Experimental study of corrosion protection of a three-layer film on AZ31B Mg alloy

Corrosion Science 65 (2012) 367–375

Contents lists available at SciVerse ScienceDirect

Corrosion Science journal homepage: www.elsevier.com/locate/corsci

Experimental study of corrosion protection of a three-layer film on AZ31B Mg alloy Xinghua Guo, Keqin Du ⇑, Quanzhong Guo, Yong Wang, Fuhiu Wang State Key Laboratory for Corrosion and Protection, Institute of Metal Research, Chinese Academy of Science, Shenyang, China

a r t i c l e

i n f o

Article history: Received 22 December 2011 Accepted 16 August 2012 Available online 30 August 2012 Keywords: A. Magnesium A. Organic coatings A. Nickel A. Metal coatings C. Anodic films

a b s t r a c t A composite three-layer film was prepared on AZ31B Mg alloy for corrosion protection by means of plasma electrolytic oxidation (PEO), self-assembled nanophase particle (SANP) and electroless nickel (EN) plating. Structure and corrosion protection were characterized by various analytical instruments and electrochemical methods. Experiment results indicated that the composite film consisted of compact EN plating and coherent bilayer film, which provided effective protection for Mg alloy against galvanic corrosion and long-term immersion corrosion. Ó 2012 Elsevier Ltd. All rights reserved.

1. Introduction It is well known that Mg alloy with poor corrosion resistance and high chemical reactivity limits its practical application in many domains. Among various surface treatment techniques of Mg alloy, EN plating is an effective way due to good chemical stability, high rigidity, good abrasion resistance and corrosion resistance [1,2]. However, galvanic corrosion between the EN plating and the Mg alloy substrate seems still to be a serious problem due to the pores in the EN plating. Thus, developing a compact EN plating process on Mg alloy and Mg alloy with good barrier is an important task. PEO can produce a ceramic film by in situ growth on the surface of Mg with excellent adhesion [3–7]. However, the porous PEO film as the barrier layer is unsuitable to provide long-term immersion protection for Mg alloy substrate. In recent years, combined technologies that produce multilayer, composite film for Mg alloy are a promising way. SANP technology is a new organic–inorganic hybrid system which can produce inorganic silica nanoparticles in an aqueous sol–gel process [8–14]. Our previous studies [15,16] have proved that these nanoparticles can penetrate into the porous PEO film to form an integrated SANP + PEO film via organic crosslink agent. Accordingly, the SANP + PEO film will be a good choice as the barrier layer between the EN plating and the Mg alloy substrate. The aim of this work is to prepare a three-layer film on AZ31 Mg alloy which consists of the PEO film as the bottom layer, the SANP film as the intermediate layer and the EN plating as the top layer.

⇑ Corresponding author. Tel.: +86 13998328471; fax: +86 24 23915894. E-mail address: [email protected] (K. Du). 0010-938X/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.corsci.2012.08.055

For the systematic study of the three-layer film property, another three films coated on Mg substrate are also prepared: a bilayer of PEO + EN film, single PEO film and single EN plating. Their properties are characterized by surface analysis measurements and electrochemical methods. It is our intention to clarify the protection property of the three-layer film against galvanic corrosion and long-term immersion corrosion through the studies mentioned above.

2. Experimental methods 2.1. Preparation AZ31 Mg alloy plates (50  50  1 mm) were used as the substrate material for the surface treatments with the following composition (wt.%): Al 2.5–3.5, Zn 0.6–1.4, Mn 0.2, Si 6 0.1, Fe 0.005, Cu 6 0.05, Ni 0.005 and Mg balance. The Mg plates were degreased ultrasonically in acetone, cleaned with distilled water, and then dried in air. SANP solution (Table 1) was prepared by the hydrolysis of 3-glycidoxypropylp- trimethoxysilane (GPTMS) and tetraethoxysilane (TEOS) in de-ionized water under stirring at room temperature for 3 h. Subsequently, a mixture solvent of H2O and ethanol was added with a mole ratio of n(GPTMS):n(TEOS):n(H2O):n(ethanol) = 3:1:10:1. Then acetic acid (0.05 mol/L, 5 ml) was added dropwise under stirring within 30 min. After being hydrolyzed for 150 h at room temperature, the crosslink agent of triethylene tetramine (TETA) was added into the solution until the pH value was between 5.5 and 6.5. After 10 min, the specimens were deposited in the SANP solution by controlling the withdrawal with a speed of 1 cm/min immediately. At last the deposited films were dried at room temperature for 30 min.

368

X. Guo et al. / Corrosion Science 65 (2012) 367–375

Table 1 Composition and operating conditions of SANP solution. Silane

Table 3 Preparation of composite films coated on AZ31 Mg alloy.

