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Electrophoretic deposition of nano-ceramics for the photo-generated cathodic corrosion protection of steel substrates
Surface & Coatings Technology 236 (2013) 172–181
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Electrophoretic deposition of nano-ceramics for the photo-generated cathodic corrosion protection of steel substrates Ji Hoon Park a, Jong Sang Kim b, Jong Myung Park a,⁎ a b
Graduate Institute of Ferrous Technology, Pohang University of Science and Technology (POSTECH), Pohang 790-784, Republic of Korea Technical Research Laboratories, POSCO, Pohang 790-785, Republic of Korea
a r t i c l e
i n f o
Article history: Received 1 July 2013 Accepted in revised form 23 September 2013 Available online 2 October 2013 Keywords: Electrophoretic deposition Photo-generated cathodic protection TiO2 WO3 Corrosion protection
a b s t r a c t Electrophoretic deposition (EPD) was employed to deposit crystalline TiO2/WO3 nanoparticles onto stainless steel to fabricate a photo-generated cathodic protection layer. The electrophoretic mobility of the colloidal particles was determined by measuring the zeta-potential of the particles in the suspension. The surface morphology of the EPD layers was observed using scanning electron microscopy (SEM), and the composition of the EPD layer was determined from the peak intensity using an X-ray diffractometer (XRD). Furthermore, the open circuit potential (OCP) of the TiO2–WO3-deposited sample was measured under UV irradiation to evaluate the photogenerated cathodic protection ability of the EPD layers on stainless steel. Through the zeta-potential measurement, it was verified that the TiO2 particle was effectively charged by phosphoric acid di-butyl ester in isopropanol, but the WO3 particle was slightly negatively charged because of the different zero point of charge. However, TiO2 and WO3 particles were deposited simultaneously via an EPD process at the same rate despite their different electrophoretic mobilities in the suspension. The TiO2–WO3 co-deposited layer was uniformly deposited, and no noticeable homo-aggregation of the same type of particle was observed. Using the OCP measurement, it was also found that stainless steel was cathodically protected from corrosion by UV light irradiation because the potential of the photo-stimulated EPD layers was more negative than the corrosion potential of stainless steel. Because the WO3 contents in the EPD layer were increased, the time of potential recovery was gradually delayed. Moreover, the addition of binders enhanced the adhesion strength between the metal and the layer that hindered the delamination and improved the cathodic protection performance. The potential recovery time of the EPD sample containing 40% TiO2–60% WO3 was approximately 6 h after 3 h of light irradiation, which implied that the steel was cathodically protected for 6 h in dark conditions. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Corrosion of steel is a destructive result of an electrochemical reaction in a corrosive environment. In general, the electrochemical reaction of steel corrosion is composed of the coupling of an anodic reaction (Fe → Fe2+ + 2e−) and a cathodic reaction (1/2 O2 + H2O + 2e− → 2 OH− or 2H+ + 2e− → H2) [1]. Protective coatings are widely applied to protect steel from corrosion. Among these protective coatings, sacrificial zinc coatings are frequently employed because of their superior anti-corrosion performance and good mechanical characteristics [2,3]. A zinc coating can be sacrificially dissolved away in a corrosive environment because the electrochemical potential of zinc is more negative than that of steel, thus preventing the anodic reaction of steel corrosion. In this manner, the corrosion of steel can be suppressed by the sacrificial action of a zinc coating, which maintains the electrochemical potential of the steel below its own corrosion potential. However, the protection lifetime of a zinc coating is limited because it is gradually consumed ⁎ Corresponding author. Tel.: +82 054 279 9017; fax: +82 054 279 9299. E-mail address: [email protected] (J.M. Park). 0257-8972/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.surfcoat.2013.09.044
during corrosion protection. As an alternative to sacrificial cathodic protection, a novel approach has recently been introduced that utilizes photocatalytic TiO2 as an anti-corrosion coating material [4–8]. A TiO2 coating is a good insulation layer because of its electrochemical stability. Furthermore, a TiO2 coating can be utilized as a photo-generated, nonsacrificial protection layer due to its photocatalytic activity. Under solar illumination, a TiO2 coating can shift the electrochemical potential of a steel substrate toward values that are lower than the corrosion potential of steel. In other words, the potential of steel is negatively shifted by the TiO2 coating into the immunity domain of the E-pH diagrams. However, TiO2 coatings only protect steel substrates that are exposed to solar irradiation during daytime, while steel suffers from corrosion around the clock. To overcome this drawback, the incorporation of various photo-electron storing materials into the coating layer has been tested to maintain the efficacy of the cathodic protection that is provided by TiO2 in the absence of sunlight [9,10]. Various metal oxide materials, such as WO3 [9–11], SnO2 [12,13], Cu2O, [14] and MoO3 [15], have been suggested as photo-electron storing materials. WO3 was the first and most frequently exploited as an effective photo-electron storing agent. Therefore, TiO2–WO3 photocatalyst systems were utilized
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Electrophoretic deposition (EPD) was performed at a constant voltage of 30 V for 1 min, and the working distance between the cathode and anode was 15 mm. The deposited specimen was dried at 60 °C in a convection-drying oven for 1 h.
in this study to impart photo-generated cathodic protection to a ceramic coating layer around the clock. Several methods, such as physical vapor deposition, chemical vapor deposition, electrochemical vapor deposition methods and plasma technologies [16–20], have been used to deposit a metal oxide layer onto steel. However, none of these methodologies are appropriate for implementation in manufacturing processes because they require a vacuum environment for operation. As such, an efficient and accessible approach for preparing a metal oxide layer on steel is a colloidal deposition route, such as dip-coating, screen-printing or slip-casting. It is clear that colloidal deposition is a simple and inexpensive method for fabricating a ceramic layer [21]. However, difficulties in controlling the deposition factor and poor adhesion strength are crucial disadvantages in using the colloidal method. Moreover, all of the above-mentioned processing methods must be followed by a high-temperature heat treatment to guarantee the adhesion and photo-catalytic activity of the deposited ceramics. As an alternative, electrophoretic deposition (EPD) was employed in the present work to deposit TiO2–WO3 photo-catalysts onto a steel substrate to construct a photo-generated, cathodic protection layer. EPD has recently attracted increasing attention as a powerful method for the fabrication of nano-structured, thin ceramic composite films on conductive substrates because of its fascinating advantages of versatility for application, cost effectiveness and simplicity [22–25]. In the EPD process, the uniform, nano-structured ceramic layer is formed by coagulation of a charged nano-particle in a liquid suspension under an applied electric field [26,27]. Various additives can be employed to control the particle charge of the EPD suspension [28]. The addition of acids has been a typical approach for obtaining positively charged colloidal particles [29]; however, the ceramic particles or the electrode could react with the acid and corrode [30,31]. The addition of inorganic cations or organic macromolecules can be utilized for colloidal particle charging [32–35]. In the present work, phosphate ester (PE) was employed as a charging additive for cathodic, electrophoretic deposition because it is an electrostatic stabilizer that positively charges particles in organic liquids by donating protons [36,37].
The zeta potential of the nanoparticles in the EPD solution was measured using a Zetasizer (Nano-ZS, Marvern Instruments) and its software (Dispersion Technology software, DTS). The surface morphology of the deposited sample was observed using scanning electron microscopy (SEM, Hitachi SU-6600). Before the SEM observations, the samples were coated with 10 nm of Pt/Pd, and the nanoparticle composition of the EPD sample was determined using X-ray diffraction (XRD, D8 Advance, Bruker AXS) with CuKa radiation. The optical properties of the electrodes were also characterized using a UV–vis diffuse reflectance spectrophotometer (Shimadzu UV2450), and the band-gap of the EPD samples was estimated using the UV–vis absorption spectra. Furthermore, the open circuit potential (OCP) measurement and the potentiodynamic polarization test of the TiO2–WO3-deposited sample were performed under UV irradiation using a Gamry Reference 600 with a PCI4 Controller. Fig. 1 shows the devised photo-electrochemical cell that was used to evaluate the photo-generated cathodic protection ability of the EPD layers on steel. The coated specimen, saturated calomel electrode (SCE) and graphite rod counter electrode were immersed in 3.5% NaCl solutions. After 3 min of immersion, the sample was illuminated for 1 or 3 h with a 200 W mercury-xenon lamp as an ultraviolet (UV) light source, and then the light was turned off. The changes in the OCP of the samples under UV light on/off conditions were monitored to investigate the cathodic protection performance of the EPD coating layers. The weight fractions of the organic-charging additive and binder material were determined by thermogravimetric analysis (TGA, Mettler-Toledo 851E). In the TGA scan, the deposited materials that were obtained from the EPD layer were heated from 100 °C to 700 °C at a rate of 10 °C·min−1 under a nitrogen atmosphere.
