Printer Friendly

Electrochemical investigation of the epoxy nanocomposites containing Mn[Al.sub.2][O.sub.4] and Co[Al.sub.2][O.sub.4] nanopigments applied on the aluminum alloy 1050.

Abstract The Mn[Al.sub.2][O.sub.4] and Co[Al.sub.2][O.sub.4] nanopigments were synthesized through solution combustion method. The nanopigments extracts in the 3.5 wt% NaCl solution were prepared. Electrochemical impedance spectroscopy (EIS) and linear polarization (LP) techniques were utilized to investigate the corrosion inhibition properties of the pigments in the solutions containing pigments extracts. The epoxy nanocomposites were prepared and applied on the aluminum specimens. EIS and pull-off measurements were performed to characterize the corrosion protection properties of the epoxy nanocomposites. It was revealed that the Mn[Al.sub.2][O.sub.4] pigment showed higher corrosion inhibition properties than Co[Al.sub.2][.sub.4] in the solution phase. In addition, Mn[Al.sub.2][O.sub.4] enhanced the corrosion resistance and decreased the adhesion loss of the epoxy coating on the aluminum substrate significantly. The Mn[Al.sub.2][O.sub.4] nanopigment improved the corrosion resistance of the epoxy coating through increasing the barrier performance and releasing inhibitive species.

Keywords Aluminum alloy, Nanostructured pigment, Epoxy nanocomposite, Corrosion resistance, EIS

Introduction

The 1xxx series aluminum alloys, which represent the purest commercially available metal, offer the highest corrosion resistance compared to other aluminum alloys. However, among the lxxx series, AA1050 can undergo localized pitting corrosion due to the presence of some alloying elements. The major alloying elements present in the AA1050 are iron and silicon which have low solubility in aluminum and form intermetallic particles. [Al.sub.6]Fe, [Al.sub.3]Fe, and [Al.sub.12][Fe.sub.3][Si.sub.2] are the main intermetallics particles present in AA1050. The intermetallic particles exhibit cathodic activity relative to the aluminum matrix. (1-4) Therefore, they are catalytic sites for the cathodic reactions and nucleation of pits. (5-7) AA1050 has been used in conventional engineering applications, i.e., architectural, automotive industries, containers, and equipment for the food and chemical industries, where corrosion resistance is required and the mechanical strength is relatively unimportant. Therefore, the improvement of the corrosion resistance of AA1050 has been under the attention of the researchers in recent years.

Attempts have been carried out to enhance the corrosion resistance of the aluminum alloys through various methods. Chemical treatment by conversion coating is an effective way of improving adhesion and corrosion protection properties of the organic coatings on the aluminum alloy. Attar et al. (8) studied the effects of surface treatment of AA1050 by zirconium conversion coating on its corrosion resistance and the adhesion properties of the epoxy coating. They revealed that surface treatment of the alloy could enhance its corrosion resistance and adhesion properties. The same results have been reported by Staffer et al. (9,10) who used cerium-based conversion coatings (CeCC) on the aluminum alloys. They showed that the CeCC can improve the corrosion resistance of aluminum alloys and therefore they have been identified as leading candidates to replace hexavalent chrome conversion coatings.

One effective way to protect the aluminum alloys from their environment is by using a coating system. (11) Also, the corrosion inhibitive pigments are embedded in the organic coatings formulations in order to enhance its protection properties on the aluminum alloy surface. Simoes et al. (12) studied the corrosion protection of aluminum substrates by using Mg-rich primer. They showed that this type of primer could provide cathodic protection at an early stage and barrier protection later as a result of magnesium oxide layer formation on the aluminum surface. In fact, the Mg-rich primer could effectively prevent the nucleation and growth of pits on the aluminum substrate. In a similar study, Bierwagen et al. (13) used multiple electrochemical techniques to characterize Mg-rich primers for Al alloys. They showed that Mg particles connection to the aluminum substrate provided cathodic protection.

Using corrosion inhibitive pigments provides longterm corrosion protection performance for the aluminum alloys. These pigments can release corrosion inhibitive species into the solution, restricting the access of the aggressive agents to the active anodic and cathodic sites on the metal surface. (14-18) Sitaram et al. (19) studied the use of conducting polymers, i.e., polyaniline, as corrosion inhibitors. They showed that these materials are proper substitutes for conventional anticorrosion materials. O'Keefe et al. (20) studied the importance of using the rare-earth materials in the organic paints as corrosion inhibitors. They reported the use of cerium salts in an electrolytically deposited coating on the corrosion protection of metals. They found that the cerium salts could effectively reduce the corrosion rate of the metals.

Zinc chromates and zinc phosphates are the most common types of inhibitive pigments which have been extensively used to enhance the corrosion protection properties of the organic coatings. Due to their slight solubility in water, they could release inhibitive species when exposed to the corrosive electrolyte. The released ions can reach the coating/metal interface and restrict the aggressive ions access to the active sites of the metal surface. (16-18) The electrolyte diffusion into the aluminum surface is responsible for the increase of pH especially around intermetallic particles as a result of the following cathodic reaction: 2[H.sub.2]O + [O.sub.2]+ 2e [??] 40[H.sup.-]. Therefore, the inhibitive species could react with O[H.sup.-] ions and form insoluble compounds on the aluminum surface blocking the active sites. However, chromates and zinc phosphates produce carcinogenic and toxic compounds. Therefore, in the last decades, it has been attempted to replace them with the more environmentally friendly alternatives. (14,21-23)

Spinel anticorrosive pigments have been developed via doping metal oxide with inhibitive components. (24-28) They showed both barrier and inhibitive actions when embedded in the organic coatings. Therefore, they could significantly enhance the corrosion protection properties of the organic coatings. Recently, researchers have found that reducing the pigment size is another useful approach to obtain coatings with enhanced corrosion resistance. Nanoparticles, due to their small size and high surface area, could provide better barrier performance than conventional pigments. (29) Tallman et al. (30) studied the corrosion protection of aluminum alloy 2024-T3 by nanocomposite of polypyrrole and alumina nanoparticles. They found that the used nanocomposite could successfully protect the aluminum alloy against corrosion. Among various nanopigments, it has been shown that nano-alumina is an anticorrosive pigment with great capability of enhancing the barrier properties of the epoxy coating. However, nano-alumina does not have proper inhibitive action due to its low solubility in water. (31) In contrast, Al-based spinel nanopigments containing various cations reveal good solubility and then show proper inhibitive performance. (32,33)

In this work, an attempt has been made to prepare nanopigments, i.e., Mn[Al.sub.2][O.sub.4] and Co[Al.sub.2][O.sub.4] through a combustion method. The effects of Mn and Co elements on the inhibition properties of the nanopigments were studied. Then, the pigments were characterized by XRD and SEM analyses. The corrosion inhibition properties of the pigments were studied in 3.5 wt% NaCl solution containing Mn[Al.sub.2][O.sub.4] and Co[Al.sub.2][O.sub.4] extracts on the aluminum alloy 1050. EIS and polarization techniques were employed for this purpose. The surface morphology of the aluminum alloy exposed to the solutions without and with pigments extracts was studied by the SEM/EDS analysis. The effects of addition of nanopigments on the anticorrosion and adhesion properties of the epoxy coating were studied on the aluminum alloy 1050.

