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Chapter 6
IN-VITRO CORROSION STUDIES
This chapter presents in-vitro corrosion behavior of the
investigated implant
materials in SBF (Ringer’s solution). Potentiodynamic
polarization and linear polarization
techniques were used to assess the corrosion susceptibilty of
HVFS coated and un-coated
substrate materials. The results of XRD, FE-SEM/EDS and
Elemental Mapping analysis
of the corroded materials have been incorporated. A part of the
chapter has already been
published elsewhere [288-289].
6.1 CORROSION BEHAVIOR
The potentiodynamic polarization curves of coated and un-coated
316L SS
specimens in Ringer’s solution at 37±1 °C temperature are shown
in Figure 6.1. These
curves are selected because their data is nearest to the mean
values of current densities of
the corresponding group of specimens. The corrosion parameters
determined from the
potentiodynamic curves of coated and un-coated 316L SS specimen
(Figure 6.1) by Tafel
extrapolation method are summarized in Table 6.1.
Figure 6.1: The potentiodynamic curves of flame-sprayed (1) HA-A
coated (2) HA-B coated
(3) HA-TiO2 coated (4) HA/TiO2 coated (5) un-coated, 316L SS
specimens in Ringer’s
solution at 37±1 °C temperature.
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The different corrosion parameters are anodic Tafel slope (βa),
cathodic Tafel slope
(βc), corrosion potential (Ecorr), corrosion current density
(ICorr) and CR. Polarization
resistance (Rp) values determined from linear polarization
tests, for coated and un-coated
316L SS specimens are also compiled in Table 6.1.
Table 6.1: Corrosion parameters of coated and un-coated 316L SS
in Ringer’s solution at
37±1 °C temperature
Parameters HA-A HA-B HA-TiO2 HA/TiO2 Un-coated
�a (e-3V/decade) 308.5 393.8 160.9 230.7 158.7
�c(e-3V/decade) 443.3 151.4 60.50 161.8 47.10
ECorr (mV) -468 -151 -442 -235 -329
ICorr (µAcm-2) 0.376 0.324 0.450 0.493 1.86
CR(mm/year) 0.0039 0.0034 0.0047 0.0051 0.0195
Rp (KΩ-cm2) 12.74 17.79 11.39 09.74 05.79
According to the analysis of Tafel slope values (Table 6.1), the
results show that
corrosion current density of un-coated 316L SS specimen in
Ringer’s solution (ICorr= 1.86
µAcm-2, ECorr= -329 mV) is higher than all of the coated
specimens. The polarization
curve, for the un-coated 316L SS specimen got shifted towards
the right in comparison
with all, the other specimens. The shift of polarization curves
of HA-B coated 316L SS to
lower ICorr values shows a lower tendency towards corrosion in
comparison with the un-
coated specimen. The higher the corrosion current density
(ICorr) at a given potential,
more prone is the material to corrode. HA-B coatings have shown
a minimum corrosion
current density (ICorr= 0.324 µAcm-2, ECorr= -151 mV) amongst
all the coated 316L SS
specimens as shown in Table 6.1.
The lower ICorr values of both HA-A (ICorr= 0.376 µAcm-2, ECorr=
-468 mV) and
HA-B coated 316L SS indicate that they are more corrosion
resistant than the HA-TiO2
composite coated (ICorr= 0.450 µAcm-2, ECorr= -442 mV) and
HA/TiO2 bond coated
(ICorr= 0.493 µAcm-2, ECorr= -235 mV) specimens. The results
indicated that un-coated
316L SS have lower corrosion resistance than all coated
specimens. After linear
polarization tests, HA-B coated 316L SS showed a highest
polarization resistance
(Rp=17.79 KΩ-cm2), while un-coated 316L SS showed a lowest
polarization resistance
(Rp=5.79 KΩ-cm2). Higher the value of Rp, lower is the corrosion
rate of specimens after
their immersion in Ringer’s solution. Furthermore, un-coated
316L SS exhibited a highest
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corrosion rate (0.0195 mm/year), while HA-B coated 316L SS
showed lowest corrosion
rate (0.0034 mm/year) amongst the all specimens. Summarizing, it
is found that all the
investigated coatings have been found to useful to enhance the
corrosion resistance of
316L SS.