Composition Mole ratio, n(GPTMS):n(TEOS) Mixture time

GPTMS and TEOS 3:1 3h

Acid

Composition Mole concentration Dropwise duration

Acetic acid 0.05 mol/L, 5 ml 30 min

Solvent

Composition Mole ratio Hydrolytic time

H2O and ethanol 10:1 150 h

Crosslink

Composition pH of SANP solution Crosslink time

TETA 5.5–6.5 10 min

EN plating on specimen was activated in a solution of 0.5 g PdCl2, 500 ml ethanol and 500 ml water and then preliminarily reduced in a solution of 30 g/L NaH2PO2H2O at room temperature. Finally, the EN plating was deposited on the specimen in a bath which contained 30 g/L NiSO46H2O, 32 g/L NaH2PO2H2O, 34 g/L NH4HF2, 6 g/L C6H8O7H2O, 17 ml/L NH3H2O and 7 g/L Na(CH3COOH) at 80 °C for 1 h. The bath compositions and other parameters which were used in the EN process are presented in Table 2. For the preparation of PEO film, AZ31B Mg alloy plate 50 cm2 in surface area was used as the working electrode, with two austenitic stainless steel piece of 100 cm2 in surface area as counter electrodes. The AZ31B Mg plate was centered between the two parallel stainless steel counter electrodes; the distance between the parallel counter electrodes was 10 cm. PEO was carried out with a bipolar current power applied for 20 min, of which the control parameters were as follows: bipolar current pulses’ frequency was 100 Hz, anodic current amplitude was 35 A, and the parameter R of the anodic to cathodic current was 1.57. Matching of R value was done by adjusting the negative current amplitude of 22.3 A. The electrolyte contained 180 g/L KOH, 40 g/L Na3PO4 and 35 g/L KF which was kept at 20 °C using a water bath. Two composite films were prepared including PEO + EN (PE) film and PEO + SANP + EN (PSE) film, and their compositions and thicknesses are shown in Table 3. For the PE film on specimen, Mg plate was first anodized by PEO treatment according to the above. Rinsing with distilled water and drying with cool air followed the PEO treatment. Subsequently, the deposition of the EN plating on the PEO film by the steps of activation, reduction and nickel deposition as shown in Table 2. For the PSE film on specimen, the Mg plate was still treated by PEO firstly. After rinsing with distilled water and drying with cool air, the specimens were deposited by the SANP treatment and

Table 2 Composition and operating conditions of Ni–P plating on AZ31B alloy. Step

Composition

Quantity

Activation

PdCl2 C2H5OH H2O Temperature

0.5 g 500 ml 500 ml Room temperature 1–2 min

Reduction

NaH2PO2H2O Temperature

30 g/L Room temperature 10 s–1 min

Nickel deposition

NiSO46H2O NaH2PO2H2O NH4HF2 C6H8O7H2O NH3H2O Na(CH3COOH) pH Temperature Holding time in bath

30 g/L 32 g/L 34 g/L 6 g/L 17 ml/L 7 g/L 6.7 80 °C 1h

Layer

Treatment

Thickness (lm)

PE

Bottom layer Top layer

PEO EN

20–30 25–30

PSE

Bottom layer Interface layer Top layer

PEO SANP EN

20–30 15–20 25–30

dried at room temperature for 30 min. And then, the EN plating was through the steps of activation, reduction and nickel deposition as shown in Table 2. In this paper, all coated specimens (EN plating, PEO film, PE film and PSE film) were rinsed thoroughly in distilled water and dried in ambient air immediately after the final surface treatments. 2.2. Characterization Surface morphologies of different films coated on AZ31 Mg substrates were characterized by scanning electron microscopy (SEM, Hitachi S-4700 system with electron beam energy of 15 keV). Energy dispersion spectrometry (EDS) attached to SEM was used to detect the elemental distribution on the cross-section of the PSE film. Hydrolytic nanoparticle state in the SANP solution was measured by transmission electron microscopy (TEM). Firstly, these nanoparticles were deposited on carbon coated electron microscope grids and allowed to dry. The electron microscopy was performed with a Hitachi H-700H TEM, operated at an acceleration voltage of 80 kV, using a low dose electron beam for observation and recording. Fourier transform infrared spectrometer (FT-IR, HORUBA FT-730) was performed in attenuated total reflectance mode for the dried SANP film without TETA. The measurement parameters were set as a wavenumber range from 4000 to 400 cm1 with a resolution of 4 cm1, utilizing deuterated triglycine sulfate as a detector. Electrochemical tests were carried out using a computer controlled potentiostat/frequency response analyser (model 273, EG&G Instrument Inc.) to evaluate the corrosion behavior of Mg alloy substrates with different films. A typical three electrode cell was used including a saturated calomel reference electrode (SCE, Leici 232–01), a platinum foil (25  25 mm) as a counter electrode and an exposed specimen (1 cm2) as a working electrode. Three different tests were performed viz., potentiodynamic polarization tests, open circuit potential (OCP) tests and electrochemical impedance spectroscopy (EIS) tests in non-deaerated 3.5 wt.% NaCl solution in the as-prepared condition. Potentiodynamic polarization tests were performed, starting at 500 mV with reference to OCP (after 30 min of exposure at OCP) at a sweep rate of 0.5 mVs1 to a final potential of 500 mV with reference to OCP. OCP tests of the Mg alloy substrates with different films exposed to 3.5 wt.% NaCl solution were measured for 10 h. EIS tests were performed at the OCP with applied 10 mV sinusoidal perturbation in the frequency ranges of 105–102Hz with 10 steps per decade and the studies were made after exposing the specimens to the 3.5 wt.% NaCl solution for 0.5, 10, 30, 50 and 50 h. Zsimpwin software was utilized to simulate the EIS experimental data. In this paper, all potentials were referred to the SCE, and all the electrochemical tests were carried out at room temperature. 3. Results and discussion 3.1. SANP solution characterization Figure 1 shows that the TEM image of nanoparticles with a diameter of 20–25 nm after 60 days can still suspend stably in

