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Author’s Accepted Manuscript
Synthesis and corrosion resistance of SiO2-TiO2ZrO2-Bi2O3 coatings spin-coated on Ti6Al4V alloy
Fabio Leonardo Alférez Vega, J.J. Olaya, Jorge
Bautista Ruiz
www.elsevier.com/locate/ceri
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DOI:
Reference:
S0272-8842(17)32367-2
https://doi.org/10.1016/j.ceramint.2017.10.161
CERI16580
To appear in: Ceramics International
Received date: 28 September 2017
Revised date: 23 October 2017
Accepted date: 23 October 2017
Cite this article as: Fabio Leonardo Alférez Vega, J.J. Olaya and Jorge Bautista
Ruiz, Synthesis and corrosion resistance of SiO2-TiO2-ZrO2-Bi2O3 coatings
spin-coated
on
Ti6Al4V
alloy, Ceramics
International,
https://doi.org/10.1016/j.ceramint.2017.10.161
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Synthesis and corrosion resistance of SiO2-TiO2-ZrO2-Bi2O3 coatings spin-coated on Ti6Al4V alloy
Fabio Leonardo Alférez Vega a J. J. Olaya a Jorge Bautista Ruiz b
a
Department of Mechanical Engineering and Mechatronics, Universidad Nacional, Bogotá, Colombia
b
Department of Physics, Universidad Francisco de Paula Santander, San José de Cúcuta, Colombia
e-mail: flalferezv@unal.edu.co; jjolayaf@unal.edu.co; jorgeh.bautista@gmail.com
Abstract
This report shows the synthesis and corrosion resistance of SiO2-TiO2-ZrO2-Bi2O3 coatings deposited by means of spincoating on Ti6Al4V alloy. The sols were prepared from a mixture of organic precursors, tetraethoxysilane (TEOS) 98%,
titanium tetrabutoxide (TBT) 97%, Zirconium (IV) butoxide (TBZ) 80% in 1-butanol solution, and bismuth nitrate
pentahydrate Bi (NO3)3 * 5H2O. The coatings were evaluated via potentiodynamic polarization and electrochemical
impedance spectroscopy (EIS) tests in an electrolytic solution of 3.5% Wt NaCl + 0.5 M H2SO4. The rheology and pH of the
prepared sols were studied through rheology, pH as a function of time, Fourier transform infrared spectroscopy (FTIR), and
differential scanning calorimetry (DSC) analysis. The coatings were characterized via X-ray diffraction (XRD), scanning
electron microscopy equipped with dispersive energy spectroscopy (SEMEDX), X-ray fluorescence (XRF), and adhesion
measurements. The results showed that the films add good corrosion resistance to the metal substrate, decreasing current
densities up to one order of magnitude.
Keywords:
Sol-gel, Ti6Al4V, Coating, (SiO2-TiO2-ZrO2- Bi2O3) composite, corrosión, adherence.
1. Introduction
Titanium alloys such as Ti6Al4V are widely used in various industrial fields, especially biomedicine and
aeronautical engineering, since they have attractive properties such as high strength-to-weight ratio, low density,
a high degree of fatigue strength, high corrosion resistance, and excellent biocompatibility, in addition to their
relatively high specific strength, and they are considered promising candidates for turbine engine components
[1], [2]. In contrast to these advantages, they have relatively low tribological properties, such as a high and
unstable coefficient of friction, low abrasion and adhesion resistance. The high corrosion resistance of titanium
is due to the spontaneous formation of a protective oxide film 1–4 nm in thickness. However, under certain
environmental conditions, the anticorrosive properties can be considerably reduced because of material
degradation or failure [3], [4]. The high corrosion resistance of titanium is due to the spontaneous formation of a
protective oxide film 1–4 nm thickness; however, body fluids contain Cl- ions that can induce the rupture of the
oxide films of titanium and initiate the process of localized corrosion, introducing foreign ions into the body that
can generate adverse biological reactions, which can give rise to cancerous-type diseases [4] [5]. The corrosive
power of sulfates (SO42- ) increases when they are associated with Cl- ions, especially in soft waters of low
alkalinity that are not saturated with CaCO3 [6], [7].
