- Email: [email protected]
TMPSi ceramic composite coating with enhanced corrosion resistance on 316L
Journal Pre-proof Efficient one-step fabrication of superhydrophobic nano-TiO2/TMPSi ceramic composite coating with enhanced corrosion resistance on 316L Seyed Masoud Emarati, Mahdi Mozammel PII:
S0272-8842(19)32668-9
DOI:
https://doi.org/10.1016/j.ceramint.2019.09.137
Reference:
CERI 22904
To appear in:
Ceramics International
Received Date: 16 August 2019 Revised Date:
14 September 2019
Accepted Date: 15 September 2019
Please cite this article as: S.M. Emarati, M. Mozammel, Efficient one-step fabrication of superhydrophobic nano-TiO2/TMPSi ceramic composite coating with enhanced corrosion resistance on 316L, Ceramics International (2019), doi: https://doi.org/10.1016/j.ceramint.2019.09.137. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.
Efficient one-step fabrication of superhydrophobic nano-TiO2/TMPSi ceramic composite coating with enhanced corrosion resistance on 316L
Seyed Masoud Emarati1, Mahdi Mozammel1* 1
Faculty of Material Engineering, Sahand University of Technology,Sahand New Town,
Tabriz, Iran, Postal Code: 5331817634 *
Corresponding Author, [email protected], Tel: +98-9125267093
Abstract TiO2 Nanoparticle / Trimethoxy(propyl)silane (TMPSi) ceramic composite coating was deposited on 316L steel using a one-step electrophoretic deposition (EPD) method. Silane coupling agent (TMPSi) was added to the EPD bath in different concentrations (from 0.5 to 15 vol. %) to decrease the surface energy of the deposited coating. TiO2 coating is hydrophilic whereas by adding varying concentrations of TMPSi, the obtained nanocomposite coating showed much better hydrophobicity. Surface wettability was measured by water contact angle (WCA) and sliding angle (SA) tests. Moreover, the effect of TMPSi concentration was determined by comparing the WCA and SA values. Surface morphology was studied through Field Emission Scanning Electron Microscopy (FESEM), and the presence of micro/nano meter roughness on the surface was confirmed. The distribution of elements were investigated by EDS analysis in which their uniform dispersion was observed. Corrosion behavior of 316L samples 1
before and after the coating process was studied by potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) tests in 3.5 wt. % NaCl solution. The polarization curve proved that the superhydrophobic ceramic nanocomposite coatings (WCA = 168 º and SA = 3.1 º) were able to decrease the corrosion rate of bare 316L (from 12.180 to 5.621 (µm per year)). Keywords: superhydrophobic, composite, EPD, TiO2, corrosion
1. Introduction Stainless steels (SS) are unique among all metals that are used industry. Because of their extensive commercial applications, they are used in clinics, public places, food industry, and kitchenwares. Above all, 316L stainless steel has attracted great attention in medical applications (including medical instruments and body implants) for its biocompatibility [1, 2]. But it has been difficult to achieve health standards due to the easy growth of bacterial colonies on the surface of stainless steel. Therefore, it is necessary to enhance the anti-bacterial and anticorrosion propertiesof 316L. One of the most effective ways to achieve this purpose is to deposit a protective hydrophobic metal oxide coating on the surface of 316L. So far, many efforts have been reported to fabricate an antibacterial coating on stainless steel using different methods, including dual magnetron sputtering, synthesis of nanoparticles (NPs), etc.[3, 4]. Recently Limei Chen et al. [5] have fabricated an antimicrobial composite coating by fixing Ag NPs on stainless steel with the use of 3- aminopropyltriethoxysilane (APTES).
2
Generally, in superhydrophobic surfaces, one of the most important problems is that the modified surface loses its anti-bacterial property in a short period of time. Therefore it would not exhibit an appropriate anti-bacterial performance during the service. Also is not easy to preserve the surface antibacterial characteristics along with high corrosion resistance simultaneously. One way to protect metals from corrosion is by exerting ceramic coatings. These coatings have special properties, including high resistance against corrosion, oxidation, and erosion [6]. E. Celik et al. [7] investigated the corrosion behavior of ceramic coating on the AISI 304L SS in 1 N H2SO4 solution and revealed the improvement of the samples’ corrosion resistance. Titanium dioxide is a well-known biomaterial that can be used as a biomaterial coating for sterilizing medical implants, surgical instruments and clinical equipment [8, 9]. Moreover, this metal oxide is one of the most prominent materials to fabricate superhydrophobic surfaces because it is low-priced, non-toxic and chemically stable in the ambient environment [10, 11]. Recently C-T. Hsieh et al. [12] have applied a superhydrophobic film on ITO using TiO2 NPs and post-treatment of fluorination. The results showed that the prepared film has high waterrepellency. The self-cleaning property has attracted a lot of attention because its production procedure is more time-efficient and cheaper [13]. For fabricating an artificial superhydrophobic surface two necessary conditions must be met. The first is surface roughening which includes methods like chemical etching, mechanical scratching, electrodeposition, sol-gel processing, and chemical/physical vapor deposition. The second condition is lowering surface energy that can be achieved by modifying the surface chemistry with various functional groups such as silane groups. These silane groups can decrease the
3
surface energy or alter the surface chemistry (such as by formation of physical binding, surface adsorption, and coating process) [14-19]. In fact, when a water droplet settles on a rough surface, it contacts with both the micro/nano protrusions, and the air gets trapped between the pillars. The trapped air acts as a repulsive force preventing the water from spreading (WCAair = 180 °). Thus by increasing the proportion of trapped air to contact point, superhydrophobicity of the surface increases [15, 20]. Electrophoretic deposition (EPD) is reported to be a highly effective method in preparing superhydrophobic coatings on a conductive substrate [21-24]. Recently M. Javidi et al. [25] have successfully fabricated hydroxyapatite on 316L using EPD in a suspension of isopropyl alcohol/ polyethyleneimine. Also A. R. Boccaccini et al. [26] electrophoretically obtained TiO2 coating on SiC fiber and Carbon-fiber substrates. Some significant advantages of this method are: 1) The possibility of using different materials capable of being coated on conductive substrates such as metal oxides. 2) This technique has high economic justifications for its simple equipment, cheap supplies, and high applying speed. 3) High capability in the deposition of coatings with delicate and complex shapes in wide dimensional ranges. Moreover, the thickness of the coatings can be easily controlled by varying the deposition parameters [27-29]. By controlling the specific parameters of EPD procedure (such as time and applied potential) the appropriate roughness can be obtained. In fact, the rough surface was achieved by aggregation of
4
deposited particles on the substrate [16]. G. X. Shen et al. [6] have improved the corrosion resistance of 316L in two steps. First, EPD of TiO2 NPs on the surface followed by chemical modification of a substrate by dip coating the sample in fluoroalkyl silane (FAS-B) solution. Furthermore, T. Ishizaki et al. [30] have fabricated a superhydrophobic coating by deposition of cerium oxide film on the magnesium alloy and modification of the coating in FAS solution. Results exhibited improvements in the corrosion resistance of magnesium alloy. To the best of our knowledge, this is the first time that synthesis of a superhydrophobic nanoTiO2/TMPSi ceramic composite coating on a 316L substrate by EPD process without post surface modification was investigated. the optimal condition exhibited outstanding results (WCA = 168 º and SA = 3.1 º). Moreover, the effect of surface hydrophobicity on the corrosion resistance of 316L was investigated by potentiodynamic polarization and EIS tests. Applying the superhydrophobic nano-TiO2/5 vol. % TMPSi ceramic composite coating in a single step on the surface results in a hydrophobic coating thoughout of the surface. If the outer layer of coating is damaged for any reason due to the presence of the silane bonds along with TiO2 nanoparticles throughout the coating, the inner layers still show the hydrophobic properties.
2. Experimental 2.1. Electrodes preparation The 316L substrates (Caspian Steel Vista) with chemical composition (in wt. %) of [65.3% Fe, 12.2% Ni, 17.4% Cr, 2% Mn, 2.1% Mo, 0.03% S, 0.1% N, 0.03% C, 0.75% Si, 0.045% P] were ground by emery paper with 100- 3000 mesh sizes and were polished by diamond paste. Then they were ultrasonically cleaned in acetone (99%, CAS No.: 67-64-1, Merck) /ethanol (99.8 %, CAS No.: 64-17-5, Sigma-Aldrich) bath with the volume ratio of 9:1 for 20 min at 35°C. 5
Afterward, electrodes were removed from the bath and ultrasonically cleaned in a deionized water bath for 10 min at 35°C to ensure the removal of any contaminations from micro groves of the surface. 2.2. Preparation of superhydrophobic nano-TiO2/TMPSi ceramic composite coating The superhydrophobic nano-TiO2/TMPSi ceramic composite coating was deposited on 316L by a one-step EPD process. The EPD bath was prepared by adding 0.5 g TiO2 (P25, CAS No.: 13463-67-7, Evonik Degussa GmbH), 0.04 g Iodine (99.99 %, CAS No.: 7553-56-2, SigmaAldrich), and different concentrations (0.5 vol. %, 1 vol. %, 2 vol. %, 3 vol. %, 4 vol. %, 5 vol. %, 6 vol. %, 7 vol. %, 8 vol. %, and 15 vol. %) of TMPSi (97%, CAS No.: 1067-25-0, SigmaAldrich) in 50 mL acetylacetone (99%, CAS No.: 123-54-6, Merck) as solvent (Fig. 1). The prepared suspension was stirred for 20 min in room temperature and then was placed in the ultrasonic bath for 15 min. Prepared 316L electrodes were used as cathode and anode with a distance of 20 mm. Working and counter electrodes were connected to the negative and positive poles of the DC power supply, respectively. By applying the potential of 5 V for the first 45 s, and increasing the voltage (with a constant rate of 1 V.s-1 until it reaches 30 V in a period of 45 s – 70 s), the superhydrophobic nano-TiO2/TMPSi ceramic composite coating was deposited on the 316L substrate. By adding the iodine to suspension, TiO2 NPs and silane agents were partially positively charged and migrated towards the cathode and were deposited under an electrical field. After the coating process, samples were removed from the cell and were placed in an oven at a temperature of 30 °C, so that the green deposited coatings dry slowly. 2.3. Characterization of the coating
6
Surface morphology and roughness of samples were studied by field-emission scanning electron microscopy (FESEM; MIRA3-XMU, TESCAN) and atomic force microscopy (AFM; NaioAFM, Nanosurf) analysis, respectively. Moreover, chemical bonds on the hydrophobic film were investigated by fourier transformed infrared spectroscopy in attenuated total reflectance mode FTIR-ATR analysis (Bruker Tensor 27 FT-IR). 2.4. Investigation of water-repellency of the coating The water-repellency of the superhydrophobic nano-TiO2/5 vol. % TMPSi ceramic composite coating was investigated by measuring WCA and SA. For determining the amount of WCA and SA, a water droplet with a volume of 1-2 µL was dropped on the surface by FIBRO SYSTEM AB (model of PG-X, Sweden) device and WCA and SA were measured optically by a digital camera mounted on the mentioned device. 2.5. Potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) tests To determine the corrosion resistance of the samples, the potentiodynamic polarization tests were performed. Two samples were prepared with dimensions of 1×1 cm2. One of them was the superhydrophobic sample (the best sample prepared under optimized conditions), and the other one was the substrate. The samples were placed in the corrosive solution of 3.5 wt. % NaCl whereas only 1×1 cm2 of the surface was in contact with the solution. The rest of the surface of the sample was insulated with Teflon. Significant corrosion parameters such as corrosion potential (Ecorr) and corrosion current density (icorr) were calculated using potentiodynamic curves. Also, the important EIS parameters were calculated using a simulated equivalent electrical circuit fitted to the real experimental data.
