Corrosion Assessment of Zr2.5Nb Alloy in Ringer’s Solution by Electrochemical Methods

Abstract

This study aims to investigate the anticorrosive properties of Zr2.5Nb alloy intended for possible applications in the human body; it was tested for 2 days in Ringer solution (an artificial analogue for human blood, considered the most corrosive body fluid). For Zr2.5Nb samples, in situ electrochemical measurements to assess the anticorrosive properties were applied, such as open circuit potential (OCP), polarization resistance (Rp), potentiodynamic polarization (PD) and cyclic voltammetry (CV). The electrochemical results show that the Zr2.5Nb alloy shows a positive and stable trend according to the open circuit potential, but with a modest corrosion rate in the form of pitting, deduced from the analysis of the polarization resistance and cyclic voltammetry data.

1. Introduction

The average human body contains about one microgram of zirconium, but the chemical element itself does not serve to any known vital biological function. In the chemical industry, it is found in commercial cosmetics, such as antiperspirants, or in water purification systems. In addition to the aforementioned benefits, some research shows that, in the short term, zirconium powder can cause irritation on the skin and granulomas when inhaling zirconium compounds, or other pathologies, such as hemoglobin and low hematocrit with lethal effects in rats and guinea pigs, but only after persistent exposure to zirconium tetrachloride [1,2].

The major use of zirconium alloys by the nuclear community as a material for radioactive fuel plating boxes in the water-cooling reactor is indispensable, as it provides a low neutron uptake rate of zirconium in the cross section, a behavior suitable for mechanical wear, as well as corrosion resistant in specific environments [3]. In addition, an experimental study found that alloying zirconium with a small amount of niobium (less than 5%) improves the corrosion resistance of the material without further increasing the hydrogenation process, thus limiting the formation of the brittle hydride phase [3,4]. It is also necessary to specify the high temperature resistance of zirconium alloys, with interest for the aerospace industry in the manufacture of components in jet engines or stationary gas turbines [5].

The clinical applicability of the original zirconium alloys as an implant material has been known since 1969, being part of the hip and knee joint device [6]. The first reference to the use of 3Y-TZP zirconium in dentistry was in the 1970s; then, in 1993, zirconium was integrated into dentistry as a basis for the manufacture of crowns, dental bridges and other surgical or prosthetic devices. It should be noted that zirconium type 3Y-TZP is a single-phase material made of tetragonal polycrystals, made by alloying zirconium with 2–3 mol % of yttrium oxide (Y₂O₃); so in this form, the first bone dental implant was inserted in 2008 [7].

According to the data available in the literature, a zirconium alloy with niobium is a bio-inert material, which does not affect the growth of bone and tissue cells and also does not cause visible morphological changes in internal organs and has a minimal adhesion of bacteria [7].

Another study illustrates the favorable applicability of Zr2.5Nb zirconium alloy as a base material in biomedical applications [8].

The purpose of this study is to evaluate the electrochemical corrosion resistance of Zr2.5Nb alloy in Ringer’s saline solution. According to existing data in the literature, this alloy, in terms of applicability in the biomedical field, presents a number of advantages, such as low cost, higher corrosion resistance and is not allergenic [9,10,11].

2. Materials and Methods

The Zr2.5Nb alloy was purchased from Evek GmbH, Mülheim a.d. Ruhr-Germany in the form of a plate made by cold rolling of zirconium alloys doped with niobium up to 2.5% of the total mass of the material. According to the manufacturer’s instructions, this initial alloy was subjected to the vacuum annealing and cold rolled process, providing the following chemical composition shown in

Table 1. Chemical composition of Zr2.5Nb —wt%.

Be	Hf	Ni	Cr	Ti	Al	O	Pb	Nb	Zr
0.003	0.01	0.02	0.02	0.007	0.008	0.06–0.1	0.005	2.4–2.7	Bal.

.

Zr2.5Nb samples for corrosion tests were prepared in the form of plates with a thickness of 1 mm and dimensions of 25 mm × 25 mm, connected to a copper wire (to have electrical contact) after which they were covered with epoxy resin in the form of the film for the subsequent delimitation of the active surface areas of 3.2 cm² [12]. The tests were performed at the body temperature of 37 °C ± 0.5 °C.

The experimental test protocol, approximately 2 days, was part of the stage of hematoma formation and inflammatory response of a physiological process of bone regeneration, with specific interest in pH variability due to bioactive molecules and other determining factors [10,11,13].

