Electrochemical corrosion behaviors and microhardness of laser thermal sprayed amorphous AlCrNi coating on S275JR steel

Optics and Laser Technology 118 (2019) 115–120

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Electrochemical corrosion behaviors and microhardness of laser thermal sprayed amorphous AlCrNi coating on S275JR steel Wu Yongzhonga, Zhang Donghuib, a b

T

⁎

School of Mechanical Engineering, Suzhou University of Science and Technology, Suzhou 215009, China ShangHai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800, China

H I GH L IG H T S

coating is composed of AlNi and NiCrFe in low crystallinity state. • Amorphous microhardness is 501.91 gf/mm higher than S275JR substrate. • Average curves shift positively with corrosion speed of 0.0058 mm/a. • Polarization • Resistance radius indicated the corrosion speed is far less than the substrate. 2

A R T I C LE I N FO

A B S T R A C T

Keywords: Laser thermal spraying (LTS) Amorphous AlCrNi coating Vickers microhardness Electrochemical corrosion

An amorphous AlCrNi coating was fabricated on S275JR structural steel using a laser thermal spraying (LTS) at the laser power of 1100 W. The surface and cross–section morphology, chemical elements and phases of the AlCrNi coating was analyzed using a scanning electron microscope (SEM), energy dispersive spectrometer (EDS), and X–ray diffractometer (XRD), respectively. The Vickers microhardness and electrochemical corrosion behavior of AlCrNi coating in 3.5% NaCl solution was investigated. The results show that the amorphous AlCrNi coatings primarily composed of AlNi, NiCrFe and AlFeNi phase in low crystallinity state, the metallurgical binding at the interface has been formed due to the mutual diffusion. The average microhardness of the laser thermal sprayed AlCrNi coating was 501.91 gf/μm2, which enhanced two times compared with S275JR substrate steel. The polarization curve of AlCrNi coating shifts positively, and the corresponding corrosion speed is 0.0058 mm/a meeting with the level 2 of international standard corrosion resistance.

1. Introduction As the development of industry in 21 century, the large-scale industrial equipment has become more and more advanced, and the technology and function of the equipment are becoming more complete. Meanwhile, industry application puts forward higher requirements for equipment maintenance [1]. Among them, the requirement of industrial materials has been a key limiting factor for many technological developments. For example, a large number of structural steels are used in aerospace, electric power, petrochemical industry, shipping, machinery, electronics, environmental protection and other industries [2]. It is difficult for traditional single steel to meet long-term service and maintenance standards. As a kind of alloy steel commonly used on chemical equipment and pressure vessel, the S275JR structural steel has good strength and stiffness [3]. In the long-term service, the failure caused by steel ⁎

fracture only accounts for ∼4%, while corrosion, wear, residual deformation and various fatigue failure accounts for ∼70%. Therefore, surface treatment of structural steel has become an important method to protect and prolong the service life of components. Commonly surface treatment methods include electroplating, coating, thermal spraying and so on. Among various surface thermal spraying methods, laser thermal spraying (LTS) uses a high–energy laser to melt alloy powders on the substrate, its thermal effect causes the thin layer of substrate melt, cool and solidify rapidly [4]. The fabrication of amorphous coatings have attracted much attention of researchers [5]. Among them, nickel-based amorphous coatings are one of the early successful amorphous coatings [6], which show unique short-range ordered arrangement (similar to clusters). Superalloys based on the Ni-Al-Cr system and comprising up to 12 alloying elements are widely used for components in the hottest sections of gas turbine engines [7]. AlCrNi has low electrode potential [8], high hardness and

Corresponding author. E-mail address: [email protected] (Z. Donghui).

https://doi.org/10.1016/j.optlastec.2019.05.004 Received 20 January 2019; Received in revised form 16 March 2019; Accepted 4 May 2019 Available online 15 May 2019 0030-3992/ © 2019 Elsevier Ltd. All rights reserved.

