New Method for the Corrosion Resistance of AZ31 Mg Alloy with a

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A New Method for the Corrosion Resistance of AZ31 Mg Alloy with Porous Micro-arc Oxidation Membrane as Ionic Corrosion Inhibitor Container Zhaoxia Li, Wenbin Yang, Qiangliang Yu, Yang Wu, Daoai Wang, Jun Liang, and Feng Zhou Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b01637 • Publication Date (Web): 07 Aug 2018 Downloaded from http://pubs.acs.org on August 9, 2018

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A New Method for the Corrosion Resistance of AZ31 Mg Alloy with Porous Micro-arc Oxidation Membrane as Ionic Corrosion Inhibitor Container Zhaoxia Lia,b, Wenbin Yanga, b, Qiangliang Yua, Yang Wua, Daoai Wang a,c*, Jun Lianga, Feng Zhoua* a

State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese

Academy of Sciences, Lanzhou 730000, China. b

University of Chinese Academy of Sciences, Beijing, 100049, China

c

Qingdao Center of Resource Chemistry and New Materials, Qingdao 266100, China

Corresponding Author * E-mail: [email protected]； [email protected].

ABSTRACT: This work introduces a new composite anticorrosion coating for the AZ31 magnesium alloy, basing on the synergistic effect of an organic/inorganic composite coating with micro- and nanoporous micro-arc oxidation (MAO) membrane as the container of ionic corrosion inhibitor (M-16). The surface morphologies and size of the micro/nano-containers in porous MAO membrane before and after filling with M-16 corrosion inhibitor were examined by scanning electron microscopy (SEM). The effectiveness of M-16 for corrosion suppression on AZ31 My alloy with and without epoxy coating as top sealing layer was demonstrated by electrochemical impedance spectroscopy (EIS) and salt spray tests. The potentiodynamic polarization and electrochemical impedance spectroscopy measurements showed that compared with the bare AZ31 Mg alloys, the

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composite coating has superior corrosion resistance with the a lower corrosion current (9.7×10-9 A/cm2) and a higher protection efficiency (99.3 %) after immersion in 3.5 wt % NaCl solution, and meanwhile has stronger salt spray resistance within 30 days. The results demonstrate the synergistic effect of the isolation protection of micro-arc oxidation layer, the inhibition of M-16 and epoxy coating contributed to the protection for AZ31 Mg substrate to some extent. Therefore, it is anticipated that the composite coating has a potential application in the protection of metals and their alloys.

KEY WORDS: AZ31 Mg alloy; Micro-arc oxidation layer; Corrosion inhibitor; EIS; Corrosion resistance; Synergistic effect

1. INTRODUCTION Corrosion of metals is a universal phenomenon which could highly influence the development of global economy and environment. The development of new strategies for the corrosion resistance and establishment of corrosion control system for many industrial applications of metallic materials has been one of the most hot and important issues. As typical light metallic structural engineering materials, Mg and its alloys have tremendous potential applications for the aerospace, biomedical engineering and automotive industries, owing to the superior strength-to-weight ratio, low density and excellent castability properties of Mg and its alloys. However, magnesium and its alloys are chemically active and susceptible to corrosion when exposed to damp environments or chloride-containing medium. For example, the following electrochemical reactions would occur when Mg or its alloy materials are immersed in 3.5 wt% NaCl solution.1 Anodic reaction: Mg →Mg2+ +2e- ሺ1ሻ Cathodic reaction: O2 +2H2O+4e-→4OH- ሺ2ሻ Overall reaction:

2Mg + O2 + 2H2O→ 2MgሺOHሻ2 ሺ3ሻ

According to these corrosion electrochemical reactions, except for magnesium metal, the existence of oxygen and water is also the necessary condition to occur the corrosion of magnesium or its alloys. Therefore, to inhibit above corrosion process, a number of surface treatment techniques have been put forward to improve the resistance of AZ31 Mg alloy basing on the surface passivation ACS Paragon Plus Environment

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and isolation effect, such as micro-arc oxidation (MAO),2-6 corrosion inhibitors,7,

