Corrosion Resistance and Durability of Superhydrophobic Surface

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Corrosion Resistance and Durability of Superhydrophobic Surface Formed on Magnesium Alloy Coated with Nanostructured Cerium Oxide Film and Fluoroalkylsilane Molecules in Corrosive NaCl Aqueous Solution Takahiro Ishizaki,* Yoshitake Masuda, and Michiru Sakamoto National Institute of Advanced Industrial Science and Technology (AIST), Materials Research Institute for Sustainable Development, 2266-98, Anagahora, Shimo-Shidami, Moriyama-ku, Nagoya 463-8560, Japan

bS Supporting Information ABSTRACT: The corrosion resistant performance and durability of the superhydrophobic surface on magnesium alloy coated with nanostructured cerium oxide film and fluoroalkylsilane molecules in corrosive NaCl aqueous solution were investigated using electrochemical and contact angle measurements. The durability of the superhydrophobic surface in corrosive 5 wt % NaCl aqueous solution was elucidated. The corrosion resistant performance of the superhydrophobic surface formed on magnesium alloy was estimated by electrochemical impedance spectroscopy (EIS) measurements. The EIS measurements and appropriate equivalent circuit models revealed that the superhydrophobic surface considerably improved the corrosion resistant performance of magnesium alloy AZ31. American Society for Testing and Materials (ASTM) standard D 335902 cross cut tape test was performed to investigate the adhesion of the superhydrophobic film to the magnesium alloy surface. The corrosion formation mechanism of the superhydrophobic surface formed on the magnesium alloy was also proposed.

1. INTRODUCTION A superhydrophobic surface has a water contact angle of more than 150°.14 In nature, there are many living things with superhydrophobic surfaces, and a lotus leaf is a representative example of the superhydrophobic surface.5,6 The lotus leaf has a wax-like component and surface roughness at micro- and nanoscales. The wax-like component provides hydrophobicity to the lotus leaf surface due to the low surface energy, and the surface roughness enhances the hydrophobic degree. These two factors, i.e., low surface energy and surface roughness, create the superhydrophobic property on the surface. These results obtained from the natural world provide a guide for constructing artificial superhydrophobic surfaces and designing surfaces with controllable wettability.7,8 Superhydrophobic surfaces have been artificially created on various material surfaces such as polymer,9 metal oxides,10,11 and metals1214 and are of special interest in both academic and industrial fields due to the their self-cleaning5,1517 and antisticking1821 properties. Recently, superhydrophobic treatments have been applied to various engineering material surfaces such as steel, copper, zinc, and aluminum, to improve their corrosion performances,2227 r 2011 American Chemical Society

because they could present a solution to the long-standing problems of environmental contamination and corrosion of metals and metal alloys.2830 Liu and co-workers developed a simple method to fabricate a superhydrophobic surface on copper substrate with n-tetradecanoic acid27 and revealed the corrosion resistant performance in seawater by electrochemical measurements. Shen and Liu reported relatively easy chemical etching methods to fabricate superhydrophobic surface on polycrystalline metal and on aluminum and its alloy,23,31 respectively. He et al. fabricated a superhydrophobic surface on anodized aluminum by modifying myristic acid and estimated the corrosion resistant performance by electrochemical impedance spectroscopy.25 On the basis of the electrochemical measurement results, they concluded that the superhydrophobic surface greatly improved the corrosion resistance of aluminum. In this way, a superhydrophobic surface has been shown to be effective for improving the corrosion resistance of engineering materials.25,27 Received: January 21, 2011 Revised: February 28, 2011 Published: March 18, 2011 4780