7
3. Results and discussion 3.1. Effect of TMPSi concentration on the hydrophobicity of the nano-TiO2/TMPSi ceramic composite coating WCA and SA of 316L bare substrate were measured, and their values were 39 º and 180 º respectively. Due to the hydrophilicity of TiO2, when a water droplet was dropped on a TiO2 coating, it immediately spread out on the surface coating whereas WCA and SA could not detected by FIBRO SYSTEM AB device. On the contrary, adding a slight of TMPSi to EPD bath, increased the WCAs significantly. Effect of TMPSi concentration of the suspension, provided for EPD process, on the wettability of nano-TiO2/TMPSi ceramic composite coating was shown in Fig. 2. As it is clear from Fig. 2, TMPSi in the concentration range of 4-6 vol. % affects the waterrepellency of the coating greatly. In the as-mentioned range, coating is superhydrophobic (WCA > 150 º); For the EPD bath with a concentration of 5 vol. % TMPSi, WCA and SA reach the significant values of 168 ° and 3.1 °, respectively. In fact, in low concentrations of TMPSi, WCAs and SAs were also low and high respectively. By gradually increasing the concentration of TMPSi, the WCA also increased and SA decreased until they reached to the highest and the lowest points respectively in 5 vol. % TMPSi (WCA = 168 º, SA = 3.1 º). Moreover, by increasing the concentration, the WCA got lower and SA 8
increased again. With attention to the mechanism of the silane agents, the reason for this phenomenon can be described as follows: it seems that both in lower and higher concentrations of TMPSi, insufficient silane bonds are formed, the phenomenon of self-condensation of silane agents together prevents the formation of sufficient and necessary silane bonds on the coating. WCA and SA of the sample with 5 vol. % TMPSi were shown in Fig. 3a and also a top view of the as mentioned coating was shown in Fig. 3b. As was concluded from the WCA and SA results, TMPSi concentration in the EPD bath has a direct effect on the wettability of the TiO2 coating. 3.2. FTIR-ATR analysis The FTIR-ATR tests were taken of TiO2-P25 NPs, TMPSi solution, TiO2 hydrophilic coating, and the superhydrophobic nano-TiO2/5 vol. % TMPSi ceramic composite coating (WCA = 168 º and SA = 3.1 º). As shown in Fig. 4a, two peaks in the range of 548-670 cm-1 were visible that related to Ti-O vibration in titanium dioxide crystal [31-34]. In superhydrophobic ceramic nanocomposite coating (curves with blue color) also three important peaks in the range of 1064.31-1219.07 cm-1 were visible that didn’t exist in other samples. The drastic peak at 1064.31 cm-1 corresponds to Si-O-C [35, 36] and the one at 1119.2 cm-1 is related to stretching vibration of Si-O-Si (siloxane) bonding [37]. Additionally, the peak at 1219.07 cm-1 is assigned to Si-CH2 bonding [38] that confirms the presence of TMPSi agent in the coating. The existence of these three mentioned peaks verified the water-repellency property of the superhydrophobic sample. Intense peaks at 2923.59 cm-1 and 2854.03 cm-1 [39, 40] are related to the symmetric stretching vibration of CH2 groups that are originated from acetylacetone in the nano-TiO2 hydrophilic
9
coating (the red curve). On the other hand, intense peaks at 2872.45 cm-1 and 2958.83 cm-1 are assigned to the CH2 stretching bonding of TMPSi in the superhydrophobic ceramic nanocomposite coating. These peaks only correspond to CH2 bonds of TMPSi at 2958 cm-1 and 2872.45 cm-1 [38] (Fig. 4b). According to the FTIR results, all the observed bonds on the surface and the bulk of the coating were the same. Therefore, it is reasonable to conclude that if the outer layer of the coating is damaged for any reason, the inner layers will still show hydrophobic properties as shown in Fig. 5. In fact, in the prepared EPD suspension, both TiO2 NPs and TMPSi agents were partially positively charged and migrated towards the working electrode and deposited simultaneously (Fig. 6a). Similarly, Ogihara et al. [16] studied the migration of charged trimethylsiloxysilicate (TMSS) under an applied electric field in the EPD process. Accordingly, in each part of the coating, the effective bonds of silane agents are present. This fact is shown schematically in Fig. 6b. 3.3. Study of the Surface morphology of the deposited TiO2 coating The high surface roughness is considered as an essential factor to obtain hydrophobicity. In other words, the simultaneous presence of both nano and micro-scaled roughness on the surface leads to hydrophobic property [16]. Surface morphology and roughness of the TiO2 coating which was obtained using an optimum amount of TMPSi in EPD bath (WCA = 168 º and SA = 3.1 º) were investigated by FESEM and AFM. The results were shown in Fig. 7.
10
All contents of suspension were influenced comprehensively by the electric field and were aggregated regularly on working electrode, thus a uniform coating and distribution of elements were observed on the coating (Figs. 7a and 8). For further investigation, the cross-section of coating and also the elemental distribution of coating and substrate were studied by EDS analysis (Fig. 8). As shown in Fig. 8a, the substrate was distinguishable from coating graphically. Fig. 8b shows the curves of elemental distribution, as can be seen in Fig. 8b, the amount of elements from the surface of the coating to the depth of the substrate were changing as explained blew: a) The titanium concentrate decreases (Fig. 8g) by reaching the substrate. The same trend in Si concentration would be expected but due to the presence of Si in the substrate as a primary element (section 2.1) no tangible decreases in Si concentration was observed (Fig. 8f). b) As the line scan curves approach to the substrate, the intensity of iron and chromium elements increases. Moreover, the distributions and concentrations of the main elements of coating, including Ti, O, and Si were shown in Fig. 9 and Table 1, respectively. Regarding the chemical formula of TiO2, the atomic percentage ratio of oxygen to titanium should be 2:1. However, regarding the data presented in Table 1, this ratio is 2.21 (
67.75 ). It is 30.64
anticipated that the additional atomic percentage of O (6.47%) was originated from external factors, including silica and oxygen. As shown in Fig .7b, the formation of the micro-pillar was evident. In fact, the arrangement of micropillars beside each other is the main factor in the formation of micrometer-roughness on the coating (Figs. 7e and f). Further investigation of FESEM images (in higher magnifications)
11
revealed that placement of particles on the substrate during the deposition process created cauliflower-shapes. In fact, the formation of a hierarchical structure on the surface was visible (Fig. 7c). Most of the previous researches on the natural superhydrophobic surfaces revealed the presence of a hierarchical structure [20]. In fact, the existent of available and accessible numerous sharp places for settling water droplets and also the presence trapped air (as a repellent force) between components of cauliflowershaped TiO2 NPs aggregation act as the effective factors in hydrophobicity of the surface. The average-size of deposited particles was determined in the highest FESEM magnification (Fig. 7d). As seen in Fig. 7d aggregated cauliflowers is made of TiO2 particles, and they almost have the same size (such as L1 and L2 = 17.6 nm, L3 = 18.4 nm in Fig. 10d). Thus a uniform roughness on the surface is expected (Figs. 7e and f). In other words, according to Figs. 7c-f, nanometer-roughness of the surface is visible. Therefore, according to Fig. 7. it could be concluded that the simultaneous presence of both nanometer and micrometer roughness on the surface is evident.
3.4. Investigation of corrosion behavior of the superhydrophobic nano-TiO2/5 vol. % TMPSi ceramic composite coating on the 316L substrate 3.4.1. Potentiodynamic polarization
The potentiodynamic polarization curves of the substrate and the sample with the superhydrophobic nano-TiO2/5 vol. % TMPSi ceramic composite coating in 3.5 wt. % NaCl solution with a scan rate of 0.5 mV.s-1 were shown in Fig. 10. Platinum was used as counter electrode and potentials of working electrodes were determined by saturated calomel electrode (SCE) as a reference electrode. 12
Important corrosion data such as corrosion potential (Ecorr), corrosion current density (icorr), cathodic slope (βcathodic), and anodic slope (βanodic) were derived from the linear part of the obtained dynamic polarization curves for both samples. Moreover, Rp (polarization resistance) was derived from the Stern-Geary equation [41] which is shown below:
Rp =
β cathodic × β anodic 2.303i corr (| β cathodic | +β anodic )
Eq. (1)
The corrosion rate of the samples was determined by Eq. (2) [42]:
Corrosion Rate (µm per year) = 315360 ×
icorr .a ρ.n.F
Eq. (2)
In Eq. (2), icorr is the corrosion current density [(µA.cm-2)], a shows the atomic weight of metallic samples [(g.mol-1)], n is the number of equivalent exchange, ρ represents the density of samples [(g.cm-3)] and F is the Faraday's constant [(coulomb.mol-1)]. The mentioned data were shown in Table 2. By comparing the polarization curves, it was observed that by applying the superhydrophobic coating the corrosion potential (Ecorr) was shifted to more positive values (i.e., from -187.454 mV vs. SCE for the substrate to 109.114 mV vs. SCE for the superhydrophobic nano-TiO2/5 vol. % TMPSi ceramic composite coating). Furthermore, icorr was decreased due to the presence of the coating (from 1.508 µA.cm-2 for the substrate to 0.696 µA.cm-2 for the superhydrophobic nanoTiO2/5 vol. % TMPSi ceramic composite coating). So it was concluded that the superhydrophobic nano-TiO2/5 vol. % TMPSi ceramic composite coating decreased the development of corrosion phenomenon and effectively protected the
13
substrate against corrosive agents. This happens because the water-repellent protective coatings prevent the penetration of corrosive agent to the metal substrate. 3.4.2. Electrochemical impedance spectroscopy (EIS)
The EIS is a powerful and suitable technique for investigating the anti-corrosion behavior of the coating during the exposure to corrosive environments [43]. The chosen samples were prepared according to section 2.5 and were placed in a corrosive solution of 3.5 wt. % NaCl for 30 min before starting the test, in order to stabilize the electrode in the solution. The reference electrode was a saturated calomel electrode, and Pt-plate was selected as the counter electrode. Also, the frequency range between 10-3 - 10+5 Hz was set to fit the equivalent electrical circuit and simulate the magnitude of its elements. The Bode plot of the superhydrophobic nano-TiO2/5 vol. % TMPSi ceramic composite coating along with an equivalent figurative model of the coated sample in 3.5 wt. % NaCl solution corresponding the EIS results were shown in Figs. 11a and b, respectively. Furthermore, the magnitude of elements of the simulated equivalent electrical circuit in Fig. 11b were reported in Table 3. In a real electrochemical test, using a constant phase element (CPE) is more accurate than using pure capacitance. Furthermore, due to the roughness of the coating caused by TiO2 NPs using CPE is more reasonable [42]. The impedance of CPE is defined as ZCPE =
1
( jω )
α
Eq. (3) Y0
[44]. Where in this Eq. (3), ω is the angular frequency, Y0 is the general admittance function, and
α is an empirical exponent of the CPE that 0 ≤ α ≤ 1. If α = 1 and α = 0, the interface has pure capacitive and pure resistance behavior respectively.
14
As shown in Figs. 11a and b, the fitted equivalent electrical circuit has two CPEs that their empirical exponents are near to 1 (α ≈ 1). Hence two time constants are expectable: a) τcoat = Rcp×Ccoat
Eq. (4), and b) τEDL = Rct×CEDL
Eq. (5) [45]. At high frequencies, two
capacitors have the lowest impedance (according to Eq. (6) of ZC =
1 [46] ; where ZC, j, ω, jωC
and C are the impedance of the capacitor, imaginary number ( ± −1 ), angular frequency , and capacitance, respectively.). Therefore, Rcp, Rct, and CPEEDL were short-circuited by the CPEcoat (these elements were represented by grey color in Figs. 11c-g). In this condition, the equivalent electrical circuit only contains Rs, as shown in Fig. 11c.
By decreasing the frequency and reaching to the values of near
1 τ coat
, Rct was short-circuited by
the CPEEDL due to the very low impedance of CPEEDL. The corresponding equivalent electrical circuit is shown in Fig. 11d.
In the moderate frequency range (
1 τ coat
˂ω˂
1 τ EDL
), according to the Table 3 and the definition
of both τcoat and τEDL and also by comparing CPEcoat and CPEEDL values, it is concluded that
1 τ EDL
<
1 τ coat
, so the impedance of CPEcoat is very high. Therefore, it is removed from the
electrical circuit and since the impedance of CPEEDL is low, hence it causes the Rct to shortcircuit. The equivalent electrical circuit in this mentioned frequency range is shown in Fig. 11e.
By approaching the frequency to
1 τ EDL
, the equivalent electrical circuit will be similar to Fig. 11f.
