Rapid Fabrication of a Crystalline Myristic Acid-Based Superhydrophobic Film with Corrosion Resistance on Magnesium Alloys by the Facile One-Step Immersion Process.

A simple, easy, and rapid process of fabricating superhydrophobic surfaces on magnesium alloy AZ31 by a one-step immersion at room temperature was developed. The myristic acid-modified micro-/nanostructured surfaces showed static water contact angles over 150° and water contact angle hysteresis below 10°, thus illustrating superhydrophobic property. The shortest treatment time for obtaining the superhydrophobic surfaces was 30 s. In addition, we demonstrated for the first time that crystalline solid myristic acid could be formed on a Mg alloy using a suitable molar ratio of Ce ions and myristic acid. The contact angle hysteresis was lowered with an increase in the immersion time. Potentiodynamic polarization curve measurements revealed that the corrosion resistance of AZ31 treated by the immersion process improved considerably by the formation of superhydrophobic surfaces. The chemical durability of the superhydrophobic surfaces fabricated on AZ31 was also examined. The static water contact angle values for the superhydrophobic surfaces after immersion in aqueous solutions at pHs 4, 7, and 10 for 12 h were estimated to be 90 ± 2°, 119 ± 2°, and 138 ± 2°, respectively, demonstrating that their chemical durability in a basic solution was high.

Introduction

Many superhydrophobic plant leaves are found in natural world, such as those of lotus, rice, and taro plants. Superhydrophobic surfaces have a static water contact angle over 150° and a sliding angle below 10°. Such natural systems can guide the construction and design of artificial superhydrophobic surfaces. Studies on these leaves revealed that a superhydrophobic surface with a large contact angle requires the cooperation of hierarchical micro- and nanostructures.[1] The hierarchical structure requires to be covered by hydrophobic groups having a low surface energy, such as −CH3 or CF3 groups. The existence of hydrophobic functional groups on such hierarchical structures results in superhydrophobicity.[2−4] Fabrication methods and potential applications on the superhydrophobic surfaces are attracting much interest.[5−12] Thus, superhydrophobic surfaces have been artificially fabricated on various material surfaces such as polymers,[13] metal oxides,[14,15] and metals[16−18] and are of special interest in both scientific and industrial fields owing to their superior characteristics such as self-cleaning,[19−21] antisticking,[22−25] anti-icing,[26,27] antifogging,[28] and oil–water separation properties.[29,30]

Recently, the corrosion resistance performance of superhydrophobic coatings fabricated on various metals and alloys has attracted much attention[31−34] because information in this regard can provide a solution to the long-standing issues on corrosion of metals and metal alloys.[35−37] Thus, to improve the corrosion resistance of them, formation of superhydrophobic surfaces on various metal surfaces, such as steel, copper, zinc, and aluminum[38−41] by various methods such as the sol–gel process,[42] electrodeposition,[43] electrospinning,[44] hydrothermal techniques,[45] chemical vapor deposition,[46] and spray coating,[47] has been developed. Liu et al. reported the fabrication of a superhydrophobic surface on a copper substrate by a simple technique of immersion into methanol solution containing perfluorooctyltrichlorosilane [CF3(CF2)5(CH2)2SiCl3] and revealed that its corrosion resistance performance in 3 mass % NaCl aqueous solution was improved by the formation of a superhydrophobic coating.[37] Zhang et al. demonstrated that anticorrosive superhydrophobic surfaces were formed on an Al alloy substrate by a facile one-step electrochemical deposition and revealed that the corrosion resistance performance was improved considerably by the superhydrophobic coating.[48]

Magnesium (Mg) is one of the lightest engineering metals among the practically applied metals. Thus, Mg and its alloys can be applied in aerospace engineering, as well as in airplanes, trains, and automobiles.[49−52] The greatest advantage of Mg alloys is their light weight, which has enabled us to achieve energy saving in the abovementioned applications through the use of steel-based hybrid materials. In such transportation-related applications, Mg has more superior density and specific strength than those of aluminum (Al) and carbon fiber-reinforced polymer, but it shows a much lower corrosion resistance. Corrosion of Mg alloys occurs when they come into contact with water. Therefore, it is essential to prevent such alloys from contacting with aqueous environments to prevent the corrosion reaction. A superhydrophobic coating would enable Mg alloys to improve the corrosion resistance performance because it would hinder the contact of surfaces with aqueous environments.

