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Synthesis and electrochemical behavior of a magnesium fluoride-polydopamine-stearic acid composite coating on AZ31 magnesium alloy
Surface & Coatings Technology 307 (2016) 56–64
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Synthesis and electrochemical behavior of a magnesium fluoride-polydopamine-stearic acid composite coating on AZ31 magnesium alloy Lijun Zhang a,b,c, Elbeshary A.A. Mohammed b, Annemie Adriaens b,⁎ a b c
College of Sciences, Zhejiang A & F University, Lin'an 311300, People's Republic of China Department of Analytical Chemistry, Ghent University, Krijgslaan 281 (S12), Ghent 9000, Belgium Key Laboratory of Chemical Utilization of Forestry Biomass of Zhejiang Province, Zhejiang A & F University, Lin'an 311300, People's Republic of China
a r t i c l e
i n f o
Article history: Received 27 June 2016 Revised 5 August 2016 Accepted in revised form 7 August 2016 Available online 08 August 2016 Keywords: AZ31 magnesium alloy Polydopamine Stearic acid Composite coating Corrosion resistance
a b s t r a c t To improve the corrosion resistance of AZ31 magnesium alloys, a magnesium fluoride-polydopamine-stearic acid (MgF2/PDA/SA) composite coating was synthesized on a AZ31 substrate in an easy and low cost way by successively forming a polydopamine (PDA) layer and a stearic acid (SA) layer on a HF-treated magnesium alloy surface. SEM observations show that the MgF2/PDA/SA coating presents a more compact and uniform morphology compared to the MgF2/SA coating. Impedance and polarization analyses conducted in 3.5 wt% NaCl aqueous solution show that the MgF2/PDA/SA coated Mg alloys are significantly improved in corrosion resistance, with a remarkably low corrosion current density and three orders of magnitude in corrosion resistance higher than in the case of the bare AZ31 alloy. It is found that the formation of PDA layer plays a key process in the formation of MgF2/ PDA/SA composite coating. The PDA layer not only acts as an efficient anchoring layer between the Mg alloy surface and stearic acid layer, but also plays a protective role on Mg alloy substrate. © 2016 Elsevier B.V. All rights reserved.
1. Introduction In recent years, magnesium and its alloys have been widely used as important structural materials in a variety of industries such as the automotive, computer and aerospace. This is due to their low density, high specific strength, good metal cutting performance and other desirable properties [1]. However, the large-scale application of magnesium and its alloys are yet hindered by their poor corrosion resistance [2]. Surface treatment is generally used as one of the most cost-effective methods to prevent magnesium and its alloys from corrosion. In this case, various surface treatments and coating techniques have been proposed and developed for magnesium and its alloys [3,4], such as surface conversion, anodizing, laser/electron/ion beam treatment, galvanizing/ plating, sol–gel coating and organic coating/plating. Among these techniques, an organic coating is regarded as the most cost-effective method for magnesium and its alloys [5]. Generally, the primary composition of an organic coating is a resin, which has various kinds, such as acrylic polyurethane, vinyl, polyvinyl butyral, epoxy and phenolic containing chromate [6]. Because of the low environmental impact, the natural substances, such as long-chain aliphatic mono-carboxylic acids, have been placed an increasing interest on anti-corrosion of magnesium ⁎ Corresponding author. E-mail addresses: [email protected] (L. Zhang), [email protected] (E.A.A. Mohammed), [email protected] (A. Adriaens).
http://dx.doi.org/10.1016/j.surfcoat.2016.08.021 0257-8972/© 2016 Elsevier B.V. All rights reserved.
and its alloys [7–12]. In brief, the anions of the mono-carboxylic acids should easily react with the oxide film on the metal surface and then, a long aliphatic chain (up to 18 °C atoms) should form well ordered, highly protective and hydrophobic layers through van der Waals (vdW) interactions [13]. However, in fact, the poor adhesion to the substrate usually weakens the anti-corrosion performance of the coating [6]. To deal with this problem, it is very necessary to make some appropriate surface pretreatments on Mg and its alloys, such as wet chemical etching, anodizing, or adding adhesion promoters [5,14–17]. Since Lee et al. reported it in 2007, polydopamine (PDA) films have attracted much attention on biomaterials surface modification by acting as a simple and effective strategy to promote cell adhesion onto substrates with various material types and shapes under non-toxic condition [18]. For example, Park's group found that with the assistance of polydopamine, inorganic hydroxyapatite crystals can be integrated on noble metals, ceramics, and synthetic polymers via interactions between side residues and Ca/P ions [19–21]. For Ti implants, PDA has been widely used to improve their antibacterial activity, bioactivity, and corrosion resistance [19,22,23]. Considering the disadvantages of the too fast degradation rate of the magnesium and its alloys as implants, some researchers studied the PDA layers not only for adhesion promotion but also for anti-corrosion performance. In these studies, Mg and its alloys coated by the composite coatings with PDA as intermediate layer exhibited significantly smaller free corrosion current density, an obviously lower degradation rate as well as good cytocompatibility
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in vitro compared to uncoated samples [17,24–27]. As shown above, PDA was proved to have very effective adhesion and anti-corrosion performance between the substrate and organic/inorganic coatings. Inspired by the above findings, this present work was devoted to study the application of the stearic acid coating with the assistance of PDA to reduce the corrosion rate of AZ31 magnesium alloy under environmental condition. So far, few reports have been published about such study. In the present work, a magnesium fluoride-polydopamine-stearic acid (MgF2/PDA/SA) composite coating was fabricated on AZ31 Mg alloy in an easy and low cost way by forming polydopamine (PDA) layer and stearic acid (SA) layer on HF-treated magnesium alloy surface successively. Moreover, the resulting coatings were characterized with the use of scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR), and water contact angle. The corrosion inhibition properties of those coatings were studied compared to uncoated sample using potentiodynamic polarization experiments and electrochemical impedance spectroscopy (EIS) in 3.5 wt% NaCl aqueous solution. The assessment of degradation behavior of the MgF2/PDA/SA coating in 3.5 wt% NaCl aqueous solution was also carried out by EIS. 2. Experimental 2.1. Materials and surface treatment AZ31 Mg alloy coupons (composition: 3.1 wt% Al, 0.03 wt% Si, 0.05 wt% Ca, 0.82 wt% Zn, 0.34 wt% Mn, 0.1 wt% Be, 0.005 wt% Fe, and the rest is Mg) with a diameter of 12.6 mm and a height of 1.05 mm were designed to fit in a special holder for the electrochemical measurements. The coupons were first ground with P120, P600, P1200 and P4000 grit SiC paper in turn. Then, the surface was polished using a polishing cloth (PSA 10/PK, Buehler, USA) with diamond polish dissolved in oil. Diamond particles of size 6 μm were used first, subsequently 1 μm (MetaDi polycrystalline diamond suspension, Buehler, USA). Thereafter the coupons were cleaned ultrasonically in isopropanol three times for 5 min each, with fresh propanol for each cleaning cycle, then dried in air and stored in a desiccator. In general, a PDA film could be formed on the Mg alloy by immersing the coupons in dopamine hydrochloride Tris-HCl buffer solution, but the Mg alloy would be corroded severely because of abundant Cl− ions in the solution. To solve this problem, a HF-converted film was coated firstly on Mg alloy to improve the corrosion resistance of the substrate by forming MgF2. For the HF conversion treatment, the coupons were immersed in a plastics bottle containing HF (48%, Emsure, Germany) at room temperature for 48 h. The coated samples (labeled as HFtreated) were rinsed with deionized water, and dried in air. PDA coatings were deposited on the HF-treated samples through simple dip-coating of objects. HF-treated substrates were immersed into dopamine solution (DA, analytical grade, Sigma-Aldrich, USA, 2 mg/mL, absolute ethanol, pH 8.5) at room temperature in darkness for 4 h. After that, the samples coated with PDA (labeled as HF-PDAtreated) were ultrasonically washed in deionized water for 5 min and dried in nitrogen. After that, the HF-PDA-treated and HF-treated AZ31 Mg alloy were both immersed in 0.05 M stearic acid (SA, 95+%, Sigma-Aldrich, USA) ethanol solution for 1 h, labeled as HF-PDA-SA-treated and HF-SA-treated respectively. Then some samples kept for SEM were picked up and dried in air without cleaning, while others were washed with absolute ethanol and deionized water, dried in air and stored in a desiccator for following experiments. 2.2. Surface characterization The surface morphologies of the bare and coated AZ31 samples were observed with a scanning electron microscope (Quantafeg, FEI, USA)
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applying a high voltage of 20 kV. The surface chemical compositions of HF-SA-treated, HF-PDA-treated and HF-PDA-SA-treated AZ31 samples were analyzed by X-ray photoelectron spectroscopy (PHI-5000C ESCA system, PerkinElmer, USA) with Mg-Kα as the radiation source. FTIR spectra were measured in the range 4400 to 600 nm in a Perkin-Elmer Spectrum 1000 spectrometer equipped with a HATR (Horizontal Attenuated Total Reflection) cell from Pike Technologies. Spectra were measured on 5 spots on bare and coated AZ31 samples, respectively. Water contact angles were observed with a video based optical contact angle measurement system (OCA 20) to examine surface wettability of all samples. 1 μL of water was dropped on the surface, and the images of the droplet on the films were captured for 5 s after dropping. Three spots were randomly chosen on each sample surface to make the test and the results were averaged. 2.3. Electrochemical experiments All electrochemical experiments were carried out in a three-electrode electrochemical glass cell with a platinum mesh as a counter electrode and a saturated calomel electrode (SCE, Radiometer Analytical) as a reference electrode. Samples were mounted in a homemade Teflon® working electrode holder. The exposed surface area of the samples in the holder was 0.19625 cm2. All fitted impedance and current values were corrected and normalized related to the surface. All electrochemical measurements were performed in 3.5 wt% NaCl (assay grade 100%, Prolabo, Belgium) aqueous solution at room temperature. EIS measurements were performed for bare, HF-treated, HF-SAtreated, HF-PDA-treated and HF-PDA-SA-treated samples on a potentiostat (Autolab PG-STAT 20, Metrohm, Switzerland) equipped with a frequency response analysis module (FRA module, EcoChemie BV, Utrecht, Netherlands). Data were acquired at the open circuit potential in the frequency range of 100 kHz to 10 MHz with a signal amplitude of 5 mV. Potentiodynamic polarization experiments were also performed for samples mentioned above on the same potentiostat. The OCP was determined firstly, and then the potentiodynamic polarization curves were recorded from −0.3 V to 0.6 V vs. the OCP value with a scan rate of 1 mV·s−1. Both EIS and potentiodynamic polarization experiments were controlled by the Nova 1.10 software and the same software was used to make potentiodynamic polarization curves fitting analysis. The impedance data were fitted by ZView 2.0 software with the corresponding equivalent circuits. 3. Results and discussion 3.1. Surface characterization Fig. 1 displays the surface morphology images (both optical and SEM) of all samples, as well as cross-sectional SEM image of the HFPDA-SA-treated sample before cleaning. The optical images shown in Fig. 1, after 48 h immersion period in 48% HF, demonstrate that the AZ31 Mg alloy turns from an initial metallic grey color to a uniform dark color, and keeps the dark color after the following PDA treatment and SA treatment. Moreover, as shown the red arrows point to, it is visible that the surface of the HF-PDA-SA-treated sample before cleaning was covered with something uneven and incomplete. As SEM images shown in Fig. 1(b), the coating surface of HF-treated sample is dense, with homogenous distributed particles. After the PDA layer deposited, the surface seems rougher as shown in Fig. 1(d). As the middle layer of the composite coating, the rougher morphology of the MgF2/PDA coating would be useful to the attachment of outer layer. After coated by SA layer, the surface of the HF-PDA-SA-treated sample after cleaning (Fig. 1e) appears smooth and more compact. Compared with the HF-PDA-SA-treated sample after cleaning, the surface of HF-SA-treated sample after cleaning (Fig. 1c) showed much more defects such as cracks and pores. These defects may be harmful for the long-term corrosion
