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Corrosion anisotropy of titanium deformed by the hydrostatic extrusion
Accepted Manuscript Title: CORROSION ANISOTROPY OF TITANIUM DEFORMED BY THE HYDROSTATIC EXTRUSION Authors: A. Chojnacka, J. Kawalko, H. Koscielny, J. Guspiel, A. Drewienkiewicz, M. Bieda, W. Pachla, M. Kulczyk, K. Sztwiertnia, E. Beltowska-Lehman PII: DOI: Reference:
S0169-4332(17)32232-8 http://dx.doi.org/doi:10.1016/j.apsusc.2017.07.231 APSUSC 36765
To appear in:
APSUSC
Received date: Revised date: Accepted date:
4-4-2017 3-7-2017 24-7-2017
Please cite this article as: A.Chojnacka, J.Kawalko, H.Koscielny, J.Guspiel, A.Drewienkiewicz, M.Bieda, W.Pachla, M.Kulczyk, K.Sztwiertnia, E.Beltowska-Lehman, CORROSION ANISOTROPY OF TITANIUM DEFORMED BY THE HYDROSTATIC EXTRUSION, Applied Surface Sciencehttp://dx.doi.org/10.1016/j.apsusc.2017.07.231 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
CORROSION ANISOTROPY OF TITANIUM DEFORMED BY THE HYDROSTATIC EXTRUSION A. Chojnackaa,b, J. Kawalkoa, H. Koscielnya, J. Guspiela, A. Drewienkiewiczc, M. Biedaa, W. Pachlad, M. Kulczykd, K. Sztwiertniaa, E. Beltowska-Lehmana,* a
Institute
of
Metallurgy
and
Materials
Science,
Polish
Academy
of
Sciences,
25 Reymonta St., 30-059 Krakow, Poland b
c
Faculty of Chemistry, Jagiellonian University, 3 Ingardena St., 30-060 Krakow, Poland.
Faculty of Metals Engineering and Industrial Computer Science, AGH University of Science
and Technology, 30 Mickiewicza Av., 30-059 Krakow, Poland. d
Institute of High Pressure Physics, Polish Academy of Sciences, Sokolowska 29/37, 01-142
Warsaw, Poland
*Corresponding author (E. Beltowska-Lehman): Tel. +48 12 295 2829, fax: +48 12 295 2804 E-mail address: [email protected]; [email protected] Graphical Abstract
1
Highlights
Hydrostatic extrusion deformation improves the corrosion resistance of titanium. Corrosion resistance of deformed titanium was affected by the extrusion direction. Surface oriented parallel to the extrusion direction reveals the best corrosivity.
ABSTRACT The corrosion behaviour of titanium rods deformed by hydrostatic extrusion (HE) in artificial saliva (Carter-Brugirard’s solution of pH 7.6) was investigated using open-circuit potentials (OCPs), (DC) potentiodynamic polarisation curves and (AC) electrochemical impedance spectroscopy (EIS) techniques. Various electrochemical parameters (corrosion potential Ecorr, corrosion current (Icorr) (icorr), polarisation resistance Rp, charge transfer resistance Rct and oxide film resistance Rf) were analysed. Significant coherence was observed between results achieved from these procedures, i.e., all applied techniques showed the same trend for corrosion resistance. The obtained electrochemical data were then related to the microstructure parameters (crystallographic texture, grain size, grain boundary distribution and density) determined using the EBSD/SEM technique. It was found that the corrosion behaviour of titanium processed by the HE method was superior compared to the unprocessed Ti, and this was clearly dependent on the extrusion direction. The highest corrosion resistance was revealed for the HE-deformed Ti rod of the surface oriented longitudinal (parallel) to the extrusion direction. Keywords: hydrostatic extrusion; titanium; corrosion; crystallographic texture; dental implants 1. Introduction For many years, scientists have devoted much attention to the search for materials suitable for use as dental implants. These materials must be biocompatible and resistant to corrosion in the physiological fluid environment. Initially ivory, pearls, shells, animal bones, ceramics and precious metals were applied as tooth replacements [1]. Currently, stainless steel and Co-Cr alloys are the most commonly used, due to excellent mechanical properties allowing the easy formation of components of complex shapes such as tooth crowns, bridges, and fixed or moveable prosthesis parts [2]. However, titanium and titanium alloys remain the most interesting materials for dental applications. A significant advantage of unalloyed titanium is the relatively low Young's modulus (of about 105–120 GPa), the most similar to bone stiffness among all metallic biomaterials presently applied in implantology [3]. This parameter (a measure of the stiffness of solid material) is extremely important since considerable differences between the elastic modulus of human bone and the implant can lead to bone resorption and 2
implant loosening. Titanium is also inert and resistant to corrosion due to the formation of the stable surface oxide film, which consists mainly of titanium oxide TiO2 of a rutile-type tetragonal structure. This feature is very important, since the environment of saliva in the oral cavity (pH change from 2 to 11) is highly corrosive to many noble metals [3, 4]. Nevertheless, the relatively poor tribological properties (due to low hardness and high friction coefficient) such as low strength, wear and abrasion resistances are the main disadvantages of titanium. Therefore, many studies today focus on improving titanium’s mechanical properties. One of the common methods is to replace the pure titanium with alloys, for instance Ti6Al4V [4]. However, this alloy has undesirable properties due to the strong cytotoxic reactions induced by vanadium and the negative impact of aluminium on nerve cells. Thus, unfavourable components of titanium alloys (e.g. Al and V) are replaced by biocompatible elements including tantalum, niobium, and zirconium. At the same time, various procedures for improving the mechanical properties of pure titanium, including microstructure refinement, have been suggested [5]. It was observed that, in accordance with the Hall-Petch relationship, the mechanical strength of pure titanium increases considerably as a result of grain size reduction [5–9]. There are many different methods of obtaining ultrafine-grained or nanocrystalline microstructures in metallic materials, including titanium. With the emergence of new possibilities in material engineering, titanium with ultrafine-grained microstructure can be obtained using Severe Plastic Deformation (SPD) methods [10–12]. The best known SPD techniques include Equal Channel Angular Pressing (ECAP), Accumulative Roll Bonding (ARB), High Pressure Torsion (HPT) and Cyclic Extrusion Compression (CEC) [13]. Regarding corrosion behaviour, the majority of data available in the literature concern titanium processed by ECAP, as the most popular SPD method. These studies have shown that ultrafine- crystalline grained titanium (after ECAP) shows better corrosion resistance in NaCl, HCl and H2SO4 environments than the coarse-grained counterpart [5, 6]. The effect was related to the higher density of defects in the crystal lattice of the surface layer in which the passivation process is initiated. Furthermore, it has been demonstrated that titanium transformed by ECAP reveals (in addition to excellent corrosion properties) higher adhesion, and fibroblast proliferation in an environment simulating human body fluid (SBF) compared to the unprocessed material [14]. Recently, the hydrostatic extrusion (HE) technique [15] and the KoBo method, e.g. [16], have been proposed as means of modifying titanium microstructure leading to the improvement of its mechanical properties. In contrast to other Severe Plastic Deformation (SPD) SPD techniques, the product dimension change accompanies both extrusion methods, and the final 3
strain is determined by cumulative cross section area reduction of processed billets. Based on literature and the authors’ own studies, it was possible to conclude that titanium deformed by the HE method showed better mechanical properties compared to the unprocessed material, due to strengthening by grain refinement. The significant improvement was observed in yield strength (about two and a half times higher) and microhardness (about one and a half times higher) values [17]. The hydrostatic extrusion (HE) method seems to be particularly effective for the development of nano and ultrafine structures microstructures, and for the enhancement of biomaterial properties for medical applications. There are little data available in the literature on the corrosion of titanium after the hydrostatic extrusion (HE) process, and none relate to corrosive properties depending on the extrusion direction (ED). This work presents the first study on the corrosion anisotropy of commercially pure CP (grade 2) titanium deformed by the HE method. The corrosion behaviour was investigated in an artificial saliva solution. The resulting data were analysed in relation to the microstructure, and will provide basis for the development of a dental implant-distractor (ID) designed to be embedded in the bone. The ID is an innovative treatment for bone tissue atrophy after tooth loss, and combines the function of vertical bone osteogenesis with implantation [18]. 2. Experimental details The material selected for testing was commercially pure titanium grade 2 supplied by Wolften, Poland. The nominal composition (wt.%) of samples was C 0.02, Fe 0.12, O 0.10, N 0.02, H 0.003, with the balance being titanium. Initial titanium samples were subjected to severe plastic deformation using the hydrostatic extrusion technique in the Institute of High Pressure Physics of the Polish Academy of Sciences. The deformation was performed at room temperature (20 °C), in five passes from a diameter of 25 mm to a diameter of 6.07 mm. The process was carried out without intermediate annealing. Extrusion pressures were in the range between 550 and 950 MPa. The final deformation was performed by means of rotary swaging, which further reduced the diameter of the product to 4.74 mm. Combined deformation steps performed after annealing resulted in equivalent strain accumulation equal to ε: 3.32. Samples from extruded and initial titanium were prepared, with surfaces oriented parallel (long crosssection) and perpendicular (trans cross-section) to the extrusion direction (ED). The two types of samples are presented schematically in Figure 1. Samples for corrosion testing and metallographic samples for microstructure and texture investigations were prepared using SiC sandpaper grinding up to 7000 grit, followed by mechanical polishing with a 1:1 solution of colloidal silica suspension and hydrogen peroxide 4
