coatings Article Hybrid Coating of Polystyrene–ZrO2 for Corrosion Protection of AM Magnesium Alloys Luis Chávez 1 , Lucien Veleva 1, * , Diana Sánchez-Ahumada 2 and Rafael Ramírez-Bon 3 1 2 3 * Applied Physics Department, Center for Research and Advanced Studies (CINVESTAV-Merida), Merida 97310, Yucatan, Mexico; luis.chavez@cinvestav.mx Facultad de Ingeniería Mochis, Universidad Autónoma de Sinaloa, Fuente de Poseidón y Prol. Angel Flores, S.N., Los Mochis 81223, Sinaloa, Mexico; diana.sanchez.fim@uas.edu.mx Centro de Investigación y de Estudios Avanzados (CINVESTAV-Querétaro), Fracc. Real de Juruquilla, Querétaro 76230, Querétaro, Mexico; rrbon@cinvestav.mx Correspondence: veleva@cinvestav.mx; Tel.: +52-999-9429477 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. PS–ZrO2 with an average thickness of ≈305 ± 20 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 (θCA = 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 Mg2+ from the PS–ZrO2 -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–Mg17 Al12 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. Citation: Chávez, L.; Veleva, L.; Sánchez-Ahumada, D.; Ramírez-Bon, R. Hybrid Coating of Keywords: hybrid organic–inorganic coating; spin-coating; sol–gel process; magnesium–aluminum alloys; corrosion test Polystyrene–ZrO2 for Corrosion Protection of AM Magnesium Alloys. Coatings 2023, 13, 1059. https:// doi.org/10.3390/coatings13061059 Academic Editor: Alexander Modestov Received: 13 May 2023 Revised: 25 May 2023 Accepted: 5 June 2023 Published: 7 June 2023 Copyright: © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 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 (CO2 and NOx ) 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–Mg17 Al12 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 Coatings 2023, 13, 1059. https://doi.org/10.3390/coatings13061059 https://www.mdpi.com/journal/coatings Coatings 2023, 13, 1059 2 of 20 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 [7–9] and lower roughness (≈15%) of the surface. The properties of the manufactured AM60–AlN nanocomposite have been previously described [10–12]. 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 [15–18], 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 Al(OH)3 corrosion products, confirmed by XPS analysis [14]. Consequently, the presence of 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 [23–25], as well as organic coatings [21,26], superhydrophobic [27,28], and organic– inorganic hybrid coating [29–31]. The hybrid coatings elaborated through the sol–gel methodology [32–34] 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 (SiO2 , TiO2 , and ZrO2 ) have been proposed [36–45]. The polystyrene (PS) polymer has participated as the organic part in combination with SiO2 , ZrO2 , Al2 O3 , MnO2 , and TiO2 in hybrid composites, which have been applied as dielectric materials and in searching for surface hydrophobicity or for better optoelectronic properties [46–53]. 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 (PS–ZrO2 ). The hybrid material was deposited by spin-coating on the AM60 alloy and AM60–AlN nanocomposite metallic substrates, which Coatings 2023, 13, 1059 3 of 20 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 PS–ZrO2 surface morphology and composition, as well as their change after the exposure to SME, were performed by scanning electron microscopy and energydispersive 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 ZrO2 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 [55–58]. The methodology for the synthesis of PS–ZrO2 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 PMMA–SiO2 , PMMA–TiO2 , and PMMA–ZrO2 [34,36–39,42,59]. In this study, zirconium isopropoxide (Zr(OPr)4 ) 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 (Zr (OPr)4 : EtOH : HNO3 ) 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:H2 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 PS–ZrO2 are summarized in Table 1. The three resulting solutions were mixed to obtain a homogeneous hybrid solution. Table 1. Reagents used for the synthesis of the hybrid PS–ZrO2 . Precursors Solvents Catalysis Anti-Inhibitor Initiator Zirconium isopropoxide (Zr (OPr)4 ) Anhydrous ethanol (EtOH) Nitric acid (HNO3 ) Sodium hydroxide (NaOH) Benzyl peroxide (BPO) Styrene monomer (ST) Deionized water (H2 O) Hydrochloric acid HCl 3(trimethoxysilyl)propyl methacrylate (TMSPM) 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 PS–ZrO2 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 Coatings 2023, 13, 1059 4 of 20 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 PS–ZrO2 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 ◦ 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 PS–ZrO2 hybrid were immersed in 20 mL of simulated marine environment (SME) solution (Table 2), 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 Mg2+ ions into the solution (Hanna Instruments, HI83200, Woonsocket, RI, USA) were measured. Table 2. Composition of simulated marine–coastal environment (SME, pH = 7.94) [14]. Reagents Concentration NaCl 5.84 g L−1 Na2 SO4 4.09 g L−1 NaHCO3 0.20 g L−1 2.5. SEM-EDS, XPS, and XRD Surface Analysis The morphology and composition of the PS–ZrO2 -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 PS–ZrO2 -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 cm2 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 Coatings 2023, 13, 1059 5 of 20 Figure 1 presents the homogeneous morphology of the spin-coated PS–ZrO2 , well dispersed on the Mg–Al alloy surfaces of AM60–AlN (Figure 1a,c,e) and AM60 (Figure 1b,d,f). Table 3 summarizes the EDS elemental analysis (wt.%) of several randomly se3. Results and Discussion lected areas, labeled as “zones” in Figure 1, in which the elemental content is mainly the 3.1. Surface Characterization PS–ZrO2 Hybrid Coating same. 3.1.1. SEM-EDS Analysis A magnification (×20,000) of zones (Figure 1c,d) suggested that there is air, probably Figure 1 presents the homogeneous morphology of thewhich spin-coated wellcontact distrapped inside the micro/nanostructures of the coating, couldPS–ZrO reduce2 , the persed on the Mg–Al alloy surfaces of AM60–AlN (Figure 1a,c,e) and AM60 (Figure 1b,d,f). between aggressive substances and alloy surface in the air sites, providing corrosion reTable 3 summarizes the EDS elemental analysis (wt.%) of several randomly selected areas, sistance to the alloy [62]. labeled as “zones” in Figure 1, in which the elemental content is mainly the same. Figure 1. 