Corrosion-resistant plasma electrolytic oxidation coating modified by Zinc phosphate and self-healing mechanism in the salt-spray environment

Surface & Coatings Technology 384 (2020) 125321

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Corrosion-resistant plasma electrolytic oxidation coating modified by Zinc phosphate and self-healing mechanism in the salt-spray environment

T

Qian Huanga,1, Liangliang Liub,1, Zhongzhen Wua,b, , Shunping Jia, Hao Wua, Pinghu Chena, Zhengyong Maa, Zhongcan Wua, Ricky K.Y. Fub, Hai Lina, Xiubo Tiana, Feng Pana, Paul K. Chub ⁎

a

School of Advanced Materials, Peking University Shenzhen Graduate School, Shenzhen 518055, China Department of Physics, Department of Materials Science and Engineering, and Department of Biomedical Engineering, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong, China

b

ARTICLE INFO

ABSTRACT

Keywords: Aluminum Zinc phosphate PEO Self-healing Corrosion

Aluminum alloys are widely used in many industrial applications. However, they are susceptible to corrosion especially in aggressive media such as high salinity which can expedite the corrosion reactions. In this work, zinc phosphate modified plasma electrolytic oxidation (PEO) coatings are fabricated and demonstrated to have excellent corrosion resistance for a much longer duration (> 11,000 h) than conventional PEO coatings (< 1,000 h). Insoluble zinc phosphate is distributed in the pores of the coating serving as a barrier against the high-salinity medium. A self-healing mechanism is discovered as zinc phosphate migrates from the surface to pores continuously wetting the corrosion droplets on the super-hydrophilic coating until all the zinc phosphate is consumed.

1. Introduction Aluminum and its alloys are widely used in the aerospace and transportation industry because of desirable properties such as the high strength-to-weight ratio, good formability, and light weight. However, despite the presence of a natural passivating film on the surface, aluminum alloys are susceptible to corrosion especially in aggressive media such as that with a large concentration of Cl− [1,2]. Surface treatment techniques such as anodic oxidation [3], plasma electrolytic oxidation (PEO) [4], chemical conversion [5], electrodeposition [6], and laser-clading [7] are viable means to mitigate corrosion of Al alloys. In particular, PEO, a plasma-assisted electrochemical process that tends to produce better corrosion resistance than other techniques due to the thick oxide films produced [8], is environmentally friendly and efficient [9]. However, the corrosion resistance of PEO coatings is inferior to that of bulk alumina due to the porous coating structure that allows penetration of corrosive ions and incomplete crystallization [10]. As a result, the corrosion resistance of PEO coatings normally only lasts for 600 to 800 h as determined by salt-spray tests [1]. To improve the corrosion properties of PEO coatings, the deposition parameters can be optimized by for instance, using bipolar pulsed power or composite pulsed power [11] and adjusting the electrode

structure as well as the distance between them to obtain a denser and thicker alumina coating with higher crystallinity [12]. The corrosion lifetime of PEO coatings on 7075 Al can exceed 7000 h in salt-spray tests by increasing the coating thickness to > 100 μm [13]. However, there are still vulnerable pores in the coatings and it takes hours to form thick coatings thus raising the cost [14]. The pores generated by PEO can be sealed at a high temperature or by introducing inert organic or inorganic materials as fillers [15], but the effectiveness tends to vary widely [16,17]. In fact, the treatment tends to be costly and complex and organic pollutants may be produced. Another means is to add nanoparticles or metal salts to the PEO electrolyte [18] so that insoluble compounds can be formed in the coatings to boost the corrosion resistance. Especially using a metal salt modified electrolyte, the doping materials are more likely to be distributed near the coating surface and in the pores homogenously. Consequently, effective protection can be produced in a high salinity environment. For example, the corrosion lifetime increases to 1800 h by doping with Fe and its phosphate [19]. Zinc phosphate is often used as an additive in organic paintings to improve the corrosion resistance because of the ultra-low solubility in water [20] (solubility constant Ksp = 9.1 × 10−33 [21]). In this work, Zn and its phosphate are incorporated into PEO coatings by adding a suitable metal salt into the electrolyte to improve the corrosion

Corresponding author at: School of Advanced Materials, Peking University Shenzhen Graduate School, Shenzhen 518055, China. E-mail address: [email protected] (Z. Wu). 1 The two authors make equal contributions. ⁎

https://doi.org/10.1016/j.surfcoat.2019.125321 Received 12 October 2019; Received in revised form 11 December 2019; Accepted 28 December 2019 Available online 07 January 2020 0257-8972/ © 2020 Elsevier B.V. All rights reserved.

