Integrated corrosion-resistant system for AZ31B Mg alloy via plasma electrolytic oxidation (PEO) and sol-gel processes

Received: 4 March 2021 Revised: 8 June 2021 Accepted: 5 August 2021 DOI: 10.1111/ijag.16536 SPECIAL ISSUE ARTICLE Integrated corrosion-resistant system for AZ31B Mg alloy via plasma electrolytic oxidation (PEO) and sol-gel processes Emilia Merino Alicia Durán Yolanda Castro Instituto de Cerámica y Vidrio, CSIC, Madrid, Spain Correspondence EmiliaMerino,InstitutodeCerámicay Vidrio,CSIC,CampusdeCantoblanco, Madrid28049,Spain. Email:[email protected] Fundinginformation EuropeanUnionťsHorizon2020 research andinnovation program,Grant/Award Number:739566 Abstract In this work, an integrated system that combines anodizing process with the deposition of a hybrid SiO2sol-gel coating was developed with the aim of improving the corrosion resistance of AZ31B Mg alloy. The optimization of the anodizing conditions using an alkaline electrolyte (NaOH) modified with different concentrations of sodium metasilicate pentahydrate was explored together with the preparation and deposition of a SiO2sol obtained by hydrolysis and condensation of TEOS (tetraethoxysilane), GPTMS (3-Glycidyloxypropyl) trimethoxysilane), colloidal SiO2nanoparticles, and 1-Methylimidazole (MI). The surface morphology, coating thickness, and composition were evaluated. The anodized coatings thickness was between 1.1 and 1.7 μm with composition corresponding to magnesium oxide and low silicon content. The corrosion performance was tested in 3.5% NaCl solution. The results revealed a good corrosion resistance behavior after the anodizing of the Mg alloys. However, the best corrosion resistance was reached when the porous PEO layer was sealed with a hybrid silica sol-gel film. A decrease in the corrosion current density of three orders of magnitude is observed between Mg alloy (1.54E-06 A/cm2) and multilayer system (2.80E8A/cm 2). Moreover, the polarization resistance for 8AF+SG samples showed a quite high value (31546.8 Ω cm2)comparedtoMgalloy(207.3Ωcm 2). KEYWORDS anodic films, corrosion performance, magnesium alloy, PEO, sol-gel coating 1 INTRODUCTION Due to environmental and energy concerns, the automotive, electronics, and aerospace industries are focusing their efforts on replacing toxic or hazardous materials and products with more environmentally acceptable solutions.1The low density and light weight, combined with its adequate strength/weight ratio, make Mg alloys attractive for different industries, especially those where This is an open access article under the terms of the Creative Commons Attribution-NonCommercial-NoDerivs License, which permits use and distribution in any medium, provided the original work is properly cited, the use is non-commercial and no modifications or adaptations are made. © 2021 The Authors. International Journal of Applied Glass Science published by American Ceramics Society and Wiley Periodicals LLC. weight reduction is necessary to decrease the fuel consumption and therefore the greenhouse gas emissions.2 However, its use on a large scale is limited by its high corrosion susceptibility.3,4 Since magnesium and its alloys show remarkable low corrosion resistances, surface modification techniques are used to avoid direct contact of the Mg surface with a corrosive environment, thus improving the resistance. One of the most efficient and low-cost coatings that provide Int J Appl Glass Sci. 2021;12:519–530. wileyonlinelibrary.com/journal/ijag 519 520 MERINO et al. self-healing effect2are the chromate conversion coatings (CCC and CCA). However, the restrictions related to the use of hexavalent chromium Cr (VI) for surface treatments are very strict due to health and environmental concerns.5Great efforts are underway to identify alternative effective surface treatments that are more environmentally friendly.6–8 In particular, the combination of several methods such as the plasma electrolytic oxidation (PEO), and the sol-gel process are identified as potential alternatives.9,10 In this case, the micro-porous oxide coating obtained by the PEO process can be further sealed by adding a hybrid inorganic-organic polymeric (Si-O-Si) sol-gel coating on top to provide an adequate corrosion