1.SEM images of of thethe hybrid coating Figure SEM images hybrid coatingdeposited depositedon ontwo twodifferent differentMg–Al Mg–Al alloy alloy surfaces: surfaces: (a) PS–ZrO –AM60–AlN (×1000-SEI mode); (b) PS–ZrO –AM60 (×1000-SEI mode); (c) Image 1a 2 2 (a) PS–ZrO 2 –AM60–AlN (×1000-SEI mode); (b) PS–ZrO2 –AM60 (×1000-SEI mode); (c) Image 1a (×20,000-SEI mode); (d) Image 1b (×20,000-SEI mode); (e) Image 1a (×15,000-LABE mode); (f) Image (×20,000-SEI mode); (d) Image 1b (×20,000-SEI mode); (e) Image 1a (×15,000-LABE mode); (f) Image 1b (×15,000-LABE mode). 1b (×15,000-LABE mode). Table of several several randomly randomlyselected selectedzone zone areas (Figure 1) the of the Table3.3.EDS EDSelemental elementalanalysis analysis (wt.%) (wt.%) of areas (Figure 1) of PS–ZrO –AM60–AlN (Zones 1–2) and PS–ZrO –AM60 (Zones 3–4) tested samples. 2 2 PS–ZrO –AM60–AlN (Zones 1–2) and PS–ZrO –AM60 (Zones 3–4) tested samples. 2 Element Element Zone 1 Zone 1 Zone 2 Zone 2 Zone 3 Zone 3 Zone 4 Zone 4 2 CC 20.24 20.24 19.87 19.87 18.17 18.17 18.19 O O 19.60 19.60 19.39 19.39 20.25 20.25 20.16 Mg 46.06 46.06 45.58 45.58 47.16 47.16 46.66 Al 2.20 2.20 3.03 3.03 1.78 1.78 2.10 Si 2.72 2.72 2.85 2.85 2.78 2.78 2.96 Zr 9.17 9.17 9.28 9.28 9.85 9.85 9.93 18.19 20.16 46.66 2.10 2.96 9.93 Mg Al Si Zr Despite the good dispersion of the PS–ZrO2 coating on the studied Mg–Al alloy surA magnification (×20,000) of zones (Figure 1c,d) suggested that(Figure there is1e,f) air, probably faces, some isolated areas of micro-defects have been observed that could trapped inside the micro/nanostructures of the coating, which could reduce contactthe generate channels connecting the SME solution with the alloy substrate andthe facilitate − between aggressive substances and alloy surface in the air sites, providing corrosion passage of Cl , for example. EDS analysis revealed a high C content (Table 3), correspondresistance to the alloy [62]. ing to the organic polystyrene (PS, (C8 H8 )n ), as a part of the hybrid coating, and a high Despite the good dispersion of the PS–ZrO2 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 Cl− , for example. EDS analysis revealed a high C content (Table 3), corresponding to the organic polystyrene (PS, (C8 H8 )n ), as a part of the hybrid coating, and a high content of O and Zr, confirming the presence of ZrO2 , 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 Coatings 2023, 13, 1059 content of O and Zr, confirming the presence of ZrO2 , the inorganic part of the coatin The presence of silicon (Si) in all zones was ascribed to the coupling agent (3-(trimetho ysilyl)ofpropyl acted as link2 ,between the organic inorganic com content O andmethacrylate), Zr, confirmingwhich the presence ofaZrO the inorganic part of and the coating. 6 of 20 ponents of of the hybrid The decrease in Mg and Al(3-(trimethoxcontents (not corr The presence silicon (Si)PS–ZrO in all zones was ascribed to the coupling agent 2 coating. ysilyl) propyl methacrylate), which acted as a link between the organic and inorganic comsponding to those of the Mg–Al tested alloys) was due to the physical barrier provided b ponents of thePS–ZrO hybrid 2PS–ZrO the hybrid coating. 2 coating. The decrease in Mg and Al contents (not correof theThe hybrid PS–ZrO coating. The decrease inwas Mg andto Althe contents (notbarrier corresponding toby sponding toC, those of the Mg–Al tested alloys) physical 2 Si, O, and Zr mappings (Figure 2)due show the distribution of provided these elements an those of the Mg–Al tested alloys) was due to the physical barrier provided by the hybrid the hybrid PS–ZrO coating. 2 along the coated alloy surfaces of PS–ZrO2 –AM60–AlN (Figure 2a) an their contribution PS–ZrO 2 coating. The C, Si, O, and Zr mappings 2) show the distribution of these elements andwere d PS–ZrO –AM60 (Figure 2b). The(Figure reported AlMn and β-Mg17 Al particles [13,14] 2C, The Si, O,along and Zrthe mappings (Figure 2) show distribution of 12 these elements and their contribution coated alloy surfaces ofthe PS–ZrO –AM60–AlN (Figure 2a) and 2 tected under thealong coating. their contribution the coated alloy surfaces of PS–ZrO2 –AM60–AlN (Figure 2a) and PS–ZrO2 –AM60 (Figure 2b). The reported AlMn and β-Mg17 Al12 particles [13,14] were dePS–ZrO2 –AM60 (Figure 2b). The reported AlMn and β–Mg17 Al12 particles [13,14] were tected under the the coating. detected under coating. Figure 2. SEM images (×1000) and maps of elements on (a) PS–ZrO2 –AM60–AlN - and (b) P Figure SEMSEM images (×1000) and maps of elements on (a)on PS–ZrO - and (b) PS– 2 –AM60–AlN Figure 2. images (×1000) and maps of elements (a) PS–ZrO ZrO22. –AM60-coated surfaces. 2 –AM60–AlN- and ZrO –AM60-coated surfaces. (b)2 PS–ZrO –AM60-coated surfaces. 2 The cross-sectional SEM images (Figure 3) showed three well-defined zones corr The 3) showed showed three threewell-defined well-definedzones zonescorrecorreThecross-sectional cross-sectionalSEM SEMimages images (Figure (Figure 3) sponding tothe the epoxy resin (upper zone), hybrid coating of PS–ZrO (central zone), an 2 zone), sponding epoxy hybrid coating PS–ZrO zone), and spondingtotothe epoxyresin resin(upper (upper zone), zone), hybrid coating ofofPS–ZrO and 2 (central 2 (central Mg matrix (lower zone). Mg matrix(lower (lowerzone). zone). Mg matrix Figure 3. Cross-sectional SEM images of Mg–Al coated with hybrid PS–ZrO2 : (a) AM60–AlN (×20,000) and (b) AM60 (×20,000). Figure3. 3. Cross-sectional Cross-sectional SEM images of Mg–Al with PS–ZrO hybrid2 :PS–ZrO Figure SEM images of Mg–Al coatedcoated with hybrid (a) AM60–AlN 2 : (a) AM60–A (×20,000) and (b) AM60 (×20,000). (×20,000) and (b) AM60 (×20,000). The thickness of the PS–ZrO2 coatings on the AM60–AlN surface was ≈280 ± 25 nm, The thickness of the PS–ZrO on the AM60–AlN surface ≈280 ± 25differnm, 2 coatings while on AM60, it was ≈330 ±PS–ZrO 16 nm; the difference was ascribed to was the roughness The thickness of the 2 coatings on the AM60–AlN surface was ≈280 ± 25 nm while on AM60, it was ≈ 330 ± 16 nm; the difference was ascribed to the roughness ence in the The EDS mappings, carried to outthe in the yellow diffe while onstudied AM60, alloy it wassurfaces. ≈330 ± 16 nm; theelement difference was ascribed roughness difference in the studied alloy surfaces. The EDS element mappings, carried out in the marked zones (Figure 3a,b), upper zonesmappings, present a high carbon content, ence in the studied alloyindicated surfaces.that Thethe EDS element carried out in the yello yellow marked zones (Figure 3a,b), indicated that the upper zones present a high carbon marked zones (Figure 3a,b), indicated that the upper zones present a high carbon conten Coatings 2023, 13, 1059 corresponding to the epoxy resin, while in the central region, there is a set of elements, 7 of 20 coatsuch as C, O, and Zr presenting the organic and inorganic components of the hybrid ing; in the lower zone, the high contents of Mg and Al correspond to the Mg–Al matrix of the AM60 and AM60–AlN alloys. 