369

X. Guo et al. / Corrosion Science 65 (2012) 367–375

3.2. Composite film structure characterization Figure 3 shows the surface morphologies of PEO film and EN plating on the surface of different films or substrate. It can be seen from Fig. 3(a) that the PEO film presents many micro pores and some micro cracks on the surface. Moreover, great deals of ceramic particles randomly scatter in the film. The single EN plating on Mg alloy substrate (Fig. 3(b)) shows that many ‘‘nodular-like’’ clusters and microcracks are not uniform and dense, which demonstrate that the nucleating centers appear preferentially on the b-phase or in the vicinity and form the non-uniform distribution and microdefect [24–26]. The compactness of the EN plating on PE film in Fig. 3(c) is increased limitedly by the formation of many pores that ranged from 2 to 10 lm in diameter. The nucleation mechanism of the EN plating on PE film may be attributed to the following

Si-O-Si 20

Absorbance Units

the SANP solution. Like most traditional sol–gel processes, the SANP processing also consists of hydrolysis reaction and condensation reaction. Our studies [16–18] indicate that the hydrolysis reaction takes place very fast in the high water concentration solution; while the condensation reaction is much too slow to neglect for most cases. Therefore, the nanoparticles can be kept stable after long term storage. Figure 2 shows the FT-IR spectroscopy of the dried SANP film without TETA, and some shifts may have happened to the peaks of the organic groups due to the effect of Mg substrate. The characteristic bonds attributed to epoxy functional groups of GPTMS present at 908 cm1 [19]. Two obvious absorption peaks of Si–O– Si asymmetric stretching are at 1047 and 1108 cm1 [20–22]. Si– O–Si symmetric stretching and Si–O–Si bending are also found at 796 cm1 [20–22] and 400–500 cm1 [22], respectively. According to the reference result given by Parkhill [23], a dense network of cyclic rings, particulate silica structure is produced in the high water concentration solution. Thus, these absorption peaks of Si– O–Si represent the siloxane linkage formed a cyclic siloxane ring structure. Two broad peaks at 3442 cm1 (corresponding to –OH group asymmetric stretching) and 1647 cm1 (assigned to H–O– H deformation) represent the water and the residual hydroxyl functional groups in the SANP solution [20–22]. In summary, it can be concluded that the hydrolytic reaction completes thoroughly and forms large numbers of SiO2 nanoparticles with the structure of cyclic rings and epoxy functional groups.

40

-OH 60

C-N

Epoxy

80

100 500

1000

1500

2000

2500

3000

3500

Wavenumber, cm-1 Fig. 2. Infrared spectrograph of dried SNAP film without TETA.

reasons: firstly, the porous PEO film with big specific surface area can adsorb more nucleating centers and active Ni particles. Secondly, the pretreatment of the PEO film may eliminate the effect of electrochemically heterogeneous Mg substrate surface and provide a good catalytic under-layer support for the subsequent EN plating [6,7]. Compared with PE film, the uniform EN plating on PSE film (fig. 3(d)) displays lots of compact crystal grains, which may be very helpful to corrosion protection of Mg alloy substrate. And this issue will be further confirmed through electrochemical analyses in the following sections. Figure 4 shows cross-section morphology and EDS spectrums of PSE film coated on Mg substrate. It is obviously seen in Fig. 4(a) that the PSE film consists of three layers. As shown in Fig. 4(b)– (d), the bottom layer of the PEO film contains 62.69 at.% O, 20.60 at.% Mg and 16.71 at.% P, which indicate that the PEO film mainly consists of MgO and Mg3(PO4)2 phases [27–29]. Although some interconnected micropores and microdefects are found on the PEO film, they do not perforate through this layer. In contrast, the intermediate layer of the SANP film with good compactness contains 51.37 at.% C, 34.02 at.% O, 6.89 at.% Mg and 7.72 at.% Si. Based on our previous studies [15,17,18,30], the cyclic siloxane ring group of the nanoparticles has more SiOH groups, they are capable of reacting with the PEO film to form an interface with a higher density of Mg–O–Si bonds, and these groups can also connect with each other to produce a large scale Si–O–Si network by TETA. On the other hand, due to the high porosity of the PEO film, more nanoparticles of cyclic siloxane ring groups can penetrate into the pores, and then the SANP film and the PEO film combine with each other to form a coherent structure on Mg alloy through the crosslink reaction [15]. The top layer of the EN plating with 14.59 at.% P and 85.41 at.% presents rather compact which may be attributed to the following formation processes: Firstly, the coherent structure of the SANP + PEO film can further eliminate electrochemically heterogeneous Mg substrate surface. Secondly, the SiOH groups of cyclic siloxane ring group of the SANP film may increase the nucleation density and provide more effective fastening sites between the EN plating and the substrate. 3.3. Potentiodynamic polarization tests

Fig. 1. TEM image of hydrolytic nanoparticles suspended in SANP solution.