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Fig. 1. SEM images: (a) Zn-0, (b) Zn-5, (c) Zn-10, (d) Zn-15, and (e) Zn-20; (f) Thickness and surface roughness of the PEO coatings prepared with different zinc acetate concentrations;

resistance in a high salinity environment. The corrosion behavior and self-healing mechanism in the salt-spray experiments are investigated systematically. Our results demonstrate that anti-corrosion materials with a long lifetime and high tolerance in high temperature, humidity, and salinity environments can be realized by introducing the proper coatings.

angle of 1°. The X-ray photoelectron spectroscopy (XPS, ESCALAB 250×, Thermo Fisher) spectra were referenced to the C1s peak (284.8 eV) and Fourier transform Infrared spectroscopy (FTIR, Frontier, Perkin Elmer, USA) was conducted using KBr pellets at room temperature. The PEO coatings were cross-sectioned with a focused ion beam (FIB, FEI Scios). A 1 μm thick Pt layer was deposited to protect the surface and the slice was thinned to ~20 nm by ion milling with the Ga ion beam. Transmission electron microscopy (TEM) was performed on the cross-sectioned samples on the FEI Tecnai G2 F30 microscope. The thickness of the coatings was determined with an eddy current thickness meter (CTY2300, SDCH. Co.; LTD) and multiple measurements were conducted to obtain averages. The surface roughness was determined on the Surface Roughness Tester (JD220, Jitai Keyi, China).

2. Material and methods 2.1. Sample preparation The commercial LY12 Al alloy used as substrates in our experiments were cut into pieces with dimensions of 50 mm × 25 mm × 1 mm, ultrasonically cleaned in ethanol and acetone for 10 min each, and dried with nitrogen. An 80 kW AC bipolar pulsed power (JCL-WH80, Chengdu Jin Chuang Li Technology. Co. Ltd.) was employed in the PEO process and the current error was < 1% of the set value. The electrolyte contained sodium hexametaphosphate ((NaPO3)6, 10 g/L) and ethylenediaminetetraacetic acid disodium salt (EDTA-2Na, 10 g/L). The concentration of zinc acetate in the electrolyte varied from 0–20 g/L. The conductivity of the solution was determined with a conductivity meter (PHS-2F, INESA Instrument, China). Five samples designated as Zn-0, Zn-5, Zn-10, Zn-15, and Zn-20 were prepared, where the number represented the concentration of zinc acetate. The anodic and cathodic current densities were 6 A dm−2 and 2 A dm−2, respectively, whereas the frequency and duty cycle were 100 Hz and 30% for 20 min. The negative square pulses were generated after the positive square pulses (as shown in Fig. S1). During PEO, the temperature of the electrolyte was controlled to be below 55 °C using an external water cooling system.

2.3. Corrosion assessment To determine the corrosion properties, polarization curves were acquired on an electrochemical workstation (1470E, Solartron Metrology) versus the saturated calomel electrode (SCE) at a scanning rate of 3 mV/s at 25 °C.platinum was chosen to be the counter electrode. The specimen with a surface area of 2.5 cm2 was exposed to the NaCl solution (3.5 wt%). The corrosion potential (Ecorr) and corrosion current density (icorr) were analyzed by the Tafel extrapolation method. After stabilization for 600 s, the electrochemical impedance spectra (EIS) were collected to investigate the electrode/solution interface. The data were recorded from 0.1 Hz to 100 kHz with a 5 mV sinusoidal perturbing signal at the open-circuit potential. Salt-spray tests were carried out on the salt spaying tester in accordance with the standard ASTMB 117 (SN-60A, Sannuo Instrument Co., Ltd., China) with 5 wt% NaCl (pH = 6.5–7.0). The samples were placed 20° from the vertical axis and parallel to the principal direction of the flow of fog. 3D confocal microscopy (VK - X200, KEYENCE) was utilized to monitor the surface changes at different time points during the salt-spray tests.

