Effect of Coating Treatment on the Properties of Extruded Mg-1.0Zn-0.3Zr-1.0Y-2.0Sn Alloys

Abstract

The impact of fluoride-based coatings on the microstructure and mechanical integrity of extruded Mg-1.0Zn-0.3Zr-1.0Y-2.0Sn alloys was assessed utilizing optical microscopy (OM), scanning electron microscopy (SEM), X-ray diffraction (XRD), immersion testing, electrochemical analysis, and tensile testing. It was observed that the magnesium alloys could be immersed in hydrofluoric acid (HF) for varying durations to achieve coatings of distinct thicknesses, with the coating thickness stabilizing at approximately 8 μm after a 48 h immersion period. The application of the fluoride coating significantly enhanced the corrosion resistance of the alloys, with a corrosion rate (CR_H) of 0.13 ± 0.012 mm/y. Upon a 20-day immersion in simulated body fluid (SBF), the degradation rates of the yield strength (YS), tensile strength (UTS), and elongation (EL) for the cast alloys were recorded as 62%, 59%, and 64%, respectively. For the extruded alloys, these rates escalated to 77%, 76%, and 95%. In contrast, the fluorine-coated alloys exhibited significantly lower degradation rates of 28%, 23%, and 39% after a 25-day immersion in SBF. Upon extrusion, the specimens exhibit a diminished corrosion resistance and a more substantial decline in mechanical properties compared to their as-cast state. Upon the application of the coating, there is a discernible reduction in the rate of mechanical property degradation observed in the specimens. This indicates that the fluorinated coating can mitigate the corrosion rate and enhance the corrosion resistance of magnesium alloys.

1. Introduction

The aging of the global population represents an inescapable social challenge, with an associated escalation in the prevalence of bone and cardiovascular diseases. Consequently, there is an escalating demand for biomaterials. Bio-magnesium alloys, characterized by their biodegradability and biocompatibility, hold promising potential in cardiovascular scaffolding and orthopedic repair applications, positioning them as candidates for the next generation of degradable biomaterials [1,2,3,4,5]. Despite their promise, Mg alloys encounter several challenges: 1. Qi et al. implanted extruded ZK60 samples into the right humerus of mice and found that the biocompatibility of magnesium alloy needs to be further improved [6]. 2. Xie et al., through the study of the development of magnesium alloys in recent years, put forward the problem of coordinating the improvement of magnesium alloy strength and corrosion resistance [7]. 3. The low standard electrode potential of Mg and the porous nature of its surface oxide layer predispose Mg alloys to an increased susceptibility to corrosion [8]. Hence, the alignment of the mechanical properties and corrosion resistance in Mg alloys is a critical issue that demands immediate resolution to facilitate the advancement and utilization of bio-magnesium alloys in the biomedical sector.

Current strategies to enhance the mechanical properties and corrosion resistance of Mg alloys encompass alloying, heat treatment, hot deformation, and surface modification, with alloying and hot extrusion being particularly prevalent. Regarding the alloying elements, Zn serves as a prevalent alloying element, which significantly ameliorates both the mechanical integrity and corrosion resistance of the alloys [9]. Zr is known to refine the grain structure of Mg alloys, thereby diminishing the brittleness and concurrently augmenting the alloys’ strength [10,11]. The addition of Y also confers improvements in the mechanical properties and corrosion resistance of Mg alloys [12,13]. Furthermore, the incorporation of Sn into Mg alloys facilitates the formation of the Mg₂Sn phase, which, due to its high thermal stability and fine-grained microstructure, contributes to the enhancement of the alloys’ mechanical characteristics [14,15,16]. These alloying elements and processes are pivotal in the ongoing development of Mg alloys for biomedical applications, where both strength and durability are paramount.

