silane nanocomposite coatings for protection of mild steel

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Fabrication and characterization of layered double hydroxide/silane nanocomposite coatings for protection of mild steel E. Alibakhshi a,b,c,∗, E. Ghasemi a, M. Mahdavian b,∗∗, B. Ramezanzadeh b a

Inorganic Pigment and Glazes Department, Institute for Color Science and Technology, P.O. Box 16765-654, Tehran, Iran Surface Coating and Corrosion Department, Institute for Color Science and Technology, P.O. Box 16765-654, Tehran, Iran c Department of Chemical Engineering, Payame Noor University, P.O. Box 19395-3697, Tehran, Iran b

a r t i c l e

i n f o

Article history: Received 19 June 2017 Revised 30 July 2017 Accepted 6 August 2017 Available online xxx Keywords: Corrosion Layered double hydroxide Silane sol–gel coating EN EIS

a b s t r a c t In this paper, environmentally friendly layered double hydroxide nanoparticles/silane hybrid nanocomposite coatings were fabricated on mild steel substrate. Zinc–aluminum layered double hydroxides (Zn–Al LDHs) pillared with different anions (nitrate, molybedate and phosphate) were synthesized and characterized by X-ray diffraction (XRD). Various LDHs was employed in a hybrid silane coating based on mixture of tetraethyl ortosilicate (TEOS) and aminopropyl triethoxy silane (APTES). The silane coatings including LDHs were characterized by scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FT-IR) and contact angle measurement. Then, the corrosion characterization of the coatings was investigated by electrochemical impedance spectroscopy (EIS), polarization and electrochemical noise (EN) techniques. Addition of LDHs to the sol–gel film increased its barrier properties due to their planar structure. The results revealed that the silane coating in the presence of phosphate could provide superior electrochemical corrosion protection compared to the sol–gel film which is solely incorporated with molybedate and nitrate. © 2017 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

1. Introduction One of the most important concerns in industries is the corrosion of metal structures due to huge economic losses. Thus, various strategies such as appropriate design of structure, the surface pretreatment of metals and alloys, anodic or cathodic protection, applying protective polymeric systems and utilization of corrosion inhibitors have been used to tackle the corrosion in industries [1–3]. Among these strategies, the surface pretreatment has received considerable attention because of its vital role in promoting adhesion between a substrate and the polymer coating as well as corrosion protection. For many years, the main effective surface pretreatment systems were based on the use of hexavalent chromate and phosphate containing conversion coatings. However, utilization of hexavalent chromium in almost all sectors except the aerospace is banned due to its poisonous nature. Also, phosphate containing conversion coating has some limitations such as high energy consumption and the production of sludge. Therefore, a va-

∗ Corresponding author at: Surface Coating and Corrosion Department, Institute for Color Science and Technology, Tehran, Iran. ∗∗ Corresponding author. E-mail addresses: [email protected], [email protected] (E. Alibakhshi), [email protected] (M. Mahdavian).

riety of non-toxic surface pretreatments including cerium, vanadium, molybdenum and zirconium have been developed. Recently, silane sol–gel coatings have been depicted as ecofriendly metal surface pretreatment. These coatings have strong chemical bonds with the substrate through the hydrolysable siloxane groups of silane molecules, conferring the good adhesion to metallic structure. These coatings also exhibit a dense network of siloxane, which can provide a barrier against electrolyte diffusion to the substrate. However, they cannot provide active corrosion protection like the chromate coatings. Increase in barrier protection of silane coatings utilizing nanoparticles has been widely reported by the group of Ferreira and co-workers [4–7]. Recently, nanoclay has been introduced in order to improve barrier protection performance of silane coatings [8,9]. Asadi et al. investigated influence of nanoclay on the corrosion protective performance of a silane coating [10]. They found considerable increase in charge transfer resistance and coating film resistance in the presence of nanoclay. Such an increase was attributed to the flake morphology of nanoclay which improved barrier protection of the silane coating. Incorporation of corrosion inhibitors into the silane coatings is a promising way to provide active corrosion inhibition [11]. For this purpose, several inorganic inhibitors like permanganate [12], molybdate [12,13] and cerium [14,15] as well as organic inhibitors

http://dx.doi.org/10.1016/j.jtice.2017.08.015 1876-1070/© 2017 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

Please cite this article as: E. Alibakhshi et al., Fabrication and characterization of layered double hydroxide/silane nanocomposite coatings for protection of mild steel, Journal of the Taiwan Institute of Chemical Engineers (2017), http://dx.doi.org/10.1016/j.jtice.2017.08.015

