Microstructures and immersion corrosion behavior of laser thermal sprayed amorphous Al-Ni coatings in 3.5 % NaCl solution

Accepted Manuscript Microstructures and immersion corrosion behavior of laser thermal sprayed amorphous Al-Ni coatings in 3.5 % NaCl solution Donghui Zhang, Dejun Kong PII:

S0925-8388(17)33800-8

DOI:

10.1016/j.jallcom.2017.11.054

Reference:

JALCOM 43747

To appear in:

Journal of Alloys and Compounds

Received Date: 16 August 2017 Revised Date:

29 October 2017

Accepted Date: 4 November 2017

Please cite this article as: D. Zhang, D. Kong, Microstructures and immersion corrosion behavior of laser thermal sprayed amorphous Al-Ni coatings in 3.5 % NaCl solution, Journal of Alloys and Compounds (2017), doi: 10.1016/j.jallcom.2017.11.054. 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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Microstructures and immersion corrosion behavior of laser thermal sprayed amorphous Al-Ni coatings in 3.5 % NaCl solution Zhang Donghui1, Kong Dejun1, 2

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School of Mechanical Engineering, Changzhou University, Changzhou 213164, China

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Changzhou high technology research key laboratory of mould advanced manufacturing, Changzhou University, Changzhou 213164, China

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*Correspondent: Kong Dejun, Ph. D., Professor, Tel: 86-051981169810, Fax: 86-051981169812, E-mail:

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[email protected].

Abstract: Amorphous Al-Ni coating with the Al and Ni mass ratios of 3:2, 3:1 and 4:1 was sprayed on S355 steel of offshore platforms using a laser thermal spraying (LTS). The surface-interface

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morphologies, chemical element distribution, phase compositions and crystallization temperature of the obtained amorphous Al coatings were analyzed using a scanning electron microscope (SEM), energy dispersive spectrometer (EDS), X-ray diffractometer (XRD), and differential scanning calorimetry (DSC),

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respectively. The immersion corrosion behaviors of the Al-Ni coatings in 3.5 % NaCl solution for 720 h

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were investigated. The results shows that the Al-Ni coating is composed of Al-Fe alloy, AlCrFe2, Ni2MnAl and amorphous Ni-Mn-Al alloy, the metallurgical bonding is formed at the Al-Ni coating interface due to the diffusions of Al, Ni and Fe elements, and the tensile residual stress induced by LTS accelerated the crack expansion. The amorphous Al-Ni coatings have a certain amount of amorphous component at the crystallization temperature of ~510 oC. The laser thermal sprayed Al-Ni coating generates the dense oxide film immersed in NaCl solution, hindering the anion from entering, which improving the corrosion resistance of Al-Ni coatings. The electrochemical corrosion resistance of Al-Ni

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ACCEPTED MANUSCRIPT coatings with the Al and Ni mass ratios of 3:2 and 4:1 is better than that with the Al and Ni mass ratio of 3:1, effectively improving the corrosion resistance of S355 steel. Keywords: laser thermal spraying (LTS); Al-Ni coating; differential scanning calorimetry (DSC);

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residual stress; corrosion mechanism

1 Introduction

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The development of offshore platforms has become a national key project, its maintenance is difficult and

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expensive, as a main structural steel, S355 steel is mainly used on offshore platforms. The heavy corrosion resistance of S355 steel has become the attention focus in recent years, surface coating technology [1-2] is generally used for heavy corrosion resistance on offshore platforms, which should keep the corrosion resistance effect more than 20 years. The coating used in heavy corrosion resistance must be a high performance coating to reduce the

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times of coating repairs and extend the maintenance cycle. In order to reach the effective protection period of the coating under severe corrosion conditions, the offshore platforms also covers many aspects such as coating system [3]

, thickness and surface treatment, in addition to the anti-corrosion effect of the coating itself. The coatings with

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high adhesion, aging resistance, salt-spray resistance

[4]

and sea water resistance are required offshore platforms,

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which can form high-quality coating to match with cathodic protection. At present, the corrosion protection of S355 steel on offshore platforms has become a hot topic in the world [5-6], which aims at the corrosion characteristics of marine splash zone, it is imperative to develop corrosion protection technology of steel used on offshore platforms. The successful application of thermal spraying coating on offshore platforms has been achieved, and the protection of marine splash zone has achieved long-lasting corrosion prevention effect

[7-8]

. The harsh environment of splash

zone on offshore platforms requires further improvement thermal sprayed coatings with the long-lasting heavy corrosion resistance [9]. In order to avoid the adverse effects of offshore platforms as far as possible and prolong its

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ACCEPTED MANUSCRIPT service life, which is very important to develop more long-lasting heavy corrosion resistance coatings. In the past, arc thermal sprayed Al coating primarily was used for anti-corrosion on offshore platforms, but now the offshore platforms puts forward higher requirements for its protection of structural steel. Laser thermal spraying

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(LTS) is an important surface coating technology, the surface layer on the substrate is melted using a high energy laser beam, and the coating powders are sprayed on the substrate surface with different feeding methods. The laser thermal effect causes the substrate surface melt and cool rapidly

[10]

, the coating dilution is very low

[11]

and the

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powder is combined with the substrate to form the metal coating after solidification. The LTS with the

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characteristic of quick heating and fast cooling, which possesses the preparation condition of amorphous alloy, when the cooling rate exceeds the critical value of metal alloy, the amorphous alloy was directly formed. The above cooling rate can be changed by adjusting laser parameters. Therefore, it is a good method of investigating the amorphization of metal alloy.

bonding

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The laser thermal sprayed coatings have many advantages such as low dilution rate [13]

[12]

, high hardness, high wear resistance and corrosion resistance performance

, strong metallurgical

[14]

. Besides, LTS can

selectively spray the metal coatings, which is a surface treatment of key parts on the equipment [15]. Because the Al [16]

and good oxidation resistance, the corrosion resistance of Al coatings on

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coating has low electrode potential

properties

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various metal substrate shows high anti-corrosion performance

[17]

and more excellent physical and chemical

[18]

. The preparation of amorphous Al coatings has attracted much attention of researchers

Ni-based amorphous coating is one of the earlier amorphous materials

[19]