Experimental

Materials

Manganese nitrate [Mn [(N[O.sub.3]).sub.3] x 6[H.sub.2]O], cobalt nitrate [Co[(N[O.sub.3]).sub.3]-6[H.sub.2]O], sodium hydroxide (NaOH), urea (CO[(N[H.sub.2]).sub.2]), and aluminum nitrate [Al[(N[O.sub.3]).sub.3] x 9[H.sub.2]O] were procured from Merck Co.

Co[(N[O.sub.3]).sub.2] + 2A1[(N[O.sub.3]).sub.3] + 6.6CO[(N[H.sub.2]).sub.2] [right arrow] Co[Al.sub.2][O.sub.4] + 6.6C[O.sub.2] + [N.sub.2] + 13.3[H.sub.2]O (1)

Mn[(N[O.sub.3]).sub.2] + 2A1[(N[O.sub.3]).sub.3] + 6.6CO[(N[H.sub.2]).sub.2] [right arrow] Mn[Al.sub.2][O.sub.4] + 6.6C[O.sub.2] + [N.sub.2] + 13.3[H.sub.2]O (2)

Aluminum alloy 1050 was provided by Aluminium Arak Co. (Iran). The AA1050 has the following composition (wt%): Al (99.5), Si (0.08), Fe (0.23), Cu (0.01), Mn (0.03), Mg (0.03), Zn (0.01), and others (0.03). Epoxy resin based on bisphenol-A (Araldite GZ7 7071X75) was prepared from Saman Co. The solid content, epoxy value, and density of the epoxy resin were 74-76%, 0.1492-0.1666 Eq/100 g, and 1.08 g [cm.sup.-3], respectively.

Synthesis of pigments

The pigments were produced through a solution combustion method. Al[(N[O.sub.3]).sub.3] x 9[H.sub.2]O (4.21 g), Co[(N[O.sub.3]).sub.2] x 6[H.sub.2]O (1.03 g), and urea (2.26 g) were added to 20 mL of deionized water. Also, Al[(N[O.sub.3]).sub.3] x 9[H.sub.2]O (4.31 g), Mn [(N[O.sub.3]).sub.2] x 6[H.sub.2]O (1.03 g), and urea (2.31 g) were added to 20 mL of deionized water. The role of urea was to fuel combustion, and its proportion was calculated according to reaction stoichiometry to ensure the complete combustion of the mixture. The resulting solution was heated until a clear gel solution appeared and then it was transferred into a microwave (Samsung, South Korea, 900 W) and kept for 2 min to obtain a fumy mass gel.

Pigments extract preparation

The corrosion inhibition properties of the nanopigments were studied in the solutions containing the pigments extracts. For this purpose, 1 g of the pigments was dissolved in 1 L of 3.5 wt% NaCl solution for 24 h. Then, the suspension was filtered to obtain the pigments extracts.

Epoxy nanocomposite preparation

First, 2 wt% of the Mn[Al.sub.2][O.sub.4] and [Al.sub.2][O.sub.3] nanopigments were mixed with epoxy resin by a laboratory-sized industrial-mimicked mixer for 2 h at 2000 RPM. Then, the appropriate amount of polyamide hardener was added to the epoxy nanocomposites. The epoxy coatings were then applied on the aluminum sheets by an adjustable film applicator. Before the coating application, the aluminum substrates were abraded with sand papers of 600, 800, and 1200 grades followed by acetone degreasing. All of the coatings were cured in a ventilated oven at 120[degrees]C for 30 min. The dry thickness of the coatings was about 40 [+ or -] 5 [micro]m.

Characterization

X-ray diffraction (XRD) was used to investigate the phase composition of the nanopigments. The XRD pattern was measured in reflection geometry using a Bruker D8 advanced instrument with a curved Ge(l 11) monochromator (CuK[alpha]1 radiation, [lambda] - 1.54060A).

Scanning electron microscope (SEM) (Philips XL30) was used to characterize the morphology of the nanopigments. The inhibitive properties of the pigments were studied in the solutions containing the extracts using Ivium Compactstat model electrochemical impedance spectroscopy (EIS) and linear polarization (LP) techniques. Also, the corrosion protection properties of the epoxy nanocomposites applied on the aluminum sheets were examined by EIS. The experiments were carried out in a three electrode cell containing graphite (auxiliary electrode), Ag/AgCl (reference electrode), and aluminum specimen (working electrode). The AA1050 specimens (1 cm x 1 cm) were dipped in the solution containing nanoparticles extracts for 1 and 4 h. Also, the coated specimens (1 [cm.sup.2]) were immersed in the 3.5 wt% NaCl solution for 40 days. The EIS measurements were performed at open circuit potential (OCP), in the frequency range from 10 mHz to 10 kHz and 10 mV amplitude sinusoidal voltage. Three parallel experiments were carried out for each sample and the middle one was selected among the three obtained values. Adhesion strength of the epoxy nanocomposites applied on the aluminum sheets was measured by a PosiTest pull-off adhesion tester (DeFelsko). For this purpose, the epoxy-polyamine glue was used to attach aluminum dollies to the coating surface. The test was conducted in accordance with ASTM D4541. Three parallel measurements were carried out for each sample and the middle one was selected among the three obtained values.

Results and discussion

XRD and SEM analyses

The XRD patterns of the pigments are given in Fig. 1. Mn[Al.sub.2][O.sub.4] and Co[Al.sub.2][O.sub.4] phases were detected according to JCPDS No. 00-029-0880 and 00-003-0896, respectively.

Figure 1 confirms that the Mn[Al.sub.2][O.sub.4] and Co[Al.sub.2][O.sub.4] nanopigments were successfully produced. The wide peaks observed in the XRD patterns of the nanoparticles can be attributed to the small size of the crystallites. The crystallite size of the nanopigments, calculated by Scherrer equation, was about 34 nm. The morphology of the nanostructured pigments was studied by SEM (Fig. 2). It can be seen in Fig. 2 that the average particle size of both nanopigments is less than 100 nm.

