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Super-hydrophobic film prepared on zinc and its effect on corrosion in simulated marine atmosphere
Corrosion Science 69 (2013) 23–30
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Super-hydrophobic film prepared on zinc and its effect on corrosion in simulated marine atmosphere Peng Wang a, Dun Zhang a,⇑, Ri Qiu a,b, Jiajia Wu a,b, Yi Wan a a b
Key Lab of Corrosion Science, Shandong Province, Institute of Oceanology, Chinese Academy of Sciences, 7 Naihai Road, Qingdao 266071, China Graduate School of the Chinese Academy of Sciences, 19 (Jia) Yuquan Road, Beijing 100039, China
a r t i c l e
i n f o
Article history: Received 2 July 2012 Accepted 29 October 2012 Available online 7 November 2012 Keywords: A. Zinc B. SEM B. XPS B. EIS C. Atmospheric corrosion
a b s t r a c t Zinc–laurylamine complex film with super-hydrophobicity is fabricated on zinc surface with electrolysis method. Super-hydrophobicity of film results from papillae and ridge-like structures, which form due to uneven corrosion of zinc at high anodic potential. The film obtained can maintain super-hydrophobic property in solution system, and can inhibit corrosion effectively. However, saline water penetrates into super-hydrophobic film during deliquescence process of NaCl particle in simulated marine environment. This behavior is induced by capillary condensation in groove of film, and it declines the advantage of using super-hydrophobic film as corrosion barrier in marine atmosphere. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction Atmospheric corrosion of zinc occurs in almost every atmospheric environment, and marine environment is the most aggressive one due to the highly corrosive nature of airborne pollutants [1]. In marine environment, the deposition of salt particles on metal surfaces plays a vital role in atmospheric corrosion. Bubbles that form from ocean waves can generate marine aerosols containing of small salt droplets or particles. These aerosols can be carried by wind and deposit on metal surface. When the relative humidity reaches a critical value, hygroscopic salts can absorb moisture and form salty droplets, which can greatly accelerate the corrosion of metals [2]. Until now, a great deal of research work has aimed at increasing lifetime of zinc. An effective method is to fabricate a compact coating on zinc surface to inhibit aggression of corrosive medium [3–8]. Generally, premature degradation of coating is due to a double negative effect of soluble salts at the metal/paint interface. On one hand, they cause the appearance of blisters of osmotic origin on paint film. On the other hand, they accelerate the corrosion process of metallic substrate under paint [9] . Thus, the anti-corrosion performance of coating is related to its effect of preventing electrolyte permeation into coating/metal interface. For its water-repellent property, fabrication of artificial superhydrophobic surface is regarded as a potential method for corrosion protection of metal in aqueous solution. It was proven that ⇑ Corresponding author. Tel./fax: +86 532 82898960. E-mail address: [email protected] (D. Zhang). 0010-938X/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.corsci.2012.10.025
air trapped in film is the essential contributor to excellent anticorrosion effect [10–17]. In atmospheric environment, air is trapped in grooves of super-hydrophobic film, and it can prevent electrolyte to penetrate for capillary effect [18,19]. Furthermore, super-hydrophobic surface can keep away the salt and water, because water droplet on super-hydrophobic surface can easily roll off from the surface and take away salt aerosol. Accordingly, it seems that modification with super-hydrophobic film is an effective corrosion protection method for metal in atmospheric environment. To the best of our knowledge, there is no report about corrosion inhibition of super-hydrophobic film to metal in atmospheric environment. In this paper, super-hydrophobic film was fabricated on zinc surface with electrolysis method, and it was characterized with contact angle test, electrochemical measurements and so on. Based on these results, formation mechanism of super-hydrophobic film on zinc was proposed, and inhibition effect of super-hydrophobic film to marine atmospheric corrosion induced by deliquescence of NaCl particle was investigated.
2. Experimental 2.1. Materials and reagents Zinc (P99.99 wt.%) foil and a series of chemicals, including laurylamine, sodium chloride (NaCl) and ethanol, were purchased from Sinopharm Chemical Reagent Co., Ltd. Water used was Milli-Q water (Milli Q, USA).
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2.2. Modification of zinc surface In a typical procedure, zinc foil was abraded with emery paper (400 grade), and rinsed with ethanol. The growth of super-hydrophobic film was performed in a two-electrode cell, in which zinc sample and a platinum wire acted as an anode and a cathode, respectively. Film was electrochemically grown at a constant potential of 30 V for 30 min in 0.1 M laurylamine/ethanol solution with direct current power supply at 25 °C. The width between anode and cathode is 3 cm, and the area of both anode and cathode exposed to electrolysis solution is 2 cm2. After that, zinc sample was brought out, washed with ethanol, and dried naturally in atmosphere. 2.3. Surface characterization Morphology of surface was characterized with FE-SEM (JSM6700F). Contact angles of 4 lL water droplet on bare and filmed zinc were measured with contact angle meter (JC2000C1, Shanghai Zhongchen Digital Technic Apparatus Co., Ltd.) at ambient temperature. X-ray photoelectron spectroscopy (XPS) measurement was carried out on a Thermo ESCALAB 250 photoelectron spectrometer equipped with an Al-anode at a total power dissipation of 150 W (15 kV, 10 mA), and the binding energies were referenced to the C 1s line at 284.8 eV from adventitious carbon. 2.4. Monitoring of deliquescence process of NaCl Marine atmospheric environment was simulated in a conditioning chamber, where the relative humidity (RH) was controlled by means of aqueous glycerin solution according to the Standard ASTM D 5032-97 (Reapproved 2003). In conditioning chamber, the filmed zinc sample was placed above aqueous glycerin solution, and a single NaCl particle was transferred gently onto sample surface to avoid destroying micro-structure of film. Then, formation process of saline water droplet due to deliquescence of NaCl was recorded with CCD camera. 2.5. Electrochemical experiments Open circuit potential (Eoc) t curves and electrochemical impedance spectroscopy (EIS) were obtained with a computercontrolled electrochemical system (CHI 760C, CH Instruments Inc.) in 3.5 wt.% NaCl solution with temperature of 25 °C. These experiments were performed in a three-electrode cell with a platinum electrode as counter electrode, zinc / filmed zinc as working electrode, and a silver/silver chloride (Ag/AgCl, 3 M KCl) electrode as reference electrode. EIS experiments were carried out after an immersion of sample in 3.5 wt.% NaCl for 30 min. They were carried out at frequencies ranging from 105 to 102/102 Hz at open circuit potential with amplitude of perturbation voltage of 10 mV. For this measurement, 12 points are recorded per decade. Each test was repeated more than three times to verify repeatability of results. EIS results were analyzed by fitting data using Zsimpwin software. 3. 3. Results and discussion 3.1. Composition, wetability and morphology Fig. 1 shows XPS spectra of filmed zinc. From XPS survey spectra (Fig. 1a), it can be found that C and N elements exist in film, indicating existence of laurylamine species. N1s spectra (Fig. 1b) exhibit two distinct peaks at 399.4 eV and 400.0 eV. The main peak at 399.4 eV corresponds to amine group coordinated with zinc ions,
Fig. 1. XPS spectra of filmed zinc (a)XPS survey spectra, (b) N1s spectra and (c) O1s spectra.
and the weak peak at 400.0 eV is consistent with that of a free amine group [20]. O 1s peak (Fig. 1c) is decomposed into two components. The oxygen peak registered at binding energy of 531.5 eV confirms presence of water molecules [21,22], a weak peak at 530.6 eV can be ascribed to zinc oxide [21]. It is implied that the film obtained is mainly composed of complex zinc–laurylamine. There are some free laurylamine and zinc oxide in film. Fig. 2 shows contact angle photographs of bare and filmed zinc specimens. It is found that bare zinc surface is hydrophilic with water contact angle of 62 ± 3° (Fig. 2a), and filmed zinc presents
P. Wang et al. / Corrosion Science 69 (2013) 23–30
Fig. 2. Contact angle photographs of (a) bare and (b) filmed zinc.
