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Benzoate intercalated Mg-Al-layered double hydroxides (LDHs) as efficient chloride traps for plasma electrolysis coatings
Journal of Alloys and Compounds 787 (2019) 772e778
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Correspondence
Benzoate intercalated Mg-Al-layered double hydroxides (LDHs) as efficient chloride traps for plasma electrolysis coatings a b s t r a c t Keywords: Smart coatings Plasma electrolysis Layered double hydroxides Benzoate release
In this work, a layered double hydroxide (LDH) film intercalated with benzoate (C6H5COO) anions was successfully fabricated on the porous surface of plasma electrolysis coating. For this purpose, the nitrateloaded LDH (LDH-NO3) film grown on the plasma electrolysis film of aluminum alloy was immersed additionally in C6H5COONa solution. The results revealed that the intercalation of C6H5COO anions into LDH film led to a more compact arrangement of the LDH flakes. The flake-like structure of LDH-C6H5COO film would act as smart reservoirs of C6H5COO anions that are released on demand upon the onset of corrosion. Therefore, the LDH-C6H5COO film was found to exhibit significantly lower permeability to Cl anions when compared to both plasma electrolysis and LDH-NO3 films, which emphasize the efficiency of LDH-C6H5COO film in delaying coating degradation and corrosion initiation. This result was attributed to the combined effects of Mg2þ and C6H5COO ions released from LDH film. © 2019 Elsevier B.V. All rights reserved.
1. Introduction Layered double hydroxides (LDHs) are attractive nanocontainer materials that serve as reservoirs and delivery carriers of functional molecules [1]. The LDH structure consists of positively-charged mixed metal hydroxide layers which separated by chargebalancing interlayer anions and water molecules [2,3]. The negatively-charged anions at the interlayers can be substituted by proper anionic inhibitors to form inhibitor-intercalated LDH films [4]. LDHs materials have been considered as environmentallyfriendly containers for active corrosion protection of Mg and Al alloys [5,6] and steel [7] due to their ability to trap corrosive anions (Cl and SO2 4 ) by releasing the corrosive inhibitors triggered by local increase of pH at cathodic sites, offering active protections of these alloys [8]. The anions intercalated into LDH layer can be inorganic, such as molybdate [7], vanadate [9], tungstate [10] or organic including benzotriazole [11], 2-mercaptobenzothiazolate [11], phytic acid [7]. For example, Buchheit et al. [9] postulated that the Al-Zn-LDHs layer intercalated with decavanadate can enhance the inhibition performance of the polymer coating deposited on AA2024 Al alloy due to the decavanadate release, accompanied by the uptake of the Cl anions in the exchange reaction. As such, Li et al. [12] demonstrated that vanadate intercalated Zn-Al LDHs film can significantly improve the long-term inhabitation properties of Al alloy. Williams and McMurray [13] intercalated different organic anions, such as benzotriazolate, ethyl xanthate and oxalate into the LDH layer grown on the AA2024Al alloy. They found that the inhibition efficiency is anion dependent, increasing in the order ethyl xanthate < oxalate < benzotriazolate. Moreover, carboxylate anions (aliphatic and aromatic) intercalated https://doi.org/10.1016/j.jallcom.2019.02.124 0925-8388/© 2019 Elsevier B.V. All rights reserved.
LDHs have received considerable attention in recent years [14,15]. On the other hand, plasma electrolysis (PE) is a widespread and eco-friendly method to protect metallic substrates by fabricating protective layers on their surfaces. The excessive temperature and pressure associated with PE, however, can inevitably generate the micro-defects that facilitate the corrosion phenomenon in extreme environments [16,17]. In view of the fact that benzoate anion is an interesting class of interlayer anion, due both to the rigidity of the molecules and the very hydrophobic nature of the phenyl ring; thus, novel structures and interesting properties are to be expected when these ions are intercalated into LDH grown on the porous PE film. Although benzoate is commonly used as an intermediate intercalate in forming other structures, the intercalation of this ions into LDH layer has been little investigated. In this study, therefore, the LDH loaded with nitrate anion was synthesized and then dope it with benzoate anions through an ion exchange process. 2. Experimental details A porous alumina was fabricated on 6061 Al alloy substrate by PE process utilizing an electrolyte composed of 0.1 M KOH and 0.05 M NaAlO2 at a current density of 100 mAcm2 for 3 min. Mg(NO3)2$6H2O (0.05 M) and NH4NO3 (0.3 M) were dissolved in deionized water, and urea was then slowly added until the pH reached 10. The PE films were then placed in the above solution in a water bath at 45 C for 24 h to form a homogenous solution. The synthesis was carried out under nitrogen atmosphere in order to avoid the contamination by carbonate anions. Finally, the specimens were removed, rinsed with ethanol, and dried at ambient temperature. The LDH-NO3 film was immersed later into the
Correspondence / Journal of Alloys and Compounds 787 (2019) 772e778
aqueous solution of C6H5COONa (0.1 M) at 50 C under constant stirring for 24 h. The reaction product (LDH-C6H5COO film) was centrifuged and then washed and dried as previously mentioned. The morphologies of the specimens were observed using fieldemission scanning electron microscope (FE-SEM, Hitachi, S-4800) equipped with an energy-dispersive X-ray spectrum (EDS). The crystal structure of the specimens was determined based on Xray diffraction pattern (XRD, Rigaku D/MAX-2500) with Cu Ka radiation source. For surface chemical analysis, X-ray photoelectron spectroscopy (XPS, VG Microtech, ESCA 2000). The fourier transform infrared (FT-IR) spectra (PerkinElmer Spectrum 100) were taken in the wavenumber range 400e4000 cm1. A typical threeelectrode cell system was used to perform the electrochemical impedance spectroscopy (EIS) tests after 10, 24, 48, and 96 h of immersion in 0.05 M NaCl solution. The Ag/AgCl electrode, PE film (with and without LDH-based treatment), and platinum foil were chosen as the reference electrode, working electrode and the counter electrode, respectively. EIS measurements were scanned from 106 Hz to 0.1 Hz using a 10mV amplitude perturbation. To verify the reproducibility of the results, each electrochemical test was repeated at least three times.