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(a)
(b)
(c)
(d)
(e)
(f) 10 mm
(c)
(b)
(a)
(e)
20
(h)
(g)
(f)
2µm
20 µm
20 µm
µm20
(d) 2µm
2µm
2µm
Epoxy resin
2µm
20 µm
500 µm
20 µm
Substrate
20 µm
Fig. 1. Optical images and SEM images of bare AZ31 alloy (a), HF-treated (b), HF-SA-treated after cleaning (c), HF-PDA-treated (d), HF-PDA-SA-treated after cleaning (e), HF-PDA-SAtreated before cleaning (f), enlarged morphology (g) of the red circled area in figure (f), cross-sectional SEM morphology of HF-PDA-SA-treated before cleaning (h). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
resistance of the MgF2/SA coating because the corrosive solution can penetrate into the film inner part through the routes provided by the defects, thus leading to an increased contact area between the corrosive solution and the coating. It is obvious that with the assistance of PDA, the MgF2/PDA/SA coating after cleaning presented a more compact and uniform morphology, which is much beneficial to enhance the corrosion resistance of the coating. It is worthy to note that, as seen in Fig. 1(f), the surface of HF-PDASA-treated sample before cleaning shows a rough morphology and a number of clusters. As shown in Fig. 1(g) (the enlarged morphology of the red circled area in Fig. 1(f)), the clusters are of petal shaped ridges forming a three-dimensional porous structure. According to the literatures [10,26,28,29], air could be trapped in this structure then form a buffer which could prevent water droplets to infiltrate the surface. Thus, a super-hydrophobic surface is formed. But in our present work, we found that these clusters were easy to remove, which led to the surface of coating was fragmentary and uneven. This may be attributed to the formation process of the composite coating. The composite coating was composed with three layers: the MgF2 film coated directly on the AZ31 substrate, the sandwiched PDA layer inserted between MgF2 film and SA layer, and the outermost SA layer. It is well known that one of the most important properties of polydopamine is its strong adhesion to virtually all types of surfaces [30]. According to the literature [30], the Mg alloy coated MgF2 film was likely to be coated with polydopamine via the coordination interaction between metal ions and the catechol group. Meanwhile, the exposed amino groups of the PDA molecules could serve as nucleation sites of stearic acid via acid-base reactions and moreover, a long aliphatic chain of stearic acid, through van der Waals interactions, would form a well ordered, highly protective and hydrophobic layer. On this layer, due to the electrostatic effect and Van der Waals interactions,
multilayers could be formed. The innermost formed by chemical reaction has high bond energy and best adhesion to the solid surface, however, the interactions among outer layers formed by van der Waals interactions are weaker, and the outermost has weakest adhesion to the substrate so it is easy to remove. Therefore, the composite layer should be composed of two parts: a close layer and a loose layer, as shown by the cross-sectional morphology (Fig. 1(h)). As seen in Fig. 1(h), the thickness of the coating (between the two red lines) is about 7.95 μm and obviously, the coating could be divided into two parts, separated by the yellow line. The part below the yellow line exhibits a homogenous compactness and is obviously an excellent adhesive to both the SA coating and the Mg alloy substrate. The thickness of this part is about 5.02 μm. The part above the yellow line shows a distinctive and totally different morphology, which is discontinuous and porous. It is evident that the compact and uniform coating can improve the anti-corrosion property of the substrate. As a result the MgF2/PDA/ SA coating after cleaning was used as effective coating in the following tests. The full XPS spectra of HF-PDA-SA-treated, HF-PDA-treated and HFtreated samples are shown in Fig. 2. The elemental compositions at the surfaces of different Mg alloy specimens are shown in Table 1. It can be observed that the fluorine is present in all XPS spectra, because of the pretreatment of HF acid. For HF-treated sample, the appearance of carbon in XPS surface scan was due to unavoidable hydrocarbon contamination and a small amount of nitrogen was probably the result of adventitious contamination from the environment. Oxygen could originate from hydrocarbons and/or from hydroxides of Mg. After PDA modification, an increase in the carbon and nitrogen contents and a concomitant decrease in the magnesium and fluorine contents are shown in Fig. 2 and Table 1, which confirms the successful anchoring of PDA on the HF-treated surface. The increase in C1s is further
L. Zhang et al. / Surface & Coatings Technology 307 (2016) 56–64
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Fig. 2. XPS full spectra of HF-PDA-SA-treated, HF-PDA-treated and HF-treated samples. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
accentuated after the immobilization of SA on the HF-PDA-treated Mg alloy substrate (Fig. 2) due to long aliphatic chain in stearic acid, demonstrating SA layer was formed on the HF-PDA-treated Mg alloy surface. Meanwhile, judging from Table 1, the C/N ratio and C/F ratio of HFPDA-SA-treated sample are 70.59 and 10.03 respectively, which are much higher than those of HF-PDA-treated sample (10.90 and 3.66). This also verifies that the surface of HF-PDA-treated sample was successfully coated with stearic acid. FTIR characterizations of the samples surfaces are shown in Fig. 3 to further ascertain the PDA layer as well as the SA layer. Adsorption bands located in the range of 1450–1625 cm−1, which originate from the C_C stretching vibration of benzene rings and the N\\H bending, as well as 3347 cm−1 from catechol groups of the PDA structure, confirm that PDA was successfully coated on HF-treated AZ31 substrate [24]. As seen from the spectra of the MgF2/PDA/SA coating, two adsorption peaks at 2920 cm−1 and 2850 cm−1 correspond to the C\\H asymmetric and symmetric stretching vibrations of –CH2– groups of stearic acid, respectively [25]. Moreover, it is known that the carboxyl group from stearic acid appears at 1702 cm−1, resulting from free COO band [26]. So the surface of HF-PDA-SA-treated sample exhibits adsorption peaks at 1561 cm−1 and 1430 cm−1 which may originate from the asymmetric and symmetric stretches of the –COO– group, suggesting that carboxylates were successfully adsorbed on the treated surfaces. This could also prove that the SA was coated onto PDA-coated Mg alloy surface via an acid-base reaction instead of an amidation reaction. Comparatively, the typical adsorption peaks of PDA and SA are not apparent from the spectra of the bare AZ31 Mg alloy and HF-treated surface, but the adsorption peaks of SA appeared in the spectrum of HF-SAtreated sample, which demonstrates SA layer was formed on the surface. Fig. 4 shows the photographs of water droplets on surfaces of the bare AZ31, HF-treated, HF-SA-treated, HF-PDA-treated, and HF-PDASA-treated samples. It can be seen that the wettability of the surface is
Table 1 The chemical element compositions of different Mg alloy specimens. Sample
Mg (at.%)
F (at.%)
C (at.%)
O (at.%)
N (at.%)
HF-PDA-SA-treated HF-PDA-treated HF-treated
3.29 8.34 22.30
7.88 15.28 44.79
79.06 55.92 18.31
8.65 15.33 14.39
1.12 5.13 0.21
obviously changed after the AZ31 substrate was modified by different treatments. The contact angles of the bare AZ31, HF-treated, HF-PDAtreated samples are 80.8° ± 0.31°, 11.8° ± 1.22°, 15.8° ± 2.25°, respectively, indicating a typical hydrophilic materials. However, after immersing in stearic acid ethanol solution for 1 h, the contact angles of the HF-treated and HF-PDA-treated samples increase markedly and reach 102.0° ± 1.3° and 109.3° ± 2.1°, respectively, implying that stearic acid was successfully adsorbed on the substrate and formed the close packed and regularly arranged alkyl tails of the C18A− ions. An increase in hydrophobicity could reduce the adhesive interaction between the water drop and the solid surface, and result in an increscent contact angle. The hydrophobic property of the MgF2/SA coating and MgF2/PDA/SA coating could isolate the substrate from a corrosive solution thus provide good corrosion protection for the Mg alloy substrate. Furthermore, the contact angle of the HF-PDA-SA-treated sample surface is quite similar to the equilibrium contact angle of the C18H35O–2 coated surface of a magnesium alloy which has been reported to be about 109° [11], implying that the MgF2/PDA/SA coating after cleaning was smooth and compact. Apparently, this conclusion is consistent with that of the SEM. 3.2. Electrochemical properties of the coatings The corrosion behavior of different samples was evaluated by electrochemical tests. Fig. 5 shows the potentiodynamic polarization curves of the bare, HF-treated, HF-SA-treated, HF-PDA-treated and HF-PDASA-treated AZ31 Mg alloy samples in 3.5 wt% NaCl aqueous solution. The polarization curve provides much useful information in determining the instantaneous corrosion rate of a substrate, anodic and cathodic behavior of a corrosion reaction. For magnesium and its alloys, the cathodic polarization curve is generally considered to represent the cathodic hydrogen evolution by water reduction, and the anodic one represents dissolution of magnesium or corrosion behaviors of the protective layer [27]. In Fig. 5, it can be observed that the cathodic polarization current densities of the coated samples are all lower than in the case of the bare AZ31, particularly for the HF-PDA-SA-treated sample, its cathodic polarization current density is about two orders of magnitude lower than that of the bare alloy, indicating the MgF2/PDA/SA coating significantly suppresses the cathodic reaction of hydrogen evolution thus the overall corrosion. The corrosion potentials Ecorr and current density Icorr were calculated by the Tafel method through linear extrapolation of cathodic polarization zone [31]. Their values are listed in
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Fig. 3. FTIR spectra of the samples by different treatments. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
a CA: 80.8º ±0.31º
b CA: 11.8º ±1.22º
d
c CA: 102.0º ±1.3º
CA: 15.8º ±2.25º
e
CA: 109.3º ±2.1º
Fig. 4. Images of water droplets on bare AZ31 surface (a), HF-treated surface (b), HF-SA-treated surface (c), HF-PDA-treated surface (d), and HF-PDA-SA-treated surface (e).