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such as benzotriazole [7], 8-hydroxyquinoline [16,17] and mercaptobenzimidazole [18,19] compounds have been embedded into sol– gel layers. Montemor et al studied the influences of addition of rare earth (cerium and lanthanum) salts on the corrosion protection of sol–gel coating as chromate replacers for pretreatments on galvanized steel [20]. It was shown that the presence of sulfur atoms on the silane molecule (bis-[triethoxysilylpropyl] tetrasulphide) is responsible for the development of covalent bonds with rare earth or zinc hydroxide groups on the galvanized steel and helps to form denser coatings with better corrosion resistance performance. Also, the best performance was obtained for the silane coating containing lanthanum. Direct inclusion of corrosion inhibitors in the silane coatings may be troublesome due to possible interference with curing reaction and uncontrolled release [5,15,21]. Storage of corrosion inhibitors in inorganic nanocontainers to avoid direct contact of corrosion inhibitors with coatings matrix and to provide on-demand release of doped corrosion inhibitors is a promising approach to overcome the mentioned problems [7,22,23]. In recent years, different nanocontainers including mesoporous nanosilica [24], halloysite nanotubes [11,17] have been introduced. Layered double hydroxides (LDHs) are of the nanocontainers with anion exchange capacity which can be used for storage of anionic corrosion inhibitors. LDHs, hydrotalcitelike materials or anionic clay, take the general formula [MII1−x MIxII (OH )2 ]x+ (Am− )x/m .nH2 O, where MII is a divalent cation, MIII is a trivalent cation, and Am − is an exchangeable anion with a charge of m [25–31]. This work is novel over the previous works as there is no study addressing and comparing the inhibitive action of LDH nanoparticles in silane coatings as surface pretreatments for mild steel. In this paper, phosphate and molybdate were separately intercalated in Zn–Al-LDH–NO3 − LDH and then resulting LDHs were characterized using x-ray diffraction (XRD). The LDHs were embedded to the silane hybrid coating and the surface characteristics of the samples were studied by scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FT-IR) and contact angle measurements. Furthermore, the corrosion protection performance of the coatings was investigated by electrochemical impedance spectroscopy (EIS), polarization and electrochemical noise (EN) measurements.

Table 1 Molybdenum and phosphorous concentration of the solution used in each washing step. Times

Mo (mg/l)

P (mg/l)

n 2n 3n 4n 5n 6n 7n 8n

453 202 7.6 3.1 0.5 0.18 0.02 0

685 290.3 38 8.2 2 0.31 0.05 0

Table 2 Composition of steel samples. Elements

wt%

Fe C Si Mn P S Cr Mo Co Cu

Balance 0.19 0.415 1.39 >0.005 >0.005 0.026 0.018 0.0559 0.0429

LDH and then dried. After each washing step, the phosphorous and molybdenum concentrations in the washing solutions of the LDH were measured by ICP-OES. According to the results presented in Table 1, it is possible to verify that the concentration of the solution decreases to zero after 8 times washing with deionized water, indicating complete removal of phosphate and molybdate adsorbed to the outer surface of LDH. However, all of the paper about LDHs synthesis, washing process was performed maximum in four times [33–36]. Crystallographic characteristics of prepared LDHs were studied by X-ray diffraction (XRD). The XRD analysis of LDHs were performed with a PW 1800 Philips X-ray spectrometer with Cu-Kα ˚ over the 2θ range from 10 to 65°. radiation (λ = 1.541874 A) 2.2. Fabrication and characterization of the LDH/silane hybrid coatings

2. Experimental 2.1. LDHs synthesis and characterization All the reagents/reactants used were of analytical grade from Sigma-Aldrich Co. Zn–Al–NO3 ¯ LDH was synthesized by co-precipitation using mixed solutions of Zn(NO3 )2 •6H2 O and Al(NO3 )3 •9H2 O with Zn/Al ratio of 2. Here, 40 ml of the metal solutions were added dropwise at a rate of 10 ml/h to 70 ml of a 0.02 M NaNO3 solution under vigorous stirring for 1.5 h. The pH of solution was maintained constant at 12.5 ± 0.5 by adding 0.2 M NaOH using a Metrohm 744 pH meter. Afterward, the precipitate was washed with boiled deionized water, centrifuged at a speed of 4500 rpm for 10 min several times and subsequently dried at 70 °C for 4 h. The synthesis was accomplished under a nitrogen atmosphere at room temperature to prevent the formation of carbonate in solution. Phosphate and molybdate forms of Zn−Al LDHs were prepared by anion-exchange method previously described by our group [25,32]. A neat Zn–Al–NO3 ¯ LDH (0.5 g) was added into a 0.4 M aqueous solution of Na3 PO4 •12H2 O or Na2 MoO4 •2H2 O with vigorous stirring in a nitrogen atmosphere for 48 h. The precipitate was then centrifuged, washed eight times with boiled deionized water to remove of phosphates or molybedate in the outer surface of

