Electrochemical impedance spectroscopy study of the passive oxide film on titanium for implant application

Electrochimica

Pergamon

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Vol. 41, Nos. 7/S, pp. 1143-l 153. 1996 Copyright c 19% Ekvier Science Ltd. Gnat Britain. All righa reserved

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ELECTROCHEMICAL IMPEDANCE SPECTROSCOPY STUDY OF THE PASSIVE OXIDE FILM ON TITANIUM FOR IMPLANT APPLICATION J. PAN,*? D. THIERRY$ and C. LEYGRAFt t Department of Materials Science and Engineering, Royal Institute of Technology, Dr Kristinas vlg 51, S-100 44 Stockholm, Sweden $ Swedish Corrosion Institute, Roslagsvagen 101, Hus 25, S-104 05 Stockholm, Sweden (Received 8 Mny 1995)

Abstract-The surface oxide film on titanium and its long-term stability in biological environments play a decisive role for the biocompatibility of titanium implants. In this study, the passive oxide film formed on titanium and its natural growth in a phosphate buffered solution with and without an H,O, addition have been investigated by electrochemical impedance spectroscopy (EIS) measurements over a period of several weeks. In the absence of H,O,, the impedance response indicated a stable thin oxide film on titanium. However, the introduction of H,O, into the solution resulted in significant changes in the EIS-spectra, which varied with exposure time. The interpretation of results is based upon a two-layer model of the oxide film, consisting of a thin barrier-type inner layer and a porous outer layer. The H,O, addition in the solution led to a significant decrease in corrosion resistance of titanium and also to a thickening of the porous outer layer. The observations may provide an explanation of the unexpected in uivo titanium oxide growth and ion incorporation into titanium implant oxide surfaces. Key words: impedance spectroscopy, titanium, oxide film, H,O,,

INTRODUCTION Titanium is one of the most important materials for biomedical and dental implant applications. This is partly due to the excellent corrosion resistance of titanium and its alloys in many aqueous environments provided by a most protective passive film, which spontaneously forms on titanium. The passive film is normally a few nm (1 nm = lOa9 m) thick and consists mainly of amorphous titanium dioxide[ 11. The physiochemical and electrochemical properties of the oxide film and its long-term stability in biological environments play a decisive role for the biocompatibility of titanium implantsC2, 33. Many investigations have shown that this type of natural surface film is essentially a thin layer of TiO,[4, S], which seems very stable in vitro[6, 73, and it is commonly believed that titanium surgical alloys have a high corrosion resistance[8]. However, a marked difference has been observed between in vitro and in vioo behaviors of titanium. It has been reported that the surface film formed on titanium implants in a human body after some years can reach a thickness far beyond the nm-range. Under these circumstances the thickness and composition of the oxide layer changed with implantation time, and some incorporation of mineral ions occurred[9, lo]. Despite a high corrosion resistance of titanium in vitro, there is also increasing evidence showing that titanium is released into and accumulated in tissue adjacent to titanium * Author to whom correspondence should be addressed.

biomaterials.

implants[l l-133. When examined with proton induced X-ray emission (PIXE), extensive titanium release was observed[14]. With prolonged implantation time, the titanium release was observed to reach a steady state rate[15]. To explain the observed high in vivo oxidation/corrosion rates, it has been suggested that H,O, generated in biological systems plays an important role. An increase in the oxidation/corrosion rate of titanium has been observed with increased H,O, concentration in a phosphate-buffered saline (PBS) solution[16, 171. Moreover, different models have been proposed to describe titanium oxide films and to account for the titanium release in viuo[18-211. However, little is known about the in situ change of the titanium oxide film due to H,O,. During the last years, various oxide films on metal surfaces have been characterized by electrochemical impedance spectroscopy (EIS), eg anodic oxides on aluminum[22, 231 and zirconium[24], as well as passive films on titanium[25-271. The spectra are frequently interpreted in terms of an “equivalent circuit” based on a plausible physical model with the circuit elements representing electrochemical properties of the metal and its oxide film. To investigate the influence of H,O, on the oxidation/corrosion behaviour of titanium, EIS has been used in the present study. The passive oxide film formed on titanium exposed in a PBS solution has been characterized in situ by the EIS measurement, and the progress of film formation due to an H,O, addition has been monitored by EIS during a period of several weeks. The main emphasis of this paper is on the interpreta-

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tion of the EIS results, which utilizes an equivalent circuit approach. In related papers, the main results from EIS were found to be in general agreement with the observations from surface analytical techniques, namely X-ray photoelectron spectroscopy (XPS) and scanning tunneling microscopy (STM)[28-301. EXPERIMENTAL General aspects and details of experimental procedures have been described previouslyC28, 291. To investigate the effect of H,Oz, two sets of experiments were performed in parallel. In one set, titanium was exposed to phosphate-buffered saline (PBS) solution without H,O, addition, while in the other set 1OOmM H,O, was introduced into the PBS solution at the beginning of the exposure. EIS was recorded once a day during the exposures, which lasted for a period of several weeks. The experiments with H,O, addition were terminated after 1 or 2 weeks when the titanium surface appeared blue due to an interference effect. This indithat the oxide film had thickened cates considerably[29]. Three parallel runs of each set of experiments were conducted.

