Electrochemical Properties of Ni47Ti49Co4 Shape Memory Alloy in

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Electrochemical properties of Ni47Ti49Co4 shape memory alloy in artificial urine for urological implant Rasha A. Ahmed Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.5b00838 • Publication Date (Web): 22 Jun 2015 Downloaded from http://pubs.acs.org on June 27, 2015

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Industrial & Engineering Chemistry Research

Electrochemical properties of Ni47Ti49Co4 shape memory alloy in artificial urine for urological implant Rasha A. Ahmed a,b* a b

Chemistry Department, Faculty of Science, Taif University, Taif, Saudi Arabia. Forensic Chemistry Laboratories, Medico Legal Department, Ministry of Justice, Cairo, Egypt.

* Corresponding author at: Chemistry Department, Faculty of Science, Taif University, Saudi Arabia. Tel.: +966 0562805809. E-mail address: [email protected] (R.A. Ahmed).

1 ACS Paragon Plus Environment


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Abstract The corrosion performances of Ni47Ti49Co4 shape memory alloys (SMA) in artificial urine solution was evaluated in comparison with Ni51Ti49 alloy as reference, at 37 °C, and pH 5.66.4. SEM results revealed less pitting attack for Ni47Ti49Co4 SMA surface after immersion in artificial urine solution. The XRD analysis demonstrated the formation of passive film on Ni47Ti49Co4 SMA. The XPS analysis indicated that the film was mainly consisted of O, Ti, Co, P, and a little amount of Ni, and the concentration of Ni ions release was greatly reduced compared to that of the Ni51Ti49 SMA. Linear polarization results illustrated that corrosion potential (Ecorr), corrosion current density (icorr), and ac polarization resistance (Rp) affected greatly by alloying Co to Nitinol alloy. Our observations indicated that the corrosion resistance of the ternary alloy, Ni47Ti49Co4 SMA, offers superior corrosion resistance in artificial urine when compared to Ni51Ti49 SMA, which was suitable for medical applications.

Keywords: Alloyed Co; Corrosion; NiTi; urology; Electrochemical impedance.


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Introduction Ureteral patency in malignant ureteral obstruction cases is a therapeutic challenge. Over the last decade, remarkable progress has been achieved in medical implants. Ureteral stents and percutaneous nephrostomy (PCN) have been widely used as solutions for the treatment of ureteral obstruction 1. Different designs of ureteral stent have been reported, and the results have varied2,3. Although, Polymeric ureteral stents, and metal mesh stents are used successfully, they may cause many side effects started from dysuria, urgency and ended to hematuria and suprapubic pain. Therefore, biomaterials are in a great need. Unfortunately, most implant biomaterials can cause electrochemical reactions inside the body leading to structural and mechanical degradation of the device 4, 5. Various problems are usually related to the mismatch between the implant and the replaced bone, and a secondary surgery will be needed to remove the implant, avoiding interface loosening and tissue inﬂammation

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.

Using NiTi shape memory alloys (SMAs), by their sponge bone like structure, can resolve all these side effects successfully

10

. The pores in the microstructure of the implanted NiTi

SMAs facilitate bone ingress and biological integration of the components, thus ensuring a harmonious bond between the implants and the surrounding tissue. Thus, NiTi SMAs with excellent mechanical properties, good corrosion resistance as well as biocompatibility, seem to be favorable for future applications as an implant materials for use in cardiovascular, orthodontics, and urology 11-14. Even with the existence of an oxide film on the surface of NiTi alloy implants, Ni ions can still be released to body fluid, which might cause allergic reaction, irritation, necrosis and toxic reactions for the surrounding tissues and living cells (liver, kidneys and spleen)

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Therefore, innovative modifications are needed to increase the corrosion resistance of NiTi alloy and make it more biocompatible in simulated body fluids.



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Many researchers have studied alloying NiTi SMA with another element to influence its microstructure, phase transformation and super elasticity 16-19. Alloying another element can enhance mechanical and/or corrosion properties of the binary alloy, by participating in the growth process of the passive film

20-23

.