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 3.1.2. X-ray Photoelectron Spectroscopy (XPS) hybrid coating; in the lower zone, the high contents of Mg and Al correspond to the Mg–Al The of XPS of the PS–ZrOalloys. 2 hybrid coating deposited on the AM60–AlN nanomatrix thespectra AM60 and AM60–AlN composite and AM60 alloy substrates were similar (Figure 4), and they were analyzed 3.1.2.on X-ray Spectroscopy based the Photoelectron binding energies of C1s, (XPS) Si2p, Zr3d, and O1s. The C1s signal revealed the The XPS of the PS–ZrO coatingbonds: deposited onand the AM60–AlN nanocomcontribution ofspectra three peaks, ascribed to several C–C C–H (at 284.8 eV) of hy2 hybrid and they were analyzed on eV) posite andand AM60 alloy groups, substratescharacteristic were similar (Figure drocarbon phenyl of the 4), polystyrene [63]; C–O–Cbased (at 286.0 the binding energies of C1s, Si2p, Zr3d, and O1s. The C1s signal revealed the contribution of the ether groups; and O–C=O (288.90 eV) of the double ester group, belonging to of three peaks, ascribed to several bonds: C–C and C–H (at 284.8 eV) of hydrocarbon and TMSPM coupling agent [29,64]. The binding energy of Si2p (at 102.3 eV) was attributed to phenyl groups, characteristic of the polystyrene [63]; C–O–C (at 286.0 eV) of the ether the Si–O bond of the coupling agent (TMSPM) [48,65]. The high-resolution spectrum of groups; and O–C=O (288.90 eV) of the double ester group, belonging to TMSPM coupling Zr3d3bond Zr3d presents doublet two spin–orbital components, Zr3d5⁄2toand ⁄2 , whose agent [29,64]. aThe bindingofenergy of Si2p (at 102.3 eV) was attributed the Si–O binding approximately 182.5 and 184.9 eV, respectively, where shift indiThe high-resolution spectrum of Zr3d the presents of the energies coupling are agent (TMSPM) [48,65]. 4+ 0+ and Zr3d3⁄2 cates the presence of Zr species [66], considering that for , thebinding Zr3d5⁄2energies a doublet of two spin–orbital components, Zr3d5/2 and Zr3d , whose 3/2Zr are approximately 182.5 and 184.9 eV, where therespectively shift indicates theThe presence components have binding energies ofrespectively, 178.7 and 181.1 eV, [67]. species of 4+ 0+ 4+ of Zr species [66], considering that for Zr , the Zr3d and Zr3d components have 5/2 2 to form 3/2 Zr–O–Zr and Zr–OH Zr have allowed the interaction of the inorganic ZrO 4+ binding energies of 178.7 and 181.1 eV, respectively [67]. The2− species of Zr have allowed bonds as a consequence of the ionization of the species O and OH− (the peaks of O1s at the interaction of the inorganic ZrO2 to form Zr–O–Zr and Zr–OH bonds as a consequence 530.1 and 531.1 eV, respectively) [68,69]. The O1s binding energy at 531.8 eV was ascribed of the ionization of the species O2− and OH− (the peaks of O1s at 530.1 and 531.1 eV, to the Si–O–Zr bonds of theenergy abundance ofeV Si–OH groups present in the silane respectively) [68,69].[70] Thebecause O1s binding at 531.8 was ascribed to the Si–O–Zr coupling agent (TMSPM) after its hydrolysis; the energies located at 532.6 eV and 533.7 bonds [70] because of the abundance of Si–OH groups present in the silane coupling agent eV(TMSPM) belong to theitscharacteristic oflocated ether and ester, respectively, as a part after hydrolysis; thegroups energies at 532.6 eV and 533.7 eV belong to theof the characteristic groups of ether and ester, respectively, part of the TMSPM [71]. The Mg1s TMSPM [71]. The Mg1s binding energy at 1304.5 as eVa corresponds to the Mg–O bonds, by binding energy at 1304.5 eV corresponds to the Mg–O bonds, by which the hybrid coating which the hybrid coating was attached to the Mg matrix [72]. was attached to the Mg matrix [72]. Figure 4. XPS spectra ofof PS–ZrO andAM60 AM60magnesium magnesium substrates: (a) Figure 4. XPS spectra PS–ZrO2 2deposited deposited of of the the AM60–AlN AM60–AlN and substrates: C1s, (b) Si2p, (c) Zr3d, (d) O1s, and (e) Mg1s. (a) C1s, (b) Si2p, (c) Zr3d, (d) O1s, and (e) Mg1s. 3.1.3. XRD Analysis 3.1.3. XRD Analysis Diffraction patterns shown in Figure 5 of the AM60–AlN and AM60 substrates, before Diffraction patterns shown in Figure 5 of the AM60–AlN and AM60 substrates, before and after being coated with the hybrid PS–ZrO2 , did not reveal the presence of crystalline and after being coated the hybrid PS–ZrO2 , peaks did not the presence of crystalline structure in the hybridwith material; the characteristic of reveal Mg, Al–Mn, and β–Mg 17 Al12 structure in previously the hybriddetected material; characteristic peaks Mg, Al–Mn, β-Mg Al12 have been andthe reported [13]. It has been of suggested that if and the ZrO 2 is17 have been previously detected and reported [13]. It has been suggested that if the ZrO2 is Coatings 2023, 13, 1059 present as an amorphous phase, it could reduce the sites for the diffusion of the Cl− ions through and, thereby, improve corrosion resistance the metal present such as anfilm amorphous phase, it couldthe reduce the sites for theofdiffusion ofsubstrate the Cl− io [73]. through such film and, thereby, improve the corrosion resistance of the −metal substra 8 of 20 present as an amorphous phase, it could reduce the sites for the diffusion of the Cl ions [73]. such film and, thereby, improve the corrosion resistance of the metal substrate [73]. through Figure 5. XRD patterns of non-coated and PS–ZrO2 -hybrid-coated substrates. Figure 5.5.XRD patterns of and non-coated and PS–ZrO substrates. 2 -hybrid-coated Figure XRD patterns of non-coated and PS–ZrO 3.2. Surface Roughness Contact Angle 2 -hybrid-coated substrates. (RaAngle ) of the AM60–AlN nanocomposite and AM60 alloy surThe average roughness 3.2. Surface Roughness and Contact 3.2. Surface Roughness and Angle faces, with and without the(Contact coating of PS–ZrO in Figure 6. It has The average roughness AM60–AlN nanocomposite and AM60 alloy surRhybrid 2 , are compared a ) of the (R ) The average roughness of the AM60–AlN nanocomposite and AM60 been reported that the introduction of aluminum nitride (AlN) nanoparticles favored faces, with and without the hybrid acoating of PS–ZrO2 , are compared in Figure 6. It has alloyasu been reported thatwithout the introduction of aluminum nanoparticles favored a of6.the reduction in and grain size, which in fact, allowedofnitride a PS–ZrO slight(AlN) decrease in the roughness faces, with the hybrid coating compared in Figure It h 2 , are 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 PS–ZrO been reported that the introduction of aluminum nitride (AlN) nanoparticles favored 2 AM60 alloyain of approximately 15% (Figure 6a,b) [13,14]. The hybrid of PS–ZrO 2 presented 5% higher roughness AM60 thancoating