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Wear 268 (2010) 1537–1541

Contents lists available at ScienceDirect

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hort communication

retting-corrosion mapping of CP-Ti in Ringer’s solution

atendra Kumara, B. Sivakumara, T.S.N. Sankara Narayanana,∗, S. Ganesh Sundara Ramanb, S.K. Seshadrib

National Metallurgical Laboratory, Madras Centre, CSIR Complex, Taramani, Chennai 600113, IndiaDepartment of Metallurgical and Materials Engineering, Indian Institute of Technology Madras, Chennai 600036, India

r t i c l e i n f o

rticle history:eceived 9 June 2009eceived in revised form 27 January 2010ccepted 27 January 2010vailable online 4 February 2010

a b s t r a c t

Fretting corrosion is a complex phenomena and a variety of factors such as load, contact area, frequency,number of fretting cycles, extent of damage of the fretted zone, trapping of debris between the contactingsurfaces, corrosivity of the medium, etc., will determine the fretting-corrosion behaviour. The presentpaper aims to develop a fretting-corrosion map of commercially pure Ti (CP-Ti) based on its fretting-

eywords:rettingorrosion–wearlectrochemistryapping

corrosion behaviour in Ringer’s solution under various combinations of load, frequency and number offretting cycles. Based on the fretting-corrosion behaviour and restoration ability, a fretting-corrosion mapof CP-Ti in Ringer’s solution is developed for the first time. The fretting-corrosion map of CP-Ti in Ringer’ssolution enables identification of various regimes depending on the nature of predominant phenomenonfor a given set of conditions. Since fretting corrosion is the dominant phenomenon at lesser duration offretting at 5 and 10 Hz for a load of 3 and 5 N, the safer use of CP-Ti, particularly for hip and knee implants,

oint prostheses is a major concern.

. Introduction

Fretting corrosion is the deterioration of a material that occurst the interface of two contacting surfaces due to small oscilla-ory movements in presence of a corrosive medium. Orthopaedicmplants, particularly hip and knee implants, exposed to the physi-logical medium, suffer a lot due to fretting corrosion that leads to aeduction in the life time of such prosthesis. A variety of factors suchs load, contact area, frequency, number of fretting cycles, extentf damage of the fretted zone, trapping of debris between the con-acting surfaces, corrosivity of the medium, etc., could influencehe fretting-corrosion behaviour [1–6]. Wear mechanism mapsere developed in the past to understand the complex phenomena

nvolved in tribological contacts. Lim [7] has reviewed the develop-ent of wear mechanism maps for metals, ceramics, metal–matrix

omposites, polymers, coatings, fretting and erosion. Williams [8]as pointed out that there is no universal mechanism of wear ando simple correlation could be established between rates of wearr surface degradation and values of friction coefficient. Accordingo him, development of wear maps would enable identification of

he possible modes of surface damage and any transition between

ild and severe regimes of wear [8]. The development of wear mapor gray cast iron, titanium nitride coated high speed steel and theliding wear behaviour of dissimilar metallic interfaces and its rel-

∗ Corresponding author. Tel.: +91 44 2254 2077; fax: +91 44 2254 1027.E-mail address: tsnsn@rediffmail.com (T.S.N. Sankara Narayanan).

043-1648/$ – see front matter © 2010 Elsevier B.V. All rights reserved.oi:10.1016/j.wear.2010.01.026

© 2010 Elsevier B.V. All rights reserved.

evance to understand the wear mechanism is established by manyresearchers [9–12]. Stack and co-workers [13–17] have devel-oped tribocorrosion maps to understand the erosion–corrosion andmicro-abrasion corrosion behaviour of metals, coatings and com-posites. Wellman and Nicholls [18] have presented an overviewof high-temperature erosion–oxidation mechanisms, maps andmodels.

The concept of fretting maps is introduced by Vingsbo andSöderberg [19] to determine the relevant fretting regimes such asstick regime, mixed stick-slip regime and gross-slip regime. Elleuchet al. [20] have developed the fretting maps for anodized aluminiumalloys. Running condition fretting maps for WC–Co and TiN coat-ings have been constructed by Carton et al. [21] and Wei et al. [22]. Amap detailing the various damage mechanisms sustained by silver-plated copper contacts is proposed by Kassman and Jacobson [23].Sankara Narayanan et al. [24] and Park et al. [25] have proposedthe fretting-corrosion maps for tin plated electrical connector con-tacts. The development of fretting corrosion maps of titanium fororthopaedic implant application has not been attempted earlier. Inthis context, the present paper aims to develop a fretting-corrosionmap of commercially pure Ti (CP-Ti) based on its fretting-corrosionbehaviour in Ringer’s solution.