The mechanical characteristics of the Mg-1.0Zn-0.3Zr-1.0Y-2.0Sn alloy are substantially enhanced through hot extrusion and deformation processes; however, this treatment concurrently diminishes its corrosion resistance, rendering it suboptimal for biomedical applications. Surface modification techniques offer a promising solution to this issue. These methods can produce a variety of coatings, including inorganic, polymeric, and composite layers, which have been extensively documented in the literature [17,18,19,20]. A specific surface modification approach involves immersing the magnesium alloy in an HF solution, which results in the formation of a fluorinated layer on the alloy’s surface. This layer serves as a protective barrier for the alloy matrix and effectively mitigates the corrosion rate [21,22,23]. Additionally, Weber et al. [24] conducted an investigation employing a sinus MgNd₂ implant coated with MgF₂, which exhibited a superior histocompatibility and controlled degradation over a six-month observation period. The study revealed that the controlled release of fluoride ions during the degradation of this surface coating is considered non-toxic and biocompatible.

In this study, we have utilized an extruded Mg-1.0Zn-0.3Zr-1.0Y-2.0Sn alloy as a substrate to investigate the impact of varying durations of HF immersion on the microstructure, topography, and chemical composition of the resulting fluoride coating. The objective is to enhance the alloy’s mechanical integrity and corrosion resistance, thereby developing a more robust biodegradable alloy.

2. Experimental Procedure

2.1. Material Preparation

Mg (99.95%), Zn (99.99%), and Mg-30%Zr, Mg-20%Y, and Mg-20%Sn were employed to fabricate the as-cast Mg-1.0Zn-0.3Zr-1.0Y-xSn (x = 0, 0.5, 1, 1.5, 2, 2.5, 3) alloy series. The alloy preparation was conducted in a vacuum induction melting furnace (ZGJL0.01-40-4) under a protective atmosphere consisting of CO₂ (99% by volume) and SF₆ (1% by volume). The melting temperature was maintained at 750 °C, while the casting temperature was set at 720 °C. Prior to casting, the mold was preheated to 200 °C to ensure uniform heat distribution. Subsequently, the ingots were sectioned into billets with dimensions of Φ49 × 36 mm. Hot extrusion of these billets was performed at various temperatures, specifically, 340 °C, 360 °C, 380 °C, and 400 °C, employing an extrusion ratio of 10:1 and an extrusion velocity of 5 mm/min. The outcome of the hot extrusion process resulted in billets with dimensions of Φ16 mm × 260 mm.

Specimens for fluoride-coating treatment were selected from the extrusion bar produced at an extrusion temperature of 360 °C with an extrusion ratio (λ) of 10. The central portion of the extrusion bar was wire-cut to obtain specimens with dimensions of Φ11.3 mm × 8 mm, ensuring that the test surface of each specimen was aligned parallel to the extrusion direction. The specimens underwent a sequential polishing process using water-saturated silicon carbide abrasive papers with grit sizes of 240#, 600#, 800#, 1500#, and 2000#. The polished specimens were ultrasonically cleaned in alcohol for 5 min, then ultrasonically cleaned in acetone solution for 5 min, and then ultrasonically cleaned in alcohol for 5 min, and, finally, the specimens were taken out and placed in a drying oven for 10 min. The final preparation step involved immersing the specimens in a 40% HF solution. The specimens were soaked in the HF solution for durations of 12, 24, 48, and 72 h to ensure adequate and uniform fluoride-coating penetration.

2.2. Performance Testing

The microstructures of the as-cast alloy and the extruded variants, aligned with the parallel extrusion axis, were examined via optical microscopy. The etching solution employed was a picric-acid-based mixture, comprising 4.5 g of picric acid, 100 mL of anhydrous ethanol, 9 mL of deionized water, and 5 mL of glacial acetic acid. Grain size measurements of the alloys under scrutiny were conducted utilizing the Nano-Measurer software (1.2.5). The morphologies and composition of the second phase of the investigated alloys were characterized by SEM (JSM-5610LV, Tokyo, Japan, JEOL Ltd.) and EDS.

The tensile properties of specimens oriented perpendicular to the extrusion direction were evaluated at ambient temperature using an electronic universal testing machine (DNS100, China, SINOTEST). The specimens adhered to the GB/T 228.1-2010 standard, with dimensions of 2 mm in thickness, 3.5 mm in width, and 15 mm in gauge length [25]. The tensile tests were conducted at a strain rate of 1 mm/min, employing five replicate specimens for each experimental trial.

