Microstructural, Mechanical and Electrochemical Characterization of Calcium Modified Mg-Zn Alloys for Biodegradable Medical Devices

1. Introduction

Orthopedic implants have traditionally been manufactured from metallic materials such as stainless steels and titanium alloys but also from ceramic and polymer based composites [1,2]. However, metallic implants can release potentially toxic metal ions, and these persist in the body for long periods of time leading to inflammatory responses or infections during long term service [3,4,5]. Conventional polymeric and ceramic implants, on other hand, generally exhibit insufficient mechanical strength and poor load bearing capability [6,7] compared with natural bone. Moreover, once the tissue healing and regeneration are completed, a secondary surgical procedure is required to remove the permanent implants [8,9], exposing the patients to additional pain, infection risk and additional expenses. To overcome these limitations, increasing attention has been directed toward the development of biodegradable implant materials [10,11] that combine good biocompatibility, mechanical properties comparable to those of human bone and a controlled degradation rate.

In this regard, magnesium Mg has emerged as one of the most promising candidates for the next generation of orthopedic implant materials [12,13]. Mg is a non/toxic and biocompatible element that is abundantly present in human bone where it plays a critical role in bone metabolism, mineral homeostasis and the regulation of bone density. Moreover, its elastic modulus and density are closer to those of natural bone compared with conventional metallic implants, which helps to minimize stress shielding effects [14]. Despite these advantages, the practical application of pure Mg as an implant material is severely limited by its excessively rapid and uncontrolled corrosion rate under physiological conditions. During Mg degradation in physiological environments, substantial amounts of hydrogen gas are generated, which accumulates around the implant and hinders tissue healing. Consequently, extensive research efforts have focused on developing biocompatible Mg alloys with tailored corrosion behavior and mechanical properties. Alloying elements such as calcium (Ca), zinc (Zn), tin (Sn), strontium (Sr) and rare earth (RE) elements have been widely explored to improve corrosion resistance, mechanical strength and biological performance [15]. An ideal Mg orthopedic implant alloy should therefore exhibit excellent biocompatibility, controlled biodegradability, low density and mechanical properties compatible with human bone.

Among the various Mg alloys systems, Mg-Ca alloys have attracted significant attention for biomedical implant applications due to excellent biocompatibility and favorable mechanical characteristics of both elements. Magnesium and calcium possess high strength/to/weight ratios and calcium is an essential mineral naturally present in human bone while magnesium plays an important role in facilitating calcium transport and bone mineralization. In the Mg/Ca binary system, calcium exhibits a limited solid solubility in magnesium [16], with a maximum value of approximately 1.34 wt.% at 789.5 K, leading to the formation of secondary phases when this limit is exceeded. Consequently, extensive research [9,10,17] has focused on the development and optimization of Mg-Ca alloys with systematic investigations into their microstructure, mechanical behavior and corrosion behavior under both in vitro and in vivo conditions. Studies [16] have shown that cast Mg-xCa alloys (x = 1-3 wt.%) typically consist of an α-Mg matrix and an intermetallic Mg₂Ca phase. As the calcium content increases, the volume fraction of Mg₂Ca phase also increases and has been associated with accelerated corrosion due to micro/galvanic coupling between phases in physiological environments [18]. Furthermore, excessive calcium addition has been reported [16,19,20] to deteriorate the mechanical properties of Mg-Ca alloys, resulting in reduced strength and ductility, thereby limiting their applicability for load-bearing orthopedic implants. Li et al. [21] studied binary Mg-Ca alloys, varying the concentration from 0.5% to 20% Ca. Increasing the calcium concentration implies a higher content of the secondary phase Mg₂Ca, distributed predominantly at the grain boundaries. This secondary phase is brittle and reduces the ductility of Mg-Ca alloys by increasing Ca concentration and also influences the corrosion properties of Mg alloys. High-volume fraction of the secondary phase Mg₂Ca reduces the corrosion resistance of Mg-Ca alloys due to the formation of galvanic micro-cells. Therefore, the beneficial effect of Ca is expected to depend strongly on its concentration and on the morphology and distribution of the resulting secondary phases.

