nanocrystalline coating

Accepted Manuscript Influence of sealing treatment on the corrosion behavior of HVAF sprayed Al-based amorphous/nanocrystalline coating

L.M. Zhang, S.D. Zhang, A.L. Ma, H.X. Hu, Y.G. Zheng, B.J. Yang, J.Q. Wang PII: DOI: Reference:

S0257-8972(18)30940-X doi:10.1016/j.surfcoat.2018.08.086 SCT 23755

To appear in:

Surface & Coatings Technology

Received date: Revised date: Accepted date:

10 April 2018 28 August 2018 29 August 2018

Please cite this article as: L.M. Zhang, S.D. Zhang, A.L. Ma, H.X. Hu, Y.G. Zheng, B.J. Yang, J.Q. Wang , Influence of sealing treatment on the corrosion behavior of HVAF sprayed Al-based amorphous/nanocrystalline coating. Sct (2018), doi:10.1016/ j.surfcoat.2018.08.086

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ACCEPTED MANUSCRIPT Influence of sealing treatment on the corrosion behavior of HVAF sprayed Al-based amorphous/nanocrystalline coating L.M. Zhang,1,2 S.D. Zhang,3 A.L. Ma,1 H.X. Hu,1 Y.G. Zheng,1,* B.J. Yang,3 and J.Q. Wang3,*

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1. CAS Key Laboratory of Nuclear Materials and Safety Assessment, Institute of Metal Research,

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Chinese Academy of Sciences, 62 Wencui Road, Shenyang 110016, PR China.

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2 School of Materials Science and Engineering, University of Science and Technology of China, 72 Wenhua Road, Shenyang 110016, PR China.

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3 Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese

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Academy of Sciences, 72 Wenhua Road, Shenyang 110016, PR China.

*

Corresponding author. E-mail: [email protected] (Y.G. Zheng), [email protected]

(J.Q. Wang)

1

ACCEPTED MANUSCRIPT Abstract Al-based amorphous/nanocrystalline coating with a low porosity (0.42 %) and a high amorphous content (80.3 %) was fabricated by high velocity air-fuel spraying on aluminum alloy 2024. This thin amorphous/nanocrystalline coating with α-Al

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nanocrystals precipitated in amorphous matrix displays good short-term corrosion

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resistance, while the corrosion resistance decreases rapidly with increased immersion

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time. Sealing treatment can effectively close the connecting pores and reduce the porosity defects of coating, thereby improving the corrosion resistance. Among the

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three selected sealants, stearic acid exhibits the best sealing effect on Al-based

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amorphous/nanocrystalline coating due to its good impermeability and hydrophobicity, followed by potassium dichromate and nickel acetate. The sealing mechanisms of the

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three sealants are discussed in terms of reactions at the coating/sealant interface and

Al-based

amorphous/nanocrystalline

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Keywords:

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hydrophobicity of stearic acid.

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Polarization; EIS; XPS; Corrosion resistance

2

coating;

Sealing

treatment;

ACCEPTED MANUSCRIPT 1. Introduction Aluminum-based amorphous alloys, as one important family of amorphous alloys, possessing the higher strength-to-weight ratio, better wear and corrosion resistance, more flexible alloy composition than many conventional crystalline

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aluminum alloys, have attracted great attention since its first discovery in 1988 [1-5].

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However, the glass forming ability (GFA) of Al-based amorphous alloys is far below

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other amorphous alloys (e.g., Zr-based, Cu-based, Fe-based, etc.), so it is extremely difficult to prepare Al-based bulk amorphous alloys [6-8]. Presently, the maximum bar

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of Al-based bulk amorphous alloys obtained is merely up to 2.5 mm in diameter,

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which is difficult to meet the size requirements of engineering and structural materials in practical applications [9]. Fortunately, Al-based amorphous coatings can overcome

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the restrictions of size as well as the poor ductility of Al-based bulk amorphous alloys,

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thereby providing a promising way to industrial applications. Nevertheless, it should be mentioned that it is almost impossible to produce pure amorphous coating without

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any crystal phases due to the unavoidable thermal effect in coating preparation

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process, and hence the Al-based amorphous coatings reported are all of amorphous/nanocrystalline structure [10-13]. Tailleart et al. prepared the Al-Co-Ce amorphous coating by pulsed thermal spraying (PTS) and demonstrated the superiority in corrosion resistance of this coating [10]. Lahiri et al. prepared the Al-based amorphous/nanocrystalline coating using cold spraying (CS) technique, and found that the coating displayed excellent wear resistance compared to aluminum alloy 6061 [11]. Similar findings in CS Al-based amorphous alloy coating were also 3

ACCEPTED MANUSCRIPT reported by He et al. [12]. Recently, Gao et al. prepared the Al-based amorphous/nanocrystalline coating by high velocity air-fuel (HVAF) spraying and also verified the high resistance to corrosion and wear of the coating [13]. Therefore, it is very promising to use Al-based amorphous/nanocrystalline coating in wear and

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corrosion environments.

