Corrosion behavior of Ti60 alloy under continuous NaCl solution spraying at 600 °C

doi:10.1016/j.jmst.2022.01.032

Abstract

Abstract:

The corrosion behavior of Ti60 alloy was investigated under continuous NaCl solution spraying at 600 °C. Results indicate that the corrosion rate of the Ti60 alloy is lower than that obtained with the solid NaCl deposit film in H₂O + O₂. The outer corrosion products are compact layers mainly containing Na₂TiO₃ and nanocrystalline TiO₂. The inner layer is mainly composed of Ti₂O, TiO and SnO₂. Ti₂O has a lamellar structure, and TiO has a strip-shaped structure. A mixed corrosion mechanism including classic oxidation and active oxidation occurred under NaCl solution spraying environment: the first stage was controlled by the classic oxidation mechanism with the promotion by the active oxidation following parabolic curves, and the second stage was controlled by the active oxidation mechanism following a linear rise in the mass gain curve. The mixed corrosion mechanism between classic oxidation and active oxidation produced a relatively dense Ti oxides scale, which decreased the corrosion rate.

Key words: Titanium alloy, High temperature corrosion, Continuous NaCl solution spraying

Rui Li, Li Liu, Yu Cui, Rui Liu, Lei Fan, Min Cao, Zhongfen Yu, Zhiyong Chen, Qingjiang Wang, Fuhui Wang. Corrosion behavior of Ti60 alloy under continuous NaCl solution spraying at 600 °C[J]. J. Mater. Sci. Technol., 2022, 124: 86-101.

Figures/Tables 21

Table 1. Chemical composition of the Ti60 alloy (wt.%).

Al	Sn	Zr	Ta	Mo	Si	C	Ti
5.7	3.7	3.5	1.0	0.4	0.4	0.05	Bal.

Fig. 1. Macrographs of the Ti60 alloy exposed to continuous NaCl solution spraying at 600 °C for 30 s, 5 min, 20 min, 1 h, 10 h, 20 h, 50 h and 100 h.

Fig. 2. Surface morphologies of the Ti60 alloy exposed to continuous NaCl solution spraying for 30 s at 600 °C: (a) SEM morphology, (b) CLSM two-dimensional morphologies, (c) three-dimensional morphologies (including the depth of z-axis focal plane), (d) statistical map of NaCl particles and (e) SEM morphology of NaCl deposition on oxide.

Fig. 3. Metallographic morphology of the Ti60 alloy with α and β phase by optical microscopy.

Fig. 4. Mass gain curves of the Ti60 alloy exposed to different environments for 100 h at 600 °C: H2O + O2 (), continuous NaCl solution spraying () and solid NaCl deposit film in H2O + O2 ().

Fig. 5. SEM morphologies of the Ti60 alloy after corrosion in continuous NaCl solution spraying for 1 h at 600 °C: (a) surface morphology and an enlarged picture, (b) EDS of surface analysis and (c) cross-section morphology.

Fig. 6. SEM morphologies of the Ti60 alloy after corrosion in continuous NaCl solution spraying for 10 h at 600 °C: (a) surface morphology and an enlarged picture, (b) EDS of surface analysis and (c) cross-section morphology.

Fig. 7. SEM morphologies of the Ti60 alloy after corrosion in continuous NaCl solution spraying for 20 h at 600 °C: (a) surface morphology and an enlarged picture, (b) EDS of surface analysis and (c) cross-section morphology.

Fig. 8. SEM morphologies of the Ti60 alloy after corrosion in continuous NaCl solution spraying for 100 h at 600 °C: (a) surface morphology and an enlarged picture, (b) cross-section morphology with the characteristics marked and (c-e) EDS analysis of area 1-3 marked in (b).

Fig. 9. XRD patterns of the corrosion products of the Ti60 alloy after corrosion in continuous NaCl solution spraying for 100 h at 600 °C: (a) the corrosion products scratched from the samples, (b) the inner layer near to the based metal.

Fig. 10. XPS spectra of the outer and the oxide layer neared the based metal, showing the corrosion products in the Ti60 alloy after corrosion in continuous NaCl solution spraying for 100 h at 600 °C: (a) Ti, (b) Al, (c) Sn, (d) Zr, (e) Na, and (f) Cl.

Fig. 11. EPMA results of the corrosion products formed in continuous NaCl solution spraying on the Ti60 alloy including main elements Ti, Al, Sn, Zr, Na, O and Cl after corrosion (a) 20 h and (b) 100 h at 600 °C.

Fig. 12. TEM morphologies of the corrosion products of the Ti60 alloy after corrosion in continuous NaCl solution spraying for 100 h at 600 °C: (a) inner layer and (b) outer layer.

Fig. 13. Observation of corrosion products with lamellar structures in the inner layer: (a) bright-field image, (b) SAED pattern and (c) EDS analysis of the selected area.

Fig. 14. Observation of corrosion products with strip-shaped structures in the inner layer: (a) bright-field image, (b) SAED pattern and (c) EDS analysis of the selected area.

Fig. 15. Observation of corrosion products with nanocrystalline structures in the inner layer: (a) bright-field image, (b) SAED pattern and (c) EDS analysis of the selected area.