15
According to Eq. (7) of C = ε 0ε
A d
where C, ε0, ε, A, and d are capacitance, the dielectric
constant of free space (vacuum), effective dielectric constant, area, and thickness of the coating, respectively [47]. Also, the fact that the thickness of the electrical double layer is far less than the thickness of the coating, it is concluded that CPEEDL ˃˃ CPEcoat. Compared to the high-frequency range, as the frequency drops to the lower magnitudes until it reaches
1 τ EDL
, the coating shows
more capacitive behavior.
In very-low-frequency range (ω ˂
1 τ EDL
), both CPEs were fully charged and so removed from the
electrical circuit as shown in Fig. 11g. In this frequency range, the coating has the highest magnitude of impedance, in other words:
ǀZǀequivalent electrical circuit = ǀRs+Rcp+Rctǀ
Eq. (8)
According to detailed discussion of the coating’s equivalent electrical circuit in the different frequency ranges, the behavior of coating during the exposure to a corrosive environment is predictable in every desired frequencies. According to the Eq. (8), in the organic coatings, the investigation of EIS plots in low frequencies is more important than other frequencies. Rct and Rcp, which were derived in this range of the frequency, reflect properties of the coating and the polarization resistance, respectively. Although Eq. (8) contains Rs, the value of which is much less than ǀRcp+Rctǀ [47]. Furthermore as a time of immersion progresses, undoubtedly the uptake of corrosive ions and water into the coating increases [48]. With their uptake into the coating, the protective property of the coating crucially decreases due to the changes of two parameters: a) the absorption of water increases the effective dielectric constant (ε) and as a result the coating’s capacitance rises and b) this uptake decreases both Rcp and Rct [47]. 16
The Bode plot of the bare 316L with its appropriate equivalent figurative model corresponding to the EIS results were shown in Figs. 11h and i, respectively. Furthermore, the magnitude of simulated equivalent electrical circuit elements of Fig. 11i were reported in Table 4. According to Table 4 and the high impedance values (ǀZǀ ≈ 2 × 103 Ω.cm2) in very low frequencies, it is obvious that the bare 316L has a good corrosion resistance as expected. By comparing Figs. 11a and h in low frequencies, using the superhydrophobic nano-TiO2/5 vol. % TMPSi ceramic composite coating could significantly improve the protection property of the bare 316L. This claim could be concluded from the increase in ǀZǀ (from 2 × 103 Ω.cm2 to 104 Ω.cm2) due to the presence of the superhydrophobic nano-TiO2/5 vol. % TMPSi ceramic composite coating. In other words, hydrophobic silane coupling agents in throughout the coating act as strong barriers against the uptake of water and ions. Moreover, using TiO2-P25 instead of larger sizes led to reducing the empty space between the coating’s particles and limited the water absorption and accumulation. Therefore, the resistance of pores (Rcp) was increased effectively. The proposed mechanism and the performance of a coating in contact with water are schematically modeled in two fashion: a) superhydrophobic ceramic nanocomposite coating (Fig 12a), and b) coating without hydrophobic property and the use of nano-sized materials (Fig. 12b). Finally, the experimental outcomes of corrosion tests emphasize that superhydrophobic coating represents much better corrosion resistance rather than bare 316L.
4. Conclusion Superhydrophobic nano-TiO2/5 vol. % TMPSi ceramic composite coating was deposited by a one-step EPD process on the 316L. According to the hydrophilicity of TiO2 NPs, the deposited 17
coating of TiO2 was also hydrophilic. Thus to achieve hydrophobicity on the surface, utilizing silane coupling agents such as TMPSi to decrease the surface energy was necessary. By adding TMPSi in EPD bath, the superhydrophobic nano-TiO2/5 vol. % TMPSi ceramic composite coating (WCA = 168 º and SA = 3.1 º) was prepared in a one-step process which possesses various advantages including high-speed implementation because the chemical modification of surface after the coating process was omitted. Results of WCA and SA showed that by adding 5 vol. % TMPSi in EPD bath and applying swipe potential (applying the potential of 5 V at the first 45 s, then increasing the voltage at a constant rate of 1 V.s-1 until it reaches 30 V in a period of 45 s - 70 s), the deposited nanocomposite coating had the highest hydrophobic property. In other words, by applying the superhydrophobic nano-TiO2/5 vol. % TMPSi ceramic composite coating, WCA of the surface was increased from 39 º to 168 º and SA decreased from 180 º to 3.1 º in optimum coating conditions. The prepared coating had high water-repellency and water droplet could easily roll off the surface. The corrosion resistance of 316L and coated samples in 3.5 wt. % NaCl solution was investigated. According to the results from the corrosion tests, superhydrophobicity of ceramic nanocomposite coating was enhanced the corrosion resistance of 316L (Ecorr was shifted from -187.544 mV vs. SCE to 109.114 mV vs. SCE and icorr changed from 1.508 µA.cm-2 to 0.696 µA.cm-2 for uncoated and optimum superhydrophobic sample, respectively). Furthermore, the EIS results showed the impedance of the superhydrophobic nanoTiO2/TMPSi ceramic composite coating was extremely higher than bare 316L.
Acknowledgement The authors wish to express their gratitude to Engineers Mohammad Alinezhadfar and Mohammad Tanhaei for their literary editing and Engineer Ali Mohseni for his help in the
18
preparation of corrosion samples. Responsibility for the information and views set out in this article lies entirely with the authors.
Disclosure statement No potential conflict of interest was reported by the authors.
References [1] D.S.R. Krishna, Y. Sun, Thermally oxidised rutile-TiO 2 coating on stainless steel for tribological properties and corrosion resistance enhancement, Applied Surface Science, 252 (2005) 1107-1116. [2] T. Fu, C. Wen, J. Lu, Y. Zhou, S. Ma, B. Dong, B. Liu, Sol-gel derived TiO 2 coating on plasma nitrided 316L stainless steel, Vacuum, 86 (2012) 1402-1407. [3] X. Tian, Z. Wang, S. Yang, Z. Luo, R.K. Fu, P.K. Chu, Antibacterial copper-containing titanium nitride films produced by dual magnetron sputtering, Surface and Coatings Technology, 201 (2007) 8606-8609. [4] Z. Dan, H. Ni, B. Xu, J. Xiong, P. Xiong, Microstructure and antibacterial properties of AISI 420 stainless steel implanted by copper ions, Thin solid films, 492 (2005) 93-100. [5] L. Chen, L. Zheng, Y. Lv, H. Liu, G. Wang, N. Ren, D. Liu, J. Wang, R.I. Boughton, Chemical assembly of silver nanoparticles on stainless steel for antimicrobial applications, Surface and Coatings Technology, 204 (2010) 3871-3875. [6] G. Shen, Y. Chen, L. Lin, C. Lin, D. Scantlebury, Study on a hydrophobic nano-TiO 2 coating and its properties for corrosion protection of metals, Electrochimica acta, 50 (2005) 5083-5089. [7] E. Celik, I. Ozdemir, E. Avci, Y. Tsunekawa, Corrosion behaviour of plasma sprayed coatings, surface and coatings technology, 193 (2005) 297-302. [8] H.-f. WANG, X.-f. SHU, X.-y. LI, T. Bin, Photocatalytic activities of N doped TiO2 coatings on 316L stainless steel by plasma surface alloying technique, Transactions of Nonferrous Metals Society of China, 22 (2012) s120-s126. [9] M. Jackson, W. Ahmed, Titanium dioxide coatings in medical device applications, Surface Engineered Surgical Tools and Medical Devices, Springer New York2007, pp. 49-63. [10] J. Lee, G. Jang, Y. Jun, Investigation and evaluation of structural color of TiO 2 coating on stainless steel, Ceramics International, 38 (2012) S661-S664. [11] H. Yaghoubi, N. Taghavinia, E.K. Alamdari, Self cleaning TiO 2 coating on polycarbonate: surface treatment, photocatalytic and nanomechanical properties, Surface and Coatings Technology, 204 (2010) 1562-1568. [12] C.-T. Hsieh, M.-H. Lai, Y.-S. Cheng, Fabrication and superhydrophobicity of fluorinated titanium dioxide nanocoatings, Journal of colloid and interface science, 340 (2009) 237-242. [13] F. Li, Q. Li, H. Kim, Spray deposition of electrospun TiO 2 nanoparticles with self-cleaning and transparent properties onto glass, Applied Surface Science, 276 (2013) 390-396.
19
[14] M. Ma, R.M. Hill, Superhydrophobic surfaces, Current opinion in colloid & interface science, 11 (2006) 193-202. [15] H. Ogihara, J. Xie, J. Okagaki, T. Saji, Simple method for preparing superhydrophobic paper: spray-deposited hydrophobic silica nanoparticle coatings exhibit high water-repellency and transparency, Langmuir, 28 (2012) 4605-4608. [16] H. Ogihara, T. Katayama, T. Saji, One-step electrophoretic deposition for the preparation of superhydrophobic silica particle/trimethylsiloxysilicate composite coatings, Journal of colloid and interface science, 362 (2011) 560-566. [17] B. Javanpour, M. Azadbeh, M. Mozammel, Effect of Hydrothermal Sealing on Wettability and Electrochemical Behavior of TMPSi Treated Anodized Aluminum, Journal of Inorganic and Organometallic Polymers and Materials, (2018) 1-11. [18] M. Mozammel, M. Khajeh, N.N. Ilkhechi, Effect of Surface Roughness of 316 L Stainless Steel Substrate on the Morphological and Super-Hydrophobic Property of TiO 2 Thin Films Coatings, Silicon, (2018) 1-5. [19] B. Javanpour, M. Azadbeh, M. Mozammel, Effect of Chemical Composition of Tetraethoxysilane and Trimethoxy (Propyl) Silane Hybrid Sol on Hydrophobicity and Corrosion Resistance of Anodized Aluminum, Silicon, (2019) 1-16. [20] Y.Y. Yan, N. Gao, W. Barthlott, Mimicking natural superhydrophobic surfaces and grasping the wetting process: A review on recent progress in preparing superhydrophobic surfaces, Advances in colloid and interface science, 169 (2011) 80-105. [21] Y.S. Joung, C.R. Buie, Electrophoretic deposition of unstable colloidal suspensions for superhydrophobic surfaces, Langmuir, 27 (2011) 4156-4163. [22] A.R. Boccaccini, I. Zhitomirsky, Application of electrophoretic and electrolytic deposition techniques in ceramics processing, Current Opinion in Solid State and Materials Science, 6 (2002) 251-260. [23] S.M. Emarati, M. Mozammel, Fabrication of superhydrophobic titanium dioxide coating on AISI 316L stainless steel by electrophoretic deposition and using trimethoxy (propyl) silane modification, Surface Engineering, (2018) 1-10. [24] M. Salehi, M. Mozammel, S.M. Emarati, Superhydrophobic and corrosion resistant properties of electrodeposited Ni-TiO2/TMPSi nanocomposite coating, Colloids and Surfaces A: Physicochemical and Engineering Aspects, 573 (2019) 196-204. [25] M. Javidi, S. Javadpour, M. Bahrololoom, J. Ma, Electrophoretic deposition of natural hydroxyapatite on medical grade 316L stainless steel, Materials Science and Engineering: C, 28 (2008) 1509-1515. [26] A. Boccaccini, P. Karapappas, J. Marijuan, C. Kaya, TiO 2 coatings on silicon carbide and carbon fibre substrates by electrophoretic deposition, Journal of materials science, 39 (2004) 851-859. [27] A. Chavez-Valdez, A. Arizmendi-Morquecho, K. Moreno, J. Roether, J. Kaschta, A. Boccaccini, TiO 2–PLLA nanocomposite coatings and free-standing films by a combined electrophoretic deposition-dip coating process, Composites Part B: Engineering, 67 (2014) 256261. [28] M. Wei, A. Ruys, B. Milthorpe, C. Sorrell, J. Evans, Electrophoretic deposition of hydroxyapatite coatings on metal substrates: a nanoparticulate dual-coating approach, Journal of Sol-Gel Science and Technology, 21 (2001) 39-48.