Table 2. It is evident that the corrosion current density (Icorr) decreases gradually from HF-treated to HF-PDA-SA-treated AZ31. The HF-PDASA-treated sample shows the lowest corrosion current density among all the samples, by about two orders of magnitude lower than that of the bare AZ31. HF-PDA-SA-treated sample underwent the lowest Icorr might be due to its lowest cathodic current density which is apparently lower than that of HF-SA-treated sample in Fig. 5. It implies that the PDA
Fig. 5. Potentiodynamic polarization curves for bare, HF-treated, HF-SA-treated, HF-PDAtreated and HF-PDA-SA-treated AZ31 Mg alloy after 10 min immersion in 3.5 wt% NaCl aqueous solution. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
layer effectively suppress the cathodic corrosion process of Mg alloy and therefore overall corrosion rate. In addition, another important finding from the polarization curves is that different treatment causes a significant change in the corrosion potentials (Ecorr) and the anodic Tafel curves in the NaCl solution. As shown in Table 2, the HF-treated sample shows the most positive Ecorr compared with the bare and other coated samples. PDA layer and SA layer formed on the HF-treated samples surface respectively/successively all make the Ecorr less noble compared with the Ecorr of HF-treated sample, and the value of the Ecorr, HF-PDA-SA-treated is similar to that of the bare Mg alloy. The more noble Ecorr means more thermodynamically stable of the sample, meanwhile, the lower Icorr means slower corrosion process in kinetics [32]. The results indicate that HF-treated sample seems most thermodynamically stable against corrosion due to the high thermodynamical stability of MgF2 but, the formation of PDA layer and SA layer significantly reduced the corrosion rate of AZ31 by obstruction to the cathodic reaction of hydrogen evolution. As a result, although the HF-PDA-treated sample and bare AZ31 have the similar value of Ecorr, the former corrodes much slower than the latter. In Fig. 5, an obvious passive behavior and a turning point appear in the anodic Tafel curve of each coated sample. When the potential reaches the potential (labeled as Epit in Fig. 5) corresponding to the turning point, the current density increases quickly, indicating the breakdown of the surface coating and the occurrence of the pitting corrosion. The values of | Epit - Ecorr | extracted from the polarization curves in Fig. 5 are also listed in Table 2. As shown in Table 2, the increment of |Epit - Ecorr |, which is as high as 70 mV, implies that the pitting corrosion tendency of the coated sample has been greatly reduced by the formation of the MgF2/PDA/SA composite coating. Meanwhile,
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Table 2 Values of Ecorr, Icorr and |Epit - Ecorr |, extracted from the polarization curves in Fig. 5. Samples
Bare AZ31
HF-treated
HF-SA-treated
HF-PDA-treated
HF-PDA-SA-treated
Ecorr (V) Icorr (μA cm−2) |Epit - Ecorr |(mV)
−1.52 ± 0.03 102.06 ± 10.5
−1.39 ± 0.02 4.71 ± 1.01 90 ± 1.10
−1.46 ± 0.03 3.09 ± 0.75 80 ± 0.91
−1.42 ± 0.02 2.18 ± 0.51 110 ± 1.85
−1.51 ± 0.02 1.85 ± 0.32 150 ± 1.76
there is no passive behavior in the anodic Tafel of bare AZ31, indicating that the original MgO film on bare AZ31 is too thin and incompact to protect the alloy substrate in 3.5 wt% NaCl solution. EIS is a useful technology way to evaluate the protective ability of a coating to the substrate [33]. Fig. 6 shows the Nyquist plots (Fig. 6a) and the impedance magnitude values (Fig. 6b) for the samples by different treatments in 3.5 wt% NaCl solution. The inset of Fig. 6(a) shows the enlarged Nyquist plots for bare, HF-treated and HF-SA-treated AZ31 Mg alloy samples. The results show quite different capacitive loops that can be attributed to the charge transfer resistance of the corrosion process. It discloses that the HF-PDA-SA-treated sample presents evidently an overall largest constant loop among all the samples. As shown in Fig. 6(b), in the low frequency region, the modulus for HF-PDA-treated and HF-PDA-SA-treated samples are about two and three orders of magnitude higher than in the case of the bare AZ31, respectively, meanwhile the modulus for HF-treated and HF-SA-treated samples are under one order of magnitude higher than in the case of the bare AZ31. The increase of the modulus for the coatings with PDA layer is much greater, indicating that the existence of the PDA layer greatly enhanced the corrosion resistance of the coating. This result is in good agreement with the results from potentiodynamic polarization curves. Furthermore, the HF-PDA-SA-treated sample possessed the highest impedance modulus, compared with the bare and other coated samples. This reveals that MgF2/PDA/SA coating has an excellent corrosion resistance for magnesium substrate attributed to protection by PDA layer and stearic acid layer. Determined through the EIS diagrams and the method developed by Wit [34], there are two capacitive loops in the high and middle frequency regions respectively in the EIS plots of coated samples. The capacitive loop in high frequency may arise from the coating and the capacitive loop in middle frequency represents interface reaction between the coating and substrate. The EIS plots of bare Mg alloy have only one capacitive loop in the high frequency region, which represents the interface reaction between the electrolytic solution and the substrate. An inductance behavior in the low frequency region is displayed in each Nyquist plots for all samples, which may be related to the change of the surface state such as the breakdown of the surface coating in NaCl solution [35] or adsorbed/desorbed of corrosion products on the sample [29].
To obtain an accurate analysis of the impedance data, different equivalent circuits are used to fit the EIS data in the high and middle frequency regions in Fig. 6. Two types of equivalent circuits are shown in Fig. 7 and the fitting results are shown in Table 3. Fig. 7 shows the equivalent circuits representing the electrochemical behaviors of the bare AZ31 and the coated samples respectively. In Fig. 7(a), Rct represents the charge transfer resistance, CPEdl is the constant phase element of the electrical double layer, and Rs is the solution resistance. Meanwhile, in Fig. 7(b), CPEc is the constant phase element related to the coating, Rc is the coating resistance. As clearly shown in Table 3, the evolution of Rct is similar to that of Rc, which may be attributed to the increase of Rc should decrease the reaction activity of the coating thus strengthen the protection for the substrate. Moreover the Rct value for HF-PDA-SA-treated sample is nearly 112 times larger than that of bare AZ31, more than 11 times larger than that of HF-PDA-treated and 15 times larger than that of HF-SA-treated. As discussed in the results of water contact angle tests and polarization curves, though both PDA and SA per se are effective to impede electrolyte penetrating into the surface of Mg alloy substrate, their effects seem less notable as compared with the composite MgF2/PDA/SA coating on AZ31 Mg alloy. The inhibition efficiency (IE %) can be calculated from the charge transfer resistance as follows [36]: IE ¼
Rct − Rt0 100 Rct
ð1Þ
where Rct is the charge transfer resistance of the coated AZ31 and Rt0 is the resistance of the bare AZ31. The IEs of the MgF2, MgF2/SA, MgF2/ PDA, MgF2/PDA/SA coatings were estimated to be 71.6%, 88.7%, 91.2%, and 99.2%, respectively. This indicates that our MgF2/PDA/SA composite coating significantly inhibited the corrosion progress. 3.3. Immersion degradation behavior In order to investigate the degradation behavior of the MgF2/PDA/SA coating, EIS measurements were carried out in 3.5 wt% NaCl solution at room temperature with different times. Fig. 8 shows the evolution of Nyquist and Bode plots for HF-PDA-SA-treated AZ31 sample immersed in 3.5 wt% NaCl solution. The inset of Fig. 8(a) shows the enlarged
Fig. 6. Nyquist plots (a) and Bode modulus (b) for bare, HF-treated, HF-SA-treated, HF-PDA-treated, and HF-PDA-SA-treated AZ31 Mg alloy samples in 3.5 wt% NaCl aqueous solution. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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Fig. 7. Equivalent circuits used for the numerical fitting of EIS data for the bare AZ31 sample (a), coated samples (b).