The potentiodynamic polarization curves of coated and un-coated
Ti-6-4 specimens
in Ringer’s solution at 37±1 °C temperature are depicted in
Figure 6.2. The corrosion
parameters determined from the potentiodynamic curves for coated
and un-coated Ti-6-4
specimens (Figure 6.2) by Tafel extrapolation method are
summarized in Table 6.2. The
analysis of Tafel slope values showed that corrosion current
density of HA-B coated Ti-6-
4 have a lowest corrosion current density (ICorr=0.167 µAcm-2,
ECorr= -340 mV), while the
un-coated specimen exhibited a higher corrosion current density
(ICorr= 1.2 µAcm-2,
ECorr= -116 mV) amongst the investigated cases.
Figure 6.2: The potentiodynamic curves of flame-sprayed (1) HA-A
coated (2) HA-B coated
(3) HA-TiO2 coated (4) HA/TiO2 coated (5) un-coated, Ti-6-4
alloy specimens in Ringer’s
solution at 37±1 °C temperature.
The potentiodynamic polarization curve for un-coated Ti-6-4
specimen was found
to got shifted towards the right, while for HA-B coated Ti-6-4
shifted towards left in
comparison with the potentiodynamic curves for the other
specimens as shown in Figure
6.2. In Ti-6-4 specimens, the lower ICorr values of the both
HA-A (ICorr= 0.275 µAcm-2,
ECorr= -205 mV) and HA-B (ICorr=0.167 µAcm-2, ECorr= -340 mV)
coated specimens
indicate that they are more corrosion resistant than the HA-TiO2
(ICorr= 0.374 µAcm-2,
ECorr= -133 mV) and HA/TiO2 bond coated (ICorr= 0.394 µAcm-2,
ECorr= -305 mV)
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specimens. When ICorr is low, passivation is more easily
obtained, and corrosion resistance
is excellent [314].
Table 6.2: Corrosion parameters of coated and un-coated Ti-6-4
in Ringer’s solution at 37±1
°C temperature
Parameters HA-A HA-B HA-TiO2 HA/TiO2 Un-coated
�a (e-3V/decade) 201.5 221.1 147.6 364.7 112.2
�c(e-3V/decade) 255.9 77.10 67.30 430.4 64.50
ECorr (mV) -205 -340 -133 -305 -116
ICorr (µAcm-2) 0.275 0.167 0.374 0.394 1.200
CR(mm/year) 0.0023 0.0014 0.0031 0.0033 0.0102
Rp (KΩ-cm2) 19.80 24.80 16.68 11.70 06.23
Though all ICorr values reflect good corrosion resistance,
including un-coated 316L
SS and Ti-6-4 specimen, however it can be deduced through this
study that all the
investigated coatings have improved the corrosion resistance of
the un-coated specimens.
It can be analyzed from Table 6.1 and 6.2 respectively, that
HA-B coated Ti-6-4
specimen showed a lowest corrosion rate (0.0014 mm/year), while
the un-coated 316L
SS specimen exhibited a highest corrosion rate (0.0195 mm/year)
among all the
specimens, irrespective of the substrate material.
The polarization resistance values (Table 6.2) for the Ti-6-4
specimens also showed
the similar trends for corrosion behavior of the specimens.
Un-coated Ti-6-4 specimens
showed a lowest polarization resistance (Rp=6.23 KΩ-cm2), while
HA-B coated Ti-6-4
specimens showed a highest polarization resistance (Rp=24.80
KΩ-cm2). Obviously the
corrosion resistance of coated specimens has got enhanced in
comparison to the un-coated
specimens. The results show the protective behavior of the
coatings.
