Annealed microstructure dependent corrosion behavior of Ti-6Al-3Nb-2Zr-1Mo alloy

doi:10.1016/j.jmst.2020.05.058

Abstract

Abstract:

Corrosion resistance of titanium (Ti) alloys is closely connected with their microstructure which can be adjusted and controlled via different annealing schemes. Herein, we systematically investigate the specific effects of annealing on the corrosion performance of Ti-6Al-3Nb-2Zr-1Mo (Ti80) alloy in 3.5 wt.% NaCl and 5 M HCl solutions, respectively, based on open circuit potential (OCP), potentiodynamic polarization, electrochemical impedance spectroscopy (EIS), static immersion tests and surface analysis. Results indicate that increasing annealing temperature endows Ti80 alloy with a higher volume fraction of β phase and finer α phase, which in turn improves its corrosion resistance. Surface characterization demonstrates that β phase is more resistant to corrosion than α phase owing to a higher content of Nb, Mo, and Zr in the former; additionally, the decreased thickness of α phase alleviates segregation of elements to further restrain the micro-galvanic couple effects between α and β phases. Meanwhile, the influential mechanisms of environmental conditions on corrosion of Ti80 alloy are discussed in detail. As the formation of a highly compact and stable oxide film on surface, annealed Ti80 alloys exhibit a low corrosion current density (10^-6 A/cm²) and high polarization impedance (10⁶ Ω? cm²) in 3.5 wt.% NaCl solution. However, they suffer severe corrosion in 5 M HCl solution, resulting from the breakdown of native oxide films (the conversion of TiO₂ to aqueous Ti³⁺), active dissolution of substrate Ti to aqueous Ti³⁺ and existence of micro-galvanic couple effects. Those findings could provide new insights to designing Ti alloys with high-corrosion resistance through microstructural optimization.

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

Baoxian Su, Liangshun Luo, Binbin Wang, Yanqing Su, Liang Wang, Robert O. Ritchie, Enyu Guo, Ting Li, Huimin Yang, Haiguang Huang, Jingjie Guo, Hengzhi Fu. Annealed microstructure dependent corrosion behavior of Ti-6Al-3Nb-2Zr-1Mo alloy[J]. J. Mater. Sci. Technol., 2021, 62: 234-248.

Figures/Tables 19

Fig. 1. (a) DSC curve and (b) a schematic diagram showing the processing route utilized for Ti80 alloy.

Fig. 2. XRD results of Ti80 alloy annealed at temperatures between 850 and 1000 °C: (a) XRD patterns, (b) volume fraction of β phase, vs. annealing temperature.

Fig. 3. Microstructure in Ti80 alloy annealed at different temperatures, as shown by OM (a to d) and SEM (e to h) images. Microstructure after annealing at (a) and (e) 850 °C, (b) and (f) 900 °C, (c) and (g) 950 °C, and (d) and (h) 1000 °C. (i) The mean thickness d of α phase, i.e., the average value of intersecting diagonals (ASTM E112-12), varies with annealing temperature. (j) Typical HAADF image and STEM elemental maps of Ti80 alloy annealed at 1000 °C.

Table 1 Concentration of alloying elements in α and β phases of annealed Ti80 alloys.

Specimen	Relative concentration (wt.%)
Specimen	Phase	Ti	Al	Nb	Zr	Mo
850 °C	α(hcp)	Balance	6.78 ± 0.19	2.75 ± 0.20	2.32 ± 0.11	0.84 ± 0.22
850 °C	β(bcc)	Balance	4.31 ± 0.08	7.35 ± 0.21	2.85 ± 0.36	4.76 ± 0.36
900 °C	α(hcp)	Balance	6.68 ± 0.16	3.04 ± 0.07	2.08 ± 0.04	1.05 ± 0.12
900 °C	β(bcc)	Balance	5.00 ± 0.12	6.34 ± 0.22	2.42 ± 0.09	3.48 ± 0.32
950 °C	α(hcp)	Balance	6.46 ± 0.09	2.78 ± 0.03	1.90 ± 0.07	1.31 ± 0.12
950 °C	β(bcc)	Balance	5.55 ± 0.08	3.96 ± 0.28	2.17 ± 0.03	2.23 ± 0.26
1000 °C	α(hcp)	Balance	6.24 ± 0.02	2.97 ± 0.05	1.96 ± 0.05	1.27 ± 0.06
1000 °C	β(bcc)	Balance	5.85 ± 0.06	3.57 ± 0.13	2.11 ± 0.02	1.48 ± 0.06