, in which

[20]

. The Ni-based amorphous coating

displays unique short-range ordered arrangement (analogous to atomic clusters), besides long-range disordered topological structure, which endows amorphous alloy isotropic and homogeneous structure and devoid of structural characteristics of crystalline materials including dislocations, grain boundaries, and stacking faults

[21-23]

. The

solidification process of immiscible alloys offers a unique opportunity in designing composites with spherical

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ACCEPTED MANUSCRIPT particles (globules) dispersed in the metal substrate. The formation of amorphous phase in immiscible alloys induces a decrease in the size of the globules due to high-degree super-cooling, which is achieved during the nucleation of minority liquid phase in the form of spheres

[24]

. Therefore, preparation of laser thermal sprayed

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amorphous Al coating has become one of the hot topics on the research of long term heavy corrosion protection. At present, Deng et al had been made to disperse nano-scale crystalline globules in Al-based amorphous phase using a liquid phase separation in Al-based immiscible alloys [25]. Nagase et al had prepared amorphous Al in water-cooled [26]

. Wang et al had added Ca in the Al-TM-RE alloy

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environment using a laser cladding on magnesium substrate

[27]

. The preparation

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system to form Al85-xY8Ni5Co2Cax system and re-melted it to prepare amorphous Al

technology of amorphous Al coating using a laser rapid heating cooling by liquid nitrogen has not been reported. In this study, an amorphous Al-Ni coating was firstly sprayed on S355 steel surface using a laser thermal spraying (LTS) under the liquid nitrogen online cooling condition, the effects of Al and Ni mass ratio on the formation

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ability of amorphous Al coating were investigated. The surface morphologies, distributions of chemical element, phase compositions and crystallization temperatures of the obtained amorphous Al-Ni coating were characterized using a SEM, EDS, XRD, and DSC, respectively. The corrosion behavior immersed in 3.5 % NaCl solution for 720

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h was investigated, which provided an experimental basis for the application of corrosion protection of amorphous

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Al coating on offshore platforms.

2 Experimental

The experimental material was European standard S355 structural steel with the mass fractions (mass, %): C 0.17, Si 0.55, Mn 0.94, P 0.035, Cr 0.065, S 0.035, Ni 0.065, Mo 0.30, Zr 0.15 and the rest was Fe. Spraying materials were three kinds of Al and Ni with different ratios amorphous powder, the mass ratio of Al and Ni was 3:2 (powder I), 3:1 (powder II), and 4:1 (powder III), respectively, and the corresponding sprayed amorphous Al

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ACCEPTED MANUSCRIPT coating was coating I, coating II, and coating III, respectively. The powders were mixed and milled using a QM3SP04L type planetary ball milling machine to ensure the uniformity of powders. Because small particles cause electrostatic attraction and affect the mobility of powders

[28]

, the particle sizes of the Al powder were selected at

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the range of 20-80 µm in the experiment. The morphologies, element compositions and phases of amorphous Al-Ni powders were analyzed using a JSM-6360LA type scanning electron microscope (SEM), energy dispersive spectrometer (EDS), and D/max2500 PC X-ray diffraction (XRD), respectively. Before LTS, the substrate surfaces

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were ground and treated using a sand blasting. The LTS equipment was conducted on an all solid state laser, and

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the argon gas was used as the power source of powder feeding to realize synchronous powder spraying. Simultaneously, the liquid nitrogen cooling system on the worktable was synchronously on-line cooled. The technological parameters of LTS are shown in Tabl.1. The Al-Ni coating Ⅰ, coating Ⅰ and coating Ⅰ obtained after LTS, the surface morphologies of the obtained Al-Ni coatings were analyzed using a field emission scanning

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electron microscopy (FESEM), and the surface energy spectra were analyzed using its configured energy dispersive spectrometer (EDS), and their microstructures were analyzed using a VHX-6000 type 3-D microscope system. The phases, crystallization temperatures and residual stresses of Al-Ni coatings were analyzed using a XRD, DSC, and

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X-ray stress tester, respectively. The test conditions of DSC: Al crucible, 40-600 oC, heat rising rate of 20 oC/min,

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argon flow with high purity was used for protection. The technological parameters of residual stress: fixed peak method for cross correlation method, target of Co target, radiation source of Co-Kα1, Prague crystal face of (400), incident angle of 0, 25, 35 and 45 °, stress constant of -130 MPa/°, 2θ scan start and termination angle of 155-145°, 2θ scanning step of 0.10 °, counting time of 0.50 s, X light tube high voltage of 22 kV, X light tube current of 6.0 mA. The immersion corrosion test equipment was a KD-60E type salt water immersion corrosion tester. The sample surface with the size of 15 mm × 15 mm × 3 mm was exposed in 3.5 ± 0.5 % NaCl solution, the other surfaces were sealed with epoxy resin, experimental parameters: continuous immersion time for 720 h, pH range of

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ACCEPTED MANUSCRIPT 6.5-7.2, box temperature of 30 ± 5 oC. After the immersion corrosion test, the morphologies and element compositions of Al-Ni coatings before and after immersion corrosion were analyzed using a JSUPRA5 type field emission scanning electron microscope (FESEM) and its configured electronic energy spectrometer (EDS),

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respectively. The electrochemical tests were performed on a CS350 type electrochemical workstation, the sample dimension was 15 mm × 15 mm × 3 mm, which were immersed in 3.5 % NaCl solution for 10 min before the immersion corrosion test, the test method was potentiodynamic measurement, scanning rate of 1 mV/s, sampling

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KCl, test potential range of -500~400 mV, and test time of 1800 s.

