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Protective behaviors of 2-mercaptobenzothiazole intercalated Zn-Al-layered double hydroxide coating.

Abstract A nanocontainer with 2-mercaptobenzothiazole (MBT)-intercalated Zn-Al-layered double hydroxides (LDHs) was synthesized through co-precipitation. The structures of the LDH and the nanocontainer were characterized by powder X-ray diffraction, Fourier transform infrared spectroscopy, scanning electron microscopy, and energy-dispersive X-ray analysis. The nanocontainer was added into 3.5 wt% NaCl solutions with different pH values to study its effect on the protection of Q345 steel. The corrosion rate of Q345 steel in 3.5 wt% NaCl solution with a pH value of 3 was lower than those in neutral and alkali solutions. Moreover, the addition of nanocontainers into the epoxy resin coating improved the performance of the coating and facilitated self-healing by releasing MBT inhibitors into the scratched area.

Keywords Nanocontainer, LDH, MBT. Ion-exchange, Self-healing

Introduction

Organic coatings are commonly used to protect metals from corrosion. However, mechanical or chemical damage of organic coatings is inevitable. In the case of any mechanical damage, it is desirable for the damaged surface of the coating to be automatically repaired, which can be achieved by the chemical

Electronic supplementary material The online version of this article (doi:10.1007/s11998-014-9568-9) contains supplementary material, which is available to authorized users. components within the coating, such as the high reactivity of hexavalent of chromium ion in chromate conversion coatings. This process, known as self-healing, can not only provide long-term protection but also extend the maintenance period for the metals. Adding inhibitors is another effective corrosion protection method. So the combination of self-healing with inhibitor should enhance the surface protection. However, direct addition of inhibitors to the coating formulations will result in either reaction of the inhibitors and the coating, (1) or fast exhaustion of the self-healing potential (2) because of uncontrollable release rate. Therefore, the development of nanometer-scale containers which can hold inhibitors is essential. The nanocontainers should be homogeneously distributed in the film matrix to maintain barrier properties and provide uniform active protection. Currently, oxide nanoparticles, (3,4) [beta]-cyclodextrin, (5,6) and layer-by-layer assembled deposition (7-9) have been explored as prospective reservoirs for corrosion inhibitors. Layered double hydroxides (LDHs), also known as anionic clay or hydrotalcite-like compounds, can also be used to immobilize anionic inhibitors.

The general equation for LDH is listed as: [[[M.sup.2+.sub.1-x] [M.sup.3+.sub.x][(OH).sub.2]].sup.x+] [A.sup.n-.sub.x/n] x m[H.sub.2]O, where [M.sup.2+] and [M.sup.3+] are the divalent and trivalent metals, respectively. The LDH structure comprised brucite-like layers consisting of edge-sharing Mg[(OH).sub.6] octahedral. The isomorphic substitution of [Mg.sup.2+] by [M3.sup.+] generates a positive charge in the hydroxyl sheet, and this net positive charge is compensated for by anions (usually C[O.sub.3.sup.2-]) and water. The hydrotalcite-like compounds have versatile applications in the corrosion protection field. In some studies, LDHs have been produced in situ, i.e., on top of metallic substrates (Mg and A1 alloys) as protective films. (10-13) LDHs are also used as containers for corrosion inhibitors and incorporated into organic coatings depending on their anion-exchanged characteristics. LDHs have been proven to be intercalated by inorganic inhibitors such as vanadate, (14-16) tungstate anions, (17) and organic inhibitors (e.g., 2-benzothiazolylthio-succinic acid), (18) 2-mercaptobenzothiazolate (MBT), quinaldate, (19-21) benzotriazole, ethyl xanthate and oxalate, (22) and phytic acid (23) as different nanocontainers. These nanocontainers are included in organic coatings as pigments. If the coating is damaged, the anion-exchange pigment will absorb the harmful chlorides and release the inhibiting ions.