2. Experimental methods
3. Results and discussion
2.1. Electrophoretic deposition
3.1. The colloidal suspension for electrophoretic deposition
TiO2 (Degussa P-25, primary particle size of 25 nm, BET surface area of ca. 50 m2·g−1, and a 8:2 anatase and rutile mixture) and WO3 particles (Ditto Technology Co., Ltd., primary particle size of 90 nm and BET surface area of ca. 10m2·g−1) were purchased for electrophoretic deposition. Iso-propanol (IPA, Sam Chun Chemical, 99.8%) was selected as the dispersion medium, and phosphoric acid di-butyl ester (PADE) was used as the charging additive for the nanoparticles. The charging additive was first added to IPA, and the solution was then mixed for approximately 1 h to obtain a homogeneous charging solution medium. Subsequently, TiO2 and WO3 nanoparticles were slowly added to the IPA solution containing the charging additive, and the suspension of nanoparticles was then ultrasonicated for 1 h to create a uniformly dispersed solution. Before dispersing the solution, the nanoparticles were dried in a convection oven at 60 °C for approximately 24 h. EPD solutions containing different compositions of particles were then prepared: TW 0 (TiO2 30 g·L−1 + WO3 0 g·L−1), TW 20 (TiO2 24 g·L−1 + WO3 6 g·L−1), TW 40 (TiO2 18 g·L−1 + WO3 12 g·L−1), TW 60 (TiO2 12 g·L−1 + WO3 18 g·L−1), TW 80 (TiO2 6 g·L−1 + WO3 24 g·L−1) and TW 100 (TiO2 0g·L−1 +WO3 30g·L−1). In some cases, polyvinyl butyral (PVB, Butvar® B-98, ACROS, USA) was included in the EPD solution as a binder material to improve the adhesion of the nanoparticles with the substrate. AISI Type 304 stainless steel was employed as the working substrate to deposit the nanoparticles and as the counter electrode. The two electrodes were immersed in the prepared EPD solution and connected to a DC power supply (ED Rectifier, DME Tech, Korea). The working and counter electrodes were then used as the cathode and anode, respectively.
Electrophoretic deposition (EPD) of nanoparticles onto a steel substrate was achieved by the electrophoretic motion of a charged particle in a liquid medium under an applied electric field. To deposit
2.2. Characterization
Fig. 1. The devised photo-electrochemical cell for evaluating the photo-generated cathodic protection ability of the EPD layers on stainless steel.
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nanoparticles onto a substrate by electrophoresis, many factors must be considered, e.g., electric field strength, surface area of the electrode, particle mass concentration in the suspension, permittivity, zeta potential and the viscosity of the suspension [38,39]. First, the colloidal suspension must be prepared for EPD with the consideration of these factors. The preparation of a stable colloidal suspension containing charged nanoparticles is the most important step in the EPD process to obtain a high-quality deposited layer on the steel substrate. Water or organic solvents can be employed as the liquid medium of the suspension. In the present work, the organic solvent iso-propyl alcohol was selected for the EPD suspension because water-based EPD could cause Joule heating of the suspension, nonhomogeneity of the deposited film and oxidation of the metallic substrate [40–42]. In addition, a charging additive was incorporated into the EPD suspension to manipulate the surface charge of the TiO2 and WO3 particles. Although the simplest approach for charging a ceramic particle is the addition of strong acids, such as hydrochloric or nitric acids, the use of these acids can corrode the particles or electrodes due to their strong electrochemical reactivities [29–31]. Inorganic cations such as Mg2+, Ca2+ and Al3+ have been widely employed to charge particles in an EPD suspension. However, inorganic cations could contaminate the deposition layer and deteriorate the intrinsic properties of the layer [32–34]. In some reported studies, organic macromolecules were employed as an alternative for these additives [35]. Among these, it was reported that phosphate ether was an effective charging additive for the particles [36,37]. In this manner, the phosphoric acid di-butyl ester (PADE) was exploited as the positive charging material for colloidal TiO2 and WO3 nanoparticles. Fig. 2(a) shows the evolution of the zeta potentials of TiO2 and WO3 nanoparticles as a function of the concentration of PADE. In general, the zeta-potential of the particles was a key parameter that represented the electrophoretic mobility of the particles [43]. The zeta-potential of colloidal TiO2 and WO3 in a pure IPA solution was approximately −10 mV, as shown in Fig. 2(a). In the case of TiO2, the zeta-potential was increased as the content of PADE in the suspension was increased, and the zeta-potential of colloidal TiO2 reached a maximum value of +45 mV at a concentration of 10−3 M. In contrast, the WO3 particles were not positively charged, even with a high concentration of PADE. In other words, in an IPA–PADE suspension system, it was practicable to obtain protonated TiO2 but not WO3. One of the main reasons for these dissimilar charging behaviors might be the difference in the zero point of charge (zpc) of the powder. The ceramic particle is generally positively charged at a pH that is below the zpc, but it is negatively charged at a pH that is above the zpc. The zpc of TiO2 is 6.25, and that of WO3 is 2.5 [44,45]. In the present study, the pH of the IPA–PADE solution ranged from 4.93 to 3.48 depending on the concentration of PADE, which was lower than the zpc of TiO2 and higher than the zpc of WO3. Therefore, the TiO2 particles were positively charged in the
EPD solution, while WO3 particles were not positively charged. Fig. 2(b) shows the deposition mass as a function of the application time at a constant electric field of 30 V. Despite a long-term application, the electrophoretic deposition of WO3 was not successful due to its lower electrophoretic mobility. On the contrary, the deposition mass of TiO2 linearly increased with the applied time of the electric field due to the positive charging of the TiO2 particle. Colloidal suspensions of the various TiO2/WO3 particle compositions were prepared to measure the zeta-potential for studying the codeposition behavior of the suspension of mixed particles through the EPD process. The weight ratio of WO3 in the mixed particles varied from 0% to 100%, and the whole particle concentration was 30 g/L in the solution. The zeta-potential of the solution was decreased when the WO3 contents were increased, as shown in Fig. 3. As explained above, the WO3 particles were barely charged in the IPA–PADE-solution system, and a mixed zeta-potential of the suspension containing positively charged TiO2 was reduced by the incorporation the slightly negatively charged WO3 particle. As mentioned frequently, the electrophoretic mobility, which mainly depended on the zeta-potential of the particle, was a key factor for determining the efficiency of EPD in this system. Hence, the deposited mass of the particle was significantly decreased by increasing the WO3 ratio in the composite suspension under the same operating conditions of EPD, such as applied voltage and time, as shown in Fig. 3(b). 3.2. Application of electrophoretic deposition The EPD process will be compared to a simple dip-coating process before the specific discussion of the EPD behavior of the colloidal suspensions of the various TiO2–WO3 particle compositions. The dipcoating method was selected as a reference method for the EPD process because it is a representative methodology of a non-electrical colloidal deposition route for the formation of ceramic particles. Dip-coating was performed using the isopropyl alcohol solution containing 30 g/L of TiO2 particles and 0.01 M PADE. In the dip-coating process, stainless steel was immersed in the colloidal solution for 5 min and then withdrawn from the solution with a speed of 2 m·min−1. Similarly, an EPD composite layer was also formed on stainless steel using the same colloidal suspension of dip-coating under an applied electrical field. To determine the optimal conditions, EPD was preliminarily conducted under various operating conditions, such as applied voltages and application times. Ceramic particles were deposited onto stainless steel even at low electrical fields of below 5 V; however, the adhesion strength between the layer and steel substrate was not sufficient. Under higher electrical field conditions, cracks appeared in the deposited layer, and the layer was thickened excessively even with a short application time. Based on a series of experiments, the EPD in this study was
Fig. 2. (a) The evolution of the zeta-potential of colloidal TiO2 or WO3 particles as a function of the PADE concentration in an EPD solution. (b) The deposited mass versus deposition time at 30 V for 30 g/L of TiO2 or WO3 particles in suspensions of isopropyl alcohol containing 0.01 M PADE.
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Fig. 3. (a) Zeta-potential of the colloidal particles as a function of the WO3 concentration in the suspension. (b) Deposited mass versus WO3 concentration for 30 g/L of TiO2 or WO3 particles in suspensions of isopropyl alcohol containing 0.01 M PADE.
conducted at a constant voltage of 30 V for 1 min as an optimal condition. Compared with the dip-coating process, the TiO2 layer from EPD adhered more strongly to the steel substrate, as shown in Fig. 4. Fig. 4(a) and (d) presents the results of the cross-cutting and tapping tests for evaluating the adhesion strength of the layer on a substrate (ASTM D 3359). The dip-coated TiO2 layer was easily removed by
tapping, which represents weak adhesion of the coated TiO2 particle. As shown in Fig. 4(b) and (c), it was verified that the TiO2 particles were attached sparsely and weakly during the dip-coating process. However, the TiO2 coating layer from the EPD process was not detached from the substrate, which indicates that the electric field in the EPD process played an important role in the construction of the firmly adherent
Fig. 4. Optical surface image of the dip coating (a) and EPD (d) layers after the cross-cutting and tapping tests, and SEM micrographs of the dip coating layer (b, c) and EPD layer (e, f).
