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Corrosion resistance of ZrO2–TiO2 nanocomposite multilayer thin films coated on carbon steel in hydrochloric acid solution
Corrosion resistance of ZrO 2 -TiO2 nanocomposite multilayer thin films coated on carbon steel in hydrochloric acid solution Hany M. Abd El-Lateef, Mai M. Khalaf PII: DOI: Reference:
S1044-5803(15)00305-8 doi: 10.1016/j.matchar.2015.08.010 MTL 8006
To appear in:
Materials Characterization
Received date: Revised date: Accepted date:
30 June 2015 8 August 2015 15 August 2015
Please cite this article as: Abd El-Lateef Hany M., Khalaf Mai M., Corrosion resistance of ZrO2 -TiO2 nanocomposite multilayer thin films coated on carbon steel in hydrochloric acid solution, Materials Characterization (2015), doi: 10.1016/j.matchar.2015.08.010
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT Corrosion resistance of ZrO2-TiO2 nanocomposite multilayer thin films coated on carbon steel in hydrochloric acid solution
T
Hany M. Abd El-Lateef*, Mai M. Khalaf
SC R
IP
Chemistry Department, Faculty of Science, Sohag University, 82524 Sohag, Egypt
Abstract
NU
This work reports the achievement of preparing of x% zirconia (ZrO2)-titania (TiO2) composite coatings with different ZrO2 percent on the carbon steel by dipping substrates in
MA
sol–gel solutions. The prepared coated samples were investigated by various surface techniques including X-ray diffraction (XRD), scanning electron microscopy (SEM),
D
transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy
TE
(EDAX). Open-circuit potential (OCP), potentiodynamic polarization, and electrochemical
CE P
impedance spectroscopy (EIS) methods were employed to investigate the corrosion resistance of the coated carbon steel substrates in 1.0 M HCl solution at 50 °C. The data showed that, the corrosion protection property is not always proportional to the percent of
AC
ZrO2. It can be inferred that there is an optimum percent (10%ZrO2) for beneficial effects of loading ZrO2 on the protection efficiency (7.890%), while higher loading percent of ZrO2 in the sol–gel coating leads to the formation of a fragile film with poor barrier properties. EDAX/SEM suggests that the metal surface was protected through coating with ZrO2-TiO2 composite films. Keywords: ZrO2-TiO2 Composite; Coating; Corrosion protection; SEM; TEM; XRD * Corresponding author: Fax: (+2)-093 -4601159, Tel: (+2)-010-92-593-198 E-mail address: [email protected] (Hany M. Abd El-Lateef) [email protected] (Mai M. Khalaf)
1
ACCEPTED MANUSCRIPT 1. Introduction Carbon steel is a common constructional material for many industrial units because of low cost and its excellent mechanical properties [1]. It has, however, limited service life
T
unless effective measures are taken to improve its corrosion and wear resistance properties
IP
[2–6]. Solutions of hydrochloric acid are use for pickling, chemical and electrochemical
SC R
etching of carbon steel alloys [7]. One of the most effective methods for protecting the surface of metals and alloys against corrosion in aggressive acidic solutions is used the
NU
coating process [8].
