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epoxy coating in seawater
Accepted Manuscript Effect of Pseudomonas sp. on the degradation of aluminum/epoxy coating in seawater
Tiantian Feng, Jinyi Wu, Ke Chai, Fuchun Liu PII: DOI: Reference:
S0167-7322(17)34788-8 doi:10.1016/j.molliq.2018.04.103 MOLLIQ 9006
To appear in:
Journal of Molecular Liquids
Received date: Revised date: Accepted date:
10 October 2017 24 March 2018 20 April 2018
Please cite this article as: Tiantian Feng, Jinyi Wu, Ke Chai, Fuchun Liu , Effect of Pseudomonas sp. on the degradation of aluminum/epoxy coating in seawater. The address for the corresponding author was captured as affiliation for all authors. Please check if appropriate. Molliq(2017), doi:10.1016/j.molliq.2018.04.103
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT Effect of Pseudomonas sp. on the degradation of aluminum/epoxy coating in seawater Tiantian Fenga#, Jinyi Wua#, Ke Chaia*, Fuchun Liub a
Key Laboratory of Advanced Materials of Tropical Island Resources (Hainan University), Ministry of
Education, Material and Chemical Engineering College, Hainan University, Haikou 570228, China b
Key Laboratory of Nuclear Materials and Safety Assessment, Institute of Metal Research, Chinese Academy
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of Sciences, Shenyang 110016, China
# First authors: these authors contributed equally to this work.
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Abstract
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* Corresponding author: Ke Chai, Tel: +86-0898-66279122, E-mail: [email protected]
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Polymeric coatings prevent metal materials from corrosion. Nevertheless, the degradation of the coatings in seawater by microorganisms is almost unknown. In this work, we explored the degradation of the aluminum/epoxy coating in sterile seawater and seawater inoculated with Pseudomonas sp. by electrochemical impedance spectroscopy, scanning electron microscopy, energy dispersive spectroscopy, Fourier transform infrared spectroscopy and contact angle test. The decreases in the corrosion resistance of the coating were significantly higher in seawater inoculated with Pseudomonas sp. than in sterile seawater. The mature biofilm formed on the coating and then the extensive under-coating corrosion occurred in seawater inoculated with Pseudomonas sp.. These results revealed that Pseudomonas sp. significantly decreased the corrosion resistance of the coating and might degrade the coating. Some bulges and tiny holes were observed on the coating in seawater inoculated with Pseudomonas sp.. The contents of Al and O significantly respectively decreased and increased on the surface of the coating in seawater inoculated with Pseudomonas sp. relative to those on the surfaces of the coating without immersion and in sterile seawater. The absorbance of the C-OH peak for the coating immersed in seawater inoculated with Pseudomonas sp. was significantly higher than that for the coating without immersion and the coating immersed in sterile seawater. Moreover, Pseudomonas sp. decreased the water contact angle of the coating surfaces. The results demonstrated that Pseudomonas sp. degraded the aluminum/epoxy coating through decomposing the aluminum and oxidizing the epoxy to forming hydroxyl. Keywords: Pseudomonas sp.; degradation; aluminum/epoxy coating; seawater.
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ACCEPTED MANUSCRIPT 1. Introduction Corrosion is a major damage to infrastructures in marine environment [1,2]. It is well known that more than 20% corrosion is attributed to microorganisms [3,4]. Bacteria in aquatic environments adhere to the metal surface and formed biofilm [5]. The biofilm produce an environment at the biofilm/metal interface that is different from the bulk water [6]. The differences can lead to electrochemical reactions on metals, namely corrosion. For example, Pseudomonas aeruginosa induced the corrosion of 2205 duplex stainless steel [7, 8], 2304 duplex stainless steel [9], the nickel-free high nitrogen stainless steel [10] and S32654 super austenitic stainless steel [11].
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Besides metals, microorganisms degrade lots of non-metallic materials. Polyimide used for electronic packaging was degraded by two fungi, Aspergillus versicolor and Chaetomiu sp. [12]. Penicillium simplicissimum YK could utilized polyethylene as a carbon source and lowered the molecular weight of the polyethylene [13]. Darby and Kaplan reported that polyurethane was susceptible to the damage by the fungi including Aspergillus niger, A. flavus, A. versicolor, Penicillium funiculosum, Pullularia pullulans, and Trichoderma sp. [14]. Three bacteria, Pseudomonas aeruginosa (UKMP-8T), Rhodococcus sp. M15-2 (UKMP-5T), and Rhodococcus sp. ZH8 (UKMP-7T), degraded 97.6-99.9% of crude oil within 7 days [15]. Kay et al. isolated 16 different bacteria degrading polyurethane, in which Corynebacterium sp., Corynebacterium sp. and Enterobacter agglomerans significantly decreased the tensile strength of polyurethane [16]. Especially, Pseudomonas could degrade more than 90 kinds of organics such as polyethylene [17], polyurethane [18], polycyclic aromatic hydrocarbons [19], and total petroleum hydrocarbon [20]. At present, the application of coatings is the major way to inhibit corrosion. Aluminum/epoxy composite coating is an excellent marine coating which has outstanding corrosion resistance, processability and chemical resistance, etc. However, the degradation of the coatings in seawater by microorganisms remains unclear.