Figure 5 shows the potentiodynamic polarization curves of the bare AZ31B Mg alloy and the substrates with different films. Corrosion potential (Ecorr) and corrosion current density (icorr) are often used to evaluate the corrosion protection property of the films,

370

X. Guo et al. / Corrosion Science 65 (2012) 367–375

Fig. 3. Surface morphologies of Mg substrates with different films: (a) single PEO film; (b) single EN plating; (c) EN plating on PE film; (d) EN plating on PSE film.

which are calculated by Tafel extrapolation from the linear polarization region according to the following equation [31,32]:

jgj ¼

2:303RT 2:303RT 0 logicorr þ logji j anF anF

ð1Þ

where g is the overpotential, a is a symmetry factor, n is the number of moles of electrons, F is the Faraday constant, i0 is the change current density. Thus, to determine the icorr value, the biasing of the potential on the anodic and cathodic branches respectively is carried out in a small range (generally 25–100 mV) where it obtains the Tafel slope of the linear i  E relation near the Ecorr. Then, extrapolating them linearly to Ecorr, the corresponding i value there is icorr. When the corrosion current density is determined, all the Tafel slopes are estimated to be about 0.18 ± 0.02 V decade1 using the anodic branches or 0.18 ± 0.02 V decade1 using the cathodic branches. Therefore, the Tafel slopes include at least about 11% human-inducible inaccuracy, which corresponds to about 30% [33]. Table 4 lists the icorr and Ecorr the specimens derived from polarization curves. It can be seen that the icorr value of the bare AZ31B Mg substrate is 3.91  104 A cm2, The icorr value (2.71  105 A cm2) of the EN plating is only one order of magnitude lower than that of the bare Mg substrate, which means that

the EN plating can only provide a general corrosion protection for Mg substrate. For the PEO, PE and PSE films, the icorr values decrease by about 2–3 orders of magnitude compared with the bare Mg substrate, indicating their superior corrosion resistance properties. It should be noted that the Ecorr values of the EN plating, PE film and PSE film shift to the positive direction except the PEO film compared with that of the bare AZ31B Mg substrate. In most cases, the negative potential shift of the Mg substrate in acid or neutral solutions is realized either due to the increased rate of the anodic reaction (Mg oxidation) or the decreased rate of cathodic reaction (hydrogen evolution and reduction of water and/or dissolved oxygen), which are shown as follows [33,34]:Anodic reaction:

Mg ! Mg2þ þ 2e

ð2Þ

Cathodic reactions:

2H2 O þ 2e ! H2 þ 2OH

ð3Þ

O2 þ 2H2 O þ 4e ! 4OH

ð4Þ

Reaction of corrosion products:

Mg2þ þ 2OH ! MgðOHÞ2

ð5Þ

371

X. Guo et al. / Corrosion Science 65 (2012) 367–375

(5) PSE film

0.0

(4) PE film Potential, E / V vs. SCE

(3) EN plating

C B A

-0.5

-1.0

(2) PEO film -1.5

-2.0 -10 10

(1) bare AZ31B Mg substrate -9

10

-8

10

-7

10

-6

10

-5

10

-4

-3

10

10

-2

10

-1

10

2

Current density, |i| / A cm

(a)

Fig. 5. Potentiodynamic polarization curves of the bare AZ31B magnesium alloy and the Mg substrates with different films in 3.5 wt.% NaCl solution: (1) bare AZ31B Mg substrate; (2) PEO film; (3) EN plating; (4) PE film; (5) PSE film.

Element

Content(at%)

O Mg P

62.69 20.60 16.71

(b)

Element

Content(at%)

C O Mg Si

51.37 34.02 6.89 7.72

(c)

Element

Content(at%)

P N

14.59 85.41

Table 4 Results of potentiodynamic polarization tests of different films coated on AZ31B Mg alloy. Specimens

Ecorr (V vs. SCE)

Icorr(A cm2)

Mg PEO EN PE PSE

1.53 1.71 0.56 0.41 0.42

3.91  104 1.04  107 2.71  105 1.61  106 2.58  107

could not penetrate through the defects in the barrier layer toward the substrate immediately. Thus, reaction (2) of Mg oxidation could not happen. Even though it would occur locally on some area, there would be little effect on the OCP. The cathodic reaction of reactions (3) and (4), on the other hand, can be caused by changed catalytic activity due to the presence of MgO and Mg3(PO4)2 phases in the PEO film [27–29], of which the hydrogen evolution and water reduction reactions can be inhibited effectively and overpotential will be occurred finally. That is the main reason why the Ecorr of PEO film shifts to the negative direction. From Figure 5, it can also be seen that the anodic branches of the polarization curves of all the PEO film, PE film and PSE film are characterized by passive regions in 3.5 wt.% NaCl solution. The existence of passive regions for these films suggests that the passive films have naturally formed on the surface when they are exposed to the corrosive electrolyte. It is also noted that the passive regions of the PE film and PSE film are much wider than those of PEO film, which are responsible to the improvement of corrosion resistance for these films. 3.4. OCP tests