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Fig. 2. EDS elemental distributions of aluminum (green color), oxygen (red color), magnesium (cyan color), and carbon (white color) on the surface of the LDH-C6H5COO film. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
3. Results and discussion 3.1. Structure and composition Fig. 1(a-c) displays the surface morphologies of PE, LDH-NO3, and LDH-C6H5COO films grown on Al alloy substrate. The corresponding cross-sectional images of the films were presented as insets. As shown in Fig. (1b and 1c), the LDH-based treatment modifies the porous surface of PE film where a flake-like morphology was developed in the case of LDH-NO3, and LDHC6H5COO films. The porous structure of PE film would be favorable sites for growing the LDHs flakes, offering an effective repair to defects of the PE film. It is worth mentioning that flakes were somewhat oriented perpendicularly to the substrate which was ascribed to the faster crystal growth rate in the direction of bulk solution [18]. Unlike LDH-NO3 film, it can be seen from Fig. 1c that the structure of LDH-C6H5COO film was more compact where thinner flakes were developed. Such reduction of the average size of flakes would
Fig. 1. (aec) SEM micrographs of the PE, LDH-NO3, and LDH-C6H5COO films, respectively, (d) EDS results of the PE, LDH-NO3, and LDH-C6H5COO films identified the presence of Al, O, Mg, and C throughout the surface. Insets reveal the cross-sectional images of the PE, LDH-NO3, and LDH-C6H5COO films.
Fig. 3. (a) XRD patterns of the (I) LDHs-NO3 and (II) LDHs-C6H5COO films. The XRD patterns were scanned from 5 to 35 with Cu Ka radiation source, and (b) FT-IR spectra in the range 4000-400 cm1 for (I) LDH-NO3 film and (II) LDHs-C6H5COO film.
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be attributed to the occurrence of fragmentation in the initial stage of fast anion exchange [19]. According to the cross-sectional images of the films shown in the insets of Fig. 1, the two-layered structure, from up to down, were an outer layer, a dense inner layer of PE on the substrate. The thickness of the PE film was ~5.5 mm. After the growth of LDHs films, the thicknesses of the films almost remain unchanged when compared to PE film. It can be seen that a relatively dense uniform film was formed on the substrate in the case of LDH-C6H5COO film. EDS results presented in Fig. 1d clearly demonstrate that the elements, such as Al, Mg, and O take part in the formation of all films. In EDS analysis of LDH-C6H5COO film, a carbon appears due to the anion-exchange process in the solution containing benzoate anions, suggesting the successful intercalation of C6H5COO anions into the interlayer galleries of LDHs. However, based on the results presented in Fig. 1d, the ratio of Mg/Al was found to be ~0.23 and ~0.20 for LDH-NO3 and LDH-C6H5COO films, respectively. Accordingly, the LDH-NO3 and LDH-C6H5COO films can be expressed by the empirical formula (1) and (2), respectively as follows [18]. 3þ ½Mg2þ 0:77 Al0:23 ðOHÞ2 3þ ½Mg2þ 0:8 Al0:2 ðOHÞ2
0:23
0:2
ðNO3 Þ 0:23 :mH2 O
ðC6 H5 COOÞ 0:2 :mH2 O
(1) (2)
The EDS-mapping analysis on the surface of the LDH-C6H5COO film confirmed the presence of Al, O, Mg, and C in this film where the carbon is found near and within the pores of the PE coating (Fig. 2(aed)). The EDS findings are consistent with the following XRD and FT-IR analyses. The XRD patterns of the LDH-NO3 and LDH-C6H5COO films are shown in Fig. 3a. The composition of PE film deposited on 6061 Al alloy using an electrolyte containing KOH and NaAlO2 was reported in our previous study and was found to be g-Al2O3 and a-
Al2O3 [17]. The XRD patterns, therefore, of the LDH-NO3 and LDHC6H5COO films at scan range from 5 to 35 only shown in the present study. As shown in Fig. 3a, the diffraction reflections of (003), (006), and (009) are observed at around 9.98, 19.92, and 30.5 (2q), respectively for the LDH-NO3 film which was corresponded to interlayer distances of ~0.88, 0.44, and 0.29 nm, respectively [20]. Interestingly, the X-ray diffractions of (003), (006), and (009) in the case of LDH-C6H5COO film moved to lower angels of ~9.5, 19.4, and 29.11 (2q) corresponding to interlayer distances of ~0.94, 0.45, and 0.3 nm, respectively. The increase of the interlayer distance was due to the intercalation of the C6H5COO anions with larger dimension than the NO 3 anions. The present findings are reasonably consistent with those obtained by Mohedano et al. [21] and Brnardic et al. [22]. The intercalation of C6H5COO anion into LDH film was confirmed by FT-IR spectra shown in Fig. 3b. The broad band at 3440 cm1 observed in the spectra of LDH-NO3 and LDHC6H5COO films was ascribed to the stretching of OH groups or water molecules [6,23]. The band at 1381 cm1 is assigned to the antisymmetric stretching vibration of NO3 or CO3 groups whereas the bands observed at 619 and 716 cm1 are generally related to M OH (M: Mg, Al) lattice vibrations. The two bands at 1538 and 1401 cm1 can be assigned to the COO group in LDH-C6H5COO film [15]. Moreover, the band at 1595 cm1 was attributed to the phenyl group (-C6H5) [24]. The disappearance of the 1381 cm1 of NO3 group in the LDHC6H5COO film would suggest that the C6H5COO anion is the only gallery anion [25].