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layer of the nanoparticles. In the EPD process, the charged particle accumulates on the surface of the substrates, removing the solvent by electroendosmosis during the application of the electric field, inducing a flocculation of the particles [26,27]. Moreover, the SEM micrographs of the TiO2 EPD layer, shown in Fig. 4(e) and (f), indicate that the layer was uniform and homogenous and that no visible cracks were present on the surface, despite the thickness of the TiO2 EPD layer being several tens of microns. Consequently, it was clearly demonstrated that the EPD process is a simple and powerful method for creating a nano-structured, ceramic particle layer on steel. Furthermore, the TiO2–WO3 composite layer was formed using the EPD process to evaluate the EPD behavior of a TiO2–WO3 particle colloidal suspension. Fig. 5 shows the optical surface image of the EPD layer from the colloidal suspension of the various TiO2–WO3 particle compositions (0%–100%). The total deposition mass and film thickness were decreased by increasing WO3 in the suspension. In fact, EPD layers were not successfully formed when using suspensions with high WO3 contents, i.e., 80% and 100%, even when they were exposed to a high electric field and after a long application time, because the WO3 particles were not effectively charged in the IPA–PADE solution, as mentioned above. The mixed zeta-potentials of the colloidal suspension of 80% and 100% WO3 were −2.89mV and −9.61mV, respectively. Therefore, the following experimental analysis was conducted on the EPD samples from the colloidal suspensions of 0%, 20%, 40% and 60% WO3. Fig. 6 shows the SEM micrograph of the EPD layers from the colloidal suspensions of various WO3 concentrations. No noticeable homoaggregation of the same types of particles in the TiO2–WO3 codeposited layers was evident. It was verified that the TiO2 and WO3 particles were homogeneously blended in the suspension and uniformly deposited on the stainless steel by electrophoretic co-deposition. In
addition, XRD analysis was performed to confirm the composition ratio of the EPD layers that were formed using the suspensions of various WO3 concentrations, and the corresponding XRD patterns are shown in Fig. 7. The XRD pattern of bare stainless steel shows sharp peaks at 44° and 51° that correspond to the (1 1 1) and (2 0 0) crystal orientations in the austenite phase. After the TiO2 particles were applied on stainless steel, distinct peaks appeared at 25.5° and 48°, which corresponded to the characteristic peaks of the (1 0 1) and (2 0 0) crystal planes of anatase, respectively, and a peak at 27.6° that corresponded to the characteristic peak of the (1 1 0) plane of rutile appeared. The XRD pattern of the EPD layer that was formed from the 20% WO3 suspension showed distinct peaks at 23°, 24°, 28°, 34°, 51°, 54° and 55°, which corresponded to the (0 2 0), (0 0 2), (1 1 2), (1 2 0), (4 2 0), (2 1 1) and (2 0 1) crystal orientations of the orthorhombic phase of WO3. This finding implies that the TiO2 and WO3 particles were successfully co-deposited via the EPD process despite the WO3 particles not being positively charged in the suspension. Under the application of an electric field, it appeared that the electrophoretic motion of the positively charged TiO2 induced the movement of the slightly negatively charged WO3 particle and allowed it to be deposited on the steel. Because the WO3 concentration in the suspension was increased, the distinct peak for WO3 in the XRD pattern was also intensified and represented the increment of WO3 that was deposited in the layer. To confirm the actual WO3 concentration of the co-deposited layer, the composition of the layer was calculated by comparing the XRD peak height with that of the standard sample. The standard powder samples were prepared by mixing exact amounts of the TiO2 and WO3 particles, and the XRD pattern was obtained. The characteristic peaks for the (1 0 1) planes of TiO2 and (1 2 0) planes of WO3 were employed as representative peaks for calculating the composition. According to the
Fig. 5. Optical surface image of the EPD layers that were deposited from suspensions composed of various WO3 concentrations on 304 STS: (a) TW 0, (b) TW 20, (c) TW 40, (d) TW 60, (e) TW 80 and (f) TW 100.
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Fig. 6. SEM micrograph of the EPD layers on 304 STS: (a) TW 0, (b) TW 20, (c) TW 40 and (d) TW 60.
calculation, the WO3 concentrations in the EPD layers for samples TW 20, TW 40 and TW 60 were 19%, 37% and 58%, respectively. Because the WO3 concentrations in the deposited layers were approximately identical to those in the suspension, it can be stated that the TiO2 and WO3 particles in the suspension were deposited simultaneously and at the same rate despite their different zeta-potentials (electrophoretic mobility). Many studies have demonstrated the co-deposition phenomenon of dissimilarly charged particles in the EPD process. If two or more particles from the same suspension are deposited simultaneously, the particles can be deposited at an equal rate if the volume fractions of those particles are comparatively high. On the other hand, the particles are moved individually under the application of an electrical field at low volume fractions [46]. The concentrations of particles in the suspension (30 g/L) in this study were sufficiently high to cause the simultaneous deposition of the TiO2 and WO3 particles. Moreover, in the present work, the weakly negatively charged WO3 particles will be attracted to the strongly positively charged TiO2 particles in the suspension and will be simultaneously deposited on the cathode.
Fig. 7. The variations in the XRD patterns of the EPD layers that were deposited from suspensions of various WO3 concentrations on 304 STS.
Fig. 8(a) presents the UV–vis absorption spectra of the EPD layers with different WO3 concentrations. The UV–vis absorption spectrum of TW 0 exhibited the onset of an absorption peak at 400 nm, which
Fig. 8. (a) The optical absorption spectra and (b) optical absorption edge (ahv)1/2 versus the photon energy (hv) of the EPD layer on stainless steel with different WO3 concentrations.
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was due to the charge transfer process from the valence band to the conduction band in crystalline TiO2. However, the absorption peaks were shifted to the lower energy region of the spectrum after the incorporation of the WO3 particles. In short, the TiO2–WO3 co-deposited samples possessed lower optical absorption edges than the pure TiO2 EPD sample (TW 0), and a higher WO3 concentration resulted in a more obvious redshift of the sample. To estimate the energy band gaps, the graph of the optical absorption edge (ahv)1/2 versus the photon energy (hv) was plotted and is shown in Fig. 8(b). The estimated band gap energy decreased when the WO3 concentration increased from 3.2 eV to 2.8 eV. The intrinsic band gap of TiO2 (anatase) was 3.2 eV, and that of WO3 was 2.8 eV. It is certain that the reduction in the band gap energy of the co-deposited sample originated from the coupling of TiO2–WO3. Through the optical absorption measurement, it was found that the photo-electrochemically active layers were successfully formed via the EPD process, and the TiO2 and WO3 particles were hetero-aggregated in the layer densely enough to exhibit integrated photocatalytic activity. 3.3. Photo-generated cathodic protection of the EPD layer The potentiodynamic polarization curves of stainless steel with or without a TiO2 EPD layer are shown in Fig. 9. The polarization test was performed in a 3.5 wt.%-NaCl solution under light on/off conditions. No significant shift of the polarization curve after light irradiation for bare stainless steel was observed, which indicates that no photo-induced effect with bare stainless steel occurred. In contrast, the polarization curve of EPD stainless steel (TW 0) was drastically shifted downward after light exposure. Light irradiation induced a shift in the photo-potential from +0.05 VSCE to −0.7 VSCE and accompanied a generation of large photo-currents (10 μA). Under light, the TiO2 EPD layer could shift the electrochemical potential of the substrate toward a more negative value than the corrosion potential of steel. In other words, under light irradiation, stainless steel could be cathodically protected from corrosion because the potential of the photo-stimulated EPD layers was more negative than the corrosion potential of stainless steel. The shift in the potential was due to the transferred photo-electron from the TiO2–WO3 layer during light irradiation. In addition, OCP measurements were conducted for an in-depth study of the photo-generated cathodic protection ability of the TiO2–WO3 EPD layers. Fig. 10 shows the OCP changes of the EPD TiO2–WO3 layers on stainless steel in a 3.5 wt.%-NaCl solution under light on/off conditions. As explained above, under light-exposed conditions, the OCP of the EPD samples immediately dropped to a potential in the range of −0.5 VSCE and −0.7 VSCE. However, when the light was off, the OCP rapidly recovered toward its corrosion potential because the EPD layer could no longer
Fig. 10. Effect of the WO3 concentration in the EPD layers on the changes of OCP in a NaCl solution after light irradiation and the potential recovery time to the corrosion potential of stainless steel under dark conditions.
generate the photo-electron. However, the corrosion potential of stainless steel was slightly altered from −0.05 VSCE to −0.15 VSCE before and after light irradiation. The pH change of the solution after light irradiation could be the main reason for the shift in the corrosion potential. Under light, the TiO2–WO3 layer simultaneously generated both photoelectron/hole pairs. Although electrons were mostly injected to the metal, many were consumed by the chemical reaction between water and oxygen in the solution. Similarly, the photo-generated hole was usually consumed by chemical reactions on the particle surface, such as during the oxidization of water [47]. After light irradiation, all of the chemical reactions that modified the environments of the solution and the corrosion potential of the stainless steel in the solution shifted slightly. However, for the pure TiO2–EPD sample (TW 0), the OCP was immediately increased to the potential of steel corrosion after the light was turned off. The potential recovery time to reach the original corrosion potential (−0.15 VSCE) of TW 0 was approximately 1 h from the time that the light was turned off. As the WO3 contents in the EPD layer increased, the potential recovery time was gradually delayed, as shown in Fig. 10. For example, the potential recovery time of TW 20 was approximately 3 h after 1 h of light irradiation due to the electron-storage ability of WO3. Tatsuma et al. demonstrated that WO3 could store photoelectrons when exposed to light and subsequently release the photoelectrons in dark conditions [9,10]. The electron storing mechanism of WO3 is closely related to electrochromism and photochromism. Specifically, the photo-generated electrons migrated from TiO2 to WO3, and MxWO3 (M_H or Na) was formed on the WO3 surfaces by the reaction between photo-electron M+ from the electrolyte and WO3 under light irradiation. In dark conditions, the reverse reaction occurred to release electrons from MxWO3, and the electrons were injected onto the steel substrate. Such reversible, photo-electron-charging and -discharging phenomena originate from the multivalence of W. The partial incorporation of WO3 into the photocatalytic TiO2 layer proved to be an effective method to extend the recovery time of OCP to its corrosion potential. A longer recovery time of potential meant longer cathodic corrosion protection of the EPD layer for stainless steel under dark conditions. Thus, the photo-generated cathodic protection layer that was successfully formed by the electrophoretic co-deposition of crystalline TiO2 and WO3 protected the stainless steel from corrosion in dark conditions due to the electron-storage ability of WO3.