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frequency of 0.5 Hz, measuring temperature of 25 oC, the reference electrode type was mercury/calomel-saturated

3 Results and discussion

3.1 Morphologies and EDS analysis of Al-Ni powders

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Fig.1 (a), (c) and (e) shows the morphologies of the Al-Ni powders with the different ratios of Al and Ni. The shape of three amorphous Al-Ni powders was approximately nodular structure with the powder sizes of 10-40 µm. The particle surfaces were relatively smooth, and a small amount of small particles were bonded on the large

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particles, which was beneficial to improving the feeding flowability of the powders. The Al and Ni ratio of three

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kinds of powders were 3:2, 3:1, and 4:1, respectively. Fig.1 (b), (d) and (f) shows the EDS results of Al-Ni powders with the different ratios of Al and Ni, the mass fractions of Al-Ni powder Ⅰ (mass, %): C 16.57, Al 43.50, Ni 37.90, O 2.03; Al-Ni powder Ⅰ: C 15.42, Al 50.85, Ni 31.51, O 2.22; and Al-Ni powder Ⅰ: C 10.35, Al 61.31, Ni 22.13, O 6.21, and the corresponding atom fractions (at, %): Al-Ni powder Ⅰ: C 36.77, Al 42.86, Ni 17.10, O 3.28; Al-Ni powder Ⅰ: C 33.33, Al 49.09, Ni 13.93, O 3.65; and Al-Ni powder Ⅰ: C 22.07, Al 58.33, Ni 9.68, O 9.91. The EDS analysis results show that the three Al-Ni powders were primarily composed of Al and Ni, the C and O were the impurities in the air attached on the Al-Ni powder surface.

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ACCEPTED MANUSCRIPT 3.2 Morphologies and EDS analysis of Al-Ni coating surfaces and interfaces 3.2.1 Morphologies and EDS analysis of Al-Ni coating surfaces The morphology of Al-Ni coating Ⅰ surface is shown in Fig.2 (a). The coating surface was smooth and the

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grains were compact. There was no obvious leak spraying or peeling, but there were dimples and staggered cracks. The laser quick heating and cooling in the LTS test caused the Al-Ni coating to form an uneven temperature gradient. The fusion pool shrank in the condensation process resulted in uneven elastic deformation to form tensile [29]

. Fig.2 (b) shows the plane scan analysis results of Al-Ni coating Ⅰ, the mass

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stress, and produced the cracks

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fractions (mass, %): Al 33.92, Ni 21.27, Fe 12.78, O 15.50, C 16.27, and Cr 0.26; the corresponding atomic fractions (at, %):Al 30.10, Ni 8.63, Fe 5.50, O 23.19, C 32.47, and Cr 0.11. In the Al-Ni coating, the Al content was the highest with the uneven distribution, there were some atom-poor zones, as shown in Fig.2 (c). The exist of atom-poor zones was due to the rapid cooling of laser spraying fusion pool in the crystallization process, the

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chemical composition of the solidified Al-Ni coating was too late to diffuse, resulting in uneven distribution and segregation. The Ni was uniformly distributed, as shown in Fig.2 (d). In the LTS process, although the argon was used to protect the Al-Ni coating, the coating surface was oxidized because of high temperature, so there was a

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certain amount of O. Compared with the C content in the powder Ⅰ in Fig.1 (b), the C had almost no change. The

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Fe content in the Al-Ni coating was high, which indicated that the chemical elements in the substrate were diffused into the Al-Ni coating in the LTS test. The morphology of Al-Ni coating Ⅰ surface is shown in Fig.3 (a). Compared with the coating Ⅰ, the coating Ⅰ surface was uneven, and accumulated many crystalline grains, no cracks. Fig.3 (b) shows the plane scanned analysis results of Al-Ni coating Ⅰ, the mass fractions (mass, %): Al 32.39, Ni 18.91, Fe 18.18, O 15.67, C 13.15, Mn 0.26, and Cr 0.18; the corresponding atomic fractions (at, %): Al 30.54, Ni 8.19, Fe 8.28, O 24.93, C 27.86, Mn 0.10, and Cr 0.10. The Al was unevenly distributed and formed the atom-rich zones on the grains accumulated

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ACCEPTED MANUSCRIPT zones, as shown in Fig.3 (c). The formed Al-Ni coating produced the fixed chippings on the coating due to the collisions, and formed a different degree of accumulation on the coating surface when the Al-Ni powders were sprayed again. The particle accumulations were mostly in the form of dots, and not associated with other surface

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defects. The Ni was still distributed uniformly, as shown in Fig.3 (d). The morphology of Al-Ni coating Ⅰ surface is shown in Fig.4 (a). The surface of coating Ⅰ was relatively smooth and the grains were compact. The cracks were reduced and the crack widths decreased compared with that

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of the coating Ⅰ. However, there was a small amount of un-dissolved particles. Fig.4 (b) shows the plane scanned

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analysis results of Al-Ni coating Ⅰ, the mass fractions (mass, %): Al 42.24, Ni 11.13, Fe 12.67, O 15.64, C 17.80, Mn 0.27, and Cr 0.14; the corresponding atomic fractions (at, %): Al 35.18, Ni 4.23, Fe 5.12, O 22.01, C 33.32, Mn 0.08, and Cr 0.06. Compared with those of coating Ⅰ and coating Ⅰ, the Al of the coating Ⅰ was the highest, the mass ratio of Al and Ni was close to 3.8. The reason was that the part of Ni was diffused into the substrate, led to

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the lower Ni content in the Al-Ni coating Ⅰ. The Al was uniformly distributed on the Al-Ni coating, as shown in Fig.4 (c). The Ni was also distributed uniformly, as shown in Fig.4 (d). 3.2.2 Plane scan of coating interface and crack analysis

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The diffusions of Al, Ni and Fe at the Al-Ni coatings were analyzed using a plane scan, and the results of plane

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scan analyses are shown in Figs.5-7. The Al-Ni coating interface was primarily composed of Al, Ni and Fe, while the O was the absorbed contaminants in the air, which was ignored. The plane scanned position of Al-Ni coating Ⅰinterface is shown in Fig.5 (a). The interface between the Al-Ni coating and the substrate had no obvious diffusion layer, the result of plane scan analysis of Al-Ni coating Ⅰ interface is shown in Fig.5 (b). The Al and Ni were distributed uniformly in the coating and was diffused at the coating interface to form the diffusion layer, as shown in Fig.5 (c) and (d). The Fe was originated from the substrate and was obviously diffused at the coating interface, as shown in Fig.5 (e).