The quality of the electrochemical tests was based on the proper processing of the active study surface before immersion in the electrolyte; the samples were cleaned with alcohol, then with hydrochloric acid, rinsed abundantly with distilled water, and finally dried in the oven. To exclude an increased margin of error, the tests were performed 3 times for repeatability, and the experimental data subsequently obtained were processed with accuracy and precision.

The Ringer electrolyte was used for electrochemical corrosion because it simulates the internal environment of the human body [14,15].

Table 2. Chemical composition and physical-chemical parameters for Ringer’s solution.

Nr. crt.	Chemical Compound	Ringer [g/L]
1	NaCl	8.402
2	KCl	0.302
3	CaCl2	0.298
4	Purified water (H2O)	Balance
	pH	6.79
	Conductivity [mS/cm]	14.1
	Salinity [ppt]	8.1

shows the chemical composition and physical-chemical parameters of the Ringer’s solution.

The corrosive electrochemical behavior of Zr2.5Nb alloy was evaluated using a conventional cell with glass walls and pre-drilled organic glass cover to which were attached 3 other electrode support systems, with the aim of sealing the components and the basic electrolyte. The electrolyte volume in the electrochemical cell for each experiment was 150 mL. The electrochemical cell used for corrosion investigation was composed by the three electrodes:

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WE—working electrode, i.e., the studied bio alloy;

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RE—Ag/AgCl reference electrode (saturated KCl) with a stable potential value of +199 mV compared to the normal hydrogen electrode (NHE);

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CE—auxiliary electrode or counter electrode—a rectangular grid of Pt (platinum) with an active surface of 4 cm² [12].

The electrochemical testing equipment is represented by the electrochemical workstation model PGZ 100 (VoltaLab), but the collection and monitoring of the raw data obtained was performed by a computer whose program runs a suitable software, VoltaMaster4.

The experimental protocol for determining the corrosion resistance of Zr2.5Nb alloy is shown schematically in Figure 1.

As can be seen in Figure 1, the electrochemical measurements used to evaluate the corrosion resistance were:

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Open circuit potential, OCP₁, during 6 h;

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Plotting 100 linear polarization curves with scan rate = 5 mV/s and an overvoltage ±50 mV, which in turn will provide 100 values of polarization resistance and corrosion rate by applying the Stern–Geary equations, Rp₁-Vcor₁;

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Open circuit potential, OCP₂, during 6 h;

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Plotting a single polarization curve in the Tafel linear range with an overvoltage of ±50 mV vs. OCP potential scanning speed of 5 mV/s, for calculating polarization resistance and corrosion rate, Rp₂-Vcor₂;

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Open circuit potential during 6 h, OCP₃;

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Plotting 100 linear polarization curves with scan rate = 5 mV/s and an overvoltage ±50 mV, which in turn will provide 100 values of polarization resistance and corrosion rate by applying the Stern–Geary equations, Rp₃-Vcor₃;

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Open circuit potential during 6 h, OCP₄;

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Plotting a single polarization curve in the Tafel linear range with an overvoltage of ±50 mV vs. OCP and potential scanning speed of 3 mV/s, for calculating polarization resistance and corrosion rate, Rp₄-Vcor₄;

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Open circuit potential during 6 h, OCP₅;

Plotting 100 linear polarization curves with scan rate = 5 mV/s and an overvoltage ±50 mV, which will further provide 100 values of polarization resistance and corrosion rate by applying the Stern–Geary equations, Rp₅-Vcor₅;

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Open circuit potential during 1 h, OCP₆;

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Potentiodynamic polarization diagram (PD) from −1500 to +1200 mV with a potential scan rate of 5 mV/s, performed on separate samples;

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Open circuit potential during 0.5 h, OCP7;

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Potentiodynamic polarization diagram (PD) from −1500 to +1200 mV with a potential scan rate of 3 mV/s, performed on separate samples;

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Open circuit potential during 0.5 h, OCP₈;

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Cyclic voltammetry diagram (CV) from -1500 mV to +1200 mV and back to −1500 mV with a potential scan rate of 5 mV/s, performed on separate samples;

Open circuit potential during 0.5 h, OCP₉;

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Cyclic voltammetry diagram (CV) from −1500 mV to +1200 mV and back to −1500 mV with a potential scan rate of 3 mV/s, performed on separate samples;

An OPTIKA XDS3 MET optical microscope and Vision Pro assistance software were used to view the samples after the corrosion tests.

