corrosion behavior of vt6s titanium alloy with oxidized ... - Springer Link

Materials Science, Vol. 48, No. 6, May, 2013 (Ukrainian Original Vol. 48, No. 6, November–December, 2012)

CORROSION BEHAVIOR OF VT6S TITANIUM ALLOY WITH OXIDIZED NITRIDE LAYERS IN 0.9% NACL AT 36°° С V. M. Fedirko,1 І. М. Pohrelyuk,1, 2 О. V. Tkachuk,1 and R. V. Proskurnyak1

UDC 621.78:669.295

We study the corrosion behavior of VT6s titanium alloy with oxidized nitride layers in a 0.9% isotonic solution of NaCl at 36°С. It is shown that the corrosion resistance of the alloy is determined not by the thickness of the oxynitride film formed on the surface but by its chemical composition and, in particular, by the oxygen content. Keywords: VT6s titanium alloy, nitrided layers, oxidation, corrosion resistance.

Titanium alloys are extensively used in medicine because they are characterized by a good combination of physicochemical properties as compared with the other materials and, in particular, with stainless steel and cobalt-chromium alloys [1–3]. Thus, they are characterized by high strength, corrosion resistance, biocompatibility, and low toxicity for the human body. The practical application of these alloys in medicine is often restricted due to the necessity of modifying their surface with an aim to improve the mechanical, corrosion, and tribological characteristics [4–7]. At present, oxynitriding proves to be a promising method for increasing the surface characteristics of titanium alloys intended for medical applications [7–9]. It is known that oxynitride coatings have not only improved physicochemical and mechanical properties but also high biomedical characteristics (in particular, high levels of adhesion of platelets and the optimal time of blood coagulation) [10, 11]. The aim of the present work is to evaluate the corrosion resistance of VT6s titanium alloy with oxidized nitride layers in an isotonic 0.9% NaCl solution at 36°С. Experimental Procedure The phase composition of the surface layers of the alloy after thermochemical treatment was determined by the method of X-ray phase diffraction analysis in a DRON-3.0 diffractometer ( CuK ! -monochromatic radiation;

focusing by the Bragg-Brentano scheme). The voltage on the anode of the X-ray tube was 30 kV at a current of 20 mA. Scanning was performed with steps of 0.05°. We used the Sietronix, Powder Cell 2.4, and FullProf software packages. The Fourier processing of the X-ray diffraction patterns was performed with the help of this software. The locations of the diffraction maxima of reflection and the lattice constants of the identified phases were found according to the data of the JCPDS–ASTM card file [12]. The quantitative characteristics of the surface microgeometry (height parameters Ra , Rz , and Rmax , step parameters S and Sm , and the mean radii of curvature of the profile peaks) were computed with the help of a 170,621 profilometer by using special programs [13].

1 2

Karpenko Physicomechanical Institute, Ukrainian National Academy of Sciences, Lviv, Ukraine. Corresponding author; e-mail: [email protected].

Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 48, No. 6, pp. 70–75, November–December, 2012. Original article submitted November 8, 2012. 1068-820X/13/4806–0769

© 2013 Springer Science+Business Media New York

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V. M. FEDIRKO, І. М. POHRELYUK, О. V. TKACHUK,

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For the electrochemical investigations of the alloy with oxidized nitride layers, we used a PI-50.1.1 potentiostat. The potential was measured relative to the silver/silver-chloride electrode. The polarization curves were taken on the potentiostat within the potential range (– 1.0)–2.5 V at a sweep rate of 2 mV/sec. We used a three-electrode glass cell with an auxiliary platinum electrode and a saturated Ag/AgCl reference electrode. The

surface of the electrode made of the investigated alloy was coated with epoxy resin. The working area (1 cm 2 )

was left uncoated. The corrosion potential and corrosion current were determined from the polarization curves by using the Tafel extrapolation method. Results and Discussion The specimens were oxynitrided by the consecutive modification with oxygen of the nitride formed on the surface of alloy as a result of its thermodiffusion saturation with nitrogen. The thermochemical treatment was realized in the following modes: І. Thermodiffusion saturation in nitrogen at 850°С for 12 h under dynamic and static conditions. In the

first case, under a pressure of nitrogen pN 2 = 1 Pа at a leakage rate I = 7 !10 – 3 Pa/sec and, in the second

case, for pN 2 = 10 5 Pa . The specimens were heated to the temperature of nitriding in a vacuum of 10 – 3 Pa .

