Electrochimica Acta 56 (2011) 6389–6396 Contents lists available at ScienceDirect Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta A comparative study on the corrosion behavior of porous and dense NiTi shape memory alloys in NaCl solution X.T Sun a , Z.X Kang b , X.L Zhang c , H.J Jiang a , R.F Guan a , X.P Zhang a,∗ a b c School of Materials Science and Engineering, South China University of Technology, Guangzhou 510640, China School of Mechanical and Automotive Engineering, South China University of Technology, Guangzhou 510640, China MEMS Center, Harbin Institute of Technology, Harbin 150001, China a r t i c l e i n f o Article history: Received 12 October 2010 Received in revised form May 2011 Accepted May 2011 Available online 13 May 2011 Keywords: Porous NiTi alloy Porous electrode Corrosion behavior Potential distribution Interconnectivity a b s t r a c t The corrosion behaviors of porous and dense NiTi shape memory alloys with the same nominal composition were investigated in a 0.9% aqueous NaCl solution using electrochemical methods The study clarified the role of the porous structure in influencing the corrosion behavior of the porous NiTi alloys, which exhibited porosity values ranging from 35.5% to 63.8% The results indicated that the porous NiTi alloy was more susceptible to localized corrosion than was the dense NiTi alloy However, the porous NiTi alloy sample with a higher porosity did not suffer more serious corrosion than the one with a lower porosity Furthermore, the potential distribution exists on the pore wall of the porous NiTi alloys as a result of current flow within the pore electrolyte Thus, the role of potential distribution inside the pore and porous structure in the corrosion behavior of the porous NiTi alloys is an important factor © 2011 Elsevier Ltd All rights reserved Introduction In recent decades, porous NiTi shape memory alloys have drawn a great deal of attention as one of the promising biomaterials for orthopedic implants and hard-tissue replacements because of the combined virtue of the shape memory effect, superelasticity and adjustable mechanical properties, in particular the tailored pore structure of promoting tissue in-growth [1,2] However, the high nickel content of the NiTi alloys might result in potentially negative effects on the surrounding tissue by inducing allergic responses [3,4] It has been reported that Ni ions released due to the corrosion process can maintain high levels for up to weeks or even for several months [5,6] In particular, compared with conventional dense NiTi alloys, the complex interface structure and larger exposed surface areas of porous NiTi alloys pose a more serious issue with regard to leaching of Ni [7–10], which indicates the need for a better understanding of the corrosion behavior of porous NiTi alloys The corrosion of metal implants in the human body is primarily driven by electrochemical reactions Consequently, electrochemical tests have been used to evaluate the corrosion resistance of NiTi alloys [7–13] In contrast with the dense electrode, the porous electrode generally suffers from non-uniform potential distribu- ∗ Corresponding author Tel.: +86 20 22236396; fax: +86 20 22236393 E-mail address: mexzhang@scut.edu.cn (X.P Zhang) 0013-4686/$ – see front matter © 2011 Elsevier Ltd All rights reserved doi:10.1016/j.electacta.2011.05.019 tion because of the ohmic potential drop and the concentration decay of the electroactive species [14–20] It is known that the role of potential distribution inside cavities or recesses in a metal surface is quite important to its crevice corrosion and pitting corrosion behavior, since the potential distribution has a strong effect on the distribution of cathodic and anodic reactions at various distances into the cavity or recess [21,22] Thus, the electrode potential distribution should be an important consideration in understanding the electrochemical corrosion behavior of porous NiTi alloy Furthermore, the electrochemical reaction is essentially a heterogeneous electron transfer reaction occurring at a solid-liquid interface The three-dimensional nature of the porous NiTi alloys significantly increases the interfacial structure complexity A previous study [23] has shown that the sintered porous titanium with a higher porosity ratio undergoes more corrosion than does a sample with a low porosity because of the larger real surface area However, the unsintered sample (i.e., the green sample of cold compacted titanium powder) with a higher porosity ratio (produced under low compaction pressure) experienced less corrosion than the one with a low porosity (produced under high compaction pressure) The discrepancy was related to the pore characteristics of the porous titanium Currently, there are many published results on the corrosion behavior of dense NiTi alloys [7–11], while only very limited studies have been published on the corrosion characteristics of porous NiTi alloys In a previous study, porous NiTi alloys were determined to be less