The corrosion rate of 316L SS decreased from 3.03 × 10−2 mm·y−1 at 6 months to 1.35 × 10−2 mm·y−1 at 36 months, which resulted from the protective effect of the biofouling barrier layer. The attachment of fouling organisms makes it difficult for oxygen to quickly and continuously contact the material surface, thereby reducing the overall corrosion rate of the sample26. The corrosion rate of 2205 DSS and 2507 SDSS maintained at a small value. The corrosion rate of 2205 DSS slightly increased at 12 months, which was caused by the occurrence of artificial crevice corrosion.

Figure 4a shows the total number of artificial crevice sites attacked in different periods, which represents the crevice corrosion susceptibility of stainless steel. The total number of artificial crevice sites attacked on 316L SS was higher than that of 2205 DSS and 2507 SDSS in each period. After immersing for 36 months, the total number of artificial crevice sites attacked on 316L SS was 58 (total of 72 artificial teeth), indicating that corrosion occurred under almost all artificial teeth, while 2507 SDSS only developed two corroded teeth. Therefore, the crevice corrosion susceptibility of the three stainless steels was ranked as 316L SS > 2205 DSS > 2507 SDSS. The maximum pit depths of 316L SS and 2205 DSS are shown in Fig. 4b, c, respectively. Maximum pit depths increased with the prolongation of immersion time. In addition, the concentrated area of the maximum crevice depth from different teeth was also gradually increased with time. After immersing for 36 months, the maximum artificial crevice depths of 316L SS and 2205 DSS could reach 1892 and 319 μm, respectively. On the contrary, 2507 SDSS suffered the lightest corrosion, and its maximum corrosion depth was only 181.45 μm.

Microscopic corrosion morphology

Figure 5 presents the artificial crevice corrosion morphologies of the 316L SS after immersing for 36 months. The maximum depth of the corrosion pit was located at the edge of the crevice (Fig. 5a, e). Meanwhile, a lace-cover morphology appeared on the edges (Fig. 5d), and there was severe corrosion that was difficult to observe below. The morphology at the crevice edge is related to IR drop and autocatalytic acidification27. Austenite grains were observed in the center of the artificial crevice (Fig. 5c), which was due to the preferential corrosion of grain boundaries in an acidic environment. These typical morphologies were consistent with the localized acidification model28. Due to the formation of an “oxygen concentration cell”, Cl- first entered the edge of the crevice, where it forms hydroxide with metal cations. The acidification of the solution at the crevice mouth led to the breakdown of the passive film, and corrosion was preferentially initiated from the edge. Since it is more difficult for the solution to enter the interior of the crevice, corrosion preferentially developed along the vertical direction at the edge and occurred slightly inside the crevice. According to Cai’s description, the whole area under the artificial tooth belonged to a severely attacked region29.

Lace-cover morphology also appeared in the corroded area of 2205 DSS (Fig. 6b), and the maximum depth appeared at the edge of the artificial crevice (Fig. 6a, e). In the center of the crevice, the austenite and ferrite phases can be clearly seen. Different from 316L SS, in the center area of the artificial crevice, the corrosion morphology seemed like stair-stepping (Fig. 6c). Under the crevice teeth, the local solution was also acidified. For duplex stainless steel, the difference in corrosion resistance between the two phases triggered a galvanic effect in an acidic solution30. Preferential dissolution of ferrite led to stair-stepping corrosion morphology.

It is worth noting that metastable pitting can be observed at the corrosion edge of 2507 SDSS (Fig. 7b), indicating that it is in the initial state of crevice corrosion29. Stockert et al. thinks that crevice corrosion occurs on the surface of stainless steel because the repassivation process of metastable pitting is hindered by the crevice occlusion environment31. Metastable pitting will randomly occur in the crevice occlusion area with limited material transmission. The severely attacked region appeared in the center of the crevice with a stair-stepping morphology (Fig. 7c), which was also due to the different corrosion resistance between the two phases. In addition, the maximum pit depth of 2507 SDSS occurred in the center, unlike the 316L SS and 2205 DSS, indicating that crevice corrosion propagated from the inside to the edge. 2507 SDSS had a higher PREN value, which indicates better pitting resistance. In the early stage of corrosion, the acidification degree of the localized solution at the crevice mouth cannot reach the conditions for the breakdown of the passive film, and the more internal solution would continue to be acidified, reaching the critical environment first. Therefore, corrosion occurred preferentially in the interior, and then continued from the interior to the edge.

