**1.3.5 Effect of current density on surface morphology and corrosion resistance of the film**

Figure 1-14 shows the morphologies of under optical microscope at 1000× of amplification of the Ti-O film cathodically-electrodeposited at different current densities. It can be seen from figure a and b that the Ti-O films obtained at 40mA/cm2 and 30mA/cm2 were uneven, lumpedly distributed on the surface of TiNi SMA, and with some big cracks; it can be seen from figure c that Ti-O film was fairly even at 20mA/ cm2, distributed as floc-like on the surface of Ti-O film, but with some considerable cracks; it can be seen from figure d that the Ti-O film obtained at 5mA/cm2 was fine and close, and with small cracks; from figure e Ti-O film's existence could hardly be seen, only the scratch on the substrate. This illustrates that the Ti-O film obtained at 5mA/cm2 was the best. Because current density got major, the driving force for Ti(IV) to hydrolyze got major too, Ti-O film quickly formed and grew on the surface of TiNi SMA within a short time. Owing to massedly-produced hydrogen at depositing time, Ti-O film continuously peeled off, thus causing surface very uneven. Besides the film produced at this time was very thick, a stress difference produced due to shrinking in the process of drying, so that many big cracks were brought about. With current density going down, the driving force for Ti(IV) to hydrolyze also getting down, the particles formed getting small, and this made the Ti-O film surface more even, cracks less. But when the current density went down to a certain extent, the driving force for Ti(IV) to hydrolyze would not be enough to form a film on the surface of TiNi SMA.

Figure 1-15 shows the corrosion potential-time curves of the samples athodicallyelectrodeposited at different current densities in Hank's solution (PH7.45). It can be seen from the figure that the sample deposited at the current density of 20mA/cm2 had a more positive corrosion potential about -0.2V; but the stable corrosion potential of non-depositing sample was -0.1V. This illustrates that the deposited sample has a better thermodynamic stability at current density of 20mA/cm2 in the solution. This is because of the Ti-O film deposited at the current density of 20mA/cm2 was thicker, bonding more closely, that made it have a better thermodynamic stability.

1. treated 2. untreated

012345

Fig. 1-13. The anodic-polarization curves of samples in Fusayama solution (PH6.13)

hydrolyze would not be enough to form a film on the surface of TiNi SMA.

Figure 1-15 shows the corrosion potential-time curves of the samples athodicallyelectrodeposited at different current densities in Hank's solution (PH7.45). It can be seen from the figure that the sample deposited at the current density of 20mA/cm2 had a more positive corrosion potential about -0.2V; but the stable corrosion potential of non-depositing sample was -0.1V. This illustrates that the deposited sample has a better thermodynamic stability at current density of 20mA/cm2 in the solution. This is because of the Ti-O film deposited at the current density of 20mA/cm2 was thicker, bonding more closely, that made

**1.3.5 Effect of current density on surface morphology and corrosion resistance of the** 

Figure 1-14 shows the morphologies of under optical microscope at 1000× of amplification of the Ti-O film cathodically-electrodeposited at different current densities. It can be seen from figure a and b that the Ti-O films obtained at 40mA/cm2 and 30mA/cm2 were uneven, lumpedly distributed on the surface of TiNi SMA, and with some big cracks; it can be seen from figure c that Ti-O film was fairly even at 20mA/ cm2, distributed as floc-like on the surface of Ti-O film, but with some considerable cracks; it can be seen from figure d that the Ti-O film obtained at 5mA/cm2 was fine and close, and with small cracks; from figure e Ti-O film's existence could hardly be seen, only the scratch on the substrate. This illustrates that the Ti-O film obtained at 5mA/cm2 was the best. Because current density got major, the driving force for Ti(IV) to hydrolyze got major too, Ti-O film quickly formed and grew on the surface of TiNi SMA within a short time. Owing to massedly-produced hydrogen at depositing time, Ti-O film continuously peeled off, thus causing surface very uneven. Besides the film produced at this time was very thick, a stress difference produced due to shrinking in the process of drying, so that many big cracks were brought about. With current density going down, the driving force for Ti(IV) to hydrolyze also getting down, the particles formed getting small, and this made the Ti-O film surface more even, cracks less. But when the current density went down to a certain extent, the driving force for Ti(IV) to

l g i / n A c m - <sup>2</sup>

1 .