Jiang et al. developed a method for introducing superhydrophobic surfaces on a Mg–Li alloy by combining the process of chemical etching to fabricate micronanoscale hierarchical structures and the immersion process using fluoroalkylsilane (FAS) molecules to impart low surface energy to the hierarchically structured surfaces.[53] The corrosion resistance of the Mg–Li alloy was improved greatly by the formation of a superhydrophobic surface. However, this is a multistep and time-consuming process that requires about 14 h for completion. In our previous work, a time-saving method for fabricating superhydrophobic surfaces on Mg alloy AZ31 was developed.[54] The procedure involved two-step immersion using cerium oxide, FAS molecules, and a catalyst[54] and required less than 1 h. Detailed electrochemical and immersion tests revealed that the corrosion resistance of the superhydrophobic Mg alloy was improved due to its superhydrophobicity and inhibitory effect of cerium oxide against corrosion.[55] However, from an industrial viewpoint, the treatment time should be further reduced. Myristic acid can be one of the candidate materials to impart hydrophobicity to alloy surfaces in short time because it has hydrophobic and carboxyl functional groups in the molecular framework and the carboxyl groups as the anchor group can bind to metal oxide surfaces for 1 h.[56] In addition, the cost for this process is high because of the use of the perfluoro reagent and the catalyst. Low-cost, easy, and high-efficiency procedures suitable for mass production are desirable for industrial applications. Thus, the development of an effective one-step fabrication method for the production of superhydrophobic surfaces without using the perfluoro molecule and the catalyst is highly desirable. By controlling the composition and concentration of the precursor solution, we can realize a metal coating having micronanoscale roughness and hydrophobicity, i.e., superhydrophobicity, in a short time. In addition, we demonstrated for the first time that crystalline solid myristic acid could be formed on a Mg alloy using a suitable molar ratio of Ce ions and myristic acid.

In this article, a rapid, simple, easy, and low-cost approach for fabricating superhydrophobic surfaces composed of CeO2 having an inhibitory effect against corrosion and crystalline solid myristic acid showing hydrophobicity on a Mg alloy at room temperature is discussed. The procedure is facile and does not require special equipment for operation. Moreover, it can be executed without heat treatment or using perfluoro molecules, and the shortest process time could be below 1 min. The corrosion resistance of superhydrophobically treated Mg alloy AZ31 was investigated. Moreover, the chemical durability of the superhydrophobic surfaces fabricated on the Mg alloy was demonstrated.

Results and Discussion

Figure shows the static water contact angles of the AZ31 surfaces after immersion for 180 s into the mixed solutions prepared with different volume ratios of solutions A and B. The sample (a180) fabricated from the solution containing only Ce(NO3)3 showed a static water contact angle of 80.7 ± 2°, demonstrating the hydrophilic property. On the other hand, when solution A was added to the solution, the static water contact angle increased. The static water contact angles tended to increase with an increase in the volume ratio of solution A in the mixed solution. When the myristic acid content in the mixed solution was 0.7, the static contact angle was found to be 156.2 ± 2° (characteristic of a superhydrophobic surface), the highest value among those for all samples. When the ratio of solution A to solution B was more than 0.8, the static water contact angles of the sample surfaces decreased gradually. The static water contact angle of the sample (k180) prepared from only solution A (myristic acid) was estimated to be 116.7 ± 2°, which was higher than a previously reported value for myristic acid on a smooth surface.[57] The pristine AZ31 surface was not smooth and showed some degree of roughness. According to the Wenzel equation,[1] the relationship between the surface roughness factor and the measured static water contact angle is described as followswhere r is the roughness factor, defined as the ratio between the real and projected contact areas at the solid–liquid interface, and θc and θ show the contact angles on the rough and smooth surfaces, respectively. The ideal water contact angle of the flat Si surface covered with a −CH3 group-terminated monolayer is in the range of 105–109°. Given that θc = 116.7° and θ =109°, r is found to be 1.38 using the Wenzel equation. When a water droplet was dropped on the AZ31 surface covered with the myristic molecules, the actual contact area at the solid–liquid interface was 1.38 times of the projected area. Thus, the static water contact angle of the sample (k180) surface is considered to be slightly higher than the previously reported value.

Figure 1

Static water contact angles of AZ31 surfaces after immersion for 180 s into mixed solutions prepared with different volume ratios of myristic acid–ethanol (solution A) and cerium nitrate aqueous solution (solution B). All plots show the averaged values of water contact angles measured at five different points on the same samples. The error bars for each plot indicate the maximum and minimum water contact angles.

