Effect of alternating voltage treatment on the corrosion resistance of pure magnesium

Corrosion Science 51 (2009) 1772–1779

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Effect of alternating voltage treatment on the corrosion resistance of pure magnesium Xiaolan Liu a, Tao Zhang a,b,*, Yawei Shao a,b, Guozhe Meng a,b, Fuhui Wang a,b a

Corrosion and Protection Laboratory, Key Laboratory of Superlight Materials and Surface Technology (Harbin Engineering University), Ministry of Education, Nantong ST 145, Harbin 150001, China b State Key Laboratory for Corrosion and Protection, Institute of Metal Research, Chinese Academy of Sciences, Wencui RD 62, Shenyang 110016, China

a r t i c l e

i n f o

Article history: Received 16 January 2009 Accepted 2 May 2009 Available online 15 May 2009 Keywords: A. Magnesium B. Polarization B. EIS B. XPS C. Pitting corrosion

a b s t r a c t Pure magnesium was treated by alternating voltage (AV) treatment technique. The optimal AV-treatment parameters for greatly improving corrosion resistance were determined by the orthogonal experiments. Polarization curves, electrochemical impedance spectroscopy (EIS), and scanning electrochemical microscopy (SECM) were performed to understand the effect of AV-treatment on the corrosion resistance of pure magnesium. AFM, contact angle measurement and XPS were employed to further investigate the influence of AV-treatment on the properties of the surface film formed on pure magnesium after AVtreatment. The results showed that a uniform and stable film was formed and the corrosion resistance of pure magnesium was greatly improved after AV-treatment. This was caused by the noticeable change of chemical structure and semi conducting properties of surface film after AV-treatment. Ó 2009 Elsevier Ltd. All rights reserved.

1. Introduction Magnesium and its alloys has some advantageous properties that make it an excellent choice for a number of application including automobile and computer parts, aerospace components, mobile phones, sporting goods, handheld tools and household equipment. Unfortunately, magnesium has poor corrosion properties hindered its widespread use in many applications [1]. There are a number of technologies including electrochemical plating, conversion coatings, anodizing, hydride coatings, ion implantation, laser treatment and vapor-phase processes available for coating magnesium and its alloys [2–18], but the high costs and rather small treatment-areas limit their applications. The alternating voltage (AV) treatment is an environmentalfriendly surface technology. This method presents attractive for industrial applications because this process can be operated at a lower temperature and uses a bath without Cr6+ ions. Some investigations revealed that AV-treatment could greatly improve the corrosion resistance of stainless steels in some corrosive Medias [19–23]. For AV-treatment, a periodical potential square wave is applied during the surface film formation process. At upper potential, the weak site of passive film will be broken-down; at lower potential, the broken-down site will be repaired because of the * Corresponding author. Address: Corrosion and Protection Laboratory, Key Laboratory of Superlight Materials and Surface Technology (Harbin Engineering University), Ministry of Education, Nantong ST 145, Harbin 150001, China. Tel./fax: +86 451 8251 9190. E-mail address: [email protected] (T. Zhang). 0010-938X/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.corsci.2009.05.002

re-formation of passive film. By using AV-treatment technology, the formation process of passive film can be controlled, and a higher density and higher corrosion resistance passive film is obtained. However, all these works were limited to stainless steels. There have been no reports about improving the corrosion resistance of magnesium and its alloys by AV-treatment technique. Therefore, the effort to obtain a superior surface film on pure magnesium by AV-treatment is very worthful. The purpose of this work was to apply AV-treatment technique on pure magnesium and investigate the corrosion resistance, morphology and properties of the surface film formed on pure magnesium after AV-treatment. 2. Experimental procedures 2.1. Materials and samples The samples for this study were commercial pure magnesium. The chemical composition of the commercial pure magnesium was (in wt.%): 99.7 Mg, 0.1 Cu, 0.1 Fe and 0.1 Mn and Be. Samples sized 10  10  10 mm were mounted in epoxy resin with an exposed area of 1 cm2, ground to 2000 grit, degreased in acetone, rinsed with distilled water and dried in air at room temperature. 2.2. AV-treatment The AV-treatment was carried out in 0.25 M Na2SO4 + 0.1 M NaOH solution at a constant room temperature of 20 °C. Fig. 1,

