Plasma Electrolytic Oxidation of Magnesium Alloy AZ31B in Electrolyte Containing Al2O3 Sol as Additives

Plasma electrolytic oxidation (PEO) coatings were produced on AZ31B magnesium alloys in alkaline electrolytes with the addition of various concentrations of Al2O3 sols. Effects of Al2O3 sol concentrations on the microstructure, phase composition, corrosion resistance and hardness of PEO coatings were evaluated by scanning electron microscopy (SEM), X-ray diffraction (XRD), microhardness testing and potentiodynamic polarization measurements, respectively. It was revealed that the Al2O3 sol mostly participated in the formation of the ceramic coating and transferred into the MgAl2O4 phase. With the increase of the Al2O3 sol concentration in the range of 0–6 vol%, the coating performance in terms of the microstructure, diffraction peak intensity of the MgAl2O4 phase, corrosion resistance and microhardness was improved. Further increase of Al2O3 sol addition did not generate better results. This indicated that 6 vol% might be the proper Al2O3 sol concentration for the formation of PEO coatings.


Introduction
Magnesium and its alloys, as structural and functional materials, have been increasingly attracting attention in a wide range of industries, e.g., aerospace, automotive communication devices and biodegradable implants [1][2][3]. Unlike the other light metal of aluminum, the spontaneous oxide film formed on the surface of the magnesium alloy cannot provide satisfactory protection from corrosion due to the decrease of about 20% in volume. Against this background, a variety of engineering options, including electroless plating, electropolymerization, chemical conversion, and anodization are available to improve the corrosion properties of magnesium alloys [4][5][6][7].