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Compared to Al-based amorphous alloys with the same composition, Al-based

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amorphous/nanocrystalline coating usually displays a higher corrosion current density and the lower pitting potential, which is mainly attributed to the porosity defects in

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coating [13, 14]. Pores, as one of the most destructive defects in spraying coatings,

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can significantly reduce the corrosion resistance of coating [14, 15]. The corrosive media can penetrate into the coating through the pores and eventually induce the

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failure of coating. There are no methods to eliminate the porosity of spraying coatings

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completely. Post-treatments (e.g., sealing treatment, laser remelting and annealing) are often necessary to improve coatings’ corrosion resistance and prolong their service

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lives [16-19]. Among them, sealing treatment has been widely applied in spraying

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coatings by virtue of its economic and technical advantages [20-22]. Aluminum phosphate (AlPO4), sodium orthosilicate (Na3SiO4), cerium salts (Ce(NO3)3, CeCl3), nickel salts (Ni(CH3COO)2, NiF2) and dichromate (K2Cr2O7) are common sealants used in various spraying coatings including amorphous/nanocrystalline coatings [20-26]. Wang et al. investigated the effect of sealing treatment on the corrosion behavior of high velocity oxygen fuel (HVOF) Fe-based amorphous/nanocrystalline coating, and found that sealing treatment obviously enhanced the corrosion resistance 4

ACCEPTED MANUSCRIPT of coating and AlPO4 sealant exhibited the best sealing effect [22]. Liu et al. further revealed the sealing mechanism of AlPO4 on the Fe-based amorphous coating, and they held that both the deep penetration of sealant and the formation of condensed phosphates (Al(PO3)3) at high sealing temperature (350oC) contributed to the superior resistance

[20].

coating

the

may

sealants

not

be

used

in

Fe-based

suitable

for

Al-based

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amorphous/nanocrystalline

However,

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corrosion

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amorphous/nanocrystalline coating due to its much lower crystallization temperature (~200oC), as a high sealing temperature or a long sealing duration time may cause the

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structural transformation of Al-based amorphous/nanocrystalline coating [21, 27, 28].

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Therefore, a low or medium sealing temperature and a relatively short sealing duration time should be one of the criteria to choose sealants for Al-based

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amorphous/nanocrystalline coating. However, the question of which sealant is more

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suitable for Al-based amorphous/nanocrystalline coating and the sealing mechanism are both vague and unanswered.

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Herein, we prepared the HVAF Al-based amorphous/nanocrystalline coating with

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a nominal composition of Al86Ni6Y4.5Co2La1.5 (at.%). The selection of this composition is based on its superior GFA, and the addition of Y and La is beneficial to the GFA [8, 9]. The microstructure of the as-sprayed coating as well as the sealed coatings was characterized. Meanwhile, the influence of three medium-temperature sealing treatments (i.e., nickel acetate (Ni(CH3COO)2), potassium dichromate (K2Cr2O7) and stearic acid (CH3(CH2)16COOH)) on the corrosion behavior of this coating was investigated in detail, and the sealing mechanisms of the different 5

ACCEPTED MANUSCRIPT sealants used was discussed. The aim of this study is to develop appropriate sealants for HVAF Al-based amorphous/nanocrystalline coatings toward prolonging their service lives.

2. Experimental

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The Al-based amorphous/nanocrystalline coating with a nominal composition of

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Al86Ni6Y4.5Co2La1.5 (at.%) was produced by AcuKote HVAF thermal spray system

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with AK-07 spray gun on aluminum alloy 2024 with the dimension of 100 mm (length)×30 mm (width)×2 mm (thickness). The detailed spraying parameters are as

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follows: fuel flow rate, hydrogen flow rate and nitrogen flow rate are 0.12–0.14, 0.035

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and 0.027 m3/min, respectively; the spraying distance is 180–240 mm and the traverse velocity is 700-800 mm/s. The series of pretreatments consisting of acetone

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degreasing, alcohol cleaning, drying in air and sand blasting were performed on

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aluminum alloy 2024 before the coating fabrication. Three medium-temperature sealants, i.e. nickel acetate, potassium dichromate and stearic acid, were selected for

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the sealing treatment of HVAF sprayed Al-based amorphous/nanocrystalline coating.

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The chosen concentration of sealant is based on some relevant investigations [29, 30], in which the optimal concentration of each sealant is obtained. The detailed steps of each sealing treatment are as follows: a) Nickel acetate The nickel acetate sealant included 1.2 g/L Ni2+, 0.5 wt.% acetic acid, and the pH was about 5.0~5.5. Prior to sealing treatment, the as-sprayed (unsealed) coating was degreased by acetone, rinsed with deionized water 6

ACCEPTED MANUSCRIPT and ultrasonically cleansed in alcohol solution in sequence. Then, the coating was transferred into the nickel acetate solution and immersed for 40 min at 90 oC. In order to reduce the evaporation of water and acetic acid, the beaker containing sealant was wrapped with plastic film.