The heating rate was equal to 0.04°С/sec. After isothermal holding, the specimens were cooled down at a rate of 100°С/h. After cooling to 500°С, the system was evacuated.

ІІ. Oxidation of titanium nitride. The preliminarily nitrided specimens were heated in a vacuum of 10 – 3 Pa up to the oxidation temperature TO = 650°С at which an oxygen-containing atmosphere ( pO 2 = 0.001 Pа)

was delivered into the chamber. Then the specimens were cooled down to 500°С and the system was evacuated.

ІІІ. Reoxidation of titanium nitride. The preliminarily oxidized specimens were additionally oxidized according to the scheme identical to the scheme of oxidation but at a temperature of 850°С. According to the data of X-ray phase diffraction analysis, a nitride film is formed on the surface of alloy as a result of nitriding. It is separated from the matrix of alloy by a diffusion zone (solid solution of nitrogen in ! -titanium) [14] and consists of ! -TiN and ! - Ti 2 N titanium nitrides whose contents are determined by the partial pressure of nitrogen (also specifying the thickness of the film). Thus, the nitride film formed during nitriding in a rarefied dynamic atmosphere of nitrogen contains 4% TiN and 65% Ti 2 N , whereas the film formed in the course of nitriding in nitrogen under atmospheric pressure contains 67% TiN and 18% Ti 2 N . Hence,

prior to the modification with oxygen, we have two versions of the nitrided layer (Fig. 1): version I is based on Ti 2 N titanium nitride ∼ 3 µm in thickness and version II is based on TiN mononitride ∼ 7 µm in thickness. For versions I and II, the lattice parameters of the surface ! -TiN are 0.4239 and 0.4248 nm, i.e., the composition of ! -titanium nitride in the nitrided layer II is closer to the stoichiometric composition. Fragments of the topography formed and growing during nitriding affect the parameters of profile of the nitrided surface (Table 1). In the nitrided layer II (in which the nitride layer is based on titanium mononitride), both the height and step parameters of the surface microprofile are higher (in the first case, 2.3 times higher) and the radius of curvature is smaller. The surface quality of the alloy in versions I and II of the layer differs by a quality class.

CORROSION BEHAVIOR OF VT6S TITANIUM ALLOY WITH OXIDIZED NITRIDE L AYERS IN 0.9% NAC L AT 36°С

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Fig. 1. Subsurface layers of VT6s titanium alloy with two versions of the nitrided layer (І and ІІ): TiN and Ti2 N are titanium nitrides; Ti(N) is a solid solution of nitrogen in ! -titanium, and Ti is the titanium matrix.

Table 1. Parameters of the Surface Profile of VT6s Titanium Alloy After Nitriding

Characteristics, µm

Versions of the nitrided layer I

II

Ra

0.184

0.422

Rz

0.734

1.662

Radius

10830.096

9102.066

Rmax

0.933

2.467

S

27.913

28.714

Sm

32.870

54.496

In the process of oxynitriding (both after oxidation and reoxidation of the preliminarily nitrided VT6s alloy), an oxynitride film is formed on the surface of alloy. This is demonstrated by the data of X-ray phase diffraction analyses: in the surface X-ray diffraction spectra of oxidized specimens, we detected (111), (200), and (220) reflexes of the TiN x O1–x oxynitride phase (Fig. 2). In the course of reoxidation of the alloy, the relative

intensity of reflexes of the oxynitride phase increases, which reveals the acceleration of the process of formation of oxynitrides.

After oxynitriding (both after oxidation and reoxidation), independently of the type of nitrided layer, the arithmetic mean deviation of the surface profile Ra increases but its values remain within the limits of the

quality class.

As a result of oxidation, the oxynitride with a larger lattice parameter, as compared with layer I, is formed in the nitrided layer II (Table 2). According to the dependence of the lattice parameter of titanium oxynitride on the nitrogen content [15, 16], this means that the content of the oxygen component in this case is lower.


