Investigating an effective model to estimate the water diffusion coefficient of a hybrid polymer-oxide coating

Progress in Organic Coatings 141 (2020) 105548

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Investigating an effective model to estimate the water diffusion coefficient of a hybrid polymer-oxide coating

T

X. Chen, S.F. Wen*, T. Feng*, X. Yuan, Z.F. Yue School of Mechanics and Civil & Architecture, Northwestern Polytechnical University, Xi’an, 710129, PR China

A R T I C LE I N FO

A B S T R A C T

Keywords: Hybrid polymer-oxide coating Diffusion coefficient EIS Water transport behavior

An effective model was developed to predict the water diffusion coefficient in a hybrid polymer-oxide coating. The composite coating consists of microarc oxidation (MAO) coating and silicone-epoxy (SE) coating. The performance of the SE/MAO coating and MAO-coated samples were studied by adhesion tests, X-ray photoemission spectroscopy (XPS), Fourier transform infrared spectrometer (FTIR) and scanning electron microscopy (SEM). The electrochemical impedance spectroscopy (EIS) was used to study the water transport behavior of the coatings. The diffusion coefficient (Dexp) obtained from the EIS experimental measurement was compared with the effective diffusivity (Deff) predicted by the mathematical model based upon the Ohm’s law and the model based on the effective validation model (EVM) which considers the water concentration jump at the SE and MAO coating interface The results demonstrated that the noncontinuity of the water concentration at the SE/MAO interface resulted in a difference between the diffusion coefficient obtained by the EIS results and mathematical model. The effective diffusion (Deff) coefficient derived from the EVM (6.47 × 10−12 cm2·s−1) was similar to the Dexp obtained by the EIS experimental method (7.22 × 10−12 cm2·s−1).

1. Introduction Corrosion has been a crucial problem that limits the further application of metallic structures in many fields, such as the aerospace industry, automobiles, electrical appliances and machinery industry. The application of an organic coating is the most effective method to improve the corrosion resistance of metallic materials [1–3]. The anticorrosion property of barrier coatings is usually measured by water diffusion. Water diffusion can be calculated by electrochemical impedance spectroscopy (EIS), Fourier transform infrared spectroscopy (FTIR) [4–6], gravimetric method [7–9] and microbalance water vapor uptake [10,11]. Krazk [12] investigated water diffusion in polymer coatings containing water-trapping particles and the values measured by microbalance and ATR-FTIR were compared with those obtained by numerical calculations. The water traps in the epoxy coating caused a significant decrease in the water diffusion coefficient. Ding [13] studied the water transport behavior of a solvent-free epoxy coating with different thicknesses on a Q235 steel surface. The diffusion coefficient, volume fraction and total water absorption at saturation of the coating were all calculated from the EIS experiment results, indicating the good water resistance and properties of the coating. Nguyen [6] compared the water uptake determined by impedance measurements and ⁎

gravimetry. The study shows that the water uptake obtained from dielectric constant values was in good agreement with that measured by gravimetry. According to the water diffusion behavior, they found that the coating thickness would affect the corrosion process. Rezaei [14] reported the evaluation of water diffusion through polyurethane coatings. The water diffusion coefficient strongly depends on the service temperature and the thickness of the coating. Liu [15] investigated the water diffusion behavior of alky coatings with micaceous iron oxide pigment at different volume concentrations. The study showed that the diffusion coefficient of water and the barrier property of pigment affected the water behavior. Shreepathi [7] studied the water diffusion and water vapor permeability of six different coatings. The chemical structure of the polymer influenced the anticorrosive properties of these coatings, which also affected the water diffusion. Generally, water diffusion through a barrier coating plays a significant role in evaluating the performance and endurance of the coating. Slowing down the water diffusion rate would increase the lifetime of the metallic structure. However, direct preparation on a metal surface remain potential risk of localized failures. The low adhesion of organic coatings to metallic cannot provide effective longterm protection as well. Microarc oxidation (MAO), developed from anodizing oxidation, has been considered as an effective and practical surface-pretreatment

Corresponding authors. E-mail addresses: [email protected] (S.F. Wen), [email protected] (T. Feng).

https://doi.org/10.1016/j.porgcoat.2020.105548 Received 19 February 2019; Received in revised form 21 December 2019; Accepted 9 January 2020 0300-9440/ © 2020 Elsevier B.V. All rights reserved.

Progress in Organic Coatings 141 (2020) 105548

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samples were kept in a desiccator for until use.

of Al alloys to slow down the corrosion rate in that the MAO coating is better adhered to Al substrates [16–18]. The MAO process is useful for preparing coatings due to the in-situ formation of coatings, which improves the adhesion between the coatings and substrates [19]. However, during the coating preparation, the gas evolution process generates many micropores in the MAO coating [20,21]. The micropores and microcracks can be inevitably generated by the MAO method. When the MAO coating is exposed in the corrosive environment, the corrosive medium can penetrate coating through the micropores and microcracks and attack the substrate [22]. To fill the micropores and microcracks in a MAO coating, sealing the coating is necessary to prevent the penetration of water and corrosive medium [23–25], such as chromate conversion coatings [26] and solgel coatings [27,28]. A hybrid silicone-epoxy coating has been used, because of the superior combined properties of the epoxy resin and solgel coating with good corrosion resistance [29]. On the one hand, the hydrolysis of silanes produces silanol groups that can react with the hydroxides on metallic surfaces, forming Si-O-Me metal bonds [30]. In turn, the remaining silanol groups facilitate the formation of Si-O-Si bonds [31]. The bonds of Si-O-Me and Si-O-Si cause the silane coating to form a strong adhesive bond [32]{Yue, 2013 #233} between the coating/metal interface and more cross-links and a denser film to hinder the penetration of corrosive medium. On the other hand, compared with the sol-gel coating, the organic resin can make up for the lack of flexibility, so that the coating will not shrink as much. Thus, the SE/MAO coating offers better corrosion protection and a decreased water diffusion rate than other coatings [33–35]. The numerical diffusion coefficient of the composite coating is different from that of a pure coating. The composite coating consists of two different phases. Each phase exhibits different transport properties. Normally, the composite coating can be seen as an arrangement of series and parallel resistances. Ramirez [36] presented an Ohm’s lawanalogy approach for the estimation of the effective diffusivity in permeable composite media. Particles are seen as an arrangement of series and parallel resistances. However, few studies have reported a mathematical model for a coating consisting of a porous ceramic-like MAO coating and a silicone-epoxy sealing coating. In this work, an effective validation model (EVM) to obtain the effective diffusion coefficient Deff of a hybrid polymer-ceramic coating was developed. The model requires the structure and water transport properties of the individual phases. Electrochemical impedance spectroscopy (EIS) was used to investigate the water transport behavior of the MAO coating and SE/MAO coating. Deff predicted by EVM and Ohm’s law were compared with the water coefficient calculated from the EIS results.