Fig. 16. TEM elemental mappings of the corrosion products of the Ti60 alloy after corrosion for 100 h in continuous NaCl solution spraying at 600 °C: (a) the inner layer and (b) the outer layer.

Fig. 17. Observation of the NaCl deposition on the surfa ce of the sample in continuous NaCl solution spraying for different time: (a) 5 min, (b) 20 min, (c) 20 h and (d) 50 h.

Fig. 18. Overlay of the theoretical predominance diagrams at 600 °C and 1 atm pressure as determined by HSC Chemistry 6.0 software from the thermodynamic databases OwnDB6. HSC and MainDB6. HSC: (a) M-O-Cl (M = Ti, Al, Sn and Zr) for pure constituents and (b) for Ti-TiCl4-Ti2O, Ti-TiCl4-TiO and Ti-TiCl4-TiO2.

Table 2. Reactions proposed for the mechanisms of the chemical reactions and their corresponding values of standard Gibbs free-energy calculated by HSC Chemistry 6.0 software from the thermodynamic databases OwnDB6. HSC and MainDB6. HSC.

Chemical reaction

ΔG^o (KJ mol^-1)

Ti + O_2(g) ↔ TiO₂
Ti + 2H₂O_(g) ↔ TiO₂ + 2H₂
2Ti + H₂O ↔ Ti₂O + H_2(g)
4NaCl_(s) + 2TiO_2(s) + O_2(g) ↔ 2Na₂TiO_3(s) + 2Cl_2(g)
4NaCl_(s) + 2TiO_2(s) + 2H₂O_(g) ↔ 2Na₂TiO_3(s) + 4HCl_(g)
4NaCl_(g) + O_2(g) + 2TiO₂ ↔ 2Na₂TiO₃ + 2Cl_2(g)
4NaCl_(g) + 2H₂O_(g) + 2TiO₂ ↔ 2Na₂TiO₃ + 4HCl_(g)
Ti + 2Cl_2(g) ↔ TiCl_4(g)
Ti + 4HCl_(g) ↔ TiCl_4(g) + 2H_2(g)
TiCl_4(g) + O_2(g) ↔ TiO₂ + 2Cl_2(g)
2TiCl_4(g) + O_2(g) ↔ 2TiO + 4Cl_2(g)
4TiCl_4(g) + O_2(g) ↔ 2Ti₂O + 8Cl_2(g)
4Al + 3O_2(g) ↔ 2Al₂O₃
2Al + 3H₂O_(g) ↔ Al₂O₃ + 3H_2(g)
2Al + 3Cl_2(g) ↔ 2AlCl_3(g)
2Al + 6HCl_(g) ↔ 2AlCl_3(g) + 3H_2(g)
4AlCl_3(g) + 3O_2(g) ↔ 2Al₂O₃ + 6Cl_2(g)
2AlCl_3(g) + 3H₂O ↔ Al₂O₃ + 6HCl_(g)
Zr + O_2(g) ↔ ZrO₂
Zr + 2H₂O_(g) ↔ ZrO₂ + 2H_2(g)
Zr + 2Cl_2(g) ↔ ZrCl_4(g)
Zr + 4HCl_(g) ↔ ZrCl_4(g) + 2H_2(g)
ZrCl_4(g) + O_2(g) ↔ ZrO₂ + 2Cl_2(g)
ZrCl_4(g) + 2H₂O ↔ ZrO₂ + 4HCl_(g)
Sn + O_2(g) ↔ SnO₂
Sn + Cl_2(g) ↔ SnCl_2(g)
Sn + 2Cl_2(g) ↔ SnCl_4(g)
Sn + 2HCl_(g) ↔ SnCl_2(g) + H_2(g)
SnCl_2(g) + O_2(g) ↔ SnO₂ + Cl_2(g)
SnCl_4(g) + O_2(g) ↔ SnO₂ + 2Cl_2(g)
SnCl_4(g) + 2H₂O ↔ SnO₂ + 4HCl_(g)
Si + O_2(g) = SiO₂
Si + 2H₂O_(g) = SiO₂ + 2H_2(g)
Si + 2Cl_2(g) = SiCl_4(g)
Si + 4HCl_(g) = SiCl_4(g) + 2H_2(g)
SiCl_4(g) + O_2(g) = SiO₂ + 2Cl_2(g)
SiCl_4(g) + 2H₂O_(g) = SiO₂ + 4HCl_(g)

-785.13
-385.82
-304.91
354.76
354.04
-36.81
-37.53
-656.29
-256.26
-128.84
396.41
1616.03
-2803.8
-802.94
-1082.05
-482.01
-639.7
-444.71
-933.2
-533.89
-768.04
-368.01
-165.16
-248.4
-399.98
-222.19
-360.46
-22.17
-177.79
-39.51
-122.75
-752.54
-353.24
-547.16
-147.14
-205.38
-206.10

Fig. 19. Diagram of the corrosion mechanism of the Ti60 alloy in a continuous NaCl solution spraying environment at 600 °C: (a) initial corrosion mechanism of Ti60 alloy under NaCl particles deposited on the surface in NaCl solution spray at 600 °C and (b) latter corrosion mechanism of Ti60 alloy under formation compact TiO2 out layer in NaCl solution spray at 600 °C.

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