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. 2020 Dec 24;72:105438. doi: 10.1016/j.ultsonch.2020.105438

The optimization of microbial influenced corrosion resistance of HVOF sprayed nanostructured WC-10Co-4Cr coatings by ultrasound-assisted sealing

Sheng Hong a,⁎, Ziyu Wei a, Kailin Wang a, Wenwen Gao a, Yuping Wu a, Jinran Lin b,c
PMCID: PMC7803796  PMID: 33388693

Highlights

  • •

    Nanostructured WC-CoCr coatings were prepared by HVOF spraying process.

  • •

    Effect of ultrasound-assisted sealing on microbial corrosion was investigated.

  • •

    Microbial corrosion processes of unsealed and sealed coatings were different.

  • •

    Ultrasound-assisted sealing significantly delayed the microbial corrosion process.

Keywords: Ultrasound-assisted sealing, Microbial influenced corrosion, High-velocity oxygen-fuel, WC-10Co-4Cr, Sulfate-reducing bacteria

Abstract

In this study, high-velocity oxygen-fuel (HVOF) sprayed nanostructured WC-10Co-4Cr coatings were subjected to seawater with sulfate-reducing bacteria (SRB) for different time. The effect of ultrasound-assisted sealing with aluminum phosphate on the microstructural features and microbial influenced corrosion (MIC) behavior was evaluated using scanning electron microscopy (SEM), potentiodynamic polarization and electrochemical impedance spectroscopy (EIS). The results showed that the ultrasound-assisted sealing promoted the infiltration of the sealant into as-sprayed coating, enhanced the resistance values of about one order of magnitude, and reduced the corrosion current density. During the whole immersion period, the MIC process of both unsealed and sealed coatings can be divided into two different stages, and the ultrasound-assisted sealing treatment significantly delayed the MIC process, suggesting that the ultrasound-assisted sealing with aluminum phosphate is an effective way for controlling the MIC of SRB on the WC-10Co-4Cr cermet coatings in marine environment.

1. Introduction

Microbial influenced corrosion (MIC) is a considerable issue of material degradation in many marine and offshore facilities, which causes environmental concerns and enormous economic losses [1]. As ubiquitous anaerobes, sulfate-reducing bacteria (SRB) is frequently studied because SRB-influenced corrosion can lead to pitting corrosion and pinhole leaks, which is usually more serious than uniform corrosion [2], [3]. On account of the combined virtues of promising toughness and high hardness, WC-Co hardmetals are attractive candidates for use in a wide range of components where erosion and wear resistance are required simultaneously [4], [5], [6], [7]. Moreover, for corrosive environmental conditions, the tribocorrosion resistance of WC-Co hardmetals can be improved through adding Ni and/or Cr into the Co binder phase and adjusting WC grain size [8], [9]. Compared with bulk materials, thermal spraying technology can offer relatively low material cost, high efficiency and good flexibility, thus it is regarded as a preferred protective measure for engineering and structural materials [10], [11], [12], [13]. In particular, high-velocity oxygen-fuel (HVOF) spraying shows potential for developing nanostructured WC-Co based coatings as a result of the suppression effect of thermal dissolution and decarburization of the nano WC grains [14], [15].

In order to further prolong the operating life of HVOF sprayed coatings those are exposed to severe environments, post-treatments (e.g., remelting, sealing, annealing) are still required for the reduction in porosity, although a relatively low porosity of less than 2% can be achieved by optimizing the HVOF spray parameters [16], [17], [18], [19]. Among all post-treatments, sealing is found to be a suitable method to offer effective surface protection because of its advantages in economy and technology. Using inorganic or organic sealants with the aid of various sealing technologies can improve the cohesion strength of the coating, which results in the obstruction of corrosive media into the coating [20], [21], [22], [23], [24], [25]. For example, sealant of aluminum phosphate is widely applied in various thermal sprayed coatings including oxide ceramic coatings and amorphous/nanocrystalline coatings [25], [26], [27]. In addition, sealant of aluminum phosphate can penetrate deeper into the coating by associating ultrasonic irradiation with conventional impregnation sealing treatment, and then enhance the anti-corrosion property of the coating [16], [28]. It is therefore worthwhile to implement the ultrasound-assisted sealing of HVOF sprayed coatings with aluminum phosphate, when the coatings are exposed to a marine environment containing microorganisms. As the principal culprits among the microorganisms, SRB can cause microbial corrosion of the coating, which is influenced by biological factors [29].