[FIGURE 1 OMITTED]

Corrosion inhibition evaluation of the Co- and Mn-doped nanopigments

EIS measurements

The corrosion inhibition properties of the Mn[Al.sub.2][O.sub.4] and Co[Al.sub.2][O.sub.4] nanopigments were studied on the AA1050 sample and compared with nano-[Al.sub.2][O.sub.3]. For this purpose, the aluminum specimens were dipped in the NaCl 3.5 wt% solutions containing [Al.sub.2][O.sub.3], Mn[Al.sub.2][O.sub.4], and Co[Al.sub.2][O.sub.4] extracts at different immersion times. The impedance data are reported in Figs. 3 and 4 in the forms of Nyquist and Bode plots.

Figures 3 and 4 show only one relaxation time in the Nyquist and Bode plots of all samples. Therefore, a simple electrochemical equivalent circuit was utilized to model the impedance data (Fig. 5). The representative example of using this equivalent circuit to fit the experimental data is shown in Fig. 5. The Chi squared ([chi square]) is used to evaluate the precision of the data fitting. For all fitted data, the [chi square] < 0.009 was obtained indicating that the fitted data have good agreement with the experimental data.

In Fig. 4, [R.sub.s], [R.sub.ct], and [CPE.sub.dl] are solution resistance, charge transfer resistance, and constant phase element of double layer ([CPE.sub.dl]), respectively. Also, the double layer capacitance values were calculated according to equation (3). (34,35)

[C.sub.dl] = [([Q.sub.dl] x [R.sup.1-n.sub.ct]).sup.1/n], (3)

where [C.sub.dl], [Q.sub.dl], [R.sub.ct], and n show double layer capacitance, admittance of CPE of double layer, charge transfer resistance ([R.sub.ct]), and the empirical exponent, respectively. The parameters extracted from the model are given in Table 1.

[FIGURE 2 OMITTED]

Moreover, the impedance values were calculated at the low frequency limit (10 mHz) from Bode plots (Fig. 6).

It is clear from the results presented in Table 1 and Fig. 6 that the [R.sub.ct] and impedance values of the aluminum samples immersed in the solutions containing pigments extracts are greater than the one dipped in the blank solution. This indicates that all nanopigments could reduce the dissolution rate of aluminum. It can be seen that the Mn[Al.sub.2][O.sub.4] and Co[Al.sub.2][O.sub.4] nanopigments showed higher inhibition properties than nano[Al.sub.2][O.sub.3]. As the immersion time elapsed, the [R.sub.ct] of the aluminum samples dipped in the solutions containing Mn[Al.sub.2][O.sub.4] and Co[Al.sub.2][O.sub.4] nanopigments extracts was increased. The increase of [R.sub.ct] was most pronounced in the presence of nano-Mn[Al.sub.2][O.sub.4] pigment extract. These observations clearly reveal higher corrosion inhibition properties of the nano-Mn[Al.sub.2][O.sub.4] pigment than the Co[Al.sub.2][O.sub.4] one. Also, the lowest double layer capacitance was obtained for the aluminum specimen exposed to the solution containing nano-Mn[Al.sub.2][O.sub.4] pigment extract after 4 h immersion. This may be attributed to the increase of double layer thickness as a result of protective film precipitation on the aluminum surface. These show that Co[Al.sub.2][O.sub.4] and Mn[Al.sub.2][O.sub.4] nanopigments could efficiently restrict the access of the aggressive agents to the aluminum surface through releasing inhibitive species and forming a protective layer on the aluminum surface.

[FIGURE 3 OMITTED]

Polarization test measurements

A polarization test was performed on the samples after 4 h immersion in order to investigate the inhibition mechanism of the nanoparticles. The polarization curves are given in Fig. 7.

According to Fig. 7, both anodic and cathodic branches shifted to lower current densities in the solutions containing Mn[Al.sub.2][O.sub.4] and Co[Al.sub.2][O.sub.4] extracts. This indicates that the extracts act as mixed inhibitors. Some electrochemical parameters, i.e., corrosion potential ([E.sub.corr]), corrosion current density ([i.sub.corr]), polarization resistance ([R.sub.p]), anodic Tafel slope ([b.sub.a]), and cathodic Tefel slope ([b.sub.c]), were extracted from the polarization curves through Tafel extrapolation technique. Results obtained are displayed in Table 2.

Table 2 shows that the [i.sub.corr] decreased and the [R.sub.p] increased in the presence of nanoparticles extracts. The order of [i.sub.corr] values for different samples is blank solution > [Al.sub.2][O.sub.3] > Co[Al.sub.2][O.sub.4] > Mn[Al.sub.2][O.sub.4]. This observation shows that the Mn[Al.sub.2][O.sub.4] extract could reduce the corrosion rate of the aluminum sample much greater than nano-[Al.sub.2][O.sub.3] and nano-Co[Al.sub.2][O.sub.4]. This finding is in accordance with the results of EIS measurements.

[FIGURE 4 OMITTED]

Studying the corrosion inhibition mechanism of the nanopigments

Results obtained from the EIS and polarization measurements reveal that the Mn- and Co-doped [Al.sub.2][O.sub.3] nanoparticles showed higher corrosion inhibitive properties than nano-[Al.sub.2][O.sub.3]. The ICP analysis (Table 3) showed the slight solubility of the nanoparticles in the 3.5 wt% NaCl solution. [Al.sub.3+], [Co.sup.2+], and [Mn.sup.2+] are the cations detected in the extracts of the nanopigments.

There are different types of intermetallic particles in the AA1050 structure. The intermetallic particles produce cathodic regions in the aluminum matrix due to their greater potential. This can result in pitting corrosion around the intermetallic particles (Fig. 8) The cations released from the nanopigments could precipitate on the intermetallic particles in the form of oxide/hydroxide films according to equations (4-7).

