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Fabrication and Electrochemical Corrosion Behavior of PEO Coatings on Strip-Cast AZ31Mg Alloy in 3.5% NaCl Solution Arumugam Madhan Kumar,† Sun Hwan Kwon,† Hwa Chul Jung,† Young Hee Park,‡ Hea Jeong Kim,‡ and Kwang Seon Shin*,† †

Magnesium Technology Innovation Center, School of Materials Science and Engineering, Seoul National University, Gwanak-ro, Seoul151-744, Republic of Korea ‡ Research Division of Magnesium, Research Institute of Industrial Science and Technology, San 32 Hyoja dong, Nam-gu, Pohang 790-330, Republic of Korea S Supporting Information *

ABSTRACT: The generation of compact plasma electrolytic oxidation (PEO) coatings with outstanding corrosion resistance is essential for the widespread application of Mg alloys. In the present investigation, PEO ceramic coatings formed at different silicate concentration and oxidation time on AZ31 Mg alloy were studied, and the resultant surface structures of the oxide films were observed using scanning electron microscopy (SEM) and atomic force microscopy (AFM) analysis. X-ray photoelectron spectroscopy and thin film-X-ray diffraction analysis of ceramic coatings showed that the surface coating is mainly composed of Mg2SiO4, and MgO with different amount based on oxidation time. Further, the potentiodynamic polarization and electrochemical impedance spectroscopic (EIS) measurements were used to characterize corrosion behavior of PEO coated substrates in 3.5% NaCl solution. The results revealed that PEO coatings processed with 0.2 M silicate showed better performance due to its dense and compact coating with fewer structural imperfections in comparison to others.

1. INTRODUCTION Since magnesium (Mg) and its alloys are the lightest structural materials, they are widely employed in aircraft, automobile, and aerospace applications, in which a reduction in the total weight could lead to fuel saving or load increase. A die-casting method has been employed to manufacture the majority of Mg-alloy components used in wide applications. However, high strength Mg alloys used in the aerospace applications have brought more attention toward the strip-casting method, which generally offer higher strength and better ductility over conventional processes. Despite their desirable properties, they are extremely susceptible to corrosion due to high chemical reactivity of Mg, leading to losses of strength and toughness, and thus restrict Mg and its alloys from being applied to the practical applications especially in some harsh environtments.1−3 In addition, impurities and second phases act as active cathodic sites leading to local galvanic corrosion over Mg surface.4 Hence, protection of Mg alloys against corrosion is of great importance to increase their usage. Various attempts have been made to prevent or retard the destructive effect of corrosion on Mg and its alloys. During the last two decades, several coating techniques such as chemical conversion, electrodeposition, physical vapor deposition, chemical vapor deposition, thermal spray, and anodic oxidation have been developed to improve the corrosion performance of Mg and its alloys.5−9 Among these coating techniques, plasma electrolytic oxidation (PEO) is the most commercially applied protection method for Mg alloys in recent years, providing an excellent bonding strength with the substrate, enhanced wear, and corrosion resistance.10−12 In this technique, the metal surface is anodically oxidized in an electrolyte at a voltage that creates spark or arc electric © 2014 American Chemical Society

discharges in the region near metal, which makes it possible to produce oxide layers. The formation of oxide layers by dielectric breakdown and associated processes involve the inclusion of compounds based on the electrolyte components. The method is easy, offers reproducible results, and is used predominantly to obtain protective oxide layers over Mg and its alloys. The microstructure and properties of PEO coatings depend on numerous important process variables, such as the power source, the electrolyte concentration and composition, coating time, the current density, and the oxidation time.13 In general, PEO coatings on Mg alloys are composed of two main layers namely outer layer and inner layer close to the substrate, in which the former is relatively thicker than the latter; however, the outer layer might be weak against corrosion attack due to many micropores. In order to improve corrosion protection, the alteration of the outer layer is required with respect to the constituent compounds which exist.14 It is wellknown that the electrolyte containing silicate and fluoride can form stable products, such as Mg2SiO4 or MgF2, that act as barrier layer and protect Mg alloy.13,15 Alkaline solutions containing silicate, phosphate, and fluoride have been widely used for PEO coatings of Mg alloys.16−18 Recent research related to PEO processes on Mg alloy are mainly focused on the PEO conditions, such as electrolyte composition19 and voltage mode.20 Nevertheless, it has already been reported that the PEO coatings oxidized on aluminum surface at different time intervals exhibit different properties.21 Received: Revised: Accepted: Published: 9703

