Formulation and study of corrosion prevention behavior of epoxy cerium nitrate--montmorillonite nanocomposite coated carbon steel.
Keywords Nanocomposite, Cerium nitrate, Salt spray, Impedance
Epoxies are widely used as the resin of different protective coatings since they show excellent adhesion, mechanical properties, and chemical resistance in different media. However, long exposures to wet and humid conditions attenuate their barrier properties. Reinforcement of epoxy coatings by means of inorganic pigments is a common way to prolong the duration of protection. Clay is a nontoxic, cheap, available, and environmentally friendly pigment which is used for this purpose. (1), (2)
In general, the barrier properties of the composites filled with micron-size filler particles are inferior to those filled with nanoparticles of the same filler. (3) Clay is one of the particles which has commonly been used for fabrication of nanocomposite coatings in recent years. Investigations show that incorporation of a small amount of clay (1-5%) into the formulation can cause great improvements in anticorrosion, barrier, thermal, and mechanical properties, and decreased permeability to liquids and gases. (4-7)
In the 1980s, the technology of clay nanocomposites was developed. While improvements in the barrier properties of the epoxy coatings by incorporation of nanoclay particles were undeniable, they have occurred to different extents in various studies. The improved barrier properties were attributed to the well-dispersed nanoparticles in the epoxy resin, providing a more tortuous path for corroding agents by incorporation of nanoclays. The optimum nanoclay was reported to range between 2% and 5% in different studies. (4-11)
Chromates have been used as corrosion inhibitors in different corrosive media. Surface pretreatments are based on chromates because of their high protection efficiency/cost ratio. These pretreatments will be prohibited because of the high toxicity of chromates. Therefore alternative pretreatments need to be used. Rare earth salts have been applied as the new corrosion inhibitors. Cerium is one of these alternatives which can reduce the corrosion rate of aluminum alloys by inhibiting, cathodic reactions. (12)
Montemor et al. (12) have shown that the corrosion rate of galvanized steel treated by immersion in cerium nitrate solution is reduced by the solution's effect on the anodic and cathodic reactions. In addition, application of cerium nitrate in silane film increases the healing ability and improves the corrosion resistance. (13)
The aim of this study was to obtain an appropriate dispersion process by which to formulate and apply an epoxy cerium nitrate-montmorillonitc (MMT) nanocomposite coating in the lab scale. Cerium nitrate was dissolved in ethanol solvent and MMT particles were dispersed into epoxy by mechanical agitation and sonication processes. The dispersion morphology and degree of agglomeration were analyzed by optical microscopy, sedimentation tests. X-ray diffraction (XRD), and transmission electron microscopy (TEM). To investigate the anticorrosive properties of nanocomposite coatings, electrochemical impedance spectroscopy (EIS) and salt spray tests were employed.
Epoxy resin, diglycidyl ether of bisphenol A (DGEBA), and epoxy hardener F-206, a low viscosity modified cycloaliphatic amine curing agent, were provided by Bajak Paint Co. (Tehran, Iran). The organically modified MMT clay (Cloisite 20A: dimethyl, dehydrogenated tallow, quaternary ammonium-modified MMT) with particle size of 2-13 [micro]m, layer thickness of 1 nm, supplied from Southern Clay Company (Gonzales, TX, USA) was dried at 80[degrees]C for 24 h. Cerium(III) nitrate was provided from Scharau. Spain. Ethanol was provided from Merck 100983 (Germany) as the solvent.
Preparation of nanocomposites
Cerium nitrate was dissolved in ethanol at 27[degrees]C, 1-3 g in 10 mL, and mixed for 15 min. In addition, micro-sized MMT was added to the epoxy. The number of particles which were added was based on a weight percent of the solid parts of the paints. The paint designations, based on a weight percent of particles added to the formulation, are shown in Table 1 (the hardener to resin balance ratio was 1:2). MMT containing epoxy and cerium nitrate-modified ethanol were mechanically mixed at 1200 rpm, and the resultant mixture was subjected to sonication for 40 min. The high-power sonication instrument used was the UIP1000 hd (Hielscher, Germany). During the sonication process, the power was gradually raised while maintaining the temperature of the mixture at 50-60[degrees]C by placing the reaction vessel in cool water. Subsequently, a hardener was added with the mass ratio of the hardener to the epoxy resin set at 50/100, and the materials were then subjected to high-shear mixing for 5 min.