Materials and Methods
Rectangular plates of AZ31B magnesium alloy with a size of 25 mm × 25 mm × 2 mm were cut from AZ31B plate (Hongdi Metal, Dongguan, China). Prior to PEO treatment, surfaces of all specimens were polished with successively finer grades of emery paper from 180 to 1000 grits. Then, the specimens were rinsed with distilled water and degreased with acetone.
The electrolyte composition used in the PEO process was based on NaOH (5 g/L) and Na 2 SiO 3 (20 g/L). The coatings were obtained in the electrolyte with and without Al 2 O 3 sol. The concentrations of Al 2 O 3 sol added to the electrolyte were 0, 2, 4, 6, 8 and 10 vol%. The PEO process was performed by using a pulse electrical source (SOYI-30010M, Soyi Power, Shanghai, China). The electrical parameters were set as follows: frequency at 200 Hz, duty cycle at 10%, current density at 1.5 A/dm 2 . The PEO process time was 15 min and the temperature of the electrolyte was kept within the range 25-35 • C. The Al 2 O 3 sol was prepared from aluminium isopropoxide, nitric acid and water with the molar ratio of 1:0.2:200. They were mixed and reacted under stirring for 24 h to prepare the Al 2 O 3 sol at 85 • C.
Surface morphology of the PEO coatings was characterized by scanning electron microscopy (SEM, HITACHI, S-4800, Tokyo, Japan). The phase compositions of the coatings were analyzed by X-ray diffraction (XRD, DX-2600, Fangyuan Instrument, Dandong, China), using a Cu Kα radiation source. The surface roughness (Ra) of the coatings was measured using a stylus-type surface profilometer (TR200, Grows Precision Instrument, Shanghai, China). Each sample was measured five times and the average values are given. The film thickness was measured using an eddy current film thickness measurement gauge (MPO, Fisher, Sindelfingen, Germany). Coating hardness was evaluated by using an HXS-1000 microhardness tester (Milite Instrument, Nanjing, China).
Potentiodynamic polarization tests were carried out to evaluate the corrosion resistance of the coatings using CHI 842B electrochemical equipment (Chenhua, Shanghai, China) in 3.5 wt.% NaCl solution at 25 • C. During the measurements, a traditional three-electrode cell was used, with a large-area platinum sheet as counter electrode and a saturated calomel electrode (SCE) as reference electrode. Potentiodynamic polarization curves were obtained after 600 s immersion in the electrolyte, at a scan rate of 1 mV/s. The area of samples exposed to the electrolyte for the electrochemical tests was 1 cm 2 . on the voltage-time curves, the PEO process can be divided into three stages. This phenomenon was similar to that proposed in the literature [8,19,22]. At Stage I (0-120 s), a small amount of gas bubbles is continuously released from the surface of the magnesium alloy substrate. The voltage increased quite sharply and almost linearly with the anodized time. It was estimated that a thin transparent coating was formed on the surface of the magnesium alloy. This passive coating can enhance the resistance of the PEO coating, which led to the rapid increase of the potential. When the voltage reached the "breakdown voltage", one or two bright and small sparks appeared on the surface of the magnesium alloy. This indicated that the PEO process entered into Stage II (about 120-720 s). Accordingly, the voltage still increased with respect to anodizing time, but the slopes of the curves decreased compared with those in Stage I. In this region, a number of white sparks in small sizes with shrill sound were observed on the surface of the anode. During Stage III (about 720-900 s), steady-state sparking occurred on the surface of the magnesium alloy. The voltages gradually reached the relatively steady value while the white sparks previously observed changed into separated orange ones. It is noteworthy that the breakdown voltages in the alkaline electrolytes with Al 2 O 3 sols are higher than that without the additive. The reason for this may be that though a nonconductive additive, the Al 2 O 3 sol particles may be adsorbed and trapped at the interface of alloy and electrolyte due to their negative charges and many hydroxyl groups [46][47][48]. This could be responsible for the enhancement of the surface resistance and thus have led to the increase of the breakdown voltages. alternations of slopes on the voltage-time curves, the PEO process can be divided into three stages. This phenomenon was similar to that proposed in the literature [8,19,22]. At Stage Ι (0-120 s), a small amount of gas bubbles is continuously released from the surface of the magnesium alloy substrate. The voltage increased quite sharply and almost linearly with the anodized time. It was estimated that a thin transparent coating was formed on the surface of the magnesium alloy. This passive coating can enhance the resistance of the PEO coating, which led to the rapid increase of the potential. When the voltage reached the "breakdown voltage", one or two bright and small sparks appeared on the surface of the magnesium alloy. This indicated that the PEO process entered into Stage ΙΙ (about 120-720 s). Accordingly, the voltage still increased with respect to anodizing time, but the slopes of the curves decreased compared with those in Stage I. In this region, a number of white sparks in small sizes with shrill sound were observed on the surface of the anode. During Stage III (about 720-900 s), steady-state sparking occurred on the surface of the magnesium alloy. The voltages gradually reached the relatively steady value while the white sparks previously observed changed into separated orange ones. It is noteworthy that the breakdown voltages in the alkaline electrolytes with Al2O3 sols are higher than that without the additive. The reason for this may be that though a nonconductive additive, the Al2O3 sol particles may be adsorbed and trapped at the interface of alloy and electrolyte due to their negative charges and many hydroxyl groups [46][47][48]. This could be responsible for the enhancement of the surface resistance and thus have led to the increase of the breakdown voltages.

Coating Thickness of PEO Coatings
The effect of Al2O3 sol concentration on the average thickness of PEO coatings formed on the magnesium alloy is shown in Figure 2. As seen from Figure 2, the addition of Al2O3 sols into the alkaline electrolyte for the PEO process resulted in a much thicker coating. In the region of 0-6 vol% Al2O3 sol, the coating thickness increases with increasing Al2O3 sol concentration and the thickest coating with the value of 17.9 μm is obtained with the addition of 6 vol% Al2O3 sol. The corresponding average growth rate of coating thickness was 1.19 μm/min, which was much slower than that reported in the literature [17]. This could be attributed to the much smaller current density in the PEO process compared with that reported elsewhere. However, with further increase of the Al2O3 sol concentration, the coating thickness drops slightly. These variations reveal that the additions of Al2O3 sol promoted the development of the PEO coating, while too much Al2O3 sol addition might have negative effects on the coating growth.