2.3. Immersion Corrosion Tests

Immersion tests were conducted on specimens of size Φ18 mm × 5 mm, submerged in SBF at a temperature of 37 ± 0.5 °C. The ratio of SBF volume to specimen surface area was maintained at 30 mL/cm². The specimens were immersed for a duration of 120 h, with the SBF solution being refreshed every 24 h. Hydrogen evolution was quantified daily using a gas collection apparatus. Post-immersion, the corrosion byproducts were eliminated using a boiling chromic acid solution composed of 3 g AgNO₃, 60 g CrO₃, and 300 mL deionized water. The mass loss was ascertained with a precision electronic analytical balance, accurate to 0.1 mg. The chemical constituents of the SBF are detailed in

Table 1. Chemical composition of the SBF (g/L).

NaCl	CaCl2	KCl	NaHCO3	MgCl2·6H2O	C6H12O6	Na2HPO4·12 H2O	KH2PO4	MgSO4·7H2O
8.00	0.14	0.40	0.35	0.10	1.00	0.06	0.06	0.06

. The corrosion rate, derived from the mass loss, was calculated according to the formula referenced in [26]

$$C{R}_{\mathrm{w}}=(K_1\times W)/(A\times T\times D)$$

(1)

$$C{R}_{\mathrm{H}}=K_2\times V_H$$

(2)

where $C{R}_{\mathrm{w}}$ is the corrosion rate (mm/y), $C{R}_{\mathrm{H}}$ is the hydrogen evolution corrosion rate (mm/y), K₁ = 8.76 × 10⁴, K₂ = 2.088, W is the weight loss value (g), A is the specimen surface area, T is the immersion time (h), D is the density of material (g/cm³), and V_H is the total amount of hydrogen change (mL/cm²/d).

Electrochemical assessments were conducted utilizing an Autolab electrochemical workstation (model AUT84580) within a three-electrode cell configuration. The working electrodes consisted of specimens with an exposed surface area of 1 cm². A graphite sheet served as the counter electrode, while a saturated calomel electrode (SCE) functioned as the reference electrode. The reported potentials are referenced to the SCE. Electrochemical impedance spectroscopy (EIS) measurements were initiated once the specimens, after being immersed in SBF for 1 h, had reached a stable open-circuit potential (OCP). The EIS was executed with an AC perturbation amplitude of 5 mV, spanning a frequency spectrum from 10 kHz to 0.1 Hz. Subsequently, the polarization curve was derived at a uniform potential scan rate of 5 mV/s, ranging from −1.9 V to −1.1 V.

3. Results and Discussion

3.1. Coating Morphology and Structure

The cross-sectional morphology of the fluorinated coating, following a 48 h immersion period, and the corresponding EDS analysis are depicted in Figure 1,the table in the figure is the red cross composition. Figure 2 illustrates the line-sweep morphologies of the cross-sections of the extruded Mg-1.0Zn-0.3Zr-1.0Y-2.0Sn alloy coatings after immersion in HF for intervals of 12, 24, 48, and 72 h. The EDS analysis (Figure 1) reveals that the coating is predominantly composed of Mg, F, and O elements, with an atomic ratio of Mg to F of approximately 1:2 and Mg to O of approximately 1:1, suggesting the presence of MgF₂ and MgO as the primary constituents. The line scan analysis (Figure 2) indicates an initial increase followed by a stabilization in the coating thickness with extended soaking times. The thickness measurements were approximately 2, 4, 8, and 8 μm at 12, 24, 48, and 72 h of immersion, respectively. Notably, at 48 and 72 h, the coating thicknesses were comparable, suggesting that the maximum thickness is achieved at 48 h. Collectively, the EDS and line-scan data indicate a progressive increase in coating thickness that plateaus with prolonged immersion. Elemental distribution patterns from the cross-sectional EDS and line-scan analyses confirm the coating’s composition as primarily MgF₂ and MgO, corroborating the findings of Shi et al. [27].

The analysis of the surface morphology and elemental composition of the fluorinated coating, following a 48 h immersion period, is presented in Figure 3. The surface scan data indicate that the coating’s surface is predominantly composed of Mg, F, and O, with the highest concentrations observed for these elements. In contrast, the concentrations of Sn, Y, Zn, and Zr are minimal and effectively negligible. By integrating the findings from the EDS, line scan, and surface scan analyses as depicted in Figure 1, Figure 2 and Figure 3, respectively, it is evident that the coating is primarily constituted by MgF₂ and MgO.