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b) Potassium dichromate

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The potassium dichromate sealant contained 60 g/L K2Cr2O7 with the pH of

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5.5~6.5. Similar pretreatment as nickel acetate sealing treatment was performed on the as-sprayed coating before the sealing treatment with

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potassium dichromate sealant. The coating sample was then immersed in

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potassium dichromate solution at 95 oC for 30 min. c) Stearic acid

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Pure stearic acid powders were heated to completely melt at 80 oC, and then

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the coating sample after the abovementioned pretreatment was immersed in the stearic acid liquid at 80 oC for 30 min.

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The structure of Al-based amorphous/nanocrystalline coating was characterized

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by X-ray diffraction (XRD) (D8 ADVANCE, Bruker) with monochromated Cu kα radiation (λ = 0.1542 nm). The test voltage is 40 kV with the test current of 40 mA. The step size and the scan rate for XRD tests are 0.02o and 2o/min, respectively. JEOL-2100 transmission electron microscope (TEM) with energy dispersive detector was also used to further elucidate the microstructure. The samples for TEM observation were prepared by ion beam thinning with liquid nitrogen cooling. Surface morphologies of the coatings after different sealing treatments were observed using 7

ACCEPTED MANUSCRIPT scanning electron microscopy (SEM, INSPECT F50), confocal laser scanning microscope (CLSM) as well as ordinary optical microscope. The porosity of the as-sprayed Al-based amorphous/nanocrystalline coating was revealed with the help of three-dimensional X-ray micro-tomography (XRT) system (ZEISS Xradia 500 Versa) sub-micron

resolution.

The

content

of

crystal

phase

in

Al-based

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with

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amorphous/nanocrystalline coating was obtained by differential scanning calorimetry

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(DSC) with a heating rate of 10 K/min. The contact angle of coatings was measured with JLC contact angle tester.

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The corrosion behavior of Al-based amorphous/nanocrystalline coating in 3.5 wt.%

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NaCl solution (pH 6) was measured by Gamry Interface 1000. Deionized water (18.2 M ) was used to prepare the aqueous solutions. A traditional three-electrode cell (i.e.,

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a platinum plate as the counter electrode, a saturated calomel electrode (SCE) as the

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reference electrode, and the sample as the working electrode) was used for all electrochemical measurements at room temperature. Prior to immersion experiments,

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potentiodynamic polarization tests were conducted at a scan rate of 0.5 mV/s from

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30 mV to 600 mV vs. open circuit potential (OCP). In order to evaluate the long-term corrosion resistance of the coatings after different sealing treatments, electrochemical impedance spectroscopy (EIS) and linear polarization resistance measurements were carried out after immersing in 3.5 wt.% NaCl solution for 1, 48, 144, 288, 480 and 720 hours. EIS tests were performed at OCPs, and the test frequency was from 100 kHz to 10 mHz. Linear polarization resistance measurements were performed after EIS test and subsequent 800 s’ OCP measurement, and the scan 8

ACCEPTED MANUSCRIPT rate was 0.167 mV/s from 10 mV to +10 mV vs. OCP. Data of EIS and linear polarization resistance were fitted using the software of Gamry Echem Analyst. The nominal area of all electrochemical samples was 1 cm2. Considering the rough surface of the coating, the real exposed area of coating was tested by CLSM. Each

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electrochemical test was repeated at least three times, and the result that could

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represent the average value was selected.

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The surface composition of the coatings after different sealing treatments was characterized by X-ray photoelectron spectroscopy (XPS, ESCALAB250) with Al Kα

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excitation (hν = 1,486.6 eV). The position of the adventitious carbon component

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(284.6 eV) was used for the calibration of binding energy. The signals were collected after sputtering with argon ions (2 mA, 3 kV) for 0 s, 20 s, 40 s, 80 s and 120 s

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respectively. The software of XPSPEAK 4.1 was used for the XPS data processing.

3.1.

Structural

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3. Results and discussion

characterization

of

the

as-sprayed

Al-based

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amorphous/nanocrystalline coating

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Fig. 1a presents the three-dimensional surface morphology of the as-sprayed Al-based amorphous/nanocrystalline coating. There exhibits a topographic map with different colors. The yellow represents a high terrain and the green a low terrain. It is obvious that the surface of coating is uneven and the maximum vertical distance is about 20 μm. Considering the rough surface with many valleys and protrusions, the real exposed area of the as-sprayed coating is tested by CLSM and the real area is 2.3 times of the nominal area. Figs. 1b and c demonstrate the cross-section information of 9

ACCEPTED MANUSCRIPT the as-sprayed coating. The thickness of this coating is about 150 μm, and there are no through-pores or through-cracks in the coating. However, some macro-pores can be clearly observed in the cross section. In thermal spraying process, feedstock powders are accelerated in a spray gun with a melting or semi-melting state, and then are

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propelled towards the substrate. The droplets will undergo serious deformation

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because of huge impact kinetic energy producing thin layer or lamellae, which is also

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called “splats” [31, 32]. As a result, pores inevitably exist in the coating due to incomplete intersplat contact, unmelted powders and thermal stresses. At present, it is

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still a major challenge to eliminate the porosity of thermal spraying coatings. In

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addition, there also exhibit some bright crystalline lamellae in the cross section due to unavoidable thermal activation in the fabrication process of coating.