8

chemical

conversion coating9, 10 and surface chemical modification11-16, and so on. Generally, for the protected metals, the most important way to inhibit the corrosion is the isolation of the contact of Mg and its alloys with the outside air and moisture by forming a passivation layer on the top surfaces of the protected metals, such as an inorganic layer. Many researchers have paid their attention to micro-arc oxidation technology, by which to apply a high-voltage on the metal surface to form a layer of oxide film with high hardness, good wear resistance and superior corrosion resistance. The performance of MAO coating is mainly depended on their microstructure and phase compositions which could be adjusted by the parameters of micro arc oxidation and the composition of electrolyte17, 18. While during the MAO process, some micro-pores and micro-cracks are formed in the MAO membranes, which might become the transportation channels and increase the chance for penetration of the aggressive medium to result in corrosion.19 On the other hand, the micro-pores and micro-cracks in the MAO coatings, to some extent, could also provide some mechanical interlocking sites to improve the adhesion of organic coating to the substrate. Therefore, it is better to balance the positive role and disadvantage of the porous structures in the MAO membrane. Corrosion inhibitor is another important way to enhance the corrosion resistance of metals, which could form a passivation or complex layer on the surface of metals to prevent them from corrosion. It is usually directly added into the outside organic protecting coating, while the adding amount of corrosion inhibitor has significant influence on the physical and chemical properties and anticorrosion effects of the organic coatings.20 Recently, researchers utilized some porous materials as reservoirs for encapsulating the corrosion inhibitors (e.g., BTA, CeO2 and 8-HQ) in the organic coatings to improve their anticorrosion properties, such as mesoporous silica,21-24 layered double hydroxides (LDH),25-27 halloysite,28-30 capsule31-34and so on, while these systems still existed some shortcomings to restrict the applications, such as high cost, short lifespan and the coating easy to peel off. Therefore, it is very important to design a new corrosion inhibitor carrier to increase the corrosion inhibitor's bearing capacity and reduce its influence on organic coatings. In this paper, we report a new strategy for the corrosion resistance of AZ31 Mg alloy with the micro- and nanopores in its MAO membrane as the corrosion inhibitor container, which can prolong the anti-corrosion performance of the metal substrate and enhance interface binding force between substrate

and

top

coating.

A

new


corrosion

inhibitor

of

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1-(3-((N-n-butyl)aminecarboxamido)propyl)-3-hexadecyl imidazolidin bromide (M-16)35 with long-chain compound containing urea groups and imidazole heterocyclic structure was filled in the porous MAO membrane to form a protective film on the metal surface and prevent the corrosion species and water molecule from contacting with the substrate. Furthermore, in order to prevent the corrosion inhibitor from spreading out of the pores, a thin epoxy anticorrosive coating is sprayed on the top surface to form the final organic/inorganic composite anticorrosion coating. Basing on the synergistic effect of the porous micro-arc oxide layer structure, corrosion inhibitors and top organic coating, the composite anticorrosion coating showed highly improved corrosion resistance performance. 2. EXPERIMENT SECTION 2.1 Material and Chemicals The AZ31 magnesium alloy, 30 mm×20 mm×5 mm (AZ31 composition, wt.%: 3.21 Al, 0.82 Zn, 0.42 Mn, 0.012 Si, 0.0011Fe, 0.0012 Cu, 0.00063 Ni, and 95.54Mg). Prior to the micro-arc oxidation treatment, the samples were ground with various grades SiC abrasive papers (up to 1000 grit), and then degreased with acetone, ethanol and rinsed with deionized water (DI), finally dried in nitrogen. Sodium silicate (Na2SiO3) and potassium hydroxide (KOH) were analytic pure and purchased from Tianjin Kermel Chemical Regent Co., Ltd. (China). 1-(3’-aminopropyl) imidazole, n-butylisocyanate and n-bromohexadecane were purchased from J&K chemical. 2.2 Preparation of the composite coating Preparation of MAO inorganic membrane. The micro-arc oxidation treatment of the AZ31 magnesium alloy samples was performed using WHYH-20 equipment (Low Energy Nuclear Physics Institute, Beijing Normal University, China). AZ31 Mg alloy samples were used as anodes and a stainless steel bi-pass cylinder container with a volume of 1 L was used as cathode during the MAO process. Besides, the circulating system of cooling water and the magnetic stirring were necessarily needed. Na2SiO3 (10.0 g) and KOH (1.0 g) were dissolved in distilled water (1.0 L), and then poured into the cylinder container as electrolyte. All the MAO treatments of AZ31 Mg alloy samples were performed using the same experiment parameters: a constant current of 100 mA/cm2 and the negative constant voltage of 110 V, and the micro-arc oxidation treatment time was varied for 1 to 10 minutes. After the MAO treatment, the samples were washed using distilled water repeatedly, and then dried


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in the air. Preparation of corrosion inhibitor M-16. 0.1 mol of 1-(3’-aminopropyl) imidazole was mixed with 100 mL of CH3CN by stirring continuously. 0.12 mol of n-butylisocyanate dissolved in 20 mL of CH3CN was added into the above stirred solution dropwise. The mixed solution was stirred and refluxed for 4 hour after the addition 0.15 mol of n-bromohexadecane. The mixture was then refluxing overnight, the crude product (M-16) was obtained by precipitation in the ethyl acetate and filtration. The 1HNMR and 13NMR spectra of M-16 were shown in Figure S1 and S2.