3.2. Physical Composition of Coatings

To elucidate the phase composition of the fluorinated coating, XRD was conducted on the specimens, with the findings presented in Figure 4. The XRD analysis revealed that the surface of the coating is predominantly composed of phases of MgF₂, MgO, ZnO, and ZrF₄. These findings are in closer alignment with the XRD results, which indicated atomic ratios of approximately 1:2 for Mg to F and 1:1 for Mg to O, as depicted in Figure 3. Consequently, it is postulated that the coating is primarily constituted by MgF₂ and MgO.

3.3. Effect of Fluorinated Coatings on Mechanical Properties of Alloys in the Extruded

Figure 5 shows the macroscopic corrosion morphology of the Mg-1.0Zn-0.3Zr-1.0Y-2.0Sn alloy in cast, extruded, and fluoride-coated tensile specimens before corrosion in SBF (0 days), 10 days, and 20 days of corrosion. The corrosion products on the surface of the alloys in different states gradually increased with the increase in immersion time. It is noteworthy that the corrosion on the surface of the uncoated specimen is more severe compared to the surface of the fluorinated-coated specimen. When the cast specimens were immersed in SBF for 10 days, a small amount of corrosion products appeared on the surface of the specimens, and continued to be immersed, and, at 20 days, the surface of the specimens was basically covered with corrosion products. For extruded specimens in SBF immersed for 10 days, most of the specimen surface was covered with corrosion products, with the extension of the immersion time; in 20 days, the specimen surface has been completely covered with corrosion products. For fluorinated-coating specimens in SBF immersed in 10 days, the surface of the coating is basically intact, with almost no corrosion products; with the extension of the immersion time, the production of H₂ led to the specimen surface coating cracking; in 20 days, the specimen surface is covered with a small amount of irregularly distributed corrosion products. The comparison revealed that the alloys corrode differently in different states, with the extruded-state specimens corroding the most severely and the fluorine-coated specimens the least. This indicates that the presence of fluoride coating on the surface can effectively improve the corrosion resistance of the alloy.

Figure 6 delineates the variation curves of YS, UTS, and EL as a function of the immersion time in SBF for Mg-1.0Zn-0.3Zr-1.0Y-2.0Sn alloys in their as-cast, extruded, and fluoride-coated conditions (data in

Table 2. Mechanical properties of as-cast Mg-1.0Zn-0.3Zr-1.0Y-2.0Sn alloy in Figure 6.

As-Cast	0 d	5 d	10 d	15 d	20 d	25 d
UTS (MPa)	211 ± 10	192 ± 9	165 ± 8	139 ± 6	86 ± 10	46 ± 9
YS (MPa)	154 ± 10	133 ± 10	126 ± 8	84 ± 5	58 ± 8	31 ± 9
EL (%)	19.1 ± 0.5	17.4 ± 0.4	14.3 ± 0.2	10.5 ± 0.2	6.8 ± 0.3	2.2 ± 0.4

,

Table 3. Mechanical properties of extruded Mg-1.0Zn-0.3Zr-1.0Y-2.0Sn alloy in Figure 6.

Extruded	0 d	5 d	10 d	15 d	20 d	25 d
UTS (MPa)	277 ± 9	235 ± 10	188 ± 10	123 ± 9	68 ± 10	-
YS (MPa)	221 ± 9	193 ± 10	162 ± 10	106 ± 9	52 ± 10	-
EL (%)	22.3 ± 0.5	18.6 ± 0.4	12.7 ± 0.5	6.9 ± 0.6	1.2 ± 0.5	-

and

Table 4. Mechanical properties of extruded Mg-1.0Zn-0.3Zr-1.0Y-2.0Sn alloy with fluorinated coating in Figure 6.