3.4. The effect of binder addition
Fig. 9. The potentiodynamic polarization curve for stainless steel with or without the TiO2 EPD layer in a 3.5 wt.% NaCl solution under light on/off conditions.
Although no delamination of the TiO2–WO3 EPD layer was observed during the anti-corrosion test, the binder material was incorporated into the EPD suspension to improve the adhesion strength of the
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deposited particles on the substrate. Various types of binders can be employed in the EPD process. Among these substances, polyvinyl butyral (PVB) has been widely used for non-aqueous EPD. Particularly, its influence on electrophoretic mobility could be minimized because PVB is a non-ionic polymer. PVB (4.5 g) was added into the 30 g/L (particle/solution) suspension, and the electrophoretic deposition was performed according to the same method as discussed in the previous section. An SEM micrograph of the EPD layers resulting from different WO3 concentrations in the suspension is shown in Fig. 11. A uniform surface was formed, and no noticeable homo-aggregation of the same types of particles was observed in the co-deposited layer. Furthermore, no evidence that PVB addition terminated the stability of the suspension or deteriorated the formation of the uniform surface was obvious. When PVB was included in the solution, the nano-particles appeared to be more densely deposited on the stainless steel than the EPD layer that contained no binder materials, as shown in Fig. 6. Thermogravimetric analysis (TGA) was performed to determine the relative amounts of the organic charging additives (PADE) and the binding material (PVB). The relative contents could be estimated from the weight loss in the TGA scans of the deposited layer, as shown in Fig. 12. In the TGA scan from 100 °C to 700 °C, the weight of pristine TiO2 (P-25) was not reduced; however, the weight of the EPD layer decreased due to the decomposition of the organic materials (PADE or PVB). For the EPD layer containing no PVB, the relative weight loss was 6.3%, based on the TiO2 weight, due to the decomposition of PADE. Because the amount of PADE in the EPD solution was 7 wt.% (2.1 g of PADE/30 g of TiO2), this result indicated that most of the charging additive was simultaneously deposited with the TiO2 particles on the substrate via the EPD process. For the EPD layer containing PVB binder, the total weight loss in the TGA scan of up to 700 °C was 8.5%. This weight loss was due to PADE and PVB decomposition. Although 15 wt.% of the PVB was incorporated into the EPD solution, it appeared as if considerably less of the PVB was co-deposited on the substrate with the TiO2 particles. It also appeared as if the poor co-deposition efficiency of the PVB originated from its non-ionic character in the IPA solution. Although a smaller amount of PVB was included in the EPD layer, the adhesion of the particles to the substrate was improved by the addition of the binder. An evaluation of adhesion using the ASTM D 3359 method (i.e., cross-cut adhesion) showed that the detachment area of
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Fig. 12. Thermogravimetric analysis (TGA) data of pristine TiO2 particles and EPD layer containing TiO2 particles, charging additive (PADE) or binding material (PVB).
the EPD sample that included the PVB (ASTM classes 4B) was decreased compared to that of the normal EPD sample (ASTM classes 3B). In addition, a tendency for the adhesion to decrease when the WO3 concentration in the EPD layer was increased was evident. As explained above, the WO3 particle was not effectively charged in the EPD system; thus, it would not be attached as strongly as TiO2 to the substrate. Although the TiO2 and WO3 particles were deposited on the substrate at the same rate, the adhesion of the resultant layers involving WO3 was deteriorated due to the lack of charging ions on the WO3 particle. Fig. 13 shows the OCP changes of the EPD layer that included PVB on stainless steel in a 3.5 wt.% NaCl solution under light on/off conditions. The transition behavior of the OCP was roughly similar between the sample with and without the PVB binder, but a small difference was observed. For the sample with the PVB, the OCP dropped sharply to a potential of −0.7 VSCE immediately after the light was turned on, regardless of the WO3 concentration. However, when the binder was not included in the layer, the OCP values when under light exposure were somewhat different. In this case, they reached the potential t in a range of −0.5 VSCE to −0.7 VSCE under light, and the width of the OCP drop was lessened with increased WO3 concentrations. Moreover, the OCP was not maintained at the potential of photo-stimulated TiO2
Fig. 11. SEM micrograph of EPD layers that included PVB binder on 304 STS; (a) TW 0, (b) TW 20, (c) TW 40 and (d) TW 60.
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TW 0 and TW 60 were observed. Namely, the concentration of WO3 did not significantly influence the photo-effect of the EPD layer. However, the potentiodynamic polarization curves for TW 0 and TW 60 were considerably different when they were carried out under dark conditions after 1 h of light irradiation. The potential of TW 60 (−0.35 VSCE) was lower than that of TW 0 (−0.2 VSCE). As mentioned above, this finding indicates that the longer, photo-generated cathodic protection of the EPD layer that originates from the electron-storage ability of WO3 occurs under dark conditions. 4. Conclusion
Fig. 13. The effect of PVB addition into an EPD layer on its photo-generated, anti-corrosion ability, and the effect of WO3 concentrations on the OCP changes in a NaCl solution and the potential recovery time under light on/off conditions.
(~ −0.7 VSCE), in spite of continuous light irradiation, as shown in Fig. 10, which means that the potential of the injected, photogenerated electron from TiO2 was lowered by certain impedance. A possible explanation was that the photo-electron could be impeded by the delamination of the EPD layer from the substrate by the oxygen evolution reaction of the transferred electron at the interface. However, the addition of binder enhanced the adhesion strength between the metal and the layer that hindered the delamination and improved the cathodic protection performance of the samples with high concentrations of WO3. Even at high concentrations of WO3, the OCP of the PVB-added sample was consistently maintained at −0.7 VSCE, which was the potential of photo-simulated TiO2, during the entire period of the light irradiation due to its enhanced adhesion. Furthermore, the time of potential recovery after the light was turned off was delayed significantly as more WO3 particles were incorporated into the EPD layer due to the electron-storing mechanism of WO3 in water. The potential recovery time of TW 60 was approximately 6 h after 3 h of light irradiation, which implied that the steel was cathodically protected for 6 h by the photo-cathodic TiO2–WO3 EPD layer in dark conditions. In addition, Fig. 14 shows the potentiodynamic polarization curves for TW 0 and TW 60 under light irradiation or dark following light irradiation. In detail, the potentiodynamic polarization tests were performed under light irradiation for 1 h in a 3.5%-NaCl solution, and subsequent polarization tests were then performed under dark conditions 1.5 h after the light was turned off. Under light irradiation, no substantial differences between the photo-potentials and photo-currents of
Electrophoretic mobility was a key factor for the efficiency of EPD, and it mainly depended on the zeta-potential of the particle. The zetapotential of the TiO2/WO3 particle was manipulated by the positive charging additive, PADE, in the IPA solution. The TiO2 particle was effectively charged in the IPA–PADE solution, but the WO3 particle was not positively charged due to its lower zpc than pH of the solution. Thus, the mixed zeta-potential of the suspension containing positively charged TiO2 was shifted negatively by the incorporation the slightly negatively charged WO3 particle into the solution. Hence, the deposited mass of the particle was significantly decreased when the WO3 ratio in the composite suspension was increased while maintaining the same operating conditions for EPD. In any case, a TiO2–WO3 composite layer was successfully formed on stainless steel via the EPD process. When compared to the dip-coating process, the EPD layer adhered more strongly to the steel substrate by the action of particle accumulation and electroendosmosis during the application of the electric field. Moreover, a thick EPD layer was uniformly deposited on the stainless steel, and no noticeable homo-aggregation of the particles was observed. In the EPD co-deposition process, TiO2 and WO3 particles were deposited simultaneously at the same rate despite their different zeta-potentials in the suspension. The particle concentration in the suspension in the present work was sufficiently high to cause the deposition of TiO2 and WO3 particles at an equal rate, and the OCP results confirmed the photo-generated cathodic protection ability of the TiO2–WO3 EPD layers. Under UV light irradiation, stainless steel was cathodically protected from corrosion because the potential of the photo-stimulated EPD layers was more negative than the corrosion potential of the stainless steel. As the WO3 contents in the EPD layer increased, the time of potential recovery was gradually delayed. Moreover, the addition of a binder enhanced the adhesion strength between the metal substrate and the deposited layer that hindered delamination and improved the cathodic protection performance of samples with high concentrations of WO3. The potential recovery time of the EPD sample containing 40% TiO2–60% WO3 was approximately 6 h after 3 h of light irradiation, which implied that the steel was cathodically protected for 6 h in dark conditions. The partial incorporation of WO3 into the photocatalytic TiO2 layer was an effective method to delay the OCP recovery time to its corrosion potential, and the longer recovery time of the potential meant that longer cathodic corrosion protection of the stainless steel by the EPD layer in dark conditions occurred. Acknowledgment Financial support from POSCO is gratefully appreciated. References
Fig. 14. Potentiodynamic polarization curve for TiO2 EPD (TW 0) and TiO2–WO3 EPD layers (TW 60) under light-on conditions and under dark conditions after light irradiation.
