*3.6. Electrochemical Evaluation*

PBS.

**Table 6.** EDS chemical analysis for BG-PLA composites after immersion in PBS. **Element BG70-PLA30 BG30-PLA70 14 days 28 days 14 days 28 days**  This section shows the in vitro results for the biomaterials in Hank's and SBF solutions; electrochemical techniques were used to identify the mass transport mechanism through the developed biomaterial applied as a coating on the 316L SS substrate.

**S.1 S.2** 

### **at.%**  3.6.1. Potentiodynamic Tests

C 18.96 35.73 34.87 16.31 O 62.28 72.89 51.23 44.62 57.6 Na 1.07 0.85 1.44 0.38 Si 7.28 22.54 4.82 0.89 1.42 P 0.7 1.37 3.08 6.85 3.7 Cl 0.82 0.39 Ca 8.88 3.2 3.9 11.33 20.6 Figure 13 shows the corrosion behaviour of the BG-PLA biomaterial samples (BG, BG70-PLA30, and BG30-PLA70) in both Hank's (Figure 13b) and SBF (Figure 13b) saline solutions at 37 ◦C. This condition of saline solutions is representative of the behaviour of the biomaterials in corporeal applications. All samples showed an activation mechanism in the early stages, followed by a pseudo-passivation or current limited behaviour associated with the inhibited corrosion due to the physical barrier formed by the coatings. The behaviour presented after activation was associated with the physical bioglass characteristics; as a semiconductor material, charge transfer is limited by this property. After that, a wide over-potential range inhibiting corrosion (as passivation behaviour) up to breakdown over-potential was observed in the coated samples. Table 7 shows the potentiodynamic parameter obtained from polarization plots (Figure 13). In general, *icorr* showed low current density values between 0.1 to 0.3 µA/cm<sup>2</sup> , which is lower than the *icorr* presented by the 316L SS (around 0.733 µA/cm<sup>2</sup> ) under similar conditions (Figure 13a, curve 4). Furthermore, the *ipass* values were observed in the same order of magnitude. The coated samples showed a corrosion potential *Ecorr* more positive than the 316L SS as a correlation of minor electrochemical activity, as indicated in Table 7. The electrochemical behaviour of the bioglass materials could be utilized in biomedical applications as biomaterial supports.



**Figure 13.** Potentiodynamic behaviour of both composite scaffolds in (**a**) Hank's saline solution and (**b**) SBF. **Figure 13.** Potentiodynamic behaviour of both composite scaffolds in (**a**) Hank's saline solution and (**b**) SBF. BG30-PLA70 0.190 −168.0 117 122 289 1.040