Coated	0 d	5 d	10 d	15 d	20 d	25 d
UTS (MPa)	272 ± 8	263 ± 10	254 ± 10	238 ± 8	229 ± 10	210 ± 8
YS (MPa)	219 ± 9	208 ± 9	193 ± 10	184 ± 9	172 ± 8	158 ± 9
EL (%)	22.1 ± 0.4	21.8 ± 0.5	18.9 ± 0.4	16.9 ± 0.4	15.7 ± 0.6	13.3 ± 0.4

). Prior to immersion, the mechanical properties of the extruded and fluoride-coated specimens were found to be similar, suggesting that the application of the fluoride coating had a negligible impact on the alloy’s microstructure. The analysis of the data presented in the figure indicates that the YS, UTS, and EL of the as-cast, extruded, and fluoride-coated Mg-1.0Zn-0.3Zr-1.0Y-2.0Sn alloy specimens exhibit a progressive decline with extended immersion in SBF. Nevertheless, the degradation of the mechanical properties in the fluoride-coated specimens was considerably less pronounced compared to the as-cast and extruded counterparts. The YS, UTS, and EL of the cast specimens decreased from 154 ± 10 MPa, 211 ± 10 MPa, and 19.1 ± 0.5% to 58 ± 10 MPa, 86 ± 10 MPa, and 6.8 ± 0.3% after 20 days of immersion in SBF, resulting in attenuation rates of 62%, 59%, and 64%, respectively. After 25 days of immersion, the YS, UTS, and EL further declined to 31 ± 9 MPa, 46 ± 9 MPa, and 2.2 ± 0.4%, with decay rates of 80%, 78%, and 88%, respectively. Consistent with the observations for the as-cast specimens, the extruded Mg-1.0Zn-0.3Zr-1.0Y-2.0Sn alloy specimens exhibited a significant reduction in YS, UTS, and EL after a 20-day immersion in SBF. Specifically, the YS, UTS, and EL values diminished from 221 ± 9 MPa, 277 ± 9 MPa, and 22.3 ± 0.5% to 52 ± 10 MPa, 68 ± 10 MPa, and 1.2 ± 0.5%, respectively, corresponding to decay rates of 77%, 76%, and 95%. After 25 days, the specimens were too severely corroded to facilitate the accurate measurement of their mechanical properties. Fluoride-coated specimens, however, displayed a more moderate decline in mechanical properties. After 20 days of immersion in SBF, the YS, UTS, and EL values decreased from 219 ± 9 MPa, 272 ± 9 MPa, and 22.1 ± 0.4% to 172 ± 8 MPa, 229 ± 10 MPa, and 15.7 ± 0.6%, with respective attenuation rates of 21%, 15%, and 29%. By the 25th day, these values further reduced to 158 ± 9 MPa, 210 ± 8 MPa, and 13.3 ± 0.4%, with decay rates of 28%, 23%, and 39%, respectively. The comparison of mechanical property attenuation between the extruded and fluoride-coated specimens indicates that the fluoride-coated specimens experienced the lowest degree of degradation. This finding underscores the protective role of the fluoride coating in reducing the corrosion rate and mitigating the degradation of the alloy’s mechanical properties.

Figure 7 presents the SEM fracture morphologies of the Mg-1.0Zn-0.3Zr-1.0Y-2.0Sn alloy specimens in the as-cast and extruded states, as well as those with fluorinated coatings, before corrosion (0 days) and after 10 and 20 days of corrosion in SBF, parallel to the extrusion direction. The surface corrosion of the specimens intensified with increasing immersion time; however, the fluoride-coated specimens exhibited significantly fewer and less severe corrosion pits compared to the as-cast and extruded specimens. Corroborated by the data in Table 2, a positive correlation is observed between the severity of surface corrosion and the degree of mechanical property attenuation in the alloy. In the as-cast specimens, Figure 7a–c reveals the presence of numerous pits of varying sizes on the fracture surface, attributed to the abundant Cl- in the SBF, which facilitate the formation of galvanic corrosion cells between the Mg matrix and secondary phases. Figure 7d–f depicts a similar, yet more severe, corrosion pattern on the extruded specimens, ascribed to the increased susceptibility to galvanic corrosion due to a higher concentration of secondary phases post-extrusion. As the immersion time extends, the number and depth of pitting corrosion sites escalate, leading to preferential fracture initiation at these severely corroded areas. The fluoride-coated specimens, as shown in Figure 7g–i, exhibit a fish-scale-like surface morphology, most pronounced after 0 days of SBF immersion. This morphology, also visible in Figure 3, is indicative of a mixture of MgF₂ and MgO. The fish-scale pattern arises from the higher elastic modulus of the coating compared to the magnesium matrix, resulting in a lack of cohesive plastic deformation during tensile stress. This disparity in elastic properties leads to a continuous rupture and the formation of fish-scale-like blocky features. Prolonged immersion reduces the coating’s coverage area, suggesting gradual delamination. This may be attributed to the accumulation of corrosion products and the release of H₂, which exerts internal pressure at the substrate–coating interface, ultimately compromising the coating integrity [28].

