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Performance of titania–silica composite coating on interstitial-free steel sheet
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Scripta Materialia 58 (2008) 473–476 www.elsevier.com/locate/scriptamat
Performance of titania–silica composite coating on interstitial-free steel sheet T.K. Rout,* N. Bandyopadhyay, R. Narayan, Nitu Rani and D.K. Sengupta Research and Development Division, Tata Steel Limited, Jamshedpur 831 007, India Received 4 October 2007; revised 22 October 2007; accepted 23 October 2007 Available online 4 December 2007
Coating titania–silica composite onto interstitial-free steel sheet has been attempted via the sol–gel process to create a corrosionprotective layer. The role of glycidoxypropyl trimethoxy silane in titania coating has been explained as an adhesion promoter for steel by using electrochemical impedance spectroscopy. 2007 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Titania; Precursors; Impedance spectroscopy; Composite
Transparent titania coating on glass substrates is an established product in the solar industry. Titania adheres well to glass substrates via Ti–O–Si covalent bonds and provides added advantages such as reduction in fragility and low solar heat loss through window glasses [1]. Titanium metal is known to be self-protective: an impervious titania layer forms that offers protection against many corrosive atmospheres. Titania as a corrosionprotective coating for other metal substrates such as tinplates, stainless steels and aluminium has not been widely reported. There is also very little work in the literature on sol–gel coating of titania on mild steel for corrosion protection in saline atmospheres [2–4]. Titania coating under solar light acts as a non-sacrificial anode and provides cathodic protection to metals. This behaviour is very prominent when an interfacial layer of alpha iron oxide exists between anatase and steel (i.e., anatase/ alpha iron oxide/steel). This effect can further be improved by using a multilayer coating, such as titania (amorphous)/anatase/Ti–Fe oxide/alpha iron oxide/ steel. The amorphous layer of titania helps to maintain the photopotential by inhibiting oxygen reduction. Therefore, titania coating (anatase) increases corrosion protection under both light and dark conditions [5]. The more stable and adherent is the coating on metal substrate, the more protection it offers. The stability as well as the adhesion of titania coatings on metal substrates may be improved by chelating agents and
* Corresponding author. Tel.: +91 657 2147445/2147446; fax: +91 657 2271510; e-mail: [email protected]
adhesion promoters such as acetyl acetone, 1,2-propanediol, ethylene glycol, ethanolamine, diethanolamine, triethanolamine, acetic acid, citrate acid, ethanol, n-butanol and many silicone additives [6–8]. This paper describes an attempt to prepare titania coating by using glycidoxy propyl trimethoxy silane (GPTMS) as an adhesion promoter. The role of GPTMS in this corrosion-protective coating has also been examined by electrochemical impedance spectroscopy (EIS) and Raman spectroscopy. A sol was prepared by mixing Ti-isopropoxide and 2methoxy glycol. GPTMS was added slowly into the sol at a temperature of 60 C. The sol particle size was analysed by transmission electron microscopy (Model H-600, Hitachi). The extent of hydrolysis to obtain metal oxide was monitored by Fourier transformation infrared spectroscopy. Interstitial-free (IF) steel sheet was taken for the present investigation. The composition of the IF steel was: C 0.0035, Mn 0.15, Si 0.015, S 0.02, P 0.018, Al 0.02, N 30, Ti 0.038, Nb 0.0089 ppm, and its mechanical properties were yield stress 160 MPa, ultimate tensile strength 320 (max) MPa. The sheets are cleaned and dipped into tri-cationic phosphate solution with concentrations of Ni 0.8, Zn 126 and Mn 0.4 g l 1 for 1 min as a pre-treatment stage to obtain a thin phosphate coating of 2–3 g m 2 [9] prior to application of composite titania–silica coating. The treated IF steel was coated and heat treated to 450 C at a rate of 20 C min 1 for 1 h to obtain a dense coating. The sintered samples were analysed by Raman spectroscopy to observe the bonding of coating to steel.