(30%) resulting in highly polished, mirror like surfaces. The samples for SEM EBSD (electron backscattered diffraction) microstructure characterisation were additionally polished electrochemically for 15 seconds in Struers A3 electrolyte at 5 °C and 35 V. The EBSD microstructure investigations of the titanium samples in the initial state and after HE deformation were carried out using the FEI Quanta 3D FEG microscope, with integrated EDAX TSL system using a Hikari camera, and OIM Data Collection software for the acquisition and indexing of electron backscattered diffraction patterns. The EBSD maps were collected using electron beam energy of 20 keV. The step sizes used for EBSD measurement of severely deformed Ti samples were 55 and 75 nm, and the sizes of the maps collected were 63μm x 63μm and 86 μm x 86 μm. The EBSD map from the titanium sample in as received state was collected over an area of 1mm2 with a 1 μm step size. The collected EBSD data were post-processed and analysed using the TSL OIM Analysis 7 software. Postprocessing consisted of a single iteration of the “grain dilation” procedure with a grain tolerance angle of 2° and minimum grain size of 2 points. Using the OIM Analysis software, microstructure parameters based on EBSD data were calculated, including distribution of grain size, and density of low angle grain boundaries (LAGB) and high angle grain boundaries (HAGB). A grain was defined a as set of at least five properly indexed points enclosed completely by boundary segments with misorientation between neighbour points of at least 2°. Points that did not meet this criterion were excluded from analysis (for example, about 4% of points were excluded from analysis in the case of titanium after hydrostatic extrusion HE). The size of crystallites grains was calculated as a diameter of circles with equivalent surface area, and as a second method by using maximum and minimum feret size function. Feret size function calculates the maximum and minimum distance between two parallel lines tangential to the boundaries of a grain, which essentially is the measurement of each crystallite’s grain’s long and short axis respectively. LAGB (low angle grain boundaries) was defined as boundary segment with a misorientation angle between neighbours from 2° to 15°, and HAGB (high angle grain boundaries) was defined as boundary segment with a misorientation angle of 15° and above. Grain boundary density was reported as a measure of total grain boundary length divided by the surface area of measured EBSD map (1/μm). The crystallographic EBSD data were also used in local texture analysis to calculate inverse pole figures. The texture from discreet EBSD data was calculated in TSL OIM Analysis software using the method of spherical harmonic series expansion. The structure of the titanium in the initial state and after the hydrostatic extrusion (HE) process was also examined by transmission electron microscope TEM (FEI
5
TECNAI G20). Thin foils for TEM observations were electropolished using a twin jet electro polisher. Electrochemical experiments were carried out in a conventional three-electrode electrochemical cell using an AUTOLAB PGSTAT302N potentiostat/galvanostat with NOVA 1.11 software to control the experiment, and to collect and analyse the data obtained. Titanium rods (unprocessed and after the HE process, respectively) were used as working electrodes. The counter electrode was platinum sheet located at a distance of 50 mm from the Ti working electrode. The saturated calomel electrode (SCE) placed in a Luggin capillary was used as a reference electrode. The corrosion resistance of the titanium samples was investigated in an aerated Carter-Brugirard synthetic saliva solution, prepared according to the PN-EN ISO 10993-15 standard. All reagents were of analytical grade purity. The reagents NaCl (0.7 g L-1), Na2HPO4 (0.26 g L-1), KSCN (0.33 g L-1), KH2PO4 (0.2 g L-1), NaHCO3 (1.5 g L-1), KCl (1.2 g L-1) were dissolved in distilled water. The pH of 6.7 was adjusted by addition of HCl or NaOH. Electrochemical measurements were conducted at a temperature of 37.0 ± 0.3 °C. After metallographic preparation, specimens were washed in distilled water in the presence of an ultrasonic field. The surface area of titanium samples exposed to the electrolyte was 0.24 cm 2. The corrosion behaviour was estimated both in parallel and perpendicular cross-sections to the extrusion direction, using the potentiodynamic polarisation and electrochemical impedance spectroscopy (EIS) techniques:
Potentiodynamic polarisation technique: titanium rod samples were immersed in the corrosive solution and the open circuit potential (OCP) was measured for two hours. The anodic and cathodic polarisation curves were recorded using a sweep potential test in the range of ±150 mV with respect to OCP, with a scanning rate of 1mV/s. Three polarisation measurements were repeated for each sample. The corrosion current (Icorr) (icorr) and corrosion potential (Ecorr) were determined based on the Tafel relation: log I i = f (E). The polarisation resistance (Rp) was calculated using the Stern-Geary equation: 𝑏𝑐 ⋅𝑏𝑎 , 𝑎 +𝑏𝑐 )𝑖𝑐𝑜𝑟𝑟
𝑅𝑝 = 2,3⋅(𝑏
where ba , bc are the slope of anodic and cathodic polarisation
curves.
Electrochemical impedance spectroscopy (EIS): the impedance measurements (single sine alternating current AC) were carried out at open circuit potential with a voltage amplitude of 10 mV and a frequency range from 0.1 Hz to 100 kHz (50 points for each frequency decade were acquired). The impedance data were analysed based on the Boukamp model. Impedance spectra were reported by Nyquist diagram. 6
Immersion test: measurements were conducted according to the ISO 10993-15:2000 standard. Tested samples (initial Ti and Ti after hydrostatic extrusion HE) were dipped in an artificial saliva solution (80 ml) and placed in an incubator at 37 °C for three months. The pH and weight of the samples were measured every seven days.
3. Results and discussion 3.1. Microstructure of titanium after hydrostatic extrusion Titanium microstructure in the as received condition is moderately deformed, which is the result of the production process. Grains are slightly elongated in the extrusion direction, grain size ranges from 5 μm to about 140 μm and overall mean grain diameter is 47.2 μm. Larger grains show often sub-grain microstructure defined by developed mesh of LAGB’s. Some tension {101̅2}〈101̅1̅〉 twins are present in the microstructure. Grain boundary density is low and equal to 0.01 1/μm for both LAGB and HAGB (low and high angle grain boundaries). The titanium samples after hydrostatic extrusion have a microstructure consisting of two distinct types of grains: relatively coarse ones, which are severely elongated in the direction of extrusion, and clusters of strongly refined grains, with much less elongated shapes. The elongation of the microstructure results in very different shapes and sizes of crystallites grains observed on transverse transversal (Trans) and longitudinal (Long) cross-sections. The two types of structures microstructures are presented in Fig. 2 by means of EBSD grain orientation spread (GOS) maps. Observations in TEM (bright field (BF) images presented in Fig 3 a, b) also revealed equiaxed grains in transverse transversal cross section and elongated in extrusion direction (ED) grains in longitudinal cross section. Moreover, analysis of selected area diffraction patterns obtained from transverse transversal cross section confirms the microstructure’s ultra-fine crystalline grain character. The crystallite grain sizes vary greatly between the two cross-sections. In the trans crosssection, crystallite grain sizes range from 70 nm (short axis) to 6.5 μm (long axis), and overall mean crystallite grain diameter (diameter of equivalent circles) is equal to 0.57 μm. On the other hand, sizes of the crystallites grains observed in the long cross-section range from 140 nm to 44.63 μm for short and long crystallite grain axis, respectively, and the average diameter of crystallites grains in this cross-section is equal to 2.00 μm. The histograms of crystallite grain sizes (short and long axes) are presented in Fig. 4. Such very large discrepancies between sizes of crystallites grains points to the substantial heterogeneity of the microstructure. Since grain size is a parameter determining macroscopic properties such as hardness or tensile strength, and can affect corrosion resistance, it is important to investigate parts of the microstructure with such different grain sizes separately. 7
