Formation of micro-arc oxidation coatings on AZ91HP magnesium alloys

Corrosion Science 51 (2009) 2820–2825

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Formation of micro-arc oxidation coatings on AZ91HP magnesium alloys R.F. Zhang *, S.F. Zhang Jiangxi Key Laboratory of Surface Engineering, Jiangxi Science and Technology Normal University, Nanchang, Jiangxi 330013, China School of Material Science and Engineering, Jiangxi Science and Technology Normal University, Nanchang 330013, China

a r t i c l e

i n f o

Article history: Received 26 April 2009 Accepted 1 August 2009 Available online 11 August 2009 Keywords: A. Magnesium B. SEM C. Anodic films C. Oxide coatings

a b s t r a c t In a neutral solution, coating formation by micro-arc oxidation (MAO) on AZ91HP magnesium alloy was studied. The process involved the substrate dissolution and coating development. During the first 5 s, the sample mass decreased, indicating substrate dissolution dominating the process. After 5 s, the sample mass began to increase and the coating development began to predominate the process. The coating was firstly nucleated on a phase and sparks initially appeared mainly on the edges of the sample. With treating proceeded, the coating was becoming uniform; meanwhile, microscopic pores of anodic coatings increased in size and decreased in number. Ó 2009 Elsevier Ltd. All rights reserved.

1. Introduction Magnesium and its alloys, an important class of engineering material with the properties of low density, high strength-toweight ratio and excellent dimensional stability, are commonly utilized in aeronautical, automotive, military and electronic industries where weight reduction is critical [1–3]. However, their applications have been restricted due to the poor corrosion and wear resistance. Therefore, many surface modification technologies, such as electrochemical plating, conversion coatings, micro-arc oxidation (MAO), gas-phase deposition processes, laser surface alloying/cladding and organic coatings, have been applied to improve the surface properties [3]. Among these technologies, MAO, recently developed under the traditional anodization, is one of the most widely used processes for magnesium alloys because it is less sensitive to the alloy type and the coatings have better paint-adhesion characteristics, excellent wear and abrasion resistance [3]. The coating properties obtained by MAO depend on many factors, such as the concentrations and compositions of the electrolytes [4,5], the alloy compositions [6], the microstructure of the substrate metal [7], the input electrical parameters [8,9]. The alloying elements and the microstructure of the substrate metal play important roles in determining the corrosion resistance and formation of anodic coatings. Shi et al. [6] reported that increasing zinc content led to increased corrosion resistance of an anodized single-phase Mg–Zn alloy, while Al played different

* Corresponding author. Address: Box 124, Jiangxi Key Laboratory of Surface Engineering, Jiangxi Science and Technology Normal University, Nanchang 330013, Jiangxi Province, China. Tel./fax: +86 791 3831266. E-mail address: [email protected] (R.F. Zhang). 0010-938X/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.corsci.2009.08.009

roles depending on the alloy purity. Anodic coatings can be evenly developed on the metal surface of single-phase Mg–Al alloys, but different results have been obtained for two-phase Mg–Al alloys. Khaselev et al. [10] found that coatings were successively formed, i.e., first on the primary a phase and then on b phase, while Verdier et al. [11] reported no evidence of successive anodizing of the primary solid solution and then the eutectic mixture on AM60 alloy despite the two-phase nature. In order to further understand the coating formation on the two-phase magnesium alloy, the mass variation with time and the morphology of the anodic coatings after different intervals were studied. In addition, the process of spark development after different intervals was addressed. 2. Experimental An ingot of AZ91HP magnesium alloy was used and the chemical composition is as follows (in wt.%): Al 8.93, Zn 0.47, Mn 0.22, Si 0.03, Cu 0.002, Ni 0.001, Fe 0.001, and Mg balance. The microstructure of AZ91HP was revealed after it was ground successively by using SiC paper up to a 1000 grit finish, polished by 1 lm diamond paste, washed with distilled water, degreased by acetone, chemically etched in a solution of 2% HNO3 alcohol and dried in a cool air stream. The alloys consist of two phases as shown in Fig. 1. The a phase (the dark areas) is a solid solution of aluminum in magnesium and the b phase (the bright areas) consists of intermetallic precipitates, Mg17Al12, along the a grain boundaries. Samples for MAO treatment were cut into 5 cm  7 cm  1 cm and were anodized after they were polished, washed, degreased and dried. For the samples treated for observing coating formation, they were pretreated as stated above but an additional step of chemical etching in the solution of 2% HNO3 alcohol followed by