In order to improve the anticorrosive properties of Ti6Al4V alloys and to take advantage of their positive
properties, metal oxide coatings are being developed via different surface treatment techniques, among which the
sol-gel process stands out. The sol-gel methodology allows the economical and efficient production of coatings.
This process is characterized by the use of relatively simple equipment, allowing one to deposit coatings with
different compositions, designed chemical properties, and good adhesion to metal surfaces [8], [9], [10].
Materials such as silicon oxide (SiO2) can improve the corrosion resistance of metals under different
temperatures, due to their high thermal and chemical resistance [8]. On the other hand, zirconium oxide (ZrO2) is
characterized by a high coefficient of expansion, which can reduce the formation of cracks during the hightemperature curing process. This material also offers good chemical stability, a high degree of hardness, and
good anticorrosive and biological properties [8], [11]. Titanium oxide (TiO2) has excellent chemical stability,
high mechanical strength, and low electrical conductivity. It is biocompatible and has good anti-corrosion
properties [8]. Finally, oxides containing bismuth, such as Bi2O3, have attracted great attention in recent years
due to their various properties and applications. For example, they have a high refractive index, electrical
permissiveness, photoconductivity, photoluminescence, and a wide energy gap. They are also an important
component in the manufacture of transparent ceramic glass, optical coatings, and ceramics, among others [12],
[13]. Recent studies have not reported positive results with respect to anticorrosive coatings of quaternary
systems on the Ti6Al4V alloy. For this reason, the aim of this paper is the synthesis and evaluation of SiO2TiO2-ZrO2-Bi2O3 thin films using the sol-gel spin coating technique, taking advantage of each of the components
as an alternative for improving the corrosion resistance of the Ti6Al4V alloy.
2. Experimental Procedure
2.1.
Substrate preparation
Ti6Al4V coin-shaped sheets were used as substrates for the deposition via a multi-component system with
surfaces prepared with silicon carbide sandpaper with different grits (400 to 2500), followed by magnesium
oxide solution (0.1 μm grain size). Before the deposition process, samples were degreased using ultrasonic
equipment for 15 minutes with acetone, followed by drying. The films were deposited via a spin coating
machine at a speed of ~2700 rpm for 10 seconds.
2.2. Sol preparation and film deposition processes
A three-step process was carried out in order to obtain the final mixtures, as follows: a) A preparation of bismuth
nitrate, bismuth(III) nitrate pentahydrate (Bi (NO3)3.5H2O), was used, and excess water was removed by means
of heating for 96 hours at 65 °C in a vacuum oven [14]. The dehydrated Bi(NO3)3 dissolved in acetic acid and
ethanolamine acts as a pH-regulating agent. b) A preparation of the base sols of (Ti-Zr) solution was prepared,
containing as a solvent 2-ethoxyethanol by adding 50% of the total volume required throughout the process and
then adding the complete amount of (2,4-pentanedione), which helps to decrease the reaction times of the
titanium precursor and finally the total volume of the titanium precursors Ti (OBu)4 and zirconium (Zr(OC3H7)4).
c) On the other hand, a solution containing TEOS, 50% of the calculated total amount of 2-ethoxyethanol, and
50% of the calculated total amount of water was prepared. The solutions were maintained under constant
magnetic stirring at 250 rpm. Finally, these solutions were mixed in the following order: adding solution (c) to
solution (b), considering that alcohol is the solvent of the precursors and 2,4 pentanedione is the substance that
will decrease the rate of hydrolysis and condensation of Zr (OC3H7)4 and Ti (OBu)4, and 50% of the missing
water volume was added to the mixture; then solution (a) was added to the system to form the mixture of the
four major precursors.
2.3.