The hybrid silane film was obtained by hydrolyzing two different organosilane sols namely aminopropyltriethoxysilane (Merck, 1.6 wt%) and tetraethylorthosilicate (Merck, 3.4 wt%) in an alcohol solution containing ethanol (Merck, 85 wt%) and deionized water (15 wt%). The pH of the silane mixture was fixed at 4.5 by acetic acid. According to literature, the pH range for silane coating is 2–6 [37–41]. At very acidic condition, better hydrolysis can take place. However, rusting of metal can occur at low pH. Thus, we selected a slightly acidic condition (4.5) as optimum pH to obtaining the best hydrolysis and condensation reactions. The obtained silane solution was stirred at room temperature for 3 h forming sol–gel precursors. The LDH nanoparticles (0.2 g) were dispersed in water (20 ml) for 3 min by homogenizer (Viggen, model 0310). This mixture was added to a silane solution and stirred for 20 min by magnetic stirrer before apply on the mild steel. Prior to silane application, the mild steel panels (with a dimension of 100 mm × 75 mm × 2 mm) were abraded by sand papers 40 0, 60 0 and 80 0 grades followed by acetone degreasing. The chemical composition (wt%) of the mild steel are presented in Table 2. The silane coating solutions were sprayed on the mild steel panels with a Sata Jet 20 0 0 HVLP Digital spray pistol using 4.5 bar on a 1.3 spray valve for 10 s. This procedure was repeated three times to ensure that a thick silane layer was formed on the

Please cite this article as: E. Alibakhshi et al., Fabrication and characterization of layered double hydroxide/silane nanocomposite coatings for protection of mild steel, Journal of the Taiwan Institute of Chemical Engineers (2017), http://dx.doi.org/10.1016/j.jtice.2017.08.015

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2.3. Electrochemical measurements The corrosion protection of the mild steel panels coated with SC, SCN, SCM and SCP was investigated in 3.5 wt% NaCl solution by EIS and EN techniques. For this purpose, an area of 1 cm2 of coated samples was exposed to the electrolytes while other area of the plates was sealed with beeswax–colophony mixture. Each specimen was immersed in 100 ml of prepared electrolyte. EIS measurements were conducted by an Ivium Compactstat (Netherlands) in a conventional three electrode cell containing Ag/AgCl (3 M KCl), graphite and the mild steel specimens as reference, counter and working electrodes, respectively. The EIS was implemented at open circuit potential using 10 mV perturbation in a frequency range of 10 kHz–10 mHz. The EIS was measured after 1 h, 2 h, 3 h and 4 h exposure of the sample to the saline solution. EIS was conducted on triplicates to ensure the repeatability. Using the same instrumentation and three electrode cell, polarization was performed after 4 h immersion in saline solution within polarization range of −200 mV to +200 mV with sweep rate of 0.5 mV/s. The EN measurements were carry out in a three electrode cell under open-circuit conditions consisting of two nominally identical mild steel electrodes (with dimension of 1 cm2 ) and an Ag/AgCl (3 M KCl) reference electrode. Ivium Compactstat (Netherlands) was used for EN measurements with sampling frequency of 20 Hz. A low pass filter of 10 Hz was used during data recording to avoid aliasing. All of the measurements were done after 4 h immersion and the duration of measurement was 800 s. The data were processed using Matlab from MathWorks. The electrochemical signals were extracted by a multi-resolution wavelet analysis and decomposed in predefined numbers of scales (or crystals) in undecimated wavelet transform (UWT) framework. 3. Results and discussion 3.1. Characterization of LDHs The Zn–Al-LDHs nanoparticles were characterized by XRD. Fig. 1 compares the XRD patterns of prepared LDHs intercalated with various anions. As demonstrated in Fig. 1, all of the LDHs presented a series of diffraction at 11.4°, 23.4°, 34.6°, 39.1° 46.5°, 60.2° and 62° corresponding to [0 03], [0 06], [012], [015], [018], [110] and [113] diffractions of the LDH phase (hydrotalcite; reference pattern JCPDS 00048-1026), respectively.

(003) (012) (015) (018)

(006)

(c)

Intenstity (a.u)

samples. Finally, the silane coated samples were cured at 120 °C for 1 h. In this paper, four types of coated samples were selected for evaluation as follows: (i) silane coating without LDH (SC), (ii) silane coating with Zn–Al–NO3 − LDH (SCN), (iii) silane coating with Zn–Al–MoO4 2− LDH (SCM) and (iv) silane coating with Zn– Al–PO4 3− LDH (SCP). LDH content and the amount of inhibitor in dry coating were adjusted around 12 ± 1 wt% and 1.1 ± 0.1 wt%, respectively. The surface morphology of silane coated samples was studied using scanning electron microscope (SEM) model Philips XL30. Chemical structure of the silane coatings was evaluated using Fourier transform infra-red (FT-IR, PerkinElmer Spectrum One) at the range of 40 0 0–40 0 cm−1 . The static contact angles of distilled water on the mild steel without and with silane coating systems were evaluated by a homemade instrument. A small drop of high purity distilled water (4 μl) was located on the surface of the samples and the image of droplet was recorded after 5 s.

