Electroless ZnO Deposition on Mg-Al Alloy for Improved Corrosion Resistance to Marine Environments

Abstract

Electroless ZnO (≈900 nm) was deposited on the surface of an Mg-Al alloy (AM60) to reduce its degradation in the marine environment. Uncoated and coated ZnO samples were exposed to an SME simulated marine solution for up to 30 days. The AFM and optical images revealed that the corrosion attack on the ZnO-AM60 surface was reduced due to an increase in the surface hydrophobicity of the ZnO coating (contact angle of ≈91.6°). The change in pH to more alkaline values over time was less pronounced for ZnO-AM60 (by ≈13%), whereas the release of ${\mathrm{Mg}}^{2+}$ ions was reduced by 34 times, attributed to the decrease in active sites on the Mg-matrix provided by the electroless ZnO coating. The OCP (free corrosion potential) of ZnO-AM60 shifted towards less negative values of ≈100 mV, indicating that electroless ZnO may serve as a good barrier for AM60 in a marine environment. The calculated polarization resistance (${\mathrm{R}}_{\mathrm{p}}$), based on EIS data, was ≈3 times greater for ZnO-AM60 than that of the uncoated substrate.

1. Introduction

Global demographic growth has resulted in an exponential increase in the consumption of natural, manufactured and energy resources, influencing the ecological footprint and emission of pollutants on our planet [1]. Environmental and energy problems are important issues considered now by various industries, including aerospace, transport and automotive [2]. In particular, the automotive industry has experienced significant growth because of the increase in population and its continuous need for greater mobility, which has influenced the air quality and has consequently motivated the development of more sustainable vehicles [3].

Lightweight materials for structural components are an option that can help address environmental and energy problems which negatively affect population health. A 10% weight reduction in automobile structures would result in a decrease in fuel consumption from 6 to 8% [4]. Furthermore, a 100 kg weight reduction would result in a reduction in the emission of pollutants of between 4 and 12 g/km of car mileage [5]. In recent years, the interest of various countries and regions in reducing pollutant emissions has been noted and is reflected in an increase in the production of light vehicles [6].

Magnesium alloys are one of the lightest structural metallic materials, whose great potential has aroused interest in industrial areas where a reduction in the metal weight of components is required [7,8,9]. Among their properties, the following stand out: low density, excellent processing capacity, recyclability, shock absorption, electromagnetic shielding and high thermal conductivity [10,11]. These alloys have found applications in the production of steering wheels, seats, pedals, interior doors, wheels, and other car parts [12,13,14]. As a result, in the search for lightweight material options, magnesium alloys have attracted great interest [15].

However, magnesium and its alloys are susceptible to corrosion, which limits their use on larger scales [16,17]. The passive layer on their surface, naturally formed in contact with air and humidity $\left(\mathrm{MgO}/\mathrm{Mg}{\left(\mathrm{OH}\right)}_2\right)$, is not sufficiently protective in aggressive urban (acid rain ambience) and marine (chloride content) environments compared to similar layers on aluminum or titanium [18,19]. To reduce the metal loss that occurs during the corrosion process and increase the useful life of Mg alloys, coatings are often used to provide a barrier against aggressive environments and to improve corrosion resistance [20,21]. Methods for the deposition of coatings that have been reported are physical vapor deposition (PVD) [22], sol-gel processing [23,24,25], atmospheric pressure plasma (AP) [26], chemical vapor deposition (CVD) [27], anodizing [28], microarc oxidation (MAO) [29,30,31,32], and chemical conversion [33,34,35]. To enhance coating adhesion and corrosion resistance, the surface of Mg alloys and other metals have been previously treated with chromates [36]. However, due to the toxicity of hexavalent chromium, which causes health problems in the nervous system and the development of cancer, the associated chromates have been unwanted [37,38].