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2. Experimental
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In this study, we detected the degradation of aluminum/epoxy coating by Pseudomonas sp. in seawater through electrochemical impedance spectroscopy (EIS) and scanning electron microscopy (SEM). Additionally, we explored the degradation mechanism of the coating by energy dispersive spectroscopy (EDS), Fourier transform infrared spectroscopy (FTIR) and contact angle test.
2.1. Materials and sample preparation
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Aluminum/epoxy composite paint was provided by the Institute of Metal Research, Chinese Academy of Sciences, Shenyang, China, which consists of mixed solvent (dimethylbenzene 7, n-butanol 3) 37, flatting agent (BYK 320) 3, curing agent polyamide 8115 37, NPEL-128 epoxy resin 60 and nano aluminum 15.2 by weight ratio. AISI1045 carbon steel plates with dimensions of Փ4 cm × 5 mm were used in EIS measurements. Glass plates with the dimension of 15 mm × 15 mm × 0.17 mm were used in FTIR and surface analyses. The carbon steel plates were sequentially polished with 120, 400, 800, 1200 and 1500 grit SiC papers in order to obtain smooth surfaces. all the plates were degreased with acetone and dried at room temperature to remove any contaminant on the surfaces and increase the coating adhesion. The coating was sprayed on the clean substrates. The thicknesses of the dry coating were 90 ± 3 μm on the carbon steel plates and 120 ± 3 μm on the glass plates after curing for 7 days at room temperature. Then the prepared samples were used in the immersion experiments. 2.2. Preparation of immersion seawaters 2
ACCEPTED MANUSCRIPT Pseudomonas sp. used in this work was isolated from the nature seawater of South China Sea with 2216E culture medium which was composed of 5.0 g peptone, 1.0 g yeast extract, and 1000 mL natural seawater. The medium pH was adjusted to pH=7.8 with NaOH solution and sterilized at 121 °C for 20 minutes (min) before use. Pseudomonas sp. was cultured under aerobic conditions at 26 ℃ for 2 days (d) in 2216E medium. 20 mL cell suspension was diluted with 2000 mL sterile seawater in the immersion box. The bacterium was cultured at 26 ℃ for 24 hours (h). Then the prepared samples were aseptically immersed in the seawater inoculated with Pseudomonas sp. and sterile seawater. The immersion was repeated in triplicate. 2.3. Characterization of the degradation of the coating
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The EIS measurements of the degradation of the coating were performed on a PARSTAT 2273 electrochemical workstation (Princeton Applied Research) with a three-electrode system, in which the saturated calomel electrode and the platinum electrode were employed as the reference electrode and the counter electrode, respectively. The coating side of the sample was the working electrode. The other side of the sample was connected to a wire and encapsulated. The test solutions were sterile seawater and seawater inoculated with Pseudomonas sp., respectively. The EIS measurements were implemented under 20 mV amplitude sinusoidal voltages and at the frequency range of 10-2-105 Hz. The equivalent electrical circuits (EEC) were determined with evaluator software (Zsimpwin).
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The surface morphology of the coating was observed by an environmental scanning electronic microscope (FEI XL30) after 30 d immersion in the two seawaters. The chemical compositions of the coating surfaces were analyzed by EDS. The sediment and biofilm had been removed from the coating surfaces before the EDS analysis..
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The FTIR spectra of the coating were measured in the range of 450-4000 cm-1 with a Fourier transform infrared spectrometer (Bruker IFS55 FTIR) after 30 d immersion in the two seawaters. The sediment and biofilm had been removed from the coating surfaces before the measure. The contact angles of the coating surfaces were examined by a water contact angle-measuring instrument (XHSCAZ-2) after 0, 7, 15, 30 d immersion in the two seawaters. The sediment and biofilm had been removed from the coating surfaces before the examination. The test solution was distilled water.
3.1. EIS analysis
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3. Results and discussion
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The different electrochemical processes occurring at the electrode-electrolyte interfaces can be characterized by the EIS data, in which the diameter of the capacitive loop in the Nyquist plot, the corrosion resistance in the Bode plot and the Rcoat value are important indicators of the corrosion resistance of coatings. The EIS data acquired for the aluminum/epoxy coating immersed in the two seawaters were shown in Fig. 1 and Fig. 2. The three sets of parallel immersion groups showed similar trends of the data. Whether the coating was immersed in sterile seawater or in the seawater inoculated with Pseudomonas sp., the Nyquist plots exhibited that the capacitive loop diameters of 1 h immersion were the same and much bigger than those of the other time immersion (Fig. 1, 2a and 2b), which illustrated that Pseudomonas sp. did not change the corrosion resistance of the coating in 1 h immersion. Hereafter, the capacitive loop diameter decreased from 1 h to 48 h of immersion in sterile seawater at first (Fig. 2a). Then the capacitive loop diameter increased from 48 h to 27 d of immersion (Fig. 2a). At last, the capacitive loop diameter decreased from 27 d to 145 d of immersion and reached the minimum (Fig. 2a). Similarly, in the seawater inoculated with Pseudomonas sp., 3
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the capacitive loop diameter decreased from 1 h to 48 h of immersion, increased from 48 h to 6 d of immersion, and decreased from 6 d to 45 d of immersion and reached the minimum (Fig. 2b). In addition, at the immersion time after the 1 h immersion, the capacitive loop diameters decreased considerably relative to that at 1 h of immersion in the two seawaters, respectively (Fig. 1, 2a and 2b). However, the decreases in the seawater inoculated with Pseudomonas sp. were significantly higher than the corresponding ones in sterile seawater (Fig. 1, 2a and 2b). The seawater and electrically conductive particles were the common components of the two seawaters, which could lead to the decreases of the capacitive loop diameters because the coating performance was influenced by them [21, 22]. The only difference between the two seawaters was Pseudomonas sp.. The significantly higher decreases of the capacitive loop diameters in the seawater inoculated with Pseudomonas sp. therefore revealed that Pseudomonas sp. significantly decreased the corrosion resistance of the coating and might degrade the aluminum/epoxy coating.