protection on the metal surfaces.11–15 Some researchers, as Tan et al,16 Malayoglu et al,17 and Castellanos et al,18 have addressed this strategy for the protection of Mg alloys, reporting an improvement of the corrosion resistance after a posttreatment including the deposition of a sol-gel coating. In particular, Guo et al19 studied a sol–gel film based on GPTMS (3-Glycidyloxypropyl) trimethoxysilane), TETA (triethylenetetramine), and TEOS (tetraethoxysilane) for sealing the porous PEO layer on magnesium. Although the sol–gel layers provide corrosion protection by a physical sealing of pores, the electrochemical studies were carried out using a diluted NaCl electrolyte (0.005 M), making the results difficult to compare with those tested with standard electrolyte (3.5% NaCl). The corrosion behavior of sol-gel coatings depends on the synthesis parameters, organic-inorganic precursors, and the mechanical and chemical features of the comprising organic and inorganic networks. GPTMS is the most used organo-functional alkoxysilane which allows obtaining hybrid materials with novel properties because its epoxy group can react with a wide variety of reactants. The epoxide ring-opening reaction using 1-Methylimidazole (MI) as initiator allows reaching organic-inorganic hybrid coatings with high crosslinking characteristics. In this work, we used this initiator, added to the incorporation of Ludox colloidal SiO2nanoparticles suspension. The nanoparticles increase the stiffness and density of the network. The simultaneous use of MI and SiO2nanoparticles allows reaching a higher crosslinking of sol-gel network with improved mechanical properties. Better corrosion properties are expected via one-pot procedure. Thus, the aim of this work is to propose an alternative route to achieve an integrated coating system using PEO and sol-gel treatment, able to enhance the corrosion behavior of the bare AZ31B Mg alloy without using toxic or hazardous substances. To attain this goal it is crucial to optimize the PEO process conditions (e.g., the electrolyte composition) as well as the synthesis and deposition parameters of the SiO2sol obtained by using TEOS, GPTMS, colloidal silica, and MI as precursors. The surface properties (e.g., morphology, cross-section, etc.) of the anodized and sol-gel coated substrates were characterized using optical microscopy, field emission scanning electron microscopy (FESEM) including EDX spectroscopy, Xray diffraction, and optical profilometry. The anticorrosive properties were evaluated using potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) on samples immersed in a 3.5% NaCl solution, the electrolyte commonly used to evaluate the corrosion resistance of alloys in the automotive and aerospace industries. Raman spectroscopy was used to provide additional information about corrosion products. 2EXPERIMENTAL 2.1 Material and pre-cleaning process Commercially AZ31B Mg alloy with composition: 10.16 % Al, 0.33% Mn, 1.07 % Zn, <0.5% Fe, and Mg balance (Dugopa S.A, Spain) was cut to size of 5 cm×2cm.The AZ31B Mg samples were ultrasonically cleaned in acetone during 3 min, dried with air and immersed in an acid solution of 10 vol% HNO3(65%, VWR), and 2 vol % H3PO4(85%, Aldrich) for 20 s. The samples were then rinsed with distilled water and immersed in 5 wt. % NaOH (99.9%, Scharlau) solution for 1 min. At the end of this process, the samples were rinsed again with deionized water and acetone and dried with air. 2.2 Anodizing process The pre-cleaned AZ31B substrates were anodized at a constant potential of 100 V during 2 min by using a DC power supply (Magna Power, 12 A, 500 V). The pre-cleaned AZ31B substrate was set up as anode and a platinum foil as cathode keeping a distance of 1 cm and an anodized area of 2 ×2cm 2. The experiments were performed using a basic electrolyte bath containing 40 g/L NaOH and different concentrations of sodium metasilicate pentahydrate Na2SiO3⋅5H2O(>95%, Aldrich). The concentration of Na2SiO3added to NaOH electrolyte was varied in the range of 4–8 g/L to explore the corrosion resistance effect on the anodized film. In order to simplify the tagging of the oxide coatings obtained under different electrolyte composition, the films are labeled