The mean and S.D values of corrosion rate (mm/year) for the
coated and un-coated
316L SS and Ti-6-4 specimens in Ringer’s solution at 37±1 °C
temperature are presented
in Table 6.3. It can be observed that mean values of corrosion
rate for HA-B and HA-A
coated 316L SS lie in the range of 0.0033±0.001 and 0.0039±0.001
(mm/year)
respectively. In HA-B and HA-A coated Ti-6-4, mean values of
corrosion rate lie
between 0.0015±0.001 and 0.0021±0.001 (mm/year). HA-TiO2
coatings on 316L SS and
Ti-6-4 have shown corrosion rate of 0.0043±0.003 and
0.0025±0.001 (mm/year)
respectively. HA/TiO2 bond coated 316L SS and Ti-6-4 have shown
corrosion rate of
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0.0064±0.001 and 0.0036±0.002 (mm/year) respectively. Whereas,
the corrosion rates for
un-coated 316L SS and Ti-6-4 were measured as 0.0172±0.005 and
0.0091±0.001
(mm/year) respectively.
Table 6.3: Mean and standard deviation values of corrosion rate
(mm/year) of coated and
un-coated 316L SS and Ti-6-4 specimens in Ringer’s solution at
37±1 °C temperature
Substrate HA-A HA-B HA-TiO2 HA/TiO2 Un-coated
316L SS 0.0039±0.001 0.0033±0.001 0.0043±0.003 0.0064±0.001
0.0172±0.005
Ti-6-4 0.0021±0.001 0.0015±0.001 0.0025±0.001 0.0036±0.002
0.0091±0.001
Based upon these values, it can again be concluded that
un-coated 316L SS has
suffered from highest corrosion, while HA-B coated Ti-6-4
specimens have shown a
lowest corrosion rate amongst all the tested specimens. Overall,
all the investigated
coated specimen showed a better corrosion resistance in
comparison with their un-coated
counterparts. Moreover, the corrosion resistance of HA coated
specimens was found to be
better than that of HA-TiO2 coated and HA/TiO2 bond coated
specimens. The corrosion
rates (Table 6.3) of coated and un-coated 316L SS and Ti-6-4
substrate materials after
their immersion in Ringer’s solution at 37±1 °C temperature are
in the following order:
Un-coated > HA/TiO2 coatings > HA-TiO2 coatings > HA-A
coatings > HA-B coatings
6.2 XRD ANALYSIS
XRD scans of HA-A coated 316L SS and Ti-6-4 after
electrochemical corrosion
testing in Ringer’s solution are shown in Figure 6.3 and 6.4
respectively. Though HA-A
coatings remained amorphous, however the intensity of XRD peaks
increased for both the
substrate cases after immersion in Ringer’s solution. Some XRD
peaks appeared on HA-
A coated Ti-6-4 substrate (Figure 6.4) after immersion in
Ringer’s solution, which
otherwise was highly amorphous in as-sprayed condition (Figure
5.10).
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Figure 6.3: XRD pattern of flame sprayed HA-A coating on 316L SS
after corrosion testing
in Ringer’s solution at 37±1 °C temperature.
Figure 6.4: XRD pattern of flame sprayed HA-A coating on Ti-6-4
alloy after corrosion testing in
Ringer’s solution at 37±1 °C temperature.
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XRD scans of flame sprayed HA-B coating on 316L SS and Ti-6-4
after immersion
in Ringer’s solution are shown in Figure 6.5 and 6.6
respectively. The analysis of HA-B
coatings on both the substrates confirmed the presence of HA,
with minor peaks for
TTCP and β-TCP phases. All the main peaks correspond to HA as a
dominant phase. HA-
B coatings appeared more crystalline, and it was found that the
intensity of XRD peaks
increased in comparison to as-sprayed HA-B coatings (Figure 5.11
and 5.12). It is
worthwhile to mention that a higher crystallinity leads to a
longer implant life [315]. It
has been reported that the biological response of HA coating
in-vivo was dominantly
affected by phase purity, crystallinity, and microstructure of
HA coatings [316].
