Hybrid Coating of Polystyrene–ZrO₂ for Corrosion Protection of AM Magnesium Alloys

Abstract

A hybrid material of polystyrene (PS)–ZrO₂ was developed by the sol–gel technique and deposited by spin-coating on AM60 and AM60–AlN nanocomposite surfaces to enhance corrosion resistance in marine environments.${\text{PS–ZrO}}_2$ with an average thickness of ≈$305 \pm 20 \mathrm{nm}$ was dispersed homogeneously, presenting isolated micro–nano-structure defects with air trapped inside, which led to an increase in roughness (≈4 times). The wettability of the coated substrates was close to the hydrophobic border (${\mathsf{\theta }}_{\mathrm{CA}}=90°–94°).$ The coated samples were exposed for 30 days to SME solution, simulating the marine–coastal ambience. The initial pH = 7.94 of the SME shifted to more alkaline pH ≈ 8.54, suggesting the corrosion of the Mg matrix through the coating defects. In the meantime, the release of ${\mathrm{Mg}}^{2+}$ from the ${\text{PS–ZrO}}_2$-coated alloy surfaces was reduced by ≈90% compared to that of non-coated. Localized pitting attacks occurred in the vicinity of Al–Mn and β–Mg₁₇Al₁₂ cathodic particles characteristic of the Mg matrix. The depth of penetration (≈23 µm) was reduced by ≈85% compared to that of non-coated substrates. The protective effect against Cl ions, attributed to the hybrid PS–ZrO₂-coated AM60 and AM60–AlN surfaces, was confirmed by the increase in their polarization resistance (Rp) in 37% and 22%, respectively, calculated from EIS data.

1. Introduction

Currently, there is a high demand for lightweight materials for the industrial manufacture of components for automobiles, airplanes, and other vehicles of transport, motivated by the needed reduction in fuel consumption and decrease in the emission of gases (${\mathrm{CO}}_2$ and ${\mathrm{NO}}_{\mathrm{x}}$) that are harmful to human health and climate change [1,2]. Studies have reported that it can stop generating emissions between 4 and 12 g/km per each 100 kg of weight reduction [3,4]. In this aspect, magnesium (Mg) and its alloys may offer solutions to increase the efficiency of vehicles, reducing their weight and emission of the pollutants generated [2]. As structural materials, they have been present in the automotive industry as several interior components such as steering wheels, pedals, and seats; structural components such as interior doors and instrument panels; and chassis components such as wheels and suspension arms, among others [5]. Although Mg and its alloys have great potential for the transportation sector, they are susceptible to localized corrosion in the presence of impurities or corrosion-active intermetallic particles in the Mg matrix [6]. In the AZ (Mg–Zn–Al) and AM (Mg–Al) alloy series, used in the automotive industry, the secondary phase of β–Mg₁₇Al₁₂ and that of Al–Mn intermetallic particles are the most common having anodic or cathodic activity. To face this problem, the incorporation of additional alloying elements and nano-reinforcement particles in the Mg matrix has allowed improvements in the corrosion resistance and mechanical properties. In this aspect, AlN nanoparticles of 1 wt.% and an average diameter of ≈80 nm have been added to the AM60 matrix as reinforcement as an excellent choice for grain refinement benefiting its ductility [7,8,9] and lower roughness (≈15%) of the surface. The properties of the manufactured AM60–AlN nanocomposite have been previously described [10,11,12].

The initial stages of electrochemical corrosion activity of AM60 alloy and AM60–AlN nanocomposite have been compared during their exposure to solutions, which simulated the formation of an aqueous layer at the metal surface at 100% air humidity of industrial acid rain (SAR) and marine–coastal (SME) aggressive environments [13,14]. The AlN nanoparticles have been observed as “attached”, forming clusters to those of Al–Mn intermetallic particles, the local efficient cathodes [15,16,17,18], which subsisted on the Mg matrix after the removal of corrosion layers, inducing localized corrosion in their vicinity. During the exposure of the AM60 alloy and the AM60–AlN nanocomposite, the pH of the model solutions shifted to alkaline values (>9), and besides the release of Mg ions, de-alloying of Al was suggested because of the instability of AlMn [18] and AlN [19], which is attributed to the formation of $\mathrm{Al}{\left(\mathrm{OH}\right)}_3$ corrosion products, confirmed by XPS analysis [14].

Consequently, the presence of ${\mathrm{Cl}}^{-}$ ions led to stronger corrosion in both area and depth penetration on the nanocomposite surface of AM60–AlN during its exposure to the SME–marine environment [14]; however, in the acid rain industrial (SAR) ambience [13], a dense and more protective corrosion layer was formed on the AM60–AlN nanocomposite. In both environments, the corrosion process was considered weakly persistent and localized in time, dominated by the fractional Gaussian noise (fGn) according to the power spectral density of free corrosion current fluctuations, and classified as electrochemical noise. The reported results recommended that the surfaces of AM60 and AM60–AlN need a posterior modification to improve their corrosion resistance to chloride ions attacks, characteristics of the marine environment.

A promising method for increasing the corrosion resistance of Mg alloys is the application of coatings on their surfaces [1,20], which may generate a physical barrier against aggressive corrosive substances present in the environment, diminishing abrasion damages, in addition to esthetic functions [21,22]. Chrome-free surface treatments and non-chromate conversion coating have been proposed for corrosion protection of magnesium and Mg alloys [23,24,25], as well as organic coatings [21,26], superhydrophobic [27,28], and organic–inorganic hybrid coating [29,30,31].

