Influence of Phosphate Concentration on Plasma Electrolytic Oxidation of AZ80 Magnesium Alloy in Alkaline Aluminate Solution

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1 Materials Transactions, Vol. 51, No. 1 (21) pp. 94 to 12 #21 The Japan Institute of Metals Influence of Phosphate Concentration on Plasma Electrolytic Oxidation of AZ8 Magnesium Alloy in Alkaline Aluminate Solution Santosh Prasad Sah, Yoshitaka Aoki and Hiroki Habazaki Graduate School of Engineering, Hokkaido University, Sapporo , Japan Plasma electrolytic oxidation of AZ8 magnesium alloy in alkaline aluminate electrolytes develops MgAl 2 O 4 -based highly crystalline oxide s with the morphology changing largely with phosphate concentration in electrolyte. The thickness of the s increases with phosphate concentration from 5 mm in phosphate-free electrolyte to 7 mm in the electrolyte containing.1 moldm 3 phosphate after anodizing for 9 s. The formation of the latter thick is associated with intense sparking during anodizing. The thick s formed in the electrolytes containing.75 and.1 moldm 3 phosphate are highly cracked. In contrast, the formed in the electrolyte containing.5 moldm 3 phosphate is relatively uniform, showing the highest corrosion protection in.5 moldm 3 NaCl solution. The s consist of two layers, comprising an outer thick layer with high concentration of aluminum and an inner thin magnesium-rich layer. [doi:1.232/matertrans.m29226] (Received July 13, 29; Accepted November 5, 29; Published December 25, 29) Keywords: plasma electrolytic oxidation, AZ8 magnesium alloy, corrosion protection 1. Introduction Magnesium and its alloys have several advantageous properties, including low density, high specific strength, high thermal conductivity, good electromagnetic shielding characteristics, high damping characteristics, good machinability and easy recyclability, which make them attractive for many industrial applications. However, the wide industrial applications are limited due to their high susceptibility to corrosion. 1) For protection of magnesium alloys from corrosion, surface treatments, forming protective layers on the alloy surface, become essential. 2) Chemical conversion s and anodizing are the most widely used processes with the latter generally providing thicker and more protective layers. In these processes, the electrolytes containing toxic chromate and fluoride have been often used to form protective s, but a process, free from chromate and fluoride, must be used recently from environmental point of view. The behavior of anodic film formation on magnesium is dependent upon the formation voltage, current density, electrolyte and the alloy. Huber showed the relationship between the formation voltage and the characteristics of anodic films formed on magnesium in 1 moldm 3 NaOH. 3) A very thin film is formed at below 3 V, while a thick anodic film is developed with simultaneous oxygen evolution between 3 and 2 V. A thin protective film is produced again above 2 V. Sparking due to dielectric breakdown occurs above 5 V. Similar filming behavior has been found in alkaline aluminate and phosphate electrolytes 4) and various fluoride-containing electrolytes. 5) Plasma electrolytic oxidation (PEO) has recently attracted attention to form ceramic-like s on aluminum, magnesium and titanium for corrosion and wear resistance. 6 8) Spark micro-discharge in PEO process modifies the structure, composition and morphology of the oxide layer; a thick, highly crystalline and melt-quenched high temperature oxide is formed by this process. Although the oxide is exposed locally and instantaneously at extremely high temperatures ( K), 6) the metal should be kept at low temperatures, not causing mechanical damage. Thus, PEO is a promising technique to form a thick protective on light metals. There are a number of studies on PEO s of magnesium alloys for corrosion protection. 9 22) The composition, thickness and morphology of the PEO s are dependent upon various parameters including electrolyte composition, current density, applied voltage and current wave form. 15,21 24) Pulsed DC or AC mode of anodizing is generally used, instead of simple DC anodizing, due to wider possibility of controlling the PEO process. 