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the area was defined as the mass gain at this time. The curve of mass gain with time was obtained by treating the samples for different intervals. During MAO treatment, the spark pictures formed on the under parts of the samples were taken by a camera in order to investigate the change regularity of sparks with treatment time. Coating thickness was measured by a 6000-FN1 eddy current instrument, which was calibrated with the same base metal. Measurements were conducted at 10 randomly selected positions on the sample surface and the readings were averaged to yield the coating thickness value. The morphologies of the treated samples were observed by an XL30FEG environmental scanning electron microscope (ESEM) after they were washed, dried and coated with gold. The compositions of anodic films were determined by EDX analysis in the ESEM. Fig. 1. The microstructure of AZ91HP.

3. Results 3.1. Evaluation of voltage and coating thickness with time

400

80 70

300

60

250

50

200

40

150

voltage thickness

100 50

30 20

Thickness(µm)

Voltage(V)

350

10

0

0 0

200

400

600

800

1000

1200

Time(s) Fig. 2. Curves of voltage and coating thickness with time.

degreasing was also used. The equipment for MAO was a WSK-020 power supply, which can provide constant or pulse current. Treatment was performed under a constant current control mode and the current density was maintained at 30 mA/cm2 after 2 s of ramping time. The samples and the wall of a stainless steel barrel were separately used as the anode and the cathode. The solution consists of 5 g/L phytic acid solution (C6H18O24P6, purity > 70.0%), 20 g/L hydrofluoric acid (40%), 58 g/L phosphoric acid (98%), 35 g/ L boric acid, 360 g/L hexamethylenetetramine and the bath pH was adjusted to 7.0 by adding ammonia. The bath temperature was maintained at 20–40 °C by a stirring and cooling system. Before MAO, the samples for measuring mass gain were weighed by BP211D electronic scales (precision 0.01 mg, Sartorius, Germany). Then, it was treated in the solution. The current was turned off immediately after the preset period was reached. The samples were weighed again after they were taken out of the solution, washed with distilled water and dried in a cool air stream. The mass difference of the samples before and after MAO divided by

Generally, anodizing could be classified into three stages and the corresponding phenomena during each stage were described comprehensively elsewhere [11]. Curves of voltage and coating thickness with treatment time are shown in Fig. 2 and spark pictures taken at different times are shown in Fig. 3. During the first 20 s, namely, in Stage I, the voltage increased linearly with a slope of 5 V/s (Fig. 2). Only intensive oxygen evolution could be seen on the sample surface, corresponding to the traditional anodizing stage. After 20 s, Stage II was started, corresponding to the working voltage at 106 V. Small, dense sparks were seen as shown in Fig. 3a. This stage was called micro-arc oxidation and the curve slope began to decrease. After 5 min, the working voltage reaching 220 V, large, white sparks could be observed (Fig. 3b), indicating the starting of Stage III, arc anodizing. After 10 min, the working voltage reached 284 V and the increase rate of working voltage became very slow, with a value of about 0.11 V/s. From Fig. 2, the coating thickness increased in the whole treating stage and reached 2, 18, 37 and 76 lm after 20 s, 5, 10 and 20 min, respectively. As shown in Fig. 3a and b, sparks initially appeared mainly on the edges of the samples both in Stage II and Stage III. With treatment time increasing, sparks were also improved in the uniformity and evenly covered the entire surface after 10 min as shown in Fig. 3c. In addition, sparks became larger and their lifetime increased. At 20 s, the diameter of the largest spark was less than 1 mm; it then increased to 1 and 3 mm at 5 and 10 min, respectively. Fig. 3a and c show that the color of sparks changed from yellow at 20 s to bluish-white1 at 10 min. 3.2. Sample mass gain The curve of mass gain with time for the treated sample is shown in Fig. 4. The mass of the sample slightly decreased to 0.54 g/m2 at 5 s and then increased to 1.4 g/m2 at 10 s. This implied that the dissolution was the main process during the first

Fig. 3. Appearance of sparks at different periods (a) 20 s, (b) 5 min and (c) 10 min.