as 4AF, 6AF, and 8AF when the concentration of Na2SiO3was 4 g/L, 6 g/L, and 8 g/L, respectively. During the anodizing process, the bath electrolyte solution was maintained under magnetic stirring, and the temperature was controlled with a thermo-control equipment. After each test, the anodized samples were carefully rinsed with distilled water and dried with air. MERINO et al. 521 2.3 Hybrid sol-gel top-coat Hybrid silica sol was prepared under acidic conditions at room temperature by mixing 0.5 mol of tetraethoxysilane (TEOS, Aldrich, 99%), 0.5 mol of 3-(glycidyloxypropyl) trimethoxysilane (GPTMS, ABCR, 98%) and 0.54 mol of a colloidal SiO2nanoparticles suspension (Ludox-4S, Aldrich, aqueous suspension 40 wt.%, particle size 20 nm, pH9). The solution was stirred for 5 min and then 0.063 mol of concentrated nitric acid HNO3(VWR, 65%) was added. Finally, absolute ethanol (EtOH, Panreac, 99.8%) containing 0.1 mol of 1-Methylimidazole (MI, Aldrich, 99%) was added to finally obtain a silica concentration sol [SiO2]of 200 g/L. The stability of the sol was studied by following the pH and viscosity evolution with the aging time. The pH was measured with pH paper indicator, and the viscosity was monitored through time using an A&D Vibro Viscosimeter SV-10. The structure of the silica sol was analysed by FTIR (Perkin Elmer Perkin Elmer Spectrum 100 spectrometer) spectrometer with an attenuated total reflectance accessory. The anodized specimens (4AF, 6AF, and 8AF) were topcoated with the hybrid silica sol by dipping, using a withdrawal rate of 30 cm/min. After the deposition, samples were heat-treated at 160◦Cfor1h.Itshouldbenotedthat the sol-gel coating deposited onto 8AF oxide coating is referred to 8AF+SG. 2.4 Characterization of as-received alloy, anodized, and coated samples The surface morphology of samples was characterized by Optical Microscopy (OM, Zeiss Axiophot) and FESEM (Hitachi S4700) equipped with energy dispersive X-ray spectroscopy (EDS) detector operating at an accelerating voltage of 15 kV. The surface roughness (Ra-parameter) was obtained with a Zeta 20 equipment (KLA Corporation) at magnifications of 20X and 0.1 nm of resolution. The water contact angle was measured with an Easy Drop Standard "Drop Shape Analysis System" Kruss DSA 100 equipment at room temperature using a volume of probing liquid around 5 μl. The crystallographic structure of anodizing films was analysed with a Panalytical XťPert PRO theta/theta diffractometer at a grazing angle of 0.5◦and 2θrange of 10–50◦with a step size of 0.05◦and accumulation time of 20 s per step, using Cu-Kαradiation (λ=0.15418 nm) as the excitation source. Potentiodynamic polarization curves and EIS were performed in a Gamry FAS2 electrochemical unit (Gamry, FIGURE 1 Optical images of Mg AZ31B surface alloy (a) as-received and (b) after pre-cleaning USA) with DC and AC signals. A saturated calomel electrode (SCE, Radiometer, France) was used as the reference electrode, and a platinum wire as the counter electrode. The testing area of sample was 0.78 cm2and all the tests were carried out in 3.5 wt. % NaCl electrolyte solution. To reach a steady state condition, the open circuit potential (Eoc) was recorded for 120 min before every measurement. Polarization curves were acquired varying the potential from an initial E(V) of −1.58 vs Eref up to a current density value of 1E-03 A cm−2and a scan rate of 1 mVs−1.EIS results were conducted in the frequency range from 1E05 Hz to 0.1 Hz applying an AC voltage at the open circuit potential with sinusoidal amplitude of 5 mV rms. Experiments were repeated at least three times choosing the most representative measurement for plotting. The potentiodynamic curves and EIS data were analyzed using Gamry Echem Analyst software (Gamry Instrument, Inc) and Zview 2 software, respectively. Raman spectroscopy provides additional information about corrosion products. Raman spectra were recorded in a confocal Raman microscope with a spectral resolution of 0.02 cm−1 coupled with an AFM instrument (ALPHA 300RA, Witec, Uim, Germany) and with 532 nm excitation laser. The images were analyzed with the WiTec Project Plus 2.08 software. 