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Vol. 8. Issue 5.
Pages 4302-4311 (September - October 2019)
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Vol. 8. Issue 5.
Pages 4302-4311 (September - October 2019)
Original Article
DOI: 10.1016/j.jmrt.2019.07.040
Open Access
Fabrication of Ti–Al–Cu new alloys by inductive sintering, characterization, and corrosion evaluation
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El-Sayed M. Sherifa,b,
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esherif@ksu.edu.sa

Corresponding author.
, Hany S. Abdoa,c, Fahamsyah H. Latiefd, Nabeel H. Alharthia,e, Sherif Zein El Abedinb
a Center of Excellence for Research in Engineering Materials (CEREM), King Saud University, PO Box 800, Al-Riyadh 11421, Saudi Arabia
b Electrochemistry and Corrosion Laboratory, Department of Physical Chemistry, National Research Centre, El-Behoth St. 33, Dokki, 12622 Cairo, Egypt
c Mechanical Design and Materials Department, Faculty of Energy Engineering, Aswan University, Aswan 81521, Egypt
d Department of Mechanical Engineering, Al Imam Mohammad Ibn Saud Islamic University (IMSIU), PO Box 5701, Riyadh 11432, Saudi Arabia
e Mechanical Engineering Department, King Saud University, PO Box 800, Al-Riyadh 11421, Saudi Arabia
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Tables (2)
Table 1. EIS data obtained for the different Ti–5Al–xCu alloys after different 1h, 24h, and 48h immersion in 3.5% NaCl solutions.
Table 2. Potentiodynamic polarization data obtained for the different Ti–5Al–xCu alloys after different 1h, 24h, and 48h immersion in 3.5% NaCl solutions.
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Abstract

The current work reports the manufacturing and electrochemical characterization of Ti–5%Al–5%Cu, Ti–5%Al–10%Cu, and Ti–5%Al–20%Cu alloys, which were fabricated using mechanical alloying technique. The corrosion of these alloys after varied immersion periods of time in 3.5% NaCl solution was studied using electrochemical impedance spectroscopy (EIS), potentiodynamic polarization, and potentiostatic current time measurements. The phase analyses of the alloys were investigated using X-ray diffraction. Scanning electron microscopy (SEM) was employed to observe the surface morphology of the corroded surfaces whereas the composition of the corrosion products formed on the alloys surfaces was examined using energy dispersive X-ray spectroscopy (EDX). The uniform corrosion of these alloys was found to greatly decrease with the increase of Cu content from 5wt% to 20wt%, this occurrence is due to decreasing the values of corrosion current (jCorr) together with corrosion rate (RCorr) and increasing the corrosion resistance (RP). Prolonging the immersion time up to 48h was found to further increase the values of RP and decrease the values of jCorr and RCorr. Both the electrochemical and the spectroscopic investigations indicated that the increase of Cu content and prolonging the immersion time have significantly decreased all corrosion parameters and increase the corrosion resistance of the alloys in NaCl solution.

Keywords:
Ti-base alloys
Corrosion
Mechanical alloying
Electrochemical techniques
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1Introduction

Various Ti alloys have become standard engineering materials for several common industrial applications. Where, Ti-base alloys are employed in marine, military and offshore applications as a result of their outstanding resistance to many corrosive media, particularly oxidizing and chloride-containing process streams, reliable mechanical properties, etc. [1–6]. Recently, the demand for high energy-efficiency and outstanding performance for offshore structures and marine industry promotes the materials selection move toward high strength, low density and superior corrosion resistance in the harsh environments such as seawater [7–11].

Furthermore, Ti alloys are considered as the most attractive metallic materials for biomedical applications. They are applied in implant devices to replace failed hard parts in the field of medicine. Nevertheless, Ti alloys have been used in various medical applications since 1970s like in the prosthetic dentistry [12]. The use of Ti alloys in dental implantation system is associated with their excellent properties such as good compatibility, high strength and the resistance against corrosion. For instance, Ti–6Al–4V alloy is so far the most applicable one among the Ti-based alloys due to its combination of physical, mechanical, and corrosion resistance, which are made them essential for demanding in the industries [13–16]. Hence, Ti and its alloys have attracted a great attention in dental applications so that nowadays commercially pure Ti is the dominant material for dental implants. Other reported representative dental Ti alloys are Ti–6Al–7Nb, Ti–6Al–4V, Ti–13Cu–4.5Ni, Ti–25Pd–5Cr, Ti–20Cr–0.2Si etc. [17].

