Biomaterials, Volume 24, Issue 2, January 2003, Pages 213-218

Electrochemical characterization of cast titanium alloys^*1

Zhuo Cai^{, , a}, Ty Shafer^a, Ikuya Watanabe^a, Martha E. Nunn^b and Toru Okabe<sup>a

a</sup> Baylor College of Dentistry, Texas A&M University System Health Science Center, 3302 Gaston Avenue, Dallas, TX 75246, USA
^b Department of Health Policy & Health Services Research, Goldman School of Dental Medicine, Boston University, 715 Albany Street, Boston, MA 02118, USA

Received 19 December 2001;  accepted 26 June 2002.  Available online 30 August 2002.

1. Abstract

A reaction layer forms on cast titanium alloy surfaces due to the reaction of the molten metal with the investment. This surface layer may affect the corrosion of the alloy in the oral environment. The objective of this study was to characterize the in vitro corrosion behavior of cast titanium alloys. ASTM Grade 2 CP titanium, Ti-6Al-4V, Ti-6Al-7Nb and Ti-13Nb-13Zr alloys were cast into a MgO-based investment. Experiments were performed on castings (N=4) with three surface conditions: (A) as-cast surface after sandblasting, (B) polished surface after removal of the reaction layer, and (C) sandblasted surface after removal of the reaction layer. Open-circuit potential (OCP) measurement, linear polarization, and potentiodynamic cathodic polarization were performed in aerated (air+10% CO₂) modified Tani-Zucchi synthetic saliva at 37°C. Potentiodynamic anodic polarization was subsequently conducted in the same medium deaerated with N₂+10% CO₂ gas 2 h before and during the experiment. Polarization resistance (R_P) and corrosion rate (I_CORR) were calculated. Numerical results were subjected to nonparametric statistical analysis at
=0.05. The OCP stabilized for all the specimens after 6×10⁴ s. Apparent differences in anodic polarization were observed among the different surfaces for all the metals. A passivation region followed by breakdown and repassivation were seen on specimens with surfaces A and C. An extensive passive region was observed on all the metals with surface B. The Kruskal-Wallis test showed no significant differences in OCP, R_p, I_CORR or break down potential for each of the three surfaces among all the metals. The Mann-Whitney test showed significantly lower R_P and higher I_CORR values for surface C compared to the other surfaces. Results indicate that the surface condition has more effect on corrosion of these alloys than the surface reaction layer. Within the oxidation potential range of the oral cavity, all the metal/surface combinations examined showed excellent corrosion resistance.

Author Keywords: Corrosion; Titanium; Alloys; Casting; Surface

2. Article Outline

1. Introduction

2. Materials and methods

3. Results

4. Discussion

5. Conclusion

Acknowledgements

References

3. 1. Introduction

Titanium is considered the most ideal metal for in vivo applications because of its excellent biocompatibility [1, 2, 3 and 4]. As a result, titanium and its alloys have been used extensively in the last several decades as materials for orthopedic implants, dental implant and medical devices.

Titanium is a relatively new metal for cast dental prostheses, primarily due to the technological difficulties in casting it. Commercially pure (CP) titanium has been used by the dental profession for more than a decade for crowns and bridges, and metal-ceramic restorations [5 and 6]. The titanium alloys tested in this study were originally developed for orthopedic applications. Ti-6Al-4V has been extensively used in orthopedics for many years [7]. Because of the concern about the biocompatibility of aluminum and vanadium ions released from this alloy [8 and 9], titanium alloys without aluminum and vanadium were developed. One of them, Ti-6Al-7Nb, is an
+
phase alloy similar to Ti-6Al-4V [10]. A "near-
" titanium alloy, Ti-13Nb-13Zr, was developed as a low-elastic modulus implant alloy [11] and was considered to have superior corrosion resistance compared to Ti-6Al-4V. All of these titanium alloys can potentially be used as casting alloys in prosthetic dentistry. Successful castings were produced from these alloys using a MgO-based investment in a dental titanium casting machine [12].