Ceramic based coatings are increasingly used for range of industrial applications to
MA
provide wear and erosion resistance, thermal insulation, and corrosion protection [9-10]. Titania and titania based composite coatings on metal surface have always been a research
D
focus for their versatile applications. They can be used as functional materials such as
TE
pollutant degradation, catalyzing, water-purifications, biomedical materials, solar cells and
CE P
protective materials such as protective layers on metals surface to improve the wear and corrosion resistance [11–15]. ZrO2-TiO2 compositions have gained use as a good thermal barrier coating due to their properties as a combined material. Titania and titania based
AC
composite coatings may be prepared by various techniques such as physical vapor deposition (PVD), chemical vapor deposition (CVD), ion assisted deposition, plasma spray and sol–gel [16]. The sol–gel method appears to be promising as it is low cost, offers good adhesion to metallic surface via chemical bonding, and easy adaptability in industries due to its simple application procedure. One of the main advantages of sol–gel method is its capacity to yield coatings with a wide range of compositions on different substrates without limitation of size or geometry of the work piece [17]. The preparation of coating by dipping substrates in sol–gel solutions is an established method to produce homogeneous coatings with uniform thickness below 2 μm [18]. In the literature, some oxide ceramic coatings with very low electronic conductance
2
ACCEPTED MANUSCRIPT such as TiO2 [19], SiO2 [20], Al2O3 [21, 22], ZrO2 [23], or mixed oxides composite coatings [24] have been reported as corrosion protection. The most of these ceramic coatings fabricated by sol–gel method, however, always shows surface cracks that come
T
from gel decomposition at high temperature. These cracks allow penetration of corrosive
IP
media through the coating to directly contact the carbon steel substrate, leading to
SC R
occurrence of severe corrosion at the coating/substrate interface. To reduce the amount of such cracks, another kind of nanoparticles of x% ZrO2 has been incorporated to form a
NU
ZrO2-TiO2 composite coating in this work.
In view of above, an attempt has been made to increase the corrosion resistance of carbon
MA
steel by depositing x% ZrO2-TiO2 composite coating. This paper reports the achievement of preparing of ZrO2-TiO2 composite coatings on carbon steel by dipping substrates in sol–gel
D
solutions. The coatings were characterized by X-ray diffraction (XRD), transmission
TE
electron microscopy (TEM), energy dispersive X-ray analysis (EDAX) and scanning
CE P
electron microscope (SEM). The performance of the coated carbon steel samples against corrosion was evaluated in 1.0 M HCl solution at 50 °C by means of open-circuit potential (OCP), potentiodynamic polarization, and electrochemical impedance spectroscopy
AC
measurements.
2. Experimental work 2.1. Carbon steel (C Steel) substrate composition Carbon steel substrate C1018 used for this study has the following composition (wt%): C 0.18 %, Si 0.24%, Mn 0.50%, P 0.05%, S 0.05%, Ni 0.01%, Cr 0.10% and Fe balance. Chemical composition of the electrode was determined by Energy dispersive Xray fluorescence (EDRF) (HORIBA XGT-7000). Before the deposition of films, the carbon steel specimens were first polished with a series of emery paper (grade 320–400–600–800–1000–1200) and then by polishing
3
ACCEPTED MANUSCRIPT machine (Buehler, Lake Bluff, Illinois USA), until their surfaces became smooth and mirror like bright. The substrates were degreased ultrasonically in acetone and subsequently dried prior to the deposition process.
T
2.2. Preparation of the x% ZrO2-TiO2 Nanocomposite samples
IP
In the present work, sol-gel method is used for the synthesis of nanostrucured ZrO2–
SC R
TiO2 films with various Zr-contents (5, 10, and 20 wt %) based on using zirconyl chloride (ZrOCl2) compound as a source of doping agent ions. Titanium isopropoxide (TTIP; Ti-
NU
(iOPr)4), (Aldrich 99.9% pure liquid) was mixed into isopropyl alcohol (iPr-OH; as organic solvent of titanium isopropoxide precursor), and stirred with a magnetic stir bar for
MA
approximately 30 min prior to synthesis thus for good dispersity of Ti4+ ions and to prepare the precursor solution of sol-gel process. Thereafter, ZrOCl2 dissolved in bi distilled water
D
were added dropwise in their stoichiometric ratio under vigorous stirring to the previous
TE
formed sol. Simultaneously nitric acid as catalyst and acetyl acetones (ACAC) as chelating agent were added (refer to Fig. 1) such that the molar ratio TTIP: iPr-OH: HNO3: ACAC
CE P
equals to 1:35.3:0.5:0.8 and stirring until complete dissolution. The obtained suspension was transparent and stirred for 2 h and then aged overnight until gel formed. Finally, the
AC
porous nanocomposite films were obtained after heat treatment for 3 h at 550 °C in air. The prepared coated ZrO2-TiO2 thin films were abbreviated as follows: x% ZT where x=5, 10 and 20% of doped Zr. Afterwards, the carbon steel substrates were coated by three times dip-coating with sample solutions film 5%ZT, film 10%ZT, and film 20%ZT, and the dip coating withdrawal rate was 10 cm/min. Consequently, the resulting coated carbon steel substrates were dried in a furnace at 90 °C for 1 h and then, thermally treated at 550 °C for 3 h with heating rate of 10 °C/min. This process was repeated in a periodically way for each coated layer. The coated layers of 5%ZT, 10%ZT and 20%ZT films were described by x%ZT1, x%ZT2 and x%ZT3, where the digits 1, 2 and 3 refer to the number of the coated layers.