Discussion

The effect of large organisms on artificial crevice corrosion

Figure 8 shows the macroscopic corrosion morphologies of three stainless steels covered with nylon meshes after immersing in natural seawater for 24 months. Since there was no adhesion of large fouling organisms on the surface of the sample, the corrosion occurred only under part of the artificial teeth. Table 1 shows the statistical results of crevice corrosion on samples without and with nylon meshes after immersion for 24 months. The corrosion rate of the 316L SS samples without nylon meshes was about 3 times that of the 316L SS samples with nylon meshes, and the corrosion rate of the 2205 DSS and 2507 SDSS samples without nylon meshes was about 2 times that of the 2205 DSS and 2507 SDSS samples with nylon meshes. This proves that the large fouling organisms have a greater impact on the corrosion of 316L SS, which is also consistent with the above results of low the biofouling coverage for 2205 DSS and 2507 SDSS. The maximum depths of 316L SS, 2205 DSS and 2507 SDSS samples without nylon meshes were 1571.55, 235.01 and 48.79 μm, while that of 316L SS, 2205 DSS and 2507 SDSS samples with nylon meshes were 493.45, 143.84 and 0 μm, respectively. In other words, the existence of the large fouling organisms also made the corrosion under the artificial teeth more serious. Luciana found that the open circuit potential (OCP) of stainless steels decreased when large fouling organisms were present, indicating that the samples were more susceptible to corrosion24. The reasons for this phenomenon may include two aspects: (1) The attachment of large organisms affected the diffusion of substances under the artificial crevices, making the corrosion under the artificial teeth serious; (2) Some life activities of the organisms lead to acidification in the localized solution, resulting in severe localized corrosion.

As shown in Fig. 8, the surface of 316L SS and 2205 DSS were covered with a layer of orange corrosion products, while the surface of 2507 SDSS attached few corrosion products. Further, the corrosion products were spilled under the artificial crevices to the crevice-free surface. Figure 9 shows the surface morphologies and energy-dispersive spectrometry results of the corrosion products of three stainless steels. Three stainless steel samples present flat films with spherical particles on the surface. Some cracks were found on the flat film of 316L SS and 2205 DSS. The elemental composition data indicated the presence of Fe and O as the major elements revealing that the corrosion product is mainly composed of an oxide of iron (Fig. 9a1, b1 and c1). Comparing the composition of the flat film and spherical particles, the P element in spherical particles was significantly higher, which was produced from the metabolism of life activities in the ocean32. In summary, the film formed on the surface of the samples was a mixture of corrosion products and biological metabolites. When the relative content of corrosion products was large, the relative content of the P element was smaller (Fig. 9a2, b2 and c2). The results corresponded exactly to the result in the macroscopic morphologies.

Barnacle-induced crevice corrosion

The existence of barnacle cement between the barnacle-base and the metal substrate is the reason why the barnacle is firmly attached to the metal surface33. According to Figs. 1 and 2, the coverages of barnacles on the 2205 DSS and 2507 SDSS are lower than 316L SS, and it did not cause obvious corrosion. So only the corrosion phenomenon caused by barnacles on the 316L SS was discussed. Table 2 shows statistical results of barnacle-induced crevice corrosion on 316L SS immersed for different time. It is found that the maximum pit diameter was maintained at 13–15 mm, which is close to the size of the adult barnacles34. In contrast, the maximum pit depth presents a gradual increase with time, indicating the severe pitting corrosion caused. To be noticed, the corrosion pit area hardly increased after the 12 months, which may be due to the fouling barrier layer that prevented the nascent barnacles from attaching. However, the corrosion under the previously attached barnacle shell became more serious, as seen from the increase of the maximum pit depth.

Figure 10 shows the typical corrosion morphology of 316L SS with a single barnacle attached. As shown in Fig. 10b, the macroscopic corrosion morphology seems like a continuous single ring along the periphery of the barnacle-base, and no obvious corrosion occurred in the center. Figure 10c illustrates the corrosion profile corresponding to the location in Fig. 10b. It is seen corrosion was spread from the outer edge of the barnacle-base to the inner center, with the deepest location occurring at the edge of the carapace. Eashwar believed that the initial step of the crevice corrosion was the decomposition of barnacle flesh by putrefactive bacteria after the barnacle died, causing the pH drop of solution in the barnacle shell. The acidic product penetrated the shell base in the thin center and transported to the edge through radial tubes in the base, triggering the initiation of crevice corrosion near the edge of shell-base followed by the propagation inward21. However, as shown in Fig. 10b, the diameter of the barnacle was only ~6 mm, indicating the non-adult barnacle is still live. In other words, crevice corrosion also occurred under living barnacles. Sangeetha discovered similar phenomena in his research22. A dense layer of cement secreted by barnacles, an unknown protein, could destroy the protective chromium oxide layer of stainless steel. Subsequently, the grain boundary with higher energy was disrupted, creating a channel for the inward flow of corrosive seawater. Thus, crevice corrosion initiated near the edge of shell-base and then propagated inward.