Many studies have been published on the corrosion of titanium alloys. In a study by Solar et al. [13] on characterization of the passive film formed on Ti-6Al-4V surfaces in Ringer's solution, passivity was observed over the potential range of −400 to +1400 mV vs. SCE without breakdown. The pulse potentiostatic capacitance technique and Auger electron spectroscopy analysis of the specimens after the corrosion tests indicated that the electrochemical reaction occurring on the surface was the growth of a passive film. Speck and Fraker [14] conducted potentiostatic anodic polarization from 0 mV (SCE) until the breakdown potential on several alloys in Hank's solution. Ti-6Al-4V showed a large passive region and a breakdown potential at ~+2000 mV (SCE) with a primary passive current density at ~0.1 
A/cm².

In a study on orthopedic alloys [15], the corrosion behavior was examined in a deaerated Ringer's solution by anodic polarization from 0 mV up to +2200 mV (SCE). Passivity was observed on Ti-6Al-7Nb, Ti-6Al-4V and CP titanium with the primary passive current density at ~10 
A/cm². In a recent study [16], the corrosion behavior of several orthopedic alloys was investigated in a phosphate-buffered saline with various protein components and pH values. The results showed that an increase in pH value had a greater effect on the corrosion of Ti-6Al-4V and Ti-6Al-7Nb than on Ti-13Nb-13Zr, and that the addition of protein to the medium reduced the influence of pH. The study suggested that proteins in the environment interact with the repassivation process on the alloy surfaces. While the corrosion of these orthopedic alloys has been studied, the electrochemical behavior of these alloys in the oral environment is not available.

The active chemical nature of titanium at elevated temperatures results in a characteristic surface structure on cast titanium. Molten titanium reacts with the investment during casting, and creates a hardened reaction layer on the cast titanium surfaces [17]. This layer forms as a result of the diffusion of O, H and N into the interstitial sites of the titanium lattice [18 and 19]. The layer is between 150 and 200 
m thick on CP titanium cast into various oxide-based investments [20, 21, 22 and 23]. It is believed that this layer forms through the decomposition of the investment oxides and diffusion of the resulting elements into the casting [20]. Compared to the bulk metal below the surface, the surface reaction layer on the cast titanium has increased hardness and reduced elongation. Such a structure is considered undesirable for dental applications [5]. When Ti-6Al-4V, Ti-6Al-7Zr, and Ti-13Nb-13Zr were cast into a MgO-based investment, surface reaction structures similar to that on CP titanium were observed [12].

Several studies have been published on the corrosion of cast titanium. In a study by Doi et al. [24], CP titanium was cast into a phosphate-bonded SiO₂ investment. The reaction layer was less than 100 
m thick. As-cast surfaces, with the reaction structure intact, showed the poorest corrosion resistance. Sandblasted and polished surfaces where the surface reaction structure was removed showed similar behavior, although the current density for sandblasted surfaces was almost one decade larger. Similar results were also reported by Geis-Gerstorfer [25]. In another study on corrosion of cast titanium in 1% NaCl solution [26], sandblasted surfaces showed a reduced passive region and a considerable increase in the passive current density. In another study by Cai et al. [27], corrosion of cast titanium with various surface finishes was compared in an artificial saliva. It was concluded that surface roughness is a more prominent factor on the anodic polarization behavior of CP titanium than is the surface reaction layer. Surface roughness and the presence of the surface reaction layer both contribute to the dissolution of titanium.

Information is currently unavailable regarding the corrosion of cast titanium alloys in a simulated oral environment. The purpose of this study was to characterize the electrochemical behavior of three cast titanium alloys with different surface conditions.

4. 2. Materials and methods

The titanium and titanium alloys examined in the present study are listed in Table 1. Specimens were cast into a MgO-based investment (Selevest CB, Selec, Osaka, Japan) in an argon arc-melting, centrifugal dental titanium casting machine (Ticast Super R, Selec). Four specimens (10 mm×10 mm×2 mm) were cast for each metal.