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Electrophoretic deposition of nano-ceramics for the photo-generated cathodic corrosion protection of steel substrates Ji Hoon Park a, Jong Sang Kim b, Jong Myung Park a,⁎ a b
Graduate Institute of Ferrous Technology, Pohang University of Science and Technology (POSTECH), Pohang 790-784, Republic of Korea Technical Research Laboratories, POSCO, Pohang 790-785, Republic of Korea
a r t i c l e
i n f o
Article history: Received 1 July 2013 Accepted in revised form 23 September 2013 Available online 2 October 2013 Keywords: Electrophoretic deposition Photo-generated cathodic protection TiO2 WO3 Corrosion protection
a b s t r a c t Electrophoretic deposition (EPD) was employed to deposit crystalline TiO2/WO3 nanoparticles onto stainless steel to fabricate a photo-generated cathodic protection layer. The electrophoretic mobility of the colloidal particles was determined by measuring the zeta-potential of the particles in the suspension. The surface morphology of the EPD layers was observed using scanning electron microscopy (SEM), and the composition of the EPD layer was determined from the peak intensity using an X-ray diffractometer (XRD). Furthermore, the open circuit potential (OCP) of the TiO2–WO3-deposited sample was measured under UV irradiation to evaluate the photogenerated cathodic protection ability of the EPD layers on stainless steel. Through the zeta-potential measurement, it was verified that the TiO2 particle was effectively charged by phosphoric acid di-butyl ester in isopropanol, but the WO3 particle was slightly negatively charged because of the different zero point of charge. However, TiO2 and WO3 particles were deposited simultaneously via an EPD process at the same rate despite their different electrophoretic mobilities in the suspension. The TiO2–WO3 co-deposited layer was uniformly deposited, and no noticeable homo-aggregation of the same type of particle was observed. Using the OCP measurement, it was also found that stainless steel was cathodically protected from corrosion by UV light irradiation because the potential of the photo-stimulated EPD layers was more negative than the corrosion potential of stainless steel. Because the WO3 contents in the EPD layer were increased, the time of potential recovery was gradually delayed. Moreover, the addition of binders enhanced the adhesion strength between the metal and the layer that hindered the delamination and improved the cathodic protection performance. The potential recovery time of the EPD sample containing 40% TiO2–60% WO3 was approximately 6 h after 3 h of light irradiation, which implied that the steel was cathodically protected for 6 h in dark conditions. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Corrosion of steel is a destructive result of an electrochemical reaction in a corrosive environment. In general, the electrochemical reaction of steel corrosion is composed of the coupling of an anodic reaction (Fe → Fe2+ + 2e−) and a cathodic reaction (1/2 O2 + H2O + 2e− → 2 OH− or 2H+ + 2e− → H2) [1]. Protective coatings are widely applied to protect steel from corrosion. Among these protective coatings, sacrificial zinc coatings are frequently employed because of their superior anti-corrosion performance and good mechanical characteristics [2,3]. A zinc coating can be sacrificially dissolved away in a corrosive environment because the electrochemical potential of zinc is more negative than that of steel, thus preventing the anodic reaction of steel corrosion. In this manner, the corrosion of steel can be suppressed by the sacrificial action of a zinc coating, which maintains the electrochemical potential of the steel below its own corrosion potential. However, the protection lifetime of a zinc coating is limited because it is gradually consumed ⁎ Corresponding author. Tel.: +82 054 279 9017; fax: +82 054 279 9299. E-mail address: [email protected] (J.M. Park). 0257-8972/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.surfcoat.2013.09.044
during corrosion protection. As an alternative to sacrificial cathodic protection, a novel approach has recently been introduced that utilizes photocatalytic TiO2 as an anti-corrosion coating material [4–8]. A TiO2 coating is a good insulation layer because of its electrochemical stability. Furthermore, a TiO2 coating can be utilized as a photo-generated, nonsacrificial protection layer due to its photocatalytic activity. Under solar illumination, a TiO2 coating can shift the electrochemical potential of a steel substrate toward values that are lower than the corrosion potential of steel. In other words, the potential of steel is negatively shifted by the TiO2 coating into the immunity domain of the E-pH diagrams. However, TiO2 coatings only protect steel substrates that are exposed to solar irradiation during daytime, while steel suffers from corrosion around the clock. To overcome this drawback, the incorporation of various photo-electron storing materials into the coating layer has been tested to maintain the efficacy of the cathodic protection that is provided by TiO2 in the absence of sunlight [9,10]. Various metal oxide materials, such as WO3 [9–11], SnO2 [12,13], Cu2O, [14] and MoO3 [15], have been suggested as photo-electron storing materials. WO3 was the first and most frequently exploited as an effective photo-electron storing agent. Therefore, TiO2–WO3 photocatalyst systems were utilized
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Electrophoretic deposition (EPD) was performed at a constant voltage of 30 V for 1 min, and the working distance between the cathode and anode was 15 mm. The deposited specimen was dried at 60 °C in a convection-drying oven for 1 h.
in this study to impart photo-generated cathodic protection to a ceramic coating layer around the clock. Several methods, such as physical vapor deposition, chemical vapor deposition, electrochemical vapor deposition methods and plasma technologies [16–20], have been used to deposit a metal oxide layer onto steel. However, none of these methodologies are appropriate for implementation in manufacturing processes because they require a vacuum environment for operation. As such, an efficient and accessible approach for preparing a metal oxide layer on steel is a colloidal deposition route, such as dip-coating, screen-printing or slip-casting. It is clear that colloidal deposition is a simple and inexpensive method for fabricating a ceramic layer [21]. However, difficulties in controlling the deposition factor and poor adhesion strength are crucial disadvantages in using the colloidal method. Moreover, all of the above-mentioned processing methods must be followed by a high-temperature heat treatment to guarantee the adhesion and photo-catalytic activity of the deposited ceramics. As an alternative, electrophoretic deposition (EPD) was employed in the present work to deposit TiO2–WO3 photo-catalysts onto a steel substrate to construct a photo-generated, cathodic protection layer. EPD has recently attracted increasing attention as a powerful method for the fabrication of nano-structured, thin ceramic composite films on conductive substrates because of its fascinating advantages of versatility for application, cost effectiveness and simplicity [22–25]. In the EPD process, the uniform, nano-structured ceramic layer is formed by coagulation of a charged nano-particle in a liquid suspension under an applied electric field [26,27]. Various additives can be employed to control the particle charge of the EPD suspension [28]. The addition of acids has been a typical approach for obtaining positively charged colloidal particles [29]; however, the ceramic particles or the electrode could react with the acid and corrode [30,31]. The addition of inorganic cations or organic macromolecules can be utilized for colloidal particle charging [32–35]. In the present work, phosphate ester (PE) was employed as a charging additive for cathodic, electrophoretic deposition because it is an electrostatic stabilizer that positively charges particles in organic liquids by donating protons [36,37].
The zeta potential of the nanoparticles in the EPD solution was measured using a Zetasizer (Nano-ZS, Marvern Instruments) and its software (Dispersion Technology software, DTS). The surface morphology of the deposited sample was observed using scanning electron microscopy (SEM, Hitachi SU-6600). Before the SEM observations, the samples were coated with 10 nm of Pt/Pd, and the nanoparticle composition of the EPD sample was determined using X-ray diffraction (XRD, D8 Advance, Bruker AXS) with CuKa radiation. The optical properties of the electrodes were also characterized using a UV–vis diffuse reflectance spectrophotometer (Shimadzu UV2450), and the band-gap of the EPD samples was estimated using the UV–vis absorption spectra. Furthermore, the open circuit potential (OCP) measurement and the potentiodynamic polarization test of the TiO2–WO3-deposited sample were performed under UV irradiation using a Gamry Reference 600 with a PCI4 Controller. Fig. 1 shows the devised photo-electrochemical cell that was used to evaluate the photo-generated cathodic protection ability of the EPD layers on steel. The coated specimen, saturated calomel electrode (SCE) and graphite rod counter electrode were immersed in 3.5% NaCl solutions. After 3 min of immersion, the sample was illuminated for 1 or 3 h with a 200 W mercury-xenon lamp as an ultraviolet (UV) light source, and then the light was turned off. The changes in the OCP of the samples under UV light on/off conditions were monitored to investigate the cathodic protection performance of the EPD coating layers. The weight fractions of the organic-charging additive and binder material were determined by thermogravimetric analysis (TGA, Mettler-Toledo 851E). In the TGA scan, the deposited materials that were obtained from the EPD layer were heated from 100 °C to 700 °C at a rate of 10 °C·min−1 under a nitrogen atmosphere.