R.F. Zhang, S.F. Zhang / Corrosion Science 51 (2009) 2820–2825

2

Mass gain(g/m )

2822

160

3.3. Changes of coating morphologies

140

Fig. 5 shows the surface morphologies of the two-phase AZ91HP alloy at 5, 10, 20 and 30 s, 1, 5, 10 and 20 min. The coating compositions at different points and areas selected on the sample surface after different durations are listed in Table 1. These elements, O, F, Mg, Al and P were derived from the alloys or the electrolytes. However, boron was not listed because it is a light element and cannot be detected by EDX. It can be seen from Fig. 5a that at the first 5 s, only some small, white points were observed dispersing mainly on the a phase and the anodic coatings were almost not formed on the base metal. EDX analysis showed that there was a trace amount of F and P (less than 1%) on both b phase (Point A) and a phase (Point B), but Al content of the former was much higher than that of the latter. At 10 s, the sample surface exhibited island-like topography and the coating began to be formed. The coating was preferentially formed mainly on the a phase adjacent to the b phase edges as shown by an arrow in Fig. 5b, and then spread to the a phase far away from the b phase, which was contrary to the electroless plating on magnesium [12,13]. It was consistent with what Khaselev

120 100 80 60 40 20 0 10

100

1000

Time(s) Fig. 4. The curve of mass gain with time for the treated sample.

5 s. After 5 s, the coating formation became the main process and the mass gain of the sample continually increased till the end of the experiment.

Fig. 5. The microstructure of anodic coatings obtained after different intervals: (a) 5 s, (b) 10 s, (c) 20 s, (d) 30 s, (e) 1 min, (f) 5 min, (g) 10 min and (h) 20 min.

R.F. Zhang, S.F. Zhang / Corrosion Science 51 (2009) 2820–2825 Table 1 Compositions of anodic coatings after different intervals. Anodizing time

5s 10 s 20 s 30 s

A B C D E F G H

1 min 5 min 10 min 20 min

Element content (wt.%) O

F

Mg

Al

P

1.7 1.8 13.2 17.5 32.0 16.4 32.3 30.2 29.2 28.4 31.0 31.2

0.4 0.8 6.2 5.2 0.9 0.7 0.7 0.7 0.7 0.5 0.8 1.2

54.4 83.3 39.5 62.9 18.6 51.4 26.7 35.1 30.1 31.5 32.2 32.0

42.6 13.2 38.1 10.1 13.6 7.2 0.6 0.6 1.2 2.1 2.6 3.4

0.9 0.9 3.0 4.3 34.9 24.3 39.7 33.4 38.8 37.5 33.4 32.2

et al. reported, the growth of anodic coatings started on a phase and continued on b phase (Mg17Al12) [10]. In addition, chemical compositions on the b phase (Point C) and the a phase (Point D) were different. Compared with Point C, Point D contained lower F and higher P (Table 1). At 20 s, the sample surface was fully covered with anodic films with low porosity, but the film morphology was heterogeneous with some regions light and others dark (shown in Fig. 5c), which might be attributed to uneven coating formation [7,10,11,14,15]. Light regions, such as Point E, contained 13.6 wt.% Al, which was much higher than dark areas (such as Point F, only 7.2 wt.% Al). Hence, the light regions were associated with b phase underneath the formed coating [7]. On this condition, the coating was thin (2 lm) and therefore the composition detected by EDX was the total amount of both the coating and the base metal. In addition, the P content at Point E (34.9%) was much higher than that at Point F (24.3%), which further indicated that anodic coatings were not evenly formed on the sample surface. After treating for 30 s, there were still light and dark areas on the sample surface (Fig. 5d). EDX analysis showed that the Al content of the anodic coatings on the light areas such as Point G was similar to that on the dark areas such as Point H, about 1 wt.%. Therefore, it was uncertain whether the light areas were formed on the b phase of the substrate. In addition, the P content of anodic coatings at Point G (39.7%) was higher than that at Point H (33.4%), which indicated that the coatings were not uniformly formed. After 1 min, there were still light and dark areas but the coating uniformity evidently increased, as shown in Fig. 5e. The chemical compositions at different points on the coating surface were almost the same (listed in Table 1), which showed that at the moment, the coatings were uniformly formed. With the treatment time prolonged, the pore size increased. For example, the largest pore of the anodic coatings obtained after 5 min was 9 lm in diameter (Fig. 5f), larger than that achieved after 1 min, which was about 5 lm (Fig. 5e). Fig. 5g shows that after 10 min, the surface was rather rough. Particles and crater-like pores were formed on the specimen surface. The contours of crater-like pores (such as Nos. 2 and 3) could be clearly observed (shown by circles) although these pores had been partly filled. At this time, the diameter of particles was about 20 lm. The particles became larger with anodizing time and reached 40 lm after 20 min (Fig. 5h).