It is well-known that Ti-base alloys have a remarkable corrosion resistance as a result of the formation of a stable, highly adherent, continuous, and protective Ti oxide films, mostly TiO2, on its surface [18]. However, this oxide film cannot withstand under anhydrous conditions in the absence of oxygen, where the oxide film is damaged and not able to be regenerated. These media include concentrated hydrochloric and sulfuric acids, sodium hydroxide and solutions containing chloride ions, which ultimately restricts its industrial applications [19–21].

Machining is one of the most difficulties that appear when dealing with Ti alloys as a result of their low thermal conductivity, which causes high cutting temperatures [17]. For that many investigators have claimed that the machining of Ti alloys is one of the main challenges for their applications [22–26]. For that there has been a need for developing new types of Ti alloys with increased machinability [27,28]. In this study, new Ti-based alloys have been produced by adding limited amounts of Cu into the Ti–5%Al alloy. Cu was believed to improve the resistance against corrosion. Therefore, the objective of this study is to fabricate a new series of Ti–5%Al–x%Cu alloys, where x=5, 10, and 20 (all in weight percentage) and also to report their corrosion behavior in 3.5% NaCl solution. The corrosion study was evaluated using various electrochemical techniques along with different spectroscopic observation and X-ray diffraction analysis.

2Experimental procedure2.1Materials, supplies, and fabrication of Ti-base alloys

The mixture of 90%Ti–5%Al–5%Cu, 85%Ti–5%Al–10%Cu and 75%Ti–5%Al–20%Cu (all in wt%) were fabricated from its initial raw powders of Ti, Al and Cu. These powders were of 99.99% in purity and were purchased for Aldrich, UK. A steel jar of 80ml capacity accommodated the powders with the required compositions and with some steel balls (the ball-to-powder ratio was 5:1). These powders were all placed in a high energy ball mill (Desktop 220V High Energy Vibratory Ball Mill that was brought from Across International Co, USA). The ball milling was conducted at a speed of 2000rpm with a milling time of 30min. The ball milling process is to obtain the homogenous distribution of the powder mixtures. After that the powder mixtures were ejected from the jar and then sintered in a graphite die with the dimensions of 1cm in diameter and 10cm in length. The die was placed in a high frequency induction heat sintering furnace (HFIHS). The sintering of the coupons was performed under a pressure of 40MPa and at a temperature of 1200°C for 5min. The sintered coupons were left to cool down in the furnace. Moreover, a salt of NaCl with 99% purity was used to prepare a solution of 3.5% NaCl, which was the corrosive medium.

2.2Electrochemical corrosion techniques

A conventional electrochemical cell with three electrodes configuration and accommodates for 300ml NaCl solution was employed for the electrochemical experiments. The different Ti–5%Al–x%Cu alloys were used as the working electrode. An Ag/AgCl was the reference electrode and a platinum wire was the counter electrode. The preparation and surface finishing of the alloys before being immersed in the NaCl solution for corrosion tests were reported elsewhere [29–31]. An Autolab Potentiostat/Galvanostat (PGSTAT302N) was employed in obtaining all electrochemical measurements. The EIS tests were measured at the corrosion potential (EOCP) value with a range of frequency started from 100000Hz to 0.10Hz was scanned as reported earlier [29–31]. The potentiodynamic polarization tests were collected via scanning the potential from the value of −800mV (Ag/AgCl) toward the positive direction up to 800mV with a scanning rate of 1.66mV/s [32,33]. The potential was scanned again in the reverse direction at the same scan rate. The current–time experiments for the different materials were obtained after soaking in NaCl solutions for 48h prior to stepping its potential at a fixed value of 0.3V (Ag/AgCl) for 30min.

2.3Surface studies

The surface studies included the use of XRD patterns, SEM and EDX analyses. The XRD test was carried out using D-8 Discover, Bruker, Germany and patterns were examined with a scanning rate of 2°/min and the different angle range was observed from 10° to 90° with locked scan type and an increment of 0.02°. The SEM images and the EDX spectra were collected at 15kV using SEM/EDX machine that was purchased from JEOL, Japan.