MBTs can serve as effective corrosion inhibitors for silver, copper, zinc, and gold. (24-26) They can also be intercalated by LDHs to improve the self-healing property of organic coatings on iron as well as magnesium and aluminum alloys. Wang et al. (27) investigated the absorption of MBT on an iron surface by confocal micro-Raman spectroscopy. MBT was found to be chemically adsorbed on the iron via the exocyclic S and N atoms in acidic and neutral solutions, but was observed to be bound electrostatically in basic media.

The present study was performed based on absorption of MBT by iron. The synthesis and characterization of Zn-Al-LDHs intercalated with MBT organic anions (named nanocontainers) is described. The protective performance of LDH intercalated with MBT inhibitor on the Q345 low-alloy steel under different pH conditions is studied. Furthermore, the nanocontainer is added to the epoxy resin to investigate its protective effect and self-healing ability.

Materials and methods

Materials

The working electrodes used for the electrochemical measurements were cold-rolled Q345 steel (0.120.20% C; 0.20-0.60% Si; 1.20-1.60% Mn; 0.030% S; [less than or equal to] 0.030% P, in mass%). The working surface had an area of 1.13 [cm.sup.2], ground with emery paper of up to 1200 grit, and washed with deionized water before painting.

Epoxy resin based on bisphenol A (with 0.48 eq/ 100 g to 0.54 eq/100 g epoxy value, [less than or equal to] 2500 mPa.s viscosity at 40[degrees]C, and 99% solid content) was chosen to formulate solvent-free paint (E-51 from Fenghuang Epoxy Resin Factory, Wuxi, China). Cardolite NX-20031) curing agent was purchased from Cardolite Chemical Co. (Zhuhai, China). MBT and relevant reagents were purchased from Beijing Chemical Reagent Company (China).

Synthesis and intercalation of Zn-Al-LDHs

The solution (200 mL) containing Zn[(N[O.sub.3]).sub.2] x 6[H.sub.2]O and Al[(N[O.sub.3]).sub.3] x 9[H.sub.2]O ([[Zn.sup.2+]] + [[Al.sup.3+]] = 10.5 mol/L, [[Zn.sup.2+]]/ [[Al.sup.3+]] = 2.0) was slowly added to 350 mL of 0.101 M NaN[0.sub.3] solution (pH 10) under vigorous stirring at room temperature. During this reaction, the pH of the solution was kept constant (pH 10 [+ or -] 0.5) by 0.325 M NaOH solution. Then, the obtained slurry was hydrothermally treated at 65[degrees]C for 24 h for the crystallization of the LDHs. The product was centrifuged and washed several times with boiled deionized water. A 58 pm size mesh was used to obtain products with smaller diameters. A small fraction of the LDHs was dried at 50[degrees]C for analysis, and the remaining LDHs were used in the anion-exchange reaction for the replacement of nitrate by MBT.

The anion-exchange reaction was conducted by dispersing LDH precursors in an aqueous solution. The sodium salt of MBT was previously prepared by neutralizing the aqueous solution of MBT with equivalent amount of NaOH. Subsequently, white gel-like LDHs precursors were dispersed in a solution of 0.1 M NaMBT (pH 10) under nitrogen atmosphere. Finally, the organic anion-loaded LDH powders were washed several times with deionized water, centrifuged, and finally dried at 60[degrees]C for 24 h to obtain MBT-LDHs.

Preparation of coatings

The MBT, LDH, and MBT-LDH nanoparticles were incorporated into the epoxy resin at a concentration of 12 wt% and mixed using a three-roll grinder. The curing agent was then added to the epoxy resin with a mass ratio of 3:10. Four kinds of coatings were prepared. The first coating was the epoxy varnish prepared by mixing epoxy resin and curing agent and the resulting coating was named EP. The second was a nanocomposite coating with MBT-LDH and denoted as EP-MBT/LDH. The third was the epoxy coating containing LDH nanoparticles and denoted as EP-LDH. The fourth was epoxy coating with MBT inhibitor and denoted as EP-MBT. The coatings were painted on the substrate with a brush at room temperature. The coatings were cured at room temperature for 2 h and at 80[degrees]C for 2 h. The thickness of each dry film coating was measured by a QuaNix4500 magnetic instrument, and the average film thickness was approximately 200 [micro]m.