Table 1: Designation of formulations based on particle composition MMT Cerium nitrate 0CN 1CN 2CN 3CN 0C 0C0CN -- -- 0C3CN 2C -- 2C1CN 2C2CN 2C3CN 3C -- 3C1CN 3C2CN 3C3CN 4C 4C0CN 4C1CN 4C2CN 4C3CN
SAE 1010 carbon steel panels with dimensions of 15 cm x 8 cm x 0.2 cm were blasted to Sa 2 1/2 (SIS 055900-1967) with a profile of 15-25 [micro]m and then degreased with toluene and acetone. The coatings were applied by means of a film applicator. Thickness measurements showed that dry film thicknesses were 60 [+ or -] 10 [micro]m for all panels. To ensure the film curing, the panels were kept in the laboratory atmosphere for a week before beginning the tests.
The optical dispersions homogeneity of epoxy resin and nanoclay was examined using an Olympus BUZZ-UMA optical microscope. Optical micrographs were obtained using samples prepared after 2 h of mechanical mixing and 40 min of sonication. The suspension stability was analyzed by a sedimentation method. The blends were placed at 128[degrees]C for 2 h to observe the amounts of clay precipitates. (14)
To evaluate the intercalation/exfoliation of nanoclay in the polymer matrix, XRD patterns were obtained from the surface of the cured nanocomposites. The XRD experiments were performed from 0.5[degrees] to 10[degrees] at a scanning rate of 0.5[degrees]/min with an X'PERT Philips diffractometer. The Cu K[alpha] radiation ([lambda]= 1.54 [Angstrom]) was generated at 40 kV and 40 mA and was used as an X-ray source. The interlayer spacing of the nanocomposites was derived from the peak position (do)r reflection) in the XRD diffractograms according to Bragg's equation (n[lambda] = 2d sin[theta]).
The TEM specimens were cut from nanocomposite blocks using an ultra-microtome, OM U3 (Reichert, Austria), equipped with a diamond knife. Thin specimens (70-100 nm) were cut from the cured film of the nanocomposite material of about 1 x 1 [mm.sub.2] The samples were placed on the 300 mesh copper grid. Transmission electron micrographs were taken with a Philips-EM208 at an acceleration voltage of 100 kV.
The EIS measurements were performed at room temperature in a Faraday cage using a frequency response analyzer and an electrochemical interface connected to a computer. A three-electrode electrochemical cell was used, consisting of the working electrode (3.15 [cm.sup.2] of exposed area), saturated calomel electrode (SCE) as a reference, and platinum as the counter electrode. The measuring frequency ranged from 100 kHz to 10 MHz with AC amplitude of 10 mV. The impedance diagrams were obtained at different exposure times up to 100 days.
A salt spray test was performed according to ASTM B-117 for 1000 h, and at the end of the test the panels were evaluated according to ASTM D714, ASTM D1654, and ASTM D610.
Results and discussions
Optical microscopy results
The compositions containing different amounts of clay and cerium nitrate were manufactured by two different steps: (I) a high-shear mixing method and (II) high-shear mixing plus sonication. Figure 1 presents the optical micrographs of 4C2CN suspensions after mechanical agitation and the sonication process. The dispersion which was mechanically agitated for 2 h contained many MMT agglomerates (shown in Fig. 1a). While the sonication process was applied to the suspension for 30 min, the size and quantity of the agglomerates were greatly decreased and distribution was improved (shown in Fig. 1b). The level of distribution of clay platelets in the epoxy matrix can be determined primarily using optical microscopy. Typically, the determination of an exfoliated nanocomposite has been based on X RD and TEM results.