super-hydrophobic property with water contact angle of 151.2 ± 3° (Fig. 2b). After exposure in ambient condition for 1 month, water contact angle of filmed zinc is 150.5 ± 3°, indicating that it can maintain super-hydrophobic property in atmospheric environment. In case of super-hydrophobicity, air is believed to be trapped in groove of film, and Cassie model is usually introduced to explain wettability of film. Assuming that water contact angle for air is 180°, apparent contact angle (hr) of filmed zinc can be expressed as follows [23]:
cos hr ¼ f1 cos h f2
ð1Þ
In Eq. (1), f1 and f2 are fractions of solid surface (zinc– laurylamine) and air in contact with water; hr and h are water contact angles on rough and smooth solid surfaces (zinc–laurylamine), respectively. According to Eq. (1), air trapped in groove can minimize real contact area between a water droplet and solid surface. Consequently, the hydrophobicity of surface is dramatically intensified. According to previous reports [23,24], contact angle is generally in the range 113°–115° when a compound with group CH3(CH2)m(m P 10) forms self-assembled monolayer on flat metal substrate. Thus, f1 value of the super-hydrophobic film with water contact angle of 151.2° (Fig. 3b) is 0.2030, and the air fraction is 0.7970.
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Fig. 3 presents morphology and cross section of film. Micrometer-sized papillae structures are found on filmed zinc surface (Fig. 3a), and they connect with each other by ridge-like structure (Fig. 3b). From SEM picture of cross section (Fig. 3c), it can be observed that film presents rough structure with a thickness of about 200 lm. Generally, super-hydrophobicity of film is assumed to be related to its morphology. In order to understand relevance of wettability to micro-morphology of film, image binarization method is employed to simulate contact interface between water droplet and filmed zinc surface, and it is performed through a self-written program running on the Visual Basic.net platform. Before image binarization process, some assumption should be made in advance. In a SEM image, the distance between a pixel point and substrate can be reflected by its gray scale ranging from 0 to 255. Generally, pixel point with low gray scale in image indicates that it is far away from substrate, and it is more likely to connect with water droplet. Thus, there must be a critical value for the gray scale to determine whether a pixel point contacts with water droplet. Base on this assumption, a black-white two-colored image can be extracted from gray scale image (SEM image) by assigning a threshold value with image binarization. In details, if the gray scale value is lower than a threshold value, the pixel point can be set as 0; otherwise, it is set as 1. In black-white two-colored image obtained, the point with white color is the most possible contact region of solid with water, and the point with black color indicates the region trapped by air. As we have calculated above, the solid fraction of super-hydrophobic film with a water contact angle of 151.2° (Fig. 3a) is 0.2030. In this case, a black–white two-colored image with the proportion of white pixel of 20.26% (Fig. 4) can be transferred from Fig. 3a by setting threshold value as 136. From this image, it can be observed that white pixels mainly come from top of papillae and ridge-like structure. When a water droplet is dropped on the surface, it connects with surface through the top of papillae
Fig. 3. Morphology and cross section of filmed zinc (a) bar = 100 lm, (b) bar = 10 lm and (c) bar = 200 lm.
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supersaturated. As a result, rough film with papillae structure is formed on zinc surface. Furthermore, during electrolysis process, oxygen in solution can induce formation of zinc oxide, and water molecule dissolved in solution can also combine with Zn2+ to form zinc oxide. These two aspects can explain the existence of zinc oxide on zinc surface (Fig. 1c). 3.3. The deliquescence process of NaCl particle on super-hydrophobic film
Fig. 4. Black-white two-colored image transferred from Fig. 3a with threshold value of 136.
and ridge-like structures, and air is trapped among these structures. So the presence of papillae and ridge-like structure on filmed zinc surface is main reason for its super-hydrophobicity. 3.2. Formation mechanism of super-hydrophobic film Fig. 5 is morphology of zinc substrate after removing superhydrophobic film with ultrasonic vibration. It can be observed that pits are dispersed on zinc surface (Fig. 5a), and space between two pits ranges from 10 to 40 lm (Fig. 5b), which is the same to the range of distance between two papillae on film (Fig. 3). This result implies that appearance of papillae structure on filmed zinc surface is related to formation of pits under film during electrolysis process. Based on these results above, the formation mechanism of super-hydrophobic film is proposed as follows: under high anodic potential, Zn2+ ions are released from zinc surface. These released Zn2+ ions can be captured by laurylamine moiety in solution immediately to form zinc–laurylamine complex according to reaction (2):
Zn2þ þ CH3 ðCH2 Þ11 NH2 ! Zn½CH3 ðCH2 Þ11 N þ 2Hþ 2+
ð2Þ
It can be inferred that the releasing speed of Zn at pits is higher than other places, and the concentration of zinc–laurylamine complex over pits is the highest in solution. Thus, zinc– laurylamine is prone to precipitate over pits at anode when it is
Generally, salt particle in the environment with RH higher than a critical value (saturated relative humidity of salt solution) can absorb water molecule to deliquesce. The deliquescence of salt particle is a common phenomenon in nature, especially in the marine atmospheric environment with high relative humidity (RH). It is regarded as one of the important reasons that induce atmospheric corrosion. Thus, to monitor deliquescence process of salt particle on super-hydrophobic surface can help to reveal possibility of using it as atmospheric corrosion barrier. Fig. 6 shows evolution process of NaCl particle on super-hydrophobic film in simulated atmospheric environment with RH of 90%, which is higher than the critical value for NaCl (76%) [25]. It can be observed that edges of NaCl particle become dark within 6 min (Fig. 6b), indicating that these areas deliquesce preferentially. After exposure of 12 min, NaCl particle surface is enwrapped by saline water droplet, which appears as hemisphere structure for action of surface tension (Fig. 6c). With process of deliquescence, more water molecule adsorbs onto salt particle surface, and saline water droplet gets larger (Fig. 6d-h). After the exposure of 60 min, NaCl particle transfers to saline droplet completely (Fig. 6h). It should be noticed that contact angle of the saline water on filmed zinc is 117.1 ± 3°, implying that water droplet penetrates into groove of film during deliquescence process, and it contacts with film in Wenzel mode [18,26]. In order to prove this point, filmed samples before and after deliquescence processes are immersed into water, and observed at an oblique angle. Their optical photographs are shown in Fig. 7, and the definition of abbreviations used is shown in Table 1. It is clear that SS appears as a silver mirror (Fig. 7a), indicating that air can be trapped in film, and water contacts with super-hydrophobic film in Cassie mode. From the side view of SD at the same oblique angle in water (Fig. 7b), it can be observed the area that was covered by saline water droplet in atmospheric environment gets dark (the area within the red circle in Fig. 7b), proving that saline water penetrates into film during the deliquescence process of NaCl particle. As we have proven in reference [27], capillary condensation occurs in groove of film in environment with high RH. This process allows vapor to condense below saturation vapor pressure of pure liquid, which can be attributed to intensified van der Waals interactions
Fig. 5. Morphology of the zinc substrate after removing super-hydrophhobic film (a) bar = 100 lm and (b) bar = 10 lm.
P. Wang et al. / Corrosion Science 69 (2013) 23–30
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Fig. 6. Formation process of water droplet on super-hydrophobic film due to deliquescence of NaCl particle in simulated marine atmospheric environment (a) 0 min, (b) 6 min, (c) 12 min, (d)18 min, (e) 30 min, (f) 40 min, (g) 50 min and (h) 60 min.