Optics and Laser Technology 118 (2019) 115–120

W. Yongzhong and Z. Donghui

corrosion resistance [9], which has great significance of carrying out the fabrication research of AlCrNi coating on S275JR steel using a LTS. At present, GenmiaoWang and WeihuaWang prepared the Al100−xFex (10 ≤ x ≤ 70) amorphous powders using ball milling [10]. Abrosimova fabricated the ribbons of amorphous Al88Ni6Y6, Al87Ni8Gd5 alloys by rapid melt quenching on a high-speed substrate, a thickness of the ribbons was 30–50 µm [11]. Wang et al. had added Ca in the Al-TMRE alloy system to form Al85−xY8Ni5Co2Cax system and re-melted it to prepare amorphous Al [12]. However, the research on amorphous AlCrNi coating prepared by LTS and liquid nitrogen rapid cooling technology has not been reported. In this study, an amorphous AlCrNi coating was fabricated on S275JR structural steel using a LTS under the liquid nitrogen cooling condition. The corrosion resistance and hardness of obtained coating was investigated using an electrochemical corrosion test in 3.5% NaCl solution and Vickers microhardness test, which provided an experimental basis for its applications on chemical equipment.

Table 1 Spraying parameters employed in the LTS process. Parameters

Values

Focal length/mm Defocusing amount/nm Laser wavelength/nm Spot diameter/mm Particle size/mesh Spray distance/mm Spraying scanning speed/mm/s Powder feeding rate/g·min−1 Argon gas velocity/L/min Overlap ratio/% Dip angle of structured light α/° Deviation angle indication light θ/°

410 +240 1064 4 < 200 70 3 6 5 50 40 2

1200–1300 K/s.

2. Experimental

3. Analysis and discussion

The substrate was S275JR structural steel with the European standard, its mass fractions (wt, %): C 0.127, Mn 0.062, P 0.245, S 0.033, N 0.010, Cu < 0.55, and the rest was Fe. The surface of the substrate was ground and sandblasted. The spraying material is AlCrNi powder, and its uniformity was guaranteed by ball milling and mixing with QM3SP04L ball milling machine. The LTS test was conducted on an Nd: YAG type LTS system, argon (Ar) gas was used as protective gas and power source of powder feeding, simultaneously, the liquid nitrogen (N) cooling system on the worktable was synchronously cooled online. The surface and cross–section morphologies and distributions of chemical elements of obtained AlCrNi coating were analyzed using a JSM–6360LA type scanning electron microscopy (SEM) and its configured energy dispersive spectrometer (EDS) after the LST test. And their phases were analyzed using a D/max 2500PC type X–ray diffractometer (XRD). Microhardness test was carried out on HMV-IT hardness tester with the load of 200 g. The electrochemical corrosion test was performed on a CS350 type electrochemical workstation, the sample dimension of 15 mm × 15 mm × 3 mm, the test method was potentiodynamic measurement, scanning rate of 1 mV/s, sampling frequency of 0.5 Hz, temperature of 25 °C, reference electrode of mercury/calomel–saturated KCl, potential range of −1600 ∼ 500 mV, and test time of 1800 s. And, the defocusing of positive quarter wavelength was corresponding to Rayleigh rule, and the peak-to-saddle ratio was more than 70%, which ensured that the laser energy was concentrated above the surface layer of the steel substrate. As shown in Fig. 1, the d (Defocusing amount) was +240 nm. The spraying parameters employed in the LTS process are shown in Table 1. And the fastest cooling speed occurred that the temperature of S275JR steel decreased from 1500 °C to 200–300 °C within one second, indicated the cooling speed can reach

3.1. Morphologies and EDS analysis of AlCrNi powder The high magnified morphology of AlCrNi powder is shown in Fig. 2(a). The powder mostly was spherical with the size of 1–5 μm. And, the dispersion and surface free energy of powders were small, and it was not easy to spontaneously adhere and agglomerate. The surface of the powders particles was smooth with low voidage, and the difference size made the smaller adhesion between particles, which further ensured the fluidity of the powder. The sprayed powder was AlCrNi with the mass fractions (wt, %): C 7.45, O 2.40, Al 40.15, Cr 31.87, Ni 18.13, as shown in Fig. 2(b).

3.2. Morphologies and EDS analysis of the AlCrNi coating surface and cross–section 3.2.1. Analysis of the AlCrNi coating surface The surface morphology of the AlCrNi coating fabricated by 1100 W LTS is shown in Fig. 3(a). The coating surface was smooth and impact with little dross, and the surface dross was due to the low fluidity of molten pool containing much O and C during spraying process. When molten metal solidified faster, it remain on the surface to form dross before it float out. As shown in Fig. 3(b), the mass fractions of the AlCrNi coating (wt, %): Al 32.39, Cr 23.80, Ni 18.91, Fe 11.87, O 6.45 and C 6.58. Plane scanned result shows that Al, Cr and Ni were the main components in the AlCrNi coating, with a small amount of Fe, O and C mixed. The substrate was the only source of Fe, and the C and O came from non-absolute vacuum environment.