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which showed an anodic polarization curve of pure magnesium in 0.25 M Na2SO4 + 0.1 M NaOH solution, gave a schematic illustration of experimental approach. Higher potential parts (Eh) and the lower potential parts (El) in square wave electric field used for AV-treatment fall into the upper parts of passive region and the lower parts of passive region/active dissolution region, respectively. To obtain the optimized parameters of AV-treatment technique, a three-factor at three-level orthogonal array experimental design Lg(33) was employed. Three factors are to be considered frequency (f), ratio of on-time to off-time (k) and the treatment time (t). The factors and the assignments of the corresponding levels were listed in Table 1. The results of orthogonal experiments were presented in Table 2. The pitting potential (Eb) of polarization curve was tested for evaluation of the results of orthogonal experiments. 2.3. Electrochemical measurements Electrochemical measurements were carried out to investigate the effect of AV-treatment on the corrosion resistance of pure magnesium. For all experiments a three-electrode cell was used, with a standard calomel electrode (SCE) being used as a reference electrode and a platinum counter electrode. All experiments were carried out at a constant room temperature of approximately 20 °C. Polarization curve experiments were carried out in the solution of 0.05 M NaCl with pH 12.0. The polarization curves were scanned with a scanning rate of 0.333 mV/s from open circuit potential (OCP) towards anodic direction. For the measurement of electrochemical impedance spectroscopy (EIS), the scanning frequency ranged from 100 to 10 mHz and the perturbing AC amplitude was of 5 mV. The above measurements were carried out on a Zahner IM6ex potentiostat in 0.05 M NaCl with pH 12.0. The samples were mounted into a scanning electrochemical microscope (SECM) (Uniscan Model SECM370, UK) equipped with a 10 lm platinum tip as the probe, standard calomel electrode (SCE) and a platinum counter electrode, all set up in a cell made of polytetrafluoroethene. Samples were mounted horizontally facing upwards. The samples were immersed in 0.05 M NaCl (pH 12) + 0.5 mM K3[Fe(CN)6] solution at room temperature. The sample was left unbiased at its open-circuit corrosion potential for all the work. The platinum tip scanned at a constant distance above the sample while experiments were performed. Selected a potential value of +0.40VSCE to follow the oxidation of ferrocyanide to ferricyanide ions as described by [24,25]: 3 FeðCNÞ4 6 ! FeðCNÞ6 þ e

0.00 -0.4

0.02

0.04

ð1Þ 0.06

0.08

0.10

0.12

0.14

E(VSCE)

Level

Frequency (Hz)

Ratio of on-time to off-time (%)

Treatment time (min)

1 2 3

10 20 40

20 40 70

15 30 40

If the tip is far from the substrate, the steady-state current, iT,1, for a disk-shaped tip, is given by:

iT;1 ¼ 4nFDA CA  a

ð2Þ

where n is the number of electrons exchanged in the redox reaction, F is the Faraday constant, DA and CA are the diffusion coefficient and the bulk concentration of the species A in the electrolyte solution, respectively, and a is the tip radius. When the tip is brought near a substrate, the tip current, iT, is perturbed by hindrance of diffusion of the species A to the tip from the bulk solution and by reactions that occur at the substrate surface. When the tip approaches a conductive sample, the Fe(CN)63 formed in reaction (1) could diffuse to the substrate and be reduced back to Fe(CN)64. When this process occurs, the flux of Fe(CN)64 to the tip is increased, and iT > iT,1. The dimension of SECM map is 1000  1000 lm. 2.4. AFM and contact angle The surface morphology of the sample after AV-treatment was observed by AFM. AFM images were obtained using a Digital Instruments NanoScopeIIIa system, operating in the tapping mode; imaging was carried out under ambient conditions. A silicon cantilever of spring constant of 0.3 N/m was used. Contact angle was measured by First Ten Angstroms (FTA) 200 instrument. A drop of distilled water was placed on the sample surface, and contact angle was determined using FTA V2.0 software package. 2.5. XPS analysis X-ray photoelectron spectroscopy (XPS) analysis was performed on a PHI 5700 ESCA spectrometer with an Al Ka (1486.6 eV) X-ray source operated at 13 kV and 300 W. The core-level spectra for films were obtained at take-off angles of 45°. Ion sputtering was performed at a pressure of 107 Pa using high-purity argon. Peak identification was performed by reference to an XPS database. The binding energy was corrected for charging effects by referencing to the Cls peak.

Eh

-0.6

Table 2 The Lg(33) matrix associated with the analytical results.

-0.8

Test no.

-1.0

a

b

-1.2

El -1.4 -1.6 -8 10

Table 1 Assignment of the levels of factors.