Film deposition process
The process of depositing the coatings was carried out through spin-coating at 2700 rpm for 10 sec,
using a sample volume on the substrate of 0.05ml. The coating systems studied correspond to a
configuration in molar concentrations for the silicon, titanium, and zirconium precursors and a volume
ratio of the bismuth nitrate solution to the precursors of 7: 1 ml. The concentrations of the sols studied
in this paper are summarized in Table 1, where the variation of the precursors of (Si-Ti-Zr) can be
observed with respect to the three concentrations of bismuth nitrate.
Insert table 1.
2.4. Heat treatment
Once the coatings were formed and deposited on the substrate, a drying process was carried out at room
temperature for 30 min, followed by continuous heat treatment in order to ensure the removal of the solvent. The
heat treatment was performed in order to reduce the internal stresses that can be generated by the accumulation
of the solvent, to decrease the porosity and cracking due to the contraction generated by the vacancies of the
molecules that evaporate from the solvent, and to consolidate the adhesion of the film to the substrate. The heat
treatment of the films consisted of heating at 42 °C for 24 hours and finally increasing to 200 °C for 3 hours at a
rate of 1 °C/min. These sintering temperatures were obtained from a differential scanning calorimetry (DSC)
analysis.
2.5. Chemical and structural characterization
The sols were characterized using different techniques, such as Fourier transform infrared spectroscopy (FTIR).
Thermo Nicolet equipment was used, and the working software was the OMNIC FTIR iS10 model. The working
conditions were: infrared 400 to 4000 cm-1, transmittance mode, resolution of 0.4 cm-1, and number of scans 32.
DSC thermograms were obtained using a TA Instruments Q20 differential scanning calorimeter with a heating
rate of 5 °C/min-1 in the range of 25 to 315 °C. The sample was purged with N2 at a flow rate of 50 ml min-1. The
sample size ranged from 4 to 8 mg. Monitoring of viscosity and pH evolution was carried out as a function of
time, using a Cannon-Fenske Routine capillary viscometer, which has a range of viscosity values between 0.8 cP
and 8 cP. Subsequently, the coatings were characterized through different surface techniques such as scanning
electron microscopy (SEM) and dispersive energy spectroscopy (EDX), using a FEI QUANTA 200 scanning
electron microscope in a high vacuum and at a voltage of 30kV. The X-ray diffraction (XRD) analysis was
performed using the Bragg-Brentano technique, using a Panalitycal-Emperean-type diffractometer with the
copper Ka line (1.540998 Å). The measurements were carried out with a current intensity of 30 mA, a potential
difference of 40 kV, and a sweep of 30° to 90° (2θ) configured with a step time of 0.50 seconds and a step size
of 0.020° (2θ) in continuous mode. X-ray fluorescence analysis (XRF) was carried out with a Philips MagixPro
PW-2440 X-ray fluorescence spectrometer, equipped with a Rhodium tube, with a maximum power of 4Kw. We
worked with a sensitivity of 200ppm (0.02%) for the detection of heavy elements.
Finally, adhesion analysis was performed, in which the type of failure during the film stripping test (SixTiyZrz) % Biw was identified according to the scratch atlas shown in ASTM C-1624 [15]. For this test, a CSM Revetest
Xpress scratch tester was used. The applied load increased progressively from 0 to 30N along a striping length of
8mm with a Rockwell C indenter of 200μm radius; the scratch speed was 10mm/min.
2.6. Electrochemical Measurements
Electrochemical potentiodynamic polarization tests were carried out with an ACM Instruments potentiostat. A
three-electrode cell was used, with a silver/silver chloride (Ag/AgCl) reference electrode, an auxiliary platinum
counter-electrode, and a working electrode, which in this case was the sample to be evaluated. Samples with an
approximate area of 0.175 cm2 were exposed to a solution of 3.5% wt NaCl + 0.5 M H2SO4 initially for 30
minutes in order to stabilize the system. Subsequently, the polarization test was carried out, in which a potential
scan was done from -200 mV to 1000 mV with respect to the open circuit potential (OCP), at a scanning speed
of 24 mV/min.