2.2. Sample characterization The surface morphology and composition of the samples were characterized by field-emission scanning electron microscopy (FE-SEM, Carl Zeiss, SUPRA® 55) and energy-dispersive X-ray spectroscopy (EDS). The structure was determined by grazing incidence X-ray diffraction (XRD, Bruker, D8 Advance, Cu target, λ = 0.15418 nm) at 2θ = 10°~80° at a scanning rate of 2θ = 5°/min with an incidence

3. Results 3.1. Materials characterization The voltage curves and evolution of discharges are displayed in Fig. 2

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Fig. 2. SEM micrographs showing cross-sections of the PEO films and EDS maps of the corresponding areas: (a) Zn-0, (b) Zn-5, (c) Zn-10, (d) Zn-15, (e) Zn-20, The EDS element distribution maps are shown on the right side of the SEM images, respectively.

S1. In general, the voltage increases with time because of the thicker oxide layer while the current density is constant. In the initial stage (0–90 s), the positive discharge voltage rises rapidly indicating swift oxidation on the surface. Both the voltage-time slope and sparking voltage decrease with increasing zinc acetate concentration, indicating that addition of zinc acetate mitigates oxidization but increases the ionic conductivity of the electrolyte. In the second stage (2–8 min), the reaction on the surface weakens because of formation of the oxide layer and the growth rate decreases. In the final stage (8–20 min), sample Zn20 with the biggest thickness shows the largest potential. All in all, the coating formation rate is promoted by addition of zinc acetate. The surface morphology the PEO coatings is shown in Figs. 1(a - e). The Zn-0 and Zn-5 samples exhibit similar surface morphology but as the zinc acetate concentration increases, some crater-like discharging cavities appear on the surface. Both the thickness and surface roughness of the PEO coatings increase with increasing zinc acetate concentration as shown in Fig. 1(f). These characteristics are common for PEO coatings on account of the violent micro-discharge in the process [22].

According to cross-sectional EDS mapping performed on the PEO coatings (Fig. 2), the zinc content in the layer in the Zn-10 sample increases significantly. Similar results are observed from the elemental composition (atomic percent) of the PEO coating surface as shown in Fig. S2. Zn and its phosphate are mainly located in the near surface to a depth of 5–8 μm as well as around the pores giving rise to good coverage and effective protection of the pores. Below the Zn-rich layer, there is a dense and hard alumina layer with a thickness of 20–25 μm to provide robust mechanical support. Al, Zn, O, P and C are observed from the survey scan spectrum (Fig. 3(a)), indicating that Zn and P are successfully incorporated into the PEO coatings. C may come from surface absorption in air and the P 2p peak of Zn-0 at 135.7 eV corresponds to phosphate [PO4]3−. As the zinc concentration increases, the peak position of P 2p shifts towards a smaller binding energy indicating that the cation bound to the phosphate group changes from Al3+ to Zn2+ [19]. Zinc phosphate has a highly depolymerized structure composed of a network of P-O-Zn bonding linking adjacent P and Zn tetrahedrons and bridging oxygen 3

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Fig. 3. XPS spectra: (a) Survey, (b) P 2p and Zn 3s, (c) Zn 2p, and (d) O 1s of Zn-20; (e) GIXRD spectra; (f) FTIR spectra of the PEO coatings prepared with different zinc acetate concentrations.