6) Since electrolyte species are usually highly incorporated into the PEO, probably due to plasma-assisted thermo-chemical reactions, the electrolyte composition markedly influences the composition. Aluminium is a beneficial element in enhancing the passivity of magnesium ) Khaselev et al. anodized magnesium alloys in alkaline electrolytes containing aluminate 29 31). They found the formation of MgAl 2 O 4 spinel phase under continuous sparking. 31) PEO s were developed on AM6 magnesium alloy in alkaline aluminatephosphate electrolytes containing different concentrations of aluminate, and improved corrosion protection of the s by increasing aluminate concentration was reported. 11) Since magnesium phosphate is insoluble in aqueous solutions, it is likely that the concentration of phosphate also influences the protective nature of the s. Thus, PEO s have been developed in this study in alkaline.15 moldm 3 aluminate electrolytes containing different concentration of Na 3 PO 4. The results have demonstrated the importance of phosphate concentration on the formation of protective s by PEO in alkaline aluminate solution. There is an appropriate phosphate concentration to develop a relatively uniform PEO. 2. Experimental AZ8 magnesium alloy specimens of 1 mm thickness were cut from an extruded commercial bar. The specimens were wet ground through successive grades of silicon carbide

2 Influence of Phosphate Concentration on Plasma Electrolytic Oxidation of AZ8 Magnesium Alloy in Alkaline Aluminate Solution 95 abrasive papers up to 1 grit, followed by ultrasonic cleaning in distilled water and in acetone. About 4 8 mm 2 area of the specimens was exposed for anodizing; the remaining area was coated with Teflon adhesive tape. The electrolytes used were.15 moldm 3 K 2 Al 2 O moldm 3 KOH solutions with varied concentrations of Na 3 PO 4 (Table 1). The PEO was carried out by applying a rectangular current wave of 6 A m 2 at a frequency of 1 Hz for 9 s using a Takasago, AA/X2 power supply. During the, voltage transient was monitored and recorded on PC. In addition, photo-intensity of sparking was recorded by Advantest TQ8215 optical power meter with a TQ8214 silicon photo-diode sensor (detectable wavelength: 4 11 nm). The thickness of the was measured by eddy current thickness meter (Kett, LH-3C). Average thickness of the s was measured from ten readings on a surface. The phases in the s were identified by grazing incidence X-ray diffraction (GIXRD) using Cu K radiation at an incident angle of 2 (Rigaku RINT2 system). Table 1 Composition and conductivity of the electrolytes used. Composition of electrolyte (moldm 3 ) K 2 Al 2 O 4 NaOH Na 3 PO 4 Conductivity (S m 1 ) Surfaces and cross-sections of the coated specimens were observed by JEOL JSM-63 LVS scanning electron microscope, equipped with EDX facilities. The protective nature of the s against corrosion was evaluated by potentiodynamic polarization in.5 moldm 3 NaCl aqueous solution at 293 K at a sweep rate of 1 mv s Results and Discussion 3.1 Formation of PEO s Figure 1 shows the changes in the anodic peak voltage (Figs. 1(a), 1(c)) and the intensity of photo emission (Figs. 1(b), 1(d)) with anodizing time. Figures 1(a) and 1(b) show the changes within the initial 1 s. In all the electrolytes the voltage increases with anodizing time, but the rise of voltage becomes slow with time of anodizing. The initial voltage rise is fastest in the electrolyte containing.1 moldm 3 Na 3 PO 4, while similar voltage transient is observed in the other three electrolytes up to 4 s. Associated with the faster voltage rise, sparking commences at anodizing for 1 s, at which the voltage reaches 15 V in the electrolyte containing.1 moldm 3 Na 3 PO 4. The photointensity in this electrolyte is always markedly higher than those in the other three electrolytes during anodizing for 9 s. These results suggest the formation of the with higher barrier nature in the electrolyte containing.1 moldm 3 Na 3 PO 4. In the latter three electrolytes the sparking also commences when the voltage reaches 15 V, indicating that the voltage of dielectric breakdown does not change with phosphate concentration in electrolyte. It is (a) Peak Voltage, V p / V (b) / arb. unit ph Photo-intensity, I mol dm -3 mol dm.75 mol dm -3 mol dm Anodizing Time / s 1 (c) p / V Peak Voltage, V (d) / arb. unit ph Photo-intensity, I -3 mol dm.75 mol dm mol dm -3 mol dm Anodizing Time / s 8 Fig. 1 Changes in (a,c) peak voltage and (b,d) photo-intensity during anodizing of AZ8 magnesium alloy at a rectangular AC current of 6 A m 2 (1 Hz) in.15 moldm 3 K 2 Al 2 O moldm 3 KOH electrolytes with different concentrations of Na 3 PO 4.