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ACCEPTED MANUSCRIPT The plane scanned position of Al-Ni coating Ⅰ interface is shown in Fig.6 (a). The interface between the Al-Ni coating and the substrate had the obvious diffusion layer, the result of plane scan analysis of Al-Ni coating Ⅰ interface is shown in Fig.6 (b). The Al was distributed with the three-layered shape in the Al-Ni coating, forming

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the obvious diffusion layer at the Al-Ni coating interface, as shown in Fig.6 (c). The Ni was uniformly distributed in the Al-Ni coating and formed the diffusion layer at the Al-Ni coating interface, as shown in Fig.6 (d). The Fe came from the substrate was obviously diffused at the coating interface, as shown in Fig.6 (e).

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The plane scanned position of Al-Ni coating Ⅰ is shown in Fig.7 (a). There was the clear diffusion layer between

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the Al-Ni coating and the substrate, and the result of plane scan analysis of Al-Ni coating Ⅰ interface is shown in Fig.7 (b). The Al was also uniformly distributed at the Al-Ni coating interface, showing the three-layered shape, as shown in Fig.7 (c). The Ni was sparsely distributed in the Al-Ni coating and the diffusion layer, as shown in Fig.7 (d). The Fe was distributed was the three-layered shape at the Al-Ni coating interface, as shown in Fig.7 (e).

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In order to investigate the cracks of Al-Ni coating at the bonding interface, the Al-Ni coating interfaces were corroded with 5% nitric acid alcohol for 5 s, their micro-structures are shown in Fig.8. There was an observable micro-crack with the wide of 0-2 µm and length of ~600 µm at the coating Ⅰ interface, as shown in Fig.8 (a), while

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there was no obvious crack at the coating Ⅰ interface, as shown in Fig.8 (b). There was a micro-crack with the

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width of ~0.6 µm and length of ~ 400 µm at the coating Ⅰ interface, as shown in Fig.8 (c). The above cracks at the Al-Ni coating interface were the dendrite structures, which were perpendicular to the interface with the epitaxial growth.

The above cracks were related with residual stresses in the LTS test, from the 2θψ-sin2ψ regression curves, the residual stress of laser thermal sprayed Al-Ni coating Ⅰ, Ⅰ and Ⅰ was 118.1 MPa (Fig.9 (a)), 0 MPa (Fig.9 (b)) and 43.3 MPa (Fig.9 (c)). The residual stress of the coating Al-Ni Ⅰ was tensile stress, bringing about the observably surface crack in Fig.2 (a). The Al-Ni coating Ⅰ had no residual stress, had no cracks in Fig.3 (a). The residual stress

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ACCEPTED MANUSCRIPT of the coating Ⅰ was slight tensile stress, bringing about small surface cracks in Fig.4 (a). The Al-Ni coating Ⅰ had no crack, and it was due to the absence of thermal stress and phase transformation stress in the crystallization process, the grains in the Al-Ni coatings were bonded to each other without splitting. The cracks caused by the

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residual stress were primarily crystal cracks, which belong to hot cracks, due to the liquid film formed by low melting point eutectic (residual liquid) weakened the bonding between the grains and producing crack under the action of residual stress. As a result, the greater the tensile stress was, the greater the produced cracks widths were.

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Compared with the above analyses of plane scan and micro-structures, the residual stress of laser thermal

tensile residual stresses increasing.

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sprayed amorphous Al-Ni coatings led to producing the cracks, of which widths and lengths increased with the

The dilution rate was determined according to the geometrical parameters of the coating interface, i.e. dilution

λ=

Si Si + S r

(1)

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rate was

where Sr was the coating thickness; and Si was the melting depth of substrate.

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Dilution rate was an important index to evaluate the micro quality of laser thermal sprayed coatings [30]. It can be obtained from Eq.(1), the dilution rate of coating

(λ ), coating

(λ ) and coating

(λ ) was 0.2, 0.36, and

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0.14, respectively. The coating thickness was the main factor of affecting the dilution rate of Al-Ni coating [31]. The larger the Al coating thickness was, the larger the thermal shielding effect on the substrate surface was, the more heat absorbed, resulting in lower dilution rate and lower corrosion rate, thus improving the corrosion performance of Al-Ni coating [32-33]. 3.3 XRD analysis The high crystalline diffraction peaks of AlCrFe2, AlFe3 and Al-Ni alloy phases were detected at 44.12 ° in the coating Ⅰ, as shown in Fig.10 (a). This shows that preferred orientation existed in the Al-Ni coatings, developing 10

ACCEPTED MANUSCRIPT to these three phases. The crystal face (422) appeared a dispersive amorphous diffuse peak with the existed fault, which was detected for low crystallinity of AlFe3 and Al-Ni alloy, showing that the grains were in the early period of nucleation process at 81.24 °. The high crystal diffraction peak of AlCrFe2, AlFe3, Ni2MnAl and Al-Fe-Ni alloy

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phases were detected at 44.12 ° in the coating Ⅰ. The crystal face (400) appeared a completely dispersed amorphous diffusion peak at 63.9 °, the AlCrFe2, Al2Fe3, Ni2MnAl and Al-Fe-Ni alloy phases were found, as shown in Fig.10 (b). The high crystalline diffraction peaks of AlCrFe2, Al2Fe3 and Ni2MnAl at 44.06 ° was

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detected in the coating Ⅰ, The crystal face (400) appeared a completely dispersed amorphous diffusion peak at

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64.06 °, the AlCrFe2, Al2Fe3 and Ni2MnAl were detected, as shown in Fig.10 (c). Three kinds of Al-Ni coatings were detected the different phases of Al-Fe alloy (AlFe3, Al2Fe3), AlCrFe2 and Ni2MnAl, which indicated that the Al, Ni, Mn, Cr and Fe in the Al-Ni coatings were not only mutually diffused, but also combined to form new phases. The metallurgical bonding between the coating and the substrate was formed, improving the binding

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strength of interface and increasing the reliability of the coatings. At the same time, the amorphous diffusion peaks appeared in the three coatings, which showed a certain amount of amorphous Al-Fe alloy and Ni-Mn-Al

3.4 DSC analysis

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alloy were found in the coatings.

