Short communication

Electrochemical behavior of intermetallic Ti3Al-based alloys

in simulated human body fluid environment

Animesh Choubey, B. Basu, R. Balasubramaniam*

Department of Materials and Metallurgical Engineering, Indian Institute of Technology, Kanpur 208016, India

Received 27 January 2004; received in revised form 14 March 2004; accepted 16 March 2004

Abstract

The electrochemical behavior of Ti-based intermetallic alloys (in wt%) Ti–15Al and Ti–13.4Al–29Nb has been evaluated in simulated

human body environment and compared with that of Ti–6Al–4V. Potentiodynamic polarization experiments conducted in Hank’s solution

at 37 8C indicated stable passive polarization behavior for all the alloys. The passive range was lower in the case of Ti–15Al. The passive

current densities and corrosion rates of the alloys were comparable.

q 2004 Elsevier Ltd. All rights reserved.

Keywords: A. Titanium aluminides, based on Ti3Al; B. Corrosion; G. Biomedical applications

1. Introduction

Ti alloys are used in orthopaedic applications owing to

their good biocompatibility, appropriate mechanical proper-

ties and excellent corrosion resistance [1]. For example, the

corrosion resistance of Ti–6Al–4V is superior to other

implant materials like stainless steel and alloys based on

cobalt–chromium [2]. There have been significant devel-

opments in intermetallic alloys based on Ti3Al [3]. The

addition of niobium is beneficial to the mechanical and

high-temperature oxidation behavior [4]. It has been

recently reported that intermetallic Ti–13Al–29Nb alloy

forms protective oxide scales over its surface in marine and

industrial environments, both at low and high temperatures

[5]. Its possible use in marine and industrial environments

has been suggested [5]. Ti-based intermetallic alloys, owing

to their excellent strength, light weight and ability to form

protective oxide scales, can be attractive materials for

human body implant applications. In the present investi-

gation, two Ti-based intermetallic alloys, Ti–15Al and Ti–

13Al–29Nb, were evaluated for their electrochemical

behavior in simulated human body fluid conditions at 37 8C.

2. Experimental

The materials used in the present investigation were Ti–

15Al, Ti–6Al–4V, Ti–13.4Al–29Nb (all compositions are

in weight %). The Ti–15Al composition corresponds to the

stoichiometric intermetallic Ti3Al. The typical interstitial

content (in ppm) in the intermetallic alloys was 1400 O, 100

N, and 20 H. The intermetallic alloys were received in the

form of pancakes from the Defence Metallurgical Research

Laboratory, Hyderabad, India. The Ti–6Al–4V alloy was

obtained in sheet form of dimensions 10 cm £ 10 cm and

1 cm thickness. Specimens for electrochemical studies were

sectioned using a diamond cutter. In case of the pancakes,

the samples for electrochemical testing were obtained from

the top region of the pancake. In case of Ti–6Al–4V, the

rolling plane was mounted. The areas to be exposed for

electrochemical studies were 1 cm £ 1 cm square. After

soldering a wire to the back of the specimen, each specimen

was cold mounted. Prior to the start of each experiment, the

cold mounted specimen was polished up to ANSI 800 grit

(size 12.2 mm) on emery paper and then finally polished

using 0.5 mm alumina powder. The specimens were then

washed in distilled water followed by ultrasonic cleaning in

acetone.

Electrochemical polarization studies were conducted in

a round bottom polarization cell. The potential of the

working electrode was measured against saturated calomel

0966-9795/$ - see front matter q 2004 Elsevier Ltd. All rights reserved.

doi:10.1016/j.intermet.2004.03.012

Intermetallics 12 (2004) 679–682

www.elsevier.com/locate/intermet

* Corresponding author.

E-mail address: bala@iitk.ac.in (R. Balasubramaniam).

electrode (SCE). A graphite rod was used as a counter

electrode. The electrochemical studies were conducted

using a EG&G 263A potentiostat, interfaced to a

computer. The electrolyte used for simulating human

body fluid conditions was Hank’s solution (pH 7.4),

prepared using laboratory grade chemicals and double

distilled water. Freshly-prepared solution was used for

each experiment. The composition of the Hanks solution

used was (in gm/l) 8 NaCl, 0.4 KCl, 0.14 CaCl2, 0.06

MgSO4·7H2O, 0.06 NaH2PO4·2H2O, 0.35 NaHCO3, 1.00

Glucose, 0.60 KH2PO4 and 0.10 MgCl2·6H2O. A constant

electrolyte temperature of 37 ^ 2 8C was maintained

Fig. 1. Optical micrographs of (a) Ti–15Al (b) Ti–6Al–4V (c) Ti–

13.4Al–29Nb. The microstructures were obtained on the area used for

electrochemical testing in simulated body fluid solution.

Fig. 2. Potentiodynamic polarization curves obtained in Hank’s solution at

pH 7.4 and 37 8C for (a) Ti–15Al, (b) Ti–6Al–4V and (c) Ti–13.4Al–

29Nb. The curves marked by bracketed numbers ‘1’ and ‘2’ are the results

from two independent tests.

A. Choubey et al. / Intermetallics 12 (2004) 679–682680

using a heating mantle. All the potentiodynamic polari-

zation studies were conducted after stabilization of the

free corrosion potential. The scan rate used was

0.166 mV/s. The corrosion rate was determined using

the Tafel extrapolation method, as per ASTM standard

[6]. All the tests were duplicated. The microstructures of

the surfaces exposed for electrochemical testing were also

studied using an optical microscope (Axiolab A, Zeiss,

Germany). The microstructures were revealed by etching

in 10% HF þ 5% HNO3 solution for 5–10 s at room

temperature.