It was recently demonstrated in our previous research that HVOF sprayed conventional WC-10Co-4Cr coating exhibited a strong ability to withstand MIC of SRB in seawater, which was close to that of the 1Cr18Ni9Ti stainless steel [30]. However, the influence of sealing treatment on the MIC resistance of HVOF sprayed nanostructured WC-10Co-4Cr coatings using ultrasonic irradiation is still unclear, which may reduce or delay the appearance of corrosion. The new/different in the present study compared to the previous one is that we attempt to further enhance the MIC resistance of HVOF sprayed WC-10Co-4Cr coating by sealing treatment using ultrasonic irradiation. In the current work, the morphologies and microstructures of two groups of HVOF sprayed nanostructured WC-10Co-4Cr coatings, i.e. as-sprayed coating (ASC) and ultrasound-assisted sealing coating with aluminum phosphate (UASC-AP) were characterized. The electrochemical impendence spectroscopy (EIS) and potentiodynamic polarization were applied to assess the variability of the MIC resistance of the coatings. Meanwhile, the current research was aimed at clarifying mechanisms guiding MIC in the presence of SRB for the ASC and UASC-AP.

2. Experimental details

Commercially nanostructured WC-10Co-4Cr cermet powder (Infralloy-7410) purchased from Inframat Advanced Materials Corp., Farmington, CT, USA was chosen as the raw materials in the present work. The nominal chemical composition (in wt. %) of the powder was: 10.0 Co, 4.0 Cr, 5.3C and W as the balance. The coatings were prepared with a thickness of around 400 μm on AISI 1045 steel substrates using a commercial Praxair Tafa-JP8000 HVOF spraying system. Spray experiments were conducted within open air at the optimum spraying parameters as listed in Table 1. The detailed spraying process as well as the microstructures of the coatings can be seen in our previous work [31]. Before the spraying process, the substrates were degreased, sandblasted by Al2O3 grits with approximately 550 µm in size, cleaned with acetone, rinsed with distilled water, and dried in a furnace at 100 °C for 3 h. After the spraying process, the coating specimens were wire cut into 1 cm × 1 cm pieces for subsequent characterization and testing.

Table 1.

Spraying parameters of nanostructured WC-10Co-4Cr coating.

Parameters Kerosene flow rate (gph) Oxygen flow rate (scfh) Carrier gas flow rate (scfh) Spray distance (mm) Powder feed rate (rpm) Spray gun speed (mm·s−1)
Value 6 2000 23 330 5 280
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The orthophosphoric acid (85 wt% H3PO4) and aluminum hydroxide (Al(OH)3) with a weight ratio of 4.2:1 were mixed together, dissolved in 20 wt% distilled water and heat-treated at 70 °C with magnetic stirring for 60 min to obtain the sealant of aluminum phosphate, ensuring the Al to P molar ratio of about 1:3 [22]. The ultrasound-assisted sealing treatment with the sealant of aluminum phosphate was done on the ASC specimens following a two-step process. Firstly, an ultrasonic irradiation treatment was performed with a frequency of 40 kHz at atmospheric pressure of 1.013 × 105 Pa and room-temperature of 25–30 °C for 4 h, as schemed by the diagram in Fig. 1. A KQ-300DE ultrasonic generator (Kunshan Ultrasonic Instruments Co., Ltd.) with internal dimensions of 30 cm × 24 cm × 15 cm was used to supply the ultrasonic bath at acoustic power of 300 W. The second stage was a heat treatment, which was conducted at 100 °C for 120 min, 200 °C for 120 min and 250 °C for 60 min in turns, and followed by furnace cool [25]. For microstructural characterization and microbial corrosion testing, the coating specimens were ground and polished using standard metallographic procedures for removing redundant sealant and ensuring final finish.