Figure a–j shows scanning electron microscopy (SEM) images of the different sample surfaces. Figure a–j shows an enlarged version of Figure a–j. When the ratio of the volumes of solution A to solution B is in the range 0.1–0.4, as shown in Figure a–d, the surfaces are covered with films that have some cracks. These relatively large cracks divide the whole surface coating into pieces. Several small microcracks exist within each piece. These cracks in the coating film might have been formed by shrinkage in volume due to evaporation of water molecules contained in the coating film, thereby allowing cracks to form. The shrinkage could occur by introduction of the samples into the vacuum chamber of SEM. Figure SI1a,b shows SEM images of the film surfaces fabricated from an aqueous solution containing Ce(NO3)3·6H2O, that is, only solution B, by the immersion process. The film also had some cracks, and the film denseness was not high. On the other hand, when the volume ratio is 0.4–0.7, the surface morphologies change considerably. Many nanoplates formed at fairly tilted angles with respect to the surface are observed on the surfaces. The occurrence of vertically aligned nanoplates becomes predominant as the ratio increases up to 0.7. The nanoplates exhibited a thickness below 100 nm and an edge length in the range 200–1000 nm. Moreover, several individual crystals having a length over 1 μm were also observed. The agglomeration of the nanoplates could result in an irregular surface morphology with nanoscale spaces composed of concavity and convexity. Such surface morphology is better suited for fabricating a superhydrophobic surface. In the case of the volume ratio of solution A to solution B being 0.8 and 0.9, partial aggregation of the nanoplates was observed on the sample surfaces, as shown in Figures h and 3h. As shown in Figure h,i, the shapes of the nanoplates in the aggregations were almost same as those in Figure g. When the volume ratio was 1.0, i.e., when only myristic acid was used, no nanoplates were observed. The formation of several fine particles was observed on the surface. Figure shows the relationship between the surface roughness, Ra, of the sample and the volume ratio of solutions A and B. When the ratio was in the range 0.5–0.8, the surface roughness was found to be in the range 2.0–2.5 μm, which was relatively high compared to those for other samples. SEM observations suggested aggregation of nanoplates on their surfaces. Thus, the aggregation of nanoplates results in an increase of the surface roughness. From these results, it can be concluded that the volume ratio of solution A to solution B that is the most suitable for fabricating the superhydrophobic surface is 0.7, i.e., samples (h60) and (h180).

Figure 2

SEM images of the sample surfaces prepared by mixing different volume ratios of myristic acid–ethanol (solution A) and cerium nitrate aqueous solution (solution B): (a) 1:9, (b) 2:8, (c) 3:7, (d) 4:6, (e) 5:5, (f) 6:4, (g) 7:3, (h) 8:2, (i) 9:1, and (j) 10:0.

Figure 3

SEM images of the enlarged version of Figure : (a) 1:9, (b) 2:8, (c) 3:7, (d) 4:6, (e) 5:5, (f) 6:4, (g) 7:3, (h) 8:2, (i) 9:1, and (j) 10:0.

Figure 4

Change in surface roughness, Ra, for samples prepared by mixing different ratios of myristic acid–ethanol (solution A) and cerium nitrate aqueous solution (solution B).

To investigate the effect of the immersion time on the water contact angles on sample (h), some samples (h) were fabricated for different immersion times. Changes in the static water contact angles on samples (h) as a function of immersion time are shown in Figure a. The field-emission scanning electron microscopy (FESEM) images and topographic images of the sample surfaces for (h10), (h30), (h60), (h180), (h300), and (h600) are shown in Figures SI2 and SI3, respectively. When the immersion time was 10 s, the static water contact angle value for the surface of sample (h10) was found to be 147.5 ± 2°, characteristic of a highly hydrophobic surface. As shown in Figures SI2a and SI3a, the surface was rough and showed a root mean roughness, Rrms, of 158.8 nm. Such a rough structure can increase the hydrophobicity. When the immersion time was more than 30 s, the static water contact angle for each sample was over 150° and all surfaces became superhydrophobic. This means that the shortest immersion time required for obtaining the superhydrophobic surface is 30 s. As shown in Figures SI2b–f and SI3b–f, the roughness of the sample surfaces seemed to increase with an increase in the immersion time and hierarchical structures on the micro- and nanometer scale could be realized, leading to the fabrication of a superhydrophobic surface. The advancing (θA) and receding (θR) water contact angles of the samples (h) as a function of immersion time are shown in Figure b. The surfaces of all of the samples (h) showed advancing water contact angles of more than 150° but receding water contact angles in the range of 143–150°, depending on the immersion time. The surfaces treated for 5 and 20 min showed low contact angle hysteresis (<5°), although they were rough, indicating that the treated surfaces gained water-repellent properties. On the other hand, the surfaces treated for less than 3 min showed contact angle hysteresis in the range of 5–10°. One plausible cause of the increase in water contact angle hysteresis might be an increase of the surface roughness. The motion of the water drop on the rough surface could be pinned by the surface roughness. As mentioned above, the Wenzel model can interpret the relationship between the surface roughness and the water contact angle.[58] According to the Wenzel model, it can be considered that on the rough surface the free energy at the interface per unit area increases depending on the change in surface area due to the morphological changes; thus, the contact line between a rough surface and a liquid could increase. Consequently, the liquid can get completely in contact with the surface, wetting the rough surface completely. This leads to a large contact area between the liquid and the surface, thus resulting in pinning of the water droplet on the rough surface. Another cause could be that the surface could not be treated uniformly because of the short immersion time.