20
[29] Q.-T. Vu, M. Pavlik, N. Hebestreit, J. Pfleger, U. Rammelt, W. Plieth, Electrophoretic deposition of nanocomposites formed from polythiophene and metal oxides, Electrochimica acta, 51 (2005) 1117-1124. [30] T. Ishizaki, Y. Masuda, M. Sakamoto, Corrosion resistance and durability of superhydrophobic surface formed on magnesium alloy coated with nanostructured cerium oxide film and fluoroalkylsilane molecules in corrosive NaCl aqueous solution, Langmuir, 27 (2011) 4780-4788. [31] J. Zhao, M. Milanova, M.M. Warmoeskerken, V. Dutschk, Surface modification of TiO 2 nanoparticles with silane coupling agents, Colloids and surfaces A: Physicochemical and engineering aspects, 413 (2012) 273-279. [32] L. Zhang, H. Li, Y. Liu, Z. Tian, B. Yang, Z. Sun, S. Yan, Adsorption-photocatalytic degradation of methyl orange over a facile one-step hydrothermally synthesized TiO 2/ZnO–NH 2–RGO nanocomposite, RSC Advances, 4 (2014) 48703-48711. [33] D. Wibowo, R. Salamba, M. Nurdin, Preparation and Characterization of Activated Carbon from Coconut Shell–Doped Tio2 in Water Solution, Oriental Journal of Chemistry, 31 (2015) 2337-2342. [34] J. Wang, Y. Zhao, X. Xu, X. Feng, J. Yu, T. Li, A facile interfacial assembling strategy for synthesizing yellow TiO 2 flakes with a narrowed bandgap, RSC Advances, 5 (2015) 5817658183. [35] R. Ramanathan, D.E. Weibel, NEXAFS and FTIR-ATR investigation of the static and dynamic superhydrophobicity of functionalized titanium dioxide nanoparticle coatings, Journal of the Brazilian Chemical Society, 24 (2013) 1041-1048. [36] G.R. Chagas, D.E. Weibel, Development of Poly (propylene) Superhydrophobic Surfaces by Functionalised TiO2 Nanoparticles: Effect of Solvents and Dipping Times, Macromolecular Symposia, Wiley Online Library, 2014, pp. 55-62. [37] C.B. Contreras, G. Chagas, M.C. Strumia, D.E. Weibel, Permanent superhydrophobic polypropylene nanocomposite coatings by a simple one-step dipping process, Applied Surface Science, 307 (2014) 234-240. [38] D.L. Pavia, G.M. Lampman, G.S. Kriz, J.A. Vyvyan, Introduction to spectroscopy, Cengage Learning2008. [39] F. Zucchi, A. Frignani, V. Grassi, G. Trabanelli, M. DalColle, The formation of a protective layer of 3-mercapto-propyl-trimethoxy-silane on copper, Corrosion science, 49 (2007) 15701583. [40] M. Sabzi, S. Mirabedini, J. Zohuriaan-Mehr, M. Atai, Surface modification of TiO 2 nanoparticles with silane coupling agent and investigation of its effect on the properties of polyurethane composite coating, Progress in Organic Coatings, 65 (2009) 222-228. [41] M. Stern, A.L. Geary, Electrochemical polarization I. A theoretical analysis of the shape of polarization curves, Journal of the electrochemical society, 104 (1957) 56-63. [42] S. Karimi, T. Nickchi, A. Alfantazi, Effects of bovine serum albumin on the corrosion behaviour of AISI 316L, Co–28Cr–6Mo, and Ti–6Al–4V alloys in phosphate buffered saline solutions, Corrosion science, 53 (2011) 3262-3272. [43] J. Hu, Q. Li, X. Zhong, L. Zhang, B. Chen, Composite anticorrosion coatings for AZ91D magnesium alloy with molybdate conversion coating and silicon sol–gel coatings, Progress in Organic Coatings, 66 (2009) 199-205. [44] P. Bonora, F. Deflorian, L. Fedrizzi, Electrochemical impedance spectroscopy as a tool for investigating underpaint corrosion, Electrochimica Acta, 41 (1996) 1073-1082. 21
[45] S.S. Madani, E. Schaltz, S.K. Kær, A Review of Different Electric Equivalent Circuit Models and Parameter Identification Methods of Lithium-Ion Batteries, Ecs Transactions, 87 (2018) 23-37. [46] B. Hinderliter, S. Croll, D. Tallman, Q. Su, G. Bierwagen, Interpretation of EIS data from accelerated exposure of coated metals based on modeling of coating physical properties, Electrochimica acta, 51 (2006) 4505-4515. [47] E. McCafferty, AC impedance, Introduction to Corrosion Science, Springer2010, pp. 427451. [48] M. Hosseini, M. Sabouri, T. Shahrabi, Corrosion protection of mild steel by polypyrrole phosphate composite coating, Progress in Organic Coatings, 60 (2007) 178-185.
List of Figures’ Caption: Fig. 1. Schematic of preparation of the EPD suspension. Fig. 2. Effect of TMPSi concentration (vol. %) in the EPD bath on WCAs and SAs of the coating. Fig. 3. The superhydrophobic nano-TiO2/5 vol. % TMPSi ceramic composite coating on the 316L
a) Water droplet on the surface (WCA = 168° and SA = 3.1°), b) View on top of the
sample. Fig. 4. FTIR-ATR spectra of (a) TiO2-P25 NPs (orange curve), the superhydrophobic nanoTiO2/5 vol. % TMPSi ceramic composite coating (blue curve), and nano-TiO2 hydrophilic coating (red curve), (b) TMPSi solution. Fig. 5. Hydrophobicity of the nano-TiO2 coating thoughout of the surface a) scratched coating, b) water droplets on the scratched area.
22
Fig. 6. (a) Schematic of TiO2 NPs and TMPSi agents migrating in EPD suspension under the applied electrical field, (b) schematic illustration of the superhydrophobicity of the nano-TiO2/5 vol. % TMPSi composite coating throughout the surface. Fig. 7. (a-d) FESEM images of the superhydrophobic nano-TiO2/5 vol. % TMPSi ceramic composite coating in different magnifications, (e and f) AFM topography of the surface of the coating. Fig. 8. Cross-section distribution of the primary elements in the coating and substrate a) crosssectional image of the coating, b) distribution of all elements through the coating and substrate, d) chromium, e) iron,
elemental distribution of c) carbon,
f) silicon,
g) titanium.
Fig. 9. Map of the elemental distribution of main elements in the coating a) oxygen, b) titanium, c) Silicon. Fig. 10. Potentiodynamic polarization curves of the samples in the corrosive medium, 3.5 wt. % NaCl solution, sample A was the 316L substrate and sample B was the sample with the superhydrophobic nano-TiO2/5 vol. % TMPSi ceramic composite coating. Fig. 11. (a) The Bode plot, (b) the figurative model of the superhydrophobic nano-TiO2/5 vol. % TMPSi ceramic composite coating in corrosive solution with its equivalent electrical circuit corresponding to EIS results, (c) The equivalent electrical circuit by simulation of EIS of the superhydrophobic nano-TiO2/5 vol. % TMPSi ceramic composite coating in high frequencies, (d) when frequency approaches to
1 τ coat
, (e) in the moderate frequency range (
), (f) when the frequency is equivalent to
1 τ EDL
1 τ coat
˂ω˂
1 τ EDL
and (g) in the very-low-frequency range (ω ˂
23
1 τ EDL
). (h and i) The Bode plot and the appropriate figurative model of the bare 316L in
corrosive solution with its equivalent electrical circuit corresponding to EIS results, respectively. Fig. 12. Schematical representation of the effect of both water-repellency and the nanostructure of the coating on its barrier property against the uptake of water and ions.
Fig. 1. Schematic of preparation of the EPD suspension.
24
Fig. 2. Effect of TMPSi concentration (vol. %) in the EPD bath on WCAs and SAs of the coating.
25
Fig. 3. The superhydrophobic nano-TiO2/5 vol. % TMPSi ceramic composite coating on the 316L
a) Water droplet on the surface (WCA = 168° and SA = 3.1°), b) View on top of the
sample.
26
Fig. 4. FTIR-ATR spectra of (a) TiO2-P25 NPs (orange curve), the superhydrophobic nanoTiO2/5 vol. % TMPSi ceramic composite coating (blue curve), and nano-TiO2 hydrophilic coating (red curve), (b) TMPSi solution.
27
Fig. 5. Hydrophobicity of the nano-TiO2 coating thoughout of the surface a) scratched coating, b) water droplets on the scratched area.
28
Fig. 6. (a) Schematic of TiO2 NPs and TMPSi agents migrating in EPD suspension under the applied electrical field, (b) schematic illustration of the superhydrophobicity of the nano-TiO2/5 vol. % TMPSi composite coating throughout the surface.
29
Fig. 7. (a-d) FESEM images of the superhydrophobic nano-TiO2/5 vol. % TMPSi ceramic composite coating in different magnifications, (e and f) AFM topography of the surface of the coating.
30
Fig. 8. Cross-section distribution of the primary elements in the coating and substrate a) crosssectional image of the coating, b) distribution of all elements through the coating and substrate, elemental distribution of c) carbon,
d) chromium, e) iron,
31
f) silicon,
g) titanium.
Fig. 9. Map of the elemental distribution of main elements in the coating a) oxygen, b) titanium, c) Silicon.
Fig. 10. Potentiodynamic polarization curves of the samples in the corrosive medium, 3.5 wt. % NaCl solution, sample A was the 316L substrate and sample B was the sample with the superhydrophobic nano-TiO2/5 vol. % TMPSi ceramic composite coating.
32
Fig. 11. (a) The Bode plot, (b) the figurative model of the superhydrophobic nano-TiO2/5 vol. % TMPSi ceramic composite coating in corrosive solution with its equivalent electrical circuit corresponding to EIS results, (c) The equivalent electrical circuit by simulation of EIS of the superhydrophobic nano-TiO2/5 vol. % TMPSi ceramic composite coating in high frequencies, (d) when frequency approaches to
1 τ coat
, (e) in the moderate frequency range (
), (f) when the frequency is equivalent to
1 τ EDL
1 τ EDL
1 τ coat
˂ω˂
1 τ EDL
and (g) in the very-low-frequency range (ω ˂
). (h and i) The Bode plot and the appropriate figurative model of the bare 316L in
corrosive solution with its equivalent electrical circuit corresponding to EIS results, respectively.
33
Fig. 12. Schematical representation of the effect of both water-repellency and the nanostructure of the coating on its barrier property against the uptake of water and ions.
Table 1. Weigh and atomic percentage of the main elements of the coating.
Element Weight percentage Atomic percentage
O 41.74 67.75
Si 1.75 1.61
34
Ti 56.51 30.64
total 100 100
Table 2. Important corrosion data of sample A and sample B in the corrosive medium of 3.5 wt. % NaCl solution.
parameters
icorr
Ecorr
(µA.cm-2)
(mV vs. SCE)
βcathodic
βanodic
(mV.decade-1)
(mV.decade-1)
Rp
Corrosion rate
(Ω.cm2)
(µm per year)
samples A (bare substrate)
1.508
-187.454
-309.519
36.326
11850.698
12.180
B (superhydrophobic nano-TiO2/5 vol. % TMPSi ceramic composite coating)
0.696
109.114
-24190
71.829
44970.624
5.621
Table 3. Determined elements of the equivalent electrical circuit by simulation of EIS of the superhydrophobic nano-TiO2/TMPSi ceramic composite coating in 3.5 wt. % NaCl solution. Parameter Fited data
Rs (Ω.cm2) 2.461
CPEcoat (F.cm-2.sα-1) 2.846 × 10-5
Rcp (Ω.cm2) 2471
α 0.94
CPEEDL (F.cm-2.sβ-1) 1.179 × 10-4
Rct (Ω.cm2) 7081
Table 4. The elements of the equivalent electrical circuit determined by simulation of EIS of the bare 316L in 3.5 wt. % NaCl solution. Farameter Fited data
Rs (Ω.cm2) 2.41
C (F.cm-2.sα-1) 1.465 × 10-4
35
Rct (Ω.cm2) 2098
β 0.97
Graphical abstract:
Highlights: •
A superhydrophobic nanocomposite coating was deposited on the 316L.
•
A one-step process was used.
