Interaction effect between different constituents in ...

Surface & Coatings Technology 307 (2016) 825–836

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Interaction effect between different constituents in silicate-containing electrolyte on PEO coatings on Mg alloy Yan Chen a,b,c, Yange Yang a, Tao Zhang a,⁎, Wei Zhang a, FuhuiWang a, Xiaopeng Lu b, Carsten Blawert b, Mikhail L. Zheludkevich b a b c

Laboratory for Corrosion and Protection, Institute of Metal Research, Chinese Academy of Sciences, Wencui RD 62, Shenyang 110016, China Institute of Materials Research, Helmholtz Zentrum Geesthacht, Max-Plank-Str. 1, Geesthacht 21502, Germany University of Chinese Academy of Sciences, Yuquan RD 19, Beijing 110049, China

a r t i c l e

i n f o

Article history: Received 29 July 2016 Revised 3 September 2016 Accepted in revised form 16 September 2016 Available online 17 September 2016 Keywords: Magnesium Plasma electrolytic oxidation Response surface methodology IR spectroscopy

a b s t r a c t The influence of different constituents in silicate containing alkaline electrolytes on PEO-coated AZ91D magnesium alloy has been systematically studied using response surface methodology (RSM) coupled with electrochemical measurements and microstructure characterization methods. The results indicate that the most dominant factor that determines the coating performance is the interaction between Na2SiO3 and NaOH, where better corrosion resistance could be achieved when their concentrations are tuned in different directions. Infrared spectrum analysis of the electrolytes innovatively indicates that the degree of polymerization of silicate ions (from one to two- and three-dimensional), varied by the concentration of Na2SiO3 and NaOH, influences the kinetic mechanism of coating formation and thermal-driven gel-forming process under sparking. When silicates species exist as a relatively higher polymerized state, which indicates lower mobility of silicates in the electrolyte and better affinity for trapping Al3+ to form gel-like networks, the resulting coating is inhomogeneous and susceptible to corrosive ions; otherwise, the coating is more homogeneous and exhibits improved corrosion resistance. The work herein aims to provide guidelines for designing PEO electrolyte via correlating the intrinsic nature of the electrolyte with coating properties. © 2016 Elsevier B.V. All rights reserved.

1. Introduction Given that poor corrosion resistance of Mg and its alloys still remains a significant barrier to wider industrial application, surface protective coatings on magnesium alloys, such as chemical conversion coating, electroplating, anodizing, laser beam surface treatment and PVD, have been increasingly used over the past decades [1–4]. Among these coatings, electrically insulating ceramic-like plasma electrolytic oxidation (PEO) coatings have shown to be promising candidates for significantly improving corrosion resistance [5,6]. The PEO processing involves coating formation in an aqueous solution at a relatively high voltage accompanied with a large quantity of short-lived microdischarges on the surface caused by localized dielectric breakdown of the initial oxide film, owing to the electro-, thermo-, and plasma-chemical interactions between metal and electrolyte components [7]. The properties of the PEO coating can be influenced by electrolyte, treatment time, temperature, and electric parameters [8–10], where the composition and concentration of electrolyte have the most determinant effect due to incorporation of species from the bath into the oxide coating [11,12].

⁎ Corresponding author. E-mail address: [email protected] (T. Zhang).

http://dx.doi.org/10.1016/j.surfcoat.2016.09.031 0257-8972/© 2016 Elsevier B.V. All rights reserved.

In previous studies, silicate-based alkaline solutions have been employed as the most common electrolytes for PEO treatment on magnesium alloys and it is reported that various electrolyte constituents contribute differently to coating properties [13–17]. For instance, Fukuda et al. [18] studied the effect of Na2SiO3 on anodization of AZ91D Mg alloy and demonstrated that the addition of Na2SiO3 in 3 M KOH solution resulted in enhanced corrosion resistance of anodized coating, correlating well with its denser, thicker and more uniform morphology, consisting of Mg2SiO4 and SiO2. Similar phenomenon has been revealed by Hsiao et al. [19] demonstrating that Na2SiO3 assisted the occurrence of uniform sparking. However, they observed a drastic decrease in corrosion resistance due to polymerization reaction between Na2SiO3 and aluminum-containing species if its concentration exceeds a critical value of 0.2 M. Regarding the effect of hydroxide in the electrolyte, Ko et al. [20] demonstrated that the amount of MgO in the coating evidently increased as the KOH concentration increased, which was attributed to the lower breakdown voltage of sparking. Furthermore, electrochemical measurements indicated that better corrosion performance could be obtained with higher MgO content. In addition, Lu et al. [12] found that varying the concentration of KOH can adjust the relative ratio of crystalline to amorphous phases in PEO coatings. Electrolytes containing higher amount of KOH tend to produce coatings composed of

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more crystalline phases which demonstrate better corrosion resistance and degradation stability. In terms of other additives, fluoride containing compounds for PEO treatment have attracted wide attention due to significantly enhanced coating properties. Liang et al. [21] have shown that the addition of fluoride containing compound in the electrolyte has positive effects on the mechanical and tribological properties of PEO coatings, whereas Wang et al. [22] suggested that KF changes the characteristics of the microdischarges and avoids excessive anodic dissolution of the substrate by facilitating the formation of compact MgF2 passivated layer on the surface in the initial stage of anodizing. However, previous studies were independently designed by varying the concentration of each electrolyte component individually, providing limited information about the interaction between different electrolyte constituents. Moreover, up to now, no common principle was established for designing new electrolytes. Hence, it is necessary to plan a systematic study to correlate PEO coating performance and electrolyte variables, in order to develop a scientifically based electrolyte selection process. In this work, the Box-Behnken design (BBD) coupled with the response surface methodology (RSM) was used to investigate the influence of Na2SiO3, KF and NaOH concentrations on the final coating morphology, composition and performance. The aim is to correlate the intrinsic nature of the electrolyte with the resultant coating properties and provide a guideline for the selection process of electrolyte composition, in order to guarantee more predictable coating performance.

