Effects of laser remelting speeds on microstructure, immersion corrosion, and electrochemical corrosion of arc–sprayed amorphous Al–Ti–Ni coatings

Accepted Manuscript Effects of laser remelting speeds on microstructure, immersion corrosion, and electrochemical corrosion of arc–sprayed amorphous Al–Ti–Ni coatings Haixiang Chen, Dejun Kong PII:

S0925-8388(18)33140-2

DOI:

10.1016/j.jallcom.2018.08.252

Reference:

JALCOM 47340

To appear in:

Journal of Alloys and Compounds

Received Date: 14 February 2018 Revised Date:

23 August 2018

Accepted Date: 26 August 2018

Please cite this article as: H. Chen, D. Kong, Effects of laser remelting speeds on microstructure, immersion corrosion, and electrochemical corrosion of arc–sprayed amorphous Al–Ti–Ni coatings, Journal of Alloys and Compounds (2018), doi: 10.1016/j.jallcom.2018.08.252. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Effects of laser remelting speeds on microstructure, immersion corrosion, and electrochemical corrosion of arc–sprayed amorphous Al–Ti–Ni coatings

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Chen Haixiang, Kong Dejun College of Mechanical Engineering, Changzhou University, Changzhou 213164, P.R. China

*Correspondent: Kong Dejun, Ph. D., Professor, Tel: 86–051981169810, Fax: 86–051981169812,

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E–mail: kong–[email protected].

Abstract: An arc–sprayed amorphous Al–Ti–Ni coating on S355 structural steel was processed using a laser remelting (LR). The surface and cross–section morphologies, chemical compositions, phases and residual stresses of obtained Al–Ti–Ni coatings at the LR speeds of 5, 10, and 15 mm/s were analyzed

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using a field emission scanning electron microscope (FESEM), energy dispersive spectrometer (EDS), X–ray diffractometer (XRD), and X–ray diffraction stress tester, respectively, and the effects of LR speeds on their immersion corrosion and potentiodynamic polarization curves of Al–Ti–Ni coatings in

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3.5% NaCl solution were also investigated to analyze the mechanism of corrosion resistance. The results

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show that the metallurgical bonding is formed at the Al–Ti–Ni coating interface due to the interdiffusions and recombinations of Al, Ti, Ni, Fe, Cr and Mn. The Al–Ti–Ni coatings at the different LR speeds obtain a certain amount of amorphous phases, which are detected as AlFe, AlFe3, AlCrFe2, Ni2MnAl, and AlNi phases. The residual stresses of as–obtained Al–Ti–Ni coatings at the LR speeds of 5, 10, and 15 mm/s are –12 ± 8, 43.9 ± 3, and –34.4 ± 8 MPa, respectively, of which the tensile residual stress exacerbates the immersion corrosion and the compressive stress restrains the crack expansion.. The corrosion potentials of Al–Ti–Ni coatings at the LR speeds of 5, 10, and 15 mm/s are –1.046, –1.106,

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ACCEPTED MANUSCRIPT and –0.986 V, respectively, which shift positively than S355 steel and effectively increase the corrosion resistance of substrate, the electrochemical corrosion resistance of Al–Ti–Ni coating at the LR speed of 15 mm/s is the best among the three kinds of coating.

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Keywords: arc spraying; laser remelting (LR); residual stress; immersion corrosion; electrochemical corrosion

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1 Introduction

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As a structural steel, S355 on offshore platforms continually serves in atrocious ocean environment, which is extremely vulnerable to pitting and further produce corrosive cracks [2, 3], therefore, immersion corrosion is one of main reasons for its failure [1]. Further more, offshore platforms are impossible to maintain regularly as a ship because they are far away from the coast, once the corrosion degradation of S355 steel occurs, which will lead to

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incalculable economic losses and casualties. Therefore, the development of anti–corrosion coating on offshore platforms has important scientific significance.

Al coating with low cost, high specific strength and good corrosion resistance is widely deposited on S355 steel

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to provide a cathodic protection on offshore platforms [4], which can rapidly form the passive film due to the high

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activity of Al. Unfortunately, a number of chloride ions are presented in seawater, the thin and instable passive film easily suffers from severe pitting. Compared with the traditional Al coating, amorphous Al coating with the unique atom arrangement of long– and short–range disorders is no corrosion sensitive defects such as grain boundary, dislocation and secondary phase, its corrosion resistance is higher than crystal coating [5, 6], which has become the new ideal material for application in anti–corrosion coating [7]. However, the amorphous Al belongs to the marginal glass formers, which requires a very rapid cooling rate (105–106 K/s) to avoid crystallization [6], its application is inevitablely restricted by the limited amorphous forming ability of conventional manufacturing

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ACCEPTED MANUSCRIPT methods [9]. At present, amorphous Al has been paid wide attentions at China and abroad [8], its fabrication methods have become the research hot spots, whose critical problem is to design appropriate alloy compositions and optimize fabrication methods for increasing the amorphous forming ability [10]. Researches have shown that

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Ti and Ni elements incorporated into the Al coating can increase its amorphous forming ability [11–13], the about researches of Al–Ti–Ni coating have rarely reported.

