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Article Cite This: ACS Omega 2019, 4, 73−78
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Effect of Fumed Silica/Chitosan/Poly(vinylpyrrolidone) Composite Coating on the Electrochemical Corrosion Resistance of Ti−6Al−4V Alloy in Artificial Saliva Solution Mohamed S. Hussein and Amany M. Fekry*
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Chemistry Department, Faculty of Science, Cairo University, Giza 12613, Egypt ABSTRACT: The paper discovers the electrochemical corrosion behavior of Ti− 6Al−4V alloy coated with fumed silica/chitosan/poly(vinylpyrrolidone) composite in artificial saliva solution. The surface analysis was carried out to assure the formation of the examined coating on the alloy surface. Electrochemical studies were performed to consider the stability of the alloy and the corrosion resistance contribution by the novel coating with respect to that of the uncoated one. The results reveal that this novel coating recovers well the deterioration of the Ti−6Al−4V alloy in artificial saliva solution by reaching to 17.9 MΩ cm2 for its corrosion resistance. The new fabricated coating greatly enhances the electrochemical corrosion resistance by giving a high inhibition efficiency of 99.85%. Generally, it was deduced that the electrochemical corrosion resistance of the uncoated and coated Ti−6Al−4V alloy is arranged in the following order: fumed silica/chitosan/poly(vinylpyrrolidone) coated alloy > chitosan/poly(vinylpyrrolidone) coated alloy > uncoated alloy.
1. INTRODUCTION Titanium alloys have great applications in implantology, such as dental, orthopedic prostheses owing to their own high mechanical properties, great corrosion prevention, and outstanding biocompatibility in biofluids. The superb corrosion resistance of titanium and its alloys in biological solutions is attributed to the formation of the high stable and protective titanium dioxide film on the surface.1−6 Ti−6Al−4V alloy has outstanding mechanical and biocompatible features for load bearing biomedical applications. Unfortunately, such type of alloys liberate vanadium and aluminum ions into the electrolyte because of corrosion and fretting corrosion, which has adverse effects on the patient health and gives rise to diseases. Fretting corrosion causes metal-ion buildup and brings debris close to the tissues, resulting in inflammatory reactions, which requires resurgery of patients. Researchers have been paying attention to the study of surface engineering as a solution to solve such problems. A great number of coatings and techniques, like sol−gel, thermochemical diffusion, thermal oxidation, plasma- and laser-based surface treatments, have been developed to create higher corrosion- and fretting-corrosion-resistant surfaces on Ti alloys for biomedical applications.7,8 Chitosan (CS) is a natural biopolymer, cationic polysaccharide that is prepared by alkaline N-deacetylation of chitin. The solubility of chitosan in acidic condition is attributed to the protonation of the amino groups that give a cationic character. This positively charged chitosan interacts with the negatively charged membrane of bacterial cells, which explains the antibacterial activity of chitosan. This unique physicochemical feature of chitosan makes it a pioneer material in many biomedical applications, such as biomedical implants, © 2019 American Chemical Society
drug delivery, scaffolds, biosensors, and other biomedical devices. For these mentioned reasons, chitosan is a good choice to synthetic coating systems to prohibit corrosive deprivation of materials.9−15 Poly(vinylpyrrolidone) (PVP) is a nontoxic and eco-friendly water soluble polymer. Therefore, it is remarkably used as a green corrosion inhibitor. In addition, PVP has typical inhibition properties as it contains the oxazole moiety, an Nheterocycle. PVP exhibits good corrosion inhibition in alkaline solutions containing NaCl and in neutral solutions.16,17 Fumed silica (FS) is a very pure form of SiO2, created by the vapor-phase hydrolysis of SiCl4 in a hydrogen−oxygen flame. FS is artificial, amorphous, colloidal silicon dioxide, with exceptional appearances, such as its particularly small particle size, vast surface area, ultra purity, and chain-forming tendencies. It has applications in pharmaceuticals, sealants, cosmetics, printing inks, coatings, and paints. Moreover, fumed silica is broadly utilized as filler for strength reinforcement. Fumed silica is characterized by the absence of organic residues because it is obtained by a flame process at 1800 °C.18 Our study aimed at the enhancement of the corrosion resistance of Ti−6Al−4V alloy in artificial saliva solution at 37 °C as a function of time. This study will be valuable and practical in dental application. It is a novel study using a novel coating. Such a study would be expected to provide supervision on the design of the coating materials of high stability. Ti− 6Al−4V alloy with CS/PVP and FS/CS/PVP hybrid coatings in artificial saliva at 37 °C was studied and investigated for Received: September 12, 2018 Accepted: December 7, 2018 Published: January 2, 2019 73
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formation. It shows a thick compact cigarette shape with two different magnifications 6000× (Figure 1a) and 200× (Figure 1b), whereas Figure 1c shows the bare alloy.19 Also, Figure 1b shows the fumed silica aggregates on the alloy surface giving thick film owing to their strong thickening effect.18 Also, PVP gives uniform distribution for silica by filling the pores and chitosan helps to adsorb well both PVP and silica on the surface giving highly protective film. EDX analysis shown in Figure 2 assures film formation by showing a high percentage of Si, N, C, and O on the alloy surface, ensuring the presence of the coating on the alloy surface.