In the present study, a Box-Behnken design (BBD) coupled with RSM was employed to investigate simultaneously the influence of different electrolyte constituents on the coating properties as well as the interactions among them. The following factors were chosen as experimental variables in this study: Na2SiO3 concentration (X1), KF concentration (X2), NaOH concentration (X3). Three characteristic values for evaluating corrosion performance of PEO coatings derived from electrochemical measurements were concurrently used as responses, i.e. corrosion current density (icorr), breakdown potential (Eb) and coating impedance (Z). It should be mentioned that respective ranges of the variables were chosen based on literature review and some preliminary studies of our research group. For the purpose of statistical calculations, the variables Xi were coded as xi according to the following relationship:

2. Experimental

Table 1 exhibits the values of experimental variables and their corresponding coded values. A three-level and three-factor design matrix of BBD is generated by Design-Expert software (version 8.0.6), which is exhibited in Table 2 with coating performance values listed in the last three columns. There are totally fifteen experimental runs with twelve cubic points and three repetitive central points to avoid singularity [24] and determine experimental error of the data in this design. By mathematical and statistical regression analysis, all the experimental data are capable of matching an empirical second-order polynomial equation:

2.1. Materials Specimens of AZ91D magnesium alloy with the size of 22 mm × 22 mm × 8 mm were prepared from gravity cast ingot material. The chemical composition of AZ91 identified by inductively coupled plasma-optical emission spectrometry (ICP-OES) was 8.70% Al, 0.64% Zn, 0.23% Mn, 0.014% Si, 0.002% Fe, 0.002% Cu, and Mg balance. The specimens were ground with SiC abrasive papers up to 2000 grit, rinsed with ethanol and then air-dried prior to PEO treatment.

xi ¼

X i −X i ΔX i

ð1Þ

where X⁎i is the value of Xi at the center point and ΔXi indicates the step change. X i ¼

X i; max þ X i; min 2

ð2Þ

X i; max −X i; min 2

ð3Þ

ΔX i ¼

Y ¼ b0 þ

3 X

bi xi þ

3 X

bii x2i þ

2 X 3 X

bij xi x j

ð4Þ

2.2. PEO treatment The PEO process was carried out using a pulsed DC power supply with a pulse ratio of ton:toff = 1 ms:1 ms. The specimen and a graphite plate were used as the anode and cathode, respectively. The processing electrolytes were composed of different concentrations of Na2SiO3·5H2O, KF·2H2O (which are referred as Na2SiO3 and KF later in this paper for simplicity) and NaOH (Tables 1 and 2). The temperature of the electrolytes was kept at 25 °C by a water cooling system. All PEO treatments were performed at a constant current density of 20 mA/cm2 for 3 min. The treated samples were rinsed thoroughly in distilled water, dried in ambient air and then kept in a drying chamber prior to testing. 2.3. Design of experiments Response surface methodology (RSM) has been found to be a more efficient method for gathering experimental results than the conventional, time-consuming one-factor-at-a-time approach and a useful tool to study the interactions of two or more variables [23]. Table 1 Variables and corresponding coded value for Box-Behnken design.

where Y is the response, xi and xj are coded experimental variables, b0 is a constant, bi, bii, bij are linear coefficient, quadratic coefficient and synergistic coefficient, respectively. The significance of the developed model for coating property prediction will be further justified through analysis of variance (ANOVA). ANOVA consists of classifying and cross-classifying statistical results and is tested by the means of a specified classification difference, which is carried out by Fisher's statistical test (F-test). The F-value is defined as the ratio of the mean square of regression to the error and the larger the magnitude of the F-value the more significant is the corresponding coefficient. The regression equation is also submitted to the F-test to determine the coefficient R2. In addition, a p-value (Probability N F) b 0.05 represents a high significance of the regression equation with 95% confidence level [25,26]. Therefore, prediction of corrosion performance of PEO coating can be achieved by the polynomial equation when using a certain electrolyte. Meanwhile, the effects of each factor and their interactions on coating performance can be analyzed according to the value of coefficient in the equation. 2.4. Microstructure observation and spectroscopy analysis

Coded value Variables

−1

0

+1

Na2SiO3·5H2O (g/l) KF·2H2O (g/l) NaOH (g/l)

20 10 0

35 15 2.5

50 20 5

A scanning electron microscope (SEM, PHILIPS, XL-30FEG) was used to observe the surface and cross section morphology of the PEO coatings. For latter, it was necessary to embed the coatings into resin. Specimens were then prepared by polishing successively up to 3000 grit SiC abrasive papers, followed by disc polishing using colloidal silica


827

Table 2 Box-Behnken design and experimental results. R Factors

Responses

Run

A

B

C

icorr (mA/cm2)***

Eb (mV)**

Impedance (kΩ∙cm2)****

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

+1 +1 0 +1 −1 −1 −1 −1 +1 0 0 0 0 0 0

0 0 +1 −1 −1 0 0 +1 +1 −1 0 −1 0 +1 0

−1 +1 −1 0 0 +1 −1 0 0 −1 0 +1 0 +1 0

(3.7 ± 1.1) × 10−3 (5.1 ± 2.0) × 10−3 (1.5 ± 0.4) × 10−3 (1.9 ± 1.4) × 10−3 (1.1 ± 0.9) × 10−3 (2.3 ± 0.8) × 10−4 (2.5 ± 1.4) × 10−3 (1.9 ± 1.3) × 10−3 (3.6 ± 1.6) × 10−3 (1.0 ± 0.2) × 10−3 (1.6 ± 1.2) × 10−3 (4.8 ± 2.8) × 10−3 (0.4 ± 1.3) × 10−3 (1.4 ± 3.0) × 10−3 (1.1 ± 0.7) × 10−3

−1268 ± 16 −1299 ± 25 −1275 ± 6 −1251 ± 10 −1331 ± 11 −1230 ± 19 −1298 ± 18 −1305 ± 20 −1282 ± 8 −1329 ± 16 −1256 ± 24 −1278 ± 21 −1260 ± 19 −1240 ± 15 −1272 ± 12

42.15 ± 5.00 22.16 ± 3.29 36.09 ± 6.10 31.80 ± 2.31 32.43 ± 4.06 194.40 ± 20.12 38.93 ± 6.41 60.01 ± 13.43 32.02 ± 5.09 18.27 ± 8.70 63.43 ± 8.63 80.77 ± 13.93 56.93 ± 10.34 37.24 ± 6.55 58.65 ± 4.82