Scheme 1. The chemical structure formula of inhibitor M-16. Preparation of MAO/ M-16 composite material. After the micro-arc oxidation treatment, some porous and irregular pits were formed on the surface of AZ31 Mg alloy samples, which could be used as the containers for filling with M-16 corrosion inhibitor. Due to the presence of air in the porous MAO membrane, it is better to fill with the corrosion inhibitor under certain negative pressure. The micro-arc oxidation samples were firstly soaked in a 5 mM M-16 ethanol solution, then it was transferred to a vacuum box with a certain negative pressure (smaller than the saturated vapor pressure of ethanol ) for 2 h to make sure the sufficient filling amount of M-16. Preparation of MAO/ M-16/ epoxy composite coating. The epoxy coating was prepared by spraying the commercial epoxy primer onto MAO/ M-16 substrate at room temperature. The distance between the airbrush and the glass slide was approximately 15 cm and the working pressure was 0.2 MPa. Before test, the coated substrate should be placed 7 days at room to ensure the epoxy coating


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completely dry. 2.3 Characterization Surface analysis. The top surface and cross-sectional morphologies of MAO membranes were investigated by Scanning Electron Microscopy (SEM, 5601F, Japan), and the elemental compositions of the samples were detected by energy dispersive spectroscopy (EDS) and the X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi). The phase composition of the ceramic coatings and inhibitor loaded coating was determined by means of X-ray diffraction (XRD, X’PERT PRO). In the XRD measurements, the range of 2θ was from 5 ° to 80 ° and the scan rate was 0.02 °/min. Meanwhile, the water contact angle (WCA) was acquired using a DSA-100 optical contact angle meter (Kruss Company, Ltd., Germany) at room temperature (25 °C). The average contact angle values were obtained by measuring five points on the substrate surface. The thickness of the composite coating was measured using micrometer (MDC-25PX, MITUTOYO, Japan) at 5 random places on the coating surface and ensure the thickness of as-prepared specimens was uniform. The adsorption properties of corrosion inhibitor M-16 were performed at room temperature by a Q-sense microbalance (Sweden) with a rate of 50 µL/min. And this experiment need to be equipped the commercial gold-coated quartz chips (QSX-301, QSense). Structure analysis of M-16 corrosion inhibitor.

1

H and 13C NMR spectra were measured on a 400

MHz spectrometer (Bruker AM-400) using CDCl3 as solvent. Electrochemical measurements. The corrosion resistance of composite coating was evaluated by electrochemical impedance spectroscopy (EIS), potentiodynamic polarization curve measurements and salt spray test. The tests were performed using a CHI660E electrochemical workstation (Chenhua, Shanghai, China). All electrochemical experiments were carried out using a conventional three-electrode system with a saturated calomel electrode (SCE) as the reference electrode, a platinum electrode as the counter electrode and the specimen as the working electrode (1 cm2 exposed area). The sample was immersed in 3.5 wt% NaCl solution for 30 minutes to reach a stable state before the measurement of electrochemical impedance spectroscopy (EIS) and the tafel curves. The EIS measurements were conducted in the potentiostatic mode at the open-circuit potential. The scan range of EIS was 100 KHz to 0.01 Hz with an AC voltage amplitude of 10 mV, and the potentiodynamic curves were conducted at a rate of 1mV/s. The obtained EIS data were carried out using the ZSimpWin software. ACS Paragon Plus Environment

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Salt spray test. The neutral salt spray test on the bare AZ31 Mg alloys and specimens by different treatments were performed according to the ASTM B117 specification, the test specimens are firstly placed in an enclosed chamber (Ascott Corrosion Test Chambers, 1000 L) at an angle of 45 °, then were exposed to a continuous salt spray with a concentration of 5 wt. % NaCl at 35 °C. The chamber climate is maintained under constant steady state conditions, and the test duration is 24 h, 5 days and 30 days, respectively. 3. RESULTS AND DISCUSSION Scheme 1 shows the fabrication processes of composite protection coating, which were mainly made up of three steps, including micro-arc oxidation (MAO) process to from an inorganic membrane with micro- and nanopores, embedment of corrosion inhibitor into pores MAO membrane, and spraying of epoxy anticorrosion coating on the top surface of inhibitor embedded MAO membrane.