dx.doi.org/10.1021/la2002783 | Langmuir 2011, 27, 4780–4788

Langmuir Magnesium and magnesium alloys show superior physical and mechanical properties such as high stiffness/weight ratios, good castability, good vibration and shock adsorption, and high damping capacity.3236 The magnesium alloys are, thus, expected to be an excellent material for reducing vehicle weight, lowering fuel consumption, and reducing CO2 emission;37 however, an extremely low corrosion resistant property of the magnesium alloys restricts larger scale use toward the various applications. The contact of magnesium alloys with water induces the corrosion reaction of magnesium alloys. To depress the progress of corrosion, it is effective to suppress the contact of magnesium alloys with water. A superhydrophobic treatment could be a promising technology for improving anticorrosion performance because it could inhibit the contact of a surface with water and environmental humidity due to the superhydrophobic property. However, there are few reports on the fabrication of a superhydrophobic surface on magnesium alloy. Liang et al. first demonstrated the fabrication of a superhydrophobic surface on magnesium alloy by microarc oxidation pretreatment followed by chemical modification.38 However, they did not estimate the corrosion resistance and chemical stability in corrosive medium. Jiang et al. also reported the fabrication of a bioinspired superhydrophobic surface on MgLi alloy by chemical etching, followed by fluoroalkylsilane (FAS) modification.39 They estimated the corrosion resistant performance by examining the relationship between the time of exposure to air and the change in static water contact angles on the superhydrophobic surface of the MgLi alloy. The surface showed stable superhydrophobic properties for over three months at atmospheric condition. However, the surface was not exposed to a corrosive environment such as a NaCl aqueous solution. The NaCl solution has been used as an aqueous solution to promote corrosive reaction. Thus, the corrosion resistance and durability of the superhydrophobic surface have not yet been revealed. Recently, we successfully fabricated a superhydrophobic surface on magnesium alloy AZ31 by a simple immersion process at room temperature in a short time using nanostructured cerium oxide and FAS molecules.40 However, the corrosion resistance and durability of the superhydrophobic surface on magnesium alloy AZ31 in corrosive NaCl aqueous solution has not yet been investigated. In particular, the behaviors for desorption of the FAS molecules and degradation of the nanostructured cerium oxide film in corrosive NaCl aqueous solution are unknown. In general, the NaCl aqueous solution is used as a corrosive medium. Thus, it is very important to reveal the durability of the superhydrophobic surface to the corrosive NaCl aqueous solution because the effect of the superhydrophobic surface against the corrosion resistance is decreased by half if the durability of the superhydrophobic surface would be low. In this paper, we report the corrosion resistant performance and durability of the superhydrophobic surface on magnesium alloy coated with nanostructured cerium oxide film and fluoroalkylsilane molecules in corrosive NaCl solution using electrochemical and contact angle measurements. In addition, the behaviors for desorption of the FAS molecules and degradation of the nanostructured cerium oxide film in corrosive NaCl aqueous solution are also investigated.

2. EXPERIMENTAL PROCEDURES Magnesium alloy AZ31 (composition: 2.98% Al, 0.88% Zn, 0.38% Mn, 0.0135% Si, 0.001% Cu, 0.002% Ni, 0.0027% Fe, and the rest is Mg)

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with a thickness of 1.5 mm was used as the substrate. The substrates were ultrasonically cleaned in absolute ethanol for 10 min. After cleaning, the substrates were dried with inert Ar gas. The cleaned magnesium alloy substrates were immersed in 50 mL of distilled water containing 1.086 g of cerium nitrate hexahydrate (Ce(NO3)3 3 6H2O). The pH of the solution was measured to be ca. 4.5 at room temperature. The solution was agitated with a magnetic stirrer at a rotation of 100 rpm during the immersion for 20 min. After immersion, the samples were thoroughly washed in absolute ethanol. Finally, the washed samples were immersed in 40 mL of toluene solution containing 400 μL of FAS (CF3(CF2)7CH2CH2Si(OCH3)3) and 40 μL of tetrakis(trimethylsiloxy)titanium ((CH3)3SiO)4Ti) (TTST) for 30 min. The TTST molecules were used as an accelerant to promote hydrolysis and/ or polymerization of FAS molecules. The samples were then cleaned ultrasonically in absolute ethanol for 10 min using an ultrasonic cleaner with a frequency of 50 kHz. The surface morphologies of the obtained samples before and after immersion in 5 wt % NaCl aqueous solution were observed by field emission scanning electron microscopy (FE-SEM; S-4300, Hitachi High-Technologies Corp.). The crystal structure of the samples was examined by XRD (Rigaku, RINT2200 V). The XRD data were recorded on a powder diffractometer with Cu KR radiation (40 kV, 40 mA) within the range 2090° at a scanning rate of 2θ = 4° (min1). Static water contact angles of the fabricated surfaces were estimated with a contact angle meter (Kyowa Interface Science, DM-501) based on the sessile drop measuring method with a water drop volume of 2 μL. The dynamic water contact angles of the superhydrophobic surfaces were determined using ultrapure water that was added and withdrawn from the drop, respectively. All the measurements on the wetting properties were conducted in air at 298 K. Durability of the superhydrophobic surfaces in 5 wt % NaCl aqueous solution was estimated by investigating the relationship between changes in static water contact angles and time evolution after immersing the samples in the solution. The static water contact angles of the sample surfaces were measured at different five points after the sample was immersed in the NaCl aqueous solution for a constant time. After the measurements, the samples were rinsed with ultrapure water and then immersed in the solution. These procedures were repeatedly performed. The XPS measurements were performed using Mg KR (1253.6 eV) radiation at 12 mA and 10 kV at a takeoff angle of 90°. All binding energies in the spectra were corrected using standard binding energy of a C 1s peak (284.6 eV) as reference.41 All electrochemical measurements were performed in 5.0 wt % NaCl aqueous solution, pH 6.2, at room temperature, using a computercontrolled potentiostat (Princeton Applied Research, VersaSTAT3) under open circuit conditions. The superhydrophobic surface formed on magnesium alloy AZ31 and a platinum plate were used as the working and counter electrodes, respectively. The working electrode was set in a homemade holder made of Teflon with a circular window whose area was 0.785 cm2. The details of the homemade holder were described elsewhere.42 A saturated calomel electrode (SCE) was used as the reference electrode. The reference electrode was set in the vicinity of the circular window. The superhydrophobic surface formed on AZ31 substrate was immersed in the NaCl solution for 30 to 180 min, allowing the system to be stabilized, and potentiodynamic polarization curves were subsequently measured with respect to the OCP at a scanning rate of 0.5 mV/s from 400 to þ800 mV. Electrochemical impedance spectroscopic (EIS) measurements were conducted in the frequency ranges between 0.1 Hz and 100 kHz, with a sinusoidal signal perturbation of 10 mV and eight points per decade. All the samples were immersed for 30 min before all the impedance measurements. The experimental EIS spectra were interpreted based on equivalent electrical analogues using the program Zplot 2.0 to obtain the fitting parameters. Adhesion testing of the superhydrophobic film to the magnesium alloy was performed according to ASTM D 3359B-02. The cutting tool 4781