suspension. Severe charging effect of the non-conducting PEO coatings was prevented by sputtering the surface with carbon. An acceleration voltage of 15 kV was applied for SEM investigation. The phase compositions of the PEO coatings were identified by X-ray diffraction (XRD, PHILIPS, PW1700, Cu Kα1 radiation, λ = 1.5406 Å, 30 mA, 40 kV). X-ray photoelectron spectroscopy (XPS) analysis was performed on an ESCALAB250 (Thermo VG) spectrometer with a monochromatic Al Kα (1486.6 eV) X-ray source. The surface was etched for 60 s by argon ion bombardment with an ion energy of 3 keV to remove the contaminated surface. The sample current was 2 μA and the sputtered area was 2 mm × 2 mm. Peak identification was performed using XPSPEAK software with an XPS database as reference and the binding energy scale was calibrated to the C1s (284.6 eV) peak. Fourier transform infrared spectroscopy (FT-IR, Nicolet iS10) was used to identify the chemical functional groups of electrolytes in the range of 400–4000 cm−1. About 0.5 mg of each electrolyte was mixed with 250 mg potassium bromide (KBr) and dried under an infrared baking lamp, then pressed into a transparent pellet prior to analysis. 2.5. Electrochemical measurements For electrochemical testing, a computer controlled electrochemical workstation (Zahner Zennium, Germany) was used in conjunction with a conventional corrosion cell comprising a platinum (plate) counter electrode, a saturated calomel reference electrode and the coated specimen as the working electrode (1cm2 exposed area). All testing was carried out in 3.5 wt% NaCl solution at room temperature. Potentiodynamic polarization tests were carried out at a sweep rate of 0.333 mV/s after 30 min immersion at open circuit potential (OCP). The anodic polarization was stopped when the current density reached 1 mA/cm2 and the cathodic polarization at a voltage of −300 mV relative to OCP. Electrochemical impedance spectroscopy (EIS) studies were performed at OCP (measured after 30 min of immersion as well) with a 10 mV sinusoidal perturbation over the frequency range from 100 kHz to 0.01 Hz at 30 °C in a water bath. Each measurement was repeated three times to ensure reproducibility. All potential values given here are referred to standard hydrogen electrode (SHE), unless stated otherwise.

Leroy [28], icorr is determined by the intersection of the straight line parallel to Y-axis through Ecorr. with the cathodic Tafel extrapolation curve. Regarding evaluation of anodized coating on magnesium alloys, icorr and Z are the most representative parameters closely associated with the overall corrosion rate and resistance of the coating with throughgoing pores, respectively [29]. In the present work, the corrosion performance determined by them shows a similar trend, which could corroborate the accuracy and reproducibility of the experiments. It is not surprising that Eb does not match the order of performance given by icorr and Z because in most cases Eb only manifests the resistance of a coating against breakdown damage at a remarkably higher potential compared to the corrosion potential. Normally, it fails to characterize the corrosion behavior near the corrosion potential which is more closely associated with the damage under service conditions. Nevertheless, for assessing coating properties comprehensively, it is still necessary to use Eb as one standard despite its deviation from results indicated by the other two parameters. From the aforementioned, Z is herein ranked as the most dominant parameter in these three responses identified with four asterisks in Table 2, with icorr and Eb ranking as the second and third. Using three responses simultaneously could improve the regression analysis in Design-Expert software and generate a more accurate and thorough model. 3.1.2. Establishment of regression equation and ANOVA analysis Based on the ranking order of the three responses, the experimental results of the design matrix were analyzed using the regression analysis. A two-factor interaction (2FI) and cubic models were suggested by the software, while the former one was selected in the present case due to its ability to establish a more concisely clear and straightforward relevance between the results and variables compared to higher order polynomial models. Additionally, the effects of each of the three variables and their influences on impedance are further justified by ANOVA. According to the ANOVA results, F-value of 6.08 indicates that the developed regression model is significant and there is only a 1.15% chance that such a large “Model F-value” could occur due to noise. Moreover, only the KF concentration is not significant and can be ignored in the regression equation. Therefore, a simplified empirical model in terms of coded factors for impedance is proposed, as shown in Eq. 5:

3. Results 3.1. The results of RSM experiments

Z ¼ 47; 160:13−20; 736:75x1 þ 27; 845:25x3 −6872:50x1 x2 −53; 558:00x1 x3 −16; 080:50x2 x3 ð5Þ

3.1.1. Experiment results of designed experiments The results of current density icorr, breakdown potential Eb and impedance Z of designed experiments are summarized in the last three columns in Table 2. Based on the theory of McCafferty [27] and L.

Accordingly, impedance is expressed as a function of three variables and can be predicted at certain conditions by the 2FI equation. The accuracy of fitting is assessed by the coefficient of determination, R2, which quantitatively evaluates the correlation between the experimental

828


results and the predicted values. The R2 of 0.8213 in this model implies that there is 82.13% of variability of the predicted results which can be explained by the developed model, indicating a high precision in the prediction of impedance. A normal probability plot of the residuals for Z is displayed in Fig. 1. The residuals fall on or are close to a straight line suggesting that the errors are distributed normally. 3.1.3. Analysis of main influence factors and interactions The Pareto analysis is used primarily to identify those factors that have a dominant cumulative effect on the responses, thus screening out the less significant factors. This provides information on the key factors involved in the process. According to Eq. (6), Pareto analysis is performed to measure the weight of each factor on the response according to the following relationship [30,31]: !

2

Pi ¼

b Xi

2

bi

Fig. 2. Pareto graphic analysis of RSM results.

100 ði≠0Þ

ð6Þ

where Pi is the contribution in percentage of each variable on the impedance and bi is the coefficient of each variable. Fig. 2 demonstrates that not all factors contribute equally to coating properties. It can be seen that KF has limited influences when interacting with other factors, contributing less than 10% in total. Also the other single variables have a relatively small impact on coating properties when compared to the interaction effect of Na2SiO3 and NaOH, with the cumulative probability exceeding 60%. Fig. 3 depicts the three-dimensional response surface curve which was constructed to show how Na2SiO3 and NaOH affect the coating properties with KF concentration fixed at zero level (15 g/l according to the RSM design in Table 1.). It is evident from the twisted plane of response surface and the elliptical nature of the symmetrical circular contours that the interaction between the individual variables is significant [24]. As can be seen from Fig. 3, impedance increases along with the change of Na2SiO3 and NaOH content towards opposite directions. The optimum coating property is obtained when Na2SiO3 and NaOH concentration reach the minimum and maximum value within the range studied, respectively. 3.2. Effect of interaction between Na2SiO3 and NaOH on coating properties The results of the RSM experiments indicate two significant aspects. On the one hand, coating property remarkably depends on the electrolyte composition, and on the other hand, the interaction between two electrolyte components plays a predominant role. Hence, in order to further understand how the intrinsic nature of these alkaline electrolytes are altered by the relative amount of Na2SiO3 and NaOH and

Fig. 1. Normal plot of residuals for impedance.

how this influences coating properties, coatings obtained from four endpoint runs in the three-dimensional response surface plot with KF concentration fixed at zero level (15 g/l) are investigated. The corresponding coatings are named LL-coating (low Na2SiO3 concentration, low NaOH concentration), LH-coating (low Na2SiO3 concentration, high NaOH concentration), HL-coating (high Na2SiO3 concentration, low NaOH concentration) and HH-coating (high Na2SiO3 concentration, high NaOH concentration), respectively. The detailed composition, pH and conductivity of these four endpoint electrolytes are presented in Table 3.