The electrochemical impedance spectroscopy (EIS) tests allowed evaluating the corrosion resistance of the
coatings as a function of time. They were carried out in a cell of three electrodes using the same electrolytic
solution that was used for the potenciodynamic polarization tests. The sweep used a frequency from 100,000 Hz
to 0.01 Hz, and a sinusoidal ac perturbation of 10 mV (rms) amplitude was applied. The tests were run according
to ASTM G3-14 [16] on a Gamry 600 potentiostat/galvanostat.
3. Results and discussion
3.1 Characterization of the sol/gel
3.1.1 Study of viscosity and pH of the sol/gel (as a function of time)
The evolution of viscosity as a function of time for film deposition showed that three regions can be delimited.
The first region corresponds on average to sols at a starting viscosity of 2.5 cP, and is where the interaction of
the molecules begins to form the process of hydrolysis and polymer chain formation. The second region is the
stabilization stage, where the equilibrium becomes notable in the reactions of hydrolysis and condensation, since
polymer chain formation is not carried out in a major way, favoring the zone for the formation of the coatings to
be found in an average time range between approximately 400 and 1000 hours of sol ageing, and a viscosity of
3.3-4.0 cP. At this stage, continuous and homogeneous morphologies are obtained. Finally, region 3 is related to
polymer chain formation, where its changes begin to become more noticeable, and where the hydrolysis reaction
becomes less noticeable than the condensation reaction, initiating the gelation stage of the sol.
The results of effect of pH variation as a function of the ageing time was divided into two zones. The first zone
ranged from 0 to 800 hours of sol formation, with a stable pH value and acid values, which allows generating
reactions of both hydrolysis and slow and controlled condensation. Therefore, the span of time recommended
according to the behavior of the pH for obtaining coatings is between 400 and 800 hour.
In the second zone (region 2), it can be seen that after 800 hours of ageing, the pH tends to increase, a zone
where it is not convenient to carry out the forming of the coatings due to the increase in the reaction rate of the
condensation. This has an important effect on the quality of the coating in terms of its final characteristics, such
as increased thickness, low homogeneity, cracking and failure during drying, and sintering. The decrease in pH
and viscosity of the sols with bismuth nitrate could be related to the Pourbaix diagram of the Bi-H2O system.
Once the bismuth nitrate is dissociated, the cation Bi3+ is liberated, which, depending on the pH, can be stable
or can generate the formation of hydroxides. According to studies by Baes and Mesmer in 1976 [17], bismuth at
low temperatures has a high affinity for OH- hydroxyls. In addition, their studies found that at low
concentrations of Bi, polynuclear species can be found that tend to favor the formation of different species of
hydroxides, and at low concentrations this formation of hydroxides is not highly favorable for the solution.
3.1.2 FTIR (Infrared spectroscopy via Fourier transform)
Figure 1 shows the infrared spectrum for the composition of (Si-Ti-Zr) - Bi (10-70-20) -10%, which according to
its content is a general representation of the synthesized samples. In general, the 1128-930cm-1 bands decreased
continuously with the reaction time and disappeared upon completion of the hydrolysis [18]. The bands located
at 1284 and 1128 cm -1 are associated with the polymerization of the Si-OH groups’ forming Si-O-Si bonds. Both
bands increased with time and are present in spectra that exhibit cross-linking of Si-OH groups, taking place in
the sun to form the gel. In the gel, the presence of Si-OH groups and broken Si-O bonds were detected by means
of the bonds located at 1017 and 930 cm-1, respectively. The bands at 826 cm-1 may be associated with the
symmetrical vibrations of (SiO4 ) groups [18]–[21].