In order to study the recovery property of super-hydrophobic film, SD is washed with water and then dried in atmospheric environment with RH of 60% until water volatilizes completely. When the SD after drying (SR) is immersed into water, the entire surface appears as silver mirror again (Fig. 7c), indicating that the film can recover super-hydrophobicity, and it connects with water in Cassie mode again.
Table 1 Definition of abbreviations used (BS, SS, SD and SR). Abbreviation
Definition
BS SS SD SR
Bare sample Super-hydrophobic film coated sample SS after deliquescence process of NaCl SD washed with water and then dried in atmospheric environment with RH of 60% until water volatilizes completely
3.4. The barrier effect of super-hydrophobic film in simulated marine atmosphere between vapor phase water molecules inside the confined space of groove filled with air. Thus, at pressures lower than the normal saturation pressure, vapor condenses in groove, and it induces replacement of air by water. Furthermore, micro-structure of film might be an important factor that affects penetration process of saline water into super-hydrophobic film. For super-hydrophobic film connected with water, air can be trapped in groove of film, and liquid forms a convex surface between interface of liquid and air for capillary. At this interface, capillary pressure prevents water to penetrate into film, and it can be calculated according to the Young–Laplace equation:
Pc ¼
2c cos h r
ð3Þ
In this equation, Pc is capillary pressure, r the characteristic value of radius of cylindrical tube, c the surface tension of the liquid– air interface, and h the contact angle of liquid to the cylindrical tube. According to Eq. (3), it is clear that the higher the value of r, the lower the capillary pressure. Thus, the high value of distance between two papillae (Fig. 3a) on film facilitates contact mode transfer of water droplet during NaCl deliquescence process.
In order to clarify barrier effect of super-hydrophobic film to the atmospheric corrosion induced by deliquescence of salt particle, electrochemical measurements for BS, SS, SD and SR are carried out in 3.5 wt.% NaCl solution. Fig. 8 shows Eoc values of the four samples. Eoc of BS is stable at -1.02 V vs. Ag/AgCl (3 M KCl). In presence of super-hydrophobic film, Eoc of SS is unstable within testing interval. This can be attributed to poor electrical conductivity for air trapped in film. Furthermore, Eoc of SS is higher than that of BS, indicating that the film that connects with corrosive medium in Cassie mode can reduce susceptibility of underlying zinc corrosion, though Eoc value does not provide any direct information on corrosion kinetics [28]. After saline water droplet penetrates into film for deliquescence of NaCl, Eoc of SD decreases to about 0.8 V vs. Ag/AgCl (3 M KCl). It is suggested that penetration of saline water into film weakens corrosion protection performance of film. After removing of saline water penetrated in film, Eoc of SR increases to around 0.5 V vs. Ag/AgCl (3 M KCl). It results from the behavior that SD recovers super-hydrophobicity and it contacts with corrosive medium in Cassie mode again. Fig. 9 presents (a) Nyquist, (b) Bode–phase angle versus frequency and (c) Bode |Z| versus frequency plots for BS, SS, SD and SR in 3.5 wt.% NaCl. EIS result of BS shows similar characteristics with zinc immersed in 0.6 M NaCl [29]. For the existence of
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Fig. 7. Optical photographs of (a) SS, (b) SD and (c) SR in water.
adsorbed species that contributes to the formation of corrosion layer [30], EIS result can be analyzed with circuit shown in Fig. 11a, in which Rs is solution resistance, Rt charge-transfer resistance, Rf resistance of film (corrosion product), and Qdl and Qf constant phase elements modeling capacitance of double and corrosion layers, respectively [11]. The impedance of Q is defined as
Z Q ¼ 1=Y 0 ðjxÞn
ð4Þ
where Y0 represents modulus, x angular frequency, and n the phase [31]. In case of SS, EIS data can only be achieved within frequency range of 105–102 Hz because of poor electro-conductivity of super-hydrophobic film. Within frequency range, interface impedance of super-hydrophobic film can be reflected. It can be observed that phase angle is almost 90° (Fig. 9b), and the slope of the impedance against the log (frequency) is about 1 (Fig. 9c). This behavior is comparable to any high-performance coating [11]. Note that impedance of SS is at least four orders higher than that of BS (Fig. 9c), confirming the excellent anti-corrosion effect of superhydrophobic film on zinc, as well as the great contribution of air in maximizing anti-corrosion property. According to the result shown in Fig. 7a and related analysis, an interface model for SS immersed in NaCl solution is proposed (Fig. 10a). Super-hydrophobic film can easily trap air within groove between ‘‘hills’’ (metal complex) in electrolyte, and liquid forms a convex surface between
interface of liquid and air for capillary. In this case, an air film forms between film and liquid interface. Thus, equivalent circuit shown in Fig. 11b is proposed to analyze EIS results of SS. In this circuit, constant phase element Qair and resistance Rair are introduced to characterize air film, and a parallel combination of constant phase element Qf and resistance Rf is used to characterize complex film. Fig. 10b presents interface model of SD in 3.5 wt.% NaCl solution. As we have proven above, after deliquescence process, a droplet forms on super-hydrophobic film and it penetrates into film. In this case, water droplet penetrated into film provides a pathway with lower resistance for electron transfer. From Bode |Z| versus frequency plots (Fig. 9c), it can be found that impedance of SD decreases for two orders of magnitude, indicating the failure of super-hydrophobic film after deliquescence process of NaCl particle. According to the Bode –phase angle versus frequency plots of SD shown in Fig. 9b, there are at least 3 time constants within the frequency range, which can be associated with air film, complex film and electron transfer at the interface. Furthermore, an obvious inductive loop can be found from Nyquist plots in Fig. 9a, and it represents pitting under super-hydrophobic film (as proven in Fig. 5). Thus, an equivalent circuit shown in Fig. 11(c) is proposed to fit EIS plots of SD. In this circuit, constant phase element Qair and resistance Rair are introduced to characterize air film. A parallel combination of constant phase element Qf and resistance Rf is used to characterize complex film. Qdl and Rt represent double layer constant phase element and charge transfer resistance, respectively. Rp and L are corresponding parameters for pitting. In case of SR, its impedance increases after water droplet that penetrates into film is removed. This is the result of behavior that SR recovers super-hydrophobicty, and it agrees with the conclusion drawn from open circuit potential results (Fig. 8). In this case, interface between liquid and super-hydrophobic film is similar with SS, and the equivalent used for SS (Fig. 11b) can be used to fit its EIS results. It is noteworthy that impedance of SR is a little lower than that of SS, this might be related to micro-structure destroy during water drying process due to high surface tension of water. Table 2 summarizes fitted electrochemical parameters for BS, SS, SD and SR. The listed inhibition efficiency (g) is calculated with following formula [32]:
gð%Þ ¼ 1 Fig. 8. Eoc t curves obtained on BS, SS, SD and SR in 3.5 wt.% NaCl solution.
R0t Rt
!
100
ð5Þ
P. Wang et al. / Corrosion Science 69 (2013) 23–30
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Fig. 10. Interface model of (a) SS and (b) SD in 3.5 wt.% NaCl solution.
Fig. 11. Equivalent circuits for samples in 3.5 wt.% NaCl solution.