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In accord with the Nyquist plots, the Bode plots of immersion in the two seawaters showed that the corrosion resistance reached the maxima and the minima at 1 h and 145 d of immersion in sterile seawater and at 1 h and 45 d of immersion in the seawater inoculated with Pseudomonas sp., respectively (Fig. 2c and d). The corrosion resistances of the coating in the two seawaters were more than 10 9 Ω cm2 at 1 h of immersion (Fig. 2c and d), which meant that the coating had well barrier properties in 1 h immersion [23, 24]. Moreover, the decreases in corrosion resistance were significantly greater in seawater inoculated with Pseudomonas sp. than in sterile seawater accordingly from the 1 h immersion to the immersion after 1 h (Fig. 2c and d). All these data were consistent with the results of the Nyquist plots and supported the above mentioned inference.
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For the sterile seawater immersion, the equivalent electrical circuit (EEC) model was composed of Rs (the resistance of the seawater), Rcoat (the coating resistance) and Qcoat (the constant-phase element related to the coating capacitance) from 1 h to 48 h of immersion, in which only one time constant could be found (Fig. 3a). Rct (the charge transfer resistance in parallel with the double-layer capacitance formed by the electrochemical reaction at the metal interface) and Qdl (the constant-phase element related to the double-layer capacitance) appeared in the EEC from 6 d to 145 d of immersion besides Rs, Rcoat and Qcoat (Fig. 3b). Two time constants were identified from the EEC (Fig. 3b). The increase of the number of the time constant from 1 to 2 suggested that the under-coating corrosion occurred on the steel surface extensively later besides the coating was on the steel substrate. In the seawater inoculated with Pseudomonas sp., when the immersion time was from 1 h to 48 h, the EEC was the same as that in the sterile seawater and still only one time constant was found (Fig. 3c). The EEC included Rs, Rcoat, Qcoat, Rbiofilm (the biofilm resistance) and Qbiofilm (the constant-phase element related to the biofilm capacitance) from 3 d to 27 d of immersion (Fig. 3d). Two time constants were also identified from the EEC (Fig. 3d). At 45 d of immersion, the EEC involved Rs, Rcoat, Rbiofilm, Qbiofilm, Rct, Ccoat (the pure coating capacitance) and Cdl (the double-layer capacitance formed by the electrochemical reaction at the metal interface), which showed three time constants (Fig. 3e). The increases of the number of the time constant from 1 to 2, and then to 3 indicated that the mature biofilm on the coating and the extensive under-coating corrosion formed sequentially. These results further supported the results of the Nyquist plots and the Bode plots and the inference that Pseudomonas sp. significantly decreased the corrosion resistance of the coating and might degrade the aluminum/epoxy coating. In both seawaters, the Rcoat values were the same and at high level at 1 h of immersion (Fig. 4). When the immersion time was after 1 h, the Rcoat values decreased significantly (Fig. 4). Nevertheless, the decreases in seawater inoculated with Pseudomonas sp. were significantly higher than the corresponding ones in sterile seawater (Fig. 4). These results were in agreement with the results of the Nyquist plots, the Bode plots and the EEC and corroborated the inference from the Nyquist plots. 4
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The SEM images showed that the coating surface was intact without immersion (Fig. 5a and b). After 30 d immersion in sterile seawater, some sediment was on the coating surface, but no obvious damage was observed on the exposed part of the coating surface (Fig. 5c and d). When the sediment was removed, still no obvious damage was observed on the coating surface (Fig. 5e and f). After 30 d immersion in seawater inoculated with Pseudomonas sp., the biofilm formed on the coating surface (Fig. 5g and h). When the biofilm was removed, some bulges and tiny holes appeared on the coating surface (Fig. 5i and j). The results were consistent with the results of the EIS and demonstrated that Pseudomonas sp. degraded the aluminum/epoxy coating. 3.3. EDS analysis
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The sediment and biofilm had been removed from the coating surfaces before the analysis. The areas of EDS were randomly selected on the coating surfaces (Fig. 5a, e and i). EDS analysis revealed that the contents of C, O and Al elements after 30 d immersion in sterile seawater were almost the same as those without immersion (Table 1). After 30 d immersion in the seawater inoculated with Pseudomonas sp., the content of Al significantly decreased (Table 1). However, the content of O significantly increased (Table 1). The results proved that Pseudomonas sp. degraded the aluminum/epoxy coating through decomposing the aluminum and oxidizing the epoxy.
