- Email: [email protected]
The effect of curing time and curing temperature on the corrosion behavior of nanosilica modified potassium silicate coatings on AA2024
The effect of curing time and curing temperature on the corrosion behavior of nanosilica modified potassium silicate coatings on AA2024 H. Bahri, I. Danaee, G.R. Rashed PII: DOI: Reference:
S0257-8972(14)00552-0 doi: 10.1016/j.surfcoat.2014.06.041 SCT 19502
To appear in:
Surface & Coatings Technology
Received date: Revised date: Accepted date:
2 April 2014 20 June 2014 22 June 2014
Please cite this article as: H. Bahri, I. Danaee, G.R. Rashed, The effect of curing time and curing temperature on the corrosion behavior of nanosilica modified potassium silicate coatings on AA2024, Surface & Coatings Technology (2014), doi: 10.1016/j.surfcoat.2014.06.041
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
PT
The effect of curing time and curing temperature on the corrosion behavior of nanosilica modified potassium silicate coatings on AA2024
SC
RI
H. Bahri , I. Danaee*, G.R. Rashed Abadan Faculty of Petroleum Engineering, Petroleum University of Technology, Abadan, Iran
MA
NU
*Corresponding author. Email address: [email protected] (I. Danaee) Tel: (0098631) 4429937
Abstract
D
Nanosilica modified potassium silicate conversion coatings were deposited on the surface of
TE
2024 aluminum alloy. The corrosion behavior of coatings was studied by electrochemical impedance spectroscopy, potentiodynamic polarization and the surface analyzing techniques.
AC CE P
The effect of curing time and the curing temperature were studied on anti-corrosion behavior of coatings. Curing temperature showed a significant effect in silicate conversion coating and higher corrosion resistance was obtained with 150 ºC curing temperature. Also the experimental results indicated that the corrosion resistance was increased with increasing the curing time. This behavior can be related to the increase of the silicate coating continuity and reinforcement of the siloxane chains which formed on the surface. Surface analysis results indicated that the coating obtained from 2.33 silica ratio was more uniform and continuous. Key words: Aluminum 2024; Silicate conversion coatings; Nanosilica; Corrosion.
1
ACCEPTED MANUSCRIPT 1. Introduction Aluminum and its alloy is one of the materials that has an important role in the engineering
PT
structures and its usage ranking stands only behind the ferrous alloys. Some special properties of
RI
this metal include: its strength, the density ratio, toughness and the corrosion resistance [1]. In
SC
order to increase the mechanical and physical properties of aluminum, some alloy elements are added to the pure aluminum such as: magnesium, silicon, copper, zinc, iron, manganese and
NU
lithium [2,3]. Aluminum alloy 2024 is used greatly in aerospace industries because of its excellent mechanical properties and low density [4,5]. Materials such as copper, magnesium,
MA
zinc and iron in this alloy decrease the corrosion resistance significantly due to the formation of the intermetallic compounds (IMC). Pure aluminum has a good corrosion resistance due to the
TE
D
formation of a strong oxide layer, but AA2024 involves intermetallic compounds which form small galvanic cells in the aluminum matrix and lead to the localized corrosion [6,7].
AC CE P
One of the main important ways for the protection of these alloys is coatings [8,9]. Chromate conversion coatings (CCCs) are the most common and the effective anti-corrosive treatments for aluminum alloys [10,11]. Although, the CCCs provide a great corrosion resistance, the materials used to produce the coating are toxic and carcinogenic due to the presence of hexavalent chromium in their structure. Therefore, usage of these coatings has restricted under strict regulations [12,13]. Due to the toxicity of Cr6+, several studies have done for the replacements of the CCCs and the extension of non-toxic coatings [13]. Silicate conversion coatings are one of the best superior alternatives for the CCCs [14,15]. These coatings have a good durability against each kind of corrosion and are safe for the human and environment [14-17]. Many studies showed that the silicate coatings provided good corrosion resistance for the structural metals [18-22]. Alkali silicates are a common and general
2
ACCEPTED MANUSCRIPT group of the silicate coatings. These coatings have polymerized particles containing silanol groups (-Si-OH) and siloxane bridges (-Si-O-Si-) in their structure. During the curing step,
PT
polymerizations continue and more silanol groups are converted to the siloxane bridges [23-25].
RI
In the present study, modified silicate coatings were applied on AA2024 and the effects of the curing time and curing temperature were studied. The corrosion behavior of these coatings
SC
was investigated by means of electrochemical impedance spectroscopy (EIS) and polarization
NU
curves. Surface morphology and chemical composition of silicate conversion coatings were analyzed by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and
2.1. Materials
D
TE
2. Materials and methods
MA
atomic force microscopy (AFM).
AC CE P
The samples were prepared from AA2024 sheets with dimensions 50×100×2 mm. The major alloying elements of this alloy are listed in Table 1. The samples were subjected to the mechanical polishing with sand papers of 400 to 2000 grades, and then cleaned with acetone. To remove any trace of the surface oxides, the samples were immersed in 3 M NaOH solution for 30 s and rinsed with deionized water, immediately. Then the samples were immersed in 1 M H2SO4 solution for 30 s and rinsed with deionized water. A 40 wt.% aqueous solution of potassium silicate with silica/alkali molar ratio of 2 was prepared from the Iran Silis Company (Iran). A 30 wt.% aqueous solution of colloid nanosilica with 20-30 nm particle size was prepared from Iranian Nanomaterials Pioneers Company. Coating solutions with the different ratios of SiO2/K2O were formed by mixing a constant concentration of potassium silicate (1 M) with different volumes of colloid nanosilica. AA2024
3
ACCEPTED MANUSCRIPT samples were immersed for 400 s in prepared silicate coating solution. In very low immersion times (<100 s), the prepared coatings were weak and very thin. Also in very high immersion
PT
times (>1000 s), the corrosion protection behavior of coatings was decreased. The medium range
RI
of immersion times was suitable for the optimum coatings preparation. After immersion, the coated samples were cured in different times and temperatures in the furnace. The coated
NU
SC
samples were stored at room temperature for 1 h before starting the test.
2.3. Electrochemical and surface morphology investigation
MA
Electrochemical measurements were carried out by Potentiostat/Galvanostat Autolab, model PGSTAT 302N (Holland). Electrochemical glass cell contained a reference electrode of
D
Ag/AgCl, an auxiliary electrode of platinum and a sample of coated 2024 aluminum alloy as the
TE
working electrode with exposed surface area equal to 1 cm2. To provide the steady state
AC CE P
condition, samples were stored for 30 min in 3.5 wt.% NaCl electrolyte solution before the test. The scan rate of polarization curves was 1 mV s-1. The frequency range of EIS was 105-10-2 Hz with AC signal amplitude of 10 mV. Fitting of experimental impedance spectroscopy data to the proposed equivalent circuit was done by means of a home written least square software based on the Marquardt method for the optimization of functions and Macdonald weighting for the real and imaginary parts of the impedance [26,27]. The morphology of the surface was studied by scanning electron microscopy (SEM, VEGA, TESCAN-LMU) equipped with an energy-dispersive x-ray spectroscopy (EDS) probe, and atomic force microscopy (AFM) Nanosurf Easyscan 2. The X-ray diffraction (XRD) was done by using x-ray diffractometer, broker, XPERT-PRO, Germany, with x-ray Tube Anode Cu and the Cu kα radiation (λ= 1.5406 ˚A) was generated at 40 kV and 40 mA and used as x-ray source.
4
ACCEPTED MANUSCRIPT The FTIR-transmission spectra were recorded on a Bruker Optics Equinox 55 spectrometer (Ettlingen, Germany) equipped with a MKII Golden gate micro attenuated total reflectance
PT
accessory. The thickness of the dry film was measured by Elcometer 456 coating thickness
RI
gauge and was found 600±50 nm.
SC
3. Results and discussion
NU
3.1. Surface analysis
MA
Fig. 1 & 2 show the SEM images and the corresponding EDS spectrum of potassium silicate coatings with different SiO2/K2O ratios. The EDS results are represented in Table 2 with the
D
atomic percentages of the coating elements. The results indicated that the Si:K atomic ratio was
TE
increased in conversion coatings with the addition of nanosilica. The silicate coating obtained without nanosilica from ratio 2 (Fig. 1a), contained discontinuities and agglomerates which
AC CE P
related to the silicate networks and the defects of these layers. The coating defects are the susceptible zones for the corrosion attacks and the initiation of the pitting corrosion. Fig. 1b shows the surface of silicate layer obtained from ratio 2.33. This coating was homogeneous without discontinuities and irregularities on its surface. The brighter zones on the surfaces with nanoscale size can be related to the nanosilica particles that remained unchanged on the surface and did not participate in the polymerization. These brighter zones cannot be seen in Fig. 1a. Structural analysis of the deposit was performed using XRD. Xray diffractograms of the bare and silicate coated aluminum in the range 1º< 2θ < 80º are shown in Fig. 3. Peaks at 2θ= 38.6º, 44.8º, 65.1º and 78.2º are due to face-centered cubic (fcc) structure oriented in (111), (200), (220) and (311) directions of aluminum substrate [28]. The intensity of aluminum peaks were decreased in the presence of silicate conversion coatings. In addition, a hump was observed 5
ACCEPTED MANUSCRIPT in the 2θ ranging from 15º to 35º in the presence of silicate coatings, which indicated the disordered structure. The X-ray diffraction in this range was mainly due to the amorphous silica
PT
particles [29-31]. No significant change was observed in the presence of nano-silica, due to the
RI
amorphous structures and very low thickness of the silicate conversion coatings. Fig. 4&5 show two and three-dimensional AFM images of the silicate coated aluminum
SC
obtained from different silica ratios. It can be seen that the surface morphology of the coatings
NU
differed from each other. The silicate conversion coating obtained from silica ratio 2 was not homogeneous and produced some pinholes and pores on the surface (Fig. 4). With the increase in
MA
the Si/K molar ratio of silicate solution, the continuity and uniformity of the produced layer was enhanced which was related to the nanoparticles and their filler capability. Moreover, the
TE
D
tendency to appearance of pores and the roughness was decreased (Fig. 5). The SEM images of the different curing temperatures are represented in Fig. 6. The silicate
AC CE P
layer could not cure completely and did not produce a compact and stable film in lower temperature (Fig. 6a). In other words, the temperature was not sufficient for the silicate polymerization and production of strong siloxane bonds. As seen, compact and uniform conversion coatings was obtained in 150 °C curing temperature (Fig. 1b). However, the water evaporation rate was increased in very high temperature and the continuity of the coating was decreased. Fig. 6b showed the blisters in silicate conversion coating which cured in 220oC. In addition, blisters were clearly observed by SEM image in low magnification (Fig. 6c). Fourier Transform Infrared method was used to investigate the structural changes of the silicate coating in different curing times. Fig. 7 shows the FTIR spectra of the silicate film with ratio 2.33 which cured at 150 ℃. The spectra of the silicate indicated a broad peak at 900 to 1200 cm-1 which was due to the asymmetric Si-O-Si stretching vibration [30,32,33]. Also peaks at 490
6
ACCEPTED MANUSCRIPT and 770 cm-1 were related to Si-O and Si-OH bonds, respectively. As seen, the intensity and positions of peaks were not significantly changed with increasing the curing time. The Si-O-Si
PT
bands centered at 1000 cm–1 to 1200 cm–1 became more intensive as the curing time increased.