3Results and discussion3.1XRD patterns

The phase analyses of the powder mixture and the sintered alloy of Ti–5%Al–20%Cu as shown in Fig. 1 were determined by XRD machine. The present results show that the powder mixture of Ti–5%Al–20%Cu alloy confirmed for the presence of Ti, Al and Cu in the compacted sample. More importantly, the Cu element was represented by three prominent peaks at two theta values of 43.6°, 50.8°, and 74.4°, which are corresponding to (111), (200), and (220) planes that can be attributed to the cubic form of metallic copper structure (FCC). While for the sintered sample, the diffraction peaks corresponding to TiO2 and TiCuAl phases were detected in the sintered Ti–5%Al–20%Cu alloy (Fig. 1). The formation of the TiCuAl intermetallic phase was possibly found in Ti–Al–Cu system at high temperature [34]. Moreover, no impurities or contaminations peaks were recorded in the XRD pattern because of the preparation or mixing process.

Fig. 1.

XRD patterns of the powders mixture and sintered Ti–5%Al–20%Cu alloy, respectively.

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3.2EIS data

The Nyquist plots collected for (1) Ti–5%Al–5%alloy, (2) Ti–5%Al–10%alloy and (3) Ti–5%Al–20%Cu alloy after 1h immersion in 3.5% NaCl solutions are shown in Fig. 2. Similar curves were collected for these alloys after prolonging the immersion time in NaCl solutions for 24h and 48h, and the Nyquist spectra are proposed in Fig. 3 and Fig. 4, respectively. The EIS experimental data were fitted to the circuit model as shown in Fig. 5 and the values of the elements of the drawn circuit are summarized in Table 1. The definition of the obtained EIS elements are as following; RS, RP1, and RP2 are the solution and polarization resistances, respectively; CPEs is the constant phase elements; and Cdl is the double layer capacitor (more explanations are reported in the earlier studies [29–31,35]).

Fig. 2.

Nyquist plots of (1) Ti–5%Al–5%Cu, (2) Ti–5%Al–10%Cu, and (3) Ti–5%Al–20%Cu alloys after 1h immersion in 3.5% NaCl solutions, respectively.

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Fig. 3.

Nyquist plots of (1) Ti–5%Al–5%Cu, (2) Ti–5%Al–10%Cu, and (3) Ti–5%Al-20%Cu alloys after 24h immersion in 3.5% NaCl solutions, respectively.

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Fig. 4.

Nyquist plots of (1) Ti–5%Al–5%Cu, (2) Ti–5%Al–10%Cu, and (3) Ti–5%Al–20%Cu alloys after 48h immersion in 3.5% NaCl solutions, respectively.

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Fig. 5.

Circuit model employed to fit the impedance data.

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Table 1.

EIS data obtained for the different Ti–5Al–xCu alloys after different 1h, 24h, and 48h immersion in 3.5% NaCl solutions.

Alloy  EIS parameters
  RScm2  QRP1cm2  Cdl/Fcm−2  RP2cm2 
    YQ/Fcm−2  n       
Ti–5%Al–5%Cu (1h)  26.79  0.002573  0.42  440.33  0.003220  545.0 
Ti–5%Al–10%Cu (1h)  33.24  0.001283  0.80  590.82  0.000343  802.2 
Ti–5%Al–20%Cu (1h)  50.2  0.000734  0.80  3189.1  0.000314  3029.1 
Ti–5%Al–5%Cu (24h)  29.97  0.000962  0.32  986.50  0.036810  457.0 
Ti–5%Al–10%Cu (24h)  35.45  0.000776  0.69  1757.1  0.001254  1360.7 
Ti–5%Al–20%Cu (24h)  65.88  0.000461  0.65  3934.3  0.000509  19950 
Ti–5%Al–5%Cu (48h)  33.46  0.000332  0.23  822.52  0.001680  419.1 
Ti–5%Al–10%Cu (48h)  49.71  0.000315  0.63  1232.2  0.000394  7232.2 
Ti–5%Al–20%Cu (48h)  70.77  0.000218  0.66  4705.4  0.000234  64710 