Figure 6.5: XRD pattern of flame-sprayed HA-B coating on 316L SS
after corrosion testing
in Ringer’s solution at 37±1 °C temperature [β-TCP (β), TTCP
(T'), and HA (unmarked
peaks)].
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Figure 6.6: XRD pattern of flame-sprayed HA-B coating on Ti-6-4
alloy after corrosion
testing in Ringer’s solution at 37±1 °C temperature [β-TCP (β),
TTCP (T'), and HA
(unmarked peaks)].
The XRD scans of HA-TiO2 composite coatings on 316L SS and
Ti-6-4 after
electrochemical corrosion testing in Ringer’s solution are shown
in Figure 6.7 and 6.8
respectively. The analysis of these coatings on 316L SS
substrate (Figure 6.7) shows that
the coating structure mainly comprises HA and TiO2 (rutile)
phases, with a minor
presence of α-TCP and β-TCP phases. The peaks corresponding to
25.22°, 27.29°,
35.94°, 41.06° and 54.18° revealed the presence of TiO2 as a
dominant phase in the
coating. In HA-TiO2 coatings on Ti-6-4 substrate (Figure 6.8),
the coating structure
consists of HA, TiO2 (rutile) and β-TCP as the main phases with
minor presence of α-
TCP and TTCP phases. The XRD peaks at 27.67°, 36.33°, 41.49° and
54.56° revealed the
presence of TiO2 phase in the coating. There was no peak
corresponding to CaTiO3 or
Ca2Ti2O5 phases in the XRD patterns of HA-TiO2 composite
coatings. After immersion in
Ringer’s solution, the intensity of HA-TiO2 coatings showed
improvement, in comparison
to the as-sprayed HA-TiO2 coatings (Figure 5.13 and 5.14).
Furthermore, these coatings
appear to be more crystalline in comparison with their
as-sprayed condition for both the
substrates. Earlier, it has been reported that in
electrophoretic deposited HA coatings on
Ti-6-4 specimens, immersed in Hank’s solution at 37 °C for four
weeks, the XRD
analysis showed a significant increase in intensity of apatite
peaks. The apatite formation
reveals the bone bioactive behavior of HA coatings [317].
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Figure 6.7: XRD pattern of flame sprayed HA-TiO2 composite
coating on 316L SS, after
corrosion testing in Ringer’s solution at 37±1 °C temperature
[HA (H), TiO2 rutile (T), α-
TCP(α) and ββββ-TCP (ββββ)].
Figure 6.8: XRD pattern of flame sprayed HA-TiO2 composite
coating on Ti-6-4 alloy after
corrosion testing in Ringer’s solution at 37±1 °C temperature
[HA (H), TiO2 rutile (T),
TTCP (T'), α-TCP (α) and ββββ-TCP (ββββ)].
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XRD analysis of HA/TiO2 bond coated 316L SS (Figure 6.9) shows
that the coating
structure consists of HA with a minor peak of TTCP, whereas on
Ti-6-4 (Figure 6.10), the
peaks show the presence of HA, TTCP and β-TCP phases. Sharp XRD
peaks of exposed
specimens in Ringer’s solution confirm the dissolution of
amorphous phases during
immersion. A reduction in the hump size (between 29o 2θ and 32o
2θ) also supports the
decrease in amorphous content. It is generally believed that
amorphous phases are more
soluble than crystalline HA and further stimulate the earlier
bone growth [318]. HA
dissolution has been reported to be beneficial of the initial
stage for the transformation of
biological equivalents that have a mediator role between
osteoclast and osteoblast
differentiation [315].
Figure 6.9: XRD pattern of flame sprayed HA/TiO2 bond coating on
316L SS after corrosion
testing in Ringer’s solution at 37±1 °C temperature [HA
(unmarked peaks), TTCP (T')].