Based on the criterion of crystallite grain diameter below 1 µm, the ultra-fine grained fraction of microstructure can be distinguished and analysed separately from the coarse-grained fraction, consisting of crystallites grains with diameter equal to or greater than 1 μm. The main difference between the two fractions can be observed in the content (density) of HAGB (Table 1). For instance, HAGB density in the fine-grained fraction is equal to 5.60 1/m in the trans cross-section, while in the coarse-grained fraction HAGB (high angle grain boundaries) density is much lower (0.15 1/µm). On the other hand, differences in LAGB (low angle grain boundaries) content are not that great, but similarly the density is higher in the fine-grained fraction. Such great differences in HAGB content between the two fractions results in the uneven spatial distribution of crystal lattice defects. Additionally, heterogeneity differs between both cross-sections, due to different ratios of surface area occupied by the two fractions. In the trans cross-section, the fine-grained fraction with high HAGB density occupies 84% of the microstructure, while in the long cross-section only 48% of the microstructure is occupied by the high-density HAGB fraction. The additional difference in overall HAGB density between the trans and long cross-sections results from severe elongation and the overall bigger grains observed in long cross-section. Titanium in the as received state (before hydrostatic extrusion) has almost random crystallographic texture, with a very weak maximum at the [0001] direction on inverse pole figure for the extrusion direction (Fig. 5a). After hydrostatic extrusion, titanium samples have a sharp, 101̅0 fibre texture typical for extruded hexagonal materials. The texture is characterised with the main maximum on the (101̅0) pole figure in the position of extrusion direction, and the (0001) pole distributed in the surface perpendicular to this direction. Such texture results in very different crystallographic planes observed on transverse transversal and longitudinal cross-sections. On the transverse transversal cross-section {101̅0}, planes of crystallites grains are almost exclusively observed (Fig. 5b), while on the longitudinal crosssection the crystallite grain orientation is distributed between the (0001) and {112̅0} planes, with the maximum shifted towards the (0001) position (Fig. 5c-d). Using the discrete crystallographic data from EBSD (electron backscattered diffraction) measurements, textures were calculated separately for ultra-fine grained and coarse-grained components of the microstructure. Inverse pole figures presented in Fig. 5 c-d indicate that the texture is similar in both fractions of the microstructure; however, the non-basal components of texture are slightly weaker in the case of the fine-grained fraction. 3.2. Corrosion behaviour 8
Titanium rod samples were prepared for corrosion tests in an artificial saliva solution according to the diagram shown in Fig. 1 (see the Experimental details section). Fig. 6 presents changes of the open circuit potentials (OCP) against time for tested Ti samples in an aerated Carter-Brugirard solution at 37 °C. It is known that the OCP value varies with time of immersion in a corrosive medium due to changes in the surface nature of the electrode, and indicates the thermodynamic tendency of a material to electrochemical oxidation. Thus, the open circuit potential can be a criterion for corrosion behaviour [19]. As can be seen in Figure 6, a substantial shift of the open circuit (OC) potential towards less negative values was observed for all tested samples. Then, the open circuit potential (OCP) increases slowly and eventually stabilises at a certain stationary value. The effect is mainly related to the formation and growth of the oxide films at the Ti sample surface in the artificial saliva medium. These results clearly indicate that unprocessed titanium rods behave worse than Ti deformed by hydrostatic extrusion (HE), after which the open circuit OC potential is less negative, about 0.15 mV – 0.22 mV (depending on the sample surface orientation to the extrusion direction) (Table 2). Thus, hydrostatic extrusion treatment improves the corrosion resistance of titanium, which reveals a more noble character and then less chemical activity. Open circuit potential variations are very similar for all unprocessed Ti samples regardless of the examined surfaces (sample code: Trans_Init and Long_Init) (Fig. 6). However, for Ti deformed by HE, it should be highlighted that OCP is influenced by the extrusion directions, showing a progressive shift towards noble values for surface parallel oriented (Long_HE). The cathodic and the anodic polarisation curves of all investigated Ti samples after two hours of immersion in an aerated Carter-Brugirard solution are reported in Figure 7. The electrochemical parameters derived from the Tafel plots, including the corrosion current density (Icorr) (icorr), corrosion potential (Ecorr), polarisation resistance (Rp), and the slope of anodic (ba) and cathodic (bc) polarisation curves are summarised in Table 2. As seen from the data in Table 2, all investigated Ti samples were characterised by very good corrosion resistance in an artificial saliva solution, as evidenced by the low corrosion currents and high values of corrosion potential and polarisation resistance. Moreover, the corrosion parameters of unprocessed titanium samples (Trans_Init and Long_Init) are very similar in both directions. In turn, for titanium deformed by hydrostatic extrusion (HE) the corrosion properties (especially corrosion potential Ecorr and polarisation resistance Rp) differ significantly for the surface perpendicular and parallel to the extrusion direction (Trans_HE and Long_HE). It is clearly shown that the Long_HE surface (oriented parallel to the extrusion direction ED) reveals the highest corrosion resistance in the artificial saliva solution of pH 7.6. 9
Those results suggest that the sharp texture and different orientation of crystallite grain planes observed on Trans_HE and Long_HE samples is more important than the crystal lattice defect density in determining corrosion behaviour. The basal (0001) planes in titanium have the highest atom density in this hcp structure. It has been reported previously that high atom density planes offer higher corrosion resistance than lower density ones [20]. In the HE titanium (0001), orientations are observed on long cross-sections, which, in synergy with increased grain boundary density after hydrostatic extrusion (HE), results in the highest corrosion resistance among tested samples. On the other hand, Trans_HE cross-section crystal lattice defect density is higher than in Long_HE, but {101̅0} crystal planes observed on Trans_HE are less corrosion resistant, thus this cross-section has effectively lower corrosion resistance. The EIS (electrochemical impedance spectroscopy) measurements support the results of open circuit potential (OCP) and potentiodynamic polarisation methods. Fig. 8 shows EIS spectra for the processed titanium samples Trans_HE and Long_HE. Collected results are presented in the form of Nyquist plots. To explain the corrosion behaviour of the system, experimental EIS data were fitted to an appropriate electrical equivalent circuit (EEC) model, presented in Fig. 9. The Nyquist plots show a depressed semi-circle, indicating that there is more than one time constant that describes the impedance response of the system [20,21]. Based on the composition of testing system and other studies concerning EIS analysis of titanium [20-24], the respective parts of the circuits were analysed. Rs is related to resistance resulting from all electrical contacts in the measuring system. The first parallel connection of the resistanceconstant phase element (Rct-CPEdl) in the high frequency range corresponds to the charge transfer processes, represented by the double layer constant phase element (CPEcl) and charge transfer resistance (Rct), respectively. The second parallel of the resistance-constant phase element (Rf-CPEf) can be associated with slower mass transport processes in the oxide phase, namely the diffusion of electroactive species [20, 21]. CPE elements are introduced to replace the capacitors due to the non-ideal behaviour of capacitive elements resulting in inhomogeneities present at the micro or nano (atomic/molecular) levels, such as surface roughness/porosity, adsorption, or diffusion [25]. Calculated values of all parameters are collected in Table 3. The additional parameter is polarisation resistance (Rp) which is given by the sum of charge transfer resistance (Rct) and film resistance (Rf). It should be noted that the Rp values calculated from EIS are consistent with the Rp values obtained using the potentiodynamic polarisation technique.
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For both samples, film resistance Rf is higher than charge transfer resistance Rct and Ndl is higher than Nf. This suggests that passive film exercises more resistance than the charge transfer in the double layer, and that the behaviour of the film is not ideal, which is consistent with earlier studies [24]. Rf (film resistance) values are at a similar level for both materials, so it can be assumed that the passive film formed on samples is of comparable quality. However, all parameters related to charge transfer and the formation of passive film for the sample in longitudinal direction to the hydrostatic extrusion (HE) process are slightly higher, which means that this material is expected to demonstrate better corrosion properties. Corrosion immersion tests for the initial and HE processed titanium samples were carried out in a Carter-Brugirard solution at 37 °C, for 90 days. Every seven days the pH changes of the blank solution and solutions with the samples immersed were measured (Fig. 10). The corrosion of titanium in artificial saliva resulted in a slight decrease in the pH value in relation to the blank solution. However, measurements carried out in the incubator showed no significant differences for either type of Ti sample up to about 60 days. After exceeding this period, changes in pH for unprocessed titanium became more apparent. It is also important that the surfaces of all samples tested revealed a well protective surface free of any corrosion attack after 90 days. 4. Conclusions Based on the results obtained in the present study on the corrosion anisotropy of titanium deformed by hydrostatic extrusion (HE) in artificial saliva, the following conclusions have been drawn.
The open-circuit potential measurements showed that hydrostatic extrusion treatment improves the corrosion resistance of titanium. Polarisation measurements and EIS (electrochemical impedance spectroscopy) analysis were in good agreement and revealed the same trend in corrosion properties. In general, this effect is related to the fast formation and growth of oxide films at the titanium surfaces of very high lattice defect density (in particular the high density of high angle grain boundaries HAGBs).
The corrosion resistance of titanium processed by HE was distinctly affected by the extrusion direction, due to sharp 101̅0 fibre texture and different crystal orientations observed on Trans_HE and Long_HE samples.
