Introduction

The corrosion problem of carbon steels is a great handicap for usage of these materials in marine environments (Ref 1,2,3). There are many factors that can induce the corrosive damage of simple steel components in the marine environment, including the chloride ion-induced corrosion and microbial influenced corrosion (MIC) (Ref 4). Preparing coatings on carbon steels is a versatile method to improve their chloride ion-induced corrosion and microbial influenced corrosion performance (Ref 5, 6).

Up to now, a great number of methods have been used to produce coatings on steel substrates used in seawater, including vapor deposition method (Ref 7), ion beam assisted deposition method (Ref 8), sputtering method (Ref 9), etc. However, these methods usually exhibit low deposition efficiency and cannot prepare thick coatings, which limit their application in prolonging the service life of steel components. Thermal spraying is one of the best technologies to deposit thick coatings with several hundreds of micrometers (Ref 10,11,12,13). Among various thermal spraying technologies, electric arc spraying technology has been widely used in metal protection because of its low cost, high efficiency, easy operation and excellent performance (Ref 14, 15). Nevertheless, the arc-sprayed coatings usually have certain porosity, resulting in permeation of corrosive medium much easier and thus degrading their corrosion performance. Hence, the pore-sealing treatment is necessary and significant for the electric arc-sprayed coatings before their application in seawater (Ref 16, 17).

To have good chloride ion-induced corrosion and microbial influenced corrosion simultaneously, the early coatings used for preventing the substrate from biological corrosion are usually not environmentally friendly, such as the coatings containing Sn, Cu, Pb or Hg (Ref 18). Recently, some advanced coatings such as bionic antifouling coatings or low surface antifouling coatings were developed. These coatings usually have superior biological corrosion resistance and do no harm to the environment. However, the preparation difficulty and cost of these coatings are both high (Ref 19). Hence, as typical anti-corrosion materials, environmentally friendly Zn and Al coatings have been widely used in the marine environment and related research concerning their corrosion behavior and mechanism has been conducted.

Liu et al. (Ref 20) studied the corrosion resistance of zinc-coated steel (ZCS) in seawater, and they found that the corrosion rate of ZCS elevated with the increase in immersion time. Besides, they also proved that the porous and loose corrosion products of the ZCS mainly consisted of ZnO, Zn5(OH)8Cl2, and Zn5(OH)6(CO3)2. Open circuit potential and zero-resistance ammetry techniques were used by Stoulil et al. (Ref 21) to monitor the electrochemical corrosion behavior of aluminum alloys in various environments, including seawater, artificial rain, and the atmosphere. The experiments showed that the aluminum alloys had promising corrosion performance and electrochemical behavior characteristics.

Though the chloride ion-induced corrosion performances of zinc coating and aluminum coating have been studied in diverse environments (including atmosphere, acids, chlorides, etc.), the research about the microbial influenced corrosion performance of these coatings still lacks (Ref 22, 23). As an anaerobic microbe widely exists in oxygen-deprived marine environments, sulfate-reducing bacteria (SRB) is taken as a main reason to induce the microbial influenced corrosion of metals in the sea. In anaerobic conditions, the SRB will grow into bio-community and form metabolites such as S2− and H2S. These metabolites can reduce the metals to sulfides and cause the corrosion of the metals (Ref 24, 25). Hence, the electrochemical behavior and relevant mechanism of electric arc-sprayed Zn coating and Al coating in seawater containing SRB and the role of SRB in corrosion process is worth researching.

In the present work, the open circuit potential (OCP), potentiodynamic polarization (PDP) and electrochemical impedance spectroscopy (EIS) techniques were applied to analyze the corrosion behavior of pore-sealed Zn coating and Al coating deposited by arc spray technology in SRB seawater. The corrosion mechanism of pore-sealed Zn coating and Al coating was also discussed by using the scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS).