The electroless deposition of coatings is another method, which allows modification of the metal surface, and is frequently used in industrial applications due to its easy application (in the absence of external voltage) and low cost [39]. During electroless deposition, metal ions (cations) of the desired coating are reduced and deposited on the surface of a metal substrate; this process is usually accompanied by a reducing agent present in the electrolyte for electroless deposition [40]. Electroless coatings offer good mechanical properties when $\mathrm{NaB}{\mathrm{H}}_4$ is used as reducing agent [41]. However, the use of DMAB allows for more stable coatings [39].

Zinc oxide (ZnO) coating is considered a good candidate for electroless deposition as it is a versatile material because of its multiple applications in various areas, such as optics [42], electronics [43], and sensor design [44], being a low-cost and environmentally friendly material.

In previous research studies [45,46,47], the corrosion behavior of AM Mg-Al alloys has been extensively characterized during their exposure to a simulated marine environment (SME). The studies confirmed the benefits of subsequent surface modification, by applying different coatings, to improve the corrosion resistance of these alloys in the presence of chloride ions. The present work seeks to further enhance the corrosion resistance of AM60 magnesium alloy by exploring the specific characteristics of ZnO electroless deposit on the surface. The corrosion resistance of the ZnO-coated AM60 surface was studied during its exposure for a 30-day period to a simulated marine environment (SME). The change over time in the SME pH, as well the concentration of released ${\mathrm{Mg}}^{2+}$ ions and open circuit potential (OCP), were considered as indicators of the progress of the corrosion process. As an additional technique, electrochemical impedance spectroscopy (EIS) was performed. The data were correlated with surface morphology and chemical composition and then analyzed using atomic force microscopy (AFM), optical imaging, and X-ray photoelectron spectroscopy (XPS). The contact angle was used to measure the wettability change in the surface.

2. Materials and Methods

2.1. AM60 Samples and SME Model Marine Solution

Extruded AM60 magnesium alloy, provided by Magnotec (Bottrop, Germany), has been proposed as a structural material for car components, and according to specifications, the nominal composition (wt.%) is 6.0 Al, 0.2–0.4 Mn and the balance is Mg. AM60 cylindrical samples were cut with a diameter of 10 mm and a thickness of 2 mm. The working area of the samples exposed to the tests was $0.78 {\mathrm{cm}}^2$. The marine model solution composition of SME [45] consisted of the following analytical grade reagents: $5.84 \mathrm{g}·{\mathrm{L}}^{-1}$ NaCl, $4.09 \mathrm{g}·{\mathrm{L}}^{-1}$ ${\mathrm{Na}}_2{\mathrm{SO}}_4$ and $0.20 \mathrm{g}·{\mathrm{L}}^{-1}$ $\mathrm{NaHC}{\mathrm{O}}_3$ dissolved in ultrapure deionized water (18.2 MΩ), and its measured pH was 7.94 (PH60 Premium Line, pH tester, Apera Instruments, LLC., Columbus, OH, USA).

2.2. Pretreatment of AM60 Surface

The removal of the naturally existing layer of oxide/hydroxide on the AM60 surface was carried out using silicon carbide (SiC) sandpaper with grain sizes up to 2000, polishing the samples using ethanol as a lubricant, which were then sonicated in an ultrasonic bath for 5 min in ethanol, dried at room temperature (21 °C) and stored. Subsequently, the sample surface was cleaned, etched and activated in solutions of three stages [48,49]: (1) $40 \mathrm{g}·{\mathrm{L}}^{-1}$ NaOH and $10 \mathrm{g}·{\mathrm{L}}^{-1}$ ${\mathrm{Na}}_2\mathrm{H}{\mathrm{PO}}_4$ for 10 min; (2) $20 \mathrm{mL}·{\mathrm{L}}^{-1}$ ${\mathrm{CH}}_3\mathrm{COOH}$ and $5 \mathrm{g}·{\mathrm{L}}^{-1}$ $\mathrm{Na}{\mathrm{NO}}_3$ for 10 s and (3) $80 \mathrm{g}·{\mathrm{L}}^{-1}$ ${\mathrm{NH}}_4{\mathrm{H}}_2{\mathrm{PO}}_4$ and $30 \mathrm{g}·{\mathrm{L}}^{-1}$ ${\mathrm{NH}}_4\mathrm{F}$ for 10 min. All solutions were at room temperature.