One of the EP and EP-MBT/LDH coatings was scratched with a scalpel throughout the coating thickness with length of 3 mm.

Characterization

The crystal structure of LDH was analyzed by a Bruker D8 Focus diffractometer with a graphite-filtered Cu K[alpha] target. The scanning range (2[theta]) was from 5[degrees] to 65[degrees] with a step of 0.02[degrees], and the scanning rate was 4[degrees]/min.

FTIR spectra were recorded from 4000 to 400 [cm.sup.-1] with 8 [cm.sup.-1] resolution on a Nicolet iSlO Fourier transform spectrometer (Thermo Fisher Scientific, USA) using the standard KBr disk method at room temperature.

The morphology and composition of LDH and corrosion products were observed with an FEI Quanta 200F scanning electron microscope equipped with an energy dispersive spectroscope. The specimen was prepared by gold sputtering.

The macro-appearance of the scratched coatings was observed by OLMPUS CK40M optical microscope.

Evaluation of anticorrosion properties

To investigate the protective performance of MBT for steel, 20 g/L of the MBT-LDH powder was added to 3.5 wt% NaCl solutions with pH values of 3, 7, and 11 by adding 2.0 mol/L HC1 or 2.0 mol/L NaOH. The polarization curves were fitted with the Elchem Analyst software from Gamry in the Tafel mode by intersecting the Tafel line. Each experiment was performed at least twice to insure the accuracy and repeatability of the data.

Electrochemical measurements were conducted on bare alloy and coated samples in a conventional three-electrode cell, with a carbon rod as the counter electrode and a saturated Hg-Hg[Cl.sub.2] electrode as the reference electrode. EIS measurements were performed within the frequency range of 0.01 to 100 kHz at 20 mV amplitude sinusoidal voltage with the Gamry Reference 600 electrochemical workstation at open circuit potential. The test solution at 70[degrees]C was prepared with analytical grade 3.5 wt% NaCl and deionized water. The polarization curve measurements started from -500 mV vs SCE open circuit potential at a constant voltage scan rate of 0.5 mV/s.

Results and discussion

XRD

The structures of LDHs before and after anion exchange were characterized by XRD (Fig. 1). The XRD patterns of LDHs indicated a single-phase structure, with well-defined peaks at low 2[theta] angles corresponding to the reflection by planes (003), (006), and (009). The basal spacing of LDH was estimated to be 0.89 nm from the d (003) position based on Bragg's law (2[theta] = 9.90[degrees]). The values obtained agreed with the results reported in literature. (19,20) After anion-exchange, a displacement in the position of peaks toward low 2[theta] angles occurred (2[theta] = 6.68[degrees]), with an increase in the basal spacing of 1.34 nm. The increase in basal spacing for MBT-LDH indicated that the MBT anions were intercalated into the interlayer of LDH and exchanged with some N[O.sub.3.sup.-] anions. However, the presence of residual reflections assigned to the nitrate-containing LDHs suggested the incomplete replacement of intercalating anions.

FTIR and SEM/EDX

The FTIR spectra of LDHs before and after modification and those of the organic inhibitor powders are shown in Fig. 2. The LDH samples showed broad bands in the range of 3200 to 3700 [cm.sup.-1], which were caused by the physically adsorbed water molecules and hydroxyl groups in the metal hydroxide layer (Fig. 2a (line a, b)) without presence in pure MBT (Fig. 2a (line c)). In the case of nitrate-loaded LDHs, the peak at 1630 [cm.sup.-1] could be ascribed to the bending mode of interlayer water molecules. Additionally, the intensive peak at 1384 and 824 [cm.sup.-1] corresponded to the symmetric and asymmetric stretching modes of nitrate. The bands at 605, 547 [cm.sup.-1] should be related to the vibrations of the oxygen atoms in the layer crystal lattice. Following the exchange of N[O.sub.3.sup.-] by MBT anions (Fig. 2b), the peak at 1384 [cm.sup.-1] significantly decreased and a new peak could be observed at 1368 [cm.sup.-1] which may be assigned to carbonate. The reason for the replacement of nitrate by carbonate in LDHs interlayer may be a result of insufficient removal of the C[O.sub.2] in air. (28) In addition, several new peaks appeared. Comparative results between the FTIR spectra of MBT-LDHs and pure MBT showed that these new peaks could be assigned to different vibration modes of intercalated MBT anions. (29)

Results of SEM analysis for the LDH powders before and after modification are shown in Fig. 3. The LDHs powders were aggregated by small particles with a plate-like morphology (Fig. 3a). The morphology of MBT-LDFT did not considerably change after the anion exchange, but the aggregation of the particles improved to an extent (Fig. 3c).