2.2. Coating preparation 2.2.1. Preparation of the MAO coating The MAO coatings were prepared in 30 g/L NaAlO2 and 8 g/L NaOH solution in distilled water at room temperature (25 °C) in a 65 kW microarc oxidation device with a cold water cooling system as shown in Fig. S1. The Al-alloy served as anode and the stainless steel served as the cathode. The samples were operated under a constantly applied potential of 420 V at 500 Hz with a duty cycle of 0.4. The AA2024 alloys were treated in electrolyte for 20 min at 25 °C to obtain the MAO coating. Under the combination of electrochemical, plasma, chemical and thermal reactions, ceramicAl2O3 accumulates on the surface of the substrate. The obtained samples were cleaned with ethanol five times. After that, the MAO-coated samples were rinsed in deionized water at 80 °C and then dried at room temperature. It was documented that boehmite (AlOOH) formed in hot water [37]. All the samples were kept in a desiccator for until use. 2.2.2. Preparation of the silicone-epoxy coating The silicone-epoxy coating (SE) was prepared from SILIKOPON EF resin (EVONIK Industries, with the epoxy equivalent weight of 450 g/ mol) and the solvent mixture (30 wt. % n-butyl alcohol and 70 wt. % butyl acetate). Dynasylan AMEO (EVONIK Industries, with an amine equivalent weight of 110 g/mol) was used as the hardener. The weight ratio of the hardener and silicone-epoxy resin was 1:4. The sealing SE coating was sprayed on the samples with MAO coatings and dried at 60 °C for 6 h and then at room temperature for 170 h to obtain the SE/ MAO coating. For comparison, the samples coated with only use SE coating were also prepared in the same preparation process and used as the reference samples. 2.3. Surface characterization The coating thickness was measured by an electromagnetic digital coating thickness gauge (TT230, TIME, China). The gauge was first calibrated by a bare Al alloy. Surface roughness (Ra) was measured with a surface roughness tester (PS1, MARSHRF, Germany). To ensure the accuracy of the roughness and thickness, six different places in the coated region of each sample were measured, and the average value was calculated. For each test method, three samples were used. Surface and cross-section morphologies of the coatings were obtained by a scanning electron microscopy (Quanta 600 FEG SEM, FEI, America and VEGA3 TESCAN, Czech) and optical microscopy (VHX-5000, Japan). The elemental composition of the coatings was also analyzed by energy dispersive spectrometry (EDS) equipped with SEM. For the surface observation samples, the large samples were cut into 10 mm × 10 mm, and the cross-section samples were polished to 1 μm. Before SEM observation, the samples were sputtered with a thin layer of Au to enhance the conductivity. FTIR measurements (Nicolet iS10, Thermo Scientific, American) were carried out on the SE/MAO coating in the mid-IR range from 400 to 4000 cm−1 with the attenuated total reflectance diamond component (ATR-FTIR).

2. Experimental 2.1. Pretreatment of the substrate AA 2024 sheets (150 mm × 70 mm × 2.5 mm) were used as the substrate. Their nominal composition is shown in Table 1. Prior to MAO treatment, all the samples were degreased in ethanol first. Then, they were cleaned in an aqueous solution of 5 wt. % sodium hydroxide to remove impurities from the alloy surface. Furthermore, 11 wt. % dilute nitric acid (69 %) solution was used as the following picking solution to remove oxide/hydroxide deposits left by the alkaline cleaning step. After that the samples were cleaned in ethanol ultrasonically for 10 min again, rinsed with purified water and dried at room temperature. All the

2.4. Adhesion test The adhesion of the coating was estimated by cross-cut tests according to ASTM D3359 (Standard Test Methods for Measuring Adhesion by Tape Test) and pull-off method according to ASTM D454117 (Standard Test Method for Pull-Off Strength of Coatings Using Portable Adhesion Testers). Eleven cuts in the vertical direction were made into the substrate by sharp razor blades and every cut was 1 mm apart. The detached flakes were wiped off from the coating surface by a soft brush. Then, Elcometer 99 adhesive tape was placed on the grid of the samples firmly, ensuring good contact with the coating. After 60 s,

Table 1 The chemical compositions of 2024 Al-alloy (wt.%). Element

Cu

Mg

Mn

Fe

Si

Zn

Ti

Cr

Al

Wt.%

4.36

1.49

0.46

0.35

0.14

0.07

0.01

< 0.01

Balance

2

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reacts with another epoxy ring, leading to the creation of a tertiary amine. The tertiary amine does not take part in the curing reaction at room temperature. During the first two steps, hanging −OH groups are created in the chemical reaction. The −OH groups react with epoxy and alkoxy groups or silanol groups again, which promotes the formation of the cross-linked structure of the coating. Specifically, asymmetric stretching of SieOeSi [39] is observed at approximately 1025 to 1131 cm−1. The bands at 801 cm−1 are assigned to the symmetric stretching vibration of the SiO-Si network, which confirms the condensation of Si−OH. In addition, Si−OH in AMEO reacts with the inherent Si−OH in the silicone resin, which contributes to a large-scale and strong Si-O-Si network. The peak at 697 cm−1 is related to the AleO bond [40] originating from the Al-OSi covalent bond, which formed during the dehydration reaction between Si−OH and Al−OH.