BG70-PLA30 0.228 −210.0 135 76 428 0.149

### *3.6. Electrochemical Evaluation*  3.6.2. LPR Measurements 3.6.2. LPR Measurements

This section shows the in vitro results for the biomaterials in Hank's and SBF solutions; electrochemical techniques were used to identify the mass transport mechanism through the developed biomaterial applied as a coating on the 316L SS substrate. 3.6.1. Potentiodynamic Tests Figure 13 shows the corrosion behaviour of the BG-PLA biomaterial samples (BG, BG70-PLA30, and BG30-PLA70) in both Hank's (Figure 13b) and SBF (Figure 13b) saline solutions at 37 °C. This condition of saline solutions is representative of the behaviour of the biomaterials in corporeal applications. All samples showed an activation mechanism in the early stages, followed by a pseudo-passivation or current limited behaviour associated with the inhibited corrosion due to the physical barrier formed by the coatings. The behaviour presented after activation was associated with the physical bioglass characteristics; as a semiconductor material, charge transfer is limited by this property. After that, To observe the behaviour of the coatings as a function of time, linear polarization resistance (LPR) measurements were made. The polarization resistance (*Rp*) and *Ecorr* kinetics obtained by the LPR measurements of the BG, BG70-PLA30, and BG30-PLA70 coatings in both saline solutions are shown in Figures 14 and 15 (the substrate was measured in Hank's solution only). In Hank's solution, the substrate alloy showed the highest *R<sup>p</sup>* values during the first 10 h of immersion (about the 2.5 M Ohm·cm<sup>2</sup> ), although it showed a decrease of around 1.2 M Ohm·cm<sup>2</sup> . This was associated with localized anodic dissolution and the breaking of the passive film present from the beginning of immersion due to the activity of chlorine in the solution. However, the BG coating displayed stability during complete immersion, with *<sup>R</sup><sup>p</sup>* values around 2 M Ohm·cm<sup>2</sup> (Figure 14, curve 1), which was associated with the homogeneous and continuous covering of the coating on the substrate. On the other hand, BG coating in the SBF solution showed the lowest *R<sup>p</sup>* kinetic values, between 0.5 to 1 M Ohm·cm<sup>2</sup> . To observe the behaviour of the coatings as a function of time, linear polarization resistance (LPR) measurements were made. The polarization resistance (*Rp*) and *Ecorr* kinetics obtained by the LPR measurements of the BG, BG70-PLA30, and BG30-PLA70 coatings in both saline solutions are shown in Figures 14 and 15 (the substrate was measured in Hank's solution only). In Hank's solution, the substrate alloy showed the highest *Rp* values during the first 10 h of immersion (about the 2.5 M Ohm·cm2), although it showed a decrease of around 1.2 M Ohm·cm2. This was associated with localized anodic dissolution and the breaking of the passive film present from the beginning of immersion due to the activity of chlorine in the solution. However, the BG coating displayed stability during complete immersion, with *Rp* values around 2 M Ohm·cm2 (Figure 14, curve 1), which was associated with the homogeneous and continuous covering of the coating on the substrate. On the other hand, BG coating in the SBF solution showed the lowest *Rp* kinetic values, between 0.5 to 1 M Ohm·cm2.

**µA/cm2 mV mV mV mV µA/cm2** BG 0.108 −211.7 123 67 545 0.593 **Figure 14.** *Rp* kinetics of the BG-PLA coatings in Hank's and SBF solutions at 37 °C. **Figure 14.** *Rp* kinetics of the BG-PLA coatings in Hank's and SBF solutions at 37 ◦C.

**Figure 15.** *Ecorr* kinetics of the BG-PLA coatings in Hank's and SBF solutions at 37 °C.

The addition of PLA to the BG phase caused variability in the Hank's and SBF solutions In particular, the hybrid coatings in the SBF solution presented increased corrosion

Hank BG70-PLA30 0.241 −257.5 111 62 394 0.817 BG30-PLA70 0.236 −181.1 92 79 448 1.540 316L SS 0.733 −301.8 75 67 396 6.310 SBF BG 0.145 −166.9 134 103 440 1.490 The addition of PLA to the BG phase caused variability in the Hank's and SBF solutions In particular, the hybrid coatings in the SBF solution presented increased corrosion resistance in the second half of immersion, as did the BG30-PLA70 in Hank's solution (Figure 14, curve 3). Although the BG70-PLA30 coating in Hank's solution did not show

BG70-PLA30 0.228 −210.0 135 76 428 0.149

*R<sup>p</sup>* kinetics as the others did, the coated samples generally showed improved corrosion resistance in a stable range between 1 to 2 M Ohm·cm<sup>2</sup> . The *R<sup>p</sup>* fluctuations were correlated with the porous characteristics of hybrid coatings that promote a finite diffusion corrosion mechanism, as described below in the EIS results. Likewise, the potential kinetics present the evolution of the activity of the coatings, as can be seen in Figure 15; as a result, the BG70- PLA30 coating in Hank's solution showed more negative potentials, as did the BG in SBF; thus, the *R<sup>p</sup>* values were the lowest. This behaviour was associated with the characteristics of the coating microstructures. However, the *Ecorr* kinetics of the other coating samples remained stable during the immersion time; the BG30-PLA70 hybrid coating kept the potentials in both solutions higher, in accordance with those shown by the metallic substrate. In addition, the BG70-PLA30 coating in the SBF solution developed exponential growth of the *Ecorr* kinetic to the more positive potentials; thus, the corrosion resistance increased. **Figure 14.** *Rp* kinetics of the BG-PLA coatings in Hank's and SBF solutions at 37 °C.