RI
Extensive cross-links were created in silicate during the curing process and therefore, more
SC
siloxane bonds could be formed with increasing the curing time.
NU
3.2. Effects of SiO2/K2O ratio
Fig. 8 shows the potentiodynamic polarization curves of different SiO2/K2O ratios of silicate
MA
coatings in 3.5 wt% NaCl solution. Coatings were obtained by 400 s immersion in coating solution and curing in 150 °C for 30 min. The corresponding corrosion parameters such as
TE
D
corrosion current density (Icorr), corrosion rate (CR), corrosion potential (Ecorr), anodic slope (βα), cathodic slope (βc) and polarization resistance (Rp) are listed in Table 3. The polarization
Rp
AC CE P
resistance is calculated by Stern–Geary equation [12,34]:
a.c 1 2.303( a c ) Icorr
According to Fig. 8, the corrosion current densities were decreased in the presence of the silicate coatings. Both the anodic and cathodic branches of the polarization curves of silicate coating were shifted to the lower corrosion current densities, indicated that both the anodic and cathodic reaction are blocked. During the curing process, the existing water in the coating was evaporated and the silicate networks (siloxane bridges) were formed. Corrosion protection was mainly due to the formation of silica rich Al-oxide film. The silanol groups were adsorbed on the aluminum surface by hydrogen bonds and metallo-siloxane (M-O-Si) bonds were produced between the metal surface and the coating layer during the curing process [16, 20, 35].
7
ACCEPTED MANUSCRIPT As can be seen, the anodic branches of bare aluminum and the coating obtained from 1 M potassium silicate without nanosilica showed pitting corrosion in the high applied anodic
PT
potential range. The current density was increased obviously in this region due to pit nucleation.
RI
However, intensity of pitting corrosion in silicate coating was lower than the bare aluminum. Addition of the nanosilica into the coating solution caused to increment of SiO2/K2O ratios.
SC
In addition, the corrosion resistance of the silicate coating was increased and its corrosion rate
NU
and the pitting corrosion probability were decreased (Fig. 8). The size of nano-silica particles was the most important effect which led to filling the pores and spaces of the silicate networks
MA
[25]. Therefore, the uniformity of the silicate conversion coatings were improved [36]. The high surface area was another benefit of these particles which provided higher contacts between the
TE
D
functional groups and more formation of silicate networks and siloxane bridges [37]. With increasing silica ratio up to 2.33, the corrosion resistance of silicate layer was
AC CE P
increased. Silicates with larger number of nanoparticles had more polymeric units (Si-O). Therefore, the possibility of siloxane bridges formation between the particles, during the curing process, was increased. Moreover, silicate surfaces are water absorbent due to the positive charge of surface, namely potassium. Colloidal silica has a negative charge and can bind to the positive charges. Therefore, in the presence of nano-silica, hydrophilic sites were decreased and the hydrophobic property of silicate layers was increased which provided greater corrosion protection. In the higher silica ratios, the corrosion rate was increased due to the internal stress which decreased its toughness and led to increasing the cracks (Fig. 1a&b). A few cracks were created on the surface due to the curing process and therefore, the corrosion resistance of the coatings was decreased.
8
ACCEPTED MANUSCRIPT 3.3. Effects of the curing temperature Fig. 9 depicts the polarization curves of the silicate coated aluminum in different curing
PT
temperatures and the corresponding electrochemical parameters are presented in Table 4.
RI
Coatings were obtained by 400 s immersion in solution with 2.33 silica ratio and then 30 min curing time. Fig. 9 showed that lower temperature couldn’t cure the waterborne coating layer
SC
completely and the coating did not have enough stability against the corrosive environment.
NU
Moreover, silicate coting was capitulated to the pitting corrosion. More elevated temperature up to 150 °C, could increase the curing rate of the coating. Therefore, stronger silicate networks
MA
(siloxane chains) were produced, which had higher stability against the corrosive solution. In higher curing temperature around 220 °C, the corrosion resistance was decreased due to the
TE
D
blisters and cracks in the coatings (Fig. 6a-c). In the curing step, the rate of water evaporation was important parameter. In very high temperatures, the evaporation rate was increased
AC CE P
dramatically and the continuity of the coating was decreased. Presence of the blisters and cracks on the surface of coatings were related to the high evaporation rate which led to increasing the penetration zones for the aggressive ions. Electrochemical impedance was used to confirm the anticorrosive properties of the silicate coatings. The Nyquist plots of silicate coated aluminum obtained in different curing temperatures are presented in Fig. 10. Coatings were obtained by 400 s immersion in solution with 2.33 silica ratio and then 30 min curing time. Impedance was measured at open circuit potential in 3.5 %wt. NaCl solution. The data showed that impedance diagrams consisted of a depressed capacitive semicircle which was due to the double layer capacitance and charge transfer resistance. Fig. 11 depicts the equivalent circuit compatible with the Nyquist diagrams. In this electrical equivalent circuit, Rs, Qdl and Rct represented solution resistance, a constant phase element corresponding to
9
ACCEPTED MANUSCRIPT the double layer capacitance and the charge transfer resistance, respectively. It was necessary to replace the capacitor (C) with a constant phase element (CPE) Q in the equivalent circuit to
PT
obtain a satisfactory impedance simulation of silicate coated aluminum. The most widely
RI
accepted explanation for the presence of CPE behavior and the depressed semicircles on solid electrodes is microscopic roughness, lead to the inhomogeneous distribution in the solution
SC
resistance as well as in the double-layer capacitance [38, 39].
NU
To corroborate the equivalent circuit, the experimental data was fitted to the equivalent circuit and the circuit elements were obtained. The equivalent circuit parameters for the
MA
impedance spectra of corrosion of silicate coated aluminum in NaCl solution are shown in Table 5. According to this table, high corrosion resistance was obtained by 150 °C curing temperature
TE
D
which was due to the presence of the more uniform silicate coating on the surface of AA2024.
AC CE P
3.4. Effects of the curing time
Fig. 12 shows the polarization curves of silicate coated AA2024 in different curing times and the corresponding electrochemical parameters are presented in Table 6. Coatings were obtained by 400 s immersion in solution with 2.33 silica ratio and then curing in 150 °C. As can be seen corrosion current was decreased with increasing curing time. During the curing step, water of the coating was evaporated and the strong siloxane bonds were produced on the surface. These bonds increased the continuity and stability of the silicate layer and excessively improved its corrosion resistance. Whatever the samples were kept for a longer period of time in the furnace at optimized temperature, more siloxane bonds were formed on the metal surface. Therefore, the properties of the coating layer were further improved. In the higher curing times, around 2 h, the anticorrosive properties of the silicate coating were decreased.
10
ACCEPTED MANUSCRIPT Nyquist plots for the silicate conversion coating obtained in different curing times are presented in Fig. 13. Impedance data were measured at open circuit potential in 3.5 %wt NaCl
PT
solution. Silicate conversion coatings were obtained by 400 s immersion in solution with 2.33
RI
silica ratio and then curing in 150 °C. All coatings showed capacitive loop attributed to the double layer capacitance and the charge transfer resistance. To obtain a satisfactory simulation,
SC
the capacitor was replaced with a constant phase element. The experimental data were fitted to
NU
the equivalent circuit (Fig. 11) and the circuit elements were obtained. Table 7 illustrates the equivalent circuit parameters for the impedance spectra of the silicate coated aluminum obtained
MA
in different curing times. With increasing curing times, the corrosion resistance was increased due to strong siloxane bonds. Also in the higher curing times, the corrosion resistance was
TE
D
decreased due to the decrease of stability of silicate layer in long curing time. With 1 h curing time, the silicate conversion coating with high corrosion resistance was obtained. This was in
AC CE P
agreement with the potentiodynamic polarization data.
4. Conclusion
The corrosion behavior of nanosilica modified potassium silicate coating applied on the 2024 aluminum alloy was investigated in 3.5 wt. % NaCl solution. This conversion coating improved the corrosion resistance of the aluminum alloy due to the formation of silicate film. But this protective barrier had some discontinuities and defects that limited its anticorrosive properties. Presence of nano-silica improved the silicate networks and decreased the existing defects. The ratio of nano-silica showed significant effect to provide a high corrosion protection. The film that formed from 1 M potassium silicate with ratio 2.33 nano-silica, was relatively more protective and continuous. The increasing corrosion resistance was mainly due to the formation
11
ACCEPTED MANUSCRIPT of the compact and perfect silicate layers on the surface. The curing temperature played an important role to provide silicate conversion coating and the best corrosion resistance was
PT
obtained with 150 ºC curing temperature. In addition, the corrosion resistance was increased with
RI
increasing the curing time due to the increasing of the silicate coating continuity and reinforcement of siloxane matrix. The results of SEM and AFM showed that the silicate coatings,
SC
obtained from 2.33 silica ratio and 150 ºC curing temperature, were more uniform and
AC CE P
TE
D
MA
increased with increasing the curing times.