3 96 S. P. Sah, Y. Aoki and H. Habazaki / µm Coating Thickness, t Phosphate Concentration, C P / mol dm -3 Fig. 2 Change in the thickness of the PEO s with concentration of phosphate. The PEO s were formed by anodizing of AZ8 magnesium alloy at a rectangular AC current of 6 A m 2 (1 Hz) in.15 moldm 3 K 2 Al 2 O moldm 3 KOH electrolytes with different concentrations of Na 3 PO 4 for 9 s. known that the voltage of dielectric breakdown changes linearly with logarithm of the conductivity of electrolyte. Thus, similar breakdown voltage should be associated with the fact that the conductivity of the electrolyte does not change largely with phosphate concentration (Table 1). The photo-intensity increases with anodizing time up to 23 s in the electrolytes containing.5 and.75 moldm 3 Na 3 PO 4, while the increase in the photo-intensity is further delayed in the phosphate-free electrolyte. However, the final photo-intensity at 9 s is similar in the electrolytes except that containing.1 moldm 3 Na 3 PO 4. Despite the high photo-intensity in the electrolyte containing.1 moldm 3 Na 3 PO 4 throughout the anodizing time, the anodic peak voltage is lowest between 4 s and 6 s. In this period, the voltage decreases with increasing phosphate concentration, except for the phosphate-free electrolyte. At 9 s the anodic peak voltage becomes 4 V in all the electrolytes. Thus, there is no clear correlation between the photo-intensity and anodic peak voltage. The s formed by anodizing for 9 s thicken with an increase in phosphate concentration (Fig. 2). The formed in the phosphate-free electrolyte is only 6 mm thick, while the formed in the electrolyte containing.1 moldm 3 Na 3 PO 4 becomes as thick as 7 mm. The formation of the thick in the latter electrolyte should be associated with the intense sparking, since the sparking induces development of film materials in PEO process. 32,33) 3.2 Phases in the PEO s The PEO developed in each electrolyte is highly crystalline, as is clearly seen in the XRD patterns shown in Fig. 3. The detected phases are MgAl 2 O 4 and MgO, with the former being a major component. Weak diffraction peaks of Mg 3 (PO 4 ) 2 are also detected in the formed in the electrolyte containing.1 moldm 3 Na 3 PO 4. In this specimen, no peaks are detected in the XRD pattern, in agreement with formation of the thick. Aluminum to Diffraction Intensity, I / arb. unit 2 MgAl 2 O 4 Mg MgO Mg 3 (PO 4 ) 2.75 mol dm -3 mol dm θ / degree (Cu Kα) 8 Fig. 3 GIXRD patterns of the PEO s formed by anodizing of AZ8 magnesium alloy at a rectangular AC current of 6 A m 2 (1 Hz) in.15 moldm 3 K 2 Al 2 O moldm 3 KOH electrolytes with different concentrations of Na 3 PO 4 for 9 s. form MgAl 2 O 4 should be derived mainly from electrolyte, since MgO is a major phase in the s formed on AZ91 and AM6 magnesium alloys in aluminate-free alkaline solutions. 13,23,34) 3.3 Morphology and composition of the PEO s The surface morphologies of the s change significantly with the concentration of phosphate (Fig. 4). In the low magnification images, the formation of relatively uniform s is evident in the electrolytes free from phosphate (Fig. 4(a)) and containing.5 moldm 3 Na 3 PO 4 (Fig. 4(b)). In contrast, the s are highly cracked when they are formed in the electrolytes containing higher concentrations of phosphate (.75 and.1 moldm 3 Na 3 PO 4 ) (Figs. 4(c), 4(d)). The cracks are developed even if the thickness was reduced to 1 mm by shorter anodizing time. A characteristic feature of PEO s that is the formation of many discharge pores is disclosed in the high magnification images. The pore sizes are in the range of.8 2. mm in the electrolytes containing.5 moldm 3 Na 3 PO 4, while the pore size as large as 7 mm is developed in the electrolytes containing.75 and.1 moldm 3 Na 3 PO 4. In the latter two electrolytes the pores of.2 2. mm are also present. The large discharge pores should be developed as a consequence of intense sparking. The intense sparking in the electrolyte containing.1 moldm 3 Na 3 PO 4 induces the development of thick and formation of large discharge pores. Cross-sectional observations (Fig. 5) reveal the thickening of the s with phosphate concentration. In the