BG70-PLA30 0.228 −210.0 135 76 428 0.149 BG30-PLA70 0.190 −168.0 117 122 289 1.040

To observe the behaviour of the coatings as a function of time, linear polarization resistance (LPR) measurements were made. The polarization resistance (*Rp*) and *Ecorr* kinetics obtained by the LPR measurements of the BG, BG70-PLA30, and BG30-PLA70 coatings in both saline solutions are shown in Figures 14 and 15 (the substrate was measured in Hank's solution only). In Hank's solution, the substrate alloy showed the highest *Rp* values during the first 10 h of immersion (about the 2.5 M Ohm·cm2), although it showed a decrease of around 1.2 M Ohm·cm2. This was associated with localized anodic dissolution and the breaking of the passive film present from the beginning of immersion due to the activity of chlorine in the solution. However, the BG coating displayed stability during complete immersion, with *Rp* values around 2 M Ohm·cm2 (Figure 14, curve 1), which was associated with the homogeneous and continuous covering of the coating on the substrate. On the other hand, BG coating in the SBF solution showed the lowest *Rp* kinetic values,

*Polymers* **2022**, *14*, x FOR PEER REVIEW 16 of 24

3.6.2. LPR Measurements

between 0.5 to 1 M Ohm·cm2.

**Figure 15.** *Ecorr* kinetics of the BG-PLA coatings in Hank's and SBF solutions at 37 °C. **Figure 15.** *Ecorr* kinetics of the BG-PLA coatings in Hank's and SBF solutions at 37 ◦C.

The addition of PLA to the BG phase caused variability in the Hank's and SBF solutions In particular, the hybrid coatings in the SBF solution presented increased corrosion The kinetic current density (*icorr*) showed the opposite behaviour in terms of *R<sup>p</sup>* kinetics because of the indirect correlation of the current density with the resistance, as described by Ohm's law; these were calculated using the Stern and Geary function [15]. Figure 16 shows the corrosion current behaviour of the coatings in both solutions. According to the *R<sup>p</sup>* results, in Hank's solution the lowest *icorr* values were observed with the BG and the BG30-PLA70 coatings and the 316L SS substrate, as well as the BG30-PLA70 hybrid coating in the SBF solution, with *icorr* values around 0.1 µA/cm<sup>2</sup> in the second half of the immersion time. However, the BG70-PLA30 hybrid coating showed *icorr* kinetic instability in the SBF solution. According to the *R<sup>p</sup>* results, the highest current densities were displayed by the BG coating in the SBF solution, followed by the BG70-PLA30 in Hank's solution (Figure 16 curves 5 and 2, respectively).

The estimation of the corrosion rate (*CR*), as described in the ASTM G102 [15] using the *icorr* kinetics (Figure 16 right scale), is valid for the data obtained by the substrate, presenting a *CR* between 0.06 to 0.12 µm/year. These lower *CR* values are a consequence of the chromium oxide protective film formed previously on the self-protected 316L SS alloy. The application of the hybrid coatings did not lead to an increase in corrosion resistance. However, this was not the main purpose of coating the metallic substrate with the BG-PLA biomaterials; rather, it was to improve their functionality due to their high bioactivity, osteoconductivity, and biodegradability for potential applications and physiological functionality as implantable devices. Therefore, the kinetics of the hybrid coatings showed *CR* values as low as the substrate and in the same order of magnitude. In general, the kinetics showed stability of current density and *CR* at around 1 to 2 µm/year during the immersion time, with the exception of the BG coating in SBF solution and the BG70-PLA30 in Hank's solution.

solution (Figure 16 curves 5 and 2, respectively).

*Polymers* **2022**, *14*, x FOR PEER REVIEW 18 of 24

**Figure 16.** *icorr* kinetics for the BG-PLA coatings in Hank's and SBF solutions at 37 °C. **Figure 16.** *icorr* kinetics for the BG-PLA coatings in Hank's and SBF solutions at 37 ◦C. film (for uncoated substrate) and Cr2O3 film/BG-PLA mixed thickness (for the coated samples).