Mg-Zn systems have been extensively investigated because zinc is an essential trace element required for numerous biological functions and is naturally present in the human body [22,23]. In addition to its nutritional role, Zn can mitigate the detrimental effects of impurities in magnesium alloys [24], thereby improving their overall corrosion behavior. The addition of Zn enhances the castability and mechanical strength of Mg alloys and contributes to surface protection through the formation of a relatively stable passive layer [25,26]. From a materials science perspective, Zn offers several advantages when alloying with Mg. It significantly improves corrosion resistance by stabilizing the surface oxide/hydroxide film and suppressing localized corrosion phenomena under physiological conditions [17,27]. Furthermore, Zn strengthens Mg alloys via solid/solution and precipitation hardening mechanism, while also promoting grain refinement, which leads to improved mechanical performance [28]. Owing to this favorable combination of biocompatibility, enhanced corrosion resistance and mechanical integrity, Mg-Zn alloys are regarded as promising biodegradable materials for orthopedic implant applications, effectively integrating the inherent benefits of magnesium with the strengthening and protective effects of zinc. Song et al. [29] also studied the effect of Zn on the corrosion behavior of Mg alloys. It has been reported that micro-galvanic effects dominate the corrosion behavior of Mg-Zn alloys, thus restricting the Zn content to levels below 5%. Above 5%Zn, a high-volume fraction of secondary Mg-Zn phases is formed, which act as cathodes, thus accelerating the corrosion of the α-Mg matrix around the Mg-Zn phases in Mg alloys.

To further enhance the performance of Mg-Zn alloys, calcium is often introduced into the Mg-Ca system as stabilizing element to effectively regulate and slow down the corrosion rate. Previous investigations have consistently demonstrated that Mg-Ca-Zn ternary alloys systems exhibit considerable potential for biomedical applications, provided that their biodegradation behavior can be precisely controlled [30,31]. In recent years, extensive research on Mg-Ca-Zn alloys has primarily focused on the influence of Zn content on microstructure, mechanical performance and corrosion resistance, typically while maintaining the Ca concentration at relatively low levels (approximately 0.5/1 wt.%). These studies indicate that Zn addition can refine the grain structure, improve the mechanical strength and enhance corrosion resistance by modifying secondary phase formation and surface film stability [32].

Although Mg-Zn and Mg-Ca alloys have been extensively investigated, the relationship between Ca modification, secondary-phase distribution, local mechanical heterogeneity and electrochemical degradation in selected Mg-Zn compositions remains insufficiently clarified. In particular, a better understanding of how moderate Ca ontents modify the local mechanical response of the Mg matrix and the subsequent electrochemical behavior may contribute to the development of Mg alloys with more controlled degradation. Furthermore, the effect of temperature is particularly relevant because the degradation linetics of Mg-based biomaterials are strongly dependent on the physicochemical conditions of the surrounding aqueous environment.

Accordingly, the objective of the present study was to characterize the microstructural, local mechanical and electrochemical response of four alloys: Mg1Zn, Mg2Zn, Mg1Ca1Zn and Mg1Ca2Zn (wt.%). The alloys were fabricated by levitation melting in protective argon atmosphere and were comprehensively evaluated by optical microscopy, SEM/EDS and Vickers microhardness. Gaussian mixture modelling was additionally applied to the microhardness distributions to evaluate local mechanical heterogeneity. Electrochemical behaviour was investigated in Ringer lactate solution at 25^oC and 40^oC using open circuit potential, potentiodynamic polarization and electrochemical impedance spectroscopy. The aim was to establish relationships between composition, microstructural heterogeneity and electrochemical degradation and to identify, among the investigated compositions, the alloy providing the most favourable combinations of the measured properties.

2. Materials and Methods

2.1. Obtaining the Mg-Ca-Zn Alloys

The design principle of the new alloys was to reduce the dissolution rate of Mg in contact with fluids in the implantation environment. It has been shown that upon reaction with water, one gram of magnesium can produce about 1.081 litters of hydrogen, which causes accelerated corrosion and early loss of mechanical integrity of implant, associated with adverse reactions through the formation of subcutaneous gas bubbles. In addition, the resulting magnesium hydroxide dissolves in water, producing a colourless alkaline solution [31]. To control the dissolution rate of magnesium-zinc alloys, 1wt.% Ca have been added.