improvement of the alloy corrosion protection. The surface morphology and characterization of the novel FS/CS/PVP coating was revealed by scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) techniques, whereas the corrosion characteristics were explored using electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization technique.
2. RESULTS AND DISCUSSION 2.1. Surface Characterization. Figure 1a−c shows the surface morphology of FS/CS/PVP coating compared to that of the bare Ti alloy using SEM analysis. Figure 1a,b shows the SEM image of FS/CS/PVP coating that proves coating
Figure 2. EDX pattern of FS/CS/PVP coating.
2.2. Electrochemical Techniques. 2.2.1. Electrochemical Impedance Spectroscopy (EIS). Electrochemical impedance spectroscopy technique was carried out for bare Ti−6Al−4V electrode, CS/PVP-coated alloy and FS/CS/PVP-coated alloy at 37 °C in artificial saliva as a function of immersion time for 14 days. Bode and Nyquist plots of bare Ti−6Al−4V alloy exhibit rise of impedance value |Z| and the phase shift θ with time due to the improved formation of passive TiO2 film on the alloy surface with time. Bode and Nyquist plots of uncoated, CS/PVP-coated, and FS/CS/PVP-coated alloys show similar trends with immersion time but with a considerable increase in impedance and phase shift than that in the case of the bare alloy. Impedance and phase shift for the uncoated and coated alloy are in the order: FS/CS/PVPcoated alloy > CS/PVP-coated alloy > uncoated alloy. Figure 3a−c demonstrates the Bode plot of uncoated, CS/PVPcoated, and FS/CS/PVP-coated alloy in artificial saliva solution at 37 °C with 2, 24, 192, and 336 h immersion time. Figure 4a−c demonstrates Nyquist plots of uncoated, CS/PVP-coated, and FS/CS/PVP-coated alloy in artificial saliva solutions at 37 °C with 2, 24, 192, and 336 h immersion time. The shape of Nyquist plots for the uncoated and coated Ti−6Al−4V alloy suggests that the corrosion process occurred by charge-transfer mechanism.20 The diameters of the Nyquist plots enlarged progressively with immersion time. The Nyquist plot diameters for the uncoated and coated alloy are in the order: FS/CS/PVP-coated alloy > CS/PVP-coated alloy > uncoated alloy. The formed film on Ti−6Al−4V-coated alloy consists of internal and external layers. The best fit model is the Randle circuit (Figure 5) for the uncoated and coated Ti− 6Al−4V alloy using Thales program software. It includes solution resistance Rs in series with the external layer circuit (where external layer resistance Ro is parallel to external layer
Figure 1. SEM images of FS/CS/PVP coating with two magnifications 6000× (a) and 200× (b) compared with the bare Ti alloy (c). 74
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Figure 4. Nyquist plots of (a) uncoated Ti−6Al−4V alloy and (b) CS/PVP- and (c) FS/CS/PVP-coated Ti−6Al−4V alloy in artificial saliva solutions at 37 °C with 2, 24, 192, and 336 h immersion time.