Arc spraying with simple operation, high efficiency, and low cost has become an economical and effective

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method to fabricate the Al coating [14]. However, the sprayed Al coating is typical lamellar structure with the

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mechanical bonding at the coating interface, the internal porosity originated from the unmelted Al particles easily forms the corrosion pitting in seawater, which greatly reduces its immersion corrosion resistance [15]. Laser remelting (LR) combines laser technology with surface treatment, which has advantages of sufficient energy, fast heating and rapid cooling rate (105 –1010 K/s) [9, 16], the Al coating is completely melted to eliminate the above

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arc spraying defects and form metallurgical bonding with the substrate. At the same time, the LR with higher cooling rate easily meets the critical cooling rate of amorphous Al and forms the nonequilibrium microstructure to promote amorphous formation [17]. LR speed directly determines the heating time of laser energy on the Al

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coating [18], the high LR speed shortens the interaction time between the laser energy and the substrate, which

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reduces the input energy and leads to the Al coating not to be completely melted; while the low LR speed leads to the Al coating suffer a severe warp. Thus, it is essential to investigate a suitable LR speed for fabricating amorphous Al coating, Zhang et al [19] analyzed the effects of laser power on the microstructure and properties of laser cladded Co–based amorphous composite coatings; Mahamood et al [20] researched the influence of scanning speed on the microstructure and microhardness of Ti alloy produced by laser deposition; Li et al [21] researched the effect of remelting scanning speed on the amorphous forming capability of Ni–based alloy using a laser cladding and LR. However, there are few reports about the effects of LR speed on the morphology and

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ACCEPTED MANUSCRIPT comprehensive properties of arc–sprayed Al–Ti–Ni coating. In this study, arc–sprayed Al–Ti–Ni coatings on S355 steel were remelted at the LR speeds of 5, 10, and 15 mm/s. The purpose was to investigate the effects of LR speeds on the microstructure, immersion corrosion and

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electrochemical corrosion of amorphous Al–Ti–Ni coatings in 3.5 % NaCl solution, which provided a theoretical basis for the application of amorphous Al–Ti–Ni coatings on offshore platforms.

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2 Experimental

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2.1 Sample preparation

The substrate was European standard S355 structural steel with the chemical composition (wt, %). C 0.17, Si 0.55, Mn 0.94. P 0.035, Cr 0.065, S 0.035, Ni 0.065, Mo 0.30, the rest was Fe. The substrate was ground with 400# sandpaper, coarsened with corundum and cleaned with acetone, and then dried in air. The Al wire packaged

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Ti–Ni powder with the diameter of 2 mm was used for arc spraying, its sketch of wire cross–section is shown in Fig.1. The arc spraying was conducted on an AS400 type intelligent digital arc spraying device, the specific parameters of arc–spraying process are shown in Tab.1. After arc spraying twice, the thickness of Al–Ti–Ni

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coating was 450–600 µm.

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The LR test was conducted on a ZKSX–2008 type fiber–coupled laser spraying system, Ar gas was employed as protective gas during the LR test, technical parameters: laser power of 1000 W, spot diameter of 4 mm, overlap ratio of 50 %, respective speed of 5, 10, and 15 mm/s, and Ar gas speed of 15 L/min. After the LR test, the sample was cut into the dimensions of 15 mm × 15 mm × 5 mm. 2.2 Characterization methods The morphologies and chemical compositions of obtained coatings were analyzed using a SUPRA55 type field–emission scanning electron microscope (FESEM) and its configured energy dispersive spectrometer (EDS),

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ACCEPTED MANUSCRIPT the phases were also analyzed using a D/MAX2500 type PC X–ray diffractometer (XRD), and the residual stresses were analyzed on an X350–A type X–ray diffraction stress tester with the inclination fixed 2ψ method, Cu target–Kα radiation was selected for measuring residual stress, diffraction face of (422), respective incident angle

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of 0, 15, 25 and 35 o, stress constant of –179 MPa/o, scanning angle 2θ of 145–162 o, scanning step angle of 0.1 o, counting time of 0.5 s, tube voltage of 25.0 kV, tube current of 10.0 mA, and collimated tube diameter of 1 mm. According to Prague equation, residual stress (σ) was

∆(2θ ) ∆ sin 2 ψ

(

)

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σ =k

(1)

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where k was the stress constant, which was related to the coating material; θ was the scanning angle; ψ was the incident angle of X–ray.

The immersion corrosion was performed in a YQW–250 type immersion corrosion chamber in accordance with the GB 6458–86 neutral salt spraying (NSS) protocol, 3.5 % NaCl solution was used to simulate the immersion

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corrosion in seawater. Technological parameters: pH of 6.5 – 7.2, temperature of 26 ± 7 oC, immersion time of 720 h. The morphologies, element distributions and phases of Al–Ti–Ni coatings after the immersion corrosion

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test were analyzed using a FESEM, EDS, and XRD, respectively. The electrochemical performance test was conducted on a CHI660E type electrochemical workstation with the

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conventional three–electrode system, and the platinum plate and mercury/calomel–saturated KCl were as the counter electrode and reference electrode, respectively. The sample with the area of 10 mm × 10 mm was exposed in 3.5 % NaCl solution as the working electrode, and the other surfaces were coated and soaked for 15 min before the electrochemical corrosion test, its specific parameters are shown in Tab.2.