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appear when the working voltage is higher than breakdown or sparking voltage (Ub), which depends on the base metal [16,17], the electrolyte composition [17–19] and the electrolyte resistivity (q) [20,21]. Other factors such as current density [18,21], the temperature [20], the rate of electrolyte stirring [17], the history of the anodic film formation [22] do not affect Ub noticeably. Recently, Verdier et al. [11] found that the breakdown voltage was related to current density and decreased with increasing current density. In the experiment, sparks both in Stage II and Stage III initially appeared mainly on the edges of the samples, which may be attributed to edge effects. The current density on the edges was larger than that at other regions, which resulted in lower breakdown voltage on these areas according to the findings of Verdier et al. [11]. Therefore, sparks initially appeared mainly on the borders of the samples. 4.2. Changes of sample mass Fig. 4 shows that the mass of the anodized sample decreased during the first 5 s and then continually increased. Several processes may accompany MAO in water solution: both oxygen evolution and MAO of magnesium alloy may take place on the anode surface. For example, the following general reactions normally occur:

Mg  2e ! Mg2þ ðanodic dissolutionÞ

ð1Þ

4OH  4e ! 2H2 O þ 2O ðO2 Þ

ð2Þ

or 2H2 O  4e ! O2 þ 4Hþ

ð3Þ

Mg2þ þ 2OH ! MgðOHÞ2 ðCoating formationÞ

ð4Þ

2þ

3Mg

2þ

Mg

þ

2PO3 4 

þ 2F

! Mg3 ðPO4 Þ2

! MgF2

ð5Þ ð6Þ

MgðOHÞ2 ! MgO þ H2 O

ð7Þ

þ 2PO3 ! P2 O4 4 þ 2H 7 þ H2 O

ð8Þ

4. Discussion

In the first several seconds, anodic coatings were not formed and only a trace amount of F and P were detected on the sample surface. In this period, Eq. (1) took place and magnesium ions entered the solution [23,24], which resulted in the base metal losing its brightness. In addition, oxygen evolution could take place either from Eq. (2) or (3) [25]. In this stage, anodic dissolution and oxygen evolution dominated the process and the mass of the sample slightly decreased. Only after 5 s, did the concentration of magnesium ions at the metal/electrolyte interface become high enough to combine with anions such as OH, PO43 or F. The concentration of OH ions in neutral solutions was uniform and low. However, during the process of MAO, anions such as OH, PO43 or F in the bulk solution moved to the anode due to electric migration. The local concentration of OH ions in water solution was no longer uniform and it became larger on the anode. Therefore, Eq. (4) could take place on the anode [24]. In addition, Mg3(PO4)2 and MgF2 may be formed in the coatings according to Eqs. (5) and (6). The instantaneous temperature in these micro-arc zones may be higher than 1000 °C resulting from spark appearance during MAO [26,27]. MgO and P2O74 in anodic coatings might result from Mg(OH)2 and PO43 dehydrating according to Eqs. (7) and (8) under the heating of the electrolyte near the electrode surface. The Al in anodic coatings may exist as MgAl2O4 or c-Al2O3 [4,10]. In this stage, the sample mass began to increase.

4.1. Sparks appearance

4.3. Formation process of anodic coatings

Some anodizing processes for magnesium alloys rely on sparking on the anode to develop anodic coating [11,14,15]. Sparks

Anodic coatings formed after different intervals shifted from the initial compact (Fig. 5b) to porous structures (Fig. 5g and h). In

