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Wetting and electrochemical properties of hydrophobic and superhydrophobic coatings on titanium
Colloids and Surfaces A: Physicochem. Eng. Aspects 383 (2011) 61–66
Contents lists available at ScienceDirect
Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa
Wetting and electrochemical properties of hydrophobic and superhydrophobic coatings on titanium S.V. Gnedenkov a , S.L. Sinebryukhov a,∗ , V.S. Egorkin a , D.V. Mashtalyar a , D.A. Alpysbaeva b , L.B. Boinovich b a b
Institute of Chemistry FEB RAS, Vladivostok, Russia A.N. Frumkin Institute of Physical Chemistry and Electrochemistry, Moscow, Russia
a r t i c l e
i n f o
Article history: Received 10 November 2010 Received in revised form 24 December 2010 Accepted 7 January 2011 Available online 18 January 2011 Keywords: Superhydrophobic coating Plasma electrolytic oxidation Anticorrosion properties Electrochemical impedance spectroscopy Wetting
a b s t r a c t The combined analysis of the evolution of sessile drop parameters in contact with surface layer jointly with the data, obtained by electrochemical methods, gives the possibility to study the variation in the state of the surface due to the electrochemical reaction, associated with a corrosion process. We have compared the peculiarities of the corrosion process on titanium samples protected by (1) native oxide, (2) plasma electrolytic oxidation, (3) joint action of plasma electrolytic oxidation and hydrophobic layer as well as (4) plasma electrolytic oxidation with nanocomposite superhydrophobic coating. Among the coatings have been studied, the most effective corrosion protection in brine solutions was demonstrated by the nanocomposite superhydrophobic coating. The mechanisms of the corrosion protection are discussed. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Titanium and its alloys are widely used as engineering materials. They also find application as implant materials for dentures and human bodies because of their corrosion resistance and compatibility with biological tissues. However, the corrosion caused by halogenides of alkaline metals is still the problem. In this study, wetting and electrochemical properties of different coatings on pure titanium were investigated in 0.5 M aqueous NaCl solution. For the investigation we have chosen three types of coatings, developed in our group: coating produced by plasma electrolytic oxidation in phosphate electrolyte (hereinafter referred to as a PEOcoating) [1], by two step treatment (referred to as a composite hydrophobic coating) including PEO process followed by adsorption of hydrophobic agent from the solution [2,3], and by two step treatment (nanocomposite superhydrophobic coating), where the superhydrophobic nanocopmosite coating [3–5] was deposited onto the PEO-coating. For the comparison, a native oxide film on titanium formed during natural oxidation of the titanium sample at ambient conditions was also studied. Improved corrosion resistance of materials underwent the PEO process, both alone and as a part of composite coatings procedure [6,7], on the one hand,
∗ Corresponding author. Tel.: +7 4232312588; fax: +7 4232312590. E-mail address: [email protected] (S.L. Sinebryukhov). 0927-7757/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2011.01.024
and extremely high anticorrosive protection for various metals demonstrated by highly hydrophobic and superhydrophobic coatings [8–13], on the other hand, determined our choice of coatings for the comparative analysis. 2. Experimental 2.1. Materials The commercially pure titanium VT1-0 (99.3 wt.% of Ti) plates with different coatings were used as samples. PEO-coatings were obtained in phosphate electrolyte, as described in [1] and demonstrated contact angles less than 90◦ (Table 1). The samples pretreated by oxidation were additionally modified by two methods. In the first one the sample was immersed in 2% solution of hydrophobic agent in 99% decane (Acros Organics) for 2 h. Chemisorption of hydrophobic agent on the sample surface and the following treatment in an ultrasonic bath in 99.9% ethyl alcohol (from Merck) and three times in distilled water for 5 min provided the hydrophobization of the sample, with the characteristic contact angle about 108◦ . Superhydrophobic coatings, characterized by contact angles higher 160◦ and rolling angles less than 7◦ , were produced at ambient conditions by deposition from nanoparticle dispersions in decane, as described in [4,5]. The same hydrophobic agent, methoxy-{3-[(2,2,3,3,4,4,5,5,6,6,7,7,8,8,8-pentadecafluorooctyl)
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Table 1 Initial contact angles (in degrees) for various samples. Sample
Titanium with PEO coating
Titanium with composite coating
Titanium with nanocomposite superhydrophobic coating
Initial contact angle
53.3 ± 3.2
107.6 ± 4.9
165.8 ± 2.7
oxy]propyl}-silane, was used to prepare both the hydrophobic and the superhydrophobic coatings on metal surface. The application of abovementioned hydrophobic agent allows one not only to decrease the surface energy of coating but at the same time to use it as a binder of a nanocomposite coating to the substrate and as a binder between nanoparticles. The designed coating is characterized by the heterogeneous regime of wetting which is secured by the multimodality of roughness due to aggregation of aerosil nanoparticles. The typical AFM image of the coating is shown in Fig. 1. The thickness of hydrophobic coating was of the order of 1 monolayer, whereas for the superhydrophobic coating the thickness was less than 1 m. 0.5 M solution (pH = 7) of chemically pure NaCl in distilled water was used as a testing liquid for both the wetting and the electrochemical studies. 2.2. Electrochemical studies Electrochemical properties of the surface layers were investigated using a Series G300 potentiostat/galvanostat (Gamry Instruments) with the computer interface. Measurements were carried out at ambient conditions in a three-electrode cell. A platinum coated niobium mesh was used as a counter electrode and a silver/silver chloride electrode filled with saturated KCl solution as a reference electrode (the potential versus the normal hydrogen electrode is equal to +0.201 V). The exposed sample surface area was 1 cm2 . Before the beginning of the electrochemical measurements, the samples were kept in the solution for 15 min in order to stabilize the free corrosion potential (EC ). The potentiodynamic polarization curves were obtained at a scan rate of 1 mV/s. Specimens were polarized from −30 mV versus EC in anodic direction up to +2 V. For the electrochemical impedance spectroscopy (EIS) measurements a 10 mV (rms) sine-wave perturbation signal was used. Impedance spectra were acquired at an open circuit potential over the frequency range from 0.02 Hz to 0.3 MHz. The experiments were controlled by DC105TM and EIS300TM software (Gamry Instruments). The corrosion characteristics such as a polarization resistance Rp , corrosion current IC , and EC were derived from potentiodynamic tests using Stearn–Geary method [14].