3RESULTS AND DISCUSSION 3.1 Pre-cleaning process Figure 1shows the optical images of as-received and pre-cleaned AZ31B substrates. The surface of the precleaned sample shows a smooth surface without the presence of scratches that indicate the rolling direction of die-cast process. Due to the etching process, the Ravalue decreases from 2.2 ±0.2 μmto0.7± 0.1 μm for as-received and pre-cleaned AZ31B substrates, respectively. It is crucial to remove grease, oil, and surface imperfections that could affect the anodization process. 522 MERINO et al. FIGURE 2 Variation of current density as a function of anodization time recorded during the anodization process at 100V in alkaline electrolytes of 1 mol L−1NaOH with different Na2SiO4 concentrations 3.2 Effect of Na2SiO3concentration on the properties of the anodized films The anodization process was performed at 100 V in an alkaline electrolyte containing 40 g/L NaOH and different concentrations of Na2SiO3: 4 g/L (4AF), 6 g/L (6AF) and 8 g/L (8AF). Figure 2shows the corresponding anodizing current density-time response as a function of Na2SiO3 concentration. During the first seconds of the anodizing procedure, all the plots show the same trend since the current density increases quickly, reaching a value higher than 1E03 mA cm−2. Visible sparking and gas bubbles appear on the anode surface as consequence of the dielectric breakdown phenomena which occur when the applied voltage surpasses the critical voltage (Ub). However, as the anodizing time increases, different current density behavior was observed for the different silicate concentration. For anodized samples with 4 g/L Na2SiO3concentration, the sparking and the current density drop to reach a steady value of ∼1E02 mA cm−2with the anodizing time. On the other hand, when the silicate concentration increases up to 6 g/L and 8 g/L, vigorous sparking phenomena was observed during the whole anodization process, and the current density decreases and then increases again up to values of 5.5E02 mA cm−2and 1E03 mA cm−2, respectively. The visible sparking phenomena and the higher steady-state current density could be associated with the increment of electrolyte conductivity, explained using the theoretical approach to electrical breakdown model proposed by Ikonopisov et al.20 Considering this model, the breakdown event is controlled by the electrochemical FIGURE 3 XRD diffractograms of anodized AZ31B alloy films obtained at 100 V using the electrolyte of 40 g/L NaOH and 4 g/L Na2SiO4 reaction at electrolyte/oxide interface, and it is observed when the electrolytic contact provides ions for the oxidation and electrons into the conduction band of oxide.20 When the current density increases, dissolution of magnesium described by reaction (1) and hydrogen evolution described by the reaction (2) take places. Due to the alkalization and the increase of the temperature of the media, as a consequence of the sparking arc,21 a direct electrochemical oxidation of Mg to MgO (reaction 3) and a dehydration process (reaction 4) also take place,22 favoring the formation of MgO in the surface.23 Mg → Mg2+ +2e (1) 2H2O+2e→H 2+2OH −(2) Mg + 2OH−→MgO+H 2O+2e (3) Mg(OH)2→MgO+H 2O(4) Mg + SiO32− →MgO⋅SiO 2+2e (5) X-ray diffraction patterns (XRD) (Figure 3) of anodized AZ31B magnesium alloy (8AF) shows that MgO (■) is the main compound of the anodic oxide film, which is in good agreement with the previously proposed mechanism. Silicon containing species were not identified in the XRD pattern due to the low silicon concentration in the coating. However, the energy-dispersive X-ray analysis (EDS) under SEM observation (Table 1) identifies the presence of O and Si elements (O =18–26 wt% and Si =7 wt%) in the MERINO et al. 523 TABLE 1 Elemental components of bare Mg alloy surface and anodic films formed in electrolytes with different Na2SiO3concentration Sample O (wt%) Mg (wt%) Si (wt%) Al, Mn, Zn (wt%) As-received Mg alloy 6.05 86.30 0.21 7.45 4g/LNa 2SiO318.96 62.77 7.27 11.0 6g/LNa 2SiO321.2 62.51 6.91 9.38 8g/LNa 2SiO326.46 62.27 6.81 4.46 FIGURE 4 FTIR spectra of SG coating (a) before adding MI and (b) after adding MI and treating at 160◦C anodized surface compared with the bare Mg surface alloy. The presence of O confirms the formation of MgO and SiO2, but the low content of Si indicates a