Figure 5

(a) Change in static water contact angles on AZ31 surfaces after immersion as a function of immersion time. (b) Advancing (θA) and receding (θR) water contact angles of the treated AZ31 surface as a function of immersion time.

The X-ray diffraction (XRD) pattern of sample (h) surface fabricated by immersion for 30 min is shown in Figure . The thickness of the superhydrophobic film prepared for 30 min was estimated to be in the range of 400–800 nm from the cross-sectional SEM image, as shown in Figure SI4. Some sharp and broad peaks are clearly observed. The sharp peaks are assigned to the Mg alloy substrate, whereas the broad peaks are attributable to myristic acid, with a monoclinic system; the peaks are indexed according to ICDD-PDF data (Card No. 00-008-0786). The several broad peaks at approximately 2θ = 6.6, 8.9, 11.3, 14.4, 18.6, 20.7, 22.1, and 22.9° were assigned to the 002, 003, 004, 005, 012, −113, 110, and −2010 reflections of myristic acid, with a monoclinic phase. These broad peaks corroborate the presence of nanosized grains on the surface. However, no peak originated from crystalline myristic acid could be detected in the XRD pattern of sample (a) surface prepared using only solution A, indicating that crystalline myristic acid could not be formed without Ce ions, as shown in Figure SI5a. This means that Ce ions or CeO2 would be required to fabricate crystalline solid myristic acid by the immersion process. Because of the existence of Ce ions or CeO2, myristic acid might be arranged orderly and three-dimensionally through the hydrogen bond between carboxyl groups and/or hydrophobic interaction between methyl groups. However, at present, the detailed formation mechanism is not clear. Further studies would be required to elucidate the formation mechanism of crystalline solid myristic acid. On the other hand, no peak attributable to CeO2 could be observed in the XRD pattern. According to our previous study,[54] a crystalline CeO2 film was formed on AZ31 by an immersion process at room temperature in an aqueous solution containing only Ce(NO3)3·6H2O. Figure SI5b shows the XRD pattern of the film fabricated from an aqueous solution at pH 2 containing only Ce(NO3)3·6H2O. Five peaks at 2θ ∼28.5, 33.0, 43.6, 56.3, and 68.4°, assigned to the 111, 200, 220, 311, and 400 reflections of cubic CeO2, can be clearly observed in the XRD pattern of Figure SI5b. This indicated that the crystalline CeO2 film was formed on AZ31 by immersion in an aqueous solution containing only Ce(NO3)3·6H2O, that is, using only solution B. However, no crystalline CeO2 film could be fabricated on AZ31 by immersion in the mixed solution of A and B at room temperature. An amorphous or very thin CeO2 film might be formed on AZ31. This difference in the crystallinity of the CeO2 film is possibly due to the presence of myristic acid in the solution.

Figure 6

XRD pattern of the surface treated for 20 min in the mixed solution at a volume ratio of 0.7.

