Exploring the influence of the microstructure on the 2 passive layer chemistry and breakdown for some 3 titanium-based alloys in normal saline solution 4

The effect of microstructure and chemistry of passive films on the kinetics of passive 24 layer growth and passivity breakdown of some Ti-based alloys, namely Ti-6Al-4V, Ti-6Al-7Nb and 25 TC21 alloys was studied. The rate of pitting corrosion was evaluated using cyclic polarization 26 measurements. Chronoamperometry was applied to assess the passive layer growth kinetics and 27 breakdown. Microstructure influence on the uniform corrosion rate of these alloys was also 28 investigated employing Tafel extrapolation and dynamic electrochemical impedance spectroscopy. 29 Corrosion studies were performed in 0.9% NaCl solution at 37 oC, and the obtained results were 30 compared with ultrapure Ti (99.99%). The different phases of the microstructure were 31 characterized by X-ray diffraction and scanning electron microscopy. Chemical composition and 32 chemistry of the corroded surfaces were studied using X-ray photoelectron analysis. For all studied 33 alloys, the microstructure consisted of α matrix, which was strengthened by β phase. The highest 34 and the lowest values of the β phase’s volume fraction were recorded for TC21 and Ti-Al-Nb alloys, 35 respectively. The uniform corrosion rate and pitting corrosion resistance (Rpit) of the studied alloys 36 were enhanced following the sequence: Ti-6Al-7Nb < Ti-6Al-4V << TC21. The corrosion resistance 37 of Ti-Al-Nb alloy approached that of pure Ti. The obvious changes in the microstructure of these 38 alloys, together with XPS findings, were adopted to interpret the pronounced variation in their 39 corrosion rates. 40


Introduction
Titanium and its alloys are widely used in many industrial applications, because of their highly desirable properties, including very good mechanical properties, excellent corrosion and erosion resistance, and favorable strength to weight ratios [1].In fact, titanium and its alloys have experienced increased use in the past years as biomaterials, because of their superior biocompatibility, high resistance to localized and generalized corrosion, and their good mechanical properties (fatigue resistance) [2].Among all titanium and its alloys, the commonly used materials in biomedical area are commercially pure titanium (cp Ti) and its (+ ) Ti6-Al4-V alloy [3][4][5].
Next to biomedical applications, aerospace sector has dominated titanium use, instead of heavy steel components, in fabricating crucial and decisive systems such as airfoils and airframes [6][7][8][9].About 50% of titanium used in the aerospace industry is the (α+β) alloy Ti-6Al-4V.This alloy possesses a perfect combination of operational and technological properties [10,11].Titanium alloys have also found widespread applications in a variety of fields such as in chemical and petrochemical sectors due to their excellent corrosion resistance [12].The outstanding characteristics (such as high specific strength, high fatigue strength, good corrosion resistance, etc.) of the titanium alloys (particularly Ti-6Al-4V) are attributed to a very stable native oxide film (1.5 -10 nm) formed on the Ti and Ti-alloy surface upon exposure to atmosphere and/or aqueous environments [13,14].
However, this thin oxide layer can be damaged and thus strongly impacts the bioactivity and other characteristics of the material.To improve the performance of Ti and Ti-alloys for biomedical and aerospace applications, oxidation (anodization) has been applied as a successful approach to improve the material properties [15].
The microstructure, formed during various processing methods, is found to greatly affect the mechanical properties of titanium alloys [16].The microstructure type (bimodal, lamellar and equiaxed) affects the mechanical properties of Ti based alloys [17].Even though, the corrosion of Ti-alloys in different environments was previously studied [18,19], to the best of our knowledge, literature data revealed no reports concerning the passive layer growth kinetics and breakdown, and subsequent initiation and propagation of pitting corrosion over the surfaces of Ti-6Al-7Nb, Ti-6Al-4V, and TC21 alloys.For this reason, the main objective of this work is to shed more light on the pitting corrosion characteristics of these alloys, employing cyclic polarization and chronoamperometry measurements.In addition, the uniform corrosion behavior of these alloys was also studied based on Tafel extrapolation and EIS methods.All measurements were conducted in 0.9% NaCl solution at 37 o C.