Figure 3. Bode plots of (a) uncoated Ti−6Al−4V alloy and (b) CS/ PVP- and (c) FS/CS/PVP-coated Ti−6Al−4V alloy in artificial saliva solutions at 37 °C for 14 days.
capacitance Qo in series with the internal-layer circuit and internal-layer resistance Ri is parallel to internal-layer capacitance Qi). The EIS data pointed out that the corrosion protection of the alloy comes mainly from the internal layer. The constant-phase element term of the external layer (Ro, Qo) and internal layer (Ri, Qi) was used because of the deviation from ideal capacitance behavior that results from nonuniform current distribution owing to surface coarseness and imperfections.20−22 The impedance Z(ω) of the proposed Randle circuit is given by23 Ro Ri Z(ω) = R s + + 1 + R o. Q o. (jω)α2 1 + R i. Q i. (jω)α1
Figure 5. Equivalent circuit used for fitting the experimental electrochemical data.
and Qi (in μF cm−2) are the external- and the internal-layer capacitance, respectively. ω = 2πf (in rad s−1) is the angular frequency, where f is the frequency in s−1. j = √(−1) is the imaginary factor. α1 and α2 are the nonideality factor for the external and the internal proposed Randle circuit, respectively. The α factor is because of the nonuniform current distribution due to coarseness and surface imperfections where 0 ≥ α ≥ 1. Table 1 shows the electrical parameters of a Randle circuit for
where Rs is the solution resistance, Ro and Ri (in Ω) are the external- and the internal-layer resistance, respectively, and Qo 23
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Table 1. Fitted EIS Data of the Uncoated Ti−6Al−4V Alloy, CS/PVP, and/or FS/CS/PVP-Coated Ti−6Al−4V Alloy Incubated at 37 °C in Artificial Saliva Solution for 14 Days alloy
time, h
Rs, Ω cm2
α2
Ro, Ω cm2
Qo, μF cm−2
Ri, MΩ cm2
Qi, μF cm−2
α1
2 24 192 336 2 24 192 336 2 24 192 336
26.7 18.2 19.5 23.7 26.3 27.5 31.6 36.3 31.6 39.8 50.1 46.4
0.89 0.91 0.93 0.93 0.91 0.91 0.94 0.92 0.95 0.96 0.92 0.95
25.9 48.7 74.1 89.5 88.2 191.4 246.7 322.9 174.1 282.4 502.5 652.6
15 9.5 8.6 7.9 11.2 7.0 6.6 4.9 8.8 6.9 4.3 3.2
0.8 1.6 1.9 2.1 1.6 4.8 5.0 7.9 3.2 10.5 13.7 17.9
5.0 2.5 2.1 1.9 2.1 1.7 1.5 1.3 1.5 1.3 1.2 1
0.84 0.86 0.86 0.87 0.82 0.87 0.83 0.85 0.85 0.89 0.85 0.84
uncoated alloy
CS/PVP-coated alloy
FS/CS/PVP-coated alloy
the fitted EIS data of the uncoated alloy, CS/PVP-coated alloy, and FS/CS/PVP-coated alloy incubated at 37 °C in artificial saliva solution for 14 days. The results reveal that the main contribution of the corrosion resistance comes from the internal-layer resistance Ri. Moreover, the EIS data show that corrosion resistance of the uncoated and coated alloys is arranged as: FS/CS/PVP-coated alloy > CS/PVP-coated alloy > uncoated alloy. The FS/CS/PVP-coated alloy exhibits excellent corrosion prevention with the highest inner layer corrosion resistance Ri = 17.9 MΩ cm2. This is because PVP is a binder containing oxazole moiety, nitrogen and oxygen atoms with pairs of electrons that adsorb well with chitosan (resorbable polymer) on the alloy surface through coordinate covalent bonds. PVP is used as a thickening agent in tooth whitening gels. Also, silica helps to strengthen the coating, where fumed silica particles condense and coagulate the coating by accumulating to form networks through hydrogen bonding between silanol groups.23 The surface hydroxyl groups in the fumed silica increase the tendency to create hydrogen bonds, which leads to the agglomerate formation and accumulation. These bonds are strong enough to keep the particles in touch even when the high shear forces are applied on the surface of the aggregates throughout the compounding route.24 This coating can be used in toothpaste owing to silica’s excellent properties in connection to PVP and chitosan adsorption properties to the film on the alloy surface. 4.1.2. Potentiodynamic Polarization. Potentiodynamic polarization test was carried out for uncoated, CS/PVP-coated, and FS/CS/PVP-coated alloy against the standard calomel electrode incubated at 37 °C in artificial saliva solution for 14 days. Figure 6 exhibits the potentiodynamic polarization curves for uncoated alloy, CS/PVP-coated alloy, and FS/CS/PVPcoated alloy incubated at 37 °C in artificial saliva solution for 14 days. By calculating the Tafel slopes ßa and ßc, the electrochemical corrosion potential Ecorr and corrosion current density icorr of the uncoated and coated alloys were identified by the interception point of the cathodic and anodic Tafel line extrapolation.25 The inhibition efficiency (IE) of the coated alloy was calculated from IE =
icorr(uncoated) − icorr(coated) icorr(uncoated)
Figure 6. Polarization curves for uncoated Ti−6Al−4V alloy, CS/ PVP, and/or FS/CS/PVP-coated Ti−6Al−4V alloy incubated at 37 °C in artificial saliva solution for 14 days.