Fig. 9. EIS results obtained on BS, SS, SD and SR in 3.5 wt.% NaCl solution (a) Nyquist plots, (b) Bode-phase angle versus frequency plots and (c) Bode |Z| versus frequency plots.
where R0t is charge transfer resistance of BS, and Rt is charge transfer resistance of SD. For SS and SR, Rf is used to substitute Rt for inhibition efficiency calculation. It should be noticed that SS presents excellent inhibition effect with an inhibition efficiency of 99.99%. The values of nair for SS and SR are both 1, indicating that air film acts as a pure capacitor that inhibits electron transfer between substrate and solution. Note that Rair decreases from 2.181 108 X cm2 to 2.049 105 X cm2, and Rf decreases from 2.085 108 X cm2 to 2.01 106 X cm2 after deliquescence process of NaCl, indicating that saline water that displaces air in film provides a pathway with low resistance for
the electron transfer. It can provide a closed circuit for electron transfer between anode and cathode, which is one of foundational conditions for electrochemical corrosion process. In this case, the advantage of applying super-hydrophobic film in marine atmospheric corrosion protection is declined. As we expect, inhibition efficiency of SR increases to 99.99% after water droplet penetrated into film is removed. This is the result of behavior that the film recovers super-hydrophobicty after drying, and it agrees with the conclusion drawn from open circuit potential results (Fig. 8). 4. Conclusions In summary, super-hydrophobic film composed of zinc–laurylamine is fabricated on zinc surface with electrolysis method. The image binarization result of SEM image proves that superhydrophobic property of film results from papillae and ridge-like structures. Formation of papillae can be attributed to preferential
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Table 2 The electrochemical parameters for BS, SS, SD and SR. Samples Rs (X cm2)
Cair (F cm2 1010)
nair
Rair (X cm2 103)
Cf nf (F cm2 1010)
Rf (X cm2 103)
Cdl (F cm2 106)
ndl
Rt (X cm2 103)
L Rp (H cm2 103) (X cm2 103)
g (%)
BS SS SD SR
0.572 2.733 0.973
1 0.94 1
21,810 204.9 13,770
475,000 0.1664 22,530 0.2728
0.188 208,500 2010 336,100
194 – 0.00145 –
0.41 – 0.93 –
0.575 – 788.6 –
– – 11.41 –
– 99.99 99.93 99.99
4.3 5.7 6.8 5.3
0.82 0.93 0.87 0.92
precipitation of zinc laurylamine over pits during uneven corrosion of zinc at high anodic potential. The super-hydrophobic film can present excellent anti-corrosion property with inhibition efficiency of 99.99% to the underlying zinc. However, during deliquescence process of NaCl in simulated marine atmosphere, capillary condensation in groove of film can induce penetration of saline water into film, and high value of distance between two papillae on film that results in low capillary pressure facilitates this process. The penetration behavior of saline water provides a closed circuit for electron transfer between anode and cathode of substrate, and declines corrosion inhibition effect of super-hydrophobic film in simulated marine atmospheric environment. After being washed with water and dried in atmospheric environment with low relative humidity, the filmed zinc can recover super-hydrophobic property and excellent anti-corrosion property. Acknowledgements This work was supported by National Natural Science Foundation of China (Grant Nos. 41106069 and 51131008), Natural Science Foundation of Shandong Province (Grant No. ZR2011EMQ008). References [1] N.S. Azmat, K.D. Ralston, B.C. Muddle, I.S. Cole, Corrosion of Zn under acidified marine droplets, Corros. Sci. 53 (2011) 1604–1615. [2] S.X. Li, L.H. Hihara, Atmospheric corrosion initiation on steel from predeposited NaCl salt particles in high humidity atmospheres, Corros. Eng. Sci. Technol. 45 (2010) 49–56. [3] N.T. Wen, C.S. Lin, C.Y. Bai, M.D. Ger, Structures and characteristics of Cr(III)based conversion coatings on electrogalvanized steels, Surf. Coat. Technol. 203 (2008) 317–323. [4] G.Z. Meng, L. Zhang, Y.W. Shao, T. Zhang, F.H. Wang, C.F. Dong, X.G. Li, Effect of refining grain size on the corrosion behavior of Cr(III) conversion layers on zinc coatings, Scr. Mater. 61 (2009) 1004–1007. [5] B. Szczygiel, J. Winiarski, W. Tylus, Effect of deposition time on morphology, corrosion resistance and mechanical properties of Ti-containing conversion coatings on zinc, Mater. Chem. Phys. 129 (2011) 1126–1131. [6] C.H.S.B. Teixeira, E.A. Alvarenga, W.L. Vasconcelos, V.F.C. Lins, Effect of porosity of phosphate coating on corrosion resistance of galvanized and phosphated steels Part : evaluation of corrosion resistance, Mater. Corros. 62 (2011) 853– 860. [7] V. Meiffren, K. Dumont, P. Lenormand, F. Ansart, S. Manov, Development of new processes to protect zinc against corrosion, suitable for on-site use, Prog. Org. Coat. 71 (2011) 329–335. [8] F. Rosalbino, G. Scavino, G. Mortarino, E. Angelini, G. Lunazzi, EIS study on the corrosion performance of a Cr(III)-based conversion coating on zinc galvanized steel for the automotive industry, J. Solid State Electrochem. 15 (2011) 703–709. [9] D. de la Fuente, B. Chico, M. Morcillo, Soluble salts and the durability of paint coatings: a new laboratory method for dosing chlorides and sulphates over steel surfaces, Anti-Corros. Methods Mater. 50 (2003) 208–216. [10] B. Yin, L.A. Fang, J. Hu, A.Q. Tang, W.H. Wei, J.A. He, Preparation and properties of super-hydrophobic coating on magnesium alloy, Appl. Surf. Sci. 257 (2010) 1666–1671. [11] P. Wang, D. Zhang, R. Qiu, B.R. Hou, Super-hydrophobic film prepared on zinc as corrosion barrier, Corros. Sci. 53 (2011) 2080–2086.
– – 307.2 –
[12] Z.H. Cui, F.Z. Zhang, L. Wang, S.L. Xu, X.X. Guo, In situ crystallized zirconium phenylphosphonate films with crystals vertically to the substrate and their hydrophobic, Dielectr. Anticorros. Propert. Langmuir 26 (2010) 179–182. [13] T. Liu, L.H. Dong, Y.S. Yin, Investigations on reducing microbiologicallyinfluenced corrosion of aluminum by using super-hydrophobic surfaces, Electrochim. Acta 55 (2010) 5281–5285. [14] H.Q. Liu, S. Szunerits, W.G. Xu, R. Boukherroub, Preparation of superhydrophobic coatings on zinc as effective corrosion barriers, ACS Appl. Mater. Interf. 1 (2009) 1150–1153. [15] T. He, Y.C. Wang, Y.J. Zhang, Q. Lv, T.G. Xu, T. Liu, Super-hydrophobic surface treatment as corrosion protection for aluminum in seawater, Corros. Sci. 51 (2009) 1757–1761. [16] P. Wang, D. Zhang, R. Qiu, Liquid/solid contact mode of super-hydrophobic film in aqueous solution and its effect on corrosion resistance, Corros. Sci. 54 (2012) 77–84. [17] T. Ishizaki, J. Hieda, 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. [18] X.J. Feng, L. Jiang, Design and creation of superwetting/antiwetting surfaces, Adv. Mater. 