Materials and Methods
The working electrodes investigated in this study consist of three Ti-based alloys, namely Ti-6Al-4V, Ti-6Al-7Nb and TC21; their chemical compositions are presented in Table 1.The microstructure of these alloys was studied by Meiji optical microscope fitted with a digital camera.A JEOL JSM5410 and Hitachi S-3400N scanning electron microscopes (SEM) were also used for microstructure studies.For this purpose, the specimens were prepared following ASTM E3-11 standard metallographic procedures, and then etched in a mixture of 5 mL HNO3, 10 mL HF and 85 mL H2O.The alloys were machined in the form of rods to perform electrochemical measurements.These rods were mounted in a polyester resin offering an active cross-sectional area of ~ 0.2 cm 2 .
Prior to conducting any electrochemical analysis, the surface of the working electrode was cleaned and polished using a silicon carbide paper (600-grit) installed on a polishing machine (Minitech 233).
The surface was then washed in distilled water.Finally, an absolute ethanol was used for rinsing.Ti-6Al-4V 5.85 3.94 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.02 0.03 0.14 Bal.Ti-6Al-7Nb 6.39 0.00 7.78 0.00 0.00 0.00 0.00 0.00 0.00 0.02 0.04 0.12 TC21 plots via sweeping the electrode potential around the Tafel potential (E = Ecorr ± 250 mV), applying a sweep rate of 1.0 mV s -1 .After that, the electrode is removed from the cell (which is cleaned properly and re-filled up with a new fresh test solution), cleaned and polished up to the mirror finish, as described above, and then inserted in the cell for cyclic polarization measurements.
Chronoamperometry technique was also applied using a new set of cleaned and polished electrodes submerged in a cleaned cell filled with a new fresh solution.
Prior to performing cyclic polarization technique, the working electrode is allowed to stabilize at the rest potential for 2 h, then swept linearly, with a sweep rate of 1.0 mV s -1 , starting from a cathodic potential of -2.0 V vs. SCE till +8.0 V vs. SCE.The potential sweep was then reversed back with the same sweep rate to reach the start point again thus, forming one complete cycle.To conduct chronoamperometry (current vs. time) measurements, a two-step route was applied.The working electrode is first held at a starting cathodic potential of -2.0 V vs. SCE for 60 s, then polarized towards the anodic direction with a sweep rate of 1.0 mV s -1 till the required anodic potential (Ea).
Finally, the anodic current was measured versus time (5.0 min) by holding the working electrode at Ea.To ensure results' reproducibility, each run was repeated at least three times, where mean values of the various electrochemical parameters and their standard deviations were calculated and reported.
The XRD diffraction patterns were collected for the bulk samples using a SmartLab SE (Rigaku Americas Corporation, USA) X-ray diffractometer with Cu Kα (λ =

Microstructure investigation
Based on the morphology of  phase, the microstructure of titanium alloys can be classified into equiaxed, lamellar and bi-modal microstructures [20].Moreover, the volume fraction of β phase in the microstructure of TC21 alloy is higher than that in the Ti-Al-V and Ti-Al-Nb alloys' microstructure, as depicted in Fig. 1.To further assess the influence of chemical composition on the microstructure and volume fraction of  and  phases, [Al]eq and [Mo]eq were calculated, where [Al]eq and [Mo]eq represent the alloying elements form  and  phases [5,22].Table 3 illustrates the calculated values of [Al]eq and [Mo]eq for the tested Ti-based alloys, following Eqs. 1 and 2 [5,22].
[Al]eq.= [Al] + 0.33[Sn] + 0.17 Table 3 -[Al]eq and [Mo]eq for the investigated alloys [22,23].depicted in Tables 4 and 5. From the results in Tables 4 and 5, it is evident that the Ti, Al, Sn and Zr elements tend to segregate to α phase than to β phase [23].However, V, Nb, Cr and Mo are β forming elements [24], meaning that higher ratios of these elements are found in β phase rather than in α phase.The line analysis through β phase is shown in