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Fig. 2. X-ray diffraction spectra taken from the surface of VT6s titanium alloy after nitriding (1), oxidation (2), and reoxidation (3) of titanium nitride formed under partial pressures of nitrogen of 1 Pa (a) and 10 5 Pa (b).

Table 2. Lattice Parameters of TiN x O1–x Oxynitride Formed on the Specimens of VT6s Titanium Alloy After Oxynitriding

Lattice parameter, nm

a

Versions of the nitrided layer I

II

oxidation

reoxidation

oxidation

reoxidation

0.4242

0.4245

0.4270

0.4274

Clearly, this should be associated with the occupancy of the nonmetallic sublattice of nitride prior to oxidation (lower degree of nonstoichiometry), as indicated by higher values of the lattice parameter for the surface ! TiN nitride: 0.4248 against 0.4239 nm for version I; in passing from version I to version II after oxidation, the nitrogen content of the composition of titanium oxynitride increases and, therefore, its oxygen content decreases. As a result of reoxidation, the content of the oxygen component of titanium oxynitride decreases independently of the type of the nitrided layer, as shown by the increase in the lattice parameter of oxynitride (Table 2). Clearly, this is connected with the acceleration of diffusion processes in the subsurface layers. We now describe the corrosion behaviors of the analyzed alloy in 0.9% NaCl at 36°С after oxidation and reoxidation depending on the version of the nitride layer. Version I. In the anodic branch of the polarization curve of the analyzed alloy after nitriding (Fig. 3a, curve 1) plotted in 0.9% NaCl, we detected two peaks of increase in the current density beyond the zone of intense dissolution (at 0.07 and 0.3 V) caused by the dissolution of the nitride film with formation (on the surface) of titanium oxynitride and titanium oxide (of the nonstoichiometric composition) inhibiting the process of dissolution [17, 18]. After the oxidation of titanium nitride at potentials of 0.1–1 V, we detect a broad passive region. This is obviously caused by the stability of the oxynitride film formed as a result of the modification of titanium nitride with oxygen (Fig. 3a, curve 2). The current of complete passivation is equal to 0.013 A/m 2 .


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Fig. 3. Potentiodynamic curves of VT6s titanium alloy in 0.9% NaCl after nitriding (1), oxidation (2), and reoxidation (3) of titanium nitride formed under partial pressures of nitrogen of 1 Pa (a) and 10 5 Pa (b).

Table 3. Corrosion Parameters of VT6s Titanium Alloys in 0.9% NaCl Nitriding

Oxidation

Reoxidation

Corrosion parameters

Version I

Version II

Version I

Version II

Version I

Version II

Ecorr , V

– 0.200

– 0.285

– 0.41

– 0.38

– 0.47

– 0.39

icorr , A/m 2

0.006

0.0015

0.002

0.060

0.024

0.058

In the potential range 1.0–1.9 V, the rate of dissolution of the surface films increases. The corrosion current

is equal to 0.002 A/m 2 , i.e., is three times lower than after nitriding (Table 3). The current of anodic dissolution is lower than after nitriding by two orders of magnitude. After the reoxidation of titanium nitride, the process of anodic dissolution runs in the same way as after oxidation (Fig. 3a, curve 3). Within the potential range (– 0.19)–0.23 V, we recorded the first passive region (the

current of complete passivation is equal to 0.1 A/m 2 ) caused by the stability of the oxynitride film. The nar-

rower potential range in this region shows that the composition of oxynitride formed in this case gives worse protective properties of the surface than in the process of oxidation. The subsequent increase in the current is connected with the dissolution of the surface film and formation of the oxide film, which corresponds to the second passive region observed within the range of potentials 0.7–1.45 V. The current of complete passivation is now equal to 0.2 A/m 2 and the corrosion current to 0.024 A/m 2 . The increase in the corrosion current and the current of anodic dissolution by an order of magnitude and the shift of the corrosion potential in the negative direction (by 0.06 V) reveal a decrease in the corrosion resistance of the alloy after reoxidation as compared with its corrosion resistance after oxidation.

