Influence of duty cycle on properties of the superhydrophobic coating on an anodized magnesium alloy fabricated by pulse electrodeposition

Accepted Manuscript Title: Influence of duty cycle on properties of the superhydrophobic coating on an anodized magnesium alloy fabricated by pulse electrodeposition Authors: Yufen Zhang, Tiegui Lin PII: DOI: Reference:

S0927-7757(19)30080-9 https://doi.org/10.1016/j.colsurfa.2019.01.078 COLSUA 23176

To appear in:

Colloids and Surfaces A: Physicochem. Eng. Aspects

Received date: Revised date: Accepted date:

15 December 2018 31 January 2019 31 January 2019

Please cite this article as: Zhang Y, Lin T, Influence of duty cycle on properties of the superhydrophobic coating on an anodized magnesium alloy fabricated by pulse electrodeposition, Colloids and Surfaces A: Physicochemical and Engineering Aspects (2019), https://doi.org/10.1016/j.colsurfa.2019.01.078 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Influence of duty cycle on properties of the superhydrophobic coating on an anodized magnesium alloy fabricated by pulse electrodeposition Yufen Zhanga,b, Tiegui Lina,* a

b

College of Engineering, Shanxi Agricultural University, Taigu 030801, Shanxi China

School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, China

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

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Corresponding author. Dr. Tiegui Lin E-mail addresses: [email protected]; [email protected] Tel: +86-354-6288339 Fax: +86-354-6289686 Graphical abstract

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The superhydrophobic coating prepared by pulse electrodeposition with 50% duty cycle

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exhibits the superior corrosion resistance.

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Abstract Superhydrophobic coatings based on calcium stearate were prepared successfully by direct current and pulse electrodeposition on an anodized magnesium alloy. The duty cycle have great effects on the morphology, surface wettability and thickness of the coatings,

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without affecting the phase composition. The corrosion resistance of the coated substrates in simulated body fluid was investigated as well. The results indicate that the coatings fabricated

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under different deposition modes and duty cycles exhibit different corrosion resistance. The

coating under pulse mode with 50% duty cycle can provide the best corrosion protection for

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the substrate.

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Keywords: Mg alloy; Superhydrophobicity; Electrodeposition; Corrosion resistance

1. Introduction

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Magnesium and magnesium alloys are considered as one of the important engineering

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materials due to the outstanding properties, for instance, low density, high strength and excellent machinability [1-3]. Besides that, magnesium and its alloys are very promising biodegradable materials for orthopedic implants and vascular stents, thanks to their

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degradation in physiological environment [4, 5]. However, because of the low standard potential and high chemical reactivity, the corrosion resistance of magnesium and its alloys is poor, especially in moist atmosphere or corrosive medium containing chloride anions [6, 7]. This weakness has greatly restricted their applications in the biomedical field and other 2

engineering areas. In order to improve the corrosion resistance of magnesium and its alloys, various coating systems have been applied, such as calcium phosphate coating [8-10], micro-arc oxidation coating [11-13], polymer coating [14] and other coatings [15-17]. In these coatings,

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superhydrophobic coating is considered as one of the promising ways to increase the corrosion resistance of magnesium and its alloys. Superhydrophobic coating inspired by the

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lotus leaf has the water contact angle larger than 150 ° and sliding angle smaller than 10 °,

which can prevent the water droplet to contact with the metallic substrate easily [18], since

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the micro/nanostructure on the surface can trap air. What’s more, superhydrophobic coating

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has special interest in both academic and industry areas due to its self-cleaning [19, 20],

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anti-icing property [21-23] and oil/water separation [24, 25].

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Up to now, a number of methods have been developed to ship superhydrophobic coating

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on magnesium and its alloys. Usually, a two step method is utilized including the formation of a rough surface with micro/nanostructure followed by the modification of the rough surface

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using low surface energy substance [26, 27]. Currently one-step method to fabricate

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superhydrophobic coating on magnesium and its alloys has been proposed, for example, immersion process [28], hydrothermal method [6, 7, 29] and electrodeposition [30]. Zhao et al. [28] prepared a superhydrophobic surface on AZ31 with immersion process. The

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magnesium alloy substrate was immersed in the prepared solution for 2 h, subsequently the specimen was dried at 60 ℃ for 2 h. Feng et al. [6] created a superhydrophobic coating on AZ91 plate using one-step hydrothermal process at 80 ℃ for 10 h. Zheng et al. [7] fabricated a superhydrophobic coating on AZ31 via one-step hydrothermal method. The 3

hydrothermal temperature was 170 ℃ and the time was varied from 4 h to 12 h. Zhang et al. [29] also manufactured a superhydrophobic coating on AZ31 by hydrothermal method with the different temperatures (50 – 80 ℃) and different time (1 – 3 h). From the above, it is easy to find that either the immersion method or hydrothermal process is time-consuming.