The best corrosion behaviour in an artificial saliva solution was exhibited by titanium samples of the longitudinal cross-sections (Long_HE). This effect is due to synergy between the high density of crystallographic lattice defects and the crystallographic
11
texture, where the corrosion resistant, close-packed crystallographic planes (0001) are mainly oriented parallel to the extrusion direction ED. Acknowledgements This work was supported by the National Centre for Research and Development in Poland through Project No PBS2/A6/18/2014. 5. References [1] C. Misch, Dental Implant Prosthetics, Ed. Elsevier 2014; eBook ISBN: 9780323263689. [2] M. Sharma, A.V. Ramesh Kumar, N. Singh, N. Adya, B. Saluja, Electrochemical Corrosion Behavior of Dental/Implant Alloys in Artificial Saliva, J. Mater. Eng. Perform. 17 (5) (2008) 695-701; https://doi.org/10.1007/s11665-008-9198-4. [3] R. Van Noort, Review Titanium: the implant material of today, J. Mater. Sci. 22 (1987) 3801-3811; https://doi.org/10.1007/BF01133326. [4] H.J. Rack, J.I. Qazi, Titanium alloys for biomedical applications, Mater. Sci. Eng., C 26 (2006)1269-1277; https://doi.org/10.1016/j.msec.2005.08.032. [5] A. Balakrishnan, B.C. Lee, Corrosion behaviour of ultra fine grained titanium in simulated body fluid for implant application, Trends Biomater. Artif. Organs 22 (2008) 58-64. [6] A. Balyanov, J. Kutnyakova, N.A. Amirkhanova, V.V. Stolyarov, R.Z. Valiev, X.Z. Liao, Y.H. Zhao, Y.B. Jiang, H.F. Xu, T.C. Lowe, Y.T. Zhu, Corrosion resistance of ultra finegrained Ti, Scripta Mater. 51 (2004) 225-229; https://doi.org/10.1016/ j. scriptamat. 2004. 04. 011. [7] M. Greger, M. Widomska, L. Kander, Mechanical properties of ultra-fine grain titanium, J. Achiev. Mater. Man. Eng. 40 (2010) 33-40. [8] V.V. Stolyarov, Y.T. Zhu, T.C. Lowe, R.Z. Valiev, Microstructure and properties of pure Ti processed by ECAP and cold extrusion, Mater. Sci. Eng., A 303 (2001) 82-89; https:// doi.org/10.1016/S0921-5093(00)01884-0. [9] A.V.Sergueeva, V.V. Stolyarow, R.Z. Valiev, A.K. Mukherjee, Advanced mechanical properties of pure titanium with ultrafine grained structure, Scripta Mater. 45 (2001) 747-752; https://doi.org/10.1016/S1359-6462(01)01089-2. [10] S.L. Assis, S. Wolynec, I. Costa, Corrosion characterization of titanium alloys by electrochemical techniques, Electrochim. Acta 51 (2006) 1815-1819; https:// doi.org/ 10. 1016/ j.electacta.2005.02.121. [11] Y. Cheng, J. Hu, C. Zhang, Z. Wang, Y. Hao, B. Gao, Corrosion behavior of novel Ti24Nb-4Zr-7.9Sn alloy for dental implant applications in vitro, J. Biomed. Mater. Res. B Appl. Biomater. 101B (2013) 287-294; https://doi.org/10.1002/jbm.b.32838. 12
[12] Y. Okazaki, Y. Ito, K. Kyo, T. Tateishi, Corrosion resistance and corrosion fatigue strength of new titanium alloys for medical implants without V and Al, Mater. Sci. Eng., A 213 (1996) 138-147; https://doi.org/10.1016/0921-5093(96)10247-1. [13] R. Cornwell, K.T. Hartwig, R.E. Goforth, S.L. Semiatin, The Equal Channel Angular Extrusion Process for Materials Processing, Mater. Charact. 37 (1996) 295-300; https:// doi. org /10.1016/S1044-5803(97)80018-6. [14] H. Maleki-Ghaleh, K. Hajizadeh, A. Hadjizadeh, M.S. Shakeri, S. Ghobadi-Alamdari, S. Masoudfar, E. Aghaie, M. Javidi, J. Zdunek, K.J. Kurzydlowski, Electrochemical and cellular behaviour of ultrafine-grained titanium in vitro, Mater. Sci. Eng., C 39 (2014) 299-304; https://doi.org/10.1016/j.msec.2014.03.001. [15] W. Pachla, M. Kulczyk, M. Sus-Ryszkowska, A. Mazur, K.J. Kurzydlowski, Nanocrystalline titanium produced by hydrostatic extrusion, J. Mater. Process. Technol. 205 (2008) 173-182; https://doi.org/10.1016/j.jmatprotec.2007.11.103. [16] K. Sztwiertnia, A. Morawiec, M. Bieda, J. Kawałko, Microstructure of titanium deformed by warm extrusion with forward-backward rotating die, IOP Conf.: Mater. Sci. Eng. 63 (2014) 012012; https://doi.org/10.1088/1757-899X/63/1/012012. [17] W. Pachla, M. Kulczyk, S. Przybysz, J. Skiba, K. Wojciechowski, M. Przybysz, K. Topolski, A. Sobolewski, M. Charkiewicz, Effect of severe plastic deformation realized by hydrostatic extrusion and rotary swaging on the properties of CP Ti grade 2, J. Mater. Process. Technol. 221 (2015) 256-268; https://doi.org/10.1016/j.jmatprotec.2015.02.027. [18] Z. Machynia, K. Koynov, K. Sztwiertnia, Dental implant-distractor (ID), Patent No. P.416 895. [19] Y.S. Jimenez, M.T. Gil, M.T. Guerra, L.S. Baltes, J.C.M. Rosca, Interpretation of open circuit potential of two titanium alloys for a long time immersion in physiological fluid, Bull. Trans. Univ. 2(51) (2009) 197-204. [20] M. Hoseini, A. Shahryari, S. Omanovic, J.A. Szpunar, Comparative effect of grain size and texture on the corrosion behaviour of commercially pure titanium processed by equal channel angular pressing, Corros. Sci. 51 (2009) 3064-3067; https://doi.org/10.1016/j.corsci. 2009.08.017. [21] A. Shahryari, S. Omanovic, J.A. Szpunar, Enhancement of biocompatibility of 316LVM stainless steel by cyclic potentiodynamic passivation, J. Biomed. Mater. Res., A 89A (2009) 1049-1062; https://doi.org/10.1002/jbm.a.32053.
13
[22] M. Sharma, A.V.R. Kumar, N. Singh, N.Adya, B. Saluja, Electrochemical Corrosion Behavior of Dental/Implant Alloys in Artificial Saliva, J. Mater. Eng. Perform. 17 (2008) 695701; https://doi.org/10.1007/s11665-008-9198-4. [23] H. Garbacz, M. Pisarek, K.J. Kurzydlowski, Corrosion resistance of nanostructured titanium, Biomol. Eng. 24 (2007) 559-563; https://doi.org/10.1016/j.bioeng.2007.08.007. [24] A. Dalmau, V. Guinon Pina, F. Devesa, V. Amigo, A. Igual Munoz, Electrochemical behavior of near-beta titanium biomedical alloys in phosphate buffer saline solution, Mater. Sci. Eng., C 48 (2015) 55-62; https://doi.org/10.1016/j.msec.2014.11.036. [25] E. Navarro-Flores, Z. Chong, S. Omanovic, Characterization of Ni, NiMo, NiW and NiFe electroactive coatings as electrocatalysts for hydrogen evolution in an acidic medium, J. Mol. Catal. A: Chem. 226 (2005) 179-197; https://doi.org/10.1016/j.molcata.2004.10.029. Figure Caption
14
Figr-14Figure captions:
Fig. 1. Schematic drawing of titanium samples (a), trans cross-section (b), and long crosssection (c). Fig. 2. Grain orientation spread (GOS) maps of titanium after hydrostatic extrusion a) transverse transversal, b) longitudinal cross-section. HAGBs are indicated by black lines, LAGBs are indicated by bright grey lines. Fig. 3. TEM BF images of titanium after hydrostatic extrusion a) transverse transversal, b) longitudinal cross-section and c) selected area diffraction pattern from transverse cross section indexed as Ti α. Fig. 4. Histograms showing distribution of crystallite sizes in titanium samples after hydrostatic extrusion, a) short axes in trans cross-section, b) short axes in long cross-section, c) long axes in trans cross-section, d) long axes in long cross-section. Fig. 5. Inverse pole figures (normal direction) calculated for titanium samples from discrete EBSD crystallographic data: a) transverse transversal cross-section - before HE, b) transverse transversal cross-section – after HE, c) longitudinal cross-section – after HE – coarse-grained fraction, d) longitudinal cross-section – after HE – fine-grained fraction. Fig. 6. Open circuit potential evolution in the first two hours, for tested Ti samples in a solution of artificial saliva of pH 7.6. Fig. 7. Polarisation curves in semi-logarithmic coordinates for tested Ti samples in a solution of artificial saliva of pH 7.6. Fig. 8. Comparison of the impedance response of Ti grade 2 samples Trans_HE and Long_HE. Fig. 9. Equivalent circuits fitted to the EIS data. Fig. 10. The pH values of artificial saliva containing titanium samples (initial and HE processed), and blank solution for 90 days.