between phosphorus atoms (P-O-P) [23]. The O 1 s spectrum of Zn-20 is deconvoluted into two Gaussian components corresponding to P-O-Zn at 532.8 eV and AleO at 531.6 eV [24] suggesting that zinc in the surface layer likely has the form of Zn3(PO4)2(OH)n. The two Zn 2p peaks at 1022.9 eV and 1044.8 eV are associated with Zn 2p3/2 and Zn 2p1/2 in Zn3(PO4)2, respectively [25,26]. As shown by the grazing incidence XRD (GIXRD) patterns in Fig. 3(e), α-Al2O3 (JCPDS NO. 011243) and γ-Al2O3 (JCPDS NO. 10-0425) is found from the PEO coatings. The background peak and overlapping peaks between 20° and 35° suggest an amorphous state and microcrystalline alumina in the coatings [19,27]. Crystalline alumina (mainly γ-Al2O3 and α-Al2O3) can be formed from molten Al2O3 when the local temperature reaches 104 °C during arcing and determines the mechanical properties of the coatings [8]. When zinc acetate is added to the electrolyte (> 10 g/L), a new weak peak attributed to Zn3(PO4)2·4H2O (JCPDS NO. 74-2275) appears at 2θ of 25.61°, which is consistent with the ZneP states revealed by XPS. Fig. 3(f) shows an absorption band at 1000–1250 cm−1 (ν3, 1124 cm−1) stemming from asymmetric vibration of phosphate (triply degenerate PeO stretching vibration) [28] and the absorption peaks reflecting O-P-O bending (ν4, 771 cm−1 and ν5, 488 cm−1) belong to [PO4]3−. The other two bands at 3000–3500 cm−1 (ν1) and 1660 cm−1 (ν2) are ascribed to absorption by H2O molecules and OeH bending of the terminal –OH groups, respectively, suggesting that crystalline hydrate exists in the PEO coatings [29,30]. The component and phases analysis shown in Fig. 3 reveal that alumina and Zn3(PO4)2·4H2O have better crystallinity with increasing concentration of the zinc acetate. The increased crystallinity arises from the larger discharge intensity as shown in Fig. 1 and Fig. S1. The TEM images of Zn-10 are shown in Fig. 4. An outer layer appearing as a bright band of about 150 nm rich in Zn and P is observed from the cross-sectional EDS maps in Figs. 4(a) and (b). The HR-TEM image of this area and corresponding SAED pattern of square 1 suggest that zinc phosphate exists as nanocrystals (Fig. 4(c)). The other typical microstructure is composed of a large area of crystalline Al2O3. The HRTEM image and SAED pattern of square 2 show regular lattice streaks of

γ- Al2O3 with good crystallinity [31] as shown in Fig. 4(d - e). 3.2. Corrosion resistance The potentiodynamic polarization curves of the PEO samples and substrate are presented in Fig. 5(a) and the corrosion potential (Ecorr), corrosion current density (icorr), cathodic Tafel slope (βc), and anodic Tafel slopes (βa) determined by Tafel extrapolation are presented in Table 1. A smaller corrosion current density icorr is observed after PEO because of passivation rendered by the alumina coating. As the zinc acetate concentration increases, the corrosion potential Ecorr shifts positively from −1.25 V vs. SCE to −0.77 V vs. SCE and the corrosion current is similar, indicating that the main effect of zinc phosphate in electrochemical corrosion is to increase the corrosion potential. Owing to the dual effects of alumina and zinc phosphate, the Zn-20 sample shows the smallest icorr of 1.60 × 10−10 A cm−2 that is at least three orders of magnitude less than that of the LY12 substrate. The largest corrosion potential is about 0.5 V larger than that of Zn-0. Hence, the corrosion resistance is improved substantially after doping. The corrosion behavior of LY12 and PEO coatings in the 3.5 wt% NaCl solution is investigated via EIS measurements, as shown in Figs. 5(b - d). The Nyquist plots of the LY12 are characterized by a capacitive loop in the high frequency range and an inductive loop in the low frequency range, corresponding to the charge transfer process and dissolution and pitting corrosion of Al alloys [32–34], respectively. The diameter of the capacitance arc at high frequencies increases with Zn concentration suggesting increased corrosion resistance (Fig. 5(b)). The Zn-20 sample shows the largest diameter of the capacitive loop among all the PEO samples and the impedance modulus |Z| of the Zn-20 sample at low frequencies (2.72 × 107 Ω·cm2) is about 10 times larger than that of Zn-0 sample (3.42 × 106 Ω·cm2), also indicating improved corrosion resistance by zinc doping (Fig. 5(c)) [35,36]. Besides, the phase angles at the intermediate frequencies become larger and wider as shown in Fig. 5(d). All in all, the EIS data demonstrate that the Zndoped PEO coatings have better corrosion resistance.