[FIGURE 5 OMITTED]

2[H.sub.2]O + [O.sub.2] + 4e [right arrow] 40[H.sup.-] (4)

[Al.sub.2][O.sub.3] + O[H.sup.-] [right arrow] ?+ 2Al[O.sup.-.sub.2] + [H.sub.2]O (pH [down arrow]) (5)

[Mn.sup.2+] + O[H.sup.-] [right arrow] Mn[(OH).sub.2] [down arrow] (6)

[Co.sup.2+] + O[H.sup.-] [right arrow] Co[(OH).sub.2] [down arrow] (7)

The precipitated layer decreases the cathodic activity of the intermetallic particles. The better corrosion inhibition properties of the Mn[Al.sub.2][O.sub.4] nanoparticle than Co[Al.sub.2][O.sub.4] may be attributed to two main reasons. First, the greater solubility of Mn[Al.sub.2][O.sub.4] nanoparticle than Co[Al.sub.2][O.sub.4] in the NaCl solution may be responsible for the higher precipitation of the Mn[(OH).sub.2] than Co[(OH).sub.2] on the aluminum surface. Second, the precipitation pH of the [Mn.sup.2+] and [Co.sup.2+] cations is different. According to the pourbaix diagrams of Mn and Co, the Mn[(OH).sub.2] and Co[(OH).sub.2] components are stable at pH regions of 7.5-13 and 9-14, respectively. (31) This means that [Mn.sup.2+] cations precipitation in the form of Mn[(OH).sub.2] occurs at lower pH than Co[(OH).sub.2]. As a result, Mn[Al.sub.2][O.sub.4] could form better inhibitive film on the cathodic regions of the aluminum surface. The SEM/EDS analysis was performed on the aluminum samples to reveal the nanopigments capability of film formation on the aluminum surface. The EDS spectra of the samples are given in Fig. 9. Also, the elemental compositions of the surface of the aluminum samples are presented in Table 4.

[FIGURE 6 OMITTED]

[FIGURE 7 OMITTED]

According to Table 4, Co and Mn elements were detected on the surface of the samples exposed to the solutions containing Mn[Al.sub.2][O.sub.4] and Co[Al.sub.2][O.sub.4] nanopigments. This may reveal that the films precipitated on the aluminum surface consist of Mn[(OH).sub.2] and Co[(OH).sub.2] components. Also, the EDS results clearly reveal that the Mn content on the aluminum sample was higher than Co. This may indicate that a better film was precipitated on the sample in the solution containing Mn[Al.sub.2][O.sub.4] extract than Co[Al.sub.2][O.sub.4]. These observations confirm that the higher inhibition properties of Mn[Al.sub.2][O.sub.4] than Co[Al.sub.2][O.sub.4] may be attributed to its greater capability of film formation.

The inhibition properties of the pigments were also examined after inclusion into the epoxy coating. For this purpose, the Mn[Al.sub.2][O.sub.4] nanopigment, which showed better inhibition properties than Co[Al.sub.2][O.sub.4], was used.

Anticorrosion performance of epoxy nanocomposites

The corrosion protection properties of the blank epoxy coating and the nanocomposites loaded with 2 wt% [Al.sub.2][O.sub.3] and Mn[Al.sub.2][O.sub.4] nanopigments were studied by EIS in the 3.5 wt% NaCl solutions after 7 and 40 days immersion. The Nyquist and Bode plots of these samples are depicted in Figs. 10, 11, 12.

It is clear from Fig. 12 that the impedance modulus at low frequency limit (10 mHz) of the coatings loaded with 2 wt% nanostructured pigments is > [10.sup.9] [OMEGA] [cm.sup.2] after 7 days immersion, and the phase angles are almost -90[degrees]. However, the blank epoxy coating showed impedance lower than the pigmented coatings. These observations indicate that the pigmented coatings acted like a perfect capacitor at a short period of immersion. Also, only one time constant was observed in the impedance spectra of these samples, indicating good barrier characteristics of these coatings. Figure lib shows the decrease in impedance and narrowing of the frequency range displaying capacitive behavior in the Bode phase diagrams after 40 days immersion. Also, the maximum phase angle of the epoxy coatings without and with [Al.sub.2][O.sub.3] nanopigment shifted to higher frequencies. Simultaneously, the impedance modulus was decreased for these samples. These mean that as the immersion time elapsed, the corrosive electrolyte gradually diffused into the epoxy coatings, resulting in coating damage. Two time constants were appeared in the Bode phase diagram of the blank epoxy coating after 40 days immersion. However, only one time constant was seen in the Bode phase plots of the pigmented coatings. This indicates that water, oxygen, and corrosive ions diffused into the coating/metal interface and the electrochemical corrosion reactions initiated beneath the coating. With prolonged immersion time, the frequency range displaying capacitive behavior shifted toward higher frequencies.

Electrochemical equivalent circuits with one and two relaxation times were utilized in order to model the impedance data that appeared in the Nyquist diagrams. The fitted and experimental data and the models used for data fitting are displayed in Fig. 10. [R.sub.s], [R.sub.c], [R.sub.ct], [CPE.sub.c], and [CPE.sub.dl] are solution resistance, coating resistance, charge transfer resistance, constant phase element of the coating, and constant phase element of double layer, respectively. The impedance data obtained from data fitting are presented in Table 5.

The electrochemical parameters including impedance at low frequency limit ([[absolute value of Z].sub.0.01 Hz]) and breakpoint frequency (fb) values were extracted from the impedance data and the results obtained are presented in Fig. 13.

From Fig. 13 and Table 5, it can be seen that the [[absolute value of Z].sub.0.01 Hz] and [R.sub.c] values of the coatings loaded with nanopigments are higher than the blank epoxy coating. However, the improvement of the corrosion protection properties of the epoxy coating was more pronounced in the presence of Mn[Al.sub.2][O.sub.4] nanopigments. This means that the Mn[Al.sub.2][O.sub.4] pigment enhanced the corrosion resistance of the epoxy coating much greater than nano-[Al.sub.2][O.sub.3].

The breakpoint frequencies ([f.sub.b]) of different samples are compared in Fig. 13b. It can be seen that the [f.sub.b] of the samples increased to higher frequencies after 40 days immersion. This can indicate that the capacitive behavior of the coatings was decreased as a result of the electrolyte diffusion into the coating matrix. Results show higher [f.sub.b] values of the blank coating and the one reinforced with [Al.sub.2][O.sub.3] nanopig ment than the coating loaded with Mn[Al.sub.2][O.sub.4]. In fact, the breakpoint frequency of the nanocomposite coating containing Mn[Al.sub.2][O.sub.4] is relatively smaller than other samples. There is a close relationship between the microscopic-delaminated areas beneath the coating and its corrosion protective performance. (36) The electrochemical reactions occur on the active sites of the metal surface as soon as the corrosive electrolyte reaches at the coating/metal interface. The hydroxyl ions (O[H.sup.-]) creation at the cathodic regions (2[H.sub.2]O + [O.sub.2] + e [right arrow] 40[H.sup.-]) leads to the increase of pH. The increase of pH beneath the coating would be responsible for the coating adhesion bonds destruction resulting in the coating delamination from the substrate. Based on these explanations, it can be said that the lowest coating

Adhesion loss(%) = (Dry adhesion strength - recovery adhesion strength)/Dry adhesion strength x 100 (8)

delamination occurred beneath the coating containing the Mn[Al.sub.2][O.sub.4] nanopigment.