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3.4. FTIR analysis
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FTIR was carried out on the coating without immersion and the coating immersed for 30 d in sterile seawater and the seawater inoculated with Pseudomonas sp. to detect the degradation of the epoxy by Pseudomonas sp., as shown in Fig. 6. The sediment and biofilm had been removed from the coating surfaces before the detection.
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The absorption bands of the fingerprint region were at 550-1700 cm-1. 831 cm-1 peak referred to the out-of-plane C-H bending vibration [25]. 1600 and 1500 cm-1 peaks were assigned to the vibrations of the aromatic ring [26]. The absorption band at 1680-1750 cm-1 corresponded to the C=O stretching [27]. The absorption band at 2900-2960 cm-1 was attributed to the asymmetric stretching vibration of –CHx. The strong absorption band at 3415-3430 cm-1 referred to the stretching vibration of C-OH bond. The intensity of the absorption bands at 3415-3430 cm-1 varied regularly among the three immersion conditions. The intensity was lowest after 30 d immersion in the seawater inoculated with Pseudomonas sp.. The intensity was higher after 30 d immersion in sterile seawater. The highest intensity was found for the coating without immersion. Especially, the absorbance of the C-OH peak for the coating immersed in the seawater inoculated with Pseudomonas sp. was significantly higher than that for the coating without immersion and the coating immersed in sterile seawater. These FTIR results supported the EDS results and further demonstrated that Pseudomonas sp. degraded the epoxy of the aluminum/epoxy coating through oxidizing the epoxy to forming hydroxyl. Generally, the first step of the aerobic metabolism of organics by microorganisms is that organics were oxidized to form hydroxyl. Then hydroxyl can be oxidized to carbonyl (aldehyde) which can be further degraded [28-30]. The oxidization of the epoxy by Pseudomonas sp. to forming hydroxyl completely fitted this aerobic degradation process of organics by microorganisms. 3.5. Contact angle analysis
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The sediment and biofilm had been removed from the coating surfaces before the contact angle examination. The contact angles of the coating in the two seawaters decreased with the immersion time increasing from 0 to 30 d (Fig. 7). However, the decreases of the coating in the seawater inoculated with Pseudomonas sp. were higher than those of the coating in the sterile seawater (Fig. 7). The test solution was distilled water. The contact angle results meant that the hydrophily of the coating surfaces increased after immersion in the two seawaters and increased more after immersion in the seawater inoculated with Pseudomonas sp., which corroborated the biodegradation mechanism of the epoxy of the aluminum/epoxy coating from the EDS and FTIR results that Pseudomonas sp. degraded the epoxy of the aluminum/epoxy coating through oxidizing the epoxy to forming hydroxyl. 4. Conclusion
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In the present study, the degradation of the aluminum/epoxy coating by Pseudomonas sp. in seawater was investigated by the EIS, SEM, EDS, FTIR and contact angle test. Pseudomonas sp. significantly decreased the capacitive loop diameter, the corrosion resistance and the Rcoat value, and the mature biofilm on the coating and the extensive under-coating corrosion formed sequentially, illustrating that Pseudomonas sp. significantly decreased the corrosion resistance of the coating and suggesting that Pseudomonas sp. might degrade the coating. The SEM results verified the degradation of the coating by Pseudomonas sp. after 30 d immersion. The EDS results revealed that Pseudomonas sp. decomposed the aluminum of the aluminum/epoxy coating. The EDS and FTIR results together demonstrated that Pseudomonas sp. degraded the epoxy of the aluminum/epoxy coating through oxidizing the epoxy to forming hydroxyl. The contact angle results showed that Pseudomonas sp. increased the hydrophily of the coating surfaces, which supported the degradation mechanism of the epoxy of the coating by Pseudomonas sp. from the EDS and FTIR results.
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Acknowledgments
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This work was supported by National Natural Science Foundation of China (No. 51761011, 51261006, 51161007 and 50761004), Natural Science Foundation of Hainan Province (No. 517064) and National Basic Research Program of China (No.2014CB643304). References
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ACCEPTED MANUSCRIPT [6] R.A. Lane, Under the microscope: Understanding, detecting, and preventing microbiologically influenced corrosion, J. Fail. Anal. Prev., 5 (2005) 10-12. [7] D. Xu, J. Xia, E. Zhou, D. Zhang, H. Li, C. Yang, Q. Li, H. Lin, X. Li, K. Yang, Accelerated corrosion of 2205 duplex stainless steel caused by marine aerobic Pseudomonas aeruginosa biofilm, Bioelectrochemistry, 113 (2017) 1-8. [8] Y. Zhao, E. Zhou, Y. Liu, S. Liao, Z. Li, D. Xu, T. Zhang, T. Gu, Comparison of different electrochemical techniques for continuous monitoring of the microbiologically influenced corrosion of 2205 duplex stainless steel by marine Pseudomonas aeruginosa biofilm, Corros. Sci., 126 (2017) 142-151. E. Zhou, H. Li, C. Yang, J. Wang, D. Xu, D. Zhang, T. Gu, Accelerated corrosion of 2304 duplex stainless steel by
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ACCEPTED MANUSCRIPT Table 1 The contents of C, O and Al elements from EDS analysis on the coating surfaces (wt. %).