3

(113) (110)

(b)

(a) 0

5

10

15

20

25

30

35

40

45

50

55

60

65

2-Theta (°) Fig. 1. XRD patterns of the Zn–Al–NO3 − (a), Zn–Al–MoO4 2− (b) and Zn–Al–PO4 3− (c) LDHs.

These diffractions are in good agreement with that previously reported; revealing highly crystallinity of synthesized LDH [42,43]. Also, two diffractions appeared at 29.5 and 32.5 for Zn–Al–NO3 − LDH, which can be related to the ZnO as a by-product [44]. The interlayer spacing values of LDHs were calculated using the 003 peak positions and Bragg’s law [45]. The interlayer spacing ˚ was lower than values of the nitrate including Zn–Al LDH (7.69 A) ˚ and Zn–Al–PO4 3− (7.79 A) ˚ LDHs. Inthat of Zn–Al MoO4 2− (7.74 A) creasing of the interlayer spacing for Zn–Al MoO4 2− and Zn–Al– PO4 3− LDHs is indicative of successful intercalation of molybedate and phosphate into the interlayer galleries of LDH by anion exchange with nitrate ions. These values are in contradiction with the previous values on the Zn–Al LDH intercalated with nitrate, phosphate and molybedate. It seems that interlayer spacing of LDH can be affected by parameters such as aging time, M2+ /M3+ molar ratio, calcination time and the amount of water in synthesis process [46–48]. In this paper, we did synthesis without aging time and heat treatment, which can affected on lattice parameters and crystal growth by itself compared to other papers [33,49]. Also, synthesis was performed at different pH compared to other papers. On the other hand, the discrepancy in peak shifts at higher diffraction angles (see dashlines in Fig. 1) can be attributed to a known crystallographic basic that as the diffraction angle increases, the plane with lower closed packed are appeared. It means that for planes which are in higher diffraction angles there is more space for entrance of external ions (loading). The slight difference in the interlayer spacing values between molybedate and phosphate could be related to the content of water in the interlayer galleries and remaining nitrate ions in the interlayer region [50]. It should be noted that as the phosphate and molybdate adsorbed onto the outer surface of LDH particles are completely removed (see Table 1), the shifts in the diffraction peaks is only due to intercalated phosphate and molybedate into LDH interlayer space. It can be seen from the Fig. 1 that the reflections at 60.2° and 62° (related to the brucite-like layer structur) of the Zn–Al MoO4 2− and Zn–Al–PO4 3− LDHs shifted after anion exchange. This means that the structure of layer changed in the presence of molybedate and phosphate.

Please cite this article as: E. Alibakhshi et al., Fabrication and characterization of layered double hydroxide/silane nanocomposite coatings for protection of mild steel, Journal of the Taiwan Institute of Chemical Engineers (2017), http://dx.doi.org/10.1016/j.jtice.2017.08.015

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Fig. 2. SEM micrographs from the surface of uncoated (blank) and coated samples.

3.2. Characterization of the LDH/silane hybrid coatings The structural and morphological characteristics of the silane coated mild steel samples (SC, SCN, SCM and SCP) were evaluated by SEM, as shown in Fig. 2. It is clear from Fig. 2 that the surface of uncoated sample (blank) includes multiple grooves and ripples created during sur-

face preparation step by sand papers. In contrast, the mild steel samples coated with the silane films presented distinctive surface morphologies. A uniform layer of silane without the presence of the surface features covered the whole mild steel surface for these samples. In the case of SCN, Zn–Al–NO3 − LDH agglomerates were seen in the coating matrix. This might be related to incomplete separation of LDH particulates during its incorporation and mixing

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Fig. 3. FT-IR spectra of silane coating with and without LDHs.

Fig. 4. Image of a water droplets on the surface of uncoated and coated samples.