corrosion resistant than the dense NiTi alloys [24] Additionally, 6390 X.T Sun et al / Electrochimica Acta 56 (2011) 6389–6396 Fig Optical macrograph of the cylindrical porous NiTi alloy sample the corrosion resistance of porous NiTi alloys decreased when the porosity of the alloy increased from 50.2% to 60.4% The present study aimed to clarify the influences of the electrode potential distribution within the pore and the pore characteristics on the corrosion behavior of the porous NiTi alloys during electrochemical tests; this was accomplished through a systematic comparison of the corrosion characteristics of the porous and dense NiTi alloys Both the porous NiTi alloy samples with porosity ranging from 35.5% to 63.8% and the dense NiTi alloy samples having the same nominal atomic composition as the porous samples were prepared, and their corrosion characteristics in a 0.9% aqueous NaCl solution at 37 ◦ C were investigated Experimental 2.1 Preparation of porous and dense NiTi alloy samples The porous NiTi alloy samples were fabricated by a pore-forming technique and powder metallurgy method using a high-purity ammonium bicarbonate (NH4 HCO3 ) powder and a blend of elemental titanium and nickel powders with a nominal atomic ratio of 50.8 at.% Ni to 49.2 at.% Ti [25–27] The pore characteristics and porosity ratios of the NiTi alloy samples were tailored by adjusting the amount of NH4 HCO3 powder added to the samples Fig shows an optical macrograph of the porous NiTi alloy sample Dimensions of the cylindrical samples are 14–15 mm in diameter and 10–20 mm in height There were four types of samples with porosity ratios of 35.5%, 44.9%, 55.8% and 63.8%, respectively, and had average pore sizes of approximately 100–200 ␮m The general porosity of the porous NiTi alloy samples, P, can be calculated by the following equation: P(%) = 1− m 0V × 100 Fig A schematic diagram of porous and dense NiTi alloy samples for electrochemical tests vacuum arc-melting furnace with the protection of argon under normal pressure Finally, the molten sample was furnace-cooled and a dense NiTi alloy ingot (nearly a hemi-sphere, with a base diameter of 18 mm and a height of mm) was obtained for followup studies 2.2 Sample preparation for corrosion testing The corrosion may have occurred throughout the thickness of the porous alloy To accurately characterize the corrosion, both sides of the sample were examined as the test surface A dense NiTi alloy rod (2.0 mm diameter) was used as the connect electrode, which was directly screwed into the threaded blind hole on the cylindrical side of the test sample, as shown in Fig The severe wear and deformation produced a pore-free tapped hole Therefore, the electrolyte was hardly in contact with the screwed rod The edges of the working electrodes were not covered by the non-conducting resin because the resin could penetrate the porous samples with different porosities The disc-shaped porous NiTi alloy samples with a diameter of 15 mm and a thickness of mm were cut from the porous NiTi alloy rods (as-fabricated) and the dense NiTi alloy ingots by electrical discharge machining The exposed surfaces of both the porous and dense NiTi alloy samples were well polished The samples were then degreased with acetone in an ultrasonic cleaner for 20 min, followed by rinsing in distilled water Finally, the dense NiTi samples were dried in air at room temperature; the porous samples were placed in a drying oven at a constant temperature of 80 ◦ C for h (1) where m and V are the mass and volume of the porous samples, respectively, and is the theoretical density of NiTi alloy (i.e., 6.45 g/cm3 for the bulk near-equiatomic NiTi alloy) For a comparative study, the dense NiTi alloy was prepared in a non-consumable vacuum arc-melting furnace In the preparation process, the elemental titanium and nickel powders, with the same nominal composition as the porous sample (i.e., Ni 50.8 at.% – Ti 49.2 at.%), were blended and cold pressed into green compacts with a geometry of 15 × 10 (diameter × length, mm) The green sample was first smelted and then remelted five times in a non-consumable 2.3 Electrochemical test procedure Electrochemical measurements were performed using a Zahner (model IM6ex) potentiostat, according to the ASTM G5 [28]; the standard three-electrode system was adopted The working electrode was a porous or dense NiTi alloy sample with both sides exposed to the solution The reference electrode was a saturated calomel electrode (SCE), which was connected to the working electrode via a Luggin capillary, and double symmetrical graphite electrodes were used as counter electrodes The test electrolyte was 0.9 wt.