An examination of the tensile fracture surfaces of the cast and extruded Mg-1.0Zn-0.3Zr-1.0Y-2.0Sn alloy specimens after various durations of immersion in SBF reveals that fracture initiation predominantly occurs at sites of severe corrosion, namely, the corrosion pits. These pits possess a more negative potential relative to the surrounding specimen surface, thereby acting as anodes in the corrosion cell. The larger surface area of the specimen in comparison to the pit area establishes a large-cathode-to-small-anode configuration, which intensifies the corrosion process. This configuration promotes the progression of pitting corrosion along the depth, culminating in a precipitous decline in the specimen’s mechanical properties. Post-coating, while the specimens exhibited reduced corrosion, fluoride-coated specimens, nonetheless, accumulated corrosion products at the tensile fracture surface following SBF immersion. An escalation in the quantity of corrosion products was observed with prolonged immersion periods, signifying a gradual diminishment in the coating’s protective efficacy. As the frequency and severity of pitting corrosion on the specimen surface increases, so does the degradation of the specimen’s mechanical properties. This progressive deterioration predisposes the specimen to premature failure under tensile stress.

3.4. Effect of Fluorinated Coatings on the Corrosion Properties of Extruded-State Alloys

Figure 8 shows the microscopic corrosion morphology of the Mg-1.0Zn-0.3Zr-1.0Y-2.0Sn alloys in the cast state, extruded state, and with fluorinated coatings with unremoved corrosion products after 15 days of immersion in SBF. The surface corrosion products of the alloys in the extruded state were significantly greater compared to the cast alloys (Figure 8a,b). With the addition of the coating (Figure 8c), the surface corrosion products of the alloy were again significantly reduced, which indicates that the fluorinated coating can effectively improve the corrosion resistance of the alloy. After the cast-state alloy was immersed in SBF for 15 days (Figure 8a), the surface was covered with small clusters of corrosion products, and the corrosion areas showed an uneven distribution. The extruded alloy (Figure 8b) after 15 days of immersion in SBF showed more severe corrosion than the cast alloy, and, at the same time, the corrosion occurred in a more uneven distribution of the area, and presented an uneven shape, and the corrosion products on the surface were mainly in the form of large clusters. The fluoride-coated alloy (Figure 8c) was immersed in SBF for 15 days, and the surface coating was retained intact, with no large areas of detachment and only a small amount of corrosion products. Cracks appeared on the surface of the corrosion layer of the alloy in the extruded state as well as the fluoride-coated treated alloy, which was due to the dehydration of the corrosion products on the surface of the alloy [29]. Figure 9 shows the amount of hydrogen precipitation and pH change of the Mg-1.0Zn-0.3Zr-1.0Y-2.0Sn alloy after 10 days of immersion in SBF in the as-cast, as-extruded, and as-fluoride-coated state. Hydrogen precipitation corrosion mainly occurs during immersion in SBF, and the reaction formula is as follows: Mg + 2H⁺ = Mg²⁺ +H₂. Therefore, the corrosion rate of the alloys can be reflected by the change in the amount of hydrogen precipitation in SBF, and the change in the pH value can reflect the degree of the corrosion reaction of the alloys [30]. In Figure 9a, the amount of H₂ analyzed in SBF of different alloys in different states shows a tendency of increasing and then leveling off with time. The amount of hydrogen precipitation was significantly reduced for the surface-covered coating specimen. The hydrogen precipitation rate of the as-cast alloy was 2.02 ± 0.06 mL/cm².