### The estimation of the corrosion rate (*CR*), as described in the ASTM G102 [15] using 3.6.3. EIS Analysis The physical barrier had a high effect on the current density, increasing the time for

increased.

in Hank's solution.

3.6.3. EIS Analysis

the *icorr* kinetics (Figure 16 right scale), is valid for the data obtained by the substrate, presenting a *CR* between 0.06 to 0.12 µm/year. These lower *CR* values are a consequence of the chromium oxide protective film formed previously on the self-protected 316L SS alloy. The application of the hybrid coatings did not lead to an increase in corrosion resistance. However, this was not the main purpose of coating the metallic substrate with the BG-PLA biomaterials; rather, it was to improve their functionality due to their high bioactivity, osteoconductivity, and biodegradability for potential applications and physiological functionality as implantable devices. Therefore, the kinetics of the hybrid coatings showed *CR* values as low as the substrate and in the same order of magnitude. In general, the Electrochemical impedance spectroscopy (EIS) was used to identify the probable corrosion mechanisms present at the solution/coating and coating/substrate interfaces and through the thickness scale. Additionally, the substrate alloy was evaluated to establish a baseline or reference curve. The 316L SS was evaluated in Hank's solution, considering that similar results could be obtained in SBF. Two EIS measurements were obtained for the substrate and the coated samples to identify the corrosion mechanisms both at the beginning and after approximately 24 h of immersion. Figure 17 shows the EIS results of the coatings and substrate at the beginning of immersion in the Hank's and SBF solutions, represented in Nyquist and Bode diagrams (Figure 17a,b, respectively). Likewise, the EIS measurements obtained after 24 h of immersion are presented in Figure 18. species diffusion through the scale thickness, retarding the activation mechanism at the metallic interface [29,30]. The phase angle Bode plots show a zone within a wide range of frequencies (from 500 0.5 Hz, approximately) with phase angle values above to 70°, which according to the *Rp* results described above correspond to the capacitive and resistive behaviour. This could be of interest in biomedical applications as a scaffold in tissue engineering, allowing the controlled transport of mass through the porous microstructure. Similar to the observed LPR results, the BG coating in the SBF solution and the BG70- PLA30 showed lower initial impedance values (Figure 17a). At 24 h of immersion, the total impedance had increased to values within the same order of magnitude as the other coatings, as shown in Figure 18b.

resistance in the second half of immersion, as did the BG30-PLA70 in Hank's solution (Figure 14, curve 3). Although the BG70-PLA30 coating in Hank's solution did not show *Rp* kinetics as the others did, the coated samples generally showed improved corrosion resistance in a stable range between 1 to 2 M Ohm·cm2. The *Rp* fluctuations were correlated with the porous characteristics of hybrid coatings that promote a finite diffusion corrosion mechanism, as described below in the EIS results. Likewise, the potential kinetics present the evolution of the activity of the coatings, as can be seen in Figure 15; as a result, the BG70-PLA30 coating in Hank's solution showed more negative potentials, as did the BG in SBF; thus, the *Rp* values were the lowest. This behaviour was associated with the characteristics of the coating microstructures. However, the *Ecorr* kinetics of the other coating samples remained stable during the immersion time; the BG30-PLA70 hybrid coating kept the potentials in both solutions higher, in accordance with those shown by the metallic substrate. In addition, the BG70-PLA30 coating in the SBF solution developed exponential growth of the *Ecorr* kinetic to the more positive potentials; thus, the corrosion resistance

The kinetic current density (*icorr*) showed the opposite behaviour in terms of *Rp* kinetics because of the indirect correlation of the current density with the resistance, as described by Ohm's law; these were calculated using the Stern and Geary function [15]. Figure 16 shows the corrosion current behaviour of the coatings in both solutions. According to the *Rp* results, in Hank's solution the lowest *icorr* values were observed with the BG and the BG30-PLA70 coatings and the 316L SS substrate, as well as the BG30-PLA70 hybrid coating in the SBF solution, with *icorr* values around 0.1 µA/cm2 in the second half of the immersion time. However, the BG70-PLA30 hybrid coating showed *icorr* kinetic instability in the SBF solution. According to the *Rp* results, the highest current densities were displayed by the BG coating in the SBF solution, followed by the BG70-PLA30 in Hank's

immersion time, with the exception of the BG coating in SBF solution and the BG70-PLA30