in the silane formulation. The smooth surface and more compact film formation was observed for SCN and SCP samples, suggesting that the embedded the Zn–Al–MoO4 2 − and Zn–Al–PO4 3− LDHs are compatible with the silane coating. The influence of LDHs on the extent of silane film curing was studied by FT-IR. The FT-IR spectra of silane coating with and without LDHs are illustrated in Fig. 3. As can be seen in Fig. 3, two peaks, corresponding to Si–O bending modes, were observed at 678 and 786 cm−1 wavenumbers [51]. Results also show absorption bands at 1645 cm−1 , which assigned to the presence of adsorbed water [52]. The presence of C–H and N–H bands at approximately 2920 cm−1 and 1610 cm−1 , respectively, alludes to the existence of APTES in the film resulted by the co-condensation of silanes [51,53]. FT-IR spectra of the silane coatings contains two strong absorption peaks, one in the range of 1050–1150 cm−1 related to Si–O–Si stretching vibration and the other in the range of 330 0–350 0 cm−1 attributed to OH stretching vibration. The curing reaction in silane film can be characterized by the formation of Si–O–Si crosslinks and silanol (Si–OH) groups [51]. The stronger the Si–O–Si vibration and the OH vibration were associated with higher crosslinking density and unreacted silanol groups, respectively. Silane coating with the LDHs revealed a depression in the intensity of the silanol as well as an increase in the intensity of the Si–O–Si vibration. Results reveal that incorporation of the LDHs into the silane coating enhances the condensation reactions among the different silanol groups. In other words, no negative effect on the curing reaction was observed by inclusion of LDHs in silane coating. So, the SCP was characterized by better condensation reaction and denser Si–O–Si network compared to other coatings (SCN and SCM). The surface chemistry of silane coated mild steel samples was studied by water contact angle measurements. The measured contact angle of water droplets on different samples are presented in Fig. 4. The contact angle of the SC is 49.5°, a decrease to that of the mild steel, and it could be related to the nature of the silane film that consists of many −NH2 groups as well as higher amount of SiOH groups according to the FT-IR results. From Fig. 4 it can be seen that presence of LDHs in silane coating increased the contact angle compared to the bare steel and SC. This could be related to the decrease of surface roughness, which could be easily observed in the SEM micrograph. The increase of contact angle was most pronounced for the SCP. This means that the surface film is less prone to water. This could

be confirmed by lower amount of SiOH groups resulting from an increase in the reticulation (Fig. 3).

3.3. Corrosion protection performances of the LDH/silane hybrid coatings Corrosion protection properties of the silane coating without and with LDHs in 3.5 wt% NaCl were investigated using electrochemical methods. Figs. 5 and 6 illustrate the Nyquist and Bode diagrams during 4 h immersion in 3.5 wt% NaCl for the mild steel specimens uncoated (blank sample as reference) and coated with the four kinds of silane layer namely silane coating without LDH (SC), silane coating containing Zn–Al–NO3 − LDH (SCN), silane coating with Zn–Al–MoO4 2− LDH (SCM) and (iv) silane coating with Zn–Al–PO4 3− LDH (SCP). It can be clearly seen from Figs. 5 and 6 that the bare mild steel substrate (blank) had the smallest diameter (Fig. 5) and impedance values (Fig. 6) in all of the times, revealing that it is easily corroded in the saline solution. This means that blank sample had the smallest Faradaic resistance and, therefore, the highest corrosion rate in saline solution. The diameter of semi-circle and impedance values for the silane coating without LDH (SC) was larger than that of blank sample, indicating the barrier effect of silane on the mild steel. However, these values are not enough for long protection performance and the electrolyte solution can reach the surface of the mild steel and corrosion takes place at this region. By introduction of LDHs into silane coating, the diameter of the Nyquist plots was changed at even early stages of immersion, i.e. 1 h, revealing that the LDH particles has almost effect on the barrier properties of silane coatings. Unlike the SCN coating, the diameter of the capacitive loop for SCP and SCM increased over time indicating corrosion inhibition of these particles. Further, phase plots (Fig. 6) of the SC and the SCN samples revealed one relaxation time whereas that of the SCM and SCP samples revealed a second relaxation time during the immersion times. For the SC and SCN, it seems that the time constant related to the silane coating (high frequency) is too close to that of corrosion (low frequency) leading to screening of the coating process by the corrosion. In order to better understand the electrochemical performance of the samples, the EIS data were fitted using equivalent circuits in the cases of one and two relaxation times (see Fig. 7).

Please cite this article as: E. Alibakhshi et al., Fabrication and characterization of layered double hydroxide/silane nanocomposite coatings for protection of mild steel, Journal of the Taiwan Institute of Chemical Engineers (2017), http://dx.doi.org/10.1016/j.jtice.2017.08.015

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Fig. 5. Nyquist diagrams for all the samples immersed in 3.5 wt% NaCl solution for 1 h (a), 2 h (b), 3 h (c) and 4 h (d).

In the circuits, Re represents the electrolyte resistance; Rct , the Faradaic resistance; Rf the film resistance; CPEdl and CPEf the constant phase elements of the electrical double-layer at the metal/solution interface and the inhibitive film, respectively. Further, Q is admittance of CPE. The chi-square (χ 2 ) values were used to judge quality of equivalent- circuit fitting. The results of the fitting procedure and chi-square are tabulated in Table 3. The χ 2 < 10−3 was obtained for all data fitting revealing that the fitted data have good agreement with the experimental data. From Table 3, the Rct values for SC are higher than the Rct values obtained for the bare mild steel (blank) in whole of the immersion times, evidencing the barrier properties of this coating. However the Rct decreases after 4 h probably due to the electrolyte uptake into the coating and coating degradation [38,54]. This is confirmed by increasing of Qdl values as time elapsed that may result from a decrease of the layer thickness and/or increase of the dielectric constant [55,56]. After 2 h immersion, the silane coating including Zn–Al–NO3 − LDH (SCN) demonstrates the highest Rct values as compared to the other coatings. The adsorption of Cl− aggressive ions from the saline solution and the slight release of zinc cations by Zn–Al LDH could be responsible for this action. However, the Rct value was diminished after 4 h exposure but still higher than that for SC. According to Table 3 for the SCM and SCP samples the Rct increased as time progressed, revealing that both coatings could provide effective corrosion protection. Also, the Qdl decreased for both samples. Increase of the Rct and decrease of the Qdl values during immersion is attributed to the inhibitive film formation on the steel surface and the increase of silane coating film barrier performance as a result of complex formation between the zinc cations and phosphate or molybedate ions, leading to pore plug-