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Fig.11 shows that the DSC curves of the three coatings are had a larger endothermic peak at ~110 °, there was a corresponding small endothermic peak at ~500 oC, showing that amorphous coatings existed two crystallization processes and two exothermic reactions. As the crystallization temperatures increasing, the transition from amorphous to crystalline occurred, the grains were nucleated and grew up, and ultimately completed the crystallization process to form the crystal structure. The heat release of the first reaction was significantly higher than that of the second reaction. The amorphous alloy had no glass transition temperature Tg or no obvious Tg. The second peak on the DSC curve in the Fig.11 had no obvious initial temperature of crystallization indicated that the

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ACCEPTED MANUSCRIPT samples had a certain amount of amorphous component. The crystallization exothermic process of amorphous coatings was ~400-500 oC, and when the temperature increased to ~510 oC, the coatings began to melt and release heat. The DSC curve of the coating II was relatively smooth, the characteristics of absorption and heat release

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were not obvious, while the crystallization characteristics of the coating I and the coating III were relatively remarkable. 3.5 Immersion corrosion test

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Fig.12 (a) shows the immersion corrosion morphology of laser thermal sprayed Al-Ni coating I. The part of

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surface grains were lost, appeared shallow holes. The Al-Ni grains were connected on the coating surface, and uniformly distributed with the dense structure. Compared with that in Fig.2 (a), the cracks and pits were replaced by small granular corrosion products, the uneven distribution of corrosion products led to surface irregularities. Fig.12 (b) shows the plane scanned analysis results of Al-Ni coating I after immersion corrosion, the mass fractions

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(mass, %): Al 35.08, Ni 20.10, Fe 22.23, O 14.03, Na 5.45, and Cl 3.11; the corresponding atomic fractions (at, %): Al 39.30, Ni 10.66, Fe 12.42, O 27.43, Na 7.41 2.78, and Cl. As shown in Fig.12 (c)-(e), the distributions of Al, Ni and Fe were uneven, mainly due to pits and grains defected. The corrosion medium of Cl- entered the Al-Ni coating

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through the porosities, and because the pits and the cracks were covered by the corrosion products, the process of

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mass transfer in the crevice was slower, resulting in forming of occluded cells. The electrode corrosion and electric field ion migration process resulted in the imbalance of the anode and cathode current density on the different parts of the coating surface, causing the autocatalytic effect of local corrosion on the coating surface. The reaction was that the Cl- in the solution was firstly adsorbed on the defective oxide film on the coating surface, and destroyed the oxide film, resulting in appearing of anodic dissolution and produced Al3+, i.e. Al→Al3++3e-

(1)

The electrons released from the anode process and reacted with the O2 around the porosities and led to cathodic

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ACCEPTED MANUSCRIPT reaction as follows. O2+H2O+4e-→4OH-

(2)

The OH- in Eq.(2) was reacted with the Al3+ to produce Al (OH)3, leading to the pH value drops to acidity. Al3++OH-→Al(OH)3+3H+

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(3)

Al(OH)3 in Eq.(3) was further converted to insoluble Al2O3·nH2O covered on the Al-Ni coating surface. Al(OH)3=Al2O3+3H2O

(4)

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The Al2O3 film in Eq.(4) also blocked the corrosion of the Al-Ni coating.

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Fig.12 (a) shows the immersion corrosion morphology of laser thermal sprayed Al-Ni coating Ⅰ. The coating surface was very compact with no cracks and obvious porosities, and the grains packing phenomenon disappeared. Fig.13 (b) shows the plane scanned analysis results of Al-Ni coating Ⅰ after immersion corrosion, the mass fractions (mass, %): Al 13.08, Ni 24.86, Fe 23.23, O 31.03, Na 5.06, and Cl 2.74; the corresponding atomic

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fractions (at, %): Al 13.62, Ni 11.84, Fe 11.66, O 54.50, Na 6.18, and Cl 2.20. The Al, Ni, and Fe were evenly distributed, as shown in Fig.13 (c)-(e). Compared with the results in Fig.3, the Al content in the coating Ⅰ decreased, while the O increased greatly, the corrosion on the coating Ⅰ surface was more serious than that on the

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coating I surface. From Fig.11, large tensile stress existed in the coating Ⅰ, under the actions of residual stress and

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corrosion medium Cl-, the oxide film on the corrosion surface was destroyed. The destroyed surface formed the anode, while the as-sprayed formed the cathode. The Al on the anode formed Al3+ and dissolved, the current was flowed to the cathode. Because the area on the anode zone was much smaller than that of the cathode, the anode had a large current density, which further eroded the destroyed surface, resulting in a large area of corrosion products in the coating Ⅰ. Fig.14 (a) shows the immersion corrosion morphology of laser thermal sprayed Al-Ni coating Ⅰ. There was no crack on the coating surface, but the grain packing disappeared, and the porosities appeared. Fig.14 (b) shows the

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ACCEPTED MANUSCRIPT plane scanned analysis results of Al-Ni corrosion coating Ⅰ, the mass fractions (mass, %): Al 35.71, Ni 11.76, Fe 17.67, O 23.72, Na 7.30, and Cl 3.84; and the corresponding atomic fractions (at, %): Al 35.31, Ni 5.32, Fe 8.42, O 39.56, Na 8.47, and Cl 2.92. The Al had many barren spots, while the Ni and Fe were much evenly distributed, as

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shown in Fig.14 (c)-(e). The pitting corrosion in the local parts was the main form of corrosion in 3.5% NaCl solution, the pitting corrosion was caused by the corrosion of oxide film to form the corrosion cell. 3.6 Electrochemical properties

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The potentiostatic polarization curves of the S355 steel in 3.5% NaCl solution are shown in Fig.15 (a). The

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fitting slope of the upper anode curve was 0.03150 V/dec. from -1.181 ~ -1.114 V, and the lower cathode curve was -0.02641 V/dec. from -1.114~-1.074 V. The self corrosion potential was -1.153 V at the intersection of the two straight, the corresponding self corrosion current density was 0.009 A/m2. The technological parameters of electrochemical corrosion in 3.5% NaCl solution are shown in Tab.2. The important electrochemical parameters

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were fitted from the polarization curves, such as corrosion potential (Ecorr), corrosion current density (icorr), passivation region (Epass), passive current density (ipass), cathode area slope (bc), anode area slope (ba), and corrosion rate (Vcorr).