3 Analysis and discussion 3.1 Characterizations of surfaces and cross–sections

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ACCEPTED MANUSCRIPT 3.1.1 Morphologies and EDS analysis of Ti–Ni powder The morphology of Ti–Ni powder is shown in Fig.2 (a). The particle A was Ti with the irregular shape, its surface was loose and rough; while the particle B was Ni with the flat–ball–like shape, its surface was compact

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and flat. The EDS analysis result of Ti–Ni powder is shown in Fig.2 (b). The Ti–Ni powder was primarily composed of Ti and Ni elements, the C and O elements were also detected, which originated form the absorptive products in the air. The EDS analysis results of Ti and Ni particles are shown in Fig.2 (c)–(d). The Ti and Ni

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particles were the constituent elements of Ti–Ni powder, while the C and O elements were the impurity elements;

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the Pt came from the sputtering gold on the powder surface to enhance its electrical conductivity during the SEM test.

3.1.2 Plane scan analysis of Al–Ti–Ni coating surfaces

The morphology of Al–Ti–Ni coating surface at the LR speed of 5 mm/s is shown in Fig.3 (a). The coating

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surface was rough and porous, which reduced the density of coating surface. The laser working modes between the conduction mode and the keyhole mode were able to convert with the critical value, when the laser energy was lower than that of conduction mode, the low LR speed increased the energy absorbed by the unit volume, which

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was conducive to melting the arc–sprayed Al–Ti–Ni coating completely and improved its density; conversely,

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when the laser energy was higher than that of conduction mode, the coating surface temperature was higher than the gasification temperature of Al and led to an intense evaporation of Al, the produced recoil pressure and Marangoni convection caused the decrease of molten pool to form the porosity [22]. At the LR speed of 5mm/s, the input energy was excessive to satisfy the melting of Al–Ti–Ni coating, and the gas originated from the evaporation of Al had no enough time to escape due to the rapid solidification of molten pool, forming the porosity on the coating surface. Some white spherical particles were presented on the coating surface, which was because the high energy led to longer residence time of molten pool, leading to more and more metal and carbon

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ACCEPTED MANUSCRIPT diffuse into the liquid molten pool and subsequently formed the super saturation. With the continuous and excessive input of laser energy, there was more and more particles precipitation in the rapid solidification process [23]. The result of plane scan analysis is shown in Fig.3 (b). The Al content was low due to the high temperature

were evenly distributed on the coating surface, as shown in Fig.3 (c)–(f).

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evaporation, while the Fe content was high due to the element diffusion from the substrate. The Al, Ti, Ni and Fe

The morphology of Al–Ti–Ni coating surface at the LR speed of 10 mm/s is shown in Fig.4 (a). The coating

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surface was uneven, showing a certain extent of structural fluctuation. Under the action of high laser energy, the

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coating surface was rapidly heated and melted to form the molten pool. The liquid metal flew from the edge of molten pool into the bottom due to the large gradient, density and viscosity between the center and the edge of molten pool, then forming the undulation after a rapid solidification process. The result of plane scan analysis is shown in Fig.4 (b). Compared to the result of Al–Ti–Ni coating at the LR speed of 5 mm/s, the Al content

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increased due to the decrease of input heat, which reduced the evaporation of Al. The Al was uniformly distributed on the coating surface, as shown in Fig.4 (c). The Ti, Ni and Fe were enriched on the small range, as shown in Fig.4 (d)–(f).

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The morphology of Al–Ti–Ni coating surface at the LR speed of 15 mm/s is shown in Fig.5 (a). The coating

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surface was flat and continuous with no obvious porosity and cracks. The high energy laser caused the liquid possess the adequate suitable energy to move to the equilibrium position, which was conducive to forming the homogeneous structure. The result of plane scan analysis is shown in Fig.5 (b). The high content of Al was detected on the coating surface, which was the same as that at the LR speed of 10 mm/s. The Al, Ti, Ni and Fe were enriched on the coating surface with the different degrees, as shown in Fig.5 (c)–(f). 3.1.3 Plane scan analysis of Al–Ti–Ni coating cross–sections The morphology of Al–Ti–Ni coating cross–section at the LR speed of 5 mm/s is shown in Fig.6 (a). The

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ACCEPTED MANUSCRIPT cross–section was flat, with no obvious cracks. The result of plane scan analysis is shown in Fig.6 (b). The Al, Ti, Ni, Fe, Cr, C and O were detected on the coating–substrate cross–section, among them, the Al, Ti and Ni were the constituent elements of Al–Ti–Ni coating, and the Fe, Cr and C originated from the substrate, while the O was the

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impurity element. The Al, Ti, and Ni were evenly distributed on the Al–Ti–Ni coating cross–section with the high content, as shown in Fig.6 (c)–(e). There was obvious diffusion of Fe element from the substrate into the Al–Ti–Ni coating, which was the result of Fe diffusion between the coating and the substrate, as shown in Fig.6 (f).

