Corrosion resistance improvement of titanium base alloys

ARTIGO

Corrosion resistance improvement of titanium base alloys

Mihai V. Popa^I; Ecaterina VasilescuI, ^*; Paula Drob^I; Cora Vasilescu^I; Silviu I. Drob^I; Daniel Mareci^II; Julia C. Mirza Rosca^III

^IInstitute of Physical Chemistry "Ilie Murgulescu", Spl. Independentei 202, 060021 Bucharest, Romania

^IITechnical University "Gh. Asachi", Bd. Mangeron 71, 700050 Iasi, Romania

^IIILas Palmas de Gran Canaria University, Mechanical Engineering Department, Despacho 11, 35017 Tafira, Spain

ABSTRACT

The corrosion resistance of the new Ti-6Al-4V-1Zr alloy in comparison with ternary Ti-6Al-4V alloy in Ringer-Brown solution and artificial Carter-Brugirard saliva of different pH values was studied. In Ringer-Brown solution, the new alloy presented an improvement of all electrochemical parameters due to the alloying with Zr; also, impedance spectra revealed better protective properties of its passive layer. In Carter-Brugirard artificial saliva, an increase of the passive film thickness was proved. Fluoride ions had a slight negative influence on the corrosion and ion release rates, without to affect the very good stability of the new Ti-6Al-4V-1Zr alloy.

Keywords: corrosion rates; EIS; Ti6Al4V1Zr alloy.

INTRODUCTION

Titanium and its Ti-6Al-4V alloy were used as implant material for many years.^1-6 These materials have very good corrosion resistance and biocompatibility. In the last years, important problems have been raised over their long-term performance. Several cases of extensive metallosis and necrosis in periprosthetic tissues of failed cemented Ti-6Al-4V prostheses were registered.^7,8 In recent years, attempts were made to develop titanium alloys with biomechanical compatibility, low modulus and biochemical compatibility by the using of non-toxic alloying elements as: Ti, Nb, Zr.^9-11 These elements are chemically very stable and highly corrosion resistant in biological fluids and are considered immune for human body.^12,13

Zirconium has excellent corrosion resistance to many types of corrosive media including strong acids and basses. Zirconium has acceptable mechanical strength and good biocompatibility.^14,15 Zirconium implants exhibit good osseointegration and the degree of bone-implant contact is higher for Zr than for Ti.^16-19 Only few alloys with zirconium were elaborated for implant use: Ti-2.2Al-2.6Zr,¹¹ Ti-13Nb-13Zr,^20,21 Ti-15Zr-4Nb-4Ta,¹⁰ Ti-15Zr-4Nb-4Ta-0.2Pd,²² Ti-50Zr and Zr-25Nb.¹⁴

The new Romanian Ti-6Al-4V-1Zr alloy was elaborated for to combine the very good mechanical properties of Ti-6Al-4V alloy with the good biocompatibility of zirconium. Its corrosion resistance in Ringer-Brown solution and Carter-Brugirard artificial saliva of different pH values was studied in this paper.

EXPERIMENTAL

The alloy synthesis was performed by melting in vacuum. The obtained composition for main constituents in wt% is: Al - 6.12; V - 3.92; Zr - 1.07; balance - Ti. The electrodes for experiments were prepared from ingots in casting state.

All measurements were carried out in Ringer-Brown solution and artificial Carter-Brugirard saliva at 37 ± 1 ºC. Ringer-Brown solution of pH = 7.0 had the following composition (g/L): NaCl - 6; KCl - 0.4; CaCl₂.2H₂O - 0.2; natrium lactate - 3.05; pH = 7. Carter-Brugirard saliva of pH = 2.54; 5.52; 8.21; 8.91 had the following composition (g/L): KCl -1.2; NaCl - 0.7; KH₂PO₄ - 0.26; KSCN - 0.33; Na₂HPO₄ - 0.19; urea - 0.13; NaHCO₃ - 1.5; neutral saliva was doped with 0.05M NaF for to reproduce the F^- concentration from the dental hygiene products.

The following experimental techniques were used: potentiodynamic and linear polarization, electrochemical impedance spectroscopy (EIS) and monitoring of the open circuit potentials, E_oc and of the open circuit potential gradients, due to the pH, ΔE_oc(pH) and content, ΔE_oc(c) non-uniformities of the artificial Carter-Brugirard saliva, versus exposure time (3000 h).

The cyclic potentiodynamic polarization was applied beginning from -0.5 to +4.0 V (vs. SCE) using a scan rate of 10 mV/sec. Voltalab 80 equipment with its VoltaMaster 4 program were used. From the voltammograms, the main electrochemical parameters were determined: E_corr - corrosion potential, like zero current potential, E_p - passivation potential at which the current density is constant; |E_corr - E_p| difference represents the tendency to passivation (low values characterise a good, easy passivation); δE_p - passive potential range of the constant current; i_p - passive current density.

The linear polarization measurements (Tafel) were carried out for a range of ± 200 mV around the open circuit potential, with a scan rate of 10 mV/sec. The same Voltalab 80 equipment with its VoltaMaster 4 program that delivered the values of the corrosion current densities (i_corr) and rates (V_corr) obtained from Tafel curves was used.