Fig. 1.

Fig. 1

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Schematic diagram of the ultrasound-assisted sealing apparatus.

The morphologies of the ASC and UASC-AP on surface and cross-section were investigated by scanning electron microscopy (SEM, Hitachi S-3400N, Japan) and energy dispersive spectroscopy (EDS, EX250). Porosity was measured using Image-J software and was determined from 15 randomly selected SEM images at a magnification of 1000 on the cross-section of the ASC and UASC-AP.

The SRB seeds were enriched from the marine mud of the Qingdao Coast, China. In order to enrich anaerobic bacteria, a sterile modified Postgate’s C (PGC) medium coupled with fresh marine sediment was used, which consisted of dipotassium phosphate (0.5 g·L−1), calcium chloride 6-hydrate (0.06 g·L−1), ammonium chloride (1.0 g·L−1), magnesium sulfate 7-hydrate (0.06 g·L−1), yeast extract (1.0 g·L−1), 70% sodium lactate (6 mL·L−1) and sodium citrate (0.3 g·L−1) dissolved in 1 L of seawater from Qingdao offshore area. Then, the sterile agar plates with the aid of several sterile inoculation loops and single colonies were used to purify the bacteria. This process was repeated several times until the single colonies of SRB strain were incubated successfully in the solid PGC medium. The adjustment of the pH value of the medium was performed with a moderate quantity of sodium hydroxide to 7.0, which was subsequently heat-sterilized at 121 °C for 20 min.

Electrochemical measurements of the ASC and UASC-AP were conducted at room temperature by the Solartron SI1287 electrochemical interface and SI1260 impedance/gain phase analyzer control systems. All tests were carried out in a traditional three-electrode cell composed of the coating specimen as the working electrode, the silver/silver chloride (Ag/AgCl, 3 M KCl) as the reference electrode and the ruthenium-titanium electrode as the counter electrode. Prior to tests, salt bridge, electrodes and rubber stoppers were cleaned with acetone in an ultrasonic bath, followed by sterilization under the ultraviolet radiation chamber for 20 min. After the open circuit potential of the working electrode hold, EIS measurements were performed on the frequency that ranges from 100 kHz to 10 mHz by an alternating current amplitude of ±10 mV. The EIS results were fitted using the ZsimpWin software (version 3.21). Potentiodynamic polarization tests were conducted at a potential sweep rate of 0.5 mV·s−1. The corrosion potential (Ecorr) and corrosion current density (icorr) were obtained by extrapolating the fitting lines of the cathodic and anodic polarization curves. All tests were repeated three consecutive times for ensuring the reproducibility of test results.

3. Results and discussion

3.1. Effect of ultrasonic irradiation on morphologies of the coatings

Fig. 2 compares the surface morphologies of the ASC and UASC-AP. As displayed in Fig. 2(a) and (b), the surface of the ASC is relatively rough with a number of open or semi-closed porosities due to the incomplete link between un-melted or semi-melted particles during the successive deposition process [32]. Regardless of porosities, some protuberant carbide particles are scattered around the metal matrix. It is thought that this may be a consequence of weak adaptability of the molten droplets to the previous deposited coating, causing non-overlapping of some droplets and thus the residual stress [33]. Zhang et al. [23] have previously noted that the feedstock powders may undergo serious plastic deformation during high velocity thermal spraying process, while porosities and microcracks were hard to eliminate. In contrast, there exist many filamentous substances on the surface of the UASC-AP (Fig. 2(c) and (d)). The quantitative analysis (wt%) of point A (marked with a yellow cross in Fig. 2(d)) is given in Fig. 2(e). The EDS result shows that the filamentous substance primarily contains element O together with the remarkable presence of elements P and Al, implying that the majority of porosities are effectively covered by sealant during the ultrasound-assisted sealing process. It also has a few porosities on the surface of the UASC-AP, which is considered as a result of the generation and solidification of the macro-molecular chain [28]. Despite this, the sealing layer can reduce the surface area of the ASC exposed to the corrosive medium and inhibit the MIC process of the coating, which is proved by the surface morphologies at the lower magnification (Fig. 2(a) and (c)) and the after-mentioned electrochemical tests.