Version II. In the anodic branch of the polarization curve of the analyzed alloy with nitrided layer II after nitriding (Fig. 3b, curve 1) recorded in 0.9% NaCl at potentials of 0.1–0.4 V, we observe two peaks of increase

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in the current density caused by the dissolution of nitride films accompanied by the formation of titanium oxide of the nonstoichiometric composition on the surface. In the anodic curve of the alloy plotted after the oxidation of titanium nitride (Fig. 3b, curve 2), we detect a passive region at potentials (– 0.31)–(– 0.10) V. The current

of complete passivation is equal to 0.2 A/m 2 . It is worth noting that the current of anodic dissolution in the

course of oxidation of titanium nitride is higher than after nitriding by two orders of magnitude. After reoxidation, the character of the anodic curve (Fig. 3b, curve 3) remains the same as after oxidation. The influence of reoxidation on the corrosion resistance of the alloy is insignificant as compared with the influence of oxidation. The corrosion current and corrosion potential remain almost constant but the current of anodic dissolution somewhat decreases. After oxynitriding, the thickness of the modified nitride layer is comparable with the thickness of the initial nitride, i.e., the shielding effect of the nitrided layer II against the corrosive medium is twice higher than the effect of the nitrided layer of version I both after oxidation and after reoxidation. Since both oxidation and reoxidation positively affect the corrosion characteristics of titanium alloy with nitrided layer I and worsen the characteristics of the alloy with layer II, the decisive role in the corrosion protection of oxynitrided VT6s titanium alloy is played not by the thickness of surface oxynitride but by its composition. CONCLUSIONS The modification of ! -titanium nitride whose composition is close to stoichiometric with oxygen intensifies the corrosion processes in physiological solutions; furthermore, the higher the oxygen content of oxynitride formed on the surface, the stronger its intensifying influence. The decrease in the partial pressure of nitrogen to 1 Pа in the process of nitriding positively affects the corrosion characteristics of alloy in physiological solutions in the course of its subsequent oxidation and this influence increases with the oxygen content of oxynitride formed on the surface. The present work was supported by the State Foundation for Fundamental Research of the State Agency on the Problems of Science, Innovations, and Informatization of Ukraine (Project No. F41.2/011). REFERENCES 1. P. Tengvall and I. Lundstrom, “Physicochemical considerations of titanium as a biomaterial,” Clin. Mater., 9, 115–134 (1992). 2. T. Chang-bin, L. Dao-xin, W. Zhan, and G. Yang, “Electrospark alloying using graphite electrode on titanium alloy surface for biomedical applications,” Appl. Surf. Sci., 257, 6364–6371 (2011). 3. M. R. Amaya-Vazquez, J. M. Sanches-Amaya, Z. Boukha, and F. J. Botana, “Microstructure, microhardness, and corrosion resistance of remelted TiG2 and Ti6Al4V by a high power diode laser,” Corr. Sci., 56, 36–48 (2012). 4. S. Piscanec, L. C. Ciacchi, E. Vesselli, G. Comelly, et al., “Bioactivity of TiN-coated titanium implants,” Acta Mater., 52, 1237– 1245 (2004). 5. C. Leinenbach and D. Eifler, “Fatigue and cyclic deformation behavior of surface-modified titanium alloys in simulated physiological media,” Biomaterials, 27, 1200–1208 (2006). 6. T. M. Manhabosco, S. M. Tamborim, C. B. dos Santos, and I. L. Müller, “Tribological, electrochemical, and triboelectrochemical characterization of bare and nitrided Ti6Al4V in simulated body fluid solution,” Corr. Sci., 53, 1786–1793 (2011). 7. B. Subramanian, C. V. Muraleedharan, R. Ananthakumar, and M. Jayachandran, “A comparative study of titanium nitride (TiN), titanium oxynitride (TiON), and titanium aluminum nitride (TiAlN), as surface coatings for bioimplants,” Surf. Coat. Tech., 205, 5014–5020 (2011). 8. A. Rizzo, M. A. Signore, L. Mirenghi, and T. Di Luccio, “Synthesis and characterization of titanium and zirconium oxynitride coatings,” Thin Solid Films, 517, 5956–5964 (2009).


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