To reveal the chemical composition of the superhydrophobic surface fabricated by the immersion process, X-ray photoelectron spectroscopy (XPS) measurements were conducted. Figure a–c shows the C 1s, O 1s, and Ce 3d XPS spectra of sample (h). The peak in the C 1s spectrum in Figure a can be deconvoluted into three peaks at 282.5, 285.0, and 288.5 eV. The peaks at 285.0 and 288.5 eV are attributed to −CH and O–C=O, respectively. Although details of the peak * at 282.5 eV are unknown, this peak might be related to the chemical bond between C and CeO. The O 1s spectrum of sample (h) is deconvoluted into four peaks corresponding to CeO2 at 528.3 eV, C=O at 530.5 eV, O–C=O at 531.5 eV, and O–C–OH at 532.4 eV. The two Ce 3d peaks contain nine components originating from the Ce oxidation states (Ce3+ and Ce4+) and their 4f configurations.[59] Characters u and v are related to Ce3+ and Ce4+, respectively. Liu et al. reported the possibility of the formation of cerium myristate (Ce(CH3(CH2)12COO)3) in an aqueous solution containing Ce ions and myristic acid.[60] Thus, the chemical bonding states of Ce4+ and Ce3+ can be attributed to the formation of CeO2 and cerium myristate, respectively.[60] The atomic compositions of C, O, Ce, and Mg obtained from XPS are estimated to be 82.9, 14.6, 1.9, and 0.6 atom %, respectively. These results indicate that the surface could be composed of myristic acid, cerium myristate, and CeO2 and the outermost surface is mainly composed of myristic acid with a low surface energy, which contributes to the fabricated superhydrophobic surface. The chemical bonding states of the superhydrophobic surface are further investigated by Fourier transform infrared (FT-IR) measurements. Figure SI6 shows the FT-IR spectrum of the superhydrophobic surface. The absorption peaks at approximately 2851 and 2916 cm–1 are attributed to C–H asymmetric and symmetric stretching vibrations in CH2 groups, respectively.[61] The weak peak at approximately 2955 cm–1 is assigned to asymmetric stretching vibrations in CH3 groups. The broad band in the range 3400–3000 cm–1 is attributable to the O–H stretching vibration in −COOH. The two weak peaks at approximately 1412 and 1659 cm–1 can be attributed to the O–H bending and C=O stretching modes of COOH dimers.[62] The two peaks at around 1526 and 1445 cm–1 can be assigned to cerium myristate.[63] The two peaks at approximately 1109 and 721 cm–1 are attributed to the C–H wagging and twisting vibrations in the CH2 groups.[62] The two weak peaks at around 542 and 881 cm–1 can be assigned to the Ce–O stretching mode.[64] The FT-IR spectrum also indicates that the surface could be composed of myristic acid, cerium myristate, and CeO2.

Figure 7

(a) C 1s, (b) O 1s, and (c) Ce 3d XPS spectra of the superhydrophobic AZ31 surface.

Surface wettability is known to be controlled by two factors, i.e., the surface energy and surface roughness.[65] The water contact angle of a smooth surface can reach a maximum value of approximately 120° by lowering the surface energy. On the other hand, on a rough surface with hierarchical structures, the static water contact angles can reach a maximum value of ∼178°,[66] indicating that the fabrication of micro-/nanostructures is essential for fabricating superhydrophobic surfaces.[67] The surfaces we fabricated by the immersion process had many micro-/nanostructures, as shown in Figures and 3. Such surfaces could trap a large fraction of air within the grooves fabricated between the nanoplates. The trapped air can prevent the water droplets from penetrating into the grooves of the surfaces. The superhydrophobic surface fabricated in this study had an advancing water contact angle of more than 150° and contact angle hysteresis of less than 10°. These results support the fact that the existence of the minute spaces created between nanoplates prevents water droplets from penetrating into the grooves of the surfaces. Therefore, the micro-/nanostructures composed of CeO2 and myristic acid can enhance surface hydrophobicity and decrease contact angle hysteresis by lowering the continuity of the three-phase contact line at the solid–liquid interface and the contact area between solid and liquid at their interface. It should be noted that the surface obtained on our sample after immersion for 30 s showed an advancing water contact angle over 150°, indicating that our process can impart superhydrophobicity to the Mg alloy surface in below 1 min.