2 .


it have a better thermodynamic stability.

E/V(SCE)

**film** 

a(40mA/cm2) b(30mA/cm2 )

c (20mA/cm2) d (5mA/cm2)

e (2mA/cm2)

Fig. 1-14. Optical microscope of Ti-O film by different current densities (1000×)

Ti-O Film Cathodically-Electrodeposited on



by different current densities in Hank's solution (PH7.45)




E/V(SCE)





E/V(SCE)

Hank's solution (PH7.45)

presented similarly a better cathodic-polarization property.

1.20mA/cm <sup>2</sup> 2.5mA/cm <sup>2</sup>

the Surface of TiNi SMA and Its Bioactivity and Blood Compatibility 17

thin, with less crystal defects, after crystallized and heavy thermalstress, it was easy to corrode, thus presenting a poor corrosion resistance. From the cathodic-polarization curve of view, the deposited sample under current density of 5mA/cm2 after crystallized

012345

1 .

2 .

 5mA/cm <sup>2</sup> 20mA/cm <sup>2</sup>

l g i / n A c m - <sup>2</sup>

0 1 0 2 0 3 0 4 0 5 0 6 0

t / m i n

Fig. 1-17. The Corrosion potential change plotted as a function of time of heat treated sample

Fig. 1-16. The anodic-polarization curves of samples with different current densities in

Fig. 1-15. The Corrosion potential change plotted as a function of time of samples with different current densities in Hank's solution (PH7.45)

Figure 1-16 shows the anodic-polarization curves of the samples athodicallyelectrodeposited at different current densities in Hank's solution (PH7.45). It can be seen from the anodic-polarization curves in the figure that the deposited sample's at current density of 20mA/cm2 was much less than the one's at current density of 5mA/cm2, but the passivation region of the deposited sample at current density of 5mA/cm2, was longer than that of the one at high current density. This is because of the Ti-O film of deposited sample at high current density was thicker, which could effectively prevent the ions from transfer, thus making the current density lower. Because the surface of the deposited sample at high current density was very rough, that would bring about the passive film not enough stable after the surface passivated, it is easy to be punctured, so that made the passive region short. But from the figure of view, the slope of the deposited sample at current density of 20mA/cm2 is large, it illustrates that the deposited sample has a better cathodic-polarization property.

Figure 1-17 shows the corrosionpotential-time curves of the deposited samples at different current densities in Hank's solution (PH7.45) after crystallized at 450C. The corrosion potential of the deposited sample at the current density of 20mA/cm2 after crystallized was not very stable, undulating much, finally the current potential was about -0.205V, which was more negative than that of the one at the current density of 5mA/cm2 after crystallized, -0.195V. This illustrates that the deposited sample at the current density of 5mA/cm2 after crystallized is with a better thermodynamic stability.

Figure 1-18 shows the polarization curves of the deposited samples at different current densities after crystallized at 450C. It can be seen from the polarization curves in the figure that the voltage of the deposited sample at current density of 5mA/cm2 after crystallized was far lower than that of the one at 20mA/cm2. The breakdown potential of the deposited sample at current density of 5mA/cm2 after crystallized was 1.2V, but the one at 5mA/cm2 had no the passivation phenomenon emerging. This illustrates that the deposited sample at current density of 5mA/cm2 after crystallized displayed a very good anti-corrosion. Because the granularity of the deposited sample at5mA/cm2 was small, the surface even and very