[FIGURE 8 OMITTED]

Pull-off adhesion test results

The effects of addition of Mn[Al.sub.2][O.sub.4] and [Al.sub.2][O.sub.3] nanopigments on the adhesion properties of the epoxy coating were studied by a pull-off test before (dry adhesion) and after (recovery adhesion) immersion in the 3.5 wt% NaCl solution. Also, the adhesion loss values were calculated according to equation (8). Results obtained are given in Table 6 and Fig. 14.

From Table 5, it can be seen that addition of the nanopigments did not affect the dry adhesion strength of the epoxy coating significantly. However, the recovery adhesion and adhesion loss values were significantly changed after immersion in the corrosive electrolyte. From Fig. 14, it can be seen that the blank coating showed higher adhesion loss than the pigmented coatings. In addition, the lowest adhesion loss was observed for the coating loaded with Mn[Al.sub.2][O.sub.4] nanopigment. Also, the coating failure mode after pulloff test on the samples immersed in the 3.5 wt% NaCl solution for 40 days is studied in Fig. 15. Adhesive failure was seen on the aluminum specimens coated with the blank coating and the one containing [Al.sub.2][O.sub.3] nanopigment. However, the coating failure on the sample coated with the epoxy coating containing Mn[Al.sub.2][O.sub.4] nanopigment was in the form of adhesive/cohesive detachment. Cohesive failure occurs when the coating adhesion to the aluminum surface is high enough. This means that the Mn[Al.sub.2][O.sub.4] nanopigment could successfully restrict the coating delamination from the aluminum surface in the corrosive electrolyte through decreasing electrolyte diffusion into the coating/metal interface and forming a protective layer on the cathodic regions.

[FIGURE 9 OMITTED]

[FIGURE 10 OMITTED]

[FIGURE 11 OMITTED]

[FIGURE 12 OMITTED]

Corrosion protection mechanism of Mn[Al.sub.2][O.sub.4] nanopigment in the epoxy coating

It has been found from the results of the EIS and pulloff test that the corrosion protection properties of the epoxy coating were significantly enhanced in the presence of Mn[Al.sub.2][O.sub.4] nanopigment. In Fig. 15, the effects of Mn[Al.sub.2][O.sub.4] and [Al.sub.2][O.sub.3] nanopigments on the corrosion performance of the coatings are schematically displayed.

It has been observed in the EIS measurements that incorporation of Mn[Al.sub.2][O.sub.4] and [Al.sub.2][O.sub.3] nanopigment into the epoxy coating resulted in the increase of corrosion resistance of the epoxy coating on the aluminum surface. These show that Mn[Al.sub.2][O.sub.4] and [Al.sub.2][O.sub.3] nanopigments can act as a physical barrier against corrosive electrolyte diffusion into the coating (Fig. 15). There are free volumes, i.e., voids and defects in the coating matrix which are preferential diffusion paths for water molecules, ions, and oxygen. Nanoparticles can occupy the free volumes and block the diffusion paths through the horizontal directions between local anodes and cathodes along the epoxy coating-aluminum substrate interface. The EIS results showed a lower extent of coating damage when using Mn[Al.sub.2][O.sub.4] compared to [Al.sub.2][O.sub.3]. The barrier action of the nanopigments depends on the particle size distribution in the coating matrix. However, it seems that the difference between the anticorrosion properties of the Mn[Al.sub.2][O.sub.4] and [Al.sub.2][O.sub.3] nanopigments cannot be only attributed to their barrier action behaviors. Results obtained from the EIS and polarization tests (performed on the aluminum samples immersed in the solutions containing Mn[Al.sub.2][O.sub.4] and Co[Al.sub.2][O.sub.4] extracts) revealed the inhibition actions of these nanopigments. This means that the higher corrosion resistance of the coating loaded with Mn[Al.sub.2][O.sub.4] nanopigment may be attributed to the inhibition properties of this pigment. It was found from the EIS, polarization test and ICP analyses that Mn[Al.sub.2][O.sub.4] nanopigment can release inhibitive species, i.e., [Mn.sup.2+] in the corrosive electrolyte. The released species reaches the active anodic and cathodic sites on the aluminum surface. In this way, the [Mn.sup.2+] cations react with O[H.sup.-] ions created at the cathodic sites. Therefore, less coating delamination and corrosion products creation occur beneath the coating. The lower [f.sub.b] values of the coating loaded with Mn[Al.sub.2][O.sub.4] nanopigment than the other samples confirm this mechanism of protection. Based on the EIS results obtained in the solution phase and coating phase, it can be concluded that Mn[Al.sub.2][O.sub.4] acts as barrier and inhibitive pigment in the epoxy coating causing the increase of coating corrosion resistance.

[FIGURE 13 OMITTED]

[FIGURE 14 OMITTED]

[FIGURE 15 OMITTED]

Conclusions

Mn[Al.sub.2][O.sub.4] and Co[Al.sub.2][O.sub.4] nanopigments were synthesized through a combustion method. The pigments were characterized by SEM and XRD analyses. The corrosion inhibition properties of the nanopigments were investigated in the 3.5 wt% NaCl solution on the A1 alloy 1050. Also, the epoxy nanocomposites containing 2 wt% Mn[Al.sub.2][O.sub.4] and [Al.sub.2][O.sub.3] nanopigments were applied on the surface of the aluminum sheets and their corrosion resistances were investigated. The results obtained are shown below:

* XRD analysis revealed that Mn[Al.sub.2][O.sub.4] and Co[Al.sub.2][O.sub.4] nanopigments were produced successfully. Also, it was found that the pigment prepared had crystallite size about 34 nm. The SEM analysis showed large spongy morphology particles.

* ICP analysis showed higher solubility of Mn[Al.sub.2][O.sub.4] and Co[Al.sub.2][O.sub.4] nanopigments in the 3.5 wt% NaCl solution than [Al.sub.2][O.sub.3]. Inhibitive species including [Mn.sup.2+], [Co.sup.2+], and [Al.sub.3]+ were released by the pigments.

* EIS and polarization analyses revealed corrosion inhibitive properties of the Mn[Al.sub.2][O.sub.4] and Co[Al.sub.2][O.sub.4] nanopigments on the A1 alloy 1050 in the 3.5 wt% NaCl solution. It was shown that Mn[Al.sub.2][O.sub.4] could provide better inhibition properties than the Co[Al.sub.2][O.sub.4] one.