Currently, there are published results on the

corrosion behavior and microstructure of NiTiCo SMA for some medical implant applications, while only very limited studies have been published on the corrosion characteristics of NiTiCo SMA in body fluids. The objective of this study therefore was to study corrosion behavior of Ni51Ti49 SMA after replacing Ni with 4 % cobalt, as an alloying element, in artificial urine solution, to be used as ureteral stent in urogenital system. 2. Experimental 2.1. Materials Melting technique under vacuum has been used for Ni51Ti49 and Ni47Ti49Co4 SMAs fabrication. After melting four times, to ensure homogeneity, they were casted in ceramic mold as cylindrical rods for the electrochemical tests. The alloys were welded to an electrical wire and fixed with Araldite epoxy resin in a glass tube leaving exposed a cross-sectional area of 0.2 cm2. The surface of the test electrode was mechanically polished with a series of abrasive papers of 400, 800, 1500 grades and degreased with ethanol before each measurement to ensure the same surface roughness. The tests were carried out in deaerated artificial urine at the temperature of 37±1 °C and pH 5.6 – 6.4. The electrolyte consisted of two solutions A and B with the following composition 1.76 g/L CaCl2.2H2O, 4.86 g/L Na2SO4, 1.46 g/L MgSO4.7H2O, 4.643 g/L NH4Cl, and 12.130 g/L KCl for solution A. 2.660 g/L NaH2PO4.2H2O, 0.869 g/L Na2HPO4, 1.168 g/L Na3Cit·2H2O, 13.545 g/L NaCl for solution B, both mixed together in the ratio of 1:1. All reagents used are Analar grade.


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2.2. Instrumental methods 2.2.1. Apparatus Corrosion resistance properties of Ni51Ti49, and Ni47Ti49Co4 SMAs were evaluated by electrochemical techniques in artificial urine at 37±1 °C. A potentiostat/galvanostat (Model 73022, Autolab Instruments, Metrohm) with a standard three electrode cell was used for this study. The alloy is the working electrode, Ag/AgCl/3M KCl (Satd.) as the reference electrode, and platinum as the counter electrode. Cathodic and anodic polarization curves were studied for Ni51Ti49, and Ni47Ti49Co4 SMAs with a scan rate of 10 mV s−1. The changes in free corrosion potential (Ecorr) were monitored as a function of time. Prior to electrochemical corrosion testing, the samples were immersed in 15 mL of artificial urine for 60 min to establish a relatively stable open circuit potential. The corrosion current densities (icorr) and kinetic parameters such as anodic and cathodic tafel slopes (ba and −bc), and corrosion potentials were obtained from tafel analyses based on the polarization curves. The corrosion potential and the corrosion current density were obtained through the linear analysis of Tafel approximation using NOVA software. To avoid the presence of some degree of nonlinearity in the Tafel slope region of the obtained polarization curves, the Tafel constants were calculated as the slope of the points after Ecorr , using a computer least squares analysis. The corrosion current was then determined by the intersection of the cathodic or the anodic Tafel line with the OCP (potential of zero current in the potentiodynamic curves or Ecorr). This point determines the potential (Ecorr) and current density (icorr) for corrosion. For all tested alloys the active dissolution parameters values, corrosion potential (Ecorr), corrosion current density (icorr), Tafel slopes (βa and βc) were calculated and presented in Table 1. 2.2.2. Electrochemical impedance spectroscopy (EIS) The impedance diagrams were recorded at the free immersion potential by applying a 10 mV sinusoidal potential through a frequency domain from 100 kHz down to 100 mHz. The real



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(Z) and imaginary (Z') parts of the impedance are calculated from the overall impedance (Z*) as in equation (1): Z*(ω) = Z (ω) + jZ'(ω)

(1)

where ω is the angular frequency that equals to 2πf (f/Hz is the frequency). The EIS were obtained in artificial urine solution and plotted in the form of complex plane diagrams (Nyquist and Bode plots). 2.2.3. Surface characterization The morphology and microstructure for alloys surfaces were examined using a JEOL JXA840A scanning electron microscope in the magnification range from X 500 to X 2,000 preand post- immersion in artificial urine for 10 days. Perkin-Elmer Optima 2100 Dual View inductively coupled plasma optical emission spectrometry (ICP-OES) instrument connected with AS 93 Plus autosampler was used for elemental analysis. The phase analysis of alloys was examined by X-Ray diffractometer (Bruker D8 Advance, Germany) equipped with CuKα radiation, λ=1.54 A°. X-ray photoelectron spectroscopy was employed to determine the composition and elemental state of the formed films pre- and post- immersion in artificial urine. The data were obtained using a monochromatic Al Kα X-ray source operating at 150 W (Kratos Axis Ultra DLD). The survey and high-resolution spectra were collected at fixed analyzer pass energies of 160 and 20 eV, respectively. Narrow scans of Co 2p, Ti 2p, and Ni 2p were carried out using pass energy of 29.50 eV. Detection limits are approximately 0.05 to 1.0 atomic %. Major factors affecting detection limits are the element itself (heavier elements generally have lower detection limits), interferences (can include photoelectron peaks and Auger electron peaks from other elements) and background (mainly caused by signal from electrons that have lost energy to the matrix).