Equation (1) delineates the corrosion rate (CR_H) of the alloy, which was determined to be 0.53 ± 0.05 mm/y. Comparatively, the as-cast alloy exhibited a reduced hydrogen precipitation rate compared to the extruded variant. Furthermore, the precipitation rate of hydrogen in the alloy declined significantly post-application of the fluoride coating, with the rate plummeting to 0.62 ± 0.012 mL/cm², and the CR_H was 0.13 ± 0.012 mm/y in combination with Equation (1). These findings suggest that the MgF₂ coating substantially mitigates the corrosion rate, thereby augmenting the alloy’s corrosion resistance. As depicted in Figure 9b, the daily hydrogen precipitation for all alloy states surged precipitously on the initial day, followed by a progressive stabilization. This pattern is attributed to the rapid formation of a corrosion layer on the alloy surface, which subsequently inhibits further corrosion progression, leading to a plateau in hydrogen precipitation levels. Figure 9c illustrates a deceleration in the pH values of all alloys after a 24 h immersion in SBF. This observation signifies a gradual deceleration of the corrosion reaction, indicative of the alloy’s adaptive response to the corrosive environment [31].

Figure 10 presents the EIS and corresponding equivalent circuit diagrams for the Mg-1.0Zn-0.3Zr-1.0Y-2.0Sn alloy in its as-cast, extruded, and fluoride-coated states following a 3600 s exposure to SBF. The Nyquist plots reveal minimal variance in the capacitive resistance arcs among the three alloy states, suggesting a comparable corrosion mechanism across these conditions [32]. The impedance magnitudes, denoted by the capacitive arcs and impedance Z in Figure 10a,b, indicate that the as-cast alloy exhibits higher impedance values than the extruded alloy. This observation implies a diminished corrosion resistance following the extrusion process. Conversely, the fluoride-coated alloy demonstrates the highest impedance values, signifying superior corrosion resistance. This enhancement is attributed to the protective fluoride coating, which shields the alloy surface, elevates the barrier to corrosion reactions, and bolsters the overall corrosion resistance of the alloy. By integrating the Bode and Nyquist diagrams (Figure 10a–c), an equivalent circuit model (Figure 10d) is employed to elucidate the alloy’s corrosion behavior. In this model, CPE₁ and CPE₂ represent the constant phase elements associated with the surface corrosion layer and the bilayer, respectively. Rs, R1, and R2 correspond to the solution resistance, the resistance of the corrosion layer, and the Faradaic impedance (charge transfer resistance at the bilayer), respectively. The electrochemical parameters, extracted from the fitting of the equivalent circuit, are tabulated in

Table 5. EIS fitting parameter values for Mg-1.0Zn-0.3Zr-1.0Y-2.0Sn alloy, as-cast, extruded, and with fluorinated coating.

Sample	Rs (Ω·cm2)	CPE1		R1 (Ω·cm2)	CPE2		R2 (Ω·cm2)
		Y01 (Ω−1·cm−2·s−n)	n1		Y02 (Ω−1·cm−2·s−n)	n2
As-cast	36.47	1.41 × 10−5	0.71	64.63	1.08 × 10−5	0.89	6.58 × 103
Extruded alloy	38.78	8.23 × 10−6	0.70	140.4	1.28 × 10−5	0.87	5.44 × 103
Coated alloy	41.35	1.29 × 10−6	0.91	44.01	4.48 × 10−6	0.91	1.72 × 105

. Notably, the fluoride-coated alloy exhibits the highest combined R1 and R2 values within the equivalent circuit. This finding indicates that the corrosion layer on the fluoride-coated alloy provides the most significant impediment to the charge transfer during the corrosion process, thereby conferring enhanced corrosion resistance in SBF.

3.5. Corrosion Mechanism

The corrosion characteristics of the Mg-1.0Zn-0.3Zr-1.0Y-2.0Sn alloy exhibited notable variations when assessed in its cast, extruded, and fluorinated-coated forms within SBF. The discrepancies were further elucidated by integrating the outcomes from weightlessness and electrochemical tests conducted on the alloy in these distinct conditions. Figure 11 delineates the proposed corrosion mechanisms for the alloy in its as-cast, extruded, and fluorinated-coated states within SBF. As depicted in Figure 11a,b, Cl- initiates the corrosion process by targeting the vulnerable regions, specifically the dislocations within the oxide film [33]. The presence of a galvanic cell established due to the potential gradient between the second-phase particles and the magnesium matrix, results in the second phase functioning as the cathode and the magnesium substrate as the anode. This arrangement facilitates the onset of localized galvanic corrosion.