18 (2006) 3063–3078. [19] A.B.D. Cassie, S. Baxter, Wettability of porous surfaces, Trans. Faraday Soc. 40 (1944) 546–550. [20] X.Y. Fu, Y. Wang, N.Z. Wu, L.L. Gui, Y.Q. Tang, Surface modification of small platinum nanoclusters with alkylamine and alkylthiol: an XPS study on the influence of organic ligands on the Pt 4f binding energies of small platinum nanoclusters, J. Colloid Interf. Sci. 243 (2001) 326–330. [21] N. Tabet, M. Faiz, A. Al-Oteibi, XPS study of nitrogen-implanted ZnO thin films obtained by DC-Magnetron reactive plasma, J. Electron. Spectrosc. Relat. Phenom. 163 (2008) 15–18. [22] L.J. Meng, C.P. Moreira de Sa, M.P. dos Santos, Study of the structural properties of ZnO thin films by X-ray photoelectron spectroscopy, Appl. Surf. Sci. 78 (1994) 57–61. [23] W.C. Wu, M. Chen, S. Liang, X.L. Wang, J.M. Chen, F. Zhou, Superhydrophobic surface from Cu–Zn alloy by one step O2 concentration dependent etching, J. Colloid Interf. Sci. 326 (2008) 478–482. [24] Y.T. Tao, Structural comparison of self-assembled monolayers of n-alkanoic acids on the surfaces of silver, copper, and aluminum, J. Am. Chem. Soc. 115 (1993) 4350–4358. [25] J. Wang, L.H. Liang, J. Jiang, The role of electrochemical polarization in microdroplets formation, Electrochem. Commun. 10 (2008) 1788–1791. [26] R.N. Wenzel, Resistance of solid surfaces to wetting by water, Ind. Eng. Chem. 28 (1936) 988–994. [27] R. Qiu, D. Zhang, P. Wang, Superhydrophobic-carbon fibre growth on a zinc surface for corrosion inhibition, Corros. Sci. 66 (2013) 350–359.. [28] M. Schem, T. Schmidt, J. Gerwann, M. Wittmar, M. Veith, G.E. Thompson, I.S. Molchan, T. Hashimoto, P. Skeldon, A.R. Phani, S. Santucci, M.L. Zheludkevich, CeO2-filled sol–gel coatings for corrosion protection of AA2024-T3 aluminium alloy, Corros. Sci. 51 (2009) 2304–2315. [29] M. Mouanga, P. Bercot, Comparison of corrosion behaviour of zinc in NaCl and in NaOH solutions; Part II: electrochemical analyses, Corros. Sci. 52 (2010) 3993–4000. [30] Y.Y. Feng, S.H. Chen, J.M. You, W.J. Guo, Investigation of alkylamine selfassembled films on iron electrodes by SEM, FT-IR, EIS and molecular simulations, Electrochim. Acta 53 (2007) 1743–1753. [31] Z. Cui, Q.J. Wang, Y. Xiao, C.H. Su, Q.M. Chen, The stability of superhydrophobic surfaces tested by high speed current scouring, Appl. Surf. Sci. 254 (2008) 2911–2916. [32] H.Y. Ma, C. Yang, S.H. Chen, Y.L. Jiao, S.X. Huang, D.G. Li, J.L. Luo, Electrochemical investigation of dynamic interfacial processes at 1octadecanethiol-modified copper electrodes in halide-containing solutions, Electrochim. Acta 48 (2003) 4277–4289.
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Corrosion Science journal homepage: www.elsevier.com/locate/corsci
Super-hydrophobic film prepared on zinc and its effect on corrosion in simulated marine atmosphere Peng Wang a, Dun Zhang a,⇑, Ri Qiu a,b, Jiajia Wu a,b, Yi Wan a a b
Key Lab of Corrosion Science, Shandong Province, Institute of Oceanology, Chinese Academy of Sciences, 7 Naihai Road, Qingdao 266071, China Graduate School of the Chinese Academy of Sciences, 19 (Jia) Yuquan Road, Beijing 100039, China
a r t i c l e
i n f o
Article history: Received 2 July 2012 Accepted 29 October 2012 Available online 7 November 2012 Keywords: A. Zinc B. SEM B. XPS B. EIS C. Atmospheric corrosion
a b s t r a c t Zinc–laurylamine complex film with super-hydrophobicity is fabricated on zinc surface with electrolysis method. Super-hydrophobicity of film results from papillae and ridge-like structures, which form due to uneven corrosion of zinc at high anodic potential. The film obtained can maintain super-hydrophobic property in solution system, and can inhibit corrosion effectively. However, saline water penetrates into super-hydrophobic film during deliquescence process of NaCl particle in simulated marine environment. This behavior is induced by capillary condensation in groove of film, and it declines the advantage of using super-hydrophobic film as corrosion barrier in marine atmosphere. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction Atmospheric corrosion of zinc occurs in almost every atmospheric environment, and marine environment is the most aggressive one due to the highly corrosive nature of airborne pollutants [1]. In marine environment, the deposition of salt particles on metal surfaces plays a vital role in atmospheric corrosion. Bubbles that form from ocean waves can generate marine aerosols containing of small salt droplets or particles. These aerosols can be carried by wind and deposit on metal surface. When the relative humidity reaches a critical value, hygroscopic salts can absorb moisture and form salty droplets, which can greatly accelerate the corrosion of metals [2]. Until now, a great deal of research work has aimed at increasing lifetime of zinc. An effective method is to fabricate a compact coating on zinc surface to inhibit aggression of corrosive medium [3–8]. Generally, premature degradation of coating is due to a double negative effect of soluble salts at the metal/paint interface. On one hand, they cause the appearance of blisters of osmotic origin on paint film. On the other hand, they accelerate the corrosion process of metallic substrate under paint [9] . Thus, the anti-corrosion performance of coating is related to its effect of preventing electrolyte permeation into coating/metal interface. For its water-repellent property, fabrication of artificial superhydrophobic surface is regarded as a potential method for corrosion protection of metal in aqueous solution. It was proven that ⇑ Corresponding author. Tel./fax: +86 532 82898960. E-mail address: [email protected] (D. Zhang). 0010-938X/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.corsci.2012.10.025
air trapped in film is the essential contributor to excellent anticorrosion effect [10–17]. In atmospheric environment, air is trapped in grooves of super-hydrophobic film, and it can prevent electrolyte to penetrate for capillary effect [18,19]. Furthermore, super-hydrophobic surface can keep away the salt and water, because water droplet on super-hydrophobic surface can easily roll off from the surface and take away salt aerosol. Accordingly, it seems that modification with super-hydrophobic film is an effective corrosion protection method for metal in atmospheric environment. To the best of our knowledge, there is no report about corrosion inhibition of super-hydrophobic film to metal in atmospheric environment. In this paper, super-hydrophobic film was fabricated on zinc surface with electrolysis method, and it was characterized with contact angle test, electrochemical measurements and so on. Based on these results, formation mechanism of super-hydrophobic film on zinc was proposed, and inhibition effect of super-hydrophobic film to marine atmospheric corrosion induced by deliquescence of NaCl particle was investigated.
2. Experimental 2.1. Materials and reagents Zinc (P99.99 wt.%) foil and a series of chemicals, including laurylamine, sodium chloride (NaCl) and ethanol, were purchased from Sinopharm Chemical Reagent Co., Ltd. Water used was Milli-Q water (Milli Q, USA).