Table 2. Polarization Data for the Tested Alloys electrodes
Ecorr, mV
icorr, nA cm−2
IE%
uncoated Ti−6Al−4V alloy CS/PVP-coated alloy FS/CS/PVP-coated alloy
−1004 −380 −306
239.88 39.8 0.1
83.4 99.95
demonstrates the outstanding corrosion prevention with the best inhibition efficiency (IE = 99.85%) granted by FS/CS/ PVP coating on Ti−6Al−4V alloy surface. This inhibition efficiency is higher than that obtained by Akram et al.27 (99.715%) for linseed oil-based polyurethane/tetraethoxyorthosilane/fumed silica NC hybrid nanocomposite coatings. Chellappa and Vijayalakshmi28 observed a corrosion current density of 0.120 mA cm2 using SiO2/TiO2 composite and 0.051 mA cm2 for silica composite coating29 on Ti alloy. Both results are higher than that obtained in the present work that is 0.1 nA cm2, which indicates a lower corrosion rate in the present work than others. Generally, the results ensure the effective protective behavior of the coating owing to the presence of oxygen and nitrogen atoms in CS and PVP that adsorb well and bind the coating well to the alloy surface in addition to silica utilized as filler for strength reinforcement.18,23 Also, the hydrogen bonds that are formed in the coating with forming silanol groups strengthen well the film. This was confirmed by Fourier transform infrared (FT-IR) analysis (Figure 7). The bands at 420 and 527 cm−1 are ascribed to bending, rocking, and stretching vibrations of
× 100, where
icorr(uncoated) and icorr(coated) are the corrosion current densities of the uncoated and coated alloys, respectively.26 Table 2 shows the potentiodynamic data (Ecorr, icorr, and IE) for the uncoated and coated alloy. The FS/CS/PVP-coated alloy gives the less negative electrochemical corrosion potential Ecorr = −320 mV and the less corrosion current density icorr = 0.1 nA cm2. This 76
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(wt %) as follows: 5.7 Al, 3.85 V, 0.18 Fe, 0.038 C, 0.106 O, 0.035 N, and the balance, titanium. A three-electrode cell was set up constituting of the working electrode (Ti−6Al−4V alloy), the counter electrode (large platinum sheet), and saturated calomel electrode (SCE) as a reference electrode. The working electrode has a cylindrical shape. It was fabricated by welding of one side to an electrical wire. Thereafter, three of its sides were covered with Araldite epoxy resin and inserted in a cylindrical glass. So, only one side (the fourth side) with a circular shape is the free surface. This surface is the bare surface that is covered with any used coating. Polishing of the outer surface of the alloy was carried out by emery paper of grades 320−2000 corresponding with a constant run time. Thereafter, the electrode surface was cleaned with a dry smooth cloth; the electrode surface was degreased in acetone, followed by rinsing by ethanol and then leaving the electrode outer surface for drying in air. 4.2. Chemicals and Reagents. Artificial saliva solution has a composition (wt %) of 0.72 potassium chloride, 0.22 calcium chloride dihydrate, 0.6 sodium chloride, 0.68 potassium phosphate monobasic, 0.866 sodium phosphate dibasic dodecahydrate, 1.5 potassium bicarbonate, 0.06 potassium thiocynate, and 0.03 citric acid g L−1 where the pH = 6.5.30 Chitosan was from crab shells (85% deacetylated), glacial acetic acid (≥99.0%) of analar grade and hydrophilic amorphous silica (AEROSIL 200 Pharma) were purchased from Evonic industries, and poly(vinylpyrrolidone) was purchased from Sigma-Aldrich with an average molecular weight of 360 000. Triple distilled water was used in all preparations. 4.3. Instrumentation. Polarization investigations were measured at a scan rate of 1 mV s−1. The electrochemical impedance spectroscopy was verified at 10 mV excitation alternating current amplitude in a frequency range of 0.1−100 Hz. The utilized electrochemical workstation is IM6e Zahnerelectrik, GmbH, Kronach, Germany operated with Thales software. Scanning electron microscopy (SEM) measurements were achieved using SEM Model Quanta 250 FEG (field emission gun) connected to an EDX Unit (energy dispersive X-ray analyses) (FEI company, Netherlands). FT-IR spectrum was achieved by Agilent technologies Cary 630. Vickers microhardness was determined using an HMV-200 microhardness tester, with a 50 g load (i.e., HV0.05) and loading time of 15 s. All experimentations were accomplished and repeated 2−3 times at a temperature of 37 °C and gave reproducible results. All potentials were measured and recorded against SCE (E = 0.241 V/SHE) as a reference electrode. 