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Fig. 4. (a) TEM image and (b) Elemental maps of Zn-10 sample; (c) HR-TEM image of the Zn-rich area and corresponding SAED pattern of square 1; (d) HR-TEM image of the Al-rich area and (e) SAED pattern of square 2.

The ECs of the Zn-0 and Zn-20 coatings are shown in Fig. 5e and f and the fitted data are listed in Table 2, where Rs is the solution resistance, CPEf is the capacitance of the deposited or passivating coating on the surface, Rpore is the sum resistance of the pores in the coating, CPEdl is the capacitance of the electric double layer, Rct is the related charge transfer resistance in the faradic process, and W is the Warburg resistance [32–34]. The increase in Rct from 1879 Ω·cm2 (LY12) to 7.41 × 107 Ω·cm2 (Zn-20) means that the Zn-doped PEO coating protects the substrate effectively [37,38] and the Zn-doped PEO coating has better corrosion resistance than the undoped coatings. The optical photographs and SEM micrographs of selected samples after the salt-spray test for 11,000 h using 5 wt% NaCl are depicted in Fig. 6. The Al alloy substrate is completely corroded after 100 h and the picture is not shown here. Zn-0 and Zn-5 show longer corrosion lifetime than the Al alloy and severe corrosion begins after 1000 h. After 11,000 h, these two samples are completely destroyed. Some spongylike features and cracks are observed from the SEM images in Fig. 6(b) and Fig. S3. Zn-10 with a large concentration of Zn shows better corrosion resistance up to 3000 h and after 11,000 h, some areas are still intact as shown in Fig. 6(c). Both Zn-15 and Zn-20 show only a few

independent corrosion pits as shown in Fig. 6(d) and nearly no corrosion can be observed from the optical pictures of these two samples even after the 11,000 h salt-spray test. The optical photographs and 3D laser confocal scanning microscopy images of all the PEO coatings during the salt-spray test are displayed in Figs. S4 and S5. 4. Discussion To determine the role of Zn and its phosphate in the corrosion process during the salt-spray test, the distribution and evolution of each element before and after the tests for 3000 h and 11,000 h are investigated. According to EDS conducted on Zn-20 before the corrosion test, Zn, P, O, and Al are distributed homogenously on the surface except the pores (Fig. 7(a)). The morphology and composition of the pores of the Zn-20 sample in different corrosion states are displayed in Fig. 7b to d, where Points 1, 3 and 5 indicate the surface (away from the pores) and 2, 4 and 6 show the pores. The elemental concentrations at these six points are presented in Table 3. Before the salt-spray test, the coating has a dense and smooth morphology and the Zn content in the pores is a slightly less than that away from the pores. There should be aggregation

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Fig. 5. (a) Potentiodynamic polarization curves acquired in 3.5 wt% NaCl from the pristine substrate and PEO coatings prepared with different zinc acetate concentrations (dE/dt = 3 mV/s); (b) Nyquist plots and enlarged part of low Z; (c) Bode plots of |Z| vs. frequency; (d) Bode plots of phase angle vs. frequency; (e) Equivalent circuit of LY12, Zn-0, Zn-5 and Zn-10; (f) Equivalent circuit of Zn-15 and Zn-20.