4
ACCEPTED MANUSCRIPT <
S1044-5803(15)00305-8 doi: 10.1016/j.matchar.2015.08.010 MTL 8006
To appear in:
Materials Characterization
Received date: Revised date: Accepted date:
30 June 2015 8 August 2015 15 August 2015
Please cite this article as: Abd El-Lateef Hany M., Khalaf Mai M., Corrosion resistance of ZrO2 -TiO2 nanocomposite multilayer thin films coated on carbon steel in hydrochloric acid solution, Materials Characterization (2015), doi: 10.1016/j.matchar.2015.08.010
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT Corrosion resistance of ZrO2-TiO2 nanocomposite multilayer thin films coated on carbon steel in hydrochloric acid solution
T
Hany M. Abd El-Lateef*, Mai M. Khalaf
SC R
IP
Chemistry Department, Faculty of Science, Sohag University, 82524 Sohag, Egypt
Abstract
NU
This work reports the achievement of preparing of x% zirconia (ZrO2)-titania (TiO2) composite coatings with different ZrO2 percent on the carbon steel by dipping substrates in
MA
sol–gel solutions. The prepared coated samples were investigated by various surface techniques including X-ray diffraction (XRD), scanning electron microscopy (SEM),
D
transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy
TE
(EDAX). Open-circuit potential (OCP), potentiodynamic polarization, and electrochemical
CE P
impedance spectroscopy (EIS) methods were employed to investigate the corrosion resistance of the coated carbon steel substrates in 1.0 M HCl solution at 50 °C. The data showed that, the corrosion protection property is not always proportional to the percent of
AC
ZrO2. It can be inferred that there is an optimum percent (10%ZrO2) for beneficial effects of loading ZrO2 on the protection efficiency (7.890%), while higher loading percent of ZrO2 in the sol–gel coating leads to the formation of a fragile film with poor barrier properties. EDAX/SEM suggests that the metal surface was protected through coating with ZrO2-TiO2 composite films. Keywords: ZrO2-TiO2 Composite; Coating; Corrosion protection; SEM; TEM; XRD * Corresponding author: Fax: (+2)-093 -4601159, Tel: (+2)-010-92-593-198 E-mail address: [email protected] (Hany M. Abd El-Lateef) [email protected] (Mai M. Khalaf)
1
ACCEPTED MANUSCRIPT 1. Introduction Carbon steel is a common constructional material for many industrial units because of low cost and its excellent mechanical properties [1]. It has, however, limited service life
T
unless effective measures are taken to improve its corrosion and wear resistance properties
IP
[2–6]. Solutions of hydrochloric acid are use for pickling, chemical and electrochemical
SC R
etching of carbon steel alloys [7]. One of the most effective methods for protecting the surface of metals and alloys against corrosion in aggressive acidic solutions is used the
NU
coating process [8].