The Mg-Ca-Zn alloys were made in a levitation furnace from Politehnica University of Bucharest, ERAMET laboratory (Aages, Tg. Mures, Romania), using magnesium granular materials of 2 to 5 mm in size (from MaTeek GmbH, Germany, with a purity of 99.5%), calcium and zinc grains having 1 to 2 mm in size, also with a purity of 99.5%. After weighting the alloy components, the granular materials were compressed using a 20 tf hydraulic press, resulting in buttons with a height of 18 mm and a width of 27 mm, which weighed approximately 15 g each.

During melting process, the following working parameters were used: the current intensity (I_mf), 83 to 85 A, the melting power (P_mf), 30.2 to 33 kW, the voltage in the medium frequency range (U_mf) 565 to 570 V, the frequency of 67 kHz and the melting time 2.6 to 3 minutes. The melted alloys were then solidified in a copper mould, resulting small ingots in the form of bars of about 65 mm long and 12 mm wide. Each sample were weighed to assess the oxidation losses, that results to be less than 2%.

Afterwards, the samples were processed by turning to remove the layers of material that has oxidized and obtain a diameter of 10 mm ± 0.1. Small discs with a thickness of 1.2 mm (± 0.2 mm) were cut using an IsoMet 4000 linear precision saw (Buehler, Düsseldorf, Germany) and the surfaces were ground and polished to facilitate the study of their chemical micro-composition using the EDAX method. No etching reagent was used to prevent changes in the chemical composition.

The samples that were tested (Mg1Zn, Mg2Zn, Mg1Ca1Zn, Mg1Ca2Zn) were put in a mix of epoxy resin and catalyst and taken out of the mould after 24 hours, when the mixture had hardened. The embedded samples were then polished in two steps using the Struers TegraPol-11 polishing system (Ballerup, Denmark). First, roughing was done with a variety of progressive-grit abrasive papers, from P280 to P2500. Then, a 0.3-micron α-alumina suspension was used to do the final polishing, which gave the surface a mirror-like finish and consistent results (see Figure 1) [26,33].

Prior to the test trials, the samples are drilled to fit a screw that is welded to a cable. This cable serves as the working electrode for the electrochemical testing. The exposed area, mass and density of each sample were determined prior to testing.

2.2. Microstructural Characterization

Microstructural characterization was performed using optical microscopy and scanning electron microscopy. ZEISS AxioVert.A1 MAT (Jena, Germany) optical microscope has been used to take pictures of the samples’ surfaces at different magnifications and analyze their microstructure. Metallographic samples were etched using a solution 3% Nital with the composition of 3% nitric acid and 97% ethanol, for a period of 8 to 12 s.

SEM characterization was performed using a QUANTA INSPECT F 50 field emission scanning electron microscope (FEI, Holland). EDS measurements were performed at three representative locations on each alloy surface to determine the local distribution of Mg, Ca and Zn. Because EDS provides compositional rather than crystallographic information, Ca and Zn rich regions are referred to as secondary phases or Ca/Zn rich phases unless their crystallographic identity is independently established. was used to explore the samples’ microstructure.

2.3. Microhardness Test and Statistical Analysis

Future Tech FM-810 microhardness tester (Kawasaki, Japan) was used to measure microhardness, following the ASTM E384-22 standard [34]. Ten indentations were made on the polished surface of each sample and load at different points, with each measurement lasting 15 seconds. The loads values were 1, 5 and 10 gf (HV0.001, HV0.005 and HV0.01). Using iVicky software, the diagonal lengths of the indentation were measured and the Vickers microhardness values were calculated.

On the other hand, a one-dimensional Gaussian mixture model (GMM) was used to evaluate experimental data and estimate the mechanical characteristics based on the microhardness values, for each combination of alloy and applied forces. The GMM was used to identify statistically distinct populations within the hardness distribution and to estimate their relative weights, mean hardness values and standard deviations. The optimum number of Gaussian components was selected using the Bayesian Information Criterion (BIC). The GMM populations were interpreted as statistically distinguishable local mechanical responses associated with different microstructural environments. Because an individual indentation may sample more than one microstructural constituent, the GMM populations were not considered direct phase identification measurements.

