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Formation and electrochemical properties of the superhydrophobic nanocomposite coating on PEO pretreated Mg–Mn–Ce magnesium alloy
SCT-18553; No of Pages 7 Surface & Coatings Technology xxx (2013) xxx–xxx
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Formation and electrochemical properties of the superhydrophobic nanocomposite coating on PEO pretreated Mg–Mn–Ce magnesium alloy S.V. Gnedenkov a, V.S. Egorkin a,⁎, S.L. Sinebryukhov a, I.E. Vyaliy a, A.S. Pashinin b, A.M. Emelyanenko b, L.B. Boinovich b a b
Institute of Chemistry, Far Eastern Branch, Russian Academy of Sciences, 159 pr. 100-letiya Vladivostoka, Vladivostok 690022, Russia A.N. Frumkin Institute of Physical Chemistry and Electrochemistry, Russian Academy of Sciences, 31 Leninskii pr., Moscow 119071, Russia
a r t i c l e
i n f o
Article history: Received 7 February 2013 Accepted in revised form 16 May 2013 Available online xxxx Keywords: Superhydrophobic coating Plasma electrolytic oxidation Corrosion resistance Electrochemical impedance spectroscopy Magnesium alloys Contact angle measurements
a b s t r a c t This paper describes the methods of preparation and electrochemical properties of hydrophobic (HP) and superhydrophobic (SHP) nanocomposite coatings on the surface of magnesium alloy pretreated using plasma electrolytic oxidation (PEO). The most effective corrosion protection in brine solutions among the coatings under study was demonstrated by the nanocomposite superhydrophobic coating. For this coating, the values of the contact and rolling angles are 166° ± 3° and 5° ± 3°, respectively. The impedance modulus (|Z|f = 0.005) value, which characterizes the anticorrosion properties, attains 2.5 ∙ 107 Ω сm2 for the SHP coating and 1.4 ∙ 106 Ω сm2 for the HP coating at the initial moment of immersion into a chloride-containing electrolyte, thus improving the resistance of the base PEO coating by almost 400 and 20 times, respectively. After 24 h of exposure, the protective properties of the SHP coatings are one order of magnitude higher than those of the HP coatings. The high corrosion resistance and considerable stability of the SHP coatings during their operation result from a small area of real contact with aggressive media due to a heterogeneous wetting regime and formation of a firm chemical bond between the molecules of the hydrophobic agent and the other coating components. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Magnesium alloys are attractive and promising for industry [1] due to their low density, high strength, and ease of processing. The main disadvantages, which substantially limit the practical application of magnesium alloys, consist in their low wear and corrosion resistances. In a dry air atmosphere, the surface of magnesium (due to its high affinity to oxygen) is always coated with a thin layer of magnesium oxide. However, even a moderate level of relative air humidity leads to hydration of this oxide and formation of a brittle layer of magnesium hydroxide Mg(OH)2 weakly adhering to the metal. The contact of magnesium alloys with aggressive solutions, which contain, in particular, chloride ions, leads to a rapid destruction of the passive film and dissolution of the metal. In this case, the corrosion process is very fast [2,3]. Detailed information on the corrosion behavior of magnesium and its alloys in different environments has been reviewed in [4]. Among the methods of forming protective coatings on magnesium and its alloys, which are discussed, for example, in [5–8], plasma electrolytic oxidation (PEO) is becoming increasingly common [9]. This is due to the properties of coatings formed by PEO: high adhesion to the substrate [10], increased microhardness, and corrosion resistance in comparison with the alloy [11–15]. Despite good performance of these types of coatings, they have porosity that can attain 20 or more percent ⁎ Corresponding author. Tel.: +7 4 23 2312588; fax: +7 4 23 2312590. E-mail address: [email protected] (V.S. Egorkin).
[16,17]. It should also be pointed out that layers with even a small number of microdefects in their morphological structure have low protective properties. It was shown [2] that if a PEO layer in contact with a chloride-containing solution had micron-size defects, destruction of the metal became inevitable and the corrosion process occurred at the coating/metal interface. Alloys of the Mg–Mn system form an important group of mediumstrength wrought alloys, which are used at temperatures up to 200 °C. Doping of this system (in addition to manganese) with rare earth elements results in an increase in resistance to stress corrosion cracking [1]. However, papers on the formation of protective layers on the alloys of this system are rare in scientific literature [18]. Hydrophobic and superhydrophobic layers were selected for the protection of the magnesium alloy Mg–Mn–Ce against corrosion as a result of their high barrier properties [19–25]. At the same time, good adhesion of the PEO layers to the metal substrate and their complex surface topography made it possible to use them as a basis for the formation of composite coatings. In this work, the anticorrosion properties of hydrophobic and superhydrophobic coatings on the Mg–Mn–Ce alloy were identified and the features of their changes depending on exposure time in 3% aqueous solution of sodium chloride were established. Three types of coatings that differ significantly from each other by wettability in chloride-containing electrolyte were selected for the study. Hydrophilic coatings were produced by plasma electrolytic oxidation in silicate-containing electrolyte as described in [18]. Hydrophobic coatings were formed by deposition of the hydrophobic agent from the solution onto the surface of the
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PEO layer by adsorption. The superhydrophobic coatings were fabricated on top of the PEO coatings by formation of a nanocomposite layer having both multimodal roughness and low surface energy through the deposition of a wetting film of a dispersion containing silica nanoparticles and molecules of the hydrophobic agent [26–29]. 2. Experimental 2.1. Materials The base material in this study was wrought magnesium alloy MA8 containing 1.5–2.5 mass% Mn and 0.15–0.35 mass% Ce, for which the hydrophobic and superhydrophobic coatings were designed. Rectangular plates of sizes 15 mm × 40 mm × 1.5 mm were used as samples for electrochemical studies. Typical plate sizes for wetting analysis were 15 mm × 10 mm × 1.5 mm. Preliminary treatment of a sample included grinding with sandpaper of different grits (600, 800 and 1200) and washing with distilled water. Abrading with the last paper was carried out with high purity ethanol rinsing. To prepare both the hydrophobic and the superhydrophobic coatings, methoxy-{3-[(2,2,3,3,4,4,5,5,6,6,7,7,8,8,8-pentadecafluorooctyl) oxy]propyl}-silane (synthesized in the laboratory) was used as the hydrophobic agent; decane and 99.9% ethyl alcohol were also used. 2.2. PEO coating formation PEO treatment was conducted in a silicate–fluoride electrolyte containing 15 g/l Na2SiO3 · 5H2O and 5 g/l NaF [18]. Coating formation was carried out using the same equipment for plasma electrolytic oxidation as in [18], consisting of a computerized control and monitoring system and connected to a computer with the appropriate software. All samples were processed for 10 min in the bipolar mode of PEO, in which the anodic pulses periodically alternate with cathodic ones. During the anodic component of the process, the voltage values increased from 30 to 300 V at a rate of 0.45 V/s; the cathodic component was in potentiostatic mode fixed at a value of 30 V. The duration ratio of the anodic and cathodic pulses was equal to 1, so the duty cycles were 50%, and the polarization frequency was 300 Hz. The root-mean square values of the voltage were used in the formation process in this work. The applied electrical regime and waveforms of the voltage are schematically presented in Fig. 1. 2.3. Formation of hydrophobic and superhydrophobic layers