Insert figure 1
Furthermore, the band at 3436 cm-1 corresponds to the O-H stretch bond of solvents such as ethanol. The band ~
2982–2914 cm -1 corresponds to the asymmetric stretching of the C-H bond, corresponding to the CH3 present in
ethanol and TEOS [22], [19], [21], [22]. Finally, a very fine band was observed at 444 cm-1, corresponding to the
formation of O-Si-O bonds of the group (SiO2) and also assigned to vibrations of Bi-O bonds in BiO6 units at
658 cm-1, which could be due to the formation of polynuclear species by increasing the concentration of Bi that
favors the stabilization of pH and viscosity. The bands at 537–748 cm-1 could be associated with the Ti-O-Ti
functional group, and bands close to 658 cm-1 could also be associated with the vibrational mode of the Ti-O
group [22], [19]. The intensities of the bands at 1380–1583 cm-1 correspond to the vibrations of the nitrate group
-
(NO3) . The appearance of a band at 945 cm-1 demonstrates the increase in the degree of crosslinking (formation
of a three-dimensional network formed by the union of different polymer chains) by Bi-O-Si bridges [20].
3.1.3 Differential scanning calorimetry (DSC) study
The results of the differential scanning calorimetry analysis for the solutions shows that the peaks of the six
sol/gel solutions are close together, which is suitable for heat treatment. The first peak can be observed at 42 °C,
characterized by the evaporation of the solvents, and this is an important process to consider, because at this
temperature the process of cracking due to movement and vacancies can begin, which can generate molecules
that tend to evaporate. For this reason, a very slow heating process is important, recommended to be at a rate of 1
°C/min. The exothermic peak observed at 200 ºC generated the sintering of the molecules and compaction of the
film to the substrate, which is undoubtedly an important process for finally promoting adhesion to the material,
and in some cases this is the temperature at which a material changes its structure from amorphous to crystalline
[23], [24].
3.2 Characterization of coatings
3.2.1 Scanning electron microscopy (SEM)
Figure 2 shows the morphology of the coatings for the composition 10-70-20 with different concentrations of Bi.
In the sample (10-70-20), as the content of Bi(NO3)3 increases, the formation or generation of branching or
cross-linking can be seen on the surface of the films. [12]
Insert figure 2
Initially with 5% of Bi(NO3)3, (Figure 2 (a)), a uniform, homogeneous film was observed, with some particles
embedded in the bright white surface. With increasing amounts of Bi(NO3)3, it was observed that branches of
different shapes and irregular pores are generated on the film, as evidenced in Figure 2 (c)), which decreases the
homogeneity of the coating [12]. Initially this could be confused with the cracking of the film, but it is the
formation of a network of oxides, mainly of bismuth. This type of morphology agrees with the study carried out
by Weidong, H. et al. [12], where they synthesized bismuth oxide films using Bi(NO3)3 as the main precursor via
the sol-gel technique. White particles were analyzed by means of dispersive energy spectroscopy (EDX). These
results are summarized in Figure 3, confirming that these white particles and frameworks are bismuth-rich
clusters. For the other samples studied, the same behavior was observed as the amount of bismuth nitrate
Bi(NO3)3 was increased.
Insert figure 3
The cross-section of SEM coatings, where the amorphous growth of the film can be observed, obtained through
the sol-gel technique. It can be seen that they have micro-cracks and pores that favor corrosive processes and
filtration of the electrolyte through these microcracks. Furthermore, these defects can become stress
concentrators that favor the formation of cracks and reduce their resistance to scratching or wear [25].
3.2.2 Profiling and roughness investigations
The thickness of the films was between 242 and 277 nm, characteristic of this type of coating deposited via the
spin-coating technique, as reported by several authors [8], [26], [27].
The roughness of the films increased as the amount of the bismuth precursor increased. This variation in
roughness could be explained by an increase in the formation of different oxide clusters on the surface of the
film, which favors protection against corrosion of the material because it hinders the diffusion of the electrolyte
towards the substrate [26]. These roughness values ranged from 9.23 nm to 62.30 nm.
3.2.3 X-ray fluorescence (XRF)
Table 2 shows a summary of the X-ray fluorescence results. In general, the presence of the components Si, Ti,
Zr, and Bi in the coating deposited on the substrate of the Ti6Al4V alloy can be observed. This guarantees the
presence of a quaternary film. It can be seen that these results, in addition to confirming the presence of the
synthesized elements, show that there is a formation of oxides due to the presence of these elements along the
surface of the films.