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Figure 6.10: XRD pattern of flame sprayed HA/TiO2 bond coating
on Ti-6-4 alloy after
corrosion testing in Ringer’s solution at 37±1 °C temperature
[HA (unmarked peaks),
TTCP (T')and ββββ-TCP (ββββ)].
6.3 SEM/EDS ANALYSIS
6.3.1 Surface Morphology
Surface oxide layers are of prime importance in biomaterials
since these oxide
layers come into direct contact with biological tissues [263].
Properties of the oxide film
such as stoichiometry, defect density, surface topography, and
crystal structures
determine long term corrosion and biological interactions with
alloy implants [319]. The
corrosion behavior of an implant material during its exposure to
human body environment
highly depends upon the stability of protective oxide layer.
The corroded specimens were further examined by SEM/EDS for
the
microstructural analysis of their surfaces and to detect the
compositional changes, if any,
in the specimens after the corrosion testing in Ringer’s
solution. There are only a few
studies, which report the microstructure of thermal sprayed HA
coatings after their
corrosion testing in SBFs. The microstructure of corroded HA-A
coating on 316L SS
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(Figure 6.11) and Ti-6-4 substrate (Figure 6.12) after immersion
in Ringer’s solution,
consists of well-flattened splats with the presence of some
spherical shaped particles.
Some voids are also seen in some places. However, the splats
seem to be fused to give a
well-bounded appearance. In other words the coatings have
retained their microstructure.
EDS point analysis confirms the presence of Ca, P, O and C
elements on both the
substrates. The presence of Cl and Na at some points in EDS
spectra of HA-A coatings
can be attributed to the Ringer’s solution as the specimens were
immersed into the
solution for their corrosion testing. Point 3 in HA-A coated
316L SS indicates Ca rich
area. Higher percentage of O and C in HA coating on both the
substrates indicates the
probable formation of oxides and carbides.
The microstructure of HA-B coating on 316L SS (Figure 6.13) and
Ti-6-4
specimens (Figure 6.14) consists of well-flattened splats with
the presence of some
spherical shaped particles. A comparison of the SEM micrographs
of the as-sprayed and
exposed HA-B coatings showed that the coating has retained its
morphology even after
exposure to the corrosion testing, which is a positive
attribute. EDS analysis confirms the
presence of Ca, P, O and C elements in both the HA-B coated 316L
SS and Ti-6-4
specimens. Ca and P have been detected as the predominant
elements in the coatings.
Both coated specimens showed marginal presence of Cl, since the
specimens were
immersed in the Ringer’s solution for their corrosion testing.
No cracks are found at the
surfaces of both the HA-B coated specimens after immersion in
Ringer’s solution,
However some dark regions in the HA coating on 316L SS specimens
might be
superficial pores in the microstructure. It is pertinent to
mention that these micro pores
may enhance the adhesion, spreading and proliferation of
osteoblasts [320]. A
comparison of EDS analysis of both HA-A and HA-B coated
specimens before and after
corrosion testing, shows that the atomic percentage of Ca and P
has decreased, whereas
that of O increased after their immersion in Ringer’s solution.
The dominance of O and C
in the composition indicates the formation of oxides and
carbides in the coatings.
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Figure 6.11: FE-SEM along with EDS point analysis of flame spray
HA-A coated 316L SS
after corrosion testing in Ringer’s solution.
Figure 6.12: FE-SEM along with EDS point analysis of flame spray
HA-A coated Ti-6-4
alloy after corrosion testing in Ringer’s solution.
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Figure 6.13: FE-SEM along with EDS point analysis of flame spray
HA-B coated 316L SS
after corrosion testing in Ringer’s solution.
Figure 6.14: FE-SEM along with EDS point analysis of flame spray
HA-B coated Ti-6-4
alloy after corrosion testing in Ringer’s solution.