2.3. Electroless Deposition of ZnO

The pretreated AM60 samples were immersed in a chemical bath, containing 5 mM $\mathrm{Zn}{\left({\mathrm{NO}}_3\right)}_2·6{\mathrm{H}}_2\mathrm{O}$ and 3 mM DMAB, for 5 min at a temperature of 50 °C [50]. After the coating deposition, the samples were removed from the solution, dried at room temperature and stored in a desiccator.

The ZnO electroless process followed the steps summarized below (reactions 1–5) [51,52]. The dissociated nitrate ions (1) are reduced using the reducing agent DMBA (2–3), with a formation of nitrite ions $\left({\mathrm{NO}}_2^{-}\right)$ and hydroxyl ions (${\mathrm{OH}}^{-})$ which interact with the ${\mathrm{Zn}}^{2+}$ ions and generate zinc hydroxide (4). This product is not stable, and it is transformed into ZnO, depositing on metal substrate (5).

$$\mathrm{Zn}{\left({\mathrm{NO}}_3\right)}_2\to {\mathrm{Zn}}^{2+}+2{\mathrm{NO}}_3^{-}$$

(1)

$${\left({\mathrm{CH}}_3\right)}_2{\mathrm{NHBH}}_3+2{\mathrm{H}}_2\mathrm{O}\to {\mathrm{HBO}}_2+{\left({\mathrm{CH}}_3\right)}_2{\mathrm{NH}}_2^{+}+5{\mathrm{H}}^{+}+6{\mathrm{e}}^{-}$$

(2)

$${\mathrm{NO}}_3^{-}+{\mathrm{H}}_2\mathrm{O}+2{\mathrm{e}}^{-}\to {\mathrm{NO}}_2^{-}+2{\mathrm{OH}}^{-}$$

(3)

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

(4)

$$\mathrm{Zn}{\left(\mathrm{OH}\right)}_2\to \mathrm{ZnO}+{\mathrm{H}}_2\mathrm{O}$$

(5)

2.4. Immersion Test and Surface Characterization

The ZnO-AM60 samples were immersed for up to 30 days in 20 mL of the model marine-environment solution (SME), in accordance with ASTM G31-12a [53]. During the immersion test, the change in pH of the solution was monitored and the concentration of the released ${\mathrm{Mg}}^{2+}$ ions into the SME solution was measured by photometry (HI83200, Hanna Instruments, Wonsocket, RI, USA).

The morphological surface characterization, before and after exposure to SME solution, was acquired using a model Nikon Eclipse LVision optical microscope (Nikon Instruments, Tokyo, Japan) and atomic force microscopy (AFM). The changes in surface wettability of the AM60 and ZnO-AM60 surfaces were assessed by obtaining the contact angle (CA) using a sessile drop of 5 μL volume (at 21 °C). The images were obtained with a digital camera (×30 magnification). The thickness of the ZnO electroless deposit was obtained using a profilometer model KLA (Tencor D-120) and the adhesion test was performed using a stylus tip while varying the force sweeps. The X-ray photoelectron spectroscopy (XPS, K-Alpha Surface Analyzer, Thermo Scientific, Waltham, MA, USA) spectra were analyzed to provide information on the surface chemical composition (monochromatic Al Kα X-ray source, hv = 1486.46 eV). The binding energies of the spectra were fitted with a C1s carbon peak at 284.8 eV.