4.4. Preparations of the Composites Coatings. 4.4.1. Preparation of Chitosan/Poly(vinylpyrrolidone) (CS/ PVP) Composite. Chitosan 1% solution was prepared by liquefying 1 g of chitosan in 100 mL of 2% (v/v) glacial acetic acid solution with nonstop dynamic movement of about 3−4 h. Meanwhile, poly(vinylpyrrolidone) (5% solution) was prepared by progressively dissolving 5 g in 95 g of distilled water with forceful inspiring for about 30−60 min. The required chitosan/poly(vinylpyrrolidone) composite is attained by mingling the prepared 1% chitosan solution with the 5% poly(vinylpyrrolidone) solution in a volume ratio of 3:1, respectively. 4.4.2. Preparation of Fumed Silica/Chitosan/Poly(vinylpyrrolidone) (FS/CS/PVP) Composite. The second
Figure 7. FT-IR analysis for CS/PVP and/or FS/CS/PVP coatings.
the Si−O−Si group. The band at 1645 cm−1 is ascribed to the H−O−H deformation bond representing water incorporation in fumed SiO2 net and a broad band at 1070.3 cm−1 showed the presence of Si−OH stretching bond on the alloy surface, which is responsible for forming silanol groups in artificial saliva with immersion.29 A sharp band at 3443.28 cm−1 was attributed to stretching vibrations of hydroxyl (OH−) groups on the surface of silica, which could be due to water absorption from atmospheric air because of the hygroscopic nature of fumed silica.29 Also, the Vickers microhardness testing was performed on the two tested coatings. The results were compared to the acceptance values for the Vickers microhardness. It was found to be 83 HV (HV0.05) for CS/PVP and 120 HV (HV0.05) for FS/CS/PVP coatings. This ensures that the hardness of the novel coating is higher and good. All resultant data obtained using all techniques and analyses ensured the high excellent corrosion resistance of the FS/CS/ PVP composite coating on Ti−6Al−4V alloy surface as a novel coating in artificial saliva solution at 37 °C.
3. CONCLUSIONS • The new fabricated FS/CS/PVP coating on Ti−6Al−4V alloy surface greatly improved the electrochemical corrosion resistance of Ti−6Al−4V alloy in artificial saliva solution at 37 °C. The inhibition efficiency of this novel FS/CS/PVP coating reaches up to 99.85% • Introducing fumed silica in CS/PVP composite for Ti− 6Al−4V alloy coating gives higher corrosion prevention than using CS/PVP composite coating on the alloy surface in artificial saliva solution at 37 °C. It reaches to Ecorr = −320 mV, and the lowest corrosion current density icorr = 0.1 nA cm2 for FS/CS/PVP coating. • This study reveals that the electrochemical corrosion resistance of the uncoated and coated Ti−6Al−4V alloy is arranged in the following order: FS/CS/PVP-coated alloy > CS/PVP-coated alloy > uncoated alloy. It reaches to 17.9 MΩ cm2 for FS/CS/PVP coating.
4. EXPERIMENTAL SECTION 4.1. Electrode and Cell Composition. Ti−6Al−4V alloy, with a cross-sectional area of 0.196 cm2, is manufactured by Johnson and Matthey (England) with elemental compositions 77
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composite was prepared by slow addition of 0.35 g of fumed silica to 100 mL of the CS/PVP composite solution under vigorous stirring for 30 min. 4.4.3. Coating Preparation. To make a coating of each composite (CS/PVP or FS/CS/PVP) on the bare alloy surface, the surface was dipped into the corresponding composite solution. Thereafter, the electrode was left vertically, where the electrode alloy surface is in upward direction, for 24 h to dry in air. The coated alloy electrode was then rinsed with distilled water before direct use in the corrosion measurements.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Amany M. Fekry: 0000-0001-9084-9275 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors would like to acknowledge the financial support by Cairo University.
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REFERENCES
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