486
T. Balusamy et al.
of the as-SMATed TLM alloy at 200 °C for 10 h has led to a slight increase in
corrosion rate from 1.28 × 10
−7 A/cm
2 to 2.13 × 10
−7 A/cm
2 . The passive current
density (i pass ) indicates the tendency of the alloy to form an intact passive film. The
i pass of the solution treated TLM alloy is 1.86 × 10
−4 A/cm
2 . SMAT increased the
tendency of the alloy to form a stable passive oxide layer with a higher thickness,
which is reflected by a twofold decrease in i pass from 1.86 × 10
−4 A/cm
2 to 1.35 ×
10
−6 A/cm
2 . However, post annealing of the as-SMATed TLM alloy at 200 °C for
10 h only slightly increased the i pass from 1.35 × 10
−6 A/cm
2 to 2.39 × 10
−6 A/cm
2 .
The improvement in corrosion resistance observed for the SMAT TLM alloy is due
to the decrease in grain size, increase in grain boundary density, dilution of segregated
alloying elements at grain boundaries and an increase in tendency of the treated
alloy to form an intact passive oxide layer with a higher thickness. SPD induced by
SMAT enables refinement of grains from microscale to nanoscale. The increase in
grain boundary density following nanocrystallization helps to dilute the segregated
alloying elements at the grain boundaries. The high density of dislocations induced
during plastic deformation and the increase in grain boundary density promotes the
formation of a stable and thicker passive oxide layer. Since the as-SMATed TLM alloy
and those subjected to post-annealing are subsequently polished using SiC coated
abrasive paper (2500 grade) followed by mirror polishing using 0.05 μm alumina
powders, the deleterious influence of higher surface roughness of the SMAT TLM
alloy on corrosion resistance is nullified.
The corrosion resistance of low modulus Ti-Nb-Ta-O alloy with two different
microstructures before and after SMAT in SBF is evaluated by Acharya et al. [54].
One set of the Ti-Nb-Ta-O alloy sample possesses equiaxed β-grains (STQ) while
the other set consists of uniform precipitation of α-platelets along the grain boundary
as well as inside the β-grains in the β-matrix (aged). Both of them were subjected
to SMAT using 500 numbers of 4.75 mm Ø hardened steel balls (STQ-SMAT and
aged-SMAT). SMAT has led to a decrease in i corr of STQ from 21.0 ± 3.0 × 10
−3
μA/cm
2 to 7.4 ± 2.7 × 10
−3
μA/cm
2 (for STQ-SMAT). Similarly, the i corr of aged
sample is decreased from 152.0 ± 9.0 × 10
−3
μA/cm
2 to 69.9 ± 0.3 × 10
−3
μA/cm
2
(for aged-SMAT). The critical current for passivation (i pass ) also follows trend similar
to that of the i corr . However, i pass appears to be more prominent than i corr . The lower
i pass of SMATed samples points out an increase in tendency of the treated alloy
to form a stable passive oxide layer with higher thickness. A stable higher phase
angle value over a wide range of frequencies in the Bode plot further substantiates
this phenomenon. The relatively higher i corr and i pass values of the untreated and
SMATed aged samples, when compared to the respective STQ samples, are due to
the difference in their microstructure. The presence of both α and β phases in the
aged samples could promote the formation of galvanic cells and non-uniform oxide
layer. Mott-Schottky analysis indicates a large decrease in the charge carrier density
for SMATed STQ and aged samples when compared to its untreated counterparts.
Since a lower charge carrier current density in Mott-Schottky plots signifies a higher
stability of the passive oxide layer, it is clear that SMAT is beneficial in improving
the corrosion resistance of the treated alloy.
T. Balusamy et al.
of the as-SMATed TLM alloy at 200 °C for 10 h has led to a slight increase in
corrosion rate from 1.28 × 10
−7 A/cm
2 to 2.13 × 10
−7 A/cm
2 . The passive current
density (i pass ) indicates the tendency of the alloy to form an intact passive film. The
i pass of the solution treated TLM alloy is 1.86 × 10
−4 A/cm
2 . SMAT increased the
tendency of the alloy to form a stable passive oxide layer with a higher thickness,
which is reflected by a twofold decrease in i pass from 1.86 × 10
−4 A/cm
2 to 1.35 ×
10
−6 A/cm
2 . However, post annealing of the as-SMATed TLM alloy at 200 °C for
10 h only slightly increased the i pass from 1.35 × 10
−6 A/cm
2 to 2.39 × 10
−6 A/cm
2 .
The improvement in corrosion resistance observed for the SMAT TLM alloy is due
to the decrease in grain size, increase in grain boundary density, dilution of segregated
alloying elements at grain boundaries and an increase in tendency of the treated
alloy to form an intact passive oxide layer with a higher thickness. SPD induced by
SMAT enables refinement of grains from microscale to nanoscale. The increase in
grain boundary density following nanocrystallization helps to dilute the segregated
alloying elements at the grain boundaries. The high density of dislocations induced
during plastic deformation and the increase in grain boundary density promotes the
formation of a stable and thicker passive oxide layer. Since the as-SMATed TLM alloy
and those subjected to post-annealing are subsequently polished using SiC coated
abrasive paper (2500 grade) followed by mirror polishing using 0.05 μm alumina
powders, the deleterious influence of higher surface roughness of the SMAT TLM
alloy on corrosion resistance is nullified.
The corrosion resistance of low modulus Ti-Nb-Ta-O alloy with two different
microstructures before and after SMAT in SBF is evaluated by Acharya et al. [54].
One set of the Ti-Nb-Ta-O alloy sample possesses equiaxed β-grains (STQ) while
the other set consists of uniform precipitation of α-platelets along the grain boundary
as well as inside the β-grains in the β-matrix (aged). Both of them were subjected
to SMAT using 500 numbers of 4.75 mm Ø hardened steel balls (STQ-SMAT and
aged-SMAT). SMAT has led to a decrease in i corr of STQ from 21.0 ± 3.0 × 10
−3
μA/cm
2 to 7.4 ± 2.7 × 10
−3
μA/cm
2 (for STQ-SMAT). Similarly, the i corr of aged
sample is decreased from 152.0 ± 9.0 × 10
−3
μA/cm
2 to 69.9 ± 0.3 × 10
−3
μA/cm
2
(for aged-SMAT). The critical current for passivation (i pass ) also follows trend similar
to that of the i corr . However, i pass appears to be more prominent than i corr . The lower
i pass of SMATed samples points out an increase in tendency of the treated alloy
to form a stable passive oxide layer with higher thickness. A stable higher phase
angle value over a wide range of frequencies in the Bode plot further substantiates
this phenomenon. The relatively higher i corr and i pass values of the untreated and
SMATed aged samples, when compared to the respective STQ samples, are due to
the difference in their microstructure. The presence of both α and β phases in the
aged samples could promote the formation of galvanic cells and non-uniform oxide
layer. Mott-Schottky analysis indicates a large decrease in the charge carrier density
for SMATed STQ and aged samples when compared to its untreated counterparts.
Since a lower charge carrier current density in Mott-Schottky plots signifies a higher
stability of the passive oxide layer, it is clear that SMAT is beneficial in improving
the corrosion resistance of the treated alloy.