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Compared with the above two methods, the electrodeposition is time-saving and high efficiency. Liu et al. [30] constructed a superhydrophobic coating by electrodepositing MB8

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magnesium plate with a constant voltage of 30 V for only 10 min. In addition, the surface

morphology and coating property can be controlled easily by adjusting the process parameters,

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for instance, deposition time, composition of the electrolyte, electrical parameters and so on.

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Due to these appealing advantages, electrodeposition has aroused huge interest among the

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researchers.

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In recent years, direct current (DC) electrodeposition has been utilized to prepare the

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superhydrophobic coatings on metal substrate [31-35]. Nevertheless, to our knowledge, only a few researches reported on the preparation of superhydrophobic coating via pulse

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electrodeposition [36-38]. Compared with the DC mode, pulse electrodeposition can realize

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intelligent regulation of the pulse parameters so as to better control the surface microstructure [36]. Although pulse electrodeposition shows superiority over the DC electrodeposition, systematic study on the morphology, wettability and electrochemical behavior of the

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superhydrophobic coating prepared by pulse electrodeposition is scarcely conducted. In this paper, the pulse electrodeposition is used to prepare calcium stearate superhydrophobic coating on AZ21 magnesium alloy. And the results of the coating under direct current electrodeposition will be used as the reference. Noteworthy is the fact that the 4

superhydrophobic coating can be electrodeposited successfully on AZ21 without any pre-treatment. Nevertheless, anodizing is applied to ship a rough surface prior to the electrodeposition so as to increase the adhesion between the magnesium alloy and the superhydrophobic coating. We will focus on the effect of duty cycle on the morphology,

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surface wettability and thickness of the coatings. Moreover, the electrochemical behavior of the superhydrophobic coatings fabricated with different modes and duty cycles will be

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reported. The corrosion resistance of these coatings will be studied and discussed in detail.

2. Experimental details

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2.1 Materials and specimen preparation

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The chemical composition of the received magnesium alloy was analyzed by using Arc

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Spark OES (Spark analyser M9, Spectro Ametek, Germany). The composition of the magnesium alloy was shown in Table 1. The magnesium alloy was cut into a size of 40 mm

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× 60 mm × 0.6 mm and mechanically ground to 1200 grit SiC using water as a lubricant,

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ultrasonically degreased with deionized water and ethanol and dried in air. Anodizing treatment was conducted in the ethanol based electrolyte containing calcium

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nitrate (0.045 mol L-1) and magnesium nitrate (0.005 mol L-1). The prepared magnesium alloy was connected to the anode, with another magnesium alloy connected to the cathode. During

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anodization, these plates were kept apart at a constant distance of 2.0 cm. The area of the magnesium alloy exposed to the electrolyte was about 16 cm2. Coating formation was carried out using the electrochemical workstation (Potentiostat/Galvanostat Model 273A, America). All samples were processed for 10 min at 100 V. The average thickness of the obtained anodized layer was about 6 μm. 5

The superhydrophobic coating was electrodeposited on the surface of the anodized magnesium alloy with the same equipment for the anodizing treatment. The ethanol based electrolyte containing calcium nitrate (0.05 mol L-1) and stearic acid (0.05 mol L-1) was prepared. Two electrodes were used in the electrodeposition: the cathode was the anodized

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magnesium alloy and the anode was a graphite sheet. The superhydrophobic coating was deposited using DC and pulse electrodeposition modes. Square wave potential with a pulse

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period T (ton ＋ toff) of 1 s was applied in the case of pulse mode. The duty cycle θ (θ= ton

/( ton ＋ toff)) varied from 80% to 20%. The total deposition time was 60 min in all cases. All