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The porosity of coating is evaluated through XRT, as shown in Fig. 2. For the

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grayscale images (Figs. 2a and b), the pores in the coating present some dark spots due to the presence of a weak X-ray absorption. The size distribution of the porosity

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indicates that the typical size of the pores is less than 10 μm in diameter (Fig. 2c) and

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the porosity of this coating is about 0.42 %, which is significantly lower than that of most thermal spraying coatings. Fig. 3a shows the XRD patterns of Al-based amorphous ribbon and the as-sprayed coating of the same composition. It is evident that the ribbon displays a smooth hump at the angle of 2θ = 38° without any nanocrystal peaks, indicating the complete amorphous structure. For the as-sprayed coating, it also displays an apparent hump at the angle of 2θ = 38°, indicating the existence of amorphous structure. 10

ACCEPTED MANUSCRIPT Meanwhile, several crystallized peaks appear in the XRD pattern, which correspond to α-Al crystal phase based on the standard PDF cards. Furthermore, the microstructure of the as-sprayed coating is characterized by TEM, as shown in Fig. 4. There are two types of distinctly different structural regions in the coating, one is the

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completely amorphous region (Fig. 4a) and the other is the partially crystallized

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region (Fig. 4b). The results of EDS demonstrate that these crystalline phases are

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primarily rich in Al element, indicating the existence of α-Al nanocrystals precipitated in amorphous matrix of the coating. In addition, the relative content of α-Al is

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estimated based on the DSC tests, as shown in Fig. 3b. According to the area ratio of

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the exothermic peaks of this coating to that of the complete amorphous ribbon, the volume fraction of α-Al crystal phase can be calculated out by the equation [33]:

H ribbon  H coating H ribbon

(1)

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Vcrystal 

Hcoating and Hribbon are the transformation enthalpies of a partially crystallized coating

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sample and a fully amorphous ribbon sample, respectively. The volume fraction of the α-Al crystal phase is about 19.7%, that is, this Al-based amorphous/nanocrystalline

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coating has a relatively high amorphous content of 80.3%. 3.2. Surface morphologies of Al-based amorphous/nanocrystalline coatings after different sealing treatments Fig. 5 shows the macroscopic surface morphologies of the coatings after different sealing treatments. For the as-sprayed coating, there exist many craters on the surface that are produced by the HVAF spraying process, and the whole surface presents a light orange color. The surface color has changed after different sealing 11

ACCEPTED MANUSCRIPT treatments. The coating with potassium dichromate sealing treatment presents a dark orange color, and the coating surface turns brown after sealing treatment with nickel acetate, while the craters on the coating surface after the two sealing treatments are still visible. In contrast, the coating surface after stearic acid sealing treatment remains

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the same color as the as-sprayed coating, but the craters become invisible because of

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the coverage of the stearic acid layer. The microscopic surface morphologies of the

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four kinds of coatings are displayed in Fig. 6. The as-sprayed coating presents many deep holes with open or semi-closed states, by which corrosive medium can easily

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permeate into the interior, thereby causing the failure of the coating. The surface holes

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are filled with sealant after potassium dichromate sealing treatment, and many filamentous substances are clearly observed on the whole surface (Fig. 6b). Similarly,

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the coating surface is also covered by the sealant after sealing treatment with nickel

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acetate. Nevertheless, there still exist a few semi-closed pores on the coating surface (Fig. 6c). As for the sealing treatment with stearic acid, the whole surface of coating is

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covered with a thin layer of stearic acid (Fig. 6d), and this stearic acid layer can

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effectively shield the corrosive media and promote the corrosion resistance of coating. 3.3. Potentiodynamic polarization tests Fig. 7 shows the potentiodynamic polarization plots of the coatings after different sealing treatments, and the electrochemical parameters derived from potentiodynamic polarization plots are displayed in Table 1. It is obvious that the as-sprayed (unsealed) coating displays the lowest corrosion potential (Ecorr) and the highest corrosion current density (icorr), which are 538 mV vs. SCE and 7.4 μA/cm2, 12

ACCEPTED MANUSCRIPT respectively. As to the coating with nickel acetate sealing treatment, there exhibits an increased Ecorr (438 mV vs. SCE) and a decreased icorr (2.84 μA/cm2), indicating the enhanced corrosion resistance. The icorr of the coating is decreased to 0.026 μA/cm2 after sealing treatment with potassium dichromate, implying the better corrosion

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resistance compared to the as-sprayed and the nickel acetate sealed coatings. It is

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noteworthy that this icorr is also obviously lower than that of HVOF/ HVAF Fe-based