Table 3 Fitting results of EIS plots in Fig. 6. Samples
Bare AZ31 HF-treated HF-SA-treated HF-PDA-treated HF-PDA-SA-treated
CPEc -Y0 F cm−2
CPEc n
Rc Ω cm2
1.81E-04 1.80E-04 1.76E-04 1.10E-04
0,980 0.969 0.971 0.966
36.691 1439.494 1512.112 6905.652
CPEdl Y0 F cm−2
CPEdl n
Rct Ω cm2
7.13E-05 2.08E-05 1.35E-05 9.17E-04 9.12E-05
0.934 0.998 0.986 0.991 0.989
39.486 139.082 349.132 446.075 5203.565
Nyquist plots for the coated sample after 24 h immersion. In Fig. 8(a), the capacitive loops at high and middle frequencies could be attributed to the charge transfer of the corrosion process, and the scatter plots at low frequencies shown as inductance or capacitive behavior indicate that the corrosion process is affected by several other surface-state variables except of the applied potential E [37]. As shown in Fig. 8, the diameter of capacitive loop as well as the impedance modulus (Fig. 8b) decreases gradually with increasing immersion time, indicating that the anticorrosion performance of the MgF2/PDA/SA composite coating is declined with immersion time. The localized corrosion might lead to the reduction of the anticorrosion performance. However, the charge transfer of the HF-PDA-SA-treated sample after immersion in 3.5 wt% NaCl solution for 24 h is still much greater than that of bare AZ31, indicating that the MgF2/PDA/SA composite coating can effectively improve the corrosion resistance of magnesium alloy in the NaCl corrosive solution. Fig. 9 shows the SEM images of the HF-PDA-SA-treated samples after immersion in 3.5 wt% NaCl aqueous solution for 1 h (a), 4 h (b), 7 h (c), 10 h (d) and 24 h (e). The inset of Fig. 9(e) is the overall surface morphology at the magnification of 250 ×. The SEM image of the bare
AZ31 Mg alloy after immersion in 3.5 wt% NaCl solution for 7 h is shown in Fig. 9(f). Compared with the surface morphology before immersion (Fig. 1e), there is almost no change in the surface morphology after immersion in 3.5 wt% NaCl aqueous solution for 1 h (Fig. 9a). After immersion for 4 h, small rips and pits appear on the surface but the coating remains uniform. With the increasing immersion time, more pits are displayed in the surface morphologies (Fig. 9c and d) and the small pits are developed into big and deep pits while more cracks appear on the surface (Fig. 9d). When immersed for 24 h, the surface morphology exhibits typical characterization of pitting corrosion (inset of Fig. 9e). The appearance and development of pitting corrosion should be responsible for the reduction of the impedance. Otherwise, it can be clearly seen in Fig. 9(e) that the corrosion products are accumulated on the pitting area and have begun to be destroyed by Cl−, as a result that many cracks and pores could be seen on the products layer. To some extent, the corrosion products could act as obstacle to prevent the corrosion solution penetrating into the inner part of the coating but, the protection can't sustain long time. The coating without pitting remains relatively uniform and continues to protect the substrate. In contrast, just after 7 h immersion, bare AZ31 Mg alloy suffers serious corrosion attack and big cracks are seen on the surface as shown in Fig. 9(f). It is strongly proved by the SEM images that our MgF2/PDA/SA composite coating can significantly enhance the anti-corrosion property of the Mg alloy substrate. In conclusion, the electrochemical behavior of the HF-PDASA-treated sample and bare AZ31 Mg alloy are in consistent with their corrosion morphologies from SEM observation. For the corrosion of magnesium and its alloys, kinetic factors are the most important determinants due to the very high activity of Mg thermodynamically [38]. Therefore, the slowest step within all kinetic processes will become the rate-determining step (RDS) of overall corrosion reaction. In the present work, the RDS for coated Mg alloy was changed with the different coatings. Because of hydrophobicity of the SA layer and insolubility of the MgF2 film, the RDSs of the corrosion processes were both the ionic pathway which was attributed to the coating barrier preventing electrolyte penetration into the Mg alloy substrate [39]. For PDA-coated, the RDS was changed to the electrical pathway due to the extremely low conductivity of PDA [40]. These could be reflected in the polarization curves of all the coated samples with characteristics of cathodic-control and diffusion-limitation. According to the discussion in the polarization curves and EIS, especially compared with HF-SA-treated sample, the HF-PDA-SA-treated sample showed much lower corrosion rate in 3.5 wt% NaCl solution. This implies that the RDS in corrosion process of Mg alloy coated MgF2/PDA/SA composite coating was likely to be the electrical pathway. Consequently, the cathodic reaction was suppressed effectively because of the delayed arrival of electrons from the anode and the overall corrosion rate slowed
Fig. 8. Time evolution of Nyquist plots (a) and Bode modulus (b) for HF-PDA-SA-treated AZ31 Mg alloy samples immersed in 3.5 wt% NaCl aqueous solution. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
L. Zhang et al. / Surface & Coatings Technology 307 (2016) 56–64
(b)
(a)
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Fig. 9. SEM images of the HF-PDA-SA-treated samples and bare AZ31 Mg alloy immersed in 3.5 wt% NaCl aqueous solution for different times.
down. Moreover, it seems easy to understand the degradation behavior of the composite coating. Table 4 shows the water contact angles on the surfaces of MgF2/PDA/SA composite coating during different immersion time. It could be seen that the hydrophobicity of coating was declined with the increasing immersion time. And the conductivity of PDA was increased gradually as water was absorbed [30]. As a result, the corrosion rate was accelerated due to the faster electron transfer. So in order to improve the long-term durability of the composite coating, how to enhance the hydrophobicity of SA layer will be the key point in future work. 4. Conclusions 1) A MgF2/PDA/SA composite coating was fabricated on AZ31 Mg alloy in an easy and low cost way by forming PDA layer and SA layer on HF-treated magnesium alloy surface successively. 2) MgF2/PDA/SA composite coatings before and after cleaning show totally different surface morphologies. The former exhibits a rough morphology with a number of clusters, and the latter is smooth and uniform. Both after cleaning, it is obvious that with the assistance of PDA, the MgF2/PDA/SA coating presents a more compact morphology compared with the MgF2/SA coating. 3) The MgF2/PDA/SA coated AZ31 Mg alloy showed dramatic improvement in corrosion resistance in 3.5 wt% NaCl aqueous solution by electrochemical tests, with a remarkably low corrosion current density Icorr and three orders of magnitude in corrosion resistance higher than in the case of the bare AZ31 alloy. 4) The formation of PDA layer plays a key process in the formation of MgF2/PDA/SA composite coating. The PDA layer not only acts as an efficient anchoring layer between the Mg alloy surface and stearic acid layer, but also plays a protective role on Mg alloy substrate. Acknowledgements This work was supported by Research Fund - Flanders (FWO) (G.0.068.08.N.10) and National Natural Science Foundation of China Table 4 Water contact angles on the surface of MgF2/PDA/SA composite coating during different immersion time. Time (h)
1
4
7
10
24
Water contact angles
107.1 ±
104.2 ±
95.2 ±
86.5 ±
70.9 ±
1.8
2.5
3.8
6.3
8.7
[grant number 51201153]. The authors would like to thank Olivier Janssens, Nico De Roo and Lara Misseeuw for their assistance with the measurements of SEM, XPS and water contact angle, respectively. References [1] B.L. Mordike, T. Ebert, Magnesium: properties - applications - potential, Mater. Sci. Eng. A. 302 (2001) 37–45. [2] G.L. Song, Recent progress in corrosion and protection of magnesium alloys, Adv. Eng. Mater. 7 (2005) 563–586. [3] C. Blawert, W. Dietzel, E. Ghali, G. Song, Anodizing treatments for magnesium alloys and their effect on corrosion resistance in various environments, Adv. Eng. Mater. 8 (2006) 511–533. [4] J.E. Gray, B. Luan, Protective coatings on magnesium and its alloys - a critical review, J. Alloys Compd. 336 (2002) 88–113. [5] G.L. Song, “Electroless” deposition of a pre-film of electrophoresis coating and its corrosion resistance on a Mg alloy, Electrochim. Acta 55 (2010) 2258–2268. [6] R.G. Hu, S. Zhang, J.F. Bu, C.J. Lin, G.L. Song, Recent progress in corrosion protection of magnesium alloys by organic coatings, Prog. Org. Coat. 73 (2012) 129–141. [7] X. Zhang, Q. Li, L. Li, P. Zhang, Z. Wang, F. Chen, Fabrication of hydroxyapatite/stearic acid composite coating and corrosion behavior of coated magnesium alloy, Mater. Lett. 88 (2012) 76–78. [8] R.K. Gupta, K. Mensah-Darkwa, J. Sankar, D. Kumar, Enhanced corrosion resistance of phytic acid coated magnesium by stearic acid treatment, Trans. Nonferrous Met. Soc. China. 23 (2013) 1237–1244. [9] W.F. Ng, M.H. Wong, F.T. Cheng, Stearic acid coating on magnesium for enhancing corrosion resistance in Hanks' solution, Surf. Coat. Technol. 204 (2010) 1823–1830. [10] M. Liang, Y. Wei, L. Hou, H. Wang, Y. Li, C. Guo, Fabrication of a super-hydrophobic surface on a magnesium alloy by a simple method, J. Alloys Compd. 656 (2016) 311–317. [11] Y. Liu, Z. Yu, S. Zhou, L. Wu, Self-assembled monolayers on magnesium alloy surfaces from carboxylate ions, Appl. Surf. Sci. 252 (2006) 3818–3827. [12] S.A. Salman, M. Okido, Self-assembled monolayers formed on AZ31 Mg alloy, J. Phys. Chem. Solids 73 (2012) 863–866. [13] A. Frignani, V. Grassi, F. Zucchi, F. Zanotto, Mono-carboxylate conversion coatings for AZ31 Mg alloy protection, Mater. Corros. 62 (2011) 995–1002. [14] G.L. Song, A dipping E-coating for Mg alloys, Prog. Org. Coat. 70 (2011) 252–258. [15] Y. Liu, X. Yin, J. Zhang, S. Yu, Z. Han, L. Ren, A electro-deposition process for fabrication of biomimetic super-hydrophobic surface and its corrosion resistance on magnesium alloy, Electrochim. Acta 125 (2014) 395–403. [16] Y. Liu, X. Yin, J. Zhang, Y. Wang, Z. Han, L. Ren, Biomimetic hydrophobic surface fabricated by chemical etching method from hierarchically structured magnesium alloy substrate, Appl. Surf. Sci. 280 (2013) 845–849. [17] Z. Wei, P. Tian, X. Liu, B. Zhou, Hemocompatibility and selective cell fate of polydopamine-assisted heparinized PEO/PLLA composite coating on biodegradable AZ31 alloy, Colloids Surf. B. Biointerf. 121 (2014) 451–460. [18] H. Lee, B.P. Lee, P.B. Messersmith, A reversible wet/dry adhesive inspired by mussels and geckos, Nature 448 (2007) 338–341. [19] M. Ramazanoglu, R. Lutz, P. Rusche, L. Trabzon, G.T. Kose, C. Prechtl, K.A. Schlegel, Bone response to biomimetic implants delivering BMP-2 and VEGF: an immunohistochemical study, J. Craniomaxillofac. Surg. 41 (2013) 826–835. [20] J.J. Lee, I.S. Park, G.S. Shin, S.K. Lyu, S.G. Ahn, T.S. Bae, M.H. Lee, Effects of polydopamine coating on the bioactivity of titanium for dental implants, Int. J. Precis. Eng. Manuf. 15 (2014) 1647–1655. [21] C. Wu, P. Han, X. Liu, M. Xu, T. Tian, J. Chang, Y. Xiao, Mussel-inspired bioceramics with self-assembled Ca-P/polydopamine composite nanolayer: preparation,