2.3. Contact angle measurements To measure the contact angles we have used the method of digital video image processing of a sessile drop of the testing liquid on the investigated surface. The typical volume of drops varied in the range from 90 to 110 l. The home made experimental setup for obtaining the optical images of sessile drops and software for subsequent determination of drop parameters using Laplace curve fitting routine were described earlier [15]. A monochrome digital camera Pixelink PL-B686MU with space resolution 1280 × 1024, color resolution 256 gray levels, and time resolution 25 frames per second was used to capture the drop images. Typical time, necessary for equilibrating the initial shape of deposited drop, does not exceed several milliseconds and in following the angle obtained in 2 s after the drop deposition we will refer to as an initial wetting angle. The measured contact angles, as follows from the behaviour of contact diameter, correspond to advancing contact angles. To characterize the wettability of coatings, initial contact angles were measured on three to five various points on the surface of each sample, with the average angle for ten consecutive images of the drop being defined for each location. The accuracy of contact angle determination was not worse than 0.1◦ 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. The electrochemical reaction associated with substrate/brine interactions was studied through the long-term time variation in the drop contact angle, contact diameter and liquid/vapour surface tension. To minimize the influence of drop evaporation on the measured values of contact angles the double wall experimental cell [5] with vapour pressure inside, equal to the pressure saturated with respect to a flat liquid vapour interface of 0.5 M NaCl aqueous solution was used. To measure the rolling angle, the 10–15 l drop was deposited onto the surface. After the initial drop shape equilibration, the manipulating with experimental cell positioner allowed us to vary smoothly the inclination of the sample surface and to detect the rolling angle.
Fig. 1. The AFM image of superhydrophobic coating on surface of titanium with PEO-coating. The image was obtained using a MultiMode atomic-force microscope with a Nanoscope IV controller (Veeco, USA) in the tapping mode.
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Fig. 2. Potentiodynamic polarization curves in 0.5 M aqueous NaCl solution for a titanium samples with different surface layers: 1 – native oxide; 2 – PEO-coating; 3 – composite hydrophobic coating; 4 – nanocomposite superhydrophobic coating.
3. Results and discussion The results of investigation of corrosion process caused by the interaction of brine solution with different coatings on titanium samples by electrochemical methods are presented in Fig. 2 and in Table 2. The analysis of potentiodynamic curves (Fig. 2) and corrosion characteristics (Table 2) indicates that PEO-coatings as well as the composite hydrophobic and nanocomposite superhydrophobic coatings significantly improve the corrosion characteristics of the substrate material. The positive shift of EC , multiple increase in Rp , and decrease in IC on several orders of magnitude, as compared to bare titanium, indicate the essential inhibition of the corrosion. The greatest protective properties are demonstrated by nanocomposite superhydrophobic sample (curve 4 in Fig. 2), for which EC is more positive than corresponding values for both the native oxide (curve 1 in Fig. 2) and the composite hydrophobic layer (curve 3 in Fig. 2). The value of IC for superhydrophobic layer is two orders of magnitude lower than one for the native oxide (Table 2) and almost order of magnitude lower than one for the PEO-coating. The difference between the values of the currents at the anodic bias can reach even larger values. Thus the value of corrosion current for the sample with the nanocomposite superhydrophobic layer at E = 1.0 V is four orders of magnitude smaller than the corrosion current for the sample with a native oxide. The specific structure of aqueous solution [13] and double electric layer in the vicinity of hydrophobic surface might be responsible for more positive EC value (+0.400 V), characteristic for the sample with PEO-coating in comparison to the correspondent values for composite hydrophobic (EC = 0.068 V) and nanocomposite superhydrophobic coatings (EC = 0.616 V). The comparison of potentiodynamic curves for samples 3 and 4 convincingly demonstrates the higher stability of superhydrophobic coating. Thus, the hydrophobic layer shows a larger rate of the current increase during anodic polarization than the superhydrophobic surface, for the potential values higher than 1.5 V.
Fig. 3. Bode plots of the investigated samples in 0.5 M aquous solution NaCl. Sample numeration corresponds to that in Fig. 2.
The corrosion behaviour of the samples in corrosive electrolyte was examined by EIS measurements. The obtained EIS data are presented in Fig. 3 in the form of Bode plots (i.e. the dependences of the impedance modulus and phase angle versus frequency). These data confirm the high dielectric properties of the composite and nanocomposite layers obtained on the basis of pre-formed PEO layer (curves 3 and 4 in Fig. 3). The best protective effect was achieved for the nanocomposite superhydrophobic coating. For this layer the impedance modulus |Z| measured at a frequency f = 0.02 Hz is almost three orders of magnitude higher than one for the sample with a native oxide film (Fig. 3 and Table 2). The graph of the phase angle (theta) versus frequency (Fig. 3) is extremely informative for the development of the model representations of the surface morphology of the studied layers. On the basis of experiment impedance data, the equivalent electric circuits (EEC), representing the charge transfer through the interface between the surface layer and the electrolyte, can be constructed (Fig. 4). In our work, the constant phase element (CPE) [7,16–18] was used in the equivalent circuits instead of the ideal electrical capacitance. This element better describes behaviour of the coatings having heterogeneities of the mesostructure and/or of the chemical composition. The impedance of this element is given as [19] ZCPE = 1/Qo (jω)n , where j is an imaginary unit, ω is an angular frequency (ω = 2f), n and Qo are an exponential coefficient and a frequency-independent constant, respectively [7,16–18]. The electrochemical behaviour of the bare titanium (curve 1 in Fig. 3b) demonstrates one maximum, which is associated with one time constant characterized by equivalent electrical circuit (Fig. 4a).