3.2.1. Discharge characteristics The change of voltage as a function of treatment time is displayed in Fig. 4. In the initial stage, all coatings basically show the same characteristic in V-T curve, with the voltage ramping up linearly at a rate of approximately 7 V/s. This region is due to the partial dissolution of the substrate as well as the formation of a thin passive film working as an insulator at the beginning of PEO coating growth [20]. For PEO process, the potential corresponding to the occurrence of sparking is designated as breakdown potential. Coatings produced from different electrolytes show different values, which are mainly attributed to variations of the electrolyte conductivity [32]. Based on a theoretical model proposed by Ikonopisov [33], an avalanche of electrons at the interface between electrolytes and oxide layers can cause a breakdown if the applied voltage is sufficiently high. Accordingly,

Fig. 3. The three-dimensional response surface curve of the impedance as the function of Na2SiO3 and NaOH concentration.


829

Table 3 Detailed properties of the four electrolytes for PEO treatment and corresponding characteristic values of PEO process.

Coating

Na2SiO3·5H2O (g/l)

KF·2H2O (g/l)

NaOH (g/l)

Conductivity (mS/cm)

pH

Ignition time (s)

Breakdown voltage (V)

Final voltage (V)

LL-coating LH-coating HL-coating HH-coating

20 20 50 50

15 15 15 15

0 5 0 5

36.9 57.0 60.0 77.9

12.81 13.13 13.07 13.21

50 28 27 18

150 124 122 100

250 230 228 178

the breakdown voltage can be estimated by the given equation, U b ¼ α þ β logð1=κ Þ

ð7Þ

where α and β are constants for certain materials and electrolyte composition, and Ub and κ are the breakdown voltage and the electrolyte conductivity, respectively. As listed in Table 3, the observed breakdown voltage is in the reverse order of the electrolyte conductivity, which is in good agreement with Eq. (7), and thus the lowest Ub is observed for HH-coating. After reaching Ub, different phenomena take place for the different electrolytes. For HL- and HH-coating, the potential reaches a plateau after a relatively slow ramp, where voltage fluctuations of several volts occur. At the same time, sparking behavior changes. The number of sparks decreases and size and lifetime of individual sparks increases. This only appears at localized sites on the specimen and causes the potential oscillation, revealing that the formation of new film and dissolution of existing film takes place simultaneously [34]. Additionally, the electrolytes reveal a higher viscosity after PEO treatment. For LL- and LH-coatings, the plateau of potential is absent in the V-T curves, which might imply an occurrence of different growth mechanisms compared to coatings formed in electrolytes containing higher amount of silicate. At least in the selected treatment time, the small-sized and short-lived discharges for these two coatings are not detrimental and may cause less through-going porosity. Obviously, the concentration of Na2SiO3 and NaOH has a huge effect on the evolution of voltage during PEO treatment. Since voltage response is one of the key indications correlated with pore size and thickness of the resultant coating [35], difference in microstructure and properties of the coatings would be expected. 3.2.2. Microstructure The surface morphology of the four coatings is shown in Fig. 5. SEM images in lower magnification show that the surface of all PEO coatings is dominated by numerous irregular pores. β phase (Mg17Al12) in the substrate can be obviously distinguished from α Mg matrix (the existence of the two phases in AZ91D Mg alloy is confirmed by the XRD pattern of the substrate in Fig. 7). This is due to relatively short treatment

time and higher breakdown voltage of β phase which causes retarded sparking on β phase [36–37]. In particular, the pores on the surface of LH-coating are smaller and uniformly distributed compared with the other coatings. The surface of LL- and HH-coating is inhomogeneous characterized by larger clusters of pores in localized regions and almost no pores in other regions. The pores on the surface of HL-coating are also uniformly distributed but bigger in size compared to LH-coating. The higher magnification images, Fig. 5(b), (d), (f) and (h), provide more detailed information of coatings on α phase. Plenty of regular pores with the average size of 0.46 μm are visible for LH-coating, which has an overall smooth surface appearance. The pores of LL- and HL-coating are bigger (average size of 0.653 and 0.603 μm, respectively) with protrusions which might be due to uneven dielectric breakdown and extreme thermal conditions. For HH-coating with the smallest pore size of 0.383 μm, some burned-like and exfoliated regions can be observed across the surface. Backscattered electron images for the cross section of PEO-coated specimens are shown in Fig. 6 and all the coatings exhibit a similar thickness in the range of 1 to 3 μm. Among the four coatings, LH-coating is the most uniform and least defective, which is in good agreement with the surface morphology. In contrast, the HH-coating is the most inhomogeneous characterized by some nearly uncoated regions which may result from film detachment as already observed in surface morphology. Therefore, the effect of concentration and composition of electrolytes is well reflected by the morphology of the coatings. 3.2.3. Chemical composition Fig. 7 demonstrates the XRD pattern of the substrate and one typical coating, since all the coatings are amorphous without any new peaks compared to substrate. Therefore, XPS study is further performed to get more information of the coating composition. XPS quantitative analysis by considering the integrated spectral intensities is displayed in Table 4. The results imply no obvious difference in elements detected, which are Mg, O, Si, F and Al. Mg and Al are the main elements of Mg alloy AZ91, while Si and F elements should come from the components of the electrolytes, Na2SiO3 and KF, respectively. O may come from Na2SiO3, NaOH or directly dissolved O2 in the electrolyte. The most prominent phenomenon is that increasing NaOH concentration alone leads to a decrease of Si content in the coating, and increasing the amount of Na2SiO3 alone enhances Si content in the coating, which is in line with published studies [16,19]. A further deconvolution analysis of high resolution spectra for Mg 1s and Si 2p (Fig. 7) was performed to identify the chemical composition of the PEO coatings which directly determines coating properties. The Mg 1s peak is the sum of three peaks: MgF2 (1304.9 eV), MgO (1303.9 eV), and Mg(OH)2 (1302.7 eV), similar to the studies of Fukuda and Hsiao et al. [18,19]. When anodizing begins, magnesium atom from the substrate loses electrons and transforms into Mg2+. Initial hydroxide and fluoride products form as a result of the following reactions: Mg→Mg2þ þ 2e−

ð8Þ

Mg2þ þ 2 F− →MgF2

ð9Þ

Mg2þ þ 2OH− →MgðOHÞ2 Fig. 4. Voltage as a function of treatment time for the four coatings.

ð10Þ

830


Fig. 5. SEM images of surface morphology of the four PEO coatings in 2 k and 10 k magnification, respectively: (a)(b) LL-coating, (c)(d) LH-coating, (e)(f) HL-coating, (g)(h) HH-coating.

