Corrosion Behavior of Some Timonb Alloys in Ringer Solution

The aim of this study was to investigate the influence of different process parameters as chemical composition, the pH value and immersion time on the corrosion of the some TiMoNb alloys, using different electrochemical techniques such as: cyclic voltammetry, open circuit potential (OCP) measurement, polarization curves and electrochemical impedance spectroscopy (EIS). The alloys were analyzed in the natural pH of the Ringer solution, but also with an acidic modification of the solution (ph = 4) and a basic modification (ph = 8). The more acidic values of pH, the more evident are differences between corrosion behavior of titanium-based alloys depending on their chemical compositions and immersion times.


Introduction
Titanium and its alloys are intensively studied in dental medicine due to their excellent biocompatibility, corrosion resistance and relatively low density [1]. Attempts to use titanium in the manufacture of implants date from the 1930 [2].
Metals such as Zr, Mo, Ni, Nb, Ta are elements that may be present in titanium alloys, as they exhibit excellent biocompatibility and belong to the group of non-toxic metals in their interaction with living tissue. Also, these elements are the most suitable alloying elements that can be added to decrease the modulus of elasticity without compromising its mechanical resistance [3][4]. In the case of titanium alloys, different methods of improving the corrosion resistance have been studied [5]. As an alloying element, niobium is known to be particularly for corrosion resistance in the simulated body fluid, also presenting a particularly good biocompatibility [6][7]. The use of molybdenum is still controversial, but some studies have shown that some Mo-containing Ti alloys can have adequate mechanical compatibility and good cytotoxic compatibility such as Ti-Mo, Ti-Mo-Ta, Ti-Mo-Zr-Fe [8][9][10][11][12][13]. The advantage of Mo is that it is a strong stabilizer of the  phase for Ti alloys in comparison to Nb, Ta or Zr [14][15]. Titanium alloys have been studied in different physiological mediums, such as artificial saliva, Hank solution, Ringer solution, NaCl solution to understand their electrochemical behavior as biomaterials subjected to corrosion in human organism [16][17][18][19][20]. In some studies, the influence of molybdenum content on corrosion resistance of Ti-Mo alloys in different simulated physiological environments has been investigated and one showed that the electrochemical behavior was improved due to the presence of molybdenum [21][22][23][24][25][26]. The superior properties of Ti-Nb and Ti-Nb-Zr systems are ascribed to addition of niobium (Nb) as the primary alloying element since it possesses distinguish attributes of improved β stability, low elastic modulus and high corrosion resistance [27][28][29][30][31][32][33][34][35][36].
A recent concern of researchers is to find ways to optimize the properties of these alloys with applications in medicine. Thus, efforts are made to find an optimal surface properties of alloys that meets excellent qualities due to which the lifetime of the alloys can be improved in the aggressive environment of the human organism. Ti-based alloys remain some of the most widely used biomaterials. These studies described above were conducted to better understand the electrochemical behavior and their possible effects on the human environment.
The purpose of this study is to highlight the influence of different process parameters, such as pH of the electrochemical medium, chemical composition of alloys, immersion time on the corrosion behavior of investigated alloys.

Materials and methods
Samples from 5 alloys were used in this study, symbolized as follows: Ti12Mo, Ti10Mo8Nb, Ti8Mo16Nb, Ti6Mo24Nb and Ti4Mo32Nb. These alloys were synthetized in the specialized laboratory of the ISCR/C-Met-Site INSA Rennes (France) using the induction levitation-melting processing [37].
From as-cast Ti alloys mini-ingots, samples for electrochemical tests having an active area of 0,28 cm 2 were prepared by machining, polishing and mirror-like polishing. Before each test, the electrochemical surface of the alloys was sanded with SiC 1000, 2000, and 4000 grains of paper, then rinsed with distilled water and placed in the ultrasonic bath.
The electrochemical tests were performed at room temperature using the Ringer solution (1L solution of Ringer solution: Sodium chloride 8.60 g, Potassium chloride 0.30 g, Calcium chloride dihydrate 0.33 g.) at the natural pH (pH = 6.5), then Ringer solution at basic pH (pH=8) and acidic pH (pH=4). HCl and NaOH were used to modify the pH of initial Ringer solution.
To conduct the electrochemical tests, a glass electrochemical cell was used, with a simple wall and a potentiostatic assembly consisting of a working electrode, a reference electrode (SCE) and a platinum auxiliary electrode. The potentiostat used was PGZ-301. The experimental data was processed with the Voltamaster 4 software. EIS data processing (electrochemical impedance spectroscopy) was performed using the Zsimwiew program.
The tests were performed by scanning the potential with scan rate of 10mV/s, the applied potential being between -500÷2000mV. Tafel analysis was applied to extract the Ecorr corrosion potential and the corrosion current density jcorr.
EIS spectra were recorded for the frequency range between 100 kHz and 25mHz, with a potential amplitude of 10mV and 20 points per decade.