The total quantity of the ions (ng/cm²) released in the solution was determined:

where: V_corr = corrosion rate in mm/year.

The electrochemical impedance spectroscopy (EIS) was performed at open circuit potential (E_oc) and at +0.4 V (a potential from passive domain) using a PAR 263A potentiostat connected with a PAR 5210 lock-in amplifier. The amplitude of the AC potential was 10 mV and single sine wave measurements at frequencies between 10^-1 and 10⁵ Hz were performed.

The open circuit potentials E_oc were registered with the exposure time (3000 h till present) using a performing Hewlett-Packard multimeter. Three potential gradients due to the pH non-uniformity, ΔE_oc(pH) of Carter-Brugirard saliva (relations 2-4) and one potential gradient (relation 5) due to the composition non-uniformity, ΔE_oc(c) of Carter-Brugirard saliva were monitored:

RESULTS AND DISCUSSION

Improvement of corrosion resistance in Ringer-Brown solution

Improvement of corrosion resistance from polarization curves

Potentiodynamic polarization curves for the new Ti-6Al-4V-1Zr alloy exhibited self-passivation, stable, very constant passive current density and a corrosion potential placed in the passive potential range of Ti, Al, V, Zr on the Pourbaix²³ diagrams. Comparing with Ti-6Al-4V alloy (Table 1) it results an improvement of all electrochemical parameters, due to the favorable influence of zirconium.

The alloy with Zr exhibited more electropositive corrosion potentials than the comparison Ti-6A-l4V alloy due to the galvanic couple effect of zirconium. The tendency to passivation |E_corr - E_p| is lower (better) for Ti-6Al-4V-1Zr alloy showing that, this alloy can be easier passivated as result of the favorable influence of Zr that participates with its passive oxide (ZrO₂)^10,22 to the formation of the passive film. This layer is more compact, more stable than of the comparison alloy, as it results from the lower values of the passive current densities, i_p.

Also, the corrosion current density, corrosion and ion release rates (Table 2) presented lower values for Ti-6Al-4V-1Zr alloy than for Ti-6Al-4V alloy showing the benefic influence of Zr.

Improvement of corrosion resistance from electrochemical impedance spectra

The behavior of the new Ti-6Al-4V-1Zr alloy was studied from Bode spectra that permit to determine the presence of a compact passive film if the phase angle is close to -90º over wide frequency range of the spectrum, if the Bode form of log Zmod - log f has a linear portion with the slope about -1 at intermediate frequency and if the parameter n is about 1.^24,25

In Figure 1, the Bode spectrum obtained for the new quaternary Ti-6Al-4V-1Zr alloy at open circuit potential exhibited a phase angle of ≈ -80º and a linear portion with a slope ≈-1. Similar results obtained S. Piazza et al.;²⁶ an electric equivalent circuit with two time constants (Figure 2) was modeled: one constant is for the inner, thin, barrier, compact, passive film illustrated by the compact layer resistance, R₁ and capacitance CPE₁ (a constant phase element to describe the deviation from an ideal capacitor due to the certain heterogeneity of the surface);²⁵ the second time constant is for the outer, porous layer and is represented by the porous layer resistance, R₂ and capacitance CPE₂.

For ternary Ti-6Al-4V alloy, Bode plot (Figure 3) at E_oc exhibited two phase angles at -70º and -80º corresponding with three time constants: the third time constant is for diffusion processes through the passive film²⁷ and is shown by the diffusion Warburg impedance, W and diffusion resistance R₃ (Figure 4).

Fitting parameters for Ti-6Al-4V-1Zr alloy (Table 3) show that the resistance of the porous layer, R₂ is lower than the resistance of the barrier layer, R₁, denoting a more protective barrier layer.²⁸ Capacitance of the porous layer, CPE₂ is higher than the capacitance of the barrier layer, CPE₁ supporting the idea that the corrosion resistance is mainly due to the barrier layer.

Higher value of resistance R₁ of the quaternary Ti-6Al-4V-1Zr alloy than of the ternary Ti-6Al-4V alloy were registered; this fact denotes better protective properties of the passive layer on the new Ti-6Al-4V-1Zr alloy. The frequency independent parameter n₃ = 0.51 for ternary Ti-6Al-4V alloy indicates diffusion processes; the diffusion resistance R₃ has lower value than R₁ or R₂, showing reduced diffusion processes through the passive layer.

In the passive potential range, at +0.4 V, Bode spectra both for Ti-6Al-4V-1Zr and Ti-6Al-4V alloys (Figure 5) presented a phase angle of ≈ -80º over the wide frequency range, a slope of -1 and n₁≈ 0.90, showing the same passive, compact film (Table 3). No reactions to this protective layer were detected and the electric equivalent circuit for the fitting of the impedance data contains one time constant (Figure 6).

The EIS data are in agreement with the results obtained from the cyclic polarization curves.