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amorphous/nanocrystalline coatings with AlPO4, Na3SiO4 or Ce(NO3)3 sealing treatments, exhibiting superior sealing effect of potassium dichromate on Al-based

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amorphous/nanocrystalline coatings [17, 20]. The highest Ecorr (74 mV vs. SCE) and

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the lowest icorr (0.0015 μA/cm2) are obtained when the coating is sealed with stearic acid. Recently, Shang et al. investigated the effect of different sealing treatments on

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anodic film of an aluminum alloy and found that the stearic acid sealing treatment

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exhibited the best sealing effect [30], which was in line with the result above. On the other hand, the pitting potential (Epit) of coating is markedly changed after different

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sealing treatments. The Epit of the as-sprayed coating is 381 mV vs. SCE. After

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sealing treatment with nickel acetate, the Epit shifted positively to 248 mV vs. SCE, which is mainly attributed to the reduced porosity on the coating surface. After sealing treatment with potassium dichromate, a similar Epit to the sample with nickel acetate sealing treatment is observed, which also indicates the enhanced resistance to pitting. The coating sealed with stearic acid has the highest Epit (> 200 mV vs. SCE) as a result of the best sealing effect of stearic acid. 3.4. Long term corrosion behavior of Al-based amorphous/nanocrystalline 13

ACCEPTED MANUSCRIPT coatings after different sealing treatments In order to evaluate the long-term corrosion behavior of Al-based amorphous/nanocrystalline coatings after different sealing treatments, linear polarization resistance and EIS tests were carried out after different immersion time.

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Fig. 8 shows the linear polarization resistance of the as-sprayed (unsealed) and the

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sealed Al-based amorphous/nanocrystalline coatings after different immersion time in

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3.5 wt.% NaCl solution. Almost all samples present the declined value of linear polarization resistance with the increased immersion time, indicating the decreased

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corrosion resistance. The as-sprayed coating shows a high linear polarization

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resistance of 7,610  cm2 after 1 h immersion. However, this value drops to 2,270  cm2 after 48 h immersion time implying obviously the deteriorated corrosion

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resistance of the as-sprayed coating. Prolonging the immersion time to 144 h reduces

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the value of linear polarization resistance to 923  cm2 and such a low value usually means the loss of coating protection. For the coating with nickel acetate sealing

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treatment, there exhibits a higher linear polarization resistance of 21,890  cm2 after

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1 h immersion, and the linear polarization resistance is still maintained at a high value of 7,200  cm2 after 144 h immersion, which is comparable to the value of the as-sprayed sample after 1 h immersion. Nevertheless, this value decreases to 915  cm2 rapidly after 288 h immersion implying the failure of this coating. The coating sealed with potassium dichromate shows a fairly high linear polarization resistance of 3.72×105  cm2 after 1 h immersion, and this value tends to decrease after 144 h immersion but yet high (18,250  cm2). In fact, the corrosion resistance of the 14

ACCEPTED MANUSCRIPT potassium dichromate sealed coating is still maintained to a certain extent after 288 h immersion demonstrating the superior sealing effect. The highest linear polarization resistance (7.27×108  cm2) is obtained for stearic acid sealed coating after 1 h immersion, and this value is still very high (9.7×107  cm2) after 720 h immersion,

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indicating the best sealing effect among the three sealing treatments.

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Fig. 9 shows the Bode plots of four kinds of coatings after different immersion

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time in 3.5 wt.% NaCl solution. As for the as-sprayed coating, Bode magnitude plots (Fig. 9a) present the decreased impedance in low frequency with the increased

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immersion time. In theory, the impedance in low frequency reflects the protectiveness

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of coating [20, 22]. After 144 h immersion, the value of impedance is less than 1,200  cm2, indicating the poor resistance to corrosion. Bode phase angle plots (Fig. 9b)

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display the reduced value of phase angle with the increased immersion time, which

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also signify the decreased corrosion resistance. Meanwhile, there display two peaks in Bode phase angle plots with different immersion time indicating two time constants.

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Thus, an equivalent circuit (Fig. 10a) is proposed to fit the Bode plots based on

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relevant investigations [20, 22, 34]. The specific meanings of these parameters in this equivalent circuit are as follows: Rs is the solution resistance; Rt is the charge transfer resistance and CPEdl is a constant phase element associated with the double layer capacitance; Rc and CPEc are related to the resistance and capacitance of the coating respectively. For the coatings sealed with nickel acetate and potassium dichromate, the Bode magnitude plots also exhibit a decreased impedance value in low frequency with the increased immersion time, and the Bode phase angle plots display the 15

ACCEPTED MANUSCRIPT reduced phase angle value with the increased immersion time as well. However, some obvious differences can also be observed compared to the as-sprayed coating. After 144 h immersion, the sealed coatings preserve the higher impedance value than the as-sprayed (unsealed) coating, indicating the enhanced corrosion resistance after

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sealing treatment. At the same time, the Bode phase angle plots display a broad peak

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spanning a wide frequency range at the initial immersion stage, which are

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significantly different from those of as-sprayed coating. Based on some previous studies [35, 36], equivalent circuit 2 (Fig. 10b) is proposed to fit these Bode plots.