2. Experimental methods
3. Results and discussion
2.1. Electrophoretic deposition
3.1. The colloidal suspension for electrophoretic deposition
TiO2 (Degussa P-25, primary particle size of 25 nm, BET surface area of ca. 50 m2·g−1, and a 8:2 anatase and rutile mixture) and WO3 particles (Ditto Technology Co., Ltd., primary particle size of 90 nm and BET surface area of ca. 10m2·g−1) were purchased for electrophoretic deposition. Iso-propanol (IPA, Sam Chun Chemical, 99.8%) was selected as the dispersion medium, and phosphoric acid di-butyl ester (PADE) was used as the charging additive for the nanoparticles. The charging additive was first added to IPA, and the solution was then mixed for approximately 1 h to obtain a homogeneous charging solution medium. Subsequently, TiO2 and WO3 nanoparticles were slowly added to the IPA solution containing the charging additive, and the suspension of nanoparticles was then ultrasonicated for 1 h to create a uniformly dispersed solution. Before dispersing the solution, the nanoparticles were dried in a convection oven at 60 °C for approximately 24 h. EPD solutions containing different compositions of particles were then prepared: TW 0 (TiO2 30 g·L−1 + WO3 0 g·L−1), TW 20 (TiO2 24 g·L−1 + WO3 6 g·L−1), TW 40 (TiO2 18 g·L−1 + WO3 12 g·L−1), TW 60 (TiO2 12 g·L−1 + WO3 18 g·L−1), TW 80 (TiO2 6 g·L−1 + WO3 24 g·L−1) and TW 100 (TiO2 0g·L−1 +WO3 30g·L−1). In some cases, polyvinyl butyral (PVB, Butvar® B-98, ACROS, USA) was included in the EPD solution as a binder material to improve the adhesion of the nanoparticles with the substrate. AISI Type 304 stainless steel was employed as the working substrate to deposit the nanoparticles and as the counter electrode. The two electrodes were immersed in the prepared EPD solution and connected to a DC power supply (ED Rectifier, DME Tech, Korea). The working and counter electrodes were then used as the cathode and anode, respectively.
Electrophoretic deposition (EPD) of nanoparticles onto a steel substrate was achieved by the electrophoretic motion of a charged particle in a liquid medium under an applied electric field. To deposit
2.2. Characterization
Fig. 1. The devised photo-electrochemical cell for evaluating the photo-generated cathodic protection ability of the EPD layers on stainless steel.
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nanoparticles onto a substrate by electrophoresis, many factors must be considered, e.g., electric field strength, surface area of the electrode, particle mass concentration in the suspension, permittivity, zeta potential and the viscosity of the suspension [38,39]. First, the colloidal suspension must be prepared for EPD with the consideration of these factors. The preparation of a stable colloidal suspension containing charged nanoparticles is the most important step in the EPD process to obtain a high-quality deposited layer on the steel substrate. Water or organic solvents can be employed as the liquid medium of the suspension. In the present work, the organic solvent iso-propyl alcohol was selected for the EPD suspension because water-based EPD could cause Joule heating of the suspension, nonhomogeneity of the deposited film and oxidation of the metallic substrate [40–42]. In addition, a charging additive was incorporated into the EPD suspension to manipulate the surface charge of the TiO2 and WO3 particles. Although the simplest approach for charging a ceramic particle is the addition of strong acids, such as hydrochloric or nitric acids, the use of these acids can corrode the particles or electrodes due to their strong electrochemical reactivities [29–31]. Inorganic cations such as Mg2+, Ca2+ and Al3+ have been widely employed to charge particles in an EPD suspension. However, inorganic cations could contaminate the deposition layer and deteriorate the intrinsic properties of the layer [32–34]. In some reported studies, organic macromolecules were employed as an alternative for these additives [35]. Among these, it was reported that phosphate ether was an effective charging additive for the particles [36,37]. In this manner, the phosphoric acid di-butyl ester (PADE) was exploited as the positive charging material for colloidal TiO2 and WO3 nanoparticles. Fig. 2(a) shows the evolution of the zeta potentials of TiO2 and WO3 nanoparticles as a function of the concentration of PADE. In general, the zeta-potential of the particles was a key parameter that represented the electrophoretic mobility of the particles [43]. The zeta-potential of colloidal TiO2 and WO3 in a pure IPA solution was approximately −10 mV, as shown in Fig. 2(a). In the case of TiO2, the zeta-potential was increased as the content of PADE in the suspension was increased, and the zeta-potential of colloidal TiO2 reached a maximum value of +45 mV at a concentration of 10−3 M. In contrast, the WO3 particles were not positively charged, even with a high concentration of PADE. In other words, in an IPA–PADE suspension system, it was practicable to obtain protonated TiO2 but not WO3. One of the main reasons for these dissimilar charging behaviors might be the difference in the zero point of charge (zpc) of the powder. The ceramic particle is generally positively charged at a pH that is below the zpc, but it is negatively charged at a pH that is above the zpc. The zpc of TiO2 is 6.25, and that of WO3 is 2.5 [44,45]. In the present study, the pH of the IPA–PADE solution ranged from 4.93 to 3.48 depending on the concentration of PADE, which was lower than the zpc of TiO2 and higher than the zpc of WO3. Therefore, the TiO2 particles were positively charged in the
EPD solution, while WO3 particles were not positively charged. Fig. 2(b) shows the deposition mass as a function of the application time at a constant electric field of 30 V. Despite a long-term application, the electrophoretic deposition of WO3 was not successful due to its lower electrophoretic mobility. On the contrary, the deposition mass of TiO2 linearly increased with the applied time of the electric field due to the positive charging of the TiO2 particle. Colloidal suspensions of the various TiO2/WO3 particle compositions were prepared to measure the zeta-potential for studying the codeposition behavior of the suspension of mixed particles through the EPD process. The weight ratio of WO3 in the mixed particles varied from 0% to 100%, and the whole particle concentration was 30 g/L in the solution. The zeta-potential of the solution was decreased when the WO3 contents were increased, as shown in Fig. 3. As explained above, the WO3 particles were barely charged in the IPA–PADE-solution system, and a mixed zeta-potential of the suspension containing positively charged TiO2 was reduced by the incorporation the slightly negatively charged WO3 particle. As mentioned frequently, the electrophoretic mobility, which mainly depended on the zeta-potential of the particle, was a key factor for determining the efficiency of EPD in this system. Hence, the deposited mass of the particle was significantly decreased by increasing the WO3 ratio in the composite suspension under the same operating conditions of EPD, such as applied voltage and time, as shown in Fig. 3(b). 3.2. Application of electrophoretic deposition The EPD process will be compared to a simple dip-coating process before the specific discussion of the EPD behavior of the colloidal suspensions of the various TiO2–WO3 particle compositions. The dipcoating method was selected as a reference method for the EPD process because it is a representative methodology of a non-electrical colloidal deposition route for the formation of ceramic particles. Dip-coating was performed using the isopropyl alcohol solution containing 30 g/L of TiO2 particles and 0.01 M PADE. In the dip-coating process, stainless steel was immersed in the colloidal solution for 5 min and then withdrawn from the solution with a speed of 2 m·min−1. Similarly, an EPD composite layer was also formed on stainless steel using the same colloidal suspension of dip-coating under an applied electrical field. To determine the optimal conditions, EPD was preliminarily conducted under various operating conditions, such as applied voltages and application times. Ceramic particles were deposited onto stainless steel even at low electrical fields of below 5 V; however, the adhesion strength between the layer and steel substrate was not sufficient. Under higher electrical field conditions, cracks appeared in the deposited layer, and the layer was thickened excessively even with a short application time. Based on a series of experiments, the EPD in this study was
Fig. 2. (a) The evolution of the zeta-potential of colloidal TiO2 or WO3 particles as a function of the PADE concentration in an EPD solution. (b) The deposited mass versus deposition time at 30 V for 30 g/L of TiO2 or WO3 particles in suspensions of isopropyl alcohol containing 0.01 M PADE.
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Fig. 3. (a) Zeta-potential of the colloidal particles as a function of the WO3 concentration in the suspension. (b) Deposited mass versus WO3 concentration for 30 g/L of TiO2 or WO3 particles in suspensions of isopropyl alcohol containing 0.01 M PADE.
conducted at a constant voltage of 30 V for 1 min as an optimal condition. Compared with the dip-coating process, the TiO2 layer from EPD adhered more strongly to the steel substrate, as shown in Fig. 4. Fig. 4(a) and (d) presents the results of the cross-cutting and tapping tests for evaluating the adhesion strength of the layer on a substrate (ASTM D 3359). The dip-coated TiO2 layer was easily removed by
tapping, which represents weak adhesion of the coated TiO2 particle. As shown in Fig. 4(b) and (c), it was verified that the TiO2 particles were attached sparsely and weakly during the dip-coating process. However, the TiO2 coating layer from the EPD process was not detached from the substrate, which indicates that the electric field in the EPD process played an important role in the construction of the firmly adherent
Fig. 4. Optical surface image of the dip coating (a) and EPD (d) layers after the cross-cutting and tapping tests, and SEM micrographs of the dip coating layer (b, c) and EPD layer (e, f).