15
Figr-1
16
Figr-2
17
Figr-3
18
Figr-4
19
Figr-5
20
Figr-6
21
Figr-7
22
Figr-8
23
Figr-9
24
Figr-10
25
Table 1. Statistical microstructure parameters. Trans_HE
Long_HE
Overall
Finegrained fraction
Coarsegrained fraction
Overall
Finegrained fraction
Coarsegrained fraction
Structure fraction
-
84%
12%
-
48%
48%
Avg. grain size (diameter) [µm] LAGB density [1/μm]
0.57 (SD 0.47) 3.30
0.42 (SD 0.20) 3.11
1.59 (SD 0.53) 2.78
2.00 (SD 2.21) 3.51
0.57 (SD 0.21) 3.15
3.42 (SD 2.38) 2.77
HAGB density [1/μm]
5.26
5.60
0.15
2.50
2.95
0.46
26
Table 2. Corrosion parameters of titanium samples determined in a solution of artificial saliva
Ti specimen code
OCP mV/SCE
Ecorr, mV/SCE
icorr, nA/cm2
ba mV/dec
bc mV/dec
Rp Ω⋅cm2
Trans_Init
-373
-383
156
132
119
134674
Long_Init
-384
-394
149
125
120
123455
Trans_HE
-230
-248
147
116
112
126644
Long_HE
-146
-168
130
124
106
237975
27
Table 3. The parameters of specific elements of equivalent circuits fitted to EIS spectra
Rs Ti sample code
Rct
CPEdl
Rf
cm
cm
s cm
Trans_HE
17
9997
3·10-5
Long_HE
17
10490
6·10-5
2
2
N
-2
28
Ndl
CPEf Nf
χ2
Rp
cm
s cm
0.93
102432
3·10-5
0.92
0.004
112429
0.98
116864
4·10-5
0.93
0.003
127354
2
N
-2
cm2
S0169-4332(17)32232-8 http://dx.doi.org/doi:10.1016/j.apsusc.2017.07.231 APSUSC 36765
To appear in:
APSUSC
Received date: Revised date: Accepted date:
4-4-2017 3-7-2017 24-7-2017
Please cite this article as: A.Chojnacka, J.Kawalko, H.Koscielny, J.Guspiel, A.Drewienkiewicz, M.Bieda, W.Pachla, M.Kulczyk, K.Sztwiertnia, E.Beltowska-Lehman, CORROSION ANISOTROPY OF TITANIUM DEFORMED BY THE HYDROSTATIC EXTRUSION, Applied Surface Sciencehttp://dx.doi.org/10.1016/j.apsusc.2017.07.231 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
CORROSION ANISOTROPY OF TITANIUM DEFORMED BY THE HYDROSTATIC EXTRUSION A. Chojnackaa,b, J. Kawalkoa, H. Koscielnya, J. Guspiela, A. Drewienkiewiczc, M. Biedaa, W. Pachlad, M. Kulczykd, K. Sztwiertniaa, E. Beltowska-Lehmana,* a
Institute
of
Metallurgy
and
Materials
Science,
Polish
Academy
of
Sciences,
25 Reymonta St., 30-059 Krakow, Poland b
c
Faculty of Chemistry, Jagiellonian University, 3 Ingardena St., 30-060 Krakow, Poland.
Faculty of Metals Engineering and Industrial Computer Science, AGH University of Science
and Technology, 30 Mickiewicza Av., 30-059 Krakow, Poland. d
Institute of High Pressure Physics, Polish Academy of Sciences, Sokolowska 29/37, 01-142
Warsaw, Poland
*Corresponding author (E. Beltowska-Lehman): Tel. +48 12 295 2829, fax: +48 12 295 2804 E-mail address: [email protected]; [email protected] Graphical Abstract
1
Highlights
Hydrostatic extrusion deformation improves the corrosion resistance of titanium. Corrosion resistance of deformed titanium was affected by the extrusion direction. Surface oriented parallel to the extrusion direction reveals the best corrosivity.
ABSTRACT The corrosion behaviour of titanium rods deformed by hydrostatic extrusion (HE) in artificial saliva (Carter-Brugirard’s solution of pH 7.6) was investigated using open-circuit potentials (OCPs), (DC) potentiodynamic polarisation curves and (AC) electrochemical impedance spectroscopy (EIS) techniques. Various electrochemical parameters (corrosion potential Ecorr, corrosion current (Icorr) (icorr), polarisation resistance Rp, charge transfer resistance Rct and oxide film resistance Rf) were analysed. Significant coherence was observed between results achieved from these procedures, i.e., all applied techniques showed the same trend for corrosion resistance. The obtained electrochemical data were then related to the microstructure parameters (crystallographic texture, grain size, grain boundary distribution and density) determined using the EBSD/SEM technique. It was found that the corrosion behaviour of titanium processed by the HE method was superior compared to the unprocessed Ti, and this was clearly dependent on the extrusion direction. The highest corrosion resistance was revealed for the HE-deformed Ti rod of the surface oriented longitudinal (parallel) to the extrusion direction. Keywords: hydrostatic extrusion; titanium; corrosion; crystallographic texture; dental implants 1. Introduction For many years, scientists have devoted much attention to the search for materials suitable for use as dental implants. These materials must be biocompatible and resistant to corrosion in the physiological fluid environment. Initially ivory, pearls, shells, animal bones, ceramics and precious metals were applied as tooth replacements [1]. Currently, stainless steel and Co-Cr alloys are the most commonly used, due to excellent mechanical properties allowing the easy formation of components of complex shapes such as tooth crowns, bridges, and fixed or moveable prosthesis parts [2]. However, titanium and titanium alloys remain the most interesting materials for dental applications. A significant advantage of unalloyed titanium is the relatively low Young's modulus (of about 105–120 GPa), the most similar to bone stiffness among all metallic biomaterials presently applied in implantology [3]. This parameter (a measure of the stiffness of solid material) is extremely important since considerable differences between the elastic modulus of human bone and the implant can lead to bone resorption and 2
implant loosening. Titanium is also inert and resistant to corrosion due to the formation of the stable surface oxide film, which consists mainly of titanium oxide TiO2 of a rutile-type tetragonal structure. This feature is very important, since the environment of saliva in the oral cavity (pH change from 2 to 11) is highly corrosive to many noble metals [3, 4]. Nevertheless, the relatively poor tribological properties (due to low hardness and high friction coefficient) such as low strength, wear and abrasion resistances are the main disadvantages of titanium. Therefore, many studies today focus on improving titanium’s mechanical properties. One of the common methods is to replace the pure titanium with alloys, for instance Ti6Al4V [4]. However, this alloy has undesirable properties due to the strong cytotoxic reactions induced by vanadium and the negative impact of aluminium on nerve cells. Thus, unfavourable components of titanium alloys (e.g. Al and V) are replaced by biocompatible elements including tantalum, niobium, and zirconium. At the same time, various procedures for improving the mechanical properties of pure titanium, including microstructure refinement, have been suggested [5]. It was observed that, in accordance with the Hall-Petch relationship, the mechanical strength of pure titanium increases considerably as a result of grain size reduction [5–9]. There are many different methods of obtaining ultrafine-grained or nanocrystalline microstructures in metallic materials, including titanium. With the emergence of new possibilities in material engineering, titanium with ultrafine-grained microstructure can be obtained using Severe Plastic Deformation (SPD) methods [10–12]. The best known SPD techniques include Equal Channel Angular Pressing (ECAP), Accumulative Roll Bonding (ARB), High Pressure Torsion (HPT) and Cyclic Extrusion Compression (CEC) [13]. Regarding corrosion behaviour, the majority of data available in the literature concern titanium processed by ECAP, as the most popular SPD method. These studies have shown that ultrafine- crystalline grained titanium (after ECAP) shows better corrosion resistance in NaCl, HCl and H2SO4 environments than the coarse-grained counterpart [5, 6]. The effect was related to the higher density of defects in the crystal lattice of the surface layer in which the passivation process is initiated. Furthermore, it has been demonstrated that titanium transformed by ECAP reveals (in addition to excellent corrosion properties) higher adhesion, and fibroblast proliferation in an environment simulating human body fluid (SBF) compared to the unprocessed material [14]. Recently, the hydrostatic extrusion (HE) technique [15] and the KoBo method, e.g. [16], have been proposed as means of modifying titanium microstructure leading to the improvement of its mechanical properties. In contrast to other Severe Plastic Deformation (SPD) SPD techniques, the product dimension change accompanies both extrusion methods, and the final 3
strain is determined by cumulative cross section area reduction of processed billets. Based on literature and the authors’ own studies, it was possible to conclude that titanium deformed by the HE method showed better mechanical properties compared to the unprocessed material, due to strengthening by grain refinement. The significant improvement was observed in yield strength (about two and a half times higher) and microhardness (about one and a half times higher) values [17]. The hydrostatic extrusion (HE) method seems to be particularly effective for the development of nano and ultrafine structures microstructures, and for the enhancement of biomaterial properties for medical applications. There are little data available in the literature on the corrosion of titanium after the hydrostatic extrusion (HE) process, and none relate to corrosive properties depending on the extrusion direction (ED). This work presents the first study on the corrosion anisotropy of commercially pure CP (grade 2) titanium deformed by the HE method. The corrosion behaviour was investigated in an artificial saliva solution. The resulting data were analysed in relation to the microstructure, and will provide basis for the development of a dental implant-distractor (ID) designed to be embedded in the bone. The ID is an innovative treatment for bone tissue atrophy after tooth loss, and combines the function of vertical bone osteogenesis with implantation [18]. 2. Experimental details The material selected for testing was commercially pure titanium grade 2 supplied by Wolften, Poland. The nominal composition (wt.%) of samples was C 0.02, Fe 0.12, O 0.10, N 0.02, H 0.003, with the balance being titanium. Initial titanium samples were subjected to severe plastic deformation using the hydrostatic extrusion technique in the Institute of High Pressure Physics of the Polish Academy of Sciences. The deformation was performed at room temperature (20 °C), in five passes from a diameter of 25 mm to a diameter of 6.07 mm. The process was carried out without intermediate annealing. Extrusion pressures were in the range between 550 and 950 MPa. The final deformation was performed by means of rotary swaging, which further reduced the diameter of the product to 4.74 mm. Combined deformation steps performed after annealing resulted in equivalent strain accumulation equal to ε: 3.32. Samples from extruded and initial titanium were prepared, with surfaces oriented parallel (long crosssection) and perpendicular (trans cross-section) to the extrusion direction (ED). The two types of samples are presented schematically in Figure 1. Samples for corrosion testing and metallographic samples for microstructure and texture investigations were prepared using SiC sandpaper grinding up to 7000 grit, followed by mechanical polishing with a 1:1 solution of colloidal silica suspension and hydrogen peroxide 4
