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Comparison of fretting corrosion behaviour ofTi6Al4V alloy and CP-Ti in Ringers solution

B. Sivakumar, S. Kumar and T. S. N. Sankara Narayanan*

The fretting corrosion behaviour of Ti6Al4V alloy in Ringers solution was evaluated and

compared with that of commercially pure titanium (CP-Ti). Free corrosion potential, morphology of

the fretted zone, extent of formation and accumulation of debris and wear volume were used as

parameters of evaluation. Both Ti6Al4V alloy and CP-Ti behave similarly in terms of change in

free corrosion potential as a function of time, morphological features and wear mechanism. Ti

6Al4V alloy, however, exhibits an increase in corrosion susceptibility, decrease in tendency for

repassivation, higher amount of formation and accumulation of debris and an increase in wear

volume compared with CP-Ti. The study points out the importance of material selection for

implants that would encounter fretting corrosion.

Keywords: Tribocorrosion, Fretting corrosion, Ti and its alloys, Implant, Joint prostheses, Repassivation

Introduction

Titanium and its alloys are widely used as orthopaedicand dental implants due to their low density, better

mechanical properties, very high strength/weight ratio(specific strength), excellent corrosion resistance and

biocompatibility.15 Among the various types of Tialloys, Ti6Al4V alloy has been the choice in manyinstances because its mechanical properties and corrosion

resistance are ideal for implant applications. Studies onthe corrosion and biocompatibility aspects of Ti and itsalloys performed in vitro proved that the passive oxide

layer is stable and offers excellent corrosion protectionand biocompatibility.15 Implant retrieval analysis, how-

ever, reveals discolouration of the implant and accumula-tion of metal ions on tissues beside the implant.6 Theinferior mechanical properties of the naturally formed

passive oxide layer that could be disrupted at very lowshear stresses, even by rubbing against soft tissues, areconsidered responsible for such an occurrence.7 Owing to

the inherent property of titanium and its alloys, thepassive oxide layer could subsequently reform upon

reaction with the local environment. However, implantretrieval analysis confirms that the capability of restora-tion of the damagedpassive film is not instantaneous, as it

is generally believed.

Fretting corrosion is the deterioration of a materialthat occurs at the interface of two contacting surfaces

due to small oscillatory movements in the presence of acorrosive medium. Manufacturing of implant materials,though involves a component geometry specific locking

mechanism, micromotion do occur,8 which enables thepenetration of the body fluid into this junction and

facilitates mechanically assisted crevice and frettingcorrosion.9,10 The modular interfaces of total joint

prosthesis, mainly at the fixation of the implant stem

bone or cement, are subjected to micromotion (,100

mm) that could result in fretting corrosion.11,12 Thefretting corrosion behaviour of untreated and surface

modified titanium and its alloys was studied by many

researchers.1320 These studies confirm the following

observations:(i) removal of the passive oxide layer induced by

fretting

(ii) formation and entrapment of debris at thefretted zone, though most of them are pushed

away towards edges

(iii) increase in wear volume if the debris possesses

an abrasive character

(iv) delay in repassivation after the fretting motion

is ceased.

The fretting corrosion behaviour of Ti and its alloyscould be different in terms of the nature of the passive

film, susceptibility for corrosion upon removal of the

passive film, hardness of the alloy, extent of fretting

wear, abrasive nature of the oxide debris, rate ofcorrosion and leaching of alloying elements. Such a

comparison will be of much help to choose the right type

of material for implants that would encounter fretting

corrosion. In this perspective, the present work aims toevaluate the fretting corrosion behaviour of Ti6Al4V

alloy in Ringers solution and compare it with that of

commercially pure titanium (CP-Ti).

Experimental

Commercially pure titanium (grade 2) [with the chemi-

cal composition of Ti0?01N0?03C0?01H0?20Fe

0?18O (wt-%)] and Ti6Al4V alloy (grade 5) [with thechemical composition of Ti0?02N0?03C0?011H

0?22Fe0?16O6?12Al3?93V (wt-%)] discs of 20 mm in

Metallurgical Laboratory, Madras Centre, CSIR Complex, Taramani,

Chennai 600 113, India

*Corresponding author, email [email protected]

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;

2011 W. S. Maney & Son LtdReceived 3 September 2011; accepted 12 October 2011

15 8 Tribology 2011 VOL 5 NO 4 DOI 10.1179/1751584X11Y. 0000000020


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diameter and 2 mm in thickness were used as substrate

materials. They were mechanically polished using

various grades of SiC paper followed by 0?3 mmdiamond paste to a mirror finish, rinsed with deionised

water and dried using a stream of compressed air.