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layer of the nanoparticles. In the EPD process, the charged particle accumulates on the surface of the substrates, removing the solvent by electroendosmosis during the application of the electric field, inducing a flocculation of the particles [26,27]. Moreover, the SEM micrographs of the TiO2 EPD layer, shown in Fig. 4(e) and (f), indicate that the layer was uniform and homogenous and that no visible cracks were present on the surface, despite the thickness of the TiO2 EPD layer being several tens of microns. Consequently, it was clearly demonstrated that the EPD process is a simple and powerful method for creating a nano-structured, ceramic particle layer on steel. Furthermore, the TiO2–WO3 composite layer was formed using the EPD process to evaluate the EPD behavior of a TiO2–WO3 particle colloidal suspension. Fig. 5 shows the optical surface image of the EPD layer from the colloidal suspension of the various TiO2–WO3 particle compositions (0%–100%). The total deposition mass and film thickness were decreased by increasing WO3 in the suspension. In fact, EPD layers were not successfully formed when using suspensions with high WO3 contents, i.e., 80% and 100%, even when they were exposed to a high electric field and after a long application time, because the WO3 particles were not effectively charged in the IPA–PADE solution, as mentioned above. The mixed zeta-potentials of the colloidal suspension of 80% and 100% WO3 were −2.89mV and −9.61mV, respectively. Therefore, the following experimental analysis was conducted on the EPD samples from the colloidal suspensions of 0%, 20%, 40% and 60% WO3. Fig. 6 shows the SEM micrograph of the EPD layers from the colloidal suspensions of various WO3 concentrations. No noticeable homoaggregation of the same types of particles in the TiO2–WO3 codeposited layers was evident. It was verified that the TiO2 and WO3 particles were homogeneously blended in the suspension and uniformly deposited on the stainless steel by electrophoretic co-deposition. In
addition, XRD analysis was performed to confirm the composition ratio of the EPD layers that were formed using the suspensions of various WO3 concentrations, and the corresponding XRD patterns are shown in Fig. 7. The XRD pattern of bare stainless steel shows sharp peaks at 44° and 51° that correspond to the (1 1 1) and (2 0 0) crystal orientations in the austenite phase. After the TiO2 particles were applied on stainless steel, distinct peaks appeared at 25.5° and 48°, which corresponded to the characteristic peaks of the (1 0 1) and (2 0 0) crystal planes of anatase, respectively, and a peak at 27.6° that corresponded to the characteristic peak of the (1 1 0) plane of rutile appeared. The XRD pattern of the EPD layer that was formed from the 20% WO3 suspension showed distinct peaks at 23°, 24°, 28°, 34°, 51°, 54° and 55°, which corresponded to the (0 2 0), (0 0 2), (1 1 2), (1 2 0), (4 2 0), (2 1 1) and (2 0 1) crystal orientations of the orthorhombic phase of WO3. This finding implies that the TiO2 and WO3 particles were successfully co-deposited via the EPD process despite the WO3 particles not being positively charged in the suspension. Under the application of an electric field, it appeared that the electrophoretic motion of the positively charged TiO2 induced the movement of the slightly negatively charged WO3 particle and allowed it to be deposited on the steel. Because the WO3 concentration in the suspension was increased, the distinct peak for WO3 in the XRD pattern was also intensified and represented the increment of WO3 that was deposited in the layer. To confirm the actual WO3 concentration of the co-deposited layer, the composition of the layer was calculated by comparing the XRD peak height with that of the standard sample. The standard powder samples were prepared by mixing exact amounts of the TiO2 and WO3 particles, and the XRD pattern was obtained. The characteristic peaks for the (1 0 1) planes of TiO2 and (1 2 0) planes of WO3 were employed as representative peaks for calculating the composition. According to the
Fig. 5. Optical surface image of the EPD layers that were deposited from suspensions composed of various WO3 concentrations on 304 STS: (a) TW 0, (b) TW 20, (c) TW 40, (d) TW 60, (e) TW 80 and (f) TW 100.
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Fig. 6. SEM micrograph of the EPD layers on 304 STS: (a) TW 0, (b) TW 20, (c) TW 40 and (d) TW 60.
calculation, the WO3 concentrations in the EPD layers for samples TW 20, TW 40 and TW 60 were 19%, 37% and 58%, respectively. Because the WO3 concentrations in the deposited layers were approximately identical to those in the suspension, it can be stated that the TiO2 and WO3 particles in the suspension were deposited simultaneously and at the same rate despite their different zeta-potentials (electrophoretic mobility). Many studies have demonstrated the co-deposition phenomenon of dissimilarly charged particles in the EPD process. If two or more particles from the same suspension are deposited simultaneously, the particles can be deposited at an equal rate if the volume fractions of those particles are comparatively high. On the other hand, the particles are moved individually under the application of an electrical field at low volume fractions [46]. The concentrations of particles in the suspension (30 g/L) in this study were sufficiently high to cause the simultaneous deposition of the TiO2 and WO3 particles. Moreover, in the present work, the weakly negatively charged WO3 particles will be attracted to the strongly positively charged TiO2 particles in the suspension and will be simultaneously deposited on the cathode.
Fig. 7. The variations in the XRD patterns of the EPD layers that were deposited from suspensions of various WO3 concentrations on 304 STS.
Fig. 8(a) presents the UV–vis absorption spectra of the EPD layers with different WO3 concentrations. The UV–vis absorption spectrum of TW 0 exhibited the onset of an absorption peak at 400 nm, which
Fig. 8. (a) The optical absorption spectra and (b) optical absorption edge (ahv)1/2 versus the photon energy (hv) of the EPD layer on stainless steel with different WO3 concentrations.
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was due to the charge transfer process from the valence band to the conduction band in crystalline TiO2. However, the absorption peaks were shifted to the lower energy region of the spectrum after the incorporation of the WO3 particles. In short, the TiO2–WO3 co-deposited samples possessed lower optical absorption edges than the pure TiO2 EPD sample (TW 0), and a higher WO3 concentration resulted in a more obvious redshift of the sample. To estimate the energy band gaps, the graph of the optical absorption edge (ahv)1/2 versus the photon energy (hv) was plotted and is shown in Fig. 8(b). The estimated band gap energy decreased when the WO3 concentration increased from 3.2 eV to 2.8 eV. The intrinsic band gap of TiO2 (anatase) was 3.2 eV, and that of WO3 was 2.8 eV. It is certain that the reduction in the band gap energy of the co-deposited sample originated from the coupling of TiO2–WO3. Through the optical absorption measurement, it was found that the photo-electrochemically active layers were successfully formed via the EPD process, and the TiO2 and WO3 particles were hetero-aggregated in the layer densely enough to exhibit integrated photocatalytic activity. 3.3. Photo-generated cathodic protection of the EPD layer The potentiodynamic polarization curves of stainless steel with or without a TiO2 EPD layer are shown in Fig. 9. The polarization test was performed in a 3.5 wt.%-NaCl solution under light on/off conditions. No significant shift of the polarization curve after light irradiation for bare stainless steel was observed, which indicates that no photo-induced effect with bare stainless steel occurred. In contrast, the polarization curve of EPD stainless steel (TW 0) was drastically shifted downward after light exposure. Light irradiation induced a shift in the photo-potential from +0.05 VSCE to −0.7 VSCE and accompanied a generation of large photo-currents (10 μA). Under light, the TiO2 EPD layer could shift the electrochemical potential of the substrate toward a more negative value than the corrosion potential of steel. In other words, under light irradiation, stainless steel could be cathodically protected from corrosion because the potential of the photo-stimulated EPD layers was more negative than the corrosion potential of stainless steel. The shift in the potential was due to the transferred photo-electron from the TiO2–WO3 layer during light irradiation. In addition, OCP measurements were conducted for an in-depth study of the photo-generated cathodic protection ability of the TiO2–WO3 EPD layers. Fig. 10 shows the OCP changes of the EPD TiO2–WO3 layers on stainless steel in a 3.5 wt.%-NaCl solution under light on/off conditions. As explained above, under light-exposed conditions, the OCP of the EPD samples immediately dropped to a potential in the range of −0.5 VSCE and −0.7 VSCE. However, when the light was off, the OCP rapidly recovered toward its corrosion potential because the EPD layer could no longer
Fig. 10. Effect of the WO3 concentration in the EPD layers on the changes of OCP in a NaCl solution after light irradiation and the potential recovery time to the corrosion potential of stainless steel under dark conditions.
generate the photo-electron. However, the corrosion potential of stainless steel was slightly altered from −0.05 VSCE to −0.15 VSCE before and after light irradiation. The pH change of the solution after light irradiation could be the main reason for the shift in the corrosion potential. Under light, the TiO2–WO3 layer simultaneously generated both photoelectron/hole pairs. Although electrons were mostly injected to the metal, many were consumed by the chemical reaction between water and oxygen in the solution. Similarly, the photo-generated hole was usually consumed by chemical reactions on the particle surface, such as during the oxidization of water [47]. After light irradiation, all of the chemical reactions that modified the environments of the solution and the corrosion potential of the stainless steel in the solution shifted slightly. However, for the pure TiO2–EPD sample (TW 0), the OCP was immediately increased to the potential of steel corrosion after the light was turned off. The potential recovery time to reach the original corrosion potential (−0.15 VSCE) of TW 0 was approximately 1 h from the time that the light was turned off. As the WO3 contents in the EPD layer increased, the potential recovery time was gradually delayed, as shown in Fig. 10. For example, the potential recovery time of TW 20 was approximately 3 h after 1 h of light irradiation due to the electron-storage ability of WO3. Tatsuma et al. demonstrated that WO3 could store photoelectrons when exposed to light and subsequently release the photoelectrons in dark conditions [9,10]. The electron storing mechanism of WO3 is closely related to electrochromism and photochromism. Specifically, the photo-generated electrons migrated from TiO2 to WO3, and MxWO3 (M_H or Na) was formed on the WO3 surfaces by the reaction between photo-electron M+ from the electrolyte and WO3 under light irradiation. In dark conditions, the reverse reaction occurred to release electrons from MxWO3, and the electrons were injected onto the steel substrate. Such reversible, photo-electron-charging and -discharging phenomena originate from the multivalence of W. The partial incorporation of WO3 into the photocatalytic TiO2 layer proved to be an effective method to extend the recovery time of OCP to its corrosion potential. A longer recovery time of potential meant longer cathodic corrosion protection of the EPD layer for stainless steel under dark conditions. Thus, the photo-generated cathodic protection layer that was successfully formed by the electrophoretic co-deposition of crystalline TiO2 and WO3 protected the stainless steel from corrosion in dark conditions due to the electron-storage ability of WO3.