Insert table 2
3.2.4 X-ray diffraction (XRD) study
The X-ray diffraction results obtained for each of the films exhibited an amorphous structure, as shown in Figure
4. The analyses carried out on the films deposited on the Ti6Al4V alloy showed that the films are amorphous,
corresponding to the different oxides formed in the coating. These spectra can be compared, and are similar to
those found for films of zirconium, titanium, and silicon oxides obtained via sol-gel in previous investigations
[28], [20].
This amorphous phase of the coatings may be associated with the low sintering temperatures; at higher
temperatures, there is a greater probability that crystalline phases will be exhibited (Duta, M. et al. [23]).
Insert figure 4
3.2.5 Adhesion test evaluation
Figure 5 shows the region where the process of degradation of the sample film (10-70-20) -10% begins, where
the presence of various forces can be seen. These normal critical forces correspond to: a) normal critical forces
(Lc1-) associated with cracks and plastic deformation of the coating, b) Lc2 with the onset of chipping failure or
local interfacial scattering, and c) Lc3, associated with the continuous penetration of the coating into the
substrate [29]. In the initial part of the test, the formation of small microcracks and plastic deformations at the
sides of the trace that are recorded as Lc1 can be seen. Then the detachment of coating material, identified as
Lc2, can be observed, and finally this leads to generalized failure with the release of constant material exposing
the substrate, recorded as Lc3. This type of behavior was also seen in several investigations, including that
carried out by Babiarczuk, B. et al. They studied the adhesion and chemical properties of thin films of TiO 2 and
ZrO2 synthesized via the sol-gel route [29].
Insert figure 5
Table 3 shows the data of the critical loads that are generated on the film, and it can be seen that the value of Lc1
increases slightly as the amount of bismuth precursor in each of the configurations increases, due to the
formation of oxides, observed in the form of clusters on the morphological surfaces, that favor adhesion and
make it difficult for the indenter to penetrate the film, causing the force to be dispersed and delaying the process
of failure.
The most significant failure mechanism of a coating is delamination, its detachment from the substrate. This
delamination is due to the fact that the adhesion forces are weaker than the cohesive forces in the film, or there is
an increase in viscosity [30].
Insert table 3
3.2.6 Potentiodynamic polarization test.
Figure 6 (a, b, c) shows the potentiodynamic polarization curves characteristic of the coatings produced. It can
be seen that the films show an improvement of the resistance to corrosion with respect to the base material,
where the corrosion current densities, which can be associated with the corrosion rate, decrease by an order of
magnitude for the case of the evaluated films. In addition, the nobility of the material increases, and therefore its
gradual yearly loss diminishes.
Insert figure 6.(a,b,c)
It can also be seen that the films form a greater area of passivity compared to the substrate, which consists of the
difference of Epic-Ecorr, increasing the range of the interval.
Insert table 4
The results obtained from the test are shown in Table 4. Numerical values of the decrease in current densities
and the positive increase of the corrosion potential for the films compared to the bare substrate are shown.
3.2.7 Electrochemical impedance spectroscopy (EIS) evaluation
In Fig. 7 (a), the Nyquist diagram for the system (10-70-20) -10% can be seen, which showed the best response
in the polarization tests and the diffusion process that occurs after an hour of the EIS test of the coatings, with
some degree of deterioration or porosity. At this stage, absorption or desorption of electro-active substances is
generated, which occurs with the electrolyte ions at the metal/coating interface characteristic of sol/gel films. A
larger amplitude is clearly seen in the curves representing the behavior of the film as a function of time with
respect to the base material, which describes a better capacitive behavior, corresponding to a passive protective
layer. This behavior is also described in several investigations of sol/gel coatings on a Ti6Al4V alloy. [31], [32],
[33], [34].