dx.doi.org/10.1021/la2002783 |Langmuir 2011, 27, 4780–4788

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Figure 1. FE-SEM image of the sample surfaces after immersing in cerium nitrate aqueous solutions at room temperature for 20 min followed by FAS modification. The inset shows water drop behavior on the superhydrophobic surface. was fitted with a blade containing 5 teeth spaced 1.0 mm apart. The cutting tool was used to make the cross-cut pattern at ca. 90° angles through the coating. LA-26 tape was applied to the cut surface and rubbed with the eraser end of pencil to ensure good contact with the film, and then removed after 90 s.

3. RESULTS AND DISCUSSION Fabrication of the Superhydrophobic Surface on Magnesium Alloy. In previous study,40 the XRD profile revealed that

the immersion of the magnesium alloy in the cerium nitrate hexahydrate aqueous solution induces the formation of cerium oxide, and all the films fabricated in this study exhibited XRD profiles similar to those in a previous report.40 As shown in Figure 1a, the broad five peaks at approximately 2θ = 28.5°, 33.1°, 47.5°, 56.3°, and 76.7° were assigned to the 111, 200, 220, 311, and 331 diffraction peaks of cerium oxide with the cubic phase based on the JCPDS data (card no. 431002). Other sharp peaks were attributed to magnesium alloy substrate. Figure 1b shows an FESEM image of nanostructured cerium oxide covered with FAS molecules. As clearly seen in Figure 1b, the surface has nanosheets aligned at a fairly steep angle relative to the surface. The nanosheets exhibited an edge length in the range of approximately 200 nm to 1 μm and a thickness of 20 to 50 nm. Inset of Figure 1b shows water drop behavior on the sample surface. The advancing and receding water contact angles of the sample surface were estimated to be 153 ( 2° and 146 ( 2°, respectively. The contact angle hysteresis was 7°, evidencing that the surface shows the superhydrophobic property. Chemical Properties of the Superhydrophobic Surface after Immersion in 5 wt % NaCl Aqueous Solution. Figure 2 shows FESEM images of the superhydrophobic surfaces (a) before and after immersion in 5 wt % NaCl solution for (b) 30, (c) 180, and (d) 1440 min. As clearly seen in Figure 2ac, no change in the surface morphologies before and after immersion in 5 wt % NaCl aqueous solution within 180 min can be observed, although several cracks are formed on all the sample surfaces. Even after immersion for 360 min, we cannot observe the change in the surface morphology (not shown here). In contrast, the immersion of the superhydrophobic surface in 5 wt % NaCl aqueous solution for 1440 min induces the significant change in the surface morphology shown in Figure 2d. The sample surface after immersion for 1440 min has granular and porous structures. In a previous report,40 we confirmed that the nanostructured cerium oxide film was composed of a three-layered structure with a slightly porous layer as an under layer intimately in contact with the magnesium alloy substrate and a compact layer as an intermediate layer, and a fibrous layer as the major top layer.

Figure 2. FE-SEM images of the sample surfaces (a) before and after immersing in 5 wt % NaCl aqueous solution for (b) 30, (c) 180, and (d) 1440 min.