NU
continuous. FTIR analysis of silicate conversion coatings showed that the siloxane bridges were
12
ACCEPTED MANUSCRIPT References [1] I. Shchedrina, A.G. Rakoch, G. Henrion, J. Martin, Non-destructive methods to control the
PT
properties of MAO coatings on the surface of 2024 aluminium alloy, Surf. Coat. Technol. 238
RI
(2014) 27-44.
SC
[2] L. Wen, Y. Wang, Y. Jin, B. Liu, Y. Zhou, D. Sun, Microarc oxidation of 2024 Al alloy using spraying polar and its influence on microstructure and corrosion behavior, Surf. Coat.
NU
Technol. 228 (2013) 92-99.
[3] V. Vitry, A. Sens, A.-F. Kanta, F. Delaunois, Wear and corrosion resistance of heat treated
MA
and as-plated Duplex NiP/NiB coatings on 2024 aluminum alloys, Surf. Coat. Technol. 206 (2012) 3421-3427.
TE
D
[4] J.C.S. Fernandes, M.G.S. Ferreira, D.B. Haddow, A. Goruppa, R. Short, D.G. Dixon, Plasma-
(2002) 8-13.
AC CE P
polymerised coatings used as pre-treatment for aluminium alloys, Surf. Coat. Technol. 154
[5] G. Yoganandan, J.N. Balaraju, Synergistic effect of V and Mn oxyanions for the corrosion protection of anodized aerospace aluminum alloy. Surf. Coat. Technol. (2014) in press, http://dx.doi.org/10.1016/j.surfcoat.2014.04.062. [6] W. Pinc, S. Maddela, M. O'Keefe, W. Fahrenholtz, Formation of subsurface crevices in aluminum alloy 2024-T3 during deposition of cerium-based conversion coatings, Surf. Coat. Technol. 204 (2010) 4095-4100. [7] E. Darmiani, I. Danaee, M.A. Golozar, M.R. Toroghinejad,Corrosion investigation of Al–SiC nano-composite fabricated by accumulative roll bonding (ARB) process, J. Alloys Comp. 552 (2013) 31-39.
13
ACCEPTED MANUSCRIPT [8] Lj.S. Živković, J.P. Popić, B.V. Jegdić, Z. Dohčević-Mitrović, J.B. Bajat, V.B. MiškovićStanković, Corrosion study of ceria coatings on AA6060 aluminum alloy obtained by cathodic
PT
electrodeposition: Effect of deposition potential, Surf. Coat. Technol. 240 (2014) 327-335.
RI
[9] J.M. Sánchez-Amaya, G. Blanco, F.J. Garcia-Garcia, M. Bethencourt, F.J. Botana, XPS and AES analyses of cerium conversion coatings generated on AA5083 by thermal activation, Surf.
SC
Coat. Technol. 213 (2012) 105-116.
NU
[10] R.L. Twite, G.P. Bierwagen, Review of Alternatives to Chromate for Corrosion Protection of Aluminum Aerospace Alloys. Prog. Org. Coat. 33, 1998, p 91-100.
MA
[11] J. Zhao, L. Xia, A. Sehgal, D. Lu, R.L. McCreery, G.S. Frankel, Effects of chromate and chromate conversion coatings on corrosion of aluminum alloy 2024-T3, Surf. Coat. Technol. 140
TE
D
(2001) 51-57.
[12] M. Kanani, I. Danaee, M. H. Maddahy, Microstructural characteristics and corrosion
AC CE P
behavior of cerium oxide conversion coatings on AA6063. Mater. Corros. (2014) DOI: 10.1002/maco.201307539
[13] X. Montero, M.C. Galetz, M. Schütze, Low-activity aluminide coatings for superalloys using a slurry process free of halide activators and chromates, Surf. Coat. Technol. 222 (2013) 914.
[14] J. Min, J.H. Park, H.K. Sohn, J.M. Park, Synergistic Effect of Potassium Metal Siliconate on Silicate Conversion Coating for Corrosion Protection of Galvanized Steel. Ind. Eng. Chem. 18 (2012) 655-660. [15] A.S. Hamdy, D.P. Butt, Environmentally Compliant Silica Conversion Coatings Prepared by Sol–Gel Method for Aluminum Alloys. Surf. Coat. Technol. 201 (2006) 401-407.
14
ACCEPTED MANUSCRIPT [16] M.R. Yuan, J.T. Lu, G. Kong, C.S. Che, Effect of Silicate Anion Distribution in Sodium Silicate Solution on Silicate Conversion Coatings of Hot-Dip Galvanized Steels. Surf. Coat.
PT
Technol. 205 (2011) 4466-4470.
RI
[17] M.R. Yuan, J.T. Lu, G. Kong, C.S. Che, Self-Healing Ability of Silicate Conversion Coatings on Hot Dip Galvanized Steels. Surf. Coat. Technol. 205 (2011) 4507-4513.
SC
[18] H.J. Kim, J. Zhang, R. H. Yoon, R. Gandour, Development of environmentally friendly
NU
nonchrome conversion coating for electrogalvanized steel, Surf. Coat. Technol. 188–189 (2004) 762– 767.
MA
[19] L. Jiang, M. Wolpers, P. Volovitch, K. Ogle, An atomic emission spectroelectrochemical study of passive film formation and dissolution on galvanized steel treated with silicate
TE
D
conversion coatings, Surf. Coat. Technol. 206 (2012) 3151–3157. [20] M. Hara, R. Ichino, M. Okido, N. Wada, Corrosion protection property of colloidal silicate
AC CE P
film on galvanized steel, Surf. Coat. Technol. 169 –170 (2003) 679–681. [21] Sandrine Dalbin, Georges Maurin, Ricardo P. Nogueira, Jacques Persello, Nicolas Pommier, Silica-based coating for corrosion protection of electrogalvanized steel, Surf. Coat. Technol. 194 (2005) 363– 371.
[22] K. Aramaki, Self-Healing Mechanism of an Organosiloxane Polymer Film Containing Sodium Silicate and Cerium (III) Nitrate for Corrosion of Scratched Zinc Surface in 0.5 M NaCl. Corros. Sci. 44 (2002) 1621-1632. [23] L.T. zhuravlev, The Surface Chemistery of Amorphous Silica. Zhuravlev Model. Colloid. Surf. 173 (2000) 1-38. [24] U.S. Bureau of Mines. Crystalline Silica Primer. Washington, DC. 1992.
15
ACCEPTED MANUSCRIPT [25] M. Yuan, J. Lu. G. Kong, Effect of SiO2:Na2O Molar Ratio of Sodium Silicate on the Corrosion Resistance of Silicate Conversion Coatings. Surf. Coat. Technol. 204 (2010) 1229-
PT
1235.
RI
[26] I. Danaee, Kinetics and Mechanism of Palladium Electrodeposition on Graphite Electrode by Impedance and Noise Measurements. J. Electroanal. Chem. 662 (2011) 415-420.
SC
[27] J.R. Macdonald, Note on the Parameterization of the Constant-Phase Admittance Element.
NU
Solid State Ion. 13 (1984) 147-149.
[28] M. Moradshahi, T. Tavakoli, S. Amiri, Sh. Shayeganmehr, Plasma nitriding of Al alloys by
MA
DC glow discharge, Surf. Coat. Technol. 201 (2006) 567-574. [29] D. Singh, R. Kumar, A. Kumar, K. N. Rai, Synthesis and characterization of rice husk
TE
D
silica, silica-carbon composite and H3PO4 activated silica. Cerâmica 54 (2008) 203-212. [30] M. Rokita, W. Mozgawa, A. Adamczyk, Transformation of silicate gels during heat
AC CE P
treatment in air and in argon– Spectroscopic studies, J. Mol. Struct. In press, http://dx.doi.org/10.1016/j.molstruc.2014.04.020 [31] M. Trevino, R.D.Mercado-Solis, R.Colas, A.Perez, J.Talamantes, A.Velasco, Erosive wear of plasma electrolytic oxidation layers on aluminium alloy 6061, Wear 301 (2013) 434-441. [32] X. Yang, P. Roonasi, A. Holmgren, A study of sodium silicate in aqueous solution and sorbed by synthetic magnetite using in situ ATR-FTIR spectroscopy. J. Colloid Interface Sci. 328 (2008) 41–47. [33] X. Yanga, P. Roonasi, R. Jolsterå, A. Holmgren, Kinetics of silicate sorption on magnetite and maghemite: An in situ ATR-FTIR study, Colloids Surf. A: Physicochem. Eng. Aspects 343 (2009) 24–29.
16
ACCEPTED MANUSCRIPT
PT
[34] O. Ghasemi, I. Danaee, G.R. Rashed, M. RashvandAvei, M.H. Maddahy, The Inhibition
in 1 M HCl, J. Mater. Eng. Perform. 22 (2013) 247-259.
RI
Effect of Synthesized 4-Hydroxybenzaldehyde-1,3propandiamine on the Corrosion of Mild Steel
using
a
mixture
of
bis-[triethoxysilylpropyl]tetrasulfide
and
bis-
NU
steel
SC
[35] D. Zhu, W.J. Van Ooij. Enhance corrosion resistance of AA2024-T3 and hot-dip galvanized
[trimethoxysilylpropyl]amine. Electrochim. Acta, 49 (2004) 1113-1125.
MA
[36] Klabunde, K. J. Nanoscale materials in chemistry. Wiley-Interscience, New York, 2001. [37] V. Padavettan, I. Ab Rahman, Synthesis of Silica Nanoparticles by Sol-Gel: SizeSurface
Modifications,
D
Properties,
and
Applications
in
Silica-Polymer
TE
Dependent
Nanocomposites. J. Nanomater. 2012 (2012) 1-15.