3.2. Electrochemical behavior Fig. 4 (a and b) depicts the Bode plots for PE, LDH-NO3, and LDHC6H5COO films immersed in 0.05 M NaCl solution for 10 h. As
Fig. 4. (a and b) Bode plots of the PE, LDH-NO3, and LDH-C6H5COO films after 10 h immersion in 0.05 M NaCl solution (a) impedance plots and (b) phase plots, and (c) the equivalent circuit model used for fitting the EIS results.
Correspondence / Journal of Alloys and Compounds 787 (2019) 772e778
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shown in Fig. 4b, the phase angle corresponding to the high frequency (106 Hz) and the impedance at the low frequency region (0.1 Hz) in the case of LDH-C6H5COO film were higher than those for PE and LDH-NO3 films. Therefore, a significant impediment to the movement of Cl anions towards substrate can be obtained in the case of LDH-C6H5COO film [26,27]. The Bode spectra were fitted using the equivalent circuit depicted in Fig. 4c. In the context of the proposed model, Rs is the solution resistance in a 0.05 M NaCl solution. CLDH and RLDH describe the capacitance and resistance of the synthesized LDH layer while CPE and RPE are the capacitance and resistance corresponding to the PE layer underneath, respectively. Moreover, Cdl and Rct represent the double layer capacitance and charge transfer resistance, respectively. Constant phase element (CPE) is used instead of capacitance to demonstrate the heterogeneity of the films. The CPE was described as follows:
ZCPE ¼ 1 ½YðjuÞn
(3)
where j is the imaginary unit, u is the angular frequency, and n and Y are the parameters of CPE. Based on the proposed model, Bode plots were best fitted, and the corresponding values of the equivalent circuit parameters are listed in Table 1. The values of RLDH, RPE and Rct in the LDH-NO3 and LDH-C6H5COO films were higher than their counterparts in PE film. This suggested that the growth of LDH would proceed not only on the top surface of PE film but also would affect the discharge channels of PE film. The present results were in good agreement with those obtained by Li et al. [12]. Among three samples, however, the LDH-C6H5COO film exhibited significant high values of RLDH, RPE, and Rct, indicating that presence of C6H5COO anions within LDH frame can significantly enhance the resistance of the substrate against corrosion. The corrosion-inhibiting efficiency could be calculated from EIS as follows;
IF% ¼ ð1
Rt0 Þ 100 Rt
(4)
Where Rt0 is the total resistance of the PE film (without LDH-based treatment) while Rt represents the total resistance of the PE film subjected to LDH-based treatment. The values of IF calculated for LDH-NO3 and LDH-C6H5COO films are ~95.0 and ~99.8%, respectively, indicating an increase in the inhibiting effects with the intercalation of C6H5COO anions into the LDH layer. Fig. 5(aec) shows the surface morphologies and the corresponding cross-sectional images of PE, LDH-NO3, and LDH-C6H5COO films after 10 h immersion in 0.05 M NaCl solution. It can be seen from Fig. 5a that PE film had suffered severe corrosion since many cracks were developed on the surface of this film. On the other, the flakelike structure in LDH-NO3 film protected the Al alloy substrate to some extent where some degraded areas are observed clearly, as shown in Fig. 5b. Interestingly, the surface and the cross-sectional image of the LDH-C6H5COO film (Fig. 5c) were similar to those before immersion (Fig. 1), confirming that the formation of the LDH-C6H5COO film on the PE film can effectively enhance the corrosion resistance of Al alloy substrate. To get more information on the stability of the films, however,
Fig. 5. Morphological features of the films after 10 h immersion in 0.05 M NaCl solution (a) PE, (b) LDH-NO3, and (c) LDH-C6H5COO. Insets reveal the corresponding cross-sectional images.
Table 1 Results of the equivalent circuit of the films immersed in 0.05 M NaCl solution for 10 h. sample PE LDH-NO3 LDH-C6H5COO
RLDH (U.cm2) 4
4.5 10 9.0 105 5.2 107
nLDH 0.62 0.68 0.73
YLDH (S.sn.cm2) 8
3.5 10 1.8 108 2.4 109
RPE(U.cm2) 5
1.1 10 4.7 107 9.3 108
nPE 0.61 0.65 0.69
YPE (S.sn.cm2) 8
2.2 10 3.4 109 1.0 109
Rct (U.cm2) 6
6.8 10 9.3 107 3.7 109
nct
Yct (S.sn.cm2)
0.86 0.88 0.91
3.2 108 5.1 108 2.4 109
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Bode plots of the PE, LDH-NO3, and LDH-C6H5COO films immersed for different times, such as 24, 48, and 96 h in 0.05 M solution are presented in Fig. 6. As shown in Fig. 6, the impedance value decreased with increasing the immersion time which was associated with the appearance of new time constants in phase angle plots. This indicated the penetration of corrosive anions (Cl) to the interface of the substrate. As well known, the overall resistance of inhibitive films can be evaluated by the low-frequency impedance values [27,28]. Thus, the variations of jZj0.1 Hz with respect to the immersion time shown in Fig. 7a were used to figure out the stability of the samples exposed to 0.05 M NaCl solution. Interestingly, the jZj0.1Hz value of LDH-C6H5COO film was still well above 106 U cm2 after 96 h, which was over two and four orders of magnitude higher than those of the LDH-NO3 and PE films, respectively. This result, attributed mainly to the release of C6H5COO anions during the immersion in 0.05 M NaCl solution, confirming again
the fact that the formation of LDH-C6H5COO film with stable flake-like structure can help to improve the stability of PE coating in 0.05 M NaCl solution, thereby prolonging the service life of the substrate. Previous investigations have reported that the microstructural changes and the release of inhibitors from LDH film were the main reasons for improving the inhibition performance of the PE film modified by LDH film. Moreover, Mg2þ ions are expected to be released from LDH-NO3 and LDH-C6H5COO films into the solution, either from the interlayer galleries or from metal hydroxide layer as a result of structural arrangement, leading to some inhibitory effects [29,30]. Although Mg2þ and NO 3 ions, released from LDH-NO3 film helped to improve the inhibition performance of PE film (Eq. (5)), the amounts of these ions would not be high enough to form a dense and inhibitive film, that is why the LDHNO3 film showed lower inhibitive performance when compared
Fig. 6. Bode plots (impedance and phase) of the PE, LDH-NO3, and LDH-C6H5COO films immersed in 0.05 M NaCl solution for different times (a and b) 24h, (c and d) 48 h, and (e and f) 96 h.