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addition, the initially developed coating was heterogeneous, but its uniformity was improved with the treatment time prolonged, which was consistent with the results reported by others [10,24,28]. The changes of the coating morphology could be described as follows. At 10 s, a thin, compact anodic film began to develop heterogeneously, first on the a phase around the b phase and then extending to the a phase distant from the b phase; at the same time, the mass of sample began to increase (Fig. 4). The coating formation instead of metal dissolution dominated the anodizing process. The anodic coating extended to the b phase once the a phase was totally covered by the anodic films. In addition, the F content on both the a phase and the b phase was about 5%, higher than that at 5 s. Fluoride may take the form of MgF2 according to Eq. (6). However, as anodizing proceeded, the F content gradually decreased to about 0.7% at 30 s. This suggests that fluoride may play an important role and have a beneficial effect as inhibitor in the film formation because of the low solubility product of MgF2 [29]. After anodizing for 20 s, the working voltage reached 106 V and small, dense sparks appeared (Fig. 3a). The coating was then flat with small pores. The pore size became larger with continued anodizing until 10 min, which may be attributed to larger sparks resulting from the higher working voltages. At a high temperature in these micro-arc zones, anodic coatings may be temporarily melted and are erupted from the coating surface along small discharge channels [30,31]. The porous structure was formed once the melted coating is cooled down by the electrolyte. With anodizing proceeded, working voltage gradually increased and sparks became larger though their number decreased (Fig. 3c), which resulted in the increased surface area for a single spark and then a larger pore after eruption. After sparks spread over the surface of the specimen, a rapid growth of the anodic oxide occurs at the site of sparking [10,32] resulting in a non-uniform coating thickness [24]. The sparks always start at the thinner or weaker spots and move to other thin or weak spots in the coating [30]. In addition, with the treatment time prolonged, coatings were gradually thickened and developed in this manner along two directions, namely, growing outward as well as spreading laterally. After 10 min treatment, some particles and crater-like pores appeared and anodic coatings did not show a porous structure as before, which may result from too large sparks and too high temperature on the sample surface. After 10 min, the sparks on the sample surface became very large (Fig. 3c). Acted by both large sparks and a high temperature, the surface area of the locally melted coating resulting from a spark became very large. After an eruption stopped, a crater-like pore such as No. 1 was formed after it was rapidly quenched to the temperature of the electrolyte. The erupted material was in the molten state and entered into crater-like pores nearby such as Nos. 2 and 3 according to the arrows shown in Fig. 5g. In the meantime, when the power was turned off, the eruption resulting from a spark had no time to finish. Locally melted anodic coating was fast quenched by the electrolyte and large particles as shown in Fig. 5g and h were formed. AZ91HP is electrochemically heterogeneous owing to the unequal distribution of aluminum within the two constituent phases, namely a phase and b phase. First, chemical compositions of the a phase and the b phase are different. The b phase and the neighbouring areas (probably eutectic a phase) consisted of higher percentage of aluminum and zinc, and the concentration decreases as the area is away from the b phase. The concentration of aluminum typically varies between 35% at the b phase to 8–6% near or within the primary a phase [33]. In the viewpoint of thermodynamics, the Gibbs free energy per equivalent for formation of MgO is lower than that for formation of Al2O3 [34,35], so magnesium is firstly oxidized. However, Fig. 5b shows that the anodic coating

was initiated on the a phase adjacent to the b phase (with high aluminum), not on the a phase distant from the b phase (with low aluminum), which implied that the formation of the anodic coatings was not determined by the chemical compositions of a and b phases. Another aspect that distinguishes a phase and b phase is the difference of their corrosion potentials. Arrabal et al. [36] reported potential differences up to 160 mV between the b-phase and the surrounding a-eutectic, which also revealed a lower potential than the primary a-Mg, namely 85 mV. Despite the higher aluminum content, the a-eutectic is less noble than the a-Mg. As a whole, the treated sample serves as the anode where there exists micro-galvanic coupling due to electrochemical heterogeneity between a phase and b phase. Because the a phase, especially the a-eutectic, is anodic compared with the b phase, the former has higher reactivity and tends to release more Mg2+ to take part in the coating formation reactions. In addition, a-eutectic is adjacent to b phase and the special location may play an important role in coating formation. As Shi et al. [7] investigation, the microstructures of the coatings on a phase and b phase were different, especially along the boundaries of the two phases, which indicated that the boundaries might play a special role in coating formation. Compared to the a phase, the b phase is protuberant and there exists a terrace on the boundary between a and b phases (Fig. 5a), where the original anodic coatings are preferentially formed. 5. Conclusions The coating formation on AZ91HP magnesium alloy was studied in the solution containing phytic acid, hydrofluoric acid, phosphoric acid, boric acid and ammonia under a constant current control mode. Conclusions can be summarized as: (1) The MAO process was a competition between the base metal dissolution and the new coating development. During the first 5 s, the sample lost its mass and the process was dominated by the substrate dissolution. After 5 s, the sample mass began to increase and the process was dominated by the coating formation. (2) For the two-phase AZ91HP, the coating is originally formed mainly on the a phase adjacent to the b phase edges. After anodizing for 1 min, the coating was then evenly formed on the sample surface. With the treatment time prolonged, the coating was thickened and the pore size increased. In addition, the coating morphologies changed from compact to porous and finally to rough with some particles. (3) Sparks initially appeared mainly on the edges of the samples both in the micro-arc and arcing anodizing stages, which may be attributed to the higher level of the current densities along the edge. With anodizing proceeded, sparks uniformity was also improved and the coating evenly covered the entire surface after 10 min.

Acknowledgement The authors appreciate the financial support from Scientific Research Fund of Jiangxi Provincial Education Department (GJJ08363). References [1] [2] [3] [4]

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