3.5. Corrosion Behavior
Although metals show good mechanical properties, they are susceptible to electrochemical attack. Amongst all biomaterials, \(Ti\) alloys (especially \(\alpha\) and \(\beta –Ti\) alloys) have a sufficient strength and the most corrosion resistance such as \(Ti-5Al-2-5Fe\), \(Ti-6Al-4V\), and \(Ti-6Al-7Nb\) etc., but the aluminum within these alloys can lead to certain bone diseases and disordering in neurological system.
The investigation of corrosion behavior for produced Ti-18Nb alloys was done in simulated body fluid at 37°C. Firstly, by recording Potential – time measurements (OCP) for 3600 sec. as illustrated in Fig. 6, the low percent of copper (5 wt% Cu) gives variation in potentials with time that began with rapid drop follwoed by semistable behavior. The low percent of Cu in \(Ti-18Nb\) alloy may give irrigularity in surface as indicated in surface roughness of AFM analysis. Also, the presence of copper with its positive potential (+ 0.337 V) behaves as sufficient cathode and presence of \({Ti}_{2}Cu\) phase play important role in the corrosion behavior. Addition 7 and 9 wt% gave more stability behavior.
Secondly, Tafel plot gives good investigation about corrosion properties, Fig. 7 shows the polarization curves of \(Ti-18Nb-xCu\) alloys indicating cathodic and anodic regions, the most important reactions that occur at electrodes are:
$${Ti}^{4+}+2{H}_{2}O\to {\left[Ti\right(OH{)}_{2}]}^{2+}+2{H}^{+}$$
3
$${Ti}^{4+}+4{H}_{2}O\to Ti(OH{)}_{4}+4{H}^{+}$$
4
While at cathodic site, the reduction of oxygen can occur as follow:
$${O}_{2}+4e+2{H}_{2}O\to 4{OH}^{-}$$
5
Finally, the Ti alloys can be protected by passive layer of titanium oxide as follow [21]:
$$Ti+{H}_{2}O\to TiO+2{H}^{+}+2e$$
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$$2Ti+{3H}_{2}O\to {Ti}_{2}{O}_{3}+6{H}^{+}+6e$$
7
$${Ti}_{2}{O}_{3}+4{OH}^{-}\to 2TiO{\left(OH\right)}_{2}+{H}_{2}O+2e$$
8
$$2TiO{\left(OH\right)}_{2}\to {TiO}_{2}+{2H}_{2}O$$
9
$${TiO}_{2}.n{H}_{2}O\to {TiO\left(OH\right)}_{2\left(aq\right)}$$
10
The polarization curve of \(Ti-18Nb-5Cu\) alloy was shifted to more current density values, i.e., more corrosion rate. The polarization curve of \(Ti-18Nb-7Cu\) and \(Ti-18Nb-9Cu\) alloys gave behavior similar to \(Ti-18Nb\) alloy with more noble potentials.
In study of Wang and Zheng, \(Ti-16Nb\) alloy showed OCP curves with stable passive layer compared with \(CP Ti\). Also the \(Ti-16Nb\) alloy had corrosion resistance due to the formation \({TiO}_{2}\) and \({Nb}_{2}{O}_{5}\), that obstruct the dissolution of metals from alloy [22].
While Mutlu and Oktay in another study illustrated the decreasing in polarization resistance with increasing \(Cu\) content of the \(Ti-Nb-Cu\) alloy and the passive layer was a barrier for dissolution and improved the resistance through the charge transfer at the surface/medium interface with low \(Cu\) content, while the higher content produce \({Ti}_{2}Cu\) phase which induce corrosion rates [23]. Also Mutlu et al. in another study showed that increasing the \(Cu\) content for \(Ti-Nb-Cu\) alloys led to get higher corrosion current density and more negative corrosion potential [24].
The data of corrosion behavior are listed in Table 3, the corrosion potential (Ecorr) became nobler after adding Cu to \(Ti-18Nb\) alloy, the corrosion current density (icorr) is varied with variation of Cu content in alloy and the lowest (icorr) was for \(Ti-18Nb-9Cu\) alloy.