surface to the pores, the cross-sectional images and elemental distributions of the pores in Zn-20 sample are measured before and after 3000 h and 11,000 h, as shown in Fig. 8. As shown in Fig. 2, Zn and P are mainly distributed on the surface of the alumina matrix before corrosion and even in the pores. A gradient transition and no obvious interface can be observed from the alumina matrix to the Zn-rich layer. After the 3000 h salt-spray test, a new layer with a thickness of about 1 μm is formed on the wall of the pores and there is smaller contrast in the SEM image. Both the EDS cross-sectional mas and the selected locations suggest that the layer is mainly composed of Zn, P and O, as shown in Figs. 8(a2-e2) and Table 4. After the 11,000 h salt spray test, the thickness of the new layer in the pores decreases significantly meaning that this layer in the pores is gradually consumed in the later stages of corrosion, as shown in Figs. 8(a3-f3). In the test, there no other source of Zn and P except the coating surface and therefore, the zinc phosphate layer formed on the wall of the pores must come from the coating surface thereby corroborating spontaneous transportation of zinc phosphate. The elemental concentrations of Zn-10, Zn-15, and Zn-20 before and after the 3000 h and 11,000 h salt-spray tests are shown in Fig. S6. The relative atomic percentages of Zn and P on Zn-15 and Zn-20 decrease with time first and then increase slightly at 11,000 h, but the other two elements (Al and O) exhibit the opposite trend. Two stages are revealed in the corrosion process, one at the expense of zinc phosphate and the

Table 1 Corrosion current densities (icorr), corrosion potentials (Ecorr), and cathodic Tafel slopes (βc), and anodic Tafel slopes (βa) of the LY12 aluminum alloy and PEO coatings prepared with different zinc acetate concentrations measured in 3.5 wt% NaCl after stabilization for 1 h at the open circuit potential. Samples

LY12 Zn-0 Zn-5 Zn-10 Zn-15 Zn-20

Ecorr

icorr

βc −2

βa −1

(V vs. SCE)

(A·cm

)

(V·dec

−1.25 −1.25 −1.22 −0.99 −0.78 −0.77

6.07 5.14 2.41 3.70 2.77 1.60

10−7 10−9 10−9 10−10 10−10 10−10

−0.089 −0.123 −0.142 −0.171 −0.149 −0.174

× × × × × ×

)

(V·dec−1) 0.125 0.054 0.091 0.063 0.087 0.064

of Zn in the pores since the measured Zn amount is less than the real one because of the pores [39]. After 3000 h, although the edge of the pore is still smooth, a sponge-like morphology and particles are observed from the pores (Fig. 7(c)). The Zn content increases significantly in the pores but decreases on the surface, possibly due to transportation of zinc phosphate from the coating surface to the pores. After 11,000 h, there is no obvious change at the edge of the pores except slightly larger roughness and more particles and corrosion features at the pores. To verify the transportation of the zinc phosphate from the coating Table 2 EIS data of the PEO coatings prepared with different zinc acetate concentrations. Samples

Rs (Ω cm2)

CPEf (Ω−1·sn cm−2)

LY12 Zn-0 Zn-5 Zn-10 Zn-15 Zn-20

32.13 29.9 27.44 29.01 38.76 29.44

6.20 5.13 3.45 4.42 2.47 6.43

× × × × × ×

10−4 10−7 10−7 10−7 10−7 10−8

n1

Rpore (Ω cm2)

CPEdl (Ω−1·sn cm−2)

0.62 0.84 0.84 0.85 0.89 0.97

1.66 × 104 1212 3644 3595 2295 1252

4.31 2.05 4.36 2.41 1.55 2.31

6

× × × × × ×

10−5 10−7 10−7 10−7 10−7 10−8

n2

Rct (Ω·cm2)

0.91 0.91 0.92 0.92 0.97 0.98

1879 3.17 × 4.54 × 1.91 × 4.28 × 7.41 ×

W (S·sec0.5·cm−1) 106 106 107 107 107

\ \ \ \ 3.22 × 10−8 1.94 × 10−8

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Fig. 6. (a) Optical photographs of the PEO samples and (b-d) SEM images of the corroded areas of Zn-0, Zn-10, and Zn-20 after the salt-spray test for 11,000 h.