Correspondence / Journal of Alloys and Compounds 787 (2019) 772e778
Fig. 7. (a) Representation of the jZj0.1Hz values for PE, LDH-NO3, and LDH-C6H5COO films as a function of immersion time in 0.05 M NaCl solution, and (b) a schematic illustration of the corrosion inhibition mechanism of the LDH-C6H5COO film. A diffusion boundary film, LDH film, and PE film were observed in this model above Al substrate.
to the LDH-C6H5COO film. In contrast to LDH-NO3 film, the LDH flakes were more compact in the case of LDH-C6H5COO film which would effectively retard the movement of Cl anions towards substrate since most of the pores in the PE film were sealed totally. LDH NO 3 þ Cl /LDH Cl þ NO3
(5)
The possible mechanism for the superior inhibitive performance of LDH-C6H5COO film is schematized in Fig. 7b. Based on the ionexchange process (Eq. (6)), the released C6H5COO anions tended to concentrate on the film surface, leading to the formation of a diffusion boundary layer containing high concentrations of C6H5COO anions (hard base), which may react with the dissolved Alþ3 (hard acid), forming aluminum benzoate (Al(C6H5COO)3) on the surface of the coating (Eq. (7)) [31,32].
LDH C6 H5 COO þ Cl /LDH Cl þ C6 H5 COO
(6)
Alþ3 þ 3C6 H5 COO /AlðC6 H5 COOÞ3
(7)
To confirm the formation of Al(C6H5COO)3, we carried out XPS measurements on the surface of LDH-C6H5COO film. Fig. 8a shows that the deconvolution of O1s led to the appearance of two peaks at 531.2 and 532.5 eV, which are assigned to the hydroxyl groups (OH)
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Fig. 8. XPS spectra of LDH-C6H5COO film in the (a) O1s and (b) C 1s regions.
and carboxylate groups (OC]O), respectively [33,34]. In the C1s region (Fig. 8b), two peaks were observed at 284.7 and 288.3 eV, matching with aromatic ring and OC]O groups from benzoate anions in Al(C6H5COO)3 [33,35]. All of the above-stated results would support the successful formation of Al(C6H5COO)3. On the other hand, the presence of C6H5COO anions in the diffusion boundary layer would impair the adsorption of Cl anions on the surface of the coating owing to the so-called competitive adsorption. Therefore, it can be concluded that the diffusion boundary film, LDH film containing C6H5COO anions, and PE film all can be responsible for the inhibiting performance of LDH-C6H5COO film.
4. Conclusion In the present work, benzoate anions intercalated to Mg-Al LDH layers has been successfully grown on the surface of porous PE coating via an ion-exchange route. The results revealed that the incorporation of benzoate anions into the LDH film would greatly influence the growth of LDH-NO3 film, leading to a more compact arrangement of LDH nanosheets. Accordingly, a superior inhibition performance was provided by LDH-C6H5COO film since the
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presence of Cl anions in the solution during the corrosion test can trigger the on-demand release of C6H5COO anions from the interlayers of LDH particles. Conflicts of interest
[20]
[21]
The authors declare no competing interests. [22]
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exchange in Zn-Al layered double hydroxides: in situ X-ray diffraction study, Chem. Phys. Lett. 495 (2010) 73e76. X. Guo, S. Xu, L. Zhao, W. Lu, F. Zhang, D.G. Evans, X. Duan, One-step hydrothermal crystallization of a layered double hydroxide/alumina bilayer film on aluminum and its corrosion resistance properties, Langmuir 25 (2009) 9894e9897. M. Mohedano, M. Serdechnova, M. Starykevich, S. Karpushenkov, A.C. Bouali, M.G.S. Ferreira, M.L. Zheludkevich, Active protective PEO coatings on AA2024: role of voltage on in-situ LDH growth, Mater. Des. 120 (2017) 36e46. si I. Brnardi c, Z. Matusinovi c, J. Sipu c, J. Macan, Thermal properties of epoxy resin/layered double hydroxide intercalated nanocomposites, Polymer 33 (2012) 52e57. F.R. Costa, A. Leuteritz, U. Wagenknecht, D. Jehnichen, L. Haussler, G. Heinrich, Intercalation of Mg-Al layered double hydroxide by anionic surfactants: preparation and characterization, Appl. Clay Sci. 38 (2008) 153e164. V. Prevot, B. Casal, E.R. Hitzky, Intracrystalline alkylation of benzoate ions into layered double hydroxides, J. Mater. Chem. 11 (2001) 554e560. S. Velu, V. Ramkumar, A. Narayanan, C.S. Swamy, Effect of interlayer anions on the physicochemical properties of zincealuminum hydrotalcite-like compounds, J. Mater. Sci. 32 (1997) 957e964. M. Kaseem, Y.G. Ko, Optimization of defect-free protective layer considering the geometrical linearity of condensed phosphates, J. Alloys Compd. 752 (2018) 155e163. V. Palanivel, D. Zhu, W.J. van Ooij, Nanoparticle-filled silane films as chromate replacements for aluminum alloys, Prog. Org. Coat. 47 (2003) 384e392. M. Kaseem, Y.G. Ko, On the compactness of the oxide layer induced by utilizing a porosification agent, Appl. Surf. Sci. 473 (2019) 715e725. Y. Zhang, J. liu, Y. Li, M. Yu, S.M. Li, B. Xue, Fabrication of inhibitor anionintercalated layered double hydroxide host films on aluminum alloy 2024 and their anticorrosion properties, J. Coat. Technol. 