Since attaining the hydrophobic and superhydrophobic states of the surface is based on chemisorption of the hydrophobic agent at the surface OH groups [30,31], the initial PEO coatings were boiled in a Na2O × SiO2 × H2O solution for 15 min to increase the surface density of chemisorption-active sites. The decrease in the contact angle of the PEO coating to 22° ± 7°, which occurs after boiling, indicates an increase in the density of surface hydroxyl groups. The same agent, methoxy-{3-[(2,2,3,3,4,4,5,5,6,6,7,7,8,8,8-pentadecafluorooctyl)oxy] propyl}-silane, was used to prepare both the hydrophobic and the superhydrophobic coatings on the PEO surface. Modification of the surface to attain hydrophobicity was performed by two methods. In the first method, in order to form the hydrophobic coating, the sample was immersed in a 2% solution of hydrophobic agent in 99% decane for 2 h, followed by washing once in an ultrasonic bath in 99.9% ethyl alcohol and three times in distilled water for 5 min. The hydrophobic coating was formed as a result of chemisorption. The contact angle of the sample increased to 131° ± 2°. In the second method, superhydrophobic coatings characterized by contact angles higher than 160° and rolling angles less than 7° were produced in ambient conditions by deposition of a wetting film of aerosil nanoparticle suspension in decane on the PEO coating, as described in [26,27]. In the latter case, the hydrophobic agent performs two functions. First, it reduces the free surface energy of the material. Second, having three reactive terminal groups, \Si(OCH3)3, it provides a chemical bond between the nanoparticles in the aggregates and between the nanoparticles and the PEO layer [27,31]. The role of the complex surface topography of the PEO coating should be emphasized. The key point for the formation of stable hydrophobic and superhydrophobic layers on top of the metal surface is the adhesion strength. The adhesion strength in turn is determined by two factors. The first one is the formation of a chemical bond between components of the coating and chemically active sites on top of the metal surface. The surface of PEO treated metal generally contains more hydroxyl groups responsible for chemical bonding with the hydrophobic agent than the native magnesium surface. Moreover, the density of hydroxyl groups on a PEO treated surface can be effectively controlled by boiling in a Na2O × SiO2 × H2O solution [30]. The second factor is the real area of contact between the metal surface and the hydrophobic or superhydrophobic layer. The more developed area of magnesium alloy surface after PEO pretreatment provides a significant increase in effective contact area, and thus in effective adhesion strength. In addition, the complex surface topography of the PEO coating facilitates the achievement of multimodal roughness, which is necessary to reach the stable superhydrophobic state [32].
The initial PEO coatings were hydrophilic and characterized by contact angles of 45.9° ± 2.9°. Then, to improve the corrosion resistance of the samples, the PEO coatings were further processed by the hydrophobic and superhydrophobic treatments.
2.4. Cross-section preparation, scanning electron microscopy and elemental analysis of the surface
Fig. 1. Schematic illustration of the applied electrical regime and waveforms of the voltage pulses.
The SHP coating cross-section was prepared by means of Ar+ etching of the sample using Ilion+™ Precision Cross-Section System model 693 (Gatan Inc., USA) according to [33]. The etching was performed for 1 h with the ion gun beam energy equal to 4 keV. The etching ion beam was oriented in the SHP-layer/PEO-coating/Mg alloy direction. The area of etching was limited by a special shield consisting of a steel sample blade. SEM images of the surface of the samples and data on their elemental composition were obtained with a Hitachi S5500 scanning electron microscope equipped with an energy dispersive X-ray microanalysis system. Taking into account the multimodal roughness of the analyzed surface and an electron beam diameter of about 10 nm, the accuracy of estimating the elemental content of a nanoscale edge object is significantly about 0.1 μm3. The nanoscale gold layers were deposited on the surfaces of the test samples to reduce image distortion associated with charging of the non-conductive coating.
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2.5. Electrochemical measurements Electrochemical measurements were performed at room temperature using a VMC-4 electrochemical station (Princeton Applied Research, USA). The values of corrosion currents were calculated according to the Stern–Geary relationship [34] on the basis of potentiodynamic polarization curves. Measurements were carried out in a three-electrode cell with 3% NaCl aqueous solution as an electrolyte. Platinum coated niobium mesh was used as a counter electrode. A silver/silver chloride electrode filled with saturated KCl solution served as a reference electrode. The exposed sample surface area was 1 cm2. Prior to the electrochemical measurements, the samples were kept in the solution for 30 min in order to stabilize the free corrosion potential (EC). The potentiodynamic polarization curves were registered at a sweep rate of 1 mV/s. The samples were polarized from −100 mV vs. EC in the anodic direction. Electrochemical impedance spectroscopy (EIS) measurements were conducted at open circuit potential in the frequency range from 1.0 MHz to 0.005 Hz (logarithmic sweep, 7 points per decade), with a 10 mV (rms) sine-wave perturbation signal. In order to investigate the corrosion behavior of the samples during their exposure to 3% NaCl for 24 h, electrochemical impedance spectra were recorded after 1, 5, 10, 15, 20 and 24 h of immersion. The experiments were controlled and analyzed with the aid of VersaStudio (Princeton Applied Research, USA), ZView and CorrView software (Scribner Associates, USA). 2.6. Contact angle and surface tension measurements To measure the contact angles and the surface tension, we used the method of digital video image processing of a sessile drop of the testing liquid on the investigated surface. The typical volume of the drops varied from 15 to 30 μl. The homemade experimental setup for obtaining the optical images of sessile drops and software for subsequent determination of drop parameters using a Laplace curve fitting routine were described earlier [35]. A Pixelink PL-B686MU monochrome digital camera with spatial resolution of 1280 × 1024, color resolution of 256 gray levels, and time resolution 25 frames per second was used to capture the drop images. To obtain reliable wetting characteristics of the coatings, the initial contact angles were measured 2 s after drop deposition at five different points on the surface of each sample, with the average angle for ten
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consecutive images of the drop being defined for each location. The accuracy of contact angle determination was not worse than 0.1º for all angles measured on various substrates and was defined as the root-mean-square (rms) deviation of angles for ten consecutive images of the drop. At the same time, the deviation of the value of the contact angle from the average value at different points along the sample was much higher. Such scattering of contact angles, defined as rms deviation from the average value, results from the chemical and topological nonuniformity of the sample. To measure the rolling angle, a 10–15 μl drop was deposited onto the surface. After initial drop shape equilibration, manipulations with an experimental cell positioner allowed smooth variation of the inclination of the sample surface and detection of the rolling angle. 3. Results and discussion Typical SEM images of the coatings are shown in Fig. 2. The thickness of the hydrophobic coating (HP) was one monolayer, while the thickness of the superhydrophobic nanocomposite coating (SHP) did not exceed 1–3 μm. Multimodal surface roughness caused by the aggregation of aerosil nanoparticles (Fig. 2 d, e and f) and low surface energy provided a heterogeneous wetting regime for the superhydrophobic coating, while the hydrophobic coating was characterized by a homogeneous wetting regime during prolonged contact with the electrolyte. Globules with a developed surface were found on the surface of the superhydrophobic nanocomposite coating (Fig. 3). The elemental composition of the globules and the surface region in which the measurements were performed are presented in Table 1 and Fig. 3c, respectively. The elemental composition of the coating and the globules suggests that they contain magnesium hydroxides and hydroxofluorines, silica, hydrophobic agent, and the remnants of the dispersion medium in the form of capillary bridges of the low volatility decane inside the pores, as well as compounds that are included in the chemical composition of the PEO layer: MgO, MgF2, Mg2SiO4 and Na2SiO3 [36]. Since the depth of penetration of the testing beam in the sample is approximately 1 μm, the difference in composition of the coatings is determined by both thickness of the coating and its intrinsic composition. Thus, the hydrophobic layer is on the order of 1 nm thick, and the data in the first row of Table 1 reflect joint composition of the hydrophobic monolayer and the PEO sublayer. At the same time, the SHP coating is 1–3 μm
Fig. 2. SEM images of MA8 alloy specimens with hydrophobic coating (a, b and c) and superhydrophobic nanocomposite coating (d, e and f). Arrows indicate the regions of the surface in which the elemental analysis was conducted.