on in AM60–AlN because reduction grain size, which invalue fact,deposited allowed aonslight decrease the roughness of t presented a 5% higher roughness valueSuch deposited on AM60 than onofAM60–AlN because of has of the initial difference. different roughness the PS–ZrO AM60 alloyroughness of approximately 15% (Figure 6a,b) [13,14]. The hybrid coating of PS–Zr 2 deposits the initial roughness difference. Such different roughness of the PS–ZrO2 deposits has led led to the presence of more or less trapped air inside the micro/nano-structured PS–ZrO 2 presented a 5% higher roughness value deposited on AM60 than on AM60–AlN to the presence of more or less trapped air inside the micro/nano-structured PS–ZrO2 as becau as sites of micro-defects (Figure 1e,f). of the initial roughness difference. Such different roughness of the PS–ZrO2 deposits h sites of micro-defects (Figure 1e,f). led to the presence of more or less trapped air inside the micro/nano-structured PS–Zr as sites of micro-defects (Figure 1e,f). Figure 6. Roughness Roughnesssurface surface values (Ra) of AM60–AlN, (a) AM60–AlN, (b) AM60, (c) PS–ZrO and Figure 6. values (Ra) of (a) (b) AM60, (c) PS–ZrO and 2 –AM60–AlN, 2 –AM60–AlN, (d) PS–ZrO –AM60. 2 (d) PS–ZrO2 –AM60. Hydrophobicity ofofany material is isa aproperty ofAM60, wide interest when it is Hydrophobicity anycoating coating material property of wide when it is deFigure 6. Roughness surface values (Ra) of (a) AM60–AlN, (b) (c)interest PS–ZrO 2 –AM60–AlN, a deposited on a metal surface as a protective material, thereby reducing contact with posited on a2 –AM60. metal surface as a protective material, thereby reducing contact with aahumid (d) PS–ZrO and aggressive aqueous environment and, thus, improving the corrosion resistance of the Hydrophobicity of any coating material is a property of wide interest when it is d posited on a metal surface as a protective material, thereby reducing contact with a hum and aggressive aqueous environment and, thus, improving the corrosion resistance of t Coatings 2023, 13, 1059 9 of 20 (θc ) [74], metal substrate. Through theenvironment measurement theimproving contact angle the material m humid and aggressive aqueous and,of thus, the corrosion resistance of metal substrate. Through of the contact (θc ) [74], (θthe (90° < angle bethe classified as hydrophilic 90°) , hydrophobic θc < 150°) , or the superhydr c < measurement ◦ ), hydrophobic (90◦ < θ < 150◦ ), or θ < 90 material may be classified as hydrophilic ( c c phobic (θc > 150°) [75–79]. ◦ [75–79]. superhydrophobic (θc > 150 Figure 7 presents the )recorded images of the contact angle (CA) of deionized wat 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 (Figu drops on non-coated AM60–AlN nanocomposite and the AM60 alloy surfaces (Figure 7a,b) 7a,b) compared thosewith coated with the hybrid deposit(Figure of PS–ZrO compared to thoseto coated the hybrid deposit of PS–ZrO 7c,d).2 (Figure 7c,d). 2 Figure angle (CA) of deionized waterwater drops drops on the on surfaces of (a) AM60–AlN, (b) AM60, (b) AM6 Figure7.7.Contact Contact angle (CA) of deionized the surfaces of (a) AM60–AlN, (c) PS–ZrO –AM60–AlN, and (d) PS–ZrO –AM60. (c) PS–ZrO 2 2 –AM60–AlN, and (d) PS–ZrO 2 2 –AM60. The CA values revealed that the nature of the uncoated surfaces of the tested Mg– The CA values revealed that the nature of the uncoated surfaces of the tested Mg– Al alloys is hydrophobic (θCA > 90◦ ) [74]: the AM60–AlN nanocomposite presented a alloys is hydrophobic (θCA◦>(Figure 90°) [74]: the AM60–AlN nanocomposite presented a conta 7a), whereas 110.70 ± 1.70◦ was obtained for the contact angle of 111.76 ± 2.93 angle alloy of 111.76 ± 2.93° (Figure 7a), 110.70 ± 1.70° was obtained for the AM60 a 7b). However, afterwhereas the deposit of the PS–ZrO AM60 (Figure 2 hybrid coating, the loy (Figure 7b). However, after the deposit of the PS–ZrO hybrid the wettabili wettability of the surfaces changed, presenting a reduction in the angle values: 2 contactcoating, ◦ for CA = 83.37 ± 0.86 (Figure 7c), which close values: to the for t of the thePS–ZrO surfaces changed, the presenting a reduction in the contactwas angle 2 –AM60–AlN, ◦ ); and for PS–ZrO –AM60, the CA value was 93.60 ± 1.87◦ hydrophobic border (θCA < 2 (Figure 7c), which was close to the hydr PS–ZrO2 –AM60–AlN, the90CA = 83.37 ± 0.86° (Figure with(θ a wettability still in the hydrophobic range. The change in the contact phobic7d), border CA < 90°); and for PS–ZrO2 –AM60, the CA value was 93.60 ± 1.87° (Fi angle of the coated surfaces may attribute to the increase in the PS–ZrO2 surface roughness ure 7d), with a wettability still in the hydrophobic range. The change in the contact ang (Figure 7c,d), according to the suggestions of Wenzel [80] and Caxie–Baxter [81,82]. In the of the coated surfaces may attribute to the increase in the PS–ZrO2 surface roughness (Fi presence of air trapped on the surface (PS–ZrO2 nonuniform surface), the contact with ure 7c,d), to the suggestions Wenzel in [80] In t liquids will according be interrupted; however, it was aofreduction theand CA,Caxie–Baxter associated with[81,82]. the presence of air trapped on the surface (PS–ZrO nonuniform surface), the contact wi hydroxyl groups of Zr–OH and Si–OH present in the inorganic components of the hybrid 2 coating, their incomplete condensation led to aindecrease in the benefits with t liquids due willtobe interrupted; however, it[48,83], was awhich reduction the CA, associated that the microstructure of the hybrid material could provide. hydroxyl groups of Zr–OH and Si–OH present in the inorganic components of the hybr coating, due to their incomplete condensation [48,83], which led to a decrease in the be efits 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 3.3. Monitoring pHSolution = 7.94 shifted to a more alkaline value of pH ≈ 8.64 after 7 days because the SME of solution corrosion 1–3) of the matrix, which occurred in those sites Thecaused change in time(Reactions of SME solution pHMg was monitored for a period of 30 days durin where the hybrid material of PS–ZrO presented some micro-defects (Figure 1e,f) and 2 the immersion of the hybrid-coated Mg–Al alloy samples (Figure 8). The initial value (Reaction 3) continued comeof out. observed behavior has been the 2 bubbles pHH=7.94 shifted to a more alkalinetovalue pHThe ≈ 8.64 after pH 7 days because the SME sol reported previously during the exposure of AM60 and AM60–AlN to SME solution [14]. tion caused corrosion (Reactions 1–3) of the Mg matrix, which occurred in those sit After this period of 7 days, the pH diminished, and this fact was associated with the where theofhybrid of PS–ZrO some micro-defects (Figureand 1e,f) and t 2 presented