Ceramic based coatings are increasingly used for range of industrial applications to
MA
provide wear and erosion resistance, thermal insulation, and corrosion protection [9-10]. Titania and titania based composite coatings on metal surface have always been a research
D
focus for their versatile applications. They can be used as functional materials such as
TE
pollutant degradation, catalyzing, water-purifications, biomedical materials, solar cells and
CE P
protective materials such as protective layers on metals surface to improve the wear and corrosion resistance [11–15]. ZrO2-TiO2 compositions have gained use as a good thermal barrier coating due to their properties as a combined material. Titania and titania based
AC
composite coatings may be prepared by various techniques such as physical vapor deposition (PVD), chemical vapor deposition (CVD), ion assisted deposition, plasma spray and sol–gel [16]. The sol–gel method appears to be promising as it is low cost, offers good adhesion to metallic surface via chemical bonding, and easy adaptability in industries due to its simple application procedure. One of the main advantages of sol–gel method is its capacity to yield coatings with a wide range of compositions on different substrates without limitation of size or geometry of the work piece [17]. The preparation of coating by dipping substrates in sol–gel solutions is an established method to produce homogeneous coatings with uniform thickness below 2 μm [18]. In the literature, some oxide ceramic coatings with very low electronic conductance
2
ACCEPTED MANUSCRIPT such as TiO2 [19], SiO2 [20], Al2O3 [21, 22], ZrO2 [23], or mixed oxides composite coatings [24] have been reported as corrosion protection. The most of these ceramic coatings fabricated by sol–gel method, however, always shows surface cracks that come
T
from gel decomposition at high temperature. These cracks allow penetration of corrosive
IP
media through the coating to directly contact the carbon steel substrate, leading to
SC R
occurrence of severe corrosion at the coating/substrate interface. To reduce the amount of such cracks, another kind of nanoparticles of x% ZrO2 has been incorporated to form a
NU
ZrO2-TiO2 composite coating in this work.
In view of above, an attempt has been made to increase the corrosion resistance of carbon
MA
steel by depositing x% ZrO2-TiO2 composite coating. This paper reports the achievement of preparing of ZrO2-TiO2 composite coatings on carbon steel by dipping substrates in sol–gel
D
solutions. The coatings were characterized by X-ray diffraction (XRD), transmission
TE
electron microscopy (TEM), energy dispersive X-ray analysis (EDAX) and scanning
CE P
electron microscope (SEM). The performance of the coated carbon steel samples against corrosion was evaluated in 1.0 M HCl solution at 50 °C by means of open-circuit potential (OCP), potentiodynamic polarization, and electrochemical impedance spectroscopy
AC
measurements.
2. Experimental work 2.1. Carbon steel (C Steel) substrate composition Carbon steel substrate C1018 used for this study has the following composition (wt%): C 0.18 %, Si 0.24%, Mn 0.50%, P 0.05%, S 0.05%, Ni 0.01%, Cr 0.10% and Fe balance. Chemical composition of the electrode was determined by Energy dispersive Xray fluorescence (EDRF) (HORIBA XGT-7000). Before the deposition of films, the carbon steel specimens were first polished with a series of emery paper (grade 320–400–600–800–1000–1200) and then by polishing
3
ACCEPTED MANUSCRIPT machine (Buehler, Lake Bluff, Illinois USA), until their surfaces became smooth and mirror like bright. The substrates were degreased ultrasonically in acetone and subsequently dried prior to the deposition process.
T
2.2. Preparation of the x% ZrO2-TiO2 Nanocomposite samples
IP
In the present work, sol-gel method is used for the synthesis of nanostrucured ZrO2–
SC R
TiO2 films with various Zr-contents (5, 10, and 20 wt %) based on using zirconyl chloride (ZrOCl2) compound as a source of doping agent ions. Titanium isopropoxide (TTIP; Ti-
NU
(iOPr)4), (Aldrich 99.9% pure liquid) was mixed into isopropyl alcohol (iPr-OH; as organic solvent of titanium isopropoxide precursor), and stirred with a magnetic stir bar for
MA
approximately 30 min prior to synthesis thus for good dispersity of Ti4+ ions and to prepare the precursor solution of sol-gel process. Thereafter, ZrOCl2 dissolved in bi distilled water
D
were added dropwise in their stoichiometric ratio under vigorous stirring to the previous
TE
formed sol. Simultaneously nitric acid as catalyst and acetyl acetones (ACAC) as chelating agent were added (refer to Fig. 1) such that the molar ratio TTIP: iPr-OH: HNO3: ACAC
CE P
equals to 1:35.3:0.5:0.8 and stirring until complete dissolution. The obtained suspension was transparent and stirred for 2 h and then aged overnight until gel formed. Finally, the
AC
porous nanocomposite films were obtained after heat treatment for 3 h at 550 °C in air. The prepared coated ZrO2-TiO2 thin films were abbreviated as follows: x% ZT where x=5, 10 and 20% of doped Zr. Afterwards, the carbon steel substrates were coated by three times dip-coating with sample solutions film 5%ZT, film 10%ZT, and film 20%ZT, and the dip coating withdrawal rate was 10 cm/min. Consequently, the resulting coated carbon steel substrates were dried in a furnace at 90 °C for 1 h and then, thermally treated at 550 °C for 3 h with heating rate of 10 °C/min. This process was repeated in a periodically way for each coated layer. The coated layers of 5%ZT, 10%ZT and 20%ZT films were described by x%ZT1, x%ZT2 and x%ZT3, where the digits 1, 2 and 3 refer to the number of the coated layers.