2.4. Electrochemical Tests

To assess corrosion behavior, the four study samples (working electrodes), with the saturated calomel electrode (SCE) and the platinum (Pt) electrode, were submerged in an electrochemical cell containing Grifols Ringer Lactate electrolyte, which simulates body fluid (SBF). The measurements were performed at 25° and at 40 °C and repeated 3 times for each sample. Upon immersion of each sample, the BioLogic Essential SP-150 potentiostat-galvanostat (Seyssinet-Pariset, France) was employed, using the following techniques: E_corr vs. Time (OCP) to ascertain the open circuit potential over time, Generalized Corrosion (GC) to measure the corrosion rate values and Potentio Electrochemical Impedance Spectroscopy (PEIS) to evaluate the impedance values of the samples.

2.4.1. Open Circuit Potential (OCP)

Using the EC-Lab software’s “E_corr vs. Time” method, the open circuit potential (OCP) of each sample was measured for thirty minutes. Data was automatically collected every 20 seconds or whenever the fluctuations went above 100 mV. The OCP evolution was used to assess the stabilization behavior of the alloy-electrolyte interface prior to subsequent electrochemical characterization.

2.4.2. Potentiodynamic Polarization

Potentiodynamic polarization measurements were performed over a potential range from -2.2 to -1.0 V vs. SCE, at a scan rate of 10 mV/min, with data acquisition every 0.50 seconds. The resulting polarization curves were analyzed to obtain the corrosion potential (Ecorr), corrosion current density (icorr), cathodic and anodic Tafel slopes and corrosion rate (Vcorr). The exposed electrode area, alloy density and equivalent weight were incorporated into the corrosion rate calculation.

2.4.3. Electrochemical Impedance Spectroscopy (EIS)

In the electrochemical impedance spectroscopy experiment, the potentiostatic mode is used to measure impedances. A sine wave is applied around a DC potential E, which can be set to a constant value or changed based on the cell’s equilibrium potential. The measurements of impedance and phase angle were done according to ISO 16773-1-4:2016 [35]. We applied a potential of 0.00 V to the open circuit potential (OCP) at frequencies between 200 KHz and 100 mHz. Nyquist and Bode diagrams were obtained and an electrical equivalent circuit (EEC) was used to fit the experimental data. The circuit incorporated solution resistance, constant phase elements, resistive contributions associated with the interfacial response and an inductive element to account for the low frequency response observed for selected alloys. The use of constant-phase elements rather than ideal capacitors accounts phenomenologically for the non-ideal capacitive behavior associated with surface heterogeneity and corrosion product layers.

3. Results and Discussions

3.1. Microstructural Investigation

The optical microstructures of the studied alloys are presented in Figure 2. All compositions exhibit an α-Mg matrix with secondary phases whose morphology and distribution depend on alloy composition. For Mg1Ca1Zn relatively coarse, equiaxed α-Mg grains can be observed, whereas increasing the Zn content to 2 wt.% in Mg1Ca2Zn produces a greater number of dark features concentrated near grain boundaries and within interdendritic regions. Due to the low solubility of calcium in Mg (max. 0.2% by weight at room temperature in equilibrium) for high concentrations, small, fine and dispersed particles of Mg₂Ca are formed [36]. Low concentrations of Zn are largely dissolved in the α-Mg matrix and the formation of ternary intermetallic compounds of the MgCaZn type is limited. Increasing the Zn concentration produces a more pronounced segregation of Ca and Zn constituents, promoting the formation of ternary intermetallic phases (Ca₂Mg₆Zn₃) [37] that preferentially precipitate along grain boundaries and clearly define them (see Figure 2b).

The microstructure of the Mg1CaxZn samples shown in Figure 2 change clearly as the Zn content increases. In the Mg1Ca1Zn alloy, a homogeneous α-Mg matrix is observed, with relatively clean grain boundaries and a low number of dark particles. This indicates that the alloying elements are more uniformly distributed. When the Zn content increases to 2 wt.% (Mg1Ca2Zn), more dark areas begin to appear at the grain boundaries and more fine particles within the matrix, suggesting that some of the Ca and Zn are concentrated in these regions during solidification.