3.2.2. Analysis of the AlCrNi coating cross–section The line scanned position and morphology of the AlCrNi coating was shown in Fig. 4, and the red horizontal line was the line scanned position of the coating. As shown in Fig. 4(a–d), the coating thickness was 600–700 μm, the internal structure of coating was impact, with no obvious defects. The internal structure of the coating was compact without obvious defects, and the macroscopic quality of the interface was excellent. The Al, Ni, Cr and Fe had obvious difference, the significant changes in element contents occurred at the interface. The Fe was the main element at the substrate which was adjacent to the interface. In the coating, the Al, Ni, Cr contents increased to a high level, and the Fe decreased substantially. From the starting change to dynamic stability of Al, Ni, Cr and Fe, it crossed ∼100 μm, indicating that the metallurgical bonding layer was formed by the mutual diffusion of Al and substrate.

¦È

d

Fig. 1. Schematic diagram of measuring defocusing distance. 116

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Al

1200

Counts/cps

1000

Cr

800 600

Ni

400 200 0

0

1

2

3

4

5

6

7

8

9

10

Energy/keV

(a) High magnification

(b) EDS of powder analysis

Fig. 2. Morphologies and EDS analysis of amorphous AlCrNi powders.

3.3. XRD analysis of the AlCrNi coating

3.4. Microhardness of the AlCrNi coating

The XRD spectra of AlCrNi coating fabricated at the laser power of 1100 W was shown in Fig. 5(a). The low crystal peaks of AlNi (JCPDS 44–1187) was detected at 16.94° on the crystal face (1 1 1). Moreover, the AlNi (JCPDS 44–1187), NiCrFe (JCPDS 35–1375) and AlFeNi (JCPDS 44–1126) was also detected at 37.02° on the crystal face (2 0 0), indicating that there existed a preferred orientation towards the two phases. The diffused amorphous peak appeared at 37.02°, indicating that some of the coating grains were not completely nucleated in the rapid cooling process, there were main of amorphous components with semi-disordered atomic structure in the interior. The crystallinity of AlNi, NiCrFe and AlFeNi detected from MDI jade 6 was 22%, 53% and 67%, respectively. In the Al-Cr-Ni ternary system, the cluster/site approximation takes into account short-range order, which is essential to satisfactorily describe the thermodynamics of order/disorder transitions such as occur between the fcc phases [13]. This is because the transition elements Fe, Al and Ni can exchange with each other so that they can exist synchronously in Fe-bearing intermetallic compounds [14]. Different degrees alloy phases of AlCrNi coating were detected in XRD spectra, which indicating that the Al, Ni, Cr and Fe had not only diffused with each other, but also formed new phases to improve its bonding strength and increase its reliability. By adding Cr, they could partly replace Al atoms in AlFeNi phase to form Al(Fe, Cr)Ni. The reasonability of the producing of Al(Fe, Cr)Ni phase was explained according to Gibbs phase rule [15].

The microhardness of laser thermal sprayed AlCrNi coating is shown in Fig. 6(a). In Vickers hardness test, the angle between two relative prisms of the Vickers indenter was diamond square quadrangular pyramid of 136°, which was the Vickers indenter. Vickers indenter was under 200 g load, and it was vertically pressing into the surface of the AlCrNi coating to result in indentation, and the strength per unit area was Vickers hardness. The calculation equation is as below:

HV =

P 2P sin(α /2) 1.8544P = = S d2 d2

(Kgf/mm2)

(1)

where HV was the Vickers hardness, and the unit was kgf/mm2; P was load, and the unit was kgf; S was indentation area, and the unit was mm2; d was diagonal length of indentation, which the unit was mm, α was angle between two relative prisms of indenter, i.e. 136°. The above equation had been extended to microhardness test [16], as follow:

HV =

1854.4P d2

(gf/μm2)