The EIS measurements were made using an Impedance/Gain-Phase Analyzer (Solarton 1260) coupled to a Potentiostat-Galvanostat System (EG&G Part, Model 273A), which was connected to a three-electrode electrochemical cell. Spectra were obtained at open-circuit potential of the titanium sample in PBS solution, with an amplitude of 10mV. The frequency span was normally from 1 kHz down to 5 mHz, and sometimes extended to 1 mHz. The impedance data at frequencies above 1 kHz seemed to be affected by manually setting the current range of the potentiostat at 1 mA (corresponding to a current measuring resistor of 1 KQ), and therefore no attempt was made to include data at higher frequencies. Data registration and analysis were performed with a computer. The spectra were interpreted using the nonlinear least square fitting procedure developed by Boukamp[31]. Because of the distributed relaxation feature which is commonly observed in titanium oxide films, a constant phase element (cpe) was utilized for data fitting instead of an ideal capacitor. For simplicity reasons, the value obtained from data fitting was taken as the capacitance in the forthcoming discussion, The quality of fitting to the equivalent circuit was judged firstly by the chi-square value, and secondly by the error dis-

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by a phase angle close to -90” over a wide frequency range. Furthermore, this does not change with exposure time (see Fig. 1) indicating that the oxide film is rather stable in the absence of H,O, . However, when titanium is exposed to PBS with an addition of H,O, , the spectrum appears very different and varies significantly with exposure time. A set of spectra at different exposure times is shown in Fig. 2. The evolution of the spectra may be divided into an earlier and a later stage, distinguished by the appearance of a blue colour on the sample surface.

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RESULTS AND DISCUSSION EIS

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When titanium is exposed to PBS solution without any addition of H,O,, its EIS spectra exhibit behaviour typical of a thin passive oxide film on titanium, ie, a near-capacitive response illustrated

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During the earlier stage, the spectrum varies slowly with exposure time, as shown in Fig. 2(A)-(B).

However, the spectrum is clearly different when the blue colour is seen on the surface during the later stage of exposure (see Fig. 2(C)). It is interesting to note that the remarkable change in the spectrum coincided with the appearance of the blue colour, although the length of time it took for the colour to appear varied significantly (eg, from 1 to 4 weeks between repeated experiments). These observations clearly show that the introduction of H20, into the solution modifies the passive oxide film on titanium. As a result, the electrochemical properties of the film also change with exposure time. Selection of the equivalent circuit Figure 3 shows the two equivalent circuits, both based on a two-layer model of an oxide film, which can be satisfactorily used for fitting the spectra

obtained at different exposure conditions. The spectra for the exposure without H,O,, and during the earlier stage of exposure with H,O, , can be well fitted to circuit I. During later stages of the exposure, when the sample surface appears blue, the spectra can no longer be fitted to circuit I with a reasonable fitting quality. Instead, they can be well fitted to circuit II. Figure 4 provides a comparison of fitting quality for the two circuits when applied to a spectrum obtained after 30 days of exposure. From the simulation, the chi-square value and error distribution vs frequency, it seems clear that circuit II can better describe the titanium oxide film when it appears blue. Previously, circuits I and II have been proposed by other authors to represent an unsealed[22], and sealed1321 anodic oxide films on aluminum, respectively. For the unsealed anodic oxide film, it is generally agreed that the pores in the outer porous layer are filled with electrolyte, while