Electrochemical impedance spectroscopy (EIS) was used to identify the probable corrosion mechanisms present at the solution/coating and coating/substrate interfaces and

**Figure 17.** EIS results for BG-PLA coatings obtained at the beginning of immersion in Hank's and SBF solutions: (**a**) Nyquist and (**b**) Bode plots. Scatter and lines indicate the experimental data and the fitting results, respectively.

The Nyquist plots obtained at both immersion times (beginning and 24 h.) show the representative form of high resistance corrosion mechanisms for both coated and uncoated samples. The impedance module (|*Z*|) for all materials showed high values above 100 k Ohm·cm<sup>2</sup> , as shown in the Bode plots in Figures 17b and 18b. The substrate as the Nyquist curves of the hybrid coatings presented characteristic capacitive and resistive elements

mixed with finite diffusion through a physical barrier composed by the Cr2O<sup>3</sup> film (for uncoated substrate) and Cr2O<sup>3</sup> film/BG-PLA mixed thickness (for the coated samples). **Figure 17.** EIS results for BG-PLA coatings obtained at the beginning of immersion in Hank's and SBF solutions: (**a**) Nyquist and (**b**) Bode plots. Scatter and lines indicate the experimental data and the fitting results, respectively.

**Figure 18.** EIS results for BG-PLA coatings obtained after 24 h of immersion in Hank's and SBF solutions: (**a**) Nyquist and (**b**) Bode plots. Scatter and lines indicate the experimental data and the fitting results, respectively. **Figure 18.** EIS results for BG-PLA coatings obtained after 24 h of immersion in Hank's and SBF solutions: (**a**) Nyquist and (**b**) Bode plots. Scatter and lines indicate the experimental data and the fitting results, respectively.

The proposed corrosion mechanisms were associated with the effect of the microstructural morphology on the coating behaviour composed of the mixed BG and PLA phases at different ratios, as an electrode with a microstructure with a superimposed porous layer [31] acts as a barrier against electron and ion diffusion, reducing the surface area for electrochemical reactions at the metallic interface [29]. Nevertheless, the microgalvanic cell formation at the coating/substrate could be increased. However, the corrosion behaviour of the substrate is associated with the activation mechanism and with diffusion through the Cr2O3 protective film. Figure 19 shows the electric circuit models (ECM) that were used as an analogy to better explain the governing corrosion mechanism at the active surfaces and the coating thickness. For the substrate, the analogue ECM correspond to Model 1 (Figure 19), which is composed of the electrolyte resistance (*Rs*), set The physical barrier had a high effect on the current density, increasing the time for species diffusion through the scale thickness, retarding the activation mechanism at the metallic interface [29,30]. The phase angle Bode plots show a zone within a wide range of frequencies (from 500 0.5 Hz, approximately) with phase angle values above to 70◦ , which according to the *R<sup>p</sup>* results described above correspond to the capacitive and resistive behaviour. This could be of interest in biomedical applications as a scaffold in tissue engineering, allowing the controlled transport of mass through the porous microstructure. Similar to the observed LPR results, the BG coating in the SBF solution and the BG70- PLA30 showed lower initial impedance values (Figure 17a). At 24 h of immersion, the total impedance had increased to values within the same order of magnitude as the other coatings, as shown in Figure 18b.