All Nyquist spectra obtained after soaking the alloys for 1h in NaCl solution, Fig. 2, showed that they have similar trend. Where, the increase of Cu content from 5% to 10% and further to 20% has largely increased the width (diameter) of the semicircle seen for these alloys. The increase of Cu content thus highly increased the corrosion resistance of the Ti-Al alloy. This was confirmed by the EIS values recorded in Table 1, which indicated that the values of RS, RP1 and RP2 increased when the concentration of Cu was increased from 5% to 10% and greatly increased when the Cu% was increased to 20%. Moreover, the phase constant elements (CPEs), Q, with their n values close to the value of 1.0 can be considered as double layer capacitors, particularly at higher percentages of Cu. The presence of Cdl in the equivalent circuit adds another confirmation on the effect of increasing Cu % on increased corrosion resistance of the alloys. According to the values seen in Table 1, the values of YQ and Cdl decreased with the Cu content, which indicates that the porous structure was formed on the surface of Ti-based alloys. The resistance against corrosion thus increases with Cu% as a result of decreasing the porosities of the formed passive layers, and this increases with the increase of Cu contents.

Fig. 3 shows the Nyquist plots for the alloys being immersed for 24h in NaCl solution; here the diameter of the obtained semicircle for all alloys increased compared to the ones obtained after only 1h immersion due to the improved corrosion resistance. The reason is the thickening of the corrosion product layer that is formed on the surface. This layer reduces the contact between the surface and the NaCl test solution. The effect of prolonging the time of immersion was even clearer when it was extended to 48h as seen from the Nyquist spectra presented in Fig. 4. It is seen that the values of Z′ and Z″ were almost duplicated for all alloys in compared to those obtained after immersion for 24h only (Fig. 3). It is thus proved that the increase of immersion time greatly increases the corrosion resistances of the alloys through the formation of a thicker corrosion product layer and/or an oxide layer onto the surfaces.

Table 1 lists the calculated values of the symbols drawn in Fig. 5, which was in turn estimated from the measured EIS data. It is noticeable from Table 1 that the increase of Cu content as well as prolonging the immersion time remarkably increased the values of RS, RP1 and RP2. Furthermore, the values of YQ and Cdl are seen to greatly decrease with the Cu content and with the immersion time, which may indicate on the decreased porosities of the corrosion products form on the surface. This effect is even more obvious when the Cu content was increased to 20% and the immersion time was extended to 48h before measurement. The EIS results thus confirm that the resistance against corrosion in 3.5% NaCl solutions for Ti–5%Al–5%Cu alloy, Ti–5%Al–10%Cu alloy and Ti–5%Al–20%Cu alloys increases with prolonging the immersion time and the increased of Cu content, particularly to 20wt% Cu content.

3.3Polarization data

The potentiodynamic polarization curves recorded for (1) Ti–5%Al–5%Cu alloy, (2) Ti–5%Al–10%Cu alloy and (3) Ti–5%Al–20%Cu alloy after 1.0h exposure in NaCl solutions are seen in Fig. 6. These measurements were also done for these alloys after 24h and 48h and the curves are presented in Fig. 7 and Fig. 8, respectively. The corrosion data collected from these curves are the corrosion current density (jCorr), corrosion potential (ECorr), cathodic Tafel slope (βc), anodic Tafel slope a), corrosion rate (RCorr), and polarization resistance (RP), which its values are tabulated in Table 1. The values of all corrosion data were obtained as reported in our earlier investigations [29–31,35,36].

Fig. 6.

Potentiodynamic polarization curves obtained for (1) Ti–5%Al–5%Cu, (2) Ti–5%Al–10%Cu, and (3) Ti–5%Al–20%Cu alloys after 1h immersion in 3.5% NaCl solutions, respectively.

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Fig. 7.

Potentiodynamic polarization curves obtained for (1) Ti–5%Al–10%Cu, (2) Ti–5%Al–10%Cu, and (3) Ti–5%Al–20%Cu alloys after 24h immersion in 3.5% NaCl solutions, respectively.

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Fig. 8.

Potentiodynamic polarization curves obtained for (1) Ti–5%Al–20%Cu, (2) Ti–5%Al–10%Cu, and (3) Ti–5%Al–20%Cu alloys after 48h immersion in 3.5% NaCl solutions, respectively.

