Microstructure and electrochemical behavior of cerium conversion coating modified with silane agent on magnesium substrates

Applied Surface Science 376 (2016) 161–171

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Microstructure and electrochemical behavior of cerium conversion coating modified with silane agent on magnesium substrates Li Lei, Jing Shi ∗ , Xin Wang ∗ , Dan Liu, Haigang Xu Institute of Materials Science and Engineering, Ocean University of China, Qingdao 266100, China

a r t i c l e

i n f o

Article history: Received 3 November 2015 Received in revised form 8 March 2016 Accepted 19 March 2016 Available online 22 March 2016 Keywords: Magnesium alloy Cerium conversion coating Silane agent Hydrophobic Corrosion resistance

a b s t r a c t The cerium conversion coating with and without different concentrations of silane agent bis-(␥triethoxysilylpropyl)-tetrasulfide (BTESPT) modification is obtained on magnesium alloys. Detailed properties of the coatings and the role of BTESPT as an additive are studied and followed with careful discussion. The coating morphology, wettability, chemical composition and corrosion resistance are characterized by scanning electronic microscope (SEM), water contact-angle, X-ray photoelectron spectroscopy (XPS), potentiodynamic measurements and electrochemical impedance spectroscopy (EIS). The electrochemical behavior of the coatings is investigated using EIS. The results indicate that the coating morphology and composition can be controlled by changing silane concentration. The combination of cerium ions and silane molecules could promote the formation of more homogenous and higher hydrophobic coating. The coating turns to be more compact and the adhesive strength between the coating and the magnesium substrate are strongly improved with the formation of Si O Si and Si O M chemical bonds. The optimum corrosion resistance of the coating in the corrosive media is obtained by 25 ml L−1 BTESPT modification. This whole study implies that the cerium conversion coating modified with certain silane agent deserves cautiousness before its application for corrosion resistance. © 2016 Elsevier B.V. All rights reserved.

1. Introduction Magnesium alloys are commercially available for various industrial fields and biomedical applications. They have been attracting much attention worldwide for their excellent physical and mechanical properties, such as low specific density, stiffness, mechanical stability, high thermal and electrical conductivities [1–6]. However, the limited corrosion resistance is still a serious drawback for magnesium alloys and restricts their industrial applications under certain conditions. Recent studies showed that desired corrosion resistance of magnesium alloys will necessitate application of suitable surface barrier, such as anodized films, polymer films and chemical conversion treatments [7–14]. Rare earth conversion coatings are some of the most promising substitutes for chromate conversion coatings, since they are nontoxic and can provide a physical barrier to enhance the anticorrosion protection [15]. The formation of the cerium conversion coating is simply achieved by dipping the substrate metal in baths containing cerium salts/hydrogen peroxide aqueous solution for a short period. The addition of H2 O2 is favorable for accelerat-

∗ Corresponding author. E-mail addresses: [email protected] (J. Shi), [email protected] (X. Wang). http://dx.doi.org/10.1016/j.apsusc.2016.03.150 0169-4332/© 2016 Elsevier B.V. All rights reserved.

ing the formation of the conversion film. In the past few decades many studies were devoted to cerium based conversion coatings and its anti-corrosion properties on different metallic substrates. Such conversion coatings improved the corrosion resistance to some degree. Nevertheless, the loose structure and weak adhesion properties are still the main defects for these coatings [16], which are prior to be attacked and accelerate the coating degradation. It is necessary to improve the morphology and microstructure of traditional cerium conversion coatings to obtain better corrosion resistance under various conditions. Despite many research works have been devoted to the optimization of the cerium conversion coatings, including phosphate post-treatment [16], alkaline cleaning and activation before conversion treatment [17], water solution gelatin added [18] and so on, the problem of porous and low adhesion has not been effectively resolved yet. Our interest is focused on using silane agent to modify the traditional cerium conversion coatings. Silane is emerging as an environmentally friendly alternative for improving the corrosion resistance of the magnesium substrates [7,19,20]. Due to their unique chemical structure, the hydrolysable groups can connect with the hydroxyl groups of metallic substrates to form covalent bonds and make the silane used for modification of a substrate/coating interface [21]. On the other hand, the silanol groups can react with each other to form siloxane bonds, thus self assem-