(30%) resulting in highly polished, mirror like surfaces. The samples for SEM EBSD (electron backscattered diffraction) microstructure characterisation were additionally polished electrochemically for 15 seconds in Struers A3 electrolyte at 5 °C and 35 V. The EBSD microstructure investigations of the titanium samples in the initial state and after HE deformation were carried out using the FEI Quanta 3D FEG microscope, with integrated EDAX TSL system using a Hikari camera, and OIM Data Collection software for the acquisition and indexing of electron backscattered diffraction patterns. The EBSD maps were collected using electron beam energy of 20 keV. The step sizes used for EBSD measurement of severely deformed Ti samples were 55 and 75 nm, and the sizes of the maps collected were 63μm x 63μm and 86 μm x 86 μm. The EBSD map from the titanium sample in as received state was collected over an area of 1mm2 with a 1 μm step size. The collected EBSD data were post-processed and analysed using the TSL OIM Analysis 7 software. Postprocessing consisted of a single iteration of the “grain dilation” procedure with a grain tolerance angle of 2° and minimum grain size of 2 points. Using the OIM Analysis software, microstructure parameters based on EBSD data were calculated, including distribution of grain size, and density of low angle grain boundaries (LAGB) and high angle grain boundaries (HAGB). A grain was defined a as set of at least five properly indexed points enclosed completely by boundary segments with misorientation between neighbour points of at least 2°. Points that did not meet this criterion were excluded from analysis (for example, about 4% of points were excluded from analysis in the case of titanium after hydrostatic extrusion HE). The size of crystallites grains was calculated as a diameter of circles with equivalent surface area, and as a second method by using maximum and minimum feret size function. Feret size function calculates the maximum and minimum distance between two parallel lines tangential to the boundaries of a grain, which essentially is the measurement of each crystallite’s grain’s long and short axis respectively. LAGB (low angle grain boundaries) was defined as boundary segment with a misorientation angle between neighbours from 2° to 15°, and HAGB (high angle grain boundaries) was defined as boundary segment with a misorientation angle of 15° and above. Grain boundary density was reported as a measure of total grain boundary length divided by the surface area of measured EBSD map (1/μm). The crystallographic EBSD data were also used in local texture analysis to calculate inverse pole figures. The texture from discreet EBSD data was calculated in TSL OIM Analysis software using the method of spherical harmonic series expansion. The structure of the titanium in the initial state and after the hydrostatic extrusion (HE) process was also examined by transmission electron microscope TEM (FEI
5
TECNAI G20). Thin foils for TEM observations were electropolished using a twin jet electro polisher. Electrochemical experiments were carried out in a conventional three-electrode electrochemical cell using an AUTOLAB PGSTAT302N potentiostat/galvanostat with NOVA 1.11 software to control the experiment, and to collect and analyse the data obtained. Titanium rods (unprocessed and after the HE process, respectively) were used as working electrodes. The counter electrode was platinum sheet located at a distance of 50 mm from the Ti working electrode. The saturated calomel electrode (SCE) placed in a Luggin capillary was used as a reference electrode. The corrosion resistance of the titanium samples was investigated in an aerated Carter-Brugirard synthetic saliva solution, prepared according to the PN-EN ISO 10993-15 standard. All reagents were of analytical grade purity. The reagents NaCl (0.7 g L-1), Na2HPO4 (0.26 g L-1), KSCN (0.33 g L-1), KH2PO4 (0.2 g L-1), NaHCO3 (1.5 g L-1), KCl (1.2 g L-1) were dissolved in distilled water. The pH of 6.7 was adjusted by addition of HCl or NaOH. Electrochemical measurements were conducted at a temperature of 37.0 ± 0.3 °C. After metallographic preparation, specimens were washed in distilled water in the presence of an ultrasonic field. The surface area of titanium samples exposed to the electrolyte was 0.24 cm 2. The corrosion behaviour was estimated both in parallel and perpendicular cross-sections to the extrusion direction, using the potentiodynamic polarisation and electrochemical impedance spectroscopy (EIS) techniques:
Potentiodynamic polarisation technique: titanium rod samples were immersed in the corrosive solution and the open circuit potential (OCP) was measured for two hours. The anodic and cathodic polarisation curves were recorded using a sweep potential test in the range of ±150 mV with respect to OCP, with a scanning rate of 1mV/s. Three polarisation measurements were repeated for each sample. The corrosion current (Icorr) (icorr) and corrosion potential (Ecorr) were determined based on the Tafel relation: log I i = f (E). The polarisation resistance (Rp) was calculated using the Stern-Geary equation: 𝑏𝑐 ⋅𝑏𝑎 , 𝑎 +𝑏𝑐 )𝑖𝑐𝑜𝑟𝑟
𝑅𝑝 = 2,3⋅(𝑏
where ba , bc are the slope of anodic and cathodic polarisation
curves.
Electrochemical impedance spectroscopy (EIS): the impedance measurements (single sine alternating current AC) were carried out at open circuit potential with a voltage amplitude of 10 mV and a frequency range from 0.1 Hz to 100 kHz (50 points for each frequency decade were acquired). The impedance data were analysed based on the Boukamp model. Impedance spectra were reported by Nyquist diagram. 6
Immersion test: measurements were conducted according to the ISO 10993-15:2000 standard. Tested samples (initial Ti and Ti after hydrostatic extrusion HE) were dipped in an artificial saliva solution (80 ml) and placed in an incubator at 37 °C for three months. The pH and weight of the samples were measured every seven days.
3. Results and discussion 3.1. Microstructure of titanium after hydrostatic extrusion Titanium microstructure in the as received condition is moderately deformed, which is the result of the production process. Grains are slightly elongated in the extrusion direction, grain size ranges from 5 μm to about 140 μm and overall mean grain diameter is 47.2 μm. Larger grains show often sub-grain microstructure defined by developed mesh of LAGB’s. Some tension {101̅2}〈101̅1̅〉 twins are present in the microstructure. Grain boundary density is low and equal to 0.01 1/μm for both LAGB and HAGB (low and high angle grain boundaries). The titanium samples after hydrostatic extrusion have a microstructure consisting of two distinct types of grains: relatively coarse ones, which are severely elongated in the direction of extrusion, and clusters of strongly refined grains, with much less elongated shapes. The elongation of the microstructure results in very different shapes and sizes of crystallites grains observed on transverse transversal (Trans) and longitudinal (Long) cross-sections. The two types of structures microstructures are presented in Fig. 2 by means of EBSD grain orientation spread (GOS) maps. Observations in TEM (bright field (BF) images presented in Fig 3 a, b) also revealed equiaxed grains in transverse transversal cross section and elongated in extrusion direction (ED) grains in longitudinal cross section. Moreover, analysis of selected area diffraction patterns obtained from transverse transversal cross section confirms the microstructure’s ultra-fine crystalline grain character. The crystallite grain sizes vary greatly between the two cross-sections. In the trans crosssection, crystallite grain sizes range from 70 nm (short axis) to 6.5 μm (long axis), and overall mean crystallite grain diameter (diameter of equivalent circles) is equal to 0.57 μm. On the other hand, sizes of the crystallites grains observed in the long cross-section range from 140 nm to 44.63 μm for short and long crystallite grain axis, respectively, and the average diameter of crystallites grains in this cross-section is equal to 2.00 μm. The histograms of crystallite grain sizes (short and long axes) are presented in Fig. 4. Such very large discrepancies between sizes of crystallites grains points to the substantial heterogeneity of the microstructure. Since grain size is a parameter determining macroscopic properties such as hardness or tensile strength, and can affect corrosion resistance, it is important to investigate parts of the microstructure with such different grain sizes separately. 7