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The curves of Fig. 6 exhibit that the cathodic current decreases with the increase of potential in the less negative side because of the reduction reaction for oxygen [36,37];

2H2O+O2+4e=4OH

It has previously reported [37–39] that the reaction that takes place on cathode for most alloys in NaCl solution is the oxygen reduction following by its adsorption;

OHads+e=OH

The anodic reaction for Al (that presents in the alloys) has been reported [39] to experience as the following:

Al(s)+3OH=Al(OH)3.ads+3e

This formed aluminum hydroxide, 2Al(OH)3.ads, is transformed into Al2O3·3H2O due to its instability as per this reaction:

2Al(OH)3.ads=Al2O3·3H2O

The Al2O3 (Eq. (5)) provides more passivation that in turn leads to a slowdown in the increase of the measured current. After that the current rapidly increases as a result of the dissolution of the formed oxide layer. This increase in the current values in the forward direction may also result from dissolution of Al that present in the alloy as the follows:

Al=Al3++3eˉ

The dissolved Al cations (Al3+) react with Cl ions to form AlCl3. This AlCl3 reacts with further Cl ions producing aluminum chloride complex, AlCl4ˉ, according to this reaction [39]:

Al3++4Clˉ=AlCl4ˉ

Here, the Cl ions are chemisorbed onto the film of oxide without entering into it [39–45]. This leads to dissolving the oxide film via producing an oxychloride complex, Al(OH)2Cl2, as following:

2Al3++2Cl+2OH=Al(OH)2Cl2

In addition, and as has been reported [21,46] that the formation of TiO2 layer could passivate the surface causing the deceleration of the increase of currents in the anodic side. Also, the increase of Cu% probably promotes the formation of cuprous oxide, Cu2O, and therefore the protection of the alloys. Cu2O formation can undergo thanks to the equation [47];

2Cu+H2O=Cu2O+2H++2e

Fig. 6 also reveals that the increase of Cu% to 10% and further to 20% remarkably decreases the values of both anodic and cathodic currents and shifts ECorr to the more negative value. Table 2 in turn confirms that magnifying the Cu content greatly decreases the value of jCorr as well as RCorr; and highly increases the value of RP. Meanwhile, prolonging the time to 24h (Fig. 7) was seen to decrease the corrosion via decreasing jCorr and RCorr and increasing RP values. This effect is more pronounced with increasing the Cu% in the alloys. Table 2 also indicates that the value of jCorr obtained for Ti–5%Al–5%Cu alloy decreases from 23μA/cm2 to 16μA/cm2 and the value of RCorr decreases from 0.200mpy to 0.139mpy with increasing the immersion time to 24h. This effect increases the value of RP from 1563Ωcm2 to 2272Ωcm2 for Ti–5%Al–20%Cu. Further increasing the time of immersion to 48h (Fig. 8) provided the minimum values for jCorr and RCorr and the maximum value for RP and according to the order of Ti–5%Al–20%Cu alloy>Ti–5%Al–10%Cu alloy>Ti–5%Al–5%Cu alloy. The polarization data are thus in good agreement with the data obtained from EIS. Both methods confirmed that the increase of Cu% as well as the increase of immersion time in NaCl solution magnified the resistance of the investigated alloys against corrosion.

Table 2.

Potentiodynamic polarization data obtained for the different Ti–5Al–xCu alloys after different 1h, 24h, and 48h immersion in 3.5% NaCl solutions.

Alloy  Parameter
  βc/V/dec−1  ECorr/V  βa/Vdec−1  jCorr/μAcm−2  RPcm2  RCorr/mpy 
Ti–5%Al–5%Cu (1h)  0.170  −0.125  0.180  23.0  1563  0.20024 
Ti–5%Al–10%Cu (1h)  0.200  −0.155  0.220  2.80  16267  0.02477 
Ti–5%Al–20%Cu (1h)  0.225  −0.270  0.220  1.50  32242  0.01305 
Ti–5%Al–5%Cu (24h)  0.170  −0.138  0.165  16.0  2275  0.13929 
Ti–5%Al–10%Cu (24h)  0.155  −0.180  0.160  2.70  12678  0.02351 
Ti–5%Al–20%Cu (24h)  0.150  −0.335  0.145  0.85  37713  0.00740 
Ti–5%Al–5%Cu (48h)  0.160  −0.200  0.140  75.0  4330  0.06529 
Ti–5%Al–10%Cu (48h)  0.150  −0.195  0.135  21.0  14710  0.01828 
Ti–5%Al–20%Cu (48h)  0.145  −0.340  0.125  0.75  51889  0.00653 
3.4Current–time and surface investigations