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The morphology of Al–Ti–Ni coating cross–section at the LR speed of 10 mm/s is shown in Fig.7 (a). There

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were also no obvious porosity and cracks on Al–Ti–Ni coating cross–section. The result of plane scan analysis is shown in Fig.7 (b). The coating–substrate cross–section was composed of Al, Ti, Ni, Fe, Cr and C, among them, the Al, Ti and Ni were the constituent elements of Al–Ti–Ni coating, and the Fe, Cr and C came form the elements of substrate. The Al, Ti and Ni appeared obvious atoms–enriched zones on the Al–Ti–Ni coating cross–section,

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which were diffused into the substrate to a lesser extent, as shown in Fig.7 (c)–(e). The Fe was distributed on the substrate cross–section, and obviously diffused into the Al–Ti–Ni coating at the interface, as shown in Fig.7 (f). The morphology of Al–Ti–Ni coating cross–section at the LR speed of 15 mm/s is shown in Fig.8 (a). The

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Al–Ti–Ni coating covered on the substrate with a good density and continuity. The result of plane scan analysis is

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shown in Fig.8 (b). The constituent elements of Al–Ti–Ni coating– substrate cross–section were the same as those at the laser LR speed of 5 mm/s. The Al with the high content was uniformly distributed on the Al–Ti–Ni coating cross–section, as shown in Fig.8 (c). The apparent enrichment of Ti and Ni on the coating cross–section exhibited a small degree of diffusion into the substrate, as shown in Fig.8 (d)–(e). The Fe was obviously diffused in the coating, which was similar to those at the LR speeds of 5 and 10 mm/s, as shown in Fig.8 (f). 3.1.4 XRD analysis The XRD analysis of Al–Ti–Ni coatings at different scanning speeds is shown in Fig.9. The Al–Fe intermetallic

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ACCEPTED MANUSCRIPT compounds (AlFe, AlFe3), AlCrFe2, Ni2MnAl, and AlNi were detected on the XRD patterns. The high energy laser led to the intermetallic diffusion at the coating interface, which formed the compounds and developed effective metallurgical reactions between the Al–Ti–Ni coating and the substrate, enhancing the interfacial bonding strength o

and 60–70 o, indicating that the

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[24–26]. The amorphous peaks of Al–Ti–Ni coatings were observed at 30–40

ordered crystal structure of coating was destroyed. The radius sequence of chemical elements was Ti (0.147 nm) > Al (0.143 nm) > Ni (0.125 nm) > Fe (0.124 nm), the significant difference among the atomic sizes increased the

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difficulty of rearranging atoms on the microscale, which favored the formation of amorphous structure [27]. The

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Al–Ti–Ni coatings had weakly diffusion peaks at 40–50 o, which superimposed with the high intensity diffraction peaks of AlNi, AlFe3, AlFe, Ni2MnAl, and AlCrFe, indicating that the arc–sprayed Al–Ti–Ni coatings after the LR test were a complex structure of crystal, amorphous and compound phases. Because the heating rate of affected zone was lower than the critical heating rate, which avoided the crystallization [28], the crystal phases existed in

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the Al–Ti–Ni coating. The diffraction peaks on the angle range of Al–Ti–Ni coatings were similar, but the peak intensity at the LR speed of 15 mm/s decreased obviously, which increased the amorphous content. For amorphous phases, the cooling rate was an important factor for inhibiting the nucleation and growth of grains. The

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cooling rate at the different LR speeds endowed the different amorphous forming ability for Al–Ti–Ni coating, the

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high cooling rate was beneficial to forming amorphous structure. This was because the formation of amorphous structure needed extremely rapid cooling rate and less retention time at the melting state [21]. The input energy for molten pool was large and the cooling rate was relatively small at the low LR speed, in which the grain had more time to form and grow up. Conversely, the high LR speed decreased the input heat, which resulted in the less residence time in molten state and increased the cooling rate, thus enlarging the tendency to form the amorphous structure [29]. As a result, the Al–Ti–Ni coating at the LR speed of 15 mm/s formed more amorphous phases during the LR test.

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ACCEPTED MANUSCRIPT 3.1.5 Residual stress analysis The residual stresses of Al–Ti–Ni coatings at the different LR speeds are shown in Fig.10. The residual stresses of amorphous Al–Ti–Ni coatings at the LR speeds of 5, 10 and 15 mm/s were –12.2 ± 8 MPa (Fig.10 (a)), 43.9 ± 3

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MPa (Fig.10 (b)) and –34.4 ± 8 MPa (Fig.10 (c)), respectively. The residual stresses after the LR test mainly came from that the temperature distribution was extremely uneven due to its local input heat, which led to the slower cooling rate in molten pool center and faster peripheral cooling, forming the temperature gradient from the centre

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to the edge of molten pool. The molten pool shrunk and restricted with the surrounding metal in the rapid

after solidification. 3.2 Corrosion characterizations 3.2.1 Corrosion morphologies

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solidification process, leading to constraining the deformation and forming the residual stresses in the coating

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Fig.11 (a) shows the corrosion morphology of Al–Ti–Ni coating at the LR speed of 5 mm/s. The obvious corrosion cracking was presented on the Al–Ti–Ni coating surface, the longitudinal cracks were tended along the surface perpendicular to the Al–Ti–Ni coating, which extended flexurally and connected with the secondary

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cracks. The cracks were mainly originated from the defects, such as pitting [31]. On one hand, the porosity on

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Al–Ti–Ni coating surface (Fig.3 (a)) provided the pathway for the penetration of corrosive media and formed the pitting in the initial period of immersion corrosion. The stress was concentrated on the local zones of pitting [32, 33]; On the other hand, the pitting was the source of corrosion crack, whose sensitivity was proportional to the sizes of pitting. With the increase of pitting sizes, the stress concentration on the pitting zones was more obvious [34]. The compressive stress of –12.2 ± 8 MPa in Fig.10 (a) was not enough to restrict the crack growth, more pitted porosity was formed and maintained growth up during the continuous immersion corrosion, eventually connected with each other. When the local stress around the pitting exceeded the certain critical stress, the stress

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ACCEPTED MANUSCRIPT concentration occurred on the groove position, which was transferred from the pitting to the crack [35]. The corrosion products on the cracks were loose, leading to the corrosion condition of local electrolyte was maintained, and the corrosion pitting was further connected. The result of plane scan analysis is shown in Fig.11 (b). The Al

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maintained a low content, which was unevenly distributed on the coating, as shown in Fig.11 (c). The Ti and Ni were also unevenly distributed on the coating surface, forming the atoms–rich zones on the Al–poor zones, as shown in Fig.11 (d)–(e).