The hybrid coatings elaborated through the sol–gel methodology [32,33,34] have offered advantages because, at low temperatures, the process controls the organic and inorganic composition of the coating and reaches a high level of purity [35]. A variety of hybrid organic–inorganic materials based on polymethylmethacrylate (PPMA) with various metal oxides $\left({\mathrm{SiO}}_2{, \mathrm{TiO}}_2{, \mathrm{and} \mathrm{ZrO}}_2\right)$ have been proposed [36,37,38,39,40,41,42,43,44,45]. The polystyrene (PS) polymer has participated as the organic part in combination with ${\mathrm{SiO}}_2$, ${\mathrm{ZrO}}_2$, ${\mathrm{Al}}_2{\mathrm{O}}_3$, ${\mathrm{MnO}}_2$, and ${\mathrm{TiO}}_2$ in hybrid composites, which have been applied as dielectric materials and in searching for surface hydrophobicity or for better optoelectronic properties [46,47,48,49,50,51,52,53].

The organic and inorganic components may present a certain level of incompatibility between them, and to face this problem, coupling agents have been used [34,35]. After polymerization and polycondensation of the organic and inorganic phases, these components are linked through molecular coupling [54]. For example, 3-(Trimethoxysilyl) propyl methacrylate (TMSPM) is the coupling agent commonly used for the formation of hybrid materials [34,35,45]. The TMSPM allows the coupling through the silane groups, with which the inorganic phase is attached, while the organic phase is attached with acrylate as a coupling agent [48].

In order to improve the corrosion resistance of AM–magnesium alloys against the presence of chlorides, in this research sol–gel method was applied for the synthesis of the polystyrene–zirconium dioxide $\left({\text{PS–ZrO}}_2\right)$. The hybrid material was deposited by spin-coating on the AM60 alloy and AM60–AlN nanocomposite metallic substrates, which were exposed to a simulated marine environment solution (SME). The hydrophobicity property of the coating and its roughness were evaluated. Immersion tests were performed to monitor the changes in time of SME pH and concentration of Mg ion release. The hybrid coating ${\text{PS–ZrO}}_2$ surface morphology and composition, as well as their change after the exposure to SME, were performed by scanning electron microscopy and energy-dispersive X-ray spectroscopy (SEM-EDS). X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) were used to characterize the hybrid coating deposited on the alloy substrates. Electrochemical impedance spectroscopy (EIS) diagrams were acquired to characterize the interface of the hybrid coating–alloy–electrolyte (SME solution) on which the corrosion process occurs.

2. Materials and Methods

2.1. PS–ZrO₂ Hybrid Synthesis

${\mathrm{ZrO}}_2$ has attractive properties, such as thermal and chemical stability, high mechanical and abrasion resistance, low thermal conductivity, and low toxicity, as well as providing corrosion protection of metal substrates [55,56,57,58]. The methodology for the synthesis of ${\text{PS–ZrO}}_2$ hybrid material by means of the sol–gel method used in this research has been previously described [48] and has been similar to other hybrid material systems studied as ${\text{PMMA–SiO}}_2$, ${\text{PMMA–TiO}}_2$, and ${\text{PMMA–ZrO}}_2$ [34,36,37,38,39,42,59].

In this study, zirconium isopropoxide $\left(\mathrm{Zr}{\left(\mathrm{OPr}\right)}_4\right)$ and styrene monomer (ST) were used as the inorganic and organic precursors, and 3-(trimethoxysilyl)propyl methacrylate (TMSPM) was used as the coupling agent. Anhydrous ethanol (EtOH) and nitric acid were employed as solvent and catalyst, respectively, with a molar relation of 1:30:1 $(\mathrm{Zr}{\left(\mathrm{OPr}\right)}_4{:\mathrm{EtOH}:\mathrm{HNO}}_3)$ for the preparation of the inorganic component (Solution 1). NaOH was used to remove the 4-tert-butylcatechol (4-TBC), which acts as a polymerization inhibitor in the styrene monomer using a molar relation of 1:0.11 (ST:OH), and then it was filtered. Benzoyl peroxide (BPO) was added to this solution with a ratio of 1:0.0006 (ST:BPO) as a polymerization initiator for the preparation of the organic component (Solution 2). EtOH and deionized water were added to TMSPM with a molar relation of 1:1:6 (TMSPM:EtOH:${\mathrm{H}}_2\mathrm{O}$). Hydrochloric acid was incorporated, obtaining a homogeneous solution due to the hydrolysis of the coupling agent (Solution 3). The reagents for the synthesis of ${\text{PS–ZrO}}_2$ are summarized in

Table 1. Reagents used for the synthesis of the hybrid PS–ZrO₂.

Precursors	Solvents	Catalysis	Anti-Inhibitor	Initiator
Zirconium isopropoxide $(\mathrm{Zr}{\left(\mathrm{OPr}\right)}_4)$	Anhydrous ethanol (EtOH)	Nitric acid $\left({\mathrm{HNO}}_3\right)$	Sodium hydroxide (NaOH)	Benzyl peroxide (BPO)
Styrene monomer (ST)	Deionized water $\left({\mathrm{H}}_2\mathrm{O}\right)$	Hydrochloric acid HCl
3-(trimethoxysilyl)propyl methacrylate (TMSPM)

. The three resulting solutions were mixed to obtain a homogeneous hybrid solution.

2.2. Deposition of PS–ZrO₂ Hybrid on AM60 and AM60–AlN Alloy Surfaces

The hybrid solution was stored for 24 h, leaving it to age before ${\text{PS–ZrO}}_2$ was deposited on the metal samples. The AM60–AlN nanocomposite and AM60 alloy used as substrates were provided in the form round bar, its nominal composition, according to the producer (Magontec, Bottrop, Germany), in weight percent, is 6.0 Al; 0.2–0.4 Mn and the remainder being Mg. The manufacturing and incorporation of aluminum nitride nanoparticles (AlN, 1.0 wt.%, average diameter of 80 nm) in the AM60 for the formation of the AM60–AlN nanocomposite has been reported previously [11,13,60]. The surface of substrates (10 mm in diameter and thickness of 2 mm) was polished (to 2000 grain size of silicon carbide), sonicated in ethanol, and dried at room temperature.