Scheme 2. Schematic diagram of the preparation of the MAO/M-16/epoxy composite anticorrosion coating. 3.1 Surface morphology of porous MAO membrane Figure 1 shows the SEM surface morphology and the relation of the coating thickness and oxidation time after micro-arc oxidation treatment of AZ31 Mg. It was found that the top surface of the MAO membrane has many micro- and nanopores and micro-cracks (Figure 1a). From Figure 1a to 1d, it is evident that with the extension of micro-arc oxidation time, the pore size increased gradually, while the number of micro-pore per unit area reduced gradually. From the cross-sectional


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image of MAO layer as shown in Figure 1e, we can obtain the thickness of micro-arc oxidation layer is about 3.1μm after the MAO treatment for 3 min. The MAO film produced with the oxidation time of 5 minutes exhibited a more flat and porous properties, and the pore size on the range of 200 -600 nm (Figure 1c), which was chosen for the following surface modification or functionalization. In addition, in order to understand clearly the relation between the thickness of micro-arc oxidation layer and oxidation time, the curves were shown in Figure 1f, indicating the thickness of MAO membrane thickening with the increase of time.

Figure 1. SEM micrographs of ceramic coatings on the AZ31 magnesium alloys surface by micro-arc oxidation at different oxidation period for (a) 1 min, (b) 3 min, (c) 5 min, (d) 10 min, (e) the cross section of MAO layer for 3 min, (f) the relation of thickness of micro-arc oxidation layer with oxidation time.

Figure 2 shows the magnified surface morphologies of MAO, MAO/M-16 and cross-section morphologies of MAO/M-16 and MAO/ M-16/ epoxy composite coating. The obtained MAO coating contains many irregular-shaped micro- and nano-pores on the bare Mg samples can be seen in Figure 2a, which did not interconnect each other and emerged after the micro-arc oxidation process using negative pressure of 110 voltage for 5 minutes. From the close view of MAO


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incorporated inhibitor shown in Figure 2b, it is obvious the inhibitor M-16 particles are deeply embedded into the porous MAO layer. After the embedment of inhibitor, the surface of sample with MAO film becomes more compactness and homogeneous compared with Figure 2a. In Figure 2 c, shows the cross-section morphologies of the MAO coatings and its thickness of MAO embedment inhibitor was about 9.86μm. When the M-16 inhibitor filled the pores and cracks, the coating became extraordinary compact. As shown in Figure 2 d, it can be observed that the MAO/M-16/epoxy coating consisted of MAO layer incorporated M-16 inhibitor and epoxy anticorrosion coating, and the thickness were about 10.2μm and 153μm, respectively. The compact epoxy coating as the sealing layer of the inhibitor facilitated the further protection for substrate material.

Figure 2. SEM micrographs of different samples for (a) MAO；(b) MAO/M-16；(c) the cross section of MAO/M-16 and (d) cross-section of MAO/M-16/epoxy coating. In order to understand the component of MAO layer, the elemental analysis results of cross


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section of MAO is shown in Figure 3. It is observed that the thickness of MAO layer is about 3μm as shown in Figure 3a. The EDS mapping patterns of MAO film in Figure 3a are shown in Figure 3b-e, which display the film mainly consisted of Si, O and Mg elements, coming from the AZ31 Mg sample and Na2SiO3 electrolyte. The elements and their proportion of MAO layer are shown in Figure 3 indication the main composition of the MAO film is metal oxides.

Figure 3. (a) SEM image of the cross-section and the corresponding elemental mapping pattern of the MAO coating from silicate electrolyte: (b) the electron image, (c) Mg, (d) O, (e) Si, (f) elemental content. To ascertain the composition of the prepared composite coatings, the bare Mg alloy, and MAO, M-16, and MAO/M-16 coatings were evaluated by XRD analysis as shown in Figure 4. It can be seen that the MAO coatings mainly composed of Mg and MgO. The sharp peaks of MgO from the MAO coatings were present on the spectra of the MAO coatings in Figure 4a. The composition of MAO/M-16 sample was mainly consisted the peaks of Mg, MgO and inhibitor M-16, indicating the addition of inhibitor did not change the phase composition of the MAO coatings. As shown in Figure 4b, the peaks at the same position of M-16 and MAO/M-16 indicate that the M-16 inhibitor molecules were embedded successfully into the pores of MAO membrane.