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Compared to equivalent circuit 1, Rc and CPEc are replaced by Rf and CPEf (the

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resistance and capacitance of the surface film, respectively) considering the good sealing effect on the coating surface, and the position of each component in the

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equivalent circuit 1 has also been changed. As for the coating sealed with stearic acid,

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the Bode plots hardly change with the increased immersion time. After 720 h immersion, the coating displays a very high impedance value of 108  cm2 in low

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frequency, indicating the superior corrosion resistance. Furthermore, the EIS fitting

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results of four kinds of coating samples after 288 h immersion were presented in Table 2. It is noteworthy that the equivalent circuit 1 (Fig. 10a) is used for the nickel acetate or potassium dichromate sealed coatings at this immersion stage because the impedance of these coatings after 288 h immersion is even lower than that of the unsealed coating after 1 h immersion. For the coating sealed with stearic acid, the equivalent circuit 2 (Fig. 10b) is used for the stearic acid sealed coating after 288 h immersion, as this sealed coating displays a very high impedance after immersing in 16

ACCEPTED MANUSCRIPT 3.5 wt.% NaCl solution for 288 h indicating the superior corrosion resistance of this coating. The EIS fitting results indicate that the coating sealed with stearic acid has the largest Rf (Rc) and Rt, followed by the potassium dichromate sealed coating, then the nickel acetate sealed coating and the as-sprayed coating in sequence. Based on the

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results of linear polarization test and EIS, it can be deduced that the sealing effects of

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the three selected sealants on Al-based amorphous/nanocrystalline coating are in the

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sequence of stearic acid > potassium dichromate > nickel acetate.

3.5. Corrosion morphologies of Al-based amorphous/nanocrystalline coatings

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after different sealing treatments

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Fig. 11 shows the cross-section morphologies of four kinds of Al-based amorphous/nanocrystalline coatings with different immersion time in 3.5 wt.% NaCl

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solution. For the as-sprayed coating (Figs. 11a and b), there exhibits obvious pitting

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after immersing in 3.5 wt.% NaCl solution for 144 h, and corrosion-induced thinning is also observed in local regions of the coating. As for the coating sealed with nickel

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acetate (Figs. 11c and d), the pitting of the coating after 480 h immersion displays the

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similar morphology to the as-sprayed coating after 144 h immersion. However, the thinning of this coating is more uniform compared to the as-sprayed sample, which can be attributed to the reduced defects on the coating surface. For the coating sealed with potassium dichromate (Figs. 11e and f), there also presents distinct pitting after 480 h immersion, while significant thinning of coating is not observed compared to the nickel acetate sealed sample. As for the stearic acid sealed coatings (Figs. 11g and h), they display a uniform cross section without pitting or thinning after 480 h and 720 17

ACCEPTED MANUSCRIPT h immersion, indicating the best corrosion resistance. 3.6.

Sealing

mechanisms

of

different

sealants

for

Al-based

amorphous/nanocrystalline coating Sealants can penetrate into the pores of coating by flow, diffusion and capillary

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action reducing the defects of coating [20-22]. Moreover, some sealants can react with

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the coating, thereby achieving the surface chemical treatment [29, 37]. The porosity of

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coating is a key parameter in the choice of sealants. As for the coating with a high porosity, sealants can infiltrate into the pores easily due to the interconnection between

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different pores and thereby attain a considerable depth of penetration. Correspondingly,

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sealing treatment can significantly enhance the long-term corrosion resistance of coating. Nevertheless, the sealants are only filled into the surface pores for the coating

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with a low porosity. In this investigation, the porosity of HVAF sprayed Al-based

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amorphous/nanocrystalline coating is about 0.42 % which is much lower than that of most thermal spraying coatings, and the sealing effect of some sealants may not be

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apparent. Thus, it is no longer a reasonable solution to improve sealing effect by

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increasing the penetration depth of sealants. However, those sealants that can bring about the surface chemical treatment or the uniform coverage on coating surface may be the best way to improve the corrosion resistance of the coatings with low porosity. The results of potentiodynamic polarization and linear polarization tests indicate that the three selected sealants all enhance the corrosion-resistant performance of Al-based amorphous/nanocrystalline coating, and stearic acid exhibits the best sealing effect, followed by potassium dichromate and nickel acetate in sequence. 18

ACCEPTED MANUSCRIPT As for nickel acetate sealant, some research has shown that there are two major reactions in the sealing process (>80oC) [30, 38, 39]. One is the formation of alumina hydrate (boehmite) (Equation 2), the other one is the deposition of nickel hydroxide in the pores (Equation 3):

(3)

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Ni2+ +2OH   Ni(OH)2

(2)

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Al2O3  H 2O  2AlOOH

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, and the latter usually plays a more important role. The results of XPS indicate that there exists obvious Ni(OH)2 peak with a binding energy of 856.1 eV in the coating