Hybrid material coatings were performed by spin-coating using 0.5 mL of ${\text{PS–ZrO}}_2$ solution with a speed of 3000 rpm for 30 s. Afterward, the coated samples were taken to a vacuum drying oven (ADP-200C, Yamato Scientific Company Ltd., Tokyo, Japan), heated to ${200 }^{\circ }\mathrm{C}$ for a period of 1 h, and stored in a desiccator to prevent the corrosion of the surfaces.

2.3. Roughness and Wettability of PS–ZrO₂ Hybrid Coating on Mg–Al Alloy Surfaces

The roughness of the coated and non-coated alloy samples was measured using a 3D optical profilometer (Contour GT-K 3D, Bruker, Madison, WI, USA), and their surface wettability was determined by the contact angle (CA) of deionized water drops (with a volume of 1 μL) in contact with the alloy surfaces, measured after 1 min with the goniometer equipment (VCA-optima, AST Products Inc., Billerica, MA, USA), according to the sessile drop method at room temperature. Recorded images were obtained by means of a camera installed in the goniometer and positioned on the tested surface.

2.4. Immersion Test

The samples of AM60–AlN nanocomposite and the AM60 alloy coated with ${\text{PS–ZrO}}_2$ hybrid were immersed in 20 mL of simulated marine environment (SME) solution (

Table 2. Composition of simulated marine–coastal environment (SME, pH = 7.94) [14].

Reagents	$\mathbf{NaCl}$	${\mathbf{Na}}_2{\mathbf{SO}}_4$	${\mathbf{NaHCO}}_3$
Concentration	$5{.84 \mathrm{g} \mathrm{L}}^{-1}$	$4{.09 \mathrm{g} \mathrm{L}}^{-1}$	$0{.20 \mathrm{g} \mathrm{L}}^{-1}$

), according to the ASTMG31-12a standard [61], for a period of up to 30 days. The change in time of SME pH solution and the concentration of the released ${\mathrm{Mg}}^{2+}$ ions into the solution (Hanna Instruments, HI83200, Woonsocket, RI, USA) were measured.

2.5. SEM-EDS, XPS, and XRD Surface Analysis

The morphology and composition of the ${\text{PS–ZrO}}_2$-hybrid-coated AM60 and AM60–AlN samples were analyzed before and after being immersed in the SME solution with a scanning electron microscope and energy dispersive spectroscopy (SEM-EDS, XL-30 ESEM-JEOL JSM-7600F, JEOL Ltd., Tokyo, Japan). Additional information was provided by X-ray photoelectron spectroscopy (XPS, K-Alpha Surface Analyzer, Thermo Scientific, Waltham, MA, USA), in which spectra binding energies were normalized to C1s carbon peak at 284.8 eV. X-ray diffraction patterns (Siemens D-500, Siemens D-5000, Munich, Germany; 2θ, 34 kV/25 mA CuKα) were used to determine possible crystal structures in the hybrid material.

2.6. Electrochemical Test

The electrochemical corrosion activity of the ${\text{PS–ZrO}}_2$-hybrid-coated AM60 and AM60–AlN samples was through electrochemical impedance spectroscopy (EIS) during the sample’s immersion for 15 days in the solution SME at 21 °C. The conventional three-electrode cell (inside a Faraday cage) of working electrode (tested sample of $0{.78 \mathrm{cm}}^2$ of area), auxiliary electrode of Pt mesh, and saturated calomel as a reference electrode (SCE, Gamry Instruments, Philadelphia, PA, USA), was connected to the potentiostat/galvanostat/ZRA (Gamry Instruments, Interface-1000E, Philadelphia, PA, USA). The EIS diagrams were potentiostat/galvanostat/ZRA (Gamry Instruments, Interface-1000E, Philadelphia, PA, USA). The EIS diagrams were collected at a perturbation amplitude of ±10 mV vs. OCP (after 1 h of stabilization) at frequencies from 100 kHz to 10 mHz. The EIS spectra were analyzed with the Gamry Echem Analyst software (Gamry Instruments, Philadelphia, PA, USA).

3. Results and Discussion

3.1. Surface Characterization PS–ZrO₂ Hybrid Coating

3.1.1. SEM-EDS Analysis

Figure 1 presents the homogeneous morphology of the spin-coated ${\text{PS–ZrO}}_2$, well dispersed on the Mg–Al alloy surfaces of AM60–AlN (Figure 1a,c,e) and AM60 (Figure 1b,d,f).

Table 3. EDS elemental analysis (wt.%) of several randomly selected zone areas (Figure 1) of the PS–ZrO₂–AM60–AlN (Zones 1–2) and PS–ZrO₂–AM60 (Zones 3–4) tested samples.

Element	C	O	Mg	Al	Si	Zr
Zone 1	20.24	19.60	46.06	2.20	2.72	9.17
Zone 2	19.87	19.39	45.58	3.03	2.85	9.28
Zone 3	18.17	20.25	47.16	1.78	2.78	9.85
Zone 4	18.19	20.16	46.66	2.10	2.96	9.93

summarizes the EDS elemental analysis (wt.%) of several randomly selected areas, labeled as “zones” in Figure 1, in which the elemental content is mainly the same.

A magnification (×20,000) of zones (Figure 1c,d) suggested that there is air, probably trapped inside the micro/nanostructures of the coating, which could reduce the contact between aggressive substances and alloy surface in the air sites, providing corrosion resistance to the alloy [62].