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3. Results and Discussion 3.1. Surfaces morphologies and film characterization 3.1.1. Surface morphology Immersion corrosion experiments for Ni51Ti49 and Ni47Ti49Co4 SMAs in artificial urine were performed in order to fully understand their surface morphology as biomaterials applied in urogenital system. Figure 1(a), (b) shows SEM micrographs of Ni51Ti49 and Ni47Ti49Co4 SMAs, respectively, pre- immersion in artificial urine. Urine usually contained various aggressive ions that significantly influence the corrosion of biomaterial alloys

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. SEM

micrographs of Ni51Ti49 SMA (with different magnifications) showed several deep pits that confirm localized corrosion has occurred due to the aggressive ions in urine solution (Figure 1(c)). On the contrary, SEM micrographs of Ni47Ti49Co4 SMA showed no corrosion pits (Figure 1(d)). This can be attributed to the high corrosion resistance of Ni47Ti49Co4 SMA in artificial urine. (Insert Figure 1) 3.1. 2. Ion release After immersion both alloys in the artificial urine for three weeks, elemental analysis technique was used to detect concentration of Ni released in solution. ICP measurements results showed that the concentrations of nickel ion released from Ni47Ti49Co4 alloy is 0.012 mg/L, while that of NiTi alloy is 0.919 mg/L 14. Ti and Co ions are not detected. 3.1.3. X-ray diffraction (XRD) It was reported that, the shape memory effect of Ni51Ti49 is due to the thermoelastic martensitic transformation

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. Moreover, the microstructure of Ni51Ti49 alloy consists of

austenite (B2) phase in addition to martensite phase while that of Ni47Ti49Co4 was completely martensite phase (B19) 23. Optical microscope (OM) was studied before to show the difference between the two microstructures

23

. Addition of Cobalt as a third alloying



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element into the binary alloy leads to minimizing Ni content and changing the grain size. Consequently, the change in alloy composition affects its phase transition and microstructure and gives rise to different oxide films and different corrosion behavior. Figure (2 (a) and ( b)) demonstrated the phase observed for the Ni51Ti49 and Ni47Ti49Co4 alloys, respectively, as well as their corresponding oxide films which formed after immersion in artificial urine. The XRD patterns of Ni51Ti49 showed a characteristic peak at 2θ= 42.45°. The diffraction peak at 2θ = 35.14° could be attributed to TiO2. Figure 2 (b) showed the XRD results for Ni47Ti49Co4 alloy, it could be seen the appearance of three diffraction peaks at 2θ = 22.5°, 45.6° and 51.6°, which characteristic for NiCo, (Figure (2 a)). Appearance of new diffraction peaks at 2θ = 26.8°, 39.67° and 45.56° in the XRD pattern of post-immersed Ni47Ti49Co4 demonstrates the formation of an oxide phase. At the same time, it could be seen that the relative intensity of the peaks were increased, which confirms the enhancement of the passive oxide film. Again, replacement of Ni by Co atoms, changes the grain size and crystal structure of the alloy, this can be attributed to the difference in their atomic radii and their electronegativity

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. Thus, it can be concluded that the addition of Co affects the phase

transition of the alloy and improves the oxide film, which act as a protective passive film against corrosive ions. (Insert Figure 2) 3.1.3. X-ray photoelectron spectroscopy (XPS) The chemical composition and surface chemical state of Ni47Ti49Co4 SMA and its corresponding oxide films pre and post- immersion in artificial urine were investigated by XPS and compared to Ni51Ti49 SMA as a reference alloy shown in Figure 3. The XPS survey spectrum of the Ni47Ti49Co4 SMA shown in Figure (4A), exhibited C 1s, O 1s, Ti 3p, Ni 2p and Co 2p peaks, which attributed to the composition of Ni47Ti49Co4. The presence of C is due to environmental contamination. After immersion in artificial urine, the elements in the