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Contents lists available at ScienceDirect
Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat
Synthesis and electrochemical behavior of a magnesium fluoride-polydopamine-stearic acid composite coating on AZ31 magnesium alloy Lijun Zhang a,b,c, Elbeshary A.A. Mohammed b, Annemie Adriaens b,⁎ a b c
College of Sciences, Zhejiang A & F University, Lin'an 311300, People's Republic of China Department of Analytical Chemistry, Ghent University, Krijgslaan 281 (S12), Ghent 9000, Belgium Key Laboratory of Chemical Utilization of Forestry Biomass of Zhejiang Province, Zhejiang A & F University, Lin'an 311300, People's Republic of China
a r t i c l e
i n f o
Article history: Received 27 June 2016 Revised 5 August 2016 Accepted in revised form 7 August 2016 Available online 08 August 2016 Keywords: AZ31 magnesium alloy Polydopamine Stearic acid Composite coating Corrosion resistance
a b s t r a c t To improve the corrosion resistance of AZ31 magnesium alloys, a magnesium fluoride-polydopamine-stearic acid (MgF2/PDA/SA) composite coating was synthesized on a AZ31 substrate in an easy and low cost way by successively forming a polydopamine (PDA) layer and a stearic acid (SA) layer on a HF-treated magnesium alloy surface. SEM observations show that the MgF2/PDA/SA coating presents a more compact and uniform morphology compared to the MgF2/SA coating. Impedance and polarization analyses conducted in 3.5 wt% NaCl aqueous solution show that the MgF2/PDA/SA coated Mg alloys are significantly improved in corrosion resistance, with a remarkably low corrosion current density and three orders of magnitude in corrosion resistance higher than in the case of the bare AZ31 alloy. It is found that the formation of PDA layer plays a key process in the formation of MgF2/ PDA/SA composite coating. The PDA layer not only acts as an efficient anchoring layer between the Mg alloy surface and stearic acid layer, but also plays a protective role on Mg alloy substrate. © 2016 Elsevier B.V. All rights reserved.
1. Introduction In recent years, magnesium and its alloys have been widely used as important structural materials in a variety of industries such as the automotive, computer and aerospace. This is due to their low density, high specific strength, good metal cutting performance and other desirable properties [1]. However, the large-scale application of magnesium and its alloys are yet hindered by their poor corrosion resistance [2]. Surface treatment is generally used as one of the most cost-effective methods to prevent magnesium and its alloys from corrosion. In this case, various surface treatments and coating techniques have been proposed and developed for magnesium and its alloys [3,4], such as surface conversion, anodizing, laser/electron/ion beam treatment, galvanizing/ plating, sol–gel coating and organic coating/plating. Among these techniques, an organic coating is regarded as the most cost-effective method for magnesium and its alloys [5]. Generally, the primary composition of an organic coating is a resin, which has various kinds, such as acrylic polyurethane, vinyl, polyvinyl butyral, epoxy and phenolic containing chromate [6]. Because of the low environmental impact, the natural substances, such as long-chain aliphatic mono-carboxylic acids, have been placed an increasing interest on anti-corrosion of magnesium ⁎ Corresponding author. E-mail addresses: [email protected] (L. Zhang), [email protected] (E.A.A. Mohammed), [email protected] (A. Adriaens).
http://dx.doi.org/10.1016/j.surfcoat.2016.08.021 0257-8972/© 2016 Elsevier B.V. All rights reserved.
and its alloys [7–12]. In brief, the anions of the mono-carboxylic acids should easily react with the oxide film on the metal surface and then, a long aliphatic chain (up to 18 °C atoms) should form well ordered, highly protective and hydrophobic layers through van der Waals (vdW) interactions [13]. However, in fact, the poor adhesion to the substrate usually weakens the anti-corrosion performance of the coating [6]. To deal with this problem, it is very necessary to make some appropriate surface pretreatments on Mg and its alloys, such as wet chemical etching, anodizing, or adding adhesion promoters [5,14–17]. Since Lee et al. reported it in 2007, polydopamine (PDA) films have attracted much attention on biomaterials surface modification by acting as a simple and effective strategy to promote cell adhesion onto substrates with various material types and shapes under non-toxic condition [18]. For example, Park's group found that with the assistance of polydopamine, inorganic hydroxyapatite crystals can be integrated on noble metals, ceramics, and synthetic polymers via interactions between side residues and Ca/P ions [19–21]. For Ti implants, PDA has been widely used to improve their antibacterial activity, bioactivity, and corrosion resistance [19,22,23]. Considering the disadvantages of the too fast degradation rate of the magnesium and its alloys as implants, some researchers studied the PDA layers not only for adhesion promotion but also for anti-corrosion performance. In these studies, Mg and its alloys coated by the composite coatings with PDA as intermediate layer exhibited significantly smaller free corrosion current density, an obviously lower degradation rate as well as good cytocompatibility
L. Zhang et al. / Surface & Coatings Technology 307 (2016) 56–64
in vitro compared to uncoated samples [17,24–27]. As shown above, PDA was proved to have very effective adhesion and anti-corrosion performance between the substrate and organic/inorganic coatings. Inspired by the above findings, this present work was devoted to study the application of the stearic acid coating with the assistance of PDA to reduce the corrosion rate of AZ31 magnesium alloy under environmental condition. So far, few reports have been published about such study. In the present work, a magnesium fluoride-polydopamine-stearic acid (MgF2/PDA/SA) composite coating was fabricated on AZ31 Mg alloy in an easy and low cost way by forming polydopamine (PDA) layer and stearic acid (SA) layer on HF-treated magnesium alloy surface successively. Moreover, the resulting coatings were characterized with the use of scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR), and water contact angle. The corrosion inhibition properties of those coatings were studied compared to uncoated sample using potentiodynamic polarization experiments and electrochemical impedance spectroscopy (EIS) in 3.5 wt% NaCl aqueous solution. The assessment of degradation behavior of the MgF2/PDA/SA coating in 3.5 wt% NaCl aqueous solution was also carried out by EIS. 2. Experimental 2.1. Materials and surface treatment AZ31 Mg alloy coupons (composition: 3.1 wt% Al, 0.03 wt% Si, 0.05 wt% Ca, 0.82 wt% Zn, 0.34 wt% Mn, 0.1 wt% Be, 0.005 wt% Fe, and the rest is Mg) with a diameter of 12.6 mm and a height of 1.05 mm were designed to fit in a special holder for the electrochemical measurements. The coupons were first ground with P120, P600, P1200 and P4000 grit SiC paper in turn. Then, the surface was polished using a polishing cloth (PSA 10/PK, Buehler, USA) with diamond polish dissolved in oil. Diamond particles of size 6 μm were used first, subsequently 1 μm (MetaDi polycrystalline diamond suspension, Buehler, USA). Thereafter the coupons were cleaned ultrasonically in isopropanol three times for 5 min each, with fresh propanol for each cleaning cycle, then dried in air and stored in a desiccator. In general, a PDA film could be formed on the Mg alloy by immersing the coupons in dopamine hydrochloride Tris-HCl buffer solution, but the Mg alloy would be corroded severely because of abundant Cl− ions in the solution. To solve this problem, a HF-converted film was coated firstly on Mg alloy to improve the corrosion resistance of the substrate by forming MgF2. For the HF conversion treatment, the coupons were immersed in a plastics bottle containing HF (48%, Emsure, Germany) at room temperature for 48 h. The coated samples (labeled as HFtreated) were rinsed with deionized water, and dried in air. PDA coatings were deposited on the HF-treated samples through simple dip-coating of objects. HF-treated substrates were immersed into dopamine solution (DA, analytical grade, Sigma-Aldrich, USA, 2 mg/mL, absolute ethanol, pH 8.5) at room temperature in darkness for 4 h. After that, the samples coated with PDA (labeled as HF-PDAtreated) were ultrasonically washed in deionized water for 5 min and dried in nitrogen. After that, the HF-PDA-treated and HF-treated AZ31 Mg alloy were both immersed in 0.05 M stearic acid (SA, 95+%, Sigma-Aldrich, USA) ethanol solution for 1 h, labeled as HF-PDA-SA-treated and HF-SA-treated respectively. Then some samples kept for SEM were picked up and dried in air without cleaning, while others were washed with absolute ethanol and deionized water, dried in air and stored in a desiccator for following experiments. 2.2. Surface characterization The surface morphologies of the bare and coated AZ31 samples were observed with a scanning electron microscope (Quantafeg, FEI, USA)