Coating Thickness of PEO Coatings
The effect of Al 2 O 3 sol concentration on the average thickness of PEO coatings formed on the magnesium alloy is shown in Figure 2. As seen from Figure 2, the addition of Al 2 O 3 sols into the alkaline electrolyte for the PEO process resulted in a much thicker coating. In the region of 0-6 vol% Al 2 O 3 sol, the coating thickness increases with increasing Al 2 O 3 sol concentration and the thickest coating with the value of 17.9 µm is obtained with the addition of 6 vol% Al 2 O 3 sol. The corresponding average growth rate of coating thickness was 1.19 µm/min, which was much slower than that reported in the literature [17]. This could be attributed to the much smaller current density in the PEO process compared with that reported elsewhere. However, with further increase of the Al 2 O 3 sol concentration, the coating thickness drops slightly. These variations reveal that the additions of Al 2 O 3 sol promoted the development of the PEO coating, while too much Al 2 O 3 sol addition might have negative effects on the coating growth.  Figure 3 presents the variation of average surface roughness of PEO coatings formed in the electrolyte with and without Al2O3 sol. Apparently, the surface roughness of the PEO coating formed in the electrolyte without Al2O3 sol was much higher than those of coatings formed in the electrolyte with various concentrations of Al2O3 sols. In the range of 0-6 vol% Al2O3 sol, the surface roughness dropped rapidly with the increase of the Al2O3 sol concentration. The roughness of the coating produced in the electrolyte with 6 vol% Al2O3 sol is about 0.9 μm. With further increase of the Al2O3 sol concentration, the surface roughness slightly increased. This indicated that the composition of the electrolyte, as well as the addition amount of Al2O3 sol, has a significant effect on the surface roughness of PEO coatings. The low surface roughness induced by Al2O3 sol might be beneficial to the corrosion resistance [49,50].    Figure 3 presents the variation of average surface roughness of PEO coatings formed in the electrolyte with and without Al2O3 sol. Apparently, the surface roughness of the PEO coating formed in the electrolyte without Al2O3 sol was much higher than those of coatings formed in the electrolyte with various concentrations of Al2O3 sols. In the range of 0-6 vol% Al2O3 sol, the surface roughness dropped rapidly with the increase of the Al2O3 sol concentration. The roughness of the coating produced in the electrolyte with 6 vol% Al2O3 sol is about 0.9 μm. With further increase of the Al2O3 sol concentration, the surface roughness slightly increased. This indicated that the composition of the electrolyte, as well as the addition amount of Al2O3 sol, has a significant effect on the surface roughness of PEO coatings. The low surface roughness induced by Al2O3 sol might be beneficial to the corrosion resistance [49,50].  Micropores, cracks and molten oxide could be observed on the surface of the PEO coatings in all samples, which were caused by the plasma discharge and gas release during the PEO process. However, the addition of Al 2 O 3 sol into the alkaline electrolyte generated a significant change in the surface morphologies of PEO coatings. As shown in Figure 4a, the PEO coating formed in the alkaline electrolyte without Al 2 O 3 sol had a rougher surface appearance. Many micropores with a diameter of 5-10 µm and several cracks could be found on the surface of the PEO coatings. This open structure of the porous and rough surface of the PEO coating could be easily penetrated by corrosive ions and thus cause corrosion. With the addition of Al 2 O 3 sol into the electrolyte, the number and diameter of micropores decreased, as shown in Figure 4b-f. As can be seen in these micrographs, the surface of the coating formed in the electrolyte with 6 vol% Al 2 O 3 sol was much more uniform compared with the others. Micropores, cracks and the molten oxide on the surface of the PEO coating will affect the surface roughness. The SEM image results of these PEO coatings are consistent with the surface roughness tests presented in Figure 3.  Figure 4 displays SEM images showing the morphologies of all PEO coatings formed in different compositions of electrolyte. Micropores, cracks and molten oxide could be observed on the surface of the PEO coatings in all samples, which were caused by the plasma discharge and gas release during the PEO process. However, the addition of Al2O3 sol into the alkaline electrolyte generated a significant change in the surface morphologies of PEO coatings. As shown in Figure 4a, the PEO coating formed in the alkaline electrolyte without Al2O3 sol had a rougher surface appearance. Many micropores with a diameter of 5-10 μm and several cracks could be found on the surface of the PEO coatings. This open structure of the porous and rough surface of the PEO coating could be easily penetrated by corrosive ions and thus cause corrosion. With the addition of Al2O3 sol into the electrolyte, the number and diameter of micropores decreased, as shown in Figures 4b-f. As can be seen in these micrographs, the surface of the coating formed in the electrolyte with 6 vol% Al2O3 sol was much more uniform compared with the others. Micropores, cracks and the molten oxide on the surface of the PEO coating will affect the surface roughness. The SEM image results of these PEO coatings are consistent with the surface roughness tests presented in Figure 3.