AC CE P
[38] I.Danaee, S. Noori, Kinetics of the Hydrogen Evolution Reaction on NiMn Graphite Modified Electrode. Int. J. Hydrogen Energy. 36 (2011) 12102-12111. [39] I. Danaee, M. NiknejadKhomami, A.A. Attar, Corrosion Behavior of AISI 4130 Steel Alloy in Ethylene Glycol–Water Mixture in Presence of Molybdate. Mater. Chem. Phys. 135 (2012) 658-667.
17
ACCEPTED MANUSCRIPT Table 1. Chemical composition of the substrate (AA2024). Al balance
Si <0.5
Fe <0.5
Cu 3.8-4.6
Mn 0.5-0.8
Mg 1.3-1.6
Cr <0.1
Zn <0.25
Ti <0.15
V <0.05
Zr <0.05
PT
Element % w/w
Table 2. EDS results of the 1 M potassium silicate surface coatings in different ratios of
Atomic % of Si
Ratio 2 Ratio 2.33
19.96 20.34
Atomic % of K
SC
Ratio
RI
SiO2/K2O.
69.36 73.75
NU
10.68 5.91
Atomic % of O
Table 3. Electrochemical parameters of different SiO2/K2O ratios of the silicate coating medium temperature and 30 min curing time. Ecorr / V
Icorr / A cm-2
Bare Ratio 2 Ratio 2.11 Ratio 2.33 Ratio 2.44
-0.668 -0.609 -1.079 -1.083 -1.235
8.47×10-7 4.19×10-7 3.88×10-9 1.26×10-10 1.74×10-9
βα / V dec-1
βc / V dec-1
Rp / ohm
Vcorr / mm y-1
0.005 0.095 0.114 0.066 0.041
0.012 0.113 0.265 0.085 0.259
1.82×103 5.34×104 8.91×106 1.28×108 0.96×107
4.14×10-3 4.92×10-3 4.56×10-5 1.48×10-6 2.04×10-5
AC CE P
TE
D
Samples
MA
in 3.5 wt. % NaCl solution. Coatings were prepared in 400s immersion time, 150oC curing
Table 4. Electrochemical parameters of different curing temperatures for the silicate coating with 2.33 ratio in 3.5 wt. % NaCl solution. Coatings were prepared in 400 s immersion time and 30 min curing time. Samples
Ecorr / V
Icorr / A cm-2
βα / V dec-1
βc / V dec-1
Rp / ohm
Vcorr / mm y-1
Bare 80 °C 150 °C 220 °C
-0.668 -1.012 -1.083 -1.192
8.47×10-7 8.03×10-9 1.26×10-10 9.87×10-9
0.005 0.089 0.066 0.045
0.012 0.142 0.085 0.216
1.82×103 2.95×106 1.28×108 1.63×106
4.14×10-3 4.23×10-5 1.48×10-6 6.63×10-5
Table 5. EIS parameters of different curing temperatures for the silicate coating with 2.33 ratio in 3.5 wt. % NaCl solution. Coatings were prepared in 400 s immersion time and 30 min curing time. Sample 80 °C 150 °C 220 °C
Rct /Ω 9.11×107 1.46×108 7.11×107
Qdl /F 5.51×10-8 4.23×10-8 8.05×10-8
18
n 0.66 0.83 0.63
Other <0.15
ACCEPTED MANUSCRIPT Table 6. Electrochemical parameters of different curing times for the silicate coating with 2.33 ratio in 3.5 wt. % NaCl solution. Coatings were prepared in 400 s immersion time and 150oC curing temperature. Icorr / A cm-2
βα / V dec-1
βc / V dec-1
Bare 15 min 30 min 1 hour 2 hour
-0.668 -1.218 -1.083 -0.751 -0.568
8.47×10-7 4.83×10-9 1.26×10-10 3.86×10-11 1.22×10-10
0.005 0.086 0.066 0.131 0.082
0.012 0.225 0.085 0.202 0.282
Rp / ohm
Vcorr / mm y-1
1.82×103 5.59×106 1.28×108 8.91×109 2.27×108
4.14×10-3 1.35×10-5 1.48×10-6 4.53×10-7 1.43×10-6
PT
Ecorr / V
SC
RI
Samples
Table 7. EIS parameters of different curing times for the silicate coating with 2.33 ratio in 3.5
NU
wt. % NaCl solution. Coatings were prepared in 400 s immersion time and 150oC curing temperature.
D
MA
Qdl /F 6.24×10-8 4.23×10-8 2.22×10-8 1.08×10-7
TE
15 min 30 min 1 hour 2 hour
Rct /Ω 7.53×107 1.46×108 7.13×108 5.82×108
AC CE P
Sample
19
n 0.88 0.83 0.89 0.85
ACCEPTED MANUSCRIPT Figure captions Fig. 1. SEM micrographs of the silicate conversion coating obtained from potassium silicate
PT
coating solution with ratio: (a) 2 and (b) 2.33.
RI
Fig. 2. EDX analysis of the silicate conversion coating obtained from potassium silicate coating
SC
solution with ratio: (a) 2 and (b) 2.33.
Fig. 3. (a) X-ray diffraction (XRD) of aluminum alloy 2024 with and without silicate conversion
NU
coating. (b) XRD of silicate coatings of AA2024 obtained from diffret ratio of potassium silicate solution.
MA
Fig. 4. 2D and 3D of AFM images of the silicate conversion coating obtained from ratio 2 potassium silicate solution.
potassium silicate solution.
TE
D
Fig. 5. 2D and 3D of AFM images of the silicate conversion coating obtained from ratio 2.33
AC CE P
Fig. 6. SEM micrographs of the silicate conversion coating obtained in different curing temperatures: (a) 80 oC and (b) 220 oC. (c) Low magnification SEM image of silicate conversion coating obtained in 220 oC curing temperatures. Fig. 7. FTIR spectra of potassium silicate coating with ratio 2.33 with different curing times: (1) 30 min and (2) 1 hour.
Fig. 8. Potentiodynamic polarization curves of different SiO2/K2O ratios of silicate coatings in 3.5 wt% NaCl solution with 400 s immersion time, 150 °C curing temperature and 30 min curing time. Coatings were obtained from 1 M potassium silicate solution in different ratios: (1) bare AA2024, (2) ratio 2 (without nanosilica), (3) ratio 2.11, (4) ratio 2.33 and (5) ratio 2.44. Fig. 9. Potentiodynamic polarization curves of different curing temperatures in 3.5 wt% NaCl solution which coated in silicate coating solution with 2.33 ratio, 400 s immersion time and 30
20
ACCEPTED MANUSCRIPT min curing time. Coatings were obtained in different curing temperatures: (1) bare AA2024, (2) 80 °C, (3) 150 °C and (4) 220 °C.
PT
Fig. 10. Electrochemical impedance spectroscopy diagrams of different curing temperatures in
RI
3.5 wt% NaCl solution which coated in silicate coating solution with 2.33 ratio, 400 s immersion time and 30 min curing time. Coatings were obtained in different curing temperatures: (1) 80 °C,
SC
(2) 150 °C and (3) 220 °C.
NU
Fig. 11. Equivalent circuit compatible with the Nyquist diagram of potassium silicate conversion coatings of AA2024.
MA
Fig. 12. Potentiodynamic polarization curves of different curing times in 3.5 wt% NaCl solution which coated in silicate coating solution with 2.33 ratio and 400 s immersion time and 150 °C
TE
D
curing temperature. Coatings were obtained in different curing times: (1) bare AA2024, (2) 15 min, (3) 30 min, (4) 1 hour and (5) 2 hour.
AC CE P
Fig. 13. Electrochemical impedance spectroscopy diagrams of different curing times in 3.5 wt% NaCl solution which coated in silicate coating solution with 2.33 ratio, 400 s immersion time and 150 °C curing temperature. Coatings were obtained in different curing times: (1) 15 min, (2) 30 min, (3) 1 hour and (4) 2 hour.
21
Fig. 1a
AC CE P
TE
D
MA
NU
SC
RI
PT
ACCEPTED MANUSCRIPT
Fig. 1b 22
Fig. 2a
AC CE P
TE
D
MA
NU
SC
RI
PT
ACCEPTED MANUSCRIPT
Fig. 2b 23
Fig. 3a
AC CE P
TE
D
MA
NU
SC
RI
PT
ACCEPTED MANUSCRIPT
Fig. 3b
24
NU
SC
RI
PT
ACCEPTED MANUSCRIPT
AC CE P
TE
D
MA
Fig. 4
Fig. 5
25
Fig. 6a
AC CE P
TE
D
MA
NU
SC
RI
PT
ACCEPTED MANUSCRIPT
Fig. 6b 26
Fig. 6c
AC CE P
TE
D
MA
NU
SC
RI
PT
ACCEPTED MANUSCRIPT
27
AC CE P
TE
D
MA
NU
SC
RI
PT
ACCEPTED MANUSCRIPT
Fig. 7
28
AC CE P
TE
D
MA
NU
SC
RI
PT
ACCEPTED MANUSCRIPT
Fig. 8
29
AC CE P
TE
D
MA
NU
SC
RI
PT
ACCEPTED MANUSCRIPT
Fig. 9
30
AC CE P
TE
D
MA
NU
SC
RI
PT
ACCEPTED MANUSCRIPT
Fig. 10
Fig. 11
31
AC CE P
TE
D
MA
NU
SC
RI
PT
ACCEPTED MANUSCRIPT
Fig. 12
32
Fig. 13
AC CE P
TE
D
MA
NU
SC
RI
PT
ACCEPTED MANUSCRIPT
33
ACCEPTED MANUSCRIPT Highlights
AC CE P
TE
D
MA
NU
SC
RI
PT
>Nanosilica modified silicate coatings were deposited on the surface of Al 2024.> The corrosion behavior of coatings was examined by means of electrochemical methods.> The best corrosion resistance was obtained with 150 ºC curing temperature.> The corrosion resistance was increased with increasing curing time.