3. Results and discussion

In the microstructures that would be presented, beta

phase appears dark and the alpha phase light. The

microstructures of the alloys are presented in Fig. 1. The

Ti–15Al alloy revealed a typical cast structure of alpha

platelets (Fig. 1a). The alloy Ti–6Al–4V possessed a two-

phase alpha–beta structure (Fig. 1b). Vanadium, niobium

and iron are beta stabilizers while aluminium is an alpha

stabilizer. Alpha is the dominant phase in all these alloys as

evident from the microstructures. It has been reported that,

owing to a two-phase equiaxed microstructure, Ti–6Al–4V

is susceptible to corrosion because the compositional

difference across the grain boundaries increases. This

leads to the galvanic cell formation [7]. The structure of

Ti–6Al–4V was fairly fine grained (Fig. 1b). Two phases

(alpha þ beta) with prior beta grain boundaries were

observed in Ti–13.4Al–29Nb (Fig. 1c).

The nature of stabilization of free corrosion potential with

time was similar for all the alloys. The potential moved

towards noble potential on immersion and stabilized in a

relatively short period of time (,1000 s). Duplicate poten-

tiodynamic polarization curves for Ti–15Al, Ti–6Al–4V

and Ti–13.4Al–29Nb alloy are shown in Fig. 2. The nature of

polarization curves indicated that all the alloys passivated

immediately on immersion in the solution. The polarization

behavior can be termed as stable passivity. This behavior was

noted for all the samples. The passivation parameters like

breakdown potential ðEbÞ; passive current density ðipassÞ and

the passive range (Eb –ZCP) were estimated from the

polarization curves and they are tabulated in Table 1. The

zero current potential (ZCP) of all Ti-alloys was in the range

of2276 to2585 mV vs SCE. Passive current densities were

obtained around the middle of the passive range (Table 1). The

passive current densities of the alloys investigated were of the

same order of magnitude. The highest breakdown potential

was exhibited by Ti–6Al–4V. The passive range in the case

of materials exhibiting stable passive behavior is provided by

the difference between breakdown and zero current potentials.

Addition of aluminum decreased the passive range signifi-

cantly, as can be noted from the data for Ti–15Al. On the

other hand niobium addition increased the passive range but

not as much as that for Ti–6Al–4V (Table 1).

The experimental Tafel plots were analyzed. The zero

current potential, the cathodic ðbcÞ and anodic ðbaÞ Tafel

slopes, the estimated corrosion current densities ðicorrÞ and

corrosion rates are tabulated in Table 2. The corrosion rates

were fairly reproducible. The corrosion rates of the alloys

were comparable.

Table 1

Passivation parameters obtained from the potentiodynamic polarization curves of the alloys in Hank’s solution at pH 7.4 and 37 8C

Sample ZCP (mV vs SCE) Eb (mV vs SCE) ipass (mA/cm2) Passive range (mV)

(1) (2) (1) (2) (1) (2) (1) (2)

Ti-15Al 2389 2548 750 750 1.6 2.5 1139 1298

Ti-6Al-4V 2276 2400 1277 1290 3.0 1.5 1553 1690

Ti-13.4Al-29Nb 2441 2585 1097 979 1.0 1.3 1538 1564

The bracketed numbers ‘1’ and ‘2’ indicate two independent experiments.

Table 2

Corrosion rates determined by Tafel extrapolation method

Sample ZCP (mV vs SCE) bc (mV/dec) ba (mV/dec) icorr (mA/cm2) Corrosion rate mils per year

(mm/year)

(1) (2) (1) (2) (1) (2) (1) (2) (1) (2)

Ti-15Al 2534 2571 2155 2188 160 124 0.08 0.04 0.028 (0.0007) 0.014 (0.0003)

Ti-6Al-4V 2231 2271 2176 2181 168 106 0.16 0.16 0.055 (0.0013) 0.055 (0.0013)

Ti-13.4Al-29Nb 2447 2616 2318 2138 203 134 0.15 0.07 0.054 (0.0013) 0.024 (0.0006)

The bracketed numbers ‘1’ and ‘2’ indicate two independent experiments. In the last column, the corrosion rate in mils/year is indicated without brackets,

while the corrosion rate in mm/year is mentioned within brackets.

A. Choubey et al. / Intermetallics 12 (2004) 679–682 681

4. Conclusions

The electrochemical behavior of Ti3Al-based interme-

tallic alloys, Ti–15Al and Ti–13.4Al–29Nb, was investi-

gated in simulated body fluid solution (Hanks solution) at

37 8C. The behaviour of Ti–6Al–4V was also evaluated for

comparison purposes. All the materials exhibited stable

passive polarization behavior. Aluminium addition was

detrimental to passivity of Ti in simulated body fluid

solution whereas niobium addition increased the passive

range. The passive ranges of Ti–6Al–4V and Ti–13.4Al–

29Nb were comparable. The passive current densities and

corrosion rates of the alloys were also comparable.

Acknowledgements

The authors thank Dr D. Banerjee, Director, Defence

Metallurgical Research Laboratory, Hyderabad, India for

providing the alloys used in the study. RB acknowledges the

equipment (potentiostat) grant by Alexander von Hamboldt

Foundation.

References

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[3] Banerjee D, Nandy TK, Gogia AK. Scripta Metall 1987;2:597.

[4] Roy TK, Balasubramaniam R, Ghosh A. High temperature oxidation of

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[5] Gurrapa I. Degradation of Ti–24Al–15Nb alloys under different

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[6] Metals test methods and analytical procedures. Annual Book of ASTM

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