Fig. 2.

Fig. 2

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SEM images of the surface morphologies of the ASC (a,b) and UASC-AP (c,d), (e) EDS analysis of the region marked with yellow cross in (d). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

In order to gain further data about the penetration of the sealant, the cross-section morphologies of the ASC and UASC-AP are shown in Fig. 3. It is observed from Fig. 3(a) that no through-pores and through-cracks are observed, although there are certain microcracks and pores in the cross-section of the ASC. Furthermore, the pores in the outer layer are obviously more than those in the inner layer, which may be attributed to the entrapped gases escaping within an extremely short time before the final process of rapid solidification [34]. In accordance with the findings of Li et al. [35], a difference in porosity and microcracks between the inner layer and the outer layer has been found to occur in the cross-section of arc sprayed zinc coating. As shown in Fig. 3(b), the pores in the cross-section are evidently closed, suggesting that the ultrasound-assisted sealing permit more sealant to infiltrate into the ASC and then improve the MIC resistance. Chemical bonding and adhesive bonding are main sealing mechanisms for aluminum phosphate, which appears to have the ability to block the defects of the coatings [28], [36]. Wang et al. [17] have previously noted that in porosity sealing treatments on the HVOF sprayed Fe-based amorphous metallic coatings, under conventional impregnation sealing conditions, a relatively limited depth of penetration was seen. In the present study, the UASC-AP has a lower porosity (0.53 ± 0.07%), with a reduction in porosity of 44.8% compared to that of the ASC (0.96 ± 0.13%). It can be explained that the ultrasonic energy not only ensures a certain level of compatibleness of coating with sealant, but also accelerates the escape of air from the pores within the sealant [36], [37]. The role of ultrasonic energy on the sealing depth and the anti-corrosion property has been indicated by Liu et al. [28] and Si et al. [38].

Fig. 3.

Fig. 3

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SEM images of the cross-section morphologies of the ASC (a) and UASC-AP (b).

3.2. Effect of ultrasonic irradiation on MIC behaviors of the coatings

3.2.1. EIS

EIS is a semi-quantitative method for in-situ and non-destructively probing the electrochemical processes during exposure to the microbial influenced corrosive environment. Impedance data can be analyzed to reveal the reactions at the interface between metal and solution as well as the formation of new corrosive products, natural oxidation films and SRB biofilms [39]. The Nyquist plots and the corresponding Bode plots of the ASC and UASC-AP after immersing for different times in seawater with SRB are shown in Fig. 4. From Fig. 4(a), it can be found that the diameter of Nyquist plots of the ASC decreases at the first 2 d, and then starts to increase with immersion time up to 15 d. While as for the UASC-AP is concerned (Fig. 4(c)), there is no obvious difference in the diameter with extending immersion time. In addition, the diameters of Nyquist plots of the ASC are always smaller than those of the UASC-AP, although the general shape of Nyquist diagrams of both coatings is almost the same. These results can be further proved by the impedance modulus values, which demonstrate that the sealants are comparatively steady during the immersion period and enhance the anti-corrosion property of the ASC with the aid of ultrasonic energy.

Fig. 4.

Fig. 4

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Nyquist (a,c) plots and Bode plots (b,d) of the ASC (a,b) and UASC-AP (c,d) in seawater with SRB for different times.

