*2.2. Anodizing Treatment*

Anodization was performed in an aqueous solution consisting of a mixture of 1.0 M NaOH and 0.5 M Na2SiO3 at a constant current density of 20 mA cm<sup>−</sup><sup>2</sup> for 5 min at room temperature. Details about the anodizing experiments can be found in our previous work [28].

#### *2.3. Coating Morphology and Corrosion Test*

The surface and cross-section morphologies of the anodized sample were acquired using scanning electron microscopy (SEM, JSM-6010LA, JEOL, Tokyo, Japan).

The corrosion behavior was assessed in a phosphate-buffered saline (PBS) solution consisting of 0.355 g L−<sup>1</sup> NaH2PO4·H2O, 8.2 g L−<sup>1</sup> NaCl, and 0.105 g L−<sup>1</sup> Na2HPO4 (anhydrous). The electrolyte was prepared with deionized water (18.2 MΩ cm) and analytical grade reagents. The measurements were carried out using a potentiostat/galvanostat (M101, Metrohm Autolab B.V., Utrecht, The Netherlands) in a conventional three-electrode cell configuration. The test cell consisted of a platinum wire as the counter-electrode, Ag/AgCl (3 M, KCl) as the reference and the investigated AZ31B alloy as the working electrode. Firstly, the open potential circuit (OCP) was monitored for 1 h. Next, potentiodynamic polarization tests were conducted in a potential range from −0.50 vs. OCP to 0 V vs. Ag/AgCl/(3 M, KCl), at a scanning rate of 1 mV s<sup>−</sup>1. The experiments were performed at room temperature and in triplicate.

The porosity of the anodized layer was calculated using an electrochemical relationship based on the assessment of polarization resistance (*Rp*) values obtained from the

potentiodynamic curves. This relationship is based on the variation of the corrosion potential (Δ*Ecorr* = *Ecorr*, *substrate* − *Ecorr*, *substrate + coating*) incited by the presence of the coating and from individual measurements of the polarization resistance (*Rp*) of the polished and anodized AZ31B, according to Equation (1) [28,29].

$$P = \left(\frac{R\_{p,s}}{R\_p}\right) \times 10^{\frac{|\Lambda F\_{\text{cover}}|}{h\_d}}\tag{1}$$

where *Rp*,*<sup>s</sup>* indicates the polarization resistance of the polished AZ31B alloy and *Rp* is related to the polarization resistance of the anodized AZ31B alloy, *ba* is the anodic Tafel slope of the bare material. *Rp*,*<sup>s</sup>* and *ba* are determined from separate analysis of the polished substrate.

## *2.4. Tensile Test*

The tensile tests were carried out following the ASTM E8M-16a [26] at a displacement rate of 12.7 mm/min at room temperature on a universal material testing machine (MTS Exceed E45, Eden Prairie, MN, USA). Five measurements were taken for the AZ31B alloy in the polished and anodized conditions.

#### *2.5. Fatigue and Corrosion Fatigue Tests*

Axial fatigue tests were conducted using a computer-controlled servo-hydraulic testing machine (MTS Landmark 370, Eden Prairie, MN, USA) with a sinusoidal loading control. The stress ratio was 0.1 (tension-tension mode) and the test frequency was 5 Hz. The fatigue test was continued until fracture or until the specimen did not fail up to 10<sup>6</sup> cycles. The procedure was defined according to ASTM F1801-97 [30], which is specific for temporary metal-based implants. Thus, the maximum stress at which the sample has not failed at 10<sup>6</sup> cycles is defined as a fatigue limit in this work. At every test load condition at least three specimens were evaluated.