X-Ray diffraction studies
Phase identification was performed by X-ray diffraction (XRD) patterns to define the phases comprising each alloy sample.The diffraction patterns recorded for the studied alloys are compared all together in Fig. 3.The phases were identified by matching the characteristic peaks with the JCPDS files [25].The phases α-Ti (JCPDS#00-044-1294), β-Ti (JCPDS#00-044-1288) were common and dominated the composition of the three studied alloys.The Ti-Al-V and TC21 alloys were found to contain solely α-Ti and β-Ti phases, respectively.On the other hand, Ti-Al-Nb alloy contained some Ti and Nb oxides, TiO (JCPDS#00-008-0117) and Nb6O (JCPDS#00-015-0258).An effective procedure for the simultaneous refinement of structural and microstructural parameters based on the integration of Fourier analysis for broadened peaks in the Rietveld method was first proposed by Lutterutti et al. [26] and is implemented in the Maud program [27].
Consequently, weight percent (wt.%), lattice parameters, isotropic crystallite size (D) and r.m.s microstrain (µε) were then regarded as fitting parameters in the Rietveld adjustments and were refined altogether simultaneously.The structural information for all the refined phases was obtained from the ICSD database [28].The results obtained for the structural and microstructural analysis are summarized in Table 6 for all alloys.It is worth to mention here that, all studied alloys were characterized with considerable degree of preferred orientation which strongly modified the relative intensities of the Bragg reflections, especially for α-Ti and β-Ti phases.The MAUD program also incorporates correction for preferred orientation [29,30] in the Rietveld adjustments in order to obtain the best fitting parameters.
The calculated diffraction patterns from the Rietveld adjustment are plotted with the observed ones for the three alloys in  This can be attributed to the behavior of the preferred orientation of the α-Ti phase observed for the reflection (100), which was relatively stronger for the Ti-6Al-4V and Ti6Al7Nb alloys than in the TC21 alloy.
As can be seen from Table 6, the last two alloys, Ti-Al-Nb and TC21, contain relatively higher portions of β-Ti than α-Ti in contrast to the first alloy, Ti-Al-V, which has α-Ti content higher than β-Ti.As known from literature, Al is an α-stabilizing while V, Nb, Mo and Fe are β-stabilizing.
Nevertheless, the results indicate that Nb, Mo and Fe have stronger capabilities to stabilize β-Ti phase than V.These findings corroborate microstructural studies (revisit section 3.1)., which revealed that TC21 alloy recorded a jcorr value of 0.32 mA cm -2 , which is 940, 640, and 320 times greater than those measured for pure Ti (3.4 × 10 -4 mA cm -2 ), Ti-Al-Nb (5 × 10 -4 mA cm -2 ), and Ti-Al-V alloys (10 -3 mA cm -2 ), respectively.These findings reveal that the rate of the uniform corrosion of the studied alloys increases following the order: Ti < Ti-Al-Nb < Ti-Al-V << TC21.EIS measurements were also conducted at the respective Ecorr throughout the exposure in 0.9% NaCl solution at 37 o C to confirm the polarization data and to assess the kinetics of the uniform corrosion process on the surfaces of the tested alloys.Fig. 6 displays the impedance plots in Nyquist projection, recorded for the studied alloys.Pure Ti (99.99%) was also included for comparison.
Plotting time of exposure on X-axis of impedance diagrams allowed for monitoring of uniform corrosion susceptibility [31][32][33].It can be observed that in each case the impedance spectra recorded at day 1 (after initial 120 min of conditioning) were highly scattered due to non-stationary conditions at the metal/electrolyte interface, which is a common problem in EIS measurements.This issue became negligible after a few hours of exposure.For this reason, results recorded at day 2-7 will be taken for further analysis.The impedance loop appeared as an open arc with a big diameter (charge-transfer resistance, Rct), hence the overall corrosion resistance of each investigated alloy is very high.Ti-6Al-4V and Ti-6Al-7Nb alloys seem to be stable over time of the exposure, with Ti-6Al-7Nb alloy being more corrosion resistant than Ti-6Al-4V alloy, while the results obtained for TC21 reveal gradual decrease of the impedance loop, corroborating DC electrochemical studies.All impedance plots showed a single time constant (capacitive loop), which can be verified on the corresponding Bode plots after 7 days of exposure (Fig. 7).An electric equivalent circuit  The aforementioned capacitance dispersion may in particular originate from geometric heterogeneity (pits, scratches, porosity) as well as diversified surface electric properties due to adsorption processes of passive layer breakdown [35].The CPE impedance ZCPE = (Q(j) n ) -1 represents a capacitor with capacitance 1/Q for a homogeneous surface n  1.Thus, it is often believed that CPE component n is the heterogeneity factor and its variation can be monitored.CPE describes quasi-capacitive behaviour of passive layer in case of its double layer perforation.The effective capacitance Ceff can be calculated on the base of CPE using Hirschorn's model for surface distribution of time constants [36].The EEC can be schematically written as RS(QRCT), where RS is electrolyte resistance.The aforementioned single time-constant EEC covers all the applied frequency range.conditions.The presence of stable corrosion pits would be visible in a form of rapid increase in Ceff [36,37] (likely observed at TC21 alloy at day 4).
The initial value of CPE exponent n depends on factors such as surface phase distribution and geometric defects remaining as a result of polishing.Its decrease throughout the exposure in corrosive electrolyte reflects the appearance of heterogeneities on analyzed sample surface, which in this case is primarily associated with initial phases of corrosion pits formation (see Fig. 8c).This effect is clearly seen on SEM micrographs further in the manuscript.Notably, the value of n factor of Ti-6Al-7Nb alloy was both: the highest as well as the least affected by exposure in corrosive media.
The aforementioned observation indicates high surface homogeneity, which may be the reason behind outstanding corrosion resistance of this alloy.The polarization curve of TC21 alloy exhibits active dissolution near Ecorr, followed by an obvious enhancement in the anodic current with the applied potential due to thinning and weakening of the passive layer as a result of the aggressive attack of Cl -anions.Also, Ti-Al-Nb and Ti-Al-V alloys show active dissolution near Ecorr, but to a much lower extent than TC21, and in addition, tend to passivate with a very low current (passive current, jpass) covering a wide range of potential.These findings reflect the weaker passivity of TC21 and its higher tendency to corrode in this solution than Ti-Al-V and Ti-Al-Nb alloys.On the contrary, as expected, the anodic polarization curve of pure Ti exhibits typical passivity near Ecorr, referring to its high corrosion resistance.
Passivity of the studied alloys persists up to a certain critical potential, designated here as the pitting potential (Epit).Remarkable changes occurr within the passive region at potentials exceeding Epit.These involve a sudden increase in jpass and formation of a hysteresis loop on the reverse potential scan.These events are a clear sign for passivity breakdown, and initiation and propagation of pitting corrosion.
In general, the electrochemical systems suffering from pitting corrosion are characterized by a hysteresis loop in their cyclic voltammograms.Such a loop refers to the continuation of pitting corrosion even after potential scan reversal (pitting corrosion's autocatalytic nature), indicating repassivation delay of the existing pits.Repassivation is only achieved when the reverse scan intersects the forward one within the passive region in a point designated here as the repassivation potential, Erp, below which the working electrode is immune against pitting.Others defined it as the potential below which no pit could grow, or in other words, a pit once initiated, will stop [38].
A current intermission can be seen on the reverse scan of the three tested alloys.This current discontinuity is quite clear on the reverse scan of the TC21 alloy, and can be observed for alloys Ti-Al-Nb and Ti-Al-V in the inset of Fig. 9 (a1).It takes place at two distinct potentials (E 1 ptp and E 2 ptp), designated here as the pit transition potential (Eptp), the potential between bare and salt-covered state, where diffusion control dominates beyond Eptp [38].We previously reported similar findings during pitting corrosion studies of Zn in nitrite solutions [39] and recently by Zakeri et al. [40], who explored the transition potential and the repassivation potential of AISI type 316 stainless steel in chloride containing media devoid of and containing 0.01 M thiosulfate.
At potentials beyond Eptp, the rate of anodic dissolution is diffusion-controlled (controlled by diffusion of metal cations from the salt/pit solution interface into the bulk solution) [38][39][40].Such a current transient relationship, when satisfied, refers to an anodic diffusion control process [40].On reversing the potential scan, the thickness of the salt (pitting corrosion product) film diminishes.
This decrease in salt film thickness enhances with back scanning till a certain potential is reached at which the cations' concentration decreases below the saturated concentration.At this stage, salt precipitation is stopped, and the remaining metal salt film will be dissolved, making the bottom of pits free from salt film.This in turn will establish an ohmic/activation control (a linear decrease of current density with potential) regime.
Ti-Al-Nb alloy's passivity seems stronger and more stable that of the Ti-Al-V alloy, Fig. 9 (b).
The latter is characterized by a higher jpass which enhances with potential till its Epit, which attained ~ 50 mV vs. SCE before that of the former.In addition, the pits existing on the surface of Ti-Al-V alloy find it much more difficult to repassivate than those on the surface of Ti-Al-Nb alloy, as the hysteresis loop of the former is much larger than that of the later.
Another important pitting corrosion controlling electrochemical parameter is the pitting corrosion resistance, Rpit (Rpit = |Ecorr -Epit|), which defines the resistance against the nucleation of new pits [38].Referring to Fig. 9 (b), it is clear that Rpit increases following the order: TC21 << Ti-Al-V < Ti-Al-Nb.The resistance against growth of the pits also controls the susceptibility toward pitting corrosion.This can be evaluated via comparing the areas of the hysteresis loops formed during the reverse potential scan of the cyclic polarization curves in its linear format, Fig. 9