4 Influence of Phosphate Concentration on Plasma Electrolytic Oxidation of AZ8 Magnesium Alloy in Alkaline Aluminate Solution 97 (a) (b) 5 µm 5 µm (c) (d) 5 µm 5 µm Fig. 4 Scanning electron micrographs of surfaces of the PEO s formed by anodizing of AZ8 magnesium alloy at a rectangular AC current of 6 A m 2 (1 Hz) in.15 moldm 3 K 2 Al 2 O moldm 3 KOH electrolytes with (a) moldm 3, (b).5 moldm 3, (c).75 moldm 3 and (d).1 moldm 3 Na 3 PO 4 for 9 s. developed in the phosphate-free electrolyte, high density of pores is present in the, but thickness is relatively uniform. The thickness becomes approximately twice when.5 moldm 3 Na 3 PO 4 is added in the electrolyte. This is also highly porous, but the porosity appears to be slightly lower than that formed in the phosphate-free electrolyte. The thickness is also relatively uniform. In contrast, the s do not cover uniformly the when the electrolytes containing.75 and.1 moldm 3 Na 3 PO 4 are used (Figs. 5(c), 5(d)). There are several regions where no material covers the, probably associated mainly with cracking in the s. The compositions of the s have been examined by EDS elemental mapping of the cross-sections (Figs. 6 8). The s consist of two layers. The major part of the s is composed of an aluminum-rich oxide layer, in which MgAl 2 O 4 is a main phase. A thin inner layer, less than 1 mm thick, developed in all the electrolytes, is deficient of aluminum. Thus, MgO may be a main component in this inner layer. From the comparison of the magnesium and aluminum maps in each, it is obvious that the composition is not uniform in the outer main layer of the s. Phosphorus species are incorporated into the formed in the phosphate-containing electrolytes, and they are enriched approximately in the inner half of the s. EDS point analysis revealed that phosphorus was contained in the inner magnesium-rich layer, and the content increased from 3 at% in.5 moldm 3 Na 3 PO 4 to 14 at% in.1 moldm 3 Na 3 PO 4. Aluminum (1 at%) was also contained in the inner layer. Compared with the inner layer, the phosphorus content in the outer layer was low. The phosphorus content in the outer layer was as low as 3 at% even when the was formed in the electrolyte containing.1 moldm 3 Na 3 PO 4. The aluminum content in the outer main layer of the s (23 32 at%) is comparable or even higher than the magnesium content in this layer (18 32 at%). 3.4 Corrosion protection of the PEO s Corrosion protection of the PEO s, examined by anodic potentiodynamic polarization in.5 moldm 3 NaCl solution (Fig. 1), reflects the morphology of the s shown in Figs. 4 and 5. The lowest anodic current density is obtained for the formed in the electrolyte containing.5 moldm 3 Na 3 PO 4. This coated specimen shows the anodic current density of 1 ma m 2 or less up to 1:4 V vs Ag/AgCl. Then, the current density increases an order of magnitude at about 1:37 V vs Ag/AgCl, probably local breakdown of the through the pores. The specimens with highly cracked s formed in the electrolytes containing higher phosphate concentrations revealed higher anodic current density. Further higher anodic current density is obtained for the specimen coated in the phosphate-free electrolyte, due to the thin with high pore density.