Despite the good dispersion of the ${\text{PS–ZrO}}_2$ coating on the studied Mg–Al alloy surfaces, some isolated areas of micro-defects have been observed (Figure 1e,f) that could generate channels connecting the SME solution with the alloy substrate and facilitate the passage of ${\mathrm{Cl}}^{-},$ for example. EDS analysis revealed a high C content (Table 3), corresponding to the organic polystyrene $\left(\mathrm{PS}, {\left({\mathrm{C}}_8{\mathrm{H}}_8\right)}_{\mathrm{n}}\right),$ as a part of the hybrid coating, and a high content of O and Zr, confirming the presence of ${\mathrm{ZrO}}_2$, the inorganic part of the coating. The presence of silicon (Si) in all zones was ascribed to the coupling agent (3-(trimethoxysilyl) propyl methacrylate), which acted as a link between the organic and inorganic components of the hybrid ${\text{PS–ZrO}}_2$ coating. The decrease in Mg and Al contents (not corresponding to those of the Mg–Al tested alloys) was due to the physical barrier provided by the hybrid ${\text{PS–ZrO}}_2$ coating.

The C, Si, O, and Zr mappings (Figure 2) show the distribution of these elements and their contribution along the coated alloy surfaces of PS–ZrO₂–AM60–AlN (Figure 2a) and PS–ZrO₂–AM60 (Figure 2b). The reported AlMn and β–Mg₁₇Al₁₂ particles [13,14] were detected under the coating.

The cross-sectional SEM images (Figure 3) showed three well-defined zones corresponding to the epoxy resin (upper zone), hybrid coating of PS–ZrO₂ (central zone), and Mg matrix (lower zone).

The thickness of the ${\text{PS–ZrO}}_2$ coatings on the AM60–AlN surface was ≈$280 \pm 25 \mathrm{nm}$, while on $\mathrm{AM}60$, it was ≈$330 \pm 16 \mathrm{nm}$; the difference was ascribed to the roughness difference in the studied alloy surfaces. The EDS element mappings, carried out in the yellow marked zones (Figure 3a,b), indicated that the upper zones present a high carbon content, corresponding to the epoxy resin, while in the central region, there is a set of elements, such as C, O, and Zr presenting the organic and inorganic components of the hybrid coating; in the lower zone, the high contents of Mg and Al correspond to the Mg–Al matrix of the AM60 and AM60–AlN alloys.

3.1.2. X-ray Photoelectron Spectroscopy (XPS)

The XPS spectra of the ${\text{PS–ZrO}}_2$ hybrid coating deposited on the AM60–AlN nanocomposite and AM60 alloy substrates were similar (Figure 4), and they were analyzed based on the binding energies of C1s, Si2p, Zr3d, and O1s. The C1s signal revealed the contribution of three peaks, ascribed to several bonds: C–C and C–H (at 284.8 eV) of hydrocarbon and phenyl groups, characteristic of the polystyrene [63]; C–O–C (at 286.0 eV) of the ether groups; and O–C=O (288.90 eV) of the double ester group, belonging to TMSPM coupling agent [29,64]. The binding energy of Si2p (at 102.3 eV) was attributed to the Si–O bond of the coupling agent (TMSPM) [48,65]. The high-resolution spectrum of Zr3d presents a doublet of two spin–orbital components, ${\mathrm{Zr}3\mathrm{d}}_{5/2}$ and ${\mathrm{Zr}3\mathrm{d}}_{3/2},$ whose binding energies are approximately 182.5 and 184.9 eV, respectively, where the shift indicates the presence of ${\mathrm{Zr}}^{4+}$ species [66], considering that for ${\mathrm{Zr}}^{0+}$, the ${\mathrm{Zr}3\mathrm{d}}_{5/2}$ and ${\mathrm{Zr}3\mathrm{d}}_{3/2}$ components have binding energies of 178.7 and 181.1 eV, respectively [67]. The species of ${\mathrm{Zr}}^{4+}$ have allowed the interaction of the inorganic ${\mathrm{ZrO}}_2$ to form Zr–O–Zr and Zr–OH bonds as a consequence of the ionization of the species ${\mathrm{O}}^{2-}$ and ${\mathrm{OH}}^{-}$ (the peaks of O1s at 530.1 and 531.1 eV, respectively) [68,69]. The O1s binding energy at 531.8 eV was ascribed to the Si–O–Zr bonds [70] because of the abundance of Si–OH groups present in the silane coupling agent (TMSPM) after its hydrolysis; the energies located at 532.6 eV and 533.7 eV belong to the characteristic groups of ether and ester, respectively, as a part of the TMSPM [71]. The Mg1s binding energy at 1304.5 eV corresponds to the Mg–O bonds, by which the hybrid coating was attached to the Mg matrix [72].

3.1.3. XRD Analysis

Diffraction patterns shown in Figure 5 of the AM60–AlN and AM60 substrates, before and after being coated with the hybrid ${\text{PS–ZrO}}_2$, did not reveal the presence of crystalline structure in the hybrid material; the characteristic peaks of Mg, Al–Mn, and β–Mg₁₇Al₁₂ have been previously detected and reported [13]. It has been suggested that if the ${\mathrm{ZrO}}_2$ is present as an amorphous phase, it could reduce the sites for the diffusion of the ${\mathrm{Cl}}^{-}$ ions through such film and, thereby, improve the corrosion resistance of the metal substrate [73].