Experimental

Fabrication of the Coatings

Pure Zn (99.9 wt.%) and Al (99.9 wt.%) wires with diameter of 3 mm were used as the raw material. The Q235 carbon steel was selected as substrate. Table 1 lists the nominal composition of the steel. The substrate was previously machined, rinsed in alcohol and sandblasted with corundum powder prior to spraying by a CD-500 electric arc spray system. The spraying parameters of both pure Zn and Al wires were controlled as 200 mm for the spray distance, 32 V for the spray voltage, 120 A for the spray current and 0.65 MPa for the air pressure.

Table 1 The chemical composition of Q235 steel
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After spraying, all the samples were pore-sealed with silicone resin to promote the corrosion performance. In this work, a silicone resin with technical index shown in Table 2 was used as sealant. Prior to sealing, the coatings were ultrasonically cleaned and dried at 60 °C. The cleaned surface of the coatings was then coated with appropriate amount of sealing agent with a brush, and dried at 115 °C for 30 min. Finally, the redundant silicone resin was polished by abrasive paper.

Table 2 Technical index of silicon resin
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SRB Cultivation and Experimental Medium

The SRB strains applied in this experiment were derived from sludge in Kiaochow Bay, Shandong. Afterward, modified Postgate’s culture medium was used to enrich and culture the SRB strains. The aforementioned medium consisted of 0.06 g MgSO4·7H2O, 0.06 g CaCl2·6H2O, 0.3 g Na-citrate, 0.5 g KH2PO4, 1.0 g NH4Cl, 1.0 g yeast cream and 6 mL 70% sodium lactate per liter seawater. The medium should be sterilized by high-pressure steam at 121 °C for 20 min, then 0.004 g/L Fe(NH4)2(SO4)2·7H2O was also added to the medium. Finally, the seawater was added with 5 vol.% pure bacterium solution to form the experimental medium.

Experimental Methods

The electrochemical system comprised a 1260A frequency response analyzer (Solartron) and a 1287 potentiostat (Solartron) was applied to measure the corrosion performance of pore-sealed Zn coating and Al coating in seawater containing SRB. Before the test, the samples were encapsulated in non-conducting colophony and only an end-surface with a square area of about 1 cm2 was exposed to the medium. A classic three electrode cell was used to conduct the electrochemical tests, with a saturated calomel system as reference electrode, a 10 × 20 mm Ruthenium-Titanium plate as auxiliary electrode, and the specimen as working electrode. These electrodes should be sterilized with an ultraviolet lamp for 30 min before conducting the experiment. In the experimental process, the corrosive medium should be replaced every 24 h to ensure the consistency of corrosive environments. The electrochemical impedance spectroscopy (EIS) test was measured at open circuit potential (OCP) with a sine wave potential perturbation of 10 mV in a frequency fluctuating from 10 mHz to 100 kHz. The potentiodynamic polarization test ranged from − 0.4 to 0.4 VSCE relative to the OCP at a constant rate of 0.5 mV/s.

The surface morphologies and elements of the pore-sealed Zn coating and Al coating after electrochemical tests were observed by scanning electron microscopy (SEM) and analyzed by energy dispersive spectroscopy (EDS). Before observation, the samples should be immersed in testing medium for 15 days, and then washed by sterile seawater three times. After that, the samples should be immersed in 5 vol.% pentanediol solution for 30 min to set the bacterium on their surfaces, finally dehydrated with different gradient of ethanol (50%, 70%, and 100%) in sequence and dried in a vacuum environment.