In the present work, the microscopic corrosion morphologies included three typical morphologies, as shown in Fig. 10a, d, f. Pitting holes with diameters of 1–3 μm were densely distributed around the edge of the corroded area (Fig. 10a), which belonged to the metastable pitting morphology12. Gulch-like morphology and austenite grain morphology appeared in the middle of the corroded area (Fig. 10d, f). Combined with the topographic profile shown in Fig. 10c, it was found that different corrosion morphology types were related to the depth of corrosion pits, which can be considered to represent different corrosion stages. In the early stage of corrosion, metastable pitting pits were first formed on the surface (Fig. 10a). Then, the pits were connected into pieces to form gulch-like morphology (Fig. 10d). Finally, a corrosion morphology with etched grain boundaries was formed, which was similar to the morphology under artificial crevices (Fig. 10f), indicating that barnacle-induced crevice corrosion maybe related to acidification of the solution.

Figure 11 shows cross-section morphologies and the main element distribution of the barnacles-induced corrosion region. The light-colored area in Fig. 11a was the barnacle calcareous shell, and there were hollow channels in the barnacle-base. Figure 11b shows a partially enlarged view of the interface between the barnacle and the matrix. For the convenience of observing the cross-section morphology, the barnacle-attached samples were covered with epoxy resin. It could be seen from Fig. 11c that the position containing the C and O elements was the position of epoxy resin, indicating that the epoxy-filled area between the barnacle-base and the matrix was the location of the crevice corrosion pits.

There are two classical theories about traditional crevice corrosion between metals, namely critical crevice solution theory (CCST) solution acidification and IR drop theory27,35,36. According to the results under artificial teeth, the maximum pit depth of 316L SS and 2205 DSS appeared at the edge of the crevice, basically in line with the IR drop theory, while that of 2507 SDSS showed in the center of the crevice, indicating that IR drop does not make the area under the crevice in the activation zone, which may be related to the higher pitting resistance. A total of 2507 SDSS is consistent with the critical crevice solution theory, that is, corrosion initiated in the crevice center because that’s where the solution acidified the most. the center is the first to corrode because the solution in the center is the first to reach the solution component of the passivation film rupture. According to SEM morphology, the outlines of grains were observed due to the preferential corrosion of grain boundaries in localized acidification solution. Large organisms such as mosses, seagrasses and barnacles form a fouling barrier layer that was attached to the edges of the crevices, blocking the flow of material over the crevices. The substrate inside the crevice acted as the anode, and corrosion was further accelerated. The schematic diagram of artificial crevice corrosion in natural seawater is shown in Fig. 12a.

Barnacles could secrete a special protein (i.e. barnacle cement) as binders to facilitate the attachment of barnacles to the surface of the metal substrate. In the early stage of corrosion, the cement secreted by the barnacles preferentially etched the grain boundaries14. The corroded area formed the initial crevice between the barnacle-base and the stainless steel matrix. The seawater then seeped into the crevice at the edge of the barnacle-base, which well explained inwards spread of corrosion from the edge to the inside and severest corrosion occurred at the edge. According to the critical crevice solution theory, the solution in the edge was continuously acidified, forming a catalytic-occluded cell that caused corrosion to occur continuously. After a long-term immersion, the morphology of the crevice under the artificial tooth becomes similar to that of barnacle-induced crevice corrosion, indicating that the development of barnacle-induced crevice corrosion was also due to the acidification of solution inside the crevice. Therefore, solution acidification played an important role in the development of both two crevice corrosion processes. The schematic diagram of barnacle-induced crevice corrosion in natural seawater is shown in Fig. 12b.

Based on the above discussion, the following conclusions can be drawn: (1) The corrosion rate of 316L SS was two orders of magnitude greater than that of 2205 DSS and 2507 SDSS in the natural seawater. The maximum pit depth of 316L SS was up to 1892 μm, much larger than that of 2205 DSS and 2507 SDSS. Meanwhile, 316L SS had a higher biofouling coverage which led to more severe biofouling-induced corrosion. In the presence of large organisms, the formation of more occluded areas on the metal surface promoted the occurrence of artificial crevice corrosion. (2) The maximum corrosion depth of 316L SS and 2205 DSS was at the crevice edge, which was caused by IR drop, while that of 2507 SDSS was located at the center of the crevice because the deepest solution reached the critical concentration first further led to the rupture of the passivation film. (3) Barnacles were the main large organisms that caused biological crevice corrosion. The typical macroscopic corrosion morphology of barnacle-induced crevice corrosion seemed like a continuous single ring along the periphery of the barnacle-base. When the barnacle cement interacted with the metal substrate to form a crevice, the seawater enters under the substrate from the crevice edge, and crevice corrosion was initiated. The acidification of the solution caused by catalytic-occluded cells was responsible for the continuous development of corrosion.