4
ACCEPTED MANUSCRIPT <
> 2.3. Coating samples Characterization
T
The degree of crystallinity and phase compositions of the x% ZrO2-TiO2 powdered
IP
samples calcined at 550 °C were studied by means of x-ray diffraction method (XRD) using
SC R
TD-3500 diffractometer at room temperature with Ni-filtered Cu Kα radiation (λ = 0.15418 Å), at 35 kv and 25 mA. The average crystallite size was calculated according to the
D
k cos
NU
Scherrer’s equation,
(1)
MA
where D is the mean crystallite size, k (0.89) is the Scherrer constant, is the X-ray wavelength (0.15418 Å), and is the relative value of the full-width at half-maximum
D
(FWHM) of the (101) diffraction peak of catalysts. Transmission electron microscopy
TE
(TEM) images of calcined ZrO2-TiO2 films with different loading ratio were obtained on
CE P
Jeol TEM-1230 electron microscope at an acceleration voltage 120 kV, and at magnifications 50000x and 80000x. The morphology and microstructure and elemental composition of the deposited films on carbon steel substrates before corrosion test were
AC
studied by means of the scanning electron microscopy equipped with the EDAX detector.
2.4. Electrochemical measurements Electrochemical measurements were carried out in a three-electrode cell in 1.0 M HCl using VersaSTAT 4 potentiostat/galvanostat with a frequency response analyzer (FRA) contained in a single unit and connected with laptop. All solutions were prepared from grade chemicals (Merck) and bidistilled water. Saturated calomel electrode (SCE), the coated and uncoated carbon steel specimens, and platinum plate electrode were chosen as reference electrode, working electrode and counter electrode, respectively8 The specimen surfaces in contact with the solution had a constant area of 4.55 cm2. It has been reported
5
ACCEPTED MANUSCRIPT that, the corrosion rate of carbon steel is temperature dependent, and the corrosion rate has the largest value at 50 °C [25]. Therefore, in order to evaluate the protection effect of the coated samples on carbon steel in HCl solution, all the experiments were performed at 50
T
°C in the following order: The open circuit potential vs. time for 0.5 hour, the polarization
IP
measurements were recorded by sweeping the potential from −250 mV to +250 mV with
SC R
respect to corrosion potential (Ecorr) at scan rate of 1 mV/s. The data obtained were analyzed by Tafel extrapolation method. Electrochemical impedance spectroscopy (EIS)
NU
measurements were performed at open circuit potential. The range of applied frequencies was from 100 kHz to 0.05 Hz using voltage perturbation amplitude of 10 mV RMS.
MA
2.5. Surface characterization after exposure to 1.0 M HCl In order to observe any changes in surface morphologies of the carbon steel
D
substrate after exposure to 1.0 M HCl, the specimens were first immersed in the corrosive
TE
media with and without coating for 5 days, then cleaned with bi-distilled water and acetone,
CE P
and dried with cool air then the topography of the tested samples was observed by using scanning electron microscope (SEM) conducted with energy-dispersive X-ray spectroscopy
AC
analysis (EDAX) (SEM-EDAX) (JEOL, model 5300).