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Figure 4. XRD patterns of (a) bare Mg, M-16, MAO and MAO/ M-16; (b)M-16 and MAO\M-16 at 5-35 ° of diffraction 2 theta.

To further detect the distribution of M-16 in MAO membrane, EDS analysis was performed on the MAO/M-16 composite structure coatings on the sample surface. Figure 5 shows the top view of SEM images of MAO/M-16 and its corresponding chemical elemental mapping images. From Figure 5a, it can be found that a large number of inhibitor molecules filled the porous MAO surface. And the elemental distribution of coating surface was shown in Figure 5b-f, evidently found the coatings contained elements of carbon, oxygen, nitrogen, and bromine, which was agreement with the M-16 inhibitor well. Meanwhile, the XPS data also demonstrate the existence of ionic state Br, which was consistent with the property of ionic corrosion inhibitor M-16, as shown in Figure S3.36, 37These results are further proof of the inhibitor was incorporated successfully into the pores of MAO membrane.

Figure 5. Chemical elemental mapping images of the MAO sample after filling of inhibitor M-16. ACS Paragon Plus Environment

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Surface wettability directly affects the penetration of water and ions into the coating, which can also have an important influence on the corrosion protection performance.38, 39 The water contact angle (WCA) photos of different samples for the bare AZ31, AZ31/inhibitor M-16, AZ31/M-16/ Epoxy coating, MAO, MAO/ M-16, MAO/M-16/ Epoxy coating, are shown in Figure 6. The water contact angle of bare AZ31 is about 64.7 ° as shown in Figure 6a. After micro-arc oxidation treatment, the water contact angle of the as-prepared MAO sample decreases, which would be ascribed to the formation of hydrophilic oxide and porous structures (Figure 6b). And as shown in Figure 6c, when the inhibitor M-16 incorporated into the porous MAO film, the contact angle was further decreased due to the hydrophilic M-16 inhibitor molecules transferred from MAO film to the AZ31 substrate surface, which would form an adsorption film between the MAO and M-16 to play an inhibition effect. In order to achieve longer corrosion resistance, the epoxy anticorrosion coating as top sealing was coated. It is observed that the water contact angle of the sample increased, which can isolate water molecule and aggressive ions to protect the substrate from corrosion.

Figure 6. The water contact angle of different samples for (a) the bare AZ31, (b) MAO, (c) MAO /M-16, (d) MAO/ M-16/Epoxy coating, (e) AZ31/M-16, (f) AZ31 / M-16/ Epoxy coating.

3.5 Electrochemical corrosion behaviour The corrosion behaviors of the AZ31 Mg, AZ31 Mg/M-16, AZ31 Mg/M-16/epoxy coating, MAO, MAO/ M-16 and MAO/ M-16/epoxy coatings were investigated by potentiodynamic polarization test and electrochemical impedance spectrum (EIS) measurement with a three electrode


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system in a 3.5 wt % NaCl solution. The polarization curves of the as-prepared samples are shown in Figure 7. The corrosion current density (icorr), corrosion potential (Ecorr), and anodic/cathodic Tafel constants (βa and βc) were derived from polarization curves in NaCl solution by Tafel extrapolation method, and the polarization resistance (Rp) and the protection efficiency (Eprotection, %) can be calculated by following equation,40, 41

R௣ =

ఉೌ ఉ೎ ଶ.ଷ଴ଷ௜೎೚ೝೝ ሺఉೌ ାఉ೎ ሻ

‫ܧ‬୮୰୭୲ୣୡ୲୧୭୬ =

(3)