Table 2 Electrochemical parameters of the samples. Samplea
EC (V)
IC (A/cm2 )
Rp ( cm2 )
|Z|f=0.02 Hz ( cm2 )
1 2 3 4
−0.430 0.400 0.068 0.616
1.4 × 10−7 4.3 × 10−9 2.8 × 10−9 1.4 × 10−9
2.6 × 105 7.2 × 105 1.3 × 107 2.4 × 107
2.7 × 105 8.2 × 105 8.6 × 106 2.1 × 108
a
Sample numeration corresponds to that in Fig. 2.
Fig. 4. Equivalent electrical circuits used for fitting the experimental impedance data for the titanium samples: (a) with native oxide; (b) with PEO-coating [7,16–18].
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The EEC with two time constants simulates the PEO layer on the titanium surface. As it was shown in [7,16–18], such layers are well described by EEC with two R-CPE circuits (Fig. 4b). In our case the spectrum for the PEO-coating having a porous and a poreless layers was also well described with this EEC. Values of 2 (chi-squared) are approximately equal to 10−4 . 2 is the square of the standard deviation between the original data and the calculated spectrum. The experimental impedance spectra for both the composite hydrophobic and the nanocomposite superhydrophobic coatings can be well fitted with the EEC shown in Fig. 4b. The high values of the impedance modulus for the coatings containing hydrophobic and superhydrophobic layers (curves 3 and 4 in Fig. 3) are related to the presence of compact layers of dielectric fluorooxysilane polymer. Besides, the air bubbles embedded inside the grooves of the superhydrophobic coating further increase the impedance modulus. Accordingly, |Z| values for the samples 3 and 4 at a frequency of 3 × 105 Hz do not reach the resistance of the electrolyte (tens of cm2 ) in contrast to the samples with the native oxide layer and the PEO coating (curves 1 and 2). Thus the data of Fig. 3 convincingly demonstrate higher ohmic barrier for corrosion reaction in the cases of the composite hydrophobic and the nanocomposite superhydrophobic coatings, leading to the inhibition of corrosion process. The alternative method used in our study to analyse the rate and mechanisms of corrosion processes for different coatings on titanium samples was the analysis of wetting dynamics. In Fig. 5 the evolutions of contact angle created by the sessile drop of 0.5 M NaCl brine solution with time elapsed after drop deposition are presented for the samples 2, 3 and 4. The comparison of data, related to different coatings gives (in agreement with electrochemical study results), that the most intensive corrosion process among three presented coatings, takes place at PEO coating. For this sample both the rate of contact angle diminishing and the time necessary to reach the passivation of process (as revealed by nearly constant contact angle) were the highest. At the same time the decrease in contact angle for the nanocomposite superhydrophobic coating during 50 h does not exceed 1–3◦ at different points along
Fig. 5. The evolution of contact angle with time after drop of brine deposition. 2 – PEO-coating; 3 – composite hydrophobic coating; 4 – nanocomposite superhydrophobic coating. Solid lines are used to guide the eye.
Fig. 6. Bode diagram for the sample 4 after different times of exposure to 0.5 M aqueous NaCl solution. Exposure time: 1 – 15 min, 2 – 1 h, 3 – 6 h, 4 – 8 h, 5 – 9 h, 6 – 12 h, 7 – 24 h, and 8 – 72 h. For the comparison, curve 9 represents the spectra of the sample with PEO-coating obtained in a phosphate electrolyte.
the sample, indicating the extremely high anticorrosion protective action of the nanocomposite superhydrophobic coating. The electrochemical impedance spectroscopy data for the same sample being immersed in sodium chloride solution for the long period indicated some minor transformation of the impedance spectra occurring after 8 h (Fig. 6). Afterwards the state of the composite layer was stabilized and the spectrum after 72 h of exposure was almost unchanged. For better understanding the data of wetting and EIS measurements we have redrawn the contact angle data in different coordinates. Taking into account that for both the PEO coating and the composite hydrophobic coating the homogeneous wetting regime was realized (as follows from the values of contact angle ), it was reasonable to represent the wettability data in the coordinates LV cos versus time (Fig. 7). Then, according to Derjaguin-Wenzel equation, LV cos = r( SV − SL ) (where LV is the surface tension of liquid and r is the surface roughness parameter), and thus the behaviour of LV cos reflects the time variation of the difference between the solid/vapour ( SV ) and the solid/liquid ( SL ) surface energies. The behaviour demonstrated by the sample 2 might be easily interpreted, as indicative of the continuous growth (along the drop/substrate contact area) of the oxide layer thickness, accompanied by decrease in SL . For composite hydrophobic coating the dependence is characterized by 3 regimes. For the first one, the intensive increase in SV − SL with sign change from negative to positive during nearly 10 first hours after drop deposition is demonstrated. Taking into account the decrease in liquid/vapour surface tension of the drop within the same temporal period such behaviour is interpreted as a result of the partial hydrolysis of those fluorooxysilane molecules weakly bonded to the surface. The second regime is associated in our study with oxide layer growth which is essentially weakened by blocking of mass transfer through the layer of hydrophobic agent. And the third regime, corresponding to contact angle constancy indicates the passive state of the surface. To analyse the mechanism of protective action of the superhydrophobic coating and taking into account the heterogeneous regime of wetting of such coating we redraw the data obtained in the coordinates cos = f(1/ LV ) (Fig. 8). On the basis of the Cassie–Baxter equation, the portion of wetted area, fw , can be eval-
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energy and hence no measurable revealing of corrosion reaction was detected. 4. Conclusions
Fig. 7. The evolution of LV cos = r( SV − SL ) with time after drop of brine deposition: 2 – PEO-coating; 3 – composite hydrophobic coating. Solid lines are used to guide the eye.