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Fig. 3. Example of globule location in the SHP coating and enlarged images of the surface with a marked section (c), in which the elemental analysis was conducted.
thick (Fig. 4). This explains the increase in fluorine content (contained in the hydrophobic agent molecules) and decrease in oxygen and magnesium (main components of PEO sublayer) in SHP compared to the HP coating. The globule apparently represents a microparticle of magnesium oxide covered by a hydrophobic monolayer. Fig. 5 presents the impedance spectra in the form of Bode plots (frequency dependencies of impedance modulus |Z| (a) and phase angle theta (b)). As can be seen from the impedance modulus plot (Fig. 5a), the hydrophobic and superhydrophobic layers have much higher corrosion resistance in comparison with the uncoated sample and the PEO coating. The value of the impedance modulus obtained at low frequency (|Z|f = 0.005), which characterizes the corrosion resistance of materials in the initial period of immersion of the samples in chloride medium, attains 1.4 ∙ 106 Ω cm2 for the hydrophobic coating and 2.5 ∙ 107 Ω cm2 for the SHP coating (Table 2), that is, 23 and 403 times higher than the resistance of the base PEO layer, respectively. It should be noted that, unlike the unprotected alloy, the dependencies of the phase angle theta on the frequency f for the coated samples have two pronounced inflections. At the same time, the presence of hydrophobic and superhydrophobic layers does not change the position of the inflection in the frequency range 100–10 Hz due to the presence of the dense sublayer in the PEO coating's structure. The position of the high-frequency time constant, which is responsible for the morphological features of the layer's structure, shifts towards higher frequencies. The maximum of the phase
angle of this time constant for the SHP coating is at a value of −85°, indicating the high dielectric properties of the coating. This shift is caused by the presence of an air layer between the coating and the electrolyte layer. It should be noted that |Z|(f) for the HP and SHP coatings (Fig. 5a, curves 3 and 4) in the high-frequency region (≥105 Hz) does not reach the value of the electrolyte resistance. This is also due to the structure of the coating/electrolyte interface (in particular, multimodality of the surface and the presence of an air layer between the surface and electrolyte for superhydrophobic coatings and a dense monolayer of hydrophobic agent with high insulating properties for hydrophobic coatings). The results of the corrosion behavior study of the samples carried out in 3% NaCl solution by potentiodynamic polarization (Fig. 6) support the conclusions drawn from the impedance measurements. The parameters characterizing the protective properties of the samples were calculated using the Stern–Geary equation and are presented in Table 2. The calculated values of the polarization resistance
Table 1 EDX analysis of the different surface regions of the samples. Coating
Element's content, at.% С
O
F
Na
Mg
Si
HP (Fig. 2с) SHP (Fig. 2f) Globule (Fig. 3с)
17.9 41.7 17.7
52.5 23.0 51.9
3.9 20.6 7.1
1.2 0.2 –
15.7 2.9 22.5
8.8 11.6 0.8
Fig. 4. SEM image of the SHP coating cross-section.
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Fig. 6. Potentiodynamic polarization curves in 3% NaCl for the investigated samples: uncoated MA8 alloy — 1; PEO coating — 2; hydrophobic coating — 3; and superhydrophobic nanocomposite coatings — 4.
Fig. 5. Bode plots of (a) impedance modulus and (b) phase angle in 3% NaCl for the investigated samples: uncoated MA8 alloy — 1; PEO coating — 2; hydrophobic coating — 3; and superhydrophobic nanocomposite coatings — 4.
of the samples with hydrophobic and superhydrophobic layers on the surface are much higher than the value for the sample with the PEO coating. For the exposure conditions in the chloride solution, the impedance spectra of samples with hydrophobic (Fig. 7) and superhydrophobic coatings (Fig. 8) undergo substantial transformations that reflect changes in the protective properties of the samples (decreasing values of the impedance modulus on |Z|(f) curves), resulting from the interaction of the coatings with an electrolyte solution. For a comparative study of the degradation of protective properties of the coatings in continuous contact with a chloride-containing electrolyte solution, we investigated the evolution of the sessile drop parameters and electrochemical characteristics of the coatings. Data on the evolution of the contact angle of initial PEO coatings boiled for 15 min, as well as hydrophobic and superhydrophobic surfaces, are shown in Fig. 9. These data suggest that the intensity of decrease in the contact angle on contact with an aqueous solution of sodium chloride varies considerably for the investigated coatings. We have previously shown that the rate of decrease in the contact angle values over time can be regarded as a parameter that reflects the intensity of corrosion processes of a material in contact with the test liquid [27–29]. Data on the evolution of the contact angle with time (Fig. 9) clearly show significant slowing of the corrosion process after the application of a hydrophobic layer (curve 2) in comparison with magnesium with a PEO coating on the surface (curve 3). However, the most significant inhibition of the corrosion process occurs in samples with a superhydrophobic coating (curve 1). Such conclusions on the protective effect of hydrophobic and superhydrophobic coatings are consistent with the values of the impedance modulus measured on samples exposed for different times to a chloride electrolyte solution (Fig. 10). For a detailed elaboration of the processes occurring at the interfaces with the hydrophobic and superhydrophobic surfaces, we analyzed Bode plots obtained after exposure of all samples in a corrosive
solution for 24 h (Fig. 11). As follows from Fig. 10, in the initial period of contact of the hydrophobic coating with the solution, the value of resistance increases and then gradually declines. Conversely, for a superhydrophobic composite coating, in the first five hours of exposure there is a threefold decrease in the impedance values followed by a decreasing rate of decline. This difference in the behavior of the protective layers in contact with a salt solution can be explained by the nature of the interaction of the coating's components with water molecules. As we have shown earlier, during the interaction with water molecules, the oxymethyl groups of the hydrophobic agent that did not form a siloxane bond with groups of neighboring molecules in adsorption are converted into terminal hydroxyl groups [37]. In turn, the hydration of these terminal groups in the aqueous solution leads to swelling of the HP layer. As a result, the volume of the layer increases and shielding of the coating's small defects occurs, which causes a slight increase in impedance modulus. However, long-term contact with an aqueous medium leads to hydrolysis of the Me\O\Si bond between the hydrophobic agent and coating material and desorption of the hydrophobic agent [31]. This mechanism of interaction with an aqueous medium is consistent with the data on the evolution of surface tension. This parameter decreases due to desorption of the hydrophobic agent molecules, initiated by hydrolysis,
Table 2 Electrochemical parameters of the samples. Sample
EC, V (vs. Ag/AgCl)
IC, А/сm2
Uncoated PEO HP SHP
−1.57 −1.54 −1.39 −1.21
6.6 4.4 1.6 1.5
∙ ∙ ∙ ∙
10−6 10−7 10−8 10−9
Rp, Ω сm2 4.0 6.0 1.6 1.8
∙ ∙ ∙ ∙
103 104 106 107
|Z|f = 0.005, Ω сm2 7.2 6.2 1.4 2.5
∙ ∙ ∙ ∙
102 104 106 107
Fig. 7. Bode plots of (a) impedance modulus and (b) phase angle for MA8 alloy with HP coating under 24-h exposure to 3% NaCl. The numbers denote the hour of exposure. Spectra of the uncoated and PEO-coated MA8 alloy are shown for comparison.