1359-6462/$ - see front matter 2007 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.scriptamat.2007.10.046
474
T. K. Rout et al. / Scripta Materialia 58 (2008) 473–476
Detail of the Raman spectroscopy techniques have been given elsewhere [10]. The corrosion performance of the coating was evaluated by EIS. Figure 1 shows the infrared spectra for the titania sol, GPTMS-modified titania sol and GPTMS with acetyl acetone complexing agent. A shifting in the wavenumber of the peaks and presence of new peaks is an indication of the breakage and formation of new compounds in the sol. The 4000–1000 cm 1 region shows the absorption bands of the O–H stretching (3700–3200 cm 1), O–H bending (1650–1200 cm 1) and other organic bond vibration modes [11]. The broad bands corresponding to Ti–O–Ti, Ti–O–Si and Si–O–Si vibrations are observed and marked in the figures. The presence of Si– O–Si band reflects the good homogeneity of the hybrid coating [12–14]. A detailed reaction scheme is proposed in Figure 2. Figure 3 shows the distribution of particles in the sol. It is found that particle sizes vary from 10 to 98 nm and are uniformly distributed. This uniformity is due to the controlled growth of particles in the sol during hydrolysis and the condensation reaction of titania precursor in water by the addition of acetyl acetone (AcAc). The AcAc forms a complex with titania precursor and inhibits hydrolysis [6]. Figure 4 shows the Raman spectrum of coatings: the peaks at 411, 417 and 615 cm 1 represent Ti–O/Si–O, O–Ti–O and Ti–O–O–Ti, respectively. The asymmetric stretching located at 1318 cm 1, which is supported by the peaks of 227 and 292 cm 1, is an indication of the presence of Fe–O–Si and Fe–O–Ti bonds. EIS (EIS-300, Gamry, USA) has been used to evaluate the coating capacitance (Cc), polarization resistance (Rp) and solution resistance (Rs) of titania and titania with 10% GPTMS coating on pretreated IF steel sheets at different immersion times in 3.5% NaCl solution. The Rp, Rs and Cc data have been derived by simulating the coating/metal interface with an equivalent electrical circuit to model the simple charge-transfer and diffusion-controlled processes [10]. Across the coating and solution interface, a nominal voltage of 10 mV over a wide range of frequencies (0.001–10 kHz) is applied through the working electrode
OH O O
Si
GPTMS
OH
OH OH
OH
+
Ti
OH
OH Ti SOL
H
OH
.. NH 2
O
Si
GPTMS
O
H
HYDROGEN BONDING
INTERACTION
OH
Si
O
O
.. NH 2
(R)
O
O
(R)
O
Si
GPTMS
O
Ti
H
OH H O 2
Free OH
O 1000-1200 cm - 1 O
.. NH
H
OH
2
O
.. NH 2
O
OH Bonded O H
Si
O
Ti
H
O 930 cm-1
Free OH
O
O
Ti
O
O
.. NH
OH
3334 cm-1
3460 cm-1
O
O
O 550-800 cm- 1 O
O
Ti
O
H
NH
CROSS LINKING DI-AM INE
H
O
Free OH
O
Free OH
OH Bonded OH
OH
O 2
O
Si
O .. NH 2
OH
O
Si
O .. NH
Ti
H
(R)
.. NH 2
O
O HYBRID STRUCTUR E O F GPTM S AND Ti SOL INTERACTION
Figure 2. The reaction scheme for Ti-precursor in the presence of GPTMS.
Figure 3. TEM analysis of Ti-sol.
2500
227.08
Intensity (au)
2000
292.451 1500
1000
411.923 615.364 497.864
1318.39
500
0 0
500
1000
Raman shift, cm-1
Figure 1. FTIR spectra of different sols.
Figure 4. Raman spectrums of titania coatings.