Compared with the ASC, the UASC-AP has higher impedance modulus values in the full range of frequency as shown in Fig. 4(b) and (d), although both coatings have the same impedance magnitude. Moreover, the difference in the impedance modulus values between the ASC and UASC-AP is obvious in the low frequency range, implying the weak protection of the ASC. From the variations in the impedance modulus values of the ASC, it can be noticed that there display an evident decrease at the initial immersion stage (1 d–2 d), suggesting that the generation of the passive film on the surface of the ASC may be inhibited under the combination of SRB metabolites and corrosive ions [30]. In contrast, the increase of the impedance modulus value for the UASC-AP in the middle frequency range is more pronounced than that in the low frequency range with extending immersion time, which reflects that the ultrasound-assisted sealing mainly affects cracks or pores within the oxidation films at the later immersion stage (8 d–15 d) [40], [41]. Furthermore, there display two time constants in Bode plots of the ASC for all immersion time. Compared with the ASC, the evolution of microbial corrosion electrochemical behavior of the UASC-AP can be divided into different stages, where three partially overlapped time constants are detected from 1 d to 8 d and two well-defined time constants are detected at 15 d. Other than the cases of the ASC where no clear changes are identified in the phase angle of Bode plots during all immersion time, the phase angle values of the UASC-AP in the middle frequency range decrease with increasing immersion time. The time constant in the middle frequency range represents a relaxation process, which may be associated with the existence of inorganic sealant as the interfacial layer [42]. It is well known that bath ultrasound can provide low intensity and uneven spreading of cavitation and facilitate enhanced mass transfer of sealants, further resulting in the formation of a mixture of AlPO4, H2(AlP3O10)·H2O, Al2P6O18 and Al(PO3)3 in a shorter time in comparison to conventional impregnation sealing [28], [43], [44], [45]. In the present study, improvement of mass transfer as well as mechanical effect can be attained since the ultrasonic irradiation treatment was carried out at a frequency of 40 kHz [46], [47].

3.2.2. Equivalent circuit

In order to further demonstrate the MIC processes of the ASC and UASC-AP after immersing for various times in seawater with SRB, equivalent circuits those proposed to fit the EIS data by ZSimpWin software are revealed in Fig. 5. The chi square (χ2) values are of the order of 10−3, ensuring the quality of data fitting. As for the ASC after immersing for 1 d to 2 d, an equivalent circuit with two time constants (Fig. 5(a)) is established, which consists of the resistance of electrolyte solution Rs, the resistance of pores Rp, the capacitance of the coating Qc, the resistance of charge transfer Rct, and the capacitance of the double layer Qdl. The capacitance is substituted by the constant phase element (CPE or Q), which is described as ZQ = Y0−1(jω)−n (0 < n <n 1) and associated with the dispersing effects of heterogeneous biofilm on the coating surface as well as surface roughness [48]. After immersing over 5 d (Fig. 5(b)), an extra element of Warburg impedance (Zw) should be necessary as shown in the equivalent circuit of the ASC, indicating that the corrosive products have accumulated in pores [49]. This suggests that there is a diffusion controlled mechanism for the ASC at the later immersion stage (5 d–15 d) based on relevant investigations [50], [51], [52]. In the case of the UASC-AP, Rsl, Rinter, Qsl, and Qinter stand for the resistance of the sealing layer, the resistance of the interfacial layer, the capacitance of the sealing layer, and the capacitance of the interfacial layer, respectively. Fig. 5(c) shows an equivalent circuit consisted of three RQ circuits, which corresponds to different frequency ranges of impedance spectroscopy as shown in Fig. 4(d) and can be gratifyingly used for fitting the EIS data of the UASC-AP after immersing for 1 d to 8 d. However, there are only two RQ circuits without the circuit RslQsl (Fig. 5(d)) when the immersion time increases up to 15 d, which demonstrates the disappearance of the anti-corrosion property of the sealing layer due to the expansion of the diffusive pathway.

Fig. 5.

Fig. 5

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Equivalent circuits used to fit EIS data of the ASC (a,b) and UASC-AP (c,d) in seawater with SRB.