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for the sealed oxide, the pores are filled with hydrated compounds. In the latter case, the hydrates inside pores need to be taken into account in the equivalent circuit for fitting the spectra and the inner layer may be approximated by a capacitor[32]. The present observations of the evolution of the oxide film on titanium due to an H,O, addition may be compared to a sealing process of the anodic oxide film. The presence of some hydrates/precipitates inside the oxide film has been verified by XPS showing that considerable amount of ions had been incorporated into the outer part of the oxide film[29]. The oxide model corresponding to different exposure conditions is schematically represented in Fig. 3, together with the equivalent circuits I and II found suitable for representing the impedance characteristics. Although the passive film which forms on titanium in aqueous solutions is frequently described as a single Ti02 layer, there is substantial evidence that this film in many exposure conditions exhibits a twolayer structure, ie, a dense inner layer and a porous outer layer[33-351. By using surface sensitive angular-resolved XPS, we also observed the two-

layer feature of the oxide film formed on titanium exposed to PBS solution, and the porous layer had thickened due to the introduction of H,0z[28, 291. The titanium oxide was found to be essentially TiOz , with a transient region between the inner and outer layers[28]. In addition, upon termination of the exposure with H,O, , ions from the PBS solution were found to be incorporated into the oxide frlm[29]. It seems likely that the outer layer basically consists of the same oxide as the inner layer, but possesses microscopic pores which may be filled by either the solution or some hydrated/precipitated compounds depending on the exposure conditions. In summary, the two-layer model of the oxide film used for the impedance data fitting is supported by the results from independent techniques. Fitting results and interpretation No H,02 addition. The EIS spectra can be well fitted to circuit I. As an example, the fit results of R, and R, (ie, the resistance of the barrier layer and porous layer/the electrolyte inside the pores, respectively), Cb and C, (ie, the capacitance of the

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barrier and the porous layer, respectively) have been plotted vs exposure time in Fig. 5. It can be seen in Fig. 5(A) that R, is very high, around 5 MRcm’. In addition, C, is relatively low and decreases slightly with exposure time, reaching a steady state value about lO~F/cn?. The slight decrease of C, may correspond to a slow growth of the titanium oxide film, indicating a long-term stability of the thin passive film in PBS solution without H,Oz. On the other hand, as shown in Fig. 5(B), R, is low and increases slightly with exposure time (roughly from 100 to 200Rcm2). This indicates that the pores are probably filled only with the solution. C, is around 25pF/cm2. Because of the open porous structure, it is difftcult to calculate the thickness of the outer layer from the C,r value; however, the XPS data and independent optical measurements indicate the outer layer could not be thick, owing to a small total thickness[29]. In this case without H202, the corrosion of titanium is prohibited mainly by the nonporous inner layer. The very high resistance R, implies a high corrosion resistance, ie, a low rate of titanium release and oxide growth. Since the outer layer seems to be thin and the pores are filled with electrolyte, the contribution from this porous layer to the electrochemical proper-

ties is rather small, and the impedance response is dominated by the inner layer. As shown in Fig. 6, when a single layer model with one time constant is assumed, the fitting quality is just slightly decreased, compared with the fitting to the above two-layer model. This also indicates that the parameters of the outer layer may not be accurately determined from the spectra. With H,O, addition. In the earlier stage of exposure, the spectra can also be well fitted to the circuit I. During the first few days after introduction of H,O, into solution, the value of R, is low and C, is high. These data are not plotted, because during this initial period the concentration of H,O, is high and decreases quickly, as measured by a VACUette ampoule test[29] and illustrated in Fig. 7, which may lead to some uncertainties in the interpretation. Nevertheless, the low frequency limit of the impedance modulus is around 100KRcm2 during this period, which is a significant decrease as compared to before the introduction of H,O,. This indicates that the native dense oxide layer may be partially dissolved and becomes more defective, due to the H,O, addition. The decreased value implies a relatively low corrosion resistance. Thus, the dissolution/ oxidation rate of titanium can be expected to

Passive oxide film on titanium for implant application

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increase upon introduction of H,O, , and as a result, a considerable amount of titanium may be released into the solution. After this initial period, the H,O, concentration becomes stable at a low level (Fig. 7). The fitting results from the spectra have been plotted vs exposure time and an example is given in Fig. 8. It can be seen from Fig. 8(A) that C, seems to reach a steady Vahe, while &, gradually increases with exposure time and approaches a high value of several MQcm’, which is about the same level as without H20,. This increase of R, may be attributed to a regrowth of the inner layer or to a self-rehealing process. These processes are known to occur in passive films on titaniurr and imply that the film becomes protective again. From Fig. 8(B) it is also evident that C, remains stable over a certain introductory period and shows a decreasing trend during later exposure times. The relatively high C, value may indicate the enhanced porous feature of the outer layer. Meanwhile, R, is in the order of several tens of Kncm2 and increases with exposure time. The R, value seems to be quite high for an electrolytic resistance inside pores and probably incorporates a contribution from some hydrates and/or precipitates generated inside the pores.