Fretting corrosion experiments were performed using a

fretting corrosion test assembly. The details of the test

assembly have already been discussed in our earlier

papers.1720 A ball on flat contact configuration that

involves a 8 mm w alumina ball (finish, G 10 grade;hardness, 1365 HV) moving against the stationary CP-

Ti/Ti6Al4V alloy flat was chosen so that large contact

stresses could be achieved under very low loads.

Normal loads of 3 and 10 N, oscillating frequencies of

5 and 10 Hz and linear peak to peak displacement

amplitude of 180 mm were used as the fretting corrosiontest parameters. The Hertzian contact pressures for the

loads used (3 and 10 N) will be ,500 and 1200 MPa.

The tests were performed for 18 000 (5 Hz) and 36 000

(10 Hz) fretting cycles. The test parameters employed in

this study imply a gross slip condition. Ringers solution,

having a chemical composition (in g L21) of 9NaCl

0?24CaCl20?43KCl0?2NaHCO3 (pH 7?8) at 310 K,

was used as the electrolyte solution. The CP-Ti/Ti

6Al4V alloy discs subjected to fretting corrosion

formed the working electrode, while a saturated calomel

electrode (SCE) and a graphite rod served as reference

and auxiliary electrodes respectively. These electrodes

were placed in the fretting corrosion cell in such a way

that only 2 cm2 area of the working electrode wasexposed to the Ringers solution. The alumina ballCP-

Ti/Ti6Al4V alloy flat contact was arranged in such a

way that they were totally immersed in the Ringers

solution. The fretting corrosion cell was connected to

a potentiostat/galvanostat/frequency response analyser

from ACM Instruments (model Gill AC) to measure the

free corrosion potential (FCP) of CP-Ti/Ti6Al4V

alloy as a function of time. Before the onset of fretting,

CP-Ti/Ti6Al4V alloy was allowed to stabilise for 1 h

in Ringers solution. The change in FCP of CP-Ti/Ti

6Al4V alloy was monitored as a function of time. The

FCP measurement was repeated at least three times to

ensure reproducibility of the test results. The morpho-logical features of the fretted zone were assessed using

SEM. Energy dispersive X-ray (EDX) analysis was

performed at selected regions of the fretted zone to

identify their chemical nature. The three-dimensional

(3D) profile of the fretted zone was assessed using anultrasonic based non-destructive testing device.

Results and discussion

Fretting corrosion behaviour of Ti6Al4V alloyFree corrosion potential measurement of Ti6Al4V alloy

The FCP is a qualitative indicator of the corrosion

regime (active or passive), in which a metal resides, andit has been used to evaluate the performance of Ti and

its alloys under fretting corrosion conditions.2024 Thechange in FCP of Ti6Al4V alloy recorded before the

onset of fretting, with the onset of fretting, during

fretting and after the fretting motion is ceased, is shownin Fig. 1a. The FCP is a mixed potential, reflecting the

state of the unworn material and those in the frettingwear track. Before the onset of fretting, the FCP of Ti

6Al4V alloy exhibits an anodic shift, suggestingthickening of the passive film during the initial

stabilisation period of 1 h. With the onset of fretting, a

sudden drop in FCP (cathodic shift) (Fig. 1a) anda surge in anodic current (Fig. 1b) are observed. A

similar observation was also made earlier by many

researchers during tribocorrosion of bare and surfacemodified Ti, Ti6Al4V alloy and stainless steel in many

environments.13,14,1723 Komotoriet al.24 have observed

these changes when Ti6Al4V alloy is scratched by a

sapphire ball in Ringers solution. According toPonthiaux et al.,25 the FCP of titanium becomes muchcloser to the freshly ground material in the electrolyte

during corrosion wear. The extent of cathodic shift in

FCP and the surge in anodic current observed in thepresent study indicates removal of the passive oxide

layer and increase in corrosion susceptibility of Ti6Al4V alloy in Ringers solution. The increase in applied

load from 3 to 10 N results in a higher cathodic shift inFCP (Fig. 1a). This is due to the effective removal of the

passive film that results in a larger active fretted area.