Results and Discussions

Coating Characterization

Figure 1 shows the typical cross-sectional morphologies of the as-sprayed Zn coating and Al coating. Both coatings exhibit similar thickness. The thickness of the Zn coating is about 230 μm, while the thickness of the Al coating is about 200 μm. As shown in Fig. 1(a), the Zn coating had distinct layered structure, and some obvious pores or micro-cracks can be observed between the layers. This is because during the process of arc spraying, most of the particles were deposited on the substrate or the former layer in a melted or half-melted state. These particles deformed under the impact effect and then formed splats. The gas porosity phenomenon and loose-packed layer structure resulted in the pores and micro-cracks between the splats. In addition, the volume shrinkage of droplets led to the pores and micro-cracks within the splats (Ref 26). However, the lamellar structure of the Al coating is not obvious, which could be ascribed to the high melting degree of the Al particles during the spraying process. The inherent characteristics of the arc-sprayed coatings, including pores and micro-cracks, could also be found in the Al coating (Ref 27). Based on the grayscale threshold method (Ref 28), the porosity of as-sprayed Zn coating and Al coating is calculated to be 3.1% and 7.6%, respectively.

Fig. 1
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Optical cross-sectional micrographs of as-sprayed Zn coating (a) and Al coating (b)

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The typical surface morphologies of the as-sprayed and pore-sealed Zn and Al coatings are illustrated in Fig. 2. It can be clearly found that some pores and micro-cracks are distributed on the surface of Zn (Fig. 2a) and Al (Fig. 2b) coatings. It is generally accepted that the pores and micro-cracks in thermal sprayed coatings are extremely detrimental to their performance (Ref 29). Hence, the pore-sealing treatment of the as-sprayed coatings is necessary for their practical applications. The commercially available silicone used in this study exhibited good sealing property because of its excellent liquidity and penetrability. After pore-sealing treatment, the surface morphologies of pore-sealed Zn and Al coatings can be seen in Fig. 2(c), (d), respectively. It could be obviously observed that the pores and micro-cracks were effectively blocked, thus the pore-sealed coatings exhibited less pores and micro-cracks compared to the as-sprayed coatings.

Fig. 2
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SEM surface micrographs of Zn coatings (a, c) and Al coatings (b, d): (a), (b) as-sprayed; (c), (d) pore-sealed

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Open Circuit Potential

Figure 3 shows the open circuit potential (OCP) of pore-sealed Zn coating and Al coating in SRB seawater for different immersion times. In the first 6 days, the OCP of the pore-sealed Zn coating fluctuated downward from − 976 to − 1025 mV. This is because with the diffusion of corrosion medium (Cl−, S2−, etc.), the ZnO film on pore-sealed Zn coating would be dissolved, and the partial failure of oxide film directly resulted in the decrease in OCP (Ref 30). From 6 to 9 days, the OCP had a slow growth from − 1025 to − 980 mV. This phenomenon could be ascribed to the formation and aggregation of corrosion products and the accumulation of SRB, which would reduce the porosity in the coatings and delay the diffusion of corrosive liquid (Ref 31, 32). After 9 days, the OCP kept at a basically stable value of − 1002 mV, which manifested the electrochemical corrosion kinetics of the specimen reached relative stability. For the pore-sealed Al coating, the OCP maintained at a stable value of about − 772 mV during the whole testing process. This is because of the protective effect of dense alumina film formed on the surface of the coating (Ref 15, 33).

Fig. 3
figure 3

Open circuit potential-time curves of pore-sealed Zn coating and Al coating in SRB seawater

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Meanwhile, it could be found that the OCP of the pore-sealed Al coating is consistently higher than that of the pore-sealed Zn coating throughout this experiment, which signified that in SRB seawater the pore-sealed Al coating showed superior corrosion resistance to the pore-sealed Zn coating.

Potentiodynamic Polarization Analyses

Figure 4 shows the Tafel polarization curves of pore-sealed Zn coatings (a) and Al coatings (b) in seawater containing SRB for different immersion times. The obtained corrosion potential (Ecorr) and current density (icorr) for each curve were tabulated in Table 3.