34
S0257-8972(14)00552-0 doi: 10.1016/j.surfcoat.2014.06.041 SCT 19502
To appear in:
Surface & Coatings Technology
Received date: Revised date: Accepted date:
2 April 2014 20 June 2014 22 June 2014
Please cite this article as: H. Bahri, I. Danaee, G.R. Rashed, The effect of curing time and curing temperature on the corrosion behavior of nanosilica modified potassium silicate coatings on AA2024, Surface & Coatings Technology (2014), doi: 10.1016/j.surfcoat.2014.06.041
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
PT
The effect of curing time and curing temperature on the corrosion behavior of nanosilica modified potassium silicate coatings on AA2024
SC
RI
H. Bahri , I. Danaee*, G.R. Rashed Abadan Faculty of Petroleum Engineering, Petroleum University of Technology, Abadan, Iran
MA
NU
*Corresponding author. Email address: [email protected] (I. Danaee) Tel: (0098631) 4429937
Abstract
D
Nanosilica modified potassium silicate conversion coatings were deposited on the surface of
TE
2024 aluminum alloy. The corrosion behavior of coatings was studied by electrochemical impedance spectroscopy, potentiodynamic polarization and the surface analyzing techniques.
AC CE P
The effect of curing time and the curing temperature were studied on anti-corrosion behavior of coatings. Curing temperature showed a significant effect in silicate conversion coating and higher corrosion resistance was obtained with 150 ºC curing temperature. Also the experimental results indicated that the corrosion resistance was increased with increasing the curing time. This behavior can be related to the increase of the silicate coating continuity and reinforcement of the siloxane chains which formed on the surface. Surface analysis results indicated that the coating obtained from 2.33 silica ratio was more uniform and continuous. Key words: Aluminum 2024; Silicate conversion coatings; Nanosilica; Corrosion.
1
ACCEPTED MANUSCRIPT 1. Introduction Aluminum and its alloy is one of the materials that has an important role in the engineering
PT
structures and its usage ranking stands only behind the ferrous alloys. Some special properties of
RI
this metal include: its strength, the density ratio, toughness and the corrosion resistance [1]. In
SC
order to increase the mechanical and physical properties of aluminum, some alloy elements are added to the pure aluminum such as: magnesium, silicon, copper, zinc, iron, manganese and
NU
lithium [2,3]. Aluminum alloy 2024 is used greatly in aerospace industries because of its excellent mechanical properties and low density [4,5]. Materials such as copper, magnesium,
MA
zinc and iron in this alloy decrease the corrosion resistance significantly due to the formation of the intermetallic compounds (IMC). Pure aluminum has a good corrosion resistance due to the
TE
D
formation of a strong oxide layer, but AA2024 involves intermetallic compounds which form small galvanic cells in the aluminum matrix and lead to the localized corrosion [6,7].
AC CE P
One of the main important ways for the protection of these alloys is coatings [8,9]. Chromate conversion coatings (CCCs) are the most common and the effective anti-corrosive treatments for aluminum alloys [10,11]. Although, the CCCs provide a great corrosion resistance, the materials used to produce the coating are toxic and carcinogenic due to the presence of hexavalent chromium in their structure. Therefore, usage of these coatings has restricted under strict regulations [12,13]. Due to the toxicity of Cr6+, several studies have done for the replacements of the CCCs and the extension of non-toxic coatings [13]. Silicate conversion coatings are one of the best superior alternatives for the CCCs [14,15]. These coatings have a good durability against each kind of corrosion and are safe for the human and environment [14-17]. Many studies showed that the silicate coatings provided good corrosion resistance for the structural metals [18-22]. Alkali silicates are a common and general
2
ACCEPTED MANUSCRIPT group of the silicate coatings. These coatings have polymerized particles containing silanol groups (-Si-OH) and siloxane bridges (-Si-O-Si-) in their structure. During the curing step,
PT
polymerizations continue and more silanol groups are converted to the siloxane bridges [23-25].
RI
In the present study, modified silicate coatings were applied on AA2024 and the effects of the curing time and curing temperature were studied. The corrosion behavior of these coatings
SC
was investigated by means of electrochemical impedance spectroscopy (EIS) and polarization
NU
curves. Surface morphology and chemical composition of silicate conversion coatings were analyzed by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and
2.1. Materials
D
TE
2. Materials and methods
MA
atomic force microscopy (AFM).
AC CE P
The samples were prepared from AA2024 sheets with dimensions 50×100×2 mm. The major alloying elements of this alloy are listed in Table 1. The samples were subjected to the mechanical polishing with sand papers of 400 to 2000 grades, and then cleaned with acetone. To remove any trace of the surface oxides, the samples were immersed in 3 M NaOH solution for 30 s and rinsed with deionized water, immediately. Then the samples were immersed in 1 M H2SO4 solution for 30 s and rinsed with deionized water. A 40 wt.% aqueous solution of potassium silicate with silica/alkali molar ratio of 2 was prepared from the Iran Silis Company (Iran). A 30 wt.% aqueous solution of colloid nanosilica with 20-30 nm particle size was prepared from Iranian Nanomaterials Pioneers Company. Coating solutions with the different ratios of SiO2/K2O were formed by mixing a constant concentration of potassium silicate (1 M) with different volumes of colloid nanosilica. AA2024
3
ACCEPTED MANUSCRIPT samples were immersed for 400 s in prepared silicate coating solution. In very low immersion times (<100 s), the prepared coatings were weak and very thin. Also in very high immersion
PT
times (>1000 s), the corrosion protection behavior of coatings was decreased. The medium range
RI
of immersion times was suitable for the optimum coatings preparation. After immersion, the coated samples were cured in different times and temperatures in the furnace. The coated
NU
SC
samples were stored at room temperature for 1 h before starting the test.
2.3. Electrochemical and surface morphology investigation
MA
Electrochemical measurements were carried out by Potentiostat/Galvanostat Autolab, model PGSTAT 302N (Holland). Electrochemical glass cell contained a reference electrode of
D
Ag/AgCl, an auxiliary electrode of platinum and a sample of coated 2024 aluminum alloy as the
TE
working electrode with exposed surface area equal to 1 cm2. To provide the steady state
AC CE P
condition, samples were stored for 30 min in 3.5 wt.% NaCl electrolyte solution before the test. The scan rate of polarization curves was 1 mV s-1. The frequency range of EIS was 105-10-2 Hz with AC signal amplitude of 10 mV. Fitting of experimental impedance spectroscopy data to the proposed equivalent circuit was done by means of a home written least square software based on the Marquardt method for the optimization of functions and Macdonald weighting for the real and imaginary parts of the impedance [26,27]. The morphology of the surface was studied by scanning electron microscopy (SEM, VEGA, TESCAN-LMU) equipped with an energy-dispersive x-ray spectroscopy (EDS) probe, and atomic force microscopy (AFM) Nanosurf Easyscan 2. The X-ray diffraction (XRD) was done by using x-ray diffractometer, broker, XPERT-PRO, Germany, with x-ray Tube Anode Cu and the Cu kα radiation (λ= 1.5406 ˚A) was generated at 40 kV and 40 mA and used as x-ray source.
4
ACCEPTED MANUSCRIPT The FTIR-transmission spectra were recorded on a Bruker Optics Equinox 55 spectrometer (Ettlingen, Germany) equipped with a MKII Golden gate micro attenuated total reflectance
PT
accessory. The thickness of the dry film was measured by Elcometer 456 coating thickness
RI
gauge and was found 600±50 nm.
SC
3. Results and discussion
NU
3.1. Surface analysis
MA
Fig. 1 & 2 show the SEM images and the corresponding EDS spectrum of potassium silicate coatings with different SiO2/K2O ratios. The EDS results are represented in Table 2 with the
D
atomic percentages of the coating elements. The results indicated that the Si:K atomic ratio was
TE
increased in conversion coatings with the addition of nanosilica. The silicate coating obtained without nanosilica from ratio 2 (Fig. 1a), contained discontinuities and agglomerates which
AC CE P
related to the silicate networks and the defects of these layers. The coating defects are the susceptible zones for the corrosion attacks and the initiation of the pitting corrosion. Fig. 1b shows the surface of silicate layer obtained from ratio 2.33. This coating was homogeneous without discontinuities and irregularities on its surface. The brighter zones on the surfaces with nanoscale size can be related to the nanosilica particles that remained unchanged on the surface and did not participate in the polymerization. These brighter zones cannot be seen in Fig. 1a. Structural analysis of the deposit was performed using XRD. Xray diffractograms of the bare and silicate coated aluminum in the range 1º< 2θ < 80º are shown in Fig. 3. Peaks at 2θ= 38.6º, 44.8º, 65.1º and 78.2º are due to face-centered cubic (fcc) structure oriented in (111), (200), (220) and (311) directions of aluminum substrate [28]. The intensity of aluminum peaks were decreased in the presence of silicate conversion coatings. In addition, a hump was observed 5
ACCEPTED MANUSCRIPT in the 2θ ranging from 15º to 35º in the presence of silicate coatings, which indicated the disordered structure. The X-ray diffraction in this range was mainly due to the amorphous silica
PT
particles [29-31]. No significant change was observed in the presence of nano-silica, due to the
RI
amorphous structures and very low thickness of the silicate conversion coatings. Fig. 4&5 show two and three-dimensional AFM images of the silicate coated aluminum
SC
obtained from different silica ratios. It can be seen that the surface morphology of the coatings
NU
differed from each other. The silicate conversion coating obtained from silica ratio 2 was not homogeneous and produced some pinholes and pores on the surface (Fig. 4). With the increase in
MA
the Si/K molar ratio of silicate solution, the continuity and uniformity of the produced layer was enhanced which was related to the nanoparticles and their filler capability. Moreover, the
TE
D
tendency to appearance of pores and the roughness was decreased (Fig. 5). The SEM images of the different curing temperatures are represented in Fig. 6. The silicate
AC CE P
layer could not cure completely and did not produce a compact and stable film in lower temperature (Fig. 6a). In other words, the temperature was not sufficient for the silicate polymerization and production of strong siloxane bonds. As seen, compact and uniform conversion coatings was obtained in 150 °C curing temperature (Fig. 1b). However, the water evaporation rate was increased in very high temperature and the continuity of the coating was decreased. Fig. 6b showed the blisters in silicate conversion coating which cured in 220oC. In addition, blisters were clearly observed by SEM image in low magnification (Fig. 6c). Fourier Transform Infrared method was used to investigate the structural changes of the silicate coating in different curing times. Fig. 7 shows the FTIR spectra of the silicate film with ratio 2.33 which cured at 150 ℃. The spectra of the silicate indicated a broad peak at 900 to 1200 cm-1 which was due to the asymmetric Si-O-Si stretching vibration [30,32,33]. Also peaks at 490
6
ACCEPTED MANUSCRIPT and 770 cm-1 were related to Si-O and Si-OH bonds, respectively. As seen, the intensity and positions of peaks were not significantly changed with increasing the curing time. The Si-O-Si
PT
bands centered at 1000 cm–1 to 1200 cm–1 became more intensive as the curing time increased.
