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Formation of titania composite coatings on carbon steel by plasma electrolytic oxidation
Applied Surface Science 256 (2010) 5818–5823
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Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Formation of titania composite coatings on carbon steel by plasma electrolytic oxidation Yunlong Wang a , Zhaohua Jiang a,b,∗ , Zhongping Yao a a b
School of Chemical Engineering and Technology, Harbin Institute of Technology, Harbin 150001, China State key Laboratory of Urban Water Resource and Environment, Harbin Institute of Technology, Harbin 150090, China
a r t i c l e
i n f o
Article history: Received 4 October 2009 Received in revised form 4 March 2010 Accepted 6 March 2010 Available online 15 March 2010 Keywords: Plasma electrolytic oxidation Titania Steel Corrosion resistance
a b s t r a c t Titania composite coatings were prepared on carbon steel by plasma electrolytic oxidation in silicate electrolyte and aluminate electrolyte with titania powers doping in the electrolytes. The microstructure of the coatings was characterized by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD). The properties of the coatings including bond strength, thickness, thermal shock resistance and corrosion resistance varying with the quantities of titania powers in the electrolytes were studied. Investigation results revealed that the coating obtained in silicate electrolyte was composed of anatase-TiO2 , rutile-TiO2 crystal phases and some Fe, Si, P elements; coating obtained in aluminate electrolyte consisted of anatase-TiO2 , Al2 TiO5 and some Fe, P elements. Coatings obtained in two types of electrolytes show porous and rough surface. With increasing the concentration of titania powers in the electrolytes, the coating surface first became more compact and less porous and then became more porous and coarse. The bond strength and thickness were not strongly affected by concentration of titania powers in electrolytes. The valves were 23 MPa and for 66 m for coatings obtained in aluminate electrolyte, and 21 MPa and 35 m for coatings obtained in silicate electrolyte. Coatings obtained in silicate electrolyte showed a little better thermal shock resistance than those obtained in aluminate electrolyte and the best coatings were obtained with middle concentration of titania powers in the electrolytes. All coated samples showed better corrosion resistance than the substrate in 3.5 wt% NaCl solution. The best coatings were also obtained with middle concentration of titania powers doping in both electrolytes whose corrosion current density was decreased by 2 orders of magnitude compared with the substrate. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Titania and titania based composite coatings on metal surface have always been a research focus for their versatile applications. They can be used as functional materials such as catalyzing, pollutant degradation, water-purifications, solar cells, biomedical materials and protective materials such as protective layers on metals surface to improve the wear and corrosion resistance [1–5]. There are many methods to prepare titania coatings on metals such as sputtering [6], spray [7], chemical or physical vapor deposition [8] and sol–gel [9], etc. Among these methods, the rapidly developed technique called plasma electrolytic oxidation (PEO) [10] was thought to be one of the most promising surface modification techniques of metals, by which titania coatings have
∗ Corresponding author at: School of Chemical Engineering and Technology, Harbin Institute of Technology, Xi Da Zhi Street 92#, Harbin 150001, China. Tel.: +86 451 86413707; fax: +86 451 86413707. E-mail address: [email protected] (Z. Jiang). 0169-4332/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2010.03.038
been obtained on many metal substrates including Mg, Ti, etc. [11,12]. TiO2 coatings by PEO have exhibited promising application prospect in many aspects. As a plasma-enhanced liquid deposition technique, PEO is very convenient for doping by adjusting the composition of the electrolytes, which can change the microstructure and characteristic of the coatings [13]. In addition, PEO is an easy operating technique with relatively high efficiency [10,14]. However, it should be pointed out that titania coating by PEO on steel has never been reported and this is thought to be very difficult. This paper reports the achievement of preparing of titania composite coatings on steel by PEO in aluminate and silicate electrolyte containing titania powers, respectively. Two types of titania composite coatings namely anatase-TiO2 –rutile-TiO2 coating and anatase-TiO2 —Al2 TiO5 were obtained. The microstructure of the coatings was studied. The properties of the coatings including bond strength, thickness, thermal shock resistance and corrosion resistance varying with the quantities of titania powers in the electrolytes were studied. Studies in this paper offer a new approach to achieve titania coating on steel, which may supply a new route to
Y. Wang et al. / Applied Surface Science 256 (2010) 5818–5823
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Fig. 1. Average current density and average anode voltage varying with treating time in PEO. Fig. 2. XRD spectra of PEO coatings.
