PSI - Issue 53

Serhii Lavrys et al. / Procedia Structural Integrity 53 (2024) 246–253

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Serhii Lavrys et al. / Structural Integrity Procedia 00 (2019) 000 – 000

Figure 4, b shows the results of the potentiodynamic test of Ti6Al4V titanium alloy produced by different technologies and after post HT in 20 wt.% HCl. The nature of the polarization curves of all Ti6Al4V titanium alloys is almost the same, where we observe the active-passive transition typical for passive materials. Probably, this transition is associated with the initial dissolution of the natural oxide film on the surface of titanium alloys with subsequent self-passivation after a potential of – 0.5 V. That is, after a potential of – 0.5 V, the corrosion current decreases and becomes stationary, which indicates that the system is in thermodynamic balance. The corrosion potential of all Ti6Al4V titanium samples is almost the same. On the other hand, the corrosion current of AM Ti6Al4V titanium alloy is higher than that of the wrought one, indicating lower corrosion resistance. For evaluation of the passivation process of titanium alloys, the passivation current density was determined (magnified drawing in Fig. 4, b). Thus, for AM Ti6Al4V titanium alloy, the passivation current is the highest (Table 2), which indicates difficult passivation and, as a result, higher corrosion rate even in the passive region. It should be noted that post HT allows to improve the electrochemical characteristics of the AM Ti6Al4V titanium alloy and approach them to the characteristics of the wrought one. The best effect of improving the corrosion resistance of AM Ti6Al4V titanium alloy is provided by post HT at lower temperature (800° С ) (Table 2). Table 2. E lectrochemical characteristics of wrought and AM Ti6Al4V titanium alloy after heat treatment .

Polarization measurements

EIS measurements

Samples

CPE 1 [Ω − 1 cm − 2 s n ] 3.28·10 − 4 6.11·10 − 4 4.31·10 − 4 7.54·10 − 4

CPE 2 [Ω − 1 cm − 2 s n ]

і corr [A·cm − 2 ] 8.13·10 − 5 9.07·10 − 5 8.47·10 − 5 9.52·10 − 5

і pass [A·cm − 2 ] 6.16·10 − 5 8.71·10 − 5 6.53·10 − 5 6.03·10 − 5

E corr [V]

R f [ Ω·cm 2 ].

R ct [ Ω·cm 2 ].

R p [ Ω·cm 2 ]

8.34·10 – 2 7.22·10 – 2 8.31·10 – 2 8.25·10 – 2

Wrought

– 0.63 – 0.62 – 0.61 – 0.61

247.4 198.3 215.4 138.0

180.4 168.0 175.3 151.0

427.8 366.4 390.8 289.1

AM

AM+HT 800°С AM+HT 850°С

To further clarify the influence of manufacturing technology and post HT of Ti6Al4V titanium alloy, the EIS measurements were performed. The results are presented as Nyquist and Bode plots in Fig. 4, c and b, respectively. On the Nyquist plots for all titanium alloys, we observe two well-expressed capacitive loops (Fig. 4, c). It is obviously that the high-frequency capacitive loop can be characterized as a capacitive loop of a porous oxide film (or film of corrosion products) on the surface of titanium alloys, while the second low-frequency capacitive loop should be associated with an electric double layer at the interface between hydrochloric acid (electrolyte) and titanium alloy (matrix) [5]. It should be noted that the loops of AM Ti6Al4V titanium alloy have smaller diameter than the wrought one, which indicates a decrease of corrosion resistance Fig. 4, c. We observe two peaks on Bode phase angle plots for all alloys, and two evident steps on Bode modulus plots (Fig. 4, d). This also confirms the fact that during EIS studies of Ti6Al4V titanium alloys, two time constants appear. In addition, both the maximum phase angle and the impedance module at low frequencies for AM Ti6Al4V titanium alloy are lower than for the wrought one, which indicates a less continuous surface oxide film and higher intensity of corrosion processes (Fig. 4, d). For quantitative evaluation of EIS studies of Ti6Al4V titanium alloys, an equivalent electric circuit (Fig. 4, c) was used, which includes: R s , R f and R ct – resistance of the solution, oxide film and charge transfer, respectively . CPE 1 and CPE 2 correspond to the constant phase element of the oxide film and the electric double layer, respectively. CPE was used instead of the capacitance in order to increase the accuracy of the fitting, taking into account the distribution of relaxation time due to the roughness or defects of the electrode surface [5]. The polarization resistance ( R p ) was also determined as the sum of resistances R f and R ct [5]. According to the obtained results, it can be concluded that the AM Ti6Al4V titanium alloy is characterized by lower corrosion resistance than the wrought one. The effect of post HT on the corrosion resistance of AM Ti6Al4V titanium alloy depends on its temperature. Post HT at lower temperature (800°C) allows to improve anti-corrosion properties, but at higher temperature (850°C ) post HT leads to a decrease of the corrosion resistance (Table 2). Immersion tests (for 522 hours) correlate well with the electrochemical tests (Fig. 5 and Table 2). According to the obtained results, the corrosion rate of AM Ti6Al4V titanium alloy is by almost twice higher than that of the wrought alloy. Post HT allows to increase the corrosion resistance of AM Ti6Al4V titanium alloy, where the best result is fixed at lower temperature (800 °C). The a nalysis of the morphology of the corroded surface of all Ti6Al4V titanium alloys showed signs of uniform corrosion. In addition, we observe traces of dissolution of both the α/α' -phase and the intergranular β -