12 (2) (2015) 293e302. S.K. Poznyak, J. Tedim, L.M. Rodrigues, A.N. Salak, M.L. Zheludkevich, L.F.P. Dick, M.G.S. Ferreira, Novel inorganic host layered double hydroxides intercalated with guest organic inhibitors for anticorrosion applications, ACS Appl. Mater. Interfaces 10 (2009) 2353e2362. Y. Liu, Z. Wei, F. Yang, Z. Zhang, Environmental friendly anodizing of AZ91D magnesium alloy in alkaline borateebenzoate electrolyte, J. Alloys Compd. 509 (2011) 6440e6446. M. Kaseem, J.H. Kwon, Y.G. Ko, Modification of a porous oxide layer formed on an AleZneMg alloy via plasma electrolytic oxidation and post treatment using oxalate ions, RSC Adv. 6 (2016) 107109e107113. M. Ma, R. ge, X. Ji, X. ren, Z. Liu, A.M. Asiri, X. Sun, Benzoate anionsintercalated layered nickel hydroxide nanobelts array: an earth-abundant electrocatalyst with greatly enhanced oxygen evolution activity, ACS Sustain. Chem. Eng. 5 (2017) 9625e9629. J. Mu, C. Shao, Z. Guo, Z. Zhang, M. Zhang, P. Zhang, B. Chen, Y. Liu, High photocatalytic activity of ZnO-carbon nanofiber heteroarchitectures, ACS Appl. Mater. Interfaces 3 (2011) 590e596. L. Huang, J. Jiang, L. Ai, Interlayer expansion of layered cobalt hydroxide nanobelts to highly improve oxygen evolution electrocatalysis, ACS Appl. Mater. Interfaces 9 (2017) 7059e7067.
Mosab Kaseem Department of Nanotechnology and Advanced Materials Engineering, Sejong University, Seoul, 05006, Republic of Korea Young Gun Ko* School of Materials Science and Engineering, Yeungnam University, Gyeongsan, 38541, Republic of Korea *
Corresponding author. E-mail address: [email protected] (Y.G. Ko). 25 October 5 February 10 February Available online 14 February
2018 2019 2019 2019
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Correspondence
Benzoate intercalated Mg-Al-layered double hydroxides (LDHs) as efficient chloride traps for plasma electrolysis coatings a b s t r a c t Keywords: Smart coatings Plasma electrolysis Layered double hydroxides Benzoate release
In this work, a layered double hydroxide (LDH) film intercalated with benzoate (C6H5COO) anions was successfully fabricated on the porous surface of plasma electrolysis coating. For this purpose, the nitrateloaded LDH (LDH-NO3) film grown on the plasma electrolysis film of aluminum alloy was immersed additionally in C6H5COONa solution. The results revealed that the intercalation of C6H5COO anions into LDH film led to a more compact arrangement of the LDH flakes. The flake-like structure of LDH-C6H5COO film would act as smart reservoirs of C6H5COO anions that are released on demand upon the onset of corrosion. Therefore, the LDH-C6H5COO film was found to exhibit significantly lower permeability to Cl anions when compared to both plasma electrolysis and LDH-NO3 films, which emphasize the efficiency of LDH-C6H5COO film in delaying coating degradation and corrosion initiation. This result was attributed to the combined effects of Mg2þ and C6H5COO ions released from LDH film. © 2019 Elsevier B.V. All rights reserved.
1. Introduction Layered double hydroxides (LDHs) are attractive nanocontainer materials that serve as reservoirs and delivery carriers of functional molecules [1]. The LDH structure consists of positively-charged mixed metal hydroxide layers which separated by chargebalancing interlayer anions and water molecules [2,3]. The negatively-charged anions at the interlayers can be substituted by proper anionic inhibitors to form inhibitor-intercalated LDH films [4]. LDHs materials have been considered as environmentallyfriendly containers for active corrosion protection of Mg and Al alloys [5,6] and steel [7] due to their ability to trap corrosive anions (Cl and SO2 4 ) by releasing the corrosive inhibitors triggered by local increase of pH at cathodic sites, offering active protections of these alloys [8]. The anions intercalated into LDH layer can be inorganic, such as molybdate [7], vanadate [9], tungstate [10] or organic including benzotriazole [11], 2-mercaptobenzothiazolate [11], phytic acid [7]. For example, Buchheit et al. [9] postulated that the Al-Zn-LDHs layer intercalated with decavanadate can enhance the inhibition performance of the polymer coating deposited on AA2024 Al alloy due to the decavanadate release, accompanied by the uptake of the Cl anions in the exchange reaction. As such, Li et al. [12] demonstrated that vanadate intercalated Zn-Al LDHs film can significantly improve the long-term inhabitation properties of Al alloy. Williams and McMurray [13] intercalated different organic anions, such as benzotriazolate, ethyl xanthate and oxalate into the LDH layer grown on the AA2024Al alloy. They found that the inhibition efficiency is anion dependent, increasing in the order ethyl xanthate < oxalate < benzotriazolate. Moreover, carboxylate anions (aliphatic and aromatic) intercalated https://doi.org/10.1016/j.jallcom.2019.02.124 0925-8388/© 2019 Elsevier B.V. All rights reserved.