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applying a high voltage of 20 kV. The surface chemical compositions of HF-SA-treated, HF-PDA-treated and HF-PDA-SA-treated AZ31 samples were analyzed by X-ray photoelectron spectroscopy (PHI-5000C ESCA system, PerkinElmer, USA) with Mg-Kα as the radiation source. FTIR spectra were measured in the range 4400 to 600 nm in a Perkin-Elmer Spectrum 1000 spectrometer equipped with a HATR (Horizontal Attenuated Total Reflection) cell from Pike Technologies. Spectra were measured on 5 spots on bare and coated AZ31 samples, respectively. Water contact angles were observed with a video based optical contact angle measurement system (OCA 20) to examine surface wettability of all samples. 1 μL of water was dropped on the surface, and the images of the droplet on the films were captured for 5 s after dropping. Three spots were randomly chosen on each sample surface to make the test and the results were averaged. 2.3. Electrochemical experiments All electrochemical experiments were carried out in a three-electrode electrochemical glass cell with a platinum mesh as a counter electrode and a saturated calomel electrode (SCE, Radiometer Analytical) as a reference electrode. Samples were mounted in a homemade Teflon® working electrode holder. The exposed surface area of the samples in the holder was 0.19625 cm2. All fitted impedance and current values were corrected and normalized related to the surface. All electrochemical measurements were performed in 3.5 wt% NaCl (assay grade 100%, Prolabo, Belgium) aqueous solution at room temperature. EIS measurements were performed for bare, HF-treated, HF-SAtreated, HF-PDA-treated and HF-PDA-SA-treated samples on a potentiostat (Autolab PG-STAT 20, Metrohm, Switzerland) equipped with a frequency response analysis module (FRA module, EcoChemie BV, Utrecht, Netherlands). Data were acquired at the open circuit potential in the frequency range of 100 kHz to 10 MHz with a signal amplitude of 5 mV. Potentiodynamic polarization experiments were also performed for samples mentioned above on the same potentiostat. The OCP was determined firstly, and then the potentiodynamic polarization curves were recorded from −0.3 V to 0.6 V vs. the OCP value with a scan rate of 1 mV·s−1. Both EIS and potentiodynamic polarization experiments were controlled by the Nova 1.10 software and the same software was used to make potentiodynamic polarization curves fitting analysis. The impedance data were fitted by ZView 2.0 software with the corresponding equivalent circuits. 3. Results and discussion 3.1. Surface characterization Fig. 1 displays the surface morphology images (both optical and SEM) of all samples, as well as cross-sectional SEM image of the HFPDA-SA-treated sample before cleaning. The optical images shown in Fig. 1, after 48 h immersion period in 48% HF, demonstrate that the AZ31 Mg alloy turns from an initial metallic grey color to a uniform dark color, and keeps the dark color after the following PDA treatment and SA treatment. Moreover, as shown the red arrows point to, it is visible that the surface of the HF-PDA-SA-treated sample before cleaning was covered with something uneven and incomplete. As SEM images shown in Fig. 1(b), the coating surface of HF-treated sample is dense, with homogenous distributed particles. After the PDA layer deposited, the surface seems rougher as shown in Fig. 1(d). As the middle layer of the composite coating, the rougher morphology of the MgF2/PDA coating would be useful to the attachment of outer layer. After coated by SA layer, the surface of the HF-PDA-SA-treated sample after cleaning (Fig. 1e) appears smooth and more compact. Compared with the HF-PDA-SA-treated sample after cleaning, the surface of HF-SA-treated sample after cleaning (Fig. 1c) showed much more defects such as cracks and pores. These defects may be harmful for the long-term corrosion
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Fig. 1. Optical images and SEM images of bare AZ31 alloy (a), HF-treated (b), HF-SA-treated after cleaning (c), HF-PDA-treated (d), HF-PDA-SA-treated after cleaning (e), HF-PDA-SAtreated before cleaning (f), enlarged morphology (g) of the red circled area in figure (f), cross-sectional SEM morphology of HF-PDA-SA-treated before cleaning (h). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
resistance of the MgF2/SA coating because the corrosive solution can penetrate into the film inner part through the routes provided by the defects, thus leading to an increased contact area between the corrosive solution and the coating. It is obvious that with the assistance of PDA, the MgF2/PDA/SA coating after cleaning presented a more compact and uniform morphology, which is much beneficial to enhance the corrosion resistance of the coating. It is worthy to note that, as seen in Fig. 1(f), the surface of HF-PDASA-treated sample before cleaning shows a rough morphology and a number of clusters. As shown in Fig. 1(g) (the enlarged morphology of the red circled area in Fig. 1(f)), the clusters are of petal shaped ridges forming a three-dimensional porous structure. According to the literatures [10,26,28,29], air could be trapped in this structure then form a buffer which could prevent water droplets to infiltrate the surface. Thus, a super-hydrophobic surface is formed. But in our present work, we found that these clusters were easy to remove, which led to the surface of coating was fragmentary and uneven. This may be attributed to the formation process of the composite coating. The composite coating was composed with three layers: the MgF2 film coated directly on the AZ31 substrate, the sandwiched PDA layer inserted between MgF2 film and SA layer, and the outermost SA layer. It is well known that one of the most important properties of polydopamine is its strong adhesion to virtually all types of surfaces [30]. According to the literature [30], the Mg alloy coated MgF2 film was likely to be coated with polydopamine via the coordination interaction between metal ions and the catechol group. Meanwhile, the exposed amino groups of the PDA molecules could serve as nucleation sites of stearic acid via acid-base reactions and moreover, a long aliphatic chain of stearic acid, through van der Waals interactions, would form a well ordered, highly protective and hydrophobic layer. On this layer, due to the electrostatic effect and Van der Waals interactions,
multilayers could be formed. The innermost formed by chemical reaction has high bond energy and best adhesion to the solid surface, however, the interactions among outer layers formed by van der Waals interactions are weaker, and the outermost has weakest adhesion to the substrate so it is easy to remove. Therefore, the composite layer should be composed of two parts: a close layer and a loose layer, as shown by the cross-sectional morphology (Fig. 1(h)). As seen in Fig. 1(h), the thickness of the coating (between the two red lines) is about 7.95 μm and obviously, the coating could be divided into two parts, separated by the yellow line. The part below the yellow line exhibits a homogenous compactness and is obviously an excellent adhesive to both the SA coating and the Mg alloy substrate. The thickness of this part is about 5.02 μm. The part above the yellow line shows a distinctive and totally different morphology, which is discontinuous and porous. It is evident that the compact and uniform coating can improve the anti-corrosion property of the substrate. As a result the MgF2/PDA/ SA coating after cleaning was used as effective coating in the following tests. The full XPS spectra of HF-PDA-SA-treated, HF-PDA-treated and HFtreated samples are shown in Fig. 2. The elemental compositions at the surfaces of different Mg alloy specimens are shown in Table 1. It can be observed that the fluorine is present in all XPS spectra, because of the pretreatment of HF acid. For HF-treated sample, the appearance of carbon in XPS surface scan was due to unavoidable hydrocarbon contamination and a small amount of nitrogen was probably the result of adventitious contamination from the environment. Oxygen could originate from hydrocarbons and/or from hydroxides of Mg. After PDA modification, an increase in the carbon and nitrogen contents and a concomitant decrease in the magnesium and fluorine contents are shown in Fig. 2 and Table 1, which confirms the successful anchoring of PDA on the HF-treated surface. The increase in C1s is further
L. Zhang et al. / Surface & Coatings Technology 307 (2016) 56–64
59
Fig. 2. XPS full spectra of HF-PDA-SA-treated, HF-PDA-treated and HF-treated samples. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
accentuated after the immobilization of SA on the HF-PDA-treated Mg alloy substrate (Fig. 2) due to long aliphatic chain in stearic acid, demonstrating SA layer was formed on the HF-PDA-treated Mg alloy surface. Meanwhile, judging from Table 1, the C/N ratio and C/F ratio of HFPDA-SA-treated sample are 70.59 and 10.03 respectively, which are much higher than those of HF-PDA-treated sample (10.90 and 3.66). This also verifies that the surface of HF-PDA-treated sample was successfully coated with stearic acid. FTIR characterizations of the samples surfaces are shown in Fig. 3 to further ascertain the PDA layer as well as the SA layer. Adsorption bands located in the range of 1450–1625 cm−1, which originate from the C_C stretching vibration of benzene rings and the N\\H bending, as well as 3347 cm−1 from catechol groups of the PDA structure, confirm that PDA was successfully coated on HF-treated AZ31 substrate [24]. As seen from the spectra of the MgF2/PDA/SA coating, two adsorption peaks at 2920 cm−1 and 2850 cm−1 correspond to the C\\H asymmetric and symmetric stretching vibrations of –CH2– groups of stearic acid, respectively [25]. Moreover, it is known that the carboxyl group from stearic acid appears at 1702 cm−1, resulting from free COO band [26]. So the surface of HF-PDA-SA-treated sample exhibits adsorption peaks at 1561 cm−1 and 1430 cm−1 which may originate from the asymmetric and symmetric stretches of the –COO– group, suggesting that carboxylates were successfully adsorbed on the treated surfaces. This could also prove that the SA was coated onto PDA-coated Mg alloy surface via an acid-base reaction instead of an amidation reaction. Comparatively, the typical adsorption peaks of PDA and SA are not apparent from the spectra of the bare AZ31 Mg alloy and HF-treated surface, but the adsorption peaks of SA appeared in the spectrum of HF-SAtreated sample, which demonstrates SA layer was formed on the surface. Fig. 4 shows the photographs of water droplets on surfaces of the bare AZ31, HF-treated, HF-SA-treated, HF-PDA-treated, and HF-PDASA-treated samples. It can be seen that the wettability of the surface is