Thus, the slightly porous layer could appear on the surface due to the partial detachment of the intermediate and top layers by the attack of the corrosive medium for a long time. The wettability properties of the superhydrophobic surface after immersion in 5 wt % NaCl aqueous solution were estimated using static water contact angle measurement. Figure 3 shows the changes in water 4782


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Figure 3. Changes in static water contact angles of the sample surfaces as a function of immersion time in 5 wt % NaCl aqueous solution.

contact angles as a function of immersion time in 5 wt % NaCl aqueous solution. All the plots in Figure 3 show the averaged values of water contact angles measured at different five points on the same sample. The error bars for each plot mean maximum and minimum water contact angles on the same sample. The averaged static water contact angles of the superhydrophobic surfaces gradually decreased with time evolution. This means that the density of the hydrophobic functional groups on the surface could be decreased by immersing the samples in the 5 wt % NaCl aqueous solution. The static water contact angles of the superhydrophobic surface after immersion in the 5 wt % NaCl aqueous solution within 360 min decreased to be in the ranges of ca. 136 ( 2° to 152 ( 2°. The differences in water contact angles between maximum and minimum values are relatively small, and the averaged static water contact angles after the immersion for 60 to 360 min hardly changed. This suggests that the immersion in the 5 wt % NaCl aqueous solution induces no change in the surface states from the wettability point of view. On the other hand, the averaged static water contact angles after immersion in the 5 wt % NaCl aqueous solution for 1440 min decreased considerably. The averaged static water contact angles of the superhydrophobic surfaces after immersion in the 5 wt % NaCl aqueous solution for 24 h were estimated to be 50 ( 2°. In addition, the differences in water contact angles between maximum (133 ( 2°) and minimum (25 ( 2°) values are more than 100° and very large in spite of the same sample surface. The values of the water contact angles on most parts of the sample surface were in the range 2535°. The area having high hydrophobicity, that is, the maximum water contact angle, was only a small part of the sample surface. This could indicate that on most parts of the sample surface desorption of the FAS molecules and degradation of the nanostructure occurred. The maximum water contact angle is almost the same as the averaged water contact angles after immersion in the 5 wt % NaCl aqueous solution within 360 min. From these results, it is speculated that the corrosion reaction of the superhydrophobic surface formed on magnesium alloy coated with nanostructured cerium oxide in 5 wt % NaCl aqueous solution could advance significantly between 360 and 1440 min. To investigate the surface states of the superhydrophobic surface for the immersion in 5 wt % NaCl aqueous solution within 360 min, surface observation was performed. Figure 4b,e shows FESEM images of the superhydrophobic surface after

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Figure 4. FE-SEM images of the sample surfaces after immersing in 5 wt % NaCl aqueous solution for (b) 180 and (e) 360 min. (a) Enlarged version of (b). (c) FESEM image of the corrosion product formed on the sample surface after immersion in 5 wt % NaCl aqueous solution for 180 min. (d) Enlarged version of (e). (f) FESEM image of the corrosion product formed on the sample surface after immersion in 5 wt % NaCl aqueous solution for 360 min.

immersion in 5 wt % NaCl aqueous solution for 180 and 360 min, respectively. On these two surfaces, relatively smooth parts and some granular products can be observed. The enlarged version of FESEM images for the smooth parts (Figure 4a,c) revealed that many nanosheets aligned at a fairly steep angle relative to the surface were densely formed. The surface morphologies after immersion in 5 wt % NaCl aqueous solution for 180 and 360 min were almost the same as that before immersion. On the other hand, the enlarged version of FESEM images (Figure 4c,f) of the granular products revealed that the products had the radiated needle-like structures and formed from the cracks. In addition, the formation amounts of the granular products on the sample surface increased with an increase in immersion time. The surface states were probed using XPS measurements. XPS Mg 2p spectra of the sample surface before and after immersion in 5 wt % NaCl aqueous solution are shown in Figure 5. Before the immersion, no peak originating from Mg can be observed in the spectrum because the Mg alloy surface was covered with CeO2 nanostructured film (film thickness: ca. 2 μm) and FAS molecules. On the other hand, for the sample surface after the immersion, a peak which is deconvoluted into two components corresponding to Mg(OH)2 and MgO can be clearly observed in the spectra.43,44 These components could be attributed to the formation of the corrosion products. The corrosion reaction could occur via the following reaction:45,46 2Mg f 2Mgþ þ 2e-

ð1Þ

2Mgþ þ 2H2 O f 2Mg2þ þ 2OH- þ H2

ð2Þ

2H2 O þ 2e- f H2 þ 2OH-

ð3Þ

2Mg2þ þ 4OH- f 2MgðOHÞ2

ð4Þ

MgO þ H2 O T MgðOHÞ2

ð5Þ

The corrosive NaCl aqueous solution could penetrate into the film through the cracks and reach the Mg alloy surface because the nanostructured CeO2 film formed on the Mg alloy had many cracks in the film. The contact of the corrosive solution with Mg 4783


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Figure 5. XPS Mg 2p spectra of the sample surfaces (a) before and after the immersion in 5 wt % NaCl aqueous solution for (b) 10, (c) 180, (d) 360, and (e) 1440 min.