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Fig. 10. The evolution of impedance modulus of the hydrophobic (1) and superhydrophobic (2) coatings during 24 h of exposure in 3% NaCl.
Fig. 8. Bode plots of (a) impedance modulus and (b) phase angle for MA8 alloy with SHP coating during 24 h of exposure to 3% NaCl. The numbers denote the hour of exposure. Spectra of the uncoated and PEO-coated MA8 alloy are shown for comparison.
from the coating/solution interface and their transition to the solution/ air interface. The decrease in concentration of hydrophobic agent on the surface of hydrophobic and superhydrophobic coatings contributes to the formation of defects in the coating and increases the surface energy.
Fig. 9. The evolution of contact angle (a) and surface tension at the liquid/vapor interface (b) with time after saline drop deposition on samples obtained for superhydrophobic (1 and 1′); hydrophobic (2 and 2′); and the initial PEO coatings (3).
The consequence of this process for the hydrophobic and superhydrophobic coatings is an increased rate of mass transfer through the coating, which is reflected in the gradual decrease in electrical resistance (Fig. 10). An intensive decrease in impedance values in the first five hours of exposure of the superhydrophobic coating in the solution is due to the formation of a wetting film on the surface of the coating's texture elements, resulting in an increase in the overall wetted area. Further, as in the case of hydrophobic coatings, the hydrolysis of Me\O\Si bond leads to desorption of the hydrophobic agent molecules most weakly bound with a coating formation of defects in the coating and reduction of electrical resistance. However, as follows from the data on the long-term evolution of the surface tension due to the low portion of wetted area and a very low proportion of hydrophobic agent molecules weakly coupled with the coating (and therefore removed by hydrolysis), the establishment of steady-state electrochemical parameters occurs. This provides the long-term protective properties of superhydrophobic coatings. Comparison of the impedance modulus values for systems with a PEO coating and hydrophobic and superhydrophobic coatings formed on the PEO layer after 24 h of exposure to the corrosive environment shows that the SHP coating retains a higher insulation value (2.4 ∙ 106 Ω cm2) than the HP coating (1.7 ∙ 105 Ω cm2).
Fig. 11. Bode plots of impedance modulus (a) and phase angle (b) in 3% NaCl for the investigated samples: with HP coating — 1; and SHP coating — 2, after 24 h of exposure to 3% NaCl. Spectra of the uncoated and PEO-coated MA8 alloy are shown for comparison.
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4. Conclusions It has been found that the superhydrophobic coating formed on the surface of magnesium alloy using a PEO process has the best corrosion properties among the coatings under investigation. The origins of the electrochemical properties of the investigated layers were established on the basis of the data obtained by electrochemical impedance spectroscopy, potentiodynamic polarization, and the results of the analysis of temporal evolution of contact angle and surface tension at the drop/vapor interface for superhydrophobic, hydrophobic and PEO coatings, depending on the time in contact with chloride-containing electrolyte. The small fraction of the wetted area of superhydrophobic coatings in a corrosive environment and the very small one of hydrophobic agent molecules weakly bound to the coating material provide the high corrosion resistance of SHP layers and their considerable stability when operated in a chloride-containing corrosive environment. Acknowledgment This work was supported by the Presidium of FEB RAS grant # 12-I-P8-06, grant for the support of leading scientific schools of the Russian Federation, project no. NSh-6299.2012.3, and the President of Russian Federation Awards for young scientists and postgraduate students SP-2086.2012.1, and SP-1565.2012.1. References [1] K.U. Keiner, Magnesium Alloys and Their Applications, Wiley-VCH Verlag GMBH, Weinheim, 2000. 798. [2] A.S. Gnedenkov, S.L. Sinebryukhov, D.V. Mashtalyar, S.V. Gnedenkov, Phys. Procedia 23 (2012) 98. [3] S.L. Sinebryukhov, A.S. Gnedenkov, D.V. Mashtalyar, S.V. Gnedenkov, Surf. Coat. Technol. 205 (2010) 1697. [4] G.L. Song, Corrosion of Magnesium Alloys, Woodhead Publishing, Cambridge, 2011. 656. [5] J.E. Gray, B. Luan, J. Alloys Compd. 336 (2002) 88. [6] A. Kuhn, Met. Finish. 101 (2003) 44. [7] C. Blawert, W. Dietzel, E. Ghali, G.L. Song, Adv. Eng. Mater. 8 (2006) 511. [8] F.C. Walsh, C.T.J. Low, R.J.K. Wood, K.T. Stevens, J. Archer, A.R. Poeton, A. Ryder, Trans. Inst. Met. Finish. 87 (2009) 122.
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[9] L.O. Snizhko, A. Yerokhin, N.L. Gurevina, D.O. Misnyankin, A.V. Ciba, A. Matthews, Surf. Coat. Technol. 205 (2010) 1527. [10] W. Mu, Y. Han, Surf. Coat. Technol. 202 (2008) 4278. [11] R. Arrabal, E. Matykina, T. Hashimoto, P. Skeldon, G.E. Thompson, Surf. Coat. Technol. 203 (2009) 2207. [12] L. Rama Krishna, G. Poshal, G. Sundararajan, Metall. Mater. Trans. A 41 (2010) 3499. [13] H.S. Ryu, S.J. Mun, T.S. Lim, H.C. Kim, K.S. Shin, S.H. Hong, J. Electrochem. Soc. 158 (2011) C266. [14] C.E. Barchiche, D. Veys-Renaux, E. Rocca, Surf. Coat. Technol. 205 (2011) 4243. [15] C.E. Barchiche, E. Rocca, C. Juers, J. Hazan, J. Steinmetz, Electrochim. Acta 53 (2007) 417. [16] J.A. Curran, T.W. Clyne, Acta Mater. 54 (2006) 1985. [17] W. Zhang, B. Tian, K.Q. Du, Hui-Xia Zhang, F.H. Wang, Int. J. Electrochem. Sci. 6 (2011) 5228. [18] S.V. Gnedenkov, O.A. Khrisanfova, A.G. Zavidnaya, S.L. Sinebryukhov, V.S. Egorkin, M.V. Nistratova, A. Yerokhin, A. Matthews, Surf. Coat. Technol. 204 (2010) 2316. [19] J.C. Tuberquia, N. Nizamidin, R.R. Harl, J. Albert, J. Hunter, B.R. Rogers, G.K. Jennings, J. Am. Chem. Soc. 132 (2010) 5725. [20] T. Ishizaki, J. Hieda, N. Saito, N. Saito, O. Takai, Electrochim. Acta 55 (2010) 7094. [21] T. Ishizaki, M. Okido, Y. Masuda, N. Saito, M. Sakamoto, Langmuir 27 (2011) 6009. [22] T. Liu, S. Chen, S. Cheng, J. Tian, X. Chang, Y. Yin, Electrochim. Acta 52 (2007) 8003. [23] L. Zhu, Y. Jin, Appl. Surf. Sci. 253 (2007) 3432. [24] H. Liu, S. Szunerits, W. Xu, R. Boukherroub, ACS Appl. Mater. Interfaces 1 (2009) 1150. [25] P.M. Barkhudarov, P.B. Shah, E.B. Watkins, D.A. Doshi, C.J. Brinker, J. Majewski, Corr. Sci. 50 (2008) 897. [26] L. Boinovich, A. Emelyanenko, Langmuir 25 (2009) 2907. [27] L. Boinovich, A.M. Emelyanenko, A.S. Pashinin, ACS Appl. Mater. Interfaces 2 (2010) 1754. [28] S.V. Gnedenkov, S.L. Sinebryukhov, V.S. Egorkin, D.V. Mashtalyar, D.A. Alpysbaeva, L.B. Boinovich, Colloids Surf. A 383 (2011) 61. [29] L.B. Boinovich, S.V. Gnedenkov, D.A. Alpysbaeva, V.S. Egorkin, A.M. Emelyanenko, S.L. Sinebryukhov, A.K. Zaretskaya, Corr. Sci. 55 (2012) 238. [30] L.B. Boinovich, A.M. Emelyanenko, A.S. Pashinin, S.V. Gnedenkov, V.S. Egorkin, S.L. Sinebryukhov, Surf. Innov. 1 (2013), http://dx.doi.org/10.1680/si.13.00001 (in press). [31] L. Boinovich, A. Emelyanenko, Adv. Colloid Interface Sci. 179 (2012) 133. [32] L.B. Boinovich, A.M. Emelyanenko, Usp. Khim. 77 (2008) 619. [33] Ilion+™ Precision Cross-Section System, Owner's Manual and User's Guide, Part Number: 693.82001, 2010 , (Revision 1). [34] M. Stern, A.L. Geary, J. Electrochem. Soc. 104 (1957) 56. [35] A.M. Emelyanenko, L.B. Boinovich, Instrum. Exp. Tech. 45 (2002) 44. [36] S.L. Sinebryukhov, M.V. Sidorova, V.S. Egorkin, P.M. Nedozorov, A.Yu. Ustinov, E.F. Volkova, S.V. Gnedenkov, Prot. Met. Phys. Chem. Surf. 48 (2012) 678. [37] L.B. Boinovich, Her. Russ. Acad. Sci. 83 (2013) 8.