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The calculated Ca/P ratio from EDS spectra for the flame sprayed
HA-A and HA-B
coatings on both the substrates after corrosion testing are
tabulated in Table 6.4. The
observed different Ca/P ratios confirm the presence of different
calcium phosphate
compounds in both the HA-A and HA-B coatings. The results
indicate reduction in Ca/P
ratios in both the coatings after immersion testing. A Ca/P
ratio of 1.37 in both the HA-A
coating (at point 1 in Figure 6.12), as well as HA-B coating (at
point 1 in Figure 6.14) is
very close to Ca/P ratio of 1.33, which supports the formation
of OCP. The Ca/P ratio of
2.00 in HA-A coating (at point 3 in Figure 6.12) and in HA-B
coatings (at point 1 in
Figure 6.13) indicates the presence of TCP compound. The Ca/P
ratio of 1.67 in HA-A
coatings (at point 2 in Figure 6.12) is a characteristic value
for HA phase. Furthermore, a
Ca/P ratio of 1.00 (at point 2 in Figure 6.14) indicates the
formation of dicalcium
phosphate anhydrous (DCPA) and dicalcium phosphate dihydrate
(DCPD).
Table 6.4: Ca/P ratio of HA-A and HA-B coatings on 316L SS and
Ti-6-4 alloy after
corrosion testing in Ringer’s solution
Type of coating Point 1 Point 2 Point 3
HA-A coating on 316L SS
(Figure 6.11)
1.16 1.14 -
HA-A coating on Ti-6-4
(Figure 6.12)
1.37 1.67 2.00
HA-B coating on 316L SS
(Figure 6.13)
2.00 1.12 1.20
HA-B coating on Ti-6-4
(Figure 6.14)
1.37 1.00 3.78
The SEM micrographs along with EDS point analysis of HA-TiO2
coatings on 316L
SS and Ti-6-4 specimens, after electrochemical corrosion testing
in Ringer’s solution are
shown in Figure 6.15 and 6.16 respectively. Once again, this
coating has almost retained
its morphology even after exposure to the corrosion testing for
both the substrate cases.
No cracks are found at the surfaces of HA-TiO2 coated specimens,
although some
micropores are clearly visible at the surface of both 316L SS
and Ti-6-4 specimens. The
presence of Ti (at point 1 and 3 in Figure 6.15 and at point 4
in Figure 6.16) has also been
confirmed along with other basic elements in composite coatings
by EDS point analysis.
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Figure 6.15: FE-SEM along with EDS point analysis of flame spray
HA-TiO2 composite
coating on 316L SS, after corrosion testing in Ringer’s
solution.
Figure 6.16: FE-SEM along with EDS point analysis of flame spray
HA-TiO2 composite
coating on Ti-6-4 alloy, after corrosion testing in Ringer’s
solution.
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EDS analysis for the coating on 316L SS confirms the presence of
Ca, P, O and C
elements with a marginal presence of Cl as shown by EDS point
analysis in Figure 6.17.
HA/TiO2 bond coating on Ti-6-4 (Figure 6.18), consists of
well-flattened splats, which
seem to be fused to each other to give a well-bounded
appearance. The composition of O
% is similar at all points in the coating.
No cracks are found at the surface of HA/TiO2 coated specimens
after immersion in
Ringer’s solution (Figure 6.17 and Figure 6.18). EDS point
analysis confirms that no Ti is
present on the surface of HA/TiO2 coated 316L SS and Ti-6-4
specimens. It shows that
bond coat of TiO2 remained covered completely with the HA top
coating even after the
exposure to Ringer’s solution. In EDS quantative analysis, a
Ca/P ratio of 1.37 in HA-
TiO2 coatings (at point 2 in Figure 6.15) indicates the
formation of OCP while a Ca/P
ratio of 1.67 (at point 1 in Figure 6.17) indicates the
formation of HA. A Ca/P ratio of
1.00 (at point 3 in Figure 6.18) corresponds to DCPA or DCPD
phases. It is pertinent to
mention that these phases are not suitable for implant
applications due to their higher
dissolution rate after implantation. The calculated Ca/P ratio
from EDS spectra for the
flame sprayed HA-TiO2 and HA/TiO2 coatings on both the
substrates after corrosion
testing are tabulated in Table 6.5.