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of the electrodeposition processes were performed at a constant voltage of 50 V. Fig. 1

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represents the typical potential square wave signal. After the deposition, the specimen was

surface

morphologies

and

cross

sectional

morphological

features

of

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The

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2.2 Sample Characterization

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taken out from the electrochemical bath and dried naturally.

superhydrophobic coatings were observed by a field emission scanning electron microscope

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(SEM Quanta 200, FEI Co., America). When prepared the cross sectional samples, the

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coatings fractured naturally and the cross sections of the deposited coatings exposed. After that, the cross sectional samples were rotated 45 ° relative to the horizontal direction for SEM observation. Prior to SEM study, all the samples were coated with a thin gold layer.

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X-ray diffraction (XRD, X’Pert, Philips, Holland) was performed using an X-ray

diffractometer with Cu-Kα radiation to determine the phase composition of the superhydrophobic coatings. The processes were carried out by the grazing incidence method at 40 kV and 40 mA, with a scanning rate of 0.1 °/min. 6

Water contact angle (WCA) on the superhydrophobic coating was measured using a contact angle meter (Dataphysics OCA20, Germany) with about 3 μL deionized water at room temperature. Totally three data were obtained on different surface parts of the sample and the reported value was the average.

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2.3 Electrochemical measurements Electrochemical studies were carried out using a Gamry (Reference 3000) computer

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controlled potentiostat. A typical three electrode system was used consisting of a graphite rod

as the counter electrode, a saturated calomel reference electrode (SCE) and the specimen (1

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cm2 exposed area) as working electrode. The experiments were conducted in a simulated body

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fluid (SBF) at 37 ± 0.5 ℃. The detailed chemical composition of SBF was listed in Table 2,

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which was prepared by dissolving reagent-grade chemicals of NaCl, NaHCO3, KCl,

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K2HPO4·3H2O, MgCl2·6H2O, CaCl2 and Na2SO4 into distilled water and buffering at pH 7.4

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with trishydroxymethyl aminomethane ((CH2OH)3CNH2) and 1.0 mol L-1 HCl at 37 ℃ [39]. Prior to potentiodynamic polarization measurement, the samples were exposed for 30

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min to electrolyte to establish a relatively stable open circuit potential. The potential was

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scanned at a scan rate of 0.5 mV s-1. Electrochemical impedance spectroscopy (EIS) experiments were performed in the frequency ranges between 10-2 and 105 Hz, with a sinusoidal signal perturbation of 10 mV at open circuit potential. The experimental EIS

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spectra were interpreted based on equivalent electrical analogues using the program ZSimpWin to obtain the fitting parameters. All electrochemical measurements were repeated three times with good reproducibility.

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3. Results and discussion 3.1 Phase composition and morphology of the coating Fig. 2 shows the phase composition of the anodized magnesium alloy substrate along with the coatings deposited on the substrate by DC and pulse electrodeposition mode with

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various duty cycles. It can be seen that the main phase composition of the anodized magnesium alloy is MgO and all of the deposited coatings are composed of calcium stearate

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(C36H70CaO4). Mg peaks which come from the substrate are seen. The intensity ratio between

C36H70CaO4 (the peak at 21.725 °) and Mg (the peak at 34.475 °) is 0.686 when the coating is

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formed by DC. The intensity ratio is 0.675, 0.308 and 0.118 respectively, when the coating is

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fabricated by pulse electrodeposition with the duty cycles of 80%, 50% and 20%. The

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variations in ratios suggest that the thickest coating appears under DC and the thickness of the

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coatings decrease with the decrease of the duty cycle under pulse mode.

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According to the analysis above, the phase composition of the fabricated coatings is calcium stearate, which is irrelevant to the deposition mode and duty cycle. In this case, the

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reaction equations can be proposed as the following equations.