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Formation and electrochemical properties of the superhydrophobic nanocomposite coating on PEO pretreated Mg–Mn–Ce magnesium alloy S.V. Gnedenkov a, V.S. Egorkin a,⁎, S.L. Sinebryukhov a, I.E. Vyaliy a, A.S. Pashinin b, A.M. Emelyanenko b, L.B. Boinovich b a b
Institute of Chemistry, Far Eastern Branch, Russian Academy of Sciences, 159 pr. 100-letiya Vladivostoka, Vladivostok 690022, Russia A.N. Frumkin Institute of Physical Chemistry and Electrochemistry, Russian Academy of Sciences, 31 Leninskii pr., Moscow 119071, Russia
a r t i c l e
i n f o
Article history: Received 7 February 2013 Accepted in revised form 16 May 2013 Available online xxxx Keywords: Superhydrophobic coating Plasma electrolytic oxidation Corrosion resistance Electrochemical impedance spectroscopy Magnesium alloys Contact angle measurements
a b s t r a c t This paper describes the methods of preparation and electrochemical properties of hydrophobic (HP) and superhydrophobic (SHP) nanocomposite coatings on the surface of magnesium alloy pretreated using plasma electrolytic oxidation (PEO). The most effective corrosion protection in brine solutions among the coatings under study was demonstrated by the nanocomposite superhydrophobic coating. For this coating, the values of the contact and rolling angles are 166° ± 3° and 5° ± 3°, respectively. The impedance modulus (|Z|f = 0.005) value, which characterizes the anticorrosion properties, attains 2.5 ∙ 107 Ω сm2 for the SHP coating and 1.4 ∙ 106 Ω сm2 for the HP coating at the initial moment of immersion into a chloride-containing electrolyte, thus improving the resistance of the base PEO coating by almost 400 and 20 times, respectively. After 24 h of exposure, the protective properties of the SHP coatings are one order of magnitude higher than those of the HP coatings. The high corrosion resistance and considerable stability of the SHP coatings during their operation result from a small area of real contact with aggressive media due to a heterogeneous wetting regime and formation of a firm chemical bond between the molecules of the hydrophobic agent and the other coating components. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Magnesium alloys are attractive and promising for industry [1] due to their low density, high strength, and ease of processing. The main disadvantages, which substantially limit the practical application of magnesium alloys, consist in their low wear and corrosion resistances. In a dry air atmosphere, the surface of magnesium (due to its high affinity to oxygen) is always coated with a thin layer of magnesium oxide. However, even a moderate level of relative air humidity leads to hydration of this oxide and formation of a brittle layer of magnesium hydroxide Mg(OH)2 weakly adhering to the metal. The contact of magnesium alloys with aggressive solutions, which contain, in particular, chloride ions, leads to a rapid destruction of the passive film and dissolution of the metal. In this case, the corrosion process is very fast [2,3]. Detailed information on the corrosion behavior of magnesium and its alloys in different environments has been reviewed in [4]. Among the methods of forming protective coatings on magnesium and its alloys, which are discussed, for example, in [5–8], plasma electrolytic oxidation (PEO) is becoming increasingly common [9]. This is due to the properties of coatings formed by PEO: high adhesion to the substrate [10], increased microhardness, and corrosion resistance in comparison with the alloy [11–15]. Despite good performance of these types of coatings, they have porosity that can attain 20 or more percent ⁎ Corresponding author. Tel.: +7 4 23 2312588; fax: +7 4 23 2312590. E-mail address: [email protected] (V.S. Egorkin).