LDHs have received considerable attention in recent years [14,15]. On the other hand, plasma electrolysis (PE) is a widespread and eco-friendly method to protect metallic substrates by fabricating protective layers on their surfaces. The excessive temperature and pressure associated with PE, however, can inevitably generate the micro-defects that facilitate the corrosion phenomenon in extreme environments [16,17]. In view of the fact that benzoate anion is an interesting class of interlayer anion, due both to the rigidity of the molecules and the very hydrophobic nature of the phenyl ring; thus, novel structures and interesting properties are to be expected when these ions are intercalated into LDH grown on the porous PE film. Although benzoate is commonly used as an intermediate intercalate in forming other structures, the intercalation of this ions into LDH layer has been little investigated. In this study, therefore, the LDH loaded with nitrate anion was synthesized and then dope it with benzoate anions through an ion exchange process. 2. Experimental details A porous alumina was fabricated on 6061 Al alloy substrate by PE process utilizing an electrolyte composed of 0.1 M KOH and 0.05 M NaAlO2 at a current density of 100 mAcm2 for 3 min. Mg(NO3)2$6H2O (0.05 M) and NH4NO3 (0.3 M) were dissolved in deionized water, and urea was then slowly added until the pH reached 10. The PE films were then placed in the above solution in a water bath at 45 C for 24 h to form a homogenous solution. The synthesis was carried out under nitrogen atmosphere in order to avoid the contamination by carbonate anions. Finally, the specimens were removed, rinsed with ethanol, and dried at ambient temperature. The LDH-NO3 film was immersed later into the
Correspondence / Journal of Alloys and Compounds 787 (2019) 772e778
aqueous solution of C6H5COONa (0.1 M) at 50 C under constant stirring for 24 h. The reaction product (LDH-C6H5COO film) was centrifuged and then washed and dried as previously mentioned. The morphologies of the specimens were observed using fieldemission scanning electron microscope (FE-SEM, Hitachi, S-4800) equipped with an energy-dispersive X-ray spectrum (EDS). The crystal structure of the specimens was determined based on Xray diffraction pattern (XRD, Rigaku D/MAX-2500) with Cu Ka radiation source. For surface chemical analysis, X-ray photoelectron spectroscopy (XPS, VG Microtech, ESCA 2000). The fourier transform infrared (FT-IR) spectra (PerkinElmer Spectrum 100) were taken in the wavenumber range 400e4000 cm1. A typical threeelectrode cell system was used to perform the electrochemical impedance spectroscopy (EIS) tests after 10, 24, 48, and 96 h of immersion in 0.05 M NaCl solution. The Ag/AgCl electrode, PE film (with and without LDH-based treatment), and platinum foil were chosen as the reference electrode, working electrode and the counter electrode, respectively. EIS measurements were scanned from 106 Hz to 0.1 Hz using a 10mV amplitude perturbation. To verify the reproducibility of the results, each electrochemical test was repeated at least three times.
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Fig. 2. EDS elemental distributions of aluminum (green color), oxygen (red color), magnesium (cyan color), and carbon (white color) on the surface of the LDH-C6H5COO film. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
3. Results and discussion 3.1. Structure and composition Fig. 1(a-c) displays the surface morphologies of PE, LDH-NO3, and LDH-C6H5COO films grown on Al alloy substrate. The corresponding cross-sectional images of the films were presented as insets. As shown in Fig. (1b and 1c), the LDH-based treatment modifies the porous surface of PE film where a flake-like morphology was developed in the case of LDH-NO3, and LDHC6H5COO films. The porous structure of PE film would be favorable sites for growing the LDHs flakes, offering an effective repair to defects of the PE film. It is worth mentioning that flakes were somewhat oriented perpendicularly to the substrate which was ascribed to the faster crystal growth rate in the direction of bulk solution [18]. Unlike LDH-NO3 film, it can be seen from Fig. 1c that the structure of LDH-C6H5COO film was more compact where thinner flakes were developed. Such reduction of the average size of flakes would
Fig. 1. (aec) SEM micrographs of the PE, LDH-NO3, and LDH-C6H5COO films, respectively, (d) EDS results of the PE, LDH-NO3, and LDH-C6H5COO films identified the presence of Al, O, Mg, and C throughout the surface. Insets reveal the cross-sectional images of the PE, LDH-NO3, and LDH-C6H5COO films.
Fig. 3. (a) XRD patterns of the (I) LDHs-NO3 and (II) LDHs-C6H5COO films. The XRD patterns were scanned from 5 to 35 with Cu Ka radiation source, and (b) FT-IR spectra in the range 4000-400 cm1 for (I) LDH-NO3 film and (II) LDHs-C6H5COO film.
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be attributed to the occurrence of fragmentation in the initial stage of fast anion exchange [19]. According to the cross-sectional images of the films shown in the insets of Fig. 1, the two-layered structure, from up to down, were an outer layer, a dense inner layer of PE on the substrate. The thickness of the PE film was ~5.5 mm. After the growth of LDHs films, the thicknesses of the films almost remain unchanged when compared to PE film. It can be seen that a relatively dense uniform film was formed on the substrate in the case of LDH-C6H5COO film. EDS results presented in Fig. 1d clearly demonstrate that the elements, such as Al, Mg, and O take part in the formation of all films. In EDS analysis of LDH-C6H5COO film, a carbon appears due to the anion-exchange process in the solution containing benzoate anions, suggesting the successful intercalation of C6H5COO anions into the interlayer galleries of LDHs. However, based on the results presented in Fig. 1d, the ratio of Mg/Al was found to be ~0.23 and ~0.20 for LDH-NO3 and LDH-C6H5COO films, respectively. Accordingly, the LDH-NO3 and LDH-C6H5COO films can be expressed by the empirical formula (1) and (2), respectively as follows [18]. 3þ ½Mg2þ 0:77 Al0:23 ðOHÞ2 3þ ½Mg2þ 0:8 Al0:2 ðOHÞ2