formation Mg(OHmaterial corrosion product, obstructing those micro-cracks )2 , an insoluble H2 bubbles (Reaction 3) continued to come out. The observed pH behavior has been r ported previously during the exposure of AM60 and AM60–AlN to SME solution [1 After this period of 7 days, the pH diminished, and this fact was associated with the fo mation of Mg(OH)2 , an insoluble corrosion product, obstructing those micro-cracks an Coatings 2023, 13, 1059 10 of 20 Mg(OH)2 product may suffer a localized attack from the chloride ions (SME solution) an 2+ the initially formed micro-defects, which area acted as Mg a physical hindering be partially dissolved, giving the origin of released ionsbarrier, (Reaction 4) andthe activatin progress of the corrosion process of the Mg matrix. However, in those sites, the insoluble 2+ the corrosion process (an increase in Mg concentration and pH after 10 days. Mg(OH)2 product may suffer a localized attack from the chloride ions (SME solution) and + be partially dissolved, giving the origin of released Mg2+ ions Mg → Mg2+ 2e− (Reaction 4) and activating (ac) (s) 2+ the corrosion process an increase in Mg concentration and pH after 10 days. 2+ − 2+ → Mg(OH) Mg(s) →+Mg OH Mg + 2e− 2↓ (ac) − 2+ − − 2H OOH + 2e H(2(g) Mg →→ Mg OH)+ 2+ 2 ↓2OH(ac) − 2H2 O + 2e− → H2(g) + 2OH− (ac) − Mg(OH)2 + 2Cl → MgCl2 + 2OH → Mg2+ +2Cl− Mg(OH)2 + 2Cl− → MgCl2 + 2OH− → Mg2+ +2Cl− (1 (1) (2 (2) (3 (3) (4 (4) Figure Changein intime time of pHpH during the immersion of PS–ZrO and 2 –AM60–AlNFigure 8. 8. Change of SME SMEsolution solution during the immersion of PS–ZrO 2 –AM60–AlN- an PS–ZrO –AM60-hybrid-coated surfaces for up to 30 days. 2 PS–ZrO2 –AM60-hybrid-coated surfaces for up to 30 days. Figure 9 compares the concentrations of Mg2+ ions released into the SME solution 2+ Figureof9the compares concentrations Mg ions released the SME solution be because progress the in Reaction (4) duringofthe exposure of PS–ZrOinto 2 –AM60–AlN and cause of the progress in Reaction (4)and during the exposure of PS–ZrO PS–ZrO alloys, non-coated coated with the hybrid PS–ZrO22 –AM60–AlN for 30 days to and PS 2 –AM60 1 ± 11.54 mg30L−days of to SM SME solution. From the non-coated alloy surface of AM60, 333.33 ZrO2 –AM60 alloys, non-coated and coated with the hybrid PS–ZrO2 for 2+ −1 Mg ions were while alloy from the AM60–AlN nanocomposite, the concentration solution. From thereleased, non-coated surface of AM60, 333.33 ± 11.54 mg L of Mg2+ ion −1 was 353.33 ± 11.54 mg L [14] because of the shift of pH to more alkaline values, which were released, while from the AM60–AlN nanocomposite, the concentration wa led to an instability−1 of the AlN particles [19]. The progress in the release of the Mg ions has 353.33 11.54 mgwith L the [14]presence becauseofofAl–Mn the shift of pH toparticles, more alkaline values, sites which led t been±associated intermetallic active cathodic an in instability of the particles [19]. The progress in vicinity the release of the Mg(active ions has bee AM60 allow andAlN AM60–AlN nanocomposite, in which the Mg matrix The results indicated that sites i anode) suffers localized corrosion attack [13–18]. associated with accelerated the presence of Al–Mn intermetallic particles, active cathodic PS–ZrO coating deposited on AM60–AlN and AM60 surfaces reduced the hybrid 2 AM60 allow and AM60–AlN nanocomposite, in which vicinity the Mg matrixthe (active an 2+ release of Mg by approximately 89% and 91%, respectively, because of the obstructed ode) suffers accelerated localized corrosion attack [13–18]. The results indicated that th micro-localized sites of defects and impaired diffusion of chloride ions through the formed hybrid PS–ZrO2 coating deposited on AM60–AlN and AM60 surfaces reduced the releas layer of corrosion product. of Mg2+ by approximately 89% and 91%, respectively, because of the obstructed micro 3.4. SEM-EDS the PS–ZrO Alloy Surfaces after Exposure to SMEthe Solution 2 -Coated localized sites ofAnalysis defectsofand impaired diffusion of chloride ions through formed laye The SEM images in Figure 10 illustrate the morphology of the AM60–AlN and AM60 of corrosion product. magnesium alloys’ surfaces coated with the PS–ZrO2 after their exposure for 30 days to SME marine–coastal simulate solution. The micro-cracks that appeared on the PS–ZrO2 hybrid coating may be considered a consequence of the exerted pressure by H2 bubbles Coatings 2023, 13, 1059 11 of 20 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. Figure 9. Change over time of the concentration of Mg2+ ions released from non-coated and coated AM60–AlN and AM60 surfaces with the hybrid deposit of PS–ZrO2 exposed for 30 days to SME solution. 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 PS–ZrO2 after their exposure for 30 days to SME marine–coastal simulate solution. The micro-cracks that appeared on the PS–ZrO2 hybrid coating may be considered a consequence of the exerted pressure by H2 bubbles 2+ had during9.theChange localized of concentration the Mg matrix, which the possibility to occur and beFigure overcorrosion time of Mg ions2+released from non-coated Figure 9. Change over timeofofthethe concentration of Mg ions released from non-coated a cause of the initialand micro-defects on the surface (Figure 1e,f).2Once formed, exposed for 30these days coated AM60–AlN AM60 surfaces withcoating the hybrid deposit of PS–ZrO AM60–AlN and AM60 surfaces with the hybrid PS–ZrO 2 exposed for 30 day new micro-cracks may favor the progress of Mg alloy deposit corrosionofactivity. to SME solution. solution. 3.4. SEM-EDS Analysis of the PS–ZrO2 -Coated Alloy Surfaces after Exposure to SME The SEM images in Figure 10 illustrate the morphology of the AM60–AlN a magnesium alloys’ surfaces coated with the PS–ZrO2 after their exposure for 3 SME marine–coastal simulate solution. The micro-cracks that appeared on the hybrid coating may be considered a consequence of the exerted pressure by H during the localized corrosion of the Mg matrix, which had the possibility to cause of the initial micro-defects on the coating surface (Figure 1e,f). Once form new micro-cracks may favor the progress of Mg alloy corrosion activity. Figure 10. 10. SEM images of of (a) (a) PS–ZrO PS–ZrO22–AM60–AlN –AM60–AlN((×500-SEI (b) PS–ZrO PS–ZrO22–AM60 –AM60 ((×500-SEI Figure SEM images ×500-SEI mode), mode), (b) ×500-SEI mode), (c) PS–ZrO –AM60–AlN (×1000-LABE mode), and (d) PS–ZrO –AM60 (×1000-LABE mode), (c) PS–ZrO22–AM60–AlN (×1000-LABE mode), and (d) PS–ZrO22 –AM60 (×1000-LABE mode) mode) surfaces after exposure for 30 days to SME solution. surfaces after exposure for 30 days to SME solution. The EDS EDS elemental elemental analysis analysis (Table (Table 4) 4) revealed revealed two two typical typical zones zones labeled labeled as “1” and and The as “1” “2” (Figure (Figure 10c,d). 