[FIGURE 1 OMITTED]
Sedimentation was visually observed for the mixtures that were held at 135[degrees]C for nearly 25 min. (14) Increasing the temperature caused reduction of the viscosity of the final mixture and acceleration of sedimentation and agglomeration. (14) However, with insufficient dispersion and intercalation of the clay particles, sediments could be seen in the matrix. Table 2 shows the sedimentation results of the mechanical mixing of particles within the matrix. An insufficient dispersion process caused sedimentation, but using ultrasonic mixing improved the distribution of particles and prevented agglomeration of the clay particles.
Table 2: Results of sedimentation test of different samples Composition Sedimentation after Sedimentation after code high shear mixing ultrasonication Neat (OCOCN) - - 0C3CN - - 4COCN + - 2C1CN - - 2C2CN - - 2C3CN - - 3C1CN + - 3C2CN + - 3C3CN + - 4C1CN + - 4C2CN + - 4C3CN + -
XRD and TEM analysis
XRD studies were carried out to assess the organic--inorganic interactions built up in the systems and to examine the degree of intercalation/exfoliation. The neat epoxy, MMT clay, cerium nitric, and epoxy nanocomposites filled with 2, 3, and 4 wt% of MMT and 1, 2, and 3 wt% of cerium nitrate were examined, and the XRD patterns are shown in Fig. 2. The d-spacing can be obtained from Braggs equation, as well as the angle of maximum intensity. The neat MMT has a d-spacing of 2.42 nm and cerium nitrate has a d-spacing of 4.204 nm.
According to the XRD patterns, neat cerium nitrate (CN) exhibits the reflection peak at 2.1[degrees]. On the other hand, the peak intensity of the sample 0C3CN is less remarkable. It is concluded that cerium nitrate is dissolved in the matrix.
The increments of d-spacing for 4COCN and 4C2CN clays caused by high-shear mixing and high-intensity ultrasound were 49.02 and 46.44 nm, respectively. However, it can be seen from Fig. 2 that the intensity of the peak is less remarkable because of the small amount of clay. In addition, the presence of these tactoids or aggregates in the nanocomposite can be assumed to be insignificant. (5)
[FIGURE 2 OMITTED]
As shown in Fig. 3, some individual crystallites of the silicate are visible as regions of narrow alternating dark and light bands within the particles. The separation of clay layers indicates intercalation of the layers. (16) This figure also shows some clay separations with more than 9nm indicating partial exfoliated dispersion. Platelet spacing indicated by TEM images shows that the trend was confirmed by XRD results.
[FIGURE 3 OMITTED]
Thus, it can be concluded that the MMT platelets in 4COCN and 4C2CN interacted with resin and were intercalated but contained only a small amount of tactoids or clay ordered structures in the matrix.
Electrochemical impedance spectroscopy
EIS was performed to investigate the corrosion behavior of the coatings with respect to time when exposed to a corrosive media. Table 3 shows the various electrochemical parameters such as the [R.sub.c] for exposure times of 1 h and 25, 50, and 100 days. Bode and Nyquist diagrams are presented for all samples in Fig. 4.
[FIGURE 4 OMITTED]
As shown in Table 3, the pristine epoxy capacitance increased from 2.26 to 33.7 [micro]F. This can be attributed to water absorption, as the water has a higher dielectric constant with respect to polymeric coating. (7), (17) On the other hand, the addition of MMT and CN into the epoxy coating causes an increase in the pore resistance during the long period of immersion in 3.5 wt% NaCl solution. Higher values of [R.sub.c] mean higher corrosion protection and lower ion permeability through the coating. This means that the protection of the steel was increased by the use of the nanocomposite coatings. (4)
Nanoparticles of MMT with a plate-like structure can make a barrier layer in the polymeric matrix, which can prevent the penetration of corrosive ions through the coating and, consequently, prevent corrosion by a barrier mechanism. Also, their nanosize increases the tortuosity of the penetration path of corrosive ions, which will cause a delay in the corrosion process. (4), (18) On the other hand, cerium salts occur in two oxidation states, the 3+ and the 4+ state. The 3+ is the insoluble state, whereas the 4+ is the moderately soluble state. (13), (19), (20) The cerium presents as [Ce.sup.3+] in the film, when the cathodic reaction starts to occur; the [Ce.sup.3+] ions are oxidized to [Ce.sup.4+]. The more soluble [Ce.sup.4+] ions would then be released from the coating which, when encountering the reducing environment in the opposite bare metal, will reduce to the [Ce.sup.3+] state. The formation of the hydroxide/oxide layer ensues, which protects the bare metal from further corrosion. (12), (19-23) In other words, [Ce.sup.4+] was present in the form of Ce[(OH).sub.2.sup.2+] in the solution, resulting from the oxidization of Ce[(OH).sub.3] in alkaline solution; then the Ce[(OH).sub.3] was converted into Ce[0.sub.2] in a dismutation solid state reaction. (13), (21)
The main reactions in this process were investigated as below (13), (21), (23).