(a1).
A specific routine of the Software was used to calculate the areas of the hysteresis loops, related to the charge consumed during the growth of such already formed pits.Here again, the hysteresis loop of the TC21 alloy recorded the highest area (charge consumed) among the studied alloys, while the lowest value of the hysteresis loop's charge consumed during was measured for Ti-Al-Nb alloy.
This in turn ranks the resistance against the growth of pre-existing pits as Ti-Al-Nb > Ti-Al-V >> TC21.These findings mean that replacing V by Nb in Ti-Al-V alloy promotes alloy's repassivation thus, enhancing its pitting corrosion resistance.declines with a rate depending upon chemical composition of the tested alloy, denoting passive layer electroformation and growth [39].This decay in current then reaches a steady-state value (jss), an almost constant passive current related to jpass (revisit Fig. 9), constituting the 2 nd stage of the current.The constancy of jss originates from a balance between the rates of the passive layer growth (current builds up) and its dissolution (current decays) [41,42].were obtained.Similar results were previously obtained in our lab [42,44].Stage I referred to the passive layer electroformation and growth, as its current falls with time [41,42,44].This stage, namely stage I ends at a certain time (ti), the incubation time, where stage I's current reached its minimum value; ti is defined as the time the adsorbed aggressive Cl -anions must acquire to locally attack and subsequently remove the passive oxide film [41].The magnitude of ti, more specifically its reciprocal value (1/ti), denotes the rate of pit initiation and growth [41,42], and measures the susceptibility of the oxide film to breakdown and initiate pit formation and growth.
Stage II begins at ti and terminates at another time τ, and its current is termed jpit (pit growth current density).jpit increases from the moment just after ti and continues in growth till τ, suggesting that the pit formation and growth dominate over passivation during this stage.Ultimately, jpit attained a steady-state just after the time τ, denoting the onset of stage III, and continues almost constant till the end of the run.The constancy of the stage III's current was attributed to the hindrance of the current flow (jpit) through the pits sealed off by the pitting corrosion products formed during the events of stage II, namely pit initiation and growth [42,44].This hindrance in jpit is balanced by a current increase due to metal dissolution, thus yielding an overall steady-state current.
Close inspection of Fig. 10 reveals that jpit increases and ti gets shorter, thus referring to accelerated pitting attack, in presence of alloyed V.These results again support the catalytic impact of alloyed V towards pitting corrosion After one-week exposure, investigated samples were reexamined using SEM in order to evaluate the susceptibility to pitting corrosion.This procedure was carried out after rinsing in ethanol using ultrasonic cleaner.The results of the analysis are exhibited on Fig. 11.Defects start to appear at the surface of each analyzed sample throughout the exposure.The micrographs in the inset of Fig. 11 were taken using back-scatter electrons (BSE) in topography mode.This allowed to bring out the geometry of aforementioned defects.As can be seen, each analyzed defect forms shape of a bulge above alloy's surface, testifying for either repassivation once formed shallow corrosion pits or at an early, preliminary stage of passive layer degradation.Ti-6Al-4V sample is characterized with both the highest amount and the largest defects, reaching 30 m in diameter.On the other hand, the surface of pure Ti and Ti-6Al-7Nb appeared the most intact.No real corrosion pits were observed on the surface of either investigated alloy at the end of exposure in 0.9% NaCl solution at 37 °C, testifying for the overall high pitting corrosion resistance.Nevertheless, the passive layer must have weakened hence it was possible for corrosion products to adsorb on the metal surface.EDS analysis was carried out for defects observed on each investigated alloy in order to qualify their chemical constitution.The exemplary results, obtained for Ti-6Al-7Nb alloy are summarized on Figure S4 (Supporting Information), while the chemistry of defects observed for each investigated alloy were similar.The defects are primarily composed of carbon and oxygen, most likely forming metal carbonates typical for early pitting corrosion stages [45].Small amount of chlorine was also recorded within defects.Its low amount is distorted by EDS depth of analysis ranging few microns.