Fig. 6 shows the EIS fitting results of the ASC and UASC-AP after immersing for different times in seawater with SRB. In the present work, the fitting values of Rct and Rsl + Rct are significant parameters for the assessment of the corrosion rate of the ASC and UASC-AP, respectively, since the values of Rct and Rsl + Rct are more than a thousand times greater in comparison with those of Rs, Rp, and Rinter. As for the ASC (Fig. 6(a)), the Rct value decreases from 57.9 kΩ·cm2 at 1 d to 29.2 kΩ·cm2 at 2 d, indicating that the corrosion media has penetrated into the ASC and triggered the decomposition of the active zones after 2 d immersion. With increasing immersion time, the Rct value increases from 29.2 kΩ·cm2 at 2 d to 58.1 kΩ·cm2 at 15 d due to the blockage in the pathway of the electron transfer, which may be related to the formation of SRB biofilms and the accumulation of corrosive products on the surface of the ASC [53]. In addition, the variation trend of Rp is similar to the characteristic in Rct shift for the ASC. These suggest that the amount of oxide films and SRB biofilms is still kept to a comparatively high level after immersing in seawater with SRB for 15 d, demonstrating that the ASC has a certain protective effect of steel substrates against continuous MIC. It is clearly observed from Fig. 6(b) that the Rsl value of the UASC-AP decreases from 823.4 kΩ·cm2 to 456.9 kΩ·cm2 during the 1 d–8 d immersion period, whereas the Rct value of the UASC-AP is fairly low of about 10−3 kΩ·cm2 after immersing for 1 d to 8 d, and then increases quickly to 499.0 kΩ·cm2 with immersion time up to 15 d. Meanwhile, the Rsl + Rct value of the UASC-AP keeps decreasing up to 8 d, and then increases slowly at the later immersion stage (8 d–15 d). Accordingly, the corrosion process of the UASC-AP may be characterized by two periods, which reflect the protective effect of the ultrasound-assisted sealing layer along with the time extension of immersion. During the first stage of immersing (up to 8 d), the sealing layer gradually degrades as the penetration of electrolyte into the interfacial layer and the coating by diffusion and capillary action [23]. After 8 d immersion, the Rsl + Rct value is still high (456.9 kΩ·cm2), which may be ascribed to the fact that the ultrasound-assisted sealing layer effectively restricts the transference of corrosive medium at a lower diffusion rate. The second stage is the immersion time of 8 d to 15 d, where a sharp increase in the Rct value accompanying with the disappearance of the Rsl value is observed. On the one hand, the protective function of the sealing layer is gradually lost, which is resulted from further expansion of the diffusive pathway and partial destruction of the ultrasound-assisted sealing layer under the action of SRB metabolic activity. On the other hand, the infiltration process of electrolyte into the coating is still difficult due to the compact microstructure with the aid of ultrasonic energy, as well as the formation of oxide films and SRB biofilms. After the sealing treatment with ultrasonic irradiation on the ASC, the resistance values of the UASC-AP are increased by one order of magnitude at the corresponding point-in-time, implying that the corrosion process in seawater with SRB of the ASC is significantly delayed.

Fig. 6.

Fig. 6

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The impedance values of the ASC (a) and UASC-AP (b) as a function of immersion time in seawater with SRB.