The EDX technique was used in the present study to assess the atomic composition of the LDH powders. EDX results revealed that Zn-Al-N[O.sub.3] contained the elements Zn, Al, and O (Fig. 3b). For Zn-Al-MBT, peaks of C, N, and S (Fig. 3d) were observed. This finding confirms that the MBT anion was actually intercalated into the interlayer.

The intercalation of MBT anions into the interlayers of the LDF1 can be verified further by studying release mechanisms of MBT anions. The results are given in Figs, s1 and s2.

Inhibitory activity of LDHs-MBT nanocontainers at different solutions

The polarization curve was used to evaluate the properties of MBT-LDHs on protecting Q345 steel from corrosion in 3.5 wt% NaCl solutions at 70[degrees]C, and the results are shown in Fig. 4. The corrosion parameters calculated from Fig. 4 are shown in Table 1. The curves have obvious characteristics of anodic polarization, and the metal is passivized in the solutions. When the electrode was immersed in the solution for 8 h, the corrosion potentials of the electrode at different solutions were higher than the corrosion potential at initial immersion time (initial time is when the electrode was first immersed into the solution and achieved stabilization). Meanwhile, the corrosion current densities were lower than those at initial time, possibly because, during the initial immersion time, the inhibitor intercalated in the LDH particles had not been released to the solution, and the electrode had been corroded. With prolonged immersion time, the MBT inhibitor exchanged with chloride ions in the solutions at different velocities, thereby protecting the electrode from being corroded. MBT-LDHs function as anodic inhibitors. The electrode immersed in the pH 3 solution had the highest corrosion potential and the lowest corrosion current density.

The corrosion morphologies of the Q345 steel immersed in 3.5 wt% NaCl solutions with different pH values are shown in Fig. 5. A thin, porous film was observed on the electrode surface immersed in the pH 3 solutions (Fig. 5a). After the corrosion products were removed (2% HC1 solution with 2 mL/L organic inhibitor), almost no corrosion was observed on the metal surface (Figs. 5al and 5a2). However, as for the electrode immersed in neutral solution, some microcracks were observed on the surface (Fig. 5b), and the metal was severely corroded (Figs. 5bl and 5b2). The electrode immersed in the basic solution was also corroded, and some corrosion pits were evident (Figs. 5c-5c2). The results indicated that MBT-LDH provides considerably better protection for the Q345 steel in acidic solutions than in neutral and basic solutions, which are consistent with the polarization curves. This phenomenon may be due to the fact that MBT presents as a unionized protonated form (HMBT), which could adopt either the thione or the thiol structure in acidic and neutral media. HMBT molecules are chemically adsorbed on the iron surface via the exocyclic sulfur and nitrogen atoms. By contrast, in a basic solution, the MBT intercalated in the interlayer is released to the solution and exists in the form of the thiol ion ([MBT.sup.-]). Attachment to the surface is electrostatically bound between surface iron atoms and the exocyclic sulfur atom of MBT. The potential of the Q345 steel in the acid solution is more positive than that in a neutral solution, leading to higher coalescence of Fe-S as well as better protection performance. (26,27)

Electrochemical impedance spectroscopy (EIS) measurements on coated samples

Impedance diagrams were obtained to characterize the corrosion resistance of the Q345 steel covered by the EP, EP-LDH, EP-MBT, and EP-MBT/LDH coatings. The diagrams obtained after different exposure times to the 3.5 wt% NaCl solution with a pH value of 3 (2.0 M HCl buffer solution) at 70[degrees]C are shown in Fig. 6.