Surface morphology and composition
The chemistry of the passive layer in each examined case is composed primarily of titanium (IV) oxides, as verified by a strong recorded Ti2p peak doublet, with Ti2p3/2 component located each time at 458.6 eV [35,46,47], Fig. 12.Furthermore, there is no sign of titanium oxides at lower oxidation states corroborating the aforementioned result.Besides the titanium, other alloying additives also take part in passivation process.The strongest signal among the alloying additives was recorded for aluminum oxide Al2O3 (Al2p3/2 peak at 74.5 eV), ranging between 3.5 and 3.8 at.% for each sample [48,49].The contribution of VO2 (V2p3/2 at 516.4 eV) in Ti-Al-V and Nb2O5 (Nb3d5/2 at 207.1 eV) in Ti-Al-Nb alloy did not exceed 0.7 at.% [47,50,51].The passive film formed on the surface of TC21 alloy is naturally more complex.Besides TiO2, it is composed of Al2O3 (3.8 at.%),Nb2O5 (0.3 at.%),ZrO2 (0.4 at.%,Zr3d5/2 at 182.4 eV), Cr2O3 (0.8 at.%,Cr2p3/2 at 576.0 eV), SnO2 (0.1 at.%,Sn3d3/2 at 486.5 eV), MoO3 and MoO2 (0.2+0.2 at.%,Mo3d5/2 at 232.9 and 229.2 eV, respectively) [52][53][54][55].8. Metal chlorides were found on the surface of each investigated sample, which confirms metal-chlorine covalent bond formation, testified by a peak doublet: Cl2p3/2 at 198.9 eV [35,42,56].Nevertheless, the amount of adsorbed chlorides is nearly 2.5 times higher for the TC21 alloy than pure titanium.The chloride concentration obtained for highly resistant Ti-Al-Nb alloy is nearly on par with Ti sample, and slightly smaller than in the case of Ti-Al-V.An interesting conclusion may be drawn based on O1s peak analysis for each investigated sample.The spectra were conventionally deconvoluted into three components.Two dominant components located at 530.2 and 531.6 eV are ascribed to Me-O and Me-OH species, respectively.The second component intensity may be further influenced by presence of C-O bonds in carbonates.Its formation may result from prolonged electrolyte exposure but also adventitious carbon formation due to air exposure [48,51].The finding regarding carbonates adsorption on the metal surface is further confirmed by third O1s component at 532.8 eV, typical for C=O bonds but also chemisorbed water molecules.For clarity purposes the analysis excluded data recorded for carbon C1s, which was found in large amounts, up to 30 at.%, at binding energies corroborating adventitious carbon and carbonates findings.Importantly, the highest amount of the adsorbed carbonate species was found on Ti-Al-V sample surface, which is in very good agreement with SEM micrographs presented on Fig. 11.The least amount of carbonate species was once more found on the surface of Ti sample.
The microstructure of Ti-based alloys can be controlled based on their chemical composition, or in other words, based on the balance between the  phase stabilizing elements, such as Al, Sn and O, and the forming  phase elements like V, Mo and Nb [21].As shown in Fig. 1, the microstructure of all studied titanium alloys consists of bimodal structure of / phases.The initial microstructure of Ti-Al-V and Ti-Al-Nb alloys in as-received (forged) state is represented by equiaxed grains of primary α-phase (dark), as well as β-transformed structure (light), as it can be seen in Fig. 1.The  phase formed in the microstructure of both alloys is globular in shape, but seems larger in size in Ti-Al-V alloy than in Ti-Al-V alloy.The particle size of  phase in Al-Ti-V alloy is about 0.5 to 1.5 m; however its size in Ti-Al-Nb alloy reaches about 0.25 to 1 m, as shown in Fig. 1 (a) and (b).