The current–time experiments were performed to add more light on the corrosion of the current Ti-base alloys after their immersion for different exposure periods of time in 3.5% NaCl solutions. The curves represented in Fig. 9 are obtained for (1) Ti–5%Al–5%Cu, (2) Ti–5%Al–10%Cu, (3) Ti–5%Al–20%Cu alloys, respectively after immersion for 1h in 3.5% NaCl solutions followed by fixing the potential at 0.3V vs. Ag/AgCl. The curves of Fig. 9 showed that the currents recorded for all alloys greatly decreased in the few minutes. This substantial drop of current resulted from an oxide film thickening, which passivates the surface with time. The current–time behavior for Ti–5%Al–5%Cu alloy indicates that the alloy does not suffer any pitting corrosion. Increasing the content of Cu shows nearly the same current–time trend with a large reduction in the absolute current values. The current–time curves thus confirm that the resistance against corrosion for the different alloys increase as following Ti–5%Al–20%Cu alloy>Ti–5%Al–10%Cu alloy>Ti–5%Al–5%Cu alloy.

Fig. 9.

Potentiostatic current–time curves obtained for (1) Ti–5%Al–5%Cu, (2) Ti–5%Al–10%Cu, and (3) Ti–5%Al–20%Cu alloys after 1h immersion in 3.5% NaCl solutions, respectively.

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Prolonging the immersion time to 24h as seen in Fig. 10 decreased the obtained absolute currents for all alloys. The decrease of current has certainly resulted from the passivation of the surface by the formed corrosion products. The decrease of current with Cu% confirms the increased corrosion resistance of the alloys, which was further indicated via decreasing its absolute current values. The effect of extending the exposure time to 48h before measurement (see Fig. 11) was also obvious, where it provided the lowest values of currents for all alloys. This occurs due to the thickening of the layer of corrosion products that perhaps isolate the alloy from being corroded under the corrosiveness action of the chloride ions. The current–time results confirm the obtained measurements by EIS and polarization that prolonging the immersion time as well as increasing the Cu content decreases the corrosion of the Ti-base alloys as following Ti–5%Al–5%Cu<Ti–5%Al–10%Cu<Ti–5%Al–20%Cu.

Fig. 10.

Potentiostatic current–time curves obtained for (1) Ti–5%Al–5%Cu, (2) Ti–5%Al–10%Cu, and (3) Ti–5%Al–20%Cu alloys after 24h immersion in 3.5% NaCl solutions, respectively.

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Fig. 11.

Potentiostatic current–time curves obtained for (1) Ti–5%Al–5%Cu, (2) Ti–5%Al–10%Cu, and (3) Ti–5%Al–20%Cu alloys after 48h immersion in 3.5% NaCl solutions, respectively.

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The SEM images together with EDX investigations were undertaken to understand the effect of prolonging the exposure period of time on the morphology and the products found onto the surface of the Ti alloys. These investigations were also performed to report the effect increasing Cu content into the Ti–Al alloys. Fig. 12 shows (a) SEM micrograph and (b) EDX analysis collected for Ti–5%Al–5%Cu alloy after its exposure for 48h in 3.5% NaCl solutions before carrying out the current–time experiment shown in Fig. 11 (curve 1). It is seen from Fig. 12, the SEM image, that most of the surface is covered with corrosion products with no evidence that the surface of Ti–5%Al–5%Cu alloy has any pits. This also confirms the data obtained from polarization curves both indicated that there is no pitting attack. The elements detected by EDX analysis (Fig. 12b) in weight percentages were Ti=40.84%, Al=1.18%, Cu=3.11%, O=25.94%%, Na=15.94%, and Cl=13.60%. The detected alloying elements (Ti, Al, and Cu) were lower than expected as a result of the coverage of the surface with a thick layer of corrosion products. The high O% indicates that the corrosion products may have an oxide or a mixture of oxides could be Al2O3, TiO2, and Cu2O. The presence of both Na and Cl at these low % confirm the presence of a layer of a deposited NaCl salt from the test solution onto the surface of the alloy.

Fig. 12.