3.2. Surface Roughness and Contact Angle

The average roughness $\left({\mathrm{R}}_{\mathrm{a}}\right)$ of the AM60–AlN nanocomposite and AM60 alloy surfaces, with and without the hybrid coating of ${\text{PS–ZrO}}_2$, are compared in Figure 6. It has been reported that the introduction of aluminum nitride (AlN) nanoparticles favored a reduction in grain size, which in fact, allowed a slight decrease in the roughness of the AM60 alloy of approximately 15% (Figure 6a,b) [13,14]. The hybrid coating of ${\text{PS–ZrO}}_2$ presented a 5% higher roughness value deposited on AM60 than on AM60–AlN because of the initial roughness difference. Such different roughness of the ${\text{PS–ZrO}}_2$ deposits has led to the presence of more or less trapped air inside the micro/nano-structured ${\text{PS–ZrO}}_2$ as sites of micro-defects (Figure 1e,f).

Hydrophobicity of any coating material is a property of wide interest when it is deposited on a metal surface as a protective material, thereby reducing contact with a humid and aggressive aqueous environment and, thus, improving the corrosion resistance of the metal substrate. Through the measurement of the contact angle $\left({\mathsf{\theta }}_{\mathrm{c}}\right)$ [74], the material may be classified as hydrophilic $\left({\mathsf{\theta }}_{\mathrm{c}}<90°\right)$, hydrophobic $\left(90°<{\mathsf{\theta }}_{\mathrm{c}}<150°\right)$, or superhydrophobic $\left({\mathsf{\theta }}_{\mathrm{c}}>150°\right)$ [75,76,77,78,79].

Figure 7 presents the recorded images of the contact angle (CA) of deionized water drops on non-coated AM60–AlN nanocomposite and the AM60 alloy surfaces (Figure 7a,b) compared to those coated with the hybrid deposit of ${\text{PS–ZrO}}_2$ (Figure 7c,d).

The CA values revealed that the nature of the uncoated surfaces of the tested Mg–Al alloys is hydrophobic $\left({\mathsf{\theta }}_{\mathrm{CA}} > 90°\right)$ [74]: the AM60–AlN nanocomposite presented a contact angle of $111.76\pm 2.93°$ (Figure 7a), whereas $110.70\pm 1.70°$ was obtained for the AM60 alloy (Figure 7b). However, after the deposit of the ${\text{PS–ZrO}}_2$ hybrid coating, the wettability of the surfaces changed, presenting a reduction in the contact angle values: for the PS–ZrO₂–AM60–AlN, the $\mathrm{CA}=83.37 \pm 0.86°$ (Figure 7c), which was close to the hydrophobic border $\left({\mathsf{\theta }}_{\mathrm{CA}} < 90°\right)$; and for ${\text{PS–ZrO}}_2$–AM60, the CA value was $93.60 \pm 1.87°$ (Figure 7d), with a wettability still in the hydrophobic range. The change in the contact angle of the coated surfaces may attribute to the increase in the ${\text{PS–ZrO}}_2$ surface roughness (Figure 7c,d), according to the suggestions of Wenzel [80] and Caxie–Baxter [81,82]. In the presence of air trapped on the surface (${\text{PS–ZrO}}_2$ nonuniform surface), the contact with liquids will be interrupted; however, it was a reduction in the CA, associated with the hydroxyl groups of Zr–OH and Si–OH present in the inorganic components of the hybrid coating, due to their incomplete condensation [48,83], which led to a decrease in the benefits that the microstructure of the hybrid material could provide.

3.3. Solution Monitoring

The change in time of SME solution pH was monitored for a period of 30 days during the immersion of the hybrid-coated Mg–Al alloy samples (Figure 8). The initial value of pH = 7.94 shifted to a more alkaline value of pH ≈ 8.64 after 7 days because the SME solution caused corrosion (Reactions 1–3) of the Mg matrix, which occurred in those sites where the hybrid material of ${\text{PS–ZrO}}_2$ presented some micro-defects (Figure 1e,f) and the ${\mathrm{H}}_2$ bubbles (Reaction 3) continued to come out. The observed pH behavior has been reported previously during the exposure of AM60 and AM60–AlN to SME solution [14]. After this period of 7 days, the pH diminished, and this fact was associated with the formation of $\mathrm{Mg}{\left(\mathrm{OH}\right)}_2$, an insoluble corrosion product, obstructing those micro-cracks and the initially formed micro-defects, which area acted as a physical barrier, hindering the progress of the corrosion process of the Mg matrix. However, in those sites, the insoluble $\mathrm{Mg}{\left(\mathrm{OH}\right)}_2$ product may suffer a localized attack from the chloride ions (SME solution) and be partially dissolved, giving the origin of released ${\mathrm{Mg}}^{2+}$ ions (Reaction 4) and activating the corrosion process an increase in Mg²⁺ concentration and pH after 10 days.

$${\mathrm{Mg}}_{\left(\mathrm{s}\right)}\to {\mathrm{Mg}}_{\left(\mathrm{ac}\right)}^{2+}+{2\mathrm{e}}^{-}$$

(1)

$${\mathrm{Mg}}^{2+}+{\mathrm{OH}}^{-}\to \mathrm{Mg}{\left(\mathrm{OH}\right)}_2\downarrow$$

(2)

$${2\mathrm{H}}_2\mathrm{O}+{2\mathrm{e}}^{-}\to {\mathrm{H}}_{2\left(\mathrm{g}\right)}+{2\mathrm{OH}}_{\left(\mathrm{ac}\right)}^{-}$$

(3)

$$\mathrm{Mg}{\left(\mathrm{OH}\right)}_2+{2\mathrm{Cl}}^{-}\to {\mathrm{MgCl}}_2+{2\mathrm{OH}}^{-}\to {\mathrm{Mg}}^{2+}{+2\mathrm{Cl}}^{-}$$

(4)