଴ ݅௖௢௥௥ ݅௖௢௥௥ି × 100 ଴ ݅௖௢௥௥

ሺ4ሻ

These parameters are pivotal to evaluate the corrosion resistance performance of the composite coatings. All electrochemical parameters of Tafel curves are summarized in Table 1. It was found that the corrosion potential (Ecorr) of bare AZ31 Mg is about -1.599 V vs. SCE, showing that AZ31 Mg is chemically active and susceptible to corrosion. Compared with bare AZ31 Mg, the sample with MAO coating displays a corrosion potential of -1.479 V, and the sample with composite coatings exhibits a corrosion potential of -1.548 V, which the corrosion potential of the as-prepared specimens all shifted in a positive direction. Owing to many factors affecting the anti-corrosion performance of MAO, such as the pore size, porousity and the number of isolated holes,18, 42 which will not be discussed in detail here. In here, the MAO/ M-16/epoxy coating displays a negative potential compared to other coatings. These mainly due to many factors affecting the corrosion potential, such as looseness of coating, polarization, accumulation of negative ion and evolution of hydrogen or oxygen reaction.43, 44 Meanwhile, it is evidently that the corrosion current density (icorr) of samples decreased from 1.473×10-6 A/cm2 to 9.700×10-9 A/cm2 after immersion in 3.5 wt % NaCl solution for 24h. And by calculating, the polarization resistance (Rp) and protection efficiency (‫ܧ‬୮୰୭୲ୣୡ୲୧୭୬ ሻ of as-prepared specimens were obtained. It was clearly that the polarization resistance of bare AZ31 Mg was 2.16×104Ω·cm2, and the polarization resistance of MAO/M-16/Epoxy coating was 3.76×106Ω·cm2, which increased two orders of magnitude. More significantly, the protection efficiency of composite coating would reach 99.3 %. All results demonstrated the corrosion resistance of AZ31 Mg substrate was enhanced owing to the formation of porous oxide micro/nano-container with inert properties, the adsorption action between the inhibitor and substrate,


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and the isolating effect of epoxy top-coating. It was concluded that the composite coating consisted of MAO film, M-16 inhibitor and epoxy top-coating formed on the Mg alloy substrate, to some extent improves the corrosion resistance of AZ31 Mg alloy.

Figure 7. Polarization curves of AZ31 Mg, AZ31 Mg/M-16, AZ31/M-16/Epoxy coating, MAO, MAO /M-16 and MAO/M-16/Epoxy coatings in 3.5 wt% NaCl solution for 24 h.

Table 1 Electrochemical parameters of the polarization curves.

To further understand the effect of composite coating on the corrosion resistance of the AZ31 Mg alloy, the electrochemical impedance spectroscopy of all the specimens was conducted at their OCP and the results are presented with the corresponding equivalent electrical circuits. Figure 8a and 8b are the Nyquist plots of AZ31, AZ31/M-16, AZ31/M-16/Epoxy, MAO, MAO/M-16 and


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MAO/M-16/Epoxy. It can be clearly found that electrochemical impedance spectra for different samples immersed in 3.5 wt % NaCl solution contained a depressed semicircle at least. In general, the lager axial radius of the semicircle arc, the better anticorrosive property of the coating.45 As shown in Figure 8a, the diameter of composite coating consists of MAO, M-16 inhibitor and epoxy coating was much larger than samples without micro-arc oxidation film, such as AZ31/M-16/epoxy sample. From the magnification image of Figure 8a at the high frequency, it is observed that the arc diameters of MAO coated specimens were greater than the AZ31 Mg with M-16 or epoxy coating. Results showed that the corrosion resistance of AZ31 Mg was improved significantly by the construction of the composite coatings comprised of MAO, M-16 inhibitor and epoxy coating. In addition, the Nyquist plot of the bare AZ31 Mg alloy (see inset in Figure 8a) consists of one capacitive loop at high frequency and one inductive loop at low frequency, which is similar to previous reports3. According to related reports,46-48 the capacitive loop represented the characteristics of the electric double layer at the electrode/electrolyte, while the inductive loop at low frequencies in the fourth quadrant is the dissolution of Mg and is the indicative of pitting corrosion of the substrate. Therefore, the AZ31 Mg has been corroded after immersion in 3.5 wt % NaCl solution. In general, the impedance at the high frequency region reflects the property of the coating of its compactness, and the impedance modulus at low frequency zone is related to the ability of corrosion resistance.49 Figure 8c and 8d show the Bode plots of different samples in 3.5 wt % NaCl solution for 24 h. From Figure 8c, the impedance modulus of bare AZ31 Mg at low frequency is about 102 Ω ·cm2. And the impedance modulus of MAO film coated AZ31 Mg is about 105 Ω ·cm2, which is three orders of magnitude higher than that of AZ31 Mg. It was demonstrated that the formation of porous oxide ceramic membrane facilitated the protection for the substrate materials. Compared with the AZ31 Mg and MAO at absence of inhibitor, the impedance modulus of AZ31 Mg/M-16 and MAO/M-16 has significant increase. These results manifested the possible adsorption action between the inhibitor and substrate played remarkable effect for corrosion resistance. However, the impedance modulus of the MAO/M-16/Epoxy coating at low frequency is about 1010 Ω ·cm2, which is five orders of magnitude higher than that of AZ31 Mg. The outstanding improvement of corrosion resistance for MAO/M-16/Epoxy coating would be ascribed to the isolating effect of the epoxy anticorrosion coating, which prevented the aggressive ions from permeating into the substrate. Figure 8d displays the phase angle diagrams of specimens after different treatments. It was found ACS Paragon Plus Environment