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Hence, it is valuable to study the growth process of PEO ceramic coatings on Mg alloys with different coating time in order to get high quality coatings. The primary intention of the current study is to evaluate the impact of coating time on PEO ceramic coatings on AZ31 magnesium alloy from silicate electrolyte. SEM, AFM, XRD, and XPS were used to examine the morphology, topography, and composition of those coatings. The present study also aims at examining the surface and corrosion protection aspects with PEO ceramic coatings prepared from different concentration of silicate electrolytes. In this regard, potentiodynamic polarization and electrochemical impedance spectroscopic (EIS) measurement were carried out to evaluate the corrosion resistance of the PEO coatings in 3.5% NaCl solution.

potential (OCP). The standard three electrode cell assembly consisted of a standard calomel electrode (SCE) as reference, a Pt counter electrode, and the coated substrates as the working electrode, respectively. Polarization curves for uncoated and coated substrates were recorded under potentiodynamic conditions in the potential range ±250 mV with respect to OCP at a sweep rate of 1 mV s−1. The EIS equipment was set up in the frequency range between 0.1 and 104 Hz with amplitude of ±10 mV. The analysis of the impedance spectra and fitting of the experimental results to equivalent circuits was performed using ZSimpWin 3.21, which allowed the chi-square (χ2) value to judge quality of the equivalent circuit fitting. In order to test the reproducibility of the results, the experiments were performed in triplicate.

2. EXPERIMENTAL SECTION 2.1. Preparation of Substrates. The substrate material used for this investigation was strip-cast AZ31 magnesium alloy with a chemical composition (wt %) of 2.5−3.5 Al, 0.6−1.4 Zn, 0.2−1 Mn, 0.1 Si, 0.05 Cu, and Mg balance. The samples with a size of 20 mm × 50 mm × 3.35 mm were cut and ground using 2000 grit silicon carbide sheets, cleaned using distilled water and acetone, and then dried in cold air. 2.2. Preparation of PEO Ceramic Coatings. The PEO process was carried out using a pulsed dc electrical power source with the AZ31 Mg-alloy substrate acting as the anode and a stainless steel cylinder container acting as the cathode. KOH (0.03 M), KF (0.05 M), and Na2SiO3 (0.05, 0.1, 0.2, and 0.3 M) were dissolved using distilled water, and then placed in the cylinder container as electrolyte. The temperature of the electrolytes during the processing was always maintained at 20 ± 2 °C by a water cooling system, and the electrolyte was stirred continuously during the treatment. The PEO coating was prepared in this solution at a current density of 7 mA/cm2, and the pulse frequency and duty ratio were 500 Hz and 50%, respectively. In order to study the formation processes of the PEO coatings, the substrates were oxidized for different oxidation time periods (300, 600, 1200, and 1800 s), and then the substrates were rinsed with deionized water and dried in the warm air. In order to assess the adhesion of PEO coated substrates more precisely, a Rockwell C diamond stylus of tip radius 50 μm was performed in a progressive mode of 0.5 N n −1 over a scratch length of 3 mm at an indenter speed of 0.6 mm/min. 2.3. Surface Characterization of PEO Ceramic Coatings. PEO coated substrates were examined on the plane view and at the cross-section by SEM, using a JSM-6360 (JEOL) instrument operated at 20 kV, coupled with energy-dispersive X-ray (EDAX) analysis. The surface topography was analyzed through AFM using the NANO Station II Surface Imaging Systems. The images were acquired by noncontact mode using Au coated silicon cantilevers with a spring constant of 1.6 N/m at a resonance frequency of 26 kHz in at room temperature. The phase composition of PEO coatings was investigated by Xray diffraction (XRD), using Cu and Co Kα radiation, using Philips X’Pert-MPD (PW 3040), with a step size 0.005° and a scan range from 10° to 80° (in 2θ). 2.4. Electrochemical Characterization of PEO Coatings. In order to evaluate the corrosion performance of the PEO coatings, both polarization and EIS tests were performed in 3.5% NaCl solution using coated substrates for each of these tests. Prior to the measurements, the substrates were immersed in the test solution for 30 min to reach a stable open circuit