Table 1. Titanium alloys examined in this study

In order to eliminate the surface reaction layer from the exterior of each corrosion specimen, the surfaces on each side (2 mm×10 mm) were ground on SiC abrasive paper until a thickness of 300 
m was removed. A brass nut was then attached to one flat surface (10 mm×10 mm) of each specimen using conductive paint to ensure electrical conductivity. The assembly was then embedded into an epoxy resin disk with one flat surface exposed.

Corrosion experiments of the cast titanium alloys were performed for the following three surface conditions: (A) surface sandblasted by 50 
m alumina particles with the reaction layer remaining; (B) reaction layer removed and surface polished up to 800 grit on wet SiC abrasive paper; (C) reaction layer removed and surface sandblasted by 50 
m alumina particles. After the surface preparation, each specimen was ultrasonically cleaned in deionized water for 5 min and dried in air. Modified Tani-Zucchi synthetic saliva [28] (1.5 g/l KCl, 1.5 g/l NaHCO₃, 0.5 g/l NaH₂PO₄, 0.5 g/l KSCN, 0.9 g/l lactic acid) maintained at 37±1°C was used as the corrosion medium. Fresh medium was used for each specimen.

The corrosion measurements for each specimen condition were conducted in the following sequence [28]: (a) open-circuit potential (OCP) measurement: OCP was measured in aerated (air+10% CO₂) medium for 6×10⁴ s using a potentiostat (Model 273A, EG&G Princeton Applied Research, Princeton, NJ, USA) controlled by a personal computer with dedicated software (352 SoftCorr III, EG&G Princeton Applied Research). A standard calomel electrode (SCE) was used as the reference electrode; (b) linear polarization: following the OCP measurement, linear polarization was conducted from −8 to +8 mV around the OCP at a rate of 0.1 mV/s in the aerated medium. A platinum counter electrode was used; (c) potentiodynamic cathodic polarization: the polarization scan was initiated from the OCP to 300 mV below the OCP at a rate of 0.167 mV/s in the aerated medium; and (d) potentiodynamic anodic polarization: after the cathodic polarization, the medium was deaerated with N₂+10% CO₂ gas for 2 h before and during the measurement. Polarization started at 200 mV below the OCP at a rate of 0.167 mV/s, and terminated at 2000 mV above the OCP. Breakdown potential (E_b) was determined from the polarization diagrams wherever applicable.

Qualitative analyses were made for the anodic polarization diagrams from each metal/surface condition. Polarization resistance, cathodic and anodic Tafel slopes were determined from the experiment using the computer software. The corrosion rate (I_CORR) of the alloy was determined according to the Stern-Geary equation [29]

where R_p is the polarization resistance,
_a and
_c are the anodic and cathodic Tafel slopes. The numerical results were statistically analyzed by a nonparametric method [30] at a significance level of
=0.05 using statistic software (SPSS 10.1 for Windows, SPSS, Chicago, IL).

5. 3. Results

Following the initial increase, the OCP stabilized (−205 to −85 mV vs. SCE) for all the metal surface conditions after 6×10⁴ s (Table 2, Table 3 and Table 4). Fig. 1 is a representative OCP vs. time diagram for sandblasted metals with the reaction layer remaining.

Table 2. Corrosion parameters of sandblasted cast titanium alloys with the surface reaction layer intact*

Table 3. Corrosion parameters of polished cast titanium alloys*

Table 4. Corrosion parameters of sandblasted cast titanium alloys with no surface reaction layer*

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Fig. 1. Representative OCP vs. time plots of sandblasted cast titanium alloys with surface reaction layer remaining.

Distinctive differences in anodic polarization behavior were observed among the different surfaces for all the metals (Fig. 2, Fig. 3, Fig. 4 and Fig. 5). For the specimens with sandblasted surfaces (surfaces A and C), a passive region followed by breakdown and repassivation was observed on the anodic polarization diagrams. A distinctly wider passive region with reduced passive current density was observed on all the specimens with polished surfaces. No visible surface changes were observed on any specimens after the anodic polarization.