10

-7

10

-6

10

-5

10

-4

10

-3

2

i(A/cm ) Fig. 1. Schematic illustration of the AV-treatment (a) anodic polarization curve of pure magnesium in the AV-treatment solution (b) locations of the square wave in comparison with anodic polarization curve.

Eb (mVSCE)

1 2 3 4 5 6 7 8 9

1 1 1 2 2 2 3 3 3

1 2 3 1 2 3 1 2 3

1 2 3 2 3 1 3 2 1

K1 Ki K3

733.33 726.67 760

756.67 730 733.33

750 733.33 736.67

33.33

26.67

16.67

760 730 710 720 710 750 790 750 740

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3. Results and discussion

8.0x10

4

6.0x10

4

4.0x10

4

2.0x10

4

a

pure Mg imm-treatement AV-treatment simulation plots

According to the orthogonal design Lg(3 ), nine series of tests were performed. The pitting potential (Eb) of polarization curve results were listed in Table 2. In the orthogonal test, K was the mean value of the pitting potential (Eb) of each level. Differences between the maximum K value and minimum one in the same column, which was in accordance with the same factor, were calculated and shown in the last row of Table 2. Considering the differences between the maximum mean value and the minimum mean value of each factor, it was obvious that the frequency (f) was the major factor, while the ratio of on-time to off-time (k) and the treatment time (t) were minor factors. In order to obtain the maximum values of K, frequency (f), ratio of on-time to off-time (k) and treatment time (t) were determined as 20 Hz, 40% and 30 min, respectively.

2

3

Z″(ohm.cm )

3.1. Optimization of AV-treatment by orthogonal array design

0.0 0.0

2.0x10

4

4.0x10

4

6.0x10

4

8.0x10

4

2

Z′(ohm.cm ) 6

10

5

10 2

|Z|(ohm.cm )

60

3.2.1. Polarization curves Fig. 2 showed the anodic polarization curves of the untreated, immersion (imm) treatment and alternating voltage (AV) treatment samples, respectively. The immersion (imm) treatment was performed by immersing the sample in 0.25 M Na2SO4 + 0.1 M NaOH solution for 30 min. The polarization curve of the imm-treatment sample showed an active dissolution behavior, which was similar with that of the untreated sample. However, the polarization curve of the AV-treatment samples exhibited the passive behavior. The passive region of the AV-treatment sample was about 600 mV, meanwhile, the pitting potential of the sample was 0.864VSCE after the AV-treatment. Moreover, the corrosion potential of the AV-treatment sample was 1.507VSCE, which was slightly higher than that of the untreated (1.609VSCE) and immtreatment (1.592VSCE) samples. These results indicated that the corrosion resistance of sample was greatly improved after AVtreatment. 3.2.2. EIS EIS measurements had been performed to investigate the effect of AV-treatment on the corrosion behavior. The Nyquist and Bode plots of the untreated, imm-treatment and AV-treatment samples were showed in Fig. 3.

pure Mg imm-treatment AV-treatment

-0.75 -0.90

E(VSCE)

-1.05 -1.20 -1.35 -1.50 -1.65 10

-9

10

-8

10

-7

10

-6

10

-5

10

-4

10

-3

2

i(A/cm ) Fig. 2. Anodic polarization curves of the untreated, imm-treatment and AVtreatment samples.

4

10

40

3

20

10

Phase angle(deg)

3.2. Effect of AV-treatment on the corrosion resistance of pure magnesium

pure Mg imm-treatment 80 AV-treatment simulation plots

b

0

2

10

-1

10

0

10

1

10

2

10

3

10

4

10

5

10

Frequency(Hz) Fig. 3. Impedance spectra of the untreated, imm-treatment and AV-treatment samples (a) Nyquist plots and (b) Bode plots.