* The SEM/EDS analysis showed that the inhibitive spices released by Mn[Al.sub.2][O.sub.4] and Co[Al.sub.2][O.sub.4] nanopigments could form a protective layer on the active sites of aluminum, restricting the access of corrosion species.

* The corrosion protection properties of the epoxy coating were significantly enhanced after addition of 2 wt% Mn[Al.sub.2][O.sub.4] nanopigment. Also, the epoxy nanocomposite containing Mn[Al.sub.2][O.sub.4] nanopigment showed lower adhesion loss on the aluminum alloy compared to the blank sample and the one containing [Al.sub.2][O.sub.3] nanopigment.

DOI 10.1007/s11998-015-9728-6

E. Ghasemi, S. Saket

Inorganic Pigment and Glazes Department, Institute for Colour Science and Technology. Tehran. Iran

B. Ramezanzadeh ([mail]), S. Ashhari

Department of Surface Coatings and Corrosion, Institute for Colour Science and Technology, Tehran, Iran

e-mail: ramezanzadeh@aut.ac.ir, ramezanzadeh-bh@icrc.ac.ir

References

(1.) Vargel, C, Corrosion of Aluminium. Elsevier, Boston, 2004

(2.) Koroleva, EV, Thompson, GE, Hollrigl, G, Bloeck, M, "Surface Morphological Changes of Aluminium Alloys in Alkaline Solution: Effect of Second Phase Material." Corros. Sci.. 41 1475-1495 (1991)

(3.) Dalmoro, V, dos Santos, JHZ, Armelin, E, Aleman, C, Azambuja, DS, "A Synergistic Combination of Tetraethylorthosilicate and Multiphosphonic Acid Offers Excellent Corrosion Protection to AA1100 Aluminum Alloy." Appl. Surf. Sci.. 273 758-768 (2013)

(4.) Witkowska, M, Thompson, GE, Hashimoto, T. Koroleva, E, "Assessment of the Surface Reactivity of AA1050 Aluminium Alloy." Surf. Interface Anal.. 45 1585-1589 (2013)

(5.) Ambat, R, Davenport, AJ, Seamans, GM, Afseth, A, "Effect of Iron-Containing Intermetallic Particles on the Corrosion Behaviour of Aluminium." Corros. Sci.. 48 3455-3471 (2006)

(6.) Allachi, H, Chaouket, F, Draoui, K, "Corrosion Inhibition of AA6060 Aluminium Alloy by Lanthanide Salts in Chloride Solution." J. Alloys Compel.. 475 300-303 (2009)

(7.) Allachi, H, Chaouket, F, Draoui, K, "Protection Against Corrosion in Marine Environments of AA6060 Aluminium Alloy by Cerium Chlorides." J. Alloys Compd., 491 223-229 (2010)

(8.) Sharifi Golru. S, Attar, MM, Ramezanzadeh, B, "Effects of Surface Treatment of Aluminium Alloy 1050 on the Adhesion and Anticorrosion Properties of the Epoxy Coating." Appl. Surf. Sci.. 345 360-368 (2015)

(9.) Staffer, JO, O'Keefe, TJ, Sitaram, SP. Yu, P, Lin, X, Morris, E, Electrodeposition of Cerium Based Coatings for Corrosion Protection of Aluminum Alloys. US Patent 5,932,083, 3 1999

(10.) Hayes, SA, Yu, P, O'Keefe, TJ, O'Keefe, MJ, Staffer, JO, "The Phase Stability of Cerium Species in Aqueous Systems. I. E-pH Diagram for the Ce-HCl[O.sub.4]-[H.sub.2]O System." J. Electrochem. Soc., 149 (12) C623-C630 (2002)

(11.) Bierwagen, GP, "Reflections on Corrosion Control by Coatings." Prog. Org. Coat.. 28 42-48 (1996)

(12.) Sirnoes, A, Battocchi, D, Tallman, D, Bierwagen, G, "Assessment of the Corrosion Protection of Aluminium Substrates by a Mg-rich Primer: E1S, SVET and SECM Study." Prog. Org. Coat.. 63 (3) 260-266 (2008)

(13.) Bierwagen, G, Battocchi, D, Simoes, A, Stamness, A, Tallman, D, "The Use of Multiple Electrochemical Techniques to Characterize Mg-rich Primers for A1 Alloys." Prog. Org. Coat.. 59 172-178 (2007)

(14.) Sinko, J, "Challenges of Chromate Inhibitor Pigments Replacement in Organic Coatings." Prog. Org. Coat.. 42 267-282 (2001)

(15.) Twite, RL, Bierwagen, P, "Review of Alternatives to Chromate for Corrosion Protection of Aluminum Aerospace Alloys." Prog. Org. Coat.. 33 91-100 (1998)

(16.) Heydarpour, MR, Zarrabi. A. Attar, MM, Ramezanzadeh, B, "Studying the Corrosion Protection Properties of an Epoxy Coating Containing Different Mixtures of Strontium Aluminum Polyphosphate (SAPP) and Zinc Aluminum Phosphate (ZPA) Pigments." Prog. Org. Coat.. 77 160-167 (2014)

(17.) Naderi, R, Arman, SY, Fouladvand, SH. "Investigation on the Inhibition Synergism of New Generations of PhosphateBased Anticorrosion Pigments." Dye Pigm.. 105 23-33 (2014)

(18.) Askari, F, Ghasemi, E, Ramezanzadeh, B, Mahdavian, M, "Mechanistic Approach for Evaluation of the Corrosion Inhibition of Potassium Zinc Phosphate Pigment on the Steel Surface: Application of Surface Analysis and Electrochemical Techniques." Dye Pigm.. (2014). doi:10.1016/j.dyepig.2014.05.024

(19.) Sitaram, SP. Staffer, JO, O'Keefe, TJ, "Application of Conducting Polymers in Corrosion Protection." J. Coat. Technol., 69 65-69 (1997)

(20.) O'Keefe, MJ, Fahrenholtz, WG, Stoffer, JO, Morris, EL, "Corrosion-Resistant Polymer Coatings Containing Rare Earth Compounds." Rare Earth-Based Corros. Inhih., 175e-182e 163-174 (2014)

(21.) Joshi. S, Kulp, EA, Fahrenholtz, WG, O'Keefe, MJ, "Dissolution of Cerium from Cerium-Based Conversion Coatings on A1 7075-T6 in 0.1 M NaCl Solutions." Corros. Sci., 60 290-295 (2012)

(22.) Hernandez, M, Genesca, J, "Effect of an Inhibitive Pigment Zinc-Aluminum-Phosphate (ZAP) on the Corrosion Mechanisms of Steel in Waterborne Coatings." Prog. Org. Coat., 56 199-206 (2006)