1500
2000
T. K. Rout et al. / Scripta Materialia 58 (2008) 473–476
475
Table 1. Comparison of components of the Bode plot for titania and titania + GPTMS coatings Samples
Exposure time, h
IzI, kohm
Rp, ohm (calculated)
Cc, F cm
Titania
3 (Uncoated) 1 24 72
0.3 3.9 2.7 1.2
0.3 3.9 1.9 1.9
1.0 · 10 2.5 · 10 2.3 · 10 3.1 · 10
1 (Uncoated) 1 24 72
1 316.2 100.0 63.0
0.9 316.1 99.6 62.5
5 · 10 5 1 · 10 6 7.9 · 10 7.9 · 10
Titania + GPTMS
3.00
2.00
1.00
0.00 -2.00
(calculated)
2 4 4 3
6 6
6.00 EIS CRCA 3hrs EIS Ti: 1 hr EIS Ti: 24 hrs EIS Ti: 72 hrs
Log Modulus (Ohm)
Log Modulus (Ohm)
4.00
2
4.00 3.00 2.00 1.00 0.00 -2.00
-1.00
0.00
1.00
2.00
3.00
4.00
5.00
EIS CRCA Bare -1 hr EIS Ti+Si: 1 hr EIS Ti+Si: 24 hrs EIS Ti+Si: 72 hrs
5.00
-1.00
0.00
1.00
6.00
2.00
3.00
4.00
5.00
4.00
5.00
6.00
Log Freq (Hz)
Log Freq (Hz) 0.00 30.00
-10.00
Phase (Degree)
Phase (Degree)
20.00 10.00 0.00 -10.00 -20.00
-20.00 -30.00 -40.00 -50.00
-30.00
-60.00
-40.00
-70.00 -2.00
-50.00 -60.00 -2.00
-1.00
0.00
1.00
2.00
3.00
6.00
Log Freq (Hz) -1.00
0.00
1.00
2.00
3.00
4.00
5.00
6.00
Log Freq (Hz)
Figure 5. Bode plot for the effect of 3.5% NaCl on titania-coated steel sheets.
to record current by measuring via counter-electrode without disturbing the interface. The voltage and current data are plotted using a Bode plot. The log IzI vs. log x curve is used to obtain Rp and Rs values. The solution resistance dominates the impedance at higher frequencies, and therefore log (Rs) can be obtained from the horizontal plateau of the high frequencies. Similarly, log (Rs + Rp) can be obtained from the horizontal plateau of the lower frequencies. The straight line curve with a slope 1 at intermediate frequencies is used to obtained Cc by extrapolation to the log IzI axis at x = 1(log x = 0, f = 0.16 Hz) from the relationship log IzI = 1/Cc. The values of IzI, Cc and Rp extracted from the curves are reported in Table 1. In the case of the titania coating, the impedance is found to deviate from the slope {d(lnz)/d(lnf) = 1} over the exposure time, indicating diffusion of water/ ions [15–17]. The impedance value decreases gradually from 4 to 1 kohm over the 72 h of exposure, indicating poor barrier coatings. This is presented in Figure 5 and Table 1. In case of titania + GPTMS, the impedance is 316 kohm after 1 h of exposure, which is very
Figure 6. Bode plot for the effect of 3.5% NaCl on titania + GPTMScoated steel sheets.
high compared to titania coating. This is presented in Figure 6 and Table 1. Impedance values remain higher compared to those obtained with titania coating over the 72 h of exposure in 3.5% NaCl solution, indicating that titania + GPTMS offers better protection than titania. The barrier properties of the coating are also evidenced by the constant coating capacitance (Cc) with exposure time as well as the constant polarization resistance values. In the case of titania, Cc increases with the exposure time from 6.2 · 10 8 to 9.7 · 10 7 F cm 2, indicating that water/ions diffuse through the coating matrix, which is therefore not acting as a barrier layer. However, with titania + GPTMS coating the Cc value is found to remain constant, i.e., 1.2–1.11 · 10 10 over the exposure time, indicating that water/ions are uniformly distributed across the coating/metal interface [18–21]; hence titania + GPTMS coating provides a protective barrier. The improvement in protection may be due to the three-dimensional network of polycondensation of Ti(OH)2(AcAc)2 on GPTMS and adherence to the substrate through H-bonds and Van der Waals bonds between functionalized titania particles and metal substrates immediately after application. These bonds
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T. K. Rout et al. / Scripta Materialia 58 (2008) 473–476
Ti
Si
Ti
Si
O
O
O
O H
H
H H
Ti
O Si
O
O
O
Me
Me
Me
H H
H
O
O
O
Me
Me
Me
H
H-bond Hydrogen bonded surface Immediately after adsorption
O
Ti O
O
Si O
Me Me Metal oxide bond
Covalent bonded surface after sintering at high temperature
Figure 7. Bonding mechanism before and after sintering of coating.