3. RESULTS AND DISCUSSION 3.1. Voltage−Time Response. Figure 1 displays the voltage profiles during PEO treatment of the substrates in

Figure 1. Voltage−time response curves during PEO processing at different silicate concentration on AZ31 Mg substrate (VBD in brackets).

different concentration of silicate electrolyte. It can be observed from the curves that four regions were found during processing. During the first 50−60 s (region I), the voltage increases slowly at a constant rate with time. In this region, the dissolution of Mg substrate occurs and loses its metal brightness; consequently, a thin barrier layer is produced on the metal surface.22 Once the voltage surpasses a critical value, visible microsparks of blue color with acoustic emission on the surfaces of the substrate can be observed evenly on the whole surface. It is well-known that the critical voltage relating to the appearance of microsparks on the substrate surface is termed as the breakdown voltage (VBD), which has a strong dependence on the composition and conductivity of the electrolyte. As can be seen from the figure, the value of VBD increased with increasing silicate concentration during the PEO process. During the PEO process, dielectric breakdown occurs when the applied voltage exceeds the critical voltage of the initial barrier layer on the substrate, and a number of visible sparks appear on the surface of the substrate. Further, Ikonopisov proposed a theoretical model of breakdown caused by an avalanche of electron injection at the electrolyte/oxide interface and found out that this phenomenon depends on the nature of the anodized metal as well as composition and resistivity of 9704

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concentration of 0.2 M silicate. The thickness of the PEO coatings was measured using an Elcometer instrument. The average thickness of PEO coatings was found to be about 7.5− 15 μm (Table 1). It is noteworthy to mention that the

electrolyte. According to this model, the relationship of the breakdown voltage with the electrolyte conductivity is represented by the following equation: VBD = aB + bB log(1/k)

(1)

Table 1. Description of PEO Coatings for Different Oxidation Time and Silicate Concentrations

Here, aB and bB are constant values for a given metal and electrolyte composition, and VBD and k are breakdown voltage and electrolyte conductivity, respectively. This equation indicates a relationship between conductivity and the breakdown voltage of the anodic process. Although there is a reverse relation between conductivity and VBD, our results show a reverse trend. Since various processes, including chemical, electrochemical, thermodynamical, and plasma-chemical reactions, occur at the discharge sites, due to the increased local temperature (103 to 104 K) and pressure (up to ∼102 MPa), breakdown voltage is not only dependent on conductivity of the electrolyte, but also dependent on the nature of solute present in the electrolytes, nature of metal under investigation, pH of the electrolyte, and chemical stability of ionic species at the electrolyte/metal interface. Moreover, the type of the ions existing in the electrolyte is a factor which influences the stability of the passive layer and final properties of the PEO coating. We have also compared the parameters including conductivity, pH, and VBD of the present investigation (Supporting Information as Table S1). From the comparison, it was concluded that the difference in the electrolytic conductivity is much smaller due to low ionic mobility of sodium silicate, and this slightly affects the voltage−time characteristics during PEO processing. In addition, the higher breakdown voltage with increasing silicate concentration may be due to the higher alkalinity of sodium silicate solution with concentration (pH 13.89) and synergetic effect between the sodium silicate and potassium hydroxide at electrolyte/metal interface. In general, in potassium hydroxide electrolytes, in order to initiate sparks on the alloy surface higher voltages needed to be applied as compared to sodium hydroxide electrolytes.23 Khaselev et al. also found that with the addition of Al(OH)3 the potassium hydroxide (KOH) electrolyte is highly related to sparking behavior during surface treatment in that the breakdown potential linearly increased as the Al(OH)3 concentration increased.24 Further, the reduction in the rate of the increase in voltage was similar in all electrolyte concentrations. However, microsparks on the substrate surfaces were established in region I. It is significant to mention that the voltage and the number of microsparks on the substrate surfaces in higher electrolyte concentration are larger than those obtained in lower one. While the process enters region II, the slope of the curve becomes smaller than region I, where the appearance of the microsparks becomes more prominent. Then, the process enters region III, and the voltage is in a more steady-state; only changes in the appearance of microsparks are observed. Different discharge sparking was found during different regions of PEO (photographic images of micro sparks are shown in the Supporting Information as Figure S1). With increasing concentrations of silicate, the third stage was longer, and the final voltage was increased. In the 1200 s treatment time, the final voltage in 0.05 M silicate electrolyte (219 V) is smaller than that in 0.3 M silicate electrolyte (254 V). In order to evaluate the influence of oxidation time on surface and corrosion performance of PEO coatings, the above-mentioned process was performed under similar conditions with different oxidation time of about 300, 600, 1200, and 1800 s with the