the tape was removed rapidly, and the grid area of the coating was examined with a microscope. The optical micrographs of the samples were recorded before and after the adhesion test. The reported pull-off data were the average of three samples with tape at the same positions on the coating surface. 2.5. X-ray photoelectron spectroscopy (XPS) analysis X-ray photoelectron spectroscopy (XPS) (Kratos Axis Ultra DLD) was used to analyze the interface between the MAO coating and SE coating. The measurements were carried out with Al Kα as the radiation source. The coated samples used in the XPS test were been tested by pull-off adhesion measurements. SE/MAO coated samples showed visible shiny region of residual SE coating and matte region where the MAO coating was visible. For each sample, three analyses were performed.

3.2. Microstructure of the MAO and SE/MAO coatings

2.6. EIS measurements

The surface morphologies of the MAO- and SE/MAO-coated samples are illustrated in Fig. 2a and b. The roughness of the samples with the MAO coating and SE/MAO coatings are shown in Fig. 2c. Many pores and melting holes with different sizes look like volcanos distributed randomly throughout the MAO coating. Moreover, microcracks formed during the cooling process [21] can be observed in the oxide coating. Ceramic particles accumulate on the coating surface, which contributes to a rough surface (Ra≈2.015 μm). In contrast, the SE/MAO coating shows a smooth surface (Ra≈1.479 μm), and there are no obvious cracks or pores. The roughness of the multilayer coating is greatly reduced, and the SE/MAO coating is relatively smooth with no obvious defects. Fig. 2d and e show the cross-section morphology of samples with the MAO coating and SE/MAO coating. Owing to the porous-structure growth characteristics of the MAO coating, the MAO coating grows on the substrate. Moreover, the interface is uneven due to chemical bonding with the metal substrate [21]. In Fig. 2d, the porous MAO coating is approximately 15 μm in thickness, which is consistent with the results of the thickness measurements shown in Fig. 2e. As shown in Fig. 2e, the SE coating with a thickness of 25.68 μm is locked into the porous surface closely through the coarse and porous layer of the MAO coating. Obviously, the sealing treatment increases the protective coating thickness of AA2024 and smooths the surface appearance. The outer pores in the MAO surface are filled by the SE coating. The backscatter electron morphology of the cross-section of the SE/ MAO coating is shown in Fig. 2g. The interface between the MAO coating/SE coating can be clearly detected due to the phase identification method. The EDS spectra of selected areas A and B of the SE/ MAO coating are presented in Fig. 2i and h. Area A is located in the MAO coating while Area B is located in the SE coating. The MAO coating is composed of Al and O (Fig. 2i) which is consistent with the literature [41,42]. The SE/MAO coating includs C, O, and Si, as shown in Fig. 2h, which depends on the chemical composition of the SE coating.

The electrochemical impedance spectroscopy measurements were performed using CS350 electrochemical system with ZView 2 testing software. The conventional three electrode system was employed as the electrochemical system in the frequency range of 10−2-105 Hz at open circuit potential, and the signal amplitude was 10 mV. A saturated calomel electrode (SCE) was used as the reference electrode and a platinum plate was used as the counter electrode. The working electrode area of the substrate in 3.5 % NaCl was 1 cm2. The test was conducted at room temperature. For more accurate results, the same procedure was repeated three times and the average value was calculated. 3. Results and discussion 3.1. FTIR analysis of SE/MAO coating The FTIR spectra of the SE/MAO coating are shown in Fig. 1, and reveal the chemical composition of the composite coating. The broad band at 3360 cm−1 corresponds to the presence of the −OH groups of silanols. The typical bands at 2860 cm−1and, 1259 cm−1 and 2932 cm−1 and 1430 cm−1 correspond to the symmetric stretching vibration and bending vibration of CH3 and CH2 [38], respectively, which provides the evidence of the alkylic chains. The formation of SiOH depends on the hydrolysis of the AMEO silane and the reaction between the silicone epoxy resin and curing agent. The curing agent AMEO containing silanol (Si−OH) and a primary amine (NH2) reacts with the EF resin containing epoxy functional groups and silicone in the following steps: first, NH2 reacts with the epoxy ring, leading to the extension of the long chain of the structure. Then, the secondary amine

3.3. Adhesion The optical microscopy images of the SE-, MAO- and SE/MAOcoated samples after the tape-adhesion test are illustrated in Fig. 3. It can be clearly observed that the edges of the cuts in the SE-coated sample (Fig. 3a) are damaged. However, the edges of the cuts are smooth, and there is no obvious damage at the cross area of the MAOcoated sample (Fig. 3b). In addition, there is slight pulverization on the edges of the cuts. Apparently in Fig. 3c, no pulverization and damage are observed on the edge of the multilayer coating. The adhesion strength of coatings on Al-alloy substrates are also studied by pull-off adhesion measurements. Optical microscopy images of the SE- and SE/MAO- coated samples after the pull-off adhesion test

Fig. 1. FTIR spectra of SE coating. 3

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Fig. 2. Surface morphology and cross section of (a, d) MAO and (b, e) SE/MAO coated samples, (c, f) thickness and roughness of the samples with different coatings； Selected areas in cross section of SE/MAO coated samples and EDS spectrum: (g) cross section image at BSE mode and area A and B in the image of SE/MAO coating for composition identification; (h) EDS spectrum of area B; (i) EDS spectrum of area A (inset on the top right was atomic percentage).