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after nickel acetate sealing treatment (Fig. 12b), while the as-sprayed coating does not

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have this feature (Fig. 12a), implying that some Ni(OH)2 is produced after nickel acetate sealing treatment. However, the nickel acetate sealed coating exhibits a poor

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sealing effect based on the results of linear polarization resistance and EIS, and this

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can be due to a few unclosed macro-pores on the coating surface. As we know, the dose of nickel acetate used in this work is relatively small (1.2 g/L Ni2+), and the

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pores of coating are of micron level (the maximum over 20 μm), so it is difficult to

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seal these pores completely. For potassium dichromate sealant, the surface pores are well covered with filamentous substances, as shown in Fig. 6b. The XPS results indicate that there exist Cr(VI) and Cr(III) in the surface film of Al-based amorphous/nanocrystalline coating after potassium dichromate sealing treatment, and their binding energy are about 579.4 eV and 576.1 eV respectively (Fig. 12c). According to relevant research, the Cr(VI) mainly corresponds to Al(OH)CrO4 and Al(OH)Cr2O7 [29, 40], and Cr(III) to 19

ACCEPTED MANUSCRIPT the reduction of Cr(VI) as shown in Equation 5. Meanwhile, some AlOOH can also be produced in this sealing process. As a result, the pores are well filled with these substances. The sealing mechanism of potassium dichromate can be expressed as follows: (4)

2Al+Cr2O72- +7H 2O  2Al3+ +2Cr 3 +14OH 

(5)

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2Al2O3 +3Cr2O72 +5H2O  2Al(OH)CrO4 +2Al(OH)Cr2O7 +6OH 

(6)

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Al2O3  H 2O  2AlOOH

Potassium dichromate sealant was commonly used in the surface treatment of

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aluminum alloys and magnesium alloys in the past decades [29, 37]. It is should be

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noted that Cr(VI) is poisonous and carcinogenic, so the usage of this sealant is less frequent in industries [3]. However, this study can provide some valuable guidance for

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further study of chromium-replaced sealant on Al-based amorphous/nanocrystalline

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coating.

As to the coating with stearic acid sealing treatment, the results of surface

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morphology indicate that there is a thin layer of stearic acid on the coating (Fig. 6d),

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and this stearic acid layer can segregate the coating and the solution due to possessing good impermeability, which hinders the invasion of corrosive ions. Shang et al. also illustrated the superior sealing effect of stearic acid on the anodic film of aluminum alloys [30]. In addition, the stearic acid also has good hydrophobicity ability. Figs. 13a and b exhibit the contact angles of the as-sprayed coating and the stearic acid sealed coating, respectively. The coating sealed with stearic acid retains a higher contact angle of 130o compared to the as-sprayed coating with a contact angle of 70o. 20

ACCEPTED MANUSCRIPT According to the contact angle formula proposed by Cassie et al.:

cosr = f1 cos  f 2

(7)

where f1 is the fraction of the contact area between water droplet and the solid accounting for the total area, f2 is the fraction of the contact area between water

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droplet and air accounting for the total area (f1 + f2 =1), θ represents the contact angle

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of the unsealed coating surface and θr represents the contact angle of the stearic acid

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sealed surface, it is easy to obtain value of f2, which is 0.734 in this case, indicating that the proportion of air in the interface is up to 73.4 %. The decreased contact area

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amorphous/nanocrystalline coating.

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between solution and coating can also reduce the corrosion rate of Al-based

Finally, we want to emphasize that sealing treatment is usually an auxiliary way

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to improve the corrosion resistance of spraying coatings. Fundamentally, improving

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the intrinsic corrosion resistance of coating should be a top priority. Therefore, in the next step, we will embark on relevant investigations of optimizing the coating

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composition and the spraying process to promote the intrinsic corrosion resistance of

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Al-based amorphous/nanocrystalline coating.

4. Conclusions

1) The HVAF sprayed Al-based amorphous/nanocrystalline coating has high amorphous content and good compactness. The amorphous content of this coating with α-Al nanocrystals precipitated in the amorphous matrix is 80.3%. The porosity of this coating is 0.42 %, and the typical size of pores is less than 10 μm in diameter, and as a result this coating displays good short-term corrosion 21

ACCEPTED MANUSCRIPT resistance with an average icorr of 7.4 μA/cm2. 2) Sealing treatment can effectively close the connected pores and reduce the porosity defects of coating, contributing to the improved corrosion resistance of the Al-based amorphous/nanocrystalline coating. Based on the long-term

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electrochemical tests, it can be inferred that the sealing effects of the three

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selected sealants on Al-based amorphous/nanocrystalline coating are in the

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sequence of stearic acid > potassium dichromate > nickel acetate. 3) Stearic acid presents the best sealing effect due to its good impermeability and

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hydrophobicity, as displayed by the largest linear polarization resistance after 720

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h immersion in 3.5 wt.% NaCl solution. Potassium dichromate also displays good sealing effect owing to the formation of AlOHCrO4 , AlOHCr2O7 and AlOOH

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in the coating surface, with which the pores are filled and thereby the penetration

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of the corrosive ions is prevented. Nickel acetate exhibits a weak sealing effect, which can be ascribed to the unclosed macro-pores on the coating surface.