Figure 9 compares the concentrations of ${\mathrm{Mg}}^{2+}$ ions released into the SME solution because of the progress in Reaction (4) during the exposure of PS–ZrO₂–AM60–AlN and PS–ZrO₂–AM60 alloys, non-coated and coated with the hybrid ${\text{PS–ZrO}}_2$ for 30 days to SME solution. From the non-coated alloy surface of AM60, $333.33 \pm 11{.54 \mathrm{mg} \mathrm{L}}^{-1}$ of ${\mathrm{Mg}}^{2+}$ ions were released, while from the AM60–AlN nanocomposite, the concentration was $353.33 \pm 11{.54 \mathrm{mg} \mathrm{L}}^{-1}$ [14] because of the shift of pH to more alkaline values, which led to an instability of the AlN particles [19]. The progress in the release of the Mg ions has been associated with the presence of Al–Mn intermetallic particles, active cathodic sites in AM60 allow and AM60–AlN nanocomposite, in which vicinity the Mg matrix (active anode) suffers accelerated localized corrosion attack [13,14,15,16,17,18]. The results indicated that the hybrid ${\text{PS–ZrO}}_2$ coating deposited on AM60–AlN and AM60 surfaces reduced the release of ${\mathrm{Mg}}^{2+}$ by approximately 89% and 91%, respectively, because of the obstructed micro-localized sites of defects and impaired diffusion of chloride ions through the formed layer of corrosion product.

3.4. SEM-EDS Analysis of the PS–ZrO₂-Coated Alloy Surfaces after Exposure to SME Solution

The SEM images in Figure 10 illustrate the morphology of the AM60–AlN and AM60 magnesium alloys’ surfaces coated with the ${\text{PS–ZrO}}_2$ after their exposure for 30 days to SME marine–coastal simulate solution. The micro-cracks that appeared on the ${\text{PS–ZrO}}_2$ hybrid coating may be considered a consequence of the exerted pressure by ${\mathrm{H}}_2$ bubbles during the localized corrosion of the Mg matrix, which had the possibility to occur because of the initial micro-defects on the coating surface (Figure 1e,f). Once formed, these new micro-cracks may favor the progress of Mg alloy corrosion activity.

The EDS elemental analysis (

Table 4. Elemental analysis (wt.%) of two zones (Figure 10) of PS–ZrO₂–AM60–AlN and PS–ZrO₂–AM60 surface layers after exposure for 30 days to SME solution.

Element	C	O	Na	Mg	Al	Si	S	Cl	Mn	Zr
Zone 1	-	19.36	5.03	2.19	18.79	1.49	-	-	41.61	11.53
Zone 2	14.56	40.22	1.13	18.62	4.04	1.43	1.19	2.05	-	16.76

) revealed two typical zones labeled as “1” and “2” (Figure 10c,d). Zone “1” corresponds to the Al–Mn intermetallic particles and cathodic active, characteristics of the AM60 Mg alloy matrix, which have been stable and resistive against attacks by chloride ions and changes in the pH of the SME solution. This fact allows us to confirm the cathodic activity of AlMn, previously reported [15,16,17]. Zone “2” presented the composition of the hybrid layer of ${\text{PS–ZrO}}_2$ deposited on the Mg alloys after immersion for 30 days in the SME solution (Table 4).

The elemental mapping of ${\text{PS–ZrO}}_2$-coated AM60–AlN nanocomposite and AM60 alloy (Figure 11) after 30 days of exposure in SME solution confirmed the presence of elements characteristics of intermetallic cathodic particles of Al–Mn and the attached to them those of the ${\mathrm{ZrO}}_2$, as well as the presence of β–Mg₁₇Al₁₂ cathodic particles, which resisted to the corrosion attacks. It has been reported [84] that the incorporation of ${\mathrm{ZrO}}_2$ in an organic coating has managed to reduce the advance of corrosive species (${\mathrm{Cl}}^{-}$ ions) towards the substrate because ${\mathrm{ZrO}}_2$ (an inorganic element) allowed the formation of a more complete network, obstructing the diffusion of species through the coating. This behavior may be related to the good chemical stability of ${\mathrm{ZrO}}_2$ [85,86].

After the chemical removal of the surface layers, according to the ASTM G1-03 [87] formed during the exposure of the coated and non-coated Mg alloys for 30 days to the SME model solution, the SEM images in Figure 12 show the surface appearance and the EDS analysis of the zone of interest is resumed in

Table 5. Elemental analysis (wt.%) of zones of interest (Figure 12) on the coated and non-coated surfaces after the removal of the layers formed during the exposure for 30 days to SME solution.

Element	C	O	Mg	Al	Si	Mn
Zone 1	2.51	1.14	2.17	36.92	1.18	56.09
Zone 2	4.46	1.82	89.06	4.66	-	-
Zone 3	5.67	2.58	57.64	34.11	-	-

. Zone 1 suggested the presence of the cathodic particles of Al–Mn, while Zone 3 indicates the particles of β–Mg₁₇Al₁₂, which were not attacked by the corrosion process. Zone 2 presents the Mg matrix. The low contents of C and Si have been parts of the organic material of the PS and the coupling agent of TMSPM. The localized attacks were more intensive in the non-coated AM60–AlN nanocomposite and AM60 alloy.

Additional mapping (Figure 13) confirmed the presence of those characteristic particles, as suggested above (Table 5).