2.5. Electrochemical Analysis

The electrochemical activity of the AM60 surfaces, uncoated and coated, was evaluated by the changes over time in open circuit potential (OCP, considered as free corrosion potential) and electrochemical impedance spectroscopy (EIS). EIS tests were carried out at a perturbation amplitude of $\pm 10 \mathrm{mV}$ with a frequency range from 100 kHz to 10 mHz. All electrochemical tests were performed using a potentiostat/galvanostat/ZRA (Interface-1000E, Gamry Instruments, Philadelphia, PA, USA) in a three-electrode configuration, with a Pt mesh (Alfa Aesar, Ward Hill, MA, USA) as the counter electrode, a saturated calomel (SCE, Gamry Instruments, Philadelphia, PA, USA) as the reference electrode and the sample with an exposed area of $0.78{\mathrm{cm}}^2$ as the working electrode.

3. Results and Discussion

3.1. Surface Characterization

The AFM topography of the AM60 alloy surface and electroless ZnO-coated AM60 are compared in Figure 1 (scanning area of 50 × 50 μm). The morphology of the uncoated substrate (Figure 1a) revealed the presence of lines associated with the sample polishing process. The surface of the electroless ZnO deposit on AM60 showed clusters larger than 10 μm in size and agglomerations (dimensions between 2.5 and 5 μm). The average roughness $\left({\mathrm{R}}_{\mathrm{a}}\right)$ of the AM60 surface was ≈53 nm, whereas that of the AM60 coated with ZnO increased by up to ≈614 nm. The AFM images suggest that the electroless ZnO coating could act as a physical barrier for the AM60 surface when exposed to a marine-coastal environment (SME).

Figure 2 shows the thickness profile of the ZnO electroless coating deposited on the AM60 surface divided into three regions associated with the zone of the AM60 substrate, the interface that joins the substrate with the ZnO coating and the third region that corresponds to the ZnO deposit. The thickness of the film was found to be $900 \pm 165 \mathrm{nm}$.

The optical images in Figure 3 show the change in the morphology of ZnO-AM60 (sample pattern, Figure 3a) after the performance of the adhesion test at force sweeps of 0.1 mN, 0.05 mN and 0.01 mN (Figure 3b–d, respectively). It can be observed that as the force exerted on the ZnO coating surface decreases, the changes begin to be imperceptible. After the adhesion test, detached agglomerations as a part of the coating were not observed.

Figure 4 presents the variation in the atomic concentration with respect to the sputtering (erosion) applied for the obtention of the XPS depth profile. The main signals were O1s, Zn2p and, in lower concentrations, Mg1s and C1s. The profile curves can be divided into two regions.

In region I, the high carbon content (C1s) was associated with a contamination layer formed during the exposure of the coated samples to air. However, after 30 s of sputtering, the C1s signal decreases abruptly, due to the erosion performed by an argon ion beam to remove this layer, maintaining this trend until 270 s of sputtering. On the other hand, the atomic concentration of the O1s and Zn2p signals remained constant after 30 s of sputtering until the end of the analysis time (Region II). The Mg1s signal showed a slightly increasing trend. The information obtained from the depth profile was specific for ZnO (of $900 \pm 165 \mathrm{nm}$ thickness) because the Mg1s signal did not exceed the O1s and Zn2p signals in the region closer to the alloy matrix. The low concentration of Mg1s suggests that the ZnO coating may provide a good physical barrier for the AM60 alloy surface.

The high-resolution spectra of Zn2p, O1s, Mg1s and C1s of the depth profile obtained after 60 s of sputtering time (Figure 4) indicated a stable concentration of these elements and, therefore, they were used to determine the chemical bonds corresponding to these signals (Figure 5). The Zn2p XPS spectrum (Figure 5a) suggested the presence of two signals located in satellites at 1021.7 eV and 1044.8 eV that correspond to the peak energy of the $\left(\mathrm{Zn}2{\mathrm{p}}_{3/2}\right)$ and $\left(\mathrm{Zn}2{\mathrm{p}}_{1/2}\right)$ satellites [54,55], respectively. The difference between these two signals was 23.1 eV, which indicates that the Zn species are in the oxidation state ${\mathrm{Zn}}^{2+}$ of ZnO [55,56,57].