other dominated by consumption of alumina. Since EDS only detects the near surface, when the eroded zinc phosphate on the surface moves to the pores in the first corrosion stage, it will be detected again after the coating surface is eroded in the second stage. The movement of zinc phosphate from the surface to pores results in a self-healing phenomenon to mitigate corrosion [17]. There are two main components in the Zn-doped PEO coatings to resist corrosion in the salt-spray test. The first one is zinc phosphate which has a smaller solubility than alumina [40]. For chemically inert materials, corrosion mainly originates from dissolution at defects [41] and hence, covering and sealing by zinc phosphate improves the corrosion resistance significantly. Secondly, movement of zinc phosphate from the surface to the pores provides continuous self-healing effects to extend the lifetime in the presence of Cl−. According to the results, the corrosion mechanism is described in Fig. 9 and the video in the supporting information. On the undoped alumina coating, there are two structures: dense multi-phase alumina and pores [42]. Corrosion of the multi-phase alumina produces a sponge-like morphology because of the different corrosion resistance of crystalline and amorphous Al2O3 [43–45], whereas in the pores, the corrosion medium penetrates the coating forming corrosion channels. When the corrosion medium reaches the substrate, large cracks and massive exfoliation occur [46], as shown in Fig. 9(a). On the other hand, on the Zn-doped PEO coatings, zinc phosphate covers the coating surface and seals the pores. Because of the smaller solubility of zinc phosphate than alumina [40], the coating has a much longer life time in the corrosion test. As time elapses, some dissolution of the zinc phosphate is inevitable. Especially in the presence of Cl−, the dissolution

process is expedited as shown in Eq. (1) [21,46,47]:

Zn3 (PO4 )2

Cl

3Zn2 + + 2PO43 Ksp = 9.1 × 10

2+

Zn exists in the hydrated form to Eq. (2) [21]:

Zn2 + + 4H2 O(l)

Zn (H2 O)42 +(aq)

33

(Zn(H2O)42+)

(1) in water according (2)

where aq refers to the aqueous solution and l is the liquid state. Instead of leaving the coating and going outside, most of the dissolved zinc phosphate moves to the pores in the coating and produces further sealing against the penetration of the corrosion medium until all the zinc phosphate is consumed later as shown in Fig. 9(b). Afterwards, the corrosion mechanism is similar to that of the undoped coating and the Zn-doped PEO coatings show long corrosion life time in the salt-spray test. In the corrosion process, the sample is placed horizontally in the salt spray box. No driving force is produced except the surface energy of the coating interacting with the corrosion medium [49]. Therefore, the water wetting property of Zn-20 is evaluated and the coating is superhydrophilic with a contact angle of 7.58° (Fig. 10). This indicates that speedy spreading of the corrosion medium occurs upon contact due to the surface tension [50]: SG

LG cos

+

SL

(3)

where γSG and γLG are the surface tensions of the solid (coating) and liquid (corrosive droplet) against gas, γSL is the interface tension between the solid and liquid, and θ (0 ≤ cosθ ≤ 1) is the contact angle. If

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Fig. 7. (a) EDS maps of Zn-20 before the salt-spray test; (b-d) SEM morphologies marked by six points before and after the salt-spray tests for 3000 h and 11,000 h, respectively. Table 3 Elemental atomic percentages of the six points marked in Fig. 7b to d. Samples

Zn-20-0 h

Zn-20-3000 h

Points

#1

#2

#3

#4

#5

#6

O Al P Zn Total

65.13( ± 2) 6.12( ± 1) 14.21( ± 2) 14.54( ± 1) 100.00

70.15( ± 2) 16.21( ± 1) 6.33( ± 1) 7.31( ± 1) 100.00

66.16( ± 2) 15.21( ± 1) 10.76( ± 1) 7.87( ± 1) 100.00

36.56( ± 2) 16.41( ± 2) 12.01( ± 2) 35.02( ± 3) 100.00

66.18( ± 2) 14.19( ± 2) 9.65( ± 2) 9.98( ± 2) 100.00

56.99( ± 3) 8.23( ± 1) 14.08( ± 2) 20.70( ± 3) 100.00

zinc phosphate dissolves in the corrosion medium, it can spread on the coating and as it enters the pores, the force direction of γSG and γLG changes as shown in Fig. 10(b1). In this case, Eq. (3) becomes: SG