3. Results and Discussions

3.1. Open Circuit Potential (OCP)

The time variation of the open circuit potential or the free potential evolution was used as a primary criterion for determining the corrosion behavior of a material in a specific used environment. In the case of our protocol, the periods of highlighting the potential in open circuit, Figure 2, were alternated with the analyzes of the polarization resistance and the corrosion rate, where later the collected data were merged and exposed in a single diagram for a more accurate assessment.

From Figure 2, it can be seen that the immersion potential has a value of E = −450 ± 21 mV vs. Ag/AgCl reaching at the end of the 38.5 ± 6 h a value of E = −241 ± 13 mV vs. Ag/AgCl. This shift of the potential to nobler values demonstrates that a layer of passive oxide is formed on the surface of the Zr2.5Nb alloy. This tendency to move towards more positive values was also observed by other authors when studying the corrosion behavior of different commercial alloys in Ringer’s solution [16].

3.2. Evaluation of Polarization Resistance, R_p, of the Zr2.5Nb Alloy

The evolution of polarization resistance, R_p, is a distinctive technique in measuring corrosion rates, namely the loss of direct thickness in real time. In conclusion, the higher values of polarization resistance lead to a lower corrosion rate [17].

Figure 3 shows the linear polarization log i vs. potential for the Zr2.5Nb samples tested in Ringer’s solution at a scanning speed of 3 mV/s.

Linear polarization was obtained by scanning the potential around the free potential with a small amplitude of ±50 mV.

From the polarization resistance diagram of the Zr2.5Nb, the 100 values of the polarization resistance and corrosion currents can be extracted from the slope of the curve, according to the Stern–Geary equation [18,19]:

$$i_{corr}=\frac{B}{R_p}$$

(1)

At the same time, the Stern–Geary parameter, B was calculated using the equation:

$$B=\frac{b_a{\left|b_c\right|}_{}}{2.303\left(b_a+\left|b_c\right|\right)}$$

(2)

where i_corr = corrosion current density; R_p =polarization resistance; and b_a and b_c are the slopes of Tafel for anodic and cathodic reactions, respectively.

From the analysis of Figure 4, a decrease in the polarization resistance is observed with the increase in the immersion time. If, after 6 h of immersion, R_p = 39.6 ± 4 kohm cm², after about 37 h, R_p decreases to a value of 33.2 ± 5 kohm cm². A lower value of the polarization resistance indicates a higher corrosion rate as the immersion time increases for the Zr2.5Nb alloy immersed in Ringer’s solution.

3.3. Corrosion Rate, V_corr, of Zr2.5Nb Alloy in Ringer’s Solution

The possibility of measuring the corrosion rate, V_corr, directly and in real time (μm/year) can be achieved exclusively by analyzing the evolution of the polarization resistance (R_p). The densities of the corrosion currents determined by the relation i_corr = I_corr/S were estimated by the inverse proportionality of the values collected from the linear polarization curves (R_p) and Equations (1) and (2), the corrosion rate as the penetration rate resulted after applying Faraday’s law [20]. In other words, the higher the polarization resistance values, the lower the corrosion rates.

The diagram with the corrosion rates at different immersion periods for the Zr2.5Nb alloy immersed in the Ringer’s solution are shown in Figure 5.

Figure 5a shows that the corrosion rate recorded for the Zr2.5Nb alloy in the first segment reaches a value of V_corr = 2.37 ± 0.9 μm/year. After about 21 h, the value of the corrosion rate reaches 2.31 ± 0.5 μm/year. At the end of the 37 h, it has a value of 2.29 μm/year. This decrease is a relatively small difference between the value of V_corr after 6 h and at the end of the 37 h, which is 0.08 μm/year.

In conclusion, the values of the corrosion rate recorded on the surface of the Zr2.5Nb alloy classify the studied material within the group with good stability [14].

3.4. Potentiodynamic Polarization

Potentiodynamic polarization testing is one of the most widely used techniques for assessing the corrosion resistance of a material. The potential difference was measured between the reference electrode and the working electrode. We applied the difference between the two electrodes and measured the current. The measured current indicates the corrosion rate that occurs on the working electrode in terms of current per unit area, known as current density. This method determines the domains of the active/passive potentials and helps to understand the different processes that take place on the surface of the tested sample [21].