0:23
0:2
ðNO3 Þ 0:23 :mH2 O
ðC6 H5 COOÞ 0:2 :mH2 O
(1) (2)
The EDS-mapping analysis on the surface of the LDH-C6H5COO film confirmed the presence of Al, O, Mg, and C in this film where the carbon is found near and within the pores of the PE coating (Fig. 2(aed)). The EDS findings are consistent with the following XRD and FT-IR analyses. The XRD patterns of the LDH-NO3 and LDH-C6H5COO films are shown in Fig. 3a. The composition of PE film deposited on 6061 Al alloy using an electrolyte containing KOH and NaAlO2 was reported in our previous study and was found to be g-Al2O3 and a-
Al2O3 [17]. The XRD patterns, therefore, of the LDH-NO3 and LDHC6H5COO films at scan range from 5 to 35 only shown in the present study. As shown in Fig. 3a, the diffraction reflections of (003), (006), and (009) are observed at around 9.98, 19.92, and 30.5 (2q), respectively for the LDH-NO3 film which was corresponded to interlayer distances of ~0.88, 0.44, and 0.29 nm, respectively [20]. Interestingly, the X-ray diffractions of (003), (006), and (009) in the case of LDH-C6H5COO film moved to lower angels of ~9.5, 19.4, and 29.11 (2q) corresponding to interlayer distances of ~0.94, 0.45, and 0.3 nm, respectively. The increase of the interlayer distance was due to the intercalation of the C6H5COO anions with larger dimension than the NO 3 anions. The present findings are reasonably consistent with those obtained by Mohedano et al. [21] and Brnardic et al. [22]. The intercalation of C6H5COO anion into LDH film was confirmed by FT-IR spectra shown in Fig. 3b. The broad band at 3440 cm1 observed in the spectra of LDH-NO3 and LDHC6H5COO films was ascribed to the stretching of OH groups or water molecules [6,23]. The band at 1381 cm1 is assigned to the antisymmetric stretching vibration of NO3 or CO3 groups whereas the bands observed at 619 and 716 cm1 are generally related to M OH (M: Mg, Al) lattice vibrations. The two bands at 1538 and 1401 cm1 can be assigned to the COO group in LDH-C6H5COO film [15]. Moreover, the band at 1595 cm1 was attributed to the phenyl group (-C6H5) [24]. The disappearance of the 1381 cm1 of NO3 group in the LDHC6H5COO film would suggest that the C6H5COO anion is the only gallery anion [25].
3.2. Electrochemical behavior Fig. 4 (a and b) depicts the Bode plots for PE, LDH-NO3, and LDHC6H5COO films immersed in 0.05 M NaCl solution for 10 h. As
Fig. 4. (a and b) Bode plots of the PE, LDH-NO3, and LDH-C6H5COO films after 10 h immersion in 0.05 M NaCl solution (a) impedance plots and (b) phase plots, and (c) the equivalent circuit model used for fitting the EIS results.
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shown in Fig. 4b, the phase angle corresponding to the high frequency (106 Hz) and the impedance at the low frequency region (0.1 Hz) in the case of LDH-C6H5COO film were higher than those for PE and LDH-NO3 films. Therefore, a significant impediment to the movement of Cl anions towards substrate can be obtained in the case of LDH-C6H5COO film [26,27]. The Bode spectra were fitted using the equivalent circuit depicted in Fig. 4c. In the context of the proposed model, Rs is the solution resistance in a 0.05 M NaCl solution. CLDH and RLDH describe the capacitance and resistance of the synthesized LDH layer while CPE and RPE are the capacitance and resistance corresponding to the PE layer underneath, respectively. Moreover, Cdl and Rct represent the double layer capacitance and charge transfer resistance, respectively. Constant phase element (CPE) is used instead of capacitance to demonstrate the heterogeneity of the films. The CPE was described as follows:
ZCPE ¼ 1 ½YðjuÞn
(3)
where j is the imaginary unit, u is the angular frequency, and n and Y are the parameters of CPE. Based on the proposed model, Bode plots were best fitted, and the corresponding values of the equivalent circuit parameters are listed in Table 1. The values of RLDH, RPE and Rct in the LDH-NO3 and LDH-C6H5COO films were higher than their counterparts in PE film. This suggested that the growth of LDH would proceed not only on the top surface of PE film but also would affect the discharge channels of PE film. The present results were in good agreement with those obtained by Li et al. [12]. Among three samples, however, the LDH-C6H5COO film exhibited significant high values of RLDH, RPE, and Rct, indicating that presence of C6H5COO anions within LDH frame can significantly enhance the resistance of the substrate against corrosion. The corrosion-inhibiting efficiency could be calculated from EIS as follows;
IF% ¼ ð1
Rt0 Þ 100 Rt
(4)
Where Rt0 is the total resistance of the PE film (without LDH-based treatment) while Rt represents the total resistance of the PE film subjected to LDH-based treatment. The values of IF calculated for LDH-NO3 and LDH-C6H5COO films are ~95.0 and ~99.8%, respectively, indicating an increase in the inhibiting effects with the intercalation of C6H5COO anions into the LDH layer. Fig. 5(aec) shows the surface morphologies and the corresponding cross-sectional images of PE, LDH-NO3, and LDH-C6H5COO films after 10 h immersion in 0.05 M NaCl solution. It can be seen from Fig. 5a that PE film had suffered severe corrosion since many cracks were developed on the surface of this film. On the other, the flakelike structure in LDH-NO3 film protected the Al alloy substrate to some extent where some degraded areas are observed clearly, as shown in Fig. 5b. Interestingly, the surface and the cross-sectional image of the LDH-C6H5COO film (Fig. 5c) were similar to those before immersion (Fig. 1), confirming that the formation of the LDH-C6H5COO film on the PE film can effectively enhance the corrosion resistance of Al alloy substrate. To get more information on the stability of the films, however,
Fig. 5. Morphological features of the films after 10 h immersion in 0.05 M NaCl solution (a) PE, (b) LDH-NO3, and (c) LDH-C6H5COO. Insets reveal the corresponding cross-sectional images.