For the untreated sample, the EIS plots consisted of one capacitive loop in the high and one inductive loop in low frequency range. However, the EIS plots for the imm-treatment and AV-treatment samples consisted of two capacitive loops in the high and low frequency range. Those in the high frequency range were associated with the charge transfer at the interface, while those in the low frequency range are associated with the film formed on the alloy surface [26]. For the untreated sample, the equivalent circuit (showed in Fig. 4a) was consisted of one (R–CPE) and one (R–L) components in series with the solution resistance (Rs). Here, Rct is the charge transfer resistance paralleled with the double layer (CPEdl), the resistance of adsorbed intermediate products RL in the paralleled with L. For the imm-treatment and AV-treatment samples, the equivalent circuit (showed in Fig. 4b) was consisted of two (R–CPE) components, the pore resistance of the surface film (Rpore) in the paralleled with film capacitance (CPEfilm). Based on equivalent circuit model in Fig. 4, the EIS curves were best fitted and the fitting results were showed in Fig. 3 as red solid lines passing through the testing results. The corresponding values of the equivalent elements were summarized in Table 3. After imm-treatment and AV-treatment, the inductive loop in the low frequency region disappeared, which indicated that the initiation of pitting corrosion was inhibited. Meanwhile, the pore electrical resistance Rpore of the AV-treatment sample was 2.398  104 X cm2, being one order of magnitude higher than that of imm-treatment (2.285  103 X cm2). The significant increase of Rpore indicated that the porosity of surface film decreased, and a dense film formed on the pure magnesium surface after AV-treatment; moreover, the AV-treatment sample revealed higher charge transfer resistance

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Fig. 4. Equivalent circuit for EIS spectra (a) untreated (b) imm-treatment and AVtreatment.

Rct than the imm-treatment and untreated samples, namely 6.689  104, 1.415  104 and 1.071  104 X cm2, respectively. The higher charge transfer meant that AV-treatment could provide significant corrosion protection [27]. The EIS results indicated that the AV-treatment creates a superior surface film and significantly improves the corrosion resistance of pure magnesium. 3.2.3. SECM SECM maps for both the untreated, imm-treatment and AVtreatment samples over a period of 10 h were given in Figs. 5–7, respectively. The maps clearly depicted surface reactivity of samples as reflected by the hill- or valley-like current fluxion in the vertical Z direction due to the various response current depended on the local conductivity of the sample. For SECM map, higher current value corresponds to the higher anodic zone and lower current value represents the lower reactivity zone. It should be mentioned that the use of the electrolyte resistance to follow the pit evolution allows performing in situ experiment without the consumption of any electro active species. Therefore, it can be believed that pit initiation and growth imply significant changes in the local chemistry of the electrolyte leading to local conductivity increasing. In fact, local conductivity changes can be regarded in the present case as evidences of pitting development (conductivity increase) or passivation/repassivation (conductivity decrease) in the present case. For the untreated sample, it could be seen that pit corrosion existed at zones A and B after 3 h of immersion (Fig. 5a). After 6 h of immersion, new pit corrosion occurred at zones C and D (Fig. 5b). With the increasing of immersion time, the pit corrosion zones A–D still remained at a higher current value level (Fig. 5c), which implied that the untreated sample undertook serious pit corrosion. For the imm-treatment sample, it could be seen that pit corrosion existed at zone E after 6 h of immersion (Fig. 6b). After 10 h of immersion, new pit corrosion occurred at zone F (Fig. 6c), mean-

Fig. 5. SECM images for the untreated sample following immersion for different times (a) 3 h (b) 6 h and (c) 10 h in 0.05 M NaCl with pH 12.0.

Table 3 Fitting results of equivalent circuit elements. Rs (X cm2) Untreated Imm-treatment AV-treatment

60.54 61.8 64.04

CPEdl (F/cm2) 6

7.769  10 1.068  105 8.329  106

Rct (X cm2) 4

1.071  10 1.415  104 6.689  104

L (H/cm2) 1.205  10 – –

3

RL (X cm2)

CPEfilm (F/cm2)

Rpore (X cm2)

1597 – –

– 1.531  104 2.584  105

– 2.285  103 2.398  104

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Fig. 6. SECM images for the imm-treatment sample following immersion for different times (a) 3 h (b) 6 h and (c) 10 h in 0.05 M NaCl with pH 12.0.

Fig. 7. SECM images for the AV-treatment sample following immersion for different times (a) 3 h (b) 6 h and (c) 10 h in 0.05 M NaCl with pH 12.0.

resistance. These results agreed with the results of polarization curve and EIS. while, the pit corrosion zone E still remained at a higher current value level (Fig. 6c). The SECM maps implied that the imm-treatment sample undertook serious pit corrosion, but the degree of the corrosion was lower than that on pure magnesium. However, during the immersion period, the SECM map (Fig. 7a–c) of AV-treatment sample was homogeneous and featureless, while the tip current of AV-treatment was far lower than that of the untreated (Fig. 5a–c) and im-treatment (Fig. 6a–c) samples, which revealed that the AV-treatment exhibited higher pit corrosion

3.3. Effect of AV-treatment on the properties of surface film 3.3.1. Morphology The micro-morphology of surface film after imm-treatment and AV-treatment was observed by AFM and the results were showed by two different kinds of magnification in Fig. 8. The AFM image revealed that a uniform film formed on pure magnesium surface after AV-treatment.