low concentration of silicon species. During the anodized film formation, the silicon species are incorporated into the oxide coating on amorphous form, and the reaction (5) can also be coupled in the mechanism, as reported by Moon et al.24 3.3 Characterization of silica sol-gel top-coat The silica sol-gel sol (SG) was prepared in two steps; an acid-catalysed step which promotes the inorganic hydrolysis and condensation reactions of TEOS, GPTMS and the cross-linking with the colloidal silica, followed by an organic polymerization step catalysed by 1Methylimidazole (MI) which promotes the epoxide-ring opening. As a result, a transparent and homogeneous SiO2 sol, with pH around 6, was obtained. High stability and an adequate viscosity of around 3 mPa.s, maintained during almost 4 days, were obtained for the silica sol. The effect of imidazole addition in the organic polymerization was analyzed by FTIR spectroscopy. Figure 4shows the FTIR spectra before (a) and after (b) the addition of imidazole precursor, respectively. The region between 850 and1300cm −1is analyzed in detail (inset). For both spectra, a strong absorption band around 1150 cm−1and two bands at 1250 and 950 cm−1are identified. The broad peak (1150 cm−1) is associated with the asymmetric vibration of the Si–O–Si bonds (LO and TO)25 indicating that the inorganic polymerization reaction took place. Meanwhile, the peaks at 910 cm−1and 1250 cm−1are associated with antisymmetric epoxy ring deformation and with the breakage of the ring, respectively.25 Both peaks decrease sharply with the addition of MI, suggesting that the epoxy rings were opened to form a poly (ethylene oxide) chain, thus increasing the cross-linking of the hybrid network. The anodized specimens (4AF, 6AF, and 8AF) were sealed with the hybrid silica sol by dipping, using a withdrawal rate of 30 cm/min after the deposition, samples were heat-treated at 160◦C for one hour. It should be noted that the sol-gel coating deposited onto 8AF oxide coating is referred to as 8AF+SG. 3.4 Morphology and cross-section analysis of anodized films and anodized/hybrid silica sol-gel coating system Figure 5shows the surface and cross-section morphology of anodized films obtained with different Na2SiO3electrolyte concentrations (Figure 5A,B,C) and the silica solgel film deposited onto 8AF anodized coating (Figure 5D). All the anodized films present a porous structure with different pore sizes and coating thickness. FESEM images revealed the formation of pores in the range of ∼1and 5μm and coating thickness ≥1.2 μm for films anodized with 4 g/L (Figure 5A) and 6 g/L (Figure 5B)ofNa 2SiO4. However, for 8 g/L Na2SiO4(Figure 5C), smaller pore sizes, with a diameter less than 1 μm, and thicker coatings (1.7 μm) were obtained. These results indicate a positive effect of increasing Na2SiO4concentration, reducing the pore size and increasing the thickness of the coating. A higher concentration of silica in the electrolyte provides more ions, favouring the injection of more electrons in the electrolyte/oxide interface and ensuring an increment on the thickness. FESEM was also used to evaluate the morphology and homogeneity of silica sol-gel films deposited onto the anodized surfaces. As shown in 524 MERINO et al. FIGURE 5 FESEM surface and cross-sectional morphology of (a–c) anodized AZ31B alloy films obtained at 100 V using the electrolyte of NaOH and various Na2SiO4concentrations (a) 4AF, (b) 6AF, (c) 8AF and (d) multilayer system (8AF+SG) (Figure 5D), a smooth and non-cracked homogeneous surface was obtained after the deposition of silica sol-gel on anodized AZ31B substrate obtained using the electrolyte 40 g/L NaOH and 8 g/L Na2SiO4(8AF+SG). The crosssection reveals a hybrid silica coating with a thickness of around 1.8 μm perfectly adhered to the anodized film. The arithmetical mean roughness values (Ra) and the water contact angle were measured for 8AF and 8AF+SG samples (Figure 6). The Ravalue of anodized coating (8AF) and the 8AF+SG were 0.7 ±0.1 μm and 0.6 ±0.1 μm, respectively. After the deposition of the silica sol-gel coating on the anodized coating (8AF+SG sample), no significant changes were observed on the Ra compared with the pre-cleaned substrate (see above item 3.1). The