Figure 6. XPS C 1s spectra of the sample surfaces (a) before and after the immersion in 5 wt % NaCl aqueous solution for (b) 5, (c) 60, (d) 180, and (e) 360 min.

ally induces the corrosion reaction. Thus, the corrosion products could come from the cracks as shown in Figure 4. These results support that the granular products are corrosion products, that is, Mg(OH)2. However, the occupied ratio of the corrosion products on the sample surface after immersion in 5 wt % NaCl aqueous solution within 360 min is very low as shown in Figure 4ac. Most of the sample surface would be covered with FAS molecules. The adsorption states of the FAS molecules on nanostructured CeO2 film were also confirmed using XPS measurements. Figure 6 shows XPS C 1s spectra of the sample surfaces before and after the immersion in the NaCl aqueous solution. Before the immersion, the sample surface clearly has three peaks that are deconvoluted into six components. A peak at a binding energy of 284.6 eV can be attributed unambiguously to CC/CH functional groups that arise from the CH2 functional groups and partly from carbon contamination due to ambient exposrure. The binding energy of the peak component from 286 to 290 eV can be deconvoluted into three components corresponding to CO, CdO, and CF2CH2 groups.

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Figure 7. Potentiodynamic curves of (a) untreated magnesium alloy, and superhydrophobic surface formed on magnesium alloy after immersion in 5 wt % NaCl aqueous solution for (b) 30, (c) 60, and (d) 180 min. The scanning rate was 0.5 mV/s.

Another peak at around 294 eV could be decomposed into two components corresponding to CF2CF2 and CF3 groups. The XPS C1s spectra including deconvolution curves for the sample before immersion in NaCl aqueous solution are shown in Supporting Information. After the immersion of the samples in the NaCl aqueous solution, some remarkable changes in XPS C1s spectra can be clearly observed. The peak at around 294 eV was decreased significantly and disappeared after the immersion within 60 min. In addition, subsequent changes in the CF2CH2 and CF2CF2 regions can also be observed after the immersion within 180 min. These results suggest that the desorption of the FAS molecules adsorbed on the nanostructured CeO2 film could occur due to corrosion reaction based on the attack of the corrosive solution. Due to the FAS desorption, the water contact angles could decreased gradually with an increase in the immersion time as shown in Figure 3. The corrosion products come from the cracks as shown in Figure 4. Thus, with the corrosion reaction, the nanostructured CeO2 film might be peeled off as the origin at the cracks. However, the XPS spectra and water contact angle measurements revealed that the FAS molecules would still be present at the sample surfaces because the surfaces exhibited high hydrophobic properties. When the immersion time was prolonged to 360 min, there is little change in the spectra. Thus, the sample surfaces after immersion in 5 wt % NaCl aqueous solution within 360 min showed high hydrophobic properties as shown in Figure 3. However, the further increase in immersion time induces significant changes in the sample surface as clearly seen in Figure 2d. Moreover, the hydrophobicity of the sample surface was lowered considerably. Corrosion Resistant Performance of the Superhydrophobic Surface on Magnesium Alloy. We investigated the corrosion resistance of the superhydrophobic surface in 5 wt % NaCl aqueous solution from the electrochemical points of view. Figure 7 shows potentiodynamic polarization curves of (a) untreated magnesium alloy after immersion in 5 wt % NaCl aqueous solution for 30 min and superhydrophobic surface formed on the magnesium alloy after immersion in 5 wt % NaCl aqueous solution for (b) 30, (c) 60, (d) 180, and (e) 1440 min. As compared to the corrosion current density (jcorr) of the untreated magnesium alloy (9.25 105 A/cm2), that of the 4784