on the MAO coating due to the cohesive failure. The matte area corresponds to the area where the MAO coating is exposed to the air while the shiny area is the residual SE coating. Thus, the concentration of Si in the matte area decreases, and Al in the matte area varies from 2.48 % to 11.86 %. The Al 2p spectra of the MAO coating, shiny area and matte area were also investigated by XPS, as shown in Fig. 4c. The results obtained from the deconvolution of Fig. 4c are listed in Table 2. Curves a, b and c refer to MAO coating, shiny area and matte area, respectively. Accordingly, the Al 2p spectra for the three coatings are curved fitted into different component peaks. The Al 2p spectrum for the MAO coating reveals the presence of Al2O3 and AlOOH peaks centered at 73.98 [43] and 74.60 eV [44], respectively. It can be concluded that the MAO coating is mainly composed of aluminum oxide and aluminum hydroxide oxide. In the case of the SE/MAO coating after the pull-off test, a peak centered at 75.5 eV corresponding to the Al-O-Si bond appears. It was documented [45] that AlOOH can provide hydroxyl groups for the sealing treatment. Rich hydroxyl groups on the surface of the oxide film provides the basis for the condensation reaction among silanols (Si−OH). The Si−OH in SE coating establishes hydrogen bonds with the OH on the surface of the MAO coating, which stimulates the Si-O-Al bond. The reaction is as follows:

are shown in Fig. 3d and e. For the pure SE coating, the coating is completely detached from the substrate after the pull-off test. The type of coating detachment from the surface is adhesive. However, the SE/ MAO-coated samples show cohesive failure after testing, and the coating is not completely detached from the oxide coating surface. From Fig. 3f (the magnification of Fig. 3e, it is clear that the failure area has two regions: a shiny region where the residual SE coating is visible and a matte region. The adhesion results are shown in Fig. 3g. Obviously, the adhesion of the SE/MAO coating (20.64 ± 0.02 Mpa) is much greater than that of the blank SE coating (2.13 ± 0.02 Mpa) to AA2024. The adhesion is enhanced significantly, and the SE coating adheres well with MAO coating. Apparently, the presence of the pores in the MAO coating leads to stronger bonding between the SE coating and MAO coating. 3.4. XPS analysis of the coatings XPS analysis was employed to confirm the chemical composition of the MAO and SE/MAO coatings and the interface bonding between the SE coating and MAO coating, as shown in Fig. 4. The Al 2p, O 1s, N 1s and C 1s spectra of the MAO coating are included. It is noted that Al and O are the main components of the MAO coating. The Al matrix was dissolved to Al3+ in the MAO process. Al2O3 and AlOOH are the main components. The C 1s and N 1s peaks for the MAO coating originate from surface adsorption rather than the coating. Obviously, the SE/MAO coating is different. Si appear,s and C, and N contents increases. For the SE/MAO sample, C, Si and N are related to the chemical composition of the epoxy resin and curing agent. Because the thickness detected by XPS is only 5–10 nm, the Al under the SE coating could not be detected. Meanwhile, the increase in C and the decrease in O also explain that the MAO coating is completely covered by the organic coating. The presence of the Si 2p peak illustrates that organo-silicone plays a significant role in film formation. After the pulloff test, some organic coating is pulled off, while some coatings remain

Al − OH (MAOsurface ) + Si − OH (SEcoating ) → Al − O − Si (interface ) + H2 O

(1)

The development of Al-O-Si bonds results in a relatively stable surface coating. It is noteworthy that the intensities of the Al-O-Si and Al2O3 component peaks in the matte area are much greater than those in the shiny area while the detected AlOOH shows a decrease. The residual coating in the matte area is so thin that the Al2O3 in MAO coating is exposed for detection. The Al-O-Si bond makes the SE coating difficult to peel off from the MAO coating. Fig. 4d shows the high-resolution for Si 2p spectra of the SE/MAO 4

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Fig. 3. Optical microscopy images of samples after tape-adhesion test and pull-off adhesion test: (a, d) SE coating, (c, e) SE/MAO coating; (b) MAO coating after adhesion test, (f) magnification of Fig. 3e, (g) pull-off adhesion results.

MAO coating has a positive impact on the adhesion as well. The hydroxides on the MAO coating facilitate condensation of the silanol groups of AMEO to form SieOeAl bonds between the interface of the silicone-epoxy coating and MAO base. Meanwhile, the silanol groups condense again and form SieOeSi, which can strengthen the barrier properties of the SE coating. The combination of SieOeAl and SieOeSi improves the adhesion.

coating (a), shiny area (b) and matte area (c). Because there is no Si in the MAO coating, it is not studied here. The board asymmetrical spectral envelope of Si2p can demonstrate the existing form of silicon in the coatings. For the SE coating, the curing reaction has a vital influence on the anti-corrosion property of the coating. The Si 2p spectrum for the SE coating reveals the presence of Si-O-Si peaks centered at 102.64 eV [46] and a small amount (11.1 %) of SiO2 centered at 103.54 eV [47], as shown in Table 3. SieOeSi is representative of the siloxane bonds formed with the curing agent of amino silane and silicone epoxy resin. The containing silanol groups in the SE coating are converted to produce a stable coating due to polymerization. The reaction is as follows:

Si − OH + Si − OH → Si − O − Si + H2 O

3.5. Corrosion resistance of the coatings 3.5.1. EIS of the MAO coating, SE coating, and SE/MAO coating EIS is employed to estimate the water diffusion in films. The typical EIS curves of the MAO coating, bare SE coating, and SE/MAO coating immersed in 3.5 wt.% NaCl solution for various durations are presented in Figs. S2–S4. Equivalent circuits for fitting the EIS plots are shown in Fig. S5. The corresponding fitted results based on the equivalent circuit are listed in Tables S1–S3. Considering the nonideal diffusion behavior of the coatings, the constant phase element (CPE) is used, instead of the pure capacitance. The impedance of CPE is defined by the equation:

(2)

After the pull-off test, a new peak centered at 102.03 eV appears in the shiny area. It can be assigned to the Si-O-Al bond [48], indicating the formation of covalent metallo-siloxane bonds in the interfacial regions. Curve-fitted Si 2p core-level spectra of matte area are displayed in Fig. 4d. The peak corresponding to SiO2 particles disappears, and the intensity for SieOeAl becomes high. The residual thin coating sticks to the MAO coating firmly as a consequence of the interconnected bonding networks composed of SieOeAl and SieOeSi bonds. The XPS results show that SieOeSi and SieOeAl bonds form on the MAO coating after SE coating sealing. The adhesion of the SE/MAO coating is improved significantly due to the combination of the SE coating and MAO coating. On the one hand, the result indicates that using the coarse and porous MAO coating as the base coating can strengthen the mechanical bonding force by increasing the contact area. On the other hand, the chemical bonding between the SE coating and

Z (jω) = (Y0)−1 (jω)−n

(3)

where Y0 is the CPE constant, and n is the empirical exponent of CPE (0≤n≤1). If n = 1, the CPE is pure capacitance; If n = 0, the CPE is pure resistance. The deviation of n from unity is due to the dispersion effect. In the equivalent circuit, Rs is the electrolyte solution, Rc and Cc are the coating resistance and capacitance, respectively. Rct and Cdl are the double-layer capacitance and charge transfer resistance, respectively. The diffusion occurs on the surface of the electrochemically 5

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Fig. 4. XPS spectra (a) and elemental percentage (b) of MAO coating, SE/MAO coating, Shiny area and Matte area (inset on the top right was the elemental weight percentage data obtained from XPS spectra); High resolution spectra of Al2p (c) and Si2p (d) for MAO coating, SE coating, Shiny and Matte area, respectively.

capacitive loop decreases. The impedance of MAO coating decreases from 7 × 106 to 1 × 106 Ω·cm2 after 24 h of immersion. Prolonging the immersion time to 48 h, the impedance decreased rapidly to 5 × 104 Ω·cm2 due to the electrolyte penetrating the pores and cracks in the MAO coating. However, for the SE coating, there is only one capacitive loop at 1 h (Fig. S3a1). For the SE/MAO coating, the Nyquist plot (Fig. S4a1) shows only one time constant from 0 h to 24 h, representing the dielectric properties of an almost complete and defect free layer. The SE and SE/MAO coatings are equivalent to a barrier layer with a high coating resistance, as is presented by Model C. The impedance of the SE/MAO-coated Al-alloy is three orders of magnitude larger (1.63 × 1010 Ω·cm2 vs 4.58 × 106 Ω·cm2) than that of the MAO coating and one order of magnitude higher than that of the SE coating (1.63 × 1010 Ω·cm2 vs 4.06×109 Ω·cm2), indicating the enhanced corrosion resistance of the SE/MAO coating. After the MAO coating was immersed for 120 h (Fig. S2b1), the time constant at low frequency shifts to medium frequency, which can be the result of the charge transfer relaxation process of mass transport at the alloy/MAO coating interface [49], suggesting that the resistance of the MAO coating is influenced by electrolyte permeation. The high frequency capacitance can be attributed to the formation of corrosion products, although it is not so clearly separated from the medium frequency semicircle. Pitting cores are formed, and the passive film is repaired continuously. The relaxation of absorbed species such as aluminum hydroxide Al(OH)3 [50,51,46] which resulted in the inductive component, is characterized by RL and L [52] in the fitted equivalent model (Model B of Rs(Cc(Rc(Cdl(Rct L))))). The reaction including anodic and cathodic components can be expressed as follows:

Table 2 Results obtained from deconvolution of XPS spectra for MAO coating, Shiny area and Matte area. Position

eV

%At

Ref

MAO

73.98 74.60 73.94 74.60 75.50 73.95 74.61 75.20

11.1 88.89 12.26 76.46 11.48 29.35 44.85 25.80

Al2O3 AlOOH Al2O3 AlOOH AleOeSi Al2O3 AlOOH AleOeSi

Shiny area

Matte Area

Table 3 Results obtained from deconvolution of XPS spectra for SE/MAO coating, Shiny area and Matte area. Position

eV

%At

Ref

SE

102.64 103.65 102.03 102.75 103.24 102.37 102.97

11.1 88.89 42.86 38.64 18.50 86.31 13.69

SiO2 SieOeSi SieOeAl SieOeSi SiO2 SieOeAl SiO2

Shiny Area

Matte Area

active site which can be described by the Warburg impedance (W). Cdiff represents the diffusion capacitance and Rdiff represents the resistance of the accumulated corrosion product at the metal/coating interface. For the MAO-coated sample, with the initial 1 h (see Fig. S2a1), two semicircles can be found at high and low frequencies. Model A of Rs(Cc(Rc(RctCdl))) was introduced to fit the experimental data. By increasing the immersion time from 1 h to 24 h, the diameter of the

O2 +2H2 O+4e → 4OH−

(4)

Al + 3H2 O → Al(OH ) 3+3H+ + 3e

(5)

In the 180 h immersion (Fig. S2c1), the capacitance can be clearly 6

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interface. The high impedance of the EIS results indicates that the corrosive ions and electrolyte are obstructed effectively. The protective function of the SE/MAO coating is primarily attributed to the compactness of the system. Generally, a MAO coating can be an effective surface treatment of AA2024 alloy before applying the SE coating.