RI
Extensive cross-links were created in silicate during the curing process and therefore, more
SC
siloxane bonds could be formed with increasing the curing time.
NU
3.2. Effects of SiO2/K2O ratio
Fig. 8 shows the potentiodynamic polarization curves of different SiO2/K2O ratios of silicate
MA
coatings in 3.5 wt% NaCl solution. Coatings were obtained by 400 s immersion in coating solution and curing in 150 °C for 30 min. The corresponding corrosion parameters such as
TE
D
corrosion current density (Icorr), corrosion rate (CR), corrosion potential (Ecorr), anodic slope (βα), cathodic slope (βc) and polarization resistance (Rp) are listed in Table 3. The polarization
Rp
AC CE P
resistance is calculated by Stern–Geary equation [12,34]:
a.c 1 2.303( a c ) Icorr
According to Fig. 8, the corrosion current densities were decreased in the presence of the silicate coatings. Both the anodic and cathodic branches of the polarization curves of silicate coating were shifted to the lower corrosion current densities, indicated that both the anodic and cathodic reaction are blocked. During the curing process, the existing water in the coating was evaporated and the silicate networks (siloxane bridges) were formed. Corrosion protection was mainly due to the formation of silica rich Al-oxide film. The silanol groups were adsorbed on the aluminum surface by hydrogen bonds and metallo-siloxane (M-O-Si) bonds were produced between the metal surface and the coating layer during the curing process [16, 20, 35].
7
ACCEPTED MANUSCRIPT As can be seen, the anodic branches of bare aluminum and the coating obtained from 1 M potassium silicate without nanosilica showed pitting corrosion in the high applied anodic
PT
potential range. The current density was increased obviously in this region due to pit nucleation.
RI
However, intensity of pitting corrosion in silicate coating was lower than the bare aluminum. Addition of the nanosilica into the coating solution caused to increment of SiO2/K2O ratios.
SC
In addition, the corrosion resistance of the silicate coating was increased and its corrosion rate
NU
and the pitting corrosion probability were decreased (Fig. 8). The size of nano-silica particles was the most important effect which led to filling the pores and spaces of the silicate networks
MA
[25]. Therefore, the uniformity of the silicate conversion coatings were improved [36]. The high surface area was another benefit of these particles which provided higher contacts between the
TE
D
functional groups and more formation of silicate networks and siloxane bridges [37]. With increasing silica ratio up to 2.33, the corrosion resistance of silicate layer was
AC CE P
increased. Silicates with larger number of nanoparticles had more polymeric units (Si-O). Therefore, the possibility of siloxane bridges formation between the particles, during the curing process, was increased. Moreover, silicate surfaces are water absorbent due to the positive charge of surface, namely potassium. Colloidal silica has a negative charge and can bind to the positive charges. Therefore, in the presence of nano-silica, hydrophilic sites were decreased and the hydrophobic property of silicate layers was increased which provided greater corrosion protection. In the higher silica ratios, the corrosion rate was increased due to the internal stress which decreased its toughness and led to increasing the cracks (Fig. 1a&b). A few cracks were created on the surface due to the curing process and therefore, the corrosion resistance of the coatings was decreased.
8
ACCEPTED MANUSCRIPT 3.3. Effects of the curing temperature Fig. 9 depicts the polarization curves of the silicate coated aluminum in different curing
PT
temperatures and the corresponding electrochemical parameters are presented in Table 4.
RI
Coatings were obtained by 400 s immersion in solution with 2.33 silica ratio and then 30 min curing time. Fig. 9 showed that lower temperature couldn’t cure the waterborne coating layer
SC
completely and the coating did not have enough stability against the corrosive environment.
NU
Moreover, silicate coting was capitulated to the pitting corrosion. More elevated temperature up to 150 °C, could increase the curing rate of the coating. Therefore, stronger silicate networks
MA
(siloxane chains) were produced, which had higher stability against the corrosive solution. In higher curing temperature around 220 °C, the corrosion resistance was decreased due to the
TE
D
blisters and cracks in the coatings (Fig. 6a-c). In the curing step, the rate of water evaporation was important parameter. In very high temperatures, the evaporation rate was increased
AC CE P
dramatically and the continuity of the coating was decreased. Presence of the blisters and cracks on the surface of coatings were related to the high evaporation rate which led to increasing the penetration zones for the aggressive ions. Electrochemical impedance was used to confirm the anticorrosive properties of the silicate coatings. The Nyquist plots of silicate coated aluminum obtained in different curing temperatures are presented in Fig. 10. Coatings were obtained by 400 s immersion in solution with 2.33 silica ratio and then 30 min curing time. Impedance was measured at open circuit potential in 3.5 %wt. NaCl solution. The data showed that impedance diagrams consisted of a depressed capacitive semicircle which was due to the double layer capacitance and charge transfer resistance. Fig. 11 depicts the equivalent circuit compatible with the Nyquist diagrams. In this electrical equivalent circuit, Rs, Qdl and Rct represented solution resistance, a constant phase element corresponding to
9
ACCEPTED MANUSCRIPT the double layer capacitance and the charge transfer resistance, respectively. It was necessary to replace the capacitor (C) with a constant phase element (CPE) Q in the equivalent circuit to
PT
obtain a satisfactory impedance simulation of silicate coated aluminum. The most widely
RI
accepted explanation for the presence of CPE behavior and the depressed semicircles on solid electrodes is microscopic roughness, lead to the inhomogeneous distribution in the solution
SC
resistance as well as in the double-layer capacitance [38, 39].
NU
To corroborate the equivalent circuit, the experimental data was fitted to the equivalent circuit and the circuit elements were obtained. The equivalent circuit parameters for the
MA
impedance spectra of corrosion of silicate coated aluminum in NaCl solution are shown in Table 5. According to this table, high corrosion resistance was obtained by 150 °C curing temperature
TE
D
which was due to the presence of the more uniform silicate coating on the surface of AA2024.
AC CE P
3.4. Effects of the curing time
Fig. 12 shows the polarization curves of silicate coated AA2024 in different curing times and the corresponding electrochemical parameters are presented in Table 6. Coatings were obtained by 400 s immersion in solution with 2.33 silica ratio and then curing in 150 °C. As can be seen corrosion current was decreased with increasing curing time. During the curing step, water of the coating was evaporated and the strong siloxane bonds were produced on the surface. These bonds increased the continuity and stability of the silicate layer and excessively improved its corrosion resistance. Whatever the samples were kept for a longer period of time in the furnace at optimized temperature, more siloxane bonds were formed on the metal surface. Therefore, the properties of the coating layer were further improved. In the higher curing times, around 2 h, the anticorrosive properties of the silicate coating were decreased.
10
ACCEPTED MANUSCRIPT Nyquist plots for the silicate conversion coating obtained in different curing times are presented in Fig. 13. Impedance data were measured at open circuit potential in 3.5 %wt NaCl
PT
solution. Silicate conversion coatings were obtained by 400 s immersion in solution with 2.33
RI
silica ratio and then curing in 150 °C. All coatings showed capacitive loop attributed to the double layer capacitance and the charge transfer resistance. To obtain a satisfactory simulation,
SC
the capacitor was replaced with a constant phase element. The experimental data were fitted to
NU
the equivalent circuit (Fig. 11) and the circuit elements were obtained. Table 7 illustrates the equivalent circuit parameters for the impedance spectra of the silicate coated aluminum obtained
MA
in different curing times. With increasing curing times, the corrosion resistance was increased due to strong siloxane bonds. Also in the higher curing times, the corrosion resistance was
TE
D
decreased due to the decrease of stability of silicate layer in long curing time. With 1 h curing time, the silicate conversion coating with high corrosion resistance was obtained. This was in
AC CE P
agreement with the potentiodynamic polarization data.
4. Conclusion
The corrosion behavior of nanosilica modified potassium silicate coating applied on the 2024 aluminum alloy was investigated in 3.5 wt. % NaCl solution. This conversion coating improved the corrosion resistance of the aluminum alloy due to the formation of silicate film. But this protective barrier had some discontinuities and defects that limited its anticorrosive properties. Presence of nano-silica improved the silicate networks and decreased the existing defects. The ratio of nano-silica showed significant effect to provide a high corrosion protection. The film that formed from 1 M potassium silicate with ratio 2.33 nano-silica, was relatively more protective and continuous. The increasing corrosion resistance was mainly due to the formation
11
ACCEPTED MANUSCRIPT of the compact and perfect silicate layers on the surface. The curing temperature played an important role to provide silicate conversion coating and the best corrosion resistance was
PT
obtained with 150 ºC curing temperature. In addition, the corrosion resistance was increased with
RI
increasing the curing time due to the increasing of the silicate coating continuity and reinforcement of siloxane matrix. The results of SEM and AFM showed that the silicate coatings,
SC
obtained from 2.33 silica ratio and 150 ºC curing temperature, were more uniform and
AC CE P
TE
D
MA
increased with increasing the curing times.