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The coating without immersion
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8.04
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ACCEPTED MANUSCRIPT Highlights •Pseudomonas sp. significantly decreased the corrosion resistance of the aluminum/epoxy coating. •Pseudomonas sp. brought about some bulges and tiny holes on the coating surface. •Pseudomonas sp. degraded the coating through decomposing the aluminum and oxidizing the epoxy to
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forming hydroxyl.
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•Pseudomonas sp. increased the hydrophily of the coating surfaces.
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Tiantian Feng, Jinyi Wu, Ke Chai, Fuchun Liu PII: DOI: Reference:
S0167-7322(17)34788-8 doi:10.1016/j.molliq.2018.04.103 MOLLIQ 9006
To appear in:
Journal of Molecular Liquids
Received date: Revised date: Accepted date:
10 October 2017 24 March 2018 20 April 2018
Please cite this article as: Tiantian Feng, Jinyi Wu, Ke Chai, Fuchun Liu , Effect of Pseudomonas sp. on the degradation of aluminum/epoxy coating in seawater. The address for the corresponding author was captured as affiliation for all authors. Please check if appropriate. Molliq(2017), doi:10.1016/j.molliq.2018.04.103
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT Effect of Pseudomonas sp. on the degradation of aluminum/epoxy coating in seawater Tiantian Fenga#, Jinyi Wua#, Ke Chaia*, Fuchun Liub a
Key Laboratory of Advanced Materials of Tropical Island Resources (Hainan University), Ministry of
Education, Material and Chemical Engineering College, Hainan University, Haikou 570228, China b
Key Laboratory of Nuclear Materials and Safety Assessment, Institute of Metal Research, Chinese Academy
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of Sciences, Shenyang 110016, China
# First authors: these authors contributed equally to this work.
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Abstract
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* Corresponding author: Ke Chai, Tel: +86-0898-66279122, E-mail: [email protected]
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Polymeric coatings prevent metal materials from corrosion. Nevertheless, the degradation of the coatings in seawater by microorganisms is almost unknown. In this work, we explored the degradation of the aluminum/epoxy coating in sterile seawater and seawater inoculated with Pseudomonas sp. by electrochemical impedance spectroscopy, scanning electron microscopy, energy dispersive spectroscopy, Fourier transform infrared spectroscopy and contact angle test. The decreases in the corrosion resistance of the coating were significantly higher in seawater inoculated with Pseudomonas sp. than in sterile seawater. The mature biofilm formed on the coating and then the extensive under-coating corrosion occurred in seawater inoculated with Pseudomonas sp.. These results revealed that Pseudomonas sp. significantly decreased the corrosion resistance of the coating and might degrade the coating. Some bulges and tiny holes were observed on the coating in seawater inoculated with Pseudomonas sp.. The contents of Al and O significantly respectively decreased and increased on the surface of the coating in seawater inoculated with Pseudomonas sp. relative to those on the surfaces of the coating without immersion and in sterile seawater. The absorbance of the C-OH peak for the coating immersed in seawater inoculated with Pseudomonas sp. was significantly higher than that for the coating without immersion and the coating immersed in sterile seawater. Moreover, Pseudomonas sp. decreased the water contact angle of the coating surfaces. The results demonstrated that Pseudomonas sp. degraded the aluminum/epoxy coating through decomposing the aluminum and oxidizing the epoxy to forming hydroxyl. Keywords: Pseudomonas sp.; degradation; aluminum/epoxy coating; seawater.
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ACCEPTED MANUSCRIPT 1. Introduction Corrosion is a major damage to infrastructures in marine environment [1,2]. It is well known that more than 20% corrosion is attributed to microorganisms [3,4]. Bacteria in aquatic environments adhere to the metal surface and formed biofilm [5]. The biofilm produce an environment at the biofilm/metal interface that is different from the bulk water [6]. The differences can lead to electrochemical reactions on metals, namely corrosion. For example, Pseudomonas aeruginosa induced the corrosion of 2205 duplex stainless steel [7, 8], 2304 duplex stainless steel [9], the nickel-free high nitrogen stainless steel [10] and S32654 super austenitic stainless steel [11].
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Besides metals, microorganisms degrade lots of non-metallic materials. Polyimide used for electronic packaging was degraded by two fungi, Aspergillus versicolor and Chaetomiu sp. [12]. Penicillium simplicissimum YK could utilized polyethylene as a carbon source and lowered the molecular weight of the polyethylene [13]. Darby and Kaplan reported that polyurethane was susceptible to the damage by the fungi including Aspergillus niger, A. flavus, A. versicolor, Penicillium funiculosum, Pullularia pullulans, and Trichoderma sp. [14]. Three bacteria, Pseudomonas aeruginosa (UKMP-8T), Rhodococcus sp. M15-2 (UKMP-5T), and Rhodococcus sp. ZH8 (UKMP-7T), degraded 97.6-99.9% of crude oil within 7 days [15]. Kay et al. isolated 16 different bacteria degrading polyurethane, in which Corynebacterium sp., Corynebacterium sp. and Enterobacter agglomerans significantly decreased the tensile strength of polyurethane [16]. Especially, Pseudomonas could degrade more than 90 kinds of organics such as polyethylene [17], polyurethane [18], polycyclic aromatic hydrocarbons [19], and total petroleum hydrocarbon [20]. At present, the application of coatings is the major way to inhibit corrosion. Aluminum/epoxy composite coating is an excellent marine coating which has outstanding corrosion resistance, processability and chemical resistance, etc. However, the degradation of the coatings in seawater by microorganisms remains unclear.