On the other hand, the O1s spectrum revealed an asymmetry in its signal, due to various forms of oxygen bonding in the ZnO coating. The signal was deconvolved into two peaks: the first located at 530.1 eV, associated with the presence of oxygen in the ZnO, whereas the second peak at 531.9 eV was ascribed to bonds with hydroxyl ions $\left(\mathrm{HO}–\mathrm{ZnO}\right)$ [54,55]. The spectrum of Mg1s presented two signals as compounds formed on the Mg-matrix during the electroless deposition of ZnO: one at 1303.8 eV, characteristic of ${\mathrm{Mg}\left(\mathrm{OH}\right)}_2$ and the other at 1304.6 eV, attributed to MgO [55,58]. The C1s signal presented two peaks: one at 284.8 eV, ascribed to adventitious carbon $\left({\mathrm{C}}_{\mathrm{adv}}\right)$ (commonly used as a reference for XPS analysis) and the other signal at 290.1 eV is characteristic of ${\mathrm{CO}}_3^{2-}$ [55] because of the presence of air ${\mathrm{CO}}_2$ dissolved in the aqueous electrolyte used for the electroless ZnO deposition. However, based on the lower C1s atomic concentration profile (Figure 4), the presence of the two carbon compounds must be very insignificant.

The measurement of the contact angle (CA) was carried out to analyze the affinity of the AM60 and ZnO-AM60 (as solid) to a liquid phase, known as wettability. This parameter could be affected by the roughness of the solid surface. The classification of the wettability of a solid surface is according to the value of the measured CA: hydrophilic surface $\left(\mathrm{CA}<90°\right),$ hydrophobic surfaces $\left(90°<\mathrm{CA}<150°\right)$ and superhydrophobic $\left(\mathrm{CA}>150°\right)$ [59,60,61].

Figure 6 shows the CA value of the AM60 surface (Figure 6a) compared to that of the ZnO-AM60 surface (Figure 6b). It can be noted that the surface of the uncoated AM60 alloy surface is hydrophilic as its CA value was 45.8° $\left(\mathrm{CA}<90°\right)$. After the electroless modification of the AM60 substrate, the CA value of the ZnO-AM60 surface increased to 91.6° (hydrophobic surface, $\mathrm{CA}>90°$).

The changes in the contact angle value before and after ZnO electroless deposition can be explained by the Wenzel [62] and Cassie–Baxter [63] models, considering that greater surface roughness corresponds to a higher CA (hydrophobic surface) due to air trapped in the liquid–air and solid–air–liquid interfaces [64]. As a consequence, the contact between the aggressive medium (SME) and the ZnO-AM60 surface was reduced. The measured CA values (Figure 6) correlate well with the average roughness values: ZnO-AM60 presented higher roughness $\left({\mathrm{R}}_{\mathrm{a}}=614 \mathrm{nm}\right)$ and was considered as a hydrophobic surface, whereas the lower ${\mathrm{R}}_{\mathrm{a}}=53 \mathrm{nm}$ of the uncoated AM60 led to a hydrophilic surface. Thus, the ZnO-AM60 surface will have reduced contact with the aggressive marine solution and the Mg-matrix will suffer from fewer corrosion attacks because of the protective effect of the electroless ZnO deposit.

The optical images below compare the surface morphology of the uncoated AM60 (Figure 7a) and ZnO-AM60 (Figure 7b) after removing the layers formed during exposure to the SME solution for 30 days. The uncoated AM60 surface has been more attacked by the corrosion process (Figure 7a), whereas that of the ZnO-AM60 (Figure 7b) was less susceptible to corrosion.

The corrosion attacks on the uncoated AM60 as Mg-Al alloy surface (Figure 7a) were influenced by the presence of the AlMn intermetallic particles (as local cathodic sites) around which the Mg-matrix (anodic active) suffered degradation (localized corrosion) [45], expressed by the ${\mathrm{Mg}}_{\left(\mathrm{ac}\right)}^{2+}$ release. The deposited electroless ZnO coating, as a physical barrier between the medium (SME) and the Mg-matrix, makes the entry of the SME into the AM60 matrix more difficult thus reducing its degradation.