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2.2. Modification of zinc surface In a typical procedure, zinc foil was abraded with emery paper (400 grade), and rinsed with ethanol. The growth of super-hydrophobic film was performed in a two-electrode cell, in which zinc sample and a platinum wire acted as an anode and a cathode, respectively. Film was electrochemically grown at a constant potential of 30 V for 30 min in 0.1 M laurylamine/ethanol solution with direct current power supply at 25 °C. The width between anode and cathode is 3 cm, and the area of both anode and cathode exposed to electrolysis solution is 2 cm2. After that, zinc sample was brought out, washed with ethanol, and dried naturally in atmosphere. 2.3. Surface characterization Morphology of surface was characterized with FE-SEM (JSM6700F). Contact angles of 4 lL water droplet on bare and filmed zinc were measured with contact angle meter (JC2000C1, Shanghai Zhongchen Digital Technic Apparatus Co., Ltd.) at ambient temperature. X-ray photoelectron spectroscopy (XPS) measurement was carried out on a Thermo ESCALAB 250 photoelectron spectrometer equipped with an Al-anode at a total power dissipation of 150 W (15 kV, 10 mA), and the binding energies were referenced to the C 1s line at 284.8 eV from adventitious carbon. 2.4. Monitoring of deliquescence process of NaCl Marine atmospheric environment was simulated in a conditioning chamber, where the relative humidity (RH) was controlled by means of aqueous glycerin solution according to the Standard ASTM D 5032-97 (Reapproved 2003). In conditioning chamber, the filmed zinc sample was placed above aqueous glycerin solution, and a single NaCl particle was transferred gently onto sample surface to avoid destroying micro-structure of film. Then, formation process of saline water droplet due to deliquescence of NaCl was recorded with CCD camera. 2.5. Electrochemical experiments Open circuit potential (Eoc) t curves and electrochemical impedance spectroscopy (EIS) were obtained with a computercontrolled electrochemical system (CHI 760C, CH Instruments Inc.) in 3.5 wt.% NaCl solution with temperature of 25 °C. These experiments were performed in a three-electrode cell with a platinum electrode as counter electrode, zinc / filmed zinc as working electrode, and a silver/silver chloride (Ag/AgCl, 3 M KCl) electrode as reference electrode. EIS experiments were carried out after an immersion of sample in 3.5 wt.% NaCl for 30 min. They were carried out at frequencies ranging from 105 to 102/102 Hz at open circuit potential with amplitude of perturbation voltage of 10 mV. For this measurement, 12 points are recorded per decade. Each test was repeated more than three times to verify repeatability of results. EIS results were analyzed by fitting data using Zsimpwin software. 3. 3. Results and discussion 3.1. Composition, wetability and morphology Fig. 1 shows XPS spectra of filmed zinc. From XPS survey spectra (Fig. 1a), it can be found that C and N elements exist in film, indicating existence of laurylamine species. N1s spectra (Fig. 1b) exhibit two distinct peaks at 399.4 eV and 400.0 eV. The main peak at 399.4 eV corresponds to amine group coordinated with zinc ions,
Fig. 1. XPS spectra of filmed zinc (a)XPS survey spectra, (b) N1s spectra and (c) O1s spectra.
and the weak peak at 400.0 eV is consistent with that of a free amine group [20]. O 1s peak (Fig. 1c) is decomposed into two components. The oxygen peak registered at binding energy of 531.5 eV confirms presence of water molecules [21,22], a weak peak at 530.6 eV can be ascribed to zinc oxide [21]. It is implied that the film obtained is mainly composed of complex zinc–laurylamine. There are some free laurylamine and zinc oxide in film. Fig. 2 shows contact angle photographs of bare and filmed zinc specimens. It is found that bare zinc surface is hydrophilic with water contact angle of 62 ± 3° (Fig. 2a), and filmed zinc presents
P. Wang et al. / Corrosion Science 69 (2013) 23–30
Fig. 2. Contact angle photographs of (a) bare and (b) filmed zinc.
super-hydrophobic property with water contact angle of 151.2 ± 3° (Fig. 2b). After exposure in ambient condition for 1 month, water contact angle of filmed zinc is 150.5 ± 3°, indicating that it can maintain super-hydrophobic property in atmospheric environment. In case of super-hydrophobicity, air is believed to be trapped in groove of film, and Cassie model is usually introduced to explain wettability of film. Assuming that water contact angle for air is 180°, apparent contact angle (hr) of filmed zinc can be expressed as follows [23]:
cos hr ¼ f1 cos h f2
ð1Þ
In Eq. (1), f1 and f2 are fractions of solid surface (zinc– laurylamine) and air in contact with water; hr and h are water contact angles on rough and smooth solid surfaces (zinc–laurylamine), respectively. According to Eq. (1), air trapped in groove can minimize real contact area between a water droplet and solid surface. Consequently, the hydrophobicity of surface is dramatically intensified. According to previous reports [23,24], contact angle is generally in the range 113°–115° when a compound with group CH3(CH2)m(m P 10) forms self-assembled monolayer on flat metal substrate. Thus, f1 value of the super-hydrophobic film with water contact angle of 151.2° (Fig. 3b) is 0.2030, and the air fraction is 0.7970.
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Fig. 3 presents morphology and cross section of film. Micrometer-sized papillae structures are found on filmed zinc surface (Fig. 3a), and they connect with each other by ridge-like structure (Fig. 3b). From SEM picture of cross section (Fig. 3c), it can be observed that film presents rough structure with a thickness of about 200 lm. Generally, super-hydrophobicity of film is assumed to be related to its morphology. In order to understand relevance of wettability to micro-morphology of film, image binarization method is employed to simulate contact interface between water droplet and filmed zinc surface, and it is performed through a self-written program running on the Visual Basic.net platform. Before image binarization process, some assumption should be made in advance. In a SEM image, the distance between a pixel point and substrate can be reflected by its gray scale ranging from 0 to 255. Generally, pixel point with low gray scale in image indicates that it is far away from substrate, and it is more likely to connect with water droplet. Thus, there must be a critical value for the gray scale to determine whether a pixel point contacts with water droplet. Base on this assumption, a black-white two-colored image can be extracted from gray scale image (SEM image) by assigning a threshold value with image binarization. In details, if the gray scale value is lower than a threshold value, the pixel point can be set as 0; otherwise, it is set as 1. In black-white two-colored image obtained, the point with white color is the most possible contact region of solid with water, and the point with black color indicates the region trapped by air. As we have calculated above, the solid fraction of super-hydrophobic film with a water contact angle of 151.2° (Fig. 3a) is 0.2030. In this case, a black–white two-colored image with the proportion of white pixel of 20.26% (Fig. 4) can be transferred from Fig. 3a by setting threshold value as 136. From this image, it can be observed that white pixels mainly come from top of papillae and ridge-like structure. When a water droplet is dropped on the surface, it connects with surface through the top of papillae
Fig. 3. Morphology and cross section of filmed zinc (a) bar = 100 lm, (b) bar = 10 lm and (c) bar = 200 lm.
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supersaturated. As a result, rough film with papillae structure is formed on zinc surface. Furthermore, during electrolysis process, oxygen in solution can induce formation of zinc oxide, and water molecule dissolved in solution can also combine with Zn2+ to form zinc oxide. These two aspects can explain the existence of zinc oxide on zinc surface (Fig. 1c). 3.3. The deliquescence process of NaCl particle on super-hydrophobic film
Fig. 4. Black-white two-colored image transferred from Fig. 3a with threshold value of 136.