Figure 2 -
Figure 2 -Morphology of  phase in TC21 alloy: (a) acicular-like structure and (b) blocky shaped structure eq. = [Mo] + 0.2[Ta] + 0.28[Nb] + 0.4[W] + 0.67[V] +1.25[Cr] + 1.25[Ni] + + 1.7[Mn] + 1.7[Co] + 2.5[Fe] ) and (/) ratios are maximum in case of TiAlNb alloy, and minimum for the TC21 alloy.The chemical composition of both phases in all microstructures of the investigated alloys was analyzed using the EDS unit attached to SEM.The EDS spectrum recorded for β phase in the microstructure of Ti-Al-V alloy is depicted in FigureS1(b) (Supporting Information).The location of the area of analysis is illustrated in FigureS1 (a).The highest peak in the spectrum belongs to the base metal (Ti), in addition to some other peaks from Al and V alloying elements.Similarly, the analyses of the two phases in other microstructures were accomplished.Results of these analyses are Preprints (www.preprints.org)| NOT PEER-REVIEWED | Posted: 7 March 2019 Preprints (www.preprints.org)| NOT PEER-REVIEWED | Posted: 7 March 2019 doi:10.20944/preprints201903.0084.v1Peer-reviewed version available at Materials 2019, 12, 1233; doi:10.3390/ma12081233 Figure S2(Supporting Information), the area of α phase is colored with a combination of red, yellow and pink, however the β phase area, with lower Ti content, has a dark color, see FigureS3(b), Supporting Information.The contrast for the partitioning of Al alloying element between α and β phases is not as clear as in the case of Ti element, Fig.S3 (c), Supporting Information.In addition to the distribution of Ti and Al, the segregation of Nb alloying element, with lower percentage, to α phase is represented by a combination of green, blue and black colors, while the area of the β phase with higher Nb content is decorated by a mixture of red and white colors, as shown in FigureS3 (d), Supporting Information.