3.2.3. Potentiodynamic polarization

Potentiodynamic polarization curves for the ASC and UASC-AP after immersing for different times in seawater with SRB are displayed in Fig. 7. Electrochemical parameters extracted from the curves are calculated and listed in Table 2. As can be seen, the potentiodynamic polarization behaviors are closely related to the adhesion of SRB biofilms and the SRB metabolic activity. Moreover, there is an obvious deviation in the response of the ASC and UASC-AP to the corrosive medium which mainly includes chloride ions and SRB metabolites. With extending immersion time, the corrosion potential (Ecorr) values of the ASC and UASC-AP both shift in a negative direction as a result of the continuous infiltration of electrolyte and the dissolution of active zones within the coatings [54]. Compared with the UASC-AP, the Ecorr values of the ASC are always lower during the whole immersion period, implying that the ASC has a greater tendency toward corrosion. The corrosion current density (icorr) value of the ASC firstly increases as the immersion time ranging from 1 d to 8 d, and then decreases at the later immersion stage (8 d–15 d). This can be ascribed to the inhibition of passive film formation under repetitive attacks of chloride ions and SRB metabolites at the early immersion stage, and the formation and accumulation of SRB biofilms and corrosive products at a later stage, respectively. As regards the UASC-AP, the icorr value decreases at the first 2 d, indicating that the adhesion and growth of SRB biofilms are suppressed by the presence of the ultrasound-assisted sealing layer. As the immersion time increases to 8 d, the icorr value of the UASC-AP is comparatively higher than that at the immersion time of 2 d. The reason is that the SRB metabolic activity accelerates the corrosion process and triggers the partial breakage of the sealing layer after a period of time [42], [55]. It will increase the transfer of corrosion medium and cause the reaction between corrosion medium and coating. Then, the icorr value decreases as the immersion time increases to 15 d, which is in accord with the results of the ASC at the later immersion stage. Furthermore, the UASC-AP exhibits a lower icorr value and, in turn, a lower corrosion rate in seawater with SRB, which can be explained by the fact that the ultrasonic energy promotes the infiltration of the sealant and blocks the micro-defects within the coating. As the immersion time increases up to 15 d, the UASC-AP has a lower icorr (0.573 μA·cm−2), with a reduction in icorr of 66.8% compared to that of the ASC (1.725 μA·cm−2). This is proved by the cross-section SEM images (Fig. 3) and the EIS analysis results (Fig. 4, Fig. 6) as mentioned before. Thus, ultrasound-assisted sealing with aluminum phosphate is a reasonable way to control the MIC of SRB on HVOF sprayed nanostructured WC-10Co-4Cr coating.

Fig. 7.

Fig. 7

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Potentiodynamic polarization curves of the ASC (a) and UASC-AP (b) in seawater with SRB for different times.

Table 2.

Corrosion current densities (icorr) and potentials (Ecorr) values obtained from the potentiodynamic polarization curves of the ASC (a,b) and UASC-AP (c,d) in seawater with SRB for different times.

Time Ecorr (mV)
 icorr (μA·cm−2)
ASC UASC-AP ASC UASC-AP
1 d −356 −262 1.364 1.006
2 d −485 −404 1.804 0.525
5 d −573 −497 2.118 0.909
8 d −618 −560 2.526 1.327
15 d −677 −607 1.725 0.573
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4. Conclusions

The primary objective of this work was targeted towards investigating the effects of ultrasound-assisted sealing with aluminum phosphate on the MIC of HVOF sprayed nanostructured WC-10Co-4Cr coating in seawater with SRB. Compared with the ASC, the UASC-AP possessed fewer micro-defects, higher resistance values of about one order of magnitude and lower corrosion current density values, and then provided better resistance to MIC. During the whole immersion period, the MIC process of both ASC and UASC-AP can be divided into two different stages, and the ultrasound-assisted sealing treatment significantly delayed the MIC process. The results of this work prove the capability of the ultrasound-assisted sealing method in control of MIC of SRB on nanostructured WC-10Co-4Cr coatings.

CRediT authorship contribution statement

Sheng Hong: Conceptualization, Methodology, Supervision, Writing - original draft, Writing - review & editing. Ziyu Wei: Investigation. Kailin Wang: Investigation. Wenwen Gao: Methodology. Yuping Wu: Supervision. Jinran Lin: Data curation.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

The research was supported by the National Natural Science Foundation of China (Grant Nos. 51979083 and 51975183), the Fundamental Research Funds for the Central Universities (Grant No. B210202100), the China Postdoctoral Science Foundation (Grant Nos. 2018T110435 and 2017M621665), the Postdoctoral Science Foundation of Jiangsu Province (Grant No. 2018K022A), the Natural Science Foundation of Jiangsu Province of China (Grant No. BK20201316), and Shuangchuang Program of Jiangsu Province. Professor Jizhou Duan of Institute of Oceanology, Chinese Academy of Sciences was also acknowledged for providing microbial influenced corrosion equipment.

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