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Fig.12 (a) shows the corrosion morphology of Al–Ti–Ni coating at the LR speed of 10 mm/s. The corrosion was

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uneven on the coating, there were more corrosion cracks and large shedded areas. Compared with that at the LR speed of 5 mm/s, the corrosion degree was severer. The penetration and diffusivity of corrosive Cl– with the small atom radius was enhanced due to the tensile stress of 43.9 ± 3 MPa in Fig.10 (b), which promoted the forming of microcracks and accelerated the local corrosion sensitivity near the defects and the dissolution of Al in

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microcracks, so the high corrosion speed led to the larger crack width [36]. The cracks on Al–Ti–Ni coating surface were connected with the increase of corrosion time, once the bonding strength among the Al–Ti–Ni coatings due to the mechanical interlocking reactions was less than the local tensile stress, the adhesive loss and

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spalling occurred on the coating surface [37]. The result of plane scan analysis is shown in Fig.12 (b). Compared

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with that in Fig.4 (b), the mass fraction of Al decreased, indicating that the Al was dissolved into 3.5% NaCl solution. The Al was unevenly distributed on the coating surface, forming the atom–rich zones, as shown in Fig.12 (c). The Ti and Ni were also unevenly distributed, and the atoms–rich zones were formed on the Al atom–poor zones, as shown in Fig.12 (d)–(e).

Fig.13 (a) shows the corrosion morphology of Al–Ti–Ni coating at the LR speed of 15 mm/s. The corrosion cracks were covered by the corrosion products, and the microdefects on the coating surface were the preferred location of corrosion. The microcracks reacted with the corrosive media and produced the dense corrosion

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The result of plane scan analysis is shown in Fig.13 (b). Compared with that in Fig.5 (b), the Al content also decreased, indicating that the Al was dissolved. The Al, Ti, and Ni contents were unevenly distributed on the coating surface, as shown in Fig.13 (c)–(e).

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3.2.2 XRD analysis after corrosion

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The XRD spectra of amorphous Al–Ti–Ni coatings at the different LR speeds after the immersion corrosion test are shown in Fig.14 (a). The detected phases of Al–Ti–Ni coatings were similar, which were composed of AlNi, AlFe3, AlFe, Ni2MnAl and AlCrFe2, and the trend of peak broadening at 30–40 o and 60–70 o showed the presence of amorphous phases. Compared with those in Fig.9, there was no new phases formed, indicating that their phase

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compositions were stable. The XRD results showed that the effect of phase compositions on the corrosion resistance was small, while the content of amorphous phase had the greater influence on their corrosion resistance. 3.2.3 Electrochemical Corrosion

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The potentiodynamic polarization curves of Al–Ti–Ni coatings at the different LR speeds and S355 steel in

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3.5 % NaCl solution are shown in Fig.15. The polarization curves of Al–Ti–Ni coatings had similar shape, indicating there was the same corrosion process in the initial immersion period. At the initial period of anodic polarization, the current density increased with the increase of potentials, the coatings occurred the anodic dissolution reaction. With the potentials continuously increased, the corrosion current changed slowly, the anodic polarization curve on the range did not obey the Talfel law. As the polarization potentials continuously increased, the anode currents increased sharply. Due to the increasing potentials, the migration rate of anions in 3.5 % NaCl solution also increased, and the Cl– easily penetrated the passivation film on the Al–Ti–Ni coating, leading to

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ACCEPTED MANUSCRIPT easily reaching the pitting potential. The corresponding electrochemical parameters of Al–Ti–Ni coatings such as corrosion potential (Ecorr) and corrosion current density (icorr) were obtained from the polarization curves. The corrosion potentials of Al–Ti–Ni coatings at the LR speeds of 5, 10, 15 mm/s were –1.046, –1.106, and –0.986 V,

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respectively. Compared with the –1.426 V of S355 steel, the corrosion potentials of Al–Ti–Ni coatings shifted positively, suggesting that the Al–Ti–Ni coating enhanced the corrosion resistance of substrate, this was attributed to the formed amorphous structures with high chemical stability and the complex phase of intermetallic

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compounds after the LR test. The corrosion occurred preferentially at the grain boundary location, while the

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amorphous structure inhibited the Cl– penetration into the coating due to the lack of defects, such as grain boundaries, dislocations and second particles at a certain extent [39], the intermetallic compounds increased the corrosion potential and delayed the corrosion rate [9], effectively improving the corrosion resistance of Al–Ti–Ni coatings.