(23.) Naderi, R, Mahdavian, M, Attar, MM, "Electrochemical Behavior of Organic and Inorganic Complexes of Zn(II) as Corrosion Inhibitors for Mild Steel: Solution Phase Study." Electrochim. Acta., 54 6892-6895 (2009)

(24.) Kalendova, A, "Alkalising and Neutralising Effects of Anticorrosive Pigments Containing Zn, Mg, Ca, and Sr Cations." Prog. Org. Coat., 38 199-206 (2000)

(25.) Palimi, MJ, Peymannia, M, Ramezanzadeh, B, "An Evaluation of the Anticorrosion Properties of the Spinel Nanopigment-Filled Epoxy Composite Coatings Applied on the Steel Surface." Prog. Org. Coat., 80 164-175 (2015)

(26.) Kalendova, A, Snuparek, J, Kalenda, P, "Nontoxic Anticorrosion Pigments of the Spinel Type Compared with Condensed Phosphates." Dye Pignt., 30 (2) 129-140 (1996)

(27.) Kalendova, A, Brodinova, J, "Spinel and Rutile Pigments Containing Mg, Ca, Zn and Other Cations for Anticorrosive Coatings." Anti-Corrosion Methods Mater., 50 (5) 352-363 (2003)

(28.) Rasouli, S, Jebeli Moeen, S, "Combustion Synthesis of Codoped Zinc Oxide Nanoparticles using Mixture of Citric Acid-Glycine Fuels."7. Alloys Compd., 509 1915-1919 (2011)

(29.) Palimi, MJ, Rostami, M, Mahdavian, M, Ramezanzadeh, B, "A Study on the Corrosion Inhibition Properties of Silane-Modified [Fe.sub.2][O.sub.2] Nanoparticle on Mild Steel and Its Effect on the Anticorrosion Properties of the Polyurethane Coating." J. Coat. Technol. Res., 12 (2) 277-292 (2015)

(30.) Tallman, DE, Levine, KL, Siripirom, C, Gelling, VG, Bierwagen, GP, Croll, SG, "Nanocomposite of Polypyrrole and Alumina Nanoparticles as a Coating Filler for the Corrosion Protection of Aluminium Alloy 2024-T3." Appl. Surf. Sci., 254 (17) 5452-5459 (2008)

(31.) Sharifi Golru, S, Attar, MM, Ramezanzadeh, B, "Studying the Influence of Nano-[Al.sub.2][O.sub.3] Particles on the Corrosion Performance and Hydrolytic Degradation Resistance of Anepoxy/Polyamide Coating on AA-1050." Prog. Org. Coat.. 77 1391-1399 (2014)

(32.) Gama, L, Ribeiro, MA, Barros, BS, Kiminami, RHA, Weber, IT, Costa, ACFM, "Synthesis and Characterization of the Ni[Al.sub.2][O.sub.4], Co[Al.sub.2][O.sub.4] and Zn[Al.sub.2][O.sub.4] Spinels by the Polymeric Precursors Method." J. Alloys Compd., 483 (1-2) 453-455 (2009)

(33.) Duan, X, Pan, M, Yu, F, Yuan, D, "Synthesis, Structure and Optical Properties of Co[Al.sub.2][O.sub.4] Spinel Nanocrystals." J. Alloys Compd, 509 (3) 1079-1083 (2011)

(34.) Motamedi, M, Tehrani-Bagha, AR, Mahdavian, M, "A Comparative Study on the Electrochemical Behavior of Mild Steel in Sulfamic Acid Solution in the Presence of Monomeric and Gemini Surfactants." Electrochim. Acta., 58 488-496 (2011)

(35.) Mahdavian, M, Ashhari, S, "Corrosion Inhibition Performance of 2- mercaptobenzimidazole and 2-mercaptoben zoxazole Compounds for Protection of Mild Steel in Hydrochloric Acid Solution." Electrochim. Acta, 55 1720-1724 (2010)

(36.) Liu, X, Xiong, J, Lv, Y, Zuo, Y, "Study on Corrosion Electrochemical Behavior of Several Different Coating Systems by EIS." Prog. Org. Coat., 64 497-503 (2009)
Table 1: Electrochemical parameters extracted from EIS data for mild
steel exposure to blank solution and those containing [Al.sub.2]
[O.sub.3], Co[Al.sub.2][O.sub.4], and Co[Al.sub.2][O.sub.4] extracts

Sample                  [R.sujb.ct] (k[ohm]
                          [cm.sup.-2]) (a)

Blank solution (1 h)            43.4
Blank solution (4 h)            72.0
Co[Al.sub.2][O.sub.4]          144.8
  extract (1 h)
Co[Al.sub.2][O.sub.4]          164.8
  extract (4 h)
Mn[Al.sub.2][O.sub.4]          153.9
  extract (1 h)
Mn[Al.sub.2][O.sub.4]          247.7
  extract (4 h)
[Al.sub.2][O.sub.3]             90.1
  extract (1 h)
[Al.sub.2][O.sub.3]            134
  extract (4 h)

Sample                     [CPE.sub.dl]
                                                      [C.sub.dl]
                             [Y.sub.O]                 ([micro]F
                        ([[ohm].sup.-1] (b)   n (c)   [cm.sup.-2])

Blank solution (1 h)     7.5 x [10.sup.-6]    0.88        6.44
Blank solution (4 h)    6.57 x [10.sup.-6]    0.84        5.7
Co[Al.sub.2][O.sub.4]    6.2 x [10.sup.-6]    0.89        6.12
  extract (1 h)
Co[Al.sub.2][O.sub.4]    4.6 x [10.sup.-6]    0.86        4.4
  extract (4 h)
Mn[Al.sub.2][O.sub.4]    6.8 x [10.sup.-6]    0.88        6.84
  extract (1 h)
Mn[Al.sub.2][O.sub.4]    4.9 x [10.sup.-6]    0.87        5.04
  extract (4 h)
[Al.sub.2][O.sub.3]      7.2 x [10.sup.-6]    0.89        6.83
  extract (1 h)
[Al.sub.2][O.sub.3]      5.9 x [10.sup.-6]    0.88        5.7
  extract (4 h)

(a) The standard deviation range for [R.sub.ct] values is between 2%
and 8.0%

(b) The standard deviation range for [Y.sub.0] values is between 0.5%
and 8.0%

(c) The standard deviation range for n values is between 0.2% and 1.5%

Table 2: Value corrosion current density, corrosion rate, and
polarization resistance for different samples