(2)

where HM was the Vickers microhardness, and the unit was kgf/μm2, P was load, and the unit was gf; d was diagonal length of indentation, which the unit was μm. The measured diagonal length of indentation on S275JR steel was ds of 41 μm. The diagonal length of three random indentation on AlCrNi coating were d1, d2, d3, the corresponding value were 27.79 μm, 27.02 μm, 26.78 μm, respectively. According to Eq. (2), the microhardness of one position on S275JR steel and three positions on AlCrNi

Al

800

Cr

Counts/cps

600

Ni 400

200

0

Ni C O

0

1

Fe

Fe

2

3

4

5

6

7

Energy/keV

(b) EDS of surface analysis

(a) Plane scanned position

Fig. 3. Plane scan analysis of AlCrNi coating surface. 117

8

9

10

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(a) Al

(b) Cr

(c) Ni

(d) Fe

Fig. 4. Line scan analysis of AlCrNi coating interface. 600

/kg/mm

2

500

Point 1 Point 2 480.37 Point 3 Substrate

508.13

517.24

400

300

220.63 200

100

0

Substrate

Point 1

Point 2

Fig. 5. XRD analysis of amorphous AlCrNi coating.

Point 3

Fig. 6. Microhardness of AlCrNi coating surface. 2

coating were 220.63, 480.37, 508.13, 517.24 gf/μm . The result showed that the average microhardness of AlCrNi coating was 501.91 gf/μm2, which enhanced more than 2 times compared with that of S275JR steel. The results indicated that the AlCrNi coating with relatively high hardness could be achieved via low concentration Cr atoms doping [17].

coating in 3.5% NaCl solution were shown in Fig. 7. The important data of electrochemical corrosion are shown in Table 2. These were fitted from the polarization curves, such as corrosion potential (Ecorr), corrosion current density (icorr), linear polarization resistance (RSP), passivation region (Epass), passive current density (ipass) and corrosion speed (Vcorr).

3.5. Electrochemical analysis

V= 3.5.1. Potentiostatic polarization curve The potentiostatic polarization curves of substrate and the AlCrNi

A × icorr n×F

(3)

where I was the current density; A was the atomic weight of metal; n 118

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Fig. 8. Nyquist spectra of AlCrNi coating at different power levels. Fig. 7. Polarization curves analysis of steel and amorphous AlCrNi coating fabricated on S275JR steel.

processes as shown in Fig. 9(a), which indicated that the steel substrate was in the process of continuous corrosion. The impedance spectra of the AlCrNi coating was consist of two relaxation processes, and the phase angle shows a maximum in the high frequency region, as shown in Fig. 9(b). The process of the AlCrNi coating started from initial corrosion to equilibrium gradually, which was due to the fact that the Al and Cr were easily oxidized to form a dense oxide film, and the formed shielding layer made the corrosion process stagnate. The phase angles less than −90° implies that the impedance spectra had not deviated from the ideal capacitive behavior. The coating gave the maximum phase angle in a low frequency region. The phase angle changed with the effects of test voltage and frequency. The test potential was between − 1.6 and 0.4 V, the phase angle on the Bode diagram decreased with the increasing of the test potential, and shifted to the high frequency region at the same time. In this process, the electrooxidation resistance of AlCrNi decreased with the increasing of test potential, which made the oxidation reaction of AlCrNi easier. As shown in Fig. 9, the Bode spectra of the coating fabricated at different powers shows that the slope of approximate linear impedance curves on the low frequency region gradually changed from −1 to 0 [20]. The process shows that the corrosion medium has penetrated into the interface of matrix metal through coating defects, and the penetration rate of corrosion medium was greatly limited due to the shielding effect of coating oxide film. The oxide shielding layer formed by Al and Cr can well insulate the medium and substrate, thus protecting S275JR structural steel from corrosion. The corrosion products of AlCrNi were formed uniformly on the whole surface, linking with the surface and cross-section analysis. Therefore the impedance spectrum of the AlCrNi coating can be explained by the equivalent circuit, as shown in Fig. 10. The impedance of the CPE (constant phase element) is given by the following equation [21].