Insert figure 7.(a,b)
The proposed equivalent circuit for this electrochemical system shown in Fig. 7 (b) is generally the one
recommended for porous coatings using the sol-gel technique [35], where Rsol corresponds to the resistance to
the solution, Rp to the resistance to the polarization, and Wd behaves as a constant phase element, simulating a
resistance.
Insert table 5
As seen from the data in Table 5, an improvement in the corrosion resistance of the film with respect to the metal
substrate is shown, with higher polarization resistance (Rp). It can be concluded that a higher concentration of
bismuth allows the possibility of generating a greater amount of interposing oxides and makes it difficult for the
electrolyte to migrate to the substrate, which is reflected in increasing resistance to corrosion of the substrate of
Ti6Al4V alloy [26], [12], [13]. This equivalent electric circuit has already been used by other authors to fit EIS
data for described the behavior of coating produced by sol-gel [36],[37],[38].
Figure 8 shows the Bode diagram (Z vs. f) of three different regions: a high-frequency region, indicating the
resistance properties of the solution, a medium frequency region, indicating the capacitive behavior, and a low
frequency region, which explains the charge transfer process at the solution/electrode interface [37]. In the
image, a small difference between the values of impedance at frequencies of 0.01 Hz can be seen, and this slight
decrease is due to the formation of pores in the film, which, once degraded by the action of the electrolytic
solution, generate localized points of corrosion, which cannot be used by the electrolyte to diffuse and attack the
substrate because the corrosion products generated and the passive layer of TiO2 offer an extra barrier of
protection to the substrate, which hinders the diffusion of the electrolyte.
Insert figure 8
This behavior is in good accordance with the results of Tiwari, S. K. et al. [37]. They evaluated the
electrochemical behavior of ZrO2 coatings in 3.5% NaCl, which provided protection to the substrate evaluated.
Generally, all the films have high impedance values at low frequencies (0.01 Hz), reflecting a corrosion
resistance for the substrate that improves the impedance of the films over the bare substrate by up to two orders
of magnitude.
In addition, Figure 9 (a) corresponds to the micrographs after the tests (EIS) 168 hours after the film (10-70-20) 10% was deposited on the alloy of Ti6Al4V. Coating faults and the generation of localized corrosion points on
the substrate can be seen, with pitting points with an average depth of 1-2 μm, according to the interferometer
results shown in Figure 9 (c). The EDX analysis of the chemical composition performed on this “pitting” shows
that it corresponds to the composition of the metallic substrate, which confirms the degradation of the coating
after the test (see Figure 9 (b)).
Insert figure 9 (a,b,c)
4.
CONCLUSIONS.
Quad systems were carefully deposited on Ti6Al4V alloy. According to the experimental results, the following
was concluded:
 The coatings showed protection against corrosive attacks on the metallic substrate, as evidenced in the results
of the potentiodynamic polarization tests, where material losses per year (mpy/year) decreased with respect to
the substrate, and EIS showed the increase of Rp from values of 2.46E + 03 Ohm uncoated to 1,242 E + 06
Ohm for the coating (10-70-20) -10% deposited on the Ti6Al4V substrate evaluated at 168 hours. These
results show that the formation of corrosion products favors the passivity of the material, as well as the
interaction of the passive films characteristic of the chromium oxide material.
 The adhesion of the films is favored with the increase of the roughness that is generated by the formation of
oxides and especially of bismuth nitrate, which acts as an additional barrier to the adherence to the metallic
substrate and makes the passage of abrasive material difficult, which can generate detachments of the
coating.
Acknowledgements
This research did not receive any specific grant from funding agencies in the public, commercial or nonprofit
sectors.
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List of figures.
Figure 1: Infrared spectrum for the composition (Si-Ti-Zr) - %Bi (10-70-20) -10%
Figure 2. Micrographs (SEM) of the coatings for the composition 10-70-20 and (5-7.5 and 10% Bi)
Figure 3. Image EDX composition of cumulus benches on the coatings.