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The Tafel potentiodynamic polarization curves of the Al-Ni coatings and the substrate in 3.5% NaCl solution are

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shown in Fig.15 (b). The polarization curves of the three Al-Ni coatings in 3.5 % NaCl solution shifted positively. From Tab.2, it can be seen that the self corrosion potential of Al-Ni coatings shifted positively by 0.418, 0.240, and 0.426 V, respectively compared with that of the substrate, and the corresponding self corrosion current density also shifted positively by 0.0030, 0.0027, and 0.0062 A/m2, respectively. According to the anodic polarization curves of the coating Ⅰ and the coating Ⅰ, the potential perpendicularly raised in a range, the current density was almost unchanged, indicating that the coatings was passivated, and the curve of the coating Ⅰ had no obvious active passive transition region. The corrosion speed

14

ACCEPTED MANUSCRIPT V=

A • icorr n•F

(2)

where Icorr was the current density; A was the atomic weight of the metal; n was the valence of the metal; F was the Faraday constant.

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The corrosion speed in Eq.(2) was positively correlated with the current densities, which was sorted as follows: current density of substrate (icorrS) > current density of coating Ⅰ (icorrⅠ) >current density of coating Ⅰ (icorrⅠ) > current density of coating Ⅰ (icorrⅠ), the corresponding calculated corrosion speed of substrate, coating Ⅰ, coating

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Ⅰ and coating Ⅰ was 0.0560, 0.0388, 0.0472, 0.0248 mm/a, respectively, and the sequence was corrosion speed of

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substrate coating Ⅰ (VS) > corrosion speed of coating Ⅰ VⅠ> corrosion speed of (VⅠ)> corrosion speed of coating Ⅰ (VⅠ), which were related with the positively shifted polarization curves. The corrosion speed was inversely related with the corrosion resistance. Therefore, the three coatings improved the corrosion resistance of the substrate obviously, and the corrosion resistance of the coating Ⅰ was the best. This result was consistent with the

4 Conclusions

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microscopic analysis of dilution rates (λⅠ>λⅠ>λⅠ).

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The amorphous Al-Ni coatings with the Al and Ni mass ratios of 3:2, 3:1 and 4:1 are sprayed on S355 structural

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steel, which are composed of alloy peaks and amorphous peaks, among them, the alloy peaks are Al-Fe alloy, AlCrFe2 and Ni2MnAl, while the amorphous peaks are Al-Fe and Ni-Mn-Al alloys. The chemical elements are mutually diffused between the Al-Ni coating and the substrate to form the compound diffusion layer, leading to form the metallurgical bonding at the coating interface. The characteristic of DSC crystallization peaks is obvious, showing the high degree of amorphous Al-Ni coating is produced by LTS, which increases the corrosion resistance of Al-Ni the coatings by strengthening the grains structures. The residual stress of laser thermal sprayed amorphous Al-Ni coatings is tensile stress, which leads to producing the cracks, of which widths and lengths increase with the

15

ACCEPTED MANUSCRIPT corresponding tensile stresses increasing. The Tafel potentiodynamic polarization curves show that the corrosion speed of Al-Ni coating Ⅰ, Ⅰ and Ⅰ is 0.0388, 0.0472, 0.0248 mm/a, respectively, while that of substrate is 0.0560 mm/a, the passive zones of Al-Ni coatings shift positively, indicating that the Al-Ni coatings improve the corrosion

Acknowledgments

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resistance of substrate effectively.

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Financial support for this research by the Key Research and Development Project of Jiangsu Province

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(BE2016052) is gratefully acknowledged.

References

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[6] Taban E, Deleu E, Dhooge A, Kaluc E. 12.2 – A New potential for welded structures in modified X2CrNi12: characterization of dissimilar arc weld with S355 steel. Design Fabrication and Economy of Welded Structures, 2008, 1058: 511-517. [7] Hou GL, Zhao XQ, Zhou HD, Lu JJ, An YL, Chen JM, Yang J. Cavitation erosion of several oxy-fuel sprayed coatings tested in

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ACCEPTED MANUSCRIPT [13] Chen CY, Xie YC, Huang RZ, Deng SH, Ren ZM, Liao HL. On the role of oxide film’s cleaning effect into the metallurgical bonding during cold spray. Materials Letters, 2017, 210: 199-202 [14] Gisario A, Puopolo M, Venettacci S, Veniali F. Improvement of thermally sprayed WC–Co/NiCr coatings by surface laser processing. International Journal of Refractory Metals and Hard Materials, 2015, 52: 123-130 [15] Xu X, Mi G, Chen L, Xiong L, Jiang P, Shao XY, Wang CM. Research on microstructures and properties of Inconel 625 coatings obtained by laser cladding with wire. Journal of Alloys & Compounds, 2017, 715: 362-373

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[16] Choudhary R K, Mishra P, Kain V, Sing K, Kumar S. Scratch behavior of aluminum anodized in oxalic acid: Effect of anodizing

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aluminium by ultra-thin atomic layer deposited alumina coatings. Corrosion Science, 2016, 106: 16-24

[18] Wang K, Dong S, Huang Y, Qiu YZ. Magnetic and thermal properties of amorphous TbFeCo alloy films. Journal of Magnetism and Magnetic Materials, 2017, 434: 169–173 Chicinaş HF, Ababei G,

Gabor M, Marinca TF, Lupu N, Chicinaş I. A comparative study of the Fe-based

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[19] Neamţu BV,

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[21] Pei Y, Zhou G, Luan N, Zong B, Qiao M, Tao FF. Synthesis and catalysis of chemically reduced metal-metalloid amorphous alloys. Chemical Society Reviews, 2012, 41 (24): 8140-8162

[22] Hu Z, Fan Y, Chen Y. Preparation and characterisation of ultrafine amorphous alloy particles. Applied Physics A Materials Science and Processing, 1999, 68 (2): 225-229.