Contuet al.26 have also observed a similar effect duringthe mechanical abrasion of Ti and Ti6Al4V alloy in

inorganic buffer. The increase in frequency, from 5 to10 Hz, seems to have a relatively lesser influence than

those caused by the load (Fig. 1a).

1 Change in a FCP and b anodic current of Ti6Al4V alloy measured as function of time (fixed condition: amplitude,

180 mm; variables: load, frequency and fretting cycles)

Sivakumar et al. Corrosion behaviour of Ti6Al4V and CP-Ti in Ringers solution

Tribology 2011 VOL 5 NO 4 15 9


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During fretting, some fluctuations in the FCP of Ti

6Al4V alloy are observed following the periodicremoval (depassivation) and growth (repassivation) of

the passive oxide layer in the fretted zone, suggesting the

existence of a dynamic equilibrium between depassiva-

tion and repassivation phenomena.14,22 The averagevalues of fluctuation in FCP for a load of 3 N are 902

and 1122 mV at 5 and 10 Hz respectively. However,

when the load is increased from 3 to 10 N, the

corresponding values become 662 and 762 mV.

The decrease in the extent of fluctuations with the

increase in load indicates the decrease in tendency of

the alloy to repassivate. This can be attributed to theincrease when the load is increased from 3 to 10 N. After

the fretting motion is ceased, the FCP of Ti6Al4V

alloy exhibits an anodic shift, suggesting the occurrenceof repassivation. This is due to the rapid regeneration of

TiO2 layer in the active areas of the fretted zone

following the reaction of the fresh Ti metal ions with the

dissolved oxygen available in Ringers solution. Asimilar behaviour was observed earlier by Komotori

et al.24 and Ponthiauxet al.25 during the repassivation of

Ti and Ti6Al4V alloy. During repassivation, two

important factors, such as the ability of the material to

return to the initial steady state potential and the time

required for such an occurrence, should be considered.Ideally, the potential should reach the initial steady state

before the onset of fretting. After the fretting motion is

ceased, Ti6Al4V alloy attained its initial steady state

potential for a load of 3 N at 5 and 10 Hz. However,

when the load is increased to 10 N, though the FCP

reaches the initial steady state potential, the time taken

for this occurrence becomes relatively higher. The

increase in contact area of the fretted zone and the

2 Morphology of Ti6Al4V alloy after subjecting it to fretting corrosion (conditions: amplitude, 180 mm; frequency,

10 Hz; load, 10 N; fretting cycles, 36 000): a fretted zone (circled area) and surrounding areas (fretting direction is indi-

cated by doubled sided arrow mark); b central region of fretted zone; c debris collected at edges

3 Change in FCP of CP-Ti and Ti6Al4V alloy measured as function of time:a amplitude, 180 mm; load, 3 N; frequency, 5 Hz;

number of fretting cycles, 18 000;bamplitude, 180 mm; load, 10 N; frequency, 5 Hz; number of fretting cycles, 18 000

4 Rate of change in FCP of CP-Ti and Ti6Al4V alloymeasured during repassivation (after fretting motion is

ceased)


16 0 Tribology 2011 VOL 5 NO 4


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extent of damage at 10 N could be considered respon-sible for this behaviour.

Surface morphology of fretted zone of Ti6Al4V alloy

The morphology of Ti6Al4V alloy after frettingcorrosion (conditions: amplitude, 180 mm; frequency,10 Hz; load, 10 N; fretting cycles, 36 000) is shown inFig. 2.

The fretted zone (circled region) has experiencedsevere damage, whereas the surrounding areas of thefretted zone are relatively smooth, in which wear debrisis smeared all around (Fig. 2a). The central region of thefretted zone reveals severe damage due to the extensiveshear deformation and the ploughing action of the

alumina ball, suggesting the involvement of adhesivegalling as the predominant wear mechanism (Fig. 2b).Microwelding of surface asperities occurs during the

initial stages, whereas the asperities get sheared andplucked away in the subsequent stages. Redeposition ofthe removed material, confirmed by the presence ofdebris within the fretted zone (Fig. 2b), enables anincrease in roughness and further accelerates the wearrate. The debris collected at the edges of the fretted zoneis shown in Fig. 2c. The surface of the Al2O3 ballcounterface reveals the transfer of material from Ti

6Al4V alloy, which confirms the occurrence of adhesivegalling.