ging [57,58]. It is clear that SCP provides the best corrosion protection performance among the various samples since the resistance values of this sample are the highest compared to other coatings as times elapsed. It is well known that introduction of inhibitors into the silane coating leads to improvement of its barrier and inhibitive action. This means that on the one hand, silane coatings can act as barrier, but on the other hand, the corrosion protection performance of this coating is not high enough. Therefore, SCP can provide better corrosion resistance due to the release of zinc and phosphate. Other important parameters are the Rf and Qf which are function of uniformity and thickness of the hybrid silane film. It was previously reported that the decrease of the pore resistance of sol– gel film is an indication of formation and growth of new cracks and pores [5]. From Table 3, the Rf values of SCM and SCP samples increased as the immersion time elapsed. This may mean that micropores in the silane film have been plugged by inhibitive species released from the LDHs. Moreover, Qf which is a function of water content in the silane coating decreased for these samples, confirming increase in silane film compactness due to pore plugging. The increase of Rf and decrease of Qf was significant for the SCP sample. The higher surface hydrophobicity as well as denser film with greater thickness of SCP sample may be responsible for its greater barrier properties. Studying the impedance magnitude at low frequency limit can demonstrate the general corrosion protection performance of each coating. Similar to the Rct values, the trend of the impedance magnitude at low frequencies was descending for the blank sample, while that for the SCM and SCP was ascending. The increase in impedance at 1 h is an indicative of the LDH role on the barrier

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Fig. 6. Bode diagrams for all the samples immersed in 3.5 wt% NaCl solution for 1 h (a), 2 h (b), 3 h (c) and 4 h (d).

Table 3 Variation of EIS electrochemical parameters of samples immersed in 3.5 wt% NaCl solution after 4 h. Coating

Time (h)

Rct a ( cm2 )

Qdl b (μsn / cm2 )

Rf a ( cm2 )

Qf b (μsn / cm2 )

log |Z| ( cm2 )c

χ2

Blank

1 2 3 4

1433 1133 655.1 566.6

732.1 752.7 665.9 740.7

– – – –

– – – –

3.05 2.91 2.83 2.74

0.0 0 08 0.0 0 08 0.0 0 09 0.0 0 08

SC

1 2 3 4

2329 1608 1508 1424

562.4 347.1 292.9 729.4

– – – –

– – – –

3.32 3.1 3.08 3.05

0.0 0 08 0.0 0 09 0.0 0 08 0.007

SCN

1 2 3 4

3375 8845 2848 2705

540 122.5 333.6 321.9

– – – –

– – – –

3.47 3.9 3.39 3.34

0.0 0 07 0.0 0 07 0.0 0 09 0.0 0 09

SCM

1 2 3 4

2878 4846 5473 6200

254.4 162.3 9.4 1.7

16.4 162.8 343.5 1543.3

524.5 284.8 176.6 62.1

3.49 3.61 3.76 3.87

0.0 0 09 0.0 0 05 0.008 0.0 0 06

SCP

1 2 3 4

3407 4268 13,110 28,450

146.6 164.3 187.2 99.3

198.6 220.5 432 1820

337 257.2 59.7 16.9

3.52 3.65 4.13 4.4

0.0 0 08 0.0 0 07 0.0 0 09 0.0 0 07

a b c

The standard deviation range for Rct and Rf values is between 1.4 and 9.7%. The standard deviation range for Qdl and Qf values is between 2.7 and 10.6%. The standard deviation range for log |Z| values is between 0.4 and 3.1%.

Please cite this article as: E. Alibakhshi et al., Fabrication and characterization of layered double hydroxide/silane nanocomposite coatings for protection of mild steel, Journal of the Taiwan Institute of Chemical Engineers (2017), http://dx.doi.org/10.1016/j.jtice.2017.08.015

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Fig. 7. Equivalent circuits used to model measured data on EIS diagrams; (a) one time-constant (blank, SC and SCN) and (b) two time-constant equivalent circuits (SCM and SCP).

Blank

SC

Fig. 9. Polarization plots of coated and uncoated samples after 4 h immersion in 3.5 wt% NaCl solution.