Phase Analysis of PEO Coatings
To understand the phase composition of the PEO coatings, XRD patterns are analyzed for the coated magnesium alloys prepared with different concentrations of Al 2 O 3 sol in the electrolyte, as presented in Figure 5. The addition of Al 2 O 3 sol to the electrolyte clearly had a great influence on the PEO coating composition. The PEO coating formed in the electrolyte without Al 2 O 3 sol was mainly composed of MgO and Mg 2 SiO 4 . As the Al 2 O 3 sol was added to the electrolyte, the MgAl 2 O 4 phase with the diffraction peak value of 18.8 • appeared in addition to the substrate peaks. As the concentration of Al 2 O 3 sol increased from 2 vol% to 6 vol%, the intensity of the diffraction peak related to the MgAl 2 O 4 phase increased significantly. With further increasing of the Al 2 O 3 sol concentration, the intensity of the diffraction peak of the MgAl 2 O 4 phase was less affected. This indicates that the Al 2 O 3 sol participated in the formation of the PEO coating and transferred to the MgAl 2 O 4 phase through some chemical reactions [35,36]. The formation mechanisms of PEO coatings with Al 2 O 3 sol could be proposed as presented below in Equations (1)-(5) [3,20].

Phase Analysis of PEO Coatings
To understand the phase composition of the PEO coatings, XRD patterns are analyzed for the coated magnesium alloys prepared with different concentrations of Al2O3 sol in the electrolyte, as presented in Figure 5. The addition of Al2O3 sol to the electrolyte clearly had a great influence on the PEO coating composition. The PEO coating formed in the electrolyte without Al2O3 sol was mainly composed of MgO and Mg2SiO4. As the Al2O3 sol was added to the electrolyte, the MgAl2O4 phase with the diffraction peak value of 18.8° appeared in addition to the substrate peaks. As the concentration of Al2O3 sol increased from 2 vol% to 6 vol%, the intensity of the diffraction peak related to the MgAl2O4 phase increased significantly. With further increasing of the Al2O3 sol concentration, the intensity of the diffraction peak of the MgAl2O4 phase was less affected. This indicates that the Al2O3 sol participated in the formation of the PEO coating and transferred to the MgAl2O4 phase through some chemical reactions [35,36]. The formation mechanisms of PEO coatings with Al2O3 sol could be proposed as presented below in Equations (1)-(5) [3,20].