Comparison of fretting corrosion behaviour ofCP-Ti and Ti6Al4V alloyThe similarity in shape of the FCPtime curves of CP-Ti

and Ti6Al4V alloy suggests the occurrence of similarphenomena during fretting corrosion of these materials(Fig. 3).

5 Comparison of morphologies of a, c, e, g CP-Ti and b, d, f, h Ti6Al4V alloy after subjecting them to fretting corro-

sion (conditions: amplitude, 180mm; load, 10 N; frequency, 5 Hz; fretting cycles, 18 000): a, b entire fretted zone

(circled region; fretting direction is indicated by double sided arrow mark); c, d central region;

e, f edge region; g, h debris at edges




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In spite of the similarity in trend, some noticeable

differences could be observed. Compared with CP-Ti,

Ti6Al4V alloy exhibits a higher cathodic shift in FCP

with onset fretting, few fluctuations during fretting and a

decrease in the rate of anodic shift in FCP after thefretting motion is ceased. These effects are well

pronounced at 10 N (Fig. 3b). The higher cathodic shift

in FCP signifies the increase in susceptibility of Ti6Al

4V alloy for corrosion in Ringers solution. The fewfluctuations in FCP indicate the decrease in tendency of

Ti6Al4V alloy to repassivate. Contu et al.26 have also

reported that CP-Ti displays a better tendency for

repassivation than Ti6Al4V alloy in inorganic buffer

solution at pH 4?0 a n d 7?0. The delay in cathodic

reaction kinetics can be considered responsible for the

poor tendency for repassivation exhibited by Ti6Al4V

alloy compared with CP-Ti.27

The rate of change in FCP after the fretting motion is

ceased confirms the decrease in ability of Ti6Al4V

alloy to revert to the initial steady state (Fig. 4). The

time to reach a threshold value of 2550 mV(SCE) is

117 s for CP-Ti, whereas Ti6Al4V alloy under similarexperimental conditions (at 10 N/5 Hz) attains this

threshold only at 672 s.

A comparison of the morphologies of CP-Ti and Ti

6Al4V alloy after subjecting them to fretting corrosion

(conditions: amplitude, 180 mm; load, 10 N; frequency,

5 Hz; fretting cycles, 18 000) is shown in Fig. 5. The

fretted zone (circled region) has experienced severe

damage due to the extensive shear deformation and

ploughing action of the alumina ball (Fig. 5a and b),

suggesting the involvement of adhesive galling as the

predominant wear mechanism in both cases. However,the central (Fig. 5cand d) and edge regions (Fig. 5eand

f) of the fretted zone reveal that the amount of

formation of debris and their entrapment is relatively

high for Ti6Al4V alloy. In addition, the extent of

accumulation of debris in the edge regions of the fretted

zone is relatively high for Ti6Al4V alloy (Fig. 5gand

h).

The 3D profile of the fretted zone of CP-Ti and Ti

6Al4V alloy, after subjecting them to fretting corrosion

(conditions: amplitude, 180 mm; load, 3 N; frequency,

5 Hz; fretting cycles, 18 000), is shown in Fig. 6a and b

respectively.

It is evident from Fig. 6 that the wear volume is higherfor Ti6Al4V alloy than for CP-Ti under similar

conditions. Masmoudi et al.28 have pointed out that

6 Three-dimensional profile of fretted zone of a CP-Ti and b Ti6Al4V alloy after fretting corrosion (conditions: ampli-

tude, 180mm; load, 3 N; frequency, 5 Hz; fretting cycles, 18 000) (X and Y axes: values are in mm; Z axis: values are

in 61023 mm)




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the wear rate of nitric acid passivated Ti6Al4V alloy is

higher than that of CP-Ti in Ringers solution.

According to them, the lower thickness of the oxidefilm and its inferior resistance to corrosive medium are