Under high temperature caused by sparking, Mg(OH)2 can dehydrate into MgO. The ratio of MgO/Mg(OH)2 which is deduced from the area integration, is as follows, HH-coating (4.131) N LH-coating (2.392) ≈ HL-coating (2.327) N LL-coating (1.362). Additionally, high resolution of Si peak is fitted into two peaks, Mg2SiO4 (103.9 eV) and

SiO2 (102.4 eV), which can be formed by the following reactions:

2Mg2þ þ 2SiO2− 3 →Mg2 SiO4 þ SiO2

ð11Þ


831

Table 4 XPS quantitative analysis of the four coatings.

Fig. 6. Backscattered electron images of cross-sectional morphology of the four PEO coatings.

− 2Mg2þ þ SiO2− 3 þ 2OH →Mg2 SiO4 þ H2 O

Coating (at %)

O 1s (%)

Si 2p (%)

F 1s (%)

Mg 1s (%)

Al 2p(%)

LL-coating LH-coating HL-coating HH-coating

50.3667 48.9063 50.6031 50.0805

18.5962 14.8306 20.8112 18.6738

3.4389 2.5001 2.0638 2.0081

21.3068 29.3288 24.0640 23.7403

5.6483 3.9282 2.2563 3.7131

effect of coatings [29]. It can be seen that the icorr value of LL-coating, HL-coating and HH-coating are in the same order of magnitude, whilst the icorr value of LH-coating is decreased by one order of magnitude. Eb also shows an increase of up to 70 mV for LH-coating, indicating lower sensibility to localized corrosion [39]. EIS spectra were recorded to get additional information of the coating performance as well as the corrosion process at the substrate/electrolyte interface. Fig. 9 presents the Bode and Nyquist plots of the four coatings and the equivalent circuit shown in Fig. 10 is employed to fit the results. The elements in the equivalent circuit include Rs (the solution resistance), Ro (resistance of the outer layer of the coating) paralleled with CPEo (a constant phase element representing the outer layer capacitance), Ri (resistance of the inner layer of the coating) paralleled with CPEi (a constant phase element representing the inner layer capacitance), Rct (charge transfer resistance associated with localized corrosion and micro-galvanic events) paralleled with a constant

ð12Þ

It should be noted that according to literature, the peak corresponding to MgO is also assigned to Mg2SiO4 [38]. In conclusion, there are mainly four magnesium compounds in the coatings, i.e. MgO, MgF2, Mg2SiO4 and Mg(OH)2, which are also observed in recent studies employing similar silicate-based electrolytes except for Mg(OH)2 [13, 14,17]. The absence of Mg(OH)2 in the PEO coatings produced in those studies may be due to longer treatment time which causes higher temperature, providing sufficient energy for the transformation of Mg(OH)2 into MgO. 3.2.4. Corrosion behavior The corrosion behavior of the four PEO coatings evaluated by potentiodynamic polarization measurements after 30 min immersion in 3.5 wt% NaCl solution is presented in Fig. 8. For all the coatings, the measurements were repeated at least three times and one representative measurement for each coating is displayed. The corrosion potential (Ecorr), corrosion current density (icorr), and breakdown potential (Eb) derived from potentiodynamic polarization curves are summarized in Table 5. Both anodic reaction and cathodic reaction are retarded compared to bare alloy, which is ascribed to the blocking and passivating

Fig. 7. XRD patterns of AZ91D Mg substrate and LH-coating.

Fig. 8. High resolution XPS analysis of (a) Mg 1s and (b) Si 2p spectrum for the four PEO coatings.

832


Table 5 Electrochemical data for PEO-coated specimens from potentiodynamic polarization and EIS tests. Coating

Ecorr (mV)

icorr (mA/cm2)

Eb (mV)

Z (kΩ∙cm2)

LL-coating LH-coating HL-coating HH-coating bare alloy

−1362 ± 7 −1364 ± 11 −1371 ± 19 −1371 ± 8 −1389 ± 15

(2.5 ± 1.4) × 10−3 (2.3 ± 0.8) × 10−4 (3.7 ± 1.1) × 10−3 (5.1 ± 2.0) × 10−3 (3.1 ± 0.9) × 10−2

−1298 ± 18 −1230 ± 19 −1268 ± 16 −1299 ± 25

38.93 ± 6.41 194.40 ± 20.12 42.15 ± 5.00 22.16 ± 3.29 0.72 ± 0.03

phase element CPEdl (representing the capacitance of electrochemical double layer at the metal/electrolyte interface) [40]. According to the investigation of Song et al. [29] and Zhang et al. [41], it is reasonable to use the quantitative parameter Z to characterize the overall resistance of the electrochemical processes involved in the corrosion at the coating/substrate interface. The impedance Z can be calculated by the following equation, Z ¼ Ro þ Ri þ Rct

ð13Þ

Therefore, the values of Z are given in Table 5, manifesting a similar tendency in corrosion resistance as obtained from the polarization plots, ranking as LH-coating N HL-coating ≈ LL-coating N HH-coating, which is in good agreement with the coating morphology. The cross-section of HH-coating is characterized by local uncoated regions, correlating well with the film attachment in the surface morphology and leading to poor corrosion resistance. Similarly, LL-coating with the most uneven thickness cannot effectively protect the substrate. As for HL-coating, the thickness is relatively uniform but more though-going pores and larger pore size can be observed, which also deteriorate the anti-corrosion property. LH-coating is characterized by smaller pore size and denser microstructure in both surface and cross-section morphology, demonstrating the best corrosion resistance.

Fig. 10. Electrochemical impedance behavior of the four PEO coatings: (a) Bode plot, (b) Nyquist plot.

4. Discussion It is obvious that the electrolyte composition decisively influences the morphology, composition and corrosion performance of the resultant coatings. RSM experiment demonstrates that the interaction effect of Na2SiO3 and NaOH is the most dominant factor. Additionally, detailed study of four endpoint runs in RSM design with KF concentration fixed at zero level indicates the straightforward relationship between coating properties and these two variables. During the PEO treatment, different conductivity of the electrolytes altered by composition induces different discharge ignition time (see Table 3), HH-coating (18 s) b HL-coating (27 s) ≈ LH-coating

Fig. 9. Potentiodynamic polarization behavior of the four PEO coatings compared with bare alloy.