10c,d). Zone Zone “1” “1” corresponds corresponds to to the the Al–Mn particles and and cathodic “2” Al–Mn intermetallic intermetallic particles cathodic active, characteristics characteristics of of the the AM60 alloy matrix, matrix, which which have have been been stable stable and and resistive resistive active, AM60 Mg Mg alloy against attacks by chloride ions and changes in the pH of the SME solution. This factfact alagainst attacks by chloride ions and changes in the pH of the SME solution. This lows us to confirm the cathodic activity of AlMn, previously reported [15–17]. Zone “2” allows us to confirm the cathodic activity of AlMn, previously reported [15–17]. Zone “2” presented the the composition composition of of the the hybrid hybrid layer layer of of PS–ZrO PS–ZrO22 deposited deposited on on the the Mg Mg alloys presented alloys after after immersion for for 30 30 days days in in the the SME SME solution solution (Table (Table 4). 4). immersion Figure 10. SEM images of (a) PS–ZrO2 –AM60–AlN (×500-SEI mode), (b) PS–ZrO2 –AM60 Coatings 2023, 13, 1059 12 of 20 Table zones (Figure (Figure10) 10)ofofPS–ZrO PS–ZrO–AM60–AlN and PS–ZrO 2 –AM60–AlN 2– Table4.4.Elemental Elementalanalysis analysis (wt.%) (wt.%) of of two two zones and PS–ZrO 2 2– AM60 surface layers after exposure for 30 days to SME solution. AM60 surface layers after exposure for 30 days to SME solution. Element C Zone 1 - Zone 2 14.56 Element C O Na Mg Zone 1 19.36 19.36 5.03 2.19 Zone 2 14.56 40.22 O 40.22 1.13 18.62 Na Al 5.03 18.79 1.13 4.04 Mg Al Si Si S 2.19 18.79 1.49 1.49 18.62 4.04 1.43 1.43 1.19 S 1.19 Cl 2.05 Cl Mn 41.61 2.05 - Mn Zr Zr 41.61 11.53 11.53 16.76 16.76 The elemental mapping of PS–ZrO2 -coated AM60–AlN nanocomposite and AM60 alloy (Figure 11) after mapping 30 days ofofexposure in SMEAM60–AlN solution confirmed the presence of eleThe elemental PS–ZrO2 -coated nanocomposite and AM60 30 days of exposure SME solution confirmed presence of alloy characteristics (Figure 11) after ments of intermetallic cathodicinparticles of Al–Mn and thethe attached to them elements of Al–Mn and thewhich attached to those of thecharacteristics ZrO2 , as wellof asintermetallic the presencecathodic of β-Mgparticles Al cathodic particles, resisted 12 17 them those of the ZrO , as well as the presence of β–Mg Al cathodic particles, which 2 12 17 incorporation of ZrO2 in an orto the corrosion attacks. It has been reported [84] that the incorporation oftowards ZrO2 resisted to the corrosion attacks. It hasthe been reported [84] that the ganic coating has managed to reduce advance of corrosive species (Cl− ions) in an organic coating has managed to reduce the advance of corrosive species (Cl− ions) the substrate because ZrO2 (an inorganic element) allowed the formation of a more comtowards the substrate because ZrO2 (an inorganic element) allowed the formation of a more plete network, obstructing the diffusion of species through the coating. This behavior may complete network, obstructing the diffusion of species through the coating. This behavior be related to the good chemical stability of ZrO2 [85,86]. may be related to the good chemical stability of ZrO2 [85,86]. Figure 11. SEM images (×1000) and maps of elements on (a) PS–ZrO2 –AM60–AlN and (b) PS– Figure 11. SEM images (×1000) and maps of elements on (a) PS–ZrO2 –AM60–AlN and ZrO2 –AM60 after exposure for 30 days to SME solution. (b) PS–ZrO2 –AM60 after exposure for 30 days to SME solution. After the surface surfacelayers, layers,according accordingtotothe the ASTM G1-03 [87] Afterthe thechemical chemical removal removal of of the ASTM G1-03 [87] formed during the exposure of the coated and non-coated Mg alloys for 30 days to formed during the exposure of the coated and non-coated Mg alloys for 30 days to thethe SME in Figure Figure12 12show showthe thesurface surfaceappearance appearance and SMEmodel modelsolution, solution,the the SEM SEM images images in and thethe EDSanalysis analysisofofthe thezone zone of of interest interest is thethe presence EDS is resumed resumedin inTable Table5.5.Zone Zone1 1suggested suggested presence thecathodic cathodic particles particles of of Al–Mn, particles of of β–Mg ofofthe Al–Mn, while while Zone Zone33indicates indicatesthe the particles β-Mg Al 12 , 12 , 17 Al 17 which were not attacked by the corrosion process. Zone 2 presents the Mg matrix. The low which were not attacked by the corrosion process. Zone 2 presents the Mg matrix. The low contentsofofCCand andSi Si have have been been parts parts of and thethe coupling contents of the the organic organicmaterial materialofofthe thePSPS and coupling agent of TMSPM. The localized attacks were more intensive in the non-coated AM60–AlN agent of TMSPM. The localized attacks were more intensive in the non-coated AM60–AlN nanocomposite and AM60 alloy. nanocomposite and AM60 alloy. 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 Zone 2 Zone 3 2.51 4.46 5.67 1.14 1.82 2.58 2.17 89.06 57.64 36.92 4.66 34.11 1.18 - 56.09 - Additional mapping (Figure 13) confirmed the presence of those characteristic particles, as suggested above (Table 5). Coatings 2023, 13, 1059 13 of 20 Figure 12. SEM images (×1000) of surfaces after removal of the formed layers during the e for 30 days to SME solution: (a) PS–ZrO2 –AM60–AlN, (b) PS–ZrO2 –AM60, (c) AM60–AlN AM60. Table 5. Elemental analysis (wt.%) of zones of interest (Figure 12) on the coated and non surfaces after the removal of the layers formed during the exposure for 30 days to SME solu Element Zone 1 Zone 2 Zone 3 C 2.51 4.46 5.67 O 1.14 1.82 2.58 Mg 2.17 89.06 57.64 Al 36.92 4.66 34.11 Si 1.18 - M 56 Figure (×(×1000) 1000) ofof surfaces after removal of the formed layers during the exposure Figure12. 12.SEM SEMimages images surfaces after removal of the formed layers the exposure Additional mapping (Figure 13) confirmed the presence ofduring those characteristi for30 30days daysto toSME SMEsolution: solution: (a) (a) PS–ZrO PS–ZrO22–AM60–AlN, –AM60–AlN,(b) (b)PS–ZrO PS–ZrO –AM60, (c) AM60–AlN, and (d) for –AM60, (c) AM60–AlN, and 2 2 cles, as suggested above (Table 5). AM60. (d) AM60. 