substrate thickness (μm) 300 s 600 s 1200 s 1800 s 0.05 M 0.1 M 0.2 M 0.3 M

7.8 8.6 10.4 10.8 10.5 11.4 12.3 15.7

LC (N)

av diameter of pores (μm)

RRMS (nm)

3.5 3.5 3.7 3.7 3.7 3.7 3.8 3.8

3.07 3.10 5.65 17.78 3.31 2.77 2.90 17.25

459 324 340 548 385 393 356 468

thickness of PEO coatings increases with increasing oxidation time and concentration of silicate in the electrolytic solution. Further, to acquire a quantitative result about adhesion of PEO coatings, a scratch test was carried out, and the results are summarized in Table 1. It can be seen that the critical forces of delamination (Lc) of all coated substrates exhibited about 3.5 N, revealing that PEO coatings possess excellent adhesive strength toward AZ31 Mg surface.25 3.2. XRD Results. XRD patterns of the PEO ceramic coatings on AZ31 Mg substrates with different silicate concentrations and oxidation time are shown in Figure 2a,b. The peaks from metallic magnesium and MgO were observed in the XRD pattern of the substrate treated for 300 s, which clearly indicated that the thin barrier MgO film was formed over substrate surface. The intensity of the peaks corresponding to the alloy substrate is very strong. This possibly results from the thin barrier film due to relatively shorter discharge time of these substrates during the PEO process. In contrast, peaks from monoclinic forsterite Mg2SiO4 are seen in the case of the substrates treated for 600 s, indicating that the outer PEO film is formed only after this oxidation time. It is well-known that the relative content of detected phases in the coatings is assessed on the basis of the intensities of diffraction peaks corresponding to the phases in the XRD pattern.26 The relative decrease in MgO content and an increase in the Mg2SiO4 phases with oxidation time and silicate concentration are clearly seen in Figure 2c,d. Moreover, the intensity of the peaks was gradually increased with oxidation time, which indicated that the formation of Mg2SiO4 in outer layer has thickened above 600 s. On the other hand, it can be seen that the coating formed at different silicate concentrations is composed of periclase MgO and forsterite Mg2SiO4 phases. The existence of the Mg2SiO4 phases indicated that the Mg2+ ions reaching the oxide/solution interface reacted with SiO44− anions in aqueous solution to form Mg2SiO4 in the local high temperature environment.27 In addition, the diffraction intensity of the peaks has sharpened and strengthened with increasing silicate concentration which obviously implied that the concentration of silicate in electrolyte plays a key role in the formation of outer PEO layer on the surface of substrate. 3.3. SEM Analysis. An SEM image of uncoated strip case AZ31 Mg-alloy substrate is shown in the Supporting Information as Figure S2. It clearly indicated that the existence of few microscratches during mechanical polishing. On the 9705

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Figure 2. XRD patterns of PEO coated AZ31 substrate for (a) different oxidation time and (b) different silicate concentrations, and relative content of MgO and Mg2SiO4 for (c) different oxidation time and (d) different silicate concentrations.