[H.sub.2]0 + 1/2 [0.sub.2] + 2[e.sup.-] [right arrow] 20[H.sup.-] (1)
[Ce.sup.3+] + 3(O[H.sup.-])) [right arrow] Ce(O[H.sub.3]) (2)
2Ce[(OH).sub.3] + 2(O[H.sup.-]) [right arrow] 2Ce[0.sub.2] + 4[H.sub.2]0 + 2[e.sup.-] (3)
As noted in the above equations, such species block the cathodic sites, hindering the reduction reaction and consequently the corrosion rate. The precipitation of cerium hydroxides is the key to cerium nitrate being an effective corrosion inhibitor. New cathodes are then formed, in order to counterbalance any anodic activity (dissolution), and the process is repeated until all the areas surrounding the anodic site become covered with cerium precipitates. (24) This indicates that these inhibitors are slowly leached out and thus further improve the corrosion protection by cerium nitrate, showing promise of self-healing of defects. (24) It is necessary to point out that this process can be done in the best way if the concentration of additive is at the correct value. Otherwise, it has a negative effect on the corrosion protection of the steel coating, and its addition in high concentrations reduces the homogeneity of the film, which also reduces the corrosion protection. (24)
The increase in the [R.sub.c] values of nanocomposites by increasing MMT content is due to the incorporation of clay into the polymer matrix, which decreases corrosive-ion permeability through the formation of tortuosities. This may be due to the greater aspect ratio (AR = area/thickness ratio) of MMT, which forces the corroding agents to travel a longer tortuous path to reach the substrate. (25) The ion permeability depends on length, orientation, and degree of delamination of layered silicates. (26) As the dispersion and delamination of clay layers were observed in the TEM and XRD patterns, better corrosion protection was predictable because of the blocking of the pores and defects of these coatings.
As can be seen in Table 3, [R.sub.c] values at first decreased by exposure time, but then increased after 25 days. This is because of film formation on the surface steps in the anodic and cathodic reactions. By increasing the amount of cerium nitrate more than 2 wt%, a dense film will be formed, which decreases the corrosion resistance.