[23] Ma Z, Zhang L, Chen R, Xing W, Xu N. Preparation of Pd-B/TiO2, amorphous alloy catalysts and their performance on liquid-phase hydrogenation of pnitrophenol. Chemical Engineering Journal, 2008, 138 (1-3): 517-522 [24] Deng Y, Yang YY, Ge LY, Yang WZ, Xie KN. Preparation of magnetic Ni-P amorphous alloy microspheres and their catalytic

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performance towards thermal decomposition of ammonium perchlorate. Applied Surface Science, 2017, 425: 261–271 [25] Nagase T, Takemura M, Matsumuro M, Matsumoto M, Fujii Y. Design and microstructure analysis of globules in Al-Co-La-Pb immiscible alloys with an amorphous phase. Materials and Design, 2016, 117: 338-345 [26] Tan C, Zhu H, Kuang T, Shi J, Liu HW, Liu ZW. Laser cladding Al-based amorphous-nanocrystalline composite coatings on AZ80 magnesium alloy under water cooling condition. Journal of Alloys and Compounds, 2017, 690: 108-115

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[27] Wang JQ, Liu YH, Imhoff S, Chen N, Louzguine-Luzgin DV, Takeuchi A, Chen MW, Kato H, Perepezko JH, Inoue A. Enhance the thermal stability and glass forming ability of Al-based metallic glass by Ca minor-alloying. Intermetallics, 2012, 29: 35-40 [28] Lapierre-Boire LP, Blais C, Pelletier S, Chagnon F. Improvement of flow of an iron-copper-graphite powder mix through

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17

ACCEPTED MANUSCRIPT Tables Tab.1 Technological parameters of LTS

Values

Particle size/mesh

<200

Spraying distance/mm

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Parameters

80

250

Powder feeding speed/g·min−1

4

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Spot diameter/mm

8

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Laser focus distance/mm

1500

Argon gas speed/L/min

15

Workbench speed/g·min−1

8

Overlap ratio/%.

75

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Laser power/W

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ACCEPTED MANUSCRIPT Tab.2 Technological Parameters of electrochemical corrosion in 3.5%NaCl solution

Ecorr /V

icorr/A/m2

Epass/V

ipass/A/m2

bc/V/dec

ba/V/dec.

Vcorr/mm/a

Substrate

-1.153

0.0090

-

-

-0.0264

0.0315

0.0560

Coating

-0.735

0.0060

-0.503±0.089

1.655±0.899

-0.0804

0.0773

0.0388

Coating

-0.913

0.0063

-0.713±0.106

0.561±0.251

Coating

-0.727

0.0028

-

-

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Samples

0.0718

0.0472

-0.0933

0.0682

0.0248

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-0.0873

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ACCEPTED MANUSCRIPT Figures 8000 Al

7000

Counts/cps

6000 5000 4000 Ni

3000

Ni

1000 C

0

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2000

Ni

0

1

2

3

4

5

6

7

8

9

10

9

10

9

10

Energy/keV

(a) Morphology of powder Ⅰ

(b) EDS of powder Ⅰ

Al 4000

Counts/cps

3000

Ni

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2000

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5000

1000

Ni Ni

C

0

0

1

2

3

4

5

6

7

8

Energy/keV

(c) Morphology of powder Ⅰ

(d) EDS of powder Ⅰ

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Al

10000

Counts/cps

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12000

8000 6000 Ni

4000 C

2000 Ni C

0

0

Ni

1

2

3

4

5

6

7

8

Energy/keV

(f) EDS of powder Ⅰ

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(e) Morphology of powder Ⅰ

Fig.1 Morphologies and EDS analysis of amorphous Al-Ni powders with different ratios of Al and Ni

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ACCEPTED MANUSCRIPT 30000

Al

Counts/cps

25000 20000 15000 10000

Ni

5000

O C Cr

0

1

Fe Pt 2

3

4

5

6

7

8

9

10

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Energy/keV

(a) Plane scanned position

(b) Result of plane scan analysis

(c) Al content

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Cracks

(d) Ni content

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Fig.2 Plane scan analysis of Al-Ni coatingⅠ surface

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ACCEPTED MANUSCRIPT 35000 30000

Al

Counts/cps

25000 20000 15000 10000 5000

C O 0

Fe

Pt 1

2

3

4 5 6 Energy/keV

7

8

(b) Result of plane scan analysis

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(c) Al content

9

10

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Ni

(d) Ni content

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Fig.3 Plane scan analysis of Al-Ni coatingⅠ surface

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ACCEPTED MANUSCRIPT 20000

Al

Counts/cps

15000

10000 Ni O

5000 C 0

Fe

Cr

0

Pt 1

2

Ni Ni 3

4

5

6

7

8

9

10

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Energy/keV

(b) Result of plane scan analysis

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(a) Plane scanned position

(c) Al content

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Cracks

(d) Ni content

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Fig.4 Plane scan analysis of Al-Ni coating Ⅰ surface

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ACCEPTED MANUSCRIPT 6000

Substrate

Fe Al

5000

Diffusion layer Counts/cps

Fe

Al-Ni coating

4000 O 3000

Element Mass% At% Al 9.97 17.01 Ni 4.92 3.86 Fe 80.77 66.61 O 4.35 12.51

2000 1000

Ni

Ni

Ni 0

0

1

2

3

4

5

6

7

8

9

10

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Energy/keV

(b) Result of plane scan analysis

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(a) Plane scanned position

(c) Al content

(d) Ni content

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Fig.5 Plane scan analysis of Al-Ni coating Ⅰ interface

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(e) Fe content

ACCEPTED MANUSCRIPT 6000

Substrate

Fe 5000

Counts/cps

Diffusion layer

Al-Ni coating

Al

4000

Fe

3000

O

Element Mass% At% Al 9.86 17.20 Ni 3.09 2.54 Fe 83.74 70.57 O 3.30 9.69

2000 1000

Ni

Ni 0

0

1

Ni 2

3

4

5

6

7

8

9

10

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Energy/keV

(b) Result of plane scan analysis

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(a) Plane scanned position

(c) Al content

(d) Ni content

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Fig.6 Plane scan analysis of Al-Ni coating Ⅰ interface

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(e) Fe content

ACCEPTED MANUSCRIPT Substrate

8000

Diffusion layer

Fe

7000

Al Fe

6000

Counts/cps

Al-Ni coating

5000

O

4000

Element Al Ni Fe O

Mass% At% 10.09 17.36 2.35 1.86 83.70 69.58 3.86 11.20

3000 2000 1000 0

Ni

Ni 0

1

Ni 2

3

4

5

6

7

8

9

10

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Energy/keV

(b) Result of plane scan analysis

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(c) Al content

(d) Ni content

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Fig.7 Plane scan analysis of Al-Ni coating Ⅰ interface