Fig. 4 .
The average R-values obtained for the refinements were about Rwp(%) = 24 to 27 and Rb(%) = 15 to 20.The simultaneous refinements of both structural and microstructural parameters produced good matching of the calculated to observed profiles of diffracted intensities.Also, the incorporation of the preferred orientation models enabled to account for the variations of the peak intensities of α and β-Ti phases.

Figure 4 -
Figure 4 -The calculated (red line) and observed (black dots) diffraction patterns for the three alloys as obtained from the Rietveld adjustments using the MAUD program; the positions of the Bragg reflections of each phase and the difference between the calculated and observed patterns are also presented at the bottom.

3. 3 . Electrochemical measurements 3 . 3 . 1 .Figure 5
Figure5illustrates the Tafel plots for the cathodic and anodic domains for the studied alloys in comparison with pure Ti, after 7 days of immersion in 0.9% NaCl solution at 37 o C. Table7depicts the various electrochemical parameters derived from such polarization measurements.It follows from Fig.5that, among the studied alloys, TC21 alloy exhibited the lowest cathodic and anodic overpotentials, corresponding to increased corrosion current density (jcorr) values.This is clear from Table7, which revealed that TC21 alloy recorded a jcorr value of 0.32 mA cm -2 , which is 940, 640, and

Figure 5 -Table 7 -
Figure 5 -Cathodic and anodic polarization curves recorded for the three tested alloys in comparison with pure Ti, after 7 days of exposure in 0.9% NaCl solution at a scan rate of 0.5 mV s -1 at 37 o C.

Figure 6 -
Figure 6 -Nyquist impedance plots recorded for the three tested alloys in comparison with pure Ti at the respective Ecorr in 0.9 % NaCl solution at 37 o C. The changes in spectra shape with exposure time (7 days) can be tracked on X-axis.
(EEC) was proposed to analyze the impedance results.Due to absence of additional time constants, a simple Randles circuit was proposed with constant phase element (CPE) selected instead of capacitance to take into consideration surface distribution of capacitance dispersion.The parallel resistance represents the charge-transfer resistance RCT through the metal/electrolyte interface [34].

Figure 7 -
Figure 7 -Bode plots of each investigated alloy on the day 7 of exposure in comparison with pure Ti (99.99%) at the respective Ecorr in 0.9 % NaCl solution at 37 o C.

Fig. 8
Fig.8depicts the electric parameters obtained on the base of RS(QRCT) EEC and their changes during the one-week long exposure.The higher the RCT the lower the corrosion current density, offering an easy comparison in uniform corrosion resistance of investigated alloys, see Fig.8a.Each investigated alloy is characterized with very high resistance, range of M, owing to a presence of a passive layer tightly covering metal surface.Nevertheless, for TC21 alloy, RCT value is one order of magnitude lower and slowly but consistently decreases throughout the exposure, revealing its lower corrosion resistance.High scatter of RCT value is inversely proportional to measured fraction of impedance semicircle (as seen on Fig.6).

Figure 8 -
Figure 8 -Charge transfer resistance RCT, effective capacitance Ceff and CPE exponent n calculated on the base of RS (QRCT) EEC for each investigated alloy.The one-week long exposure was carried out in 0.9 % NaCl solution at 37 o C.