 5mA/cm <sup>2</sup> 20mA/cm <sup>2</sup>

0 10 20 30 40 50 60

t / m i n

Fig. 1-15. The Corrosion potential change plotted as a function of time of samples with

Figure 1-16 shows the anodic-polarization curves of the samples athodicallyelectrodeposited at different current densities in Hank's solution (PH7.45). It can be seen from the anodic-polarization curves in the figure that the deposited sample's at current density of 20mA/cm2 was much less than the one's at current density of 5mA/cm2, but the passivation region of the deposited sample at current density of 5mA/cm2, was longer than that of the one at high current density. This is because of the Ti-O film of deposited sample at high current density was thicker, which could effectively prevent the ions from transfer, thus making the current density lower. Because the surface of the deposited sample at high current density was very rough, that would bring about the passive film not enough stable after the surface passivated, it is easy to be punctured, so that made the passive region short. But from the figure of view, the slope of the deposited sample at current density of 20mA/cm2 is large, it illustrates that the deposited sample has a better cathodic-polarization

Figure 1-17 shows the corrosionpotential-time curves of the deposited samples at different current densities in Hank's solution (PH7.45) after crystallized at 450C. The corrosion potential of the deposited sample at the current density of 20mA/cm2 after crystallized was not very stable, undulating much, finally the current potential was about -0.205V, which was more negative than that of the one at the current density of 5mA/cm2 after crystallized, -0.195V. This illustrates that the deposited sample at the current density of 5mA/cm2 after

Figure 1-18 shows the polarization curves of the deposited samples at different current densities after crystallized at 450C. It can be seen from the polarization curves in the figure that the voltage of the deposited sample at current density of 5mA/cm2 after crystallized was far lower than that of the one at 20mA/cm2. The breakdown potential of the deposited sample at current density of 5mA/cm2 after crystallized was 1.2V, but the one at 5mA/cm2 had no the passivation phenomenon emerging. This illustrates that the deposited sample at current density of 5mA/cm2 after crystallized displayed a very good anti-corrosion. Because the granularity of the deposited sample at5mA/cm2 was small, the surface even and very


different current densities in Hank's solution (PH7.45)

crystallized is with a better thermodynamic stability.

E/V(SCE)

property.

thin, with less crystal defects, after crystallized and heavy thermalstress, it was easy to corrode, thus presenting a poor corrosion resistance. From the cathodic-polarization curve of view, the deposited sample under current density of 5mA/cm2 after crystallized presented similarly a better cathodic-polarization property.

Fig. 1-16. The anodic-polarization curves of samples with different current densities in Hank's solution (PH7.45)

Fig. 1-17. The Corrosion potential change plotted as a function of time of heat treated sample by different current densities in Hank's solution (PH7.45)

Ti-O Film Cathodically-Electrodeposited on


Fusayama solution (PH6.13)

stability.

E/V(SCE)

the Surface of TiNi SMA and Its Bioactivity and Blood Compatibility 19

Figure 1-20 shows the polarization curves of the deposited TiNi SMA samples at different current densities in Fusayama solution of PH6.13. It can be seen from the polarization curves that in the process of polarization of the deposited sample at the current density of 5mA/cm2, the current density was far higher than that of the one at 20mA/cm2. Although the passivation region of the deposited sample at the current density of 5mA/cm2 was longer, its passivation current density was also higher than that of the deposited sample in large range. From the anodic-polarization curves of view as a whole, the deposited sample at the current density of 20mA/cm2 had a better corrosion resistance. The reason caused was similar to that the Ti-O film obtained at current density of 5mA/cm2 was very thick. From cathodic-polarization curves of view, the deposited sample at the current density of

012345

1

2

l g i / n A c m - <sup>2</sup>

Fig. 1-20. The anodic-polarization curves of samples with different current densities in

