The corrosion behavior of Ti-6Al-3Nb-2Zr-1Mo alloy: Effects of HCl concentration and temperature

doi:10.1016/j.jmst.2020.08.066

Abstract

Abstract:

Investigation about the corrosion behavior of Ti alloys in different ambient environment is of great significance for their practical application. Herein, we systematically investigate the corrosion behavior of a newfound Ti-6Al-3Nb-2Zr-1Mo (Ti80) alloy in hydrochloric acid (HCl) ranging from 1.37 to 7 M, and temperature ranging from 25 to 55 ℃, by means of electrochemical measurements, static immersion tests and surface analysis. Results manifest that increasing either HCl concentration or temperature can accelerate the corrosion of Ti80 alloy via promoting the breakdown of native protective oxide film and then further facilitating the active dissolution of Ti80 matrix. According to potentiodynamic polarization curves, Ti80 alloy displays a spontaneous passive behavior in 1.37 M HCl at 25 ℃, compared to a typical active-passive behavior under the other conditions. As indicated by cathodic Tafel slope, the rate determining step for cathodic hydrogen evolution reaction is likely the discharge reaction step. The apparent activation energies obtained from corrosion current density and maximum anodic current density for Ti80 alloy in 5 M HCl solution are 62.4 and 55.6 kJ mol^-1, respectively, which signifies that the rate determining step in the corrosion process of Ti80 alloy is mainly determined by surface-chemical reaction rather than diffusion. Besides, the electrochemical impedance spectroscopy tests demonstrate that a stable and compact oxide film exists in 1.37 M HCl at 25 ℃, whereas a porous corrosion product film forms under the other conditions. Overall, the critical HCl concentration at which Ti80 alloy can maintain passivation at 25 ℃ can be determined as a value between 1.37 and 3 M. Furthermore, the corroded surface morphology characterization reveals that equiaxed α phase is more susceptible to corrosion compared to intergranular β phase due to a lower content of Nb, Mo, and Zr in the former.

Key words: Titanium alloys, Electrochemical techniques, Weight loss, Corrosion behavior

Baoxian Su, Binbin Wang, Liangshun Luo, Liang Wang, Yanqing Su, Fuxin Wang, Yanjin Xu, Baoshuai Han, Haiguang Huang, Jingjie Guo, Hengzhi Fu. The corrosion behavior of Ti-6Al-3Nb-2Zr-1Mo alloy: Effects of HCl concentration and temperature[J]. J. Mater. Sci. Technol., 2021, 74: 143-154.

Figures/Tables 15

Table 1 Chemical composition of Ti-6Al-3Nb-2Zr-1Mo alloy (wt.%).

Ti	Al	Nb	Zr	Mo	C	H	O	N
Bal.	6.06	2.98	1.95	1.05	0.009	0.0032	0.07	0.008

Fig. 1. (a) XRD pattern of Ti80 alloy; the partial XRD pattern is highlighted in (b). (c) OM and (d) SEM images Ti80 alloy; (e) TEM bright-field and (f) HAADF images and (g-l) STEM elemental maps of Ti80 alloy.

Fig. 2. Variations of OCP with immersion time: (a) in 1.37, 3, 5 and 7 M HCl at 25 °C, and (b) at 25, 37, 45 and 55 °C in 5 M HCl.

Fig. 3. PDP curves of Ti80 alloy: (a) in 1.37, 3, 5 and 7 M HCl at 25 °C, and (b) at 25, 37, 45 and 55 °C in 5 M HCl. Schematic diagrams of cathodic Tafel fitting (c) in 3, 5 and 7 M HCl at 25 °C, and (d) at 25, 37, 45 and 55 °C in 5 M HCl.

Fig. 4. Variations of icorr as functions of (a) HCl concentration or (b) temperature; the bc variation with (c) HCl concentration or (d) temperature.

Fig. 5. Anodic curves after current correction for cathodic hydrogen evolution reaction: (a) in 1.37, 3, 5 and 7 M HCl at 25 °C, and (b) at 25, 37, 45 and 55 °C in 5 M HCl. The inserts are variations of the maximum anodic current (im) with HCl concentration or temperature, respectively.

Fig. 6. (a) Variations of the logarithm of icorr or im with the logarithm of HCl concentration at 25 °C, and (b) Arrhenius plots of icorr or im vs. 1/T in 5 M HCl solution in the temperature range of 25-55 °C.

Fig. 7. (a) Nyquist plots and (b) corresponding Bode plots of Ti80 alloy in 1.37, 3, 5 and 7 M HCl at 25 °C. (c) Nyquist plots and (d) corresponding Bode plots of Ti80 alloy at 25, 37, 45 and 55 °C in 5 M HCl. The inserts in (a) and (c) are the magnifications of the rectangle regions to show the details of the curves. The solid curves are the simulated results obtained by Zsimpwin 3.10.

Fig. 8. (a) The equivalent circuits used to fit the measured impedance data. Variations of Rp as functions of (b) HCl concentration or (c) temperature.

Table 2 Equivalent circuit parameters for Ti80 alloy in different concentration (CHCl) of 1.37, 3, 5 and 7 M HCl at 25 °C.

C_HCl (M)	R_s (Ω cm²)	CPE_f (10^-5 S sⁿ cm^-2)	n_f	R_f (Ω cm²)	CPE_dl (mS sⁿ cm^-2)	n_dl	R_ct (Ω cm²)	χ² (10^-3)
1.37	4.606	4.124	0.9452	1,680,000	-	-	-	1.273
3	3.971	31.85	0.9368	465.2	52.56	0.8958	369.3	1.487
5	3.086	33.36	0.9293	303.9	84.85	0.9997	156.5	2.922
7	7.325	28.38	0.9212	104.5	122.90	0.9506	47.78	0.468

Table 3 Equivalent circuit parameters for Ti80 alloy at 25, 37, 45 and 55 °C in 5 M HCl.

Temperature (°C)	R_s (Ω cm²)	CPE_f (10^-5 S sⁿ cm^-2)	n_f	R_f (Ω cm²)	CPE_dl (mS sⁿ cm^-2)	n_dl	R_ct (Ω cm²)	χ² (10^-3)
25	3.086	33.36	0.9293	303.9	84.85	0.997	156.5	2.922
37	1.582	38.69	0.9254	56.24	40.64	1	45.99	4.486
45	1.791	46.11	0.9288	32.11	36.70	0.9966	27.82	3.733
55	9.598	63.01	0.8922	20.89	33.65	0.9137	19.76	1.183

Fig. 9. Variations in weight loss of Ti80 alloy with immersion time: (a) in 1.37, 3, 5 and 7 M HCl solutions at 25 °C, (b) at 25, 37, 45 and 55 °C in 5 M HCl solution. Corrosion rate of Ti80 alloy varies with (c) HCl concentration or (d) temperature.

Fig. 10. SEM images of corroded surface morphologies of Ti80 alloy after 10 d of immersion: in (a) 1.37, (b) 3, (c) 5 and (d) 7 M HCl at 25 °C; at (e) 25, (f) 37, (g) 45 and (h) 55 °C in 5 M HCl.

Fig. 11. AFM images of Ti80 alloy after 1 h of immersion in (a) 1.37, (b) 3, (c) 5 M and (d) 7 M HCl at 25 °C; at (e) 25, (f) 37, (g) 45 and (h) 55 °C in 5 M HCl.

Fig. 12. Schematic illustration of corrosion process for Ti80 alloy.

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