2. Experimental details

The fretting-corrosion behaviour of CP-Ti was studied using afretting-corrosion test assembly (Wear and Friction Tech., Chen-nai, India), which is similar to those described by Barril et al.

1538 S. Kumar et al. / Wear 268 (2010) 1537–1541

Table 1Test conditions used to study the fretting-corrosion behaviour of CP-Ti in Ringer’ssolution.

[tftcubTotsmbra6afcaiobbaatitiwptocepicttSuateCif

3

3

1sf

Amplitude (�m) 180Load (N) 3, 5, 7 and 10Frequency (Hz) 3, 5, 7 and 10Number of fretting cycles 1500, 3000, 4500, 9000, 18,000, 36,000

26] and Berradja et al. [27]. The details of the fretting-corrosionest assembly were described in our earlier papers [28,29]. Sinceretting-corrosion behaviour of titanium is influenced by many fac-ors such as frequency, amplitude, normal load, number of frettingycles, etc., the fretting-corrosion experiments were conductedsing several combinations of these variables to understand theehaviour. The test conditions employed in this study are listed inable 1. Ringer’s solution, having a chemical composition (in kg/m3)f 9 NaCl, 0.24 CaCl2, 0.43 KCl and 0.2 NaHCO3 (pH 7.8), was used ashe electrolyte solution. Before performing the fretting-corrosiontudies, the CP-Ti discs (20 mm diameter and 2 mm thick) wereechanically polished using various grades of SiC paper followed

y 0.3 �m alumina paste to a mirror-like finish (Ra = 0.02 �m) andinsed with deionized water. Subsequently, they were pickled usingmixture of 35 vol.%, HNO3–5 vol.% HF–60 vol.% H2O at 313 K for0–70 s, thoroughly rinsed with deionized water and dried usingstream of compressed air. The cleaned CP-Ti discs subjected to

retting corrosion formed the working electrode while a saturatedalomel electrode (SCE) and a graphite rod served as the referencend auxiliary electrodes, respectively. These electrodes were placedn the fretting-corrosion cell in such a way that only 2 cm2 areaf the working electrode was exposed to the Ringer’s solution. Aall-on-flat contact configuration that involves an 8 mm Ø aluminaall (G 10 finish; hardness: 1365 HV) moving against the station-ry CP-Ti disc was chosen so that large contact stresses could bechieved under very low loads. The alumina ball/CP-Ti flat con-act was arranged in such a way that they were totally immersedn the Ringer’s solution. The fretting-corrosion cell was connectedo a potentiostat/galvanostat/frequency response analyzer of ACMnstruments (model: Gill AC). The fretting-corrosion experiments

ere performed under conditions involving free (open circuit)otential and no additional potential was imposed by the poten-iostat. The change in free corrosion potential (FCP) (i.e., potentialf CP-Ti measured after immersion in Ringer’s solution in openircuit conditions) measured as a function of time was used tovaluate the performance of CP-Ti under fretting conditions. Beforeerforming the fretting-corrosion experiment, the CP-Ti was kept

n Ringer’s solution and allowed to stabilize for 1 h. The fretting-orrosion experiments were repeated at least three times to checkhe reproducibility of the test results. It is important to main-ain uniformity in mechanical polishing using various grades ofiC paper and alumina paste, surface finish (Ra = 0.02 �m), etchingsing HNO3–HF–H2O mixture at the recommended temperaturend time, rinsing using deionized water, drying and stabiliza-ion by immersing the sample in Ringer’s solution for 1 h, tonsure reproducibility of the test results. The variation in FCP ofP-Ti measured as a function of time and, the restoration abil-

ty after the fretting motion ceases, were used to develop theretting-corrosion map.

. Results and discussion

.1. Change in FCP as a function of time

The change in FCP of CP-Ti (conditions: load: 3 N; frequency:0 Hz; amplitude: 180 �m; number of fretting cycles: 36,000), mea-ured before the onset of fretting, during fretting and after theretting motion ceases, as a function of time, is shown in Fig. 1.

