Co-Cr alloys due to outstanding mechanical properties, good biocompatibility, high corrosion and wear resistance are extensively used for the manufacture of prostheses [1,2]. However, metal ions released from Co-Cr alloy in body fluid can cause the toxicity problem . To overcome this obstacle, different surface treatments such as plasma spray, ion implantation and electrodeposition have been developed [4,5]. Among these techniques, electrodeposition is a simple and cost effective method for the preparation of coating on the substrates with different geometries [6-8].
Examination of published literature shows that the optimum wear and corrosion resistance of Co-Cr alloys are usually achieved in presence of 10-30 wt.% chrominum . In recent years, electrodeposition of Cr from hexavalent chromium electrolytes, because of the toxic and genotoxic effects of hexavalent chromium in environment, is substituted by trivalent chromium baths . The preparation of Co-Cr alloys coatings via electrodeposition process from Cr(III) based electrolytes has been reported by Czakonagy et al. . They found that an increase in temperature or decrease in current density led to reduction in chromium content of the alloys. Saravanan and Mohan  have studied the corrosion resistance of Co-(19-35%)Cr alloys deposited from electrolyte containing CoCl2, and CrCl3 without any Cr complexing agent. They showed that corrosion resistance of Co-35%Cr alloy was better than Co-19%Cr one.
Electrodeposited composite coatings prepared by dispersion of particles into the metal matrix have better wear and corrosion properties than pure metallic coatings. So far, the incorporation of different particles such as ZrO2, SiC, Al2O3, TiO2, La2O3 and CeO2 into the metal matrix has been reported . Among these particles, TiO2 particles have been used as bioactive ceramic for improvement of wear and corrosion resistance due to excellent biocompatibility . Therefore, the distribution of TiO2 particles in a Co-Cr matrix produced composite coating can give remarkable properties with possibilities to use them as medical prosthetic implant devices. Furthermore, to the author’s knowledge, there are few studies regarding the electrodeposition of TiO2 particles in the Co-Cr matrix. Hence, the purpose of this study is to investigate the morphology, microstructure and corrosion properties of Co-Cr/TiO2.
Co-Cr and Co-Cr/nano-TiO2 coatings were electrodeposited from Cr(III) based baths using the direct current. The bath composition has been presented in Table 1. Analytical grade chemicals and distilled water were used for preparing the baths. The current densities for electrodeposition of Co-Cr and Co-Cr/TiO2 coatings were 150 and 140 mA cm-2, respectively. The bath temperature kept constant at 30±1°C during the 30 min of deposition time. The pH was adjusted to 1.5 by using 1M NaOH or 4M H2SO4 solution. To achieve to a uniform dispersion of TiO2 nano-particles and prevent their agglomeration in the bath and within the final coatings, the composite bath was stirred for 24h and ultrasonically treated for 45 min before the electrodeposition process.
The 316L stainless steel plates with an exposed area of 9 cm2 were used as the substrate. The anode was a same sized dimensionally stable Ti/IrO2 sheet, which was maintained 3 cm away from the cathode. Prior to the electrodeposition, the substrates were abraded with different grades of emery papers (up to 2000 grit). Degreasing was performed in two steps. First, the substrates were put into acetone in an ultrasonic bath at room temperature for 10 min, and then they were immersed in an alkaline solution (containing NaOH, Na2CO3, Na2PO4.12H2O) at 70ºC for 15 min. Finally, the substrate were pickled in a solution containing hydrochloric acid and cobalt chloride, in accordance with ASTM B254.The substrates were transferred to the electrodeposition bath immediately after activation.
Chemical composition and morphology of the alloy and nano-composite coatings were evaluated using a scanning electron microscope equipped with an energy dispersive X-ray spectrometer (EDS). The structure, crystallite size, and preferred orientation of the electrodeposits were analyzed by the X-ray diffraction (XRD) technique. Philips X’Pert Pro XRD apparatus with Cu-Kα beam (λ=1.542 Å) was used for this purpose. The 2-theta range, step size, and time per step were 10-110º, 0.02º, and 0.4 s, respectively. Crystallite size was estimated by using the Williamson-Hall equation. The effect of TiO2 nano-particles incorporation on preferred orientation was investigated using the relative texture coefficient (Eq. 1) [14-17]:
In this equation, Ihkl is the diffraction intensities of the coatings (hkl) lines, and is those for randomly oriented cobalt powder (JCPDS no. 5-0727). Five reflection lines of (002), (100), (110), (101) and (112) for Co were used to calculate the RTC values.
Corrosion behavior of the coatings was studied by potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) in the simulated body fluid (Hanks’ solution). The chemical composition of the used Hanks’ solution was (in g L-1) 8 NaCl, 0.14 CaCl2, 0.4 KCl, 0.1 MgCl2.6H2O, 0.1 MgSO4.7H2O, 0.35 NaHCO3, 0.12 Na2HPO4.12H2O, 0.06 KH2PO4, and 1 C6H12O6. The tests were performed by using a Bio-Logic SP-300 potentiostat/galvanostat, while the counter and reference electrodes were a platinum sheet (1 cm2) and an Ag/AgCl (3M KCl), respectively. The scan rate during the polarization tests was 1 mV s-1. EIS measurements were carried out over a frequency range from 100 kHz to 10 mHz, using a 10 mV amplitude sinusoidal voltage. The obtained data were analyzed by EC-Lab software. For calculating the corrosion rate the Eq. 2 was used [15,18].
where α is the atomic weight (g mol-1), icorr is the corrosion current density (µA cm-2), n is the number of equivalent exchange, ρ is the density (g cm-3), and F is Faraday’s constant (C mol-1).