Table 6.5: Ca/P ratio of HA-TiO2 composite and HA/TiO2 bond
coatings on 316L SS and Ti-
6-4 alloy after corrosion testing in Ringer’s solution
Type of coating Point 1 Point 2 Point 3
HA-TiO2 coating on 316L SS
(Figure 6.15)
- 1.37 1.20
HA-TiO2 coating on Ti-6-4
(Figure 6.16)
- 1.42 1.20
HA/TiO2 bond coating on 316L SS
(Figure 6.17)
1.67 1.20 1.20
HA/TiO2 bond coating on Ti-6-4
(Figure 6.18)
1.25 1.22 1.00
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Figure 6.17: FE-SEM along with EDS point analysis of flame spray
HA/TiO2 bond coating
on 316L SS, after corrosion testing in Ringer’s solution.
Figure 6.18: FE-SEM along with EDS point analysis of flame spray
HA/TiO2 bond coating
on Ti-6-4 alloy, after corrosion testing in Ringer’s
solution.
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6.3.2 Cross-sectional Morphology/EDS Mapping Analysis
A cross-sectional microscopic view of un-coated 316L SS
substrate after
corrosion testing in Ringer’s solution is shown in Figure 6.19.
The figure shows a dense
and uniform oxide scale on the 316L substrate surface. Oxide
layer is rich in O
concentration with minor amounts of Fe, Cr and Ni elements. A
cross-sectional SEM
view of un-coated Ti-6-4 substrate (Figure 6.20) reveals the
presence of a dense and
uniform oxide scale over the substrate after corrosion testing
in Ringer’s solution.
EDS maps confirm the presence of Ti, Al and V elements in the
oxide scale, along
with a significant presence of O in outer layers of the scale.
This indicates the formation
of oxides of Ti, Al and V. There is an inner band at the
scale/matrix interface, which is
completely depleted of Al. This indicates that Al may have
preferentially oxidized to
form Al-rich oxide scale at the top of the alloy, leaving an
Al-depleted inner band. There
is presence of some Ti, as well as, V-depleted zones in the
oxide scale, however the
extent is marginal in comparision with Al. It is pertinent to
mention that such depletions
are not favorable for any alloy as they may degrade the material
properties.
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Figure 6.19: Cross-sectional SEM micrograph and EDS elemental
maps of un-coated 316L
SS after corrosion testing in Ringer’s solution (scale bar = 5
µµµµm).
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Figure 6.20: Cross-sectional EDS elemental maps of un-coated
Ti-6-4 alloy after corrosion
testing in Ringer’s solution (scale bar = 5 µµµµm).
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143
A cross-sectional micrograph along with corresponding EDS maps
of flame sprayed
HA-A coating on the 316L SS after corrosion testing in Ringer’s
solution is shown in
Figure 6.21. The micrograph reveals that the steel substrate is
completely protected by the
HA-A coating. The mappings of Ca and P elements clearly
demonstrated that the
elements are uniformly distributed in the coating. O is present
throughout the coating. Fe,
Cr and Ni are restricted to the base steel, without any
diffusion in the coating zone. This
is also a desirable feature of a good coating system.
The cross-section of HA-A coating on Ti-6-4 (Figure 6.22) shows
a lamellar
structure. The coating-substrate interface seems to be intact.
EDS maps show that Ti, Al
and V elements are present in the substrate only without showing
any diffusion into the
coating area. The EDS maps of as-sprayed (Figure 5.26) and
exposed (Figure 6.21)
specimens of HA-A coated Ti-6-4 have not shown any significant
differences.