As for the EP coating (Fig. 6a), the impedance modulus decreased quickly to lower than [10.sup.6] [cm.sup.2] after 7 days immersion, indicating rapid loss of the barrier property of the coating because of the introduction of aggressive ions. As for the EP-LDH coating (Figs. 6b and 6b1), the impedance modulus at low frequency decreased gradually. With 8 days immersion, the impedance modulus abruptly decreased by nearly two magnitudes, indicating that water diffused into the coating, and corrosion was initiated on the metal substrate. A horizontal line section appeared at the middle frequency after 20 days immersion, which indicated that the coating began to be delaminated. At the final immersion time, the EP-LDH coating already lost its protection performance. The impedance modulus of the EP-LDH was approximately three magnitudes higher than that of the EP coating, suggesting that the EP-LDH has better protective property than the EP coating. As for the EP-MBT coating (Figs. 6c and 6cl), the impedance modulus was higher than [10.sup.10] [ohm] [cm.sup.2] at the initial stage and then decreased gradually with increasing immersion time. Meanwhile, the maxima phase degree shifted to lower frequency with respect to time. When immersed for 27 days, the impedance modulus suddenly decreased by approximately two magnitudes and a horizontal line section appeared at middle frequency, indicating that the coating began to be delaminated. However, the Bode plots of the MBT-LDH coating were obviously different from those of the above mentioned coatings (Figs. 6d and 6dl). At the first 2 days of immersion time, the coating had a high impedance modulus higher than [10.sup.10] [ohm] [cm.sup.2] and then decreased to approximately [10.sup.9] [ohm] [cm.sup.2] because the water penetrated quickly into the coatings through the microspores, which were formed by solvent volatilization during the curing process. With prolonged immersion time, the impedance modulus at low frequency was almost constant up to 20 days. This condition may have been caused by the exchange reaction that would occur when the inhibitor anion might be released, and chloride would be absorbed into the LDH gallery. As the immersion time was prolonged to 27 days, the barrier properties of the coating decreased further, which accounted for the increase in corrosion rate. Afterward, the impedance modulus fluctuated because of the following two factors. Firstly, the pores within the coating layer were blocked with corrosion products. Secondly, the ionic movement in the coating layer was impeded.

Bierwagen et al. (30-32) proposed that the impedance modulus at low frequencies (such as [|Z|.sub.1Hz] or [|Z|.sub.10mHz]) measured with exposure time could be used to estimate the protection performance of a painted metal. Figure 7 plots [|Z|.sub.10mHz] with immersion time in 3.5 wt% NaCl solution with a pFI value of 3 for the Q345 steel painted for different coatings. As for the EP and EP-LDH coatings, the impedance modulus at low frequency decreased rapidly throughout the whole immersion time. Compared with the EP coating, the impedance modulus of the EP-LDH coating was three magnitudes higher than that of EP coating. As for the EP-MBT and EP-MBT/LDH coating, the impedance modulus at low frequency decreased slowly for 20 days of immersion time. The modulus of EP-MBT coating was higher than that of EP-MBT/LDH coating, which indicated that the inhibitor in the EP-MBT/LDH coating was released in a controllable manner, and at that time, the MBT-LDH nanoparticles had not fully exerted their function. With prolonged immersion time, the impedance modulus at low frequency for the EP-MBT decreased, whereas that of the EP-MBT/ LDH coating remained relatively stable. Usually, the LDH particles doped in organic coating used for protection serve a twofold function. The organic coating first serves as a physical barrier between the metal substrate and the corrosive environment, because the coating contains small pores that are still larger than the diameters of water and oxygen molecules. Therefore, the LDH particles act as a padding that partially blocks water, oxygen, and other aggressive species, thereby accelerating corrosion. The second and more important function of LDH is to serve as a container for corrosion-inhibiting pigments. The released MBT anions could provide protection to substrates after the corrosion process begins at inherent defects or inhomogeneities. These results show that the addition of MBT-LDH nanoparticles to the epoxy resin coating improves the performance of the coating.