[16,17]. It should also be pointed out that layers with even a small number of microdefects in their morphological structure have low protective properties. It was shown [2] that if a PEO layer in contact with a chloride-containing solution had micron-size defects, destruction of the metal became inevitable and the corrosion process occurred at the coating/metal interface. Alloys of the Mg–Mn system form an important group of mediumstrength wrought alloys, which are used at temperatures up to 200 °C. Doping of this system (in addition to manganese) with rare earth elements results in an increase in resistance to stress corrosion cracking [1]. However, papers on the formation of protective layers on the alloys of this system are rare in scientific literature [18]. Hydrophobic and superhydrophobic layers were selected for the protection of the magnesium alloy Mg–Mn–Ce against corrosion as a result of their high barrier properties [19–25]. At the same time, good adhesion of the PEO layers to the metal substrate and their complex surface topography made it possible to use them as a basis for the formation of composite coatings. In this work, the anticorrosion properties of hydrophobic and superhydrophobic coatings on the Mg–Mn–Ce alloy were identified and the features of their changes depending on exposure time in 3% aqueous solution of sodium chloride were established. Three types of coatings that differ significantly from each other by wettability in chloride-containing electrolyte were selected for the study. Hydrophilic coatings were produced by plasma electrolytic oxidation in silicate-containing electrolyte as described in [18]. Hydrophobic coatings were formed by deposition of the hydrophobic agent from the solution onto the surface of the
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PEO layer by adsorption. The superhydrophobic coatings were fabricated on top of the PEO coatings by formation of a nanocomposite layer having both multimodal roughness and low surface energy through the deposition of a wetting film of a dispersion containing silica nanoparticles and molecules of the hydrophobic agent [26–29]. 2. Experimental 2.1. Materials The base material in this study was wrought magnesium alloy MA8 containing 1.5–2.5 mass% Mn and 0.15–0.35 mass% Ce, for which the hydrophobic and superhydrophobic coatings were designed. Rectangular plates of sizes 15 mm × 40 mm × 1.5 mm were used as samples for electrochemical studies. Typical plate sizes for wetting analysis were 15 mm × 10 mm × 1.5 mm. Preliminary treatment of a sample included grinding with sandpaper of different grits (600, 800 and 1200) and washing with distilled water. Abrading with the last paper was carried out with high purity ethanol rinsing. To prepare both the hydrophobic and the superhydrophobic coatings, methoxy-{3-[(2,2,3,3,4,4,5,5,6,6,7,7,8,8,8-pentadecafluorooctyl) oxy]propyl}-silane (synthesized in the laboratory) was used as the hydrophobic agent; decane and 99.9% ethyl alcohol were also used. 2.2. PEO coating formation PEO treatment was conducted in a silicate–fluoride electrolyte containing 15 g/l Na2SiO3 · 5H2O and 5 g/l NaF [18]. Coating formation was carried out using the same equipment for plasma electrolytic oxidation as in [18], consisting of a computerized control and monitoring system and connected to a computer with the appropriate software. All samples were processed for 10 min in the bipolar mode of PEO, in which the anodic pulses periodically alternate with cathodic ones. During the anodic component of the process, the voltage values increased from 30 to 300 V at a rate of 0.45 V/s; the cathodic component was in potentiostatic mode fixed at a value of 30 V. The duration ratio of the anodic and cathodic pulses was equal to 1, so the duty cycles were 50%, and the polarization frequency was 300 Hz. The root-mean square values of the voltage were used in the formation process in this work. The applied electrical regime and waveforms of the voltage are schematically presented in Fig. 1. 2.3. Formation of hydrophobic and superhydrophobic layers
Since attaining the hydrophobic and superhydrophobic states of the surface is based on chemisorption of the hydrophobic agent at the surface OH groups [30,31], the initial PEO coatings were boiled in a Na2O × SiO2 × H2O solution for 15 min to increase the surface density of chemisorption-active sites. The decrease in the contact angle of the PEO coating to 22° ± 7°, which occurs after boiling, indicates an increase in the density of surface hydroxyl groups. The same agent, methoxy-{3-[(2,2,3,3,4,4,5,5,6,6,7,7,8,8,8-pentadecafluorooctyl)oxy] propyl}-silane, was used to prepare both the hydrophobic and the superhydrophobic coatings on the PEO surface. Modification of the surface to attain hydrophobicity was performed by two methods. In the first method, in order to form the hydrophobic coating, the sample was immersed in a 2% solution of hydrophobic agent in 99% decane for 2 h, followed by washing once in an ultrasonic bath in 99.9% ethyl alcohol and three times in distilled water for 5 min. The hydrophobic coating was formed as a result of chemisorption. The contact angle of the sample increased to 131° ± 2°. In the second method, superhydrophobic coatings characterized by contact angles higher than 160° and rolling angles less than 7° were produced in ambient conditions by deposition of a wetting film of aerosil nanoparticle suspension in decane on the PEO coating, as described in [26,27]. In the latter case, the hydrophobic agent performs two functions. First, it reduces the free surface energy of the material. Second, having three reactive terminal groups, \Si(OCH3)3, it provides a chemical bond between the nanoparticles in the aggregates and between the nanoparticles and the PEO layer [27,31]. The role of the complex surface topography of the PEO coating should be emphasized. The key point for the formation of stable hydrophobic and superhydrophobic layers on top of the metal surface is the adhesion strength. The adhesion strength in turn is determined by two factors. The first one is the formation of a chemical bond between components of the coating and chemically active sites on top of the metal surface. The surface of PEO treated metal generally contains more hydroxyl groups responsible for chemical bonding with the hydrophobic agent than the native magnesium surface. Moreover, the density of hydroxyl groups on a PEO treated surface can be effectively controlled by boiling in a Na2O × SiO2 × H2O solution [30]. The second factor is the real area of contact between the metal surface and the hydrophobic or superhydrophobic layer. The more developed area of magnesium alloy surface after PEO pretreatment provides a significant increase in effective contact area, and thus in effective adhesion strength. In addition, the complex surface topography of the PEO coating facilitates the achievement of multimodal roughness, which is necessary to reach the stable superhydrophobic state [32].
The initial PEO coatings were hydrophilic and characterized by contact angles of 45.9° ± 2.9°. Then, to improve the corrosion resistance of the samples, the PEO coatings were further processed by the hydrophobic and superhydrophobic treatments.
2.4. Cross-section preparation, scanning electron microscopy and elemental analysis of the surface
Fig. 1. Schematic illustration of the applied electrical regime and waveforms of the voltage pulses.
The SHP coating cross-section was prepared by means of Ar+ etching of the sample using Ilion+™ Precision Cross-Section System model 693 (Gatan Inc., USA) according to [33]. The etching was performed for 1 h with the ion gun beam energy equal to 4 keV. The etching ion beam was oriented in the SHP-layer/PEO-coating/Mg alloy direction. The area of etching was limited by a special shield consisting of a steel sample blade. SEM images of the surface of the samples and data on their elemental composition were obtained with a Hitachi S5500 scanning electron microscope equipped with an energy dispersive X-ray microanalysis system. Taking into account the multimodal roughness of the analyzed surface and an electron beam diameter of about 10 nm, the accuracy of estimating the elemental content of a nanoscale edge object is significantly about 0.1 μm3. The nanoscale gold layers were deposited on the surfaces of the test samples to reduce image distortion associated with charging of the non-conductive coating.