Fig. 1. Change in free corrosion potential (FCP) of untreated CP-Ti as a functionof time [conditions: load: 3 N; frequency: 10 Hz; amplitude: 180 �m; number offretting cycles: 36,000].

With the onset of fretting, a sudden drop (cathodic shift) in FCPwith a consequent increase in anodic current is observed. A simi-lar trend of decrease in FCP and an increase in anodic current withthe onset of fretting have been made earlier by many researcher[1,4,27,30]. It has been established that the potential of an electrodeshifts in the noble direction when a passive film grows on the sur-face with a consequent decrease in anodic current. On the contrary,the potential shifts in the negative direction upon damage and,partial or complete removal of the passive film with a consequentincrease in the anodic current [31]. Hence, the sudden drop in FCP(in about 30 s) (Fig. 1) and the consequent increase in anodic cur-rent observed for CP-Ti are due to the removal of the passive oxidelayer induced by fretting, suggesting an increase in susceptibility ofCP-Ti for corrosion. Ponthiaux et al. [32] have reported that the FCPof titanium during corrosion–wear test is quite close to the freshlyground material in the electrolyte. The extent of cathodic shift inFCP of CP-Ti further confirms the removal of the passive layer withthe onset of fretting and increase in corrosion susceptibility of CP-Tiin Ringer’s solution.

During fretting, some fluctuations in the FCP of CP-Ti areobserved. This is due to the periodic removal (depassivation) andgrowth (repassivation) of the passive film in the fretted zone. Thefluctuations in the FCP observed during fretting are due to theestablishment of a dynamic equilibrium between depassivationand repassivation [27,30–33]. When the fretting motion is ceased,the FCP of CP-Ti exhibits an anodic shift, suggesting the occurrenceof repassivation of the active area of the fretted zone. Ideally, thepotential should reach the initial steady state value (i.e., poten-tial before the onset of fretting, YFCP). Though CP-Ti tends to reachthe steady state potential, the repassivation is not instantaneousand the initial steady state potential is reached only after longerduration. A similar trend of change in FCP and anodic current isobserved at different loads, frequencies and number of frettingcycles. The extent of cathodic shift in potential is found to be higherwith increase in load and frequency. The ability to reach the initialsteady state potential is found to be a function of the extent of dam-age of the fretted zone, which is function of the load, frequency andnumber of fretting cycles.

3.2. Restoration ability of CP-Ti

Ennoblement of potential is a good indicator to study the com-bined influence of fretting wear and corrosion on the restorationability of CP-Ti after the passive film is partially or completely dam-aged due to fretting. It is evident from Fig. 1 that the ennoblement

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f potential is not instantaneous and the initial potential is reachednly after longer duration. While correlating the restoration abilityith the ennoblement of potential, two important factors, namely,

he extent and the rate of ennoblement of potential should be con-idered. The extent of ennoblement of potential is related to thextent of repair of the damaged (active) area while the rate of enno-lement of potential is related to the competition between rate oforrosion and rate of passive film formation. Calculation of the timeo reach the initial steady state potential and extent of ennoblementf potential indicate no reasonable correlation with the experimen-al variables. It might probably due to the variation in the potentialange for different experiments, which warrants normalization ofotential. The normalization of the potential was performed usinghe following equation:

′t = Yt − YFCP

�E

here Y ′t is the normalized potential; Yt is the measured poten-

ial at a given time; YFCP is the free corrosion potential measuredefore the onset of fretting; Y0 is the potential measured at theime at which the fretting motion is ceased; and �E = YFCP − Y0 (ashown in Fig. 1). Analysis of normalized potential vs. time curvesbtained using different combination of experimental conditionseveals that several factors influence the ability of CP-Ti to restoreo its initial steady state potential after the passive film is damaged.mong the experimental variables such as, normal load, frequencynd number of fretting cycles, each one of them has its own influ-nce. The extent of damage of the fretted zone due to the combinedction of fretting wear and corrosion is one of the important fac-ors that decide the restoration ability. A comparison of the extentf ennoblement of FCP obtained after 3000 and 36,000 frettingycles at 3 N, 10 Hz are shown in Fig. 2(a), the corresponding curvesbtained at 10 N, 10 Hz are shown in Fig. 2(b). When the numberf fretting cycles is limited to 3000, the extent of ennoblementf potential after the first 100 s could reach 86.50 and 63.90% atand 10 N, respectively. However, when the number of fretting

ycles is increased from 3000 to 36,000, the extent of ennoblementf FCP during the first 40–50 s is limited only to 10%. This is dueo the higher extent of damage of the fretted zone after 36,000ycles. Hence, it is evident that if the extent of damage of the fret-ed zone is higher, which is most likely at higher frequency and atigher fretting cycles, the extent of ennoblement of potential wille lower. This implies that a longer time period would be requiredo repair the damage and corrosion of the fretted zone would con-inue until the damage is repaired, which in turn will affect the ratef ennoblement.