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2. Experimental
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In this study, we detected the degradation of aluminum/epoxy coating by Pseudomonas sp. in seawater through electrochemical impedance spectroscopy (EIS) and scanning electron microscopy (SEM). Additionally, we explored the degradation mechanism of the coating by energy dispersive spectroscopy (EDS), Fourier transform infrared spectroscopy (FTIR) and contact angle test.
2.1. Materials and sample preparation
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Aluminum/epoxy composite paint was provided by the Institute of Metal Research, Chinese Academy of Sciences, Shenyang, China, which consists of mixed solvent (dimethylbenzene 7, n-butanol 3) 37, flatting agent (BYK 320) 3, curing agent polyamide 8115 37, NPEL-128 epoxy resin 60 and nano aluminum 15.2 by weight ratio. AISI1045 carbon steel plates with dimensions of Փ4 cm × 5 mm were used in EIS measurements. Glass plates with the dimension of 15 mm × 15 mm × 0.17 mm were used in FTIR and surface analyses. The carbon steel plates were sequentially polished with 120, 400, 800, 1200 and 1500 grit SiC papers in order to obtain smooth surfaces. all the plates were degreased with acetone and dried at room temperature to remove any contaminant on the surfaces and increase the coating adhesion. The coating was sprayed on the clean substrates. The thicknesses of the dry coating were 90 ± 3 μm on the carbon steel plates and 120 ± 3 μm on the glass plates after curing for 7 days at room temperature. Then the prepared samples were used in the immersion experiments. 2.2. Preparation of immersion seawaters 2
ACCEPTED MANUSCRIPT Pseudomonas sp. used in this work was isolated from the nature seawater of South China Sea with 2216E culture medium which was composed of 5.0 g peptone, 1.0 g yeast extract, and 1000 mL natural seawater. The medium pH was adjusted to pH=7.8 with NaOH solution and sterilized at 121 °C for 20 minutes (min) before use. Pseudomonas sp. was cultured under aerobic conditions at 26 ℃ for 2 days (d) in 2216E medium. 20 mL cell suspension was diluted with 2000 mL sterile seawater in the immersion box. The bacterium was cultured at 26 ℃ for 24 hours (h). Then the prepared samples were aseptically immersed in the seawater inoculated with Pseudomonas sp. and sterile seawater. The immersion was repeated in triplicate. 2.3. Characterization of the degradation of the coating
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The EIS measurements of the degradation of the coating were performed on a PARSTAT 2273 electrochemical workstation (Princeton Applied Research) with a three-electrode system, in which the saturated calomel electrode and the platinum electrode were employed as the reference electrode and the counter electrode, respectively. The coating side of the sample was the working electrode. The other side of the sample was connected to a wire and encapsulated. The test solutions were sterile seawater and seawater inoculated with Pseudomonas sp., respectively. The EIS measurements were implemented under 20 mV amplitude sinusoidal voltages and at the frequency range of 10-2-105 Hz. The equivalent electrical circuits (EEC) were determined with evaluator software (Zsimpwin).
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The surface morphology of the coating was observed by an environmental scanning electronic microscope (FEI XL30) after 30 d immersion in the two seawaters. The chemical compositions of the coating surfaces were analyzed by EDS. The sediment and biofilm had been removed from the coating surfaces before the EDS analysis..
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The FTIR spectra of the coating were measured in the range of 450-4000 cm-1 with a Fourier transform infrared spectrometer (Bruker IFS55 FTIR) after 30 d immersion in the two seawaters. The sediment and biofilm had been removed from the coating surfaces before the measure. The contact angles of the coating surfaces were examined by a water contact angle-measuring instrument (XHSCAZ-2) after 0, 7, 15, 30 d immersion in the two seawaters. The sediment and biofilm had been removed from the coating surfaces before the examination. The test solution was distilled water.