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2.5. Electrochemical measurements Electrochemical measurements were performed at room temperature using a VMC-4 electrochemical station (Princeton Applied Research, USA). The values of corrosion currents were calculated according to the Stern–Geary relationship [34] on the basis of potentiodynamic polarization curves. Measurements were carried out in a three-electrode cell with 3% NaCl aqueous solution as an electrolyte. Platinum coated niobium mesh was used as a counter electrode. A silver/silver chloride electrode filled with saturated KCl solution served as a reference electrode. The exposed sample surface area was 1 cm2. Prior to the electrochemical measurements, the samples were kept in the solution for 30 min in order to stabilize the free corrosion potential (EC). The potentiodynamic polarization curves were registered at a sweep rate of 1 mV/s. The samples were polarized from −100 mV vs. EC in the anodic direction. Electrochemical impedance spectroscopy (EIS) measurements were conducted at open circuit potential in the frequency range from 1.0 MHz to 0.005 Hz (logarithmic sweep, 7 points per decade), with a 10 mV (rms) sine-wave perturbation signal. In order to investigate the corrosion behavior of the samples during their exposure to 3% NaCl for 24 h, electrochemical impedance spectra were recorded after 1, 5, 10, 15, 20 and 24 h of immersion. The experiments were controlled and analyzed with the aid of VersaStudio (Princeton Applied Research, USA), ZView and CorrView software (Scribner Associates, USA). 2.6. Contact angle and surface tension measurements To measure the contact angles and the surface tension, we used the method of digital video image processing of a sessile drop of the testing liquid on the investigated surface. The typical volume of the drops varied from 15 to 30 μl. The homemade experimental setup for obtaining the optical images of sessile drops and software for subsequent determination of drop parameters using a Laplace curve fitting routine were described earlier [35]. A Pixelink PL-B686MU monochrome digital camera with spatial resolution of 1280 × 1024, color resolution of 256 gray levels, and time resolution 25 frames per second was used to capture the drop images. To obtain reliable wetting characteristics of the coatings, the initial contact angles were measured 2 s after drop deposition at five different points on the surface of each sample, with the average angle for ten
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consecutive images of the drop being defined for each location. The accuracy of contact angle determination was not worse than 0.1º for all angles measured on various substrates and was defined as the root-mean-square (rms) deviation of angles for ten consecutive images of the drop. At the same time, the deviation of the value of the contact angle from the average value at different points along the sample was much higher. Such scattering of contact angles, defined as rms deviation from the average value, results from the chemical and topological nonuniformity of the sample. To measure the rolling angle, a 10–15 μl drop was deposited onto the surface. After initial drop shape equilibration, manipulations with an experimental cell positioner allowed smooth variation of the inclination of the sample surface and detection of the rolling angle. 3. Results and discussion Typical SEM images of the coatings are shown in Fig. 2. The thickness of the hydrophobic coating (HP) was one monolayer, while the thickness of the superhydrophobic nanocomposite coating (SHP) did not exceed 1–3 μm. Multimodal surface roughness caused by the aggregation of aerosil nanoparticles (Fig. 2 d, e and f) and low surface energy provided a heterogeneous wetting regime for the superhydrophobic coating, while the hydrophobic coating was characterized by a homogeneous wetting regime during prolonged contact with the electrolyte. Globules with a developed surface were found on the surface of the superhydrophobic nanocomposite coating (Fig. 3). The elemental composition of the globules and the surface region in which the measurements were performed are presented in Table 1 and Fig. 3c, respectively. The elemental composition of the coating and the globules suggests that they contain magnesium hydroxides and hydroxofluorines, silica, hydrophobic agent, and the remnants of the dispersion medium in the form of capillary bridges of the low volatility decane inside the pores, as well as compounds that are included in the chemical composition of the PEO layer: MgO, MgF2, Mg2SiO4 and Na2SiO3 [36]. Since the depth of penetration of the testing beam in the sample is approximately 1 μm, the difference in composition of the coatings is determined by both thickness of the coating and its intrinsic composition. Thus, the hydrophobic layer is on the order of 1 nm thick, and the data in the first row of Table 1 reflect joint composition of the hydrophobic monolayer and the PEO sublayer. At the same time, the SHP coating is 1–3 μm
Fig. 2. SEM images of MA8 alloy specimens with hydrophobic coating (a, b and c) and superhydrophobic nanocomposite coating (d, e and f). Arrows indicate the regions of the surface in which the elemental analysis was conducted.
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Fig. 3. Example of globule location in the SHP coating and enlarged images of the surface with a marked section (c), in which the elemental analysis was conducted.
thick (Fig. 4). This explains the increase in fluorine content (contained in the hydrophobic agent molecules) and decrease in oxygen and magnesium (main components of PEO sublayer) in SHP compared to the HP coating. The globule apparently represents a microparticle of magnesium oxide covered by a hydrophobic monolayer. Fig. 5 presents the impedance spectra in the form of Bode plots (frequency dependencies of impedance modulus |Z| (a) and phase angle theta (b)). As can be seen from the impedance modulus plot (Fig. 5a), the hydrophobic and superhydrophobic layers have much higher corrosion resistance in comparison with the uncoated sample and the PEO coating. The value of the impedance modulus obtained at low frequency (|Z|f = 0.005), which characterizes the corrosion resistance of materials in the initial period of immersion of the samples in chloride medium, attains 1.4 ∙ 106 Ω cm2 for the hydrophobic coating and 2.5 ∙ 107 Ω cm2 for the SHP coating (Table 2), that is, 23 and 403 times higher than the resistance of the base PEO layer, respectively. It should be noted that, unlike the unprotected alloy, the dependencies of the phase angle theta on the frequency f for the coated samples have two pronounced inflections. At the same time, the presence of hydrophobic and superhydrophobic layers does not change the position of the inflection in the frequency range 100–10 Hz due to the presence of the dense sublayer in the PEO coating's structure. The position of the high-frequency time constant, which is responsible for the morphological features of the layer's structure, shifts towards higher frequencies. The maximum of the phase
angle of this time constant for the SHP coating is at a value of −85°, indicating the high dielectric properties of the coating. This shift is caused by the presence of an air layer between the coating and the electrolyte layer. It should be noted that |Z|(f) for the HP and SHP coatings (Fig. 5a, curves 3 and 4) in the high-frequency region (≥105 Hz) does not reach the value of the electrolyte resistance. This is also due to the structure of the coating/electrolyte interface (in particular, multimodality of the surface and the presence of an air layer between the surface and electrolyte for superhydrophobic coatings and a dense monolayer of hydrophobic agent with high insulating properties for hydrophobic coatings). The results of the corrosion behavior study of the samples carried out in 3% NaCl solution by potentiodynamic polarization (Fig. 6) support the conclusions drawn from the impedance measurements. The parameters characterizing the protective properties of the samples were calculated using the Stern–Geary equation and are presented in Table 2. The calculated values of the polarization resistance
Table 1 EDX analysis of the different surface regions of the samples. Coating
Element's content, at.% С
O
F
Na
Mg
Si
HP (Fig. 2с) SHP (Fig. 2f) Globule (Fig. 3с)
17.9 41.7 17.7
52.5 23.0 51.9
3.9 20.6 7.1
1.2 0.2 –
15.7 2.9 22.5
8.8 11.6 0.8
Fig. 4. SEM image of the SHP coating cross-section.
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Fig. 6. Potentiodynamic polarization curves in 3% NaCl for the investigated samples: uncoated MA8 alloy — 1; PEO coating — 2; hydrophobic coating — 3; and superhydrophobic nanocomposite coatings — 4.
Fig. 5. Bode plots of (a) impedance modulus and (b) phase angle in 3% NaCl for the investigated samples: uncoated MA8 alloy — 1; PEO coating — 2; hydrophobic coating — 3; and superhydrophobic nanocomposite coatings — 4.