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Fig. 2. Representative potentiodynamic anodic polarization diagrams of cast CP titanium with different surfaces.

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Fig. 3. Representative potentiodynamic anodic polarization diagrams of cast Ti-6Al-4V alloy with different surfaces.

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Fig. 4. Representative potentiodynamic anodic polarization diagrams of cast Ti-6Al-7Nb alloy with different surfaces.

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Fig. 5. Representative potentiodynamic anodic polarization diagrams of cast Ti-13Nb-13Zr alloy with different surfaces.

R_p values determined from the linear polarization diagram are listed in Table 2, Table 3 and Table 4, as are the calculated I_CORR and E_b. The Kruskal-Wallis test showed no significant differences in OCP, R_p, I_CORR or E_b among all of the four metals for each surface. There were no significant differences in OCP or E_b among the three surfaces for all the metals. The Mann-Whitney test showed significantly lower R_P and higher I_CORR values for the sandblasted surfaces after the removal of the reaction layer (Table 4) compared to the other two surfaces ( Table 2 and Table 3).

6. 4. Discussion

Since human saliva is essentially an aerated electrolyte, OCP measurement, linear polarization, and cathodic polarization were conducted in aerated artificial saliva in the present study. In order to gain additional information on the corrosion behavior of these metals, anodic polarization was performed in the deaerated medium.

An initial increase in the OCPs during the early hours followed by stabilization observed on all the specimens suggests that a protective passive film formed rapidly on the metal surfaces in the artificial saliva and remained stable during the entire immersion period.

The reduced passive current densities observed on polished specimens were similar to our previous observation [27], and were attributed to the reduced surface area compared to the sandblasted surfaces.

An earlier study [31] showed that for titanium, more rigorous surface preparation may result in a surface oxide film with reduced integrity or thickness. Significantly lower R_P and higher I_CORR values for specimens with the sandblasted surface and no reaction layer suggests that this surface is less corrosion-resistant than the other two surfaces tested in the present study. Sandblasting is a more rigorous process than polishing, which may account for the differences in results between surfaces (B) and (C) in the present study. The differences in results between the two sandblasted surfaces (A and C) can be attributed to the reaction layer. Our previous study [27] showed that the reaction layer is a less significant factor on the corrosion of titanium than surface finishes.

In the present study, all the metals examined with the same surface condition showed similar behavior. This finding indicates that the protective titanium oxide film formed on the surface is a common feature shared by these metals. In a X-ray photoelectron spectroscopy (XPS) study [32], CP titanium, Ti-6Al-4V, and Ti-6Al-7Nb alloy surfaces were analyzed after immersion in NaCl and FeCl₃ solutions. Hydrated Ti (IV) oxide was detected as the predominant form of titanium oxide on these metal surfaces. Stable Al (III) oxide was detected on the two alloy surfaces, and Nb (V) oxide was identified on the Ti-6Al-7Nb surfaces. Electron diffraction on titanium and titanium alloys after anodic polarization showed that TiO₂ (rutile) with tetragonal structure is the main oxide formed on the surfaces [33]. As reported in our earlier study [27], surface condition appears to be the determining factor in the electrochemical behavior of these metals in a simulated oral environment. Polished surfaces exhibited superior corrosion resistance compared to the sandblasted surfaces. Judging by the oxidation potential range (−58 to +212 mV vs. SCE) in normal human oral cavity [34], all the metal/surface combinations examined in the present study showed excellent corrosion resistance.

7. 5. Conclusion

Surface preparation appears to have more marked effect on the corrosion of cast titanium alloys than does the surface reaction layer which forms due to the reaction between melted metal and the investment. All the metal/surface combinations examined in the present study showed excellent corrosion resistance in a simulated oral environment.