LG cos +

SL cos

Zn-20-11,000 h

pores giving rise to the self-healing effect. 5. Conclusion

(4)

The corrosion resistance of Al alloy is improved significantly by incorporating Zn and its phosphate in the PEO coatings. The improved corrosion resistance stems from insoluble zinc phosphate moving from the surface to the pores continuously. The salt-spray experiments reveal an excellent corrosion resistance lifetime of 11,000 h. The self-healing mechanism is described. The dissolved zinc phosphate moves to the pores from the coating surface under the effects of surface energy and is deposited along the pores to protect against the corrosion medium. The dynamic process leads to self-sealing of the corrosion channels and

where α (0 ≤ cosα ≤ 1) is the angle between γSL and the reverse tangent of the solid-liquid interface. Here, the right side of the formula decreases further and hence, the corrosion droplets carrying dissolved zinc phosphate go to the pores more quickly. As spreading continues, the gas-liquid interface increases significantly to improve evaporation of water [51] producing zinc phosphate solute which re-precipitates and deposits along the pores when water is expelled or vaporized. In this way, zinc phosphate is transported from the coating surface to the

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Fig. 8. Cross-sectional SEM image of Zn-20 (a1) before and (a2) after the salt-spray test 3000 h, (a3) 11,000 h; (b1-e1) and (b2-e2) (b3-e3) are the EDS mappings of (a1-a3), respectively.

Table 4 Elemental atomic percentages of the five points 1–5 marked in Fig. 8(a1) and points 6 to 9 marked in Fig. 8(a2). Elements

P1 at.%

P2 at.%

P3 at.%

P4 at.%

P5 at.%

P6 at.%

P7 at.%

P8 at.%

P9 at.%

O Al P Zn Total:

64.0 18.0 7.8 10.2 100.0

71.2 23.1 3.1 2.6 100.0

71.5 7.3 9.2 12.0 100.0

77.1 4.1 9.0 9.8 100.0

76.3 7.7 7.6 8.4 100.0

37.1 36.2 18.1 8.6 100.00

58.6 9.9 14.4 17.1 100.00

54.4 8.7 17.8 19.1 100.00

55.1 12.3 15.6 17.0 100.00

Fig. 9. Schematic diagrams of the corrosion process of (a) PEO coating without Zn and (b) PEO coating with Zn during the salt-spray test.

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Fig. 10. Schematic diagrams of the dynamic self-healing process of the PEO coating with Zn during the salt-spray test.

ultra-long lifetime and this anti-corrosion concept can be extended to other materials. Supplementary data to this article can be found online at https:// doi.org/10.1016/j.surfcoat.2019.125321.

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Author contributions Z. W., Q.H. and L. L. proposed the research; S. J., H. W., P. C. and Z. M. designed the coating depositions; Q. H., Z.C. W. and H. W. did the coating characterizations and properties testing; Z. W., H. L., S. C., Y. L., H. L., X.T.and F. P. analyzed and discussed the data; Z. W., L. L., Q. H., S. J., P. C and R. F. wrote the paper; X. T. and P. C. polished the English writing; all authors provided feedback. 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 This work was supported jointly by National Materials Genome Project (No. 2016YFB0700600), Shenzhen Science and Technology Research Grant (No. JCYJ20170818150601930), Shenzhen - Hong Kong Research and Development Fund (No. 2017032005), City University of Hong Kong Strategic Research Grant (SRG) (No. 7005105), as well as Hong Kong Research Grants Council (RGC) General Research Funds (GRF) (No. CityU 11205617). References [1] Q. Chen, Z. Jiang, S. Tang, W. Dong, Q. Tong, W. Li, Influence of graphene particles on the micro-arc oxidation behaviors of 6063 aluminum alloy and the coating properties, Appl. Surf. Sci. 423 (2017) 939–950, https://doi.org/10.1016/j.apsusc. 2017.06.202.

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