of the samples with hydrophobic and superhydrophobic layers on the surface are much higher than the value for the sample with the PEO coating. For the exposure conditions in the chloride solution, the impedance spectra of samples with hydrophobic (Fig. 7) and superhydrophobic coatings (Fig. 8) undergo substantial transformations that reflect changes in the protective properties of the samples (decreasing values of the impedance modulus on |Z|(f) curves), resulting from the interaction of the coatings with an electrolyte solution. For a comparative study of the degradation of protective properties of the coatings in continuous contact with a chloride-containing electrolyte solution, we investigated the evolution of the sessile drop parameters and electrochemical characteristics of the coatings. Data on the evolution of the contact angle of initial PEO coatings boiled for 15 min, as well as hydrophobic and superhydrophobic surfaces, are shown in Fig. 9. These data suggest that the intensity of decrease in the contact angle on contact with an aqueous solution of sodium chloride varies considerably for the investigated coatings. We have previously shown that the rate of decrease in the contact angle values over time can be regarded as a parameter that reflects the intensity of corrosion processes of a material in contact with the test liquid [27–29]. Data on the evolution of the contact angle with time (Fig. 9) clearly show significant slowing of the corrosion process after the application of a hydrophobic layer (curve 2) in comparison with magnesium with a PEO coating on the surface (curve 3). However, the most significant inhibition of the corrosion process occurs in samples with a superhydrophobic coating (curve 1). Such conclusions on the protective effect of hydrophobic and superhydrophobic coatings are consistent with the values of the impedance modulus measured on samples exposed for different times to a chloride electrolyte solution (Fig. 10). For a detailed elaboration of the processes occurring at the interfaces with the hydrophobic and superhydrophobic surfaces, we analyzed Bode plots obtained after exposure of all samples in a corrosive
solution for 24 h (Fig. 11). As follows from Fig. 10, in the initial period of contact of the hydrophobic coating with the solution, the value of resistance increases and then gradually declines. Conversely, for a superhydrophobic composite coating, in the first five hours of exposure there is a threefold decrease in the impedance values followed by a decreasing rate of decline. This difference in the behavior of the protective layers in contact with a salt solution can be explained by the nature of the interaction of the coating's components with water molecules. As we have shown earlier, during the interaction with water molecules, the oxymethyl groups of the hydrophobic agent that did not form a siloxane bond with groups of neighboring molecules in adsorption are converted into terminal hydroxyl groups [37]. In turn, the hydration of these terminal groups in the aqueous solution leads to swelling of the HP layer. As a result, the volume of the layer increases and shielding of the coating's small defects occurs, which causes a slight increase in impedance modulus. However, long-term contact with an aqueous medium leads to hydrolysis of the Me\O\Si bond between the hydrophobic agent and coating material and desorption of the hydrophobic agent [31]. This mechanism of interaction with an aqueous medium is consistent with the data on the evolution of surface tension. This parameter decreases due to desorption of the hydrophobic agent molecules, initiated by hydrolysis,
Table 2 Electrochemical parameters of the samples. Sample
EC, V (vs. Ag/AgCl)
IC, А/сm2
Uncoated PEO HP SHP
−1.57 −1.54 −1.39 −1.21
6.6 4.4 1.6 1.5
∙ ∙ ∙ ∙
10−6 10−7 10−8 10−9
Rp, Ω сm2 4.0 6.0 1.6 1.8
∙ ∙ ∙ ∙
103 104 106 107
|Z|f = 0.005, Ω сm2 7.2 6.2 1.4 2.5
∙ ∙ ∙ ∙
102 104 106 107
Fig. 7. Bode plots of (a) impedance modulus and (b) phase angle for MA8 alloy with HP coating under 24-h exposure to 3% NaCl. The numbers denote the hour of exposure. Spectra of the uncoated and PEO-coated MA8 alloy are shown for comparison.
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Fig. 10. The evolution of impedance modulus of the hydrophobic (1) and superhydrophobic (2) coatings during 24 h of exposure in 3% NaCl.
Fig. 8. Bode plots of (a) impedance modulus and (b) phase angle for MA8 alloy with SHP coating during 24 h of exposure to 3% NaCl. The numbers denote the hour of exposure. Spectra of the uncoated and PEO-coated MA8 alloy are shown for comparison.
from the coating/solution interface and their transition to the solution/ air interface. The decrease in concentration of hydrophobic agent on the surface of hydrophobic and superhydrophobic coatings contributes to the formation of defects in the coating and increases the surface energy.
Fig. 9. The evolution of contact angle (a) and surface tension at the liquid/vapor interface (b) with time after saline drop deposition on samples obtained for superhydrophobic (1 and 1′); hydrophobic (2 and 2′); and the initial PEO coatings (3).
The consequence of this process for the hydrophobic and superhydrophobic coatings is an increased rate of mass transfer through the coating, which is reflected in the gradual decrease in electrical resistance (Fig. 10). An intensive decrease in impedance values in the first five hours of exposure of the superhydrophobic coating in the solution is due to the formation of a wetting film on the surface of the coating's texture elements, resulting in an increase in the overall wetted area. Further, as in the case of hydrophobic coatings, the hydrolysis of Me\O\Si bond leads to desorption of the hydrophobic agent molecules most weakly bound with a coating formation of defects in the coating and reduction of electrical resistance. However, as follows from the data on the long-term evolution of the surface tension due to the low portion of wetted area and a very low proportion of hydrophobic agent molecules weakly coupled with the coating (and therefore removed by hydrolysis), the establishment of steady-state electrochemical parameters occurs. This provides the long-term protective properties of superhydrophobic coatings. Comparison of the impedance modulus values for systems with a PEO coating and hydrophobic and superhydrophobic coatings formed on the PEO layer after 24 h of exposure to the corrosive environment shows that the SHP coating retains a higher insulation value (2.4 ∙ 106 Ω cm2) than the HP coating (1.7 ∙ 105 Ω cm2).
Fig. 11. Bode plots of impedance modulus (a) and phase angle (b) in 3% NaCl for the investigated samples: with HP coating — 1; and SHP coating — 2, after 24 h of exposure to 3% NaCl. Spectra of the uncoated and PEO-coated MA8 alloy are shown for comparison.
Please cite this article as: S.V. Gnedenkov, et al., Surf. Coat. Technol. (2013), http://dx.doi.org/10.1016/j.surfcoat.2013.05.020
S.V. Gnedenkov et al. / Surface & Coatings Technology xxx (2013) xxx–xxx
4. Conclusions It has been found that the superhydrophobic coating formed on the surface of magnesium alloy using a PEO process has the best corrosion properties among the coatings under investigation. The origins of the electrochemical properties of the investigated layers were established on the basis of the data obtained by electrochemical impedance spectroscopy, potentiodynamic polarization, and the results of the analysis of temporal evolution of contact angle and surface tension at the drop/vapor interface for superhydrophobic, hydrophobic and PEO coatings, depending on the time in contact with chloride-containing electrolyte. The small fraction of the wetted area of superhydrophobic coatings in a corrosive environment and the very small one of hydrophobic agent molecules weakly bound to the coating material provide the high corrosion resistance of SHP layers and their considerable stability when operated in a chloride-containing corrosive environment. Acknowledgment This work was supported by the Presidium of FEB RAS grant # 12-I-P8-06, grant for the support of leading scientific schools of the Russian Federation, project no. NSh-6299.2012.3, and the President of Russian Federation Awards for young scientists and postgraduate students SP-2086.2012.1, and SP-1565.2012.1. References [1] K.U. Keiner, Magnesium Alloys and Their Applications, Wiley-VCH Verlag GMBH, Weinheim, 2000. 798. [2] A.S. Gnedenkov, S.L. Sinebryukhov, D.V. Mashtalyar, S.V. Gnedenkov, Phys. Procedia 23 (2012) 98. [3] S.L. Sinebryukhov, A.S. Gnedenkov, D.V. Mashtalyar, S.V. Gnedenkov, Surf. Coat. Technol. 205 (2010) 1697. [4] G.L. Song, Corrosion of Magnesium Alloys, Woodhead Publishing, Cambridge, 2011. 656. [5] J.E. Gray, B. Luan, J. Alloys Compd. 336 (2002) 88. [6] A. Kuhn, Met. Finish. 101 (2003) 44. [7] C. Blawert, W. Dietzel, E. Ghali, G.L. Song, Adv. Eng. Mater. 8 (2006) 511. [8] F.C. Walsh, C.T.J. Low, R.J.K. Wood, K.T. Stevens, J. Archer, A.R. Poeton, A. Ryder, Trans. Inst. Met. Finish. 87 (2009) 122.
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Please cite this article as: S.V. Gnedenkov, et al., Surf. Coat. Technol. (2013), http://dx.doi.org/10.1016/j.surfcoat.2013.05.020