The formation mechanism of the superhydrophobic film is as follows. When the AZ31 Mg alloy was immersed in an acidic solution at pH 2, Mg dissolved as Mg2+ via the following redox reactionsThrough eq , the AZ31 surface could be etched and the surface roughness could be increased. The formation of OH– could result in an increase in the local solution pH, leading to a critical value at which Ce(OH)3 could exist steadily, so that Ce3+ could deposit on the surface through reaction The deposited Ce(OH)3 could be oxidized by H2O2 and/or NO3– because H2O2 and/or NO3– can act as an oxidant. As a result, CeO2 could be formed through the following electrochemical reactionsThe formation of CeO2 film on AZ31 could also increase the surface roughness. In addition, CeO2 can act as an effective inhibitor.[68] The increase in the surface roughness due to the formation of a CeO2 film on AZ31 by immersion for less than 60 s in the aqueous solution containing only Ce(NO3)3 has been reported.[54] In addition, crystalline myristic acid was formed on the AZ31 surface, as shown in Figure . The formation of crystalline myristic acid on the rough CeO2 film fabricated on the AZ31 surface could further increase the surface roughness, resulting in the formation of a hierarchical micro-/nanostructured surface. Moreover, the presence of crystalline myristic acid on the outermost surface could induce the lowering of the surface energy. These synergic effects could contribute to the formation of the superhydrophobic surface.

The corrosion resistance of the superhydrophobic samples (h) fabricated by the immersion process for 1–30 min was investigated using potentiodynamic polarization curve measurements. The corrosion potential (Ecorr) and corrosion current density (icorr), which are important parameters for estimating the corrosion resistance, can be obtained from the polarization curves using Tafel extrapolation. In general, better corrosion resistance indicates a lower corrosion rate, corresponding to a lower icorr or a more positive Ecorr.[69] Polarization curves of the superhydrophobic samples (h) fabricated by immersion for 1–30 min and pristine AZ31 in an aqueous solution of 5 mass % NaCl are shown in Figure . The Ecorr value of pristine AZ31 was −1.49 V versus Ag/AgCl. On the other hand, Ecorr values for the superhydrophobic AZ31 samples treated for 1, 3, 10, 20, and 30 min are estimated to be approximately −1.48, −1.19, −1.38, −1.45, −0.61, and −0.43 V, respectively. Ecorr values of the superhydrophobic AZ31 samples treated for 1 and 10 min are almost the same as those of pristine AZ31, whereas the values of the samples treated for 3, 20, and 30 min are more positive than those of pristine AZ31. This indicates that the superhydrophobic AZ31 samples treated for 3, 20, and 30 min have a lower corrosion potential than that of pristine AZ31. The icorr of pristine AZ31 was found to be 3.46 × 10–5 A/cm2. In contrast, the icorr values of the superhydrophobic AZ31 samples treated for 1, 3, 10, 20, and 30 min were found to be 1.95 × 10–5, 1.21 × 10–5, 1.08 × 10–6, 4.71 × 10–7, and 4.41 × 10–7 A/cm2, respectively. The icorr values of the superhydrophobic AZ31 surfaces are lower than those of pristine AZ31. The increase in the immersion time leads to lower icorr values, indicating improvement of the corrosion resistance. The corrosion inhibition efficiency, ηp, of the superhydrophobic AZ31 samples treated for 1, 3, 10, 20, and 30 min was calculated from polarization measurements according to the relationship given belowwhere icorr(untreated AZ31) and icorr(treated AZ31) are the corrosion current densities for the untreated and treated AZ31, respectively. The ηp values of the superhydrophobic AZ31 samples treated for 1, 3, 10, 20, and 30 min were found to be 43.6, 65.0, 96.9, 98.6, and 98.7%, respectively. The icorr and ηp values of the superhydrophobic AZ31 surface fabricated by immersion for 30 min are the lowest and highest among those obtained for all samples (h), respectively, indicating that superhydrophobic AZ31 samples fabricated for 30 min show the best corrosion resistance among all of the samples tested. In addition, the superhydrophobic AZ31 sample shows a long passive region on the anodic branch with no pitting corrosion. The presence of this long passive region could be due to effective corrosion inhibition characteristics of CeO2 or the Ce ions.[69] Thus, the protective ability of the superhydrophobic AZ31 surface against the NaCl aqueous solution is considered to be high. The main reasons for the improvement of the corrosion resistance are as follows: One is the presence of CeO2 or Ce ions having effective corrosion inhibition characteristics in the film and the other is superhydrophobicity of the film. It has been reported that superhydrophobicity could be effective in minimizing or reducing the wetted area on a solid surface because of the presence of many roughness structures, such as grooves, which could trap air at the solid–liquid interface. The wetted area on a solid surface could be minimizing or reducing if an air layer could be present stably in the grooves on the superhydrophobic surface. Therefore, the corrosion rate of the superhydrophobic film containing CeO2 or Ce ions would then be much lower than that of the bare substrate in the corrosive NaCl solution.

Figure 8

Polarization curves of the superhydrophobic surfaces fabricated by immersion for 1–30 min and bare AZ31 in 5 mass % NaCl aqueous solution.