Figure 4. DRX results for the different coatings systems studied.
Figure 5. Streaking of a film of (10-70-20) -10%.
Figure 6. Potentiodynamic polarization curves.
Figure 7. a) Nyquist diagram for the system (10-70-20) -10% evaluated on the Ti6Al4V alloy. b) Equivalent system
circuit at 168 hours.
Figure 8. Diagram of bode (Z vs f) Ti6Al4V.
Figure 9. Micrograph (SEM) after the (EIS) tests on the films of (10-70-20) 10% on the substrate. a) Pitting zone
on Ti6Al4V b) Pitting profile
Table 1. Configuration of concentrations of synthesis of the sol-gel.
Soles
(Si-Ti-Zr)
[molar]
[molar]
[molar]
Proporción
de Si
de Ti
de Zr
molar de Bi
0,1
0,7
0,2
5%
0,1
0,7
0,2
7,5%
0,1
0,7
0,2
10%
0,1
0,4
0,5
5%
0,1
0,4
0,5
7,5%
0,1
0,2
0,7
5%
(10-70-20)
(Si-Ti-Zr)
(10-70-20)
(Si-Ti-Zr)
(10-70-20)
(Si-Ti-Zr)
(10-40-50)
(Si-Ti-Zr)
(10-40-50)
(Si-Ti-Zr)
(10-20-70)
Table 2. Quantification by XRF analysis of the elements present in the coatings deposited on the Ti6Al4V alloy
Elemento y/o
10-70-20
compuesto
5% Bi(NO3)3
10-70-20
10-70-20
10-40-50
5% Bi(NO3)3
7,5%
10%
Bi(NO3)3
Bi(NO3)3
10-40-50
10-20-70
7,5%
5% Bi(NO3)3
Bi(NO3)3
Ti
89,69%
87,82%
89,93%
89,23%
89,55%
89,28%
Al
5,06%
6,70%
4,55%
5,49%
5,07%
5,39%
V
4,63%
4,69%
4,48%
4,50%
4,49%
4,53%
Si
0,33%
0,35%
0,49%
0,28%
0,32%
0,27%
Zr
0,03%
0,03%
0,04%
0,08%
0,08%
0,11%
Bi
0,14%
0,16%
0,33%
0,13%
0,20%
0,13%
Table 3. Adhesion test results.
Configuración
(10-70-20)-5%
(10-70 20)7,5%
Material
Ti6Al4V
Ti6Al4V
Lc1(N)
3,7
4,1
Lc2(N)
6
6,4
Lc3(N)
8,2
8
(10-70-20)10%
(10-40-50)-5%
(10-40-50)7,5%
Ti6Al4V
Ti6Al4V
Ti6Al4V
5,3
3,5
3,5
10,2
6,0
16,3
8,1
(10-20-70)-5%
Ti6Al4V
4,5
5,2
7,8
Table 4. Electrochemical parameters of the potentiodynamic polarization curves of the coatings.
Configuración
Icorr
2
Ti6Al4V Sustrato
(10-70-20)-5%
(10-70-20)7,5%
(10-70-20)10%
(10-40-50)-5%
(10-40-50)7,5%
(10-20-70)-5%
(A/cm )
3,521E-7
1,316E-8
3,088E-8
1,996E-8
8,234E-9
5,072E-8
1,772E-8
Ecorr
mpy
(mV)
-176,766
24,298
-9,863
126,312
24,897
-146,885
51,331
( mills/año)
0,1117
0,00372
0,00935
0,0100
0,0032
0,0019
0,0075
Table 5. Results EOR Rcor (ohm) of the coatings.
Rcor (Ohm)
Ti6Al4V
168hr
Ti6Al4V Sustrato
2,46E+03
(10-70-20)-5%
1,10E+06
(10-70-20)-7,5%
1,11E+06
(10-70-20)-10%
1,24E+06
(10-40-50)-5%
9,13E+05
(10-40-50)-7,5%
1,02E+06
(10-20-70)-5%
1,16E+06
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