(d) Fig. 4. Selected area in the cross-section morphology and EDS spectrums of Mg substrate with PSE film: (a) rectangular area A, B and C in the cross-section morphology of composition identification; (b) EDS spectrum of area A; (c) EDS spectrum of area B;(d) EDS spectrum of area C.

In this paper, since most surface of Mg substrate is covered by a continuous and relatively compact PEO film, the corrosive particles

The OCP tests of different films on Mg substrate are employed by Ecorr  t curves. This method not only estimates qualitatively the galvanic corrosion trends [35], but also represents indirectly the film integrities and the barrier properties. For the Ecorr  t curve of the bare Mg alloy in Fig. 6(a), the Ecorr value shows a positive potential shift after 160 s and then arrives at a stable value since corrosive products Mg(OH)2 produced by reaction (5) cover most surface of Mg substrate in a short time. For the PEO film in Fig. 6(a), the initial decline process of the Ecorr value lasts longer time for about 4000 s due to the decreased rate of cathodic reactions for the Mg alloy substrate. Hereafter the Ecorr

372

X. Guo et al. / Corrosion Science 65 (2012) 367–375

value continues to rise during the subsequent immersion time, this result might be caused by the formation of corrosion products which plug the pores and cracks of PEO film. For the mixed electrode system of the substrate and the films of EN plating, PE film and PSE film, their Ecorr values are determined by the corrosion potential of amorphous Ni–P, the corrosion potential of Mg and the surface area ratio of the galvanic coupling of the top layer and the magnesium substrate. The surface area ratio of the galvanic coupling is determined by the porosity of the Ni–P layer. For the EN plating in Fig. 6(a), the Ecorr value fluctuates in the range of 0.4 to 0.5 V vs. SCE and then begins to drop off significantly until around 1.3 V vs. SCE. When the immersion time is over 5500 s, the Ecorr value gradually slows down and reaches the relative stable stages. In order to obtain detailed information of EN plating, it is important to carry out corrosive observations after open circuit potential tests. As seen in Fig. 7(a), a seriously corrosive hole which has penetrated through the specimen occurs in the test region of the EN plating. Based on the result of Ecorr  t curve, it can be concluded that the galvanic corrosion may happen between the EN plating and Mg substrate: the Ni–P alloy with

Potential, E / V vs. SCE

-0.4 -0.6

(3) EN plating -0.8 -1.0

(2) PEO film

-1.2 -1.4

(1) bare AZ31B Mg substrate -1.6 0

5

10

15

20

25

30

35

40

Time / ks

(a)

Potential, E / V vs. SCE

-0.32

-0.34

-0.36

(5) PSE film

-0.38

(4) PE film -0.40

-0.42 0

5

10

15

20

25

30

35

40

Time / ks

(b) Fig. 6. Open circuit potentials of the bare AZ31B Mg alloy and the Mg substrates with different films versus immersion time in 3.5 wt.% NaCl solution: (1) bare AZ31B Mg substrate; (2) PEO film; (3) EN plating; (4) PE film; (5)PSE film.

Fig. 7. Optical images of the EN plating of different films coated AZ31B Mg alloy substrate after Ecorr  t tests: (a) EN plating; (b) PE film; (c) PSE film.

X. Guo et al. / Corrosion Science 65 (2012) 367–375

higher potential is the cathode of microcell, and the lower potential-Mg substrate is the anode. The EN plating in situ forms on the surface of Mg substrate, which corresponds to numerous Ni– P/Mg of microcell in parallel. Once the galvanic corrosion happens on the surface of some micropores or microdefects (Fig. 3(b)), it can get large corrosion current and lead to accelerate the anode dissolution rapidly. For the EN plating in Fig. 6(b), the Ecorr value of PE film decreases quickly then increases slowly to a stable Ecorr value of 0.42 V vs. SCE. The higher stable value of PE film may be attributed to the contributions of the barrier property of PEO film and improved compactness of the EN plating. The corroded image of PE film in Fig. 7(b) indicates that most part of the EN layer of the PE film is relatively intact compared with that of the EN plating in Fig. 7(a). The galvanic corrosion between EN layer and Mg substrate is inhibited by the PEO film to some extent. The Ecorr  t curve of PSE film in Fig. 6(b) shows the Ecorr value increases monotonously. Moreover, clearly evidence in Fig. 7(c) indicates that there is no galvanic corrosion between EN plating and Mg substrate, which shows an intact EN plating of the PSE film without any corrosive region after open circuit potential test. As described above, the top layer of the uniform and compact EN plating (Fig. 3(d)) can be used as the first barrier layer which can inhibit the formation of electrically conductive path between the