separated, and the semicircle at high frequency is smaller in size, indicating that the MAO coating can no longer provide corrosion protection for the AA2024 alloy. For the SE coating, after 5 h of immersion, the coating resistance decreases and the coating capacitance increases as a consequence of water uptake in the coatings. The spectra include two capacitive loops. Prolonging the immersion time to 27 h, there is a linear Warburg impedance with the slope angle of 45°at low frequency. After 30 h, the Warburg impedance is replaced by another loop almost approximately 0°with the real axis. However, after 110 h of immersion, the Warburg impedance disappears, and three capacitive characteristics are noticeable. The combination of Cdiff and Rdiff is used to fit the spectrum. For the SE/MAO coating, after 72 h of immersion, Model A is the proper equivalent circuit. In Fig. S3c2, the modulus of the SE/MAOcoated sample decreases gradually from 72 h to 144 h and then increases with the immersion time increasing from 215 h to 260 h. The modulus of impedance decreases again after immersion for 330 h. The penetration of water, oxygen and ions leads to a decrease in the corrosion resistance. However, hydrolysable alkoxy groups exist in the SE coating such as the methoxyl group (−OCH3) and ethoxyl group (−OC2H5), and condense again to form a -Si-O-Si structure, which can strengthen the barrier properties of the SE/MAO coating. At 514 h, a diffusion tail in the low-frequency region can be found in the Bode plot (Fig. S3c1). The diffusion tail at 45° can be ascribed to the slow infiltration process of the electrolyte into the MAO coating. Model D of Rs(Cc(Rc(Cdl(Rct L)))) is used to fit the experimental data. At 762 h, the diffusion process continues. After 1122 h, the diffusion line disappears, and a semicircle appears. The diffusion process can no longer be described by Warburg impedance. Model E (Rs(Cc(Rc(Cdl(Rct(Rdiff Cdiff)))))) is introduced to characterize the experiment results. The SE/ MAO coating prevents the diffusion of corrosion products, which can be the controlling procedure. As the immersion time increased to 3400 h, the Nyquist plot in the low-frequency region change from a diffusion tail to a noticeable capacitance arc, indicating that the double layer is penetrated by the solution. More Cl−are absorbed on the metal surface, which sped up the corrosion of the alloy. The SE/MAO coating was infiltrated.

3.5.3. Capacitance of SE and,SE/MAO coatings Water diffusion is an important measurement of the barrier property of a coating applied on metals. The capacitance of coatings changes with water diffusion. The water diffusion characteristics of the specimens were studied by the capacitance-time curve (Fig. 6). The coating capacitance at a fixed frequency [53] (10 kHz) was calculated using Eq. (6), and the capacitance curves of the coating are shown in Fig. 6a–c.

C= 1 (2π f|Z| sinθ)

(6)

As shown in Fig. 6a, the capacitance of the MAO coating ascends rapidly with increasing immersion time. The electrolyte penetrates the MAO coating and attacks the substrate quickly. However, the capacitance-time curve of single SE coating displays a “two-stage” electrolyte (Fig. 6b). The coating capacitance increases before 30 h, which suggests that the water permeates into the SE coating through the micropores and microcracks. After immersion for more than 30 h, the capacitance increases slowly and reaches a relatively steady value, which indicates that the water uptake of the coating reaches saturation. The curve in Fig. 6c presents the lnCc- t1/2 curve of the SE/MAO coating. The capacitance of the composite coating increases with the immersion time before 514 h and then remains stable for a long time until 1622 h. This implies that the corrosion medium, such as water, oxygen and Cl−, penetrates the coating gradually through the micropores formed by solvent evaporation of the double coating for 514 h, and then, the water uptake reaches saturation. A less porous and more organized coating structure delays electrolyte uptake and thus corrosive attack of the metal substrate. After immersion for 1622 h, the capacitance increases again, which implies the structural damage of the composite coating. Water has penetrated the interface between the substrate and coating, resulting in an increase in the corrosion rate. The interfacial corrosion plays a minor role in the corrosion control. By comparing the curves in Fig. 6, it is possible to conclude that the water uptake of the SE/MAO coating reaches saturation in a longer time (514 h vs 30 h), and the period of the saturation state of SE/MAO coating (approximately 1850 h) is longer than that of the SE coating (1100 h). It accounts that the SE coating can seal the porous MAO coating to prevent the corrosive medium from diffusion. Furthermore, the SE coating on the MAO coating shows excellent adhesion and corrosion protection performance due to the formation of stable SieOeAl chemical bonds between the AlOOH and Si−OH. Therefore, the saturation period of the coating system is longer. When the composite layer reaches saturation, the capacitance increases rapidly again, implying the degradation of the SE/MAO coating. The application of SE/MAO coating significantly increases the barrier property of the MAO coating, which decreases the water uptake of coatings. Water diffusion is important for evaluating the anti-corrosion properties of barrier coatings. Generally, the diffusion coefficient is regarded as a constant. Assuming that the swelling of the organic coating is ignored in this study, the diffusion coefficient (D) [54,55] can be given by the following equation:

3.5.2. Structure model The structure model of the MAO-coated samples is presented in Fig. 5. As depicted in Fig. 5a, several open pores grow in the MAO coating. The inner layer of the MAO coating is compact on the substrate and has less porosity. When the MAO coating is in a corrosive environment, the pores and cracks provide a channel for the diffusion of the corrosive medium, including Cl−, water and oxygen. The electrolyte starts to contact the Al2O3 constituent on the MAO coating surface. During the immersion, the electrolyte penetrates the porous layer and reaches the Al-alloy substrate. As a result, the SE coating can be an available barrier to obstruct the corrosive medium from contact with the substrate. As shown in Fig. 5b, the SE coating is locked into the MAO coating owing to the presence of a porous layer and a rough surface. The porous MAO coating enlarges the area of the surface in a contact. When applied on the outer surface, the pores and cavities are filled with the organic coating. The mechanical interlocking effect is improved. In addition, the –Si−OH of the silicone-epoxy coating and the −OH on the MAO coating surface condense to Si-O-Si and Al-O-Si structures. The cross-linking reaction results in a three-dimensional structure between the silicone-epoxy coating/MAO coating interfaces, which can further enhance the bonding strength. Compared with that of the MAO coating, the SE/MAO coating shows better corrosion resistance. The impedance of the SE/MAOcoated sample is approximately three orders of magnitude larger than that of the MAO coating (1.63 × 1010 Ω·cm2 vs 4.58 × 106 Ω·cm2). Apparently, it takes more time to penetrate the SE/MAO coating/metal