Table 1 The chemical element compositions of different Mg alloy specimens. Sample
Mg (at.%)
F (at.%)
C (at.%)
O (at.%)
N (at.%)
HF-PDA-SA-treated HF-PDA-treated HF-treated
3.29 8.34 22.30
7.88 15.28 44.79
79.06 55.92 18.31
8.65 15.33 14.39
1.12 5.13 0.21
obviously changed after the AZ31 substrate was modified by different treatments. The contact angles of the bare AZ31, HF-treated, HF-PDAtreated samples are 80.8° ± 0.31°, 11.8° ± 1.22°, 15.8° ± 2.25°, respectively, indicating a typical hydrophilic materials. However, after immersing in stearic acid ethanol solution for 1 h, the contact angles of the HF-treated and HF-PDA-treated samples increase markedly and reach 102.0° ± 1.3° and 109.3° ± 2.1°, respectively, implying that stearic acid was successfully adsorbed on the substrate and formed the close packed and regularly arranged alkyl tails of the C18A− ions. An increase in hydrophobicity could reduce the adhesive interaction between the water drop and the solid surface, and result in an increscent contact angle. The hydrophobic property of the MgF2/SA coating and MgF2/PDA/SA coating could isolate the substrate from a corrosive solution thus provide good corrosion protection for the Mg alloy substrate. Furthermore, the contact angle of the HF-PDA-SA-treated sample surface is quite similar to the equilibrium contact angle of the C18H35O–2 coated surface of a magnesium alloy which has been reported to be about 109° [11], implying that the MgF2/PDA/SA coating after cleaning was smooth and compact. Apparently, this conclusion is consistent with that of the SEM. 3.2. Electrochemical properties of the coatings The corrosion behavior of different samples was evaluated by electrochemical tests. Fig. 5 shows the potentiodynamic polarization curves of the bare, HF-treated, HF-SA-treated, HF-PDA-treated and HF-PDASA-treated AZ31 Mg alloy samples in 3.5 wt% NaCl aqueous solution. The polarization curve provides much useful information in determining the instantaneous corrosion rate of a substrate, anodic and cathodic behavior of a corrosion reaction. For magnesium and its alloys, the cathodic polarization curve is generally considered to represent the cathodic hydrogen evolution by water reduction, and the anodic one represents dissolution of magnesium or corrosion behaviors of the protective layer [27]. In Fig. 5, it can be observed that the cathodic polarization current densities of the coated samples are all lower than in the case of the bare AZ31, particularly for the HF-PDA-SA-treated sample, its cathodic polarization current density is about two orders of magnitude lower than that of the bare alloy, indicating the MgF2/PDA/SA coating significantly suppresses the cathodic reaction of hydrogen evolution thus the overall corrosion. The corrosion potentials Ecorr and current density Icorr were calculated by the Tafel method through linear extrapolation of cathodic polarization zone [31]. Their values are listed in
60
L. Zhang et al. / Surface & Coatings Technology 307 (2016) 56–64
Fig. 3. FTIR spectra of the samples by different treatments. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
a CA: 80.8º ±0.31º
b CA: 11.8º ±1.22º
d
c CA: 102.0º ±1.3º
CA: 15.8º ±2.25º
e
CA: 109.3º ±2.1º
Fig. 4. Images of water droplets on bare AZ31 surface (a), HF-treated surface (b), HF-SA-treated surface (c), HF-PDA-treated surface (d), and HF-PDA-SA-treated surface (e).
Table 2. It is evident that the corrosion current density (Icorr) decreases gradually from HF-treated to HF-PDA-SA-treated AZ31. The HF-PDASA-treated sample shows the lowest corrosion current density among all the samples, by about two orders of magnitude lower than that of the bare AZ31. HF-PDA-SA-treated sample underwent the lowest Icorr might be due to its lowest cathodic current density which is apparently lower than that of HF-SA-treated sample in Fig. 5. It implies that the PDA
Fig. 5. Potentiodynamic polarization curves for bare, HF-treated, HF-SA-treated, HF-PDAtreated and HF-PDA-SA-treated AZ31 Mg alloy after 10 min immersion in 3.5 wt% NaCl aqueous solution. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
layer effectively suppress the cathodic corrosion process of Mg alloy and therefore overall corrosion rate. In addition, another important finding from the polarization curves is that different treatment causes a significant change in the corrosion potentials (Ecorr) and the anodic Tafel curves in the NaCl solution. As shown in Table 2, the HF-treated sample shows the most positive Ecorr compared with the bare and other coated samples. PDA layer and SA layer formed on the HF-treated samples surface respectively/successively all make the Ecorr less noble compared with the Ecorr of HF-treated sample, and the value of the Ecorr, HF-PDA-SA-treated is similar to that of the bare Mg alloy. The more noble Ecorr means more thermodynamically stable of the sample, meanwhile, the lower Icorr means slower corrosion process in kinetics [32]. The results indicate that HF-treated sample seems most thermodynamically stable against corrosion due to the high thermodynamical stability of MgF2 but, the formation of PDA layer and SA layer significantly reduced the corrosion rate of AZ31 by obstruction to the cathodic reaction of hydrogen evolution. As a result, although the HF-PDA-treated sample and bare AZ31 have the similar value of Ecorr, the former corrodes much slower than the latter. In Fig. 5, an obvious passive behavior and a turning point appear in the anodic Tafel curve of each coated sample. When the potential reaches the potential (labeled as Epit in Fig. 5) corresponding to the turning point, the current density increases quickly, indicating the breakdown of the surface coating and the occurrence of the pitting corrosion. The values of | Epit - Ecorr | extracted from the polarization curves in Fig. 5 are also listed in Table 2. As shown in Table 2, the increment of |Epit - Ecorr |, which is as high as 70 mV, implies that the pitting corrosion tendency of the coated sample has been greatly reduced by the formation of the MgF2/PDA/SA composite coating. Meanwhile,
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Table 2 Values of Ecorr, Icorr and |Epit - Ecorr |, extracted from the polarization curves in Fig. 5. Samples
Bare AZ31
HF-treated
HF-SA-treated
HF-PDA-treated
HF-PDA-SA-treated
Ecorr (V) Icorr (μA cm−2) |Epit - Ecorr |(mV)
−1.52 ± 0.03 102.06 ± 10.5
−1.39 ± 0.02 4.71 ± 1.01 90 ± 1.10
−1.46 ± 0.03 3.09 ± 0.75 80 ± 0.91
−1.42 ± 0.02 2.18 ± 0.51 110 ± 1.85
−1.51 ± 0.02 1.85 ± 0.32 150 ± 1.76
there is no passive behavior in the anodic Tafel of bare AZ31, indicating that the original MgO film on bare AZ31 is too thin and incompact to protect the alloy substrate in 3.5 wt% NaCl solution. EIS is a useful technology way to evaluate the protective ability of a coating to the substrate [33]. Fig. 6 shows the Nyquist plots (Fig. 6a) and the impedance magnitude values (Fig. 6b) for the samples by different treatments in 3.5 wt% NaCl solution. The inset of Fig. 6(a) shows the enlarged Nyquist plots for bare, HF-treated and HF-SA-treated AZ31 Mg alloy samples. The results show quite different capacitive loops that can be attributed to the charge transfer resistance of the corrosion process. It discloses that the HF-PDA-SA-treated sample presents evidently an overall largest constant loop among all the samples. As shown in Fig. 6(b), in the low frequency region, the modulus for HF-PDA-treated and HF-PDA-SA-treated samples are about two and three orders of magnitude higher than in the case of the bare AZ31, respectively, meanwhile the modulus for HF-treated and HF-SA-treated samples are under one order of magnitude higher than in the case of the bare AZ31. The increase of the modulus for the coatings with PDA layer is much greater, indicating that the existence of the PDA layer greatly enhanced the corrosion resistance of the coating. This result is in good agreement with the results from potentiodynamic polarization curves. Furthermore, the HF-PDA-SA-treated sample possessed the highest impedance modulus, compared with the bare and other coated samples. This reveals that MgF2/PDA/SA coating has an excellent corrosion resistance for magnesium substrate attributed to protection by PDA layer and stearic acid layer. Determined through the EIS diagrams and the method developed by Wit [34], there are two capacitive loops in the high and middle frequency regions respectively in the EIS plots of coated samples. The capacitive loop in high frequency may arise from the coating and the capacitive loop in middle frequency represents interface reaction between the coating and substrate. The EIS plots of bare Mg alloy have only one capacitive loop in the high frequency region, which represents the interface reaction between the electrolytic solution and the substrate. An inductance behavior in the low frequency region is displayed in each Nyquist plots for all samples, which may be related to the change of the surface state such as the breakdown of the surface coating in NaCl solution [35] or adsorbed/desorbed of corrosion products on the sample [29].