uated from the y-coordinate intercept of the linear fit to above dependence. It was found that our superhydrophobic coatings are characterized by fw = 0.027. Thus due to the superhydrophobicity and heterogeneous regime of wetting, when testing liquid contacts partly with coating surface and partly with gas bubbles embedded inside the grooves, less than 3% of real surface of superhydrophobic coating actually contacts with the brine solution. Besides, due to much higher thickness of the superhydrophobic layer the surface of titanium sample treated by plasma electrolytic oxidation is better isolated from aqueous medium. As a result, no essential change in the difference between solid/vapour and solid/liquid surface
Fig. 8. The correlation between nanocomposite superhydrophobic coating wettability and liquid/vapour surface tension variation in time after drop deposition.
The combined analysis of the evolution of sessile drop parameters in contact with surface layer jointly with the data, obtained by electrochemical methods, gives the possibility to study the variation in the state of the surface due to the electrochemical reaction, associated with a corrosion process. We have compared the peculiarities of the corrosion process on titanium samples protected by native oxide, plasma electrolytic oxidation, composite hydrophobic coating, and nanocomposite superhydrophobic coating. By the analysis of wetting it was found that joint action of PEO treatment and adsorption of hydrophobic agent leads to the inhibition of the corrosion reaction and provides the anticorrosive behaviour of titanium sample. The analysis of potentiodynamic polarization data in conjunction with EIS spectra and appropriate equivalent circuit models enabled us to conclude that corrosion processes with nanocomposite superhydrophobic coating is inhibited much more effectively. The mechanisms of inhibition for nanocomposite superhydrophobic coatings, as follows from electrochemical and wettability data, is related to very low portion of coating area being in a real contact with electrolyte solution, the higher ohmic barrier for corrosion reaction, and the shift of free corrosion potential towards positive values, generated by the hydrophobic or the superhydrophobic layers. Acknowledgements This research was supported by the Presidential program of support for the Leading Scientific Schools (Project NSh-7853.2010.3), by the Federal target programme “Research and Pedagogical Cadre for Innovative Russia” for 2009-2013 (Contracts Nos. 02.740.11.0649 and 02.740.11.0261), by the Russian Foundation for Basic Research (Grant No. 09-03-00025-a), by the Presidium FEB RAS (Grant No. 09-II-UO-04-004). References [1] S.V. Gnedenkov, O.A. Khrisanfova, A.G. Zavidnaya, Plasma Electrolytic Oxidation of Metals and Alloys in Tartrate Containing Electrolytes, Dalnauka, 2008. [2] L.B. Boinovich, A.M. Emel’yanenko, A.S. Pashinin, Interactions of silicone rubbers, designed for electrical engineering applications, with aqueous media, Prot. Met. 45 (2009) 89–94. [3] L.B. Boinovich, A.M. Emelyanenko, Hydrophobic materials and coatings: principles of design, properties and applications, Usp. Khim. 77 (2008) 619–638. [4] L.B. Boinovich, A.M. Emelyanenko, Principles of design of superhydrophobic coatings by the deposition from dispersions, Langmuir 25 (2009) 2907– 2912. [5] L.B. Boinovich, A.M. Emelyanenko, A.S. Pashinin, Analysis of long-term durability of superhydrophobic properties under continuous contact with water, ACS Appl. Mater. Interfaces 2 (6) (2010) 1754–1758. [6] S.L. Sinebryukhov, A.S. Gnedenkov, O.A. Khrisanfova, S.V. Gnedenkov, The influence of plasma electrolytic oxidation on the mechanical characteristics of the NiTi alloys, Surf. Eng. 25 (2009) 565–569. [7] S.V. Gnedenkov, S.L. Sinebryukhov, Composite polymer containing coatings on the surface of metals and alloy, Compos. Interfaces 16 (2009) 387– 405. [8] T. Ishizaki, J. Hieda, N. Saito, N. Saito, O. Takai, Corrosion resistance and chemical stability of super-hydrophobic film deposited on magnesium alloy AZ31 by microwave plasma-enhanced chemical vapor deposition, Electrochim. Acta 55 (2010) 7094–7101. [9] J. Kijlstra, K. Reihs, A. Klamt, Roughness and topology of ultra-hydrophobic surfaces, Colloids Surf. A: Physicochem. Eng. Aspects 206 (2002) 521– 529. [10] T. Liu, S. Chen, S. Cheng, J. Tian, X. Chang, Y. Yin, Corrosion behaviour of super-hydrophobic surface on copper in seawater, Electrochim. Acta 52 (2007) 8003–8007. [11] L. Zhu, Y. Jin, A novel method to fabricate water-soluble hydrophobic agent and superhydrophobic film on pretreated metals, Appl. Surf. Sci. 253 (2007) 3432–3439.
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[12] H. Liu, S. Szunerits, W. Xu, R. Boukherroub, Preparation of superhydrophobic coatings on zinc as effective corrosion barriers, Appl. Mater. Interfaces 1 (2009) 1150–1153. [13] P.M. Barkhudarov, P.B. Shah, E.B. Watkins, D.A. Doshi, C.J. Brinker, J. Majewski, Corrosion inhibition using superhydrophobic films, Corros. Sci. 50 (2008) 897–902. [14] M. Stern, A.L. Geary, Electrochemical polarization: I. A theoretical analysis of the shape of polarization curves, J. Electrochem. Soc. 104 (1957) 56–63. [15] A.M. Emel’yanenko, L.B. Boinovich, The use of digital processing of video images for determining parameters of sessile and pendant droplets, Colloid J. 63 (2001) 159–172.