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urine solution as carbon; oxygen; sulfur; calcium; phosphorous; were observed in Figure (3 B) and (4 B). The appearance of peaks around 100 eV confirms the formation of calcium phosphate in the outer most layers. Figure (4 C) showed the Ni 2p XPS spectrum, there are two major peaks at 855.7 and 869.5 eV, corresponding to Ni 2p3/2 and Ni 2p1/2, respectively. The Ti 2p3/2 peak has its main component at 458.62 eV, which corresponds to TiO2, and a small component at 464.84 eV relates to Ti2O3. However, there is another main peak at 454.94 eV corresponds to metallic titanium. These indicate that the oxide and sub- surface layers of Ni47Ti49Co4 SMA consisted of mixture of Ti oxide compounds (Figure (4D)) which prevent Ni leaching. The XPS survey spectrum of the Ni47Ti49Co4 before immersion in artificial urine shows that the surface oxide film formed is composed mainly by TiO2 coexisting with NiTiO3 in the sub-layers. After immersion the alloy in the artificial urine, The Co 2p spectrum changed (Figure (4E)) and exhibited two peaks. One of them of binding energy at 776.4 eV corresponds to Co3+, while the other at the higher energy of 796.6 eV was attributed to Co2+. These results indicate that there exist two kinds of cobalt oxidation state in the oxide film: i.e. Co2+ and Co3+ 29. Replacement of Ni by Co, again leads to improvement in the compactness and stability of the passive film by forming different passive oxides film, resulting in significant improvement of the corrosion resistance of the SMAs by minimizing Ni ions release. (Insert Figure 3 and Figure 4) 3.3. Cyclic voltammetry analyses Corrosion potential is the coupling result of cathodic and anodic reactions, thus cyclic voltammograms for (a) Ni51Ti49 and (b) Ni47Ti49Co4 SMAs were performed in artificial urine solutions at 37 ◦C with scanning rate of 10 mVs−1 (Figure (5)). The different compositions of the two alloys can explain the difference between their voltammetric profiles. In Figure (5 (a)), for Ni51Ti49 the anodic reaction is promoted and the cathodic reaction is restrained for



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this reason the anodic current is enhanced causing corrosion while no change in the cathodic current. On the other hand, Ni47Ti49Co4 SMA exhibits a lower anodic peak current value, tending to have higher corrosion resistance. Observation of a cathodic peak at -0.48 V for Ni47Ti49Co4 SMAs (Figure 5 (b)) confirms the different characteristics of the oxide films for both alloys formed during the anodic scan. (Insert Figure 5) 3.4. Open circuit potential The open circuit potential (OCP) of (a) Ni51Ti49 and (b) Ni47Ti49Co4 SMAs was studied with immersion time in artificial urine solution at 37 ◦C. Figure 6 shows the variation of steady state potential (Est) for the two SMAs with time. At the beginning of immersion, Ni51Ti49 SMA, exhibit an anodic potential shift at -0.45 V which is related to anodic protection mechanism. On the other hand, the initial OCP value for Ni47Ti49Co4 alloy was noticed at + 0.068 V. The higher value of OCP for Ni47Ti49Co4 SMA indicates that the ternary alloy shifts corrosion potential to more positive value and increase affinity to corrosion resistance than binary alloy 30. (Insert Figure 6) 3.5. EIS measurements and equivalent circuit 3.5.1. Bode and Nyquist plots. Figure 7 illustrates the Bode and Nyquist plots representing the modulus of impedance and phase angle as a function of frequency and ZImaginary and ZReal, respectively at Ecorr and 37°C. It seems that the magnitude of the impedance of both plots of Ni51Ti49, and Ni47Ti49Co4 SMAs increases with increasing polarization time, which indicates the formation of passive oxide films over the alloys surfaces, capable to resist ions in the artificial urine solution.