up in series with a parallel arrangement of a constant phase element (*CPE1*, as capacitive behaviour at the double layer) and the polarization resistance (*RL1*) of the inner layer (composed of Cr2O3), which represents the activation mechanism at the Cr2O3 film/electrolyte interface. The arrangement of *CPE2* in parallel with the *Rct* represents the diffusive element presented by the Cr2O3 protective layer inherent in stainless-steel alloys. When the hybrid coating is applied, the ECM incorporates an element of Warburg diffusion impedance (*ZD*)in a serial arrangement before the charge transference resistance (*Rct*) at the coating/substrate interface (Model 2 of Figure 19), describing the diffusion through the coating thickness with enough roughness and porosity and allowing the fluid permeation and/or the ion diffusion. Consequently, the electrochemical mechanism governed by limited mass transport was observed. The proposed corrosion mechanisms were associated with the effect of the microstructural morphology on the coating behaviour composed of the mixed BG and PLA phases at different ratios, as an electrode with a microstructure with a superimposed porous layer [31] acts as a barrier against electron and ion diffusion, reducing the surface area for electrochemical reactions at the metallic interface [29]. Nevertheless, the micro-galvanic cell formation at the coating/substrate could be increased. However, the corrosion behaviour of the substrate is associated with the activation mechanism and with diffusion through the Cr2O<sup>3</sup> protective film. Figure 19 shows the electric circuit models (ECM) that were used as an analogy to better explain the governing corrosion mechanism at the active surfaces and the coating thickness. For the substrate, the analogue ECM correspond to Model 1 (Figure 19), which is composed of the electrolyte resistance (*Rs*), set up in series with a parallel arrangement of a constant phase element (*CPE*1, as capacitive behaviour at the double layer) and the polarization resistance (*RL*1) of the inner layer (composed of Cr2O3), which represents the activation mechanism at the Cr2O<sup>3</sup> film/electrolyte interface. The arrangement of *CPE*<sup>2</sup> in parallel with the *Rct* represents the diffusive element presented by the Cr2O<sup>3</sup> protective layer inherent in stainless-steel alloys. When the hybrid coating is applied, the ECM incorporates an element of Warburg diffusion impedance (*ZD*)in a serial arrangement before the charge transference resistance (*Rct*) at the coating/substrate interface (Model 2 of Figure 19), describing the diffusion through the coating thickness with enough roughness and porosity and allowing the fluid permeation and/or the ion dif-

fusion. Consequently, the electrochemical mechanism governed by limited mass transport was observed. *Polymers* **2022**, *14*, x FOR PEER REVIEW 20 of 24

**Figure 19.** Equivalent circuit models of the corrosion mechanisms observed at the active interfaces: Model 1 for uncoated substrate, Model 2 for BG-PLA coatings. **Figure 19.** Equivalent circuit models of the corrosion mechanisms observed at the active interfaces: Model 1 for uncoated substrate, Model 2 for BG-PLA coatings.

In the ECMs used here, the *ZD* element (finite-length Warburg) represents the short Warburg (*Ws*) element. In general, the ECM described here represents the analogue equivalent electrical circuit of the impedance for a coated electrode by a hybrid porous layer [31]. The elements of ECM are defined using the following equations: In the ECMs used here, the *Z<sup>D</sup>* element (finite-length Warburg) represents the short Warburg (*Ws*) element. In general, the ECM described here represents the analogue equivalent electrical circuit of the impedance for a coated electrode by a hybrid porous layer [31]. The elements of ECM are defined using the following equations:

$$\mathcal{Z}(\mathcal{C}PE\_i) = \frac{1}{T\_{\mathcal{C}PE\_i}(j\omega)^{\mathcal{A}}} \tag{3}$$

$$Z(R\_i) = R\_{S\prime} \ R\_{L1\prime} \ R\_{ct} \tag{4}$$

$$Z\_{W\_5} = \sigma \frac{\tan \text{h} \left( jT\_D \omega \right)^P}{\left( jT\_D \omega \right)^P} \tag{5}$$