% NaCl (analytical reagent) in de-ionized water Prior X.T Sun et al / Electrochimica Acta 56 (2011) 6389–6396 6391 Fig Evolution of open circuit potential over time for dense and porous NiTi alloys in a 0.9% NaCl solution to immersion of the electrodes, the electrolyte cell was heated to a constant temperature of 37 ◦ C using a water bath The potentiodynamic polarization tests and electrochemical impedance spectroscopy (EIS) were performed 24 h after immersion at open circuit Potentiodynamic curves were measured by scanning the potential from −0.4 V below the open circuit potential to +1.0 V at a scan rate of 0.02 mV/s Each electrochemical experiment was repeated three times with a fresh specimen for each test The corrosion potential and the corrosion current density were obtained through Tafel approximation The EIS measurements were obtained using a polarization of ±10 mV in the frequency range of 100 kHz to mHz and points measuring per decade EIS spectra were interpreted by the software ZSimpWin 3.10 2.4 Morphology of porous NiTi alloy samples The pore features of the fabricated NiTi alloy samples were characterized by an optical microscope (Leica, DM2500P) and via digital image analysis The surface morphologies of the porous NiTi samples were examined before and after the electrochemical tests using a scanning electron microscopy (SEM Quanta 200, FEI) Results 3.1 Open circuit potential measurement The open circuit potentials (OCP) of dense and porous NiTi alloys in 0.9% NaCl solution were measured over a period of 24 h Fig presents the evolution of the OCP as a function of time, where it is obvious the OCP of porous NiTi alloys experienced a sharper rate of change compared to the dense NiTi alloys In all cases, the porous NiTi alloys exhibited a more positive OCP than did the dense NiTi alloys 3.2 Potentiodynamic polarization measurements The corresponding changes in the behavior of the potentiodynamic polarization of the NiTi alloys with the structure variation from dense to porous are shown in Fig 4a, and presentation of the extracted electrochemical parameters are shown in Fig 4b Both the dense and porous NiTi alloys exhibited a typical passive region and were pitting attacked at the chemical breakdown potential, where the current density sharply increased The corrosion current densities of the dense and porous NiTi alloys seen in Fig 4b are cal- Fig (a) Polarization curves of porous and dense NiTi alloy samples after immersing in a 0.9% NaCl solution for 24 h and (b) evolutions of extracted electrochemical parameters from polarization curves with porosity ratio culated from the geometrical area for comparison (i.e the apparent area, not necessarily the real area) The values suggested that the current densities (icorr ) of porous NiTi alloys were markedly higher than the icorr values of the dense ones, and the breakdown potentials (Eb ) of porous NiTi alloys were clearly lower than the Eb values of the dense samples However, it was worth noting that the corrosion potentials (Ecorr ) of porous NiTi alloys exhibited a positive shift by more than 200 mV from the Ecorr value of the dense NiTi alloy The typical surface morphologies of the porous NiTi alloy samples polarized to a potential of 1.0 V from −1.0 V were examined by SEM, as shown in Fig It was shown that the observed pitting primarily existed on the edge of the porous NiTi alloy samples, as indicated by the white arrows in Figs 5(b), (d) and (f) The edge attack may have been related to a combination of geometry and metallurgical conditions (e.g., inclusion stringers [26,27]), which can complicate the test results However, it was likely that the uncertainties in the sample geometry and metallurgical conditions were consistent across the samples and, hence, conclusions based on the comparisons between the damage at the edges should still be valid In the repeated tests, the observed damage of the edge was reproducible In particular, the characteristics of the surface morphologies did not show an obvious tendency of deterioration of corrosion resistance with an increase in the porosity ratio As shown in Figs 5(g) and (h), compared with the samples with the porosity of 35.5% and 6392 X.T Sun et al / Electrochimica Acta 56 (2011) 6389–6396 Fig Typical SEM surface morphologies of the edge region of disc-shaped porous NiTi alloy samples with different porosity ratios before (a, c, e and g) and after (b, d, f and h) potentiodynamic polarization measurement in a 0.9% NaCl solution, terminated at 0.8 V: (a and b) 35.5%; (c and d) 43.9%; (e and f) 55.8%; and (g and h) 63.8% X.T Sun et al / Electrochimica Acta 56 (2011) 6389–6396 6393 Fig Bode spectra for porous and dense NiTi alloys: (a) impedance modulus plot; and (b) phase angle plot 43.9%, it was difficult to distinguish the morphologies of the sample with the porosity of 63.8% before and after the polarization test 3.3 Electrochemical impedance spectroscopy Impedance spectra for porous and dense NiTi alloy samples after immersion in a 0.9% NaCl solution at 37 ◦ C for 24 h are presented as Bode plots (Fig 6), where both porous and dense NiTi alloy samples exhibited similar spectral features A capacitive behavior, which is represented by the phase angle approaching −90◦ and typical of in passive materials [7,29], appeared in a medium to low frequency range, Fig 6(b) This meant that a passive film had formed