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The corrosion current density of Al–Ti–Ni coatings after the LR test was 10–6–10–9 A/cm2, which were more than 10–4 A/cm2 of substrate. Generally speaking, the higher Ecorr usually meant better chemical stability, while the lower icorr meant lower corrosion rate [38]. Compared with that at the LR speed of 15 mm/s, the Al–Ti–Ni coatings

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at the LR speeds of 5 and 10 mm/s had inactive Ecorr and greater icorr, indicating that their corrosion resistance was

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relatively poor. Therefore, there was severe corrosion in Figs.11 (a) and 12 (a). The Al–Ti–Ni coating at the LR speed of 15 mm/s showed the highest Ecorr and the lowest icorr, indicating the best corrosion resistance among three kinds of Al–Ti–Ni coatings, which was consistent with the lower corrosion degree in Fig.13 (a). The corrosion resistance of Al–Ti–Ni coating at the LR speed of 15 mm/s was the best among three kinds of coatings due to its high contents of amorphous structure and excellent surface morphology, which were benefit to reducing the active center of corrosion and improved its corrosion resistance.

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ACCEPTED MANUSCRIPT 4 Conclusions (1) The Al, Ti and Ni of Al–Ti–Ni coatings and the Fe, Mn, and Cr of substrate at the different LR speeds are diffused to generate the intermetallic compounds of AlFe3, AlFe, AlCrFe2, Ni2MnAl, and AlNi, which forms

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metallurgical bonding at the coating interface. (2) The phases of Al–Ti–Ni coatings at the LR speeds of 5, 10 and 15 mm/s are composed of crystal, amorphous and intermetallic compound phases, the increased LR speed is beneficial to forming amorphous

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phases.

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(3) The residual stresses of Al–Ti–Ni coating at the LR speeds of 5, 10 and 15 mm/s are –12 ± 8, 43.9 ± 3 and –34.4 ± 8 MPa, of which the compressive stress is benefit to restraining the crack expansion, while the tensile residual stress accelerates the occurrence and process of immersion corrosion.

(4) The corrosion potentials of Al–Ti–Ni coatings at the LR speeds of 5, 10 and 15 mm/s are –1.046, –1.106 and

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–0.986 V, respectively, the corrosion degree of Al–Ti–Ni coating at the LR speed of 15 mm/s is the lightest, which

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is due to the higher amorphous content and excellent surface morphology.

Acknowledgments

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Financial support for this research by the Key Research and Development Project of Jiangsu Province (BE2016052) is gratefully acknowledged.

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ACCEPTED MANUSCRIPT Tables Tab.1 Technological parameters of arc–sprayed Al–Ti–Ni coating

Parameter

Spraying voltage/V

30–32

Spraying current/A

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Item

160

Spraying distance/mm

150

0.1

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The move speed of arc spraying gun/m•s–1 Spraying angle/o

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80

0.6

Wire feed rate/cm•min–1

100

Overlap ratio/%

35

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Spraying pressure/MPa

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ACCEPTED MANUSCRIPT Tab.2 Test parameters of electrochemical corrosion test

Parameter

Electric potential

–1.5–0

Sweep segment

1

Scanning rate/V/s

0.001

Quiet time/S

2

Sensitivity/A/V

10–0.002

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EP

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Item

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ACCEPTED MANUSCRIPT Figures Al wire Φ 2 mm

Ni powder

100 µm

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Ti powder

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Fig.1 Sketch of Al wire packaged Ti–Ni powder cross–section

19

ACCEPTED MANUSCRIPT 1600

A

C

Ni Ti

Counts/cps

1200

800

BB

Ni

O

400

Pt Pt

(a) Morphology of Ti–Ni powder

0

1

2

Ti Al

Ni

3

7000

3000

Pt

CO

3

4 5 6 Energy/keV

7

8

0

9

10

(c) EDS analysis result of Ti particle

At/% 44.02 52.40 3.58

Ni

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2

10

Ni

1000

Pt

1

9

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Ti

C

0

4000

2000

1000 O

Ti

8

Element Mass/% Ni 78.80 C 19.33 O 1.76

5000

2000

0

Ni

6000

40.10 21.98 37.92

Counts/cps

Counts/cps

3000

68.69 9.49 21.81

7

(b) Plane scan analysis of Ti–Ni powder

Ti Element Mass/% At/%

Ti C O

4 5 6 Energy/keV

RI PT

0

4000

Element Mass/% At/% Ti 16.04 6.09 Ni 19.73 5.87 C 49.77 72.37 O 14.36 15.67

0

1

2

3

4 5 6 Energy/keV

7

8

9

10

(d) EDS analysis result of Ni particle

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Fig.2 Morphology and plane scan analysis of Ti–Ni powder and EDS analysis of Ni and Ti particles

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ACCEPTED MANUSCRIPT 8000 Al

Carbides Counts/cps

Element Al Ti Ni Fe C O

O Ti

6000

Fe Ni

4000 C

Mass/% 7.46 5.87 10.97 43.16 14.24 18.30

At/% 7.54 3.34 5.10 21.07 32.31 30.60

2000 Pt

0

10 µm

1

2

3

4 5 6 Energy/keV

Fe

7

8

9

10

(b) Result of plane scan analysis

(c) Al content

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(a) Plane scanned position

0

Ti

RI PT

Porosity

(d) Ti content

(e) Ni content

(f) Fe content

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Fig.3 Plane scan analysis of Al–Ti–Ni coating surface at LR speed of 5 mm/s