was the valence of metal; F was the Faraday constant. Eq. (3) shows that the corrosion speed was positively correlated with current density and negatively correlated with polarization resistance. The results showed the relationship between current density and current density was icorrs > icorr2. The polarization resistance of AlCrNi coating was RSP of 2078 Ω. The corrosion speed were calculated: the corrosion speed of the substrate and the coating were 0.0560, 0.0058 mm/a, respectively. The AlCrNi coating fabricated at the laser power of 1100 W fully met with the level 2 of international standard corrosion resistance with the corrosion speed of 0.001–0.005 mm/a, which was in accordance with the forward movement of polarization curves. The corrosion resistance of the coating prepared by 1100 W laser power was mainly due to its low crystallinity and good diffusion effect, which made it obtain appropriate element ratio and uniform distribution. In conclusion, compared with the seawater corrosion resistance of laser thermal sprayed single Al coatings, the overall corrosion resistance of AlCrNi coating had been improved. The relationship between the corrosion rate and the above analysis was coincident, and the conclusion had been further verified. 3.5.2. Electrochemical impedance As shown in Fig. 8, the Nyquist spectra of AlCrNi coating and S275JR steel substrate immersed in 3.5% NaCl solution is obviously presented. The impedance spectra of AlCrNi coating was circular arc, which was corresponded to different time constant. The time constant (τ) was the frequency (fm) corresponding to the highest point of semicircle on impedance or admittance spectra [18], i.e. τ = 1/2πfm. The diameter of capacitance arc in impedance spectroscopy reflected the resistance capacity of electric charge transferring on the coating surface [19]. The larger the impedance and diameter of capacitance arc was, the smaller the corrosion current was, the slower the electrode reaction rate and the slower the corrosion rate was. The relationship between substrate and arc resistance radius of AlCrNi coating in high frequency region: rsub ≪ r1100, indicated the corrosion speed was far less than the substrate. The experimental Bode plots of the S275JR steel substrate and AlCrNi coating fabricated at 1100 W powers are shown in Fig. 9(a and b). The impedance spectra of substrate steel shows only one relaxation

ZCPE = (jω)α Q

(4)

In Eq. (4), j was the imaginary number unit and ω was the angular frequency (ω = 2πf, f being the frequency). α and Q are the CPE parameters, and the dimension of Q is F cm−2 sα−1. The fitted and measured results were fit very well to each other, indicated that the equivalent circuit was reliable.

Table 2 Technological parameters of electrochemical corrosion in 3.5% NaCl solution. Parameter

Ecorr /V

icorr/A/cm2

RSP/Ω

Epass/V

Log(ipass)/mA/cm2

bc/mV/dec.

ba/mV/dec.

Vcorr/mm/a

Substrate At laser power of 1100 W

−1.153 −0.784

0.0090 0.0016

– 2078

– −0.586 ± 0.05

– −4.467 ± 0.01

−0.0264 −0.1327

0.0315 0.1350

0.0560 0.0058

119

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(a) Substrate

(b) At laser power of 1100 W

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Fig. 10. Electrical equivalent circuits.

4. Conclusions (1) The amorphous AlCrNi coating fabricated at the laser powers of 1100 W is primarily composed of AlNi, NiCrFe and AlFeNi in low crystallinity state, the metallurgical binding at the interface has been formed due to the mutual diffusion. (2) The average hardness of laser thermal sprayed AlCrNi coating is 501.91 gf/mm2, which is more than twice as high as that of S275JR steel substrate. (3) The polarization curves of AlCrNi coating fabricated at the laser powers of 1100 W shift positively, and the corresponding corrosion speed is 0.0058 mm/a, among them, which met with the level 2 of international standard corrosion resistance. (4) The relationship between substrate and arc resistance radius of AlCrNi coating in high frequency region: rsub ≪ r1100, indicated the corrosion speed is far less than the substrate. The process of the AlCrNi coating starts from initial corrosion to equilibrium gradually, because the shielding layer formed by dense oxide films makes the corrosion process stagnate. References [1] X.H. Mehmeti, B. Mehmeti, R.R. Sejdiu, The equipment maintenance management in manufacturing enterprises, IFAC-PapersOnLine 51 (30) (2018) 800–802. [2] M. Nagaraj, B. Ravisankar, Enhancing the strength of structural steel through severe plastic deformation based thermomechanical treatment, Mater. Sci. Eng., A 738 (2018) 420–429. [3] Brnic Josip, Turkalj Goran, Niu Jitai, Marko Canadija, Domagoj Lanc, Analysis of experimental data on the behavior of steel S275JR – Reliability of modern design, Mater. Des. 47 (2013) 497–504.

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