[16] S.V. Gnedenkov, S.L. Sinebryukhov, D.V. Mashtalyar, V.S. Egorkin, A.K. Tsvetnikov, A.N. Minaev, Charge transfer at the antiscale composite layer–electrolyte interface, Prot. Met. 43 (2007) 667–673. [17] S.V. Gnedenkov, S.L. Sinebryukhov, Electrochemical impedance spectroscopy of oxide layers on the titanium surface, Russ. J. Electrochem. 41 (2005) 858– 865. [18] S.V. Gnedenkov, S.L. Sinebryukhov, V.I. Sergienko, Electrochemical impedance simulation of a metal oxide heterostructure/electrolyte interface: a review, Russ. J. Electrochem. 42 (2006) 197–211. [19] E. Barsoukov, J. Ross Macdonald, Impedance Spectroscopy: Theory, Experiment, and Applications, John Wiley & Sons, Inc., 2005.
Contents lists available at ScienceDirect
Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa
Wetting and electrochemical properties of hydrophobic and superhydrophobic coatings on titanium S.V. Gnedenkov a , S.L. Sinebryukhov a,∗ , V.S. Egorkin a , D.V. Mashtalyar a , D.A. Alpysbaeva b , L.B. Boinovich b a b
Institute of Chemistry FEB RAS, Vladivostok, Russia A.N. Frumkin Institute of Physical Chemistry and Electrochemistry, Moscow, Russia
a r t i c l e
i n f o
Article history: Received 10 November 2010 Received in revised form 24 December 2010 Accepted 7 January 2011 Available online 18 January 2011 Keywords: Superhydrophobic coating Plasma electrolytic oxidation Anticorrosion properties Electrochemical impedance spectroscopy Wetting
a b s t r a c t The combined analysis of the evolution of sessile drop parameters in contact with surface layer jointly with the data, obtained by electrochemical methods, gives the possibility to study the variation in the state of the surface due to the electrochemical reaction, associated with a corrosion process. We have compared the peculiarities of the corrosion process on titanium samples protected by (1) native oxide, (2) plasma electrolytic oxidation, (3) joint action of plasma electrolytic oxidation and hydrophobic layer as well as (4) plasma electrolytic oxidation with nanocomposite superhydrophobic coating. Among the coatings have been studied, the most effective corrosion protection in brine solutions was demonstrated by the nanocomposite superhydrophobic coating. The mechanisms of the corrosion protection are discussed. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Titanium and its alloys are widely used as engineering materials. They also find application as implant materials for dentures and human bodies because of their corrosion resistance and compatibility with biological tissues. However, the corrosion caused by halogenides of alkaline metals is still the problem. In this study, wetting and electrochemical properties of different coatings on pure titanium were investigated in 0.5 M aqueous NaCl solution. For the investigation we have chosen three types of coatings, developed in our group: coating produced by plasma electrolytic oxidation in phosphate electrolyte (hereinafter referred to as a PEOcoating) [1], by two step treatment (referred to as a composite hydrophobic coating) including PEO process followed by adsorption of hydrophobic agent from the solution [2,3], and by two step treatment (nanocomposite superhydrophobic coating), where the superhydrophobic nanocopmosite coating [3–5] was deposited onto the PEO-coating. For the comparison, a native oxide film on titanium formed during natural oxidation of the titanium sample at ambient conditions was also studied. Improved corrosion resistance of materials underwent the PEO process, both alone and as a part of composite coatings procedure [6,7], on the one hand,
∗ Corresponding author. Tel.: +7 4232312588; fax: +7 4232312590. E-mail address: [email protected] (S.L. Sinebryukhov). 0927-7757/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2011.01.024
and extremely high anticorrosive protection for various metals demonstrated by highly hydrophobic and superhydrophobic coatings [8–13], on the other hand, determined our choice of coatings for the comparative analysis. 2. Experimental 2.1. Materials The commercially pure titanium VT1-0 (99.3 wt.% of Ti) plates with different coatings were used as samples. PEO-coatings were obtained in phosphate electrolyte, as described in [1] and demonstrated contact angles less than 90◦ (Table 1). The samples pretreated by oxidation were additionally modified by two methods. In the first one the sample was immersed in 2% solution of hydrophobic agent in 99% decane (Acros Organics) for 2 h. Chemisorption of hydrophobic agent on the sample surface and the following treatment in an ultrasonic bath in 99.9% ethyl alcohol (from Merck) and three times in distilled water for 5 min provided the hydrophobization of the sample, with the characteristic contact angle about 108◦ . Superhydrophobic coatings, characterized by contact angles higher 160◦ and rolling angles less than 7◦ , were produced at ambient conditions by deposition from nanoparticle dispersions in decane, as described in [4,5]. The same hydrophobic agent, methoxy-{3-[(2,2,3,3,4,4,5,5,6,6,7,7,8,8,8-pentadecafluorooctyl)
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Table 1 Initial contact angles (in degrees) for various samples. Sample
Titanium with PEO coating
Titanium with composite coating
Titanium with nanocomposite superhydrophobic coating
Initial contact angle
53.3 ± 3.2
107.6 ± 4.9
165.8 ± 2.7
oxy]propyl}-silane, was used to prepare both the hydrophobic and the superhydrophobic coatings on metal surface. The application of abovementioned hydrophobic agent allows one not only to decrease the surface energy of coating but at the same time to use it as a binder of a nanocomposite coating to the substrate and as a binder between nanoparticles. The designed coating is characterized by the heterogeneous regime of wetting which is secured by the multimodality of roughness due to aggregation of aerosil nanoparticles. The typical AFM image of the coating is shown in Fig. 1. The thickness of hydrophobic coating was of the order of 1 monolayer, whereas for the superhydrophobic coating the thickness was less than 1 m. 0.5 M solution (pH = 7) of chemically pure NaCl in distilled water was used as a testing liquid for both the wetting and the electrochemical studies. 2.2. Electrochemical studies Electrochemical properties of the surface layers were investigated using a Series G300 potentiostat/galvanostat (Gamry Instruments) with the computer interface. Measurements were carried out at ambient conditions in a three-electrode cell. A platinum coated niobium mesh was used as a counter electrode and a silver/silver chloride electrode filled with saturated KCl solution as a reference electrode (the potential versus the normal hydrogen electrode is equal to +0.201 V). The exposed sample surface area was 1 cm2 . Before the beginning of the electrochemical measurements, the samples were kept in the solution for 15 min in order to stabilize the free corrosion potential (EC ). The potentiodynamic polarization curves were obtained at a scan rate of 1 mV/s. Specimens were polarized from −30 mV versus EC in anodic direction up to +2 V. For the electrochemical impedance spectroscopy (EIS) measurements a 10 mV (rms) sine-wave perturbation signal was used. Impedance spectra were acquired at an open circuit potential over the frequency range from 0.02 Hz to 0.3 MHz. The experiments were controlled by DC105TM and EIS300TM software (Gamry Instruments). The corrosion characteristics such as a polarization resistance Rp , corrosion current IC , and EC were derived from potentiodynamic tests using Stearn–Geary method [14].