NU
continuous. FTIR analysis of silicate conversion coatings showed that the siloxane bridges were
12
ACCEPTED MANUSCRIPT References [1] I. Shchedrina, A.G. Rakoch, G. Henrion, J. Martin, Non-destructive methods to control the
PT
properties of MAO coatings on the surface of 2024 aluminium alloy, Surf. Coat. Technol. 238
RI
(2014) 27-44.
SC
[2] L. Wen, Y. Wang, Y. Jin, B. Liu, Y. Zhou, D. Sun, Microarc oxidation of 2024 Al alloy using spraying polar and its influence on microstructure and corrosion behavior, Surf. Coat.
NU
Technol. 228 (2013) 92-99.
[3] V. Vitry, A. Sens, A.-F. Kanta, F. Delaunois, Wear and corrosion resistance of heat treated
MA
and as-plated Duplex NiP/NiB coatings on 2024 aluminum alloys, Surf. Coat. Technol. 206 (2012) 3421-3427.
TE
D
[4] J.C.S. Fernandes, M.G.S. Ferreira, D.B. Haddow, A. Goruppa, R. Short, D.G. Dixon, Plasma-
(2002) 8-13.
AC CE P
polymerised coatings used as pre-treatment for aluminium alloys, Surf. Coat. Technol. 154
[5] G. Yoganandan, J.N. Balaraju, Synergistic effect of V and Mn oxyanions for the corrosion protection of anodized aerospace aluminum alloy. Surf. Coat. Technol. (2014) in press, http://dx.doi.org/10.1016/j.surfcoat.2014.04.062. [6] W. Pinc, S. Maddela, M. O'Keefe, W. Fahrenholtz, Formation of subsurface crevices in aluminum alloy 2024-T3 during deposition of cerium-based conversion coatings, Surf. Coat. Technol. 204 (2010) 4095-4100. [7] E. Darmiani, I. Danaee, M.A. Golozar, M.R. Toroghinejad,Corrosion investigation of Al–SiC nano-composite fabricated by accumulative roll bonding (ARB) process, J. Alloys Comp. 552 (2013) 31-39.
13
ACCEPTED MANUSCRIPT [8] Lj.S. Živković, J.P. Popić, B.V. Jegdić, Z. Dohčević-Mitrović, J.B. Bajat, V.B. MiškovićStanković, Corrosion study of ceria coatings on AA6060 aluminum alloy obtained by cathodic
PT
electrodeposition: Effect of deposition potential, Surf. Coat. Technol. 240 (2014) 327-335.
RI
[9] J.M. Sánchez-Amaya, G. Blanco, F.J. Garcia-Garcia, M. Bethencourt, F.J. Botana, XPS and AES analyses of cerium conversion coatings generated on AA5083 by thermal activation, Surf.
SC
Coat. Technol. 213 (2012) 105-116.
NU
[10] R.L. Twite, G.P. Bierwagen, Review of Alternatives to Chromate for Corrosion Protection of Aluminum Aerospace Alloys. Prog. Org. Coat. 33, 1998, p 91-100.
MA
[11] J. Zhao, L. Xia, A. Sehgal, D. Lu, R.L. McCreery, G.S. Frankel, Effects of chromate and chromate conversion coatings on corrosion of aluminum alloy 2024-T3, Surf. Coat. Technol. 140
TE
D
(2001) 51-57.
[12] M. Kanani, I. Danaee, M. H. Maddahy, Microstructural characteristics and corrosion
AC CE P
behavior of cerium oxide conversion coatings on AA6063. Mater. Corros. (2014) DOI: 10.1002/maco.201307539
[13] X. Montero, M.C. Galetz, M. Schütze, Low-activity aluminide coatings for superalloys using a slurry process free of halide activators and chromates, Surf. Coat. Technol. 222 (2013) 914.
[14] J. Min, J.H. Park, H.K. Sohn, J.M. Park, Synergistic Effect of Potassium Metal Siliconate on Silicate Conversion Coating for Corrosion Protection of Galvanized Steel. Ind. Eng. Chem. 18 (2012) 655-660. [15] A.S. Hamdy, D.P. Butt, Environmentally Compliant Silica Conversion Coatings Prepared by Sol–Gel Method for Aluminum Alloys. Surf. Coat. Technol. 201 (2006) 401-407.
14
ACCEPTED MANUSCRIPT [16] M.R. Yuan, J.T. Lu, G. Kong, C.S. Che, Effect of Silicate Anion Distribution in Sodium Silicate Solution on Silicate Conversion Coatings of Hot-Dip Galvanized Steels. Surf. Coat.
PT
Technol. 205 (2011) 4466-4470.
RI
[17] M.R. Yuan, J.T. Lu, G. Kong, C.S. Che, Self-Healing Ability of Silicate Conversion Coatings on Hot Dip Galvanized Steels. Surf. Coat. Technol. 205 (2011) 4507-4513.
SC
[18] H.J. Kim, J. Zhang, R. H. Yoon, R. Gandour, Development of environmentally friendly
NU
nonchrome conversion coating for electrogalvanized steel, Surf. Coat. Technol. 188–189 (2004) 762– 767.
MA
[19] L. Jiang, M. Wolpers, P. Volovitch, K. Ogle, An atomic emission spectroelectrochemical study of passive film formation and dissolution on galvanized steel treated with silicate
TE
D
conversion coatings, Surf. Coat. Technol. 206 (2012) 3151–3157. [20] M. Hara, R. Ichino, M. Okido, N. Wada, Corrosion protection property of colloidal silicate
AC CE P
film on galvanized steel, Surf. Coat. Technol. 169 –170 (2003) 679–681. [21] Sandrine Dalbin, Georges Maurin, Ricardo P. Nogueira, Jacques Persello, Nicolas Pommier, Silica-based coating for corrosion protection of electrogalvanized steel, Surf. Coat. Technol. 194 (2005) 363– 371.
[22] K. Aramaki, Self-Healing Mechanism of an Organosiloxane Polymer Film Containing Sodium Silicate and Cerium (III) Nitrate for Corrosion of Scratched Zinc Surface in 0.5 M NaCl. Corros. Sci. 44 (2002) 1621-1632. [23] L.T. zhuravlev, The Surface Chemistery of Amorphous Silica. Zhuravlev Model. Colloid. Surf. 173 (2000) 1-38. [24] U.S. Bureau of Mines. Crystalline Silica Primer. Washington, DC. 1992.
15
ACCEPTED MANUSCRIPT [25] M. Yuan, J. Lu. G. Kong, Effect of SiO2:Na2O Molar Ratio of Sodium Silicate on the Corrosion Resistance of Silicate Conversion Coatings. Surf. Coat. Technol. 204 (2010) 1229-
PT
1235.
RI
[26] I. Danaee, Kinetics and Mechanism of Palladium Electrodeposition on Graphite Electrode by Impedance and Noise Measurements. J. Electroanal. Chem. 662 (2011) 415-420.
SC
[27] J.R. Macdonald, Note on the Parameterization of the Constant-Phase Admittance Element.
NU
Solid State Ion. 13 (1984) 147-149.
[28] M. Moradshahi, T. Tavakoli, S. Amiri, Sh. Shayeganmehr, Plasma nitriding of Al alloys by
MA
DC glow discharge, Surf. Coat. Technol. 201 (2006) 567-574. [29] D. Singh, R. Kumar, A. Kumar, K. N. Rai, Synthesis and characterization of rice husk
TE
D
silica, silica-carbon composite and H3PO4 activated silica. Cerâmica 54 (2008) 203-212. [30] M. Rokita, W. Mozgawa, A. Adamczyk, Transformation of silicate gels during heat
AC CE P
treatment in air and in argon– Spectroscopic studies, J. Mol. Struct. In press, http://dx.doi.org/10.1016/j.molstruc.2014.04.020 [31] M. Trevino, R.D.Mercado-Solis, R.Colas, A.Perez, J.Talamantes, A.Velasco, Erosive wear of plasma electrolytic oxidation layers on aluminium alloy 6061, Wear 301 (2013) 434-441. [32] X. Yang, P. Roonasi, A. Holmgren, A study of sodium silicate in aqueous solution and sorbed by synthetic magnetite using in situ ATR-FTIR spectroscopy. J. Colloid Interface Sci. 328 (2008) 41–47. [33] X. Yanga, P. Roonasi, R. Jolsterå, A. Holmgren, Kinetics of silicate sorption on magnetite and maghemite: An in situ ATR-FTIR study, Colloids Surf. A: Physicochem. Eng. Aspects 343 (2009) 24–29.
16
ACCEPTED MANUSCRIPT
PT
[34] O. Ghasemi, I. Danaee, G.R. Rashed, M. RashvandAvei, M.H. Maddahy, The Inhibition
in 1 M HCl, J. Mater. Eng. Perform. 22 (2013) 247-259.
RI
Effect of Synthesized 4-Hydroxybenzaldehyde-1,3propandiamine on the Corrosion of Mild Steel
using
a
mixture
of
bis-[triethoxysilylpropyl]tetrasulfide
and
bis-
NU
steel
SC
[35] D. Zhu, W.J. Van Ooij. Enhance corrosion resistance of AA2024-T3 and hot-dip galvanized
[trimethoxysilylpropyl]amine. Electrochim. Acta, 49 (2004) 1113-1125.
MA
[36] Klabunde, K. J. Nanoscale materials in chemistry. Wiley-Interscience, New York, 2001. [37] V. Padavettan, I. Ab Rahman, Synthesis of Silica Nanoparticles by Sol-Gel: SizeSurface
Modifications,
D
Properties,
and
Applications
in
Silica-Polymer
TE
Dependent
Nanocomposites. J. Nanomater. 2012 (2012) 1-15.