can be transformed into stable covalent bonds during the densification/sintering process [22,23], as shown in Figure 7. The surface analysis of sintering samples supports the existence of Ti–O–Si, Fe–O–Ti and Fe–O–Si covalent linkages. 1. Titania coatings with GPTMS have been synthesized and TEM analyses show that particles are 30–98 nm. 2. EIS shows that there is a decrease in impedance and an increase in coating capacitance of titania-coated substrates with exposure time in 3.5% NaCl solution, indicating that the coating is pervious to diffusion of water/ions. However, the three-dimensional network of the titania + GPTMS system obtained by polycondensation of Ti(OH)2(AcAc)2 on GPTMS adheres to the surface, forming stable covalent bonds during the densification process. Authors would like to thank the Chief of R&D and SS, Tata Steel Ltd for permission to publish this article. [1] Y.A. Attia, D.K. Sengupta, H.A. Hamza, in: Y.A. Attia (Ed.), Sol–Gel Processing and Application, Plenum Press, New York, 1994, pp. 159–168.
[2] Jianshun Huang, Tornoyoshi Konishi, Tadashi Shinohara, Shigeo Tsujikawa, Zairyo-to-Kankyo 47 (3) (1998) 193–199. [3] Jianshun Huang, Tadashi Shinohara, Shigeo Tsujikawa, Zairyo-to-Kankyo 48 (9) (1999) 575–582. [4] G. Carbajal De La Torre, R. Nava Mendoza, M.A. Espinosamedina, A. Martinezvillafane, J.G. Gonzalezrodriguez, V.M. Catano, Br. Corros. J. 37 (4) (2002) 293– 297. [5] Jianshun Huang, Tadashi Shinohara, Shigeo Tsujikawa, Zairyo-to- Kankyo 46 (10) (1997) 651–661. [6] S.J. Bu, Z.G. Jin, X.X. Liu, L.R. Yang, Z.J. Cheng, J. Eur. Ceram. Soc. 25 (2005) 673–679. [7] B. O’Regan, M. Gratzel, Nature 353 (1991) 737–739. [8] C.L. Buscema, C. Malibert, S. Bach, Thin Solid Films 418 (2002) 79–84. [9] Tapan K. Rout, Scripta Mater. 56 (2007) 573–576. [10] T.K. Rout, N. Bandyopadhyay, T. Venugopalan, Surf. Coat. Technol. 201 (2006) 1022–1030. [11] A.N. Khramov, V.N. Balbyshev, N.N. Voevodin, M.S. Donley, Prog. Org. Coat. 47 (2003) 207. [12] M. Schraml-Marth, K.L. Walther, A. Wokan, B.E. Handy, A. Baiker, J. Non-Cryst. Solids 143 (1992) 93. [13] M. Aizawa, Y. Nosaka, N. Fujii, J. Non-Cryst. Solids 168 (1994) 49. [14] S.P. Mukherjee, J. Non-Cryst. Solids 42 (1980) 477. [15] F. Boccuzzi, S. Coluccia, G. Ghiotti, C. Morterra, A. Zecchina, J. Phys. Chem. 82 (1978) 1298. [16] D.L. Wood, E.M. Rabinovich, Appl. Spectrosc. 43 (2) (1989) 263. [17] K.A. Mauritz, R.M. Warren, Macromolecules 22 (1989) 1730. [18] M.A. Alim, J. Am. Ceram. Soc. 72 (1) (1989) 28–32. [19] F. Mansfeld, M.W. Kendig, S. Tsai, Corrosion 38 (11) (1982) 570–580. [20] F. Mansfield, M.W. Kendig, S. Tsai, Corrosion 38 (1982) 478–484. [21] M.F. Weber, L.C. Schumacher, M.J. Dignam, J. Electrochem. Soc. 129 (1982) 2022–2028. [22] Tapan K. Rout, N. Bandyopadhyay, T. Venugopalan, D. Bhattacharjee, Corros. Sci. 49 (2) (2007) 794–817. [23] M.L. Zheludkevich, I. Miranda Salvado, M.G.S. Ferreira, J. Mater. Chem. 15 (2005) 5099–5111.