Table 3: The electrochemical parameters measured from the EIS plots of the epoxy cerium nitrate-MMT nanocomposite according to exposure time and concentration of nanoparticles: (a) 1 h immersion, (b) 25-day immersion, (c) 50-day immersion, (d) 100-day immersion [R.sup.c] [Q.sup.c] (F) n ([OMEGA]) (a) Test samples 1H OCOCN 3.98 x [10.sup.5] 2.26 x [10.sup.-6] 0.77 0C3CN 8.07 x [10.sup.6] 1.19 x [10.sup.-9] 0.71 4C0CN 5.54 x [10.sup.8] 3.85 x [10.sup.-10] 0.77 2C1CN 2.03 x [10.sup.7] 5.22 x [10.sup.-10] 0.81 2C2CN 2.28 x [10.sup.8] 3.51 x [10.sup.-10] 0.78 2C3CN 1.83 x [10.sup.8] 3.00 x [10.sup.-10] 0.82 3C1CN 3.70 x [10.sup.8] 3.01 x [10.sup.-10] 0.79 3C2CN 1.15 x [10.sup.9] 2.78 x [10.sup.-10] 0.80 3C3CN 4.59 x [10.sup.7] 8.29 x [10.sup.-10] 0.69 4C1CN 1.13 x [10.sup.9] 2.52 x [10.sup.-10] 0.86 4C2CN 1.59 x [10.sup.9] 5.06 x [10.sup.-10] 0.75 4C3CN 3.35 x [10.sup.7] 1.69 x [10.sup.-10] 0.46 (b) Test samples 25D OCOCN 1.37 x [10.sup.5] 4.68 x [10.sup.-5] 0.45 0C3CN 1.60 x [10.sup.7] 1.05 x [10.sup.-9] 0.72 4C0CN 6.30 x [10.sup.7] 4.38 x [10.sup.-10] 0.89 2C1CN 8.47 x [10.sup.5] 3.07 x [10.sup.-10] 0.89 2C2CN 5.86 x [10.sup.6] 4.54 x [10.sup.-10] 0.83 2C3CN 1.32 x [10.sup.7] 1.26 x [10.sup.-9] 0.73 3C1CN 1.71 x [10.sup.7] 1.18 x [10.sup.-9] 0.65 3C2CN 6.71 x [10.sup.7] 1.69 x [10.sup.-9] 0.72 3C3CN 8.15 x [10.sup.6] 3.19 x [10.sup.-10] 0.89 4C1CN 2.06 x [10.sup.7] 6.62 x [10.sup.-10] 0.73 4C2CN 6.70 x [10.sup.7] 7.41 x [10.sup.-10] 0.76 4C3CN 4.68 x [10.sup.6] 9.73 x [10.sup.-10] 0.78 (c) Test samples 50D OCOCN 1.10 x [10.sup.5] 6.43 x [10.sup.-5] 0.54 0C3CN 6.15 x [10.sup.7] 2.19 x [10.sup.-9] 0.59 4C0CN 2.29 x [10.sup.7] 4.38 x [10.sup.-10] 0.89 2C1CN 1.78 x [10.sup.5] 5.93 x [10.sup.-10] 0.84 2C2CN 6.97 x [10.sup.6] 1.57 x [10.sup.-9] 0.67 2C3CN 7.94 x [10.sup.6] 1.38 x [10.sup.-9] 0.72 3C1CN 4.40 x [10.sup.7] 7.71 x [10.sup.-10] 0.70 3C2CN 8.07 x [10.sup.7] 4.09 x [10.sup.-9] 0.56 3C3CN 4.26 x [10.sup.7] 4.65 x [10.sup.-9] 0.62 4C1CN 2.63 x [10.sup.7] 4.23 x [10.sup.-10] 0.85 4C2CN 9.23 x [10.sup.7] 8.14 x [10.sup.-10] 0.76 4C3CN 4,03 x [10.sup.7] 1.43 x [10.sup.-9] 0.73 (d) Test samples 100D OCOCN 3.17 x [10.sup.4] 3.37 x [10.sup.-5] 0.55 0C3CN 5.87 x [10.sup.7] 1.44 x [10.sup.-9] 0.65 4C0CN 2.10 x [10.sup.7] 4.38 x [10.sup.-10] 0.89 2C1CN 5.90 x [10.sup.4] 5.59 x [10.sup.-5] 0.34 2C2CN 7.99 x [10.sup.4] 5.08 x [10.sup.-10] 0.87 2C3CN 3.48 x [10.sup.6] 8.85 x [10.sup.-10] 0.72 3C1CN 2.32 x [10.sup.6] 3.94 x [10.sup.-10] 0.83 3C2CN 2.57 x [10.sup.7] 8.84 x [10.sup.-10] 0.80 3C3CN 1.02 x [10.sup.5] 4.93 x [10.sup.-10] 0.84 4C1CN 4.20 x [10.sup.6] 1.15 x [10.sup.-10] 0.73 4C2CN 4.07 x [10.sup.7] 1.20 x [10.sup.-10] 0.70 4C3CN 2.84 x [10.sup.6] 7.24 x [10.sup.-9] 0.82
The 4C2CN is the best coating formulation according to Table 3, and the Bode and Nyquist diagrams for different exposure times are shown in Fig. 5.