Table 1 Results of the equivalent circuit of the films immersed in 0.05 M NaCl solution for 10 h. sample PE LDH-NO3 LDH-C6H5COO
RLDH (U.cm2) 4
4.5 10 9.0 105 5.2 107
nLDH 0.62 0.68 0.73
YLDH (S.sn.cm2) 8
3.5 10 1.8 108 2.4 109
RPE(U.cm2) 5
1.1 10 4.7 107 9.3 108
nPE 0.61 0.65 0.69
YPE (S.sn.cm2) 8
2.2 10 3.4 109 1.0 109
Rct (U.cm2) 6
6.8 10 9.3 107 3.7 109
nct
Yct (S.sn.cm2)
0.86 0.88 0.91
3.2 108 5.1 108 2.4 109
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Bode plots of the PE, LDH-NO3, and LDH-C6H5COO films immersed for different times, such as 24, 48, and 96 h in 0.05 M solution are presented in Fig. 6. As shown in Fig. 6, the impedance value decreased with increasing the immersion time which was associated with the appearance of new time constants in phase angle plots. This indicated the penetration of corrosive anions (Cl) to the interface of the substrate. As well known, the overall resistance of inhibitive films can be evaluated by the low-frequency impedance values [27,28]. Thus, the variations of jZj0.1 Hz with respect to the immersion time shown in Fig. 7a were used to figure out the stability of the samples exposed to 0.05 M NaCl solution. Interestingly, the jZj0.1Hz value of LDH-C6H5COO film was still well above 106 U cm2 after 96 h, which was over two and four orders of magnitude higher than those of the LDH-NO3 and PE films, respectively. This result, attributed mainly to the release of C6H5COO anions during the immersion in 0.05 M NaCl solution, confirming again
the fact that the formation of LDH-C6H5COO film with stable flake-like structure can help to improve the stability of PE coating in 0.05 M NaCl solution, thereby prolonging the service life of the substrate. Previous investigations have reported that the microstructural changes and the release of inhibitors from LDH film were the main reasons for improving the inhibition performance of the PE film modified by LDH film. Moreover, Mg2þ ions are expected to be released from LDH-NO3 and LDH-C6H5COO films into the solution, either from the interlayer galleries or from metal hydroxide layer as a result of structural arrangement, leading to some inhibitory effects [29,30]. Although Mg2þ and NO 3 ions, released from LDH-NO3 film helped to improve the inhibition performance of PE film (Eq. (5)), the amounts of these ions would not be high enough to form a dense and inhibitive film, that is why the LDHNO3 film showed lower inhibitive performance when compared
Fig. 6. Bode plots (impedance and phase) of the PE, LDH-NO3, and LDH-C6H5COO films immersed in 0.05 M NaCl solution for different times (a and b) 24h, (c and d) 48 h, and (e and f) 96 h.
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Fig. 7. (a) Representation of the jZj0.1Hz values for PE, LDH-NO3, and LDH-C6H5COO films as a function of immersion time in 0.05 M NaCl solution, and (b) a schematic illustration of the corrosion inhibition mechanism of the LDH-C6H5COO film. A diffusion boundary film, LDH film, and PE film were observed in this model above Al substrate.
to the LDH-C6H5COO film. In contrast to LDH-NO3 film, the LDH flakes were more compact in the case of LDH-C6H5COO film which would effectively retard the movement of Cl anions towards substrate since most of the pores in the PE film were sealed totally. LDH NO 3 þ Cl /LDH Cl þ NO3
(5)
The possible mechanism for the superior inhibitive performance of LDH-C6H5COO film is schematized in Fig. 7b. Based on the ionexchange process (Eq. (6)), the released C6H5COO anions tended to concentrate on the film surface, leading to the formation of a diffusion boundary layer containing high concentrations of C6H5COO anions (hard base), which may react with the dissolved Alþ3 (hard acid), forming aluminum benzoate (Al(C6H5COO)3) on the surface of the coating (Eq. (7)) [31,32].
LDH C6 H5 COO þ Cl /LDH Cl þ C6 H5 COO
(6)
Alþ3 þ 3C6 H5 COO /AlðC6 H5 COOÞ3
(7)
To confirm the formation of Al(C6H5COO)3, we carried out XPS measurements on the surface of LDH-C6H5COO film. Fig. 8a shows that the deconvolution of O1s led to the appearance of two peaks at 531.2 and 532.5 eV, which are assigned to the hydroxyl groups (OH)
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Fig. 8. XPS spectra of LDH-C6H5COO film in the (a) O1s and (b) C 1s regions.
and carboxylate groups (OC]O), respectively [33,34]. In the C1s region (Fig. 8b), two peaks were observed at 284.7 and 288.3 eV, matching with aromatic ring and OC]O groups from benzoate anions in Al(C6H5COO)3 [33,35]. All of the above-stated results would support the successful formation of Al(C6H5COO)3. On the other hand, the presence of C6H5COO anions in the diffusion boundary layer would impair the adsorption of Cl anions on the surface of the coating owing to the so-called competitive adsorption. Therefore, it can be concluded that the diffusion boundary film, LDH film containing C6H5COO anions, and PE film all can be responsible for the inhibiting performance of LDH-C6H5COO film.
4. Conclusion In the present work, benzoate anions intercalated to Mg-Al LDH layers has been successfully grown on the surface of porous PE coating via an ion-exchange route. The results revealed that the incorporation of benzoate anions into the LDH film would greatly influence the growth of LDH-NO3 film, leading to a more compact arrangement of LDH nanosheets. Accordingly, a superior inhibition performance was provided by LDH-C6H5COO film since the
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presence of Cl anions in the solution during the corrosion test can trigger the on-demand release of C6H5COO anions from the interlayers of LDH particles. Conflicts of interest
[20]
[21]
The authors declare no competing interests. [22]
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Mosab Kaseem Department of Nanotechnology and Advanced Materials Engineering, Sejong University, Seoul, 05006, Republic of Korea Young Gun Ko* School of Materials Science and Engineering, Yeungnam University, Gyeongsan, 38541, Republic of Korea *
Corresponding author. E-mail address: [email protected] (Y.G. Ko). 25 October 5 February 10 February Available online 14 February
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