3.5. EIS tests As described above, both the PE film and the PSE film have good barrier property and inhibit the galvanic corrosion between the EN plating and Mg substrate. However, it is necessary to evaluate their long-term immersion corrosion protection. Fig. 8(a) shows the impedance spectra and fitting results of PE film coated on AZ31B Mg substrate in 3.5% NaCl solution at the open circuit potential immersed for different time. Considering that the EN plating on PE film is not compact enough to inhibit water molecule and corrosive particles as shown in Fig. 3(c), Model A of R(Q(R(QR))) in Fig. 9(a) is used to fit the impedance spectra in Fig. 8(a) that contains two time constants before the immersion time of 50 h: Rs is the electrolyte resistance; REN is the resistance corresponding to the electrolyte in the defects and the underlying outer EN plating; RPEO is the resistance corresponding to the inner PEO film; CPEEN and CPEPEO are constant phase elements (CPE), corresponding to the capacitances of the outer EN plating and the inner PEO film. Constant phase element (CPE) is used instead of a pure capacitance (C) which can be given by Eq. (6):

0.5h 10h 30h 50h 60h fit lines

-30

-Z'' / kΩ cm2

EN layer and Mg substrate. The coherent SANP + PEO film with excellent barrier property also plays an important role in preventing the occurrence of EN/Mg substrate galvanic corrosion.

C ¼ Qðxmax Þn1

-40

0.1Hz -20

-10

0 0

10

20

30

40

Z' / kΩ cm2

50

(a) -80

0.5h 10h 30h 50h 60h fit lines

-Z'' / kΩ cm2

-60

0.1Hz

-40

-20

0 0

20

40

60

80

100

120

Z' / kΩ cm2

(b) Fig. 8. EIS diagrams of Mg substrate with (a) PE film and (b) PSE film versus immersion time in 3.5 wt.% NaCl solution.

373

Fig. 9. Electrical equivalent circuits used to fit the impedance spectra.

ð6Þ

374

X. Guo et al. / Corrosion Science 65 (2012) 367–375

where xmax is the frequency at which the imaginary impedance reaches its maximum for respective time constant, Q and n are the CPE value (F/cm2) and dispersion index of CPE, respectively. Phenomenally, only one capacitive loop is displayed in Fig. 8(a) since the response of the porous EN plating in the high frequency range is yet small. This leads to the difficulty to separate the time relaxation of coating’s physical impedance from that of the PEO film impedance. When the immersion time is over 50 h, water molecules and corrosive particles have passed through the EN plating and penetrated into the PEO film. Due to porous structure and high thickness of the PEO film, small diffusion channels may be formed after long-term immersion process meanwhile almost no corrosion reaction has happened in the PEO film. Therefore, model B of R(Q(R(Q(RW)))) in fig. 9(b) is introduced to fit the impedance spectra after 50 h. In such circumstances, the diffusion process in the PEO film is described by the element Warburg impedance (W). Moreover, the practical direction of the non-ideal Warburg impedance behavior is not in parallel concentration gradient, which is commonly associated with ‘‘finite-layer diffusion’’ or the tangential penetration of electrolyte or heterogeneous penetration of an electrolyte [36–38]. Fig. 8(b) shows the impedance spectra and fitting results of PSE film coated on AZ31B Mg substrate in 3.5% NaCl solution at the open circuit potential immersed for different time. Considering that the compact EN plating on PSE film in Fig. 3(c) and Fig. 4, Model C of R(QR)(QR) in Fig. 9(c) is used to fit the impedance spectra in Fig. 8(b) during the whole immersion time: Rs is the electrolyte resistance; REN is the resistance corresponding outer EN plating; RSP is the resistance corresponding to the inner SANP + PEO film; CPEEN and CPESP are the capacitances of the outer EN plating and the inner SANP + PEO film. Figure 10 shows variation with time of the resistances (REN, RPEO and Rsp) derived from the PE film and PSE film. The REN value of the PE film is always the lowest due to nondense cellular EN plating (Fig. 3(c)). In contrast, the REN value of the PSE film flirted with the 104 X cm2 indicates that the compact EN plating can obviously increase the corrosion protection of the composite PSE film. It should be noted that the RSP value of the PSE film with above 6  104 X cm2 is much higher than the RPEO value of the PE film during the whole immersion process, which demonstrates that the coherent SANP + PEO film with excellent barrier property can provide effectively corrosion protection for Mg alloy substrate during the long-term immersion.

90

Resisrance, R / kΩ cm2

80 70 60

REN of PE film

RPEO of PE film

50

REN of PSE film

RSP of PSE film

40 30 20 10 0 0

10

20

30

40

50

Immersion time / h Fig. 10. Evolution of R values versus immersion time.