The progression of corrosion within the Mg-1.0Zn-0.3Zr-1.0Y-2.0Sn alloy is characterized by an initial penetration along grain boundaries, followed by an internal expansion into the grains. The accumulation of H₂, a byproduct of the corrosion process, leads to the fracturing of corrosion products, thereby exacerbating the corrosion. Post-extrusion, the alloy’s corrosion resistance is compromised due to the fragmentation of the second phase and an increase in dislocations, which facilitates the detachment of nanoscale second-phase particles, exposing fresh surfaces and further reducing corrosion resistance [34]. Additionally, the extrusion process refines the grain structure, increasing the number of grain boundaries and, thus, the susceptibility to intergranular corrosion. Consequently, the corrosion resistance of the extruded alloy is diminished compared to its as-cast state. The application of a protective coating, as illustrated in Figure 11c, introduces a layer of MgO that shields the underlying MgF₂ from direct exposure to SBF. However, corrosive ions can still infiltrate the coating, reaching the MgF₂ layer and initiating a reaction that transforms it into Mg(OH)₂. Simultaneously, smaller ions from the SBF permeate the MgF₂ coating along its columnar grain boundaries, engaging in reactions with the alloy matrix. The accumulation of corrosion products within the microchannels at the grain boundaries of MgF₂ creates a barrier that impedes the access of SBF to the substrate, thereby mitigating further corrosion reactions and decelerating the overall corrosion process. As the immersion duration progresses, the accumulation of corrosion products and the release of H₂ escalate, exerting internal pressure at the interface between the substrate and the coating. This pressure can induce stress within the coating. Upon reaching a critical threshold, the pressure may cause the MgF₂ coating to fracture, thereby enlarging the area susceptible to corrosive attack. Furthermore, the dissolution of the MgF₂ coating results in a less compact structure compared to its original dense state, allowing corrosive ions to permeate the now porous coating and initiate reactions with the underlying substrate [35,36,37]. Despite these challenges, the presence of the coating is generally beneficial in retarding the rate of corrosion. The protective layer serves to mitigate the corrosive effects, even as it undergoes degradation over time.

4. Conclusions

In this study, we examined the impact of a fluorinated coating on the microstructure and characteristics of extruded Mg-1.0Zn-0.3Zr-1.0Y-2.0Sn alloys. The findings can be encapsulated as follows:

(1)

The fluorinated coating’s surface is predominantly constituted by MgO and MgF₂. As the duration of immersion in the HF solution extends, the coating’s thickness on the extruded alloy increases initially, and then plateaus. After a 48 h immersion, the thickness stabilizes at approximately 8 μm.

(2)

Post-coating treatment, the corrosion resistance of the extruded alloy is markedly enhanced, with a corrosion rate CR_H of 0.13 ± 0.012 mm/year. Following a 20-day immersion in SBF, the YS, UTS, and EL of the extruded alloy diminished by 77%, 76%, and 95%, respectively. In contrast, the mechanical properties of the fluoride-coated samples exhibit a more modest decline in YS, UTS, and EL by 21%, 15%, and 29%, respectively, after the same period.

(3)

The extruded Mg-1.0Zn-0.3Zr-1.0Y-2.0Sn alloy exhibits a higher dislocation density and residual stress levels than its as-cast counterpart, which results in an elevated internal dislocation rate and a subsequent reduction in corrosion resistance. However, the application of a fluorinated coating significantly augments the alloy’s corrosion resistance. This enhancement is attributed to the presence of MgO in the outer layer of the coating, which acts as a barrier to prevent direct contact between MgF₂ and SBF. Upon prolonged immersion, corrosive ions infiltrate the coating’s vulnerable regions and initiate a reaction with MgF₂. Concurrently, the smaller ions present in the SBF solution engage in reactions with the alloy matrix, particularly along the columnar grain boundaries of MgF₂.