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Fig. 1. SEM images of the cerium conversion coatings modified with various BTESPT concentrations (a) 0 ml L−1 ; (b) 15 ml L−1 ; (c) 20 ml L−1 ; (d) 25 ml L−1 .

bled robust Si-O-Si network linkages, which provides high stability and effective barrier against electrolyte uptake [22]. Electrochemical measurements and accelerated corrosion tests showed that silane molecules provided enhanced corrosion protection of different metallic substrates [23]. Recently many studies based on the formation of silane film were modified with cerium ions or ceria nanoparticles [24]. However, there is little literature researched on the silane as an addictive to the cerium conversion solution on magnesium alloys, and the action mechanism of silane needs to be further explored. The present work aims at morphology, chemical composition, electrochemical properties and behaviors of a novel cerium conversion coating modified with different BTESPT concentrations on magnesium alloys. BTESPT is a non-functional silane that can form six hydrolysable silanol groups, which would facilitate the reaction with the metallic substrates. Further research is carried out on characterization of the influence of BTESPT concentration on coatings. In order to give a comprehensive evaluation of the potential application of this new coating, electrochemical properties of the coatings during the corrosion process are investigated and correlated with the coating microstructure, which can be controlled by BTESPT concentration. On the basis of the experiment results, the role of BTESPT as an additive in the silane modified cerium conversion coating is discussed.

2. Experimental 2.1. Preparation and surface treatment The AZ31 magnesium alloy was used in this work. Each coupon was cut into 10 mm × 10 mm × 3 mm panels. The samples were

degreased in ethanol and mechanically polished with abrasive papers from 400 to 1200 grit. In preparation for coatings the coupons were immersed in 4 g L−1 NaOH solution for 5 min at room temperature and then thoroughly washed with deionised water. BTESPT was supplied by Sinopharm Chemical Reagent Co., Ltd. Prior to coating preparation, BTESPT solution was prepared for hydrolysis and condition for BTESPT hydrolysis was well established [25]. In this paper, the hydrolysis condition was employed by dissolving 5% (v/v) of BTESPT in 90% (v/v) ethanol and 5% (v/v) H2 O and kept under constant stirring for 24 h before use. Afterwards the BTESPT solution was added to the cerium conversion solution composed of 0.1 M cerium chloride and 5 ml L−1 30 wt.% H2 O2 with different concentrations of 15 ml L−1 , 20 ml L−1 and 25 ml L−1 . The cleaned samples were immersed in the above solutions for 5 min and then oven-dried at 110 ◦ C for 20 min to enhance the crosslinking of silane. For investigation of the silane action mechanism, the cerium conversion coating without silane modification was also prepared and discussed. 2.2. Characterization The morphology and corrosion surface after immersion for 72 h in 0.05 M sodium chloride solution of the unmodified and modified with different silane concentration coatings were observed using a Japanese Electronics Co., Ltd. JEOL JSM-6700F SEM (Akishima, Tokyo Japan). The contact angle of the coatings was examined using a Shanghai Zhongchen Digital Technical Apparatus Co., Ltd. JC 2000C1 contact angle meter (Shanghai China). The measurement was conducted in air with the accuracy of ±0.5◦ and each result presented here was an average of five tests. The XPS analysis was performed using a Thermo Fisher Scientific Inc. ESCALAB 250 (Waltham, Massachusetts USA) X-ray photoelectron spectroscopy

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Fig. 2. Static contact angles anddigital photograph of water droplet behavior of the cerium conversion coatings modified with various BTESPT concentrations.