the immobilization of titania coating and enlarge the application of both the substrate and the coating itself. 2. Experimental details 2.1. Preparation of coating Q235 carbon steels were processed into the dimension of 20 mm × 20 mm × 1.8 mm and served as anode in PEO. Before deposition, the specimens were first polished and washed thoroughly with distilled water. The aluminate electrolyte was an aqueous solution containing some NaAlO2, some NaH2 PO4, and some TiO2 powders. The silicate electrolyte was also an aqueous solution made up of some Na2 SiO3 , some NaH2 PO2 and some TiO2 . During PEO, the pulse frequency was kept constant, namely 2000 Hz, the average current density and average anode voltage varying accordingly. The curves of average current density and average anode voltage varying with treating time are shown in Fig. 1.
trolyte is composed of anatase-TiO2 and Al2 TiO5 , while two types of TiO2 , namely anatase and rutile are found in coating obtained in silicate electrolyte. Compared with our previous studies [15], it can be found that adding some TiO2 powers in the electrolyte has changed the main phase composition of the coating. EDS patterns of the coating surface are shown in Fig. 3. It can be seen that coating obtained in aluminate is mainly composed of Al, Fe, O, Ti and P elements, and that coating obtained in silicate is
2.2. Microstructure characterization and properties test of coatings X-ray diffraction (XRD) was used to detect the phase composition of the coating and EDS was employed to analyze the elements’ composition of the coating. The surface and cross-sectional morphologies of the ceramic coating were observed by scanning electron microscopy (SEM). The thickness of coating is observed from the SEM photo. The bonding strength of PEO coatings was measured by pulloff tests. The thermal shock resistance of the coating was evaluated by the “heat-cool cycle” tests. Potentiodynamic polarization curves were used to evaluate the corrosion resistance of the coated and uncoated samples, which were performed on Solartron 1287 potentiostat through a conventional three-electrode system (saturated calomel electrode, namely SCE as the reference, a Pt foil electrode as the auxiliary electrode and samples as the working electrode) in 3.5 wt% NaCl solution. The scan rate was 0.167 mV/s. 3. Results and discussion 3.1. Microstructure of the coatings 3.1.1. Phase composition and elemental composition Fig. 2 shows the XRD spectra of PEO coatings obtained in aluminate electrolyte (a) and silicate electrolyte (b), respectively. It can be seen from Fig. 2 that coating obtained in aluminate elec-
Fig. 3. EDS spectra of PEO coatings. (a) Coating obtained in aluminate electrolyte; (b) coating obtained in silicate electrolyte
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Y. Wang et al. / Applied Surface Science 256 (2010) 5818–5823
Fig. 4. Surface morphologies of PEO coatings obtained with different concentrations of titania powers in the electrolytes. (a–d) Morphologies of coating obtained in aluminate electrolyte; (e–h) morphologies of coating obtained silicate in electrolyte.
composed of Si, Fe, O, Ti and P. So in addition to the crystal phases, there might be a little noncrystal phase containing these elements in the coating. Both coatings obtained in aluminate electrolyte and silicate electrolyte are titania composite coatings.
3.1.2. Surface morphologies Fig. 4 shows the surface morphologies of PEO coatings obtained with different concentrations of titania powers in aluminate (a–d: 2, 4, 8, 16 g/L) and silicate electrolytes (e–h: 2, 4, 8, 16 g/L). It can be seen that coatings obtained in both types of electrolytes are porous and coarse. There are some micropores distributed on the whole coating surface. The morphology also changes with increase – the concentration of titania in the electrolyte. The number of pores on the coating surface decreased when the concentration of titania was first increased, and the surface becomes more compact. But when
further increase – 1 the concentration of titania, the coating surface becomes coarse again, and some big grains appear on the surface. The result here agrees with that in literature that adding appropriate concentration of solid powers in electrolyte could enhance the density of the coating [16,17]. Comparing the two types of coatings, those obtained in silicate electrolyte are a little looser than those obtained in aluminate electrolyte.