21

ACCEPTED MANUSCRIPT 50000

Counts/cps

Element Mass/% Al 23.20 Ti 1.82 Ni 0.74 Fe 2.56 Mn 1.19 C 25.01 O 45.48

Al

40000 30000 O Mn Ti

20000

At/% 14.55 0.64 0.21 0.78 0.47 35.25 48.11

10000

100 µm

0

1

Pt Ti

2

3

Fe Mn

4 5 6 Energy/keV

7

8

9

(b) Result of plane scan analysis

10

(c) Al content

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(a) Plane scanned position

C Fe Ni

RI PT

0

(d) Ti content

(e) Ni content

AC C

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Fig.4 Plane scan analysis of Al–Ti–Ni coating surface at LR speed of 10 mm/s

22

(f)

Fe content

ACCEPTED MANUSCRIPT 50000 Element Al Ti Ni Fe Mn C O

Al

30000 20000

100 µm

C Fe Ni

0

1

Pt Ti

2

3

4 5 6 Energy/keV

Fe Mn

7

8

9

(b) Result of plane scan analysis

10

(c) Al content

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(a) Plane scanned position

At/% 13.33 0.85 0.24 0.92 0.44 46.20 38.02

O Mn Ti

10000 0

Mass/% 21.87 2.48 0.86 3.12 0.98 33.73 36.98

RI PT

Counts/cps

40000

(d) Ti content

(e) Ni content

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Fig.5 Plane scan analysis of Al–Ti–Ni coating surface at LR speed of 15 mm/s

23

(f) Fe content

ACCEPTED MANUSCRIPT 40000 Fe Ni

Counts/cps

30000

Al–Ti–Ni coating

Al

O Cr Ti

20000

Element Mass/% Al 5.12 Ti 1.12 Ni 10.97 Fe 55.33 Cr 5.05 C 14.76 O 7.64

C

At/% 5.95 0.74 5.86 31.06 3.05 38.53 14.83

10000 Ti

0

200 µm

1

2

3

4 5 6 Energy / kev

7

8

9

10

(b) Result of plane scan analysis

(c) Al content

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(a) Cross–section morphology

0

Fe

RI PT

Pt

Substrate

(d) Ti content

(e) Ni content

(f) Fe content

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Fig.6 Plane scan analysis of Al–Ti–Ni coating cross–section at LR speed of 5 mm/s

24

ACCEPTED MANUSCRIPT

Counts/cps

12000

Al–Ti–Ni coating

Fe Ni

9000

6000

Cr

Element Al Ti Ni Fe Cr Al C

Mass/% At/% 7.94 8.02 1.71 0.97 6.21 2.88 56.63 27.62 Fe 1.13 0.68 26.38 59.83

3000 C

0

200 µm

1

2

Ti 3

4 5 6 Energy / kev

Fe

7

Ni Ni

8

9

(b) Result of plane scan analysis

10

(c) Al content

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(a) Cross–section morphology

0

Pt

RI PT

Substrate

(d) Ti content

(e) Ni content

(f) Fe content

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Fig.7 Plane scan analysis of Al–Ti–Ni coating cross–section at LR speed of 10 mm/s

25

ACCEPTED MANUSCRIPT

Counts/cps

Al–Ti–Ni coating

12000 8000 Cr

Element Mass/% At/% Al 7.40 7.52 Ti 1.59 0.91 Ni 6.13 2.86 Fe 56.81 27.89 Fe Cr 1.11 0.67 24.49 55.91 Al C O 2.47 4.23

4000 C

Fe

Pt

Substrate 0

200 µm

1

2

Ni

3

4 5 6 Energy / kev

7

8

9

(b) Result of plane scan analysis

10

(c) Al content

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(a) Cross–section morphology

0

RI PT

Ni Fe

16000

(d) Ti content

(e) Ni content

(f) Fe content

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Fig.8 Plane scan analysis of Al–Ti–Ni coating cross–section at LR speed of 15 mm/s

26

Ni2MnAl AlCrFe2 AlFe

AlNi

1200

15000

800 400 0 30 32 34 36 38 40 o 2θ/

10000 5000

0 10 20 30 40 50 60 70 80 90 o 2θ /

(a) At LR speed of 5 mm/s

Ni2MnAl

10000

AlFe

AlCrFe 2

500 0 30 32 34 36 38 40 o 2θ/

(c) At LR speed of 15 mm/s

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1000

0 10 20 30 40 50 60 70 80 90 o 2θ/

Fig.9 XRD spectra of Al–Ti–Ni coatings at different LR speeds

27

1500

5000

0 10 20 30 40 50 60 70 80 90 o 2θ/

(b) At LR speed of 10 mm/s

AlFe3

Intensity/a.u.

3

RI PT

5000

15000

1600

AlNi AlFe

Intensity/a.u.

10000

20000

Intensity/a.u.

AlFe AlCrFe2

2000 1500 1000 500 0 30 32 34 36 38 40 o 2θ /

Intensity/a.u.

15000

AlNi AlFe3 Ni2MnAl

AC C

Intensity/a.u.

20000

Intensity/a.u.