3.1. EIS analysis
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3. Results and discussion
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The different electrochemical processes occurring at the electrode-electrolyte interfaces can be characterized by the EIS data, in which the diameter of the capacitive loop in the Nyquist plot, the corrosion resistance in the Bode plot and the Rcoat value are important indicators of the corrosion resistance of coatings. The EIS data acquired for the aluminum/epoxy coating immersed in the two seawaters were shown in Fig. 1 and Fig. 2. The three sets of parallel immersion groups showed similar trends of the data. Whether the coating was immersed in sterile seawater or in the seawater inoculated with Pseudomonas sp., the Nyquist plots exhibited that the capacitive loop diameters of 1 h immersion were the same and much bigger than those of the other time immersion (Fig. 1, 2a and 2b), which illustrated that Pseudomonas sp. did not change the corrosion resistance of the coating in 1 h immersion. Hereafter, the capacitive loop diameter decreased from 1 h to 48 h of immersion in sterile seawater at first (Fig. 2a). Then the capacitive loop diameter increased from 48 h to 27 d of immersion (Fig. 2a). At last, the capacitive loop diameter decreased from 27 d to 145 d of immersion and reached the minimum (Fig. 2a). Similarly, in the seawater inoculated with Pseudomonas sp., 3
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the capacitive loop diameter decreased from 1 h to 48 h of immersion, increased from 48 h to 6 d of immersion, and decreased from 6 d to 45 d of immersion and reached the minimum (Fig. 2b). In addition, at the immersion time after the 1 h immersion, the capacitive loop diameters decreased considerably relative to that at 1 h of immersion in the two seawaters, respectively (Fig. 1, 2a and 2b). However, the decreases in the seawater inoculated with Pseudomonas sp. were significantly higher than the corresponding ones in sterile seawater (Fig. 1, 2a and 2b). The seawater and electrically conductive particles were the common components of the two seawaters, which could lead to the decreases of the capacitive loop diameters because the coating performance was influenced by them [21, 22]. The only difference between the two seawaters was Pseudomonas sp.. The significantly higher decreases of the capacitive loop diameters in the seawater inoculated with Pseudomonas sp. therefore revealed that Pseudomonas sp. significantly decreased the corrosion resistance of the coating and might degrade the aluminum/epoxy coating.
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In accord with the Nyquist plots, the Bode plots of immersion in the two seawaters showed that the corrosion resistance reached the maxima and the minima at 1 h and 145 d of immersion in sterile seawater and at 1 h and 45 d of immersion in the seawater inoculated with Pseudomonas sp., respectively (Fig. 2c and d). The corrosion resistances of the coating in the two seawaters were more than 10 9 Ω cm2 at 1 h of immersion (Fig. 2c and d), which meant that the coating had well barrier properties in 1 h immersion [23, 24]. Moreover, the decreases in corrosion resistance were significantly greater in seawater inoculated with Pseudomonas sp. than in sterile seawater accordingly from the 1 h immersion to the immersion after 1 h (Fig. 2c and d). All these data were consistent with the results of the Nyquist plots and supported the above mentioned inference.
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For the sterile seawater immersion, the equivalent electrical circuit (EEC) model was composed of Rs (the resistance of the seawater), Rcoat (the coating resistance) and Qcoat (the constant-phase element related to the coating capacitance) from 1 h to 48 h of immersion, in which only one time constant could be found (Fig. 3a). Rct (the charge transfer resistance in parallel with the double-layer capacitance formed by the electrochemical reaction at the metal interface) and Qdl (the constant-phase element related to the double-layer capacitance) appeared in the EEC from 6 d to 145 d of immersion besides Rs, Rcoat and Qcoat (Fig. 3b). Two time constants were identified from the EEC (Fig. 3b). The increase of the number of the time constant from 1 to 2 suggested that the under-coating corrosion occurred on the steel surface extensively later besides the coating was on the steel substrate. In the seawater inoculated with Pseudomonas sp., when the immersion time was from 1 h to 48 h, the EEC was the same as that in the sterile seawater and still only one time constant was found (Fig. 3c). The EEC included Rs, Rcoat, Qcoat, Rbiofilm (the biofilm resistance) and Qbiofilm (the constant-phase element related to the biofilm capacitance) from 3 d to 27 d of immersion (Fig. 3d). Two time constants were also identified from the EEC (Fig. 3d). At 45 d of immersion, the EEC involved Rs, Rcoat, Rbiofilm, Qbiofilm, Rct, Ccoat (the pure coating capacitance) and Cdl (the double-layer capacitance formed by the electrochemical reaction at the metal interface), which showed three time constants (Fig. 3e). The increases of the number of the time constant from 1 to 2, and then to 3 indicated that the mature biofilm on the coating and the extensive under-coating corrosion formed sequentially. These results further supported the results of the Nyquist plots and the Bode plots and the inference that Pseudomonas sp. significantly decreased the corrosion resistance of the coating and might degrade the aluminum/epoxy coating. In both seawaters, the Rcoat values were the same and at high level at 1 h of immersion (Fig. 4). When the immersion time was after 1 h, the Rcoat values decreased significantly (Fig. 4). Nevertheless, the decreases in seawater inoculated with Pseudomonas sp. were significantly higher than the corresponding ones in sterile seawater (Fig. 4). These results were in agreement with the results of the Nyquist plots, the Bode plots and the EEC and corroborated the inference from the Nyquist plots. 4
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The SEM images showed that the coating surface was intact without immersion (Fig. 5a and b). After 30 d immersion in sterile seawater, some sediment was on the coating surface, but no obvious damage was observed on the exposed part of the coating surface (Fig. 5c and d). When the sediment was removed, still no obvious damage was observed on the coating surface (Fig. 5e and f). After 30 d immersion in seawater inoculated with Pseudomonas sp., the biofilm formed on the coating surface (Fig. 5g and h). When the biofilm was removed, some bulges and tiny holes appeared on the coating surface (Fig. 5i and j). The results were consistent with the results of the EIS and demonstrated that Pseudomonas sp. degraded the aluminum/epoxy coating. 3.3. EDS analysis
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The sediment and biofilm had been removed from the coating surfaces before the analysis. The areas of EDS were randomly selected on the coating surfaces (Fig. 5a, e and i). EDS analysis revealed that the contents of C, O and Al elements after 30 d immersion in sterile seawater were almost the same as those without immersion (Table 1). After 30 d immersion in the seawater inoculated with Pseudomonas sp., the content of Al significantly decreased (Table 1). However, the content of O significantly increased (Table 1). The results proved that Pseudomonas sp. degraded the aluminum/epoxy coating through decomposing the aluminum and oxidizing the epoxy.