Cathodic and anodic Tafel slopes also varied due to variation of redox reactions on the surface. Generally, the hydrogen evolution reaction is a common cathodic reaction in almost corrosive media for metals and also may incorporate with the lattice structure of certain metals leading to embrittlement. Generally, cathodic Tafel slopes in Table 3 are greater than expected (bc=0.120 V.dec− 1) for the Volmer \(-\) Tafel mechanism, which suggest the formation of a film affecting the reduction reaction that obstructing charge transfer at film formed and/or limit the energy at the double layer (\(dl\)). The evolution of hydrogen molecules on the electrode that covered by oxide layer can work either as the charge carriers (\({H}_{3}{O}^{+}\) ions) or as the electrons (\(e\)), where the neutralization occurs between each two charge carriers (i.e., \({H}_{3}{O}^{+}\)and \(e\)). This model is represents a “Dual\(-\)barrier model” [25]; if the mechanism takes place by chemical desorption process, the cathodic slope (\({b}_{c}\)) will be less than (\(0.120\) V.dec−1) through the diffusion of \(H\) atoms to the metal surface. But if the mechanism takes place by electrochemical desorption, the cathodic slope (\({b}_{c}\)) will be (\(-0.05\) V.dec−1). Therefore in the current work, the mechanism is neither chemical desorption nor electrochemical desorption.
Corrosion rate (CR) in mil per year can be calculated by the following equation [26] using corrosion current densties and equivielnt weight \(e\) and density \(\rho\) of each alloy according to its composition [27, 28]:
$${C}_{R}=0.13\times {i}_{corr}\left(\frac{e}{\rho }\right)$$
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It is shown from Table 3 that the addition of copper increased the rate of dissolution and this result is agreement with the observations of Cheng et al. who showed that lamellar \({Ti}_{2}Cu\) phase reduces the corrosion resistance and leads to form micro-galvanic cells within α- matrix and increases the galvanic driving force (∆V) at \({Ti}_{2}Cu/\alpha -Ti\) interface [29]. Also they discussed the complex process of the anodic dissolution of \({Ti}_{2}Cu\) in simulated body fluid that can be simplified as:
$${Ti}_{2}Cu+{8OH}^{-}\to {2TiO}_{2}+{Cu}^{2+}+{4H}_{2}O+10e$$
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Firstly, \({Ti}_{2}Cu\) converts to Ti4+ and Cu2+ [30, 31] followed by converting Ti4+ and OH‾ to TiO2 [32]. Polarization resistance (RP) also can be calculated using Tafel slopes (bc & ba) and current densities as follow [33]:
$${R}_{p}=\frac{{b}_{c}\times {b}_{a}}{2.303\times {i}_{corr}({b}_{c}+{b}_{a})}$$
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The results of (RP) indicate that \(Ti-18Nb-9Cu\) alloy has higher resistance than other alloy which may due to the lowest interfacial area ratio (Rdr) from AFM data for this alloy that decreases the formed micro-galvanic cells.
Table 3
Corrosion data of \(Ti-18Nb-xCu\) alloy in SBF at 37°C.
Data | Alloy |
Ti-18Nb | Ti-18Nb-5Cu | Ti-18Nb-7Cu | Ti-18Nb-9Cu |
-Ecorr (V) | 0.232 | 0.197 | 0.137 | 0.151 |
icorr (A.cm− 2)⊆10− 7 | 1.284 | 8.028 | 2.434 | 1.230 |
-bc (V.dec− 1) | 0.1733 | 0.1278 | 0.3258 | 0.2077 |
+ba (V.dec− 1) | 0.3144 | 0.5535 | 0.3734 | 0.3271 |
CR (mpy)⊆10− 8 | 4.287 | 29.116 | 9.108 | 4.745 |
RP (kΩ.cm2) | 377.81 | 56.158 | 310.39 | 448.46 |
Figure 8 shows the microstructure of \(Ti-18Nb-xCu\) alloys after corrosion process. Firstly, it is clear that the microstructure is consists of \(\alpha -Ti\) phase (dark colored regions) as well as a small amount of Nb-rich regions (whitish regions), while α-phase or eutectoid transformation occurred by the decomposition of the \(\beta -Ti\) phase (thick dark gray areas). The microstructure of Cu-containing alloys has colored phases (ash gray) at the borders of the dark gray phases (\(\alpha -Ti\)). According to the binary phase diagram of Ti-Cu alloy, only the \({Ti}_{2}Cu\) phase can be formed when Cu percent is less than 40% [34] and according to the \(Ti-Nb-Cu\) triple phase diagram, it is seen that in the addition of less than (5 wt% Cu) to the \(Ti-18Nb\) alloy, the α and \(\beta -Ti\) are formed and the \({Ti}_{2}Cu\) phase will be formed [34]. In spite of the presence of passive films as \(Ti{O}_{2}\)and \({Nb}_{2}{O}_{5}\) on the surface, the localized corrosion was formed after the electrochemical test and the damage can occur at the \({Ti}_{2}Cu/\alpha -Ti\) interface.