2.3. Contact angle measurements To measure the contact angles we have used the method of digital video image processing of a sessile drop of the testing liquid on the investigated surface. The typical volume of drops varied in the range from 90 to 110 l. The home made experimental setup for obtaining the optical images of sessile drops and software for subsequent determination of drop parameters using Laplace curve fitting routine were described earlier [15]. A monochrome digital camera Pixelink PL-B686MU with space resolution 1280 × 1024, color resolution 256 gray levels, and time resolution 25 frames per second was used to capture the drop images. Typical time, necessary for equilibrating the initial shape of deposited drop, does not exceed several milliseconds and in following the angle obtained in 2 s after the drop deposition we will refer to as an initial wetting angle. The measured contact angles, as follows from the behaviour of contact diameter, correspond to advancing contact angles. To characterize the wettability of coatings, initial contact angles were measured on three to five various points on the surface of each sample, with the average angle for ten consecutive images of the drop being defined for each location. The accuracy of contact angle determination was not worse than 0.1◦ 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. The electrochemical reaction associated with substrate/brine interactions was studied through the long-term time variation in the drop contact angle, contact diameter and liquid/vapour surface tension. To minimize the influence of drop evaporation on the measured values of contact angles the double wall experimental cell [5] with vapour pressure inside, equal to the pressure saturated with respect to a flat liquid vapour interface of 0.5 M NaCl aqueous solution was used. To measure the rolling angle, the 10–15 l drop was deposited onto the surface. After the initial drop shape equilibration, the manipulating with experimental cell positioner allowed us to vary smoothly the inclination of the sample surface and to detect the rolling angle.
Fig. 1. The AFM image of superhydrophobic coating on surface of titanium with PEO-coating. The image was obtained using a MultiMode atomic-force microscope with a Nanoscope IV controller (Veeco, USA) in the tapping mode.
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Fig. 2. Potentiodynamic polarization curves in 0.5 M aqueous NaCl solution for a titanium samples with different surface layers: 1 – native oxide; 2 – PEO-coating; 3 – composite hydrophobic coating; 4 – nanocomposite superhydrophobic coating.
3. Results and discussion The results of investigation of corrosion process caused by the interaction of brine solution with different coatings on titanium samples by electrochemical methods are presented in Fig. 2 and in Table 2. The analysis of potentiodynamic curves (Fig. 2) and corrosion characteristics (Table 2) indicates that PEO-coatings as well as the composite hydrophobic and nanocomposite superhydrophobic coatings significantly improve the corrosion characteristics of the substrate material. The positive shift of EC , multiple increase in Rp , and decrease in IC on several orders of magnitude, as compared to bare titanium, indicate the essential inhibition of the corrosion. The greatest protective properties are demonstrated by nanocomposite superhydrophobic sample (curve 4 in Fig. 2), for which EC is more positive than corresponding values for both the native oxide (curve 1 in Fig. 2) and the composite hydrophobic layer (curve 3 in Fig. 2). The value of IC for superhydrophobic layer is two orders of magnitude lower than one for the native oxide (Table 2) and almost order of magnitude lower than one for the PEO-coating. The difference between the values of the currents at the anodic bias can reach even larger values. Thus the value of corrosion current for the sample with the nanocomposite superhydrophobic layer at E = 1.0 V is four orders of magnitude smaller than the corrosion current for the sample with a native oxide. The specific structure of aqueous solution [13] and double electric layer in the vicinity of hydrophobic surface might be responsible for more positive EC value (+0.400 V), characteristic for the sample with PEO-coating in comparison to the correspondent values for composite hydrophobic (EC = 0.068 V) and nanocomposite superhydrophobic coatings (EC = 0.616 V). The comparison of potentiodynamic curves for samples 3 and 4 convincingly demonstrates the higher stability of superhydrophobic coating. Thus, the hydrophobic layer shows a larger rate of the current increase during anodic polarization than the superhydrophobic surface, for the potential values higher than 1.5 V.
Fig. 3. Bode plots of the investigated samples in 0.5 M aquous solution NaCl. Sample numeration corresponds to that in Fig. 2.