X. Liu et al. / Corrosion Science 51 (2009) 1772–1779

3.3.2. Surface energy The contact angle of the untreated, imm-treatment and AV-treatment samples were illustrated by Fig. 9. The contact angle of AVtreatment sample (61.51°) was much higher than the untreated (19.74°) and imm-treatment (35.97°) samples. According to the contact angle data, the surface free energy was calculated and the results were summarized in Table 4. It was observed that the surface free energy decreased in the following order: untreated (68.52 J/cm2) > imm-treatment (58.92 J/cm2) > AV-treatment (34.73 J/cm2). This indicated that AV-treatment greatly decreased the surface energy. The lower surface free energy is, the more stability of surface film [28,29]. Therefore, the result indicated that the stability of the surface film formed on pure magnesium was greatly improved after AV-treatment. 3.3.3. Effect of AV-treatment on the chemical composition of surface film All of the above results demonstrated that the AV-treatment had a significant influence on the corrosion resistance of pure magnesium. In order to understand this effect, the chemical composition of the surface films was analyzed by means of XPS. The Auger parameter was calculated to distinguish MgO from Mg(OH)2. The Auger parameter was defined as follows [30,31]:

a ¼ KEðMg KLLÞ  KEðMg1sÞ ¼ BEðMg1sÞ  BEðMg KLLÞ

ð3Þ

1777

XPS surface analysis focused on the Mg KLL spectrum for the pure magnesium after imm-treatment and AV-treatment in 0.25 M Na2SO4 + 0.1 M NaOH solution for 30 min is represented in Fig. 10. The Auger parameter was calculated and the results were summarized in Table 5. A magnesium Auger parameter (a = 997.25 eV) attributed to Mg(OH)2 for the imm-treatment sample (Fig. 10a). For the AVtreatment sample, a magnesium Auger parameter (a = 998.5 eV) attributed to MgO (Fig. 10b). The XPS results indicated that an MgO film was formed on pure magnesium surface instead of Mg(OH)2 film after AV-treatment, which was responsible for the significantly improved corrosion resistance of sample after AV-treatment. 3.3.4. Effect of AV-treatment on the electronic structure of surface film It is well known that there are relationships between the semi conducting properties and the corrosion resistance of a passive film [32–34]. These properties can be determined by analyzing the curves of capacitance as a function of the electrode potential, which reflects the charge distribution in the passive films. The charge distribution at the interface of a semiconductor and an electrolyte is always determined by measuring the capacitance of the space charge layer (Csc) as a function of the electrode potential (E). The interfacial capacitance, C, is obtained from C = 1/xZ00 . Assuming that the capacitance of the Helmholtz layer can be neglected, the measured capacitance C is equal to the ‘space charge’ capacitance, Csc [32]. According to the Mott–Schottky

Fig. 8. AFM image (a) lower magnification and (b) higher magnification for the imm-treatment sample, (c) lower magnification and (d) higher magnification for the AVtreatment sample.

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X. Liu et al. / Corrosion Science 51 (2009) 1772–1779

8000

Mg KLL

7000

Intensity(CPS)

6000

b

5000

a

4000 3000 2000 1000 0 295

300

305

310

315

320

Binding energy(eV) Fig. 10. Mg KLL XPS peak of (a) imm-treatment and (b) AV-treatment samples.

Table 5 Results of XPS.

Imm-treatment AV-treatment

Fig. 9. The contact angle of (a) untreated (b) imm-treatment and (c) AV-treatment samples.

theory [33,34], the space charge capacitance of an N-type semiconductor are given by Eq. (4).

1 C2

¼

  2 KT E  EFB  e ee0 eND

ð4Þ

Table 4 The contact angle and surface free energy of the untreated, imm-treatment and AVtreatment samples.