3.4. The effect of binder addition
Fig. 9. The potentiodynamic polarization curve for stainless steel with or without the TiO2 EPD layer in a 3.5 wt.% NaCl solution under light on/off conditions.
Although no delamination of the TiO2–WO3 EPD layer was observed during the anti-corrosion test, the binder material was incorporated into the EPD suspension to improve the adhesion strength of the
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deposited particles on the substrate. Various types of binders can be employed in the EPD process. Among these substances, polyvinyl butyral (PVB) has been widely used for non-aqueous EPD. Particularly, its influence on electrophoretic mobility could be minimized because PVB is a non-ionic polymer. PVB (4.5 g) was added into the 30 g/L (particle/solution) suspension, and the electrophoretic deposition was performed according to the same method as discussed in the previous section. An SEM micrograph of the EPD layers resulting from different WO3 concentrations in the suspension is shown in Fig. 11. A uniform surface was formed, and no noticeable homo-aggregation of the same types of particles was observed in the co-deposited layer. Furthermore, no evidence that PVB addition terminated the stability of the suspension or deteriorated the formation of the uniform surface was obvious. When PVB was included in the solution, the nano-particles appeared to be more densely deposited on the stainless steel than the EPD layer that contained no binder materials, as shown in Fig. 6. Thermogravimetric analysis (TGA) was performed to determine the relative amounts of the organic charging additives (PADE) and the binding material (PVB). The relative contents could be estimated from the weight loss in the TGA scans of the deposited layer, as shown in Fig. 12. In the TGA scan from 100 °C to 700 °C, the weight of pristine TiO2 (P-25) was not reduced; however, the weight of the EPD layer decreased due to the decomposition of the organic materials (PADE or PVB). For the EPD layer containing no PVB, the relative weight loss was 6.3%, based on the TiO2 weight, due to the decomposition of PADE. Because the amount of PADE in the EPD solution was 7 wt.% (2.1 g of PADE/30 g of TiO2), this result indicated that most of the charging additive was simultaneously deposited with the TiO2 particles on the substrate via the EPD process. For the EPD layer containing PVB binder, the total weight loss in the TGA scan of up to 700 °C was 8.5%. This weight loss was due to PADE and PVB decomposition. Although 15 wt.% of the PVB was incorporated into the EPD solution, it appeared as if considerably less of the PVB was co-deposited on the substrate with the TiO2 particles. It also appeared as if the poor co-deposition efficiency of the PVB originated from its non-ionic character in the IPA solution. Although a smaller amount of PVB was included in the EPD layer, the adhesion of the particles to the substrate was improved by the addition of the binder. An evaluation of adhesion using the ASTM D 3359 method (i.e., cross-cut adhesion) showed that the detachment area of
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Fig. 12. Thermogravimetric analysis (TGA) data of pristine TiO2 particles and EPD layer containing TiO2 particles, charging additive (PADE) or binding material (PVB).
the EPD sample that included the PVB (ASTM classes 4B) was decreased compared to that of the normal EPD sample (ASTM classes 3B). In addition, a tendency for the adhesion to decrease when the WO3 concentration in the EPD layer was increased was evident. As explained above, the WO3 particle was not effectively charged in the EPD system; thus, it would not be attached as strongly as TiO2 to the substrate. Although the TiO2 and WO3 particles were deposited on the substrate at the same rate, the adhesion of the resultant layers involving WO3 was deteriorated due to the lack of charging ions on the WO3 particle. Fig. 13 shows the OCP changes of the EPD layer that included PVB on stainless steel in a 3.5 wt.% NaCl solution under light on/off conditions. The transition behavior of the OCP was roughly similar between the sample with and without the PVB binder, but a small difference was observed. For the sample with the PVB, the OCP dropped sharply to a potential of −0.7 VSCE immediately after the light was turned on, regardless of the WO3 concentration. However, when the binder was not included in the layer, the OCP values when under light exposure were somewhat different. In this case, they reached the potential t in a range of −0.5 VSCE to −0.7 VSCE under light, and the width of the OCP drop was lessened with increased WO3 concentrations. Moreover, the OCP was not maintained at the potential of photo-stimulated TiO2
Fig. 11. SEM micrograph of EPD layers that included PVB binder on 304 STS; (a) TW 0, (b) TW 20, (c) TW 40 and (d) TW 60.
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TW 0 and TW 60 were observed. Namely, the concentration of WO3 did not significantly influence the photo-effect of the EPD layer. However, the potentiodynamic polarization curves for TW 0 and TW 60 were considerably different when they were carried out under dark conditions after 1 h of light irradiation. The potential of TW 60 (−0.35 VSCE) was lower than that of TW 0 (−0.2 VSCE). As mentioned above, this finding indicates that the longer, photo-generated cathodic protection of the EPD layer that originates from the electron-storage ability of WO3 occurs under dark conditions. 4. Conclusion
Fig. 13. The effect of PVB addition into an EPD layer on its photo-generated, anti-corrosion ability, and the effect of WO3 concentrations on the OCP changes in a NaCl solution and the potential recovery time under light on/off conditions.
(~ −0.7 VSCE), in spite of continuous light irradiation, as shown in Fig. 10, which means that the potential of the injected, photogenerated electron from TiO2 was lowered by certain impedance. A possible explanation was that the photo-electron could be impeded by the delamination of the EPD layer from the substrate by the oxygen evolution reaction of the transferred electron at the interface. However, the addition of binder enhanced the adhesion strength between the metal and the layer that hindered the delamination and improved the cathodic protection performance of the samples with high concentrations of WO3. Even at high concentrations of WO3, the OCP of the PVB-added sample was consistently maintained at −0.7 VSCE, which was the potential of photo-simulated TiO2, during the entire period of the light irradiation due to its enhanced adhesion. Furthermore, the time of potential recovery after the light was turned off was delayed significantly as more WO3 particles were incorporated into the EPD layer due to the electron-storing mechanism of WO3 in water. The potential recovery time of TW 60 was approximately 6 h after 3 h of light irradiation, which implied that the steel was cathodically protected for 6 h by the photo-cathodic TiO2–WO3 EPD layer in dark conditions. In addition, Fig. 14 shows the potentiodynamic polarization curves for TW 0 and TW 60 under light irradiation or dark following light irradiation. In detail, the potentiodynamic polarization tests were performed under light irradiation for 1 h in a 3.5%-NaCl solution, and subsequent polarization tests were then performed under dark conditions 1.5 h after the light was turned off. Under light irradiation, no substantial differences between the photo-potentials and photo-currents of
Electrophoretic mobility was a key factor for the efficiency of EPD, and it mainly depended on the zeta-potential of the particle. The zetapotential of the TiO2/WO3 particle was manipulated by the positive charging additive, PADE, in the IPA solution. The TiO2 particle was effectively charged in the IPA–PADE solution, but the WO3 particle was not positively charged due to its lower zpc than pH of the solution. Thus, the mixed zeta-potential of the suspension containing positively charged TiO2 was shifted negatively by the incorporation the slightly negatively charged WO3 particle into the solution. Hence, the deposited mass of the particle was significantly decreased when the WO3 ratio in the composite suspension was increased while maintaining the same operating conditions for EPD. In any case, a TiO2–WO3 composite layer was successfully formed on stainless steel via the EPD process. When compared to the dip-coating process, the EPD layer adhered more strongly to the steel substrate by the action of particle accumulation and electroendosmosis during the application of the electric field. Moreover, a thick EPD layer was uniformly deposited on the stainless steel, and no noticeable homo-aggregation of the particles was observed. In the EPD co-deposition process, TiO2 and WO3 particles were deposited simultaneously at the same rate despite their different zeta-potentials in the suspension. The particle concentration in the suspension in the present work was sufficiently high to cause the deposition of TiO2 and WO3 particles at an equal rate, and the OCP results confirmed the photo-generated cathodic protection ability of the TiO2–WO3 EPD layers. Under UV light irradiation, stainless steel was cathodically protected from corrosion because the potential of the photo-stimulated EPD layers was more negative than the corrosion potential of the stainless steel. As the WO3 contents in the EPD layer increased, the time of potential recovery was gradually delayed. Moreover, the addition of a binder enhanced the adhesion strength between the metal substrate and the deposited layer that hindered delamination and improved the cathodic protection performance of samples with high concentrations of WO3. The potential recovery time of the EPD sample containing 40% TiO2–60% WO3 was approximately 6 h after 3 h of light irradiation, which implied that the steel was cathodically protected for 6 h in dark conditions. The partial incorporation of WO3 into the photocatalytic TiO2 layer was an effective method to delay the OCP recovery time to its corrosion potential, and the longer recovery time of the potential meant that longer cathodic corrosion protection of the stainless steel by the EPD layer in dark conditions occurred. Acknowledgment Financial support from POSCO is gratefully appreciated. References
Fig. 14. Potentiodynamic polarization curve for TiO2 EPD (TW 0) and TiO2–WO3 EPD layers (TW 60) under light-on conditions and under dark conditions after light irradiation.
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