Comparison of the Ca-free microstructures (Figure 2 a and b) and of Ca-containing alloys (Figure 2 c and d) indicates that Ca modifies the morphology and spatial distribution of the secondary phases. It resuls in a more clearly delineated and apparently coarser grain structure compared with the binary SEM images and spectra of Figure 3 confirm and detail the characteristics previously observed in optical metallography.

The SEM observations in Figure 3 provide higher resolution evidence of the compositional heterogeneity. Bright regions are enriched in Ca and Zn relative to the surrounding Mg matrix and their fraction and spatial continuity increase with alloy composition.

In the sample with the lowest Zn content, a relatively continuous Mg matrix can be distinguished, in which small bright particles appear scattered, associated with phases rich in Ca and Zn. As the Zn content increases, these phases become increasingly concentrated in the interdendritic zones and grain boundaries, forming a more defined network. In alloys with higher Zn content, this network becomes more continuous and corresponds to an eutectic phase rich in Ca and Zn, which is formed during solidification by their segregation and accumulation from the liquid alloy. In addition, a greater number of fine precipitates are observed within the Mg matrix, indicating that part of the Ca and Zn also precipitate from the solid solution during cooling. Overall, SEM analysis allows for clearer visualization of the distribution of these phases and confirms the evolution of the microstructure with Zn-containing.

To evaluate the chemical composition of the samples, the procedure described in Figure 4 was followed, where z1, z2 and z3 are zones on the same surface.

The EDS quantification summarizes the mean chemical compositions of the alloys under study (see Table 1). The presence and distribution of these phases in the SEM images correspond to the results of the EDS analysis, which highlights bright areas rich in Ca and Zn, confirming the presence of intermetallic compounds.

3.2. Microhardness Test

The analysis of microhardness results obtained for the studied alloys allows the evaluation of the heterogeneity of the microstructure and its mechanical behavior under various loading conditions, essential aspects for its use in the manufacture of biodegradable orthopedic implants [38]. Figure 5 shows the histograms and curves of microhardness density for each load (1, 5 and 10 gf). Also, the GMM fitting is considered for each load.

The Gaussian Mixture Model breaks the experimental distribution down into separate mechanical populations and a very important stage in the procedure is figuring out the average weight of each group. After we get the mean and standard deviation for each part, we find the best number of components using the Bayesian Information Criterion (BIC).

The model almost always converges toward K = 2 main components. The only time this doesn’t happen is when MgCa1Zn1 has a 1 gf value, in which case a third component is found.

Using the GMM made it easier to identify different microstructural constituents based on how they responded to the identification. In Table 2, two main zones were identified in the formation of the groups. Component 1, which has the highest statistical weight, is associated with the α-Mg matrix, while the Component 2 corresponds to long-lasting intermetallic phases such as Mg2Ca or ternary compounds of Mg-Zn-Ca, which precipitate preferentially in the grains boundaries [16].

An increase in Zn from 1% to 2% would act as a reinforcement agent through solidification and dispersion. This is demonstrated by the increase in the average hardness of the secondary phases (from 0.759 GPa to 0.797 GPa under 5 gf), improving the initial mechanical integrity required for the structural support (see Table 2).

The model identified a third zone of intermediate mechanical response (0.619 GPa) in the Mg1Ca1Zn mixture under ultra-low load (1 gf). It is highly likely that this value is an interface or boundary effect rather than a distinct physical phase (see Figure 6). The volume of deformation is small for such small loads. If the indentation occurs between a blank matrix and a harder precipitate the recorded value is a promise weighted by the volume of both phases [16].

According to ASTM E384, results for very low loads (25 gf) are typically regarded as qualitative because of the difficulty of optical measurement and the extreme sensitivity to local variations [5,7].