Finally, it could be concluded that the high temperature and pressure created during the large discharge time for the oxidation time of 1800 s during PEO processing may lead to deterioration of the coating. Further, to confirm the formation of MgO and Mg2SiO4 layer over AZ31 Mg surface, SEM/ EDAX line profile analysis was carried out, and the results are shown in Supporting Information Figure S3. From the results, it clearly indicated that the formation of MgO layer takes place at 300 s, whereas the formation of Mg2SiO4 is above 300 s. The cross section SEM images of PEO ceramic coated AZ31 Mg substrates with different silicate electrolyte concentration clearly showed that the coating thickness increased with increasing silicate concentrations. It was clearly noticed that, for the same conditions, increasing the concentration of silicate in the electrolytes resulted in more intensive discharging sparks at the substrate surface during the PEO processing. This is an indication that the density and size of sparks is the direct result of increased electrical conductivity of the electrolytes, and is beneficial to the kinetics of PEO coating growth. However, more cracks and local voids were observed at higher silicate concentration than at lower concentration. 3.4. AFM Topographic Results. Figure 5a−h shows the AFM topographic images of PEO ceramic coated AZ31 Mg substrates for different silicate concentrations and oxidation time. The surface of PEO coating formed at oxidation time of 300 s exhibited a nonuniform surface with uneven grains, whereas PEO formed at above 600 s was observed to be of uniform surface with a few micropores. On the other hand, AFM images of PEO coating processed using different silicate concentrations showed the typical porous topographic surface with different grain size of about 240−300 nm. The existence of pores and grain size increased with increasing silicate concentration.

other hand, the effects of oxidation time and concentration on the surface morphology of the resultant ceramic coatings can be observed in the scanning electron micrographs presented in Figure 3a−h. At the oxidation time of 300 s, a higher amount of intensive micropores is produced and evenly distributed on the thin oxide layer. This is caused by the discharge channel of the microarcs. When the oxidation time reaches 600 s, a liquid melt accumulates around the micropores thus covering the small micropores. This is as a result of high temperature caused by the result of sparking and discharge of the electric energy in the microregion discharge channel.28 When the oxidation time increases to 1800 s, a large amount of accumulated liquid melts are linked around the local large micropores. The average size/ diameter of the micropores appears to be the smallest for the PEO coating formed at 300 s, and the largest pore arises on the 1800 s coatings (Table 1). On the other hand, SEM images of the PEO coatings formed at AZ31 substrates with different concentrations of silicate electrolyte showed typical porous microstructure in all the coatings, where the diameter of the micropores in the PEO coating surface increases with increasing silicate concentration. The cross-sectional morphologies of the PEO ceramic coatings processed at various oxidation time and silicate concentrations are illustrated in Figure 4a−h. It can be seen that the thickness (approximately 7.5 μm at 300 s) increased with increasing oxidation time until 1800 s (10 μm). The substrate coated at 300 s shows the relatively nonuniform, thin barrier coating, whereas the substrate coated at above 600 s displays the thicker layer with a relatively smooth and uniform microstructure with fewer micropores. Oxidation time above 1800 s produced the thickest coatings, but with larger cracks and rough morphology with the formation of more local voids, which is evident in the vicinity of the coating surface, possibly as a result of the thermal stresses during coating growth. 9706

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Figure 3. SEM images of PEO coated AZ31 substrates at (a) 300 s, (b) 600 s, (c)1200 s, and (d) 1800 s, and (e) 0.05 M, (f) 0.1 M, (g) 0.2 M, and (h) 0.3 M.