where *Rs*, *RL1*, and *Rct* are the electrolyte resistance, inner layer, and charge transference resistance, respectively, *TCPEi* is the *i* constant phase capacitance, and *α* is a dimensionless potential number (0 < α ≤ 1, while α = 1 assumes that *CPE* is a perfect capacitance *Cdl*). Angular frequency is ω = 2π*f* with *f* = linear frequency, complex number *j* = √(−1), and *Zf* is the Faradaic impedance at the metal/scale interface. Hence, the term *TD* represents the ratio of scale thickness *L* and the effective diffusion coefficient *Deff* of that scale, *TD* =*L*<sup>2</sup> *D*eff−<sup>1</sup> power is between 0 < *P* < 1, and *σ* is the constant of diffusion or the modulus of the Warburg resistance. Here, the *CPEi* elements were applied instead of the perfect capacitance for better fitting. Tables 8 and 9 show the fitting values obtained for each equivalent electric element of the ECM used in the fitting analysis of the experimental data. The lines in figures 17 and 18 correspond to the fitting results of the experimental data using the proposed ECM. where *R<sup>s</sup>* , *RL*1, and *Rct* are the electrolyte resistance, inner layer, and charge transference resistance, respectively, *TCPEi* is the *i* constant phase capacitance, and *α* is a dimensionless potential number (0 < α ≤ 1, while α = 1 assumes that *CPE* is a perfect capacitance *Cdl*). Angular frequency is ω = 2π*f* with *f* = linear frequency, complex number *j* = √ (−1), and *Z<sup>f</sup>* is the Faradaic impedance at the metal/scale interface. Hence, the term *T<sup>D</sup>* represents the ratio of scale thickness *L* and the effective diffusion coefficient *Deff* of that scale, *T<sup>D</sup>* = *L* <sup>2</sup> *Deff* −1 power is between 0 < *P* < 1, and *σ* is the constant of diffusion or the modulus of the Warburg resistance. Here, the *CPE<sup>i</sup>* elements were applied instead of the perfect capacitance for better fitting. Tables 8 and 9 show the fitting values obtained for each equivalent electric element of the ECM used in the fitting analysis of the experimental data. The lines in Figures 17 and 18 correspond to the fitting results of the experimental data using the proposed ECM.

**Table 8.** EIS Parameters obtained by substrate experimental data fitting (Model 1, Figure 19).

**316L SS Time Immersion** 

**Model 1 Beginning 24 h**  *Rs* (Ω cm2) 78 75 *TCPE1* (µF cm2) 11.88 10.28 *α1* (\*) 0.924 0.896 *RL1* (Ω cm2) 167 80,654 *TCPE1* (µF cm2) 2.911 1.032 *α2* (\*) 0.892 0.875


**Table 8.** EIS Parameters obtained by substrate experimental data fitting (Model 1, Figure 19).

**Table 9.** EIS Parameters obtained by fitting the experimental data for the BG-PLA coatings using Model 2, Figure 19.


Note: (\*) Dimensionless Warburg element.

In the experimental data fitting, the *CPE<sup>i</sup>* elements were applied instead of the perfect capacitance for better fitting. These *CPE* elements were associated with the heterogeneous morphology of the metallic surface and the surface of the coating. For the purposes of EIS analysis, a homogeneously distributed porous microstructure of the BG-PLA coatings was considered, which was formed during the drying and sintering process after applying them to the substrate. Although the coating surface SEM images are not presented here, the fitting Model 2 matches the experimental data associated with a porous microstructure. Additionally, the surface roughness of the substrate and the coatings promotes a depression of the semicircle (Nyquist plot) of the activation process (for the typical *Randles* circuit), and the *α* parameter values (Equation (3)) are lower than unity, as shown in Tables 8 and 9. Thus, the Nyquist plots present a depression, and the phase angles have lower values (Figures 17 and 18). However, the values of parameter *P* (Equation (5)) were higher than 0.5, which is associated with the Warburg diffusion mechanism and a 45◦ angle of the phase

at low frequencies. Thus, *p* values close to 1 (Table 9) represent a mechanism associated with capacitive behaviour associated with the high resistance characteristic of the coatings. The Bode plots show a combination of the effect of the time delay of the mass transfer mechanism due to the finite diffusion of species through the coating thickness and the resistive characteristic of the Cr2O<sup>3</sup> inner layer, causing a wide range in the loop, with about 70–80◦ of the phase angle formed from the middle to the low frequencies. Similar results have been previously reported [32], and were associated with high resistance and capacitive behaviour.