The electrolyte employed in the corrosion fatigue tests was phosphate-buffered saline (PBS) solution, which contains: 0.355 g L−<sup>1</sup> NaH2PO4·H2O, 8.2 g L−<sup>1</sup> NaCl, 0.105 g L−<sup>1</sup> Na2HPO4 (anhydrous). The solution was prepared with deionized water and analytical grade reagents. An acrylic chamber was designed and mounted on MTS Landmark 370 servo-hydraulic machine, in order to safeguard the gage length of the specimen was immersed in the PBS during the fatigue test. The electrolyte was subject to room temperature, open to air, and static during the testing procedure. A new fresh solution was employed for each test.

#### *2.6. Fracture Surface Analysis*

The fatigue fractured specimens were immersed in 10 g L−<sup>1</sup> CrO3 for 5 min in order to remove corrosion products, in agreemen<sup>t</sup> to ASTM G1-90 [31]. Next, they were rinsed in deionized water, and dried in warm air stream provided by a conventional heat gun prior to fractographic analysis.

The fractured specimen surface was examined using scanning electron microscopy (SEM, JEOL JSM-6010LA, Tokyo, Japan) and stereomicroscopy (Olympus SZ61, Tokyo, Japan).

#### **3. Results and Discussion**

#### *3.1. Anodized Layer Morphology and Corrosion Test*

SEM micrographs of the top surface and cross-section of the AZ31B magnesium alloy in the anodized condition are shown in Figure 2. As seen in Figure 2a, the artificial oxide layer produced by the anodization treatment covered the whole substrate. Several cracks and carved regions are unevenly distributed over the surface. The cross section of the anodized specimen is showed in Figure 2b. The interface anodized layer/substrate displays an irregular thickness. A roughened interface greatly affects the design of the implantable devices, once it plays an important role behaving as a stress concentrator [32], thus limiting the fatigue resistance of the component.

**Figure 2.** SEM images of the anodized layer (**a**) top surface and (**b**) cross-section.

Potentiodynamic polarization curves of the polished and anodized samples tested in PBS solution at room temperature are presented in Figure 3. The values of corrosion potential (*Ecorr*) and corrosion current densities (*jcorr*) were determined from the curves using the Tafel extrapolation method. The results are shown in Table 1. The *jcorr* values were significantly affected by the presence of the anodic film. The dissolution rate of the anodized sample was reduced by one order of magnitude. The anodized layer acted as a barrier layer against electrolyte penetration. However, as seen in Table 1, the produced oxide layer presents an inherent porosity, which could be related to the cracks and cavities observed from the SEM micrographs (Figure 2).

**Figure 3.** Potentiodynamic polarization curves obtained for the as polished and anodized samples immersed in PBS solution at room temperature.

**Table 1.** Electrochemical parameters obtained from the potentiodynamic polarization curves displayed in Figure 2. The last column indicates the porosity percentage of the anodized sample.


In the cathodic branches, the value of the cathodic Tafel slope (bc) was increased after anodization, as shown in Table 1. According to Rahman et al. [33], this effect could be due to the formation of Mg(OH)2 inside the pores and cracks of the anodized layer, as shown in Equations (2) and (3).

$$2\text{H}\_2\text{O} + 2\text{e}^- \rightarrow \text{H}\_2 + 2\text{OH}^- \tag{2}$$

$$\text{Mg}^{2+} + 2\text{OH}^- \rightarrow \text{Mg(OH)}\_2 \tag{3}$$

When film breakdown occurs in the anodic part of the polarization curve, the possible sequence of reactions is depicted in Equations (4) and (5). These processes (anodic and cathodic reactions) are accompanied by the formation of a corrosion product layer, following the same reaction shown in Equation (3). Furthermore, due to the presence of chloride ions in the PBS solution, Mg(OH)2 could further react according to Equation (6).

> Mg → Mg2+ + 2e− (anodic dissolution) (4)

$$2\text{H}\_2\text{O} + 2\text{e}^- \rightarrow \text{H}\_2 + 2\text{OH}^- \text{ (cathodic reaction)}\tag{5}$$

$$\rm Mg(OH)\_2 + 2Cl^- \rightarrow MgCl\_2 + 2OH^- \tag{6}$$