8. Acknowledgements

This study was supported by NIH/NIDCR Grants R01DE11787, T35-07188, and Baylor College of Dentistry Research Funds. The Ti-13Nb-13Zr alloy used in the present study was provided by Smith & Nephew Richard, Memphis, TN, USA. The authors would like to thank Mrs. Jeanne Santa Cruz for her assistance in the preparation of the manuscript.

9. References

1. R. Adell, V. Lekholm, B. Rockler and P.I. Brĺnemark , A 15-year study of osseointegrated implants in the treatment of the edentulous jaw. Int J Oral Surg 10 (1981), pp. 387-416. Abstract-MEDLINE | Abstract-EMBASE  

2. D.F. Williams , Biological effects of titanium. In: D. Williams, Editor, CRC series in biocompatibility, systemic aspects of biocompatibility, CRC Press, Boca Raton, FL (1981), pp. 169-177.

3. H. Weber, L. Pröbster and J. Geis-Gerstorfer , Titan als prothetischer werkstoff (Titanium as a prosthodontic material). Dtsch Zahnärztl Z 47 (1992), pp. 473-481.

4. E.P. Lautenschlager and P. Monaghan , Titanium and titanium alloys as dental materials. Int Dent J 43 (1993), pp. 245-253. Abstract-MEDLINE  

5. T. Okabe and H. Herř , The use of titanium in dentistry. Cells Mater 5 (1995), pp. 211-230. Abstract-EMBASE  

6. R. Wang and A. Fenton , Titanium for prosthodontics applications: a review of the literature. Quintessence Int 27 (1996), pp. 401-408. Abstract-MEDLINE  

7. J.E. Lemons and L.C. Lucas , Properties of biomaterials. J Arthroplasty 1 (1986), pp. 143-147. Abstract-MEDLINE | Abstract-EMBASE  

8. J. Black , Does corrosion matter?. J Bone Jt Surg 70B (1988), pp. 517-520. Abstract-MEDLINE  

9. K. Wagner , Implication of metallic corrosion in total knee arthroplasty. Clin Orthop 271 (1991), pp. 12-20.

10. M. Semlitsch, F. Staub and H. Weber , Titanium-aluminium-niobium alloy, development for biocompatible, high strength surgical implants. Biomed Tech 30 (1985), pp. 334-339. Abstract-EMBASE | Abstract-MEDLINE  

11. J.A. Davidson, A.K. Mishra, P. Kovacs and R.A. Poggie , New surface-hardened, low-modulus, corrosion-resistant Ti-13Nb-13Zr alloy for total hip arthroplasty. Bio-Med Mater Eng 4 (1994), pp. 231-243. Abstract-MEDLINE  

12. T. Okabe, I. Watanabe, E. Watanabe and Z. Cai , Dental casting of commercial titanium alloys. J Dent Res 77 (1998), p. 165 (Abst. No. 478) .

13. R.I. Solar, S.R. Pollack and E. Korostoff , In-vitro corrosion testing of titanium surgical implant alloys: an approach to understand titanium release from implants. J Biomed Mater Res 13 (1979), pp. 217-250. Abstract-EMBASE | Abstract-Compendex | Abstract-MEDLINE  

14. K.M. Speck and A.C. Fraker , Anodic polarization behavior of Ti-Ni and Ti-6Al-4V in simulated physiological solution. J Dent Res 59 (1980), pp. 1590-1595. Abstract-MEDLINE  

15. M.F. Semlitsch, H. Weber, R.M. Streicher and R. Schön , Joint replacement components made of hot-forged and surface-treated Ti-6Al-7Nb alloy. Biomaterials 13 (1992), pp. 781-788. Abstract

16. M. Khan, R. Williams and D. Williams , The corrosion behaviour of Ti-6Al-4V, Ti-6Al-7Nb and Ti-13Nb-13Zr in protein solutions. Biomaterials 20 (1999), pp. 631-637. Abstract | PDF (445 K)

17. Donachie MJ, editor. Titanium: a technical guide. Metals Park, OH: ASM International, p. 28. 69-71.

18. Y. Tanaka, I. Watanabe, E. Miura, T. Shiraishi, M. Atsuta and H. Hisatsune , TEM study on the microstructure of titanium/bonding-porcelain fused interface. J Dent Res 79 (2000), p. 271 (Abst. 1024) .