60

4. Conclusions A composite film coated on AZ31B Mg alloy was studied which consisted of the bottom PEO film, intermediate SANP film and the top EN plating. The SANP solution contained large numbers of SiO2 nanoparticles with the structure of cyclic rings and epoxy functional groups, which penetrated into the porous PEO film to form a coherent SANP + PEO film. The electrochemical tests indicated that the galvanic corrosion was prone to happen on the surface of single EN plating; the PEO film on PE film inhibited the galvanic corrosion, but its corrosion protection was increased limitedly during the long-term immersion process due to the nondense cellular EN plating and the porous PEO film. Especially, the PSE film can provide an effective protection of galvanic corrosion and long-term immersion corrosion for Mg substrate due to the compact EN plating and excellent barrier property of the coherent SANP + PEO film. Acknowledgements This work was supported by project supported by National Science and Technology Ministry (Grant No. 2011BAE22B05) and Material foundation and application technology of key projects (Grant No. A0920110028). References [1] Z.M. Lium, W. Gao, A novel process of electroless Ni–P plating with plasma electrolytic oxidation pretreatment, Appl. Surf. Sci. 253 (2006) 2988–2991. [2] L.Y. Zeng, S.W. Yang, W. Zhang, Y.H. Guo, C.W. Yan, Preparation and characterization of a double-layer coating on magnesium alloy AZ91D, Electrochim. Acta 55 (2010) 3376–3383. [3] R. Arrabal, E. Matykina, F. Viejo, P. Skeldon, G.E. Thompson, Corrosion resistance of WE43 and AZ91D magnesium alloys with phosphate PEO coatings, Corros. Sci. 50 (2008) 1744–1752. [4] R. Arrabal, A. Pardo, M.C. Merino, M. Mohedano, P. Casajús, E. Matykina, P. Skeldon, G.E. Thompson, Corrosion behaviour of a magnesium matrix composite with a silicate plasma electrolytic oxidation coating, Corros. Sci. 52 (2010) 3738–3749. [5] F. Liu, D.Y. Shan, Y.W. Song, E.H. Han, W. Ke, Corrosion behavior of the composite ceramic coating containing zirconium oxides on AM30 magnesium alloy by plasma electrolytic oxidation, Corros. Sci. 53 (2011) 3845–3852. [6] J. Liang, P. Bala Srinivasan, C. Blawert, W. Dietzel, Influence of pH on the deterioration of plasma electrolytic oxidation coated AM50 magnesium alloy in NaCl solutions, Corros. Sci. 52 (2010) 540–547. [7] H.P. Duan, C.W. Yan, F.H. Wang, Effect of electrolyte additives on performance of plasma electrolytic oxidation films formed on magnesium alloy AZ91D, Electrochim. Acta 52 (2007) 3785–3793. [8] A.J. Vreugdenhil, V.N. Balbyshev, M.S. Donley, Nanostructured silicon sol–gel surface treatments for Al 2024–T3 protection, J. Coat. Technol. 73 (2001) 35– 43. [9] A.N. Khramov, V.N. Balbyshev, N.N. Voevodin, M.S. Donley, Nanostructured sol–gel derived conversion coatings based on epoxy- and amino-silanes, Prog. Org. Coat. 47 (2003) 207–213. [10] M.S. Donley, R.A. Mantz, A.N. Khramov, V.N. Balbyshev, L.S. Kasten, D.J. Gaspar, The self-assembled nanophase particle (SNAP) process: a nanoscience approach to coatings, Prog. Org. Coat. 47 (2003) 401–415. [11] L.S. Kasten, V.N. Balbyshev, M.S. Donley, Surface analytical study of selfassembled nanophase particle (SNAP) surface treatments, Prog. Org. Coat. 47 (2003) 214–224. [12] M.S. Donley, V.N. Balbyshev, N.N. Voevodin, Self-assembled NAnophase Particle (SNAP) surface treatments for corrosion protection of AA2024-T3, Prog. Org. Coat. 52 (2005) 34–38. [13] V.N. Balbyshev, K.L. Anderson, A. Sinsawat, B.L. Farmer, M.S. Donley, Modeling of nano-sized macromolecules in silane-based self-assembled nano-phase particle coatings, Prog. Org. Coat. 47 (2003) 337–341. [14] N.N. Voevodin, V.N. Balbyshev, M. Khobaib, M.S. Donley, Nanostructured coatings approach for corrosion protection, Prog. Org. Coat. 47 (2003) 416– 423. [15] X.H. Guo, M.Z. An, Experimental study of electrochemical corrosion behaviour of bilayer on AZ31B Mg alloy, Corros. Sci. 52 (2010) 4017–4027. [16] X.H. Guo, K.Q. Du, A new nanoparticle penetrant used for plasma electrolytic oxidation film coated on AZ31 Mg alloy in service environment, Surf. Coat. Technol. 206 (2012) 4833–4839. [17] X.H. Guo, M.Z. An, P.X. Yang, H.X. Li, C.N. Su, Y.H. Zhou, Property characterization and formation mechanism of anticorrosion film coated on AZ31B Mg alloy by SNAP technology, J. Sol–Gel Sci. Technol. 52 (2009) 335– 347.