5 98 S. P. Sah, Y. Aoki and H. Habazaki (a) (c) (b) (d) Fig. 5 Scanning electron micrographs of cross-sections of the PEO s formed by anodizing of AZ8 magnesium alloy at a rectangular AC current of 6 A m 2 (1 Hz) in.15 mol dm 3 K2 Al2 O4 +.1 mol dm 3 KOH electrolytes with (a) mol dm 3, (b).5 mol dm 3, (c).75 mol dm 3 and (d).1 mol dm 3 Na3 PO4 for 9 s. SEI Mg Al O Fig. 6 Scanning electron micrograph and X-ray images of magnesium, aluminum and oxygen of a cross-section of the PEO formed by anodizing of AZ8 magnesium alloy at a rectangular AC current of 6 A m 2 (1 Hz) in.15 mol dm 3 K2 Al2 O4 +.1 mol dm 3 KOH electrolyte for 9 s.

6 Influence of Phosphate Concentration on Plasma Electrolytic Oxidation of AZ8 Magnesium Alloy in Alkaline Aluminate Solution SEI 99 Mg 5 µm Al O P Fig. 7 Scanning electron micrograph and X-ray images of magnesium, aluminum, oxygen and phosphorus of a cross-section of the PEO formed by anodizing of AZ8 magnesium alloy at a rectangular AC current of 6 A m 2 (1 Hz) in.15 mol dm 3 K2 Al2 O4 +.1 mol dm 3 KOH electrolyte containing.5 mol dm 3 Na3 PO4 for 9 s. 4. Discussion Findings disclose the significant influence of phosphate concentration on thickness and morphology of the PEO s formed in alkaline aluminate electrolyte. The thickens with phosphate concentration. Since the electric charge passed during is the same in all the electrolytes, the increase in phosphate concentration enhances the efficiency of formation. Magnesium phosphate is insoluble in alkaline solution, promoting the formation. In addition, intense sparking in the electrolyte containing.1 mol dm 3 induces thick, since the development of material in PEO process is associated with sparking.6,32,33) The enhancement of occurrence of sparking by addition of phosphate was also reported recently.35) The peak voltage increases more rapidly in the electrolyte containing.1 mol dm 3 Na3 PO4 during initial anodizing (Fig. 1(a)), indicating the increased field strength in the barrier layer that supports the electric field and/or suppressed dissolution magnesium species. In general, phosphate incorporation into anodic oxide increases the field strength in growing barrier-type anodic oxide films. It is likely that the field strength in the inner barrier layer of the present oxide films is increased in the electrolytes with higher phosphate concentrations, leading to the more intense sparking. A main part of the developed contains high concentration of aluminum species, as shown in Figs From XRD patterns (Fig. 3), aluminum should be present in MgAl2 O4 spinel phase. It is known that aluminum species are incorporated into the anodic films formed on magnesium and its alloys in aluminate electrolytes.27,28) Verdier et al. formed PEO s on AM6 magnesium alloy in mixed solutions of KOH, KF, Na2 HPO4 and different concentrations of

7 1 S. P. Sah, Y. Aoki and H. Habazaki SEI Mg 3 µm Al O P Fig. 8 Scanning electron micrograph and X-ray images of magnesium, aluminum, oxygen and phosphorus of a cross-section of the PEO formed by anodizing of AZ8 magnesium alloy at a rectangular AC current of 6 A m 2 (1 Hz) in.15 mol dm 3 K2 Al2 O4 +.1 mol dm 3 KOH electrolyte containing.1 mol dm 3 Na3 PO4 for 9 s. aluminate.23) XRD patterns revealed that no MgAl2 O4 peaks are detected in the formed in the aluminate-free electrolyte. The content of MgAl2 O4 increased with the concentration of aluminate. In this context, aluminum species should be incorporated mainly from electrolyte into the present s, although aluminum derived from may also contribute to the formation of MgAl2 O4. Phosphate ions are also incorporated into the s, particularly in the inner part of the s. Their content in increases with the concentration of phosphate in electrolyte. Higher