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Considering the Bode plots (phase angle (θ) versus log |frequency|), Figure 7 (a) and (b), the experimental maximum phase angles (θmax) for Ni51Ti49 and Ni47Ti49Co4 SMAs were noticed to be increased with time to approach 68° and 71°, respectively. From the phase angle data, on the contrary to Ni51Ti49 SMA, it is thought that Ni47Ti49Co4 SMA stabilize the passive oxide film which intern act as an efficient barrier to corrosion by increasing resistance to charge transfer at the corrosion interface. Figure 7 (c) and (d), show the Bode plots (log |impedance| versus log |frequency|), for Ni51Ti49 and Ni47Ti49Co4 SMAs in artificial urine, respectively. It was observed that the maximum modulus of impedance (/Z/) is attained at a low frequency. Thus, at a frequency of 0.1 Hz, Ni47Ti49Co4 and Ni51Ti49 SMAs have experimental /Z/ values of about 10 4 and 10 5 Ω cm2, respectively, which also increases with time till 180 h. This means that the oxide film is stable for 6 days. The electrochemical results confirm that the ternary alloy could stabilizes the oxide film and enhances the corrosion resistance by preventing or minimizing Ni release to the body fluid, causing corrosion 23. The Nyquist plot of the Ni51Ti49 alloy is shown in Figure 7 (e). The electrochemical impedance spectrum presents a depressed semicircle and low magnitude of impedance associated with the deterioration of the passive oxide film and leaching of Ni ions

31

. A

different result was obtained for Ni47Ti49Co4 (Figure 7 (f)), where the Nyquist plot presents a straight line with a slope of 45°. Generally, both SMAs show that the magnitude of the impedance increases with polarization time, indicating that relatively and to some extent all Nitinol based alloys can resist aggressive ions in the artificial urine solution. (Insert Figure 7) 3.5.2. The ac impedance analyses Figure 8 (a) and (b) show the equivalent circuits models for Ni51Ti49 and Ni47Ti49Co4 SMAs, respectively, in artificial urine solution. The impedance spectra (in Nyquist plot) for Ni51Ti49



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alloy is comprised of a single semicircle, which account for large charge transfers at the alloy/solution interface. From bode plot we can represent the fitting model by a two-time constant equivalent electrical circuit (Figure (8a)). In this model, Rs refers to solution resistance and R1 and R2 refer to the resistances of the two formed layers, where R1 is the inner most layer. For the two resistive layers, there are corresponding two parallel constantphase elements (CPE1 and CPE2). The impedance of ZCPE is defined by ZCPE = 1/((jω)n(Q), where j = √−1 , ω = 2πf , and the exponent n of the CPE is related to a non-equilibrium current distribution due to the surface roughness

32, 33

. The parameter Q is a constant

representing true capacitance of the oxide film. CPE (C) (which represents the deviation from the true capacitance behavior) was used instead of ideal double layer capacitance. In Figure 8 (b) the impedance spectra (in Nyquist plot) for Ni47Ti49Co4 SMA is comprised of a large diameter semicircle which can be modeled as the model used in (Figure 8a) however, it includes Warburg impedance in series with R2 which can be linked to ion diffusion through the passive film. This Warburg impedance indicates adsorption pseudocapacitance, where the corrosion mechanism is controlled not only by a charge-transfer process but also by a diffusion process. Obviously, the larger the circuit diameter, the larger the charge transfer resistance at the alloy/solution interface.

(Insert Figure 8)

3.5.3. Potentiodynamic polarization Potentiodynamic polarization experiments were performed after 1h immersion in artificial urine solution in order to fully characterize the corrosion resistance of the Ni47Ti49Co4 SMA intended for urological implants. Figure 9 shows the representative potentiodynamic polarization results of (a) Ni51Ti49 and (b) Ni47Ti49Co4 SMAs in artificial urine at 37 °C.


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Separate scans for the anodic and cathodic arms were obtained for each alloy. The anodic branch for Ni46Ti49Co4 SMA has a wide passive region at a potential of -0.20 to + 0.60 V suggesting high corrosion resistances. In contrast, this passive region for Ni51Ti49 SMA becomes smaller. Furthermore, Corrosion current density (icorr) for Ni47Ti49Co4 SMA decreases remarkably to 0.11 µA cm-2, while Rp values increases 16-fold compared to Ni51Ti49 SMA. Rp indicates that corrosion resistance of a material is strongly dependent on the chemical composition of the alloy. Therefore, the high Rp values for Ni46Ti49Co4 SMA imply the high corrosion resistance of the alloy and taken as an index of less metallic ions release into the body fluid. The leaching of Ni ions causes a significant decrease in Rp value for Ni51Ti49 SMA, decreasing its corrosion resistance in urine solution. On the other hand, the significant increase in Rp values in case of Ni46Ti49Co4 indicates that Ni51Ti49 SMA improves the resistance towards corrosion by formation of a stable passive oxide film. The Ecorr shifts to more positive values for Ni46Ti49Co4 SMA, reflecting the high corrosion resistance of the ternary alloy. Meanwhile, Corrosion potential is the coupling result of cathode and anodic reaction, and two reasons could attribute to the positive shift of the corrosion potential Ecorr: the anodic reaction is promoted and the cathodic reaction is restrained 34. As shown in Table 1, the anodic slope (ba) increases by replacing Ni with Co, while the cathodic slope (bc) appears to be not greatly affected with this replacement, which indicates that the corrosion process is controlled by anodic reaction. The positive shift of corrosion potential is evidently caused by the restriction of anodic reaction process. Furthermore, from the Tafel slopes (ba and −bc), corrosion rates were calculated to be 0.57 and 0.19 mm/year for Ni51Ti49 and Ni46Ti49Co4 SMAs, respectively, reflecting the corrosion resistance enhancement by replacing Ni with Co. The corrosion potential (Ecorr), corrosion current densities (icorr), anodic and cathodic Tafel slopes (ba and −bc) for Ni46Ti49Co4 and