SCN

SCM

SCP

Fig. 8. Optical photographs of coated and uncoated samples after 1 and 4 h immersion in 3.5 wt% NaCl solution.

performance enhancement of silane film. Impedance data (Table 3) show that the impedance magnitude of SCM and SCP was significantly higher than that of SCN, and it tended to increase during the whole of exposure time. These observations can clearly illustrate that the SCM and SCP showed corrosion protection behavior much greater than SCN one. This result may be due to a synergetic inhibition behavior of zinc with phosphate or molybedate ions. Further, the smooth surface and more compact film formation of SCP and SCM compared to SCN (as confirmed by SEM) was another reason for this result. The optical photographs of all the coated and uncoated samples after 1 and 4 h of immersion are shown in Fig. 8. The visual appearance of the samples is in good agreement with the impedance results. It can be seen from Fig. 8 that the brown rust appeared on the surface of the mild steel panels immersed in the blank solution after 1 h. However, less corrosion products created on the mild steel panels coated with silane film implying their corrosion protection. As time passed, the blank sample shows the highest degree of corrosion with multiple pits. The corrosion was not seen after 4 h immersion when the mild steel coated with SCP, indicating the best corrosion protection.

In order to investigate the effect of the release ability of inhibitive species on the corrosion protection performance of silane coating, the concentration of the ions released from the nanoparticles during their reaction with an aggressive solution was evaluated using ICP-OES. Results represent that Zn–Al LDH nanoparticle released more phosphate (20.2 mg/l) than molyebdate (7.6 mg/l) ions in aggressive solution. This means that intercalation of Zn– Al LDH with phosphate enhanced the release ability of inhibitive species. Also, the zinc concentration for Zn–Al–MoO4 2− and Zn– Al–PO4 3− LDHs is 1.5 mg/l and 1.3 mg/l, respectively. Higher corrosion protection of the SCP compared to SCM can be due to higher release ability of phosphate as inhibitive species. The corrosion protection mechanism of coated samples after 4 h immersion in 3.5% NaCl solution was examined by polarization test, and the results are shown in Fig. 9. It can be seen that inclusion of LDHs into the silane coating led to a shift in corrosion potential (Ecorr ) to more positive values. Moreover, the current densities of the cathodic and anodic branches were decreased in the presence of LDHs. The electrochemical corrosion parameters including corrosion potential (Ecorr ), corrosion current density (icorr ), anodic (ba ) and cathodic (bc ) Tafel slopes were obtained from Tafel extrapolation method after 4 h immersion (Table 4). Results show the decrease in corrosion current density and increase of corrosion potential after application of silane coatings on mild steel. The decrease in corrosion current density is a consequence of blocking the anodic and cathodic active sites and restricting aggressive species access to the surface [54]. Comparison of corrosion current density illustrated in Table 4 indicates the effectiveness of studied samples which follows the sequence: SCP > SCM > SCN > SC > blank. The polarization results confirmed EIS results revealing betterment in corrosion protection performance of silane coatings in the presence of Zn–Al–PO4 3 − nanoparticles. Herein, the EN test was also used to evaluate the difference in corrosion protection ability among the various pretreatment layers (silane coatings). Different implications developed in the framework of wavelet transforms have been extracted to study electrochemical noise measurements. In this paper, undecimated wavelet transform (UWT) framework was selected as a useful tool to analyze electrochemical noise signals with a Daubechies 4.

Please cite this article as: E. Alibakhshi et al., Fabrication and characterization of layered double hydroxide/silane nanocomposite coatings for protection of mild steel, Journal of the Taiwan Institute of Chemical Engineers (2017), http://dx.doi.org/10.1016/j.jtice.2017.08.015

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E. Alibakhshi et al. / Journal of the Taiwan Institute of Chemical Engineers 000 (2017) 1–11

9

Table 4 Electrochemical parameters derived from polarization curves of the coated and uncoated samples immersed in the saline solution after 4 h. Solution

Ecorr versus Ag/AgCl (mV)

icorr (μA/cm2 )

ba (V/dec)

−bc (V/dec)

Blank SC SCN SCM SCP

−736.9 ± 48.2 −705.9 ± 44 −693.7 ± 41 −674.7 ± 34.5 −610.3 ± 32

18.7 ± 0.3 9.7 ± 0.2 7.4 ± 0.2 5.5 ± 0.1 1.5 ± 0.1

0.7 ± 0.01 0.1 ± 0.01 0.1 ± 0.01 0.1 ± 0.01 0.1 ± 0.01

0. 1 ± 0.02 0.1 ± 0.01 0.1 ± 0.01 0.2 ± 0.01 0.1 ± 0.01

Fig. 10. UWT spectra of the ECN signals obtained for blank (a), SC (b), SCN (c), SCM (d) and SCP (e) samples after 4 h immersion in 3.5 wt% NaCl solution.