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2CH3 (CH2 )16 COOH + Ca2+ → Ca[CH3 (CH2 )16 COO]2 + 2H +

(1)

2H + + 2e− → H2 ↑

(2)

Under DC mode or the on-time of the pulse electrodeposition, the calcium ions (Ca2+)

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near

the

cathode

react

with

CH3 (CH2 )16 COOH

leading

to

the

formation

of

Ca[CH3 (CH2 )16COO]2 . Hydrogen ions (H + ) are generated simultaneously on the cathode plate. The free hydrogen ions can capture electrons and form hydrogen bubbles, which have been proved by the experimental phenomena. 8

Surface morphology is one of the decisive factors of the surface wettability. Fig. 3 shows the surface morphologies of the anodized magnesium alloy along with the coatings prepared with DC and pulse electrodeposition. From Fig. 3a, it can be seen that many cracks form on the surface of the anodized layer, which is considered to be caused by internal stress. Fig. 3b

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reveals the formation of flower-shaped protrusion for the coating deposited by DC mode. However, the size of the protrusions is found to be very uneven at the micro scale, which has

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the average diameter of about 24.4 μm. In the case of pulse mode, flower-shaped protrusions

are observed as well (Fig. 3c-Fig. 3e). The average size of the protrusions decreases

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dramatically, from about 21.5 μm (Fig. 3c), 18.9 μm (Fig. 3d) to 12.8 μm (Fig. 3e) with the

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duty cycle reduction from 80%, 50% to 20%. Compared with the DC deposited coating, the

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pulse deposited coatings seem to be more homogeneous. The micron-scale protrusions are

porous

nanostructures

on

and

around

the

protrusions.

The

hierarchical

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the

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uniformly distributed and their sizes are finer. The inset images in Fig. 3b-Fig. 3e demonstrate

micro/nanostructure might bring about the superhydrophobicity of the deposited coatings.

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The formation of the coating is comprised of two steps: formation of the crystal nuclei

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and growth of the crystals. The size of the protrusions is related to the nucleation rate and growth rate. The cathode polarization is stronger under pulse mode than DC, resulting in the faster crystal nucleation rate and more nuclei number of the coatings formed with pulse mode

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[40]. Moreover, the off-time in pulse electrodeposition inhibits the crystal growth [36]. Thus the size of protrusions with pulse mode is finer than that deposited with DC. Furthermore, with the increase of the off-time (the decrease of the duty cycle from 80% to 20%) the protrusions become finer and finer. 9

A 45 ° rotation cross-sectional image of the coated substrates is displayed in Fig. 4 to compare the relative thickness of the deposited coatings. The observed cross-sections are natural fracture surfaces of the deposited coatings without any artificial scraping. The average thickness of the DC deposited coating is about 20.0 μm (Fig. 4a) while the average

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thicknesses of the pulse deposited coatings at 80%, 50% and 20% duty cycles are about 7.6 μm (Fig. 4b), 7.2 μm (Fig. 4c) and 4.5 μm (Fig. 4d), respectively. In general, the pulse

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deposited coatings are thinner compared to the DC deposited coating, owing to the off-time intervals of pulse electrodeposition. Furthermore, some irregular pores appear in the DC

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deposited coating (Fig. 4a), which might be disadvantageous for its corrosion resistance. By

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contrast, the pulse deposited coatings (Fig. 4b- Fig. 4d) are free of such defects.

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3.2 Surface wettability of the coating

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Water contact angle measurements were employed to research the wettability of the

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deposited coatings. As shown in Fig. 5, the water contact angle of the anodized magnesium alloy substrate is 92.4 °. The water contact angle of the DC deposited coating is 149.9 ° while

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those of the pulsed coatings are greater than 150.0 °. The duty cycle of the pulse mode ranges

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from 80%, 50% to 20%, the water contact angles of the deposited surfaces are 156.1 °, 158.0 ° and 157.8 °, respectively. Compared with the anodized layer, the deposited coatings enhance the hydrophobicity of the surface. It is obvious that the pulse deposited coatings at various

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duty cycles exhibit superhydrophobic behavior [41]. With respect to the surface morphologies of the coatings (Fig. 3b- Fig. 3e), the finer of the protrusions and the uniform of their sizes are beneficial to the superhydrophobicity. To further understand the influence of duty cycle on the wettability, Cassie-Baxter model 10

is employed to explain the phenomenon. According to the Cassie-Baxter equation [42]: cos𝜃𝑟 = 𝑓1 cos𝜃 − (1 − 𝑓1 )

(3)

In the equation above, 𝑓1 is fractional area of the solid on the coating surface, (1 − 𝑓1 ) is that of the air on the coating surface. 𝜃𝑟 and 𝜃 are the static water contact angles of the