The corrosion behaviour of the samples in corrosive electrolyte was examined by EIS measurements. The obtained EIS data are presented in Fig. 3 in the form of Bode plots (i.e. the dependences of the impedance modulus and phase angle versus frequency). These data confirm the high dielectric properties of the composite and nanocomposite layers obtained on the basis of pre-formed PEO layer (curves 3 and 4 in Fig. 3). The best protective effect was achieved for the nanocomposite superhydrophobic coating. For this layer the impedance modulus |Z| measured at a frequency f = 0.02 Hz is almost three orders of magnitude higher than one for the sample with a native oxide film (Fig. 3 and Table 2). The graph of the phase angle (theta) versus frequency (Fig. 3) is extremely informative for the development of the model representations of the surface morphology of the studied layers. On the basis of experiment impedance data, the equivalent electric circuits (EEC), representing the charge transfer through the interface between the surface layer and the electrolyte, can be constructed (Fig. 4). In our work, the constant phase element (CPE) [7,16–18] was used in the equivalent circuits instead of the ideal electrical capacitance. This element better describes behaviour of the coatings having heterogeneities of the mesostructure and/or of the chemical composition. The impedance of this element is given as [19] ZCPE = 1/Qo (jω)n , where j is an imaginary unit, ω is an angular frequency (ω = 2f), n and Qo are an exponential coefficient and a frequency-independent constant, respectively [7,16–18]. The electrochemical behaviour of the bare titanium (curve 1 in Fig. 3b) demonstrates one maximum, which is associated with one time constant characterized by equivalent electrical circuit (Fig. 4a).
Table 2 Electrochemical parameters of the samples. Samplea
EC (V)
IC (A/cm2 )
Rp ( cm2 )
|Z|f=0.02 Hz ( cm2 )
1 2 3 4
−0.430 0.400 0.068 0.616
1.4 × 10−7 4.3 × 10−9 2.8 × 10−9 1.4 × 10−9
2.6 × 105 7.2 × 105 1.3 × 107 2.4 × 107
2.7 × 105 8.2 × 105 8.6 × 106 2.1 × 108
a
Sample numeration corresponds to that in Fig. 2.
Fig. 4. Equivalent electrical circuits used for fitting the experimental impedance data for the titanium samples: (a) with native oxide; (b) with PEO-coating [7,16–18].
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The EEC with two time constants simulates the PEO layer on the titanium surface. As it was shown in [7,16–18], such layers are well described by EEC with two R-CPE circuits (Fig. 4b). In our case the spectrum for the PEO-coating having a porous and a poreless layers was also well described with this EEC. Values of 2 (chi-squared) are approximately equal to 10−4 . 2 is the square of the standard deviation between the original data and the calculated spectrum. The experimental impedance spectra for both the composite hydrophobic and the nanocomposite superhydrophobic coatings can be well fitted with the EEC shown in Fig. 4b. The high values of the impedance modulus for the coatings containing hydrophobic and superhydrophobic layers (curves 3 and 4 in Fig. 3) are related to the presence of compact layers of dielectric fluorooxysilane polymer. Besides, the air bubbles embedded inside the grooves of the superhydrophobic coating further increase the impedance modulus. Accordingly, |Z| values for the samples 3 and 4 at a frequency of 3 × 105 Hz do not reach the resistance of the electrolyte (tens of cm2 ) in contrast to the samples with the native oxide layer and the PEO coating (curves 1 and 2). Thus the data of Fig. 3 convincingly demonstrate higher ohmic barrier for corrosion reaction in the cases of the composite hydrophobic and the nanocomposite superhydrophobic coatings, leading to the inhibition of corrosion process. The alternative method used in our study to analyse the rate and mechanisms of corrosion processes for different coatings on titanium samples was the analysis of wetting dynamics. In Fig. 5 the evolutions of contact angle created by the sessile drop of 0.5 M NaCl brine solution with time elapsed after drop deposition are presented for the samples 2, 3 and 4. The comparison of data, related to different coatings gives (in agreement with electrochemical study results), that the most intensive corrosion process among three presented coatings, takes place at PEO coating. For this sample both the rate of contact angle diminishing and the time necessary to reach the passivation of process (as revealed by nearly constant contact angle) were the highest. At the same time the decrease in contact angle for the nanocomposite superhydrophobic coating during 50 h does not exceed 1–3◦ at different points along
Fig. 5. The evolution of contact angle with time after drop of brine deposition. 2 – PEO-coating; 3 – composite hydrophobic coating; 4 – nanocomposite superhydrophobic coating. Solid lines are used to guide the eye.
Fig. 6. Bode diagram for the sample 4 after different times of exposure to 0.5 M aqueous NaCl solution. Exposure time: 1 – 15 min, 2 – 1 h, 3 – 6 h, 4 – 8 h, 5 – 9 h, 6 – 12 h, 7 – 24 h, and 8 – 72 h. For the comparison, curve 9 represents the spectra of the sample with PEO-coating obtained in a phosphate electrolyte.
the sample, indicating the extremely high anticorrosion protective action of the nanocomposite superhydrophobic coating. The electrochemical impedance spectroscopy data for the same sample being immersed in sodium chloride solution for the long period indicated some minor transformation of the impedance spectra occurring after 8 h (Fig. 6). Afterwards the state of the composite layer was stabilized and the spectrum after 72 h of exposure was almost unchanged. For better understanding the data of wetting and EIS measurements we have redrawn the contact angle data in different coordinates. Taking into account that for both the PEO coating and the composite hydrophobic coating the homogeneous wetting regime was realized (as follows from the values of contact angle ), it was reasonable to represent the wettability data in the coordinates LV cos versus time (Fig. 7). Then, according to Derjaguin-Wenzel equation, LV cos = r( SV − SL ) (where LV is the surface tension of liquid and r is the surface roughness parameter), and thus the behaviour of LV cos reflects the time variation of the difference between the solid/vapour ( SV ) and the solid/liquid ( SL ) surface energies. The behaviour demonstrated by the sample 2 might be easily interpreted, as indicative of the continuous growth (along the drop/substrate contact area) of the oxide layer thickness, accompanied by decrease in SL . For composite hydrophobic coating the dependence is characterized by 3 regimes. For the first one, the intensive increase in SV − SL with sign change from negative to positive during nearly 10 first hours after drop deposition is demonstrated. Taking into account the decrease in liquid/vapour surface tension of the drop within the same temporal period such behaviour is interpreted as a result of the partial hydrolysis of those fluorooxysilane molecules weakly bonded to the surface. The second regime is associated in our study with oxide layer growth which is essentially weakened by blocking of mass transfer through the layer of hydrophobic agent. And the third regime, corresponding to contact angle constancy indicates the passive state of the surface. To analyse the mechanism of protective action of the superhydrophobic coating and taking into account the heterogeneous regime of wetting of such coating we redraw the data obtained in the coordinates cos = f(1/ LV ) (Fig. 8). On the basis of the Cassie–Baxter equation, the portion of wetted area, fw , can be eval-
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energy and hence no measurable revealing of corrosion reaction was detected. 4. Conclusions
Fig. 7. The evolution of LV cos = r( SV − SL ) with time after drop of brine deposition: 2 – PEO-coating; 3 – composite hydrophobic coating. Solid lines are used to guide the eye.