•
The superhydrophobic nanocomposite coating improved the corrosion resistance of 316L effectively.
•
The deposited nanocomposite coating showed significant WCA and SA (168 ° and 3.1 ° respectively).
36
S0272-8842(19)32668-9
DOI:
https://doi.org/10.1016/j.ceramint.2019.09.137
Reference:
CERI 22904
To appear in:
Ceramics International
Received Date: 16 August 2019 Revised Date:
14 September 2019
Accepted Date: 15 September 2019
Please cite this article as: S.M. Emarati, M. Mozammel, Efficient one-step fabrication of superhydrophobic nano-TiO2/TMPSi ceramic composite coating with enhanced corrosion resistance on 316L, Ceramics International (2019), doi: https://doi.org/10.1016/j.ceramint.2019.09.137. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.
Efficient one-step fabrication of superhydrophobic nano-TiO2/TMPSi ceramic composite coating with enhanced corrosion resistance on 316L
Seyed Masoud Emarati1, Mahdi Mozammel1* 1
Faculty of Material Engineering, Sahand University of Technology,Sahand New Town,
Tabriz, Iran, Postal Code: 5331817634 *
Corresponding Author, [email protected], Tel: +98-9125267093
Abstract TiO2 Nanoparticle / Trimethoxy(propyl)silane (TMPSi) ceramic composite coating was deposited on 316L steel using a one-step electrophoretic deposition (EPD) method. Silane coupling agent (TMPSi) was added to the EPD bath in different concentrations (from 0.5 to 15 vol. %) to decrease the surface energy of the deposited coating. TiO2 coating is hydrophilic whereas by adding varying concentrations of TMPSi, the obtained nanocomposite coating showed much better hydrophobicity. Surface wettability was measured by water contact angle (WCA) and sliding angle (SA) tests. Moreover, the effect of TMPSi concentration was determined by comparing the WCA and SA values. Surface morphology was studied through Field Emission Scanning Electron Microscopy (FESEM), and the presence of micro/nano meter roughness on the surface was confirmed. The distribution of elements were investigated by EDS analysis in which their uniform dispersion was observed. Corrosion behavior of 316L samples 1
before and after the coating process was studied by potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) tests in 3.5 wt. % NaCl solution. The polarization curve proved that the superhydrophobic ceramic nanocomposite coatings (WCA = 168 º and SA = 3.1 º) were able to decrease the corrosion rate of bare 316L (from 12.180 to 5.621 (µm per year)). Keywords: superhydrophobic, composite, EPD, TiO2, corrosion
1. Introduction Stainless steels (SS) are unique among all metals that are used industry. Because of their extensive commercial applications, they are used in clinics, public places, food industry, and kitchenwares. Above all, 316L stainless steel has attracted great attention in medical applications (including medical instruments and body implants) for its biocompatibility [1, 2]. But it has been difficult to achieve health standards due to the easy growth of bacterial colonies on the surface of stainless steel. Therefore, it is necessary to enhance the anti-bacterial and anticorrosion propertiesof 316L. One of the most effective ways to achieve this purpose is to deposit a protective hydrophobic metal oxide coating on the surface of 316L. So far, many efforts have been reported to fabricate an antibacterial coating on stainless steel using different methods, including dual magnetron sputtering, synthesis of nanoparticles (NPs), etc.[3, 4]. Recently Limei Chen et al. [5] have fabricated an antimicrobial composite coating by fixing Ag NPs on stainless steel with the use of 3- aminopropyltriethoxysilane (APTES).
2
Generally, in superhydrophobic surfaces, one of the most important problems is that the modified surface loses its anti-bacterial property in a short period of time. Therefore it would not exhibit an appropriate anti-bacterial performance during the service. Also is not easy to preserve the surface antibacterial characteristics along with high corrosion resistance simultaneously. One way to protect metals from corrosion is by exerting ceramic coatings. These coatings have special properties, including high resistance against corrosion, oxidation, and erosion [6]. E. Celik et al. [7] investigated the corrosion behavior of ceramic coating on the AISI 304L SS in 1 N H2SO4 solution and revealed the improvement of the samples’ corrosion resistance. Titanium dioxide is a well-known biomaterial that can be used as a biomaterial coating for sterilizing medical implants, surgical instruments and clinical equipment [8, 9]. Moreover, this metal oxide is one of the most prominent materials to fabricate superhydrophobic surfaces because it is low-priced, non-toxic and chemically stable in the ambient environment [10, 11]. Recently C-T. Hsieh et al. [12] have applied a superhydrophobic film on ITO using TiO2 NPs and post-treatment of fluorination. The results showed that the prepared film has high waterrepellency. The self-cleaning property has attracted a lot of attention because its production procedure is more time-efficient and cheaper [13]. For fabricating an artificial superhydrophobic surface two necessary conditions must be met. The first is surface roughening which includes methods like chemical etching, mechanical scratching, electrodeposition, sol-gel processing, and chemical/physical vapor deposition. The second condition is lowering surface energy that can be achieved by modifying the surface chemistry with various functional groups such as silane groups. These silane groups can decrease the
3
surface energy or alter the surface chemistry (such as by formation of physical binding, surface adsorption, and coating process) [14-19]. In fact, when a water droplet settles on a rough surface, it contacts with both the micro/nano protrusions, and the air gets trapped between the pillars. The trapped air acts as a repulsive force preventing the water from spreading (WCAair = 180 °). Thus by increasing the proportion of trapped air to contact point, superhydrophobicity of the surface increases [15, 20]. Electrophoretic deposition (EPD) is reported to be a highly effective method in preparing superhydrophobic coatings on a conductive substrate [21-24]. Recently M. Javidi et al. [25] have successfully fabricated hydroxyapatite on 316L using EPD in a suspension of isopropyl alcohol/ polyethyleneimine. Also A. R. Boccaccini et al. [26] electrophoretically obtained TiO2 coating on SiC fiber and Carbon-fiber substrates. Some significant advantages of this method are: 1) The possibility of using different materials capable of being coated on conductive substrates such as metal oxides. 2) This technique has high economic justifications for its simple equipment, cheap supplies, and high applying speed. 3) High capability in the deposition of coatings with delicate and complex shapes in wide dimensional ranges. Moreover, the thickness of the coatings can be easily controlled by varying the deposition parameters [27-29]. By controlling the specific parameters of EPD procedure (such as time and applied potential) the appropriate roughness can be obtained. In fact, the rough surface was achieved by aggregation of
4
deposited particles on the substrate [16]. G. X. Shen et al. [6] have improved the corrosion resistance of 316L in two steps. First, EPD of TiO2 NPs on the surface followed by chemical modification of a substrate by dip coating the sample in fluoroalkyl silane (FAS-B) solution. Furthermore, T. Ishizaki et al. [30] have fabricated a superhydrophobic coating by deposition of cerium oxide film on the magnesium alloy and modification of the coating in FAS solution. Results exhibited improvements in the corrosion resistance of magnesium alloy. To the best of our knowledge, this is the first time that synthesis of a superhydrophobic nanoTiO2/TMPSi ceramic composite coating on a 316L substrate by EPD process without post surface modification was investigated. the optimal condition exhibited outstanding results (WCA = 168 º and SA = 3.1 º). Moreover, the effect of surface hydrophobicity on the corrosion resistance of 316L was investigated by potentiodynamic polarization and EIS tests. Applying the superhydrophobic nano-TiO2/5 vol. % TMPSi ceramic composite coating in a single step on the surface results in a hydrophobic coating thoughout of the surface. If the outer layer of coating is damaged for any reason due to the presence of the silane bonds along with TiO2 nanoparticles throughout the coating, the inner layers still show the hydrophobic properties.
2. Experimental 2.1. Electrodes preparation The 316L substrates (Caspian Steel Vista) with chemical composition (in wt. %) of [65.3% Fe, 12.2% Ni, 17.4% Cr, 2% Mn, 2.1% Mo, 0.03% S, 0.1% N, 0.03% C, 0.75% Si, 0.045% P] were ground by emery paper with 100- 3000 mesh sizes and were polished by diamond paste. Then they were ultrasonically cleaned in acetone (99%, CAS No.: 67-64-1, Merck) /ethanol (99.8 %, CAS No.: 64-17-5, Sigma-Aldrich) bath with the volume ratio of 9:1 for 20 min at 35°C. 5
Afterward, electrodes were removed from the bath and ultrasonically cleaned in a deionized water bath for 10 min at 35°C to ensure the removal of any contaminations from micro groves of the surface. 2.2. Preparation of superhydrophobic nano-TiO2/TMPSi ceramic composite coating The superhydrophobic nano-TiO2/TMPSi ceramic composite coating was deposited on 316L by a one-step EPD process. The EPD bath was prepared by adding 0.5 g TiO2 (P25, CAS No.: 13463-67-7, Evonik Degussa GmbH), 0.04 g Iodine (99.99 %, CAS No.: 7553-56-2, SigmaAldrich), and different concentrations (0.5 vol. %, 1 vol. %, 2 vol. %, 3 vol. %, 4 vol. %, 5 vol. %, 6 vol. %, 7 vol. %, 8 vol. %, and 15 vol. %) of TMPSi (97%, CAS No.: 1067-25-0, SigmaAldrich) in 50 mL acetylacetone (99%, CAS No.: 123-54-6, Merck) as solvent (Fig. 1). The prepared suspension was stirred for 20 min in room temperature and then was placed in the ultrasonic bath for 15 min. Prepared 316L electrodes were used as cathode and anode with a distance of 20 mm. Working and counter electrodes were connected to the negative and positive poles of the DC power supply, respectively. By applying the potential of 5 V for the first 45 s, and increasing the voltage (with a constant rate of 1 V.s-1 until it reaches 30 V in a period of 45 s – 70 s), the superhydrophobic nano-TiO2/TMPSi ceramic composite coating was deposited on the 316L substrate. By adding the iodine to suspension, TiO2 NPs and silane agents were partially positively charged and migrated towards the cathode and were deposited under an electrical field. After the coating process, samples were removed from the cell and were placed in an oven at a temperature of 30 °C, so that the green deposited coatings dry slowly. 2.3. Characterization of the coating
6
Surface morphology and roughness of samples were studied by field-emission scanning electron microscopy (FESEM; MIRA3-XMU, TESCAN) and atomic force microscopy (AFM; NaioAFM, Nanosurf) analysis, respectively. Moreover, chemical bonds on the hydrophobic film were investigated by fourier transformed infrared spectroscopy in attenuated total reflectance mode FTIR-ATR analysis (Bruker Tensor 27 FT-IR). 2.4. Investigation of water-repellency of the coating The water-repellency of the superhydrophobic nano-TiO2/5 vol. % TMPSi ceramic composite coating was investigated by measuring WCA and SA. For determining the amount of WCA and SA, a water droplet with a volume of 1-2 µL was dropped on the surface by FIBRO SYSTEM AB (model of PG-X, Sweden) device and WCA and SA were measured optically by a digital camera mounted on the mentioned device. 2.5. Potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) tests To determine the corrosion resistance of the samples, the potentiodynamic polarization tests were performed. Two samples were prepared with dimensions of 1×1 cm2. One of them was the superhydrophobic sample (the best sample prepared under optimized conditions), and the other one was the substrate. The samples were placed in the corrosive solution of 3.5 wt. % NaCl whereas only 1×1 cm2 of the surface was in contact with the solution. The rest of the surface of the sample was insulated with Teflon. Significant corrosion parameters such as corrosion potential (Ecorr) and corrosion current density (icorr) were calculated using potentiodynamic curves. Also, the important EIS parameters were calculated using a simulated equivalent electrical circuit fitted to the real experimental data.