Fig. 4. Variations of open circuit potential (OCP) with time for annealed Ti80 alloys in (a) 3.5 wt.% NaCl solution and (b) 5 M HCl solution.

Fig. 5. Potentiodynamic polarization curves of annealed Ti80 alloys in (a) 3.5 wt.% NaCl solution and (b) 5 M HCl solution.

Table 2 Parameters deduced from potentiodynamic polarization curves.

Annealing temperature (°C)	In 3.5 wt.% NaCl solution		In 5 M HCl solution
	E_corr (V vs. SCE)	i_corr (μA cm^-2)	E_corr (V vs. SCE)	i_corr (μA cm^-2)	β_c (mV·decade^-1)	Corrosion rate (mm·year^-1)
	E_corr (V vs. SCE)	i_corr (μA cm^-2)	E_corr (V vs. SCE)	i_corr (μA cm^-2)	β_c (mV·decade^-1)			Obtained from i_corr	Obtained from weight loss
850	- 0.345 ± 0.011	0.937 ± 0.028	- 0.605 ± 0.013	162.51 ± 2.86	-131 ± 7	1.89 ± 0.04	1.91 ± 0.21
900	- 0.303 ± 0.009	0.728 ± 0.022	- 0.574 ± 0.005	146.13 ± 2.05	-153 ± 5	1.70 ± 0.02	1.64 ± 0.05
950	- 0.265 ± 0.015	0.591 ± 0.014	- 0.545 ± 0.008	131.07 ± 1.13	-152 ± 4	1.52 ± 0.02	1.54 ± 0.01
1000	- 0.223 ± 0.018	0.461 ± 0.031	- 0.484 ± 0.016	118.32 ± 0.85	-154 ± 2	1.37 ± 0.01	1.42 ± 0.02

Fig. 6. The electrochemical parameters of annealed Ti80 alloys vary with annealing temperature: (a) corrosion current density and (b) corrosion potential.

Fig. 7. Nyquist plots of annealed Ti80 alloys after 2.5 h immersion in (a) 3.5 wt.% NaCl solution, and (c) 5 M HCl solution. The corresponding Bode plots after 2.5 h immersion in (b) 3.5 wt% NaCl solution, and (d) 5 M HCl solution. The solid curves are simulated results obtained by ZSimpWin.

Fig. 8. The equivalent circuits used to fit the measured impedance data. (a) Equivalent circuit with one time constant, (b) equivalent circuit with two time constants. Variations of polarization resistance as a function of annealing temperature, in (c) 3.5 wt.% NaCl solution, and (d) 5 M HCl solution.

Table 3 Equivalent circuit parameters for annealed Ti80 alloys after 2.5 h immersion in 3.5 wt.% NaCl solution: Rs solution resistance, CPEf constant phase element, nf the exponent of CPEf, Rf oxide film resistance. The Chi-square parameter (χ2) and sums of squares (λ2) obtained for each annealed Ti80 alloy. Besides, the calculated oxide film capacitance Cf and oxide film thickness d are listed.