Figure 1-21 shows the corrosionpotential-time curves of the deposited TiNi SMA samples at different current densities after crystallized at 450C in Fusayama of PH6.13. It can be seen from the figure that the corrosion potential of deposited sample at the current density of 5mA/cm2 after crystallized went up with time, and the potential did not change basically at about -0.19V, but the one at higher current density was in a more negative potential, about - 0.21V. This illustrates that the sample depositing at the current density of 5mA/cm2 after crystallized in Fusayama solution was a continuous formation of a passivation film, its corrosion potential was more positive, and the film presented a better thermodynamic

Figure 1-22 shows the polarization curves of the deposited TiNi SMA samples at different current densities after crystallized at 450C in the fusayama solution of PH6.13. It can be seen from the polarization curves that the breakdown potential of deposited sample at the current density of 5mA/cm2 after crystallized was 0.7V, and the current density in the process of polarization was always lower than that of the one at 20mA/cm2, but there was no the passivation phenomenon emerging in depositing the sample. This illustrates that the deposited sample at current density of 5mA/cm2 after crystallized had a better property of

20mA/cm2 similarly presented a better cathodic-polarization property.

1. 20mA/cm <sup>2</sup> 2. 5mA/cm <sup>2</sup>

Fig. 1-18. The anodic-polarization curves of heat treated sample by different current densities in Hank's (PH7.45)

Figure 1-19 shows the corrosionpotential-time curves of the deposited TiNi SMA samples at different current densities in Fusayama solution of PH6.13. It can be seen that the potentials of both samples moved to more positive direction. The current density of the deposited sample at 20mA/cm2 had a positive potential, 0.15V, and the stable corrosion potential of the deposited sample at 5mA/cm2 was -0.12V. This illustrates that the deposited sample at the current density of 20mA/cm2 in Fusayama solution of PH6.13 has similarly a better thermodynamic stability.

Fig. 1-19. The Corrosion potential change plotted as a function of time of samples with different current densities in Fusyama solution (PH6.13)

1. 5mA/cm <sup>2</sup> 2. 20mA/cm <sup>2</sup>

012345

0 1 0 2 0 3 0 4 0 5 0 6 0

t / m i n

Fig. 1-19. The Corrosion potential change plotted as a function of time of samples with

Fig. 1-18. The anodic-polarization curves of heat treated sample by different current

Figure 1-19 shows the corrosionpotential-time curves of the deposited TiNi SMA samples at different current densities in Fusayama solution of PH6.13. It can be seen that the potentials of both samples moved to more positive direction. The current density of the deposited sample at 20mA/cm2 had a positive potential, 0.15V, and the stable corrosion potential of the deposited sample at 5mA/cm2 was -0.12V. This illustrates that the deposited sample at the current density of 20mA/cm2 in Fusayama solution of PH6.13 has similarly a better

l g i / n A c m - <sup>2</sup>

1 .

2 .

 5mA/cm <sup>2</sup> 20mA/cm <sup>2</sup>


E/V(SCE)

densities in Hank's (PH7.45)

thermodynamic stability.

E/V(SCE)


different current densities in Fusyama solution (PH6.13)

Figure 1-20 shows the polarization curves of the deposited TiNi SMA samples at different current densities in Fusayama solution of PH6.13. It can be seen from the polarization curves that in the process of polarization of the deposited sample at the current density of 5mA/cm2, the current density was far higher than that of the one at 20mA/cm2. Although the passivation region of the deposited sample at the current density of 5mA/cm2 was longer, its passivation current density was also higher than that of the deposited sample in large range. From the anodic-polarization curves of view as a whole, the deposited sample at the current density of 20mA/cm2 had a better corrosion resistance. The reason caused was similar to that the Ti-O film obtained at current density of 5mA/cm2 was very thick. From cathodic-polarization curves of view, the deposited sample at the current density of 20mA/cm2 similarly presented a better cathodic-polarization property.