Self-healing ability of the EP-MBT/LDH coating

EP and EP-MBT/LDH coatings with scratched lengths of 3 mm were immersed in 3.5 wt% NaCl solutions. The macro-appearance of the coatings at different periods was observed through an optical microscope (Fig. 8). The results indicated that, for the EP coating, the naked metal near the scratched area was gradually corroded, and corrosion products with different colors appeared. However, this phenomenon was not observed in the EP-MBT/LDH coating. The SEM of the coatings immersed for 13 h are shown in Fig. 9. The EP coating did not exhibit self-healing properties. The scratched portion shows the resultant accumulations of corrosion products in the scratched area (Fig. 9a). Line scanning results (Fig. 9b) demonstrated that amounts of Fe, O, and Cl elements around the scratched area were higher than those far from the scratched area. The EP-MBT/LDH coating showed good self-healing ability (Fig. 9c). N and S elements were detected by line scanning, which was attributed to the release of MBT inhibitor in the nanocontainer to the scratched area (Fig. 9d).

The electrochemical performance of the scratched coatings is given in Figs. s3 and s4. The impedance moduli of the EP coating decreased gradually. However, the impedance modulus of EP-MBT/LDH coating increased abruptly after immersion for 2 h and then kept almost constant with immersion time. It indicated that the MBT anions released from the nanocontainer to the scratched area and an inhibitor film may form on it. This phenomenon further confirmed the self-healing ability of the EP-MBT/LDH coating. Moreover, the impedance modulus of the EP-MBT/LDH is always two magnitudes higher than that of the EP coating.

The self-healing mechanism of the EP-MBT/LDH coating is shown in Fig. 10. The MBT-LDH nanocontainer particles in the coating serve as pigments that improve the performance of the coatings. When the coating is damaged, the naked metal will be corroded locally. If the MBT anions intercalated in the LDH galleries are released into the damaged area through an ion-exchange process, with [Cl.sup.-] anions and O[H.sup.-] produced by cathode reduction, the MBT will be chemically adsorbed on the surface of the Q345 steel via the exocyclic S and N atoms, either in the form of thione or in the form of thiol structure. (27) In addition, the [Cl.sup.-] anions in the vicinity of the scratched area will be absorbed by the LDH nanoparticles because of its anionic selectivity. Therefore, the scratched area in the coating will be repaired gradually by the inhibitor film as well as some of the corrosion products.

Conclusions

Anionic clay LDHs have the characteristics of an anion-exchange and physical barrier and can be used as nanocontainers to load inhibitors for improving the performance of protective coatings. LDH was intercalated by organic inhibitor MBT and its protective property on steel in different solutions was investigated in this paper. The polarization curves exhibited obvious characteristics of anodic polarization, and the metal was passivized in the solutions. The MBT-LDH particles added to the acid solution exhibited better protection performance than when the particles were added to neutral and alkali solutions, and the steel showed no obvious corrosion phenomena, contrary to the steel immersed in neutral and basic solutions. Moreover, the addition of MBT-LDH nanoparticles improved the performance of the epoxy coating. The EP-MBT/LDH coatings can heal the scratched area by releasing the MBT inhibitor from the nanocontainers to the damaged portion.

DOI 10.1007/s11998-014-9568-9

Acknowledgments This work was supported by Science Foundation of China University of Petroleum Beijing (No. KYJJ2012-06-19).

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Y. Dong ([mail]), F. Wang, Q. Zhou

Department of Material Science and Engineering, College of Science, China University of Petroleum (Beijing), Beijing, China

e-mail: dyhydt@yahoo.com

Table 1: Tafel parameters for Q345 steel in 3.5 wt%
NaCl solution containing 20 g/L LDH-MBT at different
release time

                    [E.sub.SCE]       [i.sub.corr]        [b.sub.c]
                       (mV)       ([micro]A/[cm.sup.2])    (mV/dec)

Initial time        -1030         5.01                    -118.3
Immersed for 8 h
  pH = 3            -932          0.4                     -156.2
  pH = 7            -1010         3.16                    -115.4
  pH = 11           -942          1.18                    -142.8


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Author:Dong, Yuhua; Wang, Feng; Zhou, Qiong
Publication:Journal of Coatings Technology and Research
Geographic Code:1USA
Date:Sep 1, 2014
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