Electrochemical behavior of open-cellular structured Ti-6Al-4V alloy fabricated by electron beam melting in simulated physiological fluid: the significance of pore characteristics

doi:10.1016/j.jmst.2021.05.024

Abstract

Abstract:

The cellular structured titanium alloys have attracted significant attention for implants because of their lower Youngʼs modulus, which is comparable to human bone and has the capability of providing space for bone tissue in-growth. However, there is a gap in the knowledge in regard to the relationship between the pore characteristics and the electrochemical performance of open-cellular structured titanium alloys. In this study, we elucidate the influence of pore characteristics on the electrochemical performance of open-cellular structured Ti-6Al-4V alloys produced by electron beam melting (EBM). Intriguingly, the passive film formed on cellular structured Ti-6Al-4V alloy with a larger pore size was more stable and protective, and the corrosion performance was superior compared to the samples with a smaller pore size in phosphate buffered saline (PBS), mainly because of relatively smaller exposed surface area and unlimited flow of electrolyte. However, in acidic PBS containing fluoride ions, the pore characteristics did not play an important role in the corrosion resistance. It was considered that the protective film breaks down such that the corrosion performance of cellular structured alloys was comparable to each other in this harsh environment.

Key words: Cellular structure, Ti-6Al-4V, Electron beam melting, Corrosion resistance, Fluoride ion

Xin Gai, Rui Liu, Yun Bai, Shujun Li, Yang Yang, Shenru Wang, Jianguo Zhang, Wentao Hou, Yulin Hao, Xing Zhang, Rui Yang, R.D.K. Misra. Electrochemical behavior of open-cellular structured Ti-6Al-4V alloy fabricated by electron beam melting in simulated physiological fluid: the significance of pore characteristics[J]. J. Mater. Sci. Technol., 2022, 97: 272-282.

Figures/Tables 14

Fig. 1. Schematic diagram of Ti-6Al-4V electrode for electrochemical corrosion tests.

Fig. 2. SEM images of pore morphology for cellular structured Ti-6Al-4V alloy: (a) P1, (b) P2, (c) P3.

Table 1 Morphology properties of EBM-produced Ti-6Al-4V alloys.

samples	Porosity (%)	Strut thickness (μm)	Pore size (μm)	Surface area (cm²)
Dense	-	-	-	5.3
P1	24.6	353.5±18.7	323.7±14.1	23.5
P2	53.7	462.7±15.7	509.0±38.3	16.9
P3	70.3	461.5±32.1	1081.8±21.8	9.0

Fig. 3. OM micrographs of (a) dense Ti-6Al-4V and (b) strut of cellular structure Ti-6Al-4V; SEM micrographs of (c) dense Ti-6Al-4V and (d) strut of cellular structure Ti-6Al-4V.

Fig. 4. Potentiodynamic polarization curves of EBM-produced dense and cellular structured Ti-6Al-4V alloys in PBS at 37°C.

Table 2 Corrosion parameters obtained from polarization curves of Ti-6Al-4V alloys in PBS at 37°C.

Sample	i_corr (μA cm^-2)	E_corr (V_SCE)	i_p (μA cm^-2)	E_p (V_SCE)	E_b (V_SCE)	∆E (V_SCE)
Dense	0.048±0.001	-0.20±0.03	7.12±0.03	0.75±0.02	0.99±0.01	0.24
P1	0.162±0.004	-0.16±0.02	44.54±0.02	0.69±0.01	0.99±0.01	0.30
P2	0.170±0.001	-0.18±0.04	37.18±0.04	0.70±0.03	0.94±0.02	0.24
P3	0.139±0.001	-0.19±0.03	20.86±0.03	0.68±0.01	0.97±0.03	0.29

Fig. 5. EIS spectra of Ti-6Al-4V alloys in PBS at 37°C: (a) Nyquist plots, (b, c) Bode plots, (d) the equivalent electric circuit used to fit EIS data.

Table 3 EIS fitting results of Ti-6Al-4V alloys in PBS at 37°C.

Sample	R_s(Ω cm²)	R_f(Ω cm²)	Q_f × 10^-4		R_ct(MΩ cm²)	Q_dl × 10^-4		χ²× 10^-4
Sample	R_s(Ω cm²)	R_f(Ω cm²)	Y₀(Ω^-1 cm^-2 sⁿ)	n₁	R_ct(MΩ cm²)	Y₀(Ω^-1 cm^-2 sⁿ)	n₂	χ²× 10^-4
Dense	27.20	1048	5.17	0.7079	0.685	2.30	0.9761	3.79
P1	19.68	18.81	10.66	0.5387	0.184	10.33	0.8984	3.03
P2	23.86	12.38	10.96	0.5767	0.231	8.32	0.8805	4.61
P3	21.67	8.74	5.52	0.6342	0.440	4.51	0.8939	2.21

Fig. 6. Comparison of EIS data of Ti-6Al-4V on day 1 and day 21 in PBS solution at 37°C.

Table 4 EIS fitting results of Ti-6Al-4V alloys after immersing in PBS for 21 d at 37°C.

Sample	R_s(Ω cm²)	R_f(Ω cm²)	Q_f × 10^-4		R_ct(MΩ cm²)	Q_dl × 10^-4		χ²× 10^-4
Sample	R_s(Ω cm²)	R_f(Ω cm²)	Y₀(Ω^-1 cm^-2 sⁿ)	n₁	R_ct(MΩ cm²)	Y₀(Ω^-1 cm^-2 sⁿ)	n₂	χ²× 10^-4
Dense	24.78	986.9	5.57	0.6884	0.582	3.01	0.9095	2.44
P1	25.86	22.08	16.45	0.5427	0.287	8.10	0.9075	2.13
P2	15.65	15.18	12.24	0.5554	0.333	7.00	0.9003	1.47
P3	25.67	13.88	9.61	0.5503	0.442	3.48	0.9025	3.65

Fig. 7. Potentiodynamic polarization curves of Ti-6Al-4V alloys in PBS-H-F solution.

Fig. 8. The release of Ti and V of Ti-6Al-4V alloys in PBS-H-F solution: (a) the total release of Ti, (b) the total release of V, (c) the release of Ti per day, (d) the release of V per day.

Fig. 9. Surface morphology of Ti-6Al-4V alloy after immersing in PBS-H-F solution for 21 d without corrosion products: (a1, a2) dense, (b1, b2) P1, (c1, c2) P2, (d1, d2) P3. (a2), (b2), (c2), (d2) are the enlarged figures corresponding to the areas highlighted in (a1), (b1), (c1), (d1), respectively.

Fig. 10. The finite element results of cellular structured Ti-6Al-4V alloy after immersing in PBS solution for 1 h: (a) P1, (b) P2, (c) P3.

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