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coating and anodized magnesium alloy substrate, respectively. According to the experiment results, the contact angle of the anodized substrate (𝜃) is 92.4 °. Thus the 𝑓1 value of the

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coating (𝜃𝑟 ) formed with DC is 13.9%. And those of the coatings deposited under pulse 80%,

50% and 20% duty cycle are respectively 8.9%, 7.5% and 7.7%. Obviously, 𝑓1 values of the

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pulsed coatings are smaller than that of the DC coating and the coating under pulse 50% duty

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cycle has the lowest solid fractional area, indicating that the coating under pulse 50% duty

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cycle has the highest air fractional area, which enhances the superhydrophobicity of the

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coating surface.

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3.3 Corrosion behavior

For simplification, the coated sample obtained by DC mode is named as DC and those

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deposited by pulse mode at 80%, 50% and 20% duty cycles are named as PL-80%, PL-50%

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and PL-20%, respectively.

3.3.1 Potentiodynamic polarization curve In order to investigate the corrosion resistance of the coatings fabricated by DC and

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pulse modes, the potentiodynamic polarization curves of the anodized magnesium alloy substrate and the coated substrates were analyzed. As shown in Fig. 6, all the coated samples possess an obvious passive region in the polarization curves in comparison to that of the substrate. This indicates that the corrosion resistance of the substrate is evidently exalted by 11

the deposited coatings. Comparing the polarization curves of the coated samples, all the polarization scans show similar shape over the potential domain examined, which means the similar corrosion behavior. The values of pitting potential (Epit) derived from Fig. 6 are listed in Table 3. These values of Epit are different for the coated substrates, illustrating different

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corrosion resistance. It can be found that PL-50% has the highest Epit value, followed by sample PL-80%, sample DC and sample PL-20%. This suggests that PL-50% owns the best

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corrosion resistance while PL-20% shows the worst. 3.3.2 Electrochemical impedance spectroscopy

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The electrochemical impedance spectra of the coated samples immersed into the SBF at

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37 ± 0.5 ℃ for different periods are depicted in Fig. 7. The symbols are the experimental data

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and the solid lines are fitting data. It is noted that the experimental curves are little unstable

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during the immersion period and the spectra of PL-80% and PL-50% at the initial immersion

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periods can be collected only within the high frequency range (Fig. 7b and Fig. 7c), due to the air layer entrapped by the hierarchical micro/nanostructure of the coating surface. This results

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in the poor electro-conductivity of the coating [43, 44], especially at the initial stage. In order

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to get the good fitting results, some scattering experimental points are deleted. Note that the impedance modulus of all the samples decreases over the immersion period, indicating the decrease of the corrosion resistance with the immersion time.

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Fig. 7a is the impedance spectra of the DC sample. Within 12 h immersion, two

well-defined time constants are observed from the phase angle variation. The time constant at high frequency (103-105 Hz) is related to the response of the outer superhydrophobic coating, and that at the middle frequency (1-103 Hz) can be ascribed to the inner anodized layer. From 12

24 h to 96 h, a new time constant appears at the low frequency range, which corresponds to the electrochemical double layer. Fig. 7b and Fig. 7c show the impedance spectra of the PL-80% and PL-50%, respectively. Initially, the phase angle is about 90 ° and the slope of the impedance modulus

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against the log (frequency) is around -1 at the high frequency range for both samples. The phenomena suggest the high corrosion resistance of the coating in SBF at the early immersion

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stage [45]. Only one time constant can be observed for PL-80% at 0 h-12 h and PL-50% at 0

h-24 h. From 24 h to 48 h, the time constant evolves from two to three for PL-80%.

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Meanwhile, the phase angle curve shows three time constants at 48 h for PL-50%. In any

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cases, the time constant at high frequency is associated with the superhydrophobic coating,

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and that at the middle frequency can be ascribed to the anodized layer. The time constant

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appearing at the low frequency is due to the electrochemical double layer, which demonstrates

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that the SBF solution has penetrated into the superhydrophobic coating and anodized layer and reached the magnesium alloy surface.