and ridge-like structures, and air is trapped among these structures. So the presence of papillae and ridge-like structure on filmed zinc surface is main reason for its super-hydrophobicity. 3.2. Formation mechanism of super-hydrophobic film Fig. 5 is morphology of zinc substrate after removing superhydrophobic film with ultrasonic vibration. It can be observed that pits are dispersed on zinc surface (Fig. 5a), and space between two pits ranges from 10 to 40 lm (Fig. 5b), which is the same to the range of distance between two papillae on film (Fig. 3). This result implies that appearance of papillae structure on filmed zinc surface is related to formation of pits under film during electrolysis process. Based on these results above, the formation mechanism of super-hydrophobic film is proposed as follows: under high anodic potential, Zn2+ ions are released from zinc surface. These released Zn2+ ions can be captured by laurylamine moiety in solution immediately to form zinc–laurylamine complex according to reaction (2):
Zn2þ þ CH3 ðCH2 Þ11 NH2 ! Zn½CH3 ðCH2 Þ11 N þ 2Hþ 2+
ð2Þ
It can be inferred that the releasing speed of Zn at pits is higher than other places, and the concentration of zinc–laurylamine complex over pits is the highest in solution. Thus, zinc– laurylamine is prone to precipitate over pits at anode when it is
Generally, salt particle in the environment with RH higher than a critical value (saturated relative humidity of salt solution) can absorb water molecule to deliquesce. The deliquescence of salt particle is a common phenomenon in nature, especially in the marine atmospheric environment with high relative humidity (RH). It is regarded as one of the important reasons that induce atmospheric corrosion. Thus, to monitor deliquescence process of salt particle on super-hydrophobic surface can help to reveal possibility of using it as atmospheric corrosion barrier. Fig. 6 shows evolution process of NaCl particle on super-hydrophobic film in simulated atmospheric environment with RH of 90%, which is higher than the critical value for NaCl (76%) [25]. It can be observed that edges of NaCl particle become dark within 6 min (Fig. 6b), indicating that these areas deliquesce preferentially. After exposure of 12 min, NaCl particle surface is enwrapped by saline water droplet, which appears as hemisphere structure for action of surface tension (Fig. 6c). With process of deliquescence, more water molecule adsorbs onto salt particle surface, and saline water droplet gets larger (Fig. 6d-h). After the exposure of 60 min, NaCl particle transfers to saline droplet completely (Fig. 6h). It should be noticed that contact angle of the saline water on filmed zinc is 117.1 ± 3°, implying that water droplet penetrates into groove of film during deliquescence process, and it contacts with film in Wenzel mode [18,26]. In order to prove this point, filmed samples before and after deliquescence processes are immersed into water, and observed at an oblique angle. Their optical photographs are shown in Fig. 7, and the definition of abbreviations used is shown in Table 1. It is clear that SS appears as a silver mirror (Fig. 7a), indicating that air can be trapped in film, and water contacts with super-hydrophobic film in Cassie mode. From the side view of SD at the same oblique angle in water (Fig. 7b), it can be observed the area that was covered by saline water droplet in atmospheric environment gets dark (the area within the red circle in Fig. 7b), proving that saline water penetrates into film during the deliquescence process of NaCl particle. As we have proven in reference [27], capillary condensation occurs in groove of film in environment with high RH. This process allows vapor to condense below saturation vapor pressure of pure liquid, which can be attributed to intensified van der Waals interactions
Fig. 5. Morphology of the zinc substrate after removing super-hydrophhobic film (a) bar = 100 lm and (b) bar = 10 lm.
P. Wang et al. / Corrosion Science 69 (2013) 23–30
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Fig. 6. Formation process of water droplet on super-hydrophobic film due to deliquescence of NaCl particle in simulated marine atmospheric environment (a) 0 min, (b) 6 min, (c) 12 min, (d)18 min, (e) 30 min, (f) 40 min, (g) 50 min and (h) 60 min.
In order to study the recovery property of super-hydrophobic film, SD is washed with water and then dried in atmospheric environment with RH of 60% until water volatilizes completely. When the SD after drying (SR) is immersed into water, the entire surface appears as silver mirror again (Fig. 7c), indicating that the film can recover super-hydrophobicity, and it connects with water in Cassie mode again.
Table 1 Definition of abbreviations used (BS, SS, SD and SR). Abbreviation
Definition
BS SS SD SR
Bare sample Super-hydrophobic film coated sample SS after deliquescence process of NaCl SD washed with water and then dried in atmospheric environment with RH of 60% until water volatilizes completely
3.4. The barrier effect of super-hydrophobic film in simulated marine atmosphere between vapor phase water molecules inside the confined space of groove filled with air. Thus, at pressures lower than the normal saturation pressure, vapor condenses in groove, and it induces replacement of air by water. Furthermore, micro-structure of film might be an important factor that affects penetration process of saline water into super-hydrophobic film. For super-hydrophobic film connected with water, air can be trapped in groove of film, and liquid forms a convex surface between interface of liquid and air for capillary. At this interface, capillary pressure prevents water to penetrate into film, and it can be calculated according to the Young–Laplace equation:
Pc ¼
2c cos h r
ð3Þ
In this equation, Pc is capillary pressure, r the characteristic value of radius of cylindrical tube, c the surface tension of the liquid– air interface, and h the contact angle of liquid to the cylindrical tube. According to Eq. (3), it is clear that the higher the value of r, the lower the capillary pressure. Thus, the high value of distance between two papillae (Fig. 3a) on film facilitates contact mode transfer of water droplet during NaCl deliquescence process.
In order to clarify barrier effect of super-hydrophobic film to the atmospheric corrosion induced by deliquescence of salt particle, electrochemical measurements for BS, SS, SD and SR are carried out in 3.5 wt.% NaCl solution. Fig. 8 shows Eoc values of the four samples. Eoc of BS is stable at -1.02 V vs. Ag/AgCl (3 M KCl). In presence of super-hydrophobic film, Eoc of SS is unstable within testing interval. This can be attributed to poor electrical conductivity for air trapped in film. Furthermore, Eoc of SS is higher than that of BS, indicating that the film that connects with corrosive medium in Cassie mode can reduce susceptibility of underlying zinc corrosion, though Eoc value does not provide any direct information on corrosion kinetics [28]. After saline water droplet penetrates into film for deliquescence of NaCl, Eoc of SD decreases to about 0.8 V vs. Ag/AgCl (3 M KCl). It is suggested that penetration of saline water into film weakens corrosion protection performance of film. After removing of saline water penetrated in film, Eoc of SR increases to around 0.5 V vs. Ag/AgCl (3 M KCl). It results from the behavior that SD recovers super-hydrophobicity and it contacts with corrosive medium in Cassie mode again. Fig. 9 presents (a) Nyquist, (b) Bode–phase angle versus frequency and (c) Bode |Z| versus frequency plots for BS, SS, SD and SR in 3.5 wt.% NaCl. EIS result of BS shows similar characteristics with zinc immersed in 0.6 M NaCl [29]. For the existence of
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Fig. 7. Optical photographs of (a) SS, (b) SD and (c) SR in water.
adsorbed species that contributes to the formation of corrosion layer [30], EIS result can be analyzed with circuit shown in Fig. 11a, in which Rs is solution resistance, Rt charge-transfer resistance, Rf resistance of film (corrosion product), and Qdl and Qf constant phase elements modeling capacitance of double and corrosion layers, respectively [11]. The impedance of Q is defined as
Z Q ¼ 1=Y 0 ðjxÞn
ð4Þ
where Y0 represents modulus, x angular frequency, and n the phase [31]. In case of SS, EIS data can only be achieved within frequency range of 105–102 Hz because of poor electro-conductivity of super-hydrophobic film. Within frequency range, interface impedance of super-hydrophobic film can be reflected. It can be observed that phase angle is almost 90° (Fig. 9b), and the slope of the impedance against the log (frequency) is about 1 (Fig. 9c). This behavior is comparable to any high-performance coating [11]. Note that impedance of SS is at least four orders higher than that of BS (Fig. 9c), confirming the excellent anti-corrosion effect of superhydrophobic film on zinc, as well as the great contribution of air in maximizing anti-corrosion property. According to the result shown in Fig. 7a and related analysis, an interface model for SS immersed in NaCl solution is proposed (Fig. 10a). Super-hydrophobic film can easily trap air within groove between ‘‘hills’’ (metal complex) in electrolyte, and liquid forms a convex surface between
interface of liquid and air for capillary. In this case, an air film forms between film and liquid interface. Thus, equivalent circuit shown in Fig. 11b is proposed to analyze EIS results of SS. In this circuit, constant phase element Qair and resistance Rair are introduced to characterize air film, and a parallel combination of constant phase element Qf and resistance Rf is used to characterize complex film. Fig. 10b presents interface model of SD in 3.5 wt.% NaCl solution. As we have proven above, after deliquescence process, a droplet forms on super-hydrophobic film and it penetrates into film. In this case, water droplet penetrated into film provides a pathway with lower resistance for electron transfer. From Bode |Z| versus frequency plots (Fig. 9c), it can be found that impedance of SD decreases for two orders of magnitude, indicating the failure of super-hydrophobic film after deliquescence process of NaCl particle. According to the Bode –phase angle versus frequency plots of SD shown in Fig. 9b, there are at least 3 time constants within the frequency range, which can be associated with air film, complex film and electron transfer at the interface. Furthermore, an obvious inductive loop can be found from Nyquist plots in Fig. 9a, and it represents pitting under super-hydrophobic film (as proven in Fig. 5). Thus, an equivalent circuit shown in Fig. 11(c) is proposed to fit EIS plots of SD. In this circuit, constant phase element Qair and resistance Rair are introduced to characterize air film. A parallel combination of constant phase element Qf and resistance Rf is used to characterize complex film. Qdl and Rt represent double layer constant phase element and charge transfer resistance, respectively. Rp and L are corresponding parameters for pitting. In case of SR, its impedance increases after water droplet that penetrates into film is removed. This is the result of behavior that SR recovers super-hydrophobicty, and it agrees with the conclusion drawn from open circuit potential results (Fig. 8). In this case, interface between liquid and super-hydrophobic film is similar with SS, and the equivalent used for SS (Fig. 11b) can be used to fit its EIS results. It is noteworthy that impedance of SR is a little lower than that of SS, this might be related to micro-structure destroy during water drying process due to high surface tension of water. Table 2 summarizes fitted electrochemical parameters for BS, SS, SD and SR. The listed inhibition efficiency (g) is calculated with following formula [32]:
gð%Þ ¼ 1 Fig. 8. Eoc t curves obtained on BS, SS, SD and SR in 3.5 wt.% NaCl solution.