Based on the criterion of crystallite grain diameter below 1 µm, the ultra-fine grained fraction of microstructure can be distinguished and analysed separately from the coarse-grained fraction, consisting of crystallites grains with diameter equal to or greater than 1 μm. The main difference between the two fractions can be observed in the content (density) of HAGB (Table 1). For instance, HAGB density in the fine-grained fraction is equal to 5.60 1/m in the trans cross-section, while in the coarse-grained fraction HAGB (high angle grain boundaries) density is much lower (0.15 1/µm). On the other hand, differences in LAGB (low angle grain boundaries) content are not that great, but similarly the density is higher in the fine-grained fraction. Such great differences in HAGB content between the two fractions results in the uneven spatial distribution of crystal lattice defects. Additionally, heterogeneity differs between both cross-sections, due to different ratios of surface area occupied by the two fractions. In the trans cross-section, the fine-grained fraction with high HAGB density occupies 84% of the microstructure, while in the long cross-section only 48% of the microstructure is occupied by the high-density HAGB fraction. The additional difference in overall HAGB density between the trans and long cross-sections results from severe elongation and the overall bigger grains observed in long cross-section. Titanium in the as received state (before hydrostatic extrusion) has almost random crystallographic texture, with a very weak maximum at the [0001] direction on inverse pole figure for the extrusion direction (Fig. 5a). After hydrostatic extrusion, titanium samples have a sharp, 101̅0 fibre texture typical for extruded hexagonal materials. The texture is characterised with the main maximum on the (101̅0) pole figure in the position of extrusion direction, and the (0001) pole distributed in the surface perpendicular to this direction. Such texture results in very different crystallographic planes observed on transverse transversal and longitudinal cross-sections. On the transverse transversal cross-section {101̅0}, planes of crystallites grains are almost exclusively observed (Fig. 5b), while on the longitudinal crosssection the crystallite grain orientation is distributed between the (0001) and {112̅0} planes, with the maximum shifted towards the (0001) position (Fig. 5c-d). Using the discrete crystallographic data from EBSD (electron backscattered diffraction) measurements, textures were calculated separately for ultra-fine grained and coarse-grained components of the microstructure. Inverse pole figures presented in Fig. 5 c-d indicate that the texture is similar in both fractions of the microstructure; however, the non-basal components of texture are slightly weaker in the case of the fine-grained fraction. 3.2. Corrosion behaviour 8
Titanium rod samples were prepared for corrosion tests in an artificial saliva solution according to the diagram shown in Fig. 1 (see the Experimental details section). Fig. 6 presents changes of the open circuit potentials (OCP) against time for tested Ti samples in an aerated Carter-Brugirard solution at 37 °C. It is known that the OCP value varies with time of immersion in a corrosive medium due to changes in the surface nature of the electrode, and indicates the thermodynamic tendency of a material to electrochemical oxidation. Thus, the open circuit potential can be a criterion for corrosion behaviour [19]. As can be seen in Figure 6, a substantial shift of the open circuit (OC) potential towards less negative values was observed for all tested samples. Then, the open circuit potential (OCP) increases slowly and eventually stabilises at a certain stationary value. The effect is mainly related to the formation and growth of the oxide films at the Ti sample surface in the artificial saliva medium. These results clearly indicate that unprocessed titanium rods behave worse than Ti deformed by hydrostatic extrusion (HE), after which the open circuit OC potential is less negative, about 0.15 mV – 0.22 mV (depending on the sample surface orientation to the extrusion direction) (Table 2). Thus, hydrostatic extrusion treatment improves the corrosion resistance of titanium, which reveals a more noble character and then less chemical activity. Open circuit potential variations are very similar for all unprocessed Ti samples regardless of the examined surfaces (sample code: Trans_Init and Long_Init) (Fig. 6). However, for Ti deformed by HE, it should be highlighted that OCP is influenced by the extrusion directions, showing a progressive shift towards noble values for surface parallel oriented (Long_HE). The cathodic and the anodic polarisation curves of all investigated Ti samples after two hours of immersion in an aerated Carter-Brugirard solution are reported in Figure 7. The electrochemical parameters derived from the Tafel plots, including the corrosion current density (Icorr) (icorr), corrosion potential (Ecorr), polarisation resistance (Rp), and the slope of anodic (ba) and cathodic (bc) polarisation curves are summarised in Table 2. As seen from the data in Table 2, all investigated Ti samples were characterised by very good corrosion resistance in an artificial saliva solution, as evidenced by the low corrosion currents and high values of corrosion potential and polarisation resistance. Moreover, the corrosion parameters of unprocessed titanium samples (Trans_Init and Long_Init) are very similar in both directions. In turn, for titanium deformed by hydrostatic extrusion (HE) the corrosion properties (especially corrosion potential Ecorr and polarisation resistance Rp) differ significantly for the surface perpendicular and parallel to the extrusion direction (Trans_HE and Long_HE). It is clearly shown that the Long_HE surface (oriented parallel to the extrusion direction ED) reveals the highest corrosion resistance in the artificial saliva solution of pH 7.6. 9
Those results suggest that the sharp texture and different orientation of crystallite grain planes observed on Trans_HE and Long_HE samples is more important than the crystal lattice defect density in determining corrosion behaviour. The basal (0001) planes in titanium have the highest atom density in this hcp structure. It has been reported previously that high atom density planes offer higher corrosion resistance than lower density ones [20]. In the HE titanium (0001), orientations are observed on long cross-sections, which, in synergy with increased grain boundary density after hydrostatic extrusion (HE), results in the highest corrosion resistance among tested samples. On the other hand, Trans_HE cross-section crystal lattice defect density is higher than in Long_HE, but {101̅0} crystal planes observed on Trans_HE are less corrosion resistant, thus this cross-section has effectively lower corrosion resistance. The EIS (electrochemical impedance spectroscopy) measurements support the results of open circuit potential (OCP) and potentiodynamic polarisation methods. Fig. 8 shows EIS spectra for the processed titanium samples Trans_HE and Long_HE. Collected results are presented in the form of Nyquist plots. To explain the corrosion behaviour of the system, experimental EIS data were fitted to an appropriate electrical equivalent circuit (EEC) model, presented in Fig. 9. The Nyquist plots show a depressed semi-circle, indicating that there is more than one time constant that describes the impedance response of the system [20,21]. Based on the composition of testing system and other studies concerning EIS analysis of titanium [20-24], the respective parts of the circuits were analysed. Rs is related to resistance resulting from all electrical contacts in the measuring system. The first parallel connection of the resistanceconstant phase element (Rct-CPEdl) in the high frequency range corresponds to the charge transfer processes, represented by the double layer constant phase element (CPEcl) and charge transfer resistance (Rct), respectively. The second parallel of the resistance-constant phase element (Rf-CPEf) can be associated with slower mass transport processes in the oxide phase, namely the diffusion of electroactive species [20, 21]. CPE elements are introduced to replace the capacitors due to the non-ideal behaviour of capacitive elements resulting in inhomogeneities present at the micro or nano (atomic/molecular) levels, such as surface roughness/porosity, adsorption, or diffusion [25]. Calculated values of all parameters are collected in Table 3. The additional parameter is polarisation resistance (Rp) which is given by the sum of charge transfer resistance (Rct) and film resistance (Rf). It should be noted that the Rp values calculated from EIS are consistent with the Rp values obtained using the potentiodynamic polarisation technique.
10
For both samples, film resistance Rf is higher than charge transfer resistance Rct and Ndl is higher than Nf. This suggests that passive film exercises more resistance than the charge transfer in the double layer, and that the behaviour of the film is not ideal, which is consistent with earlier studies [24]. Rf (film resistance) values are at a similar level for both materials, so it can be assumed that the passive film formed on samples is of comparable quality. However, all parameters related to charge transfer and the formation of passive film for the sample in longitudinal direction to the hydrostatic extrusion (HE) process are slightly higher, which means that this material is expected to demonstrate better corrosion properties. Corrosion immersion tests for the initial and HE processed titanium samples were carried out in a Carter-Brugirard solution at 37 °C, for 90 days. Every seven days the pH changes of the blank solution and solutions with the samples immersed were measured (Fig. 10). The corrosion of titanium in artificial saliva resulted in a slight decrease in the pH value in relation to the blank solution. However, measurements carried out in the incubator showed no significant differences for either type of Ti sample up to about 60 days. After exceeding this period, changes in pH for unprocessed titanium became more apparent. It is also important that the surfaces of all samples tested revealed a well protective surface free of any corrosion attack after 90 days. 4. Conclusions Based on the results obtained in the present study on the corrosion anisotropy of titanium deformed by hydrostatic extrusion (HE) in artificial saliva, the following conclusions have been drawn.
The open-circuit potential measurements showed that hydrostatic extrusion treatment improves the corrosion resistance of titanium. Polarisation measurements and EIS (electrochemical impedance spectroscopy) analysis were in good agreement and revealed the same trend in corrosion properties. In general, this effect is related to the fast formation and growth of oxide films at the titanium surfaces of very high lattice defect density (in particular the high density of high angle grain boundaries HAGBs).
The corrosion resistance of titanium processed by HE was distinctly affected by the extrusion direction, due to sharp 101̅0 fibre texture and different crystal orientations observed on Trans_HE and Long_HE samples.