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The impedance spectra of the sample PL-20% are displayed in Fig. 7d. At 0 h immersion,

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two time constants are respectively distributed at high and low frequency, corresponding to the superhydrophobic coating and anodized layer. From 1 h to 24 h, three well-defined time constants exist from the phase angle curves. They are distributed at high frequency (104-105

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Hz), middle frequency (10-104 Hz) and low frequency (10-2-10 Hz), which respectively correspond to the outer superhydrophobic coating, the inner anodized layer and the electrochemical double layer. The appearance of the electrochemical double layer means the corrosion of the magnesium alloy via SBF. After 48 h immersion in SBF, the time constant at 13

high frequency disappears, indicating that the superhydrophobic coating cannot protect the substrate any more. According to the analysis above, the equivalent circuit models used to fit the impedance spectra of the coated samples immersed in SBF at 37 ± 0.5 ℃ for different times are proposed

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in Fig. 8. The model in Fig. 8a is utilized to simulate the impedance of PL-80% at 0 h and 12 h, PL-50% at 0 h and 24 h. Sample DC at 0 h and 12 h, PL-80% at 24 h and PL-20% at 0 h

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can be presented as the circuit shown in Fig. 8b. Fig. 8c is proposed for the remaining time of

DC, PL-80% and PL-50% along with PL-20% at 1 h and 24 h. Sample PL-20% at 48 h and 96

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h can be indicated as the circuit shown in Fig. 8d. In order to get the better fitting results, CPE

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(the constant phase element) is used instead of the capacitor in these equivalent circuits. Rs is

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the solution resistance, CPEcoat and Rcoat describe the capacitance and resistance of the

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superhydrophobic coatings. CPEano and Rano are the capacitance and resistance of the

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anodized layer. CPEdl and Rct represent the reaction of the interface between the magnesium alloy and the SBF solution, where CPEdl is the electrochemical double layer capacitance and

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Rct stands for the transfer resistance.

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The fitting results are depicted in Fig. 9. From Fig. 9a, it can be found that the Rcoat value of PL-80% is slightly higher than that of PL-50% at the initial immersion stage. However, the Rcoat value of PL-80% decreases sharply after 12 h. Overall, the sample PL-50% shows the

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highest Rcoat value for the longer immersion process. By contrast, PL-20% presents the lowest Rcoat value. Changes in the Rano values during the immersion are also observed from Fig. 9b. It may be caused by the different characteristics of the superhydrophobic coatings. The Rct value (Fig. 9c) of PL-50% is obviously the highest for the whole immersion time while that of the 14

PL-20% is the lowest, which demonstrates that the sample PL-50% shows the best corrosion resistance and PL-20% is the worst. This result is consistent with that from potentiodynamic polarization curve (Fig. 6). Based on the analysis above, it is thought that the duty cycle affects the wettability,

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thickness and other characteristics of the coatings, which further influences the corrosion protective performance. Though DC sample has the thickest coating, the corrosion resistance

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is not the highest because of the lowest water contact angle (Fig. 5). Additionally, obvious pores appearing in the coating (Fig. 4a) will be paths of the water and aggressive ions

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penetrate into the coating. Under pulse electrodeposition, the coatings obtained with different

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duty cycles have different corrosion resistance. Sample PL-50% has the best corrosion

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resistance due to the highest water contact angle up to 158.0 ° and thicker coating. Sample

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PL-20% owns a higher water contact angle of 157.8 °, however, the coating is too thin to

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provide a long-term protection for the substrate. 4. Conclusions

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The successful deposition of calcium stearate coating on an anodized magnesium alloy

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was demonstrated by using DC and pulse electrodeposition. The effects of duty cycle on the phase composition, morphology, thickness, surface wettability and corrosion resistance of the coatings have been studied. Based on the results revealed, the main conclusions can be drawn

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as follows:

1. The duty cycle has significant effects on the morphology, surface wettability and thickness of the coatings and consequently on the corrosion resistance, however, it does not affect the phase composition of the coatings. 15

2. The protrusions of the coatings under pulse electrodeposition are finer and homogeneous. The well-distributed hierarchical micro/nanostructure and the low surface energy of calcium stearate are responsible for the superhydrophobicity. 3. The sample PL-50% owns the highest water contact angle and thicker coating, ensuring the

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superior corrosion resistance.

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Conflicts of interest

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There are no conflicts to declare.