(28 s) b LL-coating (50 s), where increased conductivity of the electrolyte could induce earlier ignition. The sparking behavior largely influences the coating morphology. HH-coating accompanied by the earliest sparking and lowest final voltage exhibits the smallest pores size of 0.383 μm and LL-coating with the longest pre-sparking time and highest final voltage has the largest pores (0.653 μm). XPS analysis reveals that the coatings are composed of three compounds, MgO, Mg(OH)2 and MgF2, where the ratio of MgO/Mg(OH)2 is ranked as, HH-coating N LH-coating ≈ HL-coating N LL-coating, which is in the opposite order of discharge ignition time. Since the morphology and composition of coatings decisively determine coating properties, the corrosion performance of the four coatings is in the order, LH-coating N HL-coating ≈ LL-coating N HH-coating. This can be explained from the viewpoint of energy input. Under constant current regime, the energy input in coating formation can be indicated by the final voltage, size and lifetime of the sparks. As for LH-coating, relatively short ignition time and high final voltage demonstrate higher energy input during PEO process, in favor of sintering and densification

Fig. 11. Equivalent circuit for a PEO-coated specimen.


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Table 6 Structure and possible reactions of polymerized silicates, n = 1,2,3,4… Polymerized equation n=1

Structure

Monomeric silicate

n=2

n≥3

nSi(OH)4 = SinOx(OH)(4n−2x) + xH2O

of a compact coating. Larger content of MgO in LH-coating contributes to better corrosion resistance, which is in agreement with Ko's findings [17]. On contrast, the HH-coating somewhat exhibited the worst corrosion resistance even with the highest value of MgO/Mg(OH)2 ratio. This is due to the long-lived, localized sparks on the surface, which generate higher temperature, facilitate the dehydration of Mg(OH)2 to MgO. However, in the region besides the localized sparking area, the energy input (related to the lowest final voltage) is too low to create/sinter a dense coating. It may be enough to cause thermal decomposition of Mg(OH)2 but not sufficient to produce an adherent and compact MgO layer, demonstrated well by the exfoliated surface morphology and film detachment on HH-coating. Based on the discussion above, electrolyte compositions have significant influence on morphology, composition and corrosion performance of resultant coatings. Nevertheless, in order to provide a systematical explanation for the interaction of Na2SiO3 and NaOH and what may inherently account for the inhomogeneous sparking on HH-coating, a further study into the intrinsic characteristics of the electrolytes was performed using FT-IR. Fig. 11 illustrates the FT-IR spectra of the four endpoint electrolytes (dried-pellet state). All the electrolytes contain three characteristic bands, namely free H2O (in the range from 3000 cm−1 to 3500 cm−1), Na-OH and SiO23 −. The presence of peaks close to 1000 cm−1 and 460 cm−1 are related to Si\\O stretching vibrations and Si\\O bending vibrations of silicate anions, respectively, and peaks near 1400 cm−1 are assigned to Na\\OH bonds [42]. Additionally, the location of each peak is annotated in Fig. 11. With altered electrolyte composition, the location of absorption peak of Si\\O stretching vibrations (from 995 cm− 1 to 1008 cm−1) and Si\\O bending vibration (from 441 cm−1 to 465 cm−1) moves as well. The highest frequency corresponding to the location of Si\\O stretching vibration peak is found for HH-electrolyte, whilst the lowest frequency for LH-electrolyte. According to literatures [43,44], higher frequency of location of the absorption peak of Si\\O stretching vibrations implies the existence of large three-dimensional anions. Table 6 shows the molecular structure and possible reactions of polymerized silicate in the solution. Moreover, it could be seen from Fig. 11 that the broad bands in the range above 3000 cm−1 assigned to free absorbed water has the highest intensity for HH-electrolyte, which is attributed to poor affinity to free water molecule to form micelle hydration layer of large dimensional silicate micelle. Combined with the aforementioned, it is rational to correlate the existence form of silicate in the electrolytes with the resultant coating properties. As for HH-electrolyte, when the absorption peak moves to higher frequency, representing lower mobility and reduced chemical reactivity of silicate ions, it is difficult for those

high dimensional silicate ions to move and participate in the reaction near metal-electrolyte interface. Therefore, the extremely non-uniform and exfoliated morphology of HH-coating may stem from the “reluctance” of high dimensional silicates towards coating formation. Similarly, Hsiao et al. [19] demonstrated that anodized coating formed in electrolyte containing high amount of silicate was characterized by a large number of defects or pores. They revealed that in the electrolyte containing high concentration of Na2SiO3, the electrolyte became highly viscose and the sparking changed to non-uniform in the later stage of anodization, which is in accordance with our observation. Furthermore, it was proposed in their work that the electrolyte with high viscosity was probably due to the formation of glassy phase of compound under extremely high energy generated by sparking, which was associated with the polymerization reaction between Na2SiO3 and Al3+ in the electrolyte. Different from the work done by Hsiao et al., in the present study no additional Al3+ was added to the electrolyte. However, in our case AZ91 was used and efficiency of PEO coating growth was reported to be typically 40–50% for voltages ranging from 50 to 220 V for Mg alloys and thus species from the substrate can be lost to the electrolyte at sites of breakdown [45]. In that sense, β phase (Mg17Al12) in the AZ91D Mg alloy, acting as anode during anodizing, should lose electrons and release Al3 + into the electrolyte. Swaddle [46] has proposed that in alkaline solution, the possible reaction between silicate- and aluminum-containing species to form

Fig. 12. FT-IR spectra of four endpoint electrolytes.

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aluminosilicate could be described as: SiðOHÞ4 þ xOH− →ðOHÞ4−x SiOx þ xH2 O x−

ð14Þ ðxþ1Þ−

− ðOHÞ4−x SiOx− x þ AlðOHÞ4 →ðHOÞ3 AlOSiOx ðOHÞ3−x

þ H2 O

ð15Þ

Besides this simple reaction which may form aluminosilicate gel-like substances, more complex thermal polycondensation reactions could take place given the high temperature at the sparking sites (≈104 K) [47]. Therefore, it is reasonable to conclude that in PEO process,

complicated reactions between silicate- and aluminum-containing species could generate short-ranged gel networks which alter the viscosity and therefore the sparking behavior in high silicate containing electrolytes. Additionally, it is reported by Swaddle that complexation of Al3+ by aqueous silicate is more likely at high pH, and the efficacy of polymerized silicate as an Al3+ trapper is evidently several orders of magnitude greater than that of the monomer [46]. This corresponds well with our FT-IR results because HH-electrolyte containing higher amount of polymerized silicates would facilitate to trap Al3 + and more easily

Fig. 13. Schematic diagram of the formation mechanism of PEO coatings in alkaline silicate system with different degree of polymerized silicate species: (a) pre-sparking and (b) aftersparking stage in low degree of polymerized silicates (e.g. LH-electrolyte); (c) pre-sparking and (d) after-sparking stage in high degree of polymerized silicates (e.g. HH-electrolyte).