From the cross-sectional image and EDS mapping analysis of HA-B
coatings on
316L SS after corrosion testing (Figure 6.23), it appears that
HA coating particles (Ca and
P) of the exposed specimen might have got detached during
specimen preparation. EDS
mapping shows that Fe, Cr and Ni elements are present in the
substrate only without
showing any diffusion into the coating area. In the HA-B coated
Ti-6-4 (Figure 6.24), the
analysis indicates a dense coating on the substrate. The coating
mainly contains Ca and P
elements. O is present throughout the coating area. The coating,
by and large, has retained
its identity.
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Figure 6.21: Cross-sectional EDS elemental maps of flame spray
HA-A coated 316L SS after
corrosion testing in Ringer’s solution (scale bar = 10
µµµµm).
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Figure 6.22: Cross-sectional EDS elemental maps of flame spray
HA-A coated Ti-6-4
alloyafter corrosion testing in Ringer’s solution (scale bar =
50 µµµµm).
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146
Figure 6.23: Cross-sectional EDS elemental maps of flame spray
HA-B coated 316L SS after
corrosion testing in Ringer’s solution (Scale bar = 20
µµµµm).
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147
Figure 6.24: Cross-sectional EDS elemental maps of flame spray
HA-B coated Ti-6-4 alloy
after corrosion testing in Ringer’s solution (scale bar = 20
µµµµm).
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148
In the cross-sectional analysis of HA-TiO2 composite coatings on
316L SS (Figure
6.25); Ca, P and Ti elements are found to exist in the coating
region. Wherever Ca and P
elements are present, Ti is found to be absent and vice-versa.
Fe and Cr elements are
mainly restricted to the base steel substrate. The presence of
Ti is apparent in both the
coating and substrate in HA-TiO2 composite coating on Ti-6-4
(Figure 6.26). O is present
throughout the coating, which indicates the possibility of
formation of different oxides.
There is no diffusion of elements from the substrate to the
coating or vice-versa as is
visible from the maps. However, Al is present in the coating
region in a scattered manner.
This may be due to the diffusion of Al from the substrate or
artifacts of Al2O3 polishing.
A comparison of EDS maps of as-sprayed (Figure 5.29 and Figure
5.30) and exposed
(Figure 6.25 and Figure 6.26) specimens of HA-TiO2 coatings has
not shown any
significant differences.
The cross-sectional SEM/EDS analysis of flame sprayed HA/TiO2
bond coating on
the 316L SS and Ti-6-4 specimens, after corrosion testing in
Ringer’s solution is shown
in Figure 6.27 and 6.28 respectively. From the analysis of both
the bond coatings, it
appears that HA/TiO2 coating on 316L SS specimen (Figure 6.27)
is relatively better
adhered in comparison with that on Ti-6-4 specimen (Figure
6.28). O is present
throughout both the coatings. Fe and Cr elements are present in
the substrate only without
showing any diffusion into the coating area. HA/TiO2 coating, in
general is dense and
homogenous. TiO2 bond layer has retained its good adhesion to
the base substrate and top
HA coating even after immersion in Ringer’s solution. There are
no cracks or
delaminations in the HA/TiO2 coatings for both the substrates.
The coating-substrate
interface seems to be intact and defect-free on both the
specimens. These coatings have
been successful to avert diffusion of various base alloy
elements during corrosion testing.
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Figure 6.25: Cross-sectional EDS elemental maps of flame spray
HA-TiO2 composite coating
on 316L SS after corrosion testing in Ringer’s solution (scale
bar = 10 µµµµm).
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150
Figure 6.26: Cross-sectional EDS elemental maps of flame spray
HA-TiO2 composite coating
on Ti-6-4 alloy after corrosion testing in Ringer’s solution
(scale bar = 50 µµµµm).
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151
Figure 6.27: Cross-sectional EDS elemental maps of flame spray
HA/TiO2 bond coating on
316L SS after corrosion testing in Ringer’s solution (scale bar
= 20 µµµµm).
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152
Figure 6.28: Cross-sectional EDS elemental maps of flame spray
HA/TiO2 bond coating on
Ti-6-4 alloy after corrosion testing in Ringer’s solution (scale
bar = 100 µµµµm).