Fig. 1-20. The anodic-polarization curves of samples with different current densities in Fusayama solution (PH6.13)

Figure 1-21 shows the corrosionpotential-time curves of the deposited TiNi SMA samples at different current densities after crystallized at 450C in Fusayama of PH6.13. It can be seen from the figure that the corrosion potential of deposited sample at the current density of 5mA/cm2 after crystallized went up with time, and the potential did not change basically at about -0.19V, but the one at higher current density was in a more negative potential, about - 0.21V. This illustrates that the sample depositing at the current density of 5mA/cm2 after crystallized in Fusayama solution was a continuous formation of a passivation film, its corrosion potential was more positive, and the film presented a better thermodynamic stability.

Figure 1-22 shows the polarization curves of the deposited TiNi SMA samples at different current densities after crystallized at 450C in the fusayama solution of PH6.13. It can be seen from the polarization curves that the breakdown potential of deposited sample at the current density of 5mA/cm2 after crystallized was 0.7V, and the current density in the process of polarization was always lower than that of the one at 20mA/cm2, but there was no the passivation phenomenon emerging in depositing the sample. This illustrates that the deposited sample at current density of 5mA/cm2 after crystallized had a better property of

Ti-O Film Cathodically-Electrodeposited on

Ti-O film to a large extent.

**resistance** 

treated)

the Surface of TiNi SMA and Its Bioactivity and Blood Compatibility 21

Figure 1-23 shows the optical microscope morphologies of the deposited samples at the current density of 20mA/cm2 for 7min before and after crystallization. It can be seen from figure 1-21a and figure 1-21b that before crystallization the surface Ti-O film, presenting bigger lump in shape, covered on the surface of TiNi SMA; it can be seen from figure 1-21c and figure 1-21d that the crystallized Ti-O film in smaller lump in shape distributed on the substrate surface. This is because of that the crystallization treatment made the hydrate (mainly Ti(OH)4) of amorphous Ti dehydrate water molecules and the crystal lattice recombinate to form the anatase type of TiO2. Due to that the current density in cathodic-electrodeposition was higher, the Ti-O film obtained was thicker too, so, in the process of crystallization, there might be uneven dehydration and stress distribution existing, thus bringing about serious crystal defects to affect the properties of

a 200× b 1000×

c 200× d 1000×

Fig. 1-23. Optical microscope result by current 20mA/cm2 for 7 min (a, b untreated; c, d heat

**1.3.6 Effect of crystallization treatment on surface morphology and corrosion** 

anti-corrosion. Similarly, this is because of that the deposited sample at current density of 5mA/cm2 was with small granularity, even surface and less crystal defects. From the cathodic-polarization curves of view, the deposited sample at current density of 5mA/cm2 after crystallized displayed a better cathodic-polarization property.

Fig. 1-21. The Corrosion potential change plotted as a function of time of heat treated samples by different current densities in Fusayama solution (PH6.13)

Fig. 1-22. The anodic-polarization curves of heat treated sample by different current densities in Fusayama solution (PH6.13)

anti-corrosion. Similarly, this is because of that the deposited sample at current density of 5mA/cm2 was with small granularity, even surface and less crystal defects. From the cathodic-polarization curves of view, the deposited sample at current density of 5mA/cm2

0 1 0 2 0 3 0 4 0 5 0 6 0

 5mA/cm <sup>2</sup> 20mA/cm <sup>2</sup>

t / m i n

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0

Fig. 1-22. The anodic-polarization curves of heat treated sample by different current

l g i / n A c m - <sup>2</sup>

1

2

Fig. 1-21. The Corrosion potential change plotted as a function of time of heat treated

samples by different current densities in Fusayama solution (PH6.13)

1. 5mA/cm <sup>2</sup> 2. 20mA/cm <sup>2</sup>

after crystallized displayed a better cathodic-polarization property.



densities in Fusayama solution (PH6.13)

0.0

0.2

0.4

E/V(SCE)

0.6

0.8

1.0

E/V(SCE)