on all samples in the electrolyte, which was consistent with the passive region determined in the polarization tests (Fig 4a) The large phase angle peak could be indicative of the interaction of at least two time constants Therefore, an equivalent circuit was proposed to model the EIS data obtained from both the porous and dense NiTi alloys, as shown Fig 7a This model was widely accepted for Ti and Ti-rich alloys on which a passive film with a double-layer structure was formed [7,29,30] In the model, Rs is the resistance of the solution, Rp is the additional resistance of the solution inside the pores, Rb is the charge transfer resistance of the barrier layer, Fig (a) Equivalent circuits used for fitting the experimental data and (b and c) experimental results and simulated data for the porous NiTi alloy with a porosity of 63.8% and dense NiTi alloy after immersion in a 0.9% NaCl solution for 24 h, respectively Qp is the capacitance of the pore wall and Qb is the capacitance of the barrier layer Here, Q is the magnitude of the constant-phase element (CPE), representing a deviation from the ideal capacitor, n −1 the impedance of which is defined as ZCPE = Y0 (jw) , where −1 ≤ n ≤1 The value of n is associated with the non-uniform distribution of current as a result of roughness and surface defects This was the case for the results presented in this study, where the n values of the porous NiTi alloy samples ranged from 0.81 to 0.87, while the dense NiTi alloy samples was approximately 0.94 The resistance, capacitance and n values from measurements upon the NiTi alloys, extracted using equivalent circuit illustrated in Fig 7(a), are tabulated in Table The impedance results were 6394 X.T Sun et al / Electrochimica Acta 56 (2011) 6389–6396 Table Electrical parameters of the equivalent circuits by fitting the experimental results of EIS Samples Cp (␮F cm−2 ) np Rp (k cm−2 ) Cdl (␮F cm−2 ) nb Rb (k cm−2 ) P = 35.5% P = 43.9% P = 55.8% P = 63.8% 279(82) (16.4 ) 202(75) (26.2) 208(64) (14.7) 263(88) (23.4) 0.84(0.22) 0.87(0.24) 0.86(0.26) 0.81(0.30) 17(12) 24(22) 25(18) 32(27) 240(62) 119(45) (13.5) 110(57) (21.7) 205(89) (33.2) 0.28(0.15) 0.59(0.18) 0.59(0.17) 0.82(0.14) 29(21) 56(37) 51(40) 41(32) × 10−4 × 10−4 × 10−4 × 10−4 Dense NiTi 3.3(0.9) 0.81(0.05) 1.9(0.6) × 10−3 13.2(7.4) (16.8) 0.96(0.1) 5.3(3.4) × 104 × 10−4 * * Mean values, with sample standard deviations in parentheses interpreted using the ZsimpWin software, and the respective fittings were evaluated by a Chi-squared value, The experimental and simulation data by the equivalent circuit are shown in Fig 7(b) and (c) The Chi-square (␹2 ) value between 10−4 and 10−5 indicated good agreement between the experimental and simulated data for both the dense and porous NiTi alloy samples Discussion varied in size from ␮m to 200 ␮m Parts of the pores were nearly circular and other parts of pores exhibited irregular shapes Moreover, some small pores existed in the walls of the bigger pores, as manifested by the interconnection between these pores; thus, the inner walls of the pores were generally unsmooth These features accounted for what may also be vulnerable sites and imperfections in any passive films, which is consistent with the EIS results (The resistance value Rb of porous NiTi alloy is three orders of magnitude lower than that of dense NiTi alloy.) 4.1 Effect of porous structure on the corrosion resistance of porous NiTi alloy The corrosion current density of the porous NiTi alloy samples were about two orders of magnitude higher compared to the dense samples The correspondingly smaller passive window for the porous NiTi alloy is also clearly seen in the polarization curves As such the porous NiTi alloy was qualitatively more susceptible to pitting corrosion than the dense NiTi alloy This is to a certain extent expected from the larger ‘real’ surface area Moreover, it is known that corrosion resistance of the NiTi alloy is associated with the formation of the resistant titanium oxide films on the alloy’s surface [7,31] The surface-finish quality, the amount of the residues left on the surface and the microstructures inhomogeneity of the porous NiTi alloy were the critical factors that affected the formation and quality of the oxide films These factors were strongly influenced by the preparation process of the alloy However, the preparation of the porous NiTi alloys generally consisted of complex thermomechanical processes Fig shows the SEM photographs of pore structure for porous NiTi alloy with the porosity ratio of 55.8% The pores Fig SEM image of pore morphology for porous NiTi alloy formed by the poreforming technique 4.2 Effect of porosity ratio on the corrosion behavior of porous NiTi alloys The changes in electrochemical parameters obtained from the polarization curves and impedance results of the porous NiTi alloys with increasing porosity ratio are summarized in Table and abridged into Fig 4b It was difficult to identify a trend in these parameters with varying porosity ratio In fact, the real