X. Guo et al. / Corrosion Science 65 (2012) 367–375 [18] X.H. Guo, M.Z. An, P.X. Yang, C.N. Su, Effect of sol compositions on corrosion protection of SNAP film coated on magnesium alloy, Chin. J. Inorg. Chem. 25 (2009) 1254–1261. [19] L.J. Liu, Study on the Preparation and Anticorrosion of the Epoxy/SiO2 Hybrid Materials, WUHAN University of Technology (WHUT), China, (2005) 19. [20] B. Karmakar, G. De, D. Ganguli, Dense silica microspheres from organic and inorganic acid hydrolysis of TEOS, J. Non-Cryst. Solids 272 (2000) 119–126. [21] G. De, B. Karmakar, D. Ganguli, Hydrolysis–condensation reactions of TEOS in the presence of acetic acid leading to the generation of glass-like silica microspheres in solution at room temperature, J. Mater. Chem. 10 (2000) 2289–2293. [22] G. De, D. Kundu, B. Karmakar, D. Ganguli, FTIR studies of gel to glass conversion in TEOS-fumed silica-derived gels, J. Non-Cryst. Solids 155 (1993) 253–258. [23] V. Uricanu, D. Donescu, A.G. Banu, S. Serban, M. Olteanu, M. Dudau, Organic– inorganic hybrids made from polymerizable precursors, Mater. Chem. Phys. 85 (2004) 120–130. [24] R. Ambat, W. Zhou, Electroless nickel-plating on AZ91D magnesium alloy: effect of substrate microstructure and plating parameters, Surf. Coat. Technol. 179 (2004) 124–134. [25] Z. Liu, W. Gao, The effect of substrate on the electroless nickel plating of Mg and Mg alloys, Surf. Coat. Technol. 200 (2006) 3553–3560. [26] C. Gu, J. Lian, G. Li, L. Niu, Z. Jiang, Electroless Ni–P plating on AZ91D magnesium alloy from a sulfate solution, J. Alloys Compds. 391 (2005) 104– 109. [27] P.B. Srinivasan, J. Liang, C. Blawert, M. Störmer, W. Dietzel, Characterization of calcium containing plasma electrolytic oxidation coatings on AM50 magnesium alloy, Appl. Surf. Sci. 256 (2010) 4017–4022. [28] L.Q. Wang, J.S. Zhou, J. Liang, J.M. Chen, Microstructure and corrosion behavior of plasma electrolytic oxidation coated magnesium alloy pre-treated by laser surface melting, Surf. Coat. Technol. 206 (2012) 3109–3115.

375

[29] P.B. Srinivasan, J. Liang, R.G. Balajeee, C. Blawert, M. Stormer, W. Dietzel, Effect of pulse frequency on the microstructure phase composition and corrosion performance of a phosphate-based plasma electrolytic oxidation coated AM50 magnesium alloy, Appl. Surf. Sci. 256 (2010) 3928–3935. [30] X.H. Guo, M.Z. An, P.X. Yang, H.X. Li, C.N. Su, Effects of benzotriazole on anodized film formed on AZ31B magnesium alloy in environmental-friendly electrolyte, J. Alloys Compds. 482 (2009) 487–497. [31] J.O’M. Bockris, A.K.N. Reddy, Modern Electrochemistry 2B Electrodics in Chemistry, Engineering, Biology, and Environmental Science, second ed., Kluwer Academic/Plenum Publishers, New York, 2001. p. 1661. [32] G. Wranglén, An Introduction to Corrosion and Protection of Metals, Chapman and Hall, London, 1985. p. 247. [33] S. Yagi, A. Sengoku, K. Kubota, E. Matsubara, Surface modification of ACM522 magnesium alloy by plasma electrolytic oxidation in phosphate electrolyte, Corros. Sci. 57 (2012) 74–80. [34] Y.J. Zhang, C.W. Yan, F.H. Wang, W.F. Li, Electrochemical behavior of anodized Mg alloy AZ91D in chloride containing aqueous solution, Corros. Sci. 47 (2005) 2816–2831. [35] C.M. Abreu, M. Izquierdo, M. Keddam, X.R. Nóvoa, H. Takenouti, Electrochemical behaviour of zinc-rich epoxy paints in 3% NaCl solution, Electrochim. Acta 41 (1996) 2405–2415. [36] J.T. Zhang, J.M. Hu, J.Q. Zhang, C.N. Cao, Studies of water transport behavior and impedance models of epoxy-coated metals in NaCl solution by EIS, Prog. Org. Coat. 51 (2004) 145–151. [37] J.T. Zhang, J.M. Hu, J.Q. Zhang, C.N. Cao, Studies of impedance models and water transport behaviors of polypropylene coated metals in NaCl solution, Prog. Org. Coat. 49 (2004) 293–301. [38] J.M. Hu, J.Q. Zhang, C.N. Cao, Determination of water uptake and diffusion of Clion in epoxy primer on aluminum alloys in NaCl solution by electrochemical impedance spectroscopy, Prog. Org. Coat. 46 (2003) 273–279.