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that the phase angle value of bare AZ31 Mg is about -60 o and has two peaks. According to above characteristics, the model can be represented as Rs(QdlRct(RLL) in Figure 9a, which were consistent with the curves characteristics of Figure 8a. In the equivalent circuit, Rs represents the solution resistance, Rct is the charge transfer resistance, and Qdl is the double electrical layer capacitance, L is the inductive impedance resistance and RL.is the passivation film resistance, However, the AZ31 Mg/M-16 and MAO have two time constants, thereby the equivalent circuit diagram can be fitting as Figure 9b. Apart from the Rs, Qdl and Rct, two other elements are introduced in the Rs(Qc(Rc(QdlRct))) model: a constant phase element (Qc) and resistance (Rs) represent the adsorption layer or oxidation layer. Nevertheless, it is evident that the phase angle of MAO/M-16/Epoxy coating is higher than any other samples, which is about -80 o, indicating a good anticorrosion property. As shown in Figure 9d, three phase angle peaks were obtained from the composite coating, such as AZ31/M-16/Epoxy, MAO/M-16

and

MAO/M-16/Epoxy.

Therefore,

the

model

can

be

interpreted

by

Rs(Qf(Rf(Qc(Rc(QdlRct))))). Herein, the Qf can be used to evaluate the degree of penetration of the corrosive ions into the membrane layer. In Figure 9c, values of Rf and Qf correspond to the epoxy anticorrosion coating resistance and capactive reactance.


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Figure 8. The Nyquist curves (a,b)

and Bode curves (c, d) for AZ31 Mg, AZ31 Mg/M-16,

AZ31/M-16/Epoxy coating, MAO, MAO /M-16 and MAO/M-16/Epoxy coating immersed in 3.5 wt % NaCl solution for 24h, the enlarged impedance spectra(Figure b) in the higher frequency range.

Figure 9. The equivalent electric circuit diagrams obtained by EIS data fitting: (a) AZ31 Mg, (b) AZ31 Mg /M-16 and MAO, (c) MAO/M-16, AZ31 Mg /M-16/Epoxy coating and MAO/M-16/Epoxy ACS Paragon Plus Environment

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coating. Rs: solution resistance, Qc: constant phase element corresponding to coating, Rc: coating resistance, Qf: constant phase element corresponding to epoxy coating, Rf : epoxy coating resistance, Qdl: constant phase element corresponding to double-layer, Rct: charge transfer resistance, RL: passivation membrane resistance and L:inductance impedance resistance.

The fitting results of the obtained EIS data by these three electrical circuits are showed in Table 2. According to the Rs values, no significant change is found in the solution resistance for all samples. Generally speaking, Rct as a pivotal parameter to evaluate the anticorrosion performance of coating, which is usually inversely proportional to the corrosion rate of sample. It is observed that MAO/M-16/Epoxy coating has low capacitance and high resistance compared with other specimens. These results demonstrated that the composite coating consisted of MAO membrane, M-16 inhibitor and epoxy top-coating has superior corrosion resistance, which on the one hand would be ascribed to the formation of porous micro/nano-container as inhibitor impedes the transmission of corrosive ions, on the other hand, the epoxy anticorrosion coating reduced the permeating opportunity of aggressive substance.

Table 2 Electrochemical parameters of various samples obtained via equivalent circuit fitting of the EIS curves after immersing in 3.5 wt % NaCl Solution for 24h.

3.6 Salt spray test To demonstrate the long-term anticorrosion performance of composite coatings, the salt spray measurements were done as shown in Figure 10. It was evident that the pitting corrosion phenomenon began to happen on the surface of bare AZ31 Mg after 24 h of exposure to the salt spray test. After 30 days, the bare Mg was completely corroded and accompanied by broken signs. At the meanwhile, the total surface of AZ31 Mg with M-16 inhibitor was also severely corroded, and ACS Paragon Plus Environment

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the thick corrosion products aggregate could be observed. It is noteworthy that the AZ31 Mg with MAO film and MAO/M-16 only appeared slight pitting corrosion, indicating that the formation of micro-arc oxidation film protected the materials from attacking. However, there are no obvious changes on the surface of MAO specimens with M-16 inhibitor and epoxy top coating after 30 days of exposure to the salt spray test. In addition, to investigate the protective effect of the composite coating after scratches, the pictures of e1 and e2 were obtained. It was found that the scratched MAO/M-16/Epoxy coating have lower corrosion than AZ31 Mg/M-16/Epoxy coating with scribe, which the boundary corrosion signs occurred on the latter sample. Therefore, the salt spray resistance was obviously improved by the formation of composite coating.