3. Results and discussion 1.3. Characterization of x% ZrO2-TiO2 films 3.1.1. XRD Analysis. The XRD patterns of blank TiO2, and doped samples of x wt % ZrO2-TiO2, (x=5, 10, and 20) calcined at 550 °C (powdered samples) are shown in Fig. 2a-d. The diffraction peaks of anatase TiO2 can be observed in all samples (Card No JCPDS 21-1272). It was noted that; the doped samples with x wt % ZrO2-TiO2 show the (101) peaks broader than that of TiO2. The present observations indicate that, the lattice structure of TiO2 is locally
6
ACCEPTED MANUSCRIPT deformed by incorporation of dopant Zr ions into TiO2 [26-28]. From XRD pattern of TiO2 blank, it was found that mixed rutile and anatase phases were formed and as a result of incorporation of Zr ions, the rutile phase slightly disappeared. It was further noted that, the
T
addition of Zr4+ has facilitated the formation of anatase phase than rutile phase. The lattice
IP
parameters of the samples were measured using (101) and (200) in anatase crystal planes to
equations (2) and (3):
d-2(hkl)= h2a-2+k2b-2+l2c-2
2 sin
NU
Braggs equation : d (hkl)
SC R
study the effect of Zr% content as dopant on the lattice structure of TiO2, by using
(2) (3)
MA
where d(hkl) is the distance between crystal planes of (hkl), is the X-ray wavelength, θ is the diffraction angle of crystal plane (hkl), hkl is the crystal index, and a, b and c are lattice
D
parameters (in anatase form, a=b≠ c tetragonal structure). The results are indexed in Table
TE
1. Lattice parameters calculation showed that, the lattice parameters of all TiO2 samples
CE P
remain almost constant along the a- and b- axes, whereas c-axis parameter differs due to the presence of Zr as the ionic radius of Zr4+ as dopant differs than that of Ti4+ [27-29]. Which allows for possible diffusion of Zr4+ along the c-axis to substitute Ti4+ in TiO2, the average
AC
crystallite size of samples (Table 1) was measured by the Scherrer’s equation from the (101) peak of anatase TiO2. The average crystallite size of TiO2 is 19.7 nm and doped titania samples with 5, 10 and 20% ZrO2-TiO2 are 15.8, 10.0 and 12.3 nm, respectively. When the comparison is made within the ZrO2-TiO2 series, increase in the Zr-content to 10% decreases the size. This is because the crystallite growth in the TiO2 lattices suppresses as Zr replaces Ti4+ ions during the sol-gel process.
<
T
The degree of crystallinity and phase compositions of the x% ZrO2-TiO2 powdered
IP
samples calcined at 550 °C were studied by means of x-ray diffraction method (XRD) using
SC R
TD-3500 diffractometer at room temperature with Ni-filtered Cu Kα radiation (λ = 0.15418 Å), at 35 kv and 25 mA. The average crystallite size was calculated according to the
D
k cos
NU
Scherrer’s equation,
(1)
MA
where D is the mean crystallite size, k (0.89) is the Scherrer constant, is the X-ray wavelength (0.15418 Å), and is the relative value of the full-width at half-maximum
D
(FWHM) of the (101) diffraction peak of catalysts. Transmission electron microscopy
TE
(TEM) images of calcined ZrO2-TiO2 films with different loading ratio were obtained on
CE P
Jeol TEM-1230 electron microscope at an acceleration voltage 120 kV, and at magnifications 50000x and 80000x. The morphology and microstructure and elemental composition of the deposited films on carbon steel substrates before corrosion test were
AC
studied by means of the scanning electron microscopy equipped with the EDAX detector.