3.2. Properties of the coatings 3.2.1. Bond strength The bond strength of PEO coatings obtained in electrolytes with different concentrations of titania powers is shown in Fig. 5, which clearly indicates that bond strength of PEO coating obtained in silicate electrolyte system approximately is 21 MPa and that of
Y. Wang et al. / Applied Surface Science 256 (2010) 5818–5823
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Fig. 5. Bond strength of PEO coatings.
Fig. 7. variation of thickness of PEO coatings with concentration of titania powers in the electrolytes. (a) Coating obtained in aluminate electrolyte; (b) coating obtained in silicate electrolyte.
the coating obtained in aluminate electrolyte is 23 MPa. The bond strength is not strongly affected by the concentration of titania in electrolytes. The bond strength of PEO coating on valve metals such as Al, Ti is reported to be in large range and the bond strength of PEO coating on steel is rarely reported. Compared to the bond strength of TiO2 coatings on steel by other techniques, the value obtained here is relatively high. This is attributed to PEO technique itself. When the composition of PEO coating contains species from both the substrate and electrolyte, the coating is usually “in-situ” grown on the substrate. It is not a coating just simply covering the surface. During the growth of the coating, the substrate is also consumed, making the interface between the coatings and the substrates interdigitate which is helpful for the bond strength of the coating. The
Table 1 Results of thermal shock tests of PEO coatings. Concentration of titania (g/L)
Fig. 6. Cross-sectional photos of PEO coatings. (a) Coating obtained in aluminate electrolyte; (b) coating obtained in silicate electrolyte.
2 4 8 16
No. of shock cycles Silicate electrolyte
Aluminate electrolyte
9 9 10 9
7 8 8 6
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Y. Wang et al. / Applied Surface Science 256 (2010) 5818–5823 Table 2 Corrosion potential and corrosion current density of the coated and uncoated samples. Sample
Ecorr (v)
icorr (A/cm2 )
Uncoated Aluminate Silicate
−0.638 −0.478 −0.523
3.722 × 10−5 2.236 × 10−7 7.325 × 10−7
potential and low corrosion current density exhibit a low corrosion rate, namely good corrosion resistance. Fig. 8 reveals that all titania coated samples exhibit high corrosion potential and low corrosion current density compared to the uncoated samples, revealing a low corrosion rate. When increasing the concentration of TiO2 power in electrolytes, the corrosion resistance first increased and then decreased. For both electrolytes, coating with best corrosion resistance is obtained with 8 g/L TiO2 power in electrolytes. The corrosion potential (Ecorr ) and corrosion current density icorr (A/cm2 ) of the best coating and the substrate are listed in Table 2. For coating obtained in aluminate electrolyte, the best one showed a potential shift of 0.160 V to the noble direction and for coating obtained in silicate electrolyte, a shift of 0.115 V to the same direction was presented. The corrosion current density of the best coatings obtained in both electrolytes was decreased by 2 orders of magnitude compared with the uncoated one, showing a considerable improvement of corrosion resistance. Improvement of corrosion resistance of Q235 substrate is due to the formation of TiO2 composition coatings on the surface. According to studies of PEO coating in literatures [10], although the coating is porous, this porous structure is only the surface structure, namely the pores are just on the surface. So the coating can act as a barrier between the substrate and the corrosive electrolyte. 4. Conclusion Fig. 8. Polarizing curves of substrate and coated samples. (a) Substrate and coating obtained in aluminate electrolyte; (b) substrate and coating obtained in silicate electrolyte.