water contact angle decreases from 93.7 ±4.5◦for the as-received Mg alloy to 73.4 ±6.6◦for the anodized Mg alloy, respectively. The lower wettability of as-received substrates is associated with the high substrate roughness; the line and pillar-like structures favor the air trapping FIGURE 6 Water contact angle and Ra μm values of as-received Mg alloy, 8AF, and 8AF+SG and thus the increment of the contact angle.1On the other hand, the lower water contact angle of the anodized film (8AF) is associated with the porous structure of the surface that allows the adsorption of water. After the deposition of a silica sol-gel coating on the anodized coating (8AF+SG), the water contact angle increases again to 91.8 ±1.8◦, associated with the pores sealing that changes the chemical surface composition. 3.5 Corrosion performance and electrochemical behavior of anodized and top-coated alloys The corrosion performance of the anodized films obtained by adding different Na2SiO4concentrations of electrolyte ((4AF), (6AF), and (8AF)) and the protecting behavior of the integrated systems (anodized and sealing with a silica sol-gel coating (8AF+SG)) were analyzed, with a good reproducibility of the experimental results. Figure 7shows the potentiodynamic polarization results after 2 h of immersion in 3.5 wt.% NaCl. The plots show a linear cathodic Tafel branch and a non-linear anodic branch. Considering that the pseudo-passive region is anodic polarized, the cathodic branch26 was used to determine the electrochemical parameters by Tafel extrapolation method (Table 2). To ensure a decade of the linearity in the E-log(i) region, the extrapolation was extracted from E-log(i) data at least 150 mV away from the corrosion potential (Ecorr).27 All the polarization curves of anodized coatings shift to lower corrosion current density values between 1E-06 - 1E-07 A cm−2respect to the as-received Mg alloy (1E-05 A cm−2). The decrease in current density shows that the corrosion resistance of Mg alloy significantly improved with the anodizing process. The anodic polarization curves also exhibited different anodic branch behavior. A very wide range of passivation MERINO et al. 525 TABLE 2 Electrochemical parameters of as-received AZ31B Mg alloy, anodized AZ31B alloy films obtained at 100 V using the electrolyte consisted of NaOH and various Na2SiO4concentration (A) 4 g/L (4AF), (B) 6 g/L (6AF), (C) 8 g/L (8AF), and the multilayer system (8AF+SG) derived from polarization tests in 3.5 wt% NaCl solution As-received Mg alloy 4AF 6AF 8AF 8AF+SG Ecorr(V/SCE) −1.49 (±4.21E-03) −1.47 (±6.51E-03) −1.45 (±7.65E-03) −1.42 (±5.62E-03) −1.29 (±8.12E-03) Icorr(A/cm2)1.61E-05 (±2.39E-6) 1.54E-06 (±3.56E-06) 6.63E-07 (±1.54E-07) 2.64E-07 (±7.41E-08) 2.80E-08 (±6.25E-09) βa(mV/decade) 10.3 39.68 29.46 21.14 22.53 βc(mV/decade) 137.6 92.13 84.12 69.95 83.31 Corr. rate (mm/year) 3.70E-01 3.53E-02 1.52E-02 6.06E-03 6.43E-04 FIGURE 7 Potentiodynamic polarization curves of as-received AZ31B Mg alloy, anodized AZ31B alloy films obtained at 100 V using the electrolyte of NaOH with various Na2SiO4concentrations (a) 4 g/L (4AF), (b) 6 g/L (6AF), (c) 8 g/L (8AF), and the multilayer system (8AF+SG) that increases with the increasing of Na2SiO4concentration, from 4 g/L to 8 g/L was observed. Lower current densities and broader passivation ranges indicate a more stable coating in terms of pitting corrosion and spontaneous degradation.28 The porous oxide layer obtained by adding 8g/LofNa 2SiO4to the NaOH electrolyte (8AF) exhibited the best corrosion resistance performance among the anodized samples likely due to its smaller pore size and its higher thickness (see above Section 3.4). Since the corrosive electrolyte can penetrate through the pores of the 8AF coating generating electrochemically active sites for metal corrosion, this anodized coating was sealed with the hybrid silica sol (8AF+SG), and the corrosion performance was further analyzed by potentiodynamic polarization technique (Figure 7). The polarization curve shows a significant change in Ecorr towards more noble value, −1.29 V / SCE, and a decrease in the corrosion current density of three-orders of magnitude (2.8E-08 A cm−2) compared with the uncoated AZ31B Mg alloy, Figure 7. The sealing of pores of the anodized (8AF) layer and the hydrophobicity effect of the silica sol-gel coating (91.8◦) significantly improved the corrosion resistance by blocking the penetration of the corrosive electrolyte through pores. The corrosion rate (υcorr) was calculated using the corrosion current density (A cm−2)obtainedbyTafelextrapolation (Table 2) and according to Equation (6) 𝜐corr =3.1𝐸08⋅ 𝐼𝑐𝑜𝑟𝑟 ⋅𝑀 𝑒𝑞 𝐹⋅𝜌 (6) where 𝐹is the Faraday constant in C/eq, ρis the density in g/cm3,and𝑀𝑒𝑞 is the equivalent weight of the alloy in (gr/eq). 