Results and discussions
Titanium and its alloys are passive metallic materials in aqueous media. On the surface of bodies from titanium-based metallic materials is spontaneously formed an oxide film characterized by compactness, adherence at substrate and thinness. In contact with aqueous media the features of oxide film changes because thermodynamic and kinetic conditions are changed. The corrosion behavior of titanium-based metallic materials in aqueous electrolytes, and therefore also in natural or simulated biological environments, is depending to passive state for the given conditions.
Using electrochemical methods, the corrosion behavior of some alloys from the Ti-Mo-Nb system in Ringer solution was investigated, thus the use potential of these alloys as biomaterials being preliminary evaluated. The results of the electrochemical tests highlighted the relationship between corrosion parameters (EOCP, Ecorr, icorr, Rp, R) and process parameters (Nb/Mo ratio, pH and immersion time (IT)).

Open circuit potential monitoring
Although by the monitoring the values of open circuit potential (EOCP) in time the information about corrosion behavior is minimal, the passivity state can be qualitatively evaluated in the case of metallic materials. In Figure 1(a,b,c) are shown the EOCP values vs. immersion time (IT), and in Table  1 the EOCP values at the end of monitoring time.   From Figure 1(a,b,c) one can observe that the EOCP values are quasi-stable in time even after 24 h of immersion, with some exception for initial immersion. This evolution indicates that passive films formed on the surface of all Ti alloys, regardless pH values and immersion times are stable.

Analysis of the corrosion behavior using the potentiodynamic polarization
Potetiodynamic polarization tests allows to estimate the values of some corrosion parameters such as corrosion potential (Ecorr), corrosion current density (icorr) and for corrosion characterized by passivity, breakdown potential (Ebd), passive current density (ipas). Figure 2(a,b,c) shows the characteristic potentiodynamic polarization curves and Table 2 and Table 3 contains the values of https://doi.org /10.37358/Rev. Chim.1949 Rev. Chim., 71 (7) The shape of polarization curves is characteristic for the titanium-based alloys immersed in simulated body fluids. The curves contain an active-passive transition followed through a passive domain of potential. It is worthy to mention that, after a 24 h immersion, regardless to alloy and pH, the passivation current density has a slight tendency to increase with the potential. This fact indicates that in time the passive films supports changes concerning structure and thickness. For short immersion time (1 h), especially for pH = 8, a well-defined potential plateau for which passivation current density has a value almost constant can be observed. It can be supposed that the passive films, as immersion time increase, change its features linked to corrosion behavior. Also, for pH = 8.0 the passivation current densities have values much closer. The values of Ecorr are less than the values of EOCP that is possible due to potentiodynamic polarization. The corrosion current density for pH = 4 and pH = 6.5 increased when immersion time has increased, and, inversely, for pH = 8 the values of this corrosion parameter has decreased.

Analysis of corrosion behavior using electrochemical impedance spectroscopy
Electrochemical impedance spectroscopy was performed under the potential conditions in the open circuit. The results were represented as the Bode diagrams, both Impedance module vs. Frequency and Phase angle vs. Frequency. Some Bode diagrams are shown in the Figure 3, and values of electrical quantities from equivalent electric circuit (EEC) are given in Table 4, Table 5, and Table 6. All EIS spectra were fitted using an R(QR) EEC.
EIS method used in corrosion bring out the structure and properties of passive films in relation with its protective features against corrosion of the substrate. For all investigated alloys, regardless the experimental condition of EIS tests, the spectra were fitted using an equivalent electric circuit identifies by R(QR). This EEC is characterized by a single time constant, which shows that passive film has a monolayer structure. Having in view the EEC (R(QR)), from its electric parameters, the thickness of passive films can be deduced using the following equations [38,39]: Using data from Tables 4, 5

Conclusions
As a result of this research on the corrosion behavior of five alloys from Ti-Mo-Nb system in Ringer solution with various values of pH and for the different immersion times can be formulated the following conclusions.
The more acidic values of pH, the more evident are differences between corrosion behavior of titanium-based alloys depending on their chemical compositions and immersion times. For pH of 4.0 and a short immersion time the corrosion current densities were the lowers. Increasing immersion time, the values of this corrosion parameter have increased. Inversely, at basic value of pH, the corrosion current densities have slightly decreased when immersion time increased.
Regardless of pH and immersion times, both lower corrosion current densities and lower thicknesses of passive films were found for alloys which have Nb/Mo values within 0.8 and 4.0.