To utilize the superhydrophobic surface in industrial sectors, it is very useful to clarify the chemical durability of the superhydrophobic surface against acidic, neutral, or basic aqueous solutions.[54] The chemical durability of the superhydrophobic surface for sample (h) was investigated by measuring the static water contact angles of the superhydrophobic surfaces before and after immersion in aqueous solutions at different pH values for predetermined times. Figure shows the relationship between the static water contact angles of the sample (h1800) surface and the immersion times in aqueous solutions at pHs 4.0, 7.0, and 10.0. After immersion in the acidic solutions at pH 4.0, the static water contact angles of the superhydrophobic surfaces gradually decreased with an increase in the immersion time, indicating that the molecular density of myristic acid with the hydrophobic functional groups on the surface could be lowered by immersing the samples in an acidic solution. The static water contact angles decreased from 150 ± 2° to 113 ± 2° within 80 min after immersion in the acidic solution. However, the static water contact angles after immersion for 80–180 min were kept almost constant. Furthermore, when the immersion time was extended beyond 180 min, the static contact angles for water lowered to 90 ± 2°, as shown in Figure a. In the case of immersion in the neutral aqueous solution (pH 7.0), the static contact angles of the superhydrophobic surfaces gradually decreased to 130 ± 2° within 80 min. When the immersion time was prolonged to 720 min, the static contact angle of the superhydrophobic surface was 119 ± 2°, as shown in Figure b. In the case of immersion in the basic solution at pH 10, the static contact angles of the superhydrophobic surfaces slightly lowered to 140 ± 2° within 180 min. The static contact angle was estimated to be 138 ± 2° after immersion for 720 min, indicating minimal decrease. The averaged static contact angle values of the superhydrophobic surfaces after immersion in the aqueous solutions at pHs 4, 7, and 10 for 12 h were found to be 90 ± 2°, 119 ± 2°, and 138 ± 2°, respectively. These results indicate that although our superhydrophobic surface shows high chemical durability in the basic aqueous solution its chemical durability in the acidic environment is low. This may be due to dissolution of Ce(CH3(CH2)12COO)3 on the AZ31 sample through pits or defects on the coating in the acidic environment. The roughness fabricated on the AZ31 surface may be destroyed as a result of dissolution.

Figure 9

Changes in static water contact angles of sample surfaces as a function of immersion time in aqueous solutions at pHs (a) 4.0, (b) 7.0, and (c) 10.0. The superhydrophobic surfaces were fabricated by immersion for 30 min. All of the plots show the averaged values of water contact angles measured at five different points on the same samples. The error bars for each plot indicate the maximum and minimum water contact angles.

Our superhydrophobic surface fabricated by the immersion process was exposed to air for 1 year, after which the water contact angles remained consistent, with values greater than 150°, as shown in Figure a,b, showing that the superhydrophobic surfaces presented here were highly stable for a long time. These results imply that our superhydrophobic surfaces have superior characteristics regarding hydrophobicity and durability. This supports the claim that the one-step immersion process presented here is a promising strategy for the cost-effective and rapid fabrication of superhydrophobic surfaces with corrosion resistance.

Figure 10

(a) Photograph of water droplets on a superhydrophobic surface after exposure to air for 1 year. (b) Water droplet behavior on the superhydrophobic surface after exposure to air for 1 year.

Conclusions

A simple, easy, and rapid process to impart superhydrophobicity to the AZ31 Mg alloy surfaces via one-step immersion in a mixed solution of myristic acid and cerium nitrate was developed. The myristic acid-modified micro-/nanostructured surface fabricated by the one-step immersion process was found to have a static contact angle over 150°, resulting in the formation of superhydrophobic surfaces. The most suitable volume ratio of myristic acid–ethanol and cerium nitrate aqueous solutions for fabricating superhydrophobic surfaces was found to be 0.7. When using the mixed solution, the shortest processing time for the fabrication of the superhydrophobic surface was 30 s. In addition, XRD patterns revealed that crystalline solid myristic acid could be formed on a Mg alloy using the suitable molar ratio of Ce ions and myristic acid. The formation mechanism of the superhydrophobic surface was discussed. The contact angle hysteresis decreased with an increase in the immersion time. The corrosion resistance of the superhydrophobic AZ31 surface was revealed to be excellent by potentiodynamic polarization curve measurements. The chemical durability of the superhydrophobic AZ31 surface was also investigated. The average static water contact angles of the superhydrophobic AZ31 surfaces after immersion in solutions at pHs 4, 7, and 10 for 12 h were found to be 90 ± 2°, 119 ± 2°, and 138 ± 2°, respectively. We believe that the immersion process presented here would be promising for the large-scale industrial production of a superhydrophobic Mg alloy having new applications.