concentration of phosphorus species in the inner part of PEO s was also found when Ti-15V-3Al-3Cr-3Sn alloy was anodized in the similar electrolyte.36) The incorporation of phosphate in anodic oxide films on magnesium is known,37 41) but the enrichment in the inner part of the present is the subject of further study. Beneath the main layer containing the high concentration of aluminum, a thin magnesium-rich layer, less than 1 mm thick, is developed (Figs. 6 8). The layer is compact and apparently pore-free. Since the aluminum content in this layer is relatively low, this layer seems to be formed without influence of dielectric breakdown of. Thus, the development of the outer main layer is associated with dielectric breakdown; new material is developed mainly near the /metal interface with electrolyte penetrating through a short-circuit path.32,42,43) The inner thin layer may be formed due to usual ionic transport in anodic oxide under the high electric field. This is

8 Influence of Phosphate Concentration on Plasma Electrolytic Oxidation of AZ8 Magnesium Alloy in Alkaline Aluminate Solution 11-2 Current Density, i / A m consistent with the estimation that an anodic film of 6 nm thickness is formed, since the formation ratio of anodic films on magnesium alloys is nm V 1. 44) In chloride-containing solutions, the formed in the electrolyte containing.5 moldm 3 phosphate showed the highest corrosion protection. This fact is in agreement with the morphology of the s (Figs. 4 and 5). The s formed in the electrolyte containing higher phosphate concentrations contain high density of cracks, such that corrosive solution penetrates readily into the surface through the cracks. The s formed in the electrolytes free from phosphate and containing.5 moldm 3 phosphate are crack-free or crack-less and the surface is covered entirely with the s of uniform thickness. However, the former is as high as 5 mm and has relatively high density of pores, showing relatively poor corrosion protectiveness. Thus, the relatively uniform of 15 mm thickness, formed in the electrolyte containing.5 moldm 3 phosphate reveals the highest protective properties. However, this also contains high porosity, such that long-term stability for corrosion was not sufficiently high. Thus, further study is necessary to reduce the porosity of the PEO. Recently, it has been reported that highly dense PEO s can be developed by adjusting current wave forms in PEO of aluminum alloys. 45) The importance of negative charge density has been pointed out. Further modification of anodizing parameters in a suitable composition of electrolyte in PEO of magnesium alloys may lead to the development of s with long-term stability against corrosion. 5. Conclusions Substrate.75 mol dm -3 mol dm Potential, E / V vs Ag/AgCl Fig. 9 Anodic polarization curves of the as-polished and PEO-coated AZ8 magnesium alloy specimens in.5 moldm 3 NaCl solution. The PEO s were formed by anodizing of AZ8 magnesium alloy at a rectangular AC current of 6 A m 2 (1 Hz) in.15 moldm 3 K 2 Al 2 O moldm 3 KOH electrolytes containing,.5,.75 and.1 moldm 3 Na 3 PO 4. (1) Plasma electrolytic oxidation of AZ8 magnesium alloy in alkaline aluminate electrolytes containing different concentrations of phosphate produces two-layered s, comprising an outer main layer with relatively high content of aluminum, and a thin inner magnesiumrich layer. MgAl 2 O 4 is a major phase in the outer layer. Phosphorus species are present in the, particularly in the inner part of the. (2) The thickness of the s increases with the concentration of phosphate in electrolyte. The formed in the electrolyte containing.1 moldm 3 Na 3 PO 4 is as thick as 7 mm. 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