To obtain an accurate analysis of the impedance data, different equivalent circuits are used to fit the EIS data in the high and middle frequency regions in Fig. 6. Two types of equivalent circuits are shown in Fig. 7 and the fitting results are shown in Table 3. Fig. 7 shows the equivalent circuits representing the electrochemical behaviors of the bare AZ31 and the coated samples respectively. In Fig. 7(a), Rct represents the charge transfer resistance, CPEdl is the constant phase element of the electrical double layer, and Rs is the solution resistance. Meanwhile, in Fig. 7(b), CPEc is the constant phase element related to the coating, Rc is the coating resistance. As clearly shown in Table 3, the evolution of Rct is similar to that of Rc, which may be attributed to the increase of Rc should decrease the reaction activity of the coating thus strengthen the protection for the substrate. Moreover the Rct value for HF-PDA-SA-treated sample is nearly 112 times larger than that of bare AZ31, more than 11 times larger than that of HF-PDA-treated and 15 times larger than that of HF-SA-treated. As discussed in the results of water contact angle tests and polarization curves, though both PDA and SA per se are effective to impede electrolyte penetrating into the surface of Mg alloy substrate, their effects seem less notable as compared with the composite MgF2/PDA/SA coating on AZ31 Mg alloy. The inhibition efficiency (IE %) can be calculated from the charge transfer resistance as follows [36]: IE ¼
Rct − Rt0 100 Rct
ð1Þ
where Rct is the charge transfer resistance of the coated AZ31 and Rt0 is the resistance of the bare AZ31. The IEs of the MgF2, MgF2/SA, MgF2/ PDA, MgF2/PDA/SA coatings were estimated to be 71.6%, 88.7%, 91.2%, and 99.2%, respectively. This indicates that our MgF2/PDA/SA composite coating significantly inhibited the corrosion progress. 3.3. Immersion degradation behavior In order to investigate the degradation behavior of the MgF2/PDA/SA coating, EIS measurements were carried out in 3.5 wt% NaCl solution at room temperature with different times. Fig. 8 shows the evolution of Nyquist and Bode plots for HF-PDA-SA-treated AZ31 sample immersed in 3.5 wt% NaCl solution. The inset of Fig. 8(a) shows the enlarged
Fig. 6. Nyquist plots (a) and Bode modulus (b) for bare, HF-treated, HF-SA-treated, HF-PDA-treated, and HF-PDA-SA-treated AZ31 Mg alloy samples in 3.5 wt% NaCl aqueous solution. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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L. Zhang et al. / Surface & Coatings Technology 307 (2016) 56–64
Fig. 7. Equivalent circuits used for the numerical fitting of EIS data for the bare AZ31 sample (a), coated samples (b).
Table 3 Fitting results of EIS plots in Fig. 6. Samples
Bare AZ31 HF-treated HF-SA-treated HF-PDA-treated HF-PDA-SA-treated
CPEc -Y0 F cm−2
CPEc n
Rc Ω cm2
1.81E-04 1.80E-04 1.76E-04 1.10E-04
0,980 0.969 0.971 0.966
36.691 1439.494 1512.112 6905.652
CPEdl Y0 F cm−2
CPEdl n
Rct Ω cm2
7.13E-05 2.08E-05 1.35E-05 9.17E-04 9.12E-05
0.934 0.998 0.986 0.991 0.989
39.486 139.082 349.132 446.075 5203.565
Nyquist plots for the coated sample after 24 h immersion. In Fig. 8(a), the capacitive loops at high and middle frequencies could be attributed to the charge transfer of the corrosion process, and the scatter plots at low frequencies shown as inductance or capacitive behavior indicate that the corrosion process is affected by several other surface-state variables except of the applied potential E [37]. As shown in Fig. 8, the diameter of capacitive loop as well as the impedance modulus (Fig. 8b) decreases gradually with increasing immersion time, indicating that the anticorrosion performance of the MgF2/PDA/SA composite coating is declined with immersion time. The localized corrosion might lead to the reduction of the anticorrosion performance. However, the charge transfer of the HF-PDA-SA-treated sample after immersion in 3.5 wt% NaCl solution for 24 h is still much greater than that of bare AZ31, indicating that the MgF2/PDA/SA composite coating can effectively improve the corrosion resistance of magnesium alloy in the NaCl corrosive solution. Fig. 9 shows the SEM images of the HF-PDA-SA-treated samples after immersion in 3.5 wt% NaCl aqueous solution for 1 h (a), 4 h (b), 7 h (c), 10 h (d) and 24 h (e). The inset of Fig. 9(e) is the overall surface morphology at the magnification of 250 ×. The SEM image of the bare
AZ31 Mg alloy after immersion in 3.5 wt% NaCl solution for 7 h is shown in Fig. 9(f). Compared with the surface morphology before immersion (Fig. 1e), there is almost no change in the surface morphology after immersion in 3.5 wt% NaCl aqueous solution for 1 h (Fig. 9a). After immersion for 4 h, small rips and pits appear on the surface but the coating remains uniform. With the increasing immersion time, more pits are displayed in the surface morphologies (Fig. 9c and d) and the small pits are developed into big and deep pits while more cracks appear on the surface (Fig. 9d). When immersed for 24 h, the surface morphology exhibits typical characterization of pitting corrosion (inset of Fig. 9e). The appearance and development of pitting corrosion should be responsible for the reduction of the impedance. Otherwise, it can be clearly seen in Fig. 9(e) that the corrosion products are accumulated on the pitting area and have begun to be destroyed by Cl−, as a result that many cracks and pores could be seen on the products layer. To some extent, the corrosion products could act as obstacle to prevent the corrosion solution penetrating into the inner part of the coating but, the protection can't sustain long time. The coating without pitting remains relatively uniform and continues to protect the substrate. In contrast, just after 7 h immersion, bare AZ31 Mg alloy suffers serious corrosion attack and big cracks are seen on the surface as shown in Fig. 9(f). It is strongly proved by the SEM images that our MgF2/PDA/SA composite coating can significantly enhance the anti-corrosion property of the Mg alloy substrate. In conclusion, the electrochemical behavior of the HF-PDASA-treated sample and bare AZ31 Mg alloy are in consistent with their corrosion morphologies from SEM observation. For the corrosion of magnesium and its alloys, kinetic factors are the most important determinants due to the very high activity of Mg thermodynamically [38]. Therefore, the slowest step within all kinetic processes will become the rate-determining step (RDS) of overall corrosion reaction. In the present work, the RDS for coated Mg alloy was changed with the different coatings. Because of hydrophobicity of the SA layer and insolubility of the MgF2 film, the RDSs of the corrosion processes were both the ionic pathway which was attributed to the coating barrier preventing electrolyte penetration into the Mg alloy substrate [39]. For PDA-coated, the RDS was changed to the electrical pathway due to the extremely low conductivity of PDA [40]. These could be reflected in the polarization curves of all the coated samples with characteristics of cathodic-control and diffusion-limitation. According to the discussion in the polarization curves and EIS, especially compared with HF-SA-treated sample, the HF-PDA-SA-treated sample showed much lower corrosion rate in 3.5 wt% NaCl solution. This implies that the RDS in corrosion process of Mg alloy coated MgF2/PDA/SA composite coating was likely to be the electrical pathway. Consequently, the cathodic reaction was suppressed effectively because of the delayed arrival of electrons from the anode and the overall corrosion rate slowed
Fig. 8. Time evolution of Nyquist plots (a) and Bode modulus (b) for HF-PDA-SA-treated AZ31 Mg alloy samples immersed in 3.5 wt% NaCl aqueous solution. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
L. Zhang et al. / Surface & Coatings Technology 307 (2016) 56–64
(b)
(a)
50 µm
(c)
50 µm
50 µm
(e)
(d)
63
(f) 500 µm
50 µm
50 µm
50 µm
Fig. 9. SEM images of the HF-PDA-SA-treated samples and bare AZ31 Mg alloy immersed in 3.5 wt% NaCl aqueous solution for different times.
down. Moreover, it seems easy to understand the degradation behavior of the composite coating. Table 4 shows the water contact angles on the surfaces of MgF2/PDA/SA composite coating during different immersion time. It could be seen that the hydrophobicity of coating was declined with the increasing immersion time. And the conductivity of PDA was increased gradually as water was absorbed [30]. As a result, the corrosion rate was accelerated due to the faster electron transfer. So in order to improve the long-term durability of the composite coating, how to enhance the hydrophobicity of SA layer will be the key point in future work. 4. Conclusions 1) A MgF2/PDA/SA composite coating was fabricated on AZ31 Mg alloy in an easy and low cost way by forming PDA layer and SA layer on HF-treated magnesium alloy surface successively. 2) MgF2/PDA/SA composite coatings before and after cleaning show totally different surface morphologies. The former exhibits a rough morphology with a number of clusters, and the latter is smooth and uniform. Both after cleaning, it is obvious that with the assistance of PDA, the MgF2/PDA/SA coating presents a more compact morphology compared with the MgF2/SA coating. 3) The MgF2/PDA/SA coated AZ31 Mg alloy showed dramatic improvement in corrosion resistance in 3.5 wt% NaCl aqueous solution by electrochemical tests, with a remarkably low corrosion current density Icorr and three orders of magnitude in corrosion resistance higher than in the case of the bare AZ31 alloy. 4) The formation of PDA layer plays a key process in the formation of MgF2/PDA/SA composite coating. The PDA layer not only acts as an efficient anchoring layer between the Mg alloy surface and stearic acid layer, but also plays a protective role on Mg alloy substrate. Acknowledgements This work was supported by Research Fund - Flanders (FWO) (G.0.068.08.N.10) and National Natural Science Foundation of China Table 4 Water contact angles on the surface of MgF2/PDA/SA composite coating during different immersion time. Time (h)
1
4
7
10
24
Water contact angles
107.1 ±
104.2 ±
95.2 ±
86.5 ±
70.9 ±
1.8
2.5
3.8
6.3
8.7
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