Langmuir superhydrophobic surface formed on the magnesium alloy (2.17 107 A/cm2) decreased by more than 1 order of magnitude. The jcorr values of the superhydrophobic surface formed on the magnesium alloy after immersion in the NaCl aqueous solution for 60, 180, and 1440 min were estimated to be 4.79 108, 6.89 108, and 2.21 107 A/cm2, respectively. It should be noted that all the jcorr values for the samples after immersion in the NaCl aqueous solution for 60 to 1440 min were much lower than that of untreated magnesium alloy. This supports the conclusion that our superhydrophobic treatment is effective for improving the corrosion resistance. The corrosion potential (Ecorr) of the AZ31 was ca. 1510 mV. Hydrogen evolution dominates at more negative potentials than Ecorr, resulting in an increase in the cathodic currents. At more positive potentials than Ecorr, magnesium oxidation prevails and the metal is continuously dissolved as a result of the absence of a passivation layer, which is not formed under these conditions of high chloride concentration. On the other hand, Ecorr of the superhydrophobic surface is ca. 967 mV and is considerably shifted to the positive direction compared to that of untreated magnesium alloy. The significant shift in the Ecorr to the positive direction could be attributed to an improvement in the protective properties of the superhydrophobic surface formed on the magnesium alloy. At more positive potentials than Ecorr, the abrupt increase in the current density at the potentials of ca. 830 mV is clearly observed. This increase in the current density could be related to the pitting corrosion, showing that the electrolyte permeated through the film and consequently initiated pitting corrosion. This might be due to the permeation of the electrolyte through the crack in the film. A similar behavior has been reported even in aluminum alloys.47 The Ecorr moved in the negative direction and the jcorr decreased with an increase in the immersion time of the superhydrophobic surface formed on the magnesium alloy. On the other hand, the passive region became wide. The presence of passive region suggests that the superhydrophobic surface exhibits protective properties in a solution containing Cl ions. In the case of the sample after immersion for 1440 min, no increase in the current density at more positive potentials than the Ecorr, that is, passive region, can be observed in the curves, and the current density values were almost kept constant at more positive potentials than ca. 1050 mV. Judging from the results of the water contact angle measurements, XPS data, and FE-SEM observation, the appearance of the passive region might be mainly due to the passive hydroxide film, that is, Mg(OH)2 formed on the magnesium alloy. To probe the corrosion mechanism and quantify the corrosion resistant performance of the super hydrophobic surface, EIS studies were performed. Figure 8a presents the evolution of the impedance spectra of the superhydrophobic surface formed on magnesium alloy after different times of immersion in 5 wt % NaCl solution. Figure 8b shows the enlarged impedance spectra. The impedance spectra of the untreated magnesium alloy and superhydrophobic surface after immersion in 5 wt % NaCl solution for more than 360 min had capacitive loops at high and medium frequencies and a tail at low frequencies. The capacitive loops can be attributed to the charge transfer of the corrosion process and the tail might be associated with a diffusion process across the corrosion layer.48 On the other hand, when the immersion time is less than 180 min, the two capacitive loops at high and medium to low frequencies are observed as shown in Figure 8a. The first loop from high to medium frequencies regions can be attributed to a protective surface film of FAS

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Figure 8. (a) Evolution of Nyquist plots of the untreated magnesium alloy and the superhydrophobic surface formed on magnesium alloy AZ31after immersion in 5 wt % NaCl aqueous solution for 30, 60, 180, 360, and 1440 min. (b) Enlarged impedance spectra.

Figure 9. Equivalent circuit models of the studied system for (a) untreated magnesium alloy, (b) superhydrophobic surface after immersion in 5 wt % NaCl aqueous solution within 360 min, and (c) superhydrophobic surface after immersion in 5 wt % NaCl aqueous solution for 1440 min.

molecules and/or nanostructured cerium oxide, and the double layer capacitance at the electrode surface, while the second one from medium to low frequencies might be attributed to the charge transfer (corrosion) resistance. The capacitive loops decreased gradually with an increase in the immersion time. This indicates that the anticorrosion performance of the superhydrophobic surface formed on magnesium alloy is reduced gradually with immersion time. The lowering of the anticorrosion performance could be due to the localized corrosion. This is in good agreement with the observed results of FESEM images and potentiodynamic polarization curves. To enable an accurate analysis of the impedance data, the equivalence circuit models were proposed. The surface states of the untreated and superhydrophobically treated magnesium alloy are greatly different from the physicochemical point of view, so two different models are used to fit the impedance spectra. The two equivalent circuit models are shown in Figure 9. Figure 9a shows the equivalent circuit model representing the electrochemical behavior of the untreated AZ31 surface, which shows one time constant. In this circuit, Rct means the charge transfer resistance, CPEdl is the constant phase element of the electrical double layer, and Rs is the solution resistance. The Rct value is an indicative of total corrosion resistance performance. The constant phase element (CPE) is often used as a substitute for the capacitor in the equivalent circuit to fit the impedance behavior of the electrical double layer more accurately. In the case of the superhydrophobic surface, the equivalent circuit model should have two time 4785


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Table 1. Electrochemical Model Impedance Parameters from Nyquist Plots CPEdl samples

Rct (kΩ cm2)

Y0(μF/cm2 Hz1-n)

n

Rs (Ω cm2)

Rc (kΩ cm2)

Untreated magnesium alloy

1.4

8.51

0.93

54.8

—

Superhydrophobic surface after immersion for 30 min

4067

0.50

0.47

671

143


1188

0.52

0.47

374

177


885

0.66

0.56

303

49


75

1.67

0.87

7.9

1.3


7.2

0.78

0.92

1101

—

Figure 11. Evolution of Bode plots of the untreated magnesium alloy and superhydrophobic surface after immersion in 5 wt % NaCl for different time.