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When the sample surface appears blue, circuit I is no longer suitable for the fitting, as demonstrated in Fig. 4. Instead the spectra in the later stage readily fit circuit II, in which C, is clearly resolved, and values of C,,, and R,, are obtained. An example of the fit parameters is plotted in Fig. 9. During this stage, C, seems to be quite stable (see Fig. 9(A)). Its low value of about 3pF/cmZ indicates a thick outer layer. Moreover, R, is about 50KRcm2, and ChO decreases with exposure time (see Fig. 9(B)). These two circuit elements probably correspond to a gradual hydration or to some precipitation process. Based on the increased low frequency limit of the impedance modulus, the corrosion resistance of the film could be expected to reach a high level again during prolonged exposure. When the sample was taken out from the solution, dried in air for 1 hour and exposed again to the same solution, R, was found to increase remarkably and reach a high level of around 400 KRcm2 while other parameters remained almost unchanged, as illustrated in Fig. 9(A) and 9(B). This observation confirms that R, is indeed somehow related to the hydrated and precipitated compounds, which can easily lose water and become dried in air. The interpretation is in accordance with a lower amount of water or OH groups

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found on the surface by two independent surface analytical methods, XPS and IR spectroscopyC361. From the fitting results it may be noticed a considerable difference in the capacitance value of the outer layer between the earlier and the later stages of exposure. To some extent the difference may be due to the different equivalent circuits used. In practice, different equivalent circuits are commonly employed for representing unsealed and sealed porous layers of anodic oxide films with their different microstructures. To reveal the capacitive behaviour of the porous layer on anodized aluminum, it has been demonstrated by other authors that the pores need to be sealed[32]. The results from the present study also indicate that the outer porous layer of the passive oxide film on titanium can be better characterized by EIS when the pores are filled by some hydrated/precipitated compounds.

Efect of H,O, on the passive oxidefilm on titanium AS shown above, the interpretation of the EIS data provides parameters of the passive oxide film on titanium, such as resistance and capacitance values. From the variation of these parameters with exposure time, it seems that the passive oxide film on

titanium can be attacked by H,O, in the solution. As a result, the film becomes more defective, probably porous and exhibits a relatively low corrosion resistance. However, as a result of prolonged exposure and probably also a decreased H20, concentration, the oxide can regrow to a larger thickness. The porous oxide may become hydrated, and ions may easily be incorporated into the pores and further precipitate. When the pores are filled by hydrated or precipitated compounds, the thickened oxide film can become highly protective again, ie, the oxide film is self-rehealed. The evolution of the spectra reflects the pronounced two-layer feature of the oxide tilm and the development of a thick outer porous layer which may facilitate the incorporation of ions from the PBS solution. It should be stressed that the time for the blue colour to appear on the sample surface may vary significantly, and the same is true for the evolution of the EIS spectra, Therefore, these observations should be regarded as qualitative rather than giving an exact time-dependent behaviour of the passive oxide film on titanium under present exposure conditions. The study may provide an explanation for the unexpectedly high rate of in vivo titanium release and

Passive oxide film on titanium for implant application

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facilitate the incorporation of mineral ions such as phosphate groups from biological fluids. When the oxide film becomes thickened and the pores are sealed, the corrosion resistance can reach a high level again and prevent further dissolution. This may also

oxide thickening on titanium implants. The results and interpretation suggest that a H,O, addition can lead to an increased corrosion rate (titanium release) and also an enhanced film thickening. The pronounced porous outer layer may be expected to 00

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the fact that the amount biological surroundings can reach a long period of implantation[lS]. discussed in more detail in another

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of titanium in the a stable level after These aspects are publication[29].

CONCLUSIONS From this electrochemical impedance spectroscopy (EIS) study the following conclusions may be drawn : 1. EIS is a powerful method for in situ characterization of the passive oxide film on titanium. The evolution of the film due to an introduction of H,02 can be monitored by EIS measurements as a function of exposure time. 2. In a phosphate-buffered saline solution without H202, the oxide film on titanium exhibits a high corrosion resistance and a long-term stability. The introduction of H202 into the solution results in a

decreased corrosion resistance and an enhanced dissolution/oxidation rate. 3. The EIS spectra can be interpreted in terms of a two-layer model of the oxide film, which consists of a thin barrier-type inner layer and a porous outer layer. The parameters obtained and their variation with exposure time indicate that the oxide film can be attacked by H,O,, which results in more defective inner layer and in a thicker and more porous outer layer. After prolonged exposure, the oxide film may reheal itself and exhibit a significantly higher corrosion resistance again due to some sealing processes inside pores in the oxide film. Acknowledgements-We would like to thank MS Ingegerd Annergren (Dept. of Materials Science and Engineering, Royal Institute of Technology, Sweden) for most valuable comments on the interpretation of EIS results and MS Sandra Brunsberg (Royal Institute of Technology, Sweden) for checking the English.

Passive oxide film on titanium for implant application

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