The best corrosion behaviour in an artificial saliva solution was exhibited by titanium samples of the longitudinal cross-sections (Long_HE). This effect is due to synergy between the high density of crystallographic lattice defects and the crystallographic
11
texture, where the corrosion resistant, close-packed crystallographic planes (0001) are mainly oriented parallel to the extrusion direction ED. Acknowledgements This work was supported by the National Centre for Research and Development in Poland through Project No PBS2/A6/18/2014. 5. References [1] C. Misch, Dental Implant Prosthetics, Ed. Elsevier 2014; eBook ISBN: 9780323263689. [2] M. Sharma, A.V. Ramesh Kumar, N. Singh, N. Adya, B. Saluja, Electrochemical Corrosion Behavior of Dental/Implant Alloys in Artificial Saliva, J. Mater. Eng. Perform. 17 (5) (2008) 695-701; https://doi.org/10.1007/s11665-008-9198-4. [3] R. Van Noort, Review Titanium: the implant material of today, J. Mater. Sci. 22 (1987) 3801-3811; https://doi.org/10.1007/BF01133326. [4] H.J. Rack, J.I. Qazi, Titanium alloys for biomedical applications, Mater. Sci. Eng., C 26 (2006)1269-1277; https://doi.org/10.1016/j.msec.2005.08.032. [5] A. Balakrishnan, B.C. Lee, Corrosion behaviour of ultra fine grained titanium in simulated body fluid for implant application, Trends Biomater. Artif. Organs 22 (2008) 58-64. [6] A. Balyanov, J. Kutnyakova, N.A. Amirkhanova, V.V. Stolyarov, R.Z. Valiev, X.Z. Liao, Y.H. Zhao, Y.B. Jiang, H.F. Xu, T.C. Lowe, Y.T. Zhu, Corrosion resistance of ultra finegrained Ti, Scripta Mater. 51 (2004) 225-229; https://doi.org/10.1016/ j. scriptamat. 2004. 04. 011. [7] M. Greger, M. Widomska, L. Kander, Mechanical properties of ultra-fine grain titanium, J. Achiev. Mater. Man. Eng. 40 (2010) 33-40. [8] V.V. Stolyarov, Y.T. Zhu, T.C. Lowe, R.Z. Valiev, Microstructure and properties of pure Ti processed by ECAP and cold extrusion, Mater. Sci. Eng., A 303 (2001) 82-89; https:// doi.org/10.1016/S0921-5093(00)01884-0. [9] A.V.Sergueeva, V.V. Stolyarow, R.Z. Valiev, A.K. Mukherjee, Advanced mechanical properties of pure titanium with ultrafine grained structure, Scripta Mater. 45 (2001) 747-752; https://doi.org/10.1016/S1359-6462(01)01089-2. [10] S.L. Assis, S. Wolynec, I. Costa, Corrosion characterization of titanium alloys by electrochemical techniques, Electrochim. Acta 51 (2006) 1815-1819; https:// doi.org/ 10. 1016/ j.electacta.2005.02.121. [11] Y. Cheng, J. Hu, C. Zhang, Z. Wang, Y. Hao, B. Gao, Corrosion behavior of novel Ti24Nb-4Zr-7.9Sn alloy for dental implant applications in vitro, J. Biomed. Mater. Res. B Appl. Biomater. 101B (2013) 287-294; https://doi.org/10.1002/jbm.b.32838. 12
[12] Y. Okazaki, Y. Ito, K. Kyo, T. Tateishi, Corrosion resistance and corrosion fatigue strength of new titanium alloys for medical implants without V and Al, Mater. Sci. Eng., A 213 (1996) 138-147; https://doi.org/10.1016/0921-5093(96)10247-1. [13] R. Cornwell, K.T. Hartwig, R.E. Goforth, S.L. Semiatin, The Equal Channel Angular Extrusion Process for Materials Processing, Mater. Charact. 37 (1996) 295-300; https:// doi. org /10.1016/S1044-5803(97)80018-6. [14] H. Maleki-Ghaleh, K. Hajizadeh, A. Hadjizadeh, M.S. Shakeri, S. Ghobadi-Alamdari, S. Masoudfar, E. Aghaie, M. Javidi, J. Zdunek, K.J. Kurzydlowski, Electrochemical and cellular behaviour of ultrafine-grained titanium in vitro, Mater. Sci. Eng., C 39 (2014) 299-304; https://doi.org/10.1016/j.msec.2014.03.001. [15] W. Pachla, M. Kulczyk, M. Sus-Ryszkowska, A. Mazur, K.J. Kurzydlowski, Nanocrystalline titanium produced by hydrostatic extrusion, J. Mater. Process. Technol. 205 (2008) 173-182; https://doi.org/10.1016/j.jmatprotec.2007.11.103. [16] K. Sztwiertnia, A. Morawiec, M. Bieda, J. Kawałko, Microstructure of titanium deformed by warm extrusion with forward-backward rotating die, IOP Conf.: Mater. Sci. Eng. 63 (2014) 012012; https://doi.org/10.1088/1757-899X/63/1/012012. [17] W. Pachla, M. Kulczyk, S. Przybysz, J. Skiba, K. Wojciechowski, M. Przybysz, K. Topolski, A. Sobolewski, M. Charkiewicz, Effect of severe plastic deformation realized by hydrostatic extrusion and rotary swaging on the properties of CP Ti grade 2, J. Mater. Process. Technol. 221 (2015) 256-268; https://doi.org/10.1016/j.jmatprotec.2015.02.027. [18] Z. Machynia, K. Koynov, K. Sztwiertnia, Dental implant-distractor (ID), Patent No. P.416 895. [19] Y.S. Jimenez, M.T. Gil, M.T. Guerra, L.S. Baltes, J.C.M. Rosca, Interpretation of open circuit potential of two titanium alloys for a long time immersion in physiological fluid, Bull. Trans. Univ. 2(51) (2009) 197-204. [20] M. Hoseini, A. Shahryari, S. Omanovic, J.A. Szpunar, Comparative effect of grain size and texture on the corrosion behaviour of commercially pure titanium processed by equal channel angular pressing, Corros. Sci. 51 (2009) 3064-3067; https://doi.org/10.1016/j.corsci. 2009.08.017. [21] A. Shahryari, S. Omanovic, J.A. Szpunar, Enhancement of biocompatibility of 316LVM stainless steel by cyclic potentiodynamic passivation, J. Biomed. Mater. Res., A 89A (2009) 1049-1062; https://doi.org/10.1002/jbm.a.32053.
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[22] M. Sharma, A.V.R. Kumar, N. Singh, N.Adya, B. Saluja, Electrochemical Corrosion Behavior of Dental/Implant Alloys in Artificial Saliva, J. Mater. Eng. Perform. 17 (2008) 695701; https://doi.org/10.1007/s11665-008-9198-4. [23] H. Garbacz, M. Pisarek, K.J. Kurzydlowski, Corrosion resistance of nanostructured titanium, Biomol. Eng. 24 (2007) 559-563; https://doi.org/10.1016/j.bioeng.2007.08.007. [24] A. Dalmau, V. Guinon Pina, F. Devesa, V. Amigo, A. Igual Munoz, Electrochemical behavior of near-beta titanium biomedical alloys in phosphate buffer saline solution, Mater. Sci. Eng., C 48 (2015) 55-62; https://doi.org/10.1016/j.msec.2014.11.036. [25] E. Navarro-Flores, Z. Chong, S. Omanovic, Characterization of Ni, NiMo, NiW and NiFe electroactive coatings as electrocatalysts for hydrogen evolution in an acidic medium, J. Mol. Catal. A: Chem. 226 (2005) 179-197; https://doi.org/10.1016/j.molcata.2004.10.029. Figure Caption
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Figr-14Figure captions:
Fig. 1. Schematic drawing of titanium samples (a), trans cross-section (b), and long crosssection (c). Fig. 2. Grain orientation spread (GOS) maps of titanium after hydrostatic extrusion a) transverse transversal, b) longitudinal cross-section. HAGBs are indicated by black lines, LAGBs are indicated by bright grey lines. Fig. 3. TEM BF images of titanium after hydrostatic extrusion a) transverse transversal, b) longitudinal cross-section and c) selected area diffraction pattern from transverse cross section indexed as Ti α. Fig. 4. Histograms showing distribution of crystallite sizes in titanium samples after hydrostatic extrusion, a) short axes in trans cross-section, b) short axes in long cross-section, c) long axes in trans cross-section, d) long axes in long cross-section. Fig. 5. Inverse pole figures (normal direction) calculated for titanium samples from discrete EBSD crystallographic data: a) transverse transversal cross-section - before HE, b) transverse transversal cross-section – after HE, c) longitudinal cross-section – after HE – coarse-grained fraction, d) longitudinal cross-section – after HE – fine-grained fraction. Fig. 6. Open circuit potential evolution in the first two hours, for tested Ti samples in a solution of artificial saliva of pH 7.6. Fig. 7. Polarisation curves in semi-logarithmic coordinates for tested Ti samples in a solution of artificial saliva of pH 7.6. Fig. 8. Comparison of the impedance response of Ti grade 2 samples Trans_HE and Long_HE. Fig. 9. Equivalent circuits fitted to the EIS data. Fig. 10. The pH values of artificial saliva containing titanium samples (initial and HE processed), and blank solution for 90 days.
15
Figr-1
16
Figr-2
17
Figr-3
18
Figr-4
19
Figr-5
20
Figr-6
21
Figr-7
22
Figr-8
23
Figr-9
24
Figr-10
25
Table 1. Statistical microstructure parameters. Trans_HE
Long_HE
Overall
Finegrained fraction
Coarsegrained fraction
Overall
Finegrained fraction
Coarsegrained fraction
Structure fraction
-
84%
12%
-
48%
48%
Avg. grain size (diameter) [µm] LAGB density [1/μm]
0.57 (SD 0.47) 3.30
0.42 (SD 0.20) 3.11
1.59 (SD 0.53) 2.78
2.00 (SD 2.21) 3.51
0.57 (SD 0.21) 3.15
3.42 (SD 2.38) 2.77
HAGB density [1/μm]
5.26
5.60
0.15
2.50
2.95
0.46
26
Table 2. Corrosion parameters of titanium samples determined in a solution of artificial saliva
Ti specimen code
OCP mV/SCE
Ecorr, mV/SCE
icorr, nA/cm2
ba mV/dec
bc mV/dec
Rp Ω⋅cm2
Trans_Init
-373
-383
156
132
119
134674
Long_Init
-384
-394
149
125
120
123455
Trans_HE
-230
-248
147
116
112
126644
Long_HE
-146
-168
130
124
106
237975
27
Table 3. The parameters of specific elements of equivalent circuits fitted to EIS spectra
Rs Ti sample code
Rct
CPEdl
Rf
cm
cm
s cm
Trans_HE
17
9997
3·10-5
Long_HE
17
10490
6·10-5
2
2
N
-2
28
Ndl
CPEf Nf
χ2
Rp
cm
s cm
0.93
102432
3·10-5
0.92
0.004
112429
0.98
116864
4·10-5
0.93
0.003
127354
2
N
-2
cm2