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 Zone 1 Zone 2 Zone 3 C 2.51 4.46 5.67 O 1.14 1.82 2.58 Mg 2.17 89.06 57.64 Al 36.92 4.66 34.11 Si 1.18 - Mn 56.09 - Additional mapping (Figure 13) confirmed the presence of those characteristic particles, as suggested above (Table 5). Figure 13. SEM images(× (×3000) andmaps maps elements on PS–ZrO (a) PS–ZrO and Figure 13. SEM images 3000) and of of elements on (a) and 2 –AM60–AlN 2 –AM60–AlN (b) PS–ZrO –AM60 surfaces after removal of the layers formed during the exposure to SME solution. ZrO –AM60 surfaces after removal of the layers formed during the exposure to SME soluti 2 2 Figure 14 groups several SEM images of the cross-sections of the surfaces of the Figure 14 groups several SEM images of the cross-sections of the surfaces AM60–AlN (Figure 14a) and AM60 (Figure 14b) magnesium alloys coated with PS–ZrO2 AM60–AlN andofAM60 (Figure 14b) magnesium alloys coated with P deposits. The (Figure visualized14a) depths the localized attack towards the matrix have average values of depth penetration depths of ≈28.08ofµm onlocalized the AM60–AlN surface and ≈17.90 µm on have a deposits. Theofvisualized the attack towards the matrix the AM60. 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 Figure 13. SEM imagesµm (×3000) and maps ofon elements on (a) PS–ZrO andfor (b) PS– 2 –AM60–AlN AM60–AlN of ≈246.40 and ≈ 178.00 µm the AM60 surfaces after the exposure ZrO –AM60 surfaces after removal of the layers formed during the exposure to SME solution. 2 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 Figure 14 groups several SEM images of the cross-sections of the surfaces of the AM60–AlN (Figure 14a) and AM60 (Figure 14b) magnesium alloys coated with PS–ZrO2 deposits. The visualized depths of the localized attack towards the matrix have average Coatings 2023, 13, 1059 values of depth of penetration of ≈28.08 µm on the AM60–AlN surface and ≈17.90 µm on the AM60. 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 14 of 20 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 PS–ZrO2 deposit against the chloride attack of the marine–coastal environthe PS–ZrO2 deposit against the chloride attack of the marine–coastal environment (SME ment (SME model solution). model solution). Figure 14. SEM (×500) of(× cross-sections on the surfaces of (a) PS–ZrO (b) PS– Figure 14. images SEM images 500) of cross-sections on the surfaces of2 –AM60–AlN, (a) PS–ZrO2 –AM60–AlN, ZrO2 –AM60, (c) AM60–AlN, and (d) AM60 (×250) after their exposure for 30 days to chloride SME (b) PS–ZrO2 –AM60, (c) AM60–AlN, and (d) AM60 (×250) after their exposure for 30 days to chloride solution. SME solution. However, because of theofavailable defects on the coating surface (Figure 1e,f), 1e,f), However, because the available defects onhybrid the hybrid coating surface (Figure the chloride ions (SME solution) and oxygen diffusion processes were facilitated, and the chloride ions (SME solution) and oxygen diffusion processes were facilitated, they and they werewere able able to penetrate through the hybrid material and attack the Mg On the to penetrate through the hybrid material and attack thematrix. Mg matrix. Onother the other hand,hand, the ZrO , a good semiconductor [88], could serve as local cathodes, which were not the 2ZrO 2 , a good semiconductor [88], could serve as local cathodes, which were not attacked during the the corrosion process and were maintained surfaces after attacked during corrosion process and were maintainedon onthe theMg Mg alloys’ alloys’ surfaces afterthe theremoval removalof ofthe thecorrosion corrosionlayers. layers. 3.5. Electrochemical Impedance Spectroscopy 3.5. Electrochemical Impedance Spectroscopy The electrochemical impedance, visualized BodeNyquist and Nyquist diagrams The electrochemical impedance, visualized usingusing Bode and diagrams (EIS, a (EIS, a non-destructive technique), was elaborated to characterize the interface the hybridnon-destructive technique), was elaborated to characterize the interface of theofhybridcoated Mg alloys exposure day 15 and 15 days to SEM marine environment coated Mg alloys afterafter theirtheir exposure for 1for day1 and days to SEM marine environment model solution (Figure 15). The Nyquist diagrams (Figure 15a,b) revealed two capacitive model solution (Figure 15). The Nyquist diagrams (Figure 15a,b) revealed two capacitive semi-circles associated two time constants at higher medium frequencies semi-circles associated with with two time constants at higher and and medium frequencies (HF (HF and MF), respectively. The diameters the HF-capacitive associated and MF), respectively. The diameters of theofHF-capacitive loopsloops werewere associated withwith the the particularities offormed the formed corrosion onMg thematrix Mg matrix the presence the hybrid particularities of the corrosion layerlayer on the in theinpresence of theofhybrid PS–ZrO while MF-capacitive loops may relatetotothe thecharge charge transfer transfer processes 2 coating, PS–ZrO while the the MF-capacitive loops may relate pro2 coating, 22+ + of the hydrogen evolution (H ) and the Mg released through the double layer[89– [89–91]. cesses of the hydrogen evolution (H2 2 ) and the Mg released through the double layer The Bode plots (Figure 15) of the phase angle were found to be in good agreement with 91]. The Bode plots (Figure 15) of the phase angle were found to be in good agreement ◦ ◦ ° 70 and ° 80 the observed changes in the Nyquist diagrams. The phase angle between with the observed changes in the Nyquist diagrams. The phase angle between 70 and 80 confirmed thatinterfaces the interfaces the studied alloys are capable of accumulating electrical confirmed that the of theofstudied alloys are capable of accumulating electrical charges, which in fact, will complicate the mass transfer process through the electrode 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 interface and, consequently, the progress in the corrosion process because of the hybrid PS–ZrO2 coating. 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, and they were compared to those of noncoated surfaces (Table 7). The EC includes the following components: Rs is the solution resistance; R1 denotes the resistance of the layer on the metal substrate, and the constant phase element CPE1 denotes the “capacitance”, representing the hybrid coating and the formed corrosion layer later with the progress in the corrosion process; and R2 and CPE2 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 = R1 + R2 . Coatings 2023, 13, 1059 The quantification of the EIS data, which characterize alloys, was carried out according to the equivalent circuit the values are summarized in Table 6, and they were com 15 of 20 surfaces (Table 7). The EC includes the following componen R1 denotes the resistance of the layer on the metal substra ment CPE1 denotes the “capacitance”, representing the h corrosion layer later with the progress in the corrosion pr pacitance” are characteristic of the charge transfer process lyte interface [90,92,93]. The values of Rp (polarization resi R1 + R2 . The comparison of the Rp values (Table 6) allowed us t ing of PS–ZrO2 