Figure 10. Rct values obtained from the fitting results as a function of immersion time in 5 wt % NaCl aqueous solution.

constants in the corresponding impedance spectra, since our fabricated superhydrophobic surface has a rough surface with many minute pores that could trap air at the solidliquid interface. Cc would normally be assigned to the capacitance of a surface film, which is based on various factors such as film thickness and defect structure. The Rct||Cdl elements in Figure 9b show the impedance with the interface reaction between the film and substrate. The parallel combination of Rc and Cc represents impedance with the interface reaction between the electrolytic solution and the film. Rair and Cair are typically associated with the resistance and capacitance of air within a minute pore, respectively. The Rair||Cair elements were arranged in parallel to the above-mentioned two elements with considering that the many minute pores would be filled with air. By applying this equivalence circuit model in Figure 9b to the impedance spectra for superhydrophobic surface formed on magnesium alloy, better fitting results could be obtained. Moreover, we applied the equivalent circuit model as shown in Figure 9c to the superhydrophobic surface after immersion in 5 wt % NaCl aqueous solution for 1440 min. This model is often used in the case of a protection film formed on metal substrate.49 The reason we used the model is that the water contact angles on the superhydrophobic surface after immersion in 5 wt % NaCl aqueous solution for 1440 min show hydrophilic properties and the air layer formed between many minute pores does not exist. The representative fitting results are listed in Table 1. The Rct values

obtained from the fitting results as a function of immersion time in 5 wt % NaCl aqueous solution are shown in Figure 10. The Rct values decrease with an increase in the immersion time. In particular, the decrease in the Rct values of the immersion time between 180 and 360 min is noticeable. To investigate this reason, the bode plots are shown in Figure 11, because the phase angle, j, is a sensitive parameter for indicating the presence of additional time constants in the impedance spectra and the whole impedance data are presented explicitly. Figure 11 shows the bode plots of the untreated and superhydrophobic treated magnesium alloy. One well-defined time constant can be observed in the EIS spectra of untreated magnesium alloy at around 100 Hz. This relaxation process is associated with electrochemical activity of the untreated magnesium alloy surface immersed in the NaCl aqueous solution and can be ascribed to the capacitance of electrochemical double layer on the solid/electrolyte interface. The resistive response at low frequencies corresponds to the polarization resistance. The EIS spectra of the superhydrophobic surface after immersion in 5 wt % NaCl solution for more than 360 min show one phase maximum at medium frequency and exhibit similar behavior to untreated magnesium alloy, while those of the superhydrophobic surface after immersion in 5 wt % NaCl solution within 180 min have two phase maxima at relatively low and high frequencies (Figure 11b). The high frequency time constant, at around 105 Hz, is related to the superhydrophobic surface formed on the magnesium alloy. Another time constant at around 3 Hz can be ascribed to the capacitance of double layer and is shifted to lower frequency than the time constant of untreated magnesium alloy because of good barrier properties of the superhydrophobic surface which suppress the penetration of the NaCl aqueous solution to the magnesium alloy surface. In addition, the impedance behaviors of the superhydrophobic surface after immersion in 5 wt % NaCl solution within 180 min show a similar shape (Figure 11a). Such 4786


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Figure 12. Photographs of the superhydrophobic surface (a) before and (b) after the cross cut tape test.