AC CE P
[38] I.Danaee, S. Noori, Kinetics of the Hydrogen Evolution Reaction on NiMn Graphite Modified Electrode. Int. J. Hydrogen Energy. 36 (2011) 12102-12111. [39] I. Danaee, M. NiknejadKhomami, A.A. Attar, Corrosion Behavior of AISI 4130 Steel Alloy in Ethylene Glycol–Water Mixture in Presence of Molybdate. Mater. Chem. Phys. 135 (2012) 658-667.
17
ACCEPTED MANUSCRIPT Table 1. Chemical composition of the substrate (AA2024). Al balance
Si <0.5
Fe <0.5
Cu 3.8-4.6
Mn 0.5-0.8
Mg 1.3-1.6
Cr <0.1
Zn <0.25
Ti <0.15
V <0.05
Zr <0.05
PT
Element % w/w
Table 2. EDS results of the 1 M potassium silicate surface coatings in different ratios of
Atomic % of Si
Ratio 2 Ratio 2.33
19.96 20.34
Atomic % of K
SC
Ratio
RI
SiO2/K2O.
69.36 73.75
NU
10.68 5.91
Atomic % of O
Table 3. Electrochemical parameters of different SiO2/K2O ratios of the silicate coating medium temperature and 30 min curing time. Ecorr / V
Icorr / A cm-2
Bare Ratio 2 Ratio 2.11 Ratio 2.33 Ratio 2.44
-0.668 -0.609 -1.079 -1.083 -1.235
8.47×10-7 4.19×10-7 3.88×10-9 1.26×10-10 1.74×10-9
βα / V dec-1
βc / V dec-1
Rp / ohm
Vcorr / mm y-1
0.005 0.095 0.114 0.066 0.041
0.012 0.113 0.265 0.085 0.259
1.82×103 5.34×104 8.91×106 1.28×108 0.96×107
4.14×10-3 4.92×10-3 4.56×10-5 1.48×10-6 2.04×10-5
AC CE P
TE
D
Samples
MA
in 3.5 wt. % NaCl solution. Coatings were prepared in 400s immersion time, 150oC curing
Table 4. Electrochemical parameters of different curing temperatures for the silicate coating with 2.33 ratio in 3.5 wt. % NaCl solution. Coatings were prepared in 400 s immersion time and 30 min curing time. Samples
Ecorr / V
Icorr / A cm-2
βα / V dec-1
βc / V dec-1
Rp / ohm
Vcorr / mm y-1
Bare 80 °C 150 °C 220 °C
-0.668 -1.012 -1.083 -1.192
8.47×10-7 8.03×10-9 1.26×10-10 9.87×10-9
0.005 0.089 0.066 0.045
0.012 0.142 0.085 0.216
1.82×103 2.95×106 1.28×108 1.63×106
4.14×10-3 4.23×10-5 1.48×10-6 6.63×10-5
Table 5. EIS parameters of different curing temperatures for the silicate coating with 2.33 ratio in 3.5 wt. % NaCl solution. Coatings were prepared in 400 s immersion time and 30 min curing time. Sample 80 °C 150 °C 220 °C
Rct /Ω 9.11×107 1.46×108 7.11×107
Qdl /F 5.51×10-8 4.23×10-8 8.05×10-8
18
n 0.66 0.83 0.63
Other <0.15
ACCEPTED MANUSCRIPT Table 6. Electrochemical parameters of different curing times for the silicate coating with 2.33 ratio in 3.5 wt. % NaCl solution. Coatings were prepared in 400 s immersion time and 150oC curing temperature. Icorr / A cm-2
βα / V dec-1
βc / V dec-1
Bare 15 min 30 min 1 hour 2 hour
-0.668 -1.218 -1.083 -0.751 -0.568
8.47×10-7 4.83×10-9 1.26×10-10 3.86×10-11 1.22×10-10
0.005 0.086 0.066 0.131 0.082
0.012 0.225 0.085 0.202 0.282
Rp / ohm
Vcorr / mm y-1
1.82×103 5.59×106 1.28×108 8.91×109 2.27×108
4.14×10-3 1.35×10-5 1.48×10-6 4.53×10-7 1.43×10-6
PT
Ecorr / V
SC
RI
Samples
Table 7. EIS parameters of different curing times for the silicate coating with 2.33 ratio in 3.5
NU
wt. % NaCl solution. Coatings were prepared in 400 s immersion time and 150oC curing temperature.
D
MA
Qdl /F 6.24×10-8 4.23×10-8 2.22×10-8 1.08×10-7
TE
15 min 30 min 1 hour 2 hour
Rct /Ω 7.53×107 1.46×108 7.13×108 5.82×108
AC CE P
Sample
19
n 0.88 0.83 0.89 0.85
ACCEPTED MANUSCRIPT Figure captions Fig. 1. SEM micrographs of the silicate conversion coating obtained from potassium silicate
PT
coating solution with ratio: (a) 2 and (b) 2.33.
RI
Fig. 2. EDX analysis of the silicate conversion coating obtained from potassium silicate coating
SC
solution with ratio: (a) 2 and (b) 2.33.
Fig. 3. (a) X-ray diffraction (XRD) of aluminum alloy 2024 with and without silicate conversion
NU
coating. (b) XRD of silicate coatings of AA2024 obtained from diffret ratio of potassium silicate solution.
MA
Fig. 4. 2D and 3D of AFM images of the silicate conversion coating obtained from ratio 2 potassium silicate solution.
potassium silicate solution.
TE
D
Fig. 5. 2D and 3D of AFM images of the silicate conversion coating obtained from ratio 2.33
AC CE P
Fig. 6. SEM micrographs of the silicate conversion coating obtained in different curing temperatures: (a) 80 oC and (b) 220 oC. (c) Low magnification SEM image of silicate conversion coating obtained in 220 oC curing temperatures. Fig. 7. FTIR spectra of potassium silicate coating with ratio 2.33 with different curing times: (1) 30 min and (2) 1 hour.
Fig. 8. Potentiodynamic polarization curves of different SiO2/K2O ratios of silicate coatings in 3.5 wt% NaCl solution with 400 s immersion time, 150 °C curing temperature and 30 min curing time. Coatings were obtained from 1 M potassium silicate solution in different ratios: (1) bare AA2024, (2) ratio 2 (without nanosilica), (3) ratio 2.11, (4) ratio 2.33 and (5) ratio 2.44. Fig. 9. Potentiodynamic polarization curves of different curing temperatures in 3.5 wt% NaCl solution which coated in silicate coating solution with 2.33 ratio, 400 s immersion time and 30
20
ACCEPTED MANUSCRIPT min curing time. Coatings were obtained in different curing temperatures: (1) bare AA2024, (2) 80 °C, (3) 150 °C and (4) 220 °C.
PT
Fig. 10. Electrochemical impedance spectroscopy diagrams of different curing temperatures in
RI
3.5 wt% NaCl solution which coated in silicate coating solution with 2.33 ratio, 400 s immersion time and 30 min curing time. Coatings were obtained in different curing temperatures: (1) 80 °C,
SC
(2) 150 °C and (3) 220 °C.
NU
Fig. 11. Equivalent circuit compatible with the Nyquist diagram of potassium silicate conversion coatings of AA2024.
MA
Fig. 12. Potentiodynamic polarization curves of different curing times in 3.5 wt% NaCl solution which coated in silicate coating solution with 2.33 ratio and 400 s immersion time and 150 °C
TE
D
curing temperature. Coatings were obtained in different curing times: (1) bare AA2024, (2) 15 min, (3) 30 min, (4) 1 hour and (5) 2 hour.
AC CE P
Fig. 13. Electrochemical impedance spectroscopy diagrams of different curing times in 3.5 wt% NaCl solution which coated in silicate coating solution with 2.33 ratio, 400 s immersion time and 150 °C curing temperature. Coatings were obtained in different curing times: (1) 15 min, (2) 30 min, (3) 1 hour and (4) 2 hour.
21
Fig. 1a
AC CE P
TE
D
MA
NU
SC
RI
PT
ACCEPTED MANUSCRIPT
Fig. 1b 22
Fig. 2a
AC CE P
TE
D
MA
NU
SC
RI
PT
ACCEPTED MANUSCRIPT
Fig. 2b 23
Fig. 3a
AC CE P
TE
D
MA
NU
SC
RI
PT
ACCEPTED MANUSCRIPT
Fig. 3b
24
NU
SC
RI
PT
ACCEPTED MANUSCRIPT
AC CE P
TE
D
MA
Fig. 4
Fig. 5
25
Fig. 6a
AC CE P
TE
D
MA
NU
SC
RI
PT
ACCEPTED MANUSCRIPT
Fig. 6b 26
Fig. 6c
AC CE P
TE
D
MA
NU
SC
RI
PT
ACCEPTED MANUSCRIPT
27
AC CE P
TE
D
MA
NU
SC
RI
PT
ACCEPTED MANUSCRIPT
Fig. 7
28
AC CE P
TE
D
MA
NU
SC
RI
PT
ACCEPTED MANUSCRIPT
Fig. 8
29
AC CE P
TE
D
MA
NU
SC
RI
PT
ACCEPTED MANUSCRIPT
Fig. 9
30
AC CE P
TE
D
MA
NU
SC
RI
PT
ACCEPTED MANUSCRIPT
Fig. 10
Fig. 11
31
AC CE P
TE
D
MA
NU
SC
RI
PT
ACCEPTED MANUSCRIPT
Fig. 12
32
Fig. 13
AC CE P
TE
D
MA
NU
SC
RI
PT
ACCEPTED MANUSCRIPT
33
ACCEPTED MANUSCRIPT Highlights
AC CE P
TE
D
MA
NU
SC
RI
PT
>Nanosilica modified silicate coatings were deposited on the surface of Al 2024.> The corrosion behavior of coatings was examined by means of electrochemical methods.> The best corrosion resistance was obtained with 150 ºC curing temperature.> The corrosion resistance was increased with increasing curing time.
34