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Ni51Ti49 SMAs in artificial urine solutions at 37 °C are listed in Table 1. Tafel results are being consistent with previous reported values35-37. (Insert Figure 9) (Table 1) 4. Conclusions The addition of 4% Cobalt to Ni51Ti49 SMA enhances greatly the corrosion resistance in artificial urine solution for application in urinary system. Table of content illustrated the surface morphology and electrochemical impedance obtained in which the results confirm that: 1) Replacement of Ni by Co atoms can significantly improve the corrosion resistance of Ni51Ti49 SMA in artificial urine by the formation of passive oxides film. 2) Addition of Cobalt into Ni51Ti49 SMA, prevent pitting corrosion and minimize or prevent the release of Ni into the body fluid. 3) XRD patterns show that addition of cobalt to Ni51Ti49 SMA affects the transition state and the crystal structure. 4) XPS patterns suggest that a mixture of TiO2 with cobalt oxide layer underneath, minimizing Ni release and protecting the alloy from corrosion. 5) Alloying of Cobalt into Ni51Ti49 SMA lowers the anodic current density icorr, and shifts Ecorr to more positive value. 6) These results thus show a significant step forward to bringing Ni47Ti49Co4 shape memory alloy much closer to real world applications.

Acknowledgements The author is grateful for the financial support of Chemistry Department (University of Taif, kingdom of Saudi Arabia). The authors would like to thank Core labs in King


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Abdullah University of science and technology (KAUST) for performing XRD and XPS analysis.



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(13) Ahmed, R. A., Fadlallah, S.A., ElBagory , N., ElRab, S. F., Improvement of corrosion resistance and antibacterial effect NiTi orthopedic materials by chitosan and gold nanoparticles , App. Sur. Sci., 2014, 292, 390. (14) Fadlallah, S.A., ElBagory, N., ElRab, S.F., Ahmed, R. A., El ousamii, G., An overview of NiTi shape memory alloy: corrosion resistance and antibacterial inhibition for dental application, J. alloys compd., 2014, 583, 455. (15) Rondelli, G., Corrosion resistance tests on NiTi shape memory alloy, Biomater. 1996, 17, 2003. (16) Ahmed, R. A. , Fekry, A. M. , Farghali, R. A. , A study of calcium carbonate /multiwalled-carbon nanotubes /chitosan composite coatings on Ti–6Al–4V alloy for orthopedic implants, App. Sur. Sci. 2013, 258, 309. (17) Xu, H.B., Jiang, C.B., Gong, S.K.. Martensitic transformation of the Ti50Ni48Fe2 alloy deformed at different temperatures. Mater. Sci. Eng. 2000, 218, 234. (18) Craciunescu, C.M., Li, J., Wuttig, M. Constrained martensitic transformations in TiNiCu films. Thin Solid Films 2003,434, 271. (19)

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Wang, Q.Y., Zheng, Y.F., The electrochemical behavior and surface analysis of Ti50Ni47.2Co2.8 alloy for orthodontic use, Dent. Mater. 2008, 24, 1207.