Fig. 4
figure 4

Polarization curves of pore-sealed Zn coatings (a) and Al coatings (b) in SRB seawater

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Table 3 Corrosion potentials (Ecorr) and corrosion current densities (icorr) of pore-sealed Zn coatings and Al coatings
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As presented in Table 3, it could be found that the icorr of the pore-sealed Zn coatings increased from 3.754 × 10−6 to 6.123×10−6 A cm−2 at first, and then decreased to 4.134 × 10−6 A cm−2 with the increase of immersion time. This is mainly because a large number of metabolites with high causticity were produced in the early immersion period. These metabolites and chloride ions kept attacking the coatings and increasing the corrosion rate of the coatings gradually. Afterward, the corrosion products formed on the surface and blocked the pores or micro-cracks in the coatings, which would protect the coatings from subsequent damage and decrease their corrosion rates (Ref 2). However, the icorr of the pore-sealed Al coatings kept decreasing from 3.093 × 10−6 to 1.191 × 10−6 A cm−2 throughout the test, this could be due to the outstanding protective effect of dense alumina film. As corrosion time goes on, the accumulation of SRB further prevented the pore-sealed Al coatings from being destroyed (Ref 34, 35).

Throughout the course of the experiment, the pore-sealed Al coating presented higher corrosion potential along with lower corrosion current density than the pore-sealed Zn coating, suggesting that the pore-sealed Al coating showed better corrosion performance than the pore-sealed Zn coating in seawater with SRB. The Tafel polarization results are in good agreement with the above OCP results.

Electrochemical Impedance Spectra

To further elucidate the corrosion behavior of pore-sealed Zn coating and Al coating in seawater containing SRB, EIS measurement was performed, and the results were illustrated in Fig. 5. As shown in Fig. 5(a), the Nyquist plots of pore-sealed Zn coatings all showed two capacitive arcs, i.e., a capacitive arc at high frequency which is corresponding to the capacitance and resistance of coating, and a capacitive arc at low frequency which is corresponding to the capacitance of double electric layer and the resistance of electrochemical reaction at solution/coating interface (Ref 36). For pore-sealed Zn coating, it can be found that the arc diameter at low frequency reduced greatly after immersion for 2 days, which is due to destruction of oxide films formed on the coatings. From 3 to 15 days, the arc diameters of the coatings fluctuated slightly. Hence, as the immersion time prolongs, the corrosion resistance of the pore-sealed Zn coatings decreased first and then remained relatively stable. Figure 5(b) shows the Nyquist plots of pore-sealed Al coating in SRB seawater. The arc diameters of impedance plots remained stable from 1 to 5 days, and then increased from 5 to 15 days. The results revealed that the corrosion rates of pore-sealed Al coatings kept relatively stable during the early period, and then decreased.

Fig. 5
figure 5

Nyquist plots of pore-sealed Zn coatings (a) and Al coatings (b) in SRB seawater

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To better explain the corrosion behavior of these coatings, an equivalent circuit (as shown in Fig. 6) was applied for the simulation of EIS data. The Rs(Qc(Rc(QdlRct))) circuit is commonly used for metal/oxide layer/electrolyte system (Ref 37), where Rs, Rc, Rct, Qc, and Qdl represent the resistance of solution, the resistance of the coating, the charge transfer resistance of the corrosion reaction, the capacitance of the coating, and the capacitance of the double layer or oxide film, respectively. A ZSimpwin software was applied to fit the EIS plots to obtain the equivalent circuit parameters, which were listed in Tables 4 and 5.