RESULTS AND DISCUSSION
According to EDS analysis results, the volume fraction of co-deposited TiO2 particles is 5.8 vol.%. These particles have been uniformly distributed within the alloy film, as indicated in the SEM micrograph from cross-section of the composite coating in Fig. 1. No agglomeration of nano-particles is also observed in this figure due to the appropriate preparation process of the composite electrodeposition bath. It should be noted that the chromium content of both the Co-Cr and Co-Cr/TiO2 coatings was about 29 wt.%.
Morphology of the Co-Cr and Co-Cr/TiO2 coatings are shown in Fig. 2. Both the coatings have nodular morphology and contain micro-cracks. However, the density of defects is reduced by incorporation of TiO2 nano-particles. This can be due to the higher toughness of composite film as compared to the alloy coating, as also reported by other investigators [19-21].
X-ray diffraction patterns of Co-Cr and Co-Cr/TiO2 coatings are demonstrated in Fig. 3. The structure of the alloy film is not changed by incorporation of TiO2 nano-particles. The detected peaks are related to the Co with hexagonal close-packed (hcp) structure. Small TiO2 peaks are also appeared in the XRD pattern of the nano-composite coating. The Cr peaks cannot be observed in these patterns, which indicates formation of substantial solid solution of Cr in Co.
Crystallite size were calculated by using the Williamson-Hall technique. Incorporation of nano-particles has a negligible effect on the crystallite size of the coatings, and both of these films have nano-crystalline structure. The calculated crystallite size for Co-Cr and Co-Cr/TiO2 coatings are 10 and 9 nm, respectively.
The relative intensity of diffraction peaks is slightly changed by incorporation of TiO2 particles. Relative texture coefficient values of five diffraction lines are presented in Table 2. It is observed that RTC(100) has the highest value between all the diffraction lines in both the samples. Therefore, the coatings have  texture, and main part of the grains are aligned with their basal plane vertical to the surface of the deposit.
Polarization curves of Co-Cr and Co-Cr/TiO2 coatings after 1 h immersion in Hanks’ solution are shown in Fig. 4. Corroison potentials and corrosion current densities were calculated by Tafel extrapolation technique. Corrosion rates (µm/year) were calculated by using the Eq.2, and the results are presented in Table 3.Corrosion potential shifts to more positive values by incorporation of TiO2nano-particles, indicating nobler behavior of composite coating as compared to the Co-Cr film in the simulated body fluid.
Corrosion current density and corrosion rate decrease with co-deposition of nano-particles. Corrosion rate of composite coating is about 22% lower than the Co-Cr film. Higher corrosion resistance of composite coating can be due to two reasons. First, decrement of the metallic surface area in contact with the corrosive media with incorporation of ceramic nano-particles. Second, lower density of defects of composite film than the alloy electrodeposit, as it is clear in Fig. 2.
EIS experiments were also performed for further investigation of corrosion behavior of the coatings in Hanks’s solution. The Nyquist plots for Co-Cr and Co-Cr/TiO2 coatings are shown in Fig. 5. The Nyquist plots of both the coatings show single depressed semi-circles, representing the presence of one capacitive time constant and deviation from ideal dielectric behavior because of the surface heterogeneities [15,22-25]. The presence of one capacitive time constant is also confirmed by Bode plots in Fig. 6. Therefore, as shown in Fig. 5, a simple Randles equivalent circuit is used to fit the experimental data of the both samples. In this circuit, Rs is the solution resistance between the working and the reference electrode, Rp is the charge transfer resistance, and CPE (constant phase element) is capacitance of the coatings. The calculated electrochemical parameters are presented in Table 4. EIS results are in good agreement with polarization data. Charge-transfer resistance of the Co-Cr/TiO2 coating is about 1.7 times higher than the Co-Cr film, which means that it has better corrosion resistance than the alloy sample in the simulated body fluid.
It is also clear from Table 4 that the Co-Cr film has higher double layer capacitance than the composite coating. The double layer capacitance is directly related to the surface area involved in the electrochemical reactions . The presence of open voids and cracks can increase this surface area and cause to higher capacitance. As the density of defects is decreased by co-deposition of nano-TiO2particles, the composite coating has lower double layer capacitance than the alloy film. Another reason can be decrement of metallic surface in contact with corrosive media in presence of the ceramic particles.
In this study, Co-Cr alloy and Co-Cr/TiO2 nano-composite coatings were produced by electrodeposition technique. The effect of TiO2 nano-particles incorporation on morphology, structure, and corrosion behavior of Co-Cr alloy film in simulated body fluid was investigated. The outcome of the results can be summarized as follows:
1- The amount of co-deposited TiO2 nano-particles was 5.8 vol.%, which were uniformly dispersed within the Co-Cr matrix. Both the alloy and composite coatings contained about 29 wt.% Cr.
2- Nodular morophology of the Co-Cr film was not changed by incorporation of nano-particles. However, the density of defects of the Co-Cr/TiO2 coating was lower than the unreinforced one.
3- Both the coatings were nano-crystalline substantial solid solution of Cr in Co with hcp structure, and  texture.
4- Corrosion potential shifted to more positive values and corrosion rate about 22% decreased with incorporation of TiO2 nano-particles.
5- According to EIS results, charge transfer resistance of Co-Cr/TiO2 coatings was about 1.7 time of that for Co-Cr film in the simulated body fluid (Hanks’ solution), indicating better corrosion resistance of the composite coating.
CONFLICT OF INTEREST
The authors declare that there are no conflicts of interest regarding the publication of this manuscript.