typical interface of the coating and the substrate can be seen from the cross-sectional photos in Fig. 6. 3.2.2. Thickness Fig. 7 represents the variation of thickness of PEO coatings with concentration of titania powers in the electrolytes. Fig. 7 reveals that the thickness of both coatings changes little with different concentrations of titania powers in the electrolytes. The thickness of coating obtained in aluminate electrolyte is about 66 m and the value of coating obtained in silicate electrolyte is about 35 m. 3.2.3. Thermal shock resistance Coated samples were heated to 300 ◦ C in high temperature furnace and kept for 2 min, then quickly taken out and immersed in cool water. The test was repeated until the ceramic coating was destroyed. Five samples were used for each type of coating. The test results of PEO coatings obtained in both electrolytes are shown in Table 1. It can be seen that coatings obtained in silicate electrolyte exhibit a little better thermal shock resistance. Coatings forming in electrolytes with middle concentration also showed a relatively good thermal shock resistance. 3.2.4. Corrosion resistance Potentiodynamic polarizing curves of titania coated and uncoated samples in 3.5 wt% NaCl solution are plotted in Fig. 8. According to electrochemical theory, samples with high corrosion
(1) Two types of titania composite coatings were prepared on carbon steel by PEO in silicate and aluminate electrolytes, respectively. Coating obtained in silicate electrolyte was composed of anatase-TiO2 , rutile-TiO2 crystal phase and Fe, P, Si elements; Coating obtained in aluminate electrolyte consisted of anatase-TiO2 and Al2 TiO5 , Fe, P is also found in the coating. Coating obtained in silicate electrolyte was about 35 m thick; coating obtained in silicate electrolyte was approximately 66 m thick. (2) Coatings obtained in two types of electrolytes showed porous and coarse surface. The bond strength was not strongly affected by concentration of titania powers in electrolytes. The valves were 23 MPa for coatings obtained in aluminate electrolyte, and 21 MPa for coatings obtained in silicate electrolyte. Coatings obtained in silicate electrolyte showed a little better thermal shock resistance than those obtained in aluminate electrolyte and the best coatings were obtained with middle concentration titania powers in the electrolytes. (3) Both coatings obtained in silicate electrolyte and aluminate electrolyte showed better corrosion resistance than uncoated sample in 3.5 wt% NaCl solution. For two types of electrolytes, coatings obtained with 8 g/L TiO2 power in electrolytes exhibited the best corrosion resistance, whose corrosion rate was decreased by 2 orders of magnitude compared with the uncoated one. References [1] A. Alem, H. Sarpoolaky, Mehrdad Keshmiri, Sol–gel preparation of titania multilayer membrane for photocatalytic applications, Ceram. Int. 5 (2009) 1837–1843.
Y. Wang et al. / Applied Surface Science 256 (2010) 5818–5823 [2] C.X. Shan, X.H.K.L. Choy, P. Choquet, Improvement in corrosion resistance of CrN coated stainless steel by conformal TiO2 deposition, Surf. Coat. Technol. 10 (2008) 2147–2151. [3] J. Huang, T. Shinohara, S. Tsujikawa, Protection of carbon steel from atmospheric corrosion by TiO2 coating, Zario-to -kankyo 48 (1999) 75–582. [4] J. Huang, T. Shinohara, S. Tsujikawa, Effects of interfacial iron oxides on corrosion protection of carbon steel by TiO2 coating under illumination, Zario-to -kankyo 46 (1997) 651–661. [5] Y. Ohko, S. Saitoh, T. Tatsuma, A. Fujishima, Photoelectrochemica anticorrosion and self-cleaning effects of a TiO2 coating for type 304 stainless steel, J. Eelectrochem. Soc. 1 (2001) B24–B28. [6] C. He, X.Z. Li, N. Graham, Y. Wang, Preparation of TiO2 /ITO and TiO2 /Ti photoelectrodes by magnetron sputtering for photocatalytic application, Appl. Catal. A: Gen. 1 (2006) 54–63. [7] H.P. Deshmukh, P.S. Shinde, P.S. Patil, Structural, optical and electrical characterization of spray-deposited TiO2 thin films, Mater. Sci. Eng. B 1–3 (2006) 220–227. [8] D.J. Byun, Y.K. Jin, B. Kim, J.K. Lee, D. Park, Photocatalytic TiO2 deposition by chemical vapor deposition, J. Hazard. Mater. 2-3 (2000) 199–206. [9] G.X. Shen, Y.C. Chen, C.J. Lin, Corrosion protection of 316 L stainless steel by a TiO2 nanoparticle coating prepared by sol–gel method, Thin Solid Films 1–2 (2005) 130–136.