19. J.A. Hautaniemi, H. Herř and J.T. Juhanoja , On the bonding of porcelain on titanium. J Mater Sci: Mater Med 3 (1992), pp. 186-191.

20. O. Miyakawa, K. Watanabe, S. Okawa, S. Nakano, M. Kobayashi and N. Shiokawa , Layered structure of cast titanium surface. Dent Mater J 8 (1989), pp. 175-185. Abstract-MEDLINE  

21. J. Takahashi, H. Kimura, E.P. Lautenschlager, J.H. Lin and E.H. Greener , Casting pure titanium into commercial phosphate-bonded SiO₂ investment molds. J Dent Res 69 (1990), pp. 1800-1805. Abstract-MEDLINE  

22. H. Brauner , Zur randschichtaufhärtung an titanwerkstoffen durch unterschiedliche formstoffe und einbettmassen (The effect of various mold and investment materials on the
case zone of titanium castings). Dtsch Zahnärztl Z 47 (1992), pp. 511-515.

23. J. Zhang, M. Okazaki and J. Takahashi , Effects of casting methods on surface state of pure titanium castings. Jpn J Dent Mater 13 (1994), pp. 221-227.

24. H. Doi, T. Yoneyama, M. Kotake and H. Hamanaka , Corrosion resistance on the reacted surface of titanium castings with a phosphate bonded silica investment. Jpn J Dent Mater 11 (1992), pp. 672-678.

25. Geis-Gerstorfer J. Corrosion behavior of titanium. In: Proceedings of the First International Meeting on Titanium in Dentistry, Geneva, Switzerland. 1994.

26. H. Mimura and Y. Miyagawa , Electrochemical corrosion behavior of titanium castings. Part 1. Effects of degree of surface polishing and kind of solution. Jpn J Dent Mater 15 (1996), pp. 282-295.

27. Z. Cai, H. Nakajima, M. Woldu, A. Berglund, M. Bergman and T. Okabe , In vitro corrosion resistance of titanium made using different fabrication methods. Biomaterials 20 (1999), pp. 183-190. SummaryPlus | Full Text + Links | PDF (153 K)

28. V. Goehlich and M. Marek , Corrosion behavior of Pd-Cu and Pd-Co alloys in synthetic saliva. Dent Mater 6 (1990), pp. 103-110. Abstract-MEDLINE  

29. M. Stern and A. Geary , Electrochemical polarization. I. A theoretical analysis of the shape of polarization curves. J Electrochem Soc 104 1 (1957), pp. 56-63.

30. Tait W. An introduction to electrochemical corrosion testing for practicing engineers and scientists. Racine, WI: PairODocs Publications. 1994. p. 33-4.

31. B. Johansson and B. Bergman , Corrosion of titanium and amalgam couples: effect of fluoride, area size, surface preparation and fabrication procedures. Dent Mater 11 (1995), pp. 41-46. SummaryPlus | Full Text + Links | PDF (998 K)

32. P. Maeusli, P. Bloch, V. Geret and S. Steinemann , Surface characterization of titanium and titanium alloys. Adv Biomater 6 (1986), pp. 57-62.

33. C. Kuphasuk, Y. Oshida, C.J. Andres, S. Hovijitra, M. Barco and D. Brown , Electrochemical corrosion of titanium and titanium-based alloys. J Prosthet Dent 85 (2001), pp. 195-202. Abstract | PDF (180 K)

34. G.J. Ewers and E.H. Greener , The electrochemical activity of the oral cavity. J Oral Rehabil 12 (1985), pp. 469-476. Abstract-EMBASE | Abstract-MEDLINE  

*1 This study was presented at the 30th Annual Meeting of the American Association for Dental Research, Chicago, IL, March 2001.

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