Figure 9
Figure9shows typical cyclic polarization curves in the linear (E vs. j) and logarithmic (E vs. log j) formats recorded for the studied alloys between −2.0 V and +8.0 V (SCE).Measurements were conducted in 0.9% NaCl solution at a scan rate of 5.0 mV s -1 at 37 o C. The logarithmic form of these curves (E vs. log j), Fig.9 (b), is also constructed to define precisely the location of the pitting potential (Epit) and repassivation potential (Erp) versus the corrosion potential (Ecorr).Fig. 9 (a1) is zoomed with a very narrow range of current around the cathodic and anodic processes covering the whole studied potential range to yield Fig. 9 (a2).It follows from Fig. 9 (a2) that, for all tested samples, the cathodic current density diminishes progressively reaching its zero value at the corrosion potential (Ecorr).

Figure 9 -
Figure 9 -Linear, (a1) and (a2), and logarithmic (b) cyclic polarization curves recorded for the studied alloys in 0.9% NaCl solutions at a scan rate of 1.0 mV s -1 at 37 o C.

3. 3 . 4 .
Chronoamperometry measurementsChronoamperometry (j/t) measurements were also carried out to confirm the above results and gain more information about the influence of alloyed V and Nb on the passive layer growth kinetics and breakdown.Figs.10 (a) and 10 (b) depict the j/t curves measured for the tested alloys at two different Ea values, far below and close to Eb. Measurements were conducted in 0.9% NaCl solution at 37 o C. The profile of the obtained curves is found to vary according to the chemical composition of the studied alloy and the position of Ea versus Eb.When Ea is located far cathodic to Eb, a j/t profile Preprints (www.preprints.org)| NOT PEER-REVIEWED | Posted: 7 March 2019 Preprints (www.preprints.org)| NOT PEER-REVIEWED | Posted: 7 March 2019 doi:10.20944/preprints201903.0084.v1Peer-reviewed version available at Materials 2019, 12, 1233; doi:10.3390/ma12081233withtwo stages is obtained, as shown in Fig.10 (a).During the first stage, the anodic current (ja)

Figure 11 -
Figure 11 -SEM micrographs taken in secondary electron mode for each investigated sample: (a) pure Ti as a reference, (b) TC21 alloy, (c) Ti-6Al-4V, (d) Ti-6Al-7Nb at the end of one-week long exposure in 0.9% NaCl at 37 °C.Magnification ×500.In the inset back-scatter electron topography mode images of selected surface defects.Magnification ×2000.

Figure 12 -
Figure 12 -High-resolution XPS spectra recorded in Ti2p, Cl2p and O1s energy range for each investigated alloy after 7 days of exposure to 0.9% NaCl solution at 37 o C.

Table 1 -
Chemical composition of investigated Ti alloys

Table 2
illustrates the volume fraction of α and β phases in the microstructure of the studied titanium alloys.The microstructure of pure Ti has the highest volume fraction of the  phase (~100%) and the lowest volume fraction of  phase (~0.0%).The presence of Al (-phase stabilizer) and V (-phase stabilizer) as alloying elements in the chemical composition of Ti-Al-V alloy influence the volume fraction of  and  phases.The values of the volume fractions of  and  phases (

Table 2 .
The volume fraction of both phases in the microstructure of TC21 alloy is also altered, most probably due to the mutual combination of the alloying elements of that alloy, revisit

Table 1 .
The volume fractions of  and  phases in the microstructure of TC21 alloy recorded almost equal values, namely 48% for  phase and 52% for  phase (Table2).

Table 2 -
Volume fraction of α and β phases in the investigated Ti based alloys.

[Al]eq [Mo]eq Ratio
It follows from Table3that TiAlNb and TC21 alloys recorded the highest values of [Al]eq, 8.59, while the lowest values were measured for the TiAlV alloy, 8.05.Additionally, TiAlNb alloy achieved the maximum value of [Mo]eq, 3.94, whilst TC21 alloy recorded 1.71.Table3also depicts the ratio [Al]eq/[Mo]eq for the tested alloys.TiAlNb alloy displayed the maximum ratio, 3.94, while a minimum ratio of 1.71 was measured for the TC21 alloy.It is obvious that the results obtained from Table 3 agree well with the results in Table 2.The calculated ([Al]eq /[Mo]eq

Table 4 -
Chemical composition (wt %) of different phases in Ti-Al-V and Ti-Al-Nb alloys.

Table 5 -
Chemical composition (at %) of different phases in TC21 alloy.

Table 8 -
XPS deconvolution results carried out in Ti2p, Cl2p and O1s energy range after 7 days of exposure to 0.9% NaCl solution at 37 o C (in at.%).