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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)].

131

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.

134

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.

135

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.

136

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.

138

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.

140

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.

141

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).

142

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).

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.

144

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).

145

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).

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).

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).

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.

149

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).

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).

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).

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).