2.4. Electrochemical measurements Electrochemical measurements were carried out in a three-electrode cell in 1.0 M HCl using VersaSTAT 4 potentiostat/galvanostat with a frequency response analyzer (FRA) contained in a single unit and connected with laptop. All solutions were prepared from grade chemicals (Merck) and bidistilled water. Saturated calomel electrode (SCE), the coated and uncoated carbon steel specimens, and platinum plate electrode were chosen as reference electrode, working electrode and counter electrode, respectively8 The specimen surfaces in contact with the solution had a constant area of 4.55 cm2. It has been reported
5
ACCEPTED MANUSCRIPT that, the corrosion rate of carbon steel is temperature dependent, and the corrosion rate has the largest value at 50 °C [25]. Therefore, in order to evaluate the protection effect of the coated samples on carbon steel in HCl solution, all the experiments were performed at 50
T
°C in the following order: The open circuit potential vs. time for 0.5 hour, the polarization
IP
measurements were recorded by sweeping the potential from −250 mV to +250 mV with
SC R
respect to corrosion potential (Ecorr) at scan rate of 1 mV/s. The data obtained were analyzed by Tafel extrapolation method. Electrochemical impedance spectroscopy (EIS)
NU
measurements were performed at open circuit potential. The range of applied frequencies was from 100 kHz to 0.05 Hz using voltage perturbation amplitude of 10 mV RMS.
MA
2.5. Surface characterization after exposure to 1.0 M HCl In order to observe any changes in surface morphologies of the carbon steel
D
substrate after exposure to 1.0 M HCl, the specimens were first immersed in the corrosive
TE
media with and without coating for 5 days, then cleaned with bi-distilled water and acetone,
CE P
and dried with cool air then the topography of the tested samples was observed by using scanning electron microscope (SEM) conducted with energy-dispersive X-ray spectroscopy
AC
analysis (EDAX) (SEM-EDAX) (JEOL, model 5300).
3. Results and discussion 1.3. Characterization of x% ZrO2-TiO2 films 3.1.1. XRD Analysis. The XRD patterns of blank TiO2, and doped samples of x wt % ZrO2-TiO2, (x=5, 10, and 20) calcined at 550 °C (powdered samples) are shown in Fig. 2a-d. The diffraction peaks of anatase TiO2 can be observed in all samples (Card No JCPDS 21-1272). It was noted that; the doped samples with x wt % ZrO2-TiO2 show the (101) peaks broader than that of TiO2. The present observations indicate that, the lattice structure of TiO2 is locally
6
ACCEPTED MANUSCRIPT deformed by incorporation of dopant Zr ions into TiO2 [26-28]. From XRD pattern of TiO2 blank, it was found that mixed rutile and anatase phases were formed and as a result of incorporation of Zr ions, the rutile phase slightly disappeared. It was further noted that, the
T
addition of Zr4+ has facilitated the formation of anatase phase than rutile phase. The lattice
IP
parameters of the samples were measured using (101) and (200) in anatase crystal planes to
equations (2) and (3):
d-2(hkl)= h2a-2+k2b-2+l2c-2
2 sin
NU
Braggs equation : d (hkl)
SC R
study the effect of Zr% content as dopant on the lattice structure of TiO2, by using
(2) (3)
MA
where d(hkl) is the distance between crystal planes of (hkl), is the X-ray wavelength, θ is the diffraction angle of crystal plane (hkl), hkl is the crystal index, and a, b and c are lattice
D
parameters (in anatase form, a=b≠ c tetragonal structure). The results are indexed in Table
TE
1. Lattice parameters calculation showed that, the lattice parameters of all TiO2 samples
CE P
remain almost constant along the a- and b- axes, whereas c-axis parameter differs due to the presence of Zr as the ionic radius of Zr4+ as dopant differs than that of Ti4+ [27-29]. Which allows for possible diffusion of Zr4+ along the c-axis to substitute Ti4+ in TiO2, the average
AC
crystallite size of samples (Table 1) was measured by the Scherrer’s equation from the (101) peak of anatase TiO2. The average crystallite size of TiO2 is 19.7 nm and doped titania samples with 5, 10 and 20% ZrO2-TiO2 are 15.8, 10.0 and 12.3 nm, respectively. When the comparison is made within the ZrO2-TiO2 series, increase in the Zr-content to 10% decreases the size. This is because the crystallite growth in the TiO2 lattices suppresses as Zr replaces Ti4+ ions during the sol-gel process.
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