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Acknowledgements

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This work was financially supported by National Key Research and Development Program of China (No. 2018YFC0808503), the Civil Aircraft Project of Ministry of Industry and Information (No. MJ2015F022), and the National Natural Science Foundation of China (No. 51471166 and No. 51131006). The authors were grateful to X.D. Lin for the TEM observation and valuable discussion.

22

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ACCEPTED MANUSCRIPT the clean surface of K2CrO4 and K2Cr2O7 due to air exposure and argon ion

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CE

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D

MA

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SC

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bombardment, Surf. Interface Anal. 25 (1997) 161-166.

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ACCEPTED MANUSCRIPT Figures Fig. 1. Three-dimensional CLSM surface morphology of the as-sprayed Al-based amorphous/nanocrystalline coating (a); (b) and (c) the cross-section SEM morphologies of the as-sprayed coating.

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Fig. 2. XRT three-dimensional reconstructed image of the as-sprayed Al-based

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amorphous/nanocrystalline coating (a); (b) a representative two-dimensional grayscale

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images parallel to the coating surface; (c) the size distribution of porosity in the coating.

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Fig. 3. XRD patterns (a) and DSC plots (b) of the as-sprayed Al-based

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amorphous/nanocrystalline coating and the fully amorphous ribbon. Fig. 4. Bright-field TEM images and selected area electron diffraction (SAED)

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patterns of the as-sprayed Al-based amorphous/nanocrystalline coating (a) A

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completely amorphous region; (b) the partially crystallized region. Fig. 5. The macroscopic surface morphologies of the coatings after different sealing

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treatments.

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Fig. 6. The microscopic surface morphologies of Al-based amorphous/nanocrystalline coatings after different sealing treatments. Fig. 7. Potentiodynamic polarization plots of Al-based amorphous/nanocrystalline coatings after different sealing treatments. Fig. 8. Linear polarization resistance of the different coatings after different immersion time in 3.5 wt.% NaCl solution. Fig. 9. Electrochemical impedance spectroscopy of the different coatings after 29

ACCEPTED MANUSCRIPT different immersion time in 3.5 wt.% NaCl solution. Fig. 10. General model of the equivalent circuits proposed to fit the EIS data. Fig. 11. Corrosion morphologies of Al-based amorphous/nanocrystalline coatings after different sealing treatments.

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Fig. 12. XPS spectra of Ni2p3/2 for the as-sprayed coating (a) and the nickel acetate

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sealed coating (b) after 40 s sputtering; (c) the spectrum of Cr2p3/2 for potassium

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dichromate sealed coating after 40 s sputtering.

Fig. 13. Contact angles of the as-sprayed coating (a) and the stearic acid sealed

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coating (b).

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ACCEPTED MANUSCRIPT Tables Table 1 Electrochemical parameters of Al-based amorphous/nanocrystalline coatings after different sealing treatments. icorr (μA/cm2)

Epit (mVSCE)

Ecorr (mVSCE)

unsealed nickel acetate potassiumdichromate stearic acid

7.40 ± 0.32 2.84 ± 0.25

381 ± 10 248 ±13

538 ± 16 438 ± 13

0.026 ± 0.002

246 ± 15

464 ± 10

0.0015 ± 0.0001

> 200

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Samples

Table 2

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74 ± 8

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EIS fitting parameters of the different sealed coatings after immersing in 3.5 wt.% NaCl solution for 288 h. Yc

Rc ac

(

cm2)

Ydl

CPEdl

(S＊sa/cm2)

Rt adl

( cm2)

2

12.45 7.996

1.80×103 1.55×103

0.837 0.730

401 709

2.52×102 9.29×103

1 0.951

484 3,442

5.1×104 2.5×103

9.733

1.81×104

0.712

1,131

3.68×104

0.883

5,246

5.3×103

Rf

2

2.04×108

8.8×104

CPEdl

Rs ( cm2)

1.17×109

0.969

2.04×105

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12.99

Ydl (S＊sa/cm2) adl

CPEf Rt

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stearic acid

(S＊sa/cm2)

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(

cm2)

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unsealed nickel acetate potassiumdichromate

CPEc

Rs

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Sample

31

Yf (S＊sa/cm2) af 1.301×109

0.521

ACCEPTED MANUSCRIPT Highlights 

HVAF Al-based amorphous/nanocrystalline coating with 0.42 % porosity was prepared The coating displays good short-term but poor long-term corrosion resistance



Sealing treatment enhances the long-term corrosion resistance of coating



Stearic acid exhibits best effect due to good anti-penetrability and hydrophobicity

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CE

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

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Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Figure 9

Figure 10

Figure 11

Figure 12

Figure 13