the roughness of the PEO coatings was evaluated through rootmean-square roughness (RRMS), and these values are presented in Table 1. The roughness values of PEO ceramic coated AZ31 Mg substrates were found to be about 300−550 nm, whereas

In order to gain deeper insight on the surface topography, line profile analysis was performed on the PEO coated AZ31 Mg substrates, and surface profile images are presented in the Supporting Information as Figure S4. In addition, the change in 9707

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Figure 4. Cross section SEM images of PEO coated AZ31 substrates at (a) 300 s, (b) 600 s, (c)1200 s, and (d) 1800 s, and (e) 0.05 M, (f) 0.1 M, (g) 0.2 M, and (h) 0.3 M.

the roughness of PEO coated AZ31 Mg at lower oxidation time exhibited a value of about 459 nm. It is noticeable that RRMS

decreases extensively with increasing oxidation time of about 1200 s and then increased at above 1200 s, which showed that 9708

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Figure 5. AFM images of PEO coated AZ31 substrates at (a) 300 s, (b) 600 s, (c)1200 s and (d) 1800 s, and (e) 0.05 M, (f) 0.1 M, (g) 0.2 M, and (h) 0.3 M.

the oxidation time significantly affected the surface roughness. The lower roughness value of PEO coating processed at 600 and 1200 s revealed a smoother, more compact, and relatively less porous coating than others. The size and number of pores in PEO coatings at silicate concentration (0.2 M) are comparatively low and small which revealed that the silicate concentration significantly influences the PEO coating topography. 3.5. Potentiodynamic Polarization. The polarization curves of uncoated and PEO ceramic coated AZ31 substrate with different oxidation time and silicate concentrations are presented in Figure 6a,b, and the determined Tafel parameters are summarized in Table 2. It can be clearly seen that the corrosion potential (Ecorr) of the coated substrates shifted to nobler direction about 150 mV compared with that of the uncoated AZ31 Mg substrate, and the corrosion current density of the coated samples was 1−2 orders of magnitude lower than

that of the uncoated substrate. These results show that the coatings produced by the PEO process provided significant corrosion protection to the magnesium alloy. The corrosion current density of the coated substrates formed in 0.05 M silicate electrolyte is 4.18 × 10−8 A cm−2, lower by 3−4 orders of magnitude than that of the uncoated substrate. Furthermore, the corrosion current densities of the coated substrates decrease in the order 0.05 > 0.1 > 0.3 > 0.2 M silicate concentration, which implied that the coating produced in 0.2 M silicate electrolyte has the highest corrosion resistance among these coatings. The protection efficiency (PE) of the ceramic coatings was calculated using the following expression29 PE (%) = 9709

R pol(coated) − R pol(uncoated) R pol(coated)

× 100% (2)

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Figure 6. Potentiodynamic polarization curves of PEO coated substrates: (a) different oxidation time and (b) different silicate concentrations, (c) porosity vs protection efficiency graph for different oxidation time and (d) porosity vs protection efficiency graph for different silicate concentrations.

Table 2. Tafel Parameters for PEO coated AZ31 Mg for Different Oxidation Time and Silicate Concentrations substrate

Ecorr mV (SCE)

Icorr μA cm−2

protection efficiency (%)

RP (Ω cm2 × 103)

corrosion rate (mm/year) × 104

uncoated 300 s 600 s 1200 s 1800 s 0.05 M 0.1 M 0.2 M 0.3 M

−1530 −1353 −1297 −1036 −1140 −1394 −1251 −1080 −1220

62.40 2.51 0.30 0.01 0.08 0.04 0.03 0.01 0.02

94.76 97.45 99.90 96.78 98.12 98.70 99.95 97.70

1.26 2.95 23.30 739.60 104.90 180.50 230.02 792.70 404.30

10.58 0.6694 0.0800 0.0026 0.0195 0.0470 0.0222 0.0073 0.0117

In eq 2, Rpol (uncoated) is the polarization resistance of uncoated substrates, and Rpol (coated) is the polarization resistance of coated substrates, respectively. The PEs of PEO ceramic coatings calculated from potentiodynamic polarization data are found to be 94.76%, 97.45%, 99.90%, and 96.78% for PEO processed at different oxidation times of about 300, 600, 1200, and 1800 s, respectively, whereas the PEs are found to be 98.12%, 98.70%, 99.95%, and 97.70% for PEO processed at 0.05, 0.1, 0.2, and 0.3 M silicate concentration, respectively. The corrosion rate (CR) of PEO coated AZ31 Mg substrate was calculated from polarization curves using the following equation:30 CR =