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[10] A.L. Yerokhin, X. Nie, A. Leyland, Plasma electrolysis for surface engineering, Surf. Coat. Technol. 122 (1999) 73–93. [11] L. Zhang, T. Kanki, N. Sano, A. Toyoda, Development of TiO2 photocatalyst reaction for water purification, Sep. Purif. Technol. 1 (2003) 105–110. [12] J. Liang, L.T. Hu, J.C. Hao, Preparation and characterization of oxide films containing crystalline TiO2 on magnesium alloy by plasma electrolytic oxidation, Electrochim. Acta 14 (2007) 4836–4840. [13] W.B. Xue, C. Wang, Z.W. Deng, R.Y. Chen, T.H. Zhang, Influence of electrolytes on the microstructure of microarc oxidation coatings on Ti–6Al–4V alloy, Mater. Sci. Technol. 18 (2002) 37–39. [14] S.G. Xin, X.T. Sun, X.H. Wu, Electrochemical impedance spectroscopy of ceramic coatings on Ti–6Al–4V by micro-plasma oxidation, Electrochim. Acta 50 (2005) 3273–3279. [15] Y.L. Wang, Z.H. Jiang, Z.P. Yao, Preparation and properties of ceramic coating on Q235 carbon steel by plasma electrolytic oxidation, Curr. Appl. Phys. 5 (2009) 1067–1071. [16] H. Tong, F. Jin, L. Shen, Fabrication of porous ceramic coatings embedded nanograins on titanium alloys, J. Korean Phys. Soc. 46 (2005) S134– S136. [17] F. Jin, P.K. Chu, H.H. Tong, J. Zhao, Improvement of surface porosity and properties of alumina films by incorporation of Fe micrograins in micro-arc oxidation, Appl. Surf. Sci. 253 (2006) 863–868.
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Formation of titania composite coatings on carbon steel by plasma electrolytic oxidation Yunlong Wang a , Zhaohua Jiang a,b,∗ , Zhongping Yao a a b
School of Chemical Engineering and Technology, Harbin Institute of Technology, Harbin 150001, China State key Laboratory of Urban Water Resource and Environment, Harbin Institute of Technology, Harbin 150090, China
a r t i c l e
i n f o
Article history: Received 4 October 2009 Received in revised form 4 March 2010 Accepted 6 March 2010 Available online 15 March 2010 Keywords: Plasma electrolytic oxidation Titania Steel Corrosion resistance
a b s t r a c t Titania composite coatings were prepared on carbon steel by plasma electrolytic oxidation in silicate electrolyte and aluminate electrolyte with titania powers doping in the electrolytes. The microstructure of the coatings was characterized by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD). The properties of the coatings including bond strength, thickness, thermal shock resistance and corrosion resistance varying with the quantities of titania powers in the electrolytes were studied. Investigation results revealed that the coating obtained in silicate electrolyte was composed of anatase-TiO2 , rutile-TiO2 crystal phases and some Fe, Si, P elements; coating obtained in aluminate electrolyte consisted of anatase-TiO2 , Al2 TiO5 and some Fe, P elements. Coatings obtained in two types of electrolytes show porous and rough surface. With increasing the concentration of titania powers in the electrolytes, the coating surface first became more compact and less porous and then became more porous and coarse. The bond strength and thickness were not strongly affected by concentration of titania powers in electrolytes. The valves were 23 MPa and for 66 m for coatings obtained in aluminate electrolyte, and 21 MPa and 35 m for coatings obtained in silicate electrolyte. Coatings obtained in silicate electrolyte showed a little better thermal shock resistance than those obtained in aluminate electrolyte and the best coatings were obtained with middle concentration of titania powers in the electrolytes. All coated samples showed better corrosion resistance than the substrate in 3.5 wt% NaCl solution. The best coatings were also obtained with middle concentration of titania powers doping in both electrolytes whose corrosion current density was decreased by 2 orders of magnitude compared with the substrate. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Titania and titania based composite coatings on metal surface have always been a research focus for their versatile applications. They can be used as functional materials such as catalyzing, pollutant degradation, water-purifications, solar cells, biomedical materials and protective materials such as protective layers on metals surface to improve the wear and corrosion resistance [1–5]. There are many methods to prepare titania coatings on metals such as sputtering [6], spray [7], chemical or physical vapor deposition [8] and sol–gel [9], etc. Among these methods, the rapidly developed technique called plasma electrolytic oxidation (PEO) [10] was thought to be one of the most promising surface modification techniques of metals, by which titania coatings have