8AF+SG coating exhibited the lowest corrosion rate (6.4E-04 mm/year) suggesting that the corrosion of magnesium alloy is controlled to the sealing of the porous PEO layer with the hybrid silica coating. In order to evaluate the barrier corrosion performance before and after the deposition of the hybrid silica coating, EIS analysis was also performed. Figure 8A1-A2 shows the bode and phase angle diagrams of 8AF and 8AF+SG coatings compared with the as-received substrate. The EIS spectra showed different trends for the as-received, AZ31B, 8AF, and 8AF+SG samples. Three different equivalent circuits were proposed to fit the EIS data (Figure 8B-D)).The anodized EIS spectrum (8AF) was simulated considering a characteristic equivalent circuit for porous films (Figure 8B) which include the resistance of the electrolyte (Rs) and the two-time constants (Rcoat, C1, Rdl, and CPE1).29–31 One-time constant is assigned to the porous outer layer structure, consisting of a capacitive element C1and the coating resistance Rcoat. The second time constant is attributed to the inner layer structure of the anodizing coating, which includes a constant phase element (CPE1) and a charge transfer resistance (Rdl) characteristic of the faradic reaction or the anodic dissolution of Mg into Mg2+on the substrate interface.32 On the other hand, EIS spectrum of the multilayer system (8AF+SG) was modeled considering a circuit of 526 MERINO et al. FIGURE 8 (a1) Bode plot and (a2) Phase Angle plot for As-received Mg alloy, (8AF), and the multilayer system (8AF+SG). Equivalent circuit for (b) 8AF, (c) multilayer system (8AF+SG), and (d) as-received Mg alloy Figure 8C, which includes an electrolyte resistance (Rs), two-time constants (Rcoat, CPEcoat, Rdl, and CPE1), and a finite length Warburg-short circuit (W1). This W is associated with a diffusive process through the silica sol-gel coating and anodizing film.10 Rcoat and CPEcoat represent the resistance and the capacitance associated with the silica hybrid film, respectively. CPE1 and Rdl represent the resistance and the capacitance of the oxide film and the charge transfer process on the coating/substrate interface region. Finally, the model selected to fit the AZ31B bare Mg alloy (Figure 8D) is the commonly used model in literature.33 The fitted values are shown in Table 3. The values of the circuit elements can provide information about the water uptake in the coating and the corrosion resistance properties for each coating system.28 To estimate the total corrosion resistance for each system, Rp (polarization resistance) values were calculated as the sum of all the faradaic resistance by using the respectively fitting data (Table 3) and considering the equivalent circuits presented in Figure 8B-D. The polarization resistance for the uncoated Mg alloy showed the lowest value of 207.3 Ωcm 2. On the contrary, 8AF and 8AF+SG samples have the highest Rp value of 31432.5 Ω cm2and 31546.8 Ω cm2, respectively. Although, the Rp of 8AF+SG and 8AF are very similar, the 8AF+SG exhibited a better corrosion resistance34 because this system includes an additional diffusive resistance (68716 Ω cm2) (non-faradaic resistance) between the sol-gel coating and PEO oxide layer, blocking the penetration of the electrolyte to the inner substrate. These results are in good agreement with the polarization curves analysis. On the other hand, when a system is influenced by surface inhomogeneity and time-constant dispersions (variations in reactivity, porosity, current and potential) along the electrode surface, as in our case, the capacitance represented in the equivalent electric circuits cannot be represented with an ideal capacitance. If a parallel