3.27 × Icorr × EW D

PEO coated substrates. The porosity in PEO coatings on AZ31 substrates was calculated by the following the relation31 and the results are given in Figure 7c,d. P=

R ps R pc

10−(|ΔEcorr| / β a) (4)

Here, P is the total porosity, Rps is the polarization resistance of the uncoated substrate, Rpc is the measured polarization resistance of ceramic coated substrates, ΔEcorr is the difference between corrosion potentials, and βa is the anodic Tafel slope for uncoated substrate. Figure 6c,d displayed the correlation between the porosity and protection efficiency of PEO coatings as the function of oxidation time and silicate concentration. From the plot, it is clearly observed that the porosity in PEO coating formed at different oxidation time was found to be in the order 300 s > 600 s > 1200 s > 1800 s, whereas in the case of different silicate concentration, it was found to be 0.05 M > 0.1 M > 0.3 M > 0.2 M, respectively. It could be clearly observed that increasing PEO oxidation time reduces the corrosion susceptibilily, although this trend changed for oxidation time above 1200 s,

(3)

Here, CR is the corrosion rate (mmpy), Icorr is the corrosion current density (μA cm−2), EW is the equivalent weight of the substrate, and D is the density of substrate (g cm−3). The porosity of the coatings strongly influences the corrosion protection behavior of the films. Hence, the determination of porosity of the ceramic coating is essential in order to estimate the overall corrosion performance of the 9710

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controlled the corrosion potential to drift toward the nobler side. Furthermore, this result revealed that the PEO coatings processed at 0.2 M have superior protection efficiency than other coatings due to its uniform surface morphology and less porous film which was confirmed by SEM and AFM analyses. Finally, PEO coatings with significantly lower values of the porosity permit considerable enhancement of the corrosion resistance by hindering the access of the electrolyte to the AZ31 substrates. The protection efficiency and porosity results have revealed that the PEO ceramic coating processed on substrates at 0.2 M silicate and for 1200 s effectively protects the substrate and improves the corrosion resistance of AZ31Mg substrate due to its homogeneous, smooth morphology and relatively less porous surface coatings in correlation with SEM and AFM analysis. 3.6. EIS Results. The electrochemical corrosion performance of the PEO ceramic coated AZ31 substrates was estimated by EIS in a 3.5% NaCl solution. It could be seen that the Nyquist plots (Figure 7a,b) exhibited a capacitive loop in the high frequency (HF) region followed by one inductive loop in the low frequency (LF) region for uncoated and coated AZ31 substrates at oxidation time of 300 s. The appearance of the inductive loop revealed that the corrosive ions have already induced the corrosion of the substrate. In contrast, the inductive loop in LF region disappeared when the oxidation time reached 600 s. From the observation of micrographs of cross-sectioned PEO ceramic coating (Figure 5a), the coating was not thick enough to prevent the penetration of the corrosive ions through the coating defects when the oxidation time was lower than 600 s. On the other hand, PEO coated AZ31 Mg substrates from different silicate concentrations showed a large capacitive loop in Nyquist plot, and the diameter of capacitive loop was continuously enlarged with increasing silicate concentration in the electrolyte. The impedance of the uncoated AZ31 Mg substrate was in the range 102−103 Ω cm2 at the LF region, whereas the impedance

Figure 7. Nyquist plots of PEO coated AZ31substrates (a) different oxidation time and (b) different silicate concentrations.