3.2. Immersion Test Results

The change in pH value of the SME solution over time during the exposure of uncoated and ZnO-coated AM60 for 30 days to the aggressive marine environment (SME) is shown in

Table 1. Change in pH of SME over time during exposure of ZnO-coated and uncoated AM60 for 30 days.

Sample	AM60	ZnO-AM60
pH	9.26	8.17

.

At the end of the immersion test, the pH of the SME solution was ≈13% less alkaline, after the exposure of ZnO-AM60, compared to that of the uncoated AM60 alloy. The change in pH over time is related to the reactions that occur during the Mg-matrix corrosion process shown in Equations (6)–(8).

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

(6)

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

(7)

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

(8)

The $\mathrm{Mg}{\left(\mathrm{OH}\right)}_2$ corrosion product is poorly soluble ($0.69 \mathrm{mg}·{\mathrm{L}}^{-1}$) [65] and it could act as a physical barrier against the progress of the Mg-matrix corrosion. However, in the presence of ${\mathrm{Cl}}^{-}$ ions (SME solution), the $\mathrm{Mg}{\left(\mathrm{OH}\right)}_2$ layer is gradually transformed into highly soluble $\mathrm{Mg}{\mathrm{Cl}}_2$ ($56 \mathrm{g}·{\mathrm{mL}}^{-1})$ and the release of ${\mathrm{Mg}}^{2+}$ ions initiates and accelerates the Mg-matrix degradation, as shown in Equation (9):

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

(9)

The release of ${\mathrm{Mg}}^{2+}$ ions serves as an indicator of the progress of the $\mathrm{Mg}{\left(\mathrm{OH}\right)}_2$ protective layer degradation in the presence of ${\mathrm{Cl}}^{-}$ ions (SME environment). The AM60 surface coated with electroless ZnO deposit presented ≈$10 \mathrm{mg}·{\mathrm{L}}^{-1}$ concentration of released ${\mathrm{Mg}}^{2+}$ ions, which was ≈34 times lower than that of the uncoated AM60 $(334 \mathrm{mg}·{\mathrm{L}}^{-1})$ exposed for 30 days to the SME solution (

Table 2. Concentration of ${\mathrm{Mg}}^{2+}$ ions released after 7 and 30 days of exposure of uncoated AlM60 and ZnO-AM60 to SME marine environment.

Sample	7 Days	30 Days
AM60	83.3	333.3
ZnO-AM60	5.0	10.0

).

3.3. Open Circuit Potential

Table 3. Changes over time in OCP values after 30 days of exposure to simulated marine environment (SME).

OCP (V vs. SCE)
Time (Days)	1	7	15	30
AM60	−1.580	−1.540	−1.520	−1.520
ZnO-AM60	−1.462	−1.491	−1.474	−1.431

summarizes the change over time in the open circuit potential (OCP, free corrosion potential) of the uncoated AM60 alloy and ZnO-AM60, after their immersion for 30 days in the SME marine environment. The initial value of the ZnO-AM60 was ≈120 mV less negative than that of the uncoated AM60 surface. At the end of 30 days, with the progress of the corrosion attacks, this difference was ≈100 mV, indicating that that electroless ZnO deposit can provide protection for the surface of AM60.

3.4. Electrochemical Impedance Spectroscopy

To characterize the activity of the interface ZnO-AM60/marine solution (SME), electrochemical impedance spectroscopy (EIS) was carried out (Figure 8). The Bode and Nyquist plots are common tools that are used to evaluate the level of protection provided by a coating.