participate in sol-gel transformation with Al3+. Therefore, HH-electrolyte is more viscous owing to higher content of gel-like substances, rendering it more difficult for PEO treatment and more inclined to induce severe localized sparks. Fig. 12 shows the schematic diagram of the formation mechanism of PEO coatings in alkaline silicate system with different degree of polymerized silicate species. The interpretations herein could account for the RSM experiment results showing that there is interaction effect between Na2SiO3 and NaOH which predominantly determines coating properties. In summation, when designing electrolytes for PEO treatment, appropriate pH and conductivity should be controlled via altering electrolyte constituents, aiming at generating small-sized, continuous sparks instead of large, long-living localized sparks (Fig. 13). As for the interaction effect between Na2SiO3 and NaOH, better corrosion resistance could be achieved when the amount of component with gel-forming ability (SiO23 −) was controlled at lower concentrations. It should be noted that this study has only examined a silicate-containing electrolyte and the results regarding the interaction effect between two electrolyte components may not apply to complicated electrolyte systems for PEO treatment on magnesium alloy. Notwithstanding its limitation, the fundamental work herein will help to assist in reasonable design and development of new PEO electrolytes by understanding the intrinsic properties of this basic system. 5. Conclusion The present work aims to provide guidelines for designing PEO electrolytes via correlating the intrinsic nature of the electrolytes with coating properties. Using the Box-Behnken design (BBD) coupled with the response surface methodology (RSM), an empirical relationship between coating performance and electrolyte components was attained and expressed by the 2FI equation. The Pareto analysis suggested that the interaction between Na2SiO3 and NaOH produces the largest effect on corrosion properties of PEO coatings, where better corrosion resistance could be achieved when their concentrations are tuned in different directions. The difference in response of microstructure and corrosion performance of final coatings along these two routes allows to tailor the electrolyte composition concerning pH, conductivity and viscosity to obtain best coating performance. The composition and concentration of the electrolyte decisively determines the discharge characteristics, voltage response and resultant coating properties. In terms of energy input in coating formation, the ideal coating is the balanced result of short ignition time and high final voltage which can be obtained only if sol-gel reactions in the electrolyte are prevented. The concentration of Na2SiO3 and NaOH varies the degree of polymerization of silicate ions (from one to two- and three-dimensional) and viscosity of electrolytes, which influences the kinetic mechanism of coating formation by changing the discharge conditions and possible thermal-driven gel-forming process. When silicates species change from monomer to polymerized state, which indicates lower mobility of silicates in the electrolyte and better affinity for trapping Al3+ to form gel-like networks, the corresponding electrolyte becomes more viscous and the resulting coating is inhomogeneous and susceptible to corrosive ions; otherwise, the coating is more homogeneous and exhibits improved corrosion resistance. Acknowledgements The authors wish to acknowledge the financial support of the program of Outstanding Young Scholars, the Program of Hundred Talents of Chinese Academy of Sciences, the National Natural Science Foundation of China (NO.51531007). Ms. Y. Chen thanks Chinese Academy of Sciences and German Academic Exchange Service for the jointed CASDAAD scholarship.

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References [1] G.L. Song, A. Atrens, Understanding magnesium corrosion—a framework for improved alloy performance, Adv. Eng. Mater. 5 (2003) 837–858. [2] C. Blawert, W. Dietzel, E. Ghali, G. Song, Anodizing treatments for magnesium alloys and their effect on corrosion resistance in various environments, Adv. Eng. Mater. 8 (2006) 511–533. [3] J.E. Gray, B. Luan, Protective coatings on magnesium and its alloys—a critical review, J. Alloys Compd. 336 (2002) 88–113. [4] X.B. Chen, N. Birbilis, T.B. Abbott, Review of corrosion-resistant conversion coatings for magnesium and its alloys, Corrosion 67 (2011) 1–16. [5] R. Arrabal, E. Matykina, F. Viejo, P. Skeldon, G.E. Thompson, Corrosion resistance of WE43 and AZ91D magnesium alloys with phosphate PEO coatings, Corros. Sci. 50 (2008) 1744–1752. [6] Y. Zhang, C. Yan, F. Wang, H. Lou, C.N. Cao, Study on the environmentally friendly anodizing of AZ91D magnesium alloy, Surf. Coat. Technol. 161 (2002) 36–43. [7] V.I. Belevantsev, O.P. Terleeva, G.A. Markov, E.K. Shulepko, A.I. Slonova, V.V. Utkin, Micro-plasma electrochemical processes, Prot. Met. 34 (1998) 416–430. [8] R. Arrabal, E. Matykina, T. Hashimoto, P. Skeldon, G.E. Thompson, Characterization of AC PEO coatings on magnesium alloys, Surf. Coat. Technol. 203 (2009) 2207–2220. [9] S.V. Gnedenkov, O.A. Khrisanfova, A.G. Zavidnaya, S.L. Sinebryukhov, V.S. Egorkin, M.V. Nistratova, et al., PEO coatings obtained on an Mg-Mn type alloy under unipolar and bipolar modes in silicate-containing electrolytes, Surf. Coat. Technol. 204 (2010) 2316–2322. [10] A.V. Timoshenko, Y.V. Magurova, Investigation of plasma electrolytic oxidation processes of magnesium alloy MA2-1 under pulse polarisation modes, Surf. Coat. Technol. 199 (2005) 135–140. [11] M. Mohedano, E. Matykina, R. Arrabal, A. Pardo, M.C. Merino, Metal release from ceramic coatings for dental implants, Dent. Mater. 30 (2014) 28–44. [12] X. Lu, S.P. Sah, N. Scharnagl, M. Störmer, M. Starykevich, M. Mohedano, et al., Degradation behavior of PEO coating on AM50 magnesium alloy produced from electrolytes with clay particle addition, Surf. Coat. Technol. 269 (2015) 155–169. [13] S. Stojadinović, R. Vasilić, J. Radić-Perić, M. Perić, Characterization of plasma electrolytic oxidation of magnesium alloy AZ31 in alkaline solution containing fluoride, Surf. Coat. Technol. 273 (2015) 1–11. [14] A. Kossenko, M. Zinigrad, A universal electrolyte for the plasma electrolytic oxidation of aluminum and magnesium alloys, Mater. Des. 88 (2015) 302–309. [15] H.F. Guo, M.Z. An, H.B. Huo, S. Xu, L.J. Wu, Microstructure characteristic of ceramic coatings fabricated on magnesium alloys by micro-arc oxidation in alkaline silicate solutions, Appl. Surf. Sci. 252 (2006) 7911–7916. [16] W. Li, L. Zhu, H. Liu, Effects of silicate concentration on anodic films formed on AZ91D magnesium alloy in solution containing silica sol, Surf. Coat. Technol. 201 (2006) 2505–2511. [17] S.L. Aktuğ, S. Durdu, I. Kutbay, M. Usta, Effect of Na2SiO3·5H2O concentration on microstructure and mechanical properties of plasma electrolytic oxide coatings on AZ31 Mg alloy produced by twin roll casting, Ceram. Int. 42 (2016) 1246–1253. [18] H. Fukuda, Y. Matsumoto, Effects of Na2SiO3 on anodization of Mg-Al-Zn alloy in 3 M KOH solution, Corros. Sci. 46 (2004) 2135–2142. [19] H.Y. Hsiao, H.C. Tsung, W.T. Tsai, Anodization of AZ91D magnesium alloy in silicatecontaining electrolytes, Surf. Coat. Technol. 199 (2005) 127–134. [20] Y.G. Ko, S. Namgung, D.H. Shin, Correlation between KOH concentration and surface properties of AZ91 magnesium alloy coated by plasma electrolytic oxidation, Surf. Coat. Technol. 2525–2531 (2010). [21] J. Liang, B. Guo, J. Tian, H. Liu, J. Zhou, T. Xu, Effect of potassium fluoride in electrolytic solution on the structure and properties of microarc oxidation coatings on magnesium alloy, Appl. Surf. Sci. 252 (2005) 345–351. [22] L. Wang, L. Chen, Z. Yan, H. Wang, J. Peng, Effect of potassium fluoride on structure and corrosion resistance of plasma electrolytic oxidation films formed on AZ31 magnesium alloy, J. Alloys Compd. 480 (2009) 469–474. [23] I. Tan, A. Ahmad, B. Hameed, Optimization of preparation conditions for activated carbons from coconut husk using response surface methodology, Chem. Eng. J. 137 (2008) 462–470. [24] K. Anupam, S. Dutta, C. Bhattacharjee, S. Datta, Adsorptive removal of chromium (VI) from aqueous solution over powdered activated carbon: optimisation through response surface methodology, Chem. Eng. J. 173 (2011) 135–143. [25] M.Y. Noordin, V.C. Venkatesh, S. Sharif, S. Elting, Application of response surface methodology in describing the performance of coated carbide tools when turning AISI 1045 steel, J. Mater. Process. Technol. 145 (2004) 46–58. [26] R. Azargohar, A.K. Dalai, Production of activated carbon from Luscar char: experimental and modeling studies, Microporous Mesoporous Mater. 85 (2005) 219–225. [27] E. McCafferty, Validation of corrosion rates measured by the Tafel extrapolation method, Corros. Sci. 47 (2005) 3202–3215. [28] G. Yoganandan, K.P. Premkumar, J.N. Balaraju, Evaluation of corrosion resistance and self-healing behavior of zirconium–cerium conversion coating developed on AA2024 alloy, Surf. Coat. Technol. 270 (2015) 249–258. [29] G.L. Song, Z. Shi, Corrosion mechanism and evaluation of anodized magnesium alloys, Corros. Sci. 85 (2014) 126–140. [30] N. Souissi, E. Triki, Modelling of phosphate inhibition of copper corrosion in aqueous chloride and sulphate media, Corros. Sci. 50 (2008) 231–241. [31] M. Zarei, A. Niaei, D. Salari, A. Khataee, Application of response surface methodology for optimization of peroxi-coagulation of textile dye solution using carbon nanotube-PTFE cathode, J. Hazard. Mater. 173 (2010) 544–551. [32] I. Han, J.H. Choi, B.H. Zhao, H.K. Baik, I.-S. Lee, Micro-arc oxidation in various concentration of KOH and structural change by different cut off potential, Curr. Appl. Phys. 7 (2007) 23–27.