Sample immersed in              [E.sub.corr] vs    [i.sub.corr]
                                Ag/AgCl (V) (a)    ([micro]A
                                                   [cm.sup.-2]) (b)

Blank solution                  -0.86              0.399
Co[Al.sub.2][O.sub.4] extract   -0.78              0.192
Mn[Al.sub.2][O.sub.4] extract   -0.85              0.048
[Al.sub.2][O.sub.3] extract     -0.86              0.205

Sample immersed in              [R.sub.p] (k[ohm]   ba (V
                                [cm.sup.2]) (c)     [dec.sup.-1]) (d)

Blank solution                  82.6                0.146
Co[Al.sub.2][O.sub.4] extract   125                 0.093
Mn[Al.sub.2][O.sub.4] extract   231                 0.088
[Al.sub.2][O.sub.3] extract     102                 0.146

Sample immersed in              bc (V
                                [dec.sup.-1]) (e)

Blank solution                  0.158
Co[Al.sub.2][O.sub.4] extract   0.226
Mn[Al.sub.2][O.sub.4] extract   0.187
[Al.sub.2][O.sub.3] extract     0.118

(a) The standard deviation range for [E.sub.corr] values is between
0.4% and 1.0%

(b) The standard deviation range for [i.sub.corr] values is between
0.5% and 2.1%

(c) The standard deviation range for [R.sub.p] values is between
0.8% and 10.2%

(d) The standard deviation range for be values is between 0.5% and
9.2.0%

(e) The standard deviation range for ba values is between 1.5% and
6.0%

Table 3: ICP-OES analysis of the solutions with nano-
[Al.sub.2][O.sub.3], nano-Co[Al.sub.2][O.sub.4], and
nano-Mn[Al.sub.2][O.sub.4] pigments extracts

Sample                       Ion consumption (mg/L)

                             [Al]    [Mn]    [CO]

Nano-[Al.sub.2][O.sub.3]     24.24   --      --
Nano-Mn[Al.sub.2][O.sub.4]   28.34   2.66    --
Nano-Co[Al.sub.2][O.sub.4]   27.43   --      2.35

Table 4: The EDS results obtained from the surface of metal plates
immersed in the 3.5 wt% NaCl solutions without and with nanopigments
extracts

Element (%)                     Mn    Co    Al    O      Fe

Blank sample                    --    --    91   3.85   2.89
Co[Al.sub.2][O.sub.4] extract   --    1.5   89   4.8    2.60
Mn[Al.sub.2][O.sub.4] extract   2.8   --    87   5.5    2.80

Element (%)                      Si     Mg     Mn     Zn     Na

Blank sample                    0.88   0.7    0.18   0.25   0.25
Co[Al.sub.2][O.sub.4] extract   0.87   0.65   0.16   0.20   0.22
Mn[Al.sub.2][O.sub.4] extract   0.67   0.62   0.18   0.22   0.21

Table 5: The electrochemical parameters extracted from impedance data
of the epoxy coatings without and with 2 wt% [Al.sub.2][O.sub.3] and
Mn[Al.sub.2][O.sub.4] nanoparticles immersed in the NaCl solution for
7 and 40 days

Sample                   [R.sub.c]              [CPE.sub.c]
                          (M[ohm]
                        [cm.sup.2])   [Y.sub.0]([[ohm].sup.-1]     n
                            (a)       [cm.sup.-2][s.sup.n]) (b)   (c)

Blank Epoxy (7 days)        110       14 x [10.sup.-9]            0.86
Blank Epoxy (40 days)       3.7       12 x [10.sup.-7]            0.84
Epoxy-[Al.sub.2]           1620       22 x [10.sup.-10]           0.82
  [O.sub.3]
Epoxy-[Al.sub.2]           13.0       6.0 x [10.sup.-7]           0.87
  [O.sub.3]
Epoxy-Mn[Al.sub.2]         7700       3.0 x [10.sup.-10]          0.84
  [O.sub.4]
Epoxy-Mn[Al.sub.2]         235.0      9.0 x [10.sup.-8]           0.89
  [O.sub.4]

Sample                  [R.sub.ct]              [CPE.sub.dl]
                          (k[ohm]
                        [cm.sup.2])   [Y.sub.0] ([micro]           n
                            (d)       [[ohm].sup.-1][cm.sup.-2]
                                      [s.sup.n])

Blank Epoxy (7 days)        --        --                          --
Blank Epoxy (40 days)       1.1       4.0 x [10.sup.-6]           0.85
Epoxy-[Al.sub.2]            --        --                          --
  [O.sub.3]
Epoxy-[Al.sub.2]            --        --                          --
  [O.sub.3]
Epoxy-Mn[Al.sub.2]          --        --                          --
  [O.sub.4]
Epoxy-Mn[Al.sub.2]          --        --                          --
  [O.sub.4]

(a) The standard deviation range for [R.sub.c] values is between 3%
and 7.0%

(b) The standard deviation range for [Y.sub.0] values is between 1.5%
and 5.0%

(c) The standard deviation range for n values is between 0.6% and 1.8%

(d) The Standard deviation range for [R.sub.ct] values is between 2%
and 4%

Table 6: Adhesion strength values obtained from
pull-off test (recovery adhesion was obtained after
40 days immersion in 3.5% w/w NaCI solution) for the
epoxy coatings without and with Mn[Al.sub.2][O.sub.4] and [Al.sub.2]
[O.sub.3] nanopigments

                              Dry pull-off      Recovery adhesion
                             strength (MPa)      strength (MPa)

Blank epoxy                 4.5 [+ or -] 0.35   2.4 [+ or -] 0.4
  coating
Epoxy-[Al.sub.2][O.sub.3]   4.2 [+ or -] 0.25   3.0 [+ or -] 0.30
Epoxy-                      4.3 [+ or -] 0.30   3.7 [+ or -] 0.28
  Mn[Al.sub.2][O.sub.4]
COPYRIGHT 2016 American Coatings Association, Inc.
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 2016 Gale, Cengage Learning. All rights reserved.

Article Details
Printer friendly Cite/link Email Feedback
Author:Ghasemi, E.; Ramezanzadeh, B.; Saket, S.; Ashhari, S.
Publication:Journal of Coatings Technology and Research
Geographic Code:1USA
Date:Jan 1, 2016
Words:7970
Previous Article:Ultrasonic coating of nanofibrons webs: a feasible approach to photocatalytic water filters.
Next Article:Dip-coating for dodecylphosphonic acid derivatization on aluminum surfaces: an easy approach to superhydrophobicity.
Topics:

Terms of use | Copyright © 2017 Farlex, Inc. | Feedback | For webmasters