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3.4. FTIR analysis
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FTIR was carried out on the coating without immersion and the coating immersed for 30 d in sterile seawater and the seawater inoculated with Pseudomonas sp. to detect the degradation of the epoxy by Pseudomonas sp., as shown in Fig. 6. The sediment and biofilm had been removed from the coating surfaces before the detection.
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The absorption bands of the fingerprint region were at 550-1700 cm-1. 831 cm-1 peak referred to the out-of-plane C-H bending vibration [25]. 1600 and 1500 cm-1 peaks were assigned to the vibrations of the aromatic ring [26]. The absorption band at 1680-1750 cm-1 corresponded to the C=O stretching [27]. The absorption band at 2900-2960 cm-1 was attributed to the asymmetric stretching vibration of –CHx. The strong absorption band at 3415-3430 cm-1 referred to the stretching vibration of C-OH bond. The intensity of the absorption bands at 3415-3430 cm-1 varied regularly among the three immersion conditions. The intensity was lowest after 30 d immersion in the seawater inoculated with Pseudomonas sp.. The intensity was higher after 30 d immersion in sterile seawater. The highest intensity was found for the coating without immersion. Especially, the absorbance of the C-OH peak for the coating immersed in the seawater inoculated with Pseudomonas sp. was significantly higher than that for the coating without immersion and the coating immersed in sterile seawater. These FTIR results supported the EDS results and further demonstrated that Pseudomonas sp. degraded the epoxy of the aluminum/epoxy coating through oxidizing the epoxy to forming hydroxyl. Generally, the first step of the aerobic metabolism of organics by microorganisms is that organics were oxidized to form hydroxyl. Then hydroxyl can be oxidized to carbonyl (aldehyde) which can be further degraded [28-30]. The oxidization of the epoxy by Pseudomonas sp. to forming hydroxyl completely fitted this aerobic degradation process of organics by microorganisms. 3.5. Contact angle analysis
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The sediment and biofilm had been removed from the coating surfaces before the contact angle examination. The contact angles of the coating in the two seawaters decreased with the immersion time increasing from 0 to 30 d (Fig. 7). However, the decreases of the coating in the seawater inoculated with Pseudomonas sp. were higher than those of the coating in the sterile seawater (Fig. 7). The test solution was distilled water. The contact angle results meant that the hydrophily of the coating surfaces increased after immersion in the two seawaters and increased more after immersion in the seawater inoculated with Pseudomonas sp., which corroborated the biodegradation mechanism of the epoxy of the aluminum/epoxy coating from the EDS and FTIR results that Pseudomonas sp. degraded the epoxy of the aluminum/epoxy coating through oxidizing the epoxy to forming hydroxyl. 4. Conclusion
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In the present study, the degradation of the aluminum/epoxy coating by Pseudomonas sp. in seawater was investigated by the EIS, SEM, EDS, FTIR and contact angle test. Pseudomonas sp. significantly decreased the capacitive loop diameter, the corrosion resistance and the Rcoat value, and the mature biofilm on the coating and the extensive under-coating corrosion formed sequentially, illustrating that Pseudomonas sp. significantly decreased the corrosion resistance of the coating and suggesting that Pseudomonas sp. might degrade the coating. The SEM results verified the degradation of the coating by Pseudomonas sp. after 30 d immersion. The EDS results revealed that Pseudomonas sp. decomposed the aluminum of the aluminum/epoxy coating. The EDS and FTIR results together demonstrated that Pseudomonas sp. degraded the epoxy of the aluminum/epoxy coating through oxidizing the epoxy to forming hydroxyl. The contact angle results showed that Pseudomonas sp. increased the hydrophily of the coating surfaces, which supported the degradation mechanism of the epoxy of the coating by Pseudomonas sp. from the EDS and FTIR results.
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Acknowledgments
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This work was supported by National Natural Science Foundation of China (No. 51761011, 51261006, 51161007 and 50761004), Natural Science Foundation of Hainan Province (No. 517064) and National Basic Research Program of China (No.2014CB643304). References
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ACCEPTED MANUSCRIPT Table 1 The contents of C, O and Al elements from EDS analysis on the coating surfaces (wt. %).
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The coating without immersion
81.83
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The coating immersed in sterile seawater for 30 d
81.75
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The coating immersed in the seawater inoculated with Pseudomonas sp. for 30 d
79.96
12.00
8.04
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ACCEPTED MANUSCRIPT Highlights •Pseudomonas sp. significantly decreased the corrosion resistance of the aluminum/epoxy coating. •Pseudomonas sp. brought about some bulges and tiny holes on the coating surface. •Pseudomonas sp. degraded the coating through decomposing the aluminum and oxidizing the epoxy to
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forming hydroxyl.
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•Pseudomonas sp. increased the hydrophily of the coating surfaces.
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