The microhardness study indicates that the inclusion of Ca in Mg-Zn alloys results in a notable variation in the distribution of local mechanical properties. The Mg1Ca1Zn alloy exhibits multimodal behavior (K = 3), with three distinct hardness zones detected. This means that the alloy is microstructurally quite heterogeneous due to the presence of several coexisting phases. With higher Zn content (Mg1Ca2Zn), however, the microhardness distributions shift toward one-dimensional behavior (K = 1), although the variation in concentrations is smaller.

These results suggest that Ca initially causes a higher degree of microstructural complexity; however, as Zn concentrations increase, this leads to greater uniformity in phase distribution and stabilizes the material’s mechanical response.

Figure 6 depicts the situations in which an indenter encounters a multifaceted microstructure.

Mg1Ca2Zn exhibits a single dominant population at all three loads and this indicates that, at the spatial scale sampled by the indentation, the hardness distribution is less clearly multimodal than that of Mg1Ca1Zn. This observation should not be interpreted as proof of a more homogeneous phase distribution of hardness rather than directly resolving the underlying microstructure.

The GMM analysis demonstrates that the addition of Ca modifies the distribution of local mechanical responses in the investigated Mg-Zn alloys. The results are therefore complementary to the SEM and EDS observations and provide evidence that alloy composition influences not only the mean hardness but also the spatial heterogeneity of the mechanical response.

3.3. Electrochemical Tests

3.3.1. Open Circuit Potential

Figure 7 presents the variation of open circuit potential (OCP) as a function of immersion time for MgxZn and Mg1CaxZn (x = 1, 2) alloys measured at room temperature and 40 ºC, respectively.

It can be observed that at 40 ºC all alloys exhibit more negative potential OCP values compared to room temperature, reflecting an accelerated kinetic of reactions at elevated temperature. Over time, the OCP values of all alloys gradually shift toward more positive potential, suggesting an establishment of a dynamic equilibrium between film formation and its dissolution. The more negative OCP values observed at 40 ºC indicate a change dependent of temperature in the interfacial electrochemical state. This behavior is consistent with previous studies showing that temperature accelerates the corrosion processes in magnesium alloys [26].

3.3.2. Polarization

Figure 8 shows the potentiodynamic curves for MgxZn and MgxZn1Ca (x = 1, 2) alloys measured at 25 ºC and 40 ºC, while Table 3 provides the electrochemical parameters derived from the Tafel fit.

A rise in temperature from 25 °C to 40 °C has a significant effect on the electrochemical response of all alloys. In general, a shift of corrosion potential (E_corr) towards more negative values is observed, along with a significant increase in the corrosion current density (i_corr). Thus, the rise in temperature accelerates both anodic and cathodic reactions which explains a significant increase in corrosion rate (V_corr).

The increase in the rate of corrosion observed at 40 °C compared to 25 °C can be elucidated by the interplay of enhanced electrochemical kinetics and intensified mass transport within the electrolyte. In this work, this second effect is discussed using the Stokes–Einstein equation, which shows that as the viscosity of the medium drops from 0.89 mPa·s (25 °C) to 0.65 mPa·s (40 °C), the term proportional to the diffusion T/η rises from 335 to 481.7.

The diffusion through Ringer solution follows the Stokes–Einstein relation:

$$D = {\displaystyle \frac{k·T}{(C·\eta ·r)}} ={\displaystyle \frac{T}{\eta }}$$

(1)

where D is the diffusion coefficient, k is the Boltzmann constant, T is the temperature, ºC is a constant depending on the interface, r is the radius of the macroscopic particle, and η is the viscosity of the fluid [38].

The approximate viscosity values of Ringer solution used in this analysis are η₁ = 0.89 mPa·s at 298.15 K (25 °C) and η₂ = 0.65 mPa·s at 313.15 K (40 °C). Relative decrease of viscosity from 25 °C to 40 °C is 27% and the relative increase of the diffusion coefficient is 43.8%.

This link between diffusion, temperature and viscosity fits with the widely accepted theoretical framework for diffusive systems (Einstein, 1905). So, the rise in D makes it easier for aggressive species (mostly Cl⁻) to get to the metal-electrolyte interface and speeds up the removal of reaction products, which makes the boundary layer less of a problem. This increase in mass transport is particularly significant in Mg alloys within chloride-rich environments, as chloride destabilizes the Mg(OH)₂-based film, resulting in dissolution-reformation cycles that lack protective qualities, as demonstrated in classical studies on the dissolution of Mg in the presence of halides [39].