Figure 14 groups several SEM images of the cross-sections of the surfaces of the AM60–AlN (Figure 14a) and $\mathrm{AM}60$ (Figure 14b) magnesium alloys coated with ${\text{PS–ZrO}}_2$ deposits. The visualized depths of the localized attack towards the matrix have average values of depth of penetration of ≈28.08 µm on the AM60–AlN surface and ≈17.90 µm on the $\mathrm{AM}60$. In the absence of the studied hybrid deposit, the average penetration depths were ≈175.70 µm and ≈121.40 µm, respectively, reaching a maximum in the nanocomposite AM60–AlN of ≈246.40 µm and ≈178.00 µm on the AM60 surfaces after the exposure for 30 days to SME model solution (Figure 14c,d) [14]. The comparison of these results allowed us to consider a reduction in the localized attack by ≈85% due to the protective effect of the ${\text{PS–ZrO}}_2$ deposit against the chloride attack of the marine–coastal environment (SME model solution).

However, because of the available defects on the hybrid coating surface (Figure 1e,f), the chloride ions (SME solution) and oxygen diffusion processes were facilitated, and they were able to penetrate through the hybrid material and attack the Mg matrix. On the other hand, the ${\mathrm{ZrO}}_2$, a good semiconductor [88], could serve as local cathodes, which were not attacked during the corrosion process and were maintained on the Mg alloys’ surfaces after the removal of the corrosion layers.

3.5. Electrochemical Impedance Spectroscopy

The electrochemical impedance, visualized using Bode and Nyquist diagrams (EIS, a non-destructive technique), was elaborated to characterize the interface of the hybrid-coated Mg alloys after their exposure for 1 day and 15 days to SEM marine environment model solution (Figure 15). The Nyquist diagrams (Figure 15a,b) revealed two capacitive semi-circles associated with two time constants at higher and medium frequencies (HF and MF), respectively. The diameters of the HF-capacitive loops were associated with the particularities of the formed corrosion layer on the Mg matrix in the presence of the hybrid ${\text{PS–ZrO}}_2$ coating, while the MF-capacitive loops may relate to the charge transfer processes of the hydrogen evolution (${\mathrm{H}}_2$) and the ${\mathrm{Mg}}^{2+}$ released through the double layer [89,90,91]. The Bode plots (Figure 15) of the phase angle were found to be in good agreement with the observed changes in the Nyquist diagrams. The phase angle between ${70}^{°}$ and ${80}^{°}$ confirmed that the interfaces of the studied alloys are capable of accumulating electrical charges, which in fact, will complicate the mass transfer process through the electrode interface and, consequently, the progress in the corrosion process because of the hybrid PS–ZrO₂ coating.

The quantification of the EIS data, which characterized the activity of the coated Mg alloys, was carried out according to the equivalent circuit (EC) present in Figure 16 and the values are summarized in

Table 6. Fitting parameters from EIS data of PS–ZrO₂-hybrid-coated AM60–AlN and AM60 surfaces after their immersion for 1 and 15 days in SME chloride solution.

	PS–ZrO2–AM60–AlN
Time (Days)	${\mathbf{R}}_{\mathbf{s}}$ $\left({\mathsf{\Omega } \mathbf{cm}}^2\right)$	${\mathbf{CPE}}_1$ $\left({\mathsf{\mu }\mathbf{S} \mathbf{s}}^{\mathbf{n}}{ \mathbf{cm}}^{-2}\right)$	${\mathbf{n}}_1$	${\mathbf{R}}_1$ $\left(\mathbf{k}\mathsf{\Omega } {\mathbf{cm}}^2\right)$	${\mathbf{CPE}}_2$ $\left({\mathsf{\mu }\mathbf{S} \mathbf{s}}^{\mathbf{n}}{ \mathbf{cm}}^{-2}\right)$	${\mathbf{n}}_2$	${\mathbf{R}}_2$ $\left(\mathbf{k}\mathsf{\Omega } {\mathbf{cm}}^2\right)$	${\mathbf{R}}_{\mathbf{p}}$ $\left(\mathbf{k}\mathsf{\Omega } {\mathbf{cm}}^2\right)$
1	69.16 ± 0.54	7.85 ± 0.21	0.91 ± 0.01	7.22 ± 0.14	0.66 ± 0.13	0.84 ± 0.11	2.10 ± 0.27	9.32 ± 0.30
15	71.35 ± 0.49	37.18 ± 0.64	0.88 ± 0.01	11.36 ± 0.24	3.97 ± 0.53	0.99 ± 0.13	2.61 ± 0.24	13.97± 0.37
	${\mathbf{PS}\mathbf{–}\mathbf{ZrO}}_{\mathbf{2}}\mathbf{–}\mathbf{AM}\mathbf{60}$
1	89.93 ± 0.73	3.76 ± 0.08	0.88 ± 0.01	10.63 ± 0.27	0.26 ± 0.02	0.84 ± 0.05	4.75 ± 0.26	15.35 ± 0.37
15	80.20 ± 0.54	39.48 ± 0.66	0.88 ± 0.01	13.23 ± 0.25	3.361 ± 0.55	0.99 ± 0.17	3.93 ± 0.22	17.16± 0.33

, and they were compared to those of non-coated surfaces (

Table 7. Fitting parameters from EIS data of non-coated AM60–AlN and AM60 after their immersion for 1 and 15 days in SME solution.