From the Nyquist diagrams (Figure 8a), it was possible to notice the presence of two capacitive arcs at high frequency (HF) and at medium frequency (MF), respectively. The diameter of the HF arc (semi-circle) is usually associated with the specific characteristics of a formed corrosion layer and/or the activity of the deposited coating, whereas that at medium frequencies, is associated with the charge transfer processes (${\mathrm{H}}_{2\left(\mathrm{g}\right)}$ evolution and release of ${\mathrm{Mg}}^{2+}$, Equations (8) and (9)) [66,67]. After the first day of exposure, the capacitive arc of ZnO-AM60 was much larger than that of the uncoated AM60 alloy, which indicated the protective capacity of the electroless ZnO deposit. This trend continued until 30 days of exposure. In the case of the uncoated AM60 surface, the tendency of the arc to increase was attributed to the corrosion layers formed during the exposure to SME for up to 30 days.

The Bode impedance ${\left|\mathrm{Z}\right|}_{0.01\mathrm{Hz}}$ values (Figure 8b) for ZnO-AM60 exposed to the marine environment were higher than those obtained for the uncoated AM60, indicating that the electroless ZnO coating possesses good efficiency as a protector of the Mg-matrix, hindering the electrochemical process of corrosion Equations (6)–(9). For this reason, the capacitive arcs observed in the Nyquist diagram were more pronounced for ZnO-AM60 (Figure 8a). In this way, the Nyquist and Bode diagrams (Figure 8) were used to measure the efficiency of ZnO electroless deposited on the surface of the AM60 alloy [68,69]. After the first day of exposure to the aggressive environment of ${\mathrm{Cl}}^{-}$ ions (SME), the impedance modulus value ${\left|\mathrm{Z}\right|}_{0.01\mathrm{Hz}}$ of the ZnO-AM60 sample was ≈3 times higher than that of the uncoated AM60 alloy surface ($9.6 \mathrm{k}\mathsf{\Omega } ·{\mathrm{cm}}^2$). As the time increased up to 30 days, the ${\left|\mathrm{Z}\right|}_{0.01\mathrm{Hz}}$ values of ZnO-AM60 tended to an increment of 20%, associated with the ${\mathrm{Mg}}^{2+}$ ions release $\left(10 \mathrm{mg}·{\mathrm{L}}^{-1}\right)$, with the ${\left|\mathrm{Z}\right|}_{0.01\mathrm{Hz}}$ values always being higher (≈2 times) than those of the uncoated AM60 surface, indicating the protective effect provided by the electroless ZnO deposit for the AM60 alloy.

The equivalent circuit (Figure 9) was used to reveal more evidence of the protective effect of the electroless ZnO coating for theAM60 alloy’s surface. The circuit is composed of the following components: solution resistance $\left({\mathrm{R}}_{\mathrm{s}}\right); {\mathrm{CPE}}_1$ associated with the coating capacitance and/or corrosion products; ${\mathrm{R}}_1$ as the resistance of the doble layer formed at the interface metal/electrolyte; ${\mathrm{CPE}}_2$ and ${\mathrm{R}}_2$ associated with the charge transfer between the metal surface and electrolyte. The polarization resistance (${\mathrm{R}}_{\mathrm{p}})$ was calculated as ${\mathrm{R}}_{\mathrm{p}}={\mathrm{R}}_1+{\mathrm{R}}_2$. The fitted EIS results are summarized in

Table 4. Fitting parameters for AM60 and ZnO-AM60, after their immersion in SME solution (for 1, 7 and 15 days), estimated from EIS experimental data.