In line with this framework, the polarization results of this article demonstrate a systematic increase in i_corr and the V_corr when the temperature is raised from 25 °C to 40 °C for all the studied alloys. This trend aligns with experimental findings regarding biodegradable Mg alloys in physiological solutions, where elevated temperatures significantly accelerate their degradation [40].

EIS fitting also shows that the resistive contributions at the interface decrease with temperature and this means that the film is less protective and the interface is more electrochemically active at high temperatures.

Finally, it should be noted that the response to temperature is modulated by microstructure, as the presence and distribution of secondary phases can induce microgalvanic couplings and promote localised attack in Mg alloys, as previously described for Mg systems, thereby reinforcing the link between microstructural heterogeneity and accelerated degradation [41].

The observed increase in corrosion rate with the temperature is due to the combined effect of enhanced electrochemical kinetics, altered surface film stability and temperature dependent mas transport.

3.3.3. Electrochemical Impedance Spectroscopy

The electrochemical impedance spectroscopy (EIS) is a non-destructive and highly sensitive technique that allows the real-time monitoring of corrosion processes without altering the metal surface. It separates and quantifies the electrochemical processes, permitting a detailed analysis of corrosion mechanisms.

Figure 9 presents the Nyquist plots of the MgxZn and MgxZn1Ca alloys measured in Ringer solution at room temperature. In the high-frequency region, the intercept close to the origin reflects the solution resistance (R_s) and it can be observed that all alloys show similar R_s, meaning that the electrolyte resistance is comparable for all tests. The diameter of the semicircle is mainly associated with the charge-transfer resistance (R_ct) and the larger the diameter of the semicircle, the higher R_ct, which means that resistance to corrosion is also higher. Mg1Zn shows a higher semicircle (extending to 2500 - 3000 Ω·cm²), closely followed by Mg1Ca1Zn, while the other three alloys show smaller semicircles, ending near 500 - 1000 Ω·cm². Therefore, Mg1Zn has the highest corrosion resistance, while decreasing Ca/Zn content (1/2, 1/3) reduces R_ct and thus corrosion resistance. It can be observed that the arcs are not perfect semicircles which indicate the surface heterogeneity and that the alloys have a non-ideal capacitive behavior and this is why a constant phase element will be used in the equivalent electrical circuit instead of an ideal capacitor. In the low-frequency region, a tail can be observed for Mg1Zn and Mg1Ca1Zn suggesting a possible diffusion process.

For Mg2Zn and Mg1Ca2Zn alloys, different behaviors can be observed at low frequencies region. In this region, both alloys exhibit a deviation from ideal capacitive behavior, manifested by a downturn of the impedance toward negative imaginary values. This behavior is commonly associated with inductive or relaxation phenomena, which are attributed to the adsorption/desorption of the intermediate species as Mg²⁺, Ca²⁺ or Zn²⁺ or to the partial breakdown and repair of surface corrosion film.

The inductive loop appearing at low frequencies in the alloys with higher concentrations of Zn, with and without Ca additions suggests the adsorption of intermediate Mg⁺ species and this indicates that Ca additions increase electrochemical activity when excessive secondary phases are formed.

The Bode plots (see Figure 10 and Figure 11) confirm that Mg1Zn exhibits superior corrosion resistance, as evidenced by its higher low-frequency impedance modulus and larger phase angle. Increasing Zn contents decrease impedance and phase angle, indicating reduced protection of the surface film and accelerated electrochemical activity.

Figure 12 shows the equivalent electrical circuit used to fit the electrochemical impedance spectroscopy (EIS) data, represented as R(CPE(R(CPERL))).