Annealing Temperature (°C)	R_s (Ω cm²)	CPE_f (μS·sⁿ cm^-2)	n_f	R_f (10⁶ Ω cm²)	χ²(10 ^-3)	λ²	C_f (μF cm^- ²)	d (nm)
850	10.12	33.38	0.9294	1.691	1.316	0.036	45.34	1.27 ± 0.09
Error%	0.7577	0.6147	0.1447	9.144
900	7.7	29.31	0.9458	1.716	1.389	0.037	36.69	1.57 ± 0.11
Error%	0.7987	0.6285	0.1399	8.044
950	9.187	24.86	0.9512	1.973	0.715	0.027	30.36	1.90 ± 0.07
Error%	0.5676	0.4505	0.1002	5.553
1000 Error%	11.79 0.5774	22.87 0.4562	0.9502 0.1041	2.181 6.011	0.725	0.027	28.07	2.05 ± 0.05

Table 4 Equivalent circuit parameters for annealed Ti80 alloys after 2.5 h immersion in 5 M HCl solution: Rs solution resistance, CPEdl and CPEf constant phase elements, ndl and nf are the exponent of CPEdl and CPEf, respectively, Rct charge transfer resistance, Rf resistance of the corrosion product. The Chi-square parameter (χ2) and sums of squares (λ2) obtained for each annealed Ti80 alloy. Moreover, the calculated electrical double layer capacitance Cdl and corrosion product film capacitance Cf are listed.

Annealing Temperature (°C)	R_s (Ω cm²)	CPE_dl (10^-5 S·sⁿ cm^-2)	n_dl	R_ct (Ω cm²)	CPE_f (10^-3 S·sⁿ cm^-2)	n_f	R_f (Ω cm²)	χ² (10^-3)	λ²	C_dl (10^-5 F cm^-2)	C_f (10^-3F cm^-2)
850	6.978	35.21	0.933	291.8	76.99	1	158.1	0.277	0.017	22.83	76.99
Error%	0.2951	1.017	0.2219	0.5178	8.462	3.485	7.152
900	7.138	50.06	0.9422	305	66.18	1	170	0.204	0.014	35.38	66.18
Error%	0.2436	0.8191	0.1924	0.498	7.124	2.907	5.659
950	6.203	47.71	0.9414	320.5	62.9	0.9989	188.3	0.285	0.017	33.16	63.07
Error%	0.2917	0.9404	0.216	0.579	8.196	3.351	6.738
1000	10.43	25.86	0.9355	429.8	79.77	1	220.7	0.103	0.01	17.17	79.77
Error%	0.1781	0.6062	0.1347	0.3081	6.865	2.755	7.078

Fig. 9. Results of static immersion tests. (a) Variations in loss of mass with time for annealed Ti80 alloys in 5 M HCl solution for 10 days. (b) Corrosion rate varies with annealing temperature. (c-f) SEM images of the morphologies of corroded surface, and (g-j) cross section BSE images of annealed Ti80 alloys in 5 M HCl solution after 10 days immersion, at (c) and (g) 850 °C, (d) and (h) 900 °C, (e) and (i) 950 °C, (f) and (j) 1000 °C.

Fig. 10. Results of XPS measurements for annealed Ti80 alloys. (a) Typical survey spectra from XPS; detailed spectra from (b) Ti 2p, (c) O 1s, (d) Nb 3d, (e) Zr 3d, (f) Mo 3d and (g) Al 2p, all measured from the surface of annealed Ti80 alloys after 10 days of immersion in 5 M HCl solution.

Fig. 11. Schematic diagrams of salient corrosion mechanisms in annealed Ti80 alloys in 5 M HCl solution, showing (a) dissolution of native oxide film, and (b) dissolution of substrate.

Fig. 12. The solutions of annealing Ti80 alloys after 10 days of immersion in 5 M HCl.

Fig. 13. The relationship between the corrosion current density and volume fraction of β phase in (a) 3.5 wt.% NaCl and (b) 5 M HCl; corresponding relationship between the corrosion current density and thickness of α phase in (c) 3.5 wt.% NaCl and (d) 5 M HCl.

Fig. 14. 3D and 2D AFM images of annealed Ti80 alloys after 4 h immersion in 5 M HCl solution, at (a) and (e) 850 °C, (b) and (f) 900 °C, (c) and (g) 950 °C, (d) and (h) 1000 °C. Line-profile analysis of relative height of β phase, at (i) 850 °C, (j) 900 °C, (k) 950 °C and (l) 1000 °C.

Fig. 15. Variations in the partition coefficient of alloying elements with the thickness of α phase, in (a) Al, (b) Nb, (c) Zr, and (d) Mo.

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