Untreated Imm-treatment AV-treatment

Contact angle (°)

Surface free energy (J/cm2)

19.74 35.97 61.51

68.52 58.92 34.73

Mg1s (eV)

Mg KLL (eV)

a (eV)

Atomic composition

130.75 1304.25

306.50 305.75

997.25 998.50

Mg(OH)2 MgO

where e is the dielectric constant of the surface film (MgO is 9.65, Mg(OH)2 is 3.8), e0 is the permittivity of free space (8.854  1014 F/cm), e is the electron charge (1.602  1019 C), K is the Boltzmann constant (1.38  1023 J/K), T is the absolute temperature and KT/e is approximately 25 mV at room temperature, EFB is the flat band potential, ND is the donor densities that can be determined from the slope of the experimental 1/C2 vs. applied potential (E). Fig. 11 showed the Mott–Schottky (MS) plots of the untreated, imm-treatment and AV-treatment samples. It could be observed that the capacitance values decreased with the applied potential, leading to the development of a straight line with positive slope in the 1/C2 vs. applied potential (E). This fact indicated that the untreated, imm-treatment and AV-treatment samples all represented as n-type semi conducting behavior. The parameter ND refers to donor concentration in the electrode/electrolyte interface and the donor were usually some defects, such as cation interstitials or anion vacancies. The samples with different treatment methods had similar semiconducting properties, but with different donor concentration showed by different slopes of (MS) curves. Linear equations of the MS curves (Fig. 11) were obtained by linear fitting for the plots and shown in Fig. 11. From these linear equations, the donor concentration was calculated from the slope of the linear portion. The results were summarized in Table 6. It could be seen that the donor concentration decreased in the following order: untreated (2.1865  1020 m3) > imm-treatment (1.1266  1020 m3) > AVtreatment (8.7134  1019 m3). As suspected, AV-treatment greatly decreased the donor concentration. The higher donor concentrations of the untreated and imm-treatment samples were resulted from the higher reaction of the substrate for weak protectiveness of the surface film. For comparative, lower donor concentration implied a superior protectiveness surface film formed after AV-treatment [35].

4. Conclusions The optimal AV-treatment parameters for greatly improved corrosion resistance of pure magnesium were determined by the orthogonal experiments. The frequency, ratio of on-time to off-

X. Liu et al. / Corrosion Science 51 (2009) 1772–1779

10

4.8x10

10

pure Mg imm-treatment AV-treatment

10

4.0x10

2

-2

4

1/C (F cm )

4.4x10

10

3.6x10

10

3.2x10

10

2.8x10

-1.76

-1.72

-1.68

-1.64

-1.60

-1.56

E(VSCE) Fig. 11. The Mott–Schottky plots for the untreated, imm-treatment and AVtreatment samples.

Table 6 Influence of the AV-treatment on the surface film on the slope and the value of donor densities. ND (1/m3)

Slope Untreated Imm-treatment AV-treatment

10

1.6969  10 3.2934  1010 4.2581  1010

2.1865  1020 1.1266  1020 8.7134  1019

time and treatment time was determined as 20 Hz, 40% and 30 min, respectively. Polarization curve, EIS and SECM results indicated that a significant improvement on the corrosion resistance of pure magnesium was achieved after AV-treatment, which should be attributed to the excellent film formed on the pure magnesium surface. AV-treatment had a strong influence on the chemical composition of the surface film. A compact MgO film formed after AV-treatment. For the electronic structure, AV-treatment decreased the donor densities in the surface film, These influences are responsibility for the enhancement of corrosion resistance. Acknowledgements The authors wish to acknowledge the financial support of the National Natural Science Foundation of China (No. 50601007), the financial support of the Key Laboratory of Superlight Material and Surface Technology (Harbin Engineering University), Ministry of Education. The authors are also grateful to Dr. Li Liu for help of AFM measurement. References [1] G. Song, A. Atrens, Understanding magnesium corrosion, Adv. Eng. Mater. 5 (2003) 837–858. [2] K. Funatania, Emerging technology in surface modification of light metals, Surf. Coat. Tech. 133–134 (2000) 264–272. [3] R. Arrabal, E. Matykina, F. Viejo, P. Skeldon, G. Thompson, Corrosion resistance of WE43 and AZ91D magnesium alloys with phosphate PEO coatings, Corros. Sci. 50 (2008) 1744–1752. [4] R. Arrabal, E. Matykina, P. Skeldon, G. Thompson, A. Pardo, Transport of species during plasma electrolytic oxidation of WE43-T6 magnesium alloy, J. Electrochem. Soc. 155 (2008) C101–C111. [5] H. Huo, Y. Li, F. Wang, Corrosion of AZ91D magnesium alloy with a chemical conversion coating and electroless nickel layer, Corros. Sci. 46 (2004) 1467– 1477.

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