ACCEPTED MANUSCRIPT

ACCEPTED MANUSCRIPT

149 0.0

0.2

0.4

0.6

0.8

Sin ϕ 2

150

150 149 0.0

200

300

0.2

0.4

0.6

0.8

2

Sin ϕ

100

153 2θ /

156

159

162

0

147

150

153 2θ/

0

(a) At LR speed of 5 mm/s

×× ×

149 0.0

200

×

150

0.2

0.4

0.6

0.8

2

Sin ϕ

156

159

0

162

147

150

(b) At LR speed of 10 mm/s

156 0

(c) At LR speed of 15 mm/s

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Fig.10 Residual stress analysis of Al–Ti–Ni coatings at different LR speeds

28

153 2θ/

0

RI PT

150

TE D

147

151

100

EP

0

σ = –34.4 ± 8 MPa ο

×× × ×

0 o 15 o 25 o 35

2 θρ/

151

Counts/cps

×

150

152

o

σ= 43.9 ± 3 MPa ο

×× ×

300

Counts/cps

2θp/

o

151

σ = –12.2 ± 8 MPa

400

152

o

0 o 25 o 35 o 45

AC C

Counts/cps

300

400

152

o

0 o 25 o 35 o 45

2 θ p/

450

159

162

ACCEPTED MANUSCRIPT Al

Element Mass/% Al 9.39 Ti 3.28 Ni 1.12 Fe 6.88 C 51.93 O 27.40

15000

Secondary cracks Counts /cps.

12000 C

9000 O Ti

6000 3000

Ti Ni

Cracks

0

1

Ti

2

3

4 5 6 Energy/ kev.

Fe

Ni

7

8

9

10

RI PT

0

At/% 5.25 1.03 0.29 2.46 65.16 25.81

(b) Result of plane scan analysis

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(a) Plane scanned position

(c) Al content

(d) Ti content

(e) Ni content

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Fig.11 Plane scan analysis of Al–Ti–Ni coating at LR speed of 5 mm/s after immersion corrosion test

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ACCEPTED MANUSCRIPT 25000 Al

Element Al Ti Ni Fe C O

Counts/cps.

20000 15000 C

10000

Mass/% At/% 11.8 6.52 2.04 0.63 1.02 0.26 4.95 1.78 51.78 64.30 28.40 26.51

O Ti

5000 Pt

0

1

2

Ti

3

Fe

4 5 6 Energy/kev.

7

8

9

10

RI PT

0

(b) Result of plane scan analysis

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(a) Plane scanned position

(c) Al content

(d) Ti content

(e) Ni content

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Fig.12 Plane scan analysis of Al–Ti–Ni coating at LR speed of 10 mm/s after immersion corrosion test

30

ACCEPTED MANUSCRIPT 20000 Al

Element Al Ti Ni Fe C O

Counts / cps.

16000 12000 8000

Mass/% 14.56 4.82 1.78 6.79 38.23 33.82

At/% 8.82 1.64 0.49 2.57 52.02 34.55

O CTi

4000 Ti

Pt Fe

0

1

Ni

2

3

Ti

4 5 6 Energy/kev.

Fe

7

8

9

10

RI PT

0

(b) Result of plane scan analysis

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(a) Plane scanned position

(c) Al content

(d) Ti content

(e) Ni content

AC C

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Fig.13 Plane scan analysis of Al–Ti–Ni coating at LR speed of 15 mm/s after immersion corrosion test

31

ACCEPTED MANUSCRIPT

5000

0 30 32 34 36 o38 40 2θ/

10000

40

50 2θ/

60

70

80

90

0 10

20

30

40

o

50 2θ/

(a) At LR speed of 5 mm/s

AlCrFe2

2000 1000 0 30 32 34 36o 38 40 2θ/

AlFe 10000

60

70

80

0 10

90

o

(b) At LR speed of 10 mm/s

20

30

40

50 2θ/

60

(c) At LR speed of 15 mm/s

EP

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Fig.14 XRD analysis results of Al–Ti–Ni coatings at different LR speeds after immersion corrosion test

32

70

0

RI PT

30

15000

3000

5000

5000

20

Ni2MnAl AlFe3 AlNi

1000

AlFe

15000

20000

lntensity/a.u.

AlCrFe2

2000

lntensity/a.u.

0 30 32 34 36 38 40 o 2θ/

10000

0 10

AlFe3

20000

1000

Intensity/a.u.

2000

Intensity/a.u.

15000

AlFe3 Ni2MnAl AlFe AlCrFe2

4000

3000

AlNi Ni2MnAl

25000

3000

AC C

Intensity/a.u.

20000

4000

AlNi

Intensity/a.u.

25000

80

90

ACCEPTED MANUSCRIPT 0.5

Al-Ti-Ni coating at LR speed of 15 mm/s

0.0

S355 structural steel

E/V

-0.5 -1.0 -1.5

Al-Ti-Ni coating at LR speed of 5 mm/s

-2.0

Al-Ti-Ni coating at LR speed of 10 mm/s -8

-6 -4 -2 Iogi / (A / cm )

-2

0

RI PT

-2.5 -10

AC C

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Fig.15 Polarization curves of Al–Ti–Ni coatings at different LR speeds and substrate

33

ACCEPTED MANUSCRIPT

Highlights (1) The compounds of Al-Fe, AlCrFe2 and Ni2MnAl formA metallurgical bonding at the coating interface. (2) The Al–Ti–Ni coatings after LR are composed of crystal, amorphous and intermetallic compounds.

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(3) The residual stresses of Al–Ti–Ni coating are compressive stress, restraining its cracking.

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(4) The corrosion potentials of Al–Ti–Ni coatings after LR shift positively, showing high corrosion resistance.