Corrosion Behavior of PEO Coatings
The anticorrosion performance of all PEO coating samples was evaluated by potentiodynamic polarization testing in 3.5 wt.% NaCl solution. The parameters, related to the corrosion properties such as corrosion potential (Ecorr), corrosion current density (jcorr) and anodic (βa) and cathodic (βc)

Corrosion Behavior of PEO Coatings
The anticorrosion performance of all PEO coating samples was evaluated by potentiodynamic polarization testing in 3.5 wt.% NaCl solution. The parameters, related to the corrosion properties such as corrosion potential (E corr ), corrosion current density (j corr ) and anodic (β a ) and cathodic (β c ) Tafel slopes are extracted directly from the potentiodynamic polarization curves displayed in Figure 6. The polarization resistances (R p ) were calculated from the Equation All corresponding parameter values are summarized in Table 1. Tafel slopes are extracted directly from the potentiodynamic polarization curves displayed in Figure  6. The polarization resistances (Rp) were calculated from the Equation All corresponding parameter values are summarized in Table 1.  Corrosion potential (Ecorr), corrosion current density (jcorr) and polarization resistances (Rp) are often used to evaluated the anticorrosion performance of PEO coatings. A higher polarization resistance (Rp), lower corrosion current density (jcorr) and more positive corrosion potential (Ecorr) indicate a better corrosion resistance. As can been seen from Figure 6 and Table 1, the Ecorr, jcorr and Rp of the PEO coating formed without Al2O3 sol are −1.58 V, 8.52 × 10 −6 A/cm 2 , and 1570 Ω•cm 2 , respectively. In terms of PEO coatings with various concentrations of Al2O3 sol, the Ecorr shifted to the positive direction, Rp increased and jcorr decreased. This indicated that the addition of Al2O3 sol promoted the enhancement of the anticorrosion properties of the PEO coatings. This improvement of the anticorrosion properties of the PEO coating may be due to the increase of the coating thickness and the decrease of the coating roughness, which result in better anticorrosion properties, as mentioned above. In addition, the Al2O3 sol might be introduced into the micropores and the cracks during the PEO process, which would promote the inhibition of corrosive ion penetration into the magnesium alloy matrix. More importantly, the Al2O3 sol might be chemically transferred into the MgAl2O4 phase which can improve the anticorrosion performance [35,36]. It is noted that the corrosion current density of the PEO coating with 6 vol% Al2O3 sol reaches the minimum value of 5.18 × 10 −8 A/cm 2 and the associated corrosion potential is 1.30 V; also, the polarization resistance  Corrosion potential (E corr ), corrosion current density (j corr ) and polarization resistances (R p ) are often used to evaluated the anticorrosion performance of PEO coatings. A higher polarization resistance (R p ), lower corrosion current density (j corr ) and more positive corrosion potential (E corr ) indicate a better corrosion resistance. As can been seen from Figure 6 and Table 1, the E corr , j corr and R p of the PEO coating formed without Al 2 O 3 sol are −1.58 V, 8.52 × 10 −6 A/cm 2 , and 1570 Ω·cm 2 , respectively. In terms of PEO coatings with various concentrations of Al 2 O 3 sol, the E corr shifted to the positive direction, R p increased and j corr decreased. This indicated that the addition of Al 2 O 3 sol promoted the enhancement of the anticorrosion properties of the PEO coatings. This improvement of the anticorrosion properties of the PEO coating may be due to the increase of the coating thickness and the decrease of the coating roughness, which result in better anticorrosion properties, as mentioned above. In addition, the Al 2 O 3 sol might be introduced into the micropores and the cracks during the PEO process, which would promote the inhibition of corrosive ion penetration into the magnesium alloy matrix. More importantly, the Al 2 O 3 sol might be chemically transferred into the MgAl 2 O 4 phase which can improve the anticorrosion performance [35,36]. It is noted that the corrosion current density of the PEO coating with 6 vol% Al 2 O 3 sol reaches the minimum value of 5.18 × 10 −8 A/cm 2 and the associated corrosion potential is 1.30 V; also, the polarization resistance reaches the maximum value of 607,446 Ω·cm 2 . Based on the results of electrochemical parameters from Table 1, it was considered that the PEO coatings formed in the electrolyte with 6 vol% Al 2 O 3 sol also showed good corrosion resistance compared with other PEO coatings of AZ31/AZ31B magnesium alloys in other reports [32,33,46,47]. In summary, 6 vol% is a proper concentration of Al 2 O 3 sol to improve the anticorrosion property of a PEO coating.