During fretting, the debris generated might be trapped betweenhe CP-Ti and the alumina ball. Trapping of debris between CP-Tind the alumina ball is possible under higher normal loads. Thenvolvement of an adhesive wear mechanism when CP-Ti is frettedgainst the alumina ball suggests that under such conditions thextent of damage of the fretted zone might reduce drastically sincehe debris could act as a lubricant.

The damage of the passive film on CP-Ti due to fretting isestricted only to a limited area (amplitude: 180 �m) while in theemaining area the passive film is intact. This condition would leado the formation of a galvanic cell between the ‘active’ fretted zonend the ‘passive’ unworn area. The cathodic to anodic area rationd the potential difference between them would induce a strongnfluence on the extent of corrosion of titanium from the frettedone. During repassivation, a dynamic increase in the cathodic to

nodic area ratio and a decrease in the potential difference betweenhem (due to the repair of the damaged area) would occur and thishenomenon will continue until the damaged area is completelyepaired. Hence, it is evident that higher extent of damage and for-ation of galvanic cells would cause a deleterious effect on the

Fig. 2. Comparison of the extent of ennoblement of FCP obtained after 3000 and36,000 fretting cycles at (a) 3 N and 10 Hz; and (b) 10 N and 10 Hz.

restoration ability of CP-Ti whereas trapping of debris would favourthe restoration ability of CP-Ti.

Restoration of the passive film in the damaged area dependson the extent of damage and corrosion of the titanium acceler-ated by the formation of galvanic cell. During the initial period ofennoblement of potential, a dynamic equilibrium exists betweenthe extent of corrosion and formation of passive film, which wouldrespectively cause a cathodic and anodic shift in potential. Once thedamage is repaired, the growth of passive film will occur at a lin-ear rate law while subsequent thickening of the passive film wouldfollow a logarithmic rate law.

3.3. Development of a fretting-corrosion map

To identify the dominant mechanism and to develop a fretting-corrosion map for CP-Ti in Ringer’s solution, regression analysis wasperformed on the repassivation region, i.e., from the point at which

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he fretting motion is ceased (Y0) until the free-corrosion potentialYFCP) is reached. For each experiment, a sample of the measuredotential data with sampling duration varied from 0 to 1000 s wasonsidered for analysis. The model used for regression analysis is= K1 × ln(X) + K2; where Y denotes the potential; X denotes theuration to repassivate; K1 indicates the extent of corrosion resis-ance; and K2 indicates the extent of damage of the fretted zone. The

odel used to study the repassivation/regeneration of the passivelm on Ti and Ti–6Al–4V alloy was already reported by Hanawa etl. [34] and Komotori et al. [35]. The regression analysis performedn the repassivation region of all the experiments carried out usingarious combinations of experimental conditions indicate that K1alues range from 0.16 to 0.34 whereas K2 values range from −0.90o −1.67. The larger the K1 value and the less negative the K2 value,he higher the repassivation capability. Based on the ratio of thextent of damage (K2) to the extent of corrosion resistance (K1),enoted as ‘(K2/K1)’, the dominance of fretting wear and frettingorrosion for a given set of experimental conditions are assessed.he values of (K2/K1) ranging from −4.20 to −6.88 are rated in thecale of 1–10 using the following relation:

anking of(

K2

K1

)=

[{Kn − Kmin

Kmax − Kmin

}× 9

]+ 1

here Kn is the nth value of (K2/K1); Kmax is the maximum valuef (K2/K1); and Kmin is the minimum value of (K2/K1). The higherhe (K2/K1) value, the greater the extent of fretting wear com-ared to fretting corrosion. Fretting wear and fretting corrosion

re interrelated and they occur simultaneously. However, duringretting-corrosion test, for a given set of experimental conditions,retting wear, fretting corrosion or a combination of both could pre-ominate. Hence, segmentation of the various regimes becomesecessary for the construction of the fretting-corrosion maps. The