a shape is related to porosity phenomena with paint layers.50 The impedance level decreases monotonically with an increase in the immersion time. In the low frequency region, a coincidence of the spectra is observed for the superhydrophobic surface after immersion in 5 wt % NaCl solution within 180 min. The impedance behaviors in the low frequency region depend on the properties of the protective layer. The high impedance level evidences that our superhydrophobic surface shows high corrosion resistance due to the superhydrophobic property if the immersion time is within 180 min. However, in the case where the immersion time is more than 360 min, different impedance behavior can be observed. The increase of immersion time causes different chemical and/or physical changes in the superhydrophobic surface formed on magnesium alloy. When the immersion time was 360 min, the formation of corrosion products increased as clearly seen in Figure4b. Due to the changes in the surface states, the |Z| values of the superhydrophobic surface gradually approach that of the untreated magnesium alloy with an increase in immersion time. However, the |Z| values at low frequency of the superhydrophobic surface formed on the magnesium alloy after immersion in 5 wt % NaCl aqueous solution for 24 h are estimated to be about 7.2 103 Ωcm2, 5 orders of magnitude higher than that of untreated magnesium alloy. This indicates that our superhydrophobic film retards the formation of the corrosion products considerably due to a synergistic effect of superhydrophobicity based on the FAS coating and protective property of the nanostructured cerium oxide film. Adhesion Performance of the Superhydrophobic Film on Magnesium Alloy. To quantify the adhesion of the superhydrophobic film to the magnesium alloy, ASTM standard D 335902 cross cut tape test was performed. Figure 12 shows a photograph of the superhydrophobic surface (a) before and (b) after the cross cut tape test. The superhydrophobic film showed good adhesion onto the magnesium alloy surface with no squares being removed after removal of the tapes. In addition, the water contact angles of the superhydrophobic surface before and after the cross cut tape test was almost same. This strongly proves that the FAS molecules attached covalently to the nanostructured cerium oxide film and the adhesion of our superhydrophobic film to the magnesium alloy surface is good. Corrosion Mechanism on Superhydrophobic Film Covered Magnesium Alloy. Finally, we propose the corrosion mechanism of the superhydrophobic surface formed on magnesium alloy coated with nanostructures cerium oxide. Figure 13 shows the schematic diagrams of the corrosion mechanism of the superhydrophobic surface formed on magnesium alloy. In step 1 corresponding to the superhydrophobic surface before immersion, the superhydrophobic surface had some cracking on the surface. At the beginning of the immersion in 5 wt % NaCl aqueous solution (step 2), the penetration of the solution through the cracks could occur due to a decrease of the hydrophobicity on the surface. By the penetration of the

Figure 13. Schematic diagrams of the corrosion mechanism of the superhydrophobic surface formed on magnesium alloy.

corrosive solution, the magnesium alloy surface is in contact with the corrosive solution, resulting in the formation of corrosion products (step 3). Longer immersion leads to partial breakdown of the protective layer, that is, superhydrophobic surface comprising nanostructured cerium oxide and FAS molecules due to the proceeding of localized corrosion attack (step 4). Even longer immersion induces the breakdown of the protective layer and the formation of many corrosion products, leading to the chemical and physical changes in surface states (step 5). The changes could alter the wetting properties of the surface from superhydrophobicity to superhydrophilicity, as shown in Figure 3. However, the corrosion resistance is much higher than that of untreated magnesium alloy. This might be not only because the parts of superhydrophobic surface that withstands the corrosive attack could act as a protective layer, but also because a passive hydroxide film, that is, Mg(OH)2, could be formed on the magnesium alloy.

4. CONCLUSIONS The corrosion resistant performance and durability of the superhydrophobic surface on magnesium alloy coated with nanostructured cerium oxide film in corrosive NaCl solution were investigated by electrochemical and contact angle measurements. FESEM images revealed that no change in the surface morphologies before and after immersion in 5 wt % NaCl aqueous solution within 180 min can be observed. In contrast, the immersion of the superhydrophobic surface in 5 wt % NaCl aqueous solution for 1440 min induces a significant change in the surface morphology. Water contact angle measurements revealed that the averaged static water contact angles of the superhydrophobic surfaces gradually decreased with an increase in immersion time in 5 wt % NaCl aqueous solution. The static water contact angles of the superhydrophobic surface after immersion in the 5 wt % NaCl aqueous solution within 360 min decreased to the ranges of ca. 136 ( 2° to 152 ( 2°. On the other hand, the average static water contact angles after immersion in the 5 wt % NaCl aqueous solution for 24 h decreased to 50 ( 2°. The anticorrosion resistance of the superhydrophobic surface formed on the magnesium alloy was estimated by potentiodynamic and EIS measurement. Electrochemical 4787


Langmuir potentiodynamic polarization curve measurements revealed that the superhydrophobic surface formed on the magnesium alloy exhibited a higher corrosive resistance than the untreated magnesium alloy. As compared to the corrosion current density of the untreated magnesium alloy, that of the superhydrophobically treated magnesium alloy decreased by more than 1 order of magnitude. The EIS measurement and appropriate equivalent circuit models revealed that the anticorrosion resistance was significantly improved by the superhydrophobic treatment. However, the magnesium surface was observed to have been corroded from the through-pores in the cracks. The corrosion mechanism of the superhydrophobic surface formed on magnesium alloy was also proposed. According to the ASTM standard D 3359-02 cross cut tape test, the adhesion of our superhydrophobic film to the magnesium alloy surface was very good. We believe that a superhydrophobic surface would be an effective strategy for improving the anticorrosion performance of various engineering materials.

’ ASSOCIATED CONTENT

bS

Supporting Information. Additional spectra. This material is available free of charge via the Internet at http://pubs.acs. org.

’ AUTHOR INFORMATION Corresponding Author

*Corresponding author. Tel.: þ81-52-736-7465. Fax: þ81-52736-7406. E-mail: [email protected].

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Corrosion Resistance and Durability of Superhydrophobic Surface

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