Sample	${\mathbf{R}}_{\mathbf{s}}$ (Ω cm2)	${\mathbf{CPE}}_1$ $\left(\mathsf{\mu }\mathbf{S}{·\mathbf{s}}^{\mathbf{n}}{\mathbf{cm}}^{-2}\right)$	${\mathbf{n}}_1$	${\mathbf{R}}_1$ $\left(\mathbf{k}\mathsf{\Omega }·{\mathbf{cm}}^2\right)$	${\mathbf{CPE}}_2$ $\left(\mathbf{mS}{·\mathbf{s}}^{\mathbf{n}}{\mathbf{cm}}^{-2}\right)$	${\mathbf{n}}_2$	${\mathbf{R}}_2$ $\left(\mathbf{k}\mathsf{\Omega }·{\mathbf{cm}}^2\right)$	${\mathbf{R}}_{\mathbf{p}}$ $\left(\mathbf{k}\mathsf{\Omega }·{\mathbf{cm}}^2\right)$
AM60-1D	63.4	9.86	0.936	6.64	0.35	0.720	3.12	9.76
ZnO-AM60-1D	69.9	10.2	0.905	19.35	0.29	0.719	17.07	36.42
AM60-7D	68.1	16.17	0.945	10.27	1.57	0.932	2.19	12.46
ZnO-AM60-7D	60.8	24.81	0.820	27.43	1.55	0.998	11.97	39.4
AM60-15D	71.2	39.86	0.930	10.06	6.33	0.970	2.45	12.51
ZnO-AM60-15D	64.3	42.28	0.843	28.64	1.4	0.967	12.83	41.47

.

It can be noted that the values of ${\mathrm{R}}_1$ and ${\mathrm{R}}_2$ resistances are several times higher for ZnO-AM60 than those of the uncoated AM60 alloy surface, indicating a good protective barrier of the electroless ZnO coating when exposed to the simulated marine environment. Furthermore, the charge transfer resistance ${(\mathrm{R}}_2)$ values suggested lower electrochemical activity on the ZnO-AM60 surface and, therefore, a lower corrosion rate. The calculated polarization resistance (${\mathrm{R}}_{\mathrm{p}}$) was ≈3 times greater due to the electroless ZnO deposit on the AM60 alloy surface.

4. Conclusions

In this work, electroless ZnO ($900 \pm 165 \mathrm{nm}$) was deposited on a Mg-Al alloy (AM60) and the corrosion activity of ZnO-AM60 was characterized during its exposure to a simulated marine environment (SME) for 30 days.

The AFM surface topography revealed an increase in the roughness (R_a = 614 nm) of the AM60 surface after the deposition of ZnO.

The XPS depth profile showed that the coating on AM60 was composed of ZnO (of $900 \pm 165 \mathrm{nm}$ thickness), which could reduce the contact between the Mg-matrix and the marine environment because the Mg1s signal did not exceed the O1s and Zn2p signals in the region closer to the alloy matrix.

The wettability of the AM60 surface changed from hydrophilic (45.8°) to hydrophobic (91.6°) when ZnO was deposited and because the increment in surface roughness led to a trapping of air, the contact angle (wettability) was influenced. These factors led to a decrease in the corrosion rate of ZnO-AM60, making it more difficult for chloride ions to attack the Mg-matrix on the surface.

These effects were reflected in the minor shift (≈13%) of the SME pH to less alkaline values and lower concentrations of released ${\mathrm{Mg}}^{2+}$ ions (≈34 times) during the exposure of the ZnO-AM60 to SME, compared to that of the uncoated AM60 alloy’s surface. Both parameters are associated with the rate of the corrosion process.

The OCP (free corrosion potential) of ZnO-AM60 shifted to less negative values (≈100 mV), indicating the protection efficiency of the ZnO coating and diminishing the degradation rate of the Mg-matrix when exposed to the SME marine ambience.

The EIS results indicated that the polarization resistance (${\mathrm{R}}_{\mathrm{p}}$) was ≈3 times higher for ZnO-AM60 exposed to the aggressive SME solution than that of the uncoated AM60 alloy. Furthermore, ${\mathrm{R}}_2$ values associated with charge transfer resistance suggested lower progress in electrochemical corrosion when the AM60 was coated with ZnO, which was confirmed by the optical images.

The results of this study suggest that the electroless ZnO deposit tested may improve the corrosion resistance of Mg-Al alloy (AM60) in the presence of chloride ions, which are characteristic of the marine environment.