Microhardness of PEO Coatings
The average microhardness of the PEO coatings formed in the electrolytes with various concentrations of Al 2 O 3 sol is shown in Figure 7. The microhardness of the PEO coating without Al 2 O 3 sol was 131 HV. The microhardness of all PEO coatings produced with Al 2 O 3 sol was much higher than that of the coating produced without Al 2 O 3 sol. Meanwhile, the microhardness of the PEO coatings increased significantly in the range of 0-6 vol% Al 2 O 3 sol concentration and slightly changed with further increase of the Al 2 O 3 sol. The PEO coating produced in the electrolyte with 6 vol% Al 2 O 3 sol exhibits a microhardness of 348 HV, which is more than twice that of the PEO coating formed in the electrolyte without Al 2 O 3 sol. The enhancement of the microhardness of the PEO coating is mainly due to the improvement of the microstructure and the increase of the MgAl 2 O 4 phase, both of which promote the enhancement of the microhardness [36].  [32,33,46,47]. In summary, 6 vol% is a proper concentration of Al2O3 sol to improve the anticorrosion property of a PEO coating.

Microhardness of PEO Coatings
The average microhardness of the PEO coatings formed in the electrolytes with various concentrations of Al2O3 sol is shown in Figure 7. The microhardness of the PEO coating without Al2O3 sol was 131 HV. The microhardness of all PEO coatings produced with Al2O3 sol was much higher than that of the coating produced without Al2O3 sol. Meanwhile, the microhardness of the PEO coatings increased significantly in the range of 0-6 vol% Al2O3 sol concentration and slightly changed with further increase of the Al2O3 sol. The PEO coating produced in the electrolyte with 6 vol% Al2O3 sol exhibits a microhardness of 348 HV, which is more than twice that of the PEO coating formed in the electrolyte without Al2O3 sol. The enhancement of the microhardness of the PEO coating is mainly due to the improvement of the microstructure and the increase of the MgAl2O4 phase, both of which promote the enhancement of the microhardness [36].

Conclusions
PEO coatings on magnesium alloy AZ31B were prepared in an alkaline electrolyte with various concentrations of Al2O3 sol. The surface morphology of the PEO coatings produced in the alkaline electrolytes with Al2O3 sol was more uniform with less structural imperfections than that of the coating produced in the electrolyte without Al2O3 sol. It was found that the Al2O3 sol mostly transferred into MgAl2O4 under certain chemical reactions, which also led to the better microhardness and anticorrosion performance. Moreover, under the same PEO conditions, the coating thickness and surface roughness were improved with the addition of Al2O3 sol in the alkaline electrolyte. However, when the Al2O3 sol exceeded 6 vol%, these trends of the effects of the addition of Al2O3 sol diminished. In sum, 6 vol% Al2O3 sol addition might be the better concentration for PEO coating formation for magnesium alloy AZ31B.

Conclusions
PEO coatings on magnesium alloy AZ31B were prepared in an alkaline electrolyte with various concentrations of Al 2 O 3 sol. The surface morphology of the PEO coatings produced in the alkaline electrolytes with Al 2 O 3 sol was more uniform with less structural imperfections than that of the coating produced in the electrolyte without Al 2 O 3 sol. It was found that the Al 2 O 3 sol mostly transferred into MgAl 2 O 4 under certain chemical reactions, which also led to the better microhardness and anticorrosion performance. Moreover, under the same PEO conditions, the coating thickness and surface roughness were improved with the addition of Al 2 O 3 sol in the alkaline electrolyte. However, when the Al 2 O 3 sol exceeded 6 vol%, these trends of the effects of the addition of Al 2 O 3 sol diminished. In sum, 6 vol% Al 2 O 3 sol addition might be the better concentration for PEO coating formation for magnesium alloy AZ31B.