The corrosion protection evolution of the samples after 4 h immersion in a 3.5 wt% NaCl solution is illustrated in Fig. 10. The electrochemical current noise (ECN) signals with a period of 800 s were selected as the base for the evaluation of the samples. The original ECN signals are presented at the backside of the wavelet spectra with their relative amplitudes. From Fig. 10, large low frequency contribution related to general corrosion mechanism can be seen in the ECN signals along the entire time axis for the blank sample, while low frequency components are decreased for SC indicating tendency toward localized corrosion at the coating defect sites. Low frequency components are considerably decreased in the SCM and SCP samples compared to the rest of samples. For these samples, Fig. 10d and e shows presence of high frequency components along the entire time axis.

In fact, these high frequency components show tendency toward localized corrosion at the coatings defect sites [59]. In UWT framework, the electrochemical signals were decomposed in predefined numbers of crystals or scales (d1, d2,…). The relative energy of detail crystals, which estimates the contribution of every crystal to the overall signal, is calculated by Eq. (1) [60,61].

E dj =

n k=1

d2j,k

 n

s2 k=1 k

(1)

where d reveals detail coefficient and s is the n-sample electrochemical signal without DC trend. The relative energy distribution diagram for the ECN signals obtained from the samples immersed in the 3.5 wt% NaCl is illustrated in Fig. 11.

Please cite this article as: E. Alibakhshi et al., Fabrication and characterization of layered double hydroxide/silane nanocomposite coatings for protection of mild steel, Journal of the Taiwan Institute of Chemical Engineers (2017), http://dx.doi.org/10.1016/j.jtice.2017.08.015

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3. Results obtained from SEM revealed that a uniform, smooth, dense and crack free for SCP coating. 4. The results showed that LDHs nanoparticles could significantly improve barrier performance and water repellency of silane sol–gel layer. There was a close relation between hydrophobicity of the coatings and corrosion protection of the samples. 5. The lowest corrosion current density and highest charge transfer resistance was seen for SCP sample compared to other samples, confirming better its barrier performance. 6. Significant corrosion protection performance was found by EIS and EN for the silane sol–gel layer containing Zn–Al–PO4 3− LDH, which could be connected to synergism effect of phosphate and zinc ions released from LDH as confirmed by ICP.

1

Relative energy contibution

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0.9

Blank

0.8

SC

0.7

SCN SCM

0.6

SCP

0.5 0.4 0.3 0.2 0.1

Acknowledgments

0 d1

d2

d3

d4

d5

d6

d7

d8

Detail crystal Fig. 11. Energy distribution plot of the detail crystals obtained from ECN signals for the samples.

From Fig. 11, the relative energy contributions of detail crystals d8 are higher than other detail crystals for SCM, SCN, SC and blank samples after 4 h immersion, indicating that the ECN signals are provided by the largest transients attributed to general corrosion. For SCP, the relative energy contribution of detail crystal d1 is higher than the rest of samples. Also, at lower extent compared to SCP, SCM demonstrated relatively high energy at d1 crystal. It is noteworthy that the large d1 coefficient is an indication of localized corrosion mechanism [62]. The tendency to non-uniform corrosion for SCM and SCP samples can be attributed to the formation of inhibitive film (Zn, Mo and P) on the coating defect sites restricting the numbers of active sites. The total absolute energy of detail crystals (ET) of ECN was estimated according to Eq. (2).

ET =

8  n 

d2j,k

(2)

j=1 k=1

The ET values were 169.7, 79.4, 34.9, 22.6 and 15.2 pA2 for the blank, SC, SCN, SCM and SCP, respectively. SCN, SCM and SCP have lower content compared to SC and blank samples. This means that the addition of LDHs to the sol–gel film increased its barrier properties due to their planar structure. The lowest ET obtained for the SCP sample confirms the higher corrosion protection ability observed in EIS study. These results demonstrate that the hybrid sol–gel coatings filled with Zn–Al–PO4 3− LDH nanoparticles confer higher corrosion protection than the Zn–Al–MoO4 2− LDH ones. The Zn–Al–PO4 3− LDH has high capability of releasing inhibitors species resulting in a protective film formation on the coating defect sites. 4. Conclusion In this work, Zn–Al–NO3 − , Zn–Al–MoO4 2− and Zn–Al–PO4 3− LDHs nanoparticles were synthesized and characterized. Protective performance of an eco-friendly silane sol–gel layer containing LDHs applied on mild steel was evaluated by EIS, polarization and EN measurements. The results could be summarized as follow: 1. XRD results showed that the molybedate and phosphate anion intercalated into the Zn–Al–NO3 − LDH gallery and the interlayer spacing of the LDH increased after anion exchange. 2. The effect of LDHs insertion on the network structure of silane coating was confirmed by FT-IR. Higher extent of Si–O–Si condensation reaction in the presence of Zn–Al–PO4 3− was obtained.

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Please cite this article as: E. Alibakhshi et al., Fabrication and characterization of layered double hydroxide/silane nanocomposite coatings for protection of mild steel, Journal of the Taiwan Institute of Chemical Engineers (2017), http://dx.doi.org/10.1016/j.jtice.2017.08.015