(22) Multanen, M., Talja, M., Hallanvuo, S., Siitonen , A., Valimaa, T., Tammela, T.L.J., Seppala, J., Tormala, P. Bacterial adherence to ofloxacin- blended polylactonecoated self reinforced – lactic acid polymer urological stents. BJU Inter., 2000, 86, 966. (23) Elbagoury, N., Microstructure and martensitic transformation and mechanical properties of cast Ni-rich NiTiCo shape memory alloys, Mater. Sci. Tech., 2014, 30, 1795. (24) Valimaa, T., Laaksovirta, S. Degradation behavior of self reinforced 80L/20G PLGA devices in vitro. Biomater. 2004, 25, 1225. 17 ACS Paragon Plus Environment


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Figure captions Figure 1. SEM micrographs of surface morphology: (a) Ni51Ti49 and (b) Ni47Ti49Co4 SMAs images before immersion. (c) Ni51Ti49 and (d) Ni47Ti49Co4 SMAs images after immersion in artificial urine for 10 days with different magnifications. Arrows in Fig. 1(c) show the formation of corrosion pits. Figure 2. XRD pattern of (a) Ni51Ti49 and (b) Ni47Ti49Co4 SMAs after and before immersion in artificial urine solution for 10 days. Figure 3. XPS survey spectra of Ni51Ti49 SMA (A) before, (B) after immersion in the artificial urine solution for 10 days. Figure 4. XPS survey spectra of Ni47Ti49Co4 SMA (A) before, (B) after immersion in the artificial urine solution for 10 days, (C) Ni 2p; (D) Ti 2p; and (E) Co 2p Figure 5. Cyclic voltammograms of (a) Ni51Ti49 and (b) Ni47Ti49Co4 SMAs in artificial urine solution, scan rate 10 mVs−1 Figure 6. Potential vs. time curves of (a) Ni51Ti49 and (b) Ni47Ti49Co4 SMAs in artificial urine at 37 °C. Figure 7. Experimental data in artificial urine at 37 °C: (a), (b) Bode-phase, (c), (d) Bode plots, and (e), (f) experimental and simulated Nyquist results for Ni51Ti49 and Ni47Ti49Co4 SMAs, respectively. Figure 8. Proposed equivalent circuit for (a) Ni51Ti49 and (b) Ni47Ti49Co4 SMAs used to obtain impedance parameters. Figure 9. Potentiodynamic polarization curves of (a) Ni51Ti49 and (b) Ni47Ti49Co4 SMAs in artificial urine at 37 °C showing positive corrosion behavior for Ni47Ti49Co4.



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a

b

c

d

c

d

Figure 1


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3000 Bare Ni51Ti49

(a)

oxide film Ni51Ti49

2500

Lin (counts)

2000 1500 1000 500 0 20

30

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50

60

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2θ θ (degree) (

3000

(b)

Bare Ni47Ti49Co4 Oxide film Ni47Ti49Co4

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Lin (counts)

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1500 1000 500 0 20

30

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2θ θ (degree) (

Figure 2


70

80


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 3


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Figure 4 23 ACS Paragon Plus Environment


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Figure 4

200 a

150

Current (µ µA)

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b

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Figure 5


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Potential (V)

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-0.1 -0.2 -0.3 -0.4 a -0.5 -0.6 0

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Figure 6


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Phase angle (θ) θ) / Degree

Phase angle (θ) θ) / Degree

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Figure 7



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Figure 8


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Table 1. Electrochemical corrosion parameter values for both SMAs in the artificial urine solution at 37 °C by Tafel plot analyses.

Samples

Ni49Ti51 Ni47Ti49Co4

a

Ecorr

b

icorr

c

βa

d

βc

e

Rp

Corr. Rate

mV

µA cm-2

mV dec-1

mV dec-1

KOhm

mm/year

-0.44 -0.38

0.29 0.11

31 57

194 196

16 260

0.57 0.19

a

E corr= corrosion potential; bicorr = corrosion current ;cβa= rate of anodic half reaction ;dβc= rate of cathodic half reaction, e Rp = polarization resistance.


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For Table of Contents only

Ni51Ti49

35000 30000

- Z /  cm2

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200

150 h

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150

Current (A)

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b

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50

10000

20000

30000

40000

50000

Z /  cm2

0

-50

80000

Ni47Ti49Co4

-100 -1.0

-0.5

0.0

0.5

1.0

Potential (V) - Z /  cm2

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1.5

f

2.0

60000 150 h

40000 1h

20000

0

0

5000

10000 15000 20000 25000 30000 35000

Z /  cm2

ACS Paragon Plus Environment

Electrochemical Properties of Ni47Ti49Co4 Shape Memory Alloy in

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