R0t Rt
!
100
ð5Þ
P. Wang et al. / Corrosion Science 69 (2013) 23–30
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Fig. 10. Interface model of (a) SS and (b) SD in 3.5 wt.% NaCl solution.
Fig. 11. Equivalent circuits for samples in 3.5 wt.% NaCl solution.
Fig. 9. EIS results obtained on BS, SS, SD and SR in 3.5 wt.% NaCl solution (a) Nyquist plots, (b) Bode-phase angle versus frequency plots and (c) Bode |Z| versus frequency plots.
where R0t is charge transfer resistance of BS, and Rt is charge transfer resistance of SD. For SS and SR, Rf is used to substitute Rt for inhibition efficiency calculation. It should be noticed that SS presents excellent inhibition effect with an inhibition efficiency of 99.99%. The values of nair for SS and SR are both 1, indicating that air film acts as a pure capacitor that inhibits electron transfer between substrate and solution. Note that Rair decreases from 2.181 108 X cm2 to 2.049 105 X cm2, and Rf decreases from 2.085 108 X cm2 to 2.01 106 X cm2 after deliquescence process of NaCl, indicating that saline water that displaces air in film provides a pathway with low resistance for
the electron transfer. It can provide a closed circuit for electron transfer between anode and cathode, which is one of foundational conditions for electrochemical corrosion process. In this case, the advantage of applying super-hydrophobic film in marine atmospheric corrosion protection is declined. As we expect, inhibition efficiency of SR increases to 99.99% after water droplet penetrated into film is removed. This is the result of behavior that the film recovers super-hydrophobicty after drying, and it agrees with the conclusion drawn from open circuit potential results (Fig. 8). 4. Conclusions In summary, super-hydrophobic film composed of zinc–laurylamine is fabricated on zinc surface with electrolysis method. The image binarization result of SEM image proves that superhydrophobic property of film results from papillae and ridge-like structures. Formation of papillae can be attributed to preferential
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P. Wang et al. / Corrosion Science 69 (2013) 23–30
Table 2 The electrochemical parameters for BS, SS, SD and SR. Samples Rs (X cm2)
Cair (F cm2 1010)
nair
Rair (X cm2 103)
Cf nf (F cm2 1010)
Rf (X cm2 103)
Cdl (F cm2 106)
ndl
Rt (X cm2 103)
L Rp (H cm2 103) (X cm2 103)
g (%)
BS SS SD SR
0.572 2.733 0.973
1 0.94 1
21,810 204.9 13,770
475,000 0.1664 22,530 0.2728
0.188 208,500 2010 336,100
194 – 0.00145 –
0.41 – 0.93 –
0.575 – 788.6 –
– – 11.41 –
– 99.99 99.93 99.99
4.3 5.7 6.8 5.3
0.82 0.93 0.87 0.92
precipitation of zinc laurylamine over pits during uneven corrosion of zinc at high anodic potential. The super-hydrophobic film can present excellent anti-corrosion property with inhibition efficiency of 99.99% to the underlying zinc. However, during deliquescence process of NaCl in simulated marine atmosphere, capillary condensation in groove of film can induce penetration of saline water into film, and high value of distance between two papillae on film that results in low capillary pressure facilitates this process. The penetration behavior of saline water provides a closed circuit for electron transfer between anode and cathode of substrate, and declines corrosion inhibition effect of super-hydrophobic film in simulated marine atmospheric environment. After being washed with water and dried in atmospheric environment with low relative humidity, the filmed zinc can recover super-hydrophobic property and excellent anti-corrosion property. Acknowledgements This work was supported by National Natural Science Foundation of China (Grant Nos. 41106069 and 51131008), Natural Science Foundation of Shandong Province (Grant No. ZR2011EMQ008). References [1] N.S. Azmat, K.D. Ralston, B.C. Muddle, I.S. Cole, Corrosion of Zn under acidified marine droplets, Corros. Sci. 53 (2011) 1604–1615. [2] S.X. Li, L.H. Hihara, Atmospheric corrosion initiation on steel from predeposited NaCl salt particles in high humidity atmospheres, Corros. Eng. Sci. Technol. 45 (2010) 49–56. [3] N.T. Wen, C.S. Lin, C.Y. Bai, M.D. Ger, Structures and characteristics of Cr(III)based conversion coatings on electrogalvanized steels, Surf. Coat. Technol. 203 (2008) 317–323. [4] G.Z. Meng, L. Zhang, Y.W. Shao, T. Zhang, F.H. Wang, C.F. Dong, X.G. Li, Effect of refining grain size on the corrosion behavior of Cr(III) conversion layers on zinc coatings, Scr. Mater. 61 (2009) 1004–1007. [5] B. Szczygiel, J. Winiarski, W. Tylus, Effect of deposition time on morphology, corrosion resistance and mechanical properties of Ti-containing conversion coatings on zinc, Mater. Chem. Phys. 129 (2011) 1126–1131. [6] C.H.S.B. Teixeira, E.A. Alvarenga, W.L. Vasconcelos, V.F.C. Lins, Effect of porosity of phosphate coating on corrosion resistance of galvanized and phosphated steels Part : evaluation of corrosion resistance, Mater. Corros. 62 (2011) 853– 860. [7] V. Meiffren, K. Dumont, P. Lenormand, F. Ansart, S. Manov, Development of new processes to protect zinc against corrosion, suitable for on-site use, Prog. Org. Coat. 71 (2011) 329–335. [8] F. Rosalbino, G. Scavino, G. Mortarino, E. Angelini, G. Lunazzi, EIS study on the corrosion performance of a Cr(III)-based conversion coating on zinc galvanized steel for the automotive industry, J. Solid State Electrochem. 15 (2011) 703–709. [9] D. de la Fuente, B. Chico, M. Morcillo, Soluble salts and the durability of paint coatings: a new laboratory method for dosing chlorides and sulphates over steel surfaces, Anti-Corros. Methods Mater. 50 (2003) 208–216. [10] B. Yin, L.A. Fang, J. Hu, A.Q. Tang, W.H. Wei, J.A. He, Preparation and properties of super-hydrophobic coating on magnesium alloy, Appl. Surf. Sci. 257 (2010) 1666–1671. [11] P. Wang, D. Zhang, R. Qiu, B.R. Hou, Super-hydrophobic film prepared on zinc as corrosion barrier, Corros. Sci. 53 (2011) 2080–2086.
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