surface area of the porous NiTi alloys generally increases with increasing porosity ratio within certain limits; however, the surface quality and structure uniformity of the porous NiTi alloys does not necessarily get worse with the porosity increasing Fig shows the optical micrographs for the polished porous NiTi alloy samples with different porosity ratios Clearly, there were two types of pores The large pores with sizes of 100–200 ␮m are primarily formed by the decomposition of the space-holder, NH4 HCO3 particles [26,27] The small pores (also called secondary pores) with sizes below 10 ␮m could be attributed to the trapped residuals of NH4 HCO3 decomposition and the non-metallic impurities existing in the raw powders, as well as the volume shrinkage and the Kirkendall diffusion between Ni and Ti atoms The small pores showed a better size-uniformity and a more uniform distribution than did the large ones When the porosity ratio increased, the interconnectivity of larger pores was clearly increased For the samples with a lower porosity, the majority of the pores were the isolated secondary pores (see Fig 9a), which is unlikely to have trapped appreciable volumes of solution This caused the samples relative resistant to induced pitting corrosion When the porosity ratio increased, the interconnectivity of the larger pores had been markedly improved The interconnected channels allowed the free flow of the liquid and fewer sites were available to induce pitting corrosion When the porosity ratio reached 63.8%, the pore morphology was dominated by wider and more highly interconnected pore structures (Fig 9d) As shown in Fig 5(g) and (h), there was no visible damage on the edge of the sample Furthermore, with the change of porosity ratio, pore size, pore size distribution and pore shape of the porous NiTi alloys change correspondingly It was difficult to provide a complete characterization of the pores by solely measuring the sample porosity This also accounted for no obvious trend in the corrosion behavior with the single factor change X.T Sun et al / Electrochimica Acta 56 (2011) 6389–6396 6395 Fig Optical micrographs of porous NiTi alloy samples with different porosity ratios: (a) 35.5%; (b) 43.9%; (c) 55.8%; and (d) 63.8% 4.3 Effect of potential distribution on the corrosion behavior of porous NiTi alloys Conclusions From the EIS results, the Rp value of porous NiTi alloys was four orders of magnitude higher comparing to the dense NiTi alloys This can prove that the typical potential distribution exists on the internal pore surface as a result of current flow within the pore electrolyte [22,32] The typical potential distribution follows that x E(x) = E(0) − I(x) R(x) dx (2) where E(x) is the electrode potential at a distance x into the pore, E(0) is the potential at the pore opening, and I(x) and R(x) are the magnitudes respectively of the current flowing at a distance x through the pore electrolyte and of the resistance of the electrolytic path within the pore [32,33] Since the electrode potential varies with distance x into the pore, the rate of metal dissolution also varies over the internal pore surface in accord with the polarization behavior of the metal under the local electrochemical conditions In the active region of anodic polarization, E(x) became less noble with increasing distance into the internal pore, and thus the rate of the NiTi alloy dissolution reaction decreased with increasing distance x This explains the discrepancy that the porous NiTi alloys with the poor protective oxide films exhibited much nobler corrosion potential than the dense NiTi alloys In the passive region of the anodic polarization, with the external surface inside the pore polarized into the passive region, E(x) decreased with increasing distance x and may be still in the active region, which depends on the dimensions of the pore and the electrochemical conditions prevailing at the whole porous structure of porous NiTi alloy electrode [33] Thus, the potential distribution is playing a role that strengthens the differences among the different parts of the porous metal surface This is the essential electrochemical condition that results in the localized metal corrosion Therefore, the role of potential distribution inside the pore and porous structure in the electrochemical corrosion behavior of the porous NiTi alloys is an important factor From the potentiodynamic polarization and EIS results, it was concluded that the porous NiTi alloy was more susceptible to localized corrosion than was the dense NiTi alloy However, the porous NiTi alloy sample with a higher porosity did not suffer obviously more corrosion than the one with a lower porosity The potential 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alloy However, the preparation of the porous NiTi alloys generally... corrosion characteristics in a 0.9% aqueous NaCl solution at 37 ◦ C were investigated Experimental 2.1 Preparation of porous and dense NiTi alloy samples The porous NiTi alloy samples were fabricated