(a) SEM micrograph and (b) EDX profile analysis obtained for Ti–5%Al–5%Cu alloy after 48h immersion in 3.5% NaCl solutions.

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The SEM micrograph and EDX profile that were obtained after performing the current–time experiment represented in Fig. 11 (curve 2) is presented in Fig. 13. The SEM image obtained after increasing the Cu content to 10% (Fig. 13(b), Ti–5%Al–10%Cu alloy) says that the surface develops a corrosion product layer without having any evidence on the occurrence of pitting corrosion. The EDX profile spectrum provided the weight percentages for the elements found on the surface of alloy as shown in Fig. 13(b). These elements were 48.08% Ti, 2.22% Al, 12.49% Cu, 22.26% O, 7.46% Na, and 7.49% Cl. Here, the presence of Ti and Al were lower than expected, while the Cu% was higher. Hence, the alloy is covered with corrosion product layer that hides the surface of the alloy and makes Ti appear with lower concentration. Also, the dissolution of Al and the formation of Al2O3 within the corrosion product take place. The high wt% of Cu reveals that the dissolution of Cu from the surface of the alloy is not possible. Moreover, the high wt% of Ti and O proves the formation of Ti oxide.

Fig. 13.

(a) SEM micrograph and (b) EDX profile analysis obtained for Ti–5%Al–10%Cu alloy after 48h immersion in 3.5% NaCl solutions.

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Fig. 14 shows (a) SEM micrograph and (b) EDX profile analysis obtained for Ti–5%Al–20%Cu alloy after its immersion for 48h in 3.5% NaCl solutions. The SEM image shows two areas, one has dense corrosion product layer and another has a thin layer but none of both areas has shown any pitting attack. The formation of these corrosion products resulted in the lower currents obtained for Ti–5%Al–20%Cu alloy compared to other alloys that have lower Cu contents. The chemical composition of the corrosion product investigated by EDX and it was found to be 19.11% Ti, 1.64% Al, 28.89% Cu, 37.03% O, 7.76% Na, and 5.57% Cl. These percentages indicated that the highest concentration amongst all alloying elements was Cu followed by Ti and the minimum concentration was Al. This means that the dissolution of Al from the surface into the solution was severe. On the other hand, the enrichment of Cu on the surface of the alloy or in the layer of corrosion products occurred. The low concentration of Ti also indicates that the surface of the alloy was fully covered by a layer of corrosion products that mostly contain Cu2O in it. The present SEM/EDX investigations agree with the electrochemical measurements and thus show that the increase of Cu content in the alloys plays an important role in the passivation of Ti–5Al–xCu alloys.

Fig. 14.

(a) SEM micrograph and (b) EDX profile analysis obtained for Ti–5%Al–20%Cu alloy after 48h immersion in 3.5% NaCl solutions.

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4Conclusions

In this study, manufacturing of three alloys, namely Ti–5%Al–5%Cu, Ti–5%Al–10%Cu, and Ti–5%Al–20%Cu, were synthesized using mechanical alloying technique. These alloys were characterized as powders mixture and after being sintered at 1200°C for 5min using XRD. The corrosion behavior in 3.5% NaCl solutions with different immersion time was carried out. The polarization data proved that the increase of the content of Cu reduced the values of jCorr and RCorr and concurrently increased the values of RP. The data of polarization along with the measurements of current–time confirmed that these materials do not suffer pitting attack even after long time immersion in NaCl solution. EIS data also revealed that Cu increases the corrosion resistance and this effect remarkably increased with the increase of Cu% as well as the immersion time to 24h and further to 48h before measurement. Furthermore, SEM images indicated that none of the fabricated alloys has shown any pits on its corroded surface, due to the formation of a thick layer shape corrosion products. EDX investigations confirmed that the corrosion occurs due to the dissolution of Al. Measurements together were in accordance with each other proving that increasing the additions of Cu as well as the time of immersion decreases the corrosion of the tested materials, whose corrosion resistances were increasing in the following order: Ti–5%Al–20%Cu>Ti–5%Al–10%Cu>Ti–5%Al–5%Cu.

Conflicts of interest

The authors declare no conflicts of interest.

Acknowledgment

The authors would like to extend their sincere appreciation to the Deanship of Scientific Research at King Saud University for its funding of this research through the Research Group Project No. RGP-160.

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