### **4. Discussion**

The electrochemical results with the porous hybrid coatings allow for mass transport and fluid permeation, which was observed in this work, suggesting potential applications in the tissue engineering area, results which are of interest for future study. Although study of the coatings' bioactivity for bone regeneration was not within the scope of this work, it is proposed for further study. Accordingly, the formation of particles with Ca and P contents during the bioactivity testing of BG-PLA in SBF and PBS solutions (described in Sections 3.4 and 3.5 above) due to the interaction of the H2O molecules with the Si-O bonds in the BG microstructure promoted the formation of Si−OH groups, which attract the ions of Ca2+, H2PO4−, HPO2<sup>−</sup> 4 , and PO3<sup>−</sup> 4 in the SBF solution. This favours the HAp nucleation sites, and they precipitate after a period of immersion time; similarly, observation has been made for a hybrid composite with a PCL matrix [29,33,34]. The amorphous inorganic formation of the nuclei crystallizes into the apatite phase [34], and the addition of nanopowders to the polymer matrix can improves apatite nucleation [29]. In accordance with these results, the BG phased improves of the formation of the HAp phase, as has previously been suggested.

Based on the *R<sup>p</sup>* kinetics behaviour, the proposed mechanisms associated with the EIS fitting results, and the formation of HAp particles, the physical characteristics of the microstructure of the coatings allowed redox reactions to take place at the porous surface. Therefore, Ca2+, H2PO4<sup>−</sup> and HPO2<sup>−</sup> 4 concentrations were increased at those sites and the associated current density was added to the total measured at the metallic surface. Because the application of the coatings formed a physical barrier at the metallic surface, the charge transfer consequently decreases and the corrosion resistance should increase. However, the presence of intermediate electrochemical reactions through the coating thickness maintains *R<sup>p</sup>* kinetics values of the coated samples slightly lower than those presented by the substrate. Thus, the electrochemical results support the potential application of the BG-PLA composite in biomedical applications. Consequently, further studies to determine the in vitro degradation behaviour and adhesion performance of the hybrid coatings will be undertaken.

### **5. Conclusions**

In summary, BG-PLA composite scaffolds with two different compositions synthetized by the sol–gel technique were evaluated and characterised. The morphology of the BG70- PLA30 composite structure was dense, with a well-distributed phase. The surface morphology of the BG30-PLA70 composite presented crack formation associated with tension stress concentrations in the polymeric phase during the drying process. Their potential as bone tissue engineering scaffolds was assessed by in vitro testing using Hank's and SBF solutions, confirming the bioactivity of the composites by their ability to form HAp on the surfaces and their adequate biodegradation when immersed in PBS after 21 days of immersion. Both properties were confirmed by SEM and FTIR characterization.

The electrochemical evaluation of the scaffolds in Hank's saline solution and SBF as a coating in a 316L SS substrate allowed us to observe that both samples showed activation mechanisms at the early stages, followed by pseudo-passivation or currentlimited behaviour due to the physical characteristics of the bioactive glass, which suggests

that improvements in the formation of HAp nucleation consequently allow redox reactions at the surface of the coating.

**Author Contributions:** Conceptualization, G.C.-D.l.T.; methodology, N.N.Z.-M.; investigation, M.A.E.-M. and G.C.-D.l.T. writing—original draft preparation, N.N.Z.-M.; resources, J.O.-O.; writing review and editing, G.C.-D.l.T. and M.E.; formal analysis, M.A.E.-M. and N.N.Z.-M.; supervision, G.C.-D.l.T., M.E. and M.A.E.-M.; project administration, M.d.L.B.-A.; funding acquisition, G.C.-D.l.T. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** The datasets generated during the current study are available from the corresponding author on reasonable request.

**Acknowledgments:** The authors acknowledge Hugo I. Medina-Vargas from the Biochemical Engineering Faculty, Technological Institute of Morelia, and the support of the Material Degradation Laboratory of the Mechanical Engineering Faculty (UMSNH) for research development. The present research was supported by research project number 243236 of CB-2014-02 from the I0017 fund of the CONACYT.

**Conflicts of Interest:** The authors declare no conflict of interest.