Scripta Materialia 58 (2008) 473–476 www.elsevier.com/locate/scriptamat
Performance of titania–silica composite coating on interstitial-free steel sheet T.K. Rout,* N. Bandyopadhyay, R. Narayan, Nitu Rani and D.K. Sengupta Research and Development Division, Tata Steel Limited, Jamshedpur 831 007, India Received 4 October 2007; revised 22 October 2007; accepted 23 October 2007 Available online 4 December 2007
Coating titania–silica composite onto interstitial-free steel sheet has been attempted via the sol–gel process to create a corrosionprotective layer. The role of glycidoxypropyl trimethoxy silane in titania coating has been explained as an adhesion promoter for steel by using electrochemical impedance spectroscopy. 2007 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Titania; Precursors; Impedance spectroscopy; Composite
Transparent titania coating on glass substrates is an established product in the solar industry. Titania adheres well to glass substrates via Ti–O–Si covalent bonds and provides added advantages such as reduction in fragility and low solar heat loss through window glasses [1]. Titanium metal is known to be self-protective: an impervious titania layer forms that offers protection against many corrosive atmospheres. Titania as a corrosionprotective coating for other metal substrates such as tinplates, stainless steels and aluminium has not been widely reported. There is also very little work in the literature on sol–gel coating of titania on mild steel for corrosion protection in saline atmospheres [2–4]. Titania coating under solar light acts as a non-sacrificial anode and provides cathodic protection to metals. This behaviour is very prominent when an interfacial layer of alpha iron oxide exists between anatase and steel (i.e., anatase/ alpha iron oxide/steel). This effect can further be improved by using a multilayer coating, such as titania (amorphous)/anatase/Ti–Fe oxide/alpha iron oxide/ steel. The amorphous layer of titania helps to maintain the photopotential by inhibiting oxygen reduction. Therefore, titania coating (anatase) increases corrosion protection under both light and dark conditions [5]. The more stable and adherent is the coating on metal substrate, the more protection it offers. The stability as well as the adhesion of titania coatings on metal substrates may be improved by chelating agents and
* Corresponding author. Tel.: +91 657 2147445/2147446; fax: +91 657 2271510; e-mail: [email protected]
adhesion promoters such as acetyl acetone, 1,2-propanediol, ethylene glycol, ethanolamine, diethanolamine, triethanolamine, acetic acid, citrate acid, ethanol, n-butanol and many silicone additives [6–8]. This paper describes an attempt to prepare titania coating by using glycidoxy propyl trimethoxy silane (GPTMS) as an adhesion promoter. The role of GPTMS in this corrosion-protective coating has also been examined by electrochemical impedance spectroscopy (EIS) and Raman spectroscopy. A sol was prepared by mixing Ti-isopropoxide and 2methoxy glycol. GPTMS was added slowly into the sol at a temperature of 60 C. The sol particle size was analysed by transmission electron microscopy (Model H-600, Hitachi). The extent of hydrolysis to obtain metal oxide was monitored by Fourier transformation infrared spectroscopy. Interstitial-free (IF) steel sheet was taken for the present investigation. The composition of the IF steel was: C 0.0035, Mn 0.15, Si 0.015, S 0.02, P 0.018, Al 0.02, N 30, Ti 0.038, Nb 0.0089 ppm, and its mechanical properties were yield stress 160 MPa, ultimate tensile strength 320 (max) MPa. The sheets are cleaned and dipped into tri-cationic phosphate solution with concentrations of Ni 0.8, Zn 126 and Mn 0.4 g l 1 for 1 min as a pre-treatment stage to obtain a thin phosphate coating of 2–3 g m 2 [9] prior to application of composite titania–silica coating. The treated IF steel was coated and heat treated to 450 C at a rate of 20 C min 1 for 1 h to obtain a dense coating. The sintered samples were analysed by Raman spectroscopy to observe the bonding of coating to steel.