7
3. Results and discussion 3.1. Effect of TMPSi concentration on the hydrophobicity of the nano-TiO2/TMPSi ceramic composite coating WCA and SA of 316L bare substrate were measured, and their values were 39 º and 180 º respectively. Due to the hydrophilicity of TiO2, when a water droplet was dropped on a TiO2 coating, it immediately spread out on the surface coating whereas WCA and SA could not detected by FIBRO SYSTEM AB device. On the contrary, adding a slight of TMPSi to EPD bath, increased the WCAs significantly. Effect of TMPSi concentration of the suspension, provided for EPD process, on the wettability of nano-TiO2/TMPSi ceramic composite coating was shown in Fig. 2. As it is clear from Fig. 2, TMPSi in the concentration range of 4-6 vol. % affects the waterrepellency of the coating greatly. In the as-mentioned range, coating is superhydrophobic (WCA > 150 º); For the EPD bath with a concentration of 5 vol. % TMPSi, WCA and SA reach the significant values of 168 ° and 3.1 °, respectively. In fact, in low concentrations of TMPSi, WCAs and SAs were also low and high respectively. By gradually increasing the concentration of TMPSi, the WCA also increased and SA decreased until they reached to the highest and the lowest points respectively in 5 vol. % TMPSi (WCA = 168 º, SA = 3.1 º). Moreover, by increasing the concentration, the WCA got lower and SA 8
increased again. With attention to the mechanism of the silane agents, the reason for this phenomenon can be described as follows: it seems that both in lower and higher concentrations of TMPSi, insufficient silane bonds are formed, the phenomenon of self-condensation of silane agents together prevents the formation of sufficient and necessary silane bonds on the coating. WCA and SA of the sample with 5 vol. % TMPSi were shown in Fig. 3a and also a top view of the as mentioned coating was shown in Fig. 3b. As was concluded from the WCA and SA results, TMPSi concentration in the EPD bath has a direct effect on the wettability of the TiO2 coating. 3.2. FTIR-ATR analysis The FTIR-ATR tests were taken of TiO2-P25 NPs, TMPSi solution, TiO2 hydrophilic coating, and the superhydrophobic nano-TiO2/5 vol. % TMPSi ceramic composite coating (WCA = 168 º and SA = 3.1 º). As shown in Fig. 4a, two peaks in the range of 548-670 cm-1 were visible that related to Ti-O vibration in titanium dioxide crystal [31-34]. In superhydrophobic ceramic nanocomposite coating (curves with blue color) also three important peaks in the range of 1064.31-1219.07 cm-1 were visible that didn’t exist in other samples. The drastic peak at 1064.31 cm-1 corresponds to Si-O-C [35, 36] and the one at 1119.2 cm-1 is related to stretching vibration of Si-O-Si (siloxane) bonding [37]. Additionally, the peak at 1219.07 cm-1 is assigned to Si-CH2 bonding [38] that confirms the presence of TMPSi agent in the coating. The existence of these three mentioned peaks verified the water-repellency property of the superhydrophobic sample. Intense peaks at 2923.59 cm-1 and 2854.03 cm-1 [39, 40] are related to the symmetric stretching vibration of CH2 groups that are originated from acetylacetone in the nano-TiO2 hydrophilic
9
coating (the red curve). On the other hand, intense peaks at 2872.45 cm-1 and 2958.83 cm-1 are assigned to the CH2 stretching bonding of TMPSi in the superhydrophobic ceramic nanocomposite coating. These peaks only correspond to CH2 bonds of TMPSi at 2958 cm-1 and 2872.45 cm-1 [38] (Fig. 4b). According to the FTIR results, all the observed bonds on the surface and the bulk of the coating were the same. Therefore, it is reasonable to conclude that if the outer layer of the coating is damaged for any reason, the inner layers will still show hydrophobic properties as shown in Fig. 5. In fact, in the prepared EPD suspension, both TiO2 NPs and TMPSi agents were partially positively charged and migrated towards the working electrode and deposited simultaneously (Fig. 6a). Similarly, Ogihara et al. [16] studied the migration of charged trimethylsiloxysilicate (TMSS) under an applied electric field in the EPD process. Accordingly, in each part of the coating, the effective bonds of silane agents are present. This fact is shown schematically in Fig. 6b. 3.3. Study of the Surface morphology of the deposited TiO2 coating The high surface roughness is considered as an essential factor to obtain hydrophobicity. In other words, the simultaneous presence of both nano and micro-scaled roughness on the surface leads to hydrophobic property [16]. Surface morphology and roughness of the TiO2 coating which was obtained using an optimum amount of TMPSi in EPD bath (WCA = 168 º and SA = 3.1 º) were investigated by FESEM and AFM. The results were shown in Fig. 7.
10
All contents of suspension were influenced comprehensively by the electric field and were aggregated regularly on working electrode, thus a uniform coating and distribution of elements were observed on the coating (Figs. 7a and 8). For further investigation, the cross-section of coating and also the elemental distribution of coating and substrate were studied by EDS analysis (Fig. 8). As shown in Fig. 8a, the substrate was distinguishable from coating graphically. Fig. 8b shows the curves of elemental distribution, as can be seen in Fig. 8b, the amount of elements from the surface of the coating to the depth of the substrate were changing as explained blew: a) The titanium concentrate decreases (Fig. 8g) by reaching the substrate. The same trend in Si concentration would be expected but due to the presence of Si in the substrate as a primary element (section 2.1) no tangible decreases in Si concentration was observed (Fig. 8f). b) As the line scan curves approach to the substrate, the intensity of iron and chromium elements increases. Moreover, the distributions and concentrations of the main elements of coating, including Ti, O, and Si were shown in Fig. 9 and Table 1, respectively. Regarding the chemical formula of TiO2, the atomic percentage ratio of oxygen to titanium should be 2:1. However, regarding the data presented in Table 1, this ratio is 2.21 (
67.75 ). It is 30.64
anticipated that the additional atomic percentage of O (6.47%) was originated from external factors, including silica and oxygen. As shown in Fig .7b, the formation of the micro-pillar was evident. In fact, the arrangement of micropillars beside each other is the main factor in the formation of micrometer-roughness on the coating (Figs. 7e and f). Further investigation of FESEM images (in higher magnifications)
11
revealed that placement of particles on the substrate during the deposition process created cauliflower-shapes. In fact, the formation of a hierarchical structure on the surface was visible (Fig. 7c). Most of the previous researches on the natural superhydrophobic surfaces revealed the presence of a hierarchical structure [20]. In fact, the existent of available and accessible numerous sharp places for settling water droplets and also the presence trapped air (as a repellent force) between components of cauliflowershaped TiO2 NPs aggregation act as the effective factors in hydrophobicity of the surface. The average-size of deposited particles was determined in the highest FESEM magnification (Fig. 7d). As seen in Fig. 7d aggregated cauliflowers is made of TiO2 particles, and they almost have the same size (such as L1 and L2 = 17.6 nm, L3 = 18.4 nm in Fig. 10d). Thus a uniform roughness on the surface is expected (Figs. 7e and f). In other words, according to Figs. 7c-f, nanometer-roughness of the surface is visible. Therefore, according to Fig. 7. it could be concluded that the simultaneous presence of both nanometer and micrometer roughness on the surface is evident.
3.4. Investigation of corrosion behavior of the superhydrophobic nano-TiO2/5 vol. % TMPSi ceramic composite coating on the 316L substrate 3.4.1. Potentiodynamic polarization
The potentiodynamic polarization curves of the substrate and the sample with the superhydrophobic nano-TiO2/5 vol. % TMPSi ceramic composite coating in 3.5 wt. % NaCl solution with a scan rate of 0.5 mV.s-1 were shown in Fig. 10. Platinum was used as counter electrode and potentials of working electrodes were determined by saturated calomel electrode (SCE) as a reference electrode. 12
Important corrosion data such as corrosion potential (Ecorr), corrosion current density (icorr), cathodic slope (βcathodic), and anodic slope (βanodic) were derived from the linear part of the obtained dynamic polarization curves for both samples. Moreover, Rp (polarization resistance) was derived from the Stern-Geary equation [41] which is shown below:
Rp =
β cathodic × β anodic 2.303i corr (| β cathodic | +β anodic )
Eq. (1)
The corrosion rate of the samples was determined by Eq. (2) [42]:
Corrosion Rate (µm per year) = 315360 ×
icorr .a ρ.n.F
Eq. (2)
In Eq. (2), icorr is the corrosion current density [(µA.cm-2)], a shows the atomic weight of metallic samples [(g.mol-1)], n is the number of equivalent exchange, ρ represents the density of samples [(g.cm-3)] and F is the Faraday's constant [(coulomb.mol-1)]. The mentioned data were shown in Table 2. By comparing the polarization curves, it was observed that by applying the superhydrophobic coating the corrosion potential (Ecorr) was shifted to more positive values (i.e., from -187.454 mV vs. SCE for the substrate to 109.114 mV vs. SCE for the superhydrophobic nano-TiO2/5 vol. % TMPSi ceramic composite coating). Furthermore, icorr was decreased due to the presence of the coating (from 1.508 µA.cm-2 for the substrate to 0.696 µA.cm-2 for the superhydrophobic nanoTiO2/5 vol. % TMPSi ceramic composite coating). So it was concluded that the superhydrophobic nano-TiO2/5 vol. % TMPSi ceramic composite coating decreased the development of corrosion phenomenon and effectively protected the
13
substrate against corrosive agents. This happens because the water-repellent protective coatings prevent the penetration of corrosive agent to the metal substrate. 3.4.2. Electrochemical impedance spectroscopy (EIS)
The EIS is a powerful and suitable technique for investigating the anti-corrosion behavior of the coating during the exposure to corrosive environments [43]. The chosen samples were prepared according to section 2.5 and were placed in a corrosive solution of 3.5 wt. % NaCl for 30 min before starting the test, in order to stabilize the electrode in the solution. The reference electrode was a saturated calomel electrode, and Pt-plate was selected as the counter electrode. Also, the frequency range between 10-3 - 10+5 Hz was set to fit the equivalent electrical circuit and simulate the magnitude of its elements. The Bode plot of the superhydrophobic nano-TiO2/5 vol. % TMPSi ceramic composite coating along with an equivalent figurative model of the coated sample in 3.5 wt. % NaCl solution corresponding the EIS results were shown in Figs. 11a and b, respectively. Furthermore, the magnitude of elements of the simulated equivalent electrical circuit in Fig. 11b were reported in Table 3. In a real electrochemical test, using a constant phase element (CPE) is more accurate than using pure capacitance. Furthermore, due to the roughness of the coating caused by TiO2 NPs using CPE is more reasonable [42]. The impedance of CPE is defined as ZCPE =
1
( jω )
α
Eq. (3) Y0
[44]. Where in this Eq. (3), ω is the angular frequency, Y0 is the general admittance function, and
α is an empirical exponent of the CPE that 0 ≤ α ≤ 1. If α = 1 and α = 0, the interface has pure capacitive and pure resistance behavior respectively.
14
As shown in Figs. 11a and b, the fitted equivalent electrical circuit has two CPEs that their empirical exponents are near to 1 (α ≈ 1). Hence two time constants are expectable: a) τcoat = Rcp×Ccoat
Eq. (4), and b) τEDL = Rct×CEDL
Eq. (5) [45]. At high frequencies, two
capacitors have the lowest impedance (according to Eq. (6) of ZC =
1 [46] ; where ZC, j, ω, jωC
and C are the impedance of the capacitor, imaginary number ( ± −1 ), angular frequency , and capacitance, respectively.). Therefore, Rcp, Rct, and CPEEDL were short-circuited by the CPEcoat (these elements were represented by grey color in Figs. 11c-g). In this condition, the equivalent electrical circuit only contains Rs, as shown in Fig. 11c.
By decreasing the frequency and reaching to the values of near
1 τ coat
, Rct was short-circuited by
the CPEEDL due to the very low impedance of CPEEDL. The corresponding equivalent electrical circuit is shown in Fig. 11d.
In the moderate frequency range (
1 τ coat
˂ω˂
1 τ EDL
), according to the Table 3 and the definition
of both τcoat and τEDL and also by comparing CPEcoat and CPEEDL values, it is concluded that
1 τ EDL
<
1 τ coat
, so the impedance of CPEcoat is very high. Therefore, it is removed from the
electrical circuit and since the impedance of CPEEDL is low, hence it causes the Rct to shortcircuit. The equivalent electrical circuit in this mentioned frequency range is shown in Fig. 11e.
By approaching the frequency to
1 τ EDL
, the equivalent electrical circuit will be similar to Fig. 11f.