Methods

Field test

The field test site (24°33′40″N, 118°9′52″E) is located near Xiamen Island, the southeast of Fujian Province, China. Figure 13a shows its geographical location. The marine environment around Xiamen Island is a typical subtropical marine climate, with an average monthly temperature of 25 °C and a salinity of 27.3 PSU37. Located at the confluence of Jiulong River, East China Sea and South China Sea, this marine area is rich in species diversity and conducive to high biological coverage38. The types of fouling organisms have the characteristics of the open sea and the inner bay of the estuary39. The physicochemical properties of natural seawater at the field test site were recorded periodically and shown in Table 3. These provide a beneficial environment for the growth of coastal microbial resources, and further facilitate the attachment of large fouling organisms. Therefore, there are severe biofouling and bio-corrosion problems in this marine area, which provides favorable conditions for the study of crevice corrosion of stainless steel in natural seawater.

The stainless steel samples designed with artificial crevices were bolted into a titanium frame, which was then mounted on the float raft. The function of the float raft was to ensure that all samples were always in the full immersion area of the ocean. As shown in Fig. 13b, the samples were invariably located 1–2 m below sea level. The test period was 6, 12, 24 and 36 months, respectively.

Materials and crevice specimen

The materials studied in this paper were three commercial stainless steels (316L SS, 2205 DSS and 2507 SDSS) provided by Shanxi Taigang Stainless Steel Co., Ltd. The chemical compositions of these stainless steels are listed in Table 4. The pitting resistance equivalent number (PREN) values were used to characterize the pitting resistance of stainless steel40,41. The PREN values of three stainless steels are calculated and listed in Table 4. Figure 14 shows electron backscattered diffraction (EBSD) images of three stainless steels. 316L SS was a single-phase austenitic stainless steel. 2205 DSS and 2507 SDSS were duplex stainless steels, and their austenite (γ) / ferrite (α) ratio was between 40 and 60%.

All materials were cut into samples with dimensions of 50 × 50 × 3 mm. A 10 mm-diameter hole was drilled in the middle to facilitate titanium bolt passage. The samples were machined to a surface finish Ra of 3.2 μm, followed by ultrasonic cleaning in alcohol to remove any adhering particles. Multi-crevice assembly was modified according to ASTM G7842. Figure 15a shows the detailed assembly diagram of the sample. The crevice formers consisted of two PTFE spacers each with 12 teeth. Two spacers were pressed onto the sample with a torque of 2 N·m through a titanium bolt and a titanium nuts. A PTFE sleeve surrounded the titanium bolt to ensure electrical insulation of the sample and titanium bolt. 24 artificial crevices were formed between these two spacers and the sample. The final multi-crevice assembly is shown in Fig. 15b.

In order to compare the influence of the presence and absence of large organisms on the crevice corrosion behavior of the stainless steel in natural seawater, the standard multi-crevice samples were covered with 500 μm nylon meshes as control. The nylon meshes can effectively isolate the attachment and growth of macro-fouling larvae on the surface of the samples. Three parallel samples were set up to ensure the accuracy of the test process.

Weight loss measurement

To evaluate the crevice corrosion resistance of three stainless steels in seawater, weight loss measurement was performed on the specimens in each immersing period (for 6, 12, 24 and 36 months, respectively). Impurities and part of the fouling organisms were removed by water-jet and non-metallic bristle brush. After that, calcareous crusts and corrosion products on the sample surface were chemically removed by pickling in the solution (50 mL H3PO4 and 20 g CrO3 in 1 L H2O) for 10 min at 80–100 °C. Finally, the samples were degreased with acetone, dried with warm air, and weighed to obtain their final weights w1 (g) in according with ASTM G1-0343. w0 was the original weight (g). The average corrosion in mm·y−1 units was calculated as following.

$$Corrosion\,Rate = \frac{{\left( {{{{\mathrm{K}}}} \times {{{\mathrm{W}}}}} \right)}}{{\left( {{{{\mathrm{A}}}} \times {{{\mathrm{T}}}} \times {{{\mathrm{D}}}}} \right)}}$$
(1)

Where K was a constant (8.76 × 104), T was the time of exposure in hours, A was the area in cm2, W was the mass loss (w0-w1) in grams, and D was the density of the materials in g/cm3.

Morphology characterization

After retrieval of the samples, the fouling and corrosion products were removed by water-jet cleaning at 100, 400, 800, and 1600 kPa. Macroscopic corrosion morphology was photographed after each stage using a digital camera (Nikon, D3500). The ratio of the area of contaminated organisms remaining on the surface to the total area was used as the biofilm coverage44. Photographs were projected over a grid of 100 dots and the organism under each dot was recorded.

The microstructure morphologies of the samples were observed by scanning electron microscopy (FEI, Quanta 250). The local components of the corrosion products were analyzed by energy dispersive spectroscopy (EDS).

The three-dimensional corrosion morphologies of three stainless steels were 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.