surface distribution of the element is assumed35, the pseudocapacitance is comparable to the classic capacitance by employing the effective capacitance (Ceff) model proposed by Hirschorn et al36 based on the Brug’s equation. The effective capacitance (Ceff) for each sample was also calculated using Equation (2) to analyze the penetration of the electrolyte through the coating (water uptake).37 Ceff =CPEcoat (1 𝛼)(Rs ∗ Rp Rs + Rp) 1−𝛼 𝛼(7) For a low Ceff value, low current flow occurs in the system as a consequence of a low water transportation through coating.38 In the case of multilayer system MERINO et al. 527 TABLE 3 Impedance parameters for as-received Az31B Mg alloy, (8AF), and the multilayer system (8AF+SG) obtained via the EIS data fitted with the equivalent circuit Rs Rcoat RdL RL Ccoat CPEcoat-T CPE1_T CPEcoat-P CPE1_P L1 W1-R W1-T W1-P Rp Ceff Sample Ω cm2Ωcm 2Ωcm 2Ωcm 2Ω−1cm−2sαcoat Ω−1cm−2sαcoat Ω−1cm−2sαcoat αcoat Hcm2Ωcm2Ωcm2Fcm2 8AF 39.61 (±0.4) 180.5 (±29.6) 31252 (±201) –1.14E-06 (±1.7E-8) –2.81E-06 (±6.44E-8) 0.77 (±0.01) – – – – 31432.5 1.14E-06 8AF+SG 20 .1 601.8 (±89.5) 30945 (±1685) –4.7E-07 (±5.9E-8) 2.9E-08 (±2.5E-9) 0.66 (±0.01) 1 – 68716 (±2067) 0.37 (±0.01) 0.51 (±0.01) 31546.8 1.21E-09 As-received Mg 21.2 (±0.1) – 690.8 (±4.7) 296.4 (±8.2) –2.34E-05 (±4.70E-7) –320.1 (±5.1) –––207.32.34E-05 (8F+SG), lower Ceff (1.21E-09 F cm2) was obtained compared to the anodized coating (8F) (Ceff of 1.14E-06 F cm2), three orders of magnitude less. The sealing of the porous PEO layer and the high cross-linking of the sol-gel coating retard the penetration of the corrosive liquid into the Mg substrate. Figure 9A1-A2 shows the EIS spectra of the 8AF+SG system with immersion time from 2 h to 24 h compared to EIS spectrum of as-received Mg alloy after 2 h of immersion. The impedance modulus │Z│at low frequency domain (f <1 Hz) is generally used to analyse the evolution of the corrosion barrier layer due to the Cl−ion attack.39,40 Figure 9shows that after 2 h of immersion, the lZl value of 8AF+SG system, at low frequency (LF), is three orders of magnitude higher than as-received alloys. The higher value of lZl (∼1E05 Ωcm2-LF) for 8AF+SG system is associated with the effective barrier of the top sol-gel coating. However, the lZl value at LF decreases down to 3.3E04 Ωcm2after 8 h of immersion. The degradation of the silica coating network advances, and the barrier is not enough to effectively stop the corrosion process produced by electrolyte penetration. After 16 h of immersion, a well-defined low frequency inductive loop appears, associated with a localized corrosion process.41 Finally, after 24 h of immersion, the lZl value at LF showed similar values as the as-received Mg Alloy (3.2E02 Ωcm2), indicating the total degradation of the multilayer system. Figure 10A shows an image of the sample after 24 h of immersion, confirming the degradation on the multilayer system. The inset image shows a detail of the corroded 8AF+SG surface where black zones are observed. The surface was further analyzed by Raman spectroscopy to identify the corrosion products. Figure 10B shows the Raman spectrum of the inset Figure 10A.Thebandsat3652cm −1, 443 cm−1, and 280 cm−1are associated with the A1g O-H stretching mode in Mg(OH)2,andA 1g and Eglattice vibrations, and attributed to microcrystalline brucite42 as one of the corrosion products. A band at 1080 cm −1is assigned to magnesium hydroxycarbonate43 due to the presence of CO2in the environment that dissolves in the surface electrolyte. Finally, at 1170 –1700 cm−1a broad band is detected, assigned to the remaining organic molecules of the sol-gel film such as the CH2group of the GPTMS.44 Processing parameters are a key to explain the corrosion performance of the samples and to determine the best conditions for reaching corrosion-improved performance. The results show that the multilayer 8AF+SG system constitutes a significant corrosion barrier. The corrosion behavior of AZ31B alloys is highly improved during the first hours of immersion (8 h). However, the barrier degrades with immersion time leading to extend the corrosion of the substrate. Further work is under development that proposes the addition of environmentally corrosion inhibitors