For the Zr2.5Nb alloy, the potentiodynamic polarization test was performed in a potential range from −1.5 V vs. Ag/AgCl at +1.2 V vs. the reference electrode with a scanning rate of potential equal to 3 mV/s. The potentiodynamic polarization curve of the Zr2.5Nb alloy immersed in Ringer’s solution is shown in Figure 6.

Figure 6 shows that the passive domain for the zirconium alloy immersed in the Ringer’s solution after approximately 39 h of immersion is on a large domain of potential between −856 ± 37 mV to +986 ± 31 mV vs. Ag/AgCl, having a ΔE = 1842 mV. In general, if the passive range covers a larger potential range and the value of the passivation current is lower, then the material has a higher corrosion resistance and the ability to passivate in the tested solution [18].

3.5. The Influence of the Ringer Solution on the Cyclic Voltammetry Curve, CV, of the Zr2.5Nb Alloy

The results obtained from a measurement of cyclic voltammetry provide us with qualitative and quantitative information of the induced corrosive process, in different electrochemical conditions [21].

Cyclic voltammetry is initiated in the negative direction (−1.5 V relative to the reference electrode) to the positive direction (+1.5 V relative to Ag/AgCl) with a scanning speed of 3 mV/s. The scanning direction is then reversed after the anodic current has reached the corresponding value at +1.5 V relative to Ag/AgCl to form a complete cycle. The recorded diagram for the Zr2.5Nb alloy immersed in Ringer’s solution is shown linearly in Figure 7a and on the logarithmic scale of the current density in Figure 7b.

According to Figure 7a, from the diagram obtained in the case of the Zr2.5Nb sample immersed in Ringer’s artificial solution, it can be seen that, at the beginning of the transpassive segment, the current density shows a large increase from a potential of E = +1023 ± 45 mV vs. Ag/AgCl and continues to increase until the potential is reversed at a current density of i = 534 ± 11 µA cm⁻².

The direction of the reverse potential shows a trajectory above the upward curve, indicating a high susceptibility to localized corrosion and a wide hysteresis that reveals a pitting corrosion.

This phenomenon is mainly due to the abundant quantitative chloride content of the test solution, Ringer, which corresponds to the aggressive environment in the human body for all implant biomaterials [17].

From Figure 7b, it can be seen that the current density recorded for the Zr2.5Nb alloy immersed in Ringer’s solution has a high value and the pitting corrosion potential is situated at E= +1023 ± 45 mV.

The size of the specific hysteresis visible in the transpassive (anodic) part of the drawn curve (Figure 7b) is very large. This behavior reveals a lower pitting corrosion resistance for the Zr2.5Nb alloy immersed in Ringer’s solution approximately 41 h after immersion.

3.6. Optical Microscopy of Zr2.5Nb Surfaces Subjected to Electrochemical Corrosion Tests

The OPTIKA XDS3 MET optical microscope, running with Vision Pro assistance software, was used to highlight the electrochemical changes suffered by the surface of the Zr2.5Nb alloy before and after the electrochemical corrosion tests. The micrographs shown in Figure 8 provide information on the surface state of the Zr2.5Nb alloy, before (a) and after (b) electrochemical testing in Ringer’s solution.

Figure 8a shows that the surface of the Zr2.5Nb alloy before corrosion has a uniform surface with slight traces of scratches due to the grinding process of the samples.

After the corrosion tests Figure 8b, it is observed the appearance of smaller and larger corrosion pits that are found on the entire surface of the sample, and can be better observed in Figure 8c,d. Thus, after analyzing the images obtained from optical microscopy, we can say that the results of electrochemical tests revealed by cyclic voltammetry are consistent with the results of optical microscopy.

4. Conclusions

In our research, the corrosion assessment of the Zr2.5Nb alloy by electrochemical methods in Ringer’s solution was performed for 2 days in order to evaluate the possible applications in the human body.

From the results of the open circuit potential, the surface of the Zr2.5Nb alloy showed that the alloy stabilizes in the first 20 h and presents in the subsequent period a state of equilibrium with a moderate tendency to move the potential towards more positive values.

From the analysis of the polarization resistance, a slight decrease in the polarization resistance is observed with the increase in the immersion time.

From the cyclic voltammetry curves, it is observed that the studied biomaterial has a high susceptibility to localized corrosion and a wide hysteresis that reveals a pitting corrosion occurring on its surface by immersion in Ringer’s solution.

Optical microscopy confirms the results of cyclic voltammetry and highlights the appearance of the pitting corrosion.