responsible for the higher wear rate of Ti6Al4V

alloy.28 This observation is also supported by other

researchers.27,29,30 Contuet al.26 have reported that with

respect to corrosion, both CP-Ti and Ti6Al4V alloy

exhibit similar behaviours during mechanical abrasion

in inorganic buffer, since oxidation of titanium is the

major reaction. The tendency for repassivation, how-

ever, is relatively higher for CP-Ti than for Ti6Al4V

alloy.26 Martinet al.31 have suggested that the hardness

of wear debris becomes the controlling factor in

determining the performance of Ti6Al4V alloy under

tribocorrosion conditions. The nanohardness of Ti6Al

4V alloy is ,5?1 GPa, whereas the hardness of its debris

layer is reported to be nearly double.32

The decrease in ability of Ti6Al4V alloy to revert to

its initial steady state (Fig. 4) could be correlated to the

extent of formation and entrapment of debris at the

fretted zone, the hard and abrasive nature of the debris

and the increase in wear volume. The morphological

features (Fig. 5) of the fretted zone of Ti6Al4V alloy

confirm the generation of higher quantities of debris and

their entrapment in the fretted zone. The EDX analysis

performed in the regions marked as % of the fretted

zone of CP-Ti and Ti6Al4V alloy reveals their

chemical nature (Fig. 7). For CP-Ti, this region contains

67?83 at-% of Ti, 25?96 at-% of oxygen and 6?21 at-% ofAl (Fig. 7a), which indicates that it is predominantly

oxides of Ti. The presence of Al could have originated

from the alumina ball used as the counterface. For Ti

6Al4V alloy, this region contains 41?53 at-% of Ti,

47?09 at-% of oxygen, 8?93 at-% of Al, 1?44 at-% of V

and 1?01 at-% of Cl (Fig. 7b).

The higher oxygen content indicates that the extent of

oxidation of the fretted zone is relatively higher for Ti

6Al4V alloy compared with that of CP-Ti. The

presence of Al and V, with a corresponding decrease

in Ti, supports the formation of oxides of Al and V in

the fretted zone of Ti6Al4V alloy. The hard and

abrasive nature of the oxides of Al would have increasedthe wear rate/wear volume of Ti6Al4V alloy, which is

confirmed by the 3D profile of the fretted zone (Fig. 6).

The higher cathodic shift in FCP of Ti6Al4V alloy

with the onset of fretting, the decrease in tendency of the

alloy to repassivate during fretting and the decrease inability of the alloy to revert to the initial steady state

after the fretting motion is ceased assume significance.

The increase in corrosion susceptibility with the removal

of the passive layer induced by fretting, the poor

tendency for repassivation during fretting and the delay

in reaching the initial steady state potential would

induce leaching of Al and V ions, which could cause

long term health problems like Alzheimer disease and

neuropathy. Osteolysis, adverse tissue reactions, kidney

lesion, cytotoxicity, hypersensitivity and carcinogenesis

have been reported to be associated with V and Al

ions.5,3335 Vanadium may elicit local or especially

systemic reactions or inhibit cellular proliferation.

Aluminium may be associated with osteomalacia,

pulmonary granulomatosis and neurotoxicity.34,35 The

accumulation of wear debris may produce an adverse

cellular response, leading to inflammation, release of

damaging enzymes, osteolysis, infection, implant loosen-

ing and pain.36,37

Conclusion

The study on the fretting corrosion behaviour of Ti

6Al4V alloy in Ringers solution and comparison of its

behaviour with that of CP-Ti lead to the following

conclusions.

1. Both CP-Ti and Ti6Al4V alloy exhibit a similartrend of cathodic shift in FCP with the onset of fretting,

fluctuations in FCP during fretting and anodic shift in

FCP after the fretting motion is ceased. Adhesive galling

is the predominant wear mechanism when they are

fretted against the alumina ball.

2. The fretting corrosion behaviour of Ti6Al4V

alloy differs from that of CP-Ti in terms of: increase in

susceptibility for corrosion upon removal of the passive

oxide layer with the onset of fretting, decrease in

tendency to repassivate during fretting, decrease in

ability to revert to the initial steady state potential after

the fretting motion is ceased, higher amount of

formation and entrapment of debris at the fretted zoneand accumulation of the same at the edges, increased

extent of oxidation leading to the formation of oxides of

7 Analysis (EDX) performed on marked region: region marked as % in Fig. 5c and d of fretted zone of a CP-Ti and b Ti

6Al4V alloy (conditions: as described in Fig. 5)




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Al and V besides Ti and increase in wear volume due tothe abrasive nature of aluminium oxide.

3. The difference in performance of Ti6Al4V alloyand CP-Ti points out that the choice of materials for

implants that would encounter fretting corrosion shouldbe made only after a thorough evaluation.

Acknowledgement

The authors express their sincere thanks to Dr S.Srikanth, Director, National Metallurgical Laboratory,Jamshedpur, for his constant encouragement and sup-port and for his permission to publish this paper.

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