D=

b2Lπ 4( ln Cs − ln C0 )2

(7)

where Cs is the coating capacitance when is the coating is saturated with water, and C0 is the capacitance at zero. L is the thickness of the coating, and b is the slope of the lnCc- t1/2 curve. Eq. (8) was built based on the following assumptions: (1) the permittivity of the polymer-water system is linearly proportional to that of the pure components, (2) the water distribution is random, and (3) the permittivity is proportional to capacitance. 7

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X. Chen, et al.

Fig. 5. Structure illustration of (a) MAO coating (b) SE/MAO coating.

The relationship between the volume of the MAO coating (Ф) and the porosity of the MAO coating (P) can be expressed through Eq. (11):

The diffusion coefficients for the SE coating and SE/MAO coating based on the lnCc- t1/2 curve are listed in Table 4. DSE/MAO is lower by one order of magnitude than DSE (7.22 × 10−12 cm2·s−1 vs 3.97 × 10−-11 cm2·s−1). The smaller DSE/MAO value implies that the water uptake process is longer in the double coating system, and the SE/MAO coating system performs well for protecting the Al-alloy against corrosive media. Due to the absence of a saturation stage for a single MAO coating, the diffusion coefficient cannot be calculated according to Eq. (7). The effective diffusion coefficient of the MAO coating cannot be calculated by the results of the lnCc- t1/2 curve due to the lack of the saturation stage. It can be given by Kalnin’s model [56,57] according to Eq. (8): eff DMAO =

DH nΦ (kD I − DH ) ⎤ ⎡1 + 1 − Φ + kΦ ⎢ kD I + (n − 1) DH − Φ (kD I − DH ) ⎥ ⎦ ⎣

The porosity of the MAO coating was measured by the Archimedes’ principle [58]. Six MAO-coated samples were first weighed in dry air (m1), and the samples were dipped into water. The weight after water impregnation in air was recorded as m2. The samples were weighed while immersed in water, which was recorded as m3. All procedures ware repeated five times to ensure accuracy. The density result for the MAO coating was the mean value of the results. The absorbed water volume can be calculated as:

V=

(8)

ρI ρH

dMAO =

(9)

DH (n − 1) DSE = Φ n−1+Φ 1+ 2

(12)

m2 − m1 m3 − m2

(13)

Therefore, the porosity of the MAO coating can be measured by Eq. (14). The theoretical density of Al2O3 is considered to be dAl2O3 = 4.00 g/cm3. The density of the prepared MAO coating calculated from the Archimedes’ principle is dMAO = 3.60 g/cm3.

where n is space dimension (1, 2, or 3) and n = 3 in this paper, DI=DMAO = 0, and ρI and ρH are the equilibrium concentrations of the MAO and SE coatings, respectively. ρI = 0, k2 = 0; DH=DSE. As a result, eff DMAO can be obtained as follows: eff DMAO =

m2 − m1 ρw

The density of the porous MAO coating can be calculated from Eq. (13).

where DI and DH are the diffusion coefficient of the dispersed medium (DI=DMAO) and continuous medium (DH=DSE), respectively, and Ф is the volume fraction of the MAO coating. The coefficient k is defined through the equilibrium concentrations in the MAO coating and SE coating:

k=

(11)

Φ= 1 − P

d P = ⎜⎛1 − MAO ⎟⎞ × 100% dAl2O3 ⎠ ⎝ The eff DMAO =

(10) 8

eff DMAO

value can be obtained from Eqs. (10), (11) and (14).

DSE 1.5 −

(14)

P 2

=

DSE 1.5 −

10 % 2

= 2.74 × 10−11 (cm2·s-1) (15)

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X. Chen, et al.

density gradient at the interfaces, the steady-state flux J can be shown as [36]:

J = Di

δρ Li

(16)

where δρ is the external density gradient. When the diffusion process reaches a relatively steady state, JSE=JMAO=JSE/MAO. The effective diffusivity of a composite medium can be given by:

L L L = 1 + 2 D D1 D2

(17)

where Li and Di are the width and diffusivity of the i-th layer, and L and D are the whole width and diffusivity of the composite medium, respectively. Thus, in this study:

D SE/MAO =

LSE/MAO LSE DSE

+

LMAO D MAO

(18)

DMAO is replaced with eff DSE/MAO =

eff DMAO ;

thus, Eq. (18) is expressed as follows:

LSE/MAO LSE DSE

+

LMAO eff D MAO

(19)

According to the thickness measurement of the SE/MAO coating, eff LSE = 25.68 μm, LMAO = 15.28 μm, and LSE/MAO = 40.26 μm. DSE/MAO −11 2 −1 can be calculated as 3.35 × 10 cm ·s . Obviously, this value is one order higher than the experimental result. The difference can be attributed to the noncontinuity of the water concentration at the interface of a heterogeneous composite film comprising a MAO coating and a SE coating. The SiOH in the siliconeepoxy resin and curing agent promotes an interface with a higher density of AleOeSi bonds. Consequently, the water diffusion is not continuous at the interface. Considering the boundary concentration of the absorbed water, the schematic can be illustrated in Fig. 7b. As shown in Fig. 7b, the interface i is the boundary of the SE coating and MAO coating. ρiSE and ρiMAO are the boundary concentrations of the absorbed water for SE coating and MAO coating, respectively. ρ0 is the surface concentration of absorbed water for the SE coating and ρm is the concentration of the absorbed water close to the AA2024 alloy:

JSE =

DSE (ρ0 − ρiSE )

JMAO =

D MAO (ρiMAO − ρm )

JSE/MAO =

ln(Cs/F)

b/(lnF h1/2)

L/μm

D/cm2·s−1

MAO SE SE/MAO

−22.372 −22.75722

−20.559 −22.22366

0.314 0.309 0.024

15.28 25.68 40.26

3.97 × 10−11 7.22 × 10−12

DSE/MAO (ρ0 − ρm ) LSE/MAO

(22)

ρiSE

be the coefficient [59] to specify the water conLet K = centration jump on the SE/MAO coating boundary. Thus, K can be obtained as follows:

Table 4 Fitting results of capacitance and water diffusion coefficients obtained from the EIS data. ln(C0/F)

(21)

LMAO

ρiMAO

Fig. 6. ln Cc∼t0.5curve for MAO coating (a), SE coating (b), SE/MAO coating (c) immersed in NaCl solution.

Sample

(20)

LSE

K=

ρiMAO ρiSE

=

(1 − Φ ) ρH + ΦρI ρiSE

(23)

In this study, because the transition layer between the SE coating and MAO coating is very thin, the absorbed water of the transition layer is the same as that of the SE coating, ρH=ρSE. The above Eq. (23) can be re-written as:

K= 3.5.4. Effective diffusion coefficient The schematic penetration model of the composite SE/MAO coating is shown in Fig. 7a. The permeable composite is formed with 2 parallel walls of different diffusivity. Water, chloride or other corrosive media are transported across the x-direction. By imposing the continuity of the

(1 − Φ ) ρH + ΦρI ρiSE

=1−Φ=P (24)

The effective diffusion coefficient can be expressed as follows: eff DSE/MAO =

LSE/MAO KDSE D MAO DSE LMAO + KDMAO LSE

(25)

According to the thickness measurement, Eq. (25) can be obtained 9

Progress in Organic Coatings 141 (2020) 105548

X. Chen, et al.

Fig. 7. Schematic illustration of the SE/MAO coating (a) parallel coatings (b) water concentration jump on the interface.

(1) The water diffusion coefficient value of SE/MAO coating obtained from EIS results is lower than that for the SE coating, indicating that the barrier property of the SE/MAO coating is better than that of the individual MAO and SE coatings. This can be attributed to the excellent barrier property of the SE coating and good adhesion between the pretreatment MAO layer and SE layer. eff (2) The effective coefficient DSE/MAO predicted by the mathematical model based upon the Ohm’s law and EVM was obviously different. The difference was caused by the noncontinuity of the water concentration at the SE and MAO coating interface. eff (3) The DSE/MAO predicted based on the EVM showed good agreement with the that obtained from EIS experiment results. The higher accuracy was ascribed to EVM considering the water jump at the closely bonded SE and MAO coating interface.

as follows: eff DSE/MAO =

eff 40.26KDSE DMAO eff 15.28DSE + 25.68KDMAO

(26)

eff From Eqs. (15), (24) and (26), DSE/MAO can be obtained as follows: eff DSE/MAO =

40.26 × 0.10 × 3.97 × 10−11 × 2.74 × 10−11 15.28 × 3.97 × 10−11 + 25.68 × 0.10 × 2.74 × 10−11

= 6.47 × 10−12cm2•s−1 eff It can be seen that DSE/MAO (6.47 × 10−12 cm2·s-1) obtained by the effective validation model (EVM) is the same order of magnitude as the exp experiment DSE/MAO (7.22 × 10-12 cm2·s-1). Compared with the value obtained from Ohm’s law, the value of the water diffusion coefficient for the SE/MAO coating obtained from EVM is more similar to the EIS experimental result. This also confirms that the water concentration jump appears in heterogeneous composite medium. As compared with the coefficient reported before (see Table S4), the effective coefficient of the SE/MAO coating is smaller than the sandwich PEO/SANP/FC composite coating [59]. In addition, compared with the sandwich coating, the diffusion coefficient of SE/MAO multicoating is much closer to its experiment result calculated from EIS plots. This may because that the number of layers of the sandwich PEO/ SANP/FC coating affects the diffusivity behavior, resulting in a lager lap between the calculated value and the experimental value. The SE/ MAO coating exhibit lower water diffusion coefficient as compared to the SE/SSG coating (silicone-epoxy coating and silane film) [60]. This indicates that the SE/MAO coating provides better corrosion resistance. Noticeably, the calculated diffusion coefficient of SE/SSG coating is better agree with the experimental value. This could be attributed that the diffusion coefficients of SSG and SE coating is directly measured by EIS method. These values used in latter calculations cause a smaller difference. Another reason of the slight discrepancy may lay in the similar composition (SieOeSi) of the SE and SSG coating.

CRediT authorship contribution statement X. Chen: Data curation, Investigation, Writing - original draft. S.F. Wen: Funding acquisition, Writing - review & editing. T. Feng: Funding acquisition, Writing - review & editing. X. Yuan: Methodology. Z.F. Yue: Resources. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements The authors gratefully acknowledge the support for this work from National Natural Science Foundation of China (51801159 and 51872237), Aeronautical Science Foundation of China (2017ZF53069), Provincial Nature Science Fund of Shaanxi (2018JM5032 and 2017JM5098).

4. Conclusions

Appendix A. Supplementary data

The study was devoted to estimating the water diffusion coefficient in the a hybrid polymer-ceramic coating, and an effective validation model (EVM) was developed to predict the effective water diffusion coefficient. The microarc oxidation (MAO) coatings were applied as the pretreatment coating, and a silicone-epoxy (SE) coating was used as the outer sealing coating. In general, three conclusions were drawn as follows:

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