∗ Corresponding author at: School of Chemical Engineering and Technology, Harbin Institute of Technology, Xi Da Zhi Street 92#, Harbin 150001, China. Tel.: +86 451 86413707; fax: +86 451 86413707. E-mail address: [email protected] (Z. Jiang). 0169-4332/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2010.03.038
been obtained on many metal substrates including Mg, Ti, etc. [11,12]. TiO2 coatings by PEO have exhibited promising application prospect in many aspects. As a plasma-enhanced liquid deposition technique, PEO is very convenient for doping by adjusting the composition of the electrolytes, which can change the microstructure and characteristic of the coatings [13]. In addition, PEO is an easy operating technique with relatively high efficiency [10,14]. However, it should be pointed out that titania coating by PEO on steel has never been reported and this is thought to be very difficult. This paper reports the achievement of preparing of titania composite coatings on steel by PEO in aluminate and silicate electrolyte containing titania powers, respectively. Two types of titania composite coatings namely anatase-TiO2 –rutile-TiO2 coating and anatase-TiO2 —Al2 TiO5 were obtained. The microstructure of the coatings was studied. The properties of the coatings including bond strength, thickness, thermal shock resistance and corrosion resistance varying with the quantities of titania powers in the electrolytes were studied. Studies in this paper offer a new approach to achieve titania coating on steel, which may supply a new route to
Y. Wang et al. / Applied Surface Science 256 (2010) 5818–5823
5819
Fig. 1. Average current density and average anode voltage varying with treating time in PEO. Fig. 2. XRD spectra of PEO coatings.
the immobilization of titania coating and enlarge the application of both the substrate and the coating itself. 2. Experimental details 2.1. Preparation of coating Q235 carbon steels were processed into the dimension of 20 mm × 20 mm × 1.8 mm and served as anode in PEO. Before deposition, the specimens were first polished and washed thoroughly with distilled water. The aluminate electrolyte was an aqueous solution containing some NaAlO2, some NaH2 PO4, and some TiO2 powders. The silicate electrolyte was also an aqueous solution made up of some Na2 SiO3 , some NaH2 PO2 and some TiO2 . During PEO, the pulse frequency was kept constant, namely 2000 Hz, the average current density and average anode voltage varying accordingly. The curves of average current density and average anode voltage varying with treating time are shown in Fig. 1.
trolyte is composed of anatase-TiO2 and Al2 TiO5 , while two types of TiO2 , namely anatase and rutile are found in coating obtained in silicate electrolyte. Compared with our previous studies [15], it can be found that adding some TiO2 powers in the electrolyte has changed the main phase composition of the coating. EDS patterns of the coating surface are shown in Fig. 3. It can be seen that coating obtained in aluminate is mainly composed of Al, Fe, O, Ti and P elements, and that coating obtained in silicate is
2.2. Microstructure characterization and properties test of coatings X-ray diffraction (XRD) was used to detect the phase composition of the coating and EDS was employed to analyze the elements’ composition of the coating. The surface and cross-sectional morphologies of the ceramic coating were observed by scanning electron microscopy (SEM). The thickness of coating is observed from the SEM photo. The bonding strength of PEO coatings was measured by pulloff tests. The thermal shock resistance of the coating was evaluated by the “heat-cool cycle” tests. Potentiodynamic polarization curves were used to evaluate the corrosion resistance of the coated and uncoated samples, which were performed on Solartron 1287 potentiostat through a conventional three-electrode system (saturated calomel electrode, namely SCE as the reference, a Pt foil electrode as the auxiliary electrode and samples as the working electrode) in 3.5 wt% NaCl solution. The scan rate was 0.167 mV/s. 3. Results and discussion 3.1. Microstructure of the coatings 3.1.1. Phase composition and elemental composition Fig. 2 shows the XRD spectra of PEO coatings obtained in aluminate electrolyte (a) and silicate electrolyte (b), respectively. It can be seen from Fig. 2 that coating obtained in aluminate elec-
Fig. 3. EDS spectra of PEO coatings. (a) Coating obtained in aluminate electrolyte; (b) coating obtained in silicate electrolyte
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Y. Wang et al. / Applied Surface Science 256 (2010) 5818–5823
Fig. 4. Surface morphologies of PEO coatings obtained with different concentrations of titania powers in the electrolytes. (a–d) Morphologies of coating obtained in aluminate electrolyte; (e–h) morphologies of coating obtained silicate in electrolyte.