probably due to the increased cracking observed in the coatings. In addition, it appears that the presence of Mg2SiO4, the predominant phase in the outer layer of PEO ceramic coating,

Figure 8. Bode plots of PEO coated AZ31substrates at (a) different oxidation time and (b) different silicate concentrations. 9711

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of PEO coatings was about 105 Ω cm2 in the same frequencies (Figure 8a,b). This indicated that the resistance of the AZ31 Mg alloy increased by 2 orders of magnitude by PEO ceramic coatings. From the Bode phase angle plot (Figure 8), at least two time constants are required to fit the EIS results of the PEO coated Mg substrates, and an equivalent circuit model was proposed as shown in Supporting Information Figure S5. In this circuit model, Rs is the solution resistance, Rf is a film resistance paralleled with a constant phase element (CPEf), and Rct is a charge transfer resistance paralleled with a constant phase element for double layer (CPEdl). The CPE (constant phase element) has been used to replace the double layer capacitance (Cdl) because of the deviation of Cdl from its ideal capacitive behavior. The CPE is defined by the following relation: Z = Y0−1(jω)−n, where Z is the impedance of the CPE, j is the imaginary number (j2 = −1), ω is the angular frequency (rad s−1), and Y0 and n are the frequency independent parameters. The n value ranging from 0 to 1 depends on the surface roughness. For the perfect surface, the value of n is 1, and the impedance of CPE is that of pure capacitor. On the basis of the proposed equivalent circuit model, the EIS spectra were analyzed, and the curve fitting was performed for all the substrates which showed an excellent agreement between the experiments and the fitting. The value of Bode resistance (|Z|) in HF region (102−105 Hz) and medium frequency (MF) region (10−102 Hz) correspond to the characteristics of outer porous layer and inner barrier layer of the PEO ceramic coatings, respectively. In Bode plots (Figure 8), the value of |Z| for the coated substrates in the HF region has increased about 1 order of magnitude when the oxidation time increased from 300 to 1200 s. This clearly indicated that the corrosion resistance of outer porous layer increased significantly with the oxidation time. The value of |Z| in the HF region gradually increased with the increase of oxidation time after the oxidation time exceeded 1200 s. The impedance magnitude (|Z|) of the coated substrate in the LF region considerably increased with the oxidation time from 300 to 1200 s. This behavior showed that the corrosion resistance of the inner barrier layer increased greatly in this oxidation time. Moreover, the impedance value of coated substrate with different silicate concentrations has increased with silicate concentration from 0.05 to 0.2 M. However, 0.3 M silicate concentration exhibited lower value than 0.2 M silicate concentration, which clearly indicated the presence of coating defects and local voids, which affected their corrosion performance. The impedance |Z| was higher than 105 Ω cm2 when the oxidation time is about 1200 s and silicate concentration is 0.2 M.

layer with better chemical stability of the Mg2SiO4. Finally, it can be concluded that the electrolyte concentration and oxidation time plays a significant role in PEO ceramic coatings toward its surface and corrosion protection performance.

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ASSOCIATED CONTENT

S Supporting Information *

Additional data and figures confirming the results obtained in the present investigation. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected], [email protected]. Phone: +82-2-880-7089. Fax: +82-2-887-6388. Notes

The authors declare no competing financial interest.

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REFERENCES

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4. CONCLUSIONS Ceramic coatings have been deposited on AZ31 Mg substrates by the PEO process in silicate electrolytes with different concentration and oxidation time. XRD showed that the coating processed at lower oxidation time is mainly composed of MgO, whereas other ceramic coatings contain both MgO and Mg2SiO4. Surface characterization results clearly revealed that the coating processed at 0.2 M silicate and 1200 s oxidation time exhibited thicker, more uniform coatings with relatively fewer micropores and cracks than others. Potentiodynamic polarization and EIS results confirmed the enhanced corrosion performance of PEO ceramic coatings processed at 1200 s and 0.2 M silicate concentration due to its dense and more compact 9712

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