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Acknowledgement

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This work was supported by the Outstanding Doctor Reward Fund of Shanxi [grant

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number SXYBKY201718]; and the Science and Technology Innovation Fund of Shanxi

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Agricultural University [grant number 2017YJ14]. References

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Figure/Table captions

ED

M

A

N

U

SC R

corrosion barrier, Corros. Sci. 53 (2011) 2080-2086.

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Fig. 1. Schematic representation of pulse potential waveform for electrodeposition at duty

A

CC E

cycle (θ) = 50%.

Fig. 2. X-ray diffraction patterns of the anodized magnesium alloy (a) and the coatings 22

deposited by (b) DC and pulse electrodeposition mode with different duty cycles: (c) 80%, (d)

SC R

IP T

50% and (e) 20%.

Fig. 3. SEM images of the anodized magnesium alloy (a) and the deposited surfaces prepared by (b) DC and pulse electrodeposition mode with different duty cycles: (c) 80%, (d) 50% and

A

CC E

PT

ED

M

A

N

U

(e) 20%. The insets are high magnification images respectively.

Fig. 4. The 45 ° rotation surface morphologies of the coated samples obtained by (a) DC and pulse electrodeposition mode with different duty cycles: (b) 80%, (c) 50% and (d) 20%.

23

IP T SC R

Fig. 5. Variation in the static water contact angles of the anodized magnesium alloy and the

U

surfaces obtained by DC electrodeposition and pulse electrodeposition mode with different

CC E

PT

ED

M

A

N

duty cycles.

Fig. 6. Polarization curves of anodized magnesium alloy substrate and the coated substrates in

A

SBF at 37 ± 0.5 ℃.

24

Fig. 7. Bode plots evolution of the coated substrates in SBF at 37 ± 0.5 ℃ (symbols are experimental data and solid lines are fitting data): (a) DC, (b) PL-80%, (c) PL-50%, (d)

A

N

U

SC R

IP T

PL-20%.

M

Fig. 8. Equivalent circuit models for the impedance spectra during different SBF immersion

ED

periods at 37 ± 0.5 ℃: (a) 0 h – 12 h for PL-80% and 0 h – 24 h for PL-50%, (b) 0 h – 12 h for DC, 24 h for PL-80% and 0 h for PL-20%, (c) 24 h – 96 h for DC, 48 h – 96 h for PL-80%,

A

CC E

PT

48 h – 96 h for PL-50% and 1 h – 24 h for PL-20%, (d) 48 h – 96 h for PL-20%.

25

IP T SC R U N A M ED PT

A

CC E

Fig. 9. EIS fitting parameters for the coated substrates: (a) Rcoat, (b) Rano, (c) Rct.

26

IP T

Table 1 Chemical composition of the received material.

SC R

Table 2 Reagents and amounts for preparing 1000 ml of simulated body fluid (SBF).

N

U

Table 3 The pitting potential of the samples as read from Fig. 6.

Al

Cu

1.920

M

Ele ment

00

0

Mn

Fe

Ni

0.377

0.0120

0.001

00

0

08

Zn

Be

Si

Ag

Ca

1.180

0.0000

0.051

<0.000

0.004

PT

Ele

0.0133

ED

wt.%

A

Table 1 Chemical composition of the received material.

ment

CC E

wt.%

00

A

Ele

4

90

10

44

Sn

Zr

Ce

La

Mg

<0.00

<0.00

0.005

0.0141

96.42

ment

wt.% 050

060

35

27

0

000

Table 2 Reagents and amounts for preparing 1000 ml of simulated body fluid (SBF). Concentration

NaCl

8.035 g

NaHCO3

0.355 g

KCl

0.225 g

K2HPO4·3H2O

0.231 g

MgCl2·6H2O

0.311 g

1.0 M-HCl

39 ml 0.292 g

Na2SO4

0.072 g

SC R

CaCl2

Tris

6.118 g 0-5 ml

N

U

1.0 M-HCl

A

Table 3 The pitting potential of the samples as read from Fig. 6. Epit (V vs. SCE) -1.62 ± 0.02 -0.47 ± 0.06

ED

DC

M

Samples Substrate

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Composition

-0.28 ± 0.04

PL-50%

1.17 ± 0.03

PL-20%

-1.15 ± 0.02

A

CC E

PT

PL-80%

28