composed of Si, Fe, O, Ti and P. So in addition to the crystal phases, there might be a little noncrystal phase containing these elements in the coating. Both coatings obtained in aluminate electrolyte and silicate electrolyte are titania composite coatings.
3.1.2. Surface morphologies Fig. 4 shows the surface morphologies of PEO coatings obtained with different concentrations of titania powers in aluminate (a–d: 2, 4, 8, 16 g/L) and silicate electrolytes (e–h: 2, 4, 8, 16 g/L). It can be seen that coatings obtained in both types of electrolytes are porous and coarse. There are some micropores distributed on the whole coating surface. The morphology also changes with increase – the concentration of titania in the electrolyte. The number of pores on the coating surface decreased when the concentration of titania was first increased, and the surface becomes more compact. But when
further increase – 1 the concentration of titania, the coating surface becomes coarse again, and some big grains appear on the surface. The result here agrees with that in literature that adding appropriate concentration of solid powers in electrolyte could enhance the density of the coating [16,17]. Comparing the two types of coatings, those obtained in silicate electrolyte are a little looser than those obtained in aluminate electrolyte.
3.2. Properties of the coatings 3.2.1. Bond strength The bond strength of PEO coatings obtained in electrolytes with different concentrations of titania powers is shown in Fig. 5, which clearly indicates that bond strength of PEO coating obtained in silicate electrolyte system approximately is 21 MPa and that of
Y. Wang et al. / Applied Surface Science 256 (2010) 5818–5823
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Fig. 5. Bond strength of PEO coatings.
Fig. 7. variation of thickness of PEO coatings with concentration of titania powers in the electrolytes. (a) Coating obtained in aluminate electrolyte; (b) coating obtained in silicate electrolyte.
the coating obtained in aluminate electrolyte is 23 MPa. The bond strength is not strongly affected by the concentration of titania in electrolytes. The bond strength of PEO coating on valve metals such as Al, Ti is reported to be in large range and the bond strength of PEO coating on steel is rarely reported. Compared to the bond strength of TiO2 coatings on steel by other techniques, the value obtained here is relatively high. This is attributed to PEO technique itself. When the composition of PEO coating contains species from both the substrate and electrolyte, the coating is usually “in-situ” grown on the substrate. It is not a coating just simply covering the surface. During the growth of the coating, the substrate is also consumed, making the interface between the coatings and the substrates interdigitate which is helpful for the bond strength of the coating. The
Table 1 Results of thermal shock tests of PEO coatings. Concentration of titania (g/L)
Fig. 6. Cross-sectional photos of PEO coatings. (a) Coating obtained in aluminate electrolyte; (b) coating obtained in silicate electrolyte.