deposited on the Mg alloy AM60 surface c sistance against the corrosion process, presenting an increa in SME model solution), compared to that of 22% for the co the case of non-coated AM surfaces, the Rp of AM60 was 9% AlN (Table 7). These facts were related to the AlN hydro raises the pH in the range of 5.5–12; during the exposure o of the SMEdiagrams model solution was ≈8.5 (Figure 8). In the prese Figure 15. and Bode of plots phase of angle of AM60 andof AM60–AlN coated with Figure 15.Nyquist Nyquist diagrams andplots Bode phase angle AM60 and AM60–AlN co the hybrid PS–ZrO black color), compared to those oftonon-coated surfaces after immersion in 2 (in the hybrid PS–ZrO black color), compared those ofinto non-coated surfaces after imm (OH )3 [19]. reported that AlN may transform Al 2 (inthe SME model model solution for (a) day1and (b)and 15 days. SME solution for1 (a) day (b) 15 days. The quantification of the EIS data, which characterized the activity of the c alloys, was carried out according to the equivalent circuit (EC) present in Figu the values are summarized in Table 6, and they were compared to those of n surfaces (Table 7). The EC includes the following components: Rs is the solution r R1 denotes the resistance of the layer on the metal substrate, and the constant ment CPE1 denotes the “capacitance”, representing the hybrid coating and th corrosion layer later with the progress in the corrosion process; and R2 and CP pacitance” are characteristic of the charge transfer process at the coated substra lyte interface [90,92,93]. The values of Rp (polarization resistance) were calculat R1 + R2 . The comparison of the Rp values (Table 6) allowed us to consider that the hy ing of PS–ZrO2 deposited on the Mg alloy AM60 surface can be attributed to g Figure 16. Equivalent circuit of PS–ZrO2 -coated AM60 and AM60–AlN surfaces, and non-coated, Figure 16. Equivalent ofpresenting PS–ZrO2an -coated AM60 and sistance the corrosion process, increase in its Rp byAM60 37% ( during theagainst exposure to SME solution. circuit in during SME model compared that of 22% for the coated composite AM6 thesolution), exposure to SMEtosolution. The comparison of the Rp values (Table 6) allowed us to consider that the hybrid the case of non-coated AM surfaces, the Rp of AM60 was 9% higher than that of t coating of PS–ZrO2 deposited on the Mg alloy AM60 surface can be attributed to greater AlN (Table 7). These factsprocess, were presenting related to AlNin hydroxide resistance against the corrosion an the increase its Rp by 37% phase’s (at 15 dayssolubili in SME model solution), compared to that of 22% for the coated composite AM60–AlN. raises the pH in the range of 5.5–12; during the exposure of the coated AM alloy the case of non-coated AM surfaces, the Rp of AM60 was 9% higher than that of the ofInthe SME model solution was ≈8.5 (Figure 8). In the presence of chloride ions, i AM60–AlN (Table 7). These facts were related to the AlN hydroxide phase’s solubility, reported that into the Al(exposure OH)3 [19]. which raises thethe pH AlN in themay rangetransform of 5.5–12; during 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 Al(OH)3 [19]. Coatings 2023, 13, 1059 16 of 20 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) Rs (Ω cm2 ) CPE1 (µS sn cm−2 ) n1 R1 (kΩ cm2 ) CPE2 (µS sn cm−2 ) n2 R2 (kΩ cm2 ) Rp (kΩ cm2 ) 1 15 69.16 ± 0.54 71.35 ± 0.49 7.85 ± 0.21 37.18 ± 0.64 0.91 ± 0.01 0.88 ± 0.01 7.22 ± 0.14 11.36 ± 0.24 0.66 ± 0.13 3.97 ± 0.53 0.84 ± 0.11 0.99 ± 0.13 2.10 ± 0.27 2.61 ± 0.24 9.32 ± 0.30 13.97± 0.37 1 15 89.93 ± 0.73 80.20 ± 0.54 3.76 ± 0.08 39.48 ± 0.66 0.88 ± 0.01 0.88 ± 0.01 10.63 ± 0.27 13.23 ± 0.25 0.84 ± 0.05 0.99 ± 0.17 4.75 ± 0.26 3.93 ± 0.22 15.35 ± 0.37 17.16± 0.33 PS–ZrO2 –AM60 0.26 ± 0.02 3.361 ± 0.55 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) Rs (Ω cm2 ) CPE1 (µS sn cm−2 ) n1 R1 (kΩ cm2 ) CPE2 (µS sn cm−2 ) n2 R2 (kΩ cm2 ) Rp (kΩ cm2 ) 1 15 59.89 ± 0.45 66.56 ± 0.43 10.49 ± 0.26 43.23 ± 0.72 0.93 ± 0.01 0.92 ± 0.04 7.00 ± 0.15 9.23 ± 0.16 0.54 ± 0.08 6.51 ± 0.38 0.87 ± 0.08 0.97 ± 0.22 2.72 ± 0.28 2.17 ± 0.22 9.77 ± 0.32 11.40 ± 0.27 1 15 68.62 ± 0.50 71.21 ± 0.46 11.45 ± 0.31 39.86 ± 0.67 0.94 ± 0.01 0.93 ± 0.01 6.12 ± 0.14 10.06 ± 0.16 0.48 ± 0.08 6.13 ± 0.47 0.86 ± 0.084 0.97 ± 0.225 2.37 ± 0.24 2.45 ± 0.25 8.49 ± 0.28 12.51 ± 0.30 AM60 4. Conclusions • • • • • • • A hybrid coating of polystyrene (PS)–ZrO2 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 PS–ZrO2 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 ± 20 nm. The XRD patterns revealed no crystalline structure of the hybrid organic–inorganic coating. The deposit of PS–ZrO2 reduced the contact angle of the Mg substrates, and their wettability was close to the hydrophobic border (θCA 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 PS–ZrO2 presented some micro-defects and the H2 bubbles continued to come out. The results indicated that the hybrid PS–ZrO2 coating reduced the release of Mg2+ 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–Mg17 Al12 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 PS–ZrO2 hybrid coating on AM magnesium alloy substrates exposed to marine–coastal simulated ambient (SME). The polarization values of Rp calculated from EIS indicated that the Rp of the PS–ZrO2 coated AM60 alloy increased by 37% and that of the composite AM60–AlN increased Coatings 2023, 13, 1059 17 of 20 • 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 PS–ZrO2 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. Author Contributions: Conceptualization and methodology, L.V. and L.C. performed the preparation of samples and the corrosion tests; L.C., D.S.-A. and R.R.-B. contributed to the synthesis and deposit of the hybrid material; L.C. and L.V. performed the formal analysis of the results and the writing of the original draft and editing. L.V. supervised the project. All correspondence should be addressed to L.V. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: The data are available upon request from the corresponding author. Acknowledgments: Luis Chávez acknowledges the Mexican National Council for Science and Technology (CONACYT) for the scholarship for his Ph.D. study. The authors gratefully thank the National Laboratory of Nano and Biomaterials (LANNBIO-CINVESTAV) for allowing the use of SEMEDS and XPS facilities; thanks also go to Victor Rejón, Daniel Aguilar, and Willian Cauich for their support in data acquisition. The authors thank Carlos Ávila and Agustin Galindo for their technical assistance and for allowing access to the Research Laboratory and Technological Development in Advanced Coating (LIDTRA-CINVESTAV Queretaro). 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