Figure 10. Images of different treatment samples after interval period (0 h, 24 h , 5 days and 30 days) salt spray test (a1: bare Mg；b1: bare Mg/M-16; c1: bare Mg /M-16/Epoxy coating; d1: bare Mg/ Epoxy coating; e1: bare Mg /M-16/Epoxy coating with scribe; a2: MAO; b2: MAO /M-16; c2: MAO/M-16/Epoxy coating; d2: MAO/Epoxy coating; e2: MAO /M-16/Epoxy coating with scribe). Scale bar: 2 cm. To illustrate the corrosion resistance process of the composite coatings in 3.5 wt % NaCl solution, a proposed mechanism diagram was shown in Figure 11. Compared with the bare AZ31 Mg, the MAO/M-16/Epoxy composite coating was comprised of inorganic ceramic membrane of MAO film, organic inhibitor corrosion M-16 and epoxy top-coating. The favorable corrosion resistance and


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salt spray resistance of composite coating would be ascribed to several aspects reasons following: (1) The corrosion resistance of inorganic ceramic membrane which could block the penetration of chloride ions and shift the corrosion potential to a more positive value; (2) The inhibition effect of M-16 corrosion inhibitor which could form an adsorption film on the surface of the protected metal substrate to impede the invasion of corrosive ions such as Cl- to a certain extent, thereby protecting the material from attack. For the adsorption action between the M-16 and substrate, the QCM measurement was demonstrated fully, as shown in Figure S4 and Figure S5. It can be concluded that negatively charged M-16 molecules are adsorbed onto the substrate by electrostatic interactions in physical adsorption.50 (3) The anticorrosion top-coating with high binding energy prolongs the transmission path of the aggressive substance into the substrate materials, thereby playing a long-term protection effect. Nevertheless, according to related literature,51 the microstructure of AZ31 Mg substrate was composed of α- Mg and AlMnSi phases. And the initiation of corrosion for bare AZ31 Mg begins at the second phase-AlMnSi particle when was attacked by aggressive ions, which would be damaged in a short time. Therefore, the above synergistic effects of composite coating contributed to the protection for AZ31 Mg for a long time.

Figure 11. The schematic illustration of corrosion process of bare AZ31 Mg (a) and composite coating (b) in 3.5 wt % NaCl electrolyte solution.

4. CONCLUSION In this study, an ideology of synergistic effect to improve corrosion resistance of the AZ31 Mg alloy was adopted by the combination of micro-arc oxidation layer, corrosion inhibitor and epoxy


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top-coating. The successful incorporation of inhibitor was demonstrated by the EDS mapping distribution. Electrochemical studies showed the MAO/M-16/Epoxy composite coating has superior anticorrosion performance in 3.5 wt% NaCl solution, more importantly, which would achieve lower corrosion current density with 9.7×10-9 A/cm2 and protection efficiency with 99.3%. Moreover, the formation of the composite coating with higher salt spray resistance, to some extent, protected the metal substrate from corrosion within 30 days. The corrosion resistance of the as-prepared composite coating would be ascribed to the formation of porous oxide ceramic with inert properties, the adsorption isolating action between the inhibitor and substrate and the inhibition effect of epoxy top coating. The entire coating preparation process is simple and can be mass-produced, and it is expected that it can be used for material protection in the near future.

ASSOCIATED CONTENT SUPPORTING INFORMATION The 1HNMR spectrum was shown in Figure S1. The

13

CNMR spectrum was shown in Figure S2.

The XPS analysis of survey (a) and high-resolution XPS spectra of Br 3d region for corrosion inhibitor (M-16) embedded into the MAO porous structure in Figure S3. The changes in frequency of QCM chip gold (The solvent of ethanol is the baseline) in Figure S4. The schematic diagram of interface adsorption of M-16 inhibitor molecule in Figure S5.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected]； [email protected].

ACKNOWLEDGMENT Thanks

for

the

financial

support

of

the

NSFC

(No.

51722510,

Key research project of Frontier Science of the Chinese Academy of Sciences


21573259),

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(QYZDY-SSW-JSC013), the outstanding youth fund of Gansu Province (1606RJDA31), Qingdao science and technology plan application foundation research project (17-1-1-70-JCH) and the “Hundred Talents Program” of Chinese Academy of Sciences (D. Wang).

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New Method for the Corrosion Resistance of AZ31 Mg Alloy with a

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