Fig. 3. Fretting-corrosion map of

(2010) 1537–1541

fretting-corrosion regimes are defined as follows:

1 ≤(

K2

K1

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3 ≤(

K2

K1

)< 5.5 Fretting corrosion-Fretting wear

5.5 ≤(

K2

K1

)< 7 Fretting wear-Fretting corrosion

7 ≤(

K2

K1

)≤ 10 Fretting wear dominated

The rationale used is quite similar to the tribocorrosion mappingmethodologies adopted by Stack and co-workers [14–17] and thefretting-corrosion maps proposed for tin plated contacts by SankaraNarayanan et al. [24] and Park et al. [25]. Fig. 3 depicts the fretting-corrosion map of CP-Ti subjected to fretting corrosion in Ringer’ssolution using various combinations of experimental conditions.The fretting-corrosion map is segmented into various regimes asfretting-corrosion dominant, fretting corrosion-fretting wear, fret-ting wear-fretting corrosion and fretting wear dominant. Theseregimes describe the mechanism of damage, i.e. dominated bytribological phenomena or corrosive phenomena, or intermediatebetween these processes for a given set of conditions. Besides, seg-mentation of these regimes also helps in understanding the extentof interaction between these mechanisms. The various regimesdescribed in the fretting-corrosion map (Fig. 3) represent a broadclassification. In each regime, various sub regimes might operate.In fretting wear regime depending on whether the alumina ball/CP-Ti pair stick to each other (‘stick regime’) or slide (‘slip regime’) orworn out by an intermediate mechanism, at least three sub regimes

are possible [19]. Similarly, complexity is also associated with thefretting-corrosion regime, as the extent of corrosion or passivationis largely determined by the reactivity of CP-Ti in the Ringer’s solu-tion, which can be predicted based on the Pourbaix diagram. Stack[14] has reviewed the modalities of bridging the gap between tri-

CP-Ti in Ringer’s solution.

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ology and corrosion by establishing a correlation between wearaps to Pourbaix diagram.It is evident from the fretting-corrosion map that at 5 Hz, fret-

ing corrosion is the dominant phenomenon at 3, 5 and 7 N and atesser duration of fretting (5 min) whereas the mechanism is tend-ng towards fretting corrosion-fretting wear at 10 N as well as withn increase in duration of fretting to 15, 30 and 60 min. At 10 Hz,retting corrosion is the dominant phenomenon at 3 and 5 N and atesser duration of fretting (5 min.) whereas the mechanism is tend-ng towards fretting corrosion-fretting wear at 7 and 10 N as well

ith an increase in duration of fretting to 15 min at 3 and 5 N. Theredominance of fretting corrosion under conditions mentionedbove raises concern for the safer use of CP-Ti, particularly for hipnd knee implants, where the occurrence of fretting corrosion isrevalent.

Scanning electron micrographs of the fretted zone of CP-Ti afterubjecting it to fretting corrosion in Ringer’s solution at 3 N, 5 Hz,80 �m for 18,000 fretting cycles reveal severe damage due to thextensive shear deformation and the ploughing action of the alu-ina ball, suggesting the involvement of adhesive galling as the

redominant wear mechanism [28,29]. Further work is required toorrelate the morphological features of the fretted zone with theretting-corrosion map.

. Conclusions

Based on the fretting-corrosion behaviour and restoration abil-ty under various combinations of load, frequency and number ofretting cycles, a fretting-corrosion map of CP-Ti in Ringer’s solutions developed. The fretting-corrosion map is segmented into variousegimes as fretting-corrosion dominant, fretting corrosion-frettingear, fretting wear-fretting corrosion and fretting wear dominant,epending on the nature of predominant phenomenon for a givenet of conditions. The dominance of fretting corrosion observed atand 10 Hz for lesser duration of fretting (5 min) at 3 and 5 N raises

oncern on the safer use of CP-Ti, particularly for hip and kneemplants, where the occurrence of fretting corrosion is prevalent.

cknowledgement

The authors express their sincere thanks to Director, Nationaletallurgical Laboratory, Jamshedpur, for his keen interest and

ermission to publish this paper.

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