This model describes the electrochemical response of magnesium alloys in Ringer’s solution, taking into account the contribution of the solution resistance (R_s), a porous surface layer (represented by R_P and CPE_P) and a more compact internal layer (modelling using R_C, CPE_C and an L_C inductive element). The presence of constant-phase elements (CPE) rather than ideal capacitances indicates non-ideal behavior associated with surface heterogeneity and the roughness of the corrosion layers and the total impedance can be calculated [42]:

$$Z_R=R Z_{CPE}={\displaystyle \frac{1}{Y^0{\left(jw\right)}^n}} Z_L=jwL$$

(2)

$$Z_{CPERL}={\left(Y_C^0{\left(jw\right)}^{n_C}+{\displaystyle \frac{1}{R_C+jwL}} \right)}^{-1}$$

(3)

$$Z_{R\left(CPERL\right)}={\mathrm{R}}_{\mathrm{p}}+{\left(Y_C^0{\left(jw\right)}^{n_C}+{\displaystyle \frac{1}{R_C+jwL}} \right)}^{-1}$$

(4)

$$Z_{CPE\left(R\left(QRL\right)\right)}={\left(Y_P^0{\left(jw\right)}^{n_P}+{\displaystyle \frac{1}{{\mathrm{R}}_{\mathrm{P}}+{\left(Y_C^0{\left(jw\right)}^{n_C}+\frac{1}{R_C+jwL} \right)}^{-1}}} \right)}^{-1}$$

(5)

$$Z\left(f\right)={\mathrm{R}}_{\mathrm{s}}+{\left(Y_P^0{\left(jw\right)}^{n_P}+{\displaystyle \frac{1}{{\mathrm{R}}_{\mathrm{P}}+{\left(Y_C^0{\left(jw\right)}^{n_C}+\frac{1}{R_C+jwL} \right)}^{-1}}} \right)}^{-1}$$

(6)

As shown in Table 4, the results of the impedance fitted data suggest that total corrosion resistance (R_T = R_P + R_C) depend on temperature and alloy composition. At 25 °C, the Mg1Zn and Mg1Ca1Zn alloys exhibit the highest R_T values, indicating that they offer the greatest corrosion resistance. This is consistent with the lower corrosion current densities (i_corr) obtained from potentiodynamic tests. However, alloys with a higher Zn concentration, such as Mg1Ca2Zn has considerably lower R_T values, suggesting that the surface layer offer poorer protection.

4. Conclusions

• Alloys from MgCaZn system can be successfully obtained using the induction melting furnace with levitation effect, if an inert argon protective environment is provided to reduce the oxidation effects. The purity of the alloys depends on the quality of the metallic materials used and the working conditions (inert gas protection during melting, adequate solidification molds, constant heating parameters, and exhaust of oxidation products during melting).
• The addition of calcium significantly modify the morphology and spatial distribution of secondary phases in the Mg-Zn alloys. Increasing Zn content from 1 to 2 wt.% in the compositions with Ca resulted in a more pronounced concentration of regions which are rich in Ca and Zn localized near grain boundaries and interdendritic areas.
• Microhardness measurements demonstrated that calcium addition altered the local mechanical response of the alloys. The Mg1Ca1Zn alloy exhibited greater microstructural heterogeneity with multimodal hardness distribution and the Gaussian Mixture Model analysis confirmed the coexistence of multiple mechanical populations regarded as microstructure-correlated mechanical responses.
• A moderate calcium addition improved the corrosion resistance of Mg-Zn alloys. Among the studied compositions, Mg1Ca1Zn exhibited the most balanced behavior, combining refined microstructure, relatively homogeneous mechanical response and enhanced corrosion resistance due to a higher interface resistance. In contrast, excessive Ca and Zn contents promoted the formation of secondary phases and microgalvanic coupling effects, which accelerated localized corrosion.
• EIS and potentiodynamic polarization provided complementary information on the electrochemical response. Although Mg1Zn exhibited the highest fitted total impedance at 25^oC, Mg1Ca1Zn showed the lowest corrosion rate. This difference highlights the dynamic and heterogeneous nature of the interface between alloy and electrolyte and the importance of using complementary electrochemical techniques.
• The results demonstrate that controlled calcium addition represents an effective approach for tailoring the microstructure, mechanical properties and corrosion performance of biodegradable Mg-Zn alloys. Therefore, Mg-Ca-Zn systems, particularly Mg1Ca1Zn, show promising potential for future biodegradable orthopedic implant applications.