15
According to Eq. (7) of C = ε 0ε
A d
where C, ε0, ε, A, and d are capacitance, the dielectric
constant of free space (vacuum), effective dielectric constant, area, and thickness of the coating, respectively [47]. Also, the fact that the thickness of the electrical double layer is far less than the thickness of the coating, it is concluded that CPEEDL ˃˃ CPEcoat. Compared to the high-frequency range, as the frequency drops to the lower magnitudes until it reaches
1 τ EDL
, the coating shows
more capacitive behavior.
In very-low-frequency range (ω ˂
1 τ EDL
), both CPEs were fully charged and so removed from the
electrical circuit as shown in Fig. 11g. In this frequency range, the coating has the highest magnitude of impedance, in other words:
ǀZǀequivalent electrical circuit = ǀRs+Rcp+Rctǀ
Eq. (8)
According to detailed discussion of the coating’s equivalent electrical circuit in the different frequency ranges, the behavior of coating during the exposure to a corrosive environment is predictable in every desired frequencies. According to the Eq. (8), in the organic coatings, the investigation of EIS plots in low frequencies is more important than other frequencies. Rct and Rcp, which were derived in this range of the frequency, reflect properties of the coating and the polarization resistance, respectively. Although Eq. (8) contains Rs, the value of which is much less than ǀRcp+Rctǀ [47]. Furthermore as a time of immersion progresses, undoubtedly the uptake of corrosive ions and water into the coating increases [48]. With their uptake into the coating, the protective property of the coating crucially decreases due to the changes of two parameters: a) the absorption of water increases the effective dielectric constant (ε) and as a result the coating’s capacitance rises and b) this uptake decreases both Rcp and Rct [47]. 16
The Bode plot of the bare 316L with its appropriate equivalent figurative model corresponding to the EIS results were shown in Figs. 11h and i, respectively. Furthermore, the magnitude of simulated equivalent electrical circuit elements of Fig. 11i were reported in Table 4. According to Table 4 and the high impedance values (ǀZǀ ≈ 2 × 103 Ω.cm2) in very low frequencies, it is obvious that the bare 316L has a good corrosion resistance as expected. By comparing Figs. 11a and h in low frequencies, using the superhydrophobic nano-TiO2/5 vol. % TMPSi ceramic composite coating could significantly improve the protection property of the bare 316L. This claim could be concluded from the increase in ǀZǀ (from 2 × 103 Ω.cm2 to 104 Ω.cm2) due to the presence of the superhydrophobic nano-TiO2/5 vol. % TMPSi ceramic composite coating. In other words, hydrophobic silane coupling agents in throughout the coating act as strong barriers against the uptake of water and ions. Moreover, using TiO2-P25 instead of larger sizes led to reducing the empty space between the coating’s particles and limited the water absorption and accumulation. Therefore, the resistance of pores (Rcp) was increased effectively. The proposed mechanism and the performance of a coating in contact with water are schematically modeled in two fashion: a) superhydrophobic ceramic nanocomposite coating (Fig 12a), and b) coating without hydrophobic property and the use of nano-sized materials (Fig. 12b). Finally, the experimental outcomes of corrosion tests emphasize that superhydrophobic coating represents much better corrosion resistance rather than bare 316L.
4. Conclusion Superhydrophobic nano-TiO2/5 vol. % TMPSi ceramic composite coating was deposited by a one-step EPD process on the 316L. According to the hydrophilicity of TiO2 NPs, the deposited 17
coating of TiO2 was also hydrophilic. Thus to achieve hydrophobicity on the surface, utilizing silane coupling agents such as TMPSi to decrease the surface energy was necessary. By adding TMPSi in EPD bath, the superhydrophobic nano-TiO2/5 vol. % TMPSi ceramic composite coating (WCA = 168 º and SA = 3.1 º) was prepared in a one-step process which possesses various advantages including high-speed implementation because the chemical modification of surface after the coating process was omitted. Results of WCA and SA showed that by adding 5 vol. % TMPSi in EPD bath and applying swipe potential (applying the potential of 5 V at the first 45 s, then increasing the voltage at a constant rate of 1 V.s-1 until it reaches 30 V in a period of 45 s - 70 s), the deposited nanocomposite coating had the highest hydrophobic property. In other words, by applying the superhydrophobic nano-TiO2/5 vol. % TMPSi ceramic composite coating, WCA of the surface was increased from 39 º to 168 º and SA decreased from 180 º to 3.1 º in optimum coating conditions. The prepared coating had high water-repellency and water droplet could easily roll off the surface. The corrosion resistance of 316L and coated samples in 3.5 wt. % NaCl solution was investigated. According to the results from the corrosion tests, superhydrophobicity of ceramic nanocomposite coating was enhanced the corrosion resistance of 316L (Ecorr was shifted from -187.544 mV vs. SCE to 109.114 mV vs. SCE and icorr changed from 1.508 µA.cm-2 to 0.696 µA.cm-2 for uncoated and optimum superhydrophobic sample, respectively). Furthermore, the EIS results showed the impedance of the superhydrophobic nanoTiO2/TMPSi ceramic composite coating was extremely higher than bare 316L.
Acknowledgement The authors wish to express their gratitude to Engineers Mohammad Alinezhadfar and Mohammad Tanhaei for their literary editing and Engineer Ali Mohseni for his help in the
18
preparation of corrosion samples. Responsibility for the information and views set out in this article lies entirely with the authors.
Disclosure statement No potential conflict of interest was reported by the authors.
References [1] D.S.R. Krishna, Y. Sun, Thermally oxidised rutile-TiO 2 coating on stainless steel for tribological properties and corrosion resistance enhancement, Applied Surface Science, 252 (2005) 1107-1116. [2] T. Fu, C. Wen, J. Lu, Y. Zhou, S. Ma, B. Dong, B. Liu, Sol-gel derived TiO 2 coating on plasma nitrided 316L stainless steel, Vacuum, 86 (2012) 1402-1407. [3] X. Tian, Z. Wang, S. Yang, Z. Luo, R.K. Fu, P.K. Chu, Antibacterial copper-containing titanium nitride films produced by dual magnetron sputtering, Surface and Coatings Technology, 201 (2007) 8606-8609. [4] Z. Dan, H. Ni, B. Xu, J. Xiong, P. Xiong, Microstructure and antibacterial properties of AISI 420 stainless steel implanted by copper ions, Thin solid films, 492 (2005) 93-100. [5] L. Chen, L. Zheng, Y. Lv, H. Liu, G. Wang, N. Ren, D. Liu, J. Wang, R.I. Boughton, Chemical assembly of silver nanoparticles on stainless steel for antimicrobial applications, Surface and Coatings Technology, 204 (2010) 3871-3875. [6] G. Shen, Y. Chen, L. Lin, C. Lin, D. Scantlebury, Study on a hydrophobic nano-TiO 2 coating and its properties for corrosion protection of metals, Electrochimica acta, 50 (2005) 5083-5089. [7] E. Celik, I. Ozdemir, E. Avci, Y. Tsunekawa, Corrosion behaviour of plasma sprayed coatings, surface and coatings technology, 193 (2005) 297-302. [8] H.-f. WANG, X.-f. SHU, X.-y. LI, T. Bin, Photocatalytic activities of N doped TiO2 coatings on 316L stainless steel by plasma surface alloying technique, Transactions of Nonferrous Metals Society of China, 22 (2012) s120-s126. [9] M. Jackson, W. Ahmed, Titanium dioxide coatings in medical device applications, Surface Engineered Surgical Tools and Medical Devices, Springer New York2007, pp. 49-63. [10] J. Lee, G. Jang, Y. Jun, Investigation and evaluation of structural color of TiO 2 coating on stainless steel, Ceramics International, 38 (2012) S661-S664. [11] H. Yaghoubi, N. Taghavinia, E.K. Alamdari, Self cleaning TiO 2 coating on polycarbonate: surface treatment, photocatalytic and nanomechanical properties, Surface and Coatings Technology, 204 (2010) 1562-1568. [12] C.-T. Hsieh, M.-H. Lai, Y.-S. Cheng, Fabrication and superhydrophobicity of fluorinated titanium dioxide nanocoatings, Journal of colloid and interface science, 340 (2009) 237-242. [13] F. Li, Q. Li, H. Kim, Spray deposition of electrospun TiO 2 nanoparticles with self-cleaning and transparent properties onto glass, Applied Surface Science, 276 (2013) 390-396.
19
[14] M. Ma, R.M. Hill, Superhydrophobic surfaces, Current opinion in colloid & interface science, 11 (2006) 193-202. [15] H. Ogihara, J. Xie, J. Okagaki, T. Saji, Simple method for preparing superhydrophobic paper: spray-deposited hydrophobic silica nanoparticle coatings exhibit high water-repellency and transparency, Langmuir, 28 (2012) 4605-4608. [16] H. Ogihara, T. Katayama, T. Saji, One-step electrophoretic deposition for the preparation of superhydrophobic silica particle/trimethylsiloxysilicate composite coatings, Journal of colloid and interface science, 362 (2011) 560-566. [17] B. Javanpour, M. Azadbeh, M. Mozammel, Effect of Hydrothermal Sealing on Wettability and Electrochemical Behavior of TMPSi Treated Anodized Aluminum, Journal of Inorganic and Organometallic Polymers and Materials, (2018) 1-11. [18] M. Mozammel, M. Khajeh, N.N. Ilkhechi, Effect of Surface Roughness of 316 L Stainless Steel Substrate on the Morphological and Super-Hydrophobic Property of TiO 2 Thin Films Coatings, Silicon, (2018) 1-5. [19] B. Javanpour, M. Azadbeh, M. Mozammel, Effect of Chemical Composition of Tetraethoxysilane and Trimethoxy (Propyl) Silane Hybrid Sol on Hydrophobicity and Corrosion Resistance of Anodized Aluminum, Silicon, (2019) 1-16. [20] Y.Y. Yan, N. Gao, W. Barthlott, Mimicking natural superhydrophobic surfaces and grasping the wetting process: A review on recent progress in preparing superhydrophobic surfaces, Advances in colloid and interface science, 169 (2011) 80-105. [21] Y.S. Joung, C.R. Buie, Electrophoretic deposition of unstable colloidal suspensions for superhydrophobic surfaces, Langmuir, 27 (2011) 4156-4163. [22] A.R. Boccaccini, I. Zhitomirsky, Application of electrophoretic and electrolytic deposition techniques in ceramics processing, Current Opinion in Solid State and Materials Science, 6 (2002) 251-260. [23] S.M. Emarati, M. Mozammel, Fabrication of superhydrophobic titanium dioxide coating on AISI 316L stainless steel by electrophoretic deposition and using trimethoxy (propyl) silane modification, Surface Engineering, (2018) 1-10. [24] M. Salehi, M. Mozammel, S.M. Emarati, Superhydrophobic and corrosion resistant properties of electrodeposited Ni-TiO2/TMPSi nanocomposite coating, Colloids and Surfaces A: Physicochemical and Engineering Aspects, 573 (2019) 196-204. [25] M. Javidi, S. Javadpour, M. Bahrololoom, J. Ma, Electrophoretic deposition of natural hydroxyapatite on medical grade 316L stainless steel, Materials Science and Engineering: C, 28 (2008) 1509-1515. [26] A. Boccaccini, P. Karapappas, J. Marijuan, C. Kaya, TiO 2 coatings on silicon carbide and carbon fibre substrates by electrophoretic deposition, Journal of materials science, 39 (2004) 851-859. [27] A. Chavez-Valdez, A. Arizmendi-Morquecho, K. Moreno, J. Roether, J. Kaschta, A. Boccaccini, TiO 2–PLLA nanocomposite coatings and free-standing films by a combined electrophoretic deposition-dip coating process, Composites Part B: Engineering, 67 (2014) 256261. [28] M. Wei, A. Ruys, B. Milthorpe, C. Sorrell, J. Evans, Electrophoretic deposition of hydroxyapatite coatings on metal substrates: a nanoparticulate dual-coating approach, Journal of Sol-Gel Science and Technology, 21 (2001) 39-48.