uated from the y-coordinate intercept of the linear fit to above dependence. It was found that our superhydrophobic coatings are characterized by fw = 0.027. Thus due to the superhydrophobicity and heterogeneous regime of wetting, when testing liquid contacts partly with coating surface and partly with gas bubbles embedded inside the grooves, less than 3% of real surface of superhydrophobic coating actually contacts with the brine solution. Besides, due to much higher thickness of the superhydrophobic layer the surface of titanium sample treated by plasma electrolytic oxidation is better isolated from aqueous medium. As a result, no essential change in the difference between solid/vapour and solid/liquid surface
Fig. 8. The correlation between nanocomposite superhydrophobic coating wettability and liquid/vapour surface tension variation in time after drop deposition.
The combined analysis of the evolution of sessile drop parameters in contact with surface layer jointly with the data, obtained by electrochemical methods, gives the possibility to study the variation in the state of the surface due to the electrochemical reaction, associated with a corrosion process. We have compared the peculiarities of the corrosion process on titanium samples protected by native oxide, plasma electrolytic oxidation, composite hydrophobic coating, and nanocomposite superhydrophobic coating. By the analysis of wetting it was found that joint action of PEO treatment and adsorption of hydrophobic agent leads to the inhibition of the corrosion reaction and provides the anticorrosive behaviour of titanium sample. The analysis of potentiodynamic polarization data in conjunction with EIS spectra and appropriate equivalent circuit models enabled us to conclude that corrosion processes with nanocomposite superhydrophobic coating is inhibited much more effectively. The mechanisms of inhibition for nanocomposite superhydrophobic coatings, as follows from electrochemical and wettability data, is related to very low portion of coating area being in a real contact with electrolyte solution, the higher ohmic barrier for corrosion reaction, and the shift of free corrosion potential towards positive values, generated by the hydrophobic or the superhydrophobic layers. Acknowledgements This research was supported by the Presidential program of support for the Leading Scientific Schools (Project NSh-7853.2010.3), by the Federal target programme “Research and Pedagogical Cadre for Innovative Russia” for 2009-2013 (Contracts Nos. 02.740.11.0649 and 02.740.11.0261), by the Russian Foundation for Basic Research (Grant No. 09-03-00025-a), by the Presidium FEB RAS (Grant No. 09-II-UO-04-004). References [1] S.V. Gnedenkov, O.A. Khrisanfova, A.G. Zavidnaya, Plasma Electrolytic Oxidation of Metals and Alloys in Tartrate Containing Electrolytes, Dalnauka, 2008. [2] L.B. Boinovich, A.M. Emel’yanenko, A.S. Pashinin, Interactions of silicone rubbers, designed for electrical engineering applications, with aqueous media, Prot. Met. 45 (2009) 89–94. [3] L.B. Boinovich, A.M. Emelyanenko, Hydrophobic materials and coatings: principles of design, properties and applications, Usp. Khim. 77 (2008) 619–638. [4] L.B. Boinovich, A.M. Emelyanenko, Principles of design of superhydrophobic coatings by the deposition from dispersions, Langmuir 25 (2009) 2907– 2912. [5] L.B. Boinovich, A.M. Emelyanenko, A.S. Pashinin, Analysis of long-term durability of superhydrophobic properties under continuous contact with water, ACS Appl. Mater. Interfaces 2 (6) (2010) 1754–1758. [6] S.L. Sinebryukhov, A.S. Gnedenkov, O.A. Khrisanfova, S.V. Gnedenkov, The influence of plasma electrolytic oxidation on the mechanical characteristics of the NiTi alloys, Surf. Eng. 25 (2009) 565–569. [7] S.V. Gnedenkov, S.L. Sinebryukhov, Composite polymer containing coatings on the surface of metals and alloy, Compos. Interfaces 16 (2009) 387– 405. [8] T. Ishizaki, J. Hieda, N. Saito, N. Saito, O. Takai, Corrosion resistance and chemical stability of super-hydrophobic film deposited on magnesium alloy AZ31 by microwave plasma-enhanced chemical vapor deposition, Electrochim. Acta 55 (2010) 7094–7101. [9] J. Kijlstra, K. Reihs, A. Klamt, Roughness and topology of ultra-hydrophobic surfaces, Colloids Surf. A: Physicochem. Eng. Aspects 206 (2002) 521– 529. [10] T. Liu, S. Chen, S. Cheng, J. Tian, X. Chang, Y. Yin, Corrosion behaviour of super-hydrophobic surface on copper in seawater, Electrochim. Acta 52 (2007) 8003–8007. [11] L. Zhu, Y. Jin, A novel method to fabricate water-soluble hydrophobic agent and superhydrophobic film on pretreated metals, Appl. Surf. Sci. 253 (2007) 3432–3439.
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