1359-6462/$ - see front matter 2007 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.scriptamat.2007.10.046
474
T. K. Rout et al. / Scripta Materialia 58 (2008) 473–476
Detail of the Raman spectroscopy techniques have been given elsewhere [10]. The corrosion performance of the coating was evaluated by EIS. Figure 1 shows the infrared spectra for the titania sol, GPTMS-modified titania sol and GPTMS with acetyl acetone complexing agent. A shifting in the wavenumber of the peaks and presence of new peaks is an indication of the breakage and formation of new compounds in the sol. The 4000–1000 cm 1 region shows the absorption bands of the O–H stretching (3700–3200 cm 1), O–H bending (1650–1200 cm 1) and other organic bond vibration modes [11]. The broad bands corresponding to Ti–O–Ti, Ti–O–Si and Si–O–Si vibrations are observed and marked in the figures. The presence of Si– O–Si band reflects the good homogeneity of the hybrid coating [12–14]. A detailed reaction scheme is proposed in Figure 2. Figure 3 shows the distribution of particles in the sol. It is found that particle sizes vary from 10 to 98 nm and are uniformly distributed. This uniformity is due to the controlled growth of particles in the sol during hydrolysis and the condensation reaction of titania precursor in water by the addition of acetyl acetone (AcAc). The AcAc forms a complex with titania precursor and inhibits hydrolysis [6]. Figure 4 shows the Raman spectrum of coatings: the peaks at 411, 417 and 615 cm 1 represent Ti–O/Si–O, O–Ti–O and Ti–O–O–Ti, respectively. The asymmetric stretching located at 1318 cm 1, which is supported by the peaks of 227 and 292 cm 1, is an indication of the presence of Fe–O–Si and Fe–O–Ti bonds. EIS (EIS-300, Gamry, USA) has been used to evaluate the coating capacitance (Cc), polarization resistance (Rp) and solution resistance (Rs) of titania and titania with 10% GPTMS coating on pretreated IF steel sheets at different immersion times in 3.5% NaCl solution. The Rp, Rs and Cc data have been derived by simulating the coating/metal interface with an equivalent electrical circuit to model the simple charge-transfer and diffusion-controlled processes [10]. Across the coating and solution interface, a nominal voltage of 10 mV over a wide range of frequencies (0.001–10 kHz) is applied through the working electrode
OH O O
Si
GPTMS
OH
OH OH
OH
+
Ti
OH
OH Ti SOL
H
OH
.. NH 2
O
Si
GPTMS
O
H
HYDROGEN BONDING
INTERACTION
OH
Si
O
O
.. NH 2
(R)
O
O
(R)
O
Si
GPTMS
O
Ti
H
OH H O 2
Free OH
O 1000-1200 cm - 1 O
.. NH
H
OH
2
O
.. NH 2
O
OH Bonded O H
Si
O
Ti
H
O 930 cm-1
Free OH
O
O
Ti
O
O
.. NH
OH
3334 cm-1
3460 cm-1
O
O
O 550-800 cm- 1 O
O
Ti
O
H
NH
CROSS LINKING DI-AM INE
H
O
Free OH
O
Free OH
OH Bonded OH
OH
O 2
O
Si
O .. NH 2
OH
O
Si
O .. NH
Ti
H
(R)
.. NH 2
O
O HYBRID STRUCTUR E O F GPTM S AND Ti SOL INTERACTION
Figure 2. The reaction scheme for Ti-precursor in the presence of GPTMS.
Figure 3. TEM analysis of Ti-sol.
2500
227.08
Intensity (au)
2000
292.451 1500
1000
411.923 615.364 497.864
1318.39
500
0 0
500
1000
Raman shift, cm-1
Figure 1. FTIR spectra of different sols.
Figure 4. Raman spectrums of titania coatings.
1500
2000
T. K. Rout et al. / Scripta Materialia 58 (2008) 473–476
475
Table 1. Comparison of components of the Bode plot for titania and titania + GPTMS coatings Samples
Exposure time, h
IzI, kohm
Rp, ohm (calculated)
Cc, F cm
Titania
3 (Uncoated) 1 24 72
0.3 3.9 2.7 1.2
0.3 3.9 1.9 1.9
1.0 · 10 2.5 · 10 2.3 · 10 3.1 · 10
1 (Uncoated) 1 24 72
1 316.2 100.0 63.0
0.9 316.1 99.6 62.5
5 · 10 5 1 · 10 6 7.9 · 10 7.9 · 10
Titania + GPTMS
3.00
2.00
1.00
0.00 -2.00
(calculated)
2 4 4 3
6 6
6.00 EIS CRCA 3hrs EIS Ti: 1 hr EIS Ti: 24 hrs EIS Ti: 72 hrs
Log Modulus (Ohm)
Log Modulus (Ohm)
4.00
2
4.00 3.00 2.00 1.00 0.00 -2.00
-1.00
0.00
1.00
2.00
3.00
4.00
5.00
EIS CRCA Bare -1 hr EIS Ti+Si: 1 hr EIS Ti+Si: 24 hrs EIS Ti+Si: 72 hrs
5.00
-1.00
0.00
1.00
6.00
2.00
3.00
4.00
5.00
4.00
5.00
6.00
Log Freq (Hz)
Log Freq (Hz) 0.00 30.00
-10.00
Phase (Degree)
Phase (Degree)
20.00 10.00 0.00 -10.00 -20.00
-20.00 -30.00 -40.00 -50.00
-30.00
-60.00
-40.00
-70.00 -2.00
-50.00 -60.00 -2.00
-1.00
0.00
1.00
2.00
3.00
6.00
Log Freq (Hz) -1.00
0.00
1.00
2.00
3.00
4.00
5.00
6.00
Log Freq (Hz)
Figure 5. Bode plot for the effect of 3.5% NaCl on titania-coated steel sheets.