20
[29] Q.-T. Vu, M. Pavlik, N. Hebestreit, J. Pfleger, U. Rammelt, W. Plieth, Electrophoretic deposition of nanocomposites formed from polythiophene and metal oxides, Electrochimica acta, 51 (2005) 1117-1124. [30] T. Ishizaki, Y. Masuda, M. Sakamoto, Corrosion resistance and durability of superhydrophobic surface formed on magnesium alloy coated with nanostructured cerium oxide film and fluoroalkylsilane molecules in corrosive NaCl aqueous solution, Langmuir, 27 (2011) 4780-4788. [31] J. Zhao, M. Milanova, M.M. Warmoeskerken, V. Dutschk, Surface modification of TiO 2 nanoparticles with silane coupling agents, Colloids and surfaces A: Physicochemical and engineering aspects, 413 (2012) 273-279. [32] L. Zhang, H. Li, Y. Liu, Z. Tian, B. Yang, Z. Sun, S. Yan, Adsorption-photocatalytic degradation of methyl orange over a facile one-step hydrothermally synthesized TiO 2/ZnO–NH 2–RGO nanocomposite, RSC Advances, 4 (2014) 48703-48711. [33] D. Wibowo, R. Salamba, M. Nurdin, Preparation and Characterization of Activated Carbon from Coconut Shell–Doped Tio2 in Water Solution, Oriental Journal of Chemistry, 31 (2015) 2337-2342. [34] J. Wang, Y. Zhao, X. Xu, X. Feng, J. Yu, T. Li, A facile interfacial assembling strategy for synthesizing yellow TiO 2 flakes with a narrowed bandgap, RSC Advances, 5 (2015) 5817658183. [35] R. Ramanathan, D.E. Weibel, NEXAFS and FTIR-ATR investigation of the static and dynamic superhydrophobicity of functionalized titanium dioxide nanoparticle coatings, Journal of the Brazilian Chemical Society, 24 (2013) 1041-1048. [36] G.R. Chagas, D.E. Weibel, Development of Poly (propylene) Superhydrophobic Surfaces by Functionalised TiO2 Nanoparticles: Effect of Solvents and Dipping Times, Macromolecular Symposia, Wiley Online Library, 2014, pp. 55-62. [37] C.B. Contreras, G. Chagas, M.C. Strumia, D.E. Weibel, Permanent superhydrophobic polypropylene nanocomposite coatings by a simple one-step dipping process, Applied Surface Science, 307 (2014) 234-240. [38] D.L. Pavia, G.M. Lampman, G.S. Kriz, J.A. Vyvyan, Introduction to spectroscopy, Cengage Learning2008. [39] F. Zucchi, A. Frignani, V. Grassi, G. Trabanelli, M. DalColle, The formation of a protective layer of 3-mercapto-propyl-trimethoxy-silane on copper, Corrosion science, 49 (2007) 15701583. [40] M. Sabzi, S. Mirabedini, J. Zohuriaan-Mehr, M. Atai, Surface modification of TiO 2 nanoparticles with silane coupling agent and investigation of its effect on the properties of polyurethane composite coating, Progress in Organic Coatings, 65 (2009) 222-228. [41] M. Stern, A.L. Geary, Electrochemical polarization I. A theoretical analysis of the shape of polarization curves, Journal of the electrochemical society, 104 (1957) 56-63. [42] S. Karimi, T. Nickchi, A. Alfantazi, Effects of bovine serum albumin on the corrosion behaviour of AISI 316L, Co–28Cr–6Mo, and Ti–6Al–4V alloys in phosphate buffered saline solutions, Corrosion science, 53 (2011) 3262-3272. [43] J. Hu, Q. Li, X. Zhong, L. Zhang, B. Chen, Composite anticorrosion coatings for AZ91D magnesium alloy with molybdate conversion coating and silicon sol–gel coatings, Progress in Organic Coatings, 66 (2009) 199-205. [44] P. Bonora, F. Deflorian, L. Fedrizzi, Electrochemical impedance spectroscopy as a tool for investigating underpaint corrosion, Electrochimica Acta, 41 (1996) 1073-1082. 21
[45] S.S. Madani, E. Schaltz, S.K. Kær, A Review of Different Electric Equivalent Circuit Models and Parameter Identification Methods of Lithium-Ion Batteries, Ecs Transactions, 87 (2018) 23-37. [46] B. Hinderliter, S. Croll, D. Tallman, Q. Su, G. Bierwagen, Interpretation of EIS data from accelerated exposure of coated metals based on modeling of coating physical properties, Electrochimica acta, 51 (2006) 4505-4515. [47] E. McCafferty, AC impedance, Introduction to Corrosion Science, Springer2010, pp. 427451. [48] M. Hosseini, M. Sabouri, T. Shahrabi, Corrosion protection of mild steel by polypyrrole phosphate composite coating, Progress in Organic Coatings, 60 (2007) 178-185.
List of Figures’ Caption: Fig. 1. Schematic of preparation of the EPD suspension. Fig. 2. Effect of TMPSi concentration (vol. %) in the EPD bath on WCAs and SAs of the coating. Fig. 3. The superhydrophobic nano-TiO2/5 vol. % TMPSi ceramic composite coating on the 316L
a) Water droplet on the surface (WCA = 168° and SA = 3.1°), b) View on top of the
sample. Fig. 4. FTIR-ATR spectra of (a) TiO2-P25 NPs (orange curve), the superhydrophobic nanoTiO2/5 vol. % TMPSi ceramic composite coating (blue curve), and nano-TiO2 hydrophilic coating (red curve), (b) TMPSi solution. Fig. 5. Hydrophobicity of the nano-TiO2 coating thoughout of the surface a) scratched coating, b) water droplets on the scratched area.
22
Fig. 6. (a) Schematic of TiO2 NPs and TMPSi agents migrating in EPD suspension under the applied electrical field, (b) schematic illustration of the superhydrophobicity of the nano-TiO2/5 vol. % TMPSi composite coating throughout the surface. Fig. 7. (a-d) FESEM images of the superhydrophobic nano-TiO2/5 vol. % TMPSi ceramic composite coating in different magnifications, (e and f) AFM topography of the surface of the coating. Fig. 8. Cross-section distribution of the primary elements in the coating and substrate a) crosssectional image of the coating, b) distribution of all elements through the coating and substrate, d) chromium, e) iron,
elemental distribution of c) carbon,
f) silicon,
g) titanium.
Fig. 9. Map of the elemental distribution of main elements in the coating a) oxygen, b) titanium, c) Silicon. Fig. 10. Potentiodynamic polarization curves of the samples in the corrosive medium, 3.5 wt. % NaCl solution, sample A was the 316L substrate and sample B was the sample with the superhydrophobic nano-TiO2/5 vol. % TMPSi ceramic composite coating. Fig. 11. (a) The Bode plot, (b) the figurative model of the superhydrophobic nano-TiO2/5 vol. % TMPSi ceramic composite coating in corrosive solution with its equivalent electrical circuit corresponding to EIS results, (c) The equivalent electrical circuit by simulation of EIS of the superhydrophobic nano-TiO2/5 vol. % TMPSi ceramic composite coating in high frequencies, (d) when frequency approaches to
1 τ coat
, (e) in the moderate frequency range (
), (f) when the frequency is equivalent to
1 τ EDL
1 τ coat
˂ω˂
1 τ EDL
and (g) in the very-low-frequency range (ω ˂
23
1 τ EDL
). (h and i) The Bode plot and the appropriate figurative model of the bare 316L in
corrosive solution with its equivalent electrical circuit corresponding to EIS results, respectively. Fig. 12. Schematical representation of the effect of both water-repellency and the nanostructure of the coating on its barrier property against the uptake of water and ions.
Fig. 1. Schematic of preparation of the EPD suspension.
24
Fig. 2. Effect of TMPSi concentration (vol. %) in the EPD bath on WCAs and SAs of the coating.
25
Fig. 3. The superhydrophobic nano-TiO2/5 vol. % TMPSi ceramic composite coating on the 316L
a) Water droplet on the surface (WCA = 168° and SA = 3.1°), b) View on top of the
sample.
26
Fig. 4. FTIR-ATR spectra of (a) TiO2-P25 NPs (orange curve), the superhydrophobic nanoTiO2/5 vol. % TMPSi ceramic composite coating (blue curve), and nano-TiO2 hydrophilic coating (red curve), (b) TMPSi solution.
27
Fig. 5. Hydrophobicity of the nano-TiO2 coating thoughout of the surface a) scratched coating, b) water droplets on the scratched area.
28
Fig. 6. (a) Schematic of TiO2 NPs and TMPSi agents migrating in EPD suspension under the applied electrical field, (b) schematic illustration of the superhydrophobicity of the nano-TiO2/5 vol. % TMPSi composite coating throughout the surface.
29
Fig. 7. (a-d) FESEM images of the superhydrophobic nano-TiO2/5 vol. % TMPSi ceramic composite coating in different magnifications, (e and f) AFM topography of the surface of the coating.
30
Fig. 8. Cross-section distribution of the primary elements in the coating and substrate a) crosssectional image of the coating, b) distribution of all elements through the coating and substrate, elemental distribution of c) carbon,
d) chromium, e) iron,
31
f) silicon,
g) titanium.
Fig. 9. Map of the elemental distribution of main elements in the coating a) oxygen, b) titanium, c) Silicon.
Fig. 10. Potentiodynamic polarization curves of the samples in the corrosive medium, 3.5 wt. % NaCl solution, sample A was the 316L substrate and sample B was the sample with the superhydrophobic nano-TiO2/5 vol. % TMPSi ceramic composite coating.
32
Fig. 11. (a) The Bode plot, (b) the figurative model of the superhydrophobic nano-TiO2/5 vol. % TMPSi ceramic composite coating in corrosive solution with its equivalent electrical circuit corresponding to EIS results, (c) The equivalent electrical circuit by simulation of EIS of the superhydrophobic nano-TiO2/5 vol. % TMPSi ceramic composite coating in high frequencies, (d) when frequency approaches to
1 τ coat
, (e) in the moderate frequency range (
), (f) when the frequency is equivalent to
1 τ EDL
1 τ EDL
1 τ coat
˂ω˂
1 τ EDL
and (g) in the very-low-frequency range (ω ˂
). (h and i) The Bode plot and the appropriate figurative model of the bare 316L in
corrosive solution with its equivalent electrical circuit corresponding to EIS results, respectively.
33
Fig. 12. Schematical representation of the effect of both water-repellency and the nanostructure of the coating on its barrier property against the uptake of water and ions.
Table 1. Weigh and atomic percentage of the main elements of the coating.
Element Weight percentage Atomic percentage
O 41.74 67.75
Si 1.75 1.61
34
Ti 56.51 30.64
total 100 100
Table 2. Important corrosion data of sample A and sample B in the corrosive medium of 3.5 wt. % NaCl solution.
parameters
icorr
Ecorr
(µA.cm-2)
(mV vs. SCE)
βcathodic
βanodic
(mV.decade-1)
(mV.decade-1)
Rp
Corrosion rate
(Ω.cm2)
(µm per year)
samples A (bare substrate)
1.508
-187.454
-309.519
36.326
11850.698
12.180
B (superhydrophobic nano-TiO2/5 vol. % TMPSi ceramic composite coating)
0.696
109.114
-24190
71.829
44970.624
5.621
Table 3. Determined elements of the equivalent electrical circuit by simulation of EIS of the superhydrophobic nano-TiO2/TMPSi ceramic composite coating in 3.5 wt. % NaCl solution. Parameter Fited data
Rs (Ω.cm2) 2.461
CPEcoat (F.cm-2.sα-1) 2.846 × 10-5
Rcp (Ω.cm2) 2471
α 0.94
CPEEDL (F.cm-2.sβ-1) 1.179 × 10-4
Rct (Ω.cm2) 7081
Table 4. The elements of the equivalent electrical circuit determined by simulation of EIS of the bare 316L in 3.5 wt. % NaCl solution. Farameter Fited data
Rs (Ω.cm2) 2.41
C (F.cm-2.sα-1) 1.465 × 10-4
35
Rct (Ω.cm2) 2098
β 0.97
Graphical abstract:
Highlights: •
A superhydrophobic nanocomposite coating was deposited on the 316L.
•
A one-step process was used.
•
The superhydrophobic nanocomposite coating improved the corrosion resistance of 316L effectively.
•
The deposited nanocomposite coating showed significant WCA and SA (168 ° and 3.1 ° respectively).
36