Experimental Section

Fabrication of Superhydrophobic Surfaces

A commercial AZ31 Mg alloy (composition: 2.98 wt % Al, 0.88 wt % Zn, 0.38 wt % Mn, 0.0135 wt % Si, 0.001 wt % Cu, 0.002 wt % Ni, 0.0027 wt % Fe, and balance Mg) with dimensions of 10 mm × 10 mm × 1.5 mm was used as the substrates. Each substrate was ultrasonically cleaned in absolute ethanol for 10 min. Solution A was prepared by introducing 5.03 g of myristic acid into 100 mL of ethanol. Solution B was prepared by introducing 1.15 g of cerium nitrate hexahydrate (Ce(NO3)3·6H2O) into 100 mL of ultrapure water. Solutions A and B were mixed in volume ratios of (a) 0:10, (b) 1:9, (c) 2:8, (d) 3:7, (e) 4:6, (f) 5:5, (g) 6:4, (h) 7:3, (i) 8:2, (j) 9:1, and (k) 10:0. The pH of each mixed solution was adjusted to 2.0 using nitric acid. The cleaned Mg alloy substrates were immersed for 10–1800 s in each mixed solution at room temperature without stirring. After immersion, the AZ31 substrate was extracted from the mixed solution and then rinsed with ethanol, followed by drying in inert N2 gas. Hereafter, we refer to the AZ31 samples obtained by immersing in each mixed solution for 60 and 180 s as sample (a60) and (a180), (b60) and (b180), (c60) and (c180), (d60) and (d180), (e60) and (e180), (f60) and (f180), (g60) and (g180), (h60) and (h180), (i60) and (i180), (j60) and (j180), and (k60) and (k180), respectively.

Surface Characterizations

The surface morphologies of the obtained samples were observed using field-emission scanning electron microscopy (FESEM; JSM-7610, JEOL Corp.). The composition of the samples was quantitatively determined using energy-dispersive X-ray analysis (FESEM-EDX) and X-ray photoelectron spectroscopy (XPS, JPS-9010MC). Charge correction for each spectrum was performed using the standard binding energy of the C 1s peak (284.6 eV) as a reference.[70] Crystal phases of the films fabricated on the surfaces were investigated by X-ray diffraction (XRD, Rigaku Ultima IV). The XRD pattern was measured at a scanning rate of 4°/min and the accelerating voltage of 40 kV in the range of 2θ = 5–80°. The chemical bonding states of the samples were acquired by FT-IR (IR Prestige-21, Shimadzu Co.). FT-IR spectra were recorded at a resolution of 4 cm–1 in ATR mode within the range 400–4000 cm–1 with 256 scans.

Static water contact angles of the fabricated surfaces were evaluated with a contact angle meter (DM-501, Kyowa Interface Science) based on a sessile drop measuring method. The water drop volume for the static contact angle measurements was 5 μL. The advancing and receding water contact angles of the fabricated surfaces were measured using ultrapure water that was added and withdrawn from the drop. The chemical durability of the superhydrophobic surfaces against aqueous solutions at pHs 4.0, 7.0, and 10.0 was evaluated by examining the relationship between the changes in the static water contact angles and the immersion time of the samples in solutions with different pH levels. All of the water contact angles were measured at five different points and were averaged. All error bars for each plot mean the maximum and minimum water contact angles. All contact angle measurements were carried out at room temperature in air. The mean roughness (roughness average: Ra) of the fabricated surfaces was estimated using a digital microscope (KH-1300-S, Hirox Co.). Ra means the roughness average of a surface composed of microscopic peaks and valleys and was calculated using the following equation: , where z(x) is the ordinate of the roughness profile.

All electrochemical measurements were performed in an aqueous solution of 5.0 mass % NaCl at room temperature using a computer-controlled potentiostat (VersaSTAT3, Princeton Applied Research). Potentiodynamic polarization curves were obtained at a scanning rate of 0.5 mV/s from −100 to +800 mV with respect to the open-circuit potential after immersing the treated AZ31 into the NaCl aqueous solution for 30 min. The treated AZ31 and a platinum plate were employed as the working and counter electrodes, respectively. An Ag/AgCl-saturated electrode was used as the reference electrode.