836


[33] S. Ikonopisov, A. Girginov, M. Machkova, Post-breakdown anodization of aluminum, Electrochim. Acta 22 (1977) 1283–1286. [34] O. Khaselev, D. Weiss, J. Yahalom, Anodizing of pure magnesium in KOH-aluminate solutions under sparking, J. Electrochem. Soc. 146 (1999) 1757–1761. [35] K.M. Lee, Y.G. Ko, D.H. Shin, Microstructural characteristics of oxide layers formed on Mg-9wt%Al-1wt%Zn alloy via two-step plasma electrolytic oxidation, J. Alloys Compd. 615 (2014) 418–422. [36] O. Khaselev, J. Yahalom, The anodic behavior of binary Mg-Al alloys in KOH-aluminate solutions, Corros. Sci. 40 (1998) 1149–1160. [37] O. Khaselev, Structure and composition of anodic films formed on binary Mg-Al alloys in KOH-aluminate solutions under continuous sparking, Corros. Sci. 43 (2001) 1295–1307. [38] JEOL, Handbook of X-ray Photoelectron Spectroscopy, JEOL Co. Ltd, Japan, 1991. [39] G.L. Song, An irreversible dipping sealing technique for anodized ZE41 Mg alloy, Surf. Coat. Technol. 203 (2009) 3618–3625. [40] X. Lu, C. Blawert, Y. Huang, H. Ovri, M.L. Zheludkevich, Plasma electrolytic oxidation coatings on Mg alloy with addition of SiO2 particles, Electrochim. Acta 187 (2016) 20–23.

[41] C. Zhang, T. Zhang, Y. Wang, F. Wei, Y. Shao, Effect of SiC particulates on the corrosion behavior of extruded AZ91/SiCp composites during the early stage of exposure, J. Electrochem. Soc. 152 (2015) 754–766. [42] D.G. Cao, Deopolymeric Mechanism, Microstructure and Characterics of Geopolymer Based on Metakaolinite, South China University of Technology, Guangzhou, PhD Dissertation, 2005. [43] J.L. Bass, G.L. Turner, Anion distributions in sodium silicate solutions. Characterization by 29Si NMR and infrared spectroscopies, and vapor phase osmometry, J. Phys. Chem. B 101 (1997) 10638–10644. [44] N.R. Yang, W.H. Yue, The Handbook of Inorganic Metalloid Materials Atlas, Wuhan University of Technology Press, Wuhan, 2000. [45] F.A. Bonilla, A. Berkani, Y. Liu, P. Skeldon, G.E. Thompson, H. Habazaki, et al., Formation of anodic films on magnesium alloys in an alkaline phosphate electrolyte, J. Electrochem. Soc. 149 (2002) B4–B13. [46] T.W. Swaddle, Silicate complexes of aluminum (III) in aqueous systems, Coord. Chem. Rev. 219–221 (2001) 665–686. [47] H.M. Nykyforchyn, W. Dietzel, M.D. Klapkiv, C. Blawert, Int. Conf. Magn. Alloys and their Appl. Wolfsburg (D) (2003) 176.