to record current by measuring via counter-electrode without disturbing the interface. The voltage and current data are plotted using a Bode plot. The log IzI vs. log x curve is used to obtain Rp and Rs values. The solution resistance dominates the impedance at higher frequencies, and therefore log (Rs) can be obtained from the horizontal plateau of the high frequencies. Similarly, log (Rs + Rp) can be obtained from the horizontal plateau of the lower frequencies. The straight line curve with a slope 1 at intermediate frequencies is used to obtained Cc by extrapolation to the log IzI axis at x = 1(log x = 0, f = 0.16 Hz) from the relationship log IzI = 1/Cc. The values of IzI, Cc and Rp extracted from the curves are reported in Table 1. In the case of the titania coating, the impedance is found to deviate from the slope {d(lnz)/d(lnf) = 1} over the exposure time, indicating diffusion of water/ ions [15–17]. The impedance value decreases gradually from 4 to 1 kohm over the 72 h of exposure, indicating poor barrier coatings. This is presented in Figure 5 and Table 1. In case of titania + GPTMS, the impedance is 316 kohm after 1 h of exposure, which is very
Figure 6. Bode plot for the effect of 3.5% NaCl on titania + GPTMScoated steel sheets.
high compared to titania coating. This is presented in Figure 6 and Table 1. Impedance values remain higher compared to those obtained with titania coating over the 72 h of exposure in 3.5% NaCl solution, indicating that titania + GPTMS offers better protection than titania. The barrier properties of the coating are also evidenced by the constant coating capacitance (Cc) with exposure time as well as the constant polarization resistance values. In the case of titania, Cc increases with the exposure time from 6.2 · 10 8 to 9.7 · 10 7 F cm 2, indicating that water/ions diffuse through the coating matrix, which is therefore not acting as a barrier layer. However, with titania + GPTMS coating the Cc value is found to remain constant, i.e., 1.2–1.11 · 10 10 over the exposure time, indicating that water/ions are uniformly distributed across the coating/metal interface [18–21]; hence titania + GPTMS coating provides a protective barrier. The improvement in protection may be due to the three-dimensional network of polycondensation of Ti(OH)2(AcAc)2 on GPTMS and adherence to the substrate through H-bonds and Van der Waals bonds between functionalized titania particles and metal substrates immediately after application. These bonds
476
T. K. Rout et al. / Scripta Materialia 58 (2008) 473–476
Ti
Si
Ti
Si
O
O
O
O H
H
H H
Ti
O Si
O
O
O
Me
Me
Me
H H
H
O
O
O
Me
Me
Me
H
H-bond Hydrogen bonded surface Immediately after adsorption
O
Ti O
O
Si O
Me Me Metal oxide bond
Covalent bonded surface after sintering at high temperature
Figure 7. Bonding mechanism before and after sintering of coating.
can be transformed into stable covalent bonds during the densification/sintering process [22,23], as shown in Figure 7. The surface analysis of sintering samples supports the existence of Ti–O–Si, Fe–O–Ti and Fe–O–Si covalent linkages. 1. Titania coatings with GPTMS have been synthesized and TEM analyses show that particles are 30–98 nm. 2. EIS shows that there is a decrease in impedance and an increase in coating capacitance of titania-coated substrates with exposure time in 3.5% NaCl solution, indicating that the coating is pervious to diffusion of water/ions. However, the three-dimensional network of the titania + GPTMS system obtained by polycondensation of Ti(OH)2(AcAc)2 on GPTMS adheres to the surface, forming stable covalent bonds during the densification process. Authors would like to thank the Chief of R&D and SS, Tata Steel Ltd for permission to publish this article. [1] Y.A. Attia, D.K. Sengupta, H.A. Hamza, in: Y.A. Attia (Ed.), Sol–Gel Processing and Application, Plenum Press, New York, 1994, pp. 159–168.
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