2 4 8 16
No. of shock cycles Silicate electrolyte
Aluminate electrolyte
9 9 10 9
7 8 8 6
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Y. Wang et al. / Applied Surface Science 256 (2010) 5818–5823 Table 2 Corrosion potential and corrosion current density of the coated and uncoated samples. Sample
Ecorr (v)
icorr (A/cm2 )
Uncoated Aluminate Silicate
−0.638 −0.478 −0.523
3.722 × 10−5 2.236 × 10−7 7.325 × 10−7
potential and low corrosion current density exhibit a low corrosion rate, namely good corrosion resistance. Fig. 8 reveals that all titania coated samples exhibit high corrosion potential and low corrosion current density compared to the uncoated samples, revealing a low corrosion rate. When increasing the concentration of TiO2 power in electrolytes, the corrosion resistance first increased and then decreased. For both electrolytes, coating with best corrosion resistance is obtained with 8 g/L TiO2 power in electrolytes. The corrosion potential (Ecorr ) and corrosion current density icorr (A/cm2 ) of the best coating and the substrate are listed in Table 2. For coating obtained in aluminate electrolyte, the best one showed a potential shift of 0.160 V to the noble direction and for coating obtained in silicate electrolyte, a shift of 0.115 V to the same direction was presented. The corrosion current density of the best coatings obtained in both electrolytes was decreased by 2 orders of magnitude compared with the uncoated one, showing a considerable improvement of corrosion resistance. Improvement of corrosion resistance of Q235 substrate is due to the formation of TiO2 composition coatings on the surface. According to studies of PEO coating in literatures [10], although the coating is porous, this porous structure is only the surface structure, namely the pores are just on the surface. So the coating can act as a barrier between the substrate and the corrosive electrolyte. 4. Conclusion Fig. 8. Polarizing curves of substrate and coated samples. (a) Substrate and coating obtained in aluminate electrolyte; (b) substrate and coating obtained in silicate electrolyte.
typical interface of the coating and the substrate can be seen from the cross-sectional photos in Fig. 6. 3.2.2. Thickness Fig. 7 represents the variation of thickness of PEO coatings with concentration of titania powers in the electrolytes. Fig. 7 reveals that the thickness of both coatings changes little with different concentrations of titania powers in the electrolytes. The thickness of coating obtained in aluminate electrolyte is about 66 m and the value of coating obtained in silicate electrolyte is about 35 m. 3.2.3. Thermal shock resistance Coated samples were heated to 300 ◦ C in high temperature furnace and kept for 2 min, then quickly taken out and immersed in cool water. The test was repeated until the ceramic coating was destroyed. Five samples were used for each type of coating. The test results of PEO coatings obtained in both electrolytes are shown in Table 1. It can be seen that coatings obtained in silicate electrolyte exhibit a little better thermal shock resistance. Coatings forming in electrolytes with middle concentration also showed a relatively good thermal shock resistance. 3.2.4. Corrosion resistance Potentiodynamic polarizing curves of titania coated and uncoated samples in 3.5 wt% NaCl solution are plotted in Fig. 8. According to electrochemical theory, samples with high corrosion
(1) Two types of titania composite coatings were prepared on carbon steel by PEO in silicate and aluminate electrolytes, respectively. Coating obtained in silicate electrolyte was composed of anatase-TiO2 , rutile-TiO2 crystal phase and Fe, P, Si elements; Coating obtained in aluminate electrolyte consisted of anatase-TiO2 and Al2 TiO5 , Fe, P is also found in the coating. Coating obtained in silicate electrolyte was about 35 m thick; coating obtained in silicate electrolyte was approximately 66 m thick. (2) Coatings obtained in two types of electrolytes showed porous and coarse surface. The bond strength was not strongly affected by concentration of titania powers in electrolytes. The valves were 23 MPa for coatings obtained in aluminate electrolyte, and 21 MPa for coatings obtained in silicate electrolyte. Coatings obtained in silicate electrolyte showed a little better thermal shock resistance than those obtained in aluminate electrolyte and the best coatings were obtained with middle concentration titania powers in the electrolytes. (3) Both coatings obtained in silicate electrolyte and aluminate electrolyte showed better corrosion resistance than uncoated sample in 3.5 wt% NaCl solution. For two types of electrolytes, coatings obtained with 8 g/L TiO2 power in electrolytes exhibited the best corrosion resistance, whose corrosion rate was decreased by 2 orders of magnitude compared with the uncoated one. References [1] A. Alem, H. Sarpoolaky, Mehrdad Keshmiri, Sol–gel preparation of titania multilayer membrane for photocatalytic applications, Ceram. Int. 5 (2009) 1837–1843.
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