Fig. 6
figure 6

The equivalent circuit used for EIS analysis in this study

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Table 4 Electrochemical parameters of pore-sealed Zn coatings in SRB seawater acquired from EIS plots
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Table 5 Electrochemical parameters of pore-sealed Al coatings in SRB seawater acquired from EIS plots
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It could be found from Table 4 that the Rct values of the pore-sealed Zn coatings were relatively large at the initial stage (1-2 days). This is because a protective film composed of oxide film and silicone resin retarded the diffusion of corrosive liquid effectively. After 2 days of immersion, the Rct values decreased rapidly, which could be attributed to the dissolution of ZnO film and the penetration of corrosive medium through these active zones (Ref 38). As corrosion time increased from 5 to 15 days, the Rct values increased slowly, which is mainly due to the combined effect of SRB and their metabolites. As corrosion time further increased, a large number of metabolites would be produced and then reacted with the dissolved Zn2+ to form corrosion products, such as ZnS. These corrosion products adhered to the coating or plugged the pores in the coating, thus retarding further corrosion (Ref 39). For the pore-sealed Al coatings, it can be seen from Table 5 that the Rct values showed a slight fluctuating rise during the first 5 days. Similar to the pore-sealed Zn coatings, the main reason could be due to the protective film which consisted of silicone resin and oxidation film. However, after immersion for 5 days, the Rct values of the pore-sealed Al coatings increased rapidly. This is because with the prolongation of the immersion time, the SRB proliferated continuously and then deposited on the dense Al2O3 film to form biofilms. The deposited biofilms can further prevent the coatings from being attacked by the corrosive medium, and thus improve the corrosion performance of the coatings (Ref 40, 41).

Besides, it is obvious that the Rct values of the pore-sealed Al coatings were much larger than that of the pore-sealed Zn coatings. This could be ascribed to the different oxidation films formed on the coatings. Considering the oxide film of the pore-sealed Al coatings is quite dense and hard to be dissolved in corrosive liquid, the film can prevent the coatings from being corroded effectively. However, the oxide film of the pore-sealed Zn coating is easy to destroy in corrosive medium (Ref 42). In consequence, the pore-sealed Al coatings exhibited better corrosion performance than the pore-sealed Zn coatings in the SRB seawater.

Surface Morphologies After Corrosion

After soaking for 15 days, the SEM images and EDS analyses of the pore-sealed Al coating and Zn coating in seawater containing SRB were shown in Fig. 7. It can be seen from Fig. 7(a) that some corrosion products and irregular particles were distributed on the eroded surface of pore-sealed Zn coating. These particles were supposed to be the coatings peeled off under the joint effect of corrosive medium and SRB. While in Fig. 7(b), a covering film with fewer corrosion products could be found on the surface of pore-sealed Al coating. The covering film on the surface of pore-sealed Al coating was relatively homogeneous, but the covering film on the surface of pore-sealed Zn coating was looser. In order to better understand the corrosion mechanism, EDS measurement was conducted. The EDS results showed that both pore-sealed Zn coating and Al coating contained C, O, P, S, and Si. It can be observed that the S content on the surface of pore-sealed Al coating is lower than that of pore-sealed Zn coating. Because the dense Al2O3 film is hard to be corroded in the corrosive medium, few aluminums can react with SRB metabolites (such as H2S, S2−, and other S-containing metabolites) to produce S-containing corrosion products (Ref 43). This result can also prove that the pore-sealed Al coating exhibited better corrosion performance than the pore-sealed Zn coating in SRB seawater.

Fig. 7
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SEM images and EDS analyses of pore-sealed Zn coating (a) and Al coating (b) after corrosion in SRB seawater

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Conclusions

  1. (1)

    Both arc-sprayed Zn coating and Al coating exhibited many pores and micro-cracks. After pore-sealing treatment, the pores and micro-cracks of the coatings were effectively blocked.

  2. (2)

    The corrosion rate of pore-sealed Zn coating increased first and then decreased, while the corrosion rate of pore-sealed Al coating kept decreasing. This is because the oxide film of pore-sealed Zn coatings is easier to destroy at the early stage of the immersion process, but the dense oxide film of pore-sealed Al coating can play a protective function from the beginning of immersion. At later stage of immersion, the corrosion products and the biological film formed on the surface of pore-sealed Zn coating and pore-sealed Al coating can prevent the coatings from being corroded.

  3. (3)

    In seawater with SRB, the pore-sealed Al coating exhibited better corrosion performance than the pore-sealed Zn coating. For pore-sealed Zn coating, the protective effect is mainly due to the sealing effect of the corrosion products. For pore-sealed Al coating, the protective effect depends on the dense Al2O3 film and the biological film which can prevent the invasion of corrosive medium.