Improvement of corrosion resistance in artificial

Carter-Brugirard saliva

Improvement of corrosion resistance from polarization curves

In artificial Carter-Brugirard saliva, the new Ti-6Al-4V-1Zr alloy is self-passivated both in acid, neutral and alkaline saliva. The most electropositive corrosion potential and lowest passive current density were registered in neutral saliva of pH = 8.21 (Table 4) showing that the passive film is the most stable and the alloy has the best resistance in this saliva. The difference between the corrosion potential, E_corr and passivation potential, E_p that provide an indication of the ease of passivation has the lowest value in neutral saliva, proving that the best passivation of the alloy appeared in this saliva.²⁹ The fluoride ion has a slow negative influence on the electrochemical parameters due its higher corrosivity.^24,30,31

In comparison with Ti-6Al-4V alloy, from Table 4 it resulted more favorable values of all electrochemical parameters for Ti-6Al-4V-1Zr alloy, revealing the beneficial effect of Zr.

Improvement of corrosion resistance from in time variations of the open circuit potentials

From Figure 7 it can be seen that the open circuit potential values of the new quaternary alloy have shown an overall increase for about 500 immersion hours and than slowly tended to more electropositive values; for Carter-Brugirard saliva of pH = 2.54, the open circuit potential values tended to a constant level after 500 immersion hours. According to Blackwood,³² the shift of E_oc to positive values shows an increase of the passive film thickness and correspondingly, a decrease of the corrosion rate. Therefore, the new Ti-6Al-4V-1Zr alloy improved its passive state and corrosion resistance.

The more electronegative values of E_oc registered at pH = 2.54 are due to the fact that, at this pH value, only Ti is in the passive state on the Pourbaix diagrams,²³ while Al, V and Zr are placed in the active state and so, some dissolution and repassivation processes take place, conducting to the variations of E_oc for about 500 exposure hours.

At pH = 5.52, only V is in the corrosion state, Ti, Al and Zr are passive; so, a more stable values of the open circuit potentials can be observed because of more compact passive layer that contains the resistant Al₂O₃ and ZrO₂ oxides.^10,22

In neutral saliva of pH = 8.21, the open circuit potentials of Ti-6Al-4V-1Zr alloy stabilized at very electropositive values (about +0.2 V) after 3000 immersion hours, showing a very resistant, compact passive layer; at this pH value, all constituent elements (Ti, Al, V, Zr) are placed in the passivation state on the Pourbaix diagrams,²³ contributing with their insoluble oxides to the formation of the passive layer.

Ternary Ti-6Al-4V alloy presented some oscillations of E_oc values (Figure 7) suggesting some dissolution and repassivation processes of the passive layer.^29,32-34 Also, all E_oc values of ternary Ti-6Al-4V alloy have more electronegative values than of quaternary Ti-6Al-4V-1Zr alloy, pointing out a more stable passive state for the new alloy, due to the passive ZrO₂ oxide^10,22 that is presented in the passive layer.

In the presence of fluoride ions, open circuit potential changed to more electronegative values due to the negative effect of F^- ion;^24,30,35 the same tendency to stabilize and to ennoble was registered, showing a good, resistant passive layer.

Improvement of corrosion resistance from in time variations of the open circuit potential gradients

From Table 5 can be observed that, till present (3000 exposure hours), the open circuit potential gradients have low values (between 0.001 and 0.228 V) which cannot generate galvanic or local corrosion; because only differences of 0.6 - 0.7 V can initiate and keep galvanic cells.^24,27

Improvement of corrosion and ion release rates from Tafel curves

From Tafel curves, the corrosion current densities and corrosion rates were obtained (Table 6); also ion release rates were calculated. The new Ti-6Al-4V-1Zr alloy has a very good anticorrosive resistance, being classified as "Perfect Stable" and "Very Stable". The ternary Ti-6Al-4V alloy presented slow higher corrosion rates than quaternary Ti-6Al-4V-1Zr alloy, situated in the "Very Stable" category.²⁷

CONCLUSIONS

The corrosion resistance improvement of the new Ti-6Al-4V-1Zr alloy in comparison with ternary Ti-6Al-4V alloy in Ringer-Brown solution was proved by the more electropositive corrosion potentials, lower passive current densities and better tendency to passivation, resulting from the beneficial influence of Zr. Impedance spectra revealed better protective properties of the passive layer on the surface of the new alloy.

In Carter-Brugirard artificial saliva it resulted the same more favorable values of all electrochemical parameters for Ti-6Al-4V-1Zr alloy. Open circuit potentials tend to more electropositive values, denoting an increase of the passive film thickness. Fluoride ions had a slight negative influence on the corrosion and ion release rates, without to affect the very good stability of the new Ti-6Al-4V-1Zr alloy.

ACKNOWLEDGMENT

Financial support from Romanian CNCSIS Program PCCE, project no. 248 is greatly appreciated.

Recebido em 4/2/10; aceito em 2/6/10; publicado na web em 24/8/10

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Publication Dates

• Publication in this collection
  13 Jan 2011
• Date of issue
  2010

History

• Received
  04 Feb 2010
• Accepted
  02 June 2010