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(e) Fe content

ACCEPTED MANUSCRIPT

Al-Ni coating

Al-Ni coating

Al-Ni coating

Crack Crack Diffusion layer

Diffusion layer Diffusion layer

Substrate

Substrate

(a) CoatingⅠ

(b) CoatingⅠ

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Substrate

(c) Coating Ⅰ

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Fig.8 Microstructures analysis of amorphous Al-Ni coating interfaces with different ratios of Al and Ni

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ACCEPTED MANUSCRIPT 1600

3000 o

1500 1000

1200 1000

Residual stress = 0 ± 2 MPa

800 600

2000 1500 1000

200

500

0

148

150

152

0

154

146

148

150

2θ/o

152

154

146

(a) CoatingⅠ

(b) CoatingⅠ

148

150

(c) Coating Ⅰ

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Fig.9 Residual stress analysis of amorphous Al-Ni coatings with different ratios of Al and Ni

28

152

2θ/o

2θ/o

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146

EP

0

Residual stress = 43.3 ± 60 MPa

0 o 25 o 35 o 45

2500

400

500

o

3000

AC C

Counts/cps

2000

0 o 25 o 35 o 45

Counts/cps

2500

3500 o

1400

Counts/cps

0 Residual stress =118.1 ± 89 MPa o 25 o 35 o 45

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ACCEPTED MANUSCRIPT




600

6000

79

80

81

2θ /

82

83

ο






3000

30




40

50

2 θ/ ο

60

70

80

600

7000

400

5000

0 61

62

63

4000 3000

1000 0 20

(a) CoatingⅠ

64

65

66

2θ / ο

♦

♦

 

 

30

♦








40

50

 

♦

800

AlMnNi

60

80

(b) Coating Ⅰ

63

64

65

66

2θ/ ο

3000

0 20

90

0 62

1000 70

400

4000

2000



600

200

5000

♦

30

♦

♦

 



 

40

2θ / ο

50

60

2θ / ο

(c) Coating Ⅰ

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Fig.10 XRD analysis of amorphous Al-Ni coatings with different ratios of Al and Ni

29

1000

(400)

 AlCrFe2  Al2Fe3

6000

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0 20




AlFeNi

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1000

8000

800

200

2000



♦

1000

EP

2000




 

70

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0

4000

♦

♦ AlCrFe2 Al2Fe3 Ni2MnAl

Counts/cps

400

200

5000

 

Intensity/a.u.

7000

Intensity/a.u.

800




AC C

Counts/cps

6000

8000

1000

 

Counts/cps




AlCrFe2 AlFe3 Ni2Al

(422)

 

C Intensity/ a.u.

7000

(400)

8000

80

90

ACCEPTED MANUSCRIPT 0.00

Al-Ni coating 1 Al-Ni coating 2 Al-Ni coating 3

Heat flow/W/g

-0.06

-0.12

-0.18

Tp1

0

100

Tp2 200

300

400

500

600

o

Temperature / C

AC C

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Fig.11 DSC analysis of amorphous Al-Ni coatings with different ratios of Al and Ni

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ACCEPTED MANUSCRIPT 12000 Al 10000

Counts/cps

8000 6000 4000

Pt

O

Fe

Ni 2000 Ni Ni

Cl 0 0

1

2

3

4

5

6

7

8

9

10

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EnergykeV

(b) Result of plane scan analysis

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(c) Al content

(d) Ni content

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Fig.12 Plane scan analysis of coating Ⅰ after immersion corrosion

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(e) Fe content

ACCEPTED MANUSCRIPT 3500 O

3000

Counts/cps

2500

Ni

2000 Na

Al

1500 Fe Ni

1000

Cl

Fe

500

Ni 0

0

1

2

3

4

5

6

7

8

9

10

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Energy/keV

(b) Result of plane scan analysis

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(c) Al content

(d) Ni content

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Fig.13 Plane scan analysis of coating II after immersion corrosion

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(e) Fe content

ACCEPTED MANUSCRIPT 8000

Al

7000

Counts/cps

6000 5000 4000 3000

O

2000

Ni Fe

Ni

1000

Cl

Fe

0 0

1

2

3

4

5

6

7

8

9

10

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Energy/keV

(b) Result of plane scan analysis

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(c) Al content

(d) Ni content

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Fig.14 Plane scan analysis of coating Ⅰ after immersion corrosion

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(e) Fe content

ACCEPTED MANUSCRIPT Equation

y = a + b*x

Adj. R-Square

-0.2

0.85956 Value

-0.6 -0.8

B

Intercept

B

Slope

Equation

y = a + b*x

Adj. R-Square

0.00454

0.03151

0.00265

E/V

-0.4 -0.6

0.72411 Value

-1.0

S355 steel Al-Ni coating 1 Al-Ni coating 2 Al-Ni coating 3

Standard Error

-1.04053

Standard Error

B

Intercept

-1.19335

0.00462

B

Slope

-0.02641

0.00282

E/V

-0.4

-0.8 -1.0

-1.2 -1.2

S355 steel linear fit of S355

-1.4 -1.6

-1.4 -1.6

-4

-3

-2

-2

-1

0

1

-8

-7

-6

-5

-4

-3

-2

-1

0

1

-2

logi/(A/cm )

(a) Linear fitting of S355 steel

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logi/(A/cm )

(b) Polarization curves of Al coatings and substrate

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Fig.15 Polarization curves analysis of amorphous Al-Ni coatings and substrate with different ratios

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ACCEPTED MANUSCRIPT

Highlights (1) The amorphous peaks of amorphous Al-Ni coating is composed of Al-Fe and Ni-Mn-Al. (2) The element diffusion of the Al-Ni coating-substrate forms metallurgical bonding at the coating interface.

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(3) The DSC peaks show that the higher degree of amorphous coating is produced by laser thermal spraying. (4) The tensile residual stress of Al-Ni coating induced by LTS produces the cracks.

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(5) The passive zone of laser thermal sprayed Al-Ni coating shifts positively, improving its corrosion performance.