[FIGURE 5 OMITTED]
Salt spray test
The salt spray test was conducted as an accelerated corrosion testing method to evaluate the corrosion performance according to ASTM B-117 for 1000 h. At the end of the test the panels were evaluated according to ASTM D714, ASTM D1654, and ASTM D610 Table 4 shows the results of the salt spray test.
Table 4: Method of evaluating corrosion behaviors according to ASTM D714, ASTM D1654, and ASTM D610 Composition Evaluation of rust Evaluation of Evaluation of creepage (rating blisters, ASTM rusting, number) ASTM D1645 D714 (percent of surface rusted) ASTM D610 OCOCN 3 Blister size No. 3.0 2, few 0C3CN 4 Blister size No. 0.1 4, few 4C0CN 7 Blister size No. 0.1 6, few 2C1CN 5 Blister size No. 0.3 4, few 2C2CN 6 Blister size No. 0.3 6, few 2C3CN 6 Blister size No. 0.1 6, medium 3C1CN 6 Blister size No. 0 1 6, few 3C2CN 7 -- -- 3C3CN 6 Blister size No. 0.1 6, few 4C1CN 7 Blister size No. 0.3 6, few 4C2CN 9 -- --
As can be seen in Fig. 6, numerous blisters were observed near the scratches and over the whole coated surface in the case of the OCOCN and 0C3CN specimens. Also, a brown adherent corrosion product (which was a mixture of iron hydroxides) was observed all over the sample surface. The blister was found in samples coated except for the 4C2CN and 3C2CN specimens. Delamination and water penetration were observed around the scratches with increased MMT and decreased CN, but by increasing CN more than 2 wt% in the coating formulation, delamination and water penetration increased. The enhancement of corrosion resistance of all the investigated nanocomposite coatings may he due to the nature, shape, and size of the nanoadditives. The nanolayers of these additives force the corroding agents to travel a tortuous path to reach the substrate.
As is known, the MMT clay has an extended surface area and a high aspect ratio. Therefore. MMT clay layers create longer barrier spots in front of the corroding agents. An increase in the clay concentration leads to a reduction of degradation and blistering density. On the other hand, increasing the amount of cerium nitrate in the coating up to 2 wt% caused an improved self-healing ability of the nanocomposite. Therefore, increasing the amount of CN to 3 wt% can cause decreasing anticorrosive features. Oxygen is reduced and the environment becomes alkaline. In this case, a cerium ion will be released and cerium oxide will he formed at defects, as can be seen in the X-ray spectroscopy (EDX) analysis. EDX analysis confirms the presence of cerium in the scratched areas for sample 4C2CN, as shown in Fig. 7. This is the cerium nitrate function of this coating.
[FIGURE 7 OMITTED]
Twelve samples of epoxy nanocomposites were prepared by the addition of MMT and CN dispersion using mechanical agitation and a sonication process. The results of optical microscopy and sedimentation showed that the combination of mechanical mixing and sonication is an effective way to de-agglomerate additives in epoxy resin. The results of XRD, TEM, and optical microscopy analyses of the cured nanocomposites indicated that the clay particles were dispersed and intercalated and did not fully exfoliate the epoxy polymers. In addition, the results of a series of electrochemical measurements indicated that the epoxy cerium nitrate--MMT films showed a greater corrosion protective effect on the steel surfaces than the pure epoxy film over 100 days of immersion in the 3.5 wt% aqueous NaCl solutions. The metals coated by the nanocomposites containing 4 wt% MMT and 2 wt% CN (4C2CN) showed the highest corrosion resistance.
[c] American Coatings Association & Oil and Colour Chemists' Association 2013
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E. Darmiani, I. Danaee (21), G. R. Rashed Abadan Faculty of Petroleum Engineering, Petroleum University of Technology, Abadan, Iran e-mail: email@example.com
Technical Faculty, South Tehran Branch, Islamic Azad University, Tehran, Iran
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|Author:||Darmiani, E.; Danaee, I.; Rashed, G.R.; Zaarei, D.|
|Date:||Jul 1, 2013|
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