	AM60–AlN
Time (Days)	${\mathbf{R}}_{\mathbf{s}}$ $\left({\mathsf{\Omega } \mathbf{cm}}^2\right)$	${\mathbf{CPE}}_1$ $\left({\mathsf{\mu }\mathbf{S} \mathbf{s}}^{\mathbf{n}}{ \mathbf{cm}}^{-2}\right)$	${\mathbf{n}}_1$	${\mathbf{R}}_1$ $\left(\mathbf{k}\mathsf{\Omega } {\mathbf{cm}}^2\right)$	${\mathbf{CPE}}_2$ $\left({\mathbf{mS} \mathbf{s}}^{\mathbf{n}}{ \mathbf{cm}}^{-2}\right)$	${\mathbf{n}}_2$	${\mathbf{R}}_2$ $\left(\mathbf{k}\mathsf{\Omega } {\mathbf{cm}}^2\right)$	${\mathbf{R}}_{\mathbf{p}}$ $\left(\mathbf{k}\mathsf{\Omega } {\mathbf{cm}}^2\right)$
1	59.89 ± 0.45	10.49 ± 0.26	0.93 ± 0.01	7.00 ± 0.15	0.54 ± 0.08	0.87 ± 0.08	2.72 ± 0.28	9.77 ± 0.32
15	66.56 ± 0.43	43.23 ± 0.72	0.92 ± 0.04	9.23 ± 0.16	6.51 ± 0.38	0.97 ± 0.22	2.17 ± 0.22	11.40 ± 0.27
	$\mathbf{AM}\mathbf{60}$
1	68.62 ± 0.50	11.45 ± 0.31	0.94 ± 0.01	6.12 ± 0.14	0.48 ± 0.08	0.86 ± 0.084	2.37 ± 0.24	8.49 ± 0.28
15	71.21 ± 0.46	39.86 ± 0.67	0.93 ± 0.01	10.06 ± 0.16	6.13 ± 0.47	0.97 ± 0.225	2.45 ± 0.25	12.51 ± 0.30

). The EC includes the following components: Rs is the solution resistance; ${\mathrm{R}}_1$ denotes the resistance of the layer on the metal substrate, and the constant phase element ${\mathrm{CPE}}_1$ denotes the “capacitance”, representing the hybrid coating and the formed corrosion layer later with the progress in the corrosion process; and ${\mathrm{R}}_2$ and ${\mathrm{CPE}}_2$ as “capacitance” are characteristic of the charge transfer process at the coated substrate/electrolyte interface [90,92,93]. The values of Rp (polarization resistance) were calculated as Rp = ${\mathrm{R}}_1+{\mathrm{R}}_2$.

The comparison of the ${\mathrm{R}}_{\mathrm{p}}$ values (Table 6) allowed us to consider that the hybrid coating of ${\text{PS–ZrO}}_2$ deposited on the Mg alloy AM60 surface can be attributed to greater resistance against the corrosion process, presenting an increase in its Rp by 37% (at 15 days in SME model solution), compared to that of 22% for the coated composite AM60–AlN. In the case of non-coated AM surfaces, the Rp of AM60 was 9% higher than that of the AM60–AlN (Table 7). These facts were related to the AlN hydroxide phase’s solubility, which raises the pH in the range of 5.5–12; during the exposure of the coated AM alloys, the pH of the SME model solution was ≈8.5 (Figure 8). In the presence of chloride ions, it has been reported that the AlN may transform into $\mathrm{Al}{\left(\mathrm{OH}\right)}_3$ [19].

4. Conclusions

• A hybrid coating of polystyrene (PS)–${\mathrm{ZrO}}_2$ material was developed by the sol–gel technique and deposited by spin-coating method on AM60 and nanocomposite AM60–AlN magnesium alloy surfaces to enhance the corrosion resistance in marine environments.
• The ${\text{PS–ZrO}}_2$ coating was dispersed homogeneously on the alloy substrates, presenting isolated micro–nano-structure defects with air trapped inside, which led to an increase in roughness of ≈4 times. The average thickness of the hybrid coating was ≈$305 \pm 20 \mathrm{nm}.$ The XRD patterns revealed no crystalline structure of the hybrid organic–inorganic coating. The deposit of ${\text{PS–ZrO}}_2$ reduced the contact angle of the Mg substrates, and their wettability was close to the hydrophobic border (θ_CA 90°–94°), associated with the hydroxyl groups of Zr–OH and Si–OH incomplete condensation.
• During the exposure of the hybrid-coated substrates for 30 days to SME solution, simulating marine–coastal environment, the initial value of pH = 7.94 shifted to a more alkaline pH ≈ 8.54 because the SME solution caused corrosion of the Mg matrix, which occurred in those sites where the hybrid material of ${\text{PS–ZrO}}_2$ presented some micro-defects and the ${\mathrm{H}}_2$ bubbles continued to come out.
• The results indicated that the hybrid ${\text{PS–ZrO}}_2$ coating reduced the release of ${\mathrm{Mg}}^{2+}$ by approximately 90% and 91% compared to that of non-coated AM magnesium alloy substrates, because of the obstructed micro-localized defects by corrosion products, which impaired the diffusion of chloride ions through the Mg matrix.
• After the chemical removal of the surface layers formed during the exposure to SME solution, the SEM images showed that the localized pitting attack occurred in the vicinity of the Al–Mn and β–Mg₁₇Al₁₂ intermetallic cathodic particles, suggested by EDS analysis.
• Cross-section images revealed that the average value of depth of penetration (≈23 µm) was reduced by ≈85% compared to that of non-coated substrates due to the protective effect of the ${\text{PS–ZrO}}_2$ hybrid coating on AM magnesium alloy substrates exposed to marine–coastal simulated ambient (SME).
• The polarization values of ${\mathrm{R}}_{\mathrm{p}}$ calculated from EIS indicated that the ${\mathrm{R}}_{\mathrm{p}}$ of the ${\text{PS–ZrO}}_2$-coated AM60 alloy increased by 37% and that of the composite AM60–AlN increased by 22%; these values were considered as a protection gain against the corrosion in the presence of chloride ions.
• The corrosion protection efficiency of the hybrid ${\text{PS–ZrO}}_2$ against the presence of chlorides should be improved by modifying the concentration of the precursors and/or applying a drying process that uses a temperature program ramp.