for the surface chemical analysis. XPS spectra were taken in constant analyzer mode (CAE = 30 eV), using an Al (k␣ = 1486.6 eV) anode. The samples were taken out from the film forming solution, oven dried at 110 ◦ C 20 min for the silane modified coating and nature drying for the unmodified coating. They were mounted inside the XPS chamber. The spot of the X-ray beam was around 3 × 3 mm. XPS experiments were carried at room temperature and pressures were typically in the order of less than 10–7 Pa. The core level binding energies were charged corrected with respect to the adventitious carbon (C1s) peak at 284.6 eV. The background subtraction was performed using the Shirley algorithm. Potentiodynamic polarization and EIS were carried out to evaluate the electrochemical properties and corrosion behavior of the coatings on an Metrohm Company AUTOLAB PG30 (Herisau, Switzerland) apparatus equipped with a classical three-electrode cell, consisting of the specimen with an exposed area of 1 cm2 as the working electrode, saturated calomel as the reference electrode and the platinum as counter electrode. All experiments were conducted in 0.05 M NaCl solution at room temperature. Potentiodynamic polarization was performed at a scanning rate of 10 mV s−1 from −250 mV in the cathodic direction to +200 mV in the anodic direction based on the open circuit potential (OCP). Ecorr , the corrosion potential and icorr , the corrosion current density were calculated according to the Tafel extrapolation method. The impedance spectra were recorded at various intervals of 10 min, 24 h, 48 h and 72 h with the frequency ranging from 100 kHz to 100 mHz after stabilization in 0.05 M NaCl solution for 10 min. The Bode impedance and Bode phase angle plots were also obtained. 3. Results and discussion 3.1. Microstructure and chemical composition of the coatings The surface morphologies of the cerium conversion coatings with various silane concentrations are shown in Fig. 1. The surface of the unmodified coating presents several cracks with the dry-mud type, as shown in Fig. 1a. Typical cracks may be attributed to the dehydration of the coating, which was reported by Brunelli [26]. The cracks with average width of 0.1–0.5 ␮m are distributed in the crisscross pattern irregularly. A large number of white spherical particles are also observed on the surface of the layer. At higher magnification, the coating presents a porous and loose structure, displaying a honeycomb-like morphology. With the addition of the silane agent, the cracks apparently reduce in the coating with 15 ml L−1 silane modification, as shown in Fig. 1b. Simultaneously, the surface is likely to be clustered nanoparticles. Fig. 1c shows

163

Fig. 3. XPS survey spectra of the cerium conversion coatings modified with various BTESPT concentrations.

Fig. 4. XPS atomic percentage at the surface of the cerium conversion coatings unmodified and modified with different BTESPT concentrations.

the surface image of the coating with 20 ml L−1 silane modification. It presents a large number of white particles, with apparent agglomerates. Increasing the silane concentration to 25 ml L−1 , a quite homogeneous surface is obtained and Fig. 1d shows a uniform distribution of the nanoparticles. Static contact angles of the coating with various silane concentrations are shown in Fig. 2. To gurantee the accuracy, all the results in the figure are the average of values measured at five different points on the same sample. The unmodified coating shows the average static water contact angles of a little more than 50◦ , which is clearly hydrophilic for water solutions. With the increase of BTESPT concentration, the water contact angles show an increasing tendency. Especially, when the silane concentration increases to 25 ml L−1 , the coating surface presents a hydrophobic feature, with static water contact angle of more than 110◦ . According to SEM images shown in Fig. 1, the coating surface becomes more uniform with the addition of silane concentration. For the coating modified with 25 ml L−1 silane, the nano particles with the size of 100 nm diameter are protruding out of the coating and uniformly spread over the surface. It was reported that such regularly-ordered structure could trap a large amount of air that pushed the water drop to the surface, which was experted when revealing this high a level of hydrophobicity on the surface [27–29]. The surface chemical composition of the cerium conversion coatings with various silane concentrations characterized by XPS are represented in Fig. 3. The unmodified coating includes Mg1s, Mg2p, Ce3d, O1s and C1s spectra, indicating Mg, O and Ce elements are the main components. After different silane concentrations

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Fig. 5. XPS spectra of C1s and Si2p of the cerium conversion coatings modified with various BTESPT concentrations (a) C1s ionization; (b) Si2p ionization (c, d) 15 ml L−1 ; (e, f) 20 ml L−1 ; (g, h) 25 ml L−1 .

modification, it is noted that silane agent probably affects the elemental composition of the coating. Besides the elements above, Si element appears. For all the coatings, Mg element comes from magnesium substrate. With Al as the excitation source, photoelectrons can be accelerated from different electron states. Both of Mg1s and Mg2p are related to Mg atom and the only difference between them is the binding energy for the different orbitals of the electrons that are participating. C1s in the unmodified coating originates from the surface adsorption. While for the silane modified coating, C associated with Si elements are related to the BTESPT silane (C18 H42 O6 Si2 S4 ). There is only the presence of Si2p peak in the silane modified coating, illustrating that silane plays a significant role in the film forming.

The atomic percent contribution of the coatings with various silane concentrations investigated by XPS is shown in Fig. 4. The calculation of atomic percentage is the peak intensity (peak integral area) divided by the sensitive factor, and calculations for atomic content of all species are in relation to the total amount of species. Theoretically, the relative atomic percentage of Mg element obtained by Mg1s and Mg2p should be the same. Significant differences in terms of the C, O, Mg, Ce and Si are observed in all samples. Compared with the silane modified coatings, the content of Mg and Ce are higher in the unmodified coating whereas the content of C and Si shows an opposite trend. In the 25 ml L−1 silane modification coating, Mg element has the lowest spectra, revealing that the coating has a good coverage with the 25 ml L−1 silane modification which makes the magnesium spectra hardly detected.

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Fig. 6. XPS spectra of O1s and Ce3d of the cerium conversion coatings modified with various BTESPT concentrations (a, b) 0 ml L−1 ; (c, d) 15 ml L−1 ; (e, f) 20 ml L−1 ; (g, h) 25 ml L−1 .

With the silane concentration increasing, the intensity of C1s and Si2p increases. For the 25 ml L−1 silane modification coating, C and Si elements have the highest spectra, illustrating that the BTESPT silane has well evolved in the film forming process. The C1s and Si2p spectra of the cerium conversion coatings modified with various silane concentrations are investigated by XPS, as shown in Fig. 5. For the unmodified coating, the detected C1s peak is derived from the surface adsorption rather than the coating itself. There is no need to analysis. While for the silane modified coating, as seen in Fig. 5c, e and g, the C1s spectra can be fitted to two peaks: one peak is at 284.8 eV in association with the presence of the CH groups [30], while the other peak is at 287.0 ± 0.1 eV corresponding to the presence of C O [31,32]. The CHn groups correspond to the C H bonds of the organic part of the coating arising from the BTESPT silane molecule. From Fig. 5d when silane concentration is

15 ml L−1 , the Si2p peak has a binding energy of 102.0 eV, which is attributed to Si O Si/Mg bonds. However, after the silane concentration reaches up to more than 20 ml L−1 (Fig. 5f and 5 h), the peak position is around 102.2 eV. The shift of the peak towards higher binding energy and the increace of the peak intensity suggest that a change in the coating composition. The formation of the new peak at higher binding energy may correspond to interactions between the silicon atomic and the ceria groups [33]. Fig. 6 is the high resolution for O1s and Ce3d spectra of the coatings with different silane concentrations. The O1s spectrum in the unmodified coating reveals the presence of three peaks centered at 529.7 eV, 531.6 eV and 533.4 eV, as shown in Fig. 6a. The high-intensity peaks at 531.6 eV can be assigned to the CeOH binding. The other two low intensity peaks originates from the Ce O and Mg OH binding [34,35]. It can be concluded that

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the coating without silane modification is mainly composed of cerium oxide/hydroxide and magnesium hydroxide. In case of the 15 ml L−1 silane modification coating, a peak centered at 533.4 eV disappears (Fig. 6c). For the 20 ml L−1 and 25 ml L−1 silane modification coating, two new peaks centered at 533.0 eV and 532.3 eV appear, which are attributed to Si O Si and Si O M bonds, respectively. (Fig. 6e and g) [23]. Si-OH groups are indicative of the hydrolysis process, Si O Si are representative of the siloxane bonds formed during the silane reticulation and Si O M bonds represent the formation of covalent metallo-siloxane bond either in Si O Mg or in Si O Ce type bond [36]. In the 15 ml L−1 silane modification coating, Si O M bonds are not detected, maybe because of the low silane concentration. With the silane concentration increasing, the intensity of Si-O-Si becomes high. In the 2 ml L−1 silane modification coating, the highest amount of Si O Si and Si O M bonds, which makes the silane modified coatings become more compact compared with the single cerium conversion coating. The previous study [37] pointed that Ce ions could cooperate with silane molecules and they were indicative of a better cross-linking. The Ce3d spectra for cerium compounds exhibit complex features due to Ce(III) and Ce(IV) ions. Based on Reddy researches [38,39], the Ce3d spectrum could be assigned into two sets of spinorbital multiplets, corresponding to the 3d5/2 and 3d3/2 . They were discussed in detail by Kotani [40]. Deconvoluted XPS of Ce(3d5/2,3/2 ) core level spectra are shown in Fig. 6. Binding energy of Ce4+ (3d5/2 ) at 884.5 eV along with its satellites at 889.3 eV and 898.3 eV and Ce4+ (3d3/2 ) seen at 901.1 eV along with its satellites at 908.1 eV and 917.1 eV [41]. Peaks assigned to Ce(III) state are also presented in the spectra which are at the binding energy of 886.0 eV and 903.5 eV, respectively. The satellite peak at approximately 917.0 eV is referred to as peak ␮”’. From the evaluation of the deconvoluted spectra in Fig. 6b, d, f, h and the calculation from Shy’s equation [42,43], Ce(IV) to Ce(III) ratio for the cerium conversion coating modified with different silane concentrations is 6.87:1, 13.7:1, 29.3:1 and 82.3:1, respectively. It is considered that all of the coatings are covered with a mixture of Ce(III)/Ce(IV), while Ce(IV) has a predominance in the content. It was reported that if the O2 content in the film forming solution was high enough, Ce(III) species could be oxidized to Ce(IV). It can be concluded that due to the dissolution of O2 into the conversion treatment solution, parts of Ce(III) could be oxidized to Ce(IV) immediately. Secondly, the redox potential of H2 O2 is 1.776 V, higher than that of Ce(III)/Ce(IV). It is possible for H2 O2 oxidizing Ce(III) to Ce(IV). In addition, when the silane concentration increasing to 25 ml L−1 (Fig. 1d), with the increase of coating coverage, the exposed area of the magnesium substrate shows an opposite tendency, and the local pH value increases slowly. For that the solubility product (Ksp ) of Ce(IV) is lower than Ce(III). Ce(IV) will be preferentially deposited, leading to the highest Ce(IV) percentage in the 25 ml L−1 silane modification coating. 3.2. Corrosion resistance The potentiodynamic curves of the coatings with different silane concentrations after immersion in 0.05 M NaCl solution for 10 min are depicted in Fig. 7. Obvious changes in the anodic and cathodic region of the polarization curves are observed in all coatings. Generally, the anodic region indicates dissolution of the substrates at an elevated potential and the cathodic part of the curve represents the cathodic hydrogen evolution reaction related to water reduction. With the silane addition, the cathodic branch of the curves towards a lower current density, indicating that the cathodic hydrogen evolution reaction is suppressed in comparison with the unmodified coating. The corrosion potential (Ecorr ) and corrosion current density (icorr ) obtained from the curves using Tafel extrapolarion method are listed in Table 1. The icorr of the unmodified coat-

Fig. 7. Potentiodynamic polarization curves of the cerium conversion coatings modified with various BTESPT concentrations in 0.05 M NaCl solution. Table 1 Parameters obtained from potentiodynamic polarization curves of the cerium conversion coatings modified with various BTESPT concentrations in 0.05 M NaCl solution. c(silane)/ (ml L−1 )

Ecorr / (mV)

icorr / (␮A cm−2 )

0 15 20 25

−1766 −1732 −1681 −1647

100 64 37 19

Fig. 8. Nyquist plots of the cerium conversion coatings modified with various BTESPT concentrations in 0.05 M NaCl solution.

ing is 100 ␮A cm−2 whereas the icorr of the coating modified with 15 ml L−1 , 20 ml L−1 and 25 ml L−1 silane is 64 ␮A cm−2 , 37 ␮A cm−2 and 19 ␮A cm−2 , respectively. It can be seen that the icorr of the 25 ml L−1 silane modification coating decreases by about 5 times compared with the unmodified coating. Moreover, with the silane addition, the Ecorr of the coatings shifts towards a negative direction, whereas the 25 ml L−1 silane modification coating presents the most negative potentials, which can be attributed to the decrease in the rate of the cathodic reaction [44]. All these suggest that the 25 ml L−1 silane modification offers the coating the optimum corrosion protection. The corrosion resistance of the samples in corrosive electrolyte is examined by EIS measurements. It is a non-destructive technique which can be proposed as a tool to follow up the stationary bahavior of metallic materials [45]. The Nyquist plots of the unmodified coating and modified with various silane concentration coatings in 0.05 M NaCl solution are recorded in Fig. 8. The general profile of the spectra obtained for all coatings are characterized by two overlapping capacitive semicircles. It can be seen that the obvious difference amongst the EIS diagrams for these coatings is the size of capacitive loops. Generally, the corrosion resistance is determined by the combined diameter of these semicircles. The larger size of the capacitive loop, the better the anticorrosion ability a coating presents [24]. In our case, the diameters of the silane modified coating are larger than those of the unmodified coating, indicat-

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Fig. 10. Bode EIS spectra of the cerium conversion coatings modified with various BTESPT concentrations in 0.05 M NaCl solution. Fig. 9. The electrical equivalent circuit of the cerium conversion coatings modified with various BTESPT concentrations in 0.05 M NaCl solution.

ing an improvement in electrochemical impedance. The capacitive loop diameter increases with increasing silane concentration. This means that the silane addition improves the coating corrosion resistance. The 25 ml L−1 silane modification coating presents the largest radius, indicating the highest improvement in corrosion resistant performance compared with the unmodified coating. A comparison of the plots suggests that the impedance offered by 25 ml L−1 silane modification coating is superior to that of the unmodified coating. For a more quantitative assessment of the EIS response in different coatings, the appropriate electric equivalent circuit (EEC) is applied for better understanding in the corrosion mechanism. Impedance of the interfaces of substrate/surface hydroxide/coating/sodium chloride solution are analyzed on the basis of the appropriate EEC. In the present study, the spectra are simulated using a Rs (Q1 (R1 (Q2 R2 ))) EEC of two time constants (Fig. 9). Each of these interfaces can be represented as a parallel combination of a resistance (R) and a capacitance. The constant phase element (Q) is used in the EEC instead of the pure capacitor, better describing the behavior of the non ideal coating. In this EEC, the solution resistance between the electrode surface and the reference electrode is represented by Rs . The high frequency capacitive loop can be attributed to the impedance with the interface reaction between the coating and surface hydroxide, where the coating modified with various silane concentrations is characterized by a constant phase element (Q1 ) and pore resistance (R1 ). The other time constant, which is represented by a constant phase element (Q2 ) and a resistance (R2 ) assumes to the combined responses of the electrical double layer/the hydroxide film and charge transfer resistance. During this period, the penetration of the electrolyte could be speculated the metal Mg2+ ions and reacted with the oxygen to form magnesium oxide [7]. The Bode impedance plots for the cerium conversion coatings modified with different silane concentrations are shown in Fig. 10. The spectra are characterized by a resistive response at high frequency ranges from 100 kHz to 1 kHz related to the solution resistance. Followed by a capacitive slope between |Z| vs. Frequency is shown in a frequency between 1 kHz and 10 Hz, corresponding to the coating capacitance. During this period, a slope is approximately 0.89 with a phase angle of −68◦ for the 25 ml L−1 silane modified coating and no clear resistive plateau can be observed, indicating that this coating behaves as a very effective barrier layer. At low frequency a resistive answer is shown, where the coating resistance domains [45]. The best corrosion resistance property is achieved for the 25 ml L−1 silane modification coating according to the above analysis. According to the equivalent circuit (Fig. 9), the electrochemical parameters can be extracted from measured EIS spectra. The

fitted electrochemical parameters (see Table 2) show that both R1 and R2 increase with the silane addition. Low frequency behavior can be assigned to the corrosion process in the interface. It is possible to conclude that this process is slowed down. Lower capacitance indicates a comparatively lower exposure of the substrate/coating interface to the electrolyte, and thus a more uniform and less damaged coating is developed on magnesium substrate. The polarization resistance, Rp = R1 + R2 , can be estimated the overall corrosion resistance of the coating [46]. From the results in Table 2, the corrosion resistance increases with increasing silane concentration. This means that the silane addition makes the film more compact and can effectively retard the dissolution of substrates exposed to the solution in the film pores. The total impedance of the 25 ml L−1 silane modification coating has the highest value, indicating the best anticorrosion performance. These EIS findings are well compatible with the polarization results. The coatings modified with different BTESPT concentrations immerse in 0.05 M NaCl solution for 10 min, 24 h, 48 h and 72 h, respectively and the Nyquist plots acquired are displayed in Fig. 11. The Nyquist plots during the entire immersion time exhibit two well-defined loops and the diameters are different, suggesting the same corrosion mechanism but different corrosion rate. The EIS spectra are fitted using a Rs (Q1 (R1 (Q2 R2 ))) EEC of two different equivalent electrical circuits the same as shown in Fig. 9. In this equivalent circuit, Rs , R1 and Q1 represent the solution resistance, charge transfer resistance and the corresponding capacitance, respectively. R2 and Q2 indicate the resistance and capacitance of the corrosion product layer. The capacitive loop at higher frequency region is related to the formation of Mg+ and Mg2+ ions, respectively during oxidation of Mg charge transfer resistance. The second time constant in the lower frequency region is attributed to mass transport in the solid phase due to diffusion of ions through the corrosion product layer [43]. The variation in the resistance Rp (R1 + R2 ) of the coatings modified with different silane concentrations in 0.05 M NaCl solution as a function of immersion time is plotted in Fig. 12. The surface morphology of the coatings at different magnifications immersed in 0.05 M NaCl solution for 72 h is depicted in Fig. 13. All of the coatings show a steady decrease in Rp from 10 min to 72 h possibly due to localized damage of the coating. During the whole immersion stage, the silane modified coating shows a higher Rp value compared with the unmodified coating. In addition, after 72 h immersion, the Rp value of the 25 ml L−1 silane modification coating is about 2 times larger than that of the unmodified coating, this might be related to the formation of an insoluble protective corrosion product layer. Meanwhile, more severe cracks are observed on the corroded surface of the unmodified coating, but not on the 25 ml L−1 silane modification coating. It can be concluded that the coating modified with 25 ml L−1 silane offers the best corrosion resistance among all the coatings.

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Table 2 Parameters obtained from EIS of the cerium conversion coatings modified with various BTESPT concentrations in 0.05 M NaCl solution. c(silane)/ (ml L−1 )

Rs /( cm2 )

Q1 /(␮F cm−2 )

n1

R1 /( cm2 )

Q2 /(␮F cm−2 )

n2

R2 /( cm2 )

Rp /( cm2 )

0 15 20 25

6.919 67.2 66.69 61.81

11.47 11.79 11.47 10.43

0.8316 0.8965 0.8907 0.8607

1505 2863 3893 5262

13.73 16.78 5.343 5.777

0.9302 0.7989 1 0.8473

65.16 91.37 476.4 2958

1570.16 2954.37 4369.4 8220

Fig. 11. Nyquist plots of the cerium conversion coatings modified with various BTESPT concentrations in 0.05 M NaCl solution at different intervals for 72 h (a) 0 ml L−1 ; (b) 15 ml L−1 ; (c) 20 ml L−1 ; (d) 25 ml L−1 .

Fig. 12. Resistance, Rp of the cerium conversion coatings modified with various BTESPT concentrations in 0.05 M NaCl solution from EIS at different intervals for 72 h.

Bode plots of the coatings modified with different BTESPT concentrations in 0.05 M NaCl solution at different intervals for 72 h are shown in Fig. 14. After immersion for 72 h, the impedance for all coatings is 702.9  cm2 , 806.2  cm2 , 1073  cm2 and 1945  cm2 , and the maximum phase angle is −50.8◦ at 146.5 Hz, −55.6◦ at 122.1 Hz, −53.8◦ at 46.42 Hz and −56.6◦ at 97.66 Hz, respectively. The higher the impendance value, the higher the maximum phase angle. It can be seen that the maximum phase angle shifts towards a lower frequency for the silane modification coating, indicating of a passive characteristic of the silane modified coating [43]. The EIS results show the same corrosion mechanism of all the coatings. Meantime, the 25 ml L−1 silane modified coating possesses the best corrosion resistance of all.

On the whole, the results indicate that the coating modified with 25 ml L−1 silane is the most effective and displays the optimum corrosion protection characteristic than others. It presents that if the silane concentration is not high enough, the morphology as well as the anticorrosion performance will not be greatly improved. As shown in the electrochemical experiments, the 25 ml L−1 silane addition clearly improves the corrosion resistance property of the coatings. In most cases, the anti-corrosion property is a consequence of good barrier effect created by the coating. Therefore, the corrosion resistance performance will depend upon the coating uniformity, hydrophobic property and chemical stability. Firstly, synergistic effect of cerium salt and silane increases the crosslinking and the coating density, leading to the improvement of the corrosion resistance. Secondly, the silane increases the hydrophobicity of the coating. This behavior is interpreted to diminish the water wetted area and prevent the electrolyte from permeating into the coating/substrate interface. The coating is less prone to water uptake and it must be more resistant to aqueous solution uptake. Additionally, The presence of Si-O-Mg bonds promotes a good adhesion between the substrates/coatings and enhances the chemical stability of the coating [47]. The experiments show that the silane plays an important role in the film formation. The cerium conversion coatings modified with BTESPT is able to form a more uniform, hydrophobic and stable protective layer. The electrochemical property is greatly improved. The mechanism can be described as follows: (1) BTESPT is hydrolyzed in alcohol-based solutions before the treatment of the metallic substrates [48]. The silane solutions become “workable” when a sufficient number of silanol groups

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Fig. 13. Surface morphology of the cerium conversion coatings modified with various BTESPT concentrations at different magnifications after 72 h immersion in 0.05 M NaCl solution (a, b) 0 ml L−1 ; (c, d) 15 ml L−1 ; (e, f) 20 ml L−1 ; (g, h) 25 ml L−1 .

(Si-OH) are generated. The action is:

The reaction is: Si − OH(solution) + Mg − OH(metal surface)

(CH3 CH2 O)3 Si(CH2 )3 S4 (CH2 )3 Si(OCH2 CH3 )3 + 6H2 O → (OH)3 Si(CH2 )3 S4 (CH2 )3 Si(OH)3 + 6H5 C2 OH

(2)

(2) The silanols (Si-OH) groups establish hydrogen bonds with the presence of hydroxyls on the native metallic surface film. The formation of Si O Mg bonds forms a more stable surface coating.

→ Si − O − Mg(interface) + H2 O

(3)

(3) Within the hydrolysis solution, polymerization of silanol groups occurs. Silicon atomic may have interaction with ceria groups [33]. The remaining silanol groups that could not approach the metallic substrate establish bonds among themselves. Upon

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Fig. 14. Bode plots of the cerium conversion coatings modified with various BTESPT concentrations in 0.05 M NaCl solution at different intervals for 72 h (a) 0 ml L−1 ; (b) 15 ml L−1 ; (c) 20 ml L−1 ; (d) 25 ml L−1 .

curing or drying, these hydrogen bonds are converted in stable siloxane (Si O Si) bonds via the subsequent reaction: Si − OH(solution) + Si − OH(solution) → Si − O − Si(silane film) + H2 O

(4)

(4) When the magnesium substrates immerse in the conversion bath (pH≈6), there is dissolution of the outer oxide layer. Simultaneously, oxygen is reduced to release hydroxyl ions as demonstrated in the previous works [49]. With local pH values increase, the precipitation of cerium hydroxides occurs [50]. The reactions can be described as follows:

Acknowledgement The authors are grateful to Natural Science Foundation of China (Grant Nos. 51172217 and 41406092) and Qingdao Science and Technology Foundation for Youths (Grant No. 14-2-4-113-jch) for financial supports to this work. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.apsusc.2016.03. 150. References

Mg − 2e− = Mg 2+

(5)

O2 + 2H2 O + 4e− → 4OH −

(6)

Ce4+ + 4OH − → Ce(OH)4

(7)

Ce3+ + 3OH − → Ce(OH)3

(8)

4. Conclusions The role of BTESPT as an additive in the silane modified cerium conversion coating on magnesium alloys has been analyzed using SEM, XPS and EIS. A comparative study suggests that compared with single cerium conversion coating, a combination of cerium ions and BTESPT silane molecules can effectively improve the uniformity, hydrophobic performance and corrosion resistance of the coatings. Si O Si linkage builds a robust structure for cerium-rich particles deposition and Si O Mg bonds are likely to strengthen the adhesion between the coating and substrate, which facilitates it to form a more protective coating. The combination of cerium and silane improves the coating corrosion resistance performance. The corrosion behavior study shows that after immersion for